α4 integrin is expressed during peripheral nerve regeneration and enhances neurite outgrowth

We have shown previously that repair in the peripheral nervous system is associated with a reversion to an embryonic pattern of alternative splicing of the extracellular matrix molecule fibronectin. One of the consequent changes is a relative increase in the number of fibronectins expressing the binding site for α4 integrins. Here we show that α4 integrins are expressed on dorsal root ganglion neuron cell bodies and growth cones in the sciatic nerve during regeneration and that the interaction of α4 integrin with alternatively spliced isoforms of recombinant fibronectins containing the α4 binding site enhances neurite outgrowth in dorsal root ganglion neurons. The pheochromocytoma (PC12) neuronal cell line, which normally extends neurites poorly on fibronectin, does so efficiently when α4 is expressed in the cells. Experiments using chimeric integrins expressed in PC12 cells show that the α4 cytoplasmic domain is necessary and sufficient for this enhanced neurite outgrowth. In both dorsal root ganglion neurons and PC12 cells the α4 cytoplasmic domain is tightly linked to the intracellular adapter protein paxillin. These experiments suggest an important role for α4 integrin and paxillin in peripheral nerve regeneration and show how alternative splicing of fibronectin may provide a mechanism to enhance repair after injury.