Satellite glial cells surrounding primary afferent neurons are activated and proliferate during monoarthritis in rats: is there a role for ATF3?

Protease-Activated Receptor 2 (PAR2) Expressed in Sensory Neurons Contributes to Signs of Pain and Neuropathy in Paclitaxel Treated Mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a common, dose-limiting side effect of cancer therapy. Protease-activated receptor 2 (PAR2) is implicated in a variety of pathologies, including CIPN. In this study, we demonstrate the role of PAR2 expressed in sensory neurons in a paclitaxel (PTX)-induced model of CIPN in mice. PAR2 knockout/wildtype (WT) mice and mice with PAR2 ablated in sensory neurons were treated with PTX administered via intraperitoneal injection. In vivo behavioral studies were done in mice using von Frey filaments and the Mouse Grimace Scale. We then examined immunohistochemical staining of dorsal root ganglion (DRG) and hind paw skin samples from CIPN mice to measure satellite cell gliosis and intra-epidermal nerve fiber (IENF) density. The pharmacological reversal of CIPN pain was tested with the PAR2 antagonist C781. Mechanical allodynia caused by PTX treatment was alleviated in PAR2 knockout mice of both sexes. In the PAR2 sensory neuronal conditional knockout (cKO) mice, both mechanical allodynia and facial grimacing were attenuated in mice of both sexes. In the DRG of the PTX-treated PAR2 cKO mice, satellite glial cell activation was reduced compared to control mice. IENF density analysis of the skin showed that the PTX-treated control mice had a reduction in nerve fiber density while the PAR2 cKO mice had a comparable skin innervation as the vehicle-treated animals. Similar results were seen with satellite cell gliosis in the DRG, where gliosis induced by PTX was absent in PAR cKO mice. Finally, C781 was able to transiently reverse established PTX-evoked mechanical allodynia. PERSPECTIVE: Our work demonstrates that PAR2 expressed in sensory neurons plays a key role in PTX-induced mechanical allodynia, spontaneous pain, and signs of neuropathy, suggesting PAR2 as a possible therapeutic target in multiple aspects of PTX CIPN.

Insights into FcγR involvement in pain-like behavior induced by an RA-derived anti-modified protein autoantibody

Joint pain is one of the most debilitating symptoms of rheumatoid arthritis (RA) and patients frequently rate improvements in pain management as their priority. RA is hallmarked by the presence of anti-modified protein autoantibodies (AMPA) against post-translationally modified citrullinated, carbamylated and acetylated proteins. It has been suggested that autoantibody-mediated processes represent distinct mechanisms contributing to pain in RA. In this study, we investigated the pronociceptive properties of monoclonal AMPA 1325:01B09 (B09 mAb) derived from the plasma cell of an RA patient. We found that B09 mAb induces pain-like behavior in mice that is not associated with any visual, histological or transcriptional signs of inflammation in the joints, and not alleviated by non-steroidal anti-inflammatory drugs (NSAIDs). Instead, we found that B09 mAb is retained in dorsal root ganglia (DRG) and alters the expression of several satellite glia cell (SGC), neuron and macrophage-related factors in DRGs. Using mice that lack activating FcγRs, we uncovered that FcγRs are critical for the development of B09-induced pain-like behavior, and partially drive the transcriptional changes in the DRGs. Finally, we observed that B09 mAb binds SGC in vitro and in combination with external stimuli like ATP enhances transcriptional changes and protein release of pronociceptive factors from SGCs. We propose that certain RA antibodies bind epitopes in the DRG, here on SGCs, form immune complexes and activate resident macrophages via FcγR cross-linking. Our work supports the growing notion that autoantibodies can alter nociceptor signaling via mechanisms that are at large independent of local inflammatory processes in the joint.

The Similar and Distinct Roles of Satellite Glial Cells and Spinal Astrocytes in Neuropathic Pain

Preclinical studies have identified glial cells as pivotal players in the genesis and maintenance of neuropathic pain after nerve injury associated with diabetes, chemotherapy, major surgeries, and virus infections. Satellite glial cells (SGCs) in the dorsal root and trigeminal ganglia of the peripheral nervous system (PNS) and astrocytes in the central nervous system (CNS) express similar molecular markers and are protective under physiological conditions. They also serve similar functions in the genesis and maintenance of neuropathic pain, downregulating some of their homeostatic functions and driving pro-inflammatory neuro-glial interactions in the PNS and CNS, i.e., "gliopathy". However, the role of SGCs in neuropathic pain is not simply as "peripheral astrocytes". We delineate how these peripheral and central glia participate in neuropathic pain by producing different mediators, engaging different parts of neurons, and becoming active at different stages following nerve injury. Finally, we highlight the recent findings that SGCs are enriched with proteins related to fatty acid metabolism and signaling such as Apo-E, FABP7, and LPAR1. Targeting SGCs and astrocytes may lead to novel therapeutics for the treatment of neuropathic pain.

Fibromyalgia patients with elevated levels of anti-satellite glia cell immunoglobulin G antibodies present with more severe symptoms

ABSTRACT

Transferring fibromyalgia patient immunoglobulin G (IgG) to mice induces pain-like behaviour, and fibromyalgia IgG binds mouse and human satellite glia cells (SGCs). These findings suggest that autoantibodies could be part of fibromyalgia pathology. However, it is unknown how frequently fibromyalgia patients have anti-SGC antibodies and how anti-SGC antibodies associate with disease severity. Here, we quantified serum or plasma anti-SGC IgG levels in 2 fibromyalgia cohorts from Sweden and Canada using an indirect immunofluorescence murine cell culture assay. Fibromyalgia serum IgG binding to human SGCs in human dorsal root ganglia tissue sections was also assessed by immunofluorescence. In the cell culture assay, anti-SGC IgG levels were increased in both fibromyalgia cohorts compared with control group. Elevated anti-SGC IgG was associated with higher levels of self-reported pain in both cohorts, and higher fibromyalgia impact questionnaire scores and increased pressure sensitivity in the Swedish cohort. Anti-SGC IgG levels were not associated with fibromyalgia duration. Swedish fibromyalgia (FM) patients were clustered into FM-severe and FM-mild groups, and the FM-severe group had elevated anti-SGC IgG compared with the FM-mild group and control group. Anti-SGC IgG levels detected in culture positively correlated with increased binding to human SGCs. Moreover, the FM-severe group had elevated IgG binding to human SGCs compared with the FM-mild and control groups. These results demonstrate that a subset of fibromyalgia patients have elevated levels of anti-SGC antibodies, and the antibodies are associated with more severe fibromyalgia symptoms. Screening fibromyalgia patients for anti-SGC antibodies could provide a path to personalized treatment options that target autoantibodies and autoantibody production.

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.

Antioxidant and Neuroprotective Effect of a Grape Pomace Extract on Oxaliplatin-Induced Peripheral Neuropathy in Rats: Biochemical, Behavioral and Histopathological Evaluation

Oxaliplatin is a widely used chemotherapeutic agent. Despite its many beneficial aspects in fighting many malignancies, it shares an aversive effect of neuropathy. Many substances have been used to limit this oxaliplatin-driven neuropathy in patients. This study evaluates the neuroprotective role of a grape pomace extract (GPE) into an oxaliplatin induced neuropathy in rats. For this reason, following the delivery of the substance into the animals prior to or simultaneously with oxaliplatin, their performance was evaluated by behavioral tests. Blood tests were also performed for the antioxidant activity of the extract, along with a histological and pathological evaluation of dorsal root ganglion (DRG) cells as the major components of the neuropathy. All behavioral tests were corrected following the use of the grape pomace. Oxidative stressors were also limited with the use of the extract. Additionally, the morphometrical analysis of the DRG cells and their immunohistochemical phenotype revealed the fidelity of the animal model and the changes into the parvalbumin and GFAP concentration indicative of the neuroprotective role of the pomace. In conclusion, the grape pomace extract with its antioxidant properties alleviates the harmful effects of the oxaliplatin induced chronic neuropathy in rats.

Effect of Pregabalin Combined with Duloxetine and Tramadol on Allodynia in Chronic Postischemic Pain and Spinal Nerve Ligation Mouse Models

Although there are various drugs for Neuropathic pain (NP), the effects of single drugs are often not very satisfactory. The analgesic effects of different combinations of pregabalin, duloxetine, and tramadol or the combination of all three are still unclear. Mixtures of two or three drugs at low and high concentrations (7.5, 10, 15, and 20 mg/kg pregabalin; 7.5, 10, 15, and 30 mg/kg duloxetine; 5 and 10 mg/kg tramadol) were administered to chronic postischemic pain (CPIP) and spinal nerve ligation (SNL) model mice. The effects of these combinations of drugs on mechanical allodynia were investigated. The expression of the glial fibrillary acidic protein (GFAP) in the spinal cord and dorsal root ganglia (DRGs) was measured. The combination of pregabalin, duloxetine, and tramadol significantly alleviated mechanical hyperalgesia in mice with CPIP and SNL. After the administration of this drug combination, the expression of GFAP in the spinal cord and DRGs was lower in the CPIP and SNL model mice than in control mice. This result suggests that the combination of these three drugs may be advantageous for the treatment of NP because it can reduce side effects by preventing the overuse of a single drug class and exert increased analgesic effects via synergism.

Pain-like behavior in the collagen antibody-induced arthritis model is regulated by lysophosphatidic acid and activation of satellite glia cells

Inflammatory and neuropathic-like components underlie rheumatoid arthritis (RA)-associated pain, and lysophosphatidic acid (LPA) is linked to both joint inflammation in RA patients and to neuropathic pain. Thus, we investigated a role for LPA signalling using the collagen antibody-induced arthritis (CAIA) model. Pain-like behavior during the inflammatory phase and the late, neuropathic-like phase of CAIA was reversed by a neutralizing antibody generated against LPA and by an LPA_1/3 receptor inhibitor, but joint inflammation was not affected. Autotaxin, an LPA synthesizing enzyme was upregulated in dorsal root ganglia (DRG) neurons during both CAIA phases, but not in joints or spinal cord. Late-phase pronociceptive neurochemical changes in the DRG were blocked in Lpar1 receptor deficient mice and reversed by LPA neutralization. In vitro and in vivo studies indicated that LPA regulates pain-like behavior via the LPA₁ receptor on satellite glia cells (SGCs), which is expressed by both human and mouse SGCs in the DRG. Furthermore, CAIA-induced SGC activity is reversed by phospholipid neutralization and blocked in Lpar1 deficient mice. Our findings suggest that the regulation of CAIA-induced pain-like behavior by LPA signalling is a peripheral event, associated with the DRGs and involving increased pronociceptive activity of SGCs, which in turn act on sensory neurons.

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.

Phenotypical changes of satellite glial cells in a murine model of GM1 -gangliosidosis

Satellite glial cells (SGCs) of dorsal root ganglia (DRG) react in response to various injuries in the nervous system. This study investigates reactive changes within SGCs in a murine model for G_M1‐gangliosidosis (G_M1). DRG of homozygous β‐galactosidase‐knockout mice and homozygous C57BL/6 wild‐type mice were investigated performing immunostaining on formalin‐fixed, paraffin‐embedded tissue. A marked upregulation of glial fibrillary acidic protein (GFAP), the progenitor marker nestin and Ki67 within SGCs of diseased mice, starting after 4 months at the earliest GFAP, along with intracytoplasmic accumulation of ganglioside within neurons and deterioration of clinical signs was identified. Interestingly, nestin‐positive SGCs were detected after 8 months only. No changes regarding inwardly rectifying potassium channel 4.1, 2, 3‐cyclic nucleotide 3‐phosphodiesterase, Sox2, doublecortin, periaxin and caspase3 were observed in SGCs. Iba1 was only detected in close vicinity of SGCs indicating infiltrating or tissue‐resident macrophages. These results indicate that SGCs of DRG show phenotypical changes during the course of G_M1, characterized by GFAP upregulation, proliferation and expression of a neural progenitor marker at a late time point. This points towards an important role of SGCs during neurodegenerative disorders and supports that SGCs represent a multipotent glial precursor cell line with high plasticity and functionality.

2D <em>vs</em> 3D morphological analysis of dorsal root ganglia in health and painful neuropathy

Dorsal root ganglia (DRGs) are clusters of sensory neurons that transmit the sensory information from the periphery to the central nervous system, and satellite glial cells (SGCs), their supporting trophic cells. Sensory neurons are pseudounipolar neurons with a heterogeneous neurochemistry reflecting their functional features. DRGs, not protected by the blood brain barrier, are vulnerable to stress and damage of different origin (i.e., toxic, mechanical, metabolic, genetic) that can involve sensory neurons, SGCs or, considering their intimate intercommunication, both cell populations. DRG damage, primary or secondary to nerve damage, produces a sensory peripheral neuropathy, characterized by neurophysiological abnormalities, numbness, paraesthesia and dysesthesia, tingling and burning sensations and neuropathic pain. DRG stress can be morphologically detected by light and electron microscope analysis with alterations in cell size (swelling/atrophy) and in different subcellular compartments (i.e., mitochondria, endoplasmic reticulum, and nucleus) of neurons and/or SGCs. In addition, neurochemical changes can be used to portray abnormalities of neurons and SGC. Conventional immunostaining, i.e., immunohistochemical detection of specific molecules in tissue slices, can be employed to detect, localize and quantify particular markers of damage in neurons (i.e., nuclear expression of ATF3) or SGCs (i.e., increased expression of GFAP), markers of apoptosis (i.e., caspases), markers of mitochondrial suffering and oxidative stress (i.e., 8-OHdG), markers of tissue inflammation (i.e., CD68 for macrophage infiltration) etc. However classical (2D) methods of immunostaining disrupt the overall organization of the DRG, thus resulting in the loss of some crucial information. Whole-mount (3D) methods have been recently developed to investigate DRG morphology and neurochemistry without tissue slicing, giving the opportunity to study the intimate relationship between SGCs and sensory neurons in health and disease. Here, we aim to compare classical (2D) vs whole-mount (3D) approaches to highlight “pros” and “cons” of the two methodologies when analysing neuropathy-induced alterations in DRGs.

Discrepancy in the Usage of GFAP as a Marker of Satellite Glial Cell Reactivity

Satellite glial cells (SGCs) surrounding the neuronal somas in peripheral sensory ganglia are sensitive to neuronal stressors, which induce their reactive state. It is believed that such induced gliosis affects the signaling properties of the primary sensory neurons and is an important component of the neuropathic phenotype leading to pain and other sensory disturbances. Efforts to understand and manipulate such gliosis relies on reliable markers to confirm induced SGC reactivity and ultimately the efficacy of targeted intervention. Glial fibrillary acidic protein (GFAP) is currently the only widely used marker for such analyses. However, we have previously described the lack of SGC upregulation of GFAP in a mouse model of sciatic nerve injury, suggesting that GFAP may not be a universally suitable marker of SGC gliosis across species and experimental models. To further explore this, we here investigate the regulation of GFAP in two different experimental models in both rats and mice. We found that whereas GFAP was upregulated in both rodent species in the applied inflammation model, only the rat demonstrated increased GFAP in SGCs following sciatic nerve injury; we did not observe any such GFAP upregulation in the mouse model at either protein or mRNA levels. Our results demonstrate an important discrepancy between species and experimental models that prevents the usage of GFAP as a universal marker for SGC reactivity.

Axon-glia interactions in the ascending auditory system

The auditory system detects and encodes sound information with high precision to provide a high-fidelity representation of the environment and communication. In mammals, detection occurs in the peripheral sensory organ (the cochlea) containing specialized mechanosensory cells (hair cells) that initiate the conversion of sound-generated vibrations into action potentials in the auditory nerve. Neural activity in the auditory nerve encodes information regarding the intensity and frequency of sound stimuli, which is transmitted to the auditory cortex through the ascending neural pathways. Glial cells are critical for precise control of neural conduction and synaptic transmission throughout the pathway, allowing for the precise detection of the timing, frequency, and intensity of sound signals, including the sub-millisecond temporal fidelity is necessary for tasks such as sound localization, and in humans, for processing complex sounds including speech and music. In this review, we focus on glia and glia-like cells that interact with hair cells and neurons in the ascending auditory pathway and contribute to the development, maintenance, and modulation of neural circuits and transmission in the auditory system. We also discuss the molecular mechanisms of these interactions, their impact on hearing and on auditory dysfunction associated with pathologies of each cell type.

Neuronal uptake transporters contribute to oxaliplatin neurotoxicity in mice

Peripheral neurotoxicity is a debilitating condition that afflicts up to 90% of patients with colorectal cancer receiving oxaliplatin-containing therapy. Although emerging evidence has highlighted the importance of various solute carriers to the toxicity of anticancer drugs, the contribution of these proteins to oxaliplatin-induced peripheral neurotoxicity remains controversial. Among candidate transporters investigated in genetically engineered mouse models, we provide evidence for a critical role of the organic cation transporter 2 (OCT2) in satellite glial cells in oxaliplatin-induced neurotoxicity, and demonstrate that targeting OCT2 using genetic and pharmacological approaches ameliorates acute and chronic forms of neurotoxicity. The relevance of this transport system was verified in transporter-deficient rats as a secondary model organism, and translational significance of preventive strategies was demonstrated in preclinical models of colorectal cancer. These studies suggest that pharmacological targeting of OCT2 could be exploited to afford neuroprotection in cancer patients requiring treatment with oxaliplatin.

Satellite glia as a critical component of diabetic neuropathy: Role of lipocalin-2 and pyruvate dehydrogenase kinase-2 axis in the dorsal root ganglion

Diabetic peripheral neuropathy (DPN) is a common complication of uncontrolled diabetes. The pathogenesis of DPN is associated with chronic inflammation in dorsal root ganglion (DRG), eventually causing structural and functional changes. Studies on DPN have primarily focused on neuronal component, and there is limited knowledge about the role of satellite glial cells (SGCs), although they completely enclose neuronal soma in DRG. Lipocalin-2 (LCN2) is a pro-inflammatory acute-phase protein found in high levels in diverse neuroinflammatory and metabolic disorders. In diabetic DRG, the expression of LCN2 was increased exclusively in the SGCs. This upregulation of LCN2 in SGCs correlated with increased inflammatory responses in DRG and sciatic nerve. Furthermore, diabetes-induced inflammation and morphological changes in DRG, as well as sciatic nerve, were attenuated in Lcn2 knockout (KO) mice. Lcn2 gene ablation also ameliorated neuropathy phenotype as determined by nerve conduction velocity and intraepidermal nerve fiber density. Mechanistically, studies using specific gene KO mice, adenovirus-mediated gene overexpression strategy, and primary cultures of DRG SGCs and neurons have demonstrated that LCN2 enhances the expression of mitochondrial gate-keeping regulator pyruvate dehydrogenase kinase-2 (PDK2) through PPARβ/δ, thereby inhibiting pyruvate dehydrogenase activity and increasing production of glycolytic end product lactic acid in DRG SGCs and neurons of diabetic mice. Collectively, our findings reveal a crucial role of glial LCN2-PPARβ/δ-PDK2-lactic acid axis in progression of DPN. Our results establish a link between pro-inflammatory LCN2 and glycolytic PDK2 in DRG SGCs and neurons and propose a novel glia-based mechanism and drug target for therapy of DPN. MAIN POINTS: Diabetes upregulates LCN2 in satellite glia, which in turn increases pyruvate dehydrogenase kinase-2 (PDK2) expression and lactic acid production in dorsal root ganglia (DRG). Glial LCN2-PDK2-lactic acid axis in DRG plays a crucial role in the pathogenesis of diabetic neuropathy.

TDAG8 deficiency reduces satellite glial number and pro-inflammatory macrophage number to relieve rheumatoid arthritis disease severity and chronic pain

Background

The autoimmune disease rheumatoid arthritis (RA) affects approximately 1% of the global population. RA is characterized with chronic joint inflammation and often associated with chronic pain. The imbalance of pro-inflammatory and anti-inflammatory macrophages is a feature of RA progression. Glial cells affecting neuronal sensitivity at both peripheral and central levels may also be important for RA progression and associated pain. Genetic variants in the T cell death-associated gene 8 (TDAG8) locus are found to associate with spondyloarthritis. TDAG8 was also found involved in RA disease progression and associated hyperalgesia in the RA mouse model. However, its modulation in RA remains unclear.

Methods

To address this question, we intra-articularly injected complete Freund’s adjuvant (CFA) into TDAG8^+/+, TDAG8^−/− or wild-type mice, followed by pain behavioral tests. Joints and dorsal root ganglia were taken, sectioned, and stained with antibodies to observe the number of immune cells, macrophages, and satellite glial cells (SGCs). For compound treatments, compounds were intraperitoneally or orally administered weekly for 9 consecutive weeks after CFA injection.

Results

We demonstrated that TDAG8 deletion slightly reduced RA pain in the early phase but dramatically attenuated RA progression and pain in the chronic phase (> 7 weeks). TDAG8 deletion inhibited an increase in SGC number and inhibition of SGC function attenuated chronic phase of RA pain, so TDAG8 could regulate SGC number to control chronic pain. TDAG8 deletion also reduced M1 pro-inflammatory macrophage number at 12 weeks, contributing to the attenuation of chronic RA pain. Such results were further confirmed by using salicylanilide derivatives, CCL-2d or LCC-09, to suppress TDAG8 expression and function.

Conclusions

This study demonstrates that TDAG8 deletion reduced SGC and M1 macrophage number to relieve RA disease severity and associated chronic pain. M1 macrophages are critical for the development and maintenance of RA disease and pain, but glial activation is also required for the chronic phase of RA pain.

Activation and functional modulation of satellite glial cells by oxaliplatin lead to hyperexcitability of sensory neurons in vitro

Platinum-based chemotherapeutics still play an important role in cancer therapy, however, severe side effects, such as painful neuropathy, occur frequently. The pathophysiologic mechanisms depend on the applied chemotherapeutic agent and are still controversial. In addition to neuronal damage, disturbance of glial cell activity may contribute to neurotoxicity. Here, we focused on the effect of oxaliplatin on satellite glial cell (SGC) function and on the activity of the dorsal root ganglion (DRG) neurons. SGCs were isolated as high-purity cultures and treated with 1 and 10 μM oxaliplatin for 2, 4 and 24 h. Subsequently, glial fibrillary acid protein (GFAP), reactive oxygen species (ROS), Connexin-43 (Cx-43), and inward rectifier potassium channel 4.1 (K_ir4.1) expression was determined by immunocytochemical staining (ICC) and Western blot analyses. Immunochemical staining and Western blot analysis showed an increase in the immune reactivity (IR) and protein levels of ROS, GFAP, and Cx-43. Furthermore, reduction of the IR and protein levels and current density were demonstrated using patch-clamp measurements, of K_ir4.1 channels after oxaliplatin exposure. Cytokine release in SGCs was measured using enzyme-linked immunosorbent assays (ELISA) after oxaliplatin exposure and indicated an increased release of IL-6 and TNFα, while IL-1β was decreased. The direct influence of SGC-secreted factors in the supernatant after oxaliplatin treatment led to the hyperexcitability of cultured DRG neurons. In summary, oxaliplatin has a direct impact on the modulation and function of different SGC proteins. Furthermore, SGC-released factors influence the excitability of sensory neurons, qualifying SGCs as potential targets for the prevention and treatment of oxaliplatin-induced polyneuropathy.

Changes in the transcriptional fingerprint of satellite glial cells following peripheral nerve injury

Satellite glial cells (SGCs) are homeostatic cells enveloping the somata of peripheral sensory and autonomic neurons. A wide variety of neuronal stressors trigger activation of SGCs, contributing to, for example, neuropathic pain through modulation of neuronal activity. However, compared to neurons and other glial cells of the nervous system, SGCs have received modest scientific attention and very little is known about SGC biology, possibly due to the experimental challenges associated with studying them in vivo and in vitro. Utilizing a recently developed method to obtain SGC RNA from dorsal root ganglia (DRG), we took a systematic approach to characterize the SGC transcriptional fingerprint by using next-generation sequencing and, for the first time, obtain an overview of the SGC injury response. Our RNA sequencing data are easily accessible in supporting information in Excel format. They reveal that SGCs are enriched in genes related to the immune system and cell-to-cell communication. Analysis of SGC transcriptional changes in a nerve injury-paradigm reveal a differential response at 3 days versus 14 days postinjury, suggesting dynamic modulation of SGC function over time. Significant downregulation of several genes linked to cholesterol synthesis was observed at both time points. In contrast, regulation of gene clusters linked to the immune system (MHC protein complex and leukocyte migration) was mainly observed after 14 days. Finally, we demonstrate that, after nerve injury, macrophages are in closer physical proximity to both small and large DRG neurons, and that previously reported injury-induced proliferation of SGCs may, in fact, be proliferating macrophages.

An Intimate Role for Adult Dorsal Root Ganglia Resident Cycling Cells in the Generation of Local Macrophages and Satellite Glial Cells

The intricate interactions between neurons, glial, and inflammatory cells within peripheral ganglia are physiologically important, but not well explored. Here, we show that adult dorsal root ganglia (DRG) contain populations of self-renewing cells, collectively referred as DRG resident cycling cells (DRCCs), that are active not only in "quiescent" ganglia but also accelerate their turnover in response to distal axotomy. An unexpected proportion of DRCCs were resident macrophages. These cells closely accompanied, and aligned with recycling satellite glial cells (SGCs) that were juxtaposed to sensory neurons and possessed stem cell-like properties. Selective inhibition of colony stimulating factor 1 receptor prevented the local proliferation of macrophages. Interestingly, DRCC turnover was accompanied by apoptosis at later intervals indicating a balanced cellular milieu in the DRGs. These findings identify a complex interactive multicellular DRG microenvironment supporting self-renewal of both macrophages and SGCs and its potential implications in the overall response of adult DRGs to injury.

Subclinical lipopolysaccharide from Salmonella Enteritidis induces neuropeptide dysregulation in the spinal cord and the dorsal root ganglia

Background

Despite increasing evidence that lipopolysaccharide (LPS) affects the biological active substances of dorsal root ganglia (DRG) we have limited knowledge of the influence of a single low dose of LPS, which does not result in any clinical symptoms of disease (subclinical LPS) on neuropeptides connected with the sensory pathway. Accordingly, in this work, we investigated the influence of subclinical LPS from Salmonella Enteritidis on selected neuropeptides: substance P (SP), galanin (GAL), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) and somatostatin (SOM) in the cervical, thoracic, lumbar and sacral regions of the DRG and spinal cord.

Methods

This study was performed on immature female pigs of the Pietrain × Duroc breed. Seven days after the intravenous injection of saline solution for control animals (n = 5) and 5 μg/kg b.w. LPS from S. Enteritidis for the experimental group (n = 5), the DRG and the spinal cord were collected to extract the neuropeptides using solid-phase extraction technology.

Results

Our results demonstrated that subclinical LPS in DRG was able to change the levels of all studied neuropeptides except SOM, whereas in the spinal cord it down-regulated all studied neuropeptides in the sacral spinal cord, maintaining the concentration of all studied neuropeptides in other regions similar to that observed in the control animals. The significant differences in the intensity and character of observed changes between particular regions of the DRG suggest that the exact functions of the studied neuropeptides and mechanisms of responses to subclinical LPS action depend on specific characteristics and functions of each examination region of DRG.

Conclusions

The mechanisms of observed changes are not fully understood and require further study of the molecular interactions between subclinical LPS from S. Enteritidis and neuronal and non-neuronal cells of DRG and spinal cord. The peripheral and central pain pathways must be analysed with the aspect of unknown long-term consequences of the influence of subclinical LPS from S. Enteritidis on neuropeptides in the spinal cord and the dorsal root ganglia.

Peripheral Glial Cells in the Development of Diabetic Neuropathy

The global prevalence of diabetes is rapidly increasing, affecting more than half a billion individuals within the next few years. As diabetes negatively affects several physiological systems, this dramatic increase represents not only impaired quality of life on the individual level but also a huge socioeconomic challenge. One of the physiological consequences affecting up to half of diabetic patients is the progressive deterioration of the peripheral nervous system, resulting in spontaneous pain and eventually loss of sensory function, motor weakness, and organ dysfunctions. Despite intense research on the consequences of hyperglycemia on nerve functions, the biological mechanisms underlying diabetic neuropathy are still largely unknown, and treatment options lacking. Research has mainly focused directly on the neuronal component, presumably from the perspective that this is the functional signal-transmitting unit of the nerve. However, it is noteworthy that each single peripheral sensory neuron is intimately associated with numerous glial cells; the neuronal soma is completely enclosed by satellite glial cells and the length of the longest axons covered by at least 1,000 Schwann cells. The glial cells are vital for the neuron, but very little is still known about these cells in general and especially how they respond to diabetes in terms of altered neuronal support. We will discuss current knowledge of peripheral glial cells and argue that increased research in these cells is imperative for a better understanding of the mechanisms underlying diabetic neuropathy.

Inhibition of neuronal FLT3 receptor tyrosine kinase alleviates peripheral neuropathic pain in mice

Peripheral neuropathic pain (PNP) is a debilitating and intractable chronic disease, for which sensitization of somatosensory neurons present in dorsal root ganglia that project to the dorsal spinal cord is a key physiopathological process. Here, we show that hematopoietic cells present at the nerve injury site express the cytokine FL, the ligand of fms-like tyrosine kinase 3 receptor (FLT3). FLT3 activation by intra-sciatic nerve injection of FL is sufficient to produce pain hypersensitivity, activate PNP-associated gene expression and generate short-term and long-term sensitization of sensory neurons. Nerve injury-induced PNP symptoms and associated-molecular changes were strongly altered in Flt3-deficient mice or reversed after neuronal FLT3 downregulation in wild-type mice. A first-in-class FLT3 negative allosteric modulator, discovered by structure-based in silico screening, strongly reduced nerve injury-induced sensory hypersensitivity, but had no effect on nociception in non-injured animals. Collectively, our data suggest a new and specific therapeutic approach for PNP.

Sensitisation of dorsal root ganglia neurons contributes to neuropathic pain. Here the authors demonstrate the cytokine FL contributes to sensitisation of DRGs via its receptor FLT3 expressed on neurons, and identify a novel FLT3 inhibitor that attenuates neuropathic pain in mice.

Glial activation in the collagenase model of nociception associated with osteoarthritis

Background

Experimental osteoarthritis entails neuropathic-like changes in dorsal root ganglia (DRG) neurons. Since glial activation has emerged as a key player in nociception, being reported in numerous models of neuropathic pain, we aimed at evaluating if glial cell activation may also occur in the DRG and spinal cord of rats with osteoarthritis induced by intra-articular injection of collagenase.

Methods

Osteoarthritis was induced by two injections, separated by three days, of 500 U of type II collagenase into the knee joint of rats. Movement-induced nociception was evaluated by the Knee-Bend and CatWalk tests during the following six weeks. Glial fibrillary acidic protein (GFAP) expression in satellite glial cells of the DRG was assessed by immunofluorescence and Western Blot analysis; the pattern of GFAP and activating transcription factor-3 (ATF-3) expression was also compared through double immunofluorescence analysis. GFAP expression in astrocytes and IBA-1 expression in microglia of the L3–L5 spinal cord segments was assessed by immunohistochemistry and Western Blot analysis. The effect of the intrathecal administration of fluorocitrate, an inhibitor of glial activation, on movement-induced nociception was evaluated six weeks after the first collagenase injection.

Results

GFAP expression in satellite glial cells of collagenase-injected animals was significantly increased six weeks after osteoarthritis induction. Double immunofluorescence showed GFAP upregulation in satellite glial cells surrounding ATF-3-positive neurons. In the spinal cord of collagenase-injected animals, an ipsilateral upregulation of GFAP and IBA-1 was also observed. The inhibition of glial activation with fluorocitrate decreased movement- and loading-induced nociception.

Conclusion

Collagenase-induced knee osteoarthritis leads to the development of nociception associated with movement of the affected joint and to the activation of glial cells in both the DRG and the spinal cord. Inhibition of glial cell activation by fluorocitrate decreases these osteoarthritis-associated nociceptive behaviours. These results suggest that glial cell activation may play a role in the development of chronic pain in this experimental model of osteoarthritis.

Glial fibrillary acidic protein promoter determines transgene expression in satellite glial cells following intraganglionic adeno-associated virus delivery in adult rats

Recombinant adeno-associated viral (AAV)-mediated therapeutic gene transfer to dorsal root ganglia (DRG) is an effective and safe tool for treating chronic pain. However, AAV with various constitutively active promoters leads to transgene expression predominantly to neurons, while glial cells are refractory to AAV transduction in the peripheral nervous system. The present study evaluated whether in vivo satellite glial cell (SGC) transduction in the DRG can be enhanced by the SGC-specific GFAP promoter and by using shH10 and shH19, which are engineered capsid variants with Müller glia-prone transduction. Titer-matched AAV6 (as control), AAVshH10, and AAVshH19, all encoding the EGFP driven by the constitutively active CMV promoter, as well as AAV6-EGFP and AAVshH10-EGFP driven by a GFAP promoter (AAV6-GFAP-EGFP and AAVshH10-GFAP-EGFP), were injected into DRG of adult male rats. Neurotropism of gene expression was determined and compared by immunohistochemistry. Results showed that injection of AAV6- and AAVshH10-GFAP-EGFP induces robust EGFP expression selectively in SGCs, whereas injection of either AAVshH10-CMV-EGFP or AAVshH19-CMV-EGFP into DRG resulted in a similar in vivo transduction profile to AAV6-CMV-EGFP, all showing efficient transduction of sensory neurons without significant transduction of glial cell populations. Coinjection of AAV6-CMV-mCherry and AAV6-GFAP-EGFP induces transgene expression in neurons and SGCs separately. This report, together with our prior studies, demonstrates that the GFAP promoter rather than capsid tropism determines selective gene expression in SGCs following intraganglionic AAV delivery in adult rats. A dual AAV system, one with GFAP promoter and the other with CMV promoter, can efficiently express transgenes selectively in neurons versus SGCs.

Drug-Induced HSP90 Inhibition Alleviates Pain in Monoarthritic Rats and Alters the Expression of New Putative Pain Players at the DRG

Purinergic receptors (P2XRs) have been widely associated with pain states mostly due to their involvement in neuron-glia communication. Interestingly, we have previously shown that satellite glial cells (SGC), surrounding dorsal root ganglia (DRG) neurons, become activated and proliferate during monoarthritis (MA) in the rat. Here, we demonstrate that P2X7R expression increases in ipsilateral DRG after 1 week of disease, while P2X3R immunoreactivity decreases. We have also reported a significant induction of the activating transcriptional factor 3 (ATF3) in MA. In this study, we show that ATF3 knocked down in DRG cell cultures does not affect the expression of P2X7R, P2X3R, or glial fibrillary acidic protein (GFAP). We suggest that P2X7R negatively regulates P2X3R, which, however, is unlikely mediated by ATF3. Interestingly, we found that ATF3 knockdown in vitro induced significant decreases in the heat shock protein 90 (HSP90) expression. Thus, we evaluated in vivo the involvement of HSP90 in MA and demonstrated that the HSP90 messenger RNA levels increase in ipsilateral DRG of inflamed animals. We also show that HSP90 is mostly found in a cleaved form in this condition. Moreover, administration of a HSP90 inhibitor, 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG), attenuated MA-induced mechanical allodynia in the first hours. The drug also reversed the HSP90 upregulation and cleavage. 17-DMAG seemed to attenuate glial activation and neuronal sensitization (as inferred by downregulation of GFAP and P2X3R in ipsilateral DRG) which might correlate with the observed pain alleviation. Our data indicate a role of HSP90 in MA pathophysiology, but further investigation is necessary to clarify the underlying mechanisms.

Transient auditory nerve demyelination as a new mechanism for hidden hearing loss

Hidden hearing loss (HHL) is a recently described auditory neuropathy believed to contribute to speech discrimination and intelligibility deficits in people with normal audiological tests. Animals and humans with HHL have normal auditory thresholds but defective cochlear neurotransmission, that is, reduced suprathreshold amplitude of the sound-evoked auditory nerve compound action potential. Currently, the only cellular mechanism known for HHL is loss of inner hair cell synapses (synaptopathy). Here we report that transient loss of cochlear Schwann cells results in permanent auditory deficits characteristic of HHL. This auditory neuropathy is not associated with synaptic loss, but rather with disruption of the first heminodes at the auditory nerve peripheral terminal. Thus, this study identifies a new mechanism for HHL, highlights the long-term consequences of transient Schwann cell loss on hearing and might provide insights into the causes of the auditory deficits reported in patients that recover from acute demyelinating diseases such as Guillain–Barré syndrome.

Hidden hearing loss (HHL) is an auditory neuropathy that impairs one's ability to hear, particularly in a noisy environment. Here the authors show that in mice, transient loss of cochlear Schwann cells results in permanent disruption of the cochlear heminodal structure, leading to auditory deficits characteristic of HHL.

Peripheral and central neuronal ATF3 precedes CD4+ T-cell infiltration in EAE

Experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis produced by immunization with myelin oligodendrocyte glycoprotein (MOG) and adjuvants, results from profound T-cell mediated CNS demyelination. EAE is characterized by progressive, ascending motor dysfunction and symptoms of ongoing pain and hypersensitivity, in some cases preceding or concomitant with the motor deficits. In this regard, the EAE model mimics major features of multiple sclerosis, where a central neuropathic pain state is common. Although the latter condition is presumed to arise from a CNS loss of inhibitory controls secondary to the demyelination, dysfunction of sensory neurons may also contribute. Based on our previous studies that demonstrated the utility of monitoring expression of activating transcription factor 3 (ATF3), a sensitive marker of injured sensory neurons, here we followed both ATF3 and CD4+ T cells invasion of sensory ganglia (as well as the CNS) at different stages of the EAE model. We found that ATF3 is induced in peripheral sensory ganglia and brainstem well before the appearance of motor deficits. Unexpectedly, the ATF3 induction always preceded T cell infiltration, typically in adjacent, but non-overlapping regions. Surprisingly, control administration of the pertussis toxin and/or Complete Freund's adjuvants, without MOG, induced ATF3 in sensory neurons. In contrast, T cell infiltration only occurred with MOG. Taken together, our results suggest that the clinical manifestations in the EAE result not only from central demyelination but also from neuronal stress and subsequent pathophysiology of sensory neurons.