Major Differences in Transcriptional Alterations in Dorsal Root Ganglia Between Spinal Cord Injury and Peripheral Neuropathic Pain Models

Identification of Potential Intervention Targets Involved in Prior Exercise that Attenuates Peripheral Neuropathic Pain by Integrating Transcriptome and Whole-genome Bisulfite Sequencing Analyses

Changes in DNA methylation and subsequent alterations in gene expression have opened a new direction in research related to the pathogenesis of peripheral neuropathic pain (PNP). This study aimed to reveal epigenetic perturbations underlying DNA methylation in the dorsal root ganglion (DRG) of rats with peripheral nerve injury in response to prior exercise and identify potential target genes involved. Male Sprague-Dawley rats were divided into three groups, namely, chronic constriction injury (CCI) of the sciatic nerve, CCI with prior 6-week swimming training (CCI_Ex), and sham operated (Sham). Mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were used as the main observation indicators to evaluate behavioral changes associated with pain. In this study, 6-week swimming training before CCI prevented later chronic pain. In particular, CCI rats with prior exercise showed a significant increase in the MWT and TWL of the injured lateral hind paw compared with CCI rats without exercise on days 14, 21, and 28 after CCI. Whole-genome bisulfite sequencing from the injured lumbar (L4-L6) DRGs on the 28th day after surgery was detected. We also generated DNA methylation maps of the two comparisons (sham group vs. CCI and CCI groups vs. CCI_Ex group), and 396 overlapping differentially methylated region-related genes were found between the two comparisons. Moreover, we integrated RNA sequencing to understand the mechanism by which differential DNA methylation after CCI may influence gene expression. Finally, Ryr1 and Xirp2 were identified through association analysis of two omics and quantitative reverse-transcription polymerase chain reaction, respectively. The methylation levels of Ryr1 and Xirp2 were upregulated with a corresponding increase in their mRNA expression in the DRGs of CCI rats, whereas prior exercise downregulated Ryr1 methylation and restore its expression level. Functional enrichment analysis of both omics found that the calcium signaling pathway was significantly enriched. Therefore, the potential intervention targets (Ryr1 and Xirp2) in L4-L6 DRGs may be involved in prior exercise that attenuates PNP induced by CCI. This study provides crucial insights into the epigenetic regulation of PNP responses to prior exercise.

The relationship between changes in inflammation and locomotor function in sensory phenotypes of central neuropathic pain after spinal cord injury

Introduction

Central neuropathic pain (CNP) commonly develops in patients after spinal cord injury (SCI), causing debilitating symptoms and sensory abnormalities to mechanical and thermal stimuli. The biological variability of pain phenotypes in individuals has limited the number of positive outcomes. Thus, it is necessary to investigate the physiological processes contributing to sensory changes that develop over time.

Objective

To investigate the physiological processes contributing to neuropathic pain sensory changes and locomotor impairments with sensory phenotypes that develop over time.

Methods

Using the tail flick and von Frey tests, we performed hierarchical clustering to determine the subpopulation of rats that developed thermal and mechanical sensory abnormalities. To measure inflammation as a potential mediator of CNP phenotypes, we used flow cytometry and immunohistochemistry. Finally, to assess the secondary effects on locomotor recovery, up to 8 weeks after injury, we used the CatWalk test to assess multiple parameters of gait.

Results

The von Frey test showed a subpopulation of SCI rats that were hyposensitive to mechanical stimuli from 6 to 8 weeks after injury. The tail flick test showed a subpopulation of SCI rats that were hypersensitive to thermal stimuli at 1 week and 3 to 8 weeks after injury. Although there were no differences in inflammatory cells between subpopulations, we did see significant changes in locomotor recovery between rats with and without sensory abnormalities.

Conclusion

The myeloid cell population at large is not affected by mechanical or thermal phenotypes of pain in this model; however, locomotor recovery is impaired depending on the pain phenotype present. Further investigation into acute inflammatory cells may be insightful for predicting the development of pain phenotypes.

Transcriptional reprogramming post-peripheral nerve injury: A systematic review

Neuropathic pain is characterised by periodic or continuous hyperalgesia, numbness, or allodynia, and results from insults to the somatosensory nervous system. Peripheral nerve injury induces transcriptional reprogramming in peripheral sensory neurons, contributing to increased spinal nociceptive input and the development of neuropathic pain. Effective treatment for neuropathic pain remains an unmet medical need as current therapeutics offer limited effectiveness and have undesirable effects. Understanding transcriptional changes in peripheral nerve injury-induced neuropathy might offer a path for novel analgesics. Our literature search identified 65 papers exploring transcriptomic changes post-peripheral nerve injury, many of which were conducted in animal models. We scrutinize their transcriptional changes data and conduct gene ontology enrichment analysis to reveal their common functional profile. Focusing on genes involved in 'sensory perception of pain' (GO:0019233), we identified transcriptional changes for different ion channels, receptors, and neurotransmitters, shedding light on its role in nociception. Examining peripheral sensory neurons subtype-specific transcriptional reprograming and regeneration-associated genes, we delved into downstream regulation of hypersensitivity. Identifying the temporal program of transcription regulatory mechanisms might help develop better therapeutics to target them effectively and selectively, thus preventing the development of neuropathic pain without affecting other physiological functions.

Mechanism of gabapentinoid potentiation of opioid effects on cyclic AMP signaling in neuropathic pain

Over half of spinal cord injury (SCI) patients develop opioid-resistant chronic neuropathic pain. Safer alternatives to opioids for treatment of neuropathic pain are gabapentinoids (e.g., pregabalin and gabapentin). Clinically, gabapentinoids appear to amplify opioid effects, increasing analgesia and overdose-related adverse outcomes, but in vitro proof of this amplification and its mechanism are lacking. We previously showed that after SCI, sensitivity to opioids is reduced by fourfold to sixfold in rat sensory neurons. Here, we demonstrate that after injury, gabapentinoids restore normal sensitivity of opioid inhibition of cyclic AMP (cAMP) generation, while reducing nociceptor hyperexcitability by inhibiting voltage-gated calcium channels (VGCCs). Increasing intracellular Ca2+ or activation of L-type VGCCs (L-VGCCs) suffices to mimic SCI effects on opioid sensitivity, in a manner dependent on the activity of the Raf1 proto-oncogene, serine/threonine-protein kinase C-Raf, but independent of neuronal depolarization. Together, our results provide a mechanism for potentiation of opioid effects by gabapentinoids after injury, via reduction of calcium influx through L-VGCCs, and suggest that other inhibitors targeting these channels may similarly enhance opioid treatment of neuropathic pain.

Comparative transcriptomic profiling reveals a role for Olig1 in promoting axon regeneration

The regenerative potential of injured axons displays considerable heterogeneity. However, the molecular mechanisms underlying the heterogeneity have not been fully elucidated. Here, we establish a method that can separate spinal motor neurons (spMNs) with low and high regenerative capacities and identify a set of transcripts revealing differential expression between two groups of neurons. Interestingly, oligodendrocyte transcription factor 1 (Olig1), which regulates the differentiation of various neuronal progenitors, exhibits recurrent expression in spMNs with enhanced regenerative capabilities. Furthermore, overexpression of Olig1 (Olig1 OE) facilitates axonal regeneration in various models, and down-regulation or deletion of Olig1 exhibits an opposite effect. By analyzing the overlapped differentially expressed genes after expressing individual Olig factor and functional validation, we find that the role of Olig1 is at least partially through the neurite extension factor 1 (Nrsn1). We therefore identify Olig1 as an intrinsic factor that promotes regenerative capacity of injured axons.

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

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.