The effects of A1/A2 astrocytes on oligodendrocyte linage cells against white matter injury under prolonged cerebral hypoperfusion

Role of astrocytes in Alzheimer's disease pathogenesis and the impact of exercise-induced remodeling

Alzheimer's disease (AD) is a prevalent and debilitating brain disorder that worsens progressively with age, characterized by cognitive decline and memory impairment. The accumulation of amyloid-beta (Aβ) leading to amyloid plaques and hyperphosphorylation of Tau, resulting in intracellular neurofibrillary tangles (NFTs), are primary pathological features of AD. Despite significant research investment and effort, therapies targeting Aβ and NFTs have proven limited in efficacy for treating or slowing AD progression. Consequently, there is a growing interest in non-invasive therapeutic strategies for AD prevention. Exercise, a low-cost and non-invasive intervention, has demonstrated promising neuroprotective potential in AD prevention. Astrocytes, among the most abundant glial cells in the brain, play essential roles in various physiological processes and are implicated in AD initiation and progression. Exercise delays pathological progression and mitigates cognitive dysfunction in AD by modulating astrocyte morphological and phenotypic changes and fostering crosstalk with other glial cells. This review aims to consolidate the current understanding of how exercise influences astrocyte dynamics in AD, with a focus on elucidating the molecular and cellular mechanisms underlying astrocyte remodeling. The review begins with an overview of the neuropathological changes observed in AD, followed by an examination of astrocyte dysfunction as a feature of the disease. Lastly, the review explores the potential therapeutic implications of exercise-induced astrocyte remodeling in the context of AD.

Crosstalk Among Glial Cells in the Blood-Brain Barrier Injury After Ischemic Stroke

Blood-brain barrier (BBB) is comprised of brain microvascular endothelial cells (ECs), astrocytes, perivascular microglia, pericytes, neuronal processes, and the basal lamina. As a complex and dynamic interface between the blood and the central nervous system (CNS), BBB is responsible for transporting nutrients essential for the normal metabolism of brain cells and hinders many toxic compounds entering into the CNS. The loss of BBB integrity following stroke induces tissue damage, inflammation, edema, and neural dysfunction. Thus, BBB disruption is an important pathophysiological process of acute ischemic stroke. Understanding the mechanism underlying BBB disruption can uncover more promising biological targets for developing treatments for ischemic stroke. Ischemic stroke-induced activation of microglia and astrocytes leads to increased production of inflammatory mediators, containing chemokines, cytokines, matrix metalloproteinases (MMPs), etc., which are important factors in the pathological process of BBB breakdown. In this review, we discussed the current knowledges about the vital and dual roles of astrocytes and microglia on the BBB breakdown during ischemic stroke. Specifically, we provided an updated overview of phenotypic transformation of microglia and astrocytes, as well as uncovered the crosstalk among astrocyte, microglia, and oligodendrocyte in the BBB disruption following ischemic stroke.

Enteric Glia and Brain Astroglia: Complex Communication in Health and Disease along the Gut-Brain Axis

Several studies have provided interesting evidence about the role of the bidirectional communication between the gut and brain in the onset and development of several pathologic conditions, including inflammatory bowel diseases (IBDs), neurodegenerative diseases, and related comorbidities. Indeed, patients with IBD can experience neurologic disorders, including depression and cognitive impairment, besides typical intestinal symptoms. In parallel, patients with neurodegenerative disease, such as Parkinson disease and Alzheimer disease, are often characterized by the occurrence of functional gastrointestinal disorders. In this context, enteric glial cells and brain astrocytes are emerging as pivotal players in the initiation/maintenance of neuroinflammatory responses, which appear to contribute to the alterations of intestinal and neurologic functions observed in patients with IBD and neurodegenerative disorders. The present review was conceived to provide a comprehensive and critical overview of the available knowledge on the morphologic, molecular, and functional changes occurring in the enteric glia and brain astroglia in IBDs and neurologic disorders. In addition, our intent is to identify whether such alterations could represent a common denominator involved in the onset of comorbidities associated with the aforementioned disorders. This might help to identify putative targets useful to develop novel pharmacologic approaches for the therapeutic management of such disturbances.

Transformation of A1/A2 Astrocytes Participates in Brain Ischemic Tolerance Induced by Cerebral Ischemic Preconditioning via Inhibiting NDRG2

Cerebral ischemic preconditioning (CIP) has been shown to improve brain ischemic tolerance against subsequent lethal ischemia. Reactive astrocytes play important roles in cerebral ischemia-reperfusion. Recent studies have shown that reactive astrocytes can be polarized into neurotoxic A1 phenotype (C3d) and neuroprotective A2 phenotype (S100A10). However, their role in CIP remains unclear. Here, we focused on the role of N-myc downstream-regulated gene 2 (NDRG2) in regulating the transformation of A1/A2 astrocytes and promoting to brain ischemic tolerance induced by CIP. A Sprague Dawley rat model of middle cerebral artery occlusion/reperfusion (MCAO/R) was used. Rats were divided into the following six groups: (1) sham group; (2) CIP group: left middle cerebral artery was blocked for 10 min; (3) MCAO/R group: left middle cerebral artery was blocked for 90 min; (4) CIP + MCAO/R group: CIP was performed 72 h before MCAO/R; (5) AAV-NDRG2 + CIP + MCAO/R group: adeno-associated virus (AAV) carrying NDRG2 was administered 14 days before CIP + MCAO/R; (6) AAV-Ctrl + CIP + MCAO/R group: empty control group. The rats were subjected to neurological evaluation 24 h after the above treatments, and then were sacrificed for 2, 3, 5-triphenyltetraolium chloride staining, thionin staining, immunofluorescence and western blot analysis. In CIP + MCAO/R group, the neurological deficit scores decreased, infarct volume reduced, and neuronal density increased compared with MCAO/R group. Notably, CIP significantly increased S100A10 expression and the number of S100A10+/GFAP+ cells, and also increased NDRG2 expression. MCAO/R significantly decreased S100A10 expression and the number of S100A10+/GFAP+ cells yet increased C3d expression and the number of C3d+/GFAP+ cells and NDRG2 expression, and these trends were reversed by CIP + MCAO/R. Furthermore, over-expression of NDRG2 before CIP + MCAO/R, the C3d expression and the number of C3d+/GFAP+ cells increased, while S100A10 expression and the number of S100A10+/GFAP+ cells decreased. Meanwhile, over-expression of NDRG2 blocked the CIP-induced brain ischemic tolerance. Taken together, these results suggest that CIP exerts neuroprotective effects against ischemic injury by suppressing A1 astrocyte polarization and promoting A2 astrocyte polarization via inhibiting NDRG2 expression.

Single-Cell Transcriptomics Revealed White Matter Repair Following Subarachnoid Hemorrhage

Existing research indicates the potential for white matter injury repair during the subacute phase following subarachnoid hemorrhage (SAH). However, elucidating the role of brain cell subpopulations in the acute and subacute phases of SAH pathogenesis remains challenging due to the cellular heterogeneity of the central nervous system. In this study, single-cell RNA sequencing was conducted on SAH model mice to delineate distinct cell populations. Gene Set Enrichment Analysis was performed to identify involved pathways, and cellular interactions were explored using the CellChat package in R software. Validation of the findings involved a comprehensive approach, including magnetic resonance imaging, immunofluorescence double staining, and Western blot analyses. This study identified ten major brain clusters with cell type-specific gene expression patterns. Notably, we observed infiltration and clonal expansion of reparative microglia in white matter-enriched regions during the subacute stage after SAH. Additionally, microglia-associated pleiotrophin (PTN) was identified as having a role in mediating the regulation of oligodendrocyte precursor cells (OPCs) in SAH model mice, implicating the activation of the mTOR signaling pathway. These findings emphasize the vital role of microglia-OPC interactions might occur via the PTN pathway, potentially contributing to white matter repair during the subacute phase after SAH. Our analysis revealed precise transcriptional changes in the acute and subacute phases after SAH, offering insights into the mechanism of SAH and for the development of drugs that target-specific cell subtypes.

H2S Regulates the Phenotypic Transformation of Astrocytes Following Cerebral Ischemia/Reperfusion via Inhibiting the RhoA/ROCK Pathway

The role of hydrogen sulfide (H2S) on the phenotypic change of astrocytes following cerebral ischemia/reperfusion (I/R) in mice was investigated in present study. We tested the expression of glial fibrillary acidic protein (GFAP), A2 phenotype marker S100a10, and A1 phenotype marker C3 protein and assessed the change of BrdU/GFAP-positive cells, GFAP/C3-positive cells, and GFAP/S100a10-positive cells in mice hippocampal tissues to evaluate the change of astrocyte phenotypes following cerebral I/R. The role of H2S on the phenotypic change of astrocytes following cerebral I/R in mice was investigated by using H2S synthase cystathionine-γ-lyase (CSE) knockout mice (KO). The results revealed that cerebral I/R injury promoted the astrocytes proliferation of both A1 and A2 phenotypes, which were more significant in mice of H2S synthase CSE KO than in mice of wild type (WT). Interestingly, supplement with H2S could inhibit the A1 phenotype proliferation but promote the proliferation of A2 phenotype, suggesting that H2S could regulate the transformation of astrocytes to A2 phenotype following cerebral I/R, which is beneficial for neuronal recovery. Besides, we found that H2S-mediated change of astrocyte phenotype is related to inhibiting the RhoA/ROCK pathway. Furthermore, both H2S and ROCK inhibitor could ameliorate the brain injury of mice at 9 days after cerebral I/R. In conclusion, H2S regulates the phenotypic transformation of astrocytes to A2 phenotype following the cerebral I/R via inhibiting RhoA/ROCK pathway and then exerts the neuroprotective effect against the subacute brain injury.

The Emerging Role of Microglial Hv1 as a Target for Immunomodulation in Myelin Repair

In the central nervous system (CNS), the myelin sheath ensures efficient interconnection between neurons and contributes to the regulation of the proper function of neuronal networks. The maintenance of myelin and the well-organized subtle process of myelin plasticity requires cooperation among myelin-forming cells, glial cells, and neural networks. The process of cooperation is fragile, and the balance is highly susceptible to disruption by microenvironment influences. Reactive microglia play a critical and complicated role in the demyelination and remyelination process. Recent studies have shown that the voltage-gated proton channel Hv1 is selectively expressed in microglia in CNS, which regulates intracellular pH and is involved in the production of reactive oxygen species, underlying multifaceted roles in maintaining microglia function. This paper begins by examining the molecular mechanisms of demyelination and emphasizes the crucial role of the microenvironment in demyelination. It focuses specifically on the role of Hv1 in myelin repair and its therapeutic potential in CNS demyelinating diseases.

Transcriptome Analysis Reveals Dynamic Microglial-Induced A1 Astrocyte Reactivity via C3/C3aR/NF-κB Signaling After Ischemic Stroke

Microglia and astrocytes are key players in neuroinflammation and ischemic stroke. A1 astrocytes are a subtype of astrocytes that are extremely neurotoxic and quickly kill neurons. Although the detrimental A1 astrocytes are present in many neurodegenerative diseases and are considered to accelerate neurodegeneration, their role in the pathophysiology of ischemic stroke is poorly understood. Here, we combined RNA-seq, molecular and immunological techniques, and behavioral tests to investigate the role of A1 astrocytes in the pathophysiology of ischemic stroke. We found that astrocyte phenotypes change from a beneficial A2 type in the acute phase to a detrimental A1 type in the chronic phase following ischemic stroke. The activated microglial IL1α, TNF, and C1q prompt commitment of A1 astrocytes. Inhibition of A1 astrocytes induction attenuates reactive gliosis and ameliorates morphological and functional defects following ischemic stroke. The crosstalk between astrocytic C3 and microglial C3aR contributes to the formation of A1 astrocytes and morphological and functional defects. In addition, NF-κB is activated following ischemic stroke and governs the formation of A1 astrocytes via direct targeting of inflammatory cytokines and chemokines. Taken together, we discovered that A2 astrocytes and A1 astrocytes are enriched in the acute and chronic phases of ischemic stroke respectively, and that the C3/C3aR/NF-κB signaling leads to A1 astrocytes induction. Therefore, the C3/C3aR/NF-κB signaling is a novel therapeutic target for ischemic stroke treatment.

Taurine ameliorates sensorimotor function by inhibiting apoptosis and activating A2 astrocytes in mice after subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is a form of severe acute stroke with very high mortality and disability rates. Early brain injury (EBI) and delayed cerebral ischemia (DCI) contribute to the poor prognosis of patients with SAH. Currently, some researchers have started to focus on changes in amino acid metabolism that occur in brain tissues after SAH. Taurine is a sulfur-containing amino acid that is semi-essential in animals, and it plays important roles in various processes, such as neurodevelopment, osmotic pressure regulation, and membrane stabilization. In acute stroke, such as cerebral hemorrhage, taurine plays a neuroprotective role. However, the role of taurine after subarachnoid hemorrhage has rarely been reported. In the present study, we established a mouse model of SAH. We found that taurine administration effectively improved the sensorimotor function of these mice. In addition, taurine treatment alleviated sensorimotor neuron damage and reduced the proportion of apoptotic cells. Furthermore, taurine treatment enhanced the polarization of astrocytes toward the neuroprotective phenotype while inhibiting their polarization toward the neurotoxic phenotype. This study is the first to reveal the relationship between taurine and astrocyte polarization and may provide a new strategy for SAH research and clinical treatment.

Enriched Environment Inhibits Neurotoxic Reactive Astrocytes via JAK2-STAT3 to Promote Glutamatergic Synaptogenesis and Cognitive Improvement in Chronic Cerebral Hypoperfusion Rats

Chronic cerebral hypoperfusion (CCH) is a primary contributor to cognitive decline in the elderly. Enriched environment (EE) is proved to improve cognitive function. However, mechanisms involved remain unclear. The purpose of the study was exploring the mechanisms of EE in alleviating cognitive deficit in rats with CCH. To create a rat model of CCH, 2-vessel occlusion (2-VO) surgery was performed. All rats lived in standard or enriched environments for 4 weeks. Cognitive function was assessed using the novel object recognition test and Morris water maze test. The protein levels of glutamatergic synapses, neurotoxic reactive astrocytes, reactive microglia, and JAK2-STAT3 signaling pathway were measured using Western blot. The mRNA levels of synaptic regulatory factors, C1q, TNF-α, and IL-1α were identified using quantitative PCR. Immunofluorescence was used to detect glutamatergic synapses, neurotoxic reactive astrocytes, and reactive microglia, as well as the expression of p-STAT3 in astrocytes in the hippocampus. The results demonstrated that the EE mitigated cognitive impairment in rats with CCH and enhanced glutamatergic synaptogenesis. EE also inhibited the activation of neurotoxic reactive astrocytes. Moreover, EE downregulated microglial activation, levels of C1q, TNF-α and IL-1α and phosphorylation of JAK2 and STAT3. Our results suggest that inhibition of neurotoxic reactive astrocytes may be one of the mechanisms by which EE promotes glutamatergic synaptogenesis and improves cognitive function in rats with CCH. The downregulation of reactive microglia and JAK2-STAT3 signaling pathway may be involved in this process.

Hexahydrocurcumin attenuated demyelination and improved cognitive impairment in chronic cerebral hypoperfusion rats

Age-related white matter lesions (WML) frequently present vascular problems by decreasing cerebral blood supply, resulting in the condition known as chronic cerebral hypoperfusion (CCH). This study aimed to investigate the effect of hexahydrocurcumin (HHC) on the processes of demyelination and remyelination induced by the model of the Bilateral Common Carotid Artery Occlusion (BCCAO) for 29 days to mimic the CCH condition. The pathological appearance of myelin integrity was significantly altered by CCH, as evidenced by Transmission Electron Microscopy (TEM) and Luxol Fast Blue (LFB) staining. In addition, CCH activated A1-astrocytes and reactive-microglia by increasing the expression of Glial fibrillary acidic protein (GFAP), complement 3 (C3d) and pro-inflammatory cytokines. However, S100a10 expression, a marker of neuroprotective astrocytes, was suppressed, as were regenerative factors including (IGF-1) and Transglutaminase 2 (TGM2). Therefore, the maturation step was obstructed as shown by decreases in the levels of myelin basic protein (MBP) and the proteins related with lipid synthesis. Cognitive function was therefore impaired in the CCH model, as evidenced by the Morris water maze test. By contrast, HHC treatment significantly improved myelin integrity, and inhibited A1-astrocytes and reactive-microglial activity. Consequently, pro-inflammatory cytokines and A1-astrocytes were attenuated, and regenerative factors increased assisting myelin maturation and hence improving cognitive performance. In conclusion, HHC improves cognitive function and also the integrity of white matter in CCH rats by reducing demyelination, and pro-inflammatory cytokine production and promoting the process of remyelination.

Physical and social environmental enrichment alleviate ferroptosis and inflammation with inhibition of TLR4/MyD88/p38MAPK pathway in chronic cerebral hypoperfusion rats

A typical enriched environment (EE), which combines physical activity and social interaction, has been proven to mitigate cognitive impairment caused by chronic cerebral hypoperfusion (CCH). However, it remains unclear how the different components of EE promote cognitive recovery after CCH. This study stripped out the different components of EE into physical environmental enrichment (PE) and social environmental enrichment (SE), and compared the neuroprotective effects of PE, SE and typical EE (PSE) in CCH. The results of novel object recognition and Morris water maze tests showed that PE, SE, and PSE improved cognitive function in CCH rats. Additionally, Nissl and TUNEL staining revealed that three EEs reduced neuronal loss in the hippocampus. PSE exhibited superior neuroprotective and functional improvement effects compared to PE and SE, while there was no significant difference between PE and SE. Furthermore, three EEs reduced lipid peroxidation in the hippocampus with decreasing the levels of MDA and increasing the activities of SOD and GSH. The expression of SLC7A11 and GPX4 was increased, while the level of p53 was reduced in three EEs. This suggested that three EEs inhibited ferroptosis by maintaining the redox homeostasis in the hippocampus. Three EEs reduced the levels of IL-β, TNF-α, and IL-6, thereby inhibiting neuroinflammation. Additionally, Western blotting and immunofluorescence results indicated that three EEs also inhibited the TLR4/MyD88/p38MAPK signaling pathway. These findings collectively demonstrated that the three EEs alleviated hippocampal ferroptosis and neuroinflammation in CCH rats, thereby reducing neuronal loss, which might be associated with the inhibition of the TLR4/MyD88/p38MAPK signaling pathway. Moreover, the study results supported that it is only through the combination of physical exercise and social interaction that the optimal neuroprotective effects can be achieved. These findings provided valuable insights for the prevention and treatment of vascular cognitive impairment.

Astrocytes in ischemic stroke: Crosstalk in central nervous system and therapeutic potential

In the central nervous system (CNS), a large group of glial cells called astrocytes play important roles in both physiological and disease conditions. Astrocytes participate in the formation of neurovascular units and interact closely with other cells of the CNS, such as microglia and neurons. Stroke is a global disease with high mortality and disability rate, most of which are ischemic stroke. Significant strides in understanding astrocytes have been made over the past few decades. Astrocytes respond strongly to ischemic stroke through a process known as activation or reactivity. Given the important role played by reactive astrocytes (RAs) in different spatial and temporal aspects of ischemic stroke, there is a growing interest in the potential therapeutic role of astrocytes. Currently, interventions targeting astrocytes, such as mediating astrocyte polarization, reducing edema, regulating glial scar formation, and reprogramming astrocytes, have been proven in modulating the progression of ischemic stroke. The aforementioned potential interventions on astrocytes and the crosstalk between astrocytes and other cells of the CNS will be summarized in this review.

Combined VEGF and bFGF loaded nanofiber membrane protects against neuronal injury and hypomyelination in a rat model of chronic cerebral hypoperfusion

Currently, there are no effective therapeutic targets for the treatment of chronic cerebral hypoperfusion(CCH)-induced cerebral ischemic injury. Vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) are discovered as the inducers of neurogenesis and angiogenesis. We previously made a nanofiber membrane (NFM), maintaining a long-term release of VEGF and bFGF up to 35 days, which might make VEGF and bFGF NFM as the potential protective agents against cerebral ischemic insult. In this study, the effects of VEGF and bFGF delivered by NFM into brain were investigated as well as their underlying mechanismsin a rat model of CCH. VEGF + bFGF NFM application increased the expressions of tight junction proteins, maintained BBB integrity, and alleviated vasogenic cerebral edema. Furthermore, VEGF + bFGF NFM sticking enhanced angiogenesis and elevated CBF. Besides, VEGF + bFGF NFM treatment inhibited neuronal apoptosis and decreased neuronal loss. Moreover, roofing of VEGF + bFGF NFM attenuated microglial activation and blocked the launch of NLRP3/caspase-1/IL-1β pathway. In addition, VEGF + bFGF NFM administration prevented disruption to the pre/postsynaptic membranes and loss of myelin sheath, relieving synaptic injury and demyelination. Oligodendrogenesis, neurogenesis and PI3K/AKT/mTOR pathway were involved in the treatment of VEGF + bFGF NFM against CCH-induced neuronal injury and hypomyelination. These findings supported that VEGF + bFGF NFM application constitutes a neuroprotective strategy for the treatment of CCH, which may be worth further clinical translational research as a novel neuroprotective approach, benifiting indirect surgical revascularization.

A brief overview of a mouse model of cerebral hypoperfusion by bilateral carotid artery stenosis

Vascular cognitive impairment (VCI) refers to all forms of cognitive disorder related to cerebrovascular diseases, including vascular mild cognitive impairment, post-stroke dementia, multi-infarct dementia, subcortical ischemic vascular dementia (SIVD), and mixed dementia. Among the causes of VCI, more attention has been paid to SIVD because the causative cerebral small vessel pathologies are frequently observed in elderly people and because the gradual progression of cognitive decline often mimics Alzheimer's disease. In most cases, small vessel diseases are accompanied by cerebral hypoperfusion. In mice, prolonged cerebral hypoperfusion is induced by bilateral carotid artery stenosis (BCAS) with surgically implanted metal micro-coils. This cerebral hypoperfusion BCAS model was proposed as a SIVD mouse model in 2004, and the spreading use of this mouse SIVD model has provided novel data regarding cognitive dysfunction and histological/genetic changes by cerebral hypoperfusion. Oxidative stress, microvascular injury, excitotoxicity, blood-brain barrier dysfunction, and secondary inflammation may be the main mechanisms of brain damage due to prolonged cerebral hypoperfusion, and some potential therapeutic targets for SIVD have been proposed by using transgenic mice or clinically used drugs in BCAS studies. This review article overviews findings from the studies that used this hypoperfused-SIVD mouse model, which were published between 2004 and 2021.

Increasing brain N-acetylneuraminic acid alleviates hydrocephalus-induced neurological deficits

AIMS

This metabolomic study aimed to evaluate the role of N-acetylneuraminic acid (Neu5Ac) in the neurological deficits of normal pressure hydrocephalus (NPH) and its potential therapeutic effect.

METHODS

We analyzed the metabolic profiles of NPH using cerebrospinal fluid with multivariate and univariate statistical analyses in a set of 42 NPH patients and 38 controls. We further correlated the levels of differential metabolites with severity-related clinical parameters, including the normal pressure hydrocephalus grading scale (NPHGS). We then established kaolin-induced hydrocephalus in mice and treated them using N-acetylmannosamine (ManNAc), a precursor of Neu5Ac. We examined brain Neu5Ac, astrocyte polarization, demyelination, and neurobehavioral outcomes to explore its therapeutic effect.

RESULTS

Three metabolites were significantly altered in NPH patients. Only decreased Neu5Ac levels were correlated with NPHGS scores. Decreased brain Neu5Ac levels have been observed in hydrocephalic mice. Increasing brain Neu5Ac by ManNAc suppressed the activation of astrocytes and promoted their transition from A1 to A2 polarization. ManNAc also attenuated the periventricular white matter demyelination and improved neurobehavioral outcomes in hydrocephalic mice.

CONCLUSION

Increasing brain Neu5Ac improved the neurological outcomes associated with the regulation of astrocyte polarization and the suppression of demyelination in hydrocephalic mice, which may be a potential therapeutic strategy for NPH.

H2S-RhoA/ROCK Pathway and Glial Cells in Axonal Remyelination After Ischemic Stroke

Ischemic stroke is one of the main reasons of disability and death. Stroke-induced functional deficits are mainly due to the secondary degeneration of the white matter characterized by axonal demyelination and injury of axon-glial integrity. Enhancement of the axonal regeneration and remyelination could promote the neural functional recovery. However, cerebral ischemia-induced activation of RhoA/Rho kinase (ROCK) pathway plays a crucial and harmful role in the process of axonal recovery and regeneration. Inhibition of this pathway could promote the axonal regeneration and remyelination. In addition, hydrogen sulfide (H2S) has the significant neuroprotective role during the recovery of ischemic stroke via inhibiting the inflammatory response and oxidative stress, regulating astrocyte function, promoting the differentiation of endogenous oligodendrocyte precursor cells (OPCs) to mature oligodendrocyte. Among all of these effects, promoting the formation of mature oligodendrocyte is a crucial part of axonal regeneration and remyelination. Furthermore, numerous studies have uncovered the crosstalk between astrocytes and oligodendrocyte, microglial cells and oligodendrocyte in the axonal remyelination following ischemic stroke. The purpose of this review was to discuss the relationship among H2S, RhoA/ROCK pathway, astrocytes, and microglial cells in the axonal remyelination following ischemic stroke to reveal new strategies for preventing and treating this devastating disease.

New insights into the roles of oligodendrocytes regulation in ischemic stroke recovery

Oligodendrocytes (OLs), the myelin-forming cells of the central nervous system, are integral to axonal integrity and function. Hypoxia-ischemia episodes can cause severe damage to these vulnerable cells through excitotoxicity, oxidative stress, inflammation, and mitochondrial dysfunction, leading to axonal dystrophy, neuronal dysfunction, and neurological impairments. OLs damage can result in demyelination and myelination disorders, severely impacting axonal function, structure, metabolism, and survival. Adult-onset stroke, periventricular leukomalacia, and post-stroke cognitive impairment primarily target OLs, making them a critical therapeutic target. Therapeutic strategies targeting OLs, myelin, and their receptors should be given more emphasis to attenuate ischemia injury and establish functional recovery after stroke. This review summarizes recent advances on the function of OLs in ischemic injury, as well as the present and emerging principles that serve as the foundation for protective strategies against OL deaths.

Chronic cerebral hypoperfusion: a critical feature in unravelling the etiology of vascular cognitive impairment

Vascular cognitive impairment (VCI) describes a wide spectrum of cognitive deficits related to cerebrovascular diseases. Although the loss of blood flow to cortical regions critically involved in cognitive processes must feature as the main driver of VCI, the underlying mechanisms and interactions with related disease processes remain to be fully elucidated. Recent clinical studies of cerebral blood flow measurements have supported the role of chronic cerebral hypoperfusion (CCH) as a major driver of the vascular pathology and clinical manifestations of VCI. Here we review the pathophysiological mechanisms as well as neuropathological changes of CCH. Potential interventional strategies for VCI are also reviewed. A deeper understanding of how CCH can lead to accumulation of VCI-associated pathology could potentially pave the way for early detection and development of disease-modifying therapies, thus allowing preventive interventions instead of symptomatic treatments.

Effects of liraglutide on astrocyte polarization and neuroinflammation in db/db mice: focus on iron overload and oxidative stress

Neuroinflammation plays a crucial role in the occurrence and development of cognitive impairment in type 2 diabetes mellitus (T2DM), but the specific injury mechanism is not fully understood. Astrocyte polarization has attracted new attention and has been shown to be directly and indirectly involved in neuroinflammation. Liraglutide has been shown to have beneficial effects on neurons and astrocytes. However, the specific protection mechanism still needs to be clarified. In this study, we assessed the levels of neuroinflammation and A1/A2-responsive astrocytes in the hippocampus of db/db mice and examined their relationships with iron overload and oxidative stress. First, in db/db mice, liraglutide alleviated the disturbance of glucose and lipid metabolism, increased the postsynaptic density, regulated the expression of NeuN and BDNF, and partially restored impaired cognitive function. Second, liraglutide upregulated the expression of S100A10 and downregulated the expression of GFAP and C3, and decreased the secretion of IL-1β, IL-18, and TNF-α, which may confirm that it regulates the proliferation of reactive astrocytes and A1/A2 phenotypes polarize and attenuate neuroinflammation. In addition, liraglutide reduced iron deposition in the hippocampus by reducing the expression of TfR1 and DMT1 and increasing the expression of FPN1; at the same time, liraglutide by up-regulating the levels of SOD, GSH, and SOD2 expression, as well as downregulation of MDA levels and NOX2 and NOX4 expression to reduce oxidative stress and lipid peroxidation. The above may attenuate A1 astrocyte activation. This study preliminarily explored the effect of liraglutide on the activation of different astrocyte phenotypes and neuroinflammation in the hippocampus of a T2DM model and further revealed its intervention effect on cognitive impairment in diabetes. Focusing on the pathological consequences of astrocytes may have important implications for the treatment of diabetic cognitive impairment.

Microbial lipopolysaccharide-induced inflammation contributes to cognitive impairment and white matter lesion progression in diet-induced obese mice with chronic cerebral hypoperfusion

AIMS

White matter lesions (WMLs) are involved in the pathological processes leading to cognitive decline and dementia. We examined the mechanisms underlying the exacerbation of ischemia-induced cognitive impairment and WMLs by diet-induced obesity, including lipopolysaccharide (LPS)-triggered neuroinflammation via toll-like receptor (TLR) 4.

METHODS

Wild-type (WT) and TLR4-knockout (KO) C57BL/6 mice were fed a high-fat diet (HFD) or low-fat diet (LFD), and subjected to bilateral carotid artery stenosis (BCAS). Diet groups were compared for changes in gut microbiota, intestinal permeability, systemic inflammation, neuroinflammation, WML severity, and cognitive dysfunction.

RESULTS

In WT mice, HFD induced obesity and increased cognitive impairment and WML severity compared with LFD-fed mice following BCAS. HFD caused gut dysbiosis and increased intestinal permeability, and plasma LPS and pro-inflammatory cytokine concentrations. Furthermore, HFD-fed mice had higher LPS levels and higher neuroinflammatory status, including increased TLR4 expression, in WMLs. In TLR4-KO mice, HFD also caused obesity and gut dysbiosis but did not increase cognitive impairment or WML severity after BCAS. No difference was found between HFD- and LFD-fed KO mice for LPS levels or inflammatory status in either plasma or WMLs.

CONCLUSION

Inflammation triggered by LPS-TLR4 signaling may mediate obesity-associated exacerbation of cognitive impairment and WMLs from brain ischemia.

Astrocytic CXCL5 hinders microglial phagocytosis of myelin debris and aggravates white matter injury in chronic cerebral ischemia

BACKGROUND

Chronic cerebral ischemia induces white matter injury (WMI) contributing to cognitive decline. Both astrocytes and microglia play vital roles in the demyelination and remyelination processes, but the underlying mechanism remains unclear. This study aimed to explore the influence of the chemokine CXCL5 on WMI and cognitive decline in chronic cerebral ischemia and the underlying mechanism.

METHODS

Bilateral carotid artery stenosis (BCAS) model was constructed to mimic chronic cerebral ischemia in 7-10 weeks old male mice. Astrocytic Cxcl5 conditional knockout (cKO) mice were constructed and mice with Cxcl5 overexpressing in astrocytes were generated by stereotactic injection of adeno-associated virus (AAV). WMI was evaluated by magnetic resonance imaging (MRI), electron microscopy, histological staining and western blotting. Cognitive function was examined by a series of neurobehavioral tests. The proliferation and differentiation of oligodendrocyte progenitor cells (OPCs), phagocytosis of microglia were analyzed via immunofluorescence staining, western blotting or flow cytometry.

RESULTS

CXCL5 was significantly elevated in the corpus callosum (CC) and serum in BCAS model, mainly expressed in astrocytes, and Cxcl5 cKO mice displayed improved WMI and cognitive performance. Recombinant CXCL5 (rCXCL5) had no direct effect on the proliferation and differentiation of OPCs in vitro. Astrocytic specific Cxcl5 overexpression aggravated WMI and cognitive decline induced by chronic cerebral ischemia, while microglia depletion counteracted this effect. Recombinant CXCL5 remarkably hindered microglial phagocytosis of myelin debris, which was rescued by inhibition of CXCL5 receptor C-X-C motif chemokine receptor 2 (CXCR2).

CONCLUSION

Our study revealed that astrocyte-derived CXCL5 aggravated WMI and cognitive decline by inhibiting microglial phagocytosis of myelin debris, suggesting a novel astrocyte-microglia circuit mediated by CXCL5-CXCR2 signaling in chronic cerebral ischemia.

Hydrogen sulfide inhibits lipopolysaccharide-based neuroinflammation-induced astrocyte polarization after cerebral ischemia/reperfusion injury

The effect of lipopolysaccharide (LPS)-based neuroinflammation following cerebral ischemia/reperfusion (I/R) on the genotypic transformation of reactive astrocytes and its relationship with endogenous hydrogen sulfide (H₂S) were investigated in present study. We found that LPS promoted the cerebral I/R-induced A1 astrocytes proliferation in mouse hippocampal tissues and deteriorated the reduction of hydrogen sulfide (H₂S) content in mouse sera, H₂S donor NaHS could inhibitA1 astrocytes proliferation. Similarly, knockout of cystathionine γ-lyase (CSE), one of endogenous H₂S synthases, likewise up-regulated the cerebral I/R-induced A1 astrocytes proliferation, which could also be blocked by NaHS. Besides, supplement with H₂S promoted the A2 astrocytes proliferation in hippocampal tissues of CSE knockout (CSE KO) mice or LPS-treated mice following cerebral I/R. In the oxygen glucose deprivation/reoxygenation (OGD/R) model of astrocytes, H₂S also promoted the transformation of astrocytes into A2 subtype. Moreover, we found that H₂S could up-regulate the expression of α-subunit of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in astrocytes, and the channel opener BMS-191011 likewise promoted the transformation of astrocyte into A2 subtype. In conclusion, H₂S inhibits the proliferation of A1 astrocytes induced by LPS-based neuroinflammation following cerebral I/R and promotes the transformation of astrocytes into A2 subtype, which may be related to up-regulation of BK_Ca channels.

GABAB receptor activation alters astrocyte phenotype changes induced by trimethyltin via ERK signaling in the dentate gyrus of mice

AIMS

We examined the effect of γ-aminobutyric acid (GABA)_B receptor activation on astrocyte phenotype changes induced by trimethyltin (TMT) in the dentate gyrus of mice.

MAIN METHODS

Male C57BL/6N mice received TMT (2.6 mg/kg, i.p.), and the expression of GABA_B receptors was evaluated in the hippocampus. The GABA_B receptor agonist baclofen (2.5, 5, or 10 mg/kg, i.p. × 5 at 12-h intervals) was administered 3-5 days after TMT treatment, and the expression of Iba-1, GFAP, and astrocyte phenotype markers was evaluated 6 days after TMT. SL327 (30 mg/kg, i.p.), an extracellular signal-related kinase (ERK) inhibitor, was administered 1 h after each baclofen treatment.

KEY FINDINGS

TMT insult significantly induced the astroglial expression of GABA_B receptors in the dentate molecular layer. Baclofen significantly promoted the expression of S100A10, EMP1, and CD109, but not that of C3, GGTA1, and MX1 induced by TMT. In addition, baclofen significantly increased the TMT-induced expression of p-ERK in the dentate molecular layer. Interestingly, p-ERK was more colocalized with S100A10 than with C3 after TMT insult, and a significant positive correlation was found between the expression of p-ERK and S100A10. Consistently, SL327 reversed the effect of baclofen on astrocyte phenotype changes. Baclofen also enhanced the TMT-induced astroglial expression of glial cell-derived neurotrophic factor (GDNF), an anti-inflammatory astrocytes-to-microglia mediator, and consequently attenuated Iba-1 expression and delayed apoptotic neuronal death.

SIGNIFICANCE

Our results suggest that GABA_B receptor activation increases S100A10-positive anti-inflammatory astrocytes and astroglial GDNF expression via ERK signaling after TMT excitotoxicity in the dentate molecular layer of mice.

Honokiol prevents chronic cerebral hypoperfusion induced astrocyte A1 polarization to alleviate neurotoxicity by targeting SIRT3-STAT3 axis

Alzheimer's Dementia (AD) and Vascular Dementia (VaD) are two main types of dementias for which no specific treatment is available. Chronic Cerebral Hypoperfusion (CCH) is a pathogenesis underlying AD and VaD that promotes neuroinflammatory responses and oxidative stress. Honokiol (HNK) is a natural compound isolated from magnolia leaves that can easily cross blood brain barrier and has anti-inflammatory and antioxidant effects. In the present study, the effects of HNK on astrocyte polarization and neurological damage in in vivo and in vitro models of chronic cerebral hypoperfusion were explored. We found that HNK was able to inhibit the phosphorylation and nuclear translocation of STAT3, A1 polarization, and reduce conditioned medium's neuronal toxicity of astrocyte under chronic hypoxia induced by cobalt chloride; STAT3 phosphorylation inhibitor C188-9 was able to mimic the above effects of HNK, suggesting that HNK may inhibit chronic hypoxia-induced A1 polarization in astrocytes via STAT3. SIRT3 inhibitor 3-TYP reversed, while Sirt3 overexpression mimicked the inhibitory effects of HNK on oxidative stress, STAT3 phosphorylation and nuclear translocation, A1 polarization and neuronal toxicity of astrocyte under chronic hypoxic conditions. For in vivo research, continuous intraperitoneal injection of HNK (1mg/kg) for 21 days ameliorated the decrease in SIRT3 activity and oxidative stress, inhibited astrocytic STAT3 nuclear translocation and A1 polarization, and prevented neuron and synaptic loss in the hippocampal of CCH rats. Besides, HNK application improved the spatial memory impairment of CCH rats, as assessed with Morris Water Maze. In conclusion, these results suggest that the phytochemical HNK can inhibit astrocyte A1 polarization via regulating SIRT3-STAT3 axis, thus improving CCH-induced neurological damage. These results highlight HNK as novel treatment for dementia with underlying vascular mechanisms.

Tetramethylpyrazine promotes stroke recovery by inducing the restoration of neurovascular unit and transformation of A1/A2 reactive astrocytes

2,3,5,6-Tetramethylpyrazine (TMP) as an active ingredient extracted from a traditional Chinese herbal medicine Ligusticum chuanxiong Hort. has been proved to penetrate blood-brain barrier (BBB) and show neuroprotective effects on cerebral ischemia. However, whether TMP could regulate astrocytic reactivity to facilitate neurovascular restoration in the subacute ischemic stroke needs to be urgently verified. In this research, permanent occlusion of the middle cerebral artery (MCAO) model was conducted and TMP (10, 20, 40 mg/kg) was intraperitoneally administrated to rats once daily for 2 weeks. Neurological function was evaluated by motor deficit score (MDS). Magnetic resonance imaging (MRI) was implemented to analyze tissue injury and cerebral blood flow (CBF). Magnetic resonance angiography (MRA) was applied to exhibit vascular signals. Transmission electron microscopy (TEM) was performed to detect the neurovascular unit (NVU) ultrastructure. Haematoxylin and eosin (HE) staining was utilized to evaluate cerebral histopathological lesions. The neurogenesis, angiogenesis, A1/A2 reactivity, aquaporin 4 (AQP4) and connexin 43 (Cx43) of astrocytes were observed with immunofluorescent staining. Then FGF2/PI3K/AKT signals were measured by western blot. Findings revealed TMP ameliorated neurological functional recovery, preserved NVU integrity, and enhanced endogenous neurogenesis and angiogenesis of rats with subacute ischemia. Shifting A1 to A2 reactivity, suppressing excessive AQP4 and Cx43 expression of astrocytes, and activating FGF2/PI3K/AKT pathway might be potential mechanisms of promoting neurovascular restoration with TMP after ischemic stroke.

C3aR in astrocytes mediates post-thoracotomy pain by inducing A1 astrocytes in male rats

BACKGROUND

Astrocyte activation, which is polarized into classical neurotoxic A1, neuroprotective A2, A-pan, etc., is thought to be involved in the transition from acute to chronic post-thoracotomy pain. The C3aR receptor associated with astrocyte-neuron and -microglia interactions is necessary for A1 astrocytes polarization. This study aimed to determine whether C3aR in astrocytes mediates post-thoracotomy pain by inducing A1 expression in a rat thoracotomy pain model.

METHODS

A rat thoracotomy pain model was employed. The mechanical withdraw threshold was measured to evaluate pain behavior. Lipopolysaccharide (LPS) was injected intraperitoneally to induce A1. Intrathecal injection of AAV2/9-rC3ar1 shRNA-GFAP was used to knock down in vivo C3aR expression in astrocytes. The expression of associated phenotypic markers before and after intervention was assessed by RT-PCR, western blot, co-immunofluorescence, and single-cell RNA sequencing.

RESULTS

C3aR downregulation was found to inhibit LPS-induced A1 astrocytes activation, decrease the expression of C3aR, C3, and GFAP, which were activated from acute to chronic pain, and alleviate the mechanical withdrawal threshold and chronic pain incidence. In addition, more A2 astrocytes were activated in the model group that did not develop chronic pain. C3aR downregulation increased the number of A2 astrocytes upon LPS exposure. Knockdown of C3aR also decreased the activation of M1 microglia induced by LPS or thoracotomy.

CONCLUSIONS

Our study confirmed that C3aR-induced A1 polarization contributes to chronic post-thoracotomy pain. Inhibition of A1 activation via C3aR downregulation increases anti-inflammatory A2 and decreases pro-inflammatory M1 activation, which may also be involved in the mechanism of chronic post-thoracotomy pain.

Total flavones of Rhododendron induce the transformation of A1/A2 astrocytes via promoting the release of CBS-produced H2S

BACKGROUND

We previously found that total flavones of Rhododendron (TFR) protected against the cerebral ischemia/reperfusion (I/R) injury. But the detailed mechanism is not clear. Recent research revealed that reactive astrocytes were divided into A1 and A2 phenotypes for their morphological and functional remodeling and neurotoxic- vs-neuroprotective effect on the injury of the central nervous system (CNS).

PURPOSE

The present study was undertaken to explore the role and mechanism of TFR on the phenotypic change of astrocytes following cerebral I/R in vivo and oxygen glucose deprivation/re-oxygenation (OGD/R) in vitro.

STUDY DESIGN AND METHODS

We tested the expression of astrocytes marker glial fibrillary acidic protein (GFAP), A1 astrocytes marker C3 protein and A2 astrocytes marker S100a10, as well as the BrdU/GFAP-positive cells, GFAP/S100a10-positive cells and GFAP/C3-positive cells in mice hippocampal tissues to evaluate the phenotypic change of astrocytes. Besides, we assessed the change of astrocyte phenotypes following OGD/R in vitro.

RESULTS

We found that mice cerebral I/R promoted the astrocytes proliferation of both A1 and A2 phenotypes in hippocampal tissues. While treatment with TFR could promote the proliferation of A2 astrocytes but inhibit the A1 astrocytes proliferation in mice hippocampal tissues, suggesting that TFR could accelerate the astrocytes transformation into A2 subtype following cerebral I/R. Whereas, in OGD/R model of astrocytes, we found that TFR inhibited the proliferation of both A1 and A2 astrocytes. Besides, we found that TFR could up-regulate the release of cystathionine β-synthase (CBS)-produced hydrogen sulfide (H₂S) and inhibit RhoA/Rho kinase pathway, and revealed that the inhibitory effect of TFR on astrocytes proliferation could be blocked by aminooxyacetic acid (AOAA), an CBS inhibitor. Furthermore, TFR could ameliorate the mice cerebral I/R injury and the OGD/R-induced astrocytic damage.

CONCLUSION

These findings suggested that TFR could affect the transformation of astrocytes subtypes following cerebral I/R, which may be related to up-regulation of CBS-produced H₂S and subsequent inhibition of RhoA/ROCK pathway.

Roles of NG2 Glia in Cerebral Small Vessel Disease

Cerebral small vessel disease (CSVD) is one of the most prevalent pathologic processes affecting 5% of people over 50 years of age and contributing to 45% of dementia cases. Increasing evidence has demonstrated the pathological roles of chronic hypoperfusion, impaired cerebral vascular reactivity, and leakage of the blood-brain barrier in CSVD. However, the pathogenesis of CSVD remains elusive thus far, and no radical treatment has been developed. NG2 glia, also known as oligodendrocyte precursor cells, are the fourth type of glial cell in addition to astrocytes, microglia, and oligodendrocytes in the mammalian central nervous system. Many novel functions for NG2 glia in physiological and pathological states have recently been revealed. In this review, we discuss the role of NG2 glia in CSVD and the underlying mechanisms.

TREM2 mediates physical exercise-promoted neural functional recovery in rats with ischemic stroke via microglia-promoted white matter repair

BACKGROUND

The repair of white matter injury is of significant importance for functional recovery after ischemic stroke, and the up-regulation of triggering receptors expressed on myeloid cells 2 (TREM2) after ischemic stroke is neuroprotective and implicated in remyelination. However, the lack of effective therapies calls for the need to investigate the regenerative process of remyelination and the role of rehabilitation therapy. This study sought to investigate whether and how moderate physical exercise (PE) promotes oligodendrogenesis and remyelination in rats with transient middle cerebral artery occlusion (tMCAO).

METHODS

Male Sprague-Dawley rats (weighing 250-280 g) were subjected to tMCAO. AAV-shRNA was injected into the lateral ventricle to silence the Trem2 gene before the operation. The rats in the physical exercise group started electric running cage training at 48 h after the operation. The Morris water maze and novel object recognition test were used to evaluate cognitive function. Luxol fast blue staining, diffusion tensor imaging, and electron microscopy were used to observe myelin injury and repair. Immunofluorescence staining was applied to observe the proliferation and differentiation of oligodendrocyte precursor cells (OPCs). Expression of key molecules were detected using immunofluorescence staining, quantitative real-time polymerase chain reaction, Western blotting, and Enzyme-linked immunosorbent assay, respectively.

RESULTS

PE exerted neuroprotective efects by modulating microglial state, promoting remyelination and recovery of neurological function of rats over 35 d after stroke, while silencing Trem2 expression in rats suppressed the aforementioned effects promoted by PE. In addition, by leveraging the activin-A neutralizing antibody, we found a direct beneficial effect of PE on microglia-derived activin-A and its subsequent role on oligodendrocyte differentiation and remyelination mediated by the activin-A/Acvr axis.

CONCLUSIONS

The present study reveals a novel regenerative role of PE in white matter injury after stroke, which is mediated by upregulation of TREM2 and microglia-derived factor for oligodendrocytes regeneration. PE is an effective therapeutic approach for improving white matter integrity and alleviating neurological function deficits after ischemic stroke.

Transplantation of A2 type astrocytes promotes neural repair and remyelination after spinal cord injury

BACKGROUND

Limited progress in terms of an effective treatment for spinal cord injury (SCI) emphasizes the urgent need for novel therapies. As a vital central nervous system component, the resident astrocytes play crucial roles in regulating recovery after SCI. In this study, recovery after SCI was compared following the transplantation of either A1 or A2 astrocytes. A1 astrocytes are harmful as they upregulate the neurotoxic classical complement cascade genes. Conversely, A2 astrocytes are characterized as neuroprotective as they upregulate the production of many neurotrophic factors.

METHODS

We used different supernatant obtained from microglia stimulated with lipopolysaccharide or interleukin-4 to generate A1 and A2 astrocytes. We detected the influence of astrocytes on neurons by co-culturing A1 and A2 astrocytes with neurons. We transplanted astrocytes into the lesion site of the spinal cord and assessed lesion progression, neural restoration, glia formation and locomotor recovery.

RESULTS

Astrocytes were polarized into A1 and A2 phenotypes following culture in the supernatant obtained from microglia stimulated with lipopolysaccharide or interleukin-4, respectively. Furthermore, co-culturing A2 astrocytes with neurons significantly suppressed glutamate-induced neuronal apoptosis and promoted the degree of neuron arborization. Transplantation of these A2 astrocytes into the lesion site of the spinal cord of mice significantly improved motor function recovery, preserved spared supraspinal pathways, decreased glia scar deposition, and increased neurofilament formation at the site of injury compared to the transplantation of A1 astrocytes. Additionally, enhanced A2 astrocytes with potentially beneficial A2-like genes were also detected in the A2 group. Moreover, luxol fast blue staining and electron microscopy indicated increased preservation of myelin with organized structure after transplantation of A2 astrocytes than of A1 astrocytes.

CONCLUSIONS

A2 astrocyte transplantation could be a promising potential therapy for SCI. Video abstract.

Optogenetic Stimulation of mPFC Alleviates White Matter Injury-Related Cognitive Decline after Chronic Ischemia through Adaptive Myelination

White matter injury (WMI), which reflects myelin loss, contributes to cognitive decline or dementia caused by cerebral vascular diseases. However, because pharmacological agents specifically for WMI are lacking, novel therapeutic strategies need to be explored. It is recently found that adaptive myelination is required for homeostatic control of brain functions. In this study, adaptive myelination-related strategies are applied to explore the treatment for ischemic WMI-related cognitive dysfunction. Here, bilateral carotid artery stenosis (BCAS) is used to model ischemic WMI-related cognitive impairment and uncover that optogenetic and chemogenetic activation of glutamatergic neurons in the medial prefrontal cortex (mPFC) promote the differentiation of oligodendrocyte precursor cells (OPCs) in the corpus callosum, leading to improvements in myelin repair and working memory. Mechanistically, these neuromodulatory techniques exert a therapeutic effect by inducing the secretion of Wnt2 from activated neuronal axons, which acts on oligodendrocyte precursor cells and drives oligodendrogenesis and myelination. Thus, this study suggests that neuromodulation is a promising strategy for directing myelin repair and cognitive recovery through adaptive myelination in the context of ischemic WMI.

T0901317, a liver X receptor agonist, ameliorates perinatal white matter injury induced by ischemia and hypoxia in neonatal rats

Perinatal white matter injury (PWMI) can lead to permanent neurological damage in preterm infants and bring a huge economic burden to their families and society. Liver X receptors (LXRs) are transcription factors that have been confirmed to mediate the myelination process under physiological conditions and are involved in regulating neurogenesis in adult animal models of acute and chronic cerebral ischemia. However, the role of LXRs in PWMI induced by both ischemic and hypoxic stimulation in the immature brain has not been reported. Herein, we investigated the role of LXRs in a neonatal rat model of white matter loss after hypoxia-ischemia (HI) injury through intraperitoneal injection of the LXR agonist T0901317 (T09) 1 day before and 15 min postinjury. The in vivo data showed that T09 treatment significantly facilitated myelination and ameliorated neurological behavior after PWMI. Moreover, T09 enhanced the proliferation of oligodendrocyte lineage cells and reduced microgliosis and astrogliosis in the microenvironment for oligodendrocytes (OLs), maintaining a healthy microenvironment for myelinating OLs. In vitro data suggested that the expression of the myelin-related genes Plp and Cnpase was increased in OLN-93 cells after T09 intervention compared with OLN-93 cells injured by oxygen and glucose deprivation (OGD). In primary mixed astrocytes/microglia cells, T09 also reduced the expression of Il6, Cox2, Tnfa and Il10 that was induced by OGD. Mechanistically, the mRNA expression level and the protein level of ATP binding cassette subfamily A member 1 (Abca1) decreased after HI injury, and the protective effect of T09 might be related to the activation of the LXRβ-ABCA1 signaling pathway. Our study revealed the protective role of LXRs in myelination and white matter homeostasis, providing a potential therapeutic option for PWMI.

Maraviroc promotes recovery from traumatic brain injury in mice by suppression of neuroinflammation and activation of neurotoxic reactive astrocytes

Neuroinflammation and the NACHT, LRR, and PYD domains-containing protein 3 inflammasome play crucial roles in secondary tissue damage following an initial insult in patients with traumatic brain injury (TBI). Maraviroc, a C-C chemokine receptor type 5 antagonist, has been viewed as a new therapeutic strategy for many neuroinflammatory diseases. We studied the effect of maraviroc on TBI-induced neuroinflammation. A moderate-TBI mouse model was subjected to a controlled cortical impact device. Maraviroc or vehicle was injected intraperitoneally 1 hour after TBI and then once per day for 3 consecutive days. Western blot, immunohistochemistry, and TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling) analyses were performed to evaluate the molecular mechanisms of maraviroc at 3 days post-TBI. Our results suggest that maraviroc administration reduced NACHT, LRR, and PYD domains-containing protein 3 inflammasome activation, modulated microglial polarization from M1 to M2, decreased neutrophil and macrophage infiltration, and inhibited the release of inflammatory factors after TBI. Moreover, maraviroc treatment decreased the activation of neurotoxic reactive astrocytes, which, in turn, exacerbated neuronal cell death. Additionally, we confirmed the neuroprotective effect of maraviroc using the modified neurological severity score, rotarod test, Morris water maze test, and lesion volume measurements. In summary, our findings indicate that maraviroc might be a desirable pharmacotherapeutic strategy for TBI, and C-C chemokine receptor type 5 might be a promising pharmacotherapeutic target to improve recovery after TBI.

What type of cell death occurs in chronic cerebral hypoperfusion? A review focusing on pyroptosis and its potential therapeutic implications

Chronic cerebral hypoperfusion (CCH) is a major global disease with chronic cerebral blood flow reduction. It is also the main cause of cognitive impairment and neurodegenerative diseases. Pyroptosis, a novel form of cell death, is characterized by the rupture of the cell membrane and the release of pro-inflammatory mediators. In recent years, an increasing number of studies have identified the involvement of pyroptosis and its mediated inflammatory response in the pathological process of CCH. Therefore, preventing the activation of pyroptosis following CCH is beneficial to inhibit the inflammatory cascade and reduce brain injury. In this review, we discuss the research progress on the relationship between pyroptosis and CCH, in order to provide a reference for research in related fields.

Neurogenesis potential of oligodendrocyte precursor cells from oligospheres and injured spinal cord

Severe traumatic spinal cord injury (SCI) leads to long-lasting oligodendrocyte death and extensive demyelination in the lesion area. Oligodendrocyte progenitor cells (OPCs) are the reservoir of new mature oligodendrocytes during damaged myelin regeneration, which also have latent potential for neurogenic regeneration and oligospheres formation. Whether oligospheres derived OPCs can differentiate into neurons and the neurogenesis potential of OPCs after SCI remains unclear. In this study, primary OPCs cultures were used to generate oligospheres and detect the differentiation and neurogenesis potential of oligospheres. In vivo, SCI models of juvenile and adult mice were constructed. Combining the single-cell RNA sequencing (scRNA-seq), bulk RNA sequencing (RNA-seq), bioinformatics analysis, immunofluorescence staining, and molecular experiment, we investigated the neurogenesis potential and mechanisms of OPCs in vitro and vivo. We found that OPCs differentiation and oligodendrocyte morphology were significantly different between brain and spinal cord. Intriguingly, we identify a previously undescribed findings that OPCs were involved in oligospheres formation which could further differentiate into neuron-like cells. We also firstly detected the intermediate states of oligodendrocytes and neurons during oligospheres differentiation. Furthermore, we found that OPCs were significantly activated after SCI. Combining scRNA-seq and bulk RNA-seq data from injured spinal cord, we confirmed the neurogenesis potential of OPCs and the activation of endoplasmic reticulum stress after SCI. Inhibition of endoplasmic reticulum stress could effectively attenuate OPCs death. Additionally, we also found that endoplasmic reticulum may regulate the stemness and differentiation of oligospheres. These findings revealed the neurogenesis potential of OPCs from oligospheres and injured spinal cord, which may provide a new source and a potential target for spinal cord repair.

From Low-Grade Inflammation in Osteoarthritis to Neuropsychiatric Sequelae: A Narrative Review

Nowadays, osteoarthritis (OA), a common, multifactorial musculoskeletal disease, is considered to have a low-grade inflammatory pathogenetic component. Lately, neuropsychiatric sequelae of the disease have gained recognition. However, a link between the peripheral inflammatory process of OA and the development of neuropsychiatric pathology is not completely understood. In this review, we provide a narrative that explores the development of neuropsychiatric disease in the presence of chronic peripheral low-grade inflammation with a focus on its signaling to the brain. We describe the development of a pro-inflammatory environment in the OA-affected joint. We discuss inflammation-signaling pathways that link the affected joint to the central nervous system, mainly using primary sensory afferents and blood circulation via circumventricular organs and cerebral endothelium. The review describes molecular and cellular changes in the brain, recognized in the presence of chronic peripheral inflammation. In addition, changes in the volume of gray matter and alterations of connectivity important for the assessment of the efficacy of treatment in OA are discussed in the given review. Finally, the narrative considers the importance of the use of neuropsychiatric diagnostic tools for a disease with an inflammatory component in the clinical setting.

Depletion of regulatory T cells exacerbates inflammatory responses after chronic cerebral hypoperfusion in mice

Vascular cognitive impairment is the second most common cause of dementia which can be induced by chronic cerebral hypoperfusion. Regulatory T cells (Tregs) have been proven to provide beneficial effects in several central nervous system (CNS) diseases, but the roles of Tregs in chronic cerebral hypoperfusion-induced white matter damage have not been explored. In this study, Foxp3-diphtheria toxin receptor (DTR) mice treated with diphtheria toxin (DT) and wild type C57BL/6 mice treated with anti-CD25 antibody were subjected to bilateral carotid artery stenosis (BCAS). Flow cytometry analysis showed Tregs were widely distributed in spleen whereas barely distributed in brain under normal conditions. The distribution of lymphocytes and Tregs did not change significantly in spleen and brain after BCAS. Depletion of Tregs decreased the numbers of mature oligodendrocytes and anti-inflammatory microglia at 14 days and 28 days following BCAS. And pro-inflammatory cytokines including tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6) and interferon-γ (IFN-γ) showed higher expression after Tregs depletion. In contrast, Tregs depletion did not change the overall severity of white matter injury as shown by the expression of myelin-associated glycoprotein (MAG), myelin basic protein (MBP), luxol fast blue (LFB) staining and electron microscopy assay. Moreover, Tregs depletion had marginal effect on cognition defects after BCAS revealed by Morris water maze and novel object recognition examination at 28 days after BCAS. In summary, our results suggest an anti-inflammatory role of Tregs with marginal effects on white matter damage in mice after BCAS-induced chronic cerebral hypoperfusion.

NLRP3 inflammasome regulates astrocyte transformation in brain injury induced by chronic intermittent hypoxia

BACKGROUND

Obstructive sleep apnea (OSA) is mainly characterized by sleep fragmentation and chronic intermittent hypoxia (CIH), the latter one being associated with multiple organ injury. Recently, OSA-induced cognition dysfunction has received extensive attention from scholars. Astrocytes are essential in neurocognitive deficits via A1/A2 phenotypic changes. Nucleotide oligomerization domain (NOD)-like receptor protein 3 (NLRP3) inflammasome is considered the most important factor inducing and maintaining neuroinflammation. However, whether the NLRP3 regulates the A1/A2 transformation of astrocytes in CIH-related brain injury remains unclear.

METHODS

We constructed an OSA-related CIH animal model and assessed the rats' learning ability in the Morris water maze; the histopathological assessment was performed by HE and Nissl staining. The expression of GFAP (astrocyte marker), C3d (A1-type astrocyte marker), and S100a10 (A2-type astrocyte marker) were detected by immunohistochemistry and immunofluorescence. Western blotting and RT-qPCR were used to evaluate the changes of A1/A2 astrocyte-related protein and NLRP3/Caspase-1/ASC/IL-1β.

RESULTS

The learning ability of rats decreased under CIH. Further pathological examination revealed that the neurocyte in the hippocampus were damaged. The cell nuclei were fragmented and dissolved, and Nissl bodies were reduced. Immunohistochemistry showed that astrocytes were activated, and morphology and number of astrocytes changed. Immunofluorescence, Western blotting and RT-qPCR showed that the expression of C3d was increased while S100a10 was decreased. Also, the expression of the inflammasome (NLRP3/Caspase-1/ASC/IL-1β) was increased. After treatment of MCC950 (a small molecule inhibitor of NLRP3), the damage of nerve cells was alleviated, the Nissl bodies increased, the activation of astrocytes was reduced, and the expression of A2-type astrocytes was increased. In contrast, A1-type astrocytes decreased, and the expression of inflammasome NLRP3/Caspase-1/ASC/IL-1β pathway-related proteins decreased.

CONCLUSION

The NLRP3 inflammasome could regulate the A1/A2 transformation of astrocytes in brain injury induced by CIH.

Reactive astrocytes transduce inflammation in a blood-brain barrier model through a TNF-STAT3 signaling axis and secretion of alpha 1-antichymotrypsin

Astrocytes are critical components of the neurovascular unit that support blood-brain barrier (BBB) function. Pathological transformation of astrocytes to reactive states can be protective or harmful to BBB function. Here, using a human induced pluripotent stem cell (iPSC)-derived BBB co-culture model, we show that tumor necrosis factor (TNF) transitions astrocytes to an inflammatory reactive state that causes BBB dysfunction through activation of STAT3 and increased expression of SERPINA3, which encodes alpha 1-antichymotrypsin (α1ACT). To contextualize these findings, we correlated astrocytic STAT3 activation to vascular inflammation in postmortem human tissue. Further, in murine brain organotypic cultures, astrocyte-specific silencing of Serpina3n reduced vascular inflammation after TNF challenge. Last, treatment with recombinant Serpina3n in both ex vivo explant cultures and in vivo was sufficient to induce BBB dysfunction-related molecular changes. Overall, our results define the TNF-STAT3-α1ACT signaling axis as a driver of an inflammatory reactive astrocyte signature that contributes to BBB dysfunction.

Recent insights into astrocytes as therapeutic targets for demyelinating diseases

Astrocytes are a group of glial cells that exhibit great morphological, transcriptional and functional diversity both in the resting brain and in response to injury. In recent years, astrocytes have attracted increasing interest as therapeutic targets for demyelinating diseases. Following a demyelinating insult, astrocytes can adopt a wide spectrum of reactive states, which can exacerbate damage, but may also facilitate oligodendrocyte progenitor cell differentiation and myelin regeneration. In this review, we provide an overview of recent literature on astrocyte-oligodendrocyte interactions in the context of demyelinating diseases. We highlight novel key roles for astrocytes both during demyelination and remyelination with a focus on potential therapeutic strategies to favor a pro-regenerative astrocyte response in (progressive) multiple sclerosis.

Crosstalk of Astrocytes and Other Cells during Ischemic Stroke

Stroke is a leading cause of death and long-term disability worldwide. Astrocytes structurally compose tripartite synapses, blood–brain barrier, and the neurovascular unit and perform multiple functions through cell-to-cell signaling of neurons, glial cells, and vasculature. The crosstalk of astrocytes and other cells is complicated and incompletely understood. Here we review the role of astrocytes in response to ischemic stroke, both beneficial and detrimental, from a cell–cell interaction perspective. Reactive astrocytes provide neuroprotection through antioxidation and antiexcitatory effects and metabolic support; they also contribute to neurorestoration involving neurogenesis, synaptogenesis, angiogenesis, and oligodendrogenesis by crosstalk with stem cells and cell lineage. In the meantime, reactive astrocytes also play a vital role in neuroinflammation and brain edema. Glial scar formation in the chronic phase hinders functional recovery. We further discuss astrocyte enriched microRNAs and exosomes in the regulation of ischemic stroke. In addition, the latest notion of reactive astrocyte subsets and astrocytic activity revealed by optogenetics is mentioned. This review discusses the current understanding of the intimate molecular conversation between astrocytes and other cells and outlines its potential implications after ischemic stroke. “Neurocentric” strategies may not be sufficient for neurological protection and recovery; future therapeutic strategies could target reactive astrocytes.

Effect of rottlerin on astrocyte phenotype polarization after trimethyltin insult in the dentate gyrus of mice

BACKGROUND

It has been demonstrated that reactive astrocytes can be polarized into pro-inflammatory A1 phenotype or anti-inflammatory A2 phenotype under neurotoxic and neurodegenerative conditions. Microglia have been suggested to play a critical role in astrocyte phenotype polarization by releasing pro- and anti-inflammatory mediators. In this study, we examined whether trimethyltin (TMT) insult can induce astrocyte polarization in the dentate gyrus of mice, and whether protein kinase Cδ (PKCδ) plays a role in TMT-induced astrocyte phenotype polarization.

METHODS

Male C57BL/6 N mice received TMT (2.6 mg/kg, i.p.), and temporal changes in the mRNA expression of A1 and A2 phenotype markers were evaluated in the hippocampus. In addition, temporal and spatial changes in the protein expression of C3, S100A10, Iba-1, and p-PKCδ were examined in the dentate gyrus. Rottlerin (5 mg/kg, i.p. × 5 at 12-h intervals) was administered 3-5 days after TMT treatment, and the expression of A1 and A2 transcripts, p-PKCδ, Iba-1, C3, S100A10, and C1q was evaluated 6 days after TMT treatment.

RESULTS

TMT treatment significantly increased the mRNA expression of A1 and A2 phenotype markers, and the increased expression of A1 markers remained longer than that of A2 markers. The immunoreactivity of the representative A1 phenotype marker, C3 and A2 phenotype marker, S100A10 peaked 6 days after TMT insult in the dentate gyrus. While C3 was expressed evenly throughout the dentate gyrus, S100A10 was highly expressed in the hilus and inner molecular layer. In addition, TMT insult induced microglial p-PKCδ expression. Treatment with rottlerin, a PKCδ inhibitor, decreased Iba-1 and C3 expression, but did not affect S100A10 expression, suggesting that PKCδ inhibition attenuates microglial activation and A1 astrocyte phenotype polarization. Consistently, rottlerin significantly reduced the expression of C1q and tumor necrosis factor-α (TNFα), which has been suggested to be released by activated microglia and induce A1 astrocyte polarization.

CONCLUSION

We demonstrated the temporal and spatial profiles of astrocyte polarization after TMT insult in the dentate gyrus of mice. Taken together, our results suggest that PKCδ plays a role in inducing A1 astrocyte polarization by promoting microglial activation and consequently increasing the expression of pro-inflammatory mediators after TMT insult.

Physical Exercise-Induced Astrocytic Neuroprotection and Cognitive Improvement Through Primary Cilia and Mitogen-Activated Protein Kinases Pathway in Rats With Chronic Cerebral Hypoperfusion

Chronic cerebral hypoperfusion (CCH) is closely related to vascular cognitive impairment and dementia (VCID) and Alzheimer’s disease (AD). The neuroinflammation involving astrocytes is an important pathogenic mechanism. Along with the advancement of the concept and technology of astrocytic biology, the astrocytes have been increasingly regarded as the key contributors to neurological diseases. It is well known that physical exercise can improve cognitive function. As a safe and effective non-drug treatment, physical exercise has attracted continuous interests in neurological research. In this study, we explored the effects of physical exercise on the response of reactive astrocytes, and its role and mechanism in CCH-induced cognitive impairment. A rat CCH model was established by 2 vessel occlusion (2VO) and the wheel running exercise was used as the intervention. The cognitive function of rats was evaluated by morris water maze and novel object recognition test. The phenotypic polarization and the primary cilia expression of astrocytes were detected by immunofluorescence staining. The activation of MAPKs cascades, including ERK, JNK, and P38 signaling pathways, were detected by western blot. The results showed that physical exercise improved cognitive function of rats 2 months after 2VO, reduced the number of C3/GFAP-positive neurotoxic astrocytes, promoted the expression of S100A10/GFAP-positive neuroprotective astrocytes, and enhanced primary ciliogenesis. Additionally, physical exercise also alleviated the phosphorylation of ERK and JNK proteins induced by CCH. These results indicate that physical exercise can improve the cognitive function of rats with CCH possible by promoting primary ciliogenesis and neuroprotective function of astrocytes. The MAPKs signaling cascade, especially ERK and JNK signaling pathways may be involved in this process.

Reactive Astrocytes Derived From Human Induced Pluripotent Stem Cells Suppress Oligodendrocyte Precursor Cell Differentiation

Astrocytes are instrumental in maintaining central nervous system (CNS) homeostasis and responding to injury. A major limitation of studying neurodegenerative diseases like multiple sclerosis (MS) is lack of human pathological specimens obtained during the acute stages, thereby relegating research to post-mortem specimens obtained years after the initiation of pathology. Rodent reactive astrocytes have been shown to be cytotoxic to neurons and oligodendrocytes but may differ from human cells, especially in diseases with genetic susceptibility. Herein, we purified human CD49f+ astrocytes from induced pluripotent stem cells derived from individual patient and control peripheral leukocytes. We compared TNF and IL1α stimulated human reactive astrocytes from seven persons with MS and six non-MS controls and show their transcriptomes are remarkably similar to those described in rodents. The functional effect of astrocyte conditioned media (ACM) was examined in a human oligodendrocyte precursor cell (OPC) line differentiation assay. ACM was not cytotoxic to the OPCs but robustly inhibited the myelin basic protein (MBP) reporter. No differences were seen between MS and control stimulated astrocytes at either the transcript level or in ACM mediated OPC suppression assays. We next used RNAseq to interrogate differentially expressed genes in the OPC lines that had suppressed differentiation from the human ACM. Remarkably, not only was OPC differentiation and myelin gene expression suppressed, but we observed induction of several immune pathways in OPCs exposed to the ACM. These data support the notion that reactive astrocytes can inhibit OPC differentiation thereby limiting their remyelination capacity, and that OPCs take on an immune profile in the context of inflammatory cues.

Reactive Astrocytes in Central Nervous System Injury: Subgroup and Potential Therapy

Traumatic central nervous system (CNS) injury, which includes both traumatic brain injury (TBI) and spinal cord injury (SCI), is associated with irreversible loss of neurological function and high medical care costs. Currently, no effective treatment exists to improve the prognosis of patients. Astrocytes comprise the largest population of glial cells in the CNS and, with the advancements in the field of neurology, are increasingly recognized as having key functions in both the brain and the spinal cord. When stimulated by disease or injury, astrocytes become activated and undergo a series of changes, including alterations in gene expression, hypertrophy, the loss of inherent functions, and the acquisition of new ones. Studies have shown that astrocytes are highly heterogeneous with respect to their gene expression profiles, and this heterogeneity accounts for their observed context-dependent phenotypic diversity. In the inured CNS, activated astrocytes play a dual role both as regulators of neuroinflammation and in scar formation. Identifying the subpopulations of reactive astrocytes that exert beneficial or harmful effects will aid in deciphering the pathological mechanisms underlying CNS injuries and ultimately provide a theoretical basis for the development of effective strategies for the treatment of associated conditions. Following CNS injury, as the disease progresses, astrocyte phenotypes undergo continuous changes. Although current research methods do not allow a comprehensive and accurate classification of astrocyte subpopulations in complex pathological contexts, they can nonetheless aid in understanding the roles of astrocytes in disease. In this review, after a brief introduction to the pathology of CNS injury, we summarize current knowledge regarding astrocyte activation following CNS injury, including: (a) the regulatory factors involved in this process; (b) the functions of different astrocyte subgroups based on the existing classification of astrocytes; and (c) attempts at astrocyte-targeted therapy.

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

Astrocytes control many processes of the nervous system in health and disease, and respond to injury quickly. Astrocytes produce neuroprotective factors in the injured brain to clear cellular debris and to orchestrate neurorestorative processes that are beneficial for neurological recovery after traumatic brain injury (TBI). However, astrocytes also become dysregulated and produce cytotoxic mediators that hinder CNS repair by induction of neuronal dysfunction and cell death. Hence, we discuss the potential role of astrocytes in neuropathological processes such as neuroinflammation, neurogenesis, synaptogenesis and blood-brain barrier repair after TBI. Thus, an improved understanding of the dual role of astrocytes may advance our knowledge of post-brain injury recovery, and provide opportunities for the development of novel therapeutic strategies for TBI.

Melatonin attenuates reactive astrogliosis and glial scar formation following cerebral ischemia and reperfusion injury mediated by GSK-3β and RIP1K

Even though astrocytes have been widely reported to support several brain functions, studies have emerged that they exert deleterious effects on the brain after ischemia and reperfusion (I/R) injury. The present study investigated the neuroprotective effects of melatonin on the processes of reactive astrogliosis and glial scar formation, as well as axonal regeneration after transient middle cerebral artery occlusion. Male Wistar rats were randomly divided into four groups: sham-operated, I/R, I/R treated with melatonin, and I/R treated with edaravone. All drugs were administered via intraperitoneal injection at the onset of reperfusion and were continued until the rats were sacrificed on Day 7 or 14 after the surgery. Melatonin presented long-term benefits on cerebral damage after I/R injury, as demonstrated by a decreased infarct volume, histopathological changes, and reduced neuronal cell death. We also found that melatonin attenuated reactive astrogliosis and glial scar formation and, consequently, enhanced axonal regeneration and promoted neurobehavioral recovery. Furthermore, glycogen synthase kinase-3 beta (GSK-3β) and receptor-interacting serine/threonine-protein 1 kinase (RIP1K), which had previously been revealed as proteins involved in astrocyte responses, were significantly reduced after melatonin administration. Taken together, melatonin effectively counteracted the deleterious effects due to astrocyte responses and improved axonal regeneration to promote functional recovery during the chronic phase of cerebral I/R injury by inhibiting GSK-3β and RIP1K activities.