FK506 and the role of immunophilins in nerve regeneration

FK506 is a new FDA-approved immunosuppressant used for prevention of allograft rejection in, for example, liver and kidney transplantations. FK506 is inactive by itself and requires binding to an FK506 binding protein-12 (FKBP-12), or immunophilin, for activation. In this regard, FK506 is analogous to cyclosporin A, which must bind to its immunophilin (cyclophilin A) to display activity. This FK506-FKBP complex inhibits the activity of the serine/threonine protein phosphatase 2B (calcineurin), the basis for the immunosuppressant action of FK506. The discovery that immunophilins are also present in the nervous system introduces a new level of complexity in the regulation of neuronal function. Two important calcineurin targets in brain are the growth-associated protein GAP-43 and nitric oxide (NO) synthase (NOS). This review focuses on studies showing that systemic administration of FK506 dose-dependently speeds nerve regeneration and functional recovery in rats following a sciatic-nerve crush injury. The effect appears to result from an increased rate of axonal regeneration. The nerve regenerative property of this class of agents is separate from their immunosuppressant action because FK506-related compounds that bind to FKBP-12 but do not inhibit calcineurin are also able to increase nerve regeneration. Thus, FK506's ability to increase nerve regeneration arises via a calcineurin-independent mechanism (i.e., one not involving an increase in GAP-43 phosphorylation). Possible mechanisms of action are discussed in relation to known actions of FKBPs: the interaction of FKBP-12 with two Ca²⁺ release-channels (the ryanodine and inositol 1,4,5-triphosphate receptors) which is disrupted by FK506, thereby increasing Ca²⁺ flux; the type 1 receptor for the transforming growth factor-β (TGF-β1), which stimulates nerve growth factor (NGF) synthesis by glial cells, and is a natural ligand for FKBP-12; and the immunophilin FKBP-52/FKBP-59, which has also been identified as a heat-shock protein (HSP-56) and is a component of the nontransformed glucocorticoid receptor. Taken together, studies of FK506 indicate broad functional roles for the immunophilins in the nervous system. Both calcineurin-dependent (e.g., neuroprotection via reduced NO formation) and calcineurin-independent mechanisms (i.e., nerve regeneration) need to be invoked to explain the many different neuronal effects of FK506. This suggests that multiple immunophilins mediate FK506's neuronal effects. Novel, nonimmunosuppressant ligands for FKBPs may represent important new drugs for the treatment of a variety of neurological disorders.