Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders

Advanced age is the main risk factor for most chronic diseases and functional deficits in humans, but the fundamental mechanisms that drive ageing remain largely unknown, impeding the development of interventions that might delay or prevent age-related disorders and maximize healthy lifespan. Cellular senescence, which halts the proliferation of damaged or dysfunctional cells, is an important mechanism to constrain the malignant progression of tumour cells. Senescent cells accumulate in various tissues and organs with ageing and have been hypothesized to disrupt tissue structure and function because of the components they secrete. However, whether senescent cells are causally implicated in age-related dysfunction and whether their removal is beneficial has remained unknown. To address these fundamental questions, we made use of a biomarker for senescence, p16(Ink4a), to design a novel transgene, INK-ATTAC, for inducible elimination of p16(Ink4a)-positive senescent cells upon administration of a drug. Here we show that in the BubR1 progeroid mouse background, INK-ATTAC removes p16(Ink4a)-positive senescent cells upon drug treatment. In tissues--such as adipose tissue, skeletal muscle and eye--in which p16(Ink4a) contributes to the acquisition of age-related pathologies, life-long removal of p16(Ink4a)-expressing cells delayed onset of these phenotypes. Furthermore, late-life clearance attenuated progression of already established age-related disorders. These data indicate that cellular senescence is causally implicated in generating age-related phenotypes and that removal of senescent cells can prevent or delay tissue dysfunction and extend healthspan.

The mechanism of action of cyclosporin A and FK506

CsA and FK506 are powerful suppressors of the immune system, most notably of T cells. They act at a point in activation that lies between receptor ligation and the transcription of early genes. Here, Stuart Schreiber and Gerald Crabtree review recent findings that indicate CsA and FK506 operate as prodrugs: they bind endogenous intracellular receptors, the immunophilins, and the resulting complex targets the protein phosphatase, calcineurin, to exert the immunosuppressive effect.

Identification of calcineurin as a key signalling enzyme in T-lymphocyte activation

The immunosuppressive drugs cyclosporin A (CsA) and FK506 both interfere with a Ca(2+)-sensitive T-cell signal transduction pathway, thereby preventing the activation of specific transcription factors (such as NF-AT and NF-IL2A) involved in lymphokine gene expression. CsA and FK506 seem to act by interaction with their cognate intracellular receptors, cyclophilin and FKBP, respectively (see ref. 11 for review). The Ca2+/calmodulin-regulated phosphatase calcineurin is a major target of drug-isomerase complexes in vitro. We have therefore tested the hypothesis that this interaction is responsible for the in vivo effects of CsA/FK506. We report here that overexpression of calcineurin in Jurkat cells renders them more resistant to the effects of CsA and FK506 and augments both NFAT- and NFIL2A-dependent transcription. These results identify calcineurin as a key enzyme in the T-cell signal transduction cascade and provide biological evidence to support the notion that the interaction of drug-isomerase complexes with calcineurin underlies the molecular basis of CsA/FK506-mediated immunosuppression.

Rapamycin-FKBP specifically blocks growth-dependent activation of and signaling by the 70 kd S6 protein kinases

The macrolide rapamycin blocks cell cycle progression in yeast and various animal cells by an unknown mechanism. We demonstrate that rapamycin blocks the phosphorylation and activation of the 70 kd S6 protein kinases (pp70S6K) in a variety of animal cells. The structurally related drug FK506 had no effect on pp70S6K activation but at high concentrations reversed the rapamycin-induced block, confirming the requirement for the rapamycin and FK506 receptor, FKBP. Rapamycin also interfered with signaling by these S6 kinases, blocking serum-stimulated S6 phosphorylation and delaying entry of Swiss 3T3 cells into S phase. Neither rapamycin nor FK506 blocked activation of a distinct family of S6 kinases (RSKs) or the MAP kinases. These studies identify a rapamycin-sensitive signaling pathway, argue for a ubiquitous role for FKBPs in signal transduction, indicate that FK506-FKBP-calcineurin complexes do not interfere with pp70S6K signaling, and show that in fibroblasts pp70S6K, not RSK, is the physiological S6 kinase.

Enforced dimerization of BAX results in its translocation, mitochondrial dysfunction and apoptosis

Expression of the pro-apoptotic molecule BAX has been shown to induce cell death. While BAX forms both homo- and heterodimers, questions remain concerning its native conformation in vivo and which moiety is functionally active. Here we demonstrate that a physiologic death stimulus, the withdrawal of interleukin-3 (IL-3), resulted in the translocation of monomeric BAX from the cytosol to the mitochondria where it could be cross-linked as a BAX homodimer. In contrast, cells protected by BCL-2 demonstrated a block in this process in that BAX did not redistribute or homodimerize in response to a death signal. To test the functional consequence of BAX dimerization, we expressed a chimeric FKBP-BAX molecule. Enforced dimerization of FKBP-BAX by the bivalent ligand FK1012 resulted in its translocation to mitochondria and induced apoptosis. Caspases were activated yet caspase inhibitors did not block death; cytochrome c was not released detectably despite the induction of mitochondrial dysfunction. Moreover, enforced dimerization of BAX overrode the protection by BCL-XL and IL-3 to kill cells. These data support a model in which a death signal results in the activation of BAX. This conformational change in BAX manifests in its translocation, mitochondrial membrane insertion and homodimerization, and a program of mitochondrial dysfunction that results in cell death.

Involvement of a calcineurin/inhibitor-1 phosphatase cascade in hippocampal long-term depression

Long-term potentiation (LTP) is a synaptic mechanism thought to be involved in learning and memory. Long-term depression (LTD), an activity-dependent decrease in synaptic efficacy, may be an equally important mechanism which permits neural networks to store information more effectively. One form of LTD that has been observed in the hippocampus requires activation of postsynaptic NMDA (N-methyl-D-aspartate) receptors, a change in postsynaptic calcium concentration, and activation of postsynaptic serine/threonine protein phosphatase 1 (PP1) or 2A (PP2A). The mechanism by which PP1 or PP2A is regulated by synaptic activity is unclear because these protein phosphatases are not directly influenced by calcium concentration. LTD induction may require activation of a more complex protein phosphatase cascade consisting of the Ca2+/calmodulin-dependent protein phosphatase, calcineurin, its phosphoprotein substrate, inhibitor-1, and PP1. We tested this hypothesis using calcineurin inhibitors as well as different forms of inhibitor-1 loaded into postsynaptic cells. Our results suggest a signalling pathway in which calcineurin dephosphorylates and inactivates inhibitor-1. This in turn increases PP1 activity and contributes to the generation of LTD.

TRANCE is a novel ligand of the tumor necrosis factor receptor family that activates c-Jun N-terminal kinase in T cells

A novel member of the tumor necrosis factor (TNF) cytokine family, designated TRANCE, was cloned during a search for apoptosis-regulatory genes using a somatic cell genetic approach in T cell hybridomas. The TRANCE gene encodes a type II membrane protein of 316 amino acids with a predicted molecular mass of 35 kDa. Its extracellular domain is most closely related to TRAIL, FasL, and TNF. TRANCE is an immediate early gene up-regulated by TCR stimulation and is controlled by calcineurin-regulated transcription factors. TRANCE is most highly expressed in thymus and lymph nodes but not in nonlymphoid tissues and is abundantly expressed in T cells but not in B cells. Cross-hybridization of the mouse cDNA to a human thymus library yielded the human homolog, which encodes a protein 83% identical to the mouse ectodomain. Human TRANCE was mapped to chromosome 13q14 while mouse TRANCE was located to the portion of mouse chromosome 14 syntenic with human chromosome 13q14. A recombinant soluble form of TRANCE composed of the entire ectodomain induced c-Jun N-terminal kinase (JNK) activation in T cells but not in splenic B cells or in bone marrow-derived dendritic cells. These results suggest a role for this TNF-related ligand in the regulation of the T cell-dependent immune response.

Neutrophil infiltration as an important factor in liver ischemia and reperfusion injury. Modulating effects of FK506 and cyclosporine

To examine the role of neutrophils, their presence and the degree of infiltration, as important determinants of ischemia and reperfusion injury of the liver, male Sprague-Dawley rats were subjected to 60 and 90 min of total-liver ischemia. The presence of neutrophils, assessed by the measurement of liver tissue myeloperoxidase (MPO), and the degree of neutrophil liver infiltration, determined by the naphthol AS-D chloroacetate esterase technique, correlated well with animal survival and response to FK506 and cyclosporine administration. Lipid peroxidation, measured by the malondialdehyde (MDA) test in liver tissue, was another factor closely linked with liver function and survival. Pretreatment with FK506 (0.3 mg/kg) and CsA (5 mg/kg) was given at 4 hr and 1 hr before ischemia and at the time of reperfusion. Control ischemic animals showed increased neutrophil liver infiltration, high MPO and MDA liver levels, and diminished overall survival. FK506 and CsA-treated animals had better survival and diminished neutrophil liver infiltration, as well as MPO and MDA levels. The mechanism by which FK506 and CsA protected the animals from severe liver ischemic injury is unknown. Our data indicated that the presence and the degree of infiltration of neutrophils were important components of liver ischemia/reperfusion injury in the rat. So it is possible that one of the fundamental effects of the FK506 and CsA might be through the inhibition of the presence and infiltration of neutrophils in liver tissue.

FK-506- and CsA-sensitive activation of the interleukin-2 promoter by calcineurin

Antigen recognition by the T-cell receptor (TCR) initiates events including lymphokine gene transcription, particularly interleukin-2, that lead to T-cell activation. The immunosuppressive drugs, cyclosporin A (CsA) and FK-506, prevent T-cell proliferation by inhibiting a Ca(2+)-dependent event required for induction of interleukin-2 transcription. Complexes of FK-506 or CsA and their respective intracellular binding proteins inhibit the calmodulin-dependent protein phosphatase, calcineurin, in vitro. The pharmacological relevance of this observation to immunosuppression or drug toxicity is undetermined. Calcineurin, although present in lymphocytes, has not been implicated in TCR-mediated activation of lymphokine genes or in transcriptional regulation in general. Here we report that transfection of a calcineurin catalytic subunit increases the 50% inhibitory concentration (IC50) of the immunosuppressants FK-506 and CsA, and that a mutant subunit acts in synergy with phorbol ester alone to activate the interleukin-2 promoter in a drug-sensitive manner. These results implicate calcineurin as a component of the TCR signal transduction pathway by demonstrating its role in the drug-sensitive activation of the interleukin-2 promoter.

Treatment with humanized monoclonal antibody against CD154 prevents acute renal allograft rejection in nonhuman primates

CD154 is the ligand for the receptor CD40. This ligand-receptor pair mediates endothelial and antigen-presenting cell activation, and facilitates the interaction of these cells with T cells and platelets. We demonstrate here that administration of a CD154-specific monoclonal antibody (hu5C8) allows for renal allotransplantation in outbred, MHC-mismatched rhesus monkeys without acute rejection. The effect persisted for more than 10 months after therapy termination, and no additional drug was required to achieve extended graft survival. Indeed, the use of tacrolimus or chronic steroids seemed to antagonize the anti-rejection effect. Monkeys treated with antibody against CD154 remained healthy during and after therapy. The mechanism of action does not require global depletion of T or B cells. Long-term survivors lost their mixed lymphocyte reactivity in a donor-specific manner, but still formed donor-specific antibody and generated T cells that infiltrated the grafted organ without any obvious effect on graft function. Thus, therapy with antibody against CD154 is a promising agent for clinical use in human allotransplantation.

Calcineurin phosphatase activity in T lymphocytes is inhibited by FK 506 and cyclosporin A

The immunosuppressive agents cyclosporin A (CsA) and FK 506 bind to distinct families of intracellular proteins (immunophilins) termed cyclophilins and FK 506-binding proteins (FKBPs). Recently, it has been shown that, in vitro, the complexes of CsA-cyclophilin and FK 506-FKBP-12 bind to and inhibit the activity of calcineurin, a calcium-dependent serine/threonine phosphatase. We have investigated the effects of drug treatment on phosphatase activity in T lymphocytes. Calcineurin is expressed in T cells, and its activity can be measured in cell lysates. Both CsA and FK 506 specifically inhibit cellular calcineurin at drug concentrations that inhibit interleukin 2 production in activated T cells. Rapamycin, which binds to FKBPs but exhibits different biological activities than FK 506, has no effect on calcineurin activity. Furthermore, excess concentrations of rapamycin prevent the effects of FK 506, apparently by displacing FK 506 from FKBPs. These results show that calcineurin is a target of drug-immunophilin complexes in vivo and establish a physiological role for calcineurin in T-cell activation.

Stabilization of calcium release channel (ryanodine receptor) function by FK506-binding protein

FK506-binding protein (FKBP12) was originally identified as the cytosolic receptor for the immunosuppressant drugs FK506 and rapamycin. The cellular function of FKBP12, a ubiquitously expressed 12,000-dalton proline isomerase, has been unknown. FKBP12 copurifies with the 565,000-dalton ryanodine receptor (RyR), four of which form intracellular Ca2+ release channels of the sarcoplasmic and endoplasmic reticula. By coexpressing the RyR and FKBP12 in insect cells, we have demonstrated that FKBP12 modulates channel gating by increasing channels with full conductance levels (by > 400%), decreasing open probability after caffeine activation (from 0.63 +/- 0.09 to 0.04 +/- 0.02), and increasing mean open time (from 4.4 +/- 0.6 ms to 75 +/- 41 ms). FK506 or rapamycin, inhibitors of FKBP12 isomerase activity, reverse these stabilizing effects. These results provide the first natural cellular function for FKBP12, and establish that the functional Ca2+ release channel complex includes FKBP12.

A mechanistic role for cardiac myocyte apoptosis in heart failure

Heart failure is a common, lethal condition whose pathogenesis is poorly understood. Recent studies have identified low levels of myocyte apoptosis (80-250 myocytes per 10(5) nuclei) in failing human hearts. It remains unclear, however, whether this cell death is a coincidental finding, a protective process, or a causal component in pathogenesis. Using transgenic mice that express a conditionally active caspase exclusively in the myocardium, we demonstrate that very low levels of myocyte apoptosis (23 myocytes per 10(5) nuclei, compared with 1.5 myocytes per 10(5) nuclei in controls) are sufficient to cause a lethal, dilated cardiomyopathy. Interestingly, these levels are four- to tenfold lower than those observed in failing human hearts. Conversely, inhibition of cardiac myocyte death in this murine model largely prevents the development of cardiac dilation and contractile dysfunction, the hallmarks of heart failure. To our knowledge, these data provide the first direct evidence that myocyte apoptosis may be a causal mechanism of heart failure, and they suggest that inhibition of this cell death process may constitute the basis for novel therapies.

Randomised trial comparing tacrolimus (FK506) and cyclosporin in prevention of liver allograft rejection. European FK506 Multicentre Liver Study Group

Studies in the USA and Japan have shown that tacrolimus (FK506) is a potent immunosuppressant. To compare the efficacy and safety of tacrolimus-based and conventional cyclosporin-based immunosuppressive regimens we recruited 545 liver transplant recipients from eight European centres into a randomised open trial. Analysis of the data at 12 months post-transplant showed that tacrolimus was associated with a significant reduction in acute, refractory acute, and chronic rejection episodes. The rates were, for acute rejection, tacrolimus 40.5% vs 49.8% cyclosporin (p = 0.040; absolute difference 9.3% [95% CI 0.9-17.8%]). For refractory acute and chronic rejections the comparisons were 0.8% vs 5.3% (p = 0.005) and 1.5% vs 5.3% (p = 0.032). There results were seen despite significantly lower corticosteroid usage. The incidence of infection was also lower in patients receiving tacrolimus. Patient and graft survival rates were not significantly different (tacrolimus 82.9% and 77.5%; cyclosporin 77.5% and 72.6%). Safety data were comparable--the most serious events being renal impairment, disturbances of glucose metabolism, and neurological complications--but these events were more common in the tacrolimus group. In this trial tacrolimus had advantages over cyclosporin in respect of lower rejection rates, even though less concurrent immunosuppression was administered.

Incidence of BK with tacrolimus versus cyclosporine and impact of preemptive immunosuppression reduction

Our purposes were to determine the incidence of BK viruria, viremia or nephropathy with tacrolimus (FK506) versus cyclosporine (CyA) and whether intensive monitoring and discontinuation of mycophenolate (MMF) or azathioprine (AZA), upon detection of BK viremia, could prevent BK nephropathy. We randomized 200 adult renal transplant recipients to FK506 (n = 134) or CyA (n = 66). Urine and blood were collected weekly for 16 weeks and at months 5, 6, 9 and 12 and analyzed for BK by polymerase chain reaction (PCR). By 1 year, 70 patients (35%) developed viruria and 23 (11.5%) viremia; neither were affected independently by FK506, CyA, MMF or AZA. Viruria was highest with FK506-MMF (46%) and lowest with CyA-MMF (13%), p = 0.005. Viruria >/= 9.5 log(10) copies/mL was associated with a 3-fold increased risk of viremia and a 13-fold increased risk of sustained viremia. After reduction of immunosuppression, viremia resolved in 95%, without increased acute rejection, allograft dysfunction or graft loss. No BK nephropathy was observed. Choice of calcineurin inhibitor or adjuvant immunosuppression, independently, did not affect BK viruria or viremia. Viruria was highest with FK506-MMF and lowest with CyA-MMF. Monitoring and preemptive withdrawal of immunosuppression were associated with resolution of viremia and absence of BK nephropathy without acute rejection or graft loss.

Rapamycin-induced inhibition of the 70-kilodalton S6 protein kinase

The immunosuppressant rapamycin inhibited proliferation of the H4IIEC hepatoma cell line. Rapamycin, but not its structural analog FK506, also inhibited the basal and insulin-stimulated activity of the p70 ribosomal protein S6 kinase. By contrast, insulin stimulation of the p85 Rsk S6 kinase and mitogen-activated protein (MAP) kinase activity were unaffected by drug. Rapamycin treatment of COS cells transfected with recombinant p70 S6 kinase completely inhibited the appearance of the hyperphosphorylated form of p70 S6 kinase concomitant with the inhibition of enzyme activity toward 40S subunits. Thus, rapamycin inhibits a signal transduction element that is necessary for the activation of p70 S6 kinase and mitogenesis but unnecessary for activation of p85 Rsk S6 kinase or MAP kinase.

The mechanism of action of cyclosporin A and FK506

The immunosuppressants cyclosporin A (CsA), FK506, and rapamycin suppress the immune response by inhibiting evolutionary conserved signal transduction pathways. CsA, FK506, and rapamycin bind to their intracellular receptors, immunophilins, creating composite surfaces that block the activity of specific targets. For CsA/cyclophilin and FK506/FKBP the target is calcineurin. Because of the large surface area of interaction of the drug-immunophilin complex with calcineurin, FK506 and CsA have a specificity for their biologic targets that is equivalent to growth factor-receptor interactions. To date, all the therapeutic as well as toxic effects of these drugs have been shown to be due to inhibition of calcineurin. Inhibition of the action of calcineurin results in a complete block in the translocation of the cytosolic component of the nuclear factor of activated T cells (NF-AT), resulting in a failure to activate the genes regulated by the NF-AT transcription factor. These genes include those required for B-cell help such as interleukin (IL-4) and CD40 ligand as well as those necessary for T-cell proliferation such as IL-2. The purpose of this article is to illustrate the means by which these drugs produce immunosuppression.

Cyclosporin A, FK-506, and rapamycin: pharmacologic probes of lymphocyte signal transduction

CsA, FK-506, and rapamycin are microbial products with potent immunosuppressive properties that result primarily from a selective inhibition of T lymphocyte activation. Although chemically unrelated, CsA and FK-506 affect a similar subset of calcium-associated signaling events involved in the regulation of lymphokine gene expression, activation-driven T-cell death and exocytosis. Rapamycin has structural similarity with FK-506 but suppresses T-cell activation at a different level, mainly through inhibition of proliferation induced by growth-promoting lymphokines. CsA interacts with an abundant 17 kDa protein, termed cyclophilin, that possesses peptidyl-prolyl cis-trans isomerase (PPIase) activity. Additional, minor cyclophilin-like molecules have been identified. Both FK-506 and rapamycin interact with FKBP, a 12 kDa protein, which, although unrelated to cyclophilin, is also abundant and ubiquitous, has a similar enzymatic activity, and is a member of a larger family of FKBPs. All three immunosuppressants inhibit the PPIase activity of their respective binding proteins. However, nonimmunosuppressive analogs of CsA and FK-506 are also inhibitory, indicating that inhibition of PPIase activity is not directly implicated in immunosuppression. Moreover, only a small fraction of the cellular pool of the major forms of cyclophilin or FKBP needs to be occupied by the drugs in order to achieve maximal immunosuppression. These observations suggest that complexes formed between the drugs and their major binding proteins may affect the function of other, unidentified, molecules or, alternatively, that minor binding proteins may play a role in the drugs' action. Further characterization of the biochemical processes altered by CsA, FK-506, and rapamycin should yield important insights into the signal transduction pathways involved in T-cell activation and should help in the development of novel immunosuppressive agents.

Role of the NF-ATc transcription factor in morphogenesis of cardiac valves and septum

In lymphocytes, the expression of early immune response genes is regulated by NF-AT transcription factors which translocate to the nucleus after dephosphorylation by the Ca2+-dependent phosphatase, calcineurin. We report here that mice bearing a disruption in the NF-ATc gene fail to develop normal cardiac valves and septa and die of circulatory failure before day 14.5 of development. NF-ATc is first expressed in the heart at day 7.5, and is restricted to the endocardium, a specialized endothelium that gives rise to the valves and septum. Within the endocardium, specific inductive events appear to activate NF-ATc: it is localized to the nucleus only in endocardial cells that are adjacent to the interface with the cardiac jelly and myocardium, which are thought to give the inductive stimulus to the valve primordia. Treatment of wild-type embryos with FK506, a specific calcineurin inhibitor, prevents nuclear localization of NF-ATc. These data indicate that the Ca2+/calcineurin/NF-ATc signalling pathway is essential for normal cardiac valve and septum morphogenesis; hence, NF-ATc and its regulatory pathways are candidates for genetic defects underlying congenital human heart disease.

FK506-binding proteins 51 and 52 differentially regulate dynein interaction and nuclear translocation of the glucocorticoid receptor in mammalian cells

We used a cellular system to elucidate the molecular determinants of the large immunophilin FK506-binding proteins (FKBP)51 and -52 for their action on the glucocorticoid receptor in mammalian cells. Increasing the levels of FKBP51 reduced the transcriptional activity of the receptor, as reported. Elevated levels of FKBP52 per se showed no effect but mitigated the inhibition of the receptor induced by FKBP51. We discovered that nuclear translocation of the glucocorticoid receptor was delayed by FKBP51. This correlates with the reduced interaction of FKBP51 with the motor protein dynein compared with FKBP52. From mutational analyses, we concluded that three features of the immunophilins are required for efficient receptor signaling in mammalian cells: hsp90 interaction, dynein association, and peptidylprolyl isomerase (PPIase) enzyme activity. The relevance of dynein for receptor function was substantiated by several experiments: 1) coexpression of dynamitin, which disrupts the transport complex and reduces receptor activity; 2) coexpression of the PPIase domain fragment of FKBP52, which is known to disrupt interaction of the receptor to dynein and reduce glucocorticoid receptor function, in contrast to the corresponding fragment of FKBP51; and 3) swapping of the PPIase domains FKBP51 and FKBP52, which reverses the respective activity. We concluded from our results that the mechanisms of the regulatory system FKBP51/FKBP52 discovered in yeast also operate in mammals to modulate hormone binding of the receptor. In addition, differential regulation of dynein association and nuclear translocation contributes to the effects of the two immunophilins on the glucocorticoid receptor in mammals.

Peptidyl-prolyl cis-trans isomerases, a superfamily of ubiquitous folding catalysts

Cyclosporine A therapy for prophylaxis against graft rejection revolutionized human organ transplantation. The immunosuppressant drugs cyclosporin A (CsA), FK506 and rapamycin block T-cell activation by interfering with the signal transduction pathway. The target proteins for CsA and FK506 were found to be cyclophilins and FK506-binding proteins, (FKBPs), respectively. They are unrelated in primary sequence, although both are peptidyl-prolyl cis-trans isomerases catalyzing the interconversion of peptidylprolyl imide bonds in peptide and protein substrates. However, the prolyl isomerase activity of these proteins is not essential for their immunosuppressive effects. Instead, the specific surfaces of the cyclophilin-CsA and FKBP-FK506 complexes mediate the immunosuppressive action. Moreover, the natural cellular functions of all but a few remain elusive. In some cases it could be demonstrated that prolyl isomerization is the rate-limiting step in protein folding in vitro, but many knockout mutants of single and multiple prolyl isomerases were viable with no detectable phenotype. Even though a direct requirement for in vivo protein folding could not be demonstrated, some important natural substrates of the prolyl isomerases are now known, and they demonstrate the great variety of prolyl isomerization functions in the living cell: (i) A human cyclophilin binds to the Gag polyprotein of the human immunodeficiency virus-1 (HIV-1) virion and was found to be essential for infection with HIV to occur, probably by removal of the virion coat. (ii) Together with heat shock protein (HSP) 90, a member of the chaperone family, high molecular weight cyclophilins and FKBPs bind and activate steroid receptors. This example also demonstrates that prolyl isomerases act together with other folding enzymes, for example the chaperones, and protein disulfide isomerases. (iii) An FKBP was found to act as a modulator of an intracellular calcium release channel. (iv) Along with the cyclophilins and FKBPs, a third class of prolyl isomerases exist, the parvulins. The human parvulin homologue Pin1 is a mitotic regulator essential for the G2/M transition of the eukaryotic cell cycle. These findings place proline isomerases at the intersection of protein folding, signal transduction, trafficking, assembly and cell cycle regulation.

Association of protein kinase A and protein phosphatase 2B with a common anchoring protein

Specificity of protein kinases and phosphatases may be achieved through compartmentalization with preferred substrates. In neurons, adenosine 3', 5'-monophosphate (cAMP)-dependent protein kinase (PKA) is localized at postsynaptic densities by association of its regulatory subunit with an A kinase anchor protein, AKAP79. Interaction cloning experiments demonstrated that AKAP79 also binds protein phosphatase 2B, or calcineurin (CaN). A ternary complex of PKA, AKAP, and CaN was isolated from bovine brain, and colocalization of the kinase and the phosphatase was established in neurites of cultured hippocampal neurons. The putative CaN-binding domain of AKAP79 is similar to that of the immunophilin FKBP-12, and AKAP79 inhibited CaN phosphatase activity. These results suggest that both PKA and CaN are targeted to subcellular sites by association with a common anchor protein and thereby regulate the phosphorylation state of key neuronal substrates.

Ubiquitination Regulates PSD-95 Degradation and AMPA Receptor Surface Expression

PSD-95 is a major scaffolding protein of the postsynaptic density, tethering NMDA- and AMPA-type glutamate receptors to signaling proteins and the neuronal cytoskeleton. Here we show that PSD-95 is regulated by the ubiquitin-proteasome pathway. PSD-95 interacts with and is ubiquitinated by the E3 ligase Mdm2. In response to NMDA receptor activation, PSD-95 is ubiquitinated and rapidly removed from synaptic sites by proteasome-dependent degradation. Mutations that block PSD-95 ubiquitination prevent NMDA-induced AMPA receptor endocytosis. Likewise, proteasome inhibitors prevent NMDA-induced AMPA receptor internalization and synaptically induced long-term depression. This is consistent with the notion that PSD-95 levels are an important determinant of AMPA receptor number at the synapse. These data suggest that ubiquitination of PSD-95 through an Mdm2-mediated pathway is critical in regulating AMPA receptor surface expression during synaptic plasticity.

Inhibition of T cell signaling by immunophilin-ligand complexes correlates with loss of calcineurin phosphatase activity

Calcineurin, a Ca2+, calmodulin-dependent protein phosphatase, was recently found to bind with high affinity to two different immunosuppressant binding proteins (immunophilins) with absolute dependence on the presence of the immunosuppressants FK506 or cyclosporin A (CsA) [Liu et al. (1991) Cell 66, 807-815]. The binding affinities of the immunophilin-drug complexes toward calcineurin and the stoichiometry of the resultant multimeric complexes have now been determined, and structural elements of FK506, CsA, and calcineurin that are critical for mediating their interactions have been identified. Analogues of FK506 (FK520, FK523, 15-O-demethyl-FK520) and CsA (MeBm2t1-CsA and MeAla6-CsA) whose affinities for their cognate immunophilins do not correlate with their immunosuppressive activities have been prepared and evaluated in biochemical and cellular assays. We demonstrate a strong correlation between the ability of these analogues, when bound to their immunophilins, to inhibit the phosphatase activity of calcineurin and their ability to inhibit transcriptional activation by NF-AT, a T cell specific transcription factor that regulates IL-2 gene synthesis in human T cells. In addition, FKBP-FK506 and CyP-CsA do not inhibit members of the PP1, PP2A, and PP2C classes of serine/threonine phosphatases. These data suggest that calcineurin is the relevant cellular target of these immunosuppressive agents and is involved in Ca(2+)-dependent signal transduction pathways in, among others, T cells and mast cells.