Astrocyte glypicans 4 and 6 promote formation of excitatory synapses via GluA1 AMPA receptors

Thoracic Spinal Cord Contusion Impacts on Lumbar Enlargement: Molecular Insights

Spinal cord injury (SCI) is characterized by macrostructural pathological changes in areas significantly distant from the primary injury site. The causes and mechanisms underlying these distant changes are still being explored. Identifying the causes and mechanisms of these changes in the lumbar spinal cord is particularly important for restoring motor function, especially in cases of injury to the proximal thoracic or cervical regions. This is because the lumbar region contains neural networks that play a crucial role in comprehensive locomotor outcomes. In our study, we investigated the changes in the rat lumbar spinal cord following a thoracic contusion injury. We observed an increased expression of osteopontin (OPN) in large neurons and a higher number of interneurons co-expressing parvalbumin and OPN within lamina IX of the ventral horns (VH) in the gray matter of the lumbar spinal cord post-injury. Additionally, here we noted an increased co-localization of the glial fibirillary acidic protein and S100A10 protein, a specific marker of reactive A2 astrocytes. Our findings also include changes in the expression and content of glypicans in the gray matter, a significant rise in neurotoxic M1 microglia/macrophages, alterations in the cytokine profile, and a decreased expression of the extracellular matrix molecules tenascin R and aggrecan. This research highlights the complex pathological processes occurring far from the site of SCI and attempts to provide insights into the mechanisms involving the entire spinal cord in the response to such an injury.

Molecular signature of primate astrocytes reveals pathways and regulatory changes contributing to human brain evolution

Astrocytes contribute to the development and regulation of the higher-level functions of the brain, the critical targets of evolution. However, how astrocytes evolve in primates is unsettled. Here, we obtain human, chimpanzee, and macaque induced pluripotent stem-cell-derived astrocytes (iAstrocytes). Human iAstrocytes are bigger and more complex than the non-human primate iAstrocytes. We identify new loci contributing to the increased human astrocyte. We show that genes and pathways implicated in long-range intercellular signaling are activated in the human iAstrocytes and partake in controlling iAstrocyte complexity. Genes downregulated in human iAstrocytes frequently relate to neurological disorders and were decreased in adult brain samples. Through regulome analysis and machine learning, we uncover that functional activation of enhancers coincides with a previously unappreciated, pervasive gain of "stripe" transcription factor binding sites. Altogether, we reveal the transcriptomic signature of primate astrocyte evolution and a mechanism driving the acquisition of the regulatory potential of enhancers.

Glycogenolysis-Induced Astrocytic Serping1 Expression Regulates Neuroinflammatory Effects on Hippocampal neuron

The bacterial pathogen, lipopolysaccharide (LPS), elicits microglial response and induces cytokine secretion that subsequently activates astrocytes. Recent findings have indicated that LPS-induced activation of postnatal glial cells has led to alterations in synapse formation in hippocampal and cortical neurons, thereby resulting in a prolonged increased risk for seizure or depression. Nevertheless, its mechanisms remain to be fully elucidated. Cellular metabolism has recently gained recognition as a critical regulatory mechanism for the activation of peripheral immune cells, as it supplies the requisite energy and metabolite for their activation. In the present study, we report that LPS did not change the expression of reported astrocyte-derived synaptogenic genes in the postnatal hippocampus; however, it induced upregulation of astrocytic complement component regulator Serping1 within the postnatal hippocampus. As a regulatory mechanism, activation of glycogen degradation (glycogenolysis) governs the expression of a subset of inflammatory-responsive genes including Serping1 through reactive oxygen species (ROS)-NF-κB axis. Our study further demonstrated that glycogenolysis is implicated in neurotoxic phenotypes of astrocytes, such as impaired neuronal synaptogenesis or cellular toxicity. These findings suggested that activation of glycogenolysis in postnatal astrocytes is an essential metabolic pathway for inducing responses in inflammatory astrocytes.

Common alterations to astrocytes across neurodegenerative disorders

Astrocytes perform multiple functions in the nervous system, many of which are altered in neurodegenerative disorders. In this review, we explore shared astrocytic alterations across neurodegenerative disorders, including Alzheimer's disease, Huntington's disease, Parkinson's disease, amyotrophic lateral sclerosis, and frontotemporal lobe degeneration. Assessing recent datasets of single-nucleus RNA-sequencing of human brains, a theme emerges of common alterations in astrocyte state across disorders including in neuroinflammation, synaptic organization, metabolic support, and the cellular stress response. Immune pathways are upregulated by astrocytes across disorders and may exacerbate neurodegeneration. Dysregulated expression of synaptogenic factors could contribute to synaptic loss, while compromised metabolic support affects neuronal homeostasis. On the other hand, upregulated responses to cellular stress may represent a protective response of astrocytes and thus mitigate pathology. Understanding these shared responses offers insights into disease progression and provides potential therapeutic targets for various neurodegenerative disorders.

Microglia and Astrocytes in Postnatal Neural Circuit Formation

Over the past two decades, microglia and astrocytes have emerged as critical mediators of neural circuit formation. Particularly during the postnatal period, both glial subtypes play essential roles in orchestrating nervous system development through communication with neurons. These functions include regulating synapse elimination, modulating neuronal density and activity, mediating synaptogenesis, facilitating axon guidance and organization, and actively promoting neuronal survival. Despite the vital roles of both microglia and astrocytes in ensuring homeostatic brain development, the extent to which the postnatal functions of these cells are regulated by sex and the manner in which these glial cells communicate with one another to coordinate nervous system development remain less well understood. Here, we review the critical functions of both microglia and astrocytes independently and synergistically in mediating neural circuit formation, focusing our exploration on the postnatal period from birth to early adulthood.

Astrocyte regulation of critical period plasticity across neural circuits

Critical periods are brief windows of heightened neural circuit plasticity that allow circuits to permanently reset their structure and function to facilitate robust organismal behavior. Understanding the cellular and molecular mechanisms that instruct critical period timing is of broad clinical interest, as altered developmental plasticity is linked to multiple neurodevelopmental disorders. While intrinsic, neuronal mechanisms shape both neural circuit remodeling and critical period timing, recent data indicate that signaling from astrocytes and surrounding glia can both promote and limit critical period plasticity. In this short review, we discuss recent breakthroughs in our understanding of astrocytes in critical period plasticity and highlight pioneering work in Drosophila.

Wnt signalling facilitates neuronal differentiation of cochlear Frizzled10-positive cells in mouse cochlea via glypican 6 modulation

Degeneration of cochlear spiral ganglion neurons (SGNs) leads to irreversible sensorineural hearing loss (SNHL), as SGNs lack regenerative capacity. Although cochlear glial cells (GCs) have some neuronal differentiation potential, their specific identities remain unclear. This study identifies a distinct subpopulation, Frizzled10 positive (FZD10+) cells, as an important type of GC responsible for neuronal differentiation in mouse cochlea. FZD10 + cells can differentiate into various SGN subtypes in vivo, adhering to natural proportions. Wnt signaling enhances the ability of FZD10 + cells to function as neural progenitors and increases the neuronal excitability of the FZD10-derived neurons. Single-cell RNA sequencing analysis characterizes FZD10-derived differentiating cell populations, while crosstalk network analysis identifies multiple signaling pathways and target genes influenced by Wnt signaling that contribute to the function of FZD10 + cells as neural progenitors. Pseudotime analysis maps the differentiation trajectory from proliferated GCs to differentiating neurons. Further experiments indicate that glypican 6 (GPC6) may regulate this neuronal lineage, while GPC6 deficiency diminishes the effects of Wnt signaling on FZD10-derived neuronal differentiation and synapse formation. These findings suggest the critical role of Wnt signaling in the neuronal differentiation derived from cochlear FZD10 + cells and provide insights into the mechanisms potentially involved in this process.

Modulating Neuroinflammation as a Prospective Therapeutic Target in Alzheimer's Disease

The recent approval of lecanemab highlights that the amyloid beta (Aβ) protein is an important pathological target in Alzheimer's disease (AD) and further emphasizes the significance of neuroinflammatory pathways in regulating Aβ accumulation. Indeed, Aβ accumulation triggers microglia activation, which are key mediators in neuroinflammation. The inflammatory responses in this process can lead to neuronal damage and functional decline. Microglia secrete proinflammatory cytokines that accelerate neuronal death and release anti-inflammatory cytokines and growth factors contributing to neuronal recovery and protection. Thus, microglia play a dual role in neurodegeneration and neuroprotection, complicating their function in AD. Therefore, elucidating the complex interactions between Aβ protein, microglia, and neuroinflammation is essential for developing new strategies for treating AD. This review investigates the receptors and pathways involved in activating microglia and aims to enhance understanding of how these processes impact neuroinflammation in AD, as well as how they can be regulated. This review also analyzed studies reported in the existing literature and ongoing clinical trials. Overall, these studies will contribute to understanding the regulatory mechanisms of neuroinflammation and developing new therapies that can slow the pathological progression of AD.

Oral vancomycin treatment alters levels of indole derivatives and secondary bile acids modulating the expression of mTOR pathway genes in astrocytes during EAE

Astrocytes play important roles in the central nervous system (CNS) during health and disease. Prior studies have shown that gut commensal-derived indole derivatives as well as secondary bile acids modulate astrocyte function during the late stage of EAE (recovery phase). Here we showed that administering vancomycin to mice starting during the early stage of EAE improved disease recovery, an effect that is mediated by the gut microbiota. We observed that 6 taxa within the Clostridia vadin BB60 group were enriched in vancomycin-treated mice compared to untreated EAE mice. Vancomycin-treated EAE mice also had elevated serum levels of the anti-inflammatory tryptophan-derived metabolite, indole-3-lactic acid and decreased levels of deoxycholic acid, a pro-inflammatory secondary bile acid. RNA sequencing revealed altered expression of several genes belonging to the mammalian target of rapamycin (mTOR) pathway in astrocytes obtained during the late stage of EAE from vancomycin-treated EAE mice. Furthermore, we observed a link between serum levels of indole derivatives and bile acids and expression of several genes belonging to the mTOR pathway. Interestingly, the mTOR signaling cascades have been implicated in several key biological processes including innate (e.g., astrocyte) immune responses as well as neuronal toxicity/degeneration. In addition, rapamycin, a specific inhibitor of mTOR, has been shown to inhibit the induction and progression of established EAE. Collectively, our findings suggest that the neuroprotective effect of vancomycin is at least partially mediated by indole derivatives and secondary bile acids modulating the expression of mTOR pathway genes in astrocytes.

Single-Cell RNA-Seq Uncovers Robust Glial Cell Transcriptional Changes in Methamphetamine-Administered Mice

Methamphetamine is a highly addictive stimulant known to cause neurotoxicity, cognitive deficits, and immune dysregulation in the brain. Despite significant research, the molecular mechanisms driving methamphetamine-induced neurotoxicity and glial cell dysfunction remain poorly understood. This study investigates how methamphetamine disrupts glial cell function and contributes to neurodevelopmental and neurodegenerative processes. Using single-cell RNA sequencing (scRNA-seq), we analyzed the transcriptomes of 4000 glial cell-associated genes from the cortical regions of mice chronically administered methamphetamine. Methamphetamine exposure altered the key pathways in astrocytes, including the circadian rhythm and cAMP signaling; in microglia, affecting autophagy, ubiquitin-mediated proteolysis, and mitophagy; and in oligodendrocytes, disrupting lysosomal function, cytoskeletal regulation, and protein processing. Notably, several transcription factors, such as Zbtb16, Hif3a, Foxo1, and Klf9, were significantly dysregulated in the glial cells. These findings reveal profound methamphetamine-induced changes in the glial transcriptomes, particularly in the cortical regions, highlighting potential molecular pathways and transcription factors as targets for therapeutic intervention. This study provides novel insights into the glial-mediated mechanisms of methamphetamine toxicity, contributing to our understanding of its effects on the central nervous system and laying the groundwork for future strategies to mitigate its neurotoxic consequences.

From Physiology to Pathology of Astrocytes: Highlighting Their Potential as Therapeutic Targets for CNS Injury

In the mammalian central nervous system (CNS), astrocytes are the ubiquitous glial cells that have complex morphological and molecular characteristics. These fascinating cells play essential neurosupportive and homeostatic roles in the healthy CNS and undergo morphological, molecular, and functional changes to adopt so-called 'reactive' states in response to CNS injury or disease. In recent years, interest in astrocyte research has increased dramatically and some new biological features and roles of astrocytes in physiological and pathological conditions have been discovered thanks to technological advances. Here, we will review and discuss the well-established and emerging astroglial biology and functions, with emphasis on their potential as therapeutic targets for CNS injury, including traumatic and ischemic injury. This review article will highlight the importance of astrocytes in the neuropathological process and repair of CNS injury.

Neuroplasticity elicited by peripheral immune challenge with a viral mimetic

Peripheral viral infections are well known to profoundly alter brain function; however detailed mechanisms of this immune-to-brain communication have not been deciphered. This review focuses on studies of cerebral effects of peripheral viral challenge employing intraperitoneal injection of a viral mimetic, polyinosinic-polycytidylic acid (PIC). In this paradigm, PIC challenge induces the acute phase response (APR) characterized by a transient surge of circulating inflammatory factors, primarily IFNβ, IL-6 and CXCL10. The blood-borne factors, in turn, elicit the generation of CXCL10 by hippocampal neurons. Neurons also express the cognate receptor of CXCL10, i.e., CXCR3 implicating the existence of autocrine/paracrine signaling. The CXCL10/CXCR3 axis mediates the ensuing neuroplastic changes manifested as neuronal hyperexcitability, seizure hypersusceptibility, and sickness behavior. Electrophysiological studies revealed that the neuroplastic changes entail the potentiation of excitatory synapses likely at both pre- and postsynaptic loci. Excitatory synaptic transmission is further augmented by PIC challenge-induced elevation of extracellular glutamate that is mediated by astrocytes. In addition, the hyperexcitability of neuronal circuits might involve the repression of inhibitory signaling. Accordingly, CXCL10 released by neurons activates microglia whose processes invade perisomatic inhibitory synapses, resulting in a partial detachment of the presynaptic terminals, and thus, de-inhibition. This process might be facilitated by the cerebral complement system, which is also upregulated and activated by PIC challenge. Moreover, CXCL10 stimulates the expression of neuronal c-fos protein, another index of hyperexcitability. The reviewed studies form a foundation for full elucidation of the fascinating intersection between peripheral viral infections and neuroplasticity. Because the activation of such pathways may constitute a serious comorbidity factor for neuropathological conditions, this research would advance the development of preventive strategies.

Astrocytic proteostasis in the tale of aging and neurodegeneration

Homeostasis of proteins (proteostasis), which governs protein processing, folding, quality control, and degradation, is a fundamental cellular process that plays a pivotal role in various neurodegenerative diseases and in the natural aging process of the mammalian brain. While the role of neuronal proteostasis in neuronal physiology is well characterized, the contribution of proteostasis of glial cells, particularly of astrocytes, has received fairly less attention in this context. Here, we summarize recent data highlighting proteostasis dysfunction in astrocytes and its putative implication to neurodegenerative diseases and aging. We discuss how distinct proteostasis nodes and pathways in astrocytes may specifically contribute to brain function and different age-associated pathologies. Finally, we argue that the understanding of astrocytic proteostasis role in neuronal physiology and functional decay may arise as a potential new avenue of intervention in neurodegenerative diseases and grant relevant data in the biology of aging.

Astrocytic Neuroligin-3 influences gene expression and social behavior, but is dispensable for synapse number

Neuroligin-3 (Nlgn3) is an autism-associated cell-adhesion molecule that interacts with neurexins and is robustly expressed in both neurons and astrocytes. Neuronal Nlgn3 is an essential regulator of synaptic transmission but the function of astrocytic Nlgn3 is largely unknown. Given the high penetrance of Nlgn3 mutations in autism and the emerging role of astrocytes in neuropsychiatric disorders, we here asked whether astrocytic Nlgn3 might shape neural circuit properties in the cerebellum similar to neuronal Nlgn3. Imaging of tagged Nlgn3 protein produced by CRISPR/Cas9-mediated genome editing showed that Nlgn3 is enriched in the cell body but not the fine processes of cerebellar astrocytes (Bergmann glia). Astrocyte-specific knockout of Nlgn3 did not detectably alter the number of synapses, synaptic transmission, or astrocyte morphology in mouse cerebellum. However, spatial transcriptomic analyses revealed a significant shift in gene expression among multiple cerebellar cell types after the deletion of astrocytic Nlgn3. Hence, in contrast to neuronal Nlgn3, astrocytic Nlgn3 in the cerebellum is not involved in shaping synapses but may modulate gene expression in specific brain areas.

(Re)building the nervous system: A review of neuron-glia interactions from development to disease

Neuron-glia interactions are fundamental to the development and function of the nervous system. During development, glia, including astrocytes, microglia, and oligodendrocytes, influence neuronal differentiation and migration, synapse formation and refinement, and myelination. In the mature brain, glia are crucial for maintaining neural homeostasis, modulating synaptic activity, and supporting metabolic functions. Neurons, inherently vulnerable to various stressors, rely on glia for protection and repair. However, glia, in their reactive state, can also promote neuronal damage, which contributes to neurodegenerative and neuropsychiatric diseases. Understanding the dual role of glia-as both protectors and potential aggressors-sheds light on their complex contributions to disease etiology and pathology. By appropriately modulating glial activity, it may be possible to mitigate neurodegeneration and restore neuronal function. In this review, which originated from the International Society for Neurochemistry (ISN) Advanced School in 2019 held in Montreal, Canada, we first describe the critical importance of glia in the development and maintenance of a healthy nervous system as well as their contributions to neuronal damage and neurological disorders. We then discuss potential strategies to modulate glial activity during disease to protect and promote a properly functioning nervous system. We propose that targeting glial cells presents a promising therapeutic avenue for rebuilding the nervous system.

Self-organizing human heart assembloids with autologous and developmentally relevant cardiac neural crest-derived tissues

Neural crest cells (NCCs) are a multipotent embryonic cell population of ectodermal origin that extensively migrate during early development and contribute to the formation of multiple tissues. Cardiac NCCs play a critical role in heart development by orchestrating outflow tract septation, valve formation, aortic arch artery patterning, parasympathetic innervation, and maturation of the cardiac conduction system. Abnormal migration, proliferation, or differentiation of cardiac NCCs can lead to severe congenital cardiovascular malformations. However, the complexity and timing of early embryonic heart development pose significant challenges to studying the molecular mechanisms underlying NCC-related cardiac pathologies. Here, we present a sophisticated functional model of human heart assembloids derived from induced pluripotent stem cells, which, for the first time, recapitulates cardiac NCC integration into the human embryonic heart in vitro . NCCs successfully integrated at developmentally relevant stages into heart organoids, and followed developmental trajectories known to occur in the human heart. They demonstrated extensive migration, differentiated into cholinergic neurons capable of generating nerve impulses, and formed mature glial cells. Additionally, they contributed to the mesenchymal populations of the developing outflow tract. Through transcriptomic analysis, we revealed that NCCs acquire molecular features of their cardiac derivatives as heart assembloids develop. NCC-derived parasympathetic neurons formed functional connections with cardiomyocytes, promoting the maturation of the cardiac conduction system. Leveraging this model's cellular complexity and functional maturity, we uncovered that early exposure of NCCs to antidepressants harms the development of NCC derivatives in the context of the developing heart. The commonly prescribed antidepressant Paroxetine disrupted the expression of a critical early neuronal transcription factor, resulting in impaired parasympathetic innervation and functional deficits in cardiac tissue. This advanced heart assembloid model holds great promise for high-throughput drug screening and unraveling the molecular mechanisms underlying NCC-related cardiac formation and congenital heart defects.

IN BRIEF

Human neural crest heart assembloids resembling the major directions of neural crest differentiation in the human embryonic heart, including parasympathetic innervation and the mesenchymal component of the outflow tract, provide a human-relevant embryonic platform for studying congenital heart defects and drug safety.

Astrocytic Alterations and Dysfunction in Down Syndrome: Focus on Neurogenesis, Synaptogenesis, and Neural Circuits Formation

Down syndrome (DS) is characterized by severe neurodevelopmental alterations that ultimately lead to the typical hallmark of DS: intellectual disability. In the DS brain, since the prenatal life stages, the number of astrocytes is disproportional compared to the healthy brain. This increase is due to a shift from neuron to astrocyte differentiation during brain development. Astrocytes are involved in numerous functions during brain development, including balancing pro-neurogenic and pro-gliogenic stimuli, sustaining synapse formation, regulating excitatory/inhibitory signal equilibrium, and supporting the maintenance and integration of functional neural circuits. The enhanced number of astrocytes in the brain of DS individuals leads to detrimental consequences for brain development. This review summarizes the mechanisms underlying astrocytic dysfunction in DS, and particularly the dysregulation of key signaling pathways, which promote astrogliogenesis at the expense of neurogenesis. It further examines the implications of astrocytic alterations on dendritic branching, spinogenesis and synaptogenesis, and the impact of the abnormal astrocytic number in neural excitability and in the maintenance of the inhibitory/excitatory balance. Identifying deregulated pathways and the consequences of astrocytic alterations in early DS brain development may help in identifying new therapeutic targets, with the ultimate aim of ameliorating the cognitive disability that affects individuals with DS.

Glia Development and Function in the Nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans is a powerful experimental setting for uncovering fundamental tenets of nervous system organization and function. Its nearly invariant and simple anatomy, coupled with a plethora of methodologies for interrogating single-gene functions at single-cell resolution in vivo, have led to exciting discoveries in glial cell biology and mechanisms of glia-neuron interactions. Findings over the last two decades reinforce the idea that insights from C. elegans can inform our understanding of glial operating principles in other species. Here, we summarize the current state-of-the-art, and describe mechanistic insights that have emerged from a concerted effort to understand C. elegans glia. The remarkable acceleration in the pace of discovery in recent years paints a portrait of striking molecular complexity, exquisite specificity, and functional heterogeneity among glia. Glial cells affect nearly every aspect of nervous system development and function, from generating neurons, to promoting neurite formation, to animal behavior, and to whole-animal traits, including longevity. We discuss emerging questions where C. elegans is poised to fill critical knowledge gaps in our understanding of glia biology.

Glial Regulation of Circuit Wiring, Firing, and Expiring in the Drosophila Central Nervous System

Molecular genetic approaches in small model organisms like Drosophila have helped to elucidate fundamental principles of neuronal cell biology. Much less is understood about glial cells, although interest in using invertebrate preparations to define their in vivo functions has increased significantly in recent years. This review focuses on our current understanding of the three major neuron-associated glial cell types found in the Drosophila central nervous system (CNS)-astrocytes, cortex glia, and ensheathing glia. Together, these cells act like mammalian astrocytes and microglia; they associate closely with neurons including surrounding neuronal cell bodies and proximal neurites, regulate synapses, and engulf neuronal debris. Exciting recent work has shown critical roles for these CNS glial cells in neural circuit formation, function, plasticity, and pathology. As we gain a more firm molecular and cellular understanding of how Drosophila CNS glial cells interact with neurons, it is clear that they share significant molecular and functional attributes with mammalian glia and will serve as an excellent platform for mechanistic studies of glial function.

Pramipexole, a D3 receptor agonist, increases cortical gamma power and biochemical correlates of cortical excitation; implications for mood disorders

Major depressive disorder (MDD) has been associated with deficits in working memory as well as underlying gamma oscillation power. Consistent with this, overall reductions in cortical excitation have also been described with MDD. In previous work, we have demonstrated that the monoamine reuptake inhibitor venlafaxine increases gamma oscillation power in ex vivo hippocampal slices and that this is associated with concomitant increases in pyramidal arbour and reduced levels of plasticity-restricting perineuronal nets (PNNs). In the present study, we have examined the effects of chronic treatment with pramipexole (PPX), a D3 dopamine receptor agonist, for its effects on gamma oscillation power as measured by in vivo electroencephalography (EEG) recordings in female BALB/c and C57Bl6 mice. We observe a modest but significant increase in 20-50 Hz gamma power with PPX in both strains. Additionally, biochemical analysis of prefrontal cortex lysates from PPX-treated BALB/c mice shows a number of changes that could contribute to, or follow from, increased pyramidal excitability and/or gamma power. PPX-associated changes include reduced levels of specific PNN components as well as tissue inhibitor of matrix metalloproteases-1 (TIMP-1), which inhibits long-term potentiation of synaptic transmission. Consistent with its effects on gamma power, PNN proteins and TIMP-1, chronic PPX treatment also improves working memory and reduces anhedonia. Together these results add to an emerging literature linking extracellular matrix and/or gamma oscillation power to both mood and cognition.

Glial Cell Development and Function in the Zebrafish Central Nervous System

Over the past decades the zebrafish has emerged as an excellent model organism with which to study the biology of all glial cell types in nervous system development, plasticity, and regeneration. In this review, which builds on the earlier work by Lyons and Talbot in 2015, we will summarize how the relative ease to manipulate the zebrafish genome and its suitability for intravital imaging have helped understand principles of glial cell biology with a focus on oligodendrocytes, microglia, and astrocytes. We will highlight recent findings on the diverse properties and functions of these glial cell types in the central nervous system and discuss open questions and future directions of the field.

Astrocyte-derived factors regulate CNS myelination

The role that astrocytes play in central nervous system (CNS) myelination is poorly understood. We investigated the contribution of astrocyte-derived factors to myelination and revealed a substantial overlap in the secretomes of human and rat astrocytes. Using in vitro myelinating co-cultures of primary retinal ganglion cells and cortical oligodendrocyte precursor cells, we discovered that factors secreted by resting astrocytes, but not reactive astrocytes, facilitated myelination. Soluble brevican emerged as a new enhancer of developmental myelination in vivo, CNS and its absence was linked to remyelination deficits following an immune-mediated damage in an EAE mouse model. The observed reduction of brevican expression in reactive astrocytes and human MS lesions suggested a potential link to the compromised remyelination characteristic of neurodegenerative diseases. Our findings suggested brevican's role in myelination may be mediated through interactions with binding partners such as contactin-1 and tenascin-R. Proteomic analysis of resting versus reactive astrocytes highlighted a shift in protein expression profiles, pinpointing candidates that either facilitate or impede CNS repair, suggesting that depending on their reactivity state, astrocytes play a dual role during myelination.

Astrocyte transcriptomic analysis identifies glypican 5 downregulation as a contributor to synaptic dysfunction in Alzheimer's disease models

Synaptic dysfunction is an early feature in Alzheimer's disease (AD) and correlates with cognitive decline. Astrocytes are essential regulators of synapses, impacting synapse formation, maturation, elimination and function. To understand if synapse-supportive functions of astrocytes are altered in AD, we used astrocyte BacTRAP mice to generate a comprehensive dataset of hippocampal astrocyte transcriptional alterations in two mouse models of Alzheimer's pathology (APPswe/PS1dE9 and Tau P301S), characterizing sex and age-dependent changes. We found that astrocytes from both models downregulate genes important for synapse regulation and function such as the synapse-maturation factor Glypican 5. This transcriptional signature is shared with human post-mortem AD patients. Manipulating a key component of this signature by in vivo overexpression of Glypican 5 in astrocytes is sufficient to prevent early synaptic dysfunction and improve spatial learning in APPswe/PS1dE9 mice. These findings open new avenues to target astrocytic factors to mitigate AD synaptic dysfunction.

Astrocyte TrkB.T1 deficiency disrupts glutamatergic synaptogenesis and astrocyte-synapse interactions

Perisynaptic astrocyte processes (PAPs) contact pre- and post-synaptic elements to provide structural and functional support to synapses. Accumulating research demonstrates that the cradling of synapses by PAPs is critical for synapse formation, stabilization, and plasticity. The specific signaling pathways that govern these astrocyte-synapse interactions, however, remain to be elucidated. Herein, we demonstrate a role for the astrocyte TrkB.T1 receptor, a truncated isoform of the canonical receptor for brain derived neurotrophic factor (BDNF), in modulating astrocyte-synapse interactions and excitatory synapse development. Neuron-astrocyte co-culture studies revealed that loss of astrocyte TrkB.T1 disrupts the formation of PAPs. To elucidate the role of TrkB.T1 in synapse development, we conditionally deleted TrkB.T1 in astrocytes in mice. Synaptosome preparations were employed to probe for TrkB.T1 localization at the PAP, and confocal three-dimensional microscopy revealed a significant reduction in synapse density and astrocyte-synapse interactions across development in the absence of astrocytic TrkB.T1. These findings suggest that BDNF/TrkB.T1 signaling in astrocytes is critical for normal excitatory synapse formation in the cortex and that astrocyte TrkB.T1 serves a requisite role in astrocyte synapse interactions. Overall, this work provides new insights into the molecular mechanisms of astrocyte-mediated synaptogenesis and may have implications for understanding neurodevelopmental disorders and developing potential therapeutic targets.

Targeting astrocytes polarization after spinal cord injury: a promising direction

Spinal cord injury (SCI) is a serious neurological injury that causes severe trauma to motor and sensory functions. Although long considered incurable, recent research has brought new hope for functional recovery from SCI. After SCI, astrocytes are activated into many polarization states. Here we discuss the two most important classical phenotypes: the 'A1' neurotoxic phenotype and the 'A2' neuroprotective phenotype, with A1 astrocytes being neurotoxic and impeding neurorecovery, and A2 astrocytes being neuroprotective. This paper discusses the changes in astrocyte responsiveness after SCI and the pros and cons of their polarization in SCI. It also elucidates the feasibility of astrocyte polarization as a therapeutic target for neuroprotection. In the future, multiple intervention strategies targeting astrocyte polarization are expected to gain wider clinical application, ultimately improving motor-sensory function and quality of life in SCI patients.

Glial Control of Cortical Neuronal Circuit Maturation and Plasticity

The brain is a highly adaptable organ that is molded by experience throughout life. Although the field of neuroscience has historically focused on intrinsic neuronal mechanisms of plasticity, there is growing evidence that multiple glial populations regulate the timing and extent of neuronal plasticity, particularly over the course of development. This review highlights recent discoveries on the role of glial cells in the establishment of cortical circuits and the regulation of experience-dependent neuronal plasticity during critical periods of neurodevelopment. These studies provide strong evidence that neuronal circuit maturation and plasticity are non-cell autonomous processes that require both glial-neuronal and glial-glial cross talk to proceed. We conclude by discussing open questions that will continue to guide research in this nascent field.

Extracellular Vesicle-Mediated Neuron-Glia Communications in the Central Nervous System

Communication between neurons and glia significantly influences the development maturation, plasticity, and disease progressions of the nervous system. As a new signaling modality, extracellular vesicles display a diverse role for robust functional regulation of neurons through their protein and nucleic acid cargoes. This review highlights recent breakthroughs in the research of signaling mechanisms between glia and neurons mediated by extracellular vesicles that are important for neural development, axonal maintenance, synaptic functions, and disease progression in the mammalian nervous system. We will discuss the biological roles of extracellular vesicles released from neurons, astroglia, microglia, and oligodendroglia in the nervous system and their implications in neurodegenerative disorders.

Astrocytes and microglia in the coordination of CNS development and homeostasis

Glia have emerged as important architects of central nervous system (CNS) development and maintenance. While traditionally glial contributions to CNS development and maintenance have been studied independently, there is growing evidence that either suggests or documents that glia may act in coordinated manners to effect developmental patterning and homeostatic functions in the CNS. In this review, we focus on astrocytes, the most abundant glia in the CNS, and microglia, the earliest glia to colonize the CNS highlighting research that documents either suggestive or established coordinated actions by these glial cells in various CNS processes including cell and/or debris clearance, neuronal survival and morphogenesis, synaptic maturation, and circuit function, angio-/vasculogenesis, myelination, and neurotransmission. Some molecular mechanisms underlying these processes that have been identified are also described. Throughout, we categorize the available evidence as either suggestive or established interactions between microglia and astrocytes in the regulation of the respective process and raise possible avenues for further research. We conclude indicating that a better understanding of coordinated astrocyte-microglial interactions in the developing and mature brain holds promise for developing effective therapies for brain pathologies where these processes are perturbed.

The role of the gut microbiome in the regulation of astrocytes in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is the most common cause of dementia. AD is characterized pathologically by proteinaceous aggregates composed of amyloid beta (Aβ) and tau as well as progressive neurodegeneration. Concurrently with the buildup of protein aggregates, a strong neuroinflammatory response, in the form of reactive astrocytosis and microgliosis, occurs in the AD brain. It has recently been shown that the gut microbiome (GMB), composed of trillions of bacteria in the human intestine, can regulate both reactive astrocytosis and microgliosis in the context of both amyloidosis and tauopathy. Many studies have implicated microglia in these processes. However, growing evidence suggests that interactions between the GMB and astrocytes have a much larger role than previously thought. In this review, we summarize evidence regarding the gut microbiome in the control of reactive astrocytosis in AD.

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

Most excitatory synapses in the mammalian brain are contacted or ensheathed by astrocyte processes, forming tripartite synapses. Astrocytes are thought to be critical regulators of the structural and functional dynamics of synapses. While the degree of synaptic coverage by astrocytes is known to vary across brain regions and animal species, the reason for and implications of this variability remains unknown. Further, how astrocyte coverage of synapses relates to in vivo functional properties of individual synapses has not been investigated. Here, we characterized astrocyte coverage of synapses of pyramidal neurons in the ferret visual cortex and, using correlative light and electron microscopy, examined their relationship to synaptic strength and sensory-evoked Ca2+ activity. Nearly, all synapses were contacted by astrocytes, and most were contacted along the axon-spine interface. Structurally, we found that the degree of synaptic astrocyte coverage directly scaled with synapse size and postsynaptic density complexity. Functionally, we found that the amount of astrocyte coverage scaled with how selectively a synapse responds to a particular visual stimulus 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 metrics of synaptic strength of excitatory synapses in the visual cortex and demonstrates a previously unknown relationship between astrocyte coverage and reliable sensory activation.

Astrocyte-Mediated Neuroinflammation in Neurological Conditions

Astrocytes are one of the key glial types of the central nervous system (CNS), accounting for over 20% of total glial cells in the brain. Extensive evidence has established their indispensable functions in the maintenance of CNS homeostasis, as well as their broad involvement in neurological conditions. In particular, astrocytes can participate in various neuroinflammatory processes, e.g., releasing a repertoire of cytokines and chemokines or specific neurotrophic factors, which result in both beneficial and detrimental effects. It has become increasingly clear that such astrocyte-mediated neuroinflammation, together with its complex crosstalk with other glial cells or immune cells, designates neuronal survival and the functional integrity of neurocircuits, thus critically contributing to disease onset and progression. In this review, we focus on the current knowledge of the neuroinflammatory responses of astrocytes, summarizing their common features in neurological conditions. Moreover, we highlight several vital questions for future research that promise novel insights into diagnostic or therapeutic strategies against those debilitating CNS diseases.

SynBot is 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 the 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.

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

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

Glial modulation of synapse development and plasticity: oligodendrocyte precursor cells as a new player in the synaptic quintet

Synaptic communication is an important process in the central nervous system that allows for the rapid and spatially specified transfer of signals. Neurons receive various synaptic inputs and generate action potentials required for information transfer, and these inputs can be excitatory or inhibitory, which collectively determines the output. Non-neuronal cells (glial cells) have been identified as crucial participants in influencing neuronal activity and synaptic transmission, with astrocytes forming tripartite synapses and microglia pruning synapses. While it has been known that oligodendrocyte precursor cells (OPCs) receive neuronal inputs, whether they also influence neuronal activity and synaptic transmission has remained unknown for two decades. Recent findings indicate that OPCs, too, modulate neuronal synapses. In this review, we discuss the roles of different glial cell types at synapses, including the recently discovered involvement of OPCs in synaptic transmission and synapse refinement, and discuss overlapping roles played by multiple glial cell types.

Derivation and transcriptional reprogramming of border-forming wound repair astrocytes after spinal cord injury or stroke in mice

Central nervous system (CNS) lesions become surrounded by neuroprotective borders of newly proliferated reactive astrocytes; however, fundamental features of these cells are poorly understood. Here we show that following spinal cord injury or stroke, 90% and 10% of border-forming astrocytes derive, respectively, from proliferating local astrocytes and oligodendrocyte progenitor cells in adult mice of both sexes. Temporal transcriptome analysis, single-nucleus RNA sequencing and immunohistochemistry show that after focal CNS injury, local mature astrocytes dedifferentiate, proliferate and become transcriptionally reprogrammed to permanently altered new states, with persisting downregulation of molecules associated with astrocyte-neuron interactions and upregulation of molecules associated with wound healing, microbial defense and interactions with stromal and immune cells. These wound repair astrocytes share morphologic and transcriptional features with perimeningeal limitans astrocytes and are the predominant source of neuroprotective borders that re-establish CNS integrity around lesions by separating neural parenchyma from stromal and immune cells as occurs throughout the healthy CNS.

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.

IL-10 and Cdc42 modulate astrocyte-mediated microglia activation in methamphetamine-induced neuroinflammation

Methamphetamine (Meth) use is known to induce complex neuroinflammatory responses, particularly involving astrocytes and microglia. Building upon our previous research, which demonstrated that Meth stimulates astrocytes to release tumor necrosis factor (TNF) and glutamate, leading to microglial activation, this study investigates the role of the anti-inflammatory cytokine interleukin-10 (IL-10) in this process. Our findings reveal that the presence of recombinant IL-10 (rIL-10) counteracts Meth-induced excessive glutamate release in astrocyte cultures, which significantly reduces microglial activation. This reduction is associated with the modulation of astrocytic intracellular calcium (Ca2+) dynamics, particularly by restricting the release of Ca2+ from the endoplasmic reticulum to the cytoplasm. Furthermore, we identify the small Rho GTPase Cdc42 as a crucial intermediary in the astrocyte-to-microglia communication pathway under Meth exposure. By employing a transgenic mouse model that overexpresses IL-10 (pMT-10), we also demonstrate in vivo that IL-10 prevents Meth-induced neuroinflammation. These findings not only enhance our understanding of Meth-related neuroinflammatory mechanisms, but also suggest IL-10 and Cdc42 as putative therapeutic targets for treating Meth-induced neuroinflammation.

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.

Astrocytes in functional recovery following central nervous system injuries

Astrocytes are increasingly recognised as partaking in complex homeostatic mechanisms critical for regulating neuronal plasticity following central nervous system (CNS) insults. Ischaemic stroke and traumatic brain injury are associated with high rates of disability and mortality. Depending on the context and type of injury, reactive astrocytes respond with diverse morphological, proliferative and functional changes collectively known as astrogliosis, which results in both pathogenic and protective effects. There is a large body of research on the negative consequences of astrogliosis following brain injuries. There is also growing interest in how astrogliosis might in some contexts be protective and help to limit the spread of the injury. However, little is known about how astrocytes contribute to the chronic functional recovery phase following traumatic and ischaemic brain insults. In this review, we explore the protective functions of astrocytes in various aspects of secondary brain injury such as oedema, inflammation and blood-brain barrier dysfunction. We also discuss the current knowledge on astrocyte contribution to tissue regeneration, including angiogenesis, neurogenesis, synaptogenesis, dendrogenesis and axogenesis. Finally, we discuss diverse astrocyte-related factors that, if selectively targeted, could form the basis of astrocyte-targeted therapeutic strategies to better address currently untreatable CNS disorders.

CNS/PNS proteoglycans functionalize neuronal and astrocyte niche microenvironments optimizing cellular activity by preserving membrane polarization dynamics, ionic microenvironments, ion fluxes, neuronal activation, and network neurotransductive capacity

Central and peripheral nervous system (CNS/PNS) proteoglycans (PGs) have diverse functional roles, this study examined how these control cellular behavior and tissue function. The CNS/PNS extracellular matrix (ECM) is a dynamic, responsive, highly interactive, space-filling, cell supportive, stabilizing structure maintaining tissue compartments, ionic microenvironments, and microgradients that regulate neuronal activity and maintain the neuron in an optimal ionic microenvironment. The CNS/PNS contains a high glycosaminoglycan content (60% hyaluronan, HA) and a diverse range of stabilizing PGs. Immobilization of HA in brain tissues by HA interactive hyalectan PGs preserves tissue hydration and neuronal activity, a paucity of HA in brain tissues results in a pro-convulsant epileptic phenotype. Diverse CS, KS, and HSPGs stabilize the blood-brain barrier and neurovascular unit, provide smart gel neurotransmitter neuron vesicle storage and delivery, organize the neuromuscular junction basement membrane, and provide motor neuron synaptic plasticity, and photoreceptor and neuron synaptic functions. PG-HA networks maintain ionic fluxes and microgradients and tissue compartments that contribute to membrane polarization dynamics essential to neuronal activation and neurotransduction. Hyalectans form neuroprotective perineuronal nets contributing to synaptic plasticity, memory, and cognitive learning. Sialoglycoprotein associated with cones and rods (SPACRCAN), an HA binding CSPG, stabilizes the inter-photoreceptor ECM. HSPGs pikachurin and eyes shut stabilize the photoreceptor synapse aiding in phototransduction and neurotransduction with retinal bipolar neurons crucial to visual acuity. This is achieved through Laminin G motifs in pikachurin, eyes shut, and neurexins that interact with the dystroglycan-cytoskeleton-ECM-stabilizing synaptic interconnections, neuronal interactive specificity, and co-ordination of regulatory action potentials in neural networks.

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.

The role of glia in the dysregulation of neuronal spinogenesis in Ube3a-dependent ASD

Overexpression of the Ube3a gene and the resulting increase in Ube3a protein are linked to autism spectrum disorder (ASD). However, the cellular and molecular processes underlying Ube3a-dependent ASD remain unclear. Using both male and female mice, we find that neurons in the somatosensory cortex of the Ube3a 2× Tg ASD mouse model display reduced dendritic spine density and increased immature filopodia density. Importantly, the increased gene dosage of Ube3a in astrocytes alone is sufficient to confer alterations in neurons as immature dendritic protrusions, as observed in primary hippocampal neuron cultures. We show that Ube3a overexpression in astrocytes leads to a loss of astrocyte-derived spinogenic protein, thrombospondin-2 (TSP2), due to a suppression of TSP2 gene transcription. By neonatal intraventricular injection of astrocyte-specific virus, we demonstrate that Ube3a overexpression in astrocytes in vivo results in a reduction in dendritic spine maturation in prelimbic cortical neurons, accompanied with autistic-like behaviors in mice. These findings reveal an astrocytic dominance in initiating ASD pathobiology at the neuronal and behavior levels. SIGNIFICANCE STATEMENT: Increased gene dosage of Ube3a is tied to autism spectrum disorders (ASDs), yet cellular and molecular alterations underlying autistic phenotypes remain unclear. We show that Ube3a overexpression leads to impaired dendritic spine maturation, resulting in reduced spine density and increased filopodia density. We find that dysregulation of spine development is not neuron autonomous, rather, it is mediated by an astrocytic mechanism. Increased gene dosage of Ube3a in astrocytes leads to reduced production of the spinogenic glycoprotein thrombospondin-2 (TSP2), leading to abnormalities in spines. Astrocyte-specific Ube3a overexpression in the brain in vivo confers dysregulated spine maturation concomitant with autistic-like behaviors in mice. These findings indicate the importance of astrocytes in aberrant neurodevelopment and brain function in Ube3a-depdendent ASD.

Running exercise improves astrocyte loss, morphological complexity and astrocyte-contacted synapses in the hippocampus of CUS-induced depression model mice

Although the antidepressant effects of running exercise have been widely reported, further research is still needed to determine the structural bases for these effects. Astrocyte processes physically contact many synapses and directly regulate the numbers of synapses, but it remains unclear whether running exercise can modulate astrocyte morphological complexity and astrocyte-contacted synapses in the hippocampus of the mice with depressive-like behavior. Male C57BL/6 J mice underwent four weeks of running exercise after four weeks of chronic unpredictable stress (CUS). The sucrose preference test (SPT), tail suspension test (TST) and forced swim test (FST) were used to assess anhedonia in mice. Western blotting was used to measure the expression of astrocyte- and synapse-related proteins. Immunofluorescence and 3D reconstruction were used to quantify the density and morphology of astrocytes, and astrocyte-contacted synapses in each hippocampal subregion. Four weeks of running exercise alleviated depressive-like symptoms in mice. The expression of astrocyte- and synapse-related proteins in the hippocampus; astrocyte process lengths, process numbers, and dendritic arborization; and the number of astrocyte-contacted PSD95 positive synapses in the CA2-3 and DG regions were significantly decreased in the mice with depressive-like behavior, and running exercise successfully reserved these changes. Running exercise improved the decreases in astrocyte morphological complexity and astrocyte-contacted PSD95 positive synapses in the CA2-3 and DG regions of the mice with depressive-like behavior, suggesting that the physical interactions between astrocytes and synapses can be increased by running exercise, which might be an important structural basis for the antidepressant effects of running exercise.

Alterations of the blood-brain barrier during aging

The blood-brain barrier (BBB) is a complex and dynamic interface that regulates the exchange of molecules and cells between the blood and the central nervous system. It undergoes structural and functional changes during aging, which may compromise its integrity and contribute to the pathogenesis of neurodegenerative diseases. In recent years, advances in microscopy and high-throughput bioinformatics have allowed a more in-depth investigation of the aging mechanisms of BBB. This review summarizes age-related alterations of the BBB structure and function from six perspectives: endothelial cells, astrocytes, pericytes, basement membrane, microglia and perivascular macrophages, and fibroblasts, ranging from the molecular level to the human multi-system level. These basic components are essential for the proper functioning of the BBB. Recent imaging methods of BBB were also reviewed. Elucidation of age-associated BBB changes may offer insights into BBB homeostasis and may provide effective therapeutic strategies to protect it during aging.

The contribution of the meningeal immune interface to neuroinflammation in traumatic brain injury

Traumatic brain injury (TBI) is a major cause of disability and mortality worldwide, particularly among the elderly, yet our mechanistic understanding of what renders the post-traumatic brain vulnerable to poor outcomes, and susceptible to neurological disease, is incomplete. It is well established that dysregulated and sustained immune responses elicit negative consequences after TBI; however, our understanding of the neuroimmune interface that facilitates crosstalk between central and peripheral immune reservoirs is in its infancy. The meninges serve as the interface between the brain and the immune system, facilitating important bi-directional roles in both healthy and disease settings. It has been previously shown that disruption of this system exacerbates neuroinflammation in age-related neurodegenerative disorders such as Alzheimer's disease; however, we have an incomplete understanding of how the meningeal compartment influences immune responses after TBI. In this manuscript, we will offer a detailed overview of the holistic nature of neuroinflammatory responses in TBI, including hallmark features observed across clinical and animal models. We will highlight the structure and function of the meningeal lymphatic system, including its role in immuno-surveillance and immune responses within the meninges and the brain. We will provide a comprehensive update on our current knowledge of meningeal-derived responses across the spectrum of TBI, and identify new avenues for neuroimmune modulation within the neurotrauma field.

Glioblastoma disrupts cortical network activity at multiple spatial and temporal scales

The emergence of glioblastoma in cortical tissue initiates early and persistent neural hyperexcitability with signs ranging from mild cognitive impairment to convulsive seizures. The influence of peritumoral synaptic density, expansion dynamics, and spatial contours of excess glutamate upon higher order neuronal network modularity is unknown. We combined cellular and widefield imaging of calcium and glutamate fluorescent reporters in two glioblastoma mouse models with distinct synaptic microenvironments and infiltration profiles. Functional metrics of neural ensembles are dysregulated during tumor invasion depending on the stage of malignant progression and tumor cell proximity. Neural activity is differentially modulated during periods of accelerated and inhibited tumor expansion. Abnormal glutamate accumulation precedes and outpaces the spatial extent of baseline neuronal calcium signaling, indicating these processes are uncoupled in tumor cortex. Distinctive excitability homeostasis patterns and functional connectivity of local and remote neuronal populations support the promise of precision genetic diagnosis and management of this devastating brain disease.

The role of glial cells in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) has traditionally been considered a neuron-centric disease. This view is now outdated, with increasing recognition of cell autonomous and non-cell autonomous contributions of central and peripheral nervous system glia to ALS pathomechanisms. With glial research rapidly accelerating, we comprehensively interrogate the roles of astrocytes, microglia, oligodendrocytes, ependymal cells, Schwann cells and satellite glia in nervous system physiology and ALS-associated pathology. Moreover, we highlight the inter-glial, glial-neuronal and inter-system polylogue which constitutes the healthy nervous system and destabilises in disease. We also propose classification based on function for complex glial reactive phenotypes and discuss the pre-requisite for integrative modelling to advance translation. Given the paucity of life-enhancing therapies currently available for ALS patients, we discuss the promising potential of harnessing glia in driving ALS therapeutic discovery.

Astrocyte-secreted neurocan controls inhibitory synapse formation and function

Astrocytes strongly promote the formation and maturation of synapses by secreted proteins. Several astrocyte-secreted synaptogenic proteins controlling excitatory synapse development were identified; however, those that induce inhibitory synaptogenesis remain elusive. Here, we identify neurocan as an astrocyte-secreted inhibitory synaptogenic protein. After secretion from astrocytes, neurocan is cleaved into N- and C-terminal fragments. We found that these fragments have distinct localizations in the extracellular matrix. The neurocan C-terminal fragment localizes to synapses and controls cortical inhibitory synapse formation and function. Neurocan knockout mice lacking the whole protein or only its C-terminal synaptogenic domain have reduced inhibitory synapse numbers and function. Through super-resolution microscopy, in vivo proximity labeling by secreted TurboID, and astrocyte-specific rescue approaches, we discovered that the synaptogenic domain of neurocan localizes to somatostatin-positive inhibitory synapses and strongly regulates their formation. Together, our results unveil a mechanism through which astrocytes control circuit-specific inhibitory synapse development in the mammalian brain.

Abnormal Morphology and Synaptogenic Signaling in Astrocytes Following Prenatal Opioid Exposure

In recent decades, there has been a dramatic rise in the rates of children being born after in utero exposure to drugs of abuse, particularly opioids. Opioids have been shown to have detrimental effects on neurons and glia in the central nervous system (CNS), but the impact of prenatal opioid exposure (POE) on still-developing synaptic circuitry is largely unknown. Astrocytes exert a powerful influence on synaptic development, secreting factors to either promote or inhibit synapse formation and neuronal maturation in the developing CNS. Here, we investigated the effects of the partial µ-opioid receptor agonist buprenorphine on astrocyte synaptogenic signaling and morphological development in cortical cell culture. Acute buprenorphine treatment had no effect on the excitatory synapse number in astrocyte-free neuron cultures. In conditions where neurons shared culture media with astrocytes, buprenorphine attenuated the synaptogenic capabilities of astrocyte-secreted factors. Neurons cultured from drug-naïve mice showed no change in synapses when treated with factors secreted by astrocytes from POE mice. However, this same treatment was synaptogenic when applied to neurons from POE mice, indicating a complex neuroadaptive response in the event of impaired astrocyte signaling. In addition to promoting morphological and connectivity changes in neurons, POE exerted a strong influence on astrocyte development, disrupting their structural maturation and promoting the accumulation of lipid droplets (LDs), suggestive of a maladaptive stress response in the developing CNS.

Neuronal Circuit Dysfunction in Amyotrophic Lateral Sclerosis

The primary neural circuit affected in Amyotrophic Lateral Sclerosis (ALS) patients is the corticospinal motor circuit, originating in upper motor neurons (UMNs) in the cerebral motor cortex which descend to synapse with the lower motor neurons (LMNs) in the spinal cord to ultimately innervate the skeletal muscle. Perturbation of these neural circuits and consequent loss of both UMNs and LMNs, leading to muscle wastage and impaired movement, is the key pathophysiology observed. Despite decades of research, we are still lacking in ALS disease-modifying treatments. In this review, we document the current research from patient studies, rodent models, and human stem cell models in understanding the mechanisms of corticomotor circuit dysfunction and its implication in ALS. We summarize the current knowledge about cortical UMN dysfunction and degeneration, altered excitability in LMNs, neuromuscular junction degeneration, and the non-cell autonomous role of glial cells in motor circuit dysfunction in relation to ALS. We further highlight the advances in human stem cell technology to model the complex neural circuitry and how these can aid in future studies to better understand the mechanisms of neural circuit dysfunction underpinning ALS.