Vascularization of the dorsal root ganglia and peripheral nerve of the mouse: implications for chemical-induced peripheral sensory neuropathies

Analysis of factors selectively related to herpes zoster involving peripheral sensory ganglia: Retrospective study

Herpes zoster (HZ), resulting from the reactivation of the varicella-zoster virus, is a significant disease. This study aimed to explore the factors influencing sensory neuron involvement in HZ at different locations and its association with postherpetic neuralgia (PHN). A total of 3143 cases were retrieved from an electronic medical record system, including 2676 cases of HZ and 467 cases of PHN. Gender, age, site of onset, past surgical history, and comorbidities were analyzed using a multifactorial logistic regression model. The results revealed correlations between age, gender, comorbidities (diabetes, coronary heart disease, percutaneous coronary intervention [PCI]), and sensory neuron involvement in HZ. Specifically, older age, female gender, and comorbid conditions such as diabetes/coronary heart disease were associated with sacral dorsal root ganglion (DRG) involvement, while PCI history was associated with lumbar DRG involvement. Additionally, sensory neuron involvement at different locations by HZ was linked to PHN. Furthermore, independent risk factors for PHN included thoracic DRG involvement, older age, and comorbidities (diabetes, surgical history, malignancy). It is crucial to prevent damage to the DRG, especially in individuals with comorbidities, through activities avoidance and active treatment, to minimize the occurrence of PHN.

Induction of long-term hyperexcitability by memory-related cAMP signaling in isolated nociceptor cell bodies

Persistent hyperactivity of nociceptors is known to contribute significantly to long-lasting sensitization and ongoing pain in many clinical conditions. It is often assumed that nociceptor hyperactivity is mainly driven by continuing stimulation from inflammatory mediators. We have tested an additional possibility: that persistent increases in excitability promoting hyperactivity can be induced by a prototypical cellular signaling pathway long known to induce late-phase long-term potentiation (LTP) of synapses in brain regions involved in memory formation. This cAMP-PKA-CREB-gene transcription-protein synthesis pathway was tested using whole-cell current clamp methods on small dissociated sensory neurons (primarily nociceptors) from dorsal root ganglia (DRGs) excised from previously uninjured ("naïve") rats. Six-hour treatment with the specific Gαs-coupled 5-HT4 receptor agonist, prucalopride, or with the adenylyl cyclase activator, forskolin, induced long-term hyperexcitability (LTH) in DRG neurons that manifested 12-24 hours later as action potential (AP) discharge (ongoing activity, OA) during artificial depolarization to -45 mV, a membrane potential that is normally subthreshold for AP generation. Prucalopride treatment also induced significant long-lasting depolarization of resting membrane potential (from -69 to -66 mV), enhanced depolarizing spontaneous fluctuations (DSFs) of membrane potential, and indications of reduced AP threshold and rheobase. LTH was prevented by co-treatment of prucalopride with inhibitors of PKA, CREB, gene transcription, and protein synthesis. As in the induction of synaptic memory, many other cellular signals are likely to be involved. However, the discovery that this prototypical memory induction pathway can induce nociceptor LTH, along with reports that cAMP signaling and CREB activity in DRGs can induce hyperalgesic priming, suggest that early, temporary, cAMP-induced transcriptional and translational mechanisms can induce nociceptor LTH that might last for long periods. An interesting possibility is that these mechanisms can also be reactivated by re-exposure to inflammatory mediators such as serotonin during subsequent challenges to bodily integrity, "reconsolidating" the cellular memory and thereby extending the duration of persistent nociceptor hyperexcitability.

Communicating pain: emerging axonal signaling in peripheral neuropathic pain

Peripheral nerve damage often leads to the onset of neuropathic pain (NeuP). This condition afflicts millions of people, significantly burdening healthcare systems and putting strain on families' financial well-being. Here, we will focus on the role of peripheral sensory neurons, specifically the Dorsal Root Ganglia neurons (DRG neurons) in the development of NeuP. After axotomy, DRG neurons activate regenerative signals of axons-soma communication to promote a gene program that activates an axonal branching and elongation processes. The results of a neuronal morphological cytoskeleton change are not always associated with functional recovery. Moreover, any axonal miss-targeting may contribute to NeuP development. In this review, we will explore the epidemiology of NeuP and its molecular causes at the level of the peripheral nervous system and the target organs, with major focus on the neuronal cross-talk between intrinsic and extrinsic factors. Specifically, we will describe how failures in the neuronal regenerative program can exacerbate NeuP.

Homeostatic Feelings and the Emergence of Consciousness

In this article, we summarize our views on the problem of consciousness and outline the current version of a novel hypothesis for how conscious minds can be generated in mammalian organisms. We propose that a mind can be considered conscious when three processes are in place: the first is a continuous generation of interoceptive feelings, which results in experiencing of the organism's internal operations; the second is the equally continuous production of images, generated according to the organism's sensory perspective relative to its surround; the third combines feeling/experience and perspective resulting in a process of subjectivity relative to the image contents. We also propose a biological basis for these three components: the peripheral and central physiology of interoception and exteroception help explain the implementation of the first two components, whereas the third depends on central nervous system integration, at multiple levels, from spinal cord, brainstem, and diencephalic nuclei, to selected regions of the mesial cerebral cortices.

Gut microbiota promotes pain chronicity in Myosin1A deficient male mice

Chronic pain is a heavily debilitating condition and a huge socio-economic burden, with no efficient treatment. Over the past decade, the gut microbiota has emerged as an important regulator of nervous system's health and disease states. Yet, its contribution to the pathogenesis of chronic somatic pain remains poorly documented. Here, we report that male but not female mice lacking Myosin1a (KO) raised under single genotype housing conditions (KO-SGH) are predisposed to develop chronic pain in response to a peripheral tissue injury. We further underscore the potential of MYO1A loss-of-function to alter the composition of the gut microbiota and uncover a functional connection between the vulnerability to chronic pain and the dysbiotic gut microbiota of KO-SGH males. As such, parental antibiotic treatment modifies gut microbiota composition and completely rescues the injury-induced pain chronicity in male KO-SGH offspring. Furthermore, in KO-SGH males, this dysbiosis is accompanied by a transcriptomic activation signature in the dorsal root ganglia (DRG) macrophage compartment, in response to tissue injury. We identify CD206+CD163- and CD206+CD163+ as the main subsets of DRG resident macrophages and show that both are long-lived and self-maintained and exhibit the capacity to monitor the vasculature. Consistently, in vivo depletion of DRG macrophages rescues KO-SGH males from injury-induced chronic pain underscoring a deleterious role for DRG macrophages in a Myo1a-loss-of function context. Together, our findings reveal gene-sex-microbiota interactions in determining the predisposition to injury-induced chronic pain and point-out DRG macrophages as potential effector cells.

Dorsal root ganglion magnetic resonance imaging biomarker correlations with pain in Fabry disease

Fabry disease is a rare monogenetic, X-linked lysosomal storage disorder with neuropathic pain as one characteristic symptom. Impairment of the enzyme alpha-galactosidase A leads to an accumulation of globotriaosylceramide in the dorsal root ganglia. Here, we investigate novel dorsal root ganglia MR imaging biomarkers and their association with Fabry genotype and pain phenotype. In this prospective study, 89 Fabry patients were examined using a standardized 3 T MRI protocol of the dorsal root ganglia. Fabry pain was assessed through a validated Fabry pain questionnaire. The genotype was determined by diagnostic sequencing of the alpha-galactosidase A gene. MR imaging end-points were dorsal root ganglia volume by voxel-wise morphometric analysis and dorsal root ganglia T2 signal. Reference groups included 55 healthy subjects and Fabry patients of different genotype categories without Fabry pain. In patients with Fabry pain, T2 signal of the dorsal root ganglia was increased by +39.2% compared to healthy controls (P = 0.001) and by +29.4% compared to painless Fabry disease (P = 0.017). This effect was pronounced in hemizygous males (+40.7% compared to healthy; P = 0.008 and +29.1% compared to painless; P = 0.032) and was consistently observed across the genotype spectrum of nonsense (+38.1% compared to healthy, P < 0.001) and missense mutations (+39.2% compared to healthy; P = 0.009). T2 signal of dorsal root ganglia and globotriaosylsphingosine levels were the only independent predictors of Fabry pain (P = 0.047; P = 0.002). Volume of dorsal root ganglia was enlarged by +46.0% in Fabry males in the nonsense compared to missense genotype category (P = 0.005) and by +34.5% compared to healthy controls (P = 0.034). In painful Fabry disease, MRI T2 signal of dorsal root ganglia is increased across different genotypes. Dorsal root ganglion MRI T2 signal as a novel in vivo imaging biomarker may help to better understand whether Fabry pain is modulated or even caused by dorsal root ganglion pathology.

NT-3 contributes to chemotherapy-induced neuropathic pain through TrkC-mediated CCL2 elevation in DRG neurons

Cancer patients undergoing treatment with antineoplastic drugs often experience chemotherapy-induced neuropathic pain (CINP), and the therapeutic options for managing CINP are limited. Here, we show that systemic paclitaxel administration upregulates the expression of neurotrophin-3 (Nt3) mRNA and NT3 protein in the neurons of dorsal root ganglia (DRG), but not in the spinal cord. Blocking NT3 upregulation attenuates paclitaxel-induced mechanical, heat, and cold nociceptive hypersensitivities and spontaneous pain without altering acute pain and locomotor activity in male and female mice. Conversely, mimicking this increase produces enhanced responses to mechanical, heat, and cold stimuli and spontaneous pain in naive male and female mice. Mechanistically, NT3 triggers tropomyosin receptor kinase C (TrkC) activation and participates in the paclitaxel-induced increases of C-C chemokine ligand 2 (Ccl2) mRNA and CCL2 protein in the DRG. Given that CCL2 is an endogenous initiator of CINP and that Nt3 mRNA co-expresses with TrkC and Ccl2 mRNAs in DRG neurons, NT3 likely contributes to CINP through TrkC-mediated activation of the Ccl2 gene in DRG neurons. NT3 may be thus a potential target for CINP treatment.

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.

The effects of Garcinia kola and curcumin on the dorsal root ganglion of the diabetic rat after peripheral nerve transection injury

OBJECTIVE

To test the protective effects of Garcinia kola and curcumin on the ganglion tissues of diabetic rats following the use of autologous vein graft in peripheral nerve transection injury.

METHODS

The sciatic nerve on the right side was transected, and anastomosis was performed between the proximal and distal ends using an autologous vein graft. Curcumin and Garcinia kola seed extract were administered daily by oral gavage. The ganglion tissues were harvested after a 90-day waiting period. Sensory neurons in the dorsal root ganglion at the L4 and L5 levels were used for stereological evaluations. Mean sensory neuron numbers were analyzed using a stereological technique. The size of the light and dark neurons was also estimated, and ultrastructural and immunohistochemical evaluations were performed.

RESULTS

A statistically significant difference in sensory neuron numbers was observed between the groups with and without Garcinia kola and curcumin applications. The immunohistochemical results showed that the s-100 protein is expressed selectively between cell types.

CONCLUSION

The results of this study show that curcumin and Garicinia kola prevented sensory neuron loss in diabetic rats following transection injury to the sciatic nerve.

Expression of MHC II in DRG neurons attenuates paclitaxel-induced cold hypersensitivity in male and female mice

Chemotherapy is often a life-saving treatment, but the development of intractable pain caused by chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting toxicity that restricts cancer survival rates. Recent reports demonstrate that paclitaxel (PTX) robustly increases anti-inflammatory CD4+ T cells in the dorsal root ganglion (DRG), and that T cells and anti-inflammatory cytokines are protective against CIPN. However, the mechanism by which CD4+ T cells are activated, and the extent cytokines released by CD4+ T cells target DRG neurons are unknown. Here, we are the first to detect major histocompatibility complex II (MHCII) protein in mouse DRG neurons and to find CD4+ T cells breaching the satellite glial cell barrier to be in close proximity to neurons, together suggesting CD4+ T cell activation and targeted cytokine release. MHCII protein is primarily expressed in small nociceptive neurons in male and female mouse DRG but increased after PTX in small nociceptive neurons in only female DRG. Reducing one copy of MHCII in small nociceptive neurons decreased anti-inflammatory IL-10 and IL-4 producing CD4+ T cells in naïve male DRG and increased their hypersensitivity to cold. Administration of PTX to male and female mice that lacked one copy of MHCII in nociceptive neurons decreased anti-inflammatory CD4+ T cells in the DRG and increased the severity of PTX-induced cold hypersensitivity. Collectively, our results demonstrate expression of MHCII protein in mouse DRG neurons, which modulates cytokine producing CD4+ T cells in the DRG and attenuates cold hypersensitivity during homeostasis and after PTX treatment.

CD163+ macrophages monitor enhanced permeability at the blood-dorsal root ganglion barrier

In dorsal root ganglia (DRG), macrophages reside close to sensory neurons and have largely been explored in the context of pain, nerve injury, and repair. However, we discovered that most DRG macrophages interact with and monitor the vasculature by sampling macromolecules from the blood. Characterization of the DRG vasculature revealed a specialized endothelial bed that transformed in molecular, structural, and permeability properties along the arteriovenous axis and was covered by macrophage-interacting pericytes and fibroblasts. Macrophage phagocytosis spatially aligned with peak endothelial permeability, a process regulated by enhanced caveolar transcytosis in endothelial cells. Profiling the DRG immune landscape revealed two subsets of perivascular macrophages with distinct transcriptome, turnover, and function. CD163+ macrophages self-maintained locally, specifically participated in vasculature monitoring, displayed distinct responses during peripheral inflammation, and were conserved in mouse and man. Our work provides a molecular explanation for the permeability of the blood-DRG barrier and identifies an unappreciated role of macrophages as integral components of the DRG-neurovascular unit.

Complement Receptor C3aR1 Contributes to Paclitaxel-Induced Peripheral Neuropathic Pain in Mice and Rats

Cancer chemotherapy-induced neuropathic pain is a devastating pain syndrome without effective therapies. We previously reported that rats deficient in complement C3, the central component of complement activation cascade, showed a reduced degree of paclitaxel-induced mechanical allodynia (PIMA), suggesting that complement is integrally involved in the pathogenesis of this model. However, the underlying mechanism was unclear. Complement activation leads to the production of C3a, which mediates inflammation through its receptor C3aR1. In this article, we report that the administration of paclitaxel induced a significantly higher expression level of C3aR1 on dorsal root ganglion (DRG) macrophages and expansion of these macrophages in DRGs in wild-type (WT) compared with in C3aR1 knockout (KO) mice. We also found that paclitaxel induced less severe PIMA, along with a reduced DRG expression of transient receptor potential channels of the vanilloid subtype 4 (TRPV4), an essential mediator for PIMA, in C3aR1 KO than in WT mice. Treating WT mice or rats with a C3aR1 antagonist markedly attenuated PIMA in association with downregulated DRG TRPV4 expression, reduced DRG macrophages expansion, suppressed DRG neuron hyperexcitability, and alleviated peripheral intraepidermal nerve fiber loss. Administration of C3aR1 antagonist to TRPV4 KO mice further protected them from PIMA. These results suggest that complement regulates PIMA development through C3aR1 to upregulate TRPV4 on DRG neurons and promote DRG macrophage expansion. Targeting C3aR1 could be a novel therapeutic approach to alleviate this debilitating pain syndrome.

Systemic administration of NIS-lncRNA antisense oligonucleotide alleviates neuropathic pain

OBJECTIVE

The antisense oligonucleotide (ASO) is an FDA-approved strategy in the treatment of neurological diseases. We have shown the viability of using intrathecal ASO to suppress nerve injury-specific long noncoding RNA (NIS-lncRNA) in dorsal root ganglion (DRG), resulting in a stable and long-lasting antinociceptive effect on NP. This study examined whether systemic administration of NIS-lncRNA ASO relieved the chronic constriction injury (CCI)-induced nociceptive hypersensitivity.

METHODS

A single subcutaneous injection of NIS-lncRNA ASO at a dose of 1,000 µg was carried out 7 days after CCI or sham surgery in male mice. Behavioral tests were performed one day before surgery and at different days after surgery. DRG and spinal cord were finally collected for quantitative real-time RT-PCR and Western blot assays.

RESULTS

NIS-lncRNA ASO significantly alleviated CCI-induced mechanical allodynia, heat hyperalgesia, and cold hyperalgesia starting on day 14 or 21 post-ASO injection and lasting for at least 7 days on the ipsilateral side. Additionally, CCI-induced spontaneous pain and ipsilateral dorsal horn neuronal and astrocyte hyperactivation were blocked on day 28 after NIS-lncRNA ASO injection. As predicted, the CCI-induced increases in the levels of NIS-lncRNA and its downstream target C-C motif chemokine ligand 2 in the ipsilateral lumbar 3 and 4 DRGs were attenuated on day 28 following NIS-lncRNA ASO injection.

CONCLUSION

Our findings indicate that systemic administration of NIS-lncRNA ASO also produces a stable and long-lasting antinociceptive effect on neuropathic pain. NIS-lncRNA ASO may have potential clinical application in the treatment of this disorder.

DRG Explant Model: Elucidating Mechanisms of Oxaliplatin-Induced Peripheral Neuropathy and Identifying Potential Therapeutic Targets

Oxaliplatin triggered chemotherapy induced peripheral neuropathy (CIPN) is a common and debilitating side effect of cancer treatment which limits the efficacy of chemotherapy and negatively impacts patients quality of life dramatically. For better understanding the mechanisms of CIPN and screen for potential therapeutic targets, it is critical to have reliable in vitro assays that effectively mirror the neuropathy in vivo . In this study, we established a dorsal root ganglia (DRG) explant model. This model displayed dose-dependent inhibition of neurite outgrowth in response to oxaliplatin, while oxalic acid exhibited no significant impact on the regrowth of DRG. The robustness of this assay was further demonstrated by the inhibition of OCT2 transporter, which facilitates oxaliplatin accumulation in neurons, fully restoring the neurite regrowth capacity. Using this model, we revealed that oxaliplatin triggered a substantial increase of oxidative stress in DRG. Notably, inhibition of TXNIP with verapamil significantly reduced oxidative stress level. Our results demonstrated the use of DRG explants as an efficient model to study the mechanisms of CIPN and screen for potential treatments.

Single-cell analysis of dorsal root ganglia reveals metalloproteinase signaling in satellite glial cells and pain

Satellite glial cells (SGCs) are among the most abundant non-neuronal cells in dorsal root ganglia (DRGs) and closely envelop sensory neurons that detect painful stimuli. However, little is still known about their homeostatic activities and their contribution to pain. Using single-cell RNA sequencing (scRNA-seq), we were able to obtain a unique transcriptional profile for SGCs. We found enriched expression of the tissue inhibitor metalloproteinase 3 (TIMP3) and other metalloproteinases in SGCs. Small interfering RNA and neutralizing antibody experiments revealed that TIMP3 modulates somatosensory stimuli. TIMP3 expression decreased after paclitaxel treatment, and its rescue by delivery of a recombinant TIMP3 protein reversed and prevented paclitaxel-induced pain. We also established that paclitaxel directly impacts metalloproteinase signaling in cultured SGCs, which may be used to identify potential new treatments for pain. Therefore, our results reveal a metalloproteinase signaling pathway in SGCs for proper processing of somatosensory stimuli and potential discovery of novel pain treatments.

Novel expression of major histocompatibility complex II in dorsal root ganglion neurons attenuates paclitaxel-induced cold hypersensitivity in male and female mice

Chemotherapy is often a life-saving treatment, but the development of intractable pain caused by chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting toxicity that restricts survival rates. Recent reports demonstrate that paclitaxel (PTX) robustly increases anti-inflammatory CD4+ T cells in the dorsal root ganglion (DRG), and that T cells and anti-inflammatory cytokines are protective against CIPN. However, the mechanism by which CD4+ T cells are activated, and the extent cytokines released by CD4+ T cells target DRG neurons are unknown. Here, we found novel expression of functional major histocompatibility complex II (MHCII) protein in DRG neurons, and CD4+ T cells in close proximity to DRG neurons, together suggesting CD4+ T cell activation and targeted cytokine release. MHCII protein is primarily expressed in small nociceptive neurons in male mouse DRG regardless of PTX, while MHCII is induced in small nociceptive neurons in female DRG after PTX. Accordingly, reducing MHCII in small nociceptive neurons increased hypersensitivity to cold only in naive male mice, but increased severity of PTX-induced cold hypersensitivity in both sexes. Collectively, our results demonstrate expression of MHCII on DRG neurons and a functional role during homeostasis and inflammation.

Satellite glial cell-secreted exosomes after in-vitro oxaliplatin treatment presents a pro-nociceptive effect for dorsal root ganglion neurons and induce mechanical hypersensitivity in naïve mice

BACKGROUND

The pathophysiological mechanism underlying chemotherapy-induced neuropathic pain (CINP) remains unclear. Sensory neuronal hypersensitivity in the dorsal root ganglion (DRG) is essential for the onset and maintenance of chronic pain. Satellite glial cells (SGCs) in the DRG potentially affect the function of sensory neurons, possibly by mediating extracellular or paracrine signaling. Exosomes play an essential role in cell-cell communication. However, the role of SGC-secreted exosomes in glia-neuron communication and CINP remains unclear.

METHODS

SGCs and sensory neurons were cultured from the DRG of mice. The SGCs were treated with 4 μM oxaliplatin for 24 h. Glial fibrillary acid protein (GFAP) and connexin-43 (Cx-43) expressions in the SGCs were examined with immunocytochemistry (ICC). Enzyme-linked immunosorbent assay (ELISA) detected cytokine release in the SGCs after oxaliplatin treatment. Subsequently, SGC-secreted exosomes were collected using ultracentrifugation and identified by nanoparticle tracking analysis, transmission electron microscopy, and western blotting. Subsequently, DRG neurons were incubated with SGC-secreted exosomes for 24 h. The percentage of reactive oxygen species (ROS)-positive neurons was detected using flow cytometry, and acid-sensing ion channel 3 (ASIC3) and transient receptor potential vanilloid 1 (TRPV1) expression were examined by western blotting. SGC-secreted exosomes were intrathecally injected into naïve mice. The mechanical withdrawal threshold was assessed 24, 48, and 72 h following the injection. TRPV1 expression in the DRG was examined 72 h after intrathecal injection. Furthermore, differentially expressed (DE) miRNAs within the SGC-secreted exosomes were detected using RNA sequencing and bioinformatics analysis. Gene Ontology (GO) enrichment, Kyoto Encyclopedia of Genes and Genomes (KEGG), and Reactome pathway analyses were performed to predict the function of the target genes of DE miRNAs. Finally, the DE miRNAs with pain regulation potential were identified in silico.

RESULTS

After in-vitro oxaliplatin treatment, ICC showed an increase in the immunoreactivity of GFAP and Cx-43 in the SGCs. ELISA results suggested an increased release of tumor necrosis factor-α and interleukin (IL)-1β, but a decreased release of IL-10. Oxaliplatin treatment increased the secretion of exosomes in the SGCs from 4.34 to 5.99 × 1011 (particles/ml). The exosome-specific markers CD9 and TSG101 were positive, whereas calnexin was negative for the obtained exosomes. Additionally, the SGC-secreted exosomes were endocytosed by DRG neurons after co-incubation. Moreover, after incubation with conditioned SGC-secreted exosomes (after 4 μM oxaliplatin treatment), the percentage of ROS-positive DRG neurons increased and ASIC3 and TRPV1 expressions were upregulated. After the intrathecal injection of the conditioned SGC-secreted exosomes, the mice presented with mechanical hypersensitivity and TRPV1 expression upregulation in the DRG. Notably, 25 and 120 significantly upregulated and downregulated miRNAs, respectively, were identified in the conditioned SGC-secreted exosomes. When predicting the function of target genes of DE miRNAs, certain GO terms, such as synapse organization, neurogenesis regulation, histone modification, and pain-related KEGG or Reactome pathways, including vascular endothelial growth factor A-vascular endothelial growth factor receptor 2, mammalian target of rapamycin, and mitogen-activated protein kinase signaling pathways, related to nervous system function were predicted. Finally, 27 pain regulation-related miRNAs, including miR-324-3p, miR-181a-5p, and miR-122-5p, were identified in silico.

CONCLUSION

Our study demonstrates that SGC-secreted exosomes after in-vitro oxaliplatin treatment present a pro-nociceptive effect for DRG neurons and induce mechanical hypersensitivity in naïve mice, possibly via the contained miRNA cargo. Identifying the candidate miRNAs and verifying their functions in vivo are required to elucidate the exosomes mediating 'glia-neuron' communication under CINP condition.

MAPK-ERK-CREB signaling pathway upregulates Nav1.6 in oxaliplatin-induced neuropathic pain in the rat

The voltage-gated sodium channel subtype Nav1.6 is involved in the electrophysiological changes of primary sensory neurons that occur in oxaliplatin-induced neuropathic pain, but its regulatory mechanism remains unclear. In this study, Western blot, RT-qPCR, immunofluorescence staining, chromatin immunoprecipitation were used to prove the mechanism of MAPK-ERK-CREB signaling pathway participating in oxaliplatin-induced neuropathic pain by regulating Nav1.6. The results showed that p-Raf1 and p-ERK, key molecules in MAPK/ERK pathway, and Nav1.6 were significantly increased in DRGs of oxaliplatin-induced neuropathic pain rats. Inhibition of p-Raf1 and p-ERK respectively not only reduced the expression of Nav1.6 protein in DRGs of OXA rats, but also caused a decrease in Nav1.6 mRNA, which led us to further explore the transcription factor CREB regulated by MAPK/ERK pathway. Results showed that CREB was co-distributed with Nav1.6. Inhibition of CREB resulted in decreased mRNA and protein expression of Nav1.6, and alleviated oxaliplatin-induced neuropathic pain. A chromatin immunoprecipitation experiment proved that OXA caused p-CREB to directly bind to the promoter region of Scn8A, which is the encoding gene for Nav1.6, and promote the transcription of Scn8A. In summary, in this study, we found that oxaliplatin can activate the MAPK/ERK pathway, which promotes the expression and activation of CREB and leads to an increase in Scn8A transcription, and then leads to an increase in Nav1.6 protein expression to enhance neuronal excitability and cause pain. This study provides an experimental basis for the molecular mechanism of sodium channel regulation in oxaliplatin-induced neuropathic 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.

Targeting dorsal root ganglia and primary sensory neurons for the treatment of chronic pain: an update

INTRODUCTION

Current treatments for chronic pain are inadequate. Here, we provide an update on the new therapeutic strategies that target dorsal root ganglia (DRGs) in the peripheral nervous system for a better and safer treatment of chronic pain.

AREAS COVERED

Despite the complex nature of chronic pain and its underlying mechanisms, we do know that changes in the plasticity and modality of neurons in DRGs play a pivotal role. DRG neurons are heterogenous and offer potential pain targets for different therapeutic interventions. We discuss the last advancements of these interventions, which include the use of systemic and local administrations, selective nerve drug delivery, and gene therapy. In particular, we provide updates and further details on the molecular characterization of primary sensory neurons, new analgesics entering the market, and future gene therapy approaches.

EXPERT OPINION

DRGs and primary sensory neurons are promising targets for chronic pain treatment due to their key role in pain signaling, unique anatomical location, and the potential for different targeted therapeutic interventions.

Scientific and Regulatory Policy Committee Technical Review: Biology and Pathology of Ganglia in Animal Species Used for Nonclinical Safety Testing

Dorsal root ganglia (DRG), trigeminal ganglia (TG), other sensory ganglia, and autonomic ganglia may be injured by some test article classes, including anti-neoplastic chemotherapeutics, adeno-associated virus-based gene therapies, antisense oligonucleotides, nerve growth factor inhibitors, and aminoglycoside antibiotics. This article reviews ganglion anatomy, cytology, and pathology (emphasizing sensory ganglia) among common nonclinical species used in assessing product safety for such test articles (TAs). Principal histopathologic findings associated with sensory ganglion injury include neuron degeneration, necrosis, and/or loss; increased satellite glial cell and/or Schwann cell numbers; and leukocyte infiltration and/or inflammation. Secondary nerve fiber degeneration and/or glial reactions may occur in nerves, dorsal spinal nerve roots, spinal cord (dorsal and occasionally lateral funiculi), and sometimes the brainstem. Ganglion findings related to TA administration may result from TA exposure and/or trauma related to direct TA delivery into the central nervous system or ganglia. In some cases, TA-related effects may need to be differentiated from a spectrum of artifactual and/or spontaneous background changes.

Scientific and Regulatory Policy Committee Points to Consider: Sampling, Processing, Evaluation, Interpretation, and Reporting of Test Article-Related Ganglion Pathology for Nonclinical Toxicity Studies

Certain biopharmaceutical products consistently affect dorsal root ganglia, trigeminal ganglia, and/or autonomic ganglia. Product classes targeting ganglia include antineoplastic chemotherapeutics, adeno-associated virus-based gene therapies, antisense oligonucleotides, and anti-nerve growth factor agents. This article outlines "points to consider" for sample collection, processing, evaluation, interpretation, and reporting of ganglion findings; these points are consistent with published best practices for peripheral nervous system evaluation in nonclinical toxicity studies. Ganglion findings often occur as a combination of neuronal injury (e.g., degeneration, necrosis, and/or loss) and/or glial effects (e.g., increased satellite glial cell cellularity) with leukocyte accumulation (e.g., mononuclear cell infiltration or inflammation). Nerve fiber degeneration and/or glial reactions may be seen in nerves, dorsal spinal nerve roots, spinal cord, and occasionally brainstem. Interpretation of test article (TA)-associated effects may be confounded by incidental background changes or experimental procedure-related changes and limited historical control data. Reports should describe findings at these sites, any TA relationship, and the criteria used for assigning severity grades. Contextualizing adversity of ganglia findings can require a weight-of-evidence approach because morphologic changes of variable severity occur in ganglia but often are not accompanied by observable overt in-life functional alterations detectable by conventional behavioral and neurological testing techniques.

Self-assembled innervated vasculature-on-a-chip to study nociception

Nociceptor sensory neurons play a key role in eliciting pain. An active crosstalk between nociceptor neurons and the vascular system at the molecular and cellular level is required to sense and respond to noxious stimuli. Besides nociception, interaction between nociceptor neurons and vasculature also contributes to neurogenesis and angiogenesis.In vitromodels of innervated vasculature can greatly help delineate these roles while facilitating disease modeling and drug screening. Herein, we report the development of a microfluidic-assisted tissue model of nociception in the presence of microvasculature. The self-assembled innervated microvasculature was engineered using endothelial cells and primary dorsal root ganglion (DRG) neurons. The sensory neurons and the endothelial cells displayed distinct morphologies in presence of each other. The neurons exhibited an elevated response to capsaicin in the presence of vasculature. Concomitantly, increased transient receptor potential cation channel subfamily V member 1 (TRPV1) receptor expression was observed in the DRG neurons in presence of vascularization. Finally, we demonstrated the applicability of this platform for modeling nociception associated with tissue acidosis. While not demonstrated here, this platform could also serve as a tool to study pain resulting from vascular disorders while also paving the way towards the development of innervated microphysiological models.

Neuro-mesodermal assembloids recapitulate aspects of peripheral nervous system development in vitro

Here we describe a novel neuro-mesodermal assembloid model that recapitulates aspects of peripheral nervous system (PNS) development such as neural crest cell (NCC) induction, migration, and sensory as well as sympathetic ganglion formation. The ganglia send projections to the mesodermal as well as neural compartment. Axons in the mesodermal part are associated with Schwann cells. In addition, peripheral ganglia and nerve fibers interact with a co-developing vascular plexus, forming a neurovascular niche. Finally, developing sensory ganglia show response to capsaicin indicating their functionality. The presented assembloid model could help to uncover mechanisms of human NCC induction, delamination, migration, and PNS development. Moreover, the model could be used for toxicity screenings or drug testing. The co-development of mesodermal and neuroectodermal tissues and a vascular plexus along with a PNS allows us to investigate the crosstalk between neuroectoderm and mesoderm and between peripheral neurons/neuroblasts and endothelial cells.

Feelings Are the Source of Consciousness

In this view, we address the problem of consciousness, and although we focus on its human presentation, we note that the phenomenon is present in numerous nonhuman species and use findings from a variety of animal studies to explain our hypothesis for how consciousness is made. Consciousness occurs when mind contents, such as perceptions and thoughts, are spontaneously identified as belonging to a specific organism/owner. Conscious minds are said to have a self that experiences mental events. We hypothesize that the automatic identification that associates minds and organisms is provided by a continuous flow of homeostatic feelings. Those feelings arise from the uninterrupted process of life regulation and correspond to both salient physiological fluctuations such as hunger, pain, well-being, or malaise, as well as to states closer to metabolic equilibrium and best described as feelings of life/existence, such as breathing or body temperature. We also hypothesize that homeostatic feelings were the inaugural phenomena of consciousness in biological evolution and venture that they were selected because the information they provided regarding the current state of life regulation conferred extraordinary advantages to the organisms so endowed. The "knowledge" carried by conscious homeostatic feelings provided "overt" guidance for life regulation, an advance over the covert regulation present in nonconscious organisms. Finally, we outline a mechanism for the generation of feelings based on a two-way interaction between interoceptive components of the nervous system and a particular set of nonneural components of the organism's interior, namely, viscera and circulating chemical molecules involved in their operations. Feelings emerge from this interaction as continuous and hybrid phenomena, related simultaneously to two series of events. The first is best described by the terms neural/representational/and mental and the second by the terms nonneural/visceral/and chemical. We note that this account offers a solution for the mind-body problem: homeostatic feelings constitute the "mental" version of bodily processes.

Post-infectious Painful Sensory Neuronopathy Following Giardia Infection Responsive to Intravenous Immunoglobulin Treatment

Sensory neuronopathy is a rare pure sensory disorder with characteristic clinical features of early-onset ataxia and a multifocal distribution of non-length-dependent sensory deficits. Diabetes is the most common cause of length-dependent peripheral neuropathy. However, in acute to subacute presentations, conditions such as autoimmune diseases, paraneoplastic syndrome, exposure to toxins, and viral infection could be common etiologies. This report presents a patient with sensory neuronopathy following a Giardia infection. Gait disturbance, neuropathic pain, ataxia, and pseudoathetosis improved by varying degrees following the monthly maintenance of intravenous immunoglobulin (IVIG). An immune-mediated or direct pathogenic attack can explain the underlying pathogenesis behind this patient's peripheral nerve dysfunction.

Best Practices for Dorsal Root Ganglion Stimulation for Chronic Pain: Guidelines from the American Society of Pain and Neuroscience

With continued innovations in neuromodulation comes the need for evolving reviews of best practices. Dorsal root ganglion stimulation (DRG-S) has significantly improved the treatment of complex regional pain syndrome (CRPS), and it has broad applicability across a wide range of other conditions. Through funding and organizational leadership by the American Society for Pain and Neuroscience (ASPN), this best practices consensus document has been developed for the selection, implantation, and use of DRG stimulation for the treatment of chronic pain syndromes. This document is composed of a comprehensive narrative literature review that has been performed regarding the role of the DRG in chronic pain and the clinical evidence for DRG-S as a treatment for multiple pain etiologies. Best practice recommendations encompass safety management, implantation techniques, and mitigation of the potential complications reported in the literature. Looking to the future of neuromodulation, DRG-S holds promise as a robust intervention for otherwise intractable pain.

Neurofilament light chain and dorsal root ganglia injury after adeno-associated virus 9 gene therapy in nonhuman primates

In nonhuman primates (NHPs), adeno-associated virus serotype 9 (AAV9) vectorized gene therapy can cause asymptomatic microscopic injury to dorsal root ganglia (DRG) and trigeminal ganglia (TG) somatosensory neurons, causing neurofilament light chain (NfL) to diffuse into cerebrospinal fluid (CSF) and blood. Data from 260 cynomolgus macaques administered vehicle or AAV9 vectors (intrathecally or intravenously) were analyzed to investigate NfL as a soluble biomarker for monitoring DRG/TG microscopic findings. The incidence of key DRG/TG findings with AAV9 vectors was 78% (maximum histopathology severity, moderate) at 2-12 weeks after the dose. When examined up to 52 weeks after the dose, the incidence was 42% (maximum histopathology severity, minimal). Terminal NfL concentrations in plasma, serum, and CSF correlated with microscopic severity. After 52 weeks, NfL returned to pre-dose baseline concentrations, correlating with microscopic findings of lesser incidence and/or severity compared with interim time points. Blood and CSF NfL concentrations correlated with asymptomatic DRG/TG injury, suggesting that monitoring serum and plasma concentrations is as useful for assessment as more invasive CSF sampling. Longitudinal assessment of NfL concentrations related to microscopic findings associated with AAV9 administration in NHPs indicates NfL could be a useful biomarker in nonclinical toxicity testing. Caution should be applied for any translation to humans.

Persistent sensory changes and sex differences in transgenic mice conditionally expressing HIV-1 Tat regulatory protein

HIV-associated sensory neuropathies (HIV-SN) are prevalent in >50% of patients aged over 45 years many of which report moderate to severe chronic pain. Previous preclinical studies have investigated the mechanisms by which HIV-1 causes sensory neuropathies and pain-like behaviors. The aim of the present study is to delineate the role of chronic HIV-1 trans-activator of transcription protein (Tat) exposure in the development of neuropathy in mice. The temporal effects of conditional Tat expression on the development of hypersensitivity to mechanical (von Frey filaments) and thermal (heat or cold) stimuli were tested in male and female mice that transgenically expressed HIV-1 Tat in a doxycycline-inducible manner. Inducing Tat expression produced an allodynic response to mechanical or cold (but not heat) stimuli that respectively persisted for at least 23-weeks (mechanical hypersensitivity) or at least 8-weeks (cold hypersensitivity). Both allodynic states were greater in magnitude among females, compared to males, and mechanical increased hypersensitivity progressively in females over time. Acute morphine or gabapentin treatment partly attenuated allodynia in males, but not females. Irrespective of sex, Tat reduced intraepidermal nerve fiber density, the mean amplitude of sensory nerve action potentials (but not conductance), engagement in some pain-related ethological behaviors (cage-hanging and rearing), and down-regulated PPAR-α gene expression in lumbar spinal cord while upregulating TNF-α expression in dorsal root ganglion. Taken together, these data reveal fundamental sex differences in mechanical and cold hypersensitivity in response to Tat and demonstrate the intractable nature in female mice to current therapeutics. Understanding the role of Tat in these pathologies may aid the design of future therapies aimed at mitigating the peripheral sensory neuropathies that accompany neuroHIV.

Macrophages and glial cells: Innate immune drivers of inflammatory arthritic pain perception from peripheral joints to the central nervous system

Millions of people suffer from arthritis worldwide, consistently struggling with daily activities due to debilitating pain evoked by this disease. Perhaps the most intensively investigated type of inflammatory arthritis is rheumatoid arthritis (RA), where, despite considerable advances in research and clinical management, gaps regarding the neuroimmune interactions that guide inflammation and chronic pain in this disease remain to be clarified. The pain and inflammation associated with arthritis are not isolated to the joints, and inflammatory mechanisms induced by different immune and glial cells in other tissues may affect the development of chronic pain that results from the disease. This review aims to provide an overview of the state-of-the-art research on the roles that innate immune, and glial cells play in the onset and maintenance of arthritis-associated pain, reviewing nociceptive pathways from the joint through the dorsal root ganglion, spinal circuits, and different structures in the brain. We will focus on the cellular mechanisms related to neuroinflammation and pain, and treatments targeting these mechanisms from the periphery and the CNS. A comprehensive understanding of the role these cells play in peripheral inflammation and initiation of pain and the central pathways in the spinal cord and brain will facilitate identifying new targets and pathways to aide in developing therapeutic strategies to treat joint pain associated with RA.

Peripartum effects of synthetic oxytocin: The good, the bad, and the unknown

The first clinical applications of oxytocin (OT) were in obstetrics as a hormone to start and speed up labor and to control postpartum hemorrhage. Discoveries in the 1960s and 1970s revealed that the effects of OT are not limited to its peripheral actions around birth and milk ejection. Indeed, OT also acts as a neuromodulator in the brain affecting fear memory, social attachment, and other forms of social behaviors. The peripheral and central effects of OT have been separately subject to extensive scrutiny. However, the effects of peripheral OT-particularly in the form of administration of synthetic OT (synOT) around birth-on the central nervous system are surprisingly understudied. Here, we provide a narrative review of the current evidence, suggest putative mechanisms of synOT action, and provide new directions and hypotheses for future studies to bridge the gaps between neuroscience, obstetrics, and psychiatry.

Changes in serum angiogenic factors among patients with acute pain and subacute pain

Screening serum biomarkers for acute and subacute pain is important for precise pain management. This study aimed to examine serum levels of angiogenic factors in patients with acute and subacute pain as potential biomarkers. Serum samples were collected from 12 healthy controls, 20 patients with postherpetic neuralgia (PHN), 4 with low back pain (LBP), and 1 with trigeminal neuralgia (TN). Pain intensity in these patients was evaluated using the visual analog scale (VAS). The serum concentrations of 11 angiogenic biomarkers were examined by Milliplex Map Human Angiogenesis Magnetic Bead Panel 2. The pain assessment from VAS showed that all patients showed moderate and severe pain. Among 11 angiogenic factors, osteopontin (OPN), thrombospondin-2 (TSP-2), soluble platelet endothelial cell adhesion molecule-1 (sPECAM-1), soluble urokinase-type plasminogen activator receptor (suPAR), and soluble epidermal growth factor receptors (sErbB2) were up-regulated and soluble interleukin-6 receptor α (sIL-6Rα) were down-regulated in patients with pain compared to the healthy participants (all P-values were < 0.005). Moreover, a linear regression model showed that the serum OPN concentration was correlated with pain intensity in patients with PHN (P = 0.03). There was no significant difference between the serum concentration of soluble epidermal growth factor receptors, sErbB3, soluble AXL, tenascin, and soluble neuropilin-1 in patients with acute and subacute pain and that of healthy controls. The results of this study provided new valuable insights into our understanding of angiogenic factors that may contribute to as mechanistic biomarkers of pain, and reveal the pathophysiological mechanism of pain.

Clinical Trial Registration:www.chictr.org.cn, identifier ChiCTR2200061775.

Macrophage Migration Inhibitory Factor (MIF) Makes Complex Contributions to Pain-Related Hyperactivity of Nociceptors after Spinal Cord Injury

Neuropathic pain is a major, inadequately treated challenge for people with spinal cord injury (SCI). While SCI pain mechanisms are often assumed to be in the CNS, rodent studies have revealed mechanistic contributions from primary nociceptors. These neurons become chronically hyperexcitable after SCI, generating ongoing electrical activity that promotes ongoing pain. A major question is whether extrinsic chemical signals help to drive ongoing electrical activity after SCI. People living with SCI exhibit acute and chronic elevation of circulating levels of macrophage migration inhibitory factor (MIF), a cytokine implicated in preclinical pain models. Probable nociceptors isolated from male rats and exposed to an MIF concentration reported in human plasma (1 ng/ml) showed hyperactivity similar to that induced by SCI, although, surprisingly, a 10-fold higher concentration failed to increase excitability. Conditioned behavioral aversion to a chamber associated with peripheral MIF injection suggested that MIF stimulates affective pain. A MIF inhibitor, Iso-1, reversed SCI-induced hyperexcitability. Unlike chronic SCI-induced hyperexcitability, acute MIF-induced hyperexcitability was only partially abrogated by inhibiting ERK signaling. Unexpectedly, MIF concentrations that induced hyperactivity in nociceptors from naive animals, after SCI induced a long-lasting conversion from a highly excitable nonaccommodating type to a rapidly accommodating, hypoexcitable type, possibly as a homeostatic response to prolonged depolarization. Treatment with conditioned medium from cultures of DRG cells obtained after SCI was sufficient to induce MIF-dependent hyperactivity in neurons from naive rats. Thus, changes in systemic and DRG levels of MIF may help to maintain SCI-induced nociceptor hyperactivity that persistently promotes pain.SIGNIFICANCE STATEMENT Chronic neuropathic pain is a major challenge for people with spinal cord injury (SCI). Pain can drastically impair quality of life, and produces substantial economic and social burdens. Available treatments, including opioids, remain inadequate. This study shows that the cytokine macrophage migration inhibitory factor (MIF) can induce pain-like behavior and plays an important role in driving persistent ongoing electrical activity in injury-detecting sensory neurons (nociceptors) in a rat SCI model. The results indicate that SCI produces an increase in MIF release within sensory ganglia. Low MIF levels potently excite nociceptors, but higher levels trigger a long-lasting hypoexcitable state. These findings suggest that therapeutic targeting of MIF in neuropathic pain states may reduce pain and sensory dysfunction by curbing nociceptor hyperactivity.

The Role of Vascular-Immune Interactions in Modulating Chemotherapy Induced Neuropathic Pain

Chemotherapy causes sensory disturbances in cancer patients that results in neuropathies and pain. As cancer survivorships has dramatically increased over the past 10 years, pain management of these patients is becoming clinically more important. Current analgesic strategies are mainly ineffective and long-term use is associated with severe side effects. The issue being that common analgesic strategies are based on ubiquitous pain mediator pathways, so when applied to clinically diverse neuropathic pain and neurological conditions, are unsuccessful. This is principally due to the lack of understanding of the driving forces that lead to chemotherapy induced neuropathies. It is well documented that chemotherapy causes sensory neurodegeneration through axonal atrophy and intraepidermal fibre degeneration causing alterations in pain perception. Despite the neuropathological alterations associated with chemotherapy-induced neuropathic pain being extensively researched, underlying causes remain elusive. Resent evidence from patient and rodent studies have indicated a prominent inflammatory cell component in the peripheral sensory nervous system in effected areas post chemotherapeutic treatment. This is accompanied by modulation of auxiliary cells of the dorsal root ganglia sensory neurons such as activation of satellite glia and capillary dysfunction. The presence of a neuroinflammatory component was supported by transcriptomic analysis of dorsal root ganglia taken from mice treated with common chemotherapy agents. With key inflammatory mediators identified, having potent immunoregulatory effects that directly influences nociception. We aim to evaluate the current understanding of these immune-neuronal interactions across different cancer therapy drug classes. In the belief this may lead to better pain management approaches for cancer survivors.

Dorsal root ganglion toll-like receptor 4 signaling contributes to oxaliplatin-induced peripheral neuropathy

ABSTRACT

Activation of toll-like receptor 4 (TLR4) in the dorsal root ganglion (DRG) and spinal cord contributes to the generation of paclitaxel-related chemotherapy-induced peripheral neuropathy (CIPN). Generalizability of TLR4 signaling in oxaliplatin-induced CIPN was tested here. Mechanical hypersensitivity developed in male SD rats by day 1 after oxaliplatin treatment, reached maximum intensity by day 14, and persisted through day 35. Western blot revealed an increase in TLR4 expression in the DRG of oxaliplatin at days 1 and 7 after oxaliplatin treatment. Cotreatment of rats with the TLR4 antagonist lipopolysaccharide derived from Rhodobacter sphaeroides ultrapure or with the nonspecific immunosuppressive minocycline with oxaliplatin resulted in significantly attenuated hyperalgesia on day 7 and 14 compared with rats that received oxaliplatin plus saline vehicle. Immunostaining of DRGs revealed an increase in the number of neurons expressing TLR4, its canonical downstream signal molecules myeloid differentiation primary response gene 88 (MyD88) and TIR-domain-containing adapter-inducing interferon-β, at both day 7 and day 14 after oxaliplatin treatment. These increases were blocked by cotreatment with either lipopolysaccharide derived from Rhodobacter sphaeroides or minocycline. Double staining showed the localization of TLR4, MyD88, and TIR-domain-containing adapter-inducing interferon-β in subsets of DRG neurons. Finally, there was no significant difference in oxaliplatin-induced mechanical hypersensitivity between male and female rats when observed for 2 weeks. Furthermore, upregulation of TLR4 was detected in both sexes when tested 14 days after treatment with oxaliplatin. These findings suggest that the activation of TLR4 signaling in DRG neurons is a common mechanism in CIPN induced by multiple cancer chemotherapy agents.

Dorsal Root Ganglion Volumetry by MR Gangliography

BACKGROUND AND PURPOSE

Dorsal root ganglion MR imaging (MR gangliography) is increasingly gaining clinical-scientific relevance. However, dorsal root ganglion morphometry by MR imaging is typically performed under the assumption of ellipsoid geometry, which remains to be validated.

MATERIALS AND METHODS

Sixty-four healthy volunteers (37 [57.8%] men; mean age, 31.5 [SD, 8.3] years) underwent MR gangliography of the bilateral L4-S2 levels (3D-T2WI TSE spectral attenuated inversion recovery-sampling perfection with application-optimized contrasts by using different flip angle evolution, isotropic voxels = 1.1 mm³, TE = 301 ms). Ground truth dorsal root ganglion volumes were bilaterally determined for 96 dorsal root ganglia (derivation cohort) by expert manual 3D segmentation by 3 independent raters. These ground truth dorsal root ganglion volumes were then compared with geometric ellipsoid dorsal root ganglion approximations as commonly practiced for dorsal root ganglion morphometry. On the basis of the deviations from ellipsoid geometry, improved volume estimation could be derived and was finally applied to a large human validation cohort (510 dorsal root ganglia).

RESULTS

Commonly used equations of ellipsoid geometry underestimate true dorsal root ganglion volume by large degrees (factor = 0.42-0.63). Ground truth segmentation enabled substantially optimizing dorsal root ganglion geometric approximation using its principal axes lengths by deriving the dorsal root ganglion volume term of [Formula: see text]. Using this optimization, the mean volumes of 510 lumbosacral healthy dorsal root ganglia were as follows: L4: 211.3 (SD, 52.5) mm³, L5: 290.7 (SD, 90.9) mm³, S1: 384.2 (SD, 145.0) mm³, and S2: 192.4 (SD, 52.6) mm³. Dorsal root ganglion volume increased from L4 to S1 and decreased from S1 to S2 (P < .001). Dorsal root ganglion volume correlated with subject height (r = . 22, P < .001) and was higher in men (P < .001).

CONCLUSIONS

Dorsal root ganglion volumetry by measuring its principal geometric axes on MR gangliography can be substantially optimized. By means of this optimization, dorsal root ganglion volume distribution was estimated in a large healthy cohort for the clinically most relevant lumbosacral levels, L4-S2.

Ultrasound localization microscopy and functional ultrasound imaging reveal atypical features of the trigeminal ganglion vasculature

The functional imaging within the trigeminal ganglion (TG) is highly challenging due to its small size and deep localization. This study combined a methodological framework able to dive into the rat trigeminal nociceptive system by jointly providing 1) imaging of the TG blood vasculature at microscopic resolution, and 2) the measurement of hemodynamic responses evoked by orofacial stimulations in anesthetized rats. Despite the small number of sensory neurons within the TG, functional ultrasound imaging was able to image and quantify a strong and highly localized hemodynamic response in the ipsilateral TG, evoked not only by mechanical or chemical stimulations of corneal nociceptive fibers, but also by cutaneous mechanical stimulations of the ophthalmic and maxillary orofacial regions using a von Frey hair. The in vivo quantitative imaging of the TG’s vasculature using ultrasound localization microscopy combined with in toto labelling reveals particular features of the vascularization of the area containing the sensory neurons, that are likely the origin of this strong vaso-trigeminal response. This innovative imaging approach opens the path for future studies on the mechanisms underlying changes in trigeminal local blood flow and evoked hemodynamic responses, key mechanisms for the understanding and treatment of debilitating trigeminal pain conditions.

Visualisation of rat trigeminal ganglia activation during ophthalmic or maxillary nociceptive stimulations shows atypical tortuous vascularisation and a somatotopic hemodynamic response.

DRGquant: A new modular AI-based pipeline for 3D analysis of the DRG

BACKGROUND

The dorsal root ganglion (DRG) is structurally complex and pivotal to systems processing nociception. Whole mount analysis allows examination of intricate microarchitectural and cellular relationships of the DRG in three-dimensional (3D) space.

NEW METHOD

We present DRGquant a set of tools and techniques optimized as a pipeline for automated image analysis and reconstruction of cells/structures within the DRG. We have developed an open source software pipeline that utilizes machine learning to identify substructures within the DRG and reliably classify and quantify them.

RESULTS

Our methods were sufficiently sensitive to isolate, analyze, and classify individual DRG substructures including macrophages. The activation of macrophages was visualized and quantified in the DRG following intrathecal injection of lipopolysaccharide, and in a model of chemotherapy induced peripheral neuropathy. The percent volume of infiltrating macrophages was similar to a commercial source in quantification. Circulating fluorescent dextran was visualized within DRG macrophages using whole mount preparations, which enabled 3D reconstruction of the DRG and DRGquant demonstrated subcellular spatial resolution within individual macrophages.

COMPARISON WITH EXISTING METHOD(S)

Here we describe a reliable and efficient methodologic pipeline to prepare cleared and whole mount DRG tissue. DRGquant allows automated image analysis without tedious manual gating to reduce bias. The quantitation of DRG macrophages was superior to commercial solutions.

CONCLUSIONS

Using machine learning to separate signal from noise and identify individual cells, DRGquant enabled us to isolate individual structures or areas of interest within the DRG for a more granular and fine-tuned analysis. Using these 3D techniques, we are better able to appreciate the biology of the DRG under experimental inflammatory conditions.

Single-Dose Intrathecal Dorsal Root Ganglia Toxicity of Onasemnogene Abeparvovec in Cynomolgus Monkeys

Intravenous onasemnogene abeparvovec is approved for the treatment of spinal muscular atrophy in children < 2 years. For later-onset patients, intrathecal onasemnogene abeparvovec may be advantageous over intravenous administration. Recently, microscopic dorsal root ganglion (DRG) changes were observed in nonhuman primates (NHPs) following intrathecal onasemnogene abeparvovec administration. To characterize these DRG findings, two NHP studies evaluating intrathecal onasemnogene abeparvovec administration were conducted: a 12-month study with a 6-week interim cohort and a 13-week study with a 2-week interim cohort. The latter investigated the potential impact of prednisolone or rituximab plus everolimus on DRG toxicity. An additional 6-month, single-dose, intravenous NHP study conducted in parallel evaluated onasemnogene abeparvovec safety (including DRG toxicity) with or without prednisolone coadministration. Intrathecal onasemnogene abeparvovec administration was well tolerated and not associated with clinical observations. Microscopic onasemnogene abeparvovec-related changes were observed in the DRG and trigeminal ganglion (TG) and included mononuclear cell inflammation and/or neuronal degeneration, which was colocalized with high vector transcript expression at 6 weeks postdose. Incidence and severity of DRG changes were generally decreased after 52 weeks compared with 6 weeks postdose. Other onasemnogene abeparvovec-related microscopic findings of axonal degeneration, mononuclear cell infiltrates and/or gliosis in the spinal cord, dorsal spinal nerve root/spinal nerves, and/or peripheral nerves were absent or found at decreased incidences and/or severities after 52 weeks. DRG and/or TG microscopic findings following intravenous onasemnogene abeparvovec dosing included minimal to slight neuronal degeneration and mononuclear cell inflammation at 6 weeks and 6 months postdose. Nervous system microscopic findings following intrathecal onasemnogene abeparvovec (≥1.2 × 10¹³ vg/animal) trended toward resolution after 52 weeks, supporting nonprogression of changes, including in the DRG. Onasemnogene abeparvovec-related DRG findings were not associated with electrophysiology changes and were not ameliorated by prednisolone or rituximab plus everolimus coadministration. The pathogenesis is possibly a consequence of increased vector genome transduction and/or transgene expression.

Lysophosphatidic acid receptor1/3 antagonist inhibits the activation of satellite glial cells and reduces acute nociceptive responses

Lysophosphatidic acid (LPA) exerts various biological activities through six characterized G protein-coupled receptors (LPA_1-6 ). While LPA-LPA₁  signaling contributes toward the demyelination and retraction of C-fiber and induces neuropathic pain, the effects of LPA-LPA₁  signaling on acute nociceptive pain is uncertain. This study investigated the role of LPA-LPA₁  signaling in acute nociceptive pain using the formalin test. The pharmacological inhibition of the LPA-LPA₁ axis significantly attenuated formalin-induced nociceptive behavior. The LPA₁  mRNA was expressed in satellite glial cells (SGCs) in dorsal root ganglion (DRG) and was particularly abundant in SGCs surrounding large DRG neurons, which express neurofilament 200. Treatment with LPA_1/3 receptor (LPA_1/3 ) antagonist inhibited the upregulation of glial markers and inflammatory cytokines in DRG following formalin injection. The LPA_1/3 antagonist also attenuated phosphorylation of extracellular signal-regulated kinase, especially in SGCs and cyclic AMP response element-binding protein in the dorsal horn following formalin injection. LPA amounts after formalin injection to the footpad were quantified by liquid chromatography/tandem mass spectrometry, and LPA levels were found to be increased in the innervated DRGs. Our results indicate that LPA produced in the innervated DRGs promotes the activation of SGCs through LPA₁ , increases the sensitivity of primary neurons, and modulates pain behavior. These results facilitate our understanding of the pathology of acute nociceptive pain and demonstrate the possibility of the LPA₁ on SGCs as a novel target for acute pain control.

Pharmacologic Management of Persistent Pain in Cancer Survivors

Improvements in screening, diagnosis and treatment of cancer has seen cancer mortality substantially diminish in the past three decades. It is estimated there are almost 20 million cancer survivors in the USA alone, but some 40% live with chronic pain after completing treatment. While a broad definition of survivorship that includes all people living with, through and beyond a cancer diagnosis—including those with active cancer—is often used, this narrative review primarily focuses on the management of pain in people who are disease-free after completing primary cancer treatment as adults. Chronic pain in this population needs a different approach to that used for people with a limited prognosis. After describing the common chronic pain syndromes caused by cancer treatment, and the pathophysiologic mechanisms involved, the pharmacologic management of entities such as post-surgical pain, chemotherapy-induced neuropathy, aromatase inhibitor musculoskeletal syndrome and checkpoint inhibitor-related pain are described. The challenges  associated with opioid prescribing in this population are given special attention. Expert guidelines on pain management in cancer survivors now recommend a combination of pharmacologic and non-pharmacologic modalities, and these are also briefly covered.

Small molecule targeting NaV1.7 via inhibition of the CRMP2-Ubc9 interaction reduces and prevents pain chronification in a mouse model of oxaliplatin-induced neuropathic pain

Treatment with anti-neoplastic agents can lead to the development of chemotherapy induced peripheral neuropathy (CIPN), which is long lasting and often refractory to treatment. This neuropathic pain develops along dermatomes innervated by peripheral nerves with cell bodies located in the dorsal root ganglia (DRG). The voltage-gated sodium channel NaV1.7 is expressed at high levels in peripheral nerve tissues and has been implicated in the development of CIPN. Efforts to develop novel analgesics directly inhibiting NaV1.7 have been unsuccessful, and our group has pioneered an alternative approach based on indirect modulation of channel trafficking by the accessory protein collapsin response mediator protein 2 (CRMP2). We have recently reported a small molecule, compound 194, that inhibits CRMP2 SUMOylation by the E2 SUMO-conjugating enzyme Ubc9 (Cai et al. , Sci. Transl. Med. 2021 13(6 1 9):eabh1314). Compound 194 is a potent and selective inhibitor of NaV1.7 currents in DRG neurons and reverses mechanical allodynia in models of surgical, inflammatory, and neuropathic pain, including spared nerve injury and paclitaxelinduced peripheral neuropathy. Here we report that, in addition to its reported effects in rats, 194 also reduces mechanical allodynia in male CD-1 mice treated with platinumcomplex agent oxaliplatin. Importantly, treatment with 194 prevented the development of mechanical allodynia when co-administered with oxaliplatin. No effects were observed on the body weight of animals treated with oxaliplatin or 194 throughout the study period. These findings support the notion that 194 is a robust inhibitor of CIPN that reduces established neuropathic pain and prevents the emergence of neuropathic pain during treatment with multiple anti-neoplastic agents in both mice and rats.

Protective effects of anandamide against cisplatin-induced peripheral neuropathy in rats

Background/aim

Cisplatin (CIS) is an effective antineoplastic agent used in the treatment of several cancer types. Peripheral neuropathy is a major dose-limiting side-effect in CIS therapy. Cannabinoids may alleviate this painful side effect. This study investigated the analgesic effects of anandamide (AN) on CIS-induced peripheral neuropathy, in vitro effects of AN in CIS neurotoxicity, and the contribution of nitric oxide (NO) in this effect.

Materials and methods

This is an experimental animal study. Primary dorsal root ganglion (DRG) cultures were prepared from one-day-old rats for in vitro investigations. DRG cells were incubated with CIS (100–300 M), and AN (10, 50, 100, and 500 μM) was administered with the submaximal concentration of CIS. Female Sprague Dawley rats were divided into control, CIS, CIS+AN, CIS+AN+L-NG-nitro arginine methyl ester (LNAME). CIS was administered 3 mg/kg i.p once weekly for 5 weeks. AN (1 mg/kg i.p) or in combination with 10 mg/kg i.p LNAME was administrated 30 min before CIS injection. Mechanical allodynia, thermal hyperalgesia, and tail clip tests were performed. After intracardiac perfusion, sciatic nerves (SN), and DRGs were isolated and semi-thin sections were stained with toluidine blue and investigated histologically. SPSS v. 21.0 and Sigma STAT 3.5 were used for statistical analysis. One/two way ANOVA, Kruskal–Wallis, and Wilcoxon signed ranks tests were used. A p-value of 0.05 was accepted as significant.

Results

CIS caused significant mechanical allodynia. AN and AN+LNAME significantly increased hind paw withdrawal latency in mechanical allodynia test. The degenerated axons significantly increased in CIS group, while decreased in AN group. The frequency of larger neurons seemed to be higher in CIS+AN group.

Conclusion

AN may be a therapeutic alternative for the treatment of CIS-induced peripheral neuropathy. However, its central adverse effects must be considered.

Cutaneous pain in disorders affecting peripheral nerves

Our ability to quickly detect and respond to harmful environmental stimuli is vital for our safety and survival. This inherent acute pain detection is a "gift" because it both protects our body from harm and allows healing of damaged tissues [1]. Damage to tissues from trauma or disease can result in distorted or amplified nociceptor signaling and sensitization of the spinal cord and brain (Central Nervous System; CNS) pathways to normal input from light touch mechanoreceptors. Together, these processes can result in nagging to unbearable chronic pain and extreme sensitivity to light skin touch (allodynia). Unlike acute protective pain, chronic pain and allodynia serve no useful purpose and can severely reduce the quality of life of an affected person. Chronic pain can arise from impairment to peripheral neurons, a phenomenon called "peripheral neuropathic pain." Peripheral neuropathic pain can be caused by many insults that directly affect peripheral sensory neurons, including mechanical trauma, metabolic imbalance (e.g., diabetes), autoimmune diseases, chemotherapeutic agents, viral infections (e.g., shingles). These insults cause "acquired" neuropathies such as small-fiber neuropathies, diabetic neuropathy, chemotherapy-induced peripheral neuropathy, and post herpetic neuralgia. Peripheral neuropathic pain can also be caused by genetic factors and result in hereditary neuropathies that include Charcot-Marie-Tooth disease, rare channelopathies and Fabry disease. Many acquired and hereditary neuropathies affect the skin, our largest organ and protector of nearly our entire body. Here we review how cutaneous nociception (pain perceived from the skin) is altered following diseases that affect peripheral nerves that innervate the skin. We provide an overview of how noxious stimuli are detected and encoded by molecular transducers on subtypes of cutaneous afferent endings and conveyed to the CNS. Next, we discuss several acquired and hereditary diseases and disorders that cause painful or insensate (lack of sensation) cutaneous peripheral neuropathies, the symptoms and percepts patients experience, and how cutaneous afferents and other peripheral cell types are altered in function in these disorders. We highlight exciting new research areas that implicate non-neuronal skin cells, particularly keratinocytes, in cutaneous nociception and peripheral neuropathies. Finally, we conclude with ideas for innovative new directions, areas of unmet need, and potential opportunities for novel cutaneous therapeutics that may avoid CNS side effects, as well as ideas for improved translation of mechanisms identified in preclinical models to patients.

Targeting strategies for oxaliplatin-induced peripheral neuropathy: clinical syndrome, molecular basis, and drug development

Oxaliplatin (OHP)-induced peripheral neurotoxicity (OIPN) is a severe clinical problem and potentially permanent side effect of cancer treatment. For the management of OIPN, accurate diagnosis and understanding of significant risk factors including genetic vulnerability are essential to improve knowledge regarding the prevalence and incidence of OIPN as well as enhance strategies for the prevention and treatment of OIPN. The molecular mechanisms underlying OIPN are complex, with multi-targets and various cells causing neuropathy. Furthermore, mechanisms of OIPN can reinforce each other, and combination therapies may be required for effective management. However, despite intense investigation in preclinical and clinical studies, no preventive therapies have shown significant clinical efficacy, and the established treatment for painful OIPN is limited. Duloxetine is the only agent currently recommended by the American Society of Clinical Oncology. The present article summarizes the most recent advances in the field of studies on OIPN, the overview of the clinical syndrome, molecular basis, therapy development, and outlook of future drug candidates. Importantly, closer links between clinical pain management teams and oncology will advance the effectiveness of OIPN treatment, and the continued close collaboration between preclinical and clinical research will facilitate the development of novel prevention and treatments for OIPN.

Biodistribution of Adeno-Associated Virus Serotype 5 Viral Vectors Following Intrathecal Injection

The pharmacokinetic profile of AAV particles following intrathecal delivery has not yet been clearly defined. The present study evaluated the distribution profile of adeno-associated virus serotype 5 (AAV5) viral vectors following lumbar intrathecal injection in mice. After a single bolus intrathecal injection, viral DNA concentrations in mouse whole blood, spinal cord, and peripheral tissues were determined using quantitative polymerase chain reaction (qPCR). The kinetics of AAV5 vector in whole blood and the concentration over time in spinal and peripheral tissues were analyzed. Distribution of the AAV5 vector to all levels of the spinal cord, dorsal root ganglia, and into systemic circulation occurred rapidly within 30 min following injection. Vector concentration in whole blood reached a maximum 6 h postinjection with a half-life of approximately 12 h. Area under the curve data revealed the highest concentration of vector distributed to dorsal root ganglia tissue. Immunohistochemical analysis revealed AAV5 particle colocalization with the pia mater at the spinal cord and macrophages in the dorsal root ganglia (DRG) 30 min after injection. These results demonstrate the widespread distribution of AAV5 particles through cerebrospinal fluid and preferential targeting of DRG tissue with possible clearance mechanisms via DRG macrophages.

Management of diabetic neuropathy

Diabetic neuropathy is a neurodegenerative disorder that may alter both the somatic and autonomic peripheral nervous systems in the context of diabetes mellitus (DM). It is a prevalent and burdensome chronic complication of DM, that requires timely management. Optimized glycemic control (mainly for type 1 DM), multifactorial intervention (mainly for type 2 DM), with lifestyle intervention/physical exercise, and weight loss represent the basis of management for diabetic distal symmetrical polyneuropathy, and should be implemented early in the disease course. Despite better understanding of the pathogenetic mechanisms of diabetic peripheral neuropathy, there is still a stringent need for more pathogenetic-based agents that would significantly modify the natural history of the disease. The paper reviews the available drugs and current recommendations for the management of distal symmetrical polyneuropathy, including pain management, and for diabetic autonomic neuropathy. Evaluation of drug combinations that would perhaps be more efficient in slowing the progression of the disease or even reversing it, and that would provide a better pain management is still needed.

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.

Antiplexin D1 Antibodies Relate to Small Fiber Neuropathy and Induce Neuropathic Pain in Animals

OBJECTIVES

To assess the prevalence of antiplexin D1 antibodies (plexin D1-immunoglobulin G [IgG]) in small fiber neuropathy (SFN) and the effects of these antibodies in vivo.

METHODS

We developed an ELISA for plexin D1-IgG using a recombinant extracellular domain of human plexin D1 containing the major epitope and sera from 58 subjects previously studied with a standard tissue-based indirect immunofluorescence assay (TBA). We screened 63 patients with probable SFN and 55 healthy controls (HCs) for serum plexin D1-IgG using ELISA. The results were confirmed by TBA. IgG from 3 plexin D1-IgG-positive patients, 2 plexin D1-IgG-negative inflammatory disease controls, and 2 HCs was intrathecally injected into mice, which were assessed for mechanical and thermal hypersensitivity 24 and 48 hours after injection.

RESULTS

The ELISA had 75% sensitivity and 100% specificity using the TBA as a standard, and the coincidence rate of ELISA to TBA was 96.6% (56/58). The frequency of plexin D1-IgG was higher in patients with SFN than in HCs (12.7% [8/63] vs 0.0% [0/55], p = 0.007). Purified IgG from all 3 plexin D1-IgG-positive patients, but not 2 plexin D1-IgG-negative patients, induced significant mechanical and/or thermal hypersensitivity compared with IgG from HCs. In mice injected with plexin D1-IgG-positive but not D1-IgG-negative patient IgG, phosphorylated extracellular signal-regulated protein kinase immunoreactivity, an activation marker, was confined to small dorsal root ganglion neurons and was significantly more abundant than in mice injected with HC IgG.

CONCLUSIONS

Plexin D1-IgG is pathogenic but with low prevalence and is a potential biomarker for immunotherapy in SFN.