Genetic removal of p70 S6K1 corrects coding sequence length-dependent alterations in mRNA translation in fragile X syndrome mice

Abstract

Loss of the fragile X mental retardation protein (FMRP) causes fragile X syndrome (FXS). FMRP is widely thought to repress protein synthesis, but its translational targets and modes of control remain in dispute. We previously showed that genetic removal of p70 S6 kinase 1 (S6K1) corrects altered protein synthesis as well as synaptic and behavioral phenotypes in FXS mice. In this study, we examined the gene specificity of altered messenger RNA (mRNA) translation in FXS and the mechanism of rescue with genetic reduction of S6K1 by carrying out ribosome profiling and RNA sequencing on cortical lysates from wild-type, FXS, S6K1 knockout, and double knockout mice. We observed reduced ribosome footprint (RF) abundance in the majority of differentially translated genes in the cortices of FXS mice. We used molecular assays to discover evidence that the reduction in RF abundance reflects an increased rate of ribosome translocation, which is captured as a decrease in the number of translating ribosomes at steady state and is normalized by inhibition of S6K1. We also found that genetic removal of S6K1 prevented a positive-to-negative gradation of alterations in translation efficiencies (RF/mRNA) with coding sequence length across mRNAs in FXS mouse cortices. Our findings reveal the identities of dysregulated mRNAs and a molecular mechanism by which reduction of S6K1 prevents altered translation in FXS.

Excess ribosomal protein production unbalances translation in Fragile X Syndrome

“…In contrast, recent studies by Das Sharma et al and Aryal et al suggest the reduced ribosome binding of long mRNAs in ribosome profiling from Fmr1 -/y cortical lysates represents an increase in the translation of these targets due to reduction in ribosome stalling by FMRP 11,59 .…”

“…Based on these results, the authors propose that FMRP is in fact an activator of translation for long mRNAs. In contrast, recent studies by Das Sharma et al and Aryal et al suggest the reduced ribosome binding of long mRNAs in ribosome profiling from Fmr1 -/y cortical lysates represents an increase in the translation of these targets due to reduction in ribosome stalling by FMRP 11,59 . Although these studies do not include a corresponding analysis of the proteome, the prediction is an increase in the production of FMRP target proteins.…”

“…4i ). However, it was not clear that this downregulation would indicate a reduction in protein expression, as recent studies indicate the reduced ribosome binding of these targets may represent accelerated elongation 11,59 . To investigate this question, we quantified FMRP targets in the Fmr1 -/y proteome, and found the same downregulation seen in the TRAP fraction ( Fig.…”

Transcriptomic and epigenomic consequences of heterozygous loss of function mutations inAKAP11, the first large-effect shared risk gene for bipolar disorder and schizophrenia

“…Furthermore, synaptic mitochondria content supplies energy for local translation in the process of synaptic plasticity 91,92 . As ribosomal proteins are predominantly located in neurites, downregulation of ribosomal protein genes may lead to reduced synthesis of synaptic proteins 93 as a part of a plasticity mechanism where neurons adjust their excitability in response to network activity 93 , an event that occurs in neuropsychiatric disorders 94,95 .…”

FMRP-dependent production of large dosage-sensitive proteins is highly conserved

“…However, FMRP is highly conserved in structure and function between Drosophila , mouse, and many other species, and the human FMR1 gene rescues neuronal defects in Drosophila Fmr1 mutants ( Coffee et al 2010 ). Moreover, long protein-coding capacity is emerging as a common property shared by most mRNA targets regulated by FMRP in multiple systems ( Greenblatt and Spradling 2018 ; Das Sharma et al 2019 ; Sears et al 2019 ; Aryal et al 2021 ).…”