Bi-directional heterologous desensitization between the major HIV-1 co-receptor CXCR4 and the kappa-opioid receptor

Oral difelikefalin reduces moderate to severe pruritus and expression of pruritic and inflammatory biomarkers in subjects with atopic dermatitis

BACKGROUND

Pruritus is the most common and burdensome symptom of atopic dermatitis (AD). Pruritus-targeted treatments in AD are lacking, particularly for patients with milder skin disease.

OBJECTIVE

We sought to evaluate the impact of the selective κ-opioid receptor agonist difelikefalin (DFK) on pruritus intensity and pruritus- and immune-related biomarkers in subjects with moderate to severe AD-related pruritus.

METHODS

A phase 2 clinical trial investigated the efficacy and safety of oral DFK 0.25, 0.5, and 1.0 mg in subjects with moderate to severe AD-related pruritus. A biomarker substudy evaluated the effects of DFK on the expression of pruritus, TH2-associated genes, and skin barrier-related genes.

RESULTS

In the clinical trial (N = 401), all DFK doses reduced itch versus placebo; however, the results were not statistically significant at week 12. In a subgroup of subjects in the trial with mild to moderate skin inflammation and moderate to severe itch (itch-dominant AD phenotype), DFK reduced itch at week 12 versus placebo. In the biomarker substudy, DFK downregulated the expression of key pruritus-related genes (eg, IL-31 and TRPV1) and the AD phenotype (eg, CCL17). Gene set variation analysis confirmed that DFK, but not placebo, downregulated pruritus-related genes and TH2 pathways. DFK improved skin barrier integrity markers and upregulated the expression of claudins and lipid metabolism-associated genes (eg, SEC14L6, ELOVL3, CYP1A2, and AKR1D1).

CONCLUSIONS

DFK treatment reduced itch in subjects with moderate to severe AD-related pruritus, particularly those with an "itch-dominant" AD phenotype, and had an impact on the expression of pruritus, TH2-associated genes, and skin barrier-related genes. DFK is a promising therapy for AD-related pruritus; further clinical studies are warranted.

The Kappa Opioid Receptor: A Promising Therapeutic Target for Multiple Pathologies

Kappa-opioid receptors (KOR) are widely expressed throughout the central nervous system, where they modulate a range of physiological processes depending on their location, including stress, mood, reward, pain, inflammation, and remyelination. However, clinical use of KOR agonists is limited by adverse effects such as dysphoria, aversion, and sedation. Within the drug-development field KOR agonists have been extensively investigated for the treatment of many centrally mediated nociceptive disorders including pruritis and pain. KOR agonists are potential alternatives to mu-opioid receptor (MOR) agonists for the treatment of pain due to their anti-nociceptive effects, lack of abuse potential, and reduced respiratory depressive effects, however, dysphoric side-effects have limited their widespread clinical use. Other diseases for which KOR agonists hold promising therapeutic potential include pruritis, multiple sclerosis, Alzheimer’s disease, inflammatory diseases, gastrointestinal diseases, cancer, and ischemia. This review highlights recent drug-development efforts targeting KOR, including the development of G-protein–biased ligands, mixed opioid agonists, and peripherally restricted ligands to reduce side-effects. We also highlight the current KOR agonists that are in preclinical development or undergoing clinical trials.

CXCR4/CXCL12 Activities in the Tumor Microenvironment and Implications for Tumor Immunotherapy

Simple Summary

Chemokines are small soluble proteins that control and regulate cell trafficking within and between tissues by binding to their receptors. Among them, CXCL12 and its receptor CXCR4 appeared with ancestral vertebrates, are expressed almost ubiquitously, and play essential roles in embryogenesis and organogenesis. In addition, CXCL12 and CXCR4 are involved in antigen recognition by T and B cells and in shaping the tumor microenvironment (TME), mainly towards dampening immune responses. New data indicate that CXCR4 interacts with the surface protein CD47 in a novel form of immunosurveillance, called ImmunoGenic Surrender (IGS). Following the co-internalization of CXCR4 and CD47 in tumor cells, macrophages phagocytose them and cross-present their antigens to the adaptive immune system, leading to tumor rejection in a fraction of mice. All of these specific activities of CXCL12 and CXCR4 in antigen presentation might be complementary to current immunotherapies.

Abstract

CXCR4 is a G-Protein coupled receptor that is expressed nearly ubiquitously and is known to control cell migration via its interaction with CXCL12, the most ancient chemokine. The functions of CXCR4/CXCL12 extend beyond cell migration and involve the recognition and disposal of unhealthy or tumor cells. The CXCR4/CXCL12 axis plays a relevant role in shaping the tumor microenvironment (TME), mainly towards dampening immune responses. Notably, CXCR4/CXCL12 cross-signal via the T and B cell receptors (TCR and BCR) and co-internalize with CD47, promoting tumor cell phagocytosis by macrophages in an anti-tumor immune process called ImmunoGenic Surrender (IGS). These specific activities in shaping the immune response might be exploited to improve current immunotherapies.

Chemokines, cytokines and substance use disorders

Efficacious pharmacotherapies for the treatment of substance use disorders need to be expanded and improved. Non-neuronal cells, particularly astrocytes and microglia, have emerged as therapeutic targets for the development of pharmacotherapies to treat dependence and relapse that accompanies chronic drug use. Cytokines and chemokines are neuroimmune factors expressed in neurons, astrocytes, and microglia that demonstrate promising clinical utility as therapeutic targets for substance use disorders. In this review, we describe a role for cytokines and chemokines in the rewarding and reinforcing effects of alcohol, opioids, and psychostimulants. We also discuss emerging cytokine- and chemokine-based therapeutic strategies that differ from conventional strategies directed toward transporters and receptors within the dopamine, glutamate, GABA, serotonin, and GABA systems.

Kappa Opioid Receptor Expression and Function in Cells of the Immune System

The kappa opioid receptor (KOR) is expressed on a number of hematopoietic cell populations, based on both protein binding analysis and the detection of kappa opioid receptor gene (Oprk1) transcripts. There are prominent Oprk1 splice variants that are expressed in the mouse and human brain cells and leukocytes. The activation of KOR results in reduced antibody production, an inhibition of phagocytic cell activity, an inhibition of T cell development, alterations in the production of various pro-inflammatory cytokines, chemokines, and the receptors for these mediators. Finally, the activation of KOR also leads to the regulation of receptor functional activity of chemokine receptors through the process of heterologous desensitization. The functional activity of KOR is important for the regulation of inflammatory responses and may provide opportunities for the development of therapeutics for the treatment of inflammatory disease states.

Opioid and neuroHIV Comorbidity - Current and Future Perspectives

With the current national opioid crisis, it is critical to examine the mechanisms underlying pathophysiologic interactions between human immunodeficiency virus (HIV) and opioids in the central nervous system (CNS). Recent advances in experimental models, methodology, and our understanding of disease processes at the molecular and cellular levels reveal opioid-HIV interactions with increasing clarity. However, despite the substantial new insight, the unique impact of opioids on the severity, progression, and prognosis of neuroHIV and HIV-associated neurocognitive disorders (HAND) are not fully understood. In this review, we explore, in detail, what is currently known about mechanisms underlying opioid interactions with HIV, with emphasis on individual HIV-1-expressed gene products at the molecular, cellular and systems levels. Furthermore, we review preclinical and clinical studies with a focus on key considerations when addressing questions of whether opioid-HIV interactive pathogenesis results in unique structural or functional deficits not seen with either disease alone. These considerations include, understanding the combined consequences of HIV-1 genetic variants, host variants, and μ-opioid receptor (MOR) and HIV chemokine co-receptor interactions on the comorbidity. Lastly, we present topics that need to be considered in the future to better understand the unique contributions of opioids to the pathophysiology of neuroHIV.

Graphical Abstract

The Endogenous Opioid System in Schizophrenia and Treatment Resistant Schizophrenia: Increased Plasma Endomorphin 2, and κ and μ Opioid Receptors Are Associated with Interleukin-6

BACKGROUND

activation of the immune-inflammatory response system (IRS) and the compensatory immune-regulatory system (CIRS) plays a key role in schizophrenia (SCZ) and treatment resistant SCZ. There are only a few data on immune and endogenous opioid system (EOS) interactions in SCZ and treatment resistant SCZ.

METHODS

we examined serum β-endorphin, endomorphin-2 (EM2), mu-opioid (MOR) and kappa-opioid (KOR) receptors, and interleukin (IL)-6 and IL-10 in 60 non responders to treatment (NRTT), 55 partial RTT (PRTT) and 43 normal controls.

RESULTS

serum EM2, KOR, MOR, IL-6 and IL-10 were significantly increased in SCZ as compared with controls. β-endorphin, EM2, MOR and IL-6 were significantly higher in NRTT than in PRTT. There were significant correlations between IL-6, on the one hand, and β-endorphin, EM2, KOR, and MOR, on the other, while IL-10 was significantly correlated with MOR only. A large part of the variance in negative symptoms, psychosis, hostility, excitation, mannerism, psychomotor retardation and formal thought disorders was explained by the combined effects of EM2 and MOR with or without IL-6 while increased KOR was significantly associated with all symptom dimensions. Increased MOR, KOR, EM2 and IL-6 were also associated with neurocognitive impairments including in episodic, semantic and working memory and executive functions.

CONCLUSION

the EOS contributes to SCZ symptomatology, neurocognitive impairments and a non-response to treatment. In SCZ, EOS peptides/receptors may exert CIRS functions, whereas increased KOR levels may contribute to the pathophysiology of SCZ and EM2 and KOR to a non-response to treatment.

New and emerging treatments for inflammatory itch

OBJECTIVE

To summarize recent therapeutic developments for chronic pruritus with a focus on allergic and type 2 inflammatory pathways.

DATA SOURCES

Literature search of PubMed, industry websites, and review of the ClinicalTrials.gov database.

STUDY SELECTIONS

Peer-reviewed publications and public disclosures by industry relating to chronic pruritus pathophysiology and therapeutics.

RESULTS

Histamine and immunoglobulin E remain primary targets for the treatment of itch in the setting of chronic urticaria. More recently, blockade of type 2 immune cell-associated cytokines, including interleukin (IL) 4, IL-13, and IL-31, and the epithelial cell-derived cytokines, specifically IL-33 and thymic stromal lymphopoietin, has and is revolutionizing the treatment of chronic pruritic dermatoses, such as atopic dermatitis and prurigo nodularis. Other novel targets include histamine receptor 4, Janus kinases, κ-opioid receptor, neurokinin 1 receptor, and phosphodiesterase 4.

CONCLUSION

Advances in our understanding of the neuroimmunology of chronic pruritus have led to the identification of new therapeutic targets and the rapid development of cutting-edge clinical trials. Although incredible advances have already been made, chronic itch continues to be an area of great unmet need.

Bidirectional Regulation of Opioid and Chemokine Function

The opioid family of GPCRs consists of the classical opioid receptors, designated μ-, κ-, and δ-opioid receptors, and the orphanin-FQ receptor, and these proteins are expressed on both neuronal and hematopoietic cells. A number of laboratories have reported that an important degree of cross-talk can occur between the opioid receptors and the chemokine and chemokine receptor families. As a part of this, the opioid receptors are known to regulate the expression of certain chemokines and chemokine receptors, including those that possess strong pro-inflammatory activity. At the level of receptor function, it is clear that certain members of the chemokine family can mediate cross-desensitization of the opioid receptors. Conversely, the opioid receptors are all able to induce heterologous desensitization of some of the chemokine receptors. Consequently, activation of one or more of the opioid receptors can selectively cross-desensitize chemokine receptors and regulate chemokine function. These cross-talk processes have significant implications for the inflammatory response, since the regulation of both the recruitment of inflammatory cells, as well as the sensation of pain, can be controlled in this way.

The Role of Opioid Receptors in Immune System Function

Research on the effects of opioids on immune responses was stimulated in the 1980s by the intersection of use of intravenous heroin and HIV infection, to determine if opioids were enhancing HIV progression. The majority of experiments administering opioid alkaloids (morphine and heroin) in vivo, or adding these drugs to cell cultures in vitro, showed that they were immunosuppressive. Immunosuppression was reported as down-regulation: of Natural Killer cell activity; of responses of T and B cells to mitogens; of antibody formation in vivo and in vitro; of depression of phagocytic and microbicidal activity of neutrophils and macrophages; of cytokine and chemokine production by macrophages, microglia, and astrocytes; by sensitization to various infections using animal models; and by enhanced replication of HIV in vitro. The specificity of the receptor involved in the immunosuppression was shown to be the mu opioid receptor (MOR) by using pharmacological antagonists and mice genetically deficient in MOR. Beginning with a paper published in 2005, evidence was presented that morphine is immune-stimulating via binding to MD2, a molecule associated with Toll-like Receptor 4 (TLR4), the receptor for bacterial lipopolysaccharide (LPS). This concept was pursued to implicate inflammation as a mechanism for the psychoactive effects of the opioid. This review considers the validity of this hypothesis and concludes that it is hard to sustain. The experiments demonstrating immunosuppression were carried out in vivo in rodent strains with normal levels of TLR4, or involved use of cells taken from animals that were wild-type for expression of TLR4. Since engagement of TLR4 is universally accepted to result in immune activation by up-regulation of NF-κB, if morphine were binding to TLR4, it would be predicted that opioids would have been found to be pro-inflammatory, which they were not. Further, morphine is immunosuppressive in mice with a defective TLR4 receptor. Morphine and morphine withdrawal have been shown to permit leakage of Gram-negative bacteria and LPS from the intestinal lumen. LPS is the major ligand for TLR4. It is proposed that an occult variable in experiments where morphine is being proposed to activate TLR4 is actually underlying sepsis induced by the opioid.

Context-Dependent Signaling of CXC Chemokine Receptor 4 and Atypical Chemokine Receptor 3

G protein-coupled receptors (GPCRs) are regulated by complex molecular mechanisms, both in physiologic and pathologic conditions, and their signaling can be intricate. Many factors influence their signaling behavior, including the type of ligand that activates the GPCR, the presence of interacting partners, the kinetics involved, or their location. The two CXC-type chemokine receptors, CXC chemokine receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3), both members of the GPCR superfamily, are important and established therapeutic targets in relation to cancer, human immunodeficiency virus infection, and inflammatory diseases. Therefore, it is crucial to understand how the signaling of these receptors works to be able to specifically target them. In this review, we discuss how the signaling pathways activated by CXCR4 and ACKR3 can vary in different situations. G protein signaling of CXCR4 depends on the cellular context, and discrepancies exist depending on the cell lines used. ACKR3, as an atypical chemokine receptor, is generally reported to not activate G proteins but can broaden its signaling spectrum upon heteromerization with other receptors, such as CXCR4, endothelial growth factor receptor, or the α ₁-adrenergic receptor (α ₁-AR). Also, CXCR4 forms heteromers with CC chemokine receptor (CCR) 2, CCR5, the Na⁺/H⁺ exchanger regulatory factor 1, CXCR3, α ₁-AR, and the opioid receptors, which results in differential signaling from that of the monomeric subunits. In addition, CXCR4 is present on membrane rafts but can go into the nucleus during cancer progression, probably acquiring different signaling properties. In this review, we also provide an overview of the currently known critical amino acids involved in CXCR4 and ACKR3 signaling.

Activated niacin receptor HCA2 inhibits chemoattractant-mediated macrophage migration via Gβγ/PKC/ERK1/2 pathway and heterologous receptor desensitization

The niacin receptor HCA2 is implicated in controlling inflammatory host responses with yet poorly understood mechanistic basis. We previously reported that HCA2 in A431 epithelial cells transduced Gβγ-protein kinase C- and Gβγ-metalloproteinase/EGFR-dependent MAPK/ERK signaling cascades. Here, we investigated the role of HCA2 in macrophage-mediated inflammation and the underlying mechanisms. We found that proinflammatory stimulants LPS, IL-6 and IL-1β up-regulated the expression of HCA2 on macrophages. Niacin significantly inhibited macrophage chemotaxis in response to chemoattractants fMLF and CCL2 by disrupting polarized distribution of F-actin and Gβ protein. Niacin showed a selected additive effect on chemoattractant-induced activation of ERK1/2, JNK and PI3K pathways, but only the MEK inhibitor UO126 reduced niacin-mediated inhibition of macrophage chemotaxis, while activation of ERK1/2 by EGF alone did not inhibit fMLF-mediated migration of HEK293T cells co-expressing HCA2 and fMLF receptor FPR1. In addition, niacin induced heterologous desensitization and internalization of FPR1. Furthermore, niacin rescued mice from septic shock by diminishing inflammatory symptoms and the effect was abrogated in HCA2^-/- mice. These results suggest that Gβγ/PKC-dependent ERK1/2 activation and heterologous desensitization of chemoattractant receptors are involved in the inhibition of chemoattractant-induced migration of macrophages by niacin. Thus, HCA2 plays a critical role in host protection against pro-inflammatory insults.

Molecular signatures of mu opioid receptor and somatostatin receptor 2 in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC), a particularly aggressive malignancy, has been linked to atypical levels, certain mutations, and aberrant signaling of G-protein-coupled receptors (GPCRs). GPCRs have been challenging to target in cancer because they organize into complex networks in tumor cells. To dissect such networks with nanometer-scale precision, here we combine traditional biochemical approaches with superresolution microscopy methods. A novel interaction specific to PDAC is identified between mu opioid receptor (MOR) and somatostatin receptor 2 (SSTR2). Although MOR and SSTR2 did not colocalize in healthy pancreatic cells or matching healthy patient tissues, the pair did significantly colocalize in pancreatic cancer cells, multicellular tumor spheroids, and cancerous patient tissues. Moreover, this association in pancreatic cancer cells correlated with functional cross-talk and increased metastatic potential of cells. Coactivation of MOR and SSTR2 in PDAC cells led to increased expression of mesenchymal markers and decreased expression of an epithelial marker. Together these results suggest that the MOR-SSTR2 heteromer may constitute a novel therapeutic target for PDAC.

Opioids and Viral Infections: A Double-Edged Sword

Opioids and their receptors have received remarkable attention because they have the ability to alter immune function, which affects disease progression. In vitro and in vivo findings as well as observations in humans indicate that opioids and their receptors positively or negatively affect viral replication and virus-mediated pathology. The present study reviews recent insights in the role of opioids and their receptors in viral infections and discusses possible therapeutic opportunities. This review supports the emerging concept that opioids and their receptors have both favorable and unfavorable effects on viral disease, depending on the type of virus. Targeting of the opioid system is a potential option for developing effective therapies; however caution is required in relation to the beneficial functions of opioid systems.

Dopamine D2 and serotonin 5-HT1A receptor interaction in the context of the effects of antipsychotics - in vitro studies

The serotonin 5-HT1A receptor (5-HT1 A R) and dopamine D2 receptor (D2 R) have been implicated as important sites of action in antipsychotics. Several lines of evidence indicate the key role of G protein-coupled receptors (GPCRs) heteromers in pathophysiology of schizophrenia and highlight these complexes as novel drug targets. Because heterodimers can form only on those cells co-expressing constituent receptors, they present a target of high pharmacological specificity in the context of biochemical effects induced by antipsychotic drugs. In studies conducted in the HEK 293 cell line, we demonstrated that 5-HT1 A R and D2 R are able to form constitutive heterodimers, and antipsychotic drugs (clozapine, olanzapine, aripiprazole, and lurasidone) enhanced this process, with clozapine being most effective. Various functional tests (cAMP and IP1 as well as ERK activation) indicated that the drugs had different effects on signal transduction by the heteromer. Interestingly, co-incubation of heterodimer-expressing HEK 293 cells with clozapine and the 5-HT1 A R agonist 8-OH DPAT potentiated post-synaptic effects, especially with respect to ERK activation. Our results indicate that the D2 -5-HT1A complex possesses biochemical, pharmacological, and functional properties distinct from those of mono- and homomers. This result has implications for the development of improved pharmacotherapy for schizophrenia or other disorders (activating the heteromer might be cognitive enhancing, since it is expressed in frontal cortex) through the specific targeting of heterodimers. We reported the constitutive formation of D2 -5-HT1A heteromers, which possess biochemical, pharmacological, and functional properties distinct from those of mono- and homomers, as revealed by antipsychotics action. We also showed that these two receptors are co-expressed in mouse cortical neurons; therefore their potential to heterodimerize may comprise an essential target for the development of novel strategies for schizophrenia treatment.

The orexin 1 receptor modulates kappa opioid receptor function via a JNK-dependent mechanism

The orexin 1 receptor (OX1R) and the kappa opioid receptor (KOR) are two G protein-coupled receptors (GPCRs) previously demonstrated to play important roles in modulating the rewarding effects of drugs of abuse such as cocaine. Using cells heterologously expressing both receptors, we investigated whether OX1R can regulate the function of KOR and vice versa. Activation of OX1R was found to attenuate agonist-activated KOR-mediated inhibition of cAMP production. In contrast, agonist-activated KOR-mediated β-arrestin recruitment and p38 activation were enhanced in the presence of activated OX1R. These effects are independent of OX1R internalization but are blocked in the presence of the JNK inhibitor SP-600125. OX1R signaling does not affect ligand binding by KOR. Taken together, these data suggest that OX1R signaling can modulate KOR function in a JNK-dependent manner, promoting preferential signaling of KOR via β-arrestin/p38 rather than Gαi. Conversely, Gαq coupling of OX1R is unaffected by activation of KOR, suggesting that this crosstalk is unidirectional. Given that KOR Gαi-mediated signaling events and β-arrestin-mediated signaling events are thought to promote distinct cellular responses and physiological outcomes downstream of KOR activation, this mechanism may have important implications on the behavioral effects of KOR activity.

Opioid and chemokine receptor crosstalk: a promising target for pain therapy?

Chemokines and opioids are important regulators of immune, inflammatory and neuronal responses in peripheral and central pain pathways. Recent studies have provided insights into the functional interactions between chemokine receptors and opioid receptors, and their role in pain modulation. In this Progress article, we discuss how crosstalk between these two systems might provide a molecular and cellular framework for the development of novel analgesic therapies for the management of acute and/or chronic pain.

Cross-desensitization of CCR1, but not CCR2, following activation of the formyl peptide receptor FPR1

The cross-regulation of G protein-coupled receptors (GPCRs) plays an important role in the immune response. Studies from several laboratories have suggested that a hierarchy of sensitivities to cross-desensitization exists for the chemoattractant GPCRs. We carried out experiments to study the capacity of the formyl peptide receptor-1 (FPR1) to desensitize chemokine receptors CCR1 and CCR2. Our results show that activation of FPR1 resulted in the desensitization and partial internalization of CCR1, but not CCR2, in both primary human monocytes and HEK293 cells coexpressing CCR1, CCR2, and FPR1 (HR1R2F cells). The desensitization of CCR1 by FPR1 stimulation was not due to the simple depletion of the Ca(2+) stores, but was dependent on activation of protein kinase C. Furthermore, we found that the cross-desensitization of CCR1 by FPR1 was associated with CCR1 phosphorylation and moderate reduction of CCR1 cell-surface expression. In contrast, CCR2 was not phosphorylated or internalized after FPR1 activation. Additional studies showed that optimal cross talk between FPR1 and CCR1 was dependent on the functional activity of protein kinase Cβ. These results provide a mechanistic basis for the capacity of certain GPCR ligands to exert rapid and selective cross-inactivation of other chemoattractant receptors, and suggest that FPR1 is able to exert "traffic control" in the migration of inflammatory cells by rapidly inhibiting the cell responses to potentially "low-priority" chemoattractants such as CCR1 agonists without inhibiting the response to "higher priority" CCR2 chemoattractants.

Src family kinases involved in CXCL12-induced loss of acute morphine analgesia

Functional interactions between the chemokine receptor CXCR4 and opioid receptors have been reported in the brain, leading to a decreased morphine analgesic activity. However the cellular mechanisms responsible for this loss of opioid analgesia are largely unknown. Here we examined whether Src family-kinases (SFK)-linked mechanisms induced by CXCR4 contributed to the loss of acute morphine analgesia and could represent a new physiological anti-opioid signaling pathway. In this way, we showed by immunohistochemistry and western blot that CXCL12 rapidly activated SFK phosphorylation in vitro in primary cultured lumbar rat dorsal root ganglia (DRG) but also in vivo in the DRG and the spinal cord. We showed that SFK activation occurred in a sub population of sensory neurons, in spinal microglia but also in spinal nerve terminals expressing mu-(MOR) and delta-opioid (DOR) receptor. In addition we described that CXCR4 is detected in MOR- and DOR-immunoreactive neurons in the DRG and spinal cord. In vivo, we demonstrated that an intrathecal administration of CXCL12 (1μg) significantly attenuated the subcutaneous morphine (4mg/kg) analgesia. Conversely, pretreatment with a potent CXCR4 antagonist (5μg) significantly enhanced morphine analgesia. Similar effects were obtained after an intrathecal injection of a specific SFK inhibitor, PP2 (10μg). Furthermore, PP2 abrogated CXCL12-induced decrease in morphine analgesia by suppressing SFK activation in the spinal cord. In conclusion, our data highlight that CXCL12-induced loss of acute morphine analgesia is linked to Src family kinases activation.

Neurocognitive and neuroinflammatory correlates of PDYN and OPRK1 mRNA expression in the anterior cingulate in postmortem brain of HIV-infected subjects

Chronic inflammation may contribute to neuropsychological impairments in individuals with HIV, and modulation of this inflammatory response by opiate receptor ligands is important in light of the prevalence of drug use in HIV populations. Exogenous MOR and KOR agonists have differential effects on central nervous system (CNS) immunity and, while some data suggest KOR agonists are immunosuppressive, the KOR agonist dynorphin has been shown to stimulate human monocyte chemotaxis. In this study, we examined mRNA levels of endogenous opioid receptors OPRK1 and OPRM1, prodynorphin (PDYN), macrophage scavenger receptor CD163, and microglia/macrophage marker CD68 in the caudate and anterior cingulate of postmortem brains from HIV-positive and HIV-negative subjects. Brain tissues of HIV-infected (n = 24) and control subjects (n = 15) were obtained from the Manhattan HIV Brain Bank. Quantification of the gene mRNA was performed using SYBR Green RT-PCR. CD68 and CD163 were increased in HIV-positive (HIV+) compared to HIV-negative (HIV-) individuals in both brain regions. There were higher OPRK1 (P <0.005), and lower PDYN mRNA (P <0.005) levels in the anterior cingulate of HIV+ compared to HIV- subjects. This difference between the clinical groups was not found in the caudate. There was no difference in the levels of OPRM1 mRNA between HIV+ and HIV- subjects. Using linear regression analysis, we examined the relationship of OPRK1 and PDYN mRNA levels in the HIV+ subjects with seven cognitive domain T scores of a neuropsychological test battery. Within the HIV+ subjects, there was a positive correlation between anterior cingulate PDYN mRNA levels and better T-scores in the motor domain. Within the HIV+ subjects there were also positive correlations of both OPRK1 and PDYN mRNA levels with the anti-inflammatory marker CD163, but not with proinflammatory CD68 levels. In this setting, decreased PDYN mRNA may reflect a homeostatic mechanism to reduce monocyte migration, accompanied by compensatory increases in the cognate receptor (KOR) to dampen pro-inflammatory responses. It is possible that enhanced neuroprotection and better motor performance are associated with higher levels of dynorphin and the recruitment of neuroprotective CD163-positive macrophages. Further studies are needed to test this hypothesis.

Neuronal ferritin heavy chain and drug abuse affect HIV-associated cognitive dysfunction

Interaction of the chemokine CXCL12 with its receptor CXCR4 promotes neuronal function and survival during embryonic development and throughout adulthood. Previous studies indicated that μ-opioid agonists specifically elevate neuronal levels of the protein ferritin heavy chain (FHC), which negatively regulates CXCR4 signaling and affects the neuroprotective function of the CXCL12/CXCR4 axis. Here, we determined that CXCL12/CXCR4 activity increased dendritic spine density, and also examined FHC expression and CXCR4 status in opiate abusers and patients with HIV-associated neurocognitive disorders (HAND), which is typically exacerbated by illicit drug use. Drug abusers and HIV patients with HAND had increased levels of FHC, which correlated with reduced CXCR4 activation, within cortical neurons. We confirmed these findings in a nonhuman primate model of SIV infection with morphine administration. Transfection of a CXCR4-expressing human cell line with an iron-deficient FHC mutant confirmed that increased FHC expression deregulated CXCR4 signaling and that this function of FHC was independent of iron binding. Furthermore, examination of morphine-treated rodents and isolated neurons expressing FHC shRNA revealed that FHC contributed to morphine-induced dendritic spine loss. Together, these data implicate FHC-dependent deregulation of CXCL12/CXCR4 as a contributing factor to cognitive dysfunction in neuroAIDS.

Differential expression of the alternatively spliced OPRM1 isoform μ-opioid receptor-1K in HIV-infected individuals

OBJECTIVE

We previously examined the expression of specific C-terminal μ-opioid receptor (MOR) splice variants in human central nervous system cell types and HIV-infected brain tissue from individuals with neurocognitive impairment ± HIV encephalitis (HIVE). In the present study, we examined the N-terminal splice variant MOR-1K, which mediates excitatory cellular signaling.

METHODS AND RESULTS

We found segregation of expression ranging from undetectable to seemingly exclusive across nervous system cell types compared to the pool of C-terminal MOR splice variants using the real-time polymerase chain reaction (RT-PCR). Expression of MOR-1K mRNA was also increased in HIV-infected individuals with combined neurocognitive impairment and HIVE compared with the other groups. MOR-1K expression correlated with the level of patient neurocognitive impairment, whereas the pool of C-terminal MOR splice variants did not. HIVE was also associated with increased expression of the inflammatory mediators MCP-1, MCP-2, and RANTES, but not the host HIV coreceptors CXCR4 and CCR5 or the CD4 receptor using qRT-PCR. Network analysis of microarray data from these same patients revealed filamin A (FLNA) as a possible interaction partner with MOR-1K, and FLNA gene expression was also found to be upregulated in HIVE using qRT-PCR. Overexpression of FLNA in HEK293 cells redistributed MOR-1K from intracellular compartments to the cell surface.

CONCLUSION

These results suggest that HIVE, and neurocognitive impairment depending on its severity, are associated with enhanced MOR-1K signaling through both increased expression and trafficking to the cell surface, which may alter the contribution of MOR receptor isoforms and exacerbate the effects of MOR activation in neuroAIDS.

Functions of the chemokine receptor CXCR4 in the central nervous system and its regulation by μ-opioid receptors

Activation of the G protein-coupled receptor CXCR4 by its chemokine ligand CXCL12 regulates a number of physiopathological functions in the central nervous system, during development as well as later in life. In addition to the more classical roles of the CXCL12/CXCR4 axis in the recruitment of immune cells or migration and proliferation of neural precursor cells, recent studies suggest that CXCR4 signaling also modulates synaptic function and neuronal survival in the mature brain, through direct and indirect effects on neurons and glia. These effects, which include regulation of glutamate receptors and uptake, and of dendritic spine density, can significantly alter the ability of neurons to face excitotoxic insults. Therefore, they are particularly relevant to neurodegenerative diseases featuring alterations of glutamate neurotransmission, such as HIV-associated neurocognitive disorders. Importantly, CXCR4 signaling can be dysregulated by HIV viral proteins, host HIV-induced factors, and opioids. Potential mechanisms of opioid regulation of CXCR4 include heterologous desensitization, transcriptional regulation and changes in receptor expression levels, opioid-chemokine receptor dimer or heteromer formation, and the newly described modulation by the protein ferritin heavy chain-all leading to inhibition of CXCR4 signaling. After reviewing major effects of chemokines and opioids in the CNS, this chapter discusses chemokine-opioid interactions in neuronal and immune cells, focusing on their potential contribution to HIV-associated neurocognitive disorders.

Interactions of HIV and drugs of abuse: the importance of glia, neural progenitors, and host genetic factors

Considerable insight has been gained into the comorbid, interactive effects of HIV and drug abuse in the brain using experimental models. This review, which considers opiates, methamphetamine, and cocaine, emphasizes the importance of host genetics and glial plasticity in driving the pathogenic neuron remodeling underlying neuro-acquired immunodeficiency syndrome and drug abuse comorbidity. Clinical findings are less concordant than experimental work, and the response of individuals to HIV and to drug abuse can vary tremendously. Host-genetic variability is important in determining viral tropism, neuropathogenesis, drug responses, and addictive behavior. However, genetic differences alone cannot account for individual variability in the brain "connectome." Environment and experience are critical determinants in the evolution of synaptic circuitry throughout life. Neurons and glia both exercise control over determinants of synaptic plasticity that are disrupted by HIV and drug abuse. Perivascular macrophages, microglia, and to a lesser extent astroglia can harbor the infection. Uninfected bystanders, especially astroglia, propagate and amplify inflammatory signals. Drug abuse by itself derails neuronal and glial function, and the outcome of chronic exposure is maladaptive plasticity. The negative consequences of coexposure to HIV and drug abuse are determined by numerous factors including genetics, sex, age, and multidrug exposure. Glia and some neurons are generated throughout life, and their progenitors appear to be targets of HIV and opiates/psychostimulants. The chronic nature of HIV and drug abuse appears to result in sustained alterations in the maturation and fate of neural progenitors, which may affect the balance of glial populations within multiple brain regions.

Chemokines and Their Receptors Are Key Players in the Orchestra That Regulates Wound Healing

SIGNIFICANCE

Normal wound healing progresses through a series of overlapping phases, all of which are coordinated and regulated by a variety of molecules, including chemokines. Because these regulatory molecules play roles during the various stages of healing, alterations in their presence or function can lead to dysregulation of the wound-healing process, potentially leading to the development of chronic, nonhealing wounds.

RECENT ADVANCES

A discovery that chemokines participate in a variety of disease conditions has propelled the study of these proteins to a level that potentially could lead to new avenues to treat disease. Their small size, exposed termini, and the fact that their only modifications are two disulfide bonds make them excellent targets for manipulation. In addition, because they bind to G-protein-coupled receptors (GPCRs), they are highly amenable to pharmacological modulation.

CRITICAL ISSUES

Chemokines are multifunctional, and in many situations, their functions are highly dependent on the microenvironment. Moreover, each specific chemokine can bind to several GPCRs to stimulate the function, and both can function as monomers, homodimers, heterodimers, and even oligomers. Activation of one receptor by any single chemokine can lead to desensitization of other chemokine receptors, or even other GPCRs in the same cell, with implications for how these proteins or their receptors could be used to manipulate function.

FUTURE DIRECTIONS

Investment in better understanding of the functions of chemokines and their receptors in a local context can reveal new ways for therapeutic intervention. Understanding how different chemokines can activate the same receptor and vice versa could identify new possibilities for drug development based on their heterotypic interactions.

Neuroendocrine modulation of the inflammatory response in common carp: adrenaline regulates leukocyte profile and activity

Inflammatory responses have to be carefully controlled, as high concentrations and/or prolonged action of inflammation-related molecules (e.g. reactive oxygen species, nitric oxide and pro-inflammatory cytokines) can be detrimental to host tissue and organs. One of the potential regulators of the inflammatory process are stress mediators including adrenaline. In vivo effects of adrenaline were studied during zymosan-induced (Z) peritoneal inflammation in the common carp Cyprinus carpio L. Adrenaline injected together with zymosan (ZA) did not change the number of inflammatory leukocytes in the peritoneal cavity, however at 24h post-injection it significantly reduced the percentage of monocytes/macrophages. Moreover, compared to cells retrieved from fish treated with PBS or zymosan only, adrenaline increased the percentage of apoptotic leukocytes in the focus of inflammation. Furthermore, adrenaline significantly reduced the expression of chemokine CXCL8_L1 (a functional homolog of mammalian IL-8) and its receptors (CXCR1 and CXCR2), indicating changes in leukocyte recruitment after stress. We conclude that adrenaline may contribute to a coordinated reaction by influencing the inflammatory response via direct regulation of leukocyte migration and/or apoptosis.

Polymorphisms of the kappa opioid receptor and prodynorphin genes: HIV risk and HIV natural history

OBJECTIVE

Studies indicate cross-desensitization between opioid receptors (eg, kappa opioid receptor, OPRK1) and chemokine receptors (eg, CXCR4) involved in HIV infection. Whether gene variants of OPRK1 and its ligand, prodynorphin (PDYN), influence the outcome of HIV therapy was tested.

METHODS

Three study points, admission to the Women's Interagency HIV Study, initiation of highly active antiretroviral therapy (HAART), and the most recent visit, were chosen for analysis as crucial events in the clinical history of the HIV patients. Regression analyses of 17 variants of OPRK1 and 11 variants of PDYN with change of viral load (VL) and CD4 count between admission and initiation of HAART and initiation of HAART to the most recent visit to Women's Interagency HIV Study were performed in 598 HIV+ subjects, including African Americans, Hispanics, and Whites. Association with HIV status was done in 1009 subjects.

RESULTS

Before HAART, greater VL decline (improvement) in carriers of PDYN IVS3+189C>T and greater increase of CD4 count (improvement) in carriers of OPRK -72C>T were found in African Americans. Also, greater increase of CD4 count in carriers of OPRK1 IVS2+7886A>G and greater decline of CD4 count (deterioration) in carriers of OPRK1 -1205G>A were found in Whites. After HAART, greater decline of VL in carriers of OPRK1 IVS2+2225G>A and greater increase of VL in carriers of OPRK1 IVS2+10658G>T and IVS2+10963A>G were found in Whites. Also, a lesser increase of CD4 count was found in Hispanic carriers of OPRK1 IVS2+2225G>A.

CONCLUSIONS

OPRK1 and PDYN polymorphisms may alter severity of HIV infection and response to treatment.

Consequences of ChemR23 heteromerization with the chemokine receptors CXCR4 and CCR7

Recent studies have shown that heteromerization of the chemokine receptors CCR2, CCR5 and CXCR4 is associated to negative binding cooperativity. In the present study, we build on these previous results, and investigate the consequences of chemokine receptor heteromerization with ChemR23, the receptor of chemerin, a leukocyte chemoattractant protein structurally unrelated to chemokines. We show, using BRET and HTRF assays, that ChemR23 forms homomers, and provide data suggesting that ChemR23 also forms heteromers with the chemokine receptors CCR7 and CXCR4. As previously described for other chemokine receptor heteromers, negative binding cooperativity was detected between ChemR23 and chemokine receptors, i.e. the ligands of one receptor competed for the binding of a specific tracer of the other. We also showed, using mouse bone marrow-derived dendritic cells prepared from wild-type and ChemR23 knockout mice, that ChemR23-specific ligands cross-inhibited CXCL12 binding on CXCR4 in a ChemR23-dependent manner, supporting the relevance of the ChemR23/CXCR4 interaction in native leukocytes. Finally, and in contrast to the situation encountered for other previously characterized CXCR4 heteromers, we showed that the CXCR4-specific antagonist AMD3100 did not cross-inhibit chemerin binding in cells co-expressing ChemR23 and CXCR4, demonstrating that cross-regulation by AMD3100 depends on the nature of receptor partners with which CXCR4 is co-expressed.

Current status of chemokines in the adult CNS

Chemokines - chemotactic cytokines - are small secreted proteins that attract and activate immune and non-immune cells in vitro and in vivo. It has been suggested that chemokines and their receptors play a role in the central nervous system (CNS), in addition to their well established role in the immune system. We focus here on three chemokines-CXCL12 (C-X-C motif ligand 12), CCL2 (C-C motif ligand 2), and CX3CL1 (C-X-3C motif ligand 1) - and their principal receptors - CXCR4 (C-X-C motif receptor 4), CCR2 (C-C motif receptor 2) and CX3CR1 (C-X-3C motif receptor 1), respectively. We first introduce the classification of chemokines and their G-protein coupled receptors and the main signaling pathways triggered by receptor activation. We then discuss the cellular distribution of CXCL12/CXCR4, CCL2/CCR2 and CX3CL1/CX3CR1 in adult brain and the neurotransmission and neuromodulation effects controlled by these chemokines in the adult CNS. Changes in the expression of CXCL12, CCL2 and CX3CL1 and their respective receptors are also increasingly being implicated in the pathogenesis of CNS disorders, such as Alzheimer's disease, Parkinson's disease, HIV-associated encephalopathy, stroke and multiple sclerosis, and are therefore plausible targets for future pharmacological intervention. The final section thus discusses the role of these chemokines in these pathophysiological states. In conclusion, the role of these chemokines in cellular communication may make it possible: (i) to identify new pathways of neuron-neuron, glia-glia or neuron-glia communications relevant to both normal brain function and neuroinflammatory and neurodegenerative diseases; (ii) to develop new therapeutic approaches for currently untreatable brain diseases.

Immunohistochemistry detection of kappa-opioid receptors in human skin

The imbalance of p- and kappa-opioid receptors in the skin or central nervous system is currently deemed to be one of the reasons of chronic pruritus. A number of studies demonstrated a positive effect of system agonists of kappa-opioid receptors in the treatment of uremic pruritus, nodular pruritus, paraneoplastic and cholestatic pruritus. This research demonstrates an expression of kappa-opioid receptors in human skin (basal keratinocytes, dendritic cells, epidermal melanocytes and fibroblasts of the upper dermis) detected with the use of different immunochemistry methods.

Decreased pain responses of C-C chemokine receptor 5 knockout mice to chemical or inflammatory stimuli

Chemokines are small chemotactic cytokines that elicit many physiological and pathological effects through binding to their corresponding receptors. Recent studies have suggested that C-C chemokine receptor (CCR) 5 interacts with μ-opioid receptor and modifies a nociceptive reaction. We examined effects of CCR5 deficiency on pain responses by employing CCR5 knockout (KO) mice. We found that pain responses of CCR5 KO mice to chemical or inflammation stimuli were milder than those of CCR5 wild type (WT) mice. However, there was no remarkable change in thermal nociception. To prove the involvement of CCR5 deletion in lowered nociception, we examined pain reactions with CCR5 WT mice following treatment of a CCR5 antagonist (D-Ala(1)-peptide T-NH(2,) DAPTA). Chemical or inflammatory pain behavior was significantly relieved by intracerebroventricular infusion of the inhibitor. When we assessed expression level of μ-opioid receptor (MOR) in the periaqueductal gray where the receptors are critical for analgesic effects, immunoreactivity of MOR was significantly higher in CCR5 KO mice than WT mice without change in phosphorylation level of the receptor. Reduced nociceptive responses in CCR5 KO mice were moderated by administration of naloxone and d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), MOR antagonists. Our data indicate that CCR5 deficiency is related to up-regulation of MOR without an increase in the receptor desensitization which might result in increased analgesic effects against chemical or inflammatory stimuli. Alternatively, higher amount of opioid ligands in CCR5 mice might be linked to these results. Therefore, CCR5 appears to be a therapeutic target for treatment of pain related diseases such as inflammatory hyperalgesia.

Morphine and gp120 toxic interactions in striatal neurons are dependent on HIV-1 strain

A rigorously controlled, cell culture paradigm was used to assess the role of HIV-1 gp120 ± morphine in mediating opioid-HIV interactive toxicity in striatal neurons. Computerized time-lapse microscopy tracked the fate of individual neurons co-cultured with mixed-glia from mouse striata during opioid and gp120 exposure. Subpopulations of neurons and astroglia displayed μ-opioid receptor, CXCR4, and CCR5 immunoreactivity. While gp120 alone was or tended to be neurotoxic irrespective of whether X4-tropic gp120(IIIB), R5-tropic gp120(ADA), or dual-tropic gp120(MN) was administered, interactive toxicity with morphine differed depending on HIV-1 strain. For example, morphine only transiently exacerbated gp120(IIIB)-induced neuronal death; however, in combination with gp120(MN), morphine caused sustained increases in the rate of neuronal death compared to gp120(MN) alone that were prevented by naloxone. Alternatively, gp120(ADA) significantly increased the rate of neuron death, but gp120(ADA) toxicity was unaffected by morphine. The transient neurotoxic interactions between morphine and gp120(IIIB) were abrogated in the absence of glia suggesting that glia contribute significantly to the interactive pathology with chronic opiate abuse and neuroAIDS. To assess how mixed-glia might contribute to the neurotoxicity, the effects of morphine and/or gp120 on the production of reactive oxygen species (ROS) and on glutamate buffering were examined. All gp120 variants, and to a lesser extent morphine, increased ROS and/or decreased glutamate buffering, but together failed to show any interaction with morphine. Our findings indicate that HIV-1 strain-specific differences in gp120 are critical determinants in shaping both the timing and pattern of neurotoxic interactions with opioid drugs.

Mechanisms regulating chemokine receptor activity

Co-ordinated movement and controlled positioning of leucocytes is key to the development, maintenance and proper functioning of the immune system. Chemokines and their receptors play an essential role in these events by mediating directed cell migration, often referred to as chemotaxis. The chemotactic property of these molecules is also thought to contribute to an array of pathologies where inappropriate recruitment of specific chemokine receptor-expressing leucocytes is observed, including cancer and inflammatory diseases. As a result, chemokine receptors have become major targets for therapeutic intervention, and during the past 15 years much research has been devoted to understanding the regulation of their biological activity. From these studies, processes which govern the availability of functional chemokine receptors at the cell surface have emerged as playing a central role. In this review, we summarize and discuss current knowledge on the molecular mechanisms contributing to the regulation of chemokine receptor surface expression, from gene transcription and protein degradation to post-translational modifications, multimerization, intracellular transport and cross-talk.

Immunomodulatory properties of kappa opioids and synthetic cannabinoids in HIV-1 neuropathogenesis

Anti-retroviral therapy (ART) has had a tremendous impact on the clinical outcomes of HIV-1 infected individuals. While ART has produced many tangible benefits, chronic, long-term consequences of HIV infection have grown in importance. HIV-1-associated neurocognitive disorder (HAND) represents a collection of neurological syndromes that have a wide range of functional cognitive impairments. HAND remains a serious threat to AIDS patients, and there currently remains no specific therapy for the neurological manifestations of HIV-1. Based upon work in other models of neuroinflammation, kappa opioid receptors (KOR) and synthetic cannabinoids have emerged as having neuroprotective properties and the ability to dampen pro-inflammatory responses of glial cells; properties that may have a positive influence in HIV-1 neuropathogenesis. The ability of KOR ligands to inhibit HIV-1 production in human microglial cells and CD4 T lymphocytes, demonstrate neuroprotection, and dampen chemokine production in astrocytes provides encouraging data to suggest that KOR ligands may emerge as potential therapeutic agents in HIV neuropathogenesis. Based upon findings that synthetic cannabinoids inhibit HIV-1 expression in human microglia and suppress production of inflammatory mediators such as nitric oxide (NO) in human astrocytes, as well as a substantial literature demonstrating neuroprotective properties of cannabinoids in other systems, synthetic cannabinoids have also emerged as potential therapeutic agents in HIV neuropathogenesis. This review focuses on these two classes of compounds and describes the immunomodulatory and neuroprotective properties attributed to each in the context of HIV neuropathogenesis.

Activation of cannabinoid type 2 receptors inhibits HIV-1 envelope glycoprotein gp120-induced synapse loss

HIV-1 infection of the central nervous system is associated with dendritic and synaptic damage that correlates with cognitive decline in patients with HIV-1-associated dementia (HAD). HAD is due in part to the release of viral proteins from infected cells. Because cannabinoids modulate neurotoxic and inflammatory processes, we investigated their effects on changes in synaptic connections induced by the HIV-1 envelope glycoprotein gp120. Morphology and synapses between cultured hippocampal neurons were visualized by confocal imaging of neurons expressing DsRed2 and postsynaptic density protein 95 fused to green fluorescent protein (PSD95-GFP). Twenty-four-hour treatment with gp120 IIIB decreased the number of PSD95-GFP puncta by 37 ± 4%. The decrease was concentration-dependent (EC₅₀ = 153 ± 50 pM). Synapse loss preceded cell death as defined by retention of DsRed2 fluorescence gp120 activated CXCR4 on microglia to evoke interleukin-1β (IL-1β) release. Pharmacological studies determined that sequential activation of CXCR4, the IL-1β receptor, and the N-methyl-d-aspartate receptor was required. Expression of alternative reading frame polypeptide, which inhibits the ubiquitin ligase murine double minute 2, protected synapses, implicating the ubiquitin-proteasome pathway. Cannabimimetic drugs are of particular relevance to HAD because of their clinical and illicit use in patients with AIDS. The cannabinoid receptor full agonist [(R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate salt] (Win55,212-2) inhibited gp120-induced IL-1β production and synapse in a manner reversed by a cannabinoid type 2 receptor antagonist. In contrast, Win55,212-2 did not inhibit synapse loss elicited by exposure to the HIV-1 protein Tat. These results indicate that cannabinoids prevent the impairment of network function produced by gp120 and, thus, might have therapeutic potential in HAD.

Receptor oligomerization: a pivotal mechanism for regulating chemokine function

Since the first reports on chemokine function, much information has been generated on the implications of these molecules in numerous physiological and pathological processes, as well as on the signaling events activated through their binding to receptors. Despite these extensive studies, no chemokine-related drugs have yet been approved for use in patients with inflammatory or autoimmune diseases. This discrepancy between efforts and results has forced a re-evaluation of the chemokine field. We have explored chemokine receptor conformations at the cell surface and found that, as is the case for other G protein-coupled receptors, chemokine receptors are not isolated entities that are activated following ligand binding; rather, they are found as dimers and/or higher order oligomers at the cell surface, even in the absence of ligands. These complexes form organized arrays that can be modified by receptor expression and ligand levels, indicating that they are dynamic structures. The way in which these receptor complexes are stabilized modulates ligand binding, as well as their pharmacological properties and the signaling events activated. These conformations thus represent a mechanism that increases the broad variety of chemokine functions. Understanding these receptor interactions and their dynamics at the cell surface is thus critical for influencing chemokine function and could open up new possibilities for drug design.

Regulation of neuronal ferritin heavy chain, a new player in opiate-induced chemokine dysfunction

The heavy chain subunit of ferritin (FHC), a ubiquitous protein best known for its iron-sequestering activity as part of the ferritin complex, has recently been described as a novel inhibitor of signaling through the chemokine receptor CXCR4. Levels of FHC as well as its effects on CXCR4 activation increase in cortical neurons exposed to mu-opioid receptor agonists such as morphine, an effect likely specific to neurons. Major actions of CXCR4 signaling in the mature brain include a promotion of neurogenesis, activation of pro-survival signals, and modulation of excitotoxic pathways; thus, FHC up-regulation may contribute to the neuronal dysfunction often associated with opiate drug abuse. This review summarizes our knowledge of neuronal CXCR4 function, its regulation by opiates and the role of FHC in this process, and known mechanisms controlling FHC production. We speculate on the mechanism involved in FHC regulation by opiates and offer FHC as a new target in opioid-induced neuropathology.

Biological Significance of GPCR Heteromerization in the Neuro-Endocrine System

Clustering of proteins in higher order complexes is a common theme in biology and profoundly influences protein function. The idea that seven-transmembrane spanning G protein-coupled receptors (GPCRs) might form dimers or higher order oligomeric complexes has been formulated more than 20 years ago. Since then, this phenomenon has been investigated with many different biochemical and biophysical techniques. The more recent notion of GPCR heteromerization describes the specific association of two different GPCRs. GPCR heteromerization may be of primary importance in neuroendocrinology, as this may explain at least some of the functional crosstalks described between different hormonal systems. Importantly, many GPCR heteromers have distinct functional properties compared to their corresponding homomers. Heteromer-specific pharmacological profiles might be exploited for drug design and open new therapeutic options. GPCR heteromerization has been first studied in heterologous expression systems. Today, increasing evidence for the existence of GPCR heteromers in endogenous systems is emerging providing crucial evidence for the physiological function of GPCR heteromerization.

Progress in elucidating the structural and dynamic character of G Protein-Coupled Receptor oligomers for use in drug discovery

G Protein-Coupled Receptors (GPCRs) are the most targeted group of proteins for the development of therapeutic drugs. Until the last decade, structural information about this family of membrane proteins was relatively scarce, and their mechanisms of ligand binding and signal transduction were modeled on the assumption that GPCRs existed and functioned as monomeric entities. New crystal structures of native and engineered GPCRs, together with important biochemical and biophysical data that reveal structural details of the activation mechanism(s) of this receptor family, provide a valuable framework to improve dynamic molecular models of GPCRs with the ultimate goal of elucidating their allostery and functional selectivity. Since the dynamic movements of single GPCR protomers are likely to be affected by the presence of neighboring interacting subunits, oligomeric arrangements should be taken into account to improve the predictive ability of computer-assisted structural models of GPCRs for effective use in drug design.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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 and thermoregulation (Section 16); and immunological responses (Section 17).

Autoantibodies to the delta-opioid receptor function as opioid agonists and display immunomodulatory activity

In this report, we show that affinity purified human anti-delta opioid receptor (DOR) autoantibodies from IVIG are specific to DOR and possess agonistic properties displayed by their ability to dramatically decrease forskolin stimulated cAMP accumulation. Anti-DOR autoantibody also caused phosphorylation of the opioid receptor. Anti-DOR autoantibody treatment showed a significant reduction in CXCR4 gene expression as well as surface protein expression. In contrast, anti-DOR autoantibody treatment significantly upregulated CCR5 gene and protein expression. The presence of anti-DOR autoantibodies in IVIG and their potent immunomodulatory activity is further evidence to support the cross-talk between the neuroendocrine and immune systems.

Pertussis toxin signals through the TCR to initiate cross-desensitization of the chemokine receptor CXCR4

Pertussis toxin (PTx) has been shown to exert a variety of effects on immune cells independent of its ability to ADP-ribosylate G proteins. Of these effects, the binding subunit of PTx (PTxB) has been shown to block signaling via the chemokine receptor CCR5, but the mechanism involved in this process is unknown. Here, we show that PTxB causes desensitization of a related chemokine receptor, CXCR4, and explore the mechanism by which this occurs. CXCR4 is the receptor for the chemokine stromal cell-derived factor 1alpha (SDF-1alpha) and elicits a number of biological effects, including stimulation of T cell migration. PTxB treatment causes a decrease in CXCR4 surface expression, inhibits G protein-associated signaling, and blocks SDF-1alpha-mediated chemotaxis. We show that PTxB mediates these effects by activating the TCR signaling network, as the effects are dependent on TCR and ZAP70 expression. Additionally, the activation of the TCR with anti-CD3 mAb elicits a similar set of effects on CXCR4 activity, supporting the idea that TCR signaling leads to cross-desensitization of CXCR4. The inhibition of CXCR4 by PTxB is rapid and transient; however, the catalytic activity of PTx prevents CXCR4 signaling in the long term. Thus, the effects of PTx holotoxin on CXCR4 signaling can be divided into two phases: short term by the B subunit, and long term by the catalytic subunit. These data suggest that TCR crosstalk with CXCR4 is likely a normal cellular process that leads to cross-desensitization, which is exploited by the B subunit of PTx.

Increasingly accurate dynamic molecular models of G-protein coupled receptor oligomers: Panacea or Pandora's box for novel drug discovery?

For years, conventional drug design at G-protein coupled receptors (GPCRs) has mainly focused on the inhibition of a single receptor at a usually well-defined ligand-binding site. The recent discovery of more and more physiologically relevant GPCR dimers/oligomers suggests that selectively targeting these complexes or designing small molecules that inhibit receptor-receptor interactions might provide new opportunities for novel drug discovery. To uncover the fundamental mechanisms and dynamics governing GPCR dimerization/oligomerization, it is crucial to understand the dynamic process of receptor-receptor association, and to identify regions that are suitable for selective drug binding. This minireview highlights current progress in the development of increasingly accurate dynamic molecular models of GPCR oligomers based on structural, biochemical, and biophysical information that has recently appeared in the literature. In view of this new information, there has never been a more exciting time for computational research into GPCRs than at present. Information-driven modern molecular models of GPCR complexes are expected to efficiently guide the rational design of GPCR oligomer-specific drugs, possibly allowing researchers to reach for the high-hanging fruits in GPCR drug discovery, i.e. more potent and selective drugs for efficient therapeutic interventions.

Modulation of chemokine receptor activity through dimerization and crosstalk

Chemokines are small, secreted proteins that bind to the chemokine receptor subfamily of class A G protein-coupled receptors. Collectively, these receptor-ligand pairs are responsible for diverse physiological responses including immune cell trafficking, development and mitogenic signaling, both in the context of homeostasis and disease. However, chemokines and their receptors are not isolated entities, but instead function in complex networks involving homo- and heterodimer formation as well as crosstalk with other signaling complexes. Here the functional consequences of chemokine receptor activity, from the perspective of both direct physical associations with other receptors and indirect crosstalk with orthogonal signaling pathways, are reviewed. Modulation of chemokine receptor activity through these mechanisms has significant implications in physiological and pathological processes, as well as drug discovery and drug efficacy. The integration of signals downstream of chemokine and other receptors will be key to understanding how cells fine-tune their response to a variety of stimuli, including therapeutics.

Morphine increases brain levels of ferritin heavy chain leading to inhibition of CXCR4-mediated survival signaling in neurons

This study focuses on the effect of mu-opioid receptor agonists on CXCR4 signaling in neurons and the mechanisms involved in regulation of neuronal CXCR4 by opiates. The data show that CXCR4 is negatively modulated by long-term morphine treatments both in vitro and in vivo; CXCR4 inhibition is caused by direct stimulation of mu-opioid receptors in neurons, leading to alterations of ligand-induced CXCR4 phosphorylation and upregulation of protein ferritin heavy chain (FHC), a negative intracellular regulator of CXCR4. Reduced coupling of CXCR4 to G-proteins was found in the brain of morphine-treated rats, primarily cortex and hippocampus. CXCR4-induced G alpha(i)/G betagamma activities were suppressed after 24 h treatment of cortical neurons with morphine or the selective mu-opioid agonist DAMGO (D-Ala2-N-Me-Phe(4)-glycol(5)-enkephalin), as shown by analysis of downstream targets of CXCR4 (i.e., cAMP, Akt, and ERK1/2). These agonists also prevented CXCL12-induced phosphorylation of CXCR4, indicating a deficit of CXCR4 activation in these conditions. Indeed, morphine (or DAMGO) inhibited prosurvival signaling in neurons. These effects are not attributable to a reduction in CXCR4 expression or surface levels but rather to upregulation of FHC by opioids. The crucial role of FHC in inhibition of neuronal CXCR4 was confirmed by in vitro and in vivo RNA interference studies. Overall, these findings suggest that opiates interfere with normal CXCR4 function in the brain. By this mechanism, opiates could reduce the neuroprotective functions of CXCR4 and exacerbate neuropathology in opiate abusers who are affected by neuroinflammatory/infectious disorders, including neuroAIDS.