The pathobiochemistry of Alport syndrome and mechanism of cell-based therapy

Alport syndrome is characterized by progressive glomerulonephritis associated with structural defects in the glomerular basement membrane. The renal disease is life- threatening for patients with Alport syndrome, who eventually rely on hemodialysis while awaiting a kidney transplant. Genetic mutations in the COL4A3, COL4A4, or COL4A5 genes, encoding for the α3-, α4-, and α5-chains of type IV collagen respectively, result in the compound loss of all three chains due to post-translational requirement for the assembly of α3α4α5(IV) protomer. Importantly, the α3α4α5(IV) protomer is required for normal glomerular filtration function. The unique primary and secondary structures of type IV collagen chains allow for their intrinsic ability to assemble into scaffolds, yet the underlying mechanism for chain selection in the protomer formation and network organization is unknown. Here we provide the first mechanistic study to establish the role of the NCI domain in specifying chain selectivity in the organization of type IV collagen network in the glomerular basement membrane (GBM). Next, we utilized a mouse model for Alport syndrome, in which the COL4A3 gene was targeted for deletion, to test the hypothesis that restoration of the α3α4α5(IV) network in the GBM of these mice will lead to improved renal function and survival of the mice. We describe that the wild-type bone marrow-derived and blood-derived cells travel to the diseased kidney, synthesize the missing chain of type IV collagen and incorporate it into the GBM, and improves the renal function and ultrastructural lesions by partially restoring the normal GBM type IV collagen composition. We determined that mature B- and T-lymphocytes as well as macrophages in the bone marrow-derived cell population are dispensable for the production and the incorporation of the missing chain of type IV collagen. Using several compound transgenic mice, we identify cell fusion involving the bone marrow-derived cells as one possible mechanism for the revival and repair of defective podocytes by enabling synthesis of the missing α3 chain of type IV collagen. Collectively, we provide new insights into the disease mechanism associated with Alport syndrome and offer new possibilities for therapy.