Metabolic dynamics in astrocytes and microglia during post-natal development and their implications for autism spectrum disorders.

Autism Spectrum Disorder (ASD) is a complex neurodevelopmental condition characterized by elusive underlying mechanisms. Recent attention has focused on the involvement of astrocytes and microglia in ASD pathology. These glial cells play pivotal roles in maintaining neuronal homeostasis, including the regulation of metabolism. Emerging evidence suggests a potential association between ASD and inborn errors of metabolism. Therefore, gaining a comprehensive understanding of the functions of microglia and astrocytes in ASD is crucial for the development of effective therapeutic interventions. This review aims to provide a summary of the metabolism of astrocytes and microglia during post-natal development and the evidence of disrupted metabolic pathways in ASD, with particular emphasis on those potentially important for the regulation of neuronal post-natal maturation by astrocytes and microglia.

Microglia at the Tripartite Synapse during Postnatal Development: Implications for Autism Spectrum Disorders and Schizophrenia.

Synapses are the fundamental structures of neural circuits that control brain functions and behavioral and cognitive processes. Synapses undergo formation, maturation, and elimination mainly during postnatal development via a complex interplay with neighboring astrocytes and microglia that, by shaping neural connectivity, may have a crucial role in the strengthening and weakening of synaptic functions, that is, the functional plasticity of synapses. Indeed, an increasing number of studies have unveiled the roles of microglia and astrocytes in synapse formation, maturation, and elimination as well as in regulating synaptic function. Over the past 15 years, the mechanisms underlying the microglia- and astrocytes-dependent regulation of synaptic plasticity have been thoroughly studied, and researchers have reported that the disruption of these glial cells in early postnatal development may underlie the cause of synaptic dysfunction that leads to neurodevelopmental disorders such as autism spectrum disorder (ASD) and schizophrenia.

Unlocking Neural Function with 3D In Vitro Models: A Technical Review of Self-Assembled, Guided, and Bioprinted Brain Organoids and Their Applications in the Study of Neurodevelopmental and Neurodegenerative Disorders.

Understanding the complexities of the human brain and its associated disorders poses a significant challenge in neuroscience. Traditional research methods have limitations in replicating its intricacies, necessitating the development of in vitro models that can simulate its structure and function. Three-dimensional in vitro models, including organoids, cerebral organoids, bioprinted brain models, and functionalized brain organoids, offer promising platforms for studying human brain development, physiology, and disease. These models accurately replicate key aspects of human brain anatomy, gene expression, and cellular behavior, enabling drug discovery and toxicology studies while providing insights into human-specific phenomena not easily studied in animal models. The use of human-induced pluripotent stem cells has revolutionized the generation of 3D brain structures, with various techniques developed to generate specific brain regions. These advancements facilitate the study of brain structure development and function, overcoming previous limitations due to the scarcity of human brain samples. This technical review provides an overview of current 3D in vitro models of the human cortex, their development, characterization, and limitations, and explores the state of the art and future directions in the field, with a specific focus on their applications in studying neurodevelopmental and neurodegenerative disorders.

Disruption of Astrocyte-Dependent Dopamine Control in the Developing Medial Prefrontal Cortex Leads to Excessive Grooming in Mice.

BACKGROUND: Astrocytes control synaptic activity by modulating perisynaptic concentrations of ions and neurotransmitters including dopamine (DA) and, as such, could be involved in the modulating aspects of mammalian behavior. METHODS: We produced a conditional deletion of the vesicular monoamine transporter 2 (VMAT2) specifically in astrocytes (aVMTA2cKO mice) and studied the effects of the lack of VMAT2 in prefrontal cortex (PFC) astrocytes on the regulation of DA levels, PFC circuit functions, and behavioral processes. RESULTS: We found a significant reduction of medial PFC (mPFC) DA levels and excessive grooming and compulsive repetitive behaviors in aVMAT2cKO mice. The mice also developed a synaptic pathology, expressed through increased relative AMPA versus NMDA receptor currents in synapses of the dorsal striatum receiving inputs from the mPFC. Importantly, behavioral and synaptic phenotypes were rescued by re-expression of mPFC VMAT2 and L-DOPA treatment, showing that the deficits were driven by mPFC astrocytes that are critically involved in developmental DA homeostasis. By analyzing human tissue samples, we found that VMAT2 is expressed in human PFC astrocytes, corroborating the potential translational relevance of our observations in mice. CONCLUSIONS: Our study shows that impairment of the astrocytic control of DA in the mPFC leads to symptoms resembling obsessive-compulsive spectrum disorders such as trichotillomania and has a profound impact on circuit function and behaviors.

Reduction of inflammation and mitochondrial degeneration in mutant SOD1 mice through inhibition of voltage-gated potassium channel Kv1.3.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with no effective therapy, causing progressive loss of motor neurons in the spinal cord, brainstem, and motor cortex. Regardless of its genetic or sporadic origin, there is currently no cure for ALS or therapy that can reverse or control its progression. In the present study, taking advantage of a human superoxide dismutase-1 mutant (hSOD1-G93A) mouse that recapitulates key pathological features of human ALS, we investigated the possible role of voltage-gated potassium channel Kv1.3 in disease progression. We found that chronic administration of the brain-penetrant Kv1.3 inhibitor, PAP-1 (40 mg/Kg), in early symptomatic mice (i) improves motor deficits and prolongs survival of diseased mice (ii) reduces astrocyte reactivity, microglial Kv1.3 expression, and serum pro-inflammatory soluble factors (iii) improves structural mitochondrial deficits in motor neuron mitochondria (iv) restores mitochondrial respiratory dysfunction. Taken together, these findings underscore the potential significance of Kv1.3 activity as a contributing factor to the metabolic disturbances observed in ALS. Consequently, targeting Kv1.3 presents a promising avenue for modulating disease progression, shedding new light on potential therapeutic strategies for ALS.

Human iPSC-Derived Cortical Neurons Display Homeostatic Plasticity.

Maintaining the excitability of neurons and circuits is fundamental for healthy brain functions. The global compensatory increase in excitatory synaptic strength, in response to decreased activity, is one of the main homeostatic mechanisms responsible for such regulation. This type of plasticity has been extensively characterized in rodents in vivo and in vitro, but few data exist on human neurons maturation. We have generated an in vitro cortical model system, based on differentiated human-induced pluripotent stem cells, chronically treated with tetrodotoxin, to investigate homeostatic plasticity at different developmental stages. Our findings highlight the presence of homeostatic plasticity in human cortical networks and show that the changes in synaptic strength are due to both pre- and post-synaptic mechanisms. Pre-synaptic plasticity involves the potentiation of neurotransmitter release machinery, associated to an increase in synaptic vesicle proteins expression. At the post-synaptic level, we report an increase in the expression of post-synaptic density proteins, involved in glutamatergic receptor anchoring. These results extend our understanding of neuronal homeostasis and reveal the developmental regulation of its expression in human cortical networks. Since induced pluripotent stem cell-derived neurons can be obtained from patients with neurodevelopmental and neurodegenerative diseases, our platform offers a versatile model for assessing human neural plasticity under physiological and pathological conditions.

Emerging evidence for astrocyte dysfunction in schizophrenia.

Schizophrenia is a complex, chronic mental health disorder whose heterogeneous genetic and neurobiological background influences early brain development, and whose precise etiology is still poorly understood. Schizophrenia is not characterized by gross brain pathology, but involves subtle pathological changes in neuronal populations and glial cells. Among the latter, astrocytes critically contribute to the regulation of early neurodevelopmental processes, and any dysfunctions in their morphological and functional maturation may lead to aberrant neurodevelopmental processes involved in the pathogenesis of schizophrenia, such as mitochondrial biogenesis, synaptogenesis, and glutamatergic and dopaminergic transmission. Studies of the mechanisms regulating astrocyte maturation may therefore improve our understanding of the cellular and molecular mechanisms underlying the pathogenesis of schizophrenia.

Synaptic Plasticity Dysfunctions in the Pathophysiology of 22q11 Deletion Syndrome: Is There a Role for Astrocytes?

The 22q11 deletion syndrome (DS) is the most common microdeletion syndrome in humans and gives a high probability of developing psychiatric disorders. Synaptic and neuronal malfunctions appear to be at the core of the symptoms presented by patients. In fact, it has long been suggested that the behavioural and cognitive impairments observed in 22q11DS are probably due to alterations in the mechanisms regulating synaptic function and plasticity. Often, synaptic changes are related to structural and functional changes observed in patients with cognitive dysfunctions, therefore suggesting that synaptic plasticity has a crucial role in the pathophysiology of the syndrome. Most interestingly, among the genes deleted in 22q11DS, six encode for mitochondrial proteins that, in mouse models, are highly expressed just after birth, when active synaptogenesis occurs, therefore indicating that mitochondrial processes are strictly related to synapse formation and maintenance of a correct synaptic signalling. Because correct synaptic functioning, not only requires correct neuronal function and metabolism, but also needs the active contribution of astrocytes, we summarize in this review recent studies showing the involvement of synaptic plasticity in the pathophysiology of 22q11DS and we discuss the relevance of mitochondria in these processes and the possible involvement of astrocytes.

Tweaking neural organoids to model human reactive astrocytes.

The study of human reactive astrocytes has been limited by resource availability. In this issue, Cvetkovic et al. (2022. J. Cell Biol.https://doi.org/10.1083/jcb.202107135) develop multicellular organoid systems containing mature astrocytes to study the dynamics of human astrocytes reactivity and its downstream effects on neuronal activity.

Mitochondrial biogenesis in developing astrocytes regulates astrocyte maturation and synapse formation.

The mechanisms controlling the post-natal maturation of astrocytes play a crucial role in ensuring correct synaptogenesis. We show that mitochondrial biogenesis in developing astrocytes is necessary for coordinating post-natal astrocyte maturation and synaptogenesis. The astrocytic mitochondrial biogenesis depends on the transient upregulation of metabolic regulator peroxisome proliferator-activated receptor gamma (PPARγ) co-activator 1α (PGC-1α), which is controlled by metabotropic glutamate receptor 5 (mGluR5). At tissue level, the loss or downregulation of astrocytic PGC-1α sustains astrocyte proliferation, dampens astrocyte morphogenesis, and impairs the formation and function of neighboring synapses, whereas its genetic re-expression is sufficient to restore the mitochondria compartment and correct astroglial and synaptic defects. Our findings show that the developmental enhancement of mitochondrial biogenesis in astrocytes is a critical mechanism controlling astrocyte maturation and supporting synaptogenesis, thus suggesting that astrocytic mitochondria may be a therapeutic target in the case of neurodevelopmental and psychiatric disorders characterized by impaired synaptogenesis.

Tau accumulation in astrocytes of the dentate gyrus induces neuronal dysfunction and memory deficits in Alzheimer's disease.

Alzheimer's disease (AD) is characterized by the accumulation of the tau protein in neurons, neurodegeneration and memory loss. However, the role of non-neuronal cells in this chain of events remains unclear. In the present study, we found accumulation of tau in hilar astrocytes of the dentate gyrus of individuals with AD. In mice, the overexpression of 3R tau specifically in hilar astrocytes of the dentate gyrus altered mitochondrial dynamics and function. In turn, these changes led to a reduction of adult neurogenesis, parvalbumin-expressing neurons, inhibitory synapses and hilar gamma oscillations, which were accompanied by impaired spatial memory performances. Together, these results indicate that the loss of tau homeostasis in hilar astrocytes of the dentate gyrus is sufficient to induce AD-like symptoms, through the impairment of the neuronal network. These results are important for our understanding of disease mechanisms and underline the crucial role of astrocytes in hippocampal function.

A New Pathway Promotes Adaptation of Human Glioblastoma Cells to Glucose Starvation.

Adaptation of glioblastoma to caloric restriction induces compensatory changes in tumor metabolism that are incompletely known. Here we show that in human glioblastoma cells maintained in exhausted medium, SHC adaptor protein 3 (SHC3) increases due to down-regulation of SHC3 protein degradation. This effect is reversed by glucose addition and is not present in normal astrocytes. Increased SHC3 levels are associated to increased glucose uptake mediated by changes in membrane trafficking of glucose transporters of the solute carrier 2A superfamily (GLUT/SLC2A). We found that the effects on vesicle trafficking are mediated by SHC3 interactions with adaptor protein complex 1 and 2 (AP), BMP-2-inducible protein kinase and a fraction of poly ADP-ribose polymerase 1 (PARP1) associated to vesicles containing GLUT/SLC2As. In glioblastoma cells, PARP1 inhibitor veliparib mimics glucose starvation in enhancing glucose uptake. Furthermore, cytosol extracted from glioblastoma cells inhibits PARP1 enzymatic activity in vitro while immunodepletion of SHC3 from the cytosol significantly relieves this inhibition. The identification of a new pathway controlling glucose uptake in high grade gliomas represents an opportunity for repositioning existing drugs and designing new ones.

Dysfunction of homeostatic control of dopamine by astrocytes in the developing prefrontal cortex leads to cognitive impairments.

Astrocytes orchestrate neural development by powerfully coordinating synapse formation and function and, as such, may be critically involved in the pathogenesis of neurodevelopmental abnormalities and cognitive deficits commonly observed in psychiatric disorders. Here, we report the identification of a subset of cortical astrocytes that are competent for regulating dopamine (DA) homeostasis during postnatal development of the prefrontal cortex (PFC), allowing for optimal DA-mediated maturation of excitatory circuits. Such control of DA homeostasis occurs through the coordinated activity of astroglial vesicular monoamine transporter 2 (VMAT2) together with organic cation transporter 3 and monoamine oxidase type B, two key proteins for DA uptake and metabolism. Conditional deletion of VMAT2 in astrocytes postnatally produces loss of PFC DA homeostasis, leading to defective synaptic transmission and plasticity as well as impaired executive functions. Our findings show a novel role for PFC astrocytes in the DA modulation of cognitive performances with relevance to psychiatric disorders.

Spatio-temporal overview of neuroinflammation in an experimental mouse stroke model.

After ischemic stroke, in the lesion core as well as in the ischemic penumbra, evolution of tissue damage and repair is strongly affected by neuroinflammatory events that involve activation of local specialized glial cells, release of inflammatory mediators, recruiting of systemic cells and vascular remodelling. To take advantage of this intricate response in the quest to devise new protective therapeutic strategies we need a better understanding of the territorial and temporal interplay between stroke-triggered inflammatory and cell death-inducing processes in both parenchymal and vascular brain cells. Our goal is to describe structural rearrangements and functional modifications occurring in glial and vascular cells early after an acute ischemic stroke. Low and high scale mapping of the glial activation on brain sections of mice subjected to 30 minutes middle cerebral artery occlusion (MCAO) was correlated with that of the neuronal cell death, with markers for microvascular changes and with markers for pro-inflammatory (IL-1ß) and reparative (TGFß1) cytokines. Our results illustrate a time-course of the neuroinflammatory response starting at early time-points (1 h) and up to one week after MCAO injury in mice, with an accurate spatial distribution of the observed phenomena.

Insights into the release mechanism of astrocytic glutamate evoking in neurons NMDA receptor-mediated slow depolarizing inward currents.

The gliotransmitter glutamate in different brain regions modulates neuronal excitability and synaptic transmission through a variety of mechanisms. Among the hallmarks of astrocytic glutamate release are the slow depolarizing inward currents (SICs) in neurons mediated by N-methyl-d-aspartate receptor activation. Different stimuli that evoke Ca2+ elevations in astrocytes induce neuronal SICs suggesting a Ca2+ -dependent exocytotic glutamate release mechanism of SIC generation. To gain new insights into this mechanism, we investigated the relationship between astrocytic Ca2+ elevations and neuronal SICs in mouse hippocampal slice preparations. Here we provide evidence that SICs, occurring either spontaneously or following a hypotonic challenge, are unchanged in the virtual absence of Ca2+ signal changes at astrocytic soma and processes, including spatially restricted Ca2+ microdomains. SICs are also unchanged in the presence of Bafilomycin A1 that after prolonged slice incubation depletes glutamate from astrocytic vesicles. We also found that hemichannels and TREK family channels-that recent studies proposed to mediate astrocytic glutamate release - are not involved in SIC generation. SICs are reduced by the volume-sensitive anion channel antagonists diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), quinine and fluoxetine, suggesting a possible contribution of these channels in SIC generation. Direct measurements of astrocytic glutamate release further confirm that hypotonicity-evoked gliotransmission is impaired following DIDS, quinine and fluoxetine while the exocytotic release of glutamate-that is proposed to mediate synaptic transmission modulation by astrocytes-remains unaffected. In conclusion, our data provide evidence that the release of glutamate generating SICs occurs independently on exocytotic Ca2+ -dependent glutamate release mechanism.

mGlu5-mediated signalling in developing astrocyte and the pathogenesis of autism spectrum disorders.

Astrocytes, the largest glial population in human and murine brains, are crucial to the regulation of synaptic connectivity. During the first three weeks of postnatal development, immature astrocytes express mGlu5 and expands several fold while undergoing a transition towards their mature phase. Although mGlu5-mediated signalling in astrocyte functions has been extensively studied in the last decades, whether this signalling is implicated in the mechanisms governing their development, as well as the effects of dysregulated astrocytic development on neurodevelopmental disorders, are still unclear. The aim of this review is to examine what is known about the mGlu5-mediated signalling in the developing astrocytes and its possible contribution to the pathophysiology of autism spectrum disorders.

Homer1 Scaffold Proteins Govern Ca2+ Dynamics in Normal and Reactive Astrocytes.

In astrocytes, the intracellular calcium (Ca2+) signaling mediated by activation of metabotropic glutamate receptor 5 (mGlu5) is crucially involved in the modulation of many aspects of brain physiology, including gliotransmission. Here, we find that the mGlu5-mediated Ca2+ signaling leading to release of glutamate is governed by mGlu5 interaction with Homer1 scaffolding proteins. We show that the long splice variants Homer1b/c are expressed in astrocytic processes, where they cluster with mGlu5 at sites displaying intense local Ca2+ activity. We show that the structural and functional significance of the Homer1b/c-mGlu5 interaction is to relocate endoplasmic reticulum (ER) to the proximity of the plasma membrane and to optimize Ca2+ signaling and glutamate release. We also show that in reactive astrocytes the short dominant-negative splice variant Homer1a is upregulated. Homer1a, by precluding the mGlu5-ER interaction decreases the intensity of Ca2+ signaling thus limiting the intensity and the duration of glutamate release by astrocytes. Hindering upregulation of Homer1a with a local injection of short interfering RNA in vivo restores mGlu5-mediated Ca2+ signaling and glutamate release and sensitizes astrocytes to apoptosis. We propose that Homer1a may represent one of the cellular mechanisms by which inflammatory astrocytic reactions are beneficial for limiting brain injury.

SHANK3 controls maturation of social reward circuits in the VTA.

Haploinsufficiency of SHANK3, encoding the synapse scaffolding protein SHANK3, leads to a highly penetrant form of autism spectrum disorder. How SHANK3 insufficiency affects specific neural circuits and how this is related to specific symptoms remains elusive. Here we used shRNA to model Shank3 insufficiency in the ventral tegmental area of mice. We identified dopamine (DA) and GABA cell-type-specific changes in excitatory synapse transmission that converge to reduce DA neuron activity and generate behavioral deficits, including impaired social preference. Administration of a positive allosteric modulator of the type 1 metabotropic glutamate receptors mGluR1 during the first postnatal week restored DA neuron excitatory synapse transmission and partially rescued the social preference defects, while optogenetic DA neuron stimulation was sufficient to enhance social preference. Collectively, these data reveal the contribution of impaired ventral tegmental area function to social behaviors and identify mGluR1 modulation during postnatal development as a potential treatment strategy.

Astrocytes and Microglia and Their Potential Link with Autism Spectrum Disorders.

The cellular mechanism(s) underlying autism spectrum disorders (ASDs) are not fully understood although it has been shown that various genetic and environmental factors contribute to their etiology. As increasing evidence indicates that astrocytes and microglial cells play a major role in synapse maturation and function, and there is evidence of deficits in glial cell functions in ASDs, one current hypothesis is that glial dysfunctions directly contribute to their pathophysiology. The aim of this review is to summarize microglia and astrocyte functions in synapse development and their contributions to ASDs.