**3. Neuroinflammatory processes**

Neuroinflammation is defined as the reactive response of CNS against elements that interfere with homeostasis, inside or outside the CNS, and this response is involved in all neurological diseases, including developmental, traumatic, ischemic, metabolic, infectious, toxic, neoplas‐ tic, and neurodegenerative diseases. Emerging evidence suggests that inflammation has a causal role in disease pathogenesis, and understanding and controlling interactions between the immune system and the nervous system might be key for the prevention or delay of most late onset CNS diseases.

The wide variety of cellular and molecular mechanisms of neuroinflammation, probably the same in aging and chronic metabolic diseases such as hypertension, diabetes, depression, dementia or traumatic brain injury, are currently considered as silent contributors to neuro‐ inflammation [1]. In addition to these chronic diseases, another major risk factor that causes tissue destruction in CNS is stroke and atherosclerosis. This disease of brain arteries is characterized by vascular inflammation caused by monocyte infiltration in the injured vessel wall and increased interleukin (IL)-6 triggering significant damage in the area of the lesion [8]. Considering the current poor quality of life in many cities and unhealthy eating habits, we must know that this can lead to a number of disorders and diseases such as cardiovascular disease, stroke, hypertension, insulin resistance, and metabolic syndrome. Thus, lipid hor‐ mones, cytokines, and adipokines play an important role in inflammatory metabolic diseases through induction of adverse regulatory responses [1]. In other way, the chronic activation of proinflammatory signals in aging CNS may contribute to an increased vulnerability to neuropsychiatric disorders [9]. For example, the group of Dr. Ouchi linked obesity and inflammation, and they demonstrated that the inflammatory state was associated with a higher concentration of proinflammatory markers including IL-6, CRP, and adipokines [10]. All these proinflammatory markers are correlated positively with symptoms of depression and anxiety, and in agreement with those findings, metabolic diseases such as obesity, hypertension, and even senescence are prevalent risk factors for depression, cognitive dysfunction, and dementia [1] favoring the neurodegenerative process. On the other hand, the mechanism linking inflammation and depression involves oxidative stress and elevated levels of proinflammatory cytokines IL-6 and IL-8 among other factors. For example, major depressive disorder, a serious psychiatric illness, is associated with increased levels of peripheral inflammatory markers and to mortality due to depression and suicide [11, 12]. Therefore, inflammatory markers, identi‐ fied in neurodegenerative diseases including psychiatric disorders, are related to increased processes of neuronal death.

It is important to consider that some biological mechanisms involved in neuroinflammatory processes, such as the participation of complement cascade and microglial cells in responses of pruning synapses, also occur in healthy brain development [13].

On the other hand, the inflammatory response in the CNS is also linked to various processes such as aging, systemic infection, metabolic syndrome, and intrinsic CNS disease. Activation of the immune system in the CNS may compromise the generation of neurotrophic factors and the secretion of cytotoxic factors from the microglial cell. In line with the hypothesis that proinflammatory molecules such as the IL-1β family of cytokines and factors that simulate Toll-like receptors (TLRs) can impair the clearance function of microglia, there are some findings showing that disrupting IRAK4, an essential kinase downstream of TLRs and receptors for IL-1β cytokines, shifts microglial cells from a proinflammatory phenotype toward an anti-inflammatory phenotype [14].

The main agent involved in neuroinflammation is the microglial cell, but other factors that are also present in the inflammatory response should be considered. In animal models of AD, it has been demonstrated in areas associated with amyloid plaques, infiltration of mononuclear cells into the CNS as part of an innate immune response, but the role and the participation of these cells is not very clear yet [15]. Evidence from a mouse model of AD showed that peripheral mononuclear phagocytes have an important role to reduce the buildup of amyloidbeta plaques, which improve local inflammatory response [16].

The astroglial cells are also involved in the inflammatory response. Astrocytes respond to all forms of CNS insults through a process referred to as reactive astrogliosis. This response is a complex, multistage and pathology-specific reaction. On the other hand, the response of astrocytes is generally aimed for neuroprotection and recovery of injured neural tissue [17]. Emerging evidence of sustained inflammatory response in the CNS supports the major contribution of microglia and astrocytes in the progression of a wide variety of diseases, suggesting an important role as effectors of neuroinflammation in neuronal dysfunction and death. Other cell types including neurons, astrocytes, endothelial cells, etc., also express receptors for cytokines and other inflammatory mediators and can be activated by these signals and participate in the coordinated inflammatory response in the brain.

#### **3.1. Microglia and neuroinflammation**

through induction of adverse regulatory responses [1]. In other way, the chronic activation of proinflammatory signals in aging CNS may contribute to an increased vulnerability to neuropsychiatric disorders [9]. For example, the group of Dr. Ouchi linked obesity and inflammation, and they demonstrated that the inflammatory state was associated with a higher concentration of proinflammatory markers including IL-6, CRP, and adipokines [10]. All these proinflammatory markers are correlated positively with symptoms of depression and anxiety, and in agreement with those findings, metabolic diseases such as obesity, hypertension, and even senescence are prevalent risk factors for depression, cognitive dysfunction, and dementia [1] favoring the neurodegenerative process. On the other hand, the mechanism linking inflammation and depression involves oxidative stress and elevated levels of proinflammatory cytokines IL-6 and IL-8 among other factors. For example, major depressive disorder, a serious psychiatric illness, is associated with increased levels of peripheral inflammatory markers and to mortality due to depression and suicide [11, 12]. Therefore, inflammatory markers, identi‐ fied in neurodegenerative diseases including psychiatric disorders, are related to increased

It is important to consider that some biological mechanisms involved in neuroinflammatory processes, such as the participation of complement cascade and microglial cells in responses

On the other hand, the inflammatory response in the CNS is also linked to various processes such as aging, systemic infection, metabolic syndrome, and intrinsic CNS disease. Activation of the immune system in the CNS may compromise the generation of neurotrophic factors and the secretion of cytotoxic factors from the microglial cell. In line with the hypothesis that proinflammatory molecules such as the IL-1β family of cytokines and factors that simulate Toll-like receptors (TLRs) can impair the clearance function of microglia, there are some findings showing that disrupting IRAK4, an essential kinase downstream of TLRs and receptors for IL-1β cytokines, shifts microglial cells from a proinflammatory phenotype toward

The main agent involved in neuroinflammation is the microglial cell, but other factors that are also present in the inflammatory response should be considered. In animal models of AD, it has been demonstrated in areas associated with amyloid plaques, infiltration of mononuclear cells into the CNS as part of an innate immune response, but the role and the participation of these cells is not very clear yet [15]. Evidence from a mouse model of AD showed that peripheral mononuclear phagocytes have an important role to reduce the buildup of amyloid-

The astroglial cells are also involved in the inflammatory response. Astrocytes respond to all forms of CNS insults through a process referred to as reactive astrogliosis. This response is a complex, multistage and pathology-specific reaction. On the other hand, the response of astrocytes is generally aimed for neuroprotection and recovery of injured neural tissue [17]. Emerging evidence of sustained inflammatory response in the CNS supports the major contribution of microglia and astrocytes in the progression of a wide variety of diseases, suggesting an important role as effectors of neuroinflammation in neuronal dysfunction and death. Other cell types including neurons, astrocytes, endothelial cells, etc., also express

of pruning synapses, also occur in healthy brain development [13].

beta plaques, which improve local inflammatory response [16].

processes of neuronal death.

20 Update on Dementia

an anti-inflammatory phenotype [14].

Microglia is widely distributed throughout the brain and spinal cord but mainly in the hippocampus and the substantia nigra [18]. These cells are approximately 5–20% of the total population of glial cells in the CNS and are considered the representatives of the immune system in the central nervous system, since they have the ability to perform phagocytosis, release cytotoxic factors, and behave as antigen presenting cells [19, 20]. These cells are derived from macrophages produced by hematopoiesis in the primitive yolk sac [21], and they migrate to the developing neural tube, where they give rise to microglia [22].

This cellular type represents the major cellular component of the innate immune system of the brain. In normal conditions, microglia protects the brain environment by initiating a quick response to changes and effectively modulates inflammation.

Numerous signs that threaten homeostasis of the CNS, such as structures and/or residues from bacteria, viruses, and fungi; abnormal endogenous proteins; complement factors; antibodies; cytokines and chemokines, among others, are sensed by the microglia and consequently induce their activation [18]. Thus, there are two major functional aspects of the microglial cell: immune defense and maintenance of the CNS. Microglial cells function as immune cells acting as sentinels, detecting the first signs of invasion of pathogens or tissue damage. Furthermore, under inflammatory conditions produced by an active immune response, the microglia should moderate potential damage to the tissues that support, help to repair, and remodel CNS [22].

Microglial cells mediate immune and inflammatory responses in the CNS. These cells become functionally polarized to execute specific effector programs and thus, express specific func‐ tional reaction programs in response to diverse microenvironmental signals. Microglia have two functional states of polarization: one of them is phenotypically polarized to develop a classical proinflammatory or an alternative phenotype is anti-inflammatory and prohealing [23]. Thus, diverse pro- and anti-inflammatory cytokines, and others stimulus, can polarize microglia toward distinct functional phenotypes.

Microglial cells are characterized by the expression of distinct cell surface receptors and also by the release of numerous soluble factors. Activated cells with inflammatory phenotype are characterized by upregulation of CD16 Fc receptors, CD32, CD64, CD86, IL-1β, IL-6, IL-12, IL-23, tumor necrosis factor (TNF)-α, inducible nitric oxide synthase (iNOS), and chemokine, whereas microglial cells with anti-inflammatory phenotype display the upregulation of arginase (Arg)-1, mannose receptor (CD206), insulin-like growth factor (IGF)-1, triggering receptor expressed on myeloid cells 2 (TREM2), chitinase 3-like 3 (Ym-1), among others [23]. All these factors contribute to microglia activation that leads to further production of cytokines and other inflammatory mediators, which may contribute to the apoptotic cell death of neurons in multiple neurodegenerative diseases.

For these characteristics, microglia is considered the resident immune cells in the brain that are sensitive to even minor disturbances in homeostasis of the central nervous system (CNS) and become readily activated during most neuropathological conditions, including PD and AD [24].

But, which are the determinants that define whether the inflammatory response from micro‐ glial cells will result in a protective or neurodegenerative effect?

An important consideration, among other things, is the timing of the disease in which microglial activity begins. For example, in the case of AD, an increase in microglial activation at early stages has been observed [25]. This could be an indication that the microglia initially tries to eliminate harmful elements involved in disease such as amyloid-beta plaques. Thus, to assess whether microglial response is harmful or has a protective effect, we must distinguish between chronic and acute stimulation. An acute injury can cause oxidative and nitrosative stress, but it is usually short lived and unlikely to be detrimental to long-term neuronal survival. Therefore, it is believed that acute neuroinflammatory response is usually beneficial to the CNS, tends to minimize damage, and helps to repair damaged tissue. Moreover, microglial cells are capable of removing glutamate, a well-known neurotoxic substance that acts at NMDA receptors from neurons and can lead to neuronal death. In the case of AD, importance of glutamate and associated microglial function has been evidenced by the therapeutic effect of the drug memantine (an antagonist of NMDA receptors) that improves cognitive ability and everyday life functions in AD patients [26].

Oppositely, chronic microglia stimulation would trigger a chronic neuroinflammatory response, which is almost always harmful and damaging to nerve tissue. Thus, if neuroin‐ flammation has beneficial or deleterious results in the brain, it depends primarily on the length of the inflammatory response given by the microglial cell. For example, in the initial phases of AD, the progressive deposition of amyloid-beta plaques could provide chronic stimulation for microglial cells [27]. The release of pathogenic tau protein (hyperphosphorylated and selfaggregated) from dying neurons would also cause a constant activation of microglia [28]. As for the relationship of the proinflammatory cytokine IL-1 and the anti-inflammatory cytokine IL-10, levels of IL-1 raises drastically in the serum of AD patients, giving these patients a defined proinflammatory long-term profile, indicating a chronic CNS neuroinflammatory state [29]. In addition, the loss of neurons that characterizes AD further contributes to the generation of waste that is liberated from degenerating neurons and maintains microglia indefinitely in a state of long-term activation. These data indicate that, in AD, inflammation may be more chronic and therefore contribute to disease progression [27].

It is important to consider that microglia can be stimulated with environmental toxins, or with endogenous proteins too, and in this way, the cell can enter an overactivated state and release reactive oxygen species (ROS) and also reactive nitrogen species (RNS), which cause environ‐ mental toxicity for surrounding neurons [30]. This information is of great interest because overactivated microglia can be detected using imaging techniques, and therefore this knowl‐ edge offers an opportunity for an early diagnosis, and eventually in the future, this could be a target for the development of targeted anti-inflammatory therapies that lead to diminish the progression of a disease or may support existing therapies.

Thus, chronic inflammation is characterized by the long-standing activation of microglia, produced by chronic stimuli, trauma, and even pathological aggregates of neuronal proteins such as tau and beta-amyloid. These stimuli induce sustained release of inflammatory mediators, leading to an increase in oxidative and nitrosative stress, which perpetuates the inflammatory cycle, causing a permanent and detrimental inflammatory state.

#### **3.2. Astrocytes and neuroinflammation**

and become readily activated during most neuropathological conditions, including PD and

But, which are the determinants that define whether the inflammatory response from micro‐

An important consideration, among other things, is the timing of the disease in which microglial activity begins. For example, in the case of AD, an increase in microglial activation at early stages has been observed [25]. This could be an indication that the microglia initially tries to eliminate harmful elements involved in disease such as amyloid-beta plaques. Thus, to assess whether microglial response is harmful or has a protective effect, we must distinguish between chronic and acute stimulation. An acute injury can cause oxidative and nitrosative stress, but it is usually short lived and unlikely to be detrimental to long-term neuronal survival. Therefore, it is believed that acute neuroinflammatory response is usually beneficial to the CNS, tends to minimize damage, and helps to repair damaged tissue. Moreover, microglial cells are capable of removing glutamate, a well-known neurotoxic substance that acts at NMDA receptors from neurons and can lead to neuronal death. In the case of AD, importance of glutamate and associated microglial function has been evidenced by the therapeutic effect of the drug memantine (an antagonist of NMDA receptors) that improves

Oppositely, chronic microglia stimulation would trigger a chronic neuroinflammatory response, which is almost always harmful and damaging to nerve tissue. Thus, if neuroin‐ flammation has beneficial or deleterious results in the brain, it depends primarily on the length of the inflammatory response given by the microglial cell. For example, in the initial phases of AD, the progressive deposition of amyloid-beta plaques could provide chronic stimulation for microglial cells [27]. The release of pathogenic tau protein (hyperphosphorylated and selfaggregated) from dying neurons would also cause a constant activation of microglia [28]. As for the relationship of the proinflammatory cytokine IL-1 and the anti-inflammatory cytokine IL-10, levels of IL-1 raises drastically in the serum of AD patients, giving these patients a defined proinflammatory long-term profile, indicating a chronic CNS neuroinflammatory state [29]. In addition, the loss of neurons that characterizes AD further contributes to the generation of waste that is liberated from degenerating neurons and maintains microglia indefinitely in a state of long-term activation. These data indicate that, in AD, inflammation

It is important to consider that microglia can be stimulated with environmental toxins, or with endogenous proteins too, and in this way, the cell can enter an overactivated state and release reactive oxygen species (ROS) and also reactive nitrogen species (RNS), which cause environ‐ mental toxicity for surrounding neurons [30]. This information is of great interest because overactivated microglia can be detected using imaging techniques, and therefore this knowl‐ edge offers an opportunity for an early diagnosis, and eventually in the future, this could be a target for the development of targeted anti-inflammatory therapies that lead to diminish the

glial cells will result in a protective or neurodegenerative effect?

cognitive ability and everyday life functions in AD patients [26].

may be more chronic and therefore contribute to disease progression [27].

progression of a disease or may support existing therapies.

AD [24].

22 Update on Dementia

These type of cells are the most abundant and heterogeneous type of glial cells in the CNS. Their morphology can change depending on their developmental stage, subtype, and locali‐ zation [31]. For example, astrocytes of the gray matter are the protoplasmic ones; they exhibit short branches, whereas the fibrous astrocytes, present in the white matter, exhibit long unbranched processes [32].

The astrocytes are supportive for neuronal cell components in neural tissue and, as well as microglia, also respond to all forms of insults to the CNS through a process known as reactive astrogliosis, and this process is a reliable and sensitive marker of diseased tissue. These cells, which are responsible for a wide variety of complex and essential functions in healthy CNS, for example, are involved in primary roles in synaptic transmission and information process‐ ing by neural circuits [17], and they can contribute to synaptogenesis and dynamically modulate information processing and signal transmission, regulate neural and synaptic plasticity, and provide trophic and metabolic support for neurons [33, 34].

In effect, astrocytes are involved in very important processes such as controlling the environ‐ ment by regulating pH, ion homeostasis, blood flow, and modulating oxidative stress [31], and they are also responsible for a massive number of homeostatic tasks in the CNS [35]. With all these capacities, astrocytes, together with microglia, act as the main effectors of the neuroin‐ flammatory response. After suffering an injury, or detecting a damage signal, astrocytes rapidly act in response to pathology and undergo important changes in their morphology and functioning [17], as occurs with microglial cells. Thus the objective of the response is to control and to remove the brain insult, but this response may also have deleterious consequences. In fact, reactive gliosis is a self-perpetuating process, which, at the end, exacerbates the injury and, on the other hand, represents a nonphysiologic state in which astrocytes lose their helpful properties [31].

The mechanisms leading to the activation of these cells are actually unclear, and many factors that are involved in neurodegenerative diseases can trigger the response of these cells. In AD, for example, it has been demonstrated that the presence of amyloid activates astrocytes. As microglial cells, astrocytes also can phagocytose and degrade amyloid-beta, and to bring this capacity, astrocytes and microglia are activated through TLRs and RAGE receptors, thus causing local inflammation [36]. When the response of astrocytes is activated, they change their morphology and increase significantly the expression of the glial fibrillary acidic protein (GFAP), a recognized marker of astrocyte reactivity [37]. All these changes cause a disturbance of normal activities in astrocytes, which are essential for normal neuronal function.

Activation of astrocytes, internally, involves the activation of transcription factor NF-κB, which controls secretions of chemokine and adhesion molecules, and thus favors peripheral lym‐ phocyte infiltration and increases inflammatory response, which leads to neurodegeneration [36]. It has been shown that blockage of NF-κB transcriptional activity in astrocytes can extensively reduce inflammation, thus suggesting that inhibition of NF-κB in astrocytes may be regarded as a potential therapy for diseases such as AD [38].

With this background, it is possible to say that activated astrocytes are able to cause neurode‐ generation; moreover, when activated astrocytes express inflammation-associated factors, such as the peptide S100β, they represent a key factor for neuroinflammation. S100β is exclusively produced by astrocytes and, under physiological conditions, it is a neurotrophin responsible for survival, development, and function of neurons [39]. In neurodegenerative diseases such as AD and PD, among others, and also in subjects with severe brain trauma, the peptide S100β is overexpressed, and its levels correlate with the progression of the pathology [36, 40].

Another evidence linking astroglial activation with the development of neurodegenerative processes is proton resonance spectroscopy. Through this technique consistent evidence of significant increase of myoinositol (characteristic marker of astroglial cells) in neurodegener‐ ative diseases has been obtained. This has been observed both in brains of patients with mild cognitive impairment (MCI) and AD patients, and according to some studies, it has been reported to correlate with progression of pathology [36, 41, 42].
