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.