Intraganglionic interactions between satellite cells and adult sensory neurons

Neurophysiological Basis of Electroacupuncture Stimulation in the Treatment of Cardiovascular-Related Diseases: Vagal Interoceptive Loops

PURPOSE

The vagal sensory nerve (VSN) is an essential interoceptive pathway that is connected to every level of the body. Its intricate genetic coding provides sustenance for physiological processes, including controlling blood pressure and respiration. Electroacupuncture (EA) is a proven surface stimulation therapy that can regulate vagal nerve activity, which can effectively prevent cardiovascular diseases. A growing number of studies have concentrated on the mapping of VSN codes, but little is known, and the physiological background of how EA influences interoceptive has not been fully explored.

METHOD

Here, we incorporate the hypothesized interaction among EA targets, VSNs, and the heart. This offers suggestions for using a versatile and focused EA strategy to modify vagal interoceptive awareness to enhance cardiovascular conditions. We first clarified the major role of vagal nerve in the control of cardiac activity. Additionally, we clarified the multidimensional coding pattern in the VSNs, revealing that the targeted control of multimodal interoceptive is the functional basis of the synchronization of cardiovascular system.

FINDING

We propose a strategy in which EA of the VSNs is employed to activate the interoceptive loop and reduce the risk of cardiovascular disease.

Satellite Glial Cells Bridge Sensory Neuron Crosstalk in Visceral Pain and Cross-Organ Sensitization

Following colonic inflammation, the uninjured bladder afferent neurons are also activated. The mechanisms and pathways underlying this sensory neuron cross-activation (from injured neurons to uninjured neurons) are not fully understood. Colonic and bladder afferent neurons reside in the same spinal segments and are separated by satellite glial cells (SGCs) and extracellular matrix in dorsal root ganglia (DRG). SGCs communicate with sensory neurons in a bidirectional fashion. This review summarizes the differentially regulated genes/proteins in the injured and uninjured DRG neurons and explores the role of SGCs in regulation of sensory neuron crosstalk in visceral cross-organ sensitization. The review also highlights the paracrine pathways in mediating neuron-SGC and SGC-neuron coupling with an emphasis on the neurotrophins and purinergic systems. Finally, I discuss the results from recent RNAseq profiling of SGCs to reveal useful molecular markers for characterization, functional study, and therapeutic targets of SGCs. SIGNIFICANCE STATEMENT: Satellite glial cells (SGCs) are the largest glial subtypes in sensory ganglia and play a critical role in mediating sensory neuron crosstalk, an underlying mechanism in colon-bladder cross-sensitization. Identification of novel and unique molecular markers of SGCs can advance the discovery of therapeutic targets in treatment of chronic pain including visceral pain comorbidity.

Collaborative Roles for RAC1, ERM Proteins and PTEN During Adult Sensory Axon Regeneration

The role of local of growth cone (GC) manipulation in adult regenerative systems is largely unexplored despite substantial translational importance. Here we investigated collaboration among Rac1 GTPase, its partnering ERM proteins and PTEN in adult sensory neurons and adult nerve regeneration. We confirmed expression of both Rac1 and ERM in adults and noted substantial impacts on neurite outgrowth in naïve and pre-injured adult sensory neurons. PTEN inhibition added to this outgrowth. Rac1 activation acted directly on adult GCs facilitating both attractive turning and advancement. In vivo regeneration indices including electrophysiological recovery, return of sensation, walking, repopulation of myelinated axons and reinnervation of the target epidermis indicated benefits of local Rac1 activation. These indices suggested maximal GC activation whereas local PTEN inhibition offered only limited added improvement. Our findings provide support for the concept of manipulating adult GCs, by emphasizing local Rac1 activation in directing therapy for nerve repair.

Prediction of cell-cell communication patterns of dorsal root ganglion cells: single-cell RNA sequencing data analysis

Dorsal root ganglion neurons transmit peripheral somatic information to the central nervous system, and dorsal root ganglion neuron excitability affects pain perception. Dorsal root ganglion stimulation is a new approach for managing pain sensation. Knowledge of the cell-cell communication among dorsal root ganglion cells may help in the development of new pain and itch management strategies. Here, we used the single-cell RNA-sequencing (scRNA-seq) database to investigate intercellular communication networks among dorsal root ganglion cells. We collected scRNA-seq data from six samples from three studies, yielding data on a total of 17,766 cells. Based on genetic profiles, we identified satellite glial cells, Schwann cells, neurons, vascular endothelial cells, immune cells, fibroblasts, and vascular smooth muscle cells. Further analysis revealed that eight types of dorsal root ganglion neurons mediated proprioceptive, itch, touch, mechanical, heat, and cold sensations. Moreover, we predicted several distinct forms of intercellular communication among dorsal root ganglion cells, including cell-cell contact, secreted signals, extracellular matrix, and neurotransmitter-mediated signals. The data mining predicted that Mrgpra3 -positive neurons robustly express the genes encoding the adenosine Adora2b (A2B) receptor and glial cell line-derived neurotrophic factor family receptor alpha 1 (GFRα-1). Our immunohistochemistry results confirmed the coexpression of the A2B receptor and GFRα-1. Intrathecal injection of the A2B receptor antagonist PSB-603 effectively prevented histamine-induced scratching behaviour in a dose-dependent manner. Our results demonstrate the involvement of the A2B receptor in the modulation of itch sensation. Furthermore, our findings provide insight into dorsal root ganglion cell-cell communication patterns and mechanisms. Our results should contribute to the development of new strategies for the regulation of dorsal root ganglion excitability.

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.

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.

Anti-Inflammatory Activity of Synaptamide in the Peripheral Nervous System in a Model of Sciatic Nerve Injury

N-docosahexaenoylethanolamine (DHEA), or synaptamide, is an endogenous metabolite of docosahexaenoic acid (DHA) that exhibits synaptogenic and neurogenic effects. In our previous studies, synaptamide administration inhibited the neuropathic pain-like behavior and reduced inflammation in the central nervous system following sciatic nerve injury. In the present study, we examine the effect of synaptamide on the peripheral nervous system in a neuropathic pain condition. The dynamics of ionized calcium-binding adapter molecule 1 (iba-1), CD68, CD163, myelin basic protein, and the production of interleukin 1β and 6 within the sciatic nerve, as well as the neuro-glial index and the activity of iba-1, CD163, glial fibrillary acidic protein (GFAP), neuronal NO synthase (nNOS), substance P (SP), activating transcription factor 3 (ATF3) in the dorsal root ganglia (DRG), are studied. According to our results, synaptamide treatment (4 mg/kg/day) (1) decreases the weight-bearing deficit after nerve trauma; (2) enhances the remyelination process in the sciatic nerve; (3) shows anti-inflammatory properties in the peripheral nervous system; (4) decreases the neuro-glial index and GFAP immunoreactivity in the DRG; (5) inhibits nNOS- and SP-ergic activity in the DRG, which might contribute to neuropathic pain attenuation. In general, the current study demonstrates the complex effect of synaptamide on nerve injury, which indicates its high potential for neuropathic pain management.

Activation of Intra-nodose Ganglion P2X7 Receptors Elicit Increases in Neuronal Activity

Vagus nerve innervates several organs including the heart, stomach, and pancreas among others. Somas of sensory neurons that project through the vagal nerve are located in the nodose ganglion. The presence of purinergic receptors has been reported in neurons and satellite glial cells in several sensory ganglia. In the nodose ganglion, calcium depletion-induced increases in neuron activity can be partly reversed by P2X7 blockers applied directly into the ganglion. The later suggest a possible role of P2X7 receptors in the modulation of neuronal activity within this sensory ganglion. We aimed to characterize the response to P2X7 activation in nodose ganglion neurons under physiological conditions. Using an ex vivo preparation for electrophysiological recordings of the neural discharges of nodose ganglion neurons, we found that treatments with ATP induce transient neuronal activity increases. Also, we found a concentration-dependent increase in neural activity in response to Bz-ATP (ED₅₀ = 0.62 mM, a selective P2X7 receptor agonist), with a clear desensitization pattern when applied every ~ 30 s. Electrophysiological recordings from isolated nodose ganglion neurons reveal no differences in the responses to Bz-ATP and ATP. Finally, we showed that the P2X7 receptor was expressed in the rat nodose ganglion, both in neurons and satellite glial cells. Additionally, a P2X7 receptor negative allosteric modulator decreased the duration of Bz-ATP-induced maximal responses without affecting their amplitude. Our results show the presence of functional P2X7 receptors under physiological conditions within the nodose ganglion of the rat, and suggest that ATP modulation of nodose ganglion activity may be in part mediated by the activation of P2X7 receptors.

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.

Survival of compromised adult sensory neurons involves macrovesicular formation

Adult neurons are recognized as post-mitotically arrested cells with limited regenerative potential. Given these restraints, it is perplexing how neurons sustain routine physiological and occasional reparative stress without compromising their density and integrity. We observed that specific insults or physiological alterations drive adult sensory neurons to attempt cell cycle entry. In this context, we demonstrate that at least a small population of sensory neurons modify their cytoskeleton as a survival mechanism in settings of growth arrest and associated stress. Most notably, among their apparent survival modifications is included a unique, and uncharacterized form of macrovesicle shedding and a subsequent neuron size adjustment. Using time-lapse imaging, we demonstrate macrovesicle shedding in some neurons subjected to growth restraint, but not associated with apoptosis. In axotomized neurons in vivo, cell cycle entry was rare to absent and macrovesicles were not observed, but we nonetheless identified changes in mRNA associated with autophagy. In vivo, neighbouring macrophages may have a role in modifying the neuron cytoskeleton after axotomy. Overall, the findings identify previously unrecognized structural adaptations in adult sensory neurons that may provide resilience to diverse insults.

Peroxisome Metabolism Contributes to PIEZO2-Mediated Mechanical Allodynia

Mutations in the peroxisomal half-transporter ABCD1 cause X-linked adrenoleukodystrophy, resulting in elevated very long-chain fatty acids (VLCFA), progressive neurodegeneration and an associated pain syndrome that is poorly understood. In the nervous system of mice, we found ABCD1 expression to be highest in dorsal root ganglia (DRG), with satellite glial cells (SGCs) displaying higher expression than neurons. We subsequently examined sensory behavior and DRG pathophysiology in mice deficient in ABCD1 compared to wild-type mice. Beginning at 8 months of age, Abcd1-/y mice developed persistent mechanical allodynia. DRG had a greater number of IB4-positive nociceptive neurons expressing PIEZO2, the mechanosensitive ion channel. Blocking PIEZO2 partially rescued the mechanical allodynia. Beyond affecting neurons, ABCD1 deficiency impacted SGCs, as demonstrated by high levels of VLCFA, increased glial fibrillary acidic protein (GFAP), as well as genes disrupting neuron-SGC connectivity. These findings suggest that lack of the peroxisomal half-transporter ABCD1 leads to PIEZO2-mediated mechanical allodynia as well as SGC dysfunction. Given the known supportive role of SGCs to neurons, this elucidates a novel mechanism underlying pain in X-linked adrenoleukodystrophy.

Glutamine Maintains Satellite Glial Cells Growth and Survival in Culture

Satellite glial cells (SGCs) tightly surround neurons and modulate sensory transmission in dorsal root ganglion (DRG). At present, the biological property of primary SGCs in culture deserves further investigation. To reveal the key factor for SGCs growth and survival, we examined the effects of different culture supplementations containing Dulbecco's Modified Eagle Medium (DMEM)/F12, DMEM high glucose (HG) or Neurobasal-A (NB). CCK-8 proliferation assay showed an increased proliferation of SGCs in DMEM/F12 and DMEM/HG, but not in NB medium. Bax, AnnexinV, and propidium iodide (PI) staining results showed that NB medium caused cell death and apoptosis. We showed that glutamine was over 2.5 mM in DMEM/F12 and DMEM/HG, whereas it was absence in NB medium. Interestingly, exogenous glutamine application significantly reversed the poor proliferation and cell death of SGCs in NB medium. These findings demonstrated that DMEM/F12 medium was optimal to get high-purity SGCs. Glutamine was the key molecule to maintain SGCs growth and survival in culture. Here, we provided a novel approach to get high-purity SGCs by changing the key component of culture medium. Our study shed a new light on understanding the biological property and modulation of glial cells of primary sensory ganglia.

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.

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.

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.

Inactivation of vimentin in satellite glial cells affects dorsal root ganglion intermediate filament expression and neuronal axon growth in vitro

Peripheral nerve trauma and regeneration are complex events, and little is known concerning how occurrences in the distal stump affect the cell body's response to injury. Intermediate filament (IF) proteins underpin cellular architecture and take part in nerve cell proliferation, differentiation and axon regeneration, but their role in these processes is not yet fully understood. The present study aimed to investigate the regulation and interrelationship of major neural IFs in adult dorsal root ganglion (DRG) neurons and satellite glial cells (SGCs) following sciatic nerve injury. We demonstrated that the expression of neural IFs in DRG neurons and SGCs after axotomy depends on vimentin activity. In intact DRGs, synemin M and peripherin proteins are detected in small neurons while neurofilament L (NFL) and synemin L characterize large neurons. Both neuronal populations are surrounded by vimentin positive- and glial fibrillary acidic protein (GFAP)-negative SGCs. In response to axotomy, synemin M and peripherin were upregulated in large wild-type DRG neurons and, to a lesser extent, in vim-/- and synm-/- DRG neurons, suggesting the role for these IFs in axon regeneration. However, an increase in the number of NFL-positive small neurons was observed in vim-/- mice, accompanied by a decrease of peripherin-positive small neurons. These findings suggest that vimentin is required for injury-induced neuronal IF remodeling. We further show that vimentin is also indispensable for nerve injury-induced GFAP upregulation in perineuronal SGCs and that inactivation of vimentin and synemin appears to accelerate the rate of DRG neurite regeneration at early stages in vitro.

Hepatocyte growth factor induces pErk and pSTAT3 (Ser 727) to promote mitochondrial activity and neurite outgrowth in primary dorsal root ganglion cultures

Hepatocyte growth factor (HGF) promotes the neurite outgrowth of sensory neurons in developmental stages, but its role in injured peripheral nerves in adult mice remains largely been unexplored. In this study, we investigated the role of HGF in the regeneration of injured peripheral nerves using cultured dorsal root ganglions (DRGs). When cells were treated with HGF protein, the length of the neurite was increased 1.4-fold compared to the untreated control group. HGF greatly increased the level of phosphorylated STAT3 at serine 727 [pSTAT3 (Ser 727)], thereby translocating the protein to the mitochondria. HGF treatment increased the activity of mitochondrial complex I. When DRGs were cultured in the presence of U0126, a pharmacological inhibitor of Erk, the HGF-mediated increase in neurite outgrowth and the level of pSTAT3 (Ser 727) were both suppressed. Taken together, these results suggest that the HGF/c-met pathway might promote neurite outgrowth by controlling mitochondrial activity through the HGF/Erk/STAT3 axis.

Regulation of Mitochondrial Function by Epac2 Contributes to Acute Inflammatory Hyperalgesia

G_αs-coupled receptors signaling through cAMP provide a key mechanism for the sensitization of nociceptive sensory neurons, and the cAMP effector Epac has been implicated in the transition from acute to chronic pain. Epac exerts its effects through Rap1 and protein kinase C (PKC). To identify targets of Epac-PKC signaling in sensory neurons of the mouse dorsal root ganglion (DRG), we profiled PKC substrate proteins phosphorylated in response to the activation of Epac with the proinflammatory prostaglandin E2 (PGE2). A prominent Epac-dependent phospho-protein band induced by PGE2 was identified by mass spectrometry as the mitochondrial enzyme pyruvate dehydrogenase (Pdha1). In dissociated DRG from both males and females, the recruitment of Pdha1 to phospho-protein fractions was rapidly induced by PGE2 and prevented by selective inhibition of Epac2. Epac activation increased mitochondrial respiration, consistent with an increase in Pdha1 function mediated by Epac2. Hindpaw injection of PGE2 induced heat hyperalgesia in males and females, but Pdha1 phosphorylation occurred only in males. Hyperalgesia was attenuated in males but not in females by systemic inhibition of Epac2, and also by a mitochondrial membrane potential uncoupler, dinitrophenol, supporting a role for mitochondrial regulation in acute hyperalgesia. These findings identify a mechanism for the regulation of mitochondrial function by Epac2 that contributes to acute inflammatory hyperalgesia in male mice. Systemic administration of the cyclooxygenase 2 inhibitor celecoxib suppressed both PGE2-induced heat hyperalgesia and Pdha1 phosphorylation in DRG of males but not females, suggesting that prostaglandin synthesis within the DRG mediates the phosphorylation of Pdha1 in response to hindpaw insult.SIGNIFICANCE STATEMENT There has been extensive investigation of mitochondrial dysfunction as a causative factor in neuropathic pain disorders. In contrast, results reported here implicate enhanced mitochondrial function as a contributing factor in the development of acute inflammatory hyperalgesia. We describe a mechanism in which Epac2 activation by prostaglandin receptors leads to phosphorylation of pyruvate dehydrogenase and an increase in mitochondrial respiration in peripheral sensory neurons. Although Epac2 activation leads to Pdha1 (pyruvate dehydrogenase) phosphorylation in dissociated neurons from mice of both sexes, induction of this pathway in vivo by hindpaw insult is restricted to males and appears to require intraganglionic prostaglandin synthesis. These findings support a model in which G_s-coupled receptor modulation of mitochondrial function promotes acute nociceptive signaling and inflammatory hyperalgesia.

In vitro priming response in dorsal root ganglia partially mimics injury-driven pre-conditioning response and reprograms neurons for enhanced outgrowth

Peripheral nerve injuries have the potential to bring about long-term disabilities in individuals. The major issue in repairing nerve injuries is the poor growth rate of axons. Although several molecules have been identified as potential candidates for improving axon growth, their potential translation into clinical practice is preliminary and largely unexplored. This necessitates identifying additional molecular candidates with superior potential to improve axon growth. Lack of a simple non-surgical screening model also poses a hurdle in rapidly screening potential candidate molecules. In this work, we developed a novel, rapid screening model for nerve regeneration therapeutics that retains a focus on adult neurons. The model involves simple incubation of sensory ganglia over a period of 24 h prior to dissociation. Surprisingly, this model features unique events that reprogram both sensory neurons and supporting glia favoring axon growth. Moreover, several associated cellular and molecular changes involved in this model partially mimic classic axotomy-induced changes in sensory ganglia. Overall, this model presents with a platform that not only allows rapid screening of drug candidates but offers opportunities in studying novel intrinsic molecular changes in both neurons and glial cells directed towards improving the pace of axon growth.

Satellite glial cells promote regenerative growth in sensory neurons

Peripheral sensory neurons regenerate their axon after nerve injury to enable functional recovery. Intrinsic mechanisms operating in sensory neurons are known to regulate nerve repair, but whether satellite glial cells (SGC), which completely envelop the neuronal soma, contribute to nerve regeneration remains unexplored. Using a single cell RNAseq approach, we reveal that SGC are distinct from Schwann cells and share similarities with astrocytes. Nerve injury elicits changes in the expression of genes related to fatty acid synthesis and peroxisome proliferator-activated receptor (PPARα) signaling. Conditional deletion of fatty acid synthase (Fasn) in SGC impairs axon regeneration. The PPARα agonist fenofibrate rescues the impaired axon regeneration in mice lacking Fasn in SGC. These results indicate that PPARα activity downstream of FASN in SGC contributes to promote axon regeneration in adult peripheral nerves and highlight that the sensory neuron and its surrounding glial coat form a functional unit that orchestrates nerve repair.

The contribution of satellite glia to peripheral nerve regeneration is unclear. Here, the authors show that satellite glia are transcriptionally distinct from Schwann cells, share similarities with astrocytes, and, upon injury, they contribute to axon regeneration via Fasn-PPARα signalling pathway.

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.

The ATP-P2X7 Signaling Pathway Participates in the Regulation of Slit1 Expression in Satellite Glial Cells

Slit1 is one of the known signaling factors of the slit family and can promote neurite growth by binding to its receptor, Robo2. Upregulation of Slit1 expression in dorsal root ganglia (DRG) after peripheral nerve injury plays an important role in nerve regeneration. Each sensory neuronal soma in the DRG is encapsulated by several surrounding satellite glial cells (SGCs) to form a neural structural unit. However, the temporal and spatial patterns of Slit1 upregulation in SGCs in DRG and its molecular mechanisms are not well understood. This study examined the spatial and temporal patterns of Slit1 expression in DRG after sciatic nerve crush by immunohistochemistry and western blotting. The effect of neuronal damage signaling on the expression of Slit1 in SGCs was studied in vivo by fluorescent gold retrograde tracing and double immunofluorescence staining. The relationship between the expression of Slit1 in SGCs and neuronal somas was also observed by culturing DRG cells and double immunofluorescence labeling. The molecular mechanism of Slit1 was further explored by immunohistochemistry and western blotting after intraperitoneal injection of Bright Blue G (BBG, P2X7R inhibitor). The results showed that after peripheral nerve injury, the expression of Slit1 in the neurons and SGCs of DRG increased. The expression of Slit1 was presented with a time lag in SGCs than in neurons. The expression of Slit1 in SGCs was induced by contact with surrounding neuronal somas. Through injured cell localization, it was found that the expression of Slit1 was stronger in SGCs surrounding injured neurons than in SGCs surrounding non-injured neurons. The expression of vesicular nucleotide transporter (VNUT) in DRG neurons was increased by injury signaling. After the inhibition of P2X7R, the expression of Slit1 in SGCs was downregulated, and the expression of VNUT in DRG neurons was upregulated. These results indicate that the ATP-P2X7R pathway is involved in signal transduction from peripheral nerve injury to SGCs, leading to the upregulation of Slit1 expression.

Intrinsic mechanisms of neuronal axon regeneration

Permanent disabilities following CNS injuries result from the failure of injured axons to regenerate and rebuild functional connections with their original targets. By contrast, injury to peripheral nerves is followed by robust regeneration, which can lead to recovery of sensory and motor functions. This regenerative response requires the induction of widespread transcriptional and epigenetic changes in injured neurons. Considerable progress has been made in recent years in understanding how peripheral axon injury elicits these widespread changes through the coordinated actions of transcription factors, epigenetic modifiers and, to a lesser extent, microRNAs. Although many questions remain about the interplay between these mechanisms, these new findings provide important insights into the pivotal role of coordinated gene expression and chromatin remodelling in the neuronal response to injury.

Nociceptor Deletion of Tsc2 Enhances Axon Regeneration by Inducing a Conditioning Injury Response in Dorsal Root Ganglia

Neurons of the PNS are able to regenerate injured axons, a process requiring significant cellular resources to establish and maintain long-distance growth. Genetic activation of mTORC1, a potent regulator of cellular metabolism and protein translation, improves axon regeneration of peripheral neurons by an unresolved mechanism. To gain insight into this process, we activated mTORC1 signaling in mouse nociceptors via genetic deletion of its negative regulator Tsc2. Perinatal deletion of Tsc2 in nociceptors enhanced initial axon growth after sciatic nerve crush, however by 3 d post-injury axon elongation rate became similar to controls. mTORC1 inhibition prior to nerve injury was required to suppress the enhanced axon growth. Gene expression analysis in purified nociceptors revealed that Tsc2-deficient nociceptors had increased activity of regeneration-associated transcription factors (RATFs), including cJun and Atf3, in the absence of injury. Additionally, nociceptor deletion of Tsc2 activated satellite glial cells and macrophages in the dorsal root ganglia (DRG) in a similar manner to nerve injury. Surprisingly, these changes improved axon length but not percentage of initiating axons in dissociated cultures. The pro-regenerative environment in naïve DRG was recapitulated by AAV8-mediated deletion of Tsc2 in adult mice, suggesting that this phenotype does not result from a developmental effect. Consistently, AAV8-mediated Tsc2 deletion did not improve behavioral recovery after a sciatic nerve crush injury despite initially enhanced axon growth. Together, these data show that neuronal mTORC1 activation induces an incomplete pro-regenerative environment in the DRG that improves initial but not later axon growth after nerve injury.

Involvement of Endothelin Receptors in Peripheral Sensory Neuropathy Induced by Oxaliplatin in Mice

The aim of this study was to evaluate the participation of the endothelin ET_A and ET_B receptors and the effects of bosentan in oxaliplatin-induced peripheral sensory neuropathy (OIN) in mice. Adult male Swiss mice received 1 mg/kg of oxaliplatin intravenously, twice a week for 5 weeks. Dorsal root ganglia (DRG) and spinal cords were removed for evaluation of the endothelin ET_A and ET_B receptor expression. Afterwards, selective (BQ-123 and BQ-788; 10 nmol in 30 μL, intraplantarly) and non-selective (bosentan, 100 mg/kg, orally) antagonists were administered in order to evaluate the involvement of the endothelin receptors in OIN. Mechanical and thermal nociception tests were performed once a week for 56 days. Oxaliplatin induced mechanical and thermal hypersensitivity and increased the endothelin ET_A receptor expression in both the DRG and spinal cord (P < 0.05). Endothelin ET_B receptor expression was increased in the DRG (P < 0.05) but not in the spinal cord. Both endothelin ET_A and ET_B receptor selective antagonists partially prevented mechanical hyperalgesia in mice with OIN (P < 0.05). Moreover, bosentan prevented mechanical and thermal hypersensitivity in oxaliplatin-treated mice (P < 0.05). In conclusion, both endothelin ET_A and ET_B receptors seem to be involved in the OIN in mice and they should be considered possible targets for the management of this clinical feature.

The secreted neurotrophin Spätzle 3 promotes glial morphogenesis and supports neuronal survival and function

Most glial functions depend on establishing intimate morphological relationships with neurons. Significant progress has been made in understanding neuron-glia signaling at synaptic and axonal contacts, but how glia support neuronal cell bodies is unclear. Here we explored the growth and functions of Drosophila cortex glia (which associate almost exclusively with neuronal cell bodies) to understand glia-soma interactions. We show that cortex glia tile with one another and with astrocytes to establish unique central nervous system (CNS) spatial domains that actively restrict glial growth, and selective ablation of cortex glia causes animal lethality. In an RNAi-based screen, we identified αSNAP (soluble NSF [N-ethylmalemeide-sensitive factor] attachment protein α) and several components of vesicle fusion and recycling machinery as essential for the maintenance of cortex glial morphology and continued contact with neurons. Interestingly, loss of the secreted neurotrophin Spätzle 3 (Spz3) phenocopied αSNAP phenotypes, which included loss of glial ensheathment of neuron cell bodies, increased neuronal cell death, and defects in animal behavior. Rescue experiments suggest that Spz3 can exert these effects only over very short distances. This work identifies essential roles for glial ensheathment of neuronal cell bodies in CNS homeostasis as well as Spz3 as a novel signaling factor required for maintenance of cortex glial morphology and neuron-glia contact.

Primary Postnatal Dorsal Root Ganglion Culture from Conventionally Slaughtered Calves

Neurological disorders in ruminants have an important impact on veterinary health, but very few host-specific in vitro models have been established to study diseases affecting the nervous system. Here we describe a primary neuronal dorsal root ganglia (DRG) culture derived from calves after being conventionally slaughtered for food consumption. The study focuses on the in vitro characterization of bovine DRG cell populations by immunofluorescence analysis. The effects of various growth factors on neuron viability, neurite outgrowth and arborisation were evaluated by morphological analysis. Bovine DRG neurons are able to survive for more than 4 weeks in culture. GF supplementation is not required for neuronal survival and neurite outgrowth. However, exogenously added growth factors promote neurite outgrowth. DRG cultures from regularly slaughtered calves represent a promising and sustainable host specific model for the investigation of pain and neurological diseases in bovines.

An improved method for in vitro morphofunctional analysis of mouse dorsal root ganglia

Sensory neurons in dorsal root ganglia (DRGs) are the first-order neurons along the pathway conveying sensory information from the periphery to the central nervous system. The analysis of the morphological and physiological features of these neurons and their alterations in pathology is the necessary prerequisite to understand pain encoding mechanisms. Here, we describe an in vitro procedure for combined morphofunctional analysis of mouse DRGs. Freshly excised DRGs obtained from adult mice were incubated in collagenase to dissolve the ensheathing connective capsule. The degradation of the connective tissue facilitates both access to the neurons by classical recording glass pipettes and the penetration of primary antibodies for immunohistochemical procedures. The entire DRGs were then imaged using a confocal microscope obtaining a fine 3D representation of their cytoarchitecture without requiring tissue sectioning. Thus, our proposed whole-mount preparation represents a flexible in vitro approach for both functional and phenotypic analysis of DRG neurons by at the same time preserving their neuroanatomical relationships.