**3. Microglia in brain homeostasis**

#### **3.1. Microglia and brain development**

Microglia are the endogenous immune cells of the central nervous system. Over the past decade, the ontogeny of microglial cells has been controversial. Their developmental progression has gone through several interesting iterations leading to our current understanding of how these peripherally derived cells come to reside in the central nervous system [3]. During development, myeloid precursors travel to the brain and then differentiate into microglia (CNS parenchymal macrophages). These tissue-specific macrophages make their way to the brain through the circulation from the embryonic yolk sac [4]. They grow concurrently with neurons, before the development of astrocytes and oligodendrocytes, participating in key neurodevelopmental events such as neurogenesis, synaptic pruning, and thus the development and remodeling of neuronal circuits. There is evidence that microglia need to adapt to their quickly changing environment and modify their functions as needed [5]. It seems logical, then, that aberrant or impaired microglial activation during development would be implicated in CNS disease later on in life.

Early brain development involves a vast amount of axon and synaptic growth—a process known as exuberant synaptogenesis. During early childhood and puberty, these synapses are slowly eliminated in a regulatory process called synaptic pruning. Interestingly, the mechanisms responsible for synaptic pruning are related to peripheral immune mediators such as major histocompatibility complex [6] and complement proteins [7, 8]. As described in a review by our group [9] and briefly summarized below, the reemergence of these molecules in the aging brain may lead to inappropriate synaptic pruning and uncontrolled neuroinflammation.

#### **3.2. Microglia and AD**

a result of multiple hits contributed by systems within and outside the brain parenchyma and thus prompt the search for novel therapies that address the multi-organ etiology of AD

The most widely accepted theory of AD etiology is the amyloid cascade hypothesis [1], which maintains that overproduction and/or decreased clearance leads to extracellular aggregation of the presumably toxic amyloid-beta (Aβ) peptide. These extracellular Aβ aggregates act to increase neuronal kinase activity, resulting in phosphorylation of the microtubule-associated protein tau. Hyperphosphorylation of tau induces formation of intracellular aggregates known as neurofibrillary tangles and alters intracellular transport along microtubule tracks. This in turn abolishes neuronal communication, resulting in cell death in a spatially conserved pattern and producing deficits in networks that subserve memory and cognition. Aggregation of Aβ and tau is well-established pathological characteristics of AD brain tissue at autopsy. It is also known that in familial forms of AD, mutations in amyloid precursor protein (APP), Presenilin 1, or Presenilin 2 accelerate Aβ production and accumulation and lead to cognitive decline at a much earlier age. Presinilins function as part of the gamma secretase protein complex, one of three proteolytic enzymes responsible for cleaving APP into Aβ or nonaggregating amyloid peptides. Autopsy samples from brain parenchyma of patients with familial AD, which account for less than 1% of all AD cases, present with exorbitant Aβ and Tau accumulation similar to sporadic AD. Additionally, since the APP gene is located on chromosome 21, individuals with Down syndrome (trisomy 21) invariably develop AD-like dementia, also at a younger age than sporadic cases. This intuitively makes sense: an extra copy of APP on chromosome 21 will inevitably lead to the generation of more Aβ. However, it is highly uncertain to what degree familial AD and Down syndrome recapitulate the initial stages of sporadic AD, which accounts for the vast majority of AD cases. This is the core of the debate surrounding the amyloid cascade hypothesis: Is Aβ aggregation the start of AD or a downstream effect of an earlier insult? Additionally, and of considerable concern, to the day of writing this chapter, multiple immunotherapy clinical trials that target and clear Aβ as well as trials to block the activity of the secretases have failed to reverse cognitive loss and, in some cases, have accelerated it [2]. In this chapter, we will describe Aβ aggregation only as surrogate for the final common pathway of multiple disease mechanisms leading to the established

end pathology of AD and not as a direct, initiating cause of clinical demise.

Microglia are the endogenous immune cells of the central nervous system. Over the past decade, the ontogeny of microglial cells has been controversial. Their developmental progression has

**3. Microglia in brain homeostasis**

**3.1. Microglia and brain development**

pathology.

**2. The amyloid cascade hypothesis**

22 Alzheimer's Disease - The 21st Century Challenge

The role of microglia in the body is the story of Goldilocks. Much like the body's peripheral immune system, diseased or dystrophic microglia have diminished capacity to fight exogenous infections, clear endogenous cellular waste products, or promote homeostasis after an injurious insult. On the other hand, too much activation can severely harm the brain, much like how autoimmunity or graft rejection occurs in the periphery. In the brain, microglia contribute to Aβ clearance [10, 11]. However, the ability of microglial clearance appears to deteriorate and, in some cases, negatively change with age [12, 13]. At late stages of AD, microglia are thought to become overstimulated and paradoxically contribute to the disease by releasing proinflammatory cytokines in response to Aβ deposition [14, 15] or actively phagocytosing damaged, but live neurons [16]. Recent studies have consistently shown complement cascade proteins C1q and C3b—both normally associated with peripheral inflammation—upregulated on synapses induced by Aβ plaques in a mouse model of AD. Microglia then eliminated these C1q- or C3b-tagged synapses, leading to neurodegeneration and behavioral impairment [17, 18]. Immunohistochemistry studies reveal that Ig-positive neurons were C1q and C5b-9 positive and appeared degenerative [19]. These data suggest that neurons in AD brains are dying from an antibody-induced classical complement process. Additionally, newly discovered genetic risk factors are based on microglial phagocytosis, including CD33 [20], TREM2 [21, 22], and complement receptor 1 [23]. A full description of these mechanisms is out of the scope of this chapter, but the reader is encouraged to read more exhaustive reviews on this topic [24–26]. Nonetheless, it is a fascinating prospect that a peripherally derived cell plays such a large part in a central nervous system disease and that many of the processes used for brain development resurface to wreak havoc during degeneration. This shall segue into our next section discussing purely systemic mechanisms of AD pathogenesis.
