Showing papers in "Frontiers in Cellular Neuroscience in 2022"

A Perspective on Nrf2 Signaling Pathway for Neuroinflammation: A Potential Therapeutic Target in Alzheimer's and Parkinson's Diseases

Abstract: Neuroinflammation plays a pivotal role in Alzheimer's disease (AD) and Parkinson's disease (PD), the leading causes of dementia. These neurological disorders are characterized by the accumulation of misfolded proteins such as amyloid-ß (Aß), tau protein and α-synuclein, contributing to mitochondrial fragmentation, oxidative stress, and neuroinflammation. Misfolded proteins activate microglia, which induces neuroinflammation, expression of pro-inflammatory cytokines and subsequently facilitates synaptic damage and neuronal loss. So far, all the proposed drugs were based on the inhibition of protein aggregation and were failed in clinical trials. Therefore, the treatment options of dementia are still a challenging issue. Thus, it is worthwhile to study alternative therapeutic strategies. In this context, there is increasing data on the pivotal role of transcription factor NF- E2 p45-related factor 2 (Nrf2) on the redox homeostasis and anti-inflammatory functions in neurodegenerative disorders. Interestingly, Nrf2 signaling pathway has shown upregulation of antioxidant genes, inhibition of microglia-mediated inflammation, and improved mitochondrial function in neurodegenerative diseases, suggesting Nrf2 activation could be a novel therapeutic approach to target pathogenesis. The present review will examine the correlation between Nrf2 signaling with neuroinflammation in AD and PD.

Modes of Brain Cell Death Following Intracerebral Hemorrhage

Abstract: Intracerebral hemorrhage (ICH) is a devastating form of stroke with high rates of mortality and morbidity. It induces cell death that is responsible for neurological deficits postinjury. There are no therapies that effectively mitigate cell death to treat ICH. This review aims to summarize our knowledge of ICH-induced cell death with a focus on apoptosis and necrosis. We also discuss the involvement of ICH in recently described modes of cell death including necroptosis, pyroptosis, ferroptosis, autophagy, and parthanatos. We summarize treatment strategies to mitigate brain injury based on particular cell death pathways after ICH.

Migraine Aura, Transient Ischemic Attacks, Stroke, and Dying of the Brain Share the Same Key Pathophysiological Process in Neurons Driven by Gibbs–Donnan Forces, Namely Spreading Depolarization

Abstract: Neuronal cytotoxic edema is the morphological correlate of the near-complete neuronal battery breakdown called spreading depolarization, or conversely, spreading depolarization is the electrophysiological correlate of the initial, still reversible phase of neuronal cytotoxic edema. Cytotoxic edema and spreading depolarization are thus different modalities of the same process, which represents a metastable universal reference state in the gray matter of the brain close to Gibbs–Donnan equilibrium. Different but merging sections of the spreading-depolarization continuum from short duration waves to intermediate duration waves to terminal waves occur in a plethora of clinical conditions, including migraine aura, ischemic stroke, traumatic brain injury, aneurysmal subarachnoid hemorrhage (aSAH) and delayed cerebral ischemia (DCI), spontaneous intracerebral hemorrhage, subdural hematoma, development of brain death, and the dying process during cardio circulatory arrest. Thus, spreading depolarization represents a prime and simultaneously the most neglected pathophysiological process in acute neurology. Aristides Leão postulated as early as the 1940s that the pathophysiological process in neurons underlying migraine aura is of the same nature as the pathophysiological process in neurons that occurs in response to cerebral circulatory arrest, because he assumed that spreading depolarization occurs in both conditions. With this in mind, it is not surprising that patients with migraine with aura have about a twofold increased risk of stroke, as some spreading depolarizations leading to the patient percept of migraine aura could be caused by cerebral ischemia. However, it is in the nature of spreading depolarization that it can have different etiologies and not all spreading depolarizations arise because of ischemia. Spreading depolarization is observed as a negative direct current (DC) shift and associated with different changes in spontaneous brain activity in the alternating current (AC) band of the electrocorticogram. These are non-spreading depression and spreading activity depression and epileptiform activity. The same spreading depolarization wave may be associated with different activity changes in adjacent brain regions. Here, we review the basal mechanism underlying spreading depolarization and the associated activity changes. Using original recordings in animals and patients, we illustrate that the associated changes in spontaneous activity are by no means trivial, but pose unsolved mechanistic puzzles and require proper scientific analysis.

Neuroinflammation as the Underlying Mechanism of Postoperative Cognitive Dysfunction and Therapeutic Strategies

Abstract: Postoperative cognitive dysfunction (POCD) is a common neurological complication following surgery and general anesthesia, especially in elderly patients. Severe cases delay patient discharge, affect the patient’s quality of life after surgery, and are heavy burdens to society. In addition, as the population ages, surgery is increasingly used for older patients and those with higher prevalences of complications. This trend presents a huge challenge to the current healthcare system. Although studies on POCD are ongoing, the underlying pathogenesis is still unclear due to conflicting results and lack of evidence. According to existing studies, the occurrence and development of POCD are related to multiple factors. Among them, the pathogenesis of neuroinflammation in POCD has become a focus of research in recent years, and many clinical and preclinical studies have confirmed the correlation between neuroinflammation and POCD. In this article, we reviewed how central nervous system inflammation occurred, and how it could lead to POCD with changes in peripheral circulation and the pathological pathways between peripheral circulation and the central nervous system (CNS). Furthermore, we proposed some potential therapeutic targets, diagnosis and treatment strategies at the cellular and molecular levels, and clinical applications. The goal of this article was to provide a better perspective for understanding the occurrence of POCD, its development, and preventive strategies to help manage these vulnerable geriatric patients.

Emerging Diversity of Channelrhodopsins and Their Structure-Function Relationships

Abstract: Cation and anion channelrhodopsins (CCRs and ACRs, respectively) from phototactic algae have become widely used as genetically encoded molecular tools to control cell membrane potential with light. Recent advances in polynucleotide sequencing, especially in environmental samples, have led to identification of hundreds of channelrhodopsin homologs in many phylogenetic lineages, including non-photosynthetic protists. Only a few CCRs and ACRs have been characterized in detail, but there are indications that ion channel function has evolved within the rhodopsin superfamily by convergent routes. The diversity of channelrhodopsins provides an exceptional platform for the study of structure-function evolution in membrane proteins. Here we review the current state of channelrhodopsin research and outline perspectives for its further development.

Red Light Optogenetics in Neuroscience

Abstract: Optogenetics, a field concentrating on controlling cellular functions by means of light-activated proteins, has shown tremendous potential in neuroscience. It possesses superior spatiotemporal resolution compared to the surgical, electrical, and pharmacological methods traditionally used in studying brain function. A multitude of optogenetic tools for neuroscience have been created that, for example, enable the control of action potential generation via light-activated ion channels. Other optogenetic proteins have been used in the brain, for example, to control long-term potentiation or to ablate specific subtypes of neurons. In in vivo applications, however, the majority of optogenetic tools are operated with blue, green, or yellow light, which all have limited penetration in biological tissues compared to red light and especially infrared light. This difference is significant, especially considering the size of the rodent brain, a major research model in neuroscience. Our review will focus on the utilization of red light-operated optogenetic tools in neuroscience. We first outline the advantages of red light for in vivo studies. Then we provide a brief overview of the red light-activated optogenetic proteins and systems with a focus on new developments in the field. Finally, we will highlight different tools and applications, which further facilitate the use of red light optogenetics in neuroscience.

The Role of c-Jun and Autocrine Signaling Loops in the Control of Repair Schwann Cells and Regeneration

Abstract: After nerve injury, both Schwann cells and neurons switch to pro-regenerative states. For Schwann cells, this involves reprogramming of myelin and Remak cells to repair Schwann cells that provide the signals and mechanisms needed for the survival of injured neurons, myelin clearance, axonal regeneration and target reinnervation. Because functional repair cells are essential for regeneration, it is unfortunate that their phenotype is not robust. Repair cell activation falters as animals get older and the repair phenotype fades during chronic denervation. These malfunctions are important reasons for the poor outcomes after nerve damage in humans. This review will discuss injury-induced Schwann cell reprogramming and the concept of the repair Schwann cell, and consider the molecular control of these cells with emphasis on c-Jun. This transcription factor is required for the generation of functional repair cells, and failure of c-Jun expression is implicated in repair cell failures in older animals and during chronic denervation. Elevating c-Jun expression in repair cells promotes regeneration, showing in principle that targeting repair cells is an effective way of improving nerve repair. In this context, we will outline the emerging evidence that repair cells are sustained by autocrine signaling loops, attractive targets for interventions aimed at promoting regeneration.

Dysregulation of GABAergic Signaling in Neurodevelomental Disorders: Targeting Cation-Chloride Co-transporters to Re-establish a Proper E/I Balance

Abstract: The construction of the brain relies on a series of well-defined genetically and experience- or activity -dependent mechanisms which allow to adapt to the external environment. Disruption of these processes leads to neurological and psychiatric disorders, which in many cases are manifest already early in postnatal life. GABA, the main inhibitory neurotransmitter in the adult brain is one of the major players in the early assembly and formation of neuronal circuits. In the prenatal and immediate postnatal period GABA, acting on GABAA receptors, depolarizes and excites targeted cells via an outwardly directed flux of chloride. In this way it activates NMDA receptors and voltage-dependent calcium channels contributing, through intracellular calcium rise, to shape neuronal activity and to establish, through the formation of new synapses and elimination of others, adult neuronal circuits. The direction of GABAA-mediated neurotransmission (depolarizing or hyperpolarizing) depends on the intracellular levels of chloride [Cl−]i, which in turn are maintained by the activity of the cation-chloride importer and exporter KCC2 and NKCC1, respectively. Thus, the premature hyperpolarizing action of GABA or its persistent depolarizing effect beyond the postnatal period, leads to behavioral deficits associated with morphological alterations and an excitatory (E)/inhibitory (I) imbalance in selective brain areas. The aim of this review is to summarize recent data concerning the functional role of GABAergic transmission in building up and refining neuronal circuits early in development and its dysfunction in neurodevelopmental disorders such as Autism Spectrum Disorders (ASDs), schizophrenia and epilepsy. In particular, we focus on novel information concerning the mechanisms by which alterations in cation-chloride co-transporters (CCC) generate behavioral and cognitive impairment in these diseases. We discuss also the possibility to re-establish a proper GABAA-mediated neurotransmission and excitatory (E)/inhibitory (I) balance within selective brain areas acting on CCC.

Protein Expression of the Microglial Marker Tmem119 Decreases in Association With Morphological Changes and Location in a Mouse Model of Traumatic Brain Injury

Abstract: The activation of microglia and the infiltration of macrophages are hallmarks of neuroinflammation after acute brain injuries, including traumatic brain injury (TBI). The two myeloid populations share many features in the post-injury inflammatory response, thus, being antigenically indistinguishable. Recently Tmem119, a type I transmembrane protein specifically expressed by microglia under physiological conditions, was proposed as a tool to differentiate resident microglia from blood-borne macrophages, not expressing it. However, the validity of Tmem119 as a specific marker of resident microglia in the context of acute brain injury, where microglia are activated and macrophages are recruited, needs validation. Our purpose was to investigate Tmem119 expression and distribution in relation to the morphology of brain myeloid cells present in the injured area after TBI. Mice underwent sham surgery or TBI by controlled cortical impact (CCI). Brains from sham-operated, or TBI mice, were analyzed by in situ hybridization to identify the cells expressing Tmem119, and by Western blot and quantitative immunofluorescence to measure Tmem119 protein levels in the entire brain regions and single cells. The morphology of Iba1+ myeloid cells was analyzed at different times (4 and 7 days after TBI) and several distances from the contused edge in order to associate Tmem119 expression with morphological evolution of active microglia. In situ hybridization indicated an increased Tmem119 RNA along with increased microglial complement C1q activation in the contused area and surrounding regions. On the contrary, the biochemical evaluation showed a drop in Tmem119 protein levels in the same areas. The Tmem119 immunoreactivity decreased in Iba1+ myeloid cells found in the contused cortex at both time points, with the cells showing the hypertrophic ameboid morphology having no Tmem119 expression. The Tmem119 was present on ramifications of resident microglia and its presence was decreased as a consequence of microglial activation in cortical areas close to contusion. Based on the data, we conclude that the decrease of Tmem119 in reactive microglia may depend on the process of microglial activation, which involves the retracting of their branchings to acquire an ameboid shape. The Tmem119 immunoreactivity decreases in reactive microglia to similar levels than the blood-borne macrophages, thus, failing to discriminate the two myeloid populations after TBI.

Endoplasmic Reticulum Stress and the Unfolded Protein Response in Cerebral Ischemia/Reperfusion Injury

Abstract: Ischemic stroke is an acute cerebrovascular disease characterized by sudden interruption of blood flow in a certain part of the brain, leading to serious disability and death. At present, treatment methods for ischemic stroke are limited to thrombolysis or thrombus removal, but the treatment window is very narrow. However, recovery of cerebral blood circulation further causes cerebral ischemia/reperfusion injury (CIRI). The endoplasmic reticulum (ER) plays an important role in protein secretion, membrane protein folding, transportation, and maintenance of intracellular calcium homeostasis. Endoplasmic reticulum stress (ERS) plays a crucial role in cerebral ischemia pathophysiology. Mild ERS helps improve cell tolerance and restore cell homeostasis; however, excessive or long-term ERS causes apoptotic pathway activation. Specifically, the protein kinase R-like endoplasmic reticulum kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring enzyme 1 (IRE1) pathways are significantly activated following initiation of the unfolded protein response (UPR). CIRI-induced apoptosis leads to nerve cell death, which ultimately aggravates neurological deficits in patients. Therefore, it is necessary and important to comprehensively explore the mechanism of ERS in CIRI to identify methods for preserving brain cells and neuronal function after ischemia.

The Specific Role of Reactive Astrocytes in Stroke

Abstract: Astrocytes are essential in maintaining normal brain functions such as blood brain barrier (BBB) homeostasis and synapse formation as the most abundant cell type in the central nervous system (CNS). After the stroke, astrocytes are known as reactive astrocytes (RAs) because they are stimulated by various damage-associated molecular patterns (DAMPs) and cytokines, resulting in significant changes in their reactivity, gene expression, and functional characteristics. RAs perform multiple functions after stroke. The inflammatory response of RAs may aggravate neuro-inflammation and release toxic factors to exert neurological damage. However, RAs also reduce excitotoxicity and release neurotrophies to promote neuroprotection. Furthermore, RAs contribute to angiogenesis and axonal remodeling to promote neurological recovery. Therefore, RAs’ biphasic roles and mechanisms make them an effective target for functional recovery after the stroke. In this review, we summarized the dynamic functional changes and internal molecular mechanisms of RAs, as well as their therapeutic potential and strategies, in order to comprehensively understand the role of RAs in the outcome of stroke disease and provide a new direction for the clinical treatment of stroke.

Neuronal Redevelopment and the Regeneration of Neuromodulatory Axons in the Adult Mammalian Central Nervous System

Abstract: One reason that many central nervous system injuries, including those arising from traumatic brain injury, spinal cord injury, and stroke, have limited recovery of function is that neurons within the adult mammalian CNS lack the ability to regenerate their axons following trauma. This stands in contrast to neurons of the adult mammalian peripheral nervous system (PNS). New evidence, provided by single-cell expression profiling, suggests that, following injury, both mammalian central and peripheral neurons can revert to an embryonic-like growth state which is permissive for axon regeneration. This “redevelopment” strategy could both facilitate a damage response necessary to isolate and repair the acute damage from injury and provide the intracellular machinery necessary for axon regrowth. Interestingly, serotonin neurons of the rostral group of raphe nuclei, which project their axons into the forebrain, display a robust ability to regenerate their axons unaided, counter to the widely held view that CNS axons cannot regenerate without experimental intervention after injury. Furthermore, initial evidence suggests that norepinephrine neurons within the locus coeruleus possess similar regenerative abilities. Several morphological characteristics of serotonin axon regeneration in adult mammals, observable using longitudinal in vivo imaging, are distinct from the known characteristics of unaided peripheral nerve regeneration, or of the regeneration seen in the spinal cord and optic nerve that occurs with experimental intervention. These results suggest that there is an alternative CNS program for axon regeneration that likely differs from that displayed by the PNS.

Critical Role of Astrocytic Polyamine and GABA Metabolism in Epileptogenesis

Abstract: Accumulating evidence indicate that astrocytes are essential players of the excitatory and inhibitory signaling during normal and epileptiform activity via uptake and release of gliotransmitters, ions, and other substances. Polyamines can be regarded as gliotransmitters since they are almost exclusively stored in astrocytes and can be released by various mechanisms. The polyamine putrescine (PUT) is utilized to synthesize GABA, which can also be released from astrocytes and provide tonic inhibition on neurons. The polyamine spermine (SPM), synthesized form PUT through spermidine (SPD), is known to unblock astrocytic Cx43 gap junction channels and therefore facilitate astrocytic synchronization. In addition, SPM released from astrocytes may also modulate neuronal NMDA, AMPA, and kainate receptors. As a consequence, astrocytic polyamines possess the capability to significantly modulate epileptiform activity. In this study, we investigated different steps in polyamine metabolism and coupled GABA release to assess their potential to control seizure generation and maintenance in two different epilepsy models: the low-[Mg2+] model of temporal lobe epilepsy in vitro and in the WAG/Rij rat model of absence epilepsy in vivo. We show that SPM is a gliotransmitter that is released from astrocytes and significantly contributes to network excitation. Importantly, we found that inhibition of SPD synthesis completely prevented seizure generation in WAG/Rij rats. We hypothesize that this antiepileptic effect is attributed to the subsequent enhancement of PUT to GABA conversion in astrocytes, leading to GABA release through GAT-2/3 transporters. This interpretation is supported by the observation that antiepileptic potential of the Food and Drug Administration (FDA)-approved drug levetiracetam can be diminished by specifically blocking astrocytic GAT-2/3 with SNAP-5114, suggesting that levetiracetam exerts its effect by increasing surface expression of GAT-2/3. Our findings conclusively suggest that the major pathway through which astrocytic polyamines contribute to epileptiform activity is the production of GABA. Modulation of astrocytic polyamine levels, therefore, may serve for a more effective antiepileptic drug development in the future.

A New Understanding of TMEM119 as a Marker of Microglia

Abstract: Neuroinflammation is a hallmark of many neurological diseases, including traumatic brain injury (TBI), which is usually characterized by two pathological hallmarks: activation of microglia and infiltration of blood-borne macrophages (Hanisch and Kettenmann, 2007; Ransohoff and Perry, 2009; Zanier et al., 2015). Until recently, the two cell populations were antigenically indistinguishable due to a shared myeloid lineage (Varol et al., 2015; Prinz et al., 2017), which complicates our understanding of their neuroinflammatory response in the brain. Recently, advanced single-cell RNA sequencing profiling technologies have uncovered several protein markers of microglia, including transmembrane protein 119 (TMEM119) (Bennett et al., 2016). Under physiological conditions, TMEM119 is specifically expressed in homeostatic human and murine microglia, but not in other brain-resident cells nor in infiltrating macrophages (Bennett et al., 2016; Satoh et al., 2016; Ruan et al., 2020), which enables the differentiation between brainresident microglia and infiltrating blood-derived macrophages. More recent studies, including the present one by Mercurio and colleagues, have tested its efficacy in reactive microglia, and their findings challenge the use of TMEM119 as a robust marker of microglia, especially under pathological conditions. Mercurio et al. (2022) reported an upregulation of Tmem119 gene expression in frozen brain sections of TBI (Day 7) by RT-PCR and using in situ hybridization experiments; they further confirmed its upregulation, specifically in the contused area and surrounding regions, along with increased microglial activation. These findings are consistent with a study showing the increased Tmem119 mRNA level in frozen tissue of the frontal cortex from patients with Alzheimer’s (Satoh et al., 2016), which suggests that increased Tmem119 gene expression in the whole population of microglia is paralleled with tissue inflammation. However, after isolation by fluorescence-activated cell sorting (FACS), Li and colleagues found a reduced Tmem119 mRNA level in CD11b+CD45Int microglia (bulk population) on Day 1 post-intracerebral hemorrhage (Li et al., 2019). In line with this finding, transcriptome studies using the RNA-seq technique also showed a reduced Tmem119 gene expression in isolated reactive microglia (Keren-Shaul et al., 2017; Krasemann et al., 2017; Masuda et al., 2020). Interestingly, transforming growth factor beta (TGF-β) induced an upregulated Tmem119 gene expression in cultured mouse microglia (Attaai et al., 2018), while lipopolysaccharide (LPS), interleukin 4 (IL-4), or interferon gamma (IFN-γ) induced a downregulated Tmem119 gene expression in cultured human microglia (Bennett et al., 2016; Satoh et al., 2016; Van Wageningen et al., 2019), although Satoh et al. (2016) also found no change of Tmem119 gene expression in human microglia HMO6 cells after treatment of LPS or TGF-β. Taken together, although the gene regulation of Tmem119 in reactive and diseased microglia needs further clarification, it is most likely that the different patterns of Tmem119 gene expression in inflamed tissues compared to isolated microglial cells are due to the presence of inflammatory milieu and/or other microenvironmental stimuli. Single-cell RNA-seq data should be helpful to characterize different subsets of microglia with differential levels of Tmem119 gene expression in health and disease.

Healthy Serum-Derived Exosomes Improve Neurological Outcomes and Protect Blood–Brain Barrier by Inhibiting Endothelial Cell Apoptosis and Reversing Autophagy-Mediated Tight Junction Protein Reduction in Rat Stroke Model

Abstract: Blood–brain barrier (BBB) dysfunction causing edema and hemorrhagic transformation is one of the pathophysiological characteristics of stroke. Protection of BBB integrity has shown great potential in improving stroke outcome. Here, we assessed the efficacy of exosomes extracted from healthy rat serum in protection against ischemic stroke in vivo and in vitro. Exosomes were isolated by gradient centrifugation and ultracentrifugation and exosomes were characterized by transmission electron microscopy (TEM) and nanoparticle tracking video microscope. Exosomes were applied to middle cerebral artery occlusion (MCAO) rats or brain microvascular endothelial cell line (bEnd.3) subjected to oxygen-glucose deprivation (OGD) injury. Serum-derived exosomes were injected intravenously into adult male rats 2 h after transient MCAO. Infarct volume and gross cognitive function were assessed 24 h after reperfusion. Poststroke rats treated with serum-derived exosomes exhibited significantly reduced infarct volumes and enhanced neurological function. Apoptosis was assessed via terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) staining and the expression of B-cell lymphoma-2 (Bcl-2), Bax, and cleaved caspase-3 24 h after injury. Our data showed that serum exosomes treatment strikingly decreased TUNEL+ cells in the striatum, enhanced the ratio of Bcl-2 to Bax, and inhibited cleaved caspase-3 production in MCAO rats and OGD/reoxygenation insulted bEnd.3 cells. Under the consistent treatment, the expression of microtubule-associated protein 1 light chain 3B-II (LC3B-II), LC3B-I, and Sequestosome-1 (SQSTM1)/p62 was detected by Western blotting. Autolysosomes were observed via TEM. We found that serum exosomes reversed the ratio of LC3B-II to LC3B-I, prevented SQSTM1/p62 degradation, autolysosome formation, and autophagic flux. Together, these results indicated that exosomes isolated from healthy serum provided neuroprotection against experimental stroke partially via inhibition of endothelial cell apoptosis and autophagy-mediated BBB breakdown. Intravenous serum-derived exosome treatment may, therefore, provide a novel clinical therapeutic strategy for ischemic stroke.

The Pathobiology of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: The Case for Neuroglial Failure

Abstract: Although myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) has a specific and distinctive profile of clinical features, the disease remains an enigma because causal explanation of the pathobiological matrix is lacking. Several potential disease mechanisms have been identified, including immune abnormalities, inflammatory activation, mitochondrial alterations, endothelial and muscular disturbances, cardiovascular anomalies, and dysfunction of the peripheral and central nervous systems. Yet, it remains unclear whether and how these pathways may be related and orchestrated. Here we explore the hypothesis that a common denominator of the pathobiological processes in ME/CFS may be central nervous system dysfunction due to impaired or pathologically reactive neuroglia (astrocytes, microglia and oligodendrocytes). We will test this hypothesis by reviewing, in reference to the current literature, the two most salient and widely accepted features of ME/CFS, and by investigating how these might be linked to dysfunctional neuroglia. From this review we conclude that the multifaceted pathobiology of ME/CFS may be attributable in a unifying manner to neuroglial dysfunction. Because the two key features – post exertional malaise and decreased cerebral blood flow – are also recognized in a subset of patients with post-acute sequelae COVID, we suggest that our findings may also be pertinent to this entity.

Cellular Physiology and Pathophysiology of EAAT Anion Channels

Abstract: Excitatory amino acid transporters (EAATs) optimize the temporal resolution and energy demand of mammalian excitatory synapses by quickly removing glutamate from the synaptic cleft into surrounding neuronal and glial cells and ensuring low resting glutamate concentrations. In addition to secondary active glutamate transport, EAATs also function as anion channels. The channel function of these transporters is conserved in all homologs ranging from archaebacteria to mammals; however, its physiological roles are insufficiently understood. There are five human EAATs, which differ in their glutamate transport rates. Until recently the high-capacity transporters EAAT1, EAAT2, and EAAT3 were believed to conduct only negligible anion currents, with no obvious function in cell physiology. In contrast, the low-capacity glutamate transporters EAAT4 and EAAT5 are thought to regulate neuronal signaling as glutamate-gated channels. In recent years, new experimental approaches and novel animal models, together with the discovery of a human genetic disease caused by gain-of-function mutations in EAAT anion channels have enabled identification of the first physiological and pathophysiological roles of EAAT anion channels.

Microglia: The Hub of Intercellular Communication in Ischemic Stroke

Abstract: Communication between microglia and other cells has recently been at the forefront of research in central nervous system (CNS) disease. In this review, we provide an overview of the neuroinflammation mediated by microglia, highlight recent studies of crosstalk between microglia and CNS resident and infiltrating cells in the context of ischemic stroke (IS), and discuss how these interactions affect the course of IS. The in-depth exploration of microglia-intercellular communication will be beneficial for therapeutic tools development and clinical translation for stroke control.

The Role of DNA Damage in Neural Plasticity in Physiology and Neurodegeneration

Abstract: Damage to DNA is generally considered to be a harmful process associated with aging and aging-related disorders such as neurodegenerative diseases that involve the selective death of specific groups of neurons. However, recent studies have provided evidence that DNA damage and its subsequent repair are important processes in the physiology and normal function of neurons. Neurons are unique cells that form new neural connections throughout life by growth and re-organisation in response to various stimuli. This “plasticity” is essential for cognitive processes such as learning and memory as well as brain development, sensorial training, and recovery from brain lesions. Interestingly, recent evidence has suggested that the formation of double strand breaks (DSBs) in DNA, the most toxic form of damage, is a physiological process that modifies gene expression during normal brain activity. Together with subsequent DNA repair, this is thought to underlie neural plasticity and thus control neuronal function. Interestingly, neurodegenerative diseases such as Alzheimer’s disease, amyotrophic lateral sclerosis, frontotemporal dementia, and Huntington’s disease, manifest by a decline in cognitive functions, which are governed by plasticity. This suggests that DNA damage and DNA repair processes that normally function in neural plasticity may contribute to neurodegeneration. In this review, we summarize current understanding about the relationship between DNA damage and neural plasticity in physiological conditions, as well as in the pathophysiology of neurodegenerative diseases.

Microglia and Microglia-Like Cells: Similar but Different

Abstract: Microglia are the tissue-resident macrophages of the central nervous parenchyma. In mammals, microglia are thought to originate from yolk sac precursors and posteriorly maintained through the entire life of the organism. However, the contribution of microglial cells from other sources should also be considered. In addition to “true” or “bona-fide” microglia, which are of embryonic origin, the so-called “microglia-like cells” are hematopoietic cells of bone marrow origin that can engraft the mature brain mainly under pathological conditions. These cells implement great parts of the microglial immune phenotype, but they do not completely adopt the “true microglia” features. Because of their pronounced similarity, true microglia and microglia-like cells are usually considered together as one population. In this review, we discuss the origin and development of these two distinct cell types and their differences. We will also review the factors determining the appearance and presence of microglia-like cells, which can vary among species. This knowledge might contribute to the development of therapeutic strategies aiming at microglial cells for the treatment of diseases in which they are involved, for example neurodegenerative disorders like Alzheimer’s and Parkinson’s diseases.

WHOPPA Enables Parallel Assessment of Leucine-Rich Repeat Kinase 2 and Glucocerebrosidase Enzymatic Activity in Parkinson’s Disease Monocytes

Abstract: Both leucine-rich repeat kinase 2 (LRRK2) and glucocerebrosidase (GCase) are promising targets for the treatment of Parkinson’s disease (PD). Evidence suggests that both proteins are involved in biological pathways involving the lysosome. However, studies to date have largely investigated the enzymes in isolation and any relationship between LRRK2 and GCase remains unclear. Both enzymes are highly expressed in peripheral blood monocytes and have been implicated in immune function and inflammation. To facilitate the standardized measurement of these readouts in large cohorts of samples collected from persons with PD across the globe, we developed and optimized a sample collection and processing protocol with parallel flow cytometry assays. Assay parameters were first optimized using healthy control peripheral blood mononuclear cells (PBMCs), and then LRRK2 and GCase activities were measured in immune cells from persons with idiopathic PD (iPD). We tested the ability of this protocol to deliver similar results across institutes across the globe, and named this protocol the Wallings-Hughes Optimized Protocol for PBMC Assessment (WHOPPA). In the application of this protocol, we found increased LRRK2 levels and stimulation-dependent enzymatic activity, and decreased GBA index in classical iPD monocytes, as well as increased cytokine release in PD PBMCs. WHOPPA also demonstrated a strong positive correlation between LRRK2 levels, pRab10 and HLA-DR in classical monocytes from subjects with iPD. These data support a role for the global use of WHOPPA and expression levels of these two PD-associated proteins in immune responses, and provide a robust assay to determine if LRRK2 and GCase activities in monocytes have potential utility as reliable and reproducible biomarkers of disease in larger cohorts of subjects with PD.

Repetitive Transcranial Magnetic Stimulation Improves Neurological Function and Promotes the Anti-inflammatory Polarization of Microglia in Ischemic Rats

Abstract: Ischemic stroke (IS) is a severe neurological disease that is difficult to recovery. Previous studies have shown that repetitive transcranial magnetic stimulation (rTMS) is a promising therapeutic approach, while the exact therapy mechanisms of rTMS in improving neural functional recovery remain unclear. Furthermore, the inflammatory environment may influence the rehabilitation efficacy. Our study shows that long-term rTMS stimulation will significantly promote neurogenesis, inhibit apoptosis, and control inflammation. rTMS inhibits the activation of transcription factors nuclear factor kappa b (NF-κB) and signal transducer and activator of transcription 6 (STAT6) and promotes the anti-inflammatory polarization of microglia. Obvious promotion of anti-inflammatory cytokines production is observed both in vitro and in vivo through rTMS stimulation on microglia. In addition, neural stem cells (NSCs) cultured in conditioned medium (CM) from microglia treated with rTMS showed downregulation of apoptosis and upregulation of neuronal differentiation. Overall, our results illustrate that rTMS can modulate microglia with anti-inflammatory polarization variation, promote neurogenesis, and improve neural function recovery.

Mapping the Plasticity of Morphology, Molecular Properties and Function in Mouse Primary Microglia

Abstract: Microglia exert diverse functions by responding in diverse ways to different stimuli, yet little is known about the plasticity of various phenotypes that microglia display. We used interferon (IFN)-γ, interleukin (IL)-4 and IL-10 to induce different phenotypes in mouse primary microglia. RNA sequencing was used to identify genes differentially expressed in response to stimulation, and the different stimulated populations were compared in terms of morphology, proliferative capacity, phagocytic ability and neurotoxicity. IFN-γ induced an “immunodefensive” phenotype characterizing both induction of filopodia and upregulation of inducible nitric oxide synthase (iNOS) and tumor necrosis factor α. Microglia with this phenotype mediated an acute inflammatory response accompanied by excellent proliferative capacity and neurotoxicity, and remained susceptible to remodeling for up to 48 h after initial stimulation. IL-4 induced an enduring “neuroimmunoregulatory” phenotype involving induction of lamellipodium and persistent upregulation of arginase (Arg)-1 and YM-1 expression. Microglia with this phenotype remained susceptible to remodeling for up to 24 h after initial stimulation. IL-10 induced an “immunosuppressive” phenotype involving induction of ameba-like morphology and upregulation of transforming growth factor β and IL-10 as well as inhibition of inflammation. This phenotype was accompanied by inhibition of self-proliferation, while its morphology, molecular properties and function were the least susceptible to remodeling. IFN-γ, IL-4, or IL-10 appear to induce substantially different phenotypes in microglia. The immunodefensive microglia induced by IFN-γ showed remarkable plasticity, which may help repair CNS inflammation damage under pathological condition. Chronic activation with IL-10 decreases microglial plasticity, which may help protect the brain form the immune response. Our research justifies and guides further studies into the molecular pathways that operate in each phenotype to help multitasking microglia regulate homeostasis in the brain.

Astrocytes: GABAceptive and GABAergic Cells in the Brain

Abstract: Astrocytes, the most numerous glial cells in the brain, play an important role in preserving normal neural functions and mediating the pathogenesis of neurological disorders. Recent studies have shown that astrocytes are GABAceptive and GABAergic astrocytes express GABAA receptors, GABAB receptors, and GABA transporter proteins to capture and internalize GABA. GABAceptive astrocytes thus influence both inhibitory and excitatory neurotransmission by controlling the levels of extracellular GABA. Furthermore, astrocytes synthesize and release GABA to directly regulate brain functions. In this review, we highlight recent research progresses that support astrocytes as GABAceptive and GABAergic cells. We also summarize the roles of GABAceptive and GABAergic astrocytes that serve as an inhibitory node in the intercellular communication in the brain. Besides, we discuss future directions for further expanding our knowledge on the GABAceptive and GABAergic astrocyte signaling.

Downregulation of Fat Mass and Obesity-Related Protein in the Anterior Cingulate Cortex Participates in Anxiety- and Depression-Like Behaviors Induced by Neuropathic Pain

Abstract: N6-methyladenosine (m6A) is the most abundant methylation modification on mRNA in mammals. Fat mass and obesity-related protein (FTO) is the main RNA m6A demethylase. FTO is involved in the occurrence and maintenance of neuropathic pain (NP). NP often induces mental disorders. We found that NP downregulated the expression of FTO in the anterior cingulate cortex (ACC), inhibited the expression of matrix metalloproteinase-9 (MMP-9) in the ACC, maladjusted the brain-derived neurotrophic factor precursor (proBDNF) and mature brain-derived neurotrophic factor (mBDNF) levels in the ACC, and induced anxiety- and depression-like behaviors in mice. Blocking the downregulation of FTO in the ACC induced by peripheral nerve injury could reverse the anxiety- and depression-like behaviors of mice. Contrarily, downregulation of simulated FTO induced anxiety- and depression-like behaviors in mice. After peripheral nerve injury, the binding of FTO to MMP-9 mRNA decreased and the enrichment of m6A on MMP-9 mRNA increased. In conclusion, downregulation of FTO in ACC by regulating MMP-9 mRNA methylation level contributes to the occurrence of anxiety- and depression-like behaviors in NP mice.

Therapeutic Low-Intensity Ultrasound for Peripheral Nerve Regeneration – A Schwann Cell Perspective

Abstract: Peripheral nerve injuries are common conditions that can arise from trauma (e.g., compression, severance) and can lead to neuropathic pain as well as motor and sensory deficits. Although much knowledge exists on the mechanisms of injury and nerve regeneration, treatments that ensure functional recovery following peripheral nerve injury are limited. Schwann cells, the supporting glial cells in peripheral nerves, orchestrate the response to nerve injury, by converting to a “repair” phenotype. However, nerve regeneration is often suboptimal in humans as the repair Schwann cells do not sustain their repair phenotype long enough to support the prolonged regeneration times required for successful nerve regrowth. Thus, numerous strategies are currently focused on promoting and extending the Schwann cells repair phenotype. Low-intensity ultrasound (LIU) is a non-destructive therapeutic approach which has been shown to facilitate peripheral nerve regeneration following nerve injury in rodents. Still, clinical trials in humans are scarce and limited to small population sizes. The benefit of LIU on nerve regeneration could possibly be mediated through the repair Schwann cells. In this review, we discuss the known and possible molecular mechanisms activated in response to LIU in repair Schwann cells to draw support and attention to LIU as a compelling regenerative treatment for peripheral nerve injury.

Microglia: Key Players in Retinal Ageing and Neurodegeneration

Abstract: Microglia are the resident immune cells of the central nervous system (CNS) and play a key role in maintaining the normal function of the retina and brain. During early development, microglia migrate into the retina, transform into a highly ramified phenotype, and scan their environment constantly. Microglia can be activated by any homeostatic disturbance that may endanger neurons and threaten tissue integrity. Once activated, the young microglia exhibit a high diversity in their phenotypes as well as their functions, which relate to either beneficial or harmful consequences. Microglial activation is associated with the release of cytokines, chemokines, and growth factors that can determine pathological outcomes. As the professional phagocytes in the retina, microglia are responsible for the clearance of pathogens, dead cells, and protein aggregates. However, their phenotypic diversity and phagocytic capacity is compromised with ageing. This may result in the accumulation of protein aggregates and myelin debris leading to retinal neuroinflammation and neurodegeneration. In this review, we describe microglial phenotypes and functions in the context of the young and ageing retina, and the mechanisms underlying changes in ageing. Additionally, we review microglia-mediated retinal neuroinflammation and discuss the mechanisms of microglial involvement in retinal neurodegenerative diseases.

Molecular mechanisms of neuronal death in brain injury after subarachnoid hemorrhage

Abstract: Subarachnoid haemorrhage (SAH) is a common cerebrovascular disease with high disability and mortality rates worldwide. The pathophysiological mechanisms involved in an aneurysm rupture in SAH are complex and can be divided into early brain injury and delayed brain injury. The initial mechanical insult results in brain tissue and vascular disruption with hemorrhages and neuronal necrosis. Following this, the secondary injury results in diffused cerebral damage in the peri-core area. However, the molecular mechanisms of neuronal death following an aneurysmal SAH are complex and currently unclear. Furthermore, multiple cell death pathways are stimulated during the pathogenesis of brain damage. Notably, particular attention should be devoted to necrosis, apoptosis, autophagy, necroptosis, pyroptosis and ferroptosis. Thus, this review discussed the mechanism of neuronal death and its influence on brain injury after SAH.

Thrombo-Inflammation and Immunological Response in Ischemic Stroke: Focusing on Platelet-Tregs Interaction

Abstract: Strokes are mainly caused by thromboembolic obstruction of a major cerebral artery. Major clinical manifestations include paralysis hemiplegia, aphasia, memory, and learning disorders. In the case of ischemic stroke (IS), hyperactive platelets contribute to advancing an acute thrombotic event progression. Therefore, the principal goal of treatment is to recanalize the occluded vessel and restore cerebral blood flow by thrombolysis or mechanical thrombectomy. However, antiplatelets or thrombolytic therapy may increase the risk of bleeding. Beyond the involvement in thrombosis, platelets also contribute to the inflammatory process induced by cerebral ischemia. Platelet-mediated thrombosis and inflammation in IS lie primarily in the interaction of platelet receptors with endothelial cells and immune cells, including T-cells, monocytes/macrophages, and neutrophils. Following revascularization, intervention with conventional antiplatelet medicines such as aspirin or clopidogrel does not substantially diminish infarct development, most likely due to the limited effects on the thrombo-inflammation process. Emerging evidence has shown that T cells, especially regulatory T cells (Tregs), maintain immune homeostasis and suppress immune responses, playing a critical immunomodulatory role in ischemia-reperfusion injury. Hence, considering the deleterious effects of inflammatory and immune responses, there is an urgent need for more targeted agents to limit the thrombotic-inflammatory activity of platelets and minimize the risk of a cerebral hemorrhage. This review highlights the involvement of platelets in neuroinflammation and the evolving role of Tregs and platelets in IS. In response to all issues, preclinical and clinical strategies should generate more viable therapeutics for preventing and managing IS with immunotherapy targeting platelets and Tregs.

Exosomes Derived From M2 Microglia Cells Attenuates Neuronal Impairment and Mitochondrial Dysfunction in Alzheimer’s Disease Through the PINK1/Parkin Pathway

Abstract: The accumulation of abnormal aggregation of amyloid-β plaques is one of the most distinguishing pathologies of Alzheimer’s disease (AD) and is highly toxic to neurons. Exosomes have demonstrated great potential for AD therapy. However, the impact and underlying mechanism of M2 microglia-derived exosomes (M2-EXOs) in AD progression and outcome are seldom explored. Therefore, we employed an Aβ1-42 oligomer (Aβ)-induced AD model in neuronal HT-22 cells and 7-month-old APP/PS1 mice to investigate the effects of M2-EXOs on AD. We revealed that the AD cell model established by Aβ was accompanied by the upregulation of Aβ1-42, neuronal death, alternation of mitochondrial function and autophagy. M2-EXOs can be internalized by HT-22 cells and MAP2-positive neuronal cells in APP/PS1 mice, and exert neuroprotective functions. Specifically, the administration of M2-EXOs in the AD cell model partially increased cell viability, restored the destruction of mitochondrial membrane potential, and reduced the accumulation of reactive oxygen species inside the mitochondria and cells in a dose-dependent manner. Moreover, we demonstrated that PINK1/Parkin mediated mitophagy was enhanced, while incubation with M2-EXOs decreased beclin1, LC3II, PINK1, and Parkin expression levels. Finally, we observed that compared with APP/PS1 mice treated with PBS, the application of M2-EXOs could decrease Aβ plaque deposition and minus Aβ oligomer expression along with improved PINK1/Parkin pathway-mediated autophagy. Overall, our results imply that M2-EXOs play a protective role in the pathogenesis of AD by ameliorating PINK1/Parkin-mediated mitophagy, indicating that it may provide a novel therapeutic strategy to treat AD.