Tsung-Ping Su

Sigma-1 Receptor Chaperones at the ER- Mitochondrion Interface Regulate Ca2+ Signaling and Cell Survival

Communication between the endoplasmic reticulum (ER) and mitochondrion is important for bioenergetics and cellular survival. The ER supplies Ca(2+) directly to mitochondria via inositol 1,4,5-trisphosphate receptors (IP3Rs) at close contacts between the two organelles referred to as mitochondrion-associated ER membrane (MAM). We found here that the ER protein sigma-1 receptor (Sig-1R), which is implicated in neuroprotection, carcinogenesis, and neuroplasticity, is a Ca(2+)-sensitive and ligand-operated receptor chaperone at MAM. Normally, Sig-1Rs form a complex at MAM with another chaperone, BiP. Upon ER Ca(2+) depletion or via ligand stimulation, Sig-1Rs dissociate from BiP, leading to a prolonged Ca(2+) signaling into mitochondria via IP3Rs. Sig-1Rs can translocate under chronic ER stress. Increasing Sig-1Rs in cells counteracts ER stress response, whereas decreasing them enhances apoptosis. These results reveal that the orchestrated ER chaperone machinery at MAM, by sensing ER Ca(2+) concentrations, regulates ER-mitochondrial interorganellar Ca(2+) signaling and cell survival.

MAM: more than just a housekeeper.

The physical association between the endoplasmic reticulum (ER) and mitochondria, which is known as the mitochondria-associated ER membrane (MAM), has important roles in various cellular housekeeping functions including the non-vesicular transports of phospholipids. It has recently become clear that the MAM also enables highly efficient transmission of Ca(2+) from the ER to mitochondria to stimulate oxidative metabolism and, conversely, might enable the metabolically energized mitochondria to regulate the ER Ca(2+) homeostasis. Recent studies have shed light on molecular chaperones such as calnexin, calreticulin, ERp44, ERp57, grp75 and the sigma-1 receptor at the MAM, which regulate the association between the two organelles. The MAM thus integrates signal transduction with metabolic pathways to regulate the communication and functional interactions between the ER and mitochondrion.

σ-1 Receptor Ligands

The σ receptor was originally proposed to be a subtype of the opioid receptor. However, it is now clear that σ receptors are unique non-opioid, non-phencyclidine brain proteins. Two types of σ receptor exist, the σ-1 receptor and the σ-2 receptor. σ-1 receptors have been cloned and their distribution, physiological functions and roles in signal transduction were recently characterised. Certain sex hormones in the brain (neurosteroids) are known to interact with σ-1 receptors. σ-1 receptors regulate glutamate NMDA receptor function and the release of neurotransmitters such as dopamine. They are thus proposed to be involved in learning and memory as well as in certain neuropsychiatric disorders.Selective σ-1 receptor ligands have been suggested to represent a new class of therapeutic agents for neuropsychiatric disorders, although none have yet been introduced into therapeutic use. Early studies showed that psychotomimetic benzomorphans, as well as several antipsychotics, can bind to σ-1 receptors. As a result of these findings, σ-1 receptor ligands have been proposed as being of potential use in the treatment of schizophrenia. Nevertheless, the relationship of σ-1 receptors to the underlying pathogenesis of schizophrenia is still unclear. σ-1 receptor ligands have failed to improve acute psychotic symptoms of schizophrenia in clinical trials, but, interestingly, a few studies have shown an improvement in negative symptoms in schizophrenic patients.A number of preclinical studies have shown that selective agonists of σ-1 receptors affect higher-ordered brain functions such as learning and memory, cognition and mood. These studies indicate that σ-1 receptor agonists may exert therapeutic effects in depression and senile dementia. Indeed, the σ-1 receptor agonist igmesine, has been shown to improve depression in a clinical trial. The most distinctive feature of the action of σ-1 receptor ligands is their ‘modulatory’ role. In behavioural studies of depression and memory, they exert beneficial effects only when brain functions are perturbed.Given the recently accumulated preclinical and clinical data, it is time to reconstruct the concept of σ-1 receptors and the associated pathophysiological conditions that ligands of these receptors target. This would allow clinical trials to be performed more efficiently, and the results may confirm a long-speculated possibility that σ-1 receptor ligands represent a new class of therapeutic agents for neuropsychiatric disorders.

Targeting ligand-operated chaperone sigma-1 receptors in the treatment of neuropsychiatric disorders

Introduction: Current drugs for the treatment of psychiatric or neurodegenerative disorders have limitations. Psychotherapeutic drugs such as typical and atypical antipsychotics, tricyclic antidepressants and selective monoamine reuptake inhibitors, aim to normalize the hyper- or hypo-neurotransmission of monoaminergic systems. Despite their contribution to the outcomes of psychiatric patients, these agents often exert severe side effects and require chronic treatments to promote amelioration of symptoms. Drugs available for the treatment of neurodegenerative disorders are severely limited. Areas covered: Recent evidence that has shed light on sigma-1 receptor ligands, which may serve as a new class of antidepressants or neuroprotective agents. Sigma-1 receptors are novel ligand-operated molecular chaperones regulating signal transduction, ER stress, cellular redox, cellular survival and synaptogenesis. Selective sigma-1 receptor ligands exert rapid antidepressant-like, anxiolytic, antinociceptive and robust neuroprotective actions in preclinical studies. Recent studies that suggest that reactive oxygen species might play a role as signal integrators downstream of Sig-1Rs are also covered. Expert opinion: The advances in sigma receptor research in the last decade have begun to elucidate the intracellular signal cascades upstream and downstream of sigma-1 receptors. The novel ligand-operated properties of the sigma-1 receptor chaperone may enable interventions by which stress-related cellular systems can be pharmacologically controlled.

Nerve growth factor-induced neurite sprouting in PC12 cells involves sigma-1 receptors: implications for antidepressants.

One theory concerning the action of antidepressants relates to the drugs ability to induce an adaptive plasticity in neurons such as neurite sprouting. Certain antidepressants are known to bind to ς-1 receptors (Sig-1R) with high affinity. Sig-1R are dynamic endoplasmic reticulum proteins that are highly concentrated at the tip of growth cones in cultured cells. We therefore tested the hypotheses that Sig-1R might participate in the neurite sprouting and that antidepressants with Sig-1R affinity may promote the neuronal sprouting via Sig-1R. The prototypic Sig-1R agonist (+)-pentazocine [(+)PTZ], as well as the Sig-1R-active antidepressants imipramine and fluvoxamine, although ineffective by themselves, were found to enhance the nerve growth factor (NGF)-induced neurite sprouting in PC12 cells in a dose-dependent manner. A Sig-1R antagonistN,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE100) blocked the enhancements caused by these Sig-1R agonists. In separate experiments, we found that NGF dose and time dependently increased Sig-1R in PC12 cells. Chronic treatment of cells with (+)PTZ, imipramine, or fluvoxamine also increased Sig-1R. These latter results suggested that NGF induces the neurite sprouting by increasing Sig-1R. Indeed, the overexpression of Sig-1R per se in PC12 cells enhanced the NGF-induced neurite sprouting. Furthermore, antisense deoxyoligonucleotides directed against Sig-1R attenuated the NGF-induced neurite sprouting. Thus, when taken together, our results indicate that Sig-1R play an important role in the NGF-induced neurite sprouting and that certain antidepressants may facilitate neuronal sprouting in the brain via Sig-1R.

Chronic Antidepressants Potentiate via Sigma-1 Receptors the Brain-derived Neurotrophic Factor-induced Signaling for Glutamate Release

Up-regulation of BDNF (brain-derived neurotrophic factor) has been suggested to contribute to the action of antidepressants. However, it is unclear whether chronic treatment with antidepressants may influence acute BDNF signaling in central nervous system neurons. Because BDNF has been shown by us to reinforce excitatory glutamatergic transmission in cultured cortical neurons via the phospholipase-γ (PLC-γ)/inositol 1,4,5-trisphosphate (IP3)/Ca2+ pathway (Numakawa, T., Yamagishi, S., Adachi, N., Matsumoto, T., Yokomaku, D., Yamada, M., and Hatanaka, H. (2002) J. Biol. Chem. 277, 6520-6529), we examined in this study the possible effects of pretreatment with antidepressants on the BDNF signaling through the PLC-γ)/IP3/Ca2+ pathway. Furthermore, because the PLC-γ/IP3/Ca2+ pathway is regulated by sigma-1 receptors (Hayashi, T., and Su, T. P. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 491-496), we examined whether the BDNF signaling is modulated by sigma-1 receptors (Sig-1R). We found that the BDNF-stimulated PLC-γ activation and the ensued increase in intracellular Ca2+ ([Ca2+]i) were potentiated by pretreatment with imipramine or fluvoxamine, so was the BDNF-induced glutamate release. Furthermore, enhancement of the interaction between PLC-γ and TrkB (receptor for BDNF) after imipramine pretreatment was observed. Interestingly, BD1047, a potent Sig-1R antagonist, blocked the imipramine-dependent potentiation on the BDNF-induced PLC-γ activation and glutamate release. In contrast, overexpression of Sig-1R per se, without antidepressant pretreatment, enhances BDNF-induced PLC-γ activation and glutamate release. These results suggest that antidepressant pretreatment selectively enhance the BDNF signaling on the PLC-γ/IP3/Ca2+ pathway via Sig-1R, and that Sig-1R plays an important role in BDNF signaling leading to glutamate release.

Sigma-1 receptor chaperone at the ER-mitochondrion interface mediates the mitochondrion-ER-nucleus signaling for cellular survival.

The membrane of the endoplasmic reticulum (ER) of a cell forms contacts directly with mitochondria whereby the contact is referred to as the mitochondrion-associated ER membrane or the MAM. Here we found that the MAM regulates cellular survival via an MAM-residing ER chaperone the sigma-1 receptor (Sig-1R) in that the Sig-1R chaperones the ER stress sensor IRE1 to facilitate inter-organelle signaling for survival. IRE1 is found in this study to be enriched at the MAM in CHO cells. We found that IRE1 is stabilized at the MAM by Sig-1Rs when cells are under ER stress. Sig-1Rs stabilize IRE1 and thus allow for conformationally correct IRE1 to dimerize into the long-lasting, activated endonuclease. The IRE1 at the MAM also responds to reactive oxygen species derived from mitochondria. Therefore, the ER-mitochondrion interface serves as an important subcellular entity in the regulation of cellular survival by enhancing the stress-responding signaling between mitochondria, ER, and nucleus.

An update on the development of drugs for neuropsychiatric disorders: focusing on the σ1 receptor ligand

The σ1 receptor is an intracellular molecule that shares no homology with any mammalian proteins. σ1 receptors normally localize at the endoplasmic reticulum and regulate a variety of signal transductions including intracellular Ca2+ dynamics and neurotrophic factor signaling. In the brain, σ1 receptors are known to regulate the activity of diverse ion channels via protein–protein interactions. Accumulated evidences strongly indicate that the activation/upregulation of σ1 receptors promotes the neuronal differentiation as well as a robust antiapoptotic action. In animals, σ1 receptor agonists exhibit an antidepressant-like action. Furthermore, the agonists enhanced neuronal survival eventhough they were administered several hours after a brain ischemia. Thus, primary clinical targets of σ1 receptor ligands are proposed to include stroke, neurodegenerative disorders and depression. Ligands for the σ1 receptor may constitute a new class of therapeutic drugs targeting an endoplasmic reticular protein.

Sigma-1 receptor chaperones regulate the secretion of brain-derived neurotrophic factor.

The sigma‐1 receptor (Sig‐1R) is a novel endoplasmic reticulum (ER) molecular chaperone that regulates protein folding and degradation. The Sig‐1R activation by agonists is known to improve memory, promote cell survival, and exert an antidepressant‐like action in animals. Cutamesine (SA4503), a selective Sig‐1R ligand, was shown to increase BDNF in the hippocampus of rats. How exactly the intracellular chaperone Sig‐1R or associated ligand causes the increase of BDNF or any other neurotrophins is unknown. We examined here whether the action of Sig‐1Rs may relate to the post‐translational processing and release of BDNF in neuroblastoma cell lines. We used in vitro assays and confirmed that cutamesine possesses the bona fide Sig‐1R agonist property by causing the dissociation of BiP from Sig‐1Rs. The C‐terminus of Sig‐1Rs exerted robust chaperone activity by completely blocking the aggregation of BDNF and GDNF in vitro. Chronic treatment with cutamesine in rat B104 neuroblastoma caused a time‐ and dose‐dependent potentiation of the secretion of BDNF without affecting the mRNA level of BDNF. Cutamesine decreased the intracellular level of pro‐BDNF and mature BDNF whereas increased the extracellular level of mature BDNF. The pulse‐chase experiment indicated that the knockdown of Sig‐1Rs decreased the secreted mature BDNF in B104 cells without affecting the synthesis of BDNF. Our findings indicate that, in contrast to clinically used antidepressants that promote the transcriptional upregulation of BDNF, the Sig‐1R agonist cutamesine potentiates the post‐translational processing of neurotrophins. This unique pharmacological profile may provide a novel therapeutic opportunity for the treatment of neuropsychiatric disorders. Synapse, 2012.

Sigma1 receptor upregulation after chronic methamphetamine self-administration in rats: a study with yoked controls

RationaleSigma1 receptors (Sig-1R) are implicated in behavioral sensitization, conditioned place preference, and cellular restructuring induced by psychostimulants. We previously reported that rats which actively self-administered methamphetamine for 5 weeks and were then withdrawn from methamphetamine for 24xa0h showed downregulation of dopamine D2 autoreceptors (approximately 30%) in the midbrain and this was not seen in rats that passively received injections of methamphetamine or saline at the same time (yoked controls). Involvement of Sig-1R in the self-administration of psychostimulants, however, has never been reported.ObjectivesThis study examined neuroadaptive changes in Sig-1R in the brains of rats self-administering methamphetamine.MethodsThree groups of rats were tested simultaneously 5 days per week, for 5 weeks (25 daily sessions). Two groups served as yoked controls and passively received an injection of either 0.1xa0mg/kg methamphetamine or saline (not contingent on responding) each time a response-contingent injection of 0.1xa0mg/kg methamphetamine was actively self-administered by the first group of rats. Protein and mRNA levels of Sig-1R were then measured by Western and Northern blottings, respectively.ResultsThere was a marked upregulation of Sig-1R proteins (50%) in the midbrain and altered levels of Sig-1R mRNA in the frontal cortex and hippocampus of rats that learned to actively self-administer methamphetamine, but not in yoked methamphetamine- or saline-control rats.ConclusionsNeuroadaptive increases in Sig-1R seen in this study may contribute to the reinforcing effects of methamphetamine. This upregulation of Sig-1R may be mediated by increased protein kinase A activity due to downregulation of dopamine D2 autoreceptors.