sigma Receptor activation blocks potassium channels and depresses neuroexcitability in rat intracardiac neurons

The proarrhythmogenic role of autonomics and emerging neuromodulation approaches to prevent sudden death in cardiac ion channelopathies

Ventricular arrhythmias in cardiac channelopathies are linked to autonomic triggers, which are sub-optimally targeted in current management strategies. Improved molecular understanding of cardiac channelopathies and cellular autonomic signalling could refine autonomic therapies to target the specific signalling pathways relevant to the specific aetiologies as well as the central nervous system centres involved in the cardiac autonomic regulation. This review summarizes key anatomical and physiological aspects of the cardiac autonomic nervous system and its impact on ventricular arrhythmias in primary inherited arrhythmia syndromes. Proarrhythmogenic autonomic effects and potential therapeutic targets in defined conditions including the Brugada syndrome, early repolarization syndrome, long QT syndrome, and catecholaminergic polymorphic ventricular tachycardia will be examined. Pharmacological and interventional neuromodulation options for these cardiac channelopathies are discussed. Promising new targets for cardiac neuromodulation include inhibitory and excitatory G-protein coupled receptors, neuropeptides, chemorepellents/attractants as well as the vagal and sympathetic nuclei in the central nervous system. Novel therapeutic strategies utilizing invasive and non-invasive deep brain/brain stem stimulation as well as the rapidly growing field of chemo-, opto-, or sonogenetics allowing cell-specific targeting to reduce ventricular arrhythmias are presented.

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.

Sigma-1 Receptor Alleviates Airway Inflammation and Airway Remodeling Through AMPK/CXCR4 Signal Pathway

Sigma non-opioid intracellular receptor 1 (Sigma-1R) has been proven to play a major role in inflammation and structural remodeling. However, its role in airway inflammation and airway remodeling remains unclear. The purpose of this study aimed to explore the role and mechanism of Sigma-1R in airway remodeling and epithelial-mesenchymal transition (EMT) process in vivo and in vitro. We observed the decrease of Sigma-1R in lung tissue of asthma model. In the mouse model of allergic airway inflammation (AAI), Sigma-1R agonist RPE-084 significantly relieved airway inflammation and airway remodeling, while Sigma-1R antagonist BD1047 (B8562) had opposite effects. Further research showed that RPE-084 treatment increased the expression of pAMPK and inhibited the expression of CXCR4. Furthermore, RPE-084 treatment suppressed the levels of IL-4, IL-5, and IL-13 in BALF. We found that RPE-084 or Sigma-1R overexpression vector treatment regulated cell cycle and inhibited cell proliferation, migration, and EMT process in TGF-β1-induced 16HBE cells. Finally, we confirmed that AMP-activated protein kinase (AMPK) inhibitor compound C or CXCR4 agonist ATI-2341 both reversed the effects of Sigma-1R on TGF-β1-induced 16 HBE cells. In a word, our research shows that Sigma-1R is helpful to improve airway remodeling of asthma, and emphasizes a new candidate molecular for asthma treatment.

Revisiting the sigma-1 receptor as a biological target to treat affective and cognitive disorders

Depression and cognitive disorders are diseases with complex and not-fully understood etiology. Unfortunately, the COVID-19 pandemic dramatically increased the prevalence of both conditions. Since the current treatments are inadequate in many patients, there is a constant need for discovering new compounds, which will be more effective in ameliorating depressive symptoms and treating cognitive decline. Proteins attracting much attention as potential targets for drugs treating these conditions are sigma-1 receptors. Sigma-1 receptors are multi-functional proteins localized in endoplasmic reticulum membranes, which play a crucial role in cellular signal transduction by interacting with receptors, ion channels, lipids, and kinases. Changes in their functions and expression may lead to various diseases, including depression or memory impairments. Thus, sigma-1 receptor modulation might be useful in treating these central nervous system diseases. Importantly, two sigma-1 receptor ligands entered clinical trials, showing that this compound group possesses therapeutic potential. Therefore, based on preclinical studies, this review discusses whether the sigma-1 receptor could be a promising target for drugs treating affective and cognitive disorders.

In Vitro and In Vivo Pharmaco-Toxicological Characterization of 1-Cyclohexyl-x-methoxybenzene Derivatives in Mice: Comparison with Tramadol and PCP

1-cyclohexyl-x-methoxybenzene is a novel psychoactive substance (NPS), first discovered in Europe in 2012 as unknown racemic mixture of its three stereoisomers: ortho, meta and para. Each of these has structural similarities with the analgesic tramadol and the dissociative anesthetic phencyclidine. In light of these structural analogies, and based on the fact that both tramadol and phencyclidine are substances that cause toxic effects in humans, the aim of this study was to investigate the in vitro and in vivo pharmacodynamic profile of these molecules, and to compare them with those caused by tramadol and phencyclidine. In vitro studies demonstrated that tramadol, ortho, meta and para were inactive at mu, kappa and delta opioid receptors. Systemic administration of the three stereoisomers impairs sensorimotor responses, modulates spontaneous motor activity, induces modest analgesia, and alters thermoregulation and cardiorespiratory responses in the mouse in some cases, with a similar profile to that of tramadol and phencyclidine. Naloxone partially prevents only the visual sensorimotor impairments caused by three stereoisomers, without preventing other effects. The present data show that 1-cyclohexyl-x-methoxybenzene derivatives cause pharmaco-toxicological effects by activating both opioid and non-opioid mechanisms and suggest that their use could potentially lead to abuse and bodily harm.

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.

Sigma-1 receptor ligands improves ventricular repolarization-related ion remodeling in rats with major depression disorder

RATIONALE

It has been reported that patients with major depressive disorder (MDD) are prone to developing ventricular arrhythmias. Moreover, the Sigma-1 receptor not only plays a crucial role in MDD but has also been shown to have antiarrhythmic properties. The Sigma-1 receptor is a common receptor related to depression and ventricular arrhythmias.

OBJECTIVE

We analyzed the effects of the Sigma-1 receptor on depression and ventricular repolarization-related ion remodeling in MDD rats.

METHODS

MDD was induced in rats by chronic unpredictable mild stress (CUMS), and 28 days later, the rats were subjected to behavior tests. Protein expression was measured by western blotting, and cardiac morphological changes were observed by Masson staining. Electrophysiological measurement of the myocardium was performed with the whole-cell patch-clamp technique.

RESULTS

Compared with the control rats, the MDD rats exhibited lower transient outward potassium current (Ito) and L-type calcium current (ICa-L) amplitudes. On the other hand, a trend of depolarization of Ito and hyperpolarization of ICa-L was observed in the MDD rats. Thus, we investigated the effect of fluvoxamine, a Sigma-1 receptor agonist, on Ito and ICa-L. Fluvoxamine enhanced Ito and altered its current kinetics, as shown by acceleration of activation and recovery from inactivation. In contrast, fluvoxamine inhibited the Ca2+ by hyperpolarizing the steady-state activation of ICa-L. All these effects were blocked by BD1047.

CONCLUSION

Taken together, our results indicate that Sigma-1 receptor modulates the functions of Ito and ICa-L to possibly exert antiarrhythmic effects.

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.

Noscapine protects the H9c2 cardiomyocytes of rats against oxygen-glucose deprivation/reperfusion injury

Noscapine is an antitumor alkaloid derived from Papaver somniferum plants. Our previous study has demonstrated that exposure of noscapine on primary murine fetal cortical neurons exposed to oxygen-glucose deprivation/reperfusion (OGD/R) has neuroprotective effects. In current study, the effects of noscapine on cardiomyocytes (H9c2 cells) damage caused by 120 minutes (min) of OGD/R were evaluated and we determined whether the addition of BD1047, sigma-one receptor antagonist, prevents the protective effects of noscapine in H9c2 cells through the production of nitric oxide (NO) and apoptosis. To initiate OGD, H9c2 cells was transferred to glucose-free DMEM, and placed in a humidified incubation chamber. Cell viability was assessed with noscapine (1-5 μM) in the presence or absence of BD1047, 24 hours (h) after OGD/R. Cell viability, NO production and apoptosis ratio were evaluated by the MTT assay, the Griess method and the quantitative real-time PCR. Noscapine considerably improved the survival of H9c2 cells compared to OGD/R. Also, noscapine was extremely capable of reducing the concentrations of NO and Bax/Bcl-2 ratio expression. While the BD1047 administration alone diminished cell viability and increased the Bax/Bcl-2 ratio and NO levels. The addition of noscapine in the presence of BD1047 did not increase the cell viability relative to noscapine alone. Noscapine exerted cardioprotective effects exposed to OGD/R-induced injury in H9c2 cells, at least partly via attenuation of NO production and Bax/Bcl-2 ratio, which indicates that the sigma-one receptor activation is involved in the protection by noscapine of H9c2 cells injured by OGD/R.

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 Sigma-1 Receptor: When Adaptive Regulation of Cell Electrical Activity Contributes to Stimulant Addiction and Cancer

The sigma-1 receptor (σ1R) is an endoplasmic reticulum (ER)-resident chaperone protein that acts like an inter-organelle signaling modulator. Among its several functions such as ER lipid metabolisms/transports and indirect regulation of genes transcription, one of its most intriguing feature is the ability to regulate the function and trafficking of a variety of functional proteins. To date, and directly relevant to the present review, σ1R has been found to regulate both voltage-gated ion channels (VGICs) belonging to distinct superfamilies (i.e., sodium, Na⁺; potassium, K⁺; and calcium, Ca²⁺ channels) and non-voltage-gated ion channels. This regulatory function endows σ1R with a powerful capability to fine tune cells’ electrical activity and calcium homeostasis—a regulatory power that appears to favor cell survival in pathological contexts such as stroke or neurodegenerative diseases. In this review, we present the current state of knowledge on σ1R’s role in the regulation of cellular electrical activity, and how this seemingly adaptive function can shift cell homeostasis and contribute to the development of very distinct chronic pathologies such as psychostimulant abuse and tumor cell growth in cancers.

The sigma-1 receptor behaves as an atypical auxiliary subunit to modulate the functional characteristics of Kv1.2 channels expressed in HEK293 cells

Expression of Kv1.2 within Kv1.x potassium channel complexes is critical in maintaining appropriate neuronal excitability and determining the threshold for action potential firing. This is attributed to the interaction of Kv1.2 with a hitherto unidentified protein that confers bimodal channel activation gating, allowing neurons to adapt to repetitive trains of stimulation and protecting against hyperexcitability. One potential protein candidate is the sigma-1 receptor (Sig-1R), which regulates other members of the Kv1.x channel family; however, the biophysical nature of the interaction between Sig-1R and Kv1.2 has not been elucidated. We hypothesized that Sig-1R may regulate Kv1.2 and may further act as the unidentified modulator of Kv1.2 activation. In transiently transfected HEK293 cells, we found that ligand activation of the Sig-1R modulates Kv1.2 current amplitude. More importantly, Sig-1R interacts with Kv1.2 in baseline conditions to influence bimodal activation gating. These effects are abolished in the presence of the auxiliary subunit Kvβ2 and when the Sig-1R mutation underlying ALS16 (Sig-1R-E102Q), is expressed. These data suggest that Kvβ2 occludes the interaction of Sig-1R with Kv1.2, and that E102 may be a residue critical for Sig-1R modulation of Kv1.2. The results of this investigation describe an important new role for Sig-1R in the regulation of neuronal excitability and introduce a novel mechanism of pathophysiology in Sig-1R dysfunction.

Tandem pore TWIK-related potassium channels and neuroprotection

TWIK-related potassium channels (TREK) belong to a subfamily of the two-pore domain potassium channels family with three members, TREK1, TREK2 and TWIK-related arachidonic acid-activated potassium channels. The two-pore domain potassium channels is the last big family of channels being discovered, therefore it is not surprising that most of the information we know about TREK channels predominantly comes from the study of heterologously expressed channels. Notwithstanding, in this review we pay special attention to the limited amount of information available on native TREK-like channels and real neurons in relation to neuroprotection. Mainly we focus on the role of free fatty acids, lysophospholipids and other neuroprotective agents like riluzole in the modulation of TREK channels, emphasizing on how important this modulation may be for the development of new therapies against neuropathic pain, depression, schizophrenia, epilepsy, ischemia and cardiac complications.

Cardiac sigma receptors - an update

More than four decades passed since sigma receptors were first mentioned. Since then, existence of at least two receptor subtypes and their tissue distributions have been proposed. Nowadays, it is clear, that sigma receptors are unique ubiquitous proteins with pluripotent function, which can interact with so many different classes of proteins. As the endoplasmic resident proteins, they work as molecular chaperones - accompany various proteins during their folding, ensure trafficking of the maturated proteins between cellular organelles and regulate their functions. In the heart, sigma receptor type 1 is more dominant. Cardiac sigma 1 receptors regulate response to endoplasmic reticulum stress, modulates calcium signaling in cardiomyocyte and can affect function of voltage-gated ion channels. They contributed in pathophysiology of cardiac hypertrophy, heart failure and many other cardiovascular disorders. Therefore, sigma receptors are potential novel targets for specific treatment of cardiovascular diseases.

Chronic stimulation of the sigma-1 receptor ameliorates autonomic nerve dysfunction and atrial fibrillation susceptibility in a rat model of depression

The present study aimed to assess the effect of sigma-1 receptor (S1R) stimulation on autonomic nerve dysfunction and susceptibility to atrial fibrillation (AF) in a rat depression model. Male rats were randomly divided into one of the following four treatment groups: saline [control (CTL)]; saline + intragastric administration of SA4503, an agonist of S1R (CTS); chronic unpredictable mild stress (CUMS) to produce depression (MDD); and CUMS + intragastric administration of SA4503 (MDS). Depression-like behaviors, such as reduced sucrose preference, decreased body weight gain, and increased immobility time during forced swimming, improved in the MDS group after 4 wk of SA4503 treatment. Compared with rats in the CTL group, rats in the MDD group showed significantly augmented sympathetic activity, reduced parasympathetic activity, decreased heart rate variability, and lowered S1R expression in the atrium and hippocampus (all P < 0.01). However, rats in the MDS group showed mitigated aforementioned alterations and improved electrical remodeling compared with rats in the MDD group (all P < 0.01). Furthermore, rats in the MDS group showed shortened activation latencies, increased effective refractory periods, and lowered frequency of AF incidence duration and fibrosis compared with rats in the MDD group (all P < 0.01). The results indicate that S1R stimulation reduces sympathetic activity and susceptibility to AF by improving depressive behaviors, modulating cardiac autonomic nerve balance, lightening nerve remodeling, and upregulating S1R and ion channel protein expression. NEW & NOTEWORTHY Chronic stimulation of the sigma-1 receptor (S1R) ameliorates depression-induced autonomic nerve dysfunction by modulating the imbalance between overactivated sympathetic activity and decreased vagal activity. Chronic S1R stimulation alleviates atrial electrical remodeling, fibrosis, and susceptibility to atrial fibrillation (AF). The S1R agonist may target the underlying mechanisms related to AF occurrence. The results indicate that the S1R could be a potential clinical target for atrial arrhythmia, especially when it is combined with major depressive disorders.

Benzomorphan skeleton, a versatile scaffold for different targets: A comprehensive review

Despite the fact that the benzomorphan skeleton has mainly been employed in medicinal chemistry for the development of opioid analgesics, it is a versatile structure. Its stereochemistry, as well as opportune modifications at the phenolic hydroxyl group and at the basic nitrogen, play a pivotal role addressing the benzomorphan-based compounds to a specific target. In this review, we describe the structure activity-relationships (SARs) of benzomorphan-based compounds acting at sigma 1 receptor (σ1R), sigma 2 receptor (σ2R), voltage-dependent sodium channel, N-Methyl-d-Aspartate (NMDA) receptor-channel complex and other targets. Collectively, the SARs data have highlighted that the benzomorphan nucleus could be regarded as a useful template for the synthesis of drug candidates for different targets.

Involvement of the delayed rectifier outward potassium channel Kv2.1 in methamphetamine-induced neuronal apoptosis via the p38 mitogen-activated protein kinase signaling pathway

Methamphetamine (Meth) is an illicit psychostimulant with high abuse potential and severe neurotoxicity. Recent studies have shown that dysfunctions in learning and memory induced by Meth may partially reveal the mechanisms of neuronal channelopathies. Kv2.1, the primary delayed rectifying potassium channel in neurons, is responsible for mediating apoptotic current surge. However, whether Kv2.1 is involved in Meth-mediated neural injury remains unknown. In the present study, the treatment of primary cultured hippocampal neurons with Meth indicated that Meth induced a time- and dose-dependent augmentation of Kv2.1 protein expression, accompanied by elevated cleaved-caspase 3 and declined bcl-2/bax ratio. The blockage of Kv2.1 with the inhibitor GxTx-1E or the knockdown of the channel noticeably abrogated the pro-apoptotic effects mediated by Meth, demonstrating the specific roles of Kv2.1 in Meth-mediated neural damage. Additionally, the p38 mitogen-activated protein kinase (MAPK) signaling was demonstrated to be involved in Meth-mediated Kv2.1 upregulation and in the subsequent pro-apoptotic effects, as treatment with a p38 MAPK inhibitor significantly attenuated Meth-mediated Kv2.1 upregulation and cell apoptosis. Of note, PRE-084, a sigma-1 receptor agonist, obviously attenuated Meth-induced upregulation of Kv2.1 expression, neural apoptosis and p38 MAPK activation. Taken together, these results reveal a novel mechanism involved in Meth-induced neural death with implications for therapeutic interventions for Meth users.

Sigma-1 receptor in brain ischemia/reperfusion: Possible role in the NR2A-induced pathway to regulate brain-derived neurotrophic factor

Sigma-1 receptor (σ1r) activation could attenuate the learning and memory deficits in the AD model, ischemia model and others. In our previous study, the activation of σ1r increased the expression of brain-derived neurotrophic factor (BDNF), possibly through the NR2A-induced pathway, and σ1r agonists might function as neuroprotectant agents in vascular dementia. Here, we used σ1r knockout mice to confirm the role of σ1r. Furthermore, an antagonist of NR2A was first used to investigate whether the NR2A-induced pathway is the necessary link between σ1r and BDNF. The operation of brain ischemia/reperfusion was induced by bilateral common carotid artery occlusion for 20min in C57BL/6 and σ1r knockout mice as the ischemic group. A σ1r agonist, PRE084 (1mg/kg, i.p.), and NR2A antagonist, PEAQX (10mg/kg, i.p.), were administered once daily throughout the experiment. Behavioral tests were performed starting on day 8. On day 22 after brain ischemia/reperfusion, mice were sacrificed and brains were immediately collected and the injured and the hippocampus was isolated and stored at -80°C for western blot analysis. After ischemic operation, contrast with the σ1r knockout mice, PRE084 significantly ameliorated learning and memory impairments in the behavioral evaluation, and prevented the protein decline of BDNF, NR2A, CaMKIV and TORC1 expression in wild-type mice. However, the effects of PRE084 on CaMKIV-TORC1-CREB and BDNF, even for learning and memory impairment, were antagonized by the co-administration of PEAQX, an antagonist of NR2A. The activation of σ1r improves the impairment of learning and memory in the ischemia/reperfusion model, and the expression of BDNF, which may have been achieved through the NR2A-CaMKIV-TORC1 pathway.

New arylsparteine derivatives as positive inotropic drugs

Positive inotropic agents are fundamental in the treatment of heart failure; however, their arrhythmogenic liability and the increased myocardial oxygen demand strongly limit their therapeutic utility. Pursuing our study on cardiovascular activities of lupin alkaloid derivatives, several 2-(4-substituted-phenyl)-2-dehydrosparteines and 2-(4-substituted-phenyl)sparteines were prepared and tested for inotropic and chronotropic activities on isolated guinea pig atria. Four compounds (6b, 6e, 7b, and 7f) exhibited significant inotropism that, at the higher concentrations, was followed by negative inotropism or toxicity. Compound 7e (2-(4-tolyl)sparteine) exhibited a steep dose-depending inotropic activity up to the highest concentration tested (300 µM) with an E_max of 116.5 ± 3.4% of basal force, proving less potent but much more active in comparison to the highest concentrations tested of digoxin and milrinone having E_max of 87.5 ± 3.1% and 52.2 ± 1.1%, respectively. Finally, docking studies suggested that the relevant sparteine derivatives could target the sigma-1 receptor, whose involvement in cardiac activity is well documented.

Sigma-1 Receptor and Neuronal Excitability

The sigma-1 receptor (Sig-1R), via interaction with various proteins, including voltage-gated and ligand-gated ion channels (VGICs and LGICs), is involved in a plethora of neuronal functions. This capability to regulate a variety of ion channel targets endows the Sig-1R with a powerful capability to fine tune neuronal excitability, and thereby the transmission of information within brain circuits. This versatility may also explain why the Sig-1R is associated to numerous diseases at both peripheral and central levels. To date, how the Sig-1R chooses its targets and how the combinations of target modulations alter overall neuronal excitability is one of the challenges in the field of Sig-1R-dependent regulation of neuronal activity. Here, we will describe and discuss the latest findings on Sig-1R-dependent modulation of VGICs and LGICs, and provide hypotheses that may explain the diverse excitability outcomes that have been reported so far.

Aberrant Subcellular Dynamics of Sigma-1 Receptor Mutants Underlying Neuromuscular Diseases

The sigma-1 receptor (σ-1R) is an endoplasmic reticulum resident chaperone protein involved in a plethora of cellular functions, and whose disruption has been implicated in a wide range of diseases. Genetic analysis has revealed two σ-1R mutants involved in neuromuscular disorders. A point mutation (E102Q) in the ligand-binding domain results in the juvenile form of amyotrophic lateral sclerosis (ALS16), and a 20 amino-acid deletion (Δ31-50) in the putative cytosolic domain leads to a form of distal hereditary motor neuropathy. We investigated the localization and functional properties of these mutants in cell lines using confocal imaging and electrophysiology. The σ-1R mutants exhibited a significant increase in mobility, aberrant localization, and enhanced block of the inwardly rectifying K(+) channel Kir2.1, compared with the wild-type σ-1R. Thus, these σ-1R mutants have different functional properties that could contribute to their disease phenotypes.

σ-1 Receptor Inhibition of ASIC1a Channels is Dependent on a Pertussis Toxin-Sensitive G-Protein and an AKAP150/Calcineurin Complex

ASIC1a channels play a major role in various pathophysiological conditions including depression, anxiety, epilepsy, and neurodegeneration following ischemic stroke. Sigma-1 (σ-1) receptor stimulation depresses the activity of ASIC1a channels in cortical neurons, but the mechanism(s) by which σ-1 receptors exert their influence on ASIC1a remains unknown. Experiments were undertaken to elucidate the signaling cascade linking σ-1 receptors to ASIC1a channels. Immunohistochemical studies showed that σ-1 receptors, ASIC1a and A-kinase anchoring peptide 150 colocalize in the plasma membrane of the cell body and processes of cortical neurons. Fluorometric Ca(2+) imaging experiments showed that disruption of the macromolecular complexes containing AKAP150 diminished the effects of the σ-1 on ASIC1a, as did application of the calcineurin inhibitors, cyclosporin A and FK-506. Moreover, whole-cell patch clamp experiments showed that σ-1 receptors were less effective at decreasing ASIC1a-mediated currents in the presence of the VIVIT peptide, which binds to calcineurin and prevents cellular effects dependent on AKAP150/calcineurin interaction. The coupling of σ-1 to ASIC1a was also disrupted by preincubation of the neurons in the G-protein inhibitor, pertussis toxin (PTX). Taken together, our data reveal that σ-1 receptor block of ASIC1a function is dependent on activation of a PTX-sensitive G-protein and stimulation of AKAP150 bound calcineurin.

Potential applications for sigma receptor ligands in cancer diagnosis and therapy

Sigma receptors (sigma-1 and sigma-2) represent two independent classes of proteins. Their endogenous ligands may include the hallucinogen N,N-dimethyltryptamine (DMT) and sphingolipid-derived amines which interact with sigma-1 receptors, besides steroid hormones (e.g., progesterone) which bind to both sigma receptor subpopulations. The sigma-1 receptor is a ligand-regulated molecular chaperone with various ion channels and G-protein-coupled membrane receptors as clients. The sigma-2 receptor was identified as the progesterone receptor membrane component 1 (PGRMC1). Although sigma receptors are over-expressed in tumors and up-regulated in rapidly dividing normal tissue, their ligands induce significant cell death only in tumor tissue. Sigma ligands may therefore be used to selectively eradicate tumors. Multiple mechanisms appear to underlie cell killing after administration of sigma ligands, and the signaling pathways are dependent both on the type of ligand and the type of tumor cell. Recent evidence suggests that the sigma-2 receptor is a potential tumor and serum biomarker for human lung cancer and an important target for inhibiting tumor invasion and cancer progression. Current radiochemical efforts are focused on the development of subtype-selective radioligands for positron emission tomography (PET) imaging. Right now, the mostpromising tracers are [18F]fluspidine and [18F]FTC-146 for sigma-1 receptors and [11C]RHM-1 and [18F]ISO-1 for the sigma-2 subtype. Nanoparticles coupled to sigma ligands have shown considerable potential for targeted delivery of antitumor drugs in animal models of cancer, but clinical studies exploring this strategy in cancer patients have not yet been reported. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

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.

Neuroprotective effects of cutamesine, a ligand of the sigma-1 receptor chaperone, against noise-induced hearing loss

The sigma-1 receptor, which is expressed throughout the brain, provides physiological benefits that include higher brain function. The sigma-1 receptor functions as a chaperone in the endoplasmic reticulum and may control cell death and regeneration within the central nervous system. Cutamesine (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl) piperazine dihydrochloride) is a ligand selective for this receptor and may mediate neuroprotective effects in the context of neurodegenerative disease. We therefore assessed whether cutamesine protects the inner ear from noise-induced or aging-associated hearing loss. Immunohistochemistry and Western blotting revealed that the sigma-1 receptor is present in adult cochlea. We treated mice with 0, 3, or 30 mg/kg cutamesine from 10 days before noise exposure until the end of the study. All subjects were exposed to a 120-dB, 4-kHz octave-band noise for 2 hr. We assessed auditory thresholds by measuring the auditory-evoked brainstem responses at 4, 8, and 16 kHz, prior to and 1 week, 1 month, or 3 months following noise exposure. For the aging study, measurements were made before treatment was initiated and after 3 or 9 months of cutamesine treatment. Damage to fibrocytes within the cochlear spiral limbus was assessed by quantitative histology. Cutamesine significantly reduced threshold shifts and cell death within the spiral limbus in response to intense noise. These effects were not dose or time dependent. Conversely, cutamesine did not prevent aging-associated hearing loss. These results suggest that cutamesine reduces noise-induced hearing loss and cochlear damage during the acute phase that follows exposure to an intense noise.

A direct interaction between the sigma-1 receptor and the hERG voltage-gated K+ channel revealed by atomic force microscopy and homogeneous time-resolved fluorescence (HTRF®)

The sigma-1 receptor is an endoplasmic reticulum chaperone protein, widely expressed in central and peripheral tissues, which can translocate to the plasma membrane and modulate the function of various ion channels. The human ether-à-go-go-related gene encodes hERG, a cardiac voltage-gated K(+) channel that is abnormally expressed in many human cancers and is known to interact functionally with the sigma-1 receptor. Our aim was to investigate the nature of the interaction between the sigma-1 receptor and hERG. We show that the two proteins can be co-isolated from a detergent extract of stably transfected HEK-293 cells, consistent with a direct interaction between them. Atomic force microscopy imaging of the isolated protein confirmed the direct binding of the sigma-1 receptor to hERG monomers, dimers, and tetramers. hERG dimers and tetramers became both singly and doubly decorated by sigma-1 receptors; however, hERG monomers were only singly decorated. The distribution of angles between pairs of sigma-1 receptors bound to hERG tetramers had two peaks, at ∼90 and ∼180° in a ratio of ∼2:1, indicating that the sigma-1 receptor interacts with hERG with 4-fold symmetry. Homogeneous time-resolved fluorescence (HTRF®) allowed the detection of the interaction between the sigma-1 receptor and hERG within the plane of the plasma membrane. This interaction was resistant to sigma ligands, but was decreased in response to cholesterol depletion of the membrane. We suggest that the sigma-1 receptor may bind to hERG in the endoplasmic reticulum, aiding its assembly and trafficking to the plasma membrane.

NMDA receptors are upregulated and trafficked to the plasma membrane after sigma-1 receptor activation in the rat hippocampus

Sigma-1 receptors (σ-1Rs) are endoplasmic reticulum resident chaperone proteins implicated in many physiological and pathological processes in the CNS. A striking feature of σ-1Rs is their ability to interact and modulate a large number of voltage- and ligand-gated ion channels at the plasma membrane. We have reported previously that agonists for σ-1Rs potentiate NMDA receptor (NMDAR) currents, although the mechanism by which this occurs is still unclear. In this study, we show that in vivo administration of the selective σ-1R agonists (+)-SKF 10,047 [2S-(2α,6α,11R*]-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol hydrochloride (N-allylnormetazocine) hydrochloride], PRE-084 (2-morpholin-4-ylethyl 1-phenylcyclohexane-1-carboxylate hydrochloride), and (+)-pentazocine increases the expression of GluN2A and GluN2B subunits, as well as postsynaptic density protein 95 in the rat hippocampus. We also demonstrate that σ-1R activation leads to an increased interaction between GluN2 subunits and σ-1Rs and mediates trafficking of NMDARs to the cell surface. These results suggest that σ-1R may play an important role in NMDAR-mediated functions, such as learning and memory. It also opens new avenues for additional studies into a multitude of pathological conditions in which NMDARs are involved, including schizophrenia, dementia, and stroke.

Lack of synergistic effect of resveratrol and sigma-1 receptor agonist (PRE-084) in SOD1G⁹³A ALS mice: overlapping effects or limited therapeutic opportunity?

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is an adult onset neurodegenerative disease characterized by the loss of motoneurons (MNs) in the spinal cord, brainstem and motor cortex, causing progressive paralysis and death. Nowadays, there is no effective therapy and most patients die 2-5 years after diagnosis. Sigma-1R is a transmembrane protein highly expressed in the CNS and specially enriched in MNs. Mutations on the Sigma-1R leading to frontotemporal lobar degeneration-ALS were recently described in human patients. We previously reported the therapeutic role of the selective sigma-1R agonist 2-(4-morpholi-nethyl)1-phenylcyclohexanecarboxylate (PRE-084) in SOD1G93A ALS mice, that promoted spinal MN preservation and extended animal survival by controlling NMDA receptor calcium influx. Resveratrol (RSV, trans-3,4',5-trihydroxystilbene) is a natural polyphenol with promising neuroprotective effects. We recently found that RSV administration to SOD1G93A mice preserves spinal MN function and increases mice survival. These beneficial effects were associated to activation of Sirtuin 1 (Sirt1) and AMP-activated protein kinase (AMPK) pathways, leading to the modulation of autophagy and an increase of mitochondrial biogenesis. The main goal of this work was to assess the effect of combined RSV and PRE-084 administration in SOD1G93A ALS mice.

METHODS

We determined the locomotor performance of the animals by rotarod test and evaluated spinal motoneuron function using electrophysiological tests.

RESULTS

RSV plus PRE-084 treatment from 8 weeks of age significantly improved locomotor performance and spinal MN function, accompanied by a significant reduction of MN degeneration and an extension of mice lifespan. In agreement with our previous findings, there was an induction of PKC-specific phosphorylation of the NMDA-NR1 subunit and an increased expression and activation of Sirt1 and AMPK in the ventral spinal cord of treated SOD1G93A animals.

CONCLUSIONS

Although combined PRE and RSV treatment significantly ameliorated SOD1G93A mice, it did not show a synergistic effect compared to RSV-only and PRE-084-only treated groups.

Accumulation of the sigma-1 receptor is common to neuronal nuclear inclusions in various neurodegenerative diseases

The sigma-1 receptor (SIGMAR1) is now known to be one of the endoplasmic reticulum (ER) chaperones, which participate in the degradation of misfolded proteins in cells via the ER-related degradation machinery linked to the ubiquitin-proteasome pathway. Mutations of the SIGMAR1 gene are implicated in the pathogenesis of familial frontotemporal lobar degeneration and motor neuron disease. Involvement of ER dysfunction in the formation of inclusion bodies in various neurodegenerative diseases has also become evident. We performed immunohistochemical staining to clarify the localization of SIGMAR1 in the brains of patients with neurodegenerative disorders, including trans-activation response DNA protein 43 (TDP-43) proteinopathy, tauopathy, α-synucleinopathy, polyglutamine disease and intranuclear inclusion body disease (INIBD). Double-immunocytofluorescence and Western blot analyses of cultured cells were also performed to investigate the role of SIGMAR1 using a specific exportin 1 inhibitor, leptomycin B and an ER stress inducer, thapsigargin. SIGMAR1 was consistently shown to be co-localized with neuronal nuclear inclusions in TDP-43 proteinopathy, five polyglutamine diseases and INIBD, as well as in intranuclear Marinesco bodies in aged normal controls. Cytoplasmic inclusions in neurons and glial cells were unreactive for SIGMAR1. In cultured cells, immunocytofluorescent study showed that leptomycin B and thapsigargin were shown to sequester SIGMAR1 within the nucleus, acting together with p62. This finding was also supported by immunoblot analysis. These results indicate that SIGMAR1 might shuttle between the nucleus and the cytoplasm. Neurodegenerative diseases characterized by neuronal nuclear inclusions might utilize the ER-related degradation machinery as a common pathway for the degradation of aberrant proteins.

Sigma (σ) receptors as potential therapeutic targets to mitigate psychostimulant effects

Many psychostimulants, including cocaine and methamphetamine, interact with sigma (σ) receptors at physiologically relevant concentrations. The potential therapeutic relevance of this interaction is underscored by the ability to selectively target σ receptors to mitigate many behavioral and physiological effects of psychostimulants in animal and cell-based model systems. This chapter begins with an overview of these enigmatic proteins. Provocative preclinical data showing that σ ligands modulate an array of cocaine and methamphetamine effects are summarized, along with emerging areas of research. Together, the literature suggests targeting of σ receptors as an innovative option for combating undesired actions of psychostimulants through both neuronal and glial mechanisms.

[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.

Stimulation of sigma receptors with afobazole blocks activation of microglia and reduces toxicity caused by amyloid-β25-35

Alzheimer's disease (AD) is a progressive neurodegenerative disease and the leading cause of senile dementia in the United States. Accumulation of amyloid-β (Aβ) and the effects of this peptide on microglial cells contribute greatly to the etiology of AD. Experiments were carried out to determine whether the pan-selective σ-receptor agonist afobazole can modulate microglial response to the cytotoxic Aβ fragment, Aβ25-35. Treatment with afobazole decreased microglial activation in response to Aβ, as indicated by reduced membrane ruffling and cell migration. The effects of afobazole on Aβ25-35-evoked migration were concentration dependent and consistent with σ-receptor activation. When afobazole was coapplied with either BD-1047 [N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine dihydrobromide] or rimcazole, which are σ-1- and σ-2-selective antagonists, respectively, the inhibition of Aβ25-35-induced migration by afobazole was reduced. Prolonged exposure of microglia to Aβ25-35 resulted in glial cell death that was associated with increased expression of the proapoptotic protein Bax and the death protease caspase-3. Coapplication of afobazole with Aβ25-35 decreased the number of cells expressing both Bax and caspase-3 and resulted in a concomitant enhancement in cell survival. Although afobazole inhibited activation of microglia cells by Aβ25-35, it preserved normal functional responses in these cells after exposure to the amyloid peptide. Intracellular calcium increases induced by ATP were depressed in microglia after 24-hour exposure to Aβ25-35. However, coincubation in afobazole returned these responses to near control levels. Therefore, stimulation of σ-1 and σ-2 receptors by afobazole prevents Aβ25-35 activation of microglia and inhibits Aβ25-35-associated cytotoxicity, suggesting that afobazole may be useful for AD therapeutics.

Interorganellar membrane microdomains: dynamic platforms in the control of calcium signaling and apoptosis

The dynamic interplay among intracellular organelles occurs at specific membrane tethering sites, where two organellar membranes come in close apposition but do not fuse. Such membrane microdomains allow for rapid and efficient interorganelle communication that contributes to the maintenance of cell physiology. Pathological conditions that interfere with the proper composition, number, and physical vicinity of the apposing membranes initiate a cascade of events resulting in cell death. Membrane contact sites have now been identified that tether the extensive network of the endoplasmic reticulum (ER) membranes with the mitochondria, the plasma membrane (PM), the Golgi and the endosomes/lysosomes. Thus far, the most extensively studied are the MAMs, or mitochondria associated ER membranes, and the ER-PM junctions that share functional properties and crosstalk to one another. Specific molecular components that define these microdomains have been shown to promote the interaction in trans between these intracellular compartments and the transfer or exchange of Ca²⁺ ions, lipids, and metabolic signaling molecules that determine the fate of the cell.

Inhibitory effects of sigma-1 ligands on handling-induced tachycardia in conscious tethered rats

We used conscious tethered Sprague-Dawley rats to evaluate the cardiovascular effects of four sigma-1 (σ1 ) agonists and five antagonists, given alone or in combination. All drugs were administered as a single intraperitoneal dose. The agonists were given at doses reported as efficacious in rodent cognition models, while the antagonists were administered at doses neutralizing agonist effects in vivo. Systolic blood pressure (SBP) and heart rate (HR) were continuously recorded for 20 min before and 60 min postadministration. Immediately after injection, a sudden, transitory increase in HR and SBP was noted in all animals, because of the stress induced by handling. For both parameters, a peak value (ΔHRmax and ΔSBPmax ) and an area under the curve of changes from baseline over the period 5-20 min postinjection (ΔHR_AUC5-20 min and ΔSBP_AUC5-20 min ) were calculated. Three of the four σ1 agonists (SKF-10,047, dehydroepiandrosterone (DHEAS), Compound 14) significantly reduced ΔHR_AUC5-20 min value without changing ΔHRmax , while the fourth one, SA-4503, had no significant effect. None of the antagonists (haloperidol, rimcazole, NE-100, and BD1047) reduced, and even one (progesterone) enhanced the stress-induced effects on HR. No changes in SBP were noted with any compound. When the antagonist NE-100 was administered just before SKF-10,047, it completely reversed the inhibitory effects of the σ1 agonist on HR increase. In conclusion, we demonstrated for the first time the involvement of σ1 receptors in the regulation of handling-induced tachycardia in the conscious rat. Although additional investigations are needed to fully understand this role, it might offer new therapeutic perspectives to σ1 ligands in the cardiovascular sphere.

MicroRNA-301a mediated regulation of Kv4.2 in diabetes: identification of key modulators

Diabetes is a metabolic disorder that ultimately results in major pathophysiological complications in the cardiovascular system. Diabetics are predisposed to higher incidences of sudden cardiac deaths (SCD). Several studies have associated diabetes as a major underlying risk for heart diseases and its complications. The diabetic heart undergoes remodeling to cope up with the underlying changes, however ultimately fails. In the present study we investigated the changes associated with a key ion channel and transcriptional factors in a diabetic heart model. In the mouse db/db model, we identified key transcriptional regulators and mediators that play important roles in the regulation of ion channel expression. Voltage-gated potassium channel (Kv4.2) is modulated in diabetes and is down regulated. We hypothesized that Kv4.2 expression is altered by potassium channel interacting protein-2 (KChIP2) which is regulated upstream by NFkB and miR-301a. We utilized qRT-PCR analysis and identified the genes that are affected in diabetes in a regional specific manner in the heart. At protein level we identified and validated differential expression of Kv4.2 and KChIP2 along with NFkB in both ventricles of diabetic hearts. In addition, we identified up-regulation of miR-301a in diabetic ventricles. We utilized loss and gain of function approaches to identify and validate the role of miR-301a in regulating Kv4.2. Based on in vivo and in vitro studies we conclude that miR-301a may be a central regulator for the expression of Kv4.2 in diabetes. This miR-301 mediated regulation of Kv4.2 is independent of NFkB and Irx5 and modulates Kv4.2 by direct binding on Kv4.2 3′untranslated region (3′-UTR). Therefore targeting miR-301a may offer new potential for developing therapeutic approaches.

Dynamic interaction between sigma-1 receptor and Kv1.2 shapes neuronal and behavioral responses to cocaine

The sigma-1 receptor (Sig-1R), an endoplasmic reticulum (ER) chaperone protein, is an interorganelle signaling modulator that potentially plays a role in drug-seeking behaviors. However, the brain site of action and underlying cellular mechanisms remain unidentified. We found that cocaine exposure triggers a Sig-1R-dependent upregulation of D-type K(+) current in the nucleus accumbens (NAc) that results in neuronal hypoactivity and thereby enhances behavioral cocaine response. Combining ex vivo and in vitro studies, we demonstrated that this neuroadaptation is caused by a persistent protein-protein association between Sig-1Rs and Kv1.2 channels, a phenomenon that is associated to a redistribution of both proteins from intracellular compartments to the plasma membrane. In conclusion, the dynamic Sig-1R-Kv1.2 complex represents a mechanism that shapes neuronal and behavioral response to cocaine. Functional consequences of Sig-1R binding to K(+) channels may have implications for other chronic diseases where maladaptive intrinsic plasticity and Sig-1Rs are engaged.

The sigma-1 receptor: roles in neuronal plasticity and disease

Sigma-1 receptors (Sig-1Rs) have been implicated in many neurological and psychiatric conditions. Sig-1Rs are intracellular chaperones that reside specifically at the endoplasmic reticulum (ER)-mitochondrion interface, referred to as the mitochondrion-associated ER membrane (MAM). Here, Sig-1Rs regulate ER-mitochondrion Ca(2+) signaling. In this review, we discuss the current understanding of Sig-1R functions. Based on this, we suggest that the key cellular mechanisms linking Sig-1Rs to neurological disorders involve the translocation of Sig-1Rs from the MAM to other parts of the cell, whereby Sig-1Rs bind and modulate the activities of various ion channels, receptors, or kinases. Thus, Sig-1Rs and their associated ligands may represent new avenues for treating aspects of neurological and psychiatric diseases.

The sigma-1 receptor binds to the Nav1.5 voltage-gated Na+ channel with 4-fold symmetry

The sigma-1 receptor (Sig1R) is up-regulated in many human tumors and plays a role in the control of cancer cell proliferation and invasiveness. At the molecular level, the Sig1R modulates the activity of various ion channels, apparently through a direct interaction. We have previously shown using atomic force microscopy imaging that the Sig1R binds to the trimeric acid-sensing ion channel 1A with 3-fold symmetry. Here, we investigated the interaction between the Sig1R and the Nav1.5 voltage-gated Na(+) channel, which has also been implicated in promoting the invasiveness of cancer cells. We show that the Sig1R and Nav1.5 can be co-isolated from co-transfected cells, consistent with an intimate association between the two proteins. Atomic force microscopy imaging of the co-isolated proteins revealed complexes in which Nav1.5 was decorated by Sig1Rs. Frequency distributions of angles between pairs of bound Sig1Rs had two peaks, at ∼90° and ∼180°, and the 90° peak was about twice the size of the 180° peak. These results demonstrate that the Sig1R binds to Nav1.5 with 4-fold symmetry. Hence, each set of six transmembrane regions in Nav1.5 likely constitutes a Sig1R binding site, suggesting that the Sig1R interacts with the transmembrane regions of its partners. Interestingly, two known Sig1R ligands, haloperidol and (+)-pentazocine, disrupted the Nav1.5/Sig1R interaction both in vitro and in living cells. Finally, we show that endogenously expressed Sig1R and Nav1.5 also functionally interact.

In vitro evaluation of guanidine analogs as sigma receptor ligands for potential anti-stroke therapeutics

Currently, the only Food and Drug Administration-approved treatment of acute stroke is recombinant tissue plasminogen activator, which must be administered within 6 hours after stroke onset. The pan-selective σ-receptor agonist N,N'-di-o-tolyl-guanidine (o-DTG) has been shown to reduce infarct volume in rats after middle cerebral artery occlusion, even when administered 24 hours after stroke. DTG derivatives were synthesized to develop novel compounds with greater potency than o-DTG. Fluorometric Ca(2+) imaging was used in cultured cortical neurons to screen compounds for their capacity to reduce ischemia- and acidosis-evoked cytosolic Ca(2+) overload, which has been linked to stroke-induced neurodegeneration. In both assays, migration of the methyl moiety produced no significant differences, but removal of the group increased potency of the compound for inhibiting acidosis-induced [Ca(2+)](i) elevations. Chloro and bromo substitution of the methyl moiety in the meta and para positions increased potency by ≤160%, but fluoro substitutions had no effect. The most potent DTG derivative tested was N,N'-di-p-bromo-phenyl-guanidine (p-BrDPhG), which had an IC(50) of 2.2 µM in the ischemia assay, compared with 74.7 μM for o-DTG. Microglial migration assays also showed that p-BrDPhG is more potent than o-DTG in this marker for microglial activation, which is also linked to neuronal injury after stroke. Radioligand binding studies showed that p-BrDPhG is a pan-selective σ ligand. Experiments using the σ-1 receptor-selective antagonist 1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine dihydrochloride (BD-1063) demonstrated that p-BrDPhG blocks Ca(2+) overload via σ-1 receptor activation. The study identified four compounds that may be more effective than o-DTG for the treatment of ischemic stroke at delayed time points.

Cyproheptadine enhances the I(K) of mouse cortical neurons through sigma-1 receptor-mediated intracellular signal pathway

Cyproheptadine (CPH) is a histamine- and serotonin-receptor antagonist, and its effects are observed recently in the modulation of multiple intracellular signals. In this study, we used cortical neurons and HEK-293 cells transfected with Kv2.1 α-subunit to address whether CPH modify neural voltage-gated K(+) channels by a mechanism independent of its serotonergic and histaminergic properties. Our results demonstrate that intracellularly delivered CPH increased the I(K) by reducing the activity of protein kinas A (PKA). Inhibition of G(i) eliminated the CPH-induced effect on both the I(K) and PKA. Blocking of 5-HT-, M-, D(2)-, H(1)- or H(2)-type GPCR receptors with relevant antagonists did not eliminate the CPH-induced effect on the I(K). Antagonists of the sigma-1 receptor, however, blocked the effect of CPH. Moreover, the inhibition of sigma-1 by siRNA knockdown significantly reduced the CPH-induced effect on the I(K). On the contrary, sigma-1 receptor agonist mimicked the effects of CPH on the induction of I(K). A ligand-receptor binding assay indicated that CPH bound to the sigma-1 receptor. Similar effect of CPH were obtained from HEK-293 cells transfected with the α-subunit of Kv2.1. In overall, we reveal for the first time that CPH enhances the I(K) by modulating activity of PKA, and that the associated activation of the sigma-1 receptor/G(i)-protein pathway might be involved. Our findings illustrate an uncharacterized effect of CPH on neuron excitability through the I(K), which is independent of histamine H(1) and serotonin receptors.

σ-1 Receptor agonist SKF10047 inhibits glutamate release in rat cerebral cortex nerve endings

σ-1 Receptors are expressed in the brain, and their activation has been shown to prevent neuronal death associated with glutamate toxicity. This study investigates the possible mechanism and effect of [2S-(2α,6α,11R*]-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol (SKF10047), a σ-1 receptor agonist, on endogenous glutamate release in the nerve terminals of rat cerebral cortex. Results show that SKF10047 inhibited the release of glutamate evoked by the K⁺ channel blocker 4-aminopyridine (4-AP), and the σ-1 receptor antagonist N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine (BD1047) blocked this phenomenon. The effects of SKF10047 on the evoked glutamate release were prevented by the chelating extracellular Ca²⁺ions and the vesicular transporter inhibitor bafilomycin A1. However, the glutamate transporter inhibitor DL-threo-β-benzyl-oxyaspartate did not have any effect on the action of SKF10047. SKF10047 decreased the depolarization-induced increase in the cytosolic free Ca²⁺ concentration ([Ca²⁺](C)), but did not alter 4-AP-mediated depolarization. Furthermore, the effects of SKF10047 on evoked glutamate release were prevented by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking the ryanodine receptors or the mitochondrial Na⁺/Ca²⁺ exchange. In addition, conventional protein kinase C (PKC) inhibitors abolished the SKF10047 effect on 4-AP-evoked glutamate release. Western blot analyses showed that SKF10047 decreased the 4-AP-induced phosphorylation of PKC and PKCα. These results show that σ-1 receptor activation inhibits glutamate release from rat cortical nerve terminals. This effect is linked to a decrease in [Ca²⁺](C) caused by Ca²⁺ entry through presynaptic voltage-dependent Ca²⁺ channels and the suppression of the PKC signaling cascade.

Examining protection from anoxic depolarization by the drugs dibucaine and carbetapentane using whole cell recording from CA1 neurons

As an immediate consequence of stroke onset, failure of the Na(+)-K(+)-ATPase pump evokes a propagating anoxic depolarization (AD) across gray matter. Acute neuronal swelling and dendritic beading arise within seconds in the future ischemic core, imaged as changes in light transmittance (ΔLT). AD is itself not a target for drug-based reduction of stroke injury because it is generated in the 1st min of stroke onset. Peri-infarct depolarizations (PIDs) are milder AD-like events that recur during the hours following AD and contribute to infarct expansion. Inhibiting PIDs with drugs could limit expansion. Two types of drugs, "caines" and σ(1)-receptor ligands, have been found to inhibit AD onset (and may also oppose PID initiation), yet their underlying actions have not been examined. Imaging ΔLT in the CA1 region simultaneously with whole cell current-clamp recording from CA1 pyramidal neurons reveal that the elevated LT front and onset of the AD are coincident. Either dibucaine or carbetapentane pretreatment significantly delays AD onset without affecting resting membrane potential or neuronal input resistance. Dibucaine decreases excitability by raising spike threshold and decreasing action potential (AP) frequency, whereas carbetapentane eliminates the fast afterhyperpolarization while accentuating the slow afterhyperpolarization to reduce AP frequency. Orthodromic and antidromic APs are eliminated by dibucaine within 15 min but not by carbetapentane. Thus both drugs reduce cortical excitability at the level of the single pyramidal neuron but through strikingly different mechanisms. In vivo, both drugs would likely inhibit recurring PIDs in the expanding penumbra and so potentially could reduce developing neuronal damage over many hours poststroke when PIDs occur.

Afobazole modulates neuronal response to ischemia and acidosis via activation of sigma-1 receptors

Afobazole is an anxiolytic medication that has been previously shown to be neuroprotective both in vitro and in vivo. However, the mechanism(s) by which afobazole can enhance neuronal survival remain poorly understood. Experiments were carried out to determine whether afobazole can decrease intracellular calcium overload associated with ischemia and acidosis and whether the effects of afobazole are mediated via interaction of the compound with σ receptors. Fluorometric Ca(2+) imaging was used to resolve how application of afobazole affects intracellular Ca(2+) handling in cortical neurons. Application of afobazole significantly depressed, in a concentration-dependent and reversible manner, the intracellular Ca(2+) overload resulting from in vitro ischemia and acidosis. The IC(50) for afobazole inhibition of ischemia-evoked intracellular Ca(2+) overload was considerably less than that for the inhibition of [Ca(2+)](i) increases induced by acidosis. However, afobazole maximally inhibited only 70% of the ischemia-evoked intracellular Ca(2+) overload but effectively abolished intracellular Ca(2+) increases produced by acidosis. The effects of afobazole on ischemia- and acidosis-induced intracellular Ca(2+) dysregulation were inhibited by preincubating the neurons in the irreversible, pan-selective σ-receptor antagonist, metaphit. Moreover, the effects of afobazole on intracellular Ca(2+) increases triggered by acidosis and ischemia were blocked by the selective σ-1-receptor antagonists, BD 1063 and BD 1047, respectively. Experiments examining the effects of afobazole on neuronal survival in response to ischemia showed that afobazole was neuroprotective. Taken together, these data suggest that afobazole regulates intracellular Ca(2+) overload during ischemia and acidosis via activation of σ-1 receptors. This mechanism is probably responsible for afobazole-mediated neuroprotection.

Functional heterogeneity of nociceptin/orphanin FQ receptors revealed by (+)-5a Compound and Ro 64-6198 in rat periaqueductal grey slices

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a non-opioid branch of the opioid receptor family implicated in several neurological and psychological disorders, such as pain, anxiety, depression, involuntary movement, addiction, seizure and dementia. Heterogeneity of NOP receptors has been proposed based on the findings of splicing variants and from binding and functional studies. We have previously reported that Ro 64-6198, a NOP receptor agonist, activated a subset, but not all, of N/OFQ-sensitive NOP receptors in midbrain ventrolateral periaqueductal grey (vlPAG). In this study, we found that a new NOP receptor ligand, (+)-5a Compound ((3aS, 6aR)-1-(cis-4-isopropylcyclohexyl)-5'-methyl-2'-phenylhexahydrospiro[piperidine-4,1'-pyrrolo[3, 4-c]pyrrole]), also activated a subset of NOP receptors in vlPAG neurons. (+)-5a Compound (0.1-30 μm) concentration-dependently activated G-protein-coupled inwardly-rectifying potassium (GIRK) channels mediated through the NOP receptors in about 35% of the recorded vlPAG neurons. (+)-5a Compound (EC50: 605 nm) was less potent (1/12) and efficacious (47%) than N/OFQ. In (+)-5a Compound-insensitive neurons, Ro 64-6198 was also ineffective, and vice versa, but N/OFQ activated GIRK channels through NOP receptors. In (+)-5a Compound-sensitive neurons, (+)-5a Compound precluded the effect of Ro 64-6198. Immunofluorecent and morphometric studies showed that most of the (+)-5a Compound-sensitive neurons were multipolar with intensive dendritic arborization and immunoreactive to glutamic acid decarboxylase-67. It is suggested that (+)-5a Compound activates a subset of NOP receptors, similar to the Ro 64-6198-sensitive subset, in the vlPAG neurons which are mostly GABAergic. These results further support the presence of functional heterogeneity of NOP receptors in the midbrain PAG.

Ethanol exposure in early adolescence inhibits intrinsic neuronal plasticity via sigma-1 receptor activation in hippocampal CA1 neurons

BACKGROUND

We demonstrated previously that rats exposed to chronic intermittent ethanol (CIE) vapors in early adolescence show increased magnitudes of long-term potentiation (LTP) of excitatory transmission when recorded at dendritic synapses in hippocampus. Large amplitude LTP following CIE exposure is mediated by sigma-1 receptors; however, not yet addressed is the role of sigma-1 receptors in modulating the intrinsic properties of neurons to alter their action potential firing during LTP.

METHODS

Activity-induced plasticity of spike firing was investigated using rat hippocampal slice recordings to measure changes in both field excitatory postsynaptic potentials (fEPSPs) and population spikes (pop. spikes) concomitantly at dendritic inputs and soma of CA1 pyramidal neurons, respectively.

RESULTS

We observed unique modifications in plasticity of action potential firing in hippocampal slices from CIE exposed adolescent rats, where the induction of large amplitude LTP by 100 Hz stimulations was accompanied by reduced CA1 neuronal excitability--reflected as decreased pop. spike efficacy and impaired activity-induced fEPSP-to-spike (E-S) potentiation. In contrast, LTP induction in ethanol-naïve control slices resulted in increased spike efficacy and robust E-S potentiation. E-S potentiation impairments emerged at 24 hours after CIE treatment cessation, but not before the alcohol withdrawal period, and were restored with bath-application of the sigma-1 receptor selective antagonist BD1047, but not the NMDA receptor antagonist d-AP5. Further evidence revealed a significantly shortened somatic fEPSP time course in adolescent CIE-withdrawn hippocampal slices during LTP; however, paired-pulse data show no apparent correspondence between E-S dissociation and altered recurrent feedback inhibition.

CONCLUSIONS

Results here suggest that acute withdrawal from adolescent CIE exposure triggers sigma-1 receptors that act to depress the efficacy of excitatory inputs in triggering action potentials during LTP. Such withdrawal-induced depression of E-S plasticity in hippocampus probably entails sigma-1 receptor modulation of 1 or several voltage-gated ion channels controlling the neuronal input-output dynamics.

Antagonist action of progesterone at σ-receptors in the modulation of voltage-gated sodium channels

σ-Receptors are integral membrane proteins that have been implicated in a number of biological functions, many of which involve the modulation of ion channels. A wide range of synthetic ligands activate σ-receptors, but endogenous σ-receptor ligands have proven elusive. One endogenous ligand, dimethyltryptamine (DMT), has been shown to act as a σ-receptor agonist. Progesterone and other steroids bind σ-receptors, but the functional consequences of these interactions are unclear. Here we investigated progesterone binding to σ(1)- and σ(2)-receptors and evaluated its effect on σ-receptor-mediated modulation of voltage-gated Na(+) channels. Progesterone binds both σ-receptor subtypes in liver membranes with comparable affinities and blocks photolabeling of both subtypes in human embryonic kidney 293 cells that stably express the human cardiac Na(+) channel Na(v)1.5. Patch-clamp recording in this cell line tested Na(+) current modulation by the σ-receptor ligands ditolylguanidine, PB28, (+)SKF10047, and DMT. Progesterone inhibited the action of these ligands to varying degrees, and some of these actions were reduced by σ(1)-receptor knockdown with small interfering RNA. Progesterone inhibition of channel modulation by drugs was consistent with stronger antagonism of σ(2)-receptors. By contrast, progesterone inhibition of channel modulation by DMT was consistent with stronger antagonism of σ(1)-receptors. Progesterone binding to σ-receptors blocks σ-receptor-mediated modulation of a voltage-gated ion channel, and this novel membrane action of progesterone may be relevant to changes in brain and cardiovascular function during endocrine transitions.

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.

The sigma-1 receptor chaperone as an inter-organelle signaling modulator

Inter-organelle signaling plays important roles in many physiological functions. Endoplasmic reticulum (ER)-mitochondrion signaling affects intramitochondrial calcium (Ca(2+)) homeostasis and cellular bioenergetics. ER-nucleus signaling attenuates ER stress. ER-plasma membrane signaling regulates cytosolic Ca(2+) homeostasis and ER-mitochondrion-plasma membrane signaling regulates hippocampal dendritic spine formation. Here, we propose that the sigma-1 receptor (Sig-1R), an ER chaperone protein, acts as an inter-organelle signaling modulator. Sig-1Rs normally reside at the ER-mitochondrion contact called the MAM (mitochondrion-associated ER membrane), where Sig-1Rs regulate ER-mitochondrion signaling and ER-nucleus crosstalk. When cells are stimulated by ligands or undergo prolonged stress, Sig-1Rs translocate from the MAM to the ER reticular network and plasmalemma/plasma membrane to regulate a variety of functional proteins, including ion channels, receptors and kinases. Thus, the Sig-1R serves as an inter-organelle signaling modulator locally at the MAM and remotely at the plasmalemma/plasma membrane. Many pharmacological/physiological effects of Sig-1Rs might relate to this unique action of Sig-1Rs.