Loss of surface transport is a main cellular pathomechanism of CRB2 variants causing podocytopathies

Abstract

Crumbs2 (CRB2) is a central component of the renal filtration barrier and part of the slit diaphragm, a unique cell contact formed by glomerular podocytes. SomeCRB2variants cause recessive inherited forms of steroid-resistant nephrotic syndrome. However, the disease-causing potential of numerousCRB2variants remains unknown. Here, we report the establishment of a live-cell imaging–based assay, allowing a quantitative evaluation of the pathogenic potential of so far non-categorizedCRB2variants. Based on in silico data analysis and protein prediction software, putative disease–associated CRB2 missense variants were selected, expressed as CRB2-GFP fusion proteins, and analyzed in reporter cell lines with BFP-labeled plasma membrane. We found that in comparison with PM-localized WT, disease-associated CRB2 variants remained predominantly at the ER. Accumulation at the ER was also present for several non-characterized CRB2 variants and variants in which putative disulfide bridge–forming cysteines were replaced. Strikingly, WT CRB2 retained inside the ER in cells lacking protein disulfide isomerase A3, indicating that posttranslational modification, especially the formation of disulfide bridges, is a crucial step for the CRB2 PM transport.