Aluminum-Induced Amyloidogenesis and Impairment in the Clearance of Amyloid Peptides from the Central Nervous System in Alzheimer’s Disease

The membrane-integral beta-amyloid pre-cursor protein (bAPP) is probably the mostintensively studied brain cell protein inneurobiology. bAPP is processed by tan-dem beta-gamma secretase cleavage into42 amino acid amyloid (Ab42) peptides,whose progressive accumulation is onedistinguishing feature of Alzheimer’s dis-ease (AD) neuropathology (1–3). WhilehomeostaticamountsofAb42peptidegen-eration and clearance seem to be toler-ated by brain cells, their over-abundance,aggregation into higher order structures,and inability of brain cells to effec-tively phagocytose and clear these intenselyhydrophobic peptides contribute to thepro-inﬂammatory and neurotoxic pathol-ogy of AD. Aluminum, as an extremelyhigh charge density cation (

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title: The aluminium-amyloid cascade hypothesis and Alzheimer's disease.
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title: β-Amyloid-aluminum complex alters cytoskeletal stability and increases ROS production in cortical neurons
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title: Chronic exposure to aluminum in drinking water increases inflammatory parameters selectively in the brain
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title: Role of metal ions in the abeta oligomerization in Alzheimer's disease and in other neurological disorders.
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title: The role of metals in the etiology of Alzheimer's disease.
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title: Rapid Microglial Response Around Amyloid Pathology after Systemic Anti-Aβ Antibody Administration in PDAPP Mice
abstract: Aggregation of amyloid-β (Aβ) peptide in the brain in the form of neuritic plaques and cerebral amyloid angiopathy (CAA) is a key feature of Alzheimer's disease (AD). Microglial cells surround aggregated Aβ and are believed to play a role in AD pathogenesis. A therapy for AD that has entered clinical trials is the administration of anti-Aβ antibodies. One mechanism by which certain anti-Aβ antibodies have been proposed to exert their effects is via antibody-mediated microglial activation. Whether, when, or to what extent microglial activation occurs after systemic administration of anti-Aβ antibodies has not been fully assessed. We administered an anti-Aβ antibody (m3D6) that binds aggregated Aβ to PDAPP mice, an AD mouse model that was bred to contain fluorescent microglia. Three days after systemic administration of m3D6, there was a marked increase in both the number of microglial cells and processes per cell visualized in vivo by multiphoton microscopy. These changes required the Fc domain of m3D6 and were not observed with an antibody specific to soluble Aβ. These findings demonstrate that some effects of antibodies that recognize aggregated Aβ are rapid, involve microglia, and provide insight into the mechanism of action of a specific passive immunotherapy for AD.

title: Phagocytosis of Microglia in the Central Nervous System Diseases
abstract: Microglia, the resident macrophages of the central nervous system, rapidly activate in nearly all kinds of neurological diseases. These activated microglia become highly motile, secreting inflammatory cytokines, migrating to the lesion area, and phagocytosing cell debris or damaged neurons. During the past decades, the secretory property and chemotaxis of microglia have been well-studied, while relatively less attention has been paid to microglial phagocytosis. So far there is no obvious concordance with whether it is beneficial or detrimental in tissue repair. This review focuses on phagocytic phenotype of microglia in neurological diseases such as Alzheimer’s disease, multiple sclerosis, Parkinson’s disease, traumatic brain injury, ischemic and other brain diseases. Microglial morphological characteristics, involved receptors and signaling pathways, distribution variation along with time and space changes, and environmental factors that affecting phagocytic function in each disease are reviewed. Moreover, a comparison of contributions between macrophages from peripheral circulation and the resident microglia to these pathogenic processes will also be discussed.

title: Regulating amyloidogenesis through the natural triggering receptor expressed in myeloid/microglial cells 2 (TREM2)
abstract: Amyloidogenesis, the progressive accumulation of amyloid-beta (Aβ) peptides into insoluble, toxic, senile plaque lesions is one of the major defining features of the Alzheimer's disease (AD) brain. Normally, Aβ42 peptides are cleared from the extracellular space by natural phagocytic mechanisms, but when this intrinsic sensing and clearance system is functionally compromised or defective, Aβ42 peptides accumulate. Largely due to their intensely hydrophobic character, under physiological conditions Aβ42 peptides have a strong tendency to self-aggregate into higher order neurotoxic and pro-inflammatory fibrillar aggregates. While the Aβ peptide-clearing mechanisms are highly complex, one particularly important molecular sensor for Aβ42 peptide clearance is the triggering receptor expressed in myeloid/microglial cells 2 (TREM2; encoded at chr6p21.1), a variably glycosylated 230 amino acid transmembrane spanning stimulatory receptor of the immunoglobulin/lectin-like gene superfamily strongly associated with innate-immune, pro-inflammatory, and neurodegenerative signaling in AD. TREM2 is highly and almost exclusively expressed on the outer plasma membrane of microglial cells, the resident phagocytic and scavenging neuroimmune macrophages of the human central nervous system (CNS). As AD progresses, microglia appear to become progressively dysfunctional; TREM2 becomes down-regulated and microglia lose their ability to clear Aβ42 peptides while producing and releasing neurotoxins, reactive oxygen species (ROS), and pro-inflammatory cytokines that further promote Aβ42 production and pathological aggregation (Schmid et al., 2002; Alexandrov et al., 2013; Boutajangout and Wisniewski, 2013; Hickman and El Khoury, 2013). 
 
Recently, much interest in the molecular biology, genetics, and epigenetics of TREM2 expression, and its potential for sensing and scavenging Aβ42 peptides in AD and other progressive neurodegenerative diseases has arisen. TREM2's critical importance is underscored by seven recent observations: (1) that relatively rare mutations of TREM2 (or of its coupling protein, DAP12, also known as TYROBP; see Figure ​Figure1)1) are currently associated with the progressive, pre-senile dementing illnesses Nasu-Hakola syndrome, polycystic lipomembranous osteodysplasia with sclerosing leucoencephalopathy (POSL), sporadic amyotrophic lateral sclerosis (ALS), and AD (Nimmerjahn et al., 2005; Neumann and Takahashi, 2007; Guerreiro and Hardy, 2013; Zhao and Lukiw, 2013; Zhao et al., 2013; Cady et al., 2014); (2) that down-regulation in the phagocytic ability of microglia to degrade Aβ42 peptides in AD, and down-regulation in TREM2 expression, has been reported in sporadic AD brain tissues (Hickman and El Khoury, 2013; Zhao et al., 2013); (3) that TREM2 knockdown has been shown to exacerbate age-related neuro-inflammation and enhance cognitive deficiency in senescence accelerated mouse prone 8 (SAMP8) mice (Jiang et al., 2014); (4) that microglial TREM2 gene expression in cell culture, both at the level of mRNA and protein, have been shown to be remarkably sensitive to external cytokine stressors such as tumor necrosis factor alpha (TNFα; a pro-inflammatory adipokine known to be up-regulated in AD brain; Zhao et al., 2013; unpublished observations); (5) that AD-relevant pro-inflammatory neurotoxins such as bacterial lipopolysaccharide (LPS) and environmentally abundant toxic metals such as aluminum strongly down-regulate TREM2 and the ability of microglial cells to phagocytose extracellular debris (Hickman and El Khoury, 2013; Zhao et al., 2013; unpublished observations); (6) that down-regulation in the expression of TREM2 appears to be regulated in part by the up-regulation of the microglial-enriched, NF-kB-sensitive microRNA-34a (miRNA-34a), and perhaps other NF-kB-sensitive miRNAs, and (7) that both anti-NF-kB and anti-microRNA (AM-RNA) strategies have been shown to be useful in the restoration of homeostatic TREM2 gene expression levels and the neutralization of pro-inflammatory signaling and amyloidogenesis, at least in vitro (Hill et al., 2009; Pogue et al., 2009, 2010; Alexandrov et al., 2012, 2013; Zhao et al., 2013; unpublished observations). From what we know so far it is tempting to speculate that (1) loss-of function of TREM2 due to genetic mutations in familial AD may have the same end effects on phagocytosis as down-regulation of a fully functional TREM2 in sporadic AD; and that (2) modest TREM2 over-expression might be useful in enhancing the scavenging and removal of cellular debris in the CNS, including neurotoxic and self-aggregating Aβ42 peptides. Importantly, TREM2 signaling has been recently shown to be selectively inducible and manipulated from outside of the cell, suggesting that the modulation of TREM2 expression may be effectively regulated using highly specific targeting via drug-based pharmacological strategies exogenously supplied (Alexandrov et al., 2013; Lukiw, 2013; Zhao et al., 2013). 
 
 
 
Figure 1 
 
The triggering receptor expressed in myeloid/microglial cells type 2 (TREM2) is a 230 amino acid, ~25.4 kDa integral trans-membrane glycoprotein sensor spanning the lipid bilayer of CNS microglial cells. Very recently, TREM2 has been shown to act as a ... 
 
 
 
AD represents a highly complex, insidious, progressive, multi-factorial brain dysfunction whose incidence is reaching epidemic proportions. Despite the billions of dollars already spent on AD research, including multiple Aβ immunization and immunotherapy strategies, there is still no adequate treatment or cure for AD, and the development and implementation of novel, more effective treatment strategies are critical. Recruitment and harvesting of the TREM2 mechanism as a potent, endogenous Aβ42-peptide scavenging activity may represent a singularly attractive new direction for the clinical management of AD. Indeed, as a natural sensor and scavenger of noxious cellular debris TREM-2 stimulation may turn out (1) to be remarkably neuroprotective against both amyloidogenesis and age-related neuro-inflammation; while (2) significantly reducing the progressive cognitive impairment associated with amyloidogenesis and inflammatory neurodegeneration in the CNS (Hickman and El Khoury, 2013; Zhao et al., 2013; Cady et al., 2014; Jiang et al., 2014). Put another way, a decline in TREM2's contribution to the innate immune response, in part driving amyloid-clearance deficits and progressive degeneration characteristic of the AD process, suggest novel therapeutic targets and treatment strategies directed at maintaining natural and homeostatic TREM2 functions. This may be accomplished not only through the direct stimulation of TREM2 itself but also through the poorly understood downstream TREM2-linked TYROBP (DAP12) signaling pathways responsible for; (1) the actual phagocytosis of extracellular molecules; (2) the production of damaging quantities of ROS; (3) the induction of pro-inflammatory signals via NF-kB; and (4) the maintenance of homeostatic microglial function that together may diminish amyloidogenesis and the intercellular propagation of pathogenic signaling in the AD affected brain (Boutajangout and Wisniewski, 2013; Guerreiro and Hardy, 2013; Zhao and Lukiw, 2013; Jiang et al., 2014; Figure ​Figure11).

title: TREM2 signaling, miRNA-34a and the extinction of phagocytosis
abstract: The triggering receptor expressed in myeloid/microglial cells 2 (TREM2; encoded at chr6p21.1) is a glycosylated type 1 transmembrane sensor-receptor of the immunoglobulin-lectin-like gene superfamily expressed in the human central nervous system (CNS). TREM2 normally functions in immune surveillance, sensing and phagocytosis, including the homeostatic clearance of deleterious extracellular debris. Perhaps not too surprising, TREM2 deficiencies have been associated with pathological deficits in phagocytosis, amyloidogenesis and a compromised innate immune system in the inflammatory, neuro-degenerative illnesses polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL) and more recently with late onset Alzheimer's disease (AD; Forabosco et al., 2013; Golde et al., 2013; Guerreiro et al., 2013; Jonsson et al., 2013; Neumann and Daly, 2013; Zhao et al., 2013). Meta-analysis from multiple genome-wide association studies (GWAS) in AD have recently identified an rs75932628 (R47H; loss of function) variant in TREM2 as a strong AD risk factor, conveying an increase in AD with an odds ratio of 1.3–8.8-fold (p = 0.0076) in recent studies, an effect size comparable to that of the APOEe4 allele (Gonzalez Murcia et al., 2013). However, TREM2 R47H mutations appear to be relatively rare in the human populations so far studied (Gonzalez Murcia et al., 2013; Guerreiro et al., 2013; Hampel and Lista, 2013; Jonsson et al., 2013; Lattante et al., 2013). 
 
Not so rare in AD, however, are significant focal increases in the abundance of a pro-inflammatory, NF-κ B-regulated miRNA-34a (encoded at chr1p36.22) in virtually all AD cells and tissues examined compared to age-matched controls, as well as in amyloid overexpressing transgenic murine models for AD (Schipper et al., 2007; Wang et al., 2009; Zhao et al., 2013). For example, miRNA-34a was recently shown to be up-regulated, and TREM2 was found to be significantly down-regulated, in short post-mortem interval (mean ~2 h) samples of sporadic AD hippocampal CA1 compared with age-matched controls. This novel epigenetic mechanism appears to be mediated by virtue of an unusually strong miRNA-34a recognition feature within the 299 nucleotide TREM2 mRNA 3′-untranslated (3′-UTR) region (energy of association, EA ≤ 16 kcal/mol; Figure ​Figure1)1) (Zhao et al., 2013). The stress- and inflammation-induced transcription factor NF-κ B, a driver for miRNA-34a expression, is also strongly up-regulated in the hippocampal CA1, and both NF-κ B inhibitors and stabilized anti-miRNA-34a are effective in restoring TREM2 back to homeostatic levels (Kaltschmidt and Kaltschmidt, 2009; Lukiw, 2013; Zhao et al., 2013). Interestingly, a pathologically up-regulated miRNA-34a has been strongly associated with progressive neurotrophic deficits (Wang et al., 2009), altered synaptogenesis (Agostini et al., 2011) and deficient immune and phagocytotic responses in inflammatory degenerative disorders such as cardiovascular disease (Boon et al., 2013), multiple sclerosis (Junker et al., 2009), and in sporadic AD mononuclear cells (Schipper et al., 2007) as well as in AD brain (Zhao et al., 2013). 
 
 
 
Figure 1 
 
A hsa-miRNA-34a-TREM2-mRNA-3′-UTR complementarity map; gene products on two independent chromosomes orchestrate a down-regulation of TREM2 and a progressive deficit in cellular debris sensing, phagocytosis and clearance in human neurodegenerative ... 
 
 
 
Abundant evidence indicates that multiple genes, through multiple genetic processes, initiate and propagate AD-type change. Collectively, emerging observations indicate that an epigenetic mechanism involving an NF-κB-mediated, miRNA-34a-regulated down-regulation of TREM2 expression may shape innate immunity, inflammation and the extinction of the phagocytic response that contributes to amyloidogenesis and inflammatory neurodegeneration. Pro-inflammatory transcription factors and miRNAs, such as NF-κ B and miRNA-34a, and their target mRNA 3′-UTRs appear to form a highly interwoven genetic regulatory network that may escape classical GWAS- and SNP-based detection. Interestingly, AD-relevant stress-mediated up-regulation of miRNA-34a in cultured microglial cells, subsequent down-regulation in the expression of TREM2-3′-UTR reporter vectors, and rescue by stabilized anti-miRNA-34a indicates that this type of pathogenic signaling can be effectively quenched, at least in vitro (Lukiw, 2013; Zhao et al., 2013). Totally novel anti-miRNA strategies involving miRNA-34a mimics (i.e., MRX34) that normally induce senescence and apoptosis, and utilizing liposome delivery technologies are just now appearing in the clinic for the treatment of metastatic liver cancer (Bouchie, 2013). In the near future these approaches may have considerable potential in also directing novel, combinatorial anti-NF-κ B- and/or anti-miRNA-based AD therapeutic strategies that target the multiple pathogenic pathways which lie at the core of the AD process.

title: Aluminum modulates brain amyloidosis through oxidative stress in APP transgenic mice
abstract: Epidemiological studies have implicated aluminum (Al) exposure in the pathogenesis of Alzheimer's disease (AD); however, other studies have failed to confirm these results. Oxidative stress is a feature of AD, and Al can exacerbate oxidative events. This biological property has been suggested as a possible mechanism by which this metal could influence the onset and/or evolution of the disease. To test this hypothesis, we fed transgenic mice that over express human amyloid precursor protein (Tg2576) with a diet enriched in Al and measured isoprostane levels, sensitive and specific markers of in vivo oxidative stress, as well as amyloid β peptide formation and deposition. Here, we show an increase in brain isoprostane levels that correlated with increased amyloid β levels and accelerated plaque deposition in Tg2576 mice but not in wild‐type (WT) littermates fed with high dietary Al. Significantly, these in vivo effects of Al were reversed by vitamin E, as judged by a reduction of isoprostane production, amyloid β levels, and plaque deposition. These results indicate that dietary Al can modulate in vivo AD‐like amyloidosis in Tg2576 by increasing brain oxidative stress.

title: Aluminum and its potential contribution to Alzheimer's disease (AD)
abstract: Alzheimer's disease (AD) is perhaps the principal example of cognitive failure in humans, and currently over 5.5 million Americans suffer from this incapacitating and progressive disorder of thought, reasoning and memory. Our laboratory has been evaluating the potential contribution of environmentally bioavailable neurotoxic metals to the onset, development and progression of AD for about 30 years (Lukiw et al., 1987). Largely because of its known multiple and potent neurotoxic effects, much of our research has focused on the potential contribution of aluminum to the AD process: (i) because of aluminum's remarkable abundance and bioavailability in the biosphere—in fact it is the most abundant naturally occurring neurotoxic element to which we are exposed; (ii) because of aluminum's remarkable cellular toxicity and genotoxicity at low nanomolar concentrations toward brain genetic processes, and (iii) because of aluminum's highly structured, specific and unique interactions with the phosphate-rich nucleic acids associated with the expression of genetic information in the human brain (Lukiw et al., 1989; Bryant et al., 2004; Alexandrov et al., 2005, 2013; Lukiw and Pogue, 2007; Pogue et al., 2009, 2012; Lukiw, 2010; Percy et al., 2011; Bhattacharjee et al., 2013; De Sole et al., 2013). 
 
Aluminum's contribution to AD is based upon at least seven independently derived observations: (i) that at physiologically realistic concentrations, aluminum strongly promotes amyloid aggregation and accumulation, a key feature of AD neuropathology (Exley, 2005; Rodella et al., 2008; Walton and Wang, 2009; Yumoto et al., 2009); (ii) that both in vitro and in vivo aluminum promotes inflammatory signaling via the pro-inflammatory transcription factor NF-kB, another prominent feature characteristic of AD brain (Bondy, 2013; Walton, 2013); (iii) that out of the many thousands of brain gene messenger RNA (mRNAs) and micro RNAs (miRNAs), the family of mRNAs and miRNAs induced by aluminum are also strikingly similar to those found to be increased in AD; (iv) that in transgenic animal models of AD dietary aluminum enhances the development of pathological markers such as lipid peroxidation, oxidative stress, apoptosis, and gene expression deficits (Pratico et al., 2002; Bharathi et al., 2008; Zhang et al., 2012); (v) that many of the observed deficits in AD such as chromatin compaction, impaired energy utilization, impaired signaling involving chemical messengers such as adenine triphosphate (ATP) are recapitulated in aluminum-treated cellular or animal models of AD (Alexandrov et al., 2005; Lukiw and Pogue, 2007; Pogue et al., 2012; Bhattacharjee et al., 2013); (vi) that a very significant number of studies link the amount of aluminum in drinking water to the incidence of AD [worldwide, aluminum is added to drinking water as hydrated aluminum potassium sulfate KAl(SO4)2·12H2O, or alum, as a clarification and “finishing” agent (Flaten, 2001; Frisardi et al., 2010; Walton, 2013)], and (vii) perhaps most importantly, that of all pharmaceutical treatment approaches directed against AD to date, chelation using the anti-oxidant and trivalent iron/aluminum chelator desferrioxamine has been shown to be one of the most effective therapeutic strategies yet devised (Crapper McLachlan et al., 1991; Percy et al., 2011). 
 
Abundant research indicates that aluminum is a particularly reactive metal toward multiple aspects of human neurobiology and the altered genetics that are associated with the development and propagation of sporadic AD (Lukiw et al., 1989; Lukiw, 2010; Bhattacharjee et al., 2013; Bondy, 2013; Shaw and Tomljenovic, 2013; Walton, 2013). Thirty years of research since the potent effects of aluminum on the genetic apparatus in AD were first described, the most recent evidence suggests a strong linkage between aluminum sulfates and induction of NF-kB-sensitive pro-inflammatory miRNAs (Lukiw et al., 1987; Alexandrov et al., 2013; Zhao et al., 2013). Aluminum has been previously shown to significantly induce the transcription factor NF-kB (Pogue et al., 2009; Bondy, 2013), and up-regulation of NF-kB drives synthesis of NF-kB-sensitive miRNAs which in turn down regulate the expression of many AD-relevant genes, including complement factor H (CFH) and neurotropic signaling in human brain cells (Pogue et al., 2009; Zhao et al., 2013). 
 
We would like here to briefly include some recent genetic data on aluminum and its effects on miRNA abundance in a highly relevant transgenic animal model for AD that shows strong parallels to miRNA profiles which are found in AD brain (Figure ​(Figure1).1). There are currently over 90 transgenic mouse models of AD (http://www.alzforum.org/research-models/). A commonly used Tg2576 mouse model overexpresses a mutant form of beta amyloid precursor protein (β APP), APPK670/671L, linked to early-onset familial AD, and develops amyloid plaques and progressive cognitive deficits as the mice age. Tg2576 mice exposed to dietary aluminum have been shown to develop oxidative stress and robust amyloidogenesis, key features of AD neuropathology (Pratico et al., 2002). Animals provided a 2 mg/kg aluminum-supplemented diet were analyzed for miRNA speciation and complexity in their brains using GeneChip and miRNA array technologies; intriguingly, the same quintet of up-regulated pro-inflammatory miRNAs (miRNA-9, miRNA-34a, miRNA-125b, miRNA-146a, and miRNA-155) as found in (i) AD and (ii) in aluminum-treated human brain cells in primary culture were also found to be amongst the most up-regulated in these aluminum-supplemented Tg2576 mice (Alexandrov et al., 2005; Lukiw and Pogue, 2007; Pogue et al., 2012; Bhattacharjee et al., 2013; Hill et al., 2014; Figure ​Figure1;1; unpublished observations). Up-regulated miRNAs are known to target susceptible mRNAs and down-regulate the expression of many AD-relevant brain genes as is widely observed in AD brain tissues (Colangelo et al., 2002; Guo et al., 2010; Ginsberg et al., 2012). Interestingly, these findings suggest some common miRNA-induced mechanism between two important in vitro and in vivo models for AD with AD itself. Indeed, the abundance of specific miRNAs are highly selective, and potential indicators and predictors of human health and disease, including progressive neurological disorders such as AD (Alexandrov et al., 2005, 2013; Lukiw and Pogue, 2007; Pogue et al., 2009; Maciotta et al., 2013; Zhao et al., 2013). 
 
 
 
Figure 1 
 
Array-based cluster analysis of miRNA abundance in aluminum-fed Tg2576 mice vs. controls. In these experiments the brain (cortex) of 3 month-old Tg2576 mice fed aluminum-enriched diets were analyzed for miRNA speciation compared to age-matched controls ... 
 
 
 
Lastly, more research into the potential contribution of aluminum to the AD process is clearly warranted. There are currently no treatments for AD that effectively prevent or cure AD's insidious onset or propagation. We think it important to emphasize that the most effective clinical treatment yet devised for moderate- to late-stage AD patients was the implementation of the first generation anti-oxidant and trivalent iron/aluminum chelator desferrioxamine to attempt to remove aluminum from the brains of AD patients (Crapper McLachlan et al., 1991; Percy et al., 2011). Second generation aluminum chelators such as Feralex-G, either alone or in combination with other chelators, has shown higher specificity, increased selectivity and higher efficacy in aluminum sequestration and chelation in preliminary in vitro studies (Kruck et al., 2004, 2008; Percy et al., 2011; unpublished observations). Certainly, laboratory experimentation in cultured primary human brain cells, in transgenic AD animal models, and clinical studies employing next-generation aluminum chelators, perhaps in combination with other drug strategies, are one of the research areas needing more focused attention—to more effectively address the exact role and mechanism of aluminum neurotoxicity in this rapidly expanding healthcare concern.

title: TREM2 and the neuroimmunology of Alzheimer's disease.
abstract: Late-onset Alzheimer's disease (AD) is a sporadic disorder with increasing prevalence in aging. The ɛ4 allele of Apolipoprotein E(ApoEɛ4) was the only known major risk factor for late onset AD. Recently, two groups of investigators independently identified variants of the TREM2 gene, encoding triggering receptor expressed on myeloid cells 2 as causing increased susceptibility to late onset AD with an odds ratio similar to that of ApoEɛ4. TREM2 is a receptor expressed on innate immune cells. Using a novel technology called Direct RNA Sequencing wedetermined the quantitative transcriptome of microglia, the principal innate neuroimmune cells and confirmed that TREM2 is a major microglia-specific gene in the central nervous system. Over the past several years we have shown that microglia play a dichotomous role in AD. Microglia can be protective and promote phagocytosis, degradation and ultimately clearance of Aβ, the pathogenic protein deposited in the brains of Alzheimer's patients. However, with disease progression, microglia become dysfunctional, release neurotoxins, lose their ability to clear Aβ and produce pro-inflammatory cytokines that promote Aβ production and accumulation. TREM2 has been shown to regulate the phagocytic ability of myeloid cells and their inflammatory response. Here we propose that the mechanism(s) by which TREM2 variants cause Alzheimer's disease are via down regulation of the Aβ phagocytic ability of microglia and by dysregulation of the pro-inflammatory response of these cells. Based on our discussion we propose that TREM2 is a potential therapeutic target for stopping ordelaying progression of AD.

title: Regulation of TREM2 expression by an NF-кB-sensitive miRNA-34a
abstract: Genetic deficits and loss of function for the triggering receptor expressed in myeloid cells 2 (TREM2; encoded at chr6p21.1), a transmembrane spanning stimulatory receptor of the immunoglobulin/lectin-like gene superfamily, have been associated with deficiencies in phagocytosis and the innate immune system in Alzheimer’s disease. In this study, we provide evidence that TREM2 is downregulated in samples of sporadic Alzheimer hippocampal CA1 compared with age-matched controls. A nuclear factor-кB (NF-кB)-sensitive miRNA-34a (encoded at chr1p36.22), upregulated in Alzheimer’s disease, was found to target the 299 nucleotide human TREM2 mRNA 3′-untranslated region (3′-UTR) and downregulate the expression of a TREM2-3′-UTR reporter vector. A stabilized anti-miRNA-34a (AM-34a) quenched this pathogenic response. The results suggest that an epigenetic mechanism involving an NF-кB-mediated, miRNA-34a-regulated downregulation of TREM2 expression may shape innate immune and phagocytic responses that contribute to inflammatory neurodegeneration.

title: Expression of the phagocytosis-essential protein TREM2 is down-regulated by an aluminum-induced miRNA-34a in a murine microglial cell line.
abstract: One of the key classical pathological features of Alzheimer's disease (AD) is the progressive accumulation of amyloid beta (Aβ42) peptides and their coalescence into highly insoluble senile plaque cores. A major factor driving Aβ42 peptide accumulation is the inability of brain cells to effectively clear excessive amounts of Aβ42 via phagocytosis. The trans-membrane spanning, sensor-receptor known as the "triggering receptor expressed in myeloid cells 2" (TREM2; chr6p21) is essential in the sensing, recognition, phagocytosis and clearance of noxious cellular debris from brain cells, including neurotoxic Aβ42 peptides. Recently, mutations in the TREM2 gene have been associated with amyloidogenesis in neurodegenerative diseases including AD. In this report, we provide evidence that aluminum-sulfate, when incubated with microglial cells, induces the up-regulation of an NF-кB-sensitive micro RNA-34a (miRNA-34a; chr1p36) that is known to target the TREM2 mRNA 3'-untranslated region (3'-UTR), significantly down-regulating TREM2 expression. The aluminum-induced up-regulation of miRNA-34a and down-regulation of TREM2 expression were effectively quenched using the natural phenolic compound and NF-kB inhibitor CAPE [2-phenylethyl-(2E)-3-(3,4-dihydroxyphenyl) acrylate; caffeic-acid phenethyl ester]. These results suggest, for the first time, that an epigenetic mechanism involving an aluminum-triggered, NF-kB-sensitive, miRNA-34a-mediated down-regulation of TREM2 expression may impair phagocytic responses that ultimately contribute to Aβ42 peptide accumulation, aggregation, amyloidogenesis and inflammatory degeneration in the brain.

title: Deficits in the miRNA-34a-regulated endogenous TREM2 phagocytosis sensor-receptor in Alzheimer's disease (AD); an update
abstract: One characteristic feature of Alzheimer's disease (AD) neuropathology is the progressive generation, aggregation and deposition of the 42 amino acid amyloid beta (Aβ42) peptide, and other related amyloidogenic molecules, into dense clumps of insoluble, pro-inflammatory senile plaque cores in the extracellular space of the brain. It is not generally appreciated that the Aβ42 peptide, derived via tandem beta-gamma secretase cleavage of the larger ~770 amino acid transmembrane beta-amyloid precursor protein (βAPP) is one of the “stickiest” peptides known, due in part to its high content of lipophillic and hydrophobic peptide domains (21.4% valine-isoleucine; Mager, 1998; Watson et al., 2005). Aβ42 peptide monomers have tremendously high potential for relatively rapid self-aggregation into Aβ42 peptide dimers and higher order fibrillar structures via long-range, non-covalent hydrophobic forces that over time promote β-pleated sheet conformations (Mager, 1998; Watson et al., 2005; Boutajangout and Wisniewski, 2013; Chang et al., 2013). There is abundant evidence that under normal, homeostatic conditions, Aβ42 peptide monomers and perhaps other higher order Aβ42 peptides are effectively “cleared” from the brain's extracellular medium by highly active and efficient innate-immune surveillance and phagocytic systems that limit excessive Aβ42 peptide dimerization, accumulation and further self-aggregation into pathological senile plaque lesions. Recently described, one of the key phagocytosis sensor-receptors responsible for Aβ42 peptide clearance from the human central nervous system (CNS) is very likely the triggering receptor expressed in myeloid/microglial cells 2 (TREM2) enriched in myeloid cells and microglial cells of the CNS (Benitez et al., 2013; Forabosco et al., 2013; Gonzalez Murcia et al., 2013; Guerreiro et al., 2013; Neumann and Daly, 2013; Zhao et al., 2013; Jones et al., 2014; ). This short paper is an update on some very recent observations on TREM2 neurobiology, on newly discovered roles for miRNA-34a-mediated signaling in human degenerative disease, including miRNA-34a-mediated effects on TREM2 expression, and how dysfunctional TREM2 signaling may contribute to amyloidogenesis in AD and in related progressive, inflammatory neurodegenerative diseases of the human CNS.