Voltage-gated sodium channel expression in mouse DRG after SNI leads to re-evaluation of projections of injured fibers

Nerve injury inhibits Oprd1 and Cnr1 transcription through REST in primary sensory neurons

The transcription repressor REST in the dorsal root ganglion (DRG) is upregulated by peripheral nerve injury and promotes the development of chronic pain. However, the genes targeted by REST in neuropathic pain development remain unclear. The expression levels of 4 opioid receptor (Oprm1, Oprd1, Oprl1, Oprk1) and the cannabinoid CB1 receptor (Cnr1) genes in the DRG regulate nociception. In this study, we determined the role of REST in the control of their expression in the DRG induced by spared nerve injury (SNI) in both male and female mice. Transcriptomic analyses of male mouse DRGs followed by quantitative reverse transcription polymerase chain reaction analyses of both male and female mouse DRGs showed that SNI upregulated expression of Rest and downregulated mRNA levels of all 4 opioid receptor and Cnr1 genes, but Oprm1 was upregulated in female mice. Analysis of publicly available bioinformatic data suggested that REST binds to the promoter regions of Oprm1 and Cnr1. Chromatin immunoprecipitation analyses indicated differing levels of REST at these promoters in male and female mice. Full-length Rest conditional knockout in primary sensory neurons reduced SNI-induced pain hypersensitivity and rescued the SNI-induced reduction in the expression of Oprd1 and Cnr1 in the DRG in both male and female mice. Our results suggest that nerve injury represses the transcription of Oprd1 and Cnr1 via REST in primary sensory neurons and that REST is a potential therapeutic target for neuropathic pain.

Potassium channel modulation in macrophages sensitizes dorsal root ganglion neurons after nerve injury

Macrophages and satellite glial cells are found between injured and uninjured neurons in the lumbar dorsal root ganglia (DRG). We explored the mechanism of neuro-immune and neuron-glia crosstalk leading to hyperexcitability of DRG neurons. After spared nerve injury (SNI), CX3CR1+ resident macrophages became activated, proliferated, and increased inward-rectifying potassium channel Kir 2.1 currents. Conditioned medium (CM) by macrophages, obtained from DRG of SNI mice, sensitized small DRG neurons from naïve mice. However, treatment with CM from GFAP+ glial cells did not affect neuronal excitability. When subjected to this macrophage-derived CM, DRG neurons had increased spontaneous activity, current-evoked responses and voltage-gated NaV 1.7 and NaV 1.8 currents. Silencing Kir 2.1 in macrophages after SNI prevented the induction of neuronal hyperexcitability from their CM. Blocking vesicular exocytosis or soluble tumor necrosis factor in CM or interfering with the downstream intracellular p38 pathway in neurons, also prevented neuronal hyperexcitability. Blocking protein trafficking in neurons reduced the effect of CM, suggesting that the hyperexcitable state resulted from changes in NaV channel trafficking. These results suggest that DRG macrophages, primed by peripheral nerve injury, contribute to neuron-glia crosstalk, NaV channel dysregulation and neuronal hyperexcitability implicated in the development of neuropathic pain.

A Perspective on Electrical Stimulation and Sympathetic Regeneration in Peripheral Nerve Injuries

Peripheral nerve injuries (PNIs) are common and devastating. The current standard of care relies on the slow and inefficient process of nerve regeneration after surgical intervention. Electrical stimulation (ES) has been shown to both experimentally and clinically result in improved regeneration and functional recovery after PNI for motor and sensory neurons; however, its effects on sympathetic regeneration have never been studied. Sympathetic neurons are responsible for a myriad of homeostatic processes that include, but are not limited to, blood pressure, immune response, sweating, and the structural integrity of the neuromuscular junction. Almost one quarter of the axons in the sciatic nerve are from sympathetic neurons, and their importance in bodily homeostasis and the pathogenesis of neuropathic pain should not be underestimated. Therefore, as ES continues to make its way into patient care, it is not only important to understand its impact on all neuron subtypes, but also to ensure that potential adverse effects are minimized. This piece gives an overview of the effects of ES in animals models and in humans while offering a perspective on the potential effects of ES on sympathetic axon regeneration.

Neat1 lncRNA organizes the inflammatory gene expressions in the dorsal root ganglion in neuropathic pain caused by nerve injury

Primary sensory neurons regulate inflammatory processes in innervated regions through neuro-immune communication. However, how their immune-modulating functions are regulated in concert remains largely unknown. Here, we show that Neat1 long non-coding RNA (lncRNA) organizes the proinflammatory gene expressions in the dorsal root ganglion (DRG) in chronic intractable neuropathic pain in rats. Neat1 was abundantly expressed in the DRG and was upregulated after peripheral nerve injury. Neat1 overexpression in primary sensory neurons caused mechanical and thermal hypersensitivity, whereas its knockdown alleviated neuropathic pain. Bioinformatics analysis of comprehensive transcriptome changes indicated the inflammatory response was the most relevant function of genes upregulated through Neat1. Consistent with this, upregulation of proinflammatory genes in the DRG following nerve injury was suppressed by Neat1 knockdown. Expression changes of these proinflammatory genes were regulated through Neat1-mRNA interaction-dependent and -independent mechanisms. Notably, Neat1 increased proinflammatory genes by stabilizing its interacting mRNAs in neuropathic pain. Finally, Neat1 in primary sensory neurons contributed to spinal inflammatory processes that mediated peripheral neuropathic pain. These findings demonstrate that Neat1 lncRNA is a key regulator of neuro-immune communication in neuropathic pain.

NMDA Receptors at Primary Afferent-Excitatory Neuron Synapses Differentially Sustain Chemotherapy- and Nerve Trauma-Induced Chronic Pain

The spinal dorsal horn contains vesicular glutamate transporter-2 (VGluT2)-expressing excitatory neurons and vesicular GABA transporter (VGAT)-expressing inhibitory neurons, which normally have different roles in nociceptive transmission. Spinal glutamate NMDAR hyperactivity is a crucial mechanism of chronic neuropathic pain. However, it is unclear how NMDARs regulate primary afferent input to spinal excitatory and inhibitory neurons in neuropathic pain. Also, the functional significance of presynaptic NMDARs in neuropathic pain has not been defined explicitly. Here we showed that paclitaxel treatment or spared nerve injury (SNI) similarly increased the NMDAR-mediated mEPSC frequency and dorsal root-evoked EPSCs in VGluT2 dorsal horn neurons in male and female mice. By contrast, neither paclitaxel nor SNI had any effect on mEPSCs or evoked EPSCs in VGAT neurons. In mice with conditional Grin1 (gene encoding GluN1) KO in primary sensory neurons (Grin1-cKO), paclitaxel treatment failed to induce pain hypersensitivity. Unexpectedly, SNI still caused long-lasting pain hypersensitivity in Grin1-cKO mice. SNI increased the amplitude of puff NMDA currents in VGluT2 neurons and caused similar depolarizing shifts in GABA reversal potentials in WT and Grin1-cKO mice. Concordantly, spinal Grin1 knockdown diminished SNI-induced pain hypersensitivity. Thus, presynaptic NMDARs preferentially amplify primary afferent input to spinal excitatory neurons in neuropathic pain. Although presynaptic NMDARs are required for chemotherapy-induced pain hypersensitivity, postsynaptic NMDARs in spinal excitatory neurons play a dominant role in traumatic nerve injury-induced chronic pain. Our findings reveal the divergent synaptic connectivity and functional significance of spinal presynaptic and postsynaptic NMDARs in regulating cell type-specific nociceptive input in neuropathic pain with different etiologies.SIGNIFICANCE STATEMENT Spinal excitatory neurons relay input from nociceptors, whereas inhibitory neurons repress spinal nociceptive transmission. Chronic nerve pain is associated with aberrant NMDAR activity in the spinal dorsal horn. This study demonstrates, for the first time, that chemotherapy and traumatic nerve injury preferentially enhance the NMDAR activity at primary afferent-excitatory neuron synapses but have no effect on primary afferent input to spinal inhibitory neurons. NMDARs in primary sensory neurons are essential for chemotherapy-induced chronic pain, whereas nerve trauma causes pain hypersensitivity predominantly via postsynaptic NMDARs in spinal excitatory neurons. Thus, presynaptic and postsynaptic NMDARs at primary afferent-excitatory neuron synapses are differentially engaged in chemotherapy- and nerve injury-induced chronic pain and could be targeted respectively for treating these painful conditions.

Synchronized activity of sensory neurons initiates cortical synchrony in a model of neuropathic pain

Increased low frequency cortical oscillations are observed in people with neuropathic pain, but the cause of such elevated cortical oscillations and their impact on pain development remain unclear. By imaging neuronal activity in a spared nerve injury (SNI) mouse model of neuropathic pain, we show that neurons in dorsal root ganglia (DRG) and somatosensory cortex (S1) exhibit synchronized activity after peripheral nerve injury. Notably, synchronized activity of DRG neurons occurs within hours after injury and 1-2 days before increased cortical oscillations. This DRG synchrony is initiated by axotomized neurons and mediated by local purinergic signaling at the site of nerve injury. We further show that synchronized DRG activity after SNI is responsible for increasing low frequency cortical oscillations and synaptic remodeling in S1, as well as for inducing animals' pain-like behaviors. In naive mice, enhancing the synchrony, not the level, of DRG neuronal activity causes synaptic changes in S1 and pain-like behaviors similar to SNI mice. Taken together, these results reveal the critical role of synchronized DRG neuronal activity in increasing cortical plasticity and oscillations in a neuropathic pain model. These findings also suggest the potential importance of detection and suppression of elevated cortical oscillations in neuropathic pain states.

Spinal cord retinoic acid receptor signaling gates mechanical hypersensitivity in neuropathic pain

Central sensitization caused by spinal disinhibition is a key mechanism of mechanical allodynia in neuropathic pain. However, the molecular mechanisms underlying spinal disinhibition after nerve injury remain unclear. Here, we show in mice that spared nerve injury (SNI), which induces mechanical hypersensitivity and neuropathic pain, triggers homeostatic reduction of inhibitory outputs from dorsal horn parvalbumin-positive (PV+) interneurons onto both primary afferent terminals and excitatory interneurons. The reduction in inhibitory outputs drives hyperactivation of the spinal cord nociceptive pathway, causing mechanical hypersensitivity. We identified the retinoic acid receptor RARα, a central regulator of homeostatic plasticity, as the key molecular mediator for this synaptic disinhibition. Deletion of RARα in spinal PV+ neurons or application of an RARα antagonist in the spinal cord prevented the development of SNI-induced mechanical hypersensitivity. Our results identify RARα as a crucial molecular effector for neuropathic pain and a potential target for its treatment.

Microglia-independent peripheral neuropathic pain in male and female mice

ABSTRACT

The dominant view in the field of pain is that peripheral neuropathic pain is driven by microglia in the somatosensory processing region of the spinal dorsal horn. Here, to the contrary, we discovered a form of neuropathic pain that is independent of microglia. Mice in which the nucleus pulposus (NP) of the intervertebral disc was apposed to the sciatic nerve developed a constellation of neuropathic pain behaviours: hypersensitivity to mechanical, cold, and heat stimuli. However, NP application caused no activation of spinal microglia nor was pain hypersensitivity reversed by microglial inhibition. Rather, NP-induced pain hypersensitivity was dependent on cells within the NP which recruited macrophages to the adjacent nerve. Eliminating macrophages systemically or locally prevented NP-induced pain hypersensitivity. Pain hypersensitivity was also prevented by genetically disrupting the neurotrophin brain-derived neurotrophic factor selectively in macrophages. Moreover, the behavioural phenotypes as well as the molecular mechanisms of NP-induced pain hypersensitivity were not different between males and females. Our findings reveal a previously unappreciated mechanism for by which a discrete peripheral nerve lesion may produce pain hypersensitivity, which may help to explain the limited success of microglial inhibitors on neuropathic pain in human clinical trials.

The Development of Mechanical Allodynia in Diabetic Rats Revealed by Single-Cell RNA-Seq

Mechanical allodynia (MA) is the main reason that patients with diabetic peripheral neuropathy (DPN) seek medical advice. It severely debilitates the quality of life. Investigating hyperglycemia-induced changes in neural transcription could provide fundamental insights into the complex pathogenesis of painful DPN (PDPN). Gene expression profiles of physiological dorsal root ganglia (DRG) have been studied. However, the transcriptomic changes in DRG neurons in PDPN remain largely unexplored. In this study, by single-cell RNA sequencing on dissociated rat DRG, we identified five physiological neuron types and a novel neuron type MAAC (Fxyd7⁺/Atp1b1⁺) in PDPN. The novel neuron type originated from peptidergic neuron cluster and was characterized by highly expressing genes related to neurofilament and cytoskeleton. Based on the inferred gene regulatory networks, we found that activated transcription factors Hobx7 and Larp1 in MAAC could enhance Atp1b1 expression. Moreover, we constructed the cellular communication network of MAAC and revealed its receptor-ligand pairs for transmitting signals with other cells. Our molecular investigation at single-cell resolution advances the understanding of the dynamic peripheral neuron changes and underlying molecular mechanisms during the development of PDPN.

Development of a common peroneal nerve injury model in domestic swine for the study of translational neuropathic pain treatments

OBJECTIVE

To date, muscular and bone pain have been studied in domestic swine models, but the only neuropathic pain model described in swine is a mixed neuritis model. Common peroneal nerve injury (CPNI) neuropathic pain models have been utilized in both mice and rats.

METHODS

The authors developed a swine surgical CPNI model of neuropathic pain. Behavioral outcomes were validated with von Frey filament testing, thermal sensitivity assessments, and social and motor scoring. Demyelination of the nerve was confirmed through standard histological assessment. The contralateral nerve served as the control.

RESULTS

CPNI induced mechanical and thermal allodynia (p < 0.001 [n = 10] and p < 0.05 [n = 4], respectively) and increased pain behavior, i.e., guarding of the painful leg (n = 12). Myelin protein zero (P0) staining revealed demyelination of the ligated nerve upstream of the ligation site.

CONCLUSIONS

In a neuropathic pain model in domestic swine, the authors demonstrated that CPNI induces demyelination of the common peroneal nerve, which the authors hypothesize is responsible for the resulting allodynic pain behavior. As the anatomical features of domestic swine resemble those of humans more closely than previously used rat and mouse models, utilizing this swine model, which is to the authors' knowledge the first of its kind, will aid in the translation of experimental treatments to clinical trials.

Heme oxygenase-1 in the spinal cord plays crucial roles in the analgesic effects of pregabalin and gabapentin in a spared nerve-injury mouse model

INTRODUCTION

Neuropathic pain remains one of the most intractable types of pain; although calcium channel α₂δ ligands, such as pregabalin and gabapentin, are classified as first-line drugs, they have only modest efficacy. Heme oxygenase-1 (HO-1) signaling attenuates glial activation during neuropathic pain. Thus, this study aimed to investigate the effects of the blood-brain barrier (BBB)-permeable HO-1 inhibitor, tin protoporphyrin IX (SnPP), or the BBB-impermeable HO-1 inhibitor, zinc (II) protoporphyrin IX (ZnPP), on the analgesic efficacy of pregabalin and gabapentin. Additionally, we examined the effects of co-administration of SnPP with pregabalin or gabapentin on the expression of glial markers or other genes.

METHODS

Neuropathic pain was induced by spared nerve injury (SNI) of the sciatic nerve. The mechanical threshold was tested using the von Frey filaments. The expression of spinal glial markers or other genes was examined using reverse transcription polymerase chain reaction.

RESULTS

Systemic HO-1 inhibition reversed the mechanical antiallodynic effects of pregabalin and gabapentin, although peripheral HO-1 inhibition did not alter the mechanical antiallodynic effects of either pregabalin or gabapentin. Intrathecal injection of SnPP or ZnPP abolished the mechanical antiallodynic effects of pregabalin and gabapentin. Pregabalin and gabapentin increased HO-1, arginase-1, and endogenous opioid precursor preproenkephalin gene expression and decreased the expression of glial markers, interleukin-1β, and inducible nitric oxide synthase.

CONCLUSIONS

This study suggests that spinal HO-1 plays a crucial role in the analgesic effects of calcium channel α₂δ ligands through the attenuation of glial activation and endogenous opioid release.

Profiling sensory neuron microenvironment after peripheral and central axon injury reveals key pathways for neural repair

Sensory neurons with cell bodies in dorsal root ganglia (DRG) represent a useful model to study axon regeneration. Whereas regeneration and functional recovery occurs after peripheral nerve injury, spinal cord injury or dorsal root injury is not followed by regenerative outcomes. Regeneration of sensory axons in peripheral nerves is not entirely cell autonomous. Whether the DRG microenvironment influences the different regenerative capacities after injury to peripheral or central axons remains largely unknown. To answer this question, we performed a single-cell transcriptional profiling of mouse DRG in response to peripheral (sciatic nerve crush) and central axon injuries (dorsal root crush and spinal cord injury). Each cell type responded differently to the three types of injuries. All injuries increased the proportion of a cell type that shares features of both immune cells and glial cells. A distinct subset of satellite glial cells (SGC) appeared specifically in response to peripheral nerve injury. Activation of the PPARα signaling pathway in SGC, which promotes axon regeneration after peripheral nerve injury, failed to occur after central axon injuries. Treatment with the FDA-approved PPARα agonist fenofibrate increased axon regeneration after dorsal root injury. This study provides a map of the distinct DRG microenvironment responses to peripheral and central injuries at the single-cell level and highlights that manipulating non-neuronal cells could lead to avenues to promote functional recovery after CNS injuries or disease.

A proteome signature for acute incisional pain in dorsal root ganglia of mice

ABSTRACT

After surgery, acute pain is still managed insufficiently and may lead to short-term and long-term complications including chronic postsurgical pain and an increased prescription of opioids. Thus, identifying new targets specifically implicated in postoperative pain is of utmost importance to develop effective and nonaddictive analgesics. Here, we used an integrated and multimethod workflow to reveal unprecedented insights into proteome dynamics in dorsal root ganglia (DRG) of mice after plantar incision (INC). Based on a detailed characterization of INC-associated pain-related behavior profiles, including a novel paradigm for nonevoked pain, we performed quantitative mass-spectrometry-based proteomics in DRG 1 day after INC. Our data revealed a hitherto unknown INC-regulated protein signature in DRG with changes in distinct proteins and cellular signaling pathways. In particular, we show the differential regulation of 44 protein candidates, many of which are annotated with pathways related to immune and inflammatory responses such as MAPK/extracellular signal-regulated kinases signaling. Subsequent orthogonal assays comprised multiplex Western blotting, bioinformatic protein network analysis, and immunolabeling in independent mouse cohorts to validate (1) the INC-induced regulation of immune/inflammatory pathways and (2) the high priority candidate Annexin A1. Taken together, our results propose novel potential targets in the context of incision and, therefore, represent a highly valuable resource for further mechanistic and translational studies of postoperative pain.

Nerve Excitability and Neuropathic Pain is Reduced by BET Protein Inhibition After Spared Nerve Injury

Neuropathic pain is a common disability produced by enhanced neuronal excitability after nervous system injury. The pathophysiological changes that underlie the generation and maintenance of neuropathic pain require modifications of transcriptional programs. In particular, there is an induction of pro-inflammatory neuromodulators levels, and changes in the expression of ion channels and other factors intervening in the determination of the membrane potential in neuronal cells. We have previously found that inhibition of the BET proteins epigenetic readers reduced neuroinflammation after spinal cord injury. Within the present study we aimed to determine if BET protein inhibition may also affect neuroinflammation after a peripheral nerve injury, and if this would beneficially alter neuronal excitability and neuropathic pain. For this purpose, C57BL/6 female mice underwent spared nerve injury (SNI), and were treated with the BET inhibitor JQ1, or vehicle. Electrophysiological and algesimetry tests were performed on these mice. We also determined the effects of JQ1 treatment after injury on neuroinflammation, and the expression of neuronal components important for the maintenance of axon membrane potential. We found that treatment with JQ1 affected neuronal excitability and mechanical hyperalgesia after SNI in mice. BET protein inhibition regulated cytokine expression and reduced microglial reactivity after injury. In addition, JQ1 treatment altered the expression of SCN3A, SCN9A, KCNA1, KCNQ2, KCNQ3, HCN1 and HCN2 ion channels, as well as the expression of the Na⁺/K⁺ ATPase pump subunits. In conclusion, both, alteration of inflammation, and neuronal transcription, could be the responsible epigenetic mechanisms for the reduction of excitability and hyperalgesia observed after BET inhibition. Inhibition of BET proteins is a promising therapy for reducing neuropathic pain after neural injury.

Nociceptor-localized cGMP-dependent protein kinase I is a critical generator for central sensitization and neuropathic pain

Patients with neuropathic pain often experience exaggerated pain and anxiety. Central sensitization has been linked with the maintenance of neuropathic pain and may become an autonomous pain generator. Conversely, emerging evidence accumulated that central sensitization is initiated and maintained by ongoing nociceptive primary afferent inputs. However, it remains elusive what mechanisms underlie this phenomenon and which peripheral candidate contributes to central sensitization that accounts for pain hypersensitivity and pain-related anxiety. Previous studies have implicated peripherally localized cGMP-dependent protein kinase I (PKG-I) in plasticity of nociceptors and spinal synaptic transmission as well as inflammatory hyperalgesia. However, whether peripheral PKG-I contributes to cortical plasticity and hence maintains nerve injury-induced pain hypersensitivity and anxiety is unknown. Here, we demonstrated significant upregulation of PKG-I in ipsilateral L3 dorsal root ganglia (DRG), no change in L4 DRG, and downregulation in L5 DRG upon spared nerve injury. Genetic ablation of PKG-I specifically in nociceptors or post-treatment with intervertebral foramen injection of PKG-I antagonist, KT5823, attenuated the development and maintenance of spared nerve injury-induced bilateral pain hypersensitivity and anxiety. Mechanistic analysis revealed that activation of PKG-I in nociceptors is responsible for synaptic potentiation in the anterior cingulate cortex upon peripheral neuropathy through presynaptic mechanisms involving brain-derived neurotropic factor signaling. Our results revealed that PKG-I expressed in nociceptors is a key determinant for cingulate synaptic plasticity after nerve injury, which contributes to the maintenance of pain hypersensitivity and anxiety. Thereby, this study presents a strong basis for opening up a novel therapeutic target, PKG-I, in nociceptors for treatment of comorbidity of neuropathic pain and anxiety with least side effects.

Longitudinal intravital imaging nerve degeneration and sprouting in the toes of spared nerve injured mice

Neuropathic pain is pain caused by damage to the somatosensory nervous system. Both degenerating injured nerves and neighboring sprouting nerves can contribute to neuropathic pain. However, the mesoscale changes in cutaneous nerve fibers over time after the loss of the parent nerve has not been investigated in detail. In this study, we followed the changes in nerve fibers longitudinally in the toe tips of mice that had undergone spared nerve injury (SNI). Nav1.8-tdTomato, Thy1-GFP and MrgD-GFP mice were used to observe the small and large cutaneous nerve fibers. We found that peripheral nerve plexuses degenerated within 3 days of nerve injury, and free nerve endings in the epidermis degenerated within 2 days. The timing of degeneration paralleled the initiation of mechanical hypersensitivity. We also found that some of the Nav1.8-positive nerve plexuses and free nerve endings in the fifth toe survived, and sprouting occurred mostly from 7 to 28 days. The timing of the sprouting of nerve fibers in the fifth toe paralleled the maintenance phase of mechanical hypersensitivity. Our results support the hypotheses that both injured and intact nerve fibers participate in neuropathic pain, and that, specifically, nerve degeneration is related to the initiation of evoked pain and nerve sprouting is related to the maintenance of evoked pain.

Loss of Prdm12 during development, but not in mature nociceptors, causes defects in pain sensation

Prdm12 is a key transcription factor in nociceptor neurogenesis. Mutations of Prdm12 cause congenital insensitivity to pain (CIP) from failure of nociceptor development. However, precisely how deletion of Prdm12 during development or adulthood affects nociception is unknown. Here, we employ tissue- and temporal-specific knockout mouse models to test the function of Prdm12 during development and in adulthood. We find that constitutive loss of Prdm12 causes deficiencies in proliferation during sensory neurogenesis. We also demonstrate that conditional knockout from dorsal root ganglia (DRGs) during embryogenesis causes defects in nociception. In contrast, we find that, in adult DRGs, Prdm12 is dispensable for most pain-sensation and injury-induced hypersensitivity. Using transcriptomic analysis, we find mostly unique changes in adult Prdm12 knockout DRGs compared with embryonic knockout and that PRDM12 is likely a transcriptional activator in the adult. Overall, we find that the function of PRDM12 changes over developmental time.

Activation of the regeneration-associated gene STAT3 and functional changes in intact nociceptors after peripheral nerve damage in mice

BACKGROUND

In the context of neuropathic pain, the contribution of regeneration to the development of positive symptoms is not completely understood. Several efforts have been done to described changes in axotomized neurons, however, there is scarce data on changes occurring in intact neurons, despite experimental evidence of functional changes. To address this issue, we analysed by immunohistochemistry the presence of phosphorylated signal transducer and activator of transcription 3 (pSTAT3), an accepted marker of regeneration, within DRGs where axotomized neurons were retrogradely labelled following peripheral nerve injury. Likewise, we have characterized abnormal electrophysiological properties in intact fibres after partial nerve injury.

METHODS/RESULTS

We showed that induction of pSTAT3 in sensory neurons was similar after partial or total transection of the sciatic nerve and to the same extent within axotomized and non-axotomized neurons. We also examined pSTAT3 presence on non-peptidergic and peptidergic nociceptors. Whereas the percentage of neurons marked by IB4 decrease after injury, the proportion of CGRP neurons did not change, but its expression switched from small- to large-diameter neurons. Besides, the percentage of CGRP+ neurons expressing pSTAT3 increased significantly 2.5-folds after axotomy, preferentially in neurons with large diameters. Electrophysiological recordings showed that after nerve damage, most of the neurons with ectopic spontaneous activity (39/46) were non-axotomized C-fibres with functional receptive fields in the skin far beyond the site of damage.

CONCLUSIONS

Neuronal regeneration after nerve injury, likely triggered from the site of injury, may explain the abnormal functional properties gained by intact neurons, reinforcing their role in neuropathic pain.

SIGNIFICANCE

Positive symptoms in patients with peripheral neuropathies correlate to abnormal functioning of different subpopulations of primary afferents. Peripheral nerve damage triggers regenerating programs in the cell bodies of axotomized but also in non-axotomized nociceptors which is in turn, develop abnormal spontaneous and evoked discharges. Therefore, intact nociceptors have a significant role in the development of neuropathic pain due to their hyperexcitable peripheral terminals. Therapeutical targets should focus on inhibiting peripheral hyperexcitability in an attempt to limit peripheral and central sensitization.

Peripheral Neuropathic Pain: From Experimental Models to Potential Therapeutic Targets in Dorsal Root Ganglion Neurons

Neuropathic pain exerts a global burden caused by the lesions in the somatosensory nerve system, including the central and peripheral nervous systems. The mechanisms of nerve injury-induced neuropathic pain involve multiple mechanisms, various signaling pathways, and molecules. Currently, poor efficacy is the major limitation of medications for treating neuropathic pain. Thus, understanding the detailed molecular mechanisms should shed light on the development of new therapeutic strategies for neuropathic pain. Several well-established in vivo pain models were used to investigate the detail mechanisms of peripheral neuropathic pain. Molecular mediators of pain are regulated differentially in various forms of neuropathic pain models; these regulators include purinergic receptors, transient receptor potential receptor channels, and voltage-gated sodium and calcium channels. Meanwhile, post-translational modification and transcriptional regulation are also altered in these pain models and have been reported to mediate several pain related molecules. In this review, we focus on molecular mechanisms and mediators of neuropathic pain with their corresponding transcriptional regulation and post-translational modification underlying peripheral sensitization in the dorsal root ganglia. Taken together, these molecular mediators and their modification and regulations provide excellent targets for neuropathic pain treatment.

Revisiting PNS Plasticity: How Uninjured Sensory Afferents Promote Neuropathic Pain

Despite the widespread study of how injured nerves contribute to chronic pain, there are still major gaps in our understanding of pain mechanisms. This is particularly true of pain resulting from nerve injury, or neuropathic pain, wherein tactile or thermal stimuli cause painful responses that are particularly difficult to treat with existing therapies. Curiously, this stimulus-driven pain relies upon intact, uninjured sensory neurons that transmit the signals that are ultimately sensed as painful. Studies that interrogate uninjured neurons in search of cell-specific mechanisms have shown that nerve injury alters intact, uninjured neurons resulting in an activity that drives stimulus-evoked pain. This review of neuropathic pain mechanisms summarizes cell-type-specific pathology of uninjured sensory neurons and the sensory ganglia that house their cell bodies. Uninjured neurons have demonstrated a wide range of molecular and neurophysiologic changes, many of which are distinct from those detected in injured neurons. These intriguing findings include expression of pain-associated molecules, neurophysiological changes that underlie increased excitability, and evidence that intercellular signaling within sensory ganglia alters uninjured neurons. In addition to well-supported findings, this review also discusses potential mechanisms that remain poorly understood in the context of nerve injury. This review highlights key questions that will advance our understanding of the plasticity of sensory neuron subpopulations and clarify the role of uninjured neurons in developing anti-pain therapies.

Transcriptional Reprogramming of Distinct Peripheral Sensory Neuron Subtypes after Axonal Injury

Primary somatosensory neurons are specialized to transmit specific types of sensory information through differences in cell size, myelination, and the expression of distinct receptors and ion channels, which together define their transcriptional and functional identity. By profiling sensory ganglia at single-cell resolution, we find that all somatosensory neuronal subtypes undergo a similar transcriptional response to peripheral nerve injury that both promotes axonal regeneration and suppresses cell identity. This transcriptional reprogramming, which is not observed in non-neuronal cells, resolves over a similar time course as target reinnervation and is associated with the restoration of original cell identity. Injury-induced transcriptional reprogramming requires ATF3, a transcription factor that is induced rapidly after injury and necessary for axonal regeneration and functional recovery. Our findings suggest that transcription factors induced early after peripheral nerve injury confer the cellular plasticity required for sensory neurons to transform into a regenerative state.

The Lipid Receptor G2A (GPR132) Mediates Macrophage Migration in Nerve Injury-Induced Neuropathic Pain

Nerve injury-induced neuropathic pain is difficult to treat and mechanistically characterized by strong neuroimmune interactions, involving signaling lipids that act via specific G-protein coupled receptors. Here, we investigated the role of the signaling lipid receptor G2A (GPR132) in nerve injury-induced neuropathic pain using the robust spared nerve injury (SNI) mouse model. We found that the concentrations of the G2A agonist 9-HODE (9-Hydroxyoctadecadienoic acid) are strongly increased at the site of nerve injury during neuropathic pain. Moreover, G2A-deficient mice show a strong reduction of mechanical hypersensitivity after nerve injury. This phenotype is accompanied by a massive reduction of invading macrophages and neutrophils in G2A-deficient mice and a strongly reduced release of the proalgesic mediators TNFα, IL-6 and VEGF at the site of injury. Using a global proteome analysis to identify the underlying signaling pathways, we found that G2A activation in macrophages initiates MyD88-PI3K-AKT signaling and transient MMP9 release to trigger cytoskeleton remodeling and migration. We conclude that G2A-deficiency reduces inflammatory responses by decreasing the number of immune cells and the release of proinflammatory cytokines and growth factors at the site of nerve injury. Inhibiting the G2A receptor after nerve injury may reduce immune cell-mediated peripheral sensitization and may thus ameliorate neuropathic pain.

Differential Expression of Acid - Sensing Ion Channels in Mouse Primary Afferents in Naïve and Injured Conditions

Injury and inflammation cause tissue acidosis, which is a common feature of various painful conditions. Acid-Sensing Ion channels (ASICs) are amongst the main excitatory channels activated by extracellular protons and expressed in the nervous system. Six transcripts of ASIC subunits including ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, and ASIC4 are encoded by four genes (Asic1–4) and have been identified in rodents. Most ASIC subunits are present at substantial levels in primary sensory neurons of dorsal root ganglia (DRG) except for ASIC4. However, their expression pattern in DRG neurons remains largely unclear, mainly due to the lack of antibodies with appropriate specificity. In this study, we examined in detail the expression pattern of ASIC1-3 subunits, including splice variants, in different populations of DRG neurons in adult mice using an in situ hybridization technique (RNAscope) with high sensitivity and specificity. We found that in naïve condition, all five subunits examined were expressed in the majority of myelinated, NF200-immunoreactive, DRG neurons (NF200⁺). However, ASIC subunits showed a very different expression pattern among non-myelinated DRG neuronal subpopulations: ASIC1 and ASIC3 were only expressed in CGRP-immunoreactive neurons (CGRP⁺), ASIC2a was mostly expressed in the majority of IB4-binding neurons (IB4⁺), while ASIC2b was expressed in almost all non-myelinated DRG neurons. We also found that at least half of sensory neurons expressed multiple types of ASIC subunits, indicating prevalence of heteromeric channels. In mice with peripheral nerve injury, the expression level of ASIC1a and ASIC1b in L4 DRG and ASIC3 in L5 DRG were altered in CGRP⁺ neurons, but not in IB4⁺ neurons. Furthermore, the pattern of change varied among DRGs depending on their segmental level, which pointed to differential regulatory mechanisms between afferent types and anatomical location. The distinct expression pattern of ASIC transcripts in naïve condition, and the differential regulation of ASIC subunits after peripheral nerve injury, suggest that ASIC subunits are involved in separate sensory modalities.

Conserved Expression of Nav1.7 and Nav1.8 Contribute to the Spontaneous and Thermally Evoked Excitability in IL-6 and NGF-Sensitized Adult Dorsal Root Ganglion Neurons In Vitro

Sensory neurons respond to noxious stimuli by relaying information from the periphery to the central nervous system via action potentials driven by voltage-gated sodium channels, specifically Nav1.7 and Nav1.8. These channels play a key role in the manifestation of inflammatory pain. The ability to screen compounds that modulate voltage-gated sodium channels using cell-based assays assumes that key channels present in vivo is maintained in vitro. Prior electrophysiological work in vitro utilized acutely dissociated tissues, however, maintaining this preparation for long periods is difficult. A potential alternative involves multi-electrode arrays which permit long-term measurements of neural spike activity and are well suited for assessing persistent sensitization consistent with chronic pain. Here, we demonstrate that the addition of two inflammatory mediators associated with chronic inflammatory pain, nerve growth factor (NGF) and interleukin-6 (IL-6), to adult DRG neurons increases their firing rates on multi-electrode arrays in vitro. Nav1.7 and Nav1.8 proteins are readily detected in cultured neurons and contribute to evoked activity. The blockade of both Nav1.7 and Nav1.8, has a profound impact on thermally evoked firing after treatment with IL-6 and NGF. This work underscores the utility of multi-electrode arrays for pharmacological studies of sensory neurons and may facilitate the discovery and mechanistic analyses of anti-nociceptive compounds.

Sigma-1 receptors control neuropathic pain and macrophage infiltration into the dorsal root ganglion after peripheral nerve injury

Neuron-immune interaction in the dorsal root ganglia (DRG) plays a pivotal role in the neuropathic pain development after nerve injury. Sigma-1 receptor (Sig-1R) is expressed by DRG neurons but its role in neuropathic pain is not fully understood. We investigated the effect of peripheral Sig-1R on neuroinflammation in the DRG after spared (sciatic) nerve injury (SNI) in mice. Nerve injury induced a decrease in NeuN staining along with the nuclear eccentricity and ATF3 expression in the injured DRG. Sig-1R was present in all DRG neurons examined, and after SNI this receptor translocated to the periphery of the soma and the vicinity of the nucleus, especially in injured ATF3 + neurons. In WT mice, injured DRG produced the chemokine CCL2, and this was followed by massive infiltration of macrophages/monocytes, which clustered mainly around sensory neurons with translocated Sig-1R, accompanied by robust IL-6 increase and mechanical allodynia. In contrast, Sig-1R knockout (Sig-1R-KO) mice showed reduced levels of CCL2, decreased macrophage/monocyte infiltration into DRG, and less IL-6 and neuropathic mechanical allodynia after SNI. Our findings point to an important role of peripheral Sig-1R in sensory neuron-macrophage/monocyte communication in the DRG after peripheral nerve injury; thus, these receptors may contribute to the neuropathic pain phenotype.

Histone methyltransferase G9a diminishes expression of cannabinoid CB1 receptors in primary sensory neurons in neuropathic pain

Type 1 cannabinoid receptors (CB₁Rs) are expressed in the dorsal root ganglion (DRG) and contribute to the analgesic effect of cannabinoids. However, the epigenetic mechanism regulating the expression of CB₁Rs in neuropathic pain is unknown. G9a (encoded by the Ehmt2 gene), a histone 3 at lysine 9 methyltransferase, is a key chromatin regulator responsible for gene silencing. In this study, we determined G9a's role in regulating CB₁R expression in the DRG and in CB₁R-mediated analgesic effects in an animal model of neuropathic pain. We show that nerve injury profoundly reduced mRNA levels of CB₁Rs but increased the expression of CB₂ receptors in the rat DRG. ChIP results indicated increased enrichment of histone 3 at lysine 9 dimethylation, a G9a-catalyzed repressive histone mark, at the promoter regions of the CB₁R genes. G9a inhibition in nerve-injured rats not only up-regulated the CB₁R expression level in the DRG but also potentiated the analgesic effect of a CB₁R agonist on nerve injury-induced pain hypersensitivity. Furthermore, in mice lacking Ehmt2 in DRG neurons, nerve injury failed to reduce CB₁R expression in the DRG and to decrease the analgesic effect of the CB₁R agonist. Moreover, nerve injury diminished the inhibitory effect of the CB₁R agonist on synaptic glutamate release from primary afferent nerves to spinal cord dorsal horn neurons in WT mice but not in mice lacking Ehmt2 in DRG neurons. Our findings reveal that nerve injury diminishes the analgesic effect of CB₁R agonists through G9a-mediated CB₁R down-regulation in primary sensory neurons.

Oral Dimethyl Fumarate Reduces Peripheral Neuropathic Pain in Rodents via NFE2L2 Antioxidant Signaling

BACKGROUND

Available treatments for neuropathic pain have modest efficacy and significant adverse effects, including abuse potential. Because oxidative stress is a key mechanistic node for neuropathic pain, the authors focused on the master regulator of the antioxidant response-nuclear factor erythroid 2-related factor 2 (NFE2L2; Nrf2)-as an alternative target for neuropathic pain. The authors tested whether dimethyl fumarate (U.S. Food and Drug Administration-approved treatment for multiple sclerosis) would activate NFE2L2 and promote antioxidant activity to reverse neuropathic pain behaviors and oxidative stress-dependent mechanisms.

METHODS

Male Sprague Dawley rats, and male and female wild type and Nfe2l2 mice were treated with oral dimethyl fumarate/vehicle for 5 days (300 mg/kg; daily) after spared nerve injury/sham surgery (n = 5 to 8 per group). Allodynia was measured in von Frey reflex tests and hyperalgesia in operant conflict-avoidance tests. Ipsilateral L4/5 dorsal root ganglia were assayed for antioxidant and cytokine/chemokine levels, and mitochondrial bioenergetic capacity.

RESULTS

Dimethyl fumarate treatment reversed mechanical allodynia (injury-vehicle, 0.45 ± 0.06 g [mean ± SD]; injury-dimethyl fumarate, 8.2 ± 0.16 g; P < 0.001) and hyperalgesia induced by nerve injury (injury-vehicle, 2 of 6 crossed noxious probes; injury-dimethyl fumarate, 6 of 6 crossed; P = 0.013). The antiallodynic effect of dimethyl fumarate was lost in nerve-injured Nfe2l2 mice, but retained in nerve-injured male and female wild type mice (wild type, 0.94 ± 0.25 g; Nfe2l2, 0.02 ± 0.01 g; P < 0.001). Superoxide dismutase activity was increased by dimethyl fumarate after nerve injury (injury-vehicle, 3.96 ± 1.28 mU/mg; injury-dimethyl fumarate, 7.97 ± 0.47 mU/mg; P < 0.001). Treatment reduced the injury-dependent increases in cytokines and chemokines, including interleukin-1β (injury-vehicle, 13.30 ± 2.95 pg/mg; injury-dimethyl fumarate, 6.33 ± 1.97 pg/mg; P = 0.022). Injury-impaired mitochondrial bioenergetics, including basal respiratory capacity, were restored by dimethyl fumarate treatment (P = 0.025).

CONCLUSIONS

Dimethyl fumarate, a nonopioid and orally-bioavailable drug, alleviated nociceptive hypersensitivity induced by peripheral nerve injury via activation of NFE2L2 antioxidant signaling. Dimethyl fumarate also resolved neuroinflammation and mitochondrial dysfunction-oxidative stress-dependent mechanisms that drive nociceptive hypersensitivity after nerve injury.

Elevated ocular pressure reduces voltage-gated sodium channel NaV1.2 protein expression in retinal ganglion cell axons

Glaucoma is an age-related neurodegenerative disease that is commonly associated with sensitivity to intraocular pressure. The disease selectively targets retinal ganglion cells (RGCs) and constituent axons. RGC axons are rich in voltage-gated sodium channels, which are essential for action potential initiation and regeneration. Here, we identified voltage-dependent sodium channel, NaV1.2, in the retina, examined how this channel contributes to RGC light responses, and monitored NaV1.2 mRNA and protein expression in the retina during progression of modeled glaucoma. We found NaV1.2 is predominately localized in ganglion cell intraretinal axons with dispersed expression in the outer and inner plexiform layers. We showed Phrixotoxin-3, a potent NaV1.2 channel blocker, significantly decreased RGC electrical activity in a dose-dependent manner with an IC50 of 40 nM. Finally, we found four weeks of raised intraocular pressure (30% above baseline) significantly increased NaV1.2 mRNA expression but reduced NaV1.2 protein level in the retina up to 57% (p < 0.001). Following prolonged intraocular pressure elevation, NaV1.2 protein expression particularly diminished at distal sections of ganglion cell intraretinal axons (p ≤ 0.01). Our results suggest NaV1.2 might be a therapeutic target during disease progression to maintain RGC excitability, preserving presynaptic connections through action potential backpropagation.

TIMP-1 Attenuates the Development of Inflammatory Pain Through MMP-Dependent and Receptor-Mediated Cell Signaling Mechanisms

Unresolved inflammation is a significant predictor for developing chronic pain, and targeting the mechanisms underlying inflammation offers opportunities for therapeutic intervention. During inflammation, matrix metalloproteinase (MMP) activity contributes to tissue remodeling and inflammatory signaling, and is regulated by tissue inhibitors of metalloproteinases (TIMPs). TIMP-1 and -2 have known roles in pain, but only in the context of MMP inhibition. However, TIMP-1 also has receptor-mediated cell signaling functions that are not well understood. Here, we examined how TIMP-1-dependent cell signaling impacts inflammatory hypersensitivity and ongoing pain. We found that hindpaw injection of complete Freund's adjuvant (CFA) increased cutaneous TIMP-1 expression that peaked prior to development of mechanical hypersensitivity, suggesting that TIMP-1 inhibits the development of inflammatory hypersensitivity. To examine this possibility, we injected TIMP-1 knockout (T1KO) mice with CFA and found that T1KO mice exhibited rapid onset thermal and mechanical hypersensitivity at the site of inflammation that was absent or attenuated in WT controls. We also found that T1KO mice exhibited hypersensitivity in adjacent tissues innervated by different sets of afferents, as well as skin contralateral to the site of inflammation. Replacement of recombinant murine (rm)TIMP-1 alleviated hypersensitivity when administered at the site and time of inflammation. Administration of either the MMP inhibiting N-terminal or the cell signaling C-terminal domains recapitulated the antinociceptive effect of full-length rmTIMP-1, suggesting that rmTIMP-1inhibits hypersensitivity through MMP inhibition and receptor-mediated cell signaling. We also found that hypersensitivity was not due to genotype-specific differences in MMP-9 activity or expression, nor to differences in cytokine expression. Administration of rmTIMP-1 prevented mechanical hypersensitivity and ongoing pain in WT mice, collectively suggesting a novel role for TIMP-1 in the attenuation of inflammatory pain.

TIMP-1 Attenuates the Development of Inflammatory Pain Through MMP-Dependent and Receptor-Mediated Cell Signaling Mechanisms

Unresolved inflammation is a significant predictor for developing chronic pain, and targeting the mechanisms underlying inflammation offers opportunities for therapeutic intervention. During inflammation, matrix metalloproteinase (MMP) activity contributes to tissue remodeling and inflammatory signaling, and is regulated by tissue inhibitors of metalloproteinases (TIMPs). TIMP-1 and -2 have known roles in pain, but only in the context of MMP inhibition. However, TIMP-1 also has receptor-mediated cell signaling functions that are not well understood. Here, we examined how TIMP-1-dependent cell signaling impacts inflammatory hypersensitivity and ongoing pain. We found that hindpaw injection of complete Freund’s adjuvant (CFA) increased cutaneous TIMP-1 expression that peaked prior to development of mechanical hypersensitivity, suggesting that TIMP-1 inhibits the development of inflammatory hypersensitivity. To examine this possibility, we injected TIMP-1 knockout (T1KO) mice with CFA and found that T1KO mice exhibited rapid onset thermal and mechanical hypersensitivity at the site of inflammation that was absent or attenuated in WT controls. We also found that T1KO mice exhibited hypersensitivity in adjacent tissues innervated by different sets of afferents, as well as skin contralateral to the site of inflammation. Replacement of recombinant murine (rm)TIMP-1 alleviated hypersensitivity when administered at the site and time of inflammation. Administration of either the MMP inhibiting N-terminal or the cell signaling C-terminal domains recapitulated the antinociceptive effect of full-length rmTIMP-1, suggesting that rmTIMP-1inhibits hypersensitivity through MMP inhibition and receptor-mediated cell signaling. We also found that hypersensitivity was not due to genotype-specific differences in MMP-9 activity or expression, nor to differences in cytokine expression. Administration of rmTIMP-1 prevented mechanical hypersensitivity and ongoing pain in WT mice, collectively suggesting a novel role for TIMP-1 in the attenuation of inflammatory pain.

RNA-Binding Proteins HuB, HuC, and HuD are Distinctly Regulated in Dorsal Root Ganglia Neurons from STZ-Sensitive Compared to STZ-Resistant Diabetic Mice

The neuron-specific Elav-like Hu RNA-binding proteins were described to play an important role in neuronal differentiation and plasticity by ensuring the post-transcriptional control of RNAs encoding for various proteins. Although Elav-like Hu proteins alterations were reported in diabetes or neuropathy, little is known about the regulation of neuron-specific Elav-like Hu RNA-binding proteins in sensory neurons of dorsal root ganglia (DRG) due to the diabetic condition. The goal of our study was to analyze the gene and protein expression of HuB, HuC, and HuD in DRG sensory neurons in diabetes. The diabetic condition was induced in CD-1 adult male mice with single-intraperitoneal injection of streptozotocin (STZ, 150 mg/kg), and 8-weeks (advanced diabetes) after induction was quantified the Elav-like proteins expression. Based on the glycemia values, we identified two types of responses to STZ, and mice were classified in STZ-resistant (diabetic resistant, glycemia < 260 mg/dL) and STZ-sensitive (diabetic, glycemia > 260 mg/dL). Body weight measurements indicated that 8-weeks after STZ-induction of diabetes, control mice have a higher increase in body weight compared to the diabetic and diabetic resistant mice. Moreover, after 8-weeks, diabetic mice (19.52 ± 3.52 s) have longer paw withdrawal latencies in the hot-plate test than diabetic resistant (11.36 ± 1.92 s) and control (11.03 ± 1.97 s) mice, that correlates with the installation of warm hypoalgesia due to the diabetic condition. Further on, we evidenced the decrease of Elav-like gene expression in DRG neurons of diabetic mice (Elavl2, 0.68 ± 0.05 fold; Elavl3, 0.65 ± 0.01 fold; Elavl4, 0.53 ± 0.07 fold) and diabetic resistant mice (Ealvl2, 0.56 ± 0.07 fold; Elavl3, 0.32 ± 0.09 fold) compared to control mice. Interestingly, Elav-like genes have a more accentuated downregulation in diabetic resistant than in diabetic mice, although hypoalgesia was evidenced only in diabetic mice. The Elav-like gene expression changes do not always correlate with the Hu protein expression changes. To detail, HuB is upregulated and HuD is downregulated in diabetic mice, while HuB, HuC, and HuD are downregulated in diabetic resistant mice compared to control mice. To resume, we demonstrated HuD downregulation and HuB upregulation in DRG sensory neurons induced by diabetes, which might be correlated with altered post-transcriptional control of RNAs involved in the regulation of thermal hypoalgesia condition caused by the advanced diabetic neuropathy.

Expression of a novel versican variant in dorsal root ganglia from spared nerve injury rats

The size and modular structure of versican and its gene suggest the existence of multiple splice variants. We have identified, cloned, and sequenced a previously unknown exon located within the noncoding gene sequence downstream of exon 8. This exon, which we have named exon 8β, specifies two stop-codons. mRNAs of the versican gene with exon 8β are predicted to be constitutively degraded by nonsense-mediated RNA decay. Here, we tested the hypothesis that these transcripts become expressed in a model of neuropathic pain.

Mechanisms Underlying Bone and Joint Pain

PURPOSE OF REVIEW

The goal of this review is to provide a broad overview of the current understanding of mechanisms underlying bone and joint pain.

RECENT FINDINGS

Bone or joint pathology is generally accompanied by local release of pro-inflammatory cytokines, growth factors, and neurotransmitters that activate and sensitize sensory nerves resulting in an amplified pain signal. Modulation of the pain signal within the spinal cord and brain that result in net increased facilitation is proposed to contribute to the development of chronic pain. Great strides have been made in our understanding of mechanisms underlying bone and joint pain that will guide development of improved therapeutic options for these patients. Continued research is required for improved understanding of mechanistic differences driving different components of bone and/or joint pain such as movement related pain compared to persistent background pain. Advances will guide development of more individualized and comprehensive therapeutic options.

AMPK activation regulates P-body dynamics in mouse sensory neurons in vitro and in vivo

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AMPK activators increase P-body formation in DRG neurons in vitro.

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Peripheral nerve injury decreases P-bodies in mouse DRG neurons.

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Treatment with metformin restores normal P-body numbers in DRG neurons from nerve injured mice.

Increased mRNA translation in sensory neurons following peripheral nerve injury contributes to the induction and maintenance of chronic neuropathic pain. Metformin, a common anti-diabetic drug and an activator of AMP-activated protein kinase (AMPK), inhibits cap-dependent mRNA translation and reverses mechanical hypersensitivity caused by a neuropathic injury in both mice and rats. P-bodies are RNA granules that comprise sites for metabolizing mRNA through the process of de-capping followed by RNA decay. These RNA granules may also sequester mRNAs for storage. We have previously demonstrated that induction of cap-dependent translation in cultured trigeminal ganglion (TG) neurons decreases P-body formation and AMPK activators increase P-body formation. Here we examined the impact of AMPK activation on protein synthesis and P-body formation in vitro and in vivo on mouse dorsal root ganglion (DRG) neurons. We demonstrate that AMPK activators inhibit nascent protein synthesis and increase P-body formation in DRG neurons. We also demonstrate that mice with a spared-nerve injury (SNI) show decreased P-bodies in the DRG, consistent with increased mRNA translation resulting from injury. Metformin treatment normalizes this effect in SNI mice and increases P-body formation in sham animals. These findings indicate that P-bodies are dynamically regulated by nerve injury in vivo and this effect can be regulated via AMPK activation.

MicroRNA-182 Alleviates Neuropathic Pain by Regulating Nav1.7 Following Spared Nerve Injury in Rats

The sodium channel 1.7 (Nav1.7), which is encoded by SCN9A gene, is involved in neuropathic pain. As crucial regulators of gene expression, many miRNAs have already gained importance in neuropathic pain, including miR-182, which is predicted to regulate the SCN9A gene. Nav1.7 expression in L4-L6 dorsal root ganglions (DRGs) can be up regulated by spared nerve injury (SNI), while miR-182 expression was down regulated following SNI model. Exploring the connection between Nav1.7 and miR-182 may facilitate the development of a better-targeted therapy. In the current study, direct pairing of miR-182 with the SCN9A gene was verified using a luciferase assay in vitro. Over-expression of miR-182 via microinjection of miR-182 agomir reversed the abnormal increase of Nav1.7 at both mRNA and protein level in L4-6 DRGs of SNI rats, and significantly attenuated the hypersensitivity to mechanical stimulus in the rats. In contrast, administration of miR-182 antagomir enhanced the Nav1.7 expression at both mRNA and protein level in L4-6 DRGs, companied with the generation of mechanical hypersensitivity in naïve rats. Collectively, we concluded that miR-182 can alleviate SNI- induced neuropathic pain through regulating Nav1.7 in rats.

Regulating nociceptive transmission by VGluT2-expressing spinal dorsal horn neurons

Vesicular glutamate transporter-2 (VGluT2) mediates the uptake of glutamate into synaptic vesicles in neurons. Spinal cord dorsal horn interneurons are highly heterogeneous and molecularly diverse. The functional significance of VGluT2-expressing dorsal horn neurons in physiological and pathological pain conditions has not been explicitly demonstrated. Designer receptors exclusively activated by designer drugs (DREADDs) are a powerful chemogenetic tool to reversibly control neuronal excitability and behavior. Here, we used transgenic mice with Cre recombinase expression driven by the VGluT2 promoter, combined with the chemogenetic approach, to determine the contribution of VGluT2-expressing dorsal horn neurons to nociceptive regulation. Adeno-associated viral vectors expressing double-floxed Cre-dependent Gαq-coupled human M3 muscarinic receptor DREADD (hM3D)-mCherry or Gαi-coupled κ-opioid receptor DREADD (KORD)-IRES-mCitrine were microinjected into the superficial spinal dorsal horn of VGluT2-Cre mice. Immunofluorescence labeling showed that VGluT2 was predominantly expressed in lamina II excitatory interneurons. Activation of excitatory hM3D in VGluT2-expressing neurons with clozapine N-oxide caused a profound increase in neuronal firing and synaptic glutamate release. Conversely, activation of inhibitory KORD in VGluT2-expressing neurons with salvinorin B markedly inhibited neuronal activity and synaptic glutamate release. In addition, chemogenetic stimulation of VGluT2-expressing neurons increased mechanical and thermal sensitivities in naive mice, whereas chemogenetic silencing of VGluT2-expressing neurons reversed pain hypersensitivity induced by tissue inflammation and peripheral nerve injury. These findings indicate that VGluT2-expressing excitatory neurons play a crucial role in mediating nociceptive transmission in the spinal dorsal horn. Targeting glutamatergic dorsal horn neurons with inhibitory DREADDs may be a new strategy for treating inflammatory pain and neuropathic pain.

Increased expression of CaV3.2 T-type calcium channels in damaged DRG neurons contributes to neuropathic pain in rats with spared nerve injury

Ion channels are very important in the peripheral sensitization in neuropathic pain. Our present study aims to investigate the possible contribution of Ca_V3.2 T-type calcium channels in damaged dorsal root ganglion neurons in neuropathic pain. We established a neuropathic pain model of rats with spared nerve injury. In these model rats, it was easy to distinguish damaged dorsal root ganglion neurons (of tibial nerve and common peroneal nerve) from intact dorsal root ganglion neurons (of sural nerves). Our results showed that Ca_V3.2 protein expression increased in medium-sized neurons from the damaged dorsal root ganglions but not in the intact ones. With whole cell patch clamp recording technique, it was found that after-depolarizing amplitudes of the damaged medium-sized dorsal root ganglion neurons increased significantly at membrane potentials of −85 mV and −95 mV. These results indicate a functional up-regulation of Ca_V3.2 T-type calcium channels in the damaged medium-sized neurons after spared nerve injury. Behaviorally, blockade of Ca_V3.2 with antisense oligodeoxynucleotides could significantly reverse mechanical allodynia. These results suggest that Ca_V3.2 T-type calcium channels in damaged medium-sized dorsal root ganglion neurons might contribute to neuropathic pain after peripheral nerve injury.

RE1-silencing transcription factor controls the acute-to-chronic neuropathic pain transition and Chrm2 receptor gene expression in primary sensory neurons

Neuropathic pain is associated with persistent changes in gene expression in primary sensory neurons, but the underlying epigenetic mechanisms that cause these changes remain unclear. The muscarinic cholinergic receptors (mAChRs), particularly the M2 subtype (encoded by the cholinergic receptor muscarinic 2 (Chrm2) gene), are critically involved in the regulation of spinal nociceptive transmission. However, little is known about how Chrm2 expression is transcriptionally regulated. Here we show that nerve injury persistently increased the expression of RE1-silencing transcription factor (REST, also known as neuron-restrictive silencing factor [NRSF]), a gene-silencing transcription factor, in the dorsal root ganglion (DRG). Remarkably, nerve injury-induced chronic but not acute pain hypersensitivity was attenuated in mice with Rest knockout in DRG neurons. Also, siRNA-mediated Rest knockdown reversed nerve injury-induced chronic pain hypersensitivity in rats. Nerve injury persistently reduced Chrm2 expression in the DRG and diminished the analgesic effect of muscarine. The RE1 binding site on the Chrm2 promoter is required for REST-mediated Chrm2 repression, and nerve injury increased the enrichment of REST in the Chrm2 promoter in the DRG. Furthermore, Rest knockdown or genetic ablation in DRG neurons normalized Chrm2 expression and augmented muscarine's analgesic effect on neuropathic pain and fully reversed the nerve injury-induced reduction in the inhibitory effect of muscarine on glutamatergic input to spinal dorsal horn neurons. Our findings indicate that nerve injury-induced REST up-regulation in DRG neurons plays an important role in the acute-to-chronic pain transition and is essential for the transcriptional repression of Chrm2 in neuropathic pain.

Heme oxygenase-1 inducer and carbon monoxide-releasing molecule enhance the effects of gabapentinoids by modulating glial activation during neuropathic pain in mice

Introduction

Neuropathic pain is one of the most difficult-to-treat symptoms. Although gabapentinoids are classified as first-line drugs, they have only modest efficacy.

Objectives

The aim of this study was to investigate whether treatment with the heme oxygenase-1 (HO-1) inducer cobalt protoporphyrin IX (CoPP) or the carbon monoxide-releasing molecule tricarbonyldichlororuthenium (II) dimer (CORM-2) can enhance the antinociceptive effects produced by gabapentinoids in mice with neuropathic pain.

Methods

Neuropathic pain was induced by spared nerve injury (SNI) of the sciatic nerve. The mechanical threshold was tested using von Frey filaments. The expression of spinal HO-1, HO-2, the Ca²⁺ channel α₂δ₁ subunit, microglial markers, and M1 or M2 microglial markers was examined using reverse transcription polymerase chain reaction.

Results

Treatment with CoPP or CORM-2 alleviated mechanical allodynia induced by SNI. CoPP or CORM-2 enhanced the antiallodynic effects of gabapentinoids (pregabalin or gabapentin) during SNI-induced mechanical allodynia. HO-1 inhibitor tin protoporphyrin IX (SnPP) prevented the antiallodynic effects of gabapentinoids (pregabalin or gabapentin) during SNI-induced mechanical allodynia. CoPP or CORM-2 increased HO-1 and Ca²⁺ channel α₂δ₁ subunit gene expression and the decreased gene expression of microglial markers, M1 microglial marker, or tumor necrosis factor in the ipsilateral spinal dorsal horn of mice with SNI. SnPP prevented HO-1 induction and glial inhibition, which were produced by gabapentinoids during SNI-induced mechanical allodynia.

Conclusions

This study suggests that HO-1 plays crucial roles in the antiallodynic effects of gabapentinoids. Gabapentinoids attenuate the glial activation induced by SNI and some of these effects are mediated by HO-1.

Analysis of the distribution of spinal NOP receptors in a chronic pain model using NOP-eGFP knock-in mice

BACKGROUND AND PURPOSE

The nociceptin/orphanin FQ opioid peptide (NOP) receptor system plays a significant role in the regulation of pain. This system functions differently in the spinal cord and brain. The mechanism by which the NOP receptor agonists regulate pain transmission in these regions is not clearly understood. Here, we investigate the peripheral and spinal NOP receptor distribution and antinociceptive effects of intrathecal nociceptin/orphanin FQ (N/OFQ) in chronic neuropathic pain.

EXPERIMENTAL APPROACH

We used immunohistochemistry to determine changes in NOP receptor distribution triggered by spinal nerve ligation (SNL) using NOP-eGFP knock-in mice. Antinociceptive effects of intrathecal N/OFQ on SNL-mediated allodynia and heat/cold hyperalgesia were assessed in wild-type mice.

KEY RESULTS

NOP-eGFP immunoreactivity was decreased by SNL in the spinal laminae I and II outer, regions that mediate noxious heat stimuli. In contrast, immunoreactivity of NOP-eGFP was unchanged in the ventral border of lamina II inner, which is an important region for the development of allodynia. NOP-eGFP expression was also decreased in a large number of primary afferents in the L4 dorsal root ganglion (DRG) of SNL mice. However, SNL mice showed increased sensitivity, compared to sham animals to the effects of i.t administered N/OFQ with respect to mechanical as well as thermal stimuli.

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that the spinal NOP receptor system attenuates injury-induced hyperalgesia by direct inhibition of the projection neurons in the spinal cord that send nociceptive signals to the brain and not by inhibiting presynaptic terminals of DRG neurons in the superficial lamina.

Downregulation of adult and neonatal Nav1.5 in the dorsal root ganglia and axon of peripheral sensory neurons of rats with spared nerve injury

Previous studies demonstrated that Nav1.5 splice variants, including Nav1.5a and Nav1.5c, were expressed in dorsal root ganglia (DRG) neurons. However, since nine Nav1.5 isoforms have been identified, whether other Nav1.5 splice variants, especially the neonatal Nav1.5 splice variant, express in the DRG neurons is still unknown. In this study, we systematically investigated the expression of adult and neonatal Nav1.5 isoforms in the DRG neurons and axon of peripheral sensory neurons of rats with spared nerve injury (SNI) by RT-PCR, DNA sequencing, restriction enzyme digestion, immunohistochemistry and immunofluorescence methods. The results demonstrated that both adult and neonatal Nav1.5 isoforms were expressed in the DRG neurons, but their expression ratio was ~2.5:1. In SNI rat models, the expression of both adult and neonatal Nav1.5 decreased by approximately a half in both mRNA and protein levels. In contrast, the expression of protein kinase C (PKC)-γ, one of the negative modulators for sodium currents, increased by ~1-fold. Taken together, this study first confirmed the expression of both adult and neonatal Nav1.5 isoforms in the DRG neurons and axon of peripheral sensory neurons of rat, but their expression level decreased in pain models. The upregulation of PKC-γ may directly or indirectly downregulate the expression of Nav1.5 isoforms in SNI rat models, which may further involve in the pathological process of neuropathic pain.

Voltage-gated Na+ currents in human dorsal root ganglion neurons

Available evidence indicates voltage-gated Na⁺ channels (VGSCs) in peripheral sensory neurons are essential for the pain and hypersensitivity associated with tissue injury. However, our understanding of the biophysical and pharmacological properties of the channels in sensory neurons is largely based on the study of heterologous systems or rodent tissue, despite evidence that both expression systems and species differences influence these properties. Therefore, we sought to determine the extent to which the biophysical and pharmacological properties of VGSCs were comparable in rat and human sensory neurons. Whole cell patch clamp techniques were used to study Na⁺ currents in acutely dissociated neurons from human and rat. Our results indicate that while the two major current types, generally referred to as tetrodotoxin (TTX)-sensitive and TTX-resistant were qualitatively similar in neurons from rats and humans, there were several differences that have important implications for drug development as well as our understanding of pain mechanisms.

DOI:http://dx.doi.org/10.7554/eLife.23235.001

Expression and Role of Voltage-Gated Sodium Channels in Human Dorsal Root Ganglion Neurons with Special Focus on Nav1.7, Species Differences, and Regulation by Paclitaxel

Voltage-gated sodium channels (Navs) play an important role in human pain sensation. However, the expression and role of Nav subtypes in native human sensory neurons are unclear. To address this issue, we obtained human dorsal root ganglion (hDRG) tissues from healthy donors. PCR analysis of seven DRG-expressed Nav subtypes revealed that the hDRG has higher expression of Nav1.7 (~50% of total Nav expression) and lower expression of Nav1.8 (~12%), whereas the mouse DRG has higher expression of Nav1.8 (~45%) and lower expression of Nav1.7 (~18%). To mimic Nav regulation in chronic pain, we treated hDRG neurons in primary cultures with paclitaxel (0.1–1 μmol/L) for 24 h. Paclitaxel increased the Nav1.7 but not Nav1.8 expression and also increased the transient Na⁺ currents and action potential firing frequency in small-diameter (<50 μm) hDRG neurons. Thus, the hDRG provides a translational model in which to study “human pain in a dish” and test new pain therapeutics.

Impact of gabapentin on neuronal high voltage-activated Ca2+ channel properties of injured-side axotomized and adjacent uninjured dorsal root ganglions in a rat model of spinal nerve ligation

The density and properties of ion channels in the injured axon and dorsal root ganglion (DRG) neuronal soma membrane change following nerve injury, which may result in the development of neuropathic pain. Gabapentin (GBP) is a drug for the first-line treatment of neuropathic pain. One of its therapeutic targets is the voltage-activated calcium channel (VACC). In the present study, the whole-cell patch clamp technique was used to examine the changes of high voltage-activated Ca²⁺ (HVA-Ca²⁺) channels in DRG neurons from sham and neuropathic rats in the absence and presence of GBP. The results demonstrated a reduction in peak current density and the ‘window current’ between activation and inactivation in adjacent and axotomized neurons from rats that had undergone L₅ spinal nerve ligation, thus attenuating the total inward Ca²⁺ current. Following the use of the specific channel blockers nifedipine, ω-conotoxin MVIIC and ω-conotoxin MVIIA, increased HVA-Ca²⁺ channels as well as an increased proportion of N-type Ca²⁺ currents were observed in axotomized neurons. GBP inhibited HVA calcium channel currents in a dose-dependent manner. The activation and steady-state inactivation curves for HVA channels were shifted in a hyperpolarizing direction by 100 µmol/l GBP. Following the application of GBP, a reduction in the ‘window current’ was observed in control and axotomized neurons, whereas the ‘window current’ was unchanged in adjacent neurons. This indicates that the inhibitory effects of GBP may be dependent on particular neuropathological or inflammatory conditions. The proportion of N-type Ca²⁺ currents and sensitivity to GBP were increased in axotomized neurons, which indicated the involvement of N-type Ca²⁺ currents in the inhibitory effect of GBP.

Posttranslational Modification of Sodium Channels

Voltage-gated sodium channels (VGSCs) are critical determinants of excitability. The properties of VGSCs are thought to be tightly controlled. However, VGSCs are also subjected to extensive modifications. Multiple posttranslational modifications that covalently modify VGSCs in neurons and muscle have been identified. These include, but are not limited to, phosphorylation, ubiquitination, palmitoylation, nitrosylation, glycosylation, and SUMOylation. Posttranslational modifications of VGSCs can have profound impact on cellular excitability, contributing to normal and abnormal physiology. Despite four decades of research, the complexity of VGSC modulation is still being determined. While some modifications have similar effects on the various VGSC isoforms, others have isoform-specific interactions. In addition, while much has been learned about how individual modifications can impact VGSC function, there is still more to be learned about how different modifications can interact. Here we review what is known about VGSC posttranslational modifications with a focus on the breadth and complexity of the regulatory mechanisms that impact VGSC properties.

Gene expression profile of sodium channel subunits in the anterior cingulate cortex during experimental paclitaxel-induced neuropathic pain in mice

Paclitaxel, a chemotherapeutic agent, causes neuropathic pain whose supraspinal pathophysiology is not fully understood. Dysregulation of sodium channel expression, studied mainly in the periphery and spinal cord level, contributes to the pathogenesis of neuropathic pain. We examined gene expression of sodium channel (Na_v) subunits by real time polymerase chain reaction (PCR) in the anterior cingulate cortex (ACC) at day 7 post first administration of paclitaxel, when mice had developed paclitaxel-induced thermal hyperalgesia. The ACC was chosen because increased activity in the ACC has been observed during neuropathic pain. In the ACC of vehicle-treated animals the threshold cycle (Ct) values for Na_v1.4, Na_v1.5, Na_v1.7, Na_v1.8 and Na_v1.9 were above 30 and/or not detectable in some samples. Thus, comparison in mRNA expression between untreated control, vehicle-treated and paclitaxel treated animals was done for Na_v1.1, Na_v1.2, Na_v1.3, Na_v1.6, Na_x as well as Na_vβ1–Na_vβ4. There were no differences in the transcript levels of Na_v1.1–Na_v1.3, Na_v1.6, Na_x, Na_vβ1–Na_vβ3 between untreated and vehicle-treated mice, however, vehicle treatment increased Na_vβ4 expression. Paclitaxel treatment significantly increased the mRNA expression of Na_v1.1, Na_v1.2, Na_v1.6 and Na_x, but not Na_v1.3, sodium channel alpha subunits compared to vehicle-treated animals. Treatment with paclitaxel significantly increased the expression of Na_vβ1 and Na_vβ3, but not Na_vβ2 and Na_vβ4, sodium channel beta subunits compared to vehicle-treated animals. These findings suggest that during paclitaxel-induced neuropathic pain (PINP) there is differential upregulation of sodium channels in the ACC, which might contribute to the increased neuronal activity observed in the area during neuropathic pain.

MicroRNA-30b regulates expression of the sodium channel Nav1.7 in nerve injury-induced neuropathic pain in the rat

Voltage-gated sodium channels, which are involved in pain pathways, have emerged as major targets for therapeutic intervention in pain disorders. Nav1.7, the tetrodotoxin-sensitive voltage-gated sodium channel isoform encoded by SCN9A and predominantly expressed in pain-sensing neurons in the dorsal root ganglion, plays a crucial role in nociception. MicroRNAs are highly conserved, small non-coding RNAs. Through binding to the 3′ untranslated region of their target mRNAs, microRNAs induce the cleavage and/or inhibition of protein translation. Based on bioinformatics analysis using TargetScan software, we determined that miR-30b directly targets SCN9A. To investigate the roles of Nav1.7 and miR-30b in neuropathic pain, we examined changes in the expression of Nav1.7 in the dorsal root ganglion by miR-30b over-expression or knockdown in rats with spared nerve injury. Our results demonstrated that the expression of miR-30b and Nav1.7 was down-regulated and up-regulated, respectively, in the dorsal root ganglion of spared nerve injury rats. MiR-30b over-expression in spared nerve injury rats inhibited SCN9A transcription, resulting in pain relief. In addition, miR-30b knockdown significantly increased hypersensitivity to pain in naive rats. We also observed that miR-30b decreased Nav1.7 expression in PC12 cells. Taken together, our results suggest that miR-30b plays an important role in neuropathic pain by regulating Nav1.7 expression. Therefore, miR-30b may be a promising target for the treatment of chronic neuropathic pain.

Standardized Profiling of The Membrane-Enriched Proteome of Mouse Dorsal Root Ganglia (DRG) Provides Novel Insights Into Chronic Pain

Chronic pain is a complex disease with limited treatment options. Several profiling efforts have been employed with the aim to dissect its molecular underpinnings. However, generated results are often inconsistent and nonoverlapping, which is largely because of inherent technical constraints. Emerging data-independent acquisition (DIA)-mass spectrometry (MS) has the potential to provide unbiased, reproducible and quantitative proteome maps - a prerequisite for standardization among experiments. Here, we designed a DIA-based proteomics workflow to profile changes in the abundance of dorsal root ganglia (DRG) proteins in two mouse models of chronic pain, inflammatory and neuropathic. We generated a DRG-specific spectral library containing 3067 DRG proteins, which enables their standardized quantification by means of DIA-MS in any laboratory. Using this resource, we profiled 2526 DRG proteins in each biological replicate of both chronic pain models and respective controls with unprecedented reproducibility. We detected numerous differentially regulated proteins, the majority of which exhibited pain model-specificity. Our approach recapitulates known biology and discovers dozens of proteins that have not been characterized in the somatosensory system before. Functional validation experiments and analysis of mouse pain behaviors demonstrate that indeed meaningful protein alterations were discovered. These results illustrate how the application of DIA-MS can open new avenues to achieve the long-awaited standardization in the molecular dissection of pathologies of the somatosensory system. Therefore, our findings provide a valuable framework to qualitatively extend our understanding of chronic pain and somatosensation.

Identification of Amino Acid Residues in Fibroblast Growth Factor 14 (FGF14) Required for Structure-Function Interactions with Voltage-gated Sodium Channel Nav1.6

The voltage-gated Na(+) (Nav) channel provides the basis for electrical excitability in the brain. This channel is regulated by a number of accessory proteins including fibroblast growth factor 14 (FGF14), a member of the intracellular FGF family. In addition to forming homodimers, FGF14 binds directly to the Nav1.6 channel C-tail, regulating channel gating and expression, properties that are required for intrinsic excitability in neurons. Seeking amino acid residues with unique roles at the protein-protein interaction interface (PPI) of FGF14·Nav1.6, we engineered model-guided mutations of FGF14 and validated their impact on the FGF14·Nav1.6 complex and the FGF14:FGF14 dimer formation using a luciferase assay. Divergence was found in the β-9 sheet of FGF14 where an alanine (Ala) mutation of Val-160 impaired binding to Nav1.6 but had no effect on FGF14:FGF14 dimer formation. Additional analysis revealed also a key role of residues Lys-74/Ile-76 at the N-terminal of FGF14 in the FGF14·Nav1.6 complex and FGF14:FGF14 dimer formation. Using whole-cell patch clamp electrophysiology, we demonstrated that either the FGF14(V160A) or the FGF14(K74A/I76A) mutation was sufficient to abolish the FGF14-dependent regulation of peak transient Na(+) currents and the voltage-dependent activation and steady-state inactivation of Nav1.6; but only V160A with a concomitant alanine mutation at Tyr-158 could impede FGF14-dependent modulation of the channel fast inactivation. Intrinsic fluorescence spectroscopy of purified proteins confirmed a stronger binding reduction of FGF14(V160A) to the Nav1.6 C-tail compared with FGF14(K74A/I76A) Altogether these studies indicate that the β-9 sheet and the N terminus of FGF14 are well positioned targets for drug development of PPI-based allosteric modulators of Nav channels.