Osteopontin: a novel axon-regulated Schwann cell gene.

Osteopontin (OPN) is a RGD-containing glycoprotein with cytokine-like, chemotactic, and pro-adhesive properties. During wound healing, OPN is abundantly expressed by infiltrating macrophages and has been implicated in posttraumatic tissue repair. To delineate a role in the regenerative response to axotomy we examined the expression of OPN in Wallerian degeneration of the sciatic nerve in rats. Unexpectedly, we found high constitutive expression of OPN by myelinating Schwann cells (SCs) in uninjured control nerves. OPN mRNA expression was confirmed in primary cultures of rat SCs. Upon axotomy, SC-expressed OPN in the degenerating distal nerve stump transiently increased during the first days after injury, but was continuously downregulated thereafter, reaching its minimum at Day 14. Macrophages invading axotomized nerves were OPN-negative. During late stages after axotomy, SC-OPN was reexpressed in regenerating but not permanently transected nerves. We also found OPN expression by myelinating SCs in human sural nerves with a dramatic reduction in severe axonal polyneuropathies. Taken together, our study identifies OPN as a novel Schwann cell gene regulated by axon-derived signals. The lack of OPN induction in infiltrating macrophages indicates fundamental differences in tissue repair between axonal injury in the peripheral nervous system and structural lesions in other organ systems.