Heterogeneity of microglia and their differential roles in white matter pathology

Examining the associations between microglia genetic capacity, early life exposures and white matter development at the level of the individual

There are inter-individual differences in susceptibility to the influence of early life experiences for which the underlying neurobiological mechanisms are poorly understood. Microglia play a role in environmental surveillance and may influence individual susceptibility to environmental factors. As an index of neurodevelopment, we estimated individual slopes of mean white matter fractional anisotropy (WM-FA) across three time-points (age 4.5, 6.0, and 7.5 years) for 351 participants. Individual variation in microglia reactivity was derived from an expression-based polygenic score(ePGS) comprised of Single Nucleotide Polymorphisms (SNPs) functionally related to the expression of microglia-enriched genes.A higher ePGS denotes an increased genetic capacity for the expression of microglia-related genes, and thus may confer a greater capacity to respond to the early environment and to influence brain development. We hypothesized that this ePGS would associate with the WM-FA index of neurodevelopment and moderate the influence of early environmental factors.Our findings show sex dependency, where a significant association between WM-FA and microglia ePGS was only obtained for females.We then examined associations with perinatal factors known to decrease (optimal birth outcomes and familial conditions) or increase (systemic inflammation) the risk for later mental health problems.In females, individuals with high microglia ePGS showed a negative association between systemic inflammation and WM-FA and a positive association between more advantageous environmental conditions and WM-FA. The microglia ePGS in females thus accounted for variations in the influence of the quality of the early environment on WM-FA.Finally, WM-FA slopes mediated the association of microglia ePGS with interpersonal problems and social hostility in females. Our findings suggest the genetic capacity for microglia function as a potential factor underlying differential susceptibility to early life exposuresthrough influences on neurodevelopment.

Microglia aggravate white matter injury via C3/C3aR pathway after experimental subarachnoid hemorrhage

The activation of glial cells is intimately associated with the pathophysiology of neuroinflammation and white matter injury (WMI) during both acute and chronic phases following subarachnoid hemorrhage (SAH). The complement C3a receptor (C3aR) has a dual role in modulating inflammation and contributes to neurodevelopment, neuroplasticity, and neurodegeneration. However, its impact on WMI in the context of SAH remains unclear. In this study, 175 male C57BL/6J mice underwent SAH through endovascular perforation. Oxyhemoglobin (oxy-Hb) was employed to simulate SAH in vitro. A suite of techniques, including immunohistochemistry, transcriptomic sequencing, and a range of molecular biotechnologies, were utilized to evaluate the activation of the C3-C3aR pathway on microglial polarization and WMI. Results revealed that post-SAH abnormal activation of microglia was accompanied by upregulation of complement C3 and C3aR. The inhibition of C3aR decreased abnormal microglial activation, attenuated neuroinflammation, and ameliorated WMI and cognitive deficits following SAH. RNA-Seq indicated that C3aR inhibition downregulated several immune and inflammatory pathways and mitigated cellular injury by reducing p53-induced death domain protein 1 (Pidd1) and Protein kinase RNA-like ER kinase (Perk) expression, two factors mainly function in sensing and responding to cellular stress and endoplasmic reticulum (ER) stress. The deleterious effects of the C3-C3aR axis in the context of SAH may be related to endoplasmic reticulum (ER) stress-dependent cellular injury and inflammasome formation. Agonists of Perk can exacerbate the cellular injury and neuroinflammation, which was attenuated by C3aR inhibition after SAH. Additionally, intranasal administration of C3a during the subacute phase of SAH was found to decrease astrocyte reactivity and alleviate cognitive deficits post-SAH. This research deepens our understanding of the complex pathophysiology of WMI following SAH and underscores the therapeutic potential of C3a treatment in promoting white matter repair and enhancing functional recovery prognosis. These insights pave the way for future clinical application of C3a-based therapies, promising significant benefits in the treatment of SAH and its related complications.

BDNF Signaling in Vascular Dementia and Its Effects on Cerebrovascular Dysfunction, Synaptic Plasticity, and Cholinergic System Abnormality

Vascular dementia (VaD) is the second most common type of dementia and is characterized by memory impairment, blood-brain barrier disruption, neuronal cell loss, glia activation, impaired synaptic plasticity, and cholinergic system abnormalities. To effectively prevent and treat VaD a good understanding of the mechanisms underlying its neuropathology is needed. Brain-derived neurotrophic factor (BDNF) is an important neurotrophic factor with multiple functions in the systemic circulation and the central nervous system and is known to regulate neuronal cell survival, synaptic formation, glia activation, and cognitive decline. Recent studies indicate that when compared with normal subjects, patients with VaD have low serum BDNF levels and that BDNF deficiency in the serum and cerebrospinal fluid is an important indicator of VaD. Here, we review current knowledge on the role of BDNF signaling in the pathology of VaD, such as cerebrovascular dysfunction, synaptic dysfunction, and cholinergic system impairment.

Immune proteins C1q and CD47 may contribute to aberrant microglia-mediated synapse loss in the aging monkey brain that is associated with cognitive impairment

Cognitive impairment in learning, memory, and executive function occurs in normal aging even in the absence of Alzheimer's disease (AD). While neurons do not degenerate in humans or monkeys free of AD, there are structural changes including synapse loss and dendritic atrophy, especially in the dorsolateral prefrontal cortex (dlPFC), and these correlate with cognitive age-related impairment. Developmental studies revealed activity-dependent neuronal properties that lead to synapse remodeling by microglia. Microglia-mediated phagocytosis that may eliminate synapses is regulated by immune "eat me" and "don't eat me" signaling proteins in an activity-dependent manner, so that less active synapses are eliminated. Whether this process contributes to age-related synapse loss remains unknown. The present study used a rhesus monkey model of normal aging to investigate the balance between the "eat me" signal, complement component C1q, and the "don't eat me" signal, transmembrane glycoprotein CD47, relative to age-related synapse loss in dlPFC Area 46. Results showed an age-related elevation of C1q and reduction of CD47 at PSD95+ synapses that is associated with cognitive impairment. Additionally, reduced neuronal CD47 RNA expression was found, indicating that aged neurons were less able to produce the protective signal CD47. Interestingly, microglia do not show the hypertrophic morphology indicative of phagocytic activity. These findings suggest that in the aging brain, changes in the balance of immunologic proteins give microglia instructions favoring synapse elimination of less active synapses, but this may occur by a process other than classic phagocytosis such as trogocytosis.

Deletion of Slc9a1 in Cx3cr1+ cells stimulated microglial subcluster CREB1 signaling and microglia-oligodendrocyte crosstalk

Microglial Na/H exchanger-1 (NHE1) protein, encoded by Slc9a1, plays a role in white matter demyelination of ischemic stroke brains. To explore underlying mechanisms, we conducted single cell RNA-seq transcriptome analysis in conditional Slc9a1 knockout (cKO) and wild-type (WT) mouse white matter tissues at 3 days post-stroke. Compared to WT, Nhe1 cKO brains expanded a microglial subgroup with elevated transcription of white matter myelination genes including Spp1, Lgals3, Gpnmb, and Fabp5. This subgroup also exhibited more acidic pHi and significantly upregulated CREB signaling detected by ingenuity pathway analysis and flow cytometry. Moreover, the Nhe1 cKO white matter tissues showed enrichment of a corresponding oligodendrocyte subgroup, with pro-phagocytosis and lactate shuffling gene expression, where activated CREB signaling is a likely upstream regulator. These findings demonstrate that attenuation of NHE1-mediated H+ extrusion acidifies microglia/macrophage and may underlie the stimulation of CREB1 signaling, giving rise to restorative microglia-oligodendrocyte interactions for remyelination.

Inhibiting CSF1R alleviates cerebrovascular white matter disease and cognitive impairment

White matter abnormalities, related to poor cerebral perfusion, are a core feature of small vessel cerebrovascular disease, and critical determinants of vascular cognitive impairment and dementia. Despite this importance there is a lack of treatment options. Proliferation of microglia producing an expanded, reactive population and associated neuroinflammatory alterations have been implicated in the onset and progression of cerebrovascular white matter disease, in patients and in animal models, suggesting that targeting microglial proliferation may exert protection. Colony-stimulating factor-1 receptor (CSF1R) is a key regulator of microglial proliferation. We found that the expression of CSF1R/Csf1r and other markers indicative of increased microglial abundance are significantly elevated in damaged white matter in human cerebrovascular disease and in a clinically relevant mouse model of chronic cerebral hypoperfusion and vascular cognitive impairment. Using the mouse model, we investigated long-term pharmacological CSF1R inhibition, via GW2580, and demonstrated that the expansion of microglial numbers in chronic hypoperfused white matter is prevented. Transcriptomic analysis of hypoperfused white matter tissue showed enrichment of microglial and inflammatory gene sets, including phagocytic genes that were the predominant expression modules modified by CSF1R inhibition. Further, CSF1R inhibition attenuated hypoperfusion-induced white matter pathology and rescued spatial learning impairments and to a lesser extent cognitive flexibility. Overall, this work suggests that inhibition of CSF1R and microglial proliferation mediates protection against chronic cerebrovascular white matter pathology and cognitive deficits. Our study nominates CSF1R as a target for the treatment of vascular cognitive disorders with broader implications for treatment of other chronic white matter diseases.

Mesenchymal stromal cells suppress microglial activation and tumor necrosis factor production

BACKGROUND AIMS

White matter diseases are commonly associated with microglial activation and neuroinflammation. Mesenchymal stromal cells (MSCs) have immunomodulatory properties and thus have the potential to be developed as cell therapy for white matter disease. MSCs interact with resident macrophages to alter the trajectory of inflammation; however, the impact MSCs have on central nervous system macrophages and the effect this has on the progression of white matter disease are unclear.

METHODS

In this study, we utilized numerous assays of varying complexity to model different aspects of white matter disease. These assays ranged from an in vivo spinal cord acute demyelination model to a simple microglial cell line activation assay. Our goal was to investigate the influence of human umbilical cord tissue MSCs on the activation of microglia.

RESULTS

MSCs reduced the production of tumor necrosis factor (TNF) by microglia and decreased demyelinated lesions in the spinal cord after acute focal injury. To determine if MSCs could directly suppress the activation of microglia and to develop an efficient potency assay, we utilized isolated primary microglia from mouse brains and the Immortalized MicroGlial Cell Line (IMG). MSCs suppressed the activation of microglia and the release of TNF after stimulation with lipopolysaccharide, a toll-like receptor agonist.

CONCLUSIONS

In this study, we demonstrated that MSCs altered the immune response after acute injury in the spinal cord. In numerous assays, MSCs suppressed activation of microglia and release of the pro-inflammatory cytokine TNF. Of these assays, IMG could be standardized and used as an effective potency assay to determine the efficacy of MSCs for treating white matter disease or other neuroinflammatory conditions associated with microglial activation.

Immunological dimensions of neuroinflammation and microglial activation: exploring innovative immunomodulatory approaches to mitigate neuroinflammatory progression

The increasing life expectancy has led to a higher incidence of age-related neurodegenerative conditions. Within this framework, neuroinflammation emerges as a significant contributing factor. It involves the activation of microglia and astrocytes, leading to the release of pro-inflammatory cytokines and chemokines and the infiltration of peripheral leukocytes into the central nervous system (CNS). These instances result in neuronal damage and neurodegeneration through activated nucleotide-binding domain and leucine-rich repeat containing (NLR) family pyrin domain containing protein 3 (NLRP3) and nuclear factor kappa B (NF-kB) pathways and decreased nuclear factor erythroid 2-related factor 2 (Nrf2) activity. Due to limited effectiveness regarding the inhibition of neuroinflammatory targets using conventional drugs, there is challenging growth in the search for innovative therapies for alleviating neuroinflammation in CNS diseases or even before their onset. Our results indicate that interventions focusing on Interleukin-Driven Immunomodulation, Chemokine (CXC) Receptor Signaling and Expression, Cold Exposure, and Fibrin-Targeted strategies significantly promise to mitigate neuroinflammatory processes. These approaches demonstrate potential anti-neuroinflammatory effects, addressing conditions such as Multiple Sclerosis, Experimental autoimmune encephalomyelitis, Parkinson's Disease, and Alzheimer's Disease. While the findings are promising, immunomodulatory therapies often face limitations due to Immune-Related Adverse Events. Therefore, the conduction of randomized clinical trials in this matter is mandatory, and will pave the way for a promising future in the development of new medicines with specific therapeutic targets.

Metformin treatment reduces inflammation, dysmyelination and disease severity in a mouse model of multiple sclerosis, experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an autoimmune disease characterized by inflammation, death or damage of oligodendrocytes, and axonal degeneration. Current MS treatments are non-curative, associated with undesired side-effects, and expensive, highlighting the need for expanded therapeutic options for patients. There is great interest in developing interventions using drugs or therapeutics to reduce symptom onset and protect pre-existing myelin. Metformin is a well-tolerated drug used to treat Type 2 diabetes that has pleiotropic effects in the central nervous system (CNS), including reducing inflammation, enhancing oligodendrogenesis, increasing the survival/proliferation of neural stem cells (NSCs), and increasing myelination. Here, we investigated whether metformin administration could improve functional outcomes, modulate oligodendrocyte precursor cells (OPCs), and reduce inflammation in a well-established mouse model of MS- experimental autoimmune encephalomyelitis (EAE). Male and female mice received metformin treatment at the time of EAE induction ("acute") or upon presentation of disease symptoms ("delayed"). We found that acute metformin treatment improved functional outcomes, concomitant with reduced microglia numbers and decreased dysmyelination. Conversely, delayed metformin treatment did not improve functional outcomes. Our findings reveal that metformin administration can improve EAE outcomes when administered before symptom onset in both sexes.

HLA-C*07:01 and HLA-DQB1*02:01 protect against white matter hyperintensities and deterioration of cognitive function: A population-based cohort study

BACKGROUND

Neuroinflammation and aberrant immune regulation are increasingly implicated in the pathophysiology of white matter hyperintensities (WMH), an imaging marker of cerebrovascular pathologies and predictor of cognitive impairment. The role of human leukocyte antigen (HLA) genes, critical in immunoregulation and associated with susceptibility to neurodegenerative diseases, in WMH pathophysiology remains unexplored.

METHODS

We performed association analyses between classical HLA alleles and WMH volume, derived from MRI scans of 38 302 participants in the UK Biobank. To identify independent functional alleles driving these associations, we conducted conditional forward stepwise regression and lasso regression. We further investigated whether these functional alleles showed consistent associations with WMH across subgroups characterized by varying levels of clinical determinants. Additionally, we validated the clinical relevance of the identified alleles by examining their association with cognitive function (n = 147 549) and dementia (n = 460 029) in a larger cohort.

FINDINGS

Four HLA alleles (DQB1*02:01, DRB1*03:01, C*07:01, and B*08:01) showed an association with reduced WMH volume after Bonferroni correction for multiple comparisons. Among these alleles, DQB1*02:01 exhibited the most significant association (β = -0.041, 95 % CI: -0.060 to -0.023, p = 1.04 × 10-5). Forward selection and lasso regression analyses indicated that DQB1*02:01 and C*07:01 primarily drove this association. The protective effect against WMH conferred by DQB1*02:01 and C*07:01 persisted in clinically relevant subgroups, with a stronger effect observed in older participants. Carrying DQB1*02:01 and C*07:01 was associated with higher cognitive function, but no association with dementia was found.

INTERPRETATION

Our population-based findings support the involvement of immune-associated mechanisms, particularly both HLA class I and class II genes, in the pathogenesis of WMH and subsequent consequence of cognitive functions.

Unique Bioactives from Zombie Fungus (Cordyceps) as Promising Multitargeted Neuroprotective Agents

Cordyceps, also known as "zombie fungus", is a non-poisonous mushroom that parasitizes insects for growth and development by manipulating the host system in a way that makes the victim behave like a "zombie". These species produce promising bioactive metabolites, like adenosine, β-glucans, cordycepin, and ergosterol. Cordyceps has been used in traditional medicine due to its immense health benefits, as it boosts stamina, appetite, immunity, longevity, libido, memory, and sleep. Neuronal loss is the typical feature of neurodegenerative diseases (NDs) (Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS)) and neurotrauma. Both these conditions share common pathophysiological features, like oxidative stress, neuroinflammation, and glutamatergic excitotoxicity. Cordyceps bioactives (adenosine, N6-(2-hydroxyethyl)-adenosine, ergosta-7, 9 (11), 22-trien-3β-ol, active peptides, and polysaccharides) exert potential antioxidant, anti-inflammatory, and anti-apoptotic activities and display beneficial effects in the management and/or treatment of neurodegenerative disorders in vitro and in vivo. Although a considerable list of compounds is available from Cordyceps, only a few have been evaluated for their neuroprotective potential and still lack information for clinical trials. In this review, the neuroprotective mechanisms and safety profile of Cordyceps extracts/bioactives have been discussed, which might be helpful in the identification of novel potential therapeutic entities in the future.

Ferroptosis of Microglia in Aging Human White Matter Injury

OBJECTIVE

Because the role of white matter (WM) degenerating microglia (DM) in remyelination failure is unclear, we sought to define the core features of this novel population of aging human microglia.

METHODS

We analyzed postmortem human brain tissue to define a population of DM in aging WM lesions. We used immunofluorescence staining and gene expression analysis to investigate molecular mechanisms related to the degeneration of DM.

RESULTS

We found that DM, which accumulated myelin debris were selectively enriched in the iron-binding protein light chain ferritin, and accumulated PLIN2-labeled lipid droplets. DM displayed lipid peroxidation injury and enhanced expression for TOM20, a mitochondrial translocase, and a sensor of oxidative stress. DM also displayed enhanced expression of the DNA fragmentation marker phospho-histone H2A.X. We identified a unique set of ferroptosis-related genes involving iron-mediated lipid dysmetabolism and oxidative stress that were preferentially expressed in WM injury relative to gray matter neurodegeneration.

INTERPRETATION

Ferroptosis appears to be a major mechanism of WM injury in Alzheimer's disease and vascular dementia. WM DM are a novel therapeutic target to potentially reduce the impact of WM injury and myelin loss on the progression of cognitive impairment. ANN NEUROL 2023;94:1048-1066.

Cytokine enrichment in deep cerebellar nuclei is contributed by multiple glial populations and linked to reduced amyloid plaque pathology

Alzheimer's disease (AD) pathology and amyloid-beta (Aβ) plaque deposition progress slowly in the cerebellum compared to other brain regions, while the entorhinal cortex (EC) is one of the most vulnerable regions. Using a knock-in AD mouse model (App KI), we show that within the cerebellum, the deep cerebellar nuclei (DCN) has particularly low accumulation of Aβ plaques. To identify factors that might underlie differences in the progression of AD-associated neuropathology across regions, we profiled gene expression in single nuclei (snRNAseq) across all cell types in the DCN and EC of wild-type (WT) and App KI male mice at age 7 months. We found differences in expression of genes associated with inflammatory activation, PI3K-AKT signalling, and neuron support functions between both regions and genotypes. In WT mice, the expression of interferon-response genes in microglia is higher in the DCN than the EC and this enrichment is confirmed by RNA in situ hybridisation, and measurement of inflammatory cytokines by protein array. Our analyses also revealed that multiple glial populations are responsible for establishing this cytokine-enriched niche. Furthermore, homogenates derived from the DCN induced inflammatory gene expression in BV2 microglia. We also assessed the relationship between the DCN microenvironment and Aβ pathology by depleting microglia using a CSF1R inhibitor PLX5622 and saw that, surprisingly, the expression of a subset of inflammatory cytokines was increased while plaque abundance in the DCN was further reduced. Overall, our study revealed the presence of a cytokine-enriched microenvironment unique to the DCN that when modulated, can alter plaque deposition.

Myelination- and migration-associated genes are downregulated after phagocytosis in cultured oligodendrocyte precursor cells

In the central nervous system, microglia are responsible for removing infectious agents, damaged/dead cells, and amyloid plaques by phagocytosis. Other cell types, such as astrocytes, are also recently recognized to show phagocytotic activity under some conditions. Oligodendrocyte precursor cells (OPCs), which belong to the same glial cell family as microglia and astrocytes, may have similar functions. However, it remains largely unknown whether OPCs exhibit phagocytic activity against foreign materials like microglia. To answer this question, we examined the phagocytosis activity of OPCs using primary rat OPC cultures. Since innate phagocytosis activity could trigger cell death pathways, we also investigated whether participating in phagocytosis activity may lead to OPC cell death. Our data shows that cultured OPCs phagocytosed myelin-debris-rich lysates prepared from rat corpus callosum, without progressing to cell death. In contrast to OPCs, mature oligodendrocytes did not show phagocytotic activity against the bait. OPCs also exhibited phagocytosis towards lysates of rat brain cortex and cell membrane debris from cultured astrocytes, but the percentage of OPCs that phagocytosed beta-amyloid was much lower than the myelin debris. We then conducted RNA-seq experiments to examine the transcriptome profile of OPC cultures and found that myelination- and migration-associated genes were downregulated 24 h after phagocytosis. On the other hand, there were a few upregulated genes in OPCs 24 h after phagocytosis. These data confirm that OPCs play a role in debris removal and suggest that OPCs may remain in a quiescent state after phagocytosis.

Periventricular Microglia Polarization and Morphological Changes Accompany NLRP3 Inflammasome-Mediated Neuroinflammation after Hypoxic-Ischemic White Matter Damage in Premature Rats

White matter damage (WMD) is a primary cause of cerebral palsy and cognitive impairment in preterm infants, and no effective treatments are available. Microglia are a major component of the innate immune system. When activated, they form typical pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes and regulate myelin development and synapse formation. Therefore, they may play a pivotal role in hypoxic-ischemic (HI) WMD. Herein, we investigated neural inflammation and long-term microglia phenotypic polarization in a neonatal rat model of hypoxia-ischemia-induced WMD and elucidated the underlying pathophysiological processes. We exposed 3-day-old (P3) Sprague-Dawley rats to hypoxia (8% oxygen) for 2.5 hr after unilateral common carotid artery ligation. The activation of NLRP3 inflammatory bodies, microglia M1/M2 polarization, myelination, and synaptic development in our model were monitored 7, 14, and 21 days after birth. In addition, the Morris water maze test was performed on postnatal Day 28. We confirmed myelination disturbance in the periventricular white matter, abnormal synaptic development, and behavioral changes in the periventricular area during the development of HI WMD. In addition, we found an association between the occurrence and development of HI WMD and activation of the NLRP3 inflammasome, microglial M1/M2 polarization, and the release of inflammatory factors. NLRP3 inhibition can play an anti-inflammatory role by inhibiting the differentiation of microglia into the M1 phenotype, thereby improving myelination and synapse formation. In conclusion, microglia are key mediators of the inflammatory response and exhibit continuous phenotypic polarization 7-21 days after HI-induced WMD. This finding can potentially lead to a new treatment regimen targeting the phenotypic polarization of microglia early after HI-induced brain injury.

The kynurenic acid analog SZR104 induces cytomorphological changes associated with the anti-inflammatory phenotype in cultured microglia

We previously showed the anti-inflammatory effects of kynurenic acid (KYNA) and its brain-penetrable analog N-(2-(dimethylamino)ethyl)-3-(morpholinomethyl)-4-hydroxyquinoline-2-carboxamide (SZR104) both in vivo and in vitro. Here, we identified the cytomorphological effects of KYNA and SZR104 in secondary microglial cultures established from newborn rat forebrains. We quantitatively analyzed selected morphological aspects of microglia in control (unchallenged), lipopolysaccharide (LPS)-treated (challenged), KYNA- or SZR104-treated, and LPS + KYNA or LPS + SZR104-treated cultures. Multicolor immunofluorescence labeling followed by morphometric analysis (area, perimeter, transformation index, lacunarity, density, span ratio, maximum span across the convex hull, hull circularity, hull area, hull perimeter, max/min radii, mean radius, diameter of bounding circle, fractal dimension, roughness, circularity) on binary (digital) silhouettes of the microglia revealed their morphological plasticity under experimental conditions. SZR104 and, to a lesser degree, KYNA inhibited proinflammatory phenotypic changes. For example, SZR104 treatment resulted in hypertrophied microglia characterized by a swollen cell body, enlarged perimeter, increased transformation index/decreased circularity, increased convex hull values (area, perimeter, mean radius, maximum span, diameter of the bounding circle and hull circularity), altered box-counting parameters (such as fractal dimension), and increased roughness/decreased density. Taken together, analysis of cytomorphological features could contribute to the characterization of the anti-inflammatory activity of SZR104 on cultured microglia.

Advanced Therapies for Traumatic Central Nervous System Injury: Delivery Strategy Reinforced Efficient Microglial Manipulation

Traumatic central nervous system (CNS) injuries, including spinal cord injury and traumatic brain injury, are challenging enemies of human health. Microglia, the main component of the innate immune system in CNS, can be activated postinjury and are key participants in the pathological procedure and development of CNS trauma. Activated microglia can be typically classified into pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes. Reducing M1 polarization while promoting M2 polarization is thought to be promising for CNS injury treatment. However, obstacles such as the low permeability of the blood-brain barrier and short retention time in circulation limit the therapeutic outcomes of administrated drugs, and rational delivery strategies are necessary for efficient microglial regulation. To this end, proper administration methods and delivery systems like nano/microcarriers and scaffolds are investigated to augment the therapeutic effects of drugs, while some of these delivery systems have self-efficacies in microglial manipulation. Besides, systems based on cell and cell-derived exosomes also show impressive effects, and some underlying targeting mechanisms of these delivery systems have been discovered. In this review, we introduce the roles of microglia play in traumatic CNS injuries, discuss the potential targets for the polarization regulation of microglial phenotype, and summarize recent studies and clinical trials about delivery strategies on enhancing the effect of microglial regulation and therapeutic outcome, as well as targeting mechanisms post CNS trauma.

Comparing the Characteristics of Microglia Preparations Generated Using Different Human iPSC-Based Differentiation Methods to Model Neurodegenerative Diseases

As the resident immune cells of the healthy nervous system, homeostatic microglia can rapidly become activated in response to injury/disease. Dysregulated microglia activation is a hallmark of nervous system disorders including neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and Alzheimer's disease. The elucidation of the biological and pathological roles of microglia has recently benefitted from the development of microglia-like cells using human induced pluripotent stem cell (iPSC)-based approaches. The success of iPSC-derived microglia preparations as a disease-relevant model system depends on their representation of the in vivo spatial and temporal heterogeneity of microglia under pathological conditions. Little is currently known about the potential of human iPSC-derived microglia generated using different methods for the study of neurodegenerative diseases. We compared the transcriptomes of human iPSC-derived microglia generated using two frequently used in vitro differentiation methods to determine whether separate strategies can generate microglia with distinct transcriptional signatures in vitro. We show that microglia derived using different differentiation methods display distinct maturation characteristics after equivalent times in culture. We also reveal that iPSC-derived microglia preparations generated using these two methods are composed of different subpopulations with transcriptomic signatures resembling those of in vivo regionally distinct microglia subtypes, specifically white-matter and gray-matter microglia. These findings highlight the need to better characterize the subtype composition of each microglia preparation prior to its use to model neurodegenerative diseases.

Reactive Microgliosis in Sepsis-Associated and Acute Hepatic Encephalopathies: An Ultrastructural Study

Sepsis and acute liver failure are associated with severe endogenous intoxication. Microglia, which are the resident immune brain cells, play diverse roles in central nervous system development, surveillance, and defense, as well as contributing to neuroinflammatory reactions. In particular, microglia are fundamental to the pathophysiology of reactive toxic encephalopathies. We analyzed microglial ultrastructure, morphotypes, and phagocytosis in the sensorimotor cortex of cecal ligation and puncture (CLP) and acetaminophen-induced liver failure (AILF) Wistar rats. A CLP model induced a gradual shift of ~50% of surveillant microglia to amoeboid hypertrophic-like and gitter cell-like reactive phenotypes with active phagocytosis and frequent contacts with damaged neurons. In contrast, AILF microglia exhibited amoeboid, rod-like, and hypertrophic-like reactive morphotypes with minimal indications for efficient phagocytosis, and were mostly in contact with edematous astrocytes. Close interactions of reactive microglia with neurons, astrocytes, and blood-brain barrier components reflect an active contribution of these cells to the tissue adaptation and cellular remodeling to toxic brain damage. Partial disability of reactive microglia may affect the integrity and metabolism in all tissue compartments, leading to failure of the compensatory mechanisms in acute endogenous toxic encephalopathies.

PGC-1α activity and mitochondrial dysfunction in preterm infants

Extremely low gestational age neonates (ELGANs) are born in a relatively hyperoxic environment with weak antioxidant defenses, placing them at high risk for mitochondrial dysfunction affecting multiple organ systems including the nervous, respiratory, ocular, and gastrointestinal systems. The brain and lungs are highly affected by mitochondrial dysfunction and dysregulation in the neonate, causing white matter injury (WMI) and bronchopulmonary dysplasia (BPD), respectively. Adequate mitochondrial function is important in providing sufficient energy for organ development as it relates to alveolarization and axonal myelination and decreasing oxidative stress via reactive oxygen species (ROS) and reactive nitrogen species (RNS) detoxification. Peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) is a master regulator of mitochondrial biogenesis and function. Since mitochondrial dysfunction is at the root of WMI and BPD pathobiology, exploring therapies that can regulate PGC-1α activity may be beneficial. This review article describes several promising therapeutic agents that can mitigate mitochondrial dysfunction through direct and indirect activation and upregulation of the PGC-1α pathway. Metformin, resveratrol, omega 3 fatty acids, montelukast, L-citrulline, and adiponectin are promising candidates that require further pre-clinical and clinical studies to understand their efficacy in decreasing the burden of disease from WMI and BPD in preterm infants.

Effects of choline containing phospholipids on the neurovascular unit: A review

The roles of choline and of choline-containing phospholipids (CCPLs) on the maintenance and progress of neurovascular unit (NVU) integrity are analyzed. NVU is composed of neurons, glial and vascular cells ensuring the correct homeostasis of the blood-brain barrier (BBB) and indirectly the function of the central nervous system. The CCPLs phosphatidylcholine (lecithin), cytidine 5′-diphosphocholine (CDP-choline), choline alphoscerate or α-glyceryl-phosphorylcholine (α-GPC) contribute to the modulation of the physiology of the NVU cells. A loss of CCPLs contributes to the development of neurodegenerative diseases such as Alzheimer’s disease, multiple sclerosis, Parkinson’s disease. Our study has characterized the cellular components of the NVU and has reviewed the effect of lecithin, of CDP-choline and α-GPC documented in preclinical studies and in limited clinical trials on these compounds. The interesting results obtained with some CCPLs, in particular with α-GPC, probably would justify reconsideration of the most promising molecules in larger attentively controlled studies. This can also contribute to better define the role of the NVU in the pathophysiology of brain disorders characterized by vascular impairment.

Systemic Beta-Hydroxybutyrate Affects BDNF and Autophagy into the Retina of Diabetic Mice

Background: Diabetic retinopathy (DR) is a neurovascular disease, characterized by a deficiency of brain-derived neurotrophic factor (BDNF), a regulator of autophagy. Beta-hydroxybutyrate (BHB), previously reported as a protective agent in DR, has been associated with BDNF promotion. Here, we investigated whether systemic BHB affects the retinal levels of BDNF and local autophagy in diabetic mice with retinopathy; Methods: C57BL/6J mice were administered with intraperitoneal (i.p.) streptozotocin (STZ) (75 mg/kg) injection to develop diabetes. After 2 weeks, they received i.p. injections of BHB (25−50−100 mg/kg) twice a week for 10 weeks. Retinal samples were collected in order to perform immunofluorescence, Western blotting, and ELISA analysis; Results: BHB 50 mg/kg and 100 mg/kg significantly improved retinal BDNF levels (p < 0.01) in diabetic mice. This improvement was negatively associated with autophagosome−lysosome formations (marked by LC3B and ATG14) and to higher levels of connexin 43 (p < 0.01), a marker of cell integrity. Moreover, BHB administration significantly reduced M1 microglial activation and autophagy (p < 0.01); Conclusions: The systemic administration of BHB in mice with DR improves the retinal levels of BDNF, with the consequent reduction of the abnormal microglial autophagy. This leads to retinal cell safety through connexin 43 restoration.

miR-214-3p Deficiency Enhances Caspase-1-Dependent Pyroptosis of Microglia in White Matter Injury

White matter injury (WMI) is the most frequent impairment of neurodevelopment in preterm infants. Here, we report that the caspase-1 inflammasome is abundantly activated in the microglia of WMI mice and results in increased pyroptosis of microglia. Pharmacology inhibition of caspase-1 cleavage alleviated the pathogenesis of WMI mice. The expression of microRNA miR-214-3p was largely reduced in the microglia of WMI mice compared to controls. Compromised expression of miR-214-3p on microglia gives rise to the inflammasome activation and microglial pyroptosis. Treatment with miR-214-3p agomir is sufficient to relieve the white matter lesion and demyelination in WMI mice. miR-214-3p is able to bind to the 3 $\prime$ region of the NLRP-3 inflammasome compartment NEK7, preventing the transcription of NEK7 mRNA. As a result, in WMI mice, the lack of miR-214-3p leads to the accumulation of NEK7 which supports NLRP 3 inflammasome activation, microglial pyroptosis, and white matter pathogenesis.

Resveratrol reduces inflammatory response and detrimental effects in chronic cerebral hypoperfusion by down-regulating stimulator of interferon genes/TANK-binding kinase 1/interferon regulatory factor 3 signaling

Inflammatory responses induced by chronic cerebral hypoperfusion (CCH) play a critical role in the progression of vascular dementia. Stimulator of interferon genes (STING) signaling function as a key mediator of inflammation and immunological responses in the central nervous system (CNS), and resveratrol (RES) exerts potent anti-inflammatory effects. However, the role of STING signaling and the relationship between RES and STING signaling in persistent hypoperfusion-induced cerebral inflammation remain unclear. In this study, Sprague–Dawley rats were subjected to either Sham or bilateral common carotid artery occlusion (2VO) surgery and received RES or vehicle daily by intraperitoneal injection for 4 or 8 weeks. Morris’s water maze was used for the analysis of cognitive function. The neuroinflammatory responses in white matter and hippocampus of the rat brain were assessed by Western blot, Immunofluorescence staining, and qRT-PCR analyses. Myelin integrity, neutrophil infiltration, and microglia proliferation were assessed by Immunohistochemistry and histologic analysis. We demonstrated that after CCH, neurons, microglia, and astrocyte under endoplasmic reticulum (ER) stress upregulated the expression of STING, TANK-binding kinase 1 (TBK1), and the transcription factor interferon regulatory factor 3 (IRF3), as well as translocation of IRF3 into the nucleus. These were accompanied by infiltration of neutrophils, activation of microglia, and overproduction of proinflammatory mediators. Improvements in cognitive deficits were related to reduced hippocampal neuronal cell death and increased myelin integrity in RES-treated rats. The neuroprotective effects of RES were associated with suppression of the expression of tumor necrosis factor-alpha (TNF-α), intercellular adhesion molecule 1 (ICAM-1), VCAM-1, interferon-β (IFN-β), and IL-1β, likely through mitigation of the STING/TBK1/IRF3 pathway. These inhibitory effects exerted by RES also inhibited the levels of myeloperoxidase, reduced excess expression of reactive astrocytes, and activated microglia. In conclusion, the STING/TBK1/IRF3 axis may be critical for proinflammatory responses in cerebral tissue with persistent hypoperfusion, and RES exerts its anti-inflammatory effects by suppressing STING/TBK1/IRF3 signaling.

Increased pyroptosis activation in white matter microglia is associated with neuronal loss in ALS motor cortex

Amyotrophic lateral sclerosis (ALS) is characterized by the degeneration of motor neurons in the motor cortex, brainstem, and spinal cord. Although ALS is considered a motor neuron disorder, neuroinflammation also plays an important role. Recent evidence in ALS disease models indicates activation of the inflammasome and subsequent initiation of pyroptosis, an inflammatory type of cell death. In this study, we determined the expression and distribution of the inflammasome and pyroptosis effector proteins in post-mortem brain and spinal cord from ALS patients (n = 25) and controls (n = 19), as well as in symptomatic and asymptomatic TDP-43^A315T transgenic and wild-type mice. Furthermore, we evaluated its correlation with the presence of TDP-43 pathological proteins and neuronal loss. Expression of the NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome, pyroptosis effector protein cleaved Gasdermin D (GSDMD), and IL-18 was detected in microglia in human ALS motor cortex and spinal cord, indicative of canonical inflammasome-triggered pyroptosis activation. The number of cleaved GSDMD-positive precentral white matter microglia was increased compared to controls and correlated with a decreased neuronal density in human ALS motor cortex. Neither of this was observed in the spinal cord. Similar results were obtained in TDP-43^A315T mice, where microglial pyroptosis activation was significantly increased in the motor cortex upon symptom onset, and correlated with neuronal loss. There was no significant correlation with the presence of TDP-43 pathological proteins both in human and mouse tissue. Our findings emphasize the importance of microglial NLRP3 inflammasome-mediated pyroptosis activation for neuronal degeneration in ALS and pave the way for new therapeutic strategies counteracting motor neuron degeneration in ALS by inhibiting microglial inflammasome/pyroptosis activation.

Transport Mechanisms at the Blood–Brain Barrier and in Cellular Compartments of the Neurovascular Unit: Focus on CNS Delivery of Small Molecule Drugs

Ischemic stroke is a primary origin of morbidity and mortality in the United States and around the world. Indeed, several research projects have attempted to discover new drugs or repurpose existing therapeutics to advance stroke pharmacotherapy. Many of these preclinical stroke studies have reported positive results for neuroprotective agents; however, only one compound (3K3A-activated protein C (3K3A-APC)) has advanced to Phase III clinical trial evaluation. One reason for these many failures is the lack of consideration of transport mechanisms at the blood–brain barrier (BBB) and neurovascular unit (NVU). These endogenous transport processes function as a “gateway” that is a primary determinant of efficacious brain concentrations for centrally acting drugs. Despite the knowledge that some neuroprotective agents (i.e., statins and memantine) are substrates for these endogenous BBB transporters, preclinical stroke studies have largely ignored the role of transporters in CNS drug disposition. Here, we review the current knowledge on specific BBB transporters that either limit drug uptake into the brain (i.e., ATP-binding cassette (ABC) transporters) or can be targeted for optimized drug delivery (i.e., solute carrier (SLC) transporters). Additionally, we highlight the current knowledge on transporter expression in astrocytes, microglia, pericytes, and neurons with an emphasis on transport mechanisms in these cell types that can influence drug distribution within the brain.

Neuroinflammation in Vascular Cognitive Impairment and Dementia: Current Evidence, Advances, and Prospects

Vascular cognitive impairment and dementia (VCID) is a major heterogeneous brain disease caused by multiple factors, and it is the second most common type of dementia in the world. It is caused by long-term chronic low perfusion in the whole brain or local brain area, and it eventually develops into severe cognitive dysfunction syndrome. Because of the disease’s ambiguous classification and diagnostic criteria, there is no clear treatment strategy for VCID, and the association between cerebrovascular pathology and cognitive impairment is controversial. Neuroinflammation is an immunological cascade reaction mediated by glial cells in the central nervous system where innate immunity resides. Inflammatory reactions could be triggered by various damaging events, including hypoxia, ischemia, and infection. Long-term chronic hypoperfusion-induced ischemia and hypoxia can overactivate neuroinflammation, causing apoptosis, blood–brain barrier damage and other pathological changes, triggering or aggravating the occurrence and development of VCID. In this review, we will explore the mechanisms of neuroinflammation induced by ischemia and hypoxia caused by chronic hypoperfusion and emphasize the important role of neuroinflammation in the development of VCID from the perspective of immune cells, immune mediators and immune signaling pathways, so as to provide valuable ideas for the prevention and treatment of the disease.

Pentoxifylline alleviates ischemic white matter injury through up-regulating Mertk-mediated myelin clearance

BACKGROUND

Vascular dementia (VAD) is the second most common type of dementia lacking effective treatments. Pentoxifylline (PTX), a nonselective phosphodiesterase inhibitor, displays protective effects in multiple cerebral diseases. In this study, we aimed to investigate the therapeutic effects and potential mechanisms of PTX in VAD.

METHODS

Bilateral common carotid artery stenosis (BCAS) mouse model was established to mimic VAD. Mouse behavior was tested by open field test, novel object recognition test, Y-maze and Morris water maze (MWM) tests. Histological staining, magnetic resonance imaging (MRI) and electron microscopy were used to define white matter integrity. The impact of PTX on microglia phagocytosis, peroxisome proliferator-activated receptors-γ (PPAR-γ) activation and Mer receptor tyrosine kinase (Mertk) expression was assessed by immunofluorescence, western blotting and flow cytometry with the application of microglia-specific Mertk knockout mice, Mertk inhibitor and PPAR-γ inhibitor.

RESULTS

Here, we found that PTX treatment alleviated cognitive impairment in novel object recognition test, Y-maze and Morris water maze tests. Furthermore, PTX alleviated white matter injury in corpus callosum (CC) and internal capsule (IC) areas as shown by histological staining and MRI analysis. PTX-treatment group presented thicker myelin sheath than vehicle group by electron microscopy. Mechanistically, PTX facilitated microglial phagocytosis of myelin debris by up-regulating the expression of Mertk in BCAS model and primary cultured microglia. Importantly, microglia-specific Mertk knockout blocked the therapeutic effects of PTX in BCAS model. Moreover, Mertk expression was regulated by the nuclear translocation of PPAR-γ. Through modulating PPAR-γ, PTX enhanced Mertk expression.

CONCLUSIONS

Collectively, our results demonstrated that PTX showed therapeutic potentials in VAD and alleviated ischemic white matter injury via modulating Mertk-mediated myelin clearance in microglia.

Cell type specific isolation of primary astrocytes and microglia from adult mouse spinal cord

BACKGROUND

Astrocytes and microglia are essential cellular elements of the CNS that are critical for normal development, function, and injury responses. Both cell types are highly pleiotropic and respond rapidly to environmental changes, making them challenging to characterize. One approach is to develop efficient isolation paradigms of distinct cell populations, allowing for characterization of their roles in distinct CNS regions and in pathological states.

NEW METHOD

We have developed an efficient and reliable protocol for isolation of astrocytes and microglia from the adult mouse spinal cord, which can be easily manipulated for immediate or future analyses. This method involves (1) rapid tissue dissociation; (2) cell release after myelin debris removal; (3) magnetic-activated cell sorting; and (4) optional downstream molecular and functional analyses.

RESULTS

High levels of viability and purity of the cells were confirmed after isolation. More importantly, characterization of cells verified their ability to proliferate and respond to external stimuli for potential use in downstream molecular and functional assays.

COMPARISON WITH EXISTING METHOD(S)

Long-term culture of cells isolated from neonatal animals and cell type specific isolation from the brain have been successful; however, isolation of spinal cord cells from adult mice has been challenging due to the large amount of myelin and limited size of the tissue compared to the brain. Our method allows for efficient isolation of astrocytes and microglia from spinal cord alone and includes simple modifications to allow for various downstream applications.

CONCLUSIONS

This technique will be a valuable tool to better understand the functions of astrocytes and microglia in spinal cord function and pathology.

Intellectual Structure and Emerging Trends of White Matter Hyperintensity Studies: A Bibliometric Analysis From 2012 to 2021

White matter hyperintensities (WMHs), which have a significant effect on human health, have received increasing attention since their number of publications has increased in the past 10 years. We aimed to explore the intellectual structure, hotspots, and emerging trends of publications on WMHs using bibliometric analysis from 2012 to 2021. Publications on WMHs from 2012 to 2021 were retrieved from the Web of Science Core Collection. CiteSpace 5.8.R3, VOSviewer 1.6.17, and an online bibliometric analysis platform (Bibliometric. com) were used to quantitatively analyze the trends of publications from multiple perspectives. A total of 29,707 publications on WMHs were obtained, and the number of annual publications generally increased from 2012 to 2021. Neurology had the most publications on WMHs. The top country and institution were the United States and Harvard University, respectively. Massimo Filippi and Stephen M. Smith were the most productive and co-cited authors, respectively. Thematic concentrations primarily included cerebral small vessel disease, diffusion magnetic resonance imaging (dMRI), schizophrenia, Alzheimer’s disease, multiple sclerosis, microglia, and oligodendrocyte. The hotspots were clustered into five groups: white matter and diffusion tensor imaging, inflammation and demyelination, small vessel disease and cognitive impairment, MRI and multiple sclerosis, and Alzheimer’s disease. Emerging trends mainly include deep learning, machine learning, perivascular space, convolutional neural network, neurovascular unit, and neurite orientation dispersion and density imaging. This study presents an overview of publications on WMHs and provides insights into the intellectual structure of WMH studies. Our study provides information to help researchers and clinicians quickly and comprehensively understand the hotspots and emerging trends within WMH studies as well as providing direction for future basic and clinical studies on WMHs.

White matter-associated microglia: New players in brain aging and neurodegenerative diseases

There has been growing interest in brain aging and rejuvenation. It is well known that brain aging is one of the leading causes of neurodegenerative diseases, such as Alzheimer's disease, but brain aging alone can cause cognitive decline. Microglia are thought to act as 'conductors' of white matter aging by modulating diverse glial cells and phagocytosing white matter-derived myelin debris. A recent study identified a specific subpopulation of microglia in the white matter of aged mice, termed white matter-associated microglia (WAM). Additionally, senescent microglia show impaired phagocytic function and altered lipid metabolism, which cause accumulation of lipid metabolites and eventually lead to myelin sheath degeneration. These results suggest that senescent WAM could be pivotal players in axonal loss during brain aging. The aim of this review is to assess the current state of knowledge on brain aging, with an emphasis on the roles of the white matter and microglia, and suggest potential approaches for rejuvenating the aged brain.

Dynamic Changes in Brain Iron Metabolism in Neonatal Rats after Hypoxia-Ischemia

OBJECTIVES

The pathogenesis of hypoxic-ischemic white matter injury (WMI) in premature infants is still unclear, and the imbalance of cerebral iron metabolism may play an important role. Our study set out to investigate the changes in iron distribution, iron content and malondialdehyde (MDA) in disparate brain regions (parietal cortex, corpus callosum, hippocampus) within 84 days after hypoxia-ischemia (HI) in neonatal rats and to clarify the role of iron metabolism in WMI.

MATERIALS AND METHODS

We adopted a rat model of hypoxic-ischemic WMI. Alterations in iron metabolism were detected by iron staining and iron assay kits, and the degree of brain injury was determined by MDA assays.

RESULTS

Our results showed that different degrees of brain iron deposition occurred within 28 days after HI, and iron staining was the most obvious 3 days after HI. The iron content increased remarkably at 1-7 d after HI in the mixed tissues, especially at 3 d after HI. While the iron content in the parietal cortex and corpus callosum elevated obviously 14 days after HI. And the change trend of MDA was almost consistent with that of the iron content.

CONCLUSIONS

Our findings revealed that brain iron metabolism changed dynamically in 3-day-old neonatal rats suffering from HI, which may cause lipid peroxidation damage to brain tissues. This process may be one of the pathogeneses of hypoxic-ischemic WMI.

N-3 PUFA Deficiency Affects the Ultrastructural Organization and Density of White Matter Microglia in the Developing Brain of Male Mice

Over the last century, westernization of dietary habits has led to a dramatic reduction in dietary intake of n-3 polyunsaturated fatty acids (n-3 PUFAs). In particular, low maternal intake of n-3 PUFAs throughout gestation and lactation causes defects in brain myelination. Microglia are recognized for their critical contribution to neurodevelopmental processes, such as myelination. These cells invade the white matter in the first weeks of the post-natal period, where they participate in oligodendrocyte maturation and myelin production. Therefore, we investigated whether an alteration of white matter microglia accompanies the myelination deficits observed in the brain of n-3 PUFA-deficient animals. Macroscopic imaging analysis shows that maternal n-3 PUFA deficiency decreases the density of white matter microglia around post-natal day 10. Microscopic electron microscopy analyses also revealed alterations of microglial ultrastructure, a decrease in the number of contacts between microglia and myelin sheet, and a decreased amount of myelin debris in their cell body. White matter microglia further displayed increased mitochondrial abundance and network area under perinatal n-3 PUFA deficiency. Overall, our data suggest that maternal n-3 PUFA deficiency alters the structure and function of microglial cells located in the white matter of pups early in life, and this could be the key to understand myelination deficits during neurodevelopment.

The immunomodulatory effect of microglia on ECM neuroinflammation via the PD-1/PD-L1 pathway

INTRODUCTION

The experimental cerebral malaria (ECM) model in C57BL/6 mice infected with Plasmodium berghei ANKA (PbA) has revealed microglia are involved in the ECM immune microenvironment. However, the regulation of microglia in the ECM immune response is not clear, and there is no safe and efficient treatment clinically for the protection of the nerve cells.

AIMS

To elucidate the negative regulation mechanism in the ECM brain mediated by microglia. Furthermore, to investigate protective effect of the appropriate enhancement of the PD-1/PD-L1 pathway in the brain against ECM through the intrathecal injection of the adenovirus expressing PDL1-IgG1Fc fusion protein.

RESULTS

The PD-1/PD-L1 pathway was induced in the ECM brain and showed an upregulation in the microglia. Deep single-cell analysis of immune niches in the ECM brainstem indicated that the microglia showed obvious heterogeneity and activation characteristics. Intrathecal injection of recombinant adenovirus expressing PD-L1 repressed the neuroinflammation and alleviated ECM symptoms. In addition, the synergistic effect of artemisinin and intracranial immunosuppression mediated by PD-L1 was more efficacious than either treatment alone.

CONCLUSION

The appropriate enhancement of the PD-1/PD-L1 pathway in the early stage of ECM has an obvious protective effect on the maintenance of immune microenvironment homeostasis in the brain. Regulating microglia and the PD-1/PD-L1 pathway could be considered as a promising approach for protection against human cerebral malaria in the future.

Caffeine treatment started before injury reduces hypoxic-ischemic white-matter damage in neonatal rats by regulating phenotypic microglia polarization

BACKGROUND

Reducing neuroinflammatory damage is an effective strategy for treating white-matter damage (WMD) in premature infants. Caffeine can ameliorate hypoxia-ischemia-induced brain WMD; however, its neuroprotective effect and mechanism against hypoxic-ischemic WMD remain unclear.

METHODS

We used 3-day-old Sprague-Dawley rats to establish a model of cerebral hypoxia-ischemia-induced brain WMD after unilateral common carotid artery ligation and hypoxia exposure (8% O₂ + 92% N₂) for 2.5 h. Mechanism experiments were conducted to detect M1/M2 polarization and activation of microglia and NLRP3 inflammasome.

RESULTS

Caffeine inhibited NLRP3 inflammasome activation, reduced microglial Iba-1 activation, inhibited microglia M1 polarization, and promoted microglia M2 polarization by downregulating CD86 and iNOS protein expression, inhibiting the transcription of the proinflammatory TNF-α and IL-1β, upregulating CD206 and Arg-1 expression, and promoting the transcription of the anti-inflammatory factors IL-10 and TGF-β. Importantly, we found that these caffeine-mediated effects could be reversed after inhibiting A2aR activity.

CONCLUSIONS

Caffeine improved long-term cognitive function in neonatal rats with hypoxic-ischemic WMD via A2aR-mediated inhibition of NLRP3 inflammasome activation, reduction of microglial activation, regulation of the phenotypic polarization of microglia and the release of inflammatory factors, and improvement of myelination development.

IMPACT

The direct protective effect of caffeine on hypoxic-ischemic white-matter damage (WMD) and its mechanism remains unclear. This study elucidated this mechanism using neonatal rats as an animal model of hypoxia-ischemia-induced cerebral WMD. The findings demonstrated caffeine as a promising therapeutic tool against immature WMD to protect neonatal cognitive function. We found that caffeine pretreatment reduced WMD in immature brains via regulation of microglial activation and polarization by adenosine A2a receptor, thereby, providing a scientific basis for future clinical application of caffeine.

Temporal patterns of microglial activation in white matter following experimental mild traumatic brain injury: a systematic literature review

Mild traumatic brain injuries (mTBIs) are a prevalent form of injury that can result in persistent neurological impairments. Microglial activation has become increasingly recognized as a key process regulating the pathology of white matter in a wide range of brain injury and disease contexts. As white matter damage is known to be a major contributor to the impairments that follow mTBI, microglia have rightfully become a common target of investigation for the development of mTBI therapies and biomarkers. Recent work has demonstrated that the efficacy of microglial manipulation as a therapeutic intervention following injury or disease is highly time-sensitive, emphasizing the importance of advancing our understanding of the dynamics of post-mTBI microglial activation from onset to resolution. Current reporting of microglial activation in experimental studies of mTBI is non-standardized, which has limited our ability to identify concrete patterns of post-mTBI microglial activation over time. In this review, we examine preclinical studies of mTBI that report on microglial activation in white matter regions to summarize our current understanding of these patterns. Specifically, we summarize timecourses of post-mTBI microglial activation in white matter regions of the brain, identify factors that influence this activation, examine the temporal relationship between microglial activation and other post-mTBI assessments, and compare the relative sensitivities of various methods for detecting microglial activation. While the lack of replicated experimental conditions has limited the extent of conclusions that can confidently be drawn, we find that microglia are activated over a wide range of timecourses following mTBI and that microglial activation is a long-lasting outcome of mTBI that may resolve after most typical post-mTBI assessments, with the exception of those measuring oligodendrocyte lineage cell integrity. We identify several understudied parameters of post-mTBI microglial activation in white matter, such as the inclusion of female subjects. This review summarizes our current understanding of the progression of microglial activation in white matter structures following experimental mTBI and offers suggestions for important future research directions.

Serum Levels of the Biomarkers Associated with Astrocytosis, Neurodegeneration, and Demyelination: Neurological Benefits of Citicoline Treatment of Patients with Ischemic Stroke and Atrial Fibrillation

Ischemic stroke is a main complication of atrial fibrillation (cardiac arrhythmia). The aim of our study was to estimate the effects of citicoline (CDP-choline) therapy on the levels of circulating neurospecific protein markers in serum of the patients with ischemic stroke and atrial fibrillation. Fiftyfour patients (mean age 76 years) treated with citicoline in a dose of 2.0 g daily intravenously for 12 to 14 days in addition to basic treatment formed the examined group. Thirty-two patients (mean age 68.5 years) obtained only standard therapy and formed the control group. Serum levels of neuronal and glial protein markers, including glial fibrillary acidic protein (GFAP), a neurofilament light subunit (NF-L), myelin basic protein (MBP), and ionized calcium-binding adaptor molecule 1 (Iba1), were measured in patients of both groups before and after treatment; an immunoblotting technique followed by densitometry analysis were used. Supplementary citicoline treatment provided significant reductions of the levels of GFAP (33%, P = 0.034), NF-L (27%, P = 0.019), and MBP (32%, P = 0.018), as compared to the initial values, while there were no marked changes in the studied parameters in the control group. The results obtained allow us to hypothesize that therapeutic benefit of citicoline in patients with ischemic stroke and atrial fibrillation can be mediated through increasing neuronal viability, protecting against axonal injury, decreasing the level of reactive astrogliosis, preventing deficiencies in the blood-brain integrity, and reducing the intensity of demyelination. However, citicoline administration exerted no effect on the blood content of microglial marker Iba-1, thus possibly preserving an important functional significance of microglia, which is needed to resolve local inflammation and clear cellular debris, and also provide protective factors to reduce cell injury in the ischemic brain. The obtained results indicate that serum levels of neurospecific biomarkers are significant and clinically relevant indices of the efficiency of treatment of the above-mentioned pathologies and can be used for further investigations of the stroke pathophysiology and molecular mechanisms of nootropic-mediated neuroprotection.

Nano-structured myelin: new nanovesicles for targeted delivery to white matter and microglia, from brain-to-brain

Neurodegenerative diseases affect millions of people worldwide and the presence of various physiological barriers limits the accessibility to the brain and reduces the efficacy of various therapies. Moreover, new carriers having targeting properties to specific brain regions and cells are needed in order to improve therapies for the brain disorder treatment. In this study, for the first time, Myelin nanoVesicles (hereafter defined MyVes) from brain-extracted myelin were produced. The MyVes have an average diameter of 100–150 ​nm, negative zeta potential, spheroidal morphology, and contain lipids and the key proteins of the myelin sheath. Furthermore, they exhibit good cytocompatibility. The MyVes were able to target the white matter and interact mainly with the microglia cells. The preliminary results here presented allow us to suppose the employment of MyVes as potential carrier to target the white matter and microglia in order to counteract white matter microglia-related diseases.

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Bio-fabrication of brain tissue derived nanovesicles: myelin nanovesicles.

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Myelin nanovesicles contain the main proteins of the myelin sheath (myelin basic protein and myelin proteolipid protein).

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Myelin nanovesicles can lade a drug/molecule and cross a blood–brain barrier model.

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Myelin nanovesicles target white matter and microglia cells.

Chronic Alcoholism and HHV-6 Infection Synergistically Promote Neuroinflammatory Microglial Phenotypes in the Substantia Nigra of the Adult Human Brain

Both chronic alcoholism and human herpesvirus-6 (HHV-6) infection have been identified as promoters of neuroinflammation and known to cause movement-related disorders. Substantia Nigra (SN), the dopaminergic neuron-rich region of the basal ganglia, is involved in regulating motor function and the reward system. Hence, we hypothesize the presence of possible synergism between alcoholism and HHV-6 infection in the SN region and report a comprehensive quantification and characterization of microglial functions and morphology in postmortem brain tissue from 44 healthy, age-matched alcoholics and chronic alcoholics. A decrease in the perivascular CD68+ microglia in alcoholics was noted in both the gray and white matter. Additionally, the CD68+/Iba1− microglial subpopulation was found to be the dominant type in the controls. Conversely, in alcoholics, dystrophic changes in microglia were seen with a significant increase in Iba1 expression and perivascular to diffuse migration. An increase in CD11b expression was noted in alcoholics, with the Iba1+/CD11b− subtype promoting inflammation. All the controls were found to be negative for HHV-6 whilst the alcoholics demonstrated HHV-6 positivity in both gray and white matter. Amongst HHV-6 positive alcoholics, all the above-mentioned changes were found to be heightened when compared with HHV-6 negative alcoholics, thereby highlighting the compounding relationship between alcoholism and HHV-6 infection that promotes microglia-mediated neuroinflammation.

The Role of Microglia in Modulating Neuroinflammation after Spinal Cord Injury

The pathobiology of traumatic and nontraumatic spinal cord injury (SCI), including degenerative myelopathy, is influenced by neuroinflammation. The neuroinflammatory response is initiated by a multitude of injury signals emanating from necrotic and apoptotic cells at the lesion site, recruiting local and infiltrating immune cells that modulate inflammatory cascades to aid in the protection of the lesion site and encourage regenerative processes. While peripheral immune cells are involved, microglia, the resident immune cells of the central nervous system (CNS), are known to play a central role in modulating this response. Microglia are armed with numerous cell surface receptors that interact with neurons, astrocytes, infiltrating monocytes, and endothelial cells to facilitate a dynamic, multi-faceted injury response. While their origin and essential nature are understood, their mechanisms of action and spatial and temporal profiles warrant extensive additional research. In this review, we describe the role of microglia and the cellular network in SCI, discuss tools for their investigation, outline their spatiotemporal profile, and propose translationally-relevant therapeutic targets to modulate neuroinflammation in the setting of SCI.

Specific microglial phagocytic phenotype and decrease of lipid oxidation in white matter areas during aging: Implications of different microenvironments

Physiological aging is characterized by an imbalance of pro-inflammatory and anti-inflammatory mediators leading to neuroinflammation. Microglial cells, which are highly regulated by the local microenvironment, undergo specific changes depending upon the brain area during aging. The aim of this study was to evaluate the influence of age over microglial cells along different brain areas and microenvironments. For this purpose, transgenic mice with overproduction of either the anti-inflammatory IL-10 cytokine or the pro-inflammatory IL-6 cytokine were used. Our results show that, during aging, microglial cells located in white matter (WM) areas maintain their phagocytic capacity but present a specific phagocytic phenotype with receptors involved in myelin recognition, arguing for aging-derived myelin damage. Whereas IL-10 overproduction anticipates the age-related microglial phagocytic phenotype, maintaining it over time, IL-6 overproduction exacerbates this phenotype in aging. These modifications were linked with a higher efficiency of myelin engulfment by microglia in aged transgenic animals. Moreover, we show, in a novel way, lower lipid oxidation during aging in WM areas, regardless of the genotype. The novelty of the insights presented in this study open a window to deeply investigate myelin lipid oxidation and the role of microglial cells in its regulation during physiological aging.

Cordycepin confers long-term neuroprotection via inhibiting neutrophil infiltration and neuroinflammation after traumatic brain injury

Background

The secondary injury caused by traumatic brain injury (TBI), especially white matter injury (WMI), is highly sensitive to neuroinflammation, which further leads to unfavored long-term outcomes. Although the cross-talk between the three active events, immune cell infiltration, BBB breakdown, and proinflammatory microglial/macrophage polarization, plays a role in the vicious cycle, its mechanisms are not fully understood. It has been reported that cordycepin, an extract from Cordyceps militaris, can inhibit TBI-induced neuroinflammation although the long-term effects of cordycepin remain unknown. Here, we report our investigation of cordycepin’s long-term neuroprotective function and its underlying immunological mechanism.

Methods

TBI mice model was established with a controlled cortical impact (CCI) method. Cordycepin was intraperitoneally administered twice daily for a week. Neurological outcomes were assessed by behavioral tests, including grid walking test, cylinder test, wire hang test, and rotarod test. Immunofluorescence staining, transmission electron microscopy, and electrophysiology recording were employed to assess histological and functional lesions. Quantitative-PCR and flow cytometry were used to detect neuroinflammation. The tracers of Sulfo-NHS-biotin and Evans blue were assessed for the blood-brain barrier (BBB) leakage. Western blot and gelatin zymography were used to analyze protein activity or expression. Neutrophil depletion in vivo was performed via using Ly6G antibody intraperitoneal injection.

Results

Cordycepin administration ameliorated long-term neurological deficits and reduced neuronal tissue loss in TBI mice. Meanwhile, the long-term integrity of white matter was also preserved, which was revealed in multiple dimensions, such as morphology, histology, ultrastructure, and electrical conductivity. Cordycepin administration inhibited microglia/macrophage pro-inflammatory polarization and promoted anti-inflammatory polarization after TBI. BBB breach was attenuated by cordycepin administration at 3 days after TBI. Cordycepin suppressed the activities of MMP-2 and MMP-9 and the neutrophil infiltration at 3 days after TBI. Moreover, neutrophil depletion provided a cordycepin-like effect, and cordycepin administration united with neutrophil depletion did not show a benefit of superposition.

Conclusions

The long-term neuroprotective function of cordycepin via suppressing neutrophil infiltration after TBI, thereby preserving BBB integrity and changing microglia/macrophage polarization. These findings provide significant clinical potentials to improve the quality of life for TBI patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02188-x.

IL33 (Interleukin 33)/ST2 (Interleukin 1 Receptor-Like 1) Axis Drives Protective Microglial Responses and Promotes White Matter Integrity After Stroke

[Figure: see text].

Juxtacortical susceptibility changes in progressive multifocal leukoencephalopathy at the gray-white matter junction correlates with iron-enriched macrophages

OBJECTIVE

Describe magnetic resonance imaging (MRI) susceptibility changes in progressive multifocal leukoencephalopathy (PML) and identify neuropathological correlates.

METHODS

PML cases and matched controls with primary central nervous system lymphoma (PCNSL) were retrospectively identified. MRI brain at 3 T and 7 T were reviewed. MRI-pathology correlations in fixed brain autopsy tissue were conducted in three subjects with confirmed PML.

RESULTS

With PML (n = 26 total, n = 5 multiple sclerosis natalizumab-associated), juxtacortical changes on susceptibility-weighted imaging (SWI) or gradient echo (GRE) sequences were noted in 3/3 cases on 7 T MRI and 14/22 cases (63.6%) on 1.5 T or 8/22 (36.4%) 3 T MRI. Similar findings were only noted in 3/25 (12.0%) of PCNSL patients (odds ratio (OR) 12.83, 95% confidence interval (CI), 2.9-56.7, p < 0.001) on 1.5 or 3 T MRI. On susceptibility sequences available prior to diagnosis of PML, 7 (87.5%) had changes present on average 2.7 ± 1.8 months (mean ± SD) prior to diagnosis. Postmortem 7 T MRI showed SWI changes corresponded to areas of increased iron density along the gray-white matter (GM-WM) junction predominantly in macrophages.

CONCLUSION

Susceptibility changes in PML along the GM-WM junction can precede noticeable fluid-attenuated inversion recovery (FLAIR) changes and correlates with iron accumulation in macrophages.

Central nervous system diseases related to pathological microglial phagocytosis

Microglia are important phagocytes of the central nervous system (CNS). They play an important role in protecting the CNS by clearing necrotic tissue and apoptotic cells in many CNS diseases. However, recent studies have found that microglia can phagocytose parts of neurons excessively, such as the neuronal cell body, synapse, or myelin sheaths, before or after the onset of CNS diseases, leading to aggravated injury and impaired tissue repair. Meanwhile, reduced phagocytosis of synapses and myelin results in abnormal circuit connections and inhibition of remyelination, respectively. Previous studies focused primarily on the positive effects of microglia phagocytosis, whereas only a few studies have focused on the negative effects. In this review, we use the term "pathological microglial phagocytosis" to refer to excessive or reduced phagocytosis by microglia that leads to structural or functional abnormalities in target cells and brain tissue. The classification of pathological microglial phagocytosis, the composition, and activation of related signaling pathways, as well as the process of pathological phagocytosis in various kinds of CNS diseases, are described in this review. We hypothesize that pathological microglial phagocytosis leads to aggravation of tissue damage and negative functional outcome. For example, excessive microglial phagocytosis of synapses can be observed in Alzheimer's disease and schizophrenia, leading to significant synapse loss and memory impairment. In Parkinson's disease, ischemic stroke, and traumatic brain injury, excessive microglial phagocytosis of neuronal cell bodies causes impaired gray matter recovery and sensory dysfunction. We therefore believe that more studies should focus on the mechanism of pathological microglial phagocytosis and activation to uncover potential targets of therapeutic intervention.

Microglial/Macrophage polarization and function in brain injury and repair after stroke

Stroke is a leading cause of disability and mortality, with limited treatment options. After stroke injury, microglia and CNS‐resident macrophages are rapidly activated and regulate neuropathological processes to steer the course of functional recovery. To accelerate this recovery, microglia can engulf dying cells and clear irreparably‐damaged tissues, thereby creating a microenvironment that is more suitable for the formation of new neural circuitry. In addition, monocyte‐derived macrophages cross the compromised blood‐brain barrier to infiltrate the injured brain. The specific functions of myeloid lineage cells in brain injury and repair are diverse and dependent on phenotypic polarization statuses. However, it remains to be determined to what degree the CNS‐invading macrophages occupy different functional niches from CNS‐resident microglia. In this review, we describe the physiological characteristics and functions of microglia in the developing and adult brain. We also review (a) the activation and phenotypic polarization of microglia and macrophages after stroke, (b) molecular mechanisms that control polarization status, and (c) the contribution of microglia to brain pathology versus repair. Finally, we summarize current breakthroughs in therapeutic strategies that calibrate microglia/macrophage responses after stroke.

The present review summarizes recent advances in microglial research in relation to stroke with emphases on microglial/macrophage phenotypic polarization and function in brain injury and repair. It also reviews the physiological characteristics and functions of microglia in the developing and adult brain, and describes current breakthroughs in therapeutic strategies that calibrate microglia/macrophage responses after stroke.

Classification of multiple sclerosis based on patterns of CNS regional atrophy covariance

There is evidence that multiple sclerosis (MS) pathology leads to distinct patterns of volume loss over time (VLOT) in different central nervous system (CNS) structures. We aimed to use such patterns to identify patient subgroups. MS patients of all classical disease phenotypes underwent annual clinical, blood, and MRI examinations over 6 years. Spinal, striatal, pallidal, thalamic, cortical, white matter, and T2‐weighted lesion volumes as well as serum neurofilament light chain (sNfL) were quantified. CNS VLOT patterns were identified using principal component analysis and patients were classified using hierarchical cluster analysis. 225 MS patients were classified into four distinct Groups A, B, C, and D including 14, 59, 141, and 11 patients, respectively). These groups did not differ in baseline demographics, disease duration, disease phenotype distribution, and lesion‐load expansion. Interestingly, Group A showed pronounced spinothalamic VLOT, Group B marked pallidal VLOT, Group C small between‐structure VLOT differences, and Group D myelocortical volume increase and pronounced white matter VLOT. Neurologic deficits were more severe and progressed faster in Group A that also had higher mean sNfL levels than all other groups. Group B experienced more frequent relapses than Group C. In conclusion, there are distinct patterns of VLOT across the CNS in MS patients, which do not overlap with clinical MS subtypes and are independent of disease duration and lesion‐load but are partially associated to sNfL levels, relapse rates, and clinical worsening. Our findings support the need for a more biologic classification of MS subtypes including volumetric and body‐fluid markers.

Gut dysbiosis in stroke and its implications on Alzheimer's disease-like cognitive dysfunction

Various neurological disorders, such as stroke and Alzheimer's disease (AD), involve neuroinflammatory responses. The advent of the gut‐brain axis enhances our understanding of neurological disease progression and secondary cell death. Gut microbiomes, especially those associated with inflammation, may reflect the dysbiosis of both the brain and the gut, opening the possibility to utilize inflammatory microbiomes as biomarkers and therapeutic targets. The gut‐brain axis may serve as a contributing factor to disease pathology and offer innovative approaches in cell‐based regenerative medicine for the treatment of neurological diseases. In reviewing the pathogenesis of stroke and AD, we also discuss the effects of gut microbiota on cognitive decline and brain pathology. Although the underlying mechanism of primary cell death from either disease is clearly distinct, both may be linked to gut‐microbial dysfunction as a consequential aberration that is unique to each disease. Targeting peripheral cell death pathways that exacerbate disease symptoms, such as those arising from the gut, coupled with conventional central therapeutic approach, may improve stroke and AD outcomes.

Pathological changes in neurovascular units: Lessons from cases of vascular dementia

Vascular dementia (VD) is the second leading cause of dementia after Alzheimer's disease (AD). The decrease of cerebral blood flow (CBF) to different degrees is one of the main causes of VD. Neurovascular unit (NVU) is a vessel‐centered concept, emphasizing all the cellular components play an integrated role in maintaining the normal physiological functions of the brain. More and more evidence shows that reduced CBF causes a series of changes in NVU, such as impaired neuronal function, abnormal activation of glial cells, and changes in vascular permeability, all of which collectively play a role in the pathogenesis of VD. In this paper, we review NVU changes as CBF decreases, focusing on each cellular component of NVU. We also highlight remote ischemic preconditioning as a promising approach for VD prevention and treatment from the NVU perspective of view.