Alzheimer's disease (AD) affects women more frequently and more severely than men, but the biological mechanisms underlying these sex differences remain poorly understood. This review integrates recent findings from neuroscience, immunology, endocrinology, and genetics to explore how sex steroid hormones, particularly estrogen, shape neuroimmune responses and influence AD risk. We highlight the pivotal roles of microglia and astrocytes, whose inflammatory and neuroprotective actions are modulated by hormonal fluctuations across the female lifespan, including pregnancy, menopause, and menopausal hormone replacement therapy. Key genetic risk factors, such as apolipoprotein E ε4, show sex-specific effects on glial activation, tau pathology, and cognitive decline. Furthermore, life-stage transitions, especially menopause, intersect with changes in brain metabolism, immune signaling, and epigenetic regulation, increasing susceptibility to neurodegeneration in women. We propose a framework for sex-aware, personalized approaches to AD prevention and treatment. By integrating hormone-immune interactions with genetic and glial biology, this review emphasizes the critical need for sex-specific models in AD research. Highlights:Women develop greater tauopathy, with more cognitive and clinical consequences in Alzheimer's disease (AD).Glial activation is adapted by estrogens to shape vulnerability or resilience to AD.Sex differences in innate and adaptive immunity could contribute to AD progression.Effects of menopausal hormone therapy on immunity in AD remain understudied.Future studies to explore sex differences in immune function during AD are needed.

We have previously reported that APP+PS1 mice can enhance tauopathy when injected with a virus over-expressing P301L tau. Here we compare the extent of tauopathy produced by the aggregation prone human P301L tau with wild type tau in the presence of mature amyloidosis. Ten-month-old transgenic mice over-expressing mutant amyloid precursor protein and presenilin 1 (APP+PS1) were injected intravenously with AAV.Cap-B10 over-expressing either wild-type (AAV-WT) or P301L (AAV-P301L) 4R2N human tau. Half of the mice were behaviorally assessed four months post-injection and euthanized five months post-injection. The remaining mice were euthanized nine months post-injection. Pathology was assessed by ELISA and immunohistochemistry. By ELISA, measures of soluble total tau showed reduced levels in mice injected with AAV-WT compared with AAV-P301L at both five- and nine-month survival times. However, in both cohorts, there was significantly more soluble pS396 tau in AAV-WT mice relative to AAV-P301L, while soluble pS199 tau was significantly increased in AAV-WT mice in only the younger cohort. Measures of insoluble total tau were reduced in AAV-WT compared to AAV-P301L mice, yet insoluble phospho-tau levels were similar in both cohorts. Similarly, histological measures of total tau (HT7) show reduced levels in mice injected with AAV-WT relative to AAV-P301L in both the cortex and hippocampus. However, we observed increased area staining of AT8-positive phospho-tau in mice injected with AAV-WT compared with AAV-P301L. In agreement with our protein measures, we observed significantly less human tau mRNA in mice injected with AAV-WT compared to AAV-P301L. ELISA measures of amyloid pathology were not significantly different between groups. Both AAV-WT and AAV-P301L mice displayed similar learning impairments compared to nontransgenic mice. Over-expression of both wild-type and P301L tau induced learning and memory deficits in middle-aged APP+PS1 mice with mature amyloid pathology. While we observed less total tau in mice injected with AAV-WT than AAV-P301L, a greater fraction of the total available pool of tau was phosphorylated in mice over-expressing wild-type tau. These data suggest that although mutant P301L tau may be more prone to aggregation in vitro , WT-tau is more prone to phosphorylation in vivo in the presence of amyloidosis.

Aging is the primary risk factor for Alzheimer's disease. With aging, senescent cells accumulate within the body, including the brain. Prior studies have identified a broad association between increased levels of cellular senescence and Alzheimer's disease-like pathology. However, the descriptive literature cannot resolve whether senescent cells result from the pathology or contribute to it. Here, we over-express the gene products of Cdkn2a , a regulator of cellular senescence, in astrocytes of both nontransgenic mice and the PS19 mouse model of tauopathy. We developed astrocyte-specific adeno-associated virus (AAV) serotype 9 vectors expressing both transcripts of the Cdkn2a gene under control of the GFAP promoter. We injected these vectors bilaterally in the anterior cortex and hippocampus of four-month-old nontransgenic (NonTg) or PS19 tauopathy mice with AAV-GFP serving as a control treatment. Mice survived four months before being behaviorally assessed and euthanized. We observed a significant effect of tauopathy on several affective behavioral tasks in mice (open field, elevated plus maze, marble burying, and nest building), but this was not altered by over-expression of Cdkn2a . Surprisingly, we did observe a significant improvement in pre-pulse inhibition of the acoustic startle response in PS19 mice over-expressing Cdkn2a in astrocytes. Finally, we did not observe significant effects of Cdkn2a over-expression on hippocampal dependent learning and memory as measured by the radial arm water maze. At tissue collection, we did not observe significant effects of Cdkn2a over-expression on brain mass but there was a significant main effect of genotype on body mass with PS19 mice having lower body mass than NonTg mice. We are presently measuring markers of senescence (dsDNA breaks and p21 expression), gliosis, tauopathy, and neurodegeneration. These data indicate selective astrocytic over-expression of both Cdkn2a transcripts has minimal effects on the behavior of NonTg mice. Over-expression of both Cdkn2a transcripts in PS19 mice improved pre-pulse inhibition but had no impact on other affective behavioral tasks or learning behavioral tasks.

A recent study of familial Alzheimer’s disease identified a mutation in the RELN gene that appeared to delay the onset of dementia. It was hypothesized that this RELN-COLBOS variant protected against dementia by enhanced signaling at reelin receptors. We previously developed a secreted, bio-active reelin fragment (R36) and packaged it into AAV. Recently, we found that intravenous AAV-tau delivered in capsids (AAV.B10) that cross the blood brain barrier led to increased tauopathy in APP+PS1 (A+P) transgenic mice compared to nontransgenic (NonTg) littermates. Nine-month old mice were intravenously injected with AAV.B10 expressing P301L 2N4R human tau to produce A+P TAU and NonTg TAU . Concurrently, these mice were injected intracerebroventricularly (ICV) with AAV9 expressing either R36 or green fluorescent protein (GFP). Learning and memory was assessed by radial arm water maze (RAWM) four months after surgery. Untreated A+P and NonTg mice were included as controls. RIPA soluble and insoluble hippocampal fractions were assayed for total tau, phospho-tau, Aβ40, Aβ42, and oligomeric Aβ by ELISA. All NonTg mice, regardless of tau treatment, learned and correctly recalled the RAWM platform location. Recall in untreated A+P mice was slightly impaired. A+P TAU mice given ICV-GFP performed at chance levels on all trials. In contrast, A+P TAU mice given ICV-R36 performed significantly better than ICV-GFP A+P TAU mice, but not as well as untreated A+P mice, indicating a partial rescue of the tau-associated memory deficit by ICV-R36. Tauopathy, as measured by ELISA, was unchanged by R36 over-expression, but insoluble Aβ42 was reduced in ICV-R36 A+P TAU mice compared to ICV-GFP A+P TAU mice. We conclude that ICV injections of AAV expressing a bio-active fragment of reelin can partially protect against cognitive impairment in a mouse model of amyloid induced tauopathy. This is consistent with a potential protective role of reelin signaling in Alzheimer’s disease. While we did not observe an effect of R36 over-expression on tau pathology, we do demonstrate a significant reduction of insoluble Aβ42. We continue to investigate additional chemical and histological markers in these mice to better understand the protective influence of reelin.

Prior work suggests that the cytokine interleukin-1β (IL-1β) may be a key regulator of tau pathology in the presence of amyloidosis. Here, we tested the possible benefits of interleukin-1 receptor antagonist (IL-1RA) gene therapy in two mouse models of tauopathy. We performed intracranial injections in the rTg4510 model, achieving approximately 300-fold over-expression in the hippocampus, and systemic injections in the PS19 model, resulting in approximately 10-fold over-expression. In neither model did we find substantial treatment effects with IL-1RA over-expression. We found large increases in Il1b gene expression in these mouse models, but considerably smaller increases in IL-1β protein. These data suggest that interleukin-1 receptor antagonist may not be a viable therapeutic strategy for pure tauopathies but cannot rule out possible benefits in amyloid-enhanced tauopathy, which appear to have larger elevations of IL-1β.

AbstractINTRODUCTIONAlzheimer's disease (AD) is a progressive neurodegenerative disease in which extracellular aggregates of the amyloid beta (Aβ) peptide precede widespread intracellular inclusions of the microtubule‐associated protein tau. The autosomal dominant form of AD requires mutations that increase production or aggregation of the Aβ peptide. This has led to the hypothesis that amyloid deposition initiates downstream responses that lead to the hyperphosphorylation and aggregation of tau.METHODSHere we use a novel approach, somatic gene transfer via intravenous adeno‐associated virus (AAV), to further explore the effects of pre‐existing amyloid deposits on tauopathy. APP+PS1 mice, which develop amyloid deposits at 3 to 6 months of age, and non‐transgenic littermates were injected at 8 months of age intravenously with AAV‐PHP.eB encoding P301L human tau. Tissue was collected at 13 months and tauopathy was assessed.RESULTSTotal human tau expression was observed to be relatively uniform throughout the brain, reflecting the vascular route of AAV administration. Phospho‐tau deposition was not equal across brain regions and significantly increased in APP+PS1 mice compared to non‐transgenic controls. Interestingly, the rank order of phospho‐tau deposition of affected brain regions in both genotypes paralleled the rank order of amyloid plaque deposits in APP+PS1 mice. We also observed significantly increased MAPT RNA expression in APP+PS1 mice compared to non‐transgenic despite equal AAV transduction efficiency between groups.DISCUSSIONThis model has advantages over prior approaches with widespread uniform human tau expression throughout the brain and the ability to specify the stage of amyloidosis when the tau pathology is initiated. These data add further support to the amyloid cascade hypothesis and suggest RNA metabolism as a potential mechanism for amyloid‐induced tauopathy.

Animal models of tauopathy help in understanding the role of mutations in tau pathobiology. Here, we used adeno-associated viral (AAV) vectors to administer three tau genetic variants (tauwild-type, tauP301L, and tauR406W) intracranially into 12-month-old C57BL/6Nia mice and collected tissue at 16 months. Vectors designed to express green fluorescent protein controlled for surgical procedures and exogenous protein expression by AAV. The tau genetic variants produced considerably different phenotypes. Tauwild-type and tauP301L caused memory impairments. The tauP301L caused increased amounts of aggregated tau, measured both neurochemically and histologically. Tauwild-type produced elevated levels of soluble tau and phosphorylated tau by ELISA and increased staining for phosphorylated forms of tau histologically. However, only the tauwild-type caused localized atrophy of brain tissue at the sites near the injection. The tauR406W had low protein expression and produced no atrophy or memory impairments. This supports the potential use of AAV expressing tauwild-type in aged mice to examine events leading to neurodegeneration in Alzheimer's disease pathology.

The limitation of cancer radiotherapy does not derive from an inability to ablate tumor, but rather to do so without excessively damaging critical tissues and organs and adversely affecting patient’s quality of life. Although cellular senescence is a normal consequence of aging, there is increasing evidence showing that the radiation-induced senescence in both tumor and adjacent normal tissues contributes to tumor recurrence, metastasis, and resistance to therapy, while chronic senescent cells in the normal tissue and organ are a source of many late damaging effects. In this review, we discuss how to identify cellular senescence using various bio-markers and the role of the so-called senescence-associated secretory phenotype characteristics on the pathogenesis of the radiation-induced late effects. We also discuss therapeutic options to eliminate cellular senescence using either senolytics and/or senostatics. Finally, a discussion of cellular reprogramming is presented, another promising avenue to improve the therapeutic gain of radiotherapy.

Aging is the primary risk factor for Alzheimer’s disease (AD). With aging, senescent cells accumulate in the body and the brain. Cells that become senescent cease contributing to homeostasis but do not typically undergo apoptosis. Prior studies have identified a broad association between increased cellular senescence and AD-like pathology and have identified senescent cells in AD patient brain and mouse models of AD-like pathology. However, contributions of senescent cells to disease progression remains uncertain. The two gene products of the senescence regulator Cdkn2a (p16-Ink4a and p19-Arf) were overexpressed using intracranial delivery of adeno-associated viral vectors (AAV). Mice were injected with 2 µL of 1×10 13 , 5×10 12 , or 1×10 12 vg/mL of AAV-Arf and tissue was collected two months post-injection. Control mice were injected with AAV empty capsid. Brain weights, NeuN IHC, and p19-Arf mRNA were assessed. A second cohort of mice was injected with AAV-Ink, AAV-Arf, AAV-Ink & AAV-Arf (co-injection), or AAV-GFP and followed for four months post-injection. We assessed body mass, brain mass, NeuN IHC, and expression of senescence-induced genes by PCR-array and qRT-PCR. A third experiment over-expressing p19-Arf in PS19 mice is ongoing. We observed that animals over-expressing p19-Arf failed to gain body mass up to four months post-injection. Post-mortem, we found that over-expression of p19-Arf is toxic to neurons, reducing gross brain mass and NeuN area staining. Furthermore, a greater number of differentially regulated genes were observed in mice over-expressing p19-Arf than in mice over-expressing p16-Ink4a. We confirmed increased expression of the senescence-regulator p21 in mice over-expressing p19-Arf, but not p16-Ink4a, by qPCR. In a separate experiment, we replicated the finding of neurotoxicity induced by over-expression of p19-Arf at only two months post-injection and determined the optimal viral titer to administer that induces significant over-expression with minimal neurotoxicity for use in future studies. As observed in cultured cells, over-expression of Cdkn2a seems to increase markers of senescence. Effects on tau pathology will be examined in the PS19 mouse model of tauopathy. Cell type-specific expression in neurons, astroglia, and microglia will be used to elucidate cell type specific contributions to senescence-induced disease progression.

Background: Advanced age is the greatest risk factor for the development of Alzheimer’s disease (AD). This implies that some aspect of the aged milieu is possibly accelerating the development of AD related pathologies. Objective: We hypothesized that intracranially injected with AAV9 tauP301L may cause a greater degree of pathology in old versus young mice. Methods: Animals were injected with viral vectors overexpressing the mutant tauP301L or control protein (green fluorescent protein, GFP) into the brains of mature, middle-aged, and old C57BL/6Nia mice. The tauopathy phenotype was monitored four months after injection using behavioral, histological, and neurochemical measures. Results: Phosphorylated-tau immunostaining (AT8) or Gallyas staining of aggregated tau increased with age, but other measures of tau accumulation were not significantly affected. Overall, AAV-tau injected mice had impaired radial arm water maze performance, increased microglial activation, and showed evidence of hippocampal atrophy. Aging impaired open field and rotarod performance in both AAV-tau and control mice. The efficiency of viral transduction and gene expression were the same at all animal ages. Conclusion: We conclude that tauP301L over expression results in a tauopathy phenotype with memory impairment and accumulation of aggregated tau. However, the effects of aging on this phenotype are modest and not detected by some markers of tau accumulation, similar to prior work on this topic. Thus, although age does influence the development of tauopathy, it is likely that other factors, such as ability to compensate for tau pathology, are more responsible for the increased risk of AD with advanced age.

Aging, familial gene mutations, and genetic, environmental, and modifiable lifestyle risk factors predispose individuals to cognitive impairment or dementia by influencing the efficacy of multiple, often interdependent cellular and molecular homeostatic pathways mediating neuronal, glial, and vascular integrity and, ultimately, cognitive status. This review summarizes data from foundational and recent breakthrough studies to highlight common and differential vascular and nonvascular pathogenic mechanisms underlying the progression of Alzheimer disease, vascular dementia, frontotemporal dementia, and dementia with Lewy bodies.

EDITORIAL article Front. Neurosci., 03 August 2022Sec. Neurodegeneration https://doi.org/10.3389/fnins.2022.995409

Mechanisms of Alzheimer's disease (AD) and its putative prodromal stage, amnestic mild cognitive impairment (aMCI), involve the dysregulation of multiple candidate molecular pathways that drive selective cellular vulnerability in cognitive brain regions. However, the spatiotemporal overlap of markers for pathway dysregulation in different brain regions and cell types presents a challenge for pinpointing causal versus epiphenomenal changes characterizing disease progression. To approach this problem, we performed Weighted Gene Co-expression Network Analysis and STRING interactome analysis of gene expression patterns quantified in frontal cortex samples (Brodmann area 10) from subjects who died with a clinical diagnosis of no cognitive impairment, aMCI, or mild/moderate AD. Frontal cortex was chosen due to the relatively protracted involvement of this region in AD, which might reveal pathways associated with disease onset. A co-expressed network correlating with clinical diagnosis was functionally associated with insulin signaling, with insulin (INS) being the most highly connected gene within the network. Co-expressed networks correlating with neuropathological diagnostic criteria (e.g., NIA-Reagan Likelihood of AD) were associated with platelet-endothelium-leucocyte cell adhesion pathways and hypoxia-oxidative stress. Dysregulation of these functional pathways may represent incipient alterations impacting disease progression and the clinical presentation of aMCI and AD.

Studies on the effective and safe therapeutic dosage of delta-9-tetrahydrocannabinol (THC) for the treatment of Alzheimer's disease (AD) have been sparse due to the concern about THC's psychotropic activity. The present study focused on demonstrating the beneficial effect of low-dose THC treatment in preclinical AD models. The effect of THC on amyloid-β (Aβ) production was examined in N2a/AβPPswe cells. An in vivo study was conducted in aged APP/PS1 transgenic mice that received an intraperitoneal injection of THC at 0.02 and 0.2 mg/kg every other day for three months. The in vitro study showed that THC inhibited Aβ aggregation within a safe dose range. Results of the radial arm water maze (RAWM) test demonstrated that treatment with 0.02 and 0.2 mg/kg of THC for three months significantly improved the spatial learning performance of aged APP/PS1 mice in a dose-dependent manner. Results of protein analyses revealed that low-dose THC treatment significantly decreased the expression of Aβ oligomers, phospho-tau and total tau, and increased the expression of Aβ monomers and phospho-GSK-3β (Ser9) in the THC-treated brain tissues. In conclusion, treatment with THC at 0.2 and 0.02 mg/kg improved the spatial learning of aged APP/PS1 mice, suggesting low-dose THC is a safe and effective treatment for AD.

THC has been used as a promising treatment approach for neurological disorders, but the highly psychoactive effects have largely warned off many scientists from pursuing it further. We conducted an intranasal treatment using low-dose THC on 12-month-old APP/PS1 mice daily for 3 months to overcome any potential psychoactive response induced by the systemic delivery. Our results demonstrate that the THC nasal treatment at 0.002 and 0.02 mg/kg significantly slowed the memory decline compared to that in the vehicle-treated transgenic mouse control group. An enzyme-linked immunosorbent assay showed that the Aβ1–40 and 1–42 peptides decreased in the THC-treated groups. The Western blot data indicate that long-term low-dose THC intranasal administration promoted p-tau level reduction and mitochondrial function marker redistribution. The blood biochemical parameter data demonstrate some insignificant changes in cytokine, immunoglobulin, and immune cell profiles during intranasal THC treatment. Intranasal delivery is a non-invasive and convenient method that rapidly targets therapeutics to the brain, minimizing systemic exposure to avoid unwanted adverse effects. Our study provides new insights into the role of low-dose THC intranasal treatment as a pharmacological strategy to counteract alterations in Alzheimer’s disease-related cognitive performance.

Fractalkine (FKN) is a membrane‐bound chemokine that can be cleaved by proteases such as ADAM 10, ADAM 17, and cathepsin S to generate soluble fragments. Studies using different forms of the soluble FKN yield conflicting results in vivo. These observations prompted us to investigate the function and pharmacology of two commonly used isoforms of FKN, a human full‐length soluble FKN (sFKN), and a human chemokine domain only FKN (cdFKN). Both are prevalent in the literature and are often assumed to be functionally equivalent. We observed that recombinant sFKN and cdFKN exhibit similar potencies in a cell‐based cAMP assay, but binding affinity for CX3CR1 was modestly different. There was a 10‐fold difference in potency between sFKN and cdFKN when assessing their ability to stimulate β‐arrestin recruitment. Interestingly, high concentrations of FKN, regardless of cleavage variant, were ineffective at reducing pro‐inflammatory microglial activation and may induce a pro‐inflammatory response. This effect was observed in mouse and rat primary microglial cells as well as microglial cell lines. The inflammatory response was exacerbated in aged microglia, which is known to exhibit age‐related inflammatory phenotypes. We observed the same effects in Cx3cr1−/− primary microglia and therefore speculate that an alternative FKN receptor may exist. Collectively, these data provide greater insights into the function and pharmacology of these common FKN reagents, which may clarify conflicting reports and urge greater caution in the selection of FKN peptides for use in in vitro and in vivo studies and the interpretation of results obtained using these differing peptides.

COVID-19 causes lasting neurological symptoms in some survivors. Like other infections, COVID-19 may increase risk of cognitive impairment. This perspective highlights four knowledge gaps about COVID-19 that need to be filled to avoid this possible health issue. The first is the need to identify the COVID-19 symptoms, genetic polymorphisms and treatment decisions associated with risk of cognitive impairment. The second is the absence of model systems in which to test hypotheses relating infection to cognition. The third is the need for consortia for studying both existing and new longitudinal cohorts in which to monitor long term consequences of COVID-19 infection. A final knowledge gap discussed is the impact of the isolation and lack of social services brought about by quarantine/lockdowns on people living with dementia and their caregivers. Research into these areas may lead to interventions that reduce the overall risk of cognitive decline for COVID-19 survivors.

Alzheimer’s disease (AD) includes several hallmarks comprised of amyloid- β (Aβ) deposition, tau neuropathology, inflammation, and memory impairment. Brain metabolism becomes uncoupled due to aging and other AD risk factors, which ultimately lead to impaired protein clearance and aggregation. Increasing evidence indicates a role of arginine metabolism in AD, where arginases are key enzymes in neurons and glia capable of depleting arginine and producing ornithine and polyamines. However, currently, it remains unknown if the reduction of arginase 1 ( Arg1) in myeloid cell impacts amyloidosis. Herein, we produced haploinsufficiency of Arg1 by the hemizygous deletion in myeloid cells using Arg1 fl/fl and LysMcre Tg/+ mice crossed with APP Tg2576 mice. Our data indicated that Arg1 haploinsufficiency promoted Aβ deposition, exacerbated some behavioral impairment, and decreased components of Ragulator-Rag complex involved in mechanistic target of rapamycin complex 1 (mTORC1) signaling and autophagy. Additionally, Arg1 repression and arginine supplementation both impaired microglial phagocytosis in vitro . These data suggest that proper function of Arg1 and arginine metabolism in myeloid cells remains essential to restrict amyloidosis.

Brain myeloid cells, include infiltrating macrophages and resident microglia, play an essential role in responding to and inducing neurodegenerative diseases, such as Alzheimer’s disease (AD). Genome-wide association studies (GWAS) implicate many AD casual and risk genes enriched in brain myeloid cells. Coordinated arginine metabolism through arginase 1 ( Arg1 ) is critical for brain myeloid cells to perform biological functions, whereas dysregulated arginine metabolism disrupts them. Altered arginine metabolism is proposed as a new biomarker pathway for AD. We previously reported Arg1 deficiency in myeloid biased cells using lysozyme M (LysM) promoter-driven deletion worsened amyloidosis-related neuropathology and behavioral impairment. However, it remains unclear how Arg1 deficiency in these cells impacts the whole brain to promote amyloidosis. Herein, we aim to determine how Arg1 deficiency driven by LysM restriction during amyloidosis affects fundamental neurodegenerative pathways at the transcriptome level. By applying several bioinformatic tools and analyses, we found that amyloid-β (Aβ) stimulated transcriptomic signatures in autophagy-related pathways and myeloid cells’ inflammatory response. At the same time, myeloid Arg1 deficiency during amyloidosis promoted gene signatures of lipid metabolism, myelination, and migration of myeloid cells. Focusing on Aβ associated glial transcriptomic signatures, we found myeloid Arg1 deficiency up-regulated glial gene transcripts that positively correlated with Aβ plaque burden. We also observed that Aβ preferentially activated disease-associated microglial signatures to increase phagocytic response, whereas myeloid Arg1 deficiency selectively promoted homeostatic microglial signature that is non-phagocytic. These transcriptomic findings suggest a critical role for proper Arg1 function during normal and pathological challenges associated with amyloidosis. Furthermore, understanding pathways that govern Arg1 metabolism may provide new therapeutic opportunities to rebalance immune function and improve microglia/macrophage fitness.

The morphological plasticity of microglia has fascinated neuroscientists for 100 years. Attempts to classify functional phenotypes are hampered by similarities between endogenous brain microglia and peripheral myeloid cells that can enter the brain under pathological conditions. Recent advances in single-cell -omic methodologies have led to an explosion of data regarding gene expression in microglia. Herein, we review the diversity of microglial phenotypes in healthy brains, aging, and Alzheimer's disease (AD); identify knowledge gaps in the body of evidence; and suggest areas in which new knowledge would be useful. Data from human samples and mouse models are compared and contrasted. Understanding the molecular complexity of the microglial response repertoire will suggest new avenues for therapeutic treatments in AD.