Collagen IV of basement membranes: I. Origin and diversification of COL4 genes enabling animal evolution

Collagen IV is a major component of basement membranes, a specialized form of extracellular matrix that enabled the assembly of multicellular epithelial tissues. In mammals, collagen IV assembles from a family of six -chains (1 to 6), forming three supramolecular scaffolds: Col-IV121, Col-IV345 and Col-IV121-556. The -chains are encoded by six genes (COL4A1-6) that occur in pairs in a head-to-head arrangement. In Alport syndrome, variants in COL4A3, 4 or 5 genes, encoding Col-IV345 scaffold in glomerular basement membrane (GBM), the kidney ultrafilter, cause progressive renal failure in millions of people worldwide. How variants cause dysfunction remains obscure. Here, we gained insights into Col-IV345 function by determining its evolutionary lineage, as revealed from phylogenetic analyses and tissue expression of COL4 gene-pairs. We found that the COL4A[<]1|2[>] gene-pair emerged in basal Ctenophores and Cnidaria phyla and is highly conserved across metazoans. The COL4A[<]1|2[>] duplicated and arose as the progenitor to the COL4A[<]3|4[>] gene-pair in cyclostomes, coinciding with emergence of kidney GBM, and expressed and conserved in jawed-vertebrates, except for amphibians, and a second duplication as the progenitor to the COL4A[<]5|6[>] gene-pair and conserved in jawed-vertebrates. These findings revealed that Col-IV121 is the progenitor scaffold, expressed ubiquitously in metazoan basement membranes, and which evolved into vertebrate Col-IV345 and expressed in GBM. The Col-IV345 scaffold, in comparison, has an increased number of cysteine residues, varying in number with osmolarity of the environment. Cysteines mediate disulfide crosslinks between protomers, an adaptation enabling a compact GBM that withstands the high hydrostatic pressure associated with glomerular ultrafiltration.