S6K1 Phosphorylates and Regulates Fragile X Mental Retardation Protein (FMRP) with the Neuronal Protein Synthesis-dependent Mammalian Target of Rapamycin (mTOR) Signaling Cascade

Stephen T. Warren,Mika Nakamoto,George Thomas,Stephanie Ceman,Usha Narayanan,Vijayalaxmi Nalavadi,Gary J. Bassell

doi:10.1074/jbc.c800055200

Abstract

Fragile X syndrome is a common form of cognitive deficit caused by the functional absence of fragile X mental retardation protein (FMRP), a dendritic RNA-binding protein that represses translation of specific messages. Although FMRP is phosphorylated in a group I metabotropic glutamate receptor (mGluR) activity-dependent manner following brief protein phosphatase 2A (PP2A)-mediated dephosphorylation, the kinase regulating FMRP function in neuronal protein synthesis is unclear. Here we identify ribosomal protein S6 kinase (S6K1) as a major FMRP kinase in the mouse hippocampus, finding that activity-dependent phosphorylation of FMRP by S6K1 requires signaling inputs from mammalian target of rapamycin (mTOR), ERK1/2, and PP2A. Further, the loss of hippocampal S6K1 and the subsequent absence of phospho-FMRP mimic FMRP loss in the increased expression of SAPAP3, a synapse-associated FMRP target mRNA. Together these data reveal a S6K1-PP2A signaling module regulating FMRP function and place FMRP phosphorylation in the mGluR-triggered signaling cascade required for protein-synthesis-dependent synaptic plasticity.

Highlights

Kinase signaling cascades (14 –16), and we recently reported that mammalian target of rapamycin (mTOR) influences the temporal pattern of fragile X mental retardation protein (FMRP) phosphorylation through phosphatase 2A (PP2A), a major FMRP phosphatase [22]
S499A FMRP L-cells were probed for S6 kinase 1 (S6K1); FMRP was used as a loading control
Recombinant His-FMRP was phosphorylated by purified, active GST-S6K1 but not by GST-S6K2 as scored by incorporation of [␥-32P]ATP; GST-S6K1 alone was used as a control

Summary

ACCELERATED PUBLICATION

S6K1 Phosphorylates and Regulates Fragile X Mental Retardation Protein (FMRP) with the Neuronal Protein Synthesis-dependent Mammalian Target of Rapamycin (mTOR) Signaling Cascade*□S. We identify ribosomal protein S6 kinase (S6K1) as a major FMRP kinase in the mouse hippocampus, finding that activity-dependent phosphorylation of FMRP by S6K1 requires signaling inputs from mammalian target of rapamycin (mTOR), ERK1/2, and PP2A. The loss of hippocampal S6K1 and the subsequent absence of phosphoFMRP mimic FMRP loss in the increased expression of SAPAP3, a synapse-associated FMRP target mRNA Together these data reveal a S6K1-PP2A signaling module regulating FMRP function and place FMRP phosphorylation in the mGluR-triggered signaling cascade required for proteinsynthesis-dependent synaptic plasticity. Less than 2 min following mGluR activation, FMRP is rephosphorylated in a PP2A- and mammalian target of rapamycin (mTOR)-dependent fashion, correlating with SAPAP3 translational repression and suggesting phosphorylation as a regulator of group I mGluR-mediated FMRP translational suppression.

RESULTS

EXPERIMENTAL PROCEDURES

The transfections were found to be efficient and relatively

To examine the signaling that leads to FMRP phosphorylation by

DISCUSSION