NO-released zinc supports the simultaneous binding of Raf-1 and PKCγ cysteine-rich domains to HINT1 protein at the mu-opioid receptor

The Many Faces of Histidine Triad Nucleotide Binding Protein 1 (HINT1)

The histidine triad nucleotide binding protein 1 (HINT1) is a nucleoside phosphoramidase that has garnered interest due to its widespread expression and participation in a broad range of biological processes. Herein, we discuss the role of HINT1 as a regulator of several CNS functions, tumor suppressor, and mast cell activator via its interactions with multiple G-protein-coupled receptors and transcription factors. Importantly, altered HINT1 expression and mutation are connected to the progression of multiple disease states, including several neuropsychiatric disorders, peripheral neuropathy, and tumorigenesis. Additionally, due to its involvement in the activation of several clinically used phosphoramidate prodrugs, tremendous efforts have been made to better understand the interactions behind nucleoside binding and phosphoramidate hydrolysis by HINT1. We detail the substrate specificity and catalytic mechanism of HINT1 hydrolysis, while highlighting the structural biology behind these efforts. The aim of this review is to summarize the multitude of biological and pharmacological functions in which HINT1 participates while addressing the areas of need for future research.

Future perspectives of emerging novel drug targets and immunotherapies to control drug addiction

Substance Use Disorder (SUD) is one of the major mental illnesses that is terrifically intensifying worldwide. It is becoming overwhelming due to limited options for treatment. The complexity of addiction disorders is the main impediment to understanding the pathophysiology of the illness. Hence, unveiling the complexity of the brain through basic research, identification of novel signaling pathways, the discovery of new drug targets, and advancement in cutting-edge technologies will help control this disorder. Additionally, there is a great hope of controlling the SUDs through immunotherapeutic measures like therapeutic antibodies and vaccines. Vaccines have played a cardinal role in eliminating many diseases like polio, measles, and smallpox. Further, vaccines have controlled many diseases like cholera, dengue, diphtheria, Haemophilus influenza type b (Hib), human papillomavirus, influenza, Japanese encephalitis, etc. Recently, COVID-19 was controlled in many countries by vaccination. Currently, continuous effort is done to develop vaccines against nicotine, cocaine, morphine, methamphetamine, and heroin. Antibody therapy against SUDs is another important area where serious attention is required. Antibodies have contributed substantially against many serious diseases like diphtheria, rabies, Crohn's disease, asthma, rheumatoid arthritis, and bladder cancer. Antibody therapy is gaining immense momentum due to its success rate in cancer treatment. Furthermore, enormous advancement has been made in antibody therapy due to the generation of high-efficiency humanized antibodies with a long half-life. The advantage of antibody therapy is its instant outcome. This article's main highlight is discussing the drug targets of SUDs and their associated mechanisms. Importantly, we have also discussed the scope of prophylactic measures to eliminate drug dependence.

Nitrosyl factors play a vital role in the ventilatory depressant effects of fentanyl in unanesthetized rats

There is an urgent need to understand the intracellular mechanisms by which synthetic opioids, such as fentanyl, depress breathing. We used L-NAME (N^G-nitro-L-arginine methyl ester), a nitric oxide synthase (NOS) inhibitor, to provide evidence for a role of nitric oxide (NO) and nitrosyl factors, including S-nitrosothiols, in fentanyl-induced suppression of breathing in rats. We measured breathing parameters using unrestrained plethysmography to record the changes produced by bolus administration of fentanyl (25 μg/kg, IV) in male Sprague Dawley rats that were pretreated with vehicle (saline), L-NAME (50 μmol/kg, IV) or the inactive D-isomer, D-NAME (50 μmol/kg, IV), 15 min previously. L-NAME produced a series of ventilatory changes that included (i) sustained elevations in breathing frequency, due to the reductions in the durations of inspiration and expiration, (ii) sustained elevations in minute ventilation, accompanied by minimal changes in tidal volume, and (iii) increases in inspiratory drive and expiratory drive, and peak inspiratory flow and peak expiratory flow. Subsequent administration of fentanyl in rats pretreated with vehicle produced negative effects on breathing, including decreases in frequency, tidal volume and therefore minute ventilation. Fentanyl elicited markedly different responses in rats that were pretreated with L-NAME, and conclusively, the negative effects of fentanyl were augmented by the NOS inhibitor. D-NAME did not alter ventilatory parameters or modulate the effects of fentanyl on breathing. Our study fully characterized the effects of L-NAME on ventilation in rats and is the first to suggest a potential role of nitrosyl factors in the ventilatory responses to fentanyl. Our data shows that nitrosyl factors reduce the expression of fentanyl-induced changes in ventilation.

Magnesium and Morphine in the Treatment of Chronic Neuropathic Pain-A Biomedical Mechanism of Action

The effectiveness of opioids in the treatment of neuropathic pain is limited. It was demonstrated that magnesium ions (Mg2+), physiological antagonists of N-methyl-D-aspartate receptor (NMDAR), increase opioid analgesia in chronic pain. Our study aimed to determine the molecular mechanism of this action. Early data indicate the cross-regulation of µ opioid receptor (MOR) and NMDAR in pain control. Morphine acting on MOR stimulates protein kinase C (PKC), while induction of NMDAR recruits protein kinase A (PKA), leading to a disruption of the MOR-NMDAR complex and promoting functional changes in receptors. The mechanical Randall-Selitto test was used to assess the effect of chronic Mg2+ and morphine cotreatment on streptozotocin-induced hyperalgesia in Wistar rats. The level of phosphorylated NMDAR NR1 subunit (pNR1) and phosphorylated MOR (pMOR) in the periaqueductal gray matter was determined with the Western blot method. The activity of PKA and PKC was examined by standard enzyme immunoassays. The experiments showed a reduction in hyperalgesia after coadministration of morphine (5 mg/kg intraperitoneally) and Mg2+ (40 mg/kg intraperitoneally). Mg2+ administered alone significantly decreased the level of pNR1, pMOR, and activity of both tested kinases. The results suggest that blocking NMDAR signaling by Mg2+ restores the MOR-NMDAR complex and thus enables morphine analgesia in neuropathic rats.

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Can Src protein tyrosine kinase inhibitors be combined with opioid analgesics? Src and opioid-induced tolerance, hyperalgesia and addiction

The clinical application of opioids may be accompanied by a series of adverse consequences, such as opioid tolerance, opioid-induced hyperalgesia, opioid dependence or addiction. In view of this issue, clinicians are faced with the dilemma of treating various types of pain with or without opioids. In this review, we discuss that Src protein tyrosine kinase plays an important role in these adverse consequences, and Src inhibitors can solve these problems well. Therefore, Src inhibitors have the potential to be used in combination with opioids to achieve synergy. How to combine them together to maximize the analgesic effect while avoiding unnecessary trouble provides a topic for follow-up research.

Human HINT1 Mutant Proteins that Cause Axonal Motor Neuropathy Exhibit Anomalous Interactions with Partner Proteins

The 14 kDa histidine triad nucleotide-binding protein 1 (HINT1) is critical to maintain the normal function of motor neurons. Thus, a series of human HINT1 mutants cause autosomal recessive axonal neuropathy with neuromyotonia. HINT1 establishes a series of regulatory interactions with signaling proteins, some of which are enriched in motor neurons, such as the type 1 sigma receptor or intracellular domain (ICD) of transmembrane teneurin 1, both of which are also implicated in motor disturbances. In a previous study, we reported the capacity of HINT1 to remove the small ubiquitin-like modifier (SUMO) from a series of substrates and the influence of HINT1 mutants on this activity. We now report how human HINT1 mutations affect the interaction of HINT1 with the regulator of its SUMOylase activity, calcium-activated calmodulin, and its substrate SUMO. Moreover, HINT1 mutants exhibited anomalous interactions with G protein coupled receptors, such as the mu-opioid, and with glutamate N-methyl-D-aspartate receptors as well. Additionally, these HINT1 mutants showed impaired associations with transcriptional regulators such as the regulator of G protein signaling Z2 protein and the cleaved N-terminal ICD of teneurin 1. Thus, the altered enzymatic activity of human HINT1 mutants and their anomalous interactions with partner proteins may disrupt signaling pathways essential to the normal function of human motor neurons.

The role of HINT1 protein in morphine addiction: An animal model-based study

Drug addiction is a recurrent, chronic brain disease. The existing treatment methods have limitations, such as poor adherence and inability to completely avoid relapse. Histidine triad nucleotide-binding protein 1 (HINT1) is involved in many neuropsychiatric diseases, such as schizophrenia, pain, and drug dependence. Studies have confirmed that there is a genetic link between HINT1 and addictions such as nicotine and cocaine. However, there is no research on the role of HINT1 protein in morphine addiction at home and abroad. Thus, we designed this project by constructing different types of morphine addiction animal models, including conditioned place preference and behavioral sensitization. We comprehensively examined the participation of HINT1 protein in key brain regions associated with addiction, including prefrontal cortex, nucleus accumbens, corpus striatum, and hippocampus, in different stages of different models. In addition, we used HINT1 knockout mice to establish the above models and physical dependence model to investigate the effect of HINT1 protein deletion on morphine addiction-related behaviors. We found that HINT1 has varying degrees of involvement in different stages of multiple addictive animal models. The absence of HINT1 can attenuate morphine-mediated addictive behavior to a certain extent and can alleviate the withdrawal symptoms of morphine.

Depolarization-Dependent C-Raf Signaling Promotes Hyperexcitability and Reduces Opioid Sensitivity of Isolated Nociceptors after Spinal Cord Injury

Chronic pain caused by spinal cord injury (SCI) is notoriously resistant to treatment, particularly by opioids. After SCI, DRG neurons show hyperactivity and chronic depolarization of resting membrane potential (RMP) that is maintained by cAMP signaling through PKA and EPAC. Importantly, SCI also reduces the negative regulation by Gαi of adenylyl cyclase and its production of cAMP, independent of alterations in G protein-coupled receptors and/or G proteins. Opioid reduction of pain depends on coupling of opioid receptors to Gαi/o family members. Combining high-content imaging and cluster analysis, we show that in male rats SCI decreases opioid responsiveness in vitro within a specific subset of small-diameter nociceptors that bind isolectin B4. This SCI effect is mimicked in nociceptors from naive animals by a modest 5 min depolarization of RMP (15 mm K⁺; -45 mV), reducing inhibition of cAMP signaling by μ-opioid receptor agonists DAMGO and morphine. Disinhibition and activation of C-Raf by depolarization-dependent phosphorylation are central to these effects. Expression of an activated C-Raf reduces sensitivity of adenylyl cyclase to opioids in nonexcitable HEK293 cells, whereas inhibition of C-Raf or treatment with the hyperpolarizing drug retigabine restores opioid responsiveness and blocks spontaneous activity of nociceptors after SCI. Inhibition of ERK downstream of C-Raf also blocks SCI-induced hyperexcitability and depolarization, without direct effects on opioid responsiveness. Thus, depolarization-dependent C-Raf and downstream ERK activity maintain a depolarized RMP and nociceptor hyperactivity after SCI, providing a self-reinforcing mechanism to persistently promote nociceptor hyperexcitability and limit the therapeutic effectiveness of opioids.SIGNIFICANCE STATEMENT Chronic pain induced by spinal cord injury (SCI) is often permanent and debilitating, and usually refractory to treatment with analgesics, including opioids. SCI-induced pain in a rat model has been shown to depend on persistent hyperactivity in primary nociceptors (injury-detecting sensory neurons), associated with a decrease in the sensitivity of adenylyl cyclase production of cAMP to inhibitory Gαi proteins in DRGs. This study shows that SCI and one consequence of SCI (chronic depolarization of resting membrane potential) decrease sensitivity to opioid-mediated inhibition of cAMP and promote hyperactivity of nociceptors by enhancing C-Raf activity. ERK activation downstream of C-Raf is necessary for maintaining ongoing depolarization and hyperactivity, demonstrating an unexpected positive feedback loop to persistently promote pain.

The Axonal Motor Neuropathy-Related HINT1 Protein Is a Zinc- and Calmodulin-Regulated Cysteine SUMO Protease

Aims: Histidine triad nucleotide-binding protein 1 (HINT1) exhibits proapoptotic and tumor-suppressive activity. HINT1 binds to transcription factors such as teneurin1 and to the regulator of G protein signaling 17 (RGS) (Z2) protein, which incorporates the small ubiquitin-like modifier (SUMO), and is implicated in several types of cancer. HINT1 interacts with proteins such as PKCγ and Raf-1 through zinc ions provided by the cysteine-rich domain of RGSZ2 and the coupled neural nitric oxide synthase (nNOS). Recently, a series of HINT1 mutants have been reported to cause human autosomal recessive axonal neuropathy with neuromyotonia (ARAN-NM). However, the specific alteration in the function of HINT1 induced by these mutants remains to be elucidated. Because sumoylation modifies protein association and transcriptional regulation, we investigated whether HINT1 exhibits zinc- and redox-regulated sumoylase activity, which may be altered in those mutants. Results: HINT1 exhibits cysteine protease activity to remove SUMO from a variety of signaling proteins. HINT1 sumoylase activity is blocked by zinc, and it is released by nitric oxide or calcium-activated calmodulin (CaM). HINT1 contains a SUMO-interacting motif (110-116 HIHLHVL) and the catalytic triad Cys84-Asp87-His114 in the C-terminal region. Thus, zinc probably provided by the RGSZ2-nNOS complex may bind to Cys84 to block HINT1 isopeptidase activity. Innovation: To date, HINT1 is the only sumoylase that is regulated by two alternate pathways, redox- and calcium-activated CaM. Conclusion: The 15 human HINT1 mutants reported to cause ARAN-NM exhibited altered sumoylase activity, which may contribute to the onset of this human motor disease.

Endocannabinoid control of glutamate NMDA receptors: the therapeutic potential and consequences of dysfunction

Glutamate is probably the most important excitatory neurotransmitter in the brain. The glutamate N-methyl-D-aspartate receptor (NMDAR) is a calcium-gated channel that coordinates with G protein-coupled receptors (GPCRs) to establish the efficiency of the synaptic transmission. Cross-regulation between these receptors requires the concerted activity of the histidine triad nucleotide-binding protein 1 (HINT1) and of the sigma receptor type 1 (σ1R). Essential brain functions like learning, memory formation and consolidation, mood and behavioral responses to exogenous stimuli depend on the activity of NMDARs. In this biological context, endocannabinoids are released to retain NMDAR activity within physiological limits. The efficacy of such control depends on HINT1/σ1R assisting in the physical coupling between cannabinoid type 1 receptors (CB1Rs) and NMDARs to dampen their activity. Subsequently, the calcium-regulated HINT1/σ1R protein tandem uncouples CB1Rs to prevent NMDAR hypofunction. Thus, early recruitment or a disproportionate cannabinoid induced response can bring about excess dampening of NMDAR activity, impeding its adequate integration with GPCR signaling. Alternatively, this control circuit can apparently be overridden in situations where bursts of NMDAR overactivity provoke convulsive syndromes. In this review we will discuss the possible relevance of the HINT1/σ1R tandem and its use by endocannabinoids to diminish NMDAR activity and their implications in psychosis/schizophrenia, as well as in NMDAR-mediated convulsive episodes.

HINT1 in Neuropsychiatric Diseases: A Potential Neuroplastic Mediator

Although many studies have investigated the functions of histidine triad nucleotide-binding protein 1 (HINT1), its roles in neurobiological processes remain to be fully elucidated. As a member of the histidine triad (HIT) enzyme superfamily, HINT1 is distributed in almost every organ and has both enzymatic and nonenzymatic activity. Accumulating clinical and preclinical evidence suggests that HINT1 may play an important role as a neuroplastic mediator in neuropsychiatric diseases, such as schizophrenia, inherited peripheral neuropathies, mood disorders, and drug addiction. Though our knowledge of HINT1 is limited, it is believed that further research on the neuropathological functions of HINT1 would eventually benefit patients with neuropsychiatric and even psychosomatic diseases.

Reactive Oxygen-Related Diseases: Therapeutic Targets and Emerging Clinical Indications

SIGNIFICANCE

Enhanced levels of reactive oxygen species (ROS) have been associated with different disease states. Most attempts to validate and exploit these associations by chronic antioxidant therapies have provided disappointing results. Hence, the clinical relevance of ROS is still largely unclear.

RECENT ADVANCES

We are now beginning to understand the reasons for these failures, which reside in the many important physiological roles of ROS in cell signaling. To exploit ROS therapeutically, it would be essential to define and treat the disease-relevant ROS at the right moment and leave physiological ROS formation intact. This breakthrough seems now within reach.

CRITICAL ISSUES

Rather than antioxidants, a new generation of protein targets for classical pharmacological agents includes ROS-forming or toxifying enzymes or proteins that are oxidatively damaged and can be functionally repaired.

FUTURE DIRECTIONS

Linking these target proteins in future to specific disease states and providing in each case proof of principle will be essential for translating the oxidative stress concept into the clinic.

The σ1 receptor engages the redox-regulated HINT1 protein to bring opioid analgesia under NMDA receptor negative control

AIMS

The in vivo pharmacology of the sigma 1 receptor (σ1R) is certainly complex; however, σ1R antagonists are of therapeutic interest, because they enhance mu-opioid receptor (MOR)-mediated antinociception and reduce neuropathic pain. Thus, we investigated whether the σ1R is involved in the negative control that glutamate N-methyl-d-aspartate acid receptors (NMDARs) exert on opioid antinociception.

RESULTS

The MOR C terminus carries the histidine triad nucleotide-binding protein 1 (HINT1) coupled to the regulator of G-protein signaling RGSZ2-neural nitric oxide synthase assembly. Activated MORs stimulate the production of nitric oxide (NO), and the redox zinc switch RGSZ2 converts this signal into free zinc ions that are required to recruit the redox sensor PKCγ to HINT1 proteins. Then, PKCγ impairs HINT1-RGSZ2 association and enables σ1R-NR1 interaction with MOR-HINT1 complexes to restrain opioid signaling. The inhibition of NOS or the absence of σ1Rs prevents HINT1-PKCγ interaction, and MOR-NMDAR cross-regulation fails. The σ1R antagonists transitorily remove the binding of σ1Rs to NR1 subunits, facilitate the entrance of negative regulators of NMDARs, likely Ca(2+)-CaM, and prevent NR1 interaction with HINT1, thereby impairing the negative feedback of glutamate on opioid analgesia.

INNOVATION

A redox-regulated process situates MOR signaling under NMDAR control, and in this context, the σ1R binds to the cytosolic C terminal region of the NMDAR NR1 subunit.

CONCLUSION

The σ1R antagonists enhance opioid analgesia in naïve mice by releasing MORs from the negative influence of NMDARs, and they also reset antinociception in morphine tolerant animals. Moreover, σ1R antagonists alleviate neuropathic pain, probably by driving the inhibition of up-regulated NMDARs.

Detecting zinc release induced by mu-opioid receptor agonists in brain slices

After iron, zinc is the second most abundant transition metal in living organisms and it is known to be a contributory factor in a series of neurological disorders. In biological systems zinc exists as either bound Zn(2+), representing the majority of the total zinc in tissues, or free (chelatable) Zn(2+). Several fluorescents dyes have been developed to detect free zinc when it is released from zinc-binding proteins, which occurs via redox mechanisms in response to the stimulation of a number of neurotransmitter receptors, including the μ opioid receptor. Here we describe a detailed protocol to detect drug stimulated intracellular zinc release in rodent brain slices using time-lapse microscopy and fluorescence imaging.

HINT1 protein cooperates with cannabinoid 1 receptor to negatively regulate glutamate NMDA receptor activity

Background

G protein-coupled receptors (GPCRs) are the targets of a large number of drugs currently in therapeutic use. Likewise, the glutamate ionotropic N-methyl-D-aspartate receptor (NMDAR) has been implicated in certain neurological disorders, such as neurodegeration, neuropathic pain and mood disorders, as well as psychosis and schizophrenia. Thus, there is now an important need to characterize the interactions between GPCRs and NMDARs. Indeed, these interactions can produce distinct effects, and whereas the activation of Mu-opioid receptor (MOR) increases the calcium fluxes associated to NMDARs, that of type 1 cannabinoid receptor (CNR1) antagonizes their permeation. Notably, a series of proteins interact with these receptors affecting their responses and interactions, and then emerge as novel therapeutic targets for the aforementioned pathologies.

Results

We found that in the presence of GPCRs, the HINT1 protein influences the activity of NMDARs, whereby NMDAR activation was enhanced in CNR1^+/+/HINT1^-/- cortical neurons and the cannabinoid agonist WIN55,212-2 provided these cells with no protection against a NMDA insult. NMDAR activity was normalized in these cells by the lentiviral expression of HINT1, which also restored the neuroprotection mediated by cannabinoids. NMDAR activity was also enhanced in CNR1^-/-/HINT1^+/+ neurons, although this activity was dampened by the expression of GPCRs like the MOR, CNR1 or serotonin 1A (5HT1AR).

Conclusions

The HINT1 protein plays an essential role in the GPCR-NMDAR connection. In the absence of receptor activation, GPCRs collaborate with HINT1 proteins to negatively control NMDAR activity. When activated, most GPCRs release the control of HINT1 and NMDAR responsiveness is enhanced. However, cannabinoids that act through CNR1 maintain the negative control of HINT1 on NMDAR function and their protection against glutamate excitotoxic insult persists.

Common trace elements alleviate pain in an experimental mouse model

Trace elements represent a group of essential metals or metaloids necessary for life, present in minute amounts. Analgesic adjuvants can enhance the effect of other pain drugs or be used for pain control themselves. Previous studies on the effects of trace elements on nociception and their potential use as analgesic adjuvants have yielded conflicting results. In this study, we tested the hypothesis that three vital trace elements (Zn²⁺, Mg²⁺, Cu²⁺) have direct antinociceptive effects. Groups of eight Swiss mice were intraperitoneally (i.p) injected with incremental concentrations of Zn²⁺ sulfate (0.5, 2.0 mg/kg), Zn²⁺ citrate (0.125, 0.5 mg/kg), Mg²⁺ chloride (37.5, 75, 150 mg/kg), Cu²⁺ chloride (0.5, 1.0, 2.0 mg/kg), and Cu²⁺ sulfate (0.5, 1.0 mg/kg) or saline (control). Evaluations were made by hot plate (HP) and tail flick (TF) tests for central antinociceptive effect, writhing test (WT) for visceral antinociceptive effect, and activity cage (AC) test for spontaneous behavior. Zn²⁺ induced pain inhibition in HP/TF tests (up to 17%) and WT (up to 25%), with no significant differences among the salts used. Mg²⁺ salts induced pain inhibition for all performed tests (up to 85% in WT). Cu²⁺ salts showed antinociceptive effects for HP/TF (up to 28.6%) and WT (57.28%). Only Mg²⁺ and Cu²⁺ salts have displayed significant effects in AC (Mg²⁺ anxiolytic/depressant effect; Cu²⁺ anxiolytic effect). We interpret these data to mean that all tested trace elements induced antinociceptive effects in central and visceral pain tests. Our data indicate the potential use of these cheap adjuvants in pain therapy.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

GPCRs promote the release of zinc ions mediated by nNOS/NO and the redox transducer RGSZ2 protein

AIMS

Morphine signaling via the μ-opioid receptor (MOR) is coupled to redox-dependent zinc release from endogenous stores. Thus, MOR activation stimulates the complex formed by RGSZ2 (a regulator of G protein signaling) and neural nitric oxide synthase (nNOS) to produce NO, and to recruit PKCγ and Raf-1 in a zinc-dependent manner. Accordingly, we investigated whether redox regulation of zinc metabolism was unique to the MOR, or if it is a signaling mechanism shared by G-protein coupled receptors (GPCRs).

RESULTS

A physical interaction with the RGSZ2-nNOS complex was detected for the following GPCRs: neuropeptides, MOR and δ-opioid (DOR); biogenic amines, 5HT1A, 5HT2A, α2A, D1 and D2; acetylcholine, muscarinic M2 and M4; excitatory amino acid glutamate, mGlu2 and mGlu5; and derivatives of arachidonic acid (anandamide), CB1. Agonist activation of these receptors induced the release of zinc ions from the RGSZ2 zinc finger via a nNOS/NO-dependent mechanism, recruiting PKCγ and Raf-1 to the C terminus or the third internal loop of the GPCR.

INNOVATION

A series of GPCRs share an unexpected mechanistic feature, the nNOS/NO-dependent regulation of zinc ion signaling via a redox mechanism. The RGSZ2 protein emerges as a potential redox zinc switch that converts NO signals into zinc signals, thereby able to modulate the function of redox sensor proteins like PKCγ or Raf-1.

CONCLUSION

Redox mechanisms are crucial for the successful propagation of GPCR signals in neurons. Thus, dysfunctions of GPCR-regulated NO/zinc signaling may contribute to neurodegenerative and mood disorders such as Alzheimer's disease and depression.

Involvement of NOX1/NADPH oxidase in morphine-induced analgesia and tolerance

The involvement of reactive oxygen species (ROS) in morphine-induced analgesia and tolerance has been suggested, yet how and where ROS take part in these processes remains largely unknown. Here, we report a novel role for the superoxide-generating enzyme NOX1/NADPH oxidase in the regulation of analgesia and acute analgesic tolerance. In mice lacking Nox1 (Nox1(-/Y)), the magnitude of the analgesia induced by morphine was significantly augmented. More importantly, analgesic tolerance induced by repeated administration of morphine was significantly suppressed compared with that in the littermates, wild-type Nox1(+/Y). In a membrane fraction obtained from the dorsal spinal cord, no difference was observed in morphine-induced [(35)S]GTPγS-binding between the genotypes, whereas morphine-stimulated GTPase activity was significantly attenuated in Nox1(-/Y). At 2 h after morphine administration, a significant decline in [(35)S]GTPγS-binding was observed in Nox1(+/Y) but not in Nox1(-/Y). No difference in the maximal binding and affinity of [(3)H]DAMGO was observed between the genotypes, but the translocation of protein kinase C isoforms to the membrane fraction following morphine administration was almost completely abolished in Nox1(-/Y). Finally, the phosphorylation of RGS9-2 and formation of a complex by Gαi2/RGS9-2 with 14-3-3 found in morphine-treated Nox1(+/Y) were significantly suppressed in Nox1(-/Y). Together, these results suggest that NOX1/NADPH oxidase attenuates the pharmacological effects of opioids by regulating GTPase activity and the phosphorylation of RGS9-2 by protein kinase C. NOX1/NADPH oxidase may thus be a novel target for the development of adjuvant therapy to retain the beneficial effects of morphine.

The mu-opioid receptor and the NMDA receptor associate in PAG neurons: implications in pain control

The capacity of opioids to alleviate inflammatory pain is negatively regulated by the glutamate-binding N-methyl-D-aspartate receptor (NMDAR). Increased activity of this receptor complicates the clinical use of opioids to treat persistent neuropathic pain. Immunohistochemical and ultrastructural studies have demonstrated the coexistence of both receptors within single neurons of the CNS, including those in the mesencephalic periaqueductal gray (PAG), a region that is implicated in the opioid control of nociception. We now report that mu-opioid receptors (MOR) and NMDAR NR1 subunits associate in the postsynaptic structures of PAG neurons. Morphine disrupts this complex by protein kinase-C (PKC)-mediated phosphorylation of the NR1 C1 segment and potentiates the NMDAR-CaMKII, pathway that is implicated in morphine tolerance. Inhibition of PKC, but not PKA or GRK2, restored the MOR-NR1 association and rescued the analgesic effect of morphine as well. The administration of N-methyl-D-aspartic acid separated the MOR-NR1 complex, increased MOR Ser phosphorylation, reduced the association of the MOR with G-proteins, and diminished the antinociceptive capacity of morphine. Inhibition of PKA, but not PKC, CaMKII, or GRK2, blocked these effects and preserved morphine antinociception. Thus, the opposing activities of the MOR and NMDAR in pain control affect their relation within neurons of structures such as the PAG. This finding could be exploited in developing bifunctional drugs that would act exclusively on those NMDARs associated with MORs.

SUMO-SIM interactions regulate the activity of RGSZ2 proteins

The RGSZ2 gene, a regulator of G protein signaling, has been implicated in cognition, Alzheimer's disease, panic disorder, schizophrenia and several human cancers. This 210 amino acid protein is a GTPase accelerating protein (GAP) on Gαi/o/z subunits, binds to the N terminal of neural nitric oxide synthase (nNOS) negatively regulating the production of nitric oxide, and binds to the histidine triad nucleotide-binding protein 1 at the C terminus of different G protein-coupled receptors (GPCRs). We now describe a novel regulatory mechanism of RGS GAP function through the covalent incorporation of Small Ubiquitin-like MOdifiers (SUMO) into RGSZ2 RGS box (RH) and the SUMO non covalent binding with SUMO-interacting motifs (SIM): one upstream of the RH and a second within this region. The covalent attachment of SUMO does not affect RGSZ2 binding to GPCR-activated GαGTP subunits but abolishes its GAP activity. By contrast, non-covalent binding of SUMO with RH SIM impedes RGSZ2 from interacting with GαGTP subunits. Binding of SUMO to the RGSZ2 SIM that lies outside the RH does not affect GαGTP binding or GAP activity, but it could lead to regulatory interactions with sumoylated proteins. Thus, sumoylation and SUMO-SIM interactions constitute a new regulatory mechanism of RGS GAP function and therefore of GPCR cell signaling as well.