How astrocytic ATP shapes neuronal activity and brain circuits

Stress-Induced Sleep Dysregulation: The Roles of Astrocytes and Microglia in Neurodegenerative and Psychiatric Disorders

Stress and sleep share a reciprocal relationship, where chronic stress often leads to sleep disturbances that worsen neurodegenerative and psychiatric conditions. Non-neuronal cells, particularly astrocytes and microglia, play critical roles in the brain's response to stress and the regulation of sleep. Astrocytes influence sleep architecture by regulating adenosine signaling and glymphatic clearance, both of which can be disrupted by chronic stress, leading to reduced restorative sleep. Microglia, activated under stress conditions, drive neuroinflammatory processes that further impair sleep and exacerbate brain dysfunction. Additionally, the gut-brain axis mediates interactions between stress, sleep, and inflammation, with microbial metabolites influencing neural pathways. Many of these effects converge on the disruption of synaptic processes, such as neurotransmitter balance, synaptic plasticity, and pruning, which in turn contribute to the pathophysiology of neurodegenerative and psychiatric disorders. This review explores how these cellular and systemic mechanisms contribute to stress-induced sleep disturbances and their implications for neurodegenerative and psychiatric disorders, offering insights into potential therapeutic strategies targeting non-neuronal cells and the gut-brain axis.

Neuropeptide-mediated activation of astrocytes improves stress resilience in mice by modulating cortical neural synapses

Astrocytes are known to modulate synaptogenesis or neuronal activities, thus participating in mental functions. It has been shown that astrocytes are involved in the antidepressant mechanism. In this study we investigated the potential hormonal mediator governing the astrocyte-neuron interplay for stress-coping behaviors. Mice were subjected to chronic restraint stress (CRS) for 14 days, and then brain tissue was harvested for analyses. We found that the expression of pituitary adenylate cyclase activating polypeptide (PACAP) and its receptor PAC1 was significantly decreased in astrocytes of the prelimbic (PrL) cortex. By conducting a combination of genetics, in vivo imaging and behavioral assays we demonstrated that PAC1 in cortical astrocytes was necessary for maintaining normal resilience of mice against chronic environmental stress like restraint stress. Furthermore, we showed the enhancement of de novo cortical spine formation and synaptic activity under PACAP-mediated astrocytic activation possibly via the ATP release. The molecular mechanisms suggested that the vesicle homeostasis mediated by PACAP-PAC1 axis in astrocytes was involved in regulating synaptic functions. This study identifies a previously unrecognized route by which neuropeptide modulates cortical functions via local regulation of astrocytes.

Immune modulation to treat Alzheimer's disease

Immune mechanisms play a fundamental role in Alzheimer's disease (AD) pathogenesis, suggesting that approaches which target immune cells and immunologically relevant molecules can offer therapeutic opportunities beyond the recently approved amyloid beta monoclonal therapies. In this review, we provide an overview of immunomodulatory therapeutics in development, including their preclinical evidence and clinical trial results. Along with detailing immune processes involved in AD pathogenesis and highlighting how these mechanisms can be therapeutically targeted to modify disease progression, we summarize knowledge gained from previous trials of immune-based interventions, and provide a series of recommendations for the development of future immunomodulatory therapeutics to treat AD.

Disrupted astrocyte-neuron signaling reshapes brain activity in epilepsy and Alzheimer's disease

Astrocytes establish dynamic interactions with surrounding neurons and synchronize neuronal networks within a specific range. However, these reciprocal astrocyte-neuronal interactions are selectively disrupted in epilepsy and Alzheimer's disease (AD), which contributes to the initiation and progression of network hypersynchrony. Deciphering how disrupted astrocyte-neuronal signaling reshapes brain activity is crucial to prevent subclinical epileptiform activity in epilepsy and AD. In this review, we provide an overview of the diverse astrocyte-neuronal crosstalk in maintaining of network activity via homeostatic control of extracellular ions and transmitters, synapse formation and elimination. More importantly, since AD and epilepsy share the common symptoms of neuronal hyperexcitability and astrogliosis, we then explore the crosstalk between astrocytes and neurons in the context of epilepsy and AD and discuss how these disrupted interactions reshape brain activity in pathological conditions. Collectively, this review sheds light on how disrupted astrocyte-neuronal signaling reshapes brain activity in epilepsy and AD, and highlights that modifying astrocyte-neuronal signaling could be a therapeutic approach to prevent epileptiform activity in AD.

Role of trigeminal ganglion satellite glial cells in masseter muscle pain hypersensitivity

OBJECTIVES

The underlying mechanism of masseter muscle pain hypersensitivity by sustained masseter muscle contraction (SMMC) is not well understood. This study aimed to examine whether the activation of satellite glial cells in the trigeminal ganglion (TG) contributes to masseter muscle pain hypersensitivity induced by SMMC.

METHODS

Electrodes were placed on the masseter muscle fascia of rats to induce strong contractions, by daily electrical stimulation. Pain sensitivity in the masseter muscle was measured and the activation level of satellite glial cells in the TG was examined. The localization of P2Y12 and the effects of P2Y12 receptor inhibition on SMMC-induced pain hypersensitivity were evaluated. The amount of tumor necrosis factor alpha (TNF-α) and TNF-α receptor localization were determined in the TG.

RESULTS

SMMC induced masseter muscle pain hypersensitivity and activation of satellite glial cells. P2Y12 receptors were expressed in satellite glial cells and masseter muscle pain hypersensitivity was suppressed by intra-TG P2Y12 receptor antagonism. TG neurons innervating the sustained-contracted masseter muscle expressed TNF-α receptor and SMMC increased TNF-α levels in TG.

CONCLUSION

SMMC-induced activation of satellite glial cells though the P2Y12 receptor signaling may contribute to masseter muscle pain hypersensitivity via the TNF-α signaling pathway.

Multifarious astrocyte-neuron dialog in shaping neural circuit architecture

Astrocytes are multifaceted glial cell types that perform structural, functional, metabolic, and homeostatic roles in the brain. Recent studies have revealed mechanisms underlying the diversity of bidirectional communication modes between astrocytes and neurons - the fundamental organizing principle shaping synaptic properties at tripartite synapses. These astrocyte-neuron interactions are critical for the proper functioning of synapses and neural circuits. This review focuses on molecular mechanisms that direct these interactions, highlighting the versatile roles of multiple adhesion-based paths that likely modulate them, often in a context-dependent manner. It also describes how astrocyte-mediated processes go awry in certain brain disorders and provides a timely insight on the pivotal roles of astrocyte-neuron interactions in synaptic integrity and their relevance to understanding and treating neurological disorders.

Cortical norepinephrine-astrocyte signaling critically mediates learned behavior

Updating behavior based on feedback from the environment is a crucial means by which organisms learn and develop optimal behavioral strategies1-3. Norepinephrine (NE) release from the locus coeruleus (LC) has been shown to mediate learned behaviors4-6 such that in a task with graded stimulus uncertainty and performance, a high level of NE released after an unexpected outcome causes improvement in subsequent behavior7. Yet, how the transient activity of LC-NE neurons, lasting tens of milliseconds, influences behavior several seconds later, is unclear. Here, we show that NE acts directly on cortical astrocytes via Adra1a adrenergic receptors to elicit sustained increases in intracellular calcium. Chemogenetic blockade of astrocytic calcium elevation prevents the improvement in behavioral performance. NE-activated calcium invokes purinergic pathways in cortical astrocytes that signal to neurons; pathway-specific astrocyte gene expression is altered in mice trained on the task, and blocking neuronal adenosine-sensitive A1 receptors also prevents post-reinforcement behavioral gain. Finally, blocking either astrocyte calcium dynamics or A1 receptors alters encoding of the task in prefrontal cortex neurons, preventing the post-reinforcement change in discriminability of rewarded and unrewarded stimuli underlying behavioral improvement. Together, these data demonstrate that astrocytes, rather than indirectly reflecting neuronal drive, play a direct, instrumental role in representing task-relevant information and signaling to neurons to mediate a fundamental component of learning in the brain.

Clustered de novo start-loss variants in GLUL result in a developmental and epileptic encephalopathy via stabilization of glutamine synthetase

Glutamine synthetase (GS), encoded by GLUL, catalyzes the conversion of glutamate to glutamine. GS is pivotal for the generation of the neurotransmitters glutamate and gamma-aminobutyric acid and is the primary mechanism of ammonia detoxification in the brain. GS levels are regulated post-translationally by an N-terminal degron that enables the ubiquitin-mediated degradation of GS in a glutamine-induced manner. GS deficiency in humans is known to lead to neurological defects and death in infancy, yet how dysregulation of the degron-mediated control of GS levels might affect neurodevelopment is unknown. We ascertained nine individuals with severe developmental delay, seizures, and white matter abnormalities but normal plasma and cerebrospinal fluid biochemistry with de novo variants in GLUL. Seven out of nine were start-loss variants and two out of nine disrupted 5' UTR splicing resulting in splice exclusion of the initiation codon. Using transfection-based expression systems and mass spectrometry, these variants were shown to lead to translation initiation of GS from methionine 18, downstream of the N-terminal degron motif, resulting in a protein that is stable and enzymatically competent but insensitive to negative feedback by glutamine. Analysis of human single-cell transcriptomes demonstrated that GLUL is widely expressed in neuro- and glial-progenitor cells and mature astrocytes but not in post-mitotic neurons. One individual with a start-loss GLUL variant demonstrated periventricular nodular heterotopia, a neuronal migration disorder, yet overexpression of stabilized GS in mice using in utero electroporation demonstrated no migratory deficits. These findings underline the importance of tight regulation of glutamine metabolism during neurodevelopment in humans.

The mechanistic effects of acupuncture in rodent neurodegenerative disease models: a literature review

Neurodegenerative diseases refer to a battery of medical conditions that affect the survival and function of neurons in the brain, which are mainly presented with progressive loss of cognitive and/or motor function. Acupuncture showed benign effects in improving neurological deficits, especially on movement and cognitive function impairment. Here, we reviewed the therapeutic mechanisms of acupuncture at the neural circuit level in movement and cognition disorders, summarizing the influence of acupuncture in the dopaminergic system, glutamatergic system, γ-amino butyric acid-ergic (GABAergic) system, serotonergic system, cholinergic system, and glial cells at the circuit and synaptic levels. These findings can provide targets for clinical treatment and perspectives for further studies.

Cortical astrocyte N-methyl-D-aspartate receptors influence whisker barrel activity and sensory discrimination in mice

Astrocytes express ionotropic receptors, including N-methyl-D-aspartate receptors (NMDARs). However, the contribution of NMDARs to astrocyte-neuron interactions, particularly in vivo, has not been elucidated. Here we show that a knockdown approach to selectively reduce NMDARs in mouse cortical astrocytes decreases astrocyte Ca2+ transients evoked by sensory stimulation. Astrocyte NMDAR knockdown also impairs nearby neuronal circuits by elevating spontaneous neuron activity and limiting neuronal recruitment, synchronization, and adaptation during sensory stimulation. Furthermore, this compromises the optimal processing of sensory information since the sensory acuity of the mice is reduced during a whisker-dependent tactile discrimination task. Lastly, we rescue the effects of astrocyte NMDAR knockdown on neurons and improve the tactile acuity of the animal by supplying exogenous ATP. Overall, our findings show that astrocytes can respond to nearby neuronal activity via their NMDAR, and that these receptors are an important component for purinergic signaling that regulate astrocyte-neuron interactions and cortical sensory discrimination in vivo.