MO011PODOCYTE NEPHRONECTIN IS REGULATED BY TRANSFORMING GROWTH FACTOR BETA VIA THE CANONICAL AND NON-CANONICAL PATHWAYS
Authors: Sopel Nina , Ohs Alexandra , Schiffer Mario , Müller-Deile Janina 
Published: May 2021

Abstract

Background and Aims
The glomerular basement membrane (GBM), podocytes and glomerular endothelial cells (GEC) are composing the glomerular filtration barrier (GFB) within the glomerulus. Both podocytes and GEC are essential components for the synthesis of the extracellular matrix (ECM) of the GBM.
One ECM protein of the GBM, which is mainly produced by podocytes is nephronectin (NPNT). An altered expression pattern of NPNT has been observed in different kidney diseases. While NPNT was shown to be down regulated in membranous glomerulonephropathy, its expression was elevated in diabetic glomerulopathy, compared to healthy controls.
Using a morpholino-induced knockdown of npnt in zebrafish larvae, proteinuria, podocyte foot process effacement and thickening of the lamia rara interna of the GBM were observed. Mice that were intra peritoneally injected with a microRNA 378a (miR-378a) mimic showed a decrease in NPNT expression in the kidneys on both mRNA and protein level, suggesting a regulatory effect of miR-378a on NPNT. In addition, in cultured human podocytes treatment with transforming growth factor beta (TGFβ), as well as transfection with miR-378a mimic down regulated NPNT mRNA and protein expression.
By blocking different parts of the TGFβ pathway, we want to further investigate the mechanisms by which TGFβ mediates the regulation of NPNT in podocytes.

Method
Our main model for this study are immortalized human podocytes, which are proliferating at 33°C and differentiating at 37°C due to a temperature sensitive SV40 large T antigen. Ten to 12 days differentiated podocytes were used for experiments. Differentiated cells were either transfected with a miR-192 or control miR mimic or pre-incubated with different inhibitors for components of both the canonical and the non-canonical TGFβ signaling pathways, followed by culture with or without additional TGFβ. Cell lysates were prepared to be used for Western Blot or qPCR analyses.

Results
Treatment of immortalized human podocytes with TGFβ decreased NPNT expression on mRNA and protein level. After transfecting immortalized human podocytes with a miR-192 mimic, a GEC-derived miR up regulated by TGFβ stimulation, we observed reduced NPNT expression, compared to control miR transfection.
Blocking TGFβ receptor I signaling with the specific inhibitor SD208, caused higher NPNT protein abundance, while NPNT mRNA expression remained unchanged.
Targeting downstream parts of both the canonical and non-canonical TGFβ pathways by using inhibitors for single molecules of the respective arms of the intricate TGFβ pathway showed ambiguous results. Suppression of either Smad2 and/or Smad3 tended to enhance NPNT protein levels, accompanied by mostly unaltered mRNA expression. Blockade of different parts of the non-canonical TGFβ pathway also up regulated protein expression of NPNT. However, NPNT mRNA expression was more variable. Therefore, regulation of podocyte NPNT might not be due to changes in mRNA transcription, but to modifications on posttranscriptional level.

Conclusion
Treating immortalized human podocytes with TGFβ or TGFβ-induced miR-192 reduced NPNT expression on both the mRNA and protein level. More detailed analysis with inhibition of different parts of the canonical and non-canonical TGFβ pathways hint that both pathways are involved in NPNT expression.
Therefore, we suggest that the regulation of podocyte NPNT by TGFβ is fine-tuned via both the canonical and non-canonical pathways with additional modulation through TGFβ dependent miRs.