Effect of Hyperbaric Oxygen Therapy on Polarization Phenotype of Rat Microglia After Traumatic Brain Injury

Immune Response in Traumatic Brain Injury

PURPOSE OF REVIEW

This review aims to comprehensively examine the immune response following traumatic brain injury (TBI) and how its disruption can impact healing and recovery.

RECENT FINDINGS

The immune response is now considered a key element in the pathophysiology of TBI, with consequences far beyond the acute phase after injury. A delicate equilibrium is crucial for a healthy recovery. When this equilibrium is disrupted, chronic inflammation and immune imbalance can lead to detrimental effects on survival and disability. Globally, traumatic brain injury (TBI) imposes a substantial burden in terms of both years of life lost and years lived with disability. Although its epidemiology exhibits dynamic trends over time and across regions, TBI disproportionally affects the younger populations, posing psychosocial and financial challenge for communities and families. Following the initial trauma, the primary injury is succeeded by an inflammatory response, primarily orchestrated by the innate immune system. The inflammasome plays a pivotal role during this stage, catalyzing both programmed cell death pathways and the up-regulation of inflammatory cytokines and transcription factors. These events trigger the activation and differentiation of microglia, thereby intensifying the inflammatory response to a systemic level and facilitating the migration of immune cells and edema. This inflammatory response, initially originated in the brain, is monitored by our autonomic nervous system. Through the vagus nerve and adrenergic and cholinergic receptors in various peripheral lymphoid organs and immune cells, bidirectional communication and regulation between the immune and nervous systems is established.

Hyperbaric oxygen preconditioning rescues prolonged underwater exercise-induced hippocampal dysfunction by regulating microglia activation and polarization

Underwater exercise is becoming increasingly prevalent, during which brain function is necessary but is also at risk. However, no study has explored how prolonged exercise affect the brain in underwater environment. Previous studies have indicated that excessive exercise in common environment causes brain dysfunction but have failed to provide appropriate interventions. Numerous evidence has indicated the neuroprotective effect of hyperbaric oxygen preconditioning (HBO-PC). The objective of this study was to investigate the cognitive effect of prolonged underwater exercise (PUE) and to explore the potential neuroprotective effect of HBO-PC in underwater environment. Rats swimming for 3 h in a simulated hyperbaric chamber (2.0 ATA) was used to establish the PUE animal model and HBO-PC (2.5 ATA for 1, 3,5 times respectively) was administrated before PUE. The results demonstrated that PUE triggers anxiety-like behaviors, cognitive impairment accompanied by hippocampal dysfunction, microglia activation and neuroinflammation. Conversely, 3 HBO-PC rescued anxiety-like behaviors and cognitive impairment. Mechanistically, 3 HBO-PC reduced microglia activation and switched the activated microglia from a pro-inflammatory to neuroprotective phenotype. These findings illustrated that PUE induces anxiety-like behaviors and cognitive impairment and HBO-PC of proper frequency may provide an appropriate and less invasive intervention for protecting the brain in underwater exercise.

ROS exhaustion reverses the effects of hyperbaric oxygen on hemorrhagic transformation through reactivating microglia in post-stroke hyperglycemic mice

Acute ischemic stroke (AIS) is a major global health concern due to its high mortality and disability rates. Hemorrhagic transformation, a common complication of AIS, leads to poor prognosis yet lacks effective treatments. Preclinical studies indicate that hyperbaric oxygen (HBO) treatment within 12 h of AIS onset alleviates ischemia/reperfusion injuries, including hemorrhagic transformation. However, clinical trials have yielded conflicting results, suggesting some underlying mechanisms remain unclear. In this study, we confirmed that HBO treatments beginning within 1 h post reperfusion significantly alleviated the haemorrhage and neurological deficits in hyperglycemic transient middle cerebral arterial occlusion (tMCAO) mice, partly due to the inhibition of the NLRP3 inflammasome-mediated pro-inflammatory response in microglia. Notably, reactive oxygen species (ROS) mediate the anti-inflammatory and protective effect of early HBO treatment, as edaravone and N-Acetyl-L-Cysteine (NAC), two commonly used antioxidants, reversed the suppressive effect of HBO treatment on NLRP3 inflammasome-mediated inflammation in microglia. Furthermore, NAC countered the protective effect of early HBO treatment in tMCAO mice with hyperglycemia. These findings support that early HBO treatment is a promising intervention for AIS, however, caution is warranted when combining antioxidants with HBO treatment. Further assessments are needed to clarify the role of antioxidants in HBO therapy for AIS.

The new insights of hyperbaric oxygen therapy: focus on inflammatory bowel disease

Inflammatory bowel diseases (IBD), with an increasing incidence, pose a significant health burden. Although there have been significant advances in the treatment of IBD, more progress is still needed. Hyperbaric oxygen therapy (HBOT) has been shown to treat a host of conditions such as carbon monoxide poisoning, decompression sickness, and gas gangrene. In the last few years, there has been an increase in research into the use of HBOT as an adjunct to conventional treatment for IBD. Related research has shown that HBOT may exert its therapeutic effects by decreasing oxidative stress, inhibiting mucosal inflammation, promoting ulcer healing, influencing gut microbes, and reducing the incidence of IBD complications. This paper aims to provide a comprehensive review of experimental and clinical trials exploring HBOT as a supplement to IBD treatment strategies.

Modulating the polarization phenotype of microglia - A valuable strategy for central nervous system diseases

Central nervous system (CNS) diseases have become one of the leading causes of death in the global population. The pathogenesis of CNS diseases is complicated, so it is important to find the patterns of the disease to improve the treatment strategy. Microglia are considered to be a double-edged sword, playing both harmful and beneficial roles in CNS diseases. Therefore, it is crucial to understand the progression of the disease and the changes in the polar phenotype of microglia to provide guidance in the treatment of CNS diseases. Microglia activation may evolve into different phenotypes: M1 and M2 types. We focused on the roles that M1 and M2 microglia play in regulating intercellular dialogues, pathological reactions and specific diseases in CNS diseases. Importantly, we summarized the strategies used to modulate the polarization phenotype of microglia, including traditional pharmacological modulation, biological therapies, and physical strategies. This review will contribute to the development of potential strategies to modulate microglia polarization phenotypes and provide new alternative therapies for CNS diseases.

Hyperbaric Oxygen Therapy Alleviates Memory and Motor Impairments Following Traumatic Brain Injury via the Modulation of Mitochondrial-Dysfunction-Induced Neuronal Apoptosis in Rats

Traumatic brain injury (TBI) is a leading cause of morbidity and mortality in young adults, characterized by primary and secondary injury. Primary injury is the immediate mechanical damage, while secondary injury results from delayed neuronal death, often linked to mitochondrial damage accumulation. Hyperbaric oxygen therapy (HBOT) has been proposed as a potential treatment for modulating secondary post-traumatic neuronal death. However, the specific molecular mechanism by which HBOT modulates secondary brain damage through mitochondrial protection remains unclear. Spatial learning, reference memory, and motor performance were measured in rats before and after Controlled Cortical Impact (CCI) injury. The HBOT (2.5 ATA) was performed 4 h following the CCI and twice daily (12 h intervals) for four consecutive days. Mitochondrial functions were assessed via high-resolution respirometry on day 5 following CCI. Moreover, IHC was performed at the end of the experiment to evaluate cortical apoptosis, neuronal survival, and glial activation. The current result indicates that HBOT exhibits a multi-level neuroprotective effect. Thus, we found that HBOT prevents cortical neuronal loss, reduces the apoptosis marker (cleaved-Caspase3), and modulates glial cell proliferation. Furthermore, HBO treatment prevents the reduction in mitochondrial respiration, including non-phosphorylation state, oxidative phosphorylation, and electron transfer capacity. Additionally, a superior motor and spatial learning performance level was observed in the CCI group treated with HBO compared to the CCI group. In conclusion, our findings demonstrate that HBOT during the critical period following the TBI improves cognitive and motor damage via regulating glial proliferation apoptosis and protecting mitochondrial function, consequently preventing cortex neuronal loss.

Electroacupuncture regulates microglial polarization via inhibiting NF-κB/COX2 pathway following traumatic brain injury

BACKGROUND

Neuroinflammation and oxidative stress are important pathological mechanisms following traumatic brain injury (TBI). The NF-κB/COX2 pathway regulates neuroinflammation and oxidative damage, while microglia also play an important role in neuroinflammation. Since NF-κB is involved in microglial polarization, targeting this pathway and microglial polarization is a critical component of TBI treatment. Currently, electroacupuncture (EA) is widely used to treat various symptoms after TBI, but the mechanisms of EA remain poorly understood. Additionally, the optimal frequency of EA remains unclear, which affects its efficacy. This study focuses on exploring the optimal frequency parameters of EA on TBI and investigating the underlying mechanisms of EA through NF-κB/COX2 pathway and microglial polarization.

METHODS

The study was divided into two parts. In Experiment 1, 42 Sprague Dawley (SD) rats were induced and randomly divided into seven groups (n = 6). Except for the sham group, all rats underwent controlled cortical impact (CCI) to establish TBI model. Four EA groups (with different frequencies) and manual acupuncture (without current stimulation) received stimulation on the acupoints of Shuigou (GV26), Fengchi (GB20) and Neiguan (PC6) once a day for 7 days. The neurological function was assessed by modified Neurological Severity Scores (mNSS), and the rats' memory and learning were examined by the Morris water maze (MWM). SOD, MDA, and GSH-Px were detected to evaluate the levels of oxidative stress. The levels of IL-1β, IL-6, and TNF-α were evaluated by Enzyme Linked Immunosorbent Assay (ELISA). Detection of the above indicators indicated a treatment group that exerted the strongest neuroprotection against TBI, we then conducted Experiment 2 using this screened acupuncture treatment to investigate the mechanism of acupuncture. 48 rats were randomly divided into four groups (n = 12): sham, TBI model, acupuncture and PDTC (NF-κB inhibitor). Evaluations of mNSS, MWM test, SOD, MDA, GSH-Px, IL-1β, IL-6, TNF-α, and IL-10 were the same as in Experiment 1. Western blot was applied for detecting the expression levels of NF-κB, p-NF-κB, COX2, and Arg-1. TUNEL was used to examine neuronal apoptosis. Brain structure was observed by H&E. Iba-1, COX2, and Arg-1 were investigated by immunofluorescence staining.

RESULTS

EA with frequency of 2/100 Hz markedly improved neuronal and cognitive function as compared to the other treatment groups. Moreover, it downregulated the expression of MDA, IL-6, IL-1β, and TNF-α and upregulated the levels of SOD and GSH-Px. In addition, Both EA with 2/100 Hz and PDTC reduced the levels of p-NF-κB, COX2 and M1 markers (COX2, IL-6, IL-1β, TNF-α) and increased the levels of M2 markers (Arg-1, IL-10). Moreover, they had similar effects on reducing inflammation, oxidative stress and apoptosis, and improving neuronal and cognitive function.

CONCLUSIONS

The collective findings strongly suggest that EA with 2/100 Hz can improve neurologic function by suppressing neuroinflammation, oxidative stress and apoptosis. Additionally, we confirm that EA promotes microglial polarization towards the M2 phenotype through the suppression of NF-κB/COX2 pathway, thus exerting neuroprotective effects after TBI.

Proteasome and Neuroprotective Effect of Hyperbaric Oxygen Preconditioning in Experimental Global Cerebral Ischemia in Rats

Objectives

We investigated the involvement of the proteasome in the mechanism of preconditioning with hyperbaric oxygen (HBO-PC).

Methods

The experiments were performed on male Wistar rats subjected to a transient global cerebral ischemia of 5 min duration (2-vessel occlusion model) and preconditioned or not with HBO for 5 preceding days (1 h HBO at 2.5 atmosphere absolute [ATA] daily). In subgroups of preconditioned rats, the proteasome inhibitor MG132 was administered 30 min prior to each preconditioning session. Twenty-four hours and 7 days post-ischemia, after neurobehavioral assessment, the brains were collected and evaluated for morphological changes and quantitative immunohistochemistry of cell markers and apoptosis-related proteins.

Results

We observed reduced damage of CA1 pyramidal cells in the HBO preconditioned group only at 7 days post-ischemia. However, both at early (24 h) and later (7 days) time points, HBO-PC enhanced the tissue expression of 20S core particle of the proteasome and of the nestin, diminished astroglial reactivity, and reduced p53, rabbit anti-p53 upregulated modulator of apoptosis (PUMA), and rabbit anti-B cell lymphoma-2 interacting mediator of cell death (Bim) expressions in the hippocampus and cerebral cortex. HBO-PC also improved T-maze performance at 7 days. Proteasome inhibitor abolished the beneficial effects of HBO-PC on post-ischemic neuronal injury and functional impairment and reduced the ischemic alterations in the expression of investigated proteins.

Significance

Preconditioning with hyperbaric oxygen-induced brain protection against severe ischemic brain insult appears to involve the proteasome, which can be linked to a depletion of apoptotic proteins and improved regenerative potential.