Human TDP-43 expression worsens FTD-related phenotypes in progranulin-insufficient mice

Loss-of-function progranulin (GRN) mutations cause frontotemporal dementia with TDP-43 pathology (FTD-TDP). Nearly all pathogenic GRN mutations cause progranulin haploinsufficiency, but it is unclear how progranulin insufficiency causes FTD-TDP. To address this question, we crossed progranulin-insufficient mice with a human TDP-43 transgenic mouse line (RRID:IMSR_JAX:012836) in which homozygous mice (hTDP++) develop TDP-43 aggregates at an early age, but hemizygous mice (hTDP+) do not develop TDP-43 aggregates. We therefore analyzed the effects of progranulin insufficiency on both hTDP+ and hTDP++ mice. Progranulin insufficiency did not induce TDP-43 aggregation in hTDP+ mice, but interacted with hTDP expression to worsen FTD-related phenotypes. Grn^+/-:hTDP+ mice exhibited more dramatic impairment of social dominance than either Grn^+/- or hTDP+ mice, which was associated with combined effects of progranulin insufficiency and hTDP expression on dendritic spines of neurons in the medial prefrontal cortex (mPFC). Despite a lack of TDP-43 aggregation, progranulin insufficiency altered the RNA splicing events induced by hTDP overexpression in frontal cortex of hTDP+ mice. Progranulin insufficiency also did not alter TDP-43 aggregation in hTDP++ mice, but Grn^-/-:hTDP++ mice exhibited an abnormal neuroinflammatory response characterized by increased markers of disease-associated microglia and signs of an impaired adaptive immune response. These results highlight dysfunction of mPFC neurons as a potential mechanism of behavioral changes in FTD-GRN and implicate dysregulated inflammation as a potential driver of disease progression in FTD-GRN.