Developmental regulation of the laminin alpha5 chain suggests a role in epithelial and endothelial cell maturation

We have previously shown that mouse and bovine endothelial cells express a novel 400-kDa laminin [Alpha] chain complexed to [Beta]l and [Gamma]1 laminin chains. We describe here purification of this laminin isoform from the conditioned medium of a mouse peripheral lymph node endothelial cell line, SVEC. The laminin [Alpha] chain was isolated from the laminin complex, subjected to Edman digestion, and the amino acid sequences of the resulting peptides were determined. Amino acid sequence revealed 100% identity to the predicted amino acid sequence of the recently reported laminin [Alpha]5 gene. A monoclonal antibody to the laminin [Alpha]5 chain was raised (4G6), allowing investigation of its distribution in embryonic, newborn, and mature mouse tissues. The laminin [Alpha]5 chain was expressed mainly by epithelial, endothelial, and myogenic cells: In both embryonic and mature tissues the laminin [Alpha]5 chain was strongly expressed by epithelial cells, the bronchi of the lungs and the developing kidney tubules being the sites of strongest expression. However, laminin [Alpha]5 was not associated with early stages of epithelial cell development, but rather with epithelial cell maturation. Widespread expression of laminin [Alpha]5 in endothelial cells was apparent only in tissues of mature mice, its appearance correlating approximately with sexual maturity. During embryogenesis and in newborn tissues, laminin [Alpha]5 occurred in basement membranes of larger blood vessels only, excluding a role in angiogenic processes. Smooth muscle and skeletal muscle cells were the only other cell types which showed considerable laminin [Alpha]5 expression, with skeletal muscle exhibiting a developmentally regulated pattern of expression: The laminin [Alpha]5 chain occurred in skeletal muscle fiber basement membranes early in embryogenesis (E13-E15) but decreased with development, remaining strongly expressed only at the neuromuscular junction. The data show that laminin [Alpha]5 expression is associated with epithelial and endothelial cell maturation, implicating a role for this laminin chain in the maintenance of differentiated epithelial and endothelial cell phenotype.