Substrate selectivity and sensitivity to inhibition by FK506 and cyclosporin A of calcineurin heterodimers composed of the alpha or beta catalytic subunit

Ca2+-PP2B-PSD-95 axis: A novel regulatory mechanism of the phosphorylation state of Serine 295 of PSD-95

The phosphorylation state of PSD-95 at Serine 295 (Ser295) is important for the regulation of synaptic plasticity. Although the activation of NMDA receptors (NMDARs), which initiates an intracellular calcium signaling cascade, decreases phosphorylated Ser295 (pS295) of PSD-95, the molecular mechanisms are not fully understood. We found that the calcium-activated protein phosphatase PP2B dephosphorylated pS295 not only in basal conditions but also in NMDAR-activated conditions in cultured neurons. The biochemical assay also revealed the dephosphorylation of pS295 by PP2B, consistently supporting the results obtained using neurons. The newly identified calcium signaling cascade "Ca2+-PP2B-PSD-95 axis" would play an important role in the molecular mechanism for NMDA receptor-dependent plasticity.

Tacrolimus induces fibroblast-to-myofibroblast transition via a TGF-β-dependent mechanism to contribute to renal fibrosis

Use of immunosuppressant calcineurin inhibitors (CNIs) is limited by irreversible kidney damage, hallmarked by renal fibrosis. CNIs directly damage many renal cell types. Given the diverse renal cell populations, additional targeted cell types and signaling mechanisms warrant further investigation. We hypothesized that fibroblasts contribute to CNI-induced renal fibrosis and propagate profibrotic effects via the transforming growth factor-β (TGF-β)/Smad signaling axis. To test this, kidney damage-resistant mice (C57BL/6) received tacrolimus (10 mg/kg) or vehicle for 21 days. Renal damage markers and signaling mediators were assessed. To investigate their role in renal damage, mouse renal fibroblasts were exposed to tacrolimus (1 nM) or vehicle for 24 h. Morphological and functional changes in addition to downstream signaling events were assessed. Tacrolimus-treated kidneys displayed evidence of renal fibrosis. Moreover, α-smooth muscle actin expression was significantly increased, suggesting the presence of fibroblast activation. TGF-β receptor activation and downstream Smad2/3 signaling were also upregulated. Consistent with in vivo findings, tacrolimus-treated renal fibroblasts displayed a phenotypic switch known as fibroblast-to-myofibroblast transition (FMT), as α-smooth muscle actin, actin stress fibers, cell motility, and collagen type IV expression were significantly increased. These findings were accompanied by concomitant induction of TGF-β signaling. Pharmacological inhibition of the downstream TGF-β effector Smad3 attenuated tacrolimus-induced phenotypic changes. Collectively, these findings suggest that 1) tacrolimus inhibits the calcineurin/nuclear factor of activated T cells axis while inducing TGF-β1 ligand secretion and receptor activation in renal fibroblasts; 2) aberrant TGF-β receptor activation stimulates Smad-mediated production of myofibroblast markers, notable features of FMT; and 3) FMT contributes to extracellular matrix expansion in tacrolimus-induced renal fibrosis. These results incorporate renal fibroblasts into the growing list of CNI-targeted cell types and identify renal FMT as a process mediated via a TGF-β-dependent mechanism.NEW & NOTEWORTHY Renal fibrosis, a detrimental feature of irreversible kidney damage, remains a sinister consequence of long-term calcineurin inhibitor (CNI) immunosuppressive therapy. Our study not only incorporates renal fibroblasts into the growing list of cell types negatively impacted by CNIs but also identifies renal fibroblast-to-myofibroblast transition as a process mediated via a TGF-β-dependent mechanism. This insight will direct future studies investigating the feasibility of inhibiting TGF-β signaling to maintain CNI-mediated immunosuppression while ultimately preserving kidney health.

Voltage-Gated T-Type Calcium Channel Modulation by Kinases and Phosphatases: The Old Ones, the New Ones, and the Missing Ones

Calcium (Ca2+) can regulate a wide variety of cellular fates, such as proliferation, apoptosis, and autophagy. More importantly, changes in the intracellular Ca2+ level can modulate signaling pathways that control a broad range of physiological as well as pathological cellular events, including those important to cellular excitability, cell cycle, gene-transcription, contraction, cancer progression, etc. Not only intracellular Ca2+ level but the distribution of Ca2+ in the intracellular compartments is also a highly regulated process. For this Ca2+ homeostasis, numerous Ca2+ chelating, storage, and transport mechanisms are required. There are also specialized proteins that are responsible for buffering and transport of Ca2+. T-type Ca2+ channels (TTCCs) are one of those specialized proteins which play a key role in the signal transduction of many excitable and non-excitable cell types. TTCCs are low-voltage activated channels that belong to the family of voltage-gated Ca2+ channels. Over decades, multiple kinases and phosphatases have been shown to modulate the activity of TTCCs, thus playing an indirect role in maintaining cellular physiology. In this review, we provide information on the kinase and phosphatase modulation of TTCC isoforms Cav3.1, Cav3.2, and Cav3.3, which are mostly described for roles unrelated to cellular excitability. We also describe possible potential modulations that are yet to be explored. For example, both mitogen-activated protein kinase and citron kinase show affinity for different TTCC isoforms; however, the effect of such interaction on TTCC current/kinetics has not been studied yet.

Calcineurin inhibitor (CNI)-associated skin cancers: New insights on exploring mechanisms by which CNIs downregulate DNA repair machinery

The use of the calcineurin inhibitors (CNI) cyclosporine (CsA) and tacrolimus remains a cornerstone in post-transplantation immunosuppression. Although these immunosuppressive agents have revolutionized the field of transplantation medicine, its increased skin cancer risk poses a major concern. A key contributor to this phenomenon is a reduced capacity to repair DNA damage caused by exposure to ultraviolet (UV) wavelengths of sunlight. CNIs decrease DNA repair by mechanisms that remain to be fully explored. Though CsA is known to decrease the abundance of key DNA repair enzymes, less is known about how tacrolimus yields this effect. CNIs hold the capacity to inhibit both of the main catalytic calcineurin isoforms (CnAα and CnAβ). However, it is unknown which isoform regulates UV-induced DNA repair, which is the focus of this review. It is with hope that this insight spurs investigative efforts that conclusively addresses these gaps in knowledge. Additionally, this research also raises the possibility that newer CNIs can be developed that effectively blunt the immune response while mitigating the incidence of skin cancers with immunosuppression.

Regulation of the Ca2+-activated chloride channel Anoctamin-1 (TMEM16A) by Ca2+-induced interaction with FKBP12 and calcineurin

Chloride fluxes through the calcium-gated chloride channel Anoctamin-1 (TMEM16A) control blood pressure, secretion of saliva, mucin, insulin, and melatonin, gastrointestinal motility, sperm capacitation and motility, and pain sensation. Calcium activates a myriad of regulatory proteins but how these proteins affect TMEM16A activity is unresolved. Here we show by co-immunoprecipitation that increasing intracellular calcium with ionomycin or by activating sphingosine-1-phosphate receptors, induces coupling of calcium/calmodulin-dependent phosphatase calcineurin and prolyl isomerase FK506-binding protein 12 (FKBP12) to TMEM16A in HEK-293 cells. Application of drugs that target either calcineurin (cyclosporine A) or FKBP12 (tacrolimus known as FK506 and sirolimus known as rapamycin) caused a decrease in TMEM16A activity. In addition, FK506 and BAPTA-AM prevented co-immunoprecipitation between FKBP12 and TMEM16A. FK506 rendered the channel insensitive to cyclosporine A without altering its apparent calcium sensitivity whereas zero intracellular calcium blocked the effect of FK506. Rapamycin decreased TMEM16A activity in cells pre-treated with cyclosporine A or FK506. These results suggest the formation of a TMEM16A-FKBP12-calcineurin complex that regulates channel function. We conclude that upon a cytosolic calcium increase the TMEM16A-FKPB12-calcineurin trimers are assembled. Such hetero-oligomerization enhances TMEM16A channel activity but is not mandatory for activation by calcium.

Axonal Protection by Tacrolimus with Inhibition of NFATc1 in TNF-Induced Optic Nerve Degeneration

Tacrolimus, a calcineurin (CaN) inhibitor, has been used for treatment of refractory allergic ocular disease, although its role in optic nerve degeneration remains to be elucidated. In this study, we investigated whether tacrolimus modulates tumor necrosis factor (TNF)-mediated axonal degeneration and whether it alters nuclear factor of activated T cells (NFATc), a downstream effector of CaN signaling. Immunoblot analysis showed no significant difference in CaNAα protein levels in optic nerve on day 3, 7, or 14 after TNF injection compared with PBS injection. However, a significant increase in NFATc1 protein level was observed in optic nerve 7 days after TNF injection. This increase was negated by simultaneous administration of tacrolimus. Administration of tacrolimus alone did not change the NFATc1 protein level in comparison to that observed after PBS injection. A significant increase in TNF protein level was observed in optic nerve 14 days after TNF injection and this increase was prevented by tacrolimus. Immunohistochemical analysis showed the immunoreactivity of NFATc1 to be increased in optic nerve after TNF injection. This increased immunoreactivity was colocalized with glial fibrillary acidic protein and was suppressed by tacrolimus. Treatment of tacrolimus significantly ameliorated the TNF-mediated axonal loss. These results suggest that tacrolimus is neuroprotective against axon loss in TNF-induced optic neuropathy and that the effect arises from suppression of the CaN/NFATc1 pathway.

Calcineurin signaling in the heart: The importance of time and place

The calcium-activated protein phosphatase, calcineurin, lies at the intersection of protein phosphorylation and calcium signaling cascades, where it provides an essential nodal point for coordination between these two fundamental modes of intracellular communication. In excitatory cells, such as neurons and cardiomyocytes, that experience rapid and frequent changes in cytoplasmic calcium, calcineurin protein levels are exceptionally high, suggesting that these cells require high levels of calcineurin activity. Yet, it is widely recognized that excessive activation of calcineurin in the heart contributes to pathological hypertrophic remodeling and the progression to failure. How does a calcium activated enzyme function in the calcium-rich environment of the continuously contracting heart without pathological consequences? This review will discuss the wide range of calcineurin substrates relevant to cardiovascular health and the mechanisms calcineurin uses to find and act on appropriate substrates in the appropriate location while potentially avoiding others. Fundamental differences in calcineurin signaling in neonatal verses adult cardiomyocytes will be addressed as well as the importance of maintaining heterogeneity in calcineurin activity across the myocardium. Finally, we will discuss how circadian oscillations in calcineurin activity may facilitate integration with other essential but conflicting processes, allowing a healthy heart to reap the benefits of calcineurin signaling while avoiding the detrimental consequences of sustained calcineurin activity that can culminate in heart failure.

A novel interaction between ATOH8 and PPP3CB

ATOH8 is a bHLH transcription factor playing roles in a variety of developmental processes such as neurogenesis, differentiation of pancreatic precursor cells, development of kidney and muscle, and differentiation of endothelial cells. PPP3CB belongs to the catalytic subunit of the serine/threonine phosphatase, calcineurin, which can dephosphorylate its substrate proteins to regulate their physiological activities. In our study, we demonstrated that ATOH8 interacts with PPP3CB in vitro with different approaches. We show that the conserved catalytic domain of PPP3CB interacts with both the N-terminus and the bHLH domain of ATOH8. Although the interaction domain of PPP3CB is conserved among all isoforms of calcineurin A, ATOH8 selectively interacts with PPP3CB instead of PPP3CA, probably due to the unique proline-rich region present in the N-terminus of PPP3CB, which controls the specificity of its interaction partners. Furthermore, we show that inhibition of the interaction with calcineurin inhibitor, cyclosporin A (CsA), leads to the retention of ATOH8 to the cytoplasm, suggesting that the interaction renders nuclear localization of ATOH8 which may be critical to control its activity as transcription factor.

Biochemical and molecular characterization of the calcineurin in Echinococcus granulosus larval stages

Calcineurin (CaN) is a Ca(2+)-calmodulin activated serine-threonine protein phosphatase that couples the local or global calcium signals, thus controlling important cellular functions in physiological and developmental processes. The aim of this study was to characterize CaN in Echinococcus granulosus (Eg-CaN), a human cestode parasite of clinical importance, both functionally and molecularly. We found that the catalytic subunit isoforms have predicted sequences of 613 and 557 amino acids and are substantially similar to those of the human counterpart, except for the C-terminal end. We also found that the regulatory subunit consists of 169 amino acids which are 87% identical to the human ortholog. We cloned a cDNA encoding for one of the two catalytic subunit isoforms of CaN (Eg-can-A1) as well as the only copy of the Eg-can-B gene, both constitutively transcribed in all Echinococcus larval stages and responsible for generating a functionally active heterodimer. Eg-CaN native enzyme has phosphatase activity, which is enhanced by Ca(2+)/Ni(2+) and reduced by cyclosporine A and Ca(2+) chelators. Participation of Eg-CaN in exocytosis was demonstrated using the FM4-64 probe and Eg-CaN-A was immunolocalized in the cytoplasm of tegumental cells, suckers and excretory bladder of protoscoleces. We also showed that the Eg-can-B transcripts were down-regulated in response to low Ca(2+) intracellular level, in agreement with decreased enzyme activity. Confocal microscopy revealed a striking pattern of Eg-CaN-A in discrete fluorescent spots in the protoscolex posterior bladder and vesicularized protoscoleces beginning the vesicular differentiation. In contrast, Eg-CaN-A was undetectable during the pre-microcyst closing stage while a high DDX-like RNA helicase expression was evidenced. Finally, we identified and analyzed the expression of CaN-related endogenous regulators.

Integration of biochemical and electrical signaling-multiscale model of the medium spiny neuron of the striatum

Neuron behavior results from the interplay between networks of biochemical processes and electrical signaling. Synaptic plasticity is one of the neuronal properties emerging from such an interaction. One of the current approaches to study plasticity is to model either its electrical aspects or its biochemical components. Among the chief reasons are the different time scales involved, electrical events happening in milliseconds while biochemical cascades respond in minutes or hours. In order to create multiscale models taking in consideration both aspects simultaneously, one needs to synchronize the two models, and exchange relevant variable values. We present a new event-driven algorithm to synchronize different neuronal models, which decreases computational time and avoids superfluous synchronizations. The algorithm is implemented in the TimeScales framework. We demonstrate its use by simulating a new multiscale model of the Medium Spiny Neuron of the Neostriatum. The model comprises over a thousand dendritic spines, where the electrical model interacts with the respective instances of a biochemical model. Our results show that a multiscale model is able to exhibit changes of synaptic plasticity as a result of the interaction between electrical and biochemical signaling. Our synchronization strategy is general enough to be used in simulations of other models with similar synchronization issues, such as networks of neurons. Moreover, the integration between the electrical and the biochemical models opens up the possibility to investigate multiscale process, like synaptic plasticity, in a more global manner, while taking into account a more realistic description of the underlying mechanisms.

Calcium input frequency, duration and amplitude differentially modulate the relative activation of calcineurin and CaMKII

NMDA receptor dependent long-term potentiation (LTP) and long-term depression (LTD) are two prominent forms of synaptic plasticity, both of which are triggered by post-synaptic calcium elevation. To understand how calcium selectively stimulates two opposing processes, we developed a detailed computational model and performed simulations with different calcium input frequencies, amplitudes, and durations. We show that with a total amount of calcium ions kept constant, high frequencies of calcium pulses stimulate calmodulin more efficiently. Calcium input activates both calcineurin and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) at all frequencies, but increased frequencies shift the relative activation from calcineurin to CaMKII. Irrespective of amplitude and duration of the inputs, the total amount of calcium ions injected adjusts the sensitivity of the system to calcium input frequencies. At a given frequency, the quantity of CaMKII activated is proportional to the total amount of calcium. Thus, an input of a small amount of calcium at high frequencies can induce the same activation of CaMKII as a larger amount, at lower frequencies. Finally, the extent of activation of CaMKII signals with high calcium frequency is further controlled by other factors, including the availability of calmodulin, and by the potency of phosphatase inhibitors.

Calcineurin inhibitors and immunosuppression - a tale of two isoforms

Organ transplantation is the state of the art for treating end-stage organ failure. Over 25000 organ transplants are performed in the USA each year. Survival rates following transplantation are now approaching 90% for 1 year and 75% for 5 years. Central to this success was the introduction of drugs that suppress the immune system and prevent rejection. The most commonly used class of immunosuppressing drugs are calcineurin inhibitors (CNIs). Calcineurin is a ubiquitous enzyme that is important for T-cell function. With more people taking CNIs for longer and longer periods of time the consequences of calcineurin inhibition on other organ systems - particularly the kidney - have become a growing concern. Virtually all people who take a CNI will develop some degree of kidney toxicity and up to 10% will progress to kidney failure. In the past 15 years, research into calcineurin action has identified distinct actions of the two main isoforms of the catalytic subunit of the enzyme. The α-isoform is required for kidney function whereas the β-isoform has a predominant role in the immune system. This review will discuss the current state of knowledge about calcineurin isoforms and how these new insights may reshape post-transplant immunosuppression.

Signaling regulation of fetoplacental angiogenesis

During normal pregnancy, dramatically increased placental blood flow is critical for fetal growth and survival as well as neonatal birth weights and survivability. This increased blood flow results from angiogenesis, vasodilatation, and vascular remodeling. Locally produced growth factors including fibroblast growth factor 2 (FGF2) and vascular endothelial growth factor A (VEGFA) are key regulators of placental endothelial functions including cell proliferation, migration, and vasodilatation. However, the precise signaling mechanisms underlying such regulation in fetoplacental endothelium are less well defined, specifically with regard to the interactions amongst protein kinases (PKs), protein phosphatase, and nitric oxide (NO). Recently, we and other researchers have obtained solid evidence showing that different signaling mechanisms participate in FGF2- and VEGFA-regulated fetoplacental endothelial cell proliferation and migration as well as NO production. This review will briefly summarize currently available data on signaling mediating fetoplacental angiogenesis with a specific emphasis on PKs, ERK1/2, AKT1, and p38 MAPK and protein phosphatases, PPP2 and PPP3.

Molecular Diagnostics of Calcineurin-Related Pathologies

BACKGROUND

The Ca2+-dependent protein phosphatase enzyme calcineurin (Cn) (protein phosphatase 3) is best known for its role as director of the adaptive immune response. One of its principal substrates is the nuclear factor of activated T cells (NFAT), which translocates to the nucleus after dephosphorylation to mediate gene transcription. Drugs targeting Cn (the Cn inhibitors tacrolimus and cyclosporin A) have revolutionized posttransplantation therapy in allograft recipients by considerably reducing rejection rates.

CONTENT

Owing primarily to intensive study of the side effects of the Cn inhibitors, the unique importance of Cn and Cn/NFAT signaling in the normal physiological processes of many other cell and tissue types is becoming more evident. During the last decade, it has become clear that an extensive and diverse array of clinical conditions can be traced back, at least in part, to a disturbed Cn-signaling axis. Hence, both diagnostics and therapeutic monitoring could benefit from a technique that conveniently reads out Cn/NFAT operative status.

SUMMARY

This review outlines the current knowledge on the pathologic conditions that have calcineurin as a common denominator and reports on the progress that has been made toward successfully applying Cn and Cn/NFAT activity markers in molecular diagnostics.

Expression of a constitutively active calcineurin encoded by an intron-retaining mRNA in follicular keratinocytes

Hair growth is a highly regulated cyclical process. Immunosuppressive immunophilin ligands such as cyclosporin A (CsA) and FK506 are known as potent hair growth modulatory agents in rodents and humans that induce active hair growth and inhibit hair follicle regression. The immunosuppressive effectiveness of these drugs has been generally attributed to inhibition of T cell activation through well-characterized pathways. Specifically, CsA and FK506 bind to intracellular proteins, principally cyclophilin A and FKBP12, respectively, and thereby inhibit the phosphatase calcineurin (Cn). The calcineurin (Cn)/NFAT pathway has an important, but poorly understood, role in the regulation of hair follicle development. Here we show that a novel-splicing variant of calcineurin Aß CnAß-FK, which is encoded by an intron-retaining mRNA and is deficient in the autoinhibitory domain, is predominantly expressed in mature follicular keratinocytes but not in the proliferating keratinocytes of rodents. CnAß-FK was weakly sensitive to Ca(2+) and dephosphorylated NFATc2 under low Ca(2+) levels in keratinocytes. Inhibition of Cn/NFAT induced hair growth in nude mice. Cyclin G2 was identified as a novel target of the Cn/NFATc2 pathway and its expression in follicular keratinocytes was reduced by inhibition of Cn/NFAT. Overexpression of cyclin G2 arrested the cell cycle in follicular keratinocytes in vitro and the Cn inhibitor, cyclosporin A, inhibited nuclear localization of NFATc2, resulting in decreased cyclin G2 expression in follicular keratinocytes of rats in vivo. We therefore suggest that the calcineurin/NFAT pathway has a unique regulatory role in hair follicle development.

Phosphorylation of Mycobacterium tuberculosis Ser/Thr phosphatase by PknA and PknB

Background

The integrated functions of 11 Ser/Thr protein kinases (STPKs) and one phosphatase manipulate the phosphorylation levels of critical proteins in Mycobacterium tuberculosis. In this study, we show that the lone Ser/Thr phosphatase (PstP) is regulated through phosphorylation by STPKs.

Principal Findings

PstP is phosphorylated by PknA and PknB and phosphorylation is influenced by the presence of Zn²⁺-ions and inorganic phosphate (Pi). PstP is differentially phosphorylated on the cytosolic domain with Thr¹³⁷, Thr¹⁴¹, Thr¹⁷⁴ and Thr²⁹⁰ being the target residues of PknB while Thr¹³⁷ and Thr¹⁷⁴ are phosphorylated by PknA. The Mn²⁺-ion binding residues Asp³⁸ and Asp²²⁹ are critical for the optimal activity of PstP and substitution of these residues affects its phosphorylation status. Native PstP and its phosphatase deficient mutant PstP_c^D38G are phosphorylated by PknA and PknB in E. coli and addition of Zn²⁺/Pi in the culture conditions affect the phosphorylation level of PstP. Interestingly, the phosphorylated phosphatase is more active than its unphosphorylated equivalent.

Conclusions and Significance

This study establishes the novel mechanisms for regulation of mycobacterial Ser/Thr phosphatase. The results indicate that STPKs and PstP may regulate the signaling through mutually dependent mechanisms. Consequently, PstP phosphorylation may play a critical role in regulating its own activity. Since, the equilibrium between phosphorylated and non-phosphorylated states of mycobacterial proteins is still unexplained, understanding the regulation of PstP may help in deciphering the signal transduction pathways mediated by STPKs and the reversibility of the phenomena.

Downregulation of dendritic HCN channel gating in epilepsy is mediated by altered phosphorylation signaling

The onset of spontaneous seizures in the pilocarpine model of epilepsy causes a hyperpolarized shift in the voltage-dependent activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated current (Ih) in CA1 hippocampal pyramidal neuron dendrites, contributing to neuronal hyperexcitability and possibly to epileptogenesis. However, the specific mechanisms by which spontaneous seizures cause downregulation of HCN channel gating are yet unknown. We asked whether the seizure-dependent downregulation of HCN channel gating was due to altered phosphorylation signaling mediated by the phosphatase calcineurin (CaN) or the kinase p38 mitogen-activated protein kinase (p38 MAPK). We first found that CaN inhibition upregulated HCN channel gating and reduced neuronal excitability under normal conditions, showing that CaN is a strong modulator of HCN channels. We then found that an in vitro model of seizures (1 h in 0 Mg2+ and 50 microM bicuculline at 35-37 degrees C) reproduced the HCN channel gating change seen in vivo. Pharmacological inhibition of CaN or activation of p38 MAPK partially reversed the in vitro seizure-induced hyperpolarized shift in HCN channel gating, and the shift was fully reversed by the combination of CaN inhibition and p38 MAPK activation. We then demonstrated enhanced CaN activity as well as reduced p38 MAPK activity in vivo in the CA1 hippocampal area of chronically epileptic animals. Pharmacological reversal of these phosphorylation changes restored HCN channel gating downregulation and neuronal hyperexcitability in epileptic tissue to control levels. Together, these results suggest that alteration of two different phosphorylation pathways in epilepsy contributes to the downregulation of HCN channel gating, which consequently produces neuronal hyperexcitability and thus may be a target for novel antiepileptic therapies.

Protein phosphatase 2B (PP2B, calcineurin) in Paramecium: partial characterization reveals that two members of the unusually large catalytic subunit family have distinct roles in calcium-dependent processes

We characterized the calcineurin (CaN) gene family, including the subunits CaNA and CaNB, based upon sequence information obtained from the Paramecium genome project. Paramecium tetraurelia has seven subfamilies of the catalytic CaNA subunit and one subfamily of the regulatory CaNB subunit, with each subfamily having two members of considerable identity on the amino acid level (>or=55% between subfamilies, >or=94% within CaNA subfamilies, and full identity in the CaNB subfamily). Within CaNA subfamily members, the catalytic domain and the CaNB binding region are highly conserved and molecular modeling revealed a three-dimensional structure almost identical to a human ortholog. At 14 members, the size of the CaNA family is unprecedented, and we hypothesized that the different CaNA subfamily members were not strictly redundant and that at least some fulfill different roles in the cell. This was tested by selecting two phylogenetically distinct members of this large family for posttranscriptional silencing by RNA interference. The two targets resulted in differing effects in exocytosis, calcium dynamics, and backward swimming behavior that supported our hypothesis that the large, highly conserved CaNA family members are not strictly redundant and that at least two members have evolved diverse but overlapping functions. In sum, the occurrence of CaN in Paramecium spp., although disputed in the past, has been established on a molecular level. Its role in exocytosis and ciliary beat regulation in a protozoan, as well as in more complex organisms, suggests that these roles for CaN were acquired early in the evolution of this protein family.

Pharmacodynamic monitoring of calcineurin inhibition therapy: principles, performance, and perspectives

The calcineurin inhibitors (CNIs) cyclosporin A and tacrolimus are immunosuppressive drugs used extensively in allograft recipients. These drugs show large interindividual pharmacokinetic variation and are associated with severe adverse affects, including nephrotoxicity and cardiovascular disease. In current practice, CNIs are combined with other immunosuppressive drugs such as steroids and mycophenolate mofetil. Dosage is titrated based on blood concentration measurement. For further optimization of calcineurin (CN) inhibition therapy, new monitoring strategies are required. Pharmacodynamic-monitoring strategies constitute novel approaches for optimization of CNIs therapy. This review focuses on the general aspects of immunosuppressive drug pharmacodynamic monitoring and describes the methodologies used for monitoring CN inhibition therapy. Two different types of pharmacodynamic-monitoring strategies can be distinguished: (1) enzymatic strategies, which monitor inhibition of drug-target enzyme activity, and (2) immunologic strategies, which measure cellular responsiveness after in vitro simulated immunologic responses. Enzymatic tests are drug type-specific markers in which CN activity is directly determined. Immunologic strategies measure immune responsiveness at several levels, such as mRNA transcripts (intracellular) concentrations/excretion of cytokines, expression of surface activation markers, and cell proliferation. This review also discusses analytical issues and clinical experience with these techniques. The call for new methodologies to evaluate immunosuppressive therapy has led to the development of a large variety of pharmacodynamic-monitoring strategies. The first reports of their clinical relevance are available, but further understanding of the analytical and clinical variables involved are required for the development of accurate, reproducible, and clinically relevant markers.

Mechanism of activation of Saccharomyces cerevisiae calcineurin by Mn2+

Saccharomyces cerevisiae calcineurin (CN) consists of a catalytic subunit CNA1 or CNA2 and a regulatory subunit CNB1. The kinetics of activation of yeast CN holoenzymes and their catalytic domains by Mn2+ were investigated. We report that the in vitro phosphatase reaction activated by Mn2+ typically has a pronounced initial lag phase caused by slow conformational rearrangement of the holoenzyme-Mn2+. A similar lag phase was detected using just the catalytic domain of yeast CN, indicating that the slowness of Mn2+-induced conformational change of CN results from a rearrangement within the catalytic domain. The Mn2+-activation of CN was reversible. The dissociation constant of the CN heterodimer containing the CNA2 subunit in the presence of Mn2+ was 3-fold higher than that of CN containing the CNA1 subunit and that of the catalytic domains of CNA1 and CNA2, pointing to differences between the residues surrounding the Mn2+-binding sites of CNA1 and CNA2.

Roles of mitophagy and the mitochondrial permeability transition in remodeling of cultured rat hepatocytes

In primary culture, hepatocytes dedifferentiate, and their cytoplasm undergoes remodeling. Here, our aim was to characterize changes of mitochondria during remodeling. Hepatocytes were cultured one to five days in complete serumcontaining Waymouth's medium. In rat hepatocytes loaded with MitoTracker Green (MTG), tetramethylrhodamine methylester (TMRM), and/or LysoTracker Red (LTR), confocal microscopy revealed that mitochondria number and mass decreased by approximately 50% between Day 1 and Day 3 of culture. As mitochondria disappeared, lysosomes/autophagosomes proliferated five-fold. Decreased mitochondrial content correlated with (a) decreased cytochrome c oxidase activity and mitochondrial number observed by electron microscopy and (b) a profound decrease of PGC-1alpha mRNA expression. By contrast, mtDNA content per cell remained constant from the first to the third day of culture, although ethidium bromide (de novo mtDNA synthesis inhibitor) caused mtDNA to decrease by half from the first to the third culture day. As mitochondria disappeared, their MTG label moved into LTR-labeled lysosomes, which was indicative of autophagic degradation. A multiwell fluorescence assay revealed a 2.5-fold increase of autophagy on Day 3 of culture, which was decreased by 3-methyladenine, an inhibitor of autophagy, and also by cyclosporin A and NIM811, both selective inhibitors of the mitochondrial permeability transition (MPT). These findings indicate that mitochondrial autophagy (mitophagy) and the MPT underlie mitochondrial remodeling in cultured hepatocytes.

Complex phosphatase regulation of Ca2+-activated Cl- currents in pulmonary arterial smooth muscle cells

The present study was undertaken to determine whether the two ubiquitously expressed Ca(2+)-independent phosphatases PP1 and PP2A regulate Ca(2+)-activated Cl(-) currents (I(Cl(Ca))) elicited by 500 nM [Ca(2+)](i) in rabbit pulmonary artery (PA) myocytes dialyzed with or without 3 mM ATP. Reverse transcription-PCR experiments revealed the expression of PP1alpha, PP1beta/delta, PP1gamma, PP2Aalpha, PP2Abeta, PP2Balpha (calcineurin (CaN) Aalpha), and PP2Bbeta (CaN Abeta) but not PP2Bgamma (CaN Agamma) in rabbit PA. Western blot and immunofluorescence experiments confirmed the presence of all three PP1 isoforms and PP2A. Intracellular dialysis with a peptide inhibitor of calcineurin (CaN-AIP); the non-selective PP1/PP2A inhibitors okadaic acid (0.5, 10, or 30 nM), calyculin A (10 nM), or cantharidin (100 nM); and the selective PP1 inhibitor NIPP-1 (100 pM) potently antagonized the recovery of I(Cl(Ca)) in cells dialyzed with no ATP, whereas the PP2A-selective antagonist fostriecin (30 or 150 nM) was ineffective. The combined application of okadaic acid (10 nM) and CaN-autoinhibitory peptide (50 microM) did not potentiate the response of I(Cl(Ca)) in 0 ATP produced by maximally inhibiting CaN or PP1/PP2A alone. Consistent with the non-additive effects of either classes of phosphatases, the PP1 inhibitor NIPP-1 (100 pM) antagonized the recovery of I(Cl(Ca)) induced by exogenous CaN Aalpha (0.5 microM). These results demonstrate that I(Cl(Ca)) in PA myocytes is regulated by CaN and PP1 and/or PP2A. Our data also suggest the existence of a functional link between these two classes of phosphatases.

The proline-rich N-terminal sequence of calcineurin Abeta determines substrate binding

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

Role of calcineurin in thrombin-mediated endothelial cell contraction

Barrier function and shape changes of endothelial cells (EC) are regulated by phosphorylation/dephosphorylation of key signaling and contractile elements. EC contraction results in intercellular gap formation and loss of the selective vascular barrier to circulating macromolecules. EC dysfunction elicited by thrombin was found to correlate with actin microfilament redistribution. It is known that calcineurin (Cn) is involved in thrombin-induced EC dysfunction because inhibition of Cn potentiates PKC activity and the phosphorylation state of EC myosin light chain is also affected by Cn activity. Immunofluorescent detection of Cn catalytic subunit (CnA) isoforms coexpressed with GFP was visualized on paraformaldehyde (PFA) fixed bovine pulmonary artery endothelial cells (BPAEC). Actin microfilaments were stained with Texas Red-phalloidin. Cytotoxic effects of transfections or treatments and the efficiency of transfections were assessed by flow cytometry. Treatment of BPAEC with Cn inhibitors (cyclosporin A and FK506) hindered recovery of the cells from thrombin-induced EC dysfunction. Inhibition of Cn in the absence of thrombin had no effect on cytoskeletal actin filaments. We detected attenuated thrombin-induced stress fiber formation and changes in cell shape only when cells were transfected with constitutively active CnA and not with various CnA isoforms. Flow cytometry (FCM) analysis has proved that cytotoxic effect of treatments is negligible. We observed that Cn is involved in the recovery from thrombin-induced EC dysfunction. Inhibition of Cn caused prolonged contractile effect, while overexpression of constitutively active CnA resulted in reduced thrombin-induced stress fiber formation.

Ca2+/calcineurin regulation of cloned vascular K ATP channels: crosstalk with the protein kinase A pathway

BACKGROUND AND PURPOSE

Vascular ATP-sensitive potassium (K(ATP)) channels are activated by cyclic AMP elevating vasodilators through protein kinase A (PKA). Direct channel phosphorylation is a critical mechanism, though the phosphatase opposing these effects is unknown. Previously, we reported that calcineurin, a Ca(2+)-dependent phosphatase, inhibits K(ATP) channels, though neither the site nor the calcineurin isoform involved is established. Given that the type-2 regulatory (RII) subunit of PKA is a substrate for calcineurin we considered whether calcineurin regulates channel activity through interacting with PKA.

EXPERIMENTAL APPROACH

Whole-cell recordings were made in HEK-293 cells stably expressing the vascular K(ATP) channel (K(IR)6.1/SUR2B). The effect of intracellular Ca(2+) and modulators of the calcineurin and PKA pathway on glibenclamide-sensitive currents were examined.

KEY RESULTS

Constitutively active calcineurin A alpha but not A beta significantly attenuated K(ATP) currents activated by low intracellular Ca(2+), whereas calcineurin inhibitors had the opposite effect. PKA inhibitors reduced basal K(ATP) currents and responses to calcineurin inhibitors, consistent with the notion that some calcineurin action involves inhibition of PKA. However, raising intracellular Ca(2+) (equivalent to increasing calcineurin activity), almost completely inhibited K(ATP) channel activation induced by the catalytic subunit of PKA, whose enzymatic activity is independent of the RII subunit. In vitro phosphorylation experiments showed calcineurin could directly dephosphorylate a site in Kir6.1 that was previously phosphorylated by PKA.

CONCLUSIONS AND IMPLICATIONS

Calcineurin A alpha regulates K(IR)6.1/SUR2B by inhibiting PKA-dependent phosphorylation of the channel as well as PKA itself. Such a mechanism is likely to directly oppose the action of vasodilators on the K(ATP) channel.

Infusion of FK506, a specific inhibitor of calcineurin, induces potent tau hyperphosphorylation in mouse brain

Calcineurin is a Ca2+/calmodulin-dependent protein phosphatase expressed at high levels in brain. Many electrophysiological and pharmacological findings have shown that calcineurin plays an important role in brain function. FK506 is always used as a specific calcineurin inhibitor in these researches. But these reports did not quantify the calcineurin activity in FK506-treated brain. Here we first investigated the inhibitory effect of FK506 injected into the mouse brain ventricle on CN activity. FK506 reduced calcineurin activity in a dose-dependent manner, without affecting its amount. Injection of 12.5 nmol FK506 also significantly enhanced the phosphorylation of tau at Ser-262 (12E8 site), Ser-198, Ser-199, and/or Ser-202 (Tau-1 site) and Ser-396 and/or Ser-404 (PHF-1 site), without affecting total tau. It is suggested that calcineurin plays an important role in tau phosphorylation, dependently of its activity. Compared with the effects of cyclosporin A, another specific inhibitor of CN in our previous study, we first evaluate that such infusion of FK506 is more effective than that of cyclosporin A on calcineurin inhibition and tau phosphorylation.

Protein phosphatase 3 differentially modulates vascular endothelial growth factor- and fibroblast growth factor 2-stimulated cell proliferation and signaling in ovine fetoplacental artery endothelial cells

A critical process for vascular endothelial growth factor (VEGF)- and fibroblast growth factor 2 (FGF2)-regulated cellular function is reversible protein phosphorylation, which is tightly controlled by a balance of protein kinases and phosphatases. We have reported that in ovine fetoplacental artery endothelial (OFPAE) cells, VEGF and FGF2 stimulate cell proliferation in part via activation of mitogen-activated protein kinase kinase 1/2 (MAP2K1/2)/mitogen-activated protein kinase 3/1 (MAPK3/1) and phosphoinositide 3-kinase (PI3K)/v-akt murine thymoma viral oncogene homolog 1 (AKT1) pathways. In the present study, we examined if protein phosphatase 3 (PPP3) mediated VEGF- and FGF2-stimulated OFPAE cell proliferation via modulating activation of MAPK3/1 and AKT1. Small interfering RNA (siRNA) targeting human PPP3 catalytic subunit alpha (PPP3CA) was used to suppress PPP3CA protein expression in OFPAE cells. Compared with the scrambled siRNA, PPP3CA siRNA decreased PPP3CA protein levels by approximately 97% without altering protein levels of protein phosphatase 2 catalytic subunit alpha, total MAPK3/1, total AKT1, or glyceraldehyde-3-phosphate dehydrogenase. Knockdown of PPP3CA protein expression enhanced VEGF-stimulated, but not FGF2-stimulated, cell proliferation. Knockdown of PPP3CA protein expression did not significantly affect VEGF-induced MAPK3/1 and AKT1 phosphorylation but attenuated FGF2-induced MAPK3/1 and AKT1 phosphorylation. Thus, to our knowledge, the present study is the first to demonstrate successful knockdown of PPP3CA protein expression in any cell model using a single pair of double-strained siRNA. Moreover, specific knockdown of PPP3CA protein expression enhances VEGF-stimulated, but not FGF2-stimulated, OFPAE cell proliferation and attenuates FGF2-induced, but not VEGF-induced, MAPK3/1 and AKT1 activation. Thus, PPP3CA differentially modulates the VEGF- and FGF2-stimulated cell proliferation and signaling cascades in OFPAE cells. These data also suggest that signaling molecules other than MAPK3/1 and AKT1 play an important role in VEGF- and FGF2-stimulated cell proliferation after knockdown of PPP3CA in OFPAE cells.

Nuclear translocation of calcineurin Aβ but not calcineurin Aα by platelet-derived growth factor in rat aortic smooth muscle

Calcineurin regulates the proliferation of many cell types through activation of the nuclear factor of activated T cells (NFAT). Two main isoforms of the calcineurin catalytic subunit [calcineurin A (CnA)α and CnAβ] have been identified, although their expression and function are largely unknown in smooth muscle. Western blot analysis and confocal imaging were performed in freshly isolated and cultured rat aortic myocytes to identify these CnA isoforms and elucidate the effect of PDGF on their cellular distribution and interaction with NFAT isoforms. CnAα and CnAβ isoforms displayed differential cellular distribution, with CnAα being evenly distributed between the nucleus and cytosol and CnAβ being restricted to the cytosol. In contrast with the rat brain, we found no evidence for particulate/membrane localization of calcineurin. PDGF caused significant nuclear translocation of CnAβ and induced smooth muscle cell proliferation, with both effects being abrogated by the calcineurin inhibitor cyclosporin A, the novel NFAT inhibitors A-285222 and inhibitor of NFAT-calcineurin association-6, and the adenylyl cyclase activator forskolin. PDGF also caused cyclosporin A-sensitive translocation of NFATc3, with no apparent effect on either CnAα or NFATc1 distribution. Moreover, ∼87% of nuclear CnAβ was found to colocalize with NFATc3, consistent with the finding that CnAβ bound more avidly than CnAα to a glutathione S-transferase-NFATc3 fusion protein. Based on their differential distribution in aortic muscle, our results suggest that CnAα and CnAβ are likely to have different cellular functions. However, CnAβ appears to be specifically activated by PDGF, and we postulate that calcineurin-dependent nuclear translocation of NFATc3 is involved in smooth muscle proliferation induced by this mitogen.

Constitutively active calcineurin mediates delayed neuronal death through Fas-ligand expression via activation of NFAT and FKHR transcriptional activities in mouse brain ischemia

We recently demonstrated that a constitutively active form of calcineurin (CaN) is generated by calpain-mediated limited proteolysis following brain ischemia. The calpain-induced CaN activation mediated delayed neuronal death through translocation of nuclear factor of activated T-cells (NFAT) into nuclei after brain ischemia. We also previously demonstrated that activation of forkhead in rhabdomyosarcoma (FKHR), a forkhead transcription factor and substrate of protein kinase-B (Akt), mediated ischemia-induced neuronal death through Fas-ligand expression in gerbil hippocampus. FKHR activation occurred through decreased Akt activity and concomitant dephosphorylation mediated by undefined phosphatases. In this study, we show that phosphorylated Ser-256 of FKHR is dephosphorylated by constitutively active CaN and that in turn FKHR forms a complex with CaN that is translocated into nuclei after brain ischemia. After nuclear translocation of NFAT and FKHR, both NFAT and FKHR stimulated expression of Fas-ligand by binding to its promoter region. Consistent with activation of the Fas-ligand promoter by FKHR dephosphorylation, Fas-ligand expression increased 2 days after ischemia/reperfusion, and treatment with the CaN inhibitor FK506 inhibited that expression. These results suggest that FKHR is a downstream target of CaN and that constitutively active CaN mediates delayed neuronal death through Fas-ligand expression via up-regulation of both NFAT and FKHR transcriptional activity in brain ischemia.

Molecular modeling of the calmodulin binding region of calcineurin

The Homology module within Insight-II was used to model residues 374-420, sequences missing in the coordinates of resolved structure of the catalytic subunit of calcineurin. The modeling was done in two segments. The calmodulin binding region from residues 389 to 420 was modeled based on the structure of two other proteins having calmodulin binding domains with the same 1-8-14 structural motif as calcineurin. The link region (residues 374-389) between the calmodulin binding region and the solved core sequence was generated as a random loop and two residues at the C-terminal end of the sequence were added to the model using the EndRepair function within Homology. The model was refined using the Discover module of Insight-II with energy minimization. The Builder module was used to merge the modeled regions with the solved structure of calcineurin (residues 14-373). A final refinement step was done for the joined calcineurin model. From the model, it was predicted that the calmodulin and cyclophilin binding regions seem to be proximal. Biochemical experiments provided evidence that cyclosporin-A influenced calmodulin binding and activation of calcineurin consistent with overlapping binding regions.

Role of calcineurin in the regulation of human lung mast cell and basophil function by cyclosporine and FK506

BACKGROUND AND PURPOSE

Cyclosporine and FK506 are thought to act by targeting the Ca2+-dependent protein phosphatase, calcineurin. The aim of the present study was to determine whether cyclosporine and FK506 stabilize mast cells and basophils by interacting with calcineurin.

EXPERIMENTAL APPROACH

The effects of cyclosporine and FK506 on the IgE-mediated release of histamine from mast cells and basophils were evaluated. The presence of calcineurin in cells was determined by Western blotting. Ca2+-dependent protein phosphatase activities were assessed in cell extracts using a synthetic phosphorylated peptide that is known to serve as a substrate for calcineurin.

KEY RESULTS

FK506 was about 100-fold more potent than cyclosporine as an inhibitor of IgE-dependent histamine release from mast cells and basophils. Immunoblotting of solubilized preparations of purified cells demonstrated the presence of calcineurin in mast cells and basophils. In enzyme assays, mast cells expressed approximately 7-fold higher Ca2+-dependent protein phosphatase activity than basophils. Whereas cyclosporine effectively inhibited Ca2+-dependent protein phosphatase activity in cell extracts, FK506 was considerably less effective.

CONCLUSIONS AND IMPLICATIONS

FK506 and cyclosporine inhibit the stimulated release of histamine from mast cells and basophils. However, the ability of cyclosporine, but not FK506, to inhibit Ca2+-dependent protein phosphatase activity questions whether FK506 stabilizes mast cells and basophils by interacting with calcineurin.

New approach to prevent myocardial hypertrophy: the import blocking peptide

Calcineurin, a serine/threonine phosphatase, plays a crucial role in the development of myocardial hypertrophy. Calcineurin is a cytosolic phosphatase that dephosphorylates the nuclear factor of activated T cells (NFAT), a transcription factor. Until now, it has been postulated that dephosphorylated NFAT is shuttled into the nucleus. Recent evidence demonstrates that not only NFAT, but also calcineurin, is localized in the nucleus. Once calcineurin and NFAT enter the nucleus of cardiomyocytes, transcription of genes that are characteristic for myocardial hypertrophy (e.g., brain natriuretic peptide and atrial natriuretic peptide) occurs. Although the exact nuclear function of calcineurin remains unclear, its co-existence with NFAT is important for the full transcriptional activity of the calcineurin/NFAT signaling cascade. The principal effect of nuclear calcineurin is likely the prolonged nuclear retention period of NFAT. Potential effects of nuclear calcineurin include an antagonistic function to glycogen synthase kinase 3beta, which phosphorylates NFAT for its export out of the nucleus, or direct antagonization of the export of NFAT, catalyzed by the chromosome region maintenance 1, which would leave NFAT nuclear. The nuclear localization sequence (NLS) region at the amino acid sequence from position 172 to 183 of calcineurin Abeta is essential for shuttling calcineurin into the nucleus by importinbeta(1). A synthetic import blocking peptide (IBP) that mimics the nuclear localization sequence of calcineurin was generated. The NLS analog on IBP saturates the calcineurin binding site of importinbeta(1). This prevents the binding of calcineurin to importin and inhibits the nuclear shuttling of calcineurin. Inhibition of the calcineurin/importinbeta(1) interaction by competing synthetic peptides represents a new approach to the inhibition of the development of myocardial hypertrophy.

Generation of constitutively active calcineurin by calpain contributes to delayed neuronal death following mouse brain ischemia

Calpain, a Ca(2+)-dependent cysteine protease, in vitro converts calcineurin (CaN) to constitutively active forms of 45 kDa and 48 kDa by cleaving the autoinhibitory domain of the 60 kDa subunit. In a mouse middle cerebral artery occlusion (MCAO) model, calpain converted the CaN A subunit to the constitutively active form with 48 kDa in vivo. We also confirmed increased Ca(2+)/CaM-independent CaN activity in brain extracts. The generation of constitutively active and Ca(2+)/CaM-independent activity of CaN peaked 2 h after reperfusion in brain extracts. Increased constitutively active CaN activity was associated with dephosphorylation of dopamine-regulated phosphoprotein-32 in the brain. Generation of constitutively active CaN was accompanied by translocation of nuclear factor of activated T-cells (NFAT) into nuclei of hippocampal CA1 pyramidal neurons. In addition, a novel calmodulin antagonist, DY-9760e, blocked the generation of constitutively active CaN by calpain, thereby inhibiting NFAT nuclear translocation. Together with previous studies indicating that NFAT plays a critical role in apoptosis, we propose that calpain-induced CaN activation in part mediates delayed neuronal death in brain ischemia.

Cyclosporin-A inhibits ERK phosphorylation in B cells by modulating the binding of Raf protein to Bcl2

Extracellular signal-related kinase (ERK) signaling is regulated by sequential phosphorylation of upstream kinases including Raf. We report herein that ERK phosphorylation is inhibited by a short incubation with Cyclosporin-A (CsA) in anti-IgM activated Daudi B cells. As Bcl2, through its BH4 domain, was previously shown to bind both Calcineurin (Can) and Raf proteins, we hypothesized that CsA inhibited Can binding to Bcl2 allowing the latter to bind more Raf at the mitochondria thereby diverting it from activating the ERK cascade. In support of this less Bcl2 coprecipitated with Can heterodimer in total lysates of cells treated with CsA as compared to controls. In parallel, Raf1 was augmented in both the mitochondrial fractions of cells treated with CsA and in Bcl2 immunoprecipitates under CsA. Finally, introduction of a Bcl2 BH4 domain into Daudi cells augmented ERK phosphorylation in unstimulated cells and this augmentation was unsensitive to CsA. We therefore suggest that CsA indirectly inhibited ERK activation through sequestration of Raf1, at the mitochondria.

Tracker dyes to probe mitochondrial autophagy (mitophagy) in rat hepatocytes

Mitochondria become targets for autophagic degradation after nutrient deprivation, a process also termed mitophagy. In this study, we used LysoTracker Red (LTR) and MitoTracker Green to characterize the kinetics of autophagosomal proliferation and mitophagy in cultured rat hepatocytes. Autophagy induced by nutrient deprivation plus glucagon increased LTR uptake assessed with a fluorescence plate reader and the number of LTR-labeled acidic organelles assessed with confocal microscopy in individual hepatocytes both by 4- to 6-fold. Serial imaging of hepatocytes coloaded with MitoTracker Green (MTG) revealed an average mitochondrial digestion time of 7.5 min after autophagic induction. In the presence of protease inhibitors, digestion time more than doubled, and the total number of LTR-labeled organelles increased about 40%, but the proportion of the LTR-labeled acidic organelles containing MTG fluorescence remained constant at about 75%. Autophagy inhibitors, 3-methyladenine, wortmannin and LY204002, suppressed the increase of LTR uptake after nutrient deprivation by up to 85%, confirming that increased LTR uptake reflected autophagy induction. Cyclosporin A and NIM811, specific inhibitors of the mitochondrial permeability transition (MPT), also decreased LTR uptake, whereas tacrolimus, an immunosuppressive reagent that does not inhibit the MPT, was without effect. In addition, the c-Jun N-terminal kinase (JNK) inhibitors, SCP25041 and SP600125, blocked LTR uptake by 47% and 61%, respectively, but ERK1, p38 and caspase inhibitors had no effect. The results show that mitochondria once selected for mitophagy are rapidly digested and support the concept that mitochondrial autophagy involves the MPT and signaling through PI3 kinase and possibly JNK.

Kinetics of calmodulin binding to calcineurin

Calcineurin (CaN) binds Ca(2+)-saturated calmodulin (CaM) with relatively high affinity; however, an accurate steady-state K(d) value has not been determined. In this report, we describe, using steady-state and stopped-flow fluorescence techniques, the rates of association and dissociation of Ca(2+)-saturated CaM from CaN heterodimer (CaNA/CaNB) and CaNA only. The rate of Ca(2+)/CaM association was determined to be 4.6 x 10(7) M(-1)s(-1). The rate of Ca(2+)/CaM dissociation from CaN was slower than previously reported and was approximately 0.0012 s(-1). In preparations of CaNA alone (no regulatory CaNB subunit), the dissociation rate was slowed further to 0.00026 s(-1). From these data we calculate a K(d) for binding of Ca(2+)-saturated CaM to CaN of 28 pM. This K(d) is significantly lower than previously reported estimates of approximately 1 nM and indicates that CaN is one of the highest affinity CaM-binding proteins identified to date.

Calcineurin Aα but Not Aβ Augments ICl(Ca) in Rabbit Pulmonary Artery Smooth Muscle Cells

Activation of Ca(2+)-dependent Cl(-) currents (I(Cl(Ca))) increases membrane excitability in vascular smooth muscle cells. Previous studies showed that Ca(2+)-dependent phosphorylation suppresses I(Cl(Ca)) in pulmonary artery myocytes, and the aim of the present study was to determine the role of the Ca(2+)-dependent phosphatase calcineurin on chloride channel activity. Immunocytochemical and Western blot studies with isoform-specific antibodies revealed that the alpha and beta forms of the CaN catalytic subunit are expressed in PA cells but that only the alpha variant translocated to the cell periphery upon a rise in intracellular [Ca(2+)]. I(Cl(Ca)) evoked by pipette solutions containing a [Ca(2+)] set at 500 nm was considerably larger when the pipette solution included constitutively active CaN containing the alpha catalytic isoform. This stimulatory effect was lost by boiling the enzyme or by the inclusion of a specific CaN inhibitory peptide and was not shared by the inclusion of the beta form of the catalytic subunit. In the absence of constitutively active CaN, cyclosporin A, an inhibitor of CaN, suppressed I(Cl(Ca)) evoked by 500 nm Ca(2+) when the current amplitude was relatively large but was ineffective in cells with smaller currents. In perforated patch recordings, cyclosporin A consistently inhibited I(Cl(Ca)) evoked as a consequence of Ca(2+) influx through voltage-dependent calcium channels. These novel data show that in PA myocytes activation of I(Cl(Ca)) is enhanced by Ca(2+)-dependent dephosphorylation and that the regulation of this conductance is highly isoform-specific.

Modulation of Ca2+-dependent Cl- channels by calcineurin in rabbit coronary arterial myocytes

The role of the Ca2+-dependent phosphatase calcineurin (CaN) in the modulation of Ca2+-dependent Cl- channels (ClCa) was studied in freshly isolated rabbit coronary arterial myocytes. Immunocytochemical experiments showed that calmodulin-dependent protein kinase II (CaMKII) and CaN were distributed evenly throughout the cytoplasm of coronary myocytes at rest and translocated to the plasmalemma when intracellular Ca2+ was increased. ClCa currents (ICl(Ca)) elicited by cell dialysis with fixed intracellular Ca2+ levels up to 500 nM were inhibited by 10 microM cyclosporin A (CsA), a specific inhibitor of CaN, in a voltage-dependent manner, whereas currents evoked by 1 microM Ca2+ were not affected. Inhibition of CaN with CsA also led to a significant reduction in Ca2+ sensitivity of the channel at +50 mV; half-maximal activation increased from 363 +/- 16 nM Ca2+ in control to 515 +/- 40 nM Ca2+ in the presence of CsA. Similar effects were observed on ICl(Ca) when a specific peptide fragment inhibitor of CaN (CaN-AF, 5 microM) was dialysed into the cell via the pipette (500 nM Ca2+). Application of KN-93 (10 microM), a specific inhibitor of CaMKII, enhanced ICl(Ca) in myocytes dialysed with 1 microM Ca2+ but produced no significant effect on this current when the cells were dialysed with 350 or 500 nM Ca2+. These results are consistent with the notion that in coronary arterial cells, the activity of ClCa is enhanced by dephosphorylation of the channel or a closely associated regulatory protein. Moreover the balance of CaN and CaMKII regulating ICl(Ca) is dependent on the level of Ca2+ used to activate ICl(Ca).

Altered skeletal muscle phenotypes in calcineurin Aalpha and Abeta gene-targeted mice

Calcineurin is a calcium-regulated serine-threonine protein phosphatase that controls developmental and inducible biological responses in diverse cell types, in part through activation of the transcription factor nuclear factor of activated T cells (NFAT). In skeletal muscle, calcineurin has been implicated in the regulation of myoblast differentiation, hypertrophy of mature myofibers, and fiber type switching in response to alterations in intracellular calcium concentration. However, considerable disagreement persists about the functional role of calcineurin signaling in each of these processes. Here we evaluated the molecular phenotypes of skeletal muscle from both calcineurin Aalpha and calcineurin Abeta gene-targeted mice. Calcineurin Aalpha was observed to be the predominant catalytic isoform expressed in nearly all skeletal muscles examined. Neither calcineurin Aalpha or Abeta null mice showed any gross growth-related alterations in skeletal muscle, nor was fiber size or number altered in glycolytic/fast muscle types. In contrast, both calcineurin Aalpha and Abeta gene-targeted mice demonstrated an alteration in myofiber number in the soleus, an oxidative/slow-type muscle. More significantly, calcineurin Aalpha and Abeta gene-targeted mice showed a dramatic down-regulation in the oxidative/slow fiber type program in multiple muscles (both slow and fast). Associated with this observation, NFAT-luciferase reporter transgenic mice showed significantly greater activity in slow fiber-containing muscles than in fast. However, only calcineurin Aalpha null mice showed a defect in NFAT nuclear occupancy or NFAT-luciferase transgene activity in vivo. Collectively, our results suggest that calcineurin signaling plays a critical role in regulating skeletal muscle fiber type switching but not hypertrophy. Our results also suggest that fiber type switching occurs through an NFAT-independent mechanism.