Biological And Biochemical Characterization Of Sigma Receptors

National Institute On Drug Abuse

Investigators

Linked publications, trials & patents

Abstract

Since my laboratory at the NIDA IRP identified the sigma-1 receptor in 1982, many preclinical studies have shown that sigma-1 receptors and associated ligands are involved in stroke, amnesia, depression, cancer, Alzheimer disease, pain, and cocaine addiction. Our lab has elucidated and concluded that the basic molecular mechanisms of action of sigma-1 receptor are three-fold: (1) binds and chaperone client proteins; (2) binds lipids to lipidate functional proteins; (3) binds RNAs to mediate RNA-related signaling. My final goal is to understand how a single-transmembrane protein like sigma-1 receptor works in such a diverse molecular actions that mediate so many important biological processes in living systems. (A) In this fiscal year from Oct 1/2024-Sept 30/2025, we have published a major article [Couly S, Yasui Y, Foncham S, Grammatikakis I, Lal A, Shi L, Su TP (2024): Benzomorphan and non-benzomorphan agonists differentially alter sigma-1 receptor quaternary structure, as does types of cellular stress. Cell Mol Life Sci. Jan 9;81(1):14. doi: 10.1007/s00018-023-05023-z.PMID: 38191696] that have been tauted by the reviewers as a future highly cited article. In this study, we utilized cellular and an in-vivo model to study changes in quaternary structure of sigma-1 receptor upon activation. We incubated cells with cellular stressors (H2O2 and thapsigargin) or exogenous ligands, then quantified monomeric and oligomeric forms. We observed that benzomorphan-based sigma-1 receptor agonists induce monomerization of sigma-1 receptor and decrease oligomerization, which was confirmed in the liver tissue of mice injected with (+)-Pentazocine. Antagonists block this effect but do not induce any changes when used alone. Oxidative stress (H2O2) increases the monomeric/oligomeric sigma-1 receptor ratio whereas ER calcium depletion (thapsigargin) has no effect. We also analyzed the oligomerization ability of various truncated sigma-1 receptor fragments and identified the fragments favorizing oligomerization. In this publication we demonstrate that quaternary structural changes differ according to the mechanism of sigma-1 receptor activation. Therefore, we offer a novel perspective on sigma-1 receptor activation as a nuanced phenomenon dependent on the type of stimulus. Following up on the above study, we are currently examining the effect of structurally different sigma-1 receptor ligands as well as distinct types of stress on the substructure of endoplasmic reticulum (ER). The ER is composed of two diverse substrates: Sheet and Tubule. It was reported by a U of Colorado group that the oligomeric form of sigma-1 receptors facilitate the formation of ER sheet. Therefore we are currently examining if sigma-1 receptor ligands or different forms of stresses may differentially alter the ER substructures and thus their functions. (B) Our continuing ongoing studies indicate that the sigma-1 receptor mediates addiction processes induced by cocaine or morphine. Cocaine. (a) The induction of neuronal excitability in the VTA DA neuron is the hallmark of the initial action of cocaine addiction. We previously showed that the sigma-1 receptor mediates this action of cocaine by two different mechanisms: (i) decreases the presynaptic GABA input from nuclear accumbens (NAc) to VTA by decreasing the intrinsic excitability of NAc D1 GABA neuron; and (ii) decreasing the presynaptic GABA interneuron's input to the VTA by increasing the retrograde inhibitory endocannabinoid signaling. We are now focusing on the effect of a single dose of cocaine on the VTA DA activity to address the very fundamental initial action of cocaine. We discover that 24 hours after a single dose of cocaine the Ca2+-impermeable GluR2-AMPAR is reduced at the plasma membrane in a sigma-1 receptor-dependent manner, causing an increase of VTA DA excitability. We are now continuing to elucidate the exact molecular mechanism whereby the sigma-1 receptor mediates this action of cocaine. The current hypothesis is that (i) The acute single dose of cocaine through the action sigma-1 receptor may increase the transport of C2+-impermeable GluR2 to the plasma membrane for an initial period of time; (ii) Yet, 24 hours later with the absence of cocaine the sigma-1 receptor cannot transport GluR2 to the plasma membrane, resulting in the activation of AMPAR on the VTA DA neuron. We are using biochemical, cellular biological, molecular biological techniques to prove this hypothesis. (b) We are discovering from preliminary data that an acute single dose of cocaine may disrupt the decision-making seen in the sensory preconditioning test (SPC). Further, this effect of cocaine seems to blocked by sigma-1 receptor antagonist. We are repeating this experiment with more animals and with more fined-tuned laser beams in the behavioral boxes in order to solidify the observations and results. Morphine. (a) We find that the hyperalgesia induced by sub-chronic use of morphine (i.e., OIH) or after the fentanyl vapor self-administration is mediated by the sigma-1 receptor. In the case of morphine we find that it is the sigma-1 receptor in the central amygdala where sigma-1 receptor co-exists with mu opioid receptor and PKC-delta that is playing a key role in the OIH. We demonstrate this result by using sigma-1 receptor genomic knockout mice as well as by using the regionally conditional knockdown mice. We are elucidating the molecular mechanism whereby sigma-1 receptor participates in this opioid-induced hyperalgesia with a hope to use sigma-1 receptor antagonist to treat the hyperalgesia in clinical setting to reduce opioid craving. Ongoing experiments include the elucidation of potential actions of sigma-1 receptor on the two signaling pathways of morphine receptors: the G-protein pathway and the beta-arrestin pathway. (b) We find that the sigma-1 receptor plays a role in the neonatal abstinence syndrome (NAS) in mouse pups whose mothers were treated with morphine during pregnancy. We find that the neuronal hyperexcitability seen in NAS pups are not seen in either sigma-1 receptor knockout pups or pups that receive a single dose of sigma-1 antagonist BD1063. We are investigating the exact molecular underpinning whereby the sigma-1 receptor is doing in the NAS which is devastating many of the American newborns.