An update on nuclear calcium signalling

NR1 and NR3B Composed Intranuclear N-methyl-d-aspartate Receptor Complexes in Human Melanoma Cells

Heterotetrameric N-methyl-d-aspartate type glutamate receptors (NMDAR) are cationic channels primarily permeable for Ca²⁺. NR1 and NR3 subunits bind glycine, while NR2 subunits bind glutamate for full activation. As NR1 may contain a nuclear localization signal (NLS) that is recognized by importin-α, our aim was to investigate if NMDARs are expressed in the nuclei of melanocytes and melanoma cells. A detailed NMDAR subunit expression pattern was examined by RT-PCRs (reverse transcription followed by polymerase chain reaction), fractionated western blots and immunocytochemistry in human epidermal melanocytes and in human melanoma cell lines A2058, HT199, HT168M1, MEL35/0 and WM35. All kind of NMDAR subunits are expressed as mRNAs in melanocytes, as well as in melanoma cells, while NR2B protein remained undetectable in any cell type. Western blots proved the exclusive presence of NR1 and NR3B in nuclear fractions and immunocytochemistry confirmed NR1-NR3B colocalization inside the nuclei of all melanoma cells. The same phenomenon was not observed in melanocytes. Moreover, protein database analysis revealed a putative NLS in NR3B subunit. Our results support that unusual, NR1-NR3B composed NMDAR complexes are present in the nuclei of melanoma cells. This may indicate a new malignancy-related histopathological feature of melanoma cells and raises the possibility of a glycine-driven, NMDA-related nuclear Ca²⁺-signalling in these cells.

Phospholipase C delta 4 (PLCδ4) is a nuclear protein involved in cell proliferation and senescence in mesenchymal stromal stem cells

Ca²⁺ is an important second messenger, and it is involved in many cellular processes such as cell death and proliferation. The rise in intracellular Ca²⁺ levels can be due to the generation of inositol 1,4,5-trisphosphate (InsP₃), which is a product of phosphatidylinositol 4,5-bisphosphate (PIP₂) hydrolysis by phospholipases C (PLCs), that leads to Ca²⁺ release from endoplasmic reticulum by InsP₃ receptors (InsP₃R). Ca²⁺ signaling patterns can vary in different regions of the cell and increases in nuclear Ca²⁺ levels have specific biological effects that differ from those of Ca²⁺ increase in the cytoplasm. There are PLCs in the cytoplasm and nucleus, but little is known about the functions of nuclear PLCs. This work aimed to characterize phenotypically the human PLCδ4 (hPLCδ4) in mesenchymal stem cells. This nuclear isoform of PLC is present in different cell types and has a possible role in proliferative processes. In this work, hPLCδ4 was found to be mainly nuclear in human adipose-derived mesenchymal stem cells (hASC). PLCδ4 knockdown demonstrated that it is essential for hASC proliferation, without inducing cell death. An increase of cells in G1, and a reduction of cells on interphase and G2/M in knockdown cells were seen. Furthermore, PLCδ4 knockdown increased the percentage of senescent cells, p16^INK4A+ and p21^Cip1 mRNAs expression, which could explain the impaired cell proliferation. The results show that hPLCδ4 is in involved in cellular proliferation and senescence in hASC.

Calcium as a signal integrator in developing epithelial tissues

Decoding how tissue properties emerge across multiple spatial and temporal scales from the integration of local signals is a grand challenge in quantitative biology. For example, the collective behavior of epithelial cells is critical for shaping developing embryos. Understanding how epithelial cells interpret a diverse range of local signals to coordinate tissue-level processes requires a systems-level understanding of development. Integration of multiple signaling pathways that specify cell signaling information requires second messengers such as calcium ions. Increasingly, specific roles have been uncovered for calcium signaling throughout development. Calcium signaling regulates many processes including division, migration, death, and differentiation. However, the pleiotropic and ubiquitous nature of calcium signaling implies that many additional functions remain to be discovered. Here we review a selection of recent studies to highlight important insights into how multiple signals are transduced by calcium transients in developing epithelial tissues. Quantitative imaging and computational modeling have provided important insights into how calcium signaling integration occurs. Reverse-engineering the conserved features of signal integration mediated by calcium signaling will enable novel approaches in regenerative medicine and synthetic control of morphogenesis.

Nuclear lipid mediators: Role of nuclear sphingolipids and sphingosine-1-phosphate signaling in epigenetic regulation of inflammation and gene expression

Phospholipids, sphingolipids, and cholesterol are integral components of eukaryotic cell organelles, including the nucleus. Recent evidence shows characteristic features of nuclear lipid composition and signaling, which are distinct from that of the cytoplasm and plasma membrane. While the nuclear phosphoinositol lipid signaling in cell cycle regulation and differentiation has been well described, there is a paucity on the role of nuclear sphingolipids and sphingolipid signaling in different physiological and pathophysiological human conditions. In this prospective, we describe the role of sphingolipids and specifically focus on the sphingoid bases, such as sphingosine, ceramide, and sphingosine-1-phosphate (S1P) generation and catabolism in nuclear signaling and function. Particularly, S1P generated in the nucleus by phosphorylation of SPHK2 modulates HDAC activity either by direct binding or through activation of nuclear reactive oxygen species and regulates cell cycle and pro-inflammatory gene expression. Potential implication of association of SPHK2 with the co-repressor complexes and generation of S1P in the nucleus on chromatin remodeling under normal and pathological conditions is discussed. A better understanding of sphingolipid signaling in the nucleus will facilitate the design and development of new and novel therapeutic approaches to modulate expression of pro-inflammatory and cell cycle dependent genes in human pathologies such as cancer, bacterial lung infection, neurodegeneration, and cystic fibrosis.

Mapping genes for calcium signaling and their associated human genetic disorders

Motivation

Signal transduction via calcium ions (Ca²⁺) represents a fundamental signaling pathway in all eukaryotic cells. A large portion of the human genome encodes proteins used to assemble signaling systems that can transduce signals with diverse spatial and temporal dynamics.

Results

Here, we provide a map of all of the genes involved in Ca²⁺ signaling and link these genes to human genetic disorders. Using Gene Ontology terms and genome databases, 1805 genes were identified as regulators or targets of intracellular Ca²⁺ signals. Associating these 1805 genes with human genetic disorders uncovered 1470 diseases with mutated ‘Ca²⁺ genes’. A network with scale-free properties appeared when the Ca²⁺ genes were mapped to their associated genetic disorders.

Availability and Implementation

The Ca²⁺ genome database is freely available at http://cagedb.uhlenlab.org and will foster studies of gene functions and genetic disorders associated with Ca²⁺ signaling.

Supplementary information

Supplementary data are available at Bioinformatics online.

The Different Facets of Extracellular Calcium Sensors: Old and New Concepts in Calcium-Sensing Receptor Signalling and Pharmacology

The current interest of the scientific community for research in the field of calcium sensing in general and on the calcium-sensing Receptor (CaR) in particular is demonstrated by the still increasing number of papers published on this topic. The extracellular calcium-sensing receptor is the best-known G-protein-coupled receptor (GPCR) able to sense external Ca2+ changes. Widely recognized as a fundamental player in systemic Ca2+ homeostasis, the CaR is ubiquitously expressed in the human body where it activates multiple signalling pathways. In this review, old and new notions regarding the mechanisms by which extracellular Ca2+ microdomains are created and the tools available to measure them are analyzed. After a survey of the main signalling pathways triggered by the CaR, a special attention is reserved for the emerging concepts regarding CaR function in the heart, CaR trafficking and pharmacology. Finally, an overview on other Ca2+ sensors is provided.

Release and uptake mechanisms of vesicular Ca2+ stores

Cells utilize calcium ions (Ca²⁺) to signal almost all aspects of cellular life, ranging from cell proliferation to cell death, in a spatially and temporally regulated manner. A key aspect of this regulation is the compartmentalization of Ca²⁺ in various cytoplasmic organelles that act as intracellular Ca²⁺ stores. Whereas Ca²⁺ release from the large-volume Ca²⁺ stores, such as the endoplasmic reticulum (ER) and Golgi apparatus, are preferred for signal transduction, Ca²⁺ release from the small-volume individual vesicular stores that are dispersed throughout the cell, such as lysosomes, may be more useful in local regulation, such as membrane fusion and individualized vesicular movements. Conceivably, these two types of Ca²⁺ stores may be established, maintained or refilled via distinct mechanisms. ER stores are refilled through sustained Ca²⁺ influx at ER-plasma membrane (PM) membrane contact sites (MCSs). In this review, we discuss the release and refilling mechanisms of intracellular small vesicular Ca²⁺ stores, with a special focus on lysosomes. Recent imaging studies of Ca²⁺ release and organelle MCSs suggest that Ca²⁺ exchange may occur between two types of stores, such that the small stores acquire Ca²⁺ from the large stores via ER-vesicle MCSs. Hence vesicular stores like lysosomes may be viewed as secondary Ca²⁺ stores in the cell.

Na+/Ca2+ exchanger 1 on nuclear envelope controls PTEN/Akt pathway via nucleoplasmic Ca2+ regulation during neuronal differentiation

Nuclear envelope (NE) is a Ca²⁺-storing organelle controlling neuronal differentiation through nuclear Ca²⁺ concentrations ([Ca²⁺]_n). However, how [Ca²⁺]_n regulates this important function remains unknown. Here, we investigated the role of the nuclear form of the Na⁺/Ca²⁺ exchanger 1(nuNCX1) during the different stages of neuronal differentiation and the involvement of PTEN/PI3'K/Akt pathway. In neuronal cells, nuNCX1 was detected on the inner membrane of the NE where protein expression and activity of the exchanger increased during NGF-induced differentiation. nuNCX1 activation by Na⁺-free perfusion induced a time-dependent activation of nuclear-resident PI3K/Akt pathway in isolated nuclei. To discriminate the contribution of nuNCX1 from those of plasma membrane NCX, we generated a chimeric protein composed of the fluorophore EYFP, the exchanger inhibitory peptide, and the nuclear localization signal, named XIP-NLS. Fura-2 measurements on single nuclei and patch-clamp experiments in whole-cell configuration showed that XIP-NLS selectively inhibited nuNCX1. Once it reached the nuclear compartment, XIP-NLS increased the nucleoplasmic Ca²⁺ peak elicited by ATP and reduced Akt phosphorylation, GAP-43 and MAP-2 expression through nuclear-resident PTEN induction. Furthermore, in accordance with the prevention of the neuronal phenotype, XIP-NLS significantly reduced TTX-sensitive Na⁺ currents and membrane potential during neuronal differentiation. The selective inhibition of nuNCX1 by XIP-NLS increased the percentage of β III tubulin-positive immature neurons in mature cultures of MAP-2-positive cortical neurons, thus unraveling a new function for nuNCX1 in regulating neuronal differentiation through [Ca²⁺]_n-dependent PTEN/PI3K/Akt pathway.

Expansion microscopy of zebrafish for neuroscience and developmental biology studies

Expansion microscopy (ExM) allows scalable imaging of preserved 3D biological specimens with nanoscale resolution on fast diffraction-limited microscopes. Here, we explore the utility of ExM in the larval and embryonic zebrafish, an important model organism for the study of neuroscience and development. Regarding neuroscience, we found that ExM enabled the tracing of fine processes of radial glia, which are not resolvable with diffraction-limited microscopy. ExM further resolved putative synaptic connections, as well as molecular differences between densely packed synapses. Finally, ExM could resolve subsynaptic protein organization, such as ring-like structures composed of glycine receptors. Regarding development, we used ExM to characterize the shapes of nuclear invaginations and channels, and to visualize cytoskeletal proteins nearby. We detected nuclear invagination channels at late prophase and telophase, potentially suggesting roles for such channels in cell division. Thus, ExM of the larval and embryonic zebrafish may enable systematic studies of how molecular components are configured in multiple contexts of interest to neuroscience and developmental biology.

Odorant Receptor 51E2 Agonist β-ionone Regulates RPE Cell Migration and Proliferation

The odorant receptor 51E2 (OR51E2), which is well-characterized in prostate cancer cells and epidermal pigment cells, was identified for the first time as the most highly expressed OR in human fetal and adult retinal pigment epithelial (RPE) cells. Immunofluorescence staining and Western blot analysis revealed OR51E2 localization throughout the cytosol and in the plasma membrane. Additionally, immunohistochemical staining of diverse layers of the eye showed that the expression of OR51E2 is restricted to the pigment cells of the RPE and choroid. The results of Ca²⁺-imaging experiments demonstrate that activation of OR51E2 triggers a Ca²⁺ dependent signal pathway in RPE cells. Downstream signaling of OR51E2 involves the activation of adenylyl cyclase, ERK1/2 and AKT. The activity of these protein kinases likely accounts for the demonstrated increase in the migration and proliferation of RPE cells upon stimulation with the OR51E2 ligand β-ionone. These findings suggest that OR51E2 is involved in the regulation of RPE cell growth. Thus, OR51E2 represents a potential target for the treatment of proliferative disorders.

Calcium signaling and cell cycle: Progression or death

Cytosolic Ca²⁺ concentration levels fluctuate in an ordered manner along the cell cycle, in line with the fact that Ca²⁺ is involved in the regulation of cell proliferation. Cell proliferation should be an error-free process, yet is endangered by mistakes. In fact, a complex network of proteins ensures that cell cycle does not progress until the previous phase has been successfully completed. Occasionally, errors occur during the cell cycle leading to cell cycle arrest. If the error is severe, and the cell cycle checkpoints work perfectly, this results into cellular demise by activation of apoptotic or non-apoptotic cell death programs. Cancer is characterized by deregulated proliferation and resistance against cell death. Ca²⁺ is a central key to these phenomena as it modulates signaling pathways that control oncogenesis and cancer progression. Here, we discuss how Ca²⁺ participates in the exogenous and endogenous signals controlling cell proliferation, as well as in the mechanisms by which cells die if irreparable cell cycle damage occurs. Moreover, we summarize how Ca²⁺ homeostasis remodeling observed in cancer cells contributes to deregulated cell proliferation and resistance to cell death. Finally, we discuss the possibility to target specific components of Ca²⁺ signal pathways to obtain cytostatic or cytotoxic effects.

Emerin plays a crucial role in nuclear invagination and in the nuclear calcium transient

Alteration of the nuclear Ca²⁺ transient is an early event in cardiac remodeling. Regulation of the nuclear Ca²⁺ transient is partly independent of the cytosolic Ca²⁺ transient in cardiomyocytes. One nuclear membrane protein, emerin, is encoded by EMD, and an EMD mutation causes Emery-Dreifuss muscular dystrophy (EDMD). It remains unclear whether emerin is involved in nuclear Ca²⁺ homeostasis. The aim of this study is to elucidate the role of emerin in rat cardiomyocytes by means of hypertrophic stimuli and in EDMD induced pluripotent stem (iPS) cell-derived cardiomyocytes in terms of nuclear structure and the Ca²⁺ transient. The cardiac hypertrophic stimuli increased the nuclear area, decreased nuclear invagination, and increased the half-decay time of the nuclear Ca²⁺ transient in cardiomyocytes. Emd knockdown cardiomyocytes showed similar properties after hypertrophic stimuli. The EDMD-iPS cell-derived cardiomyocytes showed increased nuclear area, decreased nuclear invagination, and increased half-decay time of the nuclear Ca²⁺ transient. An autopsied heart from a patient with EDMD also showed increased nuclear area and decreased nuclear invagination. These data suggest that Emerin plays a crucial role in nuclear structure and in the nuclear Ca²⁺ transient. Thus, emerin and the nuclear Ca²⁺ transient are possible therapeutic targets in heart failure and EDMD.

Deep Sequencing of the Human Retinae Reveals the Expression of Odorant Receptors

Several studies have demonstrated that the expression of odorant receptors (ORs) occurs in various tissues. These findings have served as a basis for functional studies that demonstrate the potential of ORs as drug targets for a clinical application. To the best of our knowledge, this report describes the first evaluation of the mRNA expression of ORs and the localization of OR proteins in the human retina that set a stage for subsequent functional analyses. RNA-Sequencing datasets of three individual neural retinae were generated using Next-generation sequencing and were compared to previously published but reanalyzed datasets of the peripheral and the macular human retina and to reference tissues. The protein localization of several ORs was investigated by immunohistochemistry. The transcriptome analyses detected an average of 14 OR transcripts in the neural retina, of which OR6B3 is one of the most highly expressed ORs. Immunohistochemical stainings of retina sections localized OR2W3 to the photosensitive outer segment membranes of cones, whereas OR6B3 was found in various cell types. OR5P3 and OR10AD1 were detected at the base of the photoreceptor connecting cilium, and OR10AD1 was also localized to the nuclear envelope of all of the nuclei of the retina. The cell type-specific expression of the ORs in the retina suggests that there are unique biological functions for those receptors.

Cytosolic and Nucleosolic Calcium Signaling in Response to Osmotic and Salt Stresses Are Independent of Each Other in Roots of Arabidopsis Seedlings

Calcium acts as a universal second messenger in both developmental processes and responses to environmental stresses. Previous research has shown that a number of stimuli can induce [Ca²⁺] increases in both the cytoplasm and nucleus in plants. However, the relationship between cytosolic and nucleosolic calcium signaling remains obscure. Here, we generated transgenic plants containing a fusion protein, comprising rat parvalbumin (PV) with either a nuclear export sequence (PV-NES) or a nuclear localization sequence (NLS-PV), to selectively buffer the cytosolic or nucleosolic calcium. Firstly, we found that the osmotic stress-induced cytosolic [Ca²⁺] increase (OICI_cyt) and the salt stress-induced cytosolic [Ca²⁺] increase (SICI_cyt) were impaired in the PV-NES lines compared with the Arabidopsis wildtype (WT). Similarly, the osmotic stress-induced nucleosolic [Ca²⁺] increase (OICI_nuc) and salt stress-induced nucleosolic [Ca²⁺] increase (SICI_nuc) were also disrupted in the NLS-PV lines. These results indicate that PV can effectively buffer the increase of [Ca²⁺] in response to various stimuli in Arabidopsis. However, the OICI_cyt and SICI_cyt in the NLS-PV plants were similar to those in the WT, and the OICI_nuc and SICI_nuc in the PV-NES plants were also same as those in the WT, suggesting that the cytosolic and nucleosolic calcium dynamics are mutually independent. Furthermore, we found that osmotic stress- and salt stress-inhibited root growth was reduced dramatically in the PV-NES and NLS-PV lines, while the osmotic stress-induced increase of the lateral root primordia was higher in the PV-NES plants than either the WT or NLS-PV plants. In addition, several stress-responsive genes, namely CML37, DREB2A, MYB2, RD29A, and RD29B, displayed diverse expression patterns in response to osmotic and salt stress in the PV-NES and NLS-PV lines when compared with the WT. Together, these results imply that the cytosolic and nucleosolic calcium signaling coexist to play the pivotal roles in the growth and development of plants and their responses to environment stresses.

Nanojunctions of the Sarcoplasmic Reticulum Deliver Site- and Function-Specific Calcium Signaling in Vascular Smooth Muscles

Vasoactive agents may induce myocyte contraction, dilation, and the switch from a contractile to a migratory-proliferative phenotype(s), which requires changes in gene expression. These processes are directed, in part, by Ca²⁺ signals, but how different Ca²⁺ signals are generated to select each function is enigmatic. We have previously proposed that the strategic positioning of Ca²⁺ pumps and release channels at membrane-membrane junctions of the sarcoplasmic reticulum (SR) demarcates cytoplasmic nanodomains, within which site- and function-specific Ca²⁺ signals arise. This chapter will describe how nanojunctions of the SR may: (1) define cytoplasmic nanospaces about the plasma membrane, mitochondria, contractile myofilaments, lysosomes, and the nucleus; (2) provide for functional segregation by restricting passive diffusion and by coordinating active ion transfer within a given nanospace via resident Ca²⁺ pumps and release channels; (3) select for contraction, relaxation, and/or changes in gene expression; and (4) facilitate the switch in myocyte phenotype through junctional reorganization. This should serve to highlight the need for further exploration of cellular nanojunctions and the mechanisms by which they operate, that will undoubtedly open up new therapeutic horizons.

Control of NFAT Isoform Activation and NFAT-Dependent Gene Expression through Two Coincident and Spatially Segregated Intracellular Ca2+ Signals

Excitation-transcription coupling, linking stimulation at the cell surface to changes in nuclear gene expression, is conserved throughout eukaryotes. How closely related coexpressed transcription factors are differentially activated remains unclear. Here, we show that two Ca2+-dependent transcription factor isoforms, NFAT1 and NFAT4, require distinct sub-cellular InsP3 and Ca2+ signals for physiologically sustained activation. NFAT1 is stimulated by sub-plasmalemmal Ca2+ microdomains, whereas NFAT4 additionally requires Ca2+ mobilization from the inner nuclear envelope by nuclear InsP3 receptors. NFAT1 is rephosphorylated (deactivated) more slowly than NFAT4 in both cytoplasm and nucleus, enabling a more prolonged activation phase. Oscillations in cytoplasmic Ca2+, long considered the physiological form of Ca2+ signaling, play no role in activating either NFAT protein. Instead, effective sustained physiological activation of NFAT4 is tightly linked to oscillations in nuclear Ca2+. Our results show how gene expression can be controlled by coincident yet geographically distinct Ca2+ signals, generated by a freely diffusible InsP3 message.

Phosphoinositides in the nucleus and myogenic differentiation: how a nuclear turtle with a PHD builds muscle

Phosphoinositides are a family of phospholipid messenger molecules that control various aspects of cell biology in part by interacting with and regulating downstream protein partners. Importantly, phosphoinositides are present in the nucleus. They form part of the nuclear envelope and are present within the nucleus in nuclear speckles, intra nuclear chromatin domains, the nuclear matrix and in chromatin. What their exact role is within these compartments is not completely clear, but the identification of nuclear specific proteins that contain phosphoinositide interaction domains suggest that they are important regulators of DNA topology, chromatin conformation and RNA maturation and export. The plant homeo domain (PHD) finger is a phosphoinositide binding motif that is largely present in nuclear proteins that regulate chromatin conformation. In the present study I outline how changes in the levels of the nuclear phosphoinositide PtdIns5P impact on muscle cell differentiation through the PHD finger of TAF3 (TAF, TATA box binding protein (TBP)-associated factor), which is a core component of a number of different basal transcription complexes.

Second Messengers

Second messengers are small molecules and ions that relay signals received by cell-surface receptors to effector proteins. They include a wide variety of chemical species and have diverse properties that allow them to signal within membranes (e.g., hydrophobic molecules such as lipids and lipid derivatives), within the cytosol (e.g., polar molecules such as nucleotides and ions), or between the two (e.g., gases and free radicals). Second messengers are typically present at low concentrations in resting cells and can be rapidly produced or released when cells are stimulated. The levels of second messengers are exquisitely controlled temporally and spatially, and, during signaling, enzymatic reactions or opening of ion channels ensure that they are highly amplified. These messengers then diffuse rapidly from the source and bind to target proteins to alter their properties (activity, localization, stability, etc.) to propagate signaling.

Nuclear pores enable sustained perinuclear calcium oscillations

Background

Calcium signalling relies on the flux of calcium ions across membranes yet how signals in different compartments are related remains unclear. In particular, similar calcium signals on both sides of the nuclear envelope have been reported and attributed to passive diffusion through nuclear pores. However, observed differing cytosolic and nucleosolic calcium signatures suggest that the signalling machinery in these compartments can act independently.

Results

We adapt the fire-diffuse-fire model to investigate the generation of perinuclear calcium oscillations. We demonstrate that autonomous spatio-temporal calcium patterns are still possible in the presence of nuclear and cytosolic coupling via nuclear pores. The presence or absence of this autonomy is dependent upon the strength of the coupling and the maximum firing rate of an individual calcium channel. In all cases, coupling through the nuclear pores enables robust signalling with respect to changes in the diffusion constant.

Conclusions

We show that contradictory interpretations of experimental data with respect to the autonomy of nuclear calcium oscillations can be reconciled within one model, with different observations being a consequence of varying nuclear pore permeabilities for calcium and refractory conditions of channels. Furthermore, our results provide an explanation for why calcium oscillations on both sides of the nuclear envelope may be beneficial for sustained perinuclear signaling.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-016-0289-9) contains supplementary material, which is available to authorized users.

From contraction to gene expression: nanojunctions of the sarco/endoplasmic reticulum deliver site- and function-specific calcium signals

Calcium signals determine, for example, smooth muscle contraction and changes in gene expression. How calcium signals select for these processes is enigmatic. We build on the "panjunctional sarcoplasmic reticulum" hypothesis, describing our view that different calcium pumps and release channels, with different kinetics and affinities for calcium, are strategically positioned within nanojunctions of the SR and help demarcate their respective cytoplasmic nanodomains. SERCA2b and RyR1 are preferentially targeted to the sarcoplasmic reticulum (SR) proximal to the plasma membrane (PM), i.e., to the superficial buffer barrier formed by PM-SR nanojunctions, and support vasodilation. In marked contrast, SERCA2a may be entirely restricted to the deep, perinuclear SR and may supply calcium to this sub-compartment in support of vasoconstriction. RyR3 is also preferentially targeted to the perinuclear SR, where its clusters associate with lysosome-SR nanojunctions. The distribution of RyR2 is more widespread and extends from this region to the wider cell. Therefore, perinuclear RyR3s most likely support the initiation of global calcium waves at L-SR junctions, which subsequently propagate by calcium-induced calcium release via RyR2 in order to elicit contraction. Data also suggest that unique SERCA and RyR are preferentially targeted to invaginations of the nuclear membrane. Site- and function-specific calcium signals may thus arise to modulate stimulus-response coupling and transcriptional cascades.

Functional Role of Mitochondrial and Nuclear BK Channels

BK channels are important for the regulation of many cell functions. The significance of plasma membrane BK channels in the control of action potentials, resting membrane potential, and neurotransmitter release is well established; however, the composition and functions of mitochondrial and nuclear BK (nBK) channels are largely unknown. In this chapter, we summarize the recent findings on the subcellular localization, biophysical, and pharmacological properties of mitochondrial and nBK channels and discuss their molecular identity and physiological functions.

Functional Tuning of Intrinsic Endothelial Ca2+ Dynamics in Swine Coronary Arteries

RATIONALE

Recent data from mesenteric and cerebral beds have revealed spatially restricted Ca(2+) transients occurring along the vascular intima that control effector recruitment and vasodilation. Although Ca(2+) is pivotal for coronary artery endothelial function, spatial and temporal regulation of functional Ca(2+) signals in the coronary endothelium is poorly understood.

OBJECTIVE

We aimed to determine whether a discrete spatial and temporal profile of Ca(2+) dynamics underlies endothelium-dependent relaxation of swine coronary arteries.

METHODS AND RESULTS

Using confocal imaging, custom automated image analysis, and myography, we show that the swine coronary artery endothelium generates discrete basal Ca(2+) dynamics, including isolated transients and whole-cell propagating waves. These events are suppressed by depletion of internal stores or inhibition of inositol 1,4,5-trisphosphate receptors but not by inhibition of ryanodine receptors or removal of extracellular Ca(2+). In vessel rings, inhibition of specific Ca(2+)-dependent endothelial effectors, namely, small and intermediate conductance K(+) channels (K(Ca)3.1 and K(Ca)2.3) and endothelial nitric oxide synthase, produces additive tone, which is blunted by internal store depletion or inositol 1,4,5-trisphosphate receptor blockade. Stimulation of endothelial inositol 1,4,5-trisphosphate-dependent signaling with substance P causes idiosyncratic changes in dynamic Ca(2+) signal parameters (active sites, event frequency, amplitude, duration, and spatial spread). Overall, substance P-induced vasorelaxation corresponded poorly with whole-field endothelial Ca(2+) measurements but corresponded precisely with the concentration-dependent change in Ca(2+) dynamics (linearly translated composite of dynamic parameters).

CONCLUSIONS

Our findings show that endothelium-dependent control of swine coronary artery tone is determined by spatial and temporal titration of inherent endothelial Ca(2+) dynamics that are not represented by tissue-level averaged Ca(2+) changes.

Nuclear calcium in cardiac myocytes

Calcium (Ca) is a universal second messenger involved in the regulation of various cellular processes, including electrical signaling, contraction, secretion, memory, gene transcription, and cell death. In heart, Ca governs cardiomyocyte contraction, is central in electrophysiological properties, and controls major signaling pathway implicated in gene transcription. How cardiomyocytes decode Ca signal to regulate gene expression without interfering with, or being controlled by, "contractile" Ca that floods the entire cytosol during each heartbeat is still elusive. In this review, we summarize recent findings on nuclear Ca regulation and its downstream signaling in cardiomyocytes. We will address difficulties in reliable quantification of nuclear Ca fluxes and discuss its role in the development and progression of cardiac hypertrophy and heart failure. We also point out key open questions to stimulate future work.

Cytosolic and nuclear calcium signaling in atrial myocytes: IP3-mediated calcium release and the role of mitochondria

In rabbit atrial myocytes Ca signaling has unique features due to the lack of transverse (t) tubules, the spatial arrangement of mitochondria and the contribution of inositol-1,4,5-trisphosphate (IP3) receptor-induced Ca release (IICR). During excitation-contraction coupling action potential-induced elevation of cytosolic [Ca] originates in the cell periphery from Ca released from the junctional sarcoplasmic reticulum (j-SR) and then propagates by Ca-induced Ca release from non-junctional (nj-) SR toward the cell center. The subsarcolemmal region between j-SR and the first array of nj-SR Ca release sites is devoid of mitochondria which results in a rapid propagation of activation through this domain, whereas the subsequent propagation through the nj-SR network occurs at a velocity typical for a propagating Ca wave. Inhibition of mitochondrial Ca uptake with the Ca uniporter blocker Ru360 accelerates propagation and increases the amplitude of Ca transients (CaTs) originating from nj-SR. Elevation of cytosolic IP3 levels by rapid photolysis of caged IP3 has profound effects on the magnitude of subcellular CaTs with increased Ca release from nj-SR and enhanced CaTs in the nuclear compartment. IP3 uncaging restricted to the nucleus elicites 'mini'-Ca waves that remain confined to this compartment. Elementary IICR events (Ca puffs) preferentially originate in the nucleus in close physical association with membrane structures of the nuclear envelope and the nucleoplasmic reticulum. The data suggest that in atrial myocytes the nucleus is an autonomous Ca signaling domain where Ca dynamics are primarily governed by IICR.

G protein-coupled receptor signaling in cardiac nuclear membranes

G protein-coupled receptors (GPCRs) play key physiological roles and represent a significant target for drug development. However, historically, drugs were developed with the understanding that GPCRs as a therapeutic target exist solely on cell surface membranes. More recently, GPCRs have been detected on intracellular membranes, including the nuclear membrane, and the concept that intracellular GPCRs are functional is become more widely accepted. Nuclear GPCRs couple to effectors and regulate signaling pathways, analogous to their counterparts at the cell surface, but may serve distinct biological roles. Hence, the physiological responses mediated by GPCR ligands, or pharmacological agents, result from the integration of their actions at extracellular and intracellular receptors. The net effect depends on the ability of a given ligand or drug to access intracellular receptors, as dictated by its structure, lipophilic properties, and affinity for nuclear receptors. This review will discuss angiotensin II, endothelin, and β-adrenergic receptors located on the nuclear envelope in cardiac cells in terms of their origin, activation, and role in cardiovascular function and pathology.

A stromal interaction molecule 1 variant up-regulates matrix metalloproteinase-2 expression by strengthening nucleoplasmic Ca2+ signaling

Very recent studies hold promise to reveal the role of stromal interaction molecule 1 (STIM1) in non-store-operated Ca2+ entry. Here we showed that in contrast to cytoplasmic membrane redistribution as previously noted, human umbilical vein endothelial STIM1 with a T-to-C nucleotide transition resulting in an amino acid substitution of leucine by proline in the signal peptide sequence translocated to perinuclear membrane upon intracellular Ca2+ depletion, amplified nucleoplasmic Ca2+ signaling through ryanodine receptor-dependent pathway, and enhanced the subsequent cAMP responsive element binding protein activity, matrix metalloproteinase-2 (MMP-2) gene expression, and endothelial tube forming. The abundance of mutated STIM1 and the MMP-2 expression were higher in native human umbilical vein endothelial cells of patients with gestational hypertension than controls and were significantly correlated with blood pressure. These findings broaden our understanding about structure-function bias of STIM1 and offer unique insights into its application in nucleoplasmic Ca2+, MMP-2 expression, endothelial dysfunction, and pathophysiological mechanism(s) of gestational hypertension.

Modeling calcium regulation of contraction, energetics, signaling, and transcription in the cardiac myocyte

Calcium (Ca(2+)) plays many important regulatory roles in cardiac muscle cells. In the initial phase of the action potential, influx of Ca(2+) through sarcolemmal voltage-gated L-type Ca(2+) channels (LCCs) acts as a feed-forward signal that triggers a large release of Ca(2+) from the junctional sarcoplasmic reticulum (SR). This Ca(2+) drives heart muscle contraction and pumping of blood in a process known as excitation-contraction coupling (ECC). Triggered and released Ca(2+) also feed back to inactivate LCCs, attenuating the triggered Ca(2+) signal once release has been achieved. The process of ECC consumes large amounts of ATP. It is now clear that in a process known as excitation-energetics coupling, Ca(2+) signals exert beat-to-beat regulation of mitochondrial ATP production that closely couples energy production with demand. This occurs through transport of Ca(2+) into mitochondria, where it regulates enzymes of the tricarboxylic acid cycle. In excitation-signaling coupling, Ca(2+) activates a number of signaling pathways in a feed-forward manner. Through effects on their target proteins, these interconnected pathways regulate Ca(2+) signals in complex ways to control electrical excitability and contractility of heart muscle. In a process known as excitation-transcription coupling, Ca(2+) acting primarily through signal transduction pathways also regulates the process of gene transcription. Because of these diverse and complex roles, experimentally based mechanistic computational models are proving to be very useful for understanding Ca(2+) signaling in the cardiac myocyte.

Is hydrogen ion (H(+)) the real second messenger in calcium signalling?

Most second messengers have the acknowledged ability to mobilize the segregated Ca(2+) from intracellular stores, although the mechanisms of mobilization are unclear. To study this problem, the fact that inositol 1,4,5-trisphosphate, and six other known endogenous Ca(2+) mobilizers are acids, or acid-generating compounds, is highlighted. In physiological conditions, a newly generated acid releases H(+). The transient rise of H(+) in the cytosol may induce the lowering of pH, mobilization of bound Ca(2+), protein conformational rearrangement, store depletion, and Ca(2+) influx. Accordingly, a new description of the basic mechanism for signal transduction in non-excitable cells and the related consequences is put forward.

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.

Androgen receptor and chemokine receptors 4 and 7 form a signaling axis to regulate CXCL12-dependent cellular motility

Background

Identifying cellular signaling pathways that become corrupted in the presence of androgens that increase the metastatic potential of organ-confined tumor cells is critical to devising strategies capable of attenuating the metastatic progression of hormone-naïve, organ-confined tumors. In localized prostate cancers, gene fusions that place ETS-family transcription factors under the control of androgens drive gene expression programs that increase the invasiveness of organ-confined tumor cells. C-X-C chemokine receptor type 4 (CXCR4) is a downstream target of ERG, whose upregulation in prostate-tumor cells contributes to their migration from the prostate gland. Recent evidence suggests that CXCR4-mediated proliferation and metastasis of tumor cells is regulated by CXCR7 through its scavenging of chemokine CXCL12. However, the role of androgens in regulating CXCR4-mediated motility with respect to CXCR7 function in prostate-cancer cells remains unclear.

Methods

Immunocytochemistry, western blot, and affinity-purification analyses were used to study how androgens influenced the expression, subcellular localization, and function of CXCR7, CXCR4, and androgen receptor (AR) in LNCaP prostate-tumor cells. Moreover, luciferase assays and quantitative polymerase chain reaction (qPCR) were used to study how chemokines CXCL11 and CXCL12 regulate androgen-regulated genes (ARGs) in LNCaP prostate-tumor cells. Lastly, cell motility assays were carried out to determine how androgens influenced CXCR4-dependent motility through CXCL12.

Results

Here we show that, in the LNCaP prostate-tumor cell line, androgens coordinate the expression of CXCR4 and CXCR7, thereby promoting CXCL12/CXCR4-mediated cell motility. RNA interference experiments revealed functional interactions between AR and CXCR7 in these cells. Co-localization and affinity-purification experiments support a physical interaction between AR and CXCR7 in LNCaP cells. Unexpectedly, CXCR7 resided in the nuclear compartment and modulated AR-mediated transcription. Moreover, androgen-mediated cell motility correlated positively with the co-localization of CXCR4 and CXCR7 receptors, suggesting that cell migration may be linked to functional CXCR4/CXCR7 heterodimers. Lastly, CXCL12-mediated cell motility was CXCR7-dependent, with CXCR7 expression required for optimal expression of CXCR4 protein.

Conclusions

Overall, our results suggest that inhibition of CXCR7 function might decrease the metastatic potential of organ-confined prostate cancers.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1201-5) contains supplementary material, which is available to authorized users.

Macromolecular transport in synapse to nucleus communication

Local signaling events at synapses or axon terminals must be communicated to the nucleus to elicit transcriptional responses. The lengths of neuronal processes pose a significant challenge for such intracellular communication. This challenge is met by mechanisms ranging from rapid signals encoded in calcium waves to slower macromolecular signaling complexes carried by molecular motors. Here we summarize recent findings on macromolecular signaling from the synapse to the nucleus, in comparison to those employed in injury signaling along axons. A number of common themes emerge, including combinatorial signal encoding by post-translational mechanisms such as differential phosphorylation and proteolysis, and conserved roles for importins in coordinating signaling complexes. Neurons may integrate ionic flux with motor-transported signals as a temporal code for synaptic plasticity signaling.

The type 2 inositol 1,4,5-trisphosphate receptor, emerging functions for an intriguing Ca²⁺-release channel

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) type 2 (IP3R2) is an intracellular Ca²⁺-release channel located on the endoplasmic reticulum (ER). IP3R2 is characterized by a high sensitivity to both IP3 and ATP and is biphasically regulated by Ca²⁺. Furthermore, IP3R2 is modulated by various protein kinases. In addition to its regulation by protein kinase A, IP3R2 forms a complex with adenylate cyclase 6 and is directly regulated by cAMP. Finally, in the ER, IP3R2 is less mobile than the other IP3R isoforms, while its functional properties appear dominant in heterotetramers. These properties make the IP3R2 a Ca²⁺ channel with exquisite properties for setting up intracellular Ca²⁺ signals with unique characteristics. IP3R2 plays a crucial role in the function of secretory cell types (e.g. pancreatic acinar cells, hepatocytes, salivary gland, eccrine sweat gland). In cardiac myocytes, the role of IP3R2 appears more complex, because, together with IP3R1, it is needed for normal cardiogenesis, while its aberrant activity is implicated in cardiac hypertrophy and arrhythmias. Most importantly, its high sensitivity to IP3 makes IP3R2 a target for anti-apoptotic proteins (e.g. Bcl-2) in B-cell cancers. Disrupting IP3R/Bcl-2 interaction therefore leads in those cells to increased Ca²⁺ release and apoptosis. Intriguingly, IP3R2 is not only implicated in apoptosis but also in the induction of senescence, another tumour-suppressive mechanism. These results were the first to unravel the physiological and pathophysiological role of IP3R2 and we anticipate that further progress will soon be made in understanding the function of IP3R2 in various tissues and organs.

An integrated mechanism of cardiomyocyte nuclear Ca(2+) signaling

In cardiomyocytes, Ca(2+) plays a central role in governing both contraction and signaling events that regulate gene expression. Current evidence indicates that discrimination between these two critical functions is achieved by segregating Ca(2+) within subcellular microdomains: transcription is regulated by Ca(2+) release within nuclear microdomains, and excitation-contraction coupling is regulated by cytosolic Ca(2+). Accordingly, a variety of agonists that control cardiomyocyte gene expression, such as endothelin-1, angiotensin-II or insulin-like growth factor-1, share the feature of triggering nuclear Ca(2+) signals. However, signaling pathways coupling surface receptor activation to nuclear Ca(2+) release, and the phenotypic responses to such signals, differ between agonists. According to earlier hypotheses, the selective control of nuclear Ca(2+) signals by activation of plasma membrane receptors relies on the strategic localization of inositol trisphosphate receptors at the nuclear envelope. There, they mediate Ca(2+) release from perinuclear Ca(2+) stores upon binding of inositol trisphosphate generated in the cytosol, which diffuses into the nucleus. More recently, identification of such receptors at nuclear membranes or perinuclear sarcolemmal invaginations has uncovered novel mechanisms whereby agonists control nuclear Ca(2+) release. In this review, we discuss mechanisms for the selective control of nuclear Ca(2+) signals with special focus on emerging models of agonist receptor activation.

Nuclear BK channels regulate gene expression via the control of nuclear calcium signaling

Ion channels are essential for the regulation of neuronal functions. The significance of plasma membrane, mitochondrial, endoplasmic reticulum and lysosomal ion channels in the regulation of Ca(2+) is well established. In contrast, surprisingly little is known about the function of ion channels on the nuclear envelope (NE). Here we demonstrate the presence of functional large-conductance, calcium-activated potassium channels (BK channels) on the NE of rodent hippocampal neurons. Functionally, blockade of nuclear BK channels (nBK channels) induces NE-derived Ca(2+) release, nucleoplasmic Ca(2+) elevation and cyclic AMP response element binding protein (CREB)-dependent transcription. More importantly, blockade of nBK channels regulates nuclear Ca(2+)-sensitive gene expression and promotes dendritic arborization in a nuclear Ca(2+)-dependent manner. These results suggest that the nBK channel functions as a molecular link between neuronal activity and nuclear Ca(2+) to convey signals from synapse to nucleus and is a new modulator, operating at the NE, of synaptic activity-dependent neuronal functions.

CaMKII regulation of cardiac ryanodine receptors and inositol triphosphate receptors

Ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3Rs) are structurally related intracellular calcium release channels that participate in multiple primary or secondary amplified Ca(2+) signals, triggering muscle contraction and oscillatory Ca(2+) waves, or activating transcription factors. In the heart, RyRs play an indisputable role in the process of excitation-contraction coupling as the main pathway for Ca(2+) release from sarcoplasmic reticulum (SR), and a less prominent role in the process of excitation-transcription coupling. Conversely, InsP3Rs are believed to contribute in subtle ways, only, to contraction of the heart, and in more important ways to regulation of transcription factors. Because uncontrolled activity of either RyRs or InsP3Rs may elicit life-threatening arrhythmogenic and/or remodeling Ca(2+) signals, regulation of their activity is of paramount importance for normal cardiac function. Due to their structural similarity, many regulatory factors, accessory proteins, and post-translational processes are equivalent for RyRs and InsP3Rs. Here we discuss regulation of RyRs and InsP3Rs by CaMKII phosphorylation, but touch on other kinases whenever appropriate. CaMKII is emerging as a powerful modulator of RyR and InsP3R activity but interestingly, some of the complexities and controversies surrounding phosphorylation of RyRs also apply to InsP3Rs, and a clear-cut effect of CaMKII on either channel eludes investigators for now. Nevertheless, some effects of CaMKII on global cellular activity, such as SR Ca(2+) leak or force-frequency potentiation, appear clear now, and this constrains the limits of the controversies and permits a more tractable approach to elucidate the effects of phosphorylation at the single channel level.

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.

CYCLOPS, a DNA-binding transcriptional activator, orchestrates symbiotic root nodule development

Nuclear calcium oscillations are a hallmark of symbiotically stimulated plant root cells. Activation of the central nuclear decoder, calcium- and calmodulin-dependent kinase (CCaMK), triggers the entire symbiotic program including root nodule organogenesis, but the mechanism of signal transduction by CCaMK was unknown. We show that CYCLOPS, a direct phosphorylation substrate of CCaMK, is a DNA-binding transcriptional activator. Two phosphorylated serine residues within the N-terminal negative regulatory domain of CYCLOPS are necessary for its activity. CYCLOPS binds DNA in a sequence-specific and phosphorylation-dependent manner and transactivates the NODULE INCEPTION (NIN) gene. A phosphomimetic version of CYCLOPS was sufficient to trigger root nodule organogenesis in the absence of rhizobia and CCaMK. CYCLOPS thus induces a transcriptional activation cascade, in which NIN and a heterotrimeric NF-Y complex act in hierarchical succession to initiate symbiotic root nodule development.

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.

Nuclear envelope invaginations and cancer

The nuclear envelope (NE) surrounds the nucleus and separates it from the cytoplasm. The NE is not a passive structural component, but rather contributes to various cellular processes such as genome organization, transcription, signaling, and stress responses. Although the NE is mostly a smooth surface, it also forms invaginations that can reach deep into the nucleoplasm and may even traverse the nucleus completely. Cancer cells are generally characterized by irregularities and invaginations of the NE that are of diagnostic and prognostic significance. In the current chapter, we describe the link between nuclear invaginations and irregularities with cancer and explore possible mechanistic roles they might have in tumorigenesis.

Functional organization and dynamics of the cell nucleus

The eukaryotic cell nucleus enclosed within the nuclear envelope harbors organized chromatin territories and various nuclear bodies as sub-nuclear compartments. This higher-order nuclear organization provides a unique environment to regulate the genome during replication, transcription, maintenance, and other processes. In this review, we focus on the plant four-dimensional nuclear organization, its dynamics and function in response to signals during development or stress.

Temperature-induced labelling of Fluo-3 AM selectively yields brighter nucleus in adherent cells

Fluo-3 is widely used to study cell calcium. Two traditional approaches: (1) direct injection and (2) Fluo-3 acetoxymethyl ester (AM) loading, often bring conflicting results in cytoplasmic calcium ([Ca(2+)]c) and nuclear calcium ([Ca(2+)]n) imaging. AM loading usually yields a darker nucleus than in cytoplasm, while direct injection always induces a brighter nucleus which is more responsive to [Ca(2+)]n detection. In this work, we detailedly investigated the effects of loading and de-esterification temperatures on the fluorescence intensity of Fluo-3 in response to [Ca(2+)]n and [Ca(2+)]c in adherent cells, including osteoblast, HeLa and BV2 cells. Interestingly, it showed that fluorescence intensity of nucleus in osteoblast cells was about two times larger than that of cytoplasm when cells were loaded with Fluo-3 AM at 4 °C and allowed a subsequent step for de-esterification at 20 °C. Brighter nuclei were also acquired in HeLa and BV2 cells using the same experimental condition. Furthermore, loading time and adhesion quality of cells had effect on fluorescence intensity. Taken together, cold loading and room temperature de-esterification treatment of Fluo-3 AM selectively yielded brighter nucleus in adherent cells.

A possible role of transglutaminase 2 in the nucleus of INS-1E and of cells of human pancreatic islets

Transglutaminase 2 (TG2) is a multifunctional protein with Ca^2 +-dependent transamidating and G protein activity. Previously we reported that the role of TG2 in insulin secretion may involve cytoplasmic actin remodeling and a regulative action on other proteins during granule movement. The aim of this study was to gain a better insight into the role of TG2 transamidating activity in mitochondria and in the nucleus of INS-1E rat insulinoma cell line (INS-1E) during insulin secretion. To this end we labeled INS-1E with an artificial donor (biotinylated peptide), in basal condition and after stimulus with glucose for 2, 5, and 8 min. Biotinylated proteins of the nuclear/mitochondrial-enriched fraction were analyzed using two-dimensional electrophoresis and mass spectrometry. Many mitochondrial proteins involved in Ca^2 + homeostasis (e.g. voltage-dependent anion-selective channel protein, prohibitin and different ATP synthase subunits) and many nuclear proteins involved in gene regulation (e.g. histone H3, barrier to autointegration factor and various heterogeneous nuclear ribonucleoprotein) were identified among a number of transamidating substrates of TG2 in INS-1E. The combined results provide evidence that a temporal link exists between glucose-stimulation, first phase insulin secretion and the action of TG on histone H3 both in INS-1E and human pancreatic islets.

Biological significance

Research into the role of transglutaminase 2 during insulin secretion in INS-1E rat insulinoma cellular model is depicting a complex role for this enzyme. Transglutaminase 2 acts in the different INS-1E compartments in the same way: catalyzing a post-translational modification event of its substrates. In this work we identify some mitochondrial and nuclear substrates of INS-1E during first phase insulin secretion. The finding that TG2 interacts with nuclear proteins that include BAF and histone H3 immediately after (2–5 min) glucose stimulus of INS-1E suggests that TG2 may be involved not only in insulin secretion, as suggested by our previous studies in cytoplasmic INS-1E fraction, but also in the regulation of glucose-induced gene transcription.

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Transglutaminase 2 localizes in the nucleus and in the mitochondrion of INS-1E.

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TG2 acts as a modifying enzyme in both compartments during FPIS.

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TG2 may contribute to Ca^2 + sensing in mitochondrion through its substrates.

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TG2 may contribute to chromatin condensation in nucleus through its substrates.

Nuclear calcium signaling in plants

Plant cell nuclei can generate calcium responses to a variety of inputs, tantamount among them the response to signaling molecules from symbiotic microorganisms .

Towards the Physics of Calcium Signalling in Plants

Calcium is an abundant element with a wide variety of important roles within cells. Calcium ions are inter- and intra-cellular messengers that are involved in numerous signalling pathways. Fluctuating compartment-specific calcium ion concentrations can lead to localised and even plant-wide oscillations that can regulate downstream events. Understanding the mechanisms that give rise to these complex patterns that vary both in space and time can be challenging, even in cases for which individual components have been identified. Taking a systems biology approach, mathematical and computational techniques can be employed to produce models that recapitulate experimental observations and capture our current understanding of the system. Useful models make novel predictions that can be investigated and falsified experimentally. This review brings together recent work on the modelling of calcium signalling in plants, from the scale of ion channels through to plant-wide responses to external stimuli. Some in silico results that have informed later experiments are highlighted.

Intracrine endothelin signaling evokes IP3-dependent increases in nucleoplasmic Ca²⁺ in adult cardiac myocytes

Endothelin receptors are present on the nuclear membranes in adult cardiac ventricular myocytes. The objectives of the present study were to determine 1) which endothelin receptor subtype is in cardiac nuclear membranes, 2) if the receptor and ligand traffic from the cell surface to the nucleus, and 3) the effect of increased intracellular ET-1 on nuclear Ca(2+) signaling. Confocal microscopy using fluorescently-labeled endothelin analogs confirmed the presence of ETB at the nuclear membrane of rat cardiomyocytes in skinned-cells and isolated nuclei. Furthermore, in both cardiac myocytes and aortic endothelial cells, endocytosed ET:ETB complexes translocated to lysosomes and not the nuclear envelope. Although ETA and ETB can form heterodimers, the presence or absence of ETA did not alter ETB trafficking. Treatment of isolated nuclei with peptide: N-glycosidase F did not alter the electrophoretic mobility of ETB. The absence of N-glycosylation further indicates that these receptors did not originate at the cell surface. Intracellular photolysis of a caged ET-1 analog ([Trp-ODMNB(21)]ET-1) evoked an increase in nucleoplasmic Ca(2+) ([Ca(2+)]n) that was attenuated by inositol 1,4,5-trisphosphate receptor inhibitor 2-aminoethoxydiphenyl borate and prevented by pre-treatment with ryanodine. A caged cell-permeable analog of the ETB-selective antagonist IRL-2500 blocked the ability of intracellular cET-1 to increase [Ca(2+)]n whereas extracellular application of ETA and ETB receptor antagonists did not. These data suggest that 1) the endothelin receptor in the cardiac nuclear membranes is ETB, 2) ETB traffics directly to the nuclear membrane after biosynthesis, 3) exogenous endothelins are not ligands for ETB on nuclear membranes, and 4) ETB associated with the nuclear membranes regulates nuclear Ca(2+) signaling.

Genetically encoded calcium indicators for multi-color neural activity imaging and combination with optogenetics

Genetically encoded calcium indicators (GECIs) are powerful tools for systems neuroscience. Here we describe red, single-wavelength GECIs, “RCaMPs,” engineered from circular permutation of the thermostable red fluorescent protein mRuby. High-resolution crystal structures of mRuby, the red sensor RCaMP, and the recently published red GECI R-GECO1 give insight into the chromophore environments of the Ca²⁺-bound state of the sensors and the engineered protein domain interfaces of the different indicators. We characterized the biophysical properties and performance of RCaMP sensors in vitro and in vivo in Caenorhabditis elegans, Drosophila larvae, and larval zebrafish. Further, we demonstrate 2-color calcium imaging both within the same cell (registering mitochondrial and somatic [Ca²⁺]) and between two populations of cells: neurons and astrocytes. Finally, we perform integrated optogenetics experiments, wherein neural activation via channelrhodopsin-2 (ChR2) or a red-shifted variant, and activity imaging via RCaMP or GCaMP, are conducted simultaneously, with the ChR2/RCaMP pair providing independently addressable spectral channels. Using this paradigm, we measure calcium responses of naturalistic and ChR2-evoked muscle contractions in vivo in crawling C. elegans. We systematically compare the RCaMP sensors to R-GECO1, in terms of action potential-evoked fluorescence increases in neurons, photobleaching, and photoswitching. R-GECO1 displays higher Ca²⁺ affinity and larger dynamic range than RCaMP, but exhibits significant photoactivation with blue and green light, suggesting that integrated channelrhodopsin-based optogenetics using R-GECO1 may be subject to artifact. Finally, we create and test blue, cyan, and yellow variants engineered from GCaMP by rational design. This engineered set of chromatic variants facilitates new experiments in functional imaging and optogenetics.

Ca2+-sensitive fluorescent dyes and intracellular Ca2+ imaging

Imaging Ca(2+)-sensitive fluorescent indicators provides a common approach for studying Ca(2+) signals in many contexts. Fluorescent indicators are particularly useful for measuring acute Ca(2+) changes in a relatively noninvasive manner. The availability of indicators that can be targeted to specific cellular domains, coupled with variations in affinity, brightness or spectral characteristics, provides tools for exploring spatially and temporally diverse Ca(2+) signals, and moreover, multiplexing the readout of Ca(2+) with other cellular functions. This article aims to give the novice experimentalist some insight into the considerations and potential pitfalls that impinge on the use of fluorescent Ca(2+) indicators.