Astrocytes, reactive astrogliosis, and glial scar formation in traumatic brain injury

Traumatic brain injury is a global health crisis,causing significant death and disability worldwide.Neuroinflammation that follows traumatic brain injury has serious consequences for neuronal survival and cognitive impairments,with astrocytes involved in this response.Following traumatic brain injury,astrocytes rapidly become reactive,and astrogliosis propagates from the injury core to distant brain regions.Homeostatic astroglial proteins are downregulated near the traumatic brain injury core,while pro-inflammatory astroglial genes are overexpressed.This altered gene expression is considered a pathological remodeling of astrocytes that produces serious consequences for neuronal survival and cognitive recovery.In addition,glial scar formed by reactive astrocytes is initially necessary to limit immune cell infiltration,but in the long term impedes axonal reconnection and functional recovery.Current therapeutic strategies for traumatic brain injury are focused on preventing acute complications.Statins,cannabinoids,progesterone,beta-blockers,and cerebrolysin demonstrate neuroprotective benefits but most of them have not been studied in the context of astrocytes.In this review,we discuss the cell signaling pathways activated in reactive astrocytes following traumatic brain injury and we discuss some of the potential new strategies aimed to modulate astroglial responses in traumatic brain injury,especially using cell-targeted strategies with miRNAs or lncRNA,viral vectors,and repurposed drugs.

Temporal dynamics of microglia-astrocyte interaction in neuroprotective glial scar formation after intracerebral hemorrhage

The role of glial scar after intracerebral hemorrhage(ICH)remains unclear.This study aimed to investigate whether microglia-astrocyte interaction affects glial scar formation and explore the specific function of glial scar.We used a pharmacologic approach to induce microglial depletion during different ICH stages and examine how ablating microglia affects astrocytic scar formation.Spatial transcriptomics(ST)analysis was performed to explore the potential ligand-receptor pair in the modulation of microglia-astrocyte interaction and to verify the functional changes of astrocytic scars at different periods.During the early stage,sustained microglial depletion induced disorganized astrocytic scar,enhanced neutrophil infiltration,and impaired tissue repair.ST analysis indicated that microglia-derived insulin like growth factor 1(IGF1)modulated astrocytic scar formation via mechanistic target of rapamycin(mTOR)signaling activation.Moreover,repopulating microglia(RM)more strongly activated mTOR signaling,facilitating a more protective scar formation.The combination of IGF1 and osteopontin(OPN)was necessary and sufficient for RM function,rather than IGF1 or OPN alone.At the chronic stage of ICH,the overall net effect of astrocytic scar changed from protective to destructive and delayed microglial depletion could partly reverse this.The vital insight gleaned from our data is that sustained microglial depletion may not be a reasonable treatment strategy for early-stage ICH.Inversely,early-stage IGF1/OPN treatment combined with late-stage PLX3397 treatment is a promising therapeutic strategy.This prompts us to consider the complex temporal dynamics and overall net effect of microglia and astrocytes,and develop elaborate treatment strategies at precise time points after ICH.

Dual-targeting AAV9P1-mediated neuronal reprogramming in a mouse model of traumatic brain injury

Traumatic brain injury results in neuronal loss and glial scar formation.Replenishing neurons and eliminating the consequences of glial scar formation are essential for treating traumatic brain injury.Neuronal reprogramming is a promising strategy to convert glial scars to neural tissue.However,previous studies have reported inconsistent results.In this study,an AAV9P1 vector incorporating an astrocyte-targeting P1 peptide and glial fibrillary acidic protein promoter was used to achieve dual-targeting of astrocytes and the glial scar while minimizing off-target effects.The results demonstrate that AAV9P1 provides high selectivity of astrocytes and reactive astrocytes.Moreover,neuronal reprogramming was induced by downregulating the polypyrimidine tract-binding protein 1 gene via systemic administration of AAV9P1 in a mouse model of traumatic brain injury.In summary,this approach provides an improved gene delivery vehicle to study neuronal programming and evidence of its applications for traumatic brain injury.

Glial cells in intracerebral transplantation for Parkinson’s disease

In the last few decades,intracerebral transplantation has grown from a dubious neuroscientific topic to a plausible modality for treatment of neurological disorders.The possibility for cell replacement opens a new field of perspectives in the therapy of neurodegenerative disorders,ischemia,and neurotrauma,with the most lessons learned from intracerebral transplantation in Parkinson's disease.Multiple animal studies and a few small-scale clinical trials have proven the concept of intracerebral grafting,but still have to provide a uniform and highly efficient approach to the procedure,suitable for clinical application.The success of intracerebral transplantation is highly dependent on the integration of the grafted cells with the host brain.In this process,glial cells are clearly more than passive bystanders.They provide transplanted cells with mechanical support,trophics,mediate synapse formation,and participate in graft vascularization.At the same time,glial cells mediate scarring,graft rejection,and neuroinflammation,which can be detrimental.We can use this information to try to understand the mechanisms behind the glial reaction to intracerebral transplantation.Recognizing and utilizing glial reactivity can move translational research forward and provide an insight not only to post-transplantation events but also to mechanisms of neuronal death and degeneration.Knowledge about glial reactivity to transplanted cells could also be a key for optimization of transplantation protocols,which ultimately should contribute to greater patient benefit.

Chondroitinase ABC combined with Schwann cell transplantation enhances restoration of neural connection and functional recovery following acute and chronic spinal cord injury

Schwann cell transplantation is considered one of the most promising cell-based therapy to repair injured spinal cord due to its unique growth-promoting and myelin-forming properties.A the Food and Drug Administration-approved Phase I clinical trial has been conducted to evaluate the safety of transplanted human autologous Schwann cells to treat patients with spinal cord injury.A major challenge for Schwann cell transplantation is that grafted Schwann cells are confined within the lesion cavity,and they do not migrate into the host environment due to the inhibitory barrier formed by injury-induced glial scar,thus limiting axonal reentry into the host spinal cord.Here we introduce a combinatorial strategy by suppressing the inhibitory extracellular environment with injection of lentivirus-mediated transfection of chondroitinase ABC gene at the rostral and caudal borders of the lesion site and simultaneously leveraging the repair capacity of transplanted Schwann cells in adult rats following a mid-thoracic contusive spinal cord injury.We report that when the glial scar was degraded by chondroitinase ABC at the rostral and caudal lesion borders,Schwann cells migrated for considerable distances in both rostral and caudal directions.Such Schwann cell migration led to enhanced axonal regrowth,including the serotonergic and dopaminergic axons originating from supraspinal regions,and promoted recovery of locomotor and urinary bladder functions.Importantly,the Schwann cell survival and axonal regrowth persisted up to 6 months after the injury,even when treatment was delayed for 3 months to mimic chronic spinal cord injury.These findings collectively show promising evidence for a combinatorial strategy with chondroitinase ABC and Schwann cells in promoting remodeling and recovery of function following spinal cord injury.

Downregulation of EphB2 by RNA interference attenuates glial/fibrotic scar formation and promotes axon growth

The rapid formation of a glial/fibrotic scar is one of the main factors hampering axon growth after spinal cord injury. The bidirectional Eph B2/ephrin-B2 signaling of the fibroblast-astrocyte contact-dependent interaction is a trigger for glial/fibrotic scar formation. In the present study, a new in vitro model was produced by coculture of fibroblasts and astrocytes wounded by scratching to mimic glial/fibrotic scar-like structures using an improved slide system. After treatment with RNAi to downregulate Eph B2, changes in glial/fibrotic scar formation and the growth of VSC4.1 motoneuron axons were examined. Following RNAi treatment, fibroblasts and astrocytes dispersed without forming a glial/fibrotic scar-like structure. Furthermore, the expression levels of neurocan, NG2 and collagen I in the coculture were reduced, and the growth of VSC4.1 motoneuron axons was enhanced. These findings suggest that suppression of Eph B2 expression by RNAi attenuates the formation of a glial/fibrotic scar and promotes axon growth. This study was approved by the Laboratory Animal Ethics Committee of Jiangsu Province, China(approval No. 2019-0506-002) on May 6, 2019.

Glial scar size, inhibitor concentration, and growth of regenerating axons after spinal cord transection

A mathematical model has been formulated in accordance with cell chemotaxis and relevant experimental data. A three-dimensional lattice Boltzmann method was used for numerical simulation. The present study observed the effects of glial scar size and inhibitor concentration on regenerative axonal growth following spinal cord transection. The simulation test comprised two parts： （1） when release rates of growth inhibitor and promoter were constant, the effects of glial scar size on axonal growth rate were analyzed, and concentrations of inhibitor and promoters located at the moving growth cones were recorded. （2） When the glial scar size was constant, the effects of inhibitor and promoter release rates on axonal growth rate were analyzed, and inhibitor and promoter concentrations at the moving growth cones were recorded. Results demonstrated that （1） a larger glial scar and a higher release rate of inhibitor resulted in a reduced axonal growth rate. （2） The axonal growth rate depended on the ratio of inhibitor to promoter concentrations at the growth cones. When the average ratio was 〈 1.5, regenerating axons were able to grow and successfully contact target cells.

Effect of glial cells on remyelination after spinal cord injury

Remyelination plays a key role in functional recovery of axons after spinal cord injury.Glial cells are the most abundant cells in the central nervous system.When spinal cord injury occurs,many glial cells at the lesion site are immediately activated,and different cells differentially affect inflammatory reactions after injury.In this review,we aim to discuss the core role of oligodendrocyte precursor cells and crosstalk with the rest of glia and their subcategories in the remyelination process.Activated astrocytes influence proliferation,differentiation,and maturation of oligodendrocyte precursor cells,while activated microglia alter remyelination by regulating the inflammatory reaction after spinal cord injury.Understanding the interaction between oligodendrocyte precursor cells and the rest of glia is necessary when designing a therapeutic plan of remyelination after spinal cord injury.

X-irradiation for inhibiting glial scar formation in injured spinal cord

X-irradiation has a beneficial effect in treating spinal cord injury. We supposed that X-irradiation could improve the microenvironment at the site of a spinal cord injury and inhibit glial scar formation. Thus, this study was designed to observe the effects of 8 Gy X-irradiation on the injury site at 6 hours and 2, 4, 7, and 14 days post injury, in terms of improvement in the microenvironment and hind limb motor function. Immunohistochemistry showed that the expression of macrophage marker ED-1 and the area with glial scar formation were reduced. In addition, the Basso, Beattie and Bresnahan score was higher at 7 days post injury relative to the other time points post injury. Results indicated that X-irradiation at a dose of 8 Gy can inhibit glial scar formation and alleviate the inflammatory reaction, thereby repairing spinal cord injury. X-irradiation at 7 days post spinal cord injury may be the best time window.

Anti-inflammatory properties of the glial scar

Glial cells comprise -90% of the human brain and are divided into two subtypes： microglia and astrocytes. Astrocytes play a vital role in maintaining central nervous system （CNS） homeostasis, regulating ion concentrations and providing metabolic support for neighboring neurons, stabilizing synapses, supporting the neurovascular system including maintenance of the blood-brain barrier （BBB） and also producing the extracellular matrix （ECM）.

Effects of electroacupuncture combined with hydrogel on the formation and changes in the glial scar in rats with spinal cord injury

Objective: To observe the effect of electroacupuncture(EA) combined with oriented conductive bioprotein hydrogel(OCBH) on the recovery of nerve function in rats with complete spinal cord injury(SCI)and to explore its effect and mechanism on the formation and changes of glial scars.Methods: A total of 72 female Sprague-Dawley rats were randomly divided into groups according to the treatment received. A rat model of complete SCI was constructed using a spinal cord transection.Behavioral assessments, hematoxylin-eosin(H&E) staining, immunofluorescence staining, and Western blotting were performed at a fixed period after the operation.Results: The material group and the material + EA group obtained better results in the behavioral assessments(all P <.05) and the H&E staining. In the immunofluorescence staining and Western blotting,the GFAP protein was expressed more and denser in the material group and the material + EA group than in the model group, and the density of the GFAP expression in the material + EA group was lower at week 12 than in the material group(all P <.05). The expression of complement C3 in the model, material,and material + EA groups decreased in turn. Some inflammatory factors and the NF-κB signaling pathway showed similar results in the Western blotting(all P <.05). The expression of the GDNF protein in the material + EA group was significantly higher than that in the model group and the material group(both P <.01).Conclusion: EA combined with OCBH can promote the recovery of motor functions after SCI by facilitating the formation of glial scars in the early stage, preventing the further spread of an inflammatory response that would affect the activation of A1/A2 astrocytes and change the morphology of glial scars at the spinal cord-material interface in its late stage.

Telomerase expression in the glial scar of rats with spinal cord injury

A rat model of spinal cord injury was established using the weight drop method. A cavity formed 14 days following spinal cord injury, and compact scar tissue formed by 56 days. Enzyme-linked immunosorbent assay and polymerase chain reaction enzyme-linked immunosorbent assay results demonstrated that glial fibrillary acidic protein and telomerase expression increased gradually after injury, peaked at 28 days, and then gradually decreased. Spearman rank correlation showed a positive correlation between glial fibrillary acidic protein expression and telomerase expression in the glial scar. These results suggest that telomerase promotes glial scar formation.

Neuroprotective role of Noggin in spinal cord injury

Spinal cord injury is one of the leading causes of morbidity and mortality among young adults in many countries including the United States.Difficulty in the regeneration of neurons is one of the main obstacles that leave spinal cord injury patients with permanent paralysis in most instances.Recent research has found that preventing acute and subacute secondary cellular damages to the neurons and supporting glial cells can help slow the progression of spinal cord injury pathogenesis,in part by reactivating endogenous regenerative proteins including Noggin that are normally present during spinal cord development.Noggin is a complex protein and natural inhibitor of the multifunctional bone morphogenetic proteins,and its expression is high during spinal cord development and after induction of spinal cord injury.In this review article,we first discuss the change in expression of Noggin during pathogenesis in spinal cord injury.Second,we discuss the current research knowledge about the neuroprotective role of Noggin in preclinical models of spinal cord injury.Lastly,we explain the gap in the knowledge for the use of Noggin in the treatment of spinal cord injury.The results from extensive in vitro and in vivo research have revealed that the therapeutic efficacy of Noggin treatment remains debatable due to its neuroprotective effects observed only in early phases of spinal cord injury but little to no effect on altering pathogenesis and functional recovery observed in the chronic phase of spinal cord injury.Furthermore,clinical information regarding the role of Noggin in the alleviation of progression of pathogenesis,its therapeutic efficacy,bioavailability,and safety in human spinal cord injury is still lacking and therefore needs further investigation.

Neuroprotective effects of meloxicam on transient brain ischemia in rats:the two faces of anti-inflammatory treatments

The inflammato ry response plays an important role in neuroprotection and regeneration after ischemic insult.The use of non-ste roidal anti-inflammatory drugs has been a matter of debate as to whether they have beneficial or detrimental effects.In this context,the effects of the anti-inflammatory agent meloxicam have been scarcely documented after stro ke,but its ability to inhibit both cyclooxygenase isoforms(1 and 2) could be a promising strategy to modulate postischemic inflammation.This study analyzed the effect of meloxicam in a transient focal cerebral ischemia model in rats,measuring its neuroprotective effect after 48 hours and 7 days of reperfusion and the effects of the treatment on the glial scar and regenerative events such as the generation of new progenitors in the subventricular zone and axonal sprouting at the edge of the damaged area.We show that meloxicam’s neuroprotective effects remained after 7 days of reperfusion even if its administration was restricted to the two first days after ischemia.Moreover,meloxicam treatment modulated glial scar reactivity,which matched with an increase in axonal sprouting.However,this treatment decreased the formation of neuronal progenitor cells.This study discusses the dual role of anti-inflammatory treatments after stro ke and encourages the careful analysis of both the neuroprotective and the regenerative effects in preclinical studies.

Vimentin as a potential target for diverse nervous system diseases

Vimentin is a major type Ⅲ intermediate filament protein that plays important roles in several basic cellular functions including cell migration, proliferation, and division. Although vimentin is a cytoplasmic protein, it also exists in the extracellular matrix and at the cell surface. Previous studies have shown that vimentin may exert multiple physiological effects in different nervous system injuries and diseases. For example, the studies of vimentin in spinal cord injury and stroke mainly focus on the formation of reactive astrocytes. Reduced glial scar, increased axonal regeneration, and improved motor function have been noted after spinal cord injury in vimentin and glial fibrillary acidic protein knockout(GFAPVIM) mice. However, attenuated glial scar formation in post-stroke in GFAP–/– VIM–/– mice resulted in abnormal neuronal network restoration and worse neurological recovery. These opposite results have been attributed to the multiple roles of glial scar in different temporal and spatial conditions. In addition, extracellular vimentin may be a neurotrophic factor that promotes axonal extension by interaction with the insulin-like growth factor 1 receptor. In the pathogenesis of bacterial meningitis, cell surface vimentin is a meningitis facilitator, acting as a receptor of multiple pathogenic bacteria, including E. coli K1, Listeria monocytogenes, and group B streptococcus. Compared with wild type mice, VIMmice are less susceptible to bacterial infection and exhibit a reduced inflammatory response, suggesting that vimentin is necessary to induce the pathogenesis of meningitis. Recently published literature showed that vimentin serves as a double-edged sword in the nervous system, regulating axonal regrowth, myelination, apoptosis, and neuroinflammation. This review aims to provide an overview of vimentin in spinal cord injury, stroke, bacterial meningitis, gliomas, and peripheral nerve injury and to discuss the potential therapeutic methods involving vimentin manipulation in improving axonal regeneration, alleviating infection, inhibiting brain tumor progression, and enhancing nerve myelination.

The role of monocytes in optic nerve injury

Monocytes,including monocyte-derived macrophages and resident microglia,mediate many phases of optic nerve injury pathogenesis.Resident microglia respond first,followed by infiltrating macrophages which regulate neuronal inflammation,cell proliferation and differentiation,scar formation and tissue remodeling following optic nerve injury.However,microglia and macrophages have distinct functions which can be either beneficial or detrimental to the optic nerve depending on the spatial context and temporal sequence of their activity.These divergent effects are attributed to pro-and anti-inflammatory cytokines expressed by monocytes,crosstalk between monocyte and glial cells and even microglia-macrophage communication.In this review,we describe the dynamics and functions of microglia and macrophages in neuronal inflammation and regeneration following optic nerve injury,and their possible role as therapeutic targets for axonal regeneration.

The role of purinergic receptors in neural repair and regeneration after spinal cord injury

Spinal cord injury is a serious injury of the central nervous system that results in neurological deficits.The pathophysiological mechanisms underlying spinal cord injury,as well as the mechanisms involved in neural repair and regeneration,are highly complex.Although there have been many studies on these mechanisms,there is no effective intervention for such injury.In spinal cord injury,neural repair and regeneration is an important part of improving neurological function after injury,although the low regenerative ability of nerve cells and the difficulty in axonal and myelin regeneration after spinal cord injury hamper functional recovery.Large amounts of ATP and its metabolites are released after spinal cord injury and participate in various aspects of functional regulation by acting on purinergic receptors which are widely expressed in the spinal cord.These processes mediate intracellular and extracellular signalling pathways to improve neural repair and regeneration after spinal cord injury.This article reviews research on the mechanistic roles of purinergic receptors in spinal cord injury,highlighting the potential role of purinergic receptors as interventional targets for neural repair and regeneration after spinal cord injury.

Knockdown of polypyrimidine tract binding protein facilitates motor function recovery after spinal cord injury

After spinal cord injury(SCI),a fibroblast-and microglia-mediated fibrotic scar is formed in the lesion core,and a glial scar is formed around the fibrotic scar as a res ult of the activation and proliferation of astrocytes.Simultaneously,a large number of neuro ns are lost in the injured area.Regulating the dense glial scar and re plenishing neurons in the injured area are essential for SCI repair.Polypyrimidine tra ct binding protein(PTB),known as an RNA-binding protein,plays a key role in neurogenesis.Here,we utilized short hairpin RNAs(shRNAs)and antisense oligonucleotides(ASOs)to knock down PTB expression.We found that reactive spinal astrocytes from mice were directly reprogrammed into motoneuron-like cells by PTB downregulation in vitro.In a mouse model of compressioninduced SCI,adeno-associated viral shRNA-mediated PTB knockdown replenished motoneuron-like cells around the injured area.Basso Mouse Scale scores and forced swim,inclined plate,cold allodynia,and hot plate tests showed that PTB knockdown promoted motor function recovery in mice but did not improve sensory perception after SCI.Furthermore,ASO-mediated PTB knockdown improved motor function resto ration by not only replenishing motoneuron-like cells around the injured area but also by modestly reducing the density of the glial scar without disrupting its overall structure.Together,these findings suggest that PTB knockdown may be a promising therapeutic strategy to promote motor function recovery during spinal cord repair.

Human amniotic epithelial cells combined with silk fibroin scaffold in the repair of spinal cord injury

Treatment and functional reconstruction after central nervous system injury is a major medical and social challenge. An increasing number of researchers are attempting to use neural stem cells combined with artificial scaffold materials, such as fibroin, for nerve repair. However, such approaches are challenged by ethical and practical issues. Amniotic tissue, a clinical waste product, is abundant, and amniotic epithe- lial cells are pluripotent, have low immunogenicity, and are not the subject of ethical debate. We hypothesized that amniotic epithelial cells combined with silk fibroin scaffolds would be conducive to the repair of spinal cord injury. To test this, we isolated and cultured amniotic epithelial cells, and constructed complexes of these cells and silk fibroin scaffolds. Implantation of the cell-scaffold complex into a rat model of spinal cord injury resulted in a smaller glial scar in the damaged cord tissue than in model rats that received a blank scaffold, or amniotic epithelial cells alone. In addition to a milder local immunological reaction, the rats showed less inflammatory cell infiltration at the trans- plant site, milder host-versus-graft reaction, and a marked improvement in motor function. These findings confirm that the transplantation of amniotic epithelial ceils combined with silk fibroin scaffold can promote the repair of spinal cord injury. Silk fibroin scaffold can provide a good nerve regeneration microenvironment for amniotic epithelial cells.

Dynamic reactive astrocytes after focal ischemia

Astrocytes are specialized and most numerous glial cell type in the central nervous system and play important roles in physiology. Astrocytes are also critically involved in many neural disorders including focal ischemic stroke, a leading cause of brain injury and human death. One of the prominent pathological features of focal ischemic stroke is reactive astrogliosis and glial scar formation associated with morphological changes and proliferation. This review paper discusses the recent advances in spatial and temporal dynamics of morphology and proliferation of reactive astrocytes after ischemic stroke based on results from experimental animal studies. As reactive astrocytes exhibit stem cell-like properties, knowledge of dynamics of reactive astrocytes and glial scar formation will provide important insiehts for astrocvte-based cell therapy in stroke.