Involvement of Astrocytes in the Process of Metabolic Syndrome

Astrocytes constitute a very heterogeneous population of cells, which regulate pH, extracellular levels of ions and neurotransmitters, and energy metabolism in addition to actively participating in neurotransmission. In situations of damage to the CNS, the typical response is the degree of reactive gliosis, which can form glial scars. On the other hand, chronic diseases such as obesity, type 2 diabetes, hypertension, and atherosclerosis have been causally related to low-grade chronic inflammation in various metabolic tissues. It has been pointed out that the identification of hypothalamic inflammatory alterations are triggered by overnutrition, orchestrated by the hypothalamic immune system, and sustained by the pathophysiology associated with the metabolic syndrome. We discuss here the effects of astrocytes and the main astrocyte mechanisms involved in the metabolic syndrome and its comorbidities.


Introduction
The cases of metabolic syndrome (MetS) in adults are increasing, due to several factors such as aging population, physical inactivity, obesity and chronic overnutrition [1]. Metabolic abnormalities are involved in the metabolic syndrome, such as diabetes mellitus, hypercholesterolemia and dyslipidemia, hypertension, and central obesity [2]. Several studies in neuroscience and immunology are linked to overnutrition to neuroinflammation, particularly in the hypothalamus and in the hippocampus, due to interaction between accelerated adiposity, hyperglycemia, and cognitive decline [3][4][5].
The primary risk of cognitive decline in both obese and hyperglycemic individuals is the systemic and chronic inflammatory component of MetS, due to preparation of the resident population of glial cells to establish a form of low-grade neuroinflammation [6].
Astrocytes are active agents of the dynamic central nervous system (CNS) signaling. The astrocytes participate in a variety of essential physiological processes in the healthy brain, such as providing structural support to neurons, participating in the formation and maturation of synapses, control of homeostasis of ions and metabolites, receptor trafficking, neurotransmitter clearance, and modulating the synaptic plasticity moment by moment [7]. Many studies have shown their contribution to information processing and memory formation in the brain, thus pointing to a role for astrocytes in higher integrated brain functions [8,9].
A vast arsenal at the disposal of astrocytes is being defined, as in the determination of functions and mechanisms of reactive astrogliosis, cellular hypertrophy, and glial scar formation with preservation of the cellular domains and rearrangement of the tissue structure as well as contributing to specific CNS disorders and lesions [9]. Neuroinflammation in the enteric system occurs due to the activation of enteric glial cells (EGCs) which are the most abundant cells within the enteric nervous system (ENS). EGCs are located adjacent to the neurons within the enteric ganglia and along the interganglionic connections of the myenteric and submucosal plexus but also protrude into the extraganglionic mucosal layer [10][11][12][13]. Their morphology and the expression of markers such as calcium-binding protein S100 and glial fibrillary acidic protein (GFAP) [14] resemble central nervous system astrocytes.
The EGCs can exert immunomodulatory functions; they can secrete inflammatory signaling molecules such as interleukins IL-1β and IL-6 [10,15] as well as other mediators, including nerve growth factor (NGF), S-nitrosoglutathione (GSNO), nitric oxide (NO), and S100B [16], and express class II major histocompatibility (MHC) complex molecules [16,17]. CNS astrocytes activated by inflammation are characterized by hypertrophy and proliferation, coupled with a positive regulation of the GFAP's cytoskeleton [18], due metabolic syndrome and inflammatory conditions of the intestinal disease (IBD) [19].
Many studies have shown that multiple connections between peripheral and cerebral changes involving inflammatory, metabolic, and neural components have been identified under conditions associated with obesity [20][21][22]. Therefore, identifying and treating these conditions is of primary importance to people worldwide. Obesity-related chronic inflammation provides an important link to metabolic derangements including insulin resistance, cognitive impairment affecting the hypothalamus, and other brain regions [20].
The present study was designed to discuss the effects of astrocytes and the main astrocyte and neuroinflammatory mechanisms involved in the metabolic syndrome and their comorbidities, which gave rise to the field of immunometabolism.

Neuroimmunologic mechanisms and role of glial cells
Until recently the CNS was considered to be immunologically privileged, since many antibodies and peripheral immune system cells are usually blocked through the blood-brain barrier (BBB), a specialized structure composed of endothelial cells (ECs), pericytes, astrocytes, and microglia [23]. The BBB maintains the chemical composition of the neuronal microenvironment, which is necessary for the proper functioning of neuronal circuits, synaptic transmission, synaptic remodeling, angiogenesis, and neurogenesis [24].
The immune system influences the functioning of BBB, which in turn affects the functioning of the CNS in both physiological and pathological conditions. In some cases, the BBB separates the CNS from the immune system; in others it acts as a mediator of neuroimmune interactions, and in others it may act as a target for immune system attacks [25]. In physiological conditions, immune cells cross the BBB at a very low rate, through specific interactions, promoting the endothelial junctions that control the flow of cells through them [26][27][28]. On the other hand, neuropathological diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), lateral amyotrophic sclerosis (LAS), multiple sclerosis (MS), and BBB destruction or damage, can be induced or mediated by LPS (lipopolysaccharide), cytokines, prostaglandins, and nitric oxide. The BBB is capable of responding to LPS due to the presence of Toll-like receptor 4 (TLR4) and other Toll-like receptors on the membranes of the BBB cells [29]. Similarly, these cells have receptors for cytokines, chemokines, and other immunological molecules [30,31]. As a result, the immune system is able to affect the functions of the BBB beyond those of disruption.
Perivascular cells (astrocytes and microglia) in addition to endothelial cells produce several inflammatory factors, such as the release of cytokines and chemokines that affect the BBB permeability and the expression of adhesion molecules. Cytokines (TNF-α, IL1-β, IFN-γ) can stimulate the expression of adhesion molecules (vascular cell adhesion molecule 1 (VCAM-1) and intercellular adhesion molecule 1(ICAM-1)) in endothelial cells allowing the passage of activated leukocytes into the CNS [32]. Immune cell (macrophages, lymphocytes) traffic through the BBB should initiate or contribute to a vicious cycle resulting in progressive synaptic dysfunction and neuronal loss in neurodegenerative disorders [20,24].
On the other hand, a specific subset of T cells is essential to suppress autoimmunity and maintain immune homeostasis. T regulatory cells (Treg) have been characterized with important functions. Emerging evidence shows that Treg cells are not only important for maintaining immune balance at the periphery but also contribute to the self-tolerance and immune privilege in CNS [33]. Leukocyte extravasation requires interactions between adhesion molecules in endothelial cells and leukocytes. Leukocyte adhesion molecules (LAMs) expressed by ECs include P-selectin, E-selectin, intercellular adhesion molecule 1 (ICAM-1), and vascular cell adhesion molecule 1 (VCAM-1) [34]. Selectin binds to P-selectin-binding glycoprotein (PSGL-1), while ICAM-1 and VCAM-1 bind to α4-integrins in leukocytes. After the initial binding event, immune cells roll along the vessel wall releasing chemokines that strengthen their binding interactions, promoting the state of neuroinflammation [35].
Neuroinflammation is recognized as a prominent feature of various pathological conditions [36]. Thus, several lines of evidence strongly suggest that neuroinflammation is a crucial process involved in the progression of neuronal degeneration, a common feature observed in several neurodegenerative disorders such as degenerative neuropathologies [36]. In the inflammatory process, the main cellular events are observed, such as increased blood flow and vascular permeability with consequent venular dilation and recruitment of cells to the inflamed site. A significant role played by reactive oxygen species has been observed to develop inflammation, causing endothelial cell damage and increased microvascular permeability, chemotactic factor production, neutrophil recruitment, oxidation, and lipid peroxidation [37]. Such inflammatory mediators play a regulatory role in the growth, differentiation, and activation of immune cells [38]. Glial cells (microglia, astrocytes, and oligodendrocytes) define cerebral homeostasis and are responsible for defense and preservation against neural tissue injury [14].

The role of astrocytes in neuroinflammation
Astrocytes are an important group of heterogeneous cells and play key roles in the physiology of the nervous system, including regulation of pH, extracellular levels of ions and neurotransmitters, and energy metabolism. In addition, it plays an important role in the formation and functioning of the BBB [39] and also actively participates in neurotransmission [27]. In pathological situations in the CNS, the typical response of astrocytes is the state of reactive gliosis involving positive gene regulation of cytoskeletal proteins (e.g., glial fibrillary acidic protein (GFAP)) and corresponding to the change in morphology reaching a state of hypertrophy, hyperplasia, and glial scar formation [40][41][42].
In addition, astrocytes play an important role in central immunity. The innate immune response is accurately adjusted by identifying the type of threat that is present. The molecular structures that are associated with threats are recognized by Pattern Recognition Receptors (PRRs). PRRs recognize molecular patterns associated with pathogens (PAMPs) are expressions such as microorganisms such as bacteria, viruses and viruses, and damage-associated diseases (DAMPs) that signal cellular damage and are therefore responsible for a state of stress or injury [14]. Among PRRs, one of the main classes is the family of transmembrane proteins of Toll-like receptors (TLRs). Generally pathogen response and tissue damage happen quickly, assuming some roles of the cells of the immune system, releasing cytokines (IL-1β, IL-6, TNF-α) and chemokines (MCP-1, CCR-2, COX-2), influencing other cells of the immune system (macrophages and lymphocytes), and modulating the BBB [14].
Among glial cells, astrocytes play a role in the release of Toll-like receptors [43]. Since the TLRs are expressed and detected by the binding of their binding genes, a signaling mediated by the myeloid differentiation gene 88 (Myd88) is initiated, having an activation of the nuclear transcription factor NFκB. In the activation of NFκB, the inflammatory process is released through the secretion of proinflammatory molecules (1β, IL-6, TNF-α, IL-12). In addition, there is no state of activation of astrocytes to recruit microorganisms, lymphocytes, and dendritic cells to the lesion site [44].

Implications of metabolic syndrome in the central nervous system: the role of the bowel-brain axis
The increased obesity in the last 40 years is considered a consequence of the sedentary lifestyle and adherence to diets rich in saturated fat and refined carbohydrates that induce changes in the microbiota and underlying metabolic and psychological complications [45]. Bacteria, viruses, protozoa, archaea, and fungi represent the microorganisms that inhabit the intestinal tract of mammals, with bacteria composing the majority [46,47] in concentrations between 10 1 to 10 3 cells per gram in the upper intestine and 10 11 to 10 12 cells per gram in the colon [48].
These microorganisms play a role in human physiology through various mechanisms, such as the metabolism of nutrients and the regulation of immunological and neuroendocrine functions, as they bind to the CNS through the enteric nervous system (ENS) [49]. From the microbiota, the active metabolite LPS (lipopolysaccharide), in addition to short-chain fatty acids (SCFA), from invasive and commensal bacteria, respectively [50,51], can be expressed in the intestinal lumen and influence the integrity of BBB [52], especially butyrate, in which it positively induces the expression of junction proteins, including claudin-2, occludin, cingulin, and occludens-1 and occludens-2 (ZO-1 and ZO-2), forming protein structures in the gut, like tight junction [53,54]. These proteins form a mechanical link between epithelial cells and establish paracellular diffusion of fluids and solutes in the barrier [55]. Where high fiber and fruit meals were shown to reduce the increasesinduced by meals with high saturated fat and high carbohydrate content inlevels inflammatory response [56].
In mammals, colon epithelial cells, adipocytes, and peripheral blood mononuclear cells express a pair of G-protein-coupled receptors (GPR41 and GPR43) that are activated by SCFAs through the receptors on the T enteroendocrine cells [57]. They sense the amount of AGCC produced by bacteria in the colon and secrete the glucagon-like peptide 1 (GLP-1) and the tyrosine tyrosine peptide (PYY), allowing inhibition of intestinal motility and increased absorption of nutrients, respectively [58]. Thus, the composition of the diet determines the type of nutrient that reaches the gastrointestinal tract (GI) that can alter the composition of the intestinal microbiota and the production of metabolites to, consequently, influence intestinal permeability [59].
Bacterial products, such as LPS, can display a variety of PAMPs recognized by TLRs and nucleotide-binding oligomerization domain receptors (NOD) on macrophages and dendritic cells in the innate immune system, such as flagellin, recognized by TLR5 to induce α-defensin secretion through Paneth cells, a NOD-dependent antimicrobial protein [60]. All TLRs recognize protein, lipid, or nucleotide PAMPs. TLR2, TLR4, and TLR6 recognize fungal PAMPs, while TLR9 and TLR11 recognize protozoan PAMPs [61]. Although TLRs can activate immune cell proliferation through an Akt-dependent pathway, they will all induce the expression and secretion of cytokines [61]. Although TLRs may activate immune cell proliferation via an Akt-dependent pathway, all but TLR3 will induce the expression and secretion of tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β), and interferon-γ (IFN-γ), recruiting the primary myeloid differentiation response 88 (MYD88), which recruits the kinase family associated with the interleukin-1 receptor (IRAK), leading to phosphorylation of the inhibitory protein kappa B (I-kB) to induce translocation of nuclear factor kappa B (NF-kB) and influence the expression of inflammation [62,63]. In addition, the activation of TLRs participates in the proliferation of epithelial cells and IgA secretion in the intestinal lumen, essential for intestinal barrier integrity and bacterial population balance, respectively [61]. Deregulation of these processes, or excessive activation of TLRs, can result in chronic inflammatory responses and exuberant repair [64].
In the epithelium, segmented bacterial filaments (SFB) and other commensal microbes activate dendritic cells (DCs) and macrophages in the lamina propria, inducing T helper 17 (TH17) cells through the production of interleukins IL-1β, IL-6, and IL-23 and helper T cells (TH1) by the possible production of interleukin-12 (IL-12) [21]. TH17 cells regulate the gut microbiota community by secreting the IL-22-dependent antimicrobial lectin regenerating islet-derived 3 gamma (REGIIIγ). One of the microbial derivatives, polysaccharide A (PSA), stimulates intestinal epithelial cells to secrete growth factor and β transformation (TGFβ), inducing DCs and macrophages to secrete retinoic acid and interleukin 10 (IL-10) to promote activation of regulatory T cells and forkhead box P3 (FOXP3) and, subsequently, inactivation of TH17 and TH1 cells, in a type of negative feedback between the cells of defense in relation to the balance of the gut bacteria [65]. Activation of TLRs induces B cell-activating factor (BAFF) secretion, which differentiates B cells by increasing activation-induced cytidine deaminase (AID) expression and promotes differentiation of IgA-producing plasma cells, by maturing antibodies and casting in the intestinal lumen to alter the composition and function of the microbiota [65].
In obese individuals, due to imbalance of the microbiota (dysbiosis) and oxidative stress, TLR4 is activated and recognizes the bacterial LPS and the flagellin of commensal bacteria, activating TLR5 for dendritic cell signaling and activation of innate lymphoid cells (ILCs), both processes to secrete REGIIIY [66]. ILCs communicate with the microbiota through cytokines, aryl hydrocarbon (AhR) receptors, and antimicrobial peptides and participate in the cross-talk of epithelial cells with the intestinal microbiota [67]. Divided into three groups, group ILCs (ILC1) are activated by interleukin-12 (IL-12) derived from myeloid cells, which in response secrete IFN-γ, whereas group ILCs (ILC2) interact with mast cells, eosinophils, basophils, and macrophages, and group 3 (ILC3) ILCs interact with cells of both innate and adaptive immune systems to secrete IL-22 and initiate an antimicrobial program along with restoration of the intestinal barrier [68].

Neuroinflammation from the intestine
The gut-brain axis is composed of the central nervous system, innervated by afferents and efferents of the vagus nerve and extrinsic fibers of the autonomic nervous system (ANS) interconnecting it to the hypothalamic-pituitary-adrenal (HPA) axis, in addition to the intrinsic neurons of the enteric nervous system and the intestinal microbiota. Intrinsic intestinal innervations connect the intestine to the brain via vagal and spinal fibers, whereas the brain sends efferent sympathetic and parasympathetic fibers to the intestine [69,70]. The HPA axis is part of the limbic system and is the main regulator of stress response and intestinal function during digestion which, due to the corticotrophin releasing factor (CRF) secreted by the hypothalamus, can influence the motility, permeability, and level of intestinal inflammation [47,71]. Therefore stress and emotions can influence the microbial composition of the intestine by bacterial products that gain access through the bloodstream and the postrema area and due to the release of glucocorticoids and/or sympathetic neurotransmitters that influence the physiology of the intestine and alter the habitat of the microbiota, such as noradrenaline, which may even influence bacterial gene expression or signaling among bacteria, altering the composition and activity of the microbiota [72].
After bacterial colonization, increased production of neurotransmitters, such as serotonin (5-HT) and γ-aminobutyric acid (GABA), and the expression of various cytokines are physiological implications essential for intestinal homeostasis and HPA axis programming, which plays an important role in stress responses [73]. The attention is focused on the stress due to serotonin, which is synthesized through tryptophan, in enterochromaffin cells (EC), about 90%, and in autonomic nerves, about 10%, i.e., at the level of the gastrointestinal tract [74].
These events induce the barrier permeability and increase of bacterial endotoxin through the gut, which stimulates immune cells in the lamina propria to secrete proinflammatory cytokines and prostaglandins (PGE2) to communicate the brain via afferent nerves, compromising the intestinal barrier and creating a cycle, where inflammatory cytokines will activate the SAM and HPA axes, resulting in barrier rupture, increased endotoxin translocation, and an inflammatory and stress state [78]. Where, one of the causes of this translocation, from the intestinal point of view, concerns LPS of invasive bacteria and their arrival in the intestinal lumen, in which it will induce a pro-inflammatory response in lymphoid and innate immune cells, and subsequent release of cytokines such such as IL-1β, IL-6 and TNF-α, in the bloodstream to the brain, from the axis of the gut-brain, affecting BBB integrity, causing a pro-inflammatory stress cycle in CNS cells inducing neuroinflammation [79] Figure 1.

Mechanisms of neuroinflammation and the process of metabolic syndrome
The proinflammatory cytokines (IL-1β, IL-6 and IFN-γ) and catecholamines, are able to induce IDO within CNS, exactly within the astrocyte and microglia, so the uptake and metabolism of L-tryptophan into theses cells leads to the production of KYNA, was report have neuroprotective actions in the CNS, but the catabolism of L-tryptophan in the microglia gives rise to metabolites with reactive oxidative properties, such as 3OH-KYN, 3-HAA and QUIN, that can be transported through the BBB to serve as substrates and contribute to the kinurenine pathway in the CNS, where macrophages and microglia represent the main sources of QUIN, a agonist on N glutamate receptor subtype methyl-D-aspartate (NMDA) that acts as a being able contribute to excitotoxicity and neurotoxicity [80,81].
The astrocytes do not appear to have KMO, which favors the formation of KYNA, which after being released in the presynaptic area preferentially inhibits the NMDARs and α7 nicotinic acetylcholine receptors (nAChRs) in the extra-synaptic [81] Figure 2.
This leads to more leukocyte inflammation and infiltration at the BBB, since tight junction proteins, including claudins, ZO-1, and occludins, are downregulated [82]. In response to inflammation, leukocyte extravasation increases with positive regulation of VCAM-1 and ICAM-1 [39]. Once the functional capacity of astrocytes is compromised, BBB is impaired, resulting in a significant increase in BBB permeability rate, a promotion of leukocytes in cerebrospinal fluid (CSF), and increased immune response, including pathogens and toxins in the CNS. This process favors the activation of astrocytes and microglia, which stimulates the continuous symptoms in the CNS, including the hypothalamus, with consequent response to the stress of insulin symptoms, as well as cognitive injury [83].
Within the metabolic syndrome, diabetes mellitus involves the CNS, and insulin signal transduction involves the activation of phosphatidyl-3 kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways [84]. The insulin receptor (IR) has two subunits (α and β). The α-subunit is directed toward the extracellular medium. Insulin binding to the IR receptor promotes autophosphorylation of the receptor on β subunits, located within the plasma membrane. A phosphorylation cascade of the insulin receptor substrates (IRS) 1 to 4 is then followed, and PI3K is then recruited into the membrane and induces the insertion of the GLUT glucose transporter into the plasma membrane [85]. In addition, PI3K also phosphorylates AKT, which in turn phosphorylates the glycogen synthase kinase 3 (GSK3) protein by inactivating it. This inactivation decreases the phosphorylation of Tau, which is present in neurons and has the function of stabilizing the microtubules for the transport of synaptic vesicles and other cellular components [86]. Therefore, the neuroinflammatory process comprises several mechanisms through the activation of the glial cells that leads to neuronal damage and consequent damages to the CNS.

Conclusion
In metabolic syndrome and obesity-associated conditions, immune and metabolic dysregulation results in chronic systemic inflammation, neuroinflammation, cognitive impairment, and other pathological manifestations. An understanding of this complex pathology requires providing new insight into the regulatory role of the astrocytes.
Although research to date in the fields of immunometabolism and neuroinflammation has produced encouraging preliminary results, there remains a vast expanse of unexplored questions requiring the interdisciplinary knowledge of metabolism, neuroscience, and immunology.
In summary, the descriptions of this study indicate that astrocytes play an important role in immunity by triggering neuroinflammation mediated by metabolic syndrome associated with obesity. Elucidating these mechanisms by binding the metabolism syndrome, inflammation, and CNS by astrocytes could generate potential new therapeutic targets or specific strategies to combat metabolic syndrome and obesity.