Conditional Deletion of EphA4 on Cx3cr1-Expressing Microglia Fails to Influence Histopathological Outcome and Blood Brain Barrier Disruption Following Brain Injury

Skull bone marrow-derived immune cells infiltrate the damaged cortex and exhibit anti-inflammatory properties

Identifying the origins and contributions of different immune cell populations following brain injury is crucial for understanding their roles in inflammation and tissue repair. This study investigated the infiltration and phenotypic characteristics of skull bone marrow-derived immune cells in the murine brain after TBI. We performed calvarium transplantation from GFP donor mice and subjected the recipients to controlled cortical impact (CCI) injury 14 days post-transplant. Confocal imaging at 3 days post-CCI revealed GFP+ calvarium-derived cells infiltrating the ipsilateral core lesional area, expressing CD45 and CD11b immune markers. These cells included neutrophil (Ly6G+) and monocyte (Ccr2+) identities. Calvarium-derived GFP+/Iba1+ monocyte/macrophages expressed the efferocytosis receptor MerTK and displayed engulfment of NeuN+ and caspase 3+ apoptotic cells. Phenotypic analysis showed that greater calvarium-derived monocyte/macrophages disproportionately express the anti-inflammatory arginase-1 marker than pro-inflammatory CD86. To differentiate the responses of blood- and calvarium-derived macrophages, we transplanted GFP calvarium skull bone into tdTomato bone marrow chimeric mice, then performed CCI injury 14 days post-transplant. Calvarium-derived GFP+ cells predominantly infiltrated the lesion boundary, while blood-derived TdTomato+ cells dispersed throughout the lesion and peri-lesion. Compared to calvarium-derived cells, more blood-derived cells expressed pro-inflammatory CD86 and displayed altered 3D morphologic traits. These findings uniquely demonstrate that skull bone-derived immune cells infiltrate the brain after injury and contribute to the neuroinflammatory milieu, representing a novel immune cell source that may be further investigated for their causal role in functional outcomes.

STING-Dependent Signaling in Microglia or Peripheral Immune Cells Orchestrates the Early Inflammatory Response and Influences Brain Injury Outcome

While originally identified as an antiviral pathway, recent work has implicated that cyclic GMP-AMP-synthase-Stimulator of Interferon Genes (cGAS-STING) signaling is playing a critical role in the neuroinflammatory response to traumatic brain injury (TBI). STING activation results in a robust inflammatory response characterized by the production of inflammatory cytokines called interferons, as well as hundreds of interferon stimulated genes (ISGs). Global knock-out (KO) mice inhibiting this pathway display neuroprotection with evidence that this pathway is active days after injury; yet, the early neuroinflammatory events stimulated by STING signaling remain understudied. Furthermore, the source of STING signaling during brain injury is unknown. Using a murine controlled cortical impact (CCI) model of TBI, we investigated the peripheral immune and microglial response to injury utilizing male chimeric and conditional STING KO animals, respectively. We demonstrate that peripheral and microglial STING signaling contribute to negative outcomes in cortical lesion volume, cell death, and functional outcomes postinjury. A reduction in overall peripheral immune cell and neutrophil infiltration at the injury site is STING dependent in these models at 24 h. Transcriptomic analysis at 2 h, when STING is active, reveals that microglia drive an early, distinct transcriptional program to elicit proinflammatory genes including interleukin 1-β (IL-1β), which is lost in conditional knock-out mice. The upregulation of alternative innate immune pathways also occurs after injury in these animals, which supports a complex relationship between brain-resident and peripheral immune cells to coordinate the proinflammatory response and immune cell influx to damaged tissue after injury.

Immunoregulatory and neutrophil-like monocyte subsets with distinct single-cell transcriptomic signatures emerge following brain injury

Monocytes represent key cellular elements that contribute to the neurological sequela following brain injury. The current study reveals that trauma induces the augmented release of a transcriptionally distinct CD115+/Ly6Chi monocyte population into the circulation of mice pre-exposed to clodronate depletion conditions. This phenomenon correlates with tissue protection, blood-brain barrier stability, and cerebral blood flow improvement. Uniquely, this shifted the innate immune cell profile in the cortical milieu and reduced the expression of pro-inflammatory Il6, IL1r1, MCP-1, Cxcl1, and Ccl3 cytokines. Monocytes that emerged under these conditions displayed a morphological and gene profile consistent with a subset commonly seen during emergency monopoiesis. Single-cell RNA sequencing delineated distinct clusters of monocytes and revealed a key transcriptional signature of Ly6Chi monocytes enriched for Apoe and chitinase-like protein 3 (Chil3/Ym1), commonly expressed in pro-resolving immunoregulatory monocytes, as well as granule genes Elane, Prtn3, MPO, and Ctsg unique to neutrophil-like monocytes. The predominate shift in cell clusters included subsets with low expression of transcription factors involved in monocyte conversion, Pou2f2, Na4a1, and a robust enrichment of genes in the oxidative phosphorylation pathway which favors an anti-inflammatory phenotype. Transfer of this monocyte assemblage into brain-injured recipient mice demonstrated their direct role in neuroprotection. These findings reveal a multifaceted innate immune response to brain injury and suggest targeting surrogate monocyte subsets may foster tissue protection in the brain.

Efferocytosis is restricted by axon guidance molecule EphA4 via ERK/Stat6/MERTK signaling following brain injury

BACKGROUND

Efferocytosis is a process that removes apoptotic cells and cellular debris. Clearance of these cells alleviates neuroinflammation, prevents the release of inflammatory molecules, and promotes the production of anti-inflammatory cytokines to help maintain tissue homeostasis. The underlying mechanisms by which this occurs in the brain after injury remain ill-defined.

METHODS

We used GFP bone marrow chimeric knockout (KO) mice to demonstrate that the axon guidance molecule EphA4 receptor tyrosine kinase is involved in suppressing MERTK in the brain to restrict efferocytosis of resident microglia and peripheral-derived monocyte/macrophages.

RESULTS

Single-cell RNAseq identified MERTK expression, the primary receptor involved in efferocytosis, on monocytes, microglia, and a subset of astrocytes in the damaged cortex following brain injury. Loss of EphA4 on infiltrating GFP-expressing immune cells improved functional outcome concomitant with enhanced efferocytosis and overall protein expression of p-MERTK, p-ERK, and p-Stat6. The percentage of GFP+ monocyte/macrophages and resident microglia engulfing NeuN+ or TUNEL+ cells was significantly higher in KO chimeric mice. Importantly, mRNA expression of Mertk and its cognate ligand Gas6 was significantly elevated in these mice compared to the wild-type. Analysis of cell-specific expression showed that p-ERK and p-Stat6 co-localized with MERTK-expressing GFP + cells in the peri-lesional area of the cortex following brain injury. Using an in vitro efferocytosis assay, co-culturing pHrodo-labeled apoptotic Jurkat cells and bone marrow (BM)-derived macrophages, we demonstrate that efferocytosis efficiency and mRNA expression of Mertk and Gas6 was enhanced in the absence of EphA4. Selective inhibitors of ERK and Stat6 attenuated this effect, confirming that EphA4 suppresses monocyte/macrophage efferocytosis via inhibition of the ERK/Stat6 pathway.

CONCLUSIONS

Our findings implicate the ERK/Stat6/MERTK axis as a novel regulator of apoptotic debris clearance in brain injury that is restricted by peripheral myeloid-derived EphA4 to prevent the resolution of inflammation.

Endothelial deletion of EPH receptor A4 alters single-cell profile and Tie2/Akap12 signaling to preserve blood-brain barrier integrity

Neurobiological consequences of traumatic brain injury (TBI) result from a complex interplay of secondary injury responses and sequela that mediates chronic disability. Endothelial cells are important regulators of the cerebrovascular response to TBI. Our work demonstrates that genetic deletion of endothelial cell (EC)-specific EPH receptor A4 (EphA4) using conditional EphA4f/f/Tie2-Cre and EphA4f/f/VE-Cadherin-CreERT2 knockout (KO) mice promotes blood-brain barrier (BBB) integrity and tissue protection, which correlates with improved motor function and cerebral blood flow recovery following controlled cortical impact (CCI) injury. scRNAseq of capillary-derived KO ECs showed increased differential gene expression of BBB-related junctional and actin cytoskeletal regulators, namely, A-kinase anchor protein 12, Akap12, whose presence at Tie2 clustering domains is enhanced in KO microvessels. Transcript and protein analysis of CCI-injured whole cortical tissue or cortical-derived ECs suggests that EphA4 limits the expression of Cldn5, Akt, and Akap12 and promotes Ang2. Blocking Tie2 using sTie2-Fc attenuated protection and reversed Akap12 mRNA and protein levels cortical-derived ECs. Direct stimulation of Tie2 using Vasculotide, angiopoietin-1 memetic peptide, phenocopied the neuroprotection. Finally, we report a noteworthy rise in soluble Ang2 in the sera of individuals with acute TBI, highlighting its promising role as a vascular biomarker for early detection of BBB disruption. These findings describe a contribution of the axon guidance molecule, EphA4, in mediating TBI microvascular dysfunction through negative regulation of Tie2/Akap12 signaling.

Unraveling the Potential of EphA4: A Breakthrough Target and Beacon of Hope for Neurological Diseases

Erythropoietin-producing hepatocellular carcinoma A4 (EphA4) is a transmembrane receptor protein which is a part of the most prominent family of receptor tyrosine kinases (RTKs). It serves a crucial role in both physiological, biological, and functional states binding with their ligand like Ephrins. Its abundance in the majority of the body's systems has been reported. Moreover, it draws much attention in the CNS since it influences axonal and vascular guidance. Also, it has a widespread role at the pathological state of various CNS disorders. Reports suggest it obstructs axonal regeneration in various neurodegenerative diseases and neurological disorders. Although, neuro-regeneration is still an open challenge to the modern drug discovery community.  Hence, in this review, we will provide information about the role of EphA4 in neurological diseases by which it may emerge as a therapeutic target for CNS disease. We will also provide a glance at numerous signaling pathways that activate or inhibit the EphA4-associated biological processes contributing to the course of neurodegenerative diseases. Thus, this work might serve as a basis for futuristic studies that are related to the target-based drug discovery in the field of neuro-regeneration. Pathological and physiological events associated with EphA4 and Ephrin upregulation and interaction.

Efferocytosis is restricted by axon guidance molecule EphA4 via ERK/Stat6/Mertk signaling following brain injury

Background

Efferocytosis is a process that removes apoptotic cells and cellular debris. Clearance of these cells alleviates neuroinflammation and prevents the release of inflammatory molecules and promotes the production of anti-inflammatory cytokines to help maintain tissue homeostasis. The underlying mechanisms by which this occurs in the brain after injury remains ill-defined.

Methods

We demonstrate using GFP bone marrow chimeric knockout (KO) mice, that the axon guidance molecule EphA4 receptor tyrosine kinase is involved in suppressing Mertk signaling in the brain to restrict the function of efferocytosis on resident microglia and peripheral-derived monocyte/macrophages.

Results

Single-cell RNAseq identified Mertk expression, the primary receptor involved in efferocytosis, on monocytes, microglia, and a subset of astrocytes in the damaged cortex following brain injury. Loss of EphA4 on infiltrating GFP-expressing immune cells improved functional outcome concomitant with enhanced efferocytosis, and overall protein expression of p-Mertk, p-ERK, and p-Stat6. The percentage of GFP+ monocyte/macrophages and resident microglia engulfing NeuN+ or TUNEL+ cells was significantly higher in KO chimeric mice. Importantly, mRNA expression of Mertk and its cognate ligand Gas6 was significantly elevated in these mice compared to wild-type. Analysis of cell-specific expression showed that p-ERK and p-Stat6 co-localized with Mertk-expressing GFP + cells in the peri-lesional area of the cortex following brain injury. Using an in vitro efferocytosis assay, co-culturing pHrodo-labeled apoptotic Jurkat cells and bone marrow (BM)-derived macrophages, we demonstrate that efferocytosis efficiency and mRNA expression of Mertk and Gas6 was enhanced in the absence of EphA4. Select inhibitors of ERK and Stat6 attenuated this effect confirming that EphA4 suppresses monocyte/macrophage efferocytosis via inhibition of the ERK/Stat6 pathway.

Conclusions

Our findings implicate the Mertk/ERK/Stat6 axis as a novel regulator of apoptotic debris clearance in brain injury that is restricted by peripheral myeloid-derived EphA4 to prevent the resolution of inflammation.

Atypical Neurogenesis, Astrogliosis, and Excessive Hilar Interneuron Loss Are Associated with the Development of Post-Traumatic Epilepsy

BACKGROUND

Traumatic brain injury (TBI) remains a significant risk factor for post-traumatic epilepsy (PTE). The pathophysiological mechanisms underlying the injury-induced epileptogenesis are under investigation. The dentate gyrus-a structure that is highly susceptible to injury-has been implicated in the evolution of seizure development.

METHODS

Utilizing the murine unilateral focal control cortical impact (CCI) injury, we evaluated seizure onset using 24/7 EEG video analysis at 2-4 months post-injury. Cellular changes in the dentate gyrus and hilus of the hippocampus were quantified by unbiased stereology and Imaris image analysis to evaluate Prox1-positive cell migration, astrocyte branching, and morphology, as well as neuronal loss at four months post-injury. Isolation of region-specific astrocytes and RNA-Seq were performed to determine differential gene expression in animals that developed post-traumatic epilepsy (PTE+) vs. those animals that did not (PTE-), which may be associated with epileptogenesis.

RESULTS

CCI injury resulted in 37% PTE incidence, which increased with injury severity and hippocampal damage. Histological assessments uncovered a significant loss of hilar interneurons that coincided with aberrant migration of Prox1-positive granule cells and reduced astroglial branching in PTE+ compared to PTE- mice. We uniquely identified Cst3 as a PTE+-specific gene signature in astrocytes across all brain regions, which showed increased astroglial expression in the PTE+ hilus.

CONCLUSIONS

These findings suggest that epileptogenesis may emerge following TBI due to distinct aberrant cellular remodeling events and key molecular changes in the dentate gyrus of the hippocampus.

Deletion of PTEN in microglia ameliorates chronic neuroinflammation following repetitive mTBI

Traumatic brain injury is a leading cause of morbidity and mortality in adults and children in developed nations. Following the primary injury, microglia, the resident innate immune cells of the CNS, initiate several inflammatory signaling cascades and pathophysiological responses that may persist chronically; chronic neuroinflammation following TBI has been closely linked to the development of neurodegeneration and neurological dysfunction. Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that have been shown to regulate several key mechanisms in the inflammatory response to TBI. Increasing evidence has shown that the modulation of the PI3K/AKT signaling pathway has the potential to influence the cellular response to inflammatory stimuli. However, directly targeting PI3K signaling poses several challenges due to its regulatory role in several cell survival pathways. We have previously identified that the phosphatase and tensin homolog deleted on chromosome 10 (PTEN), the major negative regulator of PI3K/AKT signaling, is dysregulated following exposure to repetitive mild traumatic brain injury (r-mTBI). Moreover, this dysregulated PI3K/AKT signaling was correlated with chronic microglial-mediated neuroinflammation. Therefore, we interrogated microglial-specific PTEN as a therapeutic target in TBI by generating a microglial-specific, Tamoxifen inducible conditional PTEN knockout model using a CX3CR1 Cre recombinase mouse line PTEN^fl/fl/CX3CR1^+/CreERT2 (mcg-PTEN^cKO), and exposed them to our 20-hit r-mTBI paradigm. Animals were treated with tamoxifen at 76 days post-last injury, and the effects of microglia PTEN deletion on immune-inflammatory responses were assessed at 90-days post last injury. We observed that the deletion of microglial PTEN ameliorated the proinflammatory response to repetitive brain trauma, not only reducing chronic microglial activation and proinflammatory cytokine production but also rescuing TBI-induced reactive astrogliosis, demonstrating that these effects extended beyond microglia alone. Additionally, we observed that the pharmacological inhibition of PTEN with BpV(HOpic) ameliorated the LPS-induced activation of microglial NFκB signaling in vitro. Together, these data provide support for the role of PTEN as a regulator of chronic neuroinflammation following repetitive mild TBI.