ABSENCE OF PS INTEGRINS OR LAMININ-A AFFECTS EXTRACELLULAR ADHESION, BUT NOT INTRACELLULAR ASSEMBLY, OF HEMIADHERENS AND NEUROMUSCULAR-JUNCTIONS IN DROSOPHILA EMBRYOS

We have examined the role of integrins in the formation of the cell ju nctions that connect muscles to epidermis (muscle attachments) and mus cles to neurons (neuromuscular junctions), To this end we have analyze d muscle attachments and neuromuscular junctions ultrastructurally in single or double mutant Drosophila embryos lacking PS1 integrin (alpha (PS1)beta(PS)), PS2 integrin (alpha(PS2)beta(PS)), and/or their potent ial extracellular ligand laminin A, At the muscle attachments PS integ rins are essential for the adhesion of hemiadherens junctions (HAJs) t o extracellular matrix, but not for their intracellular link to the cy toskeleton. The PS2 integrin is only expressed in the muscles, but it is essential far the adhesion of muscle and epidermal HAJs to electron dense extracellular matrix. It is also required far adhesion of muscl e HAJs to a less electron dense form of extracellular matrix, the base ment membrane, The PS1 integrin is expressed in epidermal cells and ca rs mediate adhesion of the epidermal HAJs to the basement membrane, Th e ligands involved in adhesion mediated by both PS integrins seem dist inct because adhesion mediated by PS1 appears to require the extracell ular matrix component laminin A, while adhesion mediated by PS2 integr in does not, Ar neuromuscular junctions the formation of functional sy napses occurs normally in embryos lacking PS integrins and/or laminin A, but the extent of contact between neuronal and muscle surfaces is a ltered significantly. We suggest that neuromuscular contact in part re quires basement membrane adhesion to the general muscle surface, and t his form of adhesion is completely abolished in the absence of laminin A. (C) 1998 Academic Press.