AXL kinase-mediated astrocytic phagocytosis modulates outcomes of traumatic brain injury

Mutual regulation of microglia and astrocytes after Gas6 inhibits spinal cord injury

JOURNAL/nrgr/04.03/01300535-202502000-00032/figure1/v/2024-05-28T214302Z/r/image-tiff Invasive inflammation and excessive scar formation are the main reasons for the difficulty in repairing nervous tissue after spinal cord injury. Microglia and astrocytes play key roles in the spinal cord injury micro-environment and share a close interaction. However, the mechanisms involved remain unclear. In this study, we found that after spinal cord injury, resting microglia (M0) were polarized into pro-inflammatory phenotypes (MG1 and MG3), while resting astrocytes were polarized into reactive and scar-forming phenotypes. The expression of growth arrest-specific 6 (Gas6) and its receptor Axl were significantly down-regulated in microglia and astrocytes after spinal cord injury. In vitro experiments showed that Gas6 had negative effects on the polarization of reactive astrocytes and pro-inflammatory microglia, and even inhibited the cross-regulation between them. We further demonstrated that Gas6 can inhibit the polarization of reactive astrocytes by suppressing the activation of the Yes-associated protein signaling pathway. This, in turn, inhibited the polarization of pro-inflammatory microglia by suppressing the activation of the nuclear factor-κB/p65 and Janus kinase/signal transducer and activator of transcription signaling pathways. In vivo experiments showed that Gas6 inhibited the polarization of pro-inflammatory microglia and reactive astrocytes in the injured spinal cord, thereby promoting tissue repair and motor function recovery. Overall, Gas6 may play a role in the treatment of spinal cord injury. It can inhibit the inflammatory pathway of microglia and polarization of astrocytes, attenuate the interaction between microglia and astrocytes in the inflammatory microenvironment, and thereby alleviate local inflammation and reduce scar formation in the spinal cord.

Bioinformatics analysis reveals multiple functional changes in astrocytes in temporal lobe epilepsy

Epilepsy is a prevalent chronic neurological disorder characterized by recurrent seizures and brain dysfunction. Existing antiepileptic drugs (AEDs) mainly act on neurons and provide symptomatic control of seizures, but they do not modify the progression of epilepsy and may cause serious adverse effects. Increasing evidence suggests that reactive astrogliosis is critical in the pathophysiology of epilepsy. However, the function of reactive astrocytes in epilepsy has not been thoroughly explored. To provide a new perspective on the role of reactive astrocytes in epileptogenesis, we identified human astrocyte-specific genes and found 131 of these genes significantly differentially expressed in human temporal lobe epilepsy (TLE) datasets. Multiple astrocytic functions, such as cell adhesion, cell morphogenesis, actin filament-based process, apoptotic cell clearance and response to oxidative stress, were found to be promoted. Moreover, multiple altered astrocyte-specific genes were enriched in phagocytosis, perisynaptic astrocyte processes (PAPs), plasticity, and synaptic functions. Nine hub genes (ERBB2, GFAP, NOTCH2, ITGAV, ABCA1, AQP4, LRP1, GJA1, and YAP1) were identified by protein-protein interaction (PPI) network analysis. The correlation between the expression of these hub genes and seizure frequency, as well as epilepsy-related factors, including inflammatory mediators, complement factors, glutamate excitotoxicity and astrocyte reactivity, were analyzed. Additionally, upstream transcription factors of the hub genes were predicted. Finally, astrogliosis and the expression of the hub genes were validated in an epileptic rat model. Our findings reveal the various changes in astrocyte function associated with epilepsy and provide candidate astrocyte-specific genes that could be potential antiepileptogenic targets.

Passive immunotherapy for Alzheimer's disease: challenges & future directions

Passive immunotherapy with specific antibodies targeting Amyloid β (Aβ) peptide or tubulin-associated unit (tau) protein has emerged as a promising therapeutic approach in Alzheimer's disease (AD). However, in a recent phase III clinical study, Sperling et al. (N Engl J Med 10.1056/NEJMoa2305032, 2023) reported that solanezumab, a monoclonal antibody targeting Aβ peptide, failed to slow cognitive decline in AD patients. Previously, three other anti-Aβ antibodies, bapineuzumab, crenezumab, and gantenerumab, have also failed to show similar beneficial effects. In addition, three humanized antibodies targeting tau protein failed in their phase II trials. However, other anti-Aβ antibodies, such as lecanemab (a humanized mAb binds to soluble Aβ protofibrils), donanemab (a humanized mAb binds to insoluble, N-terminal truncated form of Aβ peptides) and aducanumab (a human mAb binds to the aggregated form of Aβ), have been shown to slow the decline of cognitive functions in early stage AD patients. The specific targets used in passive immunotherapy in these clinical trials may explain the divergent clinical outcomes. There are several challenges and limitations of passive immunotherapy using anti-Aβ antibodies and long term longitudinal studies are needed to assess their efficacy, side effects and cost effectiveness in a wider spectrum of subjects, from pre-dementia to more advanced dementia. A combination therapeutic approach using both anti-Aβ antibodies and other pharmaceutical agents should also be explored.

The role of astrocyte in neuroinflammation in traumatic brain injury

Traumatic brain injury (TBI), a significant contributor to mortality and morbidity worldwide, is a devastating condition characterized by initial mechanical damage followed by subsequent biochemical processes, including neuroinflammation. Astrocytes, the predominant glial cells in the central nervous system, play a vital role in maintaining brain homeostasis and supporting neuronal function. Nevertheless, in response to TBI, astrocytes undergo substantial phenotypic alternations and actively contribute to the neuroinflammatory response. This article explores the multifaceted involvement of astrocytes in neuroinflammation subsequent to TBI, with a particular emphasis on their activation, release of inflammatory mediators, modulation of the blood-brain barrier, and interactions with other immune cells. A comprehensive understanding the dynamic interplay between astrocytes and neuroinflammation in the condition of TBI can provide valuable insights into the development of innovative therapeutic approaches aimed at mitigating secondary damage and fostering neuroregeneration.

Chemically Functionalized Single-Walled Carbon Nanotubes Prevent the Reduction in Plasmalemmal Glutamate Transporter EAAT1 Expression in, and Increase the Release of Selected Cytokines from, Stretch-Injured Astrocytes in Vitro

We tested the effects of water-soluble single-walled carbon nanotubes, chemically functionalized with polyethylene glycol (SWCNT-PEG), on primary mouse astrocytes exposed to a severe in vitro simulated traumatic brain injury (TBI). The application of SWCNT-PEG in the culture media of injured astrocytes did not affect cell damage levels, when compared to those obtained from injured, functionalization agent (PEG)-treated cells. Furthermore, SWCNT-PEG did not change the levels of oxidatively damaged proteins in astrocytes. However, this nanomaterial prevented the reduction in plasmalemmal glutamate transporter EAAT1 expression caused by the injury, rendering the level of EAAT1 on par with that of control, uninjured PEG-treated astrocytes; in parallel, there was no significant change in the levels of GFAP. Additionally, SWCNT-PEG increased the release of selected cytokines that are generally considered to be involved in recovery processes following injuries. As a loss of EAATs has been implicated as a culprit in the suffering of human patients from TBI, the application of SWCNT-PEG could have valuable effects at the injury site, by preventing the loss of astrocytic EAAT1 and consequently allowing for a much-needed uptake of glutamate from the extracellular space, the accumulation of which leads to unwanted excitotoxicity. Additional potential therapeutic benefits could be reaped from the fact that SWCNT-PEG stimulated the release of selected cytokines from injured astrocytes, which would promote recovery after injury and thus counteract the excess of proinflammatory cytokines present in TBI.

Neutrophil-like pH-responsive pro-efferocytic nanoparticles improve neurological recovery by promoting erythrophagocytosis after intracerebral hemorrhage

Rationale: Intracerebral hemorrhage (ICH) is a devastating cerebrovascular disease resulting from blood extravasating into the brain parenchyma. Escalation of erythrophagocytosis (a form of efferocytosis), avoiding the consequent release of the detrimental erythrocyte lysates, may be a promising target of ICH management. The ADAM17 inhibitor and liver X receptor (LXR) agonist could promote efficient efferocytosis and injury repair. Nevertheless, the poor bioavailability and restriction of the blood-brain barrier (BBB) hinder their application. Therefore, it is needed that biocompatible and smart nanoplatforms were designed and synthesized to realize effective therapy targeting erythrophagocytosis. Methods: We first assessed the synergistic effect of therapeutic GW280264X (an ADAM17 inhibitor) and desmosterol (an LXR agonist) on erythrophagocytosis in vitro. Then a pH-responsive neutrophil membrane-based nanoplatform (NPEOz) served as a carrier to accurately deliver therapeutic GW280264X and desmosterol to the damaged brain was prepared via co-extrusion. Afterwards, their pH-responsive performance was valued in vitro and targeting ability was assessed through fluorescence image in vivo. Finally, the pro-erythrophagocytic and anti-neuroinflammatory ability of the nanomedicine and related mechanisms were investigated. Results: After the synergistical effect of the above two drugs on erythrophagocytosis was confirmed, we successfully developed neutrophil-disguised pH-responsive nanoparticles to efficiently co-deliver them. The nanoparticles could responsively release therapeutic agents under acidic environments, and elicit favorable biocompatibility and ability of targeting injury sites. D&G@NPEOz nanoparticles enhanced erythrophagocytosis through inhibiting shedding of the efferocytotic receptors MERTK/AXL mediated by ADAM17 and accelerating ABCA-1/ABCG-1-mediated cholesterol efflux regulated by LXR respectively. In addition, the nano-formulation was able to modulate the inflammatory microenvironment by transforming efferocytes towards a therapeutic phenotype with reducing the release of proinflammatory cytokines while increasing the secretion of anti-inflammatory factors, and improve neurological function. Conclusions: This biomimetic nanomedicine is envisaged to offer an encouraging strategy to effectively promote hematoma and inflammation resolution, consequently alleviate ICH progression.

Growth arrest specific protein 6 alleviated white matter injury after experimental ischemic stroke

Ischemic white matter injury leads to long-term neurological deficits and lacks effective medication. Growth arrest specific protein 6 (Gas6) clears myelin debris, which is hypothesized to promote white matter integrity in experimental stroke models. By the middle cerebral artery occlusion (MCAO) stroke model, we observed that Gas6 reduced infarcted volume and behavior deficits 4 weeks after MCAO. Compared with control mice, Gas6-treatment mice represented higher FA values in the ipsilateral external capsules by MRI DTI scan. The SMI32/MBP ratio of the ipsilateral cortex and striatum was profoundly alleviated by Gas6 administration. Gas6-treatment group manifested thicker myelin sheaths than the control group by electron microscopy. We observed that Gas6 mainly promoted OPC maturation, which was closely related to microglia. Mechanically, Gas6 accelerated microglia-mediated myelin debris clearance and cholesterol transport protein expression (abca1, abcg1, apoc1, apoe) in vivo and in vitro, accordingly less myelin debris and lipid deposited in Gas6 treated stroke mice. HX531 (RXR inhibitor) administration mitigated the functions of Gas6 in speeding up debris clearance and cholesterol transport protein expression. Generally, we concluded that Gas6 cleared myelin debris and promoted cholesterol transportation protein expression through activating RXR, which could be one critical mechanism contributing to white matter repair after stroke.

Gas6 Promotes Microglia Efferocytosis and Suppresses Inflammation Through Activating Axl/Rac1 Signaling in Subarachnoid Hemorrhage Mice

Early brain injury (EBI) following  subarachnoid hemorrhage (SAH) is characterized by rapid development of neuron apoptosis and dysregulated inflammatory response. Microglia efferocytosis plays a critical role in the clearance of apoptotic cells, attenuation of inflammation, and minimizing brain injury in various pathological conditions. Here, using a mouse SAH model, we aim to investigate whether microglia efferocytosis is involved in post-SAH inflammation and to determine the underlying signaling pathway. We hypothesized that TAM receptors and their ligands regulate this process. To prove our hypothesis, the expression and cellular location of TAM (Tyro3, Axl, and Mertk) receptors and their ligands growth arrest-specific 6 (Gas6) and Protein S (ProS1) were examined by PCR, western blots, and fluorescence immunostaining. Thirty minutes after SAH, mice received an intraventricular injection of recombinant Gas6 (rGas6) or recombinant ProS1 (rPros1) and underwent evaluations of inflammatory mediator expression, neurological deficits, and blood-brain barrier integrity at 24 h. Microglia efferocytosis of apoptotic neurons was analyzed in vivo and in vitro. The potential mechanism was determined by inhibiting or knocking down TAM receptors and Rac1 by specific inhibitors or siRNA. SAH induced upregulation of Axl and its ligand Gas6. The administration of rGas6 but not rPros1 promoted microglia efferocytosis, alleviated inflammation, and ameliorated SAH-induced BBB breakdown and neurological deficits. The beneficial effects of rGas6 were arrogated by inhibiting or knocking down Axl and Rac1. We concluded that rGas6 attenuated the development of early brain injury in mice after SAH by facilitating microglia efferocytosis and preventing inflammatory response, which is partly dependent on activation of Axl and Rac1.

Ly6C-high monocytes alleviate brain injury in experimental subarachnoid hemorrhage in mice

BACKGROUND

Subarachnoid hemorrhage (SAH) is an uncommon type of potentially fatal stroke. The pathophysiological mechanisms of brain injury remain unclear, which hinders the development of drugs for SAH. We aimed to investigate the pathophysiological mechanisms of SAH and to elucidate the cellular and molecular biological response to SAH-induced injury.

METHODS

A cross-species (human and mouse) multiomics approach combining high-throughput data and bioinformatic analysis was used to explore the key pathophysiological processes and cells involved in SAH-induced brain injury. Patient data were collected from the hospital (n = 712). SAH was established in adult male mice via endovascular perforation, and flow cytometry, a bone marrow chimera model, qPCR, and microglial depletion experiments were conducted to explore the origin and chemotaxis mechanism of the immune cells. To investigate cell effects on SAH prognosis, murine neurological function was evaluated based on a modified Garcia score, pole test, and rotarod test.

RESULTS

The bioinformatics analysis confirmed that inflammatory and immune responses were the key pathophysiological processes after SAH. Significant increases in the monocyte levels were observed in both the mouse brains and the peripheral blood of patients after SAH. Ly6C-high monocytes originated in the bone marrow, and the skull bone marrow contribute a higher proportion of these monocytes than neutrophils. The mRNA level of Ccl2 was significantly upregulated after SAH and was greater in CD11b-positive than CD11b-negative cells. Microglial depletion, microglial inhibition, and CCL2 blockade reduced the numbers of Ly6C-high monocytes after SAH. With CCR2 antagonization, the neurological function of the mice exhibited a slow recovery. Three days post-SAH, the monocyte-derived dendritic cell (moDC) population had a higher proportion of TNF-α-positive cells and a lower proportion of IL-10-positive cells than the macrophage population. The ratio of moDCs to macrophages was higher on day 3 than on day 5 post-SAH.

CONCLUSIONS

Inflammatory and immune responses are significantly involved in SAH-induced brain injury. Ly6C-high monocytes derived from the bone marrow, including the skull bone marrow, infiltrated into mouse brains via CCL2 secreted from microglia. Moreover, Ly6C-high monocytes alleviated neurological dysfunction after SAH.

Src inhibition rescues FUNDC1-mediated neuronal mitophagy in ischaemic stroke

BACKGROUND

Ischaemic stroke triggers neuronal mitophagy, while the involvement of mitophagy receptors in ischaemia/reperfusion (I/R) injury-induced neuronal mitophagy remain not fully elucidated. Here, we aimed to investigate the involvement of mitophagy receptor FUN14 domain-containing 1 (FUNDC1) and its modulation in neuronal mitophagy induced by I/R injury.

METHODS

Wild-type and FUNDC1 knockout mice were generated to establish models of neuronal I/R injury, including transient middle cerebral artery occlusion (tMCAO) in vivo and oxygen glucose deprivation/reperfusion in vitro. Stroke outcomes of mice with two genotypes were assessed. Neuronal mitophagy was analysed both in vivo and in vitro. Activities of FUNDC1 and its regulator Src were evaluated. The impact of Src on FUNDC1-mediated mitophagy was assessed through administration of Src antagonist PP1.

RESULTS

To our surprise, FUNDC1 knockout mice subjected to tMCAO showed stroke outcomes comparable to those of their wild-type littermates. Although neuronal mitophagy could be activated by I/R injury, FUNDC1 deletion did not disrupt neuronal mitophagy. Transient activation of FUNDC1, represented by dephosphorylation of Tyr18, was detected in the early stages (within 3 hours) of neuronal I/R injury; however, phosphorylated Tyr18 reappeared and even surpassed baseline levels in later stages (after 6 hours), accompanied by a decrease in FUNDC1-light chain 3 interactions. Spontaneous inactivation of FUNDC1 was associated with Src activation, represented by phosphorylation of Tyr416, which changed in parallel with the level of phosphorylated FUNDC1 (Tyr18) during neuronal I/R injury. Finally, FUNDC1-mediated mitophagy in neurons under I/R conditions can be rescued by pharmacological inhibition of Src.

CONCLUSIONS

FUNDC1 is inactivated by Src during the later stage (after 6 hours) of neuronal I/R injury, and rescue of FUNDC1-mediated mitophagy may serve as a potential therapeutic strategy for treating ischaemic stroke.

Megf10-related engulfment of excitatory postsynapses by astrocytes following severe brain injury

AIMS

To investigate astrocyte-related phagocytosis of synapses in the ipsilateral hippocampus after traumatic brain injury (TBI).

METHODS

We performed controlled cortical impact to simulate TBI in mice. Seven days postinjury, we performed cognitive tests, synapse quantification, and examination of astrocytic phagocytosis in association with Megf10 expression.

RESULTS

During the subacute stage post-TBI, we found a reduction in excitatory postsynaptic materials in the ipsilateral hippocampus, which was consistent with poor performance in the cognitive test. The transcriptome data suggested that robust phagocytosis was responsible for this process. Coincidently, we identified phagocytic astrocytes containing secondary lysosomes that were wrapped around the synapses in the ipsilateral hippocampus. Moreover, a significant increase in the co-location of GFAP and PSD-95 in the CA1 region suggested astrocytic engulfment of excitatory postsynaptic proteins. After examining the reported phagocytic pathways, we found that both the transcription level and protein expression of Megf10 were elevated. Co-immunofluorescence of GFAP and Megf10 demonstrated that the expression of Megf10 was spatially upregulated in astrocytes, exclusively in the CA1 region, and was related to the astrocytic engulfment of PSD-95.

CONCLUSION

Our study elaborated that the Megf10-related astrocytic engulfment of PSD-95 in the CA1 region of the ipsilateral hippocampus aggravated cognitive dysfunction following severe TBI.

Do astrocytes act as immune cells after pediatric TBI?

Astrocytes are in contact with the vasculature, neurons, oligodendrocytes and microglia, forming a local network with various functions critical for brain homeostasis. One of the primary responders to brain injury are astrocytes as they detect neuronal and vascular damage, change their phenotype with morphological, proteomic and transcriptomic transformations for an adaptive response. The role of astrocytic responses in brain dysfunction is not fully elucidated in adult, and even less described in the developing brain. Children are vulnerable to traumatic brain injury (TBI), which represents a leading cause of death and disability in the pediatric population. Pediatric brain trauma, even with mild severity, can lead to long-term health complications, such as cognitive impairments, emotional disorders and social dysfunction later in life. To date, the underlying pathophysiology is still not fully understood. In this review, we focus on the astrocytic response in pediatric TBI and propose a potential immune role of the astrocyte in response to trauma. We discuss the contribution of astrocytes in the local inflammatory cascades and secretion of various immunomodulatory factors involved in the recruitment of local microglial cells and peripheral immune cells through cerebral blood vessels. Taken together, we propose that early changes in the astrocytic phenotype can alter normal development of the brain, with long-term consequences on neurological outcomes, as described in preclinical models and patients.

Inhibition of BACE1 attenuates microglia-induced neuroinflammation after intracerebral hemorrhage by suppressing STAT3 activation

Hematoma-induced neuroinflammation is the cause of poor prognosis in intracerebral hemorrhage (ICH); therefore, promoting blood clearance and blocking overactivated inflammation are rational approaches for ICH treatment. β-site amyloid precursor protein (APP) lyase-1 (BACE1) is a key molecule regulating the microglial phenotype transition in neurodegenerative diseases. Therefore, the aim of this study was to investigate the role of BACE1 in microglial phagocytosis and inflammatory features in ICH. Here, we demonstrated the unique advantages of targeting BACE1 in microglia using an autologous blood model and primary microglia hemoglobin stimulation. When BACE1 was inhibited early in ICH, fewer residual hematomas remained, consistent with an increase in genetic features that favor phagocytosis and anti-inflammation. In addition, inhibition of BACE1 enhanced the secretion of anti-inflammatory cytokines and substantially reduced the expression of proinflammatory genes, which was regulated by signal transduction and phosphorylation of activator of transcription 3 (STAT3). Further pharmacological inhibition of STAT3 phosphorylation effectively blocked the proinflammatory and weak phagocytic phenotype of microglia due to BACE1 induction. In summary, BACE1 is the critical molecule regulating the inflammatory and phagocytic phenotypes of microglia after ICH, and targeted inhibition of the BACE1/STAT3 pathway is an important strategy for the future treatment of ICH-induced neurological injury.

Axl promotes intracranial aneurysm rupture by regulating macrophage polarization toward M1 via STAT1/HIF-1α

Background

Macrophage infiltration and polarization are crucial for the pathogenesis of intracranial aneurysm (IA) rupture. Axl, a receptor tyrosine kinase, is involved in inflammation and efferocytosis in multiple organs. Upregulated soluble Axl in cerebrospinal fluid (CSF) and plasma is correlated with intracranial aneurysm rupture. This study aimed to investigate the role of Axl in IA rupture and macrophage polarization.

Methods

Male C57BL/6J mice were used to induce IA. The level of Axl from control vessels and unruptured and ruptured IA samples was detected. In addition, the relationship between Axl and macrophages was confirmed. The pathway of Axl-mediated macrophage polarization was explored after IA induction in vivo and in bone marrow-derived macrophages (BMDMs) stimulated by LPS/IFN-γ in vitro. The animals were randomized into three groups and treated intraperitoneally with the vehicle, selective AXL antagonist R428, and recombinant mouse growth arrest-specific 6 (rmGas6) for 21 consecutive days. Then, we evaluated the influence of Axl on IA rupture by administrating R428 to inhibit or rmGas6 to activate the Axl receptor in vivo.

Results

Compared with that in normal vessels, Axl expression was significantly upregulated in unruptured IA samples. The ruptured IA tissue exhibited significantly higher expression of Axl than the unruptured IA tissue. Axl and F4/80 were coexpressed in IA tissue and LPS/IFN-γ-stimulated BMDMs. R428 treatment significantly reduced the rate of M1-like macrophage infiltration and IA rupture. In contrast, rmGas6 treatment promoted M1 macrophage infiltration and IA rupture. Mechanistically, R428 inhibited the phosphorylation of Axl and STAT1 and the expression of hypoxia-inducible factor-1α (HIF-1α) and decreased the levels of IL-1β, NOS2, and MMP9 in LPS/IFN-γ-stimulated BMDMs. rmGas6 promoted the phosphorylation of Axl and STAT1 and the expression of HIF-1α. In addition, STAT1 knockdown abolished Axl-mediated M1 macrophage polarization.

Conclusion

The inhibition of Axl reduced macrophage polarization toward the M1 phenotype via the STAT1/HIF-1α signaling pathway and prevented IA rupture in mice. This finding suggests that pharmacological inhibition of Axl might be used to prevent the progression and rupture of IA.

Neuroinflammation of traumatic brain injury: Roles of extracellular vesicles

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

Capacity of astrocytes to promote axon growth in the injured mammalian central nervous system

Understanding the regulation of axon growth after injury to the adult central nervous system (CNS) is crucial to improve neural repair. Following acute focal CNS injury, astrocytes are one cellular component of the scar tissue at the primary lesion that is traditionally associated with inhibition of axon regeneration. Advances in genetic models and experimental approaches have broadened knowledge of the capacity of astrocytes to facilitate injury-induced axon growth. This review summarizes findings that support a positive role of astrocytes in axon regeneration and axon sprouting in the mature mammalian CNS, along with potential underlying mechanisms. It is important to recognize that astrocytic functions, including modulation of axon growth, are context-dependent. Evidence suggests that the local injury environment, neuron-intrinsic regenerative potential, and astrocytes’ reactive states determine the astrocytic capacity to support axon growth. An integrated understanding of these factors will optimize therapeutic potential of astrocyte-targeted strategies for neural repair.

Soluble Trem2 is a negative regulator of erythrophagocytosis after intracerebral hemorrhage in a CD36 receptor recycling manner

INTRODUCTION

Microglia and macrophages participate in hematoma clearance after intracerebral hemorrhage (ICH), thereby facilitating tissue restoration and neurological recovery. Triggering receptor expressed on myeloid cells 2 (Trem2) has been indicated as a major pathology-induced immune signaling hub on the microglial/macrophage surface. Soluble Trem2 (sTrem2), the proteolytic form of Trem2, is abundant in the body fluid and is positively correlated with the pathological process.

OBJECTIVES

In the present study, we aimed to investigate the potential role of sTrem2 in hematoma resolution after ICH and to elucidate its underlying mechanisms.

METHODS

We explored the biological functions of sTrem2 in the murine ICH brain by stereotaxic injection of recombinant sTrem2 protein or by adeno-associated virus-mediated expression. Erythrocyte phagocytosis was assessed using flow cytometry and immunofluorescence. Western blotting was performed to evaluate protein expression. Changes in behavior, sTrem2-induced down-stream pathway, and microglia were examined.

RESULTS

sTrem2 impedes hematoma resolution and impairs functional motor and sensory recovery. Interestingly, sTrem2 bypasses full-length Trem2, negatively regulating microglial/macrophage erythrophagocytosis, and promotes an inflammatory phenotype, which is associated with reduced retromer levels and impaired recycling of the pro-erythrophagocytic receptor CD36. Rescue of retromer Vps35 abolishes the phagocytosis-inhibiting effects and lysosome-dependent CD36 degradation caused by sTrem2.

CONCLUSION

These findings indicate sTrem2 as a negative factor against microglia/macrophage-mediated hematoma and related neuronal damage clearance, provide insight into the mechanisms by which erythrophagocytosis is regulated and how it may be impaired after ICH, and suggest that the anti-proteolytic activity of Trem2 can be explored for ICH therapy.

Focal lesion size poorly correlates with motor function after experimental traumatic brain injury in mice

Background

It remains unclear whether neurobehavioral testing adds significant information to histologic assessment of experimental traumatic brain injury (TBI) and if automated gait assessment using the CatWalk XT^®, while shown to be effective in in the acute phase, is also effective in the chronic phase after experimental TBI. Therefore, we evaluated the correlation of CatWalk XT^® parameters with histologic lesion volume and analyzed their temporal and spatial patterns over four weeks after trauma induction.

Methods

C57Bl/6 mice were subjected to controlled cortical impact (CCI). CatWalk XT^® analysis was performed one day prior to surgery and together with the histological evaluation of lesion volume on postoperative days one, three, seven, 14 and 28. Temporal and spatial profiles of gait impairment were analyzed and a total of 100 CatWalk XT^® parameters were correlated to lesion size.

Results

While in the first week after CCI, there was significant impairment of nearly all CatWalk XT^® parameters, impairment of paw prints, intensities and dynamic movement parameters resolved thereafter; however, impairment of dynamic single paw parameters persisted up to four weeks. Correlation of the CatWalk XT^® parameters with lesion volume was poor at all timepoints.

Conclusion

As CatWalk XT^® parameters do not correlate with focal lesion size after CCI, gait assessment using the CatWalk XT^® might add valuable information to solitary histologic evaluation of the injury site. While all CatWalk XT^® parameters can be used for gait assessments in the first week after CCI, dynamic single paw parameters might be more relevant in the chronic phase after experimental TBI.