FK506 and FK506-derived inhibitors of the FK506-binding protein (FKBP)-type peptidylprolyl cis/trans-isomerasesinhibition can mediate neurotrophic properties of FKBP ligands. The FKBP38-specific cycloheximide derivative, N-(N,N-dimethylcarboxamidomethyl)cycloheximide (DM-CHX) was synthesized and used in a rat model of transient focal cerebral ischemia. Accordingly, DM-CHX caused neuronal protection as well as neural stem cell proliferation and neuronal differentiation at a dosage of 27.2 g/kg. These effects were still dominant, if DM-CHX was applied 2-6 h post-insult. In parallel, sustained motor behavior deficits of diseased animals were improved by drug administration, revealing a potential therapeutic relevance. Thus, our results demonstrate that FKBP38 inhibition by DM-CHX regulates neuronal cell death and proliferation, providing a promising strategy for the treatment of acute and/or chronic neurodegenerative diseases. Interestingly, FK506 and its open chain derivatives were shown to display neuroprotective and neuroregenerative effects in a wide range of animal models mimicking Parkinson disease, dementia, stroke, and nerve damage (3-11). For example, FK506 administration resulted in protection against ischemic brain injury (12), prevention of long term depression in the rat hippocampus (13), modulation of long term potentiation (14), prevention of N-methyl-D-aspartate receptor desensitization (15), alteration in neurotransmitter release (16), and attenuation of glutamate neurotoxicity ex vivo (17). FK506 increased neurite outgrowth in SH-SY5Y and PC12 cell cultures, but also in primary cultures of chicken dorsal root ganglion and of hippocampal neurons, as well (8,18,19). However, the molecular mechanism of the FK506-mediated neuroprotection and neuroregeneration remained elusive. Members of the enzyme class of peptidyl prolyl cis/trans-isomerasesIn general, the interpretation of effects caused by FK506 in cells is difficult, because FK506 inhibits not only the enzymatic activity of FKBPs, but also the protein phosphatase activity of calcineurin (CaN, PP2B). CaN inhibition is mediated by complex formation with FK506⅐FKBP complexes and is thought to be the initial process leading to immunosuppression (20 -22). CaN inhibition by immunophilin-immunosuppressant complexes is used to prevent allograft rejection in transplantation medicine, to treat autoimmune diseases and to circumvent graft-versus-host diseases. Additionally, inhibition of the protein phosphatase was the proposed basis of FK506-mediated neuroprotection, because the FKBP ligand rapamycin, which has no effects on CaN activity, did not exhibit neuroprotective properties (12,17,23).In contrast, monofunctional inhibitors of FKBPs, such as GPI1046, GPI1048, GPI1485 (Guilford Pharmaceuticals and Amgen), and V10,367 (Vertex Pharmaceuticals) have been developed, that have no influence on CaN activity, while neuroprotective and neuroregenerative effects of FK506 remain conserved. In the central nervous system, GPI1046 promotes protection and sprouting o...

Multiple intracellular receptors of the FK506 binding protein (FKBP) family of peptidylprolyl cis/trans-isomerases are potential targets for the immunosuppressive drug FK506. Inhibition of the protein phosphatase calcineurin (CaN), which has been implicated in the FK506-mediated blockade of T cell proliferation, was shown to involve a gain of function in the FKBP12/FK506 complex. We studied the potential of six human FKBPs to contribute to CaN inhibition by comparative examination of inhibition constants of the respective FK506/FKBP complexes. Interestingly, these FKBPs form tight complexes with FK506, exhibiting comparable dissociation constants, but the resulting FK506/FKBP complexes differ greatly in their affinity for CaN, with IC50 values in the range of 0.047-17 microM. The different capacities of FK506/FKBP complexes to affect CaN activity are partially caused by substitutions corresponding to the amino acid side chains K34 and I90 of FKBP12. Only the FK506 complexes of FKBP12, FKBP12.6, and FKBP51 showed high affinity to CaN; small interfering RNA against these FKBP allowed defining the contribution of individual FKBP in an NFAT reporter gene assay. Our results allow quantitative correlation between FK506-mediated CaN effects and the abundance of the different FKBPs in the cell.

Three different genes of catalytic subunit A of the Ca(2+)-dependent serine/threonine protein phosphatase calcineurin (CaN) are encoded in the human genome forming heterodimers with regulatory subunit B. Even though physiological roles of CaN have been investigated extensively, less is known about the specific functions of the different catalytic isoforms. In this study, all human CaN holoenzymes containing either the alpha, beta, or gamma isoform of the catalytic subunit (CaN alpha, beta, or gamma, respectively) were expressed for the first time. Comparative kinetic analysis of the dephosphorylation of five specific CaN substrates provided evidence that the distinct isoforms of the catalytic subunit confer substrate specificities to the holoenzymes. CaN alpha dephosphorylates the transcription factor Elk-1 with 7- and 2-fold higher catalytic efficiencies than the beta and gamma isoforms, respectively. CaN gamma exhibits the highest k(cat)/K(m) value for DARPP-32, whereas the catalytic efficiencies for the dephosphorylation of NFAT and RII peptide were 3- and 5-fold lower, respectively, when compared with the other isoforms. Elk-1 and NFAT reporter gene activity measurements revealed even more pronounced substrate preferences of CaNA isoforms. Moreover, kinetic analysis demonstrated that CaN beta exhibits for all tested protein substrates the lowest K(m) values. Enzymatic characterization of the CaN beta(P14G/P18G) variant as well as the N-terminal truncated form CaN beta(22-524) revealed that the proline-rich sequence of CaN beta is involved in substrate recognition. CaN beta(22-524) exhibits an at least 4-fold decreased substrate affinity and a 5-fold increased turnover number. Since this study demonstrates that all CaN isoforms display the same cytoplasmic subcellular distribution and are expressed in each tested cell line, differences in substrate specificities may determine specific physiological functions of the distinct isoforms.

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The Ca 2؉ /calmodulin-dependent protein phosphatase calcineurin is a key mediator in antigen-specific T cell activation. Thus, inhibitors of calcineurin, such as cyclosporin A or FK506, can block T cell activation and are used as immunosuppressive drugs to prevent graft-versus-host reactions and autoimmune diseases. In this study we describe the identification of 2,6-diaryl-substituted pyrimidine derivatives as a new class of calcineurin inhibitors, obtained by screening of a substance library. By rational design of the parent compound we have attained the derivative 6-(3,4-dichloro-phenyl)-4-(N,N-dimethylaminoethylthio)-2-phenyl-pyrimidine (CN585) that noncompetitively and reversibly inhibits calcineurin activity with a K i value of 3.8 M. This derivative specifically inhibits calcineurin without affecting other Ser/Thr protein phosphatases or peptidyl prolyl cis/trans isomerases. CN585 shows potent immunosuppressive effects by inhibiting NFAT nuclear translocation and transactivation, cytokine production, and T cell proliferation. Moreover, the calcineurin inhibitor exhibits no cytotoxicity in the effective concentration range. Therefore, calcineurin inhibition by CN585 may represent a novel promising strategy for immune intervention.The activation and the precise interplay between signaling pathways are crucial for the successful initiation and progression of the immune response as a reaction of an antigen contact. In T cells the stimulation of the T cell receptor by a specific antigen leads to a calcium release from the intracellular stores and to a calcium release-activated Ca 2ϩ channel-mediated calcium influx into the cytoplasm which activates calmodulin and thereby the Ser/Thr-protein phosphatase calcineurin (1). Consequently, calcineurin represents a bottleneck in T cell receptor signaling and allows the modulation of T cell activation by low molecular compounds, such as cyclosporin A (CsA) 2 or tacrolimus (FK506) (2). The cyclic undecapeptide CsA and the macrolid FK506 bind to and inhibit the phosphatase activity of calcineurin only after interaction with their respective peptidyl prolyl cis/trans isomerases (PPIases), cyclophilins (Cyp), and FK506-binding proteins (FKBP) through a gain-of-function mechanism (3, 4). Based on their particular characteristic to bind the immunosuppressive drugs CsA and FK506, members of the Cyp and FKBP family of PPIases were also termed immunophilins (5). Among the many known PPIases, the most abundant isoforms, Cyp18 and FKBP12, were identified as major intracellular acceptor proteins for CsA and FK506, respectively. The PPIase activity of both enzymes is strongly inhibited by the immunosuppressive drugs (6), leading to many of serious side effects (e.g. nephrotoxicity, neurotoxicity, hypertension, fibrosis) that were observed in the prevention and therapy of graftversus-host reactions or autoimmune diseases (7-10). Therefore, CsA derivatives, such as [DAT-Sar] 3 CsA, were synthesized to obtain cyclophilin-independent calcineurin inhibitors (11). However, [DAT-Sar] ...