leupeptins and 3-4-dihydroxyphenylglycol

Fragile X syndrome is caused by the loss of fragile X mental retardation protein (FMRP), which represses and reversibly regulates the translation of a subset of mRNAs in dendrites. Protein synthesis can be rapidly stimulated by mGluR-induced and protein phosphatase 2a (PP2A)-mediated dephosphorylation of FMRP, which is coupled to the dissociation of FMRP and target mRNAs from miRNA-induced silencing complexes. Here, we report the rapid ubiquitination and ubiquitin proteasome system (UPS)-mediated degradation of FMRP in dendrites upon DHPG (3,5-dihydroxyphenylglycine) stimulation in cultured rat neurons. Using inhibitors to PP2A and FMRP phosphomutants, degradation of FMRP was observed to depend on its prior dephosphorylation. Translational induction of an FMRP target, postsynaptic density-95 mRNA, required both PP2A and UPS. Thus, control of FMRP levels at the synapse by dephosphorylation-induced and UPS-mediated degradation provides a mode to regulate protein synthesis.

Topics: Analysis of Variance; Animals; Boronic Acids; Bortezomib; Cells, Cultured; Dendrites; Disks Large Homolog 4 Protein; Drosophila Proteins; Embryo, Mammalian; Enzyme Inhibitors; Female; Fragile X Mental Retardation Protein; Gene Expression Regulation; Green Fluorescent Proteins; Hippocampus; Immunoprecipitation; Intracellular Signaling Peptides and Proteins; Leupeptins; Male; Membrane Proteins; Methoxyhydroxyphenylglycol; Mutation; Neurons; Okadaic Acid; Phosphoprotein Phosphatases; Phosphorylation; Protein Biosynthesis; Pyrazines; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; RNA, Messenger; Serine; Signal Transduction; Synapses; Transfection; Ubiquitination

Genetic deletion of fragile X mental retardation protein (FMRP) has been shown to enhance mGluR-dependent long-term depression (LTD). Herein, we demonstrate that mGluR-LTD induces a transient, translation-dependent increase in FMRP that is rapidly degraded by the ubiquitin-proteasome pathway. Moreover, proteasome inhibitors abolished mGluR-LTD, and LTD was absent in mice that overexpress human FMRP. Neither translation nor proteasome inhibitors blocked the augmentation of mGluR-LTD in FMRP-deficient mice. In addition, mGluR-LTD is associated with rapid increases in the protein levels of FMRP target mRNAs in wild-type mice. Interestingly, the basal levels of these proteins were elevated and their synthesis was improperly regulated during mGluR-LTD in FMRP-deficient mice. Our findings indicate that hippocampal mGluR-LTD requires the rapid synthesis and degradation of FMRP and that mGluR-LTD triggers the synthesis of FMRP binding mRNAs. These findings indicate that the translation, ubiquitination, and proteolysis of FMRP functions as a dynamic regulatory system for controlling synaptic plasticity.

Topics: Animals; Animals, Newborn; Anisomycin; Benzoates; Blotting, Western; Cysteine Proteinase Inhibitors; Dose-Response Relationship, Drug; Drug Interactions; Excitatory Amino Acid Antagonists; Fluorescent Antibody Technique; Fragile X Mental Retardation Protein; Glycine; In Vitro Techniques; Leupeptins; Long-Term Synaptic Depression; Male; Methoxyhydroxyphenylglycol; Mice; Mice, Knockout; Microtubule-Associated Proteins; Models, Biological; Proteasome Endopeptidase Complex; Protein Biosynthesis; Protein Synthesis Inhibitors; Pyridines; Receptors, Metabotropic Glutamate; RNA, Messenger; Signal Transduction