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Objective: 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-offunction 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. Methods: 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. Results: Grn-/mice had elevated brain EV levels and altered EV protein contents relative to wildtype 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). Interpretation: 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.

Loss of function progranulin (GRN) mutations are a major autosomal dominant cause of frontotemporal dementia (FTD). Patients with FTD due to GRN mutations (FTD-GRN) develop frontotemporal lobar degeneration with TDP-43 pathology type A (FTLD-TDP type A) and exhibit elevated levels of lysosomal proteins and storage material in frontal cortex, perhaps indicating lysosomal dysfunction as a mechanism of disease. To investigate whether patients with sporadic FTLD exhibit similar signs of lysosomal dysfunction, we compared lysosomal protein levels, transcript levels, and storage material in patients with FTD-GRN or sporadic FTLD-TDP type A. We analyzed samples from frontal cortex, a degenerated brain region, and occipital cortex, a relatively spared brain region. In frontal cortex, patients with sporadic FTLD-TDP type A exhibited similar increases in lysosomal protein levels, transcript levels, and storage material as patients with FTD-GRN. In occipital cortex of both patient groups, most lysosomal measures did not differ from controls. Frontal cortex from a transgenic mouse model of TDP-opathy had similar increases in cathepsin D and lysosomal storage material, showing that TDP-opathy and neurodegeneration can drive these changes independently of progranulin. To investigate these changes in additional FTLD subtypes, we analyzed frontal cortical samples from patients with sporadic FTLD-TDP type C or Pick’s disease, an FTLD-tau subtype. All sporadic FTLD groups had similar increases in cathepsin D activity, lysosomal membrane proteins, and storage material as FTD-GRN patients. However, patients with FTLD-TDP type C or Pick’s disease did not have similar increases in lysosomal transcripts as patients with FTD-GRN or sporadic FTLD-TDP type A. Based on these data, accumulation of lysosomal proteins and storage material may be a common aspect of end-stage FTLD. However, the unique changes in gene expression in patients with FTD-GRN or sporadic FTLD-TDP type A may indicate distinct underlying lysosomal changes among FTLD subtypes.

BackgroundLoss‐of‐function mutations in progranulin (GRN) are an autosomal dominant cause of frontotemporal dementia (FTD). Progranulin is critical for maintenance of lysosomal function. Patients with FTD due to GRN mutations (FTD‐GRN) exhibit signs of lysosomal dysfunction, which may contribute to FTD‐GRN pathogenesis. To assess the potential involvement of lysosomal dysfunction in FTD of sporadic origin, we investigated whether the lysosomal abnormalities of FTD‐GRN are also present in patients with sporadic frontotemporal lobar degeneration with TDP‐43 pathology (FTLD‐TDP) type A, the same FTLD subtype found in patients with FTD‐GRN.MethodWe used enzyme activity assays, western blot, immunostaining, and Nanostring analysis to assess changes in lysosomal protein and gene expression in orbital and occipital cortex of patients with FTD‐GRN, sporadic FTLD‐TDP type A, or controls. We measured lipofuscin using autofluorescence and staining with Sudan Black B. In follow‐up studies, we assessed these lysosomal markers in a transgenic mouse line expressing wild‐type human TDP‐43 under the Thy1 promoter.ResultWe found similar lysosomal abnormalities in orbital cortex of patients with FTD‐GRN or sporadic FTLD‐TDP type A. Both FTD patient groups exhibited elevated levels of lysosomal enzymes and membrane proteins, higher expression of many lysosomal genes, and greater lipofuscin accumulation than controls. These changes were largely absent in occipital cortex from both groups. Analysis of frontal cortex from a transgenic mouse model of TDP‐opathy also revealed elevated levels of some lysosomal proteins and elevated lipofuscin levels.ConclusionThese studies indicate that the lysosomal abnormalities of patients with FTD‐GRN may be more closely associated with TDP‐opathy or neurodegeneration than with progranulin insufficiency. Orbital cortex, an affected brain region in FTD‐GRN, exhibited elevated lysosomal protein levels and lipofuscin accumulation, which were also found in patients with sporadic FTLD‐TDP type A. In contrast, the occipital cortex, a relatively spared brain region in FTD‐GRN, exhibited much milder lysosomal abnormalities. Future studies will assess signs of lysosomal dysfunction in patients with other classes of FTLD pathology to determine whether these changes are limited to patients with FTLD‐TDP or may be found throughout the FTLD spectrum.