Frontotemporal dementia (FTD) is a neurodegenerative disease with hallmark deficits in social and emotional function. Heterozygous loss-of-function mutations in GRN, the progranulin gene, are a common genetic cause of the disorder, but the mechanisms by which progranulin haploinsufficiency causes neuronal dysfunction in FTD are unclear. Homozygous progranulin knockout (Grn−/−) mice have been studied as a model of this disorder and show behavioral deficits and a neuroinflammatory phenotype with robust microglial activation. However, homozygous GRN mutations causing complete progranulin deficiency were recently shown to cause a different neurological disorder, neuronal ceroid lipofuscinosis, suggesting that the total absence of progranulin may have effects distinct from those of haploinsufficiency. Here, we studied progranulin heterozygous (Grn+/−) mice, which model progranulin haploinsufficiency. We found that Grn+/− mice developed age-dependent social and emotional deficits potentially relevant to FTD. However, unlike Grn−/− mice, behavioral deficits in Grn+/− mice occurred in the absence of gliosis or increased expression of tumor necrosis factor–α. Instead, we found neuronal abnormalities in the amygdala, an area of selective vulnerability in FTD, in Grn+/− mice. Our findings indicate that FTD-related deficits due to progranulin haploinsufficiency can develop in the absence of detectable gliosis and neuroinflammation, thereby dissociating microglial activation from functional deficits and suggesting an important effect of progranulin deficiency on neurons.

Many neurodegenerative diseases, such as frontotemporal dementia (FTD), are associated with behavioral deficits, but the anatomical and molecular bases remain poorly understood. Here we show that forebrain-specific expression of FTD-associated mutant CHMP2B causes several age-dependent neurodegenerative phenotypes, including social behavioral impairments. The social deficits were accompanied by a change in AMPA receptor (AMPAR) composition, leading to imbalance between Ca2+-permeable and -impermeable AMPARs. Expression of most AMPAR subunits was regulated by the brain-enriched microRNA (miR-124), whose abundance was markedly decreased in the superficial layers of cerebral cortex of FTD mice. We found similar changes in miR-124 and AMPAR levels in the frontal cortex and iPSC-derived neurons of subjects with behavioral variant FTD. Moreover, miR-124 expression in the medial prefrontal cortex decreased AMPAR levels and partially rescued behavioral deficits. Knockdown of Gria2 also alleviated social impairments in FTD mice. Our results identify a novel mechanism involving miR-124 and AMAPRs in regulating social behavior in FTD and suggest a potential therapeutic avenue.