Impaired activity-dependent FMRP translation and enhanced mGluR-dependent LTD in Fragile X premutation mice

Abstract

Fragile X premutation-associated disorders, including Fragile X-associated Tremor Ataxia Syndrome, result from unmethylated CGG repeat expansions in the 5' untranslated region (UTR) of the FMR1 gene. Premutation-sized repeats increase FMR1 transcription but impair rapid translation of the Fragile X mental retardation protein (FMRP), which is absent in Fragile X Syndrome (FXS). Normally, FMRP binds to RNA and regulates metabotropic glutamate receptor (mGluR)-mediated synaptic translation, allowing for dendritic synthesis of several proteins. FMRP itself is also synthesized at synapses in response to mGluR activation. However, the role of activity-dependent translation of FMRP in synaptic plasticity and Fragile X-premutation-associated disorders is unknown. To investigate this question, we utilized a CGG knock-in mouse model of the Fragile X premutation with 120-150 CGG repeats in the mouse Fmr1 5' UTR. These mice exhibit increased Fmr1 mRNA production but impaired FMRP translational efficiency, leading to a modest reduction in basal FMRP expression. Cultured hippocampal neurons and synaptoneurosomes derived from CGG KI mice demonstrate impaired FMRP translation in response to the group I mGluR agonist 3,5-dihydroxyphenylglycine. Electrophysiological analysis reveals enhanced mGluR-mediated long-term depression (mGluR-LTD) at CA3-CA1 synapses in acute hippocampal slices prepared from CGG KI mice relative to wild-type littermates, similar to Fmr1 knockout mice. However, unlike mGluR-LTD in mice completely lacking FMRP, mGluR-LTD in CGG knock-in mice remains dependent on new protein synthesis. These studies demonstrate partially overlapping synaptic plasticity phenotypes in mouse models of FXS and Fragile X premutation disorders and support a role for activity-dependent synthesis of FMRP in enduring forms of synaptic plasticity.

Enhanced Asynchronous Ca2+ Oscillations Associated with Impaired Glutamate Transport in Cortical Astrocytes Expressing Fmr1 Gene Premutation Expansion

“…This is consistent with similar results obtained from induced pluripotent stem cell neurons derived from a patient with an FMR1 premutation expansion (48), as well as neuronal/astrocyte co-cultures and slices from preCGG mouse hippocampus that exhibit amplified type I mGluR-dependent clustered burst firing and enhanced mGluR-dependent long term depression, which is positively associated with the CGG repeat length (23,54,55).…”

F-box Only Protein 2 (Fbxo2) Regulates Amyloid Precursor Protein Levels and Processing

“…Prism software (version 6, GraphPad) was used for statistical analysis and to generate graphs. Immunofluorescence data were analyzed using ImageJ as described previously (25).…”

Effects of clonidine and methylphenidate on motor activity in Fmr1 knockout mice

“…FMR1 silencing results in a lack of fragile X mental retardation protein (FMRP) production leading to abnormal dendritic overgrowth of immature spines [2,3]. The mechanism by which these phenomena occur is unknown, but it is hypothesized to be a consequence of enhanced activity of metabotropic glutamate receptors (mGluR) [4,5]; although, FMRP has also been found to affect additional neurotransmitter case reports [16,17], clinical impressions [12], or open-label trials [18]. Only two double-blind, placebo-controlled clinical trials of psychotropic medication in FXS have been published [19,20].…”