Peeking into Sigma-1 Receptor Functions Through the Retina

Endoplasmic reticulum stress: molecular mechanism and therapeutic targets

The endoplasmic reticulum (ER) functions as a quality-control organelle for protein homeostasis, or "proteostasis". The protein quality control systems involve ER-associated degradation, protein chaperons, and autophagy. ER stress is activated when proteostasis is broken with an accumulation of misfolded and unfolded proteins in the ER. ER stress activates an adaptive unfolded protein response to restore proteostasis by initiating protein kinase R-like ER kinase, activating transcription factor 6, and inositol requiring enzyme 1. ER stress is multifaceted, and acts on aspects at the epigenetic level, including transcription and protein processing. Accumulated data indicates its key role in protein homeostasis and other diverse functions involved in various ocular diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, achromatopsia, cataracts, ocular tumors, ocular surface diseases, and myopia. This review summarizes the molecular mechanisms underlying the aforementioned ocular diseases from an ER stress perspective. Drugs (chemicals, neurotrophic factors, and nanoparticles), gene therapy, and stem cell therapy are used to treat ocular diseases by alleviating ER stress. We delineate the advancement of therapy targeting ER stress to provide new treatment strategies for ocular diseases.

Sigma-1 Receptor Agonist Fluvoxamine Ameliorates Fibrotic Response of Trabecular Meshwork Cells

Primary open-angle glaucoma remains a global issue, lacking a definitive treatment. Increased intraocular pressure (IOP) is considered the primary risk factor of the disease and it can be caused by fibrotic-like changes in the trabecular meshwork (TM) such as increased tissue stiffness and outflow resistance. Previously, we demonstrated that the sigma-1 receptor (S1R) agonist fluvoxamine (FLU) has anti-fibrotic properties in the kidney and lung. In this study, the localization of the S1R in TM cells was determined, and the anti-fibrotic efficacy of FLU was examined in both mouse and human TM cells. Treatment with FLU reduced the F-actin rearrangement, inhibited cell proliferation and migration induced by the platelet-derived growth factor and decreased the levels of fibrotic proteins. The protective role of the S1R in fibrosis was confirmed by a more pronounced increase in alpha smooth muscle actin and F-actin bundle and clump formation in primary mouse S1R knockout TM cells. Furthermore, FLU demonstrated its protective effects by increasing the production of nitric oxide and facilitating the degradation of the extracellular matrix through the elevation of cathepsin K. These findings suggest that the S1R could be a novel target for the development of anti-fibrotic drugs and offer a new therapeutic approach for glaucoma.

Retinal Photoreceptor Protection in an AMD-Related Mouse Model by Selective Sigma-1 or Sigma-2 Receptor Modulation

The structurally and genetically distinct sigma-1 receptor (S1R) and sigma-2 receptor (S2R) comprise a unique class of drug binding sites. Their alleles are associated with human diseases involving neuronal systems, such as age-related macular degeneration (AMD) characterized by photoreceptor and retinal pigment epithelium (RPE) atrophy. Previous studies have suggested neuroprotective benefits for the brain and retina from pharmacological modulation of S1R and/or S2R. However, the effect of such modulation on AMD pathology remains underexplored. Here, we evaluated S1R- or S2R-selective modulation in an AMD-related model of Abca4-/-Rdh8-/- mice with a disrupted visual cycle that predisposes RPE and photoreceptors to illumination-induced damage. For S1R modulation, we used (+)-pentazocine, which is a high-affinity S1R-selective drug. For S2R modulation, we chose CM398, a high-affinity and highly S2R-selective ligand with drug-like properties. Abca4-/-Rdh8-/- mice received a single i.p. injection of (+)-pentazocine or CM398 or vehicle 30 min before illumination. Pretreatment with (+)-pentazocine improved electroretinogram a- and b-waves compared to that with vehicle. Consistently, in another AMD-related mouse model induced by tail-vein injected NaIO3, S1R genetic ablation aggravated photoreceptor loss. In Abca4-/-Rdh8-/- mice, pretreatment with CM398 appeared to partially avert illumination-induced photoreceptor loss and autofluorescent granule formation that signals RPE damage, as revealed by optical coherence tomography. Thus, this study using AMD-related models provides evidence of photoreceptor protection afforded by selective modulation of S1R or S2R.

Sigma 1 Receptor Contributes to Astrocyte-Mediated Retinal Ganglion Cell Protection

Purpose

Sigma 1 receptor (S1R) is expressed in retinal ganglion cells (RGCs) and astrocytes, and its activation is neuroprotective. We evaluated the contribution of S1R within optic nerve head astrocytes (ONHAs) to growth and survival of RGCs in vitro.

Methods

Wild-type (WT) RGCs and WT or S1R knockout (S1R KO) ONHAs were cocultured for 2, 4, or 7 days. Total and maximal neurite length, neurite root, and extremity counts were measured. Cell death was measured using a TUNEL assay. Signal transducer and activator of transcription 3 phosphorylation levels were evaluated in ONHA-derived lysates by immunoblotting.

Results

The coculture of WT RGCs with WT or S1R KO ONHAs increased the total and maximal neurite length. Neurite root and extremity counts increased at 4 and 7 days when WT RGCs were cocultured with WT or S1R KO ONHAs. At all timepoints, the total and maximal neurite length decreased for WT RGCs in coculture with S1R KO ONHAs compared with WT ONHAs. Root and extremity counts decreased for WT RGCs in coculture with S1R KO ONHAs compared with WT ONHAs at 2 and 7, but not 4 days. RGC apoptosis increased in S1R KO ONHA coculture and S1R KO-conditioned medium, compared with WT ONHA coculture or WT-conditioned medium. S1R KO ONHA-derived lysates showed decreased phosphorylated signal transducer and activator of transcription 3 levels compared with WT ONHA-derived lysates.

Conclusions

The absence of S1R within ONHAs has a deleterious effect on RGC neurite growth and RGC survival, reflected in analysis of WT RGC + S1R KO ONHA indirect cocultures. The data suggest that S1R may enhance ganglion cell survival via glia-mediated mechanisms.

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

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

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

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

Sigma 1 Receptor Modulates Optic Nerve Head Astrocyte Reactivity

Purpose

Stimulation of Sigma 1 Receptor (S1R) is neuroprotective in retina and optic nerve. S1R is expressed in both neurons and glia. The purpose of this work is to evaluate the ability of S1R to modulate reactivity responses of optic nerve head astrocytes (ONHAs) by investigating the extent to which S1R activation alters ONHA reactivity under conditions of ischemic cellular stress.

Methods

Wild type (WT) and S1R knockout (KO) ONHAs were derived and treated with vehicle or S1R agonist, (+)-pentazocine ((+)-PTZ). Cells were subjected to six hours of oxygen glucose deprivation (OGD) followed by 18 hours of re-oxygenation (OGD/R). Astrocyte reactivity responses were measured. Molecules that regulate ONHA reactivity, signal transducer and activator of transcription 3 (STAT3) and nuclear factor kappa B (NF-kB), were evaluated.

Results

Baseline glial fibrillary acidic protein (GFAP) levels were increased in nonstressed KO ONHAs compared with WT cultures. Baseline cellular migration was also increased in nonstressed KO ONHAs compared with WT. Treatment with (+)-PTZ increased cellular migration in nonstressed WT ONHAs but not in KO ONHAs. Exposure of both WT and KO ONHAs to ischemia (OGD/R), increased GFAP levels and cellular proliferation. However, (+)-PTZ treatment of OGD/R-exposed ONHAs enhanced GFAP levels, cellular proliferation, and cellular migration in WT but not KO cultures. The (+)-PTZ treatment of WT ONHAs also enhanced the OGD/R-induced increase in cellular pSTAT3 levels. However, treatment of WT ONHAs with (+)-PTZ abrogated the OGD/R-induced rise in NF-kB(p65) activation.

Conclusions

Under ischemic stress conditions, S1R activation enhanced ONHA reactivity characteristics. Future studies should address effects of these responses on RGC survival.

Renal expression of sigma 1 receptors in diabetic rats

The purpose of this study was to determine the changes in the expression of sigma 1 receptors (σ1Rs) in the kidney of diabetic rats, which could indicate their possible role in the pathogenesis of diabetic nephropathy (DN). Sprague-Dawley rats were were given intraperitoneal injection of 55 mg/kg streptozotocin (STZ) in order to induce type I of diabetes (DM1). Control and diabetic rats were sacrificed 2 weeks or 2 months after DM1 induction. Expression of σ1Rs was determined in kidneys of the experimental rats, using immunohistochemistry. The most prominent expression of σ1Rs was found in distal tubuli (DT). Results have shown significant increase in renal σ1Rs section percentage area of rats 2 months after DM1 induction, compared to both control group at the same age and diabetic group 2 weeks after induction (P < 0.01 both). Similarly, a number of immunoreactive DT increased in diabetic group 2 months after induction, compared to DM1 group 2 weeks after induction (P < 0.001). We also found a decrease of a number of immunoreactive DT 2 weeks post DM1 induction (P < 0.01). However, the same was found during maturation of the control rats (P < 0.001). In addition, a strong co-expression of σ1R and proinflammatory factor TGFβ was seen in vacuolated DT. The results indicate to the potential role of σ1Rs in postnatal maturation of the rat kidneys and in distal tubular damage in the pathogenesis of the diabetic nephropathy. We conclude that σ1Rs could be potential target in treatment of the diabetic nephropathy.

Novel harmala-ocudelic tuning (HOT) for ocular disorders

The eye contains serotonin, and possesses serotonergic receptors and modulators. Selective serotonin reuptake inhibitors (SSRIs) may impair ocular function, while other serotonin receptor-binding drugs might improve it. A putative vision-improving drug must be safe at therapeutic doses, most preferably with long-lasting benefits from a single or very few administration(s). One drug potentially satisfying these criteria is the common, botanically-occurring indole alkaloid and "major hallucinogen," harmine. Harmine is prominent in the leaves of the Generally Recognized as Safe (GRAS) Passiflora, and in Peganum and Banisteriopsis, the two preeminent "sacred" medicinal plants of India/pre-Islamic Persia/modern Iran and the Amazon respectively. Though the possibility of drug-drug or herb-drug interactions exists, especially with SSRIs, at correct patient-individualized dosages, these plants, used alone, are minimally hallucinogenic, well tolerated, potentially exerting profound therapeutic effects on vision from a single or few administration(s) lasting months or years. The hypothesized mechanism for this extraordinary action is tuning of the serotonergic receptors in the eye in their binding affinity for serotonin, a putative neurochemical "ocudelic" homologue to the psychedelic phenomenon, likely potentiated, according to clinical experience with classical hallucinogens, by therapeutic music, positive psychological "set," and conducive environmental "setting." Blinded research with harmala alkaloid-containing plants in ocular patients using psychedelic therapy accoutrements, viz. eyeshades, high fidelity classical music, headphones, two guides, pre-dosing preparation, and post-dosing integration, could non-invasively assess visual acuity in presbyopia. Significant results would stimulate further exploration of this novel approach to ocular disorders. Deeper benefit, particularly when the retina is compromised, might follow co-ingestion of the harmala alkaloid-containing plants with plants containing dimethyltryptamine.

SIGMAR1/Sigma-1 receptor ablation impairs autophagosome clearance

Autophagosome-lysosome fusion is a common critical step in various forms of macroautophagy/autophagy including mitophagy, the selective degradation of mitochondria. Regulations of this fusion process remain poorly defined. Here we have determined the role of SIGMAR1, a unique endoplasmic reticulum membrane protein. Knockout of Sigmar1 impaired mitochondrial clearance without altering the PINK1-PRKN/Parkin signaling, in mouse retinal explants and cultured cells treated with carbonyl cyanide m-chlorophenyl hydrazone (CCCP) for induction of mitophagy. SIGMAR1 depletion also caused accumulation of autophagosome markers LC3-II and SQSTM1, but did not change the levels of BECN1 and ATG7, proteins associated with autophagosome biogenesis. Lysosomal pH and protease activities were not negatively affected. However, sigmar1 knockout partially compromised autophagosome-lysosome fusion in CCCP-treated NSC34 cells, as revealed by reduced GFP fluorescence quenching of GFP-RFP-LC3-II puncta and co-localization of lysosomes with mitochondria. Furthermore, SIGMAR1 co-immunoprecipitated with ATG14, STX17, and VAMP8 (but not SNAP29), proteins key to autophagosome-lysosome membrane fusion. Re-expressing SIGMAR1 in the null background rescued clearance of mitochondria and autophagosomes. In summary, we started out finding that sigmar1 knockout impaired the clearance of mitochondria and autophagosomes, and then narrowed down the SIGMAR1 modulation to the autophagosome-lysosome fusion step. This study may shed new light on understanding autophagy-associated cyto-protection and disease mechanisms. Abbreviations: APEX2, a genetically engineered peroxidase; BiFC, bimolecule fluorescence complementation; CCCP, a mitophagy inducing compound; CRISPR, clustered regularly interspaced short palindromic repeats; EM, electron microscopy; ER, endoplasmic reticulum; MAP1LC3/LC3, microtubule-associated protein 1 light chain 3; SIGMAR1, sigma non-opioid intracellular receptor 1.

Sigma 1 receptor: A novel therapeutic target in retinal disease

Retinal degenerative diseases are major causes of untreatable blindness worldwide and efficacious treatments for these diseases are sorely needed. A novel target for treatment of retinal disease is the transmembrane protein Sigma 1 Receptor (Sig1R). This enigmatic protein is an evolutionary isolate with no known homology to any other protein. Sig1R was originally thought to be an opioid receptor. That notion has been dispelled and more recent pharmacological and molecular studies suggest that it is a pluripotent modulator with a number of biological functions, many of which are relevant to retinal disease. This review provides an overview of the discovery of Sig1R and early pharmacologic studies that led to the cloning of the Sig1R gene and eventual elucidation of its crystal structure. Studies of Sig1R in the eye were not reported until the late 1990s, but since that time there has been increasing interest in the potential role of Sig1R as a target for retinal disease. Studies have focused on elucidating the mechanism(s) of Sig1R function in retina including calcium regulation, modulation of oxidative stress, ion channel regulation and molecular chaperone activity. Mechanistic studies have been performed in isolated retinal cells, such as Müller glial cells, microglial cells, optic nerve head astrocytes and retinal ganglion cells as well as in the intact retina. Several compelling studies have provided evidence of powerful in vivo neuroprotective effects against ganglion cell loss as well as photoreceptor cell loss. Also described are studies that have examined retinal structure/function in various models of retinal disease in which Sig1R is absent and reveal that these phenotypes are accelerated compared to retinas of animals that express Sig1R. The collective evidence from analysis of studies over the past 20 years is that Sig1R plays a key role in modulating retinal cellular stress and that it holds great promise as a target in retinal neurodegenerative disease.

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

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

Role of the sigma-1 receptor chaperone in rod and cone photoreceptor degenerations in a mouse model of retinitis pigmentosa

BACKGROUND

Retinitis pigmentosa (RP) is the most common inherited retinal degenerative disease yet with no effective treatment available. The sigma-1 receptor (S1R), a ligand-regulated chaperone, emerges as a potential retina-protective therapeutic target. In particular, pharmacological activation of S1R was recently shown to rescue cones in the rd10 mouse, a rod Pde6b mutant that recapitulates the RP pathology of autonomous rod degeneration followed by secondary death of cones. The mechanisms underlying the S1R protection for cones are not understood in detail.

METHODS

By rearing rd10/S1R-/- and rd10/S1R+/+ mice in dim light to decelerate rapid rod/cone degeneration, we were able to compare their retinal biochemistry, histology and functions throughout postnatal 3-6 weeks (3 W-6 W).

RESULTS

The receptor-interacting protein kinases (RIP1/RIP3) and their interaction (proximity ligation) dramatically up-regulated after 5 W in rd10/S1R-/- (versus rd10/S1R+/+) retinas, indicative of intensified necroptosis activation, which was accompanied by exacerbated loss of cones. Greater rod loss in rd10/S1R-/- versus rd10/S1R+/+ retinas was evidenced by more cleaved Caspase3 (4 W) and lower rod electro-retinographic a-waves (4 W-6 W), concomitant with reduced LC3-II and CHOP (4 W-6 W), markers of autophagy and endoplasmic reticulum stress response, respectively. However, the opposite occurred at 3 W.

CONCLUSION

This study reveals previously uncharacterized S1R-associated mechanisms during rd10 photoreceptor degeneration, including S1R's influences on necroptosis and autophagy as well as its biphasic role in rod degeneration upstream of cone death.

Potential independent action of sigma receptor ligands through inhibition of the Kv2.1 channel

The sigma-1 receptor (σ1-R) and sigma-2 receptor (σ2-R) are potential drug targets for treatment of cancer, pain, depression, retinal degeneration and other neuronal diseases. Previous reports show that sigma-1 receptor modulates the activities of multiple channels. We are interested in possible sigma receptor modulation of Kv2.1, a K⁺ channel abundant in retinal photoreceptors. We tested the effect of established sigma receptor ligands on Kv2.1 channels which were stably expressed in HEK293 cells. Surprisingly, σ1-R antagonists inhibited Kv2.1 currents in both wild type and σ1-R knockout HEK293 cells that we engineered using the CRISPR/Cas9 technology. Moreover, PB28, a σ1-R antagonist and also σ2-R agonist, inhibited Kv2.1 in σ1-R knockout cells, but this action was not blocked by the σ2-R antagonists that did not have an effect on Kv2.1. We also observed inhibition of electroretinogram by PB28 in wild type as well as σ1-R knockout mice. Thus, the results in this study indicate that the Kv2.1-inhibiting function of the sigma ligands is not sigma receptor dependent, suggesting a direct effect of these ligands on the Kv2.1 channel.