Homology-guided mutational analysis reveals the functional requirements for antinociceptive specificity of collapsin response mediator protein 2-derived peptides

Targeted transcriptional upregulation of SENP1 by CRISPR activation enhances deSUMOylation pathways to elicit antinociception in the spinal nerve ligation model of neuropathic pain

ABSTRACT

The voltage-gated sodium channel Na V 1.7 is an essential component of human pain signaling. Changes in Na V 1.7 trafficking are considered critical in the development of neuropathic pain. SUMOylation of collapsin response mediator protein 2 (CRMP2) regulates the membrane trafficking and function of Na V 1.7. Enhanced CRMP2 SUMOylation in neuropathic pain correlates with increased Na V 1.7 activity. Pharmacological and genetic interventions that interfere with CRMP2 SUMOylation in rodents with neuropathic pain have been shown to reverse mechanical allodynia. Sentrin or SUMO-specific proteases (SENPs) are vital for balancing SUMOylation and deSUMOylation of substrates. Overexpression of SENP1 and/or SENP2 in CRMP2-expressing cells results in increased deSUMOylation and decreased membrane expression and currents of Na V 1.7. Although SENP1 is present in the spinal cord and dorsal root ganglia, its role in regulating Na V 1.7 function and pain is not known. We hypothesized that favoring SENP1 expression can enhance CRMP2 deSUMOylation to modulate Na V 1.7 channels. In this study, we used a clustered regularly interspaced short palindromic repeats activation (CRISPRa) SENP1 lentivirus to overexpress SENP1 in dorsal root ganglia neurons. We found that SENP1 lentivirus reduced CRMP2 SUMOylation, Na V 1.7-CRMP2 interaction, and Na V 1.7 membrane expression. SENP1 overexpression decreased Na V 1.7 currents through clathrin-mediated endocytosis, directly linked to CRMP2 deSUMOylation. Moreover, enhancing SENP1 expression did not affect the activity of TRPV1 channels or voltage-gated calcium and potassium channels. Intrathecal injection of CRISPRa SENP1 lentivirus reversed mechanical allodynia in male and female rats with spinal nerve injury. These results provide evidence that the pain-regulating effects of SENP1 overexpression involve, in part, the modulation of Na V 1.7 channels through the indirect mechanism of CRMP2 deSUMOylation.

Intranasal CRMP2-Ubc9 inhibitor regulates Na V 1.7 to alleviate trigeminal neuropathic pain

ABSTRACT

Dysregulation of voltage-gated sodium Na V 1.7 channels in sensory neurons contributes to chronic pain conditions, including trigeminal neuropathic pain. We previously reported that chronic pain results in part from increased SUMOylation of collapsin response mediator protein 2 (CRMP2), leading to an increased CRMP2/Na V 1.7 interaction and increased functional activity of Na V 1.7. Targeting this feed-forward regulation, we developed compound 194 , which inhibits CRMP2 SUMOylation mediated by the SUMO-conjugating enzyme Ubc9. We further demonstrated that 194 effectively reduces the functional activity of Na V 1.7 channels in dorsal root ganglia neurons and alleviated inflammatory and neuropathic pain. Here, we used a comprehensive array of approaches, encompassing biochemical, pharmacological, genetic, electrophysiological, and behavioral analyses, to assess the functional implications of Na V 1.7 regulation by CRMP2 in trigeminal ganglia (TG) neurons. We confirmed the expression of Scn9a , Dpysl2 , and UBE2I within TG neurons. Furthermore, we found an interaction between CRMP2 and Na V 1.7, with CRMP2 being SUMOylated in these sensory ganglia. Disrupting CRMP2 SUMOylation with compound 194 uncoupled the CRMP2/Na V 1.7 interaction, impeded Na V 1.7 diffusion on the plasma membrane, and subsequently diminished Na V 1.7 activity. Compound 194 also led to a reduction in TG neuron excitability. Finally, when intranasally administered to rats with chronic constriction injury of the infraorbital nerve, 194 significantly decreased nociceptive behaviors. Collectively, our findings underscore the critical role of CRMP2 in regulating Na V 1.7 within TG neurons, emphasizing the importance of this indirect modulation in trigeminal neuropathic pain.

A peptidomimetic modulator of the CaV2.2 N-type calcium channel for chronic pain

Transmembrane Cav2.2 (N-type) voltage-gated calcium channels are genetically and pharmacologically validated, clinically relevant pain targets. Clinical block of Cav2.2 (e.g., with Prialt/Ziconotide) or indirect modulation [e.g., with gabapentinoids such as Gabapentin (GBP)] mitigates chronic pain but is encumbered by side effects and abuse liability. The cytosolic auxiliary subunit collapsin response mediator protein 2 (CRMP2) targets Cav2.2 to the sensory neuron membrane and regulates their function via an intrinsically disordered motif. A CRMP2-derived peptide (CBD3) uncouples the Cav2.2-CRMP2 interaction to inhibit calcium influx, transmitter release, and pain. We developed and applied a molecular dynamics approach to identify the A1R2 dipeptide in CBD3 as the anchoring Cav2.2 motif and designed pharmacophore models to screen 27 million compounds on the open-access server ZincPharmer. Of 200 curated hits, 77 compounds were assessed using depolarization-evoked calcium influx in rat dorsal root ganglion neurons. Nine small molecules were tested electrophysiologically, while one (CBD3063) was also evaluated biochemically and behaviorally. CBD3063 uncoupled Cav2.2 from CRMP2, reduced membrane Cav2.2 expression and Ca2+ currents, decreased neurotransmission, reduced fiber photometry-based calcium responses in response to mechanical stimulation, and reversed neuropathic and inflammatory pain across sexes in two different species without changes in sensory, sedative, depressive, and cognitive behaviors. CBD3063 is a selective, first-in-class, CRMP2-based peptidomimetic small molecule, which allosterically regulates Cav2.2 to achieve analgesia and pain relief without negative side effect profiles. In summary, CBD3063 could potentially be a more effective alternative to GBP for pain relief.

Intranasal CRMP2-Ubc9 Inhibitor Regulates Na V 1.7 to Alleviate Trigeminal Neuropathic Pain

Dysregulation of voltage-gated sodium Na V 1.7 channels in sensory neurons contributes to chronic pain conditions, including trigeminal neuropathic pain. We previously reported that chronic pain results in part from increased SUMOylation of collapsin response mediator protein 2 (CRMP2), leading to an increased CRMP2/Na V 1.7 interaction and increased functional activity of Na V 1.7. Targeting this feed-forward regulation, we developed compound 194 , which inhibits CRMP2 SUMOylation mediated by the SUMO-conjugating enzyme Ubc9. We further demonstrated that 194 effectively reduces the functional activity of Na V 1.7 channels in dorsal root ganglia neurons and alleviated inflammatory and neuropathic pain. Here, we employed a comprehensive array of investigative approaches, encompassing biochemical, pharmacological, genetic, electrophysiological, and behavioral analyses, to assess the functional implications of Na V 1.7 regulation by CRMP2 in trigeminal ganglia (TG) neurons. We confirmed the expression of Scn9a , Dpysl2 , and UBE2I within TG neurons. Furthermore, we found an interaction between CRMP2 and Na V 1.7, with CRMP2 being SUMOylated in these sensory ganglia. Disrupting CRMP2 SUMOylation with compound 194 uncoupled the CRMP2/Na V 1.7 interaction, impeded Na V 1.7 diffusion on the plasma membrane, and subsequently diminished Na V 1.7 activity. Compound 194 also led to a reduction in TG neuron excitability. Finally, when intranasally administered to rats with chronic constriction injury of the infraorbital nerve (CCI-ION), 194 significantly decreased nociceptive behaviors. Collectively, our findings underscore the critical role of CRMP2 in regulating Na V 1.7 within TG neurons, emphasizing the importance of this indirect modulation in trigeminal neuropathic pain.

The natural product argentatin C attenuates postoperative pain via inhibition of voltage-gated sodium and T-type voltage-gated calcium channels

BACKGROUND AND PURPOSE

Postoperative pain occurs in as many as 70% of surgeries performed worldwide. Postoperative pain management still relies on opioids despite their negative consequences, resulting in a public health crisis. Therefore, it is important to develop alternative therapies to treat chronic pain. Natural products derived from medicinal plants are potential sources of novel biologically active compounds for development of safe analgesics. 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 that have important roles in nociceptive sensory processing.

EXPERIMENTAL APPROACH

Fractions derived from the Native American medicinal plant, Parthenium incanum, were assessed using 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 was additionally evaluated using whole-cell voltage and current-clamp electrophysiology, and behavioural analysis in a mouse model of postsurgical pain.

KEY RESULTS

Argentatin C blocked the activity of both voltage-gated sodium and low-voltage-activated (LVA) calcium channels in calcium imaging assays. Docking analysis predicted that argentatin C may bind to Na_V 1.7-1.9 and Ca_V 3.1-3.3 channels. Furthermore, argentatin C decreased Na⁺ and T-type Ca²⁺ currents as well as excitability in rat and macaque DRG neurons, and reversed mechanical allodynia in a mouse model of postsurgical pain.

CONCLUSION AND IMPLICATIONS

These results suggest that the dual effect of argentatin C on voltage-gated sodium and calcium channels supports its potential as a novel treatment for painful conditions.

Cell specific regulation of NaV1.7 activity and trafficking in rat nodose ganglia neurons

The voltage-gated sodium NaV1.7 channel sets the threshold for electrogenesis. Mutations in the gene encoding human NaV1.7 (SCN9A) cause painful neuropathies or pain insensitivity. In 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 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; however, they were resistant to downregulation via either CRMP2 knockdown or pharmacological inhibition of CRMP2 SUMOylation by 194. CRMP2 SUMOylation and interaction with NaV1.7 was consered in NG neurons but the endocytic machinery was deficient in the endocytic adaptor protein Numb. Overexpression of Numb rescued CRMP2-dependent regulation on NaV1.7, rendering NG neurons sensitive to 194. Altogether, these data point at the existence of cell-specific mechanisms regulating NaV1.7 trafficking.

Selective targeting of NaV1.7 via inhibition of the CRMP2-Ubc9 interaction reduces pain in rodents

The voltage-gated sodium NaV1.7 channel, critical for sensing pain, has been actively targeted by drug developers; however, there are currently no effective and safe therapies targeting NaV1.7. Here, we tested whether a different approach, indirect NaV1.7 regulation, could have antinociceptive effects in preclinical models. We found that preventing addition of small ubiquitin-like modifier (SUMO) on the NaV1.7-interacting cytosolic collapsin response mediator protein 2 (CRMP2) blocked NaV1.7 functions and had antinociceptive effects in rodents. In silico targeting of the SUMOylation site in CRMP2 (Lys374) identified >200 hits, of which compound 194 exhibited selective in vitro and ex vivo NaV1.7 engagement. Orally administered 194 was not only antinociceptive in preclinical models of acute and chronic pain but also demonstrated synergy alongside other analgesics—without eliciting addiction, rewarding properties, or neurotoxicity. Analgesia conferred by 194 was opioid receptor dependent. Our results demonstrate that 194 is a first-in-class protein-protein inhibitor that capitalizes on CRMP2-NaV1.7 regulation to deliver safe analgesia in rodents.

The role of cyclin-dependent kinase 5 in neuropathic pain

The chronification of pain can be attributed to changes in membrane receptors and channels underlying neuronal plasticity and signal transduction largely within nociceptive neurons that initiate and maintain pathological pain states. These proteins are subject to dynamic modification by posttranslational modifications, creating a code that controls protein function in time and space. Phosphorylation is an important posttranslational modification that affects ∼30% of proteins in vivo. Increased phosphorylation of various nociceptive ion channels and of their modulators underlies sensitization of different pain states. Cyclin-dependent kinases are proline-directed serine/threonine kinases that impact various biological and cellular systems. Cyclin-dependent kinase 5 (Cdk5), one member of this kinase family, and its activators p35 and p39 are expressed in spinal nerves, dorsal root ganglia, and the dorsal horn of the spinal cord. In neuropathic pain conditions, expression and/or activity of Cdk5 is increased, implicating Cdk5 in nociception. Experimental evidence suggests that Cdk5 is regulated through its own phosphorylation, through increasing p35's interaction with Cdk5, and through cleavage of p35 into p25. This narrative review discusses the molecular mechanisms of Cdk5-mediated regulation of target proteins involved in neuropathic pain. We focus on Cdk5 substrates that have been linked to nociceptive pathways, including channels (eg, transient receptor potential cation channel and voltage-gated calcium channel), proteins involved in neurotransmitter release (eg, synaptophysin and collapsin response mediator protein 2), and receptors (eg, glutamate, purinergic, and opioid). By altering the phosphoregulatory "set point" of proteins involved in pain signaling, Cdk5 thus appears to be an attractive target for treating neuropathic pain conditions.

A modulator of the low-voltage-activated T-type calcium channel that reverses HIV glycoprotein 120-, paclitaxel-, and spinal nerve ligation-induced peripheral neuropathies

The voltage-gated calcium channels CaV3.1-3.3 constitute the T-type subfamily, whose dysfunctions are associated with epilepsy, psychiatric disorders, and chronic pain. The unique properties of low-voltage-activation, faster inactivation, and slower deactivation of these channels support their role in modulation of cellular excitability and low-threshold firing. Thus, selective T-type calcium channel antagonists are highly sought after. Here, we explored Ugi-azide multicomponent reaction products to identify compounds targeting T-type calcium channel. Of the 46 compounds tested, an analog of benzimidazolonepiperidine-5bk (1-{1-[(R)-{1-[(1S)-1-phenylethyl]-1H-1,2,3,4-tetrazol-5-yl}(thiophen-3-yl)methyl]piperidin-4-yl}-2,3-dihydro-1H-1,3-benzodiazol-2-one) modulated depolarization-induced calcium influx in rat sensory neurons. Modulation of T-type calcium channels by 5bk was further confirmed in whole-cell patch clamp assays in dorsal root ganglion (DRG) neurons, where pharmacological isolation of T-type currents led to a time- and concentration-dependent regulation with a low micromolar IC50. Lack of an acute effect of 5bk argues against a direct action on T-type channels. Genetic knockdown revealed CaV3.2 to be the isoform preferentially modulated by 5bk. High voltage-gated calcium, as well as tetrodotoxin-sensitive and -resistant sodium, channels were unaffected by 5bk. 5bk inhibited spontaneous excitatory postsynaptic currents and depolarization-evoked release of calcitonin gene-related peptide from lumbar spinal cord slices. Notably, 5bk did not bind human mu, delta, or kappa opioid receptors. 5bk reversed mechanical allodynia in rat models of HIV-associated neuropathy, chemotherapy-induced peripheral neuropathy, and spinal nerve ligation-induced neuropathy, without effects on locomotion or anxiety. Thus, 5bk represents a novel T-type modulator that could be used to develop nonaddictive pain therapeutics.

Studies on CRMP2 SUMOylation-deficient transgenic mice identify sex-specific Nav1.7 regulation in the pathogenesis of chronic neuropathic pain

The sodium channel Nav1.7 is a master regulator of nociceptive input into the central nervous system. Mutations in this channel can result in painful conditions and produce insensitivity to pain. Despite being recognized as a "poster child" for nociceptive signaling and human pain, targeting Nav1.7 has not yet produced a clinical drug. Recent work has illuminated the Nav1.7 interactome, offering insights into the regulation of these channels and identifying potentially new druggable targets. Among the regulators of Nav1.7 is the cytosolic collapsin response mediator protein 2 (CRMP2). CRMP2, modified at lysine 374 (K374) by addition of a small ubiquitin-like modifier (SUMO), bound Nav1.7 to regulate its membrane localization and function. Corollary to this, preventing CRMP2 SUMOylation was sufficient to reverse mechanical allodynia in rats with neuropathic pain. Notably, loss of CRMP2 SUMOylation did not compromise other innate functions of CRMP2. To further elucidate the in vivo role of CRMP2 SUMOylation in pain, we generated CRMP2 K374A knock-in (CRMP2) mice in which Lys374 was replaced with Ala. CRMP2 mice had reduced Nav1.7 membrane localization and function in female, but not male, sensory neurons. Behavioral appraisal of CRMP2 mice demonstrated no changes in depressive or repetitive, compulsive-like behaviors and a decrease in noxious thermal sensitivity. No changes were observed in CRMP2 mice to inflammatory, acute, or visceral pain. By contrast, in a neuropathic model, CRMP2 mice failed to develop persistent mechanical allodynia. Our study suggests that CRMP2 SUMOylation-dependent control of peripheral Nav1.7 is a hallmark of chronic, but not physiological, neuropathic pain.

Non-SUMOylated CRMP2 decreases NaV1.7 currents via the endocytic proteins Numb, Nedd4-2 and Eps15

Voltage-gated sodium channels are key players in neuronal excitability and pain signaling. Functional expression of the voltage-gated sodium channel Na_V1.7 is under the control of SUMOylated collapsin response mediator protein 2 (CRMP2). When not SUMOylated, CRMP2 forms a complex with the endocytic proteins Numb, the epidermal growth factor receptor pathway substrate 15 (Eps15), and the E3 ubiquitin ligase Nedd4-2 to promote clathrin-mediated endocytosis of Na_V1.7. We recently reported that CRMP2 SUMO-null knock-in (CRMP2^K374A/K374A) female mice have reduced Na_V1.7 membrane localization and currents in their sensory neurons. Preventing CRMP2 SUMOylation was sufficient to reverse mechanical allodynia in CRMP2^K374A/K374A female mice with neuropathic pain. Here we report that inhibiting clathrin assembly in nerve-injured male CRMP2^K374A/K374A mice precipitated mechanical allodynia in mice otherwise resistant to developing persistent pain. Furthermore, Numb, Nedd4-2 and Eps15 expression was not modified in basal conditions in the dorsal root ganglia (DRG) of male and female CRMP2^K374A/K374A mice. Finally, silencing these proteins in DRG neurons from female CRMP2^K374A/K374A mice, restored the loss of sodium currents. Our study shows that the endocytic complex composed of Numb, Nedd4-2 and Eps15, is necessary for non-SUMOylated CRMP2-mediated internalization of sodium channels in vivo.

SARS-CoV-2 spike protein co-opts VEGF-A/neuropilin-1 receptor signaling to induce analgesia

Global spread of severe acute respiratory syndrome coronavirus 2 continues unabated. Binding of severe acute respiratory syndrome coronavirus 2's spike protein to host angiotensin-converting enzyme 2 triggers viral entry, but other proteins may participate, including the neuropilin-1 receptor (NRP-1). Because both spike protein and vascular endothelial growth factor-A (VEGF-A)-a pronociceptive and angiogenic factor, bind NRP-1, we tested whether spike could block VEGF-A/NRP-1 signaling. VEGF-A-triggered sensory neuron firing was blocked by spike protein and NRP-1 inhibitor EG00229. Pronociceptive behaviors of VEGF-A were similarly blocked through suppression of spontaneous spinal synaptic activity and reduction of electrogenic currents in sensory neurons. Remarkably, preventing VEGF-A/NRP-1 signaling was antiallodynic in a neuropathic pain model. A "silencing" of pain through subversion of VEGF-A/NRP-1 signaling may underlie increased disease transmission in asymptomatic individuals.

SARS-CoV-2 Spike protein co-opts VEGF-A/Neuropilin-1 receptor signaling to induce analgesia

Global spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues unabated. Binding of SARS-CoV-2's Spike protein to host angiotensin converting enzyme 2 triggers viral entry, but other proteins may participate, including neuropilin-1 receptor (NRP-1). As both Spike protein and vascular endothelial growth factor-A (VEGF-A) - a pro-nociceptive and angiogenic factor, bind NRP-1, we tested if Spike could block VEGF-A/NRP-1 signaling. VEGF-A-triggered sensory neuronal firing was blocked by Spike protein and NRP-1 inhibitor EG00229. Pro-nociceptive behaviors of VEGF-A were similarly blocked via suppression of spontaneous spinal synaptic activity and reduction of electrogenic currents in sensory neurons. Remarkably, preventing VEGF-A/NRP-1 signaling was antiallodynic in a neuropathic pain model. A 'silencing' of pain via subversion of VEGF-A/NRP-1 signaling may underlie increased disease transmission in asymptomatic individuals.

Assessment of nociception and related quality-of-life measures in a porcine model of neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disorder resulting from germline mutations in the NF1 gene, which encodes neurofibromin. Patients experience a variety of symptoms, but pain in the context of NF1 remains largely underrecognized. Here, we characterize nociceptive signaling and pain behaviors in a miniswine harboring a disruptive NF1 mutation (exon 42 deletion). We present the first characterization of pain-related behaviors in a pig model of NF1, identifying unchanged agitation scores, lower tactile thresholds (allodynia), and decreased response latencies to thermal laser stimulation (hyperalgesia) in NF1 (females only) pigs. Male NF1 pigs with tumors showed reduced sleep quality and increased resting, 2 health-related quality-of-life symptoms found to be comorbid in people with NF1 pain. We explore these phenotypes in relationship to suppression of the increased activity of the N-type voltage-gated calcium (CaV2.2) channel by pharmacological antagonism of phosphorylation of a regulatory protein-the collapsin response mediator protein 2 (CRMP2), a known interactor of neurofibromin, and by targeting the interface between the α subunit of CaV2.2 and the accessory β-subunits with small molecules. Our data support the use of NF1 pigs as a large animal model for studying NF1-associated pain and for understanding the pathophysiology of NF1. Our findings demonstrate the translational potential of 2 small molecules in reversing ion channel remodeling seen in NF1. Interfering with CaV2.2, a clinically validated target for pain management, might also be a promising therapeutic strategy for NF1-related pain management.

Druggability of CRMP2 for Neurodegenerative Diseases

Collapsin response mediator proteins (CRMPs) are ubiquitously expressed phosphoproteins that coordinate cytoskeletal formation and regulate cellular division, migration, polarity, and synaptic connection. CRMP2, the most studied of the five family members, is best known for its affinity for tubulin heterodimers and function in regulating the microtubule network. Accumulating evidence has also demonstrated a key role for CRMP2 in trafficking of voltage- and ligand-gated ion channels. These functions are tightly regulated by post-translational modifications including phosphorylation and SUMOylation (addition of a small ubiquitin like modifier). Over the past decade, it has become increasingly clear that dysregulated post-translational modifications of CRMP2 contribute to the pathomechanisms of diverse diseases, including cancer, neurodegenerative diseases, chronic pain, and bipolar disorder. Here, we review the discovery, functions, and current putative preclinical and clinical therapeutics targeting CRMP2. These potential therapeutics include CRMP2-based peptides that inhibit protein-protein interactions and small-molecule compounds. Capitalizing on the availability of structural information, we identify druggable pockets on CRMP2 and predict binding modes for five known CRMP2-targeting compounds, setting the stage for optimization and de novo drug discovery targeting this multifunctional protein.

Evaluation of edonerpic maleate as a CRMP2 inhibitor for pain relief

We have previously reported that the microtubule-associated collapsin response mediator protein 2 (CRMP2) is necessary for the expression of chronic pain. CRMP2 achieves this control of nociceptive signaling by virtue of its ability to regulate voltage-gated calcium and sodium channels. To date, however, no drugs exist that target CRMP2. Recently, the small molecule edonerpic maleate (1 -{3-[2-(1-benzothiophen-5-yl)ethoxy]propyl}azetidin-3-ol maleate), a candidate therapeutic for Alzheimer’s disease was reported to be a novel CRMP2 binding compound with the potential to decrease its phosphorylation level in cortical tissues in vivo. Here we sought to determine the mechanism of action of edonerpic maleate and test its possible effect in a rodent model of chronic pain. We observed: (i) no binding between human CRMP2 and edonerpic maleate; (ii) edonerpic maleate had no effect on CRMP2 expression and phosphorylation in dorsal root ganglion (DRG) neurons; (iii) edonerpic maleate-decreased calcium but increased sodium current density in DRG neurons; and (iv) edonerpic maleate was ineffective in reversing post-surgical allodynia in male and female mice. Thus, while CRMP2 inhibiting compounds remain a viable strategy for developing new mechanism-based pain inhibitors, edonerpic maleate is an unlikely candidate.

Targeting the CaVα-CaVβ interaction yields an antagonist of the N-type CaV2.2 channel with broad antinociceptive efficacy

Inhibition of voltage-gated calcium (CaV) channels is a potential therapy for many neurological diseases including chronic pain. Neuronal CaV1/CaV2 channels are composed of α, β, γ and α2δ subunits. The β subunits of CaV channels are cytoplasmic proteins that increase the surface expression of the pore-forming α subunit of CaV. We targeted the high-affinity protein-protein interface of CaVβ's pocket within the CaVα subunit. Structure-based virtual screening of 50,000 small molecule library docked to the β subunit led to the identification of 2-(3,5-dimethylisoxazol-4-yl)-N-((4-((3-phenylpropyl)amino)quinazolin-2-yl)methyl)acetamide (IPPQ). This small molecule bound to CaVβ and inhibited its coupling with N-type voltage-gated calcium (CaV2.2) channels, leading to a reduction in CaV2.2 currents in rat dorsal root ganglion sensory neurons, decreased presynaptic localization of CaV2.2 in vivo, decreased frequency of spontaneous excitatory postsynaptic potentials and miniature excitatory postsynaptic potentials, and inhibited release of the nociceptive neurotransmitter calcitonin gene-related peptide from spinal cord. IPPQ did not target opioid receptors nor did it engage inhibitory G protein-coupled receptor signaling. IPPQ was antinociceptive in naive animals and reversed allodynia and hyperalgesia in models of acute (postsurgical) and neuropathic (spinal nerve ligation, chemotherapy- and gp120-induced peripheral neuropathy, and genome-edited neuropathy) pain. IPPQ did not cause akinesia or motor impairment, a common adverse effect of CaV2.2 targeting drugs, when injected into the brain. IPPQ, a quinazoline analog, represents a novel class of CaV2.2-targeting compounds that may serve as probes to interrogate CaVα-CaVβ function and ultimately be developed as a nonopioid therapeutic for chronic pain.

1-O-Acetylgeopyxin A, a derivative of a fungal metabolite, blocks tetrodotoxin-sensitive voltage-gated sodium, calcium channels and neuronal excitability which correlates with inhibition of neuropathic pain

Chronic pain can be the result of an underlying disease or condition, medical treatment, inflammation, or injury. The number of persons experiencing this type of pain is substantial, affecting upwards of 50 million adults in the United States. Pharmacotherapy of most of the severe chronic pain patients includes drugs such as gabapentinoids, re-uptake blockers and opioids. Unfortunately, gabapentinoids are not effective in up to two-thirds of this population and although opioids can be initially effective, their long-term use is associated with multiple side effects. Therefore, there is a great need to develop novel non-opioid alternative therapies to relieve chronic pain. For this purpose, we screened a small library of natural products and their derivatives in the search for pharmacological inhibitors of voltage-gated calcium and sodium channels, which are outstanding molecular targets due to their important roles in nociceptive pathways. We discovered that the acetylated derivative of the ent-kaurane diterpenoid, geopyxin A, 1-O-acetylgeopyxin A, blocks voltage-gated calcium and tetrodotoxin-sensitive voltage-gated sodium channels but not tetrodotoxin-resistant sodium channels in dorsal root ganglion (DRG) neurons. Consistent with inhibition of voltage-gated sodium and calcium channels, 1-O-acetylgeopyxin A reduced reduce action potential firing frequency and increased firing threshold (rheobase) in DRG neurons. Finally, we identified the potential of 1-O-acetylgeopyxin A to reverse mechanical allodynia in a preclinical rat model of HIV-induced sensory neuropathy. Dual targeting of both sodium and calcium channels may permit block of nociceptor excitability and of release of pro-nociceptive transmitters. Future studies will harness the core structure of geopyxins for the generation of antinociceptive drugs.

Activity of T-type calcium channels is independent of CRMP2 in sensory neurons

Amongst the regulators of voltage-gated ion channels is the collapsin response mediator protein 2 (CRMP2). CRMP2 regulation of the activity and trafficking of NaV1.7 voltage-gated sodium channels as well as the N-type (CaV2.2) voltage-gated calcium channel (VGCC) has been reported. On the other hand, CRMP2 does not appear to regulate L- (CaV1.x), P/Q- (CaV2.1), and R- (CaV2.3) type high VGCCs. Whether CRMP2 regulates low VGCCs remains an open question. Here, we asked if CRMP2 could regulate the low voltage-gated (T-type/CaV3.x) channels in sensory neurons. Reducing CRMP2 protein levels with short interfering RNAs yielded no change in macroscopic currents carried by T-type channels. No change in biophysical properties of the T-type currents was noted. Future studies pursuing CRMP2 druggability in neuropathic pain will benefit from the findings that CRMP2 regulates only the N-type (CaV2.2) calcium channels.

Nasal delivery of a CRMP2-derived CBD3 adenovirus improves cognitive function and pathology in APP/PS1 transgenic mice

Calcium dysregulation is a key pathological event in Alzheimer's disease (AD). In studying approaches to mitigate this calcium overload, we identified the collapsin response mediator protein 2 (CRMP2), an axonal guidance protein that participates in synapse dynamics by interacting with and regulating activity of N-methyl-D-aspartate receptors (NMDARs). We further identified a 15 amino acid peptide from CRMP2 (designated CBD3, for calcium-binding domain 3), that reduced NMDAR-mediated Ca2+ influx in cultured neurons and post-synaptic NMDAR-mediated currents in cortical slices. Whether targeting CRMP2 could be therapeutically beneficial in AD is unknown. Here, using CBD3, we tested the utility of this approach. Employing the APP/PS1 mouse model of AD which demonstrates robust pathophysiology including Aβ1-42 deposition, altered tau levels, and diminished cognitive functions, we asked if overexpression of CBD3 could rescue these events. CBD3 was engineered into an adeno-associated vector and nasally delivered into APP/PS1 mice and then biochemical (immunohistochemistry, immunoblotting), cellular (TUNEL apoptosis assays), and behavioral (Morris water maze test) assessments were performed. APP/PS1 mice administered adeno-associated virus (AAV, serotype 2) harboring CBD3 demonstrated: (i) reduced levels of Aβ1-42 and phosphorylated-tau (a marker of AD progression), (ii) reduced apoptosis in the hippocampus, and (iii) reduced cognitive decline compared with APP/PS1 mice or APP/PS1 administered a control virus. These results provide an instructive example of utilizing a peptide-based approach to unravel protein-protein interactions that are necessary for AD pathology and demonstrate the therapeutic potential of CRMP2 as a novel protein player in AD.

Cationic Arginine-Rich Peptides (CARPs): A Novel Class of Neuroprotective Agents With a Multimodal Mechanism of Action

There are virtually no clinically available neuroprotective drugs for the treatment of acute and chronic neurological disorders, hence there is an urgent need for the development of new neuroprotective molecules. Cationic arginine-rich peptides (CARPs) are an expanding and relatively novel class of compounds, which possess intrinsic neuroprotective properties. Intriguingly, CARPs possess a combination of biological properties unprecedented for a neuroprotective agent including the ability to traverse cell membranes and enter the CNS, antagonize calcium influx, target mitochondria, stabilize proteins, inhibit proteolytic enzymes, induce pro-survival signaling, scavenge toxic molecules, and reduce oxidative stress as well as, having a range of anti-inflammatory, analgesic, anti-microbial, and anti-cancer actions. CARPs have also been used as carrier molecules for the delivery of other putative neuroprotective agents across the blood-brain barrier and blood-spinal cord barrier. However, there is increasing evidence that the neuroprotective efficacy of many, if not all these other agents delivered using a cationic arginine-rich cell-penetrating peptide (CCPPs) carrier (e.g., TAT) may actually be mediated largely by the properties of the carrier molecule, with overall efficacy further enhanced according to the amino acid composition of the cargo peptide, in particular its arginine content. Therefore, in reviewing the neuroprotective mechanisms of action of CARPs we also consider studies using CCPPs fused to a putative neuroprotective peptide. We review the history of CARPs in neuroprotection and discuss in detail the intrinsic biological properties that may contribute to their cytoprotective effects and their usefulness as a broad-acting class of neuroprotective drugs.

Dynamic CRMP2 Regulation of CaV2.2 in the Prefrontal Cortex Contributes to the Reinstatement of Cocaine Seeking

Cocaine addiction remains a major health concern with limited effective treatment options. A better understanding of mechanisms underlying relapse may help inform the development of new pharmacotherapies. Emerging evidence suggests that collapsin response mediator protein 2 (CRMP2) regulates presynaptic excitatory neurotransmission and contributes to pathological changes during diseases, such as neuropathic pain and substance use disorders. We examined the role of CRMP2 and its interactions with a known binding partner, CaV2.2, in cocaine-seeking behavior. We employed the rodent self-administration model of relapse to drug seeking and focused on the prefrontal cortex (PFC) for its well-established role in reinstatement behaviors. Our results indicated that repeated cocaine self-administration resulted in a dynamic and persistent alteration in the PFC expression of CRMP2 and its binding partner, the CaV2.2 (N-type) voltage-gated calcium channel. Following cocaine self-administration and extinction training, the expression of both CRMP2 and CaV2.2 was reduced relative to yoked saline controls. By contrast, cued reinstatement potentiated CRMP2 expression and increased CaV2.2 expression above extinction levels. Lastly, we utilized the recently developed peptide myr-TAT-CBD3 to disrupt the interaction between CRMP2 and CaV2.2 in vivo. We assessed the reinstatement behavior after infusing this peptide directly into the medial PFC and found that it decreased cue-induced reinstatement of cocaine seeking. Taken together, these data suggest that neuroadaptations in the CRMP2/CaV2.2 signaling cascade in the PFC can facilitate drug-seeking behavior. Targeting such interactions has implications for the treatment of cocaine relapse behavior.

The Natural Flavonoid Naringenin Elicits Analgesia through Inhibition of NaV1.8 Voltage-Gated Sodium Channels

Naringenin (2S)-5,7-dihydroxy-2-(4-hydroxyphenyl)-3,4-dihydro-2H-1-benzopyran-4-one is a natural flavonoid found in fruits from the citrus family. Because (2S)-naringenin is known to racemize, its bioactivity might be related to one or both enantiomers. Computational studies predicted that (2R)-naringenin may act on voltage-gated ion channels, particularly the N-type calcium channel (CaV2.2) and the NaV1.7 sodium channel-both of which are key for pain signaling. Here we set out to identify the possible mechanism of action of naringenin. Naringenin inhibited depolarization-evoked Ca²⁺ influx in acetylcholine-, ATP-, and capsaicin-responding rat dorsal root ganglion (DRG) neurons. This was corroborated in electrophysiological recordings from DRG neurons. Pharmacological dissection of each of the voltage-gated Ca²⁺ channels subtypes could not pinpoint any selectivity of naringenin. Instead, naringenin inhibited NaV1.8-dependent and tetrodotoxin (TTX)-resistant while sparing tetrodotoxin sensitive (TTX-S) voltage-gated Na⁺ channels as evidenced by the lack of further inhibition by the NaV1.8 blocker A-803467. The effects of the natural flavonoid were validated ex vivo in spinal cord slices where naringenin decreased both the frequency and amplitude of sEPSC recorded in neurons within the substantia gelatinosa. The antinociceptive potential of naringenin was evaluated in male and female mice. Naringenin had no effect on the nociceptive thresholds evoked by heat. Naringenin's reversed allodynia was in mouse models of postsurgical and neuropathic pain. Here, driven by a call by the National Center for Complementary and Integrative Health's strategic plan to advance fundamental research into basic biological mechanisms of the action of natural products, we advance the antinociceptive potential of the flavonoid naringenin.

Betulinic acid, derived from the desert lavender Hyptis emoryi, attenuates paclitaxel-, HIV-, and nerve injury-associated peripheral sensory neuropathy via block of N- and T-type calcium channels

The Federal Pain Research Strategy recommended development of nonopioid analgesics as a top priority in its strategic plan to address the significant public health crisis and individual burden of chronic pain faced by >100 million Americans. Motivated by this challenge, a natural product extracts library was screened and identified a plant extract that targets activity of voltage-gated calcium channels. This profile is of interest as a potential treatment for neuropathic pain. The active extract derived from the desert lavender plant native to southwestern United States, when subjected to bioassay-guided fractionation, afforded 3 compounds identified as pentacyclic triterpenoids, betulinic acid (BA), oleanolic acid, and ursolic acid. Betulinic acid inhibited depolarization-evoked calcium influx in dorsal root ganglion (DRG) neurons predominantly through targeting low-voltage-gated (Cav3 or T-type) and CaV2.2 (N-type) calcium channels. Voltage-clamp electrophysiology experiments revealed a reduction of Ca, but not Na, currents in sensory neurons after BA exposure. Betulinic acid inhibited spontaneous excitatory postsynaptic currents and depolarization-evoked release of calcitonin gene-related peptide from lumbar spinal cord slices. Notably, BA did not engage human mu, delta, or kappa opioid receptors. Intrathecal administration of BA reversed mechanical allodynia in rat models of chemotherapy-induced peripheral neuropathy and HIV-associated peripheral sensory neuropathy as well as a mouse model of partial sciatic nerve ligation without effects on locomotion. The broad-spectrum biological and medicinal properties reported, including anti-HIV and anticancer activities of BA and its derivatives, position this plant-derived small molecule natural product as a potential nonopioid therapy for management of chronic pain.

Reversal of Peripheral Neuropathic Pain by the Small-Molecule Natural Product Physalin F via Block of CaV2.3 (R-Type) and CaV2.2 (N-Type) Voltage-Gated Calcium Channels

No universally efficacious therapy exists for chronic pain, a disease affecting one-fifth of the global population. An overreliance on the prescription of opioids for chronic pain despite their poor ability to improve function has led to a national opioid crisis. In 2018, the NIH launched a Helping to End Addiction Long-term plan to spur discovery and validation of novel targets and mechanisms to develop alternative nonaddictive treatment options. Phytochemicals with medicinal properties have long been used for various treatments worldwide. The natural product physalin F, isolated from the Physalis acutifolia (family: Solanaceae) herb, demonstrated antinociceptive effects in models of inflammatory pain, consistent with earlier reports of its anti-inflammatory and immunomodulatory activities. However, the target of action of physalin F remained unknown. Here, using whole-cell and slice electrophysiology, competition binding assays, and experimental models of neuropathic pain, we uncovered a molecular target for physalin F's antinociceptive actions. We found that physalin F (i) blocks CaV2.3 (R-type) and CaV2.2 (N-type) voltage-gated calcium channels in dorsal root ganglion (DRG) neurons, (ii) does not affect CaV3 (T-type) voltage-gated calcium channels or voltage-gated sodium or potassium channels, (iii) does not bind G-protein coupled opioid receptors, (iv) inhibits the frequency of spontaneous excitatory postsynaptic currents (EPSCs) in spinal cord slices, and (v) reverses tactile hypersensitivity in models of paclitaxel-induced peripheral neuropathy and spinal nerve ligation. Identifying CaV2.2 as a molecular target of physalin F may spur its use as a tool for mechanistic studies and position it as a structural template for future synthetic compounds.

(-)-Hardwickiic Acid and Hautriwaic Acid Induce Antinociception via Blockade of Tetrodotoxin-Sensitive Voltage-Dependent Sodium Channels

For an affliction that debilitates an estimated 50 million adults in the United States, the current chronic pain management approaches are inadequate. The Centers for Disease Control and Prevention have called for a minimization in opioid prescription and use for chronic pain conditions, and thus, it is imperative to discover alternative non-opioid based strategies. For the realization of this call, a library of natural products was screened in search of pharmacological inhibitors of both voltage-gated calcium channels and voltage-gated sodium channels, which are excellent targets due to their well-established roles in nociceptive pathways. We discovered (-)-hardwickiic acid ((-)-HDA) and hautriwaic acid (HTA) isolated from plants, Croton californicus and Eremocarpus setigerus, respectively, inhibited tetrodotoxin-sensitive sodium, but not calcium or potassium, channels in small diameter, presumptively nociceptive, dorsal root ganglion (DRG) neurons. Failure to inhibit spontaneous postsynaptic excitatory currents indicated a preferential targeting of voltage-gated sodium channels over voltage-gated calcium channels by these extracts. Neither compound was a ligand at opioid receptors. Finally, we identified the potential of both (-)-HDA and HTA to reverse chronic pain behavior in preclinical rat models of HIV-sensory neuropathy, and for (-)-HDA specifically, in chemotherapy-induced peripheral neuropathy. Our results illustrate the therapeutic potential for (-)-HDA and HTA for chronic pain management and could represent a scaffold, that, if optimized by structure-activity relationship studies, may yield novel specific sodium channel antagonists for pain relief.

Neuronal Conditional Knockout of Collapsin Response Mediator Protein 2 Ameliorates Disease Severity in a Mouse Model of Multiple Sclerosis

We previously showed that treatment with lanthionine ketimine ethyl ester (LKE) reduced disease severity and axonal damage in an experimental autoimmune encephalomyelitis (EAE) mouse model of multiple sclerosis and increased neuronal maturation and survival in vitro . A major target of LKE is collapsin response mediator protein 2 (CRMP2), suggesting this protein may mediate LKE actions. We now show that conditional knockout of CRMP2 from neurons using a CamK2a promoter to drive Cre recombinase expression reduces disease severity in the myelin oligodendrocyte glycoprotein (MOG)35–55 EAE model, associated with decreased spinal cord axonal damage, and less glial activation in the cerebellum, but not the spinal cord. Immunohistochemical staining and quantitative polymerase chain reaction show CRMP2 depletion from descending motor neurons in the motor cortex, but not from spinal cord neurons, suggesting that the benefits of CRMP2 depletion on EAE may stem from effects on upper motor neurons. In addition, mice in which CRMP2 S522 phosphorylation was prevented by substitution for an alanine residue also showed reduced EAE severity. These results show that modification of CRMP2 expression and phosphorylation can influence the course of EAE and suggests that treatment with CRMP2 modulators such as LKE act in part by reducing CRMP2 S522 phosphorylation.

Phosphorylated CRMP2 Regulates Spinal Nociceptive Neurotransmission

The collapsin response mediator protein 2 (CRMP2) has emerged as a central node in assembling nociceptive signaling complexes involving voltage-gated ion channels. Concerted actions of post-translational modifications, phosphorylation and SUMOylation, of CRMP2 contribute to regulation of pathological pain states. In the present study, we demonstrate a novel role for CRMP2 in spinal nociceptive transmission. We found that, of six possible post-translational modifications, three phosphorylation sites on CRMP2 were critical for regulating calcium influx in dorsal root ganglion sensory neurons. Of these, only CRMP2 phosphorylated at serine 522 by cyclin-dependent kinase 5 (Cdk5) contributed to spinal neurotransmission in a bidirectional manner. Accordingly, expression of a non-phosphorylatable CRMP2 (S522A) decreased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs), whereas expression of a constitutively phosphorylated CRMP2 (S522D) increased the frequency of sEPSCs. The presynaptic nature of CRMP2's actions was further confirmed by pharmacological antagonism of Cdk5-mediated CRMP2 phosphorylation with S-N-benzy-2-acetamido-3-methoxypropionamide ((S)-lacosamide; (S)-LCM) which (i) decreased sEPSC frequency, (ii) increased paired-pulse ratio, and (iii) reduced the presynaptic distribution of CaV2.2 and NaV1.7, two voltage-gated ion channels implicated in nociceptive signaling. (S)-LCM also inhibited depolarization-evoked release of the pro-nociceptive neurotransmitter calcitonin gene-related peptide (CGRP) in the spinal cord. Increased CRMP2 phosphorylation in rats with spared nerve injury (SNI) was decreased by intrathecal administration of (S)-LCM resulting in a loss of presynaptic localization of CaV2.2 and NaV1.7. Together, these findings indicate that CRMP2 regulates presynaptic excitatory neurotransmission in spinal cord and may play an important role in regulating pathological pain. Novel targeting strategies to inhibit CRMP2 phosphorylation by Cdk5 may have great potential for the treatment of chronic pain.

Dissecting the role of the CRMP2-neurofibromin complex on pain behaviors

Neurofibromatosis type 1 (NF1), a genetic disorder linked to inactivating mutations or a homozygous deletion of the Nf1 gene, is characterized by tumorigenesis, cognitive dysfunction, seizures, migraine, and pain. Omic studies on human NF1 tissues identified an increase in the expression of collapsin response mediator protein 2 (CRMP2), a cytosolic protein reported to regulate the trafficking and activity of presynaptic N-type voltage-gated calcium (Cav2.2) channels. Because neurofibromin, the protein product of the Nf1 gene, binds to and inhibits CRMP2, the neurofibromin-CRMP2 signaling cascade will likely affect Ca channel activity and regulate nociceptive neurotransmission and in vivo responses to noxious stimulation. Here, we investigated the function of neurofibromin-CRMP2 interaction on Cav2.2. Mapping of >275 peptides between neurofibromin and CRMP2 identified a 15-amino acid CRMP2-derived peptide that, when fused to the tat transduction domain of HIV-1, inhibited Ca influx in dorsal root ganglion neurons. This peptide mimics the negative regulation of CRMP2 activity by neurofibromin. Neurons treated with tat-CRMP2/neurofibromin regulating peptide 1 (t-CNRP1) exhibited a decreased Cav2.2 membrane localization, and uncoupling of neurofibromin-CRMP2 and CRMP2-Cav2.2 interactions. Proteomic analysis of a nanodisc-solubilized membrane protein library identified syntaxin 1A as a novel CRMP2-binding protein whose interaction with CRMP2 was strengthened in neurofibromin-depleted cells and reduced by t-CNRP1. Stimulus-evoked release of calcitonin gene-related peptide from lumbar spinal cord slices was inhibited by t-CNRP1. Intrathecal administration of t-CNRP1 was antinociceptive in experimental models of inflammatory, postsurgical, and neuropathic pain. Our results demonstrate the utility of t-CNRP1 to inhibit CRMP2 protein-protein interactions for the potential treatment of pain.

Recent advances in targeting ion channels to treat chronic pain

LINKED ARTICLES

This 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.

CRMP2-Neurofibromin Interface Drives NF1-related Pain

An understudied symptom of the genetic disorder Neurofibromatosis type 1 (NF1) is chronic idiopathic pain. We used targeted editing of Nf1 in rats to provide direct evidence of a causal relationship between neurofibromin, the protein product of the Nf1 gene, and pain responses. Our study data identified a protein-interaction network with collapsin response meditator protein 2 (CRMP2) as a node and neurofibromin, syntaxin 1A, and the N-type voltage-gated calcium (CaV2.2) channel as interaction edges. Neurofibromin uncouples CRMP2 from syntaxin 1A. Upon loss/mutation of neurofibromin, as seen in patients with NF1, the CRMP2/Neurofibromin interaction is uncoupled, which frees CRMP2 to interact with both syntaxin 1A and CaV2.2, culminating in increased release of the pro-nociceptive neurotransmitter calcitonin gene-related peptide (CGRP). Our work also identified the CRMP2-derived peptide CNRP1, which uncoupled CRMP2's interactions with neurofibromin, syntaxin 1A, as well as CaV2.2. Here, we tested if CRISPR/Cas9-mediated editing of the Nf1 gene, which leads to functional remodeling of peripheral nociceptors through effects on the tetrodotoxin-sensitive (TTX-S) Na⁺ voltage-gated sodium channel (NaV1.7) and CaV2.2, could be affected using CNRP1, a peptide designed to target the CRMP2-neurofibromin interface. The data presented here shows that disrupting the CRMP2-neurofibromin interface is sufficient to reverse the dysregulations of voltage-gated ion channels and neurotransmitter release elicited by Nf1 gene editing. As a consequence of these effects, the CNRP1 peptide reversed hyperalgesia to thermal stimulation of the hindpaw observed in Nf1-edited rats. Our findings support future pharmacological targeting of the CRMP2/neurofibromin interface for NF1-related pain relief.

CRMP2 and voltage-gated ion channels: potential roles in neuropathic pain

Neuropathic pain represents a significant and mounting burden on patients and society at large. Management of neuropathic pain, however, is both intricate and challenging, exacerbated by the limited quantity and quality of clinically available treatments. On this stage, dysfunctional voltage-gated ion channels, especially the presynaptic N-type voltage-gated calcium channel (VGCC) (Cav2.2) and the tetrodotoxin-sensitive voltage-gated sodium channel (VGSC) (Nav1.7), underlie the pathophysiology of neuropathic pain and serve as high profile therapeutic targets. Indirect regulation of these channels holds promise for the treatment of neuropathic pain. In this review, we focus on collapsin response mediator protein 2 (CRMP2), a protein with emergent roles in voltage-gated ion channel trafficking and discuss the therapeutic potential of targetting this protein.

Homology-guided mutational analysis reveals the functional requirements for antinociceptive specificity of collapsin response mediator protein 2-derived peptides

BACKGROUND AND PURPOSE

N-type voltage-gated calcium (Ca_v 2.2) channels are critical determinants of increased neuronal excitability and neurotransmission accompanying persistent neuropathic pain. Although Ca_v 2.2 channel antagonists are recommended as first-line treatment for neuropathic pain, calcium-current blocking gabapentinoids inadequately alleviate chronic pain symptoms and often exhibit numerous side effects. Collapsin response mediator protein 2 (CRMP2) targets Ca_v 2.2 channels to the sensory neuron membrane and allosterically modulates their function. A 15-amino-acid peptide (CBD3), derived from CRMP2, disrupts the functional protein-protein interaction between CRMP2 and Ca_v 2.2 channels to inhibit calcium influx, transmitter release and acute, inflammatory and neuropathic pain. Here, we have mapped the minimal domain of CBD3 necessary for its antinociceptive potential.

EXPERIMENTAL APPROACH

Truncated as well as homology-guided mutant versions of CBD3 were generated and assessed using depolarization-evoked calcium influx in rat dorsal root ganglion neurons, binding between CRMP2 and Ca_v 2.2 channels, whole-cell voltage clamp electrophysiology and behavioural effects in two models of experimental pain: post-surgical pain and HIV-induced sensory neuropathy induced by the viral glycoprotein 120.

KEY RESULTS

The first six amino acids within CBD3 accounted for all in vitro activity and antinociception. Spinal administration of a prototypical peptide (TAT-CBD3-L5M) reversed pain behaviours. Homology-guided mutational analyses of these six amino acids identified at least two residues, Ala1 and Arg4, as being critical for antinociception in two pain models.

CONCLUSIONS AND IMPLICATIONS

These results identify an antinociceptive scaffold core in CBD3 that can be used for development of low MW mimetics of CBD3.

LINKED ARTICLES

This 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.