New insights into the role of TREM2 in Alzheimer’s disease

Alzheimer’s disease (AD) is the leading cause of dementia. The two histopathological markers of AD are amyloid plaques composed of the amyloid-β (Aβ) peptide, and neurofibrillary tangles of aggregated, abnormally hyperphosphorylated tau protein. The majority of AD cases are late-onset, after the age of 65, where a clear cause is still unknown. However, there are likely different multifactorial contributors including age, enviornment, biology and genetics which can increase risk for the disease. Genetic predisposition is considerable, with heritability estimates of 60–80%. Genetic factors such as rare variants of TREM2 (triggering receptor expressed on myeloid cells-2) strongly increase the risk of developing AD, confirming the role of microglia in AD pathogenesis. In the last 5 years, several studies have dissected the mechanisms by which TREM2, as well as its rare variants affect amyloid and tau pathologies and their consequences in both animal models and in human studies. In this review, we summarize increases in our understanding of the involvement of TREM2 and microglia in AD development that may open new therapeutic strategies targeting the immune system to influence AD pathogenesis.     

Mechanisms of NLRP3 Inflammasome Activation: Its Role in the Treatment of Alzheimer’s Disease

Alzheimer’s disease (AD) is a common neurodegenerative disease of progressive dementia which is characterized pathologically by extracellular neuritic plaques containing aggregated amyloid beta (Aβ) and intracellular hyperphosphorylated tau protein tangles in cerebrum. It has been confirmed that microglia-specific nucleotide-binding oligomerization domain (NOD)-like receptor protein 3 (NLRP3) inflammasome-mediated chronic neuroinflammation plays a crucial role in the pathogenesis of AD. Stimulated by Aβ deposition, NLRP3 assembles and activates within microglia in the AD brain, leading to caspase-1 activation along with downstream interleukin (IL)-1β secretion, and subsequent inflammatory events. Activation of the NLRP3 inflammasome mediates microglia to exhibit inflammatory M1 phenotype, with high expression of caspase-1 and IL-1β. This leads to Aβ deposition and neuronal loss in the amyloid precursor protein (APP)/human presenilin-1 (PS1) mouse model of AD. However, NLRP3 or caspase-1 deletion in APP/PS1 mice promotes microglia to transform to an anti-inflammatory M2 phenotype, with decreased secretion of caspase-1 and IL-1β. It also results in improved cognition, enhanced Aβ clearance, and a lower cerebral inflammatory response. This result suggests that the NLRP3 inflammasome may be an appropriate target for reducing neuroinflammation and alleviating pathological processes in AD. In the present review, we summarize the generally accepted regulatory mechanisms of NLRP3 inflammasome activation, and explore its role in neuroinflammation. Furthermore, we speculate on the possible roles of microglia-specific NLRP3 activation in AD pathogenesis and consider potential therapeutic interventions targeting the NLRP3 inflammasome in AD.

     

Aging and amyloid beta-induced oxidative DNA damage and mitochondrial dysfunction in Alzheimer's disease: implications for early intervention and therapeutics

Alzheimer's disease (AD) is an age-related progressive neurodegenerative disease affecting thousands of people in the world and effective treatment is still not available. Over two decades of intense research using AD postmortem brains, transgenic mouse and cell models of amyloid precursor protein and tau revealed that amyloid beta (Aβ) and hyperphosphorylated tau are synergistically involved in triggering disease progression. Accumulating evidence also revealed that aging and amyloid beta-induced oxidative DNA damage and mitochondrial dysfunction initiate and contributes to the development and progression of the disease. The purpose of this article is to summarize the latest progress in aging and AD, with a special emphasis on the mitochondria, oxidative DNA damage including methods of its measurement. It also discusses the therapeutic approaches against oxidative DNA damage and treatment strategies in AD.     

Amyloid precursor protein cross-linking stimulates beta amyloid production and pro-inflammatory cytokine release in monocytic lineage cells

Beta amyloid peptide-containing neuritic plaques are a defining feature of Alzheimer's disease pathology. Beta amyloid are 38-43 residue peptides derived by proteolytic cleavage of amyloid precursor protein. Although much attention has focused on the proteolytic events leading to beta amyloid generation, the function of amyloid precursor protein remains poorly described. Previously, we reported that amyloid precursor protein functions as a pro-inflammatory receptor on monocytic lineage cells and defined a role for amyloid precursor protein in adhesion by demonstrating that beta(1) integrin-mediated pro-inflammatory activation of monocytes is amyloid precursor protein dependent. We demonstrated that antibody-induced cross-linking of amyloid precursor protein in human THP-1 monocytes and primary mouse microglia stimulates a tyrosine kinase-based pro-inflammatory signaling response leading to acquisition of a reactive phenotype. Here, we have identified pro-inflammatory mediators released upon amyloid precursor protein-dependent activation of monocytes and microglia. We show that amyloid precursor protein cross-linking stimulated tyrosine kinase-dependent increases in pro-inflammatory cytokine release and a tyrosine kinase-independent increase in beta amyloid 1-42 generation. These data provide much needed insight into the function of amyloid precursor protein and provide potential therapeutic targets to limit inflammatory changes associated with the progression of Alzheimer's disease.     

The emerging roles of protein homeostasis‐governing pathways in Alzheimer's disease

Pathways governing protein homeostasis are involved in maintaining the structural, quantitative, and functional stability of intracellular proteins and involve the ubiquitin–proteasome system, autophagy, endoplasmic reticulum, and mTOR pathway. Due to the broad physiological implications of protein homeostasis pathways, dysregulation of proteostasis is often involved in the development of multiple pathological conditions, including Alzheimer's disease (AD). Similar to other neurodegenerative diseases that feature pathogenic accumulation of misfolded proteins, Alzheimer's disease is characterized by two pathological hallmarks, amyloid‐β (Aβ) plaques and tau aggregates. Knockout or transgenic overexpression of various proteostatic components in mice results in AD‐like phenotypes. While both Aβ plaques and tau aggregates could in turn enhance the dysfunction of these proteostatic pathways, eventually leading to apoptotic or necrotic neuronal death and pathogenesis of Alzheimer's disease. Therefore, targeting the components of proteostasis pathways may be a promising therapeutic strategy against Alzheimer's disease.     

Transmission of pathogenic protein aggregates in Alzheimer’s disease

Deposits of amyloid peptide Aβ and intracellular aggregates of hyperphosphorylated tau protein in the brain of patients are major neuropathological features of Alzheimer’s disease (AD). For a long time, the possibility of horizontal transmission of Aβ aggregates from cell to cell and from person to person remained hypothetical, since there was no experimental evidence. However, in 1993, the formation of senile plaques was confirmed in the brains of animals after intracerebral injections of AD patient brain homogenates. or homogenates of the brain of transgenic mice enriched with Aβ aggregates Other experiments indicate that amyloid peptide Aβ and intracellular aggregates of hyperphosphorylated tau protein may be transferred from cell to cell like prions. In 2015 and 2016, it was reported that AD could be transmitted to humans during medical procedures, i.e., that this disease might be iatrogenic. This review discusses the mechanisms by which pathogenic Aβ protein can be transmitted between cells and analyzes the current evidence concerning the possibility of horizontal Aβ transmission from person to person.     

Heat Shock Proteins and Amateur Chaperones in Amyloid-Beta Accumulation and Clearance in Alzheimer’s Disease

The pathologic lesions of Alzheimer’s disease (AD) are characterized by accumulation of protein aggregates consisting of intracellular or extracellular misfolded proteins. The amyloid-β (Aβ) protein accumulates extracellularly in senile plaques and cerebral amyloid angiopathy, whereas the hyperphosphorylated tau protein accumulates intracellularly as neurofibrillary tangles. “Professional chaperones”, such as the heat shock protein family, have a function in the prevention of protein misfolding and subsequent aggregation. “Amateur” chaperones, such as apolipoproteins and heparan sulfate proteoglycans, bind amyloidogenic proteins and may affect their aggregation process. Professional and amateur chaperones not only colocalize with the pathological lesions of AD, but may also be involved in conformational changes of Aβ, and in the clearance of Aβ from the brain via phagocytosis or active transport across the blood–brain barrier. Thus, both professional and amateur chaperones may be involved in the aggregation, accumulation, persistence, and clearance of Aβ and tau and in other Aβ-associated reactions such as inflammation associated with AD lesions, and may, therefore, serve as potential targets for therapeutic intervention.     

New Age of Neuroproteomics in Alzheimer’s Disease Research

Alzheimer’s disease (AD) is the leading cause of dementia, a condition that gradually destroys brain cells and leads to progressive decline in mental functions. The disease is characterized by accumulation of misfolded neuronal proteins, amyloid and tau, into insoluble aggregates known as extracellular senile plaques and intracellular neurofibrillary tangles, respectively. However, only tau pathology appears to correlate with the progression of the disease and it is believed to play a central role in the progression of neurodegeneration. In AD, tau protein undergoes various types of posttranslational modifications, most notably hyperphosphorylation and truncation. Using four proteomics approaches we aimed to uncover the key steps leading to neurofibrillary degeneration and thus to identify therapeutic targets for AD. Functional neuroproteomics was employed to generate the first transgenic rat model of AD by expressing a truncated misordered form of tau, “Alzheimer’s tau”. The rat model showed that Alzheimer’s tau toxic gain of function is responsible for the induction of abnormal tau cascade and is the driving force in the development of neurofibrillary degeneration. Structural neuroproteomics allowed us to determine partial 3D structure of the Alzheimer’s filament core at a resolution of 1.6 Å. Signaling neuroproteomics data lead to the identification and characterization of relevant phosphosites (the tau phosphosignalome) contributing to neurodegeneration. Interaction neuroproteomics revealed links to a new group of proteins interacting with Alzheimer’s tau (tau interactome) under normal and pathological conditions, which would provide novel drug targets and novel biomarkers for treatment of AD and other tauopathies.     

Focus: The Aging Brain: Amyloid-beta Alzheimer targets — protein processing,lipid rafts,and amyloid-beta pores

Amyloid beta (Aβ), the hallmark of Alzheimer’s Disease (AD), now appears to be deleterious in its low number aggregate form as opposed to the macroscopic Aβ fibers historically seen postmortem. While Alzheimer targets, such as the tau protein, amyloid precursor protein (APP) processing, and immune system activation continue to be investigated, the recent discovery that amyloid beta aggregates at lipid rafts and likely forms neurotoxic pores has led to a new paradigm regarding why past therapeutics may have failed and how to design the next round of compounds for clinical trials. An atomic resolution understanding of Aβ aggregates, which appear to exist in multiple conformations, is most desirable for future therapeutic development. The investigative difficulties, structures of these small Aβ aggregates, and current therapeutics are summarized in this review.     

Mouse models of Alzheimer's disease: a quest for plaques and tangles

Many genetically altered mice have been designed to help understand the role of specific gene mutations in the pathogenesis of Alzheimer's disease (AD) based on the realization that specific mutations in the genes for amyloid precursor protein--the presenilins and tau--are associated with early-onset familial AD or, in the case of tau mutations, other neurodegenerative diseases with neurofibrillary tangles. However, attempts to reproduce the neuropathology of AD in the mouse have been frustrating. Transgenic designs emphasizing amyloid precursor protein produced mice that develop amyloid plaques, but neurodegeneration and neurofibrillary tangles failed to form. Strategies emphasizing tau resulted in increased phosphorylation of tau and tangle formation, although amyloid plaques were absent. Nevertheless, crossing transgenic animals expressing mutated tau and amyloid precursor protein has produced a mouse that closely recapitulates the neuropathology of AD. A review of the various murine models, their role in understanding the pathogenesis of AD and their use in testing therapeutic regimens, is provided.     

Therapeutic Strategies for Alzheimer’s Disease

Therapeutic approaches for Alzheimer's disease (AD) are guided by four disease characteristics: amyloid plaques, neurofibrillar tangles (NFT), neurodegeneration, and dementia. Amyloid plaques are composed largely of 4 kDa beta-amyloid (Abeta) peptides, with the more amyloidogenic, 42 amino acid form (Abeta42) as the primary species. Because multiple, rare mutations that cause early-onset, familial AD lead to increased production or aggregation of Abeta42, amyloid therapeutics aim to reduce the amount of toxic Abeta42 aggregates. Amyloid-based therapies include gamma-secretase inhibitors and modulators, BACE inhibitors, aggregation blockers, catabolism inducers, and anti-Abeta biologics. Tangles are composed of paired helical filaments of hyperphosphorylated tau protein. Tau-based therapeutics include kinase inhibitors, microtubule stabilizers, and catabolism inducers. Therapeutic strategies for neurodegeneration target multiple mechanisms, including excitotoxicity, mitochondrial dysfunction, oxidative damage, and inflammation or stimulation of neuronal viability. Although not disease modifying, cognition enhancers are important to treat the symptom of dementia. Strategies for cognition enhancement include cholinesterase inhibitors, and other approaches to enhance the signaling of cholinergic and glutamatergic neurons. In summary, plaques, tangles, neurodegeneration and dementia guide the development of multiple therapeutic approaches for AD and are the subject of this review.     

Murine models of Alzheimer's disease and their use in developing immunotherapies

Alzheimer's disease (AD) is one of the categories of neurodegenerative diseases characterized by a conformational change of a normal protein into a pathological conformer with a high β-sheet content that renders it resistant to degradation and neurotoxic. In AD, the normal soluble amyloid β (sAβ) peptide is converted into oligomeric/fibrillar Aβ. The oligomeric forms of Aβ are thought to be the most toxic, while fibrillar Aβ becomes deposited as amyloid plaques and congophilic angiopathy, which both serve as neuropathological markers of the disease. An additional important feature of AD is the accumulation of abnormally phosphorylated tau as soluble toxic oligomers and as neurofibrillary tangles. Many therapeutic interventions are under investigation to prevent and treat AD. The testing of these diverse approaches to ameliorate AD pathology has been made possible by the existence of numerous transgenic mouse models which each mirror specific aspects of AD pathology. None of the current murine models is a perfect match of the human disease. Perhaps the most exciting of the therapeutic approaches being developed is immunomodulation targeting the aggregating proteins, Aβ and tau. This type of AD therapy is currently being assessed in many transgenic mouse models, and promising findings have led to clinical trials. However, there is a discrepancy between results in murine models and ongoing clinical trials, which highlight the limitations of these models and also of our understanding of the underlying etiology and pathogenesis of AD. Because of these uncertainties, Tg models for AD are continuously being refined with the aim to better understand the disease and to enhance the predictive validity of potential treatments such as immunotherapies.     

PKR knockout in the 5xFAD model of Alzheimer's disease reveals beneficial effects on spatial memory and brain lesions

Brain lesions in Alzheimer's disease (AD) include amyloid plaques made of Aβ peptides and neurofibrillary tangles composed of hyperphosphorylated tau protein with synaptic and neuronal loss and neuroinflammation. Aβ oligomers can trigger tau phosphorylation and neuronal alterations through activation of neuronal kinases leading to progressive cognitive decline. PKR is a ubiquitous pro‐apoptotic serine/threonine kinase, and levels of activated PKR are increased in AD brains and AD CSF. In addition, PKR regulates negatively memory formation in mice. To assess the role of PKR in an AD in vivo model, we crossed 5xFAD transgenic mice with PKR knockout (PKRKO) mice and we explored the contribution of PKR on cognition and brain lesions in the 5xFAD mouse model of AD as well as in neuron–microglia co‐cultures exposed to the innate immunity activator lipopolysaccharide (LPS). Nine‐month‐old double‐mutant mice revealed significantly improved memory consolidation with the new object location test, starmaze test, and elevated plus maze test as compared to 5xFAD mice. Brain amyloid accumulation and BACE1 levels were statistically decreased in double‐mutant mice. Apoptosis, neurodegeneration markers, and synaptic alterations were significantly reduced in double‐mutant mice as well as neuroinflammation markers such as microglial load and brain cytokine levels. Using cocultures, we found that PKR in neurons was essential for LPS microglia‐induced neuronal death. Our results demonstrate the clear involvement of PKR in abnormal spatial memory and brain lesions in the 5xFAD model and underline its interest as a target for neuroprotection in AD.     

Bone marrow-derived microglia play a critical role in restricting senile plaque formation in Alzheimer's disease

Microglia are the immune cells of the brain. Here we show a massive infiltration of highly ramified and elongated microglia within the core of amyloid plaques in transgenic mouse models of Alzheimer's disease (AD). Many of these cells originate from the bone marrow, and the beta-amyloid-40 and -42 isoforms are able to trigger this chemoattraction. These newly recruited cells also exhibit a specific immune reaction to both exogenous and endogenous beta-amyloid in the brain. Creation of a new AD transgenic mouse that expresses the thymidine kinase protein under the control of the CD11b promoter allowed us to show that blood-derived microglia and not their resident counterparts have the ability to eliminate amyloid deposits by a cell-specific phagocytic mechanism. These bone marrow-derived microglia are thus very efficient in restricting amyloid deposits. Therapeutic strategies aiming to improve their recruitment could potentially lead to a new powerful tool for the elimination of toxic senile plaques.     

Estrogen enhances uptake of amyloid beta-protein by microglia derived from the human cortex

In recent years, inflammatory mechanisms have been increasingly appreciated as important steps in the pathology of Alzheimer's disease (AD). There are two pathological defects in AD: chronic inflammation and impaired clearance of amyloid beta-peptide (Abeta). In the periphery, estrogen both increases macrophage phagocytosis and has antiinflammatory effects. If estrogen had a similar effect in the CNS, it could reverse inflammatory defects in AD. Although microglia are a key component of the immune system and help clear Abeta deposits in the AD brain, little is known about the effects of estrogen on CNS microglia. Therefore, we sought to determine the relationship between estrogen treatment and internalization of Abeta by microglia by quantifying the internalization of aggregated Abeta by human cortical microglia. Abeta uptake was found to be dose- and time-dependent in cultured microglia. Increased Abeta uptake was observed at 1.5 and 24 h after addition of aggregated Abeta (50, 100, or 1,000 nM: Abeta), and this uptake was enhanced by pretreatment with estrogen. The expression of estrogen receptor (ER) beta (ER-beta) was also up-regulated by estrogen treatment. Cells cotreated with ICI 182,780, an ER antagonist, showed significantly reduced internalization of Abeta in cultured microglia. These results indicate that microglia express an ER-beta but that the effect of estrogen on enhancing clearance of Abeta may be related to the receptor-independent action of estrogen or to nonclassical ER effects of estrogen. Thus, stimulation of the ER might contribute to the therapeutic action of estrogen in the treatment of AD.     

Mutant presenilin‐1 deregulated peripheral immunity exacerbates Alzheimer‐like pathology

Mutations in the presenilin‐1 (PS1) gene are independent causes of familial Alzheimer's disease (AD). AD patients have dysregulated immunity, and PS1 mutant mice exhibit abnormal systemic immune responses. To test whether immune function abnormality caused by a mutant human PS1 gene (mhPS1) could modify AD‐like pathology, we reconstituted immune systems of AD model mice carrying a mutant human amyloid precursor protein gene (mhAPP; Tg2576 mice) or both mhAPP and mhPS1 genes (PSAPP mice) with allo‐geneic bone marrow cells. Here, we report a marked reduction in amyloid‐β (Aβ) levels, β‐amyloid plaques and brain inflammatory responses in PSAPP mice following strain‐matched wild‐type PS1 bone marrow reconstitution. These effects occurred with immune switching from pro‐inflammatory T helper (Th) 1 to anti‐inflammatory Th2 immune responses in the periphery and in the brain, which likely instructed microglia to phagocytose and clear Aβ in an ex vivo assay. Conversely, Tg2576 mice displayed accelerated AD‐like pathology when reconstituted with mhPS1 bone marrow. These data show that haematopoietic cells bearing the mhPS1 transgene exacerbate AD‐like pathology, suggesting a novel therapeutic strategy for AD based on targeting PS1 in peripheral immune cells.     

Maresin 1 attenuates pro-inflammatory activation induced by β-amyloid and stimulates its uptake

Alzheimer's disease (AD) is the most common dementia, characterized by pathological accumulation of β-amyloid (Aβ) and hyperphosphorylation of tau protein, together with a damaging chronic inflammation. The lack of effective treatments urgently warrants new therapeutic strategies. Resolution of inflammation, associated with beneficial and regenerative activities, is mediated by specialized pro-resolving lipid mediators (SPMs) including maresin 1 (MaR1). Decreased levels of MaR1 have been observed in AD brains. However, the pro-resolving role of MaR1 in AD has not been fully investigated. In the present study, human monocyte-derived microglia (MdM) and a differentiated human monocyte cell line (THP-1 cells) exposed to Aβ were used as models of AD neuroinflammation. We have studied the potential of MaR1 to inhibit pro-inflammatory activation of Aβ and assessed its ability to stimulate phagocytosis of Aβ₄₂. MaR1 inhibited the Aβ₄₂-induced increase in cytokine secretion and stimulated the uptake of Aβ₄₂ in both MdM and differentiated THP-1 cells. MaR1 was also found to decrease chemokine secretion and reduce the associated increase in the activation marker CD40. Activation of kinases involved in transduction of inflammation was not affected by MaR1, but the activity of nuclear factor (NF)-κB was decreased. Our data show that MaR1 exerts effects that indicate a pro-resolving role in the context of AD and thus presents itself as a potential therapeutic target for AD.