Evaluating the role of microglial cells in clearance of AÎ² from

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Evaluating the role of microglial cells in clearance of A# from Alzheimer’s brain Aparna Lakshmi Manchikalapudi, Rajasekhar Reddy Chilakala, Kiran Kalia, and Aditya Sunkaria ACS Chem. Neurosci., Just Accepted Manuscript • DOI: 10.1021/acschemneuro.8b00627 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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ACS Chemical Neuroscience

Evaluating the role of microglial cells in clearance of Aβ from Alzheimer’s brain Aparna Lakshmi Manchikalapudi*, Rajasekhar Reddy Chilakala*, Kiran Kalia, Aditya Sunkaria# Department of Biotechnology, National Institute of Pharmaceutical Education and Research – Ahmedabad, Gujarat, INDIA

* These authors contributed equally. Running Title: Microglia mediated clearance of Aβ.

Key words: Alzheimer’s disease, Innate immune response, M1 and M2 markers, Microglia, Microglial receptors, Therapeutics.

#Author

to whom all correspondence be addressed.

Dr. Aditya Sunkaria Department of Biotechnology NIPER-Ahmedabad Gandhinagar-382355 (India) Email - [email protected]

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Abstract Ever increasing incidence of Alzheimer’s diseases (AD) has been reported all over the globe and practically no drug is currently available for its treatment. Since last 15 years, not a single drug came out of clinical trials. The researchers are yet to discover a drug that could specifically target AD, in fact the drugs that are about to launch in the global market either belongs to natural compounds or are already approved drugs targeting other diseases. So, we need to shift our focus on finding novel targets which are more specific and could either detect or inhibit the disease progression at very early stage. Microglia are the only resident innate immune cells of the brain that are originated from erythromyeloid progenitors. They migrate to the brain during early embryonic development, although their number is less (~ 5 to 10%) but they could act as guardians of the brain. It has been shown that the extracellular deposits of Aβ continuously phagocytosed by microglia in healthy individuals, but this ability would decrease with the age and lead to development of AD. In this review, we have explored the possibility whether microglial cells could be utilized as an early predictor of the AD progression. Here, we have discussed about innate immune response of microglial cells, the factors affecting microglia response, microglial receptors to which Aβ could bind, and microglial phenotype markers. Lastly, we concluded with the list of available AD therapeutics along with their mechanism. Key words: Alzheimer’s disease, Innate immune response, M1 and M2 markers, Microglia, Microglial receptors, Therapeutics. Introduction Alzheimer’s disease, a common form of dementia, accompanied with loss of memory, impairment in learning and thinking, depression, delusions and other symptoms. According to a recent report, 5.7 million Americans of all age groups are suffering with Alzheimer’s. According to Alzheimer’s association, over 4 million Indians are suffering with some or other form of dementia [1]. Most accepted hallmark of Alzheimer’s is accumulation of Amyloid-beta (Aβ) in the extracellular spaces, interfering within synapse, and other one is excessive phosphorylation of Tau, microtubule associated protein, in the neurons. It has been observed that in some cases Alzheimer’s patients are also suffering from Down’s syndrome, that could be because of APP (amyloid precursor protein) gene and gene responsible for Down’s syndrome are located on chromosome 21 [2]. Aβ is a peptide of 36-43 amino acids, and is a part of transmembrane protein APP in the neurons. The peptide is derived from the cleavage of APP by β secretase (BACE- Beta amyloid convertase enzyme) followed by γ secretase (Presenilin 1 and 2) [3, 4]. Initially, Aβ monomers are formed which then aggregated into insoluble fragments and finally forms fibrils and plaques. On the other hand, tau protein accumulates intracellularly as paired helical filaments and obstructs the cellular functions that are performed in the neurons which lead to neurodegeneration with neurofibrillary tangles [5]. So, in this review we have illustrated about the innate immune response of microglial cells; factors affecting microglial response; microglial receptors involved in Aβ uptake, and 2 ACS Paragon Plus Environment

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microglial phenotype markers, by believing that Aβ accumulation is the primary cause of Alzheimer’s development. Role of glial cells in AD Glial cells or neuroglial cells of CNS (central nervous system) surrounds the neurons and hold them in place. They also maintain the homeostasis and are of about 10-50 times more than neurons in the CNS. Oligodendrocytes, astrocytes, ependymal cells, and microglia are the different types of glial cells, which maintain normal physiology during the brain development. Recent reports have shown that on knocking out of myelin associated genes Ugt8, Cnp and Plp1 in mouse model, myelin dysfunction and eventually neurodegeneration was observed. Oligodendrocytes has also shown to activate inflammatory pathways in response to Aβ [11]. It has been found that BACE-1 plays an important role in oligodendrocytes mediated myelin production, which was confirmed by BACE inhibitors treatment [12]. Thus, oligodendrocytes indirectly affect the progression of Alzheimer’s. Astrocytes are star shaped cells with fine processes of variable length that differs based on their location. The major function of astrocytes is restoration of water and ion homeostasis as well as they contribute to the BBB maintenance [13]. Aβ presence leads to disturbance in Ca+2 homeostasis of astrocytes, which lead to astrogliosis and cause neuroinflammation [11]. Few reports have revealed that unsaturated fatty acid, palmitate, induced the NLRC4 (NLR family CARD domain-containing protein 4) expression in astrocytes and promotes the AD progression [14]. Ependymal cells form membrane lining of ventricles in the brain as well as spinal cord and produce small amount of cerebrospinal fluid which helps in Aβ clearance from the brain [13]. However, in pathological conditions the lysosomal function of ependymal cells has shown to be altered, and instead of clearance, Aβ gets accumulated [15]. Microglial cells are immunocompetent cells, derived from the hemangioblastic mesoderm and lineage continues through myelomonocytic cells and finally to microglial cells [13]. These are 510% of adult brain population [16] but few reports have shown that they are 10~20% [17]. They are uniformly dispersed at a density of 6 mm3 [18] and each cell occupies approximately 50000 µm3 [13]. Microglial cells express TREM2 (triggering receptors expressed in myeloid cells 2) receptors, that helps in chemotaxis, phagocytosis (besides TLR4 and CD14 receptors), migration, and proliferation. However, recent reports have suggested that in Alzheimer’s microglial phagocytic function has been decreased due to mutations in TREM2 [19]. Microglial cells also express CD33, SRP-β1, and TREM1 receptors. CD33 receptor, expressed on hematopoietic as well as immune cells, belongs to a family of sialic acid-binding immunoglobulinlike lectins. It plays an important role in the growth of immune cells, inhibition of cytokines release from monocytes, and mediate endocytosis. Reports have shown association between increased CD33 expression and Alzheimer’s risk, as it could inhibit the microglia mediated Aβ uptake [20]. On the other hand, signal regulatory protein β1 (SRP-β1) is a DAP-12 associated transmembrane protein, expressed on macrophages, hematopoietic cells and also on microglial cells. This has a major role in clearance of Aβ fibrils through CD 47 ligand, which also expressed by microglia and other brain cells. SRP-β1 expression has found to be up-regulated on microglia cells in 3 ACS Paragon Plus Environment


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Alzheimer’s patients. However, in aged brain clearance of Aβ fibrils has shown to be decreased due to dysfunctional SRP-β1/CD 47 signaling response [21-23]. In addition, TREM1 is a 234amino acids type 1 transmembrane receptor, expressed by microglia. TREM1 often associated with tyrosine kinase binding protein to enhance inflammatory response and phagocytic functions. In aged brain, TREM1 expression has found to be decreased along with reduced microglial phagocytic activity, which could be attributed to the intronic variant of TREM1 i.e. rs6910730G expression. However, the exact mechanism behind this is still not clear [24]. Neuronal circuits have shown to be functionally diminished at the initial stages of AD [25]. Reduction in the synapse number and inhibition of LTP leads to memory decline [26]. It has been shown that in AD brain, phagocytic functions of microglia also get reduced [27, 28]. Recently, these microglial cells have shown to attain tolerance against continuous but low doses of LPS administration [29]. However, it still remains unclear that whether microglia could also become tolerant to the Aβ in AD brains or not (Figure 1). Microglia - Innate immune cells of the brain The microglia due to their amyloid lineage has an inherent and preprogrammed action [30]. The morphological and functional resilience of microglia associated with their activation varies with the nature, durability, and duration of the activating stimulus as well as depends on intercellular interactions, including cell-surface molecules and soluble mediators. Microglia expresses various receptors that act as molecular switches for initiating immune response in case of CNS insult. Microglia maintains cerebral homeostasis by regular clearance of pathogens or cellular debris [31]. In resting state (M0), microglia are characterized with extensively ramified processes that perform continuous surveillance of the surroundings for potential threats making them as guardians of the CNS [30]. Microglia could attain a diversity of functional phenotypes in conditions such as imbalance in homeostasis or tissue damage which induces various dynamic processes, which regulate changes in cell structure, surface phenotypes, secretory products and proliferation. Its phenotypic diversity is parallel to the characterization of T cell responses into Th1 and Th2 cells [32]. Activated microglia exhibits two specific polarization phenotypes, M1 (classic) and M2 (alternative) [33]. Further, M2 phenotype microglia is subcategorized into three subtypes M2a, M2b, M2c based on their particular environment [34]. Recently, another isoform known as dark microglia has been identified and described in the later section [35]. Microglial cells in Alzheimer’s Microglial cells become activated in response to any change in their surrounding habitat, they shift from resting microglia state to activated microglia with shorter and thicker outer processes (6). These types of changes have been observed when insoluble Aβ aggregates are formed or in response to neuronal injury. The phenomenon is known as microglial priming and this response is stronger than the naive response [36]. Microglia communicate with the neurons via glutamate that acts through NMDA/AMPA receptors. Several reports have shown that TLR and co-receptor CD14 get activated in response to Aβ fibrils/tangles [37] (Figure 2). 4 ACS Paragon Plus Environment

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Apart from their toxic roles, few reports have shown that Aβ oligomers (oAβ) could also perform non-toxic functions at pico or nanomolar concentrations. Aβ has shown to activate cAMP responsive element (CRE) genes in the neuron through NMDA/AMPA receptors and the resulting CRE proteins are involved in the neuronal plasticity and long-term potential (LTP). However, during initial stages of AD, higher oAβ levels do not activate CRE genes but instead they get accumulated in the extracellular spaces. Eventually, Aβ oligomers blocked the synaptic transmission and resulted in neuronal injury [4, 38-40]. In turn, the injured neuron releases various factors like ATP and chemokine motif ligand 10, which attracts the microglia via purinoreceptors and chemokine receptors, respectively. [16]. Microglia express urokinase type plasminogen activator (uPA) receptor and uPA binding results in activation of the serine proteases which in turn transforms the inactive form of plasminogen into active form of plasmin. The activated plasmin degrades the extracellular matrix (ECM) in their way to reach the injured neurons. Apart from uPA, microglia release matrix metalloproteases (MMP9 or gelatinase B) that degrades ECM, mainly collagen and degrades the Aβ [41-43]. The activated microglia not only migrate towards the antigenic stimulus but also know to increase their number by secreting various neurotrophic factors like brain derived-neurotrophic factor (BDNF) [16]. Onset of fibrillation results in decreased phagocytic activity of microglia. Moreover, it has been shown that overexpression of uPA in transgenic mice results in memory impairment [44] and some recent evidence have suggested that the expression of metalloproteases also gets decreased, which ultimately resulted in decreased production of active or mature BDNF as pro-BDNF form require MMP-9 mediated cleavage [45]. Factors affecting microglial response The phenotypic diversity of the microglia depends on the particular environment and stimulating factors that activate microglia to exhibit different polarization states along with alterations in gene expression, function, and number (Table 1) [32, 46-49]. Classically activated M1 microglia are involved with acute infections like responses against bacterial and viral infection, and ultimately acquire a ramified morphology in responses to the pro-inflammatory agents such as MAMPs (Microbe-associated molecular patterns). The MAMPs like LPS, TLR, INF-γ have shown to induce cytokines and chemokines production in microglia, which include TNF-α, IL-6, IL-1β, IL12, IL-23, and CCL2 [46]. In addition, neurotoxic mediators like matrix metalloproteinase 12 (MMP12), NADPH oxidase, which produce reactive oxygen species (ROS), iNOS, MHC-II, and CD-86 have shown increased expression during early inflammation [47, 50, 51]. INF-γ induces JAK/STAT signaling pathway for activation of M1 phenotype via IFN-γ receptors 1 and 2. However, LPS induces M1 phenotype through TLRs coupled with MD2 (TLR/MD2) by activating NF-B and STAT5 pathway. Although, the M1 microglia releases pro-inflammatory cytokines to fight against microbes/infections, but chronic release of these mediators lead to neuronal damage [52]. To counter the induction of classically activated M1 microglial population, other microglia change their status from pro-inflammatory to an anti-inflammatory status and are characterized by enlarged cell bodies (M2 phenotype). Generally, microglia transformation is attained by retraction 5 ACS Paragon Plus Environment


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and hypertrophy of branched processes. These activated M2 (known as M2a phenotype) microglial cells have an amoeboid morphology. Anti-inflammatory cytokines such as IL-4, IL-10 and IL-13 are known to be involved in the conversion of M1 to M2 phenotype. These cells secrete neuronal growth factors such as insulin like growth factor-1 and express anti-inflammatory factors like arginase-1, mannose receptor, that help in tissue regeneration and ultimately suppress the action of M1 population. M2 microglia with its enhanced phagocytic ability clear misfolded proteins, apoptotic bodies, cell debris, and maintains the balance in the CNS [53]. Therefore, alternatively activated microglia are mainly concerned with protection from diseases, tissue damages, and helps healing where the classical activation would be pathogenic [48, 49]. M2 microglia phenotype could be further subcategorized into M2a, M2b, and M2c. M2a phenotype is induced by IL-4 or IL-3 which stimulate JAK1 or JAK3 and STAT6 pathway and involved in tissue reconstruction and phagocytosis by suppressing the release of pro-inflammatory mediators and by upregulating tissue repair factors such as scavenger receptors, insulin like growth factor-1 and arginase 1 [49, 54]. Moreover, M2b (Type II alternative activation) an immunoregulatory phenotype with prototype inducers include binding of immunoglobulins Fc gamma receptors by immune complexes and Toll like receptor agonists [55]. This phenotype shows over expression of CD32, CD64 and with increased phagocytic activity with recruitment of regulatory T cells. Further, M2c an acquired deactivating microglial phenotype induced by IL-10, SPHK1 and glucocorticoids, with increased expression of TGF- and sphingosine kinase. Whereas, administration of TGF- alone, induces M0 microglial un-stimulated phenotype, which is involved in immune suppression and helps in tissue remodeling [31, 53]. Different kinds of insults like HIV infection, DAMPs signals, neurodegenerative disease specific proteins such as Aβ, α-synuclein could activate microglia through pattern recognition receptors and purinergic receptors, which are involved in establishment of a microglial phenotype from its steady state phenotype [56]. Recently, another phenotype ‘dark’ microglia has been identified under pathological conditions within hippocampus, cerebral cortex, and hypothalamus [57]. Dark microglia have exclusive properties compared to other phenotypes like they are highly active than normal microglia with extensively ramified processes reaching for synaptic clefts and specifically engulfing unwanted pre-synaptic terminals and post synaptic dendritic spines. In addition, these dark microglia exhibit high oxidative stress and has been identified by dilation of endoplasmic reticulum, condensed cytoplasm, and nucleoplasm. Due to oxidative stress, these cells showed condensation of the cytoplasmic and nucleoplasm contents hence appears dark under TEM. Despite high oxidative stress, they do not appear to be apoptotic as they lack blebbing, rounding off, and fragmentation of their nucleus. These cells have been observed in chronic stress, deficiency in fractalkine receptor, aging or Alzheimer’s disease. Dark microglia express CD11b and TREM2, CD11b is involved in developmental pruning and forms complement receptor-3, whereas, TREM2 is involved in their intense phagocytic clearance of Aβ. The origin and nature of these cells are at the initial stages for considering them as therapeutic intervention and require further research to understand more about these cells [35, 57].


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Why microglial cells could not clear the Aβ load? It is now well accepted that extracellular deposition of Aβ in the brain is a hallmark lesion of Alzheimer’s disease. As per the amyloid hypothesis, initially balance between Aβ deposition and clearance is regulated by microglia. However, during later stages these microglia could not able to maintain this balance, and have shown to get attracted towards the Aβ deposits in brain with the help of various receptors like RAGE, CD36, CR1, CR3, TLR, scavenger receptor A and secrete insulin degrading enzyme, neprilysin, and MMP for the clearance of accumulated Aβ [58]. There are a number of possible explanations for the decreased efficacy of microglia to clear Aβ in early and late onset in AD’s patients, but the actual reason is not yet being understood. Acute in vitro stimulation of microglia with Aβ, express cytokines promoting increased phagocytic uptake and clearance. Chronic stimulation of microglia by brain trauma, systemic infection, and diseases make the microglia compromised in its phagocytosis of Aβ [56]. It has been observed that microglia turn dystrophic in aged brain with abnormalities in their cell body such as atrophic, de-ramified, tortuous processes attaining spherical or bulbous swelling and finally leading to neurodegeneration with increased pathogenic Aβ aggregates [59]. There are many hypotheses for the compromised nature of microglia, one possible explanation is that on chronic stimulation with Aβ, microglia cells are overwhelmed by the excess amount of Aβ produced and cannot divide rapidly and hence maintain their numbers to tackle accumulated Aβ. Another possibility is that, early microglial accumulation to the site of Aβ deposition for clearing neurotoxic peptide is a disease delaying process, as age increased the microglia becomes dysfunctional in clearing Aβ by expressing reduced Aβ–binding receptors, Aβ-degrading enzymes, without affecting the ability to produce pro-inflammatory cytokines. These cytokines in turn promotes Aβ production in an autocrine fashion by activating β and γ secretases [60]. One more reason for the loss of phagocytic ability of microglia to clear Aβ has been directly linked to ageing [23, 61]. There is another possibility that microglia on activation changes their phenotype, become more pro-inflammatory and lose their phagocytic property against Aβ, which results in reduced Aβ uptake, degradation, and finally leads to more Aβ accumulation. Table 2 comprises the list of major microglial receptors involved in Aβ binding or uptake [62]. As the age progresses, over production of various anti-inflammatory cytokines could modulate the microglia in paracrine fashion by either negative or positive feedback loops [63]. This results in generation of excessive ROS, NO, and RNS which leads to mitochondrial respiratory chain failure in glial as well as neuronal cells [63, 64]. The interaction between cytokine signaling and phagocytosis have proven to be highly complex and inflaming, as recently emerging data has put forward that microglial phagocytosis and plaque clearance may be suppressed as a result of an over production of anti-inflammatory molecules such as IL-10 and arginase-1, rather than mediated by pro-inflammatory dysfunction [65, 66]. One more possible mechanism of increased Aβ accumulation for late-onset AD include alterations in β and γ secretase activity [67]. A recent hypothesis is that microglia shows a reduced expression of genes involved in phagocytosis that has been detected in aged AD microglia [68]. Latest reports show that microglia develop tolerance 7 ACS Paragon Plus Environment


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on prolonged exposure to the inflammatory stimulus i.e. LPS and mediates shift towards an antiinflammatory microglial phenotype with decreased phagocytic ability [29, 69]. In spite of its selfrenewing capacity and continued recruitment of microglia in brain parenchyma [56], still the question remains same after decades of research in microglia that is why Aβ continues to accumulate. To answer this question, more focused studies need to be designed which target very initial response shown by microglial cells during the course of Aβ aggregation. Limitations of current therapeutic strategies against AD The most common and currently employed therapeutic targets of AD are glutamate release inhibitors, BACE inhibitors, γ secretase inhibitors, 5HT6 receptor antagonists, HDAC inhibitors and few others (Figure 3) [70, 71]. However, recent evidences have shown that these therapies have their own limitations and some of them have serious side-effects. A  secretase inhibitor, BMS 932481, which is under clinical trial haven’t shown >50% Aβ clearance at its safer therapeutic index [72]. Recently, it has been discovered that methamphetamine exposure leads not only Parkinson’s disease but also Alzheimer’s by modulating the BACE-1 expression and GSK-3 (glycogen synthase kinase 3) activation [73]. Limitations of long term usage of β secretase inhibitors have also been observed, including behavioral abnormalities and hypo myelination, whereas  secretase inhibitors have shown to cause notch-induced side effects [74]. Recently, a phase 3 clinical trial has revealed that 5HT-6 antagonists are not effective in improving cognitive impairments. Interestingly, Idalopiridine, 5HT-6 receptor antagonist had shown no effect of cognitive recovery in AD patients [75, 76]. Memantine like NMDA antagonists also shown recovery in terms of symptoms but not the cause of disease [77]. In addition, valproate, a class I HDAC inhibitor, has found to be ineffective against neurobehavioral impairments in AD [78]. There are many adverse effects of histone deacetylase (HDAC) inhibitors and recently it has been shown that HDAC levels are decreased with the AD progression [79]. Enhancement of Aβ clearance is one of the ways to treat Alzheimer’s. Peptides (vaccine or antibodies) that are administered into the body enhance B-cell epitope and elicits immune response. At the same time, peptides containing the T-cell activation domains were removed to prevent microglia mediated inflammatory response because those inflammatory mediators can cause destruction of microglia by apoptosis [80]. Direct in-take of antibodies causes severe adverse effects like cerebral amyloid angiopathy and vasogenic edema [74]. Reports have stated that immunotherapy could clear the plaques form from the brain but could not prevent neurodegeneration [80]. Therefore, there is an urgent requirement of finding novel and reliable early diagnostic markers for AD.

Microglia as early predictors of AD Microglia could be a potential target for identification of AD by using TPSO-PET (Translocator Protein- Positron Emission Tomography) radiotracers imaging technique. The TPSO expression 8 ACS Paragon Plus Environment

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has shown to be increased during AD [81] and predominantly expressed on activated microglia. Similarly, several other TPSO based radiotracer ligands are currently being explored for the detection of activated microglia such as [18F]GE-180, [11CDPA713], [11CPBR28], and [18F-DPA714] [82-85]. However, this imaging technique limits its ability to use because of their several drawbacks which led to discovery of other molecular targets like cannabinoid receptors, nicotinic acetylcholinergic receptors, purinergic receptors, and matrix metalloproteases [81]. Recently, in vivo imaging techniques like PET, MRI have confirmed that purinergic receptors (P2X7R, P2Y12R) could be utilized as novel targets for dynamic microglial cells as they can change their phenotypes with the progression of AD [86]. CD163, Iba-1 (ionized calcium-binding adaptor molecule 1), MHC-II, ricinus communis agglutinin-1, CD11c, CD33 are some receptors mainly expressed specifically on microglial cells. These are considered as microglial markers which are increased in different brain regions of AD patients. By the levels of these receptors one could know about the microglial activation [87]. Functions of microglial activation in early stages of AD are still in controversy but it is suggested that microglial activation is the link between amyloid deposition, neuronal damage and neurofibrillary tangles formation. In vivo findings of 11CPBR28 (microglial activation marker) distribution volumes was related with higher hippocampal volume, higher grey matter density and better cognition [88]. YKL-40, also known as chitinase 3-like protein, has found to be secreted in the CSF. Increased levels of YKL-40 indicates microglial activation [89], in-vivo PET imaging of YKL-40 with radiotracer ligand sounds a novel molecular target because collecting the blood from CSF is a painful method. These inferences on novel strategies gives better understanding of the microglial activation, which could be an early predictor of AD, in the pathophysiology of Alzheimer’s disease. Conclusion Since last few decades, a debate is going on how microglial cells respond to the changing microenvironment with the progression of age. Their decreased ability to clear accumulated Aβ in aged individuals leads to the development of Alzheimer’s disease. In this review, we have discussed various possible reasons why these microglia could not clear the Aβ load. Until today, several drugs have been developed for the treatment of AD, which mainly target the APP cleavage pathway and its genetics. Hence, future studies could be focused on microglial cells that have selfrenewal capacity but lost the ability to clear Aβ load from the brain. In conclusion, modulating the microglial phagocytic activity strategically at different stages of disease progression could be a potential therapeutic approach against Alzheimer’s. Authors Contribution AS has developed the concept of this paper and all authors have jointly written the manuscript. 9 ACS Paragon Plus Environment


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Conflicts of Interests None Acknowledgments The authors acknowledge Department of Science and Technology (DST), Govt. of India, for their financial support through StartUp Research Grant for Young Scientist Scheme to Dr. Aditya Sunkaria [SB/YS/LS-172/2014], Department of Pharmaceuticals, Ministry of Chemical and Fertilizers, Govt. of India, and National Institute of Pharmaceutical Education and Research (NIPER) Ahmedabad, Gandhinagar, India. References 1.

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Table of Content Graphic



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Figure Legends 16 ACS Paragon Plus Environment

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Figure 1: Age dependent changes in human brain during AD progression. Figure 2: Illustration of oligomeric Aβ uptake by neurons and oligomeric, fibrillar uptake of microglia through various receptors. Aβ oligomeric form that was in the synapse could block the communication between the neurons and may leads to damage due to excess intake of glutamate and calcium. These oligomers can also form fibrils. Oligomeric form and fibrillar form can be phagocytosed by microglia through CD14 and TLR 4 receptors. Oligomeric Aβ can inhibits CRE gene and leads to synaptotoxicity. Figure 3: Aβ and tau formation, targets for the treatment of Alzheimer’s.

Table 1: Expression of M1/M2 phenotypic markers in various microglial cells



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Human microglial cells

Mouse primary microglial cells

BV2 cells

N9 cells

HMO6 cells

TNF α

+

+

+

+

+

IL-1β

+

+

+

ND

+

NOS-2

+

+

+

+

ND

IL-6

+

+

+

+

+

COX-2

+

+

+

+

ND

MRC1

+

+

+

+

+

IL-4

+

+

+

+

IL-10

+

+

+

+

+

TGFβ1

+

+

+

ND

ND

SOCS3

+

+

-

ND

ND

ARG1

-

+

-

ND

ND

VEGF

-

+

+

ND

ND

M1 Markers

M2 Markers

ND = Not determined; + = Expressed; - = Not expressed Table 2: Functions of Ab receptors expressed on microglial cells


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Receptor

Function

SCARA-1

Involved in phagocytosis of Aβ soluble and fibrillar form.

MARCO

In inflammatory response.

SCARB-1

Decreases amyloid fibrillar and plaque formation.

CD36

Increase ROS production and Aβ phagocytosis.

α6β1 integrin

Increase ROS production and Aβ phagocytosis.

RAGE TLR2/TLR4/CD14

Instigate pro-inflammatory mediators. Instigate inflammatory response and Aβ phagocytosis.

SCARA-1; scavenger receptor class A type 1, MARCO; macrophage receptor, SCARB-1; scavenger receptor class B type 1, CD; cluster of differentiation, RAGE; receptor for advanced glycation end products, TLR; toll like receptors.



Figure 1 43x26mm (300 x 300 DPI)

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Evaluating the role of microglial cells in clearance of AÎ² from

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