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.

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.

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.

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.

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.

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.

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.

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.

Chondroitinase ABC plus bone marrow mesenchymal stem cells for repair of spinal cord injury

As chondroitinase ABC can improve the hostile microenvironment and cell transplantation is proven to be effective after spinal cord injury, we hypothesized that their combination would be a more effective treatment option. At 5 days after T8 spinal cord crush injury, rats were injected with bone marrow mesenchymal stem cell suspension or chondroitinase ABC 1 mm from the edge of spinal cord damage zone. Chondroitinase ABC was first injected, and bone marrow mesenchymal stem cell suspension was injected on the next day in the combination group. At 14 days, the mean Basso, Beattie and Bresnahan score of the rats in the combination group was higher than other groups. Hematoxylin-eosin staining showed that the necrotic area was significantly reduced in the combination group compared with other groups. Glial fibrillary acidic protein-chondroitin sulfate proteoglycan double staining showed that the damage zone of astrocytic scars was significantly reduced without the cavity in the combination group. Glial fibrillary acidic protein/growth associated protein-43 double immunostaining revealed that positive fibers traversed the damage zone in the combination group. These results suggest that the combination of chondroitinase ABC and bone marrow mesenchymal stem cell transplantation contributes to the repair of spinal cord injury.

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.

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.

Ruxolitinib attenuates secondary injury after traumatic spinal cord injury

Excessive inflammation post-traumatic spinal cord injury(SCI)induces microglial activation,which leads to prolonged neurological dysfunction.However,the mechanism underlying microglial activation-induced neuroinflammation remains poorly understood.Ruxolitinib(RUX),a selective inhibitor of JAK1/2,was recently reported to inhibit inflammatory storms caused by SARS-CoV-2 in the lung.However,its role in disrupting inflammation post-SCI has not been confirmed.In this study,microglia were treated with RUX for 24 hours and then activated with interferon-γfor 6 hours.The results showed that interferon-γ-induced phosphorylation of JAK and STAT in microglia was inhibited,and the mRNA expression levels of pro-inflammatory cytokines tumor necrosis factor-α,interleukin-1β,interleukin-6,and cell proliferation marker Ki67 were reduced.In further in vivo experiments,a mouse model of spinal cord injury was treated intragastrically with RUX for 3 successive days,and the findings suggest that RUX can inhibit microglial proliferation by inhibiting the interferon-γ/JAK/STAT pathway.Moreover,microglia treated with RUX centripetally migrated toward injured foci,remaining limited and compacted within the glial scar,which resulted in axon preservation and less demyelination.Moreover,the protein expression levels of tumor necrosis factor-α,interleukin-1β,and interleukin-6 were reduced.The neuromotor function of SCI mice also recovered.These findings suggest that RUX can inhibit neuroinflammation through inhibiting the interferon-γ/JAK/STAT pathway,thereby reducing secondary injury after SCI and producing neuroprotective effects.

Epidural electrical stimulation for spinal cord injury

A long-standing goal of spinal cord injury research is to develop effective repair strategies,which can restore motor and sensory functions to near-normal levels.Recent advances in clinical management of spinal cord injury have significantly improved the prognosis,survival rate and quality of life in patients with spinal cord injury.In addition,a significant progress in basic science research has unraveled the underlying cellular and molecular events of spinal cord injury.Such efforts enabled the development of pharmacologic agents,biomaterials and stem-cell based therapy.Despite these efforts,there is still no standard care to regenerate axons or restore function of silent axons in the injured spinal cord.These challenges led to an increased focus on another therapeutic approach,namely neuromodulation.In multiple animal models of spinal cord injury,epidural electrical stimulation of the spinal cord has demonstrated a recovery of motor function.Emerging evidence regarding the efficacy of epidural electrical stimulation has further expanded the potential of epidural electrical stimulation for treating patients with spinal cord injury.However,most clinical studies were conducted on a very small number of patients with a wide range of spinal cord injury.Thus,subsequent studies are essential to evaluate the therapeutic potential of epidural electrical stimulation for spinal cord injury and to optimize stimulation parameters.Here,we discuss cellular and molecular events that continue to damage the injured spinal cord and impede neurological recovery following spinal cord injury.We also discuss and summarize the animal and human studies that evaluated epidural electrical stimulation in spinal cord injury.

Proteoglycans: Road Signs for Neurite Outgrowth

Proteoglycans in the central nervous system play integral roles as ＂traffic signals＂ for the direction of neurite outgrowth. This attribute of proteoglycans is a major factor in regeneration of the injured central nervous system. In this review, the structures of proteoglycans and the evidence suggesting their involvement in the response following spinal cord injury are presented. The review further describes the methods routinely used to determine the effect proteoglycans have on neurite outgrowth. The effects of proteoglycans on neurite outgrowth are not completely understood as there is disagreement on what component of the molecule is interacting with growing neurites and this ambiguity is chronicled in an historical context. Finally, the most recent findings suggesting possible receptors, interactions, and sulfation patterns that may be important in eliciting the effect of proteoglycans on neurite outgrowth are discussed. A greater understanding of the proteoglycan-neurite interaction is necessary for successfully promoting regeneration in the iniured central nervous system.

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.

Synergistic actions of olomoucine and bone morphogenetic protein-4 in axonal repair after acute spinal cord contusion

To determine whether olomoucine acts synergistically with bone morphogenetic protein-4 in the treatment of spinal cord injury, we established a rat model of acute spinal cord contusion by impacting the spinal cord at the T8 vertebra. We injected a suspension of astrocytes derived from glial-restricted precursor cells exposed to bone morphogenetic protein-4 （GDAsBMP） into the spinal cord around the site of the injury, and/or olomoucine intraperitoneally. Olomoucine effectively inhibited astrocyte proliferation and the formation of scar tissue at the injury site, but did not prevent proliferation of GDAsBMP or inhibit their effects in reducing the spinal cord lesion cavity. Furthermore, while GDAsBMP and olomoucine independently resulted in small improve- ments in locomotor function in injured rats, combined administration of both treatments had a significantly greater effect on the restoration of motor function. These data indicate that the combined use of olomoucine and GDAsBMP creates a better environment for nerve regeneration than the use of either treatment alone, and contributes to spinal cord repair after injury.