2015 Group 7 Project
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- 1 Basement Membrane
- 1.1 Introduction
- 1.2 History
- 1.3 Formation, Plasticity and Regeneration
- 1.4 Functional Layers
- 1.5 Structural Components
- 1.6 Heterogeneity of Basement Membrane
- 1.7 Basement Membrane Functions
- 1.8 Abnormalities
- 1.9 Current Research
- 1.10 References
The Basement Membrane (BM) is a thin layer of specialized extracellular matrix(ECM)which serves as a support structure for which epithelium grows upon, as well as, surrounds muscle cells, fat cells, and neural (Schwann) cells. It also provides mechanical support, compartmentalizes tissues, and influences cellular behaviour . Some of the major molecular structures involved in BM are Collagen IV, laminin, proteoglycans, and nidogen/enactin. . Currently about 50 proteins have been identified, in contributing to the structure of BM. Minor structures of the BM include agrin, SPARC/BM-40/osteopontin, fibulins, type XV collagen and type XVIII collagen . This page focuses on the structure and function of the BM and how it develops, in addition it also explores historical and current research, along with abnormalities.
The basement membrane was first described by Bowman in 1840 as a membranaceous sheath of the most exquisite delicacy in muscle. It wasn't until the 1970's that laminin, type IV collagen, nidogen and perlecan were identified. .
|1840||Bowman, Membranaceous sheath of most exquisite delicacy|
|1957||Todd and Bowmen first describe the basement membrane and suggested that they were continous sheets around tissues|
|1950-60s||Electron microscopes are used in the study of BM and it's revealed them to be separate structures to the ECM|
|1970||Laminin, type IV collagen, nidogen and perlecan|
|1972||BM formed scaffolding that nerve and muscle cells follow during regeneration|
|1983||Anchoring fibers of BM were identified - collagen type VII|
|1985||'Lamina lucida' 'lamina densa' and 'lamina fibroreticularis' become internationally recognised as terms referring to BM structure|
|1900||Lindemann mouse model of nephritis evoked intrest in BM|
Formation, Plasticity and Regeneration
- The large insoluble molecules that make up the BM form sheet-like structures through the process of 'self-assembly'. This congregation of cells is mediated by cell-surface anchors and receptors. Elements such as laminin and collagen IV are supplied from other cells within the body to form some components of the BM. Information within the primary sequences of laminin and type IV collagen initiates intermolecular self-assembly. Other minor components, such as perlecan and nidogen/entactin, do not have the capability to instruct this assembly. However, these minor components facilitate the interaction of independent type IV collagen and laminin scaffolds.
- Laminin provides the center framework inside the cell, with type IV collagen providing the attached scaffold, giving strength to the cell and surrounding BM cells.
- Laminin polymers are deposited via site-specific interactions by cell-surface proteins such as Beta1 integrins and dystroglycans. Type IV collagen polymers bind with laminin polymers on the cell surface via nidogen/entactin bridging." These interactions create the fully functional BM.
- BM specialization occurs through tissue-specific isoformes of laminin and collagen IV, as well as, particular proteoglycan populations.
- BM thickness can range between 50-100nm depending on the localization in the body as well as the ratios of different isoformes of molecular components.
The structural integrity, plasticity, and regeneration of the basement membrane are important components to note. The basement membrane is not a static structure. It is constantly regenerated creating different ratios of ECM proteins over the life of an organism.
The BM has morphological differences between the epithelial, outward facing side, and stromal, inward facing side.
- Dermal Collagen extends from the dermal layer through the reticular lamina and into the lamina densa?
- Apical Layer
- Adheres to epithelial cells
- Exhibits high levels of laminin proteins.
- 40-50nm thick?
- Hemi-desmosomes connect these two layers to the epithelium?
- Stromal side
- Adheres to connective tissue cells
- Exhibits high levels of collagen IV.
It has been found that the majority of the BM consists of water tightly joined by GAG side chains of proteoglycans(source). Nevertheless, laminin and type IV collagen are the most abundant components of the BM. Basement membranes vary in thickness and strength due to the different combinations of these component's isoformes. Isoformes of laminin and type IV collagen share significant homologies; however, their amino-acid sequence may differ by 30-50%.
-Most abundant noncollagenous protein in the BM
-Aids in epithelial attachment to BM Type IV collagen
-Compromises 50% of all BM proteins which compromises the largest portion of the BM.
-6 different type IV collagen α-chains have been identified.
-56 possible combinations of trimers.
-Collagen XV and XVIII are also present in BM.
- *Insert* information about perlecan and it's functions with the extracellular matrix
Heterogeneity of Basement Membrane
Basement membranes are highly diverse and tissue specific. Differences and varied amounts of basement membrane components such as laminin, IV collagen, perlecan and nidogen as well as the varied subtypes of these components involved, are usually what cause basement membrane heterogeneity .
For example, in kidney the α3α4α5 type IV collagen protomer is found in the glomerular basement membrane (GBM), whereas α5α5α6 (speculative) and α1α2α1 protomers are found in the Bowman’s capsule BM. Furthermore, in the kidneys there are at least 3 different types of collagen . The diversity of these collagen and laminin subtypes found in the Kidney are essential for allowing tissue specific functioning. The composition of the BM of the tubular epithelium allows for osmoregulation whilst the BM in glomerular epithelium allows for plasma filtration. The BM proteins are also “encoded by differentially regulated genes” thus particular components of the BM can be down regulated or up regulated depending on the tissue .
Basement Membrane Functions
The basement membrane serves a whole array of functions depending on the type of tissue it is located in. In general, it serves as a cell supporting matrix, organizes the cell monolayers during tissue development, acts a tissue barrier, participates in cell adhesion and has ligands for cell surface receptors. Laminin and IV collagen form the meshwork of most basement membranes while it is functions are maintained by cell surface receptors such as integrins and dystroglycans. 
The image shows the normal nephron structure including a glomerular basement lining the glomerulus.
In the kidneys, the BM acts a semi permeable selective barrier which allows for filtration of the wastes from the blood. Thus, the BM in kidneys varies from other tissue BM, it's components mainly consist of α3α4α5 type IV collagen network, the laminin 10/11 polymer, and perlecans which provide mechanical strength. It was also thought to have charge properties to allow for filtration of blood but this is not confirmed. 
In the skin, the BM is connected to the dermis via anchoring fibrils, type VII collagen of these fibrils binds to laminin 332. . There are also collagen I and II fibrils interwoven. This forms a strong adhesion complex which is necessary for protecting the structural and functional integrity of the skin . A study found that knockout of Lama3 gene which expresses laminin-332 was lethal at neonatal stage and resulted in loss of cell adhesion, in other cases defects of skin basement membrane of its associated molecules can result in skin blistering. 
Vascular basement membranes can act as barrier to transmigration of leukocytes during inflammation. In neoplasia, cancer metastasis requires that cancer cells must cross the vascular and non-vascular basement membrane. However, BM is a tight covalently cross linked mesh formed by laminin and type IV collagen polymers, bridged by nidogens so it is very difficult for cells to pass through it under they are 50nm or less . Studies have suggested that microvascular basement membranes, in pancreas blood vessels, allow for insulin gene expression and promote B islet cell proliferation via laminin binding to β1 integrin.  Furthermore, binding of α4 laminin to the α6β1 integrin modulates T lymphocyte extravasation, it can allow laminin α4 to promote and laminin α5 to inhibit the T lymphocyte extravasation at postcapillary venules in CNS. 
The BM also acts as a reservoir for growth factors. One of the growth factors vascular endothelial GF (VEGF) promotes angiogenesis at sites of injury, for example in neoplasia . XVIII collagen is cleaved and released from the basement membrane during remodeling and it is anti-angiogenic thus prevents tumor growth.
Nidogen 1 and nidogen 2 knock out mice have been used to demonstrate the importance of BM in optimal organ function. Functional redundancy of nidogen in these mice are “perinatally lethal” which highlights the importance of this glycoprotein which allows for BM formation in lung and heart. However, other than the heart and lung, other organs develop as per normal suggesting that BM of different organs have different compositional requirements for optimal tissue functioning.
The mechanical, viscoelastic properties of ligand bearing polymers on BM allows them to transmit information to cells and thus play a part in cell signalling . Basement membranes contain gel consistency polymers that can increase in viscosity as the concentration of laminin or type IV collagen is increased. The laminin-111 gels have been found to promote glandular differentiation and milk production in mammary glands through signaling.
In smooth muscle, there are linkages from the basement membrane to stromal collagen that contribute to sarcolemma stabilization . Mutations affecting any of the key links (dystrophin, dystroglycan-binding to basement membrane, laminin-211, type VI collagen) result in a muscular dystrophy.
In this portion of the page we will discuss abnormalities that may occur in the development of the basement membrane. The content will cover several diseases that are most commonly and some rare abnormalities related to the formation of the basement membrane. This will include genetic predisposition and also some that may lead to cancer progression. There are currently numerous methods used to ensure patients are able to treat the diseases early on and to even detect them among children prior to their birth. This is conducted through genetic testing to ensure best treatment is established.
The most common abnormalities of the basement membrane involve the Renal system of the body. When normal homeostatic functioning is perturbed due to diseases in the basement membrane of the glomerulus then filtration of the blood will no longer be sufficient These diseases will be covered first as there are many some are more common than others. We will also cover several other diseases associated with the basement membrane. Diabetes and hypertension also cause many complications and even cancers that are best distinguished to be malignant through the invasion of the basement membrane.
|Additional info Glomerulus|
The glomerulus is a component of the kidney nephrons, its role is to filter the blood and form the waste product (urine). The glomerulus consists of a network of capillaries which increases the surface area. It is surrounded by the Bowman's capsule. Together these two components form the Renal corpuscle . The glomerulus is composed of the glomerular basement membrane (GBM) which lies between podocytes and endothelial cells. The GBM is made up of type IV collagen, nidogen, heparan sulfate proteoglycan and laminin . Mutations associated with any of these components will cause defects in renal filtration. Diseases associated with the Glomerulus will form scarring or swelling damaging the lining, this can interfere with the filtration of waste products form the kidney. Thus a build up of waste products may occur resulting in more life threatening conditions. 
Alport syndrome is charcaterised genetically and clincally as a heterogenous nephropathy. There are several types including; X-linked (most common) with an incidence of 80% diagnosed with the syndrome, autosomal recessive is 15% and autosomal dominant is 5% . It occurs due to mutations in the collagen,type IV, alpha5 (COL4A5) gene as a result of an X-lined semidominant condition. The COL4A5 gene encodes for type IV collagen and is responsible for one of six subunits which then form networks that contribute to the structure of the basement membrane. A more severe affects occurs among males compared to females. Autosomal recessive mutations in COL4A3 or COL4A4 are seen in less than 10% of patients. Although in this form both genders are affected severely. Incidence
The clinical manifestations that may occur as a result of the syndrome include; hematuria, renal failure, hearing loss, renal flecks and lenticonus.
Genetic testing is highly recommended as the best form of diagnosis for patients. Treatment involves renin-angiotensin system blockade to treat autosomal recessive and male x-linked Alport syndrome individuals. To manage the disease there is great focus on preventing kidney failure. For patients progressing to final stages of kidney failure, transplants are advised.
Thin Glomerular Basement Membrane
Thin basement membrane disease (TBMD) also known as benign familial hematuria or thin basement nephropathy, commonly causes hematuria (urine containing red blood cells). The condition causes diffuse thinning of the glomerular basement membrane in the kidney’s. It is a disorder that affects 1% of the population and it’s the most common inherited disorder. There is some evidence to suggest that abnormalities of the basement membrane occur due to defects in type IV collagen genes COL4A3 & COL4A4. These genes are responsible for encoding alpha-3 & alpha-4 chains which are essential components in the manufacturing of type IV collagen. The disorder may occur in conjunction with Allport syndrome or IgA Nephropathy . Individuals affected by the disorder generally have normal functioning kidney’s, although some adults may develop renal failure. In most cases it is often seen in patients who have a family history of hematuria. 
IgA nephropathy also known as Berger's syndrome is the most common type of glomerulonephritis and end stage renal disorder . The disorder involves the immunoglobulin A (IgA) which attach to the glomeruli and then progressively destroy it. The kidney will loose it's filtrating function and can develop into chronic kidney failure. Treatment focuses on dialysis and transplants .
Diabetes mellitus is a metabolic condition that consists a number of causes including chronic hyperglycaemia, carbohydrate, metabolism of fat and protein with insulin secretion problems. The disorder is responsible for affecting a number of organs, especially the function of the kidney's. Diabetes affects the glomeruli, it degrades the filtration structure and so normal function is altered . This occurs due to microangiopathy in which the glomerular capillary and tubular basement membranes thicken and the mesangial matrix expands . It may lead to kidney failure and even death. The end stage of renal disease is due to diabetic nephropathy, clinically accompanied by proteinuria, increased blood pressure and glomerular filtration impairment. . Patients are often advised to undergo dialysis and in some severe cases kidney transplant.
Progression to Cancer
The endothelial basement membrane normally prevents migrating cells to enter by providing a barrier. Although metastic cells can invade the parenchyma of specific organs and proliferate causing a tumour to arise or a metastasis. Enzymes which degrade the matrix have been identified in metastatic cells, these include serine proteases, cathepsins and metalloproteinases. In addition to these collagenase IV/gelatinase enzymes and serine proteases are significant for the invasion of the basement membrane. . Most cancers which become malignant will invade through the basement membrane include;
- Any epithelial derived cancers
Epidermolysis Bullosa (EB) is a condition in which the skin becomes fragile, this makes it susceptible to injury which can cause painful blisters. It may be more problematic if the blisters are infected. The blisters may occur on the skin or inside the body in areas such as the mouth, oesophagus, stomach (GIT) or the bladder  . There are several types these include simplex EB, junctional EB, and dystrophic. The dystrophic type of epidermolysis bullosa is a hereditary disorder that causes blistering, it is an autosomal dominant and recessive inheritence. It occurs due to type VII collagen gene mutations. This gene encodes collagen proteins which are aspects of fibril anchoring in dermal-epidermal junction .
Hereditary angiography with nephropathy, aneurysms and muscle cramps is caused by heterozygous mutation in COL4A1 gene. The syndrome is associated with blood vessel disorders. The vascular structures and basement membrane are weakened then become susceptible to ruptures. Nephropathy may also occur when the basement membrane in the kidney’s can cause hematuria. There may also be complications with the eyes. This involves the arteries which can lead to bleeding and also cataract clouding. Its one of the most uncommon autosomal dominant conditions and only several families have been identified to have been affected. Many parts of the body are affected because COL4A1 is responsible for type IV collagen which is necessary for basement membrane formation . The membrane is seen throughout the body especially in the blood vessels. When the IV collagen in unable to develop proteins in the basement membrane tissues such as the brain, muscles, eyes and kidneys weaken .
Goodpasture's disease or anti-glomerular basement antibody disease, is a vasculitis of the glomerular or pulmonary capillaries, even a combination of both. The immune cells produce antibodies against collagen, thus destroying collagen in the kidney and lung regions. The incidence of the disease is about 1 case in a million a year. It's diagnosed by the presence of anti-GBM antibody within blood circulation or renal tissue.  Even thoug it is a very rare autoimmune disorder a Goodpasture antigen. The ant-GBM antibodies have been identify as the noncollagenous-1 domain of the α3 chain involving basement membrane collage type IV. It has also been noted that immune mechanisms consisting of both humoral and cellular types allow the disease to develop . Treatment involves several techniques from Immunosuppressive medication (cyclophosphamide) to ensure the immune systems continues to produce antibodies and Corticosteroids for the autoimmune response in the body to be suppressed. Another method is Plasmapheresis which involves the removal of blood form the body, to make sure that anti-GBM antibodies are removed from plasma. 
Cogan’s microcystic dystrophy
Cogan's microcystic dystrophy also know as epithelial basement membrane dystrophy (EBMD) or Map-dot-fingerprint dystrophy, is a condition that affects the cornea of the eyes. A anterior corneal dystrophy occurs bilaterally features epithelial grey lines and dots (microcysts). The corneal is occluded by microcysts which can become irritant to the occular, it may become inflammed and vision is impaired . Patients are usually asymptomatic although painful chronic recurrent corneal erosion, decreased vision, photophobia. Recurrent erosion may occur due insufficient adhesion in basal epithelium to abnormal lamellar structures .
For further information on what is currently going on in the field of cell biology regarding the basement membrane, take a look at the following recent articles published to PubMed:
- Audiopedia [Audiopedia], 4/12/2014, Basement membrane, https://www.youtube.com/watch?v=_S-DIGGYYaQ
- <pubmed> PMC3971924</pubmed>