Coupling of Ca(2+) to CREB activation and gene expression in intact cerebral arteries from mouse : roles of ryanodine receptors and voltage-dependent Ca(2+) channels

Inositol 1,4,5-Trisphosphate Receptors Regulate Vascular Smooth Muscle Cell Proliferation and Neointima Formation in Mice

BACKGROUND

Vascular smooth muscle cell (VSMC) proliferation is involved in many types of arterial diseases, including neointima hyperplasia, in which Ca2+ has been recognized as a key player. However, the physiological role of Ca2+ release via inositol 1,4,5-trisphosphate receptors (IP3Rs) from endoplasmic reticulum in regulating VSMC proliferation has not been well determined.

METHODS AND RESULTS

Both in vitro cell culture models and in vivo mouse models were generated to investigate the role of IP3Rs in regulating VSMC proliferation. Expression of all 3 IP3R subtypes was increased in cultured VSMCs upon platelet-derived growth factor-BB and FBS stimulation as well as in the left carotid artery undergoing intimal thickening after vascular occlusion. Genetic ablation of all 3 IP3R subtypes abolished endoplasmic reticulum Ca2+ release in cultured VSMCs, significantly reduced cell proliferation induced by platelet-derived growth factor-BB and FBS stimulation, and also decreased cell migration of VSMCs. Furthermore, smooth muscle-specific deletion of all IP3R subtypes in adult mice dramatically attenuated neointima formation induced by left carotid artery ligation, accompanied by significant decreases in cell proliferation and matrix metalloproteinase-9 expression in injured vessels. Mechanistically, IP3R-mediated Ca2+ release may activate cAMP response element-binding protein, a key player in controlling VSMC proliferation, via Ca2+/calmodulin-dependent protein kinase II and Akt. Loss of IP3Rs suppressed cAMP response element-binding protein phosphorylation at Ser133 in both cultured VSMCs and injured vessels, whereas application of Ca2+ permeable ionophore, ionomycin, can reverse cAMP response element-binding protein phosphorylation in IP3R triple knockout VSMCs.

CONCLUSIONS

Our results demonstrated an essential role of IP3R-mediated Ca2+ release from endoplasmic reticulum in regulating cAMP response element-binding protein activation, VSMC proliferation, and neointima formation in mouse arteries.

Neurotoxicity and behavioral disorders induced in mice by acute exposure to the diamide insecticide chlorantraniliprole

Diamide insecticides activate ryanodine receptors expressed in lepidopteran skeletal muscle and promote Ca2+ release in the sarcoplasmic reticulum, causing abnormal contractions and paralysis, leading to death of the pest. Although they had been thought not to act on nontarget organisms, including mammals, adverse effects on vertebrates were recently reported, raising concerns about their safety in humans. We investigated the neurotoxicity of the acute no-observed-adverse-effect level of chlorantraniliprole (CAP), a diamide insecticide, in mice using clothianidin (CLO), a neonicotinoid insecticide, as a positive control. The CLO-administered group showed decreased locomotor activities, increased anxiety-like behaviors, and abnormal human-audible vocalizations, while the CAP-administered group showed anxiety-like behaviors but no change in locomotor activities. The CAP-administered group had greater numbers of c-fos-immunoreactive cells in the hippocampal dentate gyrus, and similar to the results in a CLO-administered group in our previous study. Blood corticosterone levels increased in the CLO-administered group but did not change in the CAP-administered group. Additionally, CAP was found to decreased 3-Methoxytyramine and histamine in mice at the time to maximum concentration. These results suggest that CAP-administered mice are less vulnerable to stress than CLO-administered mice, and the first evidence that CAP exposure increases neuronal activity and induces anxiety-like behavior as well as neurotransmitter disturbances in mammals.

A molecular complex of Cav1.2/CaMKK2/CaMK1a in caveolae is responsible for vascular remodeling via excitation-transcription coupling

Excitation–transcription (E-T) coupling can initiate and modulate essential physiological or pathological responses in cells, such as neurons and cardiac myocytes. Although vascular myocytes also exhibit E-T coupling in response to membrane depolarization, the underlying molecular mechanisms are unknown. Our study reveals that E-T coupling in vascular myocytes converts intracellular Ca²⁺ signals into selective gene transcription related to chemotaxis, leukocyte adhesion, and inflammation. Our discovery identifies a mechanism for vascular remodeling as an adaptation to increased circumferential stretch.

Elevation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) activates Ca²⁺/calmodulin-dependent kinases (CaMK) and promotes gene transcription. This signaling pathway is referred to as excitation–transcription (E-T) coupling. Although vascular myocytes can exhibit E-T coupling, the molecular mechanisms and physiological/pathological roles are unknown. Multiscale analysis spanning from single molecules to whole organisms has revealed essential steps in mouse vascular myocyte E-T coupling. Upon a depolarizing stimulus, Ca²⁺ influx through Ca_v1.2 voltage-dependent Ca²⁺ channels activates CaMKK2 and CaMK1a, resulting in intranuclear CREB phosphorylation. Within caveolae, the formation of a molecular complex of Ca_v1.2/CaMKK2/CaMK1a is promoted in vascular myocytes. Live imaging using a genetically encoded Ca²⁺ indicator revealed direct activation of CaMKK2 by Ca²⁺ influx through Ca_v1.2 localized to caveolae. CaMK1a is phosphorylated by CaMKK2 at caveolae and translocated to the nucleus upon membrane depolarization. In addition, sustained depolarization of a mesenteric artery preparation induced genes related to chemotaxis, leukocyte adhesion, and inflammation, and these changes were reversed by inhibitors of Ca_v1.2, CaMKK2, and CaMK, or disruption of caveolae. In the context of pathophysiology, when the mesenteric artery was loaded by high pressure in vivo, we observed CREB phosphorylation in myocytes, macrophage accumulation at adventitia, and an increase in thickness and cross-sectional area of the tunica media. These changes were reduced in caveolin1-knockout mice or in mice treated with the CaMKK2 inhibitor STO609. In summary, E-T coupling depends on Ca_v1.2/CaMKK2/CaMK1a localized to caveolae, and this complex converts [Ca²⁺]_i changes into gene transcription. This ultimately leads to macrophage accumulation and media remodeling for adaptation to increased circumferential stretch.

Calcium-Dependent Ion Channels and the Regulation of Arteriolar Myogenic Tone

Arterioles in the peripheral microcirculation regulate blood flow to and within tissues and organs, control capillary blood pressure and microvascular fluid exchange, govern peripheral vascular resistance, and contribute to the regulation of blood pressure. These important microvessels display pressure-dependent myogenic tone, the steady state level of contractile activity of vascular smooth muscle cells (VSMCs) that sets resting arteriolar internal diameter such that arterioles can both dilate and constrict to meet the blood flow and pressure needs of the tissues and organs that they perfuse. This perspective will focus on the Ca²⁺-dependent ion channels in the plasma and endoplasmic reticulum membranes of arteriolar VSMCs and endothelial cells (ECs) that regulate arteriolar tone. In VSMCs, Ca²⁺-dependent negative feedback regulation of myogenic tone is mediated by Ca²⁺-activated K⁺ (BK_Ca) channels and also Ca²⁺-dependent inactivation of voltage-gated Ca²⁺ channels (VGCC). Transient receptor potential subfamily M, member 4 channels (TRPM4); Ca²⁺-activated Cl⁻ channels (CaCCs; TMEM16A/ANO1), Ca²⁺-dependent inhibition of voltage-gated K⁺ (K_V) and ATP-sensitive K⁺ (K_ATP) channels; and Ca²⁺-induced-Ca²⁺ release through inositol 1,4,5-trisphosphate receptors (IP₃Rs) participate in Ca²⁺-dependent positive-feedback regulation of myogenic tone. Calcium release from VSMC ryanodine receptors (RyRs) provide negative-feedback through Ca²⁺-spark-mediated control of BK_Ca channel activity, or positive-feedback regulation in cooperation with IP₃Rs or CaCCs. In some arterioles, VSMC RyRs are silent. In ECs, transient receptor potential vanilloid subfamily, member 4 (TRPV4) channels produce Ca²⁺ sparklets that activate IP₃Rs and intermediate and small conductance Ca²⁺ activated K⁺ (IK_Ca and sK_Ca) channels causing membrane hyperpolarization that is conducted to overlying VSMCs producing endothelium-dependent hyperpolarization and vasodilation. Endothelial IP₃Rs produce Ca²⁺ pulsars, Ca²⁺ wavelets, Ca²⁺ waves and increased global Ca²⁺ levels activating EC sK_Ca and IK_Ca channels and causing Ca²⁺-dependent production of endothelial vasodilator autacoids such as NO, prostaglandin I₂ and epoxides of arachidonic acid that mediate negative-feedback regulation of myogenic tone. Thus, Ca²⁺-dependent ion channels importantly contribute to many aspects of the regulation of myogenic tone in arterioles in the microcirculation.

PGRMC1 Inhibits Progesterone-Evoked Proliferation and Ca2+ Entry Via STIM2 in MDA-MB-231 Cells

Progesterone receptor membrane component 1 (PGRMC1) has been shown to regulate some cancer hallmarks. Progesterone (P₄) evokes intracellular calcium (Ca²⁺) changes in the triple-negative breast cancer cell lines (MDA-MB-231, MDA-MB-468, and BT-20) and in other breast cancer cell lines like the luminal MCF7 cells. PGRMC1 expression is elevated in MDA-MB-231 and MCF7 cells as compared to non-tumoral MCF10A cell line, and PGRMC1 silencing enhances P₄-evoked Ca²⁺ mobilization. Here, we found a new P₄-dependent Ca²⁺ mobilization pathway in MDA-MB-231 cells and other triple-negative breast cancer cells, as well as in MCF7 cells that involved Stromal interaction molecule 2 (STIM2), Calcium release-activated calcium channel protein 1 (Orai1), and Transient Receptor Potential Channel 1 (TRPC1). Stromal interaction molecule 1 (STIM1) was not involved in this novel Ca²⁺ pathway, as evidenced by using siRNA STIM1. PGRMC1 silencing reduced the negative effect of P₄ on cell proliferation and cell death in MDA-MB-231 cells. In line with the latter observation, Nuclear Factor of Activated T-Cells 1 (NFAT1) nuclear accumulation due to P₄ incubation for 48 h was enhanced in cells transfected with the small hairpin siRNA against PGRMC1 (shPGRMC1). These results provide evidence for a novel P₄-evoked Ca²⁺ entry pathway that is downregulated by PGRMC1.

Metabolic syndrome inhibits store-operated Ca2+ entry and calcium-induced calcium-release mechanism in coronary artery smooth muscle

BACKGROUND AND PURPOSE

Metabolic syndrome causes adverse effects on the coronary circulation including altered vascular responsiveness and the progression of coronary artery disease (CAD). However the underlying mechanisms linking obesity with CAD are intricated. Augmented vasoconstriction, mainly due to impaired Ca²⁺ homeostasis in coronary vascular smooth muscle (VSM), is a critical factor for CAD. Increased calcium-induced calcium release (CICR) mechanism has been associated to pathophysiological conditions presenting persistent vasoconstriction while increased store operated calcium (SOC) entry appears to activate proliferation and migration in coronary vascular smooth muscle (VSM). We analyze here whether metabolic syndrome might alter SOC entry as well as CICR mechanism in coronary arteries, contributing thus to a defective Ca²⁺ handling and therefore accelerating the progression of CAD.

EXPERIMENTAL APPROACH

Measurements of intracellular Ca²⁺ ([Ca²⁺]_i) and tension and of Ca²⁺ channels protein expression were performed in coronary arteries (CA) from lean Zucker rats (LZR) and obese Zucker rats (OZR).

KEY RESULTS

SOC entry stimulated by emptying sarcoplasmic reticulum (SR) Ca²⁺ store with cyclopiazonic acid (CPA) was decreased and associated to decreased STIM-1 and Orai1 protein expression in OZR CA. Further, CICR mechanism was blunted in these arteries but Ca²⁺ entry through voltage-dependent L-type channels was preserved contributing to maintain depolarization-induced increases in [Ca²⁺]_i and vasoconstriction in OZR CA. These results were associated to increased expression of voltage-operated L-type Ca²⁺ channel alpha 1C subunit (Ca_V1.2) but unaltered ryanodine receptor (RyR) and sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA) pump protein content in OZR CA.

CONCLUSION AND IMPLICATIONS

The present manuscript provides evidence of impaired Ca²⁺ handling mechanisms in coronary arteries in metabolic syndrome where a decrease in both SOC entry and CICR mechanism but preserved vasoconstriction are reported in coronary arteries from obese Zucker rats. Remarkably, OZR CA VSM at this state of metabolic syndrome seemed to have developed a compensation mechanism for impaired CICR by overexpressing Ca_V1.2 channels.

Soluble adenylyl cyclase links Ca2+ entry to Ca2+/cAMP-response element binding protein (CREB) activation in vascular smooth muscle

Ca²⁺-transcription coupling controls gene expression patterns that define vascular smooth muscle cell (VSMC) phenotype. Although not well understood this allows normally contractile VSMCs to become proliferative following vessel injury, a process essential for repair but which also contributes to vascular remodelling, atherogenesis and restenosis. Here we show that the Ca²⁺/HCO₃^--sensitive enzyme, soluble adenylyl cyclase (sAC), links Ca²⁺ influx in human coronary artery smooth muscle cells (hCASMCs) to 3',5'-cyclic adenosine monophosphate (cAMP) generation and phosphorylation of the transcription factor Ca²⁺/cAMP response element binding protein (CREB). Store-operated Ca²⁺ entry (SOCE) into hCASMCs expressing the FRET-based cAMP biosensor H187 induced a rise in cAMP that mirrored cytosolic [Ca²⁺]. SOCE also activated the cAMP effector, protein kinase A (PKA), as determined by the PKA reporter, AKAR4-NES, and induced phosphorylation of vasodilator-stimulated phosphoprotein (VASP) and CREB. Transmembrane adenylyl cyclase inhibition had no effect on the SOCE-induced rise in cAMP, while sAC inhibition abolished SOCE-generated cAMP and significantly reduced SOCE-induced VASP and CREB phosphorylation. This suggests that SOCE in hCASMCs activates sAC which in turn activates the cAMP/PKA/CREB axis. sAC, which is insensitive to G-protein modulation but responsive to Ca²⁺, pH and ATP, may therefore act as an overlooked regulatory node in vascular Ca²⁺-transcription coupling.

Vascular smooth muscle cell proliferation as a therapeutic target. Part 1: molecular targets and pathways

Cardiovascular diseases are a major cause of human death worldwide. Excessive proliferation of vascular smooth muscle cells contributes to the etiology of such diseases, including atherosclerosis, restenosis, and pulmonary hypertension. The control of vascular cell proliferation is complex and encompasses interactions of many regulatory molecules and signaling pathways. Herein, we recapitulated the importance of signaling cascades relevant for the regulation of vascular cell proliferation. Detailed understanding of the mechanism underlying this process is essential for the identification of new lead compounds (e.g., natural products) for vascular therapies.

Impaired Ca2+ handling in resistance arteries from genetically obese Zucker rats: Role of the PI3K, ERK1/2 and PKC signaling pathways

The impact of obesity on vascular smooth muscle (VSM) Ca²⁺ handling and vasoconstriction, and its regulation by the phosphatidylinositol 3-kinase (PI3K), mitogen activated protein kinase (MAPK) and protein kinase C (PKC) were assessed in mesenteric arteries (MA) from obese Zucker rats (OZR). Simultaneous measurements of intracellular Ca²⁺ ([Ca²⁺]_i) and tension were performed in MA from OZR and compared to lean Zucker rats (LZR), and the effects of selective inhibitors of PI3K, ERK-MAPK kinase and PKC were assessed on the functional responses of VSM voltage-dependent L-type Ca²⁺ channels (Ca_V1.2). Increases in [Ca²⁺]_i induced by α₁-adrenoceptor activation and high K⁺ depolarization were not different in arteries from LZR and OZR although vasoconstriction was enhanced in OZR. Blockade of the ryanodine receptor (RyR) and of Ca²⁺ release from the sarcoplasmic reticulum (SR) markedly reduced depolarization-induced Ca²⁺ responses in arteries from lean but not obese rats, suggesting impaired Ca²⁺-induced Ca²⁺ release (CICR) from SR in arteries from OZR. Enhanced Ca²⁺ influx after treatment with ryanodine was abolished by nifedipine and coupled to up-regulation of Ca_V1.2 channels in arteries from OZR. Increased activation of ERK-MAPK and up-regulation of PI3Kδ, PKCβ and δ isoforms were associated to larger inhibitory effects of PI3K, MAPK and PKC blockers on VSM L-type channel Ca²⁺ entry in OZR. Changes in arterial Ca²⁺ handling in obesity involve SR Ca²⁺ store dysfunction and enhanced VSM Ca²⁺ entry through L-type channels, linked to a compensatory up-regulation of Ca_V1.2 proteins and increased activity of the ERK-MAPK, PI3Kδ and PKCβ and δ, signaling pathways.

Grass Carp Prolactin Gene: Structural Characterization and Signal Transduction for PACAP-induced Prolactin Promoter Activity

In this study, structural analysis of grass carp prolactin (PRL) gene was performed and the signaling mechanisms for pituitary adenylate cyclase-activating peptide (PACAP) regulation of PRL promoter activity were investigated. In αT3-1 cells, PRL promoter activity could be induced by oPACAP₃₈ which was blocked by PACAP antagonist but not the VIP antagonist. The stimulatory effect of oPACAP₃₈ was mimicked by activation of AC/cAMP and voltage-sensitive Ca²⁺ channel (VSCC) signaling, or induction of Ca²⁺ entry. In parallel, PACAP-induced PRL promoter activity was negated or inhibited by suppressing cAMP production, inhibiting PKA activity, removal of extracellular Ca²⁺, VSCC blockade, calmodulin (CaM) antagonism, and inactivation of CaM kinase II. Similar sensitivity to L-type VSCC, CaM and CaM kinase II inhibition were also observed by substituting cAMP analog for oPACAP₃₈ as the stimulant for PRL promoter activity. Moreover, PACAP-induced PRL promoter activity was also blocked by inhibition of PLC signaling, attenuation of [Ca²⁺]i immobilization via IP3 receptors, and blockade of PI3K/P₇₀^S6K pathway. The PACAP-induced PRL promoter activation may involve transactivation of the transcription factor CREB. These results suggest that PACAP can stimulate PRL promoter activation by PAC1 mediated functional coupling of the Ca²⁺/CaM/CaM kinase II cascades with the AC/cAMP/PKA pathway. Apparently, other signaling pathways, including PLC/IP3 and PI3K/P₇₀^S6K cascades, may also be involved in PACAP induction of PRL gene transcription.

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

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

Pyk2 inhibition promotes contractile differentiation in arterial smooth muscle

Modulation from contractile to synthetic phenotype of vascular smooth muscle cells is a central process in disorders involving compromised integrity of the vascular wall. Phenotype modulation has been shown to include transition from voltage-dependent toward voltage-independent regulation of the intracellular calcium level, and inhibition of non-voltage dependent calcium influx contributes to maintenance of the contractile phenotype. One possible mediator of calcium-dependent signaling is the FAK-family non-receptor protein kinase Pyk2, which is activated by a number of stimuli in a calcium-dependent manner. We used the Pyk2 inhibitor PF-4594755 and Pyk2 siRNA to investigate the role of Pyk2 in phenotype modulation in rat carotid artery smooth muscle cells and in cultured intact arteries. Pyk2 inhibition promoted the expression of smooth muscle markers at the mRNA and protein levels under stimulation by FBS or PDGF-BB and counteracted phenotype shift in cultured intact carotid arteries and balloon injury ex vivo. During long-term (24-96 hr) treatment with PF-4594755, smooth muscle markers increased before cell proliferation was inhibited, correlating with decreased KLF4 expression and differing from effects of MEK inhibition. The Pyk2 inhibitor reduced Orai1 and preserved SERCA2a expression in carotid artery segments in organ culture, and eliminated the inhibitory effect of PDGF stimulation on L-type calcium channel and large-conductance calcium-activated potassium channel expression in carotid cells. Basal intracellular calcium level, calcium wave activity, and store-operated calcium influx were reduced after Pyk2 inhibition of growth-stimulated cells. Pyk2 inhibition may provide an interesting approach for preserving vascular smooth muscle differentiation under pathophysiological conditions.

Exploring poisonous mechanism of honeybee, Apis mellifera ligustica Spinola, caused by pyrethroids

As the important intracellular secondary messengers, calcium channel is the target of many neurotoxic pesticides as calcium homeostasis in the neuroplasm play important role in neuronal functions and behavior in insects. This study investigated the effect of deltamethrin (DM) on calcium channel in the brain nerve cells of adult workers of Apis mellifera ligustica Spinola that were cultured in vitro. The results showed that the intracellular calcium concentration was significantly elevated even with a very low concentration of the DM (3.125×10^-2mg/L). Further testing revealed that T-type voltage-gated calcium channels (VGCCs), except for sodium channels, was one of the target of DM on toxicity of Apis mellifera, while DM has no significant effect on the L-type VGCCs, N-methyl-d-aspartate receptor-gated calcium channels and calcium store. These results suggesting that the DM may act on T-type VGCCs in brain cells of honeybees and result in behavioral abnormalities including swarming, feeding, learning, and acquisition.

Role of Transcription Factors in Pulmonary Artery Smooth Muscle Cells: An Important Link to Hypoxic Pulmonary Hypertension

Hypoxia, namely a lack of oxygen in the blood, induces pulmonary vasoconstriction and vasoremodeling, which serve as essential pathologic factors leading to pulmonary hypertension (PH). The underlying molecular mechanisms are uncertain; however, pulmonary artery smooth muscle cells (PASMCs) play an essential role in hypoxia-induced pulmonary vasoconstriction, vasoremodeling, and PH. Hypoxia causes oxidative damage to DNAs, proteins, and lipids. This damage (oxidative stress) modulates the activity of ion channels and elevates the intracellular calcium concentration ([Ca²⁺]_i, Ca²⁺ signaling) of PASMCs. The oxidative stress and increased Ca²⁺ signaling mutually interact with each other, and synergistically results in a variety of cellular responses. These responses include functional and structural abnormalities of mitochondria, sarcoplasmic reticulum, and nucleus; cell contraction, proliferation, migration, and apoptosis, as well as generation of vasoactive substances, inflammatory molecules, and growth factors that mediate the development of PH. A number of studies reveal that various transcription factors (TFs) play important roles in hypoxia-induced oxidative stress, disrupted PAMSC Ca²⁺ signaling and the development and progress of PH. It is believed that in the pathogenesis of PH, hypoxia facilitates these roles by mediating the expression of multiple genes. Therefore, the identification of specific genes and their transcription factors implicated in PH is necessary for the complete understanding of the underlying molecular mechanisms. Moreover, this identification may aid in the development of novel and effective therapeutic strategies for PH.

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.

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.

Excitation-Contraction Coupling and Excitation-Transcription Coupling in Blood Vessels: Their Possible Interactions in Hypertensive Vascular Remodeling

Vascular smooth muscle cells (VSMC) display considerable phenotype plasticity which can be studied in vivo on vascular remodeling which occurs during acute or chronic vascular injury. In differentiated cells, which represent contractile phenotype, there are characteristic rapid transient changes of intracellular Ca2+ concentration ([Ca2+]i), while the resting cytosolic [Ca2+]i concentration is low. It is mainly caused by two components of the Ca2+ signaling pathways: Ca2+ entry via L-type voltage-dependent Ca2+ channels and dynamic involvement of intracellular stores. Proliferative VSMC phenotype is characterized by long-lasting [Ca2+]i oscillations accompanied by sustained elevation of basal [Ca2+]i. During the switch from contractile to proliferative phenotype there is a general transition from voltage-dependent Ca2+ entry to voltage-independent Ca2+ entry into the cell. These changes are due to the altered gene expression which is dependent on specific transcription factors activated by various stimuli. It is an open question whether abnormal VSMC phenotype reported in rats with genetic hypertension (such as spontaneously hypertensive rats) might be partially caused by a shift from contractile to proliferative VSMC phenotype.

Mechanisms of Vascular Smooth Muscle Contraction and the Basis for Pharmacologic Treatment of Smooth Muscle Disorders

The smooth muscle cell directly drives the contraction of the vascular wall and hence regulates the size of the blood vessel lumen. We review here the current understanding of the molecular mechanisms by which agonists, therapeutics, and diseases regulate contractility of the vascular smooth muscle cell and we place this within the context of whole body function. We also discuss the implications for personalized medicine and highlight specific potential target molecules that may provide opportunities for the future development of new therapeutics to regulate vascular function.

Calcium Homeostasis and Ionic Mechanisms in Pulmonary Fibroblasts

Fibroblasts are key cellular mediators of many chronic interstitial lung diseases, including idiopathic pulmonary fibrosis, scleroderma, sarcoidosis, drug-induced interstitial lung disease, and interstitial lung disease in connective tissue disease. A great deal of effort has been expended to understand the signaling mechanisms underlying the various cellular functions of fibroblasts. Recently, it has been shown that Ca(2+) oscillations play a central role in the regulation of gene expression in human pulmonary fibroblasts. However, the mechanisms whereby cytosolic [Ca(2+)] are regulated and [Ca(2+)] oscillations transduced are both poorly understood. In this review, we present the general concepts of [Ca(2+)] homeostasis, of ionic mechanisms responsible for various Ca(2+) fluxes, and of regulation of gene expression by [Ca(2+)]. In each case, we then also summarize the original findings that pertain specifically to pulmonary fibroblasts. From these data, we propose an overall signaling cascade by which excitation of the fibroblasts triggers pulsatile release of internally sequestered Ca(2+), which, in turn, activates membrane conductances, including voltage-dependent Ca(2+) influx pathways. Collectively, these events produce recurring Ca(2+) oscillations, the frequency of which is transduced by Ca(2+)-dependent transcription factors, which, in turn, orchestrate a variety of cellular events, including proliferation, synthesis/secretion of extracellular matrix proteins, autoactivation (production of transforming growth factor-β), and transformation into myofibroblasts. That unifying hypothesis, in turn, allows us to highlight several specific cellular targets and therapeutic intervention strategies aimed at controlling unwanted pulmonary fibrosis. The relationships between Ca(2+) signaling events and the unfolded protein response and apoptosis are also explored.

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

Local regulation of L-type Ca²⁺ channel sparklets in arterial smooth muscle

This review addresses the latest advances in our understanding of the regulation of a novel Ca²⁺ signal called L-type Ca²⁺ channel sparklets in arterial smooth muscle. L-type Ca²⁺ channel sparklets are elementary Ca²⁺ influx events produced by the opening of a single or a small cluster of L-type Ca²⁺ channels. These Ca²⁺ signals were first visualized in the vasculature in arterial smooth muscle cells. In these cells, L-type Ca²⁺ channel sparklets display two functionally distinct gating modalities that regulate local and global [Ca²⁺](i). The activity of L-type Ca²⁺ channel sparklets varies regionally within a cell depending on the dynamic activity of a cohort of protein kinases and phosphatases recruited to L-type Ca²⁺ channels in the arterial smooth muscle sarcolemma in a complex coordinated by the scaffolding molecule AKAP150. We also described a mechanism whereby clusters of L-type Ca²⁺ channels gate cooperatively to amplify intracellular Ca²⁺ signals with likely pathological consequences.

CaMKIIδ-dependent inhibition of cAMP-response element-binding protein activity in vascular smooth muscle

One transcription factor mediator of Ca(2+)-signals is cAMP response element-binding protein (CREB). CREB expression and/or activity negatively correlates with vascular smooth muscle (VSM) cell proliferation and migration. Multifunctional Ca(2+)/calmodulin-dependent protein kinases, including CaMKII, have been demonstrated to regulate CREB activity through both positive and negative phosphorylation events in vitro, but the function of CaMKII as a proximal regulator of CREB in intact cell systems, including VSM, is not clear. In this study, we used gain- and loss-of-function approaches to determine the function of CaMKIIδ in regulating CREB phosphorylation, localization, and activity in VSM. Overexpression of constitutively active CaMKIIδ specifically increased CREB phosphorylation on Ser(142) and silencing CaMKIIδ expression by siRNA or blocking endogenous CaMKII activity with KN93 abolished thrombin- or ionomycin-induced CREB phosphorylation on Ser(142) without affecting Ser(133) phosphorylation. CREB-Ser(142) phosphorylation correlated with transient nucleocytoplasmic translocation of CREB. Thrombin-induced CREB promoter activity, CREB binding to Sik1 and Rgs2 promoters, and Sik1/Rgs2 transcription were enhanced by a kinase-negative CaMKIIδ2 (K43A) mutant and inhibited by a constitutively active (T287D) mutant. Taken together, these studies establish negative regulation of CREB activity by endogenous CaMKIIδ-dependent CREB-Ser(142) phosphorylation and suggest a potential mechanism for CaMKIIδ/CREB signaling in modulating proliferation and migration in VSM cells.

Vascular smooth muscle cell phenotype is defined by Ca2+-dependent transcription factors

Ca(2+) is an important second messenger in vascular smooth muscle cells (VSMCs). Therefore, VSMCs exercise tight control of the intracellular Ca(2+) concentration ([Ca(2+)]i) by expressing a wide repertoire of Ca(2+) channels and transporters. The presence of several pathways for Ca(2+) influx and efflux provides many possibilities for controlling [Ca(2+)]i in a spatial and temporal manner. Intracellular Ca(2+) has a dual role in VSMCs; first, it is necessary for VSMC contraction; and, second, it can activate multiple transcription factors. These factors are cAMP response element-binding protein, nuclear factor of activated T lymphocytes, and serum response factor. Furthermore, it was recently reported that the C-terminus of voltage-dependent L-type Ca(2+) calcium channels can regulate transcription in VSMCs. Transcription regulation in VSMCs modulates the expression patterns of genes, including genes coding for contractile and cytoskeleton proteins, and those promoting proliferation and cell growth. Depending on their gene expression, VSMCs can exist in different functional states or phenotypes. The majority of healthy VSMCs show a contractile phenotype, characterized by high contractile ability and a low proliferative rate. However, VSMCs can undergo phenotypic modulation with different physiological and pathological stimuli, whereby they start to proliferate, migrate, and synthesize excessive extracellular matrix. These events are associated with injury repair and angiogenesis, but also with the development of cardiovascular pathologies, such as atherosclerosis and hypertension. This review discusses the currently known Ca(2+)-dependent transcription factors in VSMCs, their regulation by Ca(2+) signalling, and their role in the VSMC phenotype.

The voltage-dependent L-type Ca2+ (CaV1.2) channel C-terminus fragment is a bi-modal vasodilator

Voltage-dependent L-type Ca(2+) channels (CaV1.2) are the primary Ca(2+) entry pathway in vascular smooth muscle cells (myocytes). CaV1.2 channels control systemic blood pressure and organ blood flow and are pathologically altered in vascular diseases, which modifies vessel contractility. The CaV1.2 distal C-terminus is susceptible to proteolytic cleavage, which yields a truncated CaV1.2 subunit and a cleaved C-terminal fragment (CCt). Previous studies in cardiac myocytes and neurons have identified CCt as both a transcription factor and CaV1.2 channel inhibitor, with different signalling mechanisms proposed to underlie some of these effects. CCt existence and physiological functions in arterial myocytes are unclear, but important to study given the functional significance of CaV1.2 channels. Here, we show that CCt exists in myocytes of both rat and human resistance-size cerebral arteries, where it locates to both the nucleus and plasma membrane. Recombinant CCt expression in arterial myocytes inhibited CaV1.2 transcription and reduced CaV1.2 protein. CCt induced a depolarizing shift in the voltage dependence of both CaV1.2 current activation and inactivation, and reduced non-inactivating current in myocytes. Recombinant truncated CCt lacking a putative nuclear localization sequence (92CCt) did not locate to the nucleus and had no effect on arterial CaV1.2 transcription or protein. However, 92CCt shifted the voltage dependence of CaV1.2 activation and inactivation similarly to CCt. CCt and 92CCt both inhibited pressure- and depolarization-induced vasoconstriction, although CCt was a far more effective vasodilator. These data demonstrate that endogenous CCt exists and reduces both CaV1.2 channel expression and voltage sensitivity in arterial myocytes. Thus, CCt is a bi-modal vasodilator.

Reciprocal regulation controlling the expression of CPI-17, a specific inhibitor protein for the myosin light chain phosphatase in vascular smooth muscle cells

Cellular activity of the myosin light chain phosphatase (MLCP) determines agonist-induced force development of smooth muscle (SM). CPI-17 is an endogenous inhibitor protein for MLCP, responsible for mediating G-protein signaling into SM contraction. Fluctuations in CPI-17 expression occur in response to pathological stresses, altering excitation-contraction coupling in SM. Here, we determined the signaling pathways regulating CPI-17 expression in rat aorta tissues and the cell culture using a pharmacological approach. CPI-17 transcription was suppressed in response to the proliferative stimulus with platelet-derived growth factor (PDGF) through the ERK1/2 pathway, whereas it was elevated in response to inflammatory, stress-induced and excitatory stimuli with transforming growth factor-β, IL-1β, TNFα, sorbitol, and serotonin. CPI-17 transcription was repressed by inhibition of JNK, p38, PKC, and Rho-kinase (ROCK). The mouse and human CPI-17 gene promoters were governed by the proximal GC-boxes at the 5'-flanking region, where Sp1/Sp3 transcription factors bound. Sp1 binding to the region was more prominent in intact aorta tissues, compared with the SM cell culture, where the CPI-17 gene is repressed. The 173-bp proximal promoter activity was negatively and positively regulated through PDGF-induced ERK1/2 and sorbitol-induced p38/JNK pathways, respectively. By contrast, PKC and ROCK inhibitors failed to repress the 173-bp promoter activity, suggesting distal enhancer elements. CPI-17 transcription was insensitive to knockdown of myocardin/Kruppel-like factor 4 small interfering RNA or histone deacetylase inhibition. The reciprocal regulation of Sp1/Sp3-driven CPI-17 expression through multiple kinases may be responsible for the adaptation of MLCP signal and SM tone to environmental changes.

Calcium signaling in vascular smooth muscle cells: from physiology to pathology

Cyclic variations in calcium (Ca(2+)) concentrations, through a process called excitation-contraction coupling, allow regulation of vascular smooth muscle cells contractility and thus modulation of vascular tone and blood pressure. As a second messenger, Ca(2+) also activates signaling cascades leading to transcription factors activation in a process called excitation-transcription coupling. Furthermore, recent evidences indicate an interaction between post-transcriptional regulation by microRNAs (miRNAs) and Ca(2+) signaling. All these actors, which are frequently altered in vascular diseases, will be reviewed here.

Calcium signaling in smooth muscle

Changes in intracellular Ca(2+) are central to the function of smooth muscle, which lines the walls of all hollow organs. These changes take a variety of forms, from sustained, cell-wide increases to temporally varying, localized changes. The nature of the Ca(2+) signal is a reflection of the source of Ca(2+) (extracellular or intracellular) and the molecular entity responsible for generating it. Depending on the specific channel involved and the detection technology employed, extracellular Ca(2+) entry may be detected optically as graded elevations in intracellular Ca(2+), junctional Ca(2+) transients, Ca(2+) flashes, or Ca(2+) sparklets, whereas release of Ca(2+) from intracellular stores may manifest as Ca(2+) sparks, Ca(2+) puffs, or Ca(2+) waves. These diverse Ca(2+) signals collectively regulate a variety of functions. Some functions, such as contractility, are unique to smooth muscle; others are common to other excitable cells (e.g., modulation of membrane potential) and nonexcitable cells (e.g., regulation of gene expression).

CXCL12 secretion by bone marrow stromal cells is dependent on cell contact and mediated by connexin-43 and connexin-45 gap junctions

The chemokine CXCL12 is essential for the function of hematopoietic stem and progenitor cells. Here we report that secretion of functional CXCL12 from human bone marrow stromal cells (BMSCs) was a cell contact-dependent event mediated by connexin-43 (Cx43) and Cx45 gap junctions. Inhibition of connexin gap junctions impaired the secretion of CXCL12 and homing of leukocytes to mouse bone marrow. Purified human CD34(+) progenitor cells did not adhere to noncontacting BMSCs, which led to a much smaller pool of immature cells. Calcium conduction activated signaling by cAMP-protein kinase A (PKA) and induced CXCL12 secretion mediated by the GTPase RalA. Cx43 and Cx45 additionally controlled Cxcl12 transcription by regulating the nuclear localization of the transcription factor Sp1. We suggest that BMSCs form a dynamic syncytium via connexin gap junctions that regulates CXC12 secretion and the homeostasis of hematopoietic stem cells.

Distinct effects of voltage- and store-dependent calcium influx on stretch-induced differentiation and growth in vascular smooth muscle

Stretch of the vascular wall stimulates smooth muscle hypertrophy by activating the MAPK and Rho/Rho kinase (ROK) pathways. We investigated the role of calcium in this response. Stretch-stimulated expression of contractile and cytoskeletal proteins in mouse portal vein was inhibited at mRNA and protein levels by blockade of voltage-dependent Ca(2+) entry (VDCE). In contrast, blockade of store-operated Ca(2+) entry (SOCE) did not affect smooth muscle marker expression but decreased global protein synthesis. Activation of VDCE caused membrane translocation of RhoA followed by phosphorylation of its downstream effectors LIMK-2 and cofilin-2. Stretch-activated cofilin-2 phosphorylation depended on VDCE but not on SOCE. VDCE was associated with increased mRNA expression of myocardin, myocyte enhancer factor (MEF) -2A and -2D, and smooth muscle marker genes, all of which depended on ROK activity. SOCE increased ERK1/2 phosphorylation and c-Fos expression but had no effect on phosphorylation of LIMK-2 and cofilin-2 or on myocardin and MEF2 expression. Knockdown of MEF2A or -2D eliminated the VDCE-induced activation of myocardin expression and increased basal c-Jun and c-Fos mRNA levels. These results indicate that MEF2 mediates VDCE-dependent stimulation of myocardin expression via the Rho/ROK pathway. In addition, SOCE activates the expression of immediate-early genes, known to be regulated by MEF2 via Ca(2+)-dependent phosphorylation of histone deacetylases, but this mode of Ca(2+) entry does not affect the Rho/ROK pathway. Compartmentation of Ca(2+) entry pathways appears as one mechanism whereby extracellular and membrane signals influence smooth muscle phenotype regulation, with MEF2 as a focal point.

cGMP-dependent protein kinase type I promotes CREB/CRE-mediated gene expression in neurons of the lateral amygdala

The process transforming newly learned information into stable long-term memory is called memory consolidation and, like the underlying long-term synaptic plasticity, critically depends on de novo RNA and protein synthesis. We have shown recently that the cGMP-dependent protein kinase Type I (cGKI) plays an important role for the consolidation of amygdala-dependent fear memory and long-term potentiation (LTP) in the lateral amygdala. Signalling downstream of cGKI at the level of transcriptional regulation remained unclear. A transcription factor of major importance for learning and memory is the cAMP-response element binding protein (CREB). The representation of fear memory in the lateral amygdala strikingly depends on the activity of CREB in individual neurons. Moreover, findings from in vitro experiments demonstrate CREB phosphorylation by cGK. In the hippocampus, CREB phosphorylation increases following activation of NO/cGMP signalling contributing to the late phase of LTP. To demonstrate a link from cGKI to activation of CREB and CREB-dependent transcription in neurons of the lateral amygdala as a possible mechanism for cGKI-mediated fear memory consolidation, we examined the effect of cGMP on activation of CREB/CRE using immunohistochemical staining specific for phospho-CREB and a reporter gene in control and cGKI-deficient mice, respectively. Supporting our hypothesis, marked CREB phosphorylation and CRE-mediated transcription was induced by cGMP in the lateral amygdala of control mice, but not in cGKI-deficient mice. It has been proposed that activation of cGKI is followed by its nuclear translocation that would allow direct phosphorylation of CREB. Therefore, we examined the cellular localisation of cGKI in neurons of the lateral amygdala in the presence of cGMP by double staining for cGKI and a nuclear marker in sections from areas showing prominent CREB phosphorylation, and did not observe prominent nuclear translocation of the enzyme. In summary, we provide evidence that cytosolic cGKI can support fear memory consolidation and LTP in neurons of the lateral amygdala via activation of CREB and CRE-dependent transcription.

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.

Cardionomics: a new integrative approach for screening cardiotoxicity of drug candidates

BACKGROUND

Despite the FDA guidelines for studies to be performed to rule out potential cardiac toxicity, many drugs have nevertheless entered the market only to be later withdrawn from the market owing to cardiac toxicity. Cardiac toxicity may result from drugs causing impaired function or death of cardiomyocytes, valvular damage, myocardial ischemia and/or ventricular arrhythmias. Negative cardiovascular events have been implicated in 28% of drug withdrawals in the USA. The significance for patients, regulators and the pharmaceutical industry is immense.

OBJECTIVE

We address whether a more rigorous and integrative approach is needed for cardiovascular safety screening of all new drug candidates. Furthermore, we will present a cardionomics approach that looks at several in vitro and in vivo models that can be applied to all drugs independent of category, therapeutic area or class.

METHODS

We present examples of drugs demonstrating cardiac toxicity and provide an in-depth review of how calcium homeostasis may be a unifying theme in clinically observed cardiotoxic events. We introduce a cardionomics approach that detects clinical cardiac toxicity early in the drug discovery process, thus, preventing costly late attrition.

CONCLUSION

The consequences of a failure to detect potential cardiovascular safety issues before clinical launch can have an enormous cost for the pharmaceutical industry, when major drugs are withdrawn due to lawsuits as well as loss of time and resources. An integrated cardionomics approach may reduce the risk of drug withdrawals as a result of unexpected clinical cardiac safety issues.

Molecular and biophysical mechanisms of Ca2+ sparklets in smooth muscle

In this article, we review the biophysical basis and functional implications of a novel Ca(2+) signal (called "Ca(2+) sparklets") produced by Ca(2+) influx via L-type Ca(2+) channels (LTCCs) in smooth muscle. Ca(2+) sparklet activity is bimodal. In low activity mode, Ca(2+) sparklets are produced by random, brief openings of solitary LTCCs. In contrast, small clusters of LTCCs can function in a high activity mode that creates sites of continual Ca(2+) influx called "persistent Ca(2+) sparklets". Low activity and persistent Ca(2+) sparklets contribute to Ca(2+) influx in arterial, colonic, and venous smooth muscle. Targeting of PKCalpha by the scaffolding protein AKAP150 to specific sarcolemmal domains is required for the activation of persistent Ca(2+) sparklets. Calcineurin, which is also associated with AKAP150, opposes the actions of PKCalpha on Ca(2+) sparklets. At hyperpolarized potentials, Ca(2+) sparklet activity is low and hence does not contribute to global [Ca(2+)](i). Membrane depolarization increases low and persistent Ca(2+) sparklet activity, thereby increasing local and global [Ca(2+)](i). Ca(2+) sparklet activity is increased in arterial myocytes during hypertension, thus increasing Ca(2+) influx and activating the transcription factor NFATc3. We discuss a model for subcellular variations in Ca(2+) sparklet activity and their role in the regulation of excitation-contraction coupling and excitation-transcription coupling in smooth muscle.

Developmental control of CaV1.2 L-type calcium channel splicing by Fox proteins

CaV1.2 voltage-gated calcium channels play critical roles in the control of membrane excitability, gene expression, and muscle contraction. These channels show diverse functional properties generated by alternative splicing at multiple sites within the CaV1.2 pre-mRNA. The molecular mechanisms controlling this splicing are not understood. We find that two exons in the CaV1.2 channel are controlled in part by members of the Fox family of splicing regulators. Exons 9* and 33 confer distinct electrophysiological properties on the channel and show opposite patterns of regulation during cortical development, with exon 9* progressively decreasing its inclusion in the CaV1.2 mRNA over time and exon 33 progressively increasing. Both exons contain Fox protein binding elements within their adjacent introns, and Fox protein expression is induced in cortical neurons in parallel with the changes in CaV1.2 splicing. We show that knocking down expression of Fox proteins in tissue culture cells has opposite effects on exons 9* and 33. The loss of Fox protein increases exon 9* splicing and decreases exon 33, as predicted by the positions of the Fox binding elements and by the pattern of splicing in development. Conversely, overexpression of Fox1 and Fox2 proteins represses exon 9* and enhances exon 33 splicing in the endogenous CaV1.2 mRNA. These effects of Fox proteins on exons 9* and 33 can be recapitulated in transfected minigene reporters. Both the repressive and the enhancing effects of Fox proteins are dependent on the Fox binding elements within and adjacent to the target exons, indicating that the Fox proteins are directly regulating both exons. These results demonstrate that the Fox protein family is playing a key role in tuning the properties of CaV1.2 calcium channels during neuronal development.

Smooth muscle signalling pathways in health and disease

Smooth muscle contractile activity is a major regulator of function of the vascular system, respiratory system, gastrointestinal system and the genitourinary systems. Malfunction of contractility in these systems leads to a host of clinical disorders, and yet, we still have major gaps in our understanding of the molecular mechanisms by which contractility of the differentiated smooth muscle cell is regulated. This review will summarize recent advances in the molecular understanding of the regulation of smooth muscle myosin activity via phosphorylation/dephosphorylation of myosin, the regulation of the accessibility of actin to myosin via the actin-binding proteins calponin and caldesmon, and the remodelling of the actin cytoskeleton. Understanding of the molecular 'players' should identify target molecules that could point the way to novel drug discovery programs for the treatment of smooth muscle disorders such as cardiovascular disease, asthma, functional bowel disease and pre-term labour.

Smooth muscle signalling pathways in health and disease

Smooth muscle contractile activity is a major regulator of function of the vascular system, respiratory system, gastrointestinal system and the genitourinary systems. Malfunction of contractility in these systems leads to a host of clinical disorders, and yet, we still have major gaps in our understanding of the molecular mechanisms by which contractility of the differentiated smooth muscle cell is regulated. This review will summarize recent advances in the molecular understanding of the regulation of smooth muscle myosin activity via phosphorylation/dephosphorylation of myosin, the regulation of the accessibility of actin to myosin via the actin-binding proteins calponin and caldesmon, and the remodelling of the actin cytoskeleton. Understanding of the molecular 'players' should identify target molecules that could point the way to novel drug discovery programs for the treatment of smooth muscle disorders such as cardiovascular disease, asthma, functional bowel disease and pre-term labour.

Advanced glycation endproducts induce a proliferative response in vascular smooth muscle cells via altered calcium signaling

Advanced glycation endproducts (AGEs) are proteins that accumulate in the plasma of diabetics as a result of increased glucose concentrations and are closely linked with vascular disease. The mechanisms involved are still not clear. The aim of this study was to investigate whether AGE-induced changes in calcium (Ca2+) homeostasis could contribute to these mechanisms. Cultured porcine coronary artery vascular smooth muscle (VSM) cells were preincubated with glycated albumin for 96 h. The sphingosine 1-phosphate (S1P)-induced intracellular Ca2+ increase, although not increased in amplitude, was significantly prolonged in cells preincubated with glycated albumin. Intracellular Ca2+ imaging and electrophysiological recording of ion channel currents following release of caged Ca2+ indicated that this prolonged Ca2+ rise occurred predominantly via changes in Ca2+-induced Ca2+ release. Preincubation with glycated albumin also resulted in a threefold increase in expression of the receptor for AGE. As a consequence of the prolonged intracellular Ca2+ rise following preincubation with glycated albumin, the S1P-induced activation of the Ca2+-dependent phosphatase, calcineurin (CaN) was increased. This resulted in increased S1P-induced activation of the Ca2+-dependent transcription factor, nuclear factor of activated T cells (NFATc). BrdU incorporation in VSM cells was increased in cells preincubated with glycated albumin and was inhibited by the CaN inhibitor, cyclosporin A. In conclusion, AGE can induce VSM proliferation via a prolonged agonist-induced Ca2+ increase leading to increased activation of CaN and subsequently NFATc. This mechanism may contribute to pathogenesis of vascular disease in diabetes mellitus.

The cardiac sarcoplasmic/endoplasmic reticulum calcium ATPase: a potent target for cardiovascular diseases

The cardiac isoform of the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA2a) is a calcium ion (Ca(2+)) pump powered by ATP hydrolysis. SERCA2a transfers Ca(2+) from the cytosol of the cardiomyocyte to the lumen of the sarcoplasmic reticulum during muscle relaxation. As such, this transporter has a key role in cardiomyocyte Ca(2+) regulation. In both experimental models and human heart failure, SERCA2a expression is significantly decreased, which leads to abnormal Ca(2+) handling and a deficient contractile state. Following a long line of investigations in isolated cardiac myocytes and small and large animal models, a clinical trial is underway that is restoring SERCA2a expression in patients with heart failure by use of adeno-associated virus type 1. Beyond its role in contractile abnormalities in heart failure, SERCA2a overexpression has beneficial effects in a host of other cardiovascular diseases. Here we describe the mechanism of Ca(2+) regulation by SERCA2a, examine the beneficial effects as well as the failures, risks and complexities associated with SERCA2a overexpression, and discuss the potential of SERCA2a as a target for the treatment of cardiovascular disease.

Degradation of PEP-19, a calmodulin-binding protein, by calpain is implicated in neuronal cell death induced by intracellular Ca2+ overload

Excessive elevation of intracellular Ca2+ levels and, subsequently, hyperactivation of Ca2+/calmodulin-dependent processes might play an important role in the pathologic events following cerebral ischemia. PEP-19 is a neuronally expressed polypeptide that acts as an endogenous negative regulator of calmodulin by inhibiting the association of calmodulin with enzymes and other proteins. The aims of the present study were to investigate the effect of PEP-19 overexpression on cell death triggered by Ca2+ overload and how the polypeptide levels are affected by glutamate-induced excitotoxicity and cerebral ischemia. Expression of PEP-19 in HEK293T cells suppressed calmodulin-dependent signaling and protected against cell death elicited by Ca2+ ionophore. Likewise, primary cortical neurons overexpressing PEP-19 became resistant to glutamate-induced cell death. In immunoprecipitation assay, wild type PEP-19 associated with calmodulin, whereas mutated PEP-19, which contains mutations within the calmodulin binding site of PEP-19, failed to associate with calmodulin. We found that the mutation abrogates both the ability to suppress calmodulin-dependent signaling and to protect cells from death. Additionally, the endogenous PEP-19 levels in neurons were significantly reduced following glutamate exposure, this reduction precedes neuronal cell death and can be blocked by treatment with calpain inhibitors. These data suggest that PEP-19 is a substrate for calpain, and that the decreased PEP-19 levels result from its degradation by calpain. A similar reduction of PEP-19 also occurred in the hippocampus of gerbils subjected to transient global ischemia. In contrast to the reduction in PEP-19, no changes in calmodulin occurred following excitotoxicity, suggesting the loss of negative regulation of calmodulin by PEP-19. Taken together, these results provide evidence that PEP-19 overexpression enhances resistance to Ca2+-mediated cytotoxicity, which might be mediated through calmodulin inhibition, and also raises the possibility that PEP-19 degradation by calpain might produce an aberrant activation of calmodulin functions, which in turn causes neuronal cell death.

Signature combinatorial splicing profiles of rat cardiac- and smooth-muscle Cav1.2 channels with distinct biophysical properties

l-type (Ca(v)1.2) voltage-gated calcium channels play an essential role in muscle contraction in the cardiovascular system. Alternative splicing of the pore-forming Ca(v)1.2 subunit provides potent means to enrich the functional diversity of the channels. There are 11 alternatively spliced exons identified in rat Ca(v)1.2 gene and random rearrangements may generate up to hundreds of combinatorial splicing profiles. Due to such complexity, the real combinatorial splicing profiles of Ca(v)1.2 have not been solved. This study investigated whether the 11 alternatively spliced exons are spliced randomly or linked and if linked, how many combinatorial splicing profiles can be arranged in cardiac- and smooth-muscle cells. By examining three full-length cDNA libraries of the Ca(v)1.2 transcripts isolated from rat heart and aorta, our results showed that the arrangements of some of the alternatively spliced exons are tissue-specific and tightly linked, giving rise to only 41 alternative combinatorial profiles, of which 29 have not been reported. Interestingly, the 41 combinatorial profiles were distinctively distributed in the three Ca(v)1.2 libraries and the one named "heart 1-50" contained unexpected splice variants. Significantly, the tissue-specific cardiac- and smooth-muscle combinatorial splicing profiles of Ca(v)1.2 channels demonstrated distinct electrophysiological properties that may help rationalize the differences observed in native currents. The unique sequences in these tissue-specific splice variants may provide the potential targets for drug design and screening.

Ion channels and calcium signaling in cerebral arteries following subarachnoid hemorrhage

Entry of Ca(2+) through voltage-dependent calcium channels (VDCCs) is critical to the regulation of intracellular free calcium concentration ([Ca(2+)](i)) in vascular smooth muscle and thus the control of cerebral artery diameter. Increased VDCC activity in cerebral artery myocytes may contribute to decreased cerebral blood flow and the accompanying neurological deficits associated with subarachnoid hemorrhage (SAH). This review will focus on the impact of SAH on VDCCs and K(+)-selective ion channels, two important classes of ion channels located in the plasma membrane of cerebral artery myocytes. SAH may act through a variety of direct and indirect mechanisms to increase the activity of VDCCs promoting cerebral artery constriction and reduced cerebral blood flow. Further, SAH may lead to suppression of K(+) channel activity to cause membrane potential depolarization to enhance VDCC activity. The ability of VDCC blockers or K(+) channel activators to alleviate SAH-induced vasospasm will also be examined.

Impact of testosterone on cardiac L-type calcium channels and Ca2+ sparks: acute actions antagonize chronic effects

While androgens generally have been associated with an increased cardiovascular risk, recent studies indicate potential beneficial acute effects of testosterone. However, detailed evaluation of chronic and acute actions of testosterone on the function of cardiac I(Ca,L) and intracellular Ca2+ handling is limited. To clarify this situation we performed whole-cell and single-channel analysis of I(Ca,L), recordings of Ca2+ sparks, measurements of contractility and quantitative real-time RT-PCR in rat cardiomyocytes following testosterone pretreatment and acute testosterone application. Pretreatment with testosterone 100 nM for 24-30 h increased whole-cell I(Ca,L) from 3.8+/-0.8 pA/pF (n=10) to 10.1+/-0.31 pA/pF (n=9) at +10 mV (p<0.001). Increase of I(Ca,L) density was caused by both, increased expression levels of the alpha 1C subunit of L-type calcium channel and a pronounced increment of the single-channel activity (availability 81.8+/-3.15% versus 37.1+/-7.01%; open probability 12.8+/-3.09% versus 1.0+/-0.62%, p<0.01). Moreover, testosterone pretreatment significantly increased the frequency of Ca2+ sparks and improved myocytes contractility without altering SR Ca2+ load. All chronic effects could be inhibited by flutamide. In contrast acute testosterone administration significantly reduced I(Ca,L) density. Indeed, on the single-channel level acute testosterone application completely reversed the chronic testosterone-mediated effects, and antagonized the chronic testosterone effects on Ca2+ spark frequency, which was unaffected by flutamide. Thus, testosterone pretreatment activates I(Ca,L) via nuclear receptor-mediated pathways, while testosterone acutely blocks I(Ca,L) in a direct manner. Thus, testosterone chronically affects the basal level of intracellular Ca2+ handling, which in addition rapidly may be modulated by acute changes of hormone levels.

Calcium controls smooth muscle TRPC gene transcription via the CaMK/calcineurin-dependent pathways

Transient receptor potential protein family C (TRPC) has been proposed as a candidate for channels involved in capacitative Ca(2+) entry (CCE) mechanisms, but the modulation of their gene expression remains unexplored. In this study we show that guinea pig gallbladder smooth muscle contains mRNA encoding TRPC1, TRPC2, TRPC3, and TRPC4 proteins whose abundance depends on cytosolic Ca(2+) level ([Ca(2+)](i)). Thus lowering the levels of cellular calcium with the chelators EGTA and BAPTA AM results in a downregulation of TRPC1-TRPC4 gene and protein expression. In contrast, activation of Ca(2+) influx through L-type Ca(2+) channels and Ca(2+) release from intracellular stores induced an increase in TRPC1-TRPC4 mRNA and protein abundance. Activation of Ca(2+)/calmodulin-dependent kinases (CaMK) and phosphorylation of cAMP-response element binding protein accounts for the increase in TRPC mRNA transcription in response to L-type channel-mediated Ca(2+) influx . In addition to this mechanism, activation of TRPC gene expression by intracellular Ca(2+) release also involves calcineurin pathway. According to the proposed role for these channels, activation of CCE induced an increase in TRPC1 and TRPC3 mRNA abundance, which depends on the integrity of the calcineurin and CaMK pathways. These findings show for the first time an essential autoregulatory role of Ca(2+) in Ca(2+) homeostasis at the level of TRPC gene and protein expression.

Calcium-activated potassium channels and the regulation of vascular tone

Different calcium signals in the endothelium and smooth muscle target different types of Ca2+-sensitive K+ channels to modulate vascular function. These differential calcium signals and targets represent multilayered opportunities for prevention and/or treatment of vascular dysfunctions.

Involvement of calmodulin in neuronal cell death

A large body of evidence indicates that disturbances of Ca(2+) homeostasis may be a causative factor in the neurotoxicity following cerebral ischemia. However, the mechanisms by which Ca(2+) overload leads to neuronal cell death have not been fully elucidated. Calmodulin, a major intracellular Ca(2+)-binding protein found mainly in the central nervous system, mediates many physiological functions in response to changes in the intracellular Ca(2+) concentration, whereas Ca(2+) overload in neurons after excitotoxic insult may induce excessive activation of calmodulin signaling pathways, leading to neuronal cell death. To determine the role of calmodulin in the induction of neuronal cell death, we generated primary rat cortical neurons that express a mutant calmodulin with a defect in Ca(2+)-binding affinity. Neurons expressing the mutant had low responses of calmodulin-dependent signaling to membrane depolarization by high KCl and became resistant to glutamate-triggered excitotoxic neuronal cell death compared with the vector or wild-type calmodulin-transfected cells, indicating that blocking calmodulin function is protective against excitotoxic insult. These results suggest that calmodulin plays a crucial role in the processes of Ca(2+)-induced neuronal cell death and the possibility that the blockage of calmodulin attenuates brain injury after cerebral ischemia.