Keywords

Introduction

Schizophrenia is a severe mental disorder with 1% worldwide prevalence, which means that it affects the life quality and longevity of more than 21 million people worldwide according to the World Health Organization [1]. This pathology is characterized by profound disruption of thoughts, language, perception, and the sense of self, which often includes psychotic experiences, such as hallucinations and delusions. Together, they are called positive symptoms and were initially considered as the main psychological signs of this pathology. However, schizophrenia presents other psychological features such as social behavioral deficits, lack of motivation, and anhedonia – grouped as the negative symptoms – and cognitive dysfunction [1]. Historically, the classical neuron-centric view that has long-dominated neuroscience and the pharmacological research, fundamentally by using antipsychotic drugs, constituted the largest part of the characterization of schizophrenia etiopathology. In this sense, the bibliography is full of reports pointing out to neurotransmission misbalance – essentially dopaminergic or glutamatergic – as the main factor in the development of this pathology. Over the last years, the dopaminergic-centrist theories have given place to a more complex interpretation as schizophrenia becomes a multifactorial puzzle where glial cells are one of the new targets of interest. Glial cells (oligodendrocytes, astrocytes, and microglial cells) are essential pieces in brain microenvironment function as they play crucial roles in metabolic and ion homeostasis control, synaptic establishment and function, modulation of several neurotransmission systems, as well as neuroprotection, tissue repair, and inflammation [2, 3]. Regarding these glial functions , it is not surprising that alterations in their functionality and integrity could be related to psychiatric disorder development. In this sense, it has been reported, in human and animal research, that glial cells are involved in several mental diseases including Parkinson’s disease, major depressive disorder, addiction, and schizophrenia [2]. The intricate patterns involved in both etiopathology and symptomatology make schizophrenia an unreadable enigma for the moment. Indeed, more than 100 years after its first description by Kraepelin and other psychiatrists, there is not a theory that explains all the features observed throughout its development and the consequent symptoms. Genetic, neurodevelopmental, and environmental theories can be mentioned among the multiplicity of the hypothesis that attempts to clarify the complex scenario of this pathology. Each one of them could represent different components, of the underlying cascade, that trigger the neurotransmitter imbalance and the subsequent schizophrenic symptom expression. These series of concatenated pathological events would occur throughout life, producing deficits in dendritic spine formation, due to an interplay between genetic factors and obstetric complication, along with an excessive apoptosis and synaptic pruning during adolescence, leading to brain disconnection and the subsequent psychotic symptoms [4]. Moreover, the different stages of schizophrenia development are transversally crossed by neuroinflammation. Some of the first insights of a possible association between the schizophrenic syndrome and neuroinflammation come from a century ago, when an increase of schizophrenia diagnosis rates was reported after an influenza epidemic that had happened in 1918 [5]. Although with some inconsistencies, more recent studies are in line with this asseveration, where schizophrenia development seems to be related with influenza as well as other maternal infection like herpes simplex virus type 2, Toxoplasma gondii, and nonspecific bacterial infections [6]. Taking into account the heterogeneity of etiological infections, the increased risk to suffer schizophrenia probably involve alterations related to immune system activation and not due to specific pathogenic pathways of each microorganism. To this respect, increased pro-inflammatory cytokine levels during pregnancy have been related to higher risk to suffer schizophrenia [6]. On the other hand, high genetic contribution to schizophrenia development was observed in studies made in twin with a heritability up to 80% [7]. Located on the short arm of chromosome 6, major histocompatibility complex (MHC) was consistently related to this high contribution to schizophrenia susceptibility [8,9,10,11]. In this region, at least 250 genes that encode human leukocyte antigens and many other immune and nonimmune genes are present. Variations on many genes encoded in this genome’s region could induce an unsuitable immune reaction leading to exacerbate response and the consequent neuroinflammation throughout life. Together with the genetic vulnerability, the early neuroinflammatory insults abovementioned could imprint long-life marks over microglia cells, displaying an increased immune reactivity throughout the life of schizophrenic patients [12]. Both processes could explain the high levels of inflammatory markers, such as pro-inflammatory cytokines and C-reactive protein, that have been described in the blood and cerebrospinal fluid of schizophrenia patients [13, 14]. This immune hyperactivity during brain development could lead to less dendritic spine formation and to an enhanced mesencephalic progenitor differentiation into dopaminergic neurons, promoting some features of schizophrenia, like disruption of cortical synaptic connectivity and hyperdopaminergia [12, 15].

The central brain angiotensin II (Ang II) effects are mediated mainly by the two G protein-coupled receptors, the Ang II type 1 receptor (AT1-R) and Ang II type 2 receptor (AT2-R) . AT1-R is present in astrocytes, microglia, and brain endothelial cells pointing out its crucial role in neuroinflammatory responses [16, 17]. To this last respect, Ang II, via AT1-R, is one of the most important inflammation and oxidative stress inducers by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex activation [18]. Nowadays, a large body of evidence supports the existence of a local renin-angiotensin system (RAS) with all of its components synthesized in the central nervous system (CNS). The presence and synthesis of angiotensinogen have been described in neurons and astrocytes. Whereas angiotensinogen production in neurons is restricted to some brain regions, its astrocytes’ synthesis is the most important and widespread source [9,10,11,12]. Moreover, it has been reported an intra- and extracellular location of renin, while the Ang II converter enzyme (ACE) has been found extracellularly, as soluble and membrane-bound forms. Additionally, Karamyan [19] described alternative central pathways for Ang II synthesis that involved elastase, proteinase 3, cathepsin G, and tonin activity. Further evidence supports intraneuronal generation and activity of Ang II, as well, in brain microvessels [17, 20, 21].

In the present chapter, we attempt to summarize the role of the glial cells in the schizophrenia unmasking the AT1-R involvement in the complex glial scenario.

Glial Cells in Schizophrenia

Microglia

Microglia , resident macrophages of the brain, participates in the progressive loss of synaptic connection during normal neurodevelopment after birth . This physiological process called synaptic pruning could lead, beyond a critical threshold, to cortical disconnection and psychotic symptoms. Moreover, a similar pathological state could be triggered in normal brains after an excessive microglia activation with an excessive synaptic pruning [15]. Furthermore , it has been hypothesized that an inadequate immune activation to a predominant type 2 response could induce an excess of kynurenic acid (KYNA) formation, an endogenous N-methyl-d-aspartate (NMDA) antagonist released mainly by astrocytes, known to lead to a glutamatergic hypofunction, an important hallmark of schizophrenia [12]. Moreover, it has been observed that about 10% of patients with an initial schizophrenia diagnosis have NMDA receptor antibodies [22]. In addition, a meta-analysis performed by Miller et al. shows that some cytokines, like interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor growth factor β (TGF-β), could be markers for acute exacerbations, since their levels were high during psychotic episodes and normal after antipsychotic treatment [23].

Anti-inflammatory effects of antipsychotic drugs are another important finding that relates immune imbalance with schizophrenia etiopathology. In this sense, a meta-analysis performed by Tourjman shows that antipsychotic treatment reduces the plasma levels of pro-inflammatory cytokines IL-1β and interferon-γ and increases soluble interleukin-2 receptor [24]. Further evidence, which supports an inflammatory response, came from the benefits of nonsteroidal anti-inflammatory drugs (NSAIDs) in the treatment of schizophrenia. In this case, the addition of NSAIDs to antipsychotic treatment generated an improvement, although modest, in symptom control [25, 26].

Under physiological conditions, Ang II and AT1-R are expressed mainly in neurons and astrocytes; meanwhile, in nonactivated microglia, they are present at really low levels. However, under a pathological state, such as neuroinflammation, stroke, or multiple sclerosis, Ang II and AT1-R increase their expression in activated microglia promoting an inflammatory feedback. Bhat [27] has reported that lipopolysaccharide (LPS)-induced gliosis is associated with RAS overactivation evidenced by increased Ang II level and AT1-R expression in both astroglial and microglial cells. Moreover, in the same study, they showed that AT1-R blockade blunted the neuroinflammation via suppression of glial activation and imbalance in inflammatory cytokines in both cell types. To this last respect, a large body of evidence reported that Ang II, via AT1-R, plays a key role in oxidative stress and inflammation, promoting NADPH oxidase activation. Moreover, a cross talk between AT1-R and the microglial toll-like receptor 4 contributes to Ang II pro-inflammatory signal inducing ROS production, NF- κB, and pro-inflammatory cytokine release, including IL-1β, IL-6, and TNF-α, and leads the microglial cell activation [28, 29]. In microglial cells, the NADPH oxidase activity is the main regulator of the shift between M1/pro-inflammatory and M2/immunoregulatory microglial phenotypes, where high oxidative levels promote the pro-inflammatory and inhibit the immunoregulatory phenotype [30]. In this sense, it is known that an M1/M2 phenotype balance is necessary to prevent brain damage, so it is not a surprise that the Ang II overactivation could lead to a chronic neuroinflammatory state. In this sense, the AT1-R involvement in microglial neuroinflammation has been extensively studied in models of Parkinson’s disease and hypertension [18, 29, 31,32,33]. To this last respect, both M1 and M2 markers have been found upregulated in an Ang II-induced mouse model of hypertension, whereas the microglial depletion reduced the neuroinflammation and the blood pressure, as well as the levels of peripheral hypertensive hormones in a mice model of hypertension [34, 35]. In the same way, in an animal model of parkinsonism with MPTP, it has been shown that the Ang II/AT1-R activation of the microglia is involved in the dopaminergic degeneration. This neurotoxin induces an increase in the RhoA/Rho-kinase pathway, which plays a critical role in the inflammatory and oxidative effects of Ang II that is inhibited by AT1-R blockade or depletion [36]. Moreover, Rho-kinase activation upregulated AT1-R expression in microglial cells leading to a feedforward mechanism. AT1-R regulates the microglial response through two main pathways , Rho-kinase and NADPH oxidase, thus controlling superoxide generation, microglial motility and phagocytosis, and the release of inflammatory cytokines [29].

Astrocytes

Astrocytes are the neurovascular unit’s center of integration mediating the vascular response to couple the neuronal activity, through the activation of their metabotropic glutamatergic receptors, Ca+2 oscillations, and release of gliotransmitters and vasoactive substances. In this way, astrocytes control the blood flow in response to the neuronal function (functional hyperemia), and the metabolites exchange through the blood-brain barrier. Furthermore, astrocytes exert a critical contribution to neurotransmission systems since they are responsible for the metabolism, recycling, and/or degradation of glutamate and GABA. Classical examples of these astrocytes’ functions are glutamine-glutamate or KYNA circle. On the other hand, astroglia is involved in the immune response and tissue repair after damage [3, 37,38,39]. Taking all together, since astrocytes have a critical role in maintaining neuronal function, it is not surprising that alterations in their functions have been associated with several psychiatric disorders. However, regarding schizophrenia, the astrocytic contribution is not clear [37, 40]. Initial studies have reported signs of gliosis in a regional-dependent manner and usually closely related to the illness’ history and severity [41, 42]. In this sense, Arnold [43] reported gliosis only in a schizophrenia subgroup of patients with a high prevalence of severe cognitive impairment and functional disability. However, later studies that focused on glial fibrillary acidic protein (GFAP) mRNA or protein analyses found no changes [44,45,46,47] or decreased expression [48,49,50,51,52]. In another study, augmented astrogliosis –described for schizophrenia – was reported to be concomitant with increased neuroinflammatory marker expression, and the authors suggested that astrocyte reactivity could be due to external factors (i.e., differences in psychiatric etiopathology, illnesses coexistence, or treatments received) [44]. In line with this hypothesis , it has been shown an increase in GFAP immunoreactivity after chronic antipsychotic treatment [53]. However, in animal models (rats), the treatment with haloperidol – typical antipsychotic – has no effects over GFAP expression [51]. Focusing the attention over the astroglial enzymes, products, or gliotransmitters, the reports over astrocytes’ role in schizophrenia become more inconsistent. Among the main astrocyte’s enzymes, the glutamine synthetase (GS) is a critical component of glutamine-glutamate cycling expressed mainly in astrocytes. This enzyme catalyzes the ATP-dependent condensation of ammonia and glutamate to form glutamine, playing a fundamental role in the glutamate neurotransmission and homeostasis, as well as neurotoxicity prevention by clearing of ammonia [54]. In the available bibliography, there is no agreement about GS involvement in this pathology, considering that the expression of this enzyme has been reported to be increased, decreased, or with no changes in schizophrenic patients [51, 53, 55,56,57]. Similar results were found for glutaminase, another glutamine-glutamate cycling enzyme, responsible for glutamine-glutamate transformation [55, 57]. However, it has been suggested that these changes could be due to antipsychotic treatment. In this sense, in astrocyte cultures, risperidone increased GS as well as the glutamate uptake and glutathione content; meanwhile, haloperidol increased reactive oxygen species (ROS) but did not show any effect over these astrocyte functions [58]. Moreover, it has been shown an increase in glutamine synthetase-like protein in patients with schizophrenia, which becomes even higher after the treatment with olanzapine [59]. Other critical players in the glutamate-glutamine circle are the excitatory amino acid transporters (EAATs), expressed primarily on astrocytes. The evidence suggests that abnormalities in these glutamate transporter localization and function could underlie alterations in the kinetics of perisynaptic glutamate buffering, clearance, and cycling, contributing to the glutamatergic dysfunction described in schizophrenia [60, 61]. Regarding this last respect, it has been reported a decreased expression of these transporters in critical areas implicated in schizophrenia physiopathology [61]. Interestingly, McCullumsmith [60] showed an increased expression of these glutamate transporters in neurons and suggested that this could be a way to balance out the loss of astroglial reuptake capacity. Moreover, KYNA was found to be increased in critical regions of the CNS in schizophrenic patients, an effect that contributes to the reduction of the glutamatergic neurotransmission [62]. Studies performed in rats showed that the treatment with antipsychotics, like haloperidol, clozapine, and raclopride, caused a reduction in KYNA levels in the caudate putamen, hippocampus, and frontal cortex [63]. Another astroglial marker is the S100B, a neurotrophic factor released from several cell types, but within the CNS, it is released by astrocytes, and it is used as an astrocyte integrity indicator. Through its paracrine and autocrine role, in low concentration, S100B regulates proliferation and differentiation of neurons and glia and modulates dopamine and glutamatergic synaptic function. On the contrary, the over-release of this factor has been related to neuronal dysfunction and apoptosis due to increased expression of inducible nitric oxide synthase (iNOS) or pro-inflammatory cytokines [64, 65]. Regarding schizophrenia patients, several studies have revealed increased S100B levels in the peripheral blood and cerebrospinal fluid (CSF) of patients [65,66,67,68,69,70]. This increase in S100B has been associated to a more severe negative psychopathology and cognitive deficit, supporting the key role of astroglia in the schizophrenia etiopathology [65, 68, 69, 71]. Moreover, it has been reported higher levels of this factor at the cellular level in the early stages or acute paranoid schizophrenia that could be associated with astro- and oligodendroglial activation. These evidences suggest that glial activation and structural damage lead to a neurodegenerative-like process in schizophrenia [65, 72, 73]. Interestingly, an increased inflammatory markers’ expression has been linked to higher levels of S100B in CSF of schizophrenic patients, suggesting that inflammatory processes could lead or exacerbate the glial dysfunction in these patients [66]. Furthermore, different reports showed a decrease in the S100B levels in CSF of patients after antipsychotic treatment. However, this effect appears to be selective for patients with predominantly positive symptoms, since the patients with negative symptomatology showed high levels of S100B even after antipsychotic treatment [65, 69, 72, 74]. Indeed, Qi et al. [67] reported that S100B was significantly higher in patients with refractory schizophrenia which were treated with both clozapine and typical antipsychotics, without significant difference between these two treatment groups . In animal research (monkeys), it has been reported lower S100B expression after chronic haloperidol or olanzapine treatment [75].

Succeeding the description of Ang II activity within the CNS, the first evidence of brain locally produced components was observed as the co-localization of angiotensinogen and the main astrocytic marker – GFAP – whereas the local synthesis of the precursor by astrocytes was confirmed short after [76,77,78]. Furthermore, it was simultaneously evidenced that Ang II binds to receptors in glial cells with similar binding properties and different functionality when compared to angiotensin actions over neurons [79]. These receptors were later identified as the AT1-R subtype, promoting PLC activation and inositol-phosphate hydrolysis [80, 81]. Moreover, it was observed that Ang II, through AT1-R, increased intracellular Ca+2 levels as an initial peak (via IP3) followed by a sustained plateau (for Ca+2 influx from extracellular sources) [82, 83]. Afterward, several cellular effects through different signaling pathways have been described for these receptors in cultured astrocytes including CREB phosphorylation, JACK2/STAT3 and MAPK/ERK activation, and inducible early gene transcription [84,85,86,87]. Overall, nowadays it is widely accepted the constitutive presence and expression of AT1-R in astrocytes through which Ang II autoregulates its activity by controlling the synthesis of all RAS components [88, 89]. However, under certain pathological conditions that involve neuroinflammation, AT1-R overactivity in astrocytes has been found to be detrimental. In this sense, AT1-R activation stimulates ROS production and IL-6 synthesis and release in cultured glial cells via the NF-κB signaling pathway [87, 89], whereas Ang II promotes human astrocyte senescence by superoxide production, after membrane translocation of NADPH oxidase’s subunits. Interestingly, these effects were blunted by AT1-R blockade and the antioxidant tempol [90]. Moreover, in vitro studies showed that the inflammatory condition stimulated by LPS involves AT1-R activation for the upcome of astrogliosis, Ang II synthesis, and AT1-R upregulation, concomitant with NF-κB nuclear translocation, ROS production, and TNF-α release [27]. In the same direction, experimental autoimmune encephalomyelitis in rodents implies AT1-R overexpression in glial cells, triggering the upregulation and activation of transforming growth factor-β (TGF-β) and sustaining the inflammatory condition. Specifically, the AT1-R blockade decreased thrombospondin-1 (TSP-1) secretion from astrocytes, which later promotes TGF-β activation, and improved clinical scores in rodents [91]. Furthermore, AT1-R antagonists effectively blocked TNF-α release by astrocytes under hypoxic conditions [92]. Interestingly, the immunomodulatory actions of dopamine in neurodegenerative conditions involve the misbalance of RAS component production by astrocytes [93]. This interaction has also been observed after amphetamine exposure, where the psychostimulant-induced astrocyte reactivity involves AT1-R activation, concomitant with vascular-network rearrangement and apoptosis in cortical areas [94]. This way, accumulating evidence supports the synergic activity of AT1-R, pro-inflammatory mediators, and ROS in astrocytes contributing to the upcome of neuroinflammatory scenario [95, 96].

Oligodendrocytes

Oligodendrocytes are the glial cells responsible for the myelination processes. The appearance of myelinating oligodendrocytes facilitated the conduction of the nervous impulse, making the synaptic transmission faster and more efficient and leading to vertebrate CNS increase in complexity . In this sense, it is known that myelination processes have a critical role in cognitive functions, such as attention, learning, and memory. In humans, the myelin structure formation begins postnatally, and it is completed in young adulthood, around the time of the first psychotic episode expression and schizophrenia detection [97, 98]. Initial researches reported changes in gray and white matters and in the size of the ventricles. These findings gave a new meaning to oligodendrocytes and the myelination process in the schizophrenia etiopathology. Furthermore, several studies showed an atypical myelination in patients; the association observed in healthy controls between age, education, and the myelin water fraction in the frontal lobe’s white matter is not found in schizophrenic subjects [99]. Indeed, a decreased myelin fraction was observed in schizophrenic patients on their first episode, suggesting that these myelin changes precede the pathology occurrence and the possible pharmacological treatments [99]. Moreover, it has been reported that white matter density alterations are related to illness’ severity, where the low density of the corpus callosum and anterior commissure suggests an aberrant interhemispheric connectivity of anterior cortical and subcortical brain regions and reflecting decreased hemispheric specialization in schizophrenia [100]. All together , these results support the reduced lateralization observed in schizophrenic patients [101, 102]. On the other hand, these myelin structural alterations are companied by oligodendrocyte functional alterations, integrity loss, and/or decreased population [103]. To this last respect, it has been reported a reduction of oligodendrocyte density in the hippocampus , frontal cortex, and anterior cingulate cortex [99, 104,105,106,107,108]. In the same way, decreased oligodendrocyte- and myelin-related genes have been found reduced in schizophrenic patients [108,109,110,111,112,113]. One of the main genetic risk factors in schizophrenia are the disrupted-in-schizophrenia 1 (DISC1) genes, expressed in oligodendrocytes regulating negatively their differentiation and maturation. Bernstein et al. [114] have shown augmented oligodendrocyte-positive expression for DISC1 genes in patients with paranoid schizophrenia . Furthermore, the use of knockout mouse models, missing oligodendrocyte-, or the myelin-related genes linked to schizophrenia recreates the demyelination observed in the human disease in animal research [113]. Interestingly, it has been shown that sub-chronic olanzapine improved oligodendrocyte- and myelin-related gene expression in rats [115]. In the same way, in cellular culture and in vivo, it has been reported that antipsychotics, such as haloperidol, olanzapine, and quetiapine, promote the differentiation of oligodendrocytes through transcription factors 1 and 2 (Olig1 and Olig2), without having any effect over their proliferation [97, 116, 117]. Other studies described an oligodendrocyte development stage-dependent effect for haloperidol. In this sense, in a proliferation phase, haloperidol promotes the cellular spread but inhibits their differentiation in the maturation process [118]. It seems that there is a special susceptibility period during oligodendrocyte development, mainly in gestational and perinatal phases. In this sense, it has been reported that inflammatory processes during early gestational stages produce a decreased number of oligodendrocytes and myelination alterations in the offspring’s adult brain [119, 120]. These results are supported reciprocally with prenatal or gestational inflammation, since, as mentioned above, schizophrenia has been strongly related to maternal inflammatory insults.

Only a few studies have been focused on Ang II involvement over oligodendroglial functions. However, the confirmation of AT1-R expression on oligodendrocytes suggests undescribed physiological roles of Ang II over these cells [88]. Several lines of evidences suggest an indirect Ang II involvement in re-myelinization through its action over astrocytes or microglial cells. In this sense , angiotensinogen, the precursor of Ang II, has been suggested as a potential biomarker of progression of multiple sclerosis, an illness that results in myelin sheath damage [121]. Moreover, clearance and recycling of lipid debris are necessary processes for an adequate myelination and depend on a suitable lipid transport. The main apolipoprotein (Apo) involved in this lipid transport in the brain is Apo E, which is produced by astrocyte. Since Ang II, through its AT1-R, modulates astroglial function, this peptide could alter Apo E synthesis and indirectly myelin transport and recycling [122, 123] (Fig. 16.1).

Fig. 16.1
figure 1

The available evidences underpinning the glial system as a key player in the etiopathology of schizophrenia. The AT1-R broad role over neurotransmission systems and glial function encourages their consideration in the development of this pathology and allows to postulate them as a new target for treatment. Indeed, it is important to be conscious that the actual knowledge is the tip of the iceberg in this multifactorial pathology and more studies become necessary to fill out the blank in the schizophrenia puzzle

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