Microglia polarization in nociplastic pain: mechanisms and perspectives

Analysis of gene expression in microglial apoptotic cell clearance following spinal cord injury based on machine learning algorithms

Spinal cord injury (SCI) is a severe neurological complication following spinal fracture, which has long posed a challenge for clinicians. Microglia play a dual role in the pathophysiological process after SCI, both beneficial and detrimental. The underlying mechanisms of microglial actions following SCI require further exploration. The present study combined three different machine learning algorithms, namely weighted gene co-expression network analysis, random forest analysis and least absolute shrinkage and selection operator analysis, to screen for differentially expressed genes in the GSE96055 microglia dataset after SCI. It then used protein-protein interaction networks and gene set enrichment analysis with single genes to investigate the key genes and signaling pathways involved in microglial function following SCI. The results indicated that microglia not only participate in neuroinflammation but also serve a significant role in the clearance mechanism of apoptotic cells following SCI. Notably, bioinformatics analysis and lipopolysaccharide + UNC569 (a MerTK-specific inhibitor) stimulation of BV2 cell experiments showed that the expression levels of Anxa2, Myo1e and Spp1 in microglia were significantly upregulated following SCI, thus potentially involved in regulating the clearance mechanism of apoptotic cells. The present study suggested that Anxa2, Myo1e and Spp1 may serve as potential targets for the future treatment of SCI and provided a theoretical basis for the development of new methods and drugs for treating SCI.

Assessing Risk Factors and Comorbidities in the Treatment of Chronic Pain: A Narrative Review

PURPOSE OF REVIEW

Chronic pain affects a significant portion of the population globally, making it a leading cause of disability. Understanding the multifaceted nature of chronic pain, its various types, and the intricate relationship it shares with risk factors, comorbidities, and mental health issues like depression and anxiety is critical for comprehensive patient care. Factors such as socioeconomic status (SES), age, gender, and obesity collectively add layers of complexity to chronic pain experiences and pose management challenges.

RECENT FINDINGS

Low SES presents barriers to effective pain care, while gender differences and the prevalence of chronic pain in aging adults emphasize the need for tailored approaches. The association between chronic pain and physical comorbidities like cardiovascular disease, chronic obstructive pulmonary disease (COPD), and diabetes mellitus reveals shared risk factors and further highlights the importance of integrated treatment strategies. Chronic pain and mental health are intricately linked through biochemical mechanisms, profoundly affecting overall quality of life. This review explores pharmacologic treatment for chronic pain, particularly opioid analgesia, with attention to the risk of substance misuse and the ongoing opioid epidemic. We discuss the potential role of medical cannabis as an alternative treatment with a nuanced perspective on its impact on opioid use. Addressing the totality and complexity of pain states is crucial to individualizing chronic pain management. With different types of pain having different underlying mechanisms, considerations should be made when approaching their treatment. Moreover, the synergistic relationship that pain states can have with other comorbidities further complicates chronic pain conditions.

GDPD3 Deficiency Alleviates Neuropathic Pain and Reprograms Macrophagic Polarization Through PGE2 and PPARγ Pathway

The complex mechanism of neuropathic pain involves various aspects of both central and peripheral pain conduction pathways. An effective cure for neuropathic pain therefore remains elusive. We found that deficiency of the gene Gdpd3, encoding a lysophospholipase D enzyme, alleviates the inflammatory responses in dorsal root ganglia (DRG) of mice under neuropathic pain and reduces PE (20:4) and PGE2 in DRG. Gdpd3 deficiency had a stronger analgesic effect on neuropathic pain than Celecoxib, a nonsteroidal anti-inflammatory drug. Gdpd3 deficiency also interferes with the polarization of macrophages, switching from M1 towards M2 phenotype. The PPARγ/ FABP4 pathway was screened by RNA sequencing as functional related with Gdpd3 deficient BMDMs stimulated with LPS. Both protein and mRNA levels of PPARγ in GDPD3 deficient BMDMs were higher than those of the litter control mice. However, GW9962 (inhibitor of PPARγ) could reverse the reprogramming polarization of macrophages caused by GDPD3 deficiency. Therefore, our study suggests that GDPD3 deficiency exerts a relieving effect on neuropathic pain and alleviates neuroinflammation in DRG by switching the phenotype of macrophages from M1 to M2, which was mediated through PGE2 and PPARγ/ FABP4 pathway.

Interactions and Trends of Interleukins, PAI-1, CRP, and TNF-α in Inflammatory Responses during the Perioperative Period of Joint Arthroplasty: Implications for Pain Management-A Narrative Review

Inflammation during the perioperative period of joint arthroplasty is a critical aspect of patient outcomes, influencing both the pathophysiology of pain and the healing process. This narrative review comprehensively evaluates the roles of specific cytokines and inflammatory biomarkers in this context and their implications for pain management. Inflammatory responses are initiated and propagated by cytokines, which are pivotal in the development of both acute and chronic postoperative pain. Pro-inflammatory cytokines play essential roles in up-regulating the inflammatory response, which, if not adequately controlled, leads to sustained pain and impaired tissue healing. Anti-inflammatory cytokines work to dampen inflammatory responses and promote resolution. Our discussion extends to the genetic and molecular influences on cytokine production, which influence pain perception and recovery rates post-surgery. Furthermore, the role of PAI-1 in modulating inflammation through its impact on the fibrinolytic system highlights its potential as a therapeutic target. The perioperative modulation of these cytokines through various analgesic and anesthetic techniques, including the fascia iliac compartment block, demonstrates a significant reduction in pain and inflammatory markers, thus underscoring the importance of targeted therapeutic strategies. Our analysis suggests that a nuanced understanding of the interplay between pro-inflammatory and anti-inflammatory cytokines is required. Future research should focus on individualized pain management strategies.

Bone morphogenetic protein 4 derived from the cerebrospinal fluid in patients with postherpetic neuralgia induces allodynia via the crosstalk between microglia and astrocyte

INTRODUCTION

During postherpetic neuralgia (PHN), the cerebral spinal fluid (CSF) possesses the capability to trigger glial activation and inflammation, yet the specific changes in its composition remain unclear. Recent findings from our research indicate elevations of central bone morphogenetic protein 4 (BMP4) during neuropathic pain (NP), serving as an independent modulator of glial cells. Herein, the aim of the present study is to test the CSF-BMP4 expressions and its role in the glial modulation in the process of PHN.

METHODS

CSF samples were collected from both PHN patients and non-painful individuals (Control) to assess BMP4 and its antagonist Noggin levels. Besides, intrathecal administration of both CSF types was conducted in normal rats to evaluate the impact on pain behavior, glial activity, and inflammation.; Additionally, both Noggin and STAT3 antagonist-Stattic were employed to treat the PHN-CSF or exogenous BMP4 challenged cultured astrocytes to explore downstream signals. Finally, microglial depletion was performed prior to the PHN-CSF intervention so as to elucidate the microglia-astrocyte crosstalk.

RESULTS

BMP4 levels were significantly higher in PHN-CSF compared to Control-CSF (P < 0.001), with a positive correlation with pain duration (P < 0.05, r = 0.502). Comparing with the Control-CSF producing moderate paw withdrawal threshold (PWT) decline and microglial activation, PHN-CSF further exacerbated allodynia and triggered both microglial and astrocytic activation (P < 0.05). Moreover, PHN-CSF rather than Control-CSF evoked microglial proliferation and pro-inflammatory transformation, reinforced iron storage, and activated astrocytes possibly through both SMAD159 and STAT3 signaling, which were all mitigated by the Noggin application (P < 0.05). Next, both Noggin and Stattic effectively attenuated BMP4-induced GFAP and IL-6 upregulation, as well as SMAD159 and STAT3 phosphorylation in the cultured astrocytes (P < 0.05). Finally, microglial depletion diminished PHN-CSF induced astrogliosis, inflammation and endogenous BMP4 expression (P < 0.05).

CONCLUSION

Our study highlights the role of CSF-BMP4 elevation in glial activation and allodynia during PHN, suggesting a potential therapeutic avenue for future exploration.

How to understand the overlap of long COVID, chronic fatigue syndrome/myalgic encephalomyelitis, fibromyalgia and irritable bowel syndromes

Long COVID should be limited to patients with multiple, persistent symptoms not related to well-defined organ damage. Once redefined, a focused review of long COVID demonstrates striking similarity to chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME), fibromyalgia (FM) and irritable bowel syndrome (IBS). Research in long COVID has revealed similar findings to those noted in CFS/ME and FM, characterized by central nervous system organ dysfunction. Long COVID, like CFS/ME, FM and IBS, is best understood as a bidirectional mind-body, neuroimmune illness.

Sigma-1 Receptor Inhibition Reduces Mechanical Allodynia and Modulate Neuroinflammation in Chronic Neuropathic Pain

Neuropathic pain is one of the most debilitating forms of chronic pain, resulting from an injury or disease of the somatosensory nervous system, which induces abnormal painful sensations including allodynia and hyperalgesia. Available treatments are limited by severe side-effects and reduced efficacy in the chronic phase of the disease. Sigma-1 receptor (σ1R) has been identified as a chaperone protein, which modulate opioid receptors activities and the functioning of several ion channels, exerting a role in pain transmission. As such, it represents a druggable target to treat neuropathic pain. This study aims at investigating the therapeutic potential of the novel compound (+)-2R/S-LP2, a σ1R antagonist, in reducing painful behaviour and modulating the neuroinflammatory environment. We showed that repeated administration of the compound significantly inhibited mechanical allodynia in neuropathic rats, increasing the withdrawal threshold as compared to CCI-vehicle rats. Moreover, we found that (+)-2R/S-LP2-mediated effects resolve the neuroinflammatory microenvironment by reducing central gliosis and pro-inflammatory cytokines expression levels. This effect was coupled with a significant reduction of connexin 43 (Cx43) expression levels and gap junctions/hemichannels mediated microglia-to-astrocyte communication. These results suggest that inhibition of σ1R significantly attenuates neuropathic pain chronicization, thus representing a viable effective strategy.

Brain-Derived Neurotrophic Factor, Nociception, and Pain

This article examines the involvement of the brain-derived neurotrophic factor (BDNF) in the control of nociception and pain. BDNF, a neurotrophin known for its essential role in neuronal survival and plasticity, has garnered significant attention for its potential implications as a modulator of synaptic transmission. This comprehensive review aims to provide insights into the multifaceted interactions between BDNF and pain pathways, encompassing both physiological and pathological pain conditions. I delve into the molecular mechanisms underlying BDNF's involvement in pain processing and discuss potential therapeutic applications of BDNF and its mimetics in managing pain. Furthermore, I highlight recent advancements and challenges in translating BDNF-related research into clinical practice.

Macrophage: A key player in neuropathic pain

Research on the relationship between macrophages and neuropathic pain has flourished in the past two decades. It has long been believed that macrophages are strong immune effector cells that play well-established roles in tissue homeostasis and lesions, such as promoting the initiation and progression of tissue injury and improving wound healing and tissue remodeling in a variety of pathogenesis-related diseases. They are also heterogeneous and versatile cells that can switch phenotypically/functionally in response to the micro-environment signals. Apart from microglia (resident macrophages of both the spinal cord and brain), which are required for the neuropathic pain processing of the CNS, neuropathic pain signals in PNS are influenced by the interaction of tissue-resident macrophages and BM infiltrating macrophages with primary afferent neurons. And the current review looks at new evidence that suggests sexual dimorphism in neuropathic pain are caused by variations in the immune system, notably macrophages, rather than the neurological system.

Olfactory ensheathing cells as candidate cells for chronic pain treatment

Chronic pain is often accompanied by tissue damage and pain hypersensitivity. It easily relapses and is challenging to cure, which seriously affects the patients' quality of life and is an urgent problem to be solved. Current treatment methods primarily rely on morphine drugs, which do not address the underlying nerve injury and may cause adverse reactions. Therefore, in recent years, scientists have shifted their focus from chronic pain treatment to cell transplantation. This review describes the classification and mechanism of chronic pain through the introduction of the characteristics of olfactory ensheathing cells (OECs), an in-depth discussion of special glial cells through the phagocytosis of nerve debris, receptor-ligand interactions, providing nutrition, and other inhibition of neuroinflammation, and ultimately supporting axon regeneration and mitigation of chronic pain. This review summarizes the potential and limitations of OECs for treating chronic pain by objectively analyzing relevant clinical trials and methods to enhance efficacy and future development prospects.

Redox Regulation of Immunometabolism in Microglia Underpinning Diabetic Retinopathy

Diabetic retinopathy (DR) is the leading cause of visual impairment and blindness among the working-age population. Microglia, resident immune cells in the retina, are recognized as crucial drivers in the DR process. Microglia activation is a tightly regulated immunometabolic process. In the early stages of DR, the M1 phenotype commonly shifts from oxidative phosphorylation to aerobic glycolysis for energy production. Emerging evidence suggests that microglia in DR not only engage specific metabolic pathways but also rearrange their oxidation-reduction (redox) system. This redox adaptation supports metabolic reprogramming and offers potential therapeutic strategies using antioxidants. Here, we provide an overview of recent insights into the involvement of reactive oxygen species and the distinct roles played by key cellular antioxidant pathways, including the NADPH oxidase 2 system, which promotes glycolysis via enhanced glucose transporter 4 translocation to the cell membrane through the AKT/mTOR pathway, as well as the involvement of the thioredoxin and nuclear factor E2-related factor 2 antioxidant systems, which maintain microglia in an anti-inflammatory state. Therefore, we highlight the potential for targeting the modulation of microglial redox metabolism to offer new concepts for DR treatment.

Pain in axial spondyloarthritis: role of the JAK/STAT pathway

Axial spondyloarthritis (axSpA) is a chronic inflammatory disease that is characterized by new bone formation in the axial musculoskeletal system, with X-ray discriminating between radiographic and non-radiographic forms. Current therapeutic options include non-steroidal anti-inflammatory drugs in addition to biological disease-modifying anti-rheumatic drugs that specifically target tumor necrosis factor-alpha (TNFα) or interleukin (IL)-17. Pain is the most critical symptom for axSpA patients, significantly contributing to the burden of disease and impacting daily life. While the inflammatory process exerts a major role in determining pain in the early phases of the disease, the symptom may also result from mechanical and neuromuscular causes that require complex, multi-faceted pharmacologic and non-pharmacologic treatment, especially in the later phases. In clinical practice, pain often persists and does not respond further despite the absence of inflammatory disease activity. Cytokines involved in axSpA pathogenesis interact directly/indirectly with the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling cascade, a fundamental component in the origin and development of spondyloarthropathies. The JAK/STAT pathway also plays an important role in nociception, and new-generation JAK inhibitors have demonstrated rapid pain relief. We provide a comprehensive review of the different pain types observed in axSpA and the potential role of JAK/STAT signaling in this context, with specific focus on data from preclinical studies and data from clinical trials with JAK inhibitors.

Applying Lessons From Rheumatology to Better Understand Long COVID

Long COVID can sometimes be attributed to organ damage and well-characterized pathophysiology, but more often there is no evidence of organ damage or abnormal biomarkers. This is most evident in patients with mild to moderate initial SARS-CoV-2 infection who were not hospitalized. Their persistent symptoms are strikingly similar to those of fibromyalgia and myalgic encephalomyelitis/chronic fatigue syndrome, including fatigue, post-exertional malaise, myalgias/arthralgias, and sleep and cognitive disturbances in 50% to 100% of cases. Analogous pathophysiologic pathways in fibromyalgia (FM), myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), and long COVID include host-microbial interactions in the absence of direct tissue invasion and absence of systemic autoimmunity, with evidence for immune dysregulation as well as autonomic, peripheral, and central nervous system dysfunction. Current treatment of long COVID has been based on multidisciplinary management recommended for FM and ME/CFS and has been formalized and made widely available by funding for nationwide long COVID clinics. Long COVID and its treatment should be distinguished by the presence or absence of organ damage. The acknowledged role of patient engagement in research and open dialogue regarding work and disability noted in long COVID may have meaningful impact on patients with FM and ME/CFS. Hopefully, advances in basic long COVID research will aid in understanding FM and ME/CFS, and rheumatologists should thus be involved in such research and patient care.

The dual role of microglia in neuropathic pain after spinal cord injury: Detrimental and protective effects

Spinal cord injury (SCI) is a debilitating condition that is frequently accompanied by neuropathic pain, resulting in significant physical and psychological harm to a vast number of individuals globally. Despite the high prevalence of neuropathic pain following SCI, the precise underlying mechanism remains incompletely understood. Microglia are a type of innate immune cell that are present in the central nervous system (CNS). They have been observed to have a significant impact on neuropathic pain following SCI. This article presents a comprehensive overview of recent advances in understanding the role of microglia in the development of neuropathic pain following SCI. Specifically, the article delves into the detrimental and protective effects of microglia on neuropathic pain following SCI, as well as the mechanisms underlying their interconversion. Furthermore, the article provides a thorough overview of potential avenues for future research in this area.

An Overview of the Mechanisms Involved in Neuralgia

Neuralgia is a frequently occurring condition that causes chronic pain and burdens both patients and their families. Earlier research indicated that anti-inflammatory treatment, which was primarily utilized to address conditions like neuralgia, resulted in positive outcomes. However, recent years have witnessed the emergence of various novel mechanisms associated with pain-related disorders. This review provides a concise overview of the inflammatory mechanisms involved in neuralgia. It also examines recent advancements in research, exploring the influence of ion channels and synaptic proteins on neuralgia and its complications. Additionally, the interactions between these mechanisms are discussed with the aim of suggesting innovative therapeutic approaches and research directions for the management of neuralgia.

Targeting Members of the Chemokine Family as a Novel Approach to Treating Neuropathic Pain

Neuropathic pain is a debilitating condition that affects millions of people worldwide. Numerous studies indicate that this type of pain is a chronic condition with a complex mechanism that tends to worsen over time, leading to a significant deterioration in patients' quality of life and issues like depression, disability, and disturbed sleep. Presently used analgesics are not effective enough in neuropathy treatment and may cause many side effects due to the high doses needed. In recent years, many researchers have pointed to the important role of chemokines not only in the development and maintenance of neuropathy but also in the effectiveness of analgesic drugs. Currently, approximately 50 chemokines are known to act through 20 different seven-transmembrane G-protein-coupled receptors located on the surface of neuronal, glial, and immune cells. Data from recent years clearly indicate that more chemokines than initially thought (CCL1/2/3/5/7/8/9/11, CXCL3/9/10/12/13/14/17; XCL1, CX3CL1) have pronociceptive properties; therefore, blocking their action by using neutralizing antibodies, inhibiting their synthesis, or blocking their receptors brings neuropathic pain relief. Several of them (CCL1/2/3/7/9/XCL1) have been shown to be able to reduce opioid drug effectiveness in neuropathy, and neutralizing antibodies against them can restore morphine and/or buprenorphine analgesia. The latest research provides irrefutable evidence that chemokine receptors are promising targets for pharmacotherapy; chemokine receptor antagonists can relieve pain of different etiologies, and most of them are able to enhance opioid analgesia, for example, the blockade of CCR1 (J113863), CCR2 (RS504393), CCR3 (SB328437), CCR4 (C021), CCR5 (maraviroc/AZD5672/TAK-220), CXCR2 (NVPCXCR220/SB225002), CXCR3 (NBI-74330/AMG487), CXCR4 (AMD3100/AMD3465), and XCR1 (vMIP-II). Recent research has shown that multitarget antagonists of chemokine receptors, such as CCR2/5 (cenicriviroc), CXCR1/2 (reparixin), and CCR2/CCR5/CCR8 (RAP-103), are also very effective painkillers. A multidirectional strategy based on the modulation of neuronal-glial-immune interactions by changing the activity of the chemokine family can significantly improve the quality of life of patients suffering from neuropathic pain. However, members of the chemokine family are still underestimated pharmacological targets for pain treatment. In this article, we review the literature and provide new insights into the role of chemokines and their receptors in neuropathic pain.

Exosomes derived from M2-type microglia ameliorate oxygen-glucose deprivation/reoxygenation-induced HT22 cell injury by regulating miR-124-3p/NCOA4-mediated ferroptosis

Background

Although it has been reported that miRNA carried by M2 microglial exosomes protects neurons from ischemia-reperfusion brain injury, the mechanism of action remains poorly understood. This study aimed to explore the miRNA signaling pathway by which M2-type microglia-derived exosomes (M2-exosomes) ameliorate oxygen-glucose deprivation/reoxygenation (OGD/R)-induced cytotoxicity in HT22 cells.

Methods

BV2 microglia were induced by M2 polarization. Then, M2-exosomes were identified via transmission electron microscopy and special biomarker detection and co-cultured with HT22 cells. Cell proliferation was evaluated using the Cell Counting Kit-8 (CCK-8) assay. Intracellular concentrations of reactive oxygen species (ROS), Fe²⁺, glutathione (GSH), and malondialdehyde (MDA) were determined using dichlorofluorescein fluorescence and biochemical determination. miR-124-3p levels were determined using qRT-PCR, and protein expressions were examined via western blotting.

Results

OGD/R suppressed the proliferation and induced the accumulation of Fe²⁺, ROS, and MDA and reduction of GSH in mouse HT22 cells, suggesting ferroptosis of HT22 cells. OGD/R-induced changes in the above mentioned indexes was ameliorated by M2-exosomes but restored by the exosome inhibitor GW4869. M2-exosomes with (mimic-exo) or without miR-124-3p (inhibitor-exo) promoted and suppressed proliferation and ferroptosis-associated indexes of HT22 cells, respectively. Moreover, mimic-exo and inhibitor-exo inhibited and enhanced NCOA4 expression in HT22 cells, respectively. NCOA4 overexpression reversed the protective effects of miR-124-3p mimic-exo in OGD/R-conditioned cells. NCOA4 was targeted and regulated by miR-124-3p.

Conclusions

M2-exosome protects HT22 cells against OGD/R-induced ferroptosis injury by transferring miR-124-3p and NCOA4 into HT22 cells, with the latter being a target gene for miR-124-3p.

Targeting α7-nAChR by galantamine mitigates reserpine-induced fibromyalgia-like symptoms in rats: Involvement of cAMP/PKA, PI3K/AKT, and M1/M2 microglia polarization

Fibromyalgia (FM) is a pain disorder marked by generalized musculoskeletal pain accompanied by depression, fatigue, and sleep disturbances. Galantamine (Gal) is a positive allosteric modulator of neuronal nicotinic acetylcholine receptors (nAChRs) and a reversible inhibitor of cholinesterase. The current study aimed to explore the therapeutic potential of Gal against reserpine (Res)-induced FM-like condition along with investigating the α7-nAChR's role in Gal-mediated effects. Rats were injected with Res (1 mg/kg/day; sc) for 3 successive days then Gal (5 mg/kg/day; ip) was given alone and with the α7-nAChR blocker methyllycaconitine (3 mg/kg/day; ip), for the subsequent 5 days. Galantamine alleviated Res-induced histopathological changes and monoamines depletion in rats' spinal cord. It also exerted analgesic effect along with ameliorating Res-induced depression and motor-incoordination as confirmed by behavioral tests. Moreover, Gal produced anti-inflammatory effect through modulating AKT1/AKT2 and shifting M1/M2 macrophage polarization. The neuroprotective effects of Gal were mediated through activating cAMP/PKA and PI3K/AKT pathways in α7-nAChR-dependent manner. Thus, Gal can ameliorate Res-induced FM-like symptoms and mitigate the associated monoamines depletion, neuroinflammation, oxidative stress, apoptosis, and neurodegeneration through α7-nAChR stimulation, with the involvement of cAMP/PKA, PI3K/AKT, and M1/M2 macrophage polarization.