The Role of FMRP in Ethanol-Induced Homeostatic Plasticity in the Hippocampus

Exposure to chronic ethanol induces homeostatic alterations in glutamatergic signaling and actin polymerization that may have an important role in the development of ethanol-seeking behaviors. Acute ethanol exposure promotes excitation and dampens inhibition while extended periods of exposure induce long-term adaptations in neuronal function that require new protein synthesis to maintain homeostasis. These adaptations include not only transcription and somatic protein synthesis, but also local dendritic protein translation. One of the major mediators of activity-dependent translation is the mTORC1 signaling pathway and its downstream substrates that include kinases and mRNA-binding proteins, such as p70 S6 kinase 1 (S6K1) and fragile X mental retardation protein (FMRP). FMRP is an mRNA-binding protein that interacts with mRNAs to suppress translation. FMRP also interacts with several different mRNAs that code for proteins that are necessary for synaptic plasticity, and it may also have an important role in regulating ethanol-induced alterations in homeostasis in dendrites and dendritic spines. This dissertation addresses the hypothesis that FMRP is necessary for activity-dependent homeostatic alterations in protein expression and spine morphology following chronic ethanol exposure. First, western blot analysis was used to investigate ethanol-induced alterations in expression of FMRP and proteins that are key mediators of dendritic excitability. These studies revealed an increase in FMRP phosphorylation as well as alterations in the A-type K+-channel Kv4.2, KChIP3 and NMDA receptor subunits. Further studies examining changes in FMRP interactions with Kv4.2, KChIP3, and NMDA mRNAs showed chronic ethanol-induced changes in FMRP-mRNA binding. Additionally, inhibition of FMRP phosphorylation prevented these alterations in protein expression and FMRP-mRNA interactions following chronic ethanol exposure. Studies included in this dissertation also addressed whether alterations in protein expression are accompanied by changes in actin polymerization and spine morphology. These experiments utilized two different sub-strains of C57BL/6 mice with different polymorphisms in cyfip2, a protein regulating actin polymerization that is also implicated in regulation of protein translation. A two-bottle choice/CIE exposure paradigm revealed alterations in ethanol consumption between the two strains as well as differences in ethanol-induced changes in protein expression and spine morphology. Taken together, this dissertation reveals an integral role for FMRP in mediating ethanol-induced alterations in homeostatic protein expression, and that these alterations may influence actin polymerization and drinking behaviors