Podocytes and endothelial cells synthesize collagen α1α2α1(IV) in adult Pod-Cre fVHL mice. A–C: 37-week-old Pod-Cre fVHL proteinuric kidney tissue was lightly fixed, and frozen sections were labeled with anti-collagen α1α2α1(IV) antibodies, followed by indirect immunoperoxidase as described in Materials and Methods. Intense HRP reaction product is seen along the subendothelial portion of the GBM (A–C). In addition, Pod-Cre fVHL animals displayed HRP reaction product in subepithelial humps immediately beneath effaced podocyte foot processes (double arrows, B and C). HRP reaction product was also seen intracellularly within endothelial cells (arrowhead, A and C) and podocytes (arrow, A). In control tissue (D), HRP reaction product is mainly confined to the mesangial matrix; in areas, subendothelial segments of GBM are also weakly labeled. En = endothelium; Po = podocyte; M = mesangium.