Genetic alteration of phospholipase C beta3 expression modulates behavioral and cellular responses to mu opioids

Biasing Gβγ Downstream Signaling with Gallein Inhibits Development of Morphine Tolerance and Potentiates Morphine-Induced Nociception in a Tolerant State

Opioid analgesics are widely used as a treatment option for pain management and relief. However, the misuse of opioid analgesics has contributed to the current opioid epidemic in the United States. Prescribed opioids such as morphine, codeine, oxycodone, and fentanyl are mu-opioid receptor (MOR) agonists primarily used in the clinic to treat pain or during medical procedures, but development of tolerance limits their utility for treatment of chronic pain. Here we explored the effects of biasing Gβγ signaling on tolerance development after chronic morphine treatment in vivo. We hypothesized that biasing Gβγ signaling with gallein could prevent activation of regulatory signaling pathways that result in tolerance to antinociceptive effects of MOR agonists. Gallein has been shown to bind to Gβγ and inhibit interactions of Gβγ with phospholipase-Cβ3 (PLCβ3) or G-protein-coupled receptor kinase 2 (GRK2) but not G-protein inwardly rectifying potassium (GIRK) channels. In mice, morphine-induced antinociception was evaluated in the 55°C warm water tail withdrawal assay. We used two paradigms for gallein treatment: administration during and after three times-daily morphine administration. Our results show that gallein cotreatment during repeated administration of morphine decreased opioid tolerance development and that gallein treatment in an opioid-tolerant state enhanced the potency of morphine. Mechanistically, our data suggest that PLCβ3 is necessary for potentiating effects of gallein in an opioid-tolerant state but not in preventing the development of tolerance. These studies demonstrate that small molecules that target Gβγ signaling could reduce the need for large doses of opioid analgesics to treat pain by producing an opioid-sparing effect. SIGNIFICANCE STATEMENT: Biasing Gβγ signaling prevents tolerance to repeated morphine administration in vivo and potentiates the antinociceptive effects of morphine in an opioid-tolerant state. Mechanistically, phospholipase-Cβ is necessary for potentiating effects of gallein in an opioid-tolerant state but not in preventing the development of tolerance. This study identifies a novel treatment strategy to decrease the development of tolerance to the analgesic effects of mu-opioid receptor agonists, which are necessary to improve pain treatment and decrease the incidence of opioid use disorder.

TRPM4 and PLCβ3 contribute to normal behavioral responses to an array of sweeteners and carbohydrates but PLCβ3 is not needed for taste-driven licking for glucose

The peripheral taste system is more complex than previously thought. The novel taste-signaling proteins TRPM4 and PLCβ3 appear to function in normal taste responding as part of Type II taste cell signaling or as part of a broadly responsive (BR) taste cell that can respond to some or all classes of tastants. This work begins to disentangle the roles of intracellular components found in Type II taste cells (TRPM5, TRPM4, and IP3R3) or the BR taste cells (PLCβ3 and TRPM4) in driving behavioral responses to various saccharides and other sweeteners in brief-access taste tests. We found that TRPM4, TRPM5, TRPM4/5, and IP3R3 knockout (KO) mice show blunted or abolished responding to all stimuli compared with wild-type. IP3R3 KO mice did, however, lick more for glucose than fructose following extensive experience with the 2 sugars. PLCβ3 KO mice were largely unresponsive to all stimuli except they showed normal concentration-dependent responding to glucose. The results show that key intracellular signaling proteins associated with Type II and BR taste cells are mutually required for taste-driven responses to a wide range of sweet and carbohydrate stimuli, except glucose. This confirms and extends a previous finding demonstrating that Type II and BR cells are both necessary for taste-driven licking to sucrose. Glucose appears to engage unique intracellular taste-signaling mechanisms, which remain to be fully elucidated.

mGluR5 from Primary Sensory Neurons Promotes Opioid-Induced Hyperalgesia and Tolerance by Interacting with and Potentiating Synaptic NMDA Receptors

Aberrant activation of presynaptic NMDARs in the spinal dorsal horn is integral to opioid-induced hyperalgesia and analgesic tolerance. However, the signaling mechanisms responsible for opioid-induced NMDAR hyperactivity remain poorly identified. Here, we show that repeated treatment with morphine or fentanyl reduced monomeric mGluR5 protein levels in the dorsal root ganglion (DRG) but increased levels of mGluR5 monomers and homodimers in the spinal cord in mice and rats of both sexes. Coimmunoprecipitation analysis revealed that monomeric and dimeric mGluR5 in the spinal cord, but not monomeric mGluR5 in the DRG, directly interacted with GluN1. By contrast, mGluR5 did not interact with μ-opioid receptors in the DRG or spinal cord. Repeated morphine treatment markedly increased the mGluR5-GluN1 interaction and protein levels of mGluR5 and GluN1 in spinal synaptosomes. The mGluR5 antagonist MPEP reversed morphine treatment-augmented mGluR5-GluN1 interactions, GluN1 synaptic expression, and dorsal root-evoked monosynaptic EPSCs of dorsal horn neurons. Furthermore, CRISPR-Cas9-induced conditional mGluR5 knockdown in DRG neurons normalized mGluR5 levels in spinal synaptosomes and NMDAR-mediated EPSCs of dorsal horn neurons increased by morphine treatment. Correspondingly, intrathecal injection of MPEP or conditional mGluR5 knockdown in DRG neurons not only potentiated the acute analgesic effect of morphine but also attenuated morphine treatment-induced hyperalgesia and tolerance. Together, our findings suggest that opioid treatment promotes mGluR5 trafficking from primary sensory neurons to the spinal dorsal horn. Through dimerization and direct interaction with NMDARs, presynaptic mGluR5 potentiates and/or stabilizes NMDAR synaptic expression and activity at primary afferent central terminals, thereby maintaining opioid-induced hyperalgesia and tolerance.SIGNIFICANCE STATEMENT Opioids are essential analgesics for managing severe pain caused by cancer, surgery, and tissue injury. However, these drugs paradoxically induce pain hypersensitivity and tolerance, which can cause rapid dose escalation and even overdose mortality. This study demonstrates, for the first time, that opioids promote trafficking of mGluR5, a G protein-coupled glutamate receptor, from peripheral sensory neurons to the spinal cord; there, mGluR5 proteins dimerize and physically interact with NMDARs to augment their synaptic expression and activity. Through dynamic interactions, the two distinct glutamate receptors mutually amplify and sustain nociceptive input from peripheral sensory neurons to the spinal cord. Thus, inhibiting mGluR5 activity or disrupting mGluR5-NMDAR interactions could reduce opioid-induced hyperalgesia and tolerance and potentiate opioid analgesic efficacy.

Discrete localization patterns of PIP5Kγ and PLCβ3 working sequentially in phosphoinositide-cycle within mouse sensory neuron somata

It is known that phosphatidylinositol phosphate 5 kinase (PIP5K) γ and phospholipase C (PLC) β3, working sequentially in the phosphoinositide cycle, are localized in dorsal root ganglion (DRG) somata and are involved in the regulation of pain and related sensations. However, the sites of their involvement have remained to be clarified. In the present study, immunoreactivity for PLCβ3 was distinct only in the central process of mouse DRG, but not in its peripheral process, in contrast to distinct PIP5Kγ-immunoreactivity in both peripheral and central DRG processes. No nerve terminals showing immunoreactivity for PLCβ3 were detected in any peripheral sensory fields, similar to PIP5Kγ-immunoreactivity. In DRG somata, PIP5Kγ-immunoreactivity was rather confined to the neurolemma in which dots and threads were discerned in 3D bright field light microscopy. This feature well corresponded to its discontinuous localization along the plasma membranes in immuno-electron microscopy. In contrast, PLCβ3-immunoreactivity occurred diffusely throughout the somata, but did not take distinct appearance of immunoreaction on neurolemma or plasma membranes, unlike PIP5Kγ-immunoreactivity. In addition, satellite glial cells were immunonegative for PLCβ3, but immunopositive for PIP5Kγ. The involvement of PLCβ3 in regulation of pain and related sensations is thus suggested to be mainly exerted at levels of the DRG soma and its upstream, but to be less significant in the peripheral sensory fields, similar to PIP5Kγ. The possibility is also suggested that PIP, PIP5Kγ-target, is localized heterogeneously, but PIP2, PLCβ3-target, is localized homogenously over the plane of the neuronal plasma membranes. RESEARCH HIGHLIGHTS: PIP5Kγ, different from PLCβ3, was localized heterogeneously on neuronal membranes, and this difference was demonstrated in 3D-bright field immuno-light and electron microscopy. Either PIP5Kγ or PLCβ3 was not detected in peripheral nerve terminals.

Brief Opioid Exposure Paradoxically Augments Primary Afferent Input to Spinal Excitatory Neurons via α2δ-1-Dependent Presynaptic NMDA Receptors

Treatment with opioids not only inhibits nociceptive transmission but also elicits a rebound and persistent increase in primary afferent input to the spinal cord. Opioid-elicited long-term potentiation (LTP) from TRPV1-expressing primary afferents plays a major role in opioid-induced hyperalgesia and analgesic tolerance. Here, we determined whether opioid-elicited LTP involves vesicular glutamate transporter-2 (VGluT2) or vesicular GABA transporter (VGAT) neurons in the spinal dorsal horn of male and female mice and identified underlying signaling mechanisms. Spinal cord slice recordings revealed that µ-opioid receptor (MOR) stimulation with DAMGO initially inhibited dorsal root-evoked EPSCs in 87% VGluT2 neurons and subsequently induced LTP in 49% of these neurons. Repeated morphine treatment increased the prevalence of VGluT2 neurons displaying LTP with a short onset latency. In contrast, DAMGO inhibited EPSCs in 46% VGAT neurons but did not elicit LTP in any VGAT neurons even in morphine-treated mice. Spinal superficial laminae were densely innervated by MOR-containing nerve terminals and were occupied by mostly VGluT2 neurons and few VGAT neurons. Furthermore, conditional Grin1 knockout in dorsal root ganglion neurons diminished DAMGO-elicited LTP in lamina II neurons and attenuated hyperalgesia and analgesic tolerance induced by repeated treatment with morphine. In addition, DAMGO-elicited LTP in VGluT2 neurons was abolished by protein kinase C inhibition, gabapentin, Cacna2d1 knockout, or disrupting the α2δ-1-NMDA receptor interaction with an α2δ-1 C terminus peptide. Thus, brief MOR stimulation distinctively potentiates nociceptive primary afferent input to excitatory dorsal horn neurons via α2δ-1-coupled presynaptic NMDA receptors, thereby causing hyperalgesia and reducing analgesic actions of opioids.SIGNIFICANCE STATEMENT Opioid drugs are potent analgesics for treating severe pain and are commonly used during general anesthesia. However, opioid use often induces pain hypersensitivity, rapid loss of analgesic efficacy, and dose escalation, which can cause dependence, addiction, and even overdose fatality. This study demonstrates for the first time that brief opioid exposure preferentially augments primary sensory input to genetically identified glutamatergic excitatory, but not GABAergic/glycinergic inhibitory, neurons in nociceptive dorsal horn circuits. This opioid-elicited synaptic plasticity is cell type specific and mediated by protein kinase C-dependent and α2δ-1-dependent activation of NMDA receptors at primary sensory nerve terminals. These findings elucidate how intraoperative use of opioids for preemptive analgesia paradoxically aggravates postoperative pain and increases opioid consumption and suggest new strategies to improve opioid analgesic efficacy.

Exploring pharmacological inhibition of Gq/11 as an analgesic strategy

BACKGROUND AND PURPOSE

Misuse of opioids has greatly affected our society. One potential solution is to develop analgesics that act at targets other than opioid receptors. These can be used either as stand-alone therapeutics or to improve the safety profile of opioid drugs. Previous research showed that activation of G_q/11 proteins by G-protein coupled receptors has pro-nociceptive properties, suggesting that blockade of G_q/11 signalling could be beneficial for pain control. The aim of this study was to test this hypothesis pharmacologically by using potent and selective G_q/11 inhibitor YM-254890.

EXPERIMENTAL APPROACH

We used a series of behavioural assays to evaluate the acute responses of mice to painful thermal stimulation while administering YM-254890 alone and in combination with morphine. We then used electrophysiological recordings to evaluate the effects of YM-254890 on the excitability of dorsal root ganglion (DRG) nociceptor neurons.

KEY RESULTS

We found that systemic administration of YM-254890 produced anti-nociceptive effects and also augmented morphine analgesia in both hotplate and tail flick paradigms. However, it also caused substantial inhibition of locomotion, which may limit its therapeutic utility. To circumvent these issues, we explored the local administration of YM-254890. Intrathecal injections of YM-254890 produced lasting analgesia in a tail flick test and greatly augmented the anti-nociceptive effects of morphine without any significant effects on locomotor behaviour. Electrophysiological studies showed that YM-254890 reduced the excitability of DRG nociceptors and augmented their opioid-induced inhibition.

CONCLUSION AND IMPLICATIONS

These findings indicate that pharmacological inhibition of G_q/11 could be explored as an analgesic strategy.

Coincident Regulation of PLCβ Signaling by Gq-Coupled and μ-Opioid Receptors Opposes Opioid-Mediated Antinociception

Pain management is an important problem worldwide. The current frontline approach for pain management is the use of opioid analgesics. The primary analgesic target of opioids is the μ-opioid receptor (MOR). Deletion of phospholipase Cβ3 (PLCβ3) or selective inhibition of Gβγ regulation of PLCβ3 enhances the potency of the antinociceptive effects of morphine suggesting a novel strategy for achieving opioid-sparing effects. Here we investigated a potential mechanism for regulation of PLC signaling downstream of MOR in human embryonic kidney 293 cells and found that MOR alone could not stimulate PLC but rather required a coincident signal from a Gq-coupled receptor. Knockout of PLCβ3 or pharmacological inhibition of its upstream regulators, Gβγ or Gq, ex vivo in periaqueductal gray slices increased the potency of the selective MOR agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate salt in inhibiting presynaptic GABA release. Finally, inhibition of Gq- G protein-coupled receptor coupling in mice enhanced the antinociceptive effects of morphine. These data support a model where Gq and Gβγ-dependent signaling cooperatively regulate PLC activation to decrease MOR-dependent antinociceptive potency. Ultimately, this could lead to identification of new non-MOR targets that would allow for lower-dose utilization of opioid analgesics. SIGNIFICANCE STATEMENT: Previous work demonstrated that deletion of phospholipase Cβ3 (PLCβ3) in mice potentiates μ-opioid receptor (MOR)-dependent antinociception. How PLCβ3 is regulated downstream of MOR had not been clearly defined. We show that PLC-dependent diacylglycerol generation is cooperatively regulated by MOR-Gβγ and Gq-coupled receptor signaling through PLCβ3 and that blockade of either Gq-signaling or Gβγ signaling enhances the potency of opioids in ex vivo brain slices and in vivo. These results reveal potential novel strategies for improving opioid analgesic potency and safety.

PLCβ2 Promotes VEGF-Induced Vascular Permeability

BACKGROUND

Regulation of vascular permeability is critical to maintaining tissue metabolic homeostasis. VEGF (vascular endothelial growth factor) is a key stimulus of vascular permeability in acute and chronic diseases including ischemia reperfusion injury, sepsis, and cancer. Identification of novel regulators of vascular permeability would allow for the development of effective targeted therapeutics for patients with unmet medical need.

METHODS

In vitro and in vivo models of VEGFA-induced vascular permeability, pathological permeability, quantitation of intracellular calcium release and cell entry, and phosphatidylinositol 4,5-bisphosphate levels were evaluated with and without modulation of PLC (phospholipase C) β2.

RESULTS

Global knock-out of PLCβ2 in mice resulted in blockade of VEGFA-induced vascular permeability in vivo and transendothelial permeability in primary lung endothelial cells. Further work in an immortalized human microvascular cell line modulated with stable knockdown of PLCβ2 recapitulated the observations in the mouse model and primary cell assays. Additionally, loss of PLCβ2 limited both intracellular release and extracellular entry of calcium following VEGF stimulation as well as reduced basal and VEGFA-stimulated levels of phosphatidylinositol 4,5-bisphosphate compared to control cells. Finally, loss of PLCβ2 in both a hyperoxia-induced lung permeability model and a cardiac ischemia:reperfusion model resulted in improved animal outcomes when compared with wild-type controls.

CONCLUSIONS

The results implicate PLCβ2 as a key positive regulator of VEGF-induced vascular permeability through regulation of both calcium flux and phosphatidylinositol 4,5-bisphosphate levels at the cellular level. Targeting of PLCβ2 in a therapeutic setting may provide a novel approach to regulating vascular permeability in patients.

Proteomic Analysis of Hypothalamus and Pituitary Gland in Pre and Postpubertal Brahman Heifers

The hypothalamus and the pituitary gland are directly involved in the complex systemic changes that drive the onset of puberty in cattle. Here, we applied integrated bioinformatics to elucidate the critical proteins underlying puberty and uncover potential molecular mechanisms from the hypothalamus and pituitary gland of prepubertal (n = 6) and postpubertal (n = 6) cattle. Proteomic analysis in the hypothalamus and pituitary gland revealed 275 and 186 differentially abundant (DA) proteins, respectively (adjusted p-value < 0.01). The proteome profiles found herein were integrated with previously acquired transcriptome profiles. These transcriptomic studies used the same tissues harvested from the same heifers at pre- and post-puberty. This comparison detected a small number of matched transcripts and protein changes at puberty in each tissue, suggesting the need for multiple omics analyses for interpreting complex biological systems. In the hypothalamus, upregulated DA proteins at post-puberty were enriched in pathways related to puberty, including GnRH, calcium and oxytocin signalling pathways, whereas downregulated proteins were observed in the estrogen signalling pathway, axon guidance and GABAergic synapse. Additionally, this study revealed that ribosomal pathway proteins in the pituitary were involved in the pubertal development of mammals. The reported molecules and derived protein-protein networks are a starting point for future experimental approaches that might dissect with more detail the role of each molecule to provide new insights into the mechanisms of puberty onset in cattle.

The Role of Phospholipase C in GABAergic Inhibition and Its Relevance to Epilepsy

Epilepsy is characterized by recurrent seizures due to abnormal hyperexcitation of neurons. Recent studies have suggested that the imbalance of excitation and inhibition (E/I) in the central nervous system is closely implicated in the etiology of epilepsy. In the brain, GABA is a major inhibitory neurotransmitter and plays a pivotal role in maintaining E/I balance. As such, altered GABAergic inhibition can lead to severe E/I imbalance, consequently resulting in excessive and hypersynchronous neuronal activity as in epilepsy. Phospholipase C (PLC) is a key enzyme in the intracellular signaling pathway and regulates various neuronal functions including neuronal development, synaptic transmission, and plasticity in the brain. Accumulating evidence suggests that neuronal PLC is critically involved in multiple aspects of GABAergic functions. Therefore, a better understanding of mechanisms by which neuronal PLC regulates GABAergic inhibition is necessary for revealing an unrecognized linkage between PLC and epilepsy and developing more effective treatments for epilepsy. Here we review the function of PLC in GABAergic inhibition in the brain and discuss a pathophysiological relationship between PLC and epilepsy.

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

The G-protein βγ subunits regulate platelet function

AIMS

G-protein coupled receptors (GPCRs) tightly regulate platelet function by interacting with various physiological agonists. An essential mediator of GPCR signaling is the G protein αβγ heterotrimers, in which the βγ subunits are central players in downstream signaling. Herein, we investigated the role of Gβγ subunits in platelet function, hemostasis and thrombogenesis.

METHODS

To achieve this goal, platelets from both mice and humans were employed in the context of a small molecule inhibitor of Gβγ, namely gallein. We used an aggregometer to examine aggregation and dense granules secretion. We also used flow cytometry for P-selectin and PAC1 to determine the impact of inhibiting Gβγ on α -granule secretion and αIIbβ3 activation. Clot retraction and the platelet spreading assay were used to examine Gβγ role in outside-in platelet signaling, whereas Western blot was employed to examine its role in Akt activation. Finally, we used the bleeding time assay and the FeCl₃-induced carotid-artery injury thrombosis model to determine Gβγ contribution to in vivo platelet function.

RESULTS

We observed that gallein inhibits platelet aggregation and secretion in response to agonist stimulation, in both mouse and human platelets. Furthermore, gallein also exerted inhibitory effects on integrin αIIbβ3 activation, clot retraction, platelet spreading and Akt activation/phosphorylation. Finally, gallein's inhibitory effects manifested in vivo, as documented by its ability to modulate physiological hemostasis and delay thrombus formation.

CONCLUSION

Our findings demonstrate, for the first time, that Gβγ subunits directly regulate GPCR-dependent platelet function, in vitro and in vivo. Moreover, these data highlight Gβγ as a novel therapeutic target for managing thrombotic disorders.

A subset of broadly responsive Type III taste cells contribute to the detection of bitter, sweet and umami stimuli

Taste receptor cells use multiple signaling pathways to detect chemicals in potential food items. These cells are functionally grouped into different types: Type I cells act as support cells and have glial-like properties; Type II cells detect bitter, sweet, and umami taste stimuli; and Type III cells detect sour and salty stimuli. We have identified a new population of taste cells that are broadly tuned to multiple taste stimuli including bitter, sweet, sour, and umami. The goal of this study was to characterize these broadly responsive (BR) taste cells. We used an IP₃R3-KO mouse (does not release calcium (Ca²⁺) from internal stores in Type II cells when stimulated with bitter, sweet, or umami stimuli) to characterize the BR cells without any potentially confounding input from Type II cells. Using live cell Ca²⁺ imaging in isolated taste cells from the IP₃R3-KO mouse, we found that BR cells are a subset of Type III cells that respond to sour stimuli but also use a PLCβ signaling pathway to respond to bitter, sweet, and umami stimuli. Unlike Type II cells, individual BR cells are broadly tuned and respond to multiple stimuli across different taste modalities. Live cell imaging in a PLCβ3-KO mouse confirmed that BR cells use this signaling pathway to respond to bitter, sweet, and umami stimuli. Short term behavioral assays revealed that BR cells make significant contributions to taste driven behaviors and found that loss of either PLCβ3 in BR cells or IP₃R3 in Type II cells caused similar behavioral deficits to bitter, sweet, and umami stimuli. Analysis of c-Fos activity in the nucleus of the solitary tract (NTS) also demonstrated that functional Type II and BR cells are required for normal stimulus induced expression.

We use our taste system to decide if we are going to consume or reject a potential food item. This is critical for survival, as we need energy to live but also need to avoid potentially toxic compounds. Therefore, it is important to understand how the taste cells in our mouth detect the chemicals in food and send a message to our brain. Signals from the taste cells form a code that conveys information about the nature of the potential food item to the brain. How this taste coding works is not well understood. Currently, it is thought that taste cells are primarily selective for each taste stimuli and only detect either bitter, sweet, sour, salt, or umami (amino acids) compounds. Our study describes a new population of taste cells that can detect multiple types of stimuli, including chemicals from different taste qualities. Thus, taste cells can be either selective or generally responsive to stimuli which is similar to the cells in the brain that process taste information. The presence of these broadly responsive taste cells provides new insight into how taste information is sent to the brain for processing.

Activation of Phospholipase C β by Gβγ and Gαq Involves C-Terminal Rearrangement to Release Autoinhibition

Phospholipase C (PLC) enzymes hydrolyze phosphoinositide lipids to inositol phosphates and diacylglycerol. Direct activation of PLCβ by Gα_q and/or Gβγ subunits mediates signaling by Gq and some Gi coupled G-protein-coupled receptors (GPCRs), respectively. PLCβ isoforms contain a unique C-terminal extension, consisting of proximal and distal C-terminal domains (CTDs) separated by a flexible linker. The structure of PLCβ3 bound to Gα_q is known, however, for both Gα_q and Gβγ; the mechanism for PLCβ activation on membranes is unknown. We examined PLCβ2 dynamics on membranes using hydrogen-deuterium exchange mass spectrometry (HDX-MS). Gβγ caused a robust increase in dynamics of the distal C-terminal domain (CTD). Gα_q showed decreased deuterium incorporation at the Gα_q binding site on PLCβ. In vitro Gβγ-dependent activation of PLC is inhibited by the distal CTD. The results suggest that disruption of autoinhibitory interactions with the CTD leads to increased PLCβ hydrolase activity.

Heterotrimeric G Proteins as Therapeutic Targets in Drug Discovery

Heterotrimeric G proteins are molecular switches in GPCR signaling pathways and regulate a plethora of physiological and pathological processes. GPCRs are efficient drug targets, and more than 30% of the drugs in use target them. However, selectively targeting an individual GPCR may be undesirable in various multifactorial diseases in which multiple receptors are involved. In addition, abnormal activation or expression of G proteins is frequently associated with diseases. Furthermore, G proteins harboring mutations often result in malignant diseases. Thus, targeting G proteins instead of GPCRs might provide alternative approaches for combating these diseases. In this review, we discuss the biochemistry of heterotrimeric G proteins, describe the G protein-associated diseases, and summarize the currently known modulators that can regulate the activities of G proteins. The outlook for targeting G proteins to treat diverse diseases is also included in this manuscript.

Is MYND Domain-Mediated Assembly of SMYD3 Complexes Involved in Calcium Dependent Signaling?

Macromolecular complexes are essential to intracellular signal transduction by creating signaling niches and enabling a chain of reactions that transmit external signals into various cellular responses. Analysis of SMYD3 interactome indicates this protein lysine methyltransferase might be involved in calcium dependent signaling pathways through forming complexes with the phospholipase PLCB3, calcium/calmodulin dependent kinase CAMK2B, or calcineurin inhibitor RCAN3. SMYD3 is well-known as a histone H3K4 methyltransferase involved in epigenetic transcriptional regulation; however, any roles SMYD3 may play in signaling transduction remain unknown. KEGG pathway enrichment analysis reveals the SMYD3 interacting proteins are overrepresented in several signaling pathways such as estrogen signaling pathway, NOD-like receptor signaling pathway, and WNT signaling pathway. Sequence motif scanning reveals a significant enrichment of PXLXP motif in SMYD3 interacting proteins. The MYND domain of SMYD3 is known to bind to the PXLXP motif. The enrichment of the PXLXP motif suggests that the MYND domain is likely to be a key interaction module that mediates formation of some SMYD3 complexes. The presence of the PXLXP motifs in PLCB3 and CAMK2B indicates the potential role of the MYND domain in mediating complex formation in signaling. The structural basis of SMYD3 MYND domain-mediated interactions is unknown. The only available MYND-peptide complex structure suggests the MYND domain-mediated interaction is likely transient and dynamic. The transient nature will make this domain well-suited to mediate signaling transduction processes where it may allow rapid responses to cellular perturbations and changes in environment.

Chronic activation of Mas-related gene receptors (Mrg) reduces the potency of morphine-induced analgesia via PKC pathway in naive rats

Mas oncogene-related gene receptors (Mrg) are uniquely distributed in small and medium cells of trigeminal and dorsal root ganglia (DRG). The physiological and pharmacological properties of Mrg are unknown. We have shown that intermittent activation of MrgC prevents and reverses morphine tolerance. Now we observed that intrathecal (i.t.) administration of the MrgC agonist bovine adrenal medulla 8-22 (BAM8-22, 3 nmol) for 3 and 6 days reduced the potency of morphine analgesia by 1.5 and 3.5 folds, respectively. Daily administration of BAM8-22 for 6 days also significantly decreased the tail flick latency. The administration of another MrgC agonist (Tyr⁶)-γ2-MSH-6-12 (MSH, 3 nmol) reduced morphine potency and the reduction was abolished following the co-administration of the protein kinase C (PKC) inhibitor chelerythrine chloride (CLT, 3 nmol). The chronic treatment with BAM8-22 or MSH increased the expression of PKC-gamma (PKCγ) in the cell membrane of spinal dorsal horn neurons and PKC-epsilon (PKCε) in the cell membrane and cytosol of DRG neurons. Moreover, the BAM8-22 treatment induced an increase in the expression of calcitonin gene-related peptide (CGRP) and neuronal nitric oxide synthase (nNOS) in small and medium cells in DRG. All of these responses were not seen when BAM8-22 or MSH was co-administered with the PKC inhibitor CLT (3 nmol) or GF-109203X (10 nmol). The present study suggested that the chronic activation of MrgC upregulated expressions of pronociceptive mediators via PKC signaling pathway leading to the suppression of antinociceptive property of morphine. These effects are opposite to those occurred when MrgC is activated acutely or moderately.

Cyclic-Nucleotide- and HCN-Channel-Mediated Phototransduction in Intrinsically Photosensitive Retinal Ganglion Cells

Non-image-forming vision in mammals is mediated primarily by melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs). In mouse M1-ipRGCs, by far the best-studied subtype, melanopsin activates PLCβ4 (phospholipase C-β4) to open TRPC6,7 channels, mechanistically similar to phototransduction in fly rhabdomeric (microvillous) photoreceptors. We report here that, surprisingly, mouse M4-ipRGCs rely on a different and hitherto undescribed melanopsin-driven, ciliary phototransduction mechanism involving cyclic nucleotide as the second messenger and HCN channels rather than CNG channels as the ion channel for phototransduction. Even more surprisingly, within an individual mouse M2-ipRGC, this HCN-channel-dependent, ciliary phototransduction pathway operates in parallel with the TRPC6,7-dependent rhabdomeric pathway. These findings reveal a complex heterogeneity in phototransduction among ipRGCs and, more importantly, break a general dogma about segregation of the two phototransduction motifs, likely with strong evolutionary implications.

Yin-and-yang bifurcation of opioidergic circuits for descending analgesia at the midbrain of the mouse

In the descending analgesia pathway, opioids are known to disinhibit the projections from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM), leading to suppression of pain signals at the spinal cord level. The locus coeruleus (LC) has been proposed to engage in the descending pathway through noradrenergic inputs to the spinal cord. Nevertheless, how the LC is integrated in the descending analgesia circuit has remained unknown. Here, we show that the opioidergic analgesia pathway is bifurcated in structure and function at the PAG. A knockout as well as a PAG-specific knockdown of phospholipase C β4 (PLCβ4), a signaling molecule for G protein-coupled receptors, enhanced swim stress-induced and morphine-induced analgesia in mice. Immunostaining after simultaneous retrograde labeling from the RVM and the LC revealed two mutually exclusive neuronal populations at the PAG, each projecting either to the LC or the RVM, with PLCβ4 expression only in the PAG-LC projecting cells that provide a direct synaptic input to LC-spinal cord (SC) projection neurons. The PAG-LC projection neurons in wild-type mice turned quiescent in response to opiates, but remained active in the PLCβ4 mutant, suggesting a possibility that an increased adrenergic function induced by the persistent PAG-LC activity underlies the enhanced opioid analgesia in the mutant. Indeed, the enhanced analgesia in the mutant was reversed by blocking α2-noradrenergic receptors. These findings indicate that opioids suppress descending analgesia through the PAG-LC pathway, while enhancing it through the PAG-RVM pathway, i.e., two distinct pathways with opposing effects on opioid analgesia. These results point to a therapeutic target in pain control.

Targeting G protein-coupled receptor signalling by blocking G proteins

G protein-coupled receptors (GPCRs) are the largest class of drug targets, largely owing to their druggability, diversity and physiological efficacy. Many drugs selectively target specific subtypes of GPCRs, but high specificity for individual GPCRs may not be desirable in complex multifactorial disease states in which multiple receptors may be involved. One approach is to target G protein subunits rather than the GPCRs directly. This approach has the potential to achieve broad efficacy by blocking pathways shared by multiple GPCRs. Additionally, because many GPCRs couple to multiple G protein signalling pathways, blocking specific G protein subunits can 'bias' GPCR signals by inhibiting only a subset of these signals. Molecules that target G protein α or βγ-subunits have been developed and show strong efficacy in multiple preclinical disease models and biased inhibition of G protein signalling. In this Review, we discuss the development and characterization of G protein α and βγ-subunit ligands and the preclinical evidence that this exciting new approach has potential for therapeutic efficacy in a number of indications, such as pain, thrombosis, asthma and heart failure.

Defect in phosphoinositide signalling through a homozygous variant in PLCB3 causes a new form of spondylometaphyseal dysplasia with corneal dystrophy

BACKGROUND

Bone dysplasias are a large group of disorders affecting the growth and structure of the skeletal system.

METHODS

In the present study, we report the clinical and molecular delineation of a new form of syndromic autosomal recessive spondylometaphyseal dysplasia (SMD) in two Emirati first cousins. They displayed postnatal growth deficiency causing profound limb shortening with proximal and distal segments involvement, narrow chest, radiological abnormalities involving the spine, pelvis and metaphyses, corneal clouding and intellectual disability. Whole genome homozygosity mapping localised the genetic cause to 11q12.1-q13.1, a region spanning 19.32 Mb with ~490 genes. Using whole exome sequencing, we identified four novel homozygous variants within the shared block of homozygosity. Pathogenic variants in genes involved in phospholipid metabolism, such as PLCB4 and PCYT1A, are known to cause bone dysplasia with or without eye anomalies, which led us to select PLCB3 as a strong candidate. This gene encodes phospholipase C β 3, an enzyme that converts phosphatidylinositol 4,5 bisphosphate (PIP2) to inositol 1,4,5 triphosphate (IP3) and diacylglycerol.

RESULTS

The identified variant (c.2632G>T) substitutes a serine for a highly conserved alanine within the Ha2' element of the proximal C-terminal domain. This disrupts binding of the Ha2' element to the catalytic core and destabilises PLCB3. Here we show that this hypomorphic variant leads to elevated levels of PIP2 in patient fibroblasts, causing disorganisation of the F-actin cytoskeleton.

CONCLUSIONS

Our results connect a homozygous loss of function variant in PLCB3 with a new SMD associated with corneal dystrophy and developmental delay (SMDCD).

Synergistic Signaling by Light and Acetylcholine in Mouse Iris Sphincter Muscle

The mammalian pupillary light reflex (PLR) involves a bilateral brain circuit whereby afferent light signals in the optic nerve ultimately drive iris-sphincter-muscle contraction via excitatory cholinergic parasympathetic innervation [1, 2]. Additionally, the PLR in nocturnal and crepuscular sub-primate mammals has a "local" component in the isolated sphincter muscle [3-5], as in amphibians, fish, and bird [6-10]. In mouse, this local PLR requires the pigment melanopsin [5], originally found in intrinsically photosensitive retinal ganglion cells (ipRGCs) [11-19]. However, melanopsin's presence and effector pathway locally in the iris remain uncertain. The sphincter muscle itself may express melanopsin [5], or its cholinergic parasympathetic innervation may be modulated by suggested intraocular axonal collaterals of ipRGCs traveling to the eye's ciliary body or even to the iris [20-22]. Here, we show that the muscarinic receptor antagonist, atropine, eliminated the effect of acetylcholine (ACh), but not of light, on isolated mouse sphincter muscle. Conversely, selective genetic deletion of melanopsin in smooth muscle mostly removed the light-induced, but not the ACh-triggered, increase in isolated sphincter muscle's tension and largely suppressed the local PLR in vivo. Thus, sphincter muscle cells are bona fide, albeit unconventional, photoreceptors. We found melanopsin expression in a small subset of mouse iris sphincter muscle cells, with the light-induced contractile signal apparently spreading through gap junctions into neighboring muscle cells. Light and ACh share a common signaling pathway in sphincter muscle. In summary, our experiments have provided details of a photosignaling process in the eye occurring entirely outside the retina.

Targeting GPCR-Gβγ-GRK2 signaling as a novel strategy for treating cardiorenal pathologies

The pathologic crosstalk between the heart and kidney is known as cardiorenal syndrome (CRS). While the specific mechanisms underlying this crosstalk remain poorly understood, CRS is associated with exacerbated dysfunction of either or both organs and reduced survival. Maladaptive fibrotic remodeling is a key component of both heart and kidney failure pathogenesis and progression. G-protein coupled receptor (GPCR) signaling is a crucial regulator of cardiovascular and renal function. Chronic/pathologic GPCR signaling elicits the interaction of the G-protein Gβγ subunit with GPCR kinase 2 (GRK2), targeting the receptor for internalization, scaffolding to pathologic signals, and receptor degradation. Targeting this pathologic Gβγ-GRK2 interaction has been suggested as a possible strategy for the treatment of HF. In the current review, we discuss recent updates in understanding the role of GPCR-Gβγ-GRK2 signaling as a crucial mediator of maladaptive organ remodeling detected in HF and kidney dysfunction, with specific attention to small molecule-mediated inhibition of pathologic Gβγ-GRK2 interactions. Further, we explore the potential of GPCR-Gβγ-GRK2 signaling as a possible therapeutic target for cardiorenal pathologies.

Reversal of Ethanol-induced Intoxication by a Novel Modulator of Gβγ Protein Potentiation of the Glycine Receptor

The acute intoxicating effects of ethanol in the central nervous system result from the modulation of several molecular targets. It is widely accepted that ethanol enhances the activity of the glycine receptor (GlyR), thus enhancing inhibitory neurotransmission, leading to motor effects, sedation, and respiratory depression. We previously reported that small peptides interfered with the binding of Gβγ to the GlyR and consequently inhibited the ethanol-induced potentiation of the receptor. Now, using virtual screening, we identified a subset of small molecules capable of interacting with the binding site of Gβγ. One of these compounds, M554, inhibited the ethanol potentiation of the GlyR in both evoked currents and synaptic transmission in vitro When this compound was tested in vivo in mice treated with ethanol (1-3.5 g/kg), it was found to induce a faster recovery of motor incoordination in rotarod experiments and a shorter sedative effect in loss of righting reflex assays. This study describes a novel molecule that might be relevant for the design of useful therapeutic compounds in the treatment of acute alcohol intoxication.

Synergistic analgesic effect of propofol-alfentanil combination through detecting the inhibition of cAMP signal pathway

OBJECTIVE

The study aims to investigate the possible mechanism of the synergistic analgesic effect of propofol-alfentanil combination.

METHODS

The synergistic analgesic effects of propofol-alfentanil combination in Sprague-Dawley (SD) rats were analysed through the von Frey test. Then, we examined the activity of phospholipase C (PLC) and the intracellular levels of Ca(2+) and adenosine 3', 5'cyclic monophosphate (cAMP) in primary neuronal cells of fetal SD rats. We detected the intracellular Ca(2+) concentration by fluorescence and flow cytometry. The PLC activity of the primary neuronal cells was assayed using the EnzChek(®) Direct Phospholipase C Assay Kit. The cAMP content of the cells was assayed using the cAMP Direct Immunoassay Kit (Fluorometric).

KEY FINDINGS

Both propofol and alfentanil treatments depressed cAMP levels and PLC activity, but propofol-alfentanil combination decreased these parameters to a greater extent than alfentanil treatment alone. Propofol and alfentanil both inhibited Ca(2+) channel, but propofol-alfentanil combination suppressed this channel to a greater extent than alfentanil treatment alone. Fluorescent image analysis revealed that both propofol and alfentanil reduced the intracellular levels of Ca(2+) , and propofol-alfentanil combination showed weaker signals than alfentanil alone. Propofol-alfentanil combination significantly reduced intracellular Ca(2+) level, cAMP level and PLC activity.

CONCLUSION

Propofol and alfentanil exert synergistic analgesic effects through the adenylyl cyclase pathway.

Regulation of retinal angiogenesis by phospholipase C-β3 signaling pathway

Angiogenesis has an essential role in many pathophysiologies. Here, we show that phospholipase C-β3 (PLC-β3) isoform regulates endothelial cell function and retinal angiogenesis. Silencing of PLC-β3 in human umbilical vein endothelial cells (HUVECs) significantly delayed proliferation, migration and capillary-like tube formation. In addition, mice lacking PLC-β3 showed impaired retinal angiogenesis with delayed endothelial proliferation, reduced endothelial cell activation, abnormal vessel formation and hemorrhage. Finally, tumor formation was significantly reduced in mice lacking PLC-β3 and showed irregular size and shape of blood vessels. These results suggest that regulation of endothelial function by PLC-β3 may contribute to angiogenesis.

[ROLE PHOSPHOINOSITID SIGNALING PATHWAY IN OPIOIDS CONTROL OF P2X3 RECEPTORS IN THE PRIMARY SENSORY NEURONS]

Homomeric P2X3 receptors expressed in primary nociceptive neurons are crucial elements in the pain signal generation. In turn, opioid system regulates the intensity of this signal in both CNS and PNS. Here we describe the effects of opioids on P2X3 receptors in DRG neurons studied by using patch clamp technique. Activation of G-protein coupled opioid receptors by endogenous opioid Leu-enkephalin (Leu), resulted in the two opposite effects on P2X3 receptor-mediated currents (P2X3 currents). In particular, application of 1 µM Leu lead to the complete inhibition of P2X3 currents. However, after pretreatment of the neurons with a Gi/o-protein inhibitor pertussis toxin (PT), the same concentration of Leu caused facilitation of P2X3 currents. PLC inhibitor U-73122 at concentration of 1 µM completely eliminated both facilitating and inhibitory effects of Leu on P2X3 currents. Thus, opioid receptor agonists cause two oppositely directed effects on P2X3 receptors in DRG neurons of rats and both of them are mediated through PLC signaling pathway. Our results point to a possible molecular basis of the mechanism for the well-known transition inhibitory action of opioids (analgesia) to facilitating (hyperalgesia).

Finding directionality and gene-disease predictions in disease associations

BACKGROUND

Understanding the underlying molecular mechanisms in human diseases is important for diagnosis and treatment of complex conditions and has traditionally been done by establishing associations between disorder-genes and their associated diseases. This kind of network analysis usually includes only the interaction of molecular components and shared genes. The present study offers a network and association analysis under a bioinformatics frame involving the integration of HUGO Gene Nomenclature Committee approved gene symbols, KEGG metabolic pathways and ICD-10-CM codes for the analysis of human diseases based on the level of inclusion and hypergeometric enrichment between genes and metabolic pathways shared by the different human disorders.

METHODS

The present study offers the integration of HGNC approved gene symbols, KEGG metabolic pathways andICD-10-CM codes for the analysis of associations based on the level of inclusion and hypergeometricenrichment between genes and metabolic pathways shared by different diseases.

RESULTS

880 unique ICD-10-CM codes were mapped to the 4315 OMIM phenotypes and 3083 genes with phenotype-causing mutation. From this, a total of 705 ICD-10-CM codes were linked to 1587 genes with phenotype-causing mutations and 801 KEGG pathways creating a tripartite network composed by 15,455 code-gene-pathway interactions. These associations were further used for an inclusion analysis between diseases along with gene-disease predictions based on a hypergeometric enrichment methodology.

CONCLUSIONS

The results demonstrate that even though a large number of genes and metabolic pathways are shared between diseases of the same categories, inclusion levels between these genes and pathways are directional and independent of the disease classification. However, the gene-disease-pathway associations can be used for prediction of new gene-disease interactions that will be useful in drug discovery and therapeutic applications.

G-protein βγ subunit dimers modulate kidney repair after ischemia-reperfusion injury in rats

Heterotrimeric G-proteins play a crucial role in the control of renal epithelial cell function during homeostasis and in response to injury. In this report, G-protein βγ subunit (Gβγ) dimer activity was evaluated during the process of tubular repair after renal ischemia-reperfusion injury (IRI) in male Sprague Dawley rats. Rats were treated with a small molecule inhibitor of Gβγ activity, gallein (30 or 100 mg/kg), 1 hour after reperfusion and every 24 hours for 3 additional days. After IRI, renal dysfunction was prolonged after the high-dose gallein treatment in comparison with vehicle treatment during the 7-day recovery period. Renal tubular repair in the outer medulla 7 days after IRI was significantly (P < 0.001) attenuated after treatment with high-dose gallein (100 mg/kg) in comparison with low-dose gallein (30 mg/kg), or the vehicle and fluorescein control groups. Gallein treatment significantly reduced (P < 0.05) the number of proliferating cell nuclear antigen-positive tubular epithelial cells at 24 hours after the ischemia-reperfusion phase in vivo. In vitro application of gallein on normal rat kidney (NRK-52E) proximal tubule cells significantly reduced (P < 0.05) S-phase cell cycle entry compared with vehicle-treated cells as determined by 5'-bromo-2'-deoxyuridine incorporation. Taken together, these data suggest that Gβγ signaling contributes to the maintenance and repair of renal tubular epithelium and may be a novel therapeutic target for the development of drugs to treat acute kidney injury.

Nucleus accumbens-specific interventions in RGS9-2 activity modulate responses to morphine

Regulator of G protein signalling 9-2 (Rgs9-2) modulates the actions of a wide range of CNS-acting drugs by controlling signal transduction of several GPCRs in the striatum. RGS9-2 acts via a complex mechanism that involves interactions with Gα subunits, the Gβ5 protein, and the adaptor protein R7BP. Our recent work identified Rgs9-2 complexes in the striatum associated with acute or chronic exposures to mu opioid receptor (MOR) agonists. In this study we use several new genetic tools that allow manipulations of Rgs9-2 activity in particular brain regions of adult mice in order to better understand the mechanism via which this protein modulates opiate addiction and analgesia. We used adeno-associated viruses (AAVs) to express forms of Rgs9-2 in the dorsal and ventral striatum (nucleus accumbens, NAc) in order to examine the influence of this protein in morphine actions. Consistent with earlier behavioural findings from constitutive Rgs9 knockout mice, we show that Rgs9-2 actions in the NAc modulate morphine reward and dependence. Notably, Rgs9-2 in the NAc affects the analgesic actions of morphine as well as the development of analgesic tolerance. Using optogenetics we demonstrate that activation of Channelrhodopsin2 in Rgs9-2-expressing neurons, or in D1 dopamine receptor (Drd1)-enriched medium spiny neurons, accelerates the development of morphine tolerance, whereas activation of D2 dopamine receptor (Drd2)-enriched neurons does not significantly affect the development of tolerance. Together, these data provide new information on the signal transduction mechanisms underlying opiate actions in the NAc.

Neural processing of itch

While considerable effort has been made to investigate the neural mechanisms of pain, much less effort has been devoted to itch, at least until recently. However, itch is now gaining increasing recognition as a widespread and costly medical and socioeconomic issue. This is accompanied by increasing interest in the underlying neural mechanisms of itch, which has become a vibrant and rapidly-advancing field of research. The goal of the present forefront review is to describe the recent progress that has been made in our understanding of itch mechanisms.

Inhibition of Gβγ-subunit signaling potentiates morphine-induced antinociception but not respiratory depression, constipation, locomotion, and reward

Inhibition of Gβγ-subunit signaling to phospholipase C β3 has been shown to potentiate morphine-mediated antinociception while attenuating the development of tolerance and dependence in mice. The objective of this study was to determine the effect of Gβγ-subunit inhibition on antinociception and other pharmacological effects, such as respiratory depression, constipation, and hyperlocomotion, mediated by the μ-opioid receptor. The Gβγ-subunit inhibitor, gallein, was administered to C57BL/6J mice by intraperitoneal injection before morphine, and data were compared with mice treated with vehicle, morphine, or gallein alone. Morphine-induced antinociception was measured using the 55°C warm-water tail-withdrawal test. Pretreatment with gallein produced a dose-dependent potentiation of morphine-mediated antinociception, producing up to a 10-fold leftward shift in the morphine dose-response curve and extending the duration of antinociception induced by a single dose of morphine. Gallein pretreatment also prevented acute antinociceptive tolerance induced by morphine. In contrast, the dose-dependent respiratory depression and hyperlocomotion induced by morphine were not potentiated by gallein pretreatment. Similarly, gallein pretreatment did not potentiate morphine-conditioned place preference responses or morphine-induced constipation, as measured as a reduction in excreta. These results suggest that selectively inhibiting Gβγ-mediated signaling may selectively increase μ-opioid receptor-mediated antinociception without matching increases in adverse physiological effects.

Targeting G protein coupled receptor-related pathways as emerging molecular therapies

G protein coupled receptors (GPCRs) represent the most important targets in modern pharmacology because of the different functions they mediate, especially within brain and peripheral nervous system, and also because of their functional and stereochemical properties. In this paper, we illustrate, via a variety of examples, novel advances about the GPCR-related molecules that have been shown to play diverse roles in GPCR pathways and in pathophysiological phenomena. We have exemplified how those GPCRs’ pathways are, or might constitute, potential targets for different drugs either to stimulate, modify, regulate or inhibit the cellular mechanisms that are hypothesized to govern some pathologic, physiologic, biologic and cellular or molecular aspects both in vivo and in vitro. Therefore, influencing such pathways will, undoubtedly, lead to different therapeutical applications based on the related pharmacological implications. Furthermore, such new properties can be applied in different fields. In addition to offering fruitful directions for future researches, we hope the reviewed data, together with the elements found within the cited references, will inspire clinicians and researchers devoted to the studies on GPCR’s properties.

Structural insights into phospholipase C-β function

Phospholipase C (PLC) enzymes convert phosphatidylinositol-4,5-bisphosphate into the second messengers diacylglycerol and inositol-1,4,5-triphosphate. The production of these molecules promotes the release of intracellular calcium and activation of protein kinase C, which results in profound cellular changes. The PLCβ subfamily is of particular interest given its prominent role in cardiovascular and neuronal signaling and its regulation by G protein-coupled receptors, as PLCβ is the canonical downstream target of the heterotrimeric G protein Gαq. However, this is not the only mechanism regulating PLCβ activity. Extensive structural and biochemical evidence has revealed regulatory roles for autoinhibitory elements within PLCβ, Gβγ, small molecular weight G proteins, and the lipid membrane itself. Such complex regulation highlights the central role that this enzyme plays in cell signaling. A better understanding of the molecular mechanisms underlying the control of its activity will greatly facilitate the search for selective small molecule modulators of PLCβ.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Opioid μ-receptors as new target for insulin resistance

Type-2 diabetes is one of the fastest growing public health problems worldwide resulting from both environmental and genetic factors. Activation of μ-opioid receptor (MOR) could result in reversal of the impairment of insulin-stimulated glucose disposal in genetically obese Zucker rats via exercise training. This improvement of insulin resistance was associated with an elevation of circulating β-endorphin to ameliorate the post-receptor insulin signaling cascade, including downstream effectors of the phosphatidylinositol 3-kinase (PI3-kinase) signaling pathway. In insulin resistant rats, Loperamide treatment effected on the insulin receptor substrate (IRS)-1/PI3-kinase/Akt signaling cascade and subsequent insulin-stimulated glucose transport trafficking on skeletal muscle, which were all suppressed by MOR antagonism. In addition, induction of insulin resistance by the intake of high fructose is more rapid in MOR knockout mice than in wild-type mice. Improvements in insulin sensitivity through the peripheral MOR activation overcoming defects related to the post-receptor in IRS-1-associated PI3-kinase step have been defined. Opioid receptor activation, especially of the μ-subtype, may provide merits in the amelioration of defective insulin action. Atypical zeta (ζ) isoform of protein kinase C serves as a factor that integrates with peripheral MOR pathway and insulin signals for glucose utilization. The developments call new insights into the chemical compounds and/or herbal products that might enhance opioid peptide secretion and/or stimulate MOR in peripheral insulin-sensitive tissues to serve as potential agents or adjuvants for helping the glucose metabolism. In the present review, we update these topics and discuss the concept of targeting peripheral MOR pathway for the treatment of insulin resistance.

Molecular targeting of Gα and Gβγ subunits: a potential approach for cancer therapeutics

G-Protein-coupled receptors (GPCRs) signal through G protein α and βγ subunit families to regulate a wide range of physiological and pathophysiological processes. As such, GPCRs are major targets for therapeutic drugs. Downstream targets of GPCRs have also gained interest as a therapeutic approach to complex pathologies involving multiple GPCRs. One such approach involves targeting of the G proteins themselves. Several small molecule Gα and Gβγ modulators have been developed and been tested in various animal models of disease. Here we will discuss the requirements for targeting Gα and Gβγ subunits, the mechanisms of action of currently identified inhibitors, and focus on the potential utility of Gα and Gβγ inhibitors in the treatment of various cancers.

The expanding roles of Gβγ subunits in G protein-coupled receptor signaling and drug action

Gβγ subunits from heterotrimeric G proteins perform a vast array of functions in cells with respect to signaling, often independently as well as in concert with Gα subunits. However, the eponymous term "Gβγ" does not do justice to the fact that 5 Gβ and 12 Gγ isoforms have evolved in mammals to serve much broader roles beyond their canonical roles in cellular signaling. We explore the phylogenetic diversity of Gβγ subunits with a view toward understanding these expanded roles in different cellular organelles. We suggest that the particular content of distinct Gβγ subunits regulates cellular activity, and that the granularity of individual Gβ and Gγ action is only beginning to be understood. Given the therapeutic potential of targeting Gβγ action, this larger view serves as a prelude to more specific development of drugs aimed at individual isoforms.

Full-length Gα(q)-phospholipase C-β3 structure reveals interfaces of the C-terminal coiled-coil domain

Phospholipase C-β (PLCβ) is directly activated by Gα_q, but the molecular basis for how its distal C-terminal domain (CTD) contributes to maximal activity is poorly understood. Herein we present both the crystal structure and cryo-EM 3D reconstructions of human full-length PLCβ3 in complex with murine Gα_q. The distal CTD forms an extended, monomeric helical bundle consisting of three anti-parallel segments with structural similarity to membrane-binding bin–amphiphysin–Rvs (BAR) domains. Sequence conservation of the distal CTD identifies putative membrane and protein interaction sites, the latter of which bind the N-terminal helix of Gα_q in both the crystal structure and cryo-EM reconstructions. Functional analysis suggests the distal CTD plays roles in membrane targeting and in optimizing the orientation of the catalytic core at the membrane for maximal rates of lipid hydrolysis.

Implantation failure in mice with a disruption in Phospholipase C beta 1 gene: lack of embryonic attachment, aberrant steroid hormone signalling and defective endocannabinoid metabolism

Phospholipase C beta 1 (PLCβ1) is a downstream effector of G-protein-coupled receptor signalling and holds central roles in reproductive physiology. Mice with a disruption in the Plcβ1 gene are infertile with pleiotropic reproductive defects, the major reproductive block in females being implantation failure. Here, PLCβ1 was demonstrated at the luminal and glandular epithelia throughout the pre- and peri-implantation period, with transient stromal expression during 0.5-1.5 days post coitum (dpc). Examination of implantation sites at 4.5 dpc showed that in females lacking functional PLCβ1 (knock-out (KO) females), embryos failed to establish proper contact with the uterine epithelium. Proliferating luminal epithelial cells were evident in KO implantation sites, indicating failure to establish a receptive uterus. Real-time PCR demonstrated that KO implantation sites had aberrant ovarian steroid signalling, with high levels of estrogen receptor α, lactoferrin and amphiregulin mRNA, while immunohistochemistry revealed very low levels of estrogen receptor α protein, possibly due to rapid receptor turnover. KO implantation sites expressed markedly less fatty acid amide hydrolase and monoacylglycerol lipase, indicating that endocannabinoid metabolism was also affected. Collectively, our results show that PLCβ1 is essential for uterine preparation for implantation, and that defective PLCβ1-mediated signalling during implantation is associated with aberrant ovarian steroid signalling and endocannabinoid metabolism.

Coenzyme Q10 prevents peripheral neuropathy and attenuates neuron loss in the db-/db- mouse, a type 2 diabetes model

Diabetic peripheral neuropathy (DPN) is the most common complication in both type 1 and type 2 diabetes. Here we studied some phenotypic features of a well-established animal model of type 2 diabetes, the leptin receptor-deficient db(-)/db(-) mouse, and also the effect of long-term (6 mo) treatment with coenzyme Q10 (CoQ10), an endogenous antioxidant. Diabetic mice at 8 mo of age exhibited loss of sensation, hypoalgesia (an increase in mechanical threshold), and decreases in mechanical hyperalgesia, cold allodynia, and sciatic nerve conduction velocity. All these changes were virtually completely absent after the 6-mo, daily CoQ10 treatment in db(-)/db(-) mice when started at 7 wk of age. There was a 33% neuronal loss in the lumbar 5 dorsal root ganglia (DRGs) of the db(-)/db(-) mouse versus controls at 8 mo of age, which was significantly attenuated by CoQ10. There was no difference in neuron number in 5/6-wk-old mice between diabetic and control mice. We observed a strong down-regulation of phospholipase C (PLC) β3 in the DRGs of diabetic mice at 8 mo of age, a key molecule in pain signaling, and this effect was also blocked by the 6-mo CoQ10 treatment. Many of the phenotypic, neurochemical regulations encountered in lumbar DRGs in standard models of peripheral nerve injury were not observed in diabetic mice at 8 mo of age. These results suggest that reactive oxygen species and reduced PLCβ3 expression may contribute to the sensory deficits in the late-stage diabetic db(-)/db(-) mouse, and that early long-term administration of the antioxidant CoQ10 may represent a promising therapeutic strategy for type 2 diabetes neuropathy.

Intrathecal PLC(β3) oligodeoxynucleotides antisense potentiates acute morphine efficacy and attenuates chronic morphine tolerance

Morphine is a mainstay for chronic pain treatment, but its efficacy has been hampered by physical tolerance. The underlying mechanism for chronic morphine induced tolerance is complicated and not well understood. PLC(β3) is regarded as an important factor in the morphine tolerance signal pathway. In this study, we determined intrathecal (i.t.) administration of an antisense oligodeoxynucleotide (ODN) of PLC(β3) could quicken the on-set antinociceptive efficacy of acute morphine treatment and prolong the maximum effect up to 4h. The antisense could also attenuate the development of morphine-induced tolerance and left shift the ED50 after 7 day of coadministration with morphine. These results probably were contributed by the PLC(β3) antisense ODN as they successfully knocked down protein expression levels and reduced activity of PLC(β3) in spinal cord in rats. The mismatch group had no such effects. The results confirmed the important involvement of PLC(β3) in both acute morphine efficacy and chronic morphine tolerance at spinal level in rats. This study may provide an idea for producing a novel adjuvant for morphine treatment.

Rhes: a GTP-binding protein integral to striatal physiology and pathology

Rhes, the Ras Homolog Enriched in Striatum, is a GTP-binding protein whose gene was discovered during a screen for mRNAs preferentially expressed in rodent striatum. This 266 amino acid protein is intermediate in size between small Ras-like GTP-binding proteins and α-subunits of heterotrimeric G proteins. It is most closely related to another Ras-like GTP-binding protein termed Dexras1 or AGS1. Although subsequent studies have shown that the rhes gene is expressed in other brain areas in addition to striatum, the striatal expression level is relatively high, and Rhes protein is likely to play a vital role in striatal physiology and pathology. Indeed, it has recently been shown to interact with the Huntingtin protein and play a pivotal role in the selective vulnerability of striatum in Huntington's disease (HD). Not surprisingly, Rhes can interact with multiple proteins to affect striatal physiology at multiple levels. Functional studies have indicated that Rhes plays a role in signaling by striatal G protein-coupled receptors (GPCR), although the details of the mechanism remain to be determined. Rhes has been shown to bind to both α- and β-subunits of heterotrimeric G proteins and to affect signaling by both Gi/o- and Gs/olf-coupled receptors. In this context, Rhes can be classified as a member of the family of accessory proteins to GPCR signaling. With documented effects in dopamine- and opioid-mediated behaviors, an interaction with thyroid hormone systems and a role in HD pathology, Rhes is emerging as an important protein in striatal physiology and pathology.

Itch signaling in the nervous system

Itch is a major somatic sensation, along with pain, temperature, and touch, detected and relayed by the somatosensory system. Itch can be an acute sensation, associated with mosquito bite, or a chronic condition, like atopic dermatitis (29, 59). The origins of the stimulus can be localized in the periphery or systemic, and associated with organ failure or cancer. Itch is also a perception originating in the brain. Itch is broadly characterized as either histamine-dependent (histaminergic) or histamine-independent (nonhistaminergic), both of which are relayed by subsets of C fibers and by the second-order neurons expressing gastrin-releasing peptide receptor (GRPR) and spinothalamic track (STT) neurons in the spinal cord of rodents. Historically, itch research has been primarily limited to clinical and psychophysical studies and to histamine-mediated mechanisms. In contrast, little is known about the signaling mechanisms underlying nonhistaminergic itch, despite the fact that the majority of chronic itch are mediated by nonhistaminergic mechanisms. During the past few years, important progress has been made in understanding the molecular signaling of itch, largely due to the introduction of mouse genetics. In this review, we examine some of the molecular mechanisms underlying itch sensation with an emphasis on recent studies in rodents.

The phospholipase C isozymes and their regulation

The physiological effects of many extracellular neurotransmitters, hormones, growth factors, and other stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid signaling pathways. These signaling responses include the classically described conversion of phosphatidylinositol(4,5)P(2) to the Ca(2+)-mobilizing second messenger inositol(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. The 13 mammalian PLCs elaborate a minimal catalytic core typified by PLC-d to confer multiple modes of regulation of lipase activity. PLC-b isozymes are activated by Gaq- and Gbg-subunits of heterotrimeric G proteins, and activation of PLC-g isozymes occurs through phosphorylation promoted by receptor and non-receptor tyrosine kinases. PLC-e and certain members of the PLC-b and PLC-g subclasses of isozymes are activated by direct binding of small G proteins of the Ras, Rho, and Rac subfamilies of GTPases. Recent high resolution three dimensional structures together with biochemical studies have illustrated that the X/Y linker region of the catalytic core mediates autoinhibition of most if not all PLC isozymes. Activation occurs as a consequence of removal of this autoinhibition.

Unidirectional cross-activation of GRPR by MOR1D uncouples itch and analgesia induced by opioids

Spinal opioid-induced itch, a prevalent side effect of pain management, has been proposed to result from pain inhibition. We now report that the μ-opioid receptor (MOR) isoform MOR1D is essential for morphine-induced scratching (MIS), whereas the isoform MOR1 is required only for morphine-induced analgesia (MIA). MOR1D heterodimerizes with gastrin-releasing peptide receptor (GRPR) in the spinal cord, relaying itch information. We show that morphine triggers internalization of both GRPR and MOR1D, whereas GRP specifically triggers GRPR internalization and morphine-independent scratching. Providing potential insight into opioid-induced itch prevention, we demonstrate that molecular and pharmacologic inhibition of PLCβ3 and IP3R3, downstream effectors of GRPR, specifically block MIS but not MIA. In addition, blocking MOR1D-GRPR association attenuates MIS but not MIA. Together, these data suggest that opioid-induced itch is an active process concomitant with but independent of opioid analgesia, occurring via the unidirectional cross-activation of GRPR signaling by MOR1D heterodimerization.

A unique role of RGS9-2 in the striatum as a positive or negative regulator of opiate analgesia

The signaling molecule RGS9-2 is a potent modulator of G-protein-coupled receptor function in striatum. Our earlier work revealed a critical role for RGS9-2 in the actions of the μ-opioid receptor (MOR) agonist morphine. In this study, we demonstrate that RGS9-2 may act as a positive or negative modulator of MOR-mediated behavioral responses in mice depending on the agonist administered. Paralleling these findings we use coimmunoprecipitation assays to show that the signaling complexes formed between RGS9-2 and Gα subunits in striatum are determined by the MOR agonist, and we identify RGS9-2 containing complexes associated with analgesic tolerance. In striatum, MOR activation promotes the formation of complexes between RGS9-2 and several Gα subunits, but morphine uniquely promotes an association between RGS9-2 and Gαi3. In contrast, RGS9-2/Gαq complexes assemble after acute application of several MOR agonists but not after morphine application. Repeated morphine administration leads to the formation of distinct complexes, which contain RGS9-2, Gβ5, and Gαq. Finally, we use simple pharmacological manipulations to disrupt RGS9-2 complexes formed during repeated MOR activation to delay the development of analgesic tolerance to morphine. Our data provide a better understanding of the brain-region-specific signaling events associated with opiate analgesia and tolerance and point to pharmacological approaches that can be readily tested for improving chronic analgesic responsiveness.