Neurophilins and the nervous system

Immunosuppressive drugs are being used increasingly to suppress rejection of organ transplants and to treat autoimmune disorders. Cyclosporin A (CsA), FK506, and rapamycin are among a class of immunosuppressants whose mechanisms of action are similar and which have unique neurological toxicity. Cyclosporin A and FK506 lack structural similarity yet have similar immunosuppressive effects, whereas rapamycin is structurally similar to FK506 but has a distinct spectrum of biological activities. These immunosuppressants are membrane-permeable prodrugs which mediate their effects after first binding to a variety of cytosolic proteins called “immunophilins” due to their affinity for immunosuppressant ligands. Intriguingly, concentrations of immunophilins are often greater in the nervous system than in the immune system, in some cases 10 to 50 times more so, hence the term “neurophilins.” Interest regarding mechanisms of drug toxicity as well as the normal function of neurophilins in the nervous system is growing. Immunophilins are classified into two families: cyclophilins and FK506-binding proteins (FKBPs), ubiquitous proteins that share many functions. Within each family, multiple isoforms have differential tissue expression and distinct subcellular localization. Cyclophilin A and FKBP-12 are the bestdescribed members, although more then 30 cyclophilins and more than 20 FKBPs are reported. Immunophilins are well conserved throughout evolution, suggesting that they mediate critical cellular activities. One role is to mediate proper folding of proteins by virtue of prolyl isomerase (rotamase) activity, which is inhibited upon drug binding. Cyclosporin A inhibits cyclophilin rotamase activity, whereas FK506 and rapamycin inhibit rotamase activity of the FKBPs. Although only modest evidence supports a physiological role for immunophilin regulation of protein folding, rotamase activity might play roles in such processes as protein trafficking, collagen assembly, and chaperone transport of otherwise insoluble polypeptides. Immunosuppressive drug toxicity might be attributed in part to inhibition of rotamase activity, but drug interactions with other substrates might also result in toxicity. One such substrate is calcineurin, which interacts with CsA and FK506 but not with rapamycin. Immunophilin-mediated immunosuppression does not involve inhibition of protein folding but rather inhibition of calcineurin, a calcium and calmodulin-dependent serine/threonine protein phosphatase. Calcineurin affects gene expression in response to calcium flux and is necessary for the synthesis of several cytokines. Normal activation of the T-cell receptor leads to elevation of intracellular calcium levels, thereby activating calcineurin. Calcineurin in turn dephosphorylates a variety of proteins including transcription factors, leading to increased interleukin-2 levels and T-cell activation. By blocking these events, calcineurin inhibition mediates immunosuppression. Calcineurin comprises over 1% of the total brain protein and is widely distributed, with highest concentrations in the hippocampus and corpus striatum, suggesting that immunophilins play a role in neural function. Cyclophilin and FKBP expression overlaps, but certain regions express one or more neurophilins preferentially. It is therefore not surprising that CsA and FK506 have physiological effects in the nervous system, including wellcharacterized central nervous system (CNS) toxicities. Central nervous system localization of the cyclophilins and FKBPs generally parallels that of calcineurin, and CNS expression of all three is much greater than in the immune system. Low concentrations of CsA and FK506 enhance the phosphorylation of endogenous protein substrates in brain tissue and PC12 cells by interacting with calcineurin– neurophilin complexes. Important targets of calcineurin in brain are the growth-associated protein GAP-43, N-methyl-D-aspartate (NMDA) receptor Correspondence to: E.L. Feldman