Astrocytes regulate neuronal network activity by mediating synapse remodeling

Developmental roles of astrocytes in circuit wiring

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

A Comprehensive Review of poly(I: C) as a Tool for Investigating Astrocytic TLR3 Signaling

Astrocytes play a crucial role in regulating the structure, function, and interactions between the synaptic and vascular compartments in the brain. Toll-like receptor 3 (TLR3) is expressed in astrocytes and recognizes double-stranded RNA (dsRNA), a pathogen-associated molecular pattern (PAMP). This review examines the current understanding of TLR3 signaling, with a focus on its specific role in astrocytes, and the use of the viral mimetic polyinosinic: polycytidylic acid (poly(I: C)) to model the effects of viral infections in both in vitro and in vivo studies. Poly(I: C) is a useful tool for studying neuro-immune communication and has significantly added to our knowledge of how the brain responds to immune challenges. Upon poly(I: C) exposure, TLR3 activation in astrocytes triggers inflammatory signaling pathways, leading to both antiviral responses and neuroinflammation. However, further research is required to investigate the cell-specific impacts of TLR3 activation, along with the influence of developmental stages, brain regions, and sex-specific responses, to gain a comprehensive understanding of how immune activation shapes the development and function of the central nervous system (CNS).

Ectopic expression of Slc1a2 in the prefrontal cortex of sleep-deprived male mice counteracts the glutamate/GABA-glutamine dysfunction

BACKGROUND

The prefrontal cortex (PFC) plays a pronounced role in cognitive and emotional functions, which may be compromised by dismal sleep quality. This study intended to clarify the impact of Slc1a2 ectopic expression in the PFC on sleep deprivation (SD)-induced disturbances in the glutamate (Glu)/GABA-glutamine cycle and the role of astrocyte (AC)-neuron (Neu) communication.

METHODS

Single-cell RNA sequencing was adopted to illuminate cell-specific changes in the brainstem, cortex, and hypothalamus of mice under NS, SD, and post-SD conditions. Cell communication analysis was applied to study interactions between ACs and Neus, which altered after the SD. Slc1a2 was ectopically expressed in the PFC and subjected to SD, followed by electrophysiological, immunofluorescence staining, and [1H-13C]-nuclear magnetic resonance (NMR) assays to examine neural activity and metabolic status. Behavioral tests, including the open field, novel object recognition, and Y-maze, were conducted to examine cognitive functions and emotional states.

RESULTS

SD caused notable changes in cellular distribution and downregulation of metabolic and synaptic genes in affected brain regions. Cell communication studies highlighted a reduction in AC-Neu interactions, with corresponding metabolic disruptions in the Glu/GABA-glutamine cycle as depicted by [1H-13C]-NMR results. Behavior tests confirmed anxiety and cognitive deficits in SD mice, which were substantially alleviated by Slc1a2 ectopic expression in the PFC.

CONCLUSIONS

Slc1a2 ectopic expression in the PFC negates SD-induced GABA dysfunction through vital AC-Neu communication. This study sheds light on the mechanisms through which SD affects neural function and suggesting potential treatments for sleep-related disorders.

Calycosin regulates astrocyte reactivity and astrogliosis after spinal cord injury by targeting STAT3 phosphorylation

BACKGROUND

Astrocytes are the most populous glial cells in the central nervous system (CNS), which can exert detrimental effects through a process of reactive astrogliosis. Our previous study has indicated the potential effect of Calycosin in preventing spinal cord injury (SCI). This study aims to investigate the mechanism by which calycosin regulates the polarization of A1 astrocytes, a neurotoxic subtype of reactive astrocytes, in SCI models.

MATERIALS AND METHODS

The SCI model was induced by applying mechanical compression to the spinal cord using vascular clamps. A1 astrocyte differentiation was induced by treating astrocytes with microglia supernatant obtained after Lipopolysaccharide (LPS) stimulation. Key protein expression levels were analyzed by Western blotting, and astrocyte markers such as CS56, GFAP, C3, S100A10 were assessed through immunofluorescence staining.

RESULTS

Calycosin treatment significantly reduced glial scar formation and C3 expression in SCI rats. However, S100A10 expression remained unchanged. Further analysis showed that Calycosin inhibited A1 astrocyte activation, migration, and invasion, which was associated with STAT3 phosphorylation. Calycosin downregulated p-STAT3 levels in both A1 astrocytes and SCI rats. These effects were reversed by Colivelin (a STAT3 activator) in A1 astrocytes.

CONCLUSION

Calycosin treatment can modulate p-STAT3 expression, thereby altering the functionality of astrocytes during the recovery phase and positively impacting the treatment and rehabilitation of SCI.

Emerging Roles for MFG-E8 in Synapse Elimination

Synapse elimination is an essential process in the healthy nervous system and is dysregulated in many neuropathologies. Yet, the underlying molecular mechanisms and under what conditions they occur remain unclear. MFG-E8 is a secreted glycoprotein well known to act as an opsonin, tagging stressed and dying cells for engulfment by phagocytes. Opsonization of cells and debris by MFG-E8 for microglial phagocytosis in the CNS is well established, and its role in astrocytic phagocytosis, and trogocytosis-like engulfment of synapses is beginning to be explored. However, MFG-E8's function in other tissues is highly diverse, and evidence suggests that its role in the nervous system and on synapse elimination in particular may be more complex and varied than opsonization. In this review, we outline the documented direct and indirect effects of MFG-E8 on synapse elimination, while also proposing potential roles to be explored further, in particular, cytoskeletal reorganization of neurites and glia leading to synapse elimination by various mechanisms. Finally, we demonstrate the need for several open questions to be answered-chiefly, under what conditions might MFG-E8-mediated synapse elimination occur in favor of other mechanisms, and when might its activity be dysregulated, increasing unwanted synapse elimination and neurotoxicity?

The role of astrocytes from synaptic to non-synaptic plasticity

Information storage and transfer in the brain require a high computational power. Neuronal network display various local or global mechanisms to allow information storage and transfer in the brain. From synaptic to intrinsic plasticity, the rules of input-output function modulation have been well characterized in neurons. In the past years, astrocytes have been suggested to increase the computational power of the brain and we are only just starting to uncover their role in information processing. Astrocytes maintain a close bidirectional communication with neurons to modify neuronal network excitability, transmission, axonal conduction, and plasticity through various mechanisms including the release of gliotransmitters or local ion homeostasis. Astrocytes have been significantly studied in the context of long-term or short-term synaptic plasticity, but this is not the only mechanism involved in memory formation. Plasticity of intrinsic neuronal excitability also participates in memory storage through regulation of voltage-gated ion channels or axonal morphological changes. Yet, the contribution of astrocytes to these other forms of non-synaptic plasticity remains to be investigated. In this review, we summarized the recent advances on the role of astrocytes in different forms of plasticity and discuss new directions and ideas to be explored regarding astrocytes-neuronal communication and regulation of plasticity.

A review on the consequences of molecular and genomic alterations following exposure to electromagnetic fields: Remodeling of neuronal network and cognitive changes

The use of electromagnetic fields (EMFs) is essential in daily life. Since 1970, concerns have grown about potential health hazards from EMF. Exposure to EMF can stimulate nerves and affect the central nervous system, leading to neurological and cognitive changes. However, current research results are often vague and contradictory. These effects include changes in memory and learning through changes in neuronal plasticity in the hippocampus, synapses and hippocampal neuritis, and changes in metabolism and neurotransmitter levels. Prenatal exposure to EMFs has negative effects on memory and learning, as well as changes in hippocampal neuron density and histomorphology of hippocampus. EMF exposure also affects the structure and function of glial cells, affecting gate dynamics, ion conduction, membrane concentration, and protein expression. EMF exposure affects gene expression and may change epigenetic regulation through effects on DNA methylation, histone modification, and microRNA biogenesis, and potentially leading to biological changes. Therefore, exposure to EMFs possibly leads to changes in cellular and molecular mechanisms in central nervous system and alter cognitive function.

Reactive Gliosis in Neonatal Disorders: Friend or Foe for Neuroregeneration?

A developing nervous system is particularly vulnerable to the influence of pathophysiological clues and injuries in the perinatal period. Astrocytes are among the first cells that react to insults against the nervous tissue, the presence of pathogens, misbalance of local tissue homeostasis, and a lack of oxygen and trophic support. Under this background, it remains uncertain if induced astrocyte activation, recognized as astrogliosis, is a friend or foe for progressing neonatal neurodevelopment. Likewise, the state of astrocyte reactivity is considered one of the key factors discriminating between either the initiation of endogenous reparative mechanisms compensating for aberrations in the structures and functions of nervous tissue or the triggering of neurodegeneration. The responses of activated cells are modulated by neighboring neural cells, which exhibit broad immunomodulatory and pro-regenerative properties by secreting a plethora of active compounds (including interleukins and chemokines, neurotrophins, reactive oxygen species, nitric oxide synthase and complement components), which are engaged in cell crosstalk in a paracrine manner. As the developing nervous system is extremely sensitive to the influence of signaling molecules, even subtle changes in the composition or concentration of the cellular secretome can have significant effects on the developing neonatal brain. Thus, modulating the activity of other types of cells and their interactions with overreactive astrocytes might be a promising strategy for controlling neonatal astrogliosis.

Neuroprotective astroglial response to neural damage and its relevance to affective disorders

Astrocytes not only support neuronal function with essential roles in synaptic neurotransmission, action potential propagation, metabolic support, or neuroplastic and developmental adaptations. They also respond to damage or dysfunction in surrounding neurons and oligodendrocytes by releasing neurotrophic factors and other molecules that increase the survival of the supported cells or contribute to mechanisms of structural and molecular restoration. The neuroprotective responsiveness of astrocytes is based on their ability to sense signals of degeneration, metabolic jeopardy and structural damage, and on their aptitude to locally deliver specific molecules to remedy threats to the molecular and structural features of their cellular partners. To the extent that neuronal and other glial cell disturbances are known to occur in affective disorders, astrocyte responsiveness to those disturbances may help to better understand the roles astrocytes play in affective disorders. The astrocytic sensing apparatus supporting those responses involves receptors for neurotransmitters, purines, cell adhesion molecules and growth factors. Astrocytes also share with the immune system the capacity of responding to cytokines released upon neuronal damage. In addition, in responses to specific signals astrocytes release unique factors such as clusterin or humanin that have been shown to exert potent neuroprotective effects. Astrocytes integrate the signals above to further deliver structural lipids, removing toxic metabolites, stabilizing the osmotic environment, normalizing neurotransmitters, providing anti-oxidant protection, facilitating synaptogenesis and acting as barriers to contain varied deleterious signals, some of which have been described in brain regions relevant to affective disorders and related animal models. Since various of the injurious signals that activate astrocytes have been implicated in different aspects of the etiopathology of affective disorders, particularly in relation to the diagnosis of depression, potentiating the corresponding astrocyte neuroprotective responses may provide additional opportunities to improve or complement available pharmacological and behavioral therapies for affective disorders.