Translational approaches to understanding resilience to Alzheimer\u27s disease.

Individuals who maintain cognitive function despite high levels of Alzheimer\u27s disease (AD)-associated pathology are said to be \u27resilient\u27 to AD. Identifying mechanisms underlying resilience represents an exciting therapeutic opportunity. Human studies have identified a number of molecular and genetic factors associated with resilience, but the complexity of these cohorts prohibits a complete understanding of which factors are causal or simply correlated with resilience. Genetically and phenotypically diverse mouse models of AD provide new and translationally relevant opportunities to identify and prioritize new resilience mechanisms for further cross-species investigation. This review will discuss insights into resilience gained from both human and animal studies and highlight future approaches that may help translate these insights into therapeutics designed to prevent or delay AD-related dementia

Differential effects of partial and complete loss of TREM2 on microglial injury response and tauopathy.

Alzheimer's disease (AD), the most common form of dementia, is characterized by the abnormal accumulation of amyloid plaques and hyperphosphorylated tau aggregates, as well as microgliosis. Hemizygous missense variants in Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) are associated with elevated risk for developing late-onset AD. These variants are hypothesized to result in loss of function, mimicking TREM2 haploinsufficiency. However, the consequences of TREM2 haploinsufficiency on tau pathology and microglial function remain unknown. We report the effects of partial and complete loss of TREM2 on microglial function and tau-associated deficits. In vivo imaging revealed that microglia from aged TREM2-haploinsufficient mice show a greater impairment in their injury response compared with microglia from aged TREM2-KO mice. In transgenic mice expressing mutant human tau, TREM2 haploinsufficiency, but not complete loss of TREM2, increased tau pathology. In addition, whereas complete TREM2 deficiency protected against tau-mediated microglial activation and atrophy, TREM2 haploinsufficiency elevated expression of proinflammatory markers and exacerbated atrophy at a late stage of disease. The differential effects of partial and complete loss of TREM2 on microglial function and tau pathology provide important insights into the critical role of TREM2 in AD pathogenesis

Leveraging genetic diversity in mice to inform individual differences in brain microstructure and memory.

In human Alzheimer\u27s disease (AD) patients and AD mouse models, both differential pre-disease brain features and differential disease-associated memory decline are observed, suggesting that certain neurological features may protect against AD-related cognitive decline. The combination of these features is known as brain reserve, and understanding the genetic underpinnings of brain reserve may advance AD treatment in genetically diverse human populations. One potential source of brain reserve is brain microstructure, which is genetically influenced and can be measured with diffusion MRI (dMRI). To investigate variation of dMRI metrics in pre-disease-onset, genetically diverse AD mouse models, we utilized a population of genetically distinct AD mice produced by crossing the 5XFAD transgenic mouse model of AD to 3 inbred strains (C57BL/6J, DBA/2J, FVB/NJ) and two wild-derived strains (CAST/EiJ, WSB/EiJ). At 3 months of age, these mice underwent diffusion magnetic resonance imaging (dMRI) to probe neural microanatomy in 83 regions of interest (ROIs). At 5 months of age, these mice underwent contextual fear conditioning (CFC). Strain had a significant effect on dMRI measures in most ROIs tested, while far fewer effects of sex, sex*strain interactions, or strain*sex*5XFAD genotype interactions were observed. A main effect of 5XFAD genotype was observed in only 1 ROI, suggesting that the 5XFAD transgene does not strongly disrupt neural development or microstructure of mice in early adulthood. Strain also explained the most variance in mouse baseline motor activity and long-term fear memory. Additionally, significant effects of sex and strain*sex interaction were observed on baseline motor activity, and significant strain*sex and sex*5XFAD genotype interactions were observed on long-term memory. We are the first to study the genetic influences of brain microanatomy in genetically diverse AD mice. Thus, we demonstrated that strain is the primary factor influencing brain microstructure in young adult AD mice and that neural development and early adult microstructure are not strongly altered by the 5XFAD transgene. We also demonstrated that strain, sex, and 5XFAD genotype interact to influence memory in genetically diverse adult mice. Our results support the usefulness of the 5XFAD mouse model and convey strong relationships between natural genetic variation, brain microstructure, and memory

AD-linked R47H-TREM2 mutation induces disease-enhancing microglial states via AKT hyperactivation

The hemizygous R47H variant of triggering receptor expressed on myeloid cells 2 (TREM2), a microglia-specific gene in the brain, increases risk for late-onset Alzheimer’s disease (AD). Using transcriptomic analysis of single nuclei from brain tissues of patients with AD carrying the R47H mutation or the common variant (CV)–TREM2, we found that R47H-associated microglial subpopulations had enhanced inflammatory signatures reminiscent of previously identified disease-associated microglia (DAM) and hyperactivation of AKT, one of the signaling pathways downstream of TREM2. We established a tauopathy mouse model with heterozygous knock-in of the human TREM2 with the R47H mutation or CV and found that R47H induced and exacerbated TAU-mediated spatial memory deficits in female mice. Single-cell transcriptomic analysis of microglia from these mice also revealed transcriptomic changes induced by R47H that had substantial overlaps with R47H microglia in human AD brains, including robust increases in proinflammatory cytokines, activation of AKT signaling, and elevation of a subset of DAM signatures. Pharmacological AKT inhibition with MK-2206 largely reversed the enhanced inflammatory signatures in primary R47H microglia treated with TAU fibrils. In R47H heterozygous tauopathy mice, MK-2206 treatment abolished a tauopathy-dependent microglial subcluster and rescued tauopathy-induced synapse loss. By uncovering disease-enhancing mechanisms of the R47H mutation conserved in human and mouse, our study supports inhibitors of AKT signaling as a microglial modulating strategy to treat AD

Novel multifunctional ligands and their application in Alzheimer’s disease

Alzheimer’s disease (AD) is a fatal, neurodegenerative disorder that is the most common cause of dementia currently affecting over 35 million people. Metal ions Cu(II), Zn(II), and Fe(III) have a dual deleterious role in Alzheimer’s disease through acceleration of aggregation that leads to increased toxicity of the amyloid protein, and participation in catalytic cycles generating reactive species. The therapeutic effect of metal chelating agents is currently explored in clinical studies, as there are currently no drugs for this disease. This thesis investigates the 3-hydroxy-4-pyridinone family of chelating ligands, with attention given to its multifunctional activities. The main goal is to demonstrate that metal and amyloid-binding functionalities are compatible within this scaffold, while other functionalities are retained. Novel chelating agents 1-(benzo[d]oxazol-2-ylmethyl)-3-hydroxy-2-methylpyridin-4(1H)-one (Hmbo2p), 1-(benzo[d]thiazol-2-ylmethyl)-3-hydroxy-2-methylpyridin-4(1H)-one (Hmbt2p), 1-(2-aminobenzo[d]thiazol-6-yl)-3-hydroxy-2-methylpyridin-4(1H)-one (Habt6p), 1-(4-(benzo[d]oxazol-2-yl)phenyl)-3-hydroxy-2-methylpyridin-4(1H)-one (Hpbo2p), and 1-(4-(benzo[d]thiazol-2-yl)phenyl)-3-hydroxy-2-methylpyridin-4(1H)-one (Hpbt2p) were designed, synthesised, and structural characterised. The metal binding of Hmbo2p to Cu(II), Zn(II), and Fe(III) was confirmed with titration studies, while the solid state structures of Hmbt2p were characterised using X-ray crystallography. Amyloid binding functionalities of Hmbo2p were confirmed with microscopy, fluorescence, and binding studies. Moreover, the radical quenching ability of Hmbo2p was established in the absence and presence of Cu(II) by absorbance and fluorescence studies. As well, compounds Hmbo2p, Hmbt2p, and Habt6p were found to be relatively toxic in a mouse endothelial neuronal cell line, while Hpbt2p was visualised to permeate into the cell line. Finally, by installing a carbamoyl functionality, novel compounds 2-methyl-4-oxo-1-phenyl-1,4-dihydropyridin-3-yl dimethylcarbamate (Cppp), 4-(3-hydroxy-2-methyl-4-oxopyridin-1(4H)-yl)phenyl dimethylcarbamate (Chpp), and 4-(((2-methyl-4-oxo-1-phenyl-1,4-dihydropyridin-3-yl)oxy)methyl)phenyl dimethylcarbamate (Cbppp) were synthesised for the purpose of adding an acetylcholinesterase (AChE) inhibitory functionality. The metal chelation site was masked on Cppp and Cbppp; all three inhibited eelAChE reversibly in vitro. The work presented herein demonstrates for the first time that incorporation of benzoxazole group into the hydroxypyridinone scaffold imbues it with amyloid-binding functionality while retaining the metal chelating ability. This insertion and concomitant increase of lipophilicity leads to morphological changes of amyloid protein upon incubation, and potentially results in increased toxicity and cell permeability. Finally, masking the hydroxyl group with a carbamate functionality on the scaffold leads to generation of reversible enzyme inhibitors.Science, Faculty ofChemistry, Department ofGraduat

Endo-lysosomal dysfunction: a converging mechanism in neurodegenerative diseases.

Endo-lysosomal pathways are essential in maintaining protein homeostasis in the cell. Numerous genes in the endo-lysosomal pathways have been found to associate with neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and frontotemporal dementia (FTD). Mutations of these genes lead to dysfunction in multiple steps of the endo-lysosomal network: autophagy, endocytic trafficking and lysosomal degradation, resulting in accumulation of pathogenic proteins. Although the exact pathogenic mechanism varies for different disease-associated genes, dysfunction of the endo-lysosomal pathways represents a converging mechanism shared by these diseases. Therefore, strategies that correct or compensate for endo-lysosomal dysfunction may be promising therapeutic approaches to treat neurodegenerative diseases

Endo-lysosomal dysfunction: a converging mechanism in neurodegenerative diseases.

Endo-lysosomal pathways are essential in maintaining protein homeostasis in the cell. Numerous genes in the endo-lysosomal pathways have been found to associate with neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and frontotemporal dementia (FTD). Mutations of these genes lead to dysfunction in multiple steps of the endo-lysosomal network: autophagy, endocytic trafficking and lysosomal degradation, resulting in accumulation of pathogenic proteins. Although the exact pathogenic mechanism varies for different disease-associated genes, dysfunction of the endo-lysosomal pathways represents a converging mechanism shared by these diseases. Therefore, strategies that correct or compensate for endo-lysosomal dysfunction may be promising therapeutic approaches to treat neurodegenerative diseases

Proximal recolonization by self-renewing microglia re-establishes microglial homeostasis in the adult mouse brain.

Microglia are resident immune cells that play critical roles in maintaining the normal physiology of the central nervous system (CNS). Remarkably, microglia have an intrinsic capacity to repopulate themselves after acute ablation. However, the underlying mechanisms that drive such restoration remain elusive. Here, we characterized microglial repopulation both spatially and temporally following removal via treatment with the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622. We show that microglia were replenished via self-renewal, with no contribution from nonmicroglial lineages, including Nestin+ progenitors and the circulating myeloid population. Interestingly, spatial analyses with dual-color labeling revealed that newborn microglia recolonized the parenchyma by forming distinctive clusters that maintained stable territorial boundaries over time, indicating the proximal expansive nature of adult microgliogenesis and the stability of microglia tiling. Temporal transcriptome profiling at different repopulation stages revealed that adult newborn microglia gradually regain steady-state maturity from an immature state that is reminiscent of the neonatal stage and follow a series of maturation programs, including nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation, interferon immune activation, and apoptosis. Importantly, we show that the restoration of microglial homeostatic density requires NF-κB signaling as well as apoptotic egress of excessive cells. In summary, our study reports key events that take place from microgliogenesis to homeostasis reestablishment