Laminin α subunits and their role in C. elegansdevelopment

Laminins are heterotrimeric (α/β/γ) glycoproteins that form a major polymer within basement membranes. Different α, β andγ subunits can assemble into various laminin isoforms that have different, but often overlapping, distributions and functions. In this study,we examine the contributions of the laminin α subunits to the development of C. elegans. There are two α, one β and oneγ laminin subunit, suggesting two laminin isoforms that differ by theirα subunit assemble in C. elegans. We find that near the end of gastrulation and before other basement membrane components are detected, theα subunits are secreted between primary tissue layers and become distributed in different patterns to the surfaces of cells. Mutations in either α subunit gene cause missing or disrupted extracellular matrix where the protein normally localizes. Cell-cell adhesions are abnormal: in some cases essential cell-cell adhesions are lacking, while in other cases,cells inappropriately adhere to and invade neighboring tissues. Using electron microscopy, we observe adhesion complexes at improper cell surfaces and disoriented cytoskeletal filaments. Cells throughout the animal show defective differentiation, proliferation or migration, suggesting a general disruption of cell-cell signaling. The results suggest a receptor-mediated process localizes each secreted laminin to exposed cell surfaces and that laminin is crucial for organizing extracellular matrix, receptor and intracellular proteins at those surfaces. We propose this supramolecular architecture regulates adhesions and signaling between adjacent tissues.