Astrocytes in human central nervous system diseases: a frontier for new therapies

Selective disruption of mTORC1 and mTORC2 in VTA astrocytes induces depression and anxiety-like behaviors in mice

Dysfunction of the mechanistic target of rapamycin (mTOR) signaling pathway is implicated in neuropsychiatric disorders including depression and anxiety. Most studies have been focusing on neurons, and the function of mTOR signaling pathway in astrocytes is less investigated. mTOR forms two distinct complexes, mTORC1 and mTORC2, with key scaffolding protein Raptor and Rictor, respectively. The ventral tegmental area (VTA), a vital component of the brain reward system, is enrolled in regulating both depression and anxiety. In the present study, we aimed to examine the regulation effect of VTA astrocytic mTOR signaling pathway on depression and anxiety. We specifically deleted Raptor or Rictor in VTA astrocytes in mice and performed a series of behavioral tests for depression and anxiety. Deletion of Raptor and Rictor both decreased the immobility time in the tail suspension test and the latency to eat in the novelty suppressed feeding test, and increased the horizontal activity and the movement time in locomotor activity. Deletion of Rictor decreased the number of total arm entries in the elevated plus-maze test and the vertical activity in locomotor activity. These data suggest that VTA astrocytic mTORC1 plays a role in regulating depression-related behaviors and mTORC2 is involved in both depression and anxiety-related behaviors. Our results indicate that VTA astrocytic mTOR signaling pathway might be new targets for the treatment of psychiatric disorders.

The Role of SOX2 and SOX9 Transcription Factors in the Reactivation-Related Functional Properties of NT2/D1-Derived Astrocytes

Astrocytes are the main homeostatic cells in the central nervous system, with the unique ability to transform from quiescent into a reactive state in response to pathological conditions by reacquiring some precursor properties. This process is known as reactive astrogliosis, a compensatory response that mediates tissue damage and recovery. Although it is well known that SOX transcription factors drive the expression of phenotype-specific genetic programs during neurodevelopment, their roles in mature astrocytes have not been studied extensively. We focused on the transcription factors SOX2 and SOX9, shown to be re-expressed in reactive astrocytes, in order to study the reactivation-related functional properties of astrocytes mediated by those proteins. We performed an initial screening of SOX2 and SOX9 expression after sensorimotor cortex ablation injury in rats and conducted gain-of-function studies in vitro using astrocytes derived from the human NT2/D1 cell line. Our results revealed the direct involvement of SOX2 in the reacquisition of proliferation in mature NT2/D1-derived astrocytes, while SOX9 overexpression increased migratory potential and glutamate uptake in these cells. Our results imply that modulation of SOX gene expression may change the functional properties of astrocytes, which holds promise for the discovery of potential therapeutic targets in the development of novel strategies for tissue regeneration and recovery.

Optogenetic and chemogenetic approaches for modeling neurological disorders in vivo

Animal models of human neurological disorders provide valuable experimental tools which enable us to study various aspects of disorder pathogeneses, ranging from structural abnormalities and disrupted metabolism and signaling to motor and mental deficits, and allow us to test novel therapies in preclinical studies. To be valid, these animal models should recapitulate complex pathological features at the molecular, cellular, tissue, and behavioral levels as closely as possible to those observed in human subjects. Pathological states resembling known human neurological disorders can be induced in animal species by toxins, genetic factors, lesioning, or exposure to extreme conditions. In recent years, novel animal models recapitulating neuropathologies in humans have been introduced. These animal models are based on synthetic biology approaches: opto- and chemogenetics. In this paper, we review recent opto- and chemogenetics-based animal models of human neurological disorders. These models allow for the creation of pathological states by disrupting specific processes at the cellular level. The artificial pathological states mimic a range of human neurological disorders, such as aging-related dementia, Alzheimer's and Parkinson's diseases, amyotrophic lateral sclerosis, epilepsy, and ataxias. Opto- and chemogenetics provide new opportunities unavailable with other animal models of human neurological disorders. These techniques enable researchers to induce neuropathological states varying in severity and ranging from acute to chronic. We also discuss future directions for the development and application of synthetic biology approaches for modeling neurological disorders.

Human Glial Cells as Innovative Targets for the Therapy of Central Nervous System Pathologies

In vitro and preclinical in vivo research in the last 35 years has clearly highlighted the crucial physiopathological role of glial cells, namely astrocytes/microglia/oligodendrocytes and satellite glial cells/Schwann cells in the central and peripheral nervous system, respectively. Several possible pharmacological targets to various neurodegenerative disorders and painful conditions have therefore been successfully identified, including receptors and enzymes, and mediators of neuroinflammation. However, the translation of these promising data to a clinical setting is often hampered by both technical and biological difficulties, making it necessary to perform experiments on human cells and models of the various diseases. In this review we will, therefore, summarize the most relevant data on the contribution of glial cells to human pathologies and on their possible pharmacological modulation based on data obtained in post-mortem tissues and in iPSC-derived human brain cells and organoids. The possibility of an in vivo visualization of glia reaction to neuroinflammation in patients will be also discussed.

The Interplay between Ferroptosis and Neuroinflammation in Central Neurological Disorders

Central neurological disorders are significant contributors to morbidity, mortality, and long-term disability globally in modern society. These encompass neurodegenerative diseases, ischemic brain diseases, traumatic brain injury, epilepsy, depression, and more. The involved pathogenesis is notably intricate and diverse. Ferroptosis and neuroinflammation play pivotal roles in elucidating the causes of cognitive impairment stemming from these diseases. Given the concurrent occurrence of ferroptosis and neuroinflammation due to metabolic shifts such as iron and ROS, as well as their critical roles in central nervous disorders, the investigation into the co-regulatory mechanism of ferroptosis and neuroinflammation has emerged as a prominent area of research. This paper delves into the mechanisms of ferroptosis and neuroinflammation in central nervous disorders, along with their interrelationship. It specifically emphasizes the core molecules within the shared pathways governing ferroptosis and neuroinflammation, including SIRT1, Nrf2, NF-κB, Cox-2, iNOS/NO·, and how different immune cells and structures contribute to cognitive dysfunction through these mechanisms. Researchers' findings suggest that ferroptosis and neuroinflammation mutually promote each other and may represent key factors in the progression of central neurological disorders. A deeper comprehension of the common pathway between cellular ferroptosis and neuroinflammation holds promise for improving symptoms and prognosis related to central neurological disorders.

Recent insights from non-mammalian models of brain injuries: an emerging literature

Traumatic brain injury (TBI) is a major global health concern and is increasingly recognized as a risk factor for neurodegenerative diseases including Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). Repetitive TBIs (rTBIs), commonly observed in contact sports, military service, and intimate partner violence (IPV), pose a significant risk for long-term sequelae. To study the long-term consequences of TBI and rTBI, researchers have typically used mammalian models to recapitulate brain injury and neurodegenerative phenotypes. However, there are several limitations to these models, including: (1) lengthy observation periods, (2) high cost, (3) difficult genetic manipulations, and (4) ethical concerns regarding prolonged and repeated injury of a large number of mammals. Aquatic vertebrate model organisms, including Petromyzon marinus (sea lampreys), zebrafish (Danio rerio), and invertebrates, Caenorhabditis elegans (C. elegans), and Drosophila melanogaster (Drosophila), are emerging as valuable tools for investigating the mechanisms of rTBI and tauopathy. These non-mammalian models offer unique advantages, including genetic tractability, simpler nervous systems, cost-effectiveness, and quick discovery-based approaches and high-throughput screens for therapeutics, which facilitate the study of rTBI-induced neurodegeneration and tau-related pathology. Here, we explore the use of non-vertebrate and aquatic vertebrate models to study TBI and neurodegeneration. Drosophila, in particular, provides an opportunity to explore the longitudinal effects of mild rTBI and its impact on endogenous tau, thereby offering valuable insights into the complex interplay between rTBI, tauopathy, and neurodegeneration. These models provide a platform for mechanistic studies and therapeutic interventions, ultimately advancing our understanding of the long-term consequences associated with rTBI and potential avenues for intervention.

Trace metals and astrocytes physiology and pathophysiology

Several trace metals, including iron, copper, manganese and zinc are essential for normal function of the nervous system. Both deficiency and excessive accumulation of these metals trigger neuropathological developments. The central nervous system (CNS) is in possession of dedicated homeostatic system that removes, accumulates, stores and releases these metals to fulfil nervous tissue demand. This system is mainly associated with astrocytes that act as dynamic reservoirs for trace metals, these being a part of a global system of CNS ionostasis. Here we overview physiological and pathophysiological aspects of astrocyte-cantered trace metals regulation.

Astrocytic chloride regulates brain function in health and disease

Chloride ions (Cl-) play a pivotal role in synaptic inhibition in the central nervous system, primarily mediated through ionotropic mechanisms. A recent breakthrough emphathizes the significant influence of astrocytic intracellular chloride concentration ([Cl-]i) regulation, a field still in its early stages of exploration. Typically, the [Cl-]i in most animal cells is maintained at lower levels than the extracellular chloride [Cl-]o, a critical balance to prevent cell swelling due to osmotic pressure. Various Cl- transporters are expressed differently across cell types, fine-tuning the [Cl-]i, while Cl- gradients are utilised by several families of Cl- channels. Although the passive distribution of ions within cells is governed by basic biophysical principles, astrocytes actively expend energy to sustain [Cl-]i at much higher levels than those achieved passively, and much higher than neuronal [Cl-]i. Beyond the role in volume regulation, astrocytic [Cl-]i is dynamically linked to brain states and influences neuronal signalling in actively behaving animals. As a vital component of brain function, astrocytic [Cl-]i also plays a role in the development of disorders where inhibitory transmission is disrupted. This review synthesises the latest insights into astrocytic [Cl-]i, elucidating its role in modulating brain function and its implications in various pathophysiological conditions.

The dangerous "West Coast Swing" by hyperglycaemia and chronic stress in the mouse hippocampus: Role of kynurenine catabolism

Growing epidemiological studies highlight a bi-directional relationship between depressive symptoms and diabetes mellitus. However, the detrimental impact of their co-existence on mental health suggests the need to treat this comorbidity as a separate entity rather than the two different pathologies. Herein, we characterized the peculiar mechanisms activated in mouse hippocampus from the concurrent development of hyperglycaemia, characterizing the different diabetes subtypes, and chronic stress, recognized as a possible factor predisposing to major depression. Our work demonstrates that kynurenine overproduction, leading to apoptosis in the hippocampus, is triggered in a different way depending on hyperglycaemia or chronic stress. Indeed, in the former, kynurenine appears produced by infiltered macrophages whereas, in the latter, peripheral kynurenine preferentially promotes resident microglia activation. In this scenario, QA, derived from kynurenine catabolism, appears a key mediator causing glutamatergic synapse dysfunction and apoptosis, thus contributing to brain atrophy. We demonstrated that the coexistence of hyperglycaemia and chronic stress worsened hippocampal damage through alternative mechanisms, such as GLUT-4 and BDNF down-expression, denoting mitochondrial dysfunction and apoptosis on one hand and evoking the compromission of neurogenesis on the other. Overall, in the degeneration of neurovascular unit, hyperglycaemia and chronic stress interacted each other as the partners of a "West Coast Swing" in which the leading role can be assumed alternatively by each partner of the dance. The comprehension of these mechanisms can open novel perspectives in the management of diabetic/depressed patients, but also in the understanding the pathogenesis of other neurodegenerative disease characterized by the compromission of hippocampal function.

Astrocytes: The Stars in Neurodegeneration?

Today, neurodegenerative disorders like Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) affect millions of people worldwide, and as the average human lifespan increases, similarly grows the number of patients. For many decades, cognitive and motoric decline has been explained by the very apparent deterioration of neurons in various regions of the brain and spinal cord. However, more recent studies show that disease progression is greatly influenced by the vast population of glial cells. Astrocytes are traditionally considered star-shaped cells on which neurons rely heavily for their optimal homeostasis and survival. Increasing amounts of evidence depict how astrocytes lose their supportive functions while simultaneously gaining toxic properties during neurodegeneration. Many of these changes are similar across various neurodegenerative diseases, and in this review, we highlight these commonalities. We discuss how astrocyte dysfunction drives neuronal demise across a wide range of neurodegenerative diseases, but rather than categorizing based on disease, we aim to provide an overview based on currently known mechanisms. As such, this review delivers a different perspective on the disease causes of neurodegeneration in the hope to encourage further cross-disease studies into shared disease mechanisms, which might ultimately disclose potentially common therapeutic entry points across a wide panel of neurodegenerative diseases.

A double inducible cell ablation system for eliminating senescent astrocytes via apoptosis

PURPOSE

Cell senescence stands as a principal risk factor for various neurodegenerative diseases, with astrocytic senescence emerging as a potentially pivotal player in the pathogenesis of aging and neurodegenerative disorders. Clearing senescent astrocytes holds promise as a potential therapeutic approach for senescence-related diseases.

METHODS

In this study, we designed and constructed two plasmids aimed at inducing apoptosis in senescent astrocytes. This was achieved through the ligation of FKBP (FK506-binding protein) and FRB (FKBP and FKBP rapamycin binding domain) and the formation of caspase8 dimers, thereby achieving the purpose of eliminating senescent astrocytes.

RESULTS

The developed vector system demonstrates a specifically capability to induce apoptosis in aging astrocytes, offering a targeted approach to eliminate these cells.

CONCLUSION

The utilization of the double -inducible suicide gene system provides a versatile tool forstimulating cell apoptosis and inhibiting cellular senescence. This system proves valuable in exploring the intrinsic roles and molecular mechanisms of senescent cells in the occurrence and development of aging-related diseases. Ultimately, it offers a potential avenue for developing an efficient treatment system for such conditions.

The role of NF-κB signaling pathway in reactive astrocytes among neurodegeneration after methamphetamine exposure by integrated bioinformatics

BACKGROUND

Methamphetamine (METH) is a highly addictive stimulant that has become one of the top five risk substances cause deaths from substance abuse. METH exposure increases the risk of neurodegenerative disease (ND), such as Parkinson's disease (PD), leading to disability and death. Activation of reactive astrocytes is an essential factor in neurodegeneration, and their complex role in METH exposure remains unclear. This study explored the role of reactive astrocyte overactivation in neurodegeneration after METH exposure.

METHODS

METH bulk RNA sequencing data (GSE107015 and GSE98793) and single-cell RNA sequencing data (GSE119861) were obtained from the GEO database. We performed immune infiltration analysis on the bulk RNA data. After cell clustering using the single-cell RNA data, astrocytes were extracted for downstream analysis. Differentially expressed genes (DEGs) were identified from the bulk and single-cell RNA sequencing datasets, and GO, KEGG, and GSEA pathway analyses were performed. The PPI network and random forest methods were performed on the overlapping genes of the DEGs to screen hub genes. To explore the common ground between METH exposure and neurodegenerative diseases, we applied a random forest algorithm to PD chip data (GSE99039 and GSE72267) to establish a diagnostic model using the hub genes in METH. New object recognition and the Morris water maze were used to examine cognitive function in mice exposed to METH for 14 days in vivo. Astrocytes were cocultured with neurons for the detection of intercellular crosstalk.

RESULTS

DEGs in the METH group significantly enriched pathways related to NDs, inflammation, and the NF-κB signaling pathway. Immune infiltration analysis revealed significantly increased infiltration of monocytes, T cells, and NK cells and decreased infiltration of neutrophils in the METH group. An intersection of 44 hub genes was screened based on the PPI network and random forest algorithm. These genes suggest that there might be similar pathogenesis between METH exposure and PD. METH exposure resulted in learning memory impairment, hippocampal astrocyte activation, and upregulation of NF-κB expression in mice. Activation of reactive astrocytes cocultured with neurons causes neural damage.

CONCLUSIONS

This study explored the crosstalk between astrocytes and neurons in METH exposure, providing a potential pathogenesis to explore the altered immune microenvironment involving reactive astrocytes after METH exposure.

The Diverse Roles of Reactive Astrocytes in the Pathogenesis of Amyotrophic Lateral Sclerosis

Astrocytes displaying reactive phenotypes are characterized by their ability to remodel morphologically, molecularly, and functionally in response to pathological stimuli. This process results in the loss of their typical astrocyte functions and the acquisition of neurotoxic or neuroprotective roles. A growing body of research indicates that these reactive astrocytes play a pivotal role in the pathogenesis of amyotrophic lateral sclerosis (ALS), involving calcium homeostasis imbalance, mitochondrial dysfunction, abnormal lipid and lactate metabolism, glutamate excitotoxicity, etc. This review summarizes the characteristics of reactive astrocytes, their role in the pathogenesis of ALS, and recent advancements in astrocyte-targeting strategies.

Multitracer Approach to Understanding the Complexity of Reactive Astrogliosis in Alzheimer's Brains

A monoamine oxidase B (MAO-B) selective positron emission tomography (PET) tracer [11C]-deuterium-l-deprenyl holds promise for imaging reactive astrogliosis in neurodegenerative diseases, such as Alzheimer's disease (AD). Two novel PET tracers ([11C]-BU99008 and [18F]-SMBT-1) have recently been developed to assess the complexity of reactive astrogliosis in the AD continuum. We have investigated the binding properties of SMBT-1, l-deprenyl, and BU99008 in AD and cognitively normal control (CN) brains. Competition binding assays with [3H]-l-deprenyl and [3H]-BU99008 versus unlabeled SMBT-1 in postmortem AD and CN temporal and frontal cortex brains demonstrated that SMBT-1 interacted with [3H]-deprenyl at a single binding site (nM range) and with [3H]-BU99008 at multiple binding sites (from nM to μM). Autoradiography studies on large frozen postmortem AD and CN hemisphere brain sections demonstrated that 1 μM SMBT-1 almost completely displaced the [3H]-l-deprenyl binding (>90%), while SMBT-1 only partly displaced the [3H]-BU99008 binding (50-60% displacement) in cortical regions. In conclusion, SMBT-1, l-deprenyl, and BU99008 interact at the same MAO-B binding site, while BU99008 shows an additional independent binding site in AD and CN brains. The high translational power of our studies in human AD and CN brains suggests that the multitracer approach with SMBT-1, l-deprenyl, and BU99008 could be useful for imaging reactive astrogliosis.

Amyloid precursor protein induces reactive astrogliosis

We present in vitro and in vivo evidence demonstrating that Amyloid Precursor Protein (APP) acts as an essential instigator of reactive astrogliosis. Cell-specific overexpression of APP in cultured astrocytes led to remodelling of the intermediate filament network, enhancement of cytokine production and activation of cellular programs centred around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion in cultured astrocytes abrogated remodelling of the intermediate filament network and blunted expression of IFN stimulated gene (ISG) products in response to lipopolysaccharide (LPS). Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein (GFAP) observed canonically in astrocytes in response to TBI. Thus, APP represents a molecular inducer and regulator of reactive astrogliosis.