Calmodulin and calmodulin-binding proteins in the nucleus

New Findings: Hindlimb Unloading Causes Nucleocytoplasmic Ca2+ Overload and DNA Damage in Skeletal Muscle

Disuse atrophy of skeletal muscle is associated with a severe imbalance in cellular Ca²⁺ homeostasis and marked increase in nuclear apoptosis. Nuclear Ca²⁺ is involved in the regulation of cellular Ca²⁺ homeostasis. However, it remains unclear whether nuclear Ca²⁺ levels change under skeletal muscle disuse conditions, and whether changes in nuclear Ca²⁺ levels are associated with nuclear apoptosis. In this study, changes in Ca²⁺ levels, Ca²⁺ transporters, and regulatory factors in the nucleus of hindlimb unloaded rat soleus muscle were examined to investigate the effects of disuse on nuclear Ca²⁺ homeostasis and apoptosis. Results showed that, after hindlimb unloading, the nuclear envelope Ca²⁺ levels ([Ca²⁺]_NE) and nucleocytoplasmic Ca²⁺ levels ([Ca²⁺]_NC) increased by 78% (p < 0.01) and 106% (p < 0.01), respectively. The levels of Ca²⁺-ATPase type 2 (Ca²⁺-ATPase2), Ryanodine receptor 1 (RyR1), Inositol 1,4,5-tetrakisphosphate receptor 1 (IP₃R1), Cyclic ADP ribose hydrolase (CD38) and Inositol 1,4,5-tetrakisphosphate (IP₃) increased by 470% (p < 0.001), 94% (p < 0.05), 170% (p < 0.001), 640% (p < 0.001) and 12% (p < 0.05), respectively, and the levels of Na⁺/Ca²⁺ exchanger 3 (NCX3), Ca²⁺/calmodulin dependent protein kinase II (CaMK II) and Protein kinase A (PKA) decreased by 54% (p < 0.001), 33% (p < 0.05) and 5% (p > 0.05), respectively. In addition, DNase X is mainly localized in the myonucleus and its activity is elevated after hindlimb unloading. Overall, our results suggest that enhanced Ca²⁺ uptake from cytoplasm is involved in the increase in [Ca²⁺]_NE after hindlimb unloading. Moreover, the increase in [Ca²⁺]_NC is attributed to increased Ca²⁺ release into nucleocytoplasm and weakened Ca²⁺ uptake from nucleocytoplasm. DNase X is activated due to elevated [Ca²⁺]_NC, leading to DNA fragmentation in myonucleus, ultimately initiating myonuclear apoptosis. Nucleocytoplasmic Ca²⁺ overload may contribute to the increased incidence of myonuclear apoptosis in disused skeletal muscle.

Calmodulin7: recent insights into emerging roles in plant development and stress

Analyses of the function of Arabidopsis Calmodulin7 (CAM7) in concert with multiple regulatory proteins involved in various signal transduction processes. Calmodulin (CaM) plays various regulatory roles in multiple signaling pathways in eukaryotes. Arabidopsis CALMODULIN 7 (CAM7) is a unique member of the CAM family that works as a transcription factor in light signaling pathways. CAM7 works in concert with CONSTITUTIVE PHOTOMORPHOGENIC 1 and ELONGATED HYPOCOTYL 5, and plays an important role in seedling development. Further, it is involved in the regulation of the activity of various Ca²⁺-gated channels such as cyclic nucleotide gated channel 6 (CNGC6), CNGC14 and auto-inhibited Ca²⁺ ATPase 8. Recent studies further indicate that CAM7 is also an integral part of multiple signaling pathways including hormone, immunity and stress. Here, we review the recent advances in understanding the multifaceted role of CAM7. We highlight the open-ended questions, and also discuss the diverse aspects of CAM7 characterization that need to be addressed for comprehensive understanding of its cellular functions.

Ca2+ Signaling and Its Potential Targeting in Pancreatic Ductal Carcinoma

Pancreatic cancer (PC) is a major cause of cancer-associated mortality in Western countries (and estimated to be the second cause of cancer deaths by 2030). The main form of PC is pancreatic adenocarcinoma, which is the fourth most common cause of cancer-related death, and this situation has remained virtually unchanged for several decades. Pancreatic ductal adenocarcinoma (PDAC) is inherently linked to the unique physiology and microenvironment of the exocrine pancreas, such as pH, mechanical stress, and hypoxia. Of them, calcium (Ca2+) signals, being pivotal molecular devices in sensing and integrating signals from the microenvironment, are emerging to be particularly relevant in cancer. Mutations or aberrant expression of key proteins that control Ca2+ levels can cause deregulation of Ca2+-dependent effectors that control signaling pathways determining the cells' behavior in a way that promotes pathophysiological cancer hallmarks, such as enhanced proliferation, survival and invasion. So far, it is essentially unknown how the cancer-associated Ca2+ signaling is regulated within the characteristic landscape of PDAC. This work provides a complete overview of the Ca2+ signaling and its main players in PDAC. Special consideration is given to the Ca2+ signaling as a potential target in PDAC treatment and its role in drug resistance.

Nuclear myosins - roles for molecular transporters and anchors

The myosin family of molecular motors are well-characterised cytoskeletal proteins. However, myosins are also present in the nucleus, where they have been shown to have roles in transcription, DNA repair and viral infections. Despite their involvement in these fundamental cellular processes, our understanding of these functions and their regulation remains limited. Recently, research on nuclear myosins has been gathering pace, and this Review will evaluate the current state of the field. Here, we will focus on the variation in structure of nuclear myosins, their nuclear import and their roles within transcription, DNA damage, chromatin organisation and viral infections. We will also consider both the biochemical and biophysical properties and restraints that are placed on these multifunctional motors, and how they link to their cytoplasmic counterparts. By highlighting these properties and processes, we show just how integral nuclear myosins are for cellular survival.

Ca2+/calmodulin-dependent protein kinase IIβ and IIδ mediate TGFβ-induced transduction of fibronectin and collagen in human pulmonary fibroblasts

It is now clear that in addition to activating several complex kinase pathways (Smad, MAP kinase, PI3 kinase), TGFβ also acts by elevating cytosolic Ca2+ concentration within human pulmonary fibroblasts. Ca2+/calmodulin-dependent protein kinase II (CamK II) is also known to regulate gene expression in fibroblasts. In this study, we examined the interactions between calcium signaling, activation of CamK and other kinases, and extracellular matrix (ECM) gene expression. Human pulmonary fibroblasts were cultured and stimulated with artificially generated Ca2+ pulses in the absence of TGFβ, or with TGFβ (1 nM) or vehicle in the presence of various blockers of Ca2+ signaling. PCR and Western blotting were used to measure gene expression and protein levels, respectively. We found that Ca2+ pulses in the absence of TGFβ increased ECM gene expression in a pulse frequency-dependent manner, and that blocking Ca2+ signaling and the CamK II pathway significantly reduced TGFβ-mediated ECM gene expression, without having any effects on other kinase pathways (Smad, PI3 kinase, or MAP kinase). We also found that TGFβ elevated the expression of CamK IIβ and CamK IIδ, while siRNA silencing of those two subtypes significantly reduced TGFβ-mediated expression of collagen A1 and fibronectin 1. Our data suggest that TGFβ induces the expression of CamK IIβ and CamK IIδ, which in turn are activated by TGFβ-evoked Ca2+ waves in a frequency-dependent manner, leading to increased expression of ECM proteins.

Cross talk among PMCA, calcineurin and NFAT transcription factors in control of calmodulin gene expression in differentiating PC12 cells

Brain aging is characterized by progressive loss of plasma membrane calcium pump (PMCA) and its activator - calmodulin (CaM), but the mechanism of this phenomenon remains unresolved. CaM encoded by three genes Calm1, Calm2, Calm3, works to translate Ca²⁺ signal into changes in frequently opposite cellular activities. This unique function allows CaM to affect gene expression via stimulation of calcineurin (CaN) and its downstream target - nuclear factor of activated T-cells (NFAT) and to terminate Ca²⁺ signal by stimulation of its extrusion. PMCA, which exists in four isoforms PMCA1-4, may in turn shape the pattern of Ca²⁺ transients and control CaN activity by its direct binding. Therefore, the interplay between PMCA, CaM and CaN/NFAT is highly plausible. To verify that, we used differentiated PC12 cells with reduced expression of PMCA2 or PMCA3 to mimic the potential changes in aged brain. Manipulation in PMCAs level decreased CaM protein in PMCA2 or PMCA3-reduced lines that was accompanied by down-regulation of Calm1 and Calm2 in both lines, but Calm3 only in PMCA2-reduced cells. Further studies showed substantially higher NFATc2 nuclear accumulation and increased NFAT transcriptional activity. Blocking of CaN/NFAT signalling resulted in almost full CaM recovery, mainly due to up-regulation of Calm2 and Calm3 genes. Moreover, higher occupancy of Calm2 and Calm3 promoters by NFATc2 and increased expression of these genes in response to NFATc2 silencing were demonstrated in PMCA2 and PMCA3-reduced lines. Our results indicate that decrease in CaM level in response to PMCAs downregulation can be driven by CaN/NFAT pathway.

K-Ras Promotes Tumorigenicity through Suppression of Non-canonical Wnt Signaling

K-Ras and H-Ras share identical effectors and have similar properties; however, the high degree of tumor-type specificity associated with K-Ras and H-Ras mutations suggests that they have unique roles in oncogenesis. Here, we report that oncogenic K-Ras, but not H-Ras, suppresses non-canonical Wnt/Ca(2+) signaling, an effect that contributes strongly to its tumorigenic properties. K-Ras does this by binding to calmodulin and so reducing CaMKii activity and expression of Fzd8. Restoring Fzd8 in K-Ras mutant pancreatic cells suppresses malignancy, whereas depletion of Fzd8 in H-Ras(V12)-transformed cells enhances their tumor initiating capacity. Interrupting K-Ras-calmodulin binding using genetic means or by treatment with an orally active protein kinase C (PKC)-activator, prostratin, represses tumorigenesis in K-Ras mutant pancreatic cancer cells. These findings provide an alternative way to selectively target this "undruggable" protein.

The Key Role of Calmodulin in KRAS-Driven Adenocarcinomas

KRAS4B is a highly oncogenic splice variant of the KRAS isoform. It is the only isoform associated with initiation of adenocarcinomas. Insight into why and how KRAS4B can mediate ductal adenocarcinomas, particularly of the pancreas, is vastly important for its therapeutics. Here we point out the overlooked critical role of calmodulin (CaM). Calmodulin selectively binds to GTP-bound K-Ras4B; but not to other Ras isoforms. Cell proliferation and growth require the MAPK (Raf/MEK/ERK) and PI3K/Akt pathways. We propose that Ca(2+)/calmodulin promote PI3Kα/Akt signaling, and suggest how. The elevated calcium levels clinically observed in adenocarcinomas may explain calmodulin's involvement in recruiting and stimulating PI3Kα through interaction with its n/cSH2 domains as well as K-Ras4B; importantly, it also explains why K-Ras4B specifically is a key player in ductal carcinomas, such as pancreatic (PDAC), colorectal (CRC), and lung cancers. We hypothesize that calmodulin recruits and helps activate PI3Kα at the membrane, and that this is the likely reason for Ca(2+)/calmodulin dependence in adenocarcinomas. Calmodulin can contribute to initiation/progression of ductal cancers via both PI3Kα/Akt and Raf/MEK/ERK pathways. Blocking the K-Ras4B/MAPK pathway and calmodulin/PI3Kα binding in a K-Ras4B/calmodulin/PI3Kα trimer could be a promising adenocarcinoma-specific therapeutic strategy.

γCaMKII shuttles Ca²⁺/CaM to the nucleus to trigger CREB phosphorylation and gene expression

Activity-dependent CREB phosphorylation and gene expression are critical for long-term neuronal plasticity. Local signaling at CaV1 channels triggers these events, but how information is relayed onward to the nucleus remains unclear. Here, we report a mechanism that mediates long-distance communication within cells: a shuttle that transports Ca(2+)/calmodulin from the surface membrane to the nucleus. We show that the shuttle protein is γCaMKII, its phosphorylation at Thr287 by βCaMKII protects the Ca(2+)/CaM signal, and CaN triggers its nuclear translocation. Both βCaMKII and CaN act in close proximity to CaV1 channels, supporting their dominance, whereas γCaMKII operates as a carrier, not as a kinase. Upon arrival within the nucleus, Ca(2+)/CaM activates CaMKK and its substrate CaMKIV, the CREB kinase. This mechanism resolves long-standing puzzles about CaM/CaMK-dependent signaling to the nucleus. The significance of the mechanism is emphasized by dysregulation of CaV1, γCaMKII, βCaMKII, and CaN in multiple neuropsychiatric disorders.

Inhibition of SRY-calmodulin complex formation induces ectopic expression of ovarian cell markers in developing XY gonads

The transcription factor sex-determining region of the Y chromosome (SRY) plays a key role in human sex determination, because mutations in SRY cause disorders of sex development in XY individuals. During gonadal development, Sry in pre-Sertoli cells activates Sox9 gene transcription, committing the fate of the bipotential gonad to become a testis rather than an ovary. The high-mobility group domain of human SRY contains two independent nuclear localization signals, one bound by calmodulin (CaM) and the other by importin-β. Although XY females carry SRY mutations in these nuclear localization signals that affect SRY nuclear import in transfected cells, it is not known whether these transport mechanisms are essential for gonadal development and sex determination. Here, we show that mouse Sry protein binds CaM and that a CaM antagonist reduces CaM binding, nuclear accumulation, and transcriptional activity of Sry in transfected cells. CaM antagonist treatment of cultured, sexually indifferent XY mouse fetal gonads led to reduced expression of the Sry target gene Sox9, defects in testicular cord formation, and ectopic expression of the ovarian markers Rspondin1 and forkhead box L2. These results indicate the importance of CaM for SRY nuclear import, transcriptional activity, testis differentiation, and sex determination.

Stimulation-mediated translocation of calmodulin and neurogranin from soma to dendrites of mouse hippocampal CA1 pyramidal neurons

Calmodulin (CaM) and neurogranin (Ng) are two abundant neuronal proteins in the forebrain whose interactions are implicated in the enhancement of synaptic plasticity. To gain further insight into the actions of these two proteins we investigated whether they co-localize in principle neurons and whether they respond to high frequency stimulation in a coordinated fashion. Immunohistochemical staining of CaM and Ng in mouse hippocampal slices revealed that CaM was highly concentrated in the nucleus of CA1 pyramidal neurons, whereas Ng was more broadly localized throughout the soma and dendrites. The asymmetrical localization of CaM in the nucleus of pyramidal neurons was in sharp contrast to the distribution observed in pyramidal cells of the neighboring subiculum, where CaM was uniformly localized throughout the soma and dendrites. The somatic concentrations of CaM and Ng in CA1 pyramidal neurons were approximately 10- and two-fold greater than observed in the dendrites, respectively. High frequency stimulation (HFS) of hippocampal slices promoted mobilization of CaM and Ng from soma to dendrites. These responses were spatially restricted to the area close to the site of stimulation and were inhibited by the N-methyl-D-asparate receptor antagonist 2-amino-5-phosphonopentanoic acid. Furthermore, HFS failed to promote translocation of CaM from soma to dendrites of slices from Ng knockout mice, which also exhibited deficits in HFS-induced long-term potentiation. Translocated CaM and Ng exhibited distinct puncta decorating the apical dendrites of pyramidal neurons and appeared to be concentrated in dendritic spines. These findings suggest that mobilization of CaM and Ng to stimulated dendritic spines may enhance synaptic efficacy by increasing and prolonging the Ca2+ transients and activation of Ca2+/CaM-dependent enzymes.

Calmodulin-like protein from Entamoeba histolytica: solution structure and calcium-binding properties of a partially folded protein

The mechanism of Ca(2+)-signaling in the protozoan parasite Entamoeba histolytica is yet to be understood as many of the key regulators are still to be identified. E. histolytica encodes a number of multi-EF-hand Ca(2+)-binding proteins (EhCaBPs). Functionally only one of these molecules, EhCaBP1, has been characterized to date. The calmodulin-like protein from E. histolytica (abbreviated as EhCaM or EhCaBP3) is a 17.23 kDa monomeric protein that shows maximum sequence identity with heterologous calmodulins (CaMs). Though CaM activity has been biochemically shown in E. histolytica, there are no reports on the presence of a typical CaM. In an attempt to understand the structural and functional similarity of EhCaM with CaM, we have determined the three-dimensional (3D) solution structure of EhCaM using NMR. The EhCaM has a well-folded N-terminal domain and an unstructured C-terminal counterpart. Further, it sequentially binds only two calcium ions, an unusual mode of Ca(2+)-binding among the known CaBPs, notably both in the N-terminal domain of EhCaM. Further, EhCaM is present in the nucleus in addition to the cytoplasm as detected by immunofluorescence staining, unlike other EhCaBPs that are detected only in the cytoplasm. Therefore, this protein is likely to have a different function. The presence of unusual and a diverse set of CaBPs in E. histolytica suggests a distinct Ca(2+)-signaling process in E. histolytica. The results reported here help in understanding the structure-function relationship of CaBPs including their Ca(2+)-binding properties.

Ethanol affects calmodulin and the calmodulin-binding proteins neuronal nitric oxide synthase and alphaII-spectrin (alpha-fodrin) in the nucleus of growing and differentiated rat astrocytes in primary culture

The distribution of calmodulin (CaM) and the CaM-binding proteins neuronal nitric oxide synthase (nNOS) and alphaII-spectrin (alpha-fodrin) in the nucleus of growing and differentiated astrocytes was analysed using immunogold electronmicroscopy. We also analysed the effect of moderate ethanol exposure on these proteins. For this, female Wistar rat were fed with an alcoholic liquid diet and exposed to males after several weeks. Pregnant rats were fed with this diet and, after birth, the foetuses brains were used to establish primary cultures of astrocytes. Astrocytes from control and ethanol-exposed rats foetuses were cultured in the absence or presence of ethanol (30 mM) for 7 days (growing cells) and 21 days (differentiated astrocytes). Our results indicate that all the proteins studied appeared mainly on the condensed chromatin of both control- and alcohol-exposed cells and that there are significant variations in the amount of these proteins between quiescent and dividing astrocytes. Altogether, we have not found a co-localisation between CaM and the CaM-binding proteins.

Role of Ca2+ activation and bilobal structure of calmodulin in nuclear and nucleolar localization

Ca2+ signalling to the nucleus is thought to occur by calmodulin entry into the nucleus where calmodulin has many functions. In the present study we have investigated the role of Ca2+ and the N- and C-terminal lobes of calmodulin in its subnuclear targeting by using fluorescently labelled calmodulin and its mutants and confocal microscopy. Our data show, first, that Ca2+ stimulation induces a reorganization of subnuclear structures to which apo-calmodulin can bind. Secondly, Ca2+-independent association of the C-terminal lobe is seen with subnuclear structures such as chromatin, the nuclear envelope and the nucleoli. Thirdly, Ca2+-dependent accumulation of both calmodulin and the C-terminal calmodulin lobe occurs in the nucleoli. The N-terminal lobe of calmodulin does not show significant binding to subnuclear structures although, similarly to the C-terminal lobe, it accumulates in the nucleoplasm of wheat germ agglutinin-blocked nuclei suggesting that a facilitated nuclear export mechanism exists for calmodulin.

Presence of SERCA and calcineurin during fetal development of porcine skeletal muscle

Mechanisms involved in skeletal myofiber differentiation during fetal development of large animals are poorly understood. Studies in small animals suggest that the calcineurin (Cn) pathway is involved in myofiber differentiation. Neural activity is a prerequisite for Cn activity, implying maintenance of sustained low intracellular Ca(2+) concentrations. To study the role of Cn in fetal myofiber differentiation, we monitored the temporal and spatial distribution of Cn subunits, sarcoplasmic reticulum Ca(2+) ATPase (SERCA), phospholamban (PLB), and myosin heavy chain (MyHC) isoforms in relation to ingrowing nerves in porcine semitendinosus muscle (m. semitendinosus) at 55 and 75 days of gestation (dg) and at term. Immunofluorescence analysis revealed the presence of Cn subunits and SERCA isoforms at all analyzed stages. Cn distribution was not fiber-type specific, but expression became more prominent at term. At 75 dg, differential SERCA2 expression was accompanied by perinuclear PLB in primary fibers. SERCA1 was expressed in all fiber types at all stages. No specific MyHC isoform distribution was seen in relation to neuromuscular contacts, although neuromuscular contacts were present. From these results we speculate that in porcine m. semitendinosus differential SERCA2 expression precedes differential Cn expression. The question whether the Cn pathway is involved in prenatal myofiber differentiation needs further studies.

Ultrastructural localisation of calcium deposits in pig ovarian follicles

Calcium intracellular signaling regulates many intracellular events including oocyte maturation. This signaling is strongly dependent on the influx of calcium ions from extracellular spaces and on the state of intracellular calcium stores. In this study, intracellular calcium deposits were detected in follicle-enclosed pig oocytes using the combined oxalate-pyroantimonate method. These deposits were observed in the nucleus, the mitochondria, the cytoplasm, and on the surface of lipid droplets. The amount of calcium deposits was expressed as a percentage of the area of the respective cellular compartment, which is covered with calcium deposits on ultrathin sections. The distribution of calcium deposits in oocytes changed during folliculogenesis. The amount of calcium deposits in nuclei (1.11% of the area of oocyte nuclei) and cytoplasm (1.02%) in oocytes from secondary and early antral follicles (0.90% nuclei; 0.99% cytoplasm) is significantly lower (P < 0.05) than the amount of calcium deposits in these compartments in oocytes from primary follicles (2.51% nuclei; 2.34% cytoplasm) or antral follicles with growing oocyte (2.91% nuclei; 2.21% cytoplasm). The amount of calcium deposits in mitochondria of oocytes from primary follicles (1.27%) or antral follicles with growing oocyte (1.14%) is significantly lower (P < 0.05) than in the nucleus (2.51% in oocytes from primary follicles; 2.91% in growing oocytes from antral follicles) or cytoplasm (2.34% in oocytes from primary follicles; 2.21% in growing oocytes from antral follicles). The amount of calcium deposits in the cytoplasm of fully-grown oocytes (1.46%) dropped to levels significantly lower (P < 0.05) than those observed in the oocyte nucleus (2.29%). On the basis of these data, we can conclude that the population of follicles on pig ovaries differs in the distribution and concentration of calcium deposits in oocytes, and these changes may be involved in the regulation of the meiotic competence of oocytes.

Ca2+-dependent and -independent mechanisms of calmodulin nuclear translocation

Translocation from the cytosol to the nucleus is a major response by calmodulin (CaM) to stimulation of cells by Ca2+. However, the mechanisms involved in this process are still controversial and both passive and facilitated diffusion have been put forward. We tested nuclear translocation mechanisms in electroporated HeLa cells, rat cortical neurons and glial cells using novel calmodulin and inhibitor peptide probes and confocal microscopy. Passive diffusion of calmodulin across the nuclear membrane was measured in conditions in which facilitated transport was blocked and was compared to that of a similarly sized fluorescein-labeled dextran. Wheat germ agglutinin, which blocks facilitated transport but not passive diffusion, inhibited the nuclear entry of both wild-type and Ca2+-binding-deficient mutant calmodulin both in low and elevated [Ca2+]. Ca2+-dependent nuclear translocation was prevented by a membrane-permeant CaM inhibitor, the mTrp peptide, which indicated that it was specific to Ca2+/CaM. Diffusion of free CaM and Ca2+/CaM was considerably slower than the observed nuclear translocation by facilitated transport. Our data show that the majority of CaM nuclear entry occurred by facilitated mechanisms in all cell types examined, in part by a Ca2+-independent and in part by a Ca2+-dependent translocation mechanism.

Accumulation of βm, a structural member of X,K-ATPase β-subunit family, in nuclear envelopes of perinatal myocytes

Recently discovered muscle-specific βmprotein is structurally closely related to the X,K-ATPase β-subunits. However, it has a number of unique properties such as predominant localization in intracellular stores and lack of association with known X,K-ATPase α-subunits on heterologous coexpression. In this study, the primary structure of mouse βmwas determined and developmental regulation of the gene (ATP1B4) was analyzed. The expression is first detected at day 14 of gestation, is sharply increased at day 16, and reaches its maximum at day 18. After birth, the expression quickly decreases and is hardly detectable in adult mice. A more detailed subcellular localization study was undertaken, and its results indicate that βmnot only is located in sarcoplasmic reticulum but is concentrated in nuclear envelopes of both prenatal and postnatal skeletal muscles. Immunohistochemical studies show that βmis specific to myocytes and, at the subcellular level, many nuclear envelopes are intensively labeled in both fetal and newborn skeletal muscles. Accordingly, βmis detected by immunoblotting in purified nuclei and nuclear membranes from neonatal skeletal muscles. On transfection of human rhabdomyosarcoma cell line RD, green fluorescent protein-tagged βmresides intracellularly with significant enrichment in nuclear envelopes, whereas βmwith transmembrane domain deleted localizes in both cytoplasm and nucleoplasm. Nuclear βmapparently is not in association with Na,K-ATPase because we never detected its α-subunit in myonuclear membranes. These results indicate that βmhas a specialized function in mammalian perinatal myocytes, different from functions of other X,K-ATPase β-subunits. The unique temporospatial distribution of βmprotein expression suggests its important role in development of growing skeletal muscle.

A SOX9 defect of calmodulin-dependent nuclear import in campomelic dysplasia/autosomal sex reversal

During mammalian sex determination, SOX9 is translocated into the nuclei of Sertoli cells within the developing XY gonad. The N-terminal nuclear localization signal (NLS) is contained within a SOX consensus calmodulin (CaM) binding region, thereby implicating CaM in nuclear import of SOX9. By fluorescence spectroscopy and glutaraldehyde cross-linking, we show that the SOX9 HMG domain and CaM interact in vitro. The formation of a SOX9.CaM binary complex is calcium-dependent and is accompanied by a conformational change in SOX9. A CaM antagonist, calmidazolium chloride (CDZ), was observed to block CaM recognition of SOX9 in vitro and inhibit both nuclear import and consequent transcriptional activity of SOX9 in treated cells. The significance of the SOX9-CaM interaction was highlighted by analysis of a missense SOX9 mutation, A158T, identified from a XY female with campomelic dysplasia/autosomal sex reversal (CD/SRA). This mutant binds importin beta normally despite defective nuclear import. Fluorescence and quenching studies indicate that in the unbound state, the A158T mutant shows a similar conformation to that of the WT SOX9, but in the presence of CaM, the mutant undergoes unusual conformational changes. Furthermore, SOX9-mediated transcriptional activation by cells expressing the A158T mutant is more sensitive to CDZ than cells expressing WT SOX9. These results suggest first that CaM is involved in the nuclear transport of SOX9 in a process likely to involve direct interaction and second, that CD/SRA can arise, at least in part, from a defect in CaM recognition, ultimately leading to reduced ability of SOX9 to activate transcription of cartilage and testes-forming genes.

The molecular action and regulation of the testis-determining factors, SRY (sex-determining region on the Y chromosome) and SOX9 [SRY-related high-mobility group (HMG) box 9]

Despite 12 yr since the discovery of SRY, little is known at the molecular level about how SRY and the SRY-related protein, SOX9 [SRY-related high-mobility group (HMG) box 9], initiate the program of gene expression required to commit the bipotential embryonic gonad to develop into a testis rather than an ovary. Analysis of SRY and SOX9 clinical mutant proteins and XX mice transgenic for testis-determining genes have provided some insight into their normal functions. SRY and SOX9 contain an HMG domain, a DNA-binding motif. The HMG domain plays a central role, being highly conserved between species and the site of nearly all missense mutations causing XY gonadal dysgenesis. SRY and SOX9 are architectural transcription factors; their HMG domain is capable of directing nuclear import and DNA bending. Whether SRY and SOX9 activate testis-forming genes, repress ovary-forming genes, or both remains speculative until downstream DNA target genes are identified. However, factors that control SRY and SOX9 gene expression have been identified, as have a dozen sex-determining genes, allowing some of the pieces in this molecular genetic puzzle to be connected. Many genes, however, remain unidentified, because in the majority of cases of XY females and in all cases of XX males lacking SRY, the mutated gene is unknown.

Vasopressin-induced cytoplasmic and nuclear calcium signaling in embryonic cortical astrocytes: dynamics of calcium and calcium-dependent kinase translocation

The present study sought to determine the downstream consequences of V1a vasopressin receptor (V1aR) activation of Ca2+ signaling in cortical astrocytes. Results of these analyses demonstrated that V1aR activation led to a marked increase in both cytoplasmic and nuclear Ca2+. We also investigated V1aR activation of Ca2+-activated signaling kinases, protein kinase C (PKC), Ca2+/calmodulin-dependent protein kinase II (CaMKII), and the mitogen-activated protein (MAP) kinases [MAPK and extracellular signal-regulated kinases 1 and 2 (ERK1/2)], their localization within cytoplasmic and nuclear compartments, and activation of their downstream nuclear target, the transcription factor cAMP response element-binding protein (CREB). Results of these analyses demonstrated that V1aR activation led to a significant rise in PKC, CaMKII, and ERK1/2 activation, with CaMKII and ERK1/2 demonstrating dynamic transport between cytoplasmic and nuclear compartments. Although no evidence of PKC translocation was apparent, PKC and CaMKs were required for activation and nuclear translocation of ERK1/2. Subsequent to CaMKII and ERK1/2 translocation to the nucleus, CREB activation occurred and was found to be dependent on upstream activation of ERK1/2 and CaMKs. These data provide the first systematic analysis of the V1aR-induced Ca2+ signaling cascade in cortical astrocytes. In addition, results of this study introduce a heretofore unknown effect of vasopressin, dynamic Ca2+ signaling between the cytoplasm and nucleus that leads to comparable dynamics of kinase activation and shuttling between cytoplasmic and nuclear compartments. Implications for development and regeneration induced by V1aR activation of CREB-regulated gene expression in cortical astrocytes are discussed.

Excitation-transcription coupling: signaling by ion channels to the nucleus

Changes in the concentration of intracellular Ca2+ ([Ca2+]i) in response to various stimuli play a role in regulating numerous cellular processes, including the activation of gene expression. In neurons, the extraordinary diversity of the response to Ca2+ signaling depends on the location, intensity, and duration of the Ca2+ transient. Interestingly, Ca2+-dependent gene transcription appears to be sensitive both to increases in nuclear Ca2+, which occur after relatively intense stimuli, and to highly localized increases in Ca2+ near the sites of Ca2+ influx. Activation of intracellular signaling pathways by specific types of Ca2+ channels depends on localization of specific Ca2+ receptors close to the channel mouth. The dual regulation of signaling pathways by Ca2+ near channels and in the nucleus may permit neurons to precisely tailor transcriptional activation to specific types of electrical or chemical stimuli and at the same time ensure that only robust stimuli that generate nuclear Ca2+ elevations are converted into long-term changes in gene expression.

Ultrastructural localisation of calcium deposits in the mouse ovary

Follicle-enclosed mouse oocytes contain numerous calcium deposits. The ultrastructural distribution of calcium deposits in the nuclei, mitochondria and cytoplasm of mouse oocytes and granulosa cells of primary, secondary and antral follicles was examined using the combined oxalate–pyroantimonate method. The mitochondria of oocytes from all types of follicles had the highest levels of calcium deposits of all oocyte compartments, with the exception of primary follicles, in which oocyte nuclei contained the same level of calcium deposits as the mitochondria. Calcium deposits in the cytoplasm of oocytes from primary follicles were significantly lower than those in the cytoplasm of oocytes from secondary and antral follicles. Calcium deposits in the cytoplasm of granulosa cells were significantly lower than calcium deposits in the mitochondria of granulosa cells and this difference persisted throughout all categories of follicles. Calcium deposits in the nuclei of granulosa cells did not differ from levels in the mitochondria in primary and secondary follicles. In contrast, the nuclei of granulosa cells from antral follicles had lower levels of calcium deposits than the mitochondria. The differences observed in calcium deposits in various cellular compartments in oocytes and granulosa cells in the follicles of ovaries of adult mice can be attributed to their acquisition of meiotic competence and follicular development.

Calmodulin prevents activation of Ras by PKC in 3T3 fibroblasts

We have shown previously (Villalonga, P., López- Alcalá, C., Bosch, M., Chiloeches, A., Rocamora, N., Gil, J., Marais, R., Marshall, C. J., Bachs, O., and Agell, N. (2001) Mol. Cell. Biol. 21, 7345-7354) that calmodulin negatively regulates Ras activation in fibroblasts. Hence, anti-calmodulin drugs (such as W13, trifluoroperazine, or W7) are able to induce Ras/ERK pathway activation under low levels of growth factors. We show here that cell treatment with protein kinase C (PKC) inhibitors abolishes W13-induced activation of Ras, Raf-1, and ERK. Consequently, PKC activity is essential for achieving the synergism between calmodulin inhibition and growth factors to activate Ras. Furthermore, whereas the activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) does not induce Ras activation in 3T3 cells, activation is observed if calmodulin is simultaneously inhibited. This indicates that calmodulin is preventing Ras activation by PKC. Treatment of cells with epidermal growth factor receptor or platelet-derived growth factor receptor tyrosine kinase inhibitors does not abrogate the activation of Ras by calmodulin inhibition. This implies that epidermal growth factor receptor and platelet-derived growth factor receptor tyrosine kinase activities are dispensable for the activation of Ras by TPA plus W13, and, therefore, Ras activation is not a consequence of the transactivation of those receptors by the combination of the anti-calmodulin drug plus TPA. Furthermore, K-Ras, the isoform previously shown to bind to calmodulin, is the only one activated by TPA when calmodulin is inhibited. These data suggest that direct interaction between K-Ras and calmodulin may account for the inability of PKC to activate Ras in 3T3 fibroblasts. In vitro experiments showed that the phosphorylation of K-Ras by PKC was inhibited by calmodulin, suggesting that calmodulin-dependent modulation of K-Ras phosphorylation by PKC could be the mechanism underlying K-Ras activation in fibroblasts treated with TPA plus W13.

Vasopressin-induced cytoplasmic and nuclear calcium signaling in cultured cortical astrocytes

We sought to determine whether vasopressin V(1a) receptor (V(1a)R) mRNA detected in cortical astrocytes [Brain Res. Mol. Brain Res. 45 (1997) 138] was translated into functional receptors by investigating the effector calcium signaling cascade associated with the vasopressin V(1a) receptor subtype. Analysis of intracellular calcium dynamics using the calcium-sensitive dye fura-2 AM indicated that exposure of cortical astrocytes to V(1) vasopressin receptor agonist, [Phe(2),Orn(8)]-oxytocin, induced a marked dose-dependent increase in intracellular calcium which was abolished by depletion of extracellular calcium. V(1) agonist treatment induced a rapid increase in calcium signal in both the cytoplasm and nucleus, which was followed by an accumulation of the calcium signal in the nucleus, suggesting translocation of cytoplasmic calcium into the nucleus. The nuclear calcium signal was sustained for several seconds followed by translocation back to the cytoplasm. Following the nuclear-to-cytoplasmic calcium translocation, total free intracellular calcium concentration decreased. The dynamic calcium cytoplasmic and nuclear localization was confirmed by laser scanning confocal microscopy coupled with the calcium-sensitive dye fluo-3 AM. To determine the source of calcium, V(1) agonist-induced (45)Ca(2+) uptake and [(3)H]IP(1) accumulation were investigated. V(1) agonist induced significant and rapid uptake of (45)Ca(2+) and a significant dose-dependent increase in [(3)H]IP(1) accumulation in cortical astrocytes. To our knowledge, this is the first documentation of a vasopressin receptor-induced calcium signaling cascade in cortical astrocytes and the first documentation of vasopressin receptor induction of nuclear calcium signaling.

Differential messenger RNA gradients in the unicellular alga Acetabularia acetabulum. Role of the cytoskeleton

The unicellular green alga Acetabularia acetabulum has proven itself to be a superior model for studies of morphogenesis because of its large size and distinctive polar morphology. The giant cell forms an elongated tube (a stalk of up to 60 mm in length), which at its apical pole makes whorls of hairs, followed by one whorl of gametophores in the shape of a cap. At its basal pole, the cell extends into a rhizoid wherein the single nucleus is positioned. In this study, we have determined the level of specific messenger RNAs in the apical, middle, and basal regions using reverse transcriptase-PCR methodology. Four mRNA classes were distinguished: those that were uniformly distributed (small subunit of Rubisco, actin-1, ADP-glucose, centrin, and alpha- and beta-tubulin), those that expressed apical/basal (calmodulin-4) or basal/apical gradients (calmodulin-2 and a Ran-G protein), and those with development-specific patterns of distribution (mitogen-activated protein kinase, actin-2, and UDP-glucose-epimerase). Restoration of the apical/basal calmodulin-4 mRNA gradient after amputation of the apical region of the cell requires the nucleus and was abolished by cytochalasin D. Accumulation of actin-1 mRNA in the vicinity of the wound set by the amputation needs, likewise, the presence of the nucleus and was also inhibited by cytochalasin. This suggests that actin microfilaments of the cytoskeleton are involved in directed transport and/or anchoring of these mRNAs.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Differential calmodulin gene expression in the rodent brain

Apparently redundant members of the calmodulin (CaM) gene family encode for the same amino acid sequence. CaM, a ubiquitous cytoplasmic calcium ion receptor, regulates the function of a variety of target molecules even in a single cell. Maintenance of the fidelity of the active CaM-target interactions in different compartments of the cell requires a rather complex control of the total cellular CaM pool comprising multiple levels of regulatory circuits. Among these mechanisms, it has long been proposed that a multigene family maximizes the regulatory potentials at the level of the gene expression. CaM genes are expressed at a particularly profound level in the mammalian central nervous system (CNS), especially in the highly polarized neurons. Thus, in the search for clear evidence of the suggested differential expression of the CaM genes, much of the research has been focused on the elements of the CNS. This review aims to give a comprehensive survey on the current understanding of this field at the level of the regulation of CaM mRNA transcription and distribution in the rodent brain. The results indicate that the CaM genes are indeed expressed in a gene-specific manner in the developing and adult brain under physiological conditions. To establish local CaM pools in distant intracellular compartments (dendrites and glial processes), local protein synthesis from differentially targeted mRNAs is also employed. Moreover, the CaM genes are controlled in a unique, gene-specific fashion when responding to certain external stimuli. Additionally, putative regulatory elements have been identified on the CaM genes and mRNAs.

Isolation and characterization of a novel calmodulin-binding protein from potato

Tuberization in potato is controlled by hormonal and environmental signals. Ca(2+), an important intracellular messenger, and calmodulin (CaM), one of the primary Ca(2+) sensors, have been implicated in controlling diverse cellular processes in plants including tuberization. The regulation of cellular processes by CaM involves its interaction with other proteins. To understand the role of Ca(2+)/CaM in tuberization, we have screened an expression library prepared from developing tubers with biotinylated CaM. This screening resulted in isolation of a cDNA encoding a novel CaM-binding protein (potato calmodulin-binding protein (PCBP)). Ca(2+)-dependent binding of the cDNA-encoded protein to CaM is confirmed by (35)S-labeled CaM. The full-length cDNA is 5 kb long and encodes a protein of 1309 amino acids. The deduced amino acid sequence showed significant similarity with a hypothetical protein from another plant, Arabidopsis. However, no homologs of PCBP are found in nonplant systems, suggesting that it is likely to be specific to plants. Using truncated versions of the protein and a synthetic peptide in CaM binding assays we mapped the CaM-binding region to a 20-amino acid stretch (residues 1216-1237). The bacterially expressed protein containing the CaM-binding domain interacted with three CaM isoforms (CaM2, CaM4, and CaM6). PCBP is encoded by a single gene and is expressed differentially in the tissues tested. The expression of CaM, PCBP, and another CaM-binding protein is similar in different tissues and organs. The predicted protein contained seven putative nuclear localization signals and several strong PEST motifs. Fusion of the N-terminal region of the protein containing six of the seven nuclear localization signals to the reporter gene beta-glucuronidase targeted the reporter gene to the nucleus, suggesting a nuclear role for PCBP.

Calmodulin binds to K-Ras, but not to H- or N-Ras, and modulates its downstream signaling

Activation of Ras induces a variety of cellular responses depending on the specific effector activated and the intensity and amplitude of this activation. We have previously shown that calmodulin is an essential molecule in the down-regulation of the Ras/Raf/MEK/extracellularly regulated kinase (ERK) pathway in cultured fibroblasts and that this is due at least in part to an inhibitory effect of calmodulin on Ras activation. Here we show that inhibition of calmodulin synergizes with diverse stimuli (epidermal growth factor, platelet-derived growth factor, bombesin, or fetal bovine serum) to induce ERK activation. Moreover, even in the absence of any added stimuli, activation of Ras by calmodulin inhibition was observed. To identify the calmodulin-binding protein involved in this process, calmodulin affinity chromatography was performed. We show that Ras and Raf from cellular lysates were able to bind to calmodulin. Furthermore, Ras binding to calmodulin was favored in lysates with large amounts of GTP-bound Ras, and it was Raf independent. Interestingly, only one of the Ras isoforms, K-RasB, was able to bind to calmodulin. Furthermore, calmodulin inhibition preferentially activated K-Ras. Interaction between calmodulin and K-RasB is direct and is inhibited by the calmodulin kinase II calmodulin-binding domain. Thus, GTP-bound K-RasB is a calmodulin-binding protein, and we suggest that this binding may be a key element in the modulation of Ras signaling.

The tumor-sensitive calmodulin-like protein is a specific light chain of human unconventional myosin X

Human calmodulin-like protein (CLP) is an epithelial-specific Ca(2+)-binding protein whose expression is strongly down-regulated in cancers. Like calmodulin, CLP is thought to regulate cellular processes via Ca(2+)-dependent interactions with specific target proteins. Using gel overlays, we identified a approximately 210-kDa protein binding specifically and in a Ca(2+)-dependent manner to CLP, but not to calmodulin. Yeast two-hybrid screening yielded a CLP-interacting clone encoding the three light chain binding IQ motifs of human "unconventional" myosin X. Pull-down experiments showed CLP binding to the IQ domain to be direct and Ca(2+)-dependent. CLP interacted strongly with IQ motif 3 (K(d) approximately 0.5 nm) as determined by surface plasmon resonance. Epitope-tagged myosin X was localized preferentially at the cell periphery in MCF-7 cells, and CLP colocalized with myosin X in these cells. Myosin X was able to coprecipitate CLP and, to a lesser extent, calmodulin from transfected COS-1 cells, indicating that CLP is a specific light chain of myosin X in vivo. Because unconventional myosins participate in cellular processes ranging from membrane trafficking to signaling and cell motility, myosin X is an attractive CLP target. Altered myosin X regulation in (tumor) cells lacking CLP may have as yet unknown consequences for cell growth and differentiation.

Ultrastructural localization of calcium deposits during in vitro culture of pig oocytes

Calcium deposits were localized using the combined oxalate-pyroantimonate technique in follicle-enclosed oocytes fixed in situ. These deposits can be observed within vacuoles, mitochondria, and on the surface of yolk granules as well as in the caryoplasm, but are absent from the endoplasmic reticulum. Isolation of the oocyte from the follicle resulted in the immediate depletion of these calcium deposits. Replenishment of these deposits started during the first 8 hr of in vitro culture of the oocyte and they were gradually replenished to the levels observed before the liberation of oocytes during in vitro maturation to the stage of metaphase II.

Regulation of a recombinant pea nuclear apyrase by calmodulin and casein kinase II

A cDNA encoding a pea nuclear apyrase was previously cloned. Overexpressions of a full-length and a truncated cDNA have been successfully expressed in Escherichia coli BL21(DE3). The resulting fusion proteins, apyrase and the C-terminus (residues 315-453) of apyrase, were used for calmodulin (CaM) binding and phosphorylation studies. Fusion protein apyrase but not the C-terminus of apyrase can be recognized by polyclonal antibody pc480. This suggested that the motif recognized by pc480 was located in the N-terminal region of apyrase. The recombinant apyrase protein also showed an activity 70 times higher than that of endogenous apyrase using ATP as a substrate. The recombinant apyrase has a preference for ATP more than other nucleoside triphosphate substrates. CaM can bind to recombinant apyrase, but not to the C-terminus of apyrase. This implies that the CaM-binding domain must be in the first 315 amino acids of the N-terminal region of apyrase. We found that one segment from residue 293 to 308 was a good candidate for the CaM-binding domain. This segment 293 FNKCKNTIRKALKLNY 308 has a basic amphiphilic-helical structure, which shows the predominance of basic residues on one side and hydrophobic residues on the other when displayed on a helical wheel plot. Using the gel mobility shift binding assay, this synthetic peptide was shown to bind to CaM, indicating that it is the CaM-binding domain. Both recombinant apyrase and the C-terminus of apyrase can be phosphorylated by a recombinant human protein kinase CKII. Phosphorylation does not affect CaM binding to recombinant apyrase. However, CaM does inhibit CKII phosphorylation of recombinant apyrase and this inhibition can be blocked by 5 mM EGTA.

The calmodulin multigene family as a unique case of genetic redundancy: multiple levels of regulation to provide spatial and temporal control of calmodulin pools?

Calmodulin (CaM) is a ubiquitous, highly conserved calcium sensor protein involved in the regulation of a wide variety of cellular events. In vertebrates, an identical CaM protein is encoded by a family of non-allelic genes, raising questions concerning the evolutionary pressure responsible for the maintenance of this apparently redundant family. Here we review the evidence that the control of the spatial and temporal availability of CaM may require multiple regulatory levels to ensure the proper localization, maintenance and size of intracellular CaM pools. Differential transcription of the CaM genes provides one level of regulation to meet tissue-specific, developmental and cell-specific needs for altered CaM levels. Post-transcriptional regulation occurs at the level of mRNA stability, perhaps dependent on alternative polyadenylation and differences in the untranslated sequences of the multiple gene transcripts. Recent evidence indicates that trafficking of specific CaM mRNAs may occur to specialized cellular locales such as the dendrites of neurons. This could allow local CaM synthesis and thereby help generate local pools of CaM. Local CaM activity may be further regulated by post-translational mechanisms such as phosphorylation or storage of CaM in a 'masked' form. The spatial resolution of CaM activity is enhanced by the limited free diffusion of CaM combined with differential affinity for and availability of target proteins. Preserving multiple CaM genes with divergent noncoding sequences may be necessary in complex organisms to ensure that the many CaM-dependent processes occur with the requisite spatial and temporal resolution. Transgenic mouse models and studies on mice carrying single and double gene 'knockouts' promise to shed further light on the role of specificity versus redundancy in the evolutionary maintenance of the vertebrate CaM multigene family.

Calmodulin regulates the disassembly of cortical F-actin in mast cells but is not required for secretion

Secretion is dependent on a rise in cytosolic Ca(2+)concentration and is associated with dramatic changes in actin organization. The actin cortex may act as a barrier between secretory vesicles and plasma membrane. Thus, disassembly of this cortex should precede late steps of exocytosis. Here we investigate regulation of both the actin cytoskeleton and secretion by calmodulin. Ca(2+), together with ATP, induces cortical F-actin disassembly in permeabilized rat peritoneal mast cells. This effect is strongly inhibited by removing endogenous calmodulin (using calmodulin inhibitory peptides), and increased by exogenous calmodulin. Neither treatment, however, affects secretion. Low concentrations ( approximately 1 microM) of a specific inhibitor of myosin light chain kinase, ML-7, prevent F-actin disassembly, but not secretion. In contrast, a myosin inhibitor affecting both conventional and unconventional myosins, BDM, decreases cortical disassembly as well as secretion. Observations of fluorescein-calmodulin, introduced into permeabilized cells, confirmed a strong (Ca(2+)-independent) association of calmodulin with the actin cortex. In addition, fluorescein-calmodulin enters the nuclei in a Ca(2+)-dependent manner. In conclusion, calmodulin promotes myosin II-based contraction of the membrane cytoskeleton, which is a prerequisite for its disassembly. The late steps of exocytosis, however, require neither calmodulin nor cortical F-actin disassembly, but may be modulated by unconventional myosin(s).

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Activin A-induced HepG2 liver cell apoptosis: involvement of activin receptors and smad proteins

A balance between cell proliferation and apoptosis is important for regulating normal liver function. Proteins of the transforming growth factor-beta superfamily are known to be important mediators of apoptosis in the liver. In this study we demonstrate that activin A potently induces apoptotic cell death in a hepatoma cell line, HepG2 cells. To determine the roles of activin receptors and downstream signaling proteins in activin A-induced apoptosis in these cells, the activin signaling pathway was analyzed using the transcription of an activin-responsive reporter gene, p3TP-Lux, as an assay. Although individual activin receptors had little effect on transcriptional activity, coexpression of an activin type I receptor and a type II receptor significantly increased both basal and activin-induced transcriptional activation, with the combination ofreceptors IB and IIB being the most potent. Similarly, expression of individual Smad proteins had only a modest effect on reporter gene activity, but the combination of Smad2 and Smad4 strongly stimulated transcription. Activin signaling induced a rapid relocation of Smad2 to the nucleus, as determined using a green fluorescence protein-Smad2 fusion protein. In contrast, green fluorescence protein-Smad4 remained localized to the cytoplasm unless it was coexpressed with Smad2. In agreement with the transcriptional response assays, overexpression or suppression of activin signaling components in HepG2 cells altered apoptosis. Overexpression of receptors IB and IIB or Smad proteins 2 and 4 stimulated apoptosis, whereas dominant negative mutant forms of the activin type IIB receptor or Smad2 blocked activin-stimulated apoptosis. These studies suggest that signaling from the cell surface to the nucleus through Smad proteins is a required component of the activin A-induced cell death process in liver cells.

Differential codes for free Ca(2+)-calmodulin signals in nucleus and cytosol

BACKGROUND

Many targets of calcium signaling pathways are activated or inhibited by binding the Ca(2+)-liganded form of calmodulin (Ca(2+)-CaM). Here, we test the hypothesis that local Ca(2+)-CaM-regulated signaling processes can be selectively activated by local intracellular differences in free Ca(2+)-CaM concentration.

RESULTS

Energy-transfer confocal microscopy of a fluorescent biosensor was used to measure the difference in the concentration of free Ca(2+)-CaM between nucleus and cytoplasm. Strikingly, short receptor-induced calcium spikes produced transient increases in free Ca(2+)-CaM concentration that were of markedly higher amplitude in the cytosol than in the nucleus. In contrast, prolonged increases in calcium led to equalization of the nuclear and cytosolic free Ca(2+)-CaM concentrations over a period of minutes. Photobleaching recovery and translocation measurements with fluorescently labeled CaM showed that equalization is likely to be the result of a diffusion-mediated net translocation of CaM into the nucleus. The driving force for equalization is a higher Ca(2+)-CaM-buffering capacity in the nucleus compared with the cytosol, as the direction of the free Ca(2+)-CaM concentration gradient and of CaM translocation could be reversed by expressing a Ca(2+)-CaM-binding protein at high concentration in the cytosol.

CONCLUSIONS

Subcellular differences in the distribution of Ca(2+)-CaM-binding proteins can produce gradients of free Ca(2+)-CaM concentration that result in a net translocation of CaM. This provides a mechanism for dynamically regulating local free Ca(2+)-CaM concentrations, and thus the local activity of Ca(2+)-CaM targets. Free Ca(2+)-CaM signals in the nucleus remain low during brief or low-frequency calcium spikes, whereas high-frequency spikes or persistent increases in calcium cause translocation of CaM from the cytoplasm to the nucleus, resulting in similar concentrations of nuclear and cytosolic free Ca(2+)-CaM.

Calmodulin binds to p21(Cip1) and is involved in the regulation of its nuclear localization

p21(Cip1), first described as an inhibitor of cyclin-dependent kinases, has recently been shown to have a function in the formation of cyclin D-Cdk4 complexes and in their nuclear translocation. The dual behavior of p21(Cip1) may be due to its association with other proteins. Different evidence presented here indicate an in vitro and in vivo interaction of p21(Cip1) with calmodulin: 1) purified p21(Cip1) is able to bind to calmodulin-Sepharose in a Ca(2+)-dependent manner, and this binding is inhibited by the calmodulin-binding domain of calmodulin-dependent kinase II; 2) both molecules coimmunoprecipitate when extracted from cellular lysates; and 3) colocalization of calmodulin and p21(Cip1) can be detected in vivo by electron microscopy immunogold analysis. The carboxyl-terminal domain of p21(Cip1) is responsible for the calmodulin interaction, since p21(145-164) peptide is also able to bind calmodulin and to compete with full-length p21(Cip1) for the calmodulin binding. Because treatment of cells with anti-calmodulin drugs decreases the nuclear accumulation of p21(Cip1), we hypothesize that calmodulin interaction with p21(Cip1) is important for p21(Cip1), and in consequence for cyclin D-Cdk4, translocation into the cell nucleus.

Synthesis and properties of some novel anti-calmodulin drugs

The preparation and properties of some novel inhibitors of calmodulin function are described. The compounds are cationic derivatives of phenyl-substituted thiazoles which inhibit the calmodulin stimulation of cyclic-AMP phosphodiesterase and are active against animal tumor cells in culture. These derivatives form the basis for the preparation of new, more potent inhibitors of calmodulin function which could take advantage of the reported elevated levels of calcium-bound calmodulin in tumor cells and show preferential anti-tumor activity.

Novel low molecular weight microtubule-associated protein-2 isoforms contain a functional nuclear localization sequence

Known high and low molecular weight (LMW) MAP2 protein isoforms result from alternative splicing of the MAP2 gene. Contrary to previous reports that MAP2 is neural-specific, we recently identified MAP2 mRNA and protein in somatic and germ cells of rat testis, and showed the predominant testicular isoform is LMW. Although cytoplasmic in neural tissue, MAP2 appeared predominantly nuclear in germ cells using immunohistochemistry. We sought to determine whether this unexpected localization was due to the inclusion of exon 10 within novel LMW MAP2 isoforms. Normally excluded from the LMW MAP2c, exon 10 harbors a putative CcN motif, comprising a nuclear localization sequence (NLS) flanked by regulatory phosphorylation sites for protein kinase CK2 and cdc2 kinase. Characterization of MAP2 mRNA in adult and immature brain and testis, by reverse transcriptase-polymerase chain reaction/Southern analysis and Northern blot, identified novel LMW forms containing exons 10 and 11, previously detected only in high molecular weight MAP2a and 2b. The MAP2 NLS targeted a large heterologous protein to the nucleus, as demonstrated using bacterially expressed MAP2-CcN-beta-galactosidase fusion protein and an in vitro nuclear import assay. Antibodies raised against the fusion protein produced a testicular immunohistochemical staining pattern correlating with MAP2 protein distribution in the nucleus of most germ cells, and precipitated both approximately 70-kDa and >220-kDa proteins recognized by the commercial MAP2-specific HM2 monoclonal antibody, supporting our hypothesis of a novel LMW MAP2 isoform. These results demonstrate the presence of a functional NLS in MAP2 and indicate that novel LMW MAP2 isoforms may be targeted to the nucleus in both neural and non-neuronal tissues.

Mechanism of Ca2+-dependent nuclear accumulation of calmodulin

The intracellular Ca2+ receptor calmodulin (CaM) coordinates responses to extracellular stimuli by modulating the activities of its various binding proteins. Recent reports suggest that, in addition to its familiar functions in the cytoplasm, CaM may be directly involved in rapid signaling between cytoplasm and nucleus. Here we show that Ca2+-dependent nuclear accumulation of CaM can be reconstituted in permeabilized cells. Accumulation was blocked by M13, a CaM antagonist peptide, but did not require cytosolic factors or an ATP regenerating system. Ca2+-dependent influx of CaM into nuclei was not blocked by inhibitors of nuclear localization signal-mediated nuclear import in either permeabilized or intact cells. Fluorescence recovery after photobleaching studies of CaM in intact cells showed that influx is a first-order process with a rate constant similar to that of a freely diffusible control molecule (20-kDa dextran). Studies of CaM efflux from preloaded nuclei in permeablized cells revealed the existence of three classes of nuclear binding sites that are distinguished by their Ca2+-dependence and affinity. At high [Ca2+], efflux was enhanced by addition of a high affinity CaM-binding protein outside the nucleus. These data suggest that CaM diffuses freely through nuclear pores and that CaM-binding proteins in the nucleus act as a sink for Ca2+-CaM, resulting in accumulation of CaM in the nucleus on elevation of intracellular free Ca2+.

Functional analysis of human Smad1: role of the amino-terminal domain

The signals originating from transforming growth factor beta/activin/bone morphogenetic proteins (BMPs) are transduced by a set of evolutionarily conserved family of Smad proteins which, upon activation, directly translocate to the nucleus where they may activate transcription. Smad proteins of different species contain conserved amino- (N) and carboxy- (C) terminal domains separated by a proline-rich linker. Human, Drosophila, and Xenopus Smad1 all have been shown to mediate the biological effects of BMP-4 in Xenopus embryos. We have investigated the functional domains of human Smad1 (hSmad1) using the Xenopus embryo system. Dorsal injection of hSmad1 RNA into the 4-cell-stage embryos results in embryonic ventralization. Since the C-terminus of Smads has been shown to mediate the transcriptional activity, whereas this activity is masked by the presence of the N-terminus, we tested the effect of a hSmad1 construct lacking the C-terminal domain [hSmad1(N)] in the Xenopus embryo system. Surprisingly, we found that hSmad1(N) not only synergizes with hSmad1 in embryonic ventralization, but induces ventralization by itself. Ectopic expression of a dominant negative BMP receptor (DN-BR) as well as neural inducers noggin and chordin induce neurogenesis in the animal cap, which is inhibited by co-expression of either hSmad1 or hSmad1(N). Ventral expression of DN-BR induces formation of a second body axis at tailbud stage, which is also prevented by hSmad1 and hSmad1(N). It has recently been reported that calmodulin interacts with the N-terminal domain of Smad proteins. We demonstrate that the ventralizing activity of hSmad1 and hSmad1(N) is markedly inhibited by calmodulin. Thus, calmodulin acts as a Smad1 inhibitor. A model is proposed to accomodate these findings.

Localization of familial benign hypercalcemia, Oklahoma variant (FBHOk), to chromosome 19q13

Calcium homeostasis by the kidneys and parathyroids is mediated by the calcium-sensing receptor (CaSR), which is located on 3q21-q24 and belongs to family C of the superfamily of G-protein coupled receptors that includes those for metabotropic glutamate, certain pheromones, and gamma-amino butyric acid (GABA-B). Inactivating CaSR mutations result in familial benign hypercalcemia (FBH), or familial hypocalciuric hypercalcemia (FHH), whereas activating mutations result in hypocalcemic hypercalciuria. However, not all FBH patients have CaSR mutations, which, together with the mapping of another FBH locus to 19p13.3, suggests that additional CaSRs or second messengers may be involved. These may be identified by positional cloning, and we therefore performed a genomewide search, using chromosome-specific sets of microsatellite polymorphisms, in an Oklahoma family with an FBH variant (FBHOk), for which linkage to 3q and 19p had been excluded. Linkage was established between FBHOk and eight chromosome 19q13 loci, with the highest LOD score, 6.67 (recombination fraction.00), obtained with D19S606. Recombinants further mapped FBHOk to a <12-cM interval flanked by D19S908 and D19S866. The calmodulin III gene is located within this interval, and DNA sequence analysis of the coding region, the 5' UTR, and part of the promoter region in an individual affected with FBHOk did not detect any abnormalities, thereby indicating that this gene is unlikely to be implicated in the etiology of FBHOk. This mapping of FBHOk to chromosome 19q13 will facilitate the identification of another CaSR or a mediator of calcium homeostasis.

Calmodulin is essential for cyclin-dependent kinase 4 (Cdk4) activity and nuclear accumulation of cyclin D1-Cdk4 during G1

Although it is known that calmodulin is involved in G1 progression, the calmodulin-dependent G1 events are not well understood. We have analyzed here the role of calmodulin in the activity, the expression, and the intracellular location of proteins involved in G1 progression. The addition of anti-calmodulin drugs to normal rat kidney cells in early G1 inhibited cyclin-dependent kinase 4 (Cdk4) and Cdk2 activities, as well as retinoblastoma protein phosphorylation. Protein levels of cdk4, cyclin D1, cyclin D2, cyclin E, p21, and p27 were not affected after CaM inhibition, whereas decreases in the amount of cyclin A and Cdc2 were observed. The decrease of Cdk4 activity was due neither to changes in its association to cyclin D1 nor to changes in the amount of p21 or p27 bound to cyclin D1-Cdk4 complexes. Calmodulin inhibition also produced a translocation of nuclear cyclin D1 and Cdk4 to the cytoplasm. This translocation could be responsible for the decreased Cdk4 activity upon calmodulin inhibition. Immunoprecipitation, calmodulin affinity chromatography, and direct binding experiments indicated that calmodulin associates with Cdk4 and cyclin D1 through a calmodulin-binding protein. The facts that Hsp90 interacts with Cdk4 and that its inhibition induced Cdk4 and cyclin D1 translocation to the cytoplasm point to Hsp90 as a good candidate for being the calmodulin-binding protein involved in the nuclear accumulation of Cdk4 and cyclin D1.

Distinct subcellular localization of calcium binding S100 proteins in human smooth muscle cells and their relocation in response to rises in intracellular calcium

Changes in cytosolic Ca2+ concentration control a wide range of cellular responses, and intracellular Ca2+-binding proteins are the key molecules to transduce Ca2+ signaling via interactions with different types of target proteins. Among these, S100 Ca2+-binding proteins, characterized by a common structural motif, the EF-hand, have recently attracted major interest due to their cell- and tissue-specific expression pattern and involvement in various pathological processes. The aim of our study was to identify the subcellular localization of S100 proteins in vascular smooth muscle cell lines derived from human aorta and intestinal smooth muscles, and in primary cell cultures derived from arterial smooth muscle tissue under normal conditions and after stimulation of the intracellular Ca2+ concentration. Confocal laser scanning microscopy was used with a specially designed colocalization software. Distinct intracellular localization of S100 proteins was observed: S100A6 was present in the sarcoplasmic reticulum as well as in the cell nucleus. S100A1 and S100A4 were found predominantly in the cytosol where they were strongly associated with the sarcoplasmic reticulum and with actin stress fibers. In contrast, S100A2 was located primarily in the cell nucleus. Using a sedimentation assay and subsequent electron microscopy after negative staining, we demonstrated that S100A1 directly interacts with filamentous actin in a Ca2+-dependent manner. After thapsigargin (1 microM) induced increase of the intracellular Ca2+ concentration, specific vesicular structures in the sarcoplasmic reticulum region of the cell were formed with high S100 protein content. In conclusion, we demonstrated a distinct subcellular localization pattern of S100 proteins and their interaction with actin filaments and the sarcoplasmic reticulum in human smooth muscle cells. The specific translocation of S100 proteins after intracellular Ca2+ increase supports the hypothesis that S100 proteins exert several important functions in the regulation of Ca2+ homeostasis in smooth muscle cells.

CALMODULIN AND CALMODULIN-BINDING PROTEINS IN PLANTS

Calmodulin is a small Ca2+-binding protein that acts to transduce second messenger signals into a wide array of cellular responses. Plant calmodulins share many structural and functional features with their homologs from animals and yeast, but the expression of multiple protein isoforms appears to be a distinctive feature of higher plants. Calmodulin acts by binding to short peptide sequences within target proteins, thereby inducing structural changes, which alters their activities in response to changes in intracellular Ca2+ concentration. The spectrum of plant calmodulin-binding proteins shares some overlap with that found in animals, but a growing number of calmodulin-regulated proteins in plants appear to be unique. Ca2+-binding and enzymatic activation properties of calmodulin are discussed emphasizing the functional linkages between these processes and the diverse pathways that are dependent on Ca2+ signaling.