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

Changes in the expression of satellite glial cell-specific markers during postnatal development of rat sympathetic ganglia

The sympathetic ganglia represent a final motor pathway that mediates homeostatic "fight and flight" responses in the visceral organs. Satellite glial cells (SGCs) form a thin envelope close to the neuronal cell body and synapses in the sympathetic ganglia. This unique morphological feature suggests that neurons and SGCs form functional units for regulation of sympathetic output. In the present study, we addressed whether SGC-specific markers undergo age-dependent changes in the postnatal development of rat sympathetic ganglia. We found that fatty acid-binding protein 7 (FABP7) is an early SGC marker, whereas the S100B calcium-binding protein, inwardly rectifying potassium channel, Kir4.1 and small conductance calcium-activated potassium channel, SK3 are late SGC markers in the postnatal development of sympathetic ganglia. Unlike in sensory ganglia, FABP7 + SGC was barely detectable in adult sympathetic ganglia. The expression of connexin 43, a gap junction channel gradually increased with age, although it was detected in both SGCs and neurons in sympathetic ganglia. Glutamine synthetase was expressed in sensory, but not sympathetic SGCs. Unexpectedly, the sympathetic SGCs expressed a water-selective channel, aquaporin 1 instead of aquaporin 4, a pan-glial marker. However, aquaporin 1 was not detected in the SGCs encircling large neurons. Nerve injury and inflammation induced the upregulation of glial fibrillary acidic protein, suggesting that this protein is a hall marker of glial activation in the sympathetic ganglia. In conclusion, our findings provide basic information on the in vivo profiles of specific markers for identifying sympathetic SGCs at different stages of postnatal development in both healthy and diseased states.

Mechanisms of Action of Dorsal Root Ganglion Stimulation

The dorsal root ganglion (DRG) serves as a pivotal site for managing chronic pain through dorsal root ganglion stimulation (DRG-S). In recent years, the DRG-S has emerged as an attractive modality in the armamentarium of neuromodulation therapy due to its accessibility and efficacy in alleviating chronic pain refractory to conventional treatments. Despite its therapeutic advantages, the precise mechanisms underlying DRG-S-induced analgesia remain elusive, attributed in part to the diverse sensory neuron population within the DRG and its modulation of both peripheral and central sensory processing pathways. Emerging evidence suggests that DRG-S may alleviate pain by several mechanisms, including the reduction of nociceptive signals at the T-junction of sensory neurons, modulation of pain gating pathways within the dorsal horn, and regulation of neuronal excitability within the DRG itself. However, elucidating the full extent of DRG-S mechanisms necessitates further exploration, particularly regarding its supraspinal effects and its interactions with cognitive and affective networks. Understanding these mechanisms is crucial for optimizing neurostimulation technologies and improving clinical outcomes of DRG-S for chronic pain management. This review provides a comprehensive overview of the DRG anatomy, mechanisms of action of the DRG-S, and its significance in neuromodulation therapy for chronic pain.

The dorsal root ganglion as a target for neurorestoration in neuropathic pain

Neuropathic pain is a severe and chronic condition widely found in the general population. The reason for this is the extensive variety of damage or diseases that can spark this unpleasant constant feeling in patients. During the processing of pain, the dorsal root ganglia constitute an important region where dorsal root ganglion neurons play a crucial role in the transmission and propagation of sensory electrical stimulation. Furthermore, the dorsal root ganglia have recently exhibited a regenerative capacity that should not be neglected in the understanding of the development and resolution of neuropathic pain and in the elucidation of innovative therapies. Here, we will review the complex interplay between cells (satellite glial cells and inflammatory cells) and factors (cytokines, neurotrophic factors and genetic factors) that takes place within the dorsal root ganglia and accounts for the generation of the aberrant excitation of primary sensory neurons occurring in neuropathic pain. More importantly, we will summarize an updated view of the current pharmacologic and nonpharmacologic therapies targeting the dorsal root ganglia for the treatment of neuropathic pain.

In vivo calcium imaging shows that satellite glial cells have increased activity in painful states

Satellite glial cells are important for proper neuronal function of primary sensory neurons for which they provide homeostatic support. Most research on satellite glial cell function has been performed with in vitro studies, but recent advances in calcium imaging and transgenic mouse models have enabled this first in vivo study of single-cell satellite glial cell function in mouse models of inflammation and neuropathic pain. We found that in naïve conditions, satellite glial cells do not respond in a time-locked fashion to neuronal firing. In painful inflammatory and neuropathic states, we detected time-locked signals in a subset of satellite glial cells, but only with suprathreshold stimulation of the sciatic nerve. Surprisingly, therefore, we conclude that most calcium signals in satellite glial cells seem to develop at arbitrary intervals not directly linked to neuronal activity patterns. More in line with expectations, our experiments also revealed that the number of active satellite glial cells was increased under conditions of inflammation or nerve injury. This could reflect the increased requirement for homeostatic support across dorsal root ganglion neuron populations, which are more active during such painful states.

Store-operated calcium entry in the satellite glial cells of rat sympathetic ganglia

Satellite glial cells (SGCs), a major type of glial cell in the autonomic ganglia, closely envelop the cell body and even the synaptic regions of a single neuron with a very narrow gap. This structurally unique organization suggests that autonomic neurons and SGCs may communicate reciprocally. Glial Ca2+ signaling is critical for controlling neural activity. Here, for the first time we identified the machinery of store-operated Ca2+ entry (SOCE) which is critical for cellular Ca2+ homeostasis in rat sympathetic ganglia under normal and pathological states. Quantitative realtime PCR and immunostaining analyses showed that Orai1 and stromal interaction molecules 1 (STIM1) proteins are the primary components of SOCE machinery in the sympathetic ganglia. When the internal Ca2+ stores were depleted in the absence of extracellular Ca2+, the number of plasmalemmal Orai1 puncta was increased in neurons and SGCs, suggesting activation of the Ca2+ entry channels. Intracellular Ca2+ imaging revealed that SOCE was present in SGCs and neurons; however, the magnitude of SOCE was much larger in the SGCs than in the neurons. The SOCE was significantly suppressed by GSK7975A, a selective Orai1 blocker, and Pyr6, a SOCE blocker. Lipopolysaccharide (LPS) upregulated the glial fibrillary acidic protein and Toll-like receptor 4 in the sympathetic ganglia. Importantly, LPS attenuated SOCE via downregulating Orai1 and STIM1 expression. In conclusion, sympathetic SGCs functionally express the SOCE machinery, which is indispensable for intracellular Ca2+ signaling. The SOCE is highly susceptible to inflammation, which may affect sympathetic neuronal activity and thereby autonomic output.

Chronic Glaucoma Induced in Rats by a Single Injection of Fibronectin-Loaded PLGA Microspheres: IOP-Dependent and IOP-Independent Neurodegeneration

To evaluate a new animal model of chronic glaucoma induced using a single injection of fibronectin-loaded biodegradable PLGA microspheres (Ms) to test prolonged therapies. 30 rats received a single injection of fibronectin-PLGA-Ms suspension (MsF) in the right eye, 10 received non-loaded PLGA-Ms suspension (Control), and 17 were non-injected (Healthy). Follow-up was performed (24 weeks), evaluating intraocular pressure (IOP), optical coherence tomography (OCT), histology and electroretinography. The right eyes underwent a progressive increase in IOP, but only induced cohorts reached hypertensive values. The three cohorts presented a progressive decrease in ganglion cell layer (GCL) thickness, corroborating physiological age-related loss of ganglion cells. Injected cohorts (MsF > Control) presented greater final GCL thickness. Histological exams explain this paradox: the MsF cohort showed lower ganglion cell counts but higher astrogliosis and immune response. A sequential trend of functional damage was recorded using scotopic electroretinography (MsF > Control > Healthy). It seems to be a function-structure correlation: in significant astrogliosis, early functional damage can be detected by electroretinography, and structural damage can be detected by histological exams but not by OCT. Males presented higher IOP and retinal and GCL thicknesses and lower electroretinography. A minimally invasive chronic glaucoma model was induced by a single injection of biodegradable Ms.

Naringin inhibits P2X4 receptor expression on satellite glial cells in the neonatal rat dorsal root ganglion

Naringin inhibits inflammation and oxidative stress, the P2 purinoreceptor X4 receptor (P2X4R) is associated with glial cell activation and inflammation, the purpose of this study is to investigate the effects of naringin on P2X4 receptor expression on satellite glial cells (SGCs) and its possible mechanisms. ATP promoted the SGC activation and upregulated P2X4R expression; naringin inhibited SGC activation, decreased expression of P2X4R, P38 MAPK/ERK, and NF-κB, and reduced levels of Ca2+, TNF-α, and IL-1β in SGCs in an ATP-containing environment. These findings suggest that naringin attenuates the ATP-induced SGC activation and reduces P2X4R expression via the Ca2+-P38 MAPK/ERK-NF-κB pathway.

Characterisation of GFAP-Expressing Glial Cells in the Dorsal Root Ganglion after Spared Nerve Injury

Satellite glial cells (SGCs), enveloping primary sensory neurons' somas in the dorsal root ganglion (DRG), contribute to neuropathic pain upon nerve injury. Glial fibrillary acidic protein (GFAP) serves as an SGC activation marker, though its DRG satellite cell specificity is debated. We employed the hGFAP-CFP transgenic mouse line, designed for astrocyte studies, to explore its expression within the peripheral nervous system (PNS) after spared nerve injury (SNI). We used diverse immunostaining techniques, Western blot analysis, and electrophysiology to evaluate GFAP+ cell changes. Post-SNI, GFAP+ cell numbers increased without proliferation, and were found near injured ATF3+ neurons. GFAP+ FABP7+ SGCs increased, yet 75.5% of DRG GFAP+ cells lacked FABP7 expression. This suggests a significant subset of GFAP+ cells are non-myelinating Schwann cells (nmSC), indicated by their presence in the dorsal root but not in the ventral root which lacks unmyelinated fibres. Additionally, patch clamp recordings from GFAP+ FABP7-cells lacked SGC-specific Kir4.1 currents, instead displaying outward Kv currents expressing Kv1.1 and Kv1.6 channels specific to nmSCs. In conclusion, this study demonstrates increased GFAP expression in two DRG glial cell subpopulations post-SNI: GFAP+ FABP7+ SGCs and GFAP+ FABP7- nmSCs, shedding light on GFAP's specificity as an SGC marker after SNI.

Unbiased analysis of the dorsal root ganglion after peripheral nerve injury: no neuronal loss, no gliosis, but satellite glial cell plasticity

ABSTRACT

Pain syndromes are often accompanied by complex molecular and cellular changes in dorsal root ganglia (DRG). However, the evaluation of cellular plasticity in the DRG is often performed by heuristic manual analysis of a small number of representative microscopy image fields. In this study, we introduce a deep learning-based strategy for objective and unbiased analysis of neurons and satellite glial cells (SGCs) in the DRG. To validate the approach experimentally, we examined serial sections of the rat DRG after spared nerve injury (SNI) or sham surgery. Sections were stained for neurofilament, glial fibrillary acidic protein (GFAP), and glutamine synthetase (GS) and imaged using high-resolution large-field (tile) microscopy. After training of deep learning models on consensus information of different experts, thousands of image features in DRG sections were analyzed. We used known (GFAP upregulation), controversial (neuronal loss), and novel (SGC phenotype switch) changes to evaluate the method. In our data, the number of DRG neurons was similar 14 d after SNI vs sham. In GFAP-positive subareas, the percentage of neurons in proximity to GFAP-positive cells increased after SNI. In contrast, GS-positive signals, and the percentage of neurons in proximity to GS-positive SGCs decreased after SNI. Changes in GS and GFAP levels could be linked to specific DRG neuron subgroups of different size. Hence, we could not detect gliosis but plasticity changes in the SGC marker expression. Our objective analysis of DRG tissue after peripheral nerve injury shows cellular plasticity responses of SGCs in the whole DRG but neither injury-induced neuronal death nor gliosis.

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.

Profiling the molecular signature of satellite glial cells at the single cell level reveals high similarities between rodents and humans

ABSTRACT

Peripheral sensory neurons located in dorsal root ganglia relay sensory information from the peripheral tissue to the brain. Satellite glial cells (SGCs) are unique glial cells that form an envelope completely surrounding each sensory neuron soma. This organization allows for close bidirectional communication between the neuron and its surrounding glial coat. Morphological and molecular changes in SGC have been observed in multiple pathological conditions such as inflammation, chemotherapy-induced neuropathy, viral infection, and nerve injuries. There is evidence that changes in SGC contribute to chronic pain by augmenting the neuronal activity in various rodent pain models. Satellite glial cells also play a critical role in axon regeneration. Whether findings made in rodent model systems are relevant to human physiology have not been investigated. Here, we present a detailed characterization of the transcriptional profile of SGC in mice, rats, and humans at the single cell level. Our findings suggest that key features of SGC in rodent models are conserved in humans. Our study provides the potential to leverage rodent SGC properties and identify potential targets in humans for the treatment of nerve injuries and alleviation of painful conditions.

Morphological and phenotypical characteristics of porcine satellite glial cells of the dorsal root ganglia

Satellite glial cells (SGCs) of the dorsal root ganglia (DRG) ensure homeostasis and proportional excitability of sensory neurons and gained interest in the field of development and maintenance of neuropathic pain. Pigs represent a suitable species for translational medicine with a more similar anatomy and physiology to humans compared to rodents, and are used in research regarding treatment of neuropathic pain. Knowledge of anatomical and physiological features of porcine SGCs is prerequisite for interpreting potential alterations. However, state of knowledge is still limited. In the present study, light microscopy, ultrastructural analysis and immunofluorescence staining was performed. SGCs tightly surround DRG neurons with little vascularized connective tissue between SGC-neuron units, containing, among others, axons and Schwann cells. DRG were mainly composed of large sized neurons (∼59%), accompanied by fewer medium sized (∼36%) and small sized sensory neurons (∼6%). An increase of neuronal body size was concomitant with an increased number of surrounding SGCs. The majority of porcine SGCs expressed glutamine synthetase and inwardly rectifying potassium channel Kir 4.1, known as SGC-specific markers in other species. Similar to canine SGCs, marked numbers of porcine SGCs were immunopositive for glial fibrillary acidic protein, 2',3'-cyclic-nucleotide 3'-phosphodiesterase and the transcription factor Sox2. Low to moderate numbers of SGCs showed aquaporin 4-immunoreactivity (AQP4) as described for murine SGCs. AQP4-immunoreactivity was primarily found in SGCs ensheathing small and medium sized neuronal somata. Low numbers of SGCs were immunopositive for ionized calcium-binding adapter molecule 1, indicating a potential immune cell character. No immunoreactivity for common leukocyte antigen CD45 nor neural/glial antigen 2 was detected. The present study provides essential insights into the characteristic features of non-activated porcine SGCs, contributing to a better understanding of this cell population and its functional aspects. This will help to interpret possible changes that might occur under activating conditions such as pain.

Satellite glia modulate sympathetic neuron survival, activity, and autonomic function

Satellite glia are the major glial cells in sympathetic ganglia, enveloping neuronal cell bodies. Despite this intimate association, the extent to which sympathetic functions are influenced by satellite glia in vivo remains unclear. Here, we show that satellite glia are critical for metabolism, survival, and activity of sympathetic neurons and modulate autonomic behaviors in mice. Adult ablation of satellite glia results in impaired mTOR signaling, soma atrophy, reduced noradrenergic enzymes, and loss of sympathetic neurons. However, persisting neurons have elevated activity, and satellite glia-ablated mice show increased pupil dilation and heart rate, indicative of enhanced sympathetic tone. Satellite glia-specific deletion of Kir4.1, an inward-rectifying potassium channel, largely recapitulates the cellular defects observed in glia-ablated mice, suggesting that satellite glia act in part via K⁺-dependent mechanisms. These findings highlight neuron–satellite glia as functional units in regulating sympathetic output, with implications for disorders linked to sympathetic hyper-activity such as cardiovascular disease and hypertension.

Comparative transcriptional analysis of satellite glial cell injury response

Background: Satellite glial cells (SGCs) tightly surround and support primary sensory neurons in the peripheral nervous system and are increasingly recognized for their involvement in the development of neuropathic pain following nerve injury. SGCs are difficult to investigate due to their flattened shape and tight physical connection to neurons in vivo and their rapid changes in phenotype and protein expression when cultured in vitro. Consequently, several aspects of SGC function under normal conditions as well as after a nerve injury remain to be explored. The recent advance in single cell RNA sequencing (scRNAseq) technologies has enabled a new approach to investigate SGCs. Methods: In this study we used scRNAseq to investigate SGCs from mice subjected to sciatic nerve injury. We used a meta-analysis approach to compare the injury response with that found in other published datasets.  Furthermore, we also used scRNAseq to investigate how cells from the dorsal root ganglion (DRG) change after 3 days in culture. Results: From our meta-analysis of the injured conditions, we find that SGCs share a common signature of 18 regulated genes following sciatic nerve crush or sciatic nerve ligation, involving transcriptional regulation of cholesterol biosynthesis. We also observed a considerable transcriptional change when culturing SGCs, suggesting that some differentiate into a specialised in vitro state while others start resembling Schwann cell-like precursors. Conclusion: By using integrated analyses of new and previously published scRNAseq datasets, this study provides a consensus view of which genes are most robustly changed in SGCs after injury. Our results are available via the Broad Institute Single Cell Portal, so that readers can explore and search for genes of interest.

Satellite Glial Cells and Neurons in Trigeminal Ganglia Are Altered in an Itch Model in Mice

Itch (pruritus) is a common chronic condition with a lifetime prevalence of over 20%. The mechanisms underlying itch are poorly understood, and its therapy is difficult. There is recent evidence that following nerve injury or inflammation, intercellular communications in sensory ganglia are augmented, which may lead to abnormal neuronal activity, and hence to pain, but there is no information whether such changes take place in an itch model. We studied changes in neurons and satellite glial cells (SGCs) in trigeminal ganglia in an itch model in mice using repeated applications of 2,4,6-trinitro-1-chlorobenzene (TNCB) to the external ear over a period of 11 days. Treated mice showed augmented scratching behavior as compared with controls during the application period and for several days afterwards. Immunostaining for the activation marker glial fibrillary acidic protein in SGCs was greater by about 35% after TNCB application, and gap junction-mediated coupling between neurons increased from about 2% to 13%. The injection of gap junction blockers reduced scratching behavior, suggesting that gap junctions contribute to itch. Calcium imaging studies showed increased responses of SGCs to the pain (and presumed itch) mediator ATP. We conclude that changes in both neurons and SGCs in sensory ganglia may play a role in itch.

Investigation of the effects of berberine on bortezomib-induced sciatic nerve and spinal cord damage in rats through pathways involved in oxidative stress and neuro-inflammation

Bortezomib (BTZ), a proteasome inhibitor, causes dose-limiting peripheral neuropathy in humans. Berberine (BBR), which has various biological and pharmacological properties, is known to have neuroprotective properties. The possible protective effects of BBR on peripheral neuropathy caused by BTZ were investigated in this study. For this purpose, BTZ was intraperitoneally given to Sprague dawley rats on the 1 st, 3rd, 5th, and 7th days with a cumulative dose of 0.8 mg/kg. Moreover, animals were orally administered 50 or 100 mg/kg BBR daily from day 1 to day 10. As a result of the analyzes performed on the sciatic nerve and spinal cord, it was observed that MDA levels and NRF-2, HO-1, NQO1, GCLC and GCLM mRNA transcript levels increased due to oxidative stress caused by BTZ, and the levels of these markers decreased after BBR administration. Also, it was determined that SOD, CAT, GPx and GSH levels increased after BBR treatment. It was observed that BTZ caused inflammation by triggering NF-κB, TNF-α, IL-1β and IL-6 cytokines, on the other hand, with BBR treatment, these cytokines were suppressed and inflammation was alleviated. In addition, it was determined that the expressions of RAGE, STAT3, NLRP3 and TLR4, which have important roles in inflammation, increased with BTZ administration, but BBR suppressed the expressions of these genes. It was determined that the expressions of SIRT1, which plays an important role in neuropathic pain, and CREB-LI neurons, which has an active role in neurite outgrowth and survival, decreased with BTZ administration. It was observed that GFAP levels increased with BTZ administration and decreased with BBR administration. Given all the findings, it was concluded that BBR exhibits protective qualities in the sciatic nerve and spinal cord induced by BTZ.

How Is Peripheral Injury Signaled to Satellite Glial Cells in Sensory Ganglia?

Injury or inflammation in the peripheral branches of neurons of sensory ganglia causes changes in neuronal properties, including excessive firing, which may underlie chronic pain. The main types of glial cell in these ganglia are satellite glial cells (SGCs), which completely surround neuronal somata. SGCs undergo activation following peripheral lesions, which can enhance neuronal firing. How neuronal injury induces SGC activation has been an open question. Moreover, the mechanisms by which the injury is signaled from the periphery to the ganglia are obscure and may include electrical conduction, axonal and humoral transport, and transmission at the spinal level. We found that peripheral inflammation induced SGC activation and that the messenger between injured neurons and SGCs was nitric oxide (NO), acting by elevating cyclic guanosine monophosphate (cGMP) in SGCs. These results, together with work from other laboratories, indicate that a plausible (but not exclusive) mechanism for neuron-SGCs interactions can be formulated as follows: Firing due to peripheral injury induces NO formation in neuronal somata, which diffuses to SGCs. This stimulates cGMP synthesis in SGCs, leading to their activation and to other changes, which contribute to neuronal hyperexcitability and pain. Other mediators such as proinflammatory cytokines probably also contribute to neuron-SGC communications.