One sentence summary: Bench to bedside translation using iPSC to characterise phenotype and pharmacology in primary erythromelalgia, an inherited chronic pain condition. ABSTRACTIn common with other chronic pain conditions, inherited erythromelalgia (IEM) represents a significant unmet medical need. The peripherally expressed SCN9A encoded sodium channel Nav1.7 plays a critical role in IEM with gain-of-function leading to aberrant sensory neuronal activity and extreme pain, particularly in response to heat. In five carefully phenotyped IEM patients, a novel highly potent and selective Nav1.7 blocker reduced heat-induced pain in the majority of subjects. In four of the five subjects we used induced pluripotent stem cell (iPSC) technology to create sensory neurons which uniquely emulated the clinical phenotype of hyperexcitability and aberrant responses to heat stimuli. When we compared the severity of the clinical phenotype with the iPSC-derived sensory neuron hyperexcitability we saw a trend towards a correlation for individual mutations. The in vitro IEM phenotype was sensitive to Nav1.7 blockers, including the clinical test agent. Given the importance of peripherally expressed sodium channels in many pain conditions, this translational approach is likely to have broader utility to a wide range of pain and sensory conditions. This emphasizes the use of iPSC approaches to bridge between clinical and preclinical studies, enabling greater understanding of a disease and the response to a therapeutic agent in defined patient populations.

The transient receptor potential (subfamily M, member 8; TRPM8) is a nonselective cation channel localized in primary sensory neurons, and is a candidate for cold thermosensing, mediation of cold pain, and bladder overactivity. Studies with TRPM8 knockout mice and selective TRPM8 channel blockers demonstrate a lack of cold sensitivity and reduced cold pain in various rodent models. Furthermore, TRPM8 blockers significantly lower body temperature. We have identified a moderately potent (IC 50 5 103 nM), selective TRPM8 antagonist, PF-05105679It demonstrated activity in vivo in the guinea pig bladder ice water and menthol challenge tests with an IC 50 of 200 nM and reduced core body temperature in the rat (at concentrations .1219 nM). PF-05105679 was suitable for acute administration to humans and was evaluated for effects on core body temperature and experimentally induced cold pain, using the cold pressor test. Unbound plasma concentrations greater than the IC 50 were achieved with 600-and 900-mg doses. The compound displayed a significant inhibition of pain in the cold pressor test, with efficacy equivalent to oxycodone (20 mg) at 1.5 hours postdose. No effect on core body temperature was observed. An unexpected adverse event (hot feeling) was reported, predominantly periorally, in 23 and 36% of volunteers (600-and 900-mg dose, respectively), which in two volunteers was nontolerable. In conclusion, this study supports a role for TRPM8 in acute cold pain signaling at doses that do not cause hypothermia.

G-protein-coupled receptors (GPCRs) play a major role in cell-cell communication in the CNS. These proteins oscillate between various inactive and active conformations, the latter being stabilized by agonists. Although mutations can lead to constitutive activity, most of these destabilize inactive conformations, and none lock the receptor in an active state. Moreover, GPCRs are known to form dimers, but the role of each protomer in the activation process remains unclear. Here, we show that the heterodimeric GPCR for the main inhibitory neurotransmitter, the GABA B receptor, can be locked in its active state by introducing two cysteines expected to form a disulphide bridge to maintain the binding domain of the GABA B1 subunit in a closed form. This constitutively active receptor cannot be inhibited by antagonists, but its normal functioning, activation by agonists, and inhibition by antagonists can be restored after reduction with dithiothreitol. These data show that the closed state of the binding domain of GABA B1 is sufficient to turn ON this heterodimeric receptor and illustrate for the first time that a GPCR can be locked in an active conformation.

Background and PurposeTREK two‐pore‐domain potassium (K2P) channels play a critical role in regulating the excitability of somatosensory nociceptive neurons and are important mediators of pain perception. An understanding of the roles of TREK channels in pain perception and, indeed, in other pathophysiological conditions, has been severely hampered by the lack of potent and/or selective activators and inhibitors. In this study, we describe a new, selective opener of TREK channels, GI‐530159.Experimental ApproachThe effect of GI‐530159 on TREK channels was demonstrated using 86Rb efflux assays, whole‐cell and single‐channel patch‐clamp recordings from recombinant TREK channels. The expression of K2P2.1 (TREK1), K2P10.1 (TREK2) and K2P4.1 (TRAAK) channels was determined using transcriptome analysis from single dorsal root ganglion (DRG) cells. Current‐clamp recordings from cultured rat DRG neurons were used to measure the effect of GI‐530159 on neuronal excitability.Key ResultsFor recombinant human TREK1 channels, GI‐530159 had similar low EC50 values in Rb efflux experiments and electrophysiological recordings. It activated TREK2 channels, but it had no detectable action on TRAAK channels nor any significant effect on other K channels tested. Current‐clamp recordings from cultured rat DRG neurones showed that application of GI‐530159 at 1 μM resulted in a significant reduction in firing frequency and a small hyperpolarization of resting membrane potential.Conclusions and ImplicationsThis study provides pharmacological evidence for the presence of mechanosensitive TREK K2P channels in sensory neurones and suggests that development of selective K2P channel openers like GI‐530159 could aid in the development of novel analgesic agents.Linked ArticlesThis article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc

Non-technical summary Intellectual disability has long been attributed at the cellular level to abnormalities in the structures that receive incoming connections to the major classes of neurons in the brain. These misshaped 'dendrites' and especially misshaped 'dendritic spines' have been found in many types of intellectual disability. We have used a mouse model of one of the human intellectual disability mutations on a gene on the X-chromosome called Ophn-1. We show that, in addition to the misshaped dendritic spines, these mice have abnormal physiology in the inability of both excitatory and inhibitory inputs ('synapses') to operate repetitively as they need to in many aspects of normal brain function. A drug known as a Rho-GAP inhibitor was able to reverse the physiological impairment within 20 min, without changing the structure of dendrites or dendritic spines. This class of drug may have a role in limiting disability in this condition.Abstract Intellectual disability affects 2-3% of the population: those due to mutations of the X-chromosome are a major cause of moderate to severe cases (1.8/1000 males). Established theories ascribe the cellular aetiology of intellectual disability to malformations of dendritic spines. Recent work has identified changes in synaptic physiology in some experimental models. Here, we investigated the pathophysiology of a mouse model of intellectual disability using electrophysiological recordings combined with confocal imaging of dentate gyrus granule neurons. Lack of oligophrenin-1 resulted in reductions in dendritic tree complexity and mature dendritic spine density and in evoked and spontaneous EPSCs and IPSCs. In the case of inhibitory transmission, the physiological change was associated with a reduction in the readily releasable pool and vesicle recycling which impaired the efficiency of inhibitory synaptic transmission. Acute inhibition of the downstream signalling pathway of oligophrenin-1 fully reversed the functional changes in synaptic transmission but not the dendritic abnormalities. The impaired inhibitory (as well as excitatory) synaptic transmission at frequencies associated with cognitive function suggests a cellular mechanism for the intellectual disability, because cortical oscillations associated with cognition normally depend on inhibitory neurons firing on every cycle. Abbreviations IEI, interevent interval; RRP, readily releasable pool; XLMR, X-linked mental retardation.

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Intellectual disability affects 2–3% of the population; mutations of the X-chromosome are a major cause of moderate to severe cases. The link between the molecular consequences of the mutation and impaired cognitive function remains unclear. Loss of function mutations of oligophrenin-1 (OPHN1) disrupt Rho-GTPase signalling. Here we demonstrate abnormal neurotransmission at CA3 synapses in hippocampal slices from Ophn1 -/y mice, resulting from a substantial decrease in the readily releasable pool of vesicles. As a result, synaptic transmission fails at high frequencies required for oscillations associated with cognitive functions. Both spontaneous and KA-induced gamma oscillations were reduced in Ophn1 -/y hippocampal slices. Spontaneous oscillations were rapidly rescued by inhibition of the downstream signalling pathway of oligophrenin-1. These findings suggest that the intellectual disability due to mutations of oligophrenin-1 results from a synaptopathy and consequent network malfunction, providing a plausible mechanism for the learning disabilities. Furthermore, they raise the prospect of drug treatments for affected individuals.

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