Astrocytic Kir6.1 deletion aggravates neurodegeneration in the lipopolysaccharide-induced mouse model of Parkinson's disease via astrocyte-neuron cross talk through complement C3-C3R signaling

Transcriptome Analysis Reveals Dynamic Microglial-Induced A1 Astrocyte Reactivity via C3/C3aR/NF-κB Signaling After Ischemic Stroke

Microglia and astrocytes are key players in neuroinflammation and ischemic stroke. A1 astrocytes are a subtype of astrocytes that are extremely neurotoxic and quickly kill neurons. Although the detrimental A1 astrocytes are present in many neurodegenerative diseases and are considered to accelerate neurodegeneration, their role in the pathophysiology of ischemic stroke is poorly understood. Here, we combined RNA-seq, molecular and immunological techniques, and behavioral tests to investigate the role of A1 astrocytes in the pathophysiology of ischemic stroke. We found that astrocyte phenotypes change from a beneficial A2 type in the acute phase to a detrimental A1 type in the chronic phase following ischemic stroke. The activated microglial IL1α, TNF, and C1q prompt commitment of A1 astrocytes. Inhibition of A1 astrocytes induction attenuates reactive gliosis and ameliorates morphological and functional defects following ischemic stroke. The crosstalk between astrocytic C3 and microglial C3aR contributes to the formation of A1 astrocytes and morphological and functional defects. In addition, NF-κB is activated following ischemic stroke and governs the formation of A1 astrocytes via direct targeting of inflammatory cytokines and chemokines. Taken together, we discovered that A2 astrocytes and A1 astrocytes are enriched in the acute and chronic phases of ischemic stroke respectively, and that the C3/C3aR/NF-κB signaling leads to A1 astrocytes induction. Therefore, the C3/C3aR/NF-κB signaling is a novel therapeutic target for ischemic stroke treatment.

Metformin normalizes mitochondrial function to delay astrocyte senescence in a mouse model of Parkinson's disease through Mfn2-cGAS signaling

BACKGROUND

Senescent astrocytes play crucial roles in age-associated neurodegenerative diseases, including Parkinson's disease (PD). Metformin, a drug widely used for treating diabetes, exerts longevity effects and neuroprotective activities. However, its effect on astrocyte senescence in PD remains to be defined.

METHODS

Long culture-induced replicative senescence model and 1-methyl-4-phenylpyridinium/α-synuclein aggregate-induced premature senescence model, and a mouse model of PD were used to investigate the effect of metformin on astrocyte senescence in vivo and in vitro. Immunofluorescence staining and flow cytometric analyses were performed to evaluate the mitochondrial function. We stereotactically injected AAV carrying GFAP-promoter-cGAS-shRNA to mouse substantia nigra pars compacta regions to specifically reduce astrocytic cGAS expression to clarify the potential molecular mechanism by which metformin inhibited the astrocyte senescence in PD.

RESULTS

We showed that metformin inhibited the astrocyte senescence in vitro and in PD mice. Mechanistically, metformin normalized mitochondrial function to reduce mitochondrial DNA release through mitofusin 2 (Mfn2), leading to inactivation of cGAS-STING, which delayed astrocyte senescence and prevented neurodegeneration. Mfn2 overexpression in astrocytes reversed the inhibitory role of metformin in cGAS-STING activation and astrocyte senescence. More importantly, metformin ameliorated dopamine neuron injury and behavioral deficits in mice by reducing the accumulation of senescent astrocytes via inhibition of astrocytic cGAS activation. Deletion of astrocytic cGAS abolished the suppressive effects of metformin on astrocyte senescence and neurodegeneration.

CONCLUSIONS

This work reveals that metformin delays astrocyte senescence via inhibiting astrocytic Mfn2-cGAS activation and suggest that metformin is a promising therapeutic agent for age-associated neurodegenerative diseases.

Baicalein attenuates rotenone-induced SH-SY5Y cell apoptosis through binding to SUR1 and activating ATP-sensitive potassium channels

Dopaminergic neurons in the substantia nigra (SN) expressing SUR1/Kir6.2 type ATP-sensitive potassium channels (K-ATP) are more vulnerable to rotenone or metabolic stress, which may be an important reason for the selective degeneration of neurons in Parkinson's disease (PD). Baicalein has shown neuroprotective effects in PD animal models. In this study, we investigated the effect of baicalein on K-ATP channels and the underlying mechanisms in rotenone-induced apoptosis of SH-SY5Y cells. K-ATP currents were recorded from SH-SY5Y cells using whole-cell voltage-clamp recording. Drugs dissolved in the external solution at the final concentration were directly pipetted onto the cells. We showed that rotenone and baicalein opened K-ATP channels and increased the current amplitudes with EC50 values of 0.438 μM and 6.159 μM, respectively. K-ATP channel blockers glibenclamide (50 μM) or 5-hydroxydecanoate (5-HD, 250 μM) attenuated the protective effects of baicalein in reducing reactive oxygen species (ROS) content and increasing mitochondrial membrane potential and ATP levels in rotenone-injured SH-SY5Y cells, suggesting that baicalein protected against the apoptosis of SH-SY5Y cells by regulating the effect of rotenone on opening K-ATP channels. Administration of baicalein (150, 300 mg·kg-1·d-1, i.g.) significantly inhibited rotenone-induced overexpression of SUR1 in SN and striatum of rats. We conducted surface plasmon resonance assay and molecular docking, and found that baicalein had a higher affinity with SUR1 protein (KD = 10.39 μM) than glibenclamide (KD = 24.32 μM), thus reducing the sensitivity of K-ATP channels to rotenone. Knockdown of SUR1 subunit reduced rotenone-induced apoptosis and damage of SH-SY5Y cells, confirming that SUR1 was an important target for slowing dopaminergic neuronal degeneration in PD. Taken together, we demonstrate for the first time that baicalein attenuates rotenone-induced SH-SY5Y cell apoptosis through binding to SUR1 and activating K-ATP channels.

Amyloid-β1-42 oligomers enhance mGlu5R-dependent synaptic weakening via NMDAR activation and complement C5aR1 signaling

Synaptic weakening and loss are well-correlated with the pathology of Alzheimer's disease (AD). Oligomeric amyloid beta (oAβ) is considered a major synaptotoxic trigger for AD. Recent studies have implicated hyperactivation of the complement cascade as the driving force for loss of synapses caused by oAβ. However, the initial synaptic cues that trigger pathological complement activity remain elusive. Here, we examined a form of synaptic long-term depression (LTD) mediated by metabotropic glutamate receptors (mGluRs) that is disrupted in rodent models of AD. Exogenous application of oAβ (1-42) to mouse hippocampal slices enhanced the magnitude of mGlu subtype 5 receptor (mGlu5R)-dependent LTD. We found that the enhanced synaptic weakening occurred via both N-methyl-D-aspartate receptors (NMDARs) and complement C5aR1 signaling. Our findings reveal a mechanistic interaction between mGlu5R, NMDARs, and the complement system in aberrant synaptic weakening induced by oAβ, which could represent an early trigger of synaptic loss and degeneration in AD.

Ion Channels and Ionotropic Receptors in Astrocytes: Physiological Functions and Alterations in Alzheimer's Disease and Glioblastoma

The human brain is composed of nearly one hundred billion neurons and an equal number of glial cells, including macroglia, i.e., astrocytes and oligodendrocytes, and microglia, the resident immune cells of the brain. In the last few decades, compelling evidence has revealed that glial cells are far more active and complex than previously thought. In particular, astrocytes, the most abundant glial cell population, not only take part in brain development, metabolism, and defense against pathogens and insults, but they also affect sensory, motor, and cognitive functions by constantly modulating synaptic activity. Not surprisingly, astrocytes are actively involved in neurodegenerative diseases (NDs) and other neurological disorders like brain tumors, in which they rapidly become reactive and mediate neuroinflammation. Reactive astrocytes acquire or lose specific functions that differently modulate disease progression and symptoms, including cognitive impairments. Astrocytes express several types of ion channels, including K+, Na+, and Ca2+ channels, transient receptor potential channels (TRP), aquaporins, mechanoreceptors, and anion channels, whose properties and functions are only partially understood, particularly in small processes that contact synapses. In addition, astrocytes express ionotropic receptors for several neurotransmitters. Here, we provide an extensive and up-to-date review of the roles of ion channels and ionotropic receptors in astrocyte physiology and pathology. As examples of two different brain pathologies, we focus on Alzheimer's disease (AD), one of the most diffuse neurodegenerative disorders, and glioblastoma (GBM), the most common brain tumor. Understanding how ion channels and ionotropic receptors in astrocytes participate in NDs and tumors is necessary for developing new therapeutic tools for these increasingly common neurological conditions.

Mefloquine targets NLRP3 to reduce lipopolysaccharide-induced systemic inflammation and neural injury

The NLR family pyrin domain containing 3 (NLRP3) inflammasome plays an important role in the pathogenesis of a wide variety of human diseases. So far, drugs directly and specifically targeting the NLRP3 inflammasome are not available for clinical use since the safety and efficacy of new compounds are often unclear. A promising approach is thus to identify NLRP3 inhibitors from existing drugs that are already in clinical use. Here, we show that mefloquine, a well-known antimalarial drug, is a highly selective and potent NLRP3 inhibitor by screening a FDA-approved drug library. Mechanistically, mefloquine directly binds to the NLRP3 NACHT and LRR domains to prevent NLRP3 inflammasome activation. More importantly, mefloquine treatment attenuates the symptoms of lipopolysaccharide-induced systemic inflammation and Parkinson's disease-like neural damage in mice. Our findings identify mefloquine as a potential therapeutic agent for NLRP3-driven diseases and migth expand its clinical use considerably.

The cGAS-STING-YY1 axis accelerates progression of neurodegeneration in a mouse model of Parkinson's disease via LCN2-dependent astrocyte senescence

Recent studies provide clues that astrocyte senescence is correlated with Parkinson's disease (PD) progression, while little is known about the molecular basis for astrocyte senescence in PD. Here, we found that cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) was upregulated in senescent astrocytes of PD and aged mice. Strikingly, deletion of astrocytic cGAS significantly prevented senescence of astrocytes and neurodegeneration. Furthermore, we identified LCN2 as the effector of cGAS-STING signal by RNA-Seq analysis. Genetic manipulation of LCN2 expression proved the regulation of cGAS-STING-LCN2 axis in astrocyte senescence. Additionally, YY1 was discovered as the transcription factor of LCN2 by chromatin immunoprecipitation. Binding of STING to YY1 impedes nuclear translocation of YY1. Herein, we determine the involvement of the cGAS-STING-YY1-LCN2 signaling cascade in the control of astrocyte senescence and PD progression. Together, this work fills the gap in our understanding of astrocyte senescence, and provides potential targets for delaying PD progression.

Reducing complement activation during sleep deprivation yields cognitive improvement by dexmedetomidine

BACKGROUND

Sleep loss and its associated conditions (e.g. cognitive deficits) represent a large societal burden, but the underlying mechanisms of these cognitive deficits remain unknown. This study assessed the effect of dexmedetomidine (DEX) on cognitive decline induced by sleep loss.

METHODS

C57BL/6 mice were subjected to chronic sleep restriction (CSR) for 20 h (5 pm-1 pm the next day) daily for 7 days, and cognitive tests were subsequently carried out. The neuromolecular and cellular changes that occurred in the presence and absence of DEX (100 μg kg-1, i.v., at 1 pm and 3 pm every day) were also investigated.

RESULTS

CSR mice displayed a decline in learning and memory by 12% (P<0.05) in the Y-maze and by 18% (P<0.01) in the novel object recognition test; these changes were associated with increases in microglial activation, CD68+ microglial phagosome counts, astrocyte-derived complement C3 secretion, and microglial C3a receptor expression (all P<0.05). Synapse elimination, as indicated by a 66% decrease in synaptophysin expression (P=0.0004) and a 45% decrease in postsynaptic density protein-95 expression (P=0.0003), was associated with the occurrence of cognitive deficits. DEX activated astrocytic α2A adrenoceptors and inhibited astrocytic complement C3 release to attenuate synapse elimination through microglial phagocytosis. DEX restored synaptic connections and reversed cognitive deficits induced by CSR.

CONCLUSIONS

The results demonstrate that complement pathway activation associated with synapse elimination contributes to sleep loss-related cognitive deficits and that dexmedetomidine protects against sleep deprivation-induced complement activation. Dexmedetomidine holds potential for preventing cognitive deficits associated with sleep loss, which warrants further study.

Revisiting the critical roles of reactive astrocytes in neurodegeneration

Astrocytes, an integral component of the central nervous system (CNS), contribute to the maintenance of physiological homeostasis through their roles in synaptic function, K+ buffering, blood-brain barrier (BBB) maintenance, and neuronal metabolism. Reactive astrocytes refer to astrocytes undergoing morphological, molecular and functional remodelling in response to pathological stimuli. The activation and differentiation of astrocytes are implicated in the pathogenesis of multiple neurodegenerative diseases. However, there are still controversies regarding their subset identification, function and nomenclature in neurodegeneration. In this review, we revisit the multidimensional roles of reactive astrocytes in Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), multiple sclerosis (MS) and amyotrophic lateral sclerosis (ALS). Furthermore, we propose a precise linkage between astrocyte subsets and their functions based on single-cell sequencing analyses.

Role of astrocytes in sleep deprivation: accomplices, resisters, or bystanders?

Sleep plays an essential role in all studied animals with a nervous system. However, sleep deprivation leads to various pathological changes and neurobehavioral problems. Astrocytes are the most abundant cells in the brain and are involved in various important functions, including neurotransmitter and ion homeostasis, synaptic and neuronal modulation, and blood-brain barrier maintenance; furthermore, they are associated with numerous neurodegenerative diseases, pain, and mood disorders. Moreover, astrocytes are increasingly being recognized as vital contributors to the regulation of sleep-wake cycles, both locally and in specific neural circuits. In this review, we begin by describing the role of astrocytes in regulating sleep and circadian rhythms, focusing on: (i) neuronal activity; (ii) metabolism; (iii) the glymphatic system; (iv) neuroinflammation; and (v) astrocyte-microglia cross-talk. Moreover, we review the role of astrocytes in sleep deprivation comorbidities and sleep deprivation-related brain disorders. Finally, we discuss potential interventions targeting astrocytes to prevent or treat sleep deprivation-related brain disorders. Pursuing these questions would pave the way for a deeper understanding of the cellular and neural mechanisms underlying sleep deprivation-comorbid brain disorders.

Cntnap4 partial deficiency exacerbates α-synuclein pathology through astrocyte-microglia C3-C3aR pathway

Parkinson's disease (PD) is the most common progressive neurodegenerative movement disorder, which is characterized by dopaminergic (DA) neuron death and the aggregation of neurotoxic α-synuclein. Cntnap4, a risk gene of autism, has been implicated to participate in PD pathogenesis. Here we showed Cntnap4 lacking exacerbates α-synuclein pathology, nigrostriatal DA neuron degeneration and motor impairment, induced by injection of adeno-associated viral vector (AAV)-mediated human α-synuclein overexpression (AAV-hα-Syn). This scenario was further validated in A53T α-synuclein transgenic mice injected with AAV-Cntnap4 shRNA. Mechanistically, α-synuclein derived from damaged DA neuron stimulates astrocytes to release complement C3, activating microglial C3a receptor (C3aR), which in turn triggers microglia to secrete complement C1q and pro-inflammatory cytokines. Thus, the astrocyte-microglia crosstalk further drives DA neuron death and motor dysfunction in PD. Furthermore, we showed that in vivo depletion of microglia and microglial targeted delivery of a novel C3aR antagonist (SB290157) rescue the aggravated α-synuclein pathology resulting from Cntnap4 lacking. Together, our results indicate that Cntnap4 plays a key role in α-synuclein pathogenesis by regulating glial crosstalk and may be a potential target for PD treatment.

Microglia PKM2 Mediates Neuroinflammation and Neuron Loss in Mice Epilepsy through the Astrocyte C3-Neuron C3R Signaling Pathway

Epilepsy is a neurological disease and approximately 30% of patients have failed to respond to current anti-epilepsy drugs. The neuroinflammation mechanism has raised increasing concerns and been regarded as the novel treatment strategy in epilepsy, but the target molecules require further research. Pyruvate kinase isoform 2 (PKM2) is well studied in peripheral inflammation, but its role in epilepsy neuroinflammation remains unclear. We knocked down microglia PKM2 in the hippocampus using a stereotaxic adeno-associated virus (AAV) microinjection and established a pilocarpine-induced status epilepticus (PISE) model. Racine score was used to evaluate the seizure grade. Next, we used WB, Multiplex tyramide signal amplification (TSA) staining and other methods to determine neuroinflammation and the complement component 3 (C3)-C3aR interaction in primary microglia. Results showed that microglia PKM2 knockdown reduced epilepsy grade and rescued neuron loss. Mechanistically, PKM2 knockdown inhibited microglia activation and inflammation factor secretion through suppressing p65 expression and phosphorylation. The reduced microglia C1q, TNF-α, and IL-1α were responsible for the decreased astrocyte C3 expression and the following neuron damage caused by the C3-C3aR interaction. Our data suggest that microglia PKM2 inhibition ameliorates neuroinflammation and neuron loss through C3-C3aR interaction in epilepsy, which provides an attractive target for the intervention of damaged neuron-glia crosstalk in epilepsy.

Function and therapeutic value of astrocytes in neurological diseases

Astrocytes are abundant glial cells in the central nervous system (CNS) that perform diverse functions in health and disease. Astrocyte dysfunction is found in numerous diseases, including multiple sclerosis, Alzheimer disease, Parkinson disease, Huntington disease and neuropsychiatric disorders. Astrocytes regulate glutamate and ion homeostasis, cholesterol and sphingolipid metabolism and respond to environmental factors, all of which have been implicated in neurological diseases. Astrocytes also exhibit significant heterogeneity, driven by developmental programmes and stimulus-specific cellular responses controlled by CNS location, cell-cell interactions and other mechanisms. In this Review, we highlight general mechanisms of astrocyte regulation and their potential as therapeutic targets, including drugs that alter astrocyte metabolism, and therapies that target transporters and receptors on astrocytes. Emerging ideas, such as engineered probiotics and glia-to-neuron conversion therapies, are also discussed. We further propose a concise nomenclature for astrocyte subsets that we use to highlight the roles of astrocytes and specific subsets in neurological diseases.

Astrocyte-microglia interaction through C3/C3aR pathway modulates neuropathic pain in rats model of chronic constriction injury

Neuropathic pain (NP) is the cardinal symptom of neural injury, and its underlying molecular mechanism needs further investigation. Complements, especially complement 3 (C3), are involved in the pathophysiology of many neurological disorders, while the specific role of C3 in NP is still obscure. In this study, we found that both C3 and its receptor C3aR were upregulated in the spinal dorsal horn in a rat chronic constriction injury (CCI) model. In addition, C3 was mainly detected in astrocytes, while C3aR was expressed in microglia and neuron. Intrathecal injection of C3 antibody and C3aR antagonist alleviated NP in CCI model together with reduced M1 polarization of microglia. Our finding suggested that blockade of the C3/C3aR pathway might be a novel strategy for NP.

Homer1 promotes the conversion of A1 astrocytes to A2 astrocytes and improves the recovery of transgenic mice after intracerebral hemorrhage

Background

Inflammation induced by intracerebral hemorrhage (ICH) is one of the main causes of the high mortality and poor prognosis of patients with ICH. A1 astrocytes are closely associated with neuroinflammation and neurotoxicity, whereas A2 astrocytes are neuroprotective. Homer scaffolding protein 1 (Homer1) plays a protective role in ischemic encephalopathy and neurodegenerative diseases. However, the role of Homer1 in ICH-induced inflammation and the effect of Homer1 on the phenotypic conversion of astrocytes remain unknown.

Methods

Femoral artery autologous blood from C57BL/6 mice was used to create an ICH model. We use the A1 phenotype marker C3 and A2 phenotype marker S100A10 to detect astrocyte conversion after ICH. Homer1 overexpression/knock-down mice were constructed by adeno-associated virus (AAV) infection to explore the role of Homer1 and its mechanism of action after ICH. Finally, Homer1 protein and selumetinib were injected into in situ hemorrhage sites in the brains of Homer1^flox/flox/Nestin-Cre^+/− mice to study the efficacy of Homer1 in the treatment of ICH by using a mouse cytokine array to explore the potential mechanism.

Results

The expression of Homer1 peaked on the third day after ICH and colocalized with astrocytes. Homer1 promotes A1 phenotypic conversion in astrocytes in vivo and in vitro. Overexpression of Homer1 inhibits the activation of MAPK signaling, whereas Homer1 knock-down increases the expression of pathway-related proteins. The Homer1 protein and selumetinib, a non-ATP competitive MEK1/2 inhibitor, improved the outcome in ICH in Homer1^flox/flox/Nestin-Cre^+/− mice. The efficacy of Homer1 in the treatment of ICH is associated with reduced expression of the inflammatory factor TNFSF10 and increased expression of the anti-inflammatory factors activin A, persephin, and TWEAK.

Conclusions

Homer1 plays an important role in inhibiting inflammation after ICH by suppressing the A1 phenotype conversion in astrocytes. In situ injection of Homer1 protein may be a novel and effective method for the treatment of inflammation after ICH.

Ion Channel Dysfunction in Astrocytes in Neurodegenerative Diseases

Astrocytes play an important role in the central nervous system (CNS). Ion channels in these cells not only function in ion transport, and maintain water/ion metabolism homeostasis, but also participate in physiological processes of neurons and glial cells by regulating signaling pathways. Increasing evidence indicates the ion channel proteins of astrocytes, such as aquaporins (AQPs), transient receptor potential (TRP) channels, adenosine triphosphate (ATP)-sensitive potassium (K-ATP) channels, and P2X7 receptors (P2X7R), are strongly associated with oxidative stress, neuroinflammation and characteristic proteins in neurodegenerative disorders, including Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD) and amyotrophic lateral sclerosis (ALS). Since ion channel protein dysfunction is a significant pathological feature of astrocytes in neurodegenerative diseases, we discuss these critical proteins and their signaling pathways in order to understand the underlying molecular mechanisms, which may yield new therapeutic targets for neurodegenerative disorders.