CRB2 depletion induces YAP signaling and disrupts mechanosensing in podocytes.

Focal segmental glomerulosclerosis (FSGS) is a histologic lesion caused by a variety of injurious stimuli that lead to dysfunction/loss of glomerular visceral epithelial cells (i.e., podocytes). Pathogenic mutations in crumbs homolog-2 (CRB2), encoding the type 1 transmembrane protein crumbs homolog-2, have been shown to cause early-onset corticosteroid-resistant nephrotic syndrome (SRNS)/FSGS. Here, we identified a two-generation Indian kindred (DUK40595) with biopsy-proven SRNS/FSGS caused by a compound heterozygous mutation in CRB2 comprised of the previously described truncating mutation p.Gly1036_Alafs43 and a rare 9-bp deletion mutation p.Leu1074_Asp1076del. Because compound heterozygous mutations involving the truncating p.Gly1036_Alafs43 variant have been associated with reduced CRB2 expression in podocytes and autosomal recessive SRNS/FSGS, we sought to define the pathogenic effects of CRB2 deficiency in podocytes. We show that CRB2 knockdown induces yes-associated protein (YAP) activity and target gene expression in podocytes. It upregulates YAP-mediated mechanosignaling and increases the density of focal adhesion and F-actin. Using elastic resonator interference stress microscopy (ERISM), we demonstrate that CRB2 knockdown also enhances podocyte contractility in a substrate stiffness-dependent manner. The knockdown effect decreases with increasing substrate stiffness, indicating impaired mechanosensing in CRB2 knockdown cells at low substrate stiffness. Although the mechanical activation of CRB2 knockdown cells is associated with increased YAP activity, the enhanced cell contractility is not significantly reduced by the selective YAP inhibitors K-975 and verteporfin, suggesting that multiple pathways may be involved in mechanosignaling downstream of CRB2. Taken together, these studies provide the first evidence that CRB2 deficiency may impair podocyte mechanotransduction via disruption of YAP signaling in podocytes. We identified a rare compound heterozygous CRB2 mutation as the cause of familial SRNS/FSGS in a two-generation East Asian kindred. Modeling the effect of the mutation, we show that CRB2 knockdown in podocytes induces YAP transcriptional activity and upregulates YAP-mediated mechanosignaling. Using elastic resonator interference stress microscopy (ERISM), we demonstrate that CRB2 knockdown enhances podocyte contractility in a substrate stiffness-dependent manner. The knockdown effect decreases with increasing substrate stiffness, indicating impaired mechanosensing in CRB2deficient podocytes.