Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

In the mouse cortex, oligodendrocytes regain a plastic capacity, transforming into astrocytes after acute injury

Acute brain injuries evoke various response cascades directing the formation of the glial scar. Here, we report that acute lesions associated with hemorrhagic injuries trigger a re-programming of oligodendrocytes. Single-cell RNA sequencing highlighted a subpopulation of oligodendrocytes activating astroglial genes after acute brain injuries. By using PLP-DsRed1/GFAP-EGFP and PLP-EGFP_mem/GFAP-mRFP1 transgenic mice, we visualized this population of oligodendrocytes that we termed AO cells based on their concomitant activity of astro- and oligodendroglial genes. By fate mapping using PLP- and GFAP-split Cre complementation and repeated chronic in vivo imaging with two-photon laser-scanning microscopy, we observed the conversion of oligodendrocytes into astrocytes via the AO cell stage. Such conversion was promoted by local injection of IL-6 and was diminished by IL-6 receptor-neutralizing antibody as well as by inhibiting microglial activation with minocycline. In summary, our findings highlight the plastic potential of oligodendrocytes in acute brain trauma due to microglia-derived IL-6.

Role of Lipids in Regulation of Neuroglial Interactions

Lipids comprise an extremely heterogeneous group of compounds that perform a wide variety of biological functions. Traditional view of lipids as important structural components of the cell and compounds playing a trophic role is currently being supplemented by information on the possible participation of lipids in signaling, not only intracellular, but also intercellular. The review article discusses current data on the role of lipids and their metabolites formed in glial cells (astrocytes, oligodendrocytes, microglia) in communication of these cells with neurons. In addition to metabolic transformations of lipids in each type of glial cells, special attention is paid to the lipid signal molecules (phosphatidic acid, arachidonic acid and its metabolites, cholesterol, etc.) and the possibility of their participation in realization of synaptic plasticity, as well as in other possible mechanisms associated with neuroplasticity. All these new data can significantly expand our knowledge about the regulatory functions of lipids in neuroglial relationships.

Heterogeneity of white matter astrocytes in the human brain

Astrocytes regulate central nervous system development, maintain its homeostasis and orchestrate repair upon injury. Emerging evidence support functional specialization of astroglia, both between and within brain regions. Different subtypes of gray matter astrocytes have been identified, yet molecular and functional diversity of white matter astrocytes remains largely unexplored. Nonetheless, their important and diverse roles in maintaining white matter integrity and function are well recognized. Compelling evidence indicate that impairment of normal astrocytic function and their response to injury contribute to a wide variety of diseases, including white matter disorders. In this review, we highlight our current understanding of astrocyte heterogeneity in the white matter of the mammalian brain and how an interplay between developmental origins and local environmental cues contribute to astroglial diversification. In addition, we discuss whether, and if so, how, heterogeneous astrocytes could contribute to white matter function in health and disease and focus on the sparse human research data available. We highlight four leukodystrophies primarily due to astrocytic dysfunction, the so-called astrocytopathies. Insight into the role of astroglial heterogeneity in both healthy and diseased white matter may provide new avenues for therapies aimed at promoting repair and restoring normal white matter function.

Astrocyte Role in Temporal Lobe Epilepsy and Development of Mossy Fiber Sprouting

Epilepsy affects approximately 50 million people worldwide, with 60% of adult epilepsies presenting an onset of focal origin. The most common focal epilepsy is temporal lobe epilepsy (TLE). The role of astrocytes in the presentation and development of TLE has been increasingly studied and discussed within the literature. The most common histopathological diagnosis of TLE is hippocampal sclerosis. Hippocampal sclerosis is characterized by neuronal cell loss within the Cornu ammonis and reactive astrogliosis. In some cases, mossy fiber sprouting may be observed. Mossy fiber sprouting has been controversial in its contribution to epileptogenesis in TLE patients, and the mechanisms surrounding the phenomenon have yet to be elucidated. Several studies have reported that mossy fiber sprouting has an almost certain co-existence with reactive astrogliosis within the hippocampus under epileptic conditions. Astrocytes are known to play an important role in the survival and axonal outgrowth of central and peripheral nervous system neurons, pointing to a potential role of astrocytes in TLE and associated cellular alterations. Herein, we review the recent developments surrounding the role of astrocytes in the pathogenic process of TLE and mossy fiber sprouting, with a focus on proposed signaling pathways and cellular mechanisms, histological observations, and clinical correlations in human patients.

Aging-Related Tau Astrogliopathy in Aging and Neurodegeneration

Astrocytes are of vital importance to neuronal function and the health of the central nervous system (CNS), and astrocytic dysfunction as a primary or secondary event may predispose to neurodegeneration. Until recently, the main astrocytic tauopathies were the frontotemporal lobar degeneration with tau (FTLD-tau) group of disorders; however, aging-related tau astrogliopathy (ARTAG) has now been defined. This condition is a self-describing neuropathology mainly found in individuals over 60 years of age. Astrocytic tau accumulates with a thorny or granular/fuzzy morphology and is commonly found in normal aging as well as coexisting with diverse neurodegenerative disorders. However, there are still many unknown factors associated with ARTAG, including the cause/s, the progression, and the nature of any clinical associations. In addition to FTLD-tau, ARTAG has recently been associated with chronic traumatic encephalopathy (CTE), where it has been proposed as a potential precursor to these conditions, with the different ARTAG morphological subtypes perhaps having separate etiologies. This is an emerging area of exciting research that encompasses complex neurobiological and clinicopathological investigation.

Astrocyte phenotypes: Emphasis on potential markers in neuroinflammation

Astrocytes, the most abundant glial cells in the central nervous system (CNS), have numerous integral roles in all CNS functions. They are essential for synaptic transmission and support neurons by providing metabolic substrates, secreting growth factors and regulating extracellular concentrations of ions and neurotransmitters. Astrocytes respond to CNS insults through reactive astrogliosis, in which they go through many functional and molecular changes. In neuroinflammatory conditions reactive astrocytes exert both beneficial and detrimental functions, depending on the context and heterogeneity of astrocytic populations. In this review we profile astrocytic diversity in the context of neuroinflammation; with a specific focus on multiple sclerosis (MS) and its best-described animal model experimental autoimmune encephalomyelitis (EAE). We characterize two main subtypes, protoplasmic and fibrous astrocytes and describe the role of intermediate filaments in the physiology and pathology of these cells. Additionally, we outline a variety of markers that are emerging as important in investigating astrocytic biology in both physiological conditions and neuroinflammation.

Metabolic determinants of leukocyte pathogenicity in neurological diseases

Neuroinflammatory and neurodegenerative diseases are characterized by the recruitment of circulating blood-borne innate and adaptive immune cells into the central nervous system (CNS). These leukocytes sustain the detrimental response in the CNS by releasing pro-inflammatory mediators that induce activation of local glial cells, blood-brain barrier (BBB) dysfunction, and neural cell death. However, infiltrating peripheral immune cells could also dampen CNS inflammation and support tissue repair. Recent advances in the field of immunometabolism demonstrate the importance of metabolic reprogramming for the activation and functionality of such innate and adaptive immune cell populations. In particular, an increasing body of evidence suggests that the activity of metabolites and metabolic enzymes could influence the pathogenic potential of immune cells during neuroinflammatory and neurodegenerative disorders. In this review, we discuss the role of intracellular metabolic cues in regulating leukocyte-mediated CNS damage in Alzheimer's and Parkinson's disease, multiple sclerosis and stroke, highlighting the therapeutic potential of drugs targeting metabolic pathways for the treatment of neurological diseases.

Dynamic indentation reveals differential viscoelastic properties of white matter versus gray matter-derived astrocytes upon treatment with lipopolysaccharide

Astrocytes in white matter (WM) and gray matter (GM) brain regions have been reported to have different morphology and function. Previous single cell biomechanical studies have not differentiated between WM- and GM-derived samples. In this study, we explored the local viscoelastic properties of isolated astrocytes and show that astrocytes from rat brain WM-enriched areas are ~1.8 times softer than astrocytes from GM-enriched areas. Upon treatment with pro-inflammatory lipopolysaccharide, GM-derived astrocytes become significantly softer in the nuclear and the cytoplasmic regions, where the F-actin network appears rearranged, whereas WM-derived astrocytes preserve their initial mechanical features and show no alteration in the F-actin cytoskeletal network. We hypothesize that the flexibility in biomechanical properties of GM-derived astrocytes may contribute to promote regeneration of the brain under neuroinflammatory conditions.

Astroglia in Sepsis Associated Encephalopathy

Cellular pathophysiology of sepsis associated encephalopathy (SAE) remains poorly characterised. Brain pathology in SAE, which is manifested by impaired perception, consciousness and cognition, results from multifactorial events, including high levels of systemic cytokines, microbial components and endotoxins, which all damage the brain barriers, instigate neuroinflammation and cause homeostatic failure. Astrocytes, being the principal homeostatic cells of the central nervous system contribute to the brain defence against infection. Forming multifunctional anatomical barriers, astroglial cells maintain brain-systemic interfaces and restrict the damage to the nervous tissue. Astrocytes detect, produce and integrate inflammatory signals between immune cells and cells of brain parenchyma, thus regulating brain immune response. In SAE astrocytes are present in both reactive and astrogliopathic states; balance between these states define evolution of pathology and neurological outcomes. In humans pathophysiology of SAE is complicated by frequent presence of comorbidities, as well as age-related remodelling of the brain tissue with senescence of astroglia; these confounding factors further impact upon SAE progression and neurological deficits.

Function of Glia in Aging and the Brain Diseases

Microglia cells during aging, neurodegeneration and neuroinflammation show different morphological and transcriptional profiles (related to axonal direction and cell adhesion). Furthermore, expressions of the receptors on the surface and actin formation compared to young are also different. This review delves into the role of glia during aging and the development of the diseases. The susceptibility of different regions of the brain to disease are linked to the overstimulation of signals related to the immune system during aging, as well as the damaging impact of these cascades on the functionality of different populations of microglia present in each region of the brain. Furthermore, a decrease in microglial phagocytosis has been related to many diseases and also has been detected during aging. In this paper we also describe the role of glia in different illness, such as AD, ALS, pain related disorders, cancer, developmental disorders and the problems produced by opening of the blood brain barrier. Future studies will clarify many points planted by this review.

Biomaterial Approaches to Modulate Reactive Astroglial Response

Over several decades, biomaterial scientists have developed materials to spur axonal regeneration and limit secondary injury and tested these materials within preclinical animal models. Rarely, though, are astrocytes examined comprehensively when biomaterials are placed into the injury site. Astrocytes support neuronal function in the central nervous system. Following an injury, astrocytes undergo reactive gliosis and create a glial scar. The astrocytic glial scar forms a dense barrier which restricts the extension of regenerating axons through the injury site. However, there are several beneficial effects of the glial scar, including helping to reform the blood-brain barrier, limiting the extent of secondary injury, and supporting the health of regenerating axons near the injury site. This review provides a brief introduction to the role of astrocytes in the spinal cord, discusses astrocyte phenotypic changes that occur following injury, and highlights studies that explored astrocyte changes in response to biomaterials tested within in vitro or in vivo environments. Overall, we suggest that in order to improve biomaterial designs for spinal cord injury applications, investigators should more thoroughly consider the astrocyte response to such designs.

Fast Generation of Functional Subtype Astrocytes from Human Pluripotent Stem Cells

Differentiation of astrocytes from human pluripotent stem cells (hPSCs) is a tedious and variable process. This hampers the study of hPSC-generated astrocytes in disease processes and drug development. By using CRISPR/Cas9-mediated inducible expression of NFIA or NFIA plus SOX9 in hPSCs, we developed a method to efficiently generate astrocytes in 4-7 weeks. The astrocytic identity of the induced cells was verified by their characteristic molecular and functional properties as well as after transplantation. Furthermore, we developed a strategy to generate region-specific astrocyte subtypes by combining differentiation of regional progenitors and transgenic induction of astrocytes. This simple and efficient method offers a new opportunity to study the fundamental biology of human astrocytes and their roles in disease processes.

A bell-shaped dependence between amyloidosis and GABA accumulation in astrocytes in a mouse model of Alzheimer's disease

Functioning at the interface between the nervous and immune systems, in the amyloid-depositing brain, astrocytes become hypertrophic and accumulate around senile plaques. Moreover, hippocampal astrocytes upregulate their γ-aminobutyric acid (GABA) content and enhance tonic inhibition, likely causing local circuit imbalance. It remains, however, unclear whether this effect is hippocampus specific and how it is regulated during disease progression. Here, we studied changes in astrocytic morphology and GABA content in the frontal cortex and dentate gyrus of control and amyloid-depositing mice. Healthy aging was accompanied by a transient increase in astrocytic GABA content at middle age and region-specific alterations of soma size. In contrast, amyloid deposition caused a gradual cortex-accentuated increase in soma size. Importantly, our data uncovered a bell-shaped relationship between the mouse age and astrocytic GABA content in both brain regions. Moreover, in mice carrying an Alzheimer's disease-related mutation in presenilin 1, astrocytes accumulated GABA even in the absence of amyloidosis. These data question the proposed inhibition of astrocytic GABA synthesis as a universal strategy for treating network dysfunction in Alzheimer's disease.

Heme oxygenase-1 derived carbon monoxide suppresses Aβ1-42 toxicity in astrocytes

Neurodegeneration in Alzheimer's disease (AD) is extensively studied, and the involvement of astrocytes and other cell types in this process has been described. However, the responses of astrocytes themselves to amyloid β peptides ((Aβ; the widely accepted major toxic factor in AD) is less well understood. Here, we show that Aβ_(1-42) is toxic to primary cultures of astrocytes. Toxicity does not involve disruption of astrocyte Ca²⁺ homeostasis, but instead occurs via formation of the toxic reactive species, peroxynitrite. Thus, Aβ_(1-42) raises peroxynitrite levels in astrocytes, and Aβ_(1-42) toxicity can be inhibited by antioxidants, or by inhibition of nitric oxide (NO) formation (reactive oxygen species (ROS) and NO combine to form peroxynitrite), or by a scavenger of peroxynitrite. Increased ROS levels observed following Aβ_(1-42) application were derived from NADPH oxidase. Induction of haem oxygenase-1 (HO-1) protected astrocytes from Aβ_(1-42) toxicity, and this protective effect was mimicked by application of the carbon monoxide (CO) releasing molecule CORM-2, suggesting HO-1 protection was attributable to its formation of CO. CO suppressed the rise of NADPH oxidase-derived ROS caused by Aβ_(1-42). Under hypoxic conditions (0.5% O₂, 48 h) HO-1 was induced in astrocytes and Aβ_(1-42) toxicity was significantly reduced, an effect which was reversed by the specific HO-1 inhibitor, QC-15. Our data suggest that Aβ_(1-42) is toxic to astrocytes, but that induction of HO-1 affords protection against this toxicity due to formation of CO. HO-1 induction, or CO donors, would appear to present attractive possible approaches to provide protection of both neuronal and non-neuronal cell types from the degenerative effects of AD in the central nervous system.

Astrocytic transporters in Alzheimer's disease

Astrocytes play a fundamental role in maintaining the health and function of the central nervous system. Increasing evidence indicates that astrocytes undergo both cellular and molecular changes at an early stage in neurological diseases, including Alzheimer's disease (AD). These changes may reflect a change from a neuroprotective to a neurotoxic phenotype. Given the lack of current disease-modifying therapies for AD, astrocytes have become an interesting and viable target for therapeutic intervention. The astrocyte transport system covers a diverse array of proteins involved in metabolic support, neurotransmission and synaptic architecture. Therefore, specific targeting of individual transporter families has the potential to suppress neurodegeneration, a characteristic hallmark of AD. A small number of the 400 transporter superfamilies are expressed in astrocytes, with evidence highlighting a fraction of these are implicated in AD. Here, we review the current evidence for six astrocytic transporter subfamilies involved in AD, as reported in both animal and human studies. This review confirms that astrocytes are indeed a viable target, highlights the complexities of studying astrocytes and provides future directives to exploit the potential of astrocytes in tackling AD.

Glial markers and emotional memory in rats following acute cerebral radiofrequency exposures

The widespread mobile phone use raises concerns on the possible cerebral effects of radiofrequency electromagnetic fields (RF EMF). Reactive astrogliosis was reported in neuroanatomical structures of adaptive behaviors after a single RF EMF exposure at high specific absorption rate (SAR, 6 W/kg). Here, we aimed to assess if neuronal injury and functional impairments were related to high SAR-induced astrogliosis. In addition, the level of beta amyloid 1-40 (Aβ 1-40) peptide was explored as a possible toxicity marker. Sprague Dawley male rats were exposed for 15 min at 0, 1.5, or 6 W/kg or for 45 min at 6 W/kg. Memory, emotionality, and locomotion were tested in the fear conditioning, the elevated plus maze, and the open field. Glial fibrillary acidic protein (GFAP, total and cytosolic fractions), myelin basic protein (MBP), and Aβ1-40 were quantified in six brain areas using enzyme-linked immunosorbent assay. According to our data, total GFAP was increased in the striatum (+114 %) at 1.5 W/kg. Long-term memory was reduced, and cytosolic GFAP was increased in the hippocampus (+119 %) and in the olfactory bulb (+46 %) at 6 W/kg (15 min). No MBP or Aβ1-40 expression modification was shown. Our data corroborates previous studies indicating RF EMF-induced astrogliosis. This study suggests that RF EMF-induced astrogliosis had functional consequences on memory but did not demonstrate that it was secondary to neuronal damage.

Dopamine elevates and lowers astroglial Ca2+ through distinct pathways depending on local synaptic circuitry

Whilst astrocytes in culture invariably respond to dopamine with cytosolic Ca²⁺ rises, the dopamine sensitivity of astroglia in situ and its physiological roles remain unknown. To minimize effects of experimental manipulations on astroglial physiology, here we monitored Ca²⁺ in cells connected via gap junctions to astrocytes loaded whole‐cell with cytosolic indicators in area CA1 of acute hippocampal slices. Aiming at high sensitivity of [Ca²⁺] measurements, we also employed life‐time imaging of the Ca²⁺ indicator Oregon Green BAPTA‐1. We found that dopamine triggered a dose‐dependent, bidirectional Ca²⁺ response in stratum radiatum astroglia, a jagged elevation accompanied and followed by below‐baseline decreases. The elevation depended on D1/D2 receptors and engaged intracellular Ca²⁺ storage and removal whereas the dopamine‐induced [Ca²⁺] decrease involved D2 receptors only and was sensitive to Ca²⁺ channel blockade. In contrast, the stratum lacunosum moleculare astroglia generated higher‐threshold dopamine‐induced Ca²⁺ responses which did not depend on dopamine receptors and were uncoupled from the prominent inhibitory action of dopamine on local perforant path synapses. Our findings thus suggest that a single neurotransmitter—dopamine—could either elevate or decrease astrocyte [Ca²⁺] depending on the receptors involved, that such actions are specific to the regional neural circuitry and that they may be causally uncoupled from dopamine actions on local synapses. The results also indicate that [Ca²⁺] elevations commonly detected in astroglia can represent the variety of distinct mechanisms acting on the microscopic scale. GLIA 2017;65:447–459

The Indispensable Roles of Microglia and Astrocytes during Brain Development

Glia are essential for brain functioning during development and in the adult brain. Here, we discuss the various roles of both microglia and astrocytes, and their interactions during brain development. Although both cells are fundamentally different in origin and function, they often affect the same developmental processes such as neuro-/gliogenesis, angiogenesis, axonal outgrowth, synaptogenesis and synaptic pruning. Due to their important instructive roles in these processes, dysfunction of microglia or astrocytes during brain development could contribute to neurodevelopmental disorders and potentially even late-onset neuropathology. A better understanding of the origin, differentiation process and developmental functions of microglia and astrocytes will help to fully appreciate their role both in the developing as well as in the adult brain, in health and disease.

Are astrocytes executive cells within the central nervous system?

ABSTRACT Experimental evidence suggests that astrocytes play a crucial role in the physiology of the central nervous system (CNS) by modulating synaptic activity and plasticity. Based on what is currently known we postulate that astrocytes are fundamental, along with neurons, for the information processing that takes place within the CNS. On the other hand, experimental findings and human observations signal that some of the primary degenerative diseases of the CNS, like frontotemporal dementia, Parkinson’s disease, Alzheimer’s dementia, Huntington’s dementia, primary cerebellar ataxias and amyotrophic lateral sclerosis, all of which affect the human species exclusively, may be due to astroglial dysfunction. This hypothesis is supported by observations that demonstrated that the killing of neurons by non-neural cells plays a major role in the pathogenesis of those diseases, at both their onset and their progression. Furthermore, recent findings suggest that astrocytes might be involved in the pathogenesis of some psychiatric disorders as well.

A critical review of cell culture strategies for modelling intracortical brain implant material reactions

The capacity to predict in vivo responses to medical devices in humans currently relies greatly on implantation in animal models. Researchers have been striving to develop in vitro techniques that can overcome the limitations associated with in vivo approaches. This review focuses on a critical analysis of the major in vitro strategies being utilized in laboratories around the world to improve understanding of the biological performance of intracortical, brain-implanted microdevices. Of particular interest to the current review are in vitro models for studying cell responses to penetrating intracortical devices and their materials, such as electrode arrays used for brain computer interface (BCI) and deep brain stimulation electrode probes implanted through the cortex. A background on the neural interface challenge is presented, followed by discussion of relevant in vitro culture strategies and their advantages and disadvantages. Future development of 2D culture models that exhibit developmental changes capable of mimicking normal, postnatal development will form the basis for more complex accurate predictive models in the future. Although not within the scope of this review, innovations in 3D scaffold technologies and microfluidic constructs will further improve the utility of in vitro approaches.

Restraint Stress-Induced Morphological Changes at the Blood-Brain Barrier in Adult Rats

Stress is well-known to contribute to the development of both neurological and psychiatric diseases. While the role of the blood-brain barrier is increasingly recognized in the development of neurodegenerative disorders, such as Alzheimer's disease, dysfunction of the blood-brain barrier has been linked to stress-related psychiatric diseases only recently. In the present study the effects of restraint stress with different duration (1, 3, and 21 days) were investigated on the morphology of the blood-brain barrier in male adult Wistar rats. Frontal cortex and hippocampus sections were immunostained for markers of brain endothelial cells (claudin-5, occluding, and glucose transporter-1) and astroglia (GFAP). Staining pattern and intensity were visualized by confocal microscopy and evaluated by several types of image analysis. The ultrastructure of brain capillaries was investigated by electron microscopy. Morphological changes and intensity alterations in brain endothelial tight junction proteins claudin-5 and occludin were induced by stress. Following restraint stress significant increases in the fluorescence intensity of glucose transporter-1 were detected in brain endothelial cells in the frontal cortex and hippocampus. Significant reductions in GFAP fluorescence intensity were observed in the frontal cortex in all stress groups. As observed by electron microscopy, 1-day acute stress induced morphological changes indicating damage in capillary endothelial cells in both brain regions. After 21 days of stress thicker and irregular capillary basal membranes in the hippocampus and edema in astrocytes in both regions were seen. These findings indicate that stress exerts time-dependent changes in the staining pattern of tight junction proteins occludin, claudin-5, and glucose transporter-1 at the level of brain capillaries and in the ultrastructure of brain endothelial cells and astroglial endfeet, which may contribute to neurodegenerative processes, cognitive and behavioral dysfunctions.

Small heat shock proteins are induced during multiple sclerosis lesion development in white but not grey matter

Introduction

The important protective role of small heat-shock proteins (HSPs) in regulating cellular survival and migration, counteracting protein aggregation, preventing apoptosis, and regulating inflammation in the central nervous system is now well-recognized. Yet, their role in the neuroinflammatory disorder multiple sclerosis (MS) is largely undocumented. With the exception of alpha B-crystallin (HSPB5), little is known about the roles of small HSPs in disease.

Results

Here, we examined the expression of four small HSPs during lesion development in MS, focussing on their cellular distribution, and regional differences between white matter (WM) and grey matter (GM). It is well known that MS lesions in these areas differ markedly in their pathology, with substantially more intense blood-brain barrier damage, leukocyte infiltration and microglial activation typifying WM but not GM lesions.

We analysed transcript levels and protein distribution profiles for HSPB1, HSPB6, HSPB8 and HSPB11 in MS lesions at different stages, comparing them with normal-appearing brain tissue from MS patients and non-neurological controls. During active stages of demyelination in WM, and especially the centre of chronic active MS lesions, we found significantly increased expression of HSPB1, HSPB6 and HSPB8, but not HSPB11. When induced, small HSPs were exclusively found in astrocytes but not in oligodendrocytes, microglia or neurons. Surprisingly, while the numbers of astrocytes displaying high expression of small HSPs were markedly increased in actively demyelinating lesions in WM, no such induction was observed in GM lesions. This difference was particularly obvious in leukocortical lesions covering both WM and GM areas.

Conclusions

Since induction of small HSPs in astrocytes is apparently a secondary response to damage, their differential expression between WM and GM likely reflects differences in mediators that accompany demyelination in either WM or GM during MS. Our findings also suggest that during MS, cortical structures fail to benefit from the protective actions of small HSPs.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0267-2) contains supplementary material, which is available to authorized users.

Ascl1 Converts Dorsal Midbrain Astrocytes into Functional Neurons In Vivo

In vivo induction of non-neuronal cells into neurons by transcription factors offers potential therapeutic approaches for neural regeneration. Although generation of induced neuronal (iN) cells in vitro and in vivo has been reported, whether iN cells can be fully integrated into existing circuits remains unclear. Here we show that expression of achaete-scute complex homolog-like 1 (Ascl1) alone is sufficient to convert dorsal midbrain astrocytes of mice into functional iN cells in vitro and in vivo. Specific expression of Ascl1 in astrocytes by infection with GFAP-adeno-associated virus (AAV) vector converts astrocytes in dorsal midbrain, striatum, and somatosensory cortex of postnatal and adult mice into functional neurons in vivo. These iN cells mature progressively, exhibiting neuronal morphology and markers, action potentials, and synaptic inputs from and output to existing neurons. Thus, a single transcription factor, Ascl1, is sufficient to convert brain astrocytes into functional neurons, and GFAP-AAV is an efficient vector for generating iN cells from astrocytes in vivo.

Astrocyte morphology, heterogeneity, and density in the developing African giant rat (Cricetomys gambianus)

Astrocyte morphologies and heterogeneity were described in male African giant rats (AGR; Cricetomys gambianus, Waterhouse) across three age groups (five neonates, five juveniles, and five adults) using Silver impregnation method and immunohistochemistry against glial fibrillary acidic protein. Immunopositive cell signaling, cell size and population were least in neonates, followed by adults and juveniles, respectively. In neonates, astrocyte processes were mostly detected within the glia limitans of the mid and hind brain; their cell bodies measuring 32 ± 4.8 μm in diameter against 91 ± 5.4 μm and 75 ± 1.9 μm in juveniles and adults, respectively. Astrocyte heterogeneity in juvenile and adult groups revealed eight subtypes to include fibrous astrocytes chiefly in the corpus callosum and brain stem, protoplasmic astrocytes in the cortex and dentate gyrus (DG); radial glia were found along the olfactory bulb (OB) and subventricular zone (SVZ); velate astrocytes were mainly found in the cerebellum and hippocampus; marginal astrocytes close to the pia mater; Bergmann glia in the molecular layer of the cerebellum; perivascular and periventricular astrocytes in the cortex and third ventricle, respectively. Cell counts from twelve anatomical regions of the brain were significantly higher in juveniles than in adults (p ≤ 0.01) using unpaired student t-test in the cerebral cortex, pia, corpus callosum, rostral migratory stream, DG, and cerebellum. Highest astrocyte count was found in the DG, while the least count was in the brain stem and sub cortex. Astrocytes along the periventricular layer of the OB are believed to be part of the radial glia system that transport newly formed cells towards the hippocampus and play roles in neurogenesis migration and homeostasis in the AGR. Therefore, astrocyte heterogeneity was examined across age groups in the AGR to determine whether age influences astrocytes population in different regions of the AGR brain and discuss possible functional roles.

Astrocyte physiopathology: At the crossroads of intercellular networking, inflammation and cell death

Recent breakthroughs in neuroscience have led to the awareness that we should revise our traditional mode of thinking and studying the CNS, i.e. by isolating the privileged network of "intelligent" synaptic contacts. We may instead need to contemplate all the variegate communications occurring between the different neural cell types, and centrally involving the astrocytes. Basically, it appears that a single astrocyte should be considered as a core that receives and integrates information from thousands of synapses, other glial cells and the blood vessels. In turn, it generates complex outputs that control the neural circuitry and coordinate it with the local microcirculation. Astrocytes thus emerge as the possible fulcrum of the functional homeostasis of the healthy CNS. Yet, evidence indicates that the bridging properties of the astrocytes can change in parallel with, or as a result of, the morphological, biochemical and functional alterations these cells undergo upon injury or disease. As a consequence, they have the potential to transform from supportive friends and interactive partners for neurons into noxious foes. In this review, we summarize the currently available knowledge on the contribution of astrocytes to the functioning of the CNS and what goes wrong in various pathological conditions, with a particular focus on Amyotrophic Lateral Sclerosis, Alzheimer's Disease and ischemia. The observations described convincingly demonstrate that the development and progression of several neurological disorders involve the de-regulation of a finely tuned interplay between multiple cell populations. Thus, it seems that a better understanding of the mechanisms governing the integrated communication and detrimental responses of the astrocytes as well as their impact towards the homeostasis and performance of the CNS is fundamental to open novel therapeutic perspectives.

Interleukin-1β protects astrocytes against oxidant-induced injury via an NF-κB-dependent upregulation of glutathione synthesis

Astrocytes produce and export the antioxidant glutathione (GSH). Previously, we found that interleukin-1β (IL-1β) enhanced the expression of astrocyte system xc (-) , the transporter that delivers the rate-limiting substrate for GSH synthesis-cyst(e)ine. Herein, we demonstrate directly that IL-1β mediates a time-dependent increase in extracellular GSH levels in cortical astrocyte cultures, suggesting both enhanced synthesis and export. This increased GSH production was blocked by inhibition of nuclear factor-κB (NF-κB) activity but not by inhibition of p38 MAPK. To determine whether this increase could provide protection against oxidative stress, the oxidants tert-butyl hydroperoxide (tBOOH) and ferrous sulfate (FeSO4 ) were employed. IL-1β treatment prevented the increase in reactive oxygen species produced in astrocytes following tBOOH exposure. Additionally, the toxicity induced by tBOOH or FeSO4 exposure was significantly attenuated following treatment with IL-1β, an effect reversed by concomitant exposure to l-buthionine-S,R-sulfoximine (BSO), which prevented the IL-1β-mediated rise in GSH production. IL-1β failed to increase GSH or to provide protection against t-BOOH toxicity in astrocyte cultures derived from IL-1R1 null mutant mice. Overall, our data indicate that under certain conditions IL-1β may be an important stimulus for increasing astrocyte GSH production, and potentially, total antioxidant capacity in brain, via an NF-κB-dependent process.

Human pluripotent stem cell-derived radial glia recapitulate developmental events and provide real-time access to cortical neurons and astrocytes

Studies of human cerebral cortex development are limited by difficulties in accessing and manipulating human neural tissue at specific development stages. We have derived human radial glia (hRG), which are responsible for most cerebral cortex neurogenesis, from human pluripotent stem cells. These hRG display the hallmark morphological, cellular, and molecular features of radial glia in vitro. They can be passaged and generate layer-specific subtypes of cortical neurons in a temporal and passage-dependent fashion. In later passages, they adopt a distinct progenitor phenotype that gives rise to cortical astrocytes and GABAergic interneurons. These hRG are also capable of following developmental cues to engraft, differentiate, migrate, and integrate into the embryonic mouse cortex when injected into E14 lateral ventricles. Moreover, hRG-derived cells can be cryopreserved at specific stages and retain their stage-specific phenotypes and competence when revived. Our study demonstrates that cultured hRG maintain a cell-intrinsic clock that regulates the progressive generation of stage-specific neuronal and glial subtypes. It also describes an easily accessible cell source for studying hRG lineage specification and progression and an on-demand supply of specific cortical neuron subtypes and astrocytes.

Effect of long-term (2 years) exposure of mouse brains to global system for mobile communication (GSM) radiofrequency fields on astrocytic immunoreactivity

This study was designed to determine whether long-term (2 years) brain exposure to mobile telephone radiofrequency (RF) fields produces any astrocytic activation as these glia react to a wide range of neural perturbations by astrogliosis. Using a purpose-designed exposure system at 900 MHz, mice were given a single, far-field whole body exposure at a specific absorption rate of 4 W/kg on five successive days per week for 104 weeks. Control mice were sham-exposed or freely mobile in a cage to control any stress caused by immobilization in the exposure module. Brains were perfusion-fixed with 4% paraformaldehyde and three coronal levels immunostained for glial fibrillary acidic protein (GFAP). These brain slices were then examined by light microscopy and the amount of this immunomarker quantified using a color deconvolution method. There was no change in astrocytic GFAP immunostaining in brains after long-term exposure to mobile telephony microwaves compared to control (sham-exposed or freely moving caged mice). It was concluded that long-term (2 years) exposure of murine brains to mobile telephone RF fields did not produce any astrocytic reaction (astrogliosis) detectable by GFAP immunostaining.

Could astrocytes be the primary target of an offending agent causing the primary degenerative diseases of the human central nervous system? A hypothesis

Most of the named primary degenerative diseases of the human central nervous system have been attributed to a direct, primary damage of some particular population of neurons. Within the spectrum of these illnesses there are disorders like amyotrophic lateral sclerosis, fronto-temporal dementia, Alzheimer's dementia, Parkinson's disease, Huntington's dementia and cerebellar ataxias affecting exclusively the human species. In the last years it has been shown that non-neural cells, mainly astrocytes, have a crucial role in the starting and development of these diseases. We suggest that the causative agent of these illnesses gets home first within the astrocytes, rather than the neurons, making them sick by modifying the structure of some proteins; from these cells the abnormal process would start a trip to other astrocytes having the same genetic, metabolic, structural and functional profiles that the originally affected astrocytes have, going through the gap junctions which connect that particular population devoted to a particular set of neurons. This appears to be a likely hypothesis because the astrocytes related to a defined population of neurons have their own, private properties and characteristics needed to support one particular set of neurons performing a defined function, making them a different and unique population, a fact which would limit the spreading of the disease to those astrocytes, sparing other astrocyte populations which do not share those characteristics. If this were the mechanism underlying these illnesses, the neurons, which their health depends on those astrocytes, would be deprived of their patronage and would start all the changes that characterizes a programmed cell death, and the clinical manifestations of a defined pathology would consequently appear.

Discrepancy in CCL2 and CCR2 expression in white versus grey matter hippocampal lesions of Multiple Sclerosis patients

A remarkable pathological difference between grey matter lesions (GML) and white matter lesions (WML) in Multiple Sclerosis (MS) patients is the paucity of infiltrating leukocytes in GML. To better understand these pathological differences, we hypothesize that the chemokine monocyte chemotactic protein-1 (MCP-1 or CCL2), of importance for leukocyte migration, and its receptor CCR2 are more abundantly expressed in WML than in GML of MS patients. To this end, we analyzed CCL2 and CCR2 expression in the hippocampus, comprising WML and GML,of post-mortem MS patients, and of control subjects. CCL2 and CCR2 mRNA were significantly increased in demyelinated MS hippocampus. Semi-quantification of CCL2 and CCR2 immunoreactivity showed that CCL2 is present in astrocytes only in active WML. CCR2 is upregulated in monocytes/macrophages or amoeboid microglia in active WML, and in ramified microglia in active GML, although to a lesser extent. As a follow-up, we observed a significantly increased CCL2 production by WM-, but not GM-derived astrocytes upon stimulation with bz-ATP in vitro. Finally, upon CCL2 stimulation, GM-derived microglia significantly increased their proliferation rate. We conclude that within hippocampal lesions, CCL2 expression is mainly restricted to WML, whereas the receptor CCR2 is upregulated in both WML and GML. The relative absence of CCL2 in GML may explain the lack of infiltrating immune cells in this type of lesions. We propose that the divergent expression of CCL2 and CCR2 in WML and GML explains or contributes to the differences in WML and GML formation in MS.

Astrocytic laminin regulates pericyte differentiation and maintains blood brain barrier integrity

Blood brain barrier (BBB) breakdown is not only a consequence of but also contributes to many neurological disorders, including stroke and Alzheimer's disease. How the basement membrane (BM) contributes to the normal functioning of the BBB remains elusive. Here we use conditional knockout mice and an acute adenovirus-mediated knockdown model to show that lack of astrocytic laminin, a brain-specific BM component, induces BBB breakdown. Using functional blocking antibody and RNAi, we further demonstrate that astrocytic laminin, by binding to integrin α2 receptor, prevents pericyte differentiation from the BBB-stabilizing resting stage to the BBB-disrupting contractile stage, and thus maintains the integrity of BBB. Additionally, loss of astrocytic laminin decreases aquaporin-4 (AQP4) and tight junction protein expression. Altogether, we report a critical role for astrocytic laminin in BBB regulation and pericyte differentiation. These results indicate that astrocytic laminin maintains the integrity of BBB through, at least in part, regulation of pericyte differentiation.

Neuronal clearance of amyloid-β by endocytic receptor LRP1

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly population. Accumulation, aggregation, and deposition of amyloid-β (Aβ) peptides generated through proteolytic cleavage of amyloid precursor protein (APP) are likely initiating events in the pathogenesis of AD. While Aβ production is accelerated in familial AD, increasing evidence indicates that impaired clearance of Aβ is responsible for late-onset AD. Because Aβ is mainly generated in neurons, these cells are predicted to have the highest risk of encountering Aβ among all cell types in the brain. However, it is still unclear whether they are also involved in Aβ clearance. Here we show that receptor-mediated endocytosis in neurons by the low-density lipoprotein receptor-related protein 1 (LRP1) plays a critical role in brain Aβ clearance. LRP1 is known to be an endocytic receptor for multiple ligands including Aβ. Conditional knock-out of Lrp1 in mouse forebrain neurons leads to increased brain Aβ levels and exacerbated amyloid plaque deposition selectively in the cortex of amyloid model APP/PS1 mice without affecting Aβ production. In vivo microdialysis studies demonstrated that Aβ clearance in brain interstitial fluid is impaired in neuronal Lrp1 knock-out mice. Because the neuronal LRP1-deletion did not affect the mRNA levels of major Aβ degrading enzymes, neprilysin and insulin-degrading enzyme, the disturbed Aβ clearance is likely due to the suppression of LRP1-mediated neuronal Aβ uptake and degradation. Together, our results demonstrate that LRP1 plays an important role in receptor-mediated clearance of Aβ and indicate that neurons not only produce but also clear Aβ.

Proteome of brain glia: the molecular basis of diverse glial phenotypes

Several different types of nonneuronal glial cells with diverse phenotypes are present in the CNS, and all have distinct indispensible functions. Although glial cells primarily provide neurons with metabolic and structural support in the healthy brain, they may switch phenotype from a "resting" to a "reactive" state in response to pathological insults. Furthermore, this reactive gliosis is an invariant feature of the pathogeneses of CNS maladies. The glial proteome serves as a signature of glial phenotype, and not only executes physiological functions, but also acts as a molecular mediator of the reactive glial phenotype. The glial proteome is also involved in intra- and intercellular communications as exemplified by glia-glia and neuron-glia interactions. The utilization of authoritative proteomic tools and the bioinformatic analyses have helped to profile the brain glial proteome and explore the molecular mechanisms of diverse glial phenotypes. Furthermore, technologic innovations have equipped the field of "glioproteomics" with refined tools for studies of the expression, interaction, and function of glial proteins in the healthy and in the diseased CNS. Glioproteomics is expected to contribute to the elucidation of the molecular mechanisms of CNS pathophysiology and to the discovery of biomarkers and theragnostic targets in CNS disorders.

Astrocytes reassessment - an evolving concept part one: embryology, biology, morphology and reactivity

The goal of this review is to integrate - in its two parts - the considerable amount of information that has accumulated during these recent years over the morphology, biology and functions of astrocytes - first part - and to illustrate the active role of these cells in pathophysiological processes implicated in various psychiatric and neurologic disorders – second part.

Targeting glia cells: novel perspectives for the treatment of neuropsychiatric diseases

Neuropsychiatric disorders are devastating mental illnesses with a high economic burden. The additional morbidity associated with social issues that arises along with the course of these diseases increases the need for a clear understanding of their etiopathogenesis to allow an implementation of novel pharmacological strategies. Yet a poor knowledge about interactions occurring at the glia-neuron interface in health and disease still hampers innovative discoveries, despite the fact that glia cells have been long described to actively participate in the regulation of brain circuits.

The purpose of this review was to collect the scattered literature on the involvement of glia cells in neuropsychiatric disorders and to describe how also these cells besides neurons might be responsive to current pharmacological interventions. We hope thereby to offer alternative approaches for investigations that may open avenues to search for new potential targets for drug discovery.

Human astrocytes derived from glial restricted progenitors support regeneration of the injured spinal cord

Cellular transplantation using neural stem cells and progenitors is a promising therapeutic strategy that has the potential to replace lost cells, modulate the injury environment, and create a permissive environment for the regeneration of injured host axons. Our research has focused on the use of human glial restricted progenitors (hGRP) and derived astrocytes. In the current study, we examined the morphological and phenotypic properties of hGRP prepared from the fetal central nervous system by clinically-approved protocols, compared with astrocytes derived from hGRP prepared by treatment with ciliary neurotrophic factor or bone morphogenetic protein 4. These differentiation protocols generated astrocytes that showed morphological differences and could be classified along an immature to mature spectrum, respectively. Despite these differences, the cells retained morphological and phenotypic plasticity upon a challenge with an alternate differentiation protocol. Importantly, when hGRP and derived astrocytes were transplanted acutely into a cervical dorsal column lesion, they survived and promoted regeneration of long ascending host sensory axons into the graft/lesion site, with no differences among the groups. Further, hGRP taken directly from frozen stocks behaved similarly and also supported regeneration of host axons into the lesion. Our results underscore the dynamic and permissive properties of human fetal astrocytes to promote axonal regeneration. They also suggest that a time-consuming process of pre-differentiation may not be necessary for therapeutic efficacy, and that the banking of large quantities of readily available hGRP can be an appropriate source of permissive cells for transplantation.

SPARC-like 1 (SC1) is a diversely expressed and developmentally regulated matricellular protein that does not compensate for the absence of SPARC in the CNS

SPARC-like 1 (SC1) is a member of the SPARC family of matricellular proteins that has been implicated in the regulation of processes such as cell migration, proliferation, and differentiation. Here we show that SC1 exhibits remarkably diverse and dynamic expression in the developing and adult nervous system. During development, SC1 localizes to radial glia and pial-derived structures, including the vasculature, choroid plexus, and pial membranes. SC1 is not downregulated in postnatal development, but its expression shifts to distinct time windows in subtypes of glia and neurons, including astrocytes, large projection neurons, Bergmann glia, Schwann cells, and ganglionic satellite cells. In addition, SC1 expression levels and patterns are not altered in the SPARC null mouse, suggesting that SC1 does not compensate for the absence of SPARC. We conclude that SC1 and SPARC may share significant homology, but are likely to have distinct but complementary roles in nervous system development.

Hypothalamic astrocytes in obesity

Obesity is characterized by a chronic and low-grade inflammation in tissues including the hypothalamus. Hypothalamic inflammation is considered an early and determining factor for the onset of obesity, a factor that occurs even before body weight gain. Within the hypothalamus, microglia and astrocytes produce cytokines that drive inflammatory responses. Astrocytes are directly affected by nutrient excess and might play a unique role in promoting hypothalamic inflammatory responses in obesity. This article reviews evidence supporting the role of hypothalamic astrocytes in obesity, and suggests a new approach for neuroendocrine research designed to reveal pathogenesis and develop novel treatment strategies against obesity.

Antidepressants act directly on astrocytes: evidences and functional consequences

Post-mortem histopathological studies report on reduced glial cell numbers in various frontolimbic areas of depressed patients implying that glial loss together with abnormal functioning could contribute to the pathophysiology of mood disorders. Astrocytes are regarded as the most abundant cell type in the brain and known for their housekeeping functions, but as recent developments suggest, they are also dynamic regulators of synaptogenesis, synaptic strength and stability and they control adult hippocampal neurogenesis. The primary aim of this review was to summarize the abundant experimental evidences demonstrating that antidepressant therapies have profound effect on astrocytes. Antidepressants modify astroglial physiology, morphology and by affecting gliogenesis they probably even regulate glial cell numbers. Antidepressants affect intracellular signaling pathways and gene expression of astrocytes, as well as the expression of receptors and the release of various trophic factors. We also assess the potential functional consequences of these changes on glutamate and glucose homeostasis and on synaptic communication between the neurons. We propose here a hypothesis that antidepressant treatment not only affects neurons, but also activates astrocytes, triggering them to carry out specific functions that result in the reactivation of cortical plasticity and can lead to the readjustment of abnormal neuronal networks. We argue here that these astrocyte specific changes are likely to contribute to the therapeutic effectiveness of the currently available antidepressant treatments and the better understanding of these cellular and molecular processes could help us to identify novel targets for the development of antidepressant drugs.

Is amyotrophic lateral sclerosis a primary astrocytic disease?

Amyotrophic lateral sclerosis (ALS) is thought to be due to primary involvement of motor neurons. Pathogenic mechanisms underlying its appearance are relatively well known and include inflammation, excitotoxicity, oxidative stress, endoplasmic reticulum stress, protein damage, genetic abnormalities and type of neuronal death. Although these processes have been investigated in detail in the past two decades none of them appear to be the cause of the illness. In addition several possible environmental agents have been investigated but the results, in every case, were conflicting and therefore inconclusive. However, since the motor neurons display the features of apoptosis in this illness, the possibility remains that the motor neurons die because of a hostile environment, one that is unable to sustain their health, rather than being directly targeted themselves. The above considerations lead to an examination of astrocytes, for these cells play a key role in controlling the environment of neurons. It is known that astrocytes are exquisitely plastic, adapting their metabolism and behaviour to the needs of the neurons they contact. Each population of astrocytes is therefore unique and, were one to be adversely affected at the start of a disease process, the consequences would extend to the neurons that it normally chaperoned. The disturbed relationship might involve inappropriate production and secretion of astrocytic neurotransmitters, defective transport of glutamate and impaired trophic and metabolic support of the motor neurons. In order to explain the spread of weakness and pyramidal signs in ALS patients, which is very often from one group of muscles to a neighbouring one, it is postulated that, within the spinal cord, the brainstem and the motor cortex, the disease-causing process is also spreading-in this case, from one group of astrocytes to its neighbours. A misfolded protein, possibly a prion-like protein, would be a candidate for this type of transmission.

The protective effect of astrocyte-derived 14,15-epoxyeicosatrienoic acid on hydrogen peroxide-induced cell injury in astrocyte-dopaminergic neuronal cell line co-culture

Astrocytes perform several functions that are essential for normal neuronal activity. They play a critical role in neuronal survival during ischemia and other degenerative injuries and also modulate neuronal recovery by influencing neurite outgrowth. In this study, we investigated the neuroprotective effects of astrocyte-derived 14,15-epoxyeicosatrienoic acid (14,15-EET), metabolite of arachidonic acid by cytochrome P450 epoxygenases (CYP), against oxidative stress induced by hydrogen peroxide (H(2)O(2)). We found that dopaminergic neuronal cells (N27 cell line) stimulated with two different doses of H(2)O(2) (0.1 and 1mM) for 1h showed decreased cell viability compared to the control group, while astrocytes showed less cell death after stimulation with the same doses of H(2)O(2) for 1h. Dopaminergic neuronal cells (N27 cell line) pretreated with different doses of 14,15-EET (0.1-30 μM, 30 min) before H(2)O(2) stimulation also showed increased cell viability. Furthermore, pre-treatment of the co-cultured cells with 12-(3-adamantan-1-yl-ureido)-dodecanoic acid, an inhibitor of the EET metabolizing enzyme, soluble epoxide hydrolase (sEH), before H(2)O(2) stimulation (1mM, for 1h) increased cell viability. It also increased the endogenous level of 14,15-EET in the media compared to control group. However, pretreatment with the CYP epoxygenase inhibitor miconazole (1-20 μM, 1h) before H(2)O(2) (1mM, 1h) stimulation showed decreased cell viability. Our data suggest that 14,15-EET which is released from astrocytes, enhances cell viability against oxidant-induced injury. Further understanding of the mechanism of 14,15-EET-mediated protection in dopaminergic neurons is imperative, as it could lead to novel therapeutic approaches for treating CNS neuropathologies, such as Parkinson's disease.

Subthalamic nucleus electrical stimulation modulates calcium activity of nigral astrocytes

BACKGROUND

The substantia nigra pars reticulata (SNr) is a major output nucleus of the basal ganglia, delivering inhibitory efferents to the relay nuclei of the thalamus. Pathological hyperactivity of SNr neurons is known to be responsible for some motor disorders e.g. in Parkinson's disease. One way to restore this pathological activity is to electrically stimulate one of the SNr input, the excitatory subthalamic nucleus (STN), which has emerged as an effective treatment for parkinsonian patients. The neuronal network and signal processing of the basal ganglia are well known but, paradoxically, the role of astrocytes in the regulation of SNr activity has never been studied.

PRINCIPAL FINDINGS

In this work, we developed a rat brain slice model to study the influence of spontaneous and induced excitability of afferent nuclei on SNr astrocytes calcium activity. Astrocytes represent the main cellular population in the SNr and display spontaneous calcium activities in basal conditions. Half of this activity is autonomous (i.e. independent of synaptic activity) while the other half is dependent on spontaneous glutamate and GABA release, probably controlled by the pace-maker activity of the pallido-nigral and subthalamo-nigral loops. Modification of the activity of the loops by STN electrical stimulation disrupted this astrocytic calcium excitability through an increase of glutamate and GABA releases. Astrocytic AMPA, mGlu and GABA(A) receptors were involved in this effect.

SIGNIFICANCE

Astrocytes are now viewed as active components of neural networks but their role depends on the brain structure concerned. In the SNr, evoked activity prevails and autonomous calcium activity is lower than in the cortex or hippocampus. Our data therefore reflect a specific role of SNr astrocytes in sensing the STN-GPe-SNr loops activity and suggest that SNr astrocytes could potentially feedback on SNr neuronal activity. These findings have major implications given the position of SNr in the basal ganglia network.

Astrocytes and developmental plasticity in fragile X

A growing body of research indicates a pivotal role for astrocytes at the developing synapse. In particular, astrocytes are dynamically involved in governing synapse structure, function, and plasticity. In the postnatal brain, their appearance at synapses coincides with periods of developmental plasticity when neural circuits are refined and established. Alterations in the partnership between astrocytes and neurons have now emerged as important mechanisms that underlie neuropathology. With overall synaptic function standing as a prominent link to the expression of the disease phenotype in a number of neurodevelopmental disorders and knowing that astrocytes influence synapse development and function, this paper highlights the current knowledge of astrocyte biology with a focus on their involvement in fragile X syndrome.

Clonal identity determines astrocyte cortical heterogeneity

Astrocytes are the most numerous cell type in the brain, where they are known to play multiple important functions. While there is increasing evidence of their morphological, molecular, and functional heterogeneity, it is not clear whether their positional and morphological identities are specified during brain development. We address this problem with a novel strategy to analyze cell lineages through the combinatorial expression of fluorescent proteins. Following in utero electroporation, stochastic expression of these proteins produces inheritable marks that enable the long-term in vivo tracing of glial progenitor lineages. Analyses of clonal dispersion in the adult cortex revealed unanticipated and highly specific clonal distribution patterns. In addition to the existence of clonal arrangements in specific domains, we found that different classes of astrocytes emerge from different clones. This reinforces the view that lineage origin impinges on cell heterogeneity, unveiling a new level of astrocyte diversity likely associated with specific regional functions.

Neuron-astroglial interactions in cell-fate commitment and maturation in the central nervous system

Neuron-astroglia interactions play a key role in several events of brain development, such as neuronal generation, migration, survival, and differentiation; axonal growth; and synapse formation and function. While there is compelling evidence of the effects of astrocyte factors on neurons, their effects on astrocytes have not been fully determined. In this review, we will focus on the role of neurons in astrocyte generation and maturation. Further, we highlight the great heterogeneity and diversity of astroglial and neural progenitors such as radial glia cells, and discuss the importance of the variety of cellular interactions in controlling the structural and functional organization of the brain. Finally, we present recent data on a new role of astrocytes in neuronal maturation, as mediators of the action of biolipids in the cerebral cortex. We will argue that the functional architecture of the brain depends on an intimate neuron-glia partnership, by briefly discussing the emerging view of how neuron-astrocyte dysfunctions might be associated with neurodegenerative diseases and neurological disorders.