The Role of Cytoplasmic FMRP Interacting Protein 1 In Translational Regulation In the Synapse

Background: Copy number variation at the 15q11.2 region, which includes the gene that codes for CYFIP1 (cytoplasmic FMR1 interacting protein 1), has been implicated in neuropsychiatric phenotypes. We investigated the function of Cyfip1 in synaptic physiology and behavior, using mice with a disruption of the Cyfip1 gene. We demonstrated that metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) in the CA1 region of the hippocampus was enhanced in the heterozygous mice and was independent of protein synthesis.

Objectives: In the current study we further dissected the synaptic pathways of LTD in Cyfip1-deficient mice.

Methods: We applied a combination of biochemical, immunohistochemical and electrophysiological techniques in mouse tissue, principally focusing on hippocampal area CA1.

Results: mGluR1/5 activation recruits both the mammalian target of rapamycin (mTOR) signaling pathway and the ERK pathway. We showed that in wildtype mice mGluR-LTD was sensitive to rapamycin, a selective inhibitor of mTOR, but Cyfip1 heterozygous mice showed enhanced mGluR-LTD that could not be blocked by rapamycin. During activation, mTOR phosphorylates 4E-BP, a critical step in translation control. We investigated this by examining the localization of increased 4E-BP phoshorylation using immunohistochemistry. In slices from Cyfip1 heterozygous mice, 4E-BP phosphorylation was not detectably altered by DHPG stimulation. Our results are consistent with what has recently been described for Fmr1 knockout mice. We are currently examining MEK inhibitors to determine whether enhanced LTD can be reversed by inhibiting ERK signaling. We are also directly measuring protein synthesis in these studies.

Conclusions: Our results support a shared mechanism between Fragile X syndrome and loss of a functional copy of CYFIP1. Enhanced LTD observed in either case is insensitive to either protein synthesis inhibitors or rapamycin, suggesting that the ERK pathway, which is also activated by mGluR1/5, might be involved. We hypothesize that loss of a functional copy of Cyfip1 leads to an intermediate phenotype that increases risk and/or severity of psychiatric disorders.