Glomerular Basement Membrane Type IV Collagen in Health and Disease

Glomerular basement membrane is the major supporting structural element of the glomerular capillary wall. This is a highly complex locus which functionally serves as a filtration barrier, and has been the subject of detailed investigation. The composition of whole glomerular basement membrane suggests that collagen is a major component. Isolation and characterization of the collagenous domains has revealed that glomerular basement membrane is chiefly composed of type IV collagen. This molecule has unique properties relating to composition, apparent molecular weight, and the presence of noncollagenous (NC) domains. Following enzymatic digestion of isolated glomerular basement membrane with collagenase, the NC1 domain is released, which represents the C-terminal NC peptides of the type IV collagen molecule. These peptide monomers range in molecular weight (24-28 kD) and have corresponding dimers formed by disulfide bands between adjacent type IV collagen molecules. Two of these NC1 monomer peptides (M26 and M24) are 26kD and 24kD in size, and are identical to NC1 domains of the alpha 1(IV) and alpha 2(IV) collagen chains first characterized in the extracellular matrix of ERS mouse tumor. These are referred to as the "classical type IV collagen chains." Glomerular basement membrane NC1 domains also have two additional newly described NC1 peptides of 28kD (M28^<+++> and M28^+) which have unique composition, net charge, and antigenicity; the latter are distinct from the classical type IV collagen chains described above and have been designated "novel type IV collagen chains" (alpha 3(IV) and alpha 4(1V) chains). Specific antibody probes have been developed to the M26, M24, M28^<+++> and M28^+ peptides, which have elucidated the unique distribution of classical and novel type IV collagen chains in mature and developing kidney. Specific assembly processes of classical and novel type IV collagen chains with each other and other basement membrane components are as yet unknown. It has now been established that Goodpasture syndromesyndrome is an autoimmune disorder mediated by autoantibodies to the NC1 domain of type IV collagen. These autoantibodies react with kidney basement membranes by indirect immunofluorescence in a distribution typical of the novel type IV collagen chains. The Goodpasture epitope has been localized to the alpha 3(IV) NC1 domain. It is now known that abnormalities of novel type IV collagen chains exist in the glomerular basement membranes in Alport's syndrome (familial nephritis). The known predilection of this disorder in male patients has suggested a genetic defect on the X chromosome. Since the classical type IV collagen chains have been mapped to chromosome 13, this further suggests that the novel type IV collagen chains are distinct entities. The discovery of a third novel type IV collagen chain (alpha 5(IV) chain) based on cDNA sequence analysis, is believed to be the defective element in Alport's syndrome.