Activity-dependent modulation of synapse-regulating genes in astrocytes

Disease-relevant upregulation of P2Y1 receptor in astrocytes enhances neuronal excitability via IGFBP2

Reactive astrocytes play a pivotal role in the pathogenesis of neurological diseases; however, their functional phenotype and the downstream molecules by which they modify disease pathogenesis remain unclear. Here, we genetically increase P2Y1 receptor (P2Y1R) expression, which is upregulated in reactive astrocytes in several neurological diseases, in astrocytes of male mice to explore its function and the downstream molecule. This astrocyte-specific P2Y1R overexpression causes neuronal hyperexcitability by increasing both astrocytic and neuronal Ca2+ signals. We identify insulin-like growth factor-binding protein 2 (IGFBP2) as a downstream molecule of P2Y1R in astrocytes; IGFBP2 acts as an excitatory signal to cause neuronal excitation. In neurological disease models of epilepsy and stroke, reactive astrocytes upregulate P2Y1R and increase IGFBP2. The present findings identify a mechanism underlying astrocyte-driven neuronal hyperexcitability, which is likely to be shared by several neurological disorders, providing insights that might be relevant for intervention in diverse neurological disorders.

Astrocyte Regulation of Synapse Formation, Maturation, and Elimination

Astrocytes play an integral role in the development, maturation, and refinement of neuronal circuits. Astrocytes secrete proteins and lipids that instruct the formation of new synapses and induce the maturation of existing synapses. Through contact-mediated signaling, astrocytes can regulate the formation and state of synapses within their domain. Through phagocytosis, astrocytes participate in the elimination of excess synaptic connections. In this work, we will review key findings on the molecular mechanisms of astrocyte-synapse interaction with a focus on astrocyte-secreted factors, contact-mediated mechanisms, and synapse elimination. We will discuss this in the context of typical brain development and maintenance, as well as consider the consequences of dysfunction in these pathways in neurological disorders, highlighting a role for astrocytes in health and disease.

Shared patterns of glial transcriptional dysregulation link Huntington's disease and schizophrenia

Huntington's disease and juvenile-onset schizophrenia have long been regarded as distinct disorders. However, both manifest cell-intrinsic abnormalities in glial differentiation, with resultant astrocytic dysfunction and hypomyelination. To assess whether a common mechanism might underlie the similar glial pathology of these otherwise disparate conditions, we used comparative correlation network approaches to analyse RNA-sequencing data from human glial progenitor cells (hGPCs) produced from disease-derived pluripotent stem cells. We identified gene sets preserved between Huntington's disease and schizophrenia hGPCs yet distinct from normal controls that included 174 highly connected genes in the shared disease-associated network, focusing on genes involved in synaptic signalling. These synaptic genes were largely suppressed in both schizophrenia and Huntington's disease hGPCs, and gene regulatory network analysis identified a core set of upstream regulators of this network, of which OLIG2 and TCF7L2 were prominent. Among their downstream targets, ADGRL3, a modulator of glutamatergic synapses, was notably suppressed in both schizophrenia and Huntington's disease hGPCs. Chromatin immunoprecipitation sequencing confirmed that OLIG2 and TCF7L2 each bound to the regulatory region of ADGRL3, whose expression was then rescued by lentiviral overexpression of these transcription factors. These data suggest that the disease-associated suppression of OLIG2 and TCF7L2-dependent transcription of glutamate signalling regulators may impair glial receptivity to neuronal glutamate. The consequent loss of activity-dependent mobilization of hGPCs may yield deficient oligodendrocyte production, and hence the hypomyelination noted in these disorders, as well as the disrupted astrocytic differentiation and attendant synaptic dysfunction associated with each. Together, these data highlight the importance of convergent glial molecular pathology in both the pathogenesis and phenotypic similarities of two otherwise unrelated disorders, Huntington's disease and schizophrenia.

SynBot: An open-source image analysis software for automated quantification of synapses

The formation of precise numbers of neuronal connections, known as synapses, is crucial for brain function. Therefore, synaptogenesis mechanisms have been one of the main focuses of neuroscience. Immunohistochemistry is a common tool for visualizing synapses. Thus, quantifying the numbers of synapses from light microscopy images enables screening the impacts of experimental manipulations on synapse development. Despite its utility, this approach is paired with low throughput analysis methods that are challenging to learn and results are variable between experimenters, especially when analyzing noisy images of brain tissue. We developed an open-source ImageJ-based software, SynBot, to address these technical bottlenecks by automating the analysis. SynBot incorporates the advanced algorithms ilastik and SynQuant for accurate thresholding for synaptic puncta identification, and the code can easily be modified by users. The use of this software will allow for rapid and reproducible screening of synaptic phenotypes in healthy and diseased nervous systems.

Suppression of astrocyte BMP signaling improves fragile X syndrome molecular signatures and functional deficits

Fragile X syndrome (FXS) is a monogenic neurodevelopmental disorder with manifestations spanning molecular, neuroanatomical, and behavioral changes. Astrocytes contribute to FXS pathogenesis and show hundreds of dysregulated genes and proteins; targeting upstream pathways mediating astrocyte changes in FXS could therefore be a point of intervention. To address this, we focused on the bone morphogenetic protein (BMP) pathway, which is upregulated in FXS astrocytes. We generated a conditional KO (cKO) of Smad4 in astrocytes to suppress BMP signaling, and found this lessens audiogenic seizure severity in FXS mice. To ask how this occurs on a molecular level, we performed in vivo transcriptomic and proteomic profiling of cortical astrocytes, finding upregulation of metabolic pathways, and downregulation of secretory machinery and secreted proteins in FXS astrocytes, with these alterations no longer present when BMP signaling is suppressed. Functionally, astrocyte Smad4 cKO restores deficits in inhibitory synapses present in FXS auditory cortex. Thus, astrocytes contribute to FXS molecular and functional phenotypes, and targeting astrocytes can mitigate FXS symptoms.

Astrocyte modulation of synaptic plasticity mediated by activity-dependent Sonic hedgehog signaling

The influence of neural activity on astrocytes and their reciprocal interactions with neurons has emerged as an important modulator of synapse function. Astrocytes exhibit activity-dependent changes in gene expression, yet the molecular mechanisms by which they accomplish this have remained largely unknown. The molecular signaling pathway, Sonic hedgehog (Shh), mediates neuron-astrocyte communication and regulates the organization of cortical synapses. Here, we demonstrate that neural activity stimulates Shh signaling in cortical astrocytes and upregulates expression of Hevin and SPARC, astrocyte derived molecules that modify synapses. Whisker stimulation and chemogenetic activation both increase Shh activity in deep layers of the somatosensory cortex, where neuron-astrocyte Shh signaling is predominantly found. Experience-dependent Hevin and SPARC require intact Shh signaling and selective loss of pathway activity in astrocytes occludes experience-dependent structural plasticity. Taken together, these data identify Shh signaling as an activity-dependent, neuronal derived cue that stimulates astrocyte interactions with synapses and promotes synaptic plasticity.

Network-level encoding of local neurotransmitters in cortical astrocytes

Astrocytes, the most abundant non-neuronal cell type in the mammalian brain, are crucial circuit components that respond to and modulate neuronal activity through calcium (Ca2+) signalling1-7. Astrocyte Ca2+ activity is highly heterogeneous and occurs across multiple spatiotemporal scales-from fast, subcellular activity3,4 to slow, synchronized activity across connected astrocyte networks8-10-to influence many processes5,7,11. However, the inputs that drive astrocyte network dynamics remain unclear. Here we used ex vivo and in vivo two-photon astrocyte imaging while mimicking neuronal neurotransmitter inputs at multiple spatiotemporal scales. We find that brief, subcellular inputs of GABA and glutamate lead to widespread, long-lasting astrocyte Ca2+ responses beyond an individual stimulated cell. Further, we find that a key subset of Ca2+ activity-propagative activity-differentiates astrocyte network responses to these two main neurotransmitters, and may influence responses to future inputs. Together, our results demonstrate that local, transient neurotransmitter inputs are encoded by broad cortical astrocyte networks over a minutes-long time course, contributing to accumulating evidence that substantial astrocyte-neuron communication occurs across slow, network-level spatiotemporal scales12-14. These findings will enable future studies to investigate the link between specific astrocyte Ca2+ activity and specific functional outputs, which could build a consistent framework for astrocytic modulation of neuronal activity.

Transcriptomic alterations in cortical astrocytes following the development of post-traumatic epilepsy

Post-traumatic epilepsy (PTE) stands as one of the numerous debilitating consequences that follow traumatic brain injury (TBI). Despite its impact on many individuals, the current landscape offers only a limited array of reliable treatment options, and our understanding of the underlying mechanisms and susceptibility factors remains incomplete. Among the potential contributors to epileptogenesis, astrocytes, a type of glial cell, have garnered substantial attention as they are believed to promote hyperexcitability and the development of seizures in the brain following TBI. The current study evaluated the transcriptomic changes in cortical astrocytes derived from animals that developed seizures as a result of severe focal TBI. Using RNA-Seq and ingenuity pathway analysis (IPA), we unveil a distinct gene expression profile in astrocytes, including alterations in genes supporting inflammation, early response modifiers, and neuropeptide-amidating enzymes. The findings underscore the complex molecular dynamics in astrocytes during PTE development, offering insights into therapeutic targets and avenues for further exploration.

Multicore fiber optic imaging reveals that astrocyte calcium activity in the mouse 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 in response to a rise in cytosolic calcium. However, our knowledge about how astrocytes are recruited during different animal behaviors remains limited. To measure astrocyte activity calcium in vivo during normative behaviors, we utilize a high-resolution, long working distance multicore fiber optic imaging system that allows visualization of individual astrocyte calcium transients in the cerebral cortex of freely moving mice. We define the spatiotemporal dynamics of astrocyte calcium changes during diverse behaviors, ranging from sleep-wake cycles to the exploration of novel objects, showing that their activity is more variable and less synchronous than apparent in head-immobilized imaging conditions. In accordance with their molecular diversity, individual astrocytes often exhibit distinct thresholds and activity patterns during explorative behaviors, allowing temporal encoding across the astrocyte network. Astrocyte calcium events were induced by noradrenergic and cholinergic systems and modulated by internal state. The distinct activity patterns exhibited by astrocytes provides a means to vary their neuromodulatory influence in different behavioral contexts and internal states.

Nacc1 Mutation in Mice Models Rare Neurodevelopmental Disorder with Underlying Synaptic Dysfunction

A missense mutation in the transcription repressor Nucleus accumbens-associated 1 (NACC1) gene at c.892C>T (p.Arg298Trp) on chromosome 19 causes severe neurodevelopmental delay ( Schoch et al., 2017). To model this disorder, we engineered the first mouse model with the homologous mutation (Nacc1+/R284W ) and examined mice from E17.5 to 8 months. Both genders had delayed weight gain, epileptiform discharges and altered power spectral distribution in cortical electroencephalogram, behavioral seizures, and marked hindlimb clasping; females displayed thigmotaxis in an open field. In the cortex, NACC1 long isoform, which harbors the mutation, increased from 3 to 6 months, whereas the short isoform, which is not present in humans and lacks aaR284 in mice, rose steadily from postnatal day (P) 7. Nuclear NACC1 immunoreactivity increased in cortical pyramidal neurons and parvalbumin containing interneurons but not in nuclei of astrocytes or oligodendroglia. Glial fibrillary acidic protein staining in astrocytic processes was diminished. RNA-seq of P14 mutant mice cortex revealed over 1,000 differentially expressed genes (DEGs). Glial transcripts were downregulated and synaptic genes upregulated. Top gene ontology terms from upregulated DEGs relate to postsynapse and ion channel function, while downregulated DEGs enriched for terms relating to metabolic function, mitochondria, and ribosomes. Levels of synaptic proteins were changed, but number and length of synaptic contacts were unaltered at 3 months. Homozygosity worsened some phenotypes including postnatal survival, weight gain delay, and increase in nuclear NACC1. This mouse model simulates a rare form of autism and will be indispensable for assessing pathophysiology and targets for therapeutic intervention.

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.

Lesion-remote astrocytes govern microglia-mediated white matter repair

Spared regions of the damaged central nervous system undergo dynamic remodeling and exhibit a remarkable potential for therapeutic exploitation. Here, lesion-remote astrocytes (LRAs), which interact with viable neurons, glia and neural circuitry, undergo reactive transformations whose molecular and functional properties are poorly understood. Using multiple transcriptional profiling methods, we interrogated LRAs from spared regions of mouse spinal cord following traumatic spinal cord injury (SCI). We show that LRAs acquire a spectrum of molecularly distinct, neuroanatomically restricted reactivity states that evolve after SCI. We identify transcriptionally unique reactive LRAs in degenerating white matter that direct the specification and function of local microglia that clear lipid-rich myelin debris to promote tissue repair. Fueling this LRA functional adaptation is Ccn1 , which encodes for a secreted matricellular protein. Loss of astrocyte CCN1 leads to excessive, aberrant activation of local microglia with (i) abnormal molecular specification, (ii) dysfunctional myelin debris processing, and (iii) impaired lipid metabolism, culminating in blunted debris clearance and attenuated neurological recovery from SCI. Ccn1 -expressing white matter astrocytes are specifically induced by local myelin damage and generated in diverse demyelinating disorders in mouse and human, pointing to their fundamental, evolutionarily conserved role in white matter repair. Our findings show that LRAs assume regionally divergent reactivity states with functional adaptations that are induced by local context-specific triggers and influence disorder outcome. Astrocytes tile the central nervous system (CNS) where they serve vital roles that uphold healthy nervous system function, including regulation of synapse development, buffering of neurotransmitters and ions, and provision of metabolic substrates 1 . In response to diverse CNS insults, astrocytes exhibit disorder-context specific transformations that are collectively referred to as reactivity 2-5 . The characteristics of regionally and molecularly distinct reactivity states are incompletely understood. The mechanisms through which distinct reactivity states arise, how they evolve or resolve over time, and their consequences for local cell function and CNS disorder progression remain enigmatic. Immediately adjacent to CNS lesions, border-forming astrocytes (BFAs) undergo transcriptional reprogramming and proliferation to form a neuroprotective barrier that restricts inflammation and supports axon regeneration 6-9 . Beyond the lesion, spared but dynamic regions of the injured CNS exhibit varying degrees of synaptic circuit remodeling and progressive cellular responses to secondary damage that have profound consequences for neural repair and recovery 10,11 . Throughout these cytoarchitecturally intact, but injury-reactive regions, lesion-remote astrocytes (LRAs) intermingle with neurons and glia, undergo little to no proliferation, and exhibit varying degrees of cellular hypertrophy 7,12,13 . The molecular and functional properties of LRAs remain grossly undefined. Therapeutically harnessing spared regions of the injured CNS will require a clearer understanding of the accompanying cellular and molecular landscape. Here, we leveraged integrative transcriptional profiling methodologies to identify multiple spatiotemporally resolved, molecularly distinct states of LRA reactivity within the injured spinal cord. Computational modeling of LRA-mediated heterotypic cell interactions, astrocyte-specific conditional gene deletion, and multiple mouse models of acute and chronic CNS white matter degeneration were used to interrogate a newly identified white matter degeneration-reactive astrocyte subtype. We define how this reactivity state is induced and its role in governing the molecular and functional specification of local microglia that clear myelin debris from the degenerating white matter to promote repair.

Astrocyte CCN1 stabilizes neural circuits in the adult brain

Neural circuits in many brain regions are refined by experience. Sensory circuits support higher plasticity at younger ages during critical periods - times of circuit refinement and maturation - and limit plasticity in adulthood for circuit stability. The mechanisms underlying these differing plasticity levels and how they serve to maintain and stabilize the properties of sensory circuits remain largely unclear. By combining a transcriptomic approach with ex vivo electrophysiology and in vivo imaging techniques, we identify that astrocytes release cellular communication network factor 1 (CCN1) to maintain synapse and circuit stability in the visual cortex. By overexpressing CCN1 in critical period astrocytes, we find that it promotes the maturation of inhibitory circuits and limits ocular dominance plasticity. Conversely, by knocking out astrocyte CCN1 in adults, binocular circuits are destabilized. These studies establish CCN1 as a novel astrocyte-secreted factor that stabilizes neuronal circuits. Moreover, they demonstrate that the composition and properties of sensory circuits require ongoing maintenance in adulthood, and that these maintenance cues are provided by astrocytes.

Astrocyte ensheathment of calyx-forming axons of the auditory brainstem precedes accelerated expression of myelin genes and myelination by oligodendrocytes

Early postnatal brain development involves complex interactions among maturing neurons and glial cells that drive tissue organization. We previously analyzed gene expression in tissue from the mouse medial nucleus of the trapezoid body (MNTB) during the first postnatal week to study changes that surround rapid growth of the large calyx of Held (CH) nerve terminal. Here, we present genes that show significant changes in gene expression level during the second postnatal week, a developmental timeframe that brackets the onset of airborne sound stimulation and the early stages of myelination. Gene Ontology analysis revealed that many of these genes are related to the myelination process. Further investigation of these genes using a previously published cell type-specific bulk RNA-Seq data set in cortex and our own single-cell RNA-Seq data set in the MNTB revealed enrichment of these genes in the oligodendrocyte lineage (OL) cells. Combining the postnatal day (P)6-P14 microarray gene expression data with the previously published P0-P6 data provided fine temporal resolution to investigate the initiation and subsequent waves of gene expression related to OL cell maturation and the process of myelination. Many genes showed increasing expression levels between P2 and P6 in patterns that reflect OL cell maturation. Correspondingly, the first myelin proteins were detected by P4. Using a complementary, developmental series of electron microscopy 3D image volumes, we analyzed the temporal progression of axon wrapping and myelination in the MNTB. By employing a combination of established ultrastructural criteria to classify reconstructed early postnatal glial cells in the 3D volumes, we demonstrated for the first time that astrocytes within the mouse MNTB extensively wrap the axons of the growing CH terminal prior to OL cell wrapping and compaction of myelin. Our data revealed significant expression of several myelin genes and enrichment of multiple genes associated with lipid metabolism in astrocytes, which may subserve axon wrapping in addition to myelin formation. The transition from axon wrapping by astrocytes to OL cells occurs rapidly between P4 and P9 and identifies a potential new role of astrocytes in priming calyceal axons for subsequent myelination.

Astroglial Cells: Emerging Therapeutic Targets in the Management of Traumatic Brain Injury

Traumatic Brain Injury (TBI) represents a significant health concern, necessitating advanced therapeutic interventions. This detailed review explores the critical roles of astrocytes, key cellular constituents of the central nervous system (CNS), in both the pathophysiology and possible rehabilitation of TBI. Following injury, astrocytes exhibit reactive transformations, differentiating into pro-inflammatory (A1) and neuroprotective (A2) phenotypes. This paper elucidates the interactions of astrocytes with neurons, their role in neuroinflammation, and the potential for their therapeutic exploitation. Emphasized strategies encompass the utilization of endocannabinoid and calcium signaling pathways, hormone-based treatments like 17β-estradiol, biological therapies employing anti-HBGB1 monoclonal antibodies, gene therapy targeting Connexin 43, and the innovative technique of astrocyte transplantation as a means to repair damaged neural tissues.

Gene Expression at the Tripartite Synapse: Bridging the Gap Between Neurons and Astrocytes

Astrocytes, a major class of glial cells, are an important element at the synapse where they engage in bidirectional crosstalk with neurons to regulate numerous aspects of neurotransmission, circuit function, and behavior. Mutations in synapse-related genes expressed in both neurons and astrocytes are central factors in a vast number of neurological disorders, making the proteins that they encode prominent targets for therapeutic intervention. Yet, while the roles of many of these synaptic proteins in neurons are well established, the functions of the same proteins in astrocytes are largely unknown. This gap in knowledge must be addressed to refine therapeutic approaches. In this chapter, we integrate multiomic meta-analysis and a comprehensive overview of current literature to show that astrocytes express an astounding number of genes that overlap with the neuronal and synaptic transcriptomes. Further, we highlight recent reports that characterize the expression patterns and potential novel roles of these genes in astrocytes in both physiological and pathological conditions, underscoring the importance of considering both cell types when investigating the function and regulation of synaptic proteins.

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.

Neuron-Astrocyte Interactions: A Human Perspective

This chapter explores the intricate interactions between neurons and astrocytes within the nervous system with a particular emphasis on studies conducted in human tissue or with human cells. We specifically explore how neuron-astrocyte interactions relate to processes of cellular development, morphology, migration, synapse formation, and metabolism. These findings enrich our understanding of basic neurobiology and how disruptions in these processes are relevant to human diseases.The study of human neuron-astrocyte interactions is made possible because of transformative in vitro advancements that have facilitated the generation and sustained culture of human neural cells. In addition, the rise of techniques like sequencing at single-cell resolution has enabled the exploration of numerous human cell atlases and their comparisons to other animal model systems. Thus, the innovations outlined in this chapter illuminate the convergence and divergence of neuron-astrocyte interactions across species. As technologies progress, continually more sophisticated in vitro systems will increasingly reflect in vivo environments and deepen our command of neuron-glial interactions in human biology.

Protocol for the purification and transcriptomic analysis of mouse astrocytes using GFAT

Astrocytes are glial cells of the central nervous system that modulate neuronal function. Here, we present glyoxal-fixed astrocyte nuclei transcriptomics (GFAT), a protocol for the purification and transcriptomic analysis of astrocyte nuclei from the cortex and cerebellum of adult and aged fresh mouse brain. We describe steps for tissue dissection, glyoxal fixation, homogenization, nuclei isolation, antibody staining, fluorescence-activated cell sorting, and RT-qPCR or bulk RNA sequencing. GFAT does not require transgenic lines or viral injection and allows parallel astrocyte and neuron profiling.

Network-level encoding of local neurotransmitters in cortical astrocytes

Astrocytes-the most abundant non-neuronal cell type in the mammalian brain-are crucial circuit components that respond to and modulate neuronal activity via calcium (Ca 2+ ) signaling 1-8 . Astrocyte Ca 2+ activity is highly heterogeneous and occurs across multiple spatiotemporal scales: from fast, subcellular activity 3,4 to slow, synchronized activity that travels across connected astrocyte networks 9-11 . Furthermore, astrocyte network activity has been shown to influence a wide range of processes 5,8,12 . While astrocyte network activity has important implications for neuronal circuit function, the inputs that drive astrocyte network dynamics remain unclear. Here we used ex vivo and in vivo two-photon Ca 2+ imaging of astrocytes while mimicking neuronal neurotransmitter inputs at multiple spatiotemporal scales. We find that brief, subcellular inputs of GABA and glutamate lead to widespread, long-lasting astrocyte Ca 2+ responses beyond an individual stimulated cell. Further, we find that a key subset of Ca 2+ activity-propagative events-differentiates astrocyte network responses to these two major neurotransmitters, and gates responses to future inputs. Together, our results demonstrate that local, transient neurotransmitter inputs are encoded by broad cortical astrocyte networks over the course of minutes, contributing to accumulating evidence across multiple model organisms that significant astrocyte-neuron communication occurs across slow, network-level spatiotemporal scales 13-15 . We anticipate that this study will be a starting point for future studies investigating the link between specific astrocyte Ca 2+ activity and specific astrocyte functional outputs, which could build a consistent framework for astrocytic modulation of neuronal activity.

Astrocyte coverage of excitatory synapses correlates to measures of synapse structure and function in primary visual cortex

Most excitatory synapses in the mammalian brain are contacted by astrocytes, forming the tripartite synapse. This interface is thought to be critical for glutamate turnover and structural or functional dynamics of synapses. While the degree of synaptic contact of astrocytes is known to vary across brain regions and animal species, the implications of this variability remain unknown. Furthermore, precisely how astrocyte coverage of synapses relates to in vivo functional properties of individual dendritic spines has yet to be investigated. Here, we characterized perisynaptic astrocyte processes (PAPs) contacting synapses of pyramidal neurons of the ferret visual cortex and, using correlative light and electron microscopy, examined their relationship to synaptic strength and to sensory-evoked Ca2+ activity. Nearly all synapses were contacted by PAPs, and most were contacted along the axon-spine interface (ASI). Structurally, we found that the degree of PAP coverage scaled with synapse size and complexity. Functionally, we found that PAP coverage scaled with the selectivity of Ca2+ responses of individual synapses to visual stimuli and, at least for the largest synapses, scaled with the reliability of visual stimuli to evoke postsynaptic Ca2+ events. Our study shows astrocyte coverage is highly correlated with structural properties of excitatory synapses in the visual cortex and implicates astrocytes as a contributor to reliable sensory activation.

Absence of chordin-like 1 aids motor recovery in a mouse model of stroke

Chordin-like 1 (Chrdl1) is an astrocyte-secreted protein that regulates synaptic maturation, and limits plasticity via GluA2-containing AMPA receptors (AMPARs). It was demonstrated that Chrdl1 expression is very heterogeneous throughout the brain, and it is enriched in astrocytes in cortical layers 2/3, with peak expression in the visual cortex at postnatal day 14. In response to ischemic stroke, Chrdl1 is upregulated during the acute and sub-acute phases in the peri-infarct region, potentially hindering recovery after stroke. Here, we used photothrombosis to model ischemic stroke in the motor cortex of adult male and female mice. In this study, we demonstrate that elimination of Chrdl1 in a global knock-out mouse reduces apoptotic cell death at early post-stroke stages and prevents ischemia-driven synaptic loss of AMPA receptors at later time points, all contributing to faster motor recovery. This suggests that synapse-regulating astrocyte-secreted proteins such as Chrdl1 have therapeutic potential to aid functional recovery after an ischemic injury.

Dysregulation of astrocyte-secreted pleiotrophin contributes to neuronal structural and functional deficits in Down Syndrome

Neuronal dendrite patterning and synapse formation are tightly regulated during development to promote proper connectivity. Astrocyte-secreted proteins act as guidance and pro-synaptogenic factors during development, but little is known about how astrocytes may contribute to neurodevelopmental disorders. Here we identify down-regulation of the astrocyte-secreted molecule pleiotrophin as a major contributor to neuronal morphological alterations in the Ts65Dn mouse model of Down Syndrome. We find overlapping deficits in neuronal dendrites, spines and intracortical synapses in Ts65Dn mutant and pleiotrophin knockout mice. By targeting pleiotrophin overexpression to astrocytes in adult Ts65Dn mutant mice in vivo , we show that pleiotrophin can rescue dendrite morphology and spine density and increase excitatory synapse number. We further demonstrate functional improvements in behavior. Our findings identify pleiotrophin as a molecule that can be used in Down Syndrome to promote proper circuit connectivity, importantly at later stages of development after typical periods of circuit refinement have completed.

δ-Catenin controls astrocyte morphogenesis via layer-specific astrocyte-neuron cadherin interactions

Astrocytes control the formation of specific synaptic circuits via cell adhesion and secreted molecules. Astrocyte synaptogenic functions are dependent on the establishment of their complex morphology. However, it is unknown if distinct neuronal cues differentially regulate astrocyte morphogenesis. δ-Catenin was previously thought to be a neuron-specific protein that regulates dendrite morphology. We found δ-catenin is also highly expressed by astrocytes and required both in astrocytes and neurons for astrocyte morphogenesis. δ-Catenin is hypothesized to mediate transcellular interactions through the cadherin family of cell adhesion proteins. We used structural modeling and biochemical analyses to reveal that δ-catenin interacts with the N-cadherin juxtamembrane domain to promote N-cadherin surface expression. An autism-linked δ-catenin point mutation impaired N-cadherin cell surface expression and reduced astrocyte complexity. In the developing mouse cortex, only lower-layer cortical neurons express N-cadherin. Remarkably, when we silenced astrocytic N-cadherin throughout the cortex, only lower-layer astrocyte morphology was disrupted. These findings show that δ-catenin controls astrocyte-neuron cadherin interactions that regulate layer-specific astrocyte morphogenesis.

Heterogeneity of perivascular astrocyte endfeet depending on vascular regions in the mouse brain

Astrocytes interact with not only synapses but also brain blood vessels through perivascular astrocyte endfeet (PV-AEF) to form the neurovascular unit (NVU). However, PV-AEF components have not been fully identified. Here, we biochemically isolated blood vessels from mouse brain homogenates and purified PV-AEF. The purified PV-AEF were observed in different sizes, similar to PV-AEF on brain blood vessels. Mass spectrometry analysis identified 9,762 proteins in the purified PV-AEF, including cell adhesion molecules, nectin-2δ, Kirrel2, and podoplanin. Immunofluorescence microscopic analysis revealed that nectin-2δ and podoplanin were concentrated mainly in arteries/arterioles and veins/venules of the mouse brain, whereas Kirrel2 was mainly in arteries/arterioles. Nectin-2α/δ, Kirrel2, and podoplanin were preferentially observed in large sizes of the purified PV-AEF. Furthermore, Kirrel2 potentially has cell adhesion activity of cultured astrocytes. Collectively, these results indicate that PV-AEF have heterogeneity in sizes and molecular components, implying different roles of PV-AEF in NVU function depending on vascular regions.

Glioblastoma initiation, migration, and cell types are regulated by core bHLH transcription factors ASCL1 and OLIG2

Glioblastomas (GBMs) are highly aggressive, infiltrative, and heterogeneous brain tumors driven by complex driver mutations and glioma stem cells (GSCs). The neurodevelopmental transcription factors ASCL1 and OLIG2 are co-expressed in GBMs, but their role in regulating the heterogeneity and hierarchy of GBM tumor cells is unclear. Here, we show that oncogenic driver mutations lead to dysregulation of ASCL1 and OLIG2, which function redundantly to initiate brain tumor formation in a mouse model of GBM. Subsequently, the dynamic levels and reciprocal binding of ASCL1 and OLIG2 to each other and to downstream target genes then determine the cell types and degree of migration of tumor cells. Single-cell RNA sequencing (scRNA-seq) reveals that a high level of ASCL1 is key in defining GSCs by upregulating a collection of ribosomal protein, mitochondrial, neural stem cell (NSC), and cancer metastasis genes - all essential for sustaining the high proliferation, migration, and therapeutic resistance of GSCs.

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.

Astrocytes adjust the dynamic range of cortical network activity to control modality-specific sensory information processing

Cortical neuron-astrocyte communication in response to peripheral sensory stimulation occurs in a topographic-, frequency-, and intensity-dependent manner. However, the contribution of this functional interaction to the processing of sensory inputs and consequent behavior remains unclear. We investigate the role of astrocytes in sensory information processing at circuit and behavioral levels by monitoring and manipulating astrocytic activity in vivo. We show that astrocytes control the dynamic range of the cortical network activity, optimizing its responsiveness to incoming sensory inputs. The astrocytic modulation of sensory processing contributes to setting the detection threshold for tactile and thermal behavior responses. The mechanism of such astrocytic control is mediated through modulation of inhibitory transmission to adjust the gain and sensitivity of responding networks. These results uncover a role for astrocytes in maintaining the cortical network activity in an optimal range to control behavior associated with specific sensory modalities.

Astroglial exosome HepaCAM signaling and ApoE antagonization coordinates early postnatal cortical pyramidal neuronal axon growth and dendritic spine formation

Developing astroglia play important roles in regulating synaptogenesis through secreted and contact signals. Whether they regulate postnatal axon growth is unknown. By selectively isolating exosomes using size-exclusion chromatography (SEC) and employing cell-type specific exosome reporter mice, our current results define a secreted astroglial exosome pathway that can spread long-range in vivo and stimulate axon growth of cortical pyramidal neurons. Subsequent biochemical and genetic studies found that surface expression of glial HepaCAM protein essentially and sufficiently mediates the axon-stimulating effect of astroglial exosomes. Interestingly, apolipoprotein E (ApoE), a major astroglia-secreted cholesterol carrier to promote synaptogenesis, strongly inhibits the stimulatory effect of astroglial exosomes on axon growth. Developmental ApoE deficiency also significantly reduces spine density of cortical pyramidal neurons. Together, our study suggests a surface contact mechanism of astroglial exosomes in regulating axon growth and its antagonization by ApoE, which collectively coordinates early postnatal pyramidal neuronal axon growth and dendritic spine formation.

Induction of astrocytic Slc22a3 regulates sensory processing through histone serotonylation

Neuronal activity drives alterations in gene expression within neurons, yet how it directs transcriptional and epigenomic changes in neighboring astrocytes in functioning circuits is unknown. We found that neuronal activity induces widespread transcriptional up-regulation and down-regulation in astrocytes, highlighted by the identification of Slc22a3 as an activity-inducible astrocyte gene that encodes neuromodulator transporter Slc22a3 and regulates sensory processing in the mouse olfactory bulb. Loss of astrocytic Slc22a3 reduced serotonin levels in astrocytes, leading to alterations in histone serotonylation. Inhibition of histone serotonylation in astrocytes reduced the expression of γ-aminobutyric acid (GABA) biosynthetic genes and GABA release, culminating in olfactory deficits. Our study reveals that neuronal activity orchestrates transcriptional and epigenomic responses in astrocytes while illustrating new mechanisms for how astrocytes process neuromodulatory input to gate neurotransmitter release for sensory processing.

Decoding Natural Astrocyte Rhythms: Dynamic Actin Waves Result from Environmental Sensing by Primary Rodent Astrocytes

Astrocytes are key regulators of brain homeostasis, equilibrating ion, water, and neurotransmitter concentrations and maintaining essential conditions for proper cognitive function. Recently, it has been shown that the excitability of the actin cytoskeleton manifests in second-scale dynamic fluctuations and acts as a sensor of chemophysical environmental cues. However, it is not known whether the cytoskeleton is excitable in astrocytes and how the homeostatic function of astrocytes is linked to the dynamics of the cytoskeleton. Here it is shown that homeostatic regulation involves the excitable dynamics of actin in certain subcellular regions of astrocytes, especially near the cell boundary. The results further indicate that actin dynamics concentrate into "hotspot" regions that selectively respond to certain chemophysical stimuli, specifically the homeostatic challenges of ion or water concentration increases. Substrate topography makes the actin dynamics of astrocytes weaker. Super-resolution images demonstrate that surface topography is also associated with the predominant perpendicular alignment of actin filaments near the cell boundary, whereas flat substrates result in an actin cortex mainly parallel to the cell boundary. Additionally, coculture with neurons increases both the probability of actin dynamics and the strength of hotspots. The excitable systems character of actin thus makes astrocytes direct participants in neural cell network dynamics.

Glial progenitor heterogeneity and key regulators revealed by single-cell RNA sequencing provide insight to regeneration in spinal cord injury

Recent studies have revealed the heterogeneous nature of astrocytes; however, how diverse constituents of astrocyte-lineage cells are regulated in adult spinal cord after injury and contribute to regeneration remains elusive. We perform single-cell RNA sequencing of GFAP-expressing cells from sub-chronic spinal cord injury models and identify and compare with the subpopulations in acute-stage data. We find subpopulations with distinct functional enrichment and their identities defined by subpopulation-specific transcription factors and regulons. Immunohistochemistry, RNAscope experiments, and quantification by stereology verify the molecular signature, location, and morphology of potential resident neural progenitors or neural stem cells in the adult spinal cord before and after injury and uncover the populations of the intermediate cells enriched in neuronal genes that could potentially transition into other subpopulations. This study has expanded the knowledge of the heterogeneity and cell state transition of glial progenitors in adult spinal cord before and after injury.

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.

On the origin and nature of nongenetic information in eumetazoans

Nongenetic information implies all the forms of biological information not related to genes and DNA in general. Despite the deep scientific relevance of the concept, we currently lack reliable knowledge about its carriers and origins; hence, we still do not understand its true nature. Given that genes are the targets of nongenetic information, it appears that a parsimonious approach to find the ultimate source of that information is to trace back the sequential steps of the causal chain upstream of the target genes up to the ultimate link as the source of the nongenetic information. From this perspective, I examine seven nongenetically determined phenomena: placement of locus-specific epigenetic marks on DNA and histones, changes in snRNA expression patterns, neural induction of gene expression, site-specific alternative gene splicing, predator-induced morphological changes, and cultural inheritance. Based on the available evidence, I propose a general model of the common neural origin of all these forms of nongenetic information in eumetazoans.

Inhibitory input directs astrocyte morphogenesis through glial GABABR

Communication between neurons and glia has an important role in establishing and maintaining higher-order brain function1. Astrocytes are endowed with complex morphologies, placing their peripheral processes in close proximity to neuronal synapses and directly contributing to their regulation of brain circuits2-4. Recent studies have shown that excitatory neuronal activity promotes oligodendrocyte differentiation5-7; whether inhibitory neurotransmission regulates astrocyte morphogenesis during development is unclear. Here we show that inhibitory neuron activity is necessary and sufficient for astrocyte morphogenesis. We found that input from inhibitory neurons functions through astrocytic GABAB receptor (GABABR) and that its deletion in astrocytes results in a loss of morphological complexity across a host of brain regions and disruption of circuit function. Expression of GABABR in developing astrocytes is regulated in a region-specific manner by SOX9 or NFIA and deletion of these transcription factors results in region-specific defects in astrocyte morphogenesis, which is conferred by interactions with transcription factors exhibiting region-restricted patterns of expression. Together, our studies identify input from inhibitory neurons and astrocytic GABABR as universal regulators of morphogenesis, while further revealing a combinatorial code of region-specific transcriptional dependencies for astrocyte development that is intertwined with activity-dependent processes.

Atypical Neurogenesis, Astrogliosis, and Excessive Hilar Interneuron Loss Are Associated with the Development of Post-Traumatic Epilepsy

BACKGROUND

Traumatic brain injury (TBI) remains a significant risk factor for post-traumatic epilepsy (PTE). The pathophysiological mechanisms underlying the injury-induced epileptogenesis are under investigation. The dentate gyrus-a structure that is highly susceptible to injury-has been implicated in the evolution of seizure development.

METHODS

Utilizing the murine unilateral focal control cortical impact (CCI) injury, we evaluated seizure onset using 24/7 EEG video analysis at 2-4 months post-injury. Cellular changes in the dentate gyrus and hilus of the hippocampus were quantified by unbiased stereology and Imaris image analysis to evaluate Prox1-positive cell migration, astrocyte branching, and morphology, as well as neuronal loss at four months post-injury. Isolation of region-specific astrocytes and RNA-Seq were performed to determine differential gene expression in animals that developed post-traumatic epilepsy (PTE+) vs. those animals that did not (PTE-), which may be associated with epileptogenesis.

RESULTS

CCI injury resulted in 37% PTE incidence, which increased with injury severity and hippocampal damage. Histological assessments uncovered a significant loss of hilar interneurons that coincided with aberrant migration of Prox1-positive granule cells and reduced astroglial branching in PTE+ compared to PTE- mice. We uniquely identified Cst3 as a PTE+-specific gene signature in astrocytes across all brain regions, which showed increased astroglial expression in the PTE+ hilus.

CONCLUSIONS

These findings suggest that epileptogenesis may emerge following TBI due to distinct aberrant cellular remodeling events and key molecular changes in the dentate gyrus of the hippocampus.

Differential Effects of Astrocyte Manipulations on Learned Motor Behavior and Neuronal Ensembles in the Motor Cortex

Although motor cortex is crucial for learning precise and reliable movements, whether and how astrocytes contribute to its plasticity and function during motor learning is unknown. Here, we report that astrocyte-specific manipulations in primary motor cortex (M1) during a lever push task alter motor learning and execution, as well as the underlying neuronal population coding. Mice that express decreased levels of the astrocyte glutamate transporter 1 (GLT1) show impaired and variable movement trajectories, whereas mice with increased astrocyte Gq signaling show decreased performance rates, delayed response times, and impaired trajectories. In both groups, which include male and female mice, M1 neurons have altered interneuronal correlations and impaired population representations of task parameters, including response time and movement trajectories. RNA sequencing further supports a role for M1 astrocytes in motor learning and shows changes in astrocytic expression of glutamate transporter genes, GABA transporter genes, and extracellular matrix protein genes in mice that have acquired this learned behavior. Thus, astrocytes coordinate M1 neuronal activity during motor learning, and our results suggest that this contributes to learned movement execution and dexterity through mechanisms that include regulation of neurotransmitter transport and calcium signaling.SIGNIFICANCE STATEMENT We demonstrate for the first time that in the M1 of mice, astrocyte function is critical for coordinating neuronal population activity during motor learning. We demonstrate that knockdown of astrocyte glutamate transporter GLT1 affects specific components of learning, such as smooth trajectory formation. Altering astrocyte calcium signaling by activation of Gq-DREADD upregulates GLT1 and affects other components of learning, such as response rates and reaction times as well as trajectory smoothness. In both manipulations, neuronal activity in motor cortex is dysregulated, but in different ways. Thus, astrocytes have a crucial role in motor learning via their influence on motor cortex neurons, and they do so by mechanisms that include regulation of glutamate transport and calcium signals.

Norepinephrine links astrocytic activity to regulation of cortical state

Cortical state, defined by population-level neuronal activity patterns, determines sensory perception. While arousal-associated neuromodulators-including norepinephrine (NE)-reduce cortical synchrony, how the cortex resynchronizes remains unknown. Furthermore, general mechanisms regulating cortical synchrony in the wake state are poorly understood. Using in vivo imaging and electrophysiology in mouse visual cortex, we describe a critical role for cortical astrocytes in circuit resynchronization. We characterize astrocytes' calcium responses to changes in behavioral arousal and NE, and show that astrocytes signal when arousal-driven neuronal activity is reduced and bi-hemispheric cortical synchrony is increased. Using in vivo pharmacology, we uncover a paradoxical, synchronizing response to Adra1a receptor stimulation. We reconcile these results by demonstrating that astrocyte-specific deletion of Adra1a enhances arousal-driven neuronal activity, while impairing arousal-related cortical synchrony. Our findings demonstrate that astrocytic NE signaling acts as a distinct neuromodulatory pathway, regulating cortical state and linking arousal-associated desynchrony to cortical circuit resynchronization.

Inhibitory input directs astrocyte morphogenesis through glial GABA B R

Communication between neurons and glia plays an important role in establishing and maintaining higher order brain function. Astrocytes are endowed with complex morphologies which places their peripheral processes in close proximity to neuronal synapses and directly contributes to their regulation of brain circuits. Recent studies have shown that excitatory neuronal activity promotes oligodendrocyte differentiation; whether inhibitory neurotransmission regulates astrocyte morphogenesis during development is unknown. Here we show that inhibitory neuron activity is necessary and sufficient for astrocyte morphogenesis. We found that input from inhibitory neurons functions through astrocytic GABA B R and that its deletion in astrocytes results in a loss of morphological complexity across a host of brain regions and disruption of circuit function. Expression of GABA B R in developing astrocytes is regulated in a region-specific manner by SOX9 or NFIA and deletion of these transcription factors results in region-specific defects in astrocyte morphogenesis, which is conferred by interactions with transcription factors exhibiting region-restricted patterns of expression. Together our studies identify input from inhibitory neurons and astrocytic GABA B R as universal regulators of morphogenesis, while further revealing a combinatorial code of region-specific transcriptional dependencies for astrocyte development that is intertwined with activity-dependent processes.

Astrocyte-synapse interactions during brain development

Bidirectional communication between astrocytes and neurons is essential for proper brain development. Astrocytes, a major glial cell type, are morphologically complex cells that directly interact with neuronal synapses to regulate synapse formation, maturation, and function. Astrocyte-secreted factors bind neuronal receptors to induce synaptogenesis with regional and circuit-level precision. Cell adhesion molecules mediate the direct contact between astrocytes and neurons, which is required for both synaptogenesis and astrocyte morphogenesis. Neuron-derived signals also shape astrocyte development, function, and molecular identity. This review highlights recent findings on the topic of astrocyte-synapse interactions, and discusses the importance of these interactions for synapse and astrocyte development.

Astrocytes in memory formation and maintenance

Learning and memory are fundamental but highly complex functions of the brain. They rely on multiple mechanisms including the processing of sensory information, memory formation, maintenance of short- and long-term memory, memory retrieval and memory extinction. Recent experiments provide strong evidence that, besides neurons, astrocytes crucially contribute to these higher brain functions. However, the complex interplay of astrocytes and neurons in local neuron-glia assemblies is far from being understood. Although important basic cellular principles that govern and link neuronal and astrocytic cellular functions have been established, additional mechanisms clearly continue to emerge. In this short essay, we first review current technologies allowing the experimenter to explore the role of astrocytes in behaving animals, with focus on spatial memory. We then discuss astrocytic signaling mechanisms and their role in learning and memory. We also reveal gaps in our knowledge that currently prevent a comprehensive understanding of how astrocytes contribute to acquisition, storage and retrieval of memory by modulating neuronal signaling in local circuits.

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.

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.

Activity-dependent induction of astrocytic Slc22a3 regulates sensory processing through histone serotonylation

Neuronal activity drives global alterations in gene expression within neurons, yet how it directs transcriptional and epigenomic changes in neighboring astrocytes in functioning circuits is unknown. Here we show that neuronal activity induces widespread transcriptional upregulation and downregulation in astrocytes, highlighted by the identification of a neuromodulator transporter Slc22a3 as an activity-inducible astrocyte gene regulating sensory processing in the olfactory bulb. Loss of astrocytic Slc22a3 reduces serotonin levels in astrocytes, leading to alterations in histone serotonylation. Inhibition of histone serotonylation in astrocytes reduces expression of GABA biosynthetic genes and GABA release, culminating in olfactory deficits. Our study reveals that neuronal activity orchestrates transcriptional and epigenomic responses in astrocytes, while illustrating new mechanisms for how astrocytes process neuromodulatory input to gate neurotransmitter release for sensory processing.

Astroglial exosome HepaCAM signaling and ApoE antagonization coordinates early postnatal cortical pyramidal neuronal axon growth and dendritic spine formation

Developing astroglia play important roles in regulating synaptogenesis through secreted and contact signals. Whether they regulate postnatal axon growth is unknown. By selectively isolating exosomes using size-exclusion chromatography (SEC) and employing cell-type specific exosome reporter mice, our current results define a secreted astroglial exosome pathway that can spread long-range in vivo and stimulate axon growth of cortical pyramidal neurons. Subsequent biochemical and genetic studies found that surface expression of glial HepaCAM protein essentially and sufficiently mediates the axon-stimulating effect of astroglial exosomes. Interestingly, apolipoprotein E (ApoE), a major astroglia-secreted cholesterol carrier to promote synaptogenesis, strongly inhibits the stimulatory effect of astroglial exosomes on axon growth. Developmental ApoE deficiency also significantly reduces spine density of cortical pyramidal neurons. Together, our study suggests a surface contact mechanism of astroglial exosomes in regulating axon growth and its antagonization by ApoE, which collectively coordinates early postnatal pyramidal neuronal axon growth and dendritic spine formation.

Far from home: the role of glial mRNA localization in synaptic plasticity

Neurons and glia are highly polarized cells, whose distal cytoplasmic functional subdomains require specific proteins. Neurons have axonal and dendritic cytoplasmic extensions containing synapses whose plasticity is regulated efficiently by mRNA transport and localized translation. The principles behind these mechanisms are equally attractive for explaining rapid local regulation of distal glial cytoplasmic projections, independent of their cell nucleus. However, in contrast to neurons, mRNA localization has received little experimental attention in glia. Nevertheless, there are many functionally diverse glial subtypes containing extensive networks of long cytoplasmic projections with likely localized regulation that influence neurons and their synapses. Moreover, glia have many other neuron-like properties, including electrical activity, secretion of gliotransmitters and calcium signaling, influencing, for example, synaptic transmission, plasticity and axon pruning. Here, we review previous studies concerning glial transcripts with important roles in influencing synaptic plasticity, focusing on a few cases involving localized translation. We discuss a variety of important questions about mRNA transport and localized translation in glia that remain to be addressed, using cutting-edge tools already available for neurons.

Dopamine-induced astrocytic Ca2+ signaling in mPFC is mediated by MAO-B in young mice, but by dopamine receptors in adult mice

Dopamine (DA) plays a vital role in brain physiology and pathology such as learning and memory, motor control, neurological diseases, and psychiatric diseases. In neurons, it has been well established that DA increases or decreases intracellular cyclic AMP (cAMP) through D₁-like or D₂-like dopamine receptors, respectively. In contrast, it has been elusive how astrocytes respond to DA via Ca²⁺ signaling and regulate synaptic transmission and reward systems. Previous studies suggest various molecular targets such as MAO-B, D₁R, or D₁R-D₂R heteromer to modulate astrocytic Ca²⁺ signaling. However, which molecular target is utilized under what physiological condition remains unclear. Here, we show that DA-induced astrocytic Ca²⁺ signaling pathway switches during development: MAO-B is the major player at a young age (5-6 weeks), whereas DA receptors (DARs) are responsible for the adult period (8-12 weeks). DA-mediated Ca²⁺ response in the adult period was decreased by either D₁R or D₂R blockers, which are primarily known for cyclic AMP signaling (G_s and G_i pathway, respectively), suggesting that this Ca²⁺ response might be mediated through G_q pathway by D₁R-D₂R heterodimer. Moreover, DAR-mediated Ca²⁺ response was not blocked by TTX, implying that this response is not a secondary response caused by neuronal activation. Our study proposes an age-specific molecular target of DA-induced astrocytic Ca²⁺ signaling: MAO-B in young mice and DAR in adult mice.

Post-Injury Buprenorphine Administration Is Associated with Long-Term Region-Specific Glial Alterations in Rats

Traumatic brain injury (TBI) is a major leading cause of death and disability. While previous studies regarding focal pathologies following TBI have been done, there is a lack of information concerning the role of analgesics and their influences on injury pathology. Buprenorphine (Bup), an opioid analgesic, is a commonly used analgesic in experimental TBI models. Our previous studies investigated the acute effects of Buprenorphine-sustained release-Lab (Bup-SR-Lab) on diffuse neuronal/glial pathology, neuroinflammation, cell damage, and systemic physiology. The current study investigated the longer-term chronic outcomes of Bup-SR-Lab treatment at 4 weeks following TBI utilizing a central fluid percussion injury (cFPI) model in adult male rats. Histological assessments of physiological changes, neuronal damage, cortical and thalamic cytokine expression, microglial and astrocyte morphological changes, and myelin alterations were done, as we had done in our acute study. In the current study the Whisker Nuisance Task (WNT) was also performed pre- and 4w post-injury to assess changes in somatosensory sensitivity following saline or Bup-SR-Lab treatment. Bup-SR-Lab treatment had no impact on overall physiology or neuronal damage at 4w post-injury regardless of region or injury, nor did it have any significant effects on somatosensory sensitivity. However, greater IL-4 cytokine expression with Bup-SR-Lab treatment was observed compared to saline treated animals. Microglia and astrocytes also demonstrated region-specific morphological alterations associated with Bup-SR-Lab treatment, in which cortical microglia and thalamic astrocytes were particularly vulnerable to Bup-mediated changes. There were discernable injury-specific and region-specific differences regarding myelin integrity and changes in specific myelin basic protein (MBP) isoform expression following Bup-SR-Lab treatment. This study indicates that use of Bup-SR-Lab could impact TBI-induced glial alterations in a region-specific manner 4w following diffuse brain injury.

Aberrant astrocyte protein secretion contributes to altered neuronal development in multiple models of neurodevelopmental disorders

Astrocytes negatively impact neuronal development in many models of neurodevelopmental disorders (NDs); however, how they do this, and if mechanisms are shared across disorders, is not known. In this study, we developed a cell culture system to ask how astrocyte protein secretion and gene expression change in three mouse models of genetic NDs (Rett, Fragile X and Down syndromes). ND astrocytes increase release of Igfbp2, a secreted inhibitor of insulin-like growth factor (IGF). IGF rescues neuronal deficits in many NDs, and we found that blocking Igfbp2 partially rescues inhibitory effects of Rett syndrome astrocytes, suggesting that increased astrocyte Igfbp2 contributes to decreased IGF signaling in NDs. We identified that increased BMP signaling is upstream of protein secretion changes, including Igfbp2, and blocking BMP signaling in Fragile X and Rett syndrome astrocytes reverses inhibitory effects on neurite outgrowth. This work provides a resource of astrocyte-secreted proteins in health and ND models and identifies novel targets for intervention in diverse NDs.

Biological underpinnings of sex differences in neurological disorders

The importance of sex differences in neurological disorders has been increasingly acknowledged in recent clinical and basic research studies, but the complex biology and genetics underlying sex-linked biological heterogeneity and its brain-to-body impact remained incompletely understood. Men and women differ substantially in their susceptibility to certain neurological diseases, in the severity of symptoms, prognosis as well as the nature and efficacy of their response to treatments. The detailed mechanisms underlying these differences, especially at the molecular level, are being addressed in many studies but leave a lot to be further revealed. Here, we provide an overview of recent advances in our understanding of how sex differences in the brain and brain-body signaling contribute to neurological disorders and further present some future prospects entailed in terms of diagnostics and therapeutics.