ASTROCYTES: EMERGING STARS IN LEUKODYSTROPHY PATHOGENESIS

Hematopoietic stem cell transplantation in leukodystrophies

More than 50 leukodystrophies have been described. This group of inherited disorders affects myelin development and/or maintenance and can manifest from birth to adulthood. Neuroinflammation is a hallmark of some leukodystrophies, explaining in part the therapeutic benefit of hematopoietic stem cell transplantation (HSCT). Indeed, in addition to supplying the CNS with myelomonocyte donor cells expressing the deficient protein or enzyme, HSCT allows the restoration of normal microglia function, which may act on neuroinflammation. In this chapter, we explore the rationale, indication, and outcome of HSCT in Cerebral Adrenoleukodystrophy (CALD), Metachromatic Leukodystrophy (MLD), Krabbe Disease (KD), and Adult-onset Leukoencephalopathy with Axonal Spheroids and Pigmented Glia (ALSP), which are among the most frequent leukodystrophies. For these leukodystrophies, HSCT may modify notably the natural history and improve CNS-related deficits, provided that the procedure is performed early into the disease course. In addition, we discuss the recent development of ex vivo gene therapy for CALD and MLD as a promising alternative to allograft.

Astrocytes in human central nervous system diseases: a frontier for new therapies

Astroglia are a broad class of neural parenchymal cells primarily dedicated to homoeostasis and defence of the central nervous system (CNS). Astroglia contribute to the pathophysiology of all neurological and neuropsychiatric disorders in ways that can be either beneficial or detrimental to disorder outcome. Pathophysiological changes in astroglia can be primary or secondary and can result in gain or loss of functions. Astroglia respond to external, non-cell autonomous signals associated with any form of CNS pathology by undergoing complex and variable changes in their structure, molecular expression, and function. In addition, internally driven, cell autonomous changes of astroglial innate properties can lead to CNS pathologies. Astroglial pathophysiology is complex, with different pathophysiological cell states and cell phenotypes that are context-specific and vary with disorder, disorder-stage, comorbidities, age, and sex. Here, we classify astroglial pathophysiology into (i) reactive astrogliosis, (ii) astroglial atrophy with loss of function, (iii) astroglial degeneration and death, and (iv) astrocytopathies characterised by aberrant forms that drive disease. We review astroglial pathophysiology across the spectrum of human CNS diseases and disorders, including neurotrauma, stroke, neuroinfection, autoimmune attack and epilepsy, as well as neurodevelopmental, neurodegenerative, metabolic and neuropsychiatric disorders. Characterising cellular and molecular mechanisms of astroglial pathophysiology represents a new frontier to identify novel therapeutic strategies.

Astrocytes in Parkinson's Disease: From Role to Possible Intervention

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons. While neuronal dysfunction is central to PD, astrocytes also play important roles, both positive and negative, and such roles have not yet been fully explored. This literature review serves to highlight these roles and how the properties of astrocytes can be used to increase neuron survivability. Astrocytes normally have protective functions, such as releasing neurotrophic factors, metabolizing glutamate, transferring healthy mitochondria to neurons, or maintaining the blood-brain barrier. However, in PD, astrocytes can become dysfunctional and contribute to neurotoxicity, e.g., via impaired glutamate metabolism or the release of inflammatory cytokines. Therefore, astrocytes represent a double-edged sword. Restoring healthy astrocyte function and increasing the beneficial effects of astrocytes represents a promising therapeutic approach. Strategies such as promoting neurotrophin release, preventing harmful astrocyte reactivity, or utilizing regional astrocyte diversity may help restore neuroprotection.

Astrocytic response mediated by the CLU risk allele inhibits OPC proliferation and myelination in a human iPSC model

The C allele of rs11136000 variant in the clusterin (CLU) gene represents the third strongest known genetic risk factor for late-onset Alzheimer's disease. However, whether this single-nucleotide polymorphism (SNP) is functional and what the underlying mechanisms are remain unclear. In this study, the CLU rs11136000 SNP is identified as a functional variant by a small-scale CRISPR-Cas9 screen. Astrocytes derived from isogenic induced pluripotent stem cells (iPSCs) carrying the "C" or "T" allele of the CLU rs11136000 SNP exhibit different CLU expression levels. TAR DNA-binding protein-43 (TDP-43) preferentially binds to the "C" allele to promote CLU expression and exacerbate inflammation. The interferon response and CXCL10 expression are elevated in cytokine-treated C/C astrocytes, leading to inhibition of oligodendrocyte progenitor cell (OPC) proliferation and myelination. Accordingly, elevated CLU and CXCL10 but reduced myelin basic protein (MBP) expression are detected in human brains of C/C carriers. Our study uncovers a mechanism underlying reduced white matter integrity observed in the CLU rs11136000 risk "C" allele carriers.

The Effect of Interleukin-1 Antagonists on Brain Volume and Cognitive Function in Two Patients With Megalencephalic Leukoencephalopathy With Subcortical Cysts

BACKGROUND

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare leukodystrophy characterized by early-onset macrocephaly and progressive white matter vacuolation. The MLC1 protein plays a role in astrocyte activation during neuroinflammation and regulates volume decrease following astrocyte osmotic swelling. Loss of MLC1 function activates interleukin (IL)-1β-induced inflammatory signals. Theoretically, IL-1 antagonists (such as anakinra and canakinumab) can slow the progression of MLC. Herein, we present two boys from different families who had MLC due to biallelic MLC1 gene mutations and were treated with the anti-IL-1 drug anakinra.

METHODS

Two boys from different families presented with megalencephaly and psychomotor retardation. Brain magnetic resonance imaging findings in both patients were compatible with the diagnosis of MLC. The diagnosis of MLC was confirmed via Sanger analysis of the MLC1 gene. Anakinra was administered to both patients. Volumetric brain studies and psychometric evaluations were performed before and after anakinra treatment.

RESULTS

After anakinra therapy, brain volume in both patients decreased significantly and cognitive functions and social interactions improved. No adverse effects were observed during anakinra therapy.

CONCLUSIONS

Anakinra or other IL-1 antagonists can be used to suppress disease activity in patients with MLC; however, the present findings need to be confirmed via additional research.

Human iPSC-derived astrocytes generated from donors with globoid cell leukodystrophy display phenotypes associated with disease

Globoid cell leukodystrophy (Krabbe disease) is a fatal neurodegenerative, demyelinating disease caused by dysfunctional activity of galactosylceramidase (GALC), leading to the accumulation of glycosphingolipids including psychosine. While oligodendrocytes have been extensively studied due to their high levels of GALC, the contribution of astrocytes to disease pathogenesis remains to be fully elucidated. In the current study, we generated induced pluripotent stem cells (iPSCs) from two donors with infantile onset Krabbe disease and differentiated them into cultures of astrocytes. Krabbe astrocytes recapitulated many key findings observed in humans and rodent models of the disease, including the accumulation of psychosine and elevated expression of the pro-inflammatory cytokine IL-6. Unexpectedly, Krabbe astrocytes had higher levels of glucosylceramide and ceramide, and displayed compensatory changes in genes encoding glycosphingolipid biosynthetic enzymes, suggesting a shunting away from the galactosylceramide and psychosine pathway. In co-culture, Krabbe astrocytes negatively impacted the survival of iPSC-derived human neurons while enhancing survival of iPSC-derived human microglia. Substrate reduction approaches targeting either glucosylceramide synthase or serine palmitoyltransferase to reduce the sphingolipids elevated in Krabbe astrocytes failed to rescue their detrimental impact on neuron survival. Our results suggest that astrocytes may contribute to the progression of Krabbe disease and warrant further exploration into their role as therapeutic targets.

Dominant-acting CSF1R variants cause microglial depletion and altered astrocytic phenotype in zebrafish and adult-onset leukodystrophy

Tissue-resident macrophages of the brain, including microglia, are implicated in the pathogenesis of various CNS disorders and are possible therapeutic targets by their chemical depletion or replenishment by hematopoietic stem cell therapy. Nevertheless, a comprehensive understanding of microglial function and the consequences of microglial depletion in the human brain is lacking. In human disease, heterozygous variants in CSF1R, encoding the Colony-stimulating factor 1 receptor, can lead to adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) possibly caused by microglial depletion. Here, we investigate the effects of ALSP-causing CSF1R variants on microglia and explore the consequences of microglial depletion in the brain. In intermediate- and late-stage ALSP post-mortem brain, we establish that there is an overall loss of homeostatic microglia and that this is predominantly seen in the white matter. By introducing ALSP-causing missense variants into the zebrafish genomic csf1ra locus, we show that these variants act dominant negatively on the number of microglia in vertebrate brain development. Transcriptomics and proteomics on relatively spared ALSP brain tissue validated a downregulation of microglia-associated genes and revealed elevated astrocytic proteins, possibly suggesting involvement of astrocytes in early pathogenesis. Indeed, neuropathological analysis and in vivo imaging of csf1r zebrafish models showed an astrocytic phenotype associated with enhanced, possibly compensatory, endocytosis. Together, our findings indicate that microglial depletion in zebrafish and human disease, likely as a consequence of dominant-acting pathogenic CSF1R variants, correlates with altered astrocytes. These findings underscore the unique opportunity CSF1R variants provide to gain insight into the roles of microglia in the human brain, and the need to further investigate how microglia, astrocytes, and their interactions contribute to white matter homeostasis.

Human iPSC-Derived Astrocytes: A Powerful Tool to Study Primary Astrocyte Dysfunction in the Pathogenesis of Rare Leukodystrophies

Astrocytes are very versatile cells, endowed with multitasking capacities to ensure brain homeostasis maintenance from brain development to adult life. It has become increasingly evident that astrocytes play a central role in many central nervous system pathologies, not only as regulators of defensive responses against brain insults but also as primary culprits of the disease onset and progression. This is particularly evident in some rare leukodystrophies (LDs) where white matter/myelin deterioration is due to primary astrocyte dysfunctions. Understanding the molecular defects causing these LDs may help clarify astrocyte contribution to myelin formation/maintenance and favor the identification of possible therapeutic targets for LDs and other CNS demyelinating diseases. To date, the pathogenic mechanisms of these LDs are poorly known due to the rarity of the pathological tissue and the failure of the animal models to fully recapitulate the human diseases. Thus, the development of human induced pluripotent stem cells (hiPSC) from patient fibroblasts and their differentiation into astrocytes is a promising approach to overcome these issues. In this review, we discuss the primary role of astrocytes in LD pathogenesis, the experimental models currently available and the advantages, future evolutions, perspectives, and limitations of hiPSC to study pathologies implying astrocyte dysfunctions.

A homozygous missense variant in the MLC1 gene underlies megalencephalic leukoencephalopathy with subcortical cysts in large kindred: Heterozygous carriers show seizure and mild motor function deterioration

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare type of leukodystrophy characterized by epileptic seizures, macrocephaly, and vacuolization of myelin and astrocyte. The magnetic resonance imaging of the brain of MLC patients shows diffuse white-matter anomalies and the occurrence of subcortical cysts. MLC features have been observed in individuals having mutations in the MLC1 or HEPACAM genes. In this study, we recruited a six generation large kindred with five affected individuals manifesting clinical features of epileptic seizures, macrocephaly, ataxia, and spasticity. In order to identify the underlying genetic cause of the clinical features, we performed whole-genome genotyping using Illumina microarray followed by detection of loss of heterozygosity (LOHs) regions. One affected individual was exome sequenced as well. Homozygosity mapping detected several LOH regions due to extensive consanguinity. An unbiased and hypothesis-free exome data analysis identified a homozygous missense variant (NM_015166.3:c.278C>T) in the exon 4 of the MLC1 gene. The variant is present in the LOH region on chromosome 22q (50 Mb) and segregates perfectly with the disorder within the family in an autosomal recessive manner. The variant is present in a highly conserved first cytoplasmic domain of the MLC1 protein (NM_015166.3:p.(Ser93Leu)). Interestingly, heterozygous individuals show seizure and mild motor function deterioration. We propose that the heterozygous variant in MLC1 might disrupt the functional interaction of MLC1 with GlialCAM resulting in mild clinical features in carriers of the variant.

The multicellular interplay of microglia in health and disease: lessons from leukodystrophy

Microglia are highly dynamic cells crucial for developing and maintaining lifelong brain function and health through their many interactions with essentially all cellular components of the central nervous system. The frequent connection of microglia to leukodystrophies, genetic disorders of the white matter, has highlighted their involvement in the maintenance of white matter integrity. However, the mechanisms that underlie their putative roles in these processes remain largely uncharacterized. Microglia have also been gaining attention as possible therapeutic targets for many neurological conditions, increasing the demand to understand their broad spectrum of functions and the impact of their dysregulation. In this Review, we compare the pathological features of two groups of genetic leukodystrophies: those in which microglial dysfunction holds a central role, termed 'microgliopathies', and those in which lysosomal or peroxisomal defects are considered to be the primary driver. The latter are suspected to have notable microglia involvement, as some affected individuals benefit from microglia-replenishing therapy. Based on overlapping pathology, we discuss multiple ways through which aberrant microglia could lead to white matter defects and brain dysfunction. We propose that the study of leukodystrophies, and their extensively multicellular pathology, will benefit from complementing analyses of human patient material with the examination of cellular dynamics in vivo using animal models, such as zebrafish. Together, this will yield important insight into the cell biological mechanisms of microglial impact in the central nervous system, particularly in the development and maintenance of myelin, that will facilitate the development of new, and refinement of existing, therapeutic options for a range of brain diseases.

Emerging Concepts in Vector Development for Glial Gene Therapy: Implications for Leukodystrophies

Central Nervous System (CNS) homeostasis and function rely on intercellular synchronization of metabolic pathways. Developmental and neurochemical imbalances arising from mutations are frequently associated with devastating and often intractable neurological dysfunction. In the absence of pharmacological treatment options, but with knowledge of the genetic cause underlying the pathophysiology, gene therapy holds promise for disease control. Consideration of leukodystrophies provide a case in point; we review cell type – specific expression pattern of the disease – causing genes and reflect on genetic and cellular treatment approaches including ex vivo hematopoietic stem cell gene therapies and in vivo approaches using adeno-associated virus (AAV) vectors. We link recent advances in vectorology to glial targeting directed towards gene therapies for specific leukodystrophies and related developmental or neurometabolic disorders affecting the CNS white matter and frame strategies for therapy development in future.

Neuroglia in Psychiatric Disorders

In the twentieth century, neuropsychiatric disorders have been perceived solely from a neurone-centric point of view, which considers neurones as the key cellular elements of pathological processes. This dogma has been challenged thanks to the better comprehension of the brain functioning, which, even if far from being complete, has revealed the complexity of interactions that exist between neurones and neuroglia. Glial cells represent a highly heterogeneous population of cells of neural (astroglia and oligodendroglia) and non-neural (microglia) origin populating the central nervous system. The variety of glia reflects the innumerable functions that glial cells perform to support functions of the nervous system. Aberrant execution of glial functions contributes to the development of neuropsychiatric pathologies. Arguably, all types of glial cells are implicated in the neuropathology; however, astrocytes have received particular attention in recent years because of their pleiotropic functions that make them decisive in maintaining cerebral homeostasis. This chapter describes the multiple roles of astrocytes in the healthy central nervous system and discusses the diversity of astroglial responses in neuropsychiatric disorders suggesting that targeting astrocytes may represent an effective therapeutic strategy.

Glial cells in the driver seat of leukodystrophy pathogenesis

Glia cells are often viewed as support cells in the central nervous system, but recent discoveries highlight their importance in physiological functions and in neurological diseases. Central to this are leukodystrophies, a group of progressive, neurogenetic disease affecting white matter pathology. In this review, we take a closer look at multiple leukodystrophies, classified based on the primary glial cell type that is affected. While white matter diseases involve oligodendrocyte and myelin loss, we discuss how astrocytes and microglia are affected and impinge on oligodendrocyte, myelin and axonal pathology. We provide an overview of the leukodystrophies covering their hallmark features, clinical phenotypes, diverse molecular pathways, and potential therapeutics for clinical trials. Glial cells are gaining momentum as cellular therapeutic targets for treatment of demyelinating diseases such as leukodystrophies, currently with no treatment options. Here, we bring the much needed attention to role of glia in leukodystrophies, an integral step towards furthering disease comprehension, understanding mechanisms and developing future therapeutics.

Single-Cell Analysis for Glycogen Localization and Metabolism in Cultured Astrocytes

Cerebral glycogen is principally localized in astrocytes rather than in neurons. Glycogen metabolism has been implicated in higher brain functions, including learning and memory, yet the distribution patterns of glycogen in different types of astrocytes have not been fully described. Here, we applied a method based on the incorporation of 2-NBDG, a D-glucose fluorescent derivative that can trace glycogen, to investigate glycogen's distribution in the brain. We identified two types of astrocytes, namely, 2-NBDGI (glycogen-deficient) and 2-NBDGII (glycogen-rich) cells. Whole-cell patch-clamp and fluorescence-activated cell sorting (FACS) were used to separate 2-NBDGII astrocytes from 2-NBDGI astrocytes. The expression levels of glycogen metabolic enzymes were analyzed in 2-NBDGI and 2-NBDGII astrocytes. We found unique glycogen metabolic patterns between 2-NBDGI and 2-NBDGII astrocytes. We also observed that 2-NBDGII astrocytes were mainly identified as fibrous astrocytes but not protoplasmic astrocytes. Our data reveal cell type-dependent glycogen distribution and metabolism patterns, suggesting diverse functions of these different astrocytes.

Astrocytogenesis: where, when, and how

Astrocytes are the most abundant cell type in the central nervous system and have diverse functions in blood–brain barrier maintenance, neural circuitry formation and function, and metabolic regulation. To better understand the diverse roles of astrocytes, we will summarize what is known about astrocyte development and the challenges limiting our understanding of this process. We will also discuss new approaches and technologies advancing the field.

Astrocyte and Oligodendrocyte Cross-Talk in the Central Nervous System

Over the last decade knowledge of the role of astrocytes in central nervous system (CNS) neuroinflammatory diseases has changed dramatically. Rather than playing a merely passive role in response to damage it is clear that astrocytes actively maintain CNS homeostasis by influencing pH, ion and water balance, the plasticity of neurotransmitters and synapses, cerebral blood flow, and are important immune cells. During disease astrocytes become reactive and hypertrophic, a response that was long considered to be pathogenic. However, recent studies reveal that astrocytes also have a strong tissue regenerative role. Whilst most astrocyte research focuses on modulating neuronal function and synaptic transmission little is known about the cross-talk between astrocytes and oligodendrocytes, the myelinating cells of the CNS. This communication occurs via direct cell-cell contact as well as via secreted cytokines, chemokines, exosomes, and signalling molecules. Additionally, this cross-talk is important for glial development, triggering disease onset and progression, as well as stimulating regeneration and repair. Its critical role in homeostasis is most evident when this communication fails. Here, we review emerging evidence of astrocyte-oligodendrocyte communication in health and disease. Understanding the pathways involved in this cross-talk will reveal important insights into the pathogenesis and treatment of CNS diseases.

Glial cells in schizophrenia: a unified hypothesis

The cellular neurobiology of schizophrenia remains poorly understood. We discuss neuroimaging studies, pathological findings, and experimental work supporting the idea that glial cells might contribute to the development of schizophrenia. Experimental studies suggest that abnormalities in the differentiation competence of glial progenitor cells lead to failure in the morphological and functional maturation of oligodendrocytes and astrocytes. We propose that immune activation of microglial cells during development, superimposed upon genetic risk factors, could contribute to defective differentiation competence of glial progenitor cells. The resulting hypomyelination and disrupted white matter integrity might contribute to transmission desynchronisation and dysconnectivity, whereas the failure of astrocytic differentiation results in abnormal glial coverage and support of synapses. The delayed and deficient maturation of astrocytes might, in parallel, lead to disruption of glutamatergic, potassium, and neuromodulatory homoeostasis, resulting in dysregulated synaptic transmission. By highlighting a role for glial cells in schizophrenia, these studies potentially point to new mechanisms for disease modification.

Sanhua Decoction, a Classic Herbal Prescription, Exerts Neuroprotection Through Regulating Phosphorylated Tau Level and Promoting Adult Endogenous Neurogenesis After Cerebral Ischemia/Reperfusion Injury

Background: Ischemia stroke is the leading cause of death and long-term disability. Sanhua Decoction (SHD), a classic Chinese herbal prescription, has been used for ischemic stroke for about thousands of years. Here, we aim to investigate the neuroprotective effects of SHD on cerebral ischemia/reperfusion (CIR) injury rat models. Methods: The male Sprague-Dawley rats (body weight, 250-280 g; age, 7-8 weeks) were randomly divided into sham group, CIR group, and SHD group and were further divided into subgroups according to different time points at 6 h, 1, 3, 7, 14, 21, and 28 d, respectively. The SHD group received intragastric administration of SHD at 10 g kg-1 d-1. The focal CIR models were induced by middle cerebral artery occlusion according to Longa's method, while sham group had the same operation without suture insertion. Neurological deficit score (NDS) was evaluated using the Longa's scale. BrdU, doublecortin (DCX), and glial fibrillary acidic protein (GFAP) were used to label proliferation, migration, and differentiation of nerve cells before being observed by immunofluorescence. The expression of reelin, total tau (t-tau), and phosphorylated tau (p-tau) were evaluated by western blot and RT-qPCR. Results: SHD can significantly improve NDS at 1, 3, 7, and 14 d (p < 0.05), increase the number of BrdU positive and BrdU/DCX positive cells in subventricular zone at 3, 7, and 14 d (p < 0.05), upregulate BrdU/GFAP positive cells in the ischemic penumbra at 28 d after CIR (p < 0.05), and reduce p-tau level at 1, 3, 7, and 14 d (p < 0.05). There was no significant difference on reelin and t-tau level between three groups at each time points after CIR. Conclusions: SHD exerts neuroprotection probably by regulating p-tau level and promoting the proliferation, migration, and differentiation of endogenous neural stem cells, accompanying with neurobehavioral recovery.

The Potential Role of Astrocytes in Parkinson's Disease (PD)

Astrocytes are multi-functional cells, now recognized as critical participants in many brain functions. They play a critical physiological role in the clearance of neurotransmitters, such as glutamate and gamma-aminobutyric acid (GABA), and in the regulation of K+ from the space of synaptic clefts. Astrocytes also express the excitatory amino acid transporters (EAATs) and aquaporin-4 (AQP4) water channel, which are involved in both physiological functions and neurodegenerative diseases (ND). Some of the ND are the Alzheimer's (AD), Huntington's (HD), Parkinson's diseases (PD), Cerebral edema, amyotrophic lateral sclerosis (ALS), and epilepsy pathological conditions in specific regions of the CNS. Parkinson's disease is the second most common age-related neurodegenerative disorder, characterized by degeneration of dopaminergic neurons of the substantia nigra pars compacta (SNpc). These project to the striatum, forming an important pathway within the basal ganglia. Mostly, PD has no clear etiology, and the mechanism of dopaminergic (DA) neuron loss is not well illustrated. The results of various studies suggest that astrocytes are involved in the pathophysiology of PD. Evidence has shown that the down-regulation of EAAT-2/GLT-1 and AQP4 expression is associated with PD pathogenesis. However, controversial results were reported in different experimental studies about the expression and function of EAAT-2/GLT-1 and AQP4, as well as their colocalization in different brain regions, and their involvement in PD development. Therefore, under neurological disorders, Parkinson's disease is related to the genetic and phenotypic change of astrocytes' biology. In this review, the authors summarized recent their research findings, which revealed the involvement of EAAT-2/GLT-1 and AQP4 expression, the physical interaction between EAAT-2/GLT-1 and AQP4 in astrocyte function, and their potential role in the development of PD in SNpc and Subthalamic nucleus (STN) of the basal ganglia nuclei.

Plasma membrane localization of MLC1 regulates cellular morphology and motility

Background

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare form of infantile-onset leukodystrophy. The disorder is caused primarily by mutations of MLC1 that leads to a series of phenotypic outcomes including vacuolation of myelin and astrocytes, subcortical cysts, brain edema, and macrocephaly. Recent studies have indicated that functional interactions among MLC1, GlialCAM, and ClC-2 channels play key roles in the regulation of neuronal, glial and vascular homeostasis. However, the physiological role of MLC1 in cellular homeostatic communication remains poorly understood. In the present study, we investigated the cellular function of MLC1 and its effects on cell–cell interactions.

Methods

MLC1-dependent cellular morphology and motility were analyzed by using confocal and live cell imaging technique. Biochemical approaches such as immunoblotting, co-immunoprecipitation, and surface biotinylation were conducted to support data.

Results

We found that the altered MLC1 expression and localization led to a great alteration in cellular morphology and motility through actin remodeling. MLC1 overexpression induced filopodia formation and suppressed motility. And, MLC1 proteins expressed in patient-derived MLC1 mutants resulted in trapping in the ER although no changes in morphology or motility were observed. Interestingly knockdown of Mlc1 induced Arp3-Cortactin interaction, lamellipodia formation, and increased the membrane ruffling of the astrocytes. These data indicate that subcellular localization of expressed MLC1 at the plasma membrane is critical for changes in actin dynamics through ARP2/3 complex. Thus, our results suggest that misallocation of pathogenic mutant MLC1 may disturbs the stable cell-cell communication and the homeostatic regulation of astrocytes in patients with MLC.

Astrocytes in myelination and remyelination

Astrocytes are known to play critical roles in central nervous system development, homeostasis, and response to injury. In addition to well-defined functions in synaptic signalling and blood-brain barrier control, astrocytes are now emerging as important contributors to white matter health. Here, we review the roles of astrocytes in myelin formation and regeneration (remyelination), focusing on both direct interactions with oligodendrocyte lineage cells, and indirect influences via crosstalk with central nervous system resident macrophages, microglia.

GFAP Mutations in Astrocytes Impair Oligodendrocyte Progenitor Proliferation and Myelination in an hiPSC Model of Alexander Disease

Alexander disease (AxD) is a leukodystrophy that primarily affects astrocytes and is caused by mutations in the astrocytic filament gene GFAP. While astrocytes are thought to have important roles in controlling myelination, AxD animal models do not recapitulate critical myelination phenotypes and it is therefore not clear how AxD astrocytes contribute to leukodystrophy. Here, we show that AxD patient iPSC-derived astrocytes recapitulate key features of AxD pathology such as GFAP aggregation. Moreover, AxD astrocytes inhibit proliferation of human iPSC-derived oligodendrocyte progenitor cells (OPCs) in co-culture and reduce their myelination potential. CRISPR/Cas9-based correction of GFAP mutations reversed these phenotypes. Transcriptomic analyses of AxD astrocytes and postmortem brains identified CHI3L1 as a key mediator of AxD astrocyte-induced inhibition of OPC activity. Thus, this iPSC-based model of AxD not only recapitulates patient phenotypes not observed in animal models, but also reveals mechanisms underlying disease pathology and provides a platform for assessing therapeutic interventions.

Oleic acid is a potent inducer for lipid droplet accumulation through its esterification to glycerol by diacylglycerol acyltransferase in primary cortical astrocytes

Astrocytes exhibit an important role in neural lipid metabolism for the regulation of energy balance to supply fatty acids (FAs) and ketone bodies to other neural cells. Lipid droplets (LDs) consisting of neutral- and phospho-lipids increase in the brains of patients with neurodegenerative diseases, such as Alzheimer's disease and multiple sclerosis. However, the role of LDs and its lipid source remains largely unexplored. Here, we found that oleic acid (OA) was a potent inducer of astrocytic LD accumulation among various FAs. Lipidomic analysis using liquid chromatography equipped with tandem mass spectrometry revealed that the cellular triacylglycerol and phospholipid compositions in astrocytes during LD accumulation reflected the condition of extracellular FAs. Furthermore, the inhibition of diacylglycerol acyltransferase blocked OA-induced LD accumulation and caused lipotoxicity-induced cell death in astrocytes. The present study demonstrated that the formation of LDs, caused due to the increased extracellular OA, facilitated survival against lipotoxic condition.

Astrocyte Subtype Vulnerability in Stem Cell Models of Vanishing White Matter

Objective

Astrocytes have gained attention as important players in neurological disease. In line with their heterogeneous character, defects in specific astrocyte subtypes have been identified. Leukodystrophy vanishing white matter (VWM) shows selective vulnerability in white matter astrocytes, but the underlying mechanisms remain unclear. Induced pluripotent stem cell technology is being extensively explored in studies of pathophysiology and regenerative medicine. However, models for distinct astrocyte subtypes for VWM are lacking, thereby hampering identification of disease‐specific pathways.

Methods

Here, we characterize human and mouse pluripotent stem cell–derived gray and white matter astrocyte subtypes to generate an in vitro VWM model. We examined morphology and functionality, and used coculture methods, high‐content microscopy, and RNA sequencing to study VWM cultures.

Results

We found intrinsic vulnerability in specific astrocyte subpopulations in VWM. When comparing VWM and control cultures, white matter–like astrocytes inhibited oligodendrocyte maturation, and showed affected pathways in both human and mouse cultures, involving the immune system and extracellular matrix. Interestingly, human white matter–like astrocytes presented additional, human‐specific disease mechanisms, such as neuronal and mitochondrial functioning.

Interpretation

Astrocyte subtype cultures revealed disease‐specific pathways in VWM. Cross‐validation of human‐ and mouse‐derived protocols identified human‐specific disease aspects. This study provides new insights into VWM disease mechanisms, which helps the development of in vivo regenerative applications, and we further present strategies to study astrocyte subtype vulnerability in neurological disease. ANN NEUROL 2019;86:780–792

Astroglial Calcium Signaling in Aging and Alzheimer's Disease

Astrocytes are the homeostatic and protective cells of the central nervous system (CNS). In neurological diseases, astrocytes undergo complex changes, which are subclassified into (1) reactive astrogliosis, an evolutionary conserved defensive rearrangement of cellular phenotype aimed at neuroprotection; (2) pathological remodeling, when astrocytes acquire new features driving pathology; and (3) astrodegeneration, which is manifested by astroglial atrophy and loss of homeostatic functions. In aging brains as well as in the brains affected by Alzheimer's disease (AD), astrocytes acquire both atrophic and reactive phenotypes in a region- and disease-stage-dependent manner. Prevalence of atrophy overreactivity, observed in certain brain regions and in terminal stages of the disease, arguably facilitates the development of neurological deficits. Astrocytes exhibit ionic excitability mediated by changes in intracellular concentration of ions, most importantly of Ca²⁺ and Na⁺, with intracellular ion dynamics triggered by the activity of neural networks. AD astrocytes associated with senile plaques demonstrate Ca²⁺ hyperactivity in the form of aberrant Ca²⁺ oscillations and pathological long-range Ca²⁺ waves. Astroglial Ca²⁺ signaling originating from Ca²⁺ release from the endoplasmic reticulum is a key factor in initiating astrogliotic response; deficient Ca²⁺ signaling toolkits observed in entorhinal and prefrontal cortices of AD model animals may account for vulnerability of these regions to the pathology.

The Concept of Neuroglia

Neuroglia represent a diverse population of non-neuronal cells in the nervous systems, be that peripheral, central, enteric or autonomic nervous system. Arguably, these cells represent about half of the volume of the human brain. This volumetric ratio, and by extension glia to neurone ratio, not only widely differ depending on the size of the animal species brain and its positioning on the phylogenetic tree, but also vary between the regions of an individual brain. Neuroglia derived from a dual origin (ectoderm and mesodermal) and in an assorted morphology, yet their functional traits can be mainly classified into being keepers of homeostasis (water, ions, neurotransmitters, metabolites, fuels, etc.) and defenders (e.g., against microbial organisms, etc.) of the nervous system. As these capabilities go awry, neuroglia ultimately define their fundamental role in most, if not, all neuropathologies. This concept presented in this chapter serves as a general introduction into the world of neuroglia and subsequent topics covered by this book.

General Pathophysiology of Astroglia

Astroglial cells are involved in most if not in all pathologies of the brain. These cells can change the morpho-functional properties in response to pathology or innate changes of these cells can lead to pathologies. Overall pathological changes in astroglia are complex and diverse and often vary with different disease stages. We classify astrogliopathologies into reactive astrogliosis, astrodegeneration with astroglial atrophy and loss of function, and pathological remodelling of astrocytes. Such changes can occur in neurological, neurodevelopmental, metabolic and psychiatric disorders as well as in infection and toxic insults. Mutation in astrocyte-specific genes leads to specific pathologies, such as Alexander disease, which is a leukodystrophy. We discuss changes in astroglia in the pathological context and identify some molecular entities underlying pathology. These entities within astroglia may repent targets for novel therapeutic intervention in the management of brain pathologies.

Regulation of myelin structure and conduction velocity by perinodal astrocytes

The speed of impulse transmission is critical for optimal neural circuit function, but it is unclear how the appropriate conduction velocity is established in individual axons. The velocity of impulse transmission is influenced by the thickness of the myelin sheath and the morphology of electrogenic nodes of Ranvier along axons. Here we show that myelin thickness and nodal gap length are reversibly altered by astrocytes, glial cells that contact nodes of Ranvier. Thrombin-dependent proteolysis of a cell adhesion molecule that attaches myelin to the axon (neurofascin 155) is inhibited by vesicular release of thrombin protease inhibitors from perinodal astrocytes. Transgenic mice expressing a dominant-negative fragment of VAMP2 in astrocytes, to reduce exocytosis by 50%, exhibited detachment of adjacent paranodal loops of myelin from the axon, increased nodal gap length, and thinning of the myelin sheath in the optic nerve. These morphological changes alter the passive cable properties of axons to reduce conduction velocity and spike-time arrival in the CNS in parallel with a decrease in visual acuity. All effects were reversed by the thrombin inhibitor Fondaparinux. Similar results were obtained by viral transfection of tetanus toxin into astrocytes of rat corpus callosum. Previously, it was unknown how the myelin sheath could be thinned and the functions of perinodal astrocytes were not well understood. These findings describe a form of nervous system plasticity in which myelin structure and conduction velocity are adjusted by astrocytes. The thrombin-dependent cleavage of neurofascin 155 may also have relevance to myelin disruption and repair.

Astrogliopathology in the infectious insults of the brain

Astroglia, a heterogeneous type of neuroglia, play key homeostatic functions in the central nervous system (CNS) and represent an important defence system. Impaired homeostatic capacity of astrocytes manifests in diseases and this is mirrored in various astrocyte-based pathological features including reactive astrogliosis, astrodegeneration with astroglial atrophy and pathological remodelling of astrocytes. All of these manifestations are most prominently associated with infectious insults, mediated by bacteria, protozoa and viruses. Here we focus onto neurotropic viruses such as tick-borne encephalitis (TBEV) and Zika virus (ZIKV), both belonging to Flaviviridae and both causing severe neurological impairments. We argue that astrocytes provide a route through which neurotropic infectious agents attack the CNS, since they are anatomically associated with the blood-brain barrier and exhibit aerobic glycolysis, a metabolic specialisation of highly morphologically dynamic cells, which may provide a suitable metabolic milieu for proliferation of infectious agents, including viral bodies.

Circadian control of p75 neurotrophin receptor leads to alternate activation of Nrf2 and c-Rel to reset energy metabolism in astrocytes via brain-derived neurotrophic factor

Circadian clock genes regulate energy metabolism partly through neurotrophins in the body. The low affinity neurotrophin receptor p75^NTR is a clock component directly regulated by the transcriptional factor Clock:Bmal1 complex. Brain-derived neurotrophic factor (BDNF) is expressed in the brain and plays a key role in coordinating metabolic interactions between neurons and astrocytes. BDNF transduces signals through TrkB and p75^NTR receptors. This review highlights a novel molecular mechanism by which BDNF via circadian control of p75^NTR leads to daily resetting of glucose and glycogen metabolism in brain astrocytes to accommodate their functional interaction with neurons. Astrocytes store glycogen as an energy reservoir to provide active neurons with the glycolytic metabolite lactate. Astrocytes predominantly express the truncated receptor TrkB.T1 which lacks an intracellular receptor tyrosine kinase domain. TrkB.T1 retains the capacity to regulate cell morphology through regulation of Rho GTPases. In contrast, p75^NTR mediates generation of the bioactive lipid ceramide upon stimulation with BDNF and inhibits PKA activation. As ceramide directly activates PKCζ, we discuss the importance of the TrkB.T1-p75^NTR-ceramide-PKCζ signaling axis in the stimulation of glycogen and lipid synthesis and activation of RhoA. Ceramide-PKCζ-casein kinase 2 signaling activates Nrf2 to support oxidative phosphorylation via upregulation of antioxidant enzymes. In the absence of p75^NTR, TrkB.T1 functionally interacts with adenosine A_2AR and dopamine D1R receptors to enhance cAMP-PKA signaling and activate Rac1 and NF-κB c-Rel, favoring glycogen hydrolysis, gluconeogenesis and aerobic glycolysis. Thus, diurnal changes in p75^NTR levels in astrocytes resets energy metabolism via BDNF to accommodate their metabolic interaction with neurons.

Affected astrocytes in the spinal cord of the leukodystrophy vanishing white matter

Leukodystrophies are often devastating diseases, presented with progressive clinical signs as spasticity, ataxia and cognitive decline, and lack proper treatment options. New therapy strategies for leukodystrophies mostly focus on oligodendrocyte replacement to rescue lack of myelin in the brain, even though disease pathology also often involves other glial cells and the spinal cord. In this study we investigated spinal cord pathology in a mouse model for Vanishing White Matter disease (VWM) and show that astrocytes in the white matter are severely affected. Astrocyte pathology starts postnatally in the sensory tracts, followed by changes in the astrocytic populations in the motor tracts. Studies in post-mortem tissue of two VWM patients, a 13-year-old boy and a 6-year-old girl, confirmed astrocyte abnormalities in the spinal cord. For proper development of new treatment options for VWM and, possibly, other leukodystrophies, future studies should investigate spinal cord involvement.

Stratification of astrocytes in healthy and diseased brain

Astrocytes, a subtype of glial cells, come in variety of forms and functions. However, overarching role of these cell is in the homeostasis of the brain, be that regulation of ions, neurotransmitters, metabolism or neuronal synaptic networks. Loss of homeostasis represents the underlying cause of all brain disorders. Thus, astrocytes are likely involved in most if not all of the brain pathologies. We tabulate astroglial homeostatic functions along with pathological condition that arise from dysfunction of these glial cells. Classification of astrocytes is presented with the emphasis on evolutionary trails, morphological appearance and numerical preponderance. We note that, even though astrocytes from a variety of mammalian species share some common features, human astrocytes appear to be the largest and most complex of all astrocytes studied thus far. It is then an imperative to develop humanized models to study the role of astrocytes in brain pathologies, which is perhaps most abundantly clear in the case of glioblastoma multiforme.

Childhood leukodystrophies: A literature review of updates on new definitions, classification, diagnostic approach and management

Childhood leukodystrophies are a growing category of neurological disorders in pediatric neurology practice. With the help of new advanced genetic studies such as whole exome sequencing (WES) and whole genome sequencing (WGS), the list of childhood heritable white matter disorders has been increased to more than one hundred disorders. During the last three decades, the basic concepts and definitions, classification, diagnostic approach and medical management of these disorders much have changed. Pattern recognition based on brain magnetic resonance imaging (MRI), has played an important role in this process. We reviewed the last Global Leukodystrophy Initiative (GLIA) expert opinions in definition, new classification, diagnostic approach and medical management including emerging treatments for pediatric leukodystrophies.

Neurometabolic and neurodegenerative diseases in children

Neurometabolic and neurodegenerative diseases in children (NDDC) differ from those in adults in that most of the former are autosomal-recessively inherited - few have X-linked inheritance - while the latter are often sporadic or autosomal-dominantly inherited. NDDC may be catabolic and/or anabolic conditions, some of which combine maldevelopmental and degenerative features, for instance, peroxisomal biogenesis disorders or congenital disorders of glycosylation. NDDC are often multiorgan disorders, such as lysosomal, peroxisomal, and polyglucosan disorders. This multiorgan involvement may be marked by extracerebral formation of disease-specific neuropathologic findings, especially in lysosomal diseases allowing diagnostic biopsies in easily accessible tissues, e.g., blood lymphocytes, skin, skeletal muscle, and rectum to be investigated by electron microscopy. NDDC comprise nonvacuolar and vacuolar lysosomal, peroxisomal, polyglucosan, amino and organic acid, white-matter disorders, and congenital disorders of glycosylation.

Astrocyte Differentiation of Human Pluripotent Stem Cells: New Tools for Neurological Disorder Research

Astrocytes have a central role in brain development and function, and so have gained increasing attention over the past two decades. Consequently, our knowledge about their origin, differentiation and function has increased significantly, with new research showing that astrocytes cultured alone or co-cultured with neurons have the potential to improve our understanding of various central nervous system diseases, such as amyotrophic lateral sclerosis, Alzheimer’s disease, or Alexander disease. The generation of astrocytes derived from pluripotent stem cells (PSCs) opens up a new area for studying neurologic diseases in vitro; these models could be exploited to identify and validate potential drugs by detecting adverse effects in the early stages of drug development. However, as it is now known that a range of astrocyte populations exist in the brain, it will be important in vitro to develop standardized protocols for the in vitro generation of astrocyte subsets with defined maturity status and phenotypic properties. This will then open new possibilities for co-cultures with neurons and the generation of neural organoids for research purposes. The aim of this review article is to compare and summarize the currently available protocols and their strategies to generate human astrocytes from PSCs. Furthermore, we discuss the potential role of human-induced PSCs derived astrocytes in disease modeling.

Glial Na(+) -dependent ion transporters in pathophysiological conditions

Sodium dynamics are essential for regulating functional processes in glial cells. Indeed, glial Na(+) signaling influences and regulates important glial activities, and plays a role in neuron-glia interaction under physiological conditions or in response to injury of the central nervous system (CNS). Emerging studies indicate that Na(+) pumps and Na(+) -dependent ion transporters in astrocytes, microglia, and oligodendrocytes regulate Na(+) homeostasis and play a fundamental role in modulating glial activities in neurological diseases. In this review, we first briefly introduced the emerging roles of each glial cell type in the pathophysiology of cerebral ischemia, Alzheimer's disease, epilepsy, Parkinson's disease, Amyotrophic Lateral Sclerosis, and myelin diseases. Then, we discussed the current knowledge on the main roles played by the different glial Na(+) -dependent ion transporters, including Na(+) /K(+) ATPase, Na(+) /Ca(2+) exchangers, Na(+) /H(+) exchangers, Na(+) -K(+) -Cl(-) cotransporters, and Na(+) - HCO3- cotransporter in the pathophysiology of the diverse CNS diseases. We highlighted their contributions in cell survival, synaptic pathology, gliotransmission, pH homeostasis, and their role in glial activation, migration, gliosis, inflammation, and tissue repair processes. Therefore, this review summarizes the foundation work for targeting Na(+) -dependent ion transporters in glia as a novel strategy to control important glial activities associated with Na(+) dynamics in different neurological disorders. GLIA 2016;64:1677-1697.

Malignant transformation in a case of megalencephalic leukoencephalopathy with subcortical cysts: An extreme rarity in a rare disorder

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is an autosomal recessive inherited disorder characterized by macrocephaly, progressive motor disability, seizures, mild cognitive decline, slow progression, and typical magnetic resonance imaging (MRI) findings. Age of onset of symptoms is described from birth to 25 years. Late onset presentation is very rare, only few cases have been reported worldwide. Most important clue for diagnosis is the characteristic MRI changes that include diffuse involvement of subcortical white matter mainly in frontoparietal region with relative sparing of central white matter along with subcortical cysts mostly in anterior temporal region. Cysts are usually benign and slowly progressive. Malignant transformation of cysts has not been reported as yet. We herein report a very unusual and probably the first case of MLC who presented to us in a unique manner with late onset and malignant transformation of cyst in left temporal region leading to rapid neurological decline. This case report highlights a possible life-threatening complication of a previously known slowly progressive disease warranting urgent neurosurgical intervention.

MLC1 protein: a likely link between leukodystrophies and brain channelopathies

Megalencephalic leukoencephalopathy with subcortical cysts (MLCs) disease is a rare inherited, autosomal recessive form of childhood-onset spongiform leukodystrophy characterized by macrocephaly, deterioration of motor functions, epileptic seizures and mental decline. Brain edema, subcortical fluid cysts, myelin and astrocyte vacuolation are the histopathological hallmarks of MLC. Mutations in either the MLC1 gene (>75% of patients) or the GlialCAM gene (<20% of patients) are responsible for the disease. Recently, the GlialCAM adhesion protein was found essential for the membrane expression and function of the chloride channel ClC-2 indicating MLC disease caused by mutation in GlialCAM as the first channelopathy among leukodystrophies. On the contrary, the function of MLC1 protein, which binds GlialCAM, its functional relationship with ClC-2 and the molecular mechanisms underlying MLC1 mutation-induced functional defects are not fully understood yet. The human MLC1 gene encodes a 377-amino acid membrane protein with eight predicted transmembrane domains which shows very low homology with voltage-dependent potassium (K⁺) channel subunits. The high expression of MLC1 in brain astrocytes contacting blood vessels and meninges and brain alterations observed in MLC patients have led to hypothesize a role for MLC1 in the regulation of ion and water homeostasis. Recent studies have shown that MLC1 establishes structural and/or functional interactions with several ion/water channels and transporters and ion channel accessory proteins, and that these interactions are affected by MLC1 mutations causing MLC. Here, we review data on MLC1 functional properties obtained in in vitro and in vivo models and discuss evidence linking the effects of MLC1 mutations to brain channelopathies.