Developmental regulation of the laminin alpha 5 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 1 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. (C) 1997 Academic Press.