Elevated levels of extracellular vesicles in progranulin-deficient mice and FTD-GRN Patients

The goal of this study was to investigate the effect of progranulin insufficiency on extracellular vesicles (EVs), a heterogeneous population of vesicles that may contribute to progression of neurodegenerative disease. Loss-of-function mutations in progranulin (GRN) are a major cause of frontotemporal dementia (FTD), and brains from GRN carriers with FTD (FTD-GRN) exhibit signs of lysosomal dysfunction. Lysosomal dysfunction may induce compensatory increases in secretion of exosomes, EVs secreted from the endolysosomal system, so we hypothesized that progranulin insufficiency would increase EV levels in the brain.

We analyzed levels and protein contents of brain EVs from Grn^-/- mice, which model the lysosomal abnormalities of FTD-GRN patients. We then measured brain EVs in FTD-GRN patients. To assess the relationship of EVs with symptomatic disease, we measured plasma EVs in presymptomatic and symptomatic GRN mutation carriers.

Grn^-/- mice had elevated brain EV levels and altered EV protein contents relative to wild-type mice. These changes were age-dependent, occurring only after the emergence of pathology in Grn^-/- mice. FTD-GRN patients (n = 13) had elevated brain EV levels relative to controls (n = 5). Symptomatic (n = 12), but not presymptomatic (n = 7), GRN carriers had elevated plasma EV levels relative to controls (n = 8).

These data show that symptomatic FTD-GRN patients have elevated levels of brain and plasma EVs, and that this effect is modeled in the brain of Grn^-/- mice after the onset of pathology. This increase in EVs could influence FTD disease progression, and provides further support for EVs as potential FTD biomarkers.