A Novel FK506 Binding Protein Can Mediate the Immunosuppressive Effects of FK506 and Is Associated with the Cardiac Ryanodine Receptor

Abstract

FK506, an immunosuppressant that prolongs allograft survival, is a co-drug with its intracellular receptor, FKBP12. The FKBP12⅐FK506 complex inhibits calcineurin, a critical signaling molecule during T-cell activation. FKBP12 was, until recently, the sole FKBP known to mediate calcineurin inhibition at clinically relevant FK506 concentrations. The best characterized cellular function of FKBP12 is the modulation of ryanodine receptor isoform-1, a component of the calcium release channel of skeletal muscle sarcoplasmic reticulum.Recently, a novel protein, FKBP12.6, was found to inhibit calcineurin at clinically relevant FK506 concentrations. We have cloned the cDNA encoding human FKBP12.6 and characterized the protein. In transfected Jurkat cells, FKBP12.6 is equivalent to FKBP12 at mediating the inhibitory effects of FK506. Upon binding rapamycin, FKBP12.6 complexes with the 288-kDa mammalian target of rapamycin. In contrast to FKBP12, FKBP12.6 is not associated with ryanodine receptor isoform-1 but with the distinct ryanodine receptor isoform-2 in cardiac muscle sarcoplasmic reticulum. Our results suggest that FKBP12.6 has both a unique physiological role in excitation-contraction coupling in cardiac muscle and the potential to contribute to the immunosuppressive and toxic effects of FK506 and rapamycin.FK506 (tacrolimus) is a powerful immunosuppressive drug for treating graft rejection and autoimmune disorders. Rapamycin (RAP, 1 sirolimus) is an immunosuppressant structurally-related to FK506 but with a distinct mechanism of action. Both drugs bind to a family of intracellular receptors, the FK506 binding proteins (FKBPs), whose members include FKBPs 12, 12.6, 13, 25, 51, and 52 (for review, see Ref. 1). All FKBPs are peptidyl-prolyl isomerases, catalyzing the cis-trans isomerization of peptidyl-prolyl bonds in peptides and proteins, an activity inhibited by both FK506 and RAP.Peptidyl-prolyl isomerase inhibition is unrelated to immunosuppression. FK506 and RAP gain function upon binding FKBP12. The FKBP12⅐FK506 and FKBP12⅐RAP complexes are the actual immunosuppressive species whose targets are calcineurin (CaN) and the mammalian target of RAP (mTOR), respectively (for review, see Refs. 1 and 2). CaN is a Ca 2ϩ -dependent, serine-threonine phosphatase required during the commitment phase (G 0 3 G 1 ) of T-cell activation (3). Inhibition of CaN blocks the nuclear translocation of transcription factors such as nuclear factor of activated T-cells and NF-B, controlling the expression of cytokine genes whose products are required for immune response coordination (for review, see Ref.2). RAP, unlike FK506, does not block lymphokine production but inhibits the T-cell proliferative response to cytokines by blocking G 1 3 S-phase progression. The function of mTOR, a 288-kDa protein related to phosphatidylinositol kinases, is unknown.CaN is a ubiquitous protein, and its inhibition at unwanted sites is most responsible for the toxicity associated with FK506 therapy (4). That immunosuppression and toxicity are mechanistica...

Activation of cardiac ryanodine receptors by cardiac glycosides

“…Several possibilities might explain this effect: 1) purification might remove the glycoside binding site from the RyR2 channel protein itself or from one of the associated proteins with access to the cytoplasm (junctin or triadin); 2) removal of the ancillary Ca 2ϩ -binding proteins might prevent glycoside binding to its receptor; or 3) receptor occupation may no longer be capable of translation to the effector system responsible for increased RyR2 channel activity. It is not possible to discriminate between these possibilities at this time because all we know is that purification of the cardiac isoform of the RyR2 channel removes these proteins, with the exception of the integral protein FK-506 binding protein (FKBP12.6) which remains associated with the channel during purification (17).…”

Right Ventricular Hypertrophy Secondary to Pulmonary Hypertension Is Linked to Rat Chromosome 17

“…31,32 FKBP12.6 itself is known to bind to and modulate channel gating of the Ryr2 receptor. 33,34 Knockout mice lacking either FKBP12 or Ryr2 die during embryogenesis, 35,36 principally because of disruption of heart tube development.…”

A calcineurin-dependent transcriptional pathway controls skeletal muscle fiber type

“…Although no other transcription factors are known currently to be controlled directly by calcineurin, the activities of several muscle-specific transcription factors are regulated by phosphorylation events (Li et al 1992) that conceivably could be modified by calcineurin. In addition, other muscle proteins known to be substrates of calcineurin [ryanodine and inositol 1,4,5-trisphosphate receptors (Cameron et al 1995;Lam et al 1995), and dystrophin (Walsh et al 1995)] could participate plausibly in calcineurin-activated pathways that modulate transcription, without the involvement of NFAT proteins.…”