Adhesion G protein-coupled receptor ADGRG1 promotes protective microglial response in Alzheimer’s disease

Microglia play critical roles in maintaining brain development, homeostasis, and response to aging and diseases like Alzheimer's disease (AD). Genetic studies highlight microglia's role in AD susceptibility, but the mechanisms mediating protective responses remain largely unknown. Gpr56/Adgrg1, is one of the few genes specifically expressed in yolk-sac-derived microglia, as compared with other mononuclear phagocytes. This study reveals its role in AD pathology, particularly in modulating microglial protective responses to amyloid deposition. Utilizing constitutive and inducible microglial Adgrg1 knockouts in the 5xFAD mouse model, we demonstrate that Adgrg1 deficiency leads to increased amyloid deposition, exacerbated neuropathology, and accelerated cognitive impairment. Transcriptomic analyses reveal a distinct microglial state characterized by downregulated genes associated with homeostasis, phagocytosis, and lysosomal functions. Functional assays in mouse models and human iPSC-derived microglia support the requirement for microglial ADGRG1 in efficient Aβ phagocytosis. Collectively, these results uncover a GPCR-dependent mechanism that instructs beneficial microglial functions in response to Aβ, pointing towards potential therapeutic strategies designed to alleviate disease progression by enhancing microglial functional competence. To investigate the role of microglial Adgrg1 in AD pathology, we engineered a novel mouse model and performed snRNAseq on the neocortex of 6-month-old animal. To enrich glial cells, each animal was sorted by DAPI-positive and DAPI-positive NeuN-negative nuclei before sequencing. The genotypes for the testing animals are: Adgrg1+/+;Cx3cr1Cre/+ (Con); Adgrg1fl/fl;Cx3cr1Cre/+ (CKO); 5xFAD;Adgrg1+/+;Cx3cr1Cre/+ mice (ADCon); 5xFAD;Adgrg1fl/fl;Cx3cr1Cre/+ (ADCKO).

小胶质细胞（Microglia）在维持脑发育、内环境稳态，以及响应衰老与阿尔茨海默病（Alzheimer's disease, AD）等疾病的进程中发挥关键调控作用。遗传学研究已证实小胶质细胞与AD易感性密切相关，但介导其保护性应答的分子机制仍未得到充分阐释。
与其他单核吞噬细胞（mononuclear phagocytes）相比，Gpr56/Adgrg1是少数仅在卵黄囊衍生的小胶质细胞中特异性表达的基因之一。本研究揭示了该基因在AD病理进程中的作用，尤其是在调控小胶质细胞针对淀粉样蛋白沉积的保护性应答方面。
本研究在5xFAD小鼠模型（5xFAD mouse model）中构建了组成型及诱导型小胶质细胞Adgrg1敲除体系，实验结果显示，Adgrg1缺失会导致淀粉样蛋白沉积增多、神经病理损伤加剧以及认知功能障碍加速进展。转录组学分析（transcriptomic analyses）揭示了一种独特的小胶质细胞状态，其特征为与内环境稳态、吞噬功能及溶酶体功能相关的基因表达下调。通过小鼠模型及人类诱导多能干细胞（induced pluripotent stem cell, iPSC）衍生的小胶质细胞开展的功能实验证实，小胶质细胞ADGRG1是高效吞噬β淀粉样蛋白（Aβ）所必需的。
综上，本研究揭示了一种依赖于G蛋白偶联受体（G protein-coupled receptor, GPCR）的分子机制，该机制可指导小胶质细胞针对Aβ发挥保护性功能，这为通过增强小胶质细胞功能活性以延缓疾病进展的潜在治疗策略提供了理论依据。
为探究小胶质细胞Adgrg1在AD病理中的作用，本研究构建了一种新型小鼠模型，并对6月龄小鼠的新皮层组织开展了单细胞核RNA测序（single-nucleus RNA sequencing, snRNAseq）。为富集神经胶质细胞，测序前我们通过4',6-二脒基-2-苯基吲哚（4',6-diamidino-2-phenylindole, DAPI）阳性且神经元核抗原（Neuronal Nuclei, NeuN）阴性的标准对小鼠细胞核进行了分选。受试小鼠的基因型如下：Adgrg1+/+;Cx3cr1Cre/+（对照组，Con）；Adgrg1fl/fl;Cx3cr1Cre/+（条件性敲除组，CKO）；5xFAD;Adgrg1+/+;Cx3cr1Cre/+（AD模型对照组，ADCon）；5xFAD;Adgrg1fl/fl;Cx3cr1Cre/+（AD模型条件性敲除组，ADCKO）。