Hyperconnectivity of neuronal circuits due to increased synaptic protein synthesis is postulated to cause Autism Spectrum Disorders (ASD). The mammalian target of rapamycin (mTOR) is strongly implicated in ASD via upstream signaling. However, downstream regulatory mechanisms are ill-defined. We show that knockout (KO) of the eukaryotic translation Initiation Factor 4E-Binding Protein 2 (4E-BP2), an eIF4E-repressor downstream of mTOR, or eIF4E overexpression lead to increased translation of neuroligins, which are post-synaptic proteins that are causally linked to ASD. 4E-BP2-KO mice exhibit an increased ratio of excitatory to inhibitory synaptic inputs and autistic-like behaviors: social interaction deficits, altered communication and repetitive/stereotyped behaviors. Pharmacological inhibition of eIF4E activity or normalization of neuroligin 1, but not neuroligin 2 protein amounts, restore the normal excitation/inhibition ratio and rectify the social behavior deficits. Thus, translational control by eIF4E regulates the synthesis of neuroligins, maintaining the excitation to inhibition balance, and its dysregulation engenders ASD-like phenotypes.

The translation initiation step in eukaryotes is highly regulated and rate-limiting. During this process, the 40S ribosomal subunit is usually recruited to the 5' terminus of the mRNA. It then migrates towards the initiation codon, where it is joined by the 60S ribosomal subunit to form the 80S initiation complex. Secondary structures in the 5' untranslated region (UTR) can impede binding and movement of the 40S ribosome. The canonical eukaryotic translation initiation factor eIF4A (also known as DDX2), together with its accessory proteins eIF4B and eIF4H, is thought to act as a helicase that unwinds secondary structures in the mRNA 5' UTR. Growing evidence suggests that other helicases are also important for translation initiation and may promote the scanning processivity of the 40S subunit, synergize with eIF4A to 'melt' secondary structures or facilitate translation of a subset of mRNAs.

The subcellular position of a protein is a key determinant of its function. Mounting evidence indicates that RNA localization, where specific mRNAs are transported subcellularly and subsequently translated in response to localized signals, is an evolutionarily conserved mechanism to control protein localization. On-site synthesis confers novel signaling properties to a protein and helps to maintain local proteome homeostasis. Local translation plays particularly important roles in distal neuronal compartments, and dysregulated RNA localization and translation cause defects in neuronal wiring and survival. Here, we discuss key findings in this area and possible implications of this adaptable and swift mechanism for spatial control of gene function.

SUMMARYFragile X syndrome (FXS) is the leading genetic cause of autism. Mutations in Fmr1 (fragile X mental retardation 1 gene) engender exaggerated translation resulting in dendritic spine dysmorphogenesis, synaptic plasticity alterations, and behavioral deficits in mice, which are reminiscent of FXS pheno-types. Using postmortem brains from FXS patients and Fmr1 knockout mice (Fmr1 −/y), we show that phosphorylation of the mRNA 5′ cap binding protein, eukaryotic initiation factor 4E (eIF4E), is elevated concomitant with increased expression of matrix metalloproteinase 9 (MMP-9) protein. Genetic or pharmacological reduction of eIF4E phosphorylation rescued core behavioral deficits, synaptic plasticity alterations, and dendritic spine morphology defects via reducing exaggerated translation of Mmp9 mRNA in Fmr1 −/y mice, whereas MMP-9 overexpression produced several FXS-like phenotypes. These results uncover a mechanism of regulation of synaptic function by translational control of Mmp-9 in FXS, which opens the possibility of new treatment avenues for the diverse neurological and psychiatric aspects of FXS.

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

Injury-induced sensitization of nociceptors contributes to pain states and the development of chronic pain. Inhibiting activity-dependent mRNA translation through mechanistic target of rapamycin and mitogen-activated protein kinase (MAPK) pathways blocks the development of nociceptor sensitization. These pathways convergently signal to the eukaryotic translation initiation factor (eIF) 4F complex to regulate the sensitization of nociceptors, but the details of this process are ill defined. Here we investigated the hypothesis that phosphorylation of the 5Ј cap-binding protein eIF4E by its specific kinase MAPK interacting kinases (MNKs) 1/2 is a key factor in nociceptor sensitization and the development of chronic pain. Phosphorylation of ser209 on eIF4E regulates the translation of a subset of mRNAs. We show that pronociceptive and inflammatory factors, such as nerve growth factor (NGF), interleukin-6 (IL-6), and carrageenan, produce decreased mechanical and thermal hypersensitivity, decreased affective pain behaviors, and strongly reduced hyperalgesic priming in mice lacking eIF4E phosphorylation (eIF4E S209A ). Tests were done in both sexes, and no sex differences were found. Moreover, in patch-clamp electrophysiology and Ca 2ϩ imaging experiments on dorsal root ganglion neurons, NGF-and IL-6-induced increases in excitability were attenuated in neurons from eIF4E S209A mice. These effects were recapitulated in Mnk1/2 Ϫ/Ϫ mice and with the MNK1/2 inhibitor cercosporamide. We also find that cold hypersensitivity induced by peripheral nerve injury is reduced in eIF4E S209A and Mnk1/2 Ϫ/Ϫ mice and following cercosporamide treatment. Our findings demonstrate that the MNK1/2-eIF4E signaling axis is an important contributing factor to mechanisms of nociceptor plasticity and the development of chronic pain.

A mis-sense point mutation in the human VAPB gene is associated with a familial form of motor neuron disease that has been classified as Amyotrophic Lateral Sclerosis type VIII. Affected individuals suffer from a spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS) or an atypical slowly progressing form of ALS. Mammals have two homologous VAP genes, vapA and vapB. VAPA and VAPB share 76% similar or identical amino acid residues; both are COOH-terminally anchored membrane proteins enriched on the endoplasmic reticulum. Several functions have been ascribed to VAP proteins including membrane trafficking, cytoskeleton association and membrane docking interactions for cytoplasmic factors. It is shown here that VAPA and VAPB are expressed in tissues throughout the body but at different levels, and that they are present in overlapping but distinct regions of the endoplasmic reticulum. The disease-associated mutation in VAPB, VAPB(P56S), lies within a highly conserved N-terminal region of the protein that shares extensive structural homology with the major sperm protein (MSP) from nematodes. The MSP domain of VAPA and VAPB is found to interact with the ER-localized transcription factor ATF6. Over expression of VAPB or VAPB(P56S) attenuates the activity of ATF6-regulated transcription and the mutant protein VAPB(P56S) appears to be a more potent inhibitor of ATF6 activity. These data indicate that VAP proteins interact directly with components of ER homeostatic and stress signalling systems and may therefore be parts of a previously unidentified regulatory pathway. The mis-function of such regulatory systems may contribute to the pathological mechanisms of degenerative motor neuron disease.

SUMMARY Protein synthesis is critical for circadian clock function, but little is known on how translational regulation controls the master pacemaker in mammals, the suprachiasmatic nucleus (SCN). Here we demonstrate that the pivotal translational repressor, the eukaryotic translational initiation factor 4E binding protein 1(4E-BP1) is rhythmically regulated via the mechanistic target of rapamycin (mTOR) signaling in the SCN and preferentially represses vasoactive intestinal peptide (Vip) mRNA translation. Knockout (KO) of Eif4ebp1 (gene encoding 4E-BP1) leads to upregulation of VIP and higher amplitude of molecular rhythms in the SCN. Consequently, the 4E-BP1 null mice exhibit accelerated re-entrainment to a shifted light/dark cycle and are more resistant to the rhythm-disruptive effects of constant light. Conversely, in Mtor+/− mice VIP expression is decreased and susceptibility to the effects of constant light is increased. These results reveal a novel role for mTOR/4E-BP1-mediated translational control in regulating entrainment and synchrony of the master clock.