How the Anatomical and Molecular Structure of the Podocyte Facilitates Filtration in the Kidney - Case Study Example

Discuss how the anatomical and molecular structure of the podocyte facilitates filtration in the kidney Discuss how the anatomical and molecular structure of the podocyte facilitates filtration in the kidney Discuss how the anatomical and molecular structure of the podocyte facilitates filtration in the kidney Podocytes are the glomerular visceral epithelial cells covering the outer aspect of the glomerular basement membrane. The outer part of the glomerular tuft of the kidneys is the area which has parts of the glomerular capillaries and mesangium which face the urinary space (Kriz et al, 1998). The podocytes cover this area. The podocyte cell bodies being large and voluminous, bulge into the urinary space. The foot processes of the podocyte affixes the podocyte to the capillaries. These processes of neighbouring podocytes interdigitate allowing the formation of filtration slits covered by the slit diaphragm. The epithelial cells in the podocytes are polarized in nature with a luminal and basal cell membrane. The cell membrane corresponds to the sole plates of the foot processes. The slit diaphragm is found between the luminal and basal cell membranes (Kriz et al, 1998). A thick surface coat of sialoglycoproteins covers the luminal membrane and the slit diaphragm. The podocytes have a high negative surface charge due to the sialoglycoproteins made up of podocalyxin and podoendin among others. This surface charge helps to maintain the interdigitating pattern of the foot processes. When the surface charge is neutralized by cationic substances like protamine sulphate, the processes draw back and cause tight junctions between them (Kriz et al, 1998). The abluminal membrane contains many proteins participating in the various functions of the glomerulus. Podoplanin is a recently described protein found all over the podocyte. Galatosamine residues of glycoconjugates and A13 (podocyte-specific protein) are found. The cell body has the large nucleus, well developed Golgi system, endoplasmic reticulum and lysosmes (Kriz et al, 1998).Podocytes play a significant role in the physiology and pathology of the glomerulus and form the basis of the filtration process in the kidney along with the slit diaphragm. The glomerulus has a glomerular filtration barrier which is deeply involved in the kidney functions (Pavenstadt, 2000). The podocyte is the most differentiated cell or highly specialized cell in the glomerulus for the functions. The podocytes stabilize the glomerular architecture by preventing distensions of the glomerular basement membrane and maintain the large filtration surface at the slit diaphragm. They account for 40% of the hydraulic resistance of the filtration barrier (Pavenstadt, 2000). The foot processes are contractile in nature with actin, myosin, alpha-actinin, vinculin and talin and connected to the basement membrane through a complex of alpha3 beta1-integrin. The contractile structures can respond to vasoactive hormones and control the ultrafiltration coefficient. The size and charge characteristics are provided by the podocytes. Damage of the podocytes can contribute to the withdrawal of the foot processes resulting in proteinuria (Pavenstadt, 2000). Diseases affecting the glomerulus injure the podocytes. The podocytes then change shape, retract their foot processes and die out in the damage process. Minimal change nephropathy, membranous nephropathy, focal segmental glomerulosclerosis, chronic glomerulonephritis and diabetic nephropathy can result (Pavenstadt, 2000). The physiological and molecular levels of glomerular filtration are not yet understood in its totality. Learning more about the podocytes will one day provide sufficient information about the filtration at the glomerulus. The pathogenesis of proteinuria is centred around the podocytes (Lahdenkari et al, 2004). Proteinuric kidneys show an effacement of the foot processes. The morphological change seen is the large areas of flattened epithelial cytoplasm in place of the foot processes. This could happen as an abnormal response of the epithelium to direct injury or changes in the glomerular epithelium due to other causes. Changes could also occur to the actin cytoskeleton but the reason is unknown (Lahdenkari et al, 2004). Genetic defects of the slit diaphragm causes nephrotic syndrome. The massive proteinuria in nephrotic syndrome cannot be explained by the loss of the interdigitations in the foot processes. The cytoplasm covers the glomerular basement membrane and only few interruptions are seen (Lahdenkari et al, 2004). Here the junctions are found to be tight or close so there is insufficient cause to explain the proteinuria. Two hypotheses are propounded for relating the structural changes with the proteinuria. The first states that the proteinuria occurs due to the detachment of the basement membrane leading to focal gaps in the epithelium. The increased water flux would take the proteins along with it in the escape into the urinary space. The second hypothesis is that active endocytosis causes the escape of massive amounts of protein from the epithelium into the urine. The electron microscope pictures vouch for this hypothesis; pinocytic vesicles, phagosomes and lysosomes are found (Lahdenkari et al, 2004). Though previously podocytes were taking the blame for nephrotic syndrome, genetic reasons are now indicated for the change in the slit diaphragm. Defects in the podocyte proteins like nephrin, Neph 1, podocin, fat and CD2AP have been shown as the reason for the proteinuria in nephrotic syndrome (Donoviel et al, 2001). Lahdenkari et al (2004) confirmed that effacement and fusion were evident in the kidneys in the proteinuric NPHS1 (congenital nephrotic syndrome of the Finnish type) and nonproteinuric MCNS (minimal change nephrotic syndrome). However it was indicated that the epithelial changes were not related to the proteinuria. Detachment of the podocytes from the basement membrane was not seen in the proteinuric glomeruli. Pinocytic vesicles were equally seen in both proteinuric and control kidneys. The podocyte slit pore density was reduced by 80% very much reducing the filtration surface (Lahdenkari et al, 2004). Nephrin was found in the apex of the foot process of the podocyte in the narrow slit pores of MCNS. This showed a “vertical transfer of the slit diaphragm complex in proteinuria” (Lahdenkari et al, 2004). The study concluded that the proteinuria occurred through defective podocyte slits. Associated structural alterations were just secondary changes; they were not conducive to the proteinuria. The glomerular filtration barrier is believed to have a permselectivity of restricting passage of proteins across it into Bowman’s space (Koop et al, 2008). The loss of this quality is found in renal diseases of various origins and proteinuria is the accompanying feature. The podocyte is an important component of the glomerular filtration barrier and its damage is the reason for the pathology of glomerular illnesses (Koop et al, 2008). Koop et al have indicated that mutations in the genes that encode nephrin, podocin CD2AP and alpha-actinin-4 produce hereditary and congenital forms of glomerular diseases in their study in rats. Podocyte damage is seen in these and acquired illnesses. Disease severity in diabetic nephropathy and IgA nephropathy also has accompanying podocyte damage. The foot process effacement is the change in the morphology of the foot process of the podocyte to a flatter epithelial cell (Koop et al, 2008). Proteinuria is a risk factor for advancing renal disease. Prevention of this proteinuria can reduce the burden of the chronic renal disease. Koop et al found that increased loss of podoplanin expression was related to urinary albumin excretion (2008). Decreased podoplanin was seen slightly before the morphological changes in the podocytes in rats at the age of 6 weeks. At eight weeks, when proteinuria had been present for some time, electron microscopy showed the foot process effacement in areas with loss of podoplanin (Koop et al, 2008).Those areas with no loss of podoplanin did not have structural changes. The expression of the slit diaphragm associated proteins nephrin and podocin were not related to the expression of podoplanin. The suggestion is that downregulation of podoplanin caused proteinuria or the expression of podoplanin was down-regulated because of proteinuria (Koop et al, 2008). The foot effacement and the formation of the stress fibres are reactions of the podocyte to prevent dilatation of the capillaries and further leaking of proteins (Kriz, 1996). Focal segmental glomerulosclerosis (FSGS) is a clinicopathologic syndrome featured by podocyte injury and progressive scarring in the glomerulus (Mollet et al, 2009). The incidence of this syndrome is increasing in the United States. Understanding the pathophysiology and genetics of FSGS has gained significance due to the rising prevalence of chronic renal disease (Mollet et al, 2009). Strategies need to be developed for finding preventive methods and therapeutic interventions for the illness. The genetic bases have been established for familial forms of FSGS (Hinkes et al, 2006). It has also been indicated that genes have a role in increasing the susceptibility to glomerular disease (Kopp et al, 2008). Forty-three percent of familial and 10 % of sporadic types of nephrotic syndrome is associated with the NPHS2 gene which encodes podocin, the protein in the slit diaphragm. Genetic variants can increase the susceptibility to glomerular disease. Cellular signalling is facilitated by the podocin which interacts with the slit diaphragm proteins (Huber et al, 2006). Studies have been conducted in mice (Mollet et al, 2009). Mice with absence of podocin or podocin missense mutant present with albuminuria at birth and develop lesions of mesangiolysis and mesangial sclerosis. The affected mice die early. Follow up is by performing autopsies. Animal models have allowed the study on podocyte injury using various toxic agents, damage due to immunologic damage, renal ablation, gene targeting, and transgenesis (Mollet, 2009). Absence of podocin in the glomerulus caused albuminuria, hypercholesterolaemia, hypertension, and renal failure of nephrotic syndrome. Podocin in the glomerulus was inactivated in the adult mouse using the Cre-loxP technology. After 4 weeks, FSGS symptoms appeared. Then diffuse glomerulosclerosis set in with tubulointerstitial disease. The mice which had inactivation of podocin at birth had a gradient of lesions showing a dependence on the developmental stage (Mollet, 2009). Albuminuria was seen in these mice when diffuse involvement was reached from the foot process effacement. Mollet et al used global gene expression profiling to identify new potential mediators. Molecular pathways and cellular proliferation were explored for the purpose (Mollet et al, 2009). Toyoda et al studied the structural characteristics of podocytes and endothelial cells in diabetic nephropathy (2007). The podocyte detachment and reduced endothelial fenestration were found to be classical lesions of diabetic nephropathy in type 1diabetic patients. The findings also show that all cells of the glomerulus are affected in diabetic nephropathy. Further studies of these findings have been suggested by the researchers (Toyoda, 2006). The role of the vascular endothelial growth factor (VEGF-A) was investigated by Ku et al (2008) in diabetic glomerulopathy. “Podocyte- specific, doxycycline-inducible of soluble vasculo-endothelial growth factor” was used for the investigation of the VEGF-A (Ku et al, 2008). VEGF-A is known to participate in the glomerular barrier properties for protein filtration. Inhibition of this factor produces proteinuria. In conditions like diabetes, systemic inhibition of the VEGF-A lessens the albuminuria. The physiological permselective functions of the glomerular filtration system are maintained by a strict regulation of the VEGF expression level (Ku et al, 2008). Podocyte specific sFlt-1 receptor overexpression reduces the injury in the glomerulus of diabetics. This indicates the involvement of VEGF-A in the mechanism. Shono and colleagues investigated the role of podocin at the tight junctions at the foot processes (2007). They found that podocin co-localises with the Coxsackie and Adenovirus receptor (CAR) and the zonula occludens (ZO-1) at the tight junctions (Shono, 2007). These 3 junctional proteins form a complex. Podocin allowed the coalescence of the lipid rafts containing the CAR. The lateral mobility was restricted by the actin reorganization. The CAR-podocin complex will be attached to the cytoskeleton. The podocin could be having a function other than being a structural protein. It may also be performing a scaffolding function tightening the junction proteins to the actin (Shono, 2007). The podocyte migration in nephrotic syndrome is facilitated by the interplay between the cysteine protease Cathepsin-L and alpha 3 integrin as reported in Reiser et al’s study (2004). Cell migration was decreased when cathepsin-L deficient podocytes were present. Cathepsin –L expression was increased when alpha 3 integrin was inhibited. The conclusion was that the foot process effacement is a migratory event associated with the interplay between the cathepsin-L and alpha 3 integrin (Reiser et al, 2004). An analogue of erythropoietin, darbopoietin, reduced the proteinuria of rats which were given puromycin (Eto et al, 2007). The protective effect coincided with the disappearance of the podocyte injury. Puromycin was given to produce nephrotic syndrome in rats which then exhibited podocyte foot process retraction, effacement and actin filament rearrangement. Then the administration of darbopoietin reversed the changes (Eto et al, 2007). Conclusion Researches concerning the filtration process in the kidneys are being done on a large scale. Yet the information on the process is only partial. The podocyte seems to be the center of all researches. Society has to wait for many more years before finding a solution to the various renal illnesses which hamper the filtration process and thereby become fatal in the long run. References: Eto, N., Wada, T., Inagi, R., Takano, H., Shimizu, A and Kato, H. et al, (2007). Podocyte protection by darbopoietin: preservation of the cytoskeleton and nephrin expression. Kidney International, Vol. 72, No. 4, p. 455-463 Hinkes B, Wiggins RC, Gbadegesin R, Vlangos CN, Seelow D, Nurnberg G, et al. (2006). Positional cloning uncovers mutations in PLCE1 responsible for a nephrotic syndrome variant that may be reversible. Nat Genet 38: 1397–1405, 2006 Huber TB, Schermer B, Muller RU, Hohne M, Bartram M, Calixto A. et al. (2006). :Podocin and MEC-2 bind cholesterol to regulate the activity of associated ion channels. Proc Natl Acad Sci U S A 103: 17079–17086. Koop, K., Eikmans, M., Wehkand, M., Baelde, H., Ijpelaar, D. and Kreutz, R. et al. (2008). Selective Loss of Podoplanin Protein Expression ccompanies Proteinuria and Precedes Alterations in Podocyte Morphology in a Spontaneous Proteinuric Rat Model, The American Journal of Pathology, Vol. 173, No. 2, August 2008 American Society for Investigative Pathology Kopp JB, Smith MW, Nelson GW, Johnson RC, Freedman BI, Bowden DW et al (2008). MYH9 is a major-effect risk gene for focal segmental glomerulosclerosis. Nat Genet 40: 1175–1184, 2008 Kriz, W., Kobayashi, N. and Elger, M. (1998). New aspects of structure, function, and Pathology. Clinical Experimental Nephrology, Vol. 2, p. 85-89 Kriz W, Kretzler M, Provoost AP, Shirato I: Stability and leakiness: opposing challenges to the glomerulus. Kidney Int 1996, 49: 1570–1574 Ku, C-H., White, K.E., Cas, A.D., Hayward, A., Webster, Z. and Bilous, R.(2008). Inducible Overexpression of sFlt-1 in Podocytes Ameliorates Glomerulopathy in Diabetic Mice. (2008). Diabetes 57:2824–2833 Ladenkari, A-T. Lounatmaa, K., Patrakka, J., Holmberg, C., Wartiovaara, J. , Kestila, M. Et al. (2004). Podocytes Are Firmly Attached to Glomerular Basement Membrane in Kidneys with Heavy Proteinuria. J Am Soc Nephrol 15: 2611–2618. Mollet, G., Ratelade, J., Boyer, O., Muda, A.O., Morisset, L. and Lavin\, T.A. et al, (2009). Podocin Inactivation in Mature Kidneys Causes Focal Segmental Glomerulosclerosis and Nephrotic SyndromeJ Am Soc Nephrol 20: 2181–2189 doi: 10.1681/ASN.2009040379 Pavenstadt, H, (2000). Roles of the pododcyte in glomerular function. American Journal of Physiology Renal Physiology Vol. 278, p. 173-179 Reiser, J., Oh, J., Shirato, I., Asanuma, K., Hug, A., Mundel, T.M. et al, (2004). Podocyte migration during nephritic syndrome requires a coordinated interplay between cathepsin-L and alpha 3 integrin. Journal of Biological Chemistry, Vol. 13, p. 34827-34832 Shono, A., Tsukaguchi, H., Yaoita, E., Nameta, M. Kurihara, H. and Qin, X-S. et al, (2008). J Am Soc Nephrol 18: 2525–2533, 2007. doi: 10.1681/ASN.2006101084 Toyoda, M., Najafian, B., Kim, Y., Caramori, M.L. and Mauer, M. (2007). Podocyte Detachment and Reduced Glomerular Capillary Endothelial Fenestration in Human Type 1 Diabetic Nephropathy Diabetes 56:2155–2160, 2007 Read More