Ectopic laminin α2 accumulation in the glomerular basement membrane exacerbates podocyte injury in Alport syndrome

Abstract

Alport syndrome is a hereditary disease caused by mutations in Col4a3, Col4a4, and Col4a5, which encode the type IV collagen α3, α4, and α5 chains, respectively. Alport glomerular basement membrane (GBM), which predominantly consists of collagen α1α2α1 (IV) heterotrimers, provides less biomechanical strength than normal GBM with collagen α3α4α5 (IV) heterotrimers. In addition to type IV collagen abnormalities, laminin dysregulation is observed in Alport GBM. In this study, we aimed to investigate the mechanisms underlying laminin dysregulation in Alport GBM and their roles in disease progression using primary podocytes and Col4a4-deficient mice on DBA/2 background. Histological analysis of Col4a4-deficient mice revealed that ectopic laminin α2 deposition in GBM during postnatal nephrogenesis, followed by re-expression of laminin α1 and decreased expression of nephrin. The analysis of primary podocytes indicated that podocytes on low substrate stiffness overexpressed laminin α2. Moreover, podocytes cultured on laminin-α2β1γ1 exhibited higher laminin α1 levels and lower nephrin levels than those cultured on laminin-α5β2γ1. Cell adhesion assays showed that ectopic laminin α2 deposition in GBM may cause defective podocyte–GBM adhesion, leading to podocyte depletion. Overall, these findings suggest that insufficient GBM strength increases the mechanical stress on podocytes via daily transcapillary filtration pressures, resulting in ectopic laminin α2 deposition in GBM, which contributes to defective podocyte–GBM adhesion, GBM abnormalities, and podocyte injury in Alport syndrome.