Inhibition of calcineurin by a novel FK-506-binding protein

FKBP12.6 protects heart from AngII-induced hypertrophy through inhibiting Ca2+ /calmodulin-mediated signalling pathways in vivo and in vitro

We previously observed that disruption of FK506‐binding protein 12.6 (FKBP12.6) gene resulted in cardiac hypertrophy in male mice. Studies showed that overexpression of FKBP12.6 attenuated thoracic aortic constriction (TAC)‐induced cardiac hypertrophy in mice, whereas the adenovirus‐mediated overexpression of FKBP12.6 induced hypertrophy and apoptosis in cultured neonatal cardiomyocytes, indicating that the role of FKBP12.6 in cardiac hypertrophy is still controversial. In this study, we aimed to investigate the roles and mechanisms of FKBP12.6 in angiotensin II (AngII)‐induced cardiac hypertrophy using various transgenic mouse models in vivo and in vitro. FKBP12.6 knockout (FKBP12.6^−/−) mice and cardiac‐specific FKBP12.6 overexpressing (FKBP12.6 TG) mice were infused with AngII (1500 ng/kg/min) for 14 days subcutaneously by implantation of an osmotic mini‐pump. The results showed that FKBP12.6 deficiency aggravated AngII‐induced cardiac hypertrophy, while cardiac‐specific overexpression of FKBP12.6 prevented hearts from the hypertrophic response to AngII stimulation in mice. Consistent with the results in vivo, overexpression of FKBP12.6 in H9c2 cells significantly repressed the AngII‐induced cardiomyocyte hypertrophy, seen as reductions in the cell sizes and the expressions of hypertrophic genes. Furthermore, we demonstrated that the protection of FKBP12.6 on AngII‐induced cardiac hypertrophy was involved in reducing the concentration of intracellular Ca²⁺ ([Ca²⁺]i), in which the protein significantly inhibited the key Ca²⁺/calmodulin‐dependent signalling pathways such as calcineurin/cardiac form of nuclear factor of activated T cells 4 (NFATc4), calmodulin kinaseII (CaMKII)/MEF‐2, AKT/Glycogen synthase kinase 3β (GSK3β)/NFATc4 and AKT/mTOR signalling pathways. Our study demonstrated that FKBP12.6 protects heart from AngII‐induced cardiac hypertrophy through inhibiting Ca²⁺/calmodulin‐mediated signalling pathways.

Parallel Regulation of von Hippel-Lindau Disease by pVHL-Mediated Degradation of B-Myb and Hypoxia-Inducible Factor α

pVHL, the protein product of the von Hippel-Lindau (VHL) tumor suppressor gene, is a ubiquitin ligase that targets hypoxia-inducible factor α (HIF-α) for proteasomal degradation. Although HIF-α activation is necessary for VHL disease pathogenesis, constitutive activation of HIF-α alone did not induce renal clear cell carcinomas and pheochromocytomas in mice, suggesting the involvement of an HIF-α-independent pathway in VHL pathogenesis. Here, we show that the transcription factor B-Myb is a pVHL substrate that is degraded via the ubiquitin-proteasome pathway and that vascular endothelial growth factor (VEGF)- and/or platelet-derived growth factor (PDGF)-dependent tyrosine 15 phosphorylation of B-Myb prevents its degradation. Mice injected with B-Myb knockdown 786-O cells developed dramatically larger tumors than those bearing control cell tumors. Microarray screening of B-Myb-regulated genes showed that the expression of HIF-α-dependent genes was not affected by B-Myb knockdown, indicating that B-Myb prevents HIF-α-dependent tumorigenesis through an HIF-α-independent pathway. These data indicate that the regulation of B-Myb by pVHL plays a critical role in VHL disease.

Some transformations of tacrolimus, an immunosuppressive drug

Transformations of the macrocyclic lactone tacrolimus (1), an important immunosuppressive drug produced by Streptomyces species, are described. These transformation products are primarily of interest as reference substances for drug impurity analyses. Upon action of acid (p-toluenesulfonic acid in toluene), tacrolimus is dehydrated by loss of water from the β-hydroxyketone moiety with partial inversion of configuration at C-8, resulting in formation of 5-deoxy-Δ(5,6)-tacrolimus and 5-deoxy-Δ(5,6)-8-epitacrolimus. The structure of the latter was determined by single-crystal X-ray crystallography. The same products are formed upon action of free radicals (iodine in boiling toluene), along with formation of 8-epitacrolimus. The latter is converted by p-toluenesulfonic acid to 5-deoxy-Δ(5,6)-8-epitacrolimus. Treatment of tacrolimus with weak base (1,5-diazabicyclo[4.3.0]nonene) gives, in addition to 8-epitacrolimus, the open-chain acid corresponding to 5-deoxy-Δ(5,6)-tacrolimus, a rare non-cyclic derivative of tacrolimus. Strong base (t-butoxide) causes pronounced degradation of the molecule. Thermolysis of tacrolimus leads to ring expansion by an apparent [3,3]-sigmatropic rearrangement of the allylic ester moiety with subsequent loss of water from the β-hydroxyketone moiety. ¹H and ¹³C NMR spectra of the obtained compounds, complicated by the presence of amide bond rotamers and ketal moiety tautomers, were assigned by extensive use of 2D NMR techniques.

Differential effect of atorvastatin and tacrolimus on proliferation of vascular smooth muscle and endothelial cells

Although considered promising for use in drug-eluting stents (DES), tacrolimus failed clinically. Tacrolimus inhibits growth factor production but can also act as a growth factor on vascular smooth muscle cells (VSMC). This unexpected proliferative stimulus could reverse the beneficial effects of the drug on restenosis. We hypothesized that tacrolimus' association with statins, which lower cholesterol and impair cell proliferation, could restore tacrolimus' beneficial effect by abrogating the aberrant proliferative stimulus. Additionally, since maintenance of endothelial function represents a challenge for new-generation DES, we investigated the combined effect of tacrolimus and atorvastatin on endothelial cells. Human VSMC and umbilical vein endothelial cells (HUVEC) were incubated with 100 nM tacrolimus and increasing doses of atorvastatin (0-3.0 μM). Atorvastatin plus tacrolimus dose-dependently inhibited VSMC proliferation. The percentage of cells incorporating 5-bromo-2'-deoxyuridine (BrdU) in their DNA was 49 ± 14% under basal conditions, 62 ± 15% (P = 0.01) with tacrolimus, 40 ± 22% with 3 μM atorvastatin, and 30 ± 7% (P < 0.05) with 3 μM atorvastatin plus tacrolimus. Atorvastatin downregulated β-catenin, Erk1 and Erk2, and cyclin B in tacrolimus-stimulated VSMC. In contrast, atorvastatin plus tacrolimus did not affect proliferation of endothelial cells. The percentage of HUVEC incorporating BrdU in their DNA was 47 ± 8% under basal conditions, 58 ± 6% (P = 0.01) with tacrolimus, 45 ± 4% with 3 μM atorvastatin, and 49 ± 1% with 3 μM atorvastatin plus tacrolimus. Both agents stimulated endoglin production by HUVEC. Taken together, these results suggest that, when combined with tacrolimus, atorvastatin exerts a dose-dependent antiproliferative effect on VSMC. In contrast, atorvastatin acts in concert with tacrolimus in HUVEC to stimulate production of endoglin, a factor that has an important role in endothelial repair. Our study supports the conclusion that prevention of postcoronary in-stent restenosis and late thrombosis may benefit of concomitant association of tacrolimus and high doses of atorvastatin.

Peptidyl-prolyl cis-trans isomerase xFKBP1B induces ectopic secondary axis and is involved in eye formation during Xenopus embryogenesis

Although Xenopus FKBP1A (xFKBP1A) induces an ectopic dorsal axis in Xenopus embryos, involvement of xFKBP1B, a vertebrate paralogue of FKBP1A, in embryogenesis remains undetermined. Here, we demonstrate that xFKBP1B induces ectopic dorsal axis and involves in eye formation of Xenopus embryos. Injection of the xFKBP1B mRNA in ventral blastomeres of 4-cell stage Xenopus embryos induced a secondary axis and showed multiplier effect to that of xFKBP1A on this when xFKBP1A was co-injected. In addition, BMP4 and Smad1 mRNAs did not affect the ability of xFKBP1B to induce the ectopic secondary axis when either was co-injected with xFKBP1B in ventral blastomeres, whereas they downed out that of xFKBP1A, suggesting that xFKBP1A and xFKBP1B induce the ectopic secondary axis through affecting different pathways from each other. On the other hand, the injection of the FKBP1B mRNA in dorsal blastomeres showed eye malformation, and suppressed almost completely the expression of Rx1, Mitf, and Vax2 mRNAs. xFKBP1B was expressed in the dorsal side of the embryo including the eye during embryogenesis at least until stage 46. Injection of morpholino of the xFKBP1B mRNA in dorsal blastomeres induced additional retina or failed to close tapetum nigrum in the ventral side within the optic cap, whereas it did not affect the dorsal organ development. The injection of the morpholino reduced the expression of Xotx2 and Rx1 mRNAs in the eye. These observations suggest that xFKBP1B is a key factor that regulates the expression levels of the genes involved in eye formation during Xenopus embryogenesis.

Identification and comparative analysis of the peptidyl-prolyl cis/trans isomerase repertoires of H. sapiens, D. melanogaster, C. elegans, S. cerevisiae and Sz. pombe

The peptidyl-prolyl cis/trans isomerase (PPIase) class of proteins comprises three member families that are found throughout nature and are present in all the major compartments of the cell. Their numbers appear to be linked to the number of genes in their respective genomes, although we have found the human repertoire to be smaller than expected due to a reduced cyclophilin repertoire. We show here that whilst the members of the cyclophilin family (which are predominantly found in the nucleus and cytoplasm) and the parvulin family (which are predominantly nuclear) are largely conserved between different repertoires, the FKBPs (which are predominantly found in the cytoplasm and endoplasmic reticulum) are not. It therefore appears that the cyclophilins and parvulins have evolved to perform conserved functions, while the FKBPs have evolved to fill ever-changing niches within the constantly evolving organisms. Many orthologous subgroups within the different PPIase families appear to have evolved from a distinct common ancestor, whereas others, such as the mitochondrial cyclophilins, appear to have evolved independently of one another. We have also identified a novel parvulin within Drosophila melanogaster that is unique to the fruit fly, indicating a recent evolutionary emergence. Interestingly, the fission yeast repertoire, which contains no unique cyclophilins and parvulins, shares no PPIases solely with the budding yeast but it does share a majority with the higher eukaryotes in this study, unlike the budding yeast. It therefore appears that, in comparison with Schizosaccharomyces pombe, Saccharomyces cerevisiae is a poor representation of the higher eukaryotes for the study of PPIases.

Synthesis and characterization of an epimer of tacrolimus, an immunosuppressive drug

8-Epitacrolimus (2), a new l-pipecolic acid macrolide lactone, was obtained by base-catalyzed epimerization of tacrolimus (FK-506, 1), an important immunosuppressive drug, and its structure determined by a single-crystal X-ray diffraction method. The compound was fully characterized by spectroscopic techniques. The epimer is of importance due to its potential biological effects as well as because of its possible formation during formulation, handling, and use of tacrolimus products.

Fluorescent probes to characterise FK506-binding proteins

Talented all-rounders: Fluorescence polarisation assays were developed for members of the FK506-binding protein family by using fluorescent rapamycin analogues (demonstrated in the figure). These tracers retain medium to high affinity to all tested proteins (FKBP12, -12.6, -13, -25, -51, -52). They can be used for active-site titrations, competition assays with unlabelled ligands and enable a robust, miniaturized assay adequate for high-throughput screening.FK506-binding proteins (FKBPs) convey the immunosuppressive action of FK506 and rapamycin and mediate the neuroprotective properties of these compounds, and participate in the regulation of calcium channels. In addition, the larger homologues FKBP51 and FKBP52 act as cochaperones for Hsp90 and regulate the transactivational activity of steroid hormone receptors. To further characterize these FKBPs, we have synthesized fluorescein-coupled rapamycin analogues. In fluorescence polarization assays one of these compounds retained high affinity to all tested proteins (K(d): 0.1-20 nM) and could be used for active-site titrations. To adapt the fluorescence polarization assay for high-throughput purposes, a simplified rapamycin derivative was synthesized and labelled with fluorescein. This probe showed moderate affinity for the FK1 domains of FKBP51 (177 nM) and FKBP52 (469 nM) and allowed a highly robust, optimized, miniaturized assay (Z'>0.7) sufficient for high-throughput screening of large compound libraries.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

Signaling mechanisms of down-regulation of voltage-activated Ca2+ channels by transient receptor potential vanilloid type 1 stimulation with olvanil in primary sensory neurons

Olvanil ((N-vanillyl)-9-oleamide), a non-pungent transient receptor potential vanilloid type 1 agonist, desensitizes nociceptors and alleviates pain. But its molecular targets and signaling mechanisms are little known. Calcium influx through voltage-activated Ca(2+) channels plays an important role in neurotransmitter release and synaptic transmission. Here we determined the effect of olvanil on voltage-activated Ca(2+) channel currents and the signaling pathways in primary sensory neurons. Whole-cell voltage-clamp recordings were performed in acutely isolated rat dorsal root ganglion neurons. Olvanil (1 microM) elicited a delayed but sustained inward current, and caused a profound inhibition (approximately 60%) of N-, P/Q-, L-, and R-type voltage-activated Ca(2+) channel current. Pretreatment with a specific transient receptor potential vanilloid type 1 antagonist or intracellular application of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid abolished the inhibitory effect of olvanil on voltage-activated Ca(2+) channel current. Calmodulin antagonists (ophiobolin-A and calmodulin inhibitory peptide) largely blocked the effect of olvanil and capsaicin on voltage-activated Ca(2+) channel current. Furthermore, calcineurin (protein phosphatase 2B) inhibitors (deltamethrin and FK-506) eliminated the effect of olvanil on voltage-activated Ca(2+) channel current. Notably, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, calmodulin antagonists, and calcineurin inhibitors each alone significantly increased the amplitude of voltage-activated Ca(2+) channel current. In addition, double immunofluorescence labeling revealed that olvanil induced a rapid internalization of Ca(V)2.2 immunoreactivity from the membrane surface of dorsal root ganglion neurons. Collectively, this study suggests that stimulation of non-pungent transient receptor potential vanilloid type 1 inhibits voltage-activated Ca(2+) channels through a biochemical pathway involving intracellular Ca(2+)-calmodulin and calcineurin in nociceptive neurons. This new information is important for our understanding of the signaling mechanisms of desensitization of nociceptors by transient receptor potential vanilloid type 1 analogues and the feedback regulation of intracellular Ca(2+) and voltage-activated Ca(2+) channels in nociceptive sensory neurons.

Immunosuppression with either cyclosporine a or FK506 supports survival of transplanted fibroblasts and promotes growth of host axons into the transplant after spinal cord injury

Fibroblasts that have been genetically modified to secrete neurotrophins can stimulate axonal regeneration, rescue injured neurons, and improve function when grafted into a spinal cord injury site. These grafts are usually allografts that require immunosuppression to prevent rejection. In this study, we compared the effects of two immunophilin-ligands (cyclosporine A [CsA] and FK506) that are used clinically to prevent transplant rejection on protection of grafted fibroblasts. As there are risks associated with prolonged immunosuppression, we compared the effects of 2 or 8 weeks of administration of these drugs, in combination with our standard methylprednisolone protocol, in animals that survived for 8 weeks, to determine whether a shorter course of immunosuppression would be effective. Outcome measures included fibroblast survival, infiltration of activated macrophages and microglia into the graft, final lesion size, and growth of host axons into the graft. The graft consisted of a Vitrogen matrix into which fibroblasts were suspended; the graft was placed into a C3/C4 lateral funiculus lesion. The fibroblasts were isolated from a transgenic strain of Fischer rats that produce the marker alkaline phosphatase (Fb/AP). This enabled us to track the grafted fibroblasts and to evaluate the extent of their survival. The grafted matrix filled the lesion cavity. The density of fibroblasts within the matrix differed according to treatment. Fibroblast survival was most robust in animals that received 8 weeks of immunophilin-ligand treatment. FK506 supported greater Fb/AP survival than CsA. ED-1 immunostaining for activated microglia and macrophages showed an inverse correlation between AP immunoreactivity and the density of immune cells within the graft. Thus, prolonged administration of either FK506 or CsA was necessary for maximal fibroblast survival and for limiting the macrophage invasion of the graft. None of the FK506 or CsA protocols modified the size of the lesion, indicating that these immunophilin-ligands had little effect on secondary enlargement of the lesion and therefore little neuroprotective effect. Because immunophilin-ligands have been shown to be neurotrophic, we used RT-97 immunostaining for neurofilaments and calcitonin gene related protein (CGRP) staining for dorsal root axons to visualize axons that grew into the graft. Some axons grew into the matrix even in the absence of immunophilin-ligand treatment, suggesting that the Vitrogen matrix itself is permissive, but all of the immunophilin-ligand protocols were much more effective in eliciting axonal growth. Growth of axons into the transplants was equally increased by drug treatment for 2 or 8 weeks. Thus, both treatments improved fibroblast survival, diminished immune cell invasion, and promoted axonal growth, and a 2-week course of treatment with either immunophilin-ligand was as effective as 8 weeks in stimulating axonal growth.

Three-dimensional visualization of FKBP12.6 binding to an open conformation of cardiac ryanodine receptor

The cardiac isoform of the ryanodine receptor (RyR2) from dog binds predominantly a 12.6-kDa isoform of the FK506-binding protein (FKBP12.6), whereas RyR2 from other species binds both FKBP12.6 and the closely related isoform FKBP12. The role played by FKBP12.6 in modulating calcium release by RyR2 is unclear at present. We have used cryoelectron microscopy and three-dimensional (3D) reconstruction techniques to determine the binding position of FKBP12.6 on the surface of canine RyR2. Buffer conditions that should favor the "open" state of RyR2 were used. Quantitative comparison of 3D reconstructions of RyR2 in the presence and absence of FKBP12.6 reveals that FKBP12.6 binds along the sides of the square-shaped cytoplasmic region of the receptor, adjacent to domain 9, which forms part of the four clamp (corner-forming) structures. The location of the FKBP12.6 binding site on "open" RyR2 appears similar, but slightly displaced (by 1-2 nm) from that found previously for FKBP12 binding to the skeletal muscle ryanodine receptor that was in the buffer that favors the "closed" state. The conformation of RyR2 containing bound FKBP12.6 differs considerably from that depleted of FKBP12.6, particularly in the transmembrane region and in the clamp structures. The x-ray structure of FKBP12.6 was docked into the region of the 3D reconstruction that is attributable to bound FKBP12.6, to show the relative orientations of amino acid residues (Gln-31, Asn-32, Phe-59) that have been implicated as being critical in interactions with RyR2. A thorough understanding of the structural basis of RyR2-FKBP12.6 interaction should aid in understanding the roles that have been proposed for FKBP12.6 in heart failure and in certain forms of sudden cardiac death.

Abnormal ryanodine receptor function in heart failure

The abnormally regulated release of Ca2+ from an intracellular Ca2+ store, the sarcoplasmic reticulum (SR), is the mechanism underlying contractile and relaxation dysfunctions in heart failure (HF). According to recent reports, protein kinase A (PKA)-mediated hyperphosphorylation of ryanodine receptor (RyR) in the SR has been shown to cause the dissociation of FK506 binding protein (FKBP) 12.6 from the RyR in heart failure. This causes an abnormal Ca2+ leak through the Ca2+ channel located in the RyR, leading to an increase in the cytosolic Ca2+ during diastole, prolongation of the Ca2+ transient, and delayed/slowed diastolic Ca2+ re-uptake. More recently, a considerable number of disease-linked mutations in the RyR have been reported in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) or arrhythmogenic right ventricular dysplasia type 2. An analysis of the disposition of these mutation sites within well-defined domains of the RyR polypeptide chain has led to the new concept that interdomain interactions among these domains play a critical role in channel regulation, and an altered domain interaction causes channel dysfunction in the failing heart. The knowledge gained from the recent literature concerning the critical proteins and the changes in their properties under pathological conditions has brought us to a better position to develop new pharmacological or genetic strategies for the treatment of heart failure or cardiac arrhythmia. A considerable body of evidence reviewed here indicates that abnormal RyR function plays an important role in the pathogenesis of heart failure. This review also covers some controversial issues in the literature concerning the involvement of phosphorylation and FKBP12.6.

Transcription inhibitors in inflammation

Advances in molecular medicine have revealed a key role for altered gene expression in the aetiology of many inflammatory diseases, including asthma, rheumatoid arthritis, inflammatory bowel disease and sepsis. Until recently, however, modulation of gene transcription has not been the subject of directed pharmaceutical research efforts. Notwithstanding, it is clear that the efficacy of several well-established anti-inflammatory therapeutics is mediated through their ability to modulate gene transcription. Understanding the mechanisms of action of these therapeutics and defining new gene regulatory pathways has stimulated a new wave of anti-inflammatory drug discovery. This update aims to cover our current understanding of transcription inhibitors in inflammation, including the mechanism of action of established therapeutics and the properties of new chemical entities recently described in the literature.

A novel class of dual-family immunophilins

Immunophilins are protein chaperones with peptidylprolyl isomerase activity that belong to one of two large families, the cyclosporin-binding cyclophilins (CyPs) and the FK506-binding proteins (FKBPs). Each family displays characteristic and conserved sequence features that differ between the two families. We report a novel group of dual-family immunophilins that contain both CyP and FKBP domains for which we propose the name FCBP (FK506- and cyclosporin-binding protein). The FCBP of Toxoplasma gondii, a protozoan parasite, contained N-terminal FKBP and C-terminal CyP domains joined by tetratricopeptide repeats. Structure-function analysis revealed that both domains were functional and exhibited family-specific drug sensitivity. The individual domains of FCBP inhibited calcineurin (protein phosphatase 2B) in the presence of the appropriate drugs. In binding studies, FCBP recruited calcineurin in the presence of FK506 and a putative target of rapamycin homolog in the presence of rapamycin. Two additional FCBP sequences in Flavobacterium and one in Treponema (spirochete) were also identified in which the CyP and FKBP domains were in the reverse order. T. gondii growth was inhibited by cyclosporin and FK506 in a moderately synergistic manner. The knockdown of FCBP by RNA interference revealed its essentiality for T. gondii growth. Clearly, the FCBPs are novel chaperones and potential targets of multiple immunosuppressant drugs.

The novel calcineurin inhibitor ISA247: a more potent immunosuppressant than cyclosporine in vitro

ISA247 is a novel cyclosporine analog. In this study we compare, in vitro, the effects of ISA247 on immune function with those of cyclosporine. Whole blood from cynomolgus monkeys (n = 5) was incubated with different concentrations of ISA247 or cyclosporine and stimulated with different mitogens in culture medium. Lymphocyte proliferation was assessed by [3H]-TdR incorporation assay and by flow cytometry. Flow cytometry was also used to assess production of intracellular cytokines by T cells and expression of T cell activation surface antigens. The concentration of drug necessary to attain 50% of the maximum effect (EC50) was subsequently calculated. EC50 values for ISA247 were lower than for cyclosporine, and the differences were statistically significant for lymphocyte proliferation, T cell cytokine production, and expression of all T cell activation surface antigens but one. We conclude that ISA247 suppresses diverse immune functions more potently than cyclosporine in vitro.

The FK506-binding protein of the malaria parasite, Plasmodium falciparum, is a FK506-sensitive chaperone with FK506-independent calcineurin-inhibitory activity

We have identified an immunophilin of the FKBP family in Plasmodium falciparum that contains a conserved peptidyl prolyl isomerase (PPIase) and tetratricopeptide repeat (TPR) domains. The 35 kDa protein was named FKBP35 and expressed in bacteria. Recombinant FKBP35 exhibited potent PPIase and protein folding activities against defined substrates in vitro, suggesting that it is a parasitic chaperone. Both activities were inhibited by macrolide immunosuppressant drugs, ascomycin (a FK506 derivative) and rapamycin, but not by cyclosporin A, providing biochemical evidence of its inclusion in the FKBP family. Interestingly, FKBP35 inhibited purified plasmodial calcineurin (protein phosphatase 2B) in the absence of any drug. In the parasite's cell, FKBP35 exhibited a stage-specific nucleocytoplasmic shuttling and did not co-localize with calcineurin. FKBP35 associated with plasmodial heat shock protein 90 (Hsp90), another member of the chaperone superfamily, via the TPR domain. Geldanamycin, a Hsp90 inhibitor, and ascomycin inhibited P. falciparum growth in a synergistic fashion. Extensive search of the P. falciparum genome revealed no other FKBP sequence, implicating PfFKBP35 as a highly significant antimalarial drug target. Thus, the single FKBP of Plasmodium is an essential parasitic chaperone with a novel drug-independent calcineurin-inhibitory activity.

N-terminal region of FKBP12 is essential for binding to the skeletal ryanodine receptor

It is known that the two types of FK506-binding proteins FKBP12 and FKBP12.6 are tightly associated with the skeletal (RyR1) and cardiac ryanodine receptors (RyR2), respectively, and their interactions are important for channel functions of the RyR. In the case of cardiac muscle, three amino acid residues (Gln-31, Asn-32, and Phe-59) of FKBP12.6 could be essential for the selective binding to RyR2 (Xin, H. B., Rogers, K., Qi, Y., Kanematsu, T., and Fleischer, S. (1999) J. Biol. Chem. 274, 15315-15319). In this study to identify amino acid residues of FKBP12 that are important for the selective binding to RyR1, we mutated 9 amino acid residues of FKBP12 that differ from the counterparts of FKBP12.6 (Q3E, R18A, E31Q, D32N, M49R, R57A, W59F, H94A, and K105A), and we examined binding properties of these mutants to RyR1 by in vitro binding assay by using glutathione S-transferase-fused proteins of the mutants and Triton X-100-solubilized, FKBP12-depleted rabbit skeletal sarcoplasmic reticulum vesicles. Among the nine mutants tested, only Q3E and R18A lost their selective binding ability to RyR1. Furthermore, co-immunoprecipitation of RyR1 with 33 various mutants for the 9 positions produced by introducing different size, charge, and hydrophobicity revealed that an integration of the hydrogen bonds by the irreplaceable Gln-3 and the hydrophobic interactions by the residues Arg-18 and Met-49 could be a possible mechanism for the binding of FKBP12 to RyR1. Therefore, these results suggest that the N-terminal regions of FKBP12 (Gln-3 and Arg-18) and Met-49 are essential and unique for binding of FKBP12 to RyR1 in skeletal muscle.

Gene expression of FK506-binding proteins 12.6 and 12 during chicken development

FKBPs are cytosolic receptors for the immunosuppressive drug FK506. FKBP12.6 and FKBP12 associate with cardiac and skeletal muscle isoforms of ryanodine receptors and thereby regulate intracellular Ca(2+) release. The amino acid sequences of FKBP12.6 and FKBP12 are highly conserved among mammals and chicken. The expression profiles of FKBP12.6 and FKBP12 genes during chicken development were compared by Northern blot and in situ hybridization analyses. Transcripts of the FKBP12 gene were abundant throughout the embryo at early stages of development but subsequently underwent gradual down-regulation. Expression of the FKBP12.6 gene was mostly restricted to the heart during early embryogenesis and also underwent subsequent down-regulation, becoming localized to the atrium after hatching. Treatment of early embryos with FK506 had no apparent effect on embryogenesis. Retinoic acid induced marked abnormalities in cardiogenesis, but it did not affect FKBP gene expression. These results suggest that, even though FKBP12.6 and FKBP12 genes are expressed in chick embryos, FK506-sensitive functions of the encoded proteins do not appear to contribute to early embryogenesis or cardiogenesis.

Heart-selective expression of the chicken FK506-binding protein (FKBP) 12.6 gene during embryonic development

FKBP12.6, a member of the family of FK506-binding proteins, selectively associates with the cardiac isoform of the ryanodine receptor and thereby stabilizes this Ca(2+) release channel. A chicken FKBP12.6 (chFKBP12.6) cDNA was cloned and shown to encode a protein of 108 amino acids. The deduced amino acid sequence of chFKBP12.6 is 91-92% identical to those of mammalian FKBP12.6 proteins. Northern blot analysis revealed that chFKBP12.6 mRNA is largely restricted to the heart during embryonic development and that the abundance of this mRNA in the heart decreases, and it becomes restricted to the atrium during cardiogenesis. In situ hybridization revealed that chFKBP12.6 mRNA is localized to the precardiac mesoderm before formation of the primitive heart tube. Expression of the chFKBP12.6 gene was initially apparent throughout the developing multichambered heart but became restricted to the atria before hatching. Reverse transcription and polymerase chain reaction analysis demonstrated that chFKBP12.6 mRNA is present in the embryo from early gastrulation and is most abundant immediately after the onset of the heartbeat. These observations suggest that the chFKBP12.6 gene is expressed before heart morphogenesis to play a role in excitation-contraction coupling in cardiomyocytes and that the function of the encoded protein becomes increasingly restricted to the atrium during embryonic development.

Effects of FK506 on [Ca2+]i differ in mouse and rabbit ventricular myocytes

FK506 binding proteins (FKBPs 12 and 12.6) interact with ryanodine receptor (RyR) and modulate its functions. FK506 binds to and reverses effects of FKBP on RyR, thus increasing RyR sensitivity to Ca2+, decreasing RyR cooperativity, and increasing RyR open probability. FK506 would thus be expected to have an effect on excitation-contraction coupling, but which of these FK506 effects predominates and how the [Ca2+]i transient would be altered are difficult to predict. FK506 has been reported to increase the [Ca2+]i transient in rat myocytes, but effects in other species have not been described. We compared the effects of FK506 on [Ca2+]i transients, L-type Ca2+ channel and Na/Ca exchange currents, membrane potential, and sarcoplasmic reticulum (SR) Ca2+ content in adult mouse and rabbit ventricular myocytes (VM). FK506 (10 microM) increased the [Ca2+]i transient in mouse VM (656 +/- 116 to 945 +/- 144 nM, p < 0.001) but decreased the amplitude of [Ca2+]i transients in rabbit VM (627 +/- 61 to 401 +/- 37 nM, p < 0.001). Similar effects were observed with rapamycin. The effects of FK506 and rapamycin on [Ca2+]i transients in VM of both species were reversible upon washout. FK506 did not alter SR Ca2+ content in mouse VM (0.79 +/- 0.1 versus 0.78 +/- 0.1 pC/pF) but reduced the SR Ca2+ content in rabbit VM (0.43 +/- 0.05 versus 0.30 +/- 0.04 pC/pF, P < 0.05) [pC = the integral (pA. s) of the caffeine-induced inward I(Na/Ca) normalized by cell capacitance (pF)]. FK506 had no effects on membrane potential, I(Ca,L) and outward I(Na/Ca) in either mouse or rabbit VM. These results indicate that alteration of the functions of RyR by FK506-mediated dissociation of FKBP from RyR has different species-dependent effects on SR Ca2+ load and thus [Ca2+]i transients. This difference may result from the fact that [Na+]i is low in rabbit myocytes, allowing extrusion by Na+/Ca2+ exchange of Ca2+ released by FK506-induced dissociation of FKBP12.6 from SR RyR.

Reversible inhibition of calcineurin by the polyphenolic aldehyde gossypol

The reversible inhibition of calcineurin (CaN), which is the only Ca(2+)/calmodulin-dependent protein Ser/Thr phosphatase, is thought to be a key functional event for most cyclosporin A (CsA)- and tacrolimus (FK506)-mediated biological effects. In addition to CaN inhibition, however, CsA and FK506 have multiple biochemical effects because of their action in a gain-of-function model that requires prior binding to immunophilic proteins. We screened a small molecule library for direct inhibitors of CaN using CaN-mediated dephosphorylation of (33)P-labeled 19-residue phosphopeptide substrate (RII phosphopeptide) as an assay and found the polyphenolic aldehyde gossypol to be a novel CaN inhibitor. Unlike CsA and FK506, gossypol does not require a matchmaker protein for reversible CaN inhibition with an IC(50) value of 15 microm. Gossypolone, a gossypol analog, showed improved inhibition of both RII phosphopeptide and p-nitrophenyl phosphate dephosphorylation with an IC(50) of 9 and 6 microm, respectively. In contrast, apogossypol hexaacetate was inactive. Gossypol acts noncompetitively, interfering with the binding site for the cyclophilin 18.CsA complex in CaN. In contrast to CsA and FK506, gossypol does not inactivate the peptidyl-prolyl-cis/trans-isomerase activity of immunophilins. Similar to CsA and FK506, T cell receptor signaling induced by phorbol 12-myristate 13-acetate/ionomycin is inhibited by gossypol in a dose-dependent manner, demonstrated by the inhibition of nuclear factor of activated T cell (NFAT) c1 translocation from the cytosol into the nucleus and suppression of NFAT-luciferase reporter gene activity.

Immunophilins: switched on protein binding domains?

Peptidylprolyl isomerases (PPIases) are a group of cytosolic enzymes first characterized by their ability to catalyze the cis-trans isomerization of cis-peptidylprolyl bonds. Subsequently, some PPIases were also identified as the initial targets of the immunosuppressant drugs-cyclosporin A (CsA), FK506, and rapamycin-have been called immunophilins. Immunophilins have been found to be both widely distributed and abundantly expressed leading to suggestions that they may play a general role in cellular biochemistry. However, the nature of this role has been difficult to elucidate and is still controversial in vivo. A number of roles for these enzymes have been identified in vitro including the ability to catalyze the refolding of partly denatured proteins and stabilize multiprotein complexes such as Ca(2+) channels, inactive steroid receptor complexes, and receptor protein tyrosine kinases. Generally, these effects appear to depend on the ability of immunophilins to selectively bind to other proteins. This review will examine in detail experimental and structural investigations of the mechanism of PPIase activity for both FKBPs and cyclophilins and suggest a mechanism for these enzymes, which depends on their ability to recognize a specific peptide conformation rather than sequence. Examination of structures of immunophilin-protein complexes will then be used to further suggest that the ability of these enzymes to recognize specific peptide conformations is central to the formation of these complexes and may constitute a general function of immunophilin enzymes. The binding of ligand to immunophilins will also be shown to stabilize specific conformations in surface loops of these proteins that are observed to play a critical role in a number of immunophilin-protein complexes suggesting that the immunophilins may constitute a class of ligand-triggered selective protein binders.

Altered stoichiometry of FKBP12.6 versus ryanodine receptor as a cause of abnormal Ca(2+) leak through ryanodine receptor in heart failure

BACKGROUND

In the pathogenesis of cardiac dysfunction in heart failure, a decrease in the activity of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase is believed to be a major determinant. Here, we report a novel mechanism of cardiac dysfunction revealed by assessing the functional interaction of FK506-binding protein (FKBP12.6) with the cardiac ryanodine receptor (RyR) in a canine model of pacing-induced heart failure.

METHODS AND RESULTS

SR vesicles were isolated from left ventricular muscles (normal and heart failure). The stoichiometry of FKBP12.6 per RyR was significantly decreased in failing SR, as assessed by the ratio of the B(max) values for [(3)H]dihydro-FK506 to those for [(3)H]ryanodine binding. In normal SR, the molar ratio was 3.6 ( approximately 1 FKBP12.6 for each RyR monomer), whereas it was 1.6 in failing SR. In normal SR, FK506 caused a dose-dependent Ca(2+) leak that showed a close parallelism with the conformational change in RyR. In failing SR, a prominent Ca(2+) leak was observed even in the absence of FK506, and FK506 produced little or no further increase in Ca(2+) leak and only a slight conformational change in RyR. The level of protein expression of FKBP12.6 was indeed found to be significantly decreased in failing SR.

CONCLUSIONS

An abnormal Ca(2+) leak through the RyR is present in heart failure, and this leak is presumably caused by a partial loss of RyR-bound FKBP12.6 and the resultant conformational change in RyR. This abnormal Ca(2+) leak might possibly cause Ca(2+) overload and consequent diastolic dysfunction, as well as systolic dysfunction.

Immunophilins may limit calcineurin inhibition by cyclosporine and tacrolimus at high drug concentrations

BACKGROUND

The immunosuppressive drugs cyclosporine (CsA) and tacrolimus (FK506 or FK) are qualitatively similar but differ in molar potency. Both drugs sterically inhibit the phosphatase activity of calcineurin (CN) but differ in molar potency. In our study we explored whether differential inhibition of CN explained the differences in molar potency of FK versus CsA.

METHODS

We compared their effects on NFATC2 dephosphorylation using Western analysis, interferon-gamma production using ELISA, and CN phosphatase activity using the CN assay in human peripheral blood leukocytes (PBL) and mouse spleen cell suspension.

RESULTS

The FK concentration inhibiting 50% (IC50) of all three activities was approximately 0.2 microg/ml in human PBL, versus 5-20 microg/ml for CsA. Although inhibition of interferon-gamma secretion and NFATC2 dephosphorylation was complete, inhibition of CN phosphatase activity was incomplete with both drugs at saturation, particularly with FK. Inhibition of CN phosphatase activity was incomplete whether FK treatment was in vivo in mouse or in vitro in various human and mouse tissues, especially brain. Exogenous FKBP12 or CyPA increased CN phosphatase inhibition, suggesting that incomplete inhibition of CN phosphatase activity reflected limiting amounts of active immunophilin.

CONCLUSIONS

These data contradict the prevailing assumption that immunophilins are abundant and not limiting for inhibition of CN by CsA or FK. Further, the observation that FK and CsA completely inhibit immune function without completely inhibiting CN suggests that the inhibition of immune function is not mediated by general CN inhibition but by inhibition of a subset of CN which is critical for lymphocyte activation.

Three amino acid residues determine selective binding of FK506-binding protein 12.6 to the cardiac ryanodine receptor

FK506-binding protein (FKBP12) has been found to be associated with the skeletal muscle ryanodine receptor (RyR1) (calcium release channel), whereas FKBP12.6, a novel isoform of FKBP, is selectively associated with the cardiac ryanodine receptor (RyR2). For both RyRs, the stoichiometry is 4 FKBP/RyR. Although FKBP12.6 differs from FKBP12 by only 18 of 108 amino acids, FKBP12.6 selectively binds to RyR2 and exchanges with bound FKBP12.6 of RyR2, whereas both FKBP isoforms bind to RyR1 and exchange with bound FKBP12 of RyR1. To assess the amino acid residues of FKBP12.6 that are critical for selective binding to RyR2, the residues of FKBP12.6 that differ with FKBP12 were mutated to the respective residues of FKBP12. RyR2 of cardiac sarcoplasmic reticulum, prelabeled by exchange with [35S]FKBP12.6, was used as assay system for binding/exchange with the mutants. The triple mutant (Q31E/N32D/F59W) of FKBP12.6 was found to lack selective binding to the cardiac RyR2, comparable with that of FKBP12.0. In complementary studies, mutations of FKBP12 to the three critical amino acids of FKBP12.6, conferred selective binding to RyR2. Each of the FKBP12.6 and FKBP12 mutants retained binding to the skeletal muscle RyR1. We conclude that three amino acid residues (Gln31, Asn32, and Phe59) of human FKBP12.6 account for the selective binding to cardiac RyR2.

A dynamical investigation of acrylodan-labeled mutant phosphate binding protein

The static and dynamical behavior of a fluorescently labeled mutant of the Escherichia coli periplasmic phosphate binding protein (PBP) was investigated through steady-state and time-resolved fluorescence spectroscopy. As a means of developing a biorecognition element for inorganic phosphate (P(i)), alanine-197 of PBP was replaced with a cysteine. This site was then labeled with an environmentally sensitive fluorophore. The fluorescence emission of the mutant PBP labeled with acrylodan (MPBP-AC) proved to be sensitive to micromolar concentrations of P(i), as indicated by a 50% increase in the steady-state emission intensity. Steady-state results indicated that the labeling protocol was specific for cys-197 only and did not label the wild-type PBP; thus, a site-selective labeling protocol was developed. Time-resolved measurements were used to determine the influence of the dynamics of MPBP-AC on the process of signal transduction. Time-resolved anisotropy measurements revealed that rotational dynamics were best described by a model with two independent motions: the global motion of the protein and the local motion of the acrylodan probe. The rates of both global and local rotational reorientation of MPBP-AC were faster when the protein was P(i)-bound rather than P(i)-free. This was a result of structural changes involving or surrounding both the P(i)-binding site (global changes) and the residues in near proximity to the fluorescent reporter group (local changes). Recovery of the semiangle (theta) indicated that local structural changes in MPBP-AC took place when P(i) was bound to the protein. Acrylodan gained mobility when MPBP-AC bound P(i), as indicated by the fact that theta increased by approximately 5 degrees. In addition, dynamic quenching measurements confirmed that structural changes occurred locally near the cys-197. Acrylodan became more accessible to iodide when MPBP-AC bound P(i), as demonstrated by the 35% increase in the value of the bimolecular quenching constant.

FK-binding protein is associated with the ryanodine receptor of skeletal muscle in vertebrate animals

The ryanodine receptor/calcium release channel (RyR1) of sarcoplasmic reticulum from rabbit skeletal muscle terminal cisternae (TC) contains four tightly associated FK506-binding proteins (FKBP12). Dissociation and reconstitution studies have shown that RyR1 can be modulated by FKBP12, which helps to maintain the channel in the quiescent state. In this study, we found that the association of FKBP with RyR1 of skeletal muscle is common to each of the five classes of vertebrates. TC from skeletal muscle representing animals from different vertebrates, i.e. mammals (rabbit), birds (chicken), reptiles (turtle), fish (salmon and rainbow trout), and amphibians (frog), were isolated. For each, we find the following: 1) FKBP12 is localized to the TC (there are four FKBP binding sites/ryanodine receptor); 2) soluble FKBP exchanges with the bound form on RyR1 of TC; 3) release of FKBP from terminal cisternae by drug (FK590) treatment leads to a significant reduction in the net calcium loading rate, consistent with channel activation (the calcium loading rate is restored to the control value by reconstitution with FKBP12); and 4) RyR1 of skeletal muscle TC can bind to and exchange with either FKBP12 or FKBP12.6 (FKBP12.6 is the novel FKBP isoform found selectively associated with RyR2 of dog cardiac sarcoplasmic reticulum). We conclude that FKBP is an integral part of the RyR1 of skeletal muscle in each of the classes of vertebrate animals. The studies are consistent with a role for FKBP in skeletal muscle excitation-contraction coupling.

Independent inhibition of calcineurin and K+ currents by the immunosuppressant FK-506 in rat ventricle

FK-506 increases the cytosolic Ca2+ concentration transient in rat ventricular myocytes by prolonging the action potential through inhibition of the K+ currents Ito and IK [J. Physiol. (Lond.) 501: 509-516, 1997]. Physiological and biochemical techniques were used in parallel to examine the electrophysiological mechanisms and the role of calcineurin inhibition in these effects. FK-506 prolonged the recovery of Ito from inactivation. Thus Ito inhibition was frequency dependent, with no decrease at 0.2 Hz (recorded at +50 mV from -70 mV) but a 40% decrease at 2.0 Hz. In contrast, inhibition of IK ( approximately 60%) was time and voltage independent. At 25 microM, FK-506 (by 65%) and cyclosporin A (by 57%) inhibited calcineurin activity in myocyte extracts. However, only FK-506 increased the cytosolic Ca2+ concentration transient in field-stimulated myocytes. Furthermore, FK-506 was still active on K+ currents when cells were dialyzed with 10 mM EGTA. These results demonstrate that calcineurin inhibition is not responsible for the functional effects of FK-506 in heart and suggest that IK and Ito are modulated by FK-506-binding proteins or directly by FK-506.

Molecular cloning, characterization, and chromosomal localization of FKBP23, a novel FK506-binding protein with Ca2+-binding ability

We have identified and characterized a cDNA encoding a novel FK506-binding protein (FKBP), named FKBP23, from mouse heart by the signal sequence trap method. The deduced amino acid sequence has significant homology to other FKBP family members around the peptidylprolyl cis-trans-isomerase motifs. FKBP23 also has two Ca2+-binding (EF-hand) motifs, and purified FKBP23 protein was shown to have Ca2+-binding ability. This is the first report of a Ca2+-binding FKBP. FKBP23 is a glycoprotein retained in the endoplasmic reticulum by its carboxyl-terminal tetrapeptide His-Asp-Glu-Leu, as demonstrated by immunostaining, retention, and deglycosylation assays. FKBP23 mRNA is expressed most strongly in heart, lung, and testis, beginning at day 8.5 of embryonic development. The FKBP23 gene was mapped to mouse chromosome 2.

Mechanism of action of rapamycin: new insights into the regulation of G1-phase progression in eukaryotic cells

The immunosuppressant drug, rapamycin (RAP), is a potent inhibitor of IL-2-dependent T-cell proliferation. The antiproliferative effect of RAP is mediated through the formation of an active complex with its cytosolic receptor protein, FKBP12. The molecular target of the FKBP12.RAP complex is a putative lipid kinase termed the mammalian Target Of Rapamycin (mTOR). This review will discuss recent findings suggesting that mTOR is a novel regulator of G1- to S-phase progression in eukaryotic cells.

A tacrolimus-related immunosuppressant with biochemical properties distinct from those of tacrolimus

BACKGROUND

Tacrolimus (FK506) is an immunosuppressive drug 50-100 times more potent than cyclosporine (CsA), the current mainstay of organ transplant rejection therapy. Despite being chemically unrelated, CsA and tacrolimus exert their immunosuppressive effects through the inhibition of calcineurin (CaN), a critical signaling molecule during T-lymphocyte activation. Although numerous clinical studies have proven the therapeutic efficacy of drugs within this class, tacrolimus and CsA also have a strikingly similar profile of unwanted side effects.

METHOD

Our objective has been to identify a less toxic immunosuppressant through the modification of ascomycin (FK520). Quantitative in vitro immunosuppression and toxicity assays have demonstrated (see the accompanying article, p. 18) that we achieved our goal with L-732,531 (indolyl-ascomycin; indolyl-ASC), a 32-O-(1-hydroxyethylindol-5-yl) ascomycin derivative with an improved therapeutic index relative to tacrolimus.

RESULTS

We report that the attributes of indolyl-ASC may result from its distinctive biochemical properties. In contrast to tacrolimus, indolyl-ASC binds poorly to FK506 binding protein 12 (FKBP12), the major cytosolic receptor for tacrolimus and related compounds. However, the stability of the interaction between the FKBP12-indolyl-ASC complex and CaN is much greater than that of the FKBP12-tacrolimus complex. These distinguishing properties of indolyl-ASC result in the potent inhibition of CaN within T lymphocytes but may lower the accumulation of the drug at sites of toxicity.

CONCLUSIONS

Indolyl-ASC may define those properties needed to increase the therapeutic efficacy of a macrolactam immunoregulant for treating both human autoimmune disease and organ transplant rejection.

Comparative effects of FK-506, rapamycin and cyclosporin A, on the in vitro differentiation of dorsal root ganglia explants and septal cholinergic neurons

There is increasing evidence that immunophilins play a role in neural development and differentiation. We have studied the neurotrophic effects of FK-506, rapamycin and cyclosporin A (CsA) on dorsal root ganglia (DRG) taken from different segmental levels (cervical, thoracic and lumbar/sacral), and on rat embryonic septal cholinergic neurons in culture. At a low concentration (1 nM), FK-506 significantly increased (+83%) the number of neurites of thoracic DRG explants. At a higher concentration (100 nM), it also enhanced the neuritogenesis of thoracic (+100%) and lumbar/sacral (+57%) DRG, but not cervical DRG explants. Rapamycin displayed a converse effect, reducing the development of DRG explants from cervical and thoracic segments (-78% at 1 nM in thoracic DRG). CsA (from 1 to 100 nM) was without effect on DRG neuritogenesis. In contrast to nerve growth factor (NGF), which increased neurite length (+116% at 3 ng/ml), neither FK-506 nor rapamycin affected this parameter.

Comparative mutational analysis of peptidyl prolyl cis/trans isomerases: active sites of Escherichia coli trigger factor and human FKBP12

A low degree of amino acid sequence similarity to FK506-binding proteins (FKBPs) has been obtained for the peptidyl prolyl cis/trans isomerase (PPIase) domain of E. coli trigger factor (TF) that was thought to be significant with regard to the enzymatic properties of the bacterial enzyme. We examined whether the alteration of a negatively charged side-chain at position 37 (FKBP numbering) and a phenylalanine at position 99, both highly conserved through both types of enzymes, leads to parallel effects on the catalytic activity of both FKBP12 and TF-PPIase domain in a series of tetrapeptide substrates with different P1 subsites. For the latter enzyme, substitution of Glu178 by Val or Lys, which aligns to Asp37 in human FKBP12, enhanced the PPIase activity, whereas a strongly decreased enzymatic activity was determined for the Asp37Leu and Asp37Val variants of FKBP12. Regardless of the P1 subsite of the substrate used for the assay, mutation of Phe233Tyr generated a protein variant of the TF-PPIase domain with about 1% of the wild type PPIase activity. Dependent on the substrate nature, a moderate decrease as well as a 4.8-fold increase in k(cat)/K(M) could be determined for the corresponding Phe99Tyr FKBP12 variant. Neither of the mutations of the TF-PPIase domain was able to implant FK506 inhibition found as a major characteristic of the FKBP family of PPIases.

Neurotrophic actions of nonimmunosuppressive analogues of immunosuppressive drugs FK506, rapamycin and cyclosporin A

We show that the nonimmunosuppressive analogues of the immunosuppressive drugs FK506, rapamycin and cyclosporin A promote neurite outgrowth both in PC12 cells and sensory neuronal cultures of dorsal root ganglia with potencies resembling their immunosuppressive homologues. Neurotrophic potencies of the immunophilin ligands resemble their potencies in binding to and inhibiting the rotamase activity of FKBP-12 of cyclophilin. Since nonimmunosuppressive immunophilin ligands, which are devoid of calcineurin inhibitory activity, are equally neurotrophic, inhibition of calcineurin activity is not the mediator of the neurotrophic effects. The immunophilin ligands are neurotrophic in intact animals. FK506 and L-685,818 (the C18-hydroxy, C21-ethyl derivative of FK506) treatment of rats with crushed sciatic nerves enhances both functional and morphologic recovery. The striking potency of these agents, their bioavailability and the dissociation of neurotrophic from immunosuppressant actions argue for their therapeutic relevance in the treatment of neurodegenerative diseases.

Tissue distribution and abundance of human FKBP51, and FK506-binding protein that can mediate calcineurin inhibition

We previously described the isolation of an FK506-binding protein, FKBP51, that is predominantly expressed in murine T cells and is capable of mediating drug-dependent calcineurin inhibition in vitro. In addition, the gene for FKBP51 is induced by glucocorticoids. Screening of a human thymus cDNA library resulted in the identification of the human homologue of FKBP51. Expression of the 3.7 kb mRNA corresponding to FKBP51 is induced by glucocorticoids in the human T cell line, C7TK.4. The 51.2 kDa protein encoded by this gene shares 87% identity to murine FKBP51 and demonstrates a similar IC50 value for the FK506-mediated inhibition of calcineurin phosphatase in vitro. The distribution and abundance of FKBP51 and FKBP12 in seventeen human tissues were compared by Western analysis. Unlike its murine counterpart, the human FKBP51 is abundantly expressed in numerous tissues and in many cases, is in molar excess over FKBP12.

Cyclic ADP-ribose binds to FK506-binding protein 12.6 to release Ca2+ from islet microsomes

Cyclic ADP-ribose (cADPR) is a second messenger for Ca2+ mobilization via the ryanodine receptor (RyR) from islet microsomes for insulin secretion (Takasawa, S., Nata, K., Yonekura, H., and Okamoto, H. (1993) Science 259, 370-373). In the present study, FK506, an immunosuppressant that prolongs allograft survival, as well as cADPR were found to induce the release of Ca2+ from islet microsomes. After islet microsomes were treated with FK506, the Ca2+ release by cADPR from microsomes was reduced. cADPR as well as FK506 bound to FK506-binding protein 12.6 (FKBP12.6), which we also found occurs naturally in islet microsomes. When islet microsomes were treated with cADPR, FKBP12.6 dissociated from the microsomes and moved to the supernatant, releasing Ca2+ from the intracellular stores. The microsomes that were then devoid of FKBP12.6 did not show Ca2+ release by cADPR. These results strongly suggest that cADPR may be the ligand for FKBP12.6 in islet RyR and that the binding of cADPR to FKBP12.6 frees the RyR from FKBP12.6, causing it to release Ca2+.

Effects of FK-506 on contraction and Ca2+ transients in rat cardiac myocytes

FK-506 binding protein (FKBP) has been reported to be closely associated with the ryanodine receptor in skeletal and cardiac muscle and to modulate sarcoplasmic reticulum (SR) Ca2+ release channel gating in isolated channels. FK-506 can inhibit the activity of FKBP, thereby reversing its effects on SR Ca2+ release. We investigated the function of FKBP during normal contractions and Ca2+ transients in intact rat ventricular myocytes loaded with fluorescent Ca2+ indicators. FK-506 significantly increased steady state twitch Ca2+ transients and contraction amplitudes even under conditions in which the SR Ca2+ load and Ca2+ current were unaltered, suggesting that FK-506 increases the fraction of SR Ca2+ released during excitation-contraction (E-C) coupling. Action potentials were somewhat prolonged, consistent with the larger Ca2+ transients causing greater inward Na(+)-Ca2+ exchange current. FK-506 did not affect SR Ca2+ uptake but modestly decreased Ca2+ extrusion via Na(+)-Ca2+ exchange in intact cells (although no effect on Na(+)-Ca2+ exchange was seen in sarcolemmal vesicles). In most cells, FK-506 caused an increase in SR Ca2+ content during steady state stimulation, as assessed by caffeine-induced contractures. This was probably due to the inhibition of Ca2+ efflux via Na(+)-Ca2+ exchange. FK-506 also accelerated the rest decay of SR Ca2+ content and increased the frequency of resting Ca2+ sparks about fourfold. The increase in frequency of these basic Ca2+ release events was not associated with changes in the amplitude or duration of the Ca2+ sparks. We conclude that FK-506 increases the fraction of SR Ca2+ released during normal twitches and enhances the rate of SR Ca2+ release during rest. FK-506 also inhibits Na(+)-Ca2+ exchange, although this effect may be indirect. These effects are consistent with an important SR-stabilizing effect of FKBP in intact rat ventricular myocytes.

Selective binding of FKBP12.6 by the cardiac ryanodine receptor

The calcium release channels (CRC)/ryanodine receptors of skeletal (Sk) and cardiac (C) muscle sarcoplasmic reticulum (SR) are hetero-oligomeric complexes with the structural formulas (ryanodine recepter (RyR)1 protomer)4(FKBP12)4 and (RyR2 protomer)4(FKBP12.6)4, respectively, where FKBP12 and FKBP12.6 are isoforms of the 12-kDa receptor for the immunosuppressant drug FK506. The sequence similarity between the RyR protomers and FKBP12 isoforms is 63 and 85%, respectively. Using 35S-labeled FKBP12 and 35S-labeled FKBP12.6 as probes to study the interaction with CRC, we find that: 1) analogous to its action in skeletal muscle sarcoplasmic reticulum (SkMSR), FK506 (or analog FK590) dissociates FKBP12.6 from CSR; 2) both FKBP isoforms bind to FKBP-stripped SkMSR and exchange with endogenously bound FKBP12 of SkMSR; and 3) by contrast, only FKBP12. 6 exchanges with endogenously bound FKBP12.6 or rebinds to FKBP-stripped CSR. This selective binding appears to explain why the cardiac CRC is isolated as a complex with FKBP12.6, whereas the skeletal muscle CRC is isolated as a complex with FKBP12, although only FKBP12 is detectable in the myoplasm of both muscle types. Also, in contrast to the activation of the channel by removal of FKBP from skeletal muscle, no activation is detected in CRC activity in FKBP-stripped CSR. This differential action of FKBP may reflect a fundamental difference in the modulation of excitation-contraction coupling in heart versus skeletal muscle.