RNA profiling of human dorsal root ganglia reveals sex differences in mechanisms promoting neuropathic pain

Pathology of pain and its implications for therapeutic interventions

Pain is estimated to affect more than 20% of the global population, imposing incalculable health and economic burdens. Effective pain management is crucial for individuals suffering from pain. However, the current methods for pain assessment and treatment fall short of clinical needs. Benefiting from advances in neuroscience and biotechnology, the neuronal circuits and molecular mechanisms critically involved in pain modulation have been elucidated. These research achievements have incited progress in identifying new diagnostic and therapeutic targets. In this review, we first introduce fundamental knowledge about pain, setting the stage for the subsequent contents. The review next delves into the molecular mechanisms underlying pain disorders, including gene mutation, epigenetic modification, posttranslational modification, inflammasome, signaling pathways and microbiota. To better present a comprehensive view of pain research, two prominent issues, sexual dimorphism and pain comorbidities, are discussed in detail based on current findings. The status quo of pain evaluation and manipulation is summarized. A series of improved and innovative pain management strategies, such as gene therapy, monoclonal antibody, brain-computer interface and microbial intervention, are making strides towards clinical application. We highlight existing limitations and future directions for enhancing the quality of preclinical and clinical research. Efforts to decipher the complexities of pain pathology will be instrumental in translating scientific discoveries into clinical practice, thereby improving pain management from bench to bedside.

Sex differences in mechanisms of pain hypersensitivity

The introduction of sex-as-a-biological-variable policies at funding agencies around the world has led to an explosion of very recent observations of sex differences in the biology underlying pain. This review considers evidence of sexually dimorphic mechanisms mediating pain hypersensitivity, derived from modern assays of persistent pain in rodent animal models. Three well-studied findings are described in detail: the male-specific role of spinal cord microglia, the female-specific role of calcitonin gene-related peptide (CGRP), and the female-specific role of prolactin and its receptor. Other findings of sex-specific molecular involvement in pain are subjected to pathway analyses and reveal at least one novel hypothesis: that females may preferentially use Th1 and males Th2 T cell activity to mediate chronic pain.

Behavioral characterization of G-protein-coupled receptor 160 knockout mice

ABSTRACT

Neuropathic pain is a devastating condition where current therapeutics offer little to no pain relief. Novel nonnarcotic therapeutic targets are needed to address this growing medical problem. Our work identified the G-protein-coupled receptor 160 (GPR160) as a potential target for therapeutic intervention. However, the lack of small-molecule ligands for GPR160 hampers our understanding of its role in health and disease. To address this void, we generated a global Gpr160 knockout (KO) mouse using CRISPR-Cas9 genome editing technology to validate the contributions of GPR160 in nociceptive behaviors in mice. Gpr160 KO mice are healthy and fertile, with no observable physical abnormalities. Gpr160 KO mice fail to develop behavioral hypersensitivities in a model of neuropathic pain caused by constriction of the sciatic nerve. On the other hand, responses of Gpr160 KO mice in the hot-plate and tail-flick assays are not affected. We recently deorphanized GPR160 and identified cocaine- and amphetamine-regulated transcript peptide (CARTp) as a potential ligand. Using Gpr160 KO mice, we now report that the development of behavioral hypersensitivities after intrathecal or intraplantar injections of CARTp are dependent on GPR160. Cocaine- and amphetamine-regulated transcript peptide plays a role in various affective behaviors, such as anxiety, depression, and cognition. There are no differences in learning, memory, and anxiety between Gpr160 KO mice and their age-matched and sex-matched control floxed mice. Results from these studies support the pronociceptive roles of CARTp/GPR160 and GPR160 as a potential therapeutic target for treatment of neuropathic pain.

Electrophysiological Analyses of Human Dorsal Root Ganglia and Human Induced Pluripotent Stem Cell-derived Sensory Neurons From Male and Female Donors

Human induced pluripotent stem cell-derived sensory neurons (hiPSC-SNs) and human dorsal root ganglia neurons (hDRG-N) are popular tools in the field of pain research; however, few groups make use of both approaches. For screening and analgesic validation purposes, important characterizations can be determined of the similarities and differences between hDRG-N and hiPSC-SNs. This study focuses specifically on the electrophysiology properties of hDRG-N in comparison to hiPSC-SNs. We also compared hDRG-N and hiPSC-SNs from both male and female donors to evaluate potential sex differences. We recorded neuronal size, rheobase, resting membrane potential, input resistance, and action potential waveform properties from 83 hiPSCs-SNs (2 donors) and 108 hDRG-N neurons (8 donors). We observed several statistically significant electrophysiological differences between hDRG-N and hiPSC-SNs, such as size, rheobase, input resistance, and several action potential waveform properties. Correlation analysis also revealed many properties that were positively or negatively correlated, some of which were differentially correlated between hDRG-N and hiPSC-SNs. This study shows several differences between hDRG-N and hiPSC-SNs and allows a better understanding of the advantages and disadvantages of both for use in pain research. We hope this study will be a valuable resource for pain researchers considering the use of these human in vitro systems for mechanistic studies and/or drug development projects. PERSPECTIVE: hiPSC-SNs and hDRG-N are popular tools in the field of pain research. This study allows for a better functional understanding of the pros and cons of both tools.

RNA isoform expression landscape of the human dorsal root ganglion generated from long-read sequencing

ABSTRACT

Splicing is a posttranscriptional RNA processing mechanism that enhances genomic complexity by creating multiple isoforms from the same gene. We aimed to characterize the isoforms expressed in the human peripheral nervous system, with the goal of creating a resource to identify novel isoforms of functionally relevant genes associated with somatosensation and nociception. We used long-read sequencing to document isoform expression in the human dorsal root ganglia from 3 organ donors and validated in silico by confirming expression in short-read sequencing from 3 independent organ donors. Nineteen thousand five hundred forty-seven isoforms of protein-coding genes were detected and validated. We identified 763 isoforms with at least one previously undescribed splice junction. Previously unannotated isoforms of multiple pain-associated genes, including ASIC3, MRGPRX1, and HNRNPK, were identified. In the novel isoforms of ASIC3, a region comprising approximately 35% of the 5'UTR was excised. By contrast, a novel splice junction was used in isoforms of MRGPRX1 to include an additional exon upstream of the start codon, consequently adding a region to the 5'UTR. Novel isoforms of HNRNPK were identified, which used previously unannotated splice sites to both excise exon 14 and include a sequence in the 3' end of exon 13. This novel insertion is predicted to introduce a tyrosine phosphorylation site potentially phosphorylated by SRC. We also independently confirm a recently reported DRG-specific splicing event in WNK1 that gives insight into how painless peripheral neuropathy occurs when this gene is mutated. Our findings give a clear overview of mRNA isoform diversity in the human dorsal root ganglia obtained using long-read sequencing.

Sex Differences in Visceral Pain and Comorbidities: Clinical Outcomes, Preclinical Models, and Cellular and Molecular Mechanisms

Sexual dimorphism of visceral pain has been documented in clinics and experimental animal models. Aside from hormones, emerging evidence suggests the sex-differential intrinsic neural regulation of pain generation and maintenance. According to the International Association for the Study of Pain (IASP) and the American College of Gastroenterology (ACG), up to 25% of the population have visceral pain at any one time, and in the United States 10-15 percent of adults suffer from irritable bowel syndrome (IBS). Here we examine the preclinical and clinical evidence of sex differences in visceral pain focusing on IBS, other forms of bowel dysfunction and IBS-associated comorbidities. We summarize preclinical animal models that provide a means to investigate the underlying molecular mechanisms in the sexual dimorphism of visceral pain. Neurons and nonneuronal cells (glia and immune cells) in the peripheral and central nervous systems, and the communication of gut microbiota and neural systems all contribute to sex-dependent nociception and nociplasticity in visceral painful signal processing. Emotion is another factor in pain perception and appears to have sexual dimorphism.

Immune drivers of physiological and pathological pain

Physiological pain serves as a warning of exposure to danger and prompts us to withdraw from noxious stimuli to prevent tissue damage. Pain can also alert us of an infection or organ dysfunction and aids in locating such malfunction. However, there are instances where pain is purely pathological, such as unresolved pain following an inflammation or injury to the nervous system, and this can be debilitating and persistent. We now appreciate that immune cells are integral to both physiological and pathological pain, and that pain, in consequence, is not strictly a neuronal phenomenon. Here, we discuss recent findings on how immune cells in the skin, nerve, dorsal root ganglia, and spinal cord interact with somatosensory neurons to mediate pain. We also discuss how both innate and adaptive immune cells, by releasing various ligands and mediators, contribute to the initiation, modulation, persistence, or resolution of various modalities of pain. Finally, we propose that the neuroimmune axis is an attractive target for pain treatment, but the challenges in objectively quantifying pain preclinically, variable sex differences in pain presentation, as well as adverse outcomes associated with immune system modulation, all need to be considered in the development of immunotherapies against pain.

Female-selective mechanisms promoting migraine

Sexual dimorphism has been revealed for many neurological disorders including chronic pain. Prelicinal studies and post-mortem analyses from male and female human donors reveal sexual dimorphism of nociceptors at transcript, protein and functional levels suggesting different mechanisms that may promote pain in men and women. Migraine is a common female-prevalent neurological disorder that is characterized by painful and debilitating headache. Prolactin is a neurohormone that circulates at higher levels in females and that has been implicated clinically in migraine. Prolactin sensitizes sensory neurons from female mice, non-human primates and humans revealing a female-selective pain mechanism that is conserved evolutionarily and likely translationally relevant. Prolactin produces female-selective migraine-like pain behaviors in rodents and enhances the release of calcitonin gene-related peptide (CGRP), a neurotransmitter that is causal in promoting migraine in many patients. CGRP, like prolactin, produces female-selective migraine-like pain behaviors. Consistent with these observations, publicly available clinical data indicate that small molecule CGRP-receptor antagonists are preferentially effective in treatment of acute migraine therapy in women. Collectively, these observations support the conclusion of qualitative sex differences promoting migraine pain providing the opportunity to tailor therapies based on patient sex for improved outcomes. Additionally, patient sex should be considered in design of clinical trials for migraine as well as for pain and reassessment of past trials may be warranted.

Transcriptome Analysis of Rheumatoid Arthritis Uncovers Genes Linked to Inflammation-Induced Pain

Autoimmune diseases such as rheumatoid arthritis (RA) can promote states of chronic Inflammation with accompanying tissue destruction and pain. RA can cause inflammatory synovitis in peripheral joints, particularly within the hands and feet, but can also sometimes trigger temporomandibular joint (TMJ) arthralgia. To better understand the effects of ongoing Inflammation-induced pain signaling, dorsal root ganglia (DRGs) were acquired from individuals with RA for transcriptomic study. We conducted RNA sequencing from the L5 DRGs because it contains the soma of the sensory neurons that innervate the affected joints in the foot. DRGs from 5 RA patients were compared with 9 non-arthritic controls. RNA-seq of L5 DRGs identified 128 differentially expressed genes (DEGs) that were dysregulated in the RA subjects as compared to the non-arthritic controls. The DRG resides outside the blood brain barrier and, as such, our initial transcriptome analysis detected signs of an autoimmune disorder including the upregulated expression of immunoglobulins and other immunologically related genes within the DRGs of the RA donors. Additionally, we saw the upregulation in genes implicated in neurogenesis that could promote pain hypersensitivity. overall, our DRG analysis suggests that there are upregulated inflammatory and pain signaling pathways that can contribute to chronic pain in RA.

Multi-ancestry Whole-exome Sequencing Study of Alcohol Use Disorder in Two Cohorts

Alcohol use disorder (AUD) is a leading cause of death and disability worldwide. There has been substantial progress in identifying genetic variants underlying AUD. However, there are few whole-exome sequencing (WES) studies of AUD. We analyzed WES of 4,530 samples from the Yale-Penn cohort and 469,835 samples from the UK Biobank (UKB). After quality control, 1,420 AUD cases and 619 controls of European ancestry (EUR) and 1,142 cases and 608 controls of African ancestry (AFR) from Yale-Penn were retained for subsequent analyses. WES data from 415,617 EUR samples (12,861 cases), 6,142 AFR samples (130 cases) and 4,607 South Asian (SAS) samples (130 cases) from UKB were also analyzed. Single-variant association analysis identified the well-known functional variant rs1229984 in ADH1B ( P =4.88×10 -31 ) and several other common variants in ADH1C . Gene-based tests identified ADH1B ( P =1.00×10 -31 ), ADH1C ( P =5.23×10 -7 ), CNST ( P =1.19×10 -6 ), and IFIT5 (3.74×10 -6 ). This study extends our understanding of the genetic basis of AUD.

BDNF in Neuropathic Pain; the Culprit that Cannot be Apprehended

In males but not in females, brain derived neurotrophic factor (BDNF) plays an obligatory role in the onset and maintenance of neuropathic pain. Afferent terminals of injured peripheral nerves release colony stimulating factor (CSF-1) and other mediators into the dorsal horn. These transform the phenotype of dorsal horn microglia such that they express P2X4 purinoceptors. Activation of these receptors by neuron-derived ATP promotes BDNF release. This microglial-derived BDNF increases synaptic activation of excitatory dorsal horn neurons and decreases that of inhibitory neurons. It also alters the neuronal chloride gradient such the normal inhibitory effect of GABA is converted to excitation. By as yet undefined processes, this attenuated inhibition increases NMDA receptor function. BDNF also promotes the release of pro-inflammatory cytokines from astrocytes. All of these actions culminate in the increase dorsal horn excitability that underlies many forms of neuropathic pain. Peripheral nerve injury also alters excitability of structures in the thalamus, cortex and mesolimbic system that are responsible for pain perception and for the generation of co-morbidities such as anxiety and depression. The weight of evidence from male rodents suggests that this preferential modulation of excitably of supra-spinal pain processing structures also involves the action of microglial-derived BDNF. Possible mechanisms promoting the preferential release of BDNF in pain signaling structures are discussed. In females, invading T-lymphocytes increase dorsal horn excitability but it remains to be determined whether similar processes operate in supra-spinal structures. Despite its ubiquitous role in pain aetiology neither BDNF nor TrkB receptors represent potential therapeutic targets.

Epigenomic landscape of the human dorsal root ganglion: sex differences and transcriptional regulation of nociceptive genes

Gene expression is influenced by chromatin architecture via controlled access of regulatory factors to DNA. To better understand regulation of gene expression in the human dorsal root ganglion (hDRG) we used bulk and spatial transposase-accessible chromatin technology followed by sequencing (ATAC-seq). We detected a total of 3005 differentially accessible chromatin regions (DARs) between sexes using bulk ATAC-seq. DARs in female hDRG mapped mainly to the X chromosome. In males, DARs were found in autosomal genes. We also found differential transcription factor binding motifs within DARs. EGR1/3 and SP1/4 were abundant in females, and JUN, FOS and other AP-1 family members in males. With the aim of dissecting the open chromatin profile in hDRG neurons, we used spatial ATAC-seq. Consistent with our bulk ATAC-seq data, most of the DARs in female hDRG were located in X chromosome genes. Neuron cluster showed higher chromatin accessibility in GABAergic, glutamatergic, and interferon-related genes in females, and in Ca2+-signaling-related genes in males. Sex differences in open chromatin transcription factor binding sites in neuron-proximal barcodes were consistent with the bulk data, having EGR1 transcription factor activity in females and AP-1 family members in males. Accordingly, we showed higher expression of EGR1 in female hDRG compared to male with in-situ hybridization. Our findings point to epigenomic sex differences in the hDRG that likely underlie divergent transcriptional responses that determine mechanistic sex differences in pain.

Mechanisms of pain in aging and age-related conditions: Focus on caregivers

Pain is a complex, subjective experience that can significantly impact quality of life, particularly in aging individuals, by adversely affecting physical and emotional well-being. Whereas acute pain usually serves a protective function, chronic pain is a persistent pathological condition that contributes to functional deficits, cognitive decline, and emotional disturbances in the elderly. Despite substantial progress that has been made in characterizing age-related changes in pain, complete mechanistic details of pain processing mechanisms in the aging patient remain unknown. Pain is particularly under-recognized and under-managed in the elderly, especially among patients with Alzheimer's disease (AD), Alzheimer's disease-related dementias (ADRD), and other age-related conditions. Furthermore, difficulties in assessing pain in patients with AD/ADRD and other age-related conditions may contribute to the familial caregiver burden. The purpose of this article is to discuss the mechanisms and risk factors for chronic pain development and persistence, with a particular focus on age-related changes. Our article also highlights the importance of caregivers working with aging chronic pain patients, and emphasizes the urgent need for increased legislative awareness and improved pain management in these populations to substantially alleviate caregiver burden.

SPOCK2 modulates neuropathic pain by interacting with MT1-MMP to regulate astrocytic MMP-2 activation in rats with chronic constriction injury

BACKGROUND

Neuropathic pain (NP) is a kind of intractable pain. The pathogenesis of NP remains a complicated issue for pain management practitioners. SPARC/osteonectin, CWCV, and Kazal-like domains proteoglycan 2 (SPOCK2) are members of the SPOCK family that play a significant role in the development of the central nervous system. In this study, we investigated the role of SPOCK2 in the development of NP in a rat model of chronic constriction injury (CCI).

METHODS

Sprague-Dawley rats were randomly grouped to establish CCI models. We examined the effects of SPOCK2 on pain hpersensitivity and spinal astrocyte activation after CCI-induced NP. Paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) were used to reflects the pain behavioral degree. Molecular mechanisms involved in SPOCK2-mediated NP in vivo were examined by western blot analysis, immunofluorescence, immunohistochemistry, and co-immunoprecipitation. In addition, we examined the SPOCK2-mediated potential protein-protein interaction (PPI) in vitro coimmunoprecipitation (Co-IP) experiments.

RESULTS

We founded the expression level of SPOCK2 in rat spinal cord was markedly increased after CCI-induced NP, while SPOCK2 downregulation could partially relieve pain caused by CCI. Our research showed that SPOCK2 expressed significantly increase in spinal astrocytes when CCI-induced NP. In addition, SPOCK2 could act as an upstream signaling molecule to regulate the activation of matrix metalloproteinase-2 (MMP-2), thus affecting astrocytic ERK1/2 activation and interleukin (IL)-1β production in the development of NP. Moreover, in vitro coimmunoprecipitation (Co-IP) experiments showed that SPOCK2 could interact with membrane-type 1 matrix metalloproteinase (MT1-MMP/MMP14) to regulate MMP-2 activation by the SPARC extracellular (SPARC_EC) domain.

CONCLUSIONS

Research shows that SPOCK2 can interact with MT1-MMP to regulate MMP-2 activation, thus affecting astrocytic ERK1/2 activation and IL-1β production to achieve positive promotion of NP.

Degenerative and regenerative peripheral processes are associated with persistent painful chemotherapy-induced neuropathies in males and females

This study aimed to investigate the time course of gene expression changes during the progression of persistent painful neuropathy caused by paclitaxel (PTX) in male and female mouse hind paws and dorsal root ganglia (DRG). Bulk RNA-seq was used to investigate the gene expression changes in the paw and DRG collected at 1, 16, and 31 days post-PTX. At these time points, differentially expressed DEGs were predominantly related to reduction or increase in epithelial, skin, bone, and muscle development and to angiogenesis, myelination, axonogenesis, and neurogenesis. These processes were accompanied by regulation of DEGs related to cytoskeleton, extracellular matrix organization and cellular energy production. This gene plasticity during persistent painful neuropathy progression likely represents biological processes linked to tissue regeneration and degeneration. Unlike regeneration/degeneration, gene plasticity related to immune processes was minimal at 1-31 days post-PTX. It was also noted that despite similarities in biological processes and pain chronicity in males and females, specific DEGs showed dramatic sex-dependency. The main conclusions of this study are that gene expression plasticity in paws and DRG during PTX neuropathy progression relates to tissue regeneration and degeneration, minimally affects the immune system processes, and is heavily sex-dependent at the individual gene level.

Desipramine induces anti-inflammatory dorsal root ganglion transcriptional signatures in the murine spared nerve injury model

Monoamine-targeting antidepressants serve as frontline medications for chronic pain and associated comorbidities. While persistent anti-allodynic properties of antidepressants generally require weeks of treatment, several groups have demonstrated acute analgesic effects within hours of administration, suggesting a role in non-mesocorticolimbic pain processing regions such as the peripheral nervous system. To further explore this possibility, after four weeks of spared nerve injury or sham surgeries, we systemically administered desipramine or saline for an additional three weeks and performed whole transcriptome RNA sequencing on L3-6 dorsal root ganglia. Along with alterations in molecular pathways associated with neuronal activity, we observed a robust immunomodulatory transcriptional signature in the desipramine treated group. Cell subtype deconvolution predicted that these changes were associated with A- and C-fibers. Of note, differentially expressed genes from the dorsal root ganglia of DMI-treated, injured mice were largely unique compared to those from the nucleus accumbens of the same animals. These observations suggest that, under peripheral nerve injury conditions, desipramine induces specific gene expression changes across various regions of the nociceptive circuitry.

Targeting sensory neuron GPCRs for peripheral neuropathic pain

Despite the high prevalence of peripheral neuropathic pain (NP) conditions and significant progress in understanding its underlying mechanisms, the management of peripheral NP remains inadequate. Existing pharmacotherapies for NP act primarily on the central nervous system (CNS) and are often associated with CNS-related adverse effects, limiting their clinical effectiveness. Mounting preclinical evidence indicates that reducing the heightened activity in primary sensory neurons by targeting G-protein-coupled receptors (GPCRs), without activating these receptors in the CNS, relieves pain without central adverse effects. In this review, we focus on recent advancements in GPCR-mediated peripheral pain relief and discuss strategies to advance the development of more effective and safer therapies for peripheral NP by shifting from traditional CNS modulatory approaches toward selective targeting of GPCRs on primary sensory neurons.

Electrophysiological analyses of human dorsal root ganglia and human induced pluripotent stem cell-derived sensory neurons from male and female donors

Human induced pluripotent stem cell-derived sensory neurons (hiPSC-SNs) and human dorsal root ganglia (hDRG) neurons are popular tools in the field of pain research; however, few groups make use of both approaches. For screening and analgesic validation purposes, important characterizations can be determined of the similarities and differences between hDRG and hiPSC-SNs. This study focuses specifically on electrophysiology properties of hDRG in comparison to hiPSC-SNs. We also compared hDRG and hiPSC-SNs from both male and female donors to evaluate potential sex differences. We recorded neuronal size, rheobase, resting membrane potential, input resistance, and action potential waveform properties from 83 hiPSCs-SNs (2 donors) and 108 hDRG neurons (9 donors). We observed several statistically significant electrophysiological differences between hDRG and hiPSC-SNs, such as size, rheobase, input resistance, and several actional potential (AP) waveform properties. Correlation analysis also revealed many properties that were positively or negatively correlated, some of which were differentially correlated between hDRG and hiPSC-SNs. This study shows several differences between hDRG and hiPSC-SNs and allows better understanding of the advantages and disadvantages of both for use in pain research. We hope this study will be a valuable resource for pain researchers considering the use of these human in vitro systems for mechanistic studies and/or drug development projects.

Conserved transcriptional programming across sex and species after peripheral nerve injury predicts treatments for neuropathic pain

BACKGROUND AND PURPOSE

Chronic pain is a devastating problem affecting one in five individuals around the globe, with neuropathic pain the most debilitating and poorly treated type of chronic pain. Advances in transcriptomics have contributed to cataloguing diverse cellular pathways and transcriptomic alterations in response to peripheral nerve injury but have focused on phenomenology and classifying transcriptomic responses.

EXPERIMENTAL APPROACH

To identifying new types of pain-relieving agents, we compared transcriptional reprogramming changes in the dorsal spinal cord after peripheral nerve injury cross-sex and cross-species, and imputed commonalities, as well as differences in cellular pathways and gene regulation.

KEY RESULTS

We identified 93 transcripts in the dorsal horn that were increased by peripheral nerve injury in male and female mice and rats. Following gene ontology and transcription factor analyses, we constructed a pain interactome for the proteins encoded by the differentially expressed genes, discovering new, conserved signalling nodes. We investigated the interactome with the Drug-Gene database to predict FDA-approved medications that may modulate key nodes within the network. The top hit from the analysis was fostamatinib, the molecular target of which is the non-receptor spleen associated tyrosine kinase (Syk), which our analysis had identified as a key node in the interactome. We found that intrathecally administrating the active metabolite of fostamatinib, R406 and another Syk inhibitor P505-15, significantly reversed pain hypersensitivity in both sexes.

CONCLUSIONS AND IMPLICATIONS

Thus, we have identified and shown the efficacy of an agent that could not have been previously predicted to have analgesic properties.

RNA isoform expression landscape of the human dorsal root ganglion (DRG) generated from long read sequencing

Splicing is a post-transcriptional RNA processing mechanism that enhances genomic complexity by creating multiple isoforms from the same gene. Diversity in splicing in the mammalian nervous system is associated with neuronal development, synaptic function and plasticity, and is also associated with diseases of the nervous system ranging from neurodegeneration to chronic pain. We aimed to characterize the isoforms expressed in the human peripheral nervous system, with the goal of creating a resource to identify novel isoforms of functionally relevant genes associated with somatosensation and nociception. We used long read sequencing (LRS) to document isoform expression in the human dorsal root ganglia (hDRG) from 3 organ donors. Isoforms were validated in silico by confirming expression in hDRG short read sequencing (SRS) data from 3 independent organ donors. 19,547 isoforms of protein-coding genes were detected using LRS and validated with SRS and strict expression cutoffs. We identified 763 isoforms with at least one previously undescribed splice-junction. Previously unannotated isoforms of multiple pain-associated genes, including ASIC3, MRGPRX1 and HNRNPK were identified. In the novel isoforms of ASIC3, a region comprising ~35% of the 5'UTR was excised. In contrast, a novel splice-junction was utilized in isoforms of MRGPRX1 to include an additional exon upstream of the start-codon, consequently adding a region to the 5'UTR. Novel isoforms of HNRNPK were identified which utilized previously unannotated splice-sites to both excise exon 14 and include a sequence in the 5' end of exon 13. The insertion and deletion in the coding region was predicted to excise a serine-phosphorylation site favored by cdc2, and replace it with a tyrosine-phosphorylation site potentially phosphorylated by SRC. We also independently confirm a recently reported DRG-specific splicing event in WNK1 that gives insight into how painless peripheral neuropathy occurs when this gene is mutated. Our findings give a clear overview of mRNA isoform diversity in the hDRG obtained using LRS. Using this work as a foundation, an important next step will be to use LRS on hDRG tissues recovered from people with a history of chronic pain. This should enable identification of new drug targets and a better understanding of chronic pain that may involve aberrant splicing events.

Kinesin family member 2A gates nociception

Nociceptive axons undergo remodeling as they innervate their targets during development and in response to environmental insults and pathological conditions. How is nociceptive morphogenesis regulated? Here, we show that the microtubule destabilizer kinesin family member 2A (Kif2a) is a key regulator of nociceptive terminal structures and pain sensitivity. Ablation of Kif2a in sensory neurons causes hyperinnervation and hypersensitivity to noxious stimuli in young adult mice, whereas touch sensitivity and proprioception remain unaffected. Computational modeling predicts that structural remodeling is sufficient to explain the phenotypes. Furthermore, Kif2a deficiency triggers a transcriptional response comprising sustained upregulation of injury-related genes and homeostatic downregulation of highly specific channels and receptors at the late stage. The latter effect can be predicted to relieve the hyperexcitability of nociceptive neurons, despite persisting morphological aberrations, and indeed correlates with the resolution of pain hypersensitivity. Overall, we reveal a critical control node defining nociceptive terminal structure, which is regulating nociception.

Age-Associated Changes in Knee Osteoarthritis, Pain-Related Behaviors, and Dorsal Root Ganglia Immunophenotyping of Male and Female Mice

OBJECTIVE

Osteoarthritis (OA) is a leading cause of chronic pain, yet OA pain management remains poor. Age is the strongest predictor of OA development, and mechanisms driving OA pain are unclear. We undertook this study to characterize age-associated changes in knee OA, pain-related behaviors, and dorsal root ganglion (DRG) molecular phenotypes in mice of both sexes.

METHODS

Male or female C57BL/6 mice 6 or 20 months of age were evaluated for histopathologic knee OA, pain-related behaviors, and L3-L5 DRG immune characterization via flow cytometry. DRG gene expression in older mice and humans was also examined.

RESULTS

Male mice at 20 months of age had worse cartilage degeneration than 6-month-old mice. Older female mouse knees showed increased cartilage degeneration but to a lesser degree than those of male mice. Older mice of both sexes had worse mechanical allodynia, knee hyperalgesia, and grip strength compared to younger mice. For both sexes, DRGs from older mice showed decreased CD45+ cells and a significant increase in F4/80+ macrophages and CD11c+ dendritic cells. Older male mouse DRGs showed increased expression of Ccl2 and Ccl5, and older female mouse DRGs showed increased Cxcr4 and Ccl3 expression compared to 6-month-old mouse DRGs, among other differentially expressed genes. Human DRG analysis from 6 individuals >80 years of age revealed elevated CCL2 in men compared to women, whereas CCL3 was higher in DRGs from women.

CONCLUSION

We found that aging in male and female mice is accompanied by mild knee OA, mechanical sensitization, and changes to immune cell populations in the DRG, suggesting novel avenues for development of OA therapies.

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

Neuroimmune Mechanisms Underlying Post-acute Sequelae of SARS-CoV-2 (PASC) Pain, Predictions from a Ligand-Receptor Interactome

PURPOSE OF REVIEW

Individuals with post-acute sequelae of SARS-CoV-2 (PASC) complain of persistent musculoskeletal pain. Determining how COVID-19 infection produces persistent pain would be valuable for the development of therapeutics aimed at alleviating these symptoms.

RECENT FINDINGS

To generate hypotheses regarding neuroimmune interactions in PASC, we used a ligand-receptor interactome to make predictions about how ligands from PBMCs in individuals with COVID-19 communicate with dorsal root ganglia (DRG) neurons to induce persistent pain. In a structured literature review of -omics COVID-19 studies, we identified ligands capable of binding to receptors on DRG neurons, which stimulate signaling pathways including immune cell activation and chemotaxis, the complement system, and type I interferon signaling. The most consistent finding across immune cell types was an upregulation of genes encoding the alarmins S100A8/9 and MHC-I. This ligand-receptor interactome, from our hypothesis-generating literature review, can be used to guide future research surrounding mechanisms of PASC-induced pain.

Prospective Blood Transcriptomics Study in a Motor Vehicle Collision Cohort Identified a Protective Function of the SAMD15 Gene Against Chronic Pain

Traumatic brain injuries following motor vehicle collisions (MVCs) are ubiquitous. Surprisingly, there are no correlates between concussion impact force and long-term pain outcomes. To study the molecular underpinnings of chronic pain after MVC, we assembled a prospective cohort of 36 subjects that experienced MVC and suffered documented mild traumatic brain injuries. For each participant, a first blood sample was drawn within 72 hours of the collision, then a second one at the 6-month mark. Pain was also assessed at the second blood draw to determine if pain became chronic or resolved. Blood samples enabled transcriptomics analyses for immune cells. At the transcriptome-wide level, we found that Sterile Alpha Motif Domain Containing 15 (SAMD15) mRNA was significantly upregulated with time in subjects who resolved their pain whereas unregulated in those with persistent pain. Using several large publicly available datasets, such as the UK Biobank and the GTeX portal, we then linked elevated SAMD15 gene expression, elevated neutrophils cell counts, and decreased risk for chronic pain to increased dosage of the T allele at SNP rs4903580, situated within SAMD15's gene locus. The causality between the components of our model was established and supported by Mendelian randomization. Overall, our results support the role of SAMD15 as a potential gene effector for neutrophil-dependent chronic pain development. PERSPECTIVE: This article highlights the potential protective role of the SAMD15 gene against chronic pain following a mild traumatic brain injury. The expression of the gene is associated with a SNP rs4903580, which is itself associated with neutrophils counts as well as chronic pain in large genetic studies.

Analysis of matrisome expression patterns in murine and human dorsal root ganglia

The extracellular matrix (ECM) is a dynamic structure of molecules that can be divided into six different categories and are collectively called the matrisome. The ECM plays pivotal roles in physiological processes in many tissues, including the nervous system. Intriguingly, alterations in ECM molecules/pathways are associated with painful human conditions and murine pain models. Nevertheless, mechanistic insight into the interplay of normal or defective ECM and pain is largely lacking. The goal of this study was to integrate bulk, single-cell, and spatial RNA sequencing (RNAseq) datasets to investigate the expression and cellular origin of matrisome genes in male and female murine and human dorsal root ganglia (DRG). Bulk RNAseq showed that about 65% of all matrisome genes were expressed in both murine and human DRG, with proportionally more core matrisome genes (glycoproteins, collagens, and proteoglycans) expressed compared to matrisome-associated genes (ECM-affiliated genes, ECM regulators, and secreted factors). Single cell RNAseq on male murine DRG revealed the cellular origin of matrisome expression. Core matrisome genes, especially collagens, were expressed by fibroblasts whereas matrisome-associated genes were primarily expressed by neurons. Cell-cell communication network analysis with CellChat software predicted an important role for collagen signaling pathways in connecting vascular cell types and nociceptors in murine tissue, which we confirmed by analysis of spatial transcriptomic data from human DRG. RNAscope in situ hybridization and immunohistochemistry demonstrated expression of collagens in fibroblasts surrounding nociceptors in male and female human DRG. Finally, comparing human neuropathic pain samples with non-pain samples also showed differential expression of matrisome genes produced by both fibroblasts and by nociceptors. This study supports the idea that the DRG matrisome may contribute to neuronal signaling in both mouse and human, and that dysregulation of matrisome genes is associated with neuropathic pain.

Fibroblast-derived PI16 sustains inflammatory pain via regulation of CD206+ myeloid cells

Originally identified in fibroblasts, Protease Inhibitor (PI)16 was recently shown to be crucial for the development of neuropathic pain via effects on blood-nerve barrier permeability and leukocyte infiltration, though its impact on inflammatory pain has not been established. Using the complete Freund's Adjuvant inflammatory pain model, we show that Pi16-/- mice are protected against sustained inflammatory pain. Accordingly, intrathecal delivery of a PI16 neutralizing antibody in wild-type mice prevented sustained CFA pain. In contrast to neuropathic pain models, we did not observe any changes in blood-nerve barrier permeability due to PI16 deletion. Instead, Pi16-/- mice display reduced macrophage density in the CFA-injected hindpaw. Furthermore, there was a significant bias toward CD206hi (anti-inflammatory) macrophages in the hindpaw and associated dorsal root ganglia. Following CFA, intrathecal depletion of CD206+ macrophages using mannosylated clodronate liposomes promoted sustained pain in Pi16-/- mice. Similarly, an IL-10 neutralizing antibody also promoted sustained CFA pain in the Pi16-/ when administered intrathecally. Collectively, our results point to fibroblast-derived PI16 mediating substantial differences in macrophage phenotype in the pain neuroaxis under conditions of inflammation. The co-expression of PI16 alongside fibroblast markers in human DRG raise the likelihood that a similar mechanism operates in human inflammatory pain states. Collectively, our findings may have implications for targeting fibroblast-immune cell crosstalk for the treatment of chronic pain.

Metastatic Infiltration of Nervous Tissue and Periosteal Nerve Sprouting in Multiple Myeloma-Induced Bone Pain in Mice and Human

Multiple myeloma (MM) is a neoplasia of B plasma cells that often induces bone pain. However, the mechanisms underlying myeloma-induced bone pain (MIBP) are mostly unknown. Using a syngeneic MM mouse model, we show that periosteal nerve sprouting of calcitonin gene-related peptide (CGRP+) and growth associated protein 43 (GAP43+) fibers occurs concurrent to the onset of nociception and its blockade provides transient pain relief. MM patient samples also showed increased periosteal innervation. Mechanistically, we investigated MM induced gene expression changes in the dorsal root ganglia (DRG) innervating the MM-bearing bone of male mice and found alterations in pathways associated with cell cycle, immune response and neuronal signaling. The MM transcriptional signature was consistent with metastatic MM infiltration to the DRG, a never-before described feature of the disease that we further demonstrated histologically. In the DRG, MM cells caused loss of vascularization and neuronal injury, which may contribute to late-stage MIBP. Interestingly, the transcriptional signature of a MM patient was consistent with MM cell infiltration to the DRG. Overall, our results suggest that MM induces a plethora of peripheral nervous system alterations that may contribute to the failure of current analgesics and suggest neuroprotective drugs as appropriate strategies to treat early onset MIBP.SIGNIFICANCE STATEMENT Multiple myeloma (MM) is a painful bone marrow cancer that significantly impairs the quality of life of the patients. Analgesic therapies for myeloma-induced bone pain (MIBP) are limited and often ineffective, and the mechanisms of MIBP remain unknown. In this manuscript, we describe cancer-induced periosteal nerve sprouting in a mouse model of MIBP, where we also encounter metastasis to the dorsal root ganglia (DRG), a never-before described feature of the disease. Concomitant to myeloma infiltration, the lumbar DRGs presented blood vessel damage and transcriptional alterations, which may mediate MIBP. Explorative studies on human tissue support our preclinical findings. Understanding the mechanisms of MIBP is crucial to develop targeted analgesic with better efficacy and fewer side effects for this patient population.

Single-cell transcriptomic profiling of dorsal root ganglion: an overview

The somatosensory neurons in the dorsal root ganglion (DRG) are responsible to detect peripheral physical and noxious stimuli, and then transmit these inputs into the central nervous system. DRG neurons are composed of various subpopulations, which are suggested to respond to different stimuli, such as mechanical, thermal, and cold perception. For a long time, DRG neurons were classified based on anatomical criteria. Recently, single-cell (scRNA-seq) and single-nucleus RNA-sequencing (snRNA-seq) has advanced our understanding of the composition and functional heterogeneity of both human and rodent DRG neurons at single-cell resolution. In this review, we summarized the current literature regarding single-cell transcriptomic profiling of DRG to provide an integral understanding in the molecular transcriptomes, cell types, and functional annotations of DRG neurons in humans and rodents.

MNK inhibitor eFT508 (Tomivosertib) suppresses ectopic activity in human dorsal root ganglion neurons from dermatomes with radicular neuropathic pain

Spontaneous activity in dorsal root ganglion (DRG) neurons is a key driver of neuropathic pain in preclinical models and in patients suffering from this largely untreated disease. While many intracellular signaling mechanisms have been examined in preclinical models that drive this spontaneous activity (SA), none of these have been tested directly on spontaneously active human nociceptors. Using cultured DRG neurons recovered during thoracic vertebrectomy surgeries, we show that inhibition of mitogen activated protein kinase interacting kinase (MNK) with eFT508 (25 nM) reverses SA in human sensory neurons associated with painful dermatomes. MNK inhibition in spontaneously active nociceptors decreased action potential amplitude and produced alterations in the magnitude of afterhyperpolarizing currents suggesting modification of Na+ and K+ channel activity downstream of MNK inhibition. The effects of MNK inhibition on SA took minutes to emerge and were reversible over time with eFT508 washout. MNK inhibition with eFT508 led to a profound loss of eIF4E Serine 209 phosphorylation, a specific target of the kinase, within 2 min of drug treatment, consistent with the rapid action of the drug on SA in electrophysiology experiments. Our results create a compelling case for the future testing of MNK inhibitors in clinical trials for neuropathic pain.

Deconvolution of spatial sequencing provides accurate characterization of hESC-derived DA transplants in vivo

Cell therapy for Parkinson's disease has experienced substantial growth in the past decades with several ongoing clinical trials. Despite increasing refinement of differentiation protocols and standardization of the transplanted neural precursors, the transcriptomic analysis of cells in the transplant after its full maturation in vivo has not been thoroughly investigated. Here, we present spatial transcriptomics analysis of fully differentiated grafts in their host tissue. Unlike earlier transcriptomics analyses using single-cell technologies, we observe that cells derived from human embryonic stem cells (hESCs) in the grafts adopt mature dopaminergic signatures. We show that the presence of phenotypic dopaminergic genes, which were found to be differentially expressed in the transplants, is concentrated toward the edges of the grafts, in agreement with the immunohistochemical analyses. Deconvolution shows dopamine neurons being the dominating cell type in many features beneath the graft area. These findings further support the preferred environmental niche of TH-positive cells and confirm their dopaminergic phenotype through the presence of multiple dopaminergic markers.

Optimized Electrical Stimulation of C-Nociceptors in Humans Based on the Chronaxie of Porcine C-Fibers

Classically, to electrically excite C-nociceptors, rectangular pulses are used with a duration close to the estimated chronaxie of C-fibres (about 2 ms). Recent results using slow depolarizing stimuli suggest longer chronaxies. We therefore set out to optimize C-fiber stimulation based on recordings of single C-nociceptors in-vivo and C-fiber compound-action-potentials (C-CAP) ex-vivo using half-sine shaped stimuli of durations between 1 and 250ms. Single fiber (n = 45) recording in pigs revealed high chronaxie values for C-touch fibers (15.8 ms), polymodal- (14.2 ms) and silent-nociceptors (16.8 ms). Activation thresholds decreased 2 to 3-fold in all fibre classes when increasing the duration of half-sine pulses from 1 to 25 ms (P < .05). C-CAPs strength-duration curves of the pig saphenous nerve (n = 7) showed the highest sensitivity for half-sine durations between 10 and 25 ms. Half-maximum currents for C-CAPS were reduced 3-fold compared to rectangular pulses (P < .01) whereas the opposite was found for A-fiber compound action potentials. Psychophysics in humans (n = 23) revealed that half-sine stimulus durations >10 ms reduced detection thresholds, pain thresholds, and stimulus current amplitudes required to generate a pain rating of 3 on an 11-point Numeric Rating Scale (NRS) as compared to 1 ms rectangular pulses (P < 0.05). Increasing the duration from 1 to 25 ms led to a 4-fold amplitude reduction for pain-thresholds and stimuli caused an axon-reflex flare. Excitability of single polymodal nociceptors in animals paralleled human psychophysics and we conclude optimized half-sine pulses facilitate C-nociceptor activation. PERSPECTIVE: Electrical stimulation with longer lasting half-sine wave pulses preferentially activates C-nociceptors and changes in the strength duration curve may identify nociceptor hyperexcitability in patients with neuropathic pain.

RNA-Seq Reveals Sex Differences in Gene Expression during Peripheral Neuropathic Inflammation and in Pain Relief from a COX-2 Inhibiting Theranostic Nanoemulsion

Given decades of neuroinflammatory pain research focused only on males, there is an urgent need to better understand neuroinflammatory pain in females. This, paired with the fact that currently there is no long-term effective treatment for neuropathic pain furthers the need to evaluate how neuropathic pain develops in both sexes and how it can be relieved. Here we show that chronic constriction injury of the sciatic nerve caused comparable levels of mechanical allodynia in both sexes. Using a COX-2 inhibiting theranostic nanoemulsion with increased drug loading, both sexes achieved similar reduction in mechanical hypersensitivity. Given that both sexes have improved pain behavior, we specifically explored differential gene expression between sexes in the dorsal root ganglia (DRG) during pain and relief. Total RNA from the DRG revealed a sexually dimorphic expression for injury and relief caused by COX-2 inhibition. Of note, both males and females experience increased expression of activating transcription factor 3 (Atf3), however, only the female DRG shows decreased expression following drug treatment. Alternatively, S100A8 and S100A9 expression appear to play a sex specific role in relief in males. The sex differences in RNA expression reveal that comparable behavior does not necessitate the same gene expression.

Characterization of Fragile X Mental Retardation Protein expression in human nociceptors and their axonal projections to the spinal dorsal horn

Fragile X Mental Retardation Protein (FMRP) regulates activity-dependent RNA localization and local translation to modulate synaptic plasticity throughout the central nervous system. Mutations in the FMR1 gene that hinder or ablate FMRP function cause Fragile X Syndrome (FXS), a disorder associated with sensory processing dysfunction. FXS premutations are associated with increased FMRP expression and neurological impairments including sex dimorphic presentations of chronic pain. In mice, FMRP ablation causes dysregulated dorsal root ganglion (DRG) neuron excitability and synaptic vesicle exocytosis, spinal circuit activity, and decreased translation-dependent nociceptive sensitization. Activity-dependent, local translation is a key mechanism for enhancing primary nociceptor excitability that promotes pain in animals and humans. These works indicate that FMRP likely regulates nociception and pain at the level of the primary nociceptor or spinal cord. Therefore, we sought to better understand FMRP expression in the human DRG and spinal cord using immunostaining in organ donor tissues. We find that FMRP is highly expressed in DRG and spinal neuron subsets with substantia gelatinosa exhibiting the most abundant immunoreactivity in spinal synaptic fields. Here, it is expressed in nociceptor axons. FMRP puncta colocalized with Nav1.7 and TRPV1 receptor signals suggesting a pool of axoplasmic FMRP localizes to plasma membrane-associated loci in these branches. Interestingly, FMRP puncta exhibited notable colocalization with calcitonin gene-related peptide (CGRP) immunoreactivity selectively in female spinal cord. Our results support a regulatory role for FMRP in human nociceptor axons of the dorsal horn and implicate it in the sex dimorphic actions of CGRP signaling in nociceptive sensitization and chronic pain.

MNK1 and MNK2 Expression in the Human Dorsal Root and Trigeminal Ganglion

Mitogen activated protein kinase interacting kinases (MNK) 1 and 2 are serine/threonine protein kinases that play an important role in translation of mRNAs through their phosphorylation of the RNA 5'-cap binding protein, eukaryotic translation initiation factor (eIF) 4E. These kinases are downstream targets for mitogen activated protein kinases (MAPKs), extracellular activity regulated protein kinase (ERK) and p38. MNKs have been implicated in the sensitization of peripheral nociceptors of the dorsal root and trigeminal ganglion (DRG and TG) using transgenic mouse lines and through the use of specific inhibitors of MNK1 and MNK2. While specific knockout of the Mknk1 gene suggests that it is the key isoform for regulation of nociceptor excitability and nociceptive behaviors in mice, both MKNK1 and MKNK2 genes are expressed in the DRG and TG of mice and humans based on RNA sequencing experiments. Single cell sequencing in mice suggests that Mknk1 and Mknk2 may be expressed in different populations of nociceptors. We sought to characterize mRNA expression in human DRG and TG (N = 3 ganglia for both DRG and TG) for both MNK1 and MNK2. Our results show that both genes are expressed by nearly all neurons in both human ganglia with expression in other cell types as well. Our findings provide evidence that MNK1 and MNK2 are expressed by human nociceptors of males and females and suggest that efforts to pharmacologically target MNKs for pain would likely be translatable due its conserved expression in both species.

Sex-differences in prostaglandin signaling: a semi-systematic review and characterization of PTGDS expression in human sensory neurons

There is increasing evidence of sex differences in underlying mechanisms causing pain in preclinical models, and in clinical populations. There are also important disconnects between clinical pain populations and the way preclinical pain studies are conducted. For instance, osteoarthritis pain more frequently affects women, but most preclinical studies have been conducted using males in animal models. The most widely used painkillers, nonsteroidal anti-inflammatory drugs (NSAIDs), act on the prostaglandin pathway by inhibiting cyclooxygenase (COX) enzymes. The purpose of this study was to analyze the preclinical and clinical literature on the role of prostaglandins and COX in inflammation and pain. We aimed to specifically identify studies that used both sexes and investigate whether any sex-differences in the action of prostaglandins and COX inhibition had been reported, either in clinical or preclinical studies. We conducted a PubMed search and identified 369 preclinical studies and 100 clinical studies that matched our inclusion/exclusion criteria. Our analysis shows that only 17% of preclinical studies on prostaglandins used both sexes and, out of those, only 19% analyzed or reported data separated by sex. In contrast, 79% of the clinical studies analyzed used both sexes. However, only 6% of those reported data separated by sex. Interestingly, 14 out of 15 preclinical studies and 5 out of 6 clinical studies that analyzed data separated by sex have identified sex-differences. This builds on the increasing evidence of sex-differences in prostaglandin signaling and the importance of sex as a biological variable in data analysis. The preclinical literature identifies a sex difference in prostaglandin D2 synthase (PTGDS) expression where it is higher in female than in male rodents in the nervous system. We experimentally validated that PTGDS expression is higher in female human dorsal root ganglia (DRG) neurons recovered from organ donors. Our semi-systematic literature review reveals a need for continued inclusivity of both male and female animals in prostaglandins studies and data analysis separated by sex in preclinical and clinical studies. Our finding of sex-differences in neuronal PTGDS expression in humans exemplifies the need for a more comprehensive understanding of how the prostaglandin system functions in the DRG in rodents and humans.

Persistent nociceptor hyperactivity as a painful evolutionary adaptation

Chronic pain caused by injury or disease of the nervous system (neuropathic pain) has been linked to persistent electrical hyperactivity of the sensory neurons (nociceptors) specialized to detect damaging stimuli and/or inflammation. This pain and hyperactivity are considered maladaptive because both can persist long after injured tissues have healed and inflammation has resolved. While the assumption of maladaptiveness is appropriate in many diseases, accumulating evidence from diverse species, including humans, challenges the assumption that neuropathic pain and persistent nociceptor hyperactivity are always maladaptive. We review studies indicating that persistent nociceptor hyperactivity has undergone evolutionary selection in widespread, albeit selected, animal groups as a physiological response that can increase survival long after bodily injury, using both highly conserved and divergent underlying mechanisms.

Inducible co-stimulatory molecule (ICOS) alleviates paclitaxel-induced neuropathic pain via an IL-10-mediated mechanism in female mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a primary dose-limiting side effect caused by antineoplastic agents, such as paclitaxel. A primary symptom of this neuropathy is pain. Currently, there are no effective treatments for CIPN, which can lead to long-term morbidity in cancer patients and survivors. Neuro-immune interactions occur in CIPN pain and have been implicated both in the development and progression of pain in CIPN and the resolution of pain in CIPN. We investigated the potential role of inducible co-stimulatory molecule (ICOS) in the resolution of CIPN pain-like behaviors in mice. ICOS is an immune checkpoint molecule that is expressed on the surface of activated T cells and promotes proliferation and differentiation of T cells. We found that intrathecal administration of ICOS agonist antibody (ICOSaa) alleviates mechanical hypersensitivity caused by paclitaxel and facilitates the resolution of mechanical hypersensitivity in female mice. Administration of ICOSaa reduced astrogliosis in the spinal cord and satellite cell gliosis in the DRG of mice previously treated with paclitaxel. Mechanistically, ICOSaa intrathecal treatment promoted mechanical hypersensitivity resolution by increasing interleukin 10 (IL-10) expression in the dorsal root ganglion. In line with these observations, blocking IL-10 receptor (IL-10R) activity occluded the effects of ICOSaa treatment on mechanical hypersensitivity in female mice. Suggesting a broader activity in neuropathic pain, ICOSaa also partially resolved mechanical hypersensitivity in the spared nerve injury (SNI) model. Our findings support a model wherein ICOSaa administration induces IL-10 expression to facilitate neuropathic pain relief in female mice. ICOSaa treatment is in clinical development for solid tumors and given our observation of T cells in the human DRG, ICOSaa therapy could be developed for combination chemotherapy-CIPN clinical trials.

MNK1 and MNK2 expression in the human dorsal root and trigeminal ganglion

Mitogen activated protein kinase interacting kinases (MNK) 1 and 2 are serine/threonine protein kinases that play an important role in translation of mRNAs through their phosphorylation of the RNA 5’-cap binding protein, eukaryotic translation initiation factor (eIF) 4E. These kinases are downstream targets for mitogen activated protein kinases (MAPKs), extracellular activity regulated protein kinase (ERK) and p38. MNKs have been implicated in the sensitization of peripheral nociceptors of the dorsal root and trigeminal ganglion (DRG and TG) using transgenic mouse lines and through the use of specific inhibitors of MNK1 and MNK2. While specific knockout of the Mknk1 gene suggests that it is the key isoform for regulation of nociceptor excitability and nociceptive behaviors in mice, both MKNK1 and MKNK2 genes are expressed in the DRG and TG of mice and humans based on RNA sequencing experiments. Single cell sequencing in mice suggests that Mknk1 and Mknk2 may be expressed in different populations of nociceptors. We sought to characterize mRNA expression in human DRG and TG for both MNK1 and MNK2. Our results show that both genes are expressed by nearly all neurons in both human ganglia with expression in other cell types as well. Our findings provide evidence that MNK1 and MNK2 are expressed by human nociceptors and suggest that efforts to pharmacologically target MNKs for pain would likely be translatable due its conserved expression in both species.

Involvement of interferon gamma signaling in spinal trigeminal caudal subnucleus astrocyte in orofacial neuropathic pain in rats with infraorbital nerve injury

Background: Trigeminal nerve injury causes orofacial pain that can interfere with activities of daily life. However, the underlying mechanism remains unknown, and the appropriate treatment has not been established yet. This study aimed to examine the involvement of interferon gamma (IFN-γ) signaling in the spinal trigeminal caudal subnucleus (Vc) in orofacial neuropathic pain. Methods: Infraorbital nerve (ION) injury (IONI) was performed in rats by partial ION ligation. The head-withdrawal reflex threshold (HWT) to mechanical stimulation of the whisker pad skin was measured in IONI or sham rats, as well as following a continuous intracisterna magna administration of IFN-γ and a mixture of IFN-γ and fluorocitrate (inhibitor of astrocytes activation) in naïve rats, or an IFN-γ antagonist in IONI rats. The IFN-γ receptor immunohistochemistry and IFN-γ Western blotting were analyzed in the Vc after IONI or sham treatment. The glial fibrillary acid protein (GFAP) immunohistochemistry and Western blotting were also analyzed after administration of IFN-γ and the mixture of IFN-γ and fluorocitrate. Moreover, the change in single neuronal activity in the Vc was examined in the IONI, sham, and IONI group administered IFN-γ antagonist. Results: The HWT decreased after IONI. The IFN-γ and IFN-γ receptor were upregulated after IONI, and the IFN-γ receptor was expressed in Vc astrocytes. IFN-γ administration decreased the HWT, whereas the mixture of IFN-γ and fluorocitrate recovered the decrement of HWT. IFN-γ administration upregulated GFAP expression, while the mixture of IFN-γ and fluorocitrate recovered the upregulation of GFAP expression. IONI significantly enhanced the neuronal activity of the mechanical-evoked responses, and administration of an IFN-γ antagonist significantly inhibited these enhancements. Conclusions: IFN-γ signaling through the receptor in astrocytes is a key mechanism underlying orofacial neuropathic pain associated with trigeminal nerve injury. These findings will aid in the development of therapeutics for orofacial neuropathic pain.