We first cloned cDNAs and genomic DNAs for a4(IV) and a6(IV) chains and extended our focus on analyzing organization of the collagen IV genes. We found that the two pairs of genes, COL4A3/COL4A4 and COL4A5/COL4A6, are located on chromosome 2q34 and Xq22, respectively, and their expressions appear to be regulated by bidirectional promoters between the genes (Sado et al. 1999). By using these chain specific monoclonal antibodies, we were able to recognize the individual a(IV) chains for human and mouse tissues by immunohistochemical and western-blotting analyses. Immunostaining of the serial sections, double immunostaining, and others suggested the presence of several molecular forms in kidney, skin and other tissues; a1/a2, a3/a4/a5, a5/a6, and (a4)3. The six COL4A genes are expressed in tissue-specific manner and developmentally regulated. For instance, the major collagen IV molecules in glomerular and smooth muscle cell basement membrane was found to be the a3/a4/a5 molecule (Ariel et al, 2000) and a5/a5/a6 molecule (Borza et al, 2001). Alport syndrome is hereditary glomerular nephritis caused by mutations in COL4A5 gene. Since epidermal basement membrane contains a1, a2, a5, and a6 chains, it became possible to diagnose Alport syndrome by immunostaining the basement membrane from skin biopsy without doing kidney biopsy.
Recent finding demonstrated that regulation of GBM collagen expression was regulated by a transcription factor, LMX1b, which causes Nail-Patella syndrome in human (Morello et al, 2001). Subendothelial basement membrane of brain capillaries appeared to consist of a1/a1/a2(molecules, whereas subependymal basement membrane in choroid plexus was composed of a3/a4/a5 molecules. Considering the specific locations of these molecules such as glomerulus and choroid plexus, we hypothesize that supramolecular network containing a3/a4/a5 molecules may function as a permeability selective barrier (Urabe et al. 2002).
Key words: basement membrane/type collagen/tissue-specific expression/monoclonal antibody/ capillary/endothelial cell/kidney/brain/assembly/transcription factor/choroids plexus/ependymal cell/