Functional diversity of astrocytes in neural circuit regulation

Connexin-Mediated Functional and Metabolic Coupling Between Astrocytes and Neurons

The central nervous system (CNS) requires sophisticated regulation of neuronal activity. This modulation is partly accomplished by non-neuronal cells, characterized by the presence of transmembrane gap junctions (GJs) and hemichannels (HCs). This allows small molecule diffusion to guarantee neuronal synaptic activity and plasticity. Astrocytes are metabolically and functionally coupled to neurons by the uptake, binding and recycling of neurotransmitters. In addition, astrocytes release metabolites, such as glutamate, glutamine, D-serine, adenosine triphosphate (ATP) and lactate, regulating synaptic activity and plasticity by pre- and postsynaptic mechanisms. Uncoupling neuroglial communication leads to alterations in synaptic transmission that can be detrimental to neuronal circuit function and behavior. Therefore, understanding the pathways and mechanisms involved in this intercellular communication is fundamental for the search of new targets that can be used for several neurological disease treatments. This review will focus on molecular mechanisms mediating physiological and pathological coupling between astrocytes and neurons through GJs and HCs.

Kir4.1-Dependent Astrocyte-Fast Motor Neuron Interactions Are Required for Peak Strength

Diversified neurons are essential for sensorimotor function, but whether astrocytes become specialized to optimize circuit performance remains unclear. Large fast α-motor neurons (FαMNs) of spinal cord innervate fast-twitch muscles that generate peak strength. We report that ventral horn astrocytes express the inward-rectifying K⁺ channel Kir4.1 (a.k.a. Kcnj10) around MNs in a VGLUT1-dependent manner. Loss of astrocyte-encoded Kir4.1 selectively altered FαMN size and function and led to reduced peak strength. Overexpression of Kir4.1 in astrocytes was sufficient to increase MN size through activation of the PI3K/mTOR/pS6 pathway. Kir4.1 was downregulated cell autonomously in astrocytes derived from amyotrophic lateral sclerosis (ALS) patients with SOD1 mutation. However, astrocyte Kir4.1 was dispensable for FαMN survival even in the mutant SOD1 background. These findings show that astrocyte Kir4.1 is essential for maintenance of peak strength and suggest that Kir4.1 downregulation might uncouple symptoms of muscle weakness from MN cell death in diseases like ALS.

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Kir4.1 is upregulated in astrocytes around high-activity alpha motor neurons (MNs)

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Astrocyte Kir4.1 KO caused decreased peak strength without alpha MN loss

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ALS patient-derived astrocytes show cell-autonomous Kir4.1 downregulation

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Astrocyte Kir4.1 regulates MN size through PI3K/mTOR/pS6 activation

Computational Models for Calcium-Mediated Astrocyte Functions

The computational neuroscience field has heavily concentrated on the modeling of neuronal functions, largely ignoring other brain cells, including one type of glial cell, the astrocytes. Despite the short history of modeling astrocytic functions, we were delighted about the hundreds of models developed so far to study the role of astrocytes, most often in calcium dynamics, synchronization, information transfer, and plasticity in vitro, but also in vascular events, hyperexcitability, and homeostasis. Our goal here is to present the state-of-the-art in computational modeling of astrocytes in order to facilitate better understanding of the functions and dynamics of astrocytes in the brain. Due to the large number of models, we concentrated on a hundred models that include biophysical descriptions for calcium signaling and dynamics in astrocytes. We categorized the models into four groups: single astrocyte models, astrocyte network models, neuron-astrocyte synapse models, and neuron-astrocyte network models to ease their use in future modeling projects. We characterized the models based on which earlier models were used for building the models and which type of biological entities were described in the astrocyte models. Features of the models were compared and contrasted so that similarities and differences were more readily apparent. We discovered that most of the models were basically generated from a small set of previously published models with small variations. However, neither citations to all the previous models with similar core structure nor explanations of what was built on top of the previous models were provided, which made it possible, in some cases, to have the same models published several times without an explicit intention to make new predictions about the roles of astrocytes in brain functions. Furthermore, only a few of the models are available online which makes it difficult to reproduce the simulation results and further develop the models. Thus, we would like to emphasize that only via reproducible research are we able to build better computational models for astrocytes, which truly advance science. Our study is the first to characterize in detail the biophysical and biochemical mechanisms that have been modeled for astrocytes.

Peripheral Nerve Injury-Induced Astrocyte Activation in Spinal Ventral Horn Contributes to Nerve Regeneration

Accumulating evidences suggest that peripheral nerve injury (PNI) may initiate astrocytic responses in the central nervous system (CNS). However, the response of astrocytes in the spinal ventral horn and its potential role in nerve regeneration after PNI remain unclear. Herein, we firstly illustrated that astrocytes in the spinal ventral horn were dramatically activated in the early stage following sciatic nerve injury, and these profiles were eliminated in the chronic stage. Additionally, we found that the expression of neurotrophins, including brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3), also accompanied with astrocyte activation. In comparison with the irreversible transected subjects, astrocyte activation and the neurotrophic upregulation in the early stage were more drastic in case the transected nerve was rebridged immediately after injury. Furthermore, administering fluorocitrate to inhibit astrocyte activation resulted in decreased neurotrophin expression in the spinal ventral horn and delayed axonal regeneration in the nerve as well as motor function recovery. Overall, the present study indicates that peripheral nerve injury can initiate astrocyte activation accompanied with neurotrophin upregulation in the spinal ventral horn. The above responses mainly occur in the early stage of PNI and may contribute to nerve regeneration and motor function recovery.

Influence of white matter injury on gray matter reactive gliosis upon stab wound in the adult murine cerebral cortex

Traumatic brain injury frequently affects the cerebral cortex, yet little is known about the differential effects that occur if only the gray matter (GM) is damaged or if the injury also involves the white matter (WM). To tackle this important question and directly compare similarities and differences in reactive gliosis, we performed stab wound injury affecting GM and WM (GM+) and one restricted to the GM (GM-) in the adult murine cerebral cortex. First, we examined glial reactivity in the regions affected (WM and GM) and determined the influence of WM injury on reactive gliosis in the GM comparing the same area in the two injury paradigms. In the GM+ injury microglia proliferation is increased in the WM compared with GM, while proliferating astrocytes are more abundant in the GM than in the WM. Interestingly, WM lesion exerted a strong influence on the proliferation of the GM glial cells that was most pronounced at early stages, 3 days post lesion. While astrocyte proliferation was increased, NG2 glia proliferation was decreased in the GM+ compared with GM- lesion condition. Importantly, these differences were not observed when a lesion of the same size affected only the GM. Unbiased proteomic analyses further corroborate our findings in support of a profound difference in GM reactivity when WM is also injured and revealed MIF as a key regulator of NG2 glia proliferation.

Optic neuropathies: the tip of the neurodegeneration iceberg

The optic nerve and the cells that give origin to its 1.2 million axons, the retinal ganglion cells (RGCs), are particularly vulnerable to neurodegeneration related to mitochondrial dysfunction. Optic neuropathies may range from non-syndromic genetic entities, to rare syndromic multisystem diseases with optic atrophy such as mitochondrial encephalomyopathies, to age-related neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease where optic nerve involvement has, until recently, been a relatively overlooked feature. New tools are available to thoroughly investigate optic nerve function, allowing unparalleled access to this part of the central nervous system. Understanding the molecular pathophysiology of RGC neurodegeneration and optic atrophy, is key to broadly understanding the pathogenesis of neurodegenerative disorders, for monitoring their progression in describing the natural history, and ultimately as outcome measures to evaluate therapies. In this review, the different layers, from molecular to anatomical, that may contribute to RGC neurodegeneration and optic atrophy are tackled in an integrated way, considering all relevant players. These include RGC dendrites, cell bodies and axons, the unmyelinated retinal nerve fiber layer and the myelinated post-laminar axons, as well as olygodendrocytes and astrocytes, looked for unconventional functions. Dysfunctional mitochondrial dynamics, transport, homeostatic control of mitobiogenesis and mitophagic removal, as well as specific propensity to apoptosis may target differently cell types and anatomical settings. Ultimately, we can envisage new investigative approaches and therapeutic options that will speed the early diagnosis of neurodegenerative diseases and their cure.

BMP-Responsive Protease HtrA1 Is Differentially Expressed in Astrocytes and Regulates Astrocytic Development and Injury Response

Astrocytes perform a wide array of physiological functions, including structural support, ion exchange, and neurotransmitter uptake. Despite this diversity, molecular markers that label subpopulations of astrocytes are limited, and mechanisms that generate distinct astrocyte subtypes remain unclear. Here we identified serine protease high temperature requirement A 1 (HtrA1), a bone morphogenetic protein 4 signaling regulated protein, as a novel marker of forebrain astrocytes, but not of neural stem cells, in adult mice of both sexes. Genetic deletion of HtrA1 during gliogenesis accelerates astrocyte differentiation. In addition, ablation of HtrA1 in cultured astrocytes leads to altered chondroitin sulfate proteoglycan expression and inhibition of neurite extension, along with elevated levels of transforming growth factor-β family proteins. Brain injury induces HtrA1 expression in reactive astrocytes, and loss of HtrA1 leads to an impairment in wound closure accompanied by increased proliferation of endothelial and immune cells. Our findings demonstrate that HtrA1 is differentially expressed in adult mouse forebrain astrocytes, and that HtrA1 plays important roles in astrocytic development and injury response.SIGNIFICANCE STATEMENT Astrocytes, an abundant cell type in the brain, perform a wide array of physiological functions. Although characterized as morphologically and functionally diverse, molecular markers that label astrocyte subtypes or signaling pathways that lead to their diversity remain limited. Here, after examining the expression profile of astrocytes generated in response to bone morphogenetic protein signaling, we identify high temperature requirement A 1 (HtrA1) as an astrocyte-specific marker that is differentially expressed in distinct adult mouse brain regions. HtrA1 is a serine protease that has been linked to cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy, a small blood vessel disease in humans. Understanding the role of HtrA1 during development and after injury will provide insights into how distinct astrocyte populations are generated and their unique roles in injury and disease.

Functional and structural damage of neurons by innate immune mechanisms during neurodegeneration

Over the past decades, our view on neurodegenerative diseases has been mainly centered around neurons and their networks. Only recently it became evident that immunological processes arise alongside degenerating neurons, raising the question whether these represent just meaningless bystander reactions or in turn, contribute to pathogenesis and disease symptoms. When considering any effect of inflammatory events on the CNS one has to consider the site, duration and nature of immune activation. Likewise, one has to distinguish between mechanisms which directly impact the neuronal compartment and indirect mechanisms, which affect cells that are important for neuronal functioning and survival. As discussed in this review, both types of mechanisms may be present at the same time and additively or synergistically lead to neuronal demise. Inflammatory mediators released by the principle innate immune cells of the brain, microglia and astrocytes, can compromise the function and structure of neurons, thereby playing important roles in the pathogenesis of neurodegenerative diseases.

Interneuron-specific signaling evokes distinctive somatostatin-mediated responses in adult cortical astrocytes

The signaling diversity of GABAergic interneurons to post-synaptic neurons is crucial to generate the functional heterogeneity that characterizes brain circuits. Whether this diversity applies to other brain cells, such as the glial cells astrocytes, remains unexplored. Using optogenetics and two-photon functional imaging in the adult mouse neocortex, we here reveal that parvalbumin- and somatostatin-expressing interneurons, two key interneuron classes in the brain, differentially signal to astrocytes inducing weak and robust GABA_B receptor-mediated Ca²⁺ elevations, respectively. Furthermore, the astrocyte response depresses upon parvalbumin interneuron repetitive stimulations and potentiates upon somatostatin interneuron repetitive stimulations, revealing a distinguished astrocyte plasticity. Remarkably, the potentiated response crucially depends on the neuropeptide somatostatin, released by somatostatin interneurons, which activates somatostatin receptors at astrocytic processes. Our study unveils, in the living brain, a hitherto unidentified signaling specificity between interneuron subtypes and astrocytes opening a new perspective into the role of astrocytes as non-neuronal components of inhibitory circuits.

Single-Prolonged Stress: A Review of Two Decades of Progress in a Rodent Model of Post-traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is a common, costly, and often debilitating psychiatric condition. However, the biological mechanisms underlying this disease are still largely unknown or poorly understood. Considerable evidence indicates that PTSD results from dysfunction in highly-conserved brain systems involved in stress, anxiety, fear, and reward. Pre-clinical models of traumatic stress exposure are critical in defining the neurobiological mechanisms of PTSD, which will ultimately aid in the development of new treatments for PTSD. Single prolonged stress (SPS) is a pre-clinical model that displays behavioral, molecular, and physiological alterations that recapitulate many of the same alterations observed in PTSD, illustrating its validity and giving it utility as a model for investigating post-traumatic adaptations and pre-trauma risk and protective factors. In this manuscript, we review the present state of research using the SPS model, with the goals of (1) describing the utility of the SPS model as a tool for investigating post-trauma adaptations, (2) relating findings using the SPS model to findings in patients with PTSD, and (3) indicating research gaps and strategies to address them in order to improve our understanding of the pathophysiology of PTSD.

The special relationship: glia-neuron interactions in the neuroendocrine hypothalamus

Natural fluctuations in physiological conditions require adaptive responses involving rapid and reversible structural and functional changes in the hypothalamic neuroendocrine circuits that control homeostasis. Here, we discuss the data that implicate hypothalamic glia in the control of hypothalamic neuroendocrine circuits, specifically neuron-glia interactions in the regulation of neurosecretion as well as neuronal excitability. Mechanistically, the morphological plasticity displayed by distal processes of astrocytes, pituicytes and tanycytes modifies the geometry and diffusion properties of the extracellular space. These changes alter the relationship between glial cells of the hypothalamus and adjacent neuronal elements, especially at specialized intersections such as synapses and neurohaemal junctions. The structural alterations in turn lead to functional plasticity that alters the release and spread of neurotransmitters, neuromodulators and gliotransmitters, as well as the activity of discrete glial signalling pathways that mediate feedback by peripheral signals to the hypothalamus. An understanding of the contributions of these and other non-neuronal cell types to hypothalamic neuroendocrine function is thus critical both to understand physiological processes such as puberty, the maintenance of bodily homeostasis and ageing and to develop novel therapeutic strategies for dysfunctions of these processes, such as infertility and metabolic disorders.

Biomaterials Developments for Brain Tissue Engineering

The Central Nervous System (CNS) is a highly complex organ that works as the control centre of the body, managing vital and non-vital functions. Neuro-diseases can lead to the degeneration of neural tissue, breakage of the neuronal networks which can affect vital functions and originate cognitive deficits. The complexity of the neural networks, their components and the low regenerative capacity of the CNS are on the basis for the lack of recovery, having the need for therapies that can promote tissue repair and recovery. Most brain processes are mediated through molecules (e.g. cytokines, neurotransmitters) and cells response accordingly and to surrounding cues, either biological or physical, which offers molecule administration and/or cell transplantation a great potential for use in brain recovery. Biomaterials and in particular, of natural-origin are attractive candidates owed to their intrinsic biological cues and biocompatibility and degradability. Through the use of biomaterials, it is possible to protect the cells/molecules from body clearance, enzymatic degradation while maintaining the components in a place of interest. Moreover, by means of combining several components, it is possible to obtain a more targeted and controlled delivery, to image the biomaterial implantation and its degradation over time and tackling simultaneously occurring events (cell death and inflammation) in brain diseases. In this chapter, it is reviewed some brain-affecting diseases and the current developments on tissue engineering approaches for a functional recovery of the brain from those diseases.

The diversity and disparity of the glial scar

Injury or disease to the CNS results in multifaceted cellular and molecular responses. One such response, the glial scar, is a structural formation of reactive glia around an area of severe tissue damage. While traditionally viewed as a barrier to axon regeneration, beneficial functions of the glial scar have also been recently identified. In this Perspective, we discuss the divergent roles of the glial scar during CNS regeneration and explore the possibility that these disparities are due to functional heterogeneity within the cells of the glial scar-specifically, astrocytes, NG2 glia and microglia.

Apoptosis and necroptosis of mouse hippocampal and parenchymal astrocytes, microglia and neurons caused by Angiostrongylus cantonensis infection

BACKGROUND

Angiostrongylus cantonensis has been the only parasite among Angiostrongylidae to cause human central nervous system infection characterized by eosinophilic meningitis or meningoencephalitis. The mechanism of the extensive neurological impairments of hosts caused by A. cantonensis larvae remains unclear. The aim of the present study was to investigate apoptosis, necroptosis and autophagy in the brains of mice infected with A. cantonensis, which will be valuable for better understanding the pathogenesis of angiostrongyliasis cantonensis.

METHODS

Functional and histological neurological impairments of brain tissues from mice infected with A. cantonensis were measured by the Morris water maze test and haematoxylin and eosin (H&E) staining, respectively. The transcriptional and translational levels of apoptosis-, necroptosis- and autophagy-related genes were quantified by quantitative real-time polymerase chain reaction (RT-PCR), and assessed by western blot and immunohistochemistry (IHC) analysis. Apoptotic and necroptotic cells and their distributions in infected brain tissues were analysed by flow cytometry and transmission electron microscopy (TEM).

RESULTS

Inflammatory response in the central nervous system deteriorated as A. cantonensis infection evolved, as characterized by abundant inflammatory cell infiltration underneath the meninges, which peaked at 21 days post-infection (dpi). The learning and memory capacities of the mice were significantly decreased at 14 dpi, indicating prominent impairment of their cognitive functions. Compared with those of the control group, the mRNA levels of caspase-3, -4, -6, and RIP3 and the protein levels of caspase-4, cleaved caspase-3, cleaved caspase-6, RIP3, and pRIP3 were obviously elevated. However, no changes in the mRNA or protein levels of FADD, Beclin-1 or LC3B were evident, indicating that apoptosis and necroptosis, but not autophagy, occurred in the brain tissues of mice infected with A. cantonensis. The quantitative RT-PCR, western blot, IHC, flow cytometry and TEM results further revealed the apoptotic and necroptotic microglia, astrocytes and neurons in the parenchymal and hippocampal regions of infected mice.

CONCLUSIONS

To our knowledge, we showed for the first time that A. cantonensis infection causes the apoptosis and necroptosis of microglia and astrocytes in the parenchymal and hippocampal regions of host brain tissues, further demonstrating the pathogenesis of A. cantonensis infection and providing potential therapeutic targets for the management of angiostrongyliasis.

Involvement of Astrocytes in Alzheimer's Disease from a Neuroinflammatory and Oxidative Stress Perspective

Alzheimer disease (AD) is a frequent and devastating neurodegenerative disease in humans, but still no curative treatment has been developed. Although many explicative theories have been proposed, precise pathophysiological mechanisms are unknown. Due to the importance of astrocytes in brain homeostasis they have become interesting targets for the study of AD. Changes in astrocyte function have been observed in brains from individuals with AD, as well as in AD in vitro and in vivo animal models. The presence of amyloid beta (Aβ) has been shown to disrupt gliotransmission, neurotransmitter uptake, and alter calcium signaling in astrocytes. Furthermore, astrocytes express apolipoprotein E and are involved in the production, degradation and removal of Aβ. As well, changes in astrocytes that precede other pathological characteristics observed in AD, point to an early contribution of astroglia in this disease. Astrocytes participate in the inflammatory/immune responses of the central nervous system. The presence of Aβ activates different cell receptors and intracellular signaling pathways, mainly the advanced glycation end products receptor/nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway, responsible for the transcription of pro-inflammatory cytokines and chemokines in astrocytes. The release of these pro-inflammatory agents may induce cellular damage or even stimulate the production of Aβ in astrocytes. Additionally, Aβ induces the appearance of oxidative stress (OS) and production of reactive oxygen species and reactive nitrogen species in astrocytes, affecting among others, intracellular calcium levels, NADPH oxidase (NOX), NF-κB signaling, glutamate uptake (increasing the risk of excitotoxicity) and mitochondrial function. Excessive neuroinflammation and OS are observed in AD, and astrocytes seem to be involved in both. The Aβ/NF-κB interaction in astrocytes may play a central role in these inflammatory and OS changes present in AD. In this paper, we also discuss therapeutic measures highlighting the importance of astrocytes in AD pathology. Several new therapeutic approaches involving phenols (curcumin), phytoestrogens (genistein), neuroesteroids and other natural phytochemicals have been explored in astrocytes, obtaining some promising results regarding cognitive improvements and attenuation of neuroinflammation. Novel strategies comprising astrocytes and aimed to reduce OS in AD have also been proposed. These include estrogen receptor agonists (pelargonidin), Bambusae concretio Salicea, Monascin, and various antioxidatives such as resveratrol, tocotrienol, anthocyanins, and epicatechin, showing beneficial effects in AD models.

Reactive Astrocytes in Brain Metastasis

Brain metastasis, the secondary growth of malignant cells within the central nervous system (CNS), exceeds the incidence of primary brain tumors (i.e., gliomas) by tenfold and are seemingly on the rise owing to the emergence of novel targeted therapies that are more effective in controlling extracranial disease relatively to intracranial lesions. Despite the fact that metastasis to the brain poses a unmet clinical problem, with afflicted patients carrying significant morbidity and a fatal prognosis, our knowledge as to how metastatic cells manage to adapt to the tissue environment of the CNS remains limited. Answering this question could pave the way for novel and more specific therapeutic modalities in brain metastasis by targeting the specific makeup of the brain metastatic niche. In regard to this, astrocytes have emerged as the major host cell type that cancer cells encounter and interact with during brain metastasis formation. Similarly to other CNS disorders, astrocytes become reactive and respond to the presence of cancer cells by changing their phenotype and significantly influencing the outcome of disseminated cancer cells within the CNS. Here, we summarize the current knowledge on the contribution of reactive astrocytes in brain metastasis by focusing on the signaling pathways and types of interactions that play a crucial part in the communication with cancer cells and how these could be translated into innovative therapies.

Phagocytic clearance of presynaptic dystrophies by reactive astrocytes in Alzheimer's disease

Reactive astrogliosis, a complex process characterized by cell hypertrophy and upregulation of components of intermediate filaments, is a common feature in brains of Alzheimer's patients. Reactive astrocytes are found in close association with neuritic plaques; however, the precise role of these glial cells in disease pathogenesis is unknown. In this study, using immunohistochemical techniques and light and electron microscopy, we report that plaque-associated reactive astrocytes enwrap, engulf and may digest presynaptic dystrophies in the hippocampus of amyloid precursor protein/presenilin-1 (APP/PS1) mice. Microglia, the brain phagocytic population, was apparently not engaged in this clearance. Phagocytic reactive astrocytes were present in 35% and 67% of amyloid plaques at 6 and 12 months of age, respectively. The proportion of engulfed dystrophic neurites was low, around 7% of total dystrophies around plaques at both ages. This fact, along with the accumulation of dystrophic neurites during disease course, suggests that the efficiency of the astrocyte phagocytic process might be limited or impaired. Reactive astrocytes surrounding and engulfing dystrophic neurites were also detected in the hippocampus of Alzheimer's patients by confocal and ultrastructural analysis. We posit that the phagocytic activity of reactive astrocytes might contribute to clear dysfunctional synapses or synaptic debris, thereby restoring impaired neural circuits and reducing the inflammatory impact of damaged neuronal parts and/or limiting the amyloid pathology. Therefore, potentiation of the phagocytic properties of reactive astrocytes may represent a potential therapy in Alzheimer's disease.

Mechanisms of radial glia progenitor cell lineage progression

The mammalian cerebral cortex is responsible for higher cognitive functions such as perception, consciousness, and acquiring and processing information. The neocortex is organized into six distinct laminae, each composed of a rich diversity of cell types which assemble into highly complex cortical circuits. Radial glia progenitors (RGPs) are responsible for producing all neocortical neurons and certain glia lineages. Here, we discuss recent discoveries emerging from clonal lineage analysis at the single RGP cell level that provide us with an inaugural quantitative framework of RGP lineage progression. We further discuss the importance of the relative contribution of intrinsic gene functions and non‐cell‐autonomous or community effects in regulating RGP proliferation behavior and lineage progression.

Roles of Glial Cells in Sculpting Inhibitory Synapses and Neural Circuits

Glial cells are essential for every aspect of normal neuronal development, synapse formation, and function in the central nervous system (CNS). Astrocytes secrete a variety of factors that regulate synaptic connectivity and circuit formation. Microglia also modulate synapse development through phagocytic activity. Most of the known actions of CNS glial cells are limited to roles at excitatory synapses. Nevertheless, studies have indicated that both astrocytes and microglia shape inhibitory synaptic connections through various mechanisms, including release of regulatory molecules, direct contact with synaptic terminals, and utilization of mediators in the extracellular matrix. This review summarizes recent investigations into the mechanisms underlying CNS glial cell-mediated inhibitory synapse development.

Strong sonic hedgehog signaling in the mouse ventral spinal cord is not required for oligodendrocyte precursor cell (OPC) generation but is necessary for correct timing of its generation

In the mouse neural tube, sonic hedgehog (Shh) secreted from the floor plate (FP) and the notochord (NC) regulates ventral patterning of the neural tube, and later is essential for the generation of oligodendrocyte precursor cells (OPCs). During early development, the NC is adjacent to the neural tube and induces ventral domains in it, including the FP. In the later stage of development, during gliogenesis in the spinal cord, the pMN domain receives strong Shh signaling input. While this is considered to be essential for the generation of OPCs, the actual role of this strong input in OPC generation remains unclear. Here we studied OPC generation in bromi mutant mice which show abnormal ciliary structure. Shh signaling occurs within cilia and has been reported to be weak in bromi mutants. At E11.5, accumulation of Patched1 mRNA, a Shh signaling reporter, is observed in the pMN domain of wild type but not bromi mutants, whereas expression of Gli1 mRNA, another Shh reporter, disappeared. Thus, Shh signaling input to the pMN domain at E12.5 was reduced in bromi mutant mice. In these mutants, induction of the FP structure was delayed and its size was reduced compared to wild type mice. Furthermore, while the p3 and pMN domains were induced, the length of the Nkx2.2-positive region and the number of Olig2-positive cells decreased. The number of OPCs was also significantly decreased in the E12.5 and E14.5 bromi mutant spinal cord. In contrast, motor neuron (MN) production, detected by HB9 expression, significantly increased. It is likely that the transition from MN production to OPC generation in the pMN domain is impaired in bromi mutant mice. These results suggest that strong Shh input to the pMN domain is not required for OPC generation but is essential for producing a sufficient number of OPCs.

Oligodendroglia-lineage cells in brain plasticity, homeostasis and psychiatric disorders

Adult oligodendrocyte progenitor cells are uniformly distributed in both gray and white matter, displaying robust proliferative and migratory potential during health and disease. Recently, developments in new experimental approaches have brought about several novel insights about NG2-glia and myelinating oligodendrocytes, indicating a diverse toolkit of functions in experience-dependent myelination and homeostasis in the adult CNS. In this review, we summarize some of the topical studies that highlight newly emerging findings implicating oligodendroglia-lineage cells in brain plasticity, homeostasis and pathophysiology of neuropsychiatric disorders.

Coordinated morphogenesis of neurons and glia

Glia adopt remarkable shapes that are tightly coordinated with the morphologies of their neuronal partners. To achieve these precise shapes, glia and neurons exhibit coordinated morphological changes on the time scale of minutes and on size scales ranging from nanometers to hundreds of microns. Here, we review recent studies that reveal the highly dynamic, localized morphological changes of mammalian neuron-glia contacts. We then explore the power of Drosophila and C. elegans models to study coordinated changes at defined neuron-glia contacts, highlighting the use of innovative genetic and imaging tools to uncover the molecular mechanisms responsible for coordinated morphogenesis of neurons and glia.

Selective expression of eGFP in mouse perivascular astrocytes by modification of the Mlc1 gene using T2A-based ribosome skipping

Perivascular astrocyte end feet closely juxtapose cerebral blood vessels to regulate important developmental and physiological processes including endothelial cell proliferation and sprouting as well as the formation of the blood-brain barrier (BBB). The mechanisms underlying these events remain largely unknown due to a lack of experimental models for identifying perivascular astrocytes and distinguishing these cell types from other astroglial populations. Megalencephalic leukoencephalopathy with subcortical cysts 1 (Mlc1) is a transmembrane protein that is expressed in perivascular astrocyte end feet where it controls BBB development and homeostasis. On the basis of this knowledge, we used T2A peptide-skipping strategies to engineer a knock-in mouse model in which the endogenous Mlc1 gene drives expression of enhanced green fluorescent protein (eGFP), without impacting expression of Mlc1 protein. Analysis of fetal, neonatal and adult Mlc1-eGFP knock-in mice revealed a dynamic spatiotemporal expression pattern of eGFP in glial cells, including nestin-expressing neuroepithelial cells during development and glial fibrillary acidic protein (GFAP)-expressing perivascular astrocytes in the postnatal brain. EGFP was not expressed in neurons, microglia, oligodendroglia, or cerebral vascular cells. Analysis of angiogenesis in the neonatal retina also revealed enriched Mlc1-driven eGFP expression in perivascular astrocytes that contact sprouting blood vessels and regulate blood-retinal barrier permeability. A cortical injury model revealed that Mlc1-eGFP expression is progressively induced in reactive astrocytes that form a glial scar. Hence, Mlc1-eGFP knock-in mice are a new and powerful tool to identify perivascular astrocytes in the brain and retina and characterize how these cell types regulate cerebral blood vessel functions in health and disease.

Astrocytes release prostaglandin E2 to modify respiratory network activity

Previously (Forsberg et al., 2016), we revealed that prostaglandin E2 (PGE2), released during hypercapnic challenge, increases calcium oscillations in the chemosensitive parafacial respiratory group (pFRG/RTN). Here, we demonstrate that pFRG/RTN astrocytes are the PGE2 source. Two distinct astrocyte subtypes were found using transgenic mice expressing GFP and MrgA1 receptors in astrocytes. Although most astrocytes appeared dormant during time-lapse calcium imaging, a subgroup displayed persistent, rhythmic oscillating calcium activity. These active astrocytes formed a subnetwork within the respiratory network distinct from the neuronal network. Activation of exogenous MrgA1Rs expressed in astrocytes tripled astrocytic calcium oscillation frequency in both the preBötzinger complex and pFRG/RTN. However, neurons in the preBötC were unaffected, whereas neuronal calcium oscillatory frequency in pFRG/RTN doubled. Notably, astrocyte activation in pFRG/RTN triggered local PGE2 release and blunted the hypercapnic response. Thus, astrocytes play an active role in respiratory rhythm modulation, modifying respiratory-related behavior through PGE2 release in the pFRG/RTN.

Harmine produces antidepressant-like effects via restoration of astrocytic functions

Depression is a world-wide disease with no effective therapeutic methods. Increasing evidence indicates that astrocytic pathology contributes to the formation of depression. In this study, we investigated the effects of harmine, a natural β-carboline alkaloid and potent hallucinogen, known to modulate astrocytic glutamate transporters, on chronic unpredictable stress (CUS)-induced depressive-like behaviors and astrocytic dysfunctions. Results showed that harmine treatment (10, 20mg/kg) protected the mice against the CUS-induced increases in the immobile time in the tail suspension test (TST) and forced swimming test (FST), and also reversed the reduction in sucrose intake in the sucrose preference experiment. Harmine treatment (20mg/kg) prevented the reductions in brain-derived neurotrophic factor (BDNF) protein levels and hippocampal neurogenesis induced by CUS. In addition, harmine treatment (20mg/kg) increased the protein expression levels of glutamate transporter 1 (GLT-1) and prevented the CUS-induced decreases in glial fibrillary acidic protein (GFAP) protein expressions in the prefrontal cortex and hippocampus, suggesting that restoration of astrocytic functions may be a potential mechanism underlying the antidepressant-like effects of harmine. This opinion was proved by the results that administration of mice with l-Alpha-Aminoadipic Acid (L-AAA), a gliotoxin specific for astrocytes, attenuated the antidepressant-like effects of harmine, and prevented the improvement effects of harmine on BDNF protein levels and hippocampal neurogenesis. These results provide further evidence to confirm that astrocytic dysfunction contributes critically to the development of depression and that harmine exerts antidepressant-like effects likely through restoration of astrocytic functions.

Embracing oligodendrocyte diversity in the context of perinatal injury

Emerging evidence is fueling a new appreciation of oligodendrocyte diversity that is overturning the traditional view that oligodendrocytes are a homogenous cell population. Oligodendrocytes of distinct origins, maturational stages, and regional locations may differ in their functional capacity or susceptibility to injury. One of the most unique qualities of the oligodendrocyte is its ability to produce myelin. Myelin abnormalities have been ascribed to a remarkable array of perinatal brain injuries, with concomitant oligodendrocyte dysregulation. Within this review, we discuss new insights into the diversity of the oligodendrocyte lineage and highlight their relevance in paradigms of perinatal brain injury. Future therapeutic development will be informed by comprehensive knowledge of oligodendrocyte pathophysiology that considers the particular facets of heterogeneity that this lineage exhibits.

CNS inflammation and neurodegeneration

There is an increasing recognition that inflammation plays a critical role in neurodegenerative diseases of the CNS, including Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and the prototypic neuroinflammatory disease multiple sclerosis (MS). Differential immune responses involving the adaptive versus the innate immune system are observed at various stages of neurodegenerative diseases, and may not only drive disease processes but could serve as therapeutic targets. Ongoing investigations into the specific inflammatory mechanisms that play roles in disease causation and progression have revealed lessons about inflammation-driven neurodegeneration that can be applied to other neurodegenerative diseases. An increasing number of immunotherapeutic strategies that have been successful in MS are now being applied to other neurodegenerative diseases. Some approaches suppress CNS immune mechanisms, while others harness the immune system to clear deleterious products and cells. This Review focuses on the mechanisms by which inflammation, mediated either by the peripheral immune response or by endogenous CNS immune mechanisms, can affect CNS neurodegeneration.

Neural Circuit-Specialized Astrocytes: Transcriptomic, Proteomic, Morphological, and Functional Evidence

Astrocytes are ubiquitous in the brain and are widely held to be largely identical. However, this view has not been fully tested, and the possibility that astrocytes are neural circuit specialized remains largely unexplored. Here, we used multiple integrated approaches, including RNA sequencing (RNA-seq), mass spectrometry, electrophysiology, immunohistochemistry, serial block-face-scanning electron microscopy, morphological reconstructions, pharmacogenetics, and diffusible dye, calcium, and glutamate imaging, to directly compare adult striatal and hippocampal astrocytes under identical conditions. We found significant differences in electrophysiological properties, Ca2+ signaling, morphology, and astrocyte-synapse proximity between striatal and hippocampal astrocytes. Unbiased evaluation of actively translated RNA and proteomic data confirmed significant astrocyte diversity between hippocampal and striatal circuits. We thus report core astrocyte properties, reveal evidence for specialized astrocytes within neural circuits, and provide new, integrated database resources and approaches to explore astrocyte diversity and function throughout the adult brain. VIDEO ABSTRACT.

Molecular mechanisms of astrocyte-induced synaptogenesis

Astrocytes are morphologically complex cells that perform a wide variety of critical functions in the brain. As a structurally and functionally integrated component of the synapse, astrocytes secrete proteins, lipids, and small molecules that bind neuronal receptors to promote synaptogenesis and regulate synaptic connectivity. Additionally, astrocytes are key players in circuit formation, instructing the formation of synapses between distinct classes of neurons. This review highlights recent publications on the topic of astrocyte-mediated synaptogenesis, with a focus on the molecular mechanisms through which astrocytes orchestrate the formation of synaptic circuits.

Cell biology of spinal cord injury and repair

Spinal cord injury (SCI) lesions present diverse challenges for repair strategies. Anatomically complete injuries require restoration of neural connectivity across lesions. Anatomically incomplete injuries may benefit from augmentation of spontaneous circuit reorganization. Here, we review SCI cell biology, which varies considerably across three different lesion-related tissue compartments: (a) non-neural lesion core, (b) astrocyte scar border, and (c) surrounding spared but reactive neural tissue. After SCI, axon growth and circuit reorganization are determined by neuron-cell-autonomous mechanisms and by interactions among neurons, glia, and immune and other cells. These interactions are shaped by both the presence and the absence of growth-modulating molecules, which vary markedly in different lesion compartments. The emerging understanding of how SCI cell biology differs across lesion compartments is fundamental to developing rationally targeted repair strategies.

The Role of the Oligodendrocyte Lineage in Acute Brain Trauma

An acute brain injury is commonly characterized by an extended cellular damage. The post-injury process of scar formation is largely determined by responses of various local glial cells and blood-derived immune cells. The role of astrocytes and microglia have been frequently reviewed in the traumatic sequelae. Here, we summarize the diverse contributions of oligodendrocytes (OLs) and their precursor cells (OPCs) in acute injuries. OLs at the lesion site are highly sensitive to a damaging insult, provoked by Ca²⁺ overload after hyperexcitation originating from increased levels of transmitters. At the lesion site, differentiating OPCs can replace injured oligodendrocytes to guarantee proper myelination that is instrumental for healthy brain function. In contrast to finally differentiated and non-dividing OLs, OPCs are the most proliferative cells of the brain and their proliferation rate even increases after injury. There exist even evidence that OPCs might also generate some type of astrocyte beside OLs. Thereby, OPCs can contribute to the generation and maintenance of the glial scar. In the future, detailed knowledge of the molecular cues that help to prevent injury-evoked glial cell death and that control differentiation and myelination of the oligodendroglial lineage will be pivotal in developing novel therapeutic approaches.

Transcriptome Sequencing Reveals Astrocytes as a Therapeutic Target in Heat-Stroke

Heat-stroke is a serious form of hyperthermia with high mortality, and can induce severe central nervous system disorders. The neurovascular unit (NVU), which consists of vascular cells, glial cells, and neurons, controls blood-brain barrier (BBB) permeability and cerebral blood flow, and maintains the proper functioning of neuronal circuits. However, the detailed function of each BBB component in heat-stroke remains unknown. In order to interpret alterations caused by heat stress, we performed transcriptome comparison of neuron and astrocyte primary cultures after heat treatment. Differentially-expressed genes were then selected and underwent Gene Ontology annotation and Kyoto Encyclopedia of Genes and Genomes pathway analysis. Gene-act networks were also constructed, and the expression of pivotal genes was validated by quantitative PCR, as well as single-cell qPCR in heat-stroke rats. Our work provides valuable information on the transcriptional changes in NVU cells after heat stress, reveals the diverse regulatory mechanisms of two of these cellular components, and shows that a cell-type-specific approach may be a promising therapeutic strategy for heat-stroke treatments.

Spatiotemporal control of mitochondrial network dynamics in astroglial cells

Mitochondria are increasingly recognized for playing important roles in regulating the evolving metabolic state of mammalian cells. This is particularly true for nerve cells, as dysregulation of mitochondrial dynamics is invariably associated with a number of neuropathies. Accumulating evidence now reveals that changes in mitochondrial dynamics and structure may play equally important roles also in the cell biology of astroglial cells. Astroglial cells display significant heterogeneity in their morphology and specialized functions across different brain regions, however besides fundamental differences they seem to share a surprisingly complex meshwork of mitochondria, which is highly suggestive of tightly regulated mechanisms that contribute to maintain this unique architecture. Here, we summarize recent work performed in astrocytes in situ indicating that this may indeed be the case, with astrocytic mitochondrial networks shown to experience rapid dynamic changes in response to defined external cues. Although the mechanisms underlying this degree of mitochondrial re-shaping are far from being understood, recent data suggest that they may contribute to demarcate astrocyte territories undergoing key signalling and metabolic functions.

Astrocytes in Memory Function: Pioneering Findings and Future Directions

Astrocytes have been generally believed to perform mainly homeostatic and supportive functions for neurons in the central nervous system. Recently, a growing body of evidence suggests previously unrecognized and surprising functions for astrocytes, including regulation of synaptic formation, transmission and plasticity, all of which are considered as the infrastructure for information processing and memory formation and stabilization. This review discusses the involvement of astrocytes in memory functions and the possible mechanisms that may underlie it. We review the important breakthroughs obtained in this field, as well as some of the controversies that arose from the past difficulty to manipulate these cells in a cell type-specific and non-invasive manner. Finally, we present new research avenues based on the advanced tools becoming available in recent years: optogenetics and chemogenetics, and the potential ways in which these tools may further illuminate the role of astrocytes in memory processes.

Distribution of ELOVL4 in the Developing and Adult Mouse Brain

ELOngation of Very Long chain fatty acids (ELOVL)-4 is essential for the synthesis of very long chain-fatty acids (fatty acids with chain lengths ≥ 28 carbons). The functions of ELOVL4 and its very long-chain fatty acid products are poorly understood at present. However, mutations in ELOVL4 cause neurodevelopmental or neurodegenerative diseases that vary according to the mutation and inheritance pattern. Heterozygous inheritance of different ELOVL4 mutations causes Stargardt-like Macular Dystrophy or Spinocerebellar Ataxia type 34. Homozygous inheritance of ELOVL4 mutations causes more severe disease characterized by seizures, intellectual disability, ichthyosis, and premature death. To better understand ELOVL4 and very long chain fatty acid function in the brain, we examined ELOVL4 expression in the mouse brain between embryonic day 18 and postnatal day 60 by immunolabeling using ELOVL4 and other marker antibodies. ELOVL4 was widely expressed in a region- and cell type-specific manner, and was restricted to cell bodies, consistent with its known localization to endoplasmic reticulum. ELOVL4 labeling was most prominent in gray matter, although labeling also was present in some cells located in white matter. ELOVL4 was widely expressed in the developing brain by embryonic day 18 and was especially pronounced in regions underlying the lateral ventricles and other neurogenic regions. The basal ganglia in particular showed intense ELOVL4 labeling at this stage. In the postnatal brain, cerebral cortex, hippocampus, cerebellum, thalamus, hypothalamus, midbrain, pons, and medulla all showed prominent ELOVL4 labeling, although ELOVL4 distribution was not uniform across all cells or subnuclei within these regions. In contrast, the basal ganglia showed little ELOVL4 labeling in the postnatal brain. Double labeling studies showed that ELOVL4 was primarily expressed by neurons, although presumptive oligodendrocytes located in white matter tracts also showed labeling. Little or no ELOVL4 labeling was present in astrocytes or radial glial cells. These findings suggest that ELOVL4 and its very long chain fatty acid products are important in many parts of the brain and that they are particularly associated with neuronal function. Specific roles for ELOVL4 and its products in oligodendrocytes and myelin and in cellular proliferation, especially during development, are possible.

Local energy on demand: Are 'spontaneous' astrocytic Ca2+-microdomains the regulatory unit for astrocyte-neuron metabolic cooperation?

Astrocytes are a neural cell type critically involved in maintaining brain energy homeostasis as well as signaling. Like neurons, astrocytes are a heterogeneous cell population. Cortical astrocytes show a complex morphology with a highly branched aborization and numerous fine processes ensheathing the synapses of neighboring neurons, and typically extend one process connecting to blood vessels. Recent studies employing genetically encoded fluorescent calcium (Ca²⁺) indicators have described 'spontaneous' localized Ca²⁺-transients in the astrocyte periphery that occur asynchronously, independently of signals in other parts of the cells, and that do not involve somatic Ca²⁺ transients; however, neither it is known whether these Ca²⁺-microdomains occur at or near neuronal synapses nor have their molecular basis nor downstream effector(s) been identified. In addition to Ca²⁺ microdomains, sodium (Na⁺) transients occur in astrocyte subdomains, too, most likely as a consequence of Na⁺ co-transport with the neurotransmitter glutamate, which also regulates mitochondrial movements locally - as do cytoplasmic Ca²⁺ levels. In this review, we cover various aspects of these local signaling events and discuss how structural and biophysical properties of astrocytes might foster such compartmentation. Astrocytes metabolically interact with neurons by providing energy substrates to active neurons. As a single astrocyte branch covers hundreds to thousands of synapses, it is tempting to speculate that these metabolic interactions could occur localized to specific subdomains of astrocytes, perhaps even at the level of small groups of synapses. We discuss how astrocytic metabolism might be regulated at this scale and which signals might contribute to its regulation. We speculate that the astrocytic structures that light up transiently as Ca²⁺-microdomains might be the functional units of astrocytes linking signaling and metabolic processes to adapt astrocytic function to local energy demands. The understanding of these local regulatory and metabolic interactions will be fundamental to fully appreciate the complexity of brain energy homeostasis as well as its failure in disease and may shed new light on the controversy about neuron-glia bi-directional signaling at the tripartite synapse.

Involvement of Astrocytes in Mediating the Central Effects of Ghrelin

Although astrocytes are the most abundant cells in the mammalian brain, much remains to be learned about their molecular and functional features. Astrocytes express receptors for numerous hormones and metabolic factors, including the appetite-promoting hormone ghrelin. The metabolic effects of ghrelin are largely opposite to those of leptin, as it stimulates food intake and decreases energy expenditure. Ghrelin is also involved in glucose-sensing and glucose homeostasis. The widespread expression of the ghrelin receptor in the central nervous system suggests that this hormone is not only involved in metabolism, but also in other essential functions in the brain. In fact, ghrelin has been shown to promote cell survival and neuroprotection, with some studies exploring the use of ghrelin as a therapeutic agent against metabolic and neurodegenerative diseases. In this review, we highlight the possible role of glial cells as mediators of ghrelin's actions within the brain.

Brain repair from intrinsic cell sources: Turning reactive glia into neurons

The replacement of lost neurons in the brain due to injury or disease holds great promise for the treatment of neurological disorders. However, logistical and ethical hurdles in obtaining and maintaining viable cells for transplantation have proven difficult to overcome. In vivo reprogramming offers an alternative, to bypass many of the restrictions associated with an exogenous cell source as it relies on a source of cells already present in the brain. Recent studies have demonstrated the possibility to target and reprogram glial cells into functional neurons with high efficiency in the murine brain, using virally delivered transcription factors. In this chapter, we explore the different populations of glial cells, how they react to injury and how they can be exploited for reprogramming purposes. Further, we review the most significant publications and how they have contributed to the understanding of key aspects in direct reprogramming needed to take into consideration, like timing, cell type targeted, and regional differences. Finally, we discuss future challenges and what remains to be explored in order to determine the potential of in vivo reprogramming for future brain repair.