Structural homology of myelin basic protein and muscarinic acetylcholine receptor: Significance in the pathogenesis of complex regional pain syndrome

Cholesterol-dependent LXR transcription factor activity represses pronociceptive effects of estrogen in sensory neurons and pain induced by myelin basic protein fragments

Background

A bioactive myelin basic protein (MBP) fragment, comprising MBP84-104, is released in sciatic nerve after chronic constriction injury (CCI). Intraneural injection (IN) of MBP84-104 in an intact sciatic nerve is sufficient to induce persistent neuropathic pain-like behavior via robust transcriptional remodeling at the injection site and ipsilateral dorsal root ganglia (DRG) and spinal cord. The sex (female)-specific pronociceptive activity of MBP84-104 associates with sex-specific changes in cholesterol metabolism and activation of estrogen receptor (ESR)1 signaling.

Methods

In male and female normal and post-CCI rat sciatic nerves, we assessed: (i) cholesterol precursor and metabolite levels by lipidomics; (ii) MBP84-104 interactors by mass spectrometry of MBP84-104 pull-down; and (iii) liver X receptor (LXR)α protein expression by immunoblotting. To test the effect of LXRα stimulation on IN MBP84-104-induced mechanical hypersensitivity, the LXRα expression was confirmed along the segmental neuraxis, in DRG and spinal cord, followed by von Frey testing of the effect of intrathecally administered synthetic LXR agonist, GW3965. In cultured male and female rat DRGs exposed to MBP84-104 and/or estrogen treatments, transcriptional effect of LXR stimulation by GW3965 was assessed on downstream cholesterol transporter Abc, interleukin (IL)-6, and pronociceptive Cacna2d1 gene expression.

Results

CCI regulated LXRα ligand and receptor levels in nerves of both sexes, with cholesterol precursors, desmosterol and 7-DHC, and oxysterol elevated in females relative to males. MBP84-104 interacted with nuclear receptor coactivator (Ncoa)1, known to activate LXRα, injury-specific in nerves of both sexes. LXR stimulation suppressed ESR1-induced IL-6 and Cacna2d1 expression in cultured DRGs of both sexes and attenuated MBP84-104-induced pain in females.

Conclusion

The injury-released bioactive MBP fragments induce pronociceptive changes by selective inactivation of nuclear transcription factors, including LXRα. By Ncoa1 sequestration, bioactive MBP fragments render LXRα function to counteract pronociceptive activity of estrogen/ESR1 in sensory neurons. This effect of MBP fragments is prevalent in females due to high circulating estrogen levels in females relative to males. Restoring LXR activity presents a promising therapeutic strategy in management of neuropathic pain induced by bioactive MBP.

Analysis of the risk of traumatic brain injury and evaluation neurogranin and myelin basic protein as potential biomarkers of traumatic brain injury in postmortem examination

In forensic pathology, traumatic brain injury (TBI) is a frequently encountered cause of death. Unfortunately, the statistic autopsy data, risk investigation about injury patterns, and circumstances of TBI are still sparse. Estimates of survival time post-TBI and postmortem diagnosis of TBI are especially important implications in forensic medicine. Neurogranin (Ng) and myelin basic protein (MBP) represent potential biomarkers of TBI. The present study analyzed retrospectively the forensic autopsy records of TBI cases at a university center of medico-legal investigation from 2008 to 2020. Immunohistochemistry and enzyme-linked immunosorbent assays (ELISA) were used to investigate the expression changes of Ng and MBP in the cortical brain injury adjacent tissues and serum, respectively, from cases of TBI at autopsy with different survival times post-TBI. The results show that the major mechanism of death of TBI is assault, and accident was the major manner of death. Ng and MBP are mainly expressed in the cortical nerve cells and the myelin sheath, respectively. The serum levels of Ng and MBP in each TBI group were higher compared with those in the controls. The brain cortical levels of Ng and MBP decreased at first and then steadily increased with extended survival time post-TBI. The immunopositive ratios and serum concentration of Ng and MBP have shown significant differences among control group and all TBI group (p < 0.001). Collectively, the immunohistochemical analyses of Ng and MBP in human brain tissues may be useful to determine the survival time after TBI, and Ng and MBP level in the human blood specimens could be considered as a postmortem diagnostic tools of TBI in forensic practice.

A human coronavirus OC43-derived polypeptide causes neuropathic pain

Human coronaviruses have been recently implicated in neurological sequelae by insufficiently understood mechanisms. We here identify an amino acid sequence within the HCoV-OC43 p65-like protein homologous to the evolutionarily conserved motif of myelin basic protein (MBP). Because MBP-derived peptide exposure in the sciatic nerve produces pronociceptive activity in female rodents, we examined whether a synthetic peptide derived from the homologous region of HCoV-OC43 (OC43p) acts by molecular mimicry to promote neuropathic pain. OC43p, but not scrambled peptides, induces mechanical hypersensitivity in rats following intrasciatic injections. Transcriptome analyses of the corresponding spinal cords reveal upregulation of genes and signaling pathways with known nociception-, immune-, and cellular energy-related activities. Affinity capture shows the association of OC43p with an Na+ /K+ -transporting ATPase, providing a potential direct target and mechanistic insight into virus-induced effects on energy homeostasis and the sensory neuraxis. We propose that HCoV-OC43 polypeptides released during infection dysregulate normal nervous system functions through molecular mimicry of MBP, leading to mechanical hypersensitivity. Our findings might provide a new paradigm for virus-induced neuropathic pain.

Sex-Specific B Cell and Anti-Myelin Autoantibody Response After Peripheral Nerve Injury

Immunotherapy holds promise as a non-addictive treatment of refractory chronic pain states. Increasingly, sex is recognized to impact immune regulation of pain states, including mechanical allodynia (pain from non-painful stimulation) that follows peripheral nerve trauma. This study aims to assess the role of B cells in sex-specific responses to peripheral nerve trauma. Using a rat model of sciatic nerve chronic constriction injury (CCI), we analyzed sex differences in (i) the release of the immunodominant neural epitopes of myelin basic protein (MBP); (ii) the levels of serum immunoglobulin M (IgM)/immunoglobulin G (IgG) autoantibodies against the MBP epitopes; (iii) endoneurial B cell/CD20 levels; and (iv) mechanical sensitivity behavior after B cell/CD20 targeting with intravenous (IV) Rituximab (RTX) and control, IV immunoglobulin (IVIG), therapy. The persistent MBP epitope release in CCI nerves of both sexes was accompanied by the serum anti-MBP IgM autoantibody in female CCI rats alone. IV RTX therapy during CD20-reactive cell infiltration of nerves of both sexes reduced mechanical allodynia in females but not in males. IVIG and vehicle treatments had no effect in either sex. These findings provide strong evidence for sexual dimorphism in B-cell function after peripheral nervous system (PNS) trauma and autoimmune pathogenesis of neuropathic pain, potentially amenable to immunotherapeutic intervention, particularly in females. A myelin-targeted serum autoantibody may serve as a biomarker of such painful states. This insight into the biological basis of sex-specific response to neuraxial injury will help personalize regenerative and analgesic therapies.

Autoantibodies from patients with complex regional pain syndrome (CRPS) induce pro-inflammatory effects and functional disturbances on endothelial cells in vitro

ABSTRACT

Complex regional pain syndrome (CRPS) is an inadequate local response after a limb trauma, which leads to severe pain and autonomic and trophic changes of the affected limb. Autoantibodies directed against human β2 adrenergic and muscarinic M2-receptors (hβ2AR and hM2R) have been described in CRPS-patients previously.We analyzed sera from CRPS-patients for autoantibodies against hß2AR, hM2R and endothelial cells, and investigated the functional effects of purified IgG, derived from 13 CRPS patients, on endothelial cells. Eleven healthy controls, seven radial fracture patients without CRPS, and 10 patients with peripheral arterial vascular disease served as controls.CRPS-IgG, but not control IgG, bound to the surface of endothelial cells (P < 0.001) and to hβ2AR and hM2R (P < 0.05), the latter being reversed by adding β2AR and M2R antagonists. CRPS-IgG led to an increased cytotoxicity and a reduced proliferation rate of endothelial cells, and by adding specific antagonists, the effect was neutralized. Regarding second messenger pathways, CRPS-IgG induced ERK-1/2-, P38-, and STAT1-phosphorylation, while AKT-phosphorylation was decreased at the protein level. In addition, increased expression of adhesion molecules (ICAM-1, VCAM-1) on the mRNA-level was induced by CRPS-IgG, thus inducing a pro-inflammatory condition of the endothelial cells.Our results show that patients with CRPS not only develop autoantibodies against hβ2AR and hM2R, but these antibodies interfere with endothelial cells, inducing functional effects on these in vitro, and thus might contribute to the pathophysiology of CRPS.

Recent advancements in peripheral nerve-specific fluorescent compounds

Nerve injury is a common complication of surgery. Accidental nerve damage or transection can lead to severe clinical symptoms including pain, numbness, paralysis and even expiratory dyspnoea. In recent years, with the rise of the field of fluorescence-guided surgery, researchers have discovered that nerve-specific fluorescent agents can serve as nerve markers in animals and can be used to guide surgical procedures and reduce the incidence of intraoperative nerve damage. Currently, researchers have begun to focus on biochemistry, materials chemistry and other fields to produce more neuro-specific fluorescent agents with physiological relevance and they are expected to have clinical applications. This review discusses the agents with potential to be used in fluorescence-guided nerve imaging during surgery.

A myelin basic protein fragment induces sexually dimorphic transcriptome signatures of neuropathic pain in mice

In the peripheral nerve, mechanosensitive axons are insulated by myelin, a multilamellar membrane formed by Schwann cells. Here, we offer first evidence that a myelin degradation product induces mechanical hypersensitivity and global transcriptomics changes in a sex-specific manner. Focusing on downstream signaling events of the functionally active 84-104 myelin basic protein (MBP(84-104)) fragment released after nerve injury, we demonstrate that exposing the sciatic nerve to MBP(84-104) via endoneurial injection produces robust mechanical hypersensitivity in female, but not in male, mice. RNA-seq and systems biology analysis revealed a striking sexual dimorphism in molecular signatures of the dorsal root ganglia (DRG) and spinal cord response, not observed at the nerve injection site. Mechanistically, intra-sciatic MBP(84-104) induced phospholipase C (PLC)-driven (females) and phosphoinositide 3-kinase-driven (males) phospholipid metabolism (tier 1). PLC/inositol trisphosphate receptor (IP3R) and estrogen receptor co-regulation in spinal cord yielded Ca²⁺-dependent nociceptive signaling induction in females that was suppressed in males (tier 2). IP3R inactivation by intrathecal xestospongin C attenuated the female-specific hypersensitivity induced by MBP(84-104). According to sustained sensitization in tiers 1 and 2, T cell-related signaling spreads to the DRG and spinal cord in females, but remains localized to the sciatic nerve in males (tier 3). These results are consistent with our previous finding that MBP(84-104)-induced pain is T cell-dependent. In summary, an autoantigenic peptide endogenously released in nerve injury triggers multisite, sex-specific transcriptome changes, leading to neuropathic pain only in female mice. MBP(84-104) acts through sustained co-activation of metabolic, estrogen receptor-mediated nociceptive, and autoimmune signaling programs.

Complex syndromes of chronic pain, fatigue and cognitive impairment linked to autoimmune dysautonomia and small fiber neuropathy

Chronic fatigue syndrome, postural orthostatic tachycardia syndrome, complex regional pain syndrome and silicone implant incompatibility syndrome are a subject of debate among clinicians and researchers. Both the pathogenesis and treatment of these disorders require further study. In this paper we summarize the evidence regarding the role of autoimmunity in these four syndromes with respect to immunogenetics, autoimmune co-morbidities, alteration in immune cell subsets, production of autoantibodies and presentation in animal models. These syndromes could be incorporated in a new concept of autoimmune neurosensory dysautonomia with the common denominators of autoantibodies against G-protein coupled receptors and small fiber neuropathy. Sjogren's syndrome, which is a classical autoimmune disease, could serve as a disease model, illustrating the concept. Development of this concept aims to identify an apparently autoimmune subgroup of the disputable disorders, addressed in the review, which may most benefit from the immunotherapy.

Expansion and activation of distinct central memory T lymphocyte subsets in complex regional pain syndrome

Background

Complex regional pain syndrome (CRPS) is a debilitating condition where trauma to a limb results in devastating persistent pain that is disproportionate to the initial injury. The pathophysiology of CRPS remains unknown; however, accumulating evidence suggests it is an immunoneurological disorder, especially in light of evidence of auto-antibodies in ~ 30% of patients. Despite this, a systematic assessment of all circulating leukocyte populations in CRPS has never been performed.

Methods

We characterised 14 participants as meeting the Budapest clinical criteria for CRPS and assessed their pain ratings and psychological state using a series of questionnaires. Next, we performed immunophenotyping on blood samples from the 14 CRPS participants as well as 14 healthy pain-free controls using mass cytometry. Using a panel of 38 phenotypic and activation markers, we characterised the numbers and intracellular activation status of all major leukocyte populations using manual gating strategies and unsupervised cluster analysis.

Results

We have shown expansion and activation of several distinct populations of central memory T lymphocytes in CRPS. The number of central memory CD8⁺ T cells was increased 2.15-fold; furthermore, this cell group had increased phosphorylation of NFkB and STAT1 compared to controls. Regarding central memory CD4⁺ T lymphocytes, the number of Th1 and Treg cells was increased 4.98-fold and 2.18-fold respectively, with increased phosphorylation of NFkB in both populations. We also found decreased numbers of CD1c⁺ myeloid dendritic cells, although with increased p38 phosphorylation. These changes could indicate dendritic cell tissue trafficking, as well as their involvement in lymphocyte activation.

Conclusions

These findings represent the first mass cytometry immunophenotyping study in any chronic pain state and provide preliminary evidence of an antigen-mediated T lymphocyte response in CRPS. In particular, the presence of increased numbers of long-lived central memory CD4⁺ and CD8⁺ T lymphocytes with increased activation of pro-inflammatory signalling pathways may indicate ongoing inflammation and cellular damage in CRPS.