Peripheral motoneuron interactions with laminin and Schwann cell-derived neurite-promoting molecules: developmental regulation of laminin receptor function.

Schwann cells synthesize several neurite outgrowth-promoting molecules and localize them in either the extracellular matrix (ECM; e.g., laminin) or on the plasma membrane (e.g., L1/NgCAM and N-cadherin). Neurite outgrowth by embryonic chick ciliary ganglion (CG) neurons in response to these Schwann cell molecules largely depends on several specific neuronal cell surface receptors: integrin beta 1-class ECM receptors, L1/NgCAM, and N-cadherin (Bixby et al.: Journal of Cell Biology 107:353-361 1988). To address whether neuronal ECM receptors are regulated independently of cell surface adhesion molecules, we studied the ability of dissociated CG neurons from different developmental ages to extend neurites rapidly on 1) substrates coated with the ECM glycoprotein laminin (either from Schwann cell-conditioned medium or purified from the Engelbreth-Holm-Swarm sarcoma) or 2) the surfaces of Schwann cells or Schwannoma (RN22) cells. CG neurons gradually lost the ability between embryonic day 8 (E8) and E14 to attach to and extend neurites in an integrin-dependent fashion on purified laminin or Schwann cell-derived laminin. The inability of E14 CG neurons to respond to laminin was partially reversed after explantation for 2.5 days in vitro, which increased the percentage of responsive neurons approximately ten-fold. E14 neurons remained capable of extending neurites rapidly on the surfaces of Schwann and Schwannoma cells. Thus, the inability of E14 neurons to respond to laminin reflects a specific loss of laminin receptor function, while other receptors, most likely N-cadherin and L1/NgCAM, remain capable of promoting neurite outgrowth on Schwann cell surfaces. Since integrin beta 1-class heterodimers have been shown to function directly as receptors mediating neuronal attachment and process outgrowth on laminin, our results imply that the expression or function of laminin-binding integrin heterodimers is regulated during the development of CG neurons. The apparent loss of integrin receptor function occurs during the period when the axons of CG neurons innervate their targets. Substantial integrin receptor function is recovered when target contact is disrupted by explantation. Thus, the functions of integrin-class receptors in CG neurons may be regulated by target contact.