Sigma-1 Receptor Is Critical for Mitochondrial Activity and Unfolded Protein Response in Larval Zebrafish

Convolamine, a tropane alkaloid extracted from Convolvulus plauricalis, is a potent sigma-1 receptor-positive modulator with cognitive and neuroprotective properties

BACKGROUND AND AIM

By using an in vivo phenotypic screening assay in zebrafish, we identified Convolamine, a tropane alkaloid from Convulvus plauricalis, as a positive modulator of the sigma-1 receptor (S1R). The wfs1abKO zebrafish larva, a model of Wolfram syndrome, exhibits an increased visual-motor response due to a mutation in Wolframin, a protein involved in endoplasmic reticulum-mitochondria communication. We previously reported that ligand activating S1R, restored the cellular and behavioral deficits in patient fibroblasts and zebrafish and mouse models.

EXPERIMENTAL PROCEDURES

We screened a library of 108 repurposing and natural compounds on zebrafish motor response.

KEY RESULTS

One hit, the tropane alkaloid Convolamine, restored normal mobility in wfs1abKO larvae without affecting wfs1abWT controls. They did not bind to the S1R agonist/antagonist binding site nor dissociated S1R from BiP, an S1R activity assay in vitro, but behaved as a positive modulator by shifting the IC50 value of the reference agonist PRE-084 to lower values. Convolamine restored learning in Wfs1∆Exon8 , Dizocilpine-treated, and Aβ25-35 -treated mice. These effects were observed at low ~1 mg/kg doses, not shared by Convolvine, the desmethyl metabolite, and blocked by an S1R antagonist.

CONCLUSION AND IMPLICATIONS

Convolamine therefore acts as an S1R positive modulator and this pharmacological action is relevant to the traditional use of Shankhpushpi in memory and cognitive protection.

The Neuroprotective Effect of Activation of Sigma-1 Receptor on Neural Injury by Optic Nerve Crush

Purpose

This study aimed to explore the neuroprotective effects of sigma-1 receptor (S1R) on optic nerve crush (ONC) mice by upregulating its expression through intravitreal injection of adeno-associated virus (AAV).

Methods

The animals were divided into four groups. Mice that underwent ONC were administered an intravitreal injection with blank vector (ONC group), with AAV targeting downregulation of S1R (S1R-sh group), or with AAV targeting overexpression of S1R (S1R-AAV group). Mice in the control group underwent intravitreal injection with blank vector. The thickness of each layer of the retina was measured through optical coherence tomography, and the apoptotic rate of retinal neurons was determined using the TUNEL assay. The expression levels of brain-derived neurotrophic factor (BDNF) and S1R were quantified through western blot. Electroretinogram (ERG) was performed to evaluate the visual function.

Results

The thickness of the total retina (P = 0.001), ganglion cell layer (P = 0.017), and inner nuclear layer (P = 0.002) in S1R-AAV group was significantly thicker than that of the ONC group. The number of retinal apoptotic cells in the S1R-AAV group was 23% lower than that in the ONC group (P = 0.002). ERG results showed that, compared to the ONC group, the amplitudes of the a- and b-waves were higher in the S1R-AAV group (a-wave, P < 0.001; b-wave, P = 0.007). Western blot showed that the expression of BDNF in the S1R-AAV group was higher than that in the ONC group (P < 0.001).

Conclusions

Activation of S1R in the retina through intravitreal injection of AAV can effectively maintain the retina structure, promote neuronal cell survival, and protect visual function.

Sigma-1 receptor and seizures

Over the last decade, sigma-1 receptor (Sig1R) has been recognized as a valid target for the treatment of seizure disorders and seizure-related comorbidities. Clinical trials with Sig1R ligands are underway testing therapies for the treatment of drug-resistant seizures, developmental and epileptic encephalopathies, and photosensitive epilepsy. However, the direct molecular mechanism by which Sig1R modulates seizures and the balance between excitatory and inhibitory pathways has not been fully elucidated. This review article aims to summarize existing knowledge of Sig1R and its involvement in seizures by focusing on the evidence obtained from Sig1R knockout animals and the anti-seizure effects of Sig1R ligands. In addition, this review article includes a discussion of the advantages and disadvantages of the use of existing compounds and describes the challenges and future perspectives on the use of Sig1R as a target for the treatment of seizure disorders.

An investigation of Sigma-1 receptor expression and ligand-induced endoplasmic reticulum stress in breast cancer

Targeted therapeutic options and prognostic biomarkers for hormone receptor- or Her2 receptor-negative breast cancers are severely limited. The sigma-1 receptor, a stress-activated chaperone, is frequently dysregulated in disease. However, its significance in breast cancer (BCa) has not been adequately explored. Here, we report that the sigma-1 receptor gene (SIGMAR1) is elevated in BCa, particularly in the aggressive triple-negative (TNBC) subtype. By examining several patient datasets, we found that high expression at both the gene (SIGMAR1) and protein (Sig1R) levels associated with poor survival outcomes, specifically in ER-Her2- groups. Our data further show that high SIGMAR1 was predictive of shorter survival times in patients treated with adjuvant chemotherapy (ChT). Interestingly, in a separate cohort who received neoadjuvant taxane + anthracycline treatment, elevated SIGMAR1 associated with higher rates of pathologic complete response (pCR). Treatment with a Sig1R antagonist, 1-(4-iodophenyl)-3-(2-adamantyl)guanidine (IPAG), activated the unfolded protein response (UPR) in TNBC (high-Sig1R expressing) and ER + (low-Sig1R expressing) BCa cell lines. In tamoxifen-resistant LY2 cells, IPAG caused Sig1R to aggregate and co-localise with the stress marker BiP. These findings showcase the potential of Sig1R as a novel biomarker in TNBC as well as highlight its ligand-induced interference with the stress-coping mechanisms of BCa cells.

Overview of Sigma-1R Subcellular Specific Biological Functions and Role in Neuroprotection

For the past several years, fundamental research on Sigma-1R (S1R) protein has unveiled its necessity for maintaining proper cellular homeostasis through modulation of calcium and lipid exchange between the endoplasmic reticulum (ER) and mitochondria, ER-stress response, and many other mechanisms. Most of these processes, such as ER-stress response and autophagy, have been associated with neuroprotective roles. In fact, improving these mechanisms using S1R agonists was beneficial in several brain disorders including neurodegenerative diseases. In this review, we will examine S1R subcellular localization and describe S1R-associated biological activity within these specific compartments, i.e., the Mitochondrion-Associated ER Membrane (MAM), ER-Lipid Droplet (ER-LD) interface, ER-Plasma Membreane (ER-PM) interface, and the Nuclear Envelope (NE). We also discussed how the dysregulation of these pathways contributes to neurodegenerative diseases, while highlighting the cellular mechanisms and key binding partners engaged in these processes.

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.

A novel open-source raspberry Pi-based behavioral testing in zebrafish

The zebrafish (Danio rerio) is widely used as a promising high-throughput model organism in neurobehavioral research. The mobility of zebrafish can be dissected into multiple behavior endpoints to assess its neurobehavioral performance. However, such facilities on the market are expensive and clumsy to be used in laboratories. Here, we designed a low-cost, automatic zebrafish behavior assay apparatus, barely without unintentional human operational errors. The data acquisition part, composed of Raspberry Pi and HQ Camera, automatically performs video recording and data storage. Then, the data processing process is also on the Raspberry Pi. Water droplets and inner wall reflection of multi-well cell culture plates (used for placing zebrafish) will affect the accuracy of object recognition. And during the rapid movement of zebrafish, the probability of zebrafish tracking loss increased significantly. Thus, ROI region and related thresholds were set, and the Kalman filter algorithm was performed to estimate the best position of zebrafish in each frame. In addition, all functions of this device are realized by the custom-written behavior analysis algorithm, which makes the optimization of the setup more efficient. Furthermore, this setup was also used to analyze the behavioral changes of zebrafish under different concentrations of alcohol exposure to verify the reliability and accuracy. The alcohol exposure induced an inverted U-shape dose-dependent behavior change in zebrafish, which was consistent with previous studies, showcasing that the data obtained from the setup proposed in this study are accurate and reliable. Finally, the setup was comprehensively assessed by evaluating the accuracy of zebrafish detection (precision, recall, F-score), and predicting alcohol concentration by XGBoost. In conclusion, this study provides a simple, and low-cost package for the determination of multiple behavioral parameters of zebrafish with high accuracy, which could be easily adapted for various other fields.

Sigma-1 receptor: A potential target for the development of antidepressants

Though a great many of studies on the development of antidepressants for the therapy of major depression disorder (MDD) and the development of antidepressants have been carried out, there still lacks an efficient approach in clinical practice. The involvement of Sigma-1 receptor in the pathological process of MDD has been verified. In this review, recent research focusing on the role of Sigma-1 receptor in the etiology of MDD were summarized. Preclinical studies and clinical trials have found that stress induce the variation of Sigma-1 receptor in the blood, brain and heart. Dysfunction and absence of Sigma-1 receptor result in depressive-like behaviors in rodent animals. Agonists of Sigma-1 receptor show not only antidepressant-like activities but also therapeutical effects in complications of depression. The mechanisms underlying antidepressant-like effects of Sigma-1 receptor may include suppressing neuroinflammation, regulating neurotransmitters, ameliorating brain-derived neurotrophic factor and N-Methyl-D-Aspartate receptor, and alleviating the endoplasmic reticulum stress and mitochondria damage during stress. Therefore, Sigma-1 receptor represents a potential target for antidepressants development.

Antidepressive-like Behavior-Related Metabolomic Signatures of Sigma-1 Receptor Knockout Mice

Sigma-1 receptor (Sig1R) has been proposed as a therapeutic target for neurological, neurodegenerative, and psychiatric disorders, including depression and anxiety. Identifying metabolites that are affected by Sig1R absence and cross-referencing them with specific mood-related behaviors would be helpful for the development of new therapies for Sig1R-associated disorders. Here, we examined metabolic profiles in the blood and brains of male CD-1 background Sig1R knockout (KO) mice in adulthood and old age and correlated them with the assessment of depression- and anxiety-related behaviors. The most pronounced changes in the metabolic profile were observed in the plasma of adult Sig1R KO mice. In adult mice, the absence of Sig1R significantly influenced the amino acid, sphingolipid (sphingomyelin and ceramide (18:1)), and serotonin metabolic pathways. There were higher serotonin levels in plasma and brain tissue and higher histamine levels in the plasma of Sig1R KO mice than in their age-matched wild-type counterparts. This increase correlated with the reduced behavioral despair in the tail suspension test and lack of anhedonia in the sucrose preference test. Overall, these results suggest that Sig1R regulates behavior by altering serotonergic and histaminergic systems and the sphingolipid metabolic pathway.

Potential role of mitochondria-associated endoplasmic reticulum membrane proteins in diseases

Mitochondria-associated endoplasmic reticulum membranes (MAMs) are dynamic membrane coupling regions formed by the coupling of the mitochondrial outer membrane and endoplasmic reticulum (ER). MAMs are involved in the mitochondrial dynamics, mitophagy, Ca²⁺ exchange, and ER stress. A large number of studies indicate that many proteins are involved in the formation of MAMs, including dynamic-related protein 1 (Drp1), DJ-1, PTEN-induced putative kinase 1 (PINK), α-synuclein (α-syn), sigma-1 receptor (S1R), mitofusin-2 (Mfn2), presenilin-1 (PS1), protein kinase R (PKR)-like ER kinase (PERK), Parkin, Cyclophilin D (CypD), glucose-related protein 75 (Grp75), FUN14 domain containing 1 (Fundc1), vesicle-associated membrane-protein-associated protein B (VAPB), phosphofurin acidic cluster sorting protein 2 (PACS-2), ER oxidoreductin 1 (Ero1), and receptor expression-enhancing protein 1 (REEP1). These proteins play an important role in the structure and functions of the MAMs. Abnormalities in these MAM proteins further contribute to the occurrence and development of related diseases, such as neurodegenerative diseases, non-alcoholicfattyliverdisease (NALFD), type 2 diabetes mellitus (T2DM), and diabetic kidney (DN). In this review, we introduce important proteins involved in the structure and the functions of the MAMs. Furthermore, we effectively summarize major insights about these proteins that are involved in the physiopathology of several diseases through the effect on MAMs.