Laminin Isoforms and Epithelial Development

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Abstract
Several different approaches suggest that basement‐membrane assembly is important for epithelial development. Basement membranes contain isoforms of collagen IV, proteoglycans, and noncollagenous glycoproteins such as the laminins and nidogens. The expression and role of laminins for epithelial morphogenesis is reviewed. Laminins are large heterotrimeric proteins composed of α, β, and γ chains. Many major epithelial laminins and their receptors have been identified recently, and, the extracellular protein‐protein interactions that drive basement‐membrane assembly are beginning to be understood. Three laminin α‐chains are typically made by epithelial, α1, α3, and α5. Three major epithelial heterotrimers can at present be distinguished‐laminin‐1 (α1β1γ1), laminin‐5 (α3β3γ2), and laminin‐10 (α5β1γ1)‐but other heterotrimers may exist in epithelia. Laminins containing either α1 or α3 chains are largely limited to epithelia, whereas the α5 is also found in endothelial and muscle basement membranes, particularly in the adult. Some epithelial cell types express several laminin α‐chains, so it is relevant to test how the different laminins affect epithelial cells. Laminins interact with integrin type of receptors on the cell surface, but binding to other proteins has also recently been demonstrated. Two important recent discoveries are the identification of dystroglycan as a major laminin receptor in muscle and epithelia, and nidogen as a high‐affinity laminin‐binding protein important for basement‐membrane assembly. Antibody perturbation experiments suggest these protein‐protein interactions are important for epithelial morphogenesis.