Combined integrin activation and intracellular cAMP cause Rho GTPase dependent growth cone collapse on laminin-1

Abstract

Cyclic nucleotides regulate the response of both developing and regenerating growth cones to a wide range of guidance molecules through poorly understood mechanisms. It is not clear how cAMP levels are regulated or how they translate into altered growth cone behavior. Here, we show that intracellular cAMP levels are influenced by substrata and integrin receptors. We also show that growth cones require a substratum-specific balance between cAMP levels, integrin function and Rho GTPases to maintain motility and prevent collapse. Embryonic chick dorsal root ganglion neurons plated on different concentrations of laminin extend growth cones at similar speeds, yet have distinct levels of integrin expression, integrin activation and intracellular cAMP levels. Either increasing cAMP signaling or activating integrins enhances the rate of growth cone motility, but only on substrata where these two factors are endogenously low (i.e. low concentrations of laminin). Surprisingly, combining these two positive manipulations induces growth cone collapse and retraction on laminin but not on fibronectin. Collapse and retraction on laminin are Rho and Rac1 GTPase dependent and are associated with internalization of integrins, the primary receptors responsible for adhesion. These observations define a novel pathway through which cAMP influences growth cone motility and establish a link between integrins, cAMP and Rho GTPases in growth cones.