Tight interplay between the Ca2+ affinity of the cardiac SERCA2 Ca2+ pump and the SERCA2 expression level

The Cardiac Calcium Handling Machinery is Remodeled in Friedreich's Ataxia

Background

Friedreich's ataxia (FA) is an inherited neurodegenerative disorder that causes progressive nervous system damage resulting in impaired muscle coordination. FA is the most common autosomal recessive form of ataxia and is caused by an expansion of the DNA triplet guanine-adenine-adenine (GAA) in the first intron of the Frataxin gene (FXN), located on chromosome 9q13. In the unaffected population, the number of GAA repeats ranges from 6 to 27 repetitions. In FA patients, GAA repeat expansions range from 44 to 1,700 repeats which decreases frataxin protein expression. Frataxin is a mitochondrial protein essential for various cellular functions, including iron metabolism. Reduced frataxin expression is thought to negatively affect mitochondrial iron metabolism, leading to increased oxidative damage. Although FA is considered a neurodegenerative disorder, FA patients display heart disease that includes hypertrophy, heart failure, arrhythmias, conduction abnormalities, and cardiac fibrosis.

Objective

In this work, we investigated whether abnormal Ca 2+ handling machinery is the molecular mechanism that perpetuates cardiac dysfunction in FA.

Methods

We used the frataxin knock-out (FXN-KO) mouse model of FA as well as human heart samples from donors with FA and from unaffected donors. ECG and echocardiography were used to assess cardiac function in the mice. Expression of calcium handling machinery proteins was assessed with proteomics and western blot. In left ventricular myocytes from FXN-KO and FXN-WT mice, the IonOptix system was used for calcium imaging, the seahorse assay was utilized to measure oxygen consumption rate (OCR), and confocal imaging was used to quantify the mitochondrial membrane potential (Δψm) and reactive oxygen species (ROS).

Results

We found that major contractile proteins, including SERCA2a and Ryr2, were downregulated in human left ventricular samples from deceased donors with FA compared to unaffected donors, similar to the downregulation of these proteins in the left ventricular tissue from FXN-KO compared to FXN-WT. On the ECG, the RR, PR, QRS, and QTc were significantly longer in the FXN-KO mice compared to FXN-WT. The ejection fraction and fractional shortening were significantly decreased and left ventricular wall thickness and diameter were significantly increased in the FXN-KO mice versus FXN-WT. The mitochondrial membrane potential Δψm was depolarized, ROS levels were elevated, and OCR was decreased in ventricular myocytes from FXN-KO versus FXN-WT.

Conclusion

The development of left ventricular contractile dysfunction in FA is associated with reduced expression of calcium handling proteins and mitochondrial dysfunction.

Expression of sarco (endo) plasmic reticulum calcium ATPase (SERCA) system in normal mouse cardiovascular tissues, heart failure and atherosclerosis

The sarco(endo)plasmic reticulum Ca(2+)ATPases (SERCA) system, a key regulator of calcium cycling and signaling, is composed of several isoforms. We aimed to characterize the expression of SERCA isoforms in mouse cardiovascular tissues and their modulation in cardiovascular pathologies (heart failure and/or atherosclerosis). Five isoforms (SERCA2a, 2b, 3a, 3b and 3c) were detected in the mouse heart and thoracic aorta. Absolute mRNA quantification revealed SERCA2a as the dominant isoform in the heart (~99%). Both SERCA2 isoforms co-localized in cardiomyocytes (CM) longitudinal sarcoplasmic reticulum (SR), SERCA3b was located at the junctional SR. In the aorta, SERCA2a accounted for ~91% of total SERCA and SERCA2b for ~5%. Among SERCA3, SERCA3b was the most expressed (~3.3%), mainly found in vascular smooth muscle cells (VSMC), along with SERCA2a and 2b. In failing CM, SERCA2a was down-regulated by 2-fold and re-localized from longitudinal to junctional SR. A strong down-regulation of SERCA2a was also observed in atherosclerotic vessels containing mainly synthetic VSMCs. The proportion of both SERCA2b and SERCA3b increased to 9.5% and 8.3%, respectively.

IN CONCLUSION

1) SERCA2a is the major isoform in both cardiac and vascular myocytes; 2) the expression of SERCA2a mRNA is ~30 fold higher in the heart compared to vascular tissues; and 3) nearly half the amount of SERCA2a mRNA is measured in both failing cardiomyocytes and synthetic VSMCs compared to healthy tissues, with a relocation of SERCA2a in failing cardiomyocytes. Thus, SERCA2a is the principal regulator of excitation-contraction coupling in both CMs and contractile VSMCs.

Regulation of endoplasmic reticulum Ca(2+) oscillations in mammalian eggs

Changes in the intracellular concentration of free calcium ([Ca(2+)]i) regulate diverse cellular processes including fertilization. In mammalian eggs, the [Ca(2+)]i changes induced by the sperm unfold in a pattern of periodical rises, also known as [Ca(2+)]i oscillations. The source of Ca(2+) during oscillations is the endoplasmic reticulum ([Ca(2+)]ER), but it is presently unknown how [Ca(2+)]ER is regulated. Here, we show using mouse eggs that [Ca(2+)]i oscillations induced by a variety of agonists, including PLCζ, SrCl2 and thimerosal, provoke simultaneous but opposite changes in [Ca(2+)]ER and cause differential effects on the refilling and overall load of [Ca(2+)]ER. We also found that Ca(2+) influx is required to refill [Ca(2+)]ER, because the loss of [Ca(2+)]ER was accelerated in medium devoid of Ca(2+). Pharmacological inactivation of the function of the mitochondria and of the Ca(2+)-ATPase pumps PMCA and SERCA altered the pattern of oscillations and abruptly reduced [Ca(2+)]ER, especially after inactivation of mitochondria and SERCA functions. We also examined the expression of SERCA2b protein and found that it was expressed throughout oocyte maturation and attained a conspicuous cortical cluster organization in mature eggs. We show that its overexpression reduces the duration of inositol-1,4,5-trisphosphate-induced [Ca(2+)]i rises, promotes initiation of oscillations and enhances refilling of [Ca(2+)]ER. Collectively, our results provide novel insights on the regulation of [Ca(2+)]ER oscillations, which underlie the unique Ca(2+)-signalling system that activates the developmental program in mammalian eggs.

Fasudil protects the heart against ischemia-reperfusion injury by attenuating endoplasmic reticulum stress and modulating SERCA activity: the differential role for PI3K/Akt and JAK2/STAT3 signaling pathways

Disordered calcium homeostasis can lead to endoplasmic reticulum (ER) stress. Our previous data showed that time course activation of ER stress contributes to time-related increase in ischemia-reperfusion (I/R) injury. However, it has not been tested whether PI3K/Akt and JAK2/STAT3 pathways play differential roles in reducing ER stress to protect the heart. In the present study, using fasudil which is a specific inhibitor of ROCK, we aimed to investigate whether improved SERCA expression and activity accounts for reduced ER stress by ROCK inhibition, specifically whether PI3K/Akt and JAK2/STAT3 pathways are differentially involved in modulating SERCA activity to reduce ER stress and hence I/R injury. The results showed that during the reperfusion period following 45 min of coronary ligation the infarct size (IS) increased from 3 h of reperfusion (45.4±5.57%) to 24 h reperfusion (64.21±5.43, P<0.05), which was associated with ER stress dependent apoptosis signaling activation including CHOP, Caspase-12 and JNK (P<0.05, respectively).The dynamic ER stress activation was also related to impaired SERCA activity at 24 h of reperfusion. Administration of fasudil at 10 mg/Kg significantly attenuated ROCK activation during reperfusion and resulted in an improved SERCA activity which was closely associated with decreases in temporal activation of ER stress and IS changes. Interestingly, while both PI3K/Akt and JAK2/STAT3 signaling pathways played equal role in the protection offered by ROCK inhibition at 3 h of reperfusion, the rescued SERCA expression and activity at 24 h of reperfusion by fasudil was mainly due to JAK2/STAT3 activation, in which PI3K/Akt signaling shared much less roles.

Improving cardiac Ca⁺² transport into the sarcoplasmic reticulum in heart failure: lessons from the ubiquitous SERCA2b Ca⁺² pump

As a major Ca2+ pump in the sarcoplasmic reticulum of the cardiomyocyte, SERCA2a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 2a) controls the relaxation and contraction of the cardiomyocyte. It is meticulously regulated by adapting its expression levels and affinity for Ca2+ ions to the physiological demand of the heart. Dysregulation of the SERCA2a activity entails poor cardiomyocyte contractility, resulting in heart failure. Conversely, improving cardiac SERCA2a activity, e.g. by boosting its expression level or by increasing its affinity for Ca2+, is a promising strategy to rescue contractile dysfunction of the failing heart. The structures of the related SERCA1a Ca2+ pump and the Na+/K+-ATPase of the plasma membrane exposed the pumping mechanism and conserved domain architecture of these ion pumps. However, how the Ca2+ affinity of SERCA2a is regulated at the molecular level remained unclear. A structural and functional analysis of the closely related SERCA2b Ca2+ pump, i.e. the housekeeping Ca2+ pump found in the endoplasmic reticulum and the only SERCA isoform characterized by a high Ca2+ affinity, aimed to fill this gap. We demonstrated the existence of a novel and highly conserved site on the SERCA2 pump mediating Ca2+ affinity regulation by the unique C-terminus of SERCA2b (2b-tail). It differs from the earlier-described target site of the affinity regulator phospholamban. Targeting this novel site may provide a new approach to improve SERCA2a function in the failing heart. Strikingly, the intramembrane interaction site of the 2b-tail in SERCA2b shares sequence and structural homology with the binding site of the β-subunit on the α Na+/K+-ATPase. Thus P-type ATPases seem to have developed related mechanisms of regulation, and it is a future challenge for us to discover these general principles of P-type regulation.

Modeling Ca2+ dynamics of mouse cardiac cells points to a critical role of SERCA's affinity for Ca2+

The SERCA2a isoform of the sarco/endoplasmic reticulum Ca(2+) pumps is specifically expressed in the heart, whereas SERCA2b is the ubiquitously expressed variant. It has been shown previously that replacement of SERCA2a by SERCA2b in mice (SERCA2(b/b) mice) results in only a moderate functional impairment, whereas SERCA activity is decreased by a 40% lower SERCA protein expression and by increased inhibition by phospholamban. To find out whether the documented kinetic differences in SERCA2b relative to SERCA2a (i.e., a twofold higher apparent Ca(2+) affinity, but twofold lower maximal turnover rate) can explain these compensatory changes, we simulated Ca(2+) dynamics in mouse ventricular myocytes. The model shows that the relative Ca(2+) transport capacity of SERCA2a and SERCA2b depends on the SERCA concentration. The simulations point to a dominant effect of SERCA2b's higher Ca(2+) affinity over its lower maximal turnover rate. The results suggest that increased systolic and decreased diastolic Ca(2+) levels in unstimulated conditions could contribute to the downregulation of SERCA in SERCA2(b/b) mice. In stress conditions, Ca(2+) handling is less efficient by SERCA2b than by SERCA2a, which might contribute to the observed hypertrophy in SERCA2(b/b) mice. Altogether, SERCA2a might be a better compromise between performance in basal conditions and performance during β-adrenergic stress.

Factors controlling the activity of the SERCA2a pump in the normal and failing heart

Heart failure is the leading cause of death in western countries and is often associated with impaired Ca(2+) handling in the cardiomyocyte. In fact, cardiomyocyte relaxation and contraction are tightly controlled by the activity of the cardiac sarco(endo)plasmic reticulum (ER/SR) Ca(2+) pump SERCA2a, pumping Ca(2+) from the cytosol into the lumen of the ER/SR. This review addresses three important facets that control the SERCA2 activity in the heart. First, we focus on the alternative splicing of the SERCA2 messenger, which is strictly regulated in the developing heart. This splicing controls the formation of three SERCA2 splice variants with different enzymatic properties. Second, we will discuss the role and regulation of SERCA2a activity in the normal and failing heart. The two well-studied Ca(2+) affinity modulators phospholamban and sarcolipin control the activity of SERCA2a within a narrow window. An aberrantly high or low Ca(2+) affinity is often observed in and may even trigger cardiac failure. Correcting SERCA2a activity might therefore constitute a therapeutic approach to improve the contractility of the failing heart. Finally, we address the controversies and unanswered questions of other putative regulators of the cardiac Ca(2+) pump, such as sarcalumenin, HRC, S100A1, Bcl-2, HAX-1, calreticulin, calnexin, ERp57, IRS-1, and -2.

Compartmentalized expression of three novel sarco/endoplasmic reticulum Ca2+ATPase 3 isoforms including the switch to ER stress, SERCA3f, in non-failing and failing human heart

The human sarco/endoplasmic reticulum (ER) Ca(2+)ATPase 3 (SERCA3) gene gives rise to SERCA3a-3f isoforms, the latter inducing ER stress in vitro. Here, we first demonstrated the co-expression of SERCA3a, -3d and -3f proteins in the heart. Evidence for endogenous proteins was obtained by using isoform-specific antibodies including a new SERCA3d-specific antibody, and either Western blotting of protein lysates or immunoprecipitation of membrane proteins. An immunolocalization study of both left ventricle tissue and isolated cardiomyocytes showed a distinct compartmentalization of the SERCA3 isoforms, as a uniform distribution of SERCA3a was detected while -3d and -3f isoforms were observed around the nucleus and in close vicinity of plasma membrane, respectively. Second, we studied their expressions in failing hearts including mixed (MCM) (n=1) and idiopathic dilated (IDCM) cardiomyopathies (n=4). Compared with controls (n=5), similar expressions of SERCA3a and -3d mRNAs were observed in all patients. In contrast, SERCA3f mRNA was found to be up-regulated in failing hearts (125+/-7%). Remarkably, overexpression of SERCA3f paralleled an increase in ER stress markers including processing of X-box-binding protein-1 (XBP-1) mRNA (176+/-24%), and expression of XBP-1 protein and glucose-regulated protein (GRP)78 (232+/-21%). These findings revisit the human heart's Ca(2+)ATPase system and indicate that SERCA3f may account for the mechanism of ER stress in vivo in heart failure.

High glucose oxidizes SERCA cysteine-674 and prevents inhibition by nitric oxide of smooth muscle cell migration

Nitric oxide (NO) causes S-glutathiolation of the reactive cysteine-674 in the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA), thus increasing SERCA activity, and inhibiting Ca(2+) influx and migration of vascular smooth muscle cells (VSMC). Because increased VSMC migration contributes to accelerated neointimal growth and atherosclerosis in diabetes, the effect of culture of VSMC in high glucose (HG) was determined. Rat aortic VSMC were exposed to normal (5.5 mmol/L) or high (25 mmol/L) glucose for 3 days, and serum-induced cell migration during 6 h into a wounded cell monolayer was measured 5 min after adding the NO donor S-nitroso-N-acetylpenicillamine (SNAP) or 24 h after interleukin-1beta (IL-1beta) to express inducible nitric oxide synthase (iNOS). In normal glucose, SNAP or IL-1beta significantly inhibited migration in cells infected with adenovirus to express GFP or SERCA wild type (WT), but not with a C674S SERCA mutant. After HG, NO failed to inhibit migration, nor did it decrease calcium-dependent association of calmodulin with calcineurin, indicating that NO failed to decrease intracellular calcium levels via SERCA. In contrast, overexpression of SERCA WT, but not the SERCA C674S mutant, preserved the ability for NO to inhibit migration despite exposing the cells to HG. The antioxidant, Tempol, or overexpression of superoxide dismutase also prevented the effects of HG. Further studies showed that both biotinylated-iodoacetamide and NO-induced biotinylated glutathione labeling of SERCA C674 were decreased by HG, and a sequence-specific sulfonic acid antibody detected oxidation of the C674 SERCA thiol. These results indicate that failure of NO to inhibit migration in VSMC exposed to HG is due to oxidation of the SERCA reactive cysteine-674.

Role of sarco/endoplasmic reticulum calcium content and calcium ATPase activity in the control of cell growth and proliferation

Ca(2+), the main second messenger, is central to the regulation of cellular growth. There is increasing evidence that cellular growth and proliferation are supported by a continuous store-operated Ca(2+) influx. By controlling store refilling, the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) also controls store-operated calcium entry and, thus, cell growth. In this review, we discuss data showing the involvement of SERCA in the regulation of proliferation and hypertrophy. First, we describe the Ca(2+)-related signaling pathways involved in cell growth. Then, we present evidence that SERCA controls proliferation of differentiated cells and hypertrophic growth of cardiomyocytes, and discuss the role of SERCA isoforms. Last, we consider the potential therapeutic applications of increasing SERCA activity for the treatment of cardiovascular diseases and of modulating SERCA and SR content for the treatment of cancer.