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AbstractINTRODUCTIONRecent studies have identified important species‐dependent differences in the response of microglia to β‐amyloid (Aβ) pathology. Yet, whether human microglia also interact differently with the pathognomonic combination of amyloid and tau pathologies that occur in Alzheimer's disease (AD) remains unclear.METHODSWe generated a xenotolerant mouse model of AD that develops both plaque and tangle pathologies, transplanted stem cell‐derived microglial progenitors and examined the interactions between human microglia and AD pathologies with scRNA sequencing, immunohistochemistry, and in vitro modeling.RESULTSThe combined amyloid and tau pathologies induced robust type‐I interferon and proinflammatory cytokine responses, as well as an increased adoption of a distinct “rod” morphology in human microglia. The rod morphology could be induced with type‐I interferon treatment in vitro.DISCUSSIONWe provide new insights into human microglial responses to combined AD pathologies and a novel platform to investigate and manipulate human microglia in vivo.Highlights Amyloid pathology promotes the rapid development of neurofibrillary tangles and neuronal loss in a novel chimeric model of AD.Combined Alzheimer's disease pathologies lead to an expansion of disease‐associated microglia (DAM) and exacerbate Interferon‐responsive and cytokine/chemokine‐enriched states in xenotransplanted human microglia.The combination of amyloid and tau promotes the development of a distinctive rod microglial phenotype that closely correlates with tau pathology and neurodegeneration.Rod morphology and transcriptional changes can be modeled in vitro by treatment of induced pluripotent stem cells (iPSC) ‐microglia with type‐I interferons.

Small interfering RNAs (siRNAs) hold considerable therapeutic potential to selectively silence previously “undruggable” disease-associated targets, offering new opportunities to fight human diseases. This therapeutic strategy, however, is limited by the inability of naked siRNAs to passively diffuse across cellular membranes due to their large molecular size and negative charge. Delivery of siRNAs to liver through conjugation of siRNA to N-acetylgalactosamine (GalNAc) has been a success, providing robust and durable gene knockdown, specifically in hepatocytes. However, the poor delivery and silencing efficacy of siRNAs in other cell types has hindered their applications outside the liver. We previously reported that a genome-wide pooled knockout screen identified RAB18 as a major modulator of GalNAc-siRNA conjugates. Herein, we demonstrate RAB18 knockout/knockdown efficaciously enhances siRNA-mediated gene silencing in hepatic and extrahepatic cell lines and in vivo. Our results reveal a mechanism by which retrograde Golgi-endoplasmic reticulum (ER) transport and the intracellular lipid droplets (LDs) positively regulate siRNA-mediated gene silencing. Graphical abstract Using a variety of in vitro systems, Lu and colleagues demonstrated that knocking out RAB18 enhances siRNA-mediated gene silencing in both hepatic and extrahepatic cells. Furthermore, the authors translated this finding to an in vivo model. Altogether, the data point to a mechanism by which RAB18 regulates siRNA-mediated gene silencing in extrahepatic cells.

Several genetic risk factors for Alzheimer’s disease implicate genes involved in lipid metabolism and many of these lipid genes are highly expressed in glial cells 1 . However, the relationship between lipid metabolism in glia and Alzheimer’s disease pathology remains poorly understood. Through single-nucleus RNA sequencing of brain tissue in Alzheimer’s disease, we have identified a microglial state defined by the expression of the lipid droplet-associated enzyme ACSL1 with ACSL1-positive microglia being most abundant in patients with Alzheimer’s disease having the APOE4/4 genotype. In human induced pluripotent stem cell-derived microglia, fibrillar Aβ induces ACSL1 expression, triglyceride synthesis and lipid droplet accumulation in an APOE-dependent manner. Additionally, conditioned media from lipid droplet-containing microglia lead to Tau phosphorylation and neurotoxicity in an APOE-dependent manner. Our findings suggest a link between genetic risk factors for Alzheimer’s disease with microglial lipid droplet accumulation and neurotoxic microglia-derived factors, potentially providing therapeutic strategies for Alzheimer’s disease. Subject terms: Alzheimer's disease, Microglia, Neuroimmunology