Dual roles of modulatory calcineurin-interacting protein 1 in cardiac hypertrophy

The calcium/calmodulin-dependent protein phosphatase calcineurin stimulates cardiac hypertrophy in response to numerous stimuli. Calcineurin activity is suppressed by association with modulatory calcineurin-interacting protein (MCIP)1DSCR1, which is up-regulated by calcineurin signaling and has been proposed to function in a negative feedback loop to modulate calcineurin activity. To investigate the involvement of MCIP1 in cardiac hypertrophy in vivo, we generated MCIP1 null mice and subjected them to a variety of stress stimuli that induce cardiac hypertrophy. In the absence of stress, MCIP1(-/-) animals exhibited no overt phenotype. However, the lack of MCIP1 exacerbated the hypertrophic response to activated calcineurin expressed from a muscle-specific transgene, consistent with a role of MCIP1 as a negative regulator of calcineurin signaling. Paradoxically, however, cardiac hypertrophy in response to pressure overload or chronic adrenergic stimulation was blunted in MCIP1(-/-) mice. These findings suggest that MCIP1 can facilitate or suppress cardiac calcineurin signaling depending on the nature of the hypertrophic stimulus. These opposing roles of MCIP have important implications for therapeutic strategies to regulate cardiac hypertrophy through modulation of calcineurin-MCIP activity.

Localization of calcineurin/NFAT in human skin and psoriasis and inhibition of calcineurin/NFAT activation in human keratinocytes by cyclosporin A

Systemic cyclosporin A and tacrolimus are effective treatments for psoriasis. Cyclosporin A and tacrolimus block T cell activation by inhibiting the phosphatase calcineurin and preventing translocation from the cytoplasm to the nucleus of the transcription factor nuclear factor of activated T cells (NFAT). Inhibition of T cell activation is thought to account for their therapeutic action in psoriasis. We investigated whether nonimmune cells in human skin express calcineurin and NFAT1 and whether cyclosporin A and tacrolimus block activation of calcineurin/NFAT in epidermal keratinocytes. The expression patterns of the principal components of calcineurin/NFAT signaling pathway in normal human skin and psoriasis were determined by immunohistochemistry. We assessed calcineurin/NFAT activation in cultured keratinocytes by measuring the degree of nuclear localization of calcineurin and NFAT1 using immunofluorescence/confocal microscopy and assessed if cyclosporin A and tacrolimus blocked nuclear translocation of these proteins. A variety of cell types in normal and psoriatic skin expressed calcineurin and NFAT1, but expression was particularly prominent in keratinocytes. The principal cyclosporin A and tacrolimus binding proteins cyclophilin A and FKBP12 were also expressed by keratinocytes and nonimmune cells in skin. NFAT1 was predominantly nuclear in normal basal epidermal keratinocytes. Increased nuclear localization of NFAT1 was observed in suprabasal keratinocytes within lesional and to a lesser extent nonlesional psoriatic epidermis compared to normal skin (p = 0.001 and p = 0.03, respectively), suggesting increased activation of calcineurin in psoriatic epidermal keratinocytes. Agonists that induce keratinocyte differentiation, specifically 12-0-tetradecanoyl-phorbol-13-acetate (TPA) plus ionomycin, TPA, and raised extracellular calcium, induced nuclear translocation of NFAT1 and calcineurin in keratinocytes that was inhibited by pretreatment with cyclosporin A or tacrolimus. In contrast in human dermal fibroblasts, TPA plus ionomycin or TPA did not significantly alter the proportion of nuclear-associated NFAT1. These data provide the first evidence that calcineurin is functionally active in human keratinocytes inducing nuclear translocation of NFAT1 and also indicate that regulation of NFAT1 nuclear translocation in skin is cell type specific. Inhibition of this pathway in epidermal keratinocytes may account, in part, for the therapeutic effect of cyclosporin A and tacrolimus in skin diseases such as psoriasis.

Human skeletal muscle fibre type variations correlate with PPAR alpha, PPAR delta and PGC-1 alpha mRNA

AIMS

Studies from genetically modified animals have been instrumental in highlighting genes and their products involved in the regulation of muscle fibre type and oxidative phenotypes; however, evidence in humans is limited. Our aim was therefore to investigate expression of those genes implicated in the regulation of oxidative fibre phenotypes in humans.

METHODS

Using quantitative polymerase chain reaction we determined mRNA expression of selected genes in skeletal muscle from three different groups, displaying physiological and pathological variations in muscle fibre type, activity and skeletal muscle metabolism respectively: (i) elite athletes (cyclists), with an increased proportion of type I slow twitch, oxidative fibres, (ii) normally active subjects with an average fibre type distribution, and (iii) spinal cord-injured subjects with a low proportion of type I fibres.

RESULTS

Skeletal muscle mRNA expression of calcineurin Aalpha and Abeta, peroxisome proliferator-activated receptor (PPAR)-alpha and -delta, and PPAR gamma coactivator (PGC)-1alpha and -1beta was determined. Calcineurin Aalpha and calcineurin Abeta mRNA expression was similar between groups. In contrast, mRNA expression of PPARalpha, PPARdelta, PGC-1alpha and -1beta was increased in athletes, when compared with normally active subjects. Furthermore, mRNA expression of PPARalpha, PPARdelta, PGC-1alpha and -1beta was reduced in spinal cord-injured subjects. Additionally, PPARalpha, PPARdelta and PGC-1alpha correlated with oxidative fibre content.

CONCLUSION

Skeletal muscle mRNA expression of PPARalpha, PPARdelta, PGC-1alpha and -1beta reflects differences in type I muscle fibres associated with pathologically and physiologically induced skeletal muscle fibre type differences.

Selective changes of calcineurin (protein phosphatase 2B) activity in Alzheimer's disease cerebral cortex

Neurofibrillary tangles, which contain abnormally hyperphosphorylated forms of tau protein, are one of the neuropathological hallmarks of Alzheimer's disease (AD). This altered phosphorylation state of tau protein may be due to increased kinase activity or/and decreased phosphatase activity. In the present study, we characterized human calcineurin phosphatase activity in postmortem superior frontal cortex and sensorimotor cortex and measured calcineurin phosphatase activity in samples from individuals with moderate to severe AD (n = 7) and age-matched controls (n = 5). Basal phosphatase activity was reduced by 25% (P < 0.05) in AD frontal cortex. Nickel-stimulated calcineurin activity was decreased by 52% (P < 0.05) and 30% (P < 0.05) in P2 and total cell homogenate, respectively, compared to age-matched controls. No differences in phosphatase activities were detected in the sensorimotor cortex. The decrease in nickel-stimulated calcineurin phosphatase activity in frontal lobe correlated with the neurofibrillary tangle pathology (total cell homogenate, r = -0.77, P < 0.05; P2 fraction, r = -0.76, P < 0.02), but not with diffuse or neuritic plaques. Despite the changes in calcineurin phosphatase activity in the superior frontal cortex, calcineurin protein levels determined by immunoblot were similar in control and AD cases. In addition, no changes in calcineurin regulatory proteins (cyclophilin A and FKBP12) levels were observed. These studies suggest that decrease of calcineurin activity may play a role in paired-helical filament formation and/or stabilization, and the decrease of activity was not accompanied by a decrease of calcineurin protein expression.

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.

AQP2 is a substrate for endogenous PP2B activity within an inner medullary AKAP-signaling complex

We have demonstrated that inner medullary collecting duct (IMCD) heavy endosomes purified from rat kidney IMCD contain the type II protein kinase A (PKA) regulatory subunit (RII), protein phosphatase (PP)2B, PKCzeta, and an RII-binding protein (relative molecular mass ~90 kDa) representing a putative A kinase anchoring protein (AKAP). Affinity chromatography of detergent-solubilized endosomes on cAMP-agarose permits recovery of a protein complex consisting of the 90-kDa AKAP, RII, PP2B, and PKCzeta. With the use of small-particle flow cytometry, RII and PKCzeta were localized to an identical population of endosomes, suggesting that these proteins are components of an endosomal multiprotein complex. (32)P-labeled aquaporin-2 (AQP2) present in these PKA-phosphorylated endosomes was dephosphorylated in vitro by either addition of exogenous PP2B or by an endogenous endosomal phosphatase that was inhibited by the PP2B inhibitors EDTA and the cyclophilin-cyclosporin A complex. We conclude that IMCD heavy endosomes possess an AKAP multiprotein-signaling complex similar to that described previously in hippocampal neurons. This signaling complex potentially mediates the phosphorylation of AQP2 to regulate its trafficking into the IMCD apical membrane. In addition, the PP2B component of the AKAP-signaling complex could also dephosphorylate AQP2 in vivo.

Prednisolone‐induced changes in dystrophic skeletal muscle

Although glucocorticoids delay the progression of Duchenne muscular dystrophy (DMD) their mechanism of action is unknown. Skeletal muscle gene expression profiles of mdx mice, an animal model of DMD, treated with prednisolone were compared with control mice at 1 and 6 wk. Of the 89 early differentially regulated genes and ESTs, delta-sarcoglycan, myosin Va, FK506-binding protein 51 (FKBP51), the potassium channel regulator potassium inwardly-rectifying channel Isk-like (IRK2) and ADAM 10 were overexpressed, whereas growth hormone-releasing hormone receptor (GHRHR) and Homer-2 were underexpressed. The 58 late differentially overexpressed genes included kallikreins (13, 16, and 26), FKBP51, PI3K alpha regulatory subunit, and IGFBP6, while underexpressed genes included NeuroD and nicotinic cholinergic receptor gamma. At both time points, overexpression of a cohort of genes relating to metabolism and proteolysis was apparent, alongside the differential expression of genes relating to calcium metabolism. Treatment did not increase muscle regeneration, reduce the number of infiltrating macrophages, or alter utrophin expression or localization. However, in the treated mdx soleus muscle, the percentage of slow fibers was significantly lower compared with untreated controls after 6 wk of treatment. These results show that glucocorticoids confer their benefit to dystrophic muscle in a complex fashion, culminating in a switch to a more normal muscle fiber type.

Right ventricular hypertrophy and apoptosis after pulmonary artery banding: regulation of PKC isozymes

OBJECTIVE

Pressure overload induced by pulmonary artery banding (PAB) leads to right ventricular (RV) hypertrophy and cardiomyocyte apoptosis. The present study was performed to investigate whether protein kinase C isozymes (PKC-alpha, PKC-betaI, PKC-betaII, PKC-delta and PFC- epsilon ), calcineurin and the renin-angiotensin system (RAS) contribute to PAB-induced cardiac remodeling.

METHODS AND RESULTS

PAB in male Wistar rats for 3 weeks results in enhanced PKC activity (as determined by ELISA assay) in the cytosol and membrane fraction of the hypertrophied RV, which was accompanied by increased expression (as determined by Western blot analysis) of cytosolic PKC-delta (+72%), PKC-alpha (+49%), and PKC-betaI (+39%), but not PKC-betaII and PKC- epsilon. This differential regulation of cardiac PKC isozymes was limited to the strained ventricle and was not altered in response to chronic angiotensin-converting enzyme inhibition with ramiprilate. Furthermore, no significant changes in the expression of calcineurin alpha and beta subunits were observed in RV pressure overload compared to controls. PAB-induced cardiac apoptosis was determined using Western blot analysis by a significantly increased expression of Bax protein and caspase-3 in the hypertrophied RV, which was diminished to almost control levels by chronic ramiprilate treatment. The myocardial expression of Bcl-2 was not significantly altered in the experimental groups.

CONCLUSION

We have shown for the first time that PAB-induced RV hypertrophy is associated with a differential regulation of cardiac PKC isozymes independent of the RAS and further provide evidence for a pivotal role of the RAS in the development of PAB-induced cardiac apoptosis.

Morphological analysis of the mormyrid cerebellum using immunohistochemistry, with emphasis on the unusual neuronal organization of the valvula

This study used immunohistochemistry, Golgi impregnation, and electron microscopy to examine the circuitry of the cerebellum of mormyrid fish. We used antibodies against the following antigens: the neurotransmitters glutamate and gamma-aminobutyric acid (GABA); the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD); GABA transporter 1; the anchoring protein for GABA and glycine receptors, gephyrin; the calcium binding proteins calbindin and calretinin; the NR1 subunit of the N-methyl-D-aspartate glutamate receptor; the metabotropic glutamate receptors mGluR1alpha and mGluR2/3; the intracellular signaling molecules calcineurin and calcium calmodulin kinase IIalpha (CAMKIIalpha); and the receptor for inositol triphosphate (IP3RIalpha). Purkinje cells are immunoreactive to anti-IP3R1alpha, anticalcineurin, and anti-mGluR1alpha. Cerebellar efferent cells (eurydendroid cells) are anticalretinin and anti-NR1 positive in the valvula but not in the corpus and caudal lobe. In contrast, climbing fibers are anticalretinin and anti-NR1 immunopositive in the corpus and caudal lobe but not in the valvula. Purkinje cells, Golgi cells, and stellate cells are GABA positive, whereas efferent cells are glutamate positive. Unipolar brush cells are immunoreactive to anti-mGluR2/3, anticalretinin, and anticalbindin. We describe a "new" cell type in the mormyrid valvula, the deep stellate cell. These cells are GABA, calretinin, and calbindin positive. They are different from superficial stellate cells in having myelinated axons that terminate massively with GAD- and gephyrin-positive terminals on the cell bodies and proximal dendrites of efferent cells. We discuss how the valvula specializations described here may act in concert with the palisade pattern of Purkinje cell dendrites for analyzing spatiotemporal patterns of parallel fiber activity.

Roles of calcineurin and calcium/calmodulin-dependent protein kinase II in pressure overload-induced cardiac hypertrophy

Calcineurin and calcium/calmodulin-dependent protein kinase (CaMK) II have been suggested to be the signaling molecules in cardiac hypertrophy. It was not known, however, whether these mechanisms are involved in cardiac hypertrophy induced by pressure overload without the influences of blood-derived humoral factors, such as angiotensin II. To elucidate the roles of calcineurin and CaMK II in this situation, we examined the effects of calcineurin and CaMK II inhibitors on pressure overload-induced expression of c-fos, an immediate-early gene, and protein synthesis using heart perfusion model. The hearts isolated from Sprague-Dawley rats were perfused according to the Langendorff technique, and then subjected to the acute pressure overload by raising the perfusion pressure. The activation of calcineurin was evaluated by its complex formation with calmodulin and by its R-II phosphopeptide dephosphorylation. CaMK II activation was evaluated by its autophosphorylation. Expression of c-fos mRNA and rates of protein synthesis were measured by northern blot analysis and by 14C-phenylalanine incorporation, respectively. Acute pressure overload significantly increased calcineurin activity, CaMK II activity, c-fos expression and protein synthesis. Cyclosporin A and FK506, the calcineurin inhibitors, significantly inhibited the increases in both c-fos expression and protein synthesis. KN62, a CaMK II inhibitor, also significantly prevented the increase in protein synthesis, whereas it failed to affect the expression of c-fos. These results suggest that both calcineurin and CaMK II pathways are critical in the pressure overload-induced acceleration of protein synthesis, and that transcription of c-fos gene is regulated by calcineurin pathway but not by CaMK II pathway.

Localization of the A kinase anchoring protein AKAP79 in the human hippocampus

The phosphorylation state of the proteins, regulated by phosphatases and kinases, plays an important role in signal transduction and long-term changes in neuronal excitability. In neurons, cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and calcineurin (CN) are attached to a scaffold protein, A kinase anchoring protein (AKAP), thought to anchor these three enzymes to specific sites of action. However, the localization of AKAP, and the predicted sites of linked phosphatase and kinase activities, are still unknown at the fine structural level. In the present study, we investigated the distribution of AKAP79 in the hippocampus from postmortem human brains and lobectomy samples from patients with intractable epilepsy, using preembedding immunoperoxidase and immunogold histochemical methods. AKAP79 was found in the CA1, presubicular and subicular regions, mostly in pyramidal cell dendrites, whereas pyramidal cells in the CA3, CA2 regions and dentate granule cells were negative both in postmortem and in surgical samples. In some epileptic cases, the dentate molecular layer and hilar interneurons also became immunoreactive. At the subcellular level, AKAP79 immunoreactivity was present in postsynaptic profiles near, but not attached to, the postsynaptic density of asymmetrical (presumed excitatory) synapses. We conclude that the spatial selectivity for the action of certain kinases and phosphatases regulating various ligand- and voltage-gated channels may be ensured by the selective presence of their anchoring protein, AKAP79, at the majority of glutamatergic synapses in the CA1, but not in the CA2/CA3 regions, suggesting profound differences in signal transduction and long-term synaptic plasticity between these regions of the human hippocampus.

A modulatory role for protein phosphatase 2B (calcineurin) in the regulation of Ca2+ entry

The Ca2+/calmodulin-dependent protein phosphatase 2B (PP2B) also known as calcineurin (CN) has been implicated in the Ca2+-dependent inactivation of Ca2+ channels in several cell types. To study the role of calcineurin in the regulation of Ca2+-channel activity, phosphatase expression was altered in NG108-15 cells by transfection of sense and antisense plasmid constructs carrying the catalytic subunit of human PP2Bbeta3. Relative to mock-transfected (wild-type) controls, cells overexpressing calcineurin showed dramatically reduced high-voltage-activated Ca2+ currents which were recoverable by the inclusion of 1 microM FK506 in the patch pipette. Conversely, in cells with reduced calcineurin expression, high-voltage-activated Ca2+ currents were larger relative to controls. Additionally in these cells, low-voltage-activated currents were significantly reduced. Analysis of high-voltage-activated Ca2+ currents revealed that the kinetics of inactivation were significantly accelerated in cells overexpressing calcineurin. Following the delivery of a train of depolarizing pulses in experiments designed to produce large-scale Ca2+ influx across the cell membrane, Ca2+-dependent inactivation of high-voltage-activated Ca2+ currents was increased in sense cells, and this increase could be reduced by intracellular application of 1 mM BAPTA or 1 microM FK506. These data support a role of calcineurin in the negative feedback regulation of Ca2+ entry through voltage-operated Ca2+ channels.

Isoform-specific redistribution of calcineurin A alpha and A beta in the hippocampal CA1 region of gerbils after transient ischemia

To investigate isoform-specific roles of Ca2+/calmodulin-dependent phosphatase [calcineurin (CaN)] in ischemia-induced cell death, we raised antibodies specific to CaN A alpha and CaN A beta and localized the CaN isoforms in the hippocampal CA1 region of Mongolian gerbils subjected to a 5-min occlusion of carotid arteries. In the nonischemic gerbil, immunoreactions of both isoforms were highly enriched in CA1 regions, especially in the cytoplasm and apical dendrites of CA1 pyramidal neurons. At 4-7 days after the induced ischemia, immunoreactivities of the CaN A alpha isoform in CA1 pyramidal cells were markedly reduced, whereas they were enhanced in the CA1 radiatum and oriens layers. In contrast, CaN A beta immunoreactivities were reduced in all layers of the ischemic CA1 region, whereas they were enhanced in activated astrocytes, colocalizing with glial fibrillary acidic protein. These findings suggest that up-regulation of CaN A alpha in afferent fibers in CA1 and up-regulation of CaN A beta in reactive astrocytes may be involved in neuronal reorganization after ischemic injury.

The Calcineurin Variant CnAβ1 Controls Mouse Embryonic Stem Cell Differentiation by Directing mTORC2 Membrane Localization and Activation

Embryonic stem cells (ESC) have the potential to generate all the cell lineages that form the body. However, the molecular mechanisms underlying ESC differentiation and especially the role of alternative splicing in this process remain poorly understood. Here, we show that the alternative splicing regulator MBNL1 promotes generation of the atypical calcineurin Aβ variant CnAβ1 in mouse ESCs (mESC). CnAβ1 has a unique C-terminal domain that drives its localization mainly to the Golgi apparatus by interacting with Cog8. CnAβ1 regulates the intracellular localization and activation of the mTORC2 complex. CnAβ1 knockdown results in delocalization of mTORC2 from the membrane to the cytoplasm, inactivation of the AKT/GSK3β/β-catenin signaling pathway, and defective mesoderm specification. In summary, here we unveil the structural basis for the mechanism of action of CnAβ1 and its role in the differentiation of mESCs to the mesodermal lineage.

Calcineurin activity is required for the completion of cytokinesis

Successful completion of cytokinesis requires the spatio-temporal regulation of protein phosphorylation and the coordinated activity of protein kinases and phosphatases. Many mitotic protein kinases are well characterized while mitotic phosphatases are largely unknown. Here, we show that the Ca(2+)- and calmodulin-dependent phosphatase, calcineurin (CaN), is required for cytokinesis in mammalian cells, functioning specifically at the abscission stage. CaN inhibitors induce multinucleation in HeLa cells and prolong the time cells spend connected via an extended intracellular bridge. Upon Ca(2+) influx during cytokinesis, CaN is activated, targeting a set of proteins for dephosphorylation, including dynamin II (dynII). At the intracellular bridge, phospho-dynII and CaN are co-localized to dual flanking midbody rings (FMRs) that reside on either side of the central midbody ring. CaN activity and disassembly of the FMRs coincide with abscission. Thus, CaN activity at the midbody plays a key role in regulating the completion of cytokinesis in mammalian cells.

Differential expression of cytosolic proteins in the rat kidney cortex and medulla: preliminary proteomics

The rodent kidney is a target of many xenobiotics and is typified by regionally specific structure and function. This renders distinct regions of the kidney differentially susceptible to toxic exposure and effect. To characterize these differences at the proteome level, protein patterns from male rat kidney cortex and medulla cytosols were examined by two-dimensional electrophoresis (2-DE) and image analysis and prominent proteins identified immunologically or by matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) and electrospray/ionization-tandem mass spectrometry (ESI-MS/MS) sequence tag identification. An average of 727 protein spots were resolved and matched to the cortex cytosol reference pattern, and 716 in the medulla. Of this total, 127 proteins were found to differ in abundance (86 higher in cortex; 41 higher in medulla) (P < 0.001). Of those proteins that were detectable in both cortex and medulla, the abundance of 97 differed significantly while 30 proteins were found to be unique to one region or the other (26 in cortex, 4 in medulla). Twenty protein spots were identified and their regional differences are discussed. These results both confirm and expand our understanding of the molecular heterogeneity characterizing structurally and functionally distinct regions of the kidney and serve as a useful foundation for future nephrotoxicologic studies.

Development of an assay method for activities of serine/threonine protein phosphatase type 2B (calcineurin) in crude extracts

Despite the physiological importance of serine/threonine protein phosphatase type 2B (PP2B/calcineurin), an accurate assay method of PP2B in crude tissue extracts has not been established. By using recombinant protein phosphatase inhibitor-1 as a substrate and ascorbic acid as an antioxidant, we developed an improved assay method for PP2B activity in crude extracts from mouse tissues and investigated tissue distribution of its activity. Under the assay conditions, the PP2B activities were stable for at least 30 min with more than 100-fold higher sensitivity than those previously reported. The specific activities of PP2B were 22.3, 0.85, 2.9, 0.36, and 1.5 mU/mg protein in mouse brain, heart, spleen, liver, and testis, respectively, and furthermore in each region of the brain they were 26.1, 13.7, 42.8, 40.5, 15.1, and 8.6 mU/mg protein in cerebrum, midbrain plus interbrain, striatum, hippocampus, cerebellum, and brain stem, respectively. This is the first paper to demonstrate a close correlation between tissue distributions and content of PP2B. These results showed that the present assay method is extremely powerful for precise measurement of a wide range of PP2B activities including not only high PP2B activity in the brain but also low PP2B activities in other tissues.

Activities of calcineurin and phosphatase 2A in the hippocampus after transient forebrain ischemia

We investigated the changes in the enzyme activity and immunoreactivity of calcineurin in the rat hippocampus after transient forebrain ischemia. Immediately after 20-min transient forebrain ischemia, calcineurin activity decreased to about 40% of the control in the CA1 region and to about 55% in other regions. Protein phosphatase 2A activity showed no remarkable changes. By 12 h after ischemia, calcineurin activity recovered, more in the CA1 region than in other regions. At 24 h it decreased again, but only in the CA1 region. Immunohistochemical- and immunoblot analyses showed no remarkable change in calcineurin in any region of the hippocampus within 12 h after ischemia. Thus, the activity of calcineurin is dissociated from its immunoreactivity and quantity. Several studies have suggested that unknown inhibitory factor(s) and/or reversible changes in calcineurin act to modify enzyme activity after ischemia. In contrast, phosphatase 2A activity underwent no obvious changes during the post-ischemia period we examined. This unique time course of calcineurin activity may contribute to the mechanism of ischemic neuronal injury.

Requirement for integration of phorbol 12-myristate 13-acetate and calcium pathways is preserved in the transactivation domain of NFAT1

The transcription factor NFAT integrates signals from both calcium- and phorbol ester-stimulated signaling pathways. The calcium signal activates the calmodulin (CaM)-dependent phosphatase calcineurin, which dephosphorylates the regulatory domain of NFAT and promotes its nuclear import, while the phorbol ester signal results in synthesis and activation of Fos and Jun, transcription factors that bind cooperatively with the NFAT DNA-binding domain in the nucleus to mediate the transcription of many target genes. Here we show that transactivation by a GAL4 fusion protein containing the strong acidic N-terminal transactivation domain (TAD) of NFAT1 also requires both calcium and phorbol ester stimulation. The calcium requirement can be mimicked by coexpression of activated versions of two CaM-dependent enzymes, calcineurin and CaM kinase IV. Our data indicate that a 144-amino acid segment of NFAT1, containing the N-terminal TAD but lacking the DNA-binding and Fos/Jun interaction domains, resembles the full-length protein in requiring a combined input from two separate signaling pathways for optimal function in cells.

Analysis of the cerebellar proteome in a transgenic mouse model of inherited prion disease reveals preclinical alteration of calcineurin activity

Inherited prion diseases are linked to insertional and point mutations in the prion protein (PrP) gene, which favor conversion of PrP into a conformationally altered, pathogenic isoform. The cellular mechanism by which this process causes neurological dysfunction is unknown. Transgenic (Tg) (PG14) mice express a mouse PrP homolog of a nine-octapeptide insertion associated with an inherited prion disorder. These mice develop a progressive neurological syndrome characterized by ataxia and cerebellar atrophy due to synaptic degeneration in the molecular layer and massive apoptosis of granule neurons. To investigate the molecular events that may contribute to neurological dysfunction, we carried out a differential proteomic analysis of cerebella from Tg(PG14) mice at the preclinical, onset, and symptomatic phases of their neurological illness. 2-D maps of cerebellar proteins from Tg(PG14) mice were compared to those obtained from age-matched Tg(WT) mice that express wild-type PrP and remain healthy. Proteins whose levels were significantly modified in at least one stage of the Tg(PG14) disease were identified by PMF. Analysis detected a preclinical decrease of the calcium/calmodulin-dependent phosphatase calcineurin (CaN) in granule neurons, suggesting that dysregulation of CaN activity induced by mutant PrP may be responsible for the cerebellar dysfunction in Tg(PG14) mice.

Expression, purification, and molecular characterization of Plasmodium falciparum FK506-binding protein 35 (PfFKBP35)

The immunosuppressive drug FK506 binds its targets FK506-binding protein (FKBP) family and modulates cellular processes. Recent studies demonstrated that FK506 shows anti-malaria effects. Newly identified FK506-binding protein 35 from Plasmodium falciparum (PfFKBP35) is assumed to be the molecular target of FK506 in the parasite. Currently, molecular and structural basis of growth inhibition of the parasite by FK506 remains unclear. In this study, to examine characteristics of PfFKBP35 and also understand its molecular mechanism of the inhibition by FK506, we have cloned, expressed, and purified the full-length PfFKBP35 and its FK506-binding domain (FKBD). We demonstrate that the full-length PfFKBP35 and the FKBD were properly folded, and suitable for biochemical and biophysical studies. PfFKBP35 showed a basal activity in inhibiting the phosphatase activity of calcineurin in the absence of FK506, but the presence of FK506 greatly enhanced its calcineurin-inhibitory activity. Our NMR data indicate that the FKBD binds FK506 with a high affinity.

A reassessment of the inhibitory capacity of human FKBP38 on calcineurin

The microbial peptidomacrolide FK506 affects many eukaryotic developmental and cell signaling programs via calcineurin inhibition. Prior formation of a complex between FK506 and intracellular FK506-binding proteins (FKBPs) is the precondition for the interaction with calcineurin. A puzzling difference has emerged between the mammalian multidomain protein hFKBP38 and other FKBPs. It was shown that hFKBP38 not only binds to calcineurin but also inhibits the protein phosphatase activity of calcineurin on its own [Shirane, M. and Nakayama, K.I. (2003) Nature Cell Biol. 5, 28-37]. Inherent calcineurin inhibition by hFKBP38 would completely eliminate the need for FK506 in controlling many signal transduction pathways. To address this issue, we have characterized the functional and physical interactions between calcineurin and hFKBP38. A recombinant hFKBP38 variant and endogenous hFKBP38 were tested both in vitro and in vivo. The proteins neither directly inhibited calcineurin activity nor affected NFAT reporter gene activity in SH-SY5Y and Jurkat cells. In addition, a direct physical interaction between calcineurin and hFKBP38 was not detected in co-immunoprecipitation experiments. However, hFKBP38 indirectly affected the subcellular distribution of calcineurin by interaction with typical calcineurin ligands, as exemplified by the anti-apoptotic protein Bcl-2. Our data suggest that hFKBP38 cannot substitute for the FKBP/FK506 complex in signaling pathways controlled by the protein phosphatase activity of calcineurin.

Clues to calcineurin function in mammalian fast-twitch muscle

It is believed that brief, high amplitude Ca2+ transients, as found in fast-twitch muscles, are not sufficient to activate the calcineurin (Cn)-dependent signaling pathway involved in regulation of slow myosin and slow sarcoplasmic reticulum Ca2+-ATPase genes (Olson and Williams, Cell 101: 689-692, 2000). The results reported here try to fill the gap between this molecular knowledge, and the still fragmentary pieces of information on a possible different role of calcineurin in the same type of muscles. In the present work calcineurin was determined immunocytochemically by labeling fast- and slow-twitch fibers of representative rabbit muscles with anti-CnB antibodies, and was assessed by western blotting of isolated subcellular fractions. Calcineurin was found to be largely soluble and to be constitutively overexpressed in fast muscle as CnAalpha and CnAbeta isoforms, the latter appearing to be predominant. Particulate calcineurin was not only associated with myofibrils but also with membranes of various origins. Fluorescence microscopy showed that calcineurin was distributed in the same pattern with respect to sarcomeres in both types of fibers, and formed punctate dots spanning the I-Z-I region, rather than being exclusively located at the Z-line, a disposition described for cardiomyocytes (Frey et al., Proc Natl Acad Sci USA 97: 14,632-14,637, 2000). From knowledge that, in mammalian skeletal muscle fibers, junctional triads are located at the A-I band boundary, we explored the distribution of calcineurin between triadic components, after having verified that it was present in very low amounts in dystrophin-enriched sarcolemmal membranes. Our results demonstrate that a small but significant proportion of calcineurin coenriched with transverse tubules (TT), and copurified with the DHPR and with DHPR-associated PKA-AKAP15/18, thus suggesting that it is assembled as a multiprotein complex in the junctional membrane domain of TT. The membrane specificity of this association is further corroborated by the negative evidence for the presence of calcineurin in SR terminal cisternae. Calcineurin was separated from the DHPR and isolated as a AKAP15/18 subcomplex, including beta2 adrenergic receptor, in addition to PKA and calcineurin, following equilibrium centrifugation of detergent extracts on a linear sucrose gradient. We show that the alpha1 subunit skeletal isoform of the DHPR, is a substrate for calcineurin dephosphorylation, after previous phosphorylation by endogenous PKA.