An Alport variant illuminates the bioactivity of the collagen IV ^{α565- α121} scaffold in Bowman's capsule

Alport syndrome (AS) is a major cause of chronic kidney failure and affects millions of people worldwide. Pathogenic variants in COL4A3, COL4A4, and COL4A5, which encode the collagen IV ^α345 scaffold, compromise glomerular basement membrane (GBM) structure and function. However, the molecular mechanisms linking the >5,000 reported variants to disease pathology remain poorly understood. To address this gap, we previously examined a distinctive variant, an 8-amino acid "Z-appendage", added to the NC1 domain of the α3 chain. Knock-in mice carrying this variant developed GBM abnormalities and proteinuria, implicating the NC1 hexamer as a critical determinant of GBM function and suggesting that the hexamer surface contains bioactive sites that may mediate signaling and/or organization of macromolecular complexes. Given that approximately 80% of AS cases are associated with COL4A5 variants, including many within the NC1 hexamer, we asked whether relocating the Z-appendage from the α3 NC1 subunit to the α5 subunit produces similar pathology. Strikingly, Col4a5-Z mice did not develop proteinuria and showed only minor changes in GBM morphology. In contrast, the variant induced marked thickening of Bowman's capsule, accompanied by increased deposition of the collagen IV ^α121 scaffold, increased fibrillar collagen, and cellular deposits. Structural modeling predicts that the collagen IV ^α565-α121 scaffold bearing two Z-appendages adopts an aberrant secondary structure that may stiffen the scaffold and occlude binding sites. Together, these findings reveal a bioactive role for the collagen IV ^α565-α121 scaffold in the Bowman's capsule basement membrane, with potential implications for other α565-α121containing tissues such as the aorta and bladder.