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Thermosensitive transient receptor potential (TRP) ion channels detect changes in ambient temperature to regulate body temperature and temperature-dependent cellular activity. Rodent orthologs of TRP vanilloid 2 (TRPV2) are activated by nonphysiological heat exceeding 50 °C, and human TRPV2 is heat-insensitive. TRPV2 is required for phagocytic activity of macrophages which are rarely exposed to excessive heat, but what activates TRPV2 in vivo remains elusive. Here we describe the molecular mechanism of an oxidation-induced temperature-dependent gating of TRPV2. While high concentrations of HO induce a modest sensitization of heat-induced inward currents, the oxidant chloramine-T (ChT), ultraviolet A light, and photosensitizing agents producing reactive oxygen species (ROS) activate and sensitize TRPV2. This oxidation-induced activation also occurs in excised inside-out membrane patches, indicating a direct effect on TRPV2. The reducing agent dithiothreitol (DTT) in combination with methionine sulfoxide reductase partially reverses ChT-induced sensitization, and the substitution of the methionine (M) residues M528 and M607 to isoleucine almost abolishes oxidation-induced gating of rat TRPV2. Mass spectrometry on purified rat TRPV2 protein confirms oxidation of these residues. Finally, macrophages generate TRPV2-like heat-induced inward currents upon oxidation and exhibit reduced phagocytosis when exposed to the TRP channel inhibitor ruthenium red (RR) or to DTT. In summary, our data reveal a methionine-dependent redox sensitivity of TRPV2 which may be an important endogenous mechanism for regulation of TRPV2 activity and account for its pivotal role for phagocytosis in macrophages.
Learn More >Tactile stimuli are integrated and processed by neuronal circuits in the deep dorsal horn of the spinal cord. Several spinal interneuron populations have been implicated in tactile information processing. However, dorsal horn projection neurons that contribute to the postsynaptic dorsal column (PSDC) pathway transmitting tactile information to the brain are poorly characterized. Here, we show that spinal neurons marked by the expression of Zic2cre mediate light touch sensitivity and textural discrimination. A subset of Zic2cre neurons are PSDC neurons that project to brainstem dorsal column nuclei, and chemogenetic activation of Zic2 PSDC neurons increases sensitivity to light touch stimuli. Zic2 neurons receive direct input from the cortex and brainstem motor nuclei and are required for corrective motor movements. These results suggest that Zic2 neurons integrate sensory input from cutaneous afferents with descending signals from the brain to promote corrective movements and transmit processed touch information back to the brain.
Learn More >High frequency stimulation (HFS) of the sciatic nerve has been reported to produce long term potentiation (LTP) and long-lasting pain hypersensitivity in rats. However, the central underlying mechanism remains unclear. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) belongs to a group of electron-transporting transmembrane enzymes that produce reactive oxygen species (ROS). Here we found that NOX2 was upregulated in the lumbar spinal dorsal horn after HFS of the left sciatic nerve, which induced bilateral pain and spinal LTP in both male and female rats. Blocking NOX2 with blocking peptide or shRNA prevented the development of bilateral mechanical allodynia, the induction of spinal LTP, and the phosphorylation of N-methyl-d-aspartate (NMDA) receptor 2B (GluN2B) and nuclear factor kappaB (NF-κB) p65 following HFS. Moreover, NOX2 shRNA reduced the frequency and amplitude of both spontaneous excitatory post synaptic currents (sEPSCs) and miniature excitatory postsynaptic currents (mEPSCs) in laminar II neurons. Furthermore, 8-hydroxyguanine (8-OHG), an oxidative stress marker, was increased in the spinal dorsal horn. Spinal application of ROS scavenger, Phenyl-N-tert-butylnitrone (PBN), depressed the already established spinal LTP. Spinal application of H2O2, one ROS, induced LTP and bilateral mechanical allodynia, increased the frequency and amplitude of sEPSCs in laminar II neurons, and phosphorylated GluN2B and p65 in the dorsal horn. The present study provided electrophysiological and behavioral evidence that NOX2- derived ROS in the spinal cord contributed to persistent mirror-image pain by enhancing the synaptic transmission, which was mediated by increasing presynaptic glutamate release and activation of NMDAR and NF-κB in the spinal dorsal horn.
Learn More >Neurotensin (NT) exerts its analgesic effects through activation of the G protein-coupled receptors NTS1 and NTS2. This opioid-independent antinociception represents a potential alternative for pain management. While activation of NTS1 also induces a drop in blood pressure and body temperature, NTS2 appears to be an analgesic target free of these adverse effects. Here, we report modifications of NT at Tyr11 to increase selectivity towards NTS2, complemented by modifications at the N-terminus to impair proteolytic degradation of the biologically active NT(8-13) sequence. Replacement of Tyr11 by either 6-OH-Tic or 7-OH-Tic resulted in a significant loss of binding affinity to NTS1 and subsequent NTS2 selectivity. Incorporation of the unnatural amino acid β3hLys at position 8 increased the half-life to over 24 h in plasma. Simultaneous integration of both β3hLys8 and 6-OH-Tic11 into NT(8-13) produced a potent and NTS2-selective analogue with strong analgesic action after intrathecal delivery in the rat formalin-induced pain model with an ED50 of 1.4 nmol. Additionally, intravenous administration of this NT analogue did not produce persistent hypotension or hypothermia. These results demonstrate that NT analogues harbouring unnatural amino acids at positions 8 and 11 can enhance crucial pharmacokinetic and pharmacodynamic features for NT(8-13) analogues, i.e., proteolytic stability, NTS2 selectivity and an improved analgesic/adverse effect ratio.
Learn More >Mounting evidence suggests that antiretroviral therapy (ART) drugs may contribute to the prevalence of HIV-associated neurological dysfunction. The HIV envelope glycoprotein (gp120) is neurotoxic and has been linked to alterations in mitochondrial function and increased inflammatory gene expression, which are common neuropathological findings in HIV+ cases on ART with neurological disorders. Tenofovir disproxil fumarate (TDF) has been shown to affect neurogenesis in brains of mice and mitochondria in neurons. In this study, we hypothesized that TDF contributes to neurotoxicity by modulating mitochondrial biogenesis and inflammatory pathways. TDF administered to wild-type (wt) and GFAP-gp120 transgenic (tg) mice caused peripheral neuropathy, as indicated by nerve conduction slowing and thermal hyperalgesia. Conversely TDF protected gp120-tg mice from cognitive dysfunction. In the brains of wt and gp120-tg mice, TDF decreased expression of mitochondrial transcription factor A (TFAM). However, double immunolabelling revealed that TFAM was reduced in neurons and increased in astroglia in the hippocampi of TDF-treated wt and gp120-tg mice. TDF also increased expression of GFAP and decreased expression of IBA1 in the wt and gp120-tg mice. TDF increased tumor necrosis factor (TNF) α in wt mice. However, TDF reduced interleukin (IL) 1β and TNFα mRNA in gp120-tg mouse brains. Primary human astroglia were exposed to increasing doses of TDF for 24 hours and then analyzed for mitochondrial alterations and inflammatory gene expression. In astroglia, TDF caused a dose-dependent increase in oxygen consumption rate, extracellular acidification rate and spare respiratory capacity, changes consistent with increased metabolism. TDF also reduced IL-1β-mediated increases in IL-1β and TNFα mRNA. These data demonstrate that TDF causes peripheral neuropathy in mice and alterations in inflammatory signaling and mitochondrial activity in the brain.
Learn More >The paucity of selective agonists for TWIK-related acid-sensitive K 3 (TASK-3) channel, a member of two-pore domain K (K2P) channels, has contributed to our limited understanding of its biological functions. By targeting a druggable transmembrane cavity using a structure-based drug design approach, we discovered a biguanide compound, CHET3, as a highly selective allosteric activator for TASK-3-containing K2P channels, including TASK-3 homomers and TASK-3/TASK-1 heteromers. CHET3 displayed potent analgesic effects in vivo in a variety of acute and chronic pain models in rodents that could be abolished pharmacologically or by genetic ablation of TASK-3. We further found that TASK-3-containing channels anatomically define a unique population of small-sized, transient receptor potential cation channel subfamily M member 8 (TRPM8)-, transient receptor potential cation channel subfamily V member 1 (TRPV1)-, or tyrosine hydroxylase (TH)-positive nociceptive sensory neurons and functionally regulate their membrane excitability, supporting CHET3 analgesic effects in thermal hyperalgesia and mechanical allodynia under chronic pain. Overall, our proof-of-concept study reveals TASK-3-containing K2P channels as a druggable target for treating pain.
Learn More >The proinflammatory cytokine interleukin-17 (IL-17) is implicated in pain regulation. However, the synaptic mechanisms by which IL-17 regulates pain transmission are unknown. Here, we report that glia-produced IL-17 suppresses inhibitory synaptic transmission in the spinal cord pain circuit and drives chemotherapy-induced neuropathic pain. We find that IL-17 not only enhances excitatory postsynaptic currents (EPSCs) but also suppresses inhibitory postsynaptic synaptic currents (IPSCs) and GABA-induced currents in lamina II somatostatin-expressing neurons in mouse spinal cord slices. IL-17 mainly expresses in spinal cord astrocytes, and its receptor IL-17R is detected in somatostatin-expressing neurons. Selective knockdown of IL-17R in spinal somatostatin-expressing interneurons reduces paclitaxel-induced hypersensitivity. Overexpression of IL-17 in spinal astrocytes is sufficient to induce mechanical allodynia in naive animals. In dorsal root ganglia, IL-17R expression in nociceptive sensory neurons is sufficient and required for inducing neuronal hyperexcitability after paclitaxel. Together, our data show that IL-17/IL-17R mediate neuron-glial interactions and neuronal hyperexcitability in chemotherapy-induced peripheral neuropathy.
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