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The voltage-gated sodium NaV1.7 channel sets the voltage threshold for electrogenesis. Mutation in the gene encoding human NaV1.7 (SCN9A) cause painful neuropathies or pain insensitivity. NaV1.7. In the dorsal root ganglion (DRG) neurons, activity and trafficking of NaV1.7 are regulated by the auxiliary collapsin response mediator protein 2 (CRMP2). Specifically, preventing addition of a small ubiquitin-like modifier (SUMO), by the E2 SUMO-conjugating enzyme Ubc9, at lysine-374 (K374) of CRMP2 reduces NaV1.7 channel trafficking and activity. We previously identified a small molecule, designated 194, that prevented CRMP2 SUMOylation by the E2 SUMO-conjugating enzyme Ubc9 to reduce NaV1.7 surface expression and currents, leading to a reduction in spinal nociceptive transmission, and culminating in normalization of mechanical allodynia in models of neuropathic pain. In this study, we investigated whether NaV1.7 control via CRMP2-SUMOylation is conserved in nodose ganglion (NG) neurons. This study was motivated by our desire to develop 194 as a safe, non-opioid substitute for persistent pain, which led us to wonder how 194 would impact NaV1.7 in NG neurons, which are responsible for driving the cough reflex. We found functioning NaV1.7 channels in NG neurons, which were resistant to downregulation via either CRMP2 knockdown or pharmacological inhibition of CRMP2 SUMOylation by 194. Overexpression of NaV1.7 endocytic adapter Numb, rescued CRMP2-dependent regulation on NaV1.7, making NG neurons sensitive to 194. Altogether, these data point at the existence of cell-specific mechanisms regulating NaV1.7 trafficking.
Voltage-gated sodium and calcium channels (VGSCs and VGCCs) play an important role in the modulation of physiologically relevant processes in excitable cells that range from action potential generation to neurotransmission. Once their expression and/or function is altered in disease, specific pharmacological approaches become necessary to mitigate the negative consequences of such dysregulation. Several classes of small molecules have been developed with demonstrated effectiveness on VGSCs and VGCCs; however, off-target effects have also been described, limiting their use and spurring efforts to find more specific and safer molecules to target these channels. There are a great number of plants and herbal preparations that have been empirically used for the treatment of diseases in which VGSCs and VGCCs are involved. Some of these natural products have progressed to clinical trials, while others are under investigation for their action mechanisms on signaling pathways, including channels. In this review, we synthesize information from ~30 compounds derived from natural sources like plants and fungi and delineate their effects on VGSCs and VGCCs in human disease, particularly pain.
T-type calcium channels activate in response to subthreshold membrane depolarizations and represent an important source of Ca2+ influx near the resting membrane potential. These channels regulate neuronal excitability and have been linked to pain. For this reason, T-type calcium channels are suitable molecular targets for the development of new non-opioid analgesics. Our previous work identified an analogue of benzimidazolonepiperidine, 5bk, that preferentially inhibited CaV3.2 channels and reversed mechanical allodynia. In this study, we synthesized and screened a small library of 47 compounds derived from 5bk. We found several compounds that inhibited the Ca2+ influx in DRG neurons of all sizes. After separating the enantiomers of each active compound, we found two compounds, 3-25-R and 3-14-3-S, that potently inhibited the Ca2+ influx. Whole-cell patch clamp recordings from small- to medium-sized DRG neurons revealed that both compounds decreased total Ca2+. Application of 3-14-3-S (but not 3-25-R) blocked transiently expressed CaV3.1-3.3 channels with a similar IC50 value. 3-14-3-S decreased T-type, but not N-type, Ca2+ currents in DRG neurons. Furthermore, intrathecal delivery of 3-14-3-S relieved tonic, neuropathic, and inflammatory pain in preclinical models. 3-14-3-S did not exhibit any activity against G protein-coupled opioid receptors. Preliminary docking studies also suggest that 3-14-3-S can bind to the central pore domain of T-type channels. Together, our chemical characterization and functional and behavioral data identify a novel T-type calcium channel blocker with in vivo efficacy in experimental models of tonic, neuropathic, and inflammatory pain.
Background and purpose: Postoperative pain occurs in as many as 70% of the over 230 million surgeries performed annually worldwide. Postoperative pain management still relies on opioids despite their negative consequences, resulting in a public health crisis. Therefore, it is of utmost importance to develop alternative therapies to treat chronic pain. Natural products derived from medicinal plants are potential sources of novel and are potential sources biologically active compounds for development of novel analgesics safe analgesics. Experimental approach: Hence, in this study, we screened a library of natural products to identify small molecules that target the activity of voltage-gated sodium and calcium channels due to their important roles in nociceptive sensory processing. Key Results: We found that fractions derived from the Native American medicinal plant, Parthenium incanum, inhibited depolarization-evoked calcium influx in rat dorsal root ganglion (DRG) neurons. Further separation of these fractions yielded a cycloartane-type triterpene identified as argentatin C which blocked the activity of both voltage-gated sodium and calcium channels in calcium imaging assays. Docking analysis predicted that argentatin C may bind to NaV1.7-1.9 and CaV3.1-3.3 channels. Furthermore, voltage and current clamp electrophysiology experiments showed that argentatin C decreased Na+ and T-type Ca2+ currents as well as excitability in rat and macaque DRG neurons. Consistent with these observations, argentatin C treatment reversed mechanical allodynia in a mouse model of postsurgical pain. Conclusions & Implications: The dual effect of argentatin C on voltage-gated sodium and calcium channels supports its potential as a novel treatment for painful conditions.
Intrathecal application of contulakin-G (CGX), a conotoxin peptide and a neurotensin analogue, has been demonstrated to be safe and potentially analgesic in humans. However, the mechanism of action for CGX analgesia is unknown. We hypothesized that spinal application of CGX produces antinociception through activation of the presynaptic neurotensin receptor (NTSR)2. In this study, we assessed the mechanisms of CGX antinociception in rodent models of inflammatory and neuropathic pain. Intrathecal administration of CGX, dose dependently, inhibited thermal and mechanical hypersensitivities in rodents of both sexes. Pharmacological and clustered regularly interspaced short palindromic repeats/Cas9 editing of NTSR2 reversed CGX-induced antinociception without affecting morphine analgesia. Electrophysiological and gene editing approaches demonstrated that CGX inhibition was dependent on the R-type voltage-gated calcium channel (Cav2.3) in sensory neurons. Anatomical studies demonstrated coexpression of NTSR2 and Cav2.3 in dorsal root ganglion neurons. Finally, synaptic fractionation and slice electrophysiology recordings confirmed a predominantly presynaptic effect. Together, these data reveal a nonopioid pathway engaged by a human-tested drug to produce antinociception.