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

Ca2+-Dependent and -Independent Calmodulin Binding to the Cytoplasmic Loop of Gap Junction Connexins

Ca²⁺/calmodulin (Ca²⁺/CaM) interaction with connexins (Cx) is well-established; however, the mechanistic basis of regulation of gap junction function by Ca²⁺/CaM is not fully understood. Ca²⁺/CaM is predicted to bind to a domain in the C-terminal portion of the intracellular loop (CL2) in the vast majority of Cx isoforms and for a number of Cx-s this prediction has proved correct. In this study, we investigate and characterise both Ca²⁺/CaM and apo-CaM binding to selected representatives of each of the α, β and γ connexin family to develop a better mechanistic understanding of CaM effects on gap junction function. The affinity and kinetics Ca²⁺/CaM and apo-CaM interactions of CL2 peptides of β-Cx32, γ-Cx35, α-Cx43, α-Cx45 and α-Cx57 were investigated. All five Cx CL2 peptides were found to have high affinity for Ca²⁺/CaM with dissociation constants (K_d(+Ca)) from 20 to 150 nM. The limiting rate of binding and the rates of dissociation covered a broad range. In addition, we obtained evidence for high affinity Ca²⁺-independent interaction of all five peptides with CaM, consistent with CaM remaining anchored to gap junctions in resting cells. However, for the α-Cx45 and α-Cx57 CL2 peptides, Ca²⁺-dependent association at resting [Ca²⁺] of 50-100 nM is indicated in these complexes as one of the CaM Ca²⁺ binding sites displays high affinity with K_d of 70 and 30 nM for Ca²⁺, respectively. Furthermore, complex conformational changes were observed in peptide-apo-CaM complexes with the structure of CaM compacted or stretched by the peptide in a concentration dependent manner suggesting that the CL2 domain may undergo helix-to-coil transition and/or forms bundles, which may be relevant in the hexameric gap junction. We demonstrate inhibition of gap junction permeability by Ca²⁺/CaM in a dose dependent manner, further cementing Ca²⁺/CaM as a regulator of gap junction function. The motion of a stretched CaM-CL2 complex compacting upon Ca²⁺ binding may bring about the Ca²⁺/CaM block of the gap junction pore by a push and pull action on the CL2 C-terminal hydrophobic residues of transmembrane domain 3 (TM3) in and out of the membrane.

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.

Far-infrared irradiation inhibits breast cancer cell proliferation independently of DNA damage through increased nuclear Ca2+/calmodulin binding modulated-activation of checkpoint kinase 2

Far-infrared (FIR) irradiation is reported to inhibit cell proliferation in various types of cancer cells; the underlying mechanism, however, remains unclear. We explored the molecular mechanisms using MDA-MB-231 human breast cancer cells. FIR irradiation significantly inhibited cell proliferation and colony formation compared to hyperthermal stimulus, with no alteration in cell viability. No increase in DNA fragmentation or phosphorylation of DNA damage kinases including ataxia-telangiectasia mutated kinase, ataxia telangiectasia and Rad3-related kinase, and DNA-dependent protein kinase indicated no DNA damage. FIR irradiation increased the phosphorylation of checkpoint kinase 2 (Chk2) at Thr68 (p-Chk2-Thr⁶⁸) but not that of checkpoint kinase 1 at Ser345. Increased nuclear p-Chk2-Thr⁶⁸ and Ca²⁺/CaM accumulations were found in FIR-irradiated cells, as observed in confocal microscopic analyses and cell fractionation assays. In silico analysis predicted that Chk2 possesses a Ca²⁺/calmodulin (CaM) binding motif ahead of its kinase domain. Indeed, Chk2 physically interacted with CaM in the presence of Ca²⁺, with their binding markedly pronounced in FIR-irradiated cells. Pre-treatment with a Ca²⁺ chelator significantly reversed FIR irradiation-increased p-Chk2-Thr⁶⁸ expression. In addition, a CaM antagonist or small interfering RNA-mediated knockdown of the CaM gene expression significantly attenuated FIR irradiation-increased p-Chk2-Thr⁶⁸ expression. Finally, pre-treatment with a potent Chk2 inhibitor significantly reversed both FIR irradiation-stimulated p-Chk2-Thr⁶⁸ expression and irradiation-repressed cell proliferation. In conclusion, our results demonstrate that FIR irradiation inhibited breast cancer cell proliferation, independently of DNA damage, by activating the Ca²⁺/CaM/Chk2 signaling pathway in the nucleus. These results demonstrate a novel Chk2 activation mechanism that functions irrespective of DNA damage.

Nuclear translocation of calmodulin in pathological cardiac hypertrophy originates from ryanodine receptor bound calmodulin

In cardiac myocytes Calmodulin (CaM) bound to the ryanodine receptor (RyR2) constitutes a large pool of total myocyte CaM, but the CaM-RyR2 affinity is reduced in pathological conditions. Knock-in mice expressing RyR2 unable to bind CaM also developed hypertrophy and early death. However, it is unknown whether CaM released from this RyR2-bound pool participates in pathological cardiac hypertrophy. We found that angiotensin II (AngII) or phenylephrine (PE) both cause CaM to dissociate from the RyR2 and translocate to the nucleus. To test whether this nuclear CaM accumulation depends on CaM released from RyR2, we enhanced CaM-RyR2 binding affinity (with dantrolene), or caused CaM dissociation from RyR2 (using suramin). Dantrolene dramatically reduced AngII- and PE-induced nuclear CaM accumulation. Conversely, suramin enhanced nuclear CaM accumulation. This is consistent with nuclear CaM accumulation coming largely from the CaM-RyR2 pool. CaM lacks a nuclear localization signal (NLS), but G-protein coupled receptor kinase 5 (GRK5) binds CaM, has a NLS and translocates like CaM in response to AngII or PE. Suramin also promoted GRK5 nuclear import, and caused nuclear export of histone deacetylase 5 (HDAC5). Dantrolene prevented these effects. After 2-8 weeks of pressure overload (TAC) CaM binding to RyR2 was reduced, nuclear CaM and GRK5 were both elevated and there was enhanced nuclear export of HDAC5. Stress (acute AngII or TAC) causes CaM dissociation from RyR2 and translocation to the nucleus with GRK5 with parallel HDAC5 nuclear export. Thus CaM dissociation from RyR2 may be an important step in driving pathological hypertrophic gene transcription.

Cytosolic Uptake of Large Monofunctionalized Dextrans

Dextrans are a versatile class of polysaccharides with applications that span medicine, cell biology, food science, and consumer goods. Here, we report on a new type of large monofunctionalized dextran that exhibits unusual properties: efficient cytosolic and nuclear uptake. This dextran permeates various human cell types without the use of transfection agents, electroporation, or membrane perturbation. Cellular uptake occurs primarily through active transport via receptor-mediated processes. These monofunctionalized dextrans could serve as intracellular delivery platforms for drugs or other cargos.

Memory Takes Time

Memory is an adaptation to particular temporal properties of past events, such as the frequency of occurrence of a stimulus or the coincidence of multiple stimuli. In neurons, this adaptation can be understood in terms of a hierarchical system of molecular and cellular time windows, which collectively retain information from the past. We propose that this system makes various timescales of past experience simultaneously available for future adjustment of behavior. More generally, we propose that the ability to detect and respond to temporally structured information underlies the nervous system's capacity to encode and store a memory at molecular, cellular, synaptic, and circuit levels.

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.

Distinct roles of multiple isoforms of CaMKII in signaling to the nucleus

Long-lasting synaptic changes following information acquisition are critical steps for memory. In this process, long-term potentiation (LTP) is widely considered as one of the major cellular mechanisms modifying synaptic strength. It can be classified into early phase LTP (E-LTP) and late phase LTP (L-LTP) based on its duration. Using genetically modified mice, investigators have recognized the critical role of CaMKII in E-LTP and memory. However, its function in L-LTP, which is strongly dependent on gene transcription and protein synthesis, is still unclear. In this review, we discuss how different isoforms of CaMKII are coordinated to regulate gene expression in an activity-dependent manner, and thus contribute to L-LTP and memory. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

A novel acrosomal protein, IQCF1, involved in sperm capacitation and the acrosome reaction

On the basis of the unknown tags in the mature human sperm serial analysis of gene expression library constructed by our laboratory, some transcripts were cloned, including Iqcf1 (IQ motif containing F1). To investigate the function of sperm-retained Iqcf1 in spermatogenesis and fertilization of mice, we investigated the spatial and temporal expression of IQCF1. By using the (transcription activator-like effector nuclease) strategy, Iqcf1-knockout mice were produced, and the phenotypes of the Iqcf1(-/-) mice were analyzed. The results showed that IQCF1 was localized in the acrosome of spermatozoa and spermatids; the expression of IQCF1 in testes was associated with spermatogenic capacity. The Iqcf1(-/-) mice were significantly less fertile than the wild-type mice (p = 0.0057) because of reduced sperm motility (p = 0.0094) and the acrosome reaction (AR) (p = 0.0093). In spermatozoa, IQCF1 interacted with calmodulin (CaM) and possibly participated in the tyrosine phosphorylation of sperm proteins during capacitation. In conclusion, a newly identified acrosomal protein, IQCF1, is closely related to sperm capacitation and AR; in particular, it is involved in tyrosine phosphorylation of sperm proteins through interaction with CaM. Research into the function of IQCF1 during fertilization could facilitate the investigation of the molecular mechanism of capacitation, which is unclear.

PCP4: a regulator of aldosterone synthesis in human adrenocortical tissues

Purkinje cell protein 4 (PCP4) is a calmodulin (CaM)-binding protein that accelerates calcium association and dissociation with CaM. It has been previously detected in aldosterone-producing adenomas (APA), but details on its expression and function in adrenocortical tissues have remained unknown. Therefore, we performed the immunohistochemical analysis of PCP4 in the following tissues: normal adrenal (NA; n=15), APA (n=15), cortisol-producing adenomas (n=15), and idiopathic hyperaldosteronism cases (IHA; n=5). APA samples (n=45) were also submitted to quantitative RT-PCR of PCP4, CYP11B1, and CYP11B2, as well as DNA sequencing for KCNJ5 mutations. Transient transfection analysis using PCP4 siRNA was also performed in H295R adrenocortical carcinoma cells, following ELISA analysis, and CYP11B2 luciferase assays were also performed after PCP4 vector transfection in order to study the regulation of PCP4 protein expression. In our findings, PCP4 immunoreactivity was predominantly detected in APA and in the zona glomerulosa of NA and IHA. In APA, the mRNA levels of PCP4 were significantly correlated with those of CYP11B2 (P<0.0001) and were significantly higher in cases with KCNJ5 mutation than WT (P=0.005). Following PCP4 vector transfection, CYP11B2 luciferase reporter activity was significantly higher than controls in the presence of angiotensin-II. Knockdown of PCP4 resulted in a significant decrease in CYP11B2 mRNA levels (P=0.012) and aldosterone production (P=0.011). Our results indicate that PCP4 is a regulator of aldosterone production in normal, hyperplastic, and neoplastic human adrenocortical cells.

Visualizing CaMKII and CaM activity: a paradigm of compartmentalized signaling

Calcium (Ca(2+)) has long been recognized as a crucial intracellular messenger attaining stimuli-specific cellular outcomes via localized signaling. Ca(2+)-binding proteins, such as calmodulin (CaM), and its target proteins are key to the segregation and refinement of these Ca(2+)-dependent signaling events. This review not only summarizes the recent technological advances enabling the study of subcellular Ca(2+)-CaM and Ca(2+)-CaM-dependent protein kinase (CaMKII) signaling events but also highlights the outstanding challenges in the field.

Cyclin-dependent kinase 5 is a calmodulin-binding protein that associates with puromycin-sensitive aminopeptidase in the nucleus of Dictyostelium

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase that has been implicated in a number of cellular processes. In Dictyostelium, Cdk5 localizes to the nucleus and cytoplasm, interacts with puromycin-sensitive aminopeptidase A (PsaA), and regulates endocytosis, secretion, growth, and multicellular development. Here we show that Cdk5 is a calmodulin (CaM)-binding protein (CaMBP) in Dictyostelium. Cdk5, PsaA, and CaM were all present in isolated nuclei and Cdk5 and PsaA co-immunoprecipitated with nuclear CaM. Although nuclear CaMBPs have previously been identified in Dictyostelium, the detection of CaM in purified nuclear fractions had not previously been shown. Putative CaM-binding domains (CaMBDs) were identified in Cdk5 and PsaA. Deletion of one of the two putative CaMBDs in Cdk5 ((132)LLINRKGELKLADFGLARAFGIP(154)) prevented CaM-binding indicating that this region encompasses a functional CaMBD. This deletion also increased the nuclear distribution of Cdk5 suggesting that CaM regulates the nucleocytoplasmic transport of Cdk5. A direct binding between CaM and PsaA could not be determined since deletion of the one putative CaMBD in PsaA prevented the nuclear localization of the deletion protein. Together, this study provides the first direct evidence for nuclear CaM in Dictyostelium and the first evidence in any system for Cdk5 being a CaMBP.

Heat shock-induced biphasic Ca(2+) signature and OsCaM1-1 nuclear localization mediate downstream signalling in acquisition of thermotolerance in rice (Oryza sativa L.)

We investigated heat shock (HS)-triggered Ca(2+) signalling transduced by a Ca(2+) sensor, calmodulin (CaM), linked to early transcriptome changes of HS-responsive genes in rice. We observed a biphasic [Ca(2+) ](cyt) signature in root cells that was distinct from that in epicotyl and leaf cells, which showed a monophasic response after HS. Treatment with Ca(2+) and A23187 generated an intense and sustained increase in [Ca(2+) ](cyt) in response to HS. Conversely, treatment with Ca(2+) chelator, L-type Ca(2+) channel blocker and CaM antagonist, but not intracellular Ca(2+) release inhibitor, strongly inhibited the increased [Ca(2+) ](cyt) . HS combined with Ca(2+) and A23187 accelerated the expression of OsCaM1-1 and sHSPC/N genes, which suggests that the HS-induced apoplastic Ca(2+) influx is responsible for the [Ca(2+) ](cyt) response and downstream HS signalling. In addition, the biphasic response of OsCaM1-1 in the nucleus followed the Ca(2+) signature, which may provide the information necessary to direct HS-related gene expression. Overexpression of OsCaM1-1 induced the expression of Ca(2+) /HS-related AtCBK3, AtPP7, AtHSF and AtHSP at a non-inducing temperature and enhanced intrinsic thermotolerance in transgenic Arabidopsis. Therefore, HS-triggered rapid increases in [Ca(2+) ](cyt) , together with OsCaM1-1 expression and its nuclear localization, are important in mediating downstream HS-related gene expression for the acquisition of thermotolerance in rice.

Ion channels and transporters in cancer. 1. Ion channels and cell proliferation in cancer

Progress through the cell mitotic cycle requires precise timing of the intrinsic molecular steps and tight coordination with the environmental signals that maintain a cell into the proper physiological context. Because of their great functional flexibility, ion channels coordinate the upstream and downstream signals that converge on the cell cycle machinery. Both voltage- and ligand-gated channels have been implicated in the control of different cell cycle checkpoints in normal as well as neoplastic cells. Ion channels mediate the calcium signals that punctuate the mitotic process, the cell volume oscillations typical of cycling cells, and the exocytosis of autocrine or angiogenetic factors. Other functions of ion channels in proliferation are still matter of debate. These may or may not depend on ion transport, as the channel proteins can form macromolecular complexes with growth factor and cell adhesion receptors. Direct conformational coupling with the cytoplasmic regulatory proteins is also possible. Derangement or relaxed control of the above processes can promote neoplasia. Specific types of ion channels have turned out to participate in the different stages of the tumor progression, in which cell heterogeneity is increased by the selection of malignant cell clones expressing the ion channel types that better support unrestrained growth. However, a comprehensive mechanistic picture of the functional relations between ion channels and cell proliferation is yet not available, partly because of the considerable experimental challenges offered by studying these processes in living mammalian cells. No doubt, such studies will constitute one of the most fruitful research fields for the next generation of cell physiologists.

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.

Ca2+/calmodulin-dependent transcriptional pathways: potential mediators of skeletal muscle growth and development

The loss of muscle mass with age and disuse has a significant impact on the physiological and social well-being of the aged; this is an increasingly important problem as the population becomes skewed towards older age. Exercise has psychological benefits but it also impacts on muscle protein synthesis and degradation, increasing muscle tissue volume in both young and older individuals. Skeletal muscle hypertrophy involves an increase in muscle mass and cross-sectional area and associated increased myofibrillar protein content. Attempts to understand the molecular mechanisms that underlie muscle growth, development and maintenance, have focused on characterising the molecular pathways that initiate, maintain and regenerate skeletal muscle. Such understanding may aid in improving targeted interventional therapies for age-related muscle loss and muscle wasting associated with diseases. Two major routes through which skeletal muscle development and growth are regulated are insulin-like growth factor I (IGF-I) and Ca(2+)/calmodulin-dependent transcriptional pathways. Many reviews have focused on understanding the signalling pathways of IGF-I and its receptor, which govern skeletal muscle hypertrophy. However, alternative molecular signalling pathways such as the Ca(2+)/calmodulin-dependent transcriptional pathways should also be considered as potential mediators of muscle growth. These latter pathways have received relatively little attention and the purpose herein is to highlight the progress being made in the understanding of these pathways and associated molecules: calmodulin, calmodulin kinases (CaMKs), calcineurin and nuclear factor of activated T-cell (NFAT), which are involved in skeletal muscle regulation. We describe: (1) how conformational changes in the Ca(2+) sensor calmodulin result in the exposure of binding pockets for the target proteins (CaMKs and calcineurin). (2) How Calmodulin consequently activates either the Ca(2+)/calmodulin-dependent kinases pathways (via CaMKs) or calmodulin-dependent serine/threonine phosphatases (via calcineurin). (3) How calmodulin kinases alter transcription in the nucleus through the phosphorylation, deactivation and translocation of histone deacetylase 4 (HDAC4) from the nucleus to the cytoplasm. (4) How calcineurin transmits signals to the nucleus through the dephosphorylation and translocation of NFAT from the cytoplasm to the nucleus.

An update on nuclear calcium signalling

Over the past 15 years or so, numerous studies have sought to characterise how nuclear calcium (Ca2+) signals are generated and reversed, and to understand how events that occur in the nucleoplasm influence cellular Ca2+ activity, and vice versa. In this Commentary, we describe mechanisms of nuclear Ca2+ signalling and discuss what is known about the origin and physiological significance of nuclear Ca2+ transients. In particular, we focus on the idea that the nucleus has an autonomous Ca2+ signalling system that can generate its own Ca2+ transients that modulate processes such as gene transcription. We also discuss the role of nuclear pores and the nuclear envelope in controlling ion flux into the nucleoplasm.

Calmodulin in adult mammalian skeletal muscle: localization and effect on sarcoplasmic reticulum Ca2+ release

Calmodulin is a ubiquitous Ca2+ binding protein that binds to ryanodine rectors (RyR) and is thought to modulate its activity. Here we evaluated the effects of recombinant calmodulin on the rate of occurrence and spatial properties of Ca2+ sparks as an assay of activation in saponin-permeabilized mouse myofibers. Control myofibers exhibited a time-dependent increase and subsequent decrease in spark frequency. Recombinant wild-type calmodulin prevented the time-dependent appearance of Ca2+ sparks and decreased the derived Ca2+ flux from the sarcoplasmic reticulum during a spark by approximately 37%. A recombinant Ca2+-insensitive form of calmodulin resulted in an instantaneous increase in spark frequency as well as an increase in the derived Ca2+ flux by approximately 24%. Endogenous calmodulin was found to primarily localize to the Z-line. Surprisingly, removal of endogenous calmodulin did not alter the time dependence of Ca2+ spark appearance. These results indicate that calmodulin may not be essential for RyR1-dependent Ca2+ release in adult mammalian skeletal muscle.

Subcellular Distribution of Thymosin beta4

The localization of Oregon Green cadaverine-labeled thymosin beta(4), its fragments, and variants was investigated in cytoplasm-depleted A431 cells and in microinjected cells without and with fixation. The studied thymosin beta(4) variants included substitutions of the lysine residues within the basic cluster (14-KSKLKK-19) and the actin-binding motif (17-LKKTETQ-23). In contrast to Oregon Green cadaverine, none of the variants or fragments of thymosin beta(4) could pass the intact nuclear pore of cytoplasm-depleted cells and were hence excluded from the nucleus. However, an equal distribution of all thymosin beta(4) variants was observed in living cells. The nuclear localization is neither dependent on the actin-binding ability of thymosin beta(4) nor on its basic lysine cluster. The equal distribution of the beta-thymosins, the ability of the fragments thymosin beta(4)(1-26) and beta(4)(27-43) to enter the nucleus in intact cells immediately after injection, and their exclusion from cytoplasm-depleted nuclei make it unlikely that they are transported by a single transport protein. A passive but regulated diffusion could explain the described ability of thymosin beta(4) to shuttle into the nucleus.

The Regulation of Nuclear Membrane Permeability by Ca2+ Signaling: A Tightly Regulated Pore or a Floodgate?

The nuclear pore complex functions both to separate and to connect the nucleus and the cytoplasm. Minute-to-minute changes in gene expression depend on rapid translocation of transcription factors and other regulatory proteins from the cytosol into the nucleus. However, a controversy exists as to whether cell signaling allows large molecules to enter the nucleus through tightly regulated facilitated transport or by the opening of a floodgate. A recent report suggesting that some hormones increase nuclear permeability through changes in intracellular Ca(2+) concentration has reignited this debate. Here, I consider both the basic permeability of the nuclear membrane under resting conditions and the effects of Ca(2+) on the permeability of the nuclear pore. I discuss facilitated transport through the nuclear pore complex, with particular attention to the nuclear transport of Ca(2+)-CaM signaling complexes. Finally, I weigh the arguments in favor of a generic increase in permeability versus stimulation of facilitated transport as possible mechanisms for mediating cell signaling to the nucleus.

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.

Hormonal regulation of nuclear permeability

Transport into the nucleus is critical for regulation of gene transcription and other intranuclear events. Passage of molecules into the nucleus depends in part upon their size and the presence of appropriate targeting sequences. However, little is known about the effects of hormones or their second messengers on transport across the nuclear envelope. We used localized, two-photon activation of a photoactivatable green fluorescent protein to investigate whether hormones, via their second messengers, could alter nuclear permeability. Vasopressin and other hormones that increase cytosolic Ca2+ and activate protein kinase C increased permeability across the nuclear membrane of SKHep1 liver cells in a rapid unidirectional manner. An increase in cytosolic Ca2+ was both necessary and sufficient for this process. Furthermore, localized photorelease of caged Ca2+ near the nuclear envelope resulted in a local increase in nuclear permeability. Neither activation nor inhibition of protein kinase C affected nuclear permeability. These findings provide evidence that hormones linking to certain G protein-coupled receptors increase nuclear permeability via cytosolic Ca2+. Short term regulation of nuclear permeability may provide a novel mechanism by which such hormones permit transcription factors and other regulatory molecules to enter the nucleus, thereby regulating gene transcription in target cells.

S100A16, a novel calcium-binding protein of the EF-hand superfamily

S100A16 protein is a new and unique member of the EF-hand Ca(2+)-binding proteins. S100 proteins are cell- and tissue-specific and are involved in many intra- and extracellular processes through interacting with specific target proteins. In the central nervous system S100 proteins are implicated in cell proliferation, differentiation, migration, and apoptosis as well as in cognition. S100 proteins became of major interest because of their close association with brain pathologies, for example depression or Alzheimer's disease. Here we report for the first time the purification and biochemical characterization of human and mouse recombinant S100A16 proteins. Flow dialysis revealed that both homodimeric S100A16 proteins bind two Ca(2+) ions with the C-terminal EF-hand of each subunit, the human protein exhibiting a 2-fold higher affinity. Trp fluorescence variations indicate conformational changes in the orthologous proteins upon Ca(2+) binding, whereas formation of a hydrophobic patch, implicated in target protein recognition, only occurs in the human S100A16 protein. In situ hybridization analysis and immunohistochemistry revealed a widespread distribution in the mouse brain. Furthermore, S100A16 expression was found to be astrocyte-specific. Finally, we investigated S100A16 intracellular localization in human glioblastoma cells. The protein was found to accumulate within nucleoli and to translocate to the cytoplasm in response to Ca(2+) stimulation.

Importin-mediated nuclear transport in neurons

The polarized morphology of neurons poses a particular challenge to intracellular signal transduction. Local signals generated at distal sites must be retrogradely transported to the nucleus to produce persistent changes in neuronal function. Such communication of signals between distal neuronal compartments and the nucleus occurs during axon guidance, synapse formation, synaptic plasticity and following neuronal injury. Recent studies have begun to delineate a role for the active nuclear import pathway in transporting signals from axons and dendrites to the nucleus. In this pathway, soluble cargo proteins are recognized by nuclear transport carriers, called importins, which mediate their translocation from the cytoplasm into the nucleus. In neurons, importins might serve an additional function by carrying signals from distal sites to the soma.

The calmodulin-binding site of sphingosine kinase and its role in agonist-dependent translocation of sphingosine kinase 1 to the plasma membrane

Sphingosine kinases catalyze the formation of sphingosine 1-phosphate, a bioactive lipid involved in many aspects of cellular regulation, including the fundamental biological processes of cell growth and survival. A diverse range of cell agonists induce activation of human sphingosine kinase 1 (hSK1) and, commonly, its translocation to the plasma membrane. Although the activation of hSK1 in response to at least some agonists occurs directly via its phosphorylation at Ser225 by ERK1/2, many aspects governing the regulation of this phosphorylation and subsequent translocation remain unknown. Here, in an attempt to understand some of these processes, we have examined the known interaction of hSK1 with calmodulin (CaM). By using a combination of limited proteolysis, peptide interaction analysis, and site-directed mutagenesis, we have identified that the CaM-binding site of hSK1 resides in the region spanned by residues 191-206. Specifically, Phe197 and Leu198 are critically involved in the interaction because a version of hSK1 incorporating mutations of both Phe197 --> Ala and Leu198 --> Gln failed to bind CaM. We have also shown for the first time that human sphingosine kinase 2 (hSK2) binds CaM, and does so via a CaM binding region that is conserved with hSK1 because comparable mutations in hSK2 also ablate CaM binding to this protein. By using the CaM-binding-deficient version of hSK1, we have begun to elucidate the role of CaM in hSK1 regulation by demonstrating that disruption of the CaM-binding site ablates agonist-induced translocation of hSK1 from the cytoplasm to the plasma membrane, while having no effect on hSK1 phosphorylation and catalytic activation.

A proposed mechanism to explain the stimulatory effect of melatonin on antioxidative enzymes

Melatonin, the main secretory product of the pineal gland, is known to collaborate against oxidative stress within cells, but its mechanism of action in terms of stimulating antioxidant enzymes remains unclear. Herein, we propose that melatonin modulates antioxidant enzyme activities via its interaction with calmodulin, which in turn inhibits downstream processes that lead to the inactivation of nuclear RORalpha melatonin receptor. Eventually, this nuclear transcription factor downregulates NF-kappaB-induced antioxidant enzyme expression. Therefore, the increment in antioxidant enzyme activities induced by melatonin involves the inhibition of the RORalpha pathway. Thus, in addition to its direct free radical scavenging activities, melatonin has important actions in oxidative defense by stimulating enzymes which metabolize free radicals and radical products to innocuous metabolites.

The adaptor Grb7 is a novel calmodulin-binding protein: functional implications of the interaction of calmodulin with Grb7

We demonstrate using Ca2+-dependent calmodulin (CaM)-affinity chromatography and overlay with biotinylated CaM that the adaptor proteins growth factor receptor bound (Grb)7 and Grb7V (a naturally occurring variant lacking the Src homology 2 (SH2) domain) are CaM-binding proteins. Deletion of an amphiphilic basic amino-acid sequence (residues 243-256) predicted to form an alpha-helix located in the proximal region of its pleckstrin homology (PH) domain demonstrates the location of the CaM-binding domain. This site is identical in human and rodents Grb7, and shares great homology with similar regions of Grb10 and Grb14, and the Mig10 protein from Caenorhabditis elegans. We show that Grb7 and Grb7V are present in the cytosol and bound to membranes, while the deletion mutants (Grb7Delta and Grb7VDelta) have less capacity to be associated to membranes. Grb7Delta maintains in part the capacity to bind phosphoinositides, and CaM competes for phosphoinositide binding. Activation of ErbB2 by heregulin beta1 decreases the pool of Grb7 associated to membranes. The cell-permeable CaM antagonist W7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide), but not the CaM-dependent protein kinase II inhibitor KN93, prevents this effect. Highly specific cell-permeable CaM inhibitory peptides decrease the association of Grb7 to membranes. This suggests that CaM regulates the intracellular mobilization of Grb7 in living cells. Direct interaction between enhanced yellow fluorescent protein (EYFP)-Grb7 and enhanced cyan fluorescent protein (ECFP)-CaM chimeras at the plasma membrane of living cells was demonstrated by fluorescence resonance energy transfer (FRET). The FRET signal dramatically decreased in cells loaded with a cell-permeable Ca2+ chelator, and was significantly attenuated when enhanced yellow fluorescent protein-Grb7 chimera (EYFP-Grb7)Delta instead of EYFP-Grb7 was used. Finally, we show that conditioned media from cells transiently transfected with Grb7Delta and Grb7VDelta lost its angiogenic activity, in contrast to those from cells transiently transfected with their wild-type counterparts.