Regulation of extracellular progranulin in the medial prefrontal cortex

Abstract

AbstractBackgroundLoss‐of‐function mutations in progranulin (GRN) are a major genetic cause of frontotemporal dementia (FTD). Progranulin is a secreted protein that is trafficked to lysosomes, where it promotes lysosomal function. Progranulin also exerts neurotrophic and anti‐inflammatory effects, some of which may be mediated by extracellular signaling. Neurons and microglia express progranulin and loss of progranulin from either cell type induces FTD‐like deficits in mice. Both cell types secrete progranulin in culture, but their contribution to extracellular progranulin in the brain is unclear, as are the mechanisms regulating progranulin secretion from each cell type. In this study, we used microdialysis to gain insight into the contribution of neurons and microglia to extracellular progranulin in the medial prefrontal cortex (mPFC), a region that degenerates in FTD and develops impaired dendritic arborization in progranulin‐insufficient mice.MethodPolyethylene probes (2 mm, 1000 kDa cut‐off) were implanted in mouse mPFC and perfused with artificial cerebrospinal fluid containing 4% BSA using a push‐pull pump system (Atmos LM, Amuza). Progranulin levels in interstitial fluid (ISF) were measured by ELISA (Adipogen).ResultGrn+/– mice exhibited a roughly 50% decrease in ISF progranulin versus wild‐type, which was similar to the reduction of progranulin in mPFC tissue. In wild‐type mice, inducing cellular depolarization with KCl (100 mM) increased ISF progranulin, but stimulating synaptic activity with picrotoxin (100 μM) or NMDA (50 μM) did not. Inducing systemic inflammation with LPS (10 mg/kg i.p.) increased ISF progranulin to around 200% of baseline at eight hours after injection. Analysis of ISF progranulin from microglial progranulin knockout mice (Cx3Cr1‐Cre‐ER:Grnfl/fl) revealed a nearly 50% reduction in ISF progranulin compared to Cre– littermates, which was a larger reduction than observed in mPFC tissue. Ongoing studies are analyzing ISF progranulin in neuronal progranulin knockout mice (CamKII‐Cre:Grnfl/fl).ConclusionOur findings indicate that inflammation increases ISF progranulin in the mPFC and that nearly half of the extracellular progranulin in the mPFC may originate from microglia. In contrast, synaptic activity does not significantly increase ISF progranulin in the mPFC. Ongoing studies will investigate the contribution of neurons to extracellular progranulin in the mPFC.