Liver X Receptor β Is Involved in Formalin-Induced Spontaneous Pain

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.

Spontaneous pain as a challenge of research and management in chronic pain

Spontaneous pain occurring without apparent external stimuli, is a significant complaint of individuals with chronic pain whose mechanisms, somewhat surprisingly, remain poorly understood. Over the past decades, neuroimaging studies start to reveal brain activities accompanying spontaneous pain. Meanwhile, a variety of animal models and behavioral tests have been established, including non-reflexive tests and free-choice tests, which have been shown to be effective in assessing spontaneous pain. For the spontaneous pain mechanisms, multiple lines of research mainly focus on three aspects: (1) sensitization of peripheral nociceptor receptors and ion channels, (2) spontaneous neuronal firing and abnormal activity patterns at the dorsal root ganglion and spinal cord level, (3) functional and structural alterations in the brain, particularly the limbic system and the medial pain pathway. Despite accumulating evidence revealing distinct neuronal mechanisms from evoked pain, we are still far from full understanding of spontaneous pain, leaving a big gap between bench and bedside for chronic pain treatment. A better understanding of the neural processes in chronic pain, with specific linkage as to which anatomical structures and molecules related to spontaneous pain perception and comorbidities, will greatly improve our ability to develop novel therapeutics.

Anti-inflammatory secondary metabolites from Scrophularia kotschyana

The species belonging to Scrophularia genus grow mainly in Irano-Turanian and Mediterranean regions and have been used as folk remedy for inflammatory-related diseases since ancient times. The present study was aimed to evaluate the anti-inflammatory activity of the extracts of Scrophularia kotschyana as well as the isolated compounds. The aerial parts and the roots of the plant were separately extracted with methanol. Anti-inflammatory activities of both extracts were evaluated with formalin test in mice. As the methanolic extract of the aerial parts significantly (p < .05) inhibited inflammation, it was then submitted to successive solvent extractions with n-hexane, dichloromethane, ethyl acetate and n-butanol to yield subextracts. Anti-inflammatory activities of the subextracts were evaluated within the same test system. Among the subextracts tested, the n-butanol subextract produced a significant (p < .05) anti-inflammatory activity at all doses (5, 10, and 30 mg/kg, ip.). Sequential chromatographic separation of the n-butanol subextract yielded 8-O-acetyl-4^'-O-(E)-p-coumaroylharpagide, 8-O-acetyl-4^'-O-(Z)-p-coumaroylharpagide, β-sitosterol 3-O-β-glucopyranoside, apigenin 7-O-β-glucopyranoside, apigenin 7-O-rutinoside, luteolin 7-O-β-glucopyranoside and luteolin 7-O-rutinoside. The anti-inflammatory activities of the isolates were evaluated at 5 mg/kg dose. Luteolin 7-O-β-glucopyranoside and apigenin 7-O-rutinoside caused a significant (p < .05) inhibition of oedema formation.

Sensory Neurons, Neuroimmunity, and Pain Modulation by Sex Hormones

The inclusion of women in preclinical pain studies has become more commonplace in the last decade as the National Institutes of Health (NIH) released its "Sex as a Biological Variable" mandate. Presumably, basic researchers have not had a comprehensive understanding about neuroimmune interactions in half of the population and how hormones play a role in this. To date, we have learned that sex hormones contribute to sexual differentiation of the nervous system and sex differences in behavior throughout the lifespan; however, the cycling of sex hormones does not always explain these differences. Here, we highlight recent advances in our understanding of sex differences and how hormones and immune interactions influence sensory neuron activity to contribute to physiology and pain. Neuroimmune mechanisms may be mediated by different cell types in each sex, as the actions of immune cells are sexually dimorphic. Unfortunately, the majority of studies assessing neuronal contributions to immune function have been limited to males, so it is unclear if the mechanisms are similar in females. Finally, pathways that control cellular metabolism, like nuclear receptors, have been shown to play a regulatory role both in pain and inflammation. Overall, communication between the neuroimmune and endocrine systems modulate pain signaling in a sex-dependent manner, but more research is needed to reveal nuances of these mechanisms.

Intrathecally Administered Apelin-13 Alleviated Complete Freund's Adjuvant-Induced Inflammatory Pain in Mice

Apelin is the endogenous ligand for APJ, a G-protein-coupled receptor. Apelin gene and protein are widely distributed in the central nervous system and peripheral tissues. The role of apelin in chronic inflammatory pain is still unclear. In the present study, a mouse model of complete Freund’s adjuvant (CFA)-induced inflammatory pain was utilized, and the paw withdrawal latency/threshold in response to thermal stimulation and Von Frey filament stimulation were recorded after intrathecal (i.t.) injection of apelin-13 (0.1, 1, and 10 nmol/mouse). The mRNA and protein expression, concentration of glutamic acid (Glu), and number of c-Fos immunol staining in lumbar spinal cord (L4/5) were determined. The results demonstrated that Apln gene expression in the lumbar spinal cord was down-regulated in the CFA pain model. Apelin-13 (10 nmol/mouse, i.t.) alleviated CFA-induced inflammatory pain, and it exhibited a more potent antinociceptive effect than apelin-36 and (pyr)apelin-13. The antinociception of apelin-13 could be blocked by APJ antagonist apelin-13(F13A). I.T. apelin-13 attenuated the increased levels of Aplnr, Grin2b, Camk2d, and c-Fos genes expression, Glu concentration, and NMDA receptor 2B (GluN2B) protein expression caused by CFA. Apelin-13 significantly reduced the number of Fos-positive cells in laminae III and IV/V of the dorsal horn. This study indicated that i.t. apelin-13 exerted an analgesic effect against inflammatory pain, which was mediated by activation of APJ, and inhibition of Glu/GluN2B function and neural activity of the spinal dorsal horn.

Cerebral Ischemia-Reperfusion Injury: Lysophosphatidic Acid Mediates Inflammation by Decreasing the Expression of Liver X Receptor

Lysophosphatidic acid (LPA), a ubiquitous phospholipid, plays a crucial role in the pathogenesis and pathophysiological process of neurological diseases, which constitute the pathological course after cerebral ischemia. Nevertheless, the molecular mechanisms associated with the pathogenic roles of LPA remain elusive. In this study, we evaluated the expression of the liver X receptor (LXR) and nuclear factor kappa B (NFκB) by Western blotting, quantified the levels of IL-1β, IL-6, TNF-α, and LPA by ELISA, and evaluated apoptosis and infarct by TUNEL (terminal deoxynucleotidyl transferase (TdT) dUTP nick-end labeling) and TTC (triphenyltetrazolium chloride) staining respectively in Sprague-Dawley (SD) rats after middle cerebral artery occlusion (MCAO). The levels of LPA, an extracellular signaling molecule, increased after ischemia and caused neurological injury effect, decreased the expression level of LXR, and increased the expression level of inflammatory factors (IL-1β, IL-6, and TNF-α) via the NFκB signaling pathway. This elevated LPA-induced pathological process is one of the pathological reactions associated with ischemic brain injury. We present a direct or indirect connection between LPA and LXR in the pathophysiological process. In conclusion, we speculate that the inhibition of LPA generation and administration of LXR agonist may be explored as potential cerebral infarction treatment strategies.

Imbalanced spinal infiltration of Th17/Treg cells contributes to bone cancer pain via promoting microglial activation

Increasing evidence suggests that T cells participate in the pathology of neuropathic pain, as well as the activation of microglia. However, whether T cells infiltrate into the spinal cord and contribute to the development of bone cancer pain (BCP) remains unknown. Here, we used a mouse model of BCP to show that numbers of T cells infiltrated into the spinal cord after sarcoma cell implantation with increased BCP, and most infiltrating T cells in the spinal cord were CD3⁺CD4⁺ T cells. Both Th17 and Treg subpopulations were analyzed by immunofluorescence. Treg cells in the spinal cord were transiently up-regulated, followed by an imbalance towards Th17 afterwards, and elevated IL-17/IL-17A levels were observed in both blood and spinal cord. Meanwhile, TGF-β, IL-6, and IL-23, the factors which regulate Th17/Treg differentiation, increased their expressions during the development of BCP. Additionally, IL-17A receptor (IL-17AR) was found to be expressed on microglia, and the level of IL-17AR increased with activated microglia during BCP development. Furthermore, BCP was ameliorated when IL-17/IL-17A neutralizing antibodies were intrathecally injected, accompanied with inhibited Th17/Treg infiltration and suppressed microglial activation. In conclusion, T cells infiltrated into the spinal cord with the imbalance of Th17/Treg towards Th17 during the development of BCP, which could promote the microglial activation and further increased BCP, while neutralizing IL-17/IL-17A in the spinal cord could ameliorate BCP. Our results suggest that targeting the imbalanced Th17/Treg infiltration in the spinal cord could be a novel strategy for BCP therapy.

Epigenetic suppression of liver X receptor β in anterior cingulate cortex by HDAC5 drives CFA-induced chronic inflammatory pain

Background

Liver X receptors (LXRs), including LXRα and LXRβ, are key regulators of transcriptional programs for both cholesterol homeostasis and inflammation in the brain. Here, the modes of action of LXRs and the epigenetic mechanisms regulating LXRβ expression in anterior cingulate cortex (ACC) of chronic inflammatory pain (CIP) are investigated.

Methods

The deficit of LXR isoform and analgesic effect of LXR activation by GW3965 were evaluated using the mouse model of CIP induced by hindpaw injection of complete Freund’s adjuvant (CFA). The mechanisms involved in GW-mediated analgesic effects were analyzed with immunohistochemical methods, ELISA, co-immunoprecipitation (Co-IP), Western blot, and electrophysiological recording. The epigenetic regulation of LXRβ expression was investigated by chromatin immunoprecipitation, quantitative real-time PCR, and sequencing.

Results

We revealed that CFA insult led to LXRβ reduction in ACC, which was associated with upregulated expression of histone deacetylase 5 (HDAC5), and knockdown of LXRβ by shRNA led to thermal hyperalgesia. Co-IP showed that LXRβ interacted with NF-κB p65 physically. LXRβ activation by GW3965 exerted analgesic effects by inhibiting the nuclear translocation of NF-κB, reducing the phosphorylation of mitogen-activated protein kinases (MAPKs) in ACC, and decreasing the promoted input-output and enhanced mEPSC frequency in ACC neurons after CFA exposure. In vitro experiments confirmed that HDAC5 triggered histone deacetylation on the promoter region of Lxrβ, resulting in downregulation of Lxrβ transcription.

Conclusion

These findings highlight an epigenetic mechanism underlying LXRβ deficits linked to CIP, and LXRβ activation may represent a potential novel target for the treatment of CIP with an alteration in inflammation responses and synaptic transmission in ACC.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1507-3) contains supplementary material, which is available to authorized users.

Activation of liver x receptors prevents the spinal LTP induced by skin/muscle retraction in the thigh via SIRT1/NF-Κb pathway

It has been reported that skin/muscle incision and retraction (SMIR) in the thigh, produces mechanical allodynia in the hind paw, far from the site of incision/retraction. The mechanical allodynia lasts about 22 days, indicating chronic post-operative pain develops. The precise mechanisms, however, are largely unclear. In the current study, we further found that SMIR surgery induced LTP of c-fiber evoked field potentials that lasted at least 4 h. The mRNA and protein level of tumor necrosis factor-alpha (TNFα) and acetylated nuclear factor-kappaB p65 (ac-NF-κB p65) in the lumbar spinal dorsal horn was gradually increased during LTP development, while pretreatment with either TNFα neutralization antibody or NF-κB inhibitor PDTC completely prevented the induction of LTP. Moreover, the expression of Silent information regulator 1 (SIRT1) in the lumbar spinal dorsal horn was decreased and activation of SIRT1 by SRT1720 also prevented the induction of LTP. Importantly, the spinal expression of Liver X receptors (LXRs) was increased, both at mRNA and protein level following SMIR. Application of LXRs agonist T0901317 to the spinal dorsal horn prevented LTP induction following SMIR. Mechanistically, T0901317 enhanced the expression of SIRT1 and decreased the expression of ac-NF-κB p65 and TNFα. Spinal application of SIRT1 antagonist EX-527, 30 min before T0901317 administration, completely blocked the inhibiting effect of T0901317 on LTP, and on expression of ac-NF-κB p65 and TNFα. These results indicated that activation of LXRs prevented SMIR-induced LTP by inhibiting NF-κB/TNFα pathway via increasing SIRT1 expression.

Epothilone B Benefits Nigral Dopaminergic Neurons by Attenuating Microglia Activation in the 6-Hydroxydopamine Lesion Mouse Model of Parkinson's Disease

Parkinson’s disease (PD) is characterized by loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and a subsequent reduction in striatal DA levels. Recent studies have shown that systemic administration of subtoxic doses of epothilone B (EpoB), a microtubule stabilizing agent, enhances axonal regeneration. However, the underlying alterations in cellular mechanisms remain undetermined. In the present study, we investigated the neuroprotective effects of EpoB on DA neurons in mouse model of PD induced by 6-hydroxyDA (6-OHDA) and in vitro. The results indicated that EpoB improved behavioral deficits, protected the nigrostriatal dopaminergic projections and restored DA level in the striatum of mice exposed to 6-OHDA. Meanwhile, EpoB attenuated microglia activation in the SNc of PD mice. Furthermore, EpoB treatment ameliorated 6-OHDA induced cytotoxicity to MN9D dopaminergic cells in a co-culture transwell system of BV2/MN9D cells, and redistributed the cytoskeleton of microglial BV2 and caused the morphological transition, inhibited the polarization to the M1 phenotype by suppressing expression of pro-inflammatory factors including interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α. Overall, our study suggested that EpoB treatment protects nigral DA neurons and projections through limiting the cytotoxicity of activated microglia in 6-OHDA lesioned mice.

Epothilone B Benefits Nigrostriatal Pathway Recovery by Promoting Microtubule Stabilization After Intracerebral Hemorrhage

Background

Many previous clinical studies have demonstrated that the nigrostriatal pathway, which plays a vital role in movement adjustment, is significantly impaired after stroke, according to medical imaging and autopsies. However, the basic pathomorphological changes have been poorly investigated to date. This study was designed to explore the pathomorphological changes, mechanism, and therapeutic method of nigrostriatal impairment after intracerebral hemorrhage (ICH).

Methods and Results

Intrastriatal injection of autologous blood or microtubule depolymerization reagent nocodazole was performed to mimic the pathology of ICH in C57/BL6 mice. Immunofluorescence, Western blotting, electron microscopy, functional behavioral tests, and anterograde and retrograde neural circuit tracking techniques were used in these mice. The data showed that the number of dopamine neurons and the dopamine concentration were severely decreased and that fine motor function was impaired after ICH. Microtubule depolymerization was the main contributor to the loss of dopamine neurons and to motor function deficits after ICH, as was also proven by intrastriatal injection of nocodazole. Moreover, administration of the microtubule stabilizer epothilone B (1.5 mg/kg) improved the integrity of the nigrostriatal pathway neural circuit, increased the number of dopamine neurons (4598±896 versus 3125±355; P=0.034) and the dopamine concentration (4.28±0.99 versus 3.08±0.75 ng/mg; P=0.041), and enhanced fine motor functional recovery associated with increased acetylated α‐tubulin expression to maintain microtubule stabilization after ICH.

Conclusions

Our results clarified the pathomorphological changes of the nigrostriatal pathway after ICH and found that epothilone B helped alleviate nigrostriatal pathway injury after ICH, associated with promoting α‐tubulin acetylation to maintain microtubule stabilization, thus facilitating motor recovery.

Propofol Exposure in Early Life Induced Developmental Impairments in the Mouse Cerebellum

Propofol is a widely used anesthetic in the clinic while several studies have demonstrated that propofol exposure may cause neurotoxicity in the developing brain. However, the effects of early propofol exposure on cerebellar development are not well understood. Propofol (30 or 60 mg/kg) was administered to mice on postnatal day (P)7; Purkinje cell dendritogenesis and Bergmann glial cell development were evaluated on P8, and granule neuron migration was analyzed on P10. The results indicated that exposure to propofol on P7 resulted in a significant reduction in calbindin-labeled Purkinje cells and their dendrite length. Furthermore, propofol induced impairments in Bergmann glia development, which might be involved in the delay of granule neuron migration from the external granular layer (EGL) to the internal granular layer (IGL) during P8 to P10 at the 60 mg/kg dosage, but not at the 30 mg/kg dosage. Several reports have suggested that the Notch signaling pathway plays instructive roles in the morphogenesis of Bergmann glia. Here, it was revealed that propofol treatment decreased Jagged1 and Notch1 protein levels in the cerebellum on P8. Taken together, exposure to propofol during the neonatal period impairs Bergmann glia development and may therefore lead to cerebellum development defects. Our results may aid in the understanding of the neurotoxic effects of propofol when administrated to infants.