BACKGROUND The laminins are a family of glycoproteins that provide an integral part of the structural scaffolding of basement membranes in almost every animal tissue. Each laminin is a large (900 kDa) mosaic protein composed of heterotrimer assembled from A, B1, and B2 subunits, later renamed α, β, and γ chain subunits, secreted and incorporated into cell-associated extracellular matrices. Moreover, each subunit has various isoforms. Twelve different laminin heterotrimers have been identified so far in mammals.1 The laminins can self-assemble, bind to other matrix macromolecules, and have unique and shared cell interactions mediated by integrins, dystroglycan, and other receptors.2
In mammals they play at least three essential roles.3 First, they are major structural elements of basal laminae (BLs), forming one of two self-assembling networks (the other is composed of the collagens IV) to which other glycoproteins and proteoglycans of the BL attach. Second, they interact with cell surface components such as dystroglycan to attach cells to the extracellular matrix. Third, they are signaling molecules that interact with cellular receptors such as the integrins to convey morphogenetically important information to the cell's interior. For example, laminin promotes myogenesis in skeletal muscle, outgrowth of neurites from central and peripheral neurons, and mesenchymal to epithelial transitions in kidney. Through these interactions, laminins critically contribute to cell differentiation, cell shape and movement, maintenance of tissue phenotypes, and promotion of tissue survival. Recent advances in the characterization of genetic disruptions in humans, mice, nematodes and flies have revealed developmental roles for the different laminin subunits in diverse cell types, affecting differentiation from blastocyst formation to the post-natal period.
1. Timpl, R. et al: J. Biol. Chem. 254:9933-7, 1979
2. Martin, G.R. & Timpl, R.: Ann. Rev. Cell Biol. 3:57-85, 1987
3. Mercurio, A.M.: Trends Cell Biol. 5:419-23, 1995
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