Cholinergic Modulation of Neuroinflammation: Focus on α7 Nicotinic Receptor

All nervous system pathologies (e.g., neurodegenerative/demyelinating diseases and brain tumours) develop neuroinflammation, a beneficial process during pathological events, aimed at removing damaged cells, toxic agents, and/or pathogens. Unfortunately, excessive inflammation frequently occurs during nervous system disorders, becoming a detrimental event capable of enhancing neurons and myelinating glial cell impairment, rather than improving their survival and activity. Consequently, targeting the neuroinflammation could be relevant for reducing brain injury and rescuing neuronal and glial cell functions. Several studies have highlighted the role of acetylcholine and its receptors in the regulation of central and peripheral inflammation. In particular, α7 nicotinic receptor has been described as one of the main regulators of the “brain cholinergic anti-inflammatory pathway”. Its expression in astrocytes and microglial cells and the ability to modulate anti-inflammatory cytokines make this receptor a new interesting therapeutic target for neuroinflammation regulation. In this review, we summarize the distribution and physiological functions of the α7 nicotinic receptor in glial cells (astrocytes and microglia) and its role in the modulation of neuroinflammation. Moreover, we explore how its altered expression and function contribute to the development of different neurological pathologies and exacerbate neuroinflammatory processes.     

Modeling Neurodevelopmental and Neuropsychiatric Diseases with Astrocytes Derived from Human-Induced Pluripotent Stem Cells

Accumulating studies demonstrate the morphological and functional diversity of astrocytes, a subtype of glial cells in the central nervous system. Animal models are instrumental in advancing our understanding of the role of astrocytes in brain development and their contribution to neurological disease; however, substantial interspecies differences exist between rodent and human astrocytes, underscoring the importance of studying human astrocytes. Human pluripotent stem cell differentiation approaches allow the study of patient-specific astrocytes in the etiology of neurological disorders. In this review, we summarize the structural and functional properties of astrocytes, including the unique features of human astrocytes; demonstrate the necessity of the stem cell platform; and discuss how this platform has been applied to the research of neurodevelopmental and neuropsychiatric diseases.     

Not Just a Bystander: The Emerging Role of Astrocytes and Research Tools in Studying Cognitive Dysfunctions in Schizophrenia

Cognitive dysfunction is one of the core symptoms in schizophrenia, and it is predictive of functional outcomes and therefore useful for treatment targets. Rather than improving cognitive deficits, currently available antipsychotics mainly focus on positive symptoms, targeting dopaminergic/serotoninergic neurons and receptors in the brain. Apart from investigating the neural mechanisms underlying schizophrenia, emerging evidence indicates the importance of glial cells in brain structure development and their involvement in cognitive functions. Although the etiopathology of astrocytes in schizophrenia remains unclear, accumulated evidence reveals that alterations in gene expression and astrocyte products have been reported in schizophrenic patients. To further investigate the role of astrocytes in schizophrenia, we highlighted recent progress in the investigation of the effect of astrocytes on abnormalities in glutamate transmission and impairments in the blood–brain barrier. Recent advances in animal models and behavioral methods were introduced to examine schizophrenia-related cognitive deficits and negative symptoms. We also highlighted several experimental tools that further elucidate the role of astrocytes. Instead of focusing on schizophrenia as a neuron-specific disorder, an additional astrocytic perspective provides novel and promising insight into its causal mechanisms and treatment. The involvement of astrocytes in the pathogenesis of schizophrenia and other brain disorders is worth further investigation.     

Microglia Development and Maturation and Its Implications for Induction of Microglia-Like Cells from Human iPSCs

Microglia are resident immune cells of the central nervous system and play critical roles during the development, homeostasis, and pathologies of the brain. Originated from yolk sac erythromyeloid progenitors, microglia immigrate into the embryonic brain parenchyma to undergo final postnatal differentiation and maturation driven by distinct chemokines, cytokines, and growth factors. Among them, TGFβ1 is an important regulator of microglial functions, mediating homeostasis, anti-inflammation, and triggering the expression of microglial homeostatic signature genes. Since microglia studies are mainly based on rodent cells and the isolation of homeostatic microglia from human tissue is challenging, human-induced pluripotent stem cells have been successfully differentiated into microglia-like cells recently. However, employed differentiation protocols strongly vary regarding used cytokines and growth factors, culture conditions, time span, and cell yield. Moreover, the incomplete differentiation of human microglia can hamper the similarity to primary human microglia and dramatically influence the outcome of follow-up studies with these differentiated cells. This review summarizes the current knowledge of the molecular mechanisms driving rodent microglia differentiation in vivo, further compares published differentiation protocols, and highlights the potential of TGFβ as an essential maturation factor.     

Patient-Specific iPSCs-Based Models of Neurodegenerative Diseases: Focus on Aberrant Calcium Signaling

The development of cell reprogramming technologies became a breakthrough in the creation of new models of human diseases, including neurodegenerative pathologies. The iPSCs-based models allow for the studying of both hereditary and sporadic cases of pathologies and produce deep insight into the molecular mechanisms underlying neurodegeneration. The use of the cells most vulnerable to a particular pathology makes it possible to identify specific pathological mechanisms and greatly facilitates the task of selecting the most effective drugs. To date, a large number of studies on patient-specific models of neurodegenerative diseases has been accumulated. In this review, we focused on the alterations of such a ubiquitous and important intracellular regulatory pathway as calcium signaling. Here, we reviewed and analyzed the data obtained from iPSCs-based models of different neurodegenerative disorders that demonstrated aberrant calcium signaling.     

Emerging hiPSC Models for Drug Discovery in Neurodegenerative Diseases

Neurodegenerative diseases affect millions of people worldwide and are characterized by the chronic and progressive deterioration of neural function. Neurodegenerative diseases, such as Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington’s disease (HD), represent a huge social and economic burden due to increasing prevalence in our aging society, severity of symptoms, and lack of effective disease-modifying therapies. This lack of effective treatments is partly due to a lack of reliable models. Modeling neurodegenerative diseases is difficult because of poor access to human samples (restricted in general to postmortem tissue) and limited knowledge of disease mechanisms in a human context. Animal models play an instrumental role in understanding these diseases but fail to comprehensively represent the full extent of disease due to critical differences between humans and other mammals. The advent of human-induced pluripotent stem cell (hiPSC) technology presents an advantageous system that complements animal models of neurodegenerative diseases. Coupled with advances in gene-editing technologies, hiPSC-derived neural cells from patients and healthy donors now allow disease modeling using human samples that can be used for drug discovery.     

Blood–Brain Barrier Dysfunction and Astrocyte Senescence as Reciprocal Drivers of Neuropathology in Aging

As the most abundant cell types in the brain, astrocytes form a tissue-wide signaling network that is responsible for maintaining brain homeostasis and regulating various brain activities. Here, we review some of the essential functions that astrocytes perform in supporting neurons, modulating the immune response, and regulating and maintaining the blood–brain barrier (BBB). Given their importance in brain health, it follows that astrocyte dysfunction has detrimental effects. Indeed, dysfunctional astrocytes are implicated in age-related neuropathology and participate in the onset and progression of neurodegenerative diseases. Here, we review two mechanisms by which astrocytes mediate neuropathology in the aging brain. First, age-associated blood–brain barrier dysfunction (BBBD) causes the hyperactivation of TGFβ signaling in astrocytes, which elicits a pro-inflammatory and epileptogenic phenotype. Over time, BBBD-associated astrocyte dysfunction results in hippocampal and cortical neural hyperexcitability and cognitive deficits. Second, senescent astrocytes accumulate in the brain with age and exhibit a decreased functional capacity and the secretion of senescent-associated secretory phenotype (SASP) factors, which contribute to neuroinflammation and neurotoxicity. Both BBBD and senescence progressively increase during aging and are associated with increased risk of neurodegenerative disease, but the relationship between the two has not yet been established. Thus, we discuss the potential relationship between BBBD, TGFβ hyperactivation, and senescence with respect to astrocytes in the context of aging and disease and identify future areas of investigation in the field.     

Extracellular Vesicles Taken up by Astrocytes

Extracellular vesicles (EVs) are composed of lipid bilayer membranes and contain various molecules, such as mRNA and microRNA (miRNA), that regulate the functions of the recipient cell. Recent studies have reported the importance of EV-mediated intercellular communication in the brain. The brain contains several types of cells, including neurons and glial cells. Among them, astrocytes are the most abundant glial cells in the mammalian brain and play a wide range of roles, from structural maintenance of the brain to regulation of neurotransmission. Furthermore, since astrocytes can take up EVs, it is possible that EVs originating from inside and outside the brain affect astrocyte function, which in turn affects brain function. However, it has not been fully clarified whether the specific targeting mechanism of EVs to astrocytes as recipient cells exists. In recent years, EVs have attracted attention as a cell-targeted therapeutic approach in various organs, and elucidation of the targeting mechanism of EVs to astrocytes may pave the way for new therapies for brain diseases. In this review, we focus on EVs in the brain that affect astrocyte function and discuss the targeting mechanism of EVs to astrocytes.     

SSAO/VAP-1 in Cerebrovascular Disorders: A Potential Therapeutic Target for Stroke and Alzheimer’s Disease

The semicarbazide-sensitive amine oxidase (SSAO), also known as vascular adhesion protein-1 (VAP-1) or primary amine oxidase (PrAO), is a deaminating enzyme highly expressed in vessels that generates harmful products as a result of its enzymatic activity. As a multifunctional enzyme, it is also involved in inflammation through its ability to bind and promote the transmigration of circulating leukocytes into inflamed tissues. Inflammation is present in different systemic and cerebral diseases, including stroke and Alzheimer’s disease (AD). These pathologies show important affectations on cerebral vessels, together with increased SSAO levels. This review summarizes the main roles of SSAO/VAP-1 in human physiology and pathophysiology and discusses the mechanisms by which it can affect the onset and progression of both stroke and AD. As there is an evident interrelationship between stroke and AD, basically through the vascular system dysfunction, the possibility that SSAO/VAP-1 could be involved in the transition between these two pathologies is suggested. Hence, its inhibition is proposed to be an interesting therapeutical approach to the brain damage induced in these both cerebral pathologies.     

Reactive and Senescent Astroglial Phenotypes as Hallmarks of Brain Pathologies

Astrocytes, as the most abundant glial cells in the central nervous system, are tightly integrated into neural networks and participate in numerous aspects of brain physiology and pathology. They are the main homeostatic cells in the central nervous system, and the loss of astrocyte physiological functions and/or gain of pro-inflammatory functions, due to their reactivation or cellular senescence, can have profound impacts on the surrounding microenvironment with pathological outcomes. Although the importance of astrocytes is generally recognized, and both senescence and reactive astrogliosis have been extensively reviewed independently, there are only a few comparative overviews of these complex processes. In this review, we summarize the latest data regarding astrocyte reactivation and senescence, and outline similarities and differences between these phenotypes from morphological, functional, and molecular points of view. A special focus has been given to neurodegenerative diseases, where these phenotypic alternations of astrocytes are significantly implicated. We also summarize current perspectives regarding new advances in model systems based on astrocytes as well as data pointing to these glial cells as potential therapeutic targets.     

Murine Beta-Amyloid (1–42) Oligomers Disrupt Endothelial Barrier Integrity and VEGFR Signaling via Activating Astrocytes to Release Deleterious Soluble Factors

Transgenic mouse models of Alzheimer’s disease (AD) overexpress mutations of the human amyloid protein precursor (APP) and presenilin-1 (PSEN1) genes, which are known causes of amyloid pathology in familial AD. However, animal models for studying AD in the context of aging and age-related co-morbidities, such as blood–brain barrier (BBB) disruptions, are lacking. More recently, aged and progeroid mouse models have been proposed as alternatives to study aging-related AD, but the toxicity of murine amyloid-beta protein (Aβ) is not well defined. In this study, we aimed to study the potential toxicity of murine Aβ on brain endothelial cells and astrocytes, which are important components of the BBB, using mouse brain endothelial cells (bEnd.3) and astrocytes (C8-D1A). Murine-soluble Aβ (1–42) oligomers (sAβO42) (10 µM) induced negligible injuries in an endothelial monolayer but induced significant barrier disruptions in a bEnd.3 and C8-D1A co-culture. Similar results of endothelial perturbation were observed in a bEnd.3 monolayer treated with astrocyte-conditioned medium (ACM) generated by astrocytes exposed to sAβO42 (ACM-sAβO42), while additional exogenous sAβO42 did not cause further damage. Western blot analysis showed that ACM-sAβO42 altered the basal activities of vascular endothelial growth factor receptor 2 (VEGFR2), eNOS, and the signaling of the MEK/ERK and Akt pathways in bEnd.3. Our results showed that murine sAβO42 was moderately toxic to an endothelial and astrocyte co-culture. These damaging effects on the endothelial barrier were induced by deleterious soluble factors released from astrocytes, which disrupted endothelial VEGFR2 signaling and perturbed cell survival and barrier stabilization.     

Glia in Neurodegeneration: The Housekeeper,the Defender and the Perpetrator

Over the past decade, research has unveiled the intimate relationship between neuroinflammation and neurodegeneration. Microglia and astrocytes react to brain insult by setting up a multimodal inflammatory state and act as the primary defenders and executioners of neuroinflammatory structural and functional changes. Microglia and astrocytes also play critical roles in the maintenance of normal brain function. This intricate balance of homeostatic and neuroinflammatory functions can influence the onset and the course of neurodegenerative diseases. The emergent role of the microglial-astrocytic axis in neurodegenerative disease presents many druggable targets that may have broad therapeutic benefits across neurodegenerative disease. Here, we provide a brief review of the basal function of both microglia and astrocytes, how they are changed in disease states, the significant differences between mouse and human glia, and use of human induced pluripotent stem cells derived from patients to study cell autonomous changes in human astrocytes and microglia.     

Differential Proteomic Analysis of Astrocytes and Astrocytes-Derived Extracellular Vesicles from Control and Rai Knockout Mice: Insights into the Mechanisms of Neuroprotection

Reactive astrocytes are a hallmark of neurodegenerative disease including multiple sclerosis. It is widely accepted that astrocytes may adopt alternative phenotypes depending on a combination of environmental cues and intrinsic features in a highly plastic and heterogeneous manner. However, we still lack a full understanding of signals and associated signaling pathways driving astrocyte reaction and of the mechanisms by which they drive disease. We have previously shown in the experimental autoimmune encephalomyelitis mouse model that deficiency of the molecular adaptor Rai reduces disease severity and demyelination. Moreover, using primary mouse astrocytes, we showed that Rai contributes to the generation of a pro-inflammatory central nervous system (CNS) microenvironment through the production of nitric oxide and IL-6 and by impairing CD39 activity in response to soluble factors released by encephalitogenic T cells. Here, we investigated the impact of Rai expression on astrocyte function both under basal conditions and in response to IL-17 treatment using a proteomic approach. We found that astrocytes and astrocyte-derived extracellular vesicles contain a set of proteins, to which Rai contributes, that are involved in the regulation of oligodendrocyte differentiation and myelination, nitrogen metabolism, and oxidative stress. The HIF-1α pathway and cellular energetic metabolism were the most statistically relevant molecular pathways and were related to ENOA and HSP70 dysregulation.     

Relationship between Neuronal Damage/Death and Astrogliosis in the Cerebral Motor Cortex of Gerbil Models of Mild and Severe Ischemia and Reperfusion Injury

Neuronal loss (death) occurs selectively in vulnerable brain regions after ischemic insults. Astrogliosis is accompanied by neuronal death. It can change the molecular expression and morphology of astrocytes following ischemic insults. However, little is known about cerebral ischemia and reperfusion injury that can variously lead to damage of astrocytes according to the degree of ischemic injury, which is related to neuronal damage/death. Thus, the purpose of this study was to examine the relationship between damage to cortical neurons and astrocytes using gerbil models of mild and severe transient forebrain ischemia induced by blocking the blood supply to the forebrain for five or 15 min. Significant ischemia tFI-induced neuronal death occurred in the deep layers (layers V and VI) of the motor cortex: neuronal death occurred earlier and more severely in gerbils with severe ischemia than in gerbils with mild ischemia. Distinct astrogliosis was detected in layers V and VI. It gradually increased with time after both ischemiae. The astrogliosis was significantly higher in severe ischemia than in mild ischemia. The ischemia-induced increase of glial fibrillary acidic protein (GFAP; a maker of astrocyte) expression in severe ischemia was significantly higher than that in mild ischemia. However, GFAP-immunoreactive astrocytes were apparently damaged two days after both ischemiae. At five days after ischemiae, astrocyte endfeet around capillary endothelial cells were severely ruptured. They were more severely ruptured by severe ischemia than by mild ischemia. However, the number of astrocytes stained with S100 was significantly higher in severe ischemia than in mild ischemia. These results indicate that the degree of astrogliosis, including the disruption (loss) of astrocyte endfeet following ischemia and reperfusion in the forebrain, might depend on the severity of ischemia and that the degree of ischemia-induced neuronal damage may be associated with the degree of astrogliosis.     

Metabolome Shift in Both Metastatic Breast Cancer Cells and Astrocytes Which May Contribute to the Tumor Microenvironment

The role of astrocytes in the periphery of metastatic brain tumors is unclear. Since astrocytes regulate central nervous metabolism, we hypothesized that changes in astrocytes induced by contact with cancer cells would appear in the metabolome of both cells and contribute to malignant transformation. Coculture of astrocytes with breast cancer cell supernatants altered glutamate (Glu)-centered arginine–proline metabolism. Similarly, the metabolome of cancer cells was also altered by astrocyte culture supernatants, and the changes were further amplified in astrocytes exposed to Glu. Inhibition of Glu uptake in astrocytes reduces the variability in cancer cells. Principal component analysis of the cancer cells revealed that all these changes were in the first principal component (PC1) axis, where the responsible metabolites were involved in the metabolism of the arginine–proline, pyrimidine, and pentose phosphate pathways. The contribution of these changes to the tumor microenvironment needs to be further pursued.     

Noradrenaline in Alzheimer’s Disease: A New Potential Therapeutic Target

A growing body of evidence demonstrates the important role of the noradrenergic system in the pathogenesis of many neurodegenerative processes, especially Alzheimer’s disease, due to its ability to control glial activation and chemokine production resulting in anti-inflammatory and neuroprotective effects. Noradrenaline involvement in this disease was first proposed after finding deficits of noradrenergic neurons in the locus coeruleus from Alzheimer’s disease patients. Based on this, it has been hypothesized that the early loss of noradrenergic projections and the subsequent reduction of noradrenaline brain levels contribute to cognitive dysfunctions and the progression of neurodegeneration. Several studies have focused on analyzing the role of noradrenaline in the development and progression of Alzheimer’s disease. In this review we summarize some of the most relevant data describing the alterations of the noradrenergic system normally occurring in Alzheimer’s disease as well as experimental studies in which noradrenaline concentration was modified in order to further analyze how these alterations affect the behavior and viability of different nervous cells. The combination of the different studies here presented suggests that the maintenance of adequate noradrenaline levels in the central nervous system constitutes a key factor of the endogenous defense systems that help prevent or delay the development of Alzheimer’s disease. For this reason, the use of noradrenaline modulating drugs is proposed as an interesting alternative therapeutic option for Alzheimer’s disease.     

Key Role of Astrocytes in Postnatal Brain and Retinal Angiogenesis

Angiogenesis is a key process in various physiological and pathological conditions in the nervous system and in the retina during postnatal life. Although an increasing number of studies have addressed the role of endothelial cells in this event, the astrocytes contribution in angiogenesis has received less attention. This review is focused on the role of astrocytes as a scaffold and in the stabilization of the new blood vessels, through different molecules release, which can modulate the angiogenesis process in the brain and in the retina. Further, differences in the astrocytes phenotype are addressed in glioblastoma, one of the most devastating types of brain cancer, in order to provide potential targets involved in the cross signaling between endothelial cells, astrocytes and glioma cells, that mediate tumor progression and pathological angiogenesis. Given the relevance of astrocytes in angiogenesis in physiological and pathological conditions, future studies are required to better understand the interrelation between endothelial and astrocyte signaling pathways during this process.     

Neuroimmunological Implications of AQP4 in Astrocytes

The brain has high-order functions and is composed of several kinds of cells, such as neurons and glial cells. It is becoming clear that many kinds of neurodegenerative diseases are more-or-less influenced by astrocytes, which are a type of glial cell. Aquaporin-4 (AQP4), a membrane-bound protein that regulates water permeability is a member of the aquaporin family of water channel proteins that is expressed in the endfeet of astrocytes in the central nervous system (CNS). Recently, AQP4 has been shown to function, not only as a water channel protein, but also as an adhesion molecule that is involved in cell migration and neuroexcitation, synaptic plasticity, and learning/memory through mechanisms involved in long-term potentiation or long-term depression. The most extensively examined role of AQP4 is its ability to act as a neuroimmunological inducer. Previously, we showed that AQP4 plays an important role in neuroimmunological functions in injured mouse brain in concert with the proinflammatory inducer osteopontin (OPN). The aim of this review is to summarize the functional implication of AQP4, focusing especially on its neuroimmunological roles. This review is a good opportunity to compile recent knowledge and could contribute to the therapeutic treatment of autoimmune diseases through strategies targeting AQP4. Finally, the author would like to hypothesize on AQP4’s role in interaction between reactive astrocytes and reactive microglial cells, which might occur in neurodegenerative diseases. Furthermore, a therapeutic strategy for AQP4-related neurodegenerative diseases is proposed.     

Therapeutic Potential of AAV1-Rheb(S16H) Transduction against Neurodegenerative Diseases

Neurotrophic factors (NTFs) are essential for cell growth, survival, synaptic plasticity, and maintenance of specific neuronal population in the central nervous system. Multiple studies have demonstrated that alterations in the levels and activities of NTFs are related to the pathology and symptoms of neurodegenerative disorders, such as Parkinson’s disease (PD), Alzheimer’s disease (AD), and Huntington’s disease. Hence, the key molecule that can regulate the expression of NTFs is an important target for gene therapy coupling adeno-associated virus vector (AAV) gene. We have previously reported that the Ras homolog protein enriched in brain (Rheb)–mammalian target of rapamycin complex 1 (mTORC1) axis plays a vital role in preventing neuronal death in the brain of AD and PD patients. AAV transduction using a constitutively active form of Rheb exerts a neuroprotective effect through the upregulation of NTFs, thereby promoting the neurotrophic interaction between astrocytes and neurons in AD conditions. These findings suggest the role of Rheb as an important regulator of the regulatory system of NTFs to treat neurodegenerative diseases. In this review, we present an overview of the role of Rheb in neurodegenerative diseases and summarize the therapeutic potential of AAV serotype 1 (AAV1)-Rheb(S16H) transduction in the treatment of neurodegenerative disorders, focusing on diseases, such as AD and PD.     

Crosstalk between Neuron and Glial Cells in Oxidative Injury and Neuroprotection

To counteract oxidative stress and associated brain diseases, antioxidant systems rescue neuronal cells from oxidative stress by neutralizing reactive oxygen species and preserving gene regulation. It is necessary to understand the communication and interactions between brain cells, including neurons, astrocytes and microglia, to understand oxidative stress and antioxidant mechanisms. Here, the role of glia in the protection of neurons against oxidative injury and glia–neuron crosstalk to maintain antioxidant defense mechanisms and brain protection are reviewed. The first part of this review focuses on the role of glia in the morphological and physiological changes required for brain homeostasis under oxidative stress and antioxidant defense mechanisms. The second part focuses on the essential crosstalk between neurons and glia for redox balance in the brain for protection against oxidative stress.