Structure and Biology of Type VIII Collagen

Type VIII collagen was initially characterized as a biosynthetic product of bovine aortic endothelial cells in vitro and thus named Endothelial Collagen(EG). The protein has now been identified in avian and mammalian species as a product seaeted by cultured comeal endothelial cells, keratinocytes, fibroblasts, astrocytoma and other tumor cells, and endothelial cells derived from several classes of blood vessels. In vivo, the distribution of type VIII collagen is restricted to specialized extracellular matrices such as meninges, periosteum, perichondrium, and Descemet's membrane of the cornea. It is also present in the subendothelium of large and small blood vessels and has been localized in capillary-like structures formed by sprouting cultures of endothelial cells. Data firom protein and nucleotide sequencing have identified two distinct chains, termedal(VIII) and ot2(VIII); however, the assembly of these chains in the extracellular space is still poorly understood. Protein extracts firom Descemet's membrane indicate that, in this tissue, type VIII collagen is a heterotrimer. Our understanding of the biological role of type VIII collagen is rather limited. Although type VIII collagen seems to perform a structural role in the Descemet's membrane, its temporally and spatially restricted expression during development suggests an involvement in tissue remodeling and differentiation.