EphA4/Tie2 crosstalk regulates leptomeningeal collateral remodeling following ischemic stroke

Evolucollateral dynamics in stroke: Evolutionary pathophysiology, remodelling and emerging therapeutic strategies

Leptomeningeal collaterals (LMCs) are crucial in mitigating the impact of acute ischemic stroke (AIS) by providing alternate blood flow routes when primary arteries are obstructed. This article explores the evolutionary pathophysiology of LMCs, highlighting their critical function in stroke and the genetic and molecular mechanisms governing their development and remodelling. We address the translational challenges of applying animal model findings to human clinical scenarios, emphasizing the need for further research to validate emerging therapies-such as pharmacological agents, gene therapy and mechanical interventions-in clinical settings, aimed at enhancing collateral perfusion. Computational modelling emerges as a promising method for integrating experimental data, which requires precise parameterization and empirical validation. We introduce the 'Evolucollateral Dynamics' hypothesis, proposing a novel framework that incorporates evolutionary biology principles into therapeutic strategies, offering new perspectives on enhancing collateral circulation. This hypothesis emphasizes the role of genetic predispositions and environmental influences on collateral circulation, which may impact therapeutic strategies and optimize treatment outcomes. Future research must incorporate human clinical data to create robust treatment protocols, thereby maximizing the therapeutic potential of LMCs and improving outcomes for stroke patients.

Integrated Strategies for Targeting Arteriogenesis and Angiogenesis After Stroke

The interdependence between arteriogenesis and angiogenesis is crucial for enhancing perfusion by synchronously improving leptomeningeal collaterals (LMCs) and microvascular networks after stroke. However, current approaches often focus on promoting arteriogenesis and angiogenesis separately, neglecting the potential synergistic benefits of targeting both processes simultaneously. Therefore, it is imperative to consider both arteriogenesis and angiogenesis as integral and complementary strategies for post-stroke revascularization. To gain a deeper understanding of their relationships after stroke and to facilitate the development of targeted revascularization strategies, we compared them based on their timescale, space, and pathophysiology. The temporal differences in the occurrence of arteriogenesis and angiogenesis allow them to restore blood flow at different stages after stroke. The spatial differences in the effects of arteriogenesis and angiogenesis enable them to specifically target the ischemic penumbra and core infarct region. Additionally, the endothelial cell, as the primary effector cell in their pathophysiological processes, is promising target for enhancing both. Therefore, we provide an overview of key signals that regulate endothelium-mediated arteriogenesis and angiogenesis. Finally, we summarize current therapeutic strategies that involve these signals to promote both processes after stroke, with the aim of inspiring future therapeutic advances in revascularization.

Long non-coding RNA SNHG12 regulates leptomeningeal collateral remodeling via RGMa after ischemic stroke

Leptomeningeal anastomoses or pial collateral arteries are crucial for restoring cerebral blood flow (CBF) after an ischemic stroke. Vascular smooth muscle cells (VSMCs) are hypothesized to regulate the extent of this adaptive response, while the specific molecular mechanisms underlying this process are still being investigated. SNHG12, a long non-coding RNA, has been shown to influence several diseases related angiogenesis, including osteosarcoma and gastric cancer. However, the role of SNHG12 in contractile VSMC dedifferentiation during collateral arteriogenesis-related strokes remains unclear. Here we demonstrated that SNHG12 is a positive regulator of MMP9 and VSMC dedifferentiation, which enhances pial collateral arteriogenesis following cerebrovascular occlusion. Pial collateral remodeling is limited by the crosstalk between SNHG12-MMP9 signaling in VSMCs, which is mediated through repulsive guidance molecule a (RGMa) regulation. Thus, targeting SNHG12 may represent a therapeutic strategy for improving collateral function, neural tissue health, and functional recovery following ischemic stroke.

Transcriptomic alterations in cortical astrocytes following the development of post-traumatic epilepsy

Post-traumatic epilepsy (PTE) stands as one of the numerous debilitating consequences that follow traumatic brain injury (TBI). Despite its impact on many individuals, the current landscape offers only a limited array of reliable treatment options, and our understanding of the underlying mechanisms and susceptibility factors remains incomplete. Among the potential contributors to epileptogenesis, astrocytes, a type of glial cell, have garnered substantial attention as they are believed to promote hyperexcitability and the development of seizures in the brain following TBI. The current study evaluated the transcriptomic changes in cortical astrocytes derived from animals that developed seizures as a result of severe focal TBI. Using RNA-Seq and ingenuity pathway analysis (IPA), we unveil a distinct gene expression profile in astrocytes, including alterations in genes supporting inflammation, early response modifiers, and neuropeptide-amidating enzymes. The findings underscore the complex molecular dynamics in astrocytes during PTE development, offering insights into therapeutic targets and avenues for further exploration.

Inhibition of EphA4 reduces vasogenic edema after experimental stroke in mice by protecting the blood-brain barrier integrity

Cerebral vasogenic edema, a severe complication of ischemic stroke, aggravates neurological deficits. However, therapeutics to reduce cerebral edema still represent a significant unmet medical need. Brain microvascular endothelial cells (BMECs), vital for maintaining the blood-brain barrier (BBB), represent the first defense barrier for vasogenic edema. Here, we analyzed the proteomic profiles of the cultured mouse BMECs during oxygen-glucose deprivation and reperfusion (OGD/R). Besides the extensively altered cytoskeletal proteins, ephrin type-A receptor 4 (EphA4) expressions and its activated phosphorylated form p-EphA4 were significantly increased. Blocking EphA4 using EphA4-Fc, a specific and well-tolerated inhibitor shown in our ongoing human phase I trial, effectively reduced OGD/R-induced BMECs contraction and tight junction damage. EphA4-Fc did not protect OGD/R-induced neuronal and astrocytic death. However, administration of EphA4-Fc, before or after the onset of transient middle cerebral artery occlusion (tMCAO), reduced brain edema by about 50%, leading to improved neurological function recovery. The BBB permeability test also confirmed that cerebral BBB integrity was well maintained in tMCAO brains treated with EphA4-Fc. Therefore, EphA4 was critical in signaling BMECs-mediated BBB breakdown and vasogenic edema during cerebral ischemia. EphA4-Fc is promising for the treatment of clinical post-stroke edema.

Remote Ischemic Conditioning Enhances Collateral Circulation Through Leptomeningeal Anastomosis and Diminishes Early Ischemic Lesions and Infarct Volume in Middle Cerebral Artery Occlusion

Remote ischemic conditioning (RIC) has attracted much attention as a protective strategy for the heart and brain, although the underlying mechanisms remain unclear. We hypothesized that RIC enhances collateral circulation during cerebral ischemia through endothelial function and mitigates both early ischemic change and final infarct volume. We tested the RIC and sham procedure 30 min after permanent middle cerebral artery occlusion (MCAO) in male mice. Collateral circulation was examined during the procedure with 2D color-coded ultrasound imaging. Immediately after four cycles of RIC, early ischemic lesions on magnetic resonance imaging (MRI), diffusion-weighted imaging (DWI), and development of pial collateral vessels were examined. The neurological signs and infarct volume with TTC were examined until 48 h after daily RIC. As compared with sham procedure, RIC enhanced collateral circulation, diminished early ischemic lesions, enlarged pial collaterals, and mitigated infarct volume. Next, we examined the effect of inhibitor of nitric oxide synthase (NOS) and Akt on the beneficial effect of RIC in MCAO. Both allosteric Akt inhibitor, 8-[4-(1-Aminocyclobutyl)phenyl]-9-phenyl[1,2,4]triazolo[3,4-f][1,6]naphthyridin-3(2H)-one (MK2206), and two NOS inhibitors, N5-(1-Iminoethyl)-L-ornithine dihydrochloride (L-NIO) and NG-Nitro-L-arginine methyl ester hydrochloride (L-NAME), counteracted the beneficial effect of RIC on collateral circulation, early lesions, pial anastomosis, and infarct volume. In permanent MCAO, RIC could enhance collateral circulation through leptomeningeal anastomosis with Akt-eNOS pathway and diminish early lesion and final infarct volume.

Lipidation and PEGylation Strategies to Prolong the in Vivo Half-Life of a Nanomolar EphA4 Receptor Antagonist

The EphA4 receptor tyrosine kinase plays a role in neurodegenerative diseases, inhibition of nerve regeneration, cancer progression and other diseases. Therefore, EphA4 inhibition has potential therapeutic value. Selective EphA4 kinase inhibitors are not available, but we identified peptide antagonists that inhibit ephrin ligand binding to EphA4 with high specificity. One of these peptides is the cyclic APY-d3 (βAPYCVYRβASWSC-NH2), which inhibits ephrin-A5 ligand binding to EphA4 with low nanomolar binding affinity and is highly protease resistant. Here we describe modifications of APY-d3 that yield two different key derivatives with greatly increased half-lives in the mouse circulation, the lipidated APY-d3-laur8 and the PEGylated APY-d3-PEG4. These two derivatives inhibit ligand induced EphA4 activation in cells with sub-micromolar potency. Since they retain high potency and specificity for EphA4, lipidated and PEGylated APY-d3 derivatives represent new tools for discriminating EphA4 activities in vivo and for preclinical testing of EphA4 inhibition in animal disease models.

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.

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.

MicroRNA-204-5p Ameliorates Neurological Injury via the EphA4/PI3K/AKT Signaling Pathway in Ischemic Stroke

Ischemic stroke has extremely high mortality and disability rates worldwide. miR-204-5p has been reported to be associated with neurological diseases. However, the relationship linking miR-204-5p to ischemic stroke and its molecular mechanism remain unclear. Herein, we found that expression of miR-204-5p was significantly decreased while EphA4 increased in vivo and vitro, which reached the peak at 24 h after cerebral ischemia/reperfusion. Then, we altered miR-204-5p expression in rats by cerebroventricular injection. Our study showed that miR-204-5p overexpression obviously reduced the brain infarction area and neurological score. We successfully cultured neurons to investigate the downstream mechanism. Upregulation of miR-204-5p increased cell viability and suppressed the release of LDH. Moreover, the proportion of apoptotic cells tested by TUNEL and flow cytometry and protein expression of Cleaved Caspase3 and Bax were inhibited. The relative expression of IL-6, TNF-α, and IL-1β was repressed. In contrary, knockdown of miR-204-5p showed the opposite results. Bioinformatics and a dual luciferase assay illustrated that EphA4 was a target gene. Further research studies demonstrated that the neuroprotective effects of miR-204-5p could be partially mitigated by upregulating EphA4. Next, we proved that the miR-204-5p/EphA4 axis furtherly activated the PI3K/AKT pathway. We thoroughly illustrated the role of neuroinflammation and apoptosis. However, whether there are other mechanisms associated with the EphA4/PI3K/AKT pathway needs further investigation. Altogether, the miR-204-5p axis ameliorates neurological injury via the EphA4/PI3K/AKT pathway, which is expected to serve as an effective treatment for ischemic stroke.

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.

Radiation prevents tumor progression by inhibiting the miR‑93‑5p/EphA4/NF‑κB pathway in triple‑negative breast cancer

Breast cancer (BC) is the most common type of cancer in women. Triple‑negative BC (TNBC) constitutes 10‑15% of all BC cases and is associated with a poor prognosis. It has previously been reported that microRNA (miR)‑93‑5p is dysregulated in plasma exosomes from patients with BC and that miR‑93‑5p improves radiosensitivity in BC cells. The present study identified EphA4 as a potential target gene of miR‑93‑5p and investigated the pathway related to miR‑93‑5p in TNBC. Cell transfection and nude mouse experiments were performed to verify the role of the miR‑93‑5p/EphA4/NF‑κB pathway. Moreover, miR‑93‑5p, EphA4 and NF‑κB were detected in clinical patients. The results revealed that EphA4 and NF‑κB were downregulated in the miR‑93‑5p overexpression group. By contrast, EphA4 and NF‑κB expression levels were not significantly altered in the miR‑93‑5p overexpression + radiation group compared with those in the radiation group. Furthermore, overexpression of miR‑93‑5p with concomitant radiation therapy significantly decreased the growth of TNBC tumors in vivo. In conclusion, the present study revealed that miR‑93‑5p targeted EphA4 in TNBC through the NF‑κB pathway. However, radiation therapy prevented tumor progression by inhibiting the miR‑93‑5p/EphA4/NF‑κB pathway. Therefore, it would be interesting to elucidate the role of miR‑93‑5p in clinical research.

Elabela-APJ axis mediates angiogenesis via YAP/TAZ pathway in cerebral ischemia/reperfusion injury

Angiogenesis helps to improve neurological recovery by repairing damaged brain tissue and restoring cerebral blood flow (CBF). The role of the Elabela (ELA)-Apelin receptor (APJ) system in angiogenesis has gained much attention. We aimed to investigate the function of endothelial ELA on postischemic cerebral angiogenesis. Here, we demonstrated that the endothelial ELA expression was upregulated in the ischemic brain and treatment with ELA-32 mitigated brain injury and enhanced the restoration of CBF and newly formed functional vessels following cerebral ischemia/reperfusion (I/R) injury. Furthermore, ELA-32 incubation potentiated proliferation, migration, and tube formation abilities of the mouse brain endothelial cells (bEnd.3 cells) under oxygen-glucose deprivation/reoxygenation (OGD/R) condition. RNA sequencing analysis indicated that ELA-32 incubation had a role in the Hippo signaling pathway, and improved angiogenesis-related gene expression in OGD/R-exposed bEnd.3 cells. Mechanistically, we depicted that ELA could bind to APJ and subsequently activate YAP/TAZ signaling pathway. Silence of APJ or pharmacological blockade of YAP abolished the pro-angiogenesis effects of ELA-32. Together, these findings highlight the ELA-APJ axis as a potential therapeutic strategy for ischemic stroke by showing how activation of this pathway promotes poststroke angiogenesis.

Near infrared spectroscopy detection of hemispheric cerebral ischemia following middle cerebral artery occlusion in rats

Timely and sensitive in vivo estimation of ischemic stroke-induced brain infarction are necessary to guide diagnosis and evaluation of treatments' efficacy. The gold standard for estimation of the cerebral infarction volume is magnetic resonance imaging (MRI), which is expensive and not readily accessible. Measuring regional cerebral blood flow (rCBF) with Laser Doppler flowmetry (LDF) is the status quo for confirming reduced blood flow in experimental ischemic stroke models. However, rCBF reduction following cerebral artery occlusion often does not correlate with subsequent infarct volume. In the present study, we employed the continuous-wave near infrared spectroscopy (NIRS) technique to monitor cerebral oxygenation during 90 min of the intraluminal middle cerebral artery occlusion (MCAO) in Sprague-Dawley rats (n = 8, male). The NIRS device consisted of a controller module and an optical sensor with two LED light sources and two photodiodes making up two parallel channels for monitoring left and right cerebral hemispheres. Optical intensity measurements were converted to deoxyhemoglobin (Hb) and oxyhemoglobin (HbO2) changes relative to a 2-min window prior to MCAO. Area under the curve (auc) for Hb and HbO2 was calculated for the 90-min occlusion period for each hemisphere (ipsilateral and contralateral). To obtain a measure of total ischemia, auc of the contralateral side was subtracted from the ipsilateral side resulting in ΔHb and ΔHbO2 parameters. Infarct volume (IV) was calculated by triphenyl tetrazolium chloride (TTC) staining at 24h reperfusion. Results showed a significant negative correlation (r = -0.81, p = 0.03) between ΔHb and infarct volume. In conclusion, our results show feasibility of using a noninvasive optical imaging instrument, namely NIRS, in monitoring cerebral ischemia in a rodent stroke model. This cost-effective, non-invasive technique may improve the rigor of experimental models of ischemic stroke by enabling in vivo longitudinal assessment of cerebral oxygenation and ischemic injury.

Neutrophil dynamics and inflammaging in acute ischemic stroke: A transcriptomic review

Stroke is among the leading causes of death and disability worldwide. Restoring blood flow through recanalization is currently the only acute treatment for cerebral ischemia. Unfortunately, many patients that achieve a complete recanalization fail to regain functional independence. Recent studies indicate that activation of peripheral immune cells, particularly neutrophils, may contribute to microcirculatory failure and futile recanalization. Stroke primarily affects the elderly population, and mortality after endovascular therapies is associated with advanced age. Previous analyses of differential gene expression across injury status and age identify ischemic stroke as a complex age-related disease. It also suggests robust interactions between stroke injury, aging, and inflammation on a cellular and molecular level. Understanding such interactions is crucial in developing effective protective treatments. The global stroke burden will continue to increase with a rapidly aging human population. Unfortunately, the mechanisms of age-dependent vulnerability are poorly defined. In this review, we will discuss how neutrophil-specific gene expression patterns may contribute to poor treatment responses in stroke patients. We will also discuss age-related transcriptional changes that may contribute to poor clinical outcomes and greater susceptibility to cerebrovascular diseases.

Exosomal miR-532-5p induced by long-term exercise rescues blood-brain barrier function in 5XFAD mice via downregulation of EPHA4

The breakdown of the blood-brain barrier, which develops early in Alzheimer's disease (AD), contributes to cognitive impairment. Exercise not only reduces the risk factors for AD but also confers direct protection against cognitive decline. However, the exact molecular mechanisms remain elusive, particularly whether exercise can liberate the function of the blood-brain barrier. Here, we demonstrate that long-term exercise promotes the clearance of brain amyloid-β by improving the function of the blood-brain barrier in 5XFAD mice. Significantly, treating primary brain pericytes or endothelial cells with exosomes isolated from the brain of exercised 5XFAD mice improves cell proliferation and upregulates PDGFRβ, ZO-1, and claudin-5. Moreover, exosomes isolated from exercised mice exhibit significant changes in miR-532-5p. Administration or transfection of miR-532-5p to sedentary mice or primary brain pericytes and endothelial cells reproduces the improvement of blood-brain barrier function. Exosomal miR-532-5p targets EPHA4, and accordingly, expression of EphA4 is decreased in exercised mice and miR-532-5p overexpressed mice. A specific siRNA targeting EPHA4 recapitulates the effects on blood-brain barrier-associated cells observed in exercised 5XFAD mice. Overall, our findings suggest that exosomes released by the brain contain a specific miRNA that is altered by exercise and has an impact on blood-brain barrier function in AD.

Intermittent hypoxia conditioning as a potential prevention and treatment strategy for ischemic stroke: Current evidence and future directions

Ischemic stroke (IS) is the leading cause of disability and death worldwide. Owing to the aging population and unhealthy lifestyles, the incidence of cerebrovascular disease is high. Vascular risk factors include hypertension, diabetes, dyslipidemia, and obesity. Therefore, in addition to timely and effective reperfusion therapy for IS, it is crucial to actively control these risk factors to reduce the incidence and recurrence rates of IS. Evidence from human and animal studies suggests that moderate intermittent hypoxia (IH) exposure is a promising therapeutic strategy to ameliorate common vascular risk factors and comorbidities. Given the complex pathophysiological mechanisms underlying IS, effective treatment must focus on reducing injury in the acute phase and promoting repair in the recovery phase. Therefore, this review discusses the preclinical perspectives on IH conditioning as a potential treatment for neurovascular injury and highlights IH pre and postconditioning strategies for IS. Hypoxia conditioning reduces brain injury by increasing resistance to acute ischemic and hypoxic stress, exerting neuroprotective effects, and promoting post-injury repair and regeneration. However, whether IH produces beneficial effects depends not only on the hypoxic regimen but also on inter-subject differences. Therefore, we discuss the factors that may influence the effectiveness of IH treatment, including age, sex, comorbidities, and circadian rhythm, which can be used to help identify the optimal intervention population and treatment protocols for more accurate, individualized clinical translation. In conclusion, IH conditioning as a non-invasive, non-pharmacological, systemic, and multi-targeted intervention can not only reduce brain damage after stroke but can also be applied to the prevention and functional recovery of IS, providing brain protection at different stages of the disease. It represents a promising therapeutic strategy. For patients with IS and high-risk groups, IH conditioning is expected to develop as an adjunctive clinical treatment option to reduce the incidence, recurrence, disability, and mortality of IS and to reduce disease burden.

Monocyte proinflammatory phenotypic control by ephrin type A receptor 4 mediates neural tissue damage

Circulating monocytes have emerged as key regulators of the neuroinflammatory milieu in a number of neuropathological disorders. Ephrin type A receptor 4 (Epha4) receptor tyrosine kinase, a prominent axon guidance molecule, has recently been implicated in the regulation of neuroinflammation. Using a mouse model of brain injury and a GFP BM chimeric approach, we found neuroprotection and a lack of significant motor deficits marked by reduced monocyte/macrophage cortical infiltration and an increased number of arginase-1+ cells in the absence of BM-derived Epha4. This was accompanied by a shift in monocyte gene profile from pro- to antiinflammatory that included increased Tek (Tie2 receptor) expression. Inhibition of Tie2 attenuated enhanced expression of M2-like genes in cultured Epha4-null monocytes/macrophages. In Epha4-BM-deficient mice, cortical-isolated GFP+ monocytes/macrophages displayed a phenotypic shift from a classical to an intermediate subtype, which displayed reduced Ly6chi concomitant with increased Ly6clo- and Tie2-expressing populations. Furthermore, clodronate liposome-mediated monocyte depletion mimicked these effects in WT mice but resulted in attenuation of phenotype in Epha4-BM-deficient mice. This demonstrates that monocyte polarization not overall recruitment dictates neural tissue damage. Thus, coordination of monocyte proinflammatory phenotypic state by Epha4 is a key regulatory step mediating brain injury.

Noninvasive Low-Intensity Focused Ultrasound Mediates Tissue Protection following Ischemic Stroke

Objective and Impact Statement. This study examined the efficacy and safety of pulsed, low-intensity focused ultrasound (LIFU) and determined its ability to provide neuroprotection in a murine permanent middle cerebral artery occlusion (pMCAO) model. Introduction. Focused ultrasound (FUS) has emerged as a new therapeutic strategy for the treatment of ischemic stroke; however, its nonthrombolytic properties remain ill-defined. Therefore, we examined how LIFU influenced neuroprotection and vascular changes following stroke. Due to the critical role of leptomeningeal anastomoses or pial collateral vessels, in cerebral blood flow restoration and tissue protection following ischemic stroke, we also investigated their growth and remodeling. Methods. Mice were exposed to transcranial LIFU (fundamental frequency: 1.1 MHz, sonication duration: 300 ms, interstimulus interval: 3 s, pulse repetition frequency: 1 kHz, duty cycle per pulse: 50%, and peak negative pressure: -2.0 MPa) for 30 minutes following induction of pMCAO and then evaluated for infarct volume, blood-brain barrier (BBB) disruption, and pial collateral remodeling at 24 hrs post-pMCAO. Results. We found significant neuroprotection in mice exposed to LIFU compared to mock treatment. These findings correlated with a reduced area of IgG deposition in the cerebral cortex, suggesting attenuation of BBB breakdown under LIFU conditions. We also observed increased diameter of CD31-postive microvessels in the ischemic cortex. We observed no significant difference in pial collateral vessel size between FUS and mock treatment at 24 hrs post-pMCAO. Conclusion. Our data suggests that therapeutic use of LIFU may induce protection through microvascular remodeling that is not related to its thrombolytic activity.

Leukocytes in Cerebral Thrombus Respond to Large-Vessel Occlusion in a Time-Dependent Manner and the Association of NETs With Collateral Flow

Thrombus components are dynamically influenced by local blood flow and blood immune cells. After a large-vessel occlusion stroke, changes in the cerebral thrombus are unclear. Here we assessed a total of 206 cerebral thrombi from patients with ischemic stroke undergoing endovascular thrombectomy. The thrombi were categorized by time to reperfusion of <4 h (T4), 4–8 h (T4–8), and >8 h (T8). The cellular compositions in thrombus were analyzed, and relevant clinical features were compared. Both white blood cells and neutrophils were increased and then decreased in thrombus with time to reperfusion, which were positively correlated with those in peripheral blood. The neutrophil extracellular trap (NET) content in thrombus was correlated with the degree of neurological impairment of patients. Moreover, with prolonged time to reperfusion, the patients showed a trend of better collateral grade, which was associated with a lower NET content in the thrombus. In conclusion, the present results reveal the relationship between time-related endovascular immune response and clinical symptoms post-stroke from the perspective of thrombus and peripheral blood. The time-related pathological changes of cerebral thrombus may not be the direct cause for the difficulty in thrombolysis and thrombectomy. A low NET content in thrombi indicates excellent collateral flow, which suggests that treatments targeting NETs in thrombi might be beneficial for early neurological protection.

Formation and Maintenance of the Natural Bypass Vessels of the Brain

Ischemic diseases are the leading cause of death and disability worldwide. The main compensatory mechanism by which our body responds to reduced or blocked blood flow caused by ischemia is mediated by collateral vessels. Collaterals are present in many healthy tissues (including brain and heart) and serve as natural bypass vessels, by bridging adjacent arterial trees. This review focuses on: the definition and significance of pial collateral vessels, the described mechanism of pial collateral formation, an overview of molecular players and pathways involved in pial collateral biology and emerging approaches to prevent or mitigate risk factor-associated loss of pial collaterals. Despite their high clinical relevance and recent scientific efforts toward understanding collaterals, much of the fundamental biology of collaterals remains obscure.

Leptomeningeal anastomoses: Mechanisms of pial collateral remodeling in ischemic stroke

Arterial collateralization, as determined by leptomeningeal anastomoses or pial collateral vessels, is a well‐established vital player in cerebral blood flow restoration and neurological recovery from ischemic stroke. A secondary network of cerebral collateral circulation apart from the Circle of Willis, exist as remnants of arteriole development that connect the distal arteries in the pia mater. Recent interest lies in understanding the cellular and molecular adaptations that control the growth and remodeling, or arteriogenesis, of these pre‐existing collateral vessels. New findings from both animal models and human studies of ischemic stroke suggest a multi‐factorial and complex, temporospatial interplay of endothelium, immune and vessel‐associated cell interactions may work in concert to facilitate or thwart arteriogenesis. These valuable reports may provide critical insight into potential predictors of the pial collateral response in patients with large vessel occlusion and may aid in therapeutics to enhance collateral function and improve recovery from stroke.

This article is categorized under:

Neurological Diseases > Molecular and Cellular Physiology

Collateral Supply in Preclinical Cerebral Stroke Models

Enhancing the collateral blood supply during the acute phase of cerebral ischemia may limit both the extension of the core infarct, by rescuing the penumbra area, and the degree of disability. Many imaging techniques have been applied to rodents in preclinical studies, to evaluate the magnitude of collateral blood flow and the time course of responses during the early phase of ischemic stroke. The collateral supply follows several different routes at the base of the brain (the circle of Willis) and its surface (leptomeningeal or pial arteries), corresponding to the proximal and distal collateral pathways, respectively. In this review, we describe and illustrate the cerebral collateral systems and their modifications following pre-Willis or post-Willis occlusion in rodents. We also review the potential pharmaceutical agents for stimulating the collateral blood supply tested to date. The time taken to establish a collateral blood flow supply through the leptomeningeal anastomoses differs between young and adult animals and between different species and genetic backgrounds. Caution is required when transposing preclinical findings to humans, and clinical trials must be performed to check the added value of pharmacological agents for stimulating the collateral blood supply at appropriate time points. However, collateral recruitment appears to be a rapid, beneficial, endogenous mechanism that can be stimulated shortly after artery occlusion. It should be considered a treatment target for use in addition to recanalization strategies.

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

Erythropoietin-producing human hepatocellular receptors play a major role in central nervous system injury. Preclinical and clinical studies revealed the upregulation of erythropoietin-producing human hepatocellular A4 (EphA4) receptors in the brain after acute traumatic brain injury. We have previously reported that Cx3cr1-expressing cells in the peri-lesion show high levels of EphA4 after the induction of controlled cortical impact (CCI) injury in mice. Cx3cr1 is a fractalkine receptor expressed on both resident microglia and peripheral-derived macrophages. The current study aimed to determine the role of microglial-specific EphA4 in CCI-induced damage. We used Cx3cr1 ^CreER/+ knock-in/knock-out mice, which express EYFP in Cx3cr1-positive cells to establish microglia, EphA4-deficient mice following 1-month tamoxifen injection. Consistent with our previous findings, induction of CCI in wild-type (WT) Cx3cr1 ^CreER/+ EphA4 ^+/+ mice increased EphA4 expression on EYFP-positive cells in the peri-lesion. To distinguish between peripheral-derived macrophages and resident microglia, we exploited GFP bone marrow-chimeric mice and found that CCI injury increased EphA4 expression in microglia (TMEM119+GFP-) using immunohistochemistry. Using Cx3cr1 ^CreER/+ EphA4 ^f/f (KO) mice, we observed that the EphA4 mRNA transcript was undetected in microglia but remained present in whole blood when compared to WT. Finally, we found no difference in lesion volume or blood-brain barrier (BBB) disruption between WT and KO mice at 3 dpi. Our data demonstrate a nonessential role of microglial EphA4 in the acute histopathological outcome in response to CCI.

Human urine-derived stem cell-derived exosomal miR-21-5p promotes neurogenesis to attenuate Rett syndrome via the EPha4/TEK axis

Rett syndrome (RTT) is a rare neurodevelopmental disorder that results in multiple disabilities. Exosomal microRNA (miRs) from urine-derived stem cells (USCs) have been shown to induce neurogenesis and aid in functional recovery from brain ischemia. In the present study, we sought to determine whether that exosomal miR-21-5p from USCs could promote early neural formation in a model of RTT. USCs were isolated and evaluated by flow cytometry. Exosomes were analyzed by transmission electron microscopy, tunable resistive pulse sensing (TRPS), and western blotting. PKH26 fluorescent dyes were used to observe intake of exosomes in vivo and in vitro. An RTT mouse model was treated with exosomes for behavioral studies. Dual-luciferase report gene assays were conducted to evaluate the relationship between miR-21-5p and Eph receptor A4 (EphA4). In vitro, treatment with exosomes from human urine-derived stem cells (USC-Exos) increased the percentage of neuron-specific class III beta-tubulin (Tuj1)+ nerve cells as well as the transcription levels of β-III tubulin and doublecortin (DCX). A higher level of miR-21-5p was observed in USC-Exos, which promoted differentiation in NSCs by targeting the EPha4/TEK axis. In vivo, exosomal miR-21-5p improved the behavior, motor coordination, and cognitive ability of mice, facilitated the differentiation of NSCs in the subventricular zone of the lateral ventricle and promoted a marked rise in the number of DCX+ cells. Our data provide evidence that exosomal miR-21-5p from human USCs facilitate early nerve formation by regulating the EPha4/TEK axis.

Matrix metallopeptidase 9 and placental growth factor may correlate with collateral status based on whole-brain perfusion combined with multiphase computed tomography angiography

OBJECTIVE

The aim of this study was to find out the relationship between serum biomarkers and cerebral collateral status in acute ischemic stroke with cerebral large artery atherosclerosis.

METHODS

We enrolled patients with ischemic stroke due to large artery atherosclerosis within 7 days of symptom onset, age 18-80 years, from August 2016 to December 2017. Twelve biomarkers representing different pathophysiological mechanisms were tested after admission. Whole-brain perfusion combined with multiphase computed tomography angiography was performed to assess cerebral collateral structure and function.

RESULTS

Fifty-two patients completed the test of candidate biomarkers and recruited in this study. The mean age was 55.0 (11.1) years, 42 (80.8%) patients were male, 20 (38.5%) had poor collateral, 36 (69.2%) patients had anterior circulation stenosis or occlusion. Compared with poor collateral group, the level of MMP-9 (135,475.00 pg/ml vs. 103,612.00 pg/ml, p = 0.040) and PGF (5.75 pg/ml vs. 3.46 pg/ml, p = 0.046) was significantly higher in good collateral group. The adjusted OR (95%CI) of MMP-9 and PGF were 5.533 (1.10-27.74, p = 0.038), 7.73 (1.41-42.39, p = 0.018), respectively. sTie-2 level had a positive correlation with proportion of Tmax 4-6 (r = 0.302, p = 0.033) and HMW-KGN had negative correlation with proportion of Tmax 6-8 (r = -0.338, p = 0.02). After adjustment, the correlation of sTie-2 level and proportion of Tmax 4-6 was statistically significant (p = 0.003), and correlation of HMW-KGN and Tmax6-8 was not statistically significant (p = 0.056).

DISCUSSION

Serum PGF and MMP-9 levels may correlate with collateral status based on MP-CTA in acute ischemic stroke patients with cerebral large artery atherosclerosis. Higher PGF and MMP-9 concentration associated with good collateral status.

Abrogation of atypical neurogenesis and vascular-derived EphA4 prevents repeated mild TBI-induced learning and memory impairments

Brain injury resulting from repeated mild traumatic insult is associated with cognitive dysfunction and other chronic co-morbidities. The current study tested the effects of aberrant neurogenesis in a mouse model of repeated mild traumatic brain injury (rmTBI). Using Barnes Maze analysis, we found a significant reduction in spatial learning and memory at 24 days post-rmTBI compared to repeated sham (rSham) injury. Cell fate analysis showed a greater number of BrdU-labeled cells which co-expressed Prox-1 in the DG of rmTBI-injured mice which coincided with enhanced cFos expression for neuronal activity. We then selectively ablated dividing neural progenitor cells using a 7-day continuous infusion of Ara-C prior to rSham or rmTBI. This resulted in attenuation of cFos and BrdU-labeled cell changes and prevented associated learning and memory deficits. We further showed this phenotype was ameliorated in EphA4f./f/Tie2-Cre knockout compared to EphA4f./f wild type mice, which coincided with altered mRNA transcript levels of MCP-1, Cx43 and TGFβ. These findings demonstrate that cognitive decline is associated with an increased presence of immature neurons and gene expression changes in the DG following rmTBI. Our data also suggests that vascular EphA4-mediated neurogenic remodeling adversely affects learning and memory behavior in response to repeated insult.