Construction of a global pain systems network highlights phospholipid signaling as a regulator of heat nociception

Phosphoinositide Regulation of TRP Channels: A Functional Overview in the Structural Era

Transient receptor potential (TRP) ion channels have diverse activation mechanisms including physical stimuli, such as high or low temperatures, and a variety of intracellular signaling molecules. Regulation by phosphoinositides and their derivatives is their only known common regulatory feature. For most TRP channels, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] serves as a cofactor required for activity. Such dependence on PI(4,5)P2 has been demonstrated for members of the TRPM subfamily and for the epithelial TRPV5 and TRPV6 channels. Intracellular TRPML channels show specific activation by PI(3,5)P2. Structural studies uncovered the PI(4,5)P2 and PI(3,5)P2 binding sites for these channels and shed light on the mechanism of channel opening. PI(4,5)P2 regulation of TRPV1-4 as well as some TRPC channels is more complex, involving both positive and negative effects. This review discusses the functional roles of phosphoinositides in TRP channel regulation and molecular insights gained from recent cryo-electron microscopy structures.

Prostaglandin EP3 receptor activation is antinociceptive in sensory neurons via PI3Kγ, AMPK and GRK2

BACKGROUND AND PURPOSE

Prostaglandin E₂ is considered a major mediator of inflammatory pain, by acting on neuronal G_s protein-coupled EP2 and EP4 receptors. However, the neuronal EP3 receptor, colocalized with EP2 and EP4 receptor, is G_i protein-coupled and antagonizes the pronociceptive prostaglandin E₂ effect. Here, we investigated the cellular signalling mechanisms by which the EP3 receptor reduces EP2 and EP4 receptor-evoked pronociceptive effects in sensory neurons.

EXPERIMENTAL APPROACH

Experiments were performed on isolated and cultured dorsal root ganglion (DRG) neurons from wild type, phosphoinositide 3-kinase γ (PI3Kγ)^-/- , and PI3Kγ^{kinase dead (KD)/KD} mice. For subtype-specific stimulations, we used specific EP2, EP3, and EP4 receptor agonists from ONO Pharmaceuticals. As a functional readout, we recorded TTX-resistant sodium currents in patch-clamp experiments. Western blots were used to investigate the activation of intracellular signalling pathways. EP4 receptor internalization was measured using immunocytochemistry.

KEY RESULTS

Different pathways mediate the inhibition of EP2 and EP4 receptor-dependent pronociceptive effects by EP3 receptor stimulation. Inhibition of EP2 receptor-evoked pronociceptive effect critically depends on the kinase-independent function of the signalling protein PI3Kγ, and adenosine monophosphate activated protein kinase (AMPK) is involved. By contrast, inhibition of EP4 receptor-evoked pronociceptive effect is independent on PI3Kγ and mediated through activation of G protein-coupled receptor kinase 2 (GRK2), which enhances the internalization of the EP4 receptor after ligand binding.

CONCLUSION AND IMPLICATIONS

Activation of neuronal PI3Kγ, AMPK, and GRK2 by EP3 receptor activation limits cAMP-dependent pain generation by prostaglandin E₂ . These new insights hold the potential for a novel approach in pain therapy.

Characterization of plasma metabolites and proteins in patients with herpetic neuralgia and development of machine learning predictive models based on metabolomic profiling

Herpes zoster (HZ) is a localized, painful cutaneous eruption that occurs upon reactivation of the herpes virus. Postherpetic neuralgia (PHN) is the most common chronic complication of HZ. In this study, we examined the metabolomic and proteomic signatures of disease progression in patients with HZ and PHN. We identified differentially expressed metabolites (DEMs), differentially expressed proteins (DEPs), and key signaling pathways that transition from healthy volunteers to the acute or/and chronic phases of herpetic neuralgia. Moreover, some specific metabolites correlated with pain scores, disease duration, age, and pain in sex dimorphism. In addition, we developed and validated three optimal predictive models (AUC > 0.9) for classifying HZ and PHN from healthy individuals based on metabolic patterns and machine learning. These findings may reveal the overall metabolomics and proteomics landscapes and proposed the optimal machine learning predictive models, which provide insights into the mechanisms of HZ and PHN.

Peripheral straightjacket (α2δ Ca2+ channel subunit) expression is required for neuropathic sensitization in Drosophila

Nerve injury leads to devastating and often untreatable neuropathic pain. While acute noxious sensation (nociception) is a crucial survival mechanism and is conserved across phyla, chronic neuropathic pain is considered a maladaptive response owing to its devastating impact on a patient's quality of life. We have recently shown that a neuropathic pain-like response occurs in adult Drosophila. However, the mechanisms underlying this phenomenon are largely unknown. Previous studies have shown that the α2δ peripheral calcium channel subunit straightjacket (stj) is a conserved factor required for thermal pain perception. We demonstrate here that stj is required in peripheral ppk+ sensory neurons for acute thermal responses and that it mediates nociceptive hypersensitivity in an adult Drosophila model of neuropathic pain-like disease. Given that calcium channels are the main targets of gabapentinoids (pregabalin and gabapentin), we assessed if these drugs can alleviate nociceptive hypersensitivity. Our findings suggest that gabapentinoids may prevent nociceptive hypersensitivity by preserving central inhibition after nerve injury. Together, our data further highlight the similarity of some mechanisms for pain-like conditions across phyla and validates the scientific use of Drosophila neuropathic sensitization models for analgesic drug discovery. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Nerve injury drives a heightened state of vigilance and neuropathic sensitization in Drosophila

Injury can lead to devastating and often untreatable chronic pain. While acute pain perception (nociception) evolved more than 500 million years ago, virtually nothing is known about the molecular origin of chronic pain. Here we provide the first evidence that nerve injury leads to chronic neuropathic sensitization in insects. Mechanistically, peripheral nerve injury triggers a loss of central inhibition that drives escape circuit plasticity and neuropathic allodynia. At the molecular level, excitotoxic signaling within GABAergic (γ-aminobutyric acid) neurons required the acetylcholine receptor nAChRα1 and led to caspase-dependent death of GABAergic neurons. Conversely, disruption of GABA signaling was sufficient to trigger allodynia without injury. Last, we identified the conserved transcription factor twist as a critical downstream regulator driving GABAergic cell death and neuropathic allodynia. Together, we define how injury leads to allodynia in insects, and describe a primordial precursor to neuropathic pain may have been advantageous, protecting animals after serious injury.

BMP signaling downstream of the Highwire E3 ligase sensitizes nociceptors

A comprehensive understanding of the molecular machinery important for nociception is essential to improving the treatment of pain. Here, we show that the BMP signaling pathway regulates nociception downstream of the E3 ubiquitin ligase highwire (hiw). hiw loss of function in nociceptors caused antagonistic and pleiotropic phenotypes with simultaneous insensitivity to noxious heat but sensitized responses to optogenetic activation of nociceptors. Thus, hiw functions to both positively and negatively regulate nociceptors. We find that a sensory reception-independent sensitization pathway was associated with BMP signaling. BMP signaling in nociceptors was up-regulated in hiw mutants, and nociceptor-specific expression of hiw rescued all nociception phenotypes including the increased BMP signaling. Blocking the transcriptional output of the BMP pathway with dominant negative Mad suppressed nociceptive hypersensitivity that was induced by interfering with hiw. The up-regulated BMP signaling phenotype in hiw genetic mutants could not be suppressed by mutation in wallenda suggesting that hiw regulates BMP in nociceptors via a wallenda independent pathway. In a newly established Ca²⁺ imaging preparation, we observed that up-regulated BMP signaling caused a significantly enhanced Ca²⁺ signal in the axon terminals of nociceptors that were stimulated by noxious heat. This response likely accounts for the nociceptive hypersensitivity induced by elevated BMP signaling in nociceptors. Finally, we showed that 24-hour activation of BMP signaling in nociceptors was sufficient to sensitize nociceptive responses to optogenetically-triggered nociceptor activation without altering nociceptor morphology. Overall, this study demonstrates the previously unrevealed roles of the Hiw-BMP pathway in the regulation of nociception and provides the first direct evidence that up-regulated BMP signaling physiologically sensitizes responses of nociceptors and nociception behaviors.

Although pain is a universally experienced sensation that has a significant impact on human lives and society, the molecular mechanisms of pain remain poorly understood. Elucidating these mechanisms is particularly important to gaining insight into the clinical development of currently incurable chronic pain diseases. Taking an advantage of the powerful genetic model organism Drosophila melanogaster (fruit flies), we unveil the Highwire-BMP signaling pathway as a novel molecular pathway that regulates the sensitivity of nociceptive sensory neurons. Highwire and the molecular components of the BMP signaling pathway are known to be widely conserved among animal phyla, from nematode worms to humans. Since abnormal sensitivity of nociceptive sensory neurons can play a critical role in the development of chronic pain conditions, a deeper understanding of the regulation of nociceptor sensitivity has the potential to advance effective therapeutic strategies to treat difficult pain conditions.

Signatures of Altered Gene Expression in Dorsal Root Ganglia of a Fabry Disease Mouse Model

Fabry disease is an X-linked lysosomal storage disorder with involvement of the nervous system. Accumulation of glycosphingolipids within peripheral nerves and/or dorsal root ganglia results in pain due to small-fiber neuropathy, which affects the majority of patients already in early childhood. The α-galactosidase A deficient mouse proved to be an adequate model for Fabry disease, as it shares many symptoms including altered temperature sensitivity and pain perception. To characterize the signatures of gene expression that might underlie Fabry disease-associated sensory deficits and pain, we performed one-color based hybridization microarray expression profiling of DRG explants from adult α-galactosidase A deficient mice and age-matched wildtype controls. Protein-protein interaction (PPI) and pathway analyses were performed for differentially regulated mRNAs. We found 812 differentially expressed genes between adult α-galactosidase A deficient mice and age-matched wildtype controls, 506 of them being upregulated, and 306 being downregulated. Among the enriched pathways and processes, the disease-specific pathways “lysosome” and “ceramide metabolic process” were identified, enhancing reliability of the current analysis. Novel pathways that we identified include “G-protein coupled receptor signaling” and “retrograde transport” for the upregulated genes. From the analysis of downregulated genes, immune-related pathways, autoimmune, and infection pathways emerged. The current analysis is the first to present a differential gene expression profile of DRGs from α-galactosidase A deficient mice, thereby providing knowledge on possible mechanisms underlying neuropathic pain related symptoms in Fabry patients. Therefore, the presented data provide new insights into the development of the pain phenotype and might lead to new treatment strategies.

Car8 dorsal root ganglion expression and genetic regulation of analgesic responses are associated with a cis-eQTL in mice

Carbonic anhydrase-8 (Car8 mouse gene symbol) is devoid of enzymatic activity, but instead functions as an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1) to regulate this intracellular calcium release channel important in synaptic functions and neuronal excitability. Causative mutations in ITPR1 and carbonic anhydrase-8 in mice and humans are associated with certain subtypes of spinal cerebellar ataxia (SCA). SCA mice are genetically deficient in dorsal root ganglia (DRG) Car8 expression and display mechanical and thermal hypersensitivity and susceptibility to subacute and chronic inflammatory pain behaviors. In this report, we show that DRG Car8 expression is variable across 25 naïve-inbred strains of mice, and this cis-regulated eQTL (association between rs27660559, rs27706398, and rs27688767 and DRG Car8 expression; P < 1 × 10^-11) is correlated with nociceptive responses in mice. Next, we hypothesized that increasing DRG Car8 gene expression would inhibit intracellular calcium release required for morphine antinociception and might correlate with antinociceptive sensitivity of morphine and perhaps other analgesic agents. We show that mean DRG Car8 gene expression is directly related to the dose of morphine or clonidine needed to provide a half-maximal analgesic response (r = 0.93, P < 0.00002; r = 0.83, P < 0.0008, respectively), suggesting that greater DRG Car8 expression increases analgesic requirements. Finally, we show that morphine induces intracellular free calcium release using Fura 2 calcium imaging in a dose-dependent manner; V5-Car8 ^WT overexpression in NBL cells inhibits morphine-induced calcium increase. These findings highlight the 'morphine paradox' whereby morphine provides antinociception by increasing intracellular free calcium, while Car8 and other antinociceptive agents work by decreasing intracellular free calcium. This is the first study demonstrating that biologic variability associated with this cis-eQTL may contribute to differing analgesic responses through altered regulation of ITPR1-dependent calcium release in mice.

The TrkA receptor mediates experimental thermal hyperalgesia produced by nerve growth factor: Modulation by the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75^NTR) and its activation of the sphingomyelin signaling cascade are essential for mechanical hypersensitivity resulting from locally injected nerve growth factor (NGF). Here the roles of the same effectors, and of the tropomyosin receptor kinase A (TrkA) receptor, are evaluated for thermal hyperalgesia from NGF. Sensitivity of rat hind paw plantar skin to thermal stimulation after local sub-cutaneous injection of NGF (500ng) was measured by the latency for paw withdrawal (PWL) from a radiant heat source. PWL was reduced from baseline values at 0.5-22h by ∼40% from that in naïve or vehicle-injected rats, and recovered to pre-injection levels by 48h. Local pre-injection with a p75^NTR blocking antibody did not affect the acute thermal hyperalgesia (0.5-3.5h) but hastened its recovery so that it had reversed to baseline by 22h. In addition, GW4869 (2mM), an inhibitor of the neutral sphingomyelinase (nSMase) that is an enzyme in the p75^NTR pathway, also failed to prevent thermal hyperalgesia. However, C2-ceramide, an analog of the ceramide produced by sphingomyelinase, did cause thermal hyperalgesia. Injection of an anti-TrkA antibody known to promote dimerization and activation of that receptor, independent of NGF, also caused thermal hyperalgesia, and prevented the further reduction of PWL from subsequently injected NGF. A non-specific inhibitor of tropomyosin receptor kinases, K252a, prevented thermal hyperalgesia from NGF, but not that from the anti-TrkA antibody. These findings suggest that the TrkA receptor has a predominant role in thermal hypersensitivity induced by NGF, while p75^NTR and its pathway intermediates serve a modulatory role.

The TRP Channels Pkd2, NompC, and Trpm Act in Cold-Sensing Neurons to Mediate Unique Aversive Behaviors to Noxious Cold in Drosophila

The basic mechanisms underlying noxious cold perception are not well understood. We developed Drosophila assays for noxious cold responses. Larvae respond to near-freezing temperatures via a mutually exclusive set of singular behaviors-in particular, a full-body contraction (CT). Class III (CIII) multidendritic sensory neurons are specifically activated by cold and optogenetic activation of these neurons elicits CT. Blocking synaptic transmission in CIII neurons inhibits CT. Genetically, the transient receptor potential (TRP) channels Trpm, NompC, and Polycystic kidney disease 2 (Pkd2) are expressed in CIII neurons, where each is required for CT. Misexpression of Pkd2 is sufficient to confer cold responsiveness. The optogenetic activation level of multimodal CIII neurons determines behavioral output, and visualization of neuronal activity supports this conclusion. Coactivation of cold- and heat-responsive sensory neurons suggests that the cold-evoked response circuitry is dominant. Our Drosophila model will enable a sophisticated molecular genetic dissection of cold nociceptive genes and circuits.

Lipid kinases as therapeutic targets for chronic pain

Existing analgesics are not efficacious in treating all patients with chronic pain and have harmful side effects when used long term. A deeper understanding of pain signaling and sensitization could lead to the development of more efficacious analgesics. Nociceptor sensitization occurs under conditions of inflammation and nerve injury where diverse chemicals are released and signal through receptors to reduce the activation threshold of ion channels, leading to an overall increase in neuronal excitability. Drugs that inhibit specific receptors have so far been unsuccessful in alleviating pain, possibly because they do not simultaneously target the diverse receptors that contribute to nociceptor sensitization. Hence, the focus has shifted toward targeting downstream convergence points of nociceptive signaling. Lipid mediators, including phosphatidylinositol 4,5-bisphosphate (PIP2), are attractive targets, as these molecules are required for signaling downstream of G-protein-coupled receptors and receptor tyrosine kinases. Furthermore, PIP2 regulates the activity of various ion channels. Thus, PIP2 sits at a critical convergence point for multiple receptors, ion channels, and signaling pathways that promote and maintain chronic pain. Decreasing the amount of PIP2 in neurons was recently shown to attenuate pronociceptive signaling and could provide a novel approach for treating pain. Here, we review the lipid kinases that are known to regulate pain signaling and sensitization and speculate on which additional lipid kinases might regulate signaling in nociceptive neurons.

Phosphoinositide regulation of TRPV1 revisited

The heat- and capsaicin-sensitive transient receptor potential vanilloid 1 ion channel (TRPV1) is regulated by plasma membrane phosphoinositides. The effects of these lipids on this channel have been controversial. Recent articles re-ignited the debate and also offered resolution to place some of the data in a coherent picture. This review summarizes the literature on this topic and provides a detailed and critical discussion on the experimental evidence for the various effects of phosphatidylinositol 4,5-bisphosphayte [PI(4,5)P2 or PIP2] on TRPV1. We conclude that PI(4,5)P2 and potentially its precursor PI(4)P are positive cofactors for TRPV1, acting via direct interaction with the channel, and their depletion by Ca(2+)-induced activation of phospholipase Cδ isoforms (PLCδ) limits channel activity during capsaicin-induced desensitization. Other negatively charged lipids at higher concentrations can also support channel activity, which may explain some controversies in the literature. PI(4,5)P2 also partially inhibits channel activity in some experimental settings, and relief from this inhibition upon PLCβ activation may contribute to sensitization. The negative effect of PI(4,5)P2 is more controversial and its mechanism is less well understood. Other TRP channels from the TRPV and TRPC families may also undergo similar dual regulation by phosphoinositides, thus the complexity of TRPV1 regulation is not unique to this channel.

Subgroup-elimination transcriptomics identifies signaling proteins that define subclasses of TRPV1-positive neurons and a novel paracrine circuit

Normal and painful stimuli are detected by specialized subgroups of peripheral sensory neurons. The understanding of the functional differences of each neuronal subgroup would be strongly enhanced by knowledge of the respective subgroup transcriptome. The separation of the subgroup of interest, however, has proven challenging as they can hardly be enriched. Instead of enriching, we now rapidly eliminated the subgroup of neurons expressing the heat-gated cation channel TRPV1 from dissociated rat sensory ganglia. Elimination was accomplished by brief treatment with TRPV1 agonists followed by the removal of compromised TRPV1(+) neurons using density centrifugation. By differential microarray and sequencing (RNA-Seq) based expression profiling we compared the transcriptome of all cells within sensory ganglia versus the same cells lacking TRPV1 expressing neurons, which revealed 240 differentially expressed genes (adj. p<0.05, fold-change>1.5). Corroborating the specificity of the approach, many of these genes have been reported to be involved in noxious heat or pain sensitization. Beyond the expected enrichment of ion channels, we found the TRPV1 transcriptome to be enriched for GPCRs and other signaling proteins involved in adenosine, calcium, and phosphatidylinositol signaling. Quantitative population analysis using a recent High Content Screening (HCS) microscopy approach identified substantial heterogeneity of expressed target proteins even within TRPV1-positive neurons. Signaling components defined distinct further subgroups within the population of TRPV1-positive neurons. Analysis of one such signaling system showed that the pain sensitizing prostaglandin PGD2 activates DP1 receptors expressed predominantly on TRPV1(+) neurons. In contrast, we found the PGD2 producing prostaglandin D synthase to be expressed exclusively in myelinated large-diameter neurons lacking TRPV1, which suggests a novel paracrine neuron-neuron communication. Thus, subgroup analysis based on the elimination rather than enrichment of the subgroup of interest revealed proteins that define subclasses of TRPV1-positive neurons and suggests a novel paracrine circuit.

The lipid kinase PIP5K1C regulates pain signaling and sensitization

Numerous pain-producing (pronociceptive) receptors signal via phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis. However, it is currently unknown which lipid kinases generate PIP2 in nociceptive dorsal root ganglia (DRG) neurons and if these kinases regulate pronociceptive receptor signaling. Here, we found that phosphatidylinositol 4-phosphate 5 kinase type 1C (PIP5K1C) is expressed at higher levels than any other PIP5K and, based on experiments with Pip5k1c(+/-) mice, generates at least half of all PIP2 in DRG neurons. Additionally, Pip5k1c haploinsufficiency reduces pronociceptive receptor signaling and TRPV1 sensitization in DRG neurons as well as thermal and mechanical hypersensitivity in mouse models of chronic pain. We identified a small molecule inhibitor of PIP5K1C (UNC3230) in a high-throughput screen. UNC3230 lowered PIP2 levels in DRG neurons and attenuated hypersensitivity when administered intrathecally or into the hindpaw. Our studies reveal that PIP5K1C regulates PIP2-dependent nociceptive signaling and suggest that PIP5K1C is a therapeutic target for chronic pain.

RNAi screening in Drosophila cells and in vivo

Here, I discuss how RNAi screening can be used effectively to uncover gene function. Specifically, I discuss the types of high-throughput assays that can be done in Drosophila cells and in vivo, RNAi reagent design and available reagent collections, automated screen pipelines, analysis of screen results, and approaches to RNAi results verification.

Nature and nurture of human pain

Humans are very different when it comes to pain. Some get painful piercings and tattoos; others can not stand even a flu shot. Interindividual variability is one of the main characteristics of human pain on every level including the processing of nociceptive impulses at the periphery, modification of pain signal in the central nervous system, perception of pain, and response to analgesic strategies. As for many other complex behaviors, the sources of this variability come from both nurture (environment) and nature (genes). Here, I will discuss how these factors contribute to human pain separately and via interplay and how epigenetic mechanisms add to the complexity of their effects.