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1. GLOMERULAR FILTRATION BARRIER TOPICS ANATOMY OF GLOMERULAR FILTRATION BARRIER PHYSIOLOGY CLINICAL IMPLICATIONS

2. Filtration barrier: Fenestrated capillary endothelium Glomerular basement membrane filtration slit or slit diaphragm 2 MODELS PORE MODEL ELECTROKINETIC MODEL

3. ENDOTHELIAL CELLS The glomerular capillaries are lined by a thin fenestrated Endothelium Under normal conditions , the formed elements of the blood, including erythrocytes , leukocytes, and platelets, do not gain access to the subendothelial space . The endothelium contains pores or fenestrae that range from 70 to 100 nm in diameter Non fenestrated endothelium has no role

4. The glomerular endothelium is covered by a glycocalyx layer The glycocalyx also fills the endothelial fenestrae forming “ sieve plugs ,” the exact function of which is unknown. The glycocalyx consists of membrane-bound proteoglycans ( syndecan and glypican ) with attached glycosaminoglycans (GAGs ), secreted glycoproteins ( perlecan and versican ), and secreted GAGs ( hyaluronan ), which provide a negative charge. Endogenous albumin is largely confined to the glomerular capillary lumen and does not pass through the endothelium

5. Signaling between glomerular cells is critical for the development and maintenance of the filtration barrier. VEGF is synthesized by podocytes (glomerular visceral epithelial cells) and is an important regulator of microvascular permeability. VEGF increases endothelial cell permeability and induces the formation of endothelial fenestrations. VEGF-A is the best characterized podocyte growth factor, and its principal receptor is VEGFR2, expressed on endothelial cells

6. Podocyte -specific alterations of VEGF-A have demonstrated that it is required for normal differentiation of glomerular endothelial cells Drug inhibition of VEGF-A in patients or podocyte -specific deletion of VEGF-A in adult mice results in severe glomerular endothelial injury and thrombotic microangiopathy . Thus , VEGF produced by podocytes plays a critical role in the differentiation and maintenance of glomerular endothelial cells and is an important regulator of endothelial cell permeability

7. Thus, VEGF produced by podocytes plays a critical role in the differentiation and maintenance of glomerular endothelial cells and is an important regulator of endothelial cell permeability VEGF IS THE MAIN MOLECULE SURVIVAL/DAMAGE/PROLIFERATION/MAINTENACE

8. GLOMERULAR BASEMENT MEMBRANE By transmission electron microscopy, the GBM is composed of a central dense layer, the lamina densa , and two thinner more electron-lucent layers, the lamina rara externa and the lamina rara interna Like other basement membranes in the body, the GBM is composed primarily of type IV collagen, laminin , nidogen ( entactin ), and heparan sulfate proteoglycans (HSPGs). DIFFERENCES BETWEEN GBM AND OTHER TWO LAYERS

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10. Compared with other basement membranes, the GBM is thicker, likely at least in part from fusion of endothelial and epithelial basement membranes during development . Type IV collagen consists of six chains, α1(IV) through α6(IV ). Three α(ΙV) chains self-associate intracellularly to form triple helical molecules called protomers

11. Three types of promoters are formed: α1α2α1, α3α4α5, and α5α6α5. Upon secretion into the extracellular space, the protomers self-associate via their amino- and carboxy -terminal domains to form polymerized networks .

12. The α3α4α5(IV)-α3α4α5(IV) network predominates in the GBM, whereas the α1α2α1(IV)-α5α6α5(IV) network is in Bowman’s capsule. Whereas α1α2α1(IV) protomers are synthesized from both endothelial cells and podocytes , α3α4α5(IV) protomers are secreted only by podocytes .

13. Ultrastructural tracer studies provided evidence to suggest that the GBM constitutes both a size-selective and a charge-selective barrier. Additional studies revealed a lattice of anionic sites with a spacing between them of approximately 60 nm throughout the lamina rara interna and lamina rara externa. The anionic sites in the GBM consist of heparan sulfate GAG side chains of the proteoglycans rich in heparan sulfate . Role is minimal Importance of laminin beta 2 is increasing recently

14. PODOCYTES AND THEIR ROLE Podocytes (visceral epithelial cells) are the largest cells in the glomerulus Mature podocytes are terminally differentiated They have a prominent cell body containing nuclei , endoplasmic reticulum, Golgi apparatus, and an endocytic–lysosomal system The gap between adjacent foot processes is bridged by a thin structure called the filtration slit diaphragm.

15. Podocytes have an elaborate cytoskeleton that underlies their shape, stability, adhesion, and response to stress. Large numbers of microtubules and intermediate filaments ( vimentin ) are present in the cell body and primary processes,whereas actin filaments are especially abundant in the foot processes The filtration slits have a constant width of approximately 30 to 40 nm thus the SD has to connect the FPs over a considerable distance

16. The transmembrane proteins that establish the slit diaphragm (SD) and its connection to the actin cytoskeleton of the FPs include nephrin , P-cadherin, FAT1, NEPH 1-3,podocin , and CD2AP, among others They also include receptors for cyclic guanosine monophosphate (cGMP) signaling, stimulated by natriuretic peptides (atrial natriuretic peptide [ANP], brain natriuretic peptide [BNP], and C-type natriuretic peptide [CNP]) and nitric oxide; receptors for cyclic adenosine monophosphate ( cAMP ) signaling stimulated by prostaglandin E2 (PGE2), dopamine, VEGF, isoproterenol, parathyroid hormone (PTH ), PTH-related peptide; and receptors for Ca2+ signaling stimulated by numerous ligands, including angiotensin II, acetylcholine, PGF2, arginine vasopressin (AVP), adenosine triphosphate (ATP), endothelin, and histamine. Among the transient receptor potential (TRP) cation channels, TRPC5 and TRPC6 have received much attention

17. FILTRATION PRESSURE FILTRATION FLOW FPs , which need their cytoskeleton to continually adapt their pattern of attachment to the GBM would simultaneously function as contractile pericyte -like processes, counteracting the expansion of the GBM by increasing their tone. Consequently,it may be concluded that the principal burden for counteracting transmural pressure gradients (i.e., for developing wall tension) falls instead on the GBM. FOOT PROCEESSES ,SLIT DIAPHRAGM AND GMB ACT SYNERGESTICALLY

18. Podocytes and their processes Foot processes or pedicels Filtration slit or slit pore Slit diaphragm/modified adherens junction Cytoskeleton(microtubules and intermediate filaments like vimentin and desmin ) Actin myosin interactions

19. SUMMARY

20. Size of the particles <4 nm-freely filtered >8 nm- never filtered 4-8 nm-depends on charge Low molecular weight(<70000 daltons ),water are filtered Plasma proteins like albumin,globulins,calacium,fatty acids are not filtered

21. Type 4 collagen and laminin are the main component of GBM. PAS stains mesangial matrix, GBM, tubular basement membrane and hyaline deposits. The 3 layers of glomerular capillary membrane acts as filtration barrier and ultrafiltrate is formed. GBM has negative charge, hence negatively charged bodies are not filtered. For a give molecular radius, the filtration is positive molecules>neutral>negative.

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