Intrinsic Pleckstrin Homology (PH) Domain Motion in Phospholipase C-β Exposes a Gβγ Protein Binding Site

Sevoflurane impedes neuropathic pain by maintaining endoplasmic reticulum stress and oxidative stress homeostasis through inhibiting the activation of the PLCγ/CaMKII/IP3R signaling pathway

OBJECTIVE

To investigate the effect of sevoflurane on neuropathic pain induced by chronic constriction injury (CCI) of sciatic nerve in mice, and to elucidate its mechanism by animal experiments.

METHODS AND RESULTS

Thirty-two C57BL/6 mice were randomly divided into four groups: Sham group, Model group, Control group and Sevoflurane group. First, a mouse model of neuropathic pain was established. Then, the mice in each group were killed on Day 14 after operation to harvest the enlarged lumbosacral spinal cord. In contrast with the Model group, the Sevoflurane group displayed a significantly increased paw withdrawal mechanical threshold (PWMT) and significantly prolonged paw withdrawal thermal latency (PWTL) from Day 5 after operation. The morphological changes of lumbosacral spinal cord were observed by hematoxylin-eosin (HE) staining and transmission electron microscopy. Pathological results showed that sevoflurane reduced nuclear pyknosis in lumbosacral spinal cord tissue, with a large number of mitochondrial crista disappearance and mitochondrial swelling. The results of Western blotting showed that sevoflurane significantly decreased the protein expressions of phosphorylated phospholipase Cγ (p-PLCγ), phosphorylated calcium/calmodulin-dependent protein kinase II (p-CaMKII) and phosphorylated inositol 1,4,5-triphosphate receptor (p-IP3R), and reduced the protein expressions of endoplasmic reticulum (ER) stress proteins glucose-regulated protein 78 (GRP78) and GRP94, oxidative stress-related proteins P22 and P47 and inflammatory factors nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3), interleukin-1 β (IL-1β), and tumor necrosis factor-α (TNF-α).

CONCLUSIONS

Sevoflurane inhibits neuropathic pain by maintaining ER stress and oxidative stress homeostasis through inhibiting the activation of the PLCγ/CaMKII/IP3R signaling pathway.

Signal Transduction of Transient Receptor Potential TRPM8 Channels: Role of PIP5K, Gq-Proteins, and c-Jun

Transient receptor potential melastatin-8 (TRPM8) is a cation channel that is activated by cold and "cooling agents" such as menthol and icilin, which induce a cold sensation. The stimulation of TRPM8 activates an intracellular signaling cascade that ultimately leads to a change in the gene expression pattern of the cells. Here, we investigate the TRPM8-induced signaling pathway that links TRPM8 channel activation to gene transcription. Using a pharmacological approach, we show that the inhibition of phosphatidylinositol 4-phosphate 5 kinase α (PIP5K), an enzyme essential for the biosynthesis of phosphatidylinositol 4,5-bisphosphate, attenuates TRPM8-induced gene transcription. Analyzing the link between TRPM8 and Gq proteins, we show that the pharmacological inhibition of the βγ subunits impairs TRPM8 signaling. In addition, genetic studies show that TRPM8 requires an activated Gα subunit for signaling. In the nucleus, the TRPM8-induced signaling cascade triggers the activation of the transcription factor AP-1, a complex consisting of a dimer of basic region leucine zipper (bZIP) transcription factors. Here, we identify the bZIP protein c-Jun as an essential component of AP-1 within the TRPM8-induced signaling cascade. In summary, with PIP5K, Gq subunits, and c-Jun, we identified key molecules in TRPM8-induced signaling from the plasma membrane to the nucleus.

Gαq signalling from endosomes: A new conundrum

G-protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors, and are involved in the transmission of a variety of extracellular stimuli such as hormones, neurotransmitters, light and odorants into intracellular responses. They regulate every aspect of physiology and, for this reason, about one third of all marketed drugs target these receptors. Classically, upon binding to their agonist, GPCRs are thought to activate G-proteins from the plasma membrane and to stop signalling by subsequent desensitisation and endocytosis. However, accumulating evidence indicates that, upon internalisation, some GPCRs can continue to activate G-proteins in endosomes. Importantly, this signalling from endomembranes mediates alternative cellular responses other than signalling at the plasma membrane. Endosomal G-protein signalling and its physiological relevance have been abundantly documented for Gαs - and Gαi -coupled receptors. Recently, some Gαq -coupled receptors have been reported to activate Gαq on endosomes and mediate important cellular processes. However, several questions relative to the series of cellular events required to translate endosomal Gαq activation into cellular responses remain unanswered and constitute a new conundrum. How are these responses in endosomes mediated in the quasi absence of the substrate for the canonical Gαq -activated effector? Is there another effector? Is there another substrate? If so, how does this alternative endosomal effector or substrate produce a downstream signal? This review aims to unravel and discuss these important questions, and proposes possible routes of investigation.

Molecular regulation of PLCβ signaling

Phospholipase C (PLC) enzymes convert the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PIP2) into inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 and DAG regulate numerous downstream pathways, eliciting diverse and profound cellular changes and physiological responses. In the six PLC subfamilies in higher eukaryotes, PLCβ is intensively studied due to its prominent role in regulating crucial cellular events underlying many processes including cardiovascular and neuronal signaling, and associated pathological conditions. In addition to GαqGTP, Gβγ generated upon G protein heterotrimer dissociation also regulates PLCβ activity. Here, we not only review how Gβγ directly activates PLCβ, and also extensively modulates Gαq-mediated PLCβ activity, but also provide a structure-function overview of PLC family members. Given that Gαq and PLCβ are oncogenes, and Gβγ shows unique cell-tissue-organ specific expression profiles, Gγ subtype-dependent signaling efficacies, and distinct subcellular activities, this review proposes that Gβγ is a major regulator of Gαq-dependent and independent PLCβ signaling.

The protective effect of allicin on myocardial ischemia-reperfusion by inhibition of Ca2+ overload-induced cardiomyocyte apoptosis via the PI3K/GRK2/PLC-γ/IP3R signaling pathway

Purpose: To investigate the protective effect and mechanism of allicin on myocardial ischemia-reperfusion (MI/R) injury.

Methods: We investigated the mechanisms by which allicin attenuated the MI/R injury by focusing on phosphoinositide 3-kinase, G protein coupled receptor kinases 2, phospholipase Cγ and cardiomyocyte apoptosis. Sixty male mice were randomly assigned into three groups: repeated MI/R (model), sham-operated (control), and MI/R+ allicin group (allicin). Ultrasound examination was used to examine the cardiac function. Masson staining was used to evaluate the myocardial infarct area. TUNEL assay was performed to examine the anti-apoptotic effect of allicin. Differentially expressed genes (DEGs) and pathways were analyzed by mRNA microarray analysis. Immunofluorescence staining and western blot were carried out to detect the effect of allicin on the PI3K. A pan-PLC activator, m-3M3FBS, was applied to investigate whether allicin induced cardiomyocyte apoptosis was via the GRK2/PLC/IP3R signaling pathway.

Results: Masson staining and the TUNEL assay revealed that allicin reduced infarct size and played an anti-apoptotic role in M/IR. Ultrasound examination revealed that allicin improved cardiac function after M/IR injury. Gene ontology analysis indicated that the calcium signaling pathway and PI3KCA(PI3K) were selected. Immunofluorescence staining and western blot exposed that PI3K was activated by allicin during MI/R injury. Fura-2AM staining revealed that the PI3K -mediated GRK2/PLC-γ/IP3R pathway may be involved in the protective effect of allicin on MI/R injury.

Conclusions: Allicin has a protective effect on MI/R injury. This effect might be associated with the inhibition of Ca²⁺ overload-induced apoptosis and the inhibition of the PI3K -mediated GRK2/PLC-γ/IP3R signaling pathway.

Dissociation of the G protein βγ from the Gq-PLCβ complex partially attenuates PIP2 hydrolysis

Phospholipase C β (PLCβ), which is activated by the Gq family of heterotrimeric G proteins, hydrolyzes the inner membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2), generating diacylglycerol and inositol 1,4,5-triphosphate (IP3). Because Gq and PLCβ regulate many crucial cellular processes and have been identified as major disease drivers, activation and termination of PLCβ signaling by the Gαq subunit have been extensively studied. Gq-coupled receptor activation induces intense and transient PIP2 hydrolysis, which subsequently recovers to a low-intensity steady-state equilibrium. However, the molecular underpinnings of this equilibrium remain unclear. Here, we explored the influence of signaling crosstalk between Gq and Gi/o pathways on PIP2 metabolism in living cells using single-cell and optogenetic approaches to spatially and temporally constrain signaling. Our data suggest that the Gβγ complex is a component of the highly efficient lipase Gαq_GTP-PLCβ-Gβγ. We found that over time, Gβγ dissociates from this lipase complex, leaving the less-efficient Gαq_GTP-PLCβ lipase complex and allowing the significant partial recovery of PIP2 levels. Our findings also indicate that the subtype of the Gγ subunit in Gβγ fine-tunes the lipase activity of Gq-PLCβ, in which cells expressing Gγ with higher plasma membrane interaction show lower PIP2 recovery. Given that Gγ shows cell- and tissue-specific subtype expression, our findings suggest the existence of tissue-specific distinct Gq-PLCβ signaling paradigms. Furthermore, these results also outline a molecular process that likely safeguards cells from excessive Gq signaling.

Structure and regulation of phospholipase Cβ and ε at the membrane

Phospholipase C (PLC) β and ε enzymes hydrolyze phosphatidylinositol (PI) lipids in response to direct interactions with heterotrimeric G protein subunits and small GTPases, which are activated downstream of G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). PI hydrolysis generates second messengers that increase the intracellular Ca²⁺ concentration and activate protein kinase C (PKC), thereby regulating numerous physiological processes. PLCβ and PLCε share a highly conserved core required for lipase activity, but use different strategies and structural elements to autoinhibit basal activity, bind membranes, and engage G protein activators. In this review, we discuss recent structural insights into these enzymes and the implications for how they engage membranes alone or in complex with their G protein regulators.

Heterotrimeric G Protein Subunit Gαq Is a Master Switch for Gβγ-Mediated Calcium Mobilization by Gi-Coupled GPCRs

Mechanisms that control mobilization of cytosolic calcium [Ca2+]i are key for regulation of numerous eukaryotic cell functions. One such paradigmatic mechanism involves activation of phospholipase Cβ (PLCβ) enzymes by G protein βγ subunits from activated Gαi-Gβγ heterotrimers. Here, we report identification of a master switch to enable this control for PLCβ enzymes in living cells. We find that the Gαi-Gβγ-PLCβ-Ca2+ signaling module is entirely dependent on the presence of active Gαq. If Gαq is pharmacologically inhibited or genetically ablated, Gβγ can bind to PLCβ but does not elicit Ca2+ signals. Removal of an auto-inhibitory linker that occludes the active site of the enzyme is required and sufficient to empower "stand-alone control" of PLCβ by Gβγ. This dependence of Gi-Gβγ-Ca2+ on Gαq places an entire signaling branch of G-protein-coupled receptors (GPCRs) under hierarchical control of Gq and changes our understanding of how Gi-GPCRs trigger [Ca2+]i via PLCβ enzymes.

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.

Structure of phospholipase Cε reveals an integrated RA1 domain and previously unidentified regulatory elements

Phospholipase Cε (PLCε) generates lipid-derived second messengers at the plasma and perinuclear membranes in the cardiovascular system. It is activated in response to a wide variety of signals, such as those conveyed by Rap1A and Ras, through a mechanism that involves its C-terminal Ras association (RA) domains (RA1 and RA2). However, the complexity and size of PLCε has hindered its structural and functional analysis. Herein, we report the 2.7 Å crystal structure of the minimal fragment of PLCε that retains basal activity. This structure includes the RA1 domain, which forms extensive interactions with other core domains. A conserved amphipathic helix in the autoregulatory X-Y linker of PLCε is also revealed, which we show modulates activity in vitro and in cells. The studies provide the structural framework for the core of this critical cardiovascular enzyme that will allow for a better understanding of its regulation and roles in disease.

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.

Genome-Wide Association Study Identifies Single Nucleotide Polymorphism Markers Associated with Mycelial Growth (at 15, 20, and 25°C), Mefenoxam Resistance, and Mating Type in Phytophthora infestans

Phenotypic diversity among individuals defines the potential for evolutionary selection in a species. Phytophthora infestans epidemics are generally thought to be favored by moderate to low temperatures, but temperatures in many locations worldwide are expected to rise as a result of global climate change. Thus, we investigated variation among individuals of P. infestans for relative growth at different temperatures. Isolates of P. infestans came from three collections: (i) individual genotypes recently dominant in the United States, (ii) recently collected individuals from Central Mexico, and (iii) progeny of a recent sexual recombination event in the northeastern United States. In general, these isolates had optimal mycelial growth rates at 15 or 20°C. However, two individuals from Central Mexico grew better at higher temperatures than did most others and two individuals grew relatively less at higher temperatures than did most others. The isolates were also assessed for mefenoxam sensitivity and mating type. Each collection contained individuals of diverse sensitivities to mefenoxam and individuals of the A1 and A2 mating type. We then searched for genomic regions associated with phenotypic diversity using genotyping-by-sequencing. We found one single nucleotide polymorphism (SNP) associated with variability in mycelial growth at 20°C, two associated with variability in mycelial growth at 25°C, two associated with sensitivity to mefenoxam, and one associated with mating type. Interestingly, the SNPs associated with mefenoxam sensitivity were found in a gene-sparse region, whereas the SNPs associated with growth at the two temperatures and mating type were found both at more gene-dense regions.

Direct observation of conformational dynamics of the PH domain in phospholipases Cϵ and β may contribute to subfamily-specific roles in regulation

Phospholipase C (PLC) enzymes produce second messengers that increase the intracellular Ca2+ concentration and activate protein kinase C (PKC). These enzymes also share a highly conserved arrangement of core domains. However, the contributions of the individual domains to regulation are poorly understood, particularly in isoforms lacking high-resolution information, such as PLCϵ. Here, we used small-angle X-ray scattering (SAXS), EM, and functional assays to gain insights into the molecular architecture of PLCϵ, revealing that its PH domain is conformationally dynamic and essential for activity. We further demonstrate that the PH domain of PLCβ exhibits similar dynamics in solution that are substantially different from its conformation observed in multiple previously reported crystal structures. We propose that this conformational heterogeneity contributes to subfamily-specific differences in activity and regulation by extracellular signals.

The binding of activated Gαq to phospholipase C-β exhibits anomalous affinity

Upon activation by the G_q family of Gα subunits, Gβγ subunits, and some Rho family GTPases, phospholipase C-β (PLC-β) isoforms hydrolyze phosphatidylinositol 4,5-bisphosphate to the second messengers inositol 1,4,5-trisphosphate and diacylglycerol. PLC-β isoforms also function as GTPase-activating proteins, potentiating G_q deactivation. To elucidate the mechanism of this mutual regulation, we measured the thermodynamics and kinetics of PLC-β3 binding to Gα_q FRET and fluorescence correlation spectroscopy, two physically distinct methods, both yielded K_d values of about 200 nm for PLC-β3-Gα_q binding. This K_d is 50-100 times greater than the EC₅₀ for Gα_q-mediated PLC-β3 activation and for the Gα_q GTPase-activating protein activity of PLC-β. The measured K_d was not altered either by the presence of phospholipid vesicles, phosphatidylinositol 4,5-bisphosphate and Ca²⁺, or by the identity of the fluorescent labels. FRET-based kinetic measurements were also consistent with a K_d of 200 nm We determined that PLC-β3 hysteresis, whereby PLC-β3 remains active for some time following either Gα_q-PLC-β3 dissociation or PLC-β3-potentiated Gα_q deactivation, is not sufficient to explain the observed discrepancy between EC₅₀ and K_d These results indicate that the mechanism by which Gα_q and PLC-β3 mutually regulate each other is far more complex than a simple, two-state allosteric model and instead is probably kinetically determined.