Assembly of Type IV collagen. Insights from α3(IV) collagen-deficient mice

Type IV collagen includes six genetically distinct polypeptides named α1(IV) through α6(IV). These isoforms are speculated to organize themselves into unique networks providing mammalian basement membranes specificity and inequality. Recent studies using bovine and human glomerular and testis basement membranes have shown that unique networks of collagen comprising either α1 and α2 chains or α3, α4, and α5 chains can be identified. These studies have suggested that assembly of α5 chain into type IV collagen network is dependent on α3 expression where both chains are normally present in the tissue. In the present study, we show that in the lens and inner ear of normal mice, expression of α1, α2, α3, α4, and α5 chains of type IV collagen can be detected using α chain-specific antibodies. In the α3(IV) collagen-deficient mice, only the expression of α1, α2, and α5 chains of type IV collagen was detectable. The non-collagenous 1 domain of α5 chain was associated with α1 in the non-collagenous 1 domain hexamer structure, suggesting that network incorporation of α5 is possible in the absence of the α3 chain in these tissues. The present study proves that expression of α5 is not dependent on the expression of α3 chain in these tissues and that α5 chain can assemble into basement membranes in the absence of α3 chain. These findings support the notion that type IV collagen assembly may be regulated by tissue-specific factors.