2017 Group 1 Project

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2017 Projects: Group 1 - Delta | Group 2 - Duct | Group 3 - Beta | Group 4 - Alpha

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Histology of a section of human pancreas, stained with somatostatin antibodies to demonstrate the distribution and morphology of pancreatic delta cells throughout the islet, identifiable by their dark red cytosol.


Introduction

Pancreatic Islet Location, Structure and Function

Background

The Pancreas

The pancreas is a long, flattened gland located deep in the abdomen, with both exocrine and endocrine roles. Macroscopically, it is composed of five main parts: the head proper, the uncinate process, the neck, the body and the tail, however, microscopically it can be divided into exocrine acini and endocrine pancreatic islets, or Islets of Langerhans.[1] The purpose of the present page is to inform students about the structure and function of δ-cells- a specialised endocrine cell- in normal pancreatic activity, and the results of pathological dysfunction. However, for more information on the structure and function of the pancreas as an organ, please see the embedded video, below, or follow the link to a 3D Interactive video of the exocrine pancreas:

Histology of Pancreatic Islets
What does the pancreas do? by TED-Ed

360° Endocrine Cells Animation by Nucleus Medical Media

Pancreatic Islets

The Islets of Langerhans are tiny endocrine organs situated in the pancreas that are critical for glucose balance. Islets commonly comprise of four different types of secretory hormone-producing cells: β-cells, which produce insulin and amylin, the glucagon-containing α-cells, δ-cells producing somatostatin (SST) and also the pancreatic polypeptide-producing (PP) cells. Studies by Diabetes Research Institute claim that islets are mainly made up of approximately 70% β-cells, 20% α-cells, <10% δ-cells, and <5% PP cells, however, other studies have indicated that β-cells represent fewer than this estimate, with an increase in the reported numbers of α-cells.[2] In fact, human, monkey and mouse islets all exhibit distinctive functions that are interrelated with their different structural characteristics.[3]

Histologically, these cells cannot be discerned via convention H&E staining, and, indeed, δ-cells were not able to be discriminated from their α and β counterparts until more sophisticated staining was possible. This can be seen in the image, opposite, where the pancreatic islet cannot be properly discerned (A), and it is not until the use of SST-antibody stains that δ-cells become visibly apparent (B). For more information on pancreatic histology, this video by PathologyNOW is a good educational resource.

History

Paul Langerhans discovered pancreatic islets in 1869 as a medical student at the Friedrich Wilhelm University, Berlin; where he described them as “..small cells of almost perfect homogeneous content and of a polygonal form, with round nuclei, mostly lying together in pairs or small groups”. In 1893, Edouard Laguesse studied them in the human pancreas and established the term ‘islets of Langerhans’. He indicated that the islets might be responsible in adjusting glycaemic control by releasing internal secretions. Histological and histochemical staining techniques have been fully utilised by anatomists and pathologists to distinguish different categories of hormone-secreting cells of the pancreatic islets and their respective roles. The staining of these different endocrine cell types in the islets allows them to be differentiated visually from one another and also from other surrounding structures such as exocrine cells in the pancreas, neural tissue and stromal components.[4] A tetradecapeptide known as somatostatin (SST), a regulatory neurohormone discovered in 1973, has had a remarkable influence on the field of endocrinology. Somatostatin applied an immediate effect on the endocrine pancreas by obstructing insulin and glucagon release other than inhibiting the release of growth hormone. Introductory description of δ-cells was illustrated by Bloom in 1931. However, the function of δ-cells remains controversial ever since.[5] This history, along with other discoveries pertaining to pancreatic islets and SST is summarised in the table, below:

Year Discovery
1869 Paul Langerhans discovers islets of small, polygonal cells in pancreatic parenchyma, which had a round nucleus and no nucleolus. These cells stained differently and existed in pairs or small clusters within the surrounding acinar cells, with denser innervation than the rest of the pancreas[6]
1889 Von Mering and Minkowski found that removal of the pancreas causes diabetes in dog models[7]
1893 Gustave-Edouard Laguesse is the first to refer to pancreatic islets as the islets of Langerhans, suggests they may serve and endocrine function as reviewed by Ceranowicz et al. in 2015[8]
1902 Nichols reports first single pancreatic adenoma arising from islet tissue during autopsy[9]

Laguesse describes the detailed histological characteristics of islets preserved in an atrophied pancreas following ligation of the duct[10]

1907 Lane suggests diversity among pancreatic islet cells, discriminating α- and β-cells as reviewed by Baskin in 2015[4]
1922 Banting performed the first experiment on exogenous insulin use to treat a dog in a diabetic coma[11]
1931 Bloom is the first to describe δ-cells[5]
1957 Lacy and Davies discover β-cells produce insulin via immunohistochemistry[12]
1962 Hellman et al. used silver stains to discover α-cells are actually two distinct cell types- A1 and A2. He renamed A2 cells α-cells, and used immunohistochemistry to confirm they are the site of glucagon production[13]
1969 Extract from A1 cells from pigeon pancreases was shown to inhibit insulin in mouse models by Hellman and Lernmark [14]
1973 Braseau, Vale and Burgess discovered somatostatin-14 and its inhibitory effects on growth hormone in sheep hypothalami[15], which was re-named somatostatin (14) by Burgus et al.[16]
1974 Luft and colleagues proved the presence of somatostatin in A1 cells by immunofluorescence, and they were renamed δ-cells[17]

Somatostatin is used to treat patients with agromegaly, to block growth hormone secretion[18]

1975 Polak and colleagues demonstrate that δ-cells produce somatostatin via immunohistochemistry[19]
1977 Guillemin and Schally are awarded the Nobel Prize in Medicine and Physiology for their work on somatostatin

Larsson et al. make first clinical notation of a somatostatin-producing pancreatic tumour in a human[20] Itakura, Riggs and Boyer synthesised the somatostatin-14 gene and fused it with Escherichia coli beta-galactosidase gene on the plasmid and transformed the E. coli bacteria with chimeric plasmid DNA to produce the first functional human peptide via bacterial recombination[21]

1982 Human cDNA coding preprosomatostatin was isolated and cloned [22]

Development

Formation of the pancreas starts 26 days post-conception, with the organ tissue originating from distal foregut endoderm. Through various signalling processes, distal foregut endodermal cells differentiate into pancreatic-specified endodermal cells and finally form two buds on either side of the early duodenum. These two buds, ventral and dorsal, fuse during rotation of the gastrointestinal tract to form the head and tail respectively. The buds consist of multipotent progenitors that give rise to δ-cells as well as other endocrine, exocrine and structural components. Multipotent progenitor cells are those that can give rise to more than 1 type of cell but not all.

Figure 1. Cord-like structure consisting of murine islet cells

β-cells are the first to appear, followed by α- and δ-cells and finally PP-cells. Endocrine pancreatic cells continue to proliferate and increase in number throughout the developmental state. As stem cells are present throughout life, δ-cells along with the other pancreatic cells, are generated continually but an an increasingly slower rate. Initially, endocrine cells are dispersed but are generally situated around ducts. They begin to aggregate during weeks 13-15. At this stage, a vascular network starts to form within and around these clusters. Interaction with the extracellular matrix is attributed to formation of the distinct islets seen in an adult organ. One study suggests that adult human islets form through the fusion of two separate β-cell and α-δ-cell aggregates.[23] It states that α-δ-cell is islets fuse around β-cell islets, accounting for the centralised distribution of β-cells in mature islets. Another contrasts this with a random-fission model.[24] Branched cord-like structures of endocrine progenitor cells were identified along pancreatic blood vessels in murine embryos and neonates without presence of the spheroid islets seen in an adult organ. The cord-like structures decreased in number while spherical islets increased in number with progression of pancreatic development, suggesting that fission of branched structures was taking places. Fission occurs randomly and this may account for variation in the morphology of islets. Figure 1 illustrates the interconnected nature of islet cells prior to fission.


Notch Signalling

Figure 2. Notch signalling pathway in murine pancreatic development

The most well-accepted signalling pathway for pancreatic development is Notch signalling (Figure 2). Many reports have uncovered the crucial function of Notch signalling in specialising pancreatic features, cell proliferation, cell differentiation and also lineage commitment. This mechanism regulates the fate of pancreatic cells, especially of endocrine cells, by controlling the expression of neurogenin 3 (Ngn3), through lateral inhibition. The activation and inhibition of Notch signalling brings about different outcomes. Activation of this pathway in progenitors eliminates their ability to differentiate into endocrine and exocrine cells.[25][26] On the other hand, inhibition of this pathway results in early differentiation of the multipotent progenitor cells into endocrine cells. This, however, has been contested many times. Studies have demonstrated that differentiation of cells in the endocrine and exocrine lineage is Notch-dependent. That is, activation of Notch signalling pathways is necessary for pancreatic development.[27][28] Notch signalling, at intermediate levels, increases expression of SOX9 which is known activate the gene encoding Ngn3, thereby inducing endocrine differentiation from precursor cells.[28]

Figure 2 describes pancreatic development in mice. Endocrine cells are derivatives of Ngn-3-expressing precursors[29] referred to as multipotent progenitor cells, most of which are dedicated to the endocrine lineage. Differentiation of endocrine progenitor cells, the precursors, is regulated by Ngn3 action on multipotent progenitor cells. Ngn3-dependent differentiation of δ-cells begins at E14.5 in mice.[30] The equivalent of this is around day 47-50 in humans and is known as human embryonic stage CS20. With earlier expression of Ngn3, only α-cells are generated. There are two lineages that give rise to endocrine cells and these lie downstream of Ngn3-activated progenitor cells. δ-cells are derived from the same lineage as β-cells under the influence of paired box 4 (PAX4) while α-cell and PP-cell differentiation is regulated by aristaless related homebox gene (ARX).[31] PAX4 and ARX work in opposition, meaning that inactivation of the genes encoding either factor results in increased proliferation of cells in the opposing lineage.[32]

Variation Across Species

Figure 4. Proximity of δ-cells (cyan) to intra-islet blood vessels (blue)
Figure 3. δ-cell distribution across species (blue)

Pancreatic islet structure varies significantly across species. In the murine islet, β-cells are organised as a central core surrounded by δ-cells and the remaining endocrine cells (Figure 3C). An analysis of foetal rat pancreases at various ages showed the movement of δ-cells from an even distribution across islets at 12-13 days, to a peripheral organisation at day 17. They then migrated further, with some cells found in the pancreatic duct system. δ-cells appeared later than α-cells and around the same time as β-cells.[33] In human (Figure 3A) and monkey (Figure 3B) islets, δ-cells are found at varied, random distributions.[2] Porcine islets differ the most in that they consist of multiple units all with a β-cell core and peripherally located δ-cells (Figure 3D). Islet organisation at the cellular level has implications for signalling and function. As murine β-cells are close together, their Ca2+-dependent response is synchronised. This is not observed in the human pancreas.[2]


Literature varies in its description of the relationship between islet vasculature and cellular organisation. While one study demonstrated that all types endocrine cells are in contact with intra-islet capillaries (Figure 4)[2], another reviewed that β-cells are situated a significant distance away from vessels while δ-cells are immediately adjacent.[34] In both cases, δ-cells are situated along capillaries in no particular order relative to other cells.

Structure

Receptors

This section describes some of the most significant receptors associated with δ-cells. For explanations on their function, please follow the links provided below to the signalling section. Of those listed here, the signalling section focuses on receptors specific to δ-cells and details their function within the pancreas. β1 and β2 adrenergic receptors as well as melatonin receptors are a topic of current interest and introduce interesting phenomena associated with δ-cell function. Their applications are described in more detail under topics of interest.

δ-cell-specific Receptors

The following receptors are specific to δ-cells among the other endocrine cells of the pancreas.

Cholecystokinin B (CCKB) receptors have been found localised and in high numbers in δ-cells across porcine, rat, murine, bovine and human islets.[35][36] More specifically, the CCKB receptor protein is co-localised with somatostatin. Molecular weight differs across these species; human CCKB receptor proteins are the largest at around 115 kDa, porcine islets contained 50 kDa proteins, and both 50 and 80 kDA proteins were detected in murine δ-cells.[35] This image shows the distribution of CCKB receptors across murine, porcine and human islets. For more information about the CCKB receptor, please see this section.

Muscaranic acetylcholine receptors are part of the cholinergic signalling pathway and regulate δ-cell function when activated by endogenous acetylcholine. There are five forms of this receptor but only one, M1, is specific to δ-cells, as pictured here.[37] Acetylcholine has a positive effect on somatostatin secretion. To learn more about the M1 receptor, please see this section.

Patched1 transmembrane receptors are part of the Hedgehog pancreatic developmental pathway. In the human and mouse pancreas, they are specific to somatostatin-positive δ-cells.[38] To learn more about the Hedgehog signalling pathway and its relevance to δ-cells, please see this section.

Other Receptors

Figure 5. GRP120 (red) localised to δ-cells in pancreatic islets

β1 and β2 adrenergic receptors are found on many cells. They form part of the adrenergic system that involves adrenaline and noradrenaline release in response to changing physiological conditions. Activating this system increases synthesis and release of somatostatin by δ-cells through β-adrenergic agonism.[39]

G-protein coupled receptors 120 (GPR120) are also found on multiple cells however, in the mouse and human pancreas, they are predominantly localised to δ-cells (Figure 5). GPR120 agonists have been linked to a significant reduction in glucose-induced somatostatin secretion, indicating the receptor’s role in efficient cell function.[40] GPR120 is a free fatty acid receptor and is sometimes referred to as FFAR4.

Melatonin receptors 1 and 2 are membrane receptors non-specific to δ-cells through which melatonin is able to regulate secretion of various molecules. In human δ-cells, binding of melatonin to its receptors has an inhibitory effect on somatostatin release.[41]

Cytoplasmic Structures

Figure 6. δ-cell granules in a human pancreatic islet

Granules

δ-cells, like other endocrine islet cells, are granular. SST is stored in these granules and secreted via the process of exocytosis. Size and shape of granules is relatively consistent across species for all islet cells except δ-cells where it varies significantly between the mouse and human pancreas. Murine SST-containing granules are small relative to those in α- and β-cells, and are lozenge-shaped as visualised here[42] They have an estimated area of 0.11 ± 0.02 μm2. Human δ-cells display more spherical and electron-opaque structures that are relatively the same size as β-cells granules (Figure 6).[43] Somatostatin-containing granules in guinea pig islets are spherical structures around 150-250 nm in diameter.[44]

During the early stages of development, islet cells contain polyhormonal or multihormonal granules. That is, insulin-, glucagon- or somatostatin-producing cells contain granules that also consist of a combination of the other hormones. However, the cells’ primary hormone is found at a much greater concentration.[45]



Cytoplasmic Processes

There is extensive, constant cross-talk between cells of the pancreatic islet and this is crucial for efficient function. δ-cells communicate with other δ- and non-δ-cells throughout the islet via cytoplasmic processes.[44][46] δ-cells are able to account for their relatively low abundance by using these processes to contact multiple cells at once. As a result, a small number of δ-cells are responsible for regulating the secretory function of a great proportion of α- and β-cells.

Functions

Figure 7. 2D structure of somatostatin-14

Somatostatin Secretion

δ-cells secrete somatostatin, a peptide hormone responsible for the inhibition of glucagon and insulin release by pancreatic α- and β-cells, respectively. Somatostatin is known by various other names including growth hormone-inhibiting hormone (GHIH); growth hormone release-inhibiting hormone (GHRIH); somatotropin release-inhibiting hormone (SRIH) and somatotropin release-inhibiting factor (SRIF), but will be referred to as SST in this wiki. It should be noted that SST-secreting δ-cells exist outside the pancreas and have additional roles. The focus here will be on SST action within pancreatic islets.

Downregulation of insulin and glucagon secretion is central to hormone homeostasis and is dependent on paracrine signalling between cells of the islet. Islet cells in isolation show markedly reduced functional capacity compared to those in situ, indicating a strong association between intercellular signalling and efficient cell function.[47] There is also evidence that autocrine signalling plays a role in hormone homeostasis and this is explored later.

SST Isoforms

Somatostatin has two main isoforms; SST-14 (Figure 7) and SST-28. SST-14 is released by pancreatic δ-cells, neurons, and gastric cells while SST-28 is found in intestinal mucosal cells.[48] SST-14 and SST-28 are enzymatically cleaved from the C-terminus of prosomatosiatin (PSST) and PSST is derived from the large precursor molecule called preprosomatosiatin (PPSST). Islet cells exhibit different receptors for SST isoforms. For example, β-cells have been found to bind SST-28 while α-cells prefer SST-14.[49] When released into circulation, both have a very short half-life of less than one (1) minute.

Mechanisms of SST Action

Figure 8. Autocrine and paracrine functions of somatostatin

Somatostatin acts on a family five of G-protein coupled receptors named accordingly (sst1-sst5). SST exerts its inhibitory effects by binding to a combination of the sst1, sst2 and sst5 receptor sub-types. Each receptor is expressed at various distributions across islet cells. In the rat pancreas, sst2 and sst5 are found on all β-cells and on 80% of α-cells. The sst1 receptor is found only in 40% of α-cells but has been detected in every β-cell. Non-selective analogues of SST were found to regulate insulin and glucagon secretion without the need for co-stimulation of receptor sub-types.[50] δ-cell are thought to participate in an autocrine loop (Figure 8) to regulate their own secretions, indicating that they also express certain SST receptors.[51] This has been demonstrated using SST analogues[52] and is generally considered a possibility, but as δ-cell function is not completely understood, research is ongoing.

Secretion is triggered by changes in nutrient levels, in particular during high blood glucose states. Interestingly, insulin release is typically promoted in this state in conjunction with glucagon inhibition. The secretion of SST, an insulin-inhibiting molecule, therefore has led to hypotheses that it functions to “buffer” insulin and glucagon levels as a mechanism of maintaining hormone homeostasis.[34] Secretion of somatostatin-containing granules occurs via exocytosis. Relative to α- and β-cells, it is a rapid process that is Ca2+-dependent.[42]

Transdifferentiation

Perhaps one of the more interesting phenomena associated with δ-cells, transdifferentiation is part of three regenerative processes including reprogramming and dedifferentiation, and is described as the conversion of one cell type to another. δ-cells are able to engage this process as a means to compensate for the loss of β-cells and diminished insulin production in Type 1 Diabetes. δ-cells dedifferentiate, proliferate, and then redifferentiate into insulin-producing cells through natural mechanisms unique to juvenile (2 weeks old) mice. In adult mice, reprogramming of α-cells occurs instead.[53] It should be noted that δ-cell derived insulin-producing cells may not have identical functions to the original, endogenous pancreatic β-cells. Interestingly, only half of the dedifferentiated δ-cell progeny undergoes redifferentiation to produce insulin while the others convert back into δ-cells. This suggests that one δ-cell, through this process, is able to create one insulin-producing and one somatostatin-producing cell.[53]

Signalling processes that occur during transdifferentiation are complex. This wiki will very briefly describe the role of two significant molecules involved; Forkhead box O1 (FOXO1) and Ngn3. FOXO1 is a transcription factor involved in inhibiting cell proliferation, and the senescence stage of the cell cycle.[54] Ngn3, also a transcription factor, facilitates transdifferentiation. FOXO1 has a direct effect on Ngn3; downregulation of FOXO1 was found to upregulate expression of Ngn3 in the human foetal pancreas.[55] Although FOXO1 downregulation and subsequent reprogramming happens in juvenile mice, transient inhibition of FOXO1 in adult mice is able to trigger δ-cell conversion.[53] The difference between adult and juvenile mice is thought to be the result of developmental processes still occurring in young mice but having stopped at a certain point before adult life.[53] This is centred around the hypothesis that terminal differentiation or lineage commitment has yet to occur, facilitating an easier switch in lineage.

Although the mouse and human pancreas differ significantly, evidence of transdifferentiation as a compensatory mechanism has also been found in the pancreas of human patients with Type 1 Diabetes.[56]

Signaling

Paracrine and Autocrine Signals

Islet cells need to be alert regarding the functional significance of neighbouring cells via paracrine interactions. The recognised signalling pathways may be considered as beneficial targets for treating a harmful disease, diabetes. One of the most remarkable variations between human and mouse islets is that in the human islet, different types of endocrine cells interact more with each other. In comparison, most β-cells in the mouse islet are only in adjacent to other β-cells, whereas in the human islet, the majority of β-cells may be in association with α-cells, δ-cells, or both. Therefore, these arrangements may demonstrate that paracrine interactions perform an influential role in managing hormone release in the human pancreatic islet.[57]

To be more effective, paracrine signalling operates in a close proximity between origin and target cells as many signalling molecules are immediately deteriorated in the bloodstream and also in the interstitial space. Considering that δ-cells in human islets interact both with α- and β-cells, is can be assumed that δ-cells are influenced by paracrine signals. However, knowledge of these signals is not yet fully established and how they intend to merge with glucose stimulation to regulate somatostatin release requires further research. Various signals interfering with endocrine cells influence electrical activity of the cell membrane. Islet endocrine cells present voltage-gated ion channels and complex electrical activity in addition to action potentials. α-, β- and δ-cells in human islets exhibit comparable ion channels. However, their electrical actions rely upon a subtle balance between hyperpolarising and depolarising effects.[57]

The autocrine signals emphasise the impacts created by the initial perturbation. Additionally, autocrine loops with positive feedback are capable of making intra-islet signalling extremely unstable and bring about irrelevant responses as a result. This is blocked by negative feedback methods. Furthermore, hormone release may be actively inhibited within the islet with a decrease of the stimulus, or perturbation. In fact, somatostatin release from δ-cells has been aimed to perform this role. Somatostatin interferes with both glucagon and insulin release. Autocrine loops with positive feedback are embedded within and also regulated by comprehensive negative feedback loops. These principles supply a theoretical framework to analyse the signalling pathways associated with orchestrating hormone release from the human islet.[57]

Overview of Cell Signaling

This image represents both paracrine and autocrine pathhways in the human pancreatic islets. Please watch the embedded video above for a better understanding of the paracrine and autocrine interactions.

Glutamate

Glutamate has been known as an islet paracrine signal for nearly 20 years. However, the primary function of glutamate in the islet remains uncertain. Nevertheless, glutamate must be secreted from islet cells to be appropriate for paracrine or autocrine signalling. In recent findings, it has been proposed that glutamate secreted via membrane glutamate transporters by uptake reversal originates from metabolic pools. These findings, however, require confirmation in the human islets. Additionally, it is believed that numerous mechanisms for glutamate release may be present. Glutamate is revealed to trigger δ-cells via AMPA/kainite receptors other than being responsible for functional roles in α- and β-cells both via AMPA/kainite receptors and metabotropic receptors. However, further research is definitely needed to clearly describe the functional role of glutamate in islets.[57]

γ-Aminobutyric acid (GABA)

The high content of GABA in endocrine cells of the human islet is in correspondence with vigorous expression in these cells of glutamic acid decarboxylase type 65 (GAD65), which is a type enzyme involved in catalyzing modification of glutamate to GABA. GABA is capable of activating ionotropic GABAA receptor once it has been secreted. Human δ-cells along with α- and β-cells express functional GABAA receptors. Stimulating these receptors results in an increase of Cl conductance that propels the membrane potential in approaching the equilibrium potential for Cl. The Cl equilibrium potential differs from cell to cell and is determined by the expression and action of Cl transporters. However, it is remarkably interesting that GABA inhibits the electrical activity of α-cells although it depolarizes δ-cells and β-cells. In addition, GABA may also be responsible for monitoring the release somatostatin and glucagon in human islets. Therefore, GABA secreted from β-cells may stimulate a negative feedback loop associated with δ-cells as somatostatin is known to be a powerful inhibitor of insulin and glucagon release. However, these signals are not yet fully established and need to be further explored.[57]

2-Minute Neuroscience GABA

Hedgehog Pathways

Hedgehog Signaling Pathway

The Hedgehog pathway performs a significant function throughout pancreatic development. Its inactivation is fundamental to ensure proper organogenesis and involvement of expression of pancreatic marker genes in the process. Patched1 (Ptch1) is a transmembrane receptor which executes a major role in the hedgehog pathway. In addition, the Hedgehog pathway is associated with the maintenance of an accurate endocrine morphogenesis and function in murine adult models.[58]

Three types of hedgehog genes, Sonic hedgehog (Shh), Indian hedgehog and Desert hedgehog, have been discovered in mammals and also code for released proteins which attach to receptors, Ptch1 and Ptch2. However, hedgehog ligands are expressed distinctively in the embryonic and adult pancreas. Shh is missing entirely throughout the development of the pancreas, both in embryonic and adult. In contrast, low levels of Indian hedgehog and Ptch1 receptor are represented in the epithelium of a growing pancreas as early as E13.5 and also in the islets of adult mice.[58]

Ptch1 functions both as the receptor for hedgehog constituents and as a hedgehog signalling suppressor which leads to hedgehog signalling reduction in a negative feedback loop. Based on the findings, expression of the inhibitory Ptch1 receptor takes place in δ-cells of both mouse and human pancreas and is absolutely not influenced by streptozotocin treatment in mice. In C57BL/6J mice, the receptor is detected in every somatostatin-positive δ-cell, however, not in insulin- or glucagon-positive cells. Similar results were also observed in the human pancreas.[58]

Capacitance Measurements of Exocytosis

Capacitance measurements of exocytosis were utilised in order to functionally recognise α-, β- and δ-cells present in murine pancreatic islets. Two kinetically discrete stages of capacitance increase are displayed by exocytosis in the somatostatin-secreting δ-cells which comprised of an initial fast (600 granules s-1) component and a constant slower (60 granules s-1) component. It is established that different categories of cells in the islet display specific exocytotic structures.[59]

α- and δ-cells are not electrically connected and therefore measurements of cell capacitance in these respective cells within the islet are direct. Endocytosis is a slow process in comparison to exocytosis and that being the case, no effort was initiated to make up for it. It is essential to identify the diameter of secretory granules in the particular cells of the endocrine islet in order to translate the monitored cell capacitance increments to a number of granules secreted. δ-cells granules can be distinguished from other endocrine islet cells on a micrograph as they seem to be more lengthened. A combination of individual secretory granules will generate capacitance increment of 1.8, 2.9 and 1.1 fF in α-, β- and δ-cells respectively when a detailed membrane capacitance of 10 fF µm−2 with the assumption of spherical geometry for α- and β-cells is being used.[59]

Somatostatin restricts exocytosis equivalently regardless of whether it is recognised as a cell capacitance increment or as a hormone secreted determined by biochemical assay. δ-cells were remarkable in performing an outstanding primary component of capacitance increment that was accomplished in less than 350ms compared to the other endocrine islet cells. In this pool, the secretory granules encountered exocytosis with a minimum latency of 10 to 40ms. Therefore, it contributes to an increase in a fast component of capacitance increment with extremely fast reactions of (600 fF s−1). The elevated speed of exocytosis and efficient initiation in δ-cells indicate that several granules must be located in the close proximity of the Ca2+ channels. There is a high probability that exocytosis in δ-cells is not solely based on Ca2+ influx through plasmalemmal Ca2+ channels as it can be seen observed that exocytosis in δ-cells contradict to that in α- and β-cells in that they progress after the depolarization.[59]

Electrical Activity and Release of Somatostatin

Glucose and tolbutamide trigger somatostatin release from human islets and in contrast, somatostatin secretion is hindered by diazoxide. Irregular electrical acitivity is initiated by human δ-cells, which is further intensified by tolbutamide although uninfluenced by glucose. δ-cells comprised of a tolbutamide-insensitive, Ba2+-sensitive internally adjusting K+ current and also two different kinds of voltage-gated K+ currents. These two types of voltage-gated K+ currents are very reactive to tetraethylammonium/stromaxin and also 4-aminopyridine.[60]

The rapid upstroke of action potential is aided by voltage-gated tetrodoxin (TTX)-sensitive Na+ currents. However, TTX had no influence over somatostatin secretion. Ca2+ channels in δ-cells are obstructed by isradipine, ω-agatoxin and NNC 55-0396. The obstruction of either one of these channels restricts δ-cell electrical activity and simultaneously terminates somatostatin secretion triggered by glucose. The influence of glucose and tolbutamide in somatostatin secrection is abolished in adults diagnosed with type 1 and type 2 diabetes. This resulted in weakened prandial suppression of glucagon release.[60]

Cell-matrix interactions

Pancreatic α- and β-cells colocalise in an alignment that promotes precise allocation of glucagon and insulin release from islets. Glucagon, insulin and islet amyloid polypeptide are among the few islet indicators that fit the standard description of a hormone as a ‘factor that is distributed into the general passage to alert the body at a distance’, an abundant collection of paracrine and neural intercommunications occur within the islet to establish stable command throughout their release. Pancreatic δ-cells, however, are the third-most typical hormone-producing cell category within the islets.[61]

The peptide hormone, Urocortin3 (Ucn3) is generously exhibited by mature β-cells. However, its functional role remains unidentified. It is demonstrated that Ucn3 is reserved and co–released with insulin and subsequently potentiates glucose stimulation of somatostatin by means of an associated receptor on δ-cells. In addition, it is strongly established that islets with inadequate numbers of endogenous Ucn3 expressed less δ-cells, lessened somatostatin composition, diminished somatostatin substance released and excessive insulin secretion, and that these drawbacks are repaired by synthetic Ucn3 in vitro. A few studies demonstrate that the paracrine performances of Ucn3 trigger a negative feedback loop that stimulates somatostatin release to assure the decline of insulin release over time in order to stabilise level of plasma glucose. δ-cells contribute a fundamental negative feedback to this whole procedure by suppressing the release of both insulin and glucagon. Therefore, it guarantees that the endocrine output is readjusted simultaneously to the return of plasma glucose to its point of homeostasis.[62]

Cholecystokinin B receptor (CKKBR)

Expression of CCKB receptors was observed in every single tissue originating from the pancreas. It can also be found in the majority of extrapancreatic tissues and tumours. Quite the opposite, CCKA receptors displayed a sequence of expression particularly in brain, gall bladder, intestine, ovary, spleen, and thymus. Most importantly, CCKA receptors were expressed particularly in every pancreatic adenocarcinoma, but not by any means in any healthy pancreas specimens.[63]

In a restricted amount of studies with human tissues, the CCKB receptor has been observed on healthy pancreas and also pancreatic adenocarcinomas. Regardless, the latter is established primarily on information discovered from immortalised cancer cell lines. Further studies utilising Northern blotting and ligand binding uncovered CCKB without the presence of CCKA receptors on a normal human pancreas. Therefore, it indicates that CCKB is independently expressed in that particular tissue. Additionally, expression of CCKB receptors has been detected on malignant tissue, however, not on a normal pancreas based on studies conducted in several non-human pancreatic cancer models. Therefore, the correlation between different models of pancreatic cancer in animals and humans remains disputable.[63]

Muscarinic Acetylcholine receptors

Muscarinic Acetylcholine receptors act in response to acetylcholine. Acetylcholine is a type of neurotransmitter, which triggers muscarinic receptors that target in sustaining metabolic performance, along with glucose homeostasis. It is established that acetylcholine brings about various effects in the human islet and this is apparently more complicated than predicted as of multiple studies carried out on cell lines and rodent pancreatic islets.[64]

It is demonstrated that endogenous acetylcholine triggers β-cells and δ-cells via the muscarinic acetylcholine receptors with three different subtypes. M3 and M5 receptors are responsible for insulin-secreting β-cells. On the other hand, M1 receptors are accountable of somatostatin-secreting δ-cells which are also a powerful inhibitor of β-cells, therefore cholinergic input to the δ-cells consequentially modulates the role of β-cells. In fact, somatostatin release gradually reduced and shows an unpredictable rise in insulin secretion as soon as all muscarinic signalling was obstructed, implying a decreased inhibitory input to β-cells.[64]

Quite the reverse to the sequence of expression of M3 and M5 receptors, a few number of cells were immunostained for M1 receptors which represents somatostatin secreting δ-cells. Experiments have shown that M1 receptors are specifically expressed in δ-cells of human pancreatic islets.[64]

Ghrelin stimulates the response of intracellular calcium as indicated by GCaMP6 in intact islets

Ghrelin

Ghsr is specifically expressed in δ-cells

A depth and precise interpretation of ghrelin’s performance within pancreatic islets is remarkably significant considering that ghrelin executes primary role in monitoring energy and regulating glucose metabolism. Based on the findings, it is shown that ghrelin elevates the level of intracellular calcium in δ-cells as shown in the intact islets of a mouse, indicated by GCaMP6. The level of intracellular calcium rises as a consequence of the stimulation of GHS-1R on δ-cells, and, given that calcium is essential in hormone release, the study is further continued to ascertain whether or not ghrelin stimulates somatostatin release. According to the results achieved, it has been established that ghrelin vigorously heightens the level of glucose-stimulated somatostatin secretion (GSSS) in mouse islets and both static and perfusion analysis of human islets.[61]

Additionally, the actions of endogenous Ucn3 with the CRHR2-selective antagonist Astressin2b is blocked to deduce if the performance of ghrelin is subjected to the response conveyed by Ucn3, which is derived from β-cells. It has been revealed that Astressin2b reduced GSSS, but did not restrain the role of ghrelin in GSSS. Therefore, this implies that ghrelin is involved with δ-cells directly, uninfluenced by Ucn3.[61]

Pathology

Diagnostic Methods for Pancreatitis: A: CT Scan B: Laboratory Testing C: Macroscopic Specimen D: Microscopic Slide

The function of pancreatic δ-cells in endocrine secretion of somatostatin is not unique to this cell type, with somatostatin-producing populations within the liver, gastrointestinal tract, lungs, immune system, kidneys, adrenal glands and urogenital tracts.[65] Therefore, whilst there are pathologies relative to somatostatin dysfunction, this may not be representative of pancreatic δ-cell pathologies. The following are diseases that have been implicated to have associations either directly with pancreatic δ-cell abnormalities, or via changes in somatostatin expression in the pancreas. Whilst it is recognised that this does not represent truly the dysfunction of this one cell population, the complexities of the pathological processes are recognised, and the treatment of these diseases with somatostatin or somatostatin-analogues represent the potential for the manipulation of pancreatic δ-cells in the development of future treatment options.

Pancreatitis

About

Pancreatitis is the inflammation and autodigestion of the pancreas, which arrises when pancreatic zymogens undergo premature intra-pancreatic activation and destroy the tissues of the pancreas itself.[66] This process leads to inflammation which may be acute or represent an ongoing, chronic injury, and can be caused by many factors such as autoimmune disease, trauma, cholelithiasis and alcohol abuse or occur idiopathically.[67] Diagnosis is usually achieved through a combination of obtaining a medical history, physical examination and diagnostic testing, including- among others- a complete blood count, comprehensive metabolic panel and urinalysis[68], or imaging such as chest and abdominal radiographs, CT, MRI or ultrasound[67]. Such testing is necessary to establish a diagnosis as symptoms are non-specific, and there are many possible differential diagnoses. A summary of symptoms and their mechanisms of cause are listed in the table, below:


Signs and Symptoms

Signs and Symptoms: Underlying Pathological Mechanism:
Acute or severe epigastric pain that radiates to the back and gets worse after eating The pancreas is a richly innervated organ, receiving nerve supply from vagal, right and left splanchnic nerves and the coeliac ganglion. Autodigestion of the pancreas causes severe pain during the pathogenesis, with radiating pain to the desmosomes also innervated by these nerves (T6-9). Pain often gets worse following food as the body stimulates further zymogen production for food digestion, worsening the acute pancreatitis pathogenesis.[69]
Abdominal tenderness on palpation Abdominal tenderness occurs as a result of irritation of the parietal peritoneum during pathogenesis. Should this go untreated, irritation or inflammation stimulates pain fibers within the parietal peritoneum and abdominal wall cause muscular rigidity.[70]
Fever Inflammatory cytokines released during pathogenesis e.g. TNF-a, IL-6 act as pyrogens stimulating the release of prostaglandin E2 (PGE2) which acts on the hypothalamus. The hypothalamus releases norepinephrine to increase thermogenesis in brown adipose tissue, and acetylcholine to stimulate muscle to raise the metabolic rate. Peripheral vasoconstriction also occurs to reduce heat loss through the skin, and shivering can begin to utilise muscle movements for heat production.[71]
Rapid pulse The exact mechanisms by which tachycardia occurs during pancreatits are not fully understood, but some proposed mechanisms include metabolic disturbances e.g. endogenous insulin-induced hypoglycemia, direct injury to the pericardium, hemodynamic instability, or increased sympathetic activity.[72]
Nausea, vomiting and anorexia Peripancreatic inflammation can extend to the posterior gastric wall and localized or generalized ileus can cause nausea and anorexia.[73] Such irritation within these portions of the gastrointestinal tract generates afferent impulses to the vomiting center in the medulla oblongata, initiating efferent motor messages via CNV, VII, IX & XII to the duodenum and the stomach. The lower esophageal sphincter relaxes, and the vomitus moves from the stomach to the esophagus and inspiratory and abdominal contractions to expel into the mouth.[74]
Steatorrhea, bloating, dyspepsia, and diarrhoea Structural damage to the pancreatic parenchyma via inflammation occurs, eventually causing a marked reduction in the exocrine and endocrine function of the organ. Fat malabsorption- or steatorrhea- can result, presenting as fecal excretion of fat, abdominal discomfort, and abdominal distension from gas. The steatorrhoea stools are characterised as loose, malodorous and greasy.[75] Pancreatic enzyme deficiency-particularly pancreatic lipase deficiency- is a result of destruction of functional tissue due to inflammation, and can cause persistent dyspepsia by impairing digestion.[76]Typically, disruption of over 90 percent of the pancreatic parenchyma is needed to result in such diarrhoea.[75]
Weight loss Nausea and vomiting leads patients to ultimately eat less food. Food that is consumed is malabsorbed due to decreased exocrine function of the pancreas. Fat malabsorption means patients are not able to absorb energy from any foods that they are eating, resulting in weight loss.[75]

Pathogenesis

Video Explanation of Acute Pancreatitis Pathogenesis by Osmosis:  
Video Explanation of Chronic Pancreatitis Pathogenesis by Osmosis:  

Pancreatitis is a disease defined as acute or chronic inflammatory process of the pancreas characterized by premature activation of digestive enzymes within the pancreatic acinar cells and causing pancreatic auto-digestion. In pancreatitis, a local inflammatory process initiated by release of pro- and anti-inflammatory cytokines and chemokines recruits granulocytes, monocytes and lymphocytes. The he pathogenesis of chronic pancreatitis is not fully understood, but it is believed that repeated episode of acute damage lead chronic pancreatitis. Recurrent pancreatic injury leads to scarring and remodeling that promotes fibrosis as well as calcification, and these calcifications develop into stones found within the tissue or pancreatic duct. The main causes of pancreatitis are include obstruction in the main pancreatic duct, gallstones, alcohol misuse, smoking, hypercalcemia, hyperparathyroidism, drugs like valproate, thiazide toxicity, and genetic mutations. During pancreatic injury, atrophic acinar cells activate several inflammatory key players like macrophages and granulocytes which release a number of pro-inflammatory cytokines such as IL-1,6,8,18 and 33, and tumor necrosis factor (TNF)-α. These pro-inflammatory cytokines further activate pancreatic stellate cells (PSCs) to promote chronic pancreatitis.[77]

Treatment and the Relation to δ-Cells

As SST and its associated analogues are inhibitors of pancreatic exocrine secretion, they have been used in the treatment of pancreatitis. Administration of SST or octeotide to patients with acute pancreatitis results in a significant reduction in mortality without incurring complications, and it has been shown that SST treatment for chronic pancreatitis provides a lower risk of complications.[78]

By extension, targeting δ-cells to up-regulate their SST production could be a potential therapy for those who suffer from pancreatitis. By increasing the number or function of pancreatic δ-cells, paracrine effects of SST could inhibit pathogenesis.

Histology of Diabetic Islets

Diabetes Mellitus

About

Diabetes mellitus is a heterogeneous group of disorders characterised by hyperglycaemia due to an absolute or relative deficit in insulin production or action. The chronic hyperglycaemia of diabetes mellitus is associated with end-stage organ damage, dysfunction, and failure, with organs particularly vulnerable to damage including the retina, kidneys, nervous system, and cardiovascular system.[79] The underlying cause of the hyperglycaemia forms the basis by which diabetes mellitus types are discriminated, and whilst deficient action of insulin on target tissues is the foundation of the disease, the complex pathways of hormone action- including insulin mean the pathophysiology is similarly complex. Indeed, modern research has demonstrated that pancreatic δ-cells are innately involved in the pathogenesis of diabetes, with mouse models demonstrating δ-cell hyperplasia[80], and even age-dependent conversion of δ-cells into insulin-producing β-cells in T1DM[81]. There is more information on the relationship between diabetes mellitus and pancreatic δ-cells in Recently Published Articles and Topics of Interest, below. The following video provides a good overview of diabetes mellitus, and resulting symptoms are summarised in the table, below:

Video Explanation of Diabetes Mellitus by Osmosis:  


Signs and Symptoms

Signs and Symptoms: Underlying Pathological Mechanism:
Polyuria and Polydipsia Hyperglycaemia means there is an increased amount of glucose being filtered from the kidneys. Due to osmotic pressure, water follows this due to the concentration gradient, resulting in unusually large volumes of urine, and thus increasing urination frequency. This increased amount of water being excreted results in dehydration and polydipsia.[82]
Extreme Hunger, Fatigue and Weight-Loss Insufficient insulin prevents glucose entry into cells to use as energy, resulting in fatigue and lethargy. Similarly, those on strong diabetic medications such as insulin may experience fatigue as a symptom of low blood glucose levels. Due to this fatigue, the body begins to burn fat and muscle for energy, causing a reduction in overall body weight and and increase in hunger.[83]
Ketonuria High levels of ketone bodies in the urine occurs when cells are broken down for energy. In diabetics, insufficient insulin to transport glucose into the cells for metabolism leads to the degradation of fat and muscle tissue into ketones to use as fuel without the need of insulin. Excess ketones can be filtered out by the kidneys and detected in the urine.[84]
Slow-Healing Sores and Frequent Infections Hyperglycaemia can cause diabetic neuropathy and damage to peripheral vessels can lead to poor blood circulation, thus repair to any trauma to tissue are affected by greater damage and an impaired healing capacity. This can cause wounds to remain open and unhealed for prolonged amounts of time, increasing the risk of bacterial and fungal infections.[85]

Pathogenesis of Type 1 Diabetes Mellitus

In type 1 diabetes (T1DM), the cause is an absolute deficiency of insulin secretion. Individuals at increased risk of developing this type of diabetes can often be identified by serological evidence of an autoimmune pathologic process occurring in the pancreatic islets and by genetic markers. Immune-mediated diabetes, accounts for only 5–10% of those with diabetes, results from a cellular-mediated autoimmune destruction of the β-cells of the pancreas. There are multiple genetic predispositions which relate to the disease, and it has also been connected to environmental factors which as yet are still poorly understood. These patients are also prone to other autoimmune disorders such as Graves' disease, Hashimoto's thyroiditis, Addison's disease, vitiligo, celiac sprue, autoimmune hepatitis, myasthenia gravis, and pernicious anemia. Some forms of type 1 diabetes have no known aetiologies, and thus earning the nomenclature ‘idiopathic diabetes’. This form of diabetes is strongly inherited, lacks immunological evidence for β-cell autoimmunity, and patients can suffer from episodic ketoacidosis and exhibit varying degrees of insulin deficiency between episodes.[86]

Pathogenesis of Type 2 Diabetes Mellitus

In type 2 diabetes (T2DM), the cause is a combination of resistance to insulin action and an inadequate compensatory insulin secretory response, with T2DM accounting for ~90-95% of those with diabetes. A degree of hyperglycaemia sufficient to cause pathologic and functional changes in various target tissues, but without clinical symptoms, may be present for a long period of time before T2DM is detected. During this asymptomatic period, it is possible to demonstrate an abnormality in carbohydrate metabolism by measurement of plasma glucose in the fasting state or after a challenge with an oral glucose load. This form of diabetes frequently goes undiagnosed for many years due to this gradual development of hyperglycaemia. Autoimmune destruction of β-cells does not occur, and most patients with this form of diabetes are obese, with obesity itself causing some degree of insulin resistance. Indeed, insulin resistance may improve with weight reduction alone, but pharmacological treatment of hyperglycaemia can also be used for treatment. The risk of developing this form of diabetes increases with age, BMI, and lack of physical activity. It occurs more frequently in individuals with hypertension or dyslipidemia, and it is often associated with a strong genetic predisposition, more so than the autoimmune form of T1DM. However, the genetics of this form of diabetes are complex and not yet fully understood.[86]

Treatment and the Relation to δ-Cells

Depending on a patient’s type of diabetes and individual needs, a doctor may suggest any one of the following treatments:

  • Monitoring of blood sugar
  • Insulin treatment via injection or pump
  • Oral medications e.g. Metformin 
  • Transplantation

As obesity is a major risk factor for T2DM, sometimes healthy eating, physical activity or bariatric surgery are enough to return a patient to a normoglycaemic state. [87]

Macro and Microscopic Appearance Pancreatic Adenocarcinoma

Pancreatic Carcinomas

About

Pancreatic carcinoma is the development of malignant cells which develop within the pancreas, affecting the normal exocrine and endocrine function of the pancreas. Cancer can occur in any part of the pancreas, and approximately 70% of pancreatic cancers in Australia are located in the head of the pancreas, however it has the potential to metastasise to other locations in the body. At present, pancreatic cancer is the tenth most common cancer in men and ninth most common cancer in women, and it represents the fifth most common cause of cancer death over all in Australia.[88] There are two primary pancreatic carcinomas that pertain to δ-cells or their function; pancreatic adenocarcinomas and somatostatinomas.

Video Explanation of Pancreatic Carcinoma by Osmosis:  

Pancreatic Adenocarcinoma

Signs and Symptoms

The symptoms of pancreatic adenocarcinoma can vary depending on the specific location of the tumour.[89]Some patients may be asymptomatic, and because many of the symptoms are non-specific, even those with symptoms may go undiagnosed for a long amount of time. This is a contributing factor to the high mortality of the disease.


Signs and Symptoms: Underlying Pathological Mechanism:
Jaundice, Itchy Skin Obstruction of the biliary tract by tumour or associated fibrosis impairs free drainage of bile into the intestine, thus substances normally excreted into the bile will accumulate in the vascular system, producing obstructive jaundice and associated pruritus.[90]
Unexplained Weight Loss Nausea and vomiting leads patients to ultimately eat less food. Food that is consumed is malabsorbed due to decreased exocrine function of the pancreas caused by fibrosis. Fat malabsorption means patients are not able to absorb energy from any foods that they are eating, resulting in weight loss.[75] Additionally, tumour factors released by the cancer lead to the profound loss of adipose tissue and skeletal muscle mass, a condition known as cachexia.[91]
Dyspepsia and Anorexia Pancreatic enzyme deficiency-particularly pancreatic lipase deficiency- is a result of adenocarcinoma infiltration and destruction of functional tissue, and can cause persistent dyspepsia by impairing digestion.

[92] Such dyspepsia contributes to anorexia, which is exacerbated by the release of cytokines which inhibit the neuropeptide-Y pathway or mimic negative feedback action of leptin on the hypothalamus, which causes anorexia.[91]

Nausea and Vomiting Peripancreatic inflammation caused by adenocarcinoma can extend to the posterior gastric wall and localised or generalised ileus can cause nausea and anorexia.[73] Such irritation within these portions of the gastrointestinal tract generates afferent impulses to the vomiting centre in the medulla oblongata, initiating efferent motor messages via CNV, VII, IX & XII to the duodenum and the stomach. The lower oesophageal sphincter relaxes, and the vomitus moves from the stomach to the oesophagus and inspiratory and abdominal contractions to expel into the mouth.[74]
Steatorrhoea, Diarrhoea or Constipation Structural damage to the pancreatic parenchyma via inflammation occurs, eventually causing a marked reduction in the exocrine and endocrine function of the organ. Fat malabsorption- or steatorrhea- can result, presenting as faecal excretion of fat, abdominal discomfort, and abdominal distension from gas. The steatorrhoea stools are characterised as loose, malodorous and greasy.[75] Typically, disruption of over 90 percent of the pancreatic parenchyma is needed to result in such diarrhoea.[75]
Abdominal and Back Pain Pancreatic nerve supply from vagal, right and left splanchnic nerves and the coeliac ganglion means fibrosis can cause severe pain during the pathogenesis. This pain can radiate to the desmosomes also innervated by these nerves (T6-9), including to the back.[69]

Pathogenesis

Adenocarcinoma The pathogenesis of pancreatix adenocarcinoma is extraordinarily varied, not only between individuals, but also the vastly heterogenetic tumours themselves. Genetic changes can occur in multiple subsets of genes, such as the activation of proto-oncogenes to oncogenes and the inactivation of tumor suppressor genes. Deregulation of molecules in several cell signaling pathways, such as EGFR, Akt, NF-κB, etc, and their molecular crosstalk also plays an incredibly important role in the molecular pathogenesis of pancreatic adenocarcinoma.[93] Ultimately these mutations lead to uncontrolled proliferation of glandular-like cells, demonstrating a classic microscopic appearance which is demonstrated in the histological sections in the image, above.

Treatment and the Relation δ-Cells

SST and analogues have the capacity to directly inhibit growth factor signaling and the cell cycle as well as exert pro-apoptopic and anti-migrative effects by binding to SSTRs on tumour cells. There are also indirect effects of SST, demonstrating anti-angiogenic properties and an inhibition of growth-promoting hormones to retard tumour growth or development.[78]

Therefore, δ-cells again represent a target for future therapeutic development. Targeting δ-cells to up-regulate their SST production could be a potential therapy for those with pancreatic cancer via the paracrine effects of SST, with the effort of halting pathogenesis.

Somatostatinomas

Signs and Symptoms

Signs and Symptoms: Underlying Pathological Mechanism:
Abdominal Pain Pancreatic nerve supply from vagal, right and left splanchnic nerves and the coeliac ganglion means fibrosis can cause severe pain during the pathogenesis. This pain can radiate to the desmosomes also innervated by these nerves (T6-9), including to the back.[69]
Nausea and Anorexia Peripancreatic inflammation caused by tumour can extend to the posterior gastric wall and localised or generalised ileus can cause nausea and anorexia.[73]This is exacerbated by the release of cytokines which inhibit the neuropeptide-Y pathway or mimic negative feedback action of leptin on the hypothalamus, which causes anorexia.[91]
Unexplained Weight Loss Nausea and vomiting leads patients to ultimately eat less food. Food that is consumed is malabsorbed due to decreased exocrine function of the pancreas caused by fibrosis. Fat malabsorption means patients are not able to absorb energy from any foods that they are eating, resulting in weight loss.[75]
Jaundice, Itchy Skin Obstruction of the biliary tract by tumour or associated fibrosis impairs free drainage of bile into the intestine, thus substances normally excreted into the bile will accumulate in the vascular system, producing obstructive jaundice and associated pruritus.[90]
Cholelithiasis Gallstones are hardened deposits of the digestive fluid bile, composed of cholesterol, bilirubin, and calcium salts, and form within the gallbladder. Gallstones occur when there is an imbalance in the chemical constituents of bile that result in precipitation of one or more of the components. This occurs in pancreatic cancers when infiltrating tumour blocks the bile duct, and these constituents sit stagnant, allowing so that crystals to form and grow as stones.[94]
Steatorrhoea, Diarrhoea or Constipation Structural damage to the pancreatic parenchyma via inflammation occurs, eventually causing a marked reduction in the exocrine and endocrine function of the organ. Fat malabsorption- or steatorrhea- can result, presenting as fecal excretion of fat, abdominal discomfort, and abdominal distension from gas. The steatorrhoea stools are characterised as loose, malodorous and greasy.[75] Typically, disruption of over 90 percent of the pancreatic parenchyma is needed to result in such diarrhoea.[75]
Polyuria and Polydipsia Increased SST production by tumour has a paracrine inhibitory effect on local beta cell function and any insulin production, and can result in diabetes or diabetes-like symptoms, such as hyperglycaemia. This increase in blood glucose means there is an increased amount of glucose being filtered from the kidneys. Due to osmotic pressure, water follows this due to the concentration gradient, resulting in unusually large volumes of urine, and thus increasing urination frequency. This increased amount of water being excreted results in dehydration and polydipsia.[95]
Extreme Hunger, Fatigue and Weight-Loss Insufficient insulin due to increase in SST production by somatostatinoma prevents glucose entry into cells to use as energy, resulting in fatigue and lethargy. Similarly, those on strong diabetic medications such as insulin may experience fatigue as a symptom of low blood glucose levels. Due to this fatigue, the body begins to burn fat and muscle for energy, causing a reduction in overall body weight and and increase in hunger.[96]
Macro (A) and Microscopic (B) Appearance of Somatostatinoma
Pathogenesis

Somatostatinomas are rare neoplasms originating from δ-cells and they account for approximately 1% of gastroenteropancreatic endocrine neoplasms; about half of which originate from the pancreas, with the remainder originating from other parts of the gastrointestinal tract e.g. the duodenum. Only approximately 80 cases have been reported since the first discovery in 1977, thus much is still unknown about them, particularly the mechanisms of their pathogenesis. The most common location is the pancreatic head, followed by the pancreatic body and tail. The tumours themselves are usually large, ranging in size from 3 to 11 cm, and have often metastasized to the liver and lymph nodes by the time of diagnosis. Pancreatic somatostatinomas may cause somatostatinoma syndrome, including diabetes mellitus, cholelithiasis, steatorrhea, and hypochlorhydria, caused by the inhibitory actions of somatostatin secreted from the islet δ-cells. [97]

Treatment

Surgical resection and adjuvant chemotherapy is the treatment of choice. A distal pancreatectomy can be performed in tumors of the body or tail, but as most tumours are in the pancreatic head or in the duodenum, a subtotal pancreatectomy and prophylactic cholecystectomy may be necessary. Chemotherapy agents must be administerd as adjuvant therapy in case of metastatic disease or recurrence after operation.[98] Patients frequently require nutritional support or hyperalimentation, and successful treatment with long-acting somatostatin analogues has been reported. [97]


Current research

Recently Published Articles and Topics of Interest

Conversion of Pancreatic Acinar Cells to Pancreatic Delta Cells by Weida et al., 2014
Hhex Expression in Mouse and Human Pancreas by Zhang et al., 2014

δ-Cells and Diabetes:

Diabetic mouse models typically used to study β-cell mass reduction have been used to observe increased δ-cell number over time in pancreatic islets of diabetic mice, correlating with an increase in somatostatin mRNA. Structural disturbances of cytoarchitecture were observed, with irregular β-cells accompanied by δ-cell hyperplasia and loss of pancreatic polypeptide (PPY) positivity. Whilst somatostatin was seen to increase over time, glucose-stimulation index decreased, with a similar decrease in rat body weight and an increase in hyperglycemia. These results indicate the complexities of δ-cells and somatostatin paracrine function within pancreatic islets, particularly in the pathology of diabetes.[99]

Another study demonstrated that the homeodomain transcription factor HHEX (hematopoietically expressed homeobox)- a transcription factor that has been repeatedly linked to T2DM- is selectively expressed in the δ-cells of the adult endocrine pancreas. It appears HHEX is required for δ-cell differentiation, and decreased somatostatin levels in HHEX-deficient islets can disrupt the inhibition of insulin release from β-cells. Thus it has been suggested that such compromised paracrine control is a contributor to T2DM.[100]

Making New δ-Cells From Acinar Cells:

It has been discovered that pancreatic acinar cells can be converted into pancreatic δ-like cells with mere exposure to reprogramming factor Ngn3 in mouse models in vivo.[101] Approximately 40% of cells demonstrated somatostatin (Sst) positivity following infection, and further tests were done on these cells to investigate morphological and physiological changes. Somatostatin and cholecystokinin receptor B (Cckbr) are two δ-cell specific genes, and analysis revealed 87% (±7%) of induced δ-like cells co-expressed Sst and Cckbr 30 days after induction (Figure 1a-c). Similarly, electron microscopy revealed induced cells showed characteristic δ-cell ultrastructure, demonstrating secretory granule morphology distinct to endogenous δ-cells (Figure 1d-e). Induced δ-like cells also showed strong methylation in Amylase2, with no significant difference between their expression and expression in islet δ-cells (p=0.23), suggesting that substantial DNA methylation changes occurred during conversion by Ngn3 (Figure 1h). Lastly, researchers evaluated the ability of induced δ-cells to secrete hormones after stimulation with secretagogue Arginine. The induced cells responded to stimulus by secreting somatostatin in a manner similar to endogenous δ-cells (Figure 1i).

Turning δ-Cells Into β-Cells:

Total or near-total loss of insulin-producing β-cells is a feature of T1DM, thus restoring insulin production in patients has been the focus of much research. Young (pre-pubescent) mice with ablated β-cells demonstrated reconstitution of insulin-producing cells in the absence of autoimmunity, via the spontaneous en masse reprogramming of somatostatin-producing δ-cells. This “somatostatin-to-insulin” δ-cell conversion involves de-differentiation, proliferation and re-expression of islet developmental regulators, relying in part upon the combined action of FoxO1 and downstream effectors. This restoration of insulin producing-cells was rapid, with younglings demonstrating almost normoglycemic levels merely four months after β-cell ablation. These results have profound potential in the treatment of patients with type one diabetes mellitus. Not only do these data represent a mechanism of permanent recovery, aforementioned hyperplasia of δ-cells demonstrated in the literature indicates this is a feasible focus for future implementation in the treatment of young human patients with the disease.[53]

δ-Cells, Melatonin and Stress:

The circadian clock and melatonin have an effect on pancreatic islets, with melatonin inhibiting insulin secretion from β-cells. Recent experiments using a pancreatic δ-cell model show that melatonin also has an inhibitory effect on somatostatin secretion when in the physiological concentration range. Interestingly, in the pharmacological range, melatonin elicited slightly increased somatostatin release from δ-cells. When comparing this with somatostatin levels released in different glucose levels, there was inhibition of somatostatin secretion by melatonin at low glucose concentrations, however this effect was less pronounced at higher glucose levels. These results indicate that melatonin has a significant effect on both pancreatic δ-cells and somatostatin release.[41] Other studies into how the adrenergic system affects δ-cell function have shown that activation of the adrenergic system promoted somatostatin secretion from islet δ-cells. By applying adrenaline-specific adrenergic agonists or antagonists to pancreatic islets, it was discovered that adrenaline and isoprenaline increased somatostatin content and transcription through the activation of β1-/β2-adrenergic receptors (β1-/β2ARs), with the somatostatin content in knockout mice 50% lower than in wild-type mice. Therefore, dysfunction of β-adrenergic agonists may impair pancreatic δ-cell function. Both these studies have implications in terms of disease and treatment, and demonstrate how complex the function of δ-cells are. More research into such areas are necessary to fully comprehend pancreatic physiology.[39]

Glossary

Term Definition
Addison's disease A disorder in which the adrenal glands don't produce enough hormones.
Adenocarcinoma A malignant tumour formed from glandular structures in epithelial tissue.
Akt A group of enzymes involved in several processes related to cell growth and survival.
AMPA receptor The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor is a non-NMDA-type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system
Astressin2b Corticotropin-releasing factor receptor 2 (CRF2) antagonist.
Autoimmune hepatitis Inflammation in the liver that occurs when the immune system attacks the liver.
ARX Aristaless related homeobox is a transcription factor that plays a role in α-cell development. It opposes the action of PAX4.
C57BL/6J mice C57BL/6, commonly referred to as "C57 black 6", "C57" or "black 6", is a typical inbred strain of laboratory mouse. It is the most widely used "genetic background" for genetically modified mice for use as models of human disease.
CCKA receptor Cholecystokinin A receptor is a G-protein coupled receptor that binds sulfated members of the cholecystokinin (CCK) family of peptide hormones.
CCKB receptor Cholecystokinin B receptor is a G-protein coupled receptor present in the gastrointestinal tract that binds gastrin and cholecystokinin. It is named so because it is a type B gastrin receptor.
Celiac sprue An immune reaction to eating gluten, a protein found in wheat, barley and rye.
Cholelithiasis The formation of gallstones.
CNV, VII, IX & XII Cranial nerve 5, 7, 9 and 12, also called the trigeminal, facial, glossopharyngeal and hypoglossal nerves.
CRHR2 Corticotropin-releasing hormone receptor 2 is type 2 G protein-coupled receptors for corticotropin-releasing hormone (CRH) that reside in the plasma membranes of hormone-sensitive cells.
Dyspepsia Indigestion
E14.5 etc. 'E' is a shortened form of "embryonic day". The number following 'E' represents the embryonic day at which the related process occurs. Embryonic days are used to describe development in mice.
EGFR Epidermal growth factor receptor which regulates growth, survival, proliferation, and differentiation in mammalian cells.
fF Femtofarad is an SI unit of electrical capacitance.
FOXO1 Forkhead box O1 is a transcription factor belonging to the forkhead family.
GABA gamma-Aminobutyric acid or γ-Aminobutyric acid is the chief inhibitory neurotransmitter in the mammalian central nervous system.
GABAA The GABAA receptor (GABAAR) is an ionotropic receptor and ligand-gated ion channel.
GCaMP GCaMP is a genetically encoded calcium indicator.
GHIH, GHRIH, SRIH and SRIF These are all alternative names for somatostatin and stand for growth hormone-inhibiting hormone (GHIH); growth hormone release-inhibiting hormone (GHRIH); somatotropin release-inhibiting hormone (SRIH) and somatotropin release-inhibiting factor (SRIF), respectively.
GHS1-R Growth hormone secretagogue receptor type 1
Graves' disease An immune system disorder of the thyroid.
GRP120 and FFAR4 G-protein coupled receptor 120, also known as free fatty acid receptor 4.
Hashimoto's thyroiditis When the immune system attacks the thyroid.
IL-1,6,8,18 and 33 Classes of glycoproteins produced by leucocytes for regulating immune responses.
Isradipine Isradipine is a calcium channel blocker of the dihydropyridine class.
Kainate receptor Kainate receptors, or KARs, are ionotropic receptors that respond to the neurotransmitter glutamate.
Ketonuria The excretion of abnormally large amounts of ketone bodies in the urine, characteristic of diabetes mellitus, starvation, or other medical conditions.
Jaundice Yellowing of the skin or whites of the eyes, arising from excess of the pigment bilirubin and typically caused by obstruction of the bile duct, by liver disease, or by excessive breakdown of red blood cells.
M1 Muscarinic acetylcholine receptor 1.
Metabotropic receptor Metabotropic receptors are indirectly linked with ion channels on the plasma membrane of the cell through signal transduction mechanisms, often G proteins.
Myasthenia gravis A weakness and rapid fatigue of muscles under voluntary control.
Neuropeptide-Y pathway Acts as a neurotransmitter in the brain and in the autonomic nervous system of humans.
NF-κB   A transcription factor that regulates genes responsible for both the innate and adaptive immune response.
Ngn3 Neurogenin 3 is a basic helix-loop-hoop transcription factor that plays a significant role in endocrine pancreatic development.
NNC 55-0396 Highly selective T-type Ca2+ channel blocker.
P48 Pancreas specific transcription factor 1a. A transcription factor expressed in multipotent pancreatic progenitor cells that plays a role in Notch signalling processes. It follows the differentiation pathway of acniar cells.
PDX1 Pancreatic duodenal homeobox 1. A transcription factor expressed in multipotent pancreatic progenitor cells that plays a role in Notch signalling processes. It follows the β-cell differentiation pathway where is plays an additional role in activating the insulin gene.
PAX4 Paired box 4 is a transcription factor from the paired box (PAX) family that plays a role in β- and δ-cell development. It opposes the action of ARX.
Pernicious anaemia A decrease in red blood cells when the body can't absorb enough vitamin B12.
Polydipsia Excessive thirst usually accompanied by temporary or prolonged dryness of the mouth.
Polyuria Excessive or abnormally large production or passage of urine.
PP-cells Cells situated in pancreatic islets which produce pancreatic polypeptides.
PPSST Preprosomatostatin, a large precursor molecule from which PSST is cleaved.
PSST Prosomatostatin, a precursor from which SST-14 and SST-28 are cleaved.
Ptch1 PTCH1 is a member of the patched gene family and is the receptor for sonic hedgehog, a secreted molecule implicated in the formation of embryonic structures and in tumorigenesis.
Ptch2 Patched 2 is a protein that in humans is encoded by the PTCH2 gene. This gene encodes a transmembrane receptor of the patched gene family
Somatosatinoma An extremely rare tumour that occurs in the pancreas or part of the small intestine, arising from somatostatin-producing δ-cells.
SOX9 Sex-determining region Y box 9. A transcription factor expressed in multipotent pancreatic progenitor cells that plays a role in Notch signalling processes. It follows the differentiation pathway of ductal and centroacinar cells.
Steatorrhoea The presence of excess fat in faeces.
sst1-sst5 A family of five G-protein coupled receptors that bind somatostatin.
SST-14, SST-28 The numbers here refer to the number of amino acids that make up each isoform i.e. SST-14 has 14 amino acids and SST-28 has 28.
T1DM Type 1 diabetes mellitus, a chronic condition in which the pancreas produces little or no insulin.
T2DM Type 2 diabetes mellitus, a chronic condition that affects the way the body processes blood sugar (glucose).
TNF-α A cell signalling protein involved in systemic inflammation and is one of the cytokines that make up the acute phase reaction.
TTX Tetrodotoxin is quite specific in blocking the Na+ ion channel and therefore the flow of Na+ ions while having no effect on K+ ions.
Ucn3 Urocortin-3 is a protein that in humans is encoded by the UCN3 gene. It is structurally related to the corticotropin-releasing factor (CRF) gene.
Vitiligo A disease that causes the loss of skin colour in blotches.
ω-agatoxin ω-agatoxin blocks the presynaptic calcium channels, so that the calcium influx will reduce.

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