Neurons diversify astrocytes in the adult brain through sonic hedgehog signaling

Developmental roles of astrocytes in circuit wiring

Astrocytes, the perisynaptic glial cells of the brain, play fundamental roles in sculpting synaptic circuits and instructing their remodeling and maturation during development. Astrocytes do so through a plethora of cell adhesion and secretory signaling to neurons. This rich communication between astrocytes and neurons is critical for balancing inhibitory and excitatory synaptic connectivity. Additionally, astrocytes refine neural circuits via synaptic engulfment and elimination. Here, we will review recent findings highlighting the diversity and significance of astrocyte-to-neuron communication during developmental circuit wiring. Moreover, we will point out emerging mechanisms of how neurons instruct astrocytes' maturation and synaptic functions to spotlight the essential bidirectional communication between these two cell types in shaping synaptic circuits during neurodevelopment.

Are we there yet? Exploring astrocyte heterogeneity one cell at a time

Astrocytes are a highly abundant cell type in the brain and spinal cord. Like neurons, astrocytes can be molecularly and functionally distinct to fulfill specialized roles. Recent technical advances in sequencing-based single cell assays have driven an explosion of omics data characterizing astrocytes in the healthy, aged, injured, and diseased central nervous system. In this review, we will discuss recent studies which have furthered our understanding of astrocyte biology and heterogeneity, as well as discuss the limitations and challenges of sequencing-based single cell and spatial genomics methods and their potential future utility.

Identification of a Subpopulation of Astrocyte Progenitor Cells in the Neonatal Subventricular Zone: Evidence that Migration is Regulated by Glutamate Signaling

In mice engineered to express enhanced green fluorescent protein (eGFP) under the control of the entire glutamate transporter 1 (GLT1) gene, eGFP is found in all 'adult' cortical astrocytes. However, when 8.3 kilobases of the human GLT1/EAAT2 promoter is used to control expression of tdTomato (tdT), tdT is only found in a subpopulation of these eGFP-expressing astrocytes. The eGFP mice have been used to define mechanisms of transcriptional regulation using astrocytes cultured from cortex of 1-3 day old mice. Using the same cultures, we were never able to induce tdT+ expression. We hypothesized that these cells might not have migrated into the cortex by this age. In this study, we characterized the ontogeny of tdT+ cells, performed single-cell RNA sequencing (scRNA-seq), and tracked their migration in organotypic slice cultures. At postnatal day (PND) 1, tdT+ cells were observed in the subventricular zone and striatum but not in the cortex, and they did not express eGFP. At PND7, tdT+ cells begin to appear in the cortex with their numbers increasing with age. At PND1, scRNA-seq demonstrates that the tdT+ cells are molecularly heterogeneous, with a subpopulation expressing astrocytic markers, subsequently validated with immunofluorescence. In organotypic slices, tdT+ cells migrate into the cortex, and after 7 days they express GLT1, NF1A, and eGFP. An ionotropic glutamate receptor (iGluR) antagonist reduced by 50% the distance tdT+ cells migrate from the subventricular zone into the cortex. The pan-glutamate transport inhibitor, TFB-TBOA, increased, by sixfold, the number of tdT+ cells in the cortex. In conclusion, although tdT is expressed by non-glial cells at PND1, it is also expressed by glial progenitors that migrate into the cortex postnatally. Using this fluorescent labeling, we provide novel evidence that glutamate signaling contributes to the control of glial precursor migration.

Single-cell omics and heterogeneity of neuroglial cells

Our bodies contain a rich diversity of cell types with unique physiologic properties. Interestingly, cells within our bodies contain the same DNA content, yet they can vary dramatically with respect to their molecular, structural, and functional properties. The need to better understand cellular complexity and diversity in biologic systems has led to a technical revolution in the field through the development of sophisticated single-cell "omic" approaches. This allows the investigation of the genome, epigenome, transcriptome, and proteome of individual cells derived from complex samples or tissues, such as nervous system tissue. These methods are allowing scientists to detect distinct cell populations and cellular states in different species (including rodent and human) and molecular transitions of cell populations across the lifespan. Recent studies have revealed that astrocytes, oligodendrocytes, and microglia exhibit greater molecular and functional heterogeneity than originally thought and innovative single-cell technologies have allowed a more comprehensive and less biased view of this cellular diversity. The chapter begins by providing a primer of single-cell transcriptomic and spatial transcriptomic approaches that have been particularly influential in uncovering single-cell diversity of neuroglial cells in the brain. It then takes a closer look at how these technologies have been pivotal in defining neuroglial cell subtypes and for determining their spatial relationships within the CNS. Then, it concludes with discussion of how the recent technical advances and discoveries have provoked new questions about the origin, organization, and functional purpose of diverse neuroglial cell subtypes.

Permanent cilia loss during cerebellar granule cell neurogenesis involves withdrawal of cilia maintenance and centriole capping

Brain neurons utilize the primary cilium as a privileged compartment to detect and respond to extracellular ligands such as Sonic hedgehog (SHH). However, cilia in cerebellar granule cell (GC) neurons disassemble during differentiation through ultrastructurally unique intermediates, a process we refer to as cilia deconstruction. In addition, mature neurons do not reciliate despite having docked centrioles. Here, we identify molecular changes that accompany cilia deconstruction and centriole docking in GC neurons. We used single cell transcriptomic and immunocytological analyses to compare the transcript levels and subcellular localization of proteins between progenitor, differentiating, and mature GCs. Differentiating GCs lacked transcripts for key activators of premitotic cilia resorption, indicating that cilia disassembly in differentiating cells is distinct from premitotic cilia resorption. Instead, during differentiation, transcripts of many genes required for cilia maintenance-specifically those encoding components of intraflagellar transport, pericentrosomal material, and centriolar satellites-decreased. The abundance of several corresponding proteins in and around cilia and centrosomes also decreased. These changes coincided with downregulation of SHH signaling prior to differentiation, even in a mutant with excessive SHH activation. Finally, mother centrioles in maturing granule neurons recruited the cap complex protein, CEP97. These data suggest that a global, developmentally programmed decrease in cilium maintenance in differentiating GCs mediates cilia deconstruction, while capping of docked mother centrioles prevents cilia regrowth and dysregulated SHH signaling. Our study provides mechanistic insights expanding our understanding of permanent cilia loss in multiple tissue-specific contexts.

Controversies and insights into PTBP1-related astrocyte-neuron transdifferentiation: neuronal regeneration strategies for Parkinson's and Alzheimer's disease

Promising therapeutic strategies are being explored to replace or regenerate the neuronal populations that are lost in patients with neurodegenerative disorders. Several research groups have attempted direct reprogramming of astrocytes into neurons by manipulating the expression of polypyrimidine tract-binding protein 1 (PTBP1) and claimed putative converted neurons to be functional, which led to improved disease outcomes in animal models of several neurodegenerative disorders. However, a few other studies reported data that contradict these claims, raising doubt about whether PTBP1 suppression truly reprograms astrocytes into neurons and the therapeutic potential of this approach. This review discusses recent advances in regenerative therapeutics including stem cell transplantations for central nervous system disorders, with a particular focus on Parkinson's and Alzheimer's diseases. We also provide a perspective on this controversy by considering that astrocyte heterogeneity may be the key to understanding the discrepancy in published studies, and that certain subpopulations of these glial cells may be more readily converted into neurons.

Sonic Hedgehog Is an Early Oligodendrocyte Marker During Remyelination

Failure of myelin regeneration by oligodendrocytes contributes to progressive decline in many neurological diseases. Here, using in vitro and in vivo rodent models, functional blockade, and mouse brain demyelination, we demonstrate that Sonic hedgehog (Shh) expression in a subset of oligodendrocyte progenitor cells precedes the expression of myelin basic protein (MBP), a major myelin sheath protein. Primary cultures of rodent cortical oligodendrocytes show that Shh mRNA and protein are upregulated during oligodendrocyte maturation before the upregulation of MBP expression. Importantly, almost all MBP-positive cells are Shh positive during differentiation. During remyelination, we identify a rapid induction of Shh mRNA and peptide in oligodendroglial cells present in the demyelinated corpus callosum of mice, including a population of PDGFRα-expressing cells. Shh invalidation by an adeno-associated virus strategy demonstrates that the downregulation of Shh impairs the differentiation of oligodendrocytes in vitro and decreases MBP and myelin proteolipid protein expression in the demyelinated mouse brain at late stages of remyelination. We also report a parallel expression of Shh and MBP in oligodendroglial cells during early post-natal myelination of the mouse brain. Thus, we identify a crucial Shh signal involved in oligodendroglial cell differentiation and remyelination, with potential interest in the design of better-targeted remyelinating therapeutic strategies.

Generation of Mammalian Astrocyte Functional Heterogeneity

Mammalian astrocytes have regional roles within the brain parenchyma. Indeed, the notion that astrocytes are molecularly heterogeneous could help explain how the central nervous system (CNS) retains embryonic positional information through development into specialized regions into adulthood. A growing body of evidence supports the concept of morphological and molecular differences between astrocytes in different brain regions, which might relate to their derivation from regionally patterned radial glia and/or local neuron inductive cues. Here, we review evidence for regionally encoded functions of astrocytes to provide an integrated concept on lineage origins and heterogeneity to understand regional brain organization, as well as emerging technologies to identify and further investigate novel roles for astrocytes.

Anxiety caused by chronic exposure to methylisothiazolinone in zebrafish: Behavioral analysis, brain histology and gene responses

Methylisothiazolinones (MIT) are a class of preservatives and biocides extensively utilized in everyday products, industrial processes, and medical and healthcare applications. However, reports have indicated that MIT may cause skin irritation and neurotoxicity. Given its pervasive use, the neurotoxic potential of MIT has garnered increasing attention. Recent in vitro cellular experiments have demonstrated that MIT inhibits synaptic growth, although the neurotoxic effects and underlying mechanisms at the organismal level remain largely unexplored. In this study, it was found for the first time that long-term exposure to MIT resulted in anxiety, brain tissue inflammation, and a reduction in the number of Nissl bodies in the brain. Additionally, transcriptomic analysis indicated that exposure to 300 μg/L MIT induced a greater number of differentially expressed genes compared to 30 μg/L MIT, relative to the control group. Enrichment analysis, trend analysis, and GSEA analysis collectively identified the involvement of Steroid hormone metabolism, oxidative metabolism, and the Hedgehog pathway in MIT-induced neurotoxicity. Furthermore, a subsequent reduction in green fluorescence was observed in the MLS-EGFP zebrafish strain larvae of the HD group, suggesting that high dosage of MIT exerts an inhibitory effect on mitochondrial activity. This study confirmed the neurotoxic effects of MIT and investigated the potential genetic networks behind anxiety behavior. These findings contributed to the identification of key brain genes involved in the detection and monitoring of MIT, offering new insights into the neuroendocrine toxicity of other imidazolidinone compounds.

Astrocyte regional specialization is shaped by postnatal development

Astrocytes are an abundant class of glial cells with critical roles in neural circuit assembly and function. Though many studies have uncovered significant molecular distinctions between astrocytes from different brain regions, how this regionalization unfolds over development is not fully understood. We used single-nucleus RNA sequencing to characterize the molecular diversity of brain cells across six developmental stages and four brain regions in the mouse and marmoset brain. Our analysis of over 170,000 single astrocyte nuclei revealed striking regional heterogeneity among astrocytes, particularly between telencephalic and diencephalic regions, at all developmental time points surveyed in both species. At the stages sampled, most of the region patterning was private to astrocytes and not shared with neurons or other glial types. Though astrocytes were already regionally patterned in late embryonic stages, this region-specific astrocyte gene expression signature changed dramatically over postnatal development, and its composition suggests that regional astrocytes further specialize postnatally to support their local neuronal circuits. Comparing across species, we found divergence in the expression of astrocytic region- and age-differentially expressed genes and the timing of astrocyte maturation relative to birth between mouse and marmoset, as well as hundreds of species differentially expressed genes. Finally, we used expansion microscopy to show that astrocyte morphology is largely conserved across gray matter regions of prefrontal cortex, striatum, and thalamus in the mouse, despite substantial molecular divergence.

Permanent deconstruction of intracellular primary cilia in differentiating granule cell neurons

Primary cilia on granule cell neuron progenitors in the developing cerebellum detect sonic hedgehog to facilitate proliferation. Following differentiation, cerebellar granule cells become the most abundant neuronal cell type in the brain. While granule cell cilia are essential during early developmental stages, they become infrequent upon maturation. Here, we provide nanoscopic resolution of cilia in situ using large-scale electron microscopy volumes and immunostaining of mouse cerebella. In many granule cells, we found intracellular cilia, concealed from the external environment. Cilia were disassembled in differentiating granule cell neurons-in a process we call cilia deconstruction-distinct from premitotic cilia resorption in proliferating progenitors. In differentiating granule cells, cilia deconstruction involved unique disassembly intermediates, and, as maturation progressed, mother centriolar docking at the plasma membrane. Unlike ciliated neurons in other brain regions, our results show the deconstruction of concealed cilia in differentiating granule cells, which might prevent mitogenic hedgehog responsiveness. Ciliary deconstruction could be paradigmatic of cilia removal during differentiation in other tissues.

Neuroprotection elicited by taurine in sporadic Alzheimer-like disease: benefits on memory and control of neuroinflammation in the hippocampus of rats

This study aimed to analyze whether taurine has a nootropic effect on short-term and long-term memory in a model of sporadic dementia of the Alzheimer's type (SDAT). Moreover, we evaluated the immunoreactivity and insulin receptor (IR) distribution and markers for neurons and glial cells in the hippocampus of rats with SDAT and treated with taurine. For this, Male Wistar rats received STZ (ICV, 3 mg/kg, bilateral, 5ul per site, aCFS vehicle) and were treated with taurine (100 mg/kg orally, 1 time per day, saline vehicle) for 25 days. The animals were divided into 4 groups: vehicle (VE), taurine (TAU), ICV-STZ (STZ) and ICV-STZ plus taurine (STZ + TAU). At the end of taurine treatment, short- and long-term memory were assessed by performance on object recognition and Y-maze tasks. Insulin receptor (IR) was evaluated by immunoperoxidase while mature neurons (NeuN), astrocytes (GFAP, S100B, SOX9), and microglia (Iba-1) were evaluated by immunofluorescence. STZ induced worse spatial and recognition memory (INDEX) in YM and ORT tasks. Taurine protected against STZ-induced memory impairment. SDAT reduced the population of mature neurons as well as increased astrocytic and microglial reactivity, and taurine protected against these STZ-induced effects, mainly in the CA1 region of the hippocampus. Taurine increases IR expression in the hippocampus, and protects against the reduction in the density of this receptor in CA1 induced by STZ. In conclusion, these findings demonstrate that taurine is able to enhance memory, up-regulates IR in the hippocampus, protects the neuron population, and reduces the astrogliosis found in SDAT.

Brain stars take the lead during critical periods of early postnatal brain development: relevance of astrocytes in health and mental disorders

In the brain, astrocytes regulate shape and functions of the synaptic and vascular compartments through a variety of released factors and membrane-bound proteins. An imbalanced astrocyte activity can therefore have drastic negative impacts on brain development, leading to the onset of severe pathologies. Clinical and pre-clinical studies show alterations in astrocyte cell number, morphology, molecular makeup and astrocyte-dependent processes in different affected brain regions in neurodevelopmental (ND) and neuropsychiatric (NP) disorders. Astrocytes proliferate, differentiate and mature during the critical period of early postnatal brain development, a time window of elevated glia-dependent regulation of a proper balance between synapse formation/elimination, which is pivotal in refining synaptic connectivity. Therefore, any intrinsic and/or extrinsic factors altering these processes during the critical period may result in an aberrant synaptic remodeling and onset of mental disorders. The peculiar bridging position of astrocytes between synaptic and vascular compartments further allows them to "compute" the brain state and consequently secrete factors in the bloodstream, which may serve as diagnostic biomarkers of distinct healthy or disease conditions. Here, we collect recent advancements regarding astrogenesis and astrocyte-mediated regulation of neuronal network remodeling during early postnatal critical periods of brain development, focusing on synapse elimination. We then propose alternative hypotheses for an involvement of aberrancies in these processes in the onset of ND and NP disorders. In light of the well-known differential prevalence of certain brain disorders between males and females, we also discuss putative sex-dependent influences on these neurodevelopmental events. From a translational perspective, understanding age- and sex-dependent astrocyte-specific molecular and functional changes may help to identify biomarkers of distinct cellular (dys)functions in health and disease, favouring the development of diagnostic tools or the selection of tailored treatment options for male/female patients.

Primary cilia signaling in astrocytes mediates development and regional-specific functional specification

Astrocyte diversity is greatly influenced by local environmental modulation. Here we report that the majority of astrocytes across the mouse brain possess a singular primary cilium localized to the cell soma. Comparative single-cell transcriptomics reveals that primary cilia mediate canonical SHH signaling to modulate astrocyte subtype-specific core features in synaptic regulation, intracellular transport, energy and metabolism. Independent of canonical SHH signaling, primary cilia are important regulators of astrocyte morphology and intracellular signaling balance. Dendritic spine analysis and transcriptomics reveal that perturbation of astrocytic cilia leads to disruption of neuronal development and global intercellular connectomes in the brain. Mice with primary ciliary-deficient astrocytes show behavioral deficits in sensorimotor function, sociability, learning and memory. Our results uncover a critical role for primary cilia in transmitting local cues that drive the region-specific diversification of astrocytes within the developing brain.

Cerebellar Heterogeneity and Selective vulnerability in Spinocerebellar Ataxia Type 1 (SCA1)

Heterogeneity is one of the key features of the healthy brain and selective vulnerability characterizes many, if not all, neurodegenerative diseases. While cerebellum contains majority of brain cells, neither its heterogeneity nor selective vulnerability in disease are well understood. Here we describe molecular, cellular and functional heterogeneity in the context of healthy cerebellum as well as in cerebellar disease Spinocerebellar Ataxia Type 1 (SCA1). We first compared disease pathology in cerebellar vermis and hemispheres across anterior to posterior axis in a knock-in SCA1 mouse model. Using immunohistochemistry, we demonstrated earlier and more severe pathology of PCs and glia in the posterior cerebellar vermis of SCA1 mice. We also demonstrate heterogeneity of Bergmann glia in the unaffected, wild-type mice. Then, using RNA sequencing, we found both shared, as well as, posterior cerebellum-specific molecular mechanisms of pathogenesis that include exacerbated gene dysregulation, increased number of altered signaling pathways, and decreased pathway activity scores in the posterior cerebellum of SCA1 mice. We demonstrated unexpectedly large differences in the gene expression between posterior and anterior cerebellar vermis of wild-type mice, indicative of robust intraregional heterogeneity of gene expression in the healthy cerebellum. Additionally, we found that SCA1 disease profoundly reduces intracerebellar heterogeneity of gene expression. Further, using fiber photometry, we found that population level PC calcium activity was altered in the posterior lobules in SCA1 mice during walking. We also identified regional differences in the population level activity of Purkinje cells (PCs) in unrestrained wild-type mice that were diminished in SCA1 mice.

Astrocytic transcriptional and epigenetic mechanisms of drug addiction

Addiction is a leading cause of disease burden worldwide and remains a challenge in current neuroscience research. Drug-induced lasting changes in gene expression are mediated by transcriptional and epigenetic regulation in the brain and are thought to underlie behavioral adaptations. Emerging evidence implicates astrocytes in regulating drug-seeking behaviors and demonstrates robust transcriptional response to several substances of abuse. This review focuses on the astrocytic transcriptional and epigenetic mechanisms of drug action.

Memantine suppresses the excitotoxicity but fails to rescue the ataxic phenotype in SCA1 model mice

Spinocerebellar ataxia type 1 (SCA1) is a debilitating neurodegenerative disorder of the cerebellum and brainstem. Memantine has been proposed as a potential treatment for SCA1. It blocks N-methyl-D-aspartate (NMDA) receptors on neurons, reduces excitotoxicity and decreases neurodegeneration in Alzheimer models. However, in cerebellar neurodegenerative diseases, the potential value of memantine is still unclear. We investigated the effects of memantine on motor performance and synaptic transmission in the cerebellum in a mouse model where mutant ataxin 1 is specifically targeted to glia. Lentiviral vectors (LVV) were used to express mutant ataxin 1 selectively in Bergmann glia (BG). In mice transduced with the mutant ataxin 1, chronic treatment with memantine improved motor activity during initial tests, presumably due to preserved BG and Purkinje cell (PC) morphology and numbers. However, mice were unable to improve their rota rod scores during next days of training. Memantine also compromised improvement in the rota rod scores in control mice upon repetitive training. These effects may be due to the effects of memantine on plasticity (LTD suppression) and NMDA receptor modulation. Some effects of chronically administered memantine persisted even after its wash-out from brain slices. Chronic memantine reduced morphological signs of neurodegeneration in the cerebellum of SCA1 model mice. This resulted in an apparent initial reduction of ataxic phenotype, but memantine also affected cerebellar plasticity and ultimately compromised motor learning. We speculate that that clinical application of memantine in SCA1 might be hampered by its ability to suppress NMDA-dependent plasticity in cerebellar cortex.

Methylmercury neurotoxicity: Beyond the neurocentric view

Mercury is a highly toxic metal widely used in human activities worldwide, therefore considered a global public health problem. Many cases of mercury intoxication have occurred in history and represent a huge challenge nowadays. Of particular importance is its methylated form, methylmercury (MeHg). This mercurial species induces damage to several organs in the human body, especially to the central nervous system. Neurological impairments such as executive, memory, motor and visual deficits are associated with MeHg neurotoxicity. Molecular mechanisms involved in MeHg-induced neurotoxicity include excitotoxicity due to glutamatergic imbalance, disturbance in calcium homeostasis and oxidative balance, failure in synaptic support, and inflammatory response. Although neurons are largely affected by MeHg intoxication, they only represent half of the brain cells. Glial cells represent roughly 50 % of the brain cells and are key elements in the functioning of the central nervous system. Particularly, astrocytes and microglia are deeply involved in MeHg-induced neurotoxicity, resulting in distinct neurological outcomes depending on the context. In this review, we discuss the main findings on astroglial and microglial involvement as mediators of neuroprotective and neurotoxic responses to MeHg intoxication. The literature shows that these responses depend on chemical and morphophysiological features, thus, we present some insights for future investigations, considering the particularities of the context, including time and dose of exposure, brain region, and species of study.

Astrocytes in selective vulnerability to neurodegenerative disease

Selective vulnerability of specific brain regions and cell populations is a hallmark of neurodegenerative disorders. Mechanisms of selective vulnerability involve neuronal heterogeneity, functional specializations, and differential sensitivities to stressors and pathogenic factors. In this review we discuss the growing body of literature suggesting that, like neurons, astrocytes are heterogeneous and specialized, respond to and integrate diverse inputs, and induce selective effects on brain function. In disease, astrocytes undergo specific, context-dependent changes that promote different pathogenic trajectories and functional outcomes. We propose that astrocytes contribute to selective vulnerability through maladaptive transitions to context-divergent phenotypes that impair specific brain regions and functions. Further studies on the multifaceted roles of astrocytes in disease may provide new therapeutic approaches to enhance resilience against neurodegenerative disorders.

Astrocytes in the adult dentate gyrus-balance between adult and developmental tasks

Astrocytes, a major glial cell type in the brain, are indispensable for the integration, maintenance and survival of neurons during development and adulthood. Both life phases make specific demands on the molecular and physiological properties of astrocytes, and most research projects traditionally focus on either developmental or adult astrocyte functions. In most brain regions, the generation of brain cells and the establishment of neural circuits ends with postnatal development. However, few neurogenic niches exist in the adult brain in which new neurons and glial cells are produced lifelong, and the integration of new cells into functional circuits represent a very special form of plasticity. Consequently, in the neurogenic niche, the astrocytes must be equipped to execute both mature and developmental tasks in order to integrate newborn neurons into the circuit and yet maintain overall homeostasis without affecting the preexisting neurons. In this review, we focus on astrocytes of the hippocampal dentate gyrus (DG), and discuss specific features of the astrocytic compartment that may allow the execution of both tasks. Firstly, astrocytes of the adult DG are molecularly, morphologically and functionally diverse, and the distinct astrocytes subtypes are characterized by their localization to DG layers. This spatial separation may lead to a functional specification of astrocytes subtypes according to the neuronal structures they are embedded in, hence a division of labor. Secondly, the astrocytic compartment is not static, but steadily increasing in numbers due to lifelong astrogenesis. Interestingly, astrogenesis can adapt to environmental and behavioral stimuli, revealing an unexpected astrocyte dynamic that allows the niche to adopt to changing demands. The diversity and dynamic of astrocytes in the adult DG implicate a vital contribution to hippocampal plasticity and represent an interesting model to uncover mechanisms how astrocytes simultaneously fulfill developmental and adult tasks.

Characterization of Sonic Hedgehog transcripts in the adult mouse brain: co-expression with neuronal and oligodendroglial markers

In the adult mammalian brain, astrocytes are proposed to be the major Sonic Hedgehog (Shh)-responsive cells. However, the sources of the Shh molecule mediating activation of the pathway are still poorly characterized. The present work investigates the distribution and phenotype of cells expressing Shh mRNA in the adult mouse brain. Using single-molecule fluorescent in situ hybridization (smfISH), we report much broader expression of Shh transcripts in almost all brain regions than originally reported. We identify Shh mRNA in HuC/D+ neuronal populations, including GABAergic (glutamic acid decarboxylase 67, Gad67), cholinergic (choline acetyltransferase, ChAT), dopaminergic (tyrosine hydroxylase, TH), nitrergic (neuronal nitric oxide synthase, nNOS), and in a small population of oligodendroglial cells expressing Sox10 and Olig2 mRNA transcription factors. Further analysis of Shh mRNA in cerebral cortical and hypothalamic neurons suggests that Shh is also expressed by glutamatergic neurons. Interestingly, we did not observe substantial Desert Hedgehog and Indian Hedgehog mRNA signals, nor Shh signals in S100β+ astrocytes and Iba1+ microglial cells. Collectively, the present work provides the most robust central map of Shh-expressing cells to date and underscores the importance of nitrergic neurons in regulating Shh availability to brain cells. Thus, our study provides a framework for future experiments aimed at better understanding of the functions of Shh signaling in the brain in normal and pathological states, and the characterization of novel regulatory mechanisms of the signaling pathway.

Nedd4-2-dependent regulation of astrocytic Kir4.1 and Connexin43 controls neuronal network activity

Nedd4-2 is an E3 ubiquitin ligase in which missense mutation is related to familial epilepsy, indicating its critical role in regulating neuronal network activity. However, Nedd4-2 substrates involved in neuronal network function have yet to be identified. Using mouse lines lacking Nedd4-1 and Nedd4-2, we identified astrocytic channel proteins inwardly rectifying K+ channel 4.1 (Kir4.1) and Connexin43 as Nedd4-2 substrates. We found that the expression of Kir4.1 and Connexin43 is increased upon conditional deletion of Nedd4-2 in astrocytes, leading to an elevation of astrocytic membrane ion permeability and gap junction activity, with a consequent reduction of γ-oscillatory neuronal network activity. Interestingly, our biochemical data demonstrate that missense mutations found in familial epileptic patients produce gain-of-function of the Nedd4-2 gene product. Our data reveal a process of coordinated astrocytic ion channel proteostasis that controls astrocyte function and astrocyte-dependent neuronal network activity and elucidate a potential mechanism by which aberrant Nedd4-2 function leads to epilepsy.

Astrocyte Development in the Rodent

Astrocytes have gained increasing recognition as key elements of a broad array of nervous system functions. These include essential roles in synapse formation and elimination, synaptic modulation, maintenance of the blood-brain barrier, energetic support, and neural repair after injury or disease of the nervous system. Nevertheless, our understanding of mechanisms underlying astrocyte development and maturation remains far behind that of neurons and oligodendrocytes. Early efforts to understand astrocyte development focused primarily on their specification from embryonic progenitors and the molecular mechanisms driving the switch from neuron to glial production. Considerably, less is known about postnatal stages of astrocyte development, the period during which they are predominantly generated and mature. Notably, this period is coincident with synapse formation and the emergence of nascent neural circuits. Thus, a greater understanding of astrocyte development is likely to shed new light on the formation and maturation of synapses and circuits. Here, we highlight key foundational principles of embryonic and postnatal astrocyte development, focusing largely on what is known from rodent studies.

β-Adrenergic Signaling Promotes Morphological Maturation of Astrocytes in Female Mice

Astrocytes play essential roles in the developing nervous system, including supporting synapse function. These astrocyte support functions emerge coincident with brain maturation and may be tailored in a region-specific manner. For example, gray matter astrocytes have elaborate synapse-associated processes and are morphologically and molecularly distinct from white matter astrocytes. This raises the question of whether there are unique environmental cues that promote gray matter astrocyte identity and synaptogenic function. We previously identified adrenergic receptors as preferentially enriched in developing gray versus white matter astrocytes, suggesting that noradrenergic signaling could be a cue that promotes the functional maturation of gray matter astrocytes. We first characterized noradrenergic projections during postnatal brain development in mouse and human, finding that process density was higher in the gray matter and increased concurrently with astrocyte maturation. RNA sequencing revealed that astrocytes in both species expressed α- and β-adrenergic receptors. We found that stimulation of β-adrenergic receptors increased primary branching of rodent astrocytes in vitro Conversely, astrocyte-conditional knockout of the β1-adrenergic receptor reduced the size of gray matter astrocytes and led to dysregulated sensorimotor integration in female mice. These studies suggest that adrenergic signaling to developing astrocytes impacts their morphology and has implications for adult behavior, particularly in female animals. More broadly, they demonstrate a mechanism through which environmental cues impact astrocyte development. Given the key roles of norepinephrine in brain states, such as arousal, stress, and learning, these findings could prompt further inquiry into how developmental stressors impact astrocyte development and adult brain function.SIGNIFICANCE STATEMENT This study demonstrates a role for noradrenergic signaling in the development of gray matter astrocytes. We provide new evidence that the β1-adrenergic receptor is robustly expressed by both mouse and human astrocytes, and that conditional KO of the β1-adrenergic receptor from female mouse astrocytes impairs gray matter astrocyte maturation. Moreover, female conditional KO mice exhibit behavioral deficits in two paradigms that test sensorimotor function. Given the emerging interest in moving beyond RNA sequencing to probe specific pathways that underlie astrocyte heterogeneity, this study provides a foundation for future investigation into the effect of noradrenergic signaling on astrocyte functions in conditions where noradrenergic signaling is altered, such as stress, arousal, and learning.

Permanent deconstruction of intracellular primary cilia in differentiating granule cell neurons

Primary cilia on granule cell neuron progenitors in the developing cerebellum detect sonic hedgehog to facilitate proliferation. Following differentiation, cerebellar granule cells become the most abundant neuronal cell type in the brain. While essential during early developmental stages, the fate of granule cell cilia is unknown. Here, we provide nanoscopic resolution of ciliary dynamics in situ by studying developmental changes in granule cell cilia using large-scale electron microscopy volumes and immunostaining of mouse cerebella. We found that many granule cell primary cilia were intracellular and concealed from the external environment. Cilia were disassembed in differentiating granule cell neurons in a process we call cilia deconstruction that was distinct from pre-mitotic cilia resorption in proliferating progenitors. In differentiating granule cells, ciliary loss involved unique disassembly intermediates, and, as maturation progressed, mother centriolar docking at the plasma membrane. Cilia did not reform from the docked centrioles, rather, in adult mice granule cell neurons remained unciliated. Many neurons in other brain regions require cilia to regulate function and connectivity. In contrast, our results show that granule cell progenitors had concealed cilia that underwent deconstruction potentially to prevent mitogenic hedgehog responsiveness. The ciliary deconstruction mechanism we describe could be paradigmatic of cilia removal during differentiation in other tissues.

Developmental origin and local signals cooperate to determine septal astrocyte identity

Astrocyte specification during development is influenced by both intrinsic and extrinsic factors, but the precise contribution of each remains poorly understood. Here we show that septal astrocytes from Nkx2.1 and Zic4 expressing progenitor zones are allocated into non-overlapping domains of the medial (MS) and lateral septal nuclei (LS) respectively. Astrocytes in these areas exhibit distinctive molecular and morphological features tailored to the unique cellular and synaptic circuit environment of each nucleus. Using single-nucleus (sn) RNA sequencing, we trace the developmental trajectories of cells in the septum and find that neurons and astrocytes undergo region and developmental stage-specific local cell-cell interactions. We show that expression of the classic morphogens Sonic hedgehog (Shh) and Fibroblast growth factors (Fgfs) by MS and LS neurons respectively, functions to promote the molecular specification of local astrocytes in each region. Finally, using heterotopic cell transplantation, we show that both morphological and molecular specifications of septal astrocytes are highly dependent on the local microenvironment, regardless of developmental origins. Our data highlights the complex interplay between intrinsic and extrinsic factors shaping astrocyte identities and illustrates the importance of the local environment in determining astrocyte functional specialization.

Behavioral characterization of Capn15 conditional knockout mice

Calpain 15 (CAPN15) is an intracellular cysteine protease belonging to the non-classical small optic lobe (SOL) family of calpains, which has an important role in development. Loss of Capn15 in mice leads to developmental eye anomalies and volumetric changes in the brain. Human individuals with biallelic variants in CAPN15 have developmental delay, neurodevelopmental disorders, as well as congenital malformations. In Aplysia, a reductionist model to study learning and memory, SOL calpain is important for non-associative long-term facilitation, the cellular analog of sensitization behavior. However, how CAPN15 is involved in adult behavior or learning and memory in vertebrates is unknown. Here, using Capn15 conditional knockout mice, we show that loss of the CAPN15 protein in excitatory forebrain neurons reduces self-grooming and marble burying, decreases performance in the accelerated roto-rod and reduces pre-tone freezing after strong fear conditioning. Thus, CAPN15 plays a role in regulating behavior in the adult mouse.

Yin Yang 1 controls cerebellar astrocyte maturation

Diverse subpopulations of astrocytes tile different brain regions to accommodate local requirements of neurons and associated neuronal circuits. Nevertheless, molecular mechanisms governing astrocyte diversity remain mostly unknown. We explored the role of a zinc finger transcription factor Yin Yang 1 (YY1) that is expressed in astrocytes. We found that specific deletion of YY1 from astrocytes causes severe motor deficits in mice, induces Bergmann gliosis, and results in simultaneous loss of GFAP expression in velate and fibrous cerebellar astrocytes. Single cell RNA-seq analysis showed that YY1 exerts specific effects on gene expression in subpopulations of cerebellar astrocytes. We found that although YY1 is dispensable for the initial stages of astrocyte development, it regulates subtype-specific gene expression during astrocyte maturation. Moreover, YY1 is continuously needed to maintain mature astrocytes in the adult cerebellum. Our findings suggest that YY1 plays critical roles regulating cerebellar astrocyte maturation during development and maintaining a mature phenotype of astrocytes in the adult cerebellum.

Cooperative and competitive regulation of the astrocytic transcriptome by neurons and endothelial cells: Impact on astrocyte maturation

Astrocytes have essential roles in central nervous system (CNS) health and disease. During development, immature astrocytes show complex interactions with neurons, endothelial cells, and other glial cell types. Our work and that of others have shown that these interactions are important for astrocytic maturation. However, whether and how these cells work together to control this process remains poorly understood. Here, we test the hypothesis that cooperative interactions of astrocytes with neurons and endothelial cells promote astrocytic maturation. Astrocytes were cultured alone, with neurons, endothelial cells, or a combination of both. This was followed by astrocyte sorting, RNA sequencing, and bioinformatic analysis to detect transcriptional changes. Across culture configurations, 7302 genes were differentially expressed by 4 or more fold and organized into 8 groups that demonstrate cooperative and antagonist effects of neurons and endothelia on astrocytes. We also discovered that neurons and endothelial cells caused splicing of 200 and 781 mRNAs, respectively. Changes in gene expression were validated using quantitative PCR, western blot (WB), and immunofluorescence analysis. We found that the transcriptomic data from the three-culture configurations correlated with protein expression of three representative targets (FAM107A, GAT3, and GLT1) in vivo. Alternative splicing results also correlated with cortical tissue isoform representation of a target (Fibronectin 1) at different developmental stages. By comparing our results to published transcriptomes of immature and mature astrocytes, we found that neurons or endothelia shift the astrocytic transcriptome toward a mature state and that the presence of both cell types has a greater effect on maturation than either cell alone. These results increase our understanding of cellular interactions/pathways that contribute to astrocytic maturation. They also provide insight into how alterations to neurons and/or endothelial cells may alter astrocytes with implications for astrocytic changes in CNS disorders and diseases.

A comparison of the impact on neuronal transcriptome and cognition of rAAV5 transduction with three different doses in the mouse hippocampus

Introduction

Recombinant adeno-associated viruses (rAAVs) are widely used in genetic therapeutics. AAV5 has shown superior transduction efficiency, targeting neurons and glial cells in primate brains. Nonetheless, the comprehensive impact of AAV5 transduction on molecular and behavioral alterations remains unexplored. This study focuses on evaluating the effects of AAV5 transduction in the hippocampus, a critical region for memory formation and emotional processes.

Methods

In this experiment, fluorescence-activated cell sorting (FACS) was utilized to isolate the mCherry-labeled pyramidal neurons in the hippocampus of CaMkIIα-cre mice following three different doses rAAV5-mCherry infusion after 3 weeks, which were then subjected to RNA sequencing (RNA-seq) to assess gene expression profiles. The cytokines concentration, mRNA expression, and glial response in hippocampi were confirmed by ELASA, digital droplet PCR and immunohistochemistry respectively. Locomotion and anxiety-like behaviors were elevated by Open Field Test and Elevated Plus Maze Test, while the Y-Maze were used to assessed spatial working memory. Recognition memory and fear responses were examined by the Novel Object Recognition Test and Fear Conditioning Test, respectively.

Results

We found that 2.88 × 10¹⁰ v.g rAAV5 transduction significantly upregulated genes related to the immune response and apoptosis, and downregulated genes associated with mitochondrial function and synaptic plasticity in hippocampal pyramidal neurons, while did not induce neuronal loss and gliosis compared with 2.88 × 10⁹ v.g and 2.88 × 10⁸ v.g. Furthermore, the same doses impaired working memory and contextual fear memory, without effects on locomotion and anxiety-related behaviors.

Discussion

Our findings highlight the detrimental impact of high-dose administration compared to median-dose or low-dose, resulting in increased neural vulnerability and impaired memory. Therefore, when considering the expression effectiveness of exogenous genes, it is crucial to also take potential side effects into account in clinical settings. However, the precise molecular mechanisms underlying these drawbacks of high-dose rAAV5-mCherry still require further investigation in future studies.

Astrocyte Endfeet in Brain Function and Pathology: Open Questions

Astrocyte endfeet enwrap the entire vascular tree within the central nervous system, where they perform important functions in regulating the blood-brain barrier (BBB), cerebral blood flow, nutrient uptake, and waste clearance. Accordingly, astrocyte endfeet contain specialized organelles and proteins, including local protein translation machinery and highly organized scaffold proteins, which anchor channels, transporters, receptors, and enzymes critical for astrocyte-vascular interactions. Many neurological diseases are characterized by the loss of polarization of specific endfoot proteins, vascular dysregulation, BBB disruption, altered waste clearance, or, in extreme cases, loss of endfoot coverage. A role for astrocyte endfeet has been demonstrated or postulated in many of these conditions. This review provides an overview of the development, composition, function, and pathological changes of astrocyte endfeet and highlights the gaps in our knowledge that future research should address.

Astrocytes as master modulators of neural networks: Synaptic functions and disease-associated dysfunction of astrocytes

Astrocytes are the most abundant glial cell type in the central nervous system and are essential to the development, plasticity, and maintenance of neural circuits. Astrocytes are heterogeneous, with their diversity rooted in developmental programs modulated by the local brain environment. Astrocytes play integral roles in regulating and coordinating neural activity extending far beyond their metabolic support of neurons and other brain cell phenotypes. Both gray and white matter astrocytes occupy critical functional niches capable of modulating brain physiology on time scales slower than synaptic activity but faster than those adaptive responses requiring a structural change or adaptive myelination. Given their many associations and functional roles, it is not surprising that astrocytic dysfunction has been causally implicated in a broad set of neurodegenerative and neuropsychiatric disorders. In this review, we focus on recent discoveries concerning the contributions of astrocytes to the function of neural networks, with a dual focus on the contribution of astrocytes to synaptic development and maturation, and on their role in supporting myelin integrity, and hence conduction and its regulation. We then address the emerging roles of astrocytic dysfunction in disease pathogenesis and on potential strategies for targeting these cells for therapeutic purposes.

The developmental profile of visual cortex astrocytes

We investigated how astrocytes in layer 5 mouse visual cortex mature over postnatal days (P) 3-50. Across this age range, resting membrane potential increased, input resistance decreased, and membrane responses became more passive with age. Two-photon (2p) and confocal imaging of dye-loaded cells revealed that gap-junction coupling increased starting ∼P7. Morphological reconstructions revealed increased branch density but shorter branches after P20, suggesting that astrocyte branches may get pruned as tiling is established. Finally, we visualized spontaneous Ca²⁺ transients with 2p microscopy and found that Ca²⁺ events decorrelated, became more frequent and briefer with age. As astrocytes mature, spontaneous Ca²⁺ activity thus changes from relatively cell-wide, synchronous waves to local transients. Several astrocyte properties were stably mature from ∼P15, coinciding with eye opening, although morphology continued to develop. Our findings provide a descriptive foundation of astrocyte maturation, useful for the study of astrocytic impact on visual cortex critical period plasticity.

Astrocyte calcium signaling shifts the polarity of presynaptic plasticity

Astrocytes have been increasingly acknowledged to play active roles in regulating synaptic transmission and plasticity. Through a variety of metabotropic and ionotropic receptors expressed on their surface, astrocytes detect extracellular neurotransmitters, and in turn, release gliotransmitters to modify synaptic strength, while they can also alter neuronal membrane excitability by modulating extracellular ionic milieu. Given the seemingly large repertoire of synaptic modulation, when, where and how astrocytes interact with synapses remain to be fully understood. Previously, we have identified a role for astrocyte NMDA receptor and L-VGCC signaling in heterosynaptic presynaptic plasticity and promoting the heterogeneity of presynaptic strengths at hippocampal synapses. Here, we have sought to further clarify the mode by which astrocytes regulate presynaptic plasticity by exploiting a reduced culture system to globally evoke NMDA receptor-dependent presynaptic plasticity. Recording from a postsynaptic neuron intracellularly loaded with BAPTA, briefly bath applying NMDA and glycine induces a stable decrease in the rate of spontaneous glutamate release, which requires the presence of astrocytes and the activation of A1 adenosine receptors. Upon preventing astrocyte calcium signaling or blocking L-type VGCCs, NMDA+glycine application triggers an increase, rather than a decrease, in the rate of spontaneous glutamate release, thereby shifting the presynaptic plasticity to promote an increase in strength. Our findings point to a crucial and surprising role of astrocytes in controlling the polarity of NMDA receptor and adenosine-dependent presynaptic plasticity. Such a pivotal mechanism unveils the power of astrocytes in regulating computations performed by neural circuits and is expected to profoundly impact cognitive processes.

Oestrogen ameliorates blood-brain barrier damage after experimental subarachnoid haemorrhage via the SHH pathway in male rats

BACKGROUND

Sex differences affect the occurrence, progression and regression of subarachnoid haemorrhage (SAH). Oestrogen plays a protective role in alleviating the vasospasm and neuronal apoptosis induced by SAH. However, whether oestrogen affects blood‒brain barrier (BBB) integrity has not been fully studied. Oestrogen has been found to regulate the sonic hedgehog (SHH) signalling pathway through the oestrogen receptor in gastric cancer and adrenal glands, and the SHH signalling pathway has an important role in maintaining the BBB by upregulating the expression of tight junction proteins. In this study, we investigated the relationship between oestrogen and the SHH signalling pathway using clinical data and established an experimental SAH model to explore whether oestrogen could ameliorate BBB damage after SAH through the SHH pathway.

METHODS

Correlations between oestrogen and the SHH pathway were analysed by patients' cerebrospinal fluid (CSF) samples and the Genotype-Tissue Expression database (GTEx). Then, an experimental rat SAH model was established using the endovascular perforation method and treated with oestrogen, oestrogen inhibitors and SHH signalling pathway inhibitors. Then, the effects of oestrogen on BBB damage were analysed by western blot, immunofluorescence and neurobehavioural experiments.

RESULTS

ESLIA detection and correlation analysis showed that oestrogen levels in patients' CSF were positively correlated with the SHH pathway, which was further verified by GTEx gene-correlation analysis. SHH was found to be mainly expressed in neurons and astrocytes in rats under physiological conditions and was upregulated by oestrogen pretreatment. In the SAH model, oestrogen pretreatment was found to reverse SAH-induced decreases in the SHH pathway, which were counteracted by oestrogen receptor inhibitors. Furthermore, oestrogen pretreatment reduced SAH-induced BBB damage, brain oedema and neurological dysfunction, which were eliminated by SHH pathway inhibitors.

CONCLUSION

In conclusion, we demonstrate here that oestrogen pretreatment ameliorates brain injury after SAH, at least in part through SHH pathway-mediated BBB protection.

Multicore fiber optic imaging reveals that astrocyte calcium activity in the cerebral cortex is modulated by internal motivational state

Astrocytes are a direct target of neuromodulators and can influence neuronal activity on broad spatial and temporal scales through their close proximity to synapses. However, our knowledge about how astrocytes are functionally recruited during different animal behaviors and their diverse effects on the CNS remains limited. To enable measurement of astrocyte activity patterns in vivo during normative behaviors, we developed a high-resolution, long working distance, multi-core fiber optic imaging platform that allows visualization of cortical astrocyte calcium transients through a cranial window in freely moving mice. Using this platform, we defined the spatiotemporal dynamics of astrocytes during diverse behaviors, ranging from circadian fluctuations to novelty exploration, showing that astrocyte activity patterns are more variable and less synchronous than apparent in head-immobilized imaging conditions. Although the activity of astrocytes in visual cortex was highly synchronized during quiescence to arousal transitions, individual astrocytes often exhibited distinct thresholds and activity patterns during explorative behaviors, in accordance with their molecular diversity, allowing temporal sequencing across the astrocyte network. Imaging astrocyte activity during self-initiated behaviors revealed that noradrenergic and cholinergic systems act synergistically to recruit astrocytes during state transitions associated with arousal and attention, which was profoundly modulated by internal state. The distinct activity patterns exhibited by astrocytes in the cerebral cortex may provide a means to vary their neuromodulatory influence in response to different behaviors and internal states.

Celloscope: a probabilistic model for marker-gene-driven cell type deconvolution in spatial transcriptomics data

Spatial transcriptomics maps gene expression across tissues, posing the challenge of determining the spatial arrangement of different cell types. However, spatial transcriptomics spots contain multiple cells. Therefore, the observed signal comes from mixtures of cells of different types. Here, we propose an innovative probabilistic model, Celloscope, that utilizes established prior knowledge on marker genes for cell type deconvolution from spatial transcriptomics data. Celloscope outperforms other methods on simulated data, successfully indicates known brain structures and spatially distinguishes between inhibitory and excitatory neuron types based in mouse brain tissue, and dissects large heterogeneity of immune infiltrate composition in prostate gland tissue.

Neuron-Derived Exosomes Promote the Recovery of Spinal Cord Injury by Modulating Nerve Cells in the Cellular Microenvironment of the Lesion Area

During spinal cord injury (SCI), the homeostasis of the cellular microenvironment in the injured area is seriously disrupted, which makes it extremely difficult for injured neurons with regenerative ability to repair, emphasizing the importance of restoring the cellular microenvironment at the injury site. Neurons interact closely with other nerve cells in the central nervous system (CNS) and regulate these cells. However, the specific mechanisms by which neurons modulate the cellular microenvironment remain unclear. Exosomes were isolated from the primary neurons, and their effects on astrocytes, microglia, oligodendrocyte progenitor cells (OPCs), neurons, and neural stem cells were investigated by quantifying the expression of related proteins and mRNA. A mouse SCI model was established, and neuron-derived exosomes were injected into the mice by the caudal vein to observe the recovery of motor function in mice and the changes in the nerve cells in the lesion area. Neuron-derived exosomes could reverse the activation of microglia and astrocytes and promote the maturation of OPCs in vivo and in vitro. In addition, neuron-derived exosomes promoted neurite outgrowth of neurons and the differentiation of neural stem cells into neurons. Moreover, our experiments showed that neuron-derived exosomes enhanced motor function recovery and nerve regeneration in mice with SCI. Our findings highlight that neuron-derived exosomes could promote the repair of the injured spinal cord by regulating the cellular microenvironment of neurons and could be a promising treatment for spinal cord injury.

Genesis of a functional astrocyte syncytium in the developing mouse hippocampus

Astrocytes are increasingly shown to operate as an isopotential syncytium in brain function. Protoplasmic astrocytes acquire this ability to functionally go beyond the single-cell level by evolving into a spongiform morphology, cytoplasmically connecting into a syncytium, and expressing a high density of K+ conductance. However, none of these cellular/functional features exist in neonatal newborn astrocytes, which imposes a basic question of when a functional syncytium evolves in the developing brain. Our results show that the spongiform morphology of individual astrocytes and their spatial organization all reach stationary levels by postnatal day (P) 15 in the hippocampal CA1 region. Functionally, astrocytes begin to uniformly express a mature level of passive K+ conductance by P11. We next used syncytial isopotentiality measurement to monitor the maturation of the astrocyte syncytium. In uncoupled P1 astrocytes, the substitution of endogenous K+ by a Na+ -electrode solution ([Na+ ]p ) resulted in the total elimination of the physiological membrane potential (VM ), and outward K+ conductance as predicted by the Goldman-Hodgkin-Katz (GHK) equation. As more astrocytes are coupled to each other through gap junctions during development, the [Na+ ]p -induced loss of physiological VM and the outward K+ conductance is progressively compensated by the neighboring astrocytes. By P15, a stably established syncytial isopotentiality (-73 mV), and a fully compensated outward K+ conductance appeared in all [Na+ ]p -recorded astrocytes. Thus, in view of the developmental timeframe wherein a singular syncytium is anatomically and functionally established for intra-syncytium K+ equilibration, an astrocyte syncytium becomes fully operational at P15 in the mouse hippocampus.

Astrocyte development—More questions than answers

The past 15–20 years has seen a remarkable shift in our understanding of astrocyte contributions to central nervous system (CNS) function. Astrocytes have emerged from the shadows of neuroscience and are now recognized as key elements in a broad array of CNS functions. Astrocytes comprise a substantial fraction of cells in the human CNS. Nevertheless, fundamental questions surrounding their basic biology remain poorly understood. While recent studies have revealed a diversity of essential roles in CNS function, from synapse formation and function to blood brain barrier maintenance, fundamental mechanisms of astrocyte development, including their expansion, migration, and maturation, remain to be elucidated. The coincident development of astrocytes and synapses highlights the need to better understand astrocyte development and will facilitate novel strategies for addressing neurodevelopmental and neurological dysfunction. In this review, we provide an overview of the current understanding of astrocyte development, focusing primarily on mammalian astrocytes and highlight outstanding questions that remain to be addressed. We also include an overview of Drosophila glial development, emphasizing astrocyte-like glia given their close anatomical and functional association with synapses. Drosophila offer an array of sophisticated molecular genetic tools and they remain a powerful model for elucidating fundamental cellular and molecular mechanisms governing astrocyte development. Understanding the parallels and distinctions between astrocyte development in Drosophila and vertebrates will enable investigators to leverage the strengths of each model system to gain new insights into astrocyte function.

Molecular and metabolic heterogeneity of astrocytes and microglia

Astrocytes and microglia are central players in a myriad of processes in the healthy and diseased brain, ranging from metabolism to immunity. The crosstalk between these two cell types contributes to pathology in many if not all neuroinflammatory and neurodegenerative diseases. Recent advancements in integrative multimodal sequencing techniques have begun to highlight how heterogeneous both cell types are and the importance of metabolism to their regulation. We discuss here the transcriptomic, metabolic, and functional heterogeneity of astrocytes and microglia and highlight their interaction in health and disease.

Astrocyte heterogeneity and interactions with local neural circuits

Astrocytes are ubiquitous within the central nervous system (CNS). These cells possess many individual processes which extend out into the neuropil, where they interact with a variety of other cell types, including neurons at synapses. Astrocytes are now known to be active players in all aspects of the synaptic life cycle, including synapse formation and elimination, synapse maturation, maintenance of synaptic homeostasis and modulation of synaptic transmission. Traditionally, astrocytes have been studied as a homogeneous group of cells. However, recent studies have uncovered a surprising degree of heterogeneity in their development and function, suggesting that astrocytes may be matched to neurons to support local circuits. Hence, a better understanding of astrocyte heterogeneity and its implications are needed to understand brain function.

Astrocytes Transplanted during Early Postnatal Development Integrate, Mature, and Survive Long Term in Mouse Cortex

Astrocytes have complex structural, molecular, and physiological properties and form specialized microenvironments that support circuit-specific functions in the CNS. To better understand how astrocytes acquire their unique features, we transplanted immature mouse cortical astrocytes into the developing cortex of male and female mice and assessed their integration, maturation, and survival. Within days, transplanted astrocytes developed morphologies and acquired territories and tiling behavior typical of cortical astrocytes. At 35-47 d post-transplantation, astrocytes appeared morphologically mature and expressed levels of EAAT2/GLT1 similar to nontransplanted astrocytes. Transplanted astrocytes also supported excitatory/inhibitory (E/I) presynaptic terminals within their territories, and displayed normal Ca2+ events. Transplanted astrocytes showed initially reduced expression of aquaporin 4 (AQP4) at endfeet and elevated expression of EAAT1/GLAST, with both proteins showing normalized expression by 110 d and one year post-transplantation, respectively. To understand how specific brain regions support astrocytic integration and maturation, we transplanted cortical astrocytes into the developing cerebellum. Cortical astrocytes interlaced with Bergmann glia (BG) in the cerebellar molecular layer to establish discrete territories. However, transplanted astrocytes retained many cortical astrocytic features including higher levels of EAAT2/GLT1, lower levels of EAAT1/GLAST, and the absence of expression of the AMPAR subunit GluA1. Collectively, our findings demonstrate that immature cortical astrocytes integrate, mature, and survive (more than one year) following transplantation and retain cortical astrocytic properties. Astrocytic transplantation can be useful for investigating cell-autonomous (intrinsic) and non-cell-autonomous (environmental) mechanisms contributing to astrocytic development/diversity, and for determining the optimal timing for transplanting astrocytes for cellular delivery or replacement in regenerative medicine.SIGNIFICANCE STATEMENT The mechanisms that enable astrocytes to acquire diverse molecular and structural properties remain to be better understood. In this study, we systematically analyzed the properties of cortical astrocytes following their transplantation to the early postnatal brain. We found that immature cortical astrocytes transplanted into cerebral cortex during early postnatal mouse development integrate and establish normal astrocytic properties, and show long-term survival in vivo (more than one year). In contrast, transplanted cortical astrocytes display reduced or altered ability to integrate into the more mature cerebral cortex or developing cerebellum, respectively. This study demonstrates the developmental potential of transplanted cortical astrocytes and provides an approach to tease apart cell-autonomous (intrinsic) and non-cell-autonomous (environmental) mechanisms that determine the structural, molecular, and physiological phenotype of astrocytes.

Monitoring regional astrocyte diversity by cell type-specific proteomic labeling in vivo

Astrocytes exhibit regional heterogeneity in morphology, function and molecular composition to support and modulate neuronal function and signaling in a region-specific manner. To characterize regional heterogeneity of astrocytic proteomes of different brain regions we established an inducible Aldh1l1-methionyl-tRNA-synthetase^L274G (MetRS^L274G ) mouse line that allows astrocyte-specific metabolic labeling of newly synthesized proteins by azidonorleucine (ANL) in vivo and subsequent isolation of tagged proteins by click chemistry. We analyzed astrocytic proteins from four different brain regions by mass spectrometry. The induced expression of MetRS^L274G is restricted to astrocytes and identified proteins show a high overlap with proteins compiled in "AstroProt," a newly established database for astrocytic proteins. Gene enrichment analysis reveals a high similarity among brain regions with subtle differences in enriched biological processes and in abundances of key astrocytic proteins for hippocampus, cortex and striatum. However, the cerebellar proteome stands out with proteins being highly associated with the calcium signaling pathway or with bipolar disorder. Subregional analysis of single astrocyte TAMRA intensities in hippocampal layers indicates distinct subregional heterogeneity of astrocytes and highlights the applicability of our toolbox to study differences of astrocytic proteomes in vivo.

Differential Glial Chitotriosidase 1 and Chitinase 3-like Protein 1 Expression in the Human Primary Visual Cortex and Cerebellum after Global Hypoxia-Ischemia

Here, we studied the neuroinflammation- and ischemia-related glial markers chitotriosidase 1 (CHIT1) and chitinase-3-like protein 1 (CHI3L1, alias YKL-40) in the human striate cortex and cerebellum at different time points after global hypoxic-ischemic brain injury (HIBI). Both regions differ considerably in their glial cell population but are supplied by the posterior circulation. CHIT1 and CHI3L1 expression was compared to changes in microglial (IBA1, CD68), astrocytic (GFAP, S100β), and neuronal markers (H&E, neurofilament heavy chain, NfH; calretinin, CALR) using immunohistochemistry and multiple-label immunofluorescence. Initial striatal cortical and cerebellar Purkinje cell damage, detectable already 1/2 d after HIBI, led to delayed neuronal death, whereas loss of cerebellar NfH-positive stellate and CALR-positive granule cells was variable. During the first week post-HIBI, a transient reduction of IBA1-positive microglia was observed in both regions, and fragmented/clasmatodendritic cerebellar Bergmann glia appeared. In long-term survivors, both brain regions displayed high densities of activated IBA1-positive cells and CD68-positive macrophages, which showed CHIT1 co-localization in the striate cortex. Furthermore, enlarged GFAP- and S100β-positive astroglia emerged in both regions around 9-10 d post-HIBI, i.e., along with clearance of dead neurons from the neuropil, although GFAP-/S100β-positive gemistocytic astrocytes that co-expressed CHI3L1 were found only in the striate cortex. Thus, only GFAP-/S100β-positive astrocytes in the striate cortex, but not cerebellar Bergmann glia, differentiated into CHI3L1-positive gemistocytes. CHIT1 was co-expressed almost entirely in macrophages in the striate cortex and not cerebellum of long-term survivors, thereby indicating that CHIT1 and CHI3L1 could be valuable biomarkers for monitoring the outcome of global HIBI.

Dentate gyrus astrocytes exhibit layer-specific molecular, morphological and physiological features

Neuronal heterogeneity has been established as a pillar of higher central nervous system function, but glial heterogeneity and its implications for neural circuit function are poorly understood. Here we show that the adult mouse dentate gyrus (DG) of the hippocampus is populated by molecularly distinct astrocyte subtypes that are associated with distinct DG layers. Astrocytes localized to different DG compartments also exhibit subtype-specific morphologies. Physiologically, astrocytes in upper DG layers form large syncytia, while those in lower DG compartments form smaller networks. Astrocyte subtypes differentially express glutamate transporters, which is associated with different amplitudes of glutamate transporter-mediated currents. Key molecular and morphological features of astrocyte diversity in the mice DG are conserved in humans. This adds another layer of complexity to our understanding of brain network composition and function, which will be crucial for further studies on astrocytes in health and disease.

Single nuclei RNA sequencing investigation of the Purkinje cell and glial changes in the cerebellum of transgenic Spinocerebellar ataxia type 1 mice

Glial cells constitute half the population of the human brain and are essential for normal brain function. Most, if not all, brain diseases are characterized by reactive gliosis, a process by which glial cells respond and contribute to neuronal pathology. Spinocerebellar ataxia type 1 (SCA1) is a progressive neurodegenerative disease characterized by a severe degeneration of cerebellar Purkinje cells (PCs) and cerebellar gliosis. SCA1 is caused by an abnormal expansion of CAG repeats in the gene Ataxin1 (ATXN1). While several studies reported the effects of mutant ATXN1 in Purkinje cells, it remains unclear how cerebellar glia respond to dysfunctional Purkinje cells in SCA1. To address this question, we performed single nuclei RNA sequencing (snRNA seq) on cerebella of early stage Pcp2-ATXN1[82Q] mice, a transgenic SCA1 mouse model expressing mutant ATXN1 only in Purkinje cells. We found no changes in neuronal and glial proportions in the SCA1 cerebellum at this early disease stage compared to wild-type controls. Importantly, we observed profound non-cell autonomous and potentially neuroprotective reactive gene and pathway alterations in Bergmann glia, velate astrocytes, and oligodendrocytes in response to Purkinje cell dysfunction.

Sonic hedgehog-dependent recruitment of GABAergic interneurons into the developing visual thalamus

Axons of retinal ganglion cells (RGCs) play critical roles in the development of inhibitory circuits in visual thalamus. We previously reported that RGC axons signal astrocytes to induce the expression of fibroblast growth factor 15 (FGF15), a motogen required for GABAergic interneuron migration into visual thalamus. However, how retinal axons induce thalamic astrocytes to generate Fgf15 and influence interneuron migration remains unknown. Here, we demonstrate that impairing RGC activity had little impact on interneuron recruitment into mouse visual thalamus. Instead, our data show that retinal-derived sonic hedgehog (SHH) is essential for interneuron recruitment. Specifically, we show that thalamus-projecting RGCs express SHH and thalamic astrocytes generate downstream components of SHH signaling. Deletion of RGC-derived SHH leads to a significant decrease in Fgf15 expression, as well as in the percentage of interneurons recruited into visual thalamus. Overall, our findings identify a morphogen-dependent neuron-astrocyte signaling mechanism essential for the migration of thalamic interneurons.

Cross-regional homeostatic and reactive glial signatures in multiple sclerosis

Multiple sclerosis (MS) is a multifocal and progressive inflammatory disease of the central nervous system (CNS). However, the compartmentalized pathology of the disease affecting various anatomical regions including gray and white matter and lack of appropriate disease models impede understanding of the disease. Utilizing single-nucleus RNA-sequencing and multiplex spatial RNA mapping, we generated an integrated transcriptomic map comprising leukocortical, cerebellar and spinal cord areas in normal and MS tissues that captures regional subtype diversity of various cell types with an emphasis on astrocytes and oligodendrocytes. While we found strong cross-regional diversity among glial subtypes in control tissue, regional signatures become more obscure in MS. This suggests that patterns of transcriptomic changes in MS are shared across regions and converge on specific pathways, especially those regulating cellular stress and immune activation. In addition, we found evidence that a subtype of white matter oligodendrocytes appearing across all three CNS regions adopt pro-remyelinating gene signatures in MS. In summary, our data suggest that cross-regional transcriptomic glial signatures overlap in MS, with different reactive glial cell types capable of either exacerbating or ameliorating pathology.

Spatial and Temporal Diversity of Astrocyte Phenotypes in Spinocerebellar Ataxia Type 1 Mice

While astrocyte heterogeneity is an important feature of the healthy brain, less is understood about spatiotemporal heterogeneity of astrocytes in brain disease. Spinocerebellar ataxia type 1 (SCA1) is a progressive neurodegenerative disease caused by a CAG repeat expansion in the gene Ataxin1 (ATXN1). We characterized astrocytes across disease progression in the four clinically relevant brain regions, cerebellum, brainstem, hippocampus, and motor cortex, of Atxn1^154Q/2Q mice, a knock-in mouse model of SCA1. We found brain region-specific changes in astrocyte density and GFAP expression and area, early in the disease and prior to neuronal loss. Expression of astrocytic core homeostatic genes was also altered in a brain region-specific manner and correlated with neuronal activity, indicating that astrocytes may compensate or exacerbate neuronal dysfunction. Late in disease, expression of astrocytic homeostatic genes was reduced in all four brain regions, indicating loss of astrocyte functions. We observed no obvious correlation between spatiotemporal changes in microglia and spatiotemporal astrocyte alterations, indicating a complex orchestration of glial phenotypes in disease. These results support spatiotemporal diversity of glial phenotypes as an important feature of the brain disease that may contribute to SCA1 pathogenesis in a brain region and disease stage-specific manner.