Endothelial cell-specific collagen type IV-α₃expression does not rescue Alport syndrome inCol4a3^−/−mice

Abstract

The glomerular basement membrane (GBM) is a critical component of the kidney’s blood filtration barrier. Alport syndrome, a hereditary disease leading to kidney failure, is caused by the loss or dysfunction of the GBM’s major collagen type IV (COL4) isoform α3α4α5. The constituent COL4 α-chains assemble into heterotrimers in the endoplasmic reticulum before secretion into the extracellular space. If any one of the α3-, α4-, or α5-chains is lost due to mutation of one of the genes, then the entire heterotrimer is lost. Patients with Alport syndrome typically have mutations in the X-linked COL4A5 gene or uncommonly have the autosomal recessive form of the disease due to COL4A3 or COL4A4 mutations. Treatment for Alport syndrome is currently limited to angiotensin-converting enzyme inhibition or angiotensin receptor blockers. Experimental approaches in Alport mice have demonstrated that induced expression of COL4A3, either widely or specifically in podocytes of Col4a3−/− mice, can abrogate disease progression even after establishment of the abnormal GBM. While targeting podocytes in vivo for gene therapy is a significant challenge, the more accessible glomerular endothelium could be amenable for mutant gene repair. In the present study, we expressed COL4A3 in Col4a3−/− Alport mice using an endothelial cell-specific inducible transgenic system, but collagen-α3α4α5(IV) was not detected in the GBM or elsewhere, and the Alport phenotype was not rescued. Our results suggest that endothelial cells do not express the Col4a3/a4/a5 genes and should not be viewed as a target for gene therapy.

Complexities of the glomerular basement membrane

“…The early endothelial and podocyte basement membranes are composed of a collagen α1α1α2 network, but upon fusion of the two basement membranes an α3α4α5 network is deposited 92,93 . Whether α1α1α2 is still produced by podocytes remains unclear but immunolabelling 53 , super resolution microscopy 38 and genetic analyses 94 suggest that the α3α4α5 heterotrimer is exclusively synthesised by podocytes. The reason for this switch in the composition of the collagen IV network produced by podocytes is poorly understood; however, a reduction or absence of the α3α4α5 network causes Alport syndrome, which is characterised by kidney, hearing and ocular phenotypes 95,96 .…”

Genetic Basis of Type IV Collagen Disorders of the Kidney

“…Adenovirus-mediated gene transfer into kidney glomeruli has also been demonstrated in mouse models of Alport syndrome (58). However, inducing expression of COL4A3 in mice using an endothelial cell–specific inducible transgenic system does not result in restoration of the α 3 α 4 α 5(IV) heterotrimer or resolution of the Alport phenotype (59). Successful delivery of COL4A5 into swine kidney by an adenovirus vector has been reported with deposition of α 5(IV) into the GBM (60).…”

Gene therapy for pediatric genetic kidney diseases

“…71 However, the specific induction of Col4α3 expression in the glomerular endothelial cells of Col4α3 −/− mice cannot induce the re-expression of the α3α4α5(IV) trimer. 78 These studies propose that podocytes play a critical role in the re-expression of the α3α4α5(IV) trimer. Therefore, future research on gene replacement therapy for Alport syndrome needs to specifically deliver therapeutic transgenes to podocytes.…”