7 General discussion and future perspectives 125 Podocyte actin cytoskeletal regulation not only depends on dynamin, which is classed as a large GTPase, but also on the Rho-family of small GTPases like RhoA, Cdc42, and Rac1. They are involved in podocyte foot process motility and junctional and cytoskeletal interactions. Imbalances to the Rho GTPases are described to result in either hypo- or hypermobility of foot processes which both result in the progression of podocytopathy. (123) Rho GTPase signaling can be influenced by circulation factors such as soluble urokinase-type plasminogen activator receptor (suPAR), which activates Rac1. Inhibiting suPAR has been shown to inhibit podocyte injury in vitro. (124) The results described in this thesis, combined with other literature on actin cytoskeleton regulation, expand the understanding that the GFB is not the static barrier it was once presumed to be, but rather an intricate apparatus that is dynamically regulated depending on local circumstances and circulating factors. Transmembrane protein 14A In chapter 6, transmembrane protein 14A (TMEM14A) is reported as another important protein in the preservation of adequate GFB function and integrity. It was previously implied to be involved in suppressing Bax mediated apoptosis.(95) Other than that, TMEM14A is a relatively unknown protein. Here, we identified it to be involved in the development of proteinuria by examining the results of a microarray study in spontaneously proteinuric Dahl SS rats. There, it was found to be significantly downregulated compared to spontaneously hypertensive, non proteinuric rats. To establish whether TMEM14A plays a direct and essential role in the development of proteinuria, a zebrafish embryo knockdown model was utilized. Results from this study shows that knocking down TMEM14A translation results in loss of GFB integrity without affecting tubular reabsorption capacity. Next, we show that both mRNA and protein expression of TMEM14A is reduced before onset of proteinuria. This study also reveals that glomerular TMEM14A expression is increased in proteinuric kidney disease, except in diabetic nephropathy. This result corresponds with in vitro findings, where inducing podocyte damage also increases TMEM14A expression. A protective mechanism by TMEM14A is proposed with a potential action mechanism through inhibiting podocyte apoptosis. Further studies are required to assess whether this is indeed the case. It would be of particular interest to identify up- and downstream modulators of TMEM14A expression and function. Zebrafish embryo model Zebrafish (Danio rerio) are freshwater fish originally from Southern Asia. They have become a widespread scientific model for the investigation of various pathophysiological
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