Transient global cerebral ischemia induces rapid and sustained reorganization of synaptic structures

Rapamycin Alleviates Neuronal Injury and Modulates Microglial Activation After Cerebral Ischemia

Neurons and microglia are sensitive to cerebral microcirculation and their responses play a crucial part in the pathological processes, while they are also the main target cells of many drugs used to treat brain diseases. Rapamycin exhibits beneficial effects in many diseases; however, whether it can affect neuronal injury or alter the microglial activation after global cerebral ischemia remains unclear. In this study, we performed global cerebral ischemia combined with rapamycin treatment in CX3CR1GFP/+ mice and explored the effects of rapamycin on neuronal deficit and microglial activation. Our results showed that rapamycin reduced neuronal loss, neurodegeneration, and ultrastructural damage after ischemia by histological staining and transmission electron microscopy (TEM). Interestingly, rapamycin suppressed de-ramification and proliferation of microglia and reduced the density of microglia. Immunofluorescence staining indicated that rapamycin skewed microglial polarization toward an anti-inflammatory state. Furthermore, rapamycin as well suppressed the activation of astrocytes. Meanwhile, quantitative real-time polymerase chain reaction (qRT-PCR) analyses revealed a significant reduction of pro-inflammatory factors as well as an elevation of anti-inflammatory factors upon rapamycin treatment. As a result of these effects, behavioral tests showed that rapamycin significantly alleviated the brain injury after stroke. Together, our study suggested that rapamycin attenuated neuronal injury, altered microglial activation state, and provided a more beneficial immune microenvironment for the brain, which could be used as a promising therapeutic approach to treat ischemic cerebrovascular diseases.

Effect of Early Inhibition of Toll-Like Receptor 4 on Hippocampal Plasticity in a Neonatal Rat Model of Hypoxic-Ischemic Brain Damage

Hippocampal plasticity is closely related to physiological brain functions such as learning and memory. However, the effect of toll-like receptor 4 (TLR4) activation on hippocampal plasticity after neonatal hypoxic-ischaemic brain damage (HIBD) remains unclear. In our study, seven-day-old rat pups were randomly categorised into three groups: control, hypoxic-ischemia (HI), and HI + TAK-242 (TAK-242). The pups were ligated in the left common carotid artery and then subjected to hypoxia to establish the neonatal HIBD model.The expression of the TLR4 in the left hippocampus of the HI group was increased compared to the control group, while TAK-242 reduced the expression level at 3 days after HIBD. Additionally, TAK-242 reversed the increased Zea-Longa score, increased the left/right hippocampal weight ratio, and increased the number of Nissl-positive neurons in the hippocampal CA1 region compared to HI group at 3 days after HIBD. Pre-injection of TAK-242 alleviated the decrease in PSD95, Aggrecan and NR1, BDNF, CREB, and pCREB expression in the hippocampus at 24 h after HIBD. It also alleviated the decrease in PSD95, BDNF, and NR2A/NR1 expression in the hippocampus at 7 days after HIBD. Furthermore, Pre-injection of TAK-242 alleviated the decrease in NR2A/NR1 expression at 21 days after HIBD. Finally,TAK-242 increased the percentage of third-grade dendritic mushroom spines processes in the basal and apical segments of neurons in the hippocampal CA1 region at 21 days after HIBD.Therefore, we conclude that preinhibition of TLR4 prior to neonatal HIBD improved the plasticity of the hippocampus.

Cystatin C loaded in brain-derived extracellular vesicles rescues synapses after ischemic insult in vitro and in vivo

Synaptic loss is an early event in the penumbra area after an ischemic stroke. Promoting synaptic preservation in this area would likely improve functional neurological recovery. We aimed to detect proteins involved in endogenous protection mechanisms of synapses in the penumbra after stroke and to analyse potential beneficial effects of these candidates for a prospective stroke treatment. For this, we performed Liquid Chromatography coupled to Mass Spectrometry (LC-MS)-based proteomics of synaptosomes isolated from the ipsilateral hemispheres of mice subjected to experimental stroke at different time points (24 h, 4 and 7 days) and compared them to sham-operated mice. Proteomic analyses indicated that, among the differentially expressed proteins between the two groups, cystatin C (CysC) was significantly increased at 24 h and 4 days following stroke, before returning to steady-state levels at 7 days, thus indicating a potential transient and intrinsic rescue mechanism attempt of neurons. When CysC was applied to primary neuronal cultures subjected to an in vitro model of ischemic damage, this treatment significantly improved the preservation of synaptic structures. Notably, similar effects were observed when CysC was loaded into brain-derived extracellular vesicles (BDEVs). Finally, when CysC contained in BDEVs was administered intracerebroventricularly to stroked mice, it significantly increased the expression of synaptic markers such as SNAP25, Homer-1, and NCAM in the penumbra area compared to the group supplied with empty BDEVs. Thus, we show that CysC-loaded BDEVs promote synaptic protection after ischemic damage in vitro and in vivo, opening the possibility of a therapeutic use in stroke patients.

TXNIP/NLRP3 aggravates global cerebral ischemia-reperfusion injury-induced cognitive decline in mice

Global cerebral ischemia/reperfusion (GCI/R) injury poses a risk for cognitive decline, with neuroinflammation considered pivotal in this process. This study aimed to unravel the molecular mechanisms underlying GCI/R injury and propose a potential therapeutic strategy for associated cognitive deficits. Utilizing bioinformatics analysis of a public microarray profile (GSE30655 and GSE80681) in cerebral ischemic mice, it was observed that neuroinflammation emerged as a significant gene ontology item, with an increase in the expression of thioredoxin-interacting protein (TXNIP) and NLRP3 genes. Experimental models involving bilateral occlusion of the common carotid arteries in mice revealed that GCI/R induced cognitive impairment, along with a time-dependent increase in TXNIP and NLRP3 levels. Notably, TXNIP knockdown alleviated cognitive dysfunction in mice. Furthermore, the introduction of adeno-associated virus injection with TXNIP knockdown reduced the number of activated microglia, apoptosis neurons, and levels of oxidative stress and inflammatory cytokines in the hippocampus. Collectively, these findings underscore the significance of TXNIP/NLRP3 in the hippocampus in exacerbating cognitive decline due to GCI/R injury, suggesting that TXNIP knockdown holds promise as a therapeutic strategy.

Regulation of Microglial Activation by Wnt/β-Catenin Signaling After Global Cerebral Ischemia in Mice

Microglia are immunocompetent cells in the central nervous system. Following cerebral ischemia, microglia will be rapidly activated and undergo proliferation, morphological transformation, and changes in gene expression and function. At present, the regulatory mechanisms of microglial activation following ischemia remain largely unclear. In this study, we took advantage of CX3CR1GFP/+ fluorescent mice and a global cerebral ischemia-reperfusion model to investigate the mechanisms of microglial activation following different degrees of global ischemia. Our results showed that the proliferation of microglia was gated by the degree of ischemia. Marked microglial de-ramification and proliferation were observed after 60 min of ischemia but not in transient ischemia (20 min). Immunohistology, qRT-PCR, and Western blotting analysis showed that microglial activation was accompanied with a reduction in Wnt/β-catenin signaling after cerebral ischemia. Downregulation of Wnt/β-catenin signaling using Wnt antagonist XAV939 during 20 min ischemia promoted microglial de-ramification and proliferation. In contrast, enhancing Wnt/β-catenin signaling using Wnt agonist LiCl during 60 min ischemia-reduced microglial de-ramification and proliferation. Importantly, we found that Wnt agonist inhibited inflammation in the ischemic brain and was conducive to animal behavioral recovery. Collectively, these data demonstrated that Wnt/β-catenin signaling played a key role in microglial activation following cerebral ischemia, and regulating microglial activation may be a potential therapeutic strategy for the treatment of ischemic stroke.

Dendrite regeneration mediates functional recovery after complete dendrite removal

Unlike many cell types, neurons are not typically replaced if damaged. Therefore, regeneration of damaged cellular domains is critical for maintenance of neuronal function. While axon regeneration has been documented for several hundred years, it has only recently become possible to determine whether neurons respond to dendrite removal with regeneration. Regrowth of dendrite arbors has been documented in invertebrate and vertebrate model systems, but whether it leads to functional restoration of a circuit remains unknown. To test whether dendrite regeneration restores function, we used larval Drosophila nociceptive neurons. Their dendrites detect noxious stimuli to initiate escape behavior. Previous studies of Drosophila sensory neurons have shown that dendrites of single neurons regrow after laser severing. We removed dendrites from 16 neurons per animal to clear most of the dorsal surface of nociceptive innervation. As expected, this reduced aversive responses to noxious touch. Surprisingly, behavior was completely restored 24 ​h after injury, at the stage when dendrite regeneration has begun, but the new arbor has only covered a small portion of its former territory. This behavioral recovery required regenerative outgrowth as it was eliminated in a genetic background in which new growth is blocked. We conclude that dendrite regeneration can restore behavior.

Neural Circuit Changes in Neurological Disorders: Evidence from in vivo Two-photon Imaging

Neural circuits, such as synaptic plasticity and neural activity, are critical components of healthy brain function. The consequent dynamic remodeling of neural circuits is an ongoing procedure affecting neuronal activities. Disruption of this essential process results in diseases. Advanced microscopic applications such as two-photon laser scanning microscopy have recently been applied to understand neural circuit changes during disease since it can visualize fine structural and functional cellular activation in living animals. In this review, we have summarized the latest work assessing the dynamic rewiring of postsynaptic dendritic spines and modulation of calcium transients in neurons of the intact living brain, focusing on their potential roles in neurological disorders (e.g. Alzheimer's disease, stroke, and epilepsy). Understanding the fine changes that occurred in the brain during disease is crucial for future clinical intervention developments.

The Implications of Microglial Regulation in Neuroplasticity-Dependent Stroke Recovery

Stroke causes varying degrees of neurological deficits, leading to corresponding dysfunctions. There are different therapeutic principles for each stage of pathological development. Neuroprotection is the main treatment in the acute phase, and functional recovery becomes primary in the subacute and chronic phases. Neuroplasticity is considered the basis of functional restoration and neurological rehabilitation after stroke, including the remodeling of dendrites and dendritic spines, axonal sprouting, myelin regeneration, synapse shaping, and neurogenesis. Spatiotemporal development affects the spontaneous rewiring of neural circuits and brain networks. Microglia are resident immune cells in the brain that contribute to homeostasis under physiological conditions. Microglia are activated immediately after stroke, and phenotypic polarization changes and phagocytic function are crucial for regulating focal and global brain inflammation and neurological recovery. We have previously shown that the development of neuroplasticity is spatiotemporally consistent with microglial activation, suggesting that microglia may have a profound impact on neuroplasticity after stroke and may be a key therapeutic target for post-stroke rehabilitation. In this review, we explore the impact of neuroplasticity on post-stroke restoration as well as the functions and mechanisms of microglial activation, polarization, and phagocytosis. This is followed by a summary of microglia-targeted rehabilitative interventions that influence neuroplasticity and promote stroke recovery.

LAMP-2A ablation in hippocampal CA1 astrocytes confers cerebroprotection and ameliorates neuronal injury after global brain ischemia

Reactive astrogliosis and neuronal death are major features of brain tissue damage after transient global cerebral ischemia/reperfusion (I/R). The CA1 subfield in the hippocampus is particularly susceptible to cell death after I/R. Recently, attention has focused on the relationship between the autophagy-lysosomal pathway and cerebral ischemia. Lysosomal-associated membrane protein type-2A (LAMP-2A) is a key protein in chaperone-mediated autophagy (CMA). However, LAMP-2A expression in astrocytes of the hippocampus and its influence on brain injury following I/R remain unknown. Here, we show that LAMP-2A is elevated in astrocytes of the CA1 hippocampal subfield after I/R and in primary cultured astrocytes after transient oxygen-glucose deprivation (OGD). Conditional LAMP-2A knockdown in CA1 astrocytes inhibited astrocyte activation and prevented neuronal death by inhibiting the mitochondrial pathway of apoptosis after I/R, suggesting that elevated astrocytic LAMP-2A contributes to regional ischemic vulnerability. Furthermore, astrocytic LAMP-2A ablation ameliorated the spatial learning and memory deficits caused by I/R. Conditional astrocytic LAMP-2A knockdown also prevented the loss of hippocampal synapses and dendritic spines, improved the synaptic ultrastructure, and inhibited the reduced expression of synaptic proteins after ischemia. Thus, our findings demonstrate that astrocytic LAMP-2A expression increases upon I/R and that LAMP-2A ablation specifically in hippocampal astrocytes contributes to cerebroprotection, suggesting a novel neuroprotective strategy for patients with global ischemia.

Association of Cerebral Blood Flow With Longitudinal Changes in Cerebral Microstructural Integrity in the Coronary Artery Risk Development in Young Adults (CARDIA) Study

IMPORTANCE

Decreased cerebral tissue integrity and cerebral blood flow (CBF) are features of neurodegenerative diseases. Brain tissue maintenance is an energy-demanding process, making it particularly sensitive to hypoperfusion. However, little is known about the association between blood flow and brain microstructural integrity, including in normative aging.

OBJECTIVE

To assess associations between CBF and changes in cerebral tissue integrity in white matter and gray matter brain regions.

DESIGN, SETTING, AND PARTICIPANTS

In this longitudinal cohort study, magnetic resonance imaging was performed on 732 healthy adults from the Coronary Artery Risk Development in Young Adults (CARDIA) study, a prospective longitudinal study (baseline age of 18-30 years) that examined participants up to 8 times during 30 years (1985-1986 to 2015-2016). Cerebral blood flow was measured at baseline (year 25 of the CARDIA study), and changes in diffusion tensor indices of fractional anisotropy (FA) and mean diffusivity (MD), measures of microstructural tissue integrity, were measured at both baseline and after approximately 5 years of follow-up (year 30). Analyses were conducted from November 5, 2020, to January 29, 2022.

MAIN OUTCOMES AND MEASURES

Automated algorithms and linear mixed-effects statistical models were used to evaluate the associations between CBF at baseline and changes in FA or MD.

RESULTS

After exclusion of participants with missing or low-quality data, 654 at baseline (342 women; mean [SD] age, 50.3 [3.5] years) and 433 at follow-up (230 women; mean [SD] age, 55.1 [3.5] years) were scanned for CBF or FA and MD imaging. In the baseline cohort, 247 participants were Black (37.8%) and 394 were White (60.2%); in the follow-up cohort, 156 were Black (36.0%) and 277 were White (64.0%). Cross-sectionally, FA and MD were associated with CBF in most regions evaluated, with lower CBF values associated with lower FA or higher MD values, including the frontal white matter lobes (for CBF and MD: mean [SE] β = -1.4 [0.5] × 10-6; for CBF and FA: mean [SE] β = 2.9 [1.0] × 10-4) and the parietal white matter lobes (for CBF and MD: mean [SE] β = -2.4 [0.6] × 10-6; for CBF and FA: mean [SE] β = 4.4 [1.1] × 10-4). Lower CBF values at baseline were also significantly associated with steeper regional decreases in FA or increases in MD in most brain regions investigated, including the frontal (for CBF and MD: mean [SE] β = -1.1 [0.6] × 10-6; for CBF and FA: mean [SE] β = 2.9 [1.0] × 10-4) and parietal lobes (for CBF and MD: mean [SE] β = -1.5 [0.7] × 10-6; for CBF and FA: mean [SE] β = 4.4 [1.1] × 10-4).

CONCLUSIONS AND RELEVANCE

Results of this longitudinal cohort study of the association between CBF and diffusion tensor imaging metrics suggest that blood flow may be significantly associated with brain tissue microstructure. This work may lay the foundation for investigations to clarify the nature of early brain damage in neurodegeneration. Such studies may lead to new neuroimaging biomarkers of brain microstructure and function for disease progression.

Excitatory Synaptic Transmission in Ischemic Stroke: A New Outlet for Classical Neuroprotective Strategies

Stroke is one of the leading causes of death and disability in the world, of which ischemia accounts for the majority. There is growing evidence of changes in synaptic connections and neural network functions in the brain of stroke patients. Currently, the studies on these neurobiological alterations mainly focus on the principle of glutamate excitotoxicity, and the corresponding neuroprotective strategies are limited to blocking the overactivation of ionic glutamate receptors. Nevertheless, it is disappointing that these treatments often fail because of the unspecificity and serious side effects of the tested drugs in clinical trials. Thus, in the prevention and treatment of stroke, finding and developing new targets of neuroprotective intervention is still the focus and goal of research in this field. In this review, we focus on the whole processes of glutamatergic synaptic transmission and highlight the pathological changes underlying each link to help develop potential therapeutic strategies for ischemic brain damage. These strategies include: (1) controlling the synaptic or extra-synaptic release of glutamate, (2) selectively blocking the action of the glutamate receptor NMDAR subunit, (3) increasing glutamate metabolism, and reuptake in the brain and blood, and (4) regulating the glutamate system by GABA receptors and the microbiota–gut–brain axis. Based on these latest findings, it is expected to promote a substantial understanding of the complex glutamate signal transduction mechanism, thereby providing excellent neuroprotection research direction for human ischemic stroke (IS).

Astrocyte-secreted chordin-like 1 regulates spine density after ischemic injury

Ischemic injury occurs when the brain is deprived of blood flow, preventing cells from receiving essential nutrients. The injury core is the brain region directly deprived and is surrounded by the peri-infarct area, the region with recovery potential. In the peri-infarct area neurons undergo acute loss of dendritic spines, which modifies synaptic plasticity and determines neuronal survival. Astrocytes can be protective or detrimental to the ischemic injury response depending on the specific stage, yet we lack clear understanding of the underlying mechanisms. Chordin-like 1 (Chrdl1) is an astrocyte-secreted protein that promotes synaptic maturation and limits experience-dependent plasticity in the mouse visual cortex. Given this plasticity-limiting function we asked if Chrdl1 regulates the response to ischemic injury, modelled using photothrombosis (PT). We find that Chrdl1 mRNA is upregulated in astrocytes in the peri-infarct area in both acute and sub-acute phases post-PT. To determine the impact of increased Chrdl1 on the response to PT we analyzed Chrdl1 knock-out mice. We find that absence of Chrdl1 prevents ischemia-induced spine loss in the peri-infarct area and reduces cell death in the core, without impacting gliosis. These findings highlight the important role of astrocyte-secreted proteins in regulating structural plasticity in response to brain ischemic injuries.

Neuroprotective effects of alisol A 24-acetate on cerebral ischaemia-reperfusion injury are mediated by regulating the PI3K/AKT pathway

BACKGROUND

Neuroinflammation and apoptosis are involved in the pathogenesis of ischaemic stroke. Alisol A 24-acetate (24A) exerts a strong inhibitory effect on inflammation and cell apoptosis. The neuroprotective effect of 24A on global cerebral ischaemia/reperfusion (GCI/R) injury remains unclear.

METHODS

GCI/R mice were used to investigate the neuroprotective effect of 24A. Modified neurological deficit scores, Morris water maze and object recognition tests were used to evaluate behaviours. Metabolism in brain regions was detected using magnetic resonance spectroscopy (MRS), and changes in microglia, astrocytes and neurons were detected. Inflammation and apoptosis were measured.

RESULTS

The results showed that 24A suppressed neurological deficits scores and improved GCI/R induced cognitive dysfunction. It was also observed that 24A could alleviate neuroinflammation, which manifested as 24A inhibited microglia and astrocytes proliferation, downregulated the expression of interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and inducible nitric oxide synthase (iNOS) in the GCI/R mice brain. The apoptosis of neurons reduced, and dendritic spines of hippocampal neurons increased in the presence of 24A. In addition, 24A could up-regulate the expression of phosphorylated phosphoinositide 3-kinases (p-PI3K) and phosphorylated protein kinase B (p-AKT) in GCI/R mice brain, and all the morphological, neurological, and biochemical changes of 24A treatment were abolished by the application of PI3K/AKT pathway inhibitor LY294002.

CONCLUSIONS

Taken together, our study indicated that 24A alleviated GCI/R injury by inhibiting neuroinflammation and apoptosis through the regulation of the PI3K/AKT pathway.

Differential Regulation of Microglial Activation in Response to Different Degree of Ischemia

Microglia are primary immune cells within the brain and are rapidly activated after cerebral ischemia. The degree of microglial activation is closely associated with the severity of ischemia. However, it remains largely unclear how microglial activation is differentially regulated in response to a different degree of ischemia. In this study, we used a bilateral common carotid artery ligation (BCAL) model and induced different degrees of ischemia by varying the duration of ligation to investigate the microglial response in CX3CR1^GFP/+ mice. Confocal microscopy, immunofluorescence staining, RNA sequencing, and qRT-PCR were used to evaluate the de-ramification, proliferation, and differential gene expression associated with microglial activation. Our results showed that 30 min of ischemia induced rapid de-ramification of microglia but did not have significant influence on the microglial density. In contrast, 60 min of ischemia led to a significant decrease in microglial density and more pronounced de-ramification of microglial processes. Importantly, 30 min of ischemia did not induce proliferation of microglia, but 60 min of ischemia led to a marked increase in the density of proliferative microglia. Further analysis utilized transcriptome sequencing showed that microglial activation is differentially regulated in response to different degrees of ischemia. A total of 1,097 genes were differentially regulated after 60 min of ischemia, but only 68 genes were differentially regulated after 30 min of ischemia. Pathway enrichment analysis showed that apoptosis, cell mitosis, immune receptor activity and inflammatory-related pathways were highly regulated after 60 min of ischemia compared to 30 min of ischemia. Multiple microglia-related genes such as Cxcl10, Tlr7, Cd86, Tnfrsf1a, Nfkbia, Tgfb1, Ccl2 and Il-6, were upregulated with prolonged ischemia. Pharmacological inhibition of CSF1 receptor demonstrated that CSF1R signaling pathway contributed to microglial proliferation. Together, these results suggest that the proliferation of microglia is gated by the duration of ischemia and microglia were differentially activated in responding to different degrees of ischemia.

Trelagliptin Mitigates Macrophage Infiltration by Preventing the Breakdown of the Blood-Brain Barrier in the Brain of Middle Cerebral Artery Occlusion Mice

Stroke is a significant cardiovascular disease that influences the health of human beings all over the world, especially the elderly population. It is reported that the blood-brain barrier (BBB) can be easily destroyed by stroke, which is one of the main factors responsible for macrophage infiltration and central nervous inflammation. Here, we report the protective effects of Trelagliptin against BBB injury and macrophage infiltration. Our results indicate that the infraction volume, the neurological score, and macrophage infiltration staining with CD68 were increased in middle cerebral artery occlusion (MCAO) mice but significantly reversed by treatment with Trelagliptin. Additionally, Trelagliptin reduced the permeability of the BBB by increasing the expression of the tight junction zonula occludens protein-1 (ZO-1) in the cerebral cortex. In an in vitro hypoxia model of endothelial cells, the increased migration of macrophages, enlarged permeability of endothelial monolayer, downregulation of ZO-1, and elevated expression level of CXCL1 by hypoxic conditions were all reversed by treatment with Trelagliptin in a dose-dependent manner. Our results demonstrate that Trelagliptin might mitigate macrophage infiltration by preventing the breakdown of the blood-brain barrier in the brains of MCAO mice.

Combination of Single- and Paired-Pulse Somatosensory Evoked Potentials in Ischemic Monitoring: Preliminary Investigation in Carotid Endarterectomy

Introduction

Severe ischemia induces cerebral excitability imbalance before completion of infarct. To investigate the clinical availability of this imbalance with ischemic monitoring, paired-pulse somatosensory evoked potentials (SEPs) were performed in conjunction with conventional SEPs during carotid endarterectomy.

Methods

For carotid endarterectomy patients with hemodynamic deficits of the middle cerebral artery area (n = 34), the excitability imbalances (Q) were measured by paired-pulse SEPs, wherein the second response (A₂) was divided by the first (A₁; Q = A₂/A₁). Regional cerebral saturation (rSO₂) was also measured. Occlusion was performed twice using shunting.

Results

Each carotid occlusion induced a significant decrease in mean A₁ and rSO₂, and an increase in mean Q values (p < 0.001), which returned to the baseline level after occlusion. While neuronal imbalances were mostly transient, persistently increased Q values were observed in four cases (11.8%), all indicating postoperative abnormalities in diffusion-weighted magnetic resonance imaging (100%). Meanwhile, A₁ detected the postoperative abnormality in only one case (25%). Preoperative Q values at the time of surgery were significantly higher in symptomatic patients having the upper limb deficits than those without (p < 0.01), indicating persistent or permanent imbalances.

Conclusion

Paired-pulse SEPs reliably identified transient, persistent or permanent neuronal imbalances, depending on the ischemic severity. These preliminary results indicated that paired-pulse SEPs, in combination with conventional SEPs (A₁), may offer better ischemic monitoring.

Pharmacological Targeting of CSF1R Inhibits Microglial Proliferation and Aggravates the Progression of Cerebral Ischemic Pathology

Ischemic stroke can induce rapid activation of the microglia. It has been reported that the microglia’s survival is dependent on colony-stimulating factor 1 receptor (CSF1R) signaling and that pharmacological inhibition of CSF1R leads to morphological changes in the microglia in the healthy brain. However, the impact of CSF1R inhibition on neuronal structures and motor ability after ischemia–reperfusion remains unclear. In this study, we investigated microglial de-ramification, proliferation, and activation after inhibition of CSF1R by a tyrosine kinase inhibitor (ki20227) in a mouse model of global cerebral ischemia induced by bilateral common carotid artery ligation (BCAL). In addition to microglial morphology, we evaluated the mRNA expression of cytokines, chemokines, and inflammatory receptors. Our results show that pharmacological inhibition of CSF1R in ischemic mice resulted in the blockade of microglial proliferation and a shift in microglial morphology reflected by excessive de-ramification and a more activated phenotype accompanied by an enhanced innate immune response. Furthermore, we show that pharmacological inhibition of CSF1R in ischemic mice resulted in the aggravation of neuronal degeneration and behavioral impairment. Intravital two-photon imaging revealed that although pharmacological inhibition of CSF1R did not affect the recovery of dendritic structures, it caused a significant increase in spine elimination during reperfusion in ischemic mice. These findings suggest that pharmacological inhibition of CSF1R induces a blockade of microglial proliferation and causes acute activation of the microglia accompanied by a severe inflammatory response. It aggravates neuronal degeneration, loss of dendritic spines, and behavioral deficits after transient global cerebral ischemia.

Rapid Neuronal Ultrastructure Disruption and Recovery during Spreading Depolarization-Induced Cytotoxic Edema

Two major pathogenic events that cause acute brain damage during neurologic emergencies of stroke, head trauma, and cardiac arrest are spreading depolarizing waves and the associated brain edema that course across the cortex injuring brain cells. Virtually nothing is known about how spreading depolarization (SD)-induced cytotoxic edema evolves at the ultrastructural level immediately after insult and during recovery. In vivo 2-photon imaging followed by quantitative serial section electron microscopy was used to assess synaptic circuit integrity in the neocortex of urethane-anesthetized male and female mice during and after SD evoked by transient bilateral common carotid artery occlusion. SD triggered a rapid fragmentation of dendritic mitochondria. A large increase in the density of synapses on swollen dendritic shafts implies that some dendritic spines were overwhelmed by swelling or merely retracted. The overall synaptic density was unchanged. The postsynaptic dendritic membranes remained attached to axonal boutons, providing a structural basis for the recovery of synaptic circuits. Upon immediate reperfusion, cytotoxic edema mainly subsides as affirmed by a recovery of dendritic ultrastructure. Dendritic recuperation from swelling and reversibility of mitochondrial fragmentation suggests that neurointensive care to improve tissue perfusion should be paralleled by treatments targeting mitochondrial recovery and minimizing the occurrence of SDs.

Neurodegeneration, Myelin Loss and Glial Response in the Three-Vessel Global Ischemia Model in Rat

(1) Background: Although myelin disruption is an integral part of ischemic brain injury, it is rarely the subject of research, particularly in animal models. This study assessed for the first time, myelin and oligodendrocyte loss in a three-vessel model of global cerebral ischemia (GCI), which causes hippocampal damage. In addition, we investigated the relationships between demyelination and changes in microglia and astrocytes, as well as oligodendrogenesis in the hippocampus; (2) Methods: Adult male Wistar rats (n = 15) underwent complete interruption of cerebral blood flow for 7 min by ligation of the major arteries supplying the brain or sham-operation. At 10 and 30 days after the surgery, brain slices were stained for neurodegeneration with Fluoro-Jade C and immunohistochemically to assess myelin content (MBP+ percentage of total area), oligodendrocyte (CNP+ cells) and neuronal (NeuN+ cells) loss, neuroinflammation (Iba1+ cells), astrogliosis (GFAP+ cells) and oligodendrogenesis (NG2+ cells); (3) Results: 10 days after GCI significant myelin and oligodendrocyte loss was found only in the stratum oriens and stratum pyramidale. By the 30th day, demyelination in these hippocampal layers intensified and affected the substratum radiatum. In addition to myelin damage, activation and an increase in the number of microglia and astrocytes in the corresponding layers, a loss of the CA1 pyramidal neurons, and neurodegeneration in the neocortex and thalamus was observed. At a 10-day time point, we observed rod-shaped microglia in the substratum radiatum. Parallel with ongoing myelin loss on the 30th day after ischemia, we found significant oligodendrogenesis in demyelinated hippocampal layers; (4) Conclusions: Our study showed that GCI-simulating cardiac arrest in humans—causes not only the loss of pyramidal neurons in the CA1 field, but also the myelin loss of adjacent layers of the hippocampus.

Challenges and Improvements of Developing an Ischemia Mouse Model Through Bilateral Common Carotid Artery Occlusion

Brain ischemia is one of the principal causes of death and disability worldwide in which prevention or an effective treatment does not exist. In order to develop successful treatments, an adequate and useful ischemia model is essential. Transient global cerebral ischemia is one of the most interesting pathological conditions in stroke studies because of the observed degeneration of forebrain and delayed neuronal cell death in selective vulnerable regions such as hippocampus. Transient occlusion of both common carotid arteries is the most convenient model to induce tGCI. Although there are effective rat and gerbil models using this method, the induction of a reproducible and reliable injury after global ischemia in mouse has presented higher variations, mainly because of its size and the necessary monitoring skills in order to accomplish homogeneous and reproducible results. Further, great variability among cerebral vasculature and susceptibility of the different strains and sub-strains is observed. In recent years, some modifications have been made to the model in order to normalize the heterogenic effects. Analysis of posterior communicating artery patency has been proposed as an exclusion parameter due to the direct relationship reported with the reduction of cerebral blood flow. Another method used to significantly reduce blood flow is the induction of hypotension with isoflurane. Each protocol produces distinct injury outcomes. Further improvements are needed to attain a general, simpler, reproducible and globally accepted model that allows comparisons between research groups, progress in understanding ischemia and the consequent development of therapeutic alternatives for ischemic injury.

Transient ischemia-reperfusion induces cortical hyperactivity and AMPAR trafficking in the somatosensory cortex

Brain ischemia results from cardiac arrest, stroke or head trauma. The structural basis of rescuing the synaptic impairment and cortical dysfunctions induced in the stage of ischemic-reperfusion can occur if therapeutic interventions are applied in time, but the functional basis for this resilience remains elusive. Here, we explore the changes in cortical activity and a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) GluA1 subunit in spine (sGluA1) after transient ischemia-reperfusion in vivo for 28 days. Using in vivo two-photon microscopy in the mouse somatosensory cortex, we found that the average frequency of Ca²⁺ transients in the spine (there was an unusual synchrony) was higher after 15 min of ischemia-reperfusion. In addition, the transient ischemia-reperfusion caused a reflective enhancement of AMPARs, which eventually restored to normal. The cortical hyperactivity (Ca²⁺ transients) and the increase in AMPARs were successfully blocked by an NMDA receptor antagonist. Thus, the increase of AMPARs, cortical hyperactivity and the unusual synchrony might be the reason for reperfusion injury after short-term transient ischemia.

RyRs mediate lead-induced neurodegenerative disorders through calcium signaling pathways

Heavy metal lead (Pb) is widely distributed in the environment and can induce neurodegeneration. Accumulating evidence has shown that ryanodine receptors (RyRs) play vital roles in neurodegenerative brain. However, whether aberrant RyRs levels contribute to Pb-induced neurodegeneration has largely remained unknown. In the present study, we report the important role of elevated levels of RyRs in Pb-induced neurodegeneration. Pb was found to upregulate the levels of RyRs in the rat hippocampal tissues and rat pheochromocytoma (PC12) cells. Furthermore, exposure to Pb induced neurodegenerative cognitive impairment in rats, depressed the long-term potentiation (LTP) in the rat brain slices, increased the neuronal intracellular free calcium concentration ([Ca²⁺]i), inhibited the phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and cyclic adenosine 3',5'-monophosphate (cAMP) response element binding protein (CREB) as well as the expression of anti-apoptotic protein B-cell lymphoma 2 (Bcl2), and activated the phosphorylation of extracellular regulated protein kinases (Erk) protein both in vitro and in vivo. In addition, the knockdown of RyR3 in PC12 cells significantly decreased the [Ca²⁺]i levels, increased the CaMKIIα and CREB phosphorylation, decrease the phosphorylation of Erk, and elongated the cognitive function-related neurite outgrowth after exposure to Pb. Moreover, treatment with a RyRs agonist showed the involvement of RyRs in Pb-induced depression in LTP in the rat brain slices. In summary, we determined that Pb-mediated upregulation of RyRs led to neurodegeneration via high levels of free calcium, depression of the calcium-dependent CaMKIIα/CREB mnemonic signaling pathway, and activation of the calcium-dependent Erk/Bcl2 apoptotic signaling pathway. These findings on the impact of Pb on the levels of RyRs could further improve our understanding of Pb-induced neurotoxicity and provide a promising molecular target to antagonize Pb-induced neurodegenerative diseases.

Dendritic spines: Revisiting the physiological role

Dendritic spines are small, thin, specialized protrusions from neuronal dendrites, primarily localized in the excitatory synapses. Sophisticated imaging techniques revealed that dendritic spines are complex structures consisting of a dense network of cytoskeletal, transmembrane and scaffolding molecules, and numerous surface receptors. Molecular signaling pathways, mainly Rho and Ras family small GTPases pathways that converge on actin cytoskeleton, regulate the spine morphology and dynamics bi-directionally during synaptic activity. During synaptic plasticity the number and shapes of dendritic spines undergo radical reorganizations. Long-term potentiation (LTP) induction promote spine head enlargement and the formation and stabilization of new spines. Long-term depression (LTD) results in their shrinkage and retraction. Reports indicate increased spine density in the pyramidal neurons of autism and Fragile X syndrome patients and reduced density in the temporal gyrus loci of schizophrenic patients. Post-mortem reports of Alzheimer's brains showed reduced spine number in the hippocampus and cortex. This review highlights the spine morphogenesis process, the activity-dependent structural plasticity and mechanisms by which synaptic activity sculpts the dendritic spines, the structural and functional changes in spines during learning and memory using LTP and LTD processes. It also discusses on spine status in neurodegenerative diseases and the impact of nootropics and neuroprotective agents on the functional restoration of dendritic spines.

Long-term depression induced by endogenous cannabinoids produces neuroprotection via astroglial CB1R after stroke in rodents

Ischemia not only activates cell death pathways but also triggers endogenous protective mechanisms. However, it is largely unknown what is the essence of the endogenous neuroprotective mechanisms induced by preconditioning. In this study we demonstrated that systemic injection of JZL195, a selective inhibitor of eCB clearance enzymes, induces in vivo long-term depression at CA3-CA1 synapses and at PrL-NAc synapses produces neuroprotection. JZL195-elicited long-term depression is blocked by AM281, the antagonist of cannabinoid 1 receptor (CB₁R) and is abolished in mice lacking cannabinoid CB₁ receptor (CB₁R) in astroglial cells, but is conserved in mice lacking CB₁R in glutamatergic or GABAergic neurons. Blocking the glutamate NMDA receptor and the synaptic trafficking of glutamate AMPA receptor abolishes both long-term depression and neuroprotection induced by JZL195. Mice lacking CB₁R in astroglia show decreased neuronal death following cerebral ischemia. Thus, an acute elevation of extracellular eCB following eCB clearance inhibition results in neuroprotection through long-term depression induction after sequential activation of astroglial CB₁R and postsynaptic glutamate receptors.

Fasudil alleviates brain damage in rats after carbon monoxide poisoning through regulating neurite outgrowth inhibitor/oligodendrocytemyelin glycoprotein signalling pathway

Carbon monoxide (CO) poisoning can lead to many serious neurological symptoms. Currently, there are no effective therapies for CO poisoning. In this study, rats exposed to CO received hyperbaric oxygen therapy, and those in the Fasudil group were given additional Fasudil injection once daily. We found that the escape latency in CO poisoning group (CO group) was significantly prolonged, the T₁ /T_total was obviously decreased, and the mean escape time and the active escape latency were notably extended compared with those in normal control group (NC group, P < 0.05). After administration of Fasudil, the escape latency was significantly shortened, T₁ /T_total was gradually increased as compared with CO group (>1 week, P < 0.05). Ultrastructural damage of neurons and blood-brain barrier of rats was serious in CO group, while the structural and functional integrity of neuron and mitochondria maintained relatively well in Fasudil group. Moreover, we also noted that the expressions of neurite outgrowth inhibitor (Nogo), oligodendrocyte-myelin glycoprotein (OMgp) and Rock in brain tissue were significantly increased in CO group, and the elevated levels of the three proteins were still observed at 2 months after CO poisoning. Fasudil markedly reduced their expressions compared with those of CO group (P < 0.05). In summary, the activation of Nogo-OMgp/Rho signalling pathway is associated with brain injury in rats with CO poisoning. Fasudil can efficiently down-regulate the expressions of Nogo, OMgp and Rock proteins, paving a way for the treatment of acute brain damage after CO poisoning.

Cdh1 overexpression improves emotion and cognitive-related behaviors via regulating hippocampal neuroplasticity in global cerebral ischemia rats

Post-stroke survivors exhibited cognitive deficits and performed emotional impairment. However, the effect of global cerebral ischemia on standard behavioral measures of emotionality and underlying mechanism remain largely unknown. Our previous work identified that down-regulation of Cdh1 contributed to ischemic neuronal death in rat, thus we hypothesized that Cdh1 exerts a role in emotionality after cerebral ischemia, and we investigated the effect of Cdh1 overexpression on neurogenic behaviors and possible mechanisms in transient global cerebral ischemia reperfusion (tGCI/R) rats. A series of behavioral tests were used to evaluate emotion and cognitive related behaviors, and molecular biological techniques were employed to investigate hippocampal neuroplasticity. The results showed that tGCI/R rats displayed anxiety- and depression-like behaviors and a certain degree of cognitive impairment, and these abnormal behaviors accompanied with a loss of hippocampal synapses and dendritic spines, disruption of dendrite arborization and decline in the level of GAP-43, synaptophysin, synapsin and PSD-95. However, Cdh1 overexpression improved negative emotionality, ameliorated cognitive deficits, rescued hippocampal synapses loss, prevented dendritic network disorganization, and increased the level of synaptic-associated proteins after tGCI/R. Taken together, these findings suggest that Cdh1 overexpression exerts a neuroprotective effect by regulating hippocampal neuroplasticity thus improving negative emotionality and cognitive deficits after tGCI/R.

Increased BBB Permeability Enhances Activation of Microglia and Exacerbates Loss of Dendritic Spines After Transient Global Cerebral Ischemia

Ischemic stroke can induce rapid disruption of blood-brain barrier (BBB). It has been suggested that increased BBB permeability can affect the pathological progression of ischemic tissue. However, the impact of increased BBB permeability on microglial activation and synaptic structures following reperfusion after ischemia remains unclear. In this study, we investigated microglial activation, dendritic damage and plasticity of dendritic spines after increasing BBB permeability following transient global cerebral ischemia in the somatosensory cortices in mice. Bilateral common carotid artery ligation (BCAL) was used to induce transient global cerebral ischemia. Mannitol was used to increase the BBB permeability. Intravital two-photon imaging was performed to image the dendritic structures and BBB extravasation. Microglial morphology was quantitated using a skeletonization analysis method. To evaluate inflammation of cerebral cortex, the mRNA expression levels of integrin alpha M (CD11b), CD68, chemokine (C-X-C motif) ligand 10 (IP10) and tumor necrosis factor alpha (TNF-α) were measured by fluorescent quantitative PCR. Intravital two-photon imaging revealed that mannitol caused a drastic increase in BBB extravasation during reperfusion after transient global ischemia. Increased BBB permeability induced by mannitol had no significant effect on inflammation and dendritic spines in healthy mice but triggered a marked de-ramification of microglia; importantly, in ischemic animals, mannitol accelerated de-ramification of microglia and aggravated inflammation at 3 h but not at 3 days following reperfusion after ischemia. Although mannitol did not cause significant change in the percentage of blebbed dendrites and did not affect the reversible recovery of the dendritic structures, excessive extravasation was accompanied with significant decrease in spine formation and increase in spine elimination during reperfusion in ischemic mice. These findings suggest that increased BBB permeability induced by mannitol can lead to acute activation of microglia and cause excessive loss of dendritic spines after transient global cerebral ischemia.