Demonstration of a direct interaction between sigma-1 receptors and acid-sensing ion channels

Sig1R activates extracellular matrix-induced bladder cancer cell proliferation and angiogenesis by combing β-integrin

The extracellular matrix (ECM) regulates many biological functions involved in tumorigenesis and tumor development; however, the underlying mechanism remains unknown. Sigma 1 receptor (Sig1R), a stress-activated chaperone, regulates the crosstalk between the ECM and tumor cells and is related to the malignant characteristics of several tumors. However, the link between Sig1R overexpression and ECM during malignancy has not been established in bladder cancer (BC). Here, we analyzed the interaction of Sig1R and β-integrin in BC cells and its role in ECM-mediated cell proliferation and angiogenesis. We found that Sig1R forms a complex with β-integrin to promote ECM-mediated BC cell proliferation and angiogenesis, which enhances the aggressiveness of the tumor cells. This leads to poor survival. Our research revealed that Sig1R mediates the cross-talk between BC cells and their ECM microenvironment, thereby driving the progression of BC. Promisingly, targeting an ion channel function through Sig1R inhibition may serve as a potential approach for BC treatment.

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: A Potential Therapeutic Target for Traumatic Brain Injury

The sigma-1 receptor (Sig-1R) is a chaperone receptor that primarily resides at the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) and acts as a dynamic pluripotent modulator regulating cellular pathophysiological processes. Multiple pharmacological studies have confirmed the beneficial effects of Sig-1R activation on cellular calcium homeostasis, excitotoxicity modulation, reactive oxygen species (ROS) clearance, and the structural and functional stability of the ER, mitochondria, and MAM. The Sig-1R is expressed broadly in cells of the central nervous system (CNS) and has been reported to be involved in various neurological disorders. Traumatic brain injury (TBI)-induced secondary injury involves complex and interrelated pathophysiological processes such as cellular apoptosis, glutamate excitotoxicity, inflammatory responses, endoplasmic reticulum stress, oxidative stress, and mitochondrial dysfunction. Thus, given the pluripotent modulation of the Sig-1R in diverse neurological disorders, we hypothesized that the Sig-1R may affect a series of pathophysiology after TBI. This review summarizes the current knowledge of the Sig-1R, its mechanistic role in various pathophysiological processes of multiple CNS diseases, and its potential therapeutic role in TBI.

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.

Involvement of Chaperone Sigma1R in the Anxiolytic Effect of Fabomotizole

Sigma-1 receptor (chaperone Sigma1R) is an intracellular protein with chaperone functions, which is expressed in various organs, including the brain. Sigma1R participates in the regulation of physiological mechanisms of anxiety (Su, T. P. et al., 2016) and reactions to emotional stress (Hayashi, T., 2015). In 2006, fabomotizole (ethoxy-2-[2-(morpholino)-ethylthio]benzimidazole dihydrochloride) was registered in Russia as an anxiolytic (Seredenin S. and Voronin M., 2009). The molecular targets of fabomotizole are Sigma1R, NRH: quinone reductase 2 (NQO2), and monoamine oxidase A (MAO-A) (Seredenin S. and Voronin M., 2009). The current study aimed to clarify the dependence of fabomotizole anxiolytic action on its interaction with Sigma1R and perform a docking analysis of fabomotizole interaction with Sigma1R. An elevated plus maze (EPM) test revealed that the anxiolytic-like effect of fabomotizole (2.5 mg/kg i.p.) administered to male BALB/c mice 30 min prior EPM exposition was blocked by Sigma1R antagonists BD-1047 (1.0 mg/kg i.p.) and NE-100 (1.0 mg/kg i.p.) pretreatment. Results of initial in silico study showed that fabomotizole locates in the active center of Sigma1R, reproducing the interactions with the site's amino acids common for established Sigma1R ligands, with the ΔGbind value closer to that of agonist (+)-pentazocine in the 6DK1 binding site.

Bi-phasic dose response in the preclinical and clinical developments of sigma-1 receptor ligands for the treatment of neurodegenerative disorders

Introduction: The sigma-1 receptor (S1R) is attracting much attention for disease-modifying therapies in neurodegenerative diseases. It is a conserved protein, present in plasma and endoplasmic reticulum (ER) membranes and enriched in mitochondria-associated ER membranes (MAMs). It modulates ER-mitochondria Ca2+ transfer and ER stress pathways. Mitochondrial and MAM dysfunctions contribute to neurodegenerative processes in diseases such as Alzheimer, Parkinson, Huntington or Amyotrophic Lateral Sclerosis. Interestingly, the S1R can be activated by small druggable molecules and accumulating preclinical data suggest that S1R agonists are effective protectants in these neurodegenerative diseases.Area covered: In this review, we will present the data showing the high therapeutic potential of S1R drugs for the treatment of neurodegenerative diseases, focusing on pridopidine as a potent and selective S1R agonist under clinical development. Of particular interest is the bi-phasic (bell-shaped) dose-response effect, representing a common feature of all S1R agonists and described in numerous preclinical models in vitro, in vivo and in clinical trials.Expert opinion: S1R agonists modulate inter-organelles communication altered in neurodegenerative diseases and activate intracellular survival pathways. Research will continue growing in the future. The particular cellular nature of this chaperone protein must be better understood to facilitate the clinical developement of promising molecules.

Chaperone Sigma1R and Antidepressant Effect

This review analyzes the current scientific literature on the role of the Sigma1R chaperone in the pathogenesis of depressive disorders and pharmacodynamics of antidepressants. As a result of ligand activation, Sigma1R is capable of intracellular translocation from the endoplasmic reticulum (ER) into the region of nuclear and cellular membranes, where it interacts with resident proteins. This unique property of Sigma1R provides regulation of various receptors, ion channels, enzymes, and transcriptional factors. The current review demonstrates the contribution of the Sigma1R chaperone to the regulation of molecular mechanisms involved in the antidepressant effect.

Distinct Regulation of σ 1 Receptor Multimerization by Its Agonists and Antagonists in Transfected Cells and Rat Liver Membranes

Extensive studies have shown that the σ 1 receptor (σ 1R) interacts with and modulates the activity of multiple proteins with important biological functions. Recent crystal structures of σ 1R as a homotrimer differ from a dimer-tetramer model postulated earlier. It remains inconclusive whether ligand binding regulates σ 1R oligomerization. Here, novel nondenaturing gel methods and mutational analysis were used to examine σ 1R oligomerization. In transfected cells, σ 1R exhibited as multimers, dimers, and monomers. Overall, σ 1R agonists decreased, whereas σ 1R antagonists increased σ 1R multimers, suggesting that agonists and antagonists differentially affect the stability of σ 1R multimers. Endogenous σ 1R in rat liver membranes also showed similar regulation of oligomerization as in cells. Mutations at key residues lining the trimerization interface (Arg119, Asp195, Phe191, Trp136, and Gly91) abolished multimerization without disrupting dimerization. Intriguingly, truncation of the N terminus reduced σ 1R to apparent monomer. These results demonstrate that multiple domains play crucial roles in coordinating high-order quaternary organization of σ 1R. The E102Q σ 1R mutant implicated in juvenile amyotrophic lateral sclerosis formed dimers only, suggesting that dysregulation of σ 1R multimeric assembly may impair its function. Interestingly, oligomerization of σ 1R was pH-dependent and correlated with changes in [3H](+)-pentazocine binding affinity and Bmax Combined with mutational analysis, it is reasoned that σ 1R multimers possess high-affinity and high-capacity [3H](+)-pentazocine binding, whereas monomers likely lack binding. These results suggest that σ 1R may exist in interconvertible oligomeric states in a dynamic equilibrium. Further exploration of ligand-regulated σ 1R multimerization may provide novel approaches to modulate the function of σ 1R and its interacting proteins. SIGNIFICANCE STATEMENT: The σ 1 receptor (σ 1R) modulates the activities of various partner proteins. Recently, crystal structures of σ 1R were elucidated as homotrimers. This study used novel nondenaturing gel methods to examine σ1R oligomerization in transfected cells and rat liver membranes. Overall, agonist binding decreased, whereas antagonist binding increased σ 1R multimers, which comprised trimers and larger units. σ 1R multimers were shown to bind [3H](+)-pentazocine with high affinity and high capacity. Furthermore, mutational analysis revealed a crucial role of its N-terminal domain in σ 1R multimerization.

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 Molecular Function of σ Receptors: Past, Present, and Future

The σ1 and σ2 receptors are enigmatic proteins that have attracted attention for decades due to the chemical diversity and therapeutic potential of their ligands. However, despite ongoing clinical trials with σ receptor ligands for multiple conditions, relatively little is known regarding the molecular function of these receptors. In this review, we revisit past research on σ receptors and discuss the interpretation of these data in light of recent developments. We provide a synthesis of emerging structural and genetic data on the σ1 receptor and discuss the recent cloning of the σ2 receptor. Finally, we discuss the major questions that remain in the study of σ receptors.

Neuronal Sigma-1 Receptors: Signaling Functions and Protective Roles in Neurodegenerative Diseases

Sigma-1 receptor (S1R) is a multi-functional, ligand-operated protein situated in endoplasmic reticulum (ER) membranes and changes in its function and/or expression have been associated with various neurological disorders including amyotrophic lateral sclerosis/frontotemporal dementia, Alzheimer's (AD) and Huntington's diseases (HD). S1R agonists are broadly neuroprotective and this is achieved through a diversity of S1R-mediated signaling functions that are generally pro-survival and anti-apoptotic; yet, relatively little is known regarding the exact mechanisms of receptor functioning at the molecular level. This review summarizes therapeutically relevant mechanisms by which S1R modulates neurophysiology and implements neuroprotective functions in neurodegenerative diseases. These mechanisms are diverse due to the fact that S1R can bind to and modulate a large range of client proteins, including many ion channels in both ER and plasma membranes. We summarize the effect of S1R on its interaction partners and consider some of the cell type- and disease-specific aspects of these actions. Besides direct protein interactions in the endoplasmic reticulum, S1R is likely to function at the cellular/interorganellar level by altering the activity of several plasmalemmal ion channels through control of trafficking, which may help to reduce excitotoxicity. Moreover, S1R is situated in lipid rafts where it binds cholesterol and regulates lipid and protein trafficking and calcium flux at the mitochondrial-associated membrane (MAM) domain. This may have important implications for MAM stability and function in neurodegenerative diseases as well as cellular bioenergetics. We also summarize the structural and biochemical features of S1R proposed to underlie its activity. In conclusion, S1R is incredibly versatile in its ability to foster neuronal homeostasis in the context of several neurodegenerative disorders.

Sigma-1 Receptor Antagonists Haloperidol and Chlorpromazine Modulate the Effect of Glutoxim on Na+ Transport in Frog Skin

Using voltage-clamp technique, the involvement of sigma-1 receptors in the regulation of Na+ transport in frog skin by the immunomodulatory drug glutoxim was investigated. We have shown for the first time that preincubation of the frog skin with the sigma-1 receptor antagonists haloperidol and chlorpromazine attenuates the stimulatory effect of glutoxim on the Na+ transport. The results suggest the possible involvement of the sigma-1 receptors in the regulation of Na+ transport in frog skin epithelium by glutoxim.

Acid-Sensing Ion Channel 1a Modulates NMDA Receptor Function Through Targeting NR1/NR2A/NR2B Triheteromeric Receptors

The over-activation of N-methyl-D-aspartate receptors (NMDARs) is the main cause of neuronal death in brain ischemia. Both the NMDAR and the Acid-sensing ion channel 1a (ASIC1a) are present in the postsynaptic membrane of the central nervous system (CNS) and participate in physiological and pathological processes. However, the specific role played by ASIC1a in these processes remains elusive. We hypothesize that NMDARs are the primary mediators of normal synaptic transmission and excitatory neuronal death, while ASIC1a plays a modulatory role in facilitating NMDAR function. Using various experimental approaches including patch-clamp recordings on hippocampal slices and CHO cells, primary cultures of hippocampal neurons, calcium imaging, Western blot, cDNA transfection studies, and transient middle cerebral artery occlusion (tMCAO) mouse models, we demonstrate that stimulation of ASIC1a facilitates NMDAR function and inhibition of ASIC1a suppresses NMDAR over-activation. One of our key findings is that activation of ASIC1a selectively facilitates the NR1/NR2A/NR2B triheteromeric subtype of NMDAR currents. In accordance, inhibition of ASIC1a profoundly reduced the NMDAR-mediated EPSCs in older mouse brains, which are known to express much higher levels of triheteromeric NMDARs than younger brains. Furthermore, brain infarct sizes were reduced by a greater degree in older mice compared to younger ones when ASIC1a activity was suppressed. These data suggest that ASIC1a activity selectively enhances the function of triheteromeric NMDARs and exacerbates ischemic neuronal death especially in older animal brains. We propose ASIC1a as a novel therapeutic target for preventing and reducing the detrimental effect of brain ischemia in humans.

Sigma-1 receptor activation alleviates blood-brain barrier dysfunction in vascular dementia mice

Sigma-1 receptor (Sig-1R) activation has been shown to decrease infarct volume and enhance neuronal survival after brain ischemia-reperfusion (IR) in rodent models. The present study aims to investigate first the effect of Sig-1R activation on blood-brain barrier (BBB) disruption during experimental stroke. Male C57BL/6 mice were subjected to bilateral common carotid artery occlusion (BCCAO) for 15 min, and the worst BBB leakage was observed on the 7th day after brain IR. To confirm the BBB protective role of Sig-1R, mice were divided into five groups (sham group, BCCAO group, PRE084 group, BD1047 group, PRE084 and BD1047 group; 29-35 mice for each group), and treated with agonist PRE084 (1 mg/kg) and/or antagonist BD1047 (1 mg/kg) for 7 days intraperitoneally once a day after BCCAO. Interestingly, PRE084 administration significantly improved neurobehavioral performance as well as healing of neuron damage and white matter lesions. PRE084 also reduced the leakage of Evans blue and IgG and attenuated the disassembly of BBB structural proteins, while the neuroprotective and BBB protective functions of PRE084 were blocked by BD1047. Furthermore, in Sig-1R knockout (Sig-1R KO) mice, brain IR produced more serious IgG leakage and degradation of BBB structural proteins than in wild-type model mice. In addition, the protective effect of PRE084 against the BBB was lost in Sig-1R KO mice after brain IR. Finally, treatment with PRE084 significantly increased the expression of Sig-1R in brain microvascular endothelial cells of mice that were subjected to brain IR and increased translocation of Sig-1R to the cell plasmalemma. Thus, we identified a previously unexplored role of Sig-1R in alleviating BBB disruption in stroke processes and have demonstrated that reversing BBB rupture through Sig-1R activation may be another promising method for cerebral protection against IR injury.

Sigma Receptors as Endoplasmic Reticulum Stress "Gatekeepers" and their Modulators as Emerging New Weapons in the Fight Against Cancer

Despite the interest aroused by sigma receptors (SRs) in the area of oncology, their role in tumor biology remains enigmatic. The predominant subcellular localization and main site of activity of SRs are the endoplasmic reticulum (ER). Current literature data, including recent findings on the sigma 2 receptor subtype (S2R) identity, suggest that SRs may play a role as ER stress gatekeepers. Although SR endogenous ligands are still unknown, a wide series of structurally unrelated compounds able to bind SRs have been identified. Currently, the identification of novel antiproliferative molecules acting via SR interaction is a challenging task for both academia and industry, as shown by the fact that novel anticancer drugs targeting SRs are in the preclinical-stage pipeline of pharmaceutical companies (i.e., Anavex Corp. and Accuronix). So far, no clinically available anticancer drugs targeting SRs are still available. The present review focuses literature advancements and provides a state-of-the-art overview of SRs, with emphasis on their involvement in cancer biology and on the role of SR modulators as anticancer agents. Findings from preclinical studies on novel anticancer drugs targeting SRs are presented in brief.

TRPV1 channels and the progesterone receptor Sig-1R interact to regulate pain

The Transient Receptor Potential Vanilloid 1 (TRPV1) ion channel is expressed in nociceptors where, when activated by chemical or thermal stimuli, it functions as an important transducer of painful and itch-related stimuli. Although the interaction of TRPV1 with proteins that regulate its function has been previously explored, their modulation by chaperones has not been elucidated, as is the case for other mammalian TRP channels. Here we show that TRPV1 physically interacts with the Sigma 1 Receptor (Sig-1R), a chaperone that binds progesterone, an antagonist of Sig-1R and an important neurosteroid associated to the modulation of pain. Antagonism of Sig-1R by progesterone results in the down-regulation of TRPV1 expression in the plasma membrane of sensory neurons and, consequently, a decrease in capsaicin-induced nociceptive responses. This is observed both in males treated with a synthetic antagonist of Sig-1R and in pregnant females where progesterone levels are elevated. This constitutes a previously undescribed mechanism by which TRPV1-dependent nociception and pain can be regulated.

Sigma-1 Receptor Plays a Negative Modulation on N-type Calcium Channel

The sigma-1 receptor is a 223 amino acids molecular chaperone with a single transmembrane domain. It is resident to eukaryotic mitochondrial-associated endoplasmic reticulum and plasma membranes. By chaperone-mediated interactions with ion channels, G-protein coupled receptors and cell-signaling molecules, the sigma-1 receptor performs broad physiological and pharmacological functions. Despite sigma-1 receptors have been confirmed to regulate various types of ion channels, the relationship between the sigma-1 receptor and N-type Ca²⁺ channel is still unclear. Considering both sigma-1 receptors and N-type Ca²⁺ channels are involved in intracellular calcium homeostasis and neurotransmission, we undertake studies to explore the possible interaction between these two proteins. In the experiment, we confirmed the expression of the sigma-1 receptors and the N-type calcium channels in the cholinergic interneurons (ChIs) in rat striatum by using single-cell reverse transcription-polymerase chain reaction (scRT-PCR) and immunofluorescence staining. N-type Ca²⁺ currents recorded from ChIs in the brain slice of rat striatum was depressed when sigma-1 receptor agonists (SKF-10047 and Pre-084) were administrated. The inhibition was completely abolished by sigma-1 receptor antagonist (BD-1063). Co-expression of the sigma-1 receptors and the N-type calcium channels in Xenopus oocytes presented a decrease of N-type Ca²⁺ current amplitude with an increase of sigma-1 receptor expression. SKF-10047 could further depress N-type Ca²⁺ currents recorded from oocytes. The fluorescence resonance energy transfer (FRET) assays and co-immunoprecipitation (Co-IP) demonstrated that sigma-1 receptors and N-type Ca²⁺ channels formed a protein complex when they were co-expressed in HEK-293T (Human Embryonic Kidney -293T) cells. Our results revealed that the sigma-1 receptors played a negative modulation on N-type Ca²⁺ channels. The mechanism for the inhibition of sigma-1 receptors on N-type Ca²⁺ channels probably involved a chaperone-mediated direct interaction and agonist-induced conformational changes in the receptor-channel complexes on the cell surface.

The sigma-1 receptor modulates dopamine transporter conformation and cocaine binding and may thereby potentiate cocaine self-administration in rats

The dopamine transporter (DAT) regulates dopamine (DA) neurotransmission by recapturing DA into the presynaptic terminals and is a principal target of the psychostimulant cocaine. The sigma-1 receptor (σ₁R) is a molecular chaperone, and its ligands have been shown to modulate DA neuronal signaling, although their effects on DAT activity are unclear. Here, we report that the prototypical σ₁R agonist (+)-pentazocine potentiated the dose response of cocaine self-administration in rats, consistent with the effects of the σR agonists PRE-084 and DTG (1,3-di-o-tolylguanidine) reported previously. These behavioral effects appeared to be correlated with functional changes of DAT. Preincubation with (+)-pentazocine or PRE-084 increased the B_max values of [³H]WIN35428 binding to DAT in rat striatal synaptosomes and transfected cells. A specific interaction between σ₁R and DAT was detected by co-immunoprecipitation and bioluminescence resonance energy transfer assays. Mutational analyses indicated that the transmembrane domain of σ₁R likely mediated this interaction. Furthermore, cysteine accessibility assays showed that σ₁R agonist preincubation potentiated cocaine-induced changes in DAT conformation, which were blocked by the specific σ₁R antagonist CM304. Moreover, σ₁R ligands had distinct effects on σ₁R multimerization. CM304 increased the proportion of multimeric σ₁Rs, whereas (+)-pentazocine increased monomeric σ₁Rs. Together these results support the hypothesis that σ₁R agonists promote dissociation of σ₁R multimers into monomers, which then interact with DAT to stabilize an outward-facing DAT conformation and enhance cocaine binding. We propose that this novel molecular mechanism underlies the behavioral potentiation of cocaine self-administration by σ₁R agonists in animal models.

A Review of the Human Sigma-1 Receptor Structure

The Sigma-1 Receptor (S1R) is a small, ligand-regulated integral membrane protein involved in cell homeostasis and the cellular stress response. The receptor has a multitude of protein and small molecule interaction partners with therapeutic potential. Newly reported structures of the human S1R in ligand-bound states provides essential insights into small molecule binding in the context of the overall protein structure. The structure also raises many interesting questions and provides an excellent starting point for understanding the molecular tricks employed by this small membrane receptor to modulate a large number of signaling events. Here, we review insights from the structures of ligand-bound S1R in the context of previous biochemical studies and propose, from a structural viewpoint, a set of important future directions.

Pharmacological Modulation of the Sigma 1 Receptor and the Treatment of Pain

There is a critical need for new analgesics acting through new mechanisms of action, which could increase the efficacy with respect to existing therapies and reduce their unwanted effects. Current preclinical evidence supports the modulatory role of sigma-1 receptors (σ1R) in nociception, mainly based on the pain-attenuated phenotype of σ1R knockout mice and on the antinociceptive effect exerted by σ1R antagonists on pains of different etiologies. σ1R is highly expressed in different pain areas of the CNS and the periphery (particularly dorsal root ganglia), and interacts and modulates the functionality of different receptors and ion channels . The antagonism of σ1R leads to decreased amplification of pain signaling within the spinal cord (central sensitization), but recent data also support a role at the periphery. σ1R antagonists have consistently demonstrated efficacy in neuropathic pain , but also in other types of pain including inflammatory, orofacial, visceral, and post-operative pain. Apart from acting alone, when combined with opioids, σ1R antagonists enhance opioid analgesia but not opioid-induced unwanted effects. Interestingly, unlike opioids, σ1R antagonists do not modify normal sensory mechanical and thermal sensitivity thresholds but they exert antihypersensitive effects in sensitizing conditions, enabling the reversal of nociceptive thresholds back to normal values. Accordingly, σ1R antagonists are not strictly analgesics; they are antiallodynic and antihyperalgesic drugs acting when the system is sensitized following prolonged noxious stimulation or persistent abnormal afferent input (e.g., secondary to nerve injury). These are distinctive features allowing σ1R antagonists to exert a modulatory effect specifically in pathophysiological conditions such as chronic pain .

The Sigma-1 Receptor-A Therapeutic Target for the Treatment of ALS?

The membrane bound 223 amino acid Sigma-1 Receptor (S1R) serves as a molecular chaperone and functional regulator of many signaling proteins. Spinal cord motor neuron activation occurs, in part, via large ventral horn cholinergic synapses called C-boutons/C-terminals. Chronic excitation of motor neurons and alterations in C-terminals has been associated with Amyotrophic Lateral Sclerosis (ALS ). The S1R has an important role in regulating motor neuron function. High levels of the S1R are localized in postsynaptic endoplasmic reticulum (ER) subsurface cisternae within 10-20 nm of the plasma membrane that contain muscarinic type 2 acetylcholine receptors (M2AChR), calcium activated potassium channels (Kv2.1) and slow potassium (SK) channels. An increase in action potentials in the S1R KO mouse motor neurons indicates a critical role for the S1R as a "brake" on motor neuron function possibly via calcium dependent hyperpolarization mechanisms involving the aforementioned potassium channels. The longevity of SOD-1/S1R KO ALS mice is significantly reduced compared to SOD-1/WT ALS controls. The S1R colocalizes in C-terminals with Indole(ethyl)amine-N-methyl transferase (INMT ), the enzyme that produces the S1R agonist , N,N'- dimethyltryptamine (DMT). INMT methylation can additionally neutralize endogenous toxic sulfur and selenium derivatives thus providing functional synergism with DMT to reduce oxidative stress in motor neurons . Small molecule activation of the S1R and INMT thus provides a possible therapeutic strategy to treat ALS .

Sigma-1 Receptor in Motoneuron Disease

Amyotrophic Lateral Sclerosis (ALS ) is a neurodegenerative disease affecting spinal cord and brain motoneurons , leading to paralysis and early death. Multiple etiopathogenic mechanisms appear to contribute in the development of ALS , including glutamate excitotoxicity, oxidative stress , protein misfolding, mitochondrial defects, impaired axonal transport, inflammation and glial cell alterations. The Sigma-1 receptor is highly expressed in motoneurons of the spinal cord, particularly enriched in the endoplasmic reticulum (ER) at postsynaptic cisternae of cholinergic C-terminals. Several evidences point to participation of Sigma-1R alterations in motoneuron degeneration. Thus, mutations of the transmembrane domain of the Sigma-1R have been described in familial ALS cases. Interestingly, Sigma-1R KO mice display muscle weakness and motoneuron loss. On the other hand, Sigma-1R agonists promote neuroprotection and neurite elongation through activation of protein kinase C on motoneurons in vitro and in vivo after ventral root avulsion. Remarkably, treatment of SOD1 mice, the most usual animal model of ALS , with Sigma-1R agonists resulted in significantly enhanced motoneuron function and preservation, and increased animal survival. Sigma-1R activation also reduced microglial reactivity and increased the glial expression of neurotrophic factors. Two main interconnected mechanisms seem to underlie the effects of Sigma-1R manipulation on motoneurons: modulation of neuronal excitability and regulation of calcium homeostasis. In addition, Sigma-1R also contributes to regulating protein degradation, and reducing oxidative stress. Therefore, the multi-functional nature of the Sigma-1R represents an attractive target for treating aspects of ALS and other motoneuron diseases .

Sigma 1 Receptor and Ion Channel Dynamics in Cancer

SigmaR1 is a multitasking chaperone protein which has mainly been studied in CNS physiological and pathophysiological processes such as pain, memory, neurodegenerative diseases (amyotrophic lateral sclerosis , Parkinson's and Alzheimer's diseases, retinal neurodegeneration ), stroke and addiction . Strikingly, G-protein and ion channels are the main client protein fami lies of this atypical chaperone and the recent advances that have been performed for the last 10 years demonstrate that SigmaR1 is principally activated following tissue injury and disease development to promote cell survival. In this chapter, we synthesize the data enhancing our comprehension of the interaction between SigmaR1 and ion channels and the unexpected consequences of such functional coupling in cancer development. We also describe a model in which the pro-survival functions of SigmaR1 observed in CNS pathologies are hijacked by cancer cells to shape their electrical signature and behavior in response to the tumor microenvironment .

Sigma-1 Receptor and Pain

There is a critical need for new analgesics acting through new mechanisms of action, which could increase the efficacy respect to existing therapies and/or reduce their unwanted effects. Current preclinical evidence supports the modulatory role of the sigma-1 receptor (σ1R) in nociception, mainly based on the pain-attenuated phenotype of σ1R knockout mice and on the antinociceptive effect exerted by σ1R antagonists on pain of different etiology, very consistently in neuropathic pain, but also in nociceptive, inflammatory, and visceral pain. σ1R is highly expressed in different pain areas of the CNS and the periphery, particularly dorsal root ganglia (DRG), and interacts and modulates the functionality of different receptors and ion channels. Accordingly, antinociceptive effects of σ1R antagonists both acting alone and in combination with other analgesics have been reported at both central and peripheral sites. At the central level, behavioral, electrophysiological, neurochemical, and molecular findings support a role for σ1R antagonists in inhibiting augmented excitability secondary to sustained afferent input. Moreover, the involvement of σ1R in mechanisms regulating pain at the periphery has been recently confirmed. Unlike opioids, σ1R antagonists do not modify normal sensory mechanical and thermal sensitivity thresholds but they exert antihypersensitivity effects (antihyperalgesic and antiallodynic) in sensitizing conditions, enabling the reversal of nociceptive thresholds back to normal values. These are distinctive features allowing σ1R antagonists to exert a modulatory effect specifically in pathophysiological conditions such as chronic pain.

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.

Role of peripheral sigma-1 receptors in ischaemic pain: Potential interactions with ASIC and P2X receptors

BACKGROUND

The role of peripheral sigma-1 receptors (Sig-1Rs) in normal nociception and in pathologically induced pain conditions has not been thoroughly investigated. Since there is mounting evidence that Sig-1Rs modulate ischaemia-induced pathological conditions, we investigated the role of Sig-1Rs in ischaemia-induced mechanical allodynia (MA) and addressed their possible interaction with acid-sensing ion channels (ASICs) and P2X receptors at the ischaemic site.

METHODS

We used a rodent model of hindlimb thrombus-induced ischaemic pain (TIIP) to investigate their role. Western blot was performed to observe changes in Sig-1R expression in peripheral nervous tissues. MA was measured after intraplantar (i.pl.) injections of antagonists for the Sig-1, ASIC and P2X receptors in TIIP rats or agonists of each receptor in naïve rats.

RESULTS

Sig-1R expression significantly increased in skin, sciatic nerve and dorsal root ganglia at 3 days post-TIIP surgery. I.pl. injections of the Sig-1R antagonist, BD-1047 on post-operative days 0-3 significantly attenuated the development of MA during the induction phase, but had no effect on MA when given during the maintenance phase (days 3-6 post-surgery). BD-1047 synergistically increased amiloride (an ASICs blocker)- and TNP-ATP (a P2X antagonist)-induced analgesic effects in TIIP rats. In naïve rats, i.pl. injection of Sig-1R agonist PRE-084 alone did not produce MA; but it did induce MA when co-administered with either an acidic pH solution or a sub-effective dose of αβmeATP.

CONCLUSION

Peripheral Sig-1Rs contribute to the induction of ischaemia-induced MA via facilitation of ASICs and P2X receptors. Thus, peripheral Sig-1Rs represent a novel therapeutic target for the treatment of ischaemic pain.

The Sigma-1 Receptor as a Pluripotent Modulator in Living Systems

The sigma-1 receptor (Sig-1R) is an endoplasmic reticulum (ER) protein that resides specifically in the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM), an interface between ER and mitochondria. In addition to being able to translocate to the plasma membrane (PM) to interact with ion channels and other receptors, Sig-1R also occurs at the nuclear envelope, where it recruits chromatin-remodeling factors to affect the transcription of genes. Sig-1Rs have also been reported to interact with other membranous or soluble proteins at other loci, including the cytosol, and to be involved in several central nervous system (CNS) diseases. Here, we propose that Sig-1R is a pluripotent modulator with resultant multiple functional manifestations in living systems.

SIGMAR1 Regulates Membrane Electrical Activity in Response to Extracellular Matrix Stimulation to Drive Cancer Cell Invasiveness

The sigma 1 receptor (Sig1R) is a stress-activated chaperone that regulates ion channels and is associated with pathologic conditions, such as stroke, neurodegenerative diseases, and addiction. Aberrant expression levels of ion channels and Sig1R have been detected in tumors and cancer cells, such as myeloid leukemia and colorectal cancer, but the link between ion channel regulation and Sig1R overexpression during malignancy has not been established. In this study, we found that Sig1R dynamically controls the membrane expression of the human voltage-dependent K(+) channel human ether-à-go-go-related gene (hERG) in myeloid leukemia and colorectal cancer cell lines. Sig1R promoted the formation of hERG/β1-integrin signaling complexes upon extracellular matrix stimulation, triggering the activation of the PI3K/AKT pathway. Consequently, the presence of Sig1R in cancer cells increased motility and VEGF secretion. In vivo, Sig1R expression enhanced the aggressiveness of tumor cells by potentiating invasion and angiogenesis, leading to poor survival. Collectively, our findings highlight a novel function for Sig1R in mediating cross-talk between cancer cells and their microenvironment, thus driving oncogenesis by shaping cellular electrical activity in response to extracellular signals. Given the involvement of ion channels in promoting several hallmarks of cancer, our study also offers a potential strategy to therapeutically target ion channel function through Sig1R inhibition.

Involvement of ion channels and transporters in carcinoma angiogenesis and metastasis

Angiogenesis is a finely tuned process, which is the result of the equilibrium between pro- and antiangiogenic factors. In solid tumor angiogenesis, the balance is highly in favor of the production of new, but poorly functional blood vessels, initially intended to provide growing tumors with nutrients and oxygen. Among the numerous proteins involved in tumor development, several types of ion channels are overexpressed in tumor cells, as well as in stromal and endothelial cells. Ion channels thus actively participate in the different hallmarks of cancer, especially in tumor angiogenesis and metastasis. Indeed, from their strategic localization in the plasma membrane, ion channels are key operators of cell signaling, as they sense and respond to environmental changes. This review aims to decipher how ion channels of different families are intricately involved in the fundamental angiogenesis and metastasis hallmarks, which lead from a nascent tumor to systemic dissemination. An overview of the possible use of ion channels as therapeutic targets will also be given, showing that ion channel inhibitors or specific antibodies may provide effective tools, in the near future, in the treatment of carcinomas.

Biochemical Pharmacology of the Sigma-1 Receptor

The sigma-1 receptor (S1R) is a 223 amino acid two transmembrane (TM) pass protein. It is a non-ATP-binding nonglycosylated ligand-regulated molecular chaperone of unknown three-dimensional structure. The S1R is resident to eukaryotic mitochondrial-associated endoplasmic reticulum and plasma membranes with broad functions that regulate cellular calcium homeostasis and reduce oxidative stress. Several multitasking functions of the S1R are underwritten by chaperone-mediated direct (and indirect) interactions with ion channels, G-protein coupled receptors and cell-signaling molecules involved in the regulation of cell growth. The S1R is a promising drug target for the treatment of several neurodegenerative diseases related to cellular stress. In vitro and in vivo functional and molecular characteristics of the S1R and its interactions with endogenous and synthetic small molecules have been discovered by the use of pharmacologic, biochemical, biophysical, and molecular biology approaches. The S1R exists in monomer, dimer, tetramer, hexamer/octamer, and higher oligomeric forms that may be important determinants in defining the pharmacology and mechanism(s) of action of the S1R. A canonical GXXXG in putative TM2 is important for S1R oligomerization. The ligand-binding regions of S1R have been identified and include portions of TM2 and the TM proximal regions of the C terminus. Some client protein chaperone functions and interactions with the cochaperone 78-kDa glucose-regulated protein (binding immunoglobulin protein) involve the C terminus. Based on its biochemical features and mechanisms of chaperone action the possibility that the S1R is a member of the small heat shock protein family is discussed.

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.

Sigma-1 receptors and animal studies centered on pain and analgesia

INTRODUCTION

Neuropathic pain is difficult to relieve with standard analgesics and tends to be resistant to opioid therapy. Sigma-1 receptors activated during neuropathic injury may sustain pain. Neuropathic injury activates sigma-1 receptors, which results in activation of various kinases, modulates the activity of multiple ion channels, ligand activated ion channels and voltage-gated ion channels; alters monoamine neurotransmission and dampens opioid receptors G-protein activation. Activation of sigma-1 receptors tonically inhibits opioid receptor G-protein activation and thus dampens analgesic responses. Therefore, sigma-1 receptor antagonists are potential analgesics for neuropathic and adjuvants to opioid therapy.

AREAS COVERED

This article reviews the importance of sigma-1 receptors as pain generators in multiple animal models in order to illustrate both the importance of these unique receptors in pathologic pain and the potential benefits to sigma-1 receptor antagonists as analgesics.

EXPERT OPINION

Sigma-1 receptor antagonists have a great potential as analgesics for acute neuropathic injury (herpes zoster, acute postoperative pain and chemotherapy induced neuropathy) and may, as an additional benefit, prevent the development of chronic neuropathic pain. Antagonists are potentially effective as adjuvants to opioid therapy when used early to prevent analgesic tolerance. Drug development is complicated by the complexity of sigma-1 receptor pharmacodynamics and its multiple targets, the lack of a specific sigma-1 receptor antagonist, and potential side effects due to on-target toxicities (cognitive impairment, depression).

The sigma-1 receptors are present in monomeric and oligomeric forms in living cells in the presence and absence of ligands

The sigma-1 receptor (S1R) is a 223-amino-acid membrane protein that resides in the endoplasmic reticulum and the plasma membrane of some mammalian cells. The S1R is regulated by various synthetic molecules including (+)-pentazocine, cocaine and haloperidol and endogenous molecules such as sphingosine, dimethyltryptamine and dehydroepiandrosterone. Ligand-regulated protein chaperone functions linked to oxidative stress and neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS) and neuropathic pain have been attributed to the S1R. Several client proteins that interact with S1R have been identified including various types of ion channels and G-protein coupled receptors (GPCRs). When S1R constructs containing C-terminal monomeric GFP2 and YFP fusions were co-expressed in COS-7 cells and subjected to FRET spectrometry analysis, monomers, dimers and higher oligomeric forms of S1R were identified under non-liganded conditions. In the presence of the prototypic S1R agonist, (+)-pentazocine, however, monomers and dimers were the prevailing forms of S1R. The prototypic antagonist, haloperidol, on the other hand, favoured higher order S1R oligomers. These data, in sum, indicate that heterologously expressed S1Rs occur in vivo in COS-7 cells in multiple oligomeric forms and that S1R ligands alter these oligomeric structures. We suggest that the S1R oligomerization states may regulate its function(s).

Solution NMR studies reveal the location of the second transmembrane domain of the human sigma-1 receptor

The sigma-1 receptor (S1R) is a ligand-regulated membrane chaperone protein associated with endoplasmic reticulum stress response, and modulation of ion channel activities at the plasma membrane. We report here a solution NMR study of a S1R construct (S1R(Δ35)) in which only the first transmembrane domain and the eight-residue N-terminus have been removed. The second transmembrane helix is found to be composed of residues 91-107, which corresponds to the first steroid binding domain-like region. The cytosolic domain is found to contain three helices, and the secondary structure and backbone dynamics of the chaperone domain are consistent with that determined previously for the chaperone domain alone. The position of TM2 provides a framework for ongoing studies of S1R ligand binding and oligomerisation.

Role of the Sigma-1 receptor in Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disease affecting spinal cord motoneurons (MN) with an associative connection to Frontotemporal Lobar Dementia (FTLD). The endoplasmic reticulum (ER) bound Sigma-1 Receptor (S1R) chaperone protein localizes to specialized ER cisternae within 10 nm of the plasma membrane in spinal cord ventral horn cholinergic post synaptic C-terminals. Removal of the S1R gene in the Superoxide Dismutase-1 (SOD-1) mouse model of ALS exacerbated the neurodegenerative condition and resulted in a significantly reduced longevity when compared to the SOD-1/S1R wild type (WT) mouse. The proposed amelioration of the ALS phenotype by the S1R is likely due to a "brake" on excitation of the MN as evidenced by a reduction in action potential generation in the MN of the WT when compared to the S1R KO mouse MN. Although the precise signal transduction pathway(s) regulated by the S1R in the MN has/have not been elucidated at present, it is likely that direct or indirect functional interactions occur between the S1R in the ER cisternae with voltage gated potassium channels and/or with muscarinic M2 receptor signaling in the post synaptic plasma membrane. Possible mechanisms for regulation of MN excitability by S1R are discussed.

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.

The oligomeric states of the purified sigma-1 receptor are stabilized by ligands

Sigma-1 receptor (S1R) is a mammalian member of the ERG2 and sigma-1 receptor-like protein family (pfam04622). It has been implicated in drug addiction and many human neurological disorders, including Alzheimer and Parkinson diseases and amyotrophic lateral sclerosis. A broad range of synthetic small molecules, including cocaine, (+)-pentazocine, haloperidol, and small endogenous molecules such as N,N-dimethyltryptamine, sphingosine, and steroids, have been identified as regulators of S1R. However, the mechanism of activation of S1R remains obscure. Here, we provide evidence in vitro that S1R has ligand binding activity only in an oligomeric state. The oligomeric state is prone to decay into an apparent monomeric form when exposed to elevated temperature, with loss of ligand binding activity. This decay is suppressed in the presence of the known S1R ligands such as haloperidol, BD-1047, and sphingosine. S1R has a GXXXG motif in its second transmembrane region, and these motifs are often involved in oligomerization of membrane proteins. Disrupting mutations within the GXXXG motif shifted the fraction of the higher oligomeric states toward smaller states and resulted in a significant decrease in specific (+)-[(3)H]pentazocine binding. Results presented here support the proposal that S1R function may be regulated by its oligomeric state. Possible mechanisms of molecular regulation of interacting protein partners by S1R in the presence of small molecule ligands are discussed.

Selective sigma-1 receptor antagonists for the treatment of pain

The sigma-1 receptor (σ1R) is located in areas of the CNS key for pain control and belongs to a unique target class with chaperoning functions over different molecular targets involved in transmission and amplification of nociceptive messages. Preclinical evidence supports a role for σ1R antagonists in the treatment of pain states where hypersensitivity develops as hyperalgesia and allodynia, two common symptoms encountered in neuropathic and other chronic pain conditions. Additionally, σ1R antagonists increase opioid analgesia without increasing opioid-related unwanted effects, which point to their potential use as opioid adjuvant therapy. This review summarizes the structure and function of the σ1R as well as the medicinal chemistry and pharmacological studies directed to the identification of σ1R antagonists for the treatment of pain.

The σ-1 receptor interacts directly with GluN1 but not GluN2A in the GluN1/GluN2A NMDA receptor

The σ-1 receptor (Sig1R) is widely expressed in the CNS, where it has a neuroprotective role in ischemia and stroke and an involvement in schizophrenia. The Sig1R interacts functionally with a variety of ion channels, including the NMDA receptor (NMDAR). Here, we used atomic force microscopy (AFM) imaging to investigate the interaction between the Sig1R and the NMDAR. The Sig1R bound directly to GluN1/GluN2A NMDAR heterotetramers. Furthermore, the mean angle between pairs of bound Sig1Rs was 72°. This result suggested that the Sig1R interacts with either GluN1 or GluN2A, but not both, and supports our recent demonstration that the NMDAR subunits adopt an adjacent (i.e., 1/1/2/2) arrangement. The Sig1R could be coisolated with GluN1 but not with GluN2A, indicating that GluN1 is its specific target within the NMDAR. Consistent with this conclusion, AFM imaging of coisolated Sig1R and GluN1 revealed GluN1 dimers decorated with Sig1Rs. In situ proximity ligation assays demonstrated that the Sig1R interacts with GluN1 (but not with GluN2A) within intact cells and also that its C terminus is extracellular. We conclude that the Sig1R binds to the GluN1/GluN2A NMDAR specifically via the GluN1 subunit. This interaction likely accounts for at least some of the modulatory effects of Sig1R ligands on the NMDAR.

Sigma-1 receptor enhances neurite elongation of cerebellar granule neurons via TrkB signaling

Sigma-1 receptor (Sig-1R) is an integral membrane protein predominantly expressed in the endoplasmic reticulum. Sig-1R demonstrates a high affinity to various synthetic compounds including well-known psychotherapeutic drugs in the central nervous system (CNS). For that, it is considered as an alternative target for psychotherapeutic drugs. On the cellular level, when Sig-1R is activated, it is known to play a role in neuroprotection and neurite elongation. These effects are suggested to be mediated by its ligand-operated molecular chaperone activity, and/or upregulation of various Ca(2+) signaling. In addition, recent studies show that Sig-1R activation induces neurite outgrowth via neurotrophin signaling. Here, we tested the hypothesis that Sig-1R activation promotes neurite elongation through activation of tropomyosin receptor kinase (Trk), a family of neurotrophin receptors. We found that 2-(4-morpholinethyl)1-phenylcyclohexanecarboxylate (PRE-084), a selective Sig-1R agonist, significantly promoted neurite outgrowth, and K252a, a Trk inhibitor, attenuated Sig-1R-mediated neurite elongation in cerebellar granule neurons (CGNs). Moreover, we revealed that Sig-1R interacts with TrkB, and PRE-084 treatment enhances phosphorylation of Y515, but not Y706. Thus, our results indicate that Sig-1R activation promotes neurite outgrowth in CGNs through Y515 phosphorylation of TrkB.

The sigma-1 receptor: a regulator of cancer cell electrical plasticity?

Originally mistaken as an opioid receptor, the sigma-1 receptor (Sig1R) is a ubiquitous membrane protein that has been involved in many cellular processes. While the precise function of Sig1R has long remained mysterious, recent studies have shed light on its role and the molecular mechanisms triggered. Sig1R is in fact a stress-activated chaperone mainly associated with the ER-mitochondria interface that can regulate cell survival through the control of calcium homeostasis. Sig1R functionally regulates ion channels belonging to various molecular families and it has thus been involved in neuronal plasticity and central nervous system diseases. Interestingly, Sig1R is frequently expressed in tumors but its function in cancer has not been yet clarified. In this review, we discuss the current understanding of Sig1R. We suggest herein that Sig1R shapes cancer cell electrical signature upon environmental conditions. Thus, Sig1R may be used as a novel therapeutic target to specifically abrogate pro-invasive functions of ion channels in cancer tissue.

α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors adopt different subunit arrangements

Ionotropic glutamate receptors are widely distributed in the central nervous system and play a major role in excitatory synaptic transmission. All three ionotropic glutamate subfamilies (i.e. AMPA-type, kainate-type, and NMDA-type) assemble as tetramers of four homologous subunits. There is good evidence that both heteromeric AMPA and kainate receptors have a 2:2 subunit stoichiometry and an alternating subunit arrangement. Recent studies based on presumed structural homology have indicated that NMDA receptors adopt the same arrangement. Here, we use atomic force microscopy imaging of receptor-antibody complexes to show that whereas the GluA1/GluA2 AMPA receptor assembles with an alternating (i.e. 1/2/1/2) subunit arrangement, the GluN1/GluN2A NMDA receptor adopts an adjacent (i.e. 1/1/2/2) arrangement. We conclude that the two types of ionotropic glutamate receptor are built in different ways from their constituent subunits. This surprising finding necessitates a reassessment of the assembly of these important receptors.

Improved expression and purification of sigma 1 receptor fused to maltose binding protein by alteration of linker sequence

Sigma 1 receptor (S1R) is a eukaryotic membrane protein that functions as an inter-organelle signaling modulator and chaperone. Here we report an improved expression of S1R in Escherichia coli as a fusion to maltose binding protein (MBP) and a high-yield purification. Variants with linking amino acid sequences consisting of 0-5 alanine residues between MBP and S1R were created and tested in several E. coli expression strains in order to determine the best combination of construct and host for production of active MBP-S1R. Among the linker variations, the protein containing a 4-Ala linker exhibited superior expression characteristics (MBP-4A-S1R); this construct was most productively paired with E. coli B834-pRARE2 and a chemically defined growth and expression medium. A 3-step purification was developed, including extraction from the E. coli membrane fraction using a mixture of Triton X-100 and n-dodecyl-beta-D-maltopyranoside identified by screening constrainted by retention of binding function, and purification by amylose affinity and gel filtration chromatographies. This procedure yields ∼3.5mg of purified fusion protein per L of bacterial culture medium. Purified MBP-4A-S1R showed a 175-fold purification from the starting cellular lysate with respect to specific ligand binding activity, and is stable during concentration and freeze-thaw cycling.

Photoaffinity labeling of the sigma-1 receptor with N-[3-(4-nitrophenyl)propyl]-N-dodecylamine: evidence of receptor dimers

The sigma-1 receptor is a ligand-regulated endoplasmic reticulum (ER) resident chaperone involved in the maintenance of cellular homeostasis. Coupling of the sigma-1 receptor with various ER and/or plasma membrane ion channels is associated with its ability to regulate the locomotor activity and cellular proliferation produced in response to sigma-1 receptor ligands. A number of endogenous small molecules bind to the sigma-1 receptor and have been shown to regulate its activity; these include progesterone, N,N-dimethyltryptamine, d-erythro-sphingosine, and/or other endogenous lipids. We previously reported the synthesis of long chain N-alkylamine derivatives and the characterization of the structure-activity relationship between the chain length of N-alkylamine and affinities at the sigma-1 receptor. Here, we present data demonstrating the photoincorporation of one of these N-alkylamine derivatives, N-[3-(4-nitrophenyl)propyl]-N-dodecylamine (4-NPPC12), to the sigma-1 receptor. Matrix-assisted laser desorption ionization time-of-flight and tandem mass spectrometry showed that 4-NPPC12 photoinserted at histidine 154 of the derivatized population of the sigma-1 receptor. Interestingly, light-dependent photoinsertion of 4-NPPC12 resulted in an enhanced electrophoretic mobility of only 50% of the derivatized receptor molecules as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The proposed binding and reactivity of 4-NPPC12 evoke a ligand binding model for the sigma-1 receptor that likely involves a receptor dimer and/or oligomer.

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.