Differential roles of astrocyte and microglia in supporting oligodendrocyte development and myelination in vitro

Age-related changes in myelin and myelin water quantified with short-TR adiabatic inversion-recovery (STAIR) sequences

Myelin proton density fraction (MPDF) and myelin water fraction (MWF) measured with short-TR adiabatic inversion-recovery (STAIR) sequences are potential biomarkers of aging and neurodegeneration, but the effects of aging on MPDF/MWF remain unknown. This study aims to assess relationships between age and MPDF/MWF using the 3D STAIR ultrashort echo time (STAIR-UTE) and STAIR short echo time (STAIR-STE) sequences, respectively. 42 volunteers (29 young (<55y), 13 old (>55y)), were recruited for MPDF and MWF mapping for white matter (WM) and gray matter (GM) on a 3 T scanner. Excellent inter-reader reliability was demonstrated for MPDF and MWF measurements with ICC values of 0.97 and 0.98, respectively, between two readers. In the young group, WM MPDF and MWF range from 8-13 % and 6-13 %, respectively, while GM MPDF and MWF range from 5-7 % and 3-5 %, respectively. In the old group, WM MPDF and MWF range from 6-12 % and 5-13 %, and GM MPDF and MWF range from 3-6 % and 2-5 %, respectively. The young group's MPDF/MWF values were significantly higher than those of the old group. Altogether, the 42 volunteers display linear and quadratic associations of MPDF/MWF with age. MPDF demonstrated significant correlations with MWF in the majority of brain regions. This study demonstrates the capability of two myelin imaging biomarkers, STAIR-UTE measured MPDF and STAIR-STE measured MWF to map brain myelin and MW. These biomarkers hold the potential to differentiate normal aging from neuroinflammatory/neurodegenerative diseases.

Perturbations of CB1 receptor signalling during adolescence impair cortical myelination in female rats

The constant increase in cannabis use among adolescents raises concerns about its potential neurobehavioral effects. Adolescence is a critical period for brain development, involving significant changes in grey and white matter. Grey matter decreases as the brain undergoes synaptic pruning, while white matter increases due to myelination. Cannabis use during this developmental window, particularly its active ingredient delta-9-tetrahydrocannabinol (THC), may disrupt these processes, increasing the risk of developing psychiatric disorders later in life. While the impact of THC on grey matter has been explored, the specific role of CB1 receptors as well as the effect of THC exposure in adolescent myelination remain unclear. This study investigates how CB1 receptor blockade and THC exposure during adolescence affect myelination in the prefrontal cortex of female rats. Blocking CB1 receptors during adolescence hindered myelination in the prefrontal cortex. Behaviourally, this disruption in myelin formation was associated with increased risk-taking behaviour. Notably, our data suggest that alterations in the AKT/Hippo/YAP signalling pathway may play a crucial role in mediating these effects. Supporting the involvement of the endocannabinoid system in cortical myelination during adolescence, we found that administering exogenous THC impaired myelin formation only when given during early to mid-adolescence. Moreover, when a more intensive THC exposure protocol was applied during this developmental period, the effects on myelination were long-lasting and persisted into adulthood. Overall, these data support a role for CB1 receptors in shaping cortical myelination in adolescent female rats and show that adolescent exposure to THC might adversely impact this developmental process.

From Physiology to Pathology of Astrocytes: Highlighting Their Potential as Therapeutic Targets for CNS Injury

In the mammalian central nervous system (CNS), astrocytes are the ubiquitous glial cells that have complex morphological and molecular characteristics. These fascinating cells play essential neurosupportive and homeostatic roles in the healthy CNS and undergo morphological, molecular, and functional changes to adopt so-called 'reactive' states in response to CNS injury or disease. In recent years, interest in astrocyte research has increased dramatically and some new biological features and roles of astrocytes in physiological and pathological conditions have been discovered thanks to technological advances. Here, we will review and discuss the well-established and emerging astroglial biology and functions, with emphasis on their potential as therapeutic targets for CNS injury, including traumatic and ischemic injury. This review article will highlight the importance of astrocytes in the neuropathological process and repair of CNS injury.

Orchestrating the neuroglial compartment: Ontogeny and developmental interaction of astrocytes, oligodendrocytes, and microglia

Neuroglial cells serve as the master regulators of the central nervous system, making it imperative for glial development to be tightly regulated both spatially and temporally to ensure optimal brain function. In this chapter, we will discuss the origin and development of the three major glia cells such as astrocytes, oligodendrocytes, and microglia in the central nervous system. While much of our understanding of neuroglia development stems from studies using animal models, we will also explore recent insights into human glial development and potential differences from rodent models. Finally, the extensive crosstalk between glia cells will be highlighted, discussing how interactions among astrocyte, oligodendrocyte, and microglial influence their respective developmental pathways.

C1ql1 expression in oligodendrocyte progenitor cells promotes oligodendrocyte differentiation

Myelinating oligodendrocytes arise from the stepwise differentiation of oligodendrocyte progenitor cells (OPCs). Approximately 5% of all adult brain cells are OPCs. Why would a mature brain need such a large number of OPCs? New myelination is possibly required for higher-order functions such as cognition and learning. Additionally, this pool of OPCs represents a source of new oligodendrocytes to replace those lost during injury, inflammation, or in diseases such as multiple sclerosis (MS). How OPCs are instructed to differentiate into oligodendrocytes is poorly understood, and for reasons presently unclear, resident pools of OPCs are progressively less utilized in MS. The complement component 1, q subcomponent-like (C1QL) protein family has been studied for their functions at neuron-neuron synapses, but we show that OPCs express C1ql1. We created OPC-specific conditional knockout mice and show that C1QL1 deficiency reduces the differentiation of OPCs into oligodendrocytes and reduces myelin production during both development and recovery from cuprizone-induced demyelination. In vivo over-expression of C1QL1 causes the opposite phenotype: increased oligodendrocyte density and myelination during recovery from demyelination. We further used primary cultured OPCs to show that C1QL1 levels can bidirectionally regulate the extent of OPC differentiation in vitro. Our results suggest that C1QL1 may initiate a previously unrecognized signaling pathway to promote differentiation of OPCs into oligodendrocytes. This study has relevance for possible novel therapies for demyelinating diseases and may illuminate a previously undescribed mechanism to regulate the function of myelination in cognition and learning.

Gene-environment interactions in the influence of maternal education on adolescent neurodevelopment using ABCD study

Maternal education was strongly correlated with adolescent brain morphology, cognitive performances, and mental health. However, the molecular basis for the effects of maternal education on the structural neurodevelopment remains unknown. Here, we conducted gene-environment-wide interaction study using the Adolescent Brain Cognitive Development cohort. Seven genomic loci with significant gene-environment interactions (G×E) on regional gray matter volumes were identified, with enriched biological functions related to metabolic process, inflammatory process, and synaptic plasticity. Additionally, genetic overlapping results with behavioral and disease-related phenotypes indicated shared biological mechanism between maternal education modified neurodevelopment and related behavioral traits. Finally, by decomposing the multidimensional components of maternal education, we found that socioeconomic status, rather than family environment, played a more important role in modifying the genetic effects on neurodevelopment. In summary, our study provided analytical evidence for G×E effects regarding adolescent neurodevelopment and explored potential biological mechanisms as well as social mechanisms through which maternal education could modify the genetic effects on regional brain development.

Terra incognita of glial cell dynamics in the etiology of leukodystrophies: Broadening disease and therapeutic perspectives

Neuroglial cells, also known as glia, are primarily characterized as auxiliary cells within the central nervous system (CNS). The recent findings have shed light on their significance in numerous physiological processes and their involvement in various neurological disorders. Leukodystrophies encompass an array of rare and hereditary neurodegenerative conditions that were initially characterized by the deficiency, aberration, or degradation of myelin sheath within CNS. The primary cellular populations that experience significant alterations are astrocytes, oligodendrocytes and microglia. These glial cells are either structurally or metabolically impaired due to inherent cellular dysfunction. Alternatively, they may fall victim to the accumulation of harmful by-products resulting from metabolic disturbances. In either situation, the possible replacement of glial cells through the utilization of implanted tissue or stem cell-derived human neural or glial progenitor cells hold great promise as a therapeutic strategy for both the restoration of structural integrity through remyelination and the amelioration of metabolic deficiencies. Various emerging treatment strategies like stem cell therapy, ex-vivo gene therapy, infusion of adeno-associated virus vectors, emerging RNA-based therapies as well as long-term therapies have demonstrated success in pre-clinical studies and show promise for rapid clinical translation. Here, we addressed various leukodystrophies in a comprehensive and detailed manner as well as provide prospective therapeutic interventions that are being considered for clinical trials. Further, we aim to emphasize the crucial role of different glial cells in the pathogenesis of leukodystrophies. By doing so, we hope to advance our understanding of the disease, elucidate underlying mechanisms, and facilitate the development of potential treatment interventions.

Early Postnatal Neuroinflammation Produces Key Features of Diffuse Brain White Matter Injury in Rats

BACKGROUND

Perinatal infection is a major risk factor for diffuse white matter injury (dWMI), which remains the most common form of neurological disability among very preterm infants. The disease primarily targets oligodendrocytes (OL) lineage cells in the white matter but also involves injury and/or dysmaturation of neurons of the gray matter. This study aimed to investigate whether neuroinflammation preferentially affects the cellular compositions of the white matter or gray matter.

METHOD

Neuroinflammation was initiated by intracerebral administration of lipopolysaccharide (LPS) to rat pups at postnatal (P) day 5, and neurobiological and behavioral outcomes were assessed between P6 and P21.

RESULTS

LPS challenge rapidly activates microglia and astrocytes, which is associated with the inhibition of OL and neuron differentiation leading to myelination deficits. Specifically, neuroinflammation reduces the immature OLs but not progenitors and causes acute axonal injury (β-amyloid precursor protein immunopositivity) and impaired dendritic maturation (reduced MAP2+ neural fiber density) in the cortical area at P7. Neuroinflammation also reduces the expression of doublecortin in the hippocampus, suggesting compromise in neurogenesis. Utilizing a battery of behavioral assessments, we found that LPS-exposed animals exhibited deficits in sensorimotor, neuromuscular, and cognitive domains.

CONCLUSION

Our overall results indicate that neuroinflammation alone in the early postnatal period can produce cardinal neuropathological features of dWMI.

Trem2-deficiency aggravates and accelerates age-related myelin degeneration

Aging is the greatest known risk factor for most neurodegenerative diseases. Myelin degeneration is an early pathological indicator of these diseases and a normal part of aging; albeit, to a lesser extent. Despite this, little is known about the contribution of age-related myelin degeneration on neurodegenerative disease. Microglia participate in modulating white matter events from demyelination to remyelination, including regulation of (de)myelination by the microglial innate immune receptor triggering receptor expressed on myeloid cells 2 (TREM2). Here, we demonstrate Trem2-deficiency aggravates and accelerates age-related myelin degeneration in the striatum. We show TREM2 is necessary for remyelination by recruiting reparative glia and mediating signaling that promotes OPC differentiation/maturation. In response to demyelination, TREM2 is required for phagocytosis of large volumes of myelin debris. In addition to lysosomal regulation, we show TREM2 can modify the ER stress response, even prior to overt myelin debris, that prevents lipid accumulation and microglial dysfunction. These data support a role for Trem2-dependent interactions in age-related myelin degeneration and suggest a basis for how early dysfunctional microglia could contribute to disease pathology through insufficent repair, defective phagocytosis, and the ER stress response.

Microglia protect against age-associated brain pathologies

Microglia are brain-resident macrophages that contribute to central nervous system (CNS) development, maturation, and preservation. Here, we examine the consequences of permanent microglial deficiencies on brain aging using the Csf1rΔFIRE/ΔFIRE mouse model. In juvenile Csf1rΔFIRE/ΔFIRE mice, we show that microglia are dispensable for the transcriptomic maturation of other brain cell types. By contrast, with advancing age, pathologies accumulate in Csf1rΔFIRE/ΔFIRE brains, macroglia become increasingly dysregulated, and white matter integrity declines, mimicking many pathological features of human CSF1R-related leukoencephalopathy. The thalamus is particularly vulnerable to neuropathological changes in the absence of microglia, with atrophy, neuron loss, vascular alterations, macroglial dysregulation, and severe tissue calcification. We show that populating Csf1rΔFIRE/ΔFIRE brains with wild-type microglia protects against many of these pathological changes. Together with the accompanying study by Chadarevian and colleagues1, our results indicate that the lifelong absence of microglia results in an age-related neurodegenerative condition that can be counteracted via transplantation of healthy microglia.

Monomeric Amyloid Peptide-induced Toxicity in Human Oligodendrocyte Cell Line and Mouse Brain Primary Mixed-glial Cell Cultures: Evidence for a Neuroprotective Effect of Neurosteroid 3α-O-allyl-allopregnanolone

Amyloid-peptide (Aβ) monomeric forms (ABM) occurring in presymptomatic Alzheimer's disease (AD) brain are thought to be devoid of neurotoxicity while the transition/aggregation of ABM into oligomers is determinant for Aβ-induced toxicity since Aβ is predominantly monomeric up to 3 µM and aggregates over this concentration. However, recent imaging and/or histopathological investigations revealed alterations of myelin in prodromal AD brain in absence of aggregated Aβ oligomers, suggesting that ABM may induce toxicity in myelin-producing cells in early AD-stages. To check this hypothesis, here we studied ABM effects on the viability of the Human oligodendrocyte cell line (HOG), a reliable oligodendrocyte model producing myelin proteins. Furthermore, to mimic closely interactions between oligodendrocytes and other glial cells regulating myelination, we investigated also ABM effects on mouse brain primary mixed-glial cell cultures. Various methods were combined to show that ABM concentrations (600 nM-1 µM), extremely lower than 3 µM, significantly decreased HOG cell and mouse brain primary mixed-glial cell survival. Interestingly, flow-cytometry studies using specific cell-type markers demonstrated that oligodendrocytes represent the most vulnerable glial cell population affected by ABM toxicity. Our work also shows that the neurosteroid 3α-O-allyl-allopregnanolone BR351 (250 and 500 nM) efficiently prevented ABM-induced HOG and brain primary glial cell toxicity. Bicuculline (50-100 nM), the GABA-A-receptor antagonist, was unable to block/reduce BR351 effect against ABM-induced HOG and primary glial cell toxicity, suggesting that BR351-evoked neuroprotection of these cells may not depend on GABA-A-receptor allosterically modulated by neurosteroids. Altogether, our results suggest that further exploration of BR351 therapeutic potential may offer interesting perspectives to develop effective neuroprotective strategies.

The complement system in neurodegenerative and inflammatory diseases of the central nervous system

Neurodegenerative and neuroinflammatory diseases, including Alzheimer's disease, Parkinson's disease, and multiple sclerosis, affect millions of people globally. As aging is a major risk factor for neurodegenerative diseases, the continuous increase in the elderly population across Western societies is also associated with a rising prevalence of these debilitating conditions. The complement system, a crucial component of the innate immune response, has gained increasing attention for its multifaceted involvement in the normal development of the central nervous system (CNS) and the brain but also as a pathogenic driver in several neuroinflammatory disease states. Although complement is generally understood as a liver-derived and blood or interstitial fluid operative system protecting against bloodborne pathogens or threats, recent research, particularly on the role of complement in the healthy and diseased CNS, has demonstrated the importance of locally produced and activated complement components. Here, we provide a succinct overview over the known beneficial and pathological roles of complement in the CNS with focus on local sources of complement, including a discussion on the potential importance of the recently discovered intracellularly active complement system for CNS biology and on infection-triggered neurodegeneration.

The role of glial cells in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) has traditionally been considered a neuron-centric disease. This view is now outdated, with increasing recognition of cell autonomous and non-cell autonomous contributions of central and peripheral nervous system glia to ALS pathomechanisms. With glial research rapidly accelerating, we comprehensively interrogate the roles of astrocytes, microglia, oligodendrocytes, ependymal cells, Schwann cells and satellite glia in nervous system physiology and ALS-associated pathology. Moreover, we highlight the inter-glial, glial-neuronal and inter-system polylogue which constitutes the healthy nervous system and destabilises in disease. We also propose classification based on function for complex glial reactive phenotypes and discuss the pre-requisite for integrative modelling to advance translation. Given the paucity of life-enhancing therapies currently available for ALS patients, we discuss the promising potential of harnessing glia in driving ALS therapeutic discovery.

Necrostatin-1s Suppresses RIPK1-driven Necroptosis and Inflammation in Periventricular Leukomalacia Neonatal Mice

Periventricular leukomalacia (PVL), a predominant form of brain injury in preterm survivors, is characterized by hypomyelination and microgliosis and is also the major cause of long-term neurobehavioral abnormalities in premature infants. Receptor-interacting protein kinase 1 (RIPK1) plays a pivotal role in mediating cell death and inflammatory signaling cascade. However, very little is known about the potential effect of RIPK1 in PVL and the underlying mechanism. Herein, we found that the expression level of RIPK1 was drastically increased in the brain of PVL neonatal mice models, and treatment of PVL neonatal mice with Necrostatin-1s (Nec-1s), an inhibitor of RIPK1, greatly ameliorated cerebral ischemic injury, exhibiting an increase of body weights, reduction of cerebral infarct size, neuronal loss, and occurrence of necrosis-like cells, and significantly improved the long-term abnormal neurobehaviors of PVL. In addition, Nec-1s significantly suppressed hypomyelination and promoted the differentiation of oligodendrocyte precursor cells (OPCs), as demonstrated by the increased expression levels of MBP and Olig2, the decreased expression level of GPR17, a significant increase in the number of CC-1-positive cells, and suppression of myelin ultrastructure impairment. Moreover, the mechanism of neuroprotective effects of Nec-1s against PVL is closely associated with its suppression of the RIPK1-mediated necrosis signaling molecules, RIPK1, PIPK3, and MLKL. More importantly, inhibition of RIPK1 could reduce microglial inflammatory injury by triggering the M1 to M2 microglial phenotype, appreciably decreasing the levels of M1 marker CD86 and increasing the levels of M2 markers Arg1 or CD206 in PVL mice. Taken together, inhibition of RIPK1 markedly ameliorates the brain injury and long-term neurobehavioral abnormalities of PVL mice through the reduction of neural cell necroptosis and reversing neuroinflammation.

Neuroprotective astroglial response to neural damage and its relevance to affective disorders

Astrocytes not only support neuronal function with essential roles in synaptic neurotransmission, action potential propagation, metabolic support, or neuroplastic and developmental adaptations. They also respond to damage or dysfunction in surrounding neurons and oligodendrocytes by releasing neurotrophic factors and other molecules that increase the survival of the supported cells or contribute to mechanisms of structural and molecular restoration. The neuroprotective responsiveness of astrocytes is based on their ability to sense signals of degeneration, metabolic jeopardy and structural damage, and on their aptitude to locally deliver specific molecules to remedy threats to the molecular and structural features of their cellular partners. To the extent that neuronal and other glial cell disturbances are known to occur in affective disorders, astrocyte responsiveness to those disturbances may help to better understand the roles astrocytes play in affective disorders. The astrocytic sensing apparatus supporting those responses involves receptors for neurotransmitters, purines, cell adhesion molecules and growth factors. Astrocytes also share with the immune system the capacity of responding to cytokines released upon neuronal damage. In addition, in responses to specific signals astrocytes release unique factors such as clusterin or humanin that have been shown to exert potent neuroprotective effects. Astrocytes integrate the signals above to further deliver structural lipids, removing toxic metabolites, stabilizing the osmotic environment, normalizing neurotransmitters, providing anti-oxidant protection, facilitating synaptogenesis and acting as barriers to contain varied deleterious signals, some of which have been described in brain regions relevant to affective disorders and related animal models. Since various of the injurious signals that activate astrocytes have been implicated in different aspects of the etiopathology of affective disorders, particularly in relation to the diagnosis of depression, potentiating the corresponding astrocyte neuroprotective responses may provide additional opportunities to improve or complement available pharmacological and behavioral therapies for affective disorders.

Oligodendroglia Confer Neuroprotection to NSC-34 Motor Neuronal Cells Against the Toxic Insults of Cerebrospinal Fluid from Sporadic Amyotrophic Lateral Sclerosis Patients

Amyotrophic lateral sclerosis (ALS) is a complex neurodegenerative disorder with multifactorial pathomechanisms affecting not only motor neurons but also glia. Both astrocytes and microglia get activated and contribute significantly to neurodegeneration. The role of oligodendroglia in such a situation remains obscure, especially in the sporadic form of ALS (SALS), which contributes to 90% of cases. Here, we have investigated the role of oligodendroglia in SALS pathophysiology using a human oligodendroglial cell line, MO3.13, by exposing the cells to cerebrospinal fluid from SALS patients (ALS-CSF; 10% v/v for 48 h). ALS-CSF significantly reduced the viability of MO3.13 cells and down-regulated the expression of oligodendroglia-specific proteins, namely, CNPase and Olig2. Furthermore, to investigate the effect of the observed oligodendroglial changes on motor neurons, NSC-34 motor neuronal cells were co-cultured/supplemented with conditioned/spent medium of MO3.13 cells upon exposure to ALS-CSF. Live cell imaging experiments revealed protection to NSC-34 cells against ALS-CSF toxicity upon co-culture with MO3.13 cells. This was evidenced by the absence of neuronal cytoplasmic vacuolation and beading of neurites, which instead resulted in better neuronal differentiation. Enhanced lactate levels and increased expression of its transporter, MCT-1, with sustained expression of trophic factors, namely, GDNF and BDNF, by MO3.13 cells hint towards metabolic and trophic support provided by the surviving oligodendroglia. Similar metabolic changes were seen in the lumbar spinal cord oligodendroglia of rat neonates intrathecally injected with ALS-CSF. The findings indicate that oligodendroglia are indeed rescuer to the degenerating motor neurons when the astrocytes and microglia turn topsy-turvy.

Emerging roles of oligodendrocyte precursor cells in neural circuit development and remodeling

Oligodendrocyte precursor cells (OPCs) are non-neuronal brain cells that give rise to oligodendrocytes, glia that myelinate the axons of neurons in the brain. Classically known for their contributions to myelination via oligodendrogenesis, OPCs are increasingly appreciated to play diverse roles in the nervous system, ranging from blood vessel formation to antigen presentation. Here, we review emerging literature suggesting that OPCs may be essential for the establishment and remodeling of neural circuits in the developing and adult brain via mechanisms that are distinct from the production of oligodendrocytes. We discuss the specialized features of OPCs that position these cells to integrate activity-dependent and molecular cues to shape brain wiring. Finally, we place OPCs within the context of a growing field focused on understanding the importance of communication between neurons and glia in the contexts of both health and disease.

Emerging non-proinflammatory roles of microglia in healthy and diseased brains

Microglia, the resident myeloid cells of the central nervous system, are the first line of defense against foreign pathogens, thereby confining the extent of brain injury. However, the role of microglia is not limited to macrophage-like functions. In addition to proinflammatory response mediation, microglia are involved in neurodevelopmental remodeling and homeostatic maintenance in the absence of disease. An increasing number of studies have also elucidated microglia-mediated regulation of tumor growth and neural repair in diseased brains. Here, we review the non-proinflammatory roles of microglia, with the aim of promoting a deeper understanding of the functions of microglia in healthy and diseased brains and contributing to the development of novel therapeutics that target microglia in neurological disorders.

Microglia Mediated Neuroinflammation in Parkinson’s Disease

Parkinson’s Disease (PD) is the second most common neurodegenerative disorder seen, especially in the elderly. Tremor, shaking, movement problems, and difficulty with balance and coordination are among the hallmarks, and dopaminergic neuronal loss in substantia nigra pars compacta of the brain and aggregation of intracellular protein α-synuclein are the pathological characterizations. Neuroinflammation has emerged as an involving mechanism at the initiation and development of PD. It is a complex network of interactions comprising immune and non-immune cells in addition to mediators of the immune response. Microglia, the resident macrophages in the CNS, take on the leading role in regulating neuroinflammation and maintaining homeostasis. Under normal physiological conditions, they exist as “homeostatic” but upon pathological stimuli, they switch to the “reactive state”. Pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes are used to classify microglial activity with each phenotype having its own markers and released mediators. When M1 microglia are persistent, they will contribute to various inflammatory diseases, including neurodegenerative diseases, such as PD. In this review, we focus on the role of microglia mediated neuroinflammation in PD and also signaling pathways, receptors, and mediators involved in the process, presenting the studies that associate microglia-mediated inflammation with PD. A better understanding of this complex network and interactions is important in seeking new therapies for PD and possibly other neurodegenerative diseases.

Deletion of Nox4 enhances remyelination following cuprizone-induced demyelination by increasing phagocytic capacity of microglia and macrophages in mice

NOX4 is a major reactive oxygen species-producing enzyme that modulates cell stress responses. We here examined the effect of Nox4 deletion on demyelination-remyelination, the most common pathological change in the brain. We used a model of cuprizone (CPZ)-associated demyelination-remyelination in wild-type and Nox4-deficient (Nox4^-/- ) mice. While the CPZ-induced demyelination in the corpus callosum after 4 weeks of CPZ intoxication was slightly less pronounced in Nox4^-/- mice than that in wild-type mice, remyelination following CPZ withdrawal was significantly enhanced in Nox4^-/- mice with an increased accumulation of IBA1-positive microglia/macrophages in the demyelinating corpus callosum. Consistently, locomotor function, as assessed by the beam walking test, was significantly better during the remyelination phase in Nox4^-/- mice. Nox4 deletion did not affect autonomous growth of primary-culture oligodendrocyte precursor cells. Although Nox4 expression was higher in cultured macrophages than in microglia, Nox4^-/- microglia and macrophages both showed enhanced phagocytic capacity of myelin debris and produced increased amounts of trophic factors upon phagocytosis. The expression of trophic factors was higher, in parallel with the accumulation of IBA1-positive cells, in the corpus callosum in Nox4^-/- mice than that in wild-type mice. Nox4 deletion suppressed phagocytosis-induced increase in mitochondrial membrane potential, enhancing phagocytic capacity of macrophages. Treatment with culture medium of Nox4^-/- macrophages engulfing myelin debris, but not that of Nox4^-/- astrocytes, enhanced cell growth and expression of myelin-associated proteins in cultured oligodendrocyte precursor cells. Collectively, Nox4 deletion promoted remyelination after CPZ-induced demyelination by enhancing microglia/macrophage-mediated clearance of myelin debris and the production of trophic factors leading to oligodendrogenesis.

Targeting the AKT/mTOR/p70S6K Pathway for Oligodendrocyte Differentiation and Myelin Regeneration in Neurological Disorders

BACKGROUND

The AKT/mTOR/p70S6K pathway has been shown to potentially promote spinal cord injury (SCI) repair in rats. However, its exact mechanism and beyond needs to be further explored.

OBJECTIVE

This study aims to explore the AKT/mTOR/p70S6K pathway in oligodendrocyte precursor cell (OPC) differentiation, microglial polarization differentiation, and the role of these in myelin regeneration in vitro.

METHODS

The isolation, induction and characterization of rat primary neuronal stem cells, OPCs and oligodendrocytes were investigated with immunofluorescence and RT-qPCR. Then, the role of AKT/mTOR/p70S6K signaling was explored using western blotting and immunofluorescence, the effect on myelination was examined with OPC-dorsal root ganglion (DRG) neurons co-culture, and the influence of M1/M2 polarization status of microglia on myelin formation was also observed by adding M1/M2 supernatants into OPC-DRG neurons co-culture.

RESULTS

Activation of the AKT/mTOR/p70S6K pathway elevated the expression of oligodendrocyte differentiation markers, including MBP, PLP and MOG, which also promoted the colocalization of MBP and NFH in OPC-DRG neurons co-culture. More interestingly, stimulation of the AKT/mTOR/p70S6K pathway facilitated M2 polarization of rat microglia. M2 polarization of microglia enhanced OPC differentiation to oligodendrocytes and myelin formation.

CONCLUSION

Our findings highlight the potential of targeting the AKT/mTOR/p70S6K pathway in promoting oligodendrocyte differentiation and myelin regeneration in neurological disorders such as SCI.

NG2-glia crosstalk with microglia in health and disease

Neurodegenerative diseases are increasingly becoming a global problem. However, the pathological mechanisms underlying neurodegenerative diseases are not fully understood. NG2‐glia abnormalities and microglia activation are involved in the development and/or progression of neurodegenerative disorders, such as multiple sclerosis, Alzheimer's disease, Parkinson's disease, and cerebrovascular diseases. In this review, we summarize the present understanding of the interaction between NG2‐glia and microglia in physiological and pathological states and discuss unsolved questions concerning their fate and potential fate. First, we introduce the NG2‐glia and microglia in health and disease. Second, we formulate the interaction between NG2‐glia and microglia. NG2‐glia proliferation, migration, differentiation, and apoptosis are influenced by factors released from the microglia. On the other hand, NG2‐glia also regulate microglia actions. We conclude that NG2‐glia and microglia are important immunomodulatory cells in the brain. Understanding the interaction between NG2‐glia and microglia will help provide a novel method to modulate myelination and treat neurodegenerative disorders.

Reduced and delayed myelination and volume of corpus callosum in an animal model of Fetal Alcohol Spectrum Disorders partially benefit from voluntary exercise

1 in 20 live births in the United States is affected by prenatal alcohol exposure annually, creating a major public health crisis. The teratogenic impact of alcohol on physical growth, neurodevelopment, and behavior is extensive, together resulting in clinical disorders which fall under the umbrella term of Fetal Alcohol Spectrum Disorders (FASD). FASD-related impairments to executive function and perceptual learning are prevalent among affected youth and are linked to disruptions to corpus callosum growth and myelination in adolescence. Targeted interventions that support neurodevelopment in FASD-affected youth are nonexistent. We evaluated the capacity of an adolescent exercise intervention, a stimulator of myelinogenesis, to upregulate corpus callosum myelination in a rat model of FASD (third trimester-equivalent alcohol exposure). This study employs in vivo diffusion tensor imaging (DTI) scanning to investigate the effects of: (1) neonatal alcohol exposure and (2) an adolescent exercise intervention on corpus callosum myelination in a rodent model of FASD. DTI scans were acquired twice longitudinally (pre- and post-intervention) in male and female rats using a 9.4 Tesla Bruker Biospec scanner to assess alterations to corpus callosum myelination noninvasively. Fractional anisotropy values as well as radial/axial diffusivity values were compared within-animal in a longitudinal study design. Analyses using mixed repeated measures ANOVA’s confirm that neonatal alcohol exposure in a rodent model of FASD delays the trajectory of corpus callosum growth and myelination across adolescence, with a heightened vulnerability in the male brain. Alterations to corpus callosum volume are correlated with reductions to forebrain volume which mediates an indirect relationship between body weight gain and corpus callosum growth. While we did not observe any significant effects of voluntary aerobic exercise on corpus callosum myelination immediately after completion of the 12-day intervention, we did observe a beneficial effect of exercise intervention on corpus callosum volume growth in all rats. In line with clinical findings, we have shown that prenatal alcohol exposure leads to hypomyelination of the corpus callosum in adolescence and that the severity of damage is sexually dimorphic. Further, exercise intervention improves corpus callosum growth in alcohol-exposed and control rats in adolescence.

Obesity and the Brain

Innate and adaptive immunity are essential for neurodevelopment and central nervous system (CNS) homeostasis; however, the fragile equilibrium between immune and brain cells can be disturbed by any immune dysregulation and cause detrimental effects. Accumulating evidence indicates that, despite the blood-brain barrier (BBB), overactivation of the immune system leads to brain vulnerability that increases the risk of neuropsychiatric disorders, particularly upon subsequent exposure later in life. Disruption of microglial function in later life can be triggered by various environmental and psychological factors, including obesity-driven chronic low-grade inflammation and gut dysbiosis. Increased visceral adiposity has been recognized as an important risk factor for multiple neuropsychiatric conditions. The review aims to present our current understanding of the topic.

White Matter Injury in Preterm Infants: Pathogenesis and Potential Therapy From the Aspect of the Gut–Brain Axis

Very preterm infants who survive are at high risk of white matter injury (WMI). With a greater understanding of the pathogenesis of WMI, the gut microbiota has recently drawn increasing attention in this field. This review tries to clarify the possible mechanisms behind the communication of the gut bacteria and the immature brain via the gut–brain axis. The gut microbiota releases signals, such as microbial metabolites. These metabolites regulate inflammatory and immune responses characterized by microglial activation, which ultimately impact the differentiation of pre-myelinating oligodendrocytes (pre-OLs) and lead to WMI. Moreover, probiotics and prebiotics emerge as a promising therapy to improve the neurodevelopmental outcome. However, future studies are required to clarify the function of these above products and the optimal time for their administration within a larger population. Based on the existing evidence, it is still too early to recommend probiotics and prebiotics as effective treatments for WMI.

Platelet-derived growth factor signalling in neurovascular function and disease

Platelet-derived growth factors are critical for cerebrovascular development and homeostasis. Abnormalities in this signalling pathway are implicated in neurological diseases, especially those where neurovascular dysfunction and neuroinflammation plays a prominent role in disease pathologies, such as stroke and Alzheimer's disease; the angiogenic nature of this pathway also draws its significance in brain malignancies such as glioblastoma where tumour angiogenesis is profuse. In this review, we provide an updated overview of the actions of the platelet-derived growth factors on neurovascular function, their role in the regulation of perivascular cell types expressing the cognate receptors, neurological diseases associated with aberrance in signalling, and highlight the clinical relevance and therapeutic potentials of this pathway for central nervous system diseases.

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.

Regulation of myelination by microglia

Interactions between microglia, the resident macrophages of the central nervous system (CNS), and myelin, the glial sheath on nerve fibers essential for rapid neural impulse transmission, are commonly studied in the context of neurotrauma and disease. However, interactions between microglia and myelin under normal physiological conditions have been largely overlooked. This review summarizes recent research indicating that the unique properties of microglia evident in disease states also enable microglia to regulate myelination during development and throughout life. This includes phagocytosis of cells and myelin membrane as well as the release of trophic factors, cytokines, and chemokines. The ability of microglia to sense neuronal activity and molecular features of the microenvironment enables them to optimize myelination by influencing early oligodendrogenesis, myelin formation, and removal of aberrantly targeted myelin. Understanding how microglia participate in myelination under normal conditions provides a new perspective that will increase understanding of developmental abnormalities.

Glia-Driven Brain Circuit Refinement Is Altered by Early-Life Adversity: Behavioral Outcomes

Early-life adversity (ELA), often clinically referred to as "adverse childhood experiences (ACE)," is the exposure to stress-inducing events in childhood that can result in poor health outcomes. ELA negatively affects neurodevelopment in children and adolescents resulting in several behavioral deficits and increasing the risk of developing a myriad of neuropsychiatric disorders later in life. The neurobiological mechanisms by which ELA alters neurodevelopment in childhood have been the focus of numerous reviews. However, a comprehensive review of the mechanisms affecting adolescent neurodevelopment (i.e., synaptic pruning and myelination) is lacking. Synaptic pruning and myelination are glia-driven processes that are imperative for brain circuit refinement during the transition from adolescence to adulthood. Failure to optimize brain circuitry between key brain structures involved in learning and memory, such as the hippocampus and prefrontal cortex, leads to the emergence of maladaptive behaviors including increased anxiety or reduced executive function. As such, we review preclinical and clinical literature to explore the immediate and lasting effects of ELA on brain circuit development and refinement. Finally, we describe a number of therapeutic interventions best-suited to support adolescent neurodevelopment in children with a history of ELA.

Plasticity of microglia

Microglial cells are the scions of foetal macrophages which invade the neural tube early during embryogenesis. The nervous tissue environment instigates the phenotypic metamorphosis of foetal macrophages into idiosyncratic surveilling microglia, which are generally characterised by a small cell body and highly ramified motile processes that constantly scan the nervous tissue for signs of changes in homeostasis and allow microglia to perform crucial homeostatic functions. The surveilling microglial phenotype is evolutionarily conserved from early invertebrates to humans. Despite this evolutionary conservation, microglia show substantial heterogeneity in their gene and protein expression, as well as morphological appearance. These differences are age, region and context specific and reflect a high degree of plasticity underlying the life-long adaptation of microglia, supporting the exceptional adaptive capacity of the central nervous system. Microgliocytes are essential elements of cellular network formation and refinement in the developing nervous tissue. Several distinct patrolling modes of microglial processes contribute to the formation, modification, and pruning of synapses; to the support and protection of neurones through microglial-somatic junctions; and to the control of neuronal and axonal excitability by specific microglia-axonal contacts. In pathology, microglia undergo proliferation and reactive remodelling known as microgliosis, which is context dependent, yet represents an evolutionarily conserved defence response. Microgliosis results in the emergence of multiple disease and context-specific reactive states; in addition, neuropathology is associated with the appearance of specific protective or recovery microglial forms. In summary, the plasticity of microglia supports the development and functional activity of healthy nervous tissue and provides highly sophisticated defences against disease.

The emerging tale of microglia in psychiatric disorders

As the professional phagocytes of the brain, microglia orchestrate the immunological response and play an increasingly important role in maintaining homeostatic brain functions. Microglia are activated by pathological events or slight alterations in brain homeostasis. This activation is dependent on the context and type of stressor or pathology. Through secretion of cytokines, chemokines and growth factors, microglia can strongly influence the response to a stressor and can, therefore, determine the pathological outcome. Psychopathologies have repeatedly been associated with long-lasting priming and sensitization of cerebral microglia. This review focuses on the diversity of microglial phenotype and function in health and psychiatric disease. We first discuss the diverse homeostatic functions performed by microglia and then elaborate on context-specific spatial and temporal microglial heterogeneity. Subsequently, we summarize microglia involvement in psychopathologies, namely major depressive disorder, schizophrenia and bipolar disorder, with a particular focus on post-mortem studies. Finally, we postulate microglia as a promising novel therapeutic target in psychiatry through antidepressant and antipsychotic treatment.

Understanding Abnormal c-JNK/p38MAPK Signaling Overactivation Involved in the Progression of Multiple Sclerosis: Possible Therapeutic Targets and Impact on Neurodegenerative Diseases

Demyelination, immune dysregulation, and neuroinflammation are the most common triggers of motor neuron disorders such as multiple sclerosis (MS). MS is a chronic demyelinating neurodegenerative disease of the central nervous system caused by abnormal immune activation, which causes myelin sheath damage. Cell signal transduction pathways are required for a variety of physiological and pathological processes in the brain. When these signaling systems become overactive, they can lead to disease progression. In various physiological conditions, abnormal mitogen-activated protein kinase (MAPK) activation is associated with several physiological dysfunctions that cause neurodegeneration. Previous research indicates that c-JNK and p38MAPK signaling play critical roles in neuronal growth and differentiation. c-JNK/p38MAPK is a member of the MAPK family, which regulates metabolic pathways, cell proliferation, differentiation, and apoptosis that control certain neurological activities. During brain injuries, c-JNK/p38MAPK also affects neuronal elastic properties, nerve growth, and cognitive processing. This review systematically linked abnormal c-JNK/p38MAPK signaling activation to multiple neuropathological pathways in MS and related neurological dysfunctions. MS progression is linked to genetic defects, oligodendrocyte destruction, glial overactivation, and immune dysregulation. We concluded that inhibiting both the c-JNK/p38MAPK signaling pathways can promote neuroprotection and neurotrophic effects against the clinical-pathological presentation of MS and influence other neurological disorders. As a result, the potential benefits of c-JNK/p38MAPK downregulation for the development of disease-modifying treatment interventions in the future could include MS prevention and related neurocomplications.

Piperine Improves Experimental Autoimmune Encephalomyelitis (EAE) in Lewis Rats Through its Neuroprotective, Anti-inflammatory, and Antioxidant Effects

Inflammation, demyelination, glial activation, and oxidative damage are the most pathological hallmarks of multiple sclerosis (MS). Piperine, a main bioactive alkaloid of black pepper, possesses antioxidant, anti-inflammatory, and neuroprotective properties whose therapeutic potential has been less studied in the experimental autoimmune encephalomyelitis (EAE) models. In this study, the efficiency of piperine on progression of EAE model and myelin repair mechanisms was investigated. EAE was induced in female Lewis rats and piperine and its vehicle were daily administrated intraperitoneally from day 8 to 29 post immunization. We found that piperine alleviated neurological deficits and EAE disease progression. Luxol fast blue and H&E staining and immunostaining of lumbar spinal cord cross sections confirmed that piperine significantly reduced the extent of demyelination, inflammation, immune cell infiltration, microglia, and astrocyte activation. Gene expression analysis in lumbar spinal cord showed that piperine treatment decreased the level of pro-inflammatory cytokines (TNF-α, IL-1β) and iNOS and enhanced IL-10, Nrf2, HO-1, and MBP expressions. Piperine supplementation also enhanced the total antioxidant capacity (FRAP) and reduced the level of oxidative stress marker (MDA) in the CNS of EAE rats. Finally, we found that piperine has anti-apoptotic and neuroprotective effect in EAE through reducing caspase-3 (apoptosis marker) and enhancing BDNF and NeuN expressing cells. This study strongly indicates that piperine has a beneficial effect on the EAE progression and could be considered as a potential therapeutic target for MS treatment. Upcoming clinical trials will provide a deeper understanding of piperine's role for the treatment of the MS.

Oligodendrocytes and Microglia: Key Players in Myelin Development, Damage and Repair

Oligodendrocytes, the myelin-making cells of the CNS, regulate the complex process of myelination under physiological and pathological conditions, significantly aided by other glial cell types such as microglia, the brain-resident, macrophage-like innate immune cells. In this review, we summarize how oligodendrocytes orchestrate myelination, and especially myelin repair after damage, and present novel aspects of oligodendroglial functions. We emphasize the contribution of microglia in the generation and regeneration of myelin by discussing their beneficial and detrimental roles, especially in remyelination, underlining the cellular and molecular components involved. Finally, we present recent findings towards human stem cell-derived preclinical models for the study of microglia in human pathologies and on the role of microbiome on glial cell functions.

Innate immunity at the crossroads of healthy brain maturation and neurodevelopmental disorders

The immune and nervous systems have unique developmental trajectories that individually build intricate networks of cells with highly specialized functions. These two systems have extensive mechanistic overlap and frequently coordinate to accomplish the proper growth and maturation of an organism. Brain resident innate immune cells - microglia - have the capacity to sculpt neural circuitry and coordinate copious and diverse neurodevelopmental processes. Moreover, many immune cells and immune-related signalling molecules are found in the developing nervous system and contribute to healthy neurodevelopment. In particular, many components of the innate immune system, including Toll-like receptors, cytokines, inflammasomes and phagocytic signals, are critical contributors to healthy brain development. Accordingly, dysfunction in innate immune signalling pathways has been functionally linked to many neurodevelopmental disorders, including autism and schizophrenia. This review discusses the essential roles of microglia and innate immune signalling in the assembly and maintenance of a properly functioning nervous system.

Hemoglobin induces oxidative stress and mitochondrial dysfunction in oligodendrocyte progenitor cells

Oligodendrocyte progenitor cells (OPCs) in the infant brain give rise to mature oligodendrocytes that myelinate CNS axons. OPCs are particularly vulnerable to oxidative stress that occurs in many forms of brain injury. One common cause of infant brain injury is neonatal intraventricular hemorrhage (IVH), which releases blood into the CSF and brain parenchyma of preterm infants. Although blood contains the powerful oxidant hemoglobin, the direct effects of hemoglobin on OPCs have not been studied. We utilized a cell culture system to test if hemoglobin induced free radical production and mitochondrial dysfunction in OPCs. We also tested if phenelzine (PLZ), an FDA-approved antioxidant drug, could protect OPCs from hemoglobin-induced oxidative stress. OPCs were isolated from Sprague Dawley rat pups and exposed to hemoglobin with and without PLZ. Outcomes assessed included intracellular reactive oxygen species levels using 2',7'-dichlorodihydrofluorescein diacetate (DCF-DA) fluorescent dye, oxygen consumption using the XFe96 Seahorse assay, and proliferation measured by BrdU incorporation assay. Hemoglobin induced oxidative stress and impaired mitochondrial function in OPCs. PLZ treatment reduced hemoglobin-induced oxidative stress and improved OPC mitochondrial bioenergetics. The effects of hemoglobin and PLZ on OPC proliferation were not statistically significant, but showed trends towards hemoglobin reducing OPC proliferation and PLZ increasing OPC proliferation (P=0.06 for both effects). Collectively, our results indicate that hemoglobin induces mitochondrial dysfunction in OPCs and that antioxidant therapy reduces these effects. Therefore, antioxidant therapy may hold promise for white matter diseases in which hemoglobin plays a role, such as neonatal IVH.

Neonatal Mesenchymal Stem Cell Treatment Improves Myelination Impaired by Global Perinatal Asphyxia in Rats

The effect of perinatal asphyxia (PA) on oligodendrocyte (OL), neuroinflammation, and cell viability was evaluated in telencephalon of rats at postnatal day (P)1, 7, and 14, a period characterized by a spur of neuronal networking, evaluating the effect of mesenchymal stem cell (MSCs)-treatment. The issue was investigated with a rat model of global PA, mimicking a clinical risk occurring under labor. PA was induced by immersing fetus-containing uterine horns into a water bath for 21 min (AS), using sibling-caesarean-delivered fetuses (CS) as controls. Two hours after delivery, AS and CS neonates were injected with either 5 μL of vehicle (10% plasma) or 5 × 10⁴ MSCs into the lateral ventricle. Samples were assayed for myelin-basic protein (MBP) levels; Olig-1/Olig-2 transcriptional factors; Gglial phenotype; neuroinflammation, and delayed cell death. The main effects were observed at P7, including: (i) A decrease of MBP-immunoreactivity in external capsule, corpus callosum, cingulum, but not in fimbriae of hippocampus; (ii) an increase of Olig-1-mRNA levels; (iii) an increase of IL-6-mRNA, but not in protein levels; (iv) an increase in cell death, including OLs; and (v) MSCs treatment prevented the effect of PA on myelination, OLs number, and cell death. The present findings show that PA induces regional- and developmental-dependent changes on myelination and OLs maturation. Neonatal MSCs treatment improves survival of mature OLs and myelination in telencephalic white matter.

Enduring neuroimmunological consequences of developmental experiences: From vulnerability to resilience

The immune system is crucial for normal neuronal development and function (neuroimmune system). Both immune and neuronal systems undergo significant postnatal development and are sensitive to developmental programming by environmental experiences. Negative experiences from infection to psychological stress at a range of different time points (in utero to adolescence) can permanently alter the function of the neuroimmune system: given its prominent role in normal brain development and function this dysregulation may increase vulnerability to psychiatric illness. In contrast, positive experiences such as exercise and environmental enrichment are protective and can promote resilience, even restoring the detrimental effects of negative experiences on the neuroimmune system. This suggests the neuroimmune system is a viable therapeutic target for treatment and prevention of psychiatric illnesses, especially those related to stress. In this review we will summarise the main cells, molecules and functions of the immune system in general and with specific reference to central nervous system development and function. We will then discuss the effects of negative and positive environmental experiences, especially during development, in programming the long-term functioning of the neuroimmune system. Finally, we will review the sparse but growing literature on sex differences in neuroimmune development and response to environmental experiences.

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The immune system is essential for development and function of the central nervous system (neuroimmune system)

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Environmental experiences can permanently alter neuroimmune function and associated brain development

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Altered neuroimmune function following negative developmental experiences may play a role in psychiatric illnesses

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Positive experiences can promote resilience and rescue the effects of negative experiences on the neuroimmune system

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The neuroimmune system is therefore a viable therapeutic target for preventing and treating psychiatric illnesses

Tuberous Sclerosis Complex as Disease Model for Investigating mTOR-Related Gliopathy During Epileptogenesis

Tuberous sclerosis complex (TSC) represents the prototypic monogenic disorder of the mammalian target of rapamycin (mTOR) pathway dysregulation. It provides the rational mechanistic basis of a direct link between gene mutation and brain pathology (structural and functional abnormalities) associated with a complex clinical phenotype including epilepsy, autism, and intellectual disability. So far, research conducted in TSC has been largely neuron-oriented. However, the neuropathological hallmarks of TSC and other malformations of cortical development also include major morphological and functional changes in glial cells involving astrocytes, oligodendrocytes, NG2 glia, and microglia. These cells and their interglial crosstalk may offer new insights into the common neurobiological mechanisms underlying epilepsy and the complex cognitive and behavioral comorbidities that are characteristic of the spectrum of mTOR-associated neurodevelopmental disorders. This review will focus on the role of glial dysfunction, the interaction between glia related to mTOR hyperactivity, and its contribution to epileptogenesis in TSC. Moreover, we will discuss how understanding glial abnormalities in TSC might give valuable insight into the pathophysiological mechanisms that could help to develop novel therapeutic approaches for TSC or other pathologies characterized by glial dysfunction and acquired mTOR hyperactivation.

Cell type-specific evaluation of ADGRG1/GPR56 function in developmental central nervous system myelination

Myelination of axons in the central nervous system (CNS) is a concerted effort between many cell types, resulting in significant cross-talk and communication among cells. Adhesion G protein-coupled receptor ADGRG1 (GPR56) is expressed in all major glial cells and regulates a wide variety of physiological processes by mediating cell-cell and cell-matrix communications. Previous literature has demonstrated the requirement of ADGRG1 in oligodendrocyte precursor cells (OPCs) during developmental myelination. However, it is unknown if ADGRG1 is responsible for myelin formation in a cell-type-specific manner. To that end, here we profiled myelin status in response to deletion of Adgrg1 specifically in OPCs, microglia, astrocytes, and neurons. Interestingly, we find that knocking out Adgrg1 in OPCs significantly decreases OPC proliferation and reduced number of myelinated axons. However, deleting Adgrg1 in microglia, astrocytes, and neurons does not impact developmental myelination. These data support an autonomous functional role for Adgrg1 in OPCs related to myelination.

The contribution of glial cells to Huntington's disease pathogenesis

Glial cells play critical roles in the normal development and function of neural circuits, but in many neurodegenerative diseases, they become dysregulated and may contribute to the development of brain pathology. In Huntington's disease (HD), glial cells both lose normal functions and gain neuropathic phenotypes. In addition, cell-autonomous dysfunction elicited by mutant huntingtin (mHTT) expression in specific glial cell types is sufficient to induce both pathology and Huntington's disease-related impairments in motor and cognitive performance, suggesting that these cells may drive the development of certain aspects of Huntington's disease pathogenesis. In support of this imaging studies in pre-symptomatic HD patients and work on mouse models have suggested that glial cell dysfunction occurs at a very early stage of the disease, prior to the onset of motor and cognitive deficits. Furthermore, selectively ablating mHTT from specific glial cells or correcting for HD-induced changes in their transcriptional profile rescues some HD-related phenotypes, demonstrating the potential of targeting these cells for therapeutic intervention. Here we review emerging research focused on understanding the involvement of different glial cell types in specific aspects of HD pathogenesis. This work is providing new insight into how HD impacts biological functions of glial cells in the healthy brain as well as how HD induced dysfunction in these cells might change the way they integrate into biological circuits.

Embryonic microglia influence developing hypothalamic glial populations

Background

Although historically microglia were thought to be immature in the fetal brain, evidence of purposeful interactions between these immune cells and nearby neural progenitors is becoming established. Here, we examined the influence of embryonic microglia on gliogenesis within the developing tuberal hypothalamus, a region later important for energy balance, reproduction, and thermoregulation.

Methods

We used immunohistochemistry to quantify the location and numbers of glial cells in the embryonic brain (E13.5–E17.5), as well as a pharmacological approach (i.e., PLX5622) to knock down fetal microglia. We also conducted cytokine and chemokine analyses on embryonic brains in the presence or absence of microglia, and a neurosphere assay to test the effects of the altered cytokines on hypothalamic progenitor behaviors.

Results

We identified a subpopulation of activated microglia that congregated adjacent to the third ventricle alongside embryonic Olig2+ neural progenitor cells (NPCs) that are destined to give rise to oligodendrocyte and astrocyte populations. In the absence of microglia, we observed an increase in Olig2+ glial progenitor cells that remained at the ventricle by E17.5 and a concomitant decrease of these Olig2+ cells in the mantle zone, indicative of a delay in migration of these precursor cells. A further examination of maturing oligodendrocytes in the hypothalamic grey and white matter area in the absence of microglia revealed migrating oligodendrocyte progenitor cells (OPCs) within the grey matter at E17.5, a time point when OPCs begin to slow their migration. Finally, quantification of cytokine and chemokine signaling in ex vivo E15.5 hypothalamic cultures +/− microglia revealed decreases in the protein levels of several cytokines in the absence of microglia. We assayed the influence of two downregulated cytokines (CCL2 and CXCL10) on neurosphere-forming capacity and lineage commitment of hypothalamic NPCs in culture and showed an increase in NPC proliferation as well as neuronal and oligodendrocyte differentiation.

Conclusion

These data demonstrate that microglia influence gliogenesis in the developing tuberal hypothalamus.

Novel Treatment Strategies Targeting Myelin and Oligodendrocyte Dysfunction in Schizophrenia

Oligodendrocytes are the glial cells responsible for the formation of the myelin sheath around axons. During neurodevelopment, oligodendrocytes undergo maturation and differentiation, and later remyelination in adulthood. Abnormalities in these processes have been associated with behavioral and cognitive dysfunctions and the development of various mental illnesses like schizophrenia. Several studies have implicated oligodendrocyte dysfunction and myelin abnormalities in the disorder, together with altered expression of myelin-related genes such as Olig2, CNP, and NRG1. However, the molecular mechanisms subjacent of these alterations remain elusive. Schizophrenia is a severe, chronic psychiatric disorder affecting more than 23 million individuals worldwide and its symptoms usually appear at the beginning of adulthood. Currently, the major therapeutic strategy for schizophrenia relies on the use of antipsychotics. Despite their widespread use, the effects of antipsychotics on glial cells, especially oligodendrocytes, remain unclear. Thus, in this review we highlight the current knowledge regarding oligodendrocyte dysfunction in schizophrenia, compiling data from (epi)genetic studies and up-to-date models to investigate the role of oligodendrocytes in the disorder. In addition, we examined potential targets currently investigated for the improvement of schizophrenia symptoms. Research in this area has been investigating potential beneficial compounds, including the D-amino acids D-aspartate and D-serine, that act as NMDA receptor agonists, modulating the glutamatergic signaling; the antioxidant N-acetylcysteine, a precursor in the synthesis of glutathione, protecting against the redox imbalance; as well as lithium, an inhibitor of glycogen synthase kinase 3β (GSK3β) signaling, contributing to oligodendrocyte survival and functioning. In conclusion, there is strong evidence linking oligodendrocyte dysfunction to the development of schizophrenia. Hence, a better understanding of oligodendrocyte differentiation, as well as the effects of antipsychotic medication in these cells, could have potential implications for understanding the development of schizophrenia and finding new targets for drug development.

Astrocytes and Microglia as Major Players of Myelin Production in Normal and Pathological Conditions

Myelination is an essential process that consists of the ensheathment of axons by myelin. In the central nervous system (CNS), myelin is synthesized by oligodendrocytes. The proliferation, migration, and differentiation of oligodendrocyte precursor cells constitute a prerequisite before mature oligodendrocytes extend their processes around the axons and progressively generate a multilamellar lipidic sheath. Although myelination is predominately driven by oligodendrocytes, the other glial cells including astrocytes and microglia, also contribute to this process. The present review is an update of the most recent emerging mechanisms involving astrocyte and microglia in myelin production. The contribution of these cells will be first described during developmental myelination that occurs in the early postnatal period and is critical for the proper development of cognition and behavior. Then, we will report the novel findings regarding the beneficial or deleterious effects of astroglia and microglia, which respectively promote or impair the endogenous capacity of oligodendrocyte progenitor cells (OPCs) to induce spontaneous remyelination after myelin loss. Acute delineation of astrocyte and microglia activities and cross-talk should uncover the way towards novel therapeutic perspectives aimed at recovering proper myelination during development or at breaking down the barriers impeding the regeneration of the damaged myelin that occurs in CNS demyelinating diseases.

Control of Astrocyte Quiescence and Activation in a Synthetic Brain Hydrogel

Bioengineers have designed numerous instructive brain extracellular matrix (ECM) environments with tailored and tunable protein compositions and biomechanical properties in vitro to study astrocyte reactivity during trauma and inflammation. However, a major limitation of both protein-based and synthetic model microenvironments is that astrocytes within fail to retain their characteristic stellate morphology and quiescent state without becoming activated under "normal" culture conditions. Here, a synthetic hydrogel is introduced, which for the first time demonstrates maintenance of astrocyte quiescence and activation on demand. With this synthetic brain hydrogel, the brain-specific integrin-binding and matrix metalloprotease-degradable domains of proteins are shown to control astrocyte star-shaped morphologies, and an ECM condition that maintains astrocyte quiescence with minimal activation can be achieved. In addition, activation can be induced in a dose-dependent manner via both defined cytokine cocktails and low molecular weight hyaluronic acid. This synthetic brain hydrogel is envisioned as a new tool to study the physiological role of astrocytes in health and disease.

Extrinsic Factors Driving Oligodendrocyte Lineage Cell Progression in CNS Development and Injury

Oligodendrocytes (OLs) generate myelin membranes for the rapid propagation of electrical signals along axons in the central nervous system (CNS) and provide metabolites to support axonal integrity and function. Differentiation of OLs from oligodendrocyte progenitor cells (OPCs) is orchestrated by a multitude of intrinsic and extrinsic factors in the CNS. Disruption of this process, or OL loss in the developing or adult brain, as observed in various neurological conditions including hypoxia/ischemia, stroke, and demyelination, results in axonal dystrophy, neuronal dysfunction, and severe neurological impairments. While much is known regarding the intrinsic regulatory signals required for OL lineage cell progression in development, studies from pathological conditions highlight the importance of the CNS environment and external signals in regulating OL genesis and maturation. Here, we review the recent findings in OL biology in the context of the CNS physiological and pathological conditions, focusing on extrinsic factors that facilitate OL development and regeneration.

Glial Factors Regulating White Matter Development and Pathologies of the Cerebellum

The cerebellum is a brain region that undergoes extremely dynamic growth during perinatal and postnatal development which is regulated by the proper interaction between glial cells and neurons with a complex concert of growth factors, chemokines, cytokines, neurotransmitters and transcriptions factors. The relevance of cerebellar functions for not only motor performance but also for cognition, emotion, memory and attention is increasingly being recognized and acknowledged. Since perturbed circuitry of cerebro-cerebellar trajectories can play a role in many central nervous system pathologies and thereby contribute to neurological symptoms in distinct neurodevelopmental and neurodegenerative diseases, is it the aim with this mini-review to highlight the pathways of glia–glia interplay being involved. The designs of future treatment strategies may hence be targeted to molecular pathways also playing a role in development and disease of the cerebellum.

Neuroimmunological effects of early life experiences

Exposure to adverse experiences during development increases the risk of psychiatric illness later in life. Growing evidence suggests a role for the neuroimmune system in this relationship. There is now substantial evidence that the immune system is critical for normal brain development and behaviour, and responds to environmental perturbations experienced early in life. Severe or chronic stress results in dysregulated neuroimmune function, concomitant with abnormal brain morphology and function. Positive experiences including environmental enrichment and exercise exert the opposite effect, promoting normal brain and immune function even in the face of early life stress. The neuroimmune system may therefore provide a viable target for prevention and treatment of psychiatric illness. This review will briefly summarise the neuroimmune system in brain development and function, and review the effects of stress and positive environmental experiences during development on neuroimmune function. There are also significant sex differences in how the neuroimmune system responds to environmental experiences early in life, which we will briefly review.