THE LAMININ ALPHA-CHAINS - EXPRESSION, DEVELOPMENTAL TRANSITIONS, ANDCHROMOSOMAL LOCATIONS OF ALPHA-1-5, IDENTIFICATION OF HETEROTRIMERIC LAMININS 8-11, AND CLONING OF A NOVEL ALPHA-3 ISOFORM

Laminin trimers composed of alpha, beta, and gamma chains are major co mponents of basal laminae (BLs) throughout the body. To date, three ct chains (alpha 1-3) have been shown to assemble into at least seven he terotrimers (called laminins 1-7). Genes encoding two additional alpha chains (alpha 4 and alpha 5) have been cloned, but little is known ab out their expression, and their protein products have not been identif ied. Here we generated antisera to recombinant alpha 4 and alpha 5 and used them to identify authentic proteins in tissue extracts. Immunopr ecipitation and immunoblotting showed that alpha 4 and alpha 5 assembl e into four novel laminin heterotrimers (laminins 8-11: alpha 4 beta 1 gamma 1, alpha 4 beta 2 gamma 1, alpha 5 beta 1 gamma 1, and alpha 5 beta 2 gamma 1, respectively). Using a panel of nucleotide and antibod y probes, we surveyed the expression of alpha 1-5 in murine tissues. A ll five chains were expressed in both embryos and adults, but each was distributed in a distinct pattern at both RNA and protein levels. Ove rall, alpha 4 and alpha 5 exhibited the broadest patterns of expressio n, while expression of alpha 1 was the most restricted. Immunohistoche mical analysis of kidney, lung, and heart showed that the ct chains we re confined to extracellular matrix and, with few exceptions, to BLs. All developing and adult BLs examined contained at least one alpha cha in, all alpha chains were present in multiple BLs, and some BLs contai ned two or three a chains. Detailed analysis of developing kidney reve aled that some individual BLs, including those of the tubule and glome rulus, changed in laminin chain composition as they matured, expressin g up to three different alpha chains and two different beta chains in an elaborate and dynamic progression. Interspecific backcross mapping of the five alpha chain genes revealed that they are distributed on fo ur mouse chromosomes. Finally, we identified a novel full-length alpha 3 isoform encoded by the Lama3 gene, which was previously believed to encode only truncated chains. Together, these results reveal remarkab le diversity in BL composition and complexity in BL development.