Excessive proteostasis contributes to pathology in fragile X syndrome.

In fragile X syndrome (FX), the leading monogenic cause of autism, excessive neuronal protein synthesis is a core pathophysiology; however, an overall increase in protein expression is not observed. Here, we tested whether excessive protein synthesis drives a compensatory rise in protein degradation that is protective for FX mouse model (Fmr1-/y) neurons. Surprisingly, although we find a significant increase in protein degradation through ubiquitin proteasome system (UPS), this contributes to pathological changes. Normalizing proteasome activity with bortezomib corrects excessive hippocampal protein synthesis and hyperactivation of neurons in the inferior colliculus (IC) in response to auditory stimulation. Moreover, systemic administration of bortezomib significantly reduces the incidence and severity of audiogenic seizures (AGS) in the Fmr1-/y mouse, as does genetic reduction of proteasome, specifically in the IC. Together, these results identify excessive activation of the UPS pathway in Fmr1-/y neurons as a contributor to multiple phenotypes that can be targeted for therapeutic intervention.

Lovastatin, not Simvastatin, Corrects Core Phenotypes in the Fragile X Mouse Model.

The cholesterol-lowering drug lovastatin corrects neurological phenotypes in animal models of fragile X syndrome (FX), a commonly identified genetic cause of autism and intellectual disability (ID). The therapeutic efficacy of lovastatin is being tested in clinical trials for FX; however, the structurally similar drug simvastatin has been proposed as an alternative due to an increased potency and brain penetrance. Here, we perform a side-by-side comparison of the effects of lovastatin and simvastatin treatment on two core phenotypes in Fmr1-/y mice versus WT littermates: excessive hippocampal protein synthesis and susceptibility to audiogenic seizures (AGSs). We find that simvastatin does not correct excessive hippocampal protein synthesis in the Fmr1-/y hippocampus at any dose tested. In fact, simvastatin significantly increases protein synthesis in both Fmr1-/y and WT. Moreover, injection of simvastatin does not reduce AGS in the Fmr1-/y mouse, while lovastatin significantly reduces AGS incidence and severity versus vehicle-treated animals. These results show that unlike lovastatin, simvastatin does not correct core phenotypes in the Fmr1-/y mouse model.

Cell-Type-Specific Translation Profiling Reveals a Novel Strategy for Treating Fragile X Syndrome.

Excessive mRNA translation downstream of group I metabotropic glutamate receptors (mGlu1/5) is a core pathophysiology of fragile X syndrome (FX); however, the differentially translating mRNAs that contribute to altered neural function are not known. We used translating ribosome affinity purification (TRAP) and RNA-seq to identify mistranslating mRNAs in CA1 pyramidal neurons of the FX mouse model (Fmr1-/y) hippocampus, which exhibit exaggerated mGlu1/5-induced long-term synaptic depression (LTD). In these neurons, we find that the Chrm4 transcript encoding muscarinic acetylcholine receptor 4 (M4) is excessively translated, and synthesis of M4 downstream of mGlu5 activation is mimicked and occluded. Surprisingly, enhancement rather than inhibition of M4 activity normalizes core phenotypes in the Fmr1-/y, including excessive protein synthesis, exaggerated mGluR-LTD, and audiogenic seizures. These results suggest that not all excessively translated mRNAs in the Fmr1-/y brain are detrimental, and some may be candidates for enhancement to correct pathological changes in the FX brain.