LAMININ ALPHA-2 CHAIN (M-CHAIN) IS FOUND WITHIN THE PATHWAY OF AVIAN AND MURINE RETINAL PROJECTIONS

Laminin-1 is found at the end-feet of neuroepithelial cells along the outer margin of the optic pathway during early stages of development. Prior to the establishment of most retinal projections in vivo, lamini n-1 expression becomes restricted to basement membranes associated wit h the eye and optic pathway. We report that, in contrast to the alpha 1, beta 1,, and gamma 1 chains of laminin-1, laminin alpha 2 chain (fo rmerly laminin M chain) is expressed within the pathway of avian and m urine retinal ganglion cell (RGC) growth cones as they extend into the optic nerve, across the optic chiasm and into the brain. Expression o f laminin alpha 2 chain is reduced soon after formation of the visual projections but nevertheless maintained at non-basal lamina sites with in the adult optic nerve. Laminin alpha 2 chain, in contrast to lamini n-1 chains, is also highly expressed in the developing avian tectobulb ar pathway. Chick optic nerve-derived type-1 astrocytes in culture exp ress laminin alpha 2 chains as extracellular fibrils on their surface. Laminin alpha 2 chain was also detected on the surface of cultured em bryonic retinal neurons and developing RGCs. These results suggest tha t astrocytes and/or RGCs may synthesize laminin alpha 2 chain along th e developing optic pathway, and imply that laminin alpha 2 chain-in a complex with non-beta 1 and non-gamma 1 laminin chains-may serve as an adhesive substrate and possibly as a guidance cue for elongating RGC growth cones in vivo.