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Tight Junctions

Introduction

Schematic of the various cell-cell junctions

Cell junctions are essential for cells in tissues to function in an integrated manner. The four major classes of junctions are: tight junctions, gap junctions, desmosomes, and hemidesmosomes organised into a manner otherwise known as the junctional complex.[1] Each is specialized to preform a specific function. Desmosomes and hemidesmosomes have a structural role and serve to anchor cells to each other or to the extracellular matrix, respectively. They do this by mediating the connection of a cell's internal cytoskeleton with that of another cell or to the extracellular matrix. [2] Gap junctions are channels between adjacent cells that help facilitate the passage of molecules between cells, and are therefore involved in cell-cell communication. [3] Tight junctions, the subject of this page, are also an extremely important type of cell junction.

Tight junctions, or zona occludens, are a branching network of anastomosing transmembrane protein strands that encircle the apical ends of epithelial cells.[4] Tight junctional proteins associate with similar proteins on adjacent cells which seals the gaps between them.[5] Tight junctions serve to create a barrier to diffusion across the epithelia as well as separating the membrane into apical and basolateral regions.[6] Tight junctions are only found in vertebrates.However, analogous forms of tight junctions are found in invertebrates which are called septate junctions. [7]

History

Dr. Hans Ussing

Below is a timeline of important discoveries that have led to our current understanding of tight junctions.


Early 1940’s: Hans Ussing and associates propose the “two membrane model” to address the question of how sodium ions are moved in a directional fashion across the epithelium. This model stated that the apical and basal membrane surfaces have different conductance properties, namely that sodium enters the cell across the apical membrane down its concentration gradient. [8]

1963: Farquhar and Palade first visualizes the tight junctions at the ultrastructural level. [9]

1964: Hans Ussing and associates discover inconsistencies within their findings leading to the understanding that Chloride ions follow sodium ions in a passive fashion through the tight junction which they called the “shunt pathway”. [10]


Gradually, it was accepted that intact normal epithelia display a wide range of electrical resistances and that it was the tight junctions that determined whether the transepithelial resistance is high or low[11]


1986: Stevenson and associates isolate and localize a component of the cytoplasmic face of the tight junction naming the protein Zona occludens-1 (ZO-1). [12]

1988-1991: Citi and associates discover and localise major tight junction plaque protein cingulin. [13][14][15]

Shoichiro Tsukita

1991: Gumbiner and associates identify the tight junction protein ZO-2.[16]

1993: Furuse and associates immunolocalised a tetraspan transmembrane protein of the Zona Occludens naming it "occludin".[17]

1994-1997: Furuse and associates transfect "occludin" encoding cDNA. Transfection of cDNA and expression caused the construction of tight junction possessing cells with occludin integrated into pre-existing tight junction structures. When injected into tight junction lacking cells, it was able to form de novo tight junction typical intramembranous “ridges” as well as cytoplasmic stacks of lamellae resembling tight junction membrane structures. [18][19]

1995: Fasano and associates discover the Zonula occludens toxin which is thought to activate a complex cascade of events ultimately regulating tight junction permeability. [20]

Schematic of "tricellulin"

1998: Furuse and associates identify 2 smaller polypeptides. Immunolocalisation and incorporation into tight junctions of living cells formed de novo intramembranous TJ typical ridge structures, even in TJ lacking cells naming these proteins as “claudins” nos. 1 and 2. [21][22]

1998: Haskins and associates identify the tight junction protein ZO-3. [23]

2001-2002: Tsukita and associates discovery a total of 24 different claudin proteins encoded in the human genome.[24][25]

2002: Bradner & Furuse and associates observe typical tight junction structures, positive for claudins and occluding in the uppermost layers of the epidermis and other squamous stratified epithelia. [26][27]

2005: Ikenouchi & associates and Tsukita & associates– Discover small intercellular “gaps” formed by three cells meeting together posing a “three corner state problem” (Ikenouchi). The answer to this problem came in the form of a protein, which Tsukita named “tricellulin”. This protein was found to assemble exactly at those “three corner” sites. [28]

Structure

Tight junctions consist of a branching network of sealing strands of protein. Each strand is assembled from a series of transmembrane proteins(JAMs/Junctional Adhesion Molecules,Claudins and Occludin) embedded in the plasma membrane. The extracellular domains of tight junction proteins associate with the extracellular domains of proteins on adjacent cells. The intracellular domains are linked to peripheral membrane proteins which link the transmembrane protein strands to actin cytoskeleton to create a functional network that plays a role in many cellular processes. Each strand functions as an individual or linear barrier and, therefore, the number of transmembrane protein strands is related to the degree of paracellular electrical resistance and impedance to solute flux in tight junctions[29]. Although tight junctions are composed of many different proteins, occludin and claudin are the major structural components.

Associated molecular components of tight junctions play an important role in creating and maintaining the structure of tight junctions as well as giving functions to them.Molecular components of tight junctions can be categorised into two groups, namely, transmembrane proteins and peripheral membrane proteins

Transmembrane Proteins

Transmembrane proteins serve as important components of the tight junction that span across the junction, connecting two adjacent cells and making a seal tight at the junction.The group has three members, occludins, claudins, and junctional adhesion molecules (JAMs).

Schematic representation of the tight junction transmembrane proteins between two adjacent cells

Occludins

Occludins are family of transmembrane proteins which were the first tight junction associated transmembrane proteins identified in chicken liver.[30].Structurally ,they have four transmembrane domains,two extracellular loops and two intracellular domains[31].

Roles:

  • interacting directly with proteins zonula occludens,ZO-1 and ZO-2 and ZO-3 at the tight junction to localize occludins
  • interacting indirectly with the actin cytoskeleton and junctional adhesion molecules(JAMs) via interacting with ZO proteins[32].
  • involving in cell-cell adhesion with extracellular domains of occludin[33]
  • vital in tight junction assembly in Xenopus embryo development[34]
  • regulating various signaling events initiating from the tight junction
  • may involve in RhoA activation via a tight junction–associated guanine nucleotide exchange factor,GEF-H1/Lfc[35][36]
  • targeting TGF-ß receptors to tight junctions[37].

Claudins

Claudins belong to transmembrane protein type and are also the main constituent of the tight junction intercellular strands.The claudin family is made up of a number of members.Among them,claudin-1 and claudin-2 were the first claudin family members identified in a chicken liver fraction[38].The family has been claimed important for creating and maintaining the barrier function of tight junctions[39].

Roles:

  • forms tight junction strands via interacting with each other between different tight junction strands or within individual strands in a homotypic and heterotypic manner [40]
  • structural components are similar to occuldin.Four transmembrane domains, two extracellular loops, and two intracellular domains,however,it does not have sequence similarity to occludin[41]
  • mediate calcium-independent cell-cell adhesion[42]
  • interacting directly with peripheral PDZ-domain-containing proteins(ZO-1, ZO-2, ZO-3,and protein associated with Lin seven 1 (PALS1)-associated tight junction protein (PATJ))[43]

Junctional Adhesion Molecules (JAMs)

JAMs belong to the immunoglobulin superfamily of proteins and are expressed in several cell types ,epithelial cells,leukocytes, endothelia, and platelets.The family is made up of four members JAM-A, JAM-B, JAM-C, and JAM4/JAML[44],with each serves different function.In epithelia,JAM-A are directed to tight junctions, whereas,JAM-B are directed to the lateral membrane [45] .In terms of general structure,the JAMs have a single transmembrane domain, an extracellular domain containing two Ig-like motifs, and a cytoplasmic tail[46].

Roles:

  • may participate in cell adhesion via homophilic interactions [47]
  • interacting with each other. JAM-B interacts with JAM-C and integrins through heterophilic interactions [48]
  • forming intercellular junctions and epithelial barrier function[49].

Peripheral Membrane Proteins

Peripheral membrane proteins serve as intracellular binding sites for transmembrane proteins to allow transmembrane proteins to be organized in membrane and attached to the cytoskeleton to initiate cell signaling.The group has four members,namely Zonula occludens(ZOs),Cingulin ,ZO-1-associated nucleic acid–binding protein(ZONAB) and Rab13.

Schematic model of the basic structural transmembrane and peripheral membrane components of tight junctions

Zonula occludens(ZOs)

ZOs are tight junction proteins in the membrane-associated guanylate kinase (MAGUK) family of proteins containing a core structure consisting of one or more PDZ domains, an Src homology 3 (SH3) domain, and a guanylate kinase (GUK) domain.ZOs appear as three isoforms of ZO proteins, ZO-1, ZO-2, and ZO-3[50].

Roles:

  • serving as peripheral membrane scaffolding proteins
  • interacting with many binding partners at the tight junction
  • involving in the formation of tight junction[51].

Cingulin

Cingulin is a group of protein weighing 140–160kD and appear on the cytoplasmic surface of epithelial tight junctions.It was first identified as a peripheral membrane protein at the tight junction in avian brush border cells[52].It has globular head and tail domains as well as a central a-helical rod domain[53].

Roles:

  • potential role in embryogenesis and epithelial maturation[54]
  • linking tight junction proteins to the actin cytoskeleton[55]
  • involving in transcriptional regulation and cell proliferation[56]
  • playing a role in cell-cycle progression[57].

ZO-1-associated nucleic acid–binding protein(ZONAB)

ZONAB was first identified by its function , as a binding partner of ZO-1.It has a Y-box transcription factor protein [58] .

Roles:

  • interacting with cell division kinase 4 (CDK4) to regulate cell proliferation
  • serving as a sensor of cell density [59].

Rab13

Rab 13 is member of the small GTPase Rab family of proteins.It was first identified as a mammalian homolog of the yeast secretory protein, Sec4 [60] .

Roles:

  • involved in the regulation of exocytic and endocytic pathways, including vesicle movement and fusion [61].
  • involving in early junctional formation [62].

Tight Junction Function

Tight junctions are specialized to perform several specific and roles in epithelia. They are extremely important not only to the structure and function of individual epithelial cells but also to the epithelium as a whole.

Barrier Function

The tight junction is responsible for the selective permeability of epithelia. They serve to create a barrier which separates the fluid on either side of the epithelium by joining together the plasma membranes of adjacent epithelial cells to form a tight seal near the apical surfaces of the cells. This serves the crucial function of preventing the free diffusion of solutes across the epithelium via the inter-cellular spaces between the basolateral surfaces of the cells and helps to maintain the differences in chemical composition of one side of an epithelial sheet versus the other. [63] If the tight junction were not present then solutes from could freely diffuse across the epthilium and into the space surrounding the basolateral surfaces and vice versa. This would result in a loss of control of solute transport which would have disastrous consequences for an organism. The presence of the tight junction barrier means that most substances cannot passively cross the epithelia.

In the brain (left), the peroxidase reaction product (top, black) cannot get past endothelial junctions (arrow), but in the heart (right) peroxidase flows down a cleft (C) between endothelial cells.

Polarity of Epithelial Cells

The polarized nature of epithelial sheets can also be attributed to tight junctions. The tight seal between adjacent cells that prevents free diffusion of fluid from one side of the epithelia to the other also creates a barrier that prevents the diffusion of lipids and proteins between the apical and basolateral surfaces of individual epithelial cells. This allows for the localization of specific transmembrane proteins in the plasma membrane which, for many proteins in epithelial sheets, is very important for their function. [64] In the intestinal epthilia for example, transport across the epthilia involves the importation of substances from the lumen into the cell via the apical surface, followed by exportation from the basolateral surface. These processes are mediated by different sets of membrane transport proteins and, therefore, the different proteins must be restricted to either the apical or basolateral surfaces of the cells depending on their function. [65] Tight junctions create this barrier to the free migration of transmembrane proteins and thus maintain the polarity of the plasma membranes of epithelial cells.

In addition to their role in polarizing the plasma membrane, tight junctions also serve to maintain intracellular polarity. Claudins and occuldins, for example, can associate with certain proteins which serve as anchors for the actin cytoskeleton which is involved in directing organelle movement within the cell. In addition, many other intracellular proteins have been found to associate with tight junction proteins which are involved in directing the movement of proteins and substances within the cell and thus maintaining intracellular polarity. [66]

Segregation of transmembrane proteins in the apical and basolateral surfaces of the plasma membrane is maintained by tight junctions

Transcellular and Paracellular Transport

Solute transport across an epithelium can occur via two different pathways; paracellular, which is direct transport across the tight junction; or trancellular, which is transport through the cell mediated by vesicles through the apical and basolateral membranes. These two pathways are highly integrated and regulated so that transepithelial transport can respond properly to changes in the environment [67]. Experiments by Ussing and Windhager demonstrated that the diffusion of Cl- ions through the tight junction was coupled to the electrochemical gradient generated by the transcellular active transport of Na+ ions.[68] This coupling showed that transcellular and paracellular transport are physiologically linked.[69] Specifically, tight junctions help facilitate the process of solute across the epithelium while simultaneously serving as a barrier to the free diffusion of potential toxins. [70] Tight junctions mediate the selective diffusion of solutes through differences in the spatial distributions of their transmembrane component proteins: occludin, claudins and junctional adhesion molecules (JAMs). [71] The main barrier protein in tight junctions are the claudins, which form a family of about 24 different proteins in mammals and are essential to the structure of tight junctions. [72] The spatial arrangement of the extracellular loops of claudin molecules from adjacent cells is the main determinant of paracellular solute selectivity. [73] The claudins form selective pores that only allow molecules which are of appropriate size and charge to diffuse via the paracellular pathway. [74] Additionally, tight junction pores can open or close in response through interactions with cytoskeletal elements and other proteins in response to environmental factors.[75] These various forms of regulation allows the tight junctions to be very sensitive to changing environmental circumstances with regard to solute transport.

Schematic drawing of a tight junction that shows the paracellular and transcellular transport pathways

Examples of Tight Junctions in the Body

While tight junctions are a ubiquitous characteristic of epithelia in vertebrates, there are several examples of epithelial barriers that particularly exemplify the importance of tight junctions within the body.

The Blood-Brain Barrier

The blood–brain barrier is an endothelial barrier made up of capillary walls that separates the blood and the interstitial fluid of the brain.[76] The barrier is sealed by an extensive network of tight junctions which provide a continuous cellular barrier to the free diffusion of polar molecules and has no transjunctional pores. [77] The morphology of the blood-brain barrier tight junctions creates a barrier that is much more highly selective than the barriers found elsewhere in the body. [78] Some areas of the brain have capillaries whose tight junctions do not have this barrier function, and these non-barrier areas are the sites of some transport across the capillary walls. [79] This high selectivity mediated by the tight junctions is crucial to brain function because it discriminates against compounds in the blood that are unsuitable for unique cerebral metabolism as well as facilitates the import of substrates that fulfill the brain’s specific requirements. [80]

An EM image of the blood-brain barrier in mouse cerebral tissue

The Intestinal Wall

The intestinal wall is composed of an epithelial sheet whose structure and function is largely dependent on tight junctions. The seals between the epithelial cells create a barrier that prevents free diffusion of material across the epithelia form the intestinal lumen. [81] This is extremely important to prevent the invasion of toxins or bacteria into the bloodstream that could be harmful to the organism. Additionally, tight junctions are crucial for the transport of materials across the epithelia. The proteins required for endocytosis of nutrients from the lumen must be localized on the apical surface of the cell. Conversely, exocytotic proteins must be located on the basolateral surface in order to release nutrients so that they can enter the bloodstream. This type of vectoral transport is mediated by the membrane polarity induced by tight junctions. [82] Also, tight junctions regulate paracellular transport which, in addition to transcellular transport, is a significant mechanism of transport across the intestinal wall. [83]

Classification of Epithelia Using Tight Junctions

‎Circuit Model for the Measure of TER

Epithelia can be classified into two separate categories - “tight” or “leaky” depending on their ability to pass solutes across their surface. To put it simply, “leaky” epithelia are able to pass a large amount of solute and for “tight”- only a small amout of solute is able to be passed through. It has been observed, through freeze-fracture studies, that the number of tight junctions correlated with the "tightness" or "leakiness" of the epithelia [84]. The tight junctions, as mentioned previously, are a number of anastomosing strands. These strands were seen to be more prevalent in “tight” epithelia typically consisting of approximately five or more strands whereas “leaky” epithelia consisted of only a single strand[85].

Transepithelial Resistance

Another method by which we can classify epithelia is through the measure of transepithelial resistance (TER). The measure of TER in epithelia can be done through the measure of the transepithelial electical resistance (TEER).Calculating TEER is done through the use of simplified circuit models and calculating the varying resistances. In these circuit models, the epithelium is viewed as a circuit which consists of two arms with resistors of varying resistance. These two arms constitute the paracellular and transcellular resistance respectively. For the paracellular arm, the resistance of the tight junction and the resistance of the subjunctional lateral space is calculated and for the transcellular pathway, the apical and basolateral resistance is calculated[86]. Thus, using this model, a microelectrode is impaled on the tissue being studied. By applying a transepithelial current and calculating the generated potential the resistance to current flow can be calculated by using Ohm’s law. [87]


For “tight” epithelia, they possess a very high transepithelial resistance. This is because they are able to maintain a high electrochemical gradient through active transcellular transport. This is maintained by the resistance to free diffusion by the action of the tight junction[88]. Through this, these junctions function to produce either a highly concentrated or highly diluted secretions. A good example of “tight” epithelia is the epithelium of the mammalian proximal tubule. This “tight” conformation allows for the distal nephron to produce urine which is several fold higher or lower than plasma[89].


“leaky” epithelia on the other hand have a very low transepithelial resistance which is essential in the movement of large amounts of isosmotic fluids [90]. A good example of “leaky” epithelia are found in the human gastrointestinal tract which generally absorbs and reabsorbs approximately 10 L of fluid daily[91]. However, further down the tract towards the distal colon, greater electrochemical resistance is found which is necessary for the reabsorbtion of NaCl and water to form stool [92].

Tight Junction Assembly and Regulation

Tight junction assembly appears to be regulated, in part, by signal transduction pathways involving heterotrimeric G proteins, release of intracellular Ca2+, and activation of protein kinase C. [93] There does not, however, appear to be a single universal pathway for tight junction assembly and it is likely that the process varies between different tissue types. [94] Due to environmental factors or differences in tissue type, tight junction permeability must be very dynamic. Therefore, tight junction assembly and disassembly changes in response to many signaling agents such as hormones, growth factors, and cytokines. [95]

Extracellular Ca2+ is essential for the formation of inter-cellular junctions, including tight junctions. It is required for Ca2+ dependent cell-cell adhesion proteins to properly associate. [96] Additionally, as noted above, Ca2+ is also an important component of the signal transduction pathways that lead to TJ formation. [97]

If Ca2+ is removed from the growth medium containing epithelial cells, tight junctions begin to dissociate and become "leaky" meaning they are they begin to lose their barrier function. Conversely, addition of Ca2+ results in tight junctions reassembly and return of the epithelial barrier. [98] Many experiments have been designed based on this calcium dependency when studying the role of tight junctions in epithelia. For example, in order to study the effects of the tight junction disruption in epithelia, cells can be grown in media containing low levels of calcium in order to destroy these intercellular contacts. [99]

Comparison of the Ca2+ switch and ATP depletion-repletion models of TJ biogenesis.

Diseases

Dysfunction of tight junctions (TJ) leads to hyperpermeability of epithelia and is associated with numerous diseases. This inability to exclude certain macromolecules from traversing the paracellular space means that tissues are exposed to pathogens from the external environment. Because these pathogens are able to traverse the intercellular barrier, an inflammatory response is often induced, and as demonstrated in the table below, inflammation is a common theme in many TJ related diseases. These diseases come about when the expression of one or more TJ-associated proteins is altered. This can be induced by pathogens, toxins, proinflammatory cytokines, and environmental factors [100][101]. Studies have suggested that some people are genetically predisposed to contracting illnesses related to barrier dysfunction [102].

Zonulin has been found to play a significant role in increased TJ permeability, and has been implicated in the pathogenesis of several autoimmune diseases, including celiac disease and type 1 diabetes, as well as various types of cancer. [103][104]. It seems likely that further research will reveal that zonulin modulates TJ permeability in numerous other diseases as well [105] [106].


A comprehensive review of zonulin signalling and it’s role in disease can be found here- Zonulin and its regulation of intestinal barrier function: the biological door to inflammation, autoimmunity, and cancer


Studies have implicated tight junctions in the following diseases, however this list is by no means exhaustive-


Disease TJ protein affected Description
Asthma Occludin, Claudin-1-Wan et al. 1999 Characterised by chronic inflammation of the airways, causing airway obstruction as well as a heightened response to external pathogens, resulting in broncospasms. Severe cases can result in death. [107]
Autism de Magistris et al. 2010 Autism is described as a neurodevelopmental disorder where chief symptoms are an inability to socialise appropriately, communicate effectively (including difficulty with speech), and a propensity for repetitive or unusual behaviour. [108]
Breast cancer Claudin-1, ZO-1, claudin-7- Hough et al. 2000 Malignant neoplasm of the breast with several different types. The main symptom is the appearance of a lump in the breast. Inflammation, soreness, and alteration of breast size and shape may also occur depending on the type of cancer.
Celiac disease ZO-1,

Occludin- Elli et al. 2011

Chronic inflammation of the mucosa in the gastrointestinal tract, leading to the breakdown of intestinal villi and nutrient malabsorption. Basic symptoms include distention, abdominal cramping, vomiting, diarrhoea or constipation, depression, irritability, and lethargy. If untreated, celiac disease can lead to a vast number of other illnesses, chiefly vitamin and mineral deficiencies and osteoporosis. [109]
Cholera ZO-1,

Occludin- Wu et al. 2000

A result of infection with bacteria Vibrio cholerae, which causes the intestines to release excess fluid, producing watery faeces. The resulting diarrhoea is so extreme that it quickly brings on dehydration, shock, and if untreated, death. [110]
Cholestasis 7H6, mrp2 - Kawaguchi et al. 2000 Prevention of bile flow from the liver to the duodenum, leading to bile build up in the liver. This can be caused by either a physical block in the duct, or insufficient bile production. Symptoms include discoloured faeces, nausea, vomiting, itching, and jaundice. [111]
Chronic cholestatic liver diseases 7H6, ZO-1- Sakisaka et al. 2001 Primary biliary cirrhosis and primary sclerosing cholangitis are both chronic cholestatic liver diseases where TJs hae been implicated in their pathogenesis. Both conditions cause inflammation and damage to the bile ducts. [112]
Clostridium perfringens enterotoxin Claudin-3, Claudin-4- McClane 2001 A bacteria that commonly causes diarrhoea from food contamination and antibiotics. It causes fluid and electrolyte loss by binding to epithelia in the ileum, which in turn kills these cells.[113]
Collagenous colitis Occludin, Claudin-2 Claudin-4- Burgel et al. 2002 Inflammation and thickening of lamina propria in the mucosa of the rectum and colon, causing severe diarrhoea.[114]
Crohn's Disease Claudin-2, Claudin-3, Claudin-5, Claudin-8, ZO-1- Zeissig et al. 2007 Causes transmural inflammation of the mucosa anywhere along the entire gastrointestinal tract. Inflammation is accompanied by diarrhoea, abdominal pain, bowel obstruction, fever, and stools containing blood or mucous. Examination may also reveal abdominal masses, colonic obstruction, fistulas, fissures, cryptitis, perianal disease, and abscesses. Due to a weakened immune system, and loss of nutrients and fluids, patients with Crohn’s disease can also demonstrate a vast range of extraintestinal symptoms, including pyoderma gangrenosum (see picture), osteoporosis, and erythema nodosum.[115][116]
Cystic fibrosis Occludin, Claudin-1, Claudin-4, JAM, ZO-1- Coyne et al. 2002 Brought about by impaired liver function caused by autolysis of pancreatic tissue and obstruction of pancreatic ducts. This results in an increased incidence of infections, commonly in the lungs, which leads to breathing difficulties and may prove fatal. Excessively salty sweat is an unusual and distinctive symptom of the disease. Impaired growth, infertility, and diarrhoea, have all been shown to follow on from cystic fibrosis.[117][118]
Diabetic retinopathy Occludin, ZO-1- Felinski and Antonetti 2005 One of the chief reasons for vision loss, diabetic retinopathy is a complication of diabetes. The duration of diabetes, systolic blood pressure, glycemic control, and urinary albumin concentrations are all factors that determine the likelihood of developing diabetic retinopathy.[119]
Familial hypomagnesemia Claudin-16- Simon et al. 1999 Describes a deficiency of magnesium in the body. This can be brought about by diarrhoea, short bowel syndrome and bowel fistulas. Lack of magnesium results in impaired protein synthesis, metabolic processes and phosphorylation.[120]
Barrett’s esophagus Claudin-18- Jovov et al. 2007 Lesions that appear in the esophagus in response to chronic inflammation caused by gastroesophageal reflux disease. This results in pain that causes difficulty when swallowing, and the patient may vomit blood. Lesions that appear in the esophagus in response to chronic inflammation caused by gastroesophageal reflux disease. This results in pain that causes difficulty when swallowing, and the patient may vomit blood. [121]
Hereditary deafness Claudin-14- Ben-Yosef et al. 2003 Impaired TJ are unable to separate ions within the Organ of Corti, resulting in profound deafness. [122]
Multiple sclerosis Occludin, Claudin-5- Förster et al. 2007 A disorder characterised by inflammation of the brain and spinal cord, which causes myelin and axon damage. Symptoms are numerous and depend on the region of the central nervous system that is affected. Some of these symptoms include, cognitive impairment, tremors, impaired motor function, vertigo, weakness, bladder dysfunction, fatigue, depression, and double vision.[123]
Ovarian cancer Claudin-3, Claudin-4- Hough et al. 2000 Cancerous growth in the ovaries. Symptoms include nausea, diarrhoea, pelvic or abdominal pain, bloating, and urinary frequency. Because these are very common for a vast array of disorders, early detection of ovarian cancer is difficult.[124]
Prostate cancer- prostatic adenocarcinomas Claudin-1, Claudin-3, Claudin-4 claudin-7- Sheehan et al. 2007 Cancer located in the prostate gland. The majority of cases do not demonstrate any symptoms, making it difficult to detect. When symptoms are present, they may include painful urination, blood in the urine, and frequent urination. Heightened concentrations of prostate-specific antigen indicates the presence of prostate cancer.[125]
Pulmonary edema Claudin-4, Claudin-18, Occludin- Cohen et al. 2010 In this context, pulmonary edema describes the lungs filling with fluid as a result of sepsis. Difficulty breathing and coughing up blood are the chief symptoms. [126]
Thyroid neoplasm Occludin, Claudin-1, Claudin-4, claudin-7- Tzelepi et al. 2007 Tumor of the thyroid gland, which is usually asymptomatic. Nodules are sometimes palpable and may cause pain or alter the voice as they increase in size. There are four different types of thyroid neoplasm- papillary thyroid carcinoma, follicular thyroid carcinoma, medullary thyroid carcinoma, and anaplastic carcinoma.[127]
Type 1 diabetes Claudin-1, Claudin-2- Visser et al. 2010, Uibo et al. 2011 Destruction of beta-cells within the pancreas leads to type 1 diabetes. Beta-cells detect glucose concentrations and release insulin in order to maintain appropriate glucose levels. Loss of this function leads to hypoglycaemia and ketoacidosis. Type 1 diabetes also causes a higher incidence of heart disease, blindness, and kidney failure.[128]


The literature has suggested treatment methods that inhibit proinflammatory cytokines and increase TJ exposure to growth factors and probiotics may be useful in treating TJ dysfunction diseases[129]. It has also been observed that a gluten-casein-free diet may reduce the effects of autism, Crohn’s disease, and ulcerative colitis[130]. Due to the vast array of diseases associated with impaired TJ function and relatively new techniques for examining TJ, extensive research is still required to ascertain all causes of dysfunction, as well as potential treatments for these diseases.

Normal (left) versus cancerous (right) mammography image.Courtesy of the National Cancer Institute.[131]
Scanning electron microscope image of Vibrio cholerae bacteria, which infect the digestive system. [132]
Micrograph showing bile (yellow) stasis, i.e. cholestasis. H&E stain. [133]
High magnification micrograph of collagenous colitis. H&E stain.[134]
This endoscopic image is of Crohn's colitis showing diffuse loss of mucosal architecture, friability of mucosa in sigmoid colon and exudate on wall.[135]‎
Pyoderma gangrenosum on the leg of a person with Crohn's disease[136]
Endoscopic image of Barrett's esophagus[137]
Photomicrograph of a demyelinating MS-Lesion. Klüver-Barerra-Stain. Original Magnification 10x.[138]

Current & Future Research

Berberine

Currently, studies are being conducted in finding the mechanisms by which berberine works. Berberine is a compound which is found in Coptidis rhizoma, a substance used in modern and complementary medicines for the treatment of gastrointestinal disorders[139] . Various studies have made breakthrough discoveries in the workings of berberine. Berberine was found to significantly prevent the decrease in TEER, which is associated with “leaky” epithelia[140]. Furthermore, they were found to prevent the distortion of tight junction morphology and redistribution of the protein “occludin” [141]. However, the exact mechanism by which berbeine may work is still currently not well understood. Thus, in the near future, through various studies, berberine may be used as a therapeutic agent to restore the function of the epithelial barrier function in intestinal disease states.

Gliadin in Celiac disease

Celiac disease is an autoimmune disease of the small intestine. It is still a very ambiguous disease, in that, the mechanisms by which this disease operates is still not well understood. Recently, a glycoprotein, gliadin, found in wheat and various other forms of cereal, is currently drawing interest in the topic of this disease. Various studies have discovered that gliadin had a detrimental effect on tight junction proteins[142].. It was found that gliadin activated an inflammatory response on the epithelial wall of the gut through the damaging of tight junction proteins, as well as activating the proliferation of epithelial cells- a hallmark for celiac disease[143]. Thus, through future studies, a potential mechanism by which celiac disease may be elucidated, and hopefully, a treatment.


Glossary

Barrier:a material object or set of objects that serves as a barricade.

Domain:a three dimensional subunit that combines with other subunits to make up a tertiary structure of a protein .

Exocytic pathway:any cellular process involving in transporting intracellular substances to outside of the cell.

Endocytic pathway:any cellular process involving in transporting extracellular substances to inside of the cell.

Homolog:something that is homologous.

Impedance:an opposition to flows in a circulatory system.

JAMs:Junctional Adhesion Molecules.

Peripheral membrane protein:proteins that temporarily adhere to the surface of a cell membrane via a variety of molecular interactions with integral membrane proteins and lipid bilayer.

Scaffolding protein:a group of proteins with a crucial role of regulating key signaling pathways.

TER:Transepithelial resistance: a measure of resistance to passive ion flow. Generally gives a good outline of the permeability of the cell.

TEER:Transepithelial electrical resistance - a method used to measure TER through the use of simplified circuit models.

Transmembrane proteins:proteins that span from one side of a cell membrane to the other .

ZONAB:ZO-1 Associated Nucleic Acid–binding Protein

ZOs:Zonula Occludens

Reference list

  1. ↑ Molecular Cell Biology. 4th edition. Lodish H, Berk A, Zipursky SL, et al. New York: W. H. Freeman; 2000.
  2. ↑ Molecular Cell Biology. 4th edition. Lodish H, Berk A, Zipursky SL, et al. New York: W. H. Freeman; 2000.
  3. ↑ Molecular Cell Biology. 4th edition. Lodish H, Berk A, Zipursky SL, et al. New York: W. H. Freeman; 2000.
  4. ↑ The Cell: A Molecular Approach. 2nd edition. Cooper GM. Sunderland (MA): Sinauer Associates; 2000.
  5. ↑ The Cell: A Molecular Approach. 2nd edition. Cooper GM. Sunderland (MA): Sinauer Associates; 2000.
  6. ↑ The Cell: A Molecular Approach. 2nd edition. Cooper GM. Sunderland (MA): Sinauer Associates; 2000.
  7. ↑ Molecular Biology of the Cell. 4th edition. Alberts B, Johnson A, Lewis J, et al. New York: Garland Science; 2002.
  8. ↑ Ussing H.H. & Koefoed-Johnsen V. The nature of the frog skin potential. Acta Physiol Scand. 1958 Jun 2;42(3-4):298-308.
  9. ↑ Farquhar M.G. & Palade G.E. Junctional complexes in various epithelia. J Cell Biol. 1963 May;17:375-412.
  10. ↑ Ussing H.H. & Windhager E.E. Nature of shunt path and active sodium transport path through frog skin epithelium Acta Physiol Scand. 1964 Aug;61:484-504.
  11. ↑ Anderson J.M., Van Itallie C.M. Physiology and function of the tight junction. Cold Spring Harb Perspect Biol. 2009 Aug;1(2):a002584.
  12. ↑ Stevenson BR et al. Identification of ZO-1: a high molecular weight polypeptide associated with the tight junction (zonula occludens) in a variety of epithelia. J Cell Biol. 1986 Sep;103(3):755-66.
  13. ↑ Citi S et al. Cingulin, a new peripheral component of tight junctions. Nature. 1988 May 19;333(6170):272-6.
  14. ↑ Citi S et al. Cingulin: characterization and localization. J Cell Sci. 1989 May;93 ( Pt 1):107-22.
  15. ↑ Citi et al. Cingulin, a specific protein component of tight junctions, is expressed in normal and neoplastic human epithelial tissues. Am J Pathol. 1991 Apr;138(4):781-9.
  16. ↑ Gumbiner B et al. Identification of a 160-kDa polypeptide that binds to the tight junction protein ZO-1. Proc Natl Acad Sci U S A. 1991 Apr 15;88(8):3460-4.
  17. ↑ Furuse M et al. Occludin: a novel integral membrane protein localizing at tight junctions. J Cell Biol. 1993 Dec;123(6 Pt 2):1777-88.
  18. ↑ Furuse M et al. Overexpression of occludin, a tight junction-associated integral membrane protein, induces the formation of intracellular multilamellar bodies bearing tight junction-like structures. J Cell Sci. 1996 Feb;109 ( Pt 2):429-35.
  19. ↑ Furuse M et al. Direct association of occludin with ZO-1 and its possible involvement in the localization of occludin at tight junctions. J Cell Biol. 1994 Dec;127(6 Pt 1):1617-26.
  20. ↑ Fasano A. et al. Zonula occludens toxin modulates tight junctions through protein kinase C-dependent actin reorganization, in vitro. J Clin Invest. 1995 Aug;96(2):710-20.
  21. ↑ Furuse M. Et al. Claudin-1 and -2: novel integral membrane proteins localizing at tight junctions with no sequence similarity to occludin. J Cell Biol. 1998 Jun 29;141(7):1539-50.
  22. ↑ Furuse M. Et al. A single gene product, claudin-1 or -2, reconstitutes tight junction strands and recruits occludin in fibroblasts. J Cell Biol. 1998 Oct 19;143(2):391-401.
  23. ↑ Haskins J. Et al. ZO-3, a novel member of the MAGUK protein family found at the tight junction, interacts with ZO-1 and occludin. J Cell Biol. 1998 Apr 6;141(1):199-208.
  24. ↑ Tsukita S et al. Pores in the wall: claudins constitute tight junction strands containing aqueous pores. J Cell Biol. 2000 Apr 3;149(1):13-6.
  25. ↑ Tsukita S et al. Multifunctional strands in tight junctions. Nat Rev Mol Cell Biol. 2001 Apr;2(4):285-93.
  26. ↑ Brandner J. M. Et al. Organization and formation of the tight junction system in human epidermis and cultured keratinocytes. Eur J Cell Biol. 2002 May;81(5):253-63.
  27. ↑ Furuse M. Et al. Claudin-based tight junctions are crucial for the mammalian epidermal barrier: a lesson from claudin-1-deficient mice. J Cell Biol. 2002 Mar 18;156(6):1099-111. Epub 2002 Mar 11.
  28. ↑ Ikenouchi J. Et al. Tricellulin constitutes a novel barrier at tricellular contacts of epithelial cells. J Cell Biol. 2005 Dec 19;171(6):939-45.
  29. ↑ <pubmed>641977</pubmed>
  30. ↑ <pubmed>8276896</pubmed>
  31. ↑ <pubmed>15820558</pubmed>
  32. ↑ <pubmed>10966866</pubmed>
  33. ↑ <pubmed>9175707</pubmed>
  34. ↑ <pubmed>9265654</pubmed>
  35. ↑ <pubmed>15866167</pubmed>
  36. ↑ <pubmed>12604587</pubmed>
  37. ↑ <pubmed>15761153</pubmed>
  38. ↑ <pubmed>9647647</pubmed>
  39. ↑ <pubmed>16771626</pubmed>
  40. ↑ <pubmed>10562289</pubmed>
  41. ↑ <pubmed>16771626</pubmed>
  42. ↑ <pubmed>9786950</pubmed>
  43. ↑ <pubmed>11283726</pubmed>
  44. ↑ <pubmed>15820556</pubmed>
  45. ↑ <pubmed>11739175</pubmed>
  46. ↑ <pubmed>11500366</pubmed>
  47. ↑ <pubmed>10913139</pubmed>
  48. ↑ <pubmed>12070135</pubmed>
  49. ↑ <pubmed>10852816</pubmed>
  50. ↑ <pubmed>10966866</pubmed>
  51. ↑ <pubmed>12482754</pubmed>
  52. ↑ <pubmed>3285223</pubmed>
  53. ↑ <pubmed>11042084</pubmed>
  54. ↑ <pubmed>8325238</pubmed>
  55. ↑ <pubmed>12023291</pubmed>
  56. ↑ <pubmed>15454572</pubmed>
  57. ↑ <pubmed>15866167</pubmed>
  58. ↑ Balda MS, Matter K The tight junction protein ZO-1 and an interacting transcription factor regulate ErbB-2 expression.EMBO J: 2000 May 2;19(9):2024-33 PMID:10790369
  59. ↑ Balda MS, Garrett MD, Matter K The ZO-1-associated Y-box factor ZONAB regulates epithelial cell proliferation and cell density. J Cell Biol: 2003 Feb 3;160(3):423-32 PMID: 12566432
  60. ↑ Zahraoui A, Joberty G, Arpin M, Fontaine JJ, Hellio R, Tavitian A, Louvard D A small rab GTPase is distributed in cytoplasmic vesicles in non polarized cells but colocalizes with the tight junction marker ZO-1 in polarized epithelial cells. J Cell Biol:1994 Jan;124(1-2):101-15 PMID:8294494
  61. ↑ Zahraoui A, Joberty G, Arpin M, Fontaine JJ, Hellio R, Tavitian A, Louvard D A small rab GTPase is distributed in cytoplasmic vesicles in non polarized cells but colocalizes with the tight junction marker ZO-1 in polarized epithelial cells. J Cell Biol:1994 Jan;124(1-2):101-15 PMID: 8294494
  62. ↑ Sheth B, Fontaine JJ, Ponza E, McCallum A, Page A, Citi S, Louvard D, Zahraoui A, Fleming TP Differentiation of the epithelial apical junctional complex during mouse preimplantation development: a role for rab13 in the early maturation of the tight junction. Mech Dev. 2000 Oct;97(1-2):93-104 PMID:11025210
  63. ↑ Molecular Biology of the Cell. 4th edition. Alberts B, Johnson A, Lewis J, et al. New York: Garland Science; 2002
  64. ↑ Molecular Cell Biology. 4th edition. Lodish H, Berk A, Zipursky SL, et al. New York: W. H. Freeman; 2000.
  65. ↑ Molecular Cell Biology. 4th edition. Lodish H, Berk A, Zipursky SL, et al. New York: W. H. Freeman; 2000.
  66. ↑ Molecular Biology of the Cell. 4th edition. Alberts B, Johnson A, Lewis J, et al. New York: Garland Science; 2002.
  67. ↑ <pubmed>9558458</pubmed>
  68. ↑ <pubmed>20066090</pubmed>
  69. ↑ <pubmed>20066090</pubmed>
  70. ↑ <pubmed>9558458</pubmed>
  71. ↑ <pubmed>21349151</pubmed>
  72. ↑ <pubmed>21349151</pubmed>
  73. ↑ <pubmed>21349151</pubmed>
  74. ↑ <pubmed>20066090</pubmed>
  75. ↑ <pubmed>8527224</pubmed>
  76. ↑ <The Journal of Neuroscience, 21 March 2007, 27(12): 3260-3267; doi: 10.1523/JNEUROSCI.4033-06.2007
  77. ↑ Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Siegel GJ, Agranoff BW, Albers RW, et al., editors. Philadelphia: Lippincott-Raven; 1999.
  78. ↑ Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Siegel GJ, Agranoff BW, Albers RW, et al., editors. Philadelphia: Lippincott-Raven; 1999.
  79. ↑ Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Siegel GJ, Agranoff BW, Albers RW, et al., editors. Philadelphia: Lippincott-Raven; 1999.
  80. ↑ Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Siegel GJ, Agranoff BW, Albers RW, et al., editors. Philadelphia: Lippincott-Raven; 1999.
  81. ↑ Molecular Cell Biology. 4th edition. Lodish H, Berk A, Zipursky SL, et al. New York: W. H. Freeman; 2000.
  82. ↑ Molecular Cell Biology. 4th edition. Lodish H, Berk A, Zipursky SL, et al. New York: W. H. Freeman; 2000.
  83. ↑ James L. Madara REGULATION OF THE MOVEMENT OF SOLUTES ACROSS TIGHT JUNCTIONS. Annual Review of Physiology. Vol. 60: 143-159 (Volume publication date March 1998) DOI: 10.1146/annurev.physiol.60.1.143
  84. ↑ <pubmed>20936941</pubmed>
  85. ↑ <pubmed>20936941</pubmed>
  86. ↑ <pubmed>9558458</pubmed>
  87. ↑ <pubmed>20936941</pubmed>
  88. ↑ <pubmed>20936941</pubmed>
  89. ↑ <pubmed>20936941</pubmed>
  90. ↑ <pubmed>20936941</pubmed>
  91. ↑ <pubmed>20936941</pubmed>
  92. ↑ <pubmed>20936941</pubmed>
  93. ↑ <pubmed>9458817</pubmed>
  94. ↑ <pubmed>9458817</pubmed>
  95. ↑ <pubmed>14973266</pubmed>
  96. ↑ <pubmed>9458817</pubmed>
  97. ↑ <pubmed>9458817</pubmed>
  98. ↑ <pubmed>14973266</pubmed>
  99. ↑ <pubmed>14718565</pubmed>
  100. ↑ Coyne,C., Vanhook,M., Gambling, T., Carson, J., Boucher, R., Johnson, L. (2002). Molecular Biology of the Cell 13(9): 3218-3234
  101. ↑ Catalioto, R., Maggi, C., Giuliani, S. (2011). Intestinal Epithelial Barrier Dysfunction in Disease and Possible Therapeutical Interventions. Current Medicinal Chemistry 18(3): 398-426
  102. ↑ Visser, J., Rozing, J., Sapone, A., Lammers, K., Fasano, A. (2009). Tight junctions, intestinal permeability, and autoimmunity. Annals of the New York Academy of Sciences 1165: 195-205.
  103. ↑ Visser, J., Rozing, J., Sapone, A., Lammers, K., Fasano, A. (2009). Tight junctions, intestinal permeability, and autoimmunity. Annals of the New York Academy of Sciences 1165: 195-205.
  104. ↑ Sapone, A., De Magistris, L., Pietzak, M., Clemente, MG., Tripathi, A., Cucca, F., Lampis, R., Kryszak, D et al. (2006). Zonulin upregulation is associated with increased gut permeability in subjects with type 1 diabetes and their relatives. Diabetes 55 (5): 1443–1449
  105. ↑ Wang, W., Uzzau, S., Goldblum, S., Fasano, A. (2000) Human zonulin, a potential modulator of intestinal tight junctions. The Journal of Cell Science 113: 4435-4440
  106. ↑ Fassano, A. (2011). Zonulin and Its Regulation of Intestinal Barrier Function: The Biological Door to Inflammation, Autoimmunity, and Cancer. Physiological Reviews 91(1): 151-175
  107. ↑ Expert Panel Report 3: Guidelines for the Diagnosis and Management of Asthma (2007). National Heart, Lung, and Blood Institute, U.S Department of Health and Human Services.
  108. ↑ Levy, S., Mandell, D., Schultz, R. (2009). Lancet 374:1627.
  109. ↑ Proceedings of National Institute of Health Consensus Development Conference on Celiac Disease (2004). U.S Department of Health and Services.
  110. ↑ Sack, A., Sack, B., Balakrish, G., Siddique, A (2004). Cholera. The Lancet 363(9404): 223.
  111. ↑ [ http://www.nlm.nih.gov/medlineplus/ency/article/000215.htm Cholestasis] (2010). U.S National Institute of Health.
  112. ↑ Sakisaka, S., Kawaguchi, T., Taniguchi, E., Hanada, S., Sasatomi, K., Koga, H., Harada, M., Kimura, R., Sata, M., Sawada, N., Mori, M., Todo, S., Kurohiji, T. (2001). Alterations in tight junctions differ between primary biliary cirrhosis and primary sclerosing cholangitis. Hepatology 33(6): 1460.
  113. ↑ Long, H., Crean, CD., Lee, WH., Cummings, OW., Gabig, TG. (2001). Expression of Clostridium perfringens enterotoxin receptors claudin-3 and claudin-4 in prostate cancer epithelium. Cancer Research 61(21): 7878.
  114. ↑ Madisch, A., Miehlke, S., et al. (2007). Boswellia serrata extract for the treatment of collagenous colitis. A double-blind, randomized, placebo-controlled, multicenter trial. International Journal of Colorectal Disease 22:1445.
  115. ↑ Danese, S., Semeraro, S., Papa, A., et al. (2005). Extraintestinal manifestations in inflammatory bowel disease. World Journal of Gastroenterology 11(46):7227.
  116. ↑ Baumgart, D., Sandborn, W. (2007). Inflammatory bowel disease: clinical aspects and established and evolving therapies. The Lancet 369(9573): 1641
  117. ↑ Hardin, D. (2004). GH improves growth and clinical status in children with cystic fibrosis – a review of published studies. European Journal of Endocrinology 151.
  118. ↑ Quinton, P. (2007). Cystic Fibrosis: Lessons from the Sweat Gland. Physiology 22:212-225.
  119. ↑ Tapp, R., et al. (2003). Diabetes Care 26(6): 1731.
  120. ↑ al-Ghamdi, M., Cameron, E., Sutton, R. (1994). Magnesium deficiency: pathophysiologic and clinical overview. American Journal of Kidney Diseases 24(5): 737-752.
  121. ↑ Jovov, B. et al. (2007). Do Tight Junction Structure and Function Play a Role in the Acid Resistance of Barrett’s Esophagus? The FASEB Journal 21: 711.
  122. ↑ Ben-Yosef, T., et al. (2003). Claudin 14 knockout mice, a model for autosomal recessive deafness DFNB29, are deaf due to cochlear hair cell degeneration. Human Molecular Genetics 12(16): 2049-2061.
  123. ↑ Compston, A., Alasdair Coles, A. (2008). Multiple Sclerosis. Lancet 372: 1502-1517.
  124. ↑ Rossing, M., et al. (2009). Predictive Value of Symptoms for Early Detection of Ovarian Cancer. Journal of the National Cancer Institute 102(4): 1.
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  126. ↑ Cohen, S., Gray Lawrence, G., Margulies, S., (2010). Cultured alveolar epithelial cells from septic rats mimic in vivo septic lung. PLoS One 5(6):11322.
  127. ↑ Alford, E., Hu, M., Ahn, P., Lamont, J. (2001). Thyroid and Parathyroid Cancers. Cancer Management 13.
  128. ↑ Bluestone, J. et al. (2010). Genetics, pathogenesis and clinical interventions in type 1 diabetes. Nature 464:1293.
  129. ↑ Catalioto, R., Maggi, C., Giuliani, S. (2011). Intestinal Epithelial Barrier Dysfunction in Disease and Possible Therapeutical Interventions. Current Medicinal Chemistry 18(3): 398-426
  130. ↑ de Magistris, L., Familiari, V., et al. (2010). Alterations of the Intestinal Barrier in Patients With Autism Spectrum Disorders and in Their First-degree Relatives. Journal of Pediatric Gastroenterology and Nutrition 51(4): 418-424
  131. ↑ http://en.wikipedia.org/wiki/File:Mammo_breast_cancer.jpg
  132. ↑ http://en.wikipedia.org/wiki/File:Cholera_bacteria_SEM.jpg
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  138. ↑ http://en.wikipedia.org/wiki/File:MS_Demyelinisation_KB_10x.jpg
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  142. ↑ Luca et al. Imaging analysis of the gliadin direct effect on tight junctions in an in vitro three-dimensional Lovo cell line culture system Toxicol In Vitro. 2011 Feb;25(1):45-50. 2010 Sep 17.
  143. ↑ Luca et al. Imaging analysis of the gliadin direct effect on tight junctions in an in vitro three-dimensional Lovo cell line culture system Toxicol In Vitro. 2011 Feb;25(1):45-50. 2010 Sep 17.


Coordinator Comment to all Groups

I will add a general comment that will be the same to all groups under this heading.

Referencing Extension Problem

--Mark Hill 13:16, 3 May 2011 (EST) As mentioned in the lecture, I am aware of the referencing extension problem on your project pages. I have the following temporary solution, of removing the extension, so that groups can continue to add content to their project pages. I am also giving everyone a 1 week extension before the peer assessment.

This should only be done if your project page is not allowing you to save changes!

A. The Easy Way....

The following 4 steps can be done on the webpage or select all content in edit mode, copy and paste into a text editor. All steps must be completed before you attempt to save.

  1. In page edit mode, find all <pubmed> reference tags.
  2. Replace this tag with [http://www.ncbi.nlm.nih.gov/pubmed/ Note, there should be no spaces between the internet address and the pmid number.
  3. Now find all </pubmed> reference tags.
  4. Replace this second tag with ]

This will generate a numbered reference list that we can later fix up.


B. The Better Looking Result....

Whatever is between the <ref> </ref></pubmed> tags is what will appear in your reference list, so you can format the reference and link to appear in your reference list.

2011 Projects: Synaptic Junctions | Gap Junctions | Tight Junctions | Desmosomes | Adherens Junctions | Neuromuscular Junction
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