Cardiomyocyte-specific disruption of Serca2 in adult mice causes sarco(endo)plasmic reticulum stress and apoptosis

Acute endoplasmic reticulum stress-induced mitochondria respiratory chain damage: The role of activated calpains

The induction of acute endoplasmic reticulum (ER) stress damages the electron transport chain (ETC) in cardiac mitochondria. Activation of mitochondria-localized calpain 1 (CPN1) and calpain 2 (CPN2) impairs the ETC in pathological conditions, including aging and ischemia-reperfusion in settings where ER stress is increased. We asked if the activation of calpains causes the damage to the ETC during ER stress. Control littermate and CPNS1 (calpain small regulatory subunit 1) deletion mice were used in the current study. CPNS1 is an essential subunit required to maintain CPN1 and CPN2 activities, and deletion of CPNS1 prevents their activation. Tunicamycin (TUNI, 0.4 mg/kg) was used to induce ER stress in C57BL/6 mice. Cardiac mitochondria were isolated after 72 h of TUNI treatment. ER stress was increased in both control littermate and CPNS1 deletion mice with TUNI treatment. The TUNI treatment activated both cytosolic and mitochondrial CPN1 and 2 (CPN1/2) in control but not in CPNS1 deletion mice. TUNI treatment led to decreased oxidative phosphorylation and complex I activity in control but not in CPNS1 deletion mice compared to vehicle. The contents of complex I subunits, including NDUFV2 and ND5, were decreased in control but not in CPNS1 deletion mice. TUNI treatment also led to decreased oxidation through cytochrome oxidase (COX) only in control mice. Proteomic study showed that subunit 2 of COX was decreased in control but not in CPNS1 deletion mice. Our results provide a direct link between activation of CPN1/2 and complex I and COX damage during acute ER stress.

ER stress and calcium-dependent arrhythmias

The sarcoplasmic reticulum (SR) plays the key role in cardiac function as the major source of Ca²⁺ that activates cardiomyocyte contractile machinery. Disturbances in finely-tuned SR Ca²⁺ release by SR Ca²⁺ channel ryanodine receptor (RyR2) and SR Ca²⁺ reuptake by SR Ca²⁺-ATPase (SERCa2a) not only impair contraction, but also contribute to cardiac arrhythmia trigger and reentry. Besides being the main Ca²⁺ storage organelle, SR in cardiomyocytes performs all the functions of endoplasmic reticulum (ER) in other cell types including protein synthesis, folding and degradation. In recent years ER stress has become recognized as an important contributing factor in many cardiac pathologies, including deadly ventricular arrhythmias. This brief review will therefore focus on ER stress mechanisms in the heart and how these changes can lead to pro-arrhythmic defects in SR Ca²⁺ handling machinery.

Sphingosine-1-phosphate Attenuates Endoplasmic Reticulum Stress-induced Cardiomyocyte Apoptosis Through Sphingosine-1-phosphate Receptor 1

BACKGROUND

Endoplasmic reticulum stress (ER stress) is involved in the development and progression of various forms of heart disease and may lead to myocardial apoptosis. Sphingosine-1-phosphate (S1P) possesses cardioprotective properties, including anti-apoptosis. However, little is known about the link between S1P and ER stress-induced myocardial apoptosis. This study investigated the regulatory role of S1P in ER stress-induced apoptosis in cardiomyocytes.

METHODS

ER stress and myocardial apoptosis were induced by transverse aortic constriction (TAC) or tunicamycin in mice, which were then treated with 2-acetyl-5-tetrahydroxybutyl imidazole (THI) or S1P. AC16 cells were treated with tunicamycin or thapsigargin, or pretreated with S1P, sphingosine-1-phosphate receptor (S1PR) subtype antagonists, S1PR1 agonist, and PI3K and MEK inhibitors. Cardiac function, the level of S1P in plasma and heart, ER stress markers, cell viability, and apoptosis were detected.

RESULTS

S1P reduced the expression of ER stress-related molecules and ER stress-induced myocardial apoptosis in mice subjected to TAC or an injection of tunicamycin. Furthermore, in AC16 cells exposed to thapsigargin or tunicamycin, S1P decreased the expression of ER stress-related molecules, promoting cell viability and survival. Nevertheless, the S1PR1 antagonist abrogated the protection of S1P. Subsequently, in TAC S1PR1 heterozygous (S1PR1^+/-) mice, S1P had no effect on ER stress and apoptosis in cardiomyocytes. Notably, in vitro, the impact of anti-ER stress-induced myocardial apoptosis by the S1PR1 agonist was reversed by PI3K and MEK inhibitors.

CONCLUSION

This study is the first to demonstrate that S1P relieves ER stress-induced myocardial apoptosis via S1PR1/AKT and S1PR1/ERK1/2, which are potential therapeutic targets for heart disease.

Estrogen inhibits endoplasmic reticulum stress and ameliorates myocardial ischemia/reperfusion injury in rats by upregulating SERCA2a

Background

The incidence of coronary heart disease (CHD) in premenopausal women is significantly lower than that of men of the same age, suggesting protective roles of estrogen for the cardiovascular system against CHD. This study aimed to confirm the protective effect of estrogen on myocardium during myocardial ischemia/reperfusion (MI/R) injury and explore the underlying mechanisms.

Methods

Neonatal rat cardiomyocytes and Sprague–Dawley rats were used in this study. Different groups were treated by bilateral ovariectomy, 17β-estradiol (E2), adenoviral infection, or siRNA transfection. The expression of sarcoplasmic reticulum Ca²⁺ ATPase pump (SERCA2a) and endoplasmic reticulum (ER) stress-related proteins were measured in each group to examine the effect of different E2 levels and determine the relationship between SERCA2a and ER stress. The cell apoptosis, myocardial infarction size, levels of apoptosis and serum cardiac troponin I, ejection fraction, calcium transient, and morphology changes of the myocardium and ER were examined to verify the effects of E2 on the myocardium.

Results

Bilateral ovariectomy resulted in reduced SERCA2a levels and more severe MI/R injury. E2 treatment increased SERCA2a expression. Both E2 treatment and exogenous SERCA2a overexpression decreased levels of ER stress-related proteins and alleviated myocardial damage. In contrast, SERCA2a knockdown exacerbated ER stress and myocardial damage. Addition of E2 after SERCA2a knockdown did not effectively inhibit ER stress or reduce myocardial injury.

Conclusions

Our data demonstrate that estrogen inhibits ER stress and attenuates MI/R injury by upregulating SERCA2a. These results provide a new potential target for therapeutic intervention and drug discovery in CHD.

Sarcoplasmic Reticulum Calcium Release Is Required for Arrhythmogenesis in the Mouse

Dysfunctional sarcoplasmic reticulum Ca²⁺ handling is commonly observed in heart failure, and thought to contribute to arrhythmogenesis through several mechanisms. Some time ago we developed a cardiomyocyte-specific inducible SERCA2 knockout mouse, which is remarkable in the degree to which major adaptations to sarcolemmal Ca²⁺ entry and efflux overcome the deficit in SR reuptake to permit relatively normal contractile function. Conventionally, those adaptations would also be expected to dramatically increase arrhythmia susceptibility. However, that susceptibility has never been tested, and it is possible that the very rapid repolarization of the murine action potential (AP) allows for large changes in sarcolemmal Ca²⁺ transport without substantially disrupting electrophysiologic stability. We investigated this hypothesis through telemetric ECG recording in the SERCA2-KO mouse, and patch-clamp electrophysiology, Ca²⁺ imaging, and mathematical modeling of isolated SERCA2-KO myocytes. While the SERCA2-KO animals exhibit major (and unique) electrophysiologic adaptations at both the organ and cell levels, they remain resistant to arrhythmia. A marked increase in peak L-type calcium (I_CaL) current and slowed I_CaL decay elicited pronounced prolongation of initial repolarization, but faster late repolarization normalizes overall AP duration. Early afterdepolarizations were seldom observed in KO animals, and those that were observed exhibited a mechanism intermediate between murine and large mammal dynamical properties. As expected, spontaneous SR Ca²⁺ sparks and waves were virtually absent. Together these findings suggest that intact SR Ca²⁺ handling is an absolute requirement for triggered arrhythmia in the mouse, and that in its absence, dramatic changes to the major inward currents can be resisted by the substantial K⁺ current reserve, even at end-stage disease.

Doxorubicin-induced delayed-onset subclinical cardiotoxicity in mice

Subclinical cardiotoxicity at low total cumulative doxorubicin (DOX) doses can manifest into cardiomyopathy in long-term cancer survivors. However, the underlying mechanisms are poorly understood. In male B6C3F₁ mice, assessment of cardiac function by echocardiography was performed at 1, 4, 10, 17, and 24 weeks after exposure to 6, 9, 12, and 24 mg/kg total cumulative DOX doses or saline (SAL) to monitor development of delayed-onset cardiotoxicity. The 6- or 9-mg/kg total cumulative doses resulted in a significant time-dependent decline in systolic function (left ventricular ejection fraction (LVEF) and fractional shortening (FS)) during the 24-week recovery although there was not a significant alteration in % LVEF or % FS at any specific time point during the recovery. A significant decline in systolic function was elicited by the cardiotoxic cumulative DOX dose (24 mg/kg) during the 4- to 24-week period after treatment compared to SAL-treated counterparts. At 24 weeks after DOX treatment, a significant dose-related decrease in the expression of genes and proteins involved in sarcoplasmic reticulum (SR) calcium homeostasis (Ryr2 and Serca2) was associated with a dose-related increase in the transcript level of Casp12 (SR-specific apoptosis) in hearts. These mice also showed enhanced apoptotic activity in hearts indicated by a significant dose-related elevation in the number of apoptotic cardiomyocytes compared to SAL-treated counterparts. These findings collectively suggest that a steady decline in SR calcium handling and apoptosis might be involved in the development of subclinical cardiotoxicity that can evolve into irreversible cardiomyopathy later in life.

Attenuation of the upregulation of NF‑κB and AP‑1 DNA‑binding activities induced by tunicamycin or hypoxia/reoxygenation in neonatal rat cardiomyocytes by SERCA2a overexpression

The present study aimed to investigate the effects of the overexpression of sarco/endoplasmic reticulum Ca2+‑ATPase (SERCA2a) on endoplasmic reticulum (ER) stress (ERS)‑associated inflammation in neonatal rat cardiomyocytes (NRCMs) induced by tunicamycin (TM) or hypoxia/reoxygenation (H/R). The optimal multiplicity of infection (MOI) was 2 pfu/cell. Neonatal Sprague‑Dawley rat cardiomyocytes cultured in vitro were infected with adenoviral vectors carrying SERCA2a or enhanced green fluorescent protein genes, the latter used as a control. At 48 h following gene transfer, the NRCMs were treated with TM (10 µg/ml) or subjected to H/R to induce ERS. The results of electrophoretic mobility shift assay (EMSA) revealed that overexpression of SERCA2a attenuated the upregulation of nuclear factor (NF)‑κB and activator protein‑1 (AP‑1) DNA‑binding activities induced by TM or H/R. Western blot analysis and semi‑quantitative RT‑PCR revealed that the overexpression of SERCA2a attenuated the activation of the inositol‑requiring 1α (IRE1α) signaling pathway and ERS‑associated apoptosis induced by TM. The overexpression of SERCA2a also decreased the level of phospho‑p65 (Ser536) in the nucleus, as assessed by western blot analysis. However, the overexpression of SERCA2a induced the further nuclear translocation of NF‑κB p65 and higher levels of tumor necrosis factor (TNF)‑α transcripts in the NRCMs, indicating the occurrence of the ER overload response (EOR). Therefore, the overexpression of SERCA2a has a 'double‑edged sword' effect on ERS‑associated inflammation. On the one hand, it attenuates ERS and the activation of the IRE1α signaling pathway induced by TM, resulting in the attenuation of the upregulation of NF‑κB and AP‑1 DNA‑binding activities in the nucleus, and on the other hand, it induces EOR, leading to the further nuclear translocation of NF‑κB and the transcription of TNF‑α. The preceding EOR may precondition the NRCMs against subsequent ERS induced by TM. Further studies using adult rat cardiomyocytes are required to prevent the interference of EOR. The findings of the present study may enhance the current understanding of the role of SERCA2a in cardiomyocytes.

Darier disease is associated with heart failure: a cross-sectional case-control and population based study

Human data supporting a role for endoplasmic reticulum (ER) stress and calcium dyshomeostasis in heart disease is scarce. Darier disease (DD) is a hereditary skin disease caused by mutations in the ATP2A2 gene encoding the sarcoendoplasmic-reticulum Ca2⁺ ATPase isoform 2 (SERCA2), which causes calcium dyshomeostasis and ER stress. We hypothesized that DD patients would have an increased risk for common heart disease. We performed a cross-sectional case-control clinical study on 25 DD patients and 25 matched controls; and a population-based cohort study on 935 subjects with DD and matched comparison subjects. Main outcomes and measures were N-terminal pro-brain natriuretic peptide, ECG and heart diagnosis (myocardial infarction, heart failure and arrythmia). DD subjects showed normal clinical heart phenotype including heart failure markers and ECG. The risk for heart failure was 1.59 (1,16-2,19) times elevated in DD subjects, while no major differences were found in myocardial infarcation or arrhythmias. Risk for heart failure when corrected for cardivascular risk factors or alcohol misuse was 1.53 (1.11–2.11) and 1.58 (1,15–2,18) respectively. Notably, heart failure occurred several years earlier in DD patients as compared to controls. We conclude that DD patients show a disease specific increased risk of heart failure which should be taken into account in patient management. The observation also strenghtens the clinical evidence on the important role of SERCA2 in heart failure pathophysiology.

GCN2 deficiency ameliorates cardiac dysfunction in diabetic mice by reducing lipotoxicity and oxidative stress

Excessive myocardial lipid accumulation is a major feature of diabetic cardiomyopathy (DCM). Although general control nonderepressible 2 (GCN2) has been identified as a sensor of amino acid availability, it also functions as an important regulator of hepatic lipid metabolism. Our previous studies have reported that GCN2 promotes pressure overload or doxorubicin-induced cardiac dysfunction by increasing cardiomyocyte apoptosis and myocardial oxidative stress. However, the impact of GCN2 on the development of DCM remains unclear. In this study, we investigated the effect of GCN2 on DCM in type 1 and type 2 diabetes animal models. After streptozotocin (STZ) or high-fat diet (HFD) plus low-dose STZ treatments, GCN2^-/- mice developed less cardiac dysfunction, hyperlipidemia, myocardial hypertrophy, fibrosis, lipid accumulation, oxidative stress, inflammation and apoptosis compared with wild-type (WT) mice. In diabetic hearts, GCN2 deficiency attenuated the upregulation of peroxisome proliferator-activated receptor alpha (PPARα) and gamma (PPARγ), the phosphorylation of eIF2α and the induction of activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP), as well as the reduction of Bcl-2. Furthermore, we found that knockdown of GCN2 attenuated, whereas overexpression of GCN2 exacerbated, high glucose or palmitic acid-induced cell death, oxidative and endoplasmic reticulum stress and lipid accumulation in H9C2 cells. Collectively, our data provide evidence that GCN2 deficiency protects cardiac function by reducing lipid accumulation, oxidative stress and cell death. Our findings suggest that strategies to inhibit GCN2 activity in the heart may be novel approaches for DCM therapy.

AQP4-knockout aggravation of isoprenaline-induced myocardial injury is mediated by p66Shc and endoplasmic reticulum stress

Aquaporin 4 (AQP4) is a type of water channel protein that maintains the water balance of cardiomyocytes. However, the physiological role of AQP4 in cardiovascular disease is poorly understood. We wanted to explore whether p66Shc and endoplasmic reticulum stress participates in AQP4 knockout (KO)-mediated cardiac injury. There were two types of mice: AQP4 knockout and wild-type mice. Each type was randomly divided into three groups: Control group, isoprenaline stimulation group (ISO, 1 mg/kg, s.c., 5 days), and apocynin treatment group (APO, 100 mg/kg, p.o., 3 days). H9c2 rat cardiomyocytes were cultured for RNA interference of AQP4. Results showed increased left ventricular weight index and more severe myocardial inflammation were induced in AQP4 knockout mice relative to wild-type mice, accompanied by significantly increased levels of the oxidative stress biomarkers MDA and NOX4. In addition, the expressions of p66Shc, ER stress markers PERK, GRP78 and CHOP and proinflammatory factors such as ET_A , IL6 and TNFα were upregulated in the myocardium of AQP4 knockout mice or AQP4 siRNA treated cardiomyocytes, whereas CASQ2 was downregulated. ISO stimulation aggravated these abnormalities, which were significantly attenuated by apocynin. This study showed that AQP4 knockout mice were susceptible to cardiac injury induced by ISO. The mechanism was closely connected with p66Shc and proinflammatory factors. Endoplasmic reticulum stress was also involved in the pathological process.

Endoplasmic reticulum stress in the heart: insights into mechanisms and drug targets

The endoplasmic reticulum (ER) serves several essential cellular functions including protein synthesis, protein folding, protein translocation, calcium homoeostasis and lipid biosynthesis. Physiological or pathological stimuli, which disrupt ER homoeostasis and disturb its functions, lead to an accumulation of misfolded and unfolded proteins, a condition referred to as ER stress. ER stress triggers the unfolded protein response to restore the homoeostasis of ER, through activating transcriptional and translational pathways. However, prolonged ER stress will lead to cell dysfunction and apoptosis. Recent evidence revealed that ER stress is involved in the development and progression of various heart diseases, such as cardiac hypertrophy, ischaemic heart diseases and heart failure. Therefore, improved understanding of the molecular mechanisms of ER stress in heart disease will help to investigate more potential targets for new therapeutic interventions and drug discovery.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Targeting cardiomyocyte Ca2+ homeostasis in heart failure

Improved treatments for heart failure patients will require the development of novel therapeutic strategies that target basal disease mechanisms. Disrupted cardiomyocyte Ca²⁺ homeostasis is recognized as a major contributor to the heart failure phenotype, as it plays a key role in systolic and diastolic dysfunction, arrhythmogenesis, and hypertrophy and apoptosis signaling. In this review, we outline existing knowledge of the involvement of Ca²⁺ homeostasis in these deficits, and identify four promising targets for therapeutic intervention: the sarcoplasmic reticulum Ca²⁺ ATPase, the Na⁺-Ca²⁺ exchanger, the ryanodine receptor, and t-tubule structure. We discuss experimental data indicating the applicability of these targets that has led to recent and ongoing clinical trials, and suggest future therapeutic approaches.

αB-Crystallin R120G variant causes cardiac arrhythmias and alterations in the expression of Ca(2+) -handling proteins and endoplasmic reticulum stress in mice

Mutations of αB-crystallin (CryαB), a small heat shock protein abundantly expressed in cardiac and skeletal muscles, are known to cause desmin-related myopathies. The CryαB R120G allele has been linked to a familial desminopathy and, in transgenic mice, causes a sudden death at about 28 weeks of age. To investigate the mechanisms of the sudden cardiac arrest of CryαB R120G transgenic mice, we prepared protein samples from left ventricular tissues of two different age groups (10 and 28 weeks) and examined Ca(2+) -handling proteins. Expression of sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) 2, phospholamban, ryanodine receptor 2 and calsequestrin 2 was significantly decreased in 28- versus 10-week-old CryαB R120G transgenic mice. In addition, low heart rate variability, including heart rate, total power and low frequency, was observed and continuous electrocardiogram monitoring revealed cardiac arrhythmias, such as ventricular tachycardia, atrioventricular block and atrial flutter, in 28-week-old CryαB R120G transgenic mice. In contrast, expression of endoplasmic reticulum (ER) degradation enhancing α-mannosidase-like protein, inositol requirement 1 and X-box binding protein 1 were increased significantly in 28- versus 10-week-old CryαBR120G transgenic mice, suggesting that the CryαBR120G transgenic mice exhibit increased ER stress compared with wild-type mice. Together, the data suggest that the CryαB R120G dominant variant induces ER stress and impairs Ca(2+) regulation, leading to ageing-related cardiac dysfunction, arrhythmias and decreased autonomic tone with shortened lifespan.

Endoplasmic reticulum stress sensor protein kinase R-like endoplasmic reticulum kinase (PERK) protects against pressure overload-induced heart failure and lung remodeling

Studies have reported that development of congestive heart failure is associated with increased endoplasmic reticulum stress. Double stranded RNA-activated protein kinase R-like endoplasmic reticulum kinase (PERK) is a major transducer of the endoplasmic reticulum stress response and directly phosphorylates eukaryotic initiation factor 2α, resulting in translational attenuation. However, the physiological effect of PERK on congestive heart failure development is unknown. To study the effect of PERK on ventricular structure and function, we generated inducible cardiac-specific PERK knockout mice. Under unstressed conditions, cardiac PERK knockout had no effect on left ventricular mass, or its ratio to body weight, cardiomyocyte size, fibrosis, or left ventricular function. However, in response to chronic transverse aortic constriction, PERK knockout mice exhibited decreased ejection fraction, increased left ventricular fibrosis, enhanced cardiomyocyte apoptosis, and exacerbated lung remodeling in comparison with wild-type mice. PERK knockout also dramatically attenuated cardiac sarcoplasmic reticulum Ca(2+)-ATPase expression in response to aortic constriction. Our findings suggest that PERK is required to protect the heart from pressure overload-induced congestive heart failure.

Panax quinquefolium saponin attenuates ventricular remodeling after acute myocardial infarction by inhibiting chop-mediated apoptosis

Panax quinquefolium saponin (PQS) alleviates hypoxia-reoxygenation injury of cardiomyocytes in vitro by inhibiting excessive endoplasmic reticulum stress (ERS)-related apoptosis. We hypothesized that inhibition of excessive ERS-related apoptosis contributes to cardioprotection in ventricular remodeling after acute myocardial infarction (AMI). Sprague-Dawley rats subjected to AMI were randomly treated with water, PQS (50 mg/kg per day, 100 mg/kg per day, or 200 mg/kg per day), or taurine (300 mg/kg per day), an ERS inhibitor, for 4 weeks. Left ventricular (LV) fractional shortening, ejection fraction, and structure were then evaluated using echocardiography. Myocardial infarct size was measured by Evans blue and 2,3,5-triphenyhetrazolium chloride staining. The hydroxyproline level was assayed using the colorimetric method. Cardiomyocyte apoptosis was detected using terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labeling. In addition, expression of ERS molecules in the noninfarcted myocardium was detected using Western blotting. We found that PQS treatment significantly reduced infarct size and LV dilation and improved LV ejection fraction and fractional shortening in rat hearts. Panax quinquefolium saponin treatment also decreased hydroxyproline level in noninfarcted myocardium. Panax quinquefolium saponin treatment significantly decreased expression of glucose regulating protein 78, calreticulin, C/EBP homologous protein (CHOP), and Bax protein, as well as increased Bcl-2 protein expression in noninfarcted myocardium. Panax quinquefolium saponin treatment (200 mg/kg per day) mimicked the results achieved from the taurine-treated rats. Expression of CHOP positively correlated with the apoptosis index of cardiomyocytes in the noninfarcted myocardium (r = 0.797, P < 0.01). Taken together, PQS treatment significantly improves AMI-induced LV remodeling, and this may be attributed to inhibiting CHOP-mediated ERS-related apoptosis.

The deadly connection between endoplasmic reticulum, Ca2+, protein synthesis, and the endoplasmic reticulum stress response in malignant glioma cells

BACKGROUND

The endoplasmic reticulum (ER) is involved in Ca(2+) signaling and protein processing. Accumulation of unfolded proteins following ER Ca(2+) depletion triggers the ER stress response (ERSR), which facilitates protein folding and removal of damaged proteins and can induce cell death. Unfolded proteins bind to chaperones, such as the glucose-regulated protein (GRP)78 and cause the release of GRP78-repressed proteins executing ERSR.

METHODS

Several glioma cell lines and primary astrocytes were used to analyze ERSR using standard western blots, reverse transcription-PCR, viability assays, and single cell Ca(2+) imaging.

RESULTS

ERSR induction with thapsigargin results in a more intense ERSR associated with a larger loss of ER Ca(2+), activation of ER-associated caspases (4/12) and caspase 3, and a higher rate of malignant glioma cell death than in normal glial cells. Malignant glioma cells have higher levels of protein synthesis and expression of the translocon (a component of the ribosomal complex, guiding protein entry in the ER), the activity of which is associated with the loss of ER Ca(2+). Our experiments confirm increased expression of the translocon in malignant glioma cells. In addition, blockade of the ribosome-translocon complex with agents differently affecting translocon Ca(2+) permeability causes opposite effects on ERSR deployment and death of malignant glioma cells.

CONCLUSIONS

Excessive ER Ca(2+) loss due to translocon activity appears to be responsible for the enhancement of ERSR, leading to the death of glioma cells. The results reveal a characteristic of malignant glioma cells that could be exploited to develop new therapeutic strategies to treat incurable glial malignancies.

Loss of the eukaryotic initiation factor 2α kinase general control nonderepressible 2 protects mice from pressure overload-induced congestive heart failure without affecting ventricular hypertrophy

In response to several stresses, including nutrient deprivation, general control nonderepressible 2 kinase (GCN2) attenuates mRNA translation by phosphorylating eukaryotic initiation factor 2α(Ser51). Energy starvation is known to exacerbate congestive heart failure, and eukaryotic initiation factor 2α(Ser51) phosphorylation is increased in the failing heart. However, the effect of GCN2 during the evolution of congestive heart failure has not been tested. In this study, we examined the influence of GCN2 expression in response to a cardiac stress by inducing chronic pressure overload with transverse aortic constriction in wild-type and GCN2 knockout mice. Under basal conditions, GCN2 knockout mice had normal left ventricular structure and function, but after transverse aortic constriction, they demonstrated less contractile dysfunction, less increase in lung weight, less increase in lung inflammation and vascular remodeling, and less myocardial apoptosis and fibrosis compared with wild-type mice, despite an equivalent degree of left ventricular hypertrophy. As expected, GCN2 knockout attenuated transverse aortic constriction-induced cardiac eukaryotic initiation factor 2α(Ser51) phosphorylation and preserved sarcoplasmic reticulum Ca(2+) ATPase expression compared with wild-type mice. Interestingly, the expression of the antiapoptotic protein Bcl-2 was significantly elevated in GCN2 knockout hearts, whereas in isolated neonatal cardiomyocytes, selective knockdown of GCN2 increased Bcl-2 protein expression and enhanced myocyte resistance to an apoptotic stress. Collectively, our data support the notion that GCN2 impairs the ventricular adaptation to chronic pressure overload by reducing Bcl-2 expression and increasing cardiomyocyte susceptibility to apoptotic stimuli. Our findings suggest that strategies to reduce GCN2 activity in cardiac tissue may be a novel approach to attenuate congestive heart failure development.

Comparative differential proteomic profiles of nonfailing and failing hearts after in vivo thoracic aortic constriction in mice overexpressing FKBP12.6

Chronic pressure overload (PO) induces pathological left ventricular hypertrophy (LVH) leading to congestive heart failure (HF). Overexpression of FKBP12.6 (FK506-binding protein [K]) in mice should prevent Ca2+-leak during diastole and may improve overall cardiac function. In order to decipher molecular mechanisms involved in thoracic aortic constriction (TAC)-induced cardiac remodeling and the influence of gender and genotype, we performed a proteomic analysis using two-dimensional differential in-gel electrophoresis (2D-DIGE), mass spectrometry, and bioinformatics techniques to identify alterations in characteristic biological networks. Wild-type (W) and K mice of both genders underwent TAC. Thirty days post-TAC, the altered cardiac remodeling was accompanied with systolic and diastolic dysfunction in all experimental groups. A gender difference in inflammatory protein expression (fibrinogen, α-1-antitrypsin isoforms) and in calreticulin occurred (males > females). Detoxification enzymes and cytoskeletal proteins were noticeably increased in K mice. Both non- and congestive failing mouse heart exhibited down- and upregulation of proteins related to mitochondrial function and purine metabolism, respectively. HF was characterized by a decrease in enzymes related to iron homeostasis, and altered mitochondrial protein expression related to fatty acid metabolism, glycolysis, and redox balance. Moreover, two distinct differential protein profiles characterized TAC-induced pathological LVH and congestive HF in all TAC mice. FKBP12.6 overexpression did not influence TAC-induced deleterious effects. Huntingtin was revealed as a potential mediator for HF. A broad dysregulation of signaling proteins associated with congestive HF suggested that different sets of proteins could be selected as useful biomarkers for HF progression and might predict outcome in PO-induced pathological LVH.

Depressed calcium-handling proteins due to endoplasmic reticulum stress and apoptosis in the diabetic heart are attenuated by argirein

Diabetic cardiomyopathy (DC) is a unique disease frequently complicated to diabetes mellitus, manifesting endoplasmic reticulum (ER) stress and depressed calcium-handling proteins. We hypothesized that the abnormal FKBP12.6, SERCA2a, and CASQ2 are consequent to ER stress and apoptosis that are likely due to an entity of inflammation. These abnormalities may be attributed to reactive oxygen species genesis from activated NADPH oxidase which could respond to argirein (AR) through its anti-inflammatory activity. Sprague Dawley rats were randomly divided into six groups. Except the normal group, rats were injected with streptozotocin (STZ; 60 mg/kg, i.p.) once. During weeks 5 to 8 following STZ injection, rats were treated (in milligrams per kilogram per day, i.g.) with aminoguanidine (AMG, 100; an inducible nitric oxide synthase and AGEs inhibitor) or three doses of AR (50, 100, and 200). FKBP12.6, SERCA2a, and CASQ2 and ER stress chaperones Bip and PERK and apoptotic molecules were monitored in vivo and in vitro. Impaired cardiac performance and downregulated FKBP12.6, SERCA2a, and CASQ2 were significant in DC in vivo, and abnormal calcium-handling proteins were also found in high-glucose-incubated myocytes in vitro. ER stress manifested by upregulated Bip and PERK was predominant in association with DNA ladder and upregulated Bax and downregulated BCL-2 in vivo and in vitro. AR is effective to attenuate these abnormalities compared to AMG. Diabetic myocardium has inflammatory entity expressed as ER stress contributing to downregulated calcium-handling proteins. AR has potential in managing DC through attenuating depressed calcium-handling proteins, activated ER stress, and apoptosis in the myocardium.

Endoplasmic reticulum and the unfolded protein response: dynamics and metabolic integration

The endoplasmic reticulum (ER) is a dynamic intracellular organelle with multiple functions essential for cellular homeostasis, development, and stress responsiveness. In response to cellular stress, a well-established signaling cascade, the unfolded protein response (UPR), is activated. This intricate mechanism is an important means of re-establishing cellular homeostasis and alleviating the inciting stress. Now, emerging evidence has demonstrated that the UPR influences cellular metabolism through diverse mechanisms, including calcium and lipid transfer, raising the prospect of involvement of these processes in the pathogenesis of disease, including neurodegeneration, cancer, diabetes mellitus and cardiovascular disease. Here, we review the distinct functions of the ER and UPR from a metabolic point of view, highlighting their association with prevalent pathologies.

Coping with endoplasmic reticulum stress in the cardiovascular system

The endoplasmic reticulum (ER) is a multifunctional intracellular organelle, a component of the cellular reticular network that allows cells to adjust to a wide variety of conditions. The cardiomyocyte reticular network is the ideal location of sensors for both intrinsic and extrinsic factors that disrupt energy and/or nutrient homeostasis and lead to ER stress, a disturbance in ER function. ER stress has been linked to both physiological and pathological states in the cardiovascular system; such states include myocardial infarction, oxygen starvation (hypoxia) and fuel starvation, ischemia, pressure overload, dilated cardiomyopathy, hypertrophy, and heart failure. The ER stress coping response (e.g., the unfolded protein response) is composed of discrete pathways that are controlled by a collection of common regulatory components that may function as a single entity involved in reacting to ER stress. These corrective strategies allow the cardiomyocyte reticular network to restore energy and/or nutrient homeostasis and to avoid cell death. Therefore, the identities of the ER stress corrective strategies are important targets for the development of therapeutic approaches for cardiovascular and other acquired disorders.

A network-oriented perspective on cardiac calcium signaling

The normal contractile, electrical, and energetic function of the heart depends on the synchronization of biological oscillators and signal integrators that make up cellular signaling networks. In this review we interpret experimental data from molecular, cellular, and transgenic models of cardiac signaling behavior in the context of established concepts in cell network architecture and organization. Focusing on the cellular Ca(2+) handling machinery, we describe how the plasticity and adaptability of normal Ca(2+) signaling is dependent on dynamic network configurations that operate across a wide range of functional states. We consider how (mal)adaptive changes in signaling pathways restrict the dynamic range of the network such that it cannot respond appropriately to physiologic stimuli or perturbation. Based on these concepts, a model is proposed in which pathologic abnormalities in cardiac rhythm and contractility (e.g., arrhythmias and heart failure) arise as a consequence of progressive desynchronization and reduction in the dynamic range of the Ca(2+) signaling network. We discuss how a systems-level understanding of the network organization, cellular noise, and chaotic behavior may inform the design of new therapeutic modalities that prevent or reverse the disease-linked unraveling of the Ca(2+) signaling network.

Extreme sarcoplasmic reticulum volume loss and compensatory T-tubule remodeling after Serca2 knockout

Cardiomyocyte contraction and relaxation are controlled by Ca(2+) handling, which can be regulated to meet demand. Indeed, major reduction in sarcoplasmic reticulum (SR) function in mice with Serca2 knockout (KO) is compensated by enhanced plasmalemmal Ca(2+) fluxes. Here we investigate whether altered Ca(2+) fluxes are facilitated by reorganization of cardiomyocyte ultrastructure. Hearts were fixed for electron microscopy and enzymatically dissociated for confocal microscopy and electrophysiology. SR relative surface area and volume densities were reduced by 63% and 76%, indicating marked loss and collapse of the free SR in KO. Although overall cardiomyocyte dimensions were unaltered, total surface area was increased. This resulted from increased T-tubule density, as revealed by confocal images. Fourier analysis indicated a maintained organization of transverse T-tubules but an increased presence of longitudinal T-tubules. This demonstrates a remarkable plasticity of the tubular system in the adult myocardium. Immunocytochemical data showed that the newly grown longitudinal T-tubules contained Na(+)/Ca(2+)-exchanger proximal to ryanodine receptors in the SR but did not contain Ca(2+)-channels. Ca(2+) measurements demonstrated a switch from SR-driven to Ca(2+) influx-driven Ca(2+) transients in KO. Still, SR Ca(2+) release constituted 20% of the Ca(2+) transient in KO. Mathematical modeling suggested that Ca(2+) influx via Na(+)/Ca(2+)-exchange in longitudinal T-tubules triggers release from apposing ryanodine receptors in KO, partially compensating for reduced SERCA by allowing for local Ca(2+) release near the myofilaments. T-tubule proliferation occurs without loss of the original ordered transverse orientation and thus constitutes the basis for compensation of the declining SR function without structural disarrangement.