Fragile X Mental Retardation Protein (FMRP) controls diacylglycerol kinase activity in neurons

Significance Fragile X syndrome (FXS), the most frequent form of inherited intellectual disability, is caused by the absence of the protein Fragile X Mental Retardation Protein (FMRP) in neurons. In the absence of FMRP, the translation of a high number of mRNAs is increased in glutamatergic synapses, leading to abnormal synaptic function. It is unclear whether FMRP individually controls each of these mRNAs and whether some mRNAs are more important for the pathology. This study shows that FMRP mostly associates with and controls one main mRNA target in neurons, diacylglycerol kinase kappa (Dgkκ), a master regulator that controls two key signaling pathways activating protein synthesis. The deregulation of Dgkκ could account for many of the symptoms associated with FXS and could represent a novel therapeutic target. Fragile X syndrome (FXS) is caused by the absence of the Fragile X Mental Retardation Protein (FMRP) in neurons. In the mouse, the lack of FMRP is associated with an excessive translation of hundreds of neuronal proteins, notably including postsynaptic proteins. This local protein synthesis deregulation is proposed to underlie the observed defects of glutamatergic synapse maturation and function and to affect preferentially the hundreds of mRNA species that were reported to bind to FMRP. How FMRP impacts synaptic protein translation and which mRNAs are most important for the pathology remain unclear. Here we show by cross-linking immunoprecipitation in cortical neurons that FMRP is mostly associated with one unique mRNA: diacylglycerol kinase kappa (Dgkκ), a master regulator that controls the switch between diacylglycerol and phosphatidic acid signaling pathways. The absence of FMRP in neurons abolishes group 1 metabotropic glutamate receptor-dependent DGK activity combined with a loss of Dgkκ expression. The reduction of Dgkκ in neurons is sufficient to cause dendritic spine abnormalities, synaptic plasticity alterations, and behavior disorders similar to those observed in the FXS mouse model. Overexpression of Dgkκ in neurons is able to rescue the dendritic spine defects of the Fragile X Mental Retardation 1 gene KO neurons. Together, these data suggest that Dgkκ deregulation contributes to FXS pathology and support a model where FMRP, by controlling the translation of Dgkκ, indirectly controls synaptic proteins translation and membrane properties by impacting lipid signaling in dendritic spine.