Stress granules formation and cell survival are differently modulated by group-I metabotropic glutamate receptor activation in wild type and fmr1 knockout astrocytes

Fragile X syndrome (FXS) is the most common form of inherited mental retardation and autism and is caused by the lack of Fragile X Mental Retardation Protein (FMRP), an RNA binding protein involved in RNA metabolism and protein synthesis. Several studies report that FMRP is associated with the pool of mRNAs that go into stress granules (SGs) upon cellular stress. Stress granules are cytoplasmic aggregates that are formed under stress conditions such as exposure to heat, oxidative agents, UV irradiation; these foci are thought to serve as sites of mRNA storage or triage during the cell stress response. SGs contain translation initiation components and many additional proteins affecting mRNA function. After environmental stress, mRNAs stored in this abortive translation initiation complex can be routed to either translation initiation or degradation. The link between SG formation and cell survival after stress is poorly understood. In this study we tested the appearance of SGs and the cell survival in wild type (WT) and Fmr1 knockout (KO) cultured astrocytes after exposure to sodium arsenite (200 uM for 1 hour or 500 µM for 30 minutes) and hydrogen peroxide (500 uM for 1 hour), by immunocytochemistry using an anti-TIA antibody and MTT assay, respectively. We also tested the effect of (S)-3,5-Dihydroxyphenylglycine (DHPG, 100 uM for 5 minutes), a selective agonist of group-I metabotropic glutamate (mGlu) receptor subtype1/5 on both SG formation and oxidative stress induced damage in both WT and Fmr1 KO astrocytes. We observed an increased number of cells with granules after exposure to oxidative stress, as expected; however, we detected a lower number of cells with granules in Fmr1 KO than WT astrocytes both under basal condition and after exposure to oxidative stress. Interestingly, under the same oxidative stress conditions cell survival was lower in Fmr1 KO than WT astrocytes. The activation of mGlu receptors before oxidative stress reduced both SG formation and cell survival in WT but had no effect in Fmr1 KO astrocytes. These results indicate that the lack of FMRP impairs the formation of SGs and sensitize to oxidative stress-induced damage. Furthermore, the observed differential effect of group-I mGlu receptors activation on both SG formation and cell survival in the presence and absence of FMRP suggests an interesting signalling interplay among SGs and mGlu receptors which may be related to their involvement in FMRP-mediated RNA transport and protein synthesis.