Surgical Trauma Exacerbates Cognitive Deficits and Neuroinflammation in Aged Rats: The Role of CX3CL1-CX3CR1 Signaling

Cx3cr1 deficiency interferes with learning- and direct current stimulation-mediated neuroplasticity of the motor cortex

Microglia are essential contributors to synaptic transmission and stability and communicate with neurons via the fractalkine pathway. Transcranial direct current stimulation [(t)DCS], a form of non-invasive electrical brain stimulation, modulates cortical excitability and promotes neuroplasticity, which has been extensively demonstrated in the motor cortex and for motor learning. The role of microglia and their fractalkine receptor CX3CR1 in motor cortical neuroplasticity mediated by DCS or motor learning requires further elucidation. We demonstrate the effects of pharmacological microglial depletion and genetic Cx3cr1 deficiency on the induction of DCS-induced long-term potentiation (DCS-LTP) ex vivo. The relevance of microglia-neuron communication for DCS response and structural neuroplasticity underlying motor learning are assessed via 2-photon in vivo imaging. The behavioural consequences of impaired CX3CR1 signalling are investigated for both gross and fine motor learning. We show that DCS-mediated neuroplasticity in the motor cortex depends on the presence of microglia and is driven in part by CX3CR1 signalling ex vivo and provide the first evidence of microglia interacting with neurons during DCS in vivo. Furthermore, CX3CR1 signalling is required for motor learning and underlying structural neuroplasticity in concert with microglia interaction. Although we have recently demonstrated the microglial response to DCS in vivo, we now provide a link between microglial integrity and neuronal activity for the expression of DCS-dependent neuroplasticity. In addition, we extend the knowledge on the relevance of CX3CR1 signalling for motor learning and structural neuroplasticity. The underlying molecular mechanisms and the potential impact of DCS in rescuing CX3CR1 deficits remain to be addressed in the future.

Implications of fractalkine on glial function, ablation and glial proteins/receptors/markers—understanding its therapeutic usefulness in neurological settings: a narrative review

Background

Fractalkine (CX3CL1) is a chemokine predominantly released by neurons. As a signaling molecule, CX3CL1 facilitates talk between neurons and glia. CX3CL1 is considered as a potential target which could alleviate neuroinflammation. However, certain controversial results and ambiguous role of CX3CL1 make it inexorable to decipher the overall effects of CX3CL1 on the physiopathology of glial cells.

Main body of the abstract

Implications of cross-talk between CX3CL1 and different glial proteins/receptors/markers will give a bird eye view of the therapeutic significance of CX3CL1. Keeping with the need, this review identifies the effects of CX3CL1 on glial physiopathology, glial ablation, and gives a wide coverage on the effects of CX3CL1 on certain glial proteins/receptors/markers.

Short conclusion

Pinpoint prediction of the therapeutic effect of CX3CL1 on neuroinflammation needs further research. This is owing to certain obscure roles and implications of CX3CL1 on different glial proteins/receptors/markers, which are crucial under neurological settings. Further challenges are imposed due to the dichotomous roles played by CX3CL1. The age-old chemokine shows many newer scopes of research in near future. Thus, overall assessment of the effect of CX3CL1 becomes crucial prior to its administration in neuroinflammation.

Brain fractalkine-CX3CR1 signalling is anti-obesity system as anorexigenic and anti-inflammatory actions in diet-induced obese mice

Fractalkine is one of the CX3C chemokine family, and it is widely expressed in the brain including the hypothalamus. In the brain, fractalkine is expressed in neurons and binds to a CX3C chemokine receptor 1 (CX3CR1) in microglia. The hypothalamus regulates energy homeostasis of which dysregulation is associated with obesity. Therefore, we examined whether fractalkine-CX3CR1 signalling involved in regulating food intake and hypothalamic inflammation associated with obesity pathogenesis. In the present study, fractalkine significantly reduced food intake induced by several experimental stimuli and significantly increased brain-derived neurotrophic factor (BDNF) mRNA expression in the hypothalamus. Moreover, tyrosine receptor kinase B (TrkB) antagonist impaired fractalkine-induced anorexigenic actions. In addition, compared with wild-type mice, CX3CR1-deficient mice showed a significant increase in food intake and a significant decrease in BDNF mRNA expression in the hypothalamus. Mice fed a high-fat diet (HFD) for 16 weeks showed hypothalamic inflammation and reduced fractalkine mRNA expression in the hypothalamus. Intracerebroventricular administration of fractalkine significantly suppressed HFD-induced hypothalamic inflammation in mice. HFD intake for 4 weeks caused hypothalamic inflammation in CX3CR1-deficient mice, but not in wild-type mice. These findings suggest that fractalkine-CX3CR1 signalling induces anorexigenic actions via activation of the BDNF-TrkB pathway and suppresses HFD-induced hypothalamic inflammation in mice.

TREM2 ameliorates anesthesia and surgery-induced cognitive impairment by regulating mitophagy and NLRP3 inflammasome in aged C57/BL6 mice

Postoperative cognitive dysfunction (POCD) is a major postoperative complication. Triggering receptor expressed on myeloid cells 2 (TREM2) exerts a neuroprotective function against neuro-inflammatory responses. The present study investigated the role of TREM2 in anesthesia and surgery-induced cognitive impairment and the potential related mechanism. Our results revealed that TREM2 was downregulated, coupled with activation of the NLRP3 inflammasome and subsequent IL-1β expression on postoperative day 3. A corresponding decline in PSD-95 and BDNF was found at the same time point. The key regulator of mitophagy PINK1 and Parkin protein levels were significantly decreased following surgery and anesthesia. TREM2 overexpression partially reversed postoperative cognitive impairment and enhanced PSD-95 and BDNF expression. TREM2 overexpression also improved mitophagy function and inhibited activation of the NLRP3 inflammasome and associated production of IL-1β. Our findings demonstrate that TREM2 rescues anesthesia and surgery-induced spatial learning and memory impairment and neuro-inflammation in aged C57/BL6 mice, which may be at least partially mediated through the activation of mitophagy and subsequent inhibition of the NLRP3 inflammasome.

TREM2 Ameliorates Lipopolysaccharide-Induced Oxidative Stress Response and Neuroinflammation by Promoting Sirtuin3 in BV2 Cells

Triggering receptor expressed on myeloid cells 2 (TREM2) plays a crucial role in modulating microglial-mediated neuroinflammation. The NAD-dependent deacetylase protein Sirtuin 3 (SIRT3) regulates mitochondrial oxidative stress response and neuroinflammation. TREM2 deficiency impairs the denovo synthesis pathway of NAD⁺. Therefore, the aim of this study was to investigate the potential role of TREM2 and SIRT3 in LPS-induced oxidative stress and neuroinflammation in BV2 cells. Lentivirus vector-mediated TREM2 overexpression (TREM2-OE) and corresponding negative control vector (TREM2-NC) were synthesized. BV2 cells were treated with LPS and/or TREM2-OE. 3-TYP, a selective SIRT3 inhibitor, was applied to determine the role of SIRT3 in the anti-oxidant and anti-inflammatory effects of TREM2. TREM2, SIRT3, NLRP3 inflammasome, caspase-1, postsynaptic density-95 (PSD-95), and brain derived neurotrophic factor (BDNF) were measured by Western blot analysis. Superoxide dismutase (SOD) was tested by SOD Assay Kit. Reactive oxygen species (ROS) expression was examined by immunofluorescence. Interleukin 1β (IL-1β) was determined by ELISA. Contents of NAD⁺ and NADH were detected by WST-8 method. LPS (1ug/ml for 24 h) significantly decreased TREM2 expression at both RNA and protein levels (p < 0.01 and p < 0.05, respectively). Lower levels of SIRT3 protein and NAD⁺ were also detected following LPS stimulation (p < 0.05 and p < 0.05, respectively). LPS significantly enhanced ROS, NLRP3, caspase-1, and IL-1β expression (p < 0.01, p < 0.05, p < 0.05, and p < 0.01, respectively). PSD-95 and BDNF expression were decreased triggered by LPS (p < 0.05 and p < 0.05, respectively). TREM2 overexpression enhanced NAD⁺ and SIRT3 protein expression following LPS challenge in BV2 cells (p < 0.01 and p < 0.05, respectively). TREM2 alleviated LPS-induced oxidative stress and neuroinflammation (p < 0.01 and p < 0.05, respectively). Similarly, TREM2 overexpression upregulated PSD-95 and BDNF expression (p < 0.05 and p < 0.05, respectively). The anti-oxidant and anti-inflammatory effects of TREM2 were partially abrogated by SIRT3 antagonist 3-TYP (p < 0.05 and p < 0.05, respectively). Similarly, selective SIRT3 inhibition also partially abrogated TREM2-induced BDNF protein upregulation (p < 0.05) but failed to influence PSD-95 protein expression following LPS stimulation. LPS induces oxidative stress and neuroinflammation in BV2 cells, which may be mediated in part by the downregulation of TREM2 and SIRT3. TREM2 overexpression ameliorates LPS-induced oxidative stress and neuroinflammation through enhancing SIRT3 function via NAD⁺.

Neuroinflammation in perioperative neurocognitive disorders: From bench to the bedside

The perioperative neurocognitive disorders (PNDs) are one of the most common complications in elderly patients characterized by various forms of cognitive decline after anesthesia and surgery. Although the etiology for PNDs remained unclear, neuroinflammation has been characterized as one of the major causes, especially in the elderly patients. The activation of glial cells including microglia and astrocytes plays a significant role in the inflammatory responses in central nerve system (CNS). Although carefully designed, clinical studies on PNDs showed controversial results. Meanwhile, preclinical studies provided evidence from various levels, including behavior performance, protein levels, and gene expression. In this review, we summarize high‐quality studies and recent advances from both clinical and preclinical studies and provide a broad view from the onset of PNDs to its potential therapeutic targets. Future studies are needed to investigate the signaling pathways in PNDs for prevention and treatment, as well as the relationship of PNDs and future neurocognitive dysfunction.

Gut Microbial Metabolite Short-Chain Fatt Acids Partially Reverse Surgery and Anesthesia-Induced Behavior Deficits in C57BL/6J Mice

Accumulating evidence has demonstrated that damages of gut microbiota are strongly associated with central nervous system (CNS) diseases, such as perioperative neurocognitive disorders (PND). The present study investigated the role of gut microbial metabolite short-chain fatty acids (SCFAs) in surgery-induced cognitive deficits and neuroinflammation in the hippocampus. Adult male C57BL/6J mice received either SCFA mixture or saline orally for 4 weeks, and then partial hepatectomy was performed. The fecal supernatant of surgical mice was transplanted to normal mice for 3 weeks. The Morris water maze (MWM) and open-field tests were used to evaluate behavioral performance on postoperative or post-transplantation days 3 and 7. In the MWM test, pretreatment with exogenous SCFAs partially reversed surgery-induced impairments in crossing times and the time spent in the target quadrant on postoperative day 3 (p < 0.05, p < 0.05, respectively). In the open-field test, compared with the surgical mice, exogenous SCFA administration prior to surgery partially improved the locomotor activity (p < 0.05) and anxiety-like behavior (p < 0.05) on postoperative day 3. Surgical trauma and anesthesia enhanced ionized calcium-binding adapter molecule 1 (Iba-1) expression (p < 0.001), increased the levels of interleukin (IL)-1β (p < 0.001) and IL-6 (p < 0.001), and inhibited SCFA production (p < 0.001) on postoperative day 3. The expression of the brain-derived neurotrophic factor (BDNF) was also decreased (p < 0.001). Overall, surgical trauma and anesthesia exacerbated cognitive impairment, enhanced neuroinflammatory responses, and inhibited SCFA production. Pretreatment with SCFAs attenuated these effects partially by reversing microglial overactivation, inhibiting neuroinflammatory responses, and enhancing BDNF expression.

Orthopedic surgery-induced cognitive dysfunction is mediated by CX3CL1/R1 signaling

Background

Postoperative pain is a common phenomenon after surgery and is closely associated with the development of postoperative cognitive dysfunction (POCD). Persistent pain and systemic inflammation caused by surgery have been suggested as key factors for the development of POCD. Fractalkine (CX3CL1) and its receptor, the CX3C chemokine receptor 1 (CX3CR1), are known to play a key role in pain and inflammation signaling pathways. Recent studies have shown that the regulation of CX3CR1/L1 signaling influences the development of various diseases including neuronal diseases. We determined whether CX3CR1/L1 signaling is a putative therapeutic target for POCD in a mouse model.

Methods

Adult (9–11 weeks) male mice were treated with neutralizing antibody to block CX3CR1/L1 signaling both before and after surgery. Inflammatory and behavioral responses including pain were assessed postoperatively. Also, CX3CR1 mRNA level was assessed. Hippocampal astrocyte activation, Mao B expression, and GABA expression were assessed at 2 days after surgery following neutralizing antibody administration.

Results

The behavioral response indicated cognitive dysfunction and development of pain in the surgery group compared with the control group. Also, increased levels of pro-inflammatory cytokines and CX3CR1 mRNA were observed in the surgery group. In addition, increased levels of GABA and increased Mao B expression were observed in reactive astrocytes in the surgery group; these responses were attenuated by neutralizing antibody administration.

Conclusions

Increased CX3CR1 after surgery is both necessary and sufficient to induce cognitive dysfunction. CX3CR1 could be an important target for therapeutic strategies to prevent the development of POCD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02150-x.

Downregulation of LncRNA TUG1 Inhibited TLR4 Signaling Pathway-Mediated Inflammatory Damage After Spinal Cord Ischemia Reperfusion in Rats via Suppressing TRIL Expression

Toll-like receptor 4 (TLR4) and TLR4 interactor with leucine-rich repeats (TRIL) play a crucial role in the inflammatory response. This study investigated the role of long noncoding RNA taurine-upregulated gene 1 (lncRNA TUG1) in TRIL/TLR4 signaling in spinal cord ischemia reperfusion (IR) injury. IR injury was induced in experimental rats; knockdown of TUG1 and TRIL was induced by intrathecal injection of siRNAs and overexpression of TRIL was induced by pcDNA3.3-TRIL. The results showed that the mRNA levels of TUG1 were increased at 12 hours after IR; this was accompanied by increased expression of the TRIL- and TLR4-mediated NF-κB/IL-1β signaling pathway. Activated microglia, detected with increased ionized calcium-binding adapter molecule 1 as a marker, exacerbated the hind-limb neurological impairment and blood-spinal cord barrier (BSCB) leakage after IR. TUG1 knockdown inhibited expression of TRIL and TLR4 signaling proinflammatory cytokines and microglial activation, and attenuated neurological deficit and BSCB leakage. TRIL knockdown inhibited the TLR4-mediated inflammatory response, while TRIL expression reversed the inhibited inflammatory effect caused by TUG1 knockdown. These data suggest that TUG1 knockdown inhibited inflammatory damage of the TLR4-mediated NF-κB/IL-1β signaling pathway after IR via suppressing TRIL expression.