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Human genetics and preclinical studies have identified key contributions of TREM2 to several neurodegenerative conditions, inspiring efforts to modulate TREM2 therapeutically. Here, we characterize the activities of three TREM2 agonist antibodies in multiple mixed-sex mouse models of Alzheimer's disease (AD) pathology and remyelination. Receptor activation and downstream signaling are explored in vitro, and active dose ranges are determined in vivo based on pharmacodynamic responses from microglia. For mice bearing amyloid-β (Aβ) pathology (PS2APP) or combined Aβ and tau pathology (TauPS2APP), chronic TREM2 agonist antibody treatment had limited impact on microglia engagement with pathology, overall pathology burden, or downstream neuronal damage. For mice with demyelinating injuries triggered acutely with lysolecithin, TREM2 agonist antibodies unexpectedly disrupted injury resolution. Likewise, TREM2 agonist antibodies limited myelin recovery for mice experiencing chronic demyelination from cuprizone. We highlight the contributions of dose timing and frequency across models. These results introduce important considerations for future TREM2-targeting approaches., Competing Interests: The authors declare no competing financial interests., (Copyright © 2024 the authors.)

Non-neuronal responses in neurodegenerative disease have received increasing attention as important contributors to disease pathogenesis and progression. Here we utilize single-cell RNA sequencing to broadly profile 13 cell types in three different mouse models of Alzheimer disease (AD), capturing the effects of tau-only, amyloid-only, or combined tau-amyloid pathology. We highlight microglia, oligodendrocyte, astrocyte, and T cell responses and compare them across these models. Notably, we identify two distinct transcriptional states for oligodendrocytes emerging differentially across disease models, and we determine their spatial distribution. Furthermore, we explore the impact of Trem2 deletion in the context of combined pathology. Trem2 knockout mice exhibit severely blunted microglial responses to combined tau and amyloid pathology, but responses from non-microglial cell types (oligodendrocytes, astrocytes, and T cells) are relatively unchanged. These results delineate core transcriptional states that are engaged in response to AD pathology, and how they are influenced by a key AD risk gene, Trem2., Competing Interests: Declaration of interests All authors are current or former employees of Genentech., (Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.)

Loss-of-function TREM2 mutations strongly increase Alzheimer's disease (AD) risk. Trem2 deletion has revealed protective Trem2 functions in preclinical models of β-amyloidosis, a prominent feature of pre-diagnosis AD stages. How TREM2 influences later AD stages characterized by tau-mediated neurodegeneration is unclear. To understand Trem2 function in the context of both β-amyloid and tau pathologies, we examined Trem2 deficiency in the pR5-183 mouse model expressing mutant tau alone or in TauPS2APP mice, in which β-amyloid pathology exacerbates tau pathology and neurodegeneration. Single-cell RNA sequencing in these models revealed robust disease-associated microglia (DAM) activation in TauPS2APP mice that was amyloid-dependent and Trem2-dependent. In the presence of β-amyloid pathology, Trem2 deletion further exacerbated tau accumulation and spreading and promoted brain atrophy. Without β-amyloid pathology, Trem2 deletion did not affect these processes. Therefore, TREM2 may slow AD progression and reduce tau-driven neurodegeneration by restricting the degree to which β-amyloid facilitates the spreading of pathogenic tau., Competing Interests: Declaration of interests All authors are current or former employees of Genentech, Inc., with interests in developing novel therapeutic drugs for neurodegenerative diseases at the time of conducting this research., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Damage-associated microglia (DAM) profiles observed in Alzheimer's disease (AD)-related mouse models reflect an activation state that could modulate AD risk or progression. To learn whether human AD microglia (HAM) display a similar profile, we develop a method for purifying cell types from frozen cerebrocortical tissues for RNA-seq analysis, allowing better transcriptome coverage than typical single-nucleus RNA-seq approaches. The HAM profile we observe bears little resemblance to the DAM profile. Instead, HAM display an enhanced human aging profile, in addition to other disease-related changes such as APOE upregulation. Analyses of whole-tissue RNA-seq and single-cell/nucleus RNA-seq datasets corroborate our findings and suggest that the lack of DAM response in human microglia occurs specifically in AD tissues, not other neurodegenerative settings. These results, which can be browsed at http://research-pub.gene.com/BrainMyeloidLandscape, provide a genome-wide picture of microglial activation in human AD and highlight considerable differences between mouse models and human disease., Competing Interests: Declaration of Interests T.G.B. and G.E.S. participated in this study under a contracted research agreement. All other authors are current or former employees of the pharmaceutical company Genentech, Inc.., (Copyright © 2020 The Author(s). Published by Elsevier Inc. All rights reserved.)

TREM2 is an Alzheimer's disease (AD) risk gene expressed in microglia. To study the role of Trem2 in a mouse model of β-amyloidosis, we compared PS2APP transgenic mice versus PS2APP mice lacking Trem2 (PS2APP;Trem2 ko ) at ages ranging from 4 to 22 months. Microgliosis was impaired in PS2APP;Trem2 ko mice, with Trem2 -deficient microglia showing compromised expression of proliferation/Wnt-related genes and marked accumulation of ApoE. Plaque abundance was elevated in PS2APP;Trem2 ko females at 6-7 months; but by 12 or 19-22 months of age, it was notably diminished in female and male PS2APP;Trem2 ko mice, respectively. Across all ages, plaque morphology was more diffuse in PS2APP;Trem2 ko brains, and the Aβ42:Aβ40 ratio was elevated. The amount of soluble, fibrillar Aβ oligomers also increased in PS2APP;Trem2 ko hippocampi. Associated with these changes, axonal dystrophy was exacerbated from 6 to 7 months onward in PS2APP;Trem2 ko mice, notwithstanding the reduced plaque load at later ages. PS2APP;Trem2 ko mice also exhibited more dendritic spine loss around plaque and more neurofilament light chain in CSF. Thus, aggravated neuritic dystrophy is a more consistent outcome of Trem2 deficiency than amyloid plaque load, suggesting that the microglial packing of Aβ into dense plaque is an important neuroprotective activity. SIGNIFICANCE STATEMENT Genetic studies indicate that TREM2 gene mutations confer increased Alzheimer's disease (AD) risk. We studied the effects of Trem2 deletion in the PS2APP mouse AD model, in which overproduction of Aβ peptide leads to amyloid plaque formation and associated neuritic dystrophy. Interestingly, neuritic dystrophies were intensified in the brains of Trem2 -deficient mice, despite these mice displaying reduced plaque accumulation at later ages (12-22 months). Microglial clustering around plaques was impaired, plaques were more diffuse, and the Aβ42:Aβ40 ratio and amount of soluble, fibrillar Aβ oligomers were elevated in Trem2 -deficient brains. These results suggest that the Trem2-dependent compaction of Aβ into dense plaques is a protective microglial activity, limiting the exposure of neurons to toxic Aβ species., (Copyright © 2020 Meilandt et al.)

Complement pathway overactivation can lead to neuronal damage in various neurological diseases. Although Alzheimer's disease (AD) is characterized by β-amyloid plaques and tau tangles, previous work examining complement has largely focused on amyloidosis models. We find that glial cells show increased expression of classical complement components and the central component C3 in mouse models of amyloidosis (PS2APP) and more extensively tauopathy (TauP301S). Blocking complement function by deleting C3 rescues plaque-associated synapse loss in PS2APP mice and ameliorates neuron loss and brain atrophy in TauP301S mice, improving neurophysiological and behavioral measurements. In addition, C3 protein is elevated in AD patient brains, including at synapses, and levels and processing of C3 are increased in AD patient CSF and correlate with tau. These results demonstrate that complement activation contributes to neurodegeneration caused by tau pathology and suggest that blocking C3 function might be protective in AD and other tauopathies., (Copyright © 2019 The Author(s). Published by Elsevier Inc. All rights reserved.)

Paired Immunoglobulin-like Type 2 Receptor Alpha (PILRA) is a cell surface inhibitory receptor that recognizes specific O-glycosylated proteins and is expressed on various innate immune cell types including microglia. We show here that a common missense variant (G78R, rs1859788) of PILRA is the likely causal allele for the confirmed Alzheimer's disease risk locus at 7q21 (rs1476679). The G78R variant alters the interaction of residues essential for sialic acid engagement, resulting in >50% reduced binding for several PILRA ligands including a novel ligand, complement component 4A, and herpes simplex virus 1 (HSV-1) glycoprotein B. PILRA is an entry receptor for HSV-1 via glycoprotein B, and macrophages derived from R78 homozygous donors showed significantly decreased levels of HSV-1 infection at several multiplicities of infection compared to homozygous G78 macrophages. We propose that PILRA G78R protects individuals from Alzheimer's disease risk via reduced inhibitory signaling in microglia and reduced microglial infection during HSV-1 recurrence., Competing Interests: NR, SRR, HP, JP, TB, AW, YS, SBK, LG, AAZ, DVH, TWB and RRG were full-time employees at Genentech while the work was conducted. A patent has been submitted for this work.

Proliferation and activation of microglia in the brain, concentrated around amyloid plaques, is a prominent feature of Alzheimer's disease (AD). Human genetics data point to a key role for microglia in the pathogenesis of AD. The majority of risk genes for AD are highly expressed (and many are selectively expressed) by microglia in the brain. There is mounting evidence that microglia protect against the incidence of AD, as impaired microglial activities and altered microglial responses to β-amyloid are associated with increased AD risk. On the other hand, there is also abundant evidence that activated microglia can be harmful to neurons. Microglia can mediate synapse loss by engulfment of synapses, likely via a complement-dependent mechanism; they can also exacerbate tau pathology and secrete inflammatory factors that can injure neurons directly or via activation of neurotoxic astrocytes. Gene expression profiles indicate multiple states of microglial activation in neurodegenerative disease settings, which might explain the disparate roles of microglia in the development and progression of AD pathology., (© 2018 Hansen et al.)

Microglia, the CNS-resident immune cells, play important roles in disease, but the spectrum of their possible activation states is not well understood. We derived co-regulated gene modules from transcriptional profiles of CNS myeloid cells of diverse mouse models, including new tauopathy model datasets. Using these modules to interpret single-cell data from an Alzheimer's disease (AD) model, we identified microglial subsets-distinct from previously reported "disease-associated microglia"-expressing interferon-related or proliferation modules. We then analyzed whole-tissue RNA profiles from human neurodegenerative diseases, including a new AD dataset. Correcting for altered cellular composition of AD tissue, we observed elevated expression of the neurodegeneration-related modules, but also modules not implicated using expression profiles from mouse models alone. We provide a searchable, interactive database for exploring gene expression in all these datasets (http://research-pub.gene.com/BrainMyeloidLandscape). Understanding the dimensions of CNS myeloid cell activation in human disease may reveal opportunities for therapeutic intervention., (Copyright © 2017 The Author(s). Published by Elsevier Inc. All rights reserved.)

Alzheimer's disease (AD) is the most common form of dementia and the 6th leading cause of death in the US. The neuropathological hallmarks of the disease are extracellular amyloid-β (Aβ) plaques and intraneuronal hyperphosphorylated tau aggregates. Genetic variants of TREM2 (triggering receptor expressed on myeloid cells 2), a cell-surface receptor expressed selectively in myeloid cells, greatly increase the risk of AD, implicating microglia and the innate immune system as pivotal factors in AD pathogenesis. Recent studies have advanced our understanding of TREM2 biology and microglial activities in aging and neurodegenerative brains, providing new insights into TREM2 functions in amyloid plaque maintenance, microglial envelopment of plaque, microglia viability, and the identification of novel TREM2 ligands. Our increased understanding of TREM2 and microglia has opened new avenues for therapeutic intervention to delay or prevent the progression of AD., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

The common p.D358A variant (rs2228145) in IL-6R is associated with risk for multiple diseases and with increased levels of soluble IL-6R in the periphery and central nervous system (CNS). Here, we show that the p.D358A allele leads to increased proteolysis of membrane bound IL-6R and demonstrate that IL-6R peptides with A358 are more susceptible to cleavage by ADAM10 and ADAM17. IL-6 responsive genes were identified in primary astrocytes and microglia and an IL-6 gene signature was increased in the CNS of late onset Alzheimer's disease subjects in an IL6R allele dependent manner. We conducted a screen to identify variants associated with the age of onset of Alzheimer's disease in APOE ɛ4 carriers. Across five datasets, p.D358A had a meta P = 3 ×10-4 and an odds ratio = 1.3, 95% confidence interval 1.12 -1.48. Our study suggests that a common coding region variant of the IL-6 receptor results in neuroinflammatory changes that may influence the age of onset of Alzheimer's disease in APOE ɛ4 carriers.

The spread of tau pathology correlates with cognitive decline in Alzheimer's disease. In vitro, tau antibodies can block cell-to-cell tau spreading. Although mechanisms of anti-tau function in vivo are unknown, effector function might promote microglia-mediated clearance. In this study, we investigated whether antibody effector function is required for targeting tau. We compared efficacy in vivo and in vitro of two versions of the same tau antibody, with and without effector function, measuring tau pathology, neuron health, and microglial function. Both antibodies reduced accumulation of tau pathology in Tau-P301L transgenic mice and protected cultured neurons against extracellular tau-induced toxicity. Only the full-effector antibody enhanced tau uptake in cultured microglia, which promoted release of proinflammatory cytokines. In neuron-microglia co-cultures, only effectorless anti-tau protected neurons, suggesting full-effector tau antibodies can induce indirect toxicity via microglia. We conclude that effector function is not required for efficacy, and effectorless tau antibodies may represent a safer approach to targeting tau., (Copyright © 2016 The Author(s). Published by Elsevier Inc. All rights reserved.)

A common approach to understanding neurodegenerative disease is comparing gene expression in diseased versus healthy tissues. We illustrate that expression profiles derived from whole tissue RNA highly reflect the degenerating tissues' altered cellular composition, not necessarily transcriptional regulation. To accurately understand transcriptional changes that accompany neuropathology, we acutely purify neurons, astrocytes and microglia from single adult mouse brains and analyse their transcriptomes by RNA sequencing. Using peripheral endotoxemia to establish the method, we reveal highly specific transcriptional responses and altered RNA processing in each cell type, with Tnfr1 required for the astrocytic response. Extending the method to an Alzheimer's disease model, we confirm that transcriptomic changes observed in whole tissue are driven primarily by cell type composition, not transcriptional regulation, and identify hundreds of cell type-specific changes undetected in whole tissue RNA. Applying similar methods to additional models and patient tissues will transform our understanding of aberrant gene expression in neurological disease.

We have identified a rare coding mutation, T835M (rs137875858), in the UNC5C netrin receptor gene that segregated with disease in an autosomal dominant pattern in two families enriched for late-onset Alzheimer's disease and that was associated with disease across four large case-control cohorts (odds ratio = 2.15, Pmeta = 0.0095). T835M alters a conserved residue in the hinge region of UNC5C, and in vitro studies demonstrate that this mutation leads to increased cell death in human HEK293T cells and in rodent neurons. Furthermore, neurons expressing T835M UNC5C are more susceptible to cell death from multiple neurotoxic stimuli, including β-amyloid (Aβ), glutamate and staurosporine. On the basis of these data and the enriched hippocampal expression of UNC5C in the adult nervous system, we propose that one possible mechanism in which T835M UNC5C contributes to the risk of Alzheimer's disease is by increasing susceptibility to neuronal cell death, particularly in vulnerable regions of the Alzheimer's disease brain.