Fragile X syndrome is the leading monogenic cause of ASD. Trinucleotide repeats in the FMR1 gene abolish FMRP protein expression, leading to hyperactivation of ERK and mTOR signaling, upstream of mRNA translation. Here we show that metformin, the most widely used anti-type 2 diabetes drug, rescues core phenotypes in Fmr1-/y mice and selectively normalizes Erk signaling, Eif4e phosphorylation and the expression of Mmp9. Thus, metformin is a potential FXS therapeutic. Dysregulated mRNA translation is linked to core pathologies diagnosed in the Fragile X neurodevelopmental Syndrome (FXS), such as social and behavior problems, developmental delays and learning disabilities 1,2. In the brains of FXS patients and knockout mice (Fmr1-/y ; X-linked Fmr1 deletion in male mice), loss of Fragile X mental retardation protein (FMRP) results in hyperactivation of the mammalian/mechanistic target of rapamycin complex 1 (mTORC1) and the extracellular signal-regulated kinase (ERK) signaling pathways 1,2. Consistent with increased ERK activity, eukaryotic initiation factor 4E (eIF4E) phosphorylation is elevated in the brain of FXS patients and Fmr1-/y mice, thereby promoting translation of the mRNA encoding for matrix metalloproteinase 9 (MMP-9), which is elevated in the brains of both FXS patients and the Fmr1-/y mice 1-5. In accordance with these findings, knockout of Mmp9 rescues the majority of phenotypes in Fmr1-/y mice. MMP-9 degrades components of the extracellular matrix, including proteins important for synaptic function and maturation, which are implicated in FXS and autism spectrum disorders (ASD). Recent observations indicate that metformin, a first-line therapy for type 2 diabetes, imparts numerous health benefits beyond its original therapeutic use, such as decreased cancer risk and improved cancer prognosis 6. Metformin inhibits the mitochondrial respiratory chain complex 1, leading to a decrease in cellular energy state and thus activation of the energy sensor AMP-activated protein kinase (AMPK) 6. Several AMPK-independent activities of metformin have also been reported 7,8. Since metformin suppresses translation by inhibiting

The MAPK/ERK (mitogen-activated protein kinases/extracellular signal-regulated kinase) pathway is a cardinal regulator of synaptic plasticity, learning, and memory in the hippocampus. One of major endpoints of this signaling cascade is the 5′ mRNA cap binding protein eIF4E (eukaryotic Initiation Factor 4E), which is phosphorylated on Ser 209 by MNK (MAPK-interacting protein kinases) and controls mRNA translation. The precise role of phospho-eIF4E in the brain is yet to be determined. Herein, we demonstrate that ablation of eIF4E phosphorylation in male mice (4Eki mice) does not impair long-term spatial or contextual fear memory, or the late phase of LTP. Using unbiased translational profiling in mouse brain, we show that phospho-eIF4E differentially regulates the translation of a subset of mRNAs linked to inflammation, the extracellular matrix, pituitary hormones, and the serotonin pathway. Consequently, 4Eki male mice display exaggerated inflammatory responses and reduced levels of serotonin, concomitant with depression and anxiety-like behaviors. Remarkably, eIF4E phosphorylation is required for the chronic antidepressant action of the selective serotonin reuptake inhibitor fluoxetine. Finally, we propose a novel phospho-eIF4E-dependent translational control mechanism in the brain, via the GAIT complex (gamma IFN activated inhibitor of translation). In summary, our work proposes a novel translational control mechanism involved in the regulation of inflammation and depression, which could be exploited to design novel therapeutics.SIGNIFICANCE STATEMENT We demonstrate that downstream of the MAPK (mitogen-activated protein kinase) pathway, eukaryotic Initiation Factor 4E (eIF4E) Ser209 phosphorylation is not required for classical forms of hippocampal LTP and memory. We reveal a novel role for eIF4E phosphorylation in inflammatory responses and depression-like behaviors. eIF4E phosphorylation is required for the chronic action of antidepressants, such as fluoxetine in mice. These phenotypes are accompanied by selective translation of extracellular matrix, pituitary hormones, and serotonin pathway genes, in eIF4E phospho-mutant mice. We also describe a previously unidentified translational control mechanism in the brain, whereby eIF4E phosphorylation is required for inhibiting the translation of gamma IFN activated inhibitor of translation element-containing mRNAs. These findings can be used to design novel therapeutics for depression.

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Existing assays of social interaction are suboptimal, and none measures propinquity, the tendency of rodents to maintain close physical proximity. These assays are ubiquitously performed using inbred mouse strains and mutations placed on inbred genetic backgrounds. We developed the automatable tube cooccupancy test (TCOT) based on propinquity, the tendency of freely mobile rodents to maintain close physical proximity, and assessed TCOT behavior on a variety of genotypes and social and environmental conditions. In outbred mice and rats, familiarity determined willingness to cooccupy the tube, with siblings and/or cagemates of both sexes exhibiting higher cooccupancy behavior than strangers. Subsequent testing using multiple genotypes revealed that inbred strain siblings do not cooccupy at higher rates than strangers, in marked contrast to both outbred and rederived wild mice. Mutant mouse strains with "autistic-like" phenotypes (Fmr1 −/y and Eif4e Ser209Ala) displayed significantly decreased cooccupancy.propinquity | autism | social interaction | rodent behavior | genetics