Targeting sigma-1 receptor with fluvoxamine ameliorates pressure-overload-induced hypertrophy and dysfunctions

Sigma-1 Receptor Specific Biological Functions, Protective Role, and Therapeutic Potential in Cardiovascular Diseases

Cardiovascular disease (CVD) is the leading cause of morbidity and mortality worldwide, and there is an urgent need for efficient and cost-effective treatments to decrease the risk of CVD. The sigma-1 receptor (S1R) plays a role in the development of cardiac hypertrophy, heart failure, ventricular remodeling, and various other cardiac diseases. Preclinical studies have shown that S1R activation has considerable beneficial effects on the cardiovascular system, and this knowledge might contribute to informing clinical trials associated with the prevention and treatment of CVDs. Therefore, the objective of this review was to investigate the mechanisms of S1R in CVD and how modulation of pathways contributes to cardiovascular protection to facilitate the development of new therapeutic agents targeting the cardiovascular system.

Blocking Sigmar1 exacerbates methamphetamine-induced hypertension

AIM

Methamphetamine (METH) chronic exposure is an important risk factor for hypertension development. However, the mechanisms behind METH-induced hypertension remain unclear. Therefore, we aimed to reveal the potential mechanisms underlying METH-induced hypertension.

METHODS AND RESULTS

We structured the mouse hypertension model by METH, and observed that METH-treated mice have presented vascular remodeling (large-and small-size arteries) with collagen deposit around the vessel and increasing blood pressure (BP) and Sigma1 receptor (Sigmar1) in vascular tissue. We hypothesized that Sigmar1 is crucial in METH-induced hypertension and vascular remodeling. Sigmar1 knockout (KO) mice and antagonist (BD1047) pretreated mice exposed to METH for six-week showed higher BP and more collagen deposited around vessels than wild-type (WT) mice exposed to METH for six-week, in contrast, mice pretreated with Sigmar1 agonist (PRE-084) had unchanged BP and perivascular collagen despite the six-week METH exposure. Furthermore, we found that METH exposure induced vascular smooth muscle cells (VSMCs) and mesenchymal stem cells to differentiate into the myofibroblast-like cell and secrete collagen into surrounding vessels. Mechanically, Sigmar1 can suppress the COL1A1 expression by blocking the classical fibrotic TGF-β/Smad2/3 signaling pathway in METH-exposed VSMCs and mesenchymal stem cells.

CONCLUSION

Our results suggest that Sigmar1 is involved in METH-induced hypertension and vascular fibrosis by blocking the activation of the TGF-β/Smad2/3 signaling pathway. Accordingly, Sigmar1 may be a novel therapeutic target for METH-induced hypertension and vascular fibrosis.

Dialogue between mitochondria and endoplasmic reticulum-potential therapeutic targets for age-related cardiovascular diseases

Mitochondria-associated endoplasmic reticulum membranes (MAMs) act as physical membrane contact sites facilitating material exchange and signal transmission between mitochondria and endoplasmic reticulum (ER), thereby regulating processes such as Ca2+/lipid transport, mitochondrial dynamics, autophagy, ER stress, inflammation, and apoptosis, among other pathological mechanisms. Emerging evidence underscores the pivotal role of MAMs in cardiovascular diseases (CVDs), particularly in aging-related pathologies. Aging significantly influences the structure and function of the heart and the arterial system, possibly due to the accumulation of reactive oxygen species (ROS) resulting from reduced antioxidant capacity and the age-related decline in organelle function, including mitochondria. Therefore, this paper begins by describing the composition, structure, and function of MAMs, followed by an exploration of the degenerative changes in MAMs and the cardiovascular system during aging. Subsequently, it discusses the regulatory pathways and approaches targeting MAMs in aging-related CVDs, to provide novel treatment strategies for managing CVDs in aging populations.

Deletion of Sigmar1 leads to increased arterial stiffness and altered mitochondrial respiration resulting in vascular dysfunction

Sigmar1 is a ubiquitously expressed, multifunctional protein known for its cardioprotective roles in cardiovascular diseases. While accumulating evidence indicate a critical role of Sigmar1 in cardiac biology, its physiological function in the vasculature remains unknown. In this study, we characterized the expression of Sigmar1 in the vascular wall and assessed its physiological function in the vascular system using global Sigmar1 knockout (Sigmar1-/-) mice. We determined the expression of Sigmar1 in the vascular tissue using immunostaining and biochemical experiments in both human and mouse blood vessels. Deletion of Sigmar1 globally in mice (Sigmar1-/-) led to blood vessel wall reorganizations characterized by nuclei disarray of vascular smooth muscle cells, altered organizations of elastic lamina, and higher collagen fibers deposition in and around the arteries compared to wildtype littermate controls (Wt). Vascular function was assessed in mice using non-invasive time-transit method of aortic stiffness measurement and flow-mediated dilation (FMD) of the left femoral artery. Sigmar1-/- mice showed a notable increase in arterial stiffness in the abdominal aorta and failed to increase the vessel diameter in response to reactive-hyperemia compared to Wt. This was consistent with reduced plasma and tissue nitric-oxide bioavailability (NOx) and decreased phosphorylation of endothelial nitric oxide synthase (eNOS) in the aorta of Sigmar1-/- mice. Ultrastructural analysis by transmission electron microscopy (TEM) of aorta sections showed accumulation of elongated shaped mitochondria in both vascular smooth muscle and endothelial cells of Sigmar1-/- mice. In accordance, decreased mitochondrial respirometry parameters were found in ex-vivo aortic rings from Sigmar1 deficient mice compared to Wt controls. These data indicate a potential role of Sigmar1 in maintaining vascular homeostasis.

An Emerging Role for Sigma Receptor 1 in Personalized Treatment of Breast Cancer

Despite the major progress in treating breast cancer, recurrence remains a problem and types such as triple-negative breast cancer still lack targeted medicine. The orphan Sigma receptor1 (SigmaR1) has emerged as a target in breast cancer, but its mechanism of action is unclear and hinders clinical utility. SigmaR1 is widely expressed in organ tissues and localized to various sub-cellular compartments, particularly the endoplasmic reticulum (ER), the mitochondrial-associated membranes (MAMs) and the nuclear envelope. As such, it involves diverse cellular functions, including protein quality control/ER stress, calcium signaling, cholesterol homeostasis, mitochondrial integrity and energy metabolism. Consequently, SigmaR1 has been implicated in a number of cancers and degenerative diseases and thus has been intensively pursued as a therapeutic target. Because SigmaR1 binds a number of structurally unrelated ligands, it presents an excellent context-dependent therapeutic target. Here, we review its role in breast cancer and the current therapies that have been considered based on its known functions. As SigmaR1 is not classified as an oncoprotein, we propose a model in which it serves as an oligomerization adaptor in key cellular pathways, which may help illuminate its association with variable diseases and pave the way for clinical utility in personalized medicine.

Sigmar1 ablation leads to lung pathological changes associated with pulmonary fibrosis, inflammation, and altered surfactant proteins levels

Sigma1 receptor protein (Sigmar1) is a small, multifunctional molecular chaperone protein ubiquitously expressed in almost all body tissues. This protein has previously shown its cardioprotective roles in rodent models of cardiac hypertrophy, heart failure, and ischemia-reperfusion injury. Extensive literature also suggested its protective functions in several central nervous system disorders. Sigmar1's molecular functions in the pulmonary system remained unknown. Therefore, we aimed to determine the expression of Sigmar1 in the lungs. We also examined whether Sigmar1 ablation results in histological, ultrastructural, and biochemical changes associated with lung pathology over aging in mice. In the current study, we first confirmed the presence of Sigmar1 protein in human and mouse lungs using immunohistochemistry and immunostaining. We used the Sigmar1 global knockout mouse (Sigmar1-/-) to determine the pathophysiological role of Sigmar1 in lungs over aging. The histological staining of lung sections showed altered alveolar structures, higher immune cells infiltration, and upregulation of inflammatory markers (such as pNFκB) in Sigmar1-/- mice compared to wildtype (Wt) littermate control mice (Wt). This indicates higher pulmonary inflammation resulting from Sigmar1 deficiency in mice, which was associated with increased pulmonary fibrosis. The protein levels of some fibrotic markers, fibronectin, and pSMAD2 Ser 245/250/255 and Ser 465/467, were also elevated in mice lungs in the absence of Sigmar1 compared to Wt. The ultrastructural analysis of lungs in Wt mice showed numerous multilamellar bodies of different sizes with densely packed lipid lamellae and mitochondria with a dark matrix and dense cristae. In contrast, the Sigmar1-/- mice lung tissues showed altered multilamellar body structures in alveolar epithelial type-II pneumocytes with partial loss of lipid lamellae structures in the lamellar bodies. This was further associated with higher protein levels of all four surfactant proteins, SFTP-A, SFTP-B, SFTP-C, and SFTP-D, in the Sigmar1-/- mice lungs. This is the first study showing Sigmar1's expression pattern in human and mouse lungs and its association with lung pathophysiology. Our findings suggest that Sigmar1 deficiency leads to increased pulmonary inflammation, higher pulmonary fibrosis, alterations of the multilamellar body stuructures, and elevated levels of lung surfactant proteins.

Sigma-1 Receptor Signaling: In Search of New Therapeutic Alternatives for Cardiovascular and Renal Diseases

Cardiovascular and renal diseases are among the leading causes of death worldwide, and regardless of current efforts, there is a demanding need for therapeutic alternatives to reduce their progression to advanced stages. The stress caused by diseases leads to the activation of protective mechanisms in the cell, including chaperone proteins. The Sigma-1 receptor (Sig-1R) is a ligand-operated chaperone protein that modulates signal transduction during cellular stress processes. Sig-1R interacts with various ligands and proteins to elicit distinct cellular responses, thus, making it a potential target for pharmacological modulation. Furthermore, Sig-1R ligands activate signaling pathways that promote cardioprotection, ameliorate ischemic injury, and drive myofibroblast activation and fibrosis. The role of Sig-1R in diseases has also made it a point of interest in developing clinical trials for pain, neurodegeneration, ischemic stroke, depression in patients with heart failure, and COVID-19. Sig-1R ligands in preclinical models have significantly beneficial effects associated with improved cardiac function, ventricular remodeling, hypertrophy reduction, and, in the kidney, reduced ischemic damage. These basic discoveries could inform clinical trials for heart failure (HF), myocardial hypertrophy, acute kidney injury (AKI), and chronic kidney disease (CKD). Here, we review Sig-1R signaling pathways and the evidence of Sig-1R modulation in preclinical cardiac and renal injury models to support the potential therapeutic use of Sig-1R agonists and antagonists in these diseases.

The role of mitochondria-associated membranes mediated ROS on NLRP3 inflammasome in cardiovascular diseases

Reactive oxygen species (ROS) metabolism is essential for the homeostasis of cells. Appropriate production of ROS is an important signaling molecule, but excessive ROS production can damage cells. ROS and ROS-associated proteins can act as damage associated molecular pattern molecules (DAMPs) to activate the NACHT, LRR, and PYD domains-containing protein 3 (NLRP3) inflammasome in cardiovascular diseases. Previous studies have shown that there are connected sites, termed mitochondria-associated membranes (MAMs), between mitochondria and the endoplasmic reticulum. In cardiovascular disease progression, MAMs play multiple roles, the most important of which is the ability to mediate ROS generation, which further activates the NLPR3 inflammasome, exacerbating the progression of disease. In this review, the following topics will be covered: 1. Molecular structures on MAMs that can mediate ROS generation; 2. Specific mechanisms of molecule-mediated ROS generation and the molecules' roles in cardiovascular disease, 3. The effects of MAMs-mediated ROS on the NLRP3 inflammasome in cardiovascular disease. The purpose of this review is to provide a basis for subsequent clinical treatment development.

Sigma-1 Receptor in Retina: Neuroprotective Effects and Potential Mechanisms

Retinal degenerative diseases are the major factors leading to severe visual impairment and even irreversible blindness worldwide. The therapeutic approach for retinal degenerative diseases is one extremely urgent and hot spot in science research. The sigma-1 receptor is a novel, multifunctional ligand-mediated molecular chaperone residing in endoplasmic reticulum (ER) membranes and the ER-associated mitochondrial membrane (ER-MAM); it is widely distributed in numerous organs and tissues of various species, providing protective effects on a variety of degenerative diseases. Over three decades, considerable research has manifested the neuroprotective function of sigma-1 receptor in the retina and has attempted to explore the molecular mechanism of action. In the present review, we will discuss neuroprotective effects of the sigma-1 receptor in retinal degenerative diseases, mainly in aspects of the following: the localization in different types of retinal neurons, the interactions of sigma-1 receptors with other molecules, the correlated signaling pathways, the influence of sigma-1 receptors to cellular functions, and the potential therapeutic effects on retinal degenerative diseases.

Cell death regulation by MAMs: from molecular mechanisms to therapeutic implications in cardiovascular diseases

The endoplasmic reticulum (ER) and mitochondria are interconnected intracellular organelles with vital roles in the regulation of cell signaling and function. While the ER participates in a number of biological processes including lipid biosynthesis, Ca2+ storage and protein folding and processing, mitochondria are highly dynamic organelles governing ATP synthesis, free radical production, innate immunity and apoptosis. Interplay between the ER and mitochondria plays a crucial role in regulating energy metabolism and cell fate control under stress. The mitochondria-associated membranes (MAMs) denote physical contact sites between ER and mitochondria that mediate bidirectional communications between the two organelles. Although Ca2+ transport from ER to mitochondria is vital for mitochondrial homeostasis and energy metabolism, unrestrained Ca2+ transfer may result in mitochondrial Ca2+ overload, mitochondrial damage and cell death. Here we summarize the roles of MAMs in cell physiology and its impact in pathological conditions with a focus on cardiovascular disease. The possibility of manipulating ER-mitochondria contacts as potential therapeutic approaches is also discussed.

Methamphetamine-Associated Cardiomyopathy: Addressing the Clinical Challenges

The growth in methamphetamine usage worldwide continues to present increasing societal and health care challenges. With the escalation of its usage in a variety of social demographics, the entity of methamphetamine-associated cardiomyopathy (MA-CMP) has emerged. This entity is increasingly responsible for an important proportion of heart failure burden in both admissions to hospital and in those individuals requiring chronic heart failure care. MA-CMP poses some unique challenges including its recognition, particularly in younger patients presenting with new-onset heart failure, its severity at presentation and complications as well as management options. The challenging nature of methamphetamine addiction and the necessity to achieve abstinence is a fundamental aspect of management of this condition. As methamphetamine use continues at high levels in Australia, the burden of MA-CMP will inevitably increase and, therefore, all clinicians responsible for heart failure management require an awareness of this disease entity and the specific clinical challenges of its care.

Chronic Sigma 1 receptor activation alleviates right ventricular dysfunction secondary to pulmonary arterial hypertension

Sigma 1 receptor (S1R) has shown a preferable protective effect on left ventricular function, but whether it protects right ventricular (RV) function is still elusive.This study aimed to determine the effects of S1R on RV dysfunction secondary to pulmonary arterial hypertension.Sixty wild-type male Sprague–Dawley rats were randomly divided into the control group, the fluvoxamine group, the pulmonary arterial hypertension group and the pulmonary arterial hypertension combined with fluvoxamine group. Monocrotaline (60 mg/kg) was administered to induce pulmonary arterial hypertension, and fluvoxamine was given for 21 consecutive days to activate S1R after one week of monocrotaline administration. Echocardiographic, serologic, and histologic parameters, qRT-PCR, and western blotting were conducted after 4 weeks of monocrotaline administration.The expression of S1R was decreased in the right ventricle in pulmonary arterial hypertension. TAPSE, and the FAC of the right ventricle were significantly decreased, and RV EDP and the plasma concentration of N-terminal pro-B-type natriuretic peptide was increased in the pulmonary arterial hypertension group, but fluvoxamine partly restored those abnormalities (all P < 0.05). Moreover, pulmonary arteriole remodeling, and fibrosis and hypertrophy in the RV were shown in the pulmonary arterial hypertension group; interestingly, fluvoxamine recovered RV structural remodeling (all P < 0.05) but neither alleviated pulmonary arteriole remodeling nor reduced pulmonary artery pressure. Furthermore, S1R activation protects RV function by upgrading the NRF 2/HO 1-mediated antioxidant stress pathway. In conclusion, chronic S1R activation ameliorates structural remodeling and RV dysfunction secondary to pulmonary arterial hypertension without altering pulmonary artery pressure.

The molecular role of Sigmar1 in regulating mitochondrial function through mitochondrial localization in cardiomyocytes

Sigmar1 is a widely expressed molecular chaperone protein in mammalian cell systems. Accumulating research demonstrated the cardioprotective roles of pharmacologic Sigmar1 activation by ligands in preclinical rodent models of cardiac injury. Extensive biochemical and immuno-electron microscopic research demonstrated Sigmar1's sub-cellular localization largely depends on cell and organ types. Despite comprehensive studies, Sigmar1's direct molecular role in cardiomyocytes remains elusive. In the present study, we determined Sigmar1's subcellular localization, transmembrane topology, and function using complementary microscopy, biochemical, and functional assays in cardiomyocytes. Quantum dots in transmission electron microscopy showed Sigmar1 labeled quantum dots on the mitochondrial membranes, lysosomes, and sarcoplasmic reticulum-mitochondrial interface. Subcellular fractionation of heart cell lysates confirmed Sigmar1's localization in purified mitochondria fraction and lysosome fraction. Immunocytochemistry confirmed Sigmar1 colocalization with mitochondrial proteins in isolated adult mouse cardiomyocytes. Sigmar1's mitochondrial localization was further confirmed by Sigmar1 colocalization with Mito-Tracker in isolated mouse heart mitochondria. A series of biochemical experiments, including alkaline extraction and proteinase K treatment of purified heart mitochondria, demonstrated Sigmar1 as an integral mitochondrial membrane protein. Sigmar1's structural requirement for mitochondrial localization was determined by expressing FLAG-tagged Sigmar1 fragments in cells. Full-length Sigmar1 and Sigmar1's C terminal-deletion fragments were able to localize to the mitochondrial membrane, whereas N-terminal deletion fragment was unable to incorporate into the mitochondria. Finally, functional assays using extracellular flux analyzer and high-resolution respirometry showed Sigmar1 siRNA knockdown significantly altered mitochondrial respiration in cardiomyocytes. Overall, we found that Sigmar1 localizes to mitochondrial membranes and is indispensable for maintaining mitochondrial respiratory homeostasis in cardiomyocytes.

Molecular Characterization of Skeletal Muscle Dysfunction in Sigma 1 Receptor (Sigmar1) Knockout Mice

Sigma 1 receptor (Sigmar1) is a widely expressed, multitasking molecular chaperone protein that plays functional roles in several cellular processes. Mutations in the Sigmar1 gene are associated with several distal neuropathies with strong manifestation in skeletal muscle dysfunction with phenotypes like muscle wasting and atrophy. However, the physiological function of Sigmar1 in skeletal muscle remains unknown. Herein, the physiological role of Sigmar1 in skeletal muscle structure and function in gastrocnemius, quadriceps, soleus, extensor digitorum longus, and tibialis anterior muscles was determined. Quantification of myofiber cross-sectional area showed altered myofiber size distribution and changes in myofiber type in the skeletal muscle of the Sigmar1-/- mice. Interestingly, ultrastructural analysis by transmission electron microscopy showed the presence of abnormal mitochondria, and immunostaining showed derangements in dystrophin localization in skeletal muscles from Sigmar1-/- mice. In addition, myopathy in Sigmar1-/- mice was associated with an increased number of central nuclei, increased collagen deposition, and fibrosis. Functional studies also showed reduced endurance and exercise capacity in the Sigmar1-/- mice without any changes in voluntary locomotion, markers for muscle denervation, and muscle atrophy. Overall, this study shows, for the first time, a potential physiological function of Sigmar1 in maintaining healthy skeletal muscle structure and function.

Visualization of Dynamic Expression of Myocardial Sigma-1 Receptor After Myocardial Ischemia and Reperfusion Using Radioiodine-Labeled 2-[4-(2-iodophenyl)piperidino]cyclopentanol (OI5V) Imaging

BACKGROUND

This study chronologically evaluated the expression of the intensity and distribution of the sigma-1 receptor (σ1R) demonstrated by radiolabeled 2-[4-(2-iodophenyl)piperidino]cyclopentanol (OI5V) in a rat model of myocardial ischemia and reperfusion.

METHODS AND RESULTS

The left coronary artery was occluded for 30 min, followed by reperfusion. Dual-tracer autoradiography with 125I-OI5V and 99 mTc-MIBI was performed to assess the spatiotemporal changes in 125I-OI5V uptake (n=5-6). Significant and peaked 125I-OI5V uptake in the ischemic area was observed at 3 days after reperfusion, and the 125I-OI5V uptake ratio of ischemic area to normally perfused left ventricular area decreased gradually from 3 to 28 days (mean value±SD; 0.90±0.12 at 1 day, 1.89±0.19 at 3 days, 1.52±0.17 at 7 days, 1.34±0.13 at 14 days, and 1.16±0.14 at 28 days, respectively). Triple-tracer autoradiography with 125I-OI5V, 99 mTc-MIBI, and 201TlCl was performed to evaluate 125I-OI5V uptake in the ischemic area in relation to the residual perfusion at 7 days (n=4). The 125I-OI5V uptake ratio of the non-salvaged area was higher compared to that of the salvaged area in the ischemic area. 123I-OI5V and 99 mTc-MIBI SPECT/CT was performed 3 days after reperfusion (n=3), and the in vivo images showed clear uptake of 123I-OI5V in the perfusion defect area.

CONCLUSIONS

The present study confirmed the spatiotemporal expression pattern of σ1R expression. Non-invasive σ1R imaging with 123I or 125I-OI5V was feasible to monitor the expression of σ1R after myocardial ischemia and reperfusion.

Yiqi Huoxue preserves heart function by upregulating the Sigma-1 receptor in rats with myocardial infarction

Yiqi Huoxue (YQHX) is widely used in traditional Chinese medical practice due to its reported cardioprotective effects. The aim of the present study was to investigate the mechanism underlying these effects of YQHX via the regulation of the Sigma-1 receptor. The Sigma-1 receptor is a chaperone protein located on the mitochondrion-associated endoplasmic reticulum (ER) membrane. It serves an important role in heart function by regulating intracellular Ca²⁺ homeostasis and enhancing cellular bioenergetics. In the present study, male Sprague Dawley rats with myocardial infarction (MI)-induced heart failure were used. MI rats were administered different treatments, including normal saline, YQHX and fluvoxamine, an agonist of the Sigma-1 receptor. Following four weeks of treatment, YQHX was revealed to improve heart function and attenuate myocardial hypertrophy in MI rats. Additionally, YQHX increased the ATP content and improved the mitochondrial ultrastructure in the heart tissues of MI rats in comparison with acontrol. Treatment was revealed to attenuate the decreased expression of the Sigma-1 receptor and increase the expression of inositol triphosphate type 2 receptors (IP₃R2) in MI rats. By exposing H9c2 cells to angiotensin II (Ang II), YQHX prevented cell hypertrophy and normalized the decreased ATP content. However, these positive effects were partially inhibited when the Sigma-1 receptor was knocked down via small interfering RNA transfection. The results of the present study suggested that the Sigma-1 receptor serves an important role in the cardioprotective efficacy of YQHX by increasing ATP content and attenuating cardiomyocyte hypertrophy.

Sigmar1's Molecular, Cellular, and Biological Functions in Regulating Cellular Pathophysiology

The Sigma 1 receptor (Sigmar1) is a ubiquitously expressed multifunctional inter-organelle signaling chaperone protein playing a diverse role in cellular survival. Recessive mutation in Sigmar1 have been identified as a causative gene for neuronal and neuromuscular disorder. Since the discovery over 40 years ago, Sigmar1 has been shown to contribute to numerous cellular functions, including ion channel regulation, protein quality control, endoplasmic reticulum-mitochondrial communication, lipid metabolism, mitochondrial function, autophagy activation, and involved in cellular survival. Alterations in Sigmar1’s subcellular localization, expression, and signaling has been implicated in the progression of a wide range of diseases, such as neurodegenerative diseases, ischemic brain injury, cardiovascular diseases, diabetic retinopathy, cancer, and drug addiction. The goal of this review is to summarize the current knowledge of Sigmar1 biology focusing the recent discoveries on Sigmar1’s molecular, cellular, pathophysiological, and biological functions.

Mitochondria-associated membrane-modulated Ca2+ transfer: A potential treatment target in cardiac ischemia reperfusion injury and heart failure

Effective Ca²⁺ dependent mitochondrial energy supply is imperative for proper cardiac contractile activity, while disruption of Ca²⁺ homeostasis participates in the pathogenesis of multiple human diseases. This phenomenon is particularly prominent in cardiac ischemia and reperfusion (I/R) and heart failure, both of which require strict clinical intervention. The interface between endoplasmic reticula (ER) and mitochondria, designated the mitochondria-associated membrane (MAM), is now regarded as a crucial mediator of Ca²⁺ transportation. Thus, interventions targeting this physical and functional coupling between mitochondria and the ER are highly desirable. Increasing evidence supports the notion that restoration, and maintenance, of the physiological contact between these two organelles can improve mitochondrial function, while inhibiting cell death, thereby sufficiently ameliorating I/R injury and heart failure development. A better understanding regarding the underlying mechanism of MAM-mediated transport will pave the way for identification of novel treatment approaches for heart disease. Therefore, in this review, we summarize the crucial functions and potential mechanisms of MAMs in the pathogenesis of I/R and heart failure.

Stimulation of Sigma-1 Receptor Protects against Cardiac Fibrosis by Alleviating IRE1 Pathway and Autophagy Impairment

Sigma-1 receptor (Sig1R), a chaperone in the endoplasmic reticulum (ER) membrane, has been implicated in cardiac hypertrophy; however, its role in cardiac fibroblast activation has not been established. This study investigated the possible association between Sig1R and this activation by subjecting mice to sham, transverse aortic constriction (TAC), and TAC plus fluvoxamine (an agonist of Sig1R) treatments. Cardiac function and fibrosis were evaluated four weeks later by echocardiography and histological staining. In an in vitro study, neonatal rat cardiac fibroblasts were treated with fluvoxamine or NE-100 (an antagonist of Sig1R) in the presence or absence of transforming growth factor beta1 (TGF-β1). Fibrotic markers, ER stress pathways, and autophagy were then investigated by qPCR, western blotting, immunofluorescence, confocal microscopy, and transmission electron microscopy. Fluvoxamine treatment reduced cardiac fibrosis, preserved cardiac function, and attenuated cardiac fibroblast activation. Inhibition of the IRE1/XBP1 pathway, a branch of ER stress, by a specific inhibitor of IRE1 endonuclease activity, attenuated the pathological process. Fluvoxamine stimulation of Sig1R restored autophagic flux in cardiac fibroblasts, indicating that Sig1R appears to play a protective role in the activation of cardiac fibroblasts by inhibiting the IRE1 pathway and restoring autophagic flux. Sig1R may therefore represent a therapeutic target for cardiac fibrosis.

Chronic sigma-1 receptor activation ameliorates ventricular remodeling and decreases susceptibility to ventricular arrhythmias after myocardial infarction in rats

The present study aimed to assess the effect of sigma-1 receptor (S1R) stimulation on ventricular remodeling and susceptibility to ventricular arrhythmias (VAs) after myocardial infarction (MI) in rats. Wild-type male rats were placed into one of the following four treatment groups. For four weeks, animals in the Sham group and MI group received intraperitoneal (i.p.) injections of 0.9% saline (1 ml/kg/day); those in the MI + F group received fluvoxamine (FLV) (0.3 mg/kg/day); and those in the MI + F + BD group received FLV plus BD1047 (0.3 mg/kg/day). After that, the ventricular electrophysiological parameters were measured via the langendorff system. Ventricular fibrosis quantification was determined with Masson staining. Cardiac function was evaluated by echocardiography. The protein levels of S1R, connexin (Cx)43, Cav1.2, Kv4.2, Kv4.3, tyrosine hydroxylase (TH), nerve growth factor (NGF), growth-associated protein 43 (GAP43) were detected by Western blot assays. Our results indicated that fluvoxamine significantly prolonged the ventricular effective refractory period (ERP), shortened action potential duration (APD), reduced susceptibility to VAs after MI. Masson staining showed a decrease in ventricular fibrosis in the MI + F group. Furthermore, the contents of Cx43, S1R, Cav1.2, Kv4.2, Kv4.3 were increased in the MI + F group compared with the MI group (all P < 0.05). The contents of TH, NGF, GAP43 were reduced in the MI + F group compared with the MI group. (all P < 0.05). However, BD1047 reduces all of these effects of FLV. The results suggest that S1R stimulation reduces susceptibility to VAs and improves cardiac function by improving myocardial fibrosis, lightning sympathetic remodeling, electrical remodeling, gap junction remodeling and upregulating S1R content.

Methamphetamine induces cardiomyopathy by Sigmar1 inhibition-dependent impairment of mitochondrial dynamics and function

Methamphetamine-associated cardiomyopathy is the leading cause of death linked with illicit drug use. Here we show that Sigmar1 is a therapeutic target for methamphetamine-associated cardiomyopathy and defined the molecular mechanisms using autopsy samples of human hearts, and a mouse model of "binge and crash" methamphetamine administration. Sigmar1 expression is significantly decreased in the hearts of human methamphetamine users and those of "binge and crash" methamphetamine-treated mice. The hearts of methamphetamine users also show signs of cardiomyopathy, including cellular injury, fibrosis, and enlargement of the heart. In addition, mice expose to "binge and crash" methamphetamine develop cardiac hypertrophy, fibrotic remodeling, and mitochondrial dysfunction leading to contractile dysfunction. Methamphetamine treatment inhibits Sigmar1, resulting in inactivation of the cAMP response element-binding protein (CREB), decreased expression of mitochondrial fission 1 protein (FIS1), and ultimately alteration of mitochondrial dynamics and function. Therefore, Sigmar1 is a viable therapeutic agent for protection against methamphetamine-associated cardiomyopathy.

PRE-084 as a tool to uncover potential therapeutic applications for selective sigma-1 receptor activation

The discovery of a highly selective putative sigma-1 (σ1) receptor agonist, PRE-084, has revealed the numerous potential uses of this receptor subtype as a therapeutic target. While much work has been devoted to determining the role of σ1 receptors in normal and pathophysiological states in the nervous system, recent work suggests that σ1 receptors may be important for modulating functions of other tissues. These discoveries have provided novel insights into σ1 receptor structure, function, and importance in multiple intracellular signaling mechanisms. These discoveries were made possible by σ1 receptor-selective agonists such as PRE-084. The chemical properties and pharmacological actions of PRE-084 will be reviewed here, along with the expanding list of potential therapeutic applications for selective activation of σ1 receptors.

ER membranes associated with mitochondria: Possible therapeutic targets in heart-associated diseases

Cardiovascular system cell biology is tightly regulated and mitochondria play a relevant role in maintaining heart function. In recent decades, associations between such organelles and the sarco/endoplasmic reticulum (SR) have been raised great interest. Formally identified as mitochondria-associated SR membranes (MAMs), these structures regulate different cellular functions, including calcium management, lipid metabolism, autophagy, oxidative stress, and management of unfolded proteins. In this review, we highlight MAMs' alterations mainly in cardiomyocytes, linked with cardiovascular diseases, such as cardiac ischemia-reperfusion, heart failure, and dilated cardiomyopathy. We also describe proteins that are part of the MAMs' machinery, as the FUN14 domain containing 1 (FUNDC1), the sigma 1 receptor (Sig-1R) and others, which might be new molecular targets to preserve the function and structure of the heart in such diseases. Understanding the machinery of MAMs and its function demands our attention, as such knowledge might contribute to strengthen the role of these relative novel structures in heart diseases.

Is the sigma-1 receptor a potential pharmacological target for cardiac pathologies? A systematic review

Sigma-1 receptors are ligand-regulated chaperone proteins, involved in several cellular mechanisms. The aim of this systematic review was to examine the effects that the sigma-1 receptor has on the cardiovascular system. The interaction targets and proposed mechanisms of action of sigma-1 receptors were explored, with the aim of determining if the sigma-1 receptor is a potential pharmacological target for cardiac pathologies. This systematic review was conducted according to the PRISMA guidelines and these were used to critically appraise eligible studies. Pubmed and Scopus were systematically searched for articles investigating sigma-1 receptors in the cardiovascular system. Papers identified by the search terms were then subject to analysis against pre-determined inclusion criteria. 23 manuscripts met the inclusion criteria and were included in this review. The experimental platforms, experimental techniques utilised and the results of the studies were summarised. The sigma-1 receptor is found to be implicated in cardioprotection, via various mechanisms including stimulating the Akt-eNOS pathway, and reduction of Ca2 + leakage into the cytosol via modulating certain calcium channels. Sigma-1 receptors are also found to modulate other cardiac ion channels including different subtypes of potassium and sodium channels and have been shown to modulate intracardiac neuron excitability. The sigma-1 receptor is a potential therapeutic target for treatment of cardiac pathologies, particularly cardiac hypertrophy. We therefore suggest investigating the cardioprotective mechanisms of sigma-1 receptor function, alongside proposed potential ligands that can stimulate these functions.

The Reversal Effect of Sigma-1 Receptor (S1R) Agonist, SA4503, on Atrial Fibrillation After Depression and Its Underlying Mechanism

Aim

Sigma-1 receptors have been investigated and shown to play a protective role in both depression and cardiovascular disease. SA4503, known as a σ1 receptor agonist, regulates cardiac calcium and potassium channels in rat models of depression. However, it remains unknown whether SA4503 can alleviate myocardial inflammation or conduction junctions in the atrium after exposure to chronic mild stress.

Methods and Results

Sprague-Dawley male rats received 28-day treatment with SA4503, simultaneously with chronic mild stress. Behavior measurements were assessed after the daily doses. Additionally, a multielectrode array assessment, electrophysiological study, immunohistochemistry analysis, histological analysis, and Western blot analysis were performed. Depression rats’ hearts showed abnormal electrical activity, including disordered excitation propagation and prolonged total activation time (TAT). In addition, atrial arrhythmias (AAs), induced by burst stimulation, showed higher incidence and longer duration in the depression group compared to the control group. These changes were related to reduced conduction junctions and enhanced spatial heterogeneity. Importantly, depressed rat hearts showed greater expression of inflammatory factors (TGF-α, IL-6, and TGF-β), more collagen distribution in the extracellular matrix, and lower expression of gap junction proteins (CX40 and CX43). Furthermore, SA4503 partially mitigated the above indices in the depression group (P < 0.01 for all groups).

Conclusion

These findings show the effects of the σ1R agonist SA4503; it alleviates atrial myocardial inflammation and conduction junctions after chronic mild stress. SA4503 may be the promising pharmacological agent to treat depression-related AAs by increasing conduction function, improving the expression of connexin 40 and 43, and reducing cardiac myocardial inflammation.

Anti-Ischemic Activity of Fabomotizole Hydrochloride under Conditions of Endothelial Dysfunction

Anti-ischemic activity of fabomotizole hydrochloride was studied on the model of subendocardial ischemia in rats with endothelial dysfunction. Endothelial dysfunction was modeled by intragastric administration of methionine (3 g/kg, once a day for 7 days). Acute subendocardial ischemia was induced in narcotized rats by intraperitoneal injection of isoproterenol (20 μg/kg/min over 5 min). Fabomotizole hydrochloride (intraperitoneally, 15 mg/kg) significantly reduced isoproterenol-induced ST segment depression in animals with endothelial dysfunction and with intact vasculature.

Cardiac 11C-Donepezil Binding Increases With Age in Healthy Humans: Potentially Signifying Sigma-1 Receptor Upregulation

BACKGROUND

Donepezil may have cardioprotective properties, but the mechanism is unclear. Using positron-emission tomography (PET), we explored 11C-donepezil uptake in the heart of humans in relation to age. The results are discussed in the context of the cardioprotective property of donepezil.

METHODS

We included data from 57 patients with cardiac 11C-donepezil PET scans. Linear regression analyses were performed to explore the correlation between cardiac 11C-donepezil standardized uptake value (SUV) and age. Subgroup analyses were performed for healthy controls, patients with prodromal or diagnosed Parkinson disease (PD), males, and females.

RESULTS

In the total group of 57 patients, linear regression analysis revealed a significant positive correlation between cardiac 11C-donepezil uptake and age ( r2 = .63, P < .0001). The average increase was ≈1.25 SUV per decade and a 2-fold increase in SUV from age 30 to 65 years. Subgroup analyses also showed significant correlations: healthy control patients alone (n = 28, r2 = .73, P < .0001), prodromal or diagnosed PD (n = 29, r2 = .28, P = .03), male patients (n = 34, r2 = .49, P < .0001), and female patients (n = 23, r2 = .82, P < .0001). No other organs showed increased 11C-donepezil binding with age.

CONCLUSIONS

11C-donepezil SUV increases robustly with age in the normal human heart. We speculate that the increased donepezil binding is caused primarily by sigma-1 receptor upregulation. If our interpretation is correct, it shows that sigma-1 receptors are dynamically regulated and may represent an overlooked target for pharmacological intervention studies.

May a sigma-1 antagonist improve neuropathic signs induced by cisplatin and vincristine in rats?

BACKGROUND

The antineoplastic drugs cisplatin and vincristine induce peripheral neuropathies. The sigma-1 receptor (σ1R) is expressed in areas of pain control, and its blockade with the novel selective antagonist MR-309 has shown efficacy in nociceptive and neuropathic pain models. Our goal was to test whether this compound reduces neuropathic signs provoked by these antitumoural drugs.

METHODS

Rats were treated with cisplatin or vincristine to induce neuropathies. The effects of acute or repeated administration of MR-309 were tested on mechanical and thermal sensitivity, electrophysiological activity of Aδ-primary afferents in the rat skin-saphenous nerve preparation, and gastrointestinal or cardiovascular functions.

RESULTS

Rats treated with antitumourals developed tactile allodynia, while those treated with vincristine also developed mechanical hyperalgesia. These in vivo modifications correlated with electrophysiological hyperactivity (increased spontaneous activity and hyperresponsiveness to innocuous and noxious mechanical stimulation). Animals treated with cisplatin showed gastrointestinal impairment and those receiving vincristine showed cardiovascular toxicity. A single dose of MR-309 strongly reduced both nociceptive behaviour and electrophysiological changes. Moreover, its concomitant administration with the antitumourals blocked the development of neuropathic symptoms, thus restoring mechanical sensitivity, improving the impairment of feeding behaviour and gastrointestinal transit in the cisplatin-treated group along with ameliorating the altered vascular reactivity recorded in rats treated with vincristine.

CONCLUSION

σ1R antagonist, MR-309, reduces sensorial and electrophysiological neuropathic signs in rats treated with cisplatin or vincristine and, in addition, reduces gastrointestinal and cardiovascular side effects.

SIGNIFICANCE

σ1R antagonism could be an interesting and new option to palliate antitumoural neuropathies.

Cardiac Dysfunction in the Sigma 1 Receptor Knockout Mouse Associated With Impaired Mitochondrial Dynamics and Bioenergetics

Background The Sigma 1 receptor (Sigmar1) functions as an interorganelle signaling molecule and elicits cytoprotective functions. The presence of Sigmar1 in the heart was first reported on the basis of a ligand-binding assay, and all studies to date have been limited to pharmacological approaches using less-selective ligands for Sigmar1. However, the physiological function of cardiac Sigmar1 remains unknown. We investigated the physiological function of Sigmar1 in regulating cardiac hemodynamics using the Sigmar1 knockout mouse (Sigmar1-/-). Methods and Results Sigmar1-/- hearts at 3 to 4 months of age showed significantly increased contractility as assessed by left ventricular catheterization with stimulation by increasing doses of a β1-adrenoceptor agonist. Noninvasive echocardiographic measurements were also used to measure cardiac function over time, and the data showed the development of cardiac contractile dysfunction in Sigmar1 -/- hearts as the animals aged. Histochemistry demonstrated significant cardiac fibrosis, collagen deposition, and increased periostin in the Sigmar1 -/- hearts compared with wild-type hearts. Ultrastructural analysis of Sigmar1-/- cardiomyocytes revealed an irregularly shaped, highly fused mitochondrial network with abnormal cristae. Mitochondrial size was larger in Sigmar1-/- hearts, resulting in decreased numbers of mitochondria per microscopic field. In addition, Sigmar1-/- hearts showed altered expression of mitochondrial dynamics regulatory proteins. Real-time oxygen consumption rates in isolated mitochondria showed reduced respiratory function in Sigmar1-/- hearts compared with wild-type hearts. Conclusions We demonstrate a potential function of Sigmar1 in regulating normal mitochondrial organization and size in the heart. Sigmar1 loss of function led to mitochondrial dysfunction, abnormal mitochondrial architecture, and adverse cardiac remodeling, culminating in cardiac contractile dysfunction.

Sigmar1 regulates endoplasmic reticulum stress-induced C/EBP-homologous protein expression in cardiomyocytes

C/EBP-homologous protein (CHOP) is a ubiquitously expressed stress-inducible transcription factor robustly induced by maladaptive endoplasmic reticulum (ER) stresses in a wide variety of cells. Here, we examined a novel function of Sigma 1 receptor (Sigmar1) in regulating CHOP expression under ER stress in cardiomyocytes. We also defined Sigmar1-dependent activation of the adaptive ER-stress pathway in regulating CHOP expression. We used adenovirus-mediated Sigmar1 overexpression as well as Sigmar1 knockdown by siRNA in neonatal rat ventricular cardiomyocytes (NRCs); to induce ER stress, cardiomyocytes were treated with tunicamycin. Sigmar1-siRNA knockdown significantly increased the expression of CHOP and significantly induced cellular toxicity by sustained activation of ER stress in cardiomyocytes. Sigmar1 overexpression decreased the expression of CHOP and significantly decreased cellular toxicity in cells. Using biochemical and immunocytochemical experiments, we also defined the specific ER-stress pathway associated with Sigmar1-dependent regulation of CHOP expression and cellular toxicity. We found that Sigmar1 overexpression significantly increased inositol requiring kinase 1α (IRE1α) phosphorylation and increased spliced X-box-binding proteins (XBP1s) expression as well as nuclear localization. In contrast, Sigmar1 knockdown significantly decreased IRE1α phosphorylation and decreased XBP1s expression as well as nuclear transport. Taken together, these results indicate that Sigmar1-dependent activation of IRE1α-XBP1s ER-stress response pathways are associated with inhibition of CHOP expression and suppression of cellular toxicity. Hence, Sigmar1 is an essential component of the adaptive ER-stress response pathways eliciting cellular protection in cardiomyocytes.

MicroRNA-297 promotes cardiomyocyte hypertrophy via targeting sigma-1 receptor

AIMS

Sigma-1 receptor (Sig-1R) is a ligand-regulated endoplasmic reticulum (ER) chaperone involved in cardiac hypertrophy, but it is not known whether Sig-1R is regulated by microRNAs (miRNAs). According to bioinformatic analysis, miR-297 was suggested as a potential target miRNA for Sig-1R. Therefore, we verified whether miR-297 could target Sig-1R and investigated the possible mechanisms underlying the role of miR-297 in cardiac hypertrophy.

MAIN METHODS

Bioinformatic analysis combined with laboratory experiments, including quantitative RT-PCR, Western blotting, and luciferase assay, were performed to identify the target miRNA of Sig-1R. Transverse aortic constriction (TAC) model and neonatal rat cardiomyocytes (NCMs) stimulated with angiotensin II (AngII) were used to explore the relationship between miR-297 and Sig-1R. Additionally, the function of miR-297 in cardiomyocyte hypertrophy and ER stress/unfolded protein response (UPR) signaling pathway was investigated by transfecting miR-297 mimics/inhibitor.

KEY FINDINGS

miR-297 levels were increased in both TAC-induced hypertrophic heart tissue and AngII-induced cardiomyocyte hypertrophy. Up-regulation of miR-297 by specific mimics exacerbated AngII-induced cardiomyocyte hypertrophy, whereas inhibition of miR-297 suppressed the process. During cardiomyocyte hypertrophy, Sig-1R expression, which was negatively regulated by miR-297 by directly targeting its 3'untranslated region (UTR), was decreased. Furthermore, attenuation of miR-297 inhibited the activation of X-box binding protein 1 (Xbp1) and activating transcriptional factor 4 (ATF4) signaling pathways in NCMs.

SIGNIFICANCE

Our data demonstrate that miR-297 promotes cardiomyocyte hypertrophy by inhibiting the expression of Sig-1R and activation of ER stress signaling, which provides a novel interpretation for cardiac hypertrophy.

Blockade of sigma 1 receptors alleviates sensory signs of diabetic neuropathy in rats

BACKGROUND

E-52862 (S1RA, 4-[2-[[5-methyl-1-(2-naphthalenyl)-1H-pyrazol-3-yl]oxy]ethyl]-morpholine), a novel selective sigma 1 receptor (σ1R) antagonist, has demonstrated efficacy in nociceptive and neuropathic pain models. Our aim was to test if σ1R blockade with E-52862 may modify the signs of neuropathy in Zucker diabetic fatty (ZDF) rats, a type 2 diabetes model.

METHODS

Mechanical and thermal response thresholds were tested on 7-, 13-, 14- and 15-week-old ZDF rats treated with saline or with E-52862 acutely administered on week 13, followed by sub-chronic administration (14 days). Axonal peripheral activity (skin-saphenous nerve preparation) and isolated aorta or mesenteric bed reactivity were analysed in 15-week-old ZDF rats treated with saline or E-52862 and in LEAN rats.

RESULTS

Zucker diabetic fatty rats showed significantly decreased thermal withdrawal latency and threshold to mechanical stimulation on week 13 compared to week 7 (prediabetes) and with LEAN animals; single-dose and sub-chronic E-52862 administration restored both parameters to those recorded on week 7. Regarding axonal peripheral activity, E-52862 treatment increased the mean mechanical threshold (77.3 ± 21 mN vs. 19.6 ± 1.5 mN, saline group) and reduced the response evoked by mechanical increasing stimulation (86.4 ± 36.5 vs. 352.8 ± 41.4 spikes) or by repeated mechanical supra-threshold steps (39.4 ± 1.4 vs. 83.5 ± 0.9). E-52862 treatment also restored contractile response to phenylephrine in aorta and mesenteric bed.

CONCLUSIONS

E-52862 administration reverses neuropathic (behavioural and electrophysiological) and vascular signs in the ZDF rat.

SIGNIFICANCE

Blockade of σ1R avoids the development of diabetic neuropathy in rats, and may represent a potentially useful therapeutic approach to peripheral neuropathies in diabetic patients. WHAT DOES THIS STUDY ADD?: This study presents evidences for the potential usefulness of sigma receptor blockade on diabetic neuropathy in rats. The methodology includes behavioural evidences, electrophysiological data and vascular-isolated models.

Stimulation of the Sigma-1 Receptor and the Effects on Neurogenesis and Depressive Behaviors in Mice

Sigma-1 receptor (Sig-1R) is molecular chaperone regulating calcium efflux from the neuronal endoplasmic reticulum to mitochondria. Recent studies show that Sig-1R stimulation antagonizes depressive-like behaviors in animal models, but molecular mechanisms underlying this effect remain unclear. Here, we focus on the effects of Sig-1R ligands on hippocampal neurogenesis and depressive-like behaviors. Sig-1R stimulation also enhances CaMKII /CaMKIV and protein kinase B (Akt) activities in hippocampus. Therefore, we discuss the fundamental roles of Sig-1R, CaMKII /CaMKIV and protein kinase B (Akt) signaling in amelioration of depressive-like behaviors following Sig-1R stimulation.

Corticosteroids Mediate Heart Failure-Induced Depression through Reduced σ1-Receptor Expression

Cardiovascular diseases are risk factors for depression in humans. We recently proposed that σ1 receptor (σ1R) stimulation rescued cardiac hypertrophy and heart failure induced by transverse aortic constriction (TAC) in mice. Importantly, σ1R stimulation reportedly ameliorates depression-like behaviors in rodents. Thus, we hypothesized that impaired σ1R activity in brain triggers depression-like behaviors in animals with cardiovascular disease. Indeed, here we found that cardiac hypertrophy and heart failure induced by TAC were associated with depression-like behaviors concomitant with downregulation of σ1R expression in brain 6 weeks after surgery. σ1R levels significantly decreased in astrocytes in both the hippocampal CA1 region and dentate gyrus. Oral administration of the specific σ1R agonist SA4503 (0.3-1.0mg/kg) significantly improved TAC-induced depression-like behaviors concomitant with rescued astrocytic σ1R expression in CA1 and the dentate gyrus. Plasma corticosterone levels significantly increased 6 weeks after TAC, and chronic treatment of mice with corticosterone for 3 weeks elicited depression-like behaviors concomitant with reduced astrocytic σ1R expression in hippocampus. Furthermore, the glucocorticoid receptor antagonist mifepristone antagonized depressive-like behaviors and ameliorated decreased hippocampal σ1R expression in TAC mice. We conclude that elevated corticosterone levels trigger hippocampal σ1R downregulation and that σ1R stimulation with SA4503 is an attractive therapy to improve not only cardiac dysfunction but depression-like behaviors associated with heart failure.

Haloperidol aggravates transverse aortic constriction-induced heart failure via mitochondrial dysfunction

Haloperidol is an antipsychotic drug that inhibits the dopamine D2 receptor among others. Haloperidol also binds the sigma-1 receptor (σ1R) and inhibits it irreversibly. A serious outcome of haloperidol treatment of schizophrenia patients is death due to sudden cardiac failure. Although the cause remains unclear, we hypothesized that these effects were mediated by chronic haloperidol inhibition of cardiac σ1R. To test this, we treated neonatal rat cardiomyocytes with haloperidol, exposed them to angiotensin II and assessed hypertrophy, σ1R expression, mitochondrial Ca(2+) transport and ATP levels. In this context, haloperidol treatment altered mitochondrial Ca(2+) transport resulting in decreased ATP content by inactivating cardiac σ1R and/or reducing its expression. We also performed transverse aortic constriction (TAC) and then treated mice with haloperidol. After two weeks, haloperidol-treated mice showed enhanced heart failure marked by deteriorated cardiac function, reduced ATP production and increasing mortality relative to TAC only mice. ATP supplementation via sodium pyruvate rescued phenotypes seen in haloperidol-treated TAC mice. We conclude that σ1R inactivation or downregulation in response to haloperidol treatment impairs mitochondrial Ca(2+) mobilization, depleting ATP depletion from cardiomyocytes. These findings suggest a novel approach to mitigate haloperidol-related adverse effects in schizophrenia patients by ATP supplementation.

Long-Term Haloperidol Treatment Prolongs QT Interval and Increases Expression of Sigma 1 and IP3 Receptors in Guinea Pig Hearts

Haloperidol is a neuroleptic drug used for a medication of various psychoses and deliria. Its administration is frequently accompanied by cardiovascular side effects, expressed as QT interval prolongation and occurrence of even lethal arrhythmias. Despite these side effects, haloperidol is still prescribed in Europe in clinical practice. Haloperidol binds to sigma receptors that are coupled with inositol 1,4,5-trisphosphate (IP3) receptors. Sigma receptors are expressed in various tissues, including heart muscle, and they modulate potassium channels. Together with IP3 receptors, sigma receptors are also involved in calcium handling in various tissues. Therefore, the present work aimed to study the effects of long-term haloperidol administration on the cardiac function. Haloperidol (2 mg/kg once a day) or vehiculum was administered by intraperitoneal injection to guinea pigs for 21 consecutive days. We measured the responsiveness of the hearts isolated from the haloperidol-treated animals to additional application of haloperidol. Expression of the sigma 1 receptor and IP3 receptors was studied by real time-PCR and immunohistochemical analyses. Haloperidol treatment caused the significant decrease in the relative heart rate and the prolongation of QT interval of the isolated hearts from the haloperidol-treated animals, compared to the hearts isolated from control animals. The expression of sigma 1 and IP3 type 1 and type 2 receptors was increased in both atria of the haloperidol-treated animals but not in ventricles. The modulation of sigma 1 and IP3 receptors may lead to altered calcium handling in cardiomyocytes and thus contribute to changed sensitivity of cardiac cells to arrhythmias.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

[Cardioprotective effect of the selective sigma-1 receptor agonist, SA4503]

We previously reported that the sigma-1 receptor is down-regulated in cardiomyocytes following heart failure in transverse aortic constriction (TAC) mice. In this review, we summarized the anti-hypertrophic action of selective sigma-1 receptor agonist, SA4503 in the hypertrophied cultured cardiomyocytes and discussed its possible mechanism of cardioprotection. Treatment with SA4503 (0.1-1 μM) dose-dependently inhibited hypertrophy in cultured cardiomyocytes induced by angiotensin II (Ang II). We also found that α1 receptor stimulation by phenylephrine (PE) promotes ATP production through IP3 receptor-mediated Ca(2+) mobilization into mitochondria in cultured cardiomyocytes. Interestingly, the PE-induced ATP production was impaired after Ang II-induced hypertrophy and SA4503 treatment largely restored PE-induced ATP production. The impaired PE-induced ATP production was associated with reduced mitochondrial size. The SA4503 treatment completely restored mitochondrial size concomitant with restored ATP production. These effects were blocked by sigma-1 receptor antagonist, NE-100 and sigma-1 receptor siRNA. We also confirmed that chronic SA4503 administration also significantly attenuates myocardial hypertrophy and restores ATP production in transverse aortic constriction mice. Taken together, sigma-1 receptor stimulation with selective agonist SA4503 ameliorates cardiac hypertrophy and dysfunction by restoring both mitochondrial Ca(2+) mobilization and ATP production via sigma-1 receptor stimulation. Sigma-1 receptor stimulation represents a new therapeutic strategy to rescue heart from hypertrophic dysfunction in heart failure.

Fluvoxamine rescues mitochondrial Ca2+ transport and ATP production through σ(1)-receptor in hypertrophic cardiomyocytes

AIMS

We previously reported that fluvoxamine, a selective serotonin reuptake inhibitor with high affinity for the σ1-receptor (σ1R), ameliorates cardiac hypertrophy and dysfunction via σ1R stimulation. Although σ1R on non-cardiomyocytes interacts with the IP3 receptor (IP3R) to promote mitochondrial Ca(2+) transport, little is known about its physiological and pathological relevance in cardiomyocytes.

MAIN METHODS

Here we performed Ca(2+) imaging and measured ATP production to define the role of σ1Rs in regulating sarcoplasmic reticulum (SR)-mitochondrial Ca(2+) transport in neonatal rat ventricular cardiomyocytes treated with angiotensin II to promote hypertrophy.

KEY FINDING

These cardiomyocytes exhibited imbalances in expression levels of σ1R and IP3R and impairments in both phenylephrine-induced mitochondrial Ca(2+) mobilization from the SR and ATP production. Interestingly, σ1R stimulation with fluvoxamine rescued impaired mitochondrial Ca(2+) mobilization and ATP production, an effect abolished by treatment of cells with the σ1R antagonist, NE-100. Under physiological conditions, fluvoxamine stimulation of σ1Rs suppressed intracellular Ca(2+) mobilization through IP3Rs and ryanodine receptors (RyRs). In vivo, chronic administration of fluvoxamine to TAC mice also rescued impaired ATP production.

SIGNIFICANCE

These results suggest that σ1R stimulation with fluvoxamine promotes SR-mitochondrial Ca(2+) transport and mitochondrial ATP production, whereas σ1R stimulation suppresses intracellular Ca(2+) overload through IP3Rs and RyRs. These mechanisms likely underlie in part the anti-hypertrophic and cardioprotective action of the σ1R agonists including fluvoxamine.

Diverse regulation of IP3 and ryanodine receptors by pentazocine through σ1-receptor in cardiomyocytes

Although pentazocine binds to σ1-receptor (σ1R) with high affinity, the physiological relevance of its binding remains unclear. We first confirmed that σ1R stimulation with pentazocine rescues contractile dysfunction following pressure overload (PO)-induced cardiac hypertrophy ovariectomized (OVX) female rats. In in vivo studies, vehicle, pentazocine (0.5-1.0 mg/kg ip), and NE-100 (1.0 mg/kg po), a σ1R antagonist, were administered for 4 wk (once daily) starting from the onset of aortic banding after OVX. We also examined antihypertrophic effects of pentazocine (0.5-1 μM) in cultured cardiomyocytes exposed to angiotensin II. Pentazocine administration significantly inhibited PO-induced cardiac hypertrophy and rescued hypertrophy-induced impairment of cardiac dysfunctions such as left ventricular end-diastolic pressure, left ventricular developed pressure, and left ventricular contraction and relaxation (±dp/dt) rates. Coadministration of NE-100 with pentazocine eliminated pentazocine-induced amelioration of heart dysfunction. Interestingly, pentazocine administration inhibited PO-induced σ1R reduction and inositol-1,4,5-trisphosphate (IP3) receptor type 2 (IP3R2) upregulation in heart. Therefore, the reduced mitochondrial ATP production following PO was restored by pentazocine administration. Furthermore, we found that σ1R binds to the ryanodine receptor (RyR) in addition to IP3 receptor (IP3R) in cardiomyocytes. The σ1R/RyR complexes were decreased following OVX-PO and restored by pentazocine administration. We noticed that pentazocine inhibits the ryanodine-induced Ca(2+) release from sarcoplasmic reticulum (SR) in cultured cardiomyocytes. Taken together, the stimulation of σ1R by pentazocine rescues cardiac dysfunction by restoring IP3R-mediated mitochondrial ATP production and by suppressing RyR-mediated Ca(2+) leak from SR in cardiomyocytes.

Vascular endothelial σ1-receptor stimulation with SA4503 rescues aortic relaxation via Akt/eNOS signaling in ovariectomized rats with aortic banding

BACKGROUND

We previously reported that σ1-receptor (σ1R) expression in the thoracic aorta decreased after pressure overload (PO) induced by abdominal aortic banding in ovariectomized (OVX) rats. Here, we asked whether stimulation of σ1R with the selective agonist SA4503 elicits functional recovery of aortic vasodilation and constriction following vascular injury in OVX rats with PO.

METHODS AND RESULTS

 SA4503 (0.3-1.0mg/kg) and NE-100 (a σ1R antagonist, 1.0mg/kg) were administered orally for 4 weeks (once daily) to OVX-PO rats. Vascular functions of isolated descending aorta were measured following phenylephrine (PE)- or endothelin-1 (ET-1)-induced vasoconstriction and acetylcholine (ACh)- or clonidine-induced vasodilation. SA4503 administration rescued PO-induced σ1R decreases in aortic smooth muscle and endothelial cells. SA4503 treatment also rescued PO-induced impairments in ACh- and clonidine-induced vasodilation without affecting PE- and ET-1-induced vasoconstriction. Ameliorated ACh- and clonidine-induced vasodilation was closely associated with increased Akt activity and in turn endothelial nitric oxide synthase (eNOS) phosphorylation. The SA4503-mediated improvement of vasodilation was blocked by NE-100 treatment.

CONCLUSIONS

 σ1R is downregulated following PO-induced endothelial injury in OVX rats. The selective σ1R agonist SA4503 rescues impaired endothelium-dependent vasodilation in the aorta from OVX-PO rats through σ1R stimulation, enhancing eNOS-cGMP signaling in vascular endothelial cells. These observations encourage development of novel therapeutics targeting σ1R to prevent vascular endothelial injury in vascular diseases.

Crucial interactions between selective serotonin uptake inhibitors and sigma-1 receptor in heart failure

Depression is associated with a substantial increase in the risk of developing heart failure and is independently associated with increased cardiovascular morbidity and mortality. Inversely, cardiovascular disease can lead to severe depression. Thus, therapy with selective serotonin reuptake inhibitors (SSRIs) is strongly recommended to reduce cardiovascular disease-induced morbidity and mortality. However, molecular mechanisms to support evidence-based SSRI treatment of cardiovascular disease have not been elucidated. We recently found very high expression of the sigma-1 receptor, an orphan receptor, in rat heart tissue and defined the cardiac sigma-1 receptor as a direct SSRI target in eliciting cardioprotection in both pressure overload (PO)induced and transverse aortic constriction (TAC)-induced myocardial hypertrophy models in rodents. Our findings suggest that SSRIs such as fluvoxamine protect against PO- and TAC-induced cardiac dysfunction by upregulating sigma-1 receptor expression and stimulating sigma-1 receptor-mediated Akt-eNOS signaling. Here, we discuss the association of depression and cardiovascular diseases, the protective mechanism of SSRIs in heart failure patients, and the pathophysiological relevance of sigma-1 receptors to progression of heart failure. These findings should promote development of clinical therapeutics targeting the sigma-1 receptor in cardiovascular diseases.

Stimulation of σ1-receptor restores abnormal mitochondrial Ca²⁺ mobilization and ATP production following cardiac hypertrophy

BACKGROUND

We previously reported that the σ1-receptor (σ1R) is down-regulated following cardiac hypertrophy and dysfunction in transverse aortic constriction (TAC) mice. Here we address how σ1R stimulation with the selective σ1R agonist SA4503 restores hypertrophy-induced cardiac dysfunction through σ1R localized in the sarcoplasmic reticulum (SR).

METHODS

We first confirmed anti-hypertrophic effects of SA4503 (0.1-1μM) in cultured cardiomyocytes exposed to angiotensin II (Ang II). Then, to confirm the ameliorative effects of σ1R stimulation in vivo, we administered SA4503 (1.0mg/kg) and the σ1R antagonist NE-100 (1.0mg/kg) orally to TAC mice for 4weeks (once daily).

RESULTS

σ1R stimulation with SA4503 significantly inhibited Ang II-induced cardiomyocyte hypertrophy. Ang II exposure for 72h impaired phenylephrine (PE)-induced Ca(2+) mobilization from the SR into both the cytosol and mitochondria. Treatment of cardiomyocytes with SA4503 largely restored PE-induced Ca(2+) mobilization into mitochondria. Exposure of cardiomyocytes to Ang II for 72h decreased basal ATP content and PE-induced ATP production concomitant with reduced mitochondrial size, while SA4503 treatment completely restored ATP production and mitochondrial size. Pretreatment with NE-100 or siRNA abolished these effects. Chronic SA4503 administration also significantly attenuated myocardial hypertrophy and restored ATP production in TAC mice. SA4503 administration also decreased hypertrophy-induced impairments in LV contractile function.

CONCLUSIONS

σ1R stimulation with the specific agonist SA4503 ameliorates cardiac hypertrophy and dysfunction by restoring both mitochondrial Ca(2+) mobilization and ATP production via σ1R stimulation.

GENERAL SIGNIFICANCE

Our observations suggest that σ1R stimulation represents a new therapeutic strategy to rescue the heart from hypertrophic dysfunction.

Sigma-1 receptor chaperone and brain-derived neurotrophic factor: emerging links between cardiovascular disease and depression

Epidemiological studies have demonstrated a close relationship between depression and cardiovascular disease (CVD). Although it is known that the central nervous system (CNS) contributes to this relationship, the detailed mechanisms involved in this process remain unclear. Recent studies suggest that the endoplasmic reticulum (ER) molecular chaperone sigma-1 receptor and brain-derived neurotrophic factor (BDNF) play a role in the pathophysiology of CVD and depression. Several meta-analysis studies have showed that levels of BDNF in the blood of patients with major depressive disorder (MDD) are lower than normal controls, indicating that blood BDNF might be a biomarker for depression. Furthermore, blood levels of BDNF in patients with CVD are also lower than normal controls. A recent study using conditional BDNF knock-out mice in animal models of myocardial infarction highlighted the role of CNS-mediated mechanisms in the cardioprotective effects of BDNF. In addition, a recent study shows that decreased levels of sigma-1 receptor in the mouse brain contribute to the association between heart failure and depression. Moreover, sigma-1 receptor agonists, including the endogenous neurosteroid dehydroepiandosterone (DHEA) and the selective serotonin reuptake inhibitor (SSRI) fluvoxamine, show potent cardioprotective and antidepressive effects in rodents, via sigma-1 receptor stimulation. Interestingly, agonist activation of sigma-1 receptors increased the secretion of mature BDNF from its precursor proBDNF via chaperone activity in the ER. Given the role of ER stress in the pathophysiology of CVD and MDD, the author will discuss the potential link between sigma-1 receptors and BDNF-TrkB pathway in the pathophysiology of these two diseases. Finally, the author will make a case for potent sigma-1 receptor agonists and TrkB agonists as new potential therapeutic drugs for depressive patients with CVD.

[Cardioprotective effect of fluvoxamine, sigma-1 receptor high affinity agonist]

Selective serotonin reuptake inhibitors (SSRIs) are known to reduce post-myocardial infarction (MI)-induced morbidity and mortality. However, the molecular mechanism underlying SSRI-induced cardioprotection remains unclear. Here, we investigated the role of sigma-1 receptor (Sig-1R) stimulation with fluvoxamine on myocardial hypertrophy and cardioprotection. Male ICR mice were subjected to transverse aortic constriction (TAC) in the cardiac aortic arch. To confirm the cardioprotective role of Sig-1R stimulation by fluvoxamine, we treated mice with fluvoxamine (0.5 or 1 mg/kg) orally once a day for 4 weeks after onset of aortic banding. Interestingly, in untreated mice, Sig-1R expression in the left ventricle (LV) markedly decreased over 4 weeks with increased hypertrophy. By contrast, fluvoxamine administration significantly attenuated TAC-induced myocardial hypertrophy concomitant with recovery of Sig-1R expression in LV. Fluvoxamine also attenuated hypertrophy-induced impaired LV fractional shortening. The fluvoxamine cardioprotective effect was nullified by treatment with a Sig-1R antagonist, NE-100 (1 mg/kg). Importantly, another SSRI with very low affinity for Sig-1R, paroxetine, did not exhibit antihypertrophic effects in TAC mice and in cultured cardiomyocyte treated with angiotensin II. Fluvoxamine treatment significantly restored TAC-induced impaired Akt and eNOS phosphorylation in LV. Our findings suggest that fluvoxamine protects heart against TAC-induced cardiac dysfunction via upregulation of Sig-1R and stimulation of Sig-1R-mediated Akt-eNOS signaling in mice. This is the first report of a potential role of Sig-1R stimulation by fluvoxamine in preventing cardiac hypertrophy and myocardial injury in TAC mice.

Distinct cardioprotective effects of 17β-estradiol and dehydroepiandrosterone on pressure overload-induced hypertrophy in ovariectomized female rats

OBJECTIVE

We recently reported decreased σ1 receptor expression in the heart after abdominal aortic stenosis in bilateral ovariectomized rats. Here, we use ovariectomized female rats to investigate the distinct cardioprotective effects of 17β-estradiol (E2) and dehydroepiandrosterone (DHEA) in pressure overload (PO)-induced cardiac dysfunction.

METHODS

E2 (0.1 mg/kg) and DHEA (30 mg/kg) were administered to rats subcutaneously and orally, respectively, for 14 days starting 2 weeks after aortic banding.

RESULTS

Both E2 and DHEA treatments significantly inhibited PO-induced increases both in heart weight/body weight ratio and lung weight/body weight ratios. Both E2 and DHEA also ameliorated hypertrophy-induced impairment of left ventricular end-diastolic pressure, left ventricular-developed pressure, left ventricular contraction and relaxation (± dp/dt) rates, heart rate, and mean arterial blood pressure. Notably, DHEA but not E2 administration rescued decreased PO-induced σ1 receptor reduction in the heart. Coadministration with N,N-Dipropyl-2-[4-methoxy-3-(2-phenylethoxy) phenyl]-ethylamine monohydrochloride, an σ1 receptor antagonist, inhibited DHEA-induced amelioration of heart dysfunction without altering E2-induced cardioprotection. Mechanistically, both E2 and DHEA treatments significantly restored PO-induced decreases in protein kinase B (Akt) phosphorylation and Akt-mediated endothelial nitric oxide synthase (eNOS) phosphorylation (Ser1179). N,N-Dipropyl-2-[4-methoxy-3-(2-phenylethoxy) phenyl]-ethylamine monohydrochloride treatment totally abolished DHEA-induced Akt and eNOS phosphorylation without altering E2-induced Akt-eNOS activation.

CONCLUSIONS

Taken together, these results from an ovariectomized rat model of PO-induced cardiac dysfunction show that DHEA but not E2 elicits a cardioprotective action through σ1 receptor activation. DHEA-induced Akt-eNOS activation through σ1 receptors is probably associated with its cardioprotective activity.

Synthesis and in vitro biological evaluation of carbonyl group-containing analogues for σ1 receptors

To identify the ligands for σ(1) receptors that are potent and selective, analogues of prezamicol and trozamicol scaffolds of carbonyl-containing vesicular acetylcholine transporter (VAChT) inhibitors were explored. Of the 23 analogues synthesized and tested, 5 displayed very high affinity for σ(1) (K(i) = 0.48-4.05 nM) and high selectivity for σ(1) relative to σ(2) receptors (σ(1)/σ(2) selectivity of >749-fold). Four of the five compounds (14a, 14b, 14c, and 14e) showed very low affinity for VAChT (K(i) > 290 nM), and the fifth compound (14g) showed moderate affinity for VAChT (K(i) = 44.2 nM). The compound [1'-(4-fluorobenzyl)-3'-hydroxy[1,4']bipiperidinyl-4-yl]-(4-fluorophenyl)methanone (14a) displayed very high affinity and selectivity for σ(1) receptor (K(i) = 0.48 nM, σ(1)/σ(2) > 3600). All four of these most promising compounds (14a, 14b, 14c, and 14e) can be radiosynthesized with fluorine-18 or carbon-11, which will allow further evaluation of their properties as PET probes for imaging σ(1) receptor in vivo.

Targeting ligand-operated chaperone sigma-1 receptors in the treatment of neuropsychiatric disorders

INTRODUCTION

Current drugs for the treatment of psychiatric or neurodegenerative disorders have limitations. Psychotherapeutic drugs such as typical and atypical antipsychotics, tricyclic antidepressants and selective monoamine reuptake inhibitors, aim to normalize the hyper- or hypo-neurotransmission of monoaminergic systems. Despite their contribution to the outcomes of psychiatric patients, these agents often exert severe side effects and require chronic treatments to promote amelioration of symptoms. Drugs available for the treatment of neurodegenerative disorders are severely limited.

AREAS COVERED

Recent evidence that has shed light on sigma-1 receptor ligands, which may serve as a new class of antidepressants or neuroprotective agents. Sigma-1 receptors are novel ligand-operated molecular chaperones regulating signal transduction, ER stress, cellular redox, cellular survival and synaptogenesis. Selective sigma-1 receptor ligands exert rapid antidepressant-like, anxiolytic, antinociceptive and robust neuroprotective actions in preclinical studies. Recent studies that suggest that reactive oxygen species might play a role as signal integrators downstream of Sig-1Rs are also covered.

EXPERT OPINION

The advances in sigma receptor research in the last decade have begun to elucidate the intracellular signal cascades upstream and downstream of sigma-1 receptors. The novel ligand-operated properties of the sigma-1 receptor chaperone may enable interventions by which stress-related cellular systems can be pharmacologically controlled.

Targeting sigma-1 receptor signaling by endogenous ligands for cardioprotection

INTRODUCTION

The sigma receptors, initially described as a subtype of opioid receptors, are now considered to be a unique receptor expressed in neonatal rat cardiomyocytes and in the plasma membrane of adult rat cardiomyocytes. A number of sigma receptor ligands influence cardiovascular function and the heart has binding sites for sigma receptor ligands that alter contractility both in vivo and in vitro. The human sigma-1 receptor gene contains a steroid-binding component and gonadal steroid dehydroepiandrosterone (DHEA) which interacts with the sigma-1 receptor.

AREAS COVERED

We recently documented that the pathophysiological role of the sigma-1 receptor in the heart and its modulation using DHEA, was cardioprotective. Moreover, agonist-induced activation of the sigma-1 receptor modulates diverse ion channels and thereby regulates heart function. Novel concepts for understanding the pathophysiological relevance of sigma-1 receptors in the progression of heart failure, and developing clinical therapeutics targeting for the receptor in cardiovascular diseases are discussed.

EXPERT OPINION

Future studies should attempt to develop cardiac-specific knockdown of the sigma-1 receptor to observe its downstream signaling. We expect that these observations will lead to a novel therapeutic target for which a new class of antihypertrophic drugs can be designed.