Progranulin deficiency in the brain activates an insulin signaling pathway that may promote neurodegeneration

Molecular mechanisms in frontotemporal dementia (FTD) and Alzheimer's disease (AD) are obscure. FTD can result from loss-of-function progranulin mutations, although pathogenetic consequences are uncertain. Progranulin insufficiency also increases human AD risk, and progranulin treatment improves mouse AD. Furthermore, AD and FTD risks are abetted by obesity/diabetes-induced hyperinsulinemia and hyperactivation of brain insulin signaling, and progranulin deficiency activates insulin signaling in fat and liver. Here, we found progranulin deletion in mouse brain increased activation of IRS-1 and activities of downstream PKC-λ/ι, NF-κB and mTOR, but diminished IRS-2 and Akt. Similarly, in microglial cells, progranulin deletion increased, and progranulin treatment diminished, activation of IRS-1, PKC-λ/ι, NF-κB, and mTOR. These progranulin-related changes in IRS-1 activation were due to JNK-mediated phosphorylation of inhibitory serine-302/307 residues in IRS-1. Progranulin deficiency in brain selectively activates an IRS-1-dependent insulin signaling pathway, and the resultant increases in inflammation and impaired autophagy/lysosomal function may augment progranulin deficiency-related neuropathology.