Elevated progranulin contributes to synaptic and learning deficit due to loss of fragile X mental retardation protein

Fragile X syndrome is an inheritable form of intellectual disability caused by loss of fragile X mental retardation protein (FMRP, encoded by the FMR1 gene). Absence of FMRP caused overexpression of progranulin (PGRN, encoded by GRN), a putative tumour necrosis factor receptor ligand. In the present study, we found that progranulin mRNA and protein were upregulated in the medial prefrontal cortex of Fmr1 knock-out mice. In Fmr1 knock-out mice, elevated progranulin caused insufficient dendritic spine pruning and late-phase long-term potentiation in the medial prefrontal cortex of Fmr1 knock-out mice. Partial progranulin knock-down restored spine morphology and reversed behavioural deficits, including impaired fear memory, hyperactivity, and motor inflexibility in Fmr1 knock-out mice. Progranulin increased levels of phosphorylated glutamate ionotropic receptor GluA1 and nuclear factor kappa B in cultured wild-type neurons. Tumour necrosis factor receptor 2 antibody perfusion blocked the effects of progranulin on GluA1 phosphorylation; this result indicates that tumour necrosis factor receptor 2 is required for progranulin-mediated GluA1 phosphorylation and late-phase long-term potentiation expression. However, high basal level of progranulin in Fmr1 knock-out mice prevented further facilitation of synaptic plasticity by exogenous progranulin. Partial downregulation of progranulin or tumour necrosis factor receptor 2/nuclear factor kappa B signalling restored synaptic plasticity and memory deficits in Fmr1 knock-out mice. These findings suggest that elevated PGRN is linked to cognitive deficits of fragile X syndrome, and the progranulin/tumour necrosis factor receptor 2 signalling pathway may be a putative therapeutic target for improving cognitive deficits in fragile X syndrome.