Role of the Hippo signaling pathway in the maintenance of podocyte function

• Applicant: Privatdozentin Dr. Beate Vollenbröker
• Subject Area: Nephrology
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405968954

Regulation of the highly conserved Hippo pathway, which controls the activity of the co-transcriptional factor YAP, plays a critical role in the functional preservation of the slit diaphragm (SD) of podocytes, which together with the glomerular basement membrane and the endothelium form the blood-urine filtration barrier in the kidney. Podocyte-specific knockout of YAP in mice initially leads to development of proteinuria due to damage to the SD, subsequently to scarring in the glomeruli and finally complete loss of kidney function. Exactly why damage to the SM occurs has not yet been elucidated. However, we have shown that Hippo activation, and thus loss of transcriptional activity of YAP, leads to reduced expression of the Ajuba family of proteins (Ajuba, LIMD1 WTIP) and prevents their localization at cell-cell contacts. Reduction of the Ajuba ortholog Djub in nephrocytes from Drosophila melanogaster (model system for the glomerular filtration barrier) showed direct consequences for SD, such as a) reduced number, b) increased width, and c) increased permeability. Interestingly, the Ajuba proteins themselves control the activity of the Hippo signaling pathway by inactivating the Hippo kinases LATS1/2. This interdependence, when misregulated, leads to a self-reinforcing feedback mechanism that progressively drives SD damage, with fatal consequences for the preservation of podocyte function.The aim of our application is to find ways by which this feedback mechanism can be stopped. Different approaches are planned. On the one hand, we could show that LATS2, probably by interaction with another protein, can release the Ajuba proteins from the cell-cell contacts. We would like to determine the region in the LATS2 molecule that is important for rearrangement and the interaction partner so that we can intervene here. Our further hypothesis is that increased Hippo activity alters not only the SD but also the basement membrane surrounding the nephrocyte, which is also thought to have a crucial influence on the permeability of the glomerular filtration barrier. Therefore, we would like to investigate the damage caused by Hippo activation to the basement membrane, how this affects filtration, and how it can be improved. Furthermore, we observed an accumulation of Ajuba in the nucleus dependent on the activity of the Hippo signaling pathway. Because Ajuba can act as a co-repressor for several transcription factors, we postulate that it plays an important role in transcription in podocytes. With a better understanding of Ajuba as a co-transcription factor, it should be possible to work out factors through which the damage can be reversed or at least ameliorated

• DFG Programme: Research Grants