The Laminin Alpha Chains - Expression, Developmental Transitions, and Chromosomal 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 components of basal laminae (BLs) throughout the body. To date, three ct chains (alpha 1-3) have been shown to assemble into at least seven heterotrimers (called laminins 1-7). Genes encoding two additional alpha chains (alpha 4 and alpha 5) have been cloned, but little is known about their expression, and their protein products have not been identified. Here we generated antisera to recombinant alpha 4 and alpha 5 and used them to identify authentic proteins in tissue extracts. Immunoprecipitation and immunoblotting showed that alpha 4 and alpha 5 assemble 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 antibody probes, we surveyed the expression of alpha 1-5 in murine tissues. All five chains were expressed in both embryos and adults, but each was distributed in a distinct pattern at both RNA and protein levels. Overall, alpha 4 and alpha 5 exhibited the broadest patterns of expression, while expression of alpha 1 was the most restricted. Immunohistochemical analysis of kidney, lung, and heart showed that the ct chains were confined to extracellular matrix and, with few exceptions, to BLs. All developing and adult BLs examined contained at least one alpha chain, all alpha chains were present in multiple BLs, and some BLs contained two or three a chains. Detailed analysis of developing kidney revealed that some individual BLs, including those of the tubule and glomerulus, changed in laminin chain composition as they matured, expressing 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 four 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 remarkable diversity in BL composition and complexity in BL development. [References: 91]

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