Opposing role of phagocytic receptors MERTK and AXL in Progranulin deficient FTD

Genetic mutations in the progranulin gene, GRN, cause frontotemporal dementia and a lysosomal storage disorder. Using single-nuclei RNA sequencing of the post-mortem brain tissue from heterozygous pathogenic granulin variant (GRN+/-) carriers we found dysregulation of microglia, phagocytosis and the phagocytic receptors MERTK and AXL. Exogenous progranulin regulated MERTK and AXL RNA expression in human microglia induced from iPSCs irrespective of GRN mutation status, without directly binding to MERTK or AXL proteins. We generated double knock-out mice and found that constitutive homozygous loss of Grn and Mertk (Grn-/-;Mertk-/-) rescued microglial disease signature while constitutive homozygous loss of Grn and Axl (Grn-/-;Axl-/-) worsened the microglial disease signature. Higher levels of AXL protein, but not MERTK protein, in the post-mortem superior temporal gyrus correlated with worse cross-sectional functional impairment (CDR sum of boxes) by both GRN+/- mutation carriers and non-carrier controls. These data explain in part the inflammation seen in GRN-FTD and are applicable to other inflammatory states in which PGRN, MERTK and AXL play regulatory roles, including cancer, sepsis, stroke, diabetes and obesity. The interaction between GRN, MERTK and AXL opens potential new therapeutic avenues to intervene on this inflammatory axis. Droplet-based single nuclear RNA sequencing from superior frontal gyrus from 7 patients with GRN+/- loss of function mutations and 8 GRN+/+ aged controls without neurological disease; thalamic cells from WT control, single KO mice (Grn-/-, Mertk-/- and Axl-/- mice) and double KO mice (Grn-/-;Mertk-/- and Grn-/-;Axl-/-).前颗粒蛋白基因 (progranulin gene, GRN) 的致病性突变可引发额颞叶痴呆 (frontotemporal dementia) 与溶酶体贮积症 (lysosomal storage disorder)。我们对携带杂合致病性颗粒蛋白变异 (GRN+/-) 的死者脑组织开展单细胞核RNA测序 (single-nuclei RNA sequencing)，发现小胶质细胞 (microglia)、吞噬作用 (phagocytosis) 及吞噬受体MERTK与AXL的表达失调。外源性前颗粒蛋白可在由诱导多能干细胞 (induced pluripotent stem cells, iPSCs) 诱导分化得到的人类小胶质细胞中调控MERTK与AXL的RNA表达，且不受GRN突变状态影响，亦未直接结合MERTK或AXL蛋白。我们构建了双基因敲除小鼠模型，发现Grn与Mertk的纯合组成型敲除 (Grn-/-;Mertk-/-) 可挽救小胶质细胞疾病特征，而Grn与Axl的纯合组成型敲除 (Grn-/-;Axl-/-) 则会加重该小胶质细胞疾病特征。在死者颞上回 (superior temporal gyrus) 脑组织中，AXL蛋白（而非MERTK蛋白）的高表达与更严重的横断面功能损伤 (CDR sum of boxes，即临床痴呆评定量表总分) 相关，该关联在GRN+/-突变携带者与非携带者对照人群中均成立。上述数据部分解释了GRN相关额颞叶痴呆 (GRN-FTD) 中的炎症反应，且可推广至前颗粒蛋白 (PGRN)、MERTK与AXL发挥调控作用的其他炎症状态，包括癌症、脓毒症、脑卒中、糖尿病与肥胖。GRN、MERTK与AXL之间的相互作用为干预该炎症轴提供了潜在的全新治疗方向。本数据集包含7例携带GRN+/-功能丧失突变患者与8例无神经系统疾病的年龄匹配GRN+/+对照者的额上回 (superior frontal gyrus) 液滴式单细胞核RNA测序 (Droplet-based single nuclear RNA sequencing) 数据；同时涵盖野生型 (wild type, WT) 对照、单基因敲除小鼠 (Grn-/-、Mertk-/-与Axl-/-小鼠) 及双基因敲除小鼠 (Grn-/-;Mertk-/-与Grn-/-;Axl-/-小鼠) 的丘脑细胞 (thalamic cells) 测序数据。