Molecular diversity of diencephalic astrocytes reveals adult astrogenesis regulated by Smad4

Dissecting the spatiotemporal diversity of adult neural stem cells

Adult stem cells are important for tissue turnover and regeneration. However, in most adult systems it remains elusive how stem cells assume different functional states and support spatially patterned tissue architecture. Here, we dissected the diversity of neural stem cells in the adult zebrafish brain, an organ that is characterized by pronounced zonation and high regenerative capacity. We combined single-cell transcriptomics of dissected brain regions with massively parallel lineage tracing and in vivo RNA metabolic labeling to analyze the regulation of neural stem cells in space and time. We detected a large diversity of neural stem cells, with some subtypes being restricted to a single brain region, while others were found globally across the brain. Global stem cell states are linked to neurogenic differentiation, with different states being involved in proliferative and non-proliferative differentiation. Our work reveals principles of adult stem cell organization and establishes a resource for the functional manipulation of neural stem cell subtypes.

High-resolution omics of vascular ageing and inflammatory pathways in neurodegeneration

High-resolution omics, particularly single-cell and spatial transcriptomic profiling, are rapidly enhancing our comprehension of the normal molecular diversity of gliovascular cells, as well as their age-related changes that contribute to neurodegeneration. With more omic profiling studies being conducted, it is becoming increasingly essential to synthesise valuable information from the rapidly accumulating findings. In this review, we present an overview of the molecular features of neurovascular and glial cells that have been recently discovered through omic profiling, with a focus on those that have potentially significant functional implications and/or show cross-species differences between human and mouse, and that are linked to vascular deficits and inflammatory pathways in ageing and neurodegenerative disorders. Additionally, we highlight the translational applications of omic profiling, and discuss omic-based strategies to accelerate biomarker discovery and facilitate disease course-modifying therapeutics development for neurodegenerative conditions.

Specialized astrocytes mediate glutamatergic gliotransmission in the CNS

Multimodal astrocyte-neuron communications govern brain circuitry assembly and function1. For example, through rapid glutamate release, astrocytes can control excitability, plasticity and synchronous activity2,3 of synaptic networks, while also contributing to their dysregulation in neuropsychiatric conditions4-7. For astrocytes to communicate through fast focal glutamate release, they should possess an apparatus for Ca2+-dependent exocytosis similar to neurons8-10. However, the existence of this mechanism has been questioned11-13 owing to inconsistent data14-17 and a lack of direct supporting evidence. Here we revisited the astrocyte glutamate exocytosis hypothesis by considering the emerging molecular heterogeneity of astrocytes18-21 and using molecular, bioinformatic and imaging approaches, together with cell-specific genetic tools that interfere with glutamate exocytosis in vivo. By analysing existing single-cell RNA-sequencing databases and our patch-seq data, we identified nine molecularly distinct clusters of hippocampal astrocytes, among which we found a notable subpopulation that selectively expressed synaptic-like glutamate-release machinery and localized to discrete hippocampal sites. Using GluSnFR-based glutamate imaging22 in situ and in vivo, we identified a corresponding astrocyte subgroup that responds reliably to astrocyte-selective stimulations with subsecond glutamate release events at spatially precise hotspots, which were suppressed by astrocyte-targeted deletion of vesicular glutamate transporter 1 (VGLUT1). Furthermore, deletion of this transporter or its isoform VGLUT2 revealed specific contributions of glutamatergic astrocytes in cortico-hippocampal and nigrostriatal circuits during normal behaviour and pathological processes. By uncovering this atypical subpopulation of specialized astrocytes in the adult brain, we provide insights into the complex roles of astrocytes in central nervous system (CNS) physiology and diseases, and identify a potential therapeutic target.

Longitudinal scRNA-seq analysis in mouse and human informs optimization of rapid mouse astrocyte differentiation protocols

Macroglia (astrocytes and oligodendrocytes) are required for normal development and function of the central nervous system, yet many questions remain about their emergence during the development of the brain and spinal cord. Here we used single-cell/single-nucleus RNA sequencing (scRNA-seq/snRNA-seq) to analyze over 298,000 cells and nuclei during macroglia differentiation from mouse embryonic and human-induced pluripotent stem cells. We computationally identify candidate genes involved in the fate specification of glia in both species and report heterogeneous expression of astrocyte surface markers across differentiating cells. We then used our transcriptomic data to optimize a previous mouse astrocyte differentiation protocol, decreasing the overall protocol length and complexity. Finally, we used multi-omic, dual single-nuclei (sn)RNA-seq/snATAC-seq analysis to uncover potential genomic regulatory sites mediating glial differentiation. These datasets will enable future optimization of glial differentiation protocols and provide insight into human glial differentiation.

The secreted neuronal signal Spock1 promotes blood-brain barrier development

The blood-brain barrier (BBB) is a unique set of properties of the brain vasculature which severely restrict its permeability to proteins and small molecules. Classic chick-quail chimera studies have shown that these properties are not intrinsic to the brain vasculature but rather are induced by surrounding neural tissue. Here, we identify Spock1 as a candidate neuronal signal for regulating BBB permeability in zebrafish and mice. Mosaic genetic analysis shows that neuronally expressed Spock1 is cell non-autonomously required for a functional BBB. Leakage in spock1 mutants is associated with altered extracellular matrix (ECM), increased endothelial transcytosis, and altered pericyte-endothelial interactions. Furthermore, a single dose of recombinant SPOCK1 partially restores BBB function in spock1 mutants by quenching gelatinase activity and restoring vascular expression of BBB genes including mcamb. These analyses support a model in which neuronally secreted Spock1 initiates BBB properties by altering the ECM, thereby regulating pericyte-endothelial interactions and downstream vascular gene expression.

Astrocyte-mediated mechanisms contribute to traumatic brain injury pathology

Astrocytes respond to traumatic brain injury (TBI) with changes to their molecular make-up and cell biology, which results in changes in astrocyte function. These changes can be adaptive, initiating repair processes in the brain, or detrimental, causing secondary damage including neuronal death or abnormal neuronal activity. The response of astrocytes to TBI is often-but not always-accompanied by the upregulation of intermediate filaments, including glial fibrillary acidic protein (GFAP) and vimentin. Because GFAP is often upregulated in the context of nervous system disturbance, reactive astrogliosis is sometimes treated as an "all-or-none" process. However, the extent of astrocytes' cellular, molecular, and physiological adjustments is not equal for each TBI type or even for each astrocyte within the same injured brain. Additionally, new research highlights that different neurological injuries and diseases result in entirely distinctive and sometimes divergent astrocyte changes. Thus, extrapolating findings on astrocyte biology from one pathological context to another is problematic. We summarize the current knowledge about astrocyte responses specific to TBI and point out open questions that the field should tackle to better understand how astrocytes shape TBI outcomes. We address the astrocyte response to focal versus diffuse TBI and heterogeneity of reactive astrocytes within the same brain, the role of intermediate filament upregulation, functional changes to astrocyte function including potassium and glutamate homeostasis, blood-brain barrier maintenance and repair, metabolism, and reactive oxygen species detoxification, sex differences, and factors influencing astrocyte proliferation after TBI. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology.

Postnatal expression of CD38 in astrocytes regulates synapse formation and adult social memory

Social behavior is essential for health, survival, and reproduction of animals; however, the role of astrocytes in social behavior remains largely unknown. The transmembrane protein CD38, which acts both as a receptor and ADP-ribosyl cyclase to produce cyclic ADP-ribose (cADPR) regulates social behaviors by promoting oxytocin release from hypothalamic neurons. CD38 is also abundantly expressed in astrocytes in the postnatal brain and is important for astroglial development. Here, we demonstrate that the astroglial-expressed CD38 plays an important role in social behavior during development. Selective deletion of CD38 in postnatal astrocytes, but not in adult astrocytes, impairs social memory without any other behavioral abnormalities. Morphological analysis shows that depletion of astroglial CD38 in the postnatal brain interferes with synapse formation in the medial prefrontal cortex (mPFC) and hippocampus. Moreover, astroglial CD38 expression promotes synaptogenesis of excitatory neurons by increasing the level of extracellular SPARCL1 (also known as Hevin), a synaptogenic protein. The release of SPARCL1 from astrocytes is regulated by CD38/cADPR/calcium signaling. These data demonstrate a novel developmental role of astrocytes in neural circuit formation and regulation of social behavior in adults.

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.

Diverse but unique astrocytic phenotypes during embryonic stem cell differentiation, culturing and development

Astrocytes are resident glial cells of the central nervous system (CNS) that play complex and heterogeneous roles in brain development, homeostasis and disease. Since their vast involvement in health and disease is becoming increasingly recognized, suitable and reliable tools for studying these cells in vivo and in vitro are of utmost importance. One of the key challenges hereby is to adequately mimic their context-dependent in vivo phenotypes and functions in vitro. To better understand the spectrum of astrocytic variations in defined settings we performed a side-by-side-comparison of murine embryonic stem cell (ESC)-derived astrocytes as well as primary neonatal and adult astrocytes, revealing major differences on a functional and transcriptomic level, specifically on proliferation, migration, calcium signaling and cilium activity. Our results highlight the need to carefully consider the choice of astrocyte origin and phenotype with respect to age, isolation and culture protocols based on the respective biological question.

Striatal spatial heterogeneity, clustering, and white matter association of GFAP+ astrocytes in a mouse model of Huntington's disease

Introduction

Huntington's disease (HD) is a neurodegenerative disease that primarily affects the striatum, a brain region that controls movement and some forms of cognition. Neuronal dysfunction and loss in HD is accompanied by increased astrocyte density and astrocyte pathology. Astrocytes are a heterogeneous population classified into multiple subtypes depending on the expression of different gene markers. Studying whether mutant Huntingtin (HTT) alters specific subtypes of astrocytes is necessary to understand their relative contribution to HD.

Methods

Here, we studied whether astrocytes expressing two different markers; glial fibrillary acidic protein (GFAP), associated with astrocyte activation, and S100 calcium-binding protein B (S100B), a marker of matured astrocytes and inflammation, were differentially altered in HD.

Results

First, we found three distinct populations in the striatum of WT and symptomatic zQ175 mice: GFAP⁺, S100B⁺, and dual GFAP⁺S100B⁺. The number of GFAP⁺ and S100B⁺ astrocytes throughout the striatum was increased in HD mice compared to WT, coinciding with an increase in HTT aggregation. Overlap between GFAP and S100B staining was expected, but dual GFAP⁺S100B⁺ astrocytes only accounted for less than 10% of all tested astrocytes and the number of GFAP⁺S100B⁺ astrocytes did not differ between WT and HD, suggesting that GFAP⁺ astrocytes and S100B⁺ astrocytes are distinct types of astrocytes. Interestingly, a spatial characterization of these astrocyte subtypes in HD mice showed that while S100B⁺ were homogeneously distributed throughout the striatum, GFAP⁺ preferentially accumulated in "patches" in the dorsomedial (dm) striatum, a region associated with goal-directed behaviors. In addition, GFAP⁺ astrocytes in the dm striatum of zQ175 mice showed increased clustering and association with white matter fascicles and were preferentially located in areas with low HTT aggregate load.

Discussion

In summary, we showed that GFAP⁺ and S100B⁺ astrocyte subtypes are distinctly affected in HD and exist in distinct spatial arrangements that may offer new insights to the function of these specific astrocytes subtypes and their potential implications in HD pathology.

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.

Mitochondrial dysfunction compromises ciliary homeostasis in astrocytes

Astrocytes, often considered as secondary responders to neurodegeneration, are emerging as primary drivers of brain disease. Here we show that mitochondrial DNA depletion in astrocytes affects their primary cilium, the signaling organelle of a cell. The progressive oxidative phosphorylation deficiency in astrocytes induces FOXJ1 and RFX transcription factors, known as master regulators of motile ciliogenesis. Consequently, a robust gene expression program involving motile cilia components and multiciliated cell differentiation factors are induced. While the affected astrocytes still retain a single cilium, these organelles elongate and become remarkably distorted. The data suggest that chronic activation of the mitochondrial integrated stress response (ISRmt) in astrocytes drives anabolic metabolism and promotes ciliary elongation. Collectively, our evidence indicates that an active signaling axis involving mitochondria and primary cilia exists and that ciliary signaling is part of ISRmt in astrocytes. We propose that metabolic ciliopathy is a novel pathomechanism for mitochondria-related neurodegenerative diseases.

Diet triggers specific responses of hypothalamic astrocytes in time and region dependent manner

Hypothalamic astrocytes are particularly affected by energy-dense food consumption. How the anatomical location of these glial cells and their spatial molecular distribution in the arcuate nucleus of the hypothalamus (ARC) determine the cellular response to a high caloric diet remains unclear. In this study, we investigated their distinctive molecular responses following exposure to a high-fat high-sugar (HFHS) diet, specifically in the ARC. Using RNA sequencing and proteomics, we showed that astrocytes have a distinct transcriptomic and proteomic profile dependent on their anatomical location, with a major proteomic reprogramming in hypothalamic astrocytes. By ARC single-cell sequencing, we observed that a HFHS diet dictates time- and cell- specific transcriptomic responses, revealing that astrocytes have the most distinct regulatory pattern compared to other cell types. Lastly, we topographically and molecularly characterized astrocytes expressing glial fibrillary acidic protein and/or aldehyde dehydrogenase 1 family member L1 in the ARC, of which the abundance was significantly increased, as well as the alteration in their spatial and molecular profiles, with a HFHS diet. Together, our results provide a detailed multi-omics view on the spatial and temporal changes of astrocytes particularly in the ARC during different time points of adaptation to a high calorie diet.

TGF-β as a Key Modulator of Astrocyte Reactivity: Disease Relevance and Therapeutic Implications

Astrocytes are essential for normal brain development and functioning. They respond to brain injury and disease through a process referred to as reactive astrogliosis, where the reactivity is highly heterogenous and context-dependent. Reactive astrocytes are active contributors to brain pathology and can exert beneficial, detrimental, or mixed effects following brain insults. Transforming growth factor-β (TGF-β) has been identified as one of the key factors regulating astrocyte reactivity. The genetic and pharmacological manipulation of the TGF-β signaling pathway in animal models of central nervous system (CNS) injury and disease alters pathological and functional outcomes. This review aims to provide recent understanding regarding astrocyte reactivity and TGF-β signaling in brain injury, aging, and neurodegeneration. Further, it explores how TGF-β signaling modulates astrocyte reactivity and function in the context of CNS disease and injury.

Astrogenesis in the murine dentate gyrus is a life-long and dynamic process

Astrocytes are highly abundant in the mammalian brain, and their functions are of vital importance for all aspects of development, adaption, and aging of the central nervous system (CNS). Mounting evidence indicates the important contributions of astrocytes to a wide range of neuropathies. Still, our understanding of astrocyte development significantly lags behind that of other CNS cells. We here combine immunohistochemical approaches with genetic fate-mapping, behavioral paradigms, single-cell transcriptomics, and in vivo two-photon imaging, to comprehensively assess the generation and the proliferation of astrocytes in the dentate gyrus (DG) across the life span of a mouse. Astrogenesis in the DG is initiated by radial glia-like neural stem cells giving rise to locally dividing astrocytes that enlarge the astrocyte compartment in an outside-in-pattern. Also in the adult DG, the vast majority of astrogenesis is mediated through the proliferation of local astrocytes. Interestingly, locally dividing astrocytes were able to adapt their proliferation to environmental and behavioral stimuli revealing an unexpected plasticity. Our study establishes astrocytes as enduring plastic elements in DG circuits, implicating a vital contribution of astrocyte dynamics to hippocampal plasticity.

Identification of Region-Specific Cytoskeletal and Molecular Alterations in Astrocytes of Mecp2 Deficient Animals

Rett syndrome (RTT) is a neurodevelopmental disorder that represents the most common genetic cause of severe intellectual disability in females. Most patients carry mutations in the X-linked MECP2 gene, coding for the methyl-CpG-binding protein 2 (MeCP2), originally isolated as an epigenetic transcriptional factor able to bind methylated DNA and repress transcription. Recent data implicated a role for glia in RTT, showing that astrocytes express Mecp2 and that its deficiency affects their ability to support neuronal maturation by non-cell autonomous mechanisms. To date, some molecular, structural and functional alterations have been attributed to Mecp2 null astrocytes, but how they evolve over time and whether they follow a spatial heterogeneity are two aspects which deserve further investigations. In this study, we assessed cytoskeletal features of astrocytes in Mecp2 deficient brains by analyzing their arbor complexity and processes in reconstructed GFAP⁺ cells at different ages, corresponding to peculiar stages of the disorder, and in different cerebral regions (motor and somatosensory cortices and CA1 layer of hippocampus). Our findings demonstrate the presence of defects in Mecp2 null astrocytes that worsen along disease progression and strictly depend on the brain area, highlighting motor and somatosensory cortices as the most affected regions. Of relevance, astrocyte cytoskeleton is impaired also in the somatosensory cortex of symptomatic heterozygous animals, with Mecp2 ⁺ astrocytes showing slightly more pronounced defects with respect to the Mecp2 null cells, emphasizing the importance of non-cell autonomous effects. We reported a temporal correlation between the progressive thinning of layer I and the atrophy of astrocytes, suggesting that their cytoskeletal dysfunctions might contribute to cortical defects. Considering the reciprocal link between morphology and function in astrocytes, we analyzed the effect of Mecp2 deficiency on the expression of selected astrocyte-enriched genes, which describe typical astrocytic features. qRT-PCR data corroborated our results, reporting an overall decrement of gene expression, which is area and age-dependent. In conclusion, our data show that Mecp2 deficiency causes structural and molecular alterations in astrocytes, which progress along with the severity of symptoms and diversely occur in the different cerebral regions, highlighting the importance of considering heterogeneity when studying astrocytes in RTT.

Reprogramming cellular identity in vivo

Cellular identity is established through complex layers of genetic regulation, forged over a developmental lifetime. An expanding molecular toolbox is allowing us to manipulate these gene regulatory networks in specific cell types in vivo. In principle, if we found the right molecular tricks, we could rewrite cell identity and harness the rich repertoire of possible cellular functions and attributes. Recent work suggests that this rewriting of cell identity is not only possible, but that newly induced cells can mitigate disease phenotypes in animal models of major human diseases. So, is the sky the limit, or do we need to keep our feet on the ground? This Spotlight synthesises key concepts emerging from recent efforts to reprogramme cellular identity in vivo. We provide our perspectives on recent controversies in the field of glia-to-neuron reprogramming and identify important gaps in our understanding that present barriers to progress.

Direct neuronal reprogramming: Fast forward from new concepts toward therapeutic approaches

Differentiated cells have long been considered fixed in their identity. However, about 20 years ago, the first direct conversion of glial cells into neurons in vitro opened the field of "direct neuronal reprogramming." Since then, neuronal reprogramming has achieved the generation of fully functional, mature neurons with remarkable efficiency, even in diseased brain environments. Beyond their clinical implications, these discoveries provided basic insights into crucial mechanisms underlying conversion of specific cell types into neurons and maintenance of neuronal identity. Here we discuss such principles, including the importance of the starter cell for shaping the outcome of neuronal reprogramming. We further highlight technical concerns for in vivo reprogramming and propose a code of conduct to avoid artifacts and pitfalls. We end by pointing out next challenges for development of less invasive cell replacement therapies for humans.

Molecular diversity of diencephalic astrocytes reveals adult astrogenesis regulated by Smad4

Astrocytes regulate brain‐wide functions and also show region‐specific differences, but little is known about how general and region‐specific functions are aligned at the single‐cell level. To explore this, we isolated adult mouse diencephalic astrocytes by ACSA‐2‐mediated magnetic‐activated cell sorting (MACS). Single‐cell RNA‐seq revealed 7 gene expression clusters of astrocytes, with 4 forming a supercluster. Within the supercluster, cells differed by gene expression related to ion homeostasis or metabolism, with the former sharing gene expression with other regions and the latter being restricted to specific regions. All clusters showed expression of proliferation‐related genes, and proliferation of diencephalic astrocytes was confirmed by immunostaining. Clonal analysis demonstrated low level of astrogenesis in the adult diencephalon, but not in cerebral cortex grey matter. This led to the identification of Smad4 as a key regulator of diencephalic astrocyte in vivo proliferation and in vitro neurosphere formation. Thus, astrocytes show diverse gene expression states related to distinct functions with some subsets being more widespread while others are more regionally restricted. However, all share low‐level proliferation revealing the novel concept of adult astrogenesis in the diencephalon.