Putting the pieces together : elucidation of podocyte biology in the homeostasis of the kidney filtration barrier

Abstract

End-stage renal disease (ESRD) is a devastating condition that can only be treated with chronic dialysis or kidney transplantation. ESRD treatment costs account for up to 10% of healthcare budgets in the Western world. The 5-year survival for patients in dialysis is only about 40%. Glomerular disease processes are the main cause of ESRD. Despite this, our basic knowledge on the biology and disease processes of the glomerulus is poor. As a result, we still lack effective therapy options to stop the progression of glomerular diseases.

In this thesis we have identified a number of candidate genes and proteins that could have an essential role in the glomerular homeostasis. In the first project, we identified a group of neural proteins, Hip1, Nfasc and Olfml2, which are enriched in podocytes. We used these markers to provide further evidence that podocytes are present in glomerular crescent lesions that occur in inflammatory diseases of the glomerulus.

In project 2, we studied the functional role of another neural protein, dendrin, in the kidney by generating and characterizing a knockout (KO) mouse line. Previously, we and others have shown that dendrin is very specific to podocytes and interacts with cd2ap and nephrin, two podocyte proteins imperative for the maintenance of the kidney barrier. The KO mouse model showed that dendrin is not needed for the development or maintenance of the glomerulus filtration barrier. Furthermore, the outcome of glomerular disease in two injury models was unaffected by the absence of dendrin. This suggests that dendrin does not have a role in the development of glomerular damage in these two models.

In project 3, we identified Tmem234, Slfn5, Lrrc49 and Znf185 as highly podocyte-enriched molecules. Morpholino knockdown experiments in zebrafish showed that the silencing of Tmem234 results in podocyte foot effacement and proteinuria in pronephros. Thus, Tmem234 seems to have an important role in the formation of functional filtration barrier in zebrafish pronephros, and therefore it is reasonable to speculate that it can have also an important role in the mammalian kidney.

Lastly, in project 4 we identified a mediator protein subunit, Med22, to be essential for the kidney filtration barrier. In zebrafish pronephros, Med22 morphants exhibit defective capillary loop formation and leak large proteins to tubuli. In mice, full KO mice die during embryonic development. In podocyte-specific KO animals kidney development proceeds normally. However, these mice exhibit proteinuria starting from 8 weeks of age that progresses to ESRD by 16 weeks of age. Histological analysis shows the accumulation of caveolin-positive vesicles in the cytoplasm of podocytes. As these vesicles became larger, we detected the loss of podocytes that leads to glomerulosclerosis and ESRD. Thus, Med22 seems to regulate vesicular trafficking in podocytes and be essential for podocyte survival.

This thesis provides novel insights into podocyte biology and obviously opens up new possibilities to study these candidate genes in glomerular function and pathology.