Connections count : excitation-contraction meets excitation-transcription coupling

Activation of sarcolipin expression and altered calcium cycling in LMNA cardiomyopathy

Cardiomyopathy caused by A-type lamins gene (LMNA) mutations (LMNA cardiomyopathy) is associated with dysfunction of the heart, often leading to heart failure. LMNA cardiomyopathy is highly penetrant with bad prognosis with no specific therapy available. Searching for alternative ways to halt the progression of LMNA cardiomyopathy, we studied the role of calcium homeostasis in the evolution of this disease. We showed that sarcolipin, an inhibitor of the sarco/endoplasmic reticulum (SR) Ca²⁺ ATPase (SERCA) was abnormally elevated in the ventricular cardiomyocytes of mutated mice compared with wild type mice, leading to an alteration of calcium handling. This occurs early in the progression of the disease, when the left ventricular function was not altered. We further demonstrated that down regulation of sarcolipin using adeno-associated virus (AAV) 9-mediated RNA interference delays cardiac dysfunction in mouse model of LMNA cardiomyopathy. These results showed a novel role for sarcolipin on calcium homeostasis in heart and open perspectives for future therapeutic interventions to LMNA cardiomyopathy.

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Sarcolipin, an inhibitor of the sarco/endoplasmic reticulum (SR) Ca²⁺ ATPase (SERCA) was abnormally elevated in the cardiac muscle of a mouse model of cardiomyopathy caused by LMNA mutations.

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The elevation of sarcolipin expression leads to an alteration of calcium handling.

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Down regulation of sarcolipin using adeno-associated virus (AAV) 9-mediated RNA interference delays cardiac dysfunction in mouse model of cardiomyopathy caused by LMNA mutations.

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

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

Muscle-type specific autophosphorylation of CaMKII isoforms after paced contractions

We explored to what extent isoforms of the regulator of excitation-contraction and excitation-transcription coupling, calcium/calmodulin protein kinase II (CaMKII) contribute to the specificity of myocellular calcium sensing between muscle types and whether concentration transients in its autophosphorylation can be simulated. CaMKII autophosphorylation at Thr287 was assessed in three muscle compartments of the rat after slow or fast motor unit-type stimulation and was compared against a computational model (CaMuZclE) coupling myocellular calcium dynamics with CaMKII Thr287 phosphorylation. Qualitative differences existed between fast- (gastrocnemius medialis) and slow-type muscle (soleus) for the expression pattern of CaMKII isoforms. Phospho-Thr287 content of δA CaMKII, associated with nuclear functions, demonstrated a transient and compartment-specific increase after excitation, which contrasted to the delayed autophosphorylation of the sarcoplasmic reticulum-associated βM CaMKII. In soleus muscle, excitation-induced δA CaMKII autophosphorylation demonstrated frequency dependence (P = 0.02). In the glycolytic compartment of gastrocnemius medialis, CaMKII autophosphorylation after excitation was blunted. In silico assessment emphasized the importance of mitochondrial calcium buffer capacity for excitation-induced CaMKII autophosphorylation but did not predict its isoform specificity. The findings expose that CaMKII autophosphorylation with paced contractions is regulated in an isoform and muscle type-specific fashion and highlight properties emerging for phenotype-specific regulation of CaMKII.

Calcineurin-NFATc regulates type 2 inositol 1,4,5-trisphosphate receptor (InsP3R2) expression during cardiac remodeling

In heart, the type 2 inositol 1,4,5-triphosphate receptor (InsP3R2) is the predominant isoform expressed and is localized in the nuclear membrane of ventricular myocytes. InsP3R2-mediated Ca(2+) release regulates hypertrophy specific gene expression by modulating CaMKIIδ, histone deacetylase, and calcineurin-NFATc signaling pathways. InsP3R2 protein is a hypertrophy specific marker and is overexpressed in heart failure animal models and in humans. However, the regulation of InsP3R2 mRNA and protein expression during cardiac hypertrophy and heart failure is not known. Here we show the transcriptional regulation of the Itpr2 gene in adult cardiomyocytes. Our data demonstrates that, InsP3R2 mRNA and protein expression is activated by hypertrophic agonists and attenuated by InsP3R inhibitors 2-aminoethoxyldiphenyl borate and xestospongin-C. The Itpr2 promoter is regulated by the calcineurin-NFATc signaling pathway. NFATc1 regulates Itpr2 gene expression by directly binding to the Itpr2 promoter. The calcineurin-NFATc mediated up-regulation of the Itpr2 promoter was attenuated by cyclosporine-A. InsP3R2 mRNA and protein expression was up-regulated in calcineurin-A transgenic mice and in human heart failure. Collectively, our data suggests that ITPR2 and hypertrophy specific gene expression is regulated, in part, by a positive feedback regulation between InsP3R2 and calcineurin-NFATc signaling pathways.

Epac in cardiac calcium signaling

Epac, exchange protein directly activated by cAMP, is emerging as a new regulator of cardiac physiopathology. Although its effects are much less known than the classical cAMP effector, PKA, several studies have investigated the cardiac role of Epac, providing evidences that Epac modulates intracellular Ca(2+). In one of the first analyses, it was shown that Epac can increase the frequency of spontaneous Ca(2+) oscillations in cultured rat cardiomyocytes. Later on, in adult cardiomyocytes, it was shown that Epac can induce sarcoplasmic reticulum (SR) Ca(2+) release in a PKA independent manner. The pathway identified involved phospholipase C (PLC) and Ca(2+)/calmodulin kinase II (CaMKII). The latter phosphorylates the ryanodine receptor (RyR), increasing the Ca(2+) spark probability. The RyR, Ca(2+) release channel located in the SR membrane, is a key element in the excitation-contraction coupling. Thus Epac participates in the excitation-contraction coupling. Moreover, by inducing RyR phosphorylation, Epac is arrhythmogenic. A detailed analysis of Ca(2+) mobilization in different microdomains showed that Epac preferently elevated Ca(2+) in the nucleoplasm ([Ca(2+)]n). This effect, besides PLC and CaMKII, required inositol 1,4,5 trisphosphate receptor (IP3R) activation. IP3R is other Ca(2+) release channel located mainly in the perinuclear area in the adult ventricular myocytes, where it has been shown to participate in the excitation-transcription coupling (the process by which Ca(2+) activates transcription). If Epac activation is maintained for some time, the histone deacetylase (HDAC) is translocated out of the nucleus de-repressing the transcription factor myocyte enhancer factor (MEF2). These evidences also pointed to Epac role in activating the excitation-transcription coupling. In fact, it has been shown that Epac induces cardiomyocyte hypertrophy. Epac activation for several hours, even before the cell hypertrophies, induces a profound modulation of the excitation-contraction coupling: increasing the [Ca(2+)]i transient amplitude and cellular contraction. Thus Epac actions are rapid but time and microdomain dependent in the cardiac myocyte. Taken together the results collected indicate that Epac may have an important role in the cardiac response to stress.

The other side of cardiac Ca(2+) signaling: transcriptional control

Ca²⁺ is probably the most versatile signal transduction element used by all cell types. In the heart, it is essential to activate cellular contraction in each heartbeat. Nevertheless Ca²⁺ is not only a key element in excitation-contraction coupling (EC coupling), but it is also a pivotal second messenger in cardiac signal transduction, being able to control processes such as excitability, metabolism, and transcriptional regulation. Regarding the latter, Ca²⁺ activates Ca²⁺-dependent transcription factors by a process called excitation-transcription coupling (ET coupling). ET coupling is an integrated process by which the common signaling pathways that regulate EC coupling activate transcription factors. Although ET coupling has been extensively studied in neurons and other cell types, less is known in cardiac muscle. Some hints have been found in studies on the development of cardiac hypertrophy, where two Ca²⁺-dependent enzymes are key actors: Ca²⁺/Calmodulin kinase II (CaMKII) and phosphatase calcineurin, both of which are activated by the complex Ca²⁺/Calmodulin. The question now is how ET coupling occurs in cardiomyocytes, where intracellular Ca²⁺ is continuously oscillating. In this focused review, we will draw attention to location of Ca²⁺ signaling: intranuclear ([Ca²⁺]_n) or cytoplasmic ([Ca²⁺]_c), and the specific ionic channels involved in the activation of cardiac ET coupling. Specifically, we will highlight the role of the 1,4,5 inositol triphosphate receptors (IP₃Rs) in the elevation of [Ca²⁺]_n levels, which are important to locally activate CaMKII, and the role of transient receptor potential channels canonical (TRPCs) in [Ca²⁺]_c, needed to activate calcineurin (Cn).

Sustained Epac activation induces calmodulin dependent positive inotropic effect in adult cardiomyocytes

Cardiac actions of Epac (exchange protein directly activated by cAMP) are not completely elucidated. Epac induces cardiomyocytes hypertrophy, Ca(2+)/calmodulin protein kinase II (CaMKII) and excitation-transcription coupling in rat cardiac myocytes. Here we aimed to elucidate the pathway cascade involved in Epac sustained actions, as during the initiation of hypertrophy development, where β-adrenergic signaling is chronically stimulated. Rats were treated with the Epac selective activator 8-pCPT during 4 weeks and Ca(2+) signaling was analyzed in isolated cardiac myocytes by confocal microscopy. We observed a positive inotropic effect manifested by increased [Ca(2+)](i) transient amplitudes. In order to further analyze these actions, we incubated adult cardiomyocytes in the presence of 8-pCPT. The effects were similar to those obtained in-vivo and are blunted by Epac1 knock down. Interestingly, the increase in [Ca(2+)] transients was abolished by protein synthesis blockade or when the downstream effectors of calmodulin (CaMKII or calcineurin) were inhibited, pointing to calmodulin (CaM) as an important downstream protein in Epac sustained actions. In fact, CaM expression was enhanced by 8-pCPT treatment in isolated cells, as found by Western blots. Moreover, the 8-pCPT-induced, PKA-independent, positive inotropic effect was favored by enhanced extracellular Ca(2+) influx via L-type Ca(2+) channels. However, 8-pCPT also induced aberrant Ca(2+) release as Ca(2+) waves and extra [Ca(2+)](i) transients, suggesting proarrhythmic effect. These results provide new insights regarding Epac cardiac actions, suggesting an important role in the initial compensation of the heart to pathological stimuli during the initiation of cardiac hypertrophy, favoring contraction but also arrhythmia risk.

c-Fos expression in ouabain-treated vascular smooth muscle cells from rat aorta: evidence for an intracellular-sodium-mediated, calcium-independent mechanism

In this study, we examined the effect of Na(+)-K(+) pump inhibition on the expression of early response genes in vascular smooth muscle cells (VSMC) as possible intermediates of the massive RNA synthesis and protection against apoptosis seen in ouabain-treated VSMC in our previous experiments. Incubation of VSMC with ouabain resulted in rapid induction of c-Fos protein expression with an approximately sixfold elevation after 2 h of incubation. c-Jun expression was increased by approximately fourfold after 12 h, whereas expression of activating transcription factor 2, cAMP/Ca(2+) response element binding protein (CREB)-1 and c-Myc was not altered. Markedly augmented c-Fos expression was also observed under Na(+)-K(+) pump inhibition in potassium-depleted medium. Na(+)-K(+) pump inhibition triggered c-Fos expression via elevation of the [Na(+)](i)/[K(+)](i) ratio. This conclusion follows from experiments showing the lack of effect of ouabain on c-Fos expression in high-potassium-low-sodium medium and from the comparison of dose responses of Na(+)-K(+) pump activity, [Na(+)](i) and [K(+)](i) content and c-Fos expression to ouabain. A fourfold increment of c-Fos mRNA was revealed 30 min following addition of ouabain to the incubation medium. At this time point, treatment with ouabain resulted in an approximately fourfold elevation of [Na(+)](i) but did not affect [K(+)](i). Augmented c-Fos expression was also observed under VSMC depolarization in high-potassium medium. Increments in both c-Fos expression and (45)Ca uptake in depolarized VSMC were abolished under inhibition of L-type Ca(2+) channels with 0.1 microM nicardipine. Ouabain did not affect the free [Ca(2+)](i) or the content of exchangeable [Ca(2+)](i). Ouabain-induced c-Fos expression was also insensitive to the presence of nicardipine and [Ca(2+)](o), as well as chelators of [Ca(2+)](o) (EGTA) and [Ca(2+)](i) (BAPTA). The effect of ouabain and serum on c-Fos expression was additive. In contrast to serum, however, ouabain failed to activate the Elk-1, serum response factor, CREB and activator protein-1 transcription factors identified within the c-Fos promoter. These results suggest that Na(+)-K(+) pump inhibition triggers c-Fos expression via [Na(+)](i)-sensitive [Ca(2+)](i)-independent transcription factor(s) distinct from factors interacting with known response elements of this gene promoter.

Ca2+ release through ryanodine receptors regulates skeletal muscle L-type Ca2+ channel expression

Skeletal muscle obtained from mice that lack the type 1 ryanodine receptor (RyR-1), termed dyspedic mice, exhibit a 2-fold reduction in the number of dihydropyridine binding sites (DHPRs) compared with skeletal muscle obtained from wild-type mice (Buck, E. D., Nguyen, H. T., Pessah, I. N., and Allen, P. D. (1997) J. Biol. Chem. 272, 7360-7367 and Fleig, A., Takeshima, H., and Penner, R. (1996) J. Physiol. (Lond.) 496, 339-345). To probe the role of RyR-1 in influencing L-type Ca(2+) channel (L-channel) expression, we have monitored functional L-channel expression in the sarcolemma using the whole-cell patch clamp technique in normal, dyspedic, and RyR-1-expressing dyspedic myotubes. Our results indicate that dyspedic myotubes exhibit a 45% reduction in maximum immobilization-resistant charge movement (Q(max)) and a 90% reduction in peak Ca(2+) current density. Calcium current density was significantly increased in dyspedic myotubes 3 days after injection of cDNA encoding either wild-type RyR-1 or E4032A, a mutant RyR-1 that is unable to restore robust voltage-activated release of Ca(2+) from the sarcoplasmic reticulum (SR) following expression in dyspedic myotubes (O'Brien, J. J., Allen, P. D., Beam, K., and Chen, S. R. W. (1999) Biophys. J. 76, A302 (abstr.)). The increase in L-current density 3 days after expression of either RyR-1 or E4032A occurred in the absence of a change in Q(max). However, Q(max) was increased 85% 6 days after injection of dyspedic myotubes with cDNA encoding the wild-type RyR-1 but not E4032A. Because normal and dyspedic myotubes exhibited a similar density of T-type Ca(2+) current (T-current), the presence of RyR-1 does not appear to cause a general overall increase in protein synthesis. Thus, long-term expression of L-channels in skeletal myotubes is promoted by Ca(2+) released through RyRs occurring either spontaneously or during excitation-contraction coupling.