Mechanism regulating nuclear calcium signalingThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

The basis of nuclear phospholipase C in cell proliferation

Ca²⁺ is a highly versatile intracellular signal that regulates many biological processes such as cell death and proliferation. Broad Ca²⁺-signaling machinery is used to assemble signaling systems with a precise spatial and temporal resolution to achieve this versatility. Ca²⁺-signaling components can be organized in different regions of the cell and local increases in Ca²⁺ within the nucleus can regulate different cellular functions from the increases in cytosolic Ca²⁺. However, the mechanisms and pathways that promote localized increases in Ca²⁺ levels in the nucleus are still under investigation. This review presents evidence that the nucleus has its own Ca²⁺ stores and signaling machinery, which modulate processes such as cell proliferation and tumor growth. We focus on what is known about the functions of nuclear Phospholipase C (PLC) in the generation of nuclear Ca²⁺ transients that are involved in cell proliferation.

Nuclear pore complex tethers to the cytoskeleton

The nuclear envelope is tethered to the cytoskeleton. The best known attachments of all elements of the cytoskeleton are via the so-called LINC complex. However, the nuclear pore complexes, which mediate the transport of soluble and membrane bound molecules, are also linked to the microtubule network, primarily via motor proteins (dynein and kinesins) which are linked, most importantly, to the cytoplasmic filament protein of the nuclear pore complex, Nup358, by the adaptor BicD2. The evidence for such linkages and possible roles in nuclear migration, cell cycle control, nuclear transport and cell architecture are discussed.

A C-terminally truncated mouse Best3 splice variant targets and alters the ion balance in lysosome-endosome hybrids and the endoplasmic reticulum

The Bestrophin family has been characterized as Cl(-) channels in mammals and Na(+) channels in bacteria, but their exact physiological roles remian unknown. In this study, a natural C-terminally truncated variant of mouse Bestrophin 3 (Best3V2) expression in myoblasts and muscles is demonstrated. Unlike full-length Best3, Best3V2 targets the two important intracellular Ca stores: the lysosome and the ER. Heterologous overexpression leads to lysosome swelling and renders it less acidic. Best3V2 overexpression also results in compromised Ca(2+) release from the ER. Knocking down endogenous Best3 expression in myoblasts makes these cells more excitable in response to Ca(2+) mobilizing reagents, such as caffeine. We propose that Best3V2 in myoblasts may work as a tuner to control Ca(2+) release from intracellular Ca(2+) stores.

The wavy Mutation Maps to the Inositol 1,4,5-Trisphosphate 3-Kinase 2 (IP3K2) Gene of Drosophila and Interacts with IP3R to Affect Wing Development

Inositol 1,4,5-trisphosphate (IP3) regulates a host of biological processes from egg activation to cell death. When IP3-specific receptors (IP3Rs) bind to IP3, they release calcium from the ER into the cytoplasm, triggering a variety of cell type- and developmental stage-specific responses. Alternatively, inositol polyphosphate kinases can phosphorylate IP3; this limits IP3R activation by reducing IP3 levels, and also generates new signaling molecules altogether. These divergent pathways draw from the same IP3 pool yet cause very different cellular responses. Therefore, controlling the relative rates of IP3R activation vs. phosphorylation of IP3 is essential for proper cell functioning. Establishing a model system that sensitively reports the net output of IP3 signaling is crucial for identifying the controlling genes. Here we report that mutant alleles of wavy (wy), a classic locus of the fruit fly Drosophila melanogaster, map to IP3 3-kinase 2 (IP3K2), a member of the inositol polyphosphate kinase gene family. Mutations in wy disrupt wing structure in a highly specific pattern. RNAi experiments using GAL4 and GAL80(ts) indicated that IP3K2 function is required in the wing discs of early pupae for normal wing development. Gradations in the severity of the wy phenotype provide high-resolution readouts of IP3K2 function and of overall IP3 signaling, giving this system strong potential as a model for further study of the IP3 signaling network. In proof of concept, a dominant modifier screen revealed that mutations in IP3R strongly suppress the wy phenotype, suggesting that the wy phenotype results from reduced IP4 levels, and/or excessive IP3R signaling.

Nuclear membrane R-type calcium channels mediate cytosolic ET-1-induced increase of nuclear calcium in human vascular smooth muscle cells

The objective of this work was to verify whether, as in the case of the plasma membrane of human vascular smooth muscle cells (hVSMCs), cytosolic ET-1-induced increase of nuclear calcium is mediated via the activation of calcium influx through the steady-state R-type calcium channel. Pharmacological tools to identify the R-type calcium channels, as well as real 3-D confocal microscopy imaging techniques coupled to calcium fluorescent probes, were used to study the effect of cytosolic ET-1 on nuclear calcium in isolated nuclei of human hepatocytes and plasma membrane perforated hVSMCs. Our results showed that pre-treatment with pertussis toxin (PTX) or cholera toxin (CTX) prevented cytosolic ET-1 (10−9 mol/L) from inducing a sustained increase in nuclear calcium. Furthermore, the L-type calcium channel blocker nifedipine did not prevent cytosolic ET-1 from inducing an increase in nuclear calcium, as opposed to the dual L- and R-type calcium channel blocker isradipine (PN200-110) (in the presence of nifedipine). In conclusion, the preventative effect with PTX and CTX, and the absence of an effect with nifedipine, as well as the blockade by isradipine on cytosolic ET-1-induced increase in nuclear calcium, suggest that this nuclear calcium influx in hVSMCs is due to activation of the steady-state R-type calcium channel. The sarcolemmal and nuclear membrane R-type calcium channels in hVSMCs are involved in ET-1 modulation of vascular tone in physiology and pathology.

Calcium and IP3 dynamics in cardiac myocytes: experimental and computational perspectives and approaches

Calcium plays a crucial role in excitation-contraction coupling (ECC), but it is also a pivotal second messenger activating Ca²⁺-dependent transcription factors in a process termed excitation-transcription coupling (ETC). Evidence accumulated over the past decade indicates a pivotal role of inositol 1,4,5-trisphosphate receptor (IP₃R)-mediated Ca²⁺ release in the regulation of cytosolic and nuclear Ca²⁺ signals. IP₃ is generated by stimulation of plasma membrane receptors that couple to phospholipase C (PLC), liberating IP₃ from phosphatidylinositol 4,5-bisphosphate (PIP₂). An intriguing aspect of IP₃ signaling is the presence of the entire PIP₂-PLC-IP₃ signaling cascade as well as the presence of IP₃Rs at the inner and outer membranes of the nuclear envelope (NE) which functions as a Ca²⁺ store. The observation that the nucleus is surrounded by its own putative Ca²⁺ store raises the possibility that nuclear IP₃-dependent Ca²⁺ release plays a critical role in ETC. This provides a potential mechanism of regulation that acts locally and autonomously from the global cytosolic Ca²⁺ signal underlying ECC. Moreover, there is evidence that: (i) the sarcoplasmic reticulum (SR) and NE are a single contiguous Ca²⁺ store; (ii) the nuclear pore complex is the major gateway for Ca²⁺ and macromolecules to pass between the cytosol and the nucleoplasm; (iii) the inner membrane of the NE hosts key Ca²⁺ handling proteins including the Na⁺/Ca²⁺ exchanger (NCX)/GM1 complex, ryanodine receptors (RyRs), nicotinic acid adenine dinucleotide phosphate receptors (NAADPRs), Na⁺/K⁺ ATPase, and Na⁺/H⁺ exchanger. Thus, it appears that the nucleus represents a Ca²⁺ signaling domain equipped with its own ion channels and transporters that allow for complex local Ca²⁺ signals. Many experimental and modeling approaches have been used for the study of intracellular Ca²⁺ signaling but the key to the understanding of the dual role of Ca²⁺ mediating ECC and ECT lays in quantitative differences of local [Ca²⁺] in the nuclear and cytosolic compartment. In this review, we discuss the state of knowledge regarding the origin and the physiological implications of nuclear Ca²⁺ transients in different cardiac cell types (adult atrial and ventricular myocytes) as well as experimental and mathematical approaches to study Ca²⁺ and IP₃ signaling in the cytosol and nucleus. In particular, we focus on the concept that highly localized Ca²⁺ signals are required to translocate and activate Ca²⁺-dependent transcription factors (e.g., nuclear factor of activated T-cells, NFAT; histone deacetylase, HDAC) through phosphorylation/dephosphorylation processes.

Update on the Angiotensin converting enzyme 2-Angiotensin (1-7)-MAS receptor axis: fetal programing, sex differences, and intracellular pathways

The renin-angiotensin-system (RAS) constitutes an important hormonal system in the physiological regulation of blood pressure. Indeed, dysregulation of the RAS may lead to the development of cardiovascular pathologies including kidney injury. Moreover, the blockade of this system by the inhibition of angiotensin converting enzyme (ACE) or antagonism of the angiotensin type 1 receptor (AT1R) constitutes an effective therapeutic regimen. It is now apparent with the identification of multiple components of the RAS that the system is comprised of different angiotensin peptides with diverse biological actions mediated by distinct receptor subtypes. The classic RAS can be defined as the ACE-Ang II-AT1R axis that promotes vasoconstriction, sodium retention, and other mechanisms to maintain blood pressure, as well as increased oxidative stress, fibrosis, cellular growth, and inflammation in pathological conditions. In contrast, the non-classical RAS composed of the ACE2-Ang-(1-7)-Mas receptor axis generally opposes the actions of a stimulated Ang II-AT1R axis through an increase in nitric oxide and prostaglandins and mediates vasodilation, natriuresis, diuresis, and oxidative stress. Thus, a reduced tone of the Ang-(1-7) system may contribute to these pathologies as well. Moreover, the non-classical RAS components may contribute to the effects of therapeutic blockade of the classical system to reduce blood pressure and attenuate various indices of renal injury. The review considers recent studies on the ACE2-Ang-(1-7)-Mas receptor axis regarding the precursor for Ang-(1-7), the intracellular expression and sex differences of this system, as well as an emerging role of the Ang1-(1-7) pathway in fetal programing events and cardiovascular dysfunction.

Calcium signaling in the liver

Intracellular free Ca(2+) ([Ca(2+)]i) is a highly versatile second messenger that regulates a wide range of functions in every type of cell and tissue. To achieve this versatility, the Ca(2+) signaling system operates in a variety of ways to regulate cellular processes that function over a wide dynamic range. This is particularly well exemplified for Ca(2+) signals in the liver, which modulate diverse and specialized functions such as bile secretion, glucose metabolism, cell proliferation, and apoptosis. These Ca(2+) signals are organized to control distinct cellular processes through tight spatial and temporal coordination of [Ca(2+)]i signals, both within and between cells. This article will review the machinery responsible for the formation of Ca(2+) signals in the liver, the types of subcellular, cellular, and intercellular signals that occur, the physiological role of Ca(2+) signaling in the liver, and the role of Ca(2+) signaling in liver disease.

The many faces of calmodulin in cell proliferation, programmed cell death, autophagy, and cancer

Calmodulin (CaM) is a ubiquitous Ca(2+) receptor protein mediating a large number of signaling processes in all eukaryotic cells. CaM plays a central role in regulating a myriad of cellular functions via interaction with multiple target proteins. This review focuses on the action of CaM and CaM-dependent signaling systems in the control of vertebrate cell proliferation, programmed cell death and autophagy. The significance of CaM and interconnected CaM-regulated systems for the physiology of cancer cells including tumor stem cells, and processes required for tumor progression such as growth, tumor-associated angiogenesis and metastasis are highlighted. Furthermore, the potential targeting of CaM-dependent signaling processes for therapeutic use is discussed.

Biochemical Evidence for a Putative Inositol 1,3,4,5-Tetrakisphosphate Receptor in the Olfactory System of Atlantic Salmon (Salmo salar)

Olfactory receptor neurons in Atlantic salmon (Salmo salar) appear to use a phosphoinositide-directed phospholipase C (PLC) in odorant signal transduction. The consequences of odor-activated PLC depend on its product, inositol 1,4,5-trisphosphate (IP₃). Therefore, a plasma membrane rich (PMR) fraction, previously characterized from salmon olfactory rosettes, was used to study binding sites for IP₃ and its phosphorylation product, inositol 1,3,4,5-tetrakisphosphate (IP₄). Binding sites for IP₃ were present at the lower limit for detection in the PMR fraction but were abundant in a microsomal fraction. Binding sites for IP₄ were abundant in the PMR fraction and thus colocalized in the same subcellular fraction with odorant receptors for amino acids and bile acids. Binding of IP₄ was saturable and high affinity (K_d = 83 nM). The rank order for potency of inhibition of IP₄ by other inositol polyphosphates (InsP_x) followed the phosphorylation number with InsP₆ > InsP₅ > other InsP₄ isomers > InsP₃ isomers > InsP₂ isomers, with the latter showing no activity. The consequences of PLC activity in this system may be dictated in part by a putative receptor for IP₄.

Calcium regulation of nucleocytoplasmic transport

Bidirectional trafficking of macromolecules between the cytoplasm and the nucleus is mediated by the nuclear pore complexes (NPCs) embedded in the nuclear envelope (NE) of eukaryotic cell. The NPC functions as the sole pathway to allow for the passive diffusion of small molecules and the facilitated translocation of larger molecules. Evidence shows that these two transport modes and the conformation of NPC can be regulated by calcium stored in the lumen of nuclear envelope and endoplasmic reticulum. However, the mechanism of calcium regulation remains poorly understood. In this review, we integrate data on the observations of calciumregulated structure and function of the NPC over the past years. Furthermore, we highlight challenges in the measurements of dynamic conformational changes and transient transport kinetics in the NPC. Finally, an innovative imaging approach, single-molecule superresolution fluorescence microscopy, is introduced and expected to provide more insights into the mechanism of calcium-regulated nucleocytoplasmic transport.

Human inositol 1,4,5-trisphosphate 3-kinase isoform B (IP3KB) is a nucleocytoplasmic shuttling protein specifically enriched at cortical actin filaments and at invaginations of the nuclear envelope

Recent studies have shown that inositol 1,4,5-trisphosphate 3-kinase isoform B (IP3KB) possesses important roles in the development of immune cells. IP3KB can be targeted to multiple cellular compartments, among them nuclear localization and binding in close proximity to the plasma membrane. The B isoform is the only IP3K that is almost ubiquitously expressed in mammalian cells. Detailed mechanisms of its targeting regulation will be important in understanding the role of Ins(1,4,5)P(3) phosphorylation on subcellular calcium signaling and compartment-specific initiation of pathways leading to regulatory active higher phosphorylated inositol phosphates. Here, we identified an exportin 1-dependent nuclear export signal ((134)LQRELQNVQV) and characterized the amino acids responsible for nuclear localization of IP3KB ((129)RKLR). These two targeting domains regulate the amount of nuclear IP3KB in cells. We also demonstrated that the localization of IP3KB at the plasma membrane is due to its binding to cortical actin structures. Intriguingly, all three of these targeting activities reside in one small polypeptide segment (amino acids 104-165), which acts as a multitargeting domain (MTD). Finally, a hitherto unknown subnuclear localization of IP3KB could be demonstrated in rapidly growing H1299 cells. IP3KB is specifically enriched at nuclear invaginations extending perpendicular between the apical and basal surface of the nucleus of these flat cells. Such nuclear invaginations are known to be involved in Ins(1,4,5)P(3)-mediated Ca(2+) signaling of the nucleus. Our findings indicate that IP3KB not only regulates cytoplasmic Ca(2+) signals by phosphorylation of subplasmalemmal and cytoplasmic Ins(1,4,5)P(3) but may also be involved in modulating nuclear Ca(2+) signals generated from these nuclear envelope invaginations.

Nuclear membrane receptors for ET-1 in cardiovascular function

Plasma membrane endothelin type A (ETA) receptors are internalized and recycled to the plasma membrane, whereas endothelin type B (ETB) receptors undergo degradation and subsequent nuclear translocation. Recent studies show that G protein-coupled receptors (GPCRs) and ion transporters are also present and functional at the nuclear membranes of many cell types. Similarly to other GPCRs, ETA and ETB are present at both the plasma and nuclear membranes of several cardiovascular cell types, including human cardiac, vascular smooth muscle, endocardial endothelial, and vascular endothelial cells. The distribution and density of ETARs in the cytosol (including the cell membrane) and the nucleus (including the nuclear membranes) differ between these cell types. However, the localization and density of ET-1 and ETB receptors are similar in these cell types. The extracellular ET-1-induced increase in cytosolic ([Ca]c) and nuclear ([Ca]n) free Ca2+ is associated with an increase of cytosolic and nuclear reactive oxygen species. The extracellular ET-1-induced increase of [Ca]c and [Ca]n as well as intracellular ET-1-induced increase of [Ca]n are cell-type dependent. The type of ET-1 receptor mediating the extracellular ET-1-induced increase of [Ca]c and [Ca]n depends on the cell type. However, the cytosolic ET-1-induced increase of [Ca]n does not depend on cell type. In conclusion, nuclear membranes' ET-1 receptors may play an important role in overall ET-1 action. These nuclear membrane ET-1 receptors could be targets for a new generation of antagonists.

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.

Nuclear localization of vasoactive intestinal peptide (VIP) receptors in human breast cancer

Vasoactive intestinal peptide (VIP) and its receptors (VPACs) are involved in proliferation, survival, and differentiation in human breast cancer cells. Its mechanism of action is traditionally thought to be through specific plasma membrane receptors. There is compelling evidence for a novel intracrine mode of genomic regulation by G-protein-coupled receptors (GPCRs) that implies both endocytosis and nuclear translocation of peripheral GPCR and/or the activation of nuclear-located GPCRs by endogenously-produced, non-secreted ligands. Regarding to VPAC receptors, which are GPCRs, there is only a report suggesting them as a dynamic system for signaling from plasma membrane and nuclear membrane complex. In this study, we show that VPAC(1) receptor is localized in cell nuclear fraction whereas VPAC(2) receptor presents an extranuclear localization and its protein expression is lower than that of VPAC(1) receptor in human breast tissue samples. Both receptors as well as VIP are overexpressed in breast cancer as compared to non-tumor tissue. Moreover, we report the markedly nuclear localization of VPAC(1) receptors in estrogen-dependent (T47D) and independent (MDA-MB-468) human breast cancer cell lines. VPAC(1) receptors are functional in plasma membrane and nucleus as shown by VIP stimulation of cAMP production in both cell lines. In addition, VIP increases its own intracellular and extracellular levels, and could be involved in the regulation of VPAC(1)-receptor traffic from the plasma membrane to the nucleus. These results support new concepts on function and regulation of nuclear GPCRs which could have an impact on development of new therapeutic drugs.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Lipid Mediators in the Neural Cell Nucleus: Their Metabolism, Signaling, and Association with Neurological Disorders

Lipid mediators are important endogenous regulators of neural cell proliferation, differentiation, oxidative stress, inflammation, and apoptosis. They originate from enzymic degradation of glycerophospholipids, sphingolipids, and cholesterol by phospholipases, sphingomyelinases, and cytochrome P450 hydroxylases, respectively. Arachidonic acid-derived lipid mediators are called eicosanoids. Eicosanoids have emerged as key regulators of cell proliferation, differentiation, oxidative stress, and neuroinflammation. Another arachidonic acid-derived lipid mediator is lipoxin. Eicosanoids have proinflammatory effects, whereas lipoxins produce antiinflammatrory effects. The crossponding lipid mediators of docosahexaenoic acid metabolism are named docosanoids. They include resolvins, protectins, and neuroprotectins. Docosanoids produce antioxidant, anti-inflammatory, and antiapoptotic effects in the brain tissue. Other glycerophospholipid-derived lipid mediators are platelet-activating factor, lysophosphatidic acid, and endocannabinoids. Degradation of sphingolipids also results in the generation of sphingolipid-derived lipid mediators. Sphingolipid-derived lipid mediators are ceramide, ceramide 1-phosphate, sphingosine, and sphingosine 1-phosphate. They mediate cellular differentiation, cell growth, and apoptosis. Similarly, cholesterol-derived lipid mediators hydroxycholesterol and oxycholesterol produce apoptosis. Most of these mediators originate from the plasma membrane. The nucleus has its own set of enzymes and lipid mediators that originate from the nuclear envelope and matrix. The purpose of this commentary is to describe basic and clinical information on lipid mediators in 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.

Calcium homeostasis in plant cell nuclei

In plant cells, calcium-based signaling pathways are involved in a large array of biological processes, including cell division, polarity, growth, development and adaptation to changing biotic and abiotic environmental conditions. Free calcium changes are known to proceed in a nonstereotypical manner and produce a specific signature, which mirrors the nature, strength and frequency of a stimulus. The temporal aspects of calcium signatures are well documented, but their vectorial aspects also have a profound influence on biological output. Here, we will focus on the regulation of calcium homeostasis in the nucleus. We will discuss data and present hypotheses suggesting that, while interacting with other organelles, the nucleus has the potential to generate and regulate calcium signals on its own.

Crosstalk between calcium and redox signaling: from molecular mechanisms to health implications

Studies done many years ago established unequivocally the key role of calcium as a universal second messenger. In contrast, the second messenger roles of reactive oxygen and nitrogen species have emerged only recently. Therefore, their contributions to physiological cell signaling pathways have not yet become universally accepted, and many biological researchers still regard them only as cellular noxious agents. Furthermore, it is becoming increasingly apparent that there are significant interactions between calcium and redox species, and that these interactions modify a variety of proteins that participate in signaling transduction pathways and in other fundamental cellular functions that determine cell life or death. This review article addresses first the central aspects of calcium and redox signaling pathways in animal cells, and continues with the molecular mechanisms that underlie crosstalk between calcium and redox signals under a number of physiological or pathological conditions. To conclude, the review focuses on conditions that, by promoting cellular oxidative stress, lead to the generation of abnormal calcium signals, and how this calcium imbalance may cause a variety of human diseases including, in particular, degenerative diseases of the central nervous system and cardiac pathologies.

Expression of endogenous nuclear bradykinin B2 receptors mediating signaling in immediate early gene activation

Bradykinin (BK) represents a pro-inflammatory mediator that partakes in many inflammatory diseases. The mechanism of action of BK is thought to be primarily mediated by specific cell surface membrane B2 receptors (B2Rs). Some evidence has suggested, however, the existence of an intracellular/nuclear B2R population. Whether these receptors are functional and contribute to BK signaling remains to be determined. In this study, by mean of Western blotting, 3D-confocal microscopy, receptor autoradiography and radioligand binding analysis, we showed that plasma membrane and highly purified nuclei from isolated rat hepatocytes contain specific B2R that bind BK. The results depicting B2R nuclear expression in isolated nuclear organelles were reproduced in situ on hepatic sections by immunogold labeling and transmission electron microscopy. Functional tests on single nuclei, by means of confocal microscopy and the calcium-sensitive probe fluo-4AM, showed that BK induces concentration-dependent transitory mobilization of nucleoplasmic calcium; these responses were blocked by B2R antagonist HOE 140, not by the B1R antagonist R954 and, were also found in wild-type C57/Bl6 mice, but not in B2R-KO mice. In isolated nuclei, BK elicited activation/phosphorylation of Akt, acetylation of histone H3 and ensuing pro-inflammatory iNOS gene induction as determined by Western blot and RT-PCR. ChIP assay confirmed binding of acetylated-histone H3 complexes, but not B2R, to promoter region of iNOS gene suggesting that B2R-mediated gene expression is bridged with accessory downstream effectors. This study discloses a previously undescribed mechanism in BK-induced transcriptional events, via intracrine B2R-mediated signaling, occurring in rat autologous hepatic cells.

Redox control of calcium channels: from mechanisms to therapeutic opportunities

Calcium plays an integral role in cellular function. It is a well-recognized second messenger necessary for signaling cellular responses, but in excessive amounts can be deleterious to function, causing cell death. The main route by which calcium enters the cytoplasm is either from the extracellular compartment or internal addistores via calcium channels. There is good evidence that calcium channels can respond to pharmacological compounds that reduce or oxidize thiol groups on the channel protein. In addition, reactive oxygen species such as hydrogen peroxide and superoxide that can mediate oxidative pathology also mediate changes in channel function via alterations of thiol groups. This review looks at the structure and function of calcium channels, the evidence that changes in cellular redox state mediate changes in channel function, and the role of redox modification of channels in disease processes. Understanding how redox modification of the channel protein alters channel structure and function is providing leads for the design of therapeutic interventions that target oxidative stress responses.

Mechanisms controlling the acquisition of a cardiac phenotype by liver stem cells

The mechanisms underlying stem cell acquisition of a cardiac phenotype are unresolved. We studied early events during the acquisition of a cardiac phenotype by a cloned adult liver stem cell line (WB F344) in a cardiac microenvironment. WB F344 cells express a priori the transcription factors GATA4 and SRF, connexin 43 in the cell membrane, and myoinositol 1,4,5-triphosphate receptor in the perinuclear region. Functional cell-cell communication developed between WB F344 cells and adjacent cocultured cardiomyocytes in 24 h. De novo cytoplasmic [Ca(2+)](c) and nuclear [Ca(2+)](nu) oscillations appeared in WB F344 cells, synchronous with [Ca(2+)](i) transients in adjacent cardiomyocytes. The [Ca(2+)] oscillations in the WB F344 cells, but not those in the cardiomyocytes, were eliminated by a gap junction uncoupler and reappeared with its removal. By 24 h, WB F344 cells began expressing the cardiac transcription factors Nkx2.5, Tbx5, and cofactor myocardin; cardiac proteins 24 h later; and a sarcomeric pattern 4-6 days later. Myoinositol 1,4,5-triphosphate receptor inhibition suppressed WB F344 cell [Ca(2+)](nu) oscillations but not [Ca(2+)](c) oscillations, and L-type calcium channel inhibition eliminated [Ca(2+)] oscillations in cardiomyocytes and WB F344 cells. The use of these inhibitors was associated with a decrease in Nkx2.5, Tbx5, and myocardin expression in the WB F344 cells. Our findings suggest that signals from cardiomyocytes diffuse through shared channels, inducing [Ca(2+)] oscillations in the WB F344 cells. We hypothesize that the WB F344 cell [Ca(2+)](nu) oscillations activate the expression of a cardiac specifying gene program, ushering in a cardiac phenotype.

Nuclear localization of functional metabotropic glutamate receptor mGlu1 in HEK293 cells and cortical neurons: role in nuclear calcium mobilization and development

The Group I metabotropic glutamate receptor (mGlu1) plays an important role in neuromodulation, development, and synaptic plasticity. Using immunocytochemistry, subcellular fractionation, and western blot analysis, the present study shows that mGlu1a receptors are present on nuclear membranes in stably transfected human embryonic kidney 293 (HEK293) cells as well as being endogenously expressed on rat cortical nuclei. Both glutamate and the group I agonist, quisqualate, directly activate nuclear mGlu1 receptors leading to a characteristic oscillatory pattern of calcium flux in isolated HEK nuclei and a slow rise to plateau in isolated cortical nuclei. In either case calcium responses could be terminated upon application of the mGlu1-selective antagonist, 7-(hydroxyamino)cyclopropa[b]chromen-1a-carboxylate ethyl ester. Responses could also be blocked by ryanodine and inositol 1,4,5-triphosphate receptor inhibitors, demonstrating the involvement of these calcium channels. Agonist activation of intracellular receptors was driven by Na(+)-dependent and -independent processes in nuclei isolated from either HEK or cortical neurons. Finally, mGlu1 nuclear receptors were dramatically up-regulated in the course of post-natal development. Therefore, like the other Group I receptor, mGlu5, mGlu1 can function as an intracellular receptor, suggesting a more encompassing role for nuclear G protein-coupled receptors and downstream signaling elements in the regulation of nuclear events.

PIKE GTPase are phosphoinositide-3-kinase enhancers, suppressing programmed cell death

Phosphoinositide-3-kinase enhancers (PIKE) are GTP-binding proteins that posses anti-apoptotic functions. The PIKE family includes three members, PIKE-L, PIKE-S and PIKE-A, which are originated from a single gene (CENTG1) through alternative splicing or differential transcription initiation. Both PIKE-S and PIKE-L bind to phosphoinositide-3-kinase (PI3K) and enhance its activity. PIKE-A does not interplay with PI3K. Instead, it interacts with the downstream effector Akt and promotes its activity. These actions are mediated by their GTPase activity. Because both PI3K and Akt are important effectors in the growth factor-mediated signaling which triggers cellular growth and acts against apoptosis, PIKEs therefore serve as the molecular switch that their activation are crucial for growth factors to exert their physiological functions. In this review, the current understanding of different PIKE isoforms in growth factors-induced anti-apoptotic function will be discussed. Moreover, the role of PIKE in the survival and invasion activity of cancer cells will also be introduced.

Current concepts in nuclear pore electrophysiology

Over 4 decades ago, microelectrode studies of in situ nuclei showed that, under certain conditions, the nuclear envelope (NE) behaves as a barrier opposing the nucleocytoplasmic flow of physiological ions. As the nuclear pore complexes (NPCs) of the NE are the only pathways for direct nucleocytoplasmic flow, those experiments implied that the NPCs are capable of restricting ion flow. These early studies validated electrophysiology as a useful approach to quantify some of the mechanisms by which NPCs mediate gene activity and expression. Since electron microscopy (EM) and other non-electrophysiological investigations, showed that the NPC lumen is a nanochannel, the opinion prevailed that the NPC could not oppose the flow of ions and, therefore, that electrophysiological observations resulted from technical artifacts. Consequently, the initial enthusiasm with nuclear electrophysiology faded out in less than a decade. In 1990, nuclear electrophysiology was revisited with patch-clamp, the most powerful electrophysiological technique to date. Patch-clamp has consistently demonstrated that the NE has intrinsic ion channel activity. Direct demonstrations of the NPC on-off ion channel gating behavior were published for artificial conditions in 1995 and for intact living nuclei in 2002. This on-off switching/gating behavior can be interpreted in terms of a metastable energy barrier. In the hope of advancing nuclear electrophysiology, and to complement the other papers contained in this special issue of the journal, here I review some of the main technical, experimental, and theoretical issues of the field, with special focus on NPCs.