Astrocytes in the Traumatic Brain Injury: the Good and the Bad

Impacts of PI3K/protein kinase B pathway activation in reactive astrocytes: from detrimental effects to protective functions

Astrocytes are the most abundant type of glial cell in the central nervous system. Upon injury and inflammation, astrocytes become reactive and undergo morphological and functional changes. Depending on their phenotypic classification as A1 or A2, reactive astrocytes contribute to both neurotoxic and neuroprotective responses, respectively. However, this binary classification does not fully capture the diversity of astrocyte responses observed across different diseases and injuries. Transcriptomic analysis has revealed that reactive astrocytes have a complex landscape of gene expression profiles, which emphasizes the heterogeneous nature of their reactivity. Astrocytes actively participate in regulating central nervous system inflammation by interacting with microglia and other cell types, releasing cytokines, and influencing the immune response. The phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) signaling pathway is a central player in astrocyte reactivity and impacts various aspects of astrocyte behavior, as evidenced by in silico , in vitro , and in vivo results. In astrocytes, inflammatory cues trigger a cascade of molecular events, where nuclear factor-κB serves as a central mediator of the pro-inflammatory responses. Here, we review the heterogeneity of reactive astrocytes and the molecular mechanisms underlying their activation. We highlight the involvement of various signaling pathways that regulate astrocyte reactivity, including the PI3K/AKT/mammalian target of rapamycin (mTOR), α v β 3 integrin/PI3K/AKT/connexin 43, and Notch/PI3K/AKT pathways. While targeting the inactivation of the PI3K/AKT cellular signaling pathway to control reactive astrocytes and prevent central nervous system damage, evidence suggests that activating this pathway could also yield beneficial outcomes. This dual function of the PI3K/AKT pathway underscores its complexity in astrocyte reactivity and brain function modulation. The review emphasizes the importance of employing astrocyte-exclusive models to understand their functions accurately and these models are essential for clarifying astrocyte behavior. The findings should then be validated using in vivo models to ensure real-life relevance. The review also highlights the significance of PI3K/AKT pathway modulation in preventing central nervous system damage, although further studies are required to fully comprehend its role due to varying factors such as different cell types, astrocyte responses to inflammation, and disease contexts. Specific strategies are clearly necessary to address these variables effectively.

Enhancing neurogenesis after traumatic brain injury: The role of adenosine kinase inhibition in promoting neuronal survival and differentiation

Traumatic brain injury (TBI) presents a significant public health challenge, necessitating innovative interventions for effective treatment. Recent studies have challenged conventional perspectives on neurogenesis, unveiling endogenous repair mechanisms within the adult brain following injury. However, the intricate mechanisms governing post-TBI neurogenesis remain unclear. The microenvironment of an injured brain, characterized by astrogliosis, neuroinflammation, and excessive cell death, significantly influences the fate of newly generated neurons. Adenosine kinase (ADK), the key metabolic regulator of adenosine, emerges as a crucial factor in brain development and cell proliferation after TBI. This study investigates the hypothesis that targeting ADK could enhance brain repair, promote neuronal survival, and facilitate differentiation. In a TBI model induced by controlled cortical impact, C57BL/6 male mice received intraperitoneal injections of the small molecule ADK inhibitor 5-iodotubercidin (ITU) for three days following TBI. To trace the fate of TBI-associated proliferative cells, animals received intraperitoneal injections of BrdU for seven days, beginning immediately after TBI. Our results show that ADK inhibition by ITU improved brain repair 14 days after injury as evidenced by a diminished injury size. Additionally, the number of mature neurons generated after TBI was increased in ITU-treated mice. Remarkably, the TBI-associated pathological events including astrogliosis, neuroinflammation, and cell death were arrested in ITU-treated mice. Finally, ADK inhibition modulated cell death by regulating the PERK signaling pathway. Together, these findings demonstrate a novel therapeutic approach to target multiple pathological mechanisms involved in TBI. This research contributes valuable insights into the intricate molecular mechanisms underlying neurogenesis and gliosis after TBT.

Candidate Molecular Biomarkers of Traumatic Brain Injury: A Systematic Review

Traumatic brain injury (TBI) is one of the leading causes of mortality and disability among young and middle-aged individuals. Adequate and timely diagnosis of primary brain injuries, as well as the prompt prevention and treatment of secondary injury mechanisms, significantly determine the potential for reducing mortality and severe disabling consequences. Therefore, it is crucial to have objective markers that indicate the severity of the injury. A number of molecular factors-proteins and metabolites-detected in the blood immediately after trauma and associated with the development and severity of TBI can serve in this role. TBI is a heterogeneous condition with respect to its etiology, clinical form, and genesis, being accompanied by brain cell damage and disruption of blood-brain barrier permeability. Two oppositely directed flows of substances and signals are observed: one is the flow of metabolites, proteins, and nucleic acids from damaged brain cells into the bloodstream through the damaged blood-brain barrier; the other is the infiltration of immune cells (neutrophils and macrophages) and serological proteins. Both flows aggravate brain tissue damage after TBI. Therefore, it is extremely important to study the key signaling events that regulate these flows and repair the damaged tissues, as well as to enhance the effectiveness of treatments for patients after TBI.

Targeting astrocytic TDAG8 with delayed CO2 postconditioning improves functional outcomes after controlled cortical impact injury in mice

T-cell death-associated gene 8 (TDAG8), a G-protein-coupled receptor sensing physiological or weak acids, regulates inflammatory responses. However, its role in traumatic brain injury (TBI) remains unknown. Our recent study showed that delayed CO2 postconditioning (DCPC) has neuroreparative effects after TBI. We hypothesized that activating astrocytic TDAG8 is a key mechanism for DCPC. WT and TDAG8-/- mice received DCPC daily by transiently inhaling 10% CO2 after controlled cortical impact (CCI). HBAAV2/9-GFAP-m-TDAG8-3xflag-EGFP was used to overexpress TDAG8 in astrocytes. The beam walking test, mNSS, immunofluorescence and Golgi-Cox staining were used to evaluate motor function, glial activation and dendritic plasticity. DCPC significantly improved motor function; increased total dendritic length, neuronal complexity and spine density; inhibited overactivation of astrocytes and microglia; and promoted the expression of astrocytic brain-derived neurotrophic factor in WT but not TDAG8-/- mice. Overexpressing TDAG8 in astrocytes surrounding the lesion in TDAG8-/- mice restored the beneficial effects of DCPC. Although the effects of DCPC on Days 14-28 were much weaker than those of DCPC on Days 3-28 in WT mice, these effects were further enhanced by overexpressing astrocytic TDAG8. Astrocytic TDAG8 is a key target of DCPC for TBI rehabilitation. Its overexpression is a strategy that broadens the therapeutic window and enhances the effects of DCPC.

Enhancing axonal myelination: Clemastine attenuates cognitive impairment in a rat model of diffuse traumatic brain injury

Traumatic brain injury (TBI) has a significant impact on cognitive function, affecting millions of people worldwide. Myelin loss is a prominent pathological feature of TBI, while well-functioning myelin is crucial for memory and cognition. Utilizing drug repurposing to identify effective drug candidates for TBI treatment has gained attention. Notably, recent research has highlighted the potential of clemastine, an FDA-approved allergy medication, as a promising pro-myelinating drug. Therefore, in this study, we aim to investigate whether clemastine can enhance myelination and alleviate cognitive impairment following mild TBI using a clinically relevant rat model of TBI. Mild diffuse TBI was induced using the Closed-Head Impact Model of Engineered Rotational Acceleration (CHIMERA). Animals were treated with either clemastine or an equivalent volume of the vehicle from day 1 to day 14 post-injury. Following treatment, memory-related behavioral tests were conducted, and myelin pathology in the cortex and hippocampus was assessed through immunofluorescence staining and ProteinSimple® capillary-based immunoassay. Our results showed that TBI leads to significant myelin loss, axonal damage, glial activation, and a decrease in mature oligodendrocytes in both the cortex and hippocampus. The TBI animals also exhibited notable deficits in memory-related tests. In contrast, animals treated with clemastine showed an increase in mature oligodendrocytes, enhanced myelination, and improved performance in the behavioral tests. These preliminary findings support the therapeutic value of clemastine in alleviating TBI-induced cognitive impairment, with substantial clinical translational potential. Our findings also underscore the potential of remyelinating therapies for TBI.

Neurodegenerative Diseases: Unraveling the Heterogeneity of Astrocytes

The astrocyte population, around 50% of human brain cells, plays a crucial role in maintaining the overall health and functionality of the central nervous system (CNS). Astrocytes are vital in orchestrating neuronal development by releasing synaptogenic molecules and eliminating excessive synapses. They also modulate neuronal excitability and contribute to CNS homeostasis, promoting neuronal survival by clearance of neurotransmitters, transporting metabolites, and secreting trophic factors. Astrocytes are highly heterogeneous and respond to CNS injuries and diseases through a process known as reactive astrogliosis, which can contribute to both inflammation and its resolution. Recent evidence has revealed remarkable alterations in astrocyte transcriptomes in response to several diseases, identifying at least two distinct phenotypes called A1 or neurotoxic and A2 or neuroprotective astrocytes. However, due to the vast heterogeneity of these cells, it is limited to classify them into only two phenotypes. This review explores the various physiological and pathophysiological roles, potential markers, and pathways that might be activated in different astrocytic phenotypes. Furthermore, we discuss the astrocyte heterogeneity in the main neurodegenerative diseases and identify potential therapeutic strategies. Understanding the underlying mechanisms in the differentiation and imbalance of the astrocytic population will allow the identification of specific biomarkers and timely therapeutic approaches in various neurodegenerative diseases.

Neuroinflammation and neurodegeneration following traumatic brain injuries

Traumatic brain injuries (TBI) commonly occur following head trauma. TBI may result in short- and long-term complications which may lead to neurodegenerative consequences, including cognitive impairment post-TBI. When investigating the neurodegeneration following TBI, studies have highlighted the role reactive astrocytes have in the neuroinflammation and degeneration process. This review showcases a variety of markers that show reactive astrocyte presence under pathological conditions, including glial fibrillary acidic protein (GFAP), Crystallin Alpha-B (CRYA-B), Complement Component 3 (C3) and S100A10. Astrocyte activation may lead to white-matter inflammation, expressed as white-matter hyperintensities. Other white-matter changes in the brain following TBI include increased cortical thickness in the white matter. This review addresses the gaps in the literature regarding post-mortem human studies focussing on reactive astrocytes, alongside the potential uses of these proteins as markers in the future studies that investigate the proportions of astrocytes in the post-TBI brain has been discussed. This research may benefit future studies that focus on the role reactive astrocytes play in the post-TBI brain and may assist clinicians in managing patients who have suffered TBI.

The cellular senescence response and neuroinflammation in juvenile mice following controlled cortical impact and repetitive mild traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of disability and increases the risk of developing neurodegenerative diseases. The mechanisms linking TBI to neurodegeneration remain to be defined. It has been proposed that the induction of cellular senescence after injury could amplify neuroinflammation and induce long-term tissue changes. The induction of a senescence response post-injury in the immature brain has yet to be characterised. We carried out two types of brain injury in juvenile CD1 mice: invasive TBI using controlled cortical impact (CCI) and repetitive mild TBI (rmTBI) using weight drop injury. The analysis of senescence-related signals showed an increase in γH2AX-53BP1 nuclear foci, p53, p19ARF, and p16INK4a expression in the CCI group, 5 days post-injury (dpi). At 35 days, the difference was no longer statistically significant. Gene expression showed the activation of different senescence pathways in the ipsilateral and contralateral hemispheres in the injured mice. CCI-injured mice showed a neuroinflammatory early phase after injury (increased Iba1 and GFAP expression), which persisted for GFAP. After CCI, there was an increase at 5 days in p16INK4, whereas in rmTBI, a significant increase was seen at 35 dpi. Both injuries caused a decrease in p21 at 35 dpi. In rmTBI, other markers showed no significant change. The PCR array data predicted the activation of pathways connected to senescence after rmTBI. These results indicate the induction of a complex cellular senescence and glial reaction in the immature mouse brain, with clear differences between an invasive brain injury and a repetitive mild injury.

Glial cells react to closed head injury in a distinct and spatiotemporally orchestrated manner

Traumatic brain injury (TBI) is a leading cause of mortality and disability worldwide. Acute neuroinflammation is a prominent reaction after TBI and is mostly initiated by brain-resident glial cells such as microglia, NG2-glia and astrocytes. The magnitude of this reaction paves the way for long-lasting consequences such as chronic neurological pathologies, for which therapeutic options remain limited. The neuroinflammatory response to TBI is mostly studied with craniotomy-based animal models that are very robust but also rather artificial. Here, we aimed to analyze the reaction of glial cells in a highly translational but variable closed head injury (CHI) model and were able to correlate the severity of the trauma to the degree of glial response. Furthermore, we could show that the different glial cell types react in a temporally and spatially orchestrated manner in terms of morphological changes, proliferation, and cell numbers in the first 15 days after the lesion. Interestingly, NG2-glia, the only proliferating cells in the healthy brain parenchyma, divided at a rate that was correlated with the size of the injury. Our findings describe the previously uncharacterized posttraumatic response of the major brain glial cell types in CHI in order to gain a detailed understanding of the course of neuroinflammatory events; such knowledge may open novel avenues for future therapeutic approaches in TBI.

Astrocyte-targeted siRNA delivery by adenosine-functionalized LNP in mouse TBI model

Traumatic brain injury (TBI) induces pro-inflammatory polarization of astrocytes and causes secondary disruption of the blood-brain barrier (BBB) and brain damage. Herein, we report a successful astrocyte-targeted delivery of small interfering RNA (siRNA) by ligand functionalized lipid nanoparticles (LNPs) formulated from adenosine-conjugated lipids and a novel ionizable lipid (denoted by Ad4 LNPs). Systemic administration of Ad4 LNPs carrying siRNA against TLR4 to the mice TBI model resulted in the specific internalization of the LNPs by astrocytes in the vicinity of damaged brain tissue. A substantial knockdown of TLR4 at both mRNA and protein levels in the brain was observed, which led to a significant decrease of key pro-inflammatory cytokines and an increase of key anti-inflammatory cytokines in serum. Dye leakage measurement suggested that the Ad4-LNP-mediated knockdown of TLR4 attenuated the TBI-induced BBB disruption. Together, our data suggest that Ad4 LNP is a promising vehicle for astrocyte-specific delivery of nucleic acid therapeutics.

Astrocyte-derived extracellular vesicles inhibit the abnormal activation of immune function in neonatal mice with hypoxic-ischemic brain damage by carrying miR-124-3p

OBJECTIVE

Hypoxic-ischemic brain damage (HIBD) is among the leading causes of neonatal death worldwide. miR-124-3p can be utilized as a potential diagnostic and prognostic biomarker for perinatal asphyxia and HI encephalopathy in newborns. This study investigated the protective effect and mechanism of miR-124-3p in astrocyte-derived extracellular vesicles (ADEVs) in HIBD.

METHODS

    The neonatal mouse model of HIBD was established. Astrocytes were transfected with the miR-124-3p inhibitor, followed by isolation and identification of ADEVs (ADEVs + inhi miR). HIBD mice were injected with ADEVs or ADEVs + inhi miR through the lateral ventricle, and neurological function was evaluated based on the modified neurological severity score (mNSS). The infarct volume of mice and the morphological modifications of neurons were observed by TTC staining and hematoxylin-eosin staining. The contents of SOD, GSH-Px, CAT, and MDA in the hippocampus were measured. The neuronal apoptosis, the activation of MPO+ neutrophils, NK cells, and CD3+ cells in CA1 region of the hippocampus was determined by means of TUNEL staining and immunofluorescence.

RESULTS

ADEVs alleviated HIBD in neonatal mice. ADEVs could intrinsically protect mice from HIBD by reducing oxidative stress and apoptosis in hippocampal tissue. ADEVs inhibited the positive expression of MPO+ neutrophils, NK cells, and CD3+ cells in HIBD neonatal mice. ADEVs inhibited the hippocampal immune cells by delivering miR-124-3p in neonatal HIBD mice.

CONCLUSION

ADEVs can inhibit the abnormal activation of immune function in HIBD by delivering miR-124-3p, thereby eliciting a protective effect on brain damage in neonatal mice.

Efficacy of extracellular vesicles of different cell origins in traumatic brain injury: A systematic review and network meta-analysis

Background

There was still no effective treatment for traumatic brain injury (TBI). Recently, many preclinical studies had shown promising efficacy of extracellular vesicles (EVs) from various cell sources. Our aim was to compare which cell-derived EVs were most effective in treating TBI through a network meta-analysis.

Methods

We searched four databases and screened various cell-derived EVs for use in preclinical studies of TBI treatment. A systematic review and network meta-analysis were conducted for two outcome indicators, modified Neurological Severity Score (mNSS) and Morris Water Maze (MWM), and they were ranked by the surface under the cumulative ranking curves (SUCRA). Bias risk assessment was performed with SYRCLE. R software (version 4.1.3, Boston, MA, USA) was used for data analysis.

Results

A total of 20 studies were included in this study, involving 383 animals. Astrocyte-derived extracellular vesicles (AEVs) ranked first in response to mNSS at day 1 (SUCRA: 0.26%), day 3 (SUCRA: 16.32%), and day 7 (SUCRA: 9.64%) post-TBI. Extracellular vesicles derived from mesenchymal stem cells (MSCEVs) were most effective in mNSS assessment on day 14 (SUCRA: 21.94%) and day 28 (SUCRA: 6.26%), as well as MWM’s escape latency (SUCRA: 6.16%) and time spent in the target quadrant (SUCRA: 86.52%). The result of mNSS analysis on day 21 showed that neural stem cell-derived extracellular vesicles (NSCEVs) had the best curative effect (SUCRA: 6.76%).

Conclusion

AEVs may be the best choice to improve early mNSS recovery after TBI. The efficacy of MSCEVs may be the best in the late mNSS and MWM after TBI.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, identifier CRD42023377350.

Fish oil fat emulsion alleviates traumatic brain injury in mice by regulation of microglia polarization

Microglia activation, a hallmark of brain neuroinflammation, contributes to the secondary damage following traumatic brain injury (TBI). To explore the potential roles of different fat emulsions-long chain triglyceride (LCT) / medium chain triglyceride (MCT) and fish oil (FO) fat emulsion in neuroprotection and neuroinflammation in TBI, in this study, we first generated the controlled cortical impact (CCI) model of TBI mice. Then either LCT/MCT or FO fat emulsion treated mice were studied by Nissl staining to assess the lesion volume. Sham and TBI mice treated with 0.9% saline were used as controls. The fatty acid composition in different TBI mouse brains was further evaluated by gas chromatography. Immunofluorescent staining and quantitative RT-PCR both demonstrated the suppression of pro-inflammatory microglia and upregulated anti-inflammatory microglia in FO fat emulsion treated TBI brain or primary microglia induced by lipopolysaccharide (LPS) in vitro. Furthermore, motor and cognitive behavioral tests showed FO fat emulsion could partially improve the motor function in TBI mice. Together, our results indicate that FO fat emulsion significantly alleviates the TBI injury and neuroinflammation probably by regulating microglia polarization.

Neuroinflammation of traumatic brain injury: Roles of extracellular vesicles

Traumatic brain injury (TBI) is a major cause of neurological disorder or death, with a heavy burden on individuals and families. While sustained primary insult leads to damage, subsequent secondary events are considered key pathophysiological characteristics post-TBI, and the inflammatory response is a prominent contributor to the secondary cascade. Neuroinflammation is a multifaceted physiological response and exerts both positive and negative effects on TBI. Extracellular vesicles (EVs), as messengers for intercellular communication, are involved in biological and pathological processes in central nervous system (CNS) diseases and injuries. The number and characteristics of EVs and their cargo in the CNS and peripheral circulation undergo tremendous changes in response to TBI, and these EVs regulate neuroinflammatory reactions by activating prominent receptors on receptor cells or delivering pro- or anti-inflammatory cargo to receptor cells. The purpose of this review is to discuss the possible neuroinflammatory mechanisms of EVs and loading in the context of TBI. Furthermore, we summarize the potential role of diverse types of cell-derived EVs in inflammation following TBI.

Autophagy and Apoptosis in Acute Brain Injuries: From Mechanism to Treatment

Significance: Autophagy and apoptosis are two important cellular mechanisms behind brain injuries, which are severe clinical situations with increasing incidences worldwide. To search for more and better treatments for brain injuries, it is essential to deepen the understanding of autophagy, apoptosis, and their interactions in brain injuries. This article first analyzes how autophagy and apoptosis participate in the pathogenetic processes of brain injuries respectively and mutually, then summarizes some promising treatments targeting autophagy and apoptosis to show the potential clinical applications in personalized medicine and precision medicine in the future. Recent Advances: Most current studies suggest that apoptosis is detrimental to brain recovery. Several studies indicate that autophagy can cause unnecessary death of neurons after brain injuries, while others show that autophagy is beneficial for acute brain injuries (ABIs) by facilitating the removal of damaged proteins and organelles. Whether autophagy is beneficial or detrimental in ABIs depends on many factors, and the results from different research groups are diverse or even controversial, making this topic more appealing to be explored further. Critical Issues: Neuronal autophagy and apoptosis are two primary pathological processes in ABIs. How they interact with each other and how their regulations affect the outcome and prognosis of brain injuries remain uncertain, making these answers more critical. Future Directions: Insights into the interplay between autophagy and apoptosis and the accurate regulations of their balance in ABIs may promote personalized and precise treatments in the field of brain injuries. Antioxid. Redox Signal. 38, 234-257.

TGF-β as a Key Modulator of Astrocyte Reactivity: Disease Relevance and Therapeutic Implications

Astrocytes are essential for normal brain development and functioning. They respond to brain injury and disease through a process referred to as reactive astrogliosis, where the reactivity is highly heterogenous and context-dependent. Reactive astrocytes are active contributors to brain pathology and can exert beneficial, detrimental, or mixed effects following brain insults. Transforming growth factor-β (TGF-β) has been identified as one of the key factors regulating astrocyte reactivity. The genetic and pharmacological manipulation of the TGF-β signaling pathway in animal models of central nervous system (CNS) injury and disease alters pathological and functional outcomes. This review aims to provide recent understanding regarding astrocyte reactivity and TGF-β signaling in brain injury, aging, and neurodegeneration. Further, it explores how TGF-β signaling modulates astrocyte reactivity and function in the context of CNS disease and injury.