The Ca V 3.2 T-Type Ca 2+ Channel Is Required for Pressure Overload–Induced Cardiac Hypertrophy in Mice

Voltage-gated calcium channel blockers deregulate macroautophagy in cardiomyocytes

Voltage-gated calcium channel blockers are widely used for the management of cardiovascular diseases, however little is known about their effects on cardiac cells in vitro. We challenged neonatal ventricular cardiomyocytes (CMs) with therapeutic L-type and T-type Ca(2+) channel blockers (nifedipine and mibefradil, respectively), and measured their effects on cell stress and survival, using fluorescent microscopy, Q-PCR and Western blot. Both nifedipine and mibefradil induced a low-level and partially transient up-regulation of three key mediators of the Unfolded Protein Response (UPR), indicative of endoplasmic (ER) reticulum stress. Furthermore, nifedipine triggered the activation of macroautophagy, as evidenced by increased lipidation of microtubule-associated protein 1 light chain 3 (LC3), decreased levels of polyubiquitin-binding protein p62/SQSTM1 and ubiquitinated protein aggregates, that was followed by cell death. In contrast, mibefradil inhibited CMs constitutive macroautophagy and did not promote cell death. The siRNA-mediated gene silencing approach confirmed the pharmacological findings for T-type channels. We conclude that L-type and T-type Ca(2+) channel blockers induce ER stress, which is divergently transduced into macroautophagy induction and inhibition, respectively, with relevance for cell viability. Our work identifies VGCCs as novel regulators of autophagy in the heart muscle and provides new insights into the effects of VGCC blockers on CMs homeostasis, that may underlie both noxious and cardioprotective effects.

Caveolin-3 Overexpression Attenuates Cardiac Hypertrophy via Inhibition of T-type Ca2+ Current Modulated by Protein Kinase Cα in Cardiomyocytes

Pathological cardiac hypertrophy is characterized by subcellular remodeling of the ventricular myocyte with a reduction in the scaffolding protein caveolin-3 (Cav-3), altered Ca(2+) cycling, increased protein kinase C expression, and hyperactivation of calcineurin/nuclear factor of activated T cell (NFAT) signaling. However, the precise role of Cav-3 in the regulation of local Ca(2+) signaling in pathological cardiac hypertrophy is unclear. We used cardiac-specific Cav-3-overexpressing mice and in vivo and in vitro cardiac hypertrophy models to determine the essential requirement for Cav-3 expression in protection against pharmacologically and pressure overload-induced cardiac hypertrophy. Transverse aortic constriction and angiotensin-II (Ang-II) infusion in wild type (WT) mice resulted in cardiac hypertrophy characterized by significant reduction in fractional shortening, ejection fraction, and a reduced expression of Cav-3. In addition, association of PKCα and angiotensin-II receptor, type 1, with Cav-3 was disrupted in the hypertrophic ventricular myocytes. Whole cell patch clamp analysis demonstrated increased expression of T-type Ca(2+) current (ICa, T) in hypertrophic ventricular myocytes. In contrast, the Cav-3-overexpressing mice demonstrated protection from transverse aortic constriction or Ang-II-induced pathological hypertrophy with inhibition of ICa, T and intact Cav-3-associated macromolecular signaling complexes. siRNA-mediated knockdown of Cav-3 in the neonatal cardiomyocytes resulted in enhanced Ang-II stimulation of ICa, T mediated by PKCα, which caused nuclear translocation of NFAT. Overexpression of Cav-3 in neonatal myocytes prevented a PKCα-mediated increase in ICa, T and nuclear translocation of NFAT. In conclusion, we show that stable Cav-3 expression is essential for protecting the signaling mechanisms in pharmacologically and pressure overload-induced cardiac hypertrophy.

A pathway and network review on beta-adrenoceptor signaling and beta blockers in cardiac remodeling

It is well established that cardiac remodeling plays a pivotal role in the development of heart failure, a leading cause of death worldwide. Meanwhile, sympathetic hyperactivity is an important factor in inducing cardiac remodeling. Therefore, an in-depth understanding of beta-adrenoceptor signaling pathways would help to find better ways to reverse the adverse remodeling. Here, we reviewed five pathways, namely mitogen-activated protein kinase signaling, Gs-AC-cAMP signaling, Ca(2+)-calcineurin-NFAT/CaMKII-HDACs signaling, PI3K signaling and beta-3 adrenergic signaling, in cardiac remodeling. Furthermore, we constructed a cardiac-remodeling-specific regulatory network including miRNA, transcription factors and target genes within the five pathways. Both experimental and clinical studies have documented beneficial effects of beta blockers in cardiac remodeling; nevertheless, different blockers show different extent of therapeutic effect. Exploration of the underlying mechanisms could help developing more effective drugs. Current evidence of treatment effect of beta blockers in remodeling was also reviewed based upon information from experimental data and clinical trials. We further discussed the mechanism of how beta blockers work and why some beta blockers are more potent than others in treating cardiac remodeling within the framework of cardiac remodeling network.

Genetic ablation of CaV3.2 channels enhances the arterial myogenic response by modulating the RyR-BKCa axis

OBJECTIVE

In resistance arteries, there is an emerging view that smooth muscle CaV3.2 channels restrain arterial constriction through a feedback response involving the large-conductance Ca(2+)-activated K(+) channel (BKCa). Here, we used wild-type and CaV3.2 knockout (CaV3.2(-/-)) mice to definitively test whether CaV3.2 moderates myogenic tone in mesenteric arteries via the CaV3.2-ryanodine receptor-BKCa axis and whether this regulatory mechanism influences blood pressure regulation.

APPROACH AND RESULTS

Using pressurized vessel myography, CaV3.2(-/-) mesenteric arteries displayed enhanced myogenic constriction to pressure but similar K(+)-induced vasoconstriction compared with wild-type C57BL/6 arteries. Electrophysiological and myography experiments subsequently confirmed the inability of micromolar Ni(2+), a CaV3.2 blocker, to either constrict arteries or suppress T-type currents in CaV3.2(-/-) smooth muscle cells. The frequency of BKCa-induced spontaneous transient outward K(+) currents dropped in wild-type but not in knockout arterial smooth muscle cells upon the pharmacological suppression of CaV3.2 channel. Line scan analysis performed on en face arteries loaded with Fluo-4 revealed the presence of Ca(2+) sparks in all arteries, with the subsequent application of Ni(2+) only affecting wild-type arteries. Although CaV3.2 channel moderated myogenic constriction of resistance arteries, the blood pressure measurements of CaV3.2(-/-) and wild-type animals were similar.

CONCLUSIONS

Overall, our findings establish a negative feedback mechanism of the myogenic response in which CaV3.2 channel modulates downstream ryanodine receptor-BKCa to hyperpolarize and relax arteries.

The Ca(2+)-activated cation channel TRPM4 is a negative regulator of angiotensin II-induced cardiac hypertrophy

Cardiac muscle adapts to hemodynamic stress by altering myocyte size and function, resulting in cardiac hypertrophy. Alteration in myocyte calcium homeostasis is known to be an initial signal in cardiac hypertrophy signaling. Transient receptor potential melastatin 4 protein (TRPM4) is a calcium-activated non-selective cation channel, which plays a role in regulating calcium influx and calcium-dependent cell functions in many cell types including cardiomyocytes. Selective deletion of TRPM4 from the heart muscle in mice resulted in an increased hypertrophic growth after chronic angiotensin (AngII) treatment, compared to WT mice. The enhanced hypertrophic response was also traceable by the increased expression of hypertrophy-related genes like Rcan1, ANP, and α-Actin. Intracellular calcium measurements on isolated ventricular myocytes showed significantly increased store-operated calcium entry upon AngII treatment in myocytes lacking the TRPM4 channel. Elevated intracellular calcium is a key factor in the development of pathological cardiac hypertrophy, leading to the activation of intracellular signaling pathways. In agreement with this, we observed significantly higher Rcan1 mRNA level, calcineurin enzyme activity and protein level in lysates from TRPM4-deficient mice heart compared to WT after AngII treatment. Collectively, these observations are consistent with a model in which TRPM4 is a regulator of calcium homeostasis in cardiomyocytes after AngII stimulation. TRPM4 contributes to the regulation of driving force for store-operated calcium entry and thereby the activation of the calcineurin–NFAT pathway and the development of pathological hypertrophy.

Endostatin inhibits T-type Ca2+ channel current in guinea pig ventricular myocyte

Endostatin, a fragment of collagen XVIII, is known as an endogenous angiogenesis inhibitor, and its serum concentration increases in various cardiovascular diseases. T-type Ca²⁺ channel, low voltage-activated Ca²⁺ channel, is not expressed in adult ventricular myocytes. Re-expression of T-type Ca²⁺ channels in cardiac myocytes is thought to be involved in the development of cardiac hypertrophy. We examined the effects of endostatin on T-type Ca²⁺ channel current by whole-cell patch clamp technique in freshly isolated adult guinea pig ventricular myocytes, which exceptionally express T-type Ca²⁺ channels. Although endostatin 300 ng/ml had no effect on L-type Ca²⁺ current, it significantly inhibited T-type Ca²⁺ current. These data indicate that endostatin can be an endogenous inhibitor of T-type Ca²⁺ channels in the cardiac myocytes.

Recurrent gain of function mutation in calcium channel CACNA1H causes early-onset hypertension with primary aldosteronism

Many Mendelian traits are likely unrecognized owing to absence of traditional segregation patterns in families due to causation by de novo mutations, incomplete penetrance, and/or variable expressivity. Genome-level sequencing can overcome these complications. Extreme childhood phenotypes are promising candidates for new Mendelian traits. One example is early onset hypertension, a rare form of a global cause of morbidity and mortality. We performed exome sequencing of 40 unrelated subjects with hypertension due to primary aldosteronism by age 10. Five subjects (12.5%) shared the identical, previously unidentified, heterozygous CACNA1H^M1549V mutation. Two mutations were demonstrated to be de novo events, and all mutations occurred independently. CACNA1H encodes a voltage-gated calcium channel (Ca_V3.2) expressed in adrenal glomerulosa. CACNA1H^M1549V showed drastically impaired channel inactivation and activation at more hyperpolarized potentials, producing increased intracellular Ca²⁺, the signal for aldosterone production. This mutation explains disease pathogenesis and provides new insight into mechanisms mediating aldosterone production and hypertension.

DOI:http://dx.doi.org/10.7554/eLife.06315.001

The consequence of mutations to the large majority of human genes is unknown. Most mutations that are currently known were discovered by tracing their effects through families. This allows the locations of mutations to be pinpointed on chromosomes—the structures that genetic material is packaged into. Other mutations are harder to trace because individuals with these mutations may develop very different signs and symptoms, or not develop clinical abnormalities at all. Alternatively, a trait may appear sporadically in a family because the mutation arises anew in the affected subject.

Recently developed technologies that allow scientists to rapidly sequence all the gene-encoding regions of an individual's DNA—their genome—offer a new way to identify harmful genetic variants. Comparing the genomes of individuals with rare disorders can reveal if the individuals share any genetic mutations in common that could cause their symptoms.

Scholl et al. used this strategy to sequence the genomes of 40 individuals with a rare type of hypertension—a condition that causes high blood pressure, and increases the risk of strokes, kidney failure and heart attacks—that develops early in childhood. In this form of the disease, high blood pressure is caused by the adrenal glands above the kidneys producing too much of a hormone called aldosterone. Some genetic causes of this form of the disease have already been identified. Now, Scholl et al. have found a new genetic mutation present in five families with this condition. Two of the individuals were the first in their families to develop this mutation, while three others inherited it. Some of the family members with this mutation had hypertension and some did not.

The mutation is in a gene that encodes a type of calcium channel—a protein found in the membrane that surrounds cells, and which can open and close to control the amount of calcium in the cell. This particular calcium channel is abundant in the cells of the adrenal gland. Scholl et al. found that the mutation causes the calcium channels to be more likely to open and take longer to close. This increases the number of calcium ions that move into the cell, which causes the adrenal gland to produce more aldosterone. These new insights have provided a new way of diagnosing early-onset hypertension, and suggest that targeting calcium channels could help to develop new treatments for this disease.

DOI:http://dx.doi.org/10.7554/eLife.06315.002

Eps15 Homology Domain-containing Protein 3 Regulates Cardiac T-type Ca2+ Channel Targeting and Function in the Atria

Proper trafficking of membrane-bound ion channels and transporters is requisite for normal cardiac function. Endosome-based protein trafficking of membrane-bound ion channels and transporters in the heart is poorly understood, particularly in vivo. In fact, for select cardiac cell types such as atrial myocytes, virtually nothing is known regarding endosomal transport. We previously linked the C-terminal Eps15 homology domain-containing protein 3 (EHD3) with endosome-based protein trafficking in ventricular cardiomyocytes. Here we sought to define the roles and membrane protein targets for EHD3 in atria. We identify the voltage-gated T-type Ca(2+) channels (CaV3.1, CaV3.2) as substrates for EHD3-dependent trafficking in atria. Mice selectively lacking EHD3 in heart display reduced expression and targeting of both Cav3.1 and CaV3.2 in the atria. Furthermore, functional experiments identify a significant loss of T-type-mediated Ca(2+) current in EHD3-deficient atrial myocytes. Moreover, EHD3 associates with both CaV3.1 and CaV3.2 in co-immunoprecipitation experiments. T-type Ca(2+) channel function is critical for proper electrical conduction through the atria. Consistent with these roles, EHD3-deficient mice demonstrate heart rate variability, sinus pause, and atrioventricular conduction block. In summary, our findings identify CaV3.1 and CaV3.2 as substrates for EHD3-dependent protein trafficking in heart, provide in vivo data on endosome-based trafficking pathways in atria, and implicate EHD3 as a key player in the regulation of atrial myocyte excitability and cardiac conduction.

Calcium signalling in developing cardiomyocytes: implications for model systems and disease

Adult cardiomyocytes exhibit complex Ca(2+) homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca(2+) sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca(2+) homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as cultured neonatal cells and stem cells.

Microdomain-specific localization of functional ion channels in cardiomyocytes: an emerging concept of local regulation and remodelling

Cardiac excitation involves the generation of action potential by individual cells and the subsequent conduction of the action potential from cell to cell through intercellular gap junctions. Excitation of the cellular membrane results in opening of the voltage-gated L-type calcium ion (Ca2+) channels, thereby allowing a small amount of Ca2+ to enter the cell, which in turn triggers the release of a much greater amount of Ca2+ from the sarcoplasmic reticulum, the intracellular Ca2+ store, and gives rise to the systolic Ca2+ transient and contraction. These processes are highly regulated by the autonomic nervous system, which ensures the acute and reliable contractile function of the heart and the short-term modulation of this function upon changes in heart rate or workload. It has recently become evident that discrete clusters of different ion channels and regulatory receptors are present in the sarcolemma, where they form an interacting network and work together as a part of a macro-molecular signalling complex which in turn allows the specificity, reliability and accuracy of the autonomic modulation of the excitation-contraction processes by a variety of neurohormonal pathways. Disruption in subcellular targeting of ion channels and associated signalling proteins may contribute to the pathophysiology of a variety of cardiac diseases, including heart failure and certain arrhythmias. Recent methodological advances have made it possible to routinely image the topography of live cardiomyocytes, allowing the study of clustering functional ion channels and receptors as well as their coupling within a specific microdomain. In this review we highlight the emerging understanding of the functionality of distinct subcellular microdomains in cardiac myocytes (e.g. T-tubules, lipid rafts/caveolae, costameres and intercalated discs) and their functional role in the accumulation and regulation of different subcellular populations of sodium, Ca2+ and potassium ion channels and their contributions to cellular signalling and cardiac pathology.

Functional importance of T-type voltage-gated calcium channels in the cardiovascular and renal system: news from the world of knockout mice

Over the years, it has been discussed whether T-type calcium channels Cav3 play a role in the cardiovascular and renal system. T-type channels have been reported to play an important role in renal hemodynamics, contractility of resistance vessels, and pacemaker activity in the heart. However, the lack of highly specific blockers cast doubt on the conclusions. As new T-type channel antagonists are being designed, the roles of T-type channels in cardiovascular and renal pathology need to be elucidated before T-type blockers can be clinically useful. Two types of T-type channels, Cav3.1 and Cav3.2, are expressed in blood vessels, the kidney, and the heart. Studies with gene-deficient mice have provided a way to investigate the Cav3.1 and Cav3.2 channels and their role in the cardiovascular system. This review discusses the results from these knockout mice. Evaluation of the literature leads to the conclusion that Cav3.1 and Cav3.2 channels have important, but different, functions in mice. T-type Cav3.1 channels affect heart rate, whereas Cav3.2 channels are involved in cardiac hypertrophy. In the vascular system, Cav3.2 activation leads to dilation of blood vessels, whereas Cav3.1 channels are mainly suggested to affect constriction. The Cav3.1 channel is also involved in neointima formation following vascular damage. In the kidney, Cav3.1 regulates plasma flow and Cav3.2 plays a role setting glomerular filtration rate. In conclusion, Cav3.1 and Cav3.2 are new therapeutic targets in several cardiovascular pathologies, but the use of T-type blockers should be specifically directed to the disease and to the channel subtype.

The organ-protective effect of N-type Ca(2+) channel blockade

The six subtypes of voltage-dependent Ca(2+) channels (VDCCs) mediate a wide range of physiological responses. N-type VDCCs (NCCs) were originally identified as a high voltage-activated Ca(2+) channel selectively blocked by omega-conotoxin (ω-CTX)-GVIA. Predominantly localized in the nervous system, NCCs are key regulators of neurotransmitter release. Both pharmacological blockade with ω-CTX-GVIA and, more recently, mice lacking CNCNA1B, encoding the α1B subunit of NCC, have been used to assess the physiological and pathophysiological functions of NCCs, revealing in part their significant roles in sympathetic nerve activation and nociceptive transmission. The evidence now available indicates that NCCs are a potentially useful therapeutic target for the treatment of several pathological conditions. Efforts are therefore being made to develop effective NCC blockers, including both synthetic ω-CTX-GVIA derivatives and small-molecule inhibitors. Cilnidipine, for example, is a dihydropyridine L-type VDCC blocking agent that also possesses significant NCC blocking ability. As over-activation of the sympathetic nervous system appears to contribute to the pathological processes underlying cardiovascular, renal and metabolic diseases, NCC blockade could be a useful approach to treating these ailments. In this review article, we provide an overview of what is currently known about the physiological and pathophysiological activities of NCCs and the potentially beneficial effects of NCC blockade in several disease conditions, in particular cardiovascular diseases.

Prion protein- and cardiac troponin T-marked interstitial cells from the adult myocardium spontaneously develop into beating cardiomyocytes

Atypically-shaped cardiomyocytes (ACMs) constitute a novel subpopulation of beating heart cells found in the cultures of cardiac myocyte-removed crude fraction cells obtained from adult mouse cardiac ventricles. Although ~500 beating ACMs are observed under microscope in the cell cultures obtained from the hearts of either male or female mice, the origin of these cells in cardiac tissue has yet to be elucidated due to the lack of exclusive markers. In the present study, we demonstrate the efficacy of cellular prion protein (PrP) as a surface marker of ACMs. Cells expressing PrP at the plasma membrane in the culture of the crude fraction cells were found to develop into beating ACMs by themselves or fuse with each other to become larger multinuclear beating ACMs. Combining PrP with a cardiac-specific contractile protein cardiac troponin T (cTnT) allowed us to identify native ACMs in the mouse cardiac ventricles as either clustered or solitary cells. PrP- and cTnT-marked cells were also found in the adult, even aged, human cardiac ventricles. These findings suggest that interstitial cells marked by PrP and cTnT, native ACMs, exhibit life-long survival in the cardiac ventricles of both mice and humans.

Differential effect of T-type voltage-gated Ca2+ channel disruption on renal plasma flow and glomerular filtration rate in vivo

Voltage-gated Ca(2+) (Cav) channels play an essential role in the regulation of renal blood flow and glomerular filtration rate (GFR). Because T-type Cav channels are differentially expressed in pre- and postglomerular vessels, it was hypothesized that they impact renal blood flow and GFR differentially. The question was addressed with the use of two T-type Cav knockout (Cav3.1(-/-) and Cav3.2(-/-)) mouse strains. Continuous recordings of blood pressure and heart rate, para-aminohippurate clearance (renal plasma flow), and inulin clearance (GFR) were performed in conscious, chronically catheterized, wild-type (WT) and Cav3.1(-/-) and Cav3.2(-/-) mice. The contractility of afferent and efferent arterioles was determined in isolated perfused blood vessels. Efferent arterioles from Cav3.2(-/-) mice constricted significantly more in response to a depolarization compared with WT mice. GFR was increased in Cav3.2(-/-) mice with no significant changes in renal plasma flow, heart rate, and blood pressure. Cav3.1(-/-) mice had a higher renal plasma flow compared with WT mice, whereas GFR was indistinguishable from WT mice. No difference in the concentration response to K(+) was observed in isolated afferent and efferent arterioles from Cav3.1(-/-) mice compared with WT mice. Heart rate was significantly lower in Cav3.1(-/-) mice compared with WT mice with no difference in blood pressure. T-type antagonists significantly inhibited the constriction of human intrarenal arteries in response to a small depolarization. In conclusion, Cav3.2 channels support dilatation of efferent arterioles and affect GFR, whereas Cav3.1 channels in vivo contribute to renal vascular resistance. It is suggested that endothelial and nerve localization of Cav3.2 and Cav3.1, respectively, may account for the observed effects.

Emerging roles of junctophilin-2 in the heart and implications for cardiac diseases

Cardiomyocytes rely on a highly specialized subcellular architecture to maintain normal cardiac function. In a little over a decade, junctophilin-2 (JPH2) has become recognized as a cardiac structural protein critical in forming junctional membrane complexes (JMCs), which are subcellular domains essential for excitation-contraction coupling within the heart. While initial studies described the structure of JPH2 and its role in anchoring junctional sarcoplasmic reticulum and transverse-tubule (T-tubule) membrane invaginations, recent research has an expanded role of JPH2 in JMC structure and function. For example, JPH2 is necessary for the development of postnatal T-tubule in mammals. It is also critical for the maintenance of the complex JMC architecture and stabilization of local ion channels in mature cardiomyocytes. Loss of this function by mutations or down-regulation of protein expression has been linked to hypertrophic cardiomyopathy, arrhythmias, and progression of disease in failing hearts. In this review, we summarize current views on the roles of JPH2 within the heart and how JPH2 dysregulation may contribute to a variety of cardiac diseases.

NFAT1 and NFAT3 cooperate with HDAC4 during regulation of alternative splicing of PMCA isoforms in PC12 cells

Background

The bulk of human genes undergo alternative splicing (AS) upon response to physiological stimuli. AS is a great source of protein diversity and biological processes and is associated with the development of many diseases. Pheochromocytoma is a neuroendocrine tumor, characterized by an excessive Ca²⁺-dependent secretion of catecholamines. This underlines the importance of balanced control of calcium transport via regulation of gene expression pattern, including different calcium transport systems, such as plasma membrane Ca²⁺-ATPases (PMCAs), abundantly expressed in pheochromocytoma chromaffin cells (PC12 cells). PMCAs are encoded by four genes (Atp2b1, Atp2b2, Atp2b3, Atp2b4), whose transcript products undergo alternative splicing giving almost 30 variants.

Results

In this scientific report, we propose a novel mechanism of regulation of PMCA alternative splicing in PC12 cells through cooperation of the nuclear factor of activated T-cells (NFAT) and histone deacetylases (HDACs). Luciferase assays showed increased activity of NFAT in PC12 cells, which was associated with altered expression of PMCA. RT-PCR experiments suggested that inhibition of the transcriptional activity of NFAT might result in the rearrangement of PMCA splicing variants in PC12 cells. NFAT inhibition led to dominant expression of 2x/c, 3x/a and 4x/a PMCA variants, while in untreated cells the 2w,z/b, 3z,x/b,c,e,f, and 4x/b variants were found as well. Furthermore, chromatin immunoprecipitation experiments showed that NFAT1-HDAC4 or NFAT3-HDAC4 complexes might be involved in regulation of PMCA2x splicing variant generation.

Conclusions

We suggest that the influence of NFAT/HDAC on PMCA isoform composition might be important for altered dopamine secretion by PC12 cells.

Cav3.2 T-type calcium channel is required for the NFAT-dependent Sox9 expression in tracheal cartilage

Intracellular Ca(2+) transient is crucial in initiating the differentiation of mesenchymal cells into chondrocytes, but whether voltage-gated Ca(2+) channels are involved remains uncertain. Here, we show that the T-type voltage-gated Ca(2+) channel Cav3.2 is essential for tracheal chondrogenesis. Mice lacking this channel (Cav3.2(-/-)) show congenital tracheal stenosis because of incomplete formation of cartilaginous tracheal support. Conversely, Cav3.2 overexpression in ATDC5 cells enhances chondrogenesis, which could be blunted by both blocking T-type Ca(2+) channels and inhibiting calcineurin and suggests that Cav3.2 is responsible for Ca(2+) influx during chondrogenesis. Finally, the expression of sex determination region of Y chromosome (SRY)-related high-mobility group-Box gene 9 (Sox9), one of the earliest markers of committed chondrogenic cells, is reduced in Cav3.2(-/-) tracheas. Mechanistically, Ca(2+) influx via Cav3.2 activates the calcineurin/nuclear factor of the activated T-cell (NFAT) signaling pathway, and a previously unidentified NFAT binding site is identified within the mouse Sox9 promoter using a luciferase reporter assay and gel shift and ChIP studies. Our findings define a previously unidentified mechanism that Ca(2+) influx via the Cav3.2 T-type Ca(2+) channel regulates Sox9 expression through the calcineurin/NFAT signaling pathway during tracheal chondrogenesis.

Role of T-type channels in vasomotor function: team player or chameleon?

Low-voltage-activated T-type calcium channels play an important role in regulating cellular excitability and are implicated in conditions, such as epilepsy and neuropathic pain. T-type channels, especially Cav3.1 and Cav3.2, are also expressed in the vasculature, although patch clamp studies of isolated vascular smooth muscle cells have in general failed to demonstrate these low-voltage-activated calcium currents. By contrast, the channels which are blocked by T-type channel antagonists are high-voltage activated but distinguishable from their L-type counterparts by their T-type biophysical properties and small negative shifts in activation and inactivation voltages. These changes in T-channel properties may result from vascular-specific expression of splice variants of Cav3 genes, particularly in exon 25/26 of the III-IV linker region. Recent physiological studies suggest that T-type channels make a small contribution to vascular tone at low intraluminal pressures, although the relevance of this contribution is unclear. By contrast, these channels play a larger role in vascular tone of small arterioles, which would be expected to function at lower intra-vascular pressures. Upregulation of T-type channel function following decrease in nitric oxide bioavailability and increase in oxidative stress, which occurs during cardiovascular disease, suggests that a more important role could be played by these channels in pathophysiological situations. The ability of T-type channels to be rapidly recruited to the plasma membrane, coupled with their subtype-specific localisation in signalling microdomains where they could modulate the function of calcium-dependent ion channels and pathways, provides a mechanism for rapid up- and downregulation of vasoconstriction. Future investigation into the molecules which govern these changes may illuminate novel targets for the treatment of conditions such as therapy-resistant hypertension and vasospasm.

Tolerability and pharmacokinetics of ACT-280778, a novel nondihydropyridine dual L/T-type calcium channel blocker: early clinical studies in healthy male subjects using adaptive designs

ACT-280778 is a novel nondihydropyridine dual L/T-type calcium channel blocker. Two clinical studies (AC-067-101 and AC-067-102) were conducted to characterize its safety, tolerability, and pharmacokinetics in healthy male subjects after oral administration of single and multiple doses. Both trials were single-center, randomized, double-blind, placebo-controlled, adaptive design, ascending-dose studies, in which ACT-280778 was administrated as single doses of 2, 5, 15, or 40 mg, or as once-daily doses of 5 or 15 mg for 7 days. Single and multiple doses up to and including 15 mg were well tolerated, and no serious or severe adverse event was reported in either study. A single dose of 40 mg was associated with abnormal electrocardiogram findings resulting in the discontinuation of further treatment at this dose or higher doses. ACT-280778 was rapidly absorbed, and larger than dose-proportional increases of the maximum plasma concentration and area under the plasma concentration-time curve were observed. Food intake delayed the time to maximum plasma concentration and doubled exposure. Urinary excretion of unchanged ACT-280778 was negligible, and accumulation at steady state was modest. Overall, pharmacokinetic and tolerability profiles of ACT-280778 observed in these 2 studies warranted further evaluation of ACT-280778 in a proof-of-concept study in patients with hypertension.

Changes in the function and expression of T-type and N-type calcium channels in the rat bladder after bladder outlet obstruction

PURPOSE

We evaluated possible changes in the function and expression of T-type and N-type Ca(2+) channels in the bladder of rats with bladder outlet obstruction.

MATERIALS AND METHODS

Female Sprague Dawley® rats were divided into a group with bladder outlet obstruction created by partial urethral ligation and a sham operated group. Six weeks postoperatively we determined the mRNA expression of T-type and N-type Ca(2+) channels in the bladder, dorsal root ganglion and spinal cord. We also cystometrically investigated expression by intravenous administration of the T-Ca blocker RQ-00311610 or the N-type Ca(2+) channel blocker ω-conotoxin GVIA. We then performed in vitro functional studies of detrusor strips using these blockers.

RESULTS

mRNA expression of T-type Ca(2+) channels in the bladder detrusor and mucosa layers, and the spinal cord dorsal horn, and N-type Ca(2+) channels in the whole bladder and detrusor layer, and the spinal cord dorsal horn was greater in the obstructed group than the sham operated group. In obstructed rats bladder capacity and voided volume increased after RQ-00311610 administration but the number of nonvoiding contractions decreased after ω-conotoxin GVIA administration. Detrusor strips from obstructed rats showed weaker contractile responses to electrical field stimulation, particularly in regard to the purinergic component. ω-Conotoxin GVIA suppressed electrical field stimulation induced contractions only in the detrusor of obstructed rats, especially the cholinergic component.

CONCLUSIONS

Blocking T-type Ca(2+) channels increased bladder capacity while N-type Ca(2+) channel blockade inhibited nonvoiding contractions in rats with bladder outlet obstruction. Decreased bladder efferent neurotransmission occurred after bladder outlet obstruction, predominantly in its purinergic component and detrusor contractions via cholinergic neurotransmission were activated in a compensatory manner, probably via N-type Ca(2+) channel up-regulation.

Quantitative phosphoproteomic study of pressure-overloaded mouse heart reveals dynamin-related protein 1 as a modulator of cardiac hypertrophy

Pressure-overload stress to the heart causes pathological cardiac hypertrophy, which increases the risk of cardiac morbidity and mortality. However, the detailed signaling pathways induced by pressure overload remain unclear. Here we used phosphoproteomics to delineate signaling pathways in the myocardium responding to acute pressure overload and chronic hypertrophy in mice. Myocardial samples at 4 time points (10, 30, 60 min and 2 weeks) after transverse aortic banding (TAB) in mice underwent quantitative phosphoproteomics assay. Temporal phosphoproteomics profiles showed 360 phosphorylation sites with significant regulation after TAB. Multiple mechanical stress sensors were activated after acute pressure overload. Gene ontology analysis revealed differential phosphorylation between hearts with acute pressure overload and chronic hypertrophy. Most interestingly, analysis of the cardiac hypertrophy pathway revealed phosphorylation of the mitochondrial fission protein dynamin-related protein 1 (DRP1) by prohypertrophic kinases. Phosphorylation of DRP1 S622 was confirmed in TAB-treated mouse hearts and phenylephrine (PE)-treated rat neonatal cardiomyocytes. TAB-treated mouse hearts showed phosphorylation-mediated mitochondrial translocation of DRP1. Inhibition of DRP1 with the small-molecule inhibitor mdivi-1 reduced the TAB-induced hypertrophic responses. Mdivi-1 also prevented PE-induced hypertrophic growth and oxygen consumption in rat neonatal cardiomyocytes. We reveal the signaling responses of the heart to pressure stress in vivo and in vitro. DRP1 may be important in the development of cardiac hypertrophy.

Capturing single L-type Ca(2+) channel function with optics

Advances in imaging technology have allowed optical analysis of Ca(2+)-permeable ion channel activity. Here, we briefly review novel developments in optical recording of L-type voltage-dependent Ca(2+) channel (LTCC) function with high spatial and temporal resolution. Underlying principles supporting the use of total internal reflection fluorescence (TIRF) microscopy for optical measurement of channel activity and new functional characteristics of LTCCs revealed by application of this approach are discussed. Visualization of Ca(2+) influx through single LTCCs ("LTCC sparklets") has demonstrated that channel activity is regionally heterogeneous and that clustered channels are capable of operating in a cooperative, or "coupled" manner. In light of these findings, we describe a current molecular model for the local control of LTCC activity and coupled gating in physiological and pathological contexts. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Calcineurin-dependent ion channel regulation in heart

Calcineurin, a serine-threonine-specific, Ca(2+)-calmodulin-activated protein phosphatase, conserved from yeast to humans, plays a key role in regulating cardiac development, hypertrophy, and pathological remodeling. Recent studies demonstrate that calcineurin regulates cardiomyocyte ion channels and receptors in a manner which often entails direct interaction with these target proteins. Here, we review the current state of knowledge of calcineurin-mediated regulation of ion channels in the myocardium with emphasis on the transient outward potassium current (Ito) and L-type calcium current (ICa,L). We go on to discuss unanswered questions that surround these observations and provide perspective on future directions in this exciting field.

T-type Ca²⁺ channels regulate the exit of cardiac myocytes from the cell cycle after birth

T-type Ca(2+) channels (TTCCs) are expressed in the fetal heart and then disappear from ventricular myocytes after birth. The hypothesis examined in this study was the α1G TTCCs' influence in myocyte maturation and their rapid withdrawal from the cell cycle after birth.

METHODS

Cardiac myocytes were isolated from neonatal and adult wild type (WT), α1G-/- and α1G over expressing (α1GDT) mice. Bromodeoxyuridine (BrdU) uptake, myocyte nucleation, cell cycle analysis, and T-type Ca(2+) currents were measured.

RESULTS

All myocytes were mono-nucleated at birth and 35% of WT myocytes expressed functional TTCCs. Very few neonatal myocytes had functional TTCCs in α1G-/- hearts. By the end of the first week after birth no WT or α1G-/- had functional TTCCs. During the first week after birth about 25% of WT myocytes were BrdU+ and became bi-nucleated. Significantly fewer α1G-/- myocytes became bi-nucleated and fewer of these myocytes were BrdU+. Neonatal α1G-/- myocytes were also smaller than WT. Adult WT and α1G-/- hearts were similar in size, but α1G-/- myocytes were smaller and a greater % were mono-nucleated. α1G over expressing hearts were smaller than WT but their myocytes were larger.

CONCLUSIONS

The studies performed show that loss of functional TTCCs is associated with bi-nucleation and myocyte withdrawal from the cell cycle. Loss of α1G TTCCs slowed the transition from mono- to bi-nucleation and resulted in an adult heart with a greater number of small cardiac myocytes. These results suggest that TTCCs are involved in the regulation of myocyte size and the exit of myocytes from the cell cycle during the first week after birth.

L-type calcium channel targeting and local signalling in cardiac myocytes

In the heart, Ca(2+) influx via Ca(V)1.2 L-type calcium channels (LTCCs) is a multi-functional signal that triggers muscle contraction, controls action potential duration, and regulates gene expression. The use of LTCC Ca(2+) as a multi-dimensional signalling molecule in the heart is complicated by several aspects of cardiac physiology. Cytosolic Ca(2+) continuously cycles between ~100 nM and ~1 μM with each heartbeat due to Ca(2+) linked signalling from LTCCs to ryanodine receptors. This rapid cycling raises the question as to how cardiac myocytes distinguish the Ca(2+) fluxes originating through L-type channels that are dedicated to contraction from Ca(2+) fluxes originating from other L-type channels that are used for non-contraction-related signalling. In general, disparate Ca(2+) sources in cardiac myocytes such as current through differently localized LTCCs as well as from IP3 receptors can signal selectively to Ca(2+)-dependent effectors in local microdomains that can be impervious to the cytoplasmic Ca(2+) transients that drive contraction. A particular challenge for diversified signalling via cardiac LTCCs is that they are voltage-gated and, therefore, open and presumably flood their microdomains with Ca(2+) with each action potential. Thus spatial localization of Cav1.2 channels to different types of microdomains of the ventricular cardiomyocyte membrane as well as the existence of particular macromolecular complexes in each Cav1.2 microdomain are important to effect different types of Cav1.2 signalling. In this review we examine aspects of Cav1.2 structure, targeting and signalling in two specialized membrane microdomains--transverse tubules and caveolae.

Window current through the T-type Ca2+ channel triggers the mechanism for cellular apoptosis via mitochondrial pathways

We hypothesized that Ca(2+) entry through the window T-type Ca(2+) current causes apoptosis. To test this hypothesis, we transfected human embryonic kidney (HEK) 293 cells to express recombinant Cav3.2 T-type Ca(2+) channels (hereafter called HEK-Cav3.2 cells). After incubation in media containing a high concentration (7.2 mM) of Ca(2+), intracellular Ca(2+) levels increased in HEK-Cav3.2 cells without electrical stimulation but not in untransfected HEK293 cells. In quiescent HEK-Cav3.2 cells exposed to high Ca(2+) media, apoptosis, as indicated by the appearance of hypodiploid cells, loss of mitochondrial transmembrane potential, and activation of caspases-3 and -9 was observed, while caspase-8 was not activated. These apoptosis-associated changes were blunted by pretreatment with the R(-)-isomer of efonidipine, a selective blocker of T-type Ca(2+) channels. High Ca(2+) did not induce apoptosis in untransfected HEK293 cells. Our findings show that Ca(2+) entry through the steady-state window current of T-type Ca(2+) channels causes apoptosis via mitochondrial pathways, and suggests that T-type Ca(2+) channels may be novel therapeutic targets for several diseases associated with abnormal apoptosis.

Pharmacoresistant Cav 2·3 (E-type/R-type) voltage-gated calcium channels influence heart rate dynamics and may contribute to cardiac impulse conduction

Voltage-gated Ca(2+) channels regulate cardiac automaticity, rhythmicity and excitation-contraction coupling. Whereas L-type (Cav 1·2, Cav 1·3) and T-type (Cav 3·1, Cav 3·2) channels are widely accepted for their functional relevance in the heart, the role of Cav 2·3 Ca(2+) channels expressing R-type currents remains to be elucidated. We have investigated heart rate dynamics in control and Cav 2·3-deficient mice using implantable electrocardiogram radiotelemetry and pharmacological injection experiments. Autonomic block revealed that the intrinsic heart rate does not differ between both genotypes. Systemic administration of isoproterenol resulted in a significant reduction in interbeat interval in both genotypes. It remained unaffected after administering propranolol in Cav 2·3(-|-) mice. Heart rate from isolated hearts as well as atrioventricular conduction for both genotypes differed significantly. Additionally, we identified and analysed the developmental expression of two splice variants, i.e. Cav 2·3c and Cav 2·3e. Using patch clamp technology, R-type currents could be detected in isolated prenatal cardiomyocytes and be related to R-type Ca(2+) channels. Our results indicate that on the systemic level, the pharmacologically inducible heart rate range and heart rate reserve are impaired in Cav 2·3 (-|-) mice. In addition, experiments on Langendorff perfused hearts elucidate differences in basic properties between both genotypes. Thus, Cav 2·3 does not only contribute to the cardiac autonomous nervous system but also to intrinsic rhythm propagation.

Ca(2+) influx through L-type Ca(2+) channels and transient receptor potential channels activates pathological hypertrophy signaling

Common cardiovascular diseases such as hypertension and myocardial infarction require that myocytes develop greater than normal force to maintain cardiac pump function. This requires increases in [Ca(2+)]. These diseases induce cardiac hypertrophy and increases in [Ca(2+)] are known to be an essential proximal signal for activation of hypertrophic genes. However, the source of "hypertrophic" [Ca(2+)] is not known and is the topic of this study. The role of Ca(2+) influx through L-type Ca(2+) channels (LTCC), T-type Ca(2+) channels (TTCC) and transient receptor potential (TRP) channels on the activation of calcineurin (Cn)-nuclear factor of activated T cells (NFAT) signaling and myocyte hypertrophy was studied. Neonatal rat ventricular myocytes (NRVMs) and adult feline ventricular myocytes (AFVMs) were infected with an adenovirus containing NFAT-GFP, to determine factors that could induce NFAT nuclear translocation. Four millimolar Ca(2+) or pacing induced NFAT nuclear translocation. This effect was blocked by Cn inhibitors. In NRVMs Nifedipine (Nif, LTCC antagonist) blocked high Ca(2+)-induced NFAT nuclear translocation while SKF-96365 (TRP channel antagonist) and Nickel (Ni, TTCC antagonist) were less effective. The relative potency of these antagonists against Ca(2+) induced NFAT nuclear translocation (Nif>SKF-96365>Ni) was similar to their effects on Ca(2+) transients and the LTCC current. Infection of NRVM with viruses containing TRP channels also activated NFAT-GFP nuclear translocation and caused myocyte hypertrophy. TRP effects were reduced by SKF-96365, but were more effectively antagonized by Nif. These experiments suggest that Ca(2+) influx through LTCCs is the primary source of Ca(2+) to activate Cn-NFAT signaling in NRVMs and AFVMs. While TRP channels cause hypertrophy, they appear to do so through a mechanism involving Ca(2+) entry via LTCCs.

Relevance of calmodulin/CaMKII activation for arrhythmogenesis in the AV block dog

BACKGROUND

The calcium-dependent signaling molecules calcineurin and calcium/calmodulin-dependent protein kinase II (CaMKII) both have been linked to decompensated hypertrophy and arrhythmias. CaMKII is also believed to be involved in acute modulation of ion channels.

OBJECTIVE

The purpose of this study was to determine the role of calcineurin and CaMKII in a dog model of compensated hypertrophy and a long QT phenotype.

METHODS

AV block was created in dogs to induce ventricular remodeling, including enhanced susceptibility to dofetilide-induced torsades de pointes arrhythmias. Dogs were treated with cyclosporin A for 3 weeks, which reduced calcineurin activity, as determined by mRNA expression levels of regulator of calcineurin 1 exon 4, but which was unable to prevent structural, contractile, or electrical remodeling and arrhythmias. Biopsies were taken before and at 2 or 9 weeks after AV block. Western blots were performed against phosphorylated and total CaMKII, phospholamban, Akt, and histone deacetylase 4 (HDAC4).

RESULTS

Chronic AV block showed an increase in Akt, CaMKII and phospholamban phosphorylation levels, but HDAC4 phosphorylation remained unaltered. Dofetilide induced torsades de pointes in vivo and early afterdepolarizations in cardiomyocytes, and increased [Ca(2+)](i) and CaMKII autophosphorylation. Both W-7 and KN-93 treatment counteracted this.

CONCLUSION

The calcineurin pathway seems not to be involved in long-term cardiac remodeling of the chronic AV block dog. Although CaMKII is chronically activated, this does not translate to HDAC4 phosphorylation. However, acute CaMKII overactivation is able to initiate arrhythmias based on triggered activity.

ZnT-1 enhances the activity and surface expression of T-type calcium channels through activation of Ras-ERK signaling

Zinc transporter-1 (ZnT-1) is a putative zinc transporter that confers cellular resistance from zinc toxicity. In addition, ZnT-1 has important regulatory functions, including inhibition of L-type calcium channels and activation of Raf-1 kinase. Here we studied the effects of ZnT-1 on the expression and function of T-type calcium channels. In Xenopus oocytes expressing voltage-gated calcium channel (CaV) 3.1 or CaV3.2, ZnT-1 enhanced the low-threshold calcium currents ( IcaT) to 182 ± 15 and 167.95 ± 9.27% of control, respectively ( P < 0.005 for both channels). As expected, ZnT-1 also enhanced ERK phosphorylation. Coexpression of ZnT-1 and nonactive Raf-1 blocked the ZnT-1-mediated ERK phosphorylation and abolished the ZnT-1-induced augmentation of IcaT. In mammalian cells (Chinese hamster ovary), coexpression of CaV3.1 and ZnT-1 increased the IcaT to 166.37 ± 6.37% compared with cells expressing CaV3.1 alone ( P < 0.01). Interestingly, surface expression measurements using biotinylation or total internal reflection fluorescence microscopy indicated marked ZnT-1-induced enhancement of CaV3.1 surface expression. The MEK inhibitor PD-98059 abolished the ZnT-1-induced augmentation of surface expression of CaV3.1. In cultured murine cardiomyocytes (HL-1 cells), transient exposure to zinc, leading to enhanced ZnT-1 expression, also enhanced the surface expression of endogenous CaV3.1 channels. Consistently, in these cells, endothelin-1, a potent activator of Ras-ERK signaling, enhanced the surface expression of CaV3.1 channels in a PD-98059-sensitive manner. Our findings indicate that ZnT-1 enhances the activity of CaV3.1 and CaV3.2 through activation of Ras-ERK signaling. The augmentation of CaV3.1 currents by Ras-ERK activation is associated with enhanced trafficking of the channel to the plasma membrane.

T-type calcium channel blockade improves survival and cardiovascular function in thalassemic mice

OBJECTIVES

Iron-overload cardiomyopathy is a major cause of morbidity and mortality in patients with thalassemia. However, the precise mechanisms of iron entry and sequestration in the heart are still unclear. Our previous study showed that Fe(2+) uptake in thalassemic cardiomyocytes are mainly mediated by T-type calcium channels (TTCC). Nevertheless, the role of TTCC as well as other transporters such as divalent metal transporter1 (DMT1) and L-type calcium channels (LTCC) as possible portals for iron entry into the heart in in vivo thalassemic mice under an iron-overload condition has not been investigated.

METHODS

An iron-overload condition was induced in genetically altered β-thalassemic mice and adult wild-type mice by feeding them with an iron diet (0.2% ferrocene w/w) for 3 months. Then, blockers for LTCC (verapamil and nifedipine), TTCC (efonidipine), and DMT1 (ebselen) as well as iron chelator desferoxamine (DFO) were given for 1 month with continuous iron feeding.

RESULTS

Treatment with LTCC, TTCC, DMT1 blockers, and DFO reduced cardiac iron deposit, cardiac malondialdehyde (MDA), plasma non-transferrin-bound iron, and improved heart rate variability and left ventricular (LV) function in thalassemic mice with iron overload. Only TTCC and DMT1 blockers and DFO reduced liver iron accumulation, liver MDA, plasma MDA, and decreased mortality rate in iron-overloaded thalassemic mice.

CONCLUSIONS

DMT1, LTCC, and TTCC played important roles for iron entry in the thalassemic heart under an iron-overloaded condition. Unlike LTCC blocker, TTCC blocker provided all benefits including attenuating iron deposit in both the heart and liver, reduced oxidative stress, and decreased mortality in iron-overloaded mice.

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.

Calcineurin and electrical remodeling in pathologic cardiac hypertrophy

Calcineurin is a cytoplasmic Ca(2+)/calmodulin-dependent protein phosphatase that contributes to cardiac hypertrophy. Numerous studies have demonstrated that calcineurin/nuclear factor of activated T cell pathway affects the architecture of the heart under pathologic conditions, and the effects of calcineurin/nuclear factor of activated T cell pathway on cardiac hypertrophy have been well reviewed. Cardiac electrical remodeling is generally accompanied with the cardiac hypertrophy, and alteration of cardiac ion channel activity also leads to the changes of calcineurin activity and cardiac hypertrophy. Many studies have linked calcineurin with changes of a variety of ion channels, but the therapeutic approaches to target calcineurin for correcting cardiac electrical disturbance have not been formulated. Here, we review the recent progress in calcineurin and electrical remodeling in pathologic cardiac hypertrophy.

Calcium signaling in cardiac myocytes

Calcium (Ca(2+)) is a critical regulator of cardiac myocyte function. Principally, Ca(2+) is the link between the electrical signals that pervade the heart and contraction of the myocytes to propel blood. In addition, Ca(2+) controls numerous other myocyte activities, including gene transcription. Cardiac Ca(2+) signaling essentially relies on a few critical molecular players--ryanodine receptors, voltage-operated Ca(2+) channels, and Ca(2+) pumps/transporters. These moieties are responsible for generating Ca(2+) signals upon cellular depolarization, recovery of Ca(2+) signals following cellular contraction, and setting basal conditions. Whereas these are the central players underlying cardiac Ca(2+) fluxes, networks of signaling mechanisms and accessory proteins impart complex regulation on cardiac Ca(2+) signals. Subtle changes in components of the cardiac Ca(2+) signaling machinery, albeit through mutation, disease, or chronic alteration of hemodynamic demand, can have profound consequences for the function and phenotype of myocytes. Here, we discuss mechanisms underlying Ca(2+) signaling in ventricular and atrial myocytes. In particular, we describe the roles and regulation of key participants involved in Ca(2+) signal generation and reversal.

Cardiomyocyte calcineurin signaling in subcellular domains: from the sarcolemma to the nucleus and beyond

The serine-threonine phosphatase calcineurin is activated in cardiac myocytes in the diseased heart and induces pathological hypertrophy. Calcineurin activity is mainly triggered by calcium/calmodulin binding but also through calpain mediated cleavage. How controlled calcineurin activation is possible in cardiac myocytes, which typically show a 10-fold difference in cytosolic calcium concentration with every heartbeat, has remained enigmatic. It is now emerging that calcineurin activation and signaling occur in subcellular microdomains, in which it is brought together with target proteins and exceedingly high concentrations of calcium in order to induce downstream signaling. We review current evidence of subcellular calcineurin mainly at the sarcolemma and the nucleus, but also in association with the sarcoplasmic reticulum and mitochondria. We also suggest that knowledge about subcellular signaling could help to develop inhibitors of calcineurin in specific microdomains to avoid side-effects that may arise from complete calcineurin inhibition.

Calcium signaling dysfunction in heart disease

In the heart, Ca(2+) is crucial for the regulation of contraction and intracellular signaling, processes, which are vital to the functioning of the healthy heart. Ca(2+) -activated signaling pathways must function against a background of large, rapid, and tightly regulated changes in intracellular free Ca(2+) concentrations during each contraction and relaxation cycle. This review highlights a number of proteins that regulate signaling Ca(2+) in both normal and pathological conditions including cardiac hypertrophy and heart failure, and discusses how these pathways are not regulated by the marked elevation in free intracellular calcium ([Ca(2+) ](i)) during contraction but require smaller sustained increases in Ca(2+) concentration. In addition, we present published evidence that the pool of Ca(2+) that regulates signaling is compartmentalized into distinct cellular microdomains and is thus distinct from that regulating contraction.

Contribution of sarcoplasmic reticulum Ca²+ release and Ca²+ transporters on sarcolemmal channels to Ca²+ transient in fetal mouse heart

Sarcoplasmic reticulum (SR) Ca release has been shown not to be the predominant mechanism responsible for excitation-contraction (E-C) coupling in fetal myocytes. However, most of the studies have been conducted either on primary cultures or acutely isolated cells, in which an apparent reduction of ryanodine receptor density have been reported. We aimed to elucidate the contribution of SR Ca release and Ca transporters on sarcolemmal channels to Ca transients in fetal mouse whole hearts. On embryonic day 13.5, ryanodine significantly reduced the amplitude of the Ca transient to 27.2 ± 4.4% of the control, and both nickel and SEA0400 significantly prolonged the time to peak from 84 ± 2 ms to 140 ± 5 ms and 129 ± 6 ms, respectively, whereas nifedipine did not alter it. Therefore, at early fetal stages, SR Ca release should be an important component of E-C coupling, and T-type Ca channel and reverse mode sodium-calcium exchanger (NCX)-mediated SR Ca release could be the predominant contributors. Using embryonic mouse cultured cardiomyocytes, we showed that both nifedipine and nickel inhibited the ability of NCX to extrude Ca from the cytosol. From these results, we propose a novel idea concerning E-C coupling in immature heart.

T-type Ca²+ signalling regulates aldosterone-induced CREB activation and cell death through PP2A activation in neonatal cardiomyocytes

AIMS

We have investigated Ca²(+) signalling generated by aldosterone-induced T-type current (I(CaT)), the effects of I(CaT) in neonatal cardiomyocytes, and a putative role for I(CaT) in cardiomyocytes during cardiac pathology induced by stenosis in an adult rat.

METHODS AND RESULTS

Neonatal rat cardiomyocytes treated with aldosterone showed an increase in I(CaT) density, principally due to the upregulation of the T-type channel Ca(v)3.1 (by 80%). Aldosterone activated cAMP-response element-binding protein (CREB), and this activation was enhanced by blocking I(CaT) or by inhibiting protein phosphatase 2A (PP2A) activity. Aldosterone induced PP2A activity, an induction that was prevented upon I(CaT) blockade. I(CaT) exerted a negative feedback regulation on the transcription of the Ca(v)3.1 gene, and the activation of PP2A by I(CaT) led to increased levels of the pro-apoptotic markers caspase 9 and Bcl-x(S) and decreased levels of the anti-apoptotic marker Bcl-2. These findings were corroborated by flow cytometry analysis for apoptosis and necrosis. Similarly, in a rat model of cardiac disease, I(CaT) re-emergence was associated with a decrease in CREB activation and was correlated with increases in caspase 9 and Bcl-x(S) and a decrease in Bcl-2 levels.

CONCLUSION

Our findings establish PP2A/CREB as targets of I(CaT)-generated Ca²(+) signalling and identify an important role for I(CaT) in cardiomyocyte cell death.

What causes a broken heart--molecular insights into heart failure

Our understanding of the molecular processes which regulate cardiac function has grown immeasurably in recent years. Even with the advent of β-blockers, angiotensin inhibitors and calcium modulating agents, heart failure (HF) still remains a seriously debilitating and life-threatening condition. Here, we review the molecular changes which occur in the heart in response to increased load and the pathways which control cardiac hypertrophy, calcium homeostasis, and immune activation during HF. These can occur as a result of genetic mutation in the case of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) or as a result of ischemic or hypertensive heart disease. In the majority of cases, calcineurin and CaMK respond to dysregulated calcium signaling and adrenergic drive is increased, each of which has a role to play in controlling blood pressure, heart rate, and left ventricular function. Many major pathways for pathological remodeling converge on a set of transcriptional regulators such as myocyte enhancer factor 2 (MEF2), nuclear factors of activated T cells (NFAT), and GATA4 and these are opposed by the action of the natriuretic peptides ANP and BNP. Epigenetic modification has emerged in recent years as a major influence cardiac physiology and histone acetyl transferases (HATs) and histone deacetylases (HDACs) are now known to both induce and antagonize hypertrophic growth. The newly emerging roles of microRNAs in regulating left ventricular dysfunction and fibrosis also has great potential for novel therapeutic intervention. Finally, we discuss the role of the immune system in mediating left ventricular dysfunction and fibrosis and ways this can be targeted in the setting of viral myocarditis.

T-type calcium channels and vascular function: the new kid on the block?

While L-type voltage-dependent calcium channels have long been considered the predominant source of calcium for myogenic constriction, recent studies of both cerebral and systemic circulations have provided evidence for the prominent expression of other members of the voltage-dependent calcium channel family, in particular the low voltage activated T-type channels. Although physiological studies have not supported the involvement of a classical low voltage activated, T-type channel in vascular function, evidence is accumulating that points to the involvement of a non-L-type, high voltage activated channel with sensitivity to T-type channel antagonists. We propose that this may arise due to expression of a T-type channel splice variant with unique biophysical characteristics resulting in a more depolarised profile. Expression of these channels in smooth muscle cells would broaden the voltage range over which sustained calcium influx occurs, while expression of T-type channels in endothelial cells could provide a feedback mechanism to prevent excessive vasoconstriction. Perturbation of this balance during pathophysiological conditions by upregulation of channel expression and endothelial dysfunction could contribute to vasospastic conditions and therapy-refractory hypertension.