Early Alzheimer's disease pathology in human cortex involves transient cell states.

Cellular perturbations underlying Alzheimer's disease (AD) are primarily studied in human postmortem samples and model organisms. Here, we generated a single-nucleus atlas from a rare cohort of cortical biopsies from living individuals with varying degrees of AD pathology. We next performed a systematic cross-disease and cross-species integrative analysis to identify a set of cell states that are specific to early AD pathology. These changes-which we refer to as the early cortical amyloid response-were prominent in neurons, wherein we identified a transitional hyperactive state preceding the loss of excitatory neurons, which we confirmed by acute slice physiology on independent biopsy specimens. Microglia overexpressing neuroinflammatory-related processes also expanded as AD pathology increased. Finally, both oligodendrocytes and pyramidal neurons upregulated genes associated with ß-amyloid production and processing during this early hyperactive phase. Our integrative analysis provides an organizing framework for targeting circuit dysfunction, neuroinflammation, and amyloid production early in AD pathogenesis.

Exposure of iPSC-derived human microglia to brain substrates enables the generation and manipulation of diverse transcriptional states in vitro.

Microglia, the macrophages of the brain parenchyma, are key players in neurodegenerative diseases such as Alzheimer's disease. These cells adopt distinct transcriptional subtypes known as states. Understanding state function, especially in human microglia, has been elusive owing to a lack of tools to model and manipulate these cells. Here, we developed a platform for modeling human microglia transcriptional states in vitro. We found that exposure of human stem-cell-differentiated microglia to synaptosomes, myelin debris, apoptotic neurons or synthetic amyloid-beta fibrils generated transcriptional diversity that mapped to gene signatures identified in human brain microglia, including disease-associated microglia, a state enriched in neurodegenerative diseases. Using a new lentiviral approach, we demonstrated that the transcription factor MITF drives a disease-associated transcriptional signature and a highly phagocytic state. Together, these tools enable the manipulation and functional interrogation of human microglial states in both homeostatic and disease-relevant contexts.

Immune Signaling in Neurodegeneration.

Neurodegenerative diseases of the central nervous system progressively rob patients of their memory, motor function, and ability to perform daily tasks. Advances in genetics and animal models are beginning to unearth an unexpected role of the immune system in disease onset and pathogenesis; however, the role of cytokines, growth factors, and other immune signaling pathways in disease pathogenesis is still being examined. Here we review recent genetic risk and genome-wide association studies and emerging mechanisms for three key immune pathways implicated in disease, the growth factor TGF-ß, the complement cascade, and the extracellular receptor TREM2. These immune signaling pathways are important under both healthy and neurodegenerative conditions, and recent work has highlighted new functional aspects of their signaling. Finally, we assess future directions for immune-related research in neurodegeneration and potential avenues for immune-related therapies.

Single-Cell RNA Sequencing of Microglia throughout the Mouse Lifespan and in the Injured Brain Reveals Complex Cell-State Changes.

Microglia, the resident immune cells of the brain, rapidly change states in response to their environment, but we lack molecular and functional signatures of different microglial populations. Here, we analyzed the RNA expression patterns of more than 76,000 individual microglia in mice during development, in old age, and after brain injury. Our analysis uncovered at least nine transcriptionally distinct microglial states, which expressed unique sets of genes and were localized in the brain using specific markers. The greatest microglial heterogeneity was found at young ages; however, several states-including chemokine-enriched inflammatory microglia-persisted throughout the lifespan or increased in the aged brain. Multiple reactive microglial subtypes were also found following demyelinating injury in mice, at least one of which was also found in human multiple sclerosis lesions. These distinct microglia signatures can be used to better understand microglia function and to identify and manipulate specific subpopulations in health and disease.

Microglia and Astrocytes in Disease: Dynamic Duo or Partners in Crime?

Microglia-astrocyte interactions represent a delicate balance affecting neural cell functions in health and disease. Tightly controlled to maintain homeostasis during physiological conditions, rapid and prolonged departures during disease, infection, and following trauma drive multiple outcomes: both beneficial and detrimental. Recent sequencing studies at the bulk and single-cell level in humans and rodents provide new insight into microglia-astrocyte communication in homeostasis and disease. However, the complex changing ways these two cell types functionally interact has been a barrier to understanding disease initiation, progression, and disease mechanisms. Single cell sequencing is providing new insights; however, many questions remain. Here, we discuss how to bridge transcriptional states to specific functions so we can develop therapies to mediate negative effects of altered microglia-astrocyte interactions.

APOE and immunity: Research highlights.

INTRODUCTION: At the Alzheimer's Association's APOE and Immunity virtual conference, held in October 2021, leading neuroscience experts shared recent research advances on and inspiring insights into the various roles that both the apolipoprotein E gene (APOE) and facets of immunity play in neurodegenerative diseases, including Alzheimer's disease and other dementias. METHODS: The meeting brought together more than 1200 registered attendees from 62 different countries, representing the realms of academia and industry. RESULTS: During the 4-day meeting, presenters illuminated aspects of the cross-talk between APOE and immunity, with a focus on the roles of microglia, triggering receptor expressed on myeloid cells 2 (TREM2), and components of inflammation (e.g., tumor necrosis factor α [TNFα]). DISCUSSION: This manuscript emphasizes the importance of diversity in current and future research and presents an integrated view of innate immune functions in Alzheimer's disease as well as related promising directions in drug development.

CD11a expression distinguishes infiltrating myeloid cells from plaque-associated microglia in Alzheimer's disease.

Alzheimer's disease (AD) is the leading cause of age-related neurodegeneration and is characterized neuropathologically by the accumulation of insoluble beta-amyloid (Aß) peptides. In AD brains, plaque-associated myeloid (PAM) cells cluster around Aß plaques but fail to effectively clear Aß by phagocytosis. PAM cells were originally thought to be brain-resident microglia. However, several studies have also suggested that Aß-induced inflammation causes peripheral monocytes to enter the otherwise immune-privileged brain. The relationship between AD progression and inflammation in the brain remains ambiguous because microglia and monocyte-derived macrophages are extremely difficult to distinguish from one another in an inflamed brain. Whether PAM cells are microglia, peripheral macrophages, or a mixture of both remains unclear. CD11a is a component of the ß2 integrin LFA1. We have determined that CD11a is highly expressed on peripheral immune cells, including macrophages, but is not expressed by mouse microglia. These expression patterns remain consistent in LPS-treated inflamed mice, as well as in two mouse models of AD. Thus, CD11a can be used as a marker to distinguish murine microglia from infiltrating peripheral immune cells. Using CD11a, we show that PAM cells in AD transgenic brains are comprised entirely of microglia. We also demonstrate a novel fluorescence-assisted quantification technique (FAQT), which reveals a significant increase in T lymphocytes, especially in the brains of female AD mice. Our findings support the notion that microglia are the lead myeloid players in AD and that rejuvenating their phagocytic potential may be an important therapeutic strategy.

The adaptive immune system restrains Alzheimer's disease pathogenesis by modulating microglial function.

The innate immune system is strongly implicated in the pathogenesis of Alzheimer's disease (AD). In contrast, the role of adaptive immunity in AD remains largely unknown. However, numerous clinical trials are testing vaccination strategies for AD, suggesting that T and B cells play a pivotal role in this disease. To test the hypothesis that adaptive immunity influences AD pathogenesis, we generated an immune-deficient AD mouse model that lacks T, B, and natural killer (NK) cells. The resulting "Rag-5xfAD" mice exhibit a greater than twofold increase in ß-amyloid (Aß) pathology. Gene expression analysis of the brain implicates altered innate and adaptive immune pathways, including changes in cytokine/chemokine signaling and decreased Ig-mediated processes. Neuroinflammation is also greatly exacerbated in Rag-5xfAD mice as indicated by a shift in microglial phenotype, increased cytokine production, and reduced phagocytic capacity. In contrast, immune-intact 5xfAD mice exhibit elevated levels of nonamyloid reactive IgGs in association with microglia, and treatment of Rag-5xfAD mice or microglial cells with preimmune IgG enhances Aß clearance. Last, we performed bone marrow transplantation studies in Rag-5xfAD mice, revealing that replacement of these missing adaptive immune populations can dramatically reduce AD pathology. Taken together, these data strongly suggest that adaptive immune cell populations play an important role in restraining AD pathology. In contrast, depletion of B cells and their appropriate activation by T cells leads to a loss of adaptive-innate immunity cross talk and accelerated disease progression.

The neuronal retromer can regulate both neuronal and microglial phenotypes of Alzheimer's disease.

Disruption of retromer-dependent endosomal trafficking is considered pathogenic in late-onset Alzheimer's disease (AD). Here, to investigate this disruption in the intact brain, we turn to a genetic mouse model where the retromer core protein VPS35 is depleted in hippocampal neurons, and then we replete VPS35 using an optimized viral vector protocol. The VPS35 depletion-repletion studies strengthen the causal link between the neuronal retromer and AD-associated neuronal phenotypes, including the acceleration of amyloid precursor protein cleavage and the loss of synaptic glutamate receptors. Moreover, the studies show that the neuronal retromer can regulate a distinct, dystrophic, microglia morphology, phenotypic of hippocampal microglia in AD. Finally, the neuronal and, in part, the microglia responses to VPS35 depletion were found to occur independent of tau. Showing that the neuronal retromer can regulate AD-associated pathologies in two of AD's principal cell types strengthens the link, and clarifies the mechanism, between endosomal trafficking and late-onset sporadic AD.

Dissection of artifactual and confounding glial signatures by single-cell sequencing of mouse and human brain.

A key aspect of nearly all single-cell sequencing experiments is dissociation of intact tissues into single-cell suspensions. While many protocols have been optimized for optimal cell yield, they have often overlooked the effects that dissociation can have on ex vivo gene expression. Here, we demonstrate that use of enzymatic dissociation on brain tissue induces an aberrant ex vivo gene expression signature, most prominently in microglia, which is prevalent in published literature and can substantially confound downstream analyses. To address this issue, we present a rigorously validated protocol that preserves both in vivo transcriptional profiles and cell-type diversity and yield across tissue types and species. We also identify a similar signature in postmortem human brain single-nucleus RNA-sequencing datasets, and show that this signature is induced in freshly isolated human tissue by exposure to elevated temperatures ex vivo. Together, our results provide a methodological solution for preventing artifactual gene expression changes during fresh tissue digestion and a reference for future deeper analysis of the potential confounding states present in postmortem human samples.

Neural stem cell therapy for neurodegenerative disorders: The role of neurotrophic support.

Neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and Huntington's disease currently affect tens of millions of people worldwide. Unfortunately, as the world's population ages, the incidence of many of these diseases will continue to rise and is expected to more than double by 2050. Despite significant research and a growing understanding of disease pathogenesis, only a handful of therapies are currently available and all of them provide only transient benefits. Thus, there is an urgent need to develop novel disease-modifying therapies to prevent the development or slow the progression of these debilitating disorders. A growing number of pre-clinical studies have suggested that transplantation of neural stem cells (NSCs) could offer a promising new therapeutic approach for neurodegeneration. While much of the initial excitement about this strategy focused on the use of NSCs to replace degenerating neurons, more recent studies have implicated NSC-mediated changes in neurotrophins as a major mechanism of therapeutic efficacy. In this mini-review we will discuss recent work that examines the ability of NSCs to provide trophic support to disease-effected neuronal populations and synapses in models of neurodegeneration. We will then also discuss some of key challenges that remain before NSC-based therapies for neurodegenerative diseases can be translated toward potential clinical testing.

Human Neural Progenitor Transplantation Rescues Behavior and Reduces α-Synuclein in a Transgenic Model of Dementia with Lewy Bodies.

Synucleinopathies are a group of neurodegenerative disorders sharing the common feature of misfolding and accumulation of the presynaptic protein α-synuclein (α-syn) into insoluble aggregates. Within this diverse group, Dementia with Lewy Bodies (DLB) is characterized by the aberrant accumulation of α-syn in cortical, hippocampal, and brainstem neurons, resulting in multiple cellular stressors that particularly impair dopamine and glutamate neurotransmission and related motor and cognitive function. Recent studies show that murine neural stem cell (NSC) transplantation can improve cognitive or motor function in transgenic models of Alzheimer's and Huntington's disease, and DLB. However, examination of clinically relevant human NSCs in these models is hindered by the challenges of xenotransplantation and the confounding effects of immunosuppressant drugs on pathology and behavior. To address this challenge, we developed an immune-deficient transgenic model of DLB that lacks T-, B-, and NK-cells, yet exhibits progressive accumulation of human α-syn (h-α-syn)-laden inclusions and cognitive and motor impairments. We demonstrate that clinically relevant human neural progenitor cells (line CNS10-hNPCs) survive, migrate extensively and begin to differentiate preferentially into astrocytes following striatal transplantation into this DLB model. Critically, grafted CNS10-hNPCs rescue both cognitive and motor deficits after 1 and 3 months and, furthermore, restore striatal dopamine and glutamate systems. These behavioral and neurochemical benefits are likely achieved by reducing α-syn oligomers. Collectively, these results using a new model of DLB demonstrate that hNPC transplantation can impact a broad array of disease mechanisms and phenotypes and suggest a cellular therapeutic strategy that should be pursued. Stem Cells Translational Medicine 2017;6:1477-1490.

HuCNS-SC Human NSCs Fail to Differentiate, Form Ectopic Clusters, and Provide No Cognitive Benefits in a Transgenic Model of Alzheimer's Disease.

Transplantation of neural stem cells (NSCs) can improve cognition in animal models of Alzheimer's disease (AD). However, AD is a protracted disorder, and prior studies have examined only short-term effects. We therefore used an immune-deficient model of AD (Rag-5xfAD mice) to examine long-term transplantation of human NSCs (StemCells Inc.; HuCNS-SCs). Five months after transplantation, HuCNS-SCs had engrafted and migrated throughout the hippocampus and exhibited no differences in survival or migration in response to ß-amyloid pathology. Despite robust engraftment, HuCNS-SCs failed to terminally differentiate and over a quarter of the animals exhibited ectopic human cell clusters within the lateral ventricle. Unlike prior short-term experiments with research-grade HuCNS-SCs, we also found no evidence of improved cognition, no changes in brain-derived neurotrophic factor, and no increase in synaptic density. These data, while disappointing, reinforce the notion that individual human NSC lines need to be carefully assessed for efficacy and safety in appropriate long-term models.

Activation of the STING-Dependent Type I Interferon Response Reduces Microglial Reactivity and Neuroinflammation.

Brain aging and neurodegeneration are associated with prominent microglial reactivity and activation of innate immune response pathways, commonly referred to as neuroinflammation. One such pathway, the type I interferon response, recognizes viral or mitochondrial DNA in the cytoplasm via activation of the recently discovered cyclic dinucleotide synthetase cGAS and the cyclic dinucleotide receptor STING. Here we show that the FDA-approved antiviral drug ganciclovir (GCV) induces a type I interferon response independent of its canonical thymidine kinase target. Inhibition of components of the STING pathway, including STING, IRF3, Tbk1, extracellular IFNß, and the Jak-Stat pathway resulted in reduced activity of GCV and its derivatives. Importantly, functional STING was necessary for GCV to inhibit inflammation in cultured myeloid cells and in a mouse model of multiple sclerosis. Collectively, our findings uncover an unexpected new activity of GCV and identify the STING pathway as a regulator of microglial reactivity and neuroinflammation.

iPSC-Derived Human Microglia-like Cells to Study Neurological Diseases.

Microglia play critical roles in brain development, homeostasis, and neurological disorders. Here, we report that human microglial-like cells (iMGLs) can be differentiated from iPSCs to study their function in neurological diseases, like Alzheimer's disease (AD). We find that iMGLs develop in vitro similarly to microglia in vivo, and whole-transcriptome analysis demonstrates that they are highly similar to cultured adult and fetal human microglia. Functional assessment of iMGLs reveals that they secrete cytokines in response to inflammatory stimuli, migrate and undergo calcium transients, and robustly phagocytose CNS substrates. iMGLs were used to examine the effects of Aß fibrils and brain-derived tau oligomers on AD-related gene expression and to interrogate mechanisms involved in synaptic pruning. Furthermore, iMGLs transplanted into transgenic mice and human brain organoids resemble microglia in vivo. Together, these findings demonstrate that iMGLs can be used to study microglial function, providing important new insight into human neurological disease.

Examining the mechanisms that link ß-amyloid and α-synuclein pathologies.

ß-amyloid (Aß) and α-synuclein (α-syn) are aggregation-prone proteins typically associated with two distinct neurodegenerative disorders: Alzheimer's disease (AD) and Parkinson's disease. Yet α-syn was first found in association with AD plaques several years before being linked to Parkinson's disease or Lewy body formation. Nowadays, a large subset of AD patients (~50%) is well recognized to co-exhibit significant α-syn Lewy body pathology. Unfortunately, these AD Lewy body variant patients suffer from additional symptoms and an accelerated disease course. Basic research has begun to show that Aß and α-syn may act synergistically to promote the aggregation and accumulation of each other. While the exact mechanisms by which these proteins interact remain unclear, growing evidence suggests that Aß may drive α-syn pathology by impairing protein clearance, activating inflammation, enhancing phosphorylation, or directly promoting aggregation. This review examines the interactions between Aß and α-syn and proposes potential mechanistic links between Aß accumulation and α-syn pathogenesis.