Characterization of the human sigma-1 receptor chaperone domain structure and binding immunoglobulin protein (BiP) interactions

Sigma-1 receptor regulates the endoplasmic reticulum stress pathway in the protective mechanism of dexmedetomidine against hyperoxia-induced lung injury

Perioperative hyperoxia therapy is of great significance to save the lives of patients, but little is known about the possible mechanisms that induce hyperoxia-induced acute lung injury (HALI) and the measures for clinical prevention and treatment. In this experiment, the models were established with a feeding chamber with automatic regulation of oxygen concentration. The results showed that with the increase in inhaled oxygen concentration and the prolongation of exposure time, the severity of lung injury also increases significantly, reaching the diagnostic indication of HALI after 48 h of inhaling 95 % oxygen concentration. Subsequently, according to the dynamic changes of apoptosis in lung specimens, and the expression changes in Sig-1R-regulated ER stress pathway proteins (Sig-1R, GRP78, p-PERK, ATF6, IRE1, Caspase-12, ATF4, CHOP, Caspase-3 and p-JNK), it was confirmed that the Sig-1R-regulated ER stress signaling pathway was involved in the occurrence of HALI. To explore the preventive and therapeutic effects of routine clinical medication on HALI during the perioperative period, our research group selected dexmedetomidine (Dex) with lung protection. The experimental results revealed that Dex partially reversed the changes in the expression levels of Sig-1R-regulated ER stress pathway proteins. These results preliminarily confirmed that Dex may inhibit apoptosis induced by high oxygen concentration through the Sig-1R-regulated ER stress signaling pathway, thus playing a protective role in HALI.

The Protective Role of Mitochondria-Associated Endoplasmic Reticulum Membrane (MAM) Protein Sigma-1 Receptor in Regulating Endothelial Inflammation and Permeability Associated with Acute Lung Injury

Earlier studies from our lab identified endoplasmic reticulum (ER) chaperone BiP/GRP78, an important component of MAM, to be a novel determinant of endothelial cell (EC) dysfunction associated with acute lung injury (ALI). Sigma1R (Sig1R) is another unique ER receptor chaperone that has been identified to associate with BiP/GRP78 at the MAM and is known to be a pluripotent modulator of cellular homeostasis. However, it is unclear if Sig1R also plays a role in regulating the EC inflammation and permeability associated with ALI. Our data using human pulmonary artery endothelial cells (HPAECs) showed that siRNA-mediated knockdown of Sig1R potentiated LPS-induced the expression of proinflammatory molecules ICAM-1, VCAM-1 and IL-8. Consistent with this, Sig1R agonist, PRE-084, known to activate Sig1R by inducing its dissociation from BiP/GRP78, blunted the above response. Notably, PRE-084 failed to blunt LPS-induced inflammatory responses in Sig1R-depleted cells, confirming that the effect of PRE-084 is driven by Sig1R. Furthermore, Sig1R antagonist, NE-100, known to inactivate Sig1R by blocking its dissociation from BiP/GRP78, failed to block LPS-induced inflammatory responses, establishing that dissociation from BiP/GRP78 is required for Sig1R to exert its anti-inflammatory action. Unlike Sig1R, the siRNA-mediated knockdown or Subtilase AB-mediated inactivation of BiP/GRP78 protected against LPS-induced EC inflammation. Interestingly, the protective effect of BiP/GRP78 knockdown or inactivation was abolished in cells that were depleted of Sig1R, confirming that BiP/GRP78 knockdown/inactivation-mediated suppression of EC inflammation is mediated via Sig1R. In view of these findings, we determined the in vivo relevance of Sig1R in a mouse model of sepsis-induced ALI. The intraperitoneal injection of PRE-084 mitigated sepsis-induced ALI, as evidenced by a decrease in ICAM-1, IL-6 levels, lung PMN infiltration, and lung vascular leakage. Together, these data evidence a protective role of Sig1R against endothelial dysfunction associated with ALI and identify it as a viable target in terms of controlling ALI in sepsis.

Hybrid neural network approaches to predict drug-target binding affinity for drug repurposing: screening for potential leads for Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease that primarily affects elderly individuals. Recent studies have found that sigma-1 receptor (S1R) agonists can maintain endoplasmic reticulum stress homeostasis, reduce neuronal apoptosis, and enhance mitochondrial function and autophagy, making S1R a target for AD therapy. Traditional experimental methods are costly and inefficient, and rapid and accurate prediction methods need to be developed, while drug repurposing provides new ways and options for AD treatment. In this paper, we propose HNNDTA, a hybrid neural network for drug-target affinity (DTA) prediction, to facilitate drug repurposing for AD treatment. The study combines protein-protein interaction (PPI) network analysis, the HNNDTA model, and molecular docking to identify potential leads for AD. The HNNDTA model was constructed using 13 drug encoding networks and 9 target encoding networks with 2506 FDA-approved drugs as the candidate drug library for S1R and related proteins. Seven potential drugs were identified using network pharmacology and DTA prediction results of the HNNDTA model. Molecular docking simulations were further performed using the AutoDock Vina tool to screen haloperidol and bromperidol as lead compounds for AD treatment. Absorption, distribution, metabolism, excretion, and toxicity (ADMET) evaluation results indicated that both compounds had good pharmacokinetic properties and were virtually non-toxic. The study proposes a new approach to computer-aided drug design that is faster and more economical, and can improve hit rates for new drug compounds. The results of this study provide new lead compounds for AD treatment, which may be effective due to their multi-target action. HNNDTA is freely available at https://github.com/lizhj39/HNNDTA.

Pharmacological Analysis of GABAA Receptor and Sigma1R Chaperone Interaction: Research Report I-Investigation of the Anxiolytic, Anticonvulsant and Hypnotic Effects of Allosteric GABAA Receptors' Ligands

Two groups of facts have been established in previous drug development studies of the non-benzodiazepine anxiolytic fabomotizole. First, fabomotizole prevents stress-induced decrease in binding ability of the GABA_A receptor's benzodiazepine site. Second, fabomotizole is a Sigma1R chaperone agonist, and exposure to Sigma1R antagonists blocks its anxiolytic effect. To prove our main hypothesis of Sigma1R involvement in GABA_A receptor-dependent pharmacological effects, we performed a series of experiments on BALB/c and ICR mice using Sigma1R ligands to study anxiolytic effects of benzodiazepine tranquilizers diazepam (1 mg/kg i.p.) and phenazepam (0.1 mg/kg i.p.) in the elevated plus maze test, the anticonvulsant effects of diazepam (1 mg/kg i.p.) in the pentylenetetrazole-induced seizure model, and the hypnotic effects of pentobarbital (50 mg/kg i.p.). Sigma1R antagonists BD-1047 (1, 10, and 20 mg/kg i.p.), NE-100 (1 and 3 mg/kg i.p.), and Sigma1R agonist PRE-084 (1, 5, and 20 mg/kg i.p.) were used in the experiments. Sigma1R antagonists have been found to attenuate while Sigma1R agonists can enhance GABA_ARs-dependent pharmacological effects.

Sphingoid Bases Regulate the Sigma-1 Receptor-Sphingosine and N,N'-Dimethylsphingosine Are Endogenous Agonists

Both bioactive sphingolipids and Sigma-1 receptor (S1R) chaperones occur ubiquitously in mammalian cell membranes. Endogenous compounds that regulate the S1R are important for controlling S1R responses to cellular stress. Herein, we interrogated the S1R in intact Retinal Pigment Epithelial cells (ARPE-19) with the bioactive sphingoid base, sphingosine (SPH), or the pain-provoking dimethylated SPH derivative, N,N'-dimethylsphingosine (DMS). As informed by a modified native gel approach, the basal and antagonist (BD-1047)-stabilized S1R oligomers dissociated to protomeric forms in the presence of SPH or DMS (PRE-084 as control). We, thus, posited that SPH and DMS are endogenous S1R agonists. Consistently, in silico docking of SPH and DMS to the S1R protomer showed strong associations with Asp126 and Glu172 in the cupin beta barrel and extensive van der Waals interactions of the C18 alkyl chains with the binding site including residues in helices 4 and 5. Mean docking free energies were 8.73-8.93 kcal/mol for SPH and 8.56-8.15 kcal/mol for DMS, and calculated binding constants were ~40 nM for SPH and ~120 nM for DMS. We hypothesize that SPH, DMS, and similar sphingoid bases access the S1R beta barrel via a membrane bilayer pathway. We further propose that the enzymatic control of ceramide concentrations in intracellular membranes as the primary sources of SPH dictates availability of endogenous SPH and DMS to the S1R and the subsequent control of S1R activity within the same cell and/or in cellular environments.

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.

Chaperone-Dependent Mechanisms as a Pharmacological Target for Neuroprotection

Modern pharmacotherapy of neurodegenerative diseases is predominantly symptomatic and does not allow vicious circles causing disease development to break. Protein misfolding is considered the most important pathogenetic factor of neurodegenerative diseases. Physiological mechanisms related to the function of chaperones, which contribute to the restoration of native conformation of functionally important proteins, evolved evolutionarily. These mechanisms can be considered promising for pharmacological regulation. Therefore, the aim of this review was to analyze the mechanisms of endoplasmic reticulum stress (ER stress) and unfolded protein response (UPR) in the pathogenesis of neurodegenerative diseases. Data on BiP and Sigma1R chaperones in clinical and experimental studies of Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease are presented. The possibility of neuroprotective effect dependent on Sigma1R ligand activation in these diseases is also demonstrated. The interaction between Sigma1R and BiP-associated signaling in the neuroprotection is discussed. The performed analysis suggests the feasibility of pharmacological regulation of chaperone function, possibility of ligand activation of Sigma1R in order to achieve a neuroprotective effect, and the need for further studies of the conjugation of cellular mechanisms controlled by Sigma1R and BiP chaperones.

Ligand-Guided Investigation of a Series of Formamidine-Based Thiuram Disulfides as Potential Dual-Inhibitors of COX-1and COX-2

A series of thiuram disulfides 1-6 which had been previously synthesized and characterized,^[1] were studied for their potential therapeutic properties. Target-fishing analyses through HitPick and SwissTarget prediction identified COX1 and COX2, which are essential biomolecules in cancer-related inflammations, as the possible targets for compounds 1 and 4 among all the compounds tested. These two proteins have enjoyed interest as targets for treating some neoplastic cancer types such as breast, colorectal, skin, pancreatic, haematological and head cancers. The inhibitory potency of 1 and 4 as lead anticancer drug candidates with dual-target ability against COX1 and COX2 was examined through molecular docking, molecular dynamics simulation and post-MD analyses such as binding energy calculation, RMSD, RMSF, and RoG. The two compounds had better docking scores and binding energies than the known inhibitors of COX1 and COX2. Insights from the RMSD, RMSF, and RoG suggested that both 1 and 4 showed observable influence on the structural stability of these targets throughout the simulation. The reported observations of the effects of 1 and 4 on the structures of COX1 and COX2 indicate their probable inhibitory properties against these target proteins and their potential as lead anticancer drug candidates.

αN-Acetyl β-Endorphin Is an Endogenous Ligand of σ1Rs That Regulates Mu-Opioid Receptor Signaling by Exchanging G Proteins for σ2Rs in σ1R Oligomers

The opioid peptide β-endorphin coexists in the pituitary and brain in its ^αN-acetylated form, which does not bind to opioid receptors. We now report that these neuropeptides exhibited opposite effects in in vivo paradigms, in which ligands of the sigma type 1 receptor (σ1R) displayed positive effects. Thus, ^αN-acetyl β-Endorphin reduced vascular infarct caused by permanent unilateral middle cerebral artery occlusion and diminished the incidence of N-methyl-D-aspartate acid-promoted convulsive syndrome and mechanical allodynia caused by unilateral chronic constriction of the sciatic nerve. Moreover, ^αN-acetyl β-Endorphin reduced the analgesia of morphine, β-Endorphin and clonidine but enhanced that of DAMGO. All these effects were counteracted by β-Endorphin and absent in σ1R^-/- mice. We observed that σ1Rs negatively regulate mu-opioid receptor (MOR)-mediated morphine analgesia by binding and sequestering G proteins. In this scenario, β-Endorphin promoted the exchange of σ2Rs by G proteins at σ1R oligomers and increased the regulation of G proteins by MORs. The opposite was observed for the ^αN-acetyl derivative, as σ1R oligomerization decreased and σ2R binding was favored, which displaced G proteins; thus, MOR-regulated transduction was reduced. Our findings suggest that the pharmacological β-Endorphin-specific epsilon receptor is a σ1R-regulated MOR and that β-Endorphin and ^αN-acetyl β-Endorphin are endogenous ligands of σ1R.

Interleukin 24 promotes cell death in renal epithelial cells and is associated with acute renal injury

Ischemia-reperfusion injury is a major cause of acute kidney injury. Many cytokines are involved in the pathogenesis of renal ischemia-reperfusion injury. IL24 is a member of the IL10 family and has gained importance because of its apoptosis-inducing effects in tumor disease besides its immunoregulative function. Littles is known about the role of IL24 in kidney disease. Using a mouse model, we found that IL24 is upregulated in the kidney after renal ischemia-reperfusion injury and that tubular epithelial cells and infiltrating inflammatory cells are the source of IL24. Mice lacking IL24 are protected from renal injury and inflammation. Cell culture studies showed that IL24 induces apoptosis in renal tubular epithelial cells, which is accompanied by an increased endoplasmatic reticulum stress response. Moreover, IL24 induces robust expression of endogenous IL24 in tubular cells, fostering ER-stress and apoptosis. In kidney transplant recipients with delayed graft function and patients at high risk to develop acute kidney injury after cardiac surgery IL24 is upregulated in the kidney and serum. Taken together, IL24 can serve as a biomarker, plays an important mechanistic role involving both extracellular and intracellular targets, and is a promising therapeutic target in patients at risk of or with ischemia-induced acute kidney injury.

Haloperidol Prevents Oxytosis/Ferroptosis by Targeting Lysosomal Ferrous Ions in a Manner Independent of Dopamine D2 and Sigma-1 Receptors

Haloperidol is a widely used antipsychotic agent that exerts antipsychotic effects through a strong antagonism of dopamine D2 receptors. In addition, haloperidol is classified as a sigma-1 receptor (S1R) antagonist that prevents endogenous oxidative stress in cultured cells. However, pharmacological activities of haloperidol against oxidative stress remain unclear. Oxytosis/ferroptosis are iron-dependent nonapoptotic oxidative cell deaths that are regarded as two names for the same cell death pathway and the potential physiological relevance of oxytosis/ferroptosis in multiple diseases is suggested. In the present study, the effects of haloperidol on oxytosis/ferroptosis were investigated in S1R-knockdown mouse hippocampal HT22 cells. The results indicate that haloperidol is a strong inhibitor of oxytosis/ferroptosis independent of S1R. Imaging of HT22 cells with a newly developed fluorescent probe showed that haloperidol was localized to late endosomes and lysosomes and reduced the accumulation of lysosomal ferrous ions, resulting in reduced production of intracellular reactive oxygen species and inhibition of cell death. These results indicate that haloperidol is useful not only as an antipsychotic agent but also as a neuroprotective agent against endogenous oxidative stress via distinct mechanisms. Furthermore, lysosome-targeting ferroptosis inhibitors could be useful for the treatment of various diseases, including cancers, ischemia-reperfusion injury, and neurodegenerative disorders, which have been associated with ferroptosis.

Structures of the SARS-CoV-2 spike glycoprotein and applications for novel drug development

COVID-19 caused by SARS-CoV-2 has raised a health crisis worldwide. The high morbidity and mortality associated with COVID-19 and the lack of effective drugs or vaccines for SARS-CoV-2 emphasize the urgent need for standard treatment and prophylaxis of COVID-19. The receptor-binding domain (RBD) of the glycosylated spike protein (S protein) is capable of binding to human angiotensin-converting enzyme 2 (hACE2) and initiating membrane fusion and virus entry. Hence, it is rational to inhibit the RBD activity of the S protein by blocking the RBD interaction with hACE2, which makes the glycosylated S protein a potential target for designing and developing antiviral agents. In this study, the molecular features of the S protein of SARS-CoV-2 are highlighted, such as the structures, functions, and interactions of the S protein and ACE2. Additionally, computational tools developed for the treatment of COVID-19 are provided, for example, algorithms, databases, and relevant programs. Finally, recent advances in the novel development of antivirals against the S protein are summarized, including screening of natural products, drug repurposing and rational design. This study is expected to provide novel insights for the efficient discovery of promising drug candidates against the S protein and contribute to the development of broad-spectrum anti-coronavirus drugs to fight against SARS-CoV-2.

Repurposing the Antiplatelet Agent Ticlopidine to Counteract the Acute Phase of ER Stress Condition: An Opportunity for Fighting Coronavirus Infections and Cancer

Different pathological conditions, including viral infections and cancer, can have a massive impact on the endoplasmic reticulum (ER), causing severe damage to the cell and exacerbating the disease. In particular, coronavirus infections, including SARS coronavirus-2 (SARS-CoV-2), responsible for COVID-19, cause ER stress as a consequence of the enormous amounts of viral glycoproteins synthesized, the perturbation of ER homeostasis and the modification of ER membranes. Therefore, ER has a central role in the viral life cycle, thus representing one of the Achilles’ heels on which to focus therapeutic intervention. On the other hand, prolonged ER stress has been demonstrated to promote many pro-tumoral attributes in cancer cells, having a key role in tumor growth, metastasis and response to therapies. In this report, adopting a repurposing approach of approved drugs, we identified the antiplatelet agent ticlopidine as an interferent of the unfolded protein response (UPR) via sigma receptors (SRs) modulation. The promising results obtained suggest the potential use of ticlopidine to counteract ER stress induced by viral infections, such as COVID-19, and cancer.

EST79232 and EST79376, Two Novel Sigma-1 Receptor Ligands, Exert Neuroprotection on Models of Motoneuron Degeneration

Motor neuron diseases (MNDs) include sporadic and hereditary neurological disorders characterized by progressive degeneration of motor neurons (MNs). Sigma-1 receptor (Sig-1R) is a protein enriched in MNs, and mutations on its gene lead to various types of MND. Previous studies have suggested that Sig-1R is a target to prevent MN degeneration. In this study, two novel synthesized Sig-1R ligands, coded EST79232 and EST79376, from the same chemical series, with the same scaffold and similar physicochemical properties but opposite functionality on Sig-1R, were evaluated as neuroprotective compounds to prevent MN degeneration. We used an in vitro model of spinal cord organotypic cultures under chronic excitotoxicity and two in vivo models, the spinal nerve injury and the superoxide dismutase 1 (SOD1)G93A mice, to characterize the effects of these Sig-1R ligands on MN survival and modulation of glial reactivity. The antagonist EST79376 preserved MNs in vitro and after spinal nerve injury but was not able to improve MN death in SOD1G93A mice. In contrast, the agonist EST79232 significantly increased MN survival in the three models of MN degeneration evaluated and had a mild beneficial effect on motor function in SOD1G93A mice. In vivo, Sig-1R ligand EST79232 had a more potent effect on preventing MN degeneration than EST79376. These data further support the interest in Sig-1R as a therapeutic target for neurodegeneration.

Mitochondria Fusion upon SERCA Inhibition Prevents Activation of the NLRP3 Inflammasome in Human Monocytes

Sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) is a crucial component of the cellular machinery responsible for Ca²⁺ homeostasis. The selective inhibition of SERCA by thapsigargin (TG) leads to perturbations in Ca²⁺ signaling, which can trigger endoplasmic reticulum (ER) stress. The unfolded protein response (UPR) pathway is activated in response to ER stress and induces an adaptive response to preserve cell survival or committee cells to programmed death, depending on stress duration and/or level. Early stages of ER stress stimulate mitochondrial metabolism to preserve survival but under chronic ER stress conditions, mitochondrial dysfunction is induced, which, in turn, can enhance inflammation through NLRP3 inflammasome activation. This study was aimed at investigating the role of SERCA inhibition on NLRP3 inflammasome activation in human monocytes, which was evaluated in primary monocytes isolated from healthy individuals and in the THP-1 human monocytic cell line. Findings obtained in both THP-1 and primary monocytes demonstrate that SERCA inhibition triggered by TG does not activate the NLRP3 inflammasome in these innate immune cells since IL-1β secretion was not affected. Results from THP-1 monocytes showing that SERCA inhibition increases mitochondrial Ca²⁺ content and fusion, in the absence of changes in ROS levels and membrane potential, support the view that human monocytes counteract ER stress that arises from inhibition of SERCA through modulation of mitochondrial morphology towards mitochondria fusion, thus preventing NLRP3 inflammasome activation. Overall, this work contributes to a better understanding of the molecular mechanisms that modulate the activity of the NLRP3 inflammasome leading to sterile inflammation, which are still poorly understood.

Synthesis of aminoethyl substituted piperidine derivatives as σ1 receptor ligands with antiproliferative properties

A series of novel σ₁ receptor ligands with a 4‐(2‐aminoethyl)piperidine scaffold was prepared and biologically evaluated. The underlying concept of our project was the improvement of the lipophilic ligand efficiency of previously synthesized potent σ₁ ligands. The key steps of the synthesis comprise the conjugate addition of phenylboronic acid at dihydropyridin‐4(1H)‐ones 7, homologation of the ketones 8 and introduction of diverse amino moieties and piperidine N‐substituents. 1‐Methylpiperidines showed particular high σ₁ receptor affinity and selectivity over the σ₂ subtype, whilst piperidines with a proton, a tosyl moiety or an ethyl moiety exhibited considerably lower σ₁ affinity. Molecular dynamics simulations with per‐residue binding free energy deconvolution demonstrated that different interactions of the basic piperidine‐N‐atom and its substituents (or the cyclohexane ring) with the lipophilic binding pocket consisting of Leu105, Thr181, Leu182, Ala185, Leu186, Thr202 and Tyr206 are responsible for the different σ₁ receptor affinities. Recorded logD_7.4 and calculated clogP values of 4a and 18a indicate low lipophilicity and thus high lipophilic ligand efficiency. Piperidine 4a inhibited the growth of human non‐small cell lung cancer cells A427 to a similar extent as the σ₁ antagonist haloperidol. 1‐Methylpiperidines 20a, 21a and 22a showed stronger antiproliferative effects on androgen negative human prostate cancer cells DU145 than the σ₁ ligands NE100 and S1RA.

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.

Superoxide dismutase and the sigma1 receptor as key elements of the antioxidant system in human gastrointestinal tract cancers

This long-term research was designed to evaluate whether superoxide dismutase (SOD) isoenzymes participate in the development of human gastrointestinal neoplasms and the potential influence of the sigma1 receptor (Sig1R) on the regulation of SOD gene expression during the neoplastic process. The experiments included human tissues from selected gastrointestinal tract tumors (liver cancer, colorectal adenocarcinoma, and colorectal cancer liver metastases). Activity, protein levels, and mRNA levels were determined for SOD isoenzymes and Sig1R. Additionally, markers of oxidative stress (glutathione, lipid peroxidation) were measured. The results showed significant changes in the antioxidant system activity in all examined types of tumors. SOD changed both in healthy cells and in neoplastic cells. The activity and expression of all studied enzymes significantly changed due to the advancement of tumor development. The Sig1R might be an additional regulator of the antioxidant system on which activity might depend on the survival and proliferation of cancer cells. Overall, the study shows that SOD1 and SOD2 are involved not only in the formation of neoplastic changes in the human gastrointestinal tissues (healthy intestine - colon tumor; healthy liver - liver cirrhosis - liver cancer) but also in the development of tumors in the sequence: benign tumor - malignant tumor - metastasis.

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.

Role of Sigma-1 Receptor in Calcium Modulation: Possible Involvement in Cancer

Ca2+ signaling plays a pivotal role in the control of cellular homeostasis and aberrant regulation of Ca2+ fluxes have a strong impact on cellular functioning. As a consequence of this ubiquitous role, Ca2+ signaling dysregulation is involved in the pathophysiology of multiple diseases including cancer. Indeed, multiple studies have highlighted the role of Ca2+ fluxes in all the steps of cancer progression. In particular, the transfer of Ca2+ at the ER-mitochondrial contact sites, also known as mitochondrial associated membranes (MAMs), has been shown to be crucial for cancer cell survival. One of the proteins enriched at this site is the sigma-1 receptor (S1R), a protein that has been described as a Ca2+-sensitive chaperone that exerts a protective function in cells in various ways, including the modulation of Ca2+ signaling. Interestingly, S1R is overexpressed in many types of cancer even though the exact mechanisms by which it promotes cell survival are not fully elucidated. This review summarizes the findings describing the roles of S1R in the control of Ca2+ signaling and its involvement in cancer progression.

PB28, the Sigma-1 and Sigma-2 Receptors Modulator With Potent Anti-SARS-CoV-2 Activity: A Review About Its Pharmacological Properties and Structure Affinity Relationships

These unprecedented times have forced the scientific community to gather to face the COVID-19 pandemic. Efforts in diverse directions have been made. A multi-university team has focused on the identification of the host (human) proteins interacting with SARS-CoV-2 viral proteins, with the aim of hampering these interactions that may cause severe COVID-19 symptoms. Sigma-1 and sigma-2 receptors surprisingly belong to the “druggable” host proteins found, with the pan-sigma receptor modulator PB28 displaying the most potent anti–SARS-CoV-2 activity in in vitro assays. Being 20-fold more active than hydroxychloroquine, without cardiac side effects, PB28 is a promising antiviral candidate worthy of further investigation. Our research group developed PB28 in 1996 and have thoroughly characterized its biological properties since then. Structure–affinity relationship (SAfiR) studies at the sigma receptor subtypes were also undertaken with PB28 as the lead compound. We herein report our knowledge of PB28 to share information that may help to gain insight into the antiviral action of this compound and sigma receptors, while providing structural hints that may speed up the translation into therapeutics of this class of ligands.

Repurposing Sigma-1 Receptor Ligands for COVID-19 Therapy?

Outbreaks of emerging infections, such as COVID-19 pandemic especially, confront health professionals with the unique challenge of treating patients. With no time to discover new drugs, repurposing of approved drugs or in clinical development is likely the only solution. Replication of coronaviruses (CoVs) occurs in a modified membranous compartment derived from the endoplasmic reticulum (ER), causes host cell ER stress and activates pathways to facilitate adaptation of the host cell machinery to viral needs. Accordingly, modulation of ER remodeling and ER stress response might be pivotal in elucidating CoV-host interactions and provide a rationale for new therapeutic, host-based antiviral approaches. The sigma-1 receptor (Sig-1R) is a ligand-operated, ER membrane-bound chaperone that acts as an upstream modulator of ER stress and thus a candidate host protein for host-based repurposing approaches to treat COVID-19 patients. Sig-1R ligands are frequently identified in in vitro drug repurposing screens aiming to identify antiviral compounds against CoVs, including severe acute respiratory syndrome CoV-2 (SARS-CoV-2). Sig-1R regulates key mechanisms of the adaptive host cell stress response and takes part in early steps of viral replication. It is enriched in lipid rafts and detergent-resistant ER membranes, where it colocalizes with viral replicase proteins. Indeed, the non-structural SARS-CoV-2 protein Nsp6 interacts with Sig-1R. The activity of Sig-1R ligands against COVID-19 remains to be specifically assessed in clinical trials. This review provides a rationale for targeting Sig-1R as a host-based drug repurposing approach to treat COVID-19 patients. Evidence gained using Sig-1R ligands in unbiased in vitro antiviral drug screens and the potential mechanisms underlying the modulatory effect of Sig-1R on the host cell response are discussed. Targeting Sig-1R is not expected to reduce dramatically established viral replication, but it might interfere with early steps of virus-induced host cell reprogramming, aid to slow down the course of infection, prevent the aggravation of the disease and/or allow a time window to mature a protective immune response. Sig-1R-based medicines could provide benefit not only as early intervention, preventive but also as adjuvant therapy.

Deferred Administration of Afobazole Induces Sigma1R-Dependent Restoration of Striatal Dopamine Content in a Mouse Model of Parkinson's Disease

Previously, we demonstrated that the immediate administration of multitarget anxiolytic afobazole slows down the progression of neuronal damage in a 6-hydroxidodamine (6-OHDA) model of Parkinson's disease due to the activation of chaperone Sigma1R. The aim of the present study is to evaluate the therapeutic potential of deferred afobazole administration in this model. Male ICR mice received a unilateral 6-OHDA lesion of the striatum. Fourteen days after the surgery, mice were treated with afobazole, selective Sigma1R agonist PRE-084, selective Sigma1R antagonist BD-1047, and a combination of BD-1047 with afobazole or PRE-084 for another 14 days. The deferred administration of afobazole restored the intrastriatal dopamine content in the 6-OHDA-lesioned striatum and facilitated motor behavior in rotarod tests. The action of afobazole accorded with the effect of Sigma1R selective agonist PRE-084 and was blocked by Sigma1R selective antagonist BD-1047. The present study illustrates the Sigma1R-dependent effects of afobazole in a 6-OHDA model of Parkinson's disease and reveals the therapeutic potential of Sigma1R agonists in treatment of the condition.

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.

The multi-faceted impact of methamphetamine on Alzheimer's disease: From a triggering role to a possible therapeutic use

Although it has been initially synthesized for therapeutic purposes and currently FDA-approved and prescribed for obesity, attention-deficit/hyperactivity disorder, narcolepsy and depression, methamphetamine became a recreational drug that is nowadays massively manufactured illegally. Because it is a powerful and extremely addictive psychotropic agent, its abuse has turned out to become a major health problem worldwide. Importantly, the numerous effects triggered by this drug induce neurotoxicity in the brain ultimately leading to serious neurological impairments, tissue damage and neuropsychological disturbances that are reminiscent to most of the symptoms observed in Alzheimer's disease and other pathological manifestations in aging brain. In this context, there is a growing number of compelling evidence linking methamphetamine abuse with a higher probability of developing premature Alzheimer's disease and consequent neurodegeneration. This review proposes to establish a broad assessment of the effects that this drug can generate at the cellular and molecular levels in connection with the development of the age-related Alzheimer's disease. Altogether, the objective is to warn against the long-term effects that methamphetamine abuse may convey on young consumers and the increased risk of developing this devastating brain disorder at later stages of their lives, but also to discuss a more recently emerging concept suggesting a possible use of methamphetamine for treating this pathology under proper and strictly controlled conditions.

Sigma-1 receptor is involved in diminished ovarian reserve possibly by influencing endoplasmic reticulum stress-mediated granulosa cells apoptosis

Sigma non-opioid intracellular receptor 1 (sigma-1 receptor), a non-opioid transmembrane protein, is located on cellular mitochondrial membranes and endoplasmic reticulum. Current research has demonstrated that sigma-1 receptor is related to human degenerative diseases. This study is focused on the effects of sigma-1 receptor on the pathophysiological process of diminished ovarian reserve (DOR) and granulosa cells (GCs) apoptosis. Sigma-1 receptor concentration in follicular fluid (FF) and serum were negatively correlated with basal follicle-stimulating hormone (FSH) and positively correlated with anti-mullerian hormone (AMH), antral follicle count (AFC). Sigma-1 receptor reduction in GCs was accompanied by endoplasmic reticulum stress (ERS)-mediated apoptosis in women with DOR. Plasmid transfection was used to establish SIGMAR1-overexpressed and SIGMAR1-knockdown human granulosa-like tumor (KGN) cell and thapsigargin (TG) was used to induce ERS KGN cells. We found that KGN cells treated with endogenous sigma-1 receptor ligand dehydroepiandrosterone (DHEA) and sigma-1 receptor agonist PRE-084 showed similar biological effects to SIGMAR1-overexpressed KGN cells and opposite effects to SIGMAR1-knockdown KGN cells. DHEA may improve DOR patients' pregnancy outcomes by upregulating sigma-1 receptor and downregulating ERS-mediated apoptotic genes in GCs. Thus, sigma-1 receptor may be a potential ovarian reserve biomarker, and ligand-mediated sigma-1 receptor activation could be a future approach for DOR therapy.

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.

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.

Chaperone Sigma1R mediates the neuroprotective action of afobazole in the 6-OHDA model of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease with limited treatment options. Therefore, the identification of therapeutic targets is urgently needed. Previous studies have shown that the ligand activation of the sigma-1 chaperone (Sigma1R) promotes neuroprotection. The multitarget drug afobazole (5-ethoxy-2-[2-(morpholino)-ethylthio]benzimidazole dihydrochloride) was shown to interact with Sigma1Rs and prevent decreases in striatal dopamine in the 6-hydroxydopamine (6-OHDA)-induced parkinsonism model. The aim of the present study was to elucidate the role of Sigma1Rs in afobazole pharmacological activity. Using ICR mice we found that administration of afobazole (2.5 mg/kg, i.p.) or selective agonist of Sigma1R PRE-084 (1.0 mg/kg, i.p.) over 14 days normalizes motor disfunction and prevents decreases in dopamine in the 6-OHDA-lesioned striatum. Afobazole administration also prevents the loss of TH + neurons in the substantia nigra. The pre-administration of selective Sigma1R antagonist BD-1047 (3.0 mg/kg, i.p.) abolishes the activity of either afobazole or PRE-084, as determined using the rotarod test and the analysis of striatal dopamine content. The current study demonstrates the contribution of Sigma1Rs in the neuroprotective effect of afobazole in the 6-OHDA model of Parkinson's disease and defines the therapeutic perspective of Sigma1R agonists in the clinic.

Novel missense alleles of SIGMAR1 as tools to understand emerin-dependent gene silencing in response to cocaine

Sigma-1 receptor (Sigma1R; SIGMAR1 ), an integral membrane protein of the endoplasmic reticulum and nuclear envelope, has a hydrophobic drug-binding pocket that binds with high affinity to addictive drugs (cocaine, methamphetamine) and therapeutics used to treat a wide spectrum of neurological disorders. Cocaine enhances Sigma1R association with three nuclear lamina proteins (emerin, lamin A/C, BANF1), causing Sigma1R-dependent and emerin-dependent recruitment and transcriptional repression of a gene, MAOB1 , involved in dopamine removal from neural synapses. The mechanism of Sigma1R association with emerin and the molecular impact of cocaine on their association are unknown. Mutations in Sigma1R, as a proposed regulator or mis-regulator of the nuclear lamina, have the potential to alter nuclear lamina function in brain or other tissues. We examined the frequency of SIGMAR1 missense alleles among 60,706 unrelated individuals in the ExAC database. We identified two novel SIGMAR1 missense variants of particular interest due to their frequency and potential to impact molecular association with emerin or other nuclear lamina proteins. Variant p.Q2P was widespread in ExAC (overall allele frequency 18.4%) with broad ethnic distribution among non-Finnish Europeans, Africans, South Asians, Latinx (allele frequencies ∼15% to 23%), and East Asians (∼38%). The p.R208W allele was identified in ∼0.78% of individuals overall with enrichment in Africans, Latinx, and East Asians (∼1.9–2.9%). These and other novel Sigma1R variants provide tools for future studies to determine the molecular basis of Sigma1R association with emerin and the mechanism of nuclear lamina misregulation by cocaine and potentially other Sigma1R agonists.

Cocaine-induced endocannabinoid signaling mediated by sigma-1 receptors and extracellular vesicle secretion

Cocaine is an addictive drug that acts in brain reward areas. Recent evidence suggests that cocaine stimulates synthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG) in midbrain, increasing dopamine neuron activity via disinhibition. Although a mechanism for cocaine-stimulated 2-AG synthesis is known, our understanding of 2-AG release is limited. In NG108 cells and mouse midbrain tissue, we find that 2-AG is localized in non-synaptic extracellular vesicles (EVs) that are secreted in the presence of cocaine via interaction with the chaperone protein sigma-1 receptor (Sig-1R). The release of EVs occurs when cocaine causes dissociation of the Sig-1R from ADP-ribosylation factor (ARF6), a G-protein regulating EV trafficking, leading to activation of myosin light chain kinase (MLCK). Blockade of Sig-1R function, or inhibition of ARF6 or MLCK also prevented cocaine-induced EV release and cocaine-stimulated 2-AG-modulation of inhibitory synapses in DA neurons. Our results implicate the Sig-1R-ARF6 complex in control of EV release and demonstrate that cocaine-mediated 2-AG release can occur via EVs.

Mutational Analysis of Sigma-1 Receptor's Role in Synaptic Stability

Sigma-1 receptor (S1R) is an endoplasmic reticulum (ER) resident transmembrane protein. In our previous experiments, we demonstrated neuroprotective effects of pridopidine, an agonist of S1R, in cellular and animal models of Huntington’s disease (HD) and Alzheimer’s disease (AD). Consistent with previous observations, deletion of endogenous S1R with CRISPR/Cas9 in cultured hippocampal neurons resulted in fewer mushroom-shaped dendritic spines. Overexpression of human S1R restored mushroom spine density to control levels. In contrast, overexpression of S1R with the Δ31–50 deletion (linked to distal hereditary motor neuropathy) or the E102Q mutation (linked to amyotrophic lateral sclerosis) destabilized mushroom spines. Recently a crystal structure of S1R was determined in lipidic cubic phase. In the present study, we took an advantage of this structural information and performed docking studies with pridopidine and the S1R structural model. We generated a series of S1R point mutations based on residues predicted to be involved in direct association with pridopidine. We discovered that all ligand binding-site mutants were able to compensate for loss of endogenous S1R. However, most of these mutants were not able to support pridopidine-induced rescue of mushroom spines in presenilin-1-mutant cultures. Our mutational analysis was in agreement with in silico docking based on the published S1R crystal structure, with an exception of R119 residue. Our data also suggest that basal S1R activity is required for mature spine stability, whereas agonist-mediated S1R activity is required for stabilization of mushroom spines in the context of disease-causing mutations.

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.

The Sigma-1 Receptor in Cellular Stress Signaling

After decades of research, the sigma-1 receptor (Sig-1R)’s structure, and molecular functions are being unveiled. Sig-1R is an integral endoplasmic reticulum (ER) membrane protein which forms an oligomer and binds a variety of psychotropic drugs. It forms a complex with the ER chaperone BiP that controls specific signaling molecules’ stability and function at the ER to regulate Ca²⁺ signaling, bioenergetics, and ER stress. Sig-1R is highly enriched in ER subdomains that are physically linked to outer mitochondrial membranes, reflecting its role in regulating ER–mitochondria communications. Thus, Sig-1R ligands are expected to serve as novel neuroprotective agents which treat certain psychiatric and neurodegenerative disorders. In this short review, the cell biological aspects of Sig-1R are discussed, with a particular focus on its role in fundamental ER functions.

The Roles of Intracellular Chaperone Proteins, Sigma Receptors, in Parkinson's Disease (PD) and Major Depressive Disorder (MDD)

Sigma receptors, including Sigma-1 receptors and Sigma-2 receptors, are highly expressed in the CNS. They are intracellular chaperone proteins. Sigma-1 receptors localize mainly at the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM). Upon stimulation, they translocate from MAM to plasma membrane (PM) and nucleus, where they interact with many proteins and ion channels. Sigma-1 receptor could interact with itself to form oligomers, its oligomerization states affect its ability to interact with client proteins including ion channels and BiP. Sigma-1 receptor shows high affinity for many unrelated and structurally diverse ligands, but the mechanism for this diverse drug receptor interaction remains unknown. Sigma-1 receptors also directly bind many proteins including G protein-coupled receptors (GPCRs) and ion channels. In recent years, significant progress has been made in our understanding of roles of the Sigma-1 receptors in normal and pathological conditions, but more studies are still required for the Sigma-2 receptors. The physiological roles of Sigma-1 receptors in the CNS are discussed. They can modulate the activity of many ion channels including voltage-dependent ion channels including Ca²⁺, Na⁺, K⁺ channels and NMDAR, thus affecting neuronal excitability and synaptic activity. They are also involved in synaptic plasticity and learning and memory. Moreover, the activation of Sigma receptors protects neurons from death via the modulation of ER stress, neuroinflammation, and Ca²⁺ homeostasis. Evidences about the involvement of Sigma-1 receptors in Parkinson’s disease (PD) and Major Depressive Disorder (MDD) are also presented, indicating Sigma-1 receptors might be promising targets for pharmacologically treating PD and MDD.

Molecular Interplay Between the Sigma-1 Receptor, Steroids, and Ion Channels

Cell excitability is tightly regulated by the activity of ion channels that allow for the passage of ions across cell membranes. Ion channel activity is controlled by different mechanisms that change their gating properties, expression or abundance in the cell membrane. The latter can be achieved by forming complexes with a diversity of proteins like chaperones such as the Sigma-1 receptor (Sig-1R), which is one with unique features and exhibits a role as a ligand-operated chaperone. This molecule also displays high intracellular mobility according to its activation level since, depletion of internal Ca⁺² stores or the presence of specific ligands, produce Sig-1R’s mobilization from the endoplasmic reticulum toward the plasma membrane or nuclear envelope. The function of the Sig-1R as a chaperone is regulated by synthetic and endogenous ligands, with some of these compounds being a steroids and acting as key endogenous modifiers of the actions of the Sig-1R. There are cases in the literature that exemplify the close relationship between the actions of steroids on the Sig-1R and the resulting negative or positive effects on ion channel function/abundance. Such interactions have been shown to importantly influence the physiology of mammalian cells leading to changes in their excitability. The present review focuses on describing how the Sig-1R regulates the functional properties and the expression of some sodium, calcium, potassium, and TRP ion channels in the presence of steroids and the physiological consequences of these interplays at the cellular level are also discussed.

Selective serotonin reuptake inhibitor fluvoxamine ameliorates stress- and NSAID-induced peptic ulcer possibly by involving Hsp70

Background Selective serotonin reuptake inhibitors (SSRIs) have recently become potential candidates for a new therapeutic approach to ulcer and gastric bleeding. Heat shock protein 70 (Hsp70) plays an important role in cellular resistance to nonsteroidal anti-inflammatory drugs (NSAIDs). However, there is lack of evidence that fluvoxamine recruits Hsp70 to affect stress-induced gastric ulcer. Therefore, we investigated the effect of fluvoxamine on NSAID- and stress-induced gastric ulcer and the possible involvement of Hsp70. Methods ICR mice were used in the study. Stress induction was made by the water-immersion-plus-restraint method. NSAID-induced gastric ulcer was produced by oral administration of indomethacin. Fluvoxamine was given orally 30 min before stress induction and indomethacin treatment. Results Stress and indomethacin treatment significantly increased the ulcer index and intraluminal bleeding score. Stress and indomethacin treatment also significantly increased the expression of Hsp70. Fluvoxamine significantly decreased the ulcer index and intraluminal bleeding in both ulcer models. Moreover, fluvoxamine further increased the expression of Hsp70 in the gastric tissue of stress- and indomethacin-treated mice. Conclusions Our results indicate that fluvoxamine may have a protective effect against stress- as well as NSAID-induced gastric ulcer. In addition, the present study suggests the possible involvement of Hsp70 in the amelioration of gastric ulcer by fluvoxamine.

Pharmacological profiling of sigma 1 receptor ligands by novel receptor homomer assays

The sigma 1 receptor (σ1R) is a structurally unique transmembrane protein that functions as a molecular chaperone in the endoplasmic reticulum (ER), and has been implicated in cancer, neuropathic pain, and psychostimulant abuse. Despite physiological and pharmacological significance, mechanistic underpinnings of structure-function relationships of σ1R are poorly understood, and molecular interactions of selective ligands with σ1R have not been elucidated. The recent crystallographic determination of σ1R as a homo-trimer provides the foundation for mechanistic elucidation at the molecular level. Here we report novel bioluminescence resonance energy transfer (BRET) assays that enable analyses of ligand-induced multimerization of σ1R and its interaction with BiP. Haloperidol, PD144418, and 4-PPBP enhanced σ1R homomer BRET signals in a dose dependent manner, suggesting their significant effects in stabilizing σ1R multimerization, whereas (+)-pentazocine and several other ligands do not. In non-denaturing gels, (+)-pentazocine significantly decreased whereas haloperidol increased the fraction of σ1R multimers, consistent with the results from the homomer BRET assay. Further, BRET assays examining heteromeric σ1R-BiP interaction revealed that (+)-pentazocine and haloperidol induced opposite trends of signals. From molecular modeling and simulations of σ1R in complex with the tested ligands, we identified initial clues that may lead to the differed responses of σ1R upon binding of structurally diverse ligands. By combining multiple in vitro pharmacological and in silico molecular biophysical methods, we propose a novel integrative approach to analyze σ1R-ligand binding and its impact on interaction of σ1R with client proteins.

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.

Endocannabinoid control of glutamate NMDA receptors: the therapeutic potential and consequences of dysfunction

Glutamate is probably the most important excitatory neurotransmitter in the brain. The glutamate N-methyl-D-aspartate receptor (NMDAR) is a calcium-gated channel that coordinates with G protein-coupled receptors (GPCRs) to establish the efficiency of the synaptic transmission. Cross-regulation between these receptors requires the concerted activity of the histidine triad nucleotide-binding protein 1 (HINT1) and of the sigma receptor type 1 (σ1R). Essential brain functions like learning, memory formation and consolidation, mood and behavioral responses to exogenous stimuli depend on the activity of NMDARs. In this biological context, endocannabinoids are released to retain NMDAR activity within physiological limits. The efficacy of such control depends on HINT1/σ1R assisting in the physical coupling between cannabinoid type 1 receptors (CB1Rs) and NMDARs to dampen their activity. Subsequently, the calcium-regulated HINT1/σ1R protein tandem uncouples CB1Rs to prevent NMDAR hypofunction. Thus, early recruitment or a disproportionate cannabinoid induced response can bring about excess dampening of NMDAR activity, impeding its adequate integration with GPCR signaling. Alternatively, this control circuit can apparently be overridden in situations where bursts of NMDAR overactivity provoke convulsive syndromes. In this review we will discuss the possible relevance of the HINT1/σ1R tandem and its use by endocannabinoids to diminish NMDAR activity and their implications in psychosis/schizophrenia, as well as in NMDAR-mediated convulsive episodes.

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.

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.

The effect of the sigma-1 receptor selective compound LS-1-137 on the DOI-induced head twitch response in mice

Several receptor mediated pathways have been shown to modulate the murine head twitch response (HTR). However, the role of sigma receptors in the murine (±)-2,5-dimethoxy-4-iodoamphetamine (DOI)-induced HTR has not been previously investigated. We examined the ability of LS-1-137, a novel sigma-1 vs. sigma-2 receptor selective phenylacetamide, to modulate the DOI-induced HTR in DBA/2J mice. We also assessed the in vivo efficacy of reference sigma-1 receptor antagonists and agonists PRE-084 and PPCC. The effect of the sigma-2 receptor selective antagonist RHM-1-86 was also examined. Rotarod analysis was performed to monitor motor coordination after LS-1-137 administration. Radioligand binding techniques were used to determine the affinity of LS-1-137 at 5-HT2A and 5-HT2C receptors. LS-1-137 and the sigma-1 receptor antagonists haloperidol and BD 1047 were able to attenuate a DOI-induced HTR, indicating that LS-1-137 was acting in vivo as a sigma-1 receptor antagonist. LS-1-137 did not compromise rotarod performance within a dose range capable of attenuating the effects of DOI. Radioligand binding studies indicate that LS-1-137 exhibits low affinity binding at both 5-HT2A and 5-HT2C receptors. Based upon the results from these and our previous studies, LS-1-137 is a neuroprotective agent that attenuates the murine DOI-induced HTR independent of activity at 5-HT2 receptor subtypes, D2-like dopamine receptors, sigma-2 receptors and NMDA receptors. LS-1-137 appears to act as a sigma-1 receptor antagonist to inhibit the DOI-induced HTR. Therefore, the DOI-induced HTR can be used to assess the in vivo efficacy of sigma-1 receptor selective compounds.

Loss-of-function mutations in the SIGMAR1 gene cause distal hereditary motor neuropathy by impairing ER-mitochondria tethering and Ca2+ signalling

Distal hereditary motor neuropathies (dHMNs) are clinically and genetically heterogeneous neurological conditions characterized by degeneration of the lower motor neurons. So far, 18 dHMN genes have been identified, however, about 80% of dHMN cases remain without a molecular diagnosis. By a combination of autozygosity mapping, identity-by-descent segment detection and whole-exome sequencing approaches, we identified two novel homozygous mutations in the SIGMAR1 gene (p.E138Q and p.E150K) in two distinct Italian families affected by an autosomal recessive form of HMN. Functional analyses in several neuronal cell lines strongly support the pathogenicity of the mutations and provide insights into the underlying pathomechanisms involving the regulation of ER-mitochondria tethering, Ca2+ homeostasis and autophagy. Indeed, in vitro, both mutations reduce cell viability, the formation of abnormal protein aggregates preventing the correct targeting of sigma-1R protein to the mitochondria-associated ER membrane (MAM) and thus impinging on the global Ca2+ signalling. Our data definitively demonstrate the involvement of SIGMAR1 in motor neuron maintenance and survival by correlating, for the first time in the Caucasian population, mutations in this gene to distal motor dysfunction and highlight the chaperone activity of sigma-1R at the MAM as a critical aspect in dHMN pathology.

Aβ mediates Sigma receptor degradation via CaN/NFAT pathway

Sigma receptor is an endoplasmic reticulum protein and belongs to non-opioid receptor. Increasing evidence shows that Sigma receptor activation can significantly attenuate AD induced neurological dysfunction and the functional deficiency of Sigma receptor plays an important role in the Aβ induced neuronal loss. This study aimed to investigate the influence of extracellular accumulation of Aβ on the Sigma receptor expression. Our results showed the increase in extracellular Aβ had little influence on the mRNA expression of Sigma receptor, but gradually reduced its protein expression. Co-immunoprecipitation was employed to evaluate the interaction of Sigma receptor with other proteins. Results showed BIP could bind to Sigma receptor to affect the ubiquitination of Sigma receptor. Further investigation showed there was a NFAT binding site at the promoter of BIP. Then, Western blot assay was performed to detect NFAT expression. Results showed extracellular Aβ affected the nuclear translocation of NFAT and the CaN activity of NFAT also increased with the accumulation of extracellular Aβ. In this study, NFAT-BIP luciferase reporter gene system was constructed. Results showed NFAT was able to regulate the transcription of BIP. Thus, we speculate that extracellular Aβ accumulation may activate CaN/NFAT signaling pathway to induce chaperone BIP expression, which results in Sigma receptor ubiquitination and its degradation.

Mechanism of Action and Applications of Interleukin 24 in Immunotherapy

Interleukin 24 (IL-24) is an important pleiotropic immunoregulatory cytokine, whose gene is located in human chromosome 1q32-33. IL-24's signaling pathways have diverse biological functions related to cell differentiation, proliferation, development, apoptosis, and inflammation, placing it at the center of an active area of research. IL-24 is well known for its apoptotic effect in cancer cells while having no such effect on normal cells. IL-24 can also be secreted by both immune and non-immune cells. Downstream effects of IL-24, after binding to the IL-20 receptor, can occur dependently or independently of the JAK/STAT signal transduction pathway, which is classically involved in cytokine-mediated activities. After exogenous addition of IL-24, apoptosis is induced in tumor cells independently of the JAK/STAT pathway. We have shown that IL-24 binds to Sigma 1 Receptor and this event induces endoplasmic reticulum stress, calcium mobilization, reactive oxygen species generation, p38MAPK activity, and ceramide production. Here we review IL-24's role in autoimmunity, infectious disease response, wound repair, and vascular disease. Detailed understanding of the pleiotropic roles of IL-24 signaling can assist in the selection of more accurate therapeutic approaches, as well as targeting of appropriate cell types in treatment strategy development, and ultimately achieve desired therapeutic effects.