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Journal of Pharmacological Sciences 127 (2015) 2e5

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Current perspective

Sigma-1 receptor: The novel intracellular target ofneuropsychotherapeutic drugs

Teruo Hayashi*

Seiwakai Nishikawa Hospital, 293-2 Minato-machi, Hamada, Shimane 697-0052, Japan

a r t i c l e i n f o

Article history:Received 23 May 2014Received in revised form18 July 2014Accepted 23 July 2014Available online 9 December 2014

Keywords:Sigma receptorSigma-1 receptorER stressMAMChaperone

* Tel.: þ1 81 855 22 2390; fax: þ1 81 855 22 3680E-mail address: thayashi2r@gmail.com.

Peer review under responsibility of Japanese Pharm

http://dx.doi.org/10.1016/j.jphs.2014.07.0011347-8613/© 2014 Japanese Pharmacological Societycreativecommons.org/licenses/by-nc-nd/4.0/).

a b s t r a c t

Sigma-1 receptor ligands have been long expected to serve as drugs for treatment of human diseasessuch as neurodegenerative disorders, depression, idiopathic pain, drug abuse, and cancer. Recentresearch exploring the molecular function of the sigma-1 receptor started unveiling underlying mech-anisms of the therapeutic activity of those ligands. Via the molecular chaperone activity, the sigma-1receptor regulates protein folding/degradation, ER/oxidative stress, and cell survival. The chaperoneactivity is activated or inhibited by synthetic sigma-1 receptor ligands in an agonist-antagonist manner.Sigma-1 receptors are localized at the endoplasmic reticulum (ER) membranes that are physicallyassociated with the mitochondria (MAM: mitochondria-associated ER membrane). In specific types ofneurons (e.g., those at the spinal cord), sigma-1 receptors are also clustered at ER membranes thatjuxtapose postsynaptic plasma membranes. Recent studies indicate that sigma-1 receptors, partly in sakeof its unique subcellular localization, regulate the mitochondria function that involves bioenergetics andfree radical generation. The sigma-1 receptor may thus provide an intracellular drug target that enablescontrolling ER stress and free radical generation under pathological conditions.© 2014 Japanese Pharmacological Society. Production and hosting by Elsevier B.V. This is an open access

article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

In the early 1970s, Martin proposed the sigma receptor as one ofthe opioid receptor subtypes (i.e., sigma opioid receptor), by whichthe psychotomimetic action of benzomorphans is mediated (1,2).However, following studies had confirmed that the sigma receptor isa non-opioid, non-G protein-coupled, intracellular protein (2,3).Binding assay studies found that the sigma receptor consists of atleast two subtypes: sigma-1 and sigma-2 receptor (4). Although themolecular entity and structure were totally unclear until the late1990s (3), early studies indicated that the sigma-1 receptor mayexert therapeutic activities by interacting with several psychoactivedrugs, such as haloperidol and selective serotonin uptake inhibitors(SSRIs) (1,5). Further, preclinical studies demonstrated that sigmareceptor ligands modulate neuroprotection, cancer growth, ionchannel activities, and animal behaviors implicated in memory/cognition, mood, pain, and drug abuse (6e8). Recent advances inmolecular biologyof the sigma receptor begin elucidatingmolecular

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mechanismsbywhich sigma-1 receptor ligands exert these varietiesof actions (2). In this review, firstly molecular pharmacological rolesof the sigma-1 receptor are summarized, and then a potential futuredirection of this research field is discussed.

2. Molecular biology of the sigma-1 receptor

From the 1970s to 1990s, hypotheses that tempted to explain theentity of the sigma receptor were proposed: e.g., the opioid re-ceptor subtype hypothesis and the MK801-binding site hypothesis(1,9). However, since radio-ligand binding assays were the only toolto assay the sigma-1 receptor in those days, it was difficult toexperimentally and conclusively prove those hypotheses. There-fore, even the existence of the sigma-1 receptor protein was notfully confirmed and accepted. But the debate on the entity of thesigma-1 receptor finally ended in 1996 by the successful cloning ofthe sigma-1 receptor gene (3). Thanks to the groundbreaking dis-covery, molecular biological approaches were since then activelyintroduced to this research field.

The cloning study demonstrated that the sigma-1 receptorconsists of 223 amino acids with two potential trans-membranedomains (3). In agreement with the fact that the protein

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T. Hayashi / Journal of Pharmacological Sciences 127 (2015) 2e5 3

possesses a double-arginine ER retention signal at the N-terminus,several recent studies confirmed the ER localization of sigma-1receptors (10,11). So far, only few electron microscopic data isavailable that clearly demonstrates the significant plasma mem-brane localization of the sigma-1 receptor (12,13). The secondtrans-membrane domain and the C-terminus of the sigma-1 re-ceptor in the ER lumen are proposed to form the ligand-binding site(14). Therefore, in contrast to the majority of ligand-binding sites ofneurotransmitter receptors, the binding site of the sigma-1 recep-tor is located in the inner ER membrane or the lumenal surface ofthe ER membrane. The unique hydrophobic environment of theligand-binding site may be enabling hydrophobic molecules toassociate with the binding site. Indeed, the most sigma-1 receptorligands possess hydrophobic or amphipathic property (e.g., halo-peridol and fluvoxamine) (1,5). Postulated endogenous ligands ofthe sigma-1 receptor include steroids (e.g., progesterone, DHEA-sulfate), hallucinogen N,N-dimethyltrypatamine, and sphingosine(15e17). Furthermore, a recent study suggested a possibility thatmonoglycosylated ceramide might possess a high affinity for thesigma-1 receptor (18). Among sigma-1 receptor-binding lipids,steroids (e.g., progesterone and testosterone, DHEA) seem to haverelatively low affinities (0.3e10 mM) when compared with those ofendogenous sphingolipids and monoglycosyated ceramides(15,17,18). Certain sphingolipids appear to possess affinities highenough to bind sigma-1 receptors at their physiologically relevantconcentrations.

A recent study found that, upon its binding to sigma-1 receptors,the highly selective sigma-1 agonist (þ) pentazocine promotes theassociation of sigma-1 receptors with Insig-1, the SREBP escortingprotein localized at the ER (19). Notably, 25-dehydorxylcholesterol

Fig. 1. Molecular functions of the sigma-1 receptor. The sigma-1 receptor possesses two traclustered at the mitochondria-associated ER membrane (MAM) and ER membranes juxtaposigma-1 receptor exerts chaperone activities by which ER membrane proteins are stabilizedSigma-1 receptors associating with BiP stabilizes IP3 receptors type-3 (IP3R) at the MAM, leSigma-1 receptors stabilize the ER stress sensor IRE1 at the the MAM in an ROS-dependereceptors suppress generation of reactive oxigen species (ROS) and following activationknown); 4) Sterols such as 25-hydroxycholesterol promote the association of sigma-1 recepdegradation (ERAD) of HMG-CoA reductase and galactosylceramide synthase at the ER]; 5) Sthe plasma membrane or processing/secretion of brain-derived trophic factor (BDNF). Sigmaspinal neurons, sigma-1 receptors, which colocalize with a K channel subunit are clusteredregulates processing/secretion of BDNF is unknown.

exerts the same effect at nM concentrations (19). Though furtherstudies are necessary for confirmation, the data suggests that 25-hydroxycholesterol might serve as a high-affinity endogenousligand for the sigma-1 receptor.

3. The sigma-1 receptor is a novel ligand-operated molecularchaperone (Fig. 1)

The sigma-1 receptor shares no homology with any mammalianprotein (3). This fact made it difficult to predict the molecularfunction of the sigma-1 receptor from the amino acid sequence.Nonetheless, a combination of subcellular localization studies,protein purification studies, coupling proteins identification, andin vitro protein activity assays, has begun to reveal the molecularfunction of the sigma-1 receptor. It was demonstrated that the C-terminus of the sigma-1 receptor possesses a molecular chaperoneactivity that stabilizes ER proteins, thus being able to regulate theirdegradation (10,20). The association of another ER chaperone BiPregulates the chaperone activity of the sigma-1 receptor (10). Thesigma-1 receptor forming a complex with BiP is in a dormant state,whereas the free sigma-1 receptor that dissociates from BiP exertsmaximum chaperone activity (10). The association between sigma-1 receptors and BiP is Ca2þ-dependent, and thus the depletion of ERCa2þ activates the sigma-1 receptor chaperone (10). Importantly,even in the presence of ER Ca2þ, sigma-1 receptor agonists causethe dissociation of BiP from sigma-1 receptors, leading to activationof sigma-1 receptor chaperones (10). It was also demonstrated thatsigma-1 receptor antagonists inhibit the action of agonists (10).Therefore, synthetic drug that can associate with sigma-1 receptorsactivate or inhibit the sigma-1 receptor's chaperone activity.

nsmembrane domains and mainly localize at the ER membrane. Sigma-1 receptors aresing postsynaptic density of specific types of neurons. The ER lumenal domain of the. The figure depicts the recently reported actions of the sigma-1 receptors including: 1)ading to regulation of Ca2þ influx into mitochondria and following ATP production; 2)nt manner, leading to prolongation of the IRE1-XBP1 cell survival signal; 3) Sigma-1of the NFkB signaling (How the sigma-1 receptor regulates ROS generation is un-tors with Insig-1 [Collaborating with Insig-1, sigma-1 receptors regulate ER-associatedigma-1 receptors regulate the trafficking of potassium channel subunits from the ER to-1 receptors likely associate with potassium channel subunits or pro-BDNF at the ER. Inat the ER membrane apposing postsynaptic densities (PSD). How the sigma-1 receptor

T. Hayashi / Journal of Pharmacological Sciences 127 (2015) 2e54

4. The sigma-1 receptor is the regulator of the inter-organellecommunication (21)

As previously mentioned, the primary location of sigma-1 re-ceptors is the ERmembrane (2). A line of studies demonstrated thatsigma-1 receptors tend to form clusters and are highly enriched atspecialized subdomains of ER membranes. For example, in Chinesehamster ovary (CHO) cells, sigma-1 receptors localize onto the ERmembrane that is physically associated with mitochondrial outer-membrane (MAM: the mitochondria-associated ER membrane)(10). Recent studies indicated that membrane lipids, such ascholesterol and sphingolipids, play a role in recruiting sigma-1receptors to the MAM (18). In contrast to the bulk ER membranesthat contain few cholesterol and sphingolipids, the MAM containsconsiderable levels of ceramides and cholesterol, thus forming lipidraft-like microdomains (18,22,23). Certain sterols and sphingoli-pids, which bind to sigma-1 receptors, may thus contribute topromoting sigma-1 receptors to the MAM (Fig. 2).

The MAM is known to play a role as a center that directly pro-vides Ca2þ or phospholipids to the mitochondria. Ca2þ provided byMAM-localized IP3 receptors activates the TCA cycle, thus causingATP production (20). One of proven mechanisms of the sigma-1receptor is to stabilize IP3 receptors at the MAM for proper Ca2þ

flux from the MAM into mitochondria (Fig. 1) (10).Most recently, it is found that IRE1, one of the ER stress sensor

proteins, are enriched at the MAM and stabilized by sigma-1 re-ceptors, especially during the early stage of ER stress (Fig.1) (24). Thestabilizationof IRE1bysigma-1 receptorsprolongs theER-to-nucleussignal, mediated by XBP1 for cellular survival (24). The study alsosuggests that the IRE1-sigma-1 receptor complex is equipped to theMAM to monitor mitochondria-generated reactive oxygen species(ROS) (24). It is shown that upon generation of ROS at the mito-chondria sigma-1 receptors begin to associate with IRE1 for thestabilization of IRE1 and prolonging the IRE1-XBP1 signaling (24).

Though the underlying mechanism is unclear, a line of studiesdemonstrated that sigma-1 receptors function as a suppressor ofROS generation. The knockdown of sigma-1 receptor per se causesthe accumulation of ROS and activation of the downstreampathway involving NFkB (25). In contrast, sigma-1 receptor upre-gulation are shown to cause inhibition of ROS generation underpathological conditions (25).

It is noteworthy that a recent study found that sigma-1 re-ceptors in spinal mouse motor neurons are localized to subsurface

Fig. 2. The hypothetical scheme depicting topology of membrane lipids and the sigma-1 rtransbilayer lipid distribution, the MAM may comprise more heterogeneous lipid distribformation of lipid raft-like microdomains. The ceramide-rich lipid microdomains may be prthe recruitment of specific ER proteins such as the sigma-1 receptor (red) to the MAM. In spthe ER lumen, might substitute with ceramides in the MAM-localizing lipid microdomains.PtChol, phosphatidylcholine.

structures of cholinergic postsynaptic densities (12), suggestingthat ER membranes juxtaposing postsynaptic plasma membranesare highly enriched with sigma-1 receptors in those neurons.Interestingly, those sigma-1 receptors are co-localized with Kv2.1potassium channels (12). Another set of studies recently found thatsigma-1 receptors regulate cocaine-induced behavioral sensitiza-tion by regulating expression of cell surface Kv1.2 in the nucleusaccumbens (26), postulating that sigma-1 receptors at the ER-plasma membrane interface might regulate the trafficking of po-tassium channels (Fig. 1). An in vitro study demonstrated thatsigma-1 receptors regulate processing/trafficking of pro-brain-derived neurotrophic factor (BDNF) (Fig. 1) (27).

More studies are certainly necessary to elucidate molecularmechanisms by which sigma-1 receptors at the ER preferentiallylocalize to interfaces between the ER and other organelles. None-theless, the unique subcellular localization of sigma-1 receptorsand their regulating signaling pathways at organelle-to-organelleinterfaces suggest that the sigma-1 receptor serves as a modu-lator of inter-organelle communications that aremediated by directmembrane associations.

5. Why does the sigma-1 receptor possess two trans-membrane domains?

Molecular chaperones are mostly water-soluble proteins lackingtrans-membrane domains. The water-soluble property is advanta-geous for them to move freely in the cytoplasm or in organelle lu-mens aswell as in accessing freely tomiss-folded proteins. One of thefundamental questions regarding themolecular biologyof the sigma-1 receptor is, “Why does the sigma-1 receptor possess two trans-membrane domains that may potentially restrict their movementat the ER”. One possible answer to this question is that sigma-1 re-ceptors are localized to the ER membrane to stabilize ER membraneproteins that mediate the signaling/transport across the ER mem-branes. For instance, sigma-1 receptors at the ER membranes chap-erone IP3 receptors and IRE1, both of which are transmitters of ER-derived signals (i.e., Ca2þ released via IP3 receptors, XBP1 mRNAsplicing mediated by IRE1's endonuclease activity) (10,24). Perhapssigma-1 receptors, by associating with membrane proteins, directlycommit to regulations of signals across the ER membranes (Fig. 3).The other possible answer is that the trans-membrane domain of thesigma-1 receptor is possibly being utilized to formprotein complexes(Fig. 3). A recent study found that the second trans-membrane

eceptor at the MAM. In contrast to the bulk of ER membranes that has the symmetricution by accommodating simple sphingolipids and cholesterols, thus leading to theedominantly localized at the lumenal leaflet of the MAM. Microdomains may promoteecific cell types (e.g., oligodendrocytes), galactosylceramides, which are synthesized atGlcCer, glucosylceramide; PtSer, phosphatidylserine; PtEt, phosphatidylethanolamine;

Fig. 3. The potential mechanism of the membrane-associated sigma-1 receptorchaperone. The membrane-spanning structure of the sigma-1 receptor chaperone maybe advantageous to stabilize ER membrane proteins that are involved in regulation ofthe signaling/transport across ER membranes (e.g., IP3 receptors, IRE1). As shown inthe case of its association with Insig-1, the second trans-membrane domain of thesigma-1 receptor is possibly being utilized for the association with client proteins.Residues 116e176 may contain a putative chaperone domain (red), whereas resides198e206 comprise a putative membrane attachment region (light blue) (28). Numbersplaced on the sigma-1 receptor indicate positions of respective amino acid residues.

T. Hayashi / Journal of Pharmacological Sciences 127 (2015) 2e5 5

domain of the sigma-1 receptor is necessary for its association withInsig-1, the ER sterol-sensor regulating SREBP-mediated transcrip-tions of lipid enzymes (19). The sigma-1 receptor, by forming acomplex with Insig-1, mediates ER-associated degradation (ERAD) ofspecific sets of ER proteins that bind to sterols (e.g., HMG-CoAreductase, galactosylceramide synthase) (19).

6. Conclusion

In the last fifteen years, the understanding of the function of thesigma-1 receptor was significantly advanced, particularly at themolecular level. The sigma-1 receptor localized at the ERmodulatesvia its chaperone activity inter-organelle communications. Sigma-1receptors thus regulate a variety of cellular events, such as neuronaldifferentiation, cellular survival, and bioenergetics. By numerousanimal studies, these actions of the sigma-1 receptors have beenlinked to the pathophysiology of certain human diseases such asdepression, ischemia, drug abuse, pain, and cancer. Considering thecurrent pharmacotherapy of neuropsychiatric diseases that largelydepends on drugs developed based on the monoamine theory, thesigma-1 receptor is expected to serve as amolecule, which providesa novel target of “post-monoamine” drugs, thus bringing a newapproach for treatment of patients suffering from neuropsychiatricdiseases.

Conflicts of interest

The author indicated no potential conflicts of interests.

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