Dysregulation and restoration of translational homeostasis in fragile X syndrome

Abstract

Fragile X syndrome (FXS), the most-frequently inherited form of intellectual disability and the most-prevalent single-gene cause of autism, results from a lack of fragile X mental retardation protein (FMRP), an RNA-binding protein that acts, in most cases, to repress translation. Multiple pharmacological and genetic manipulations that target receptors, scaffolding proteins, kinases and translational control proteins can rescue neuronal morphology, synaptic function and behavioural phenotypes in FXS model mice, presumably by reducing excessive neuronal translation to normal levels. Such rescue strategies might also be explored in the future to identify the mRNAs that are critical for FXS pathophysiology.

Figure 1. Translational control pathways that are dysregulated in FXS

Normally, stimulation of cell surface receptors, including NMDA receptors (NMDARs) and group 1 metabotropic glutamate receptors (mGluRs; such as mGluR5), results in the activation of the phosphoinositide 3-kinase (PI3K)^,, mammalian target of rapamycin complex 1 (mTORC1; which comprises mTOR bound to regulatory-associated protein of mTOR (RAPTOR)) and extracellular signal-regulated kinase (ERK) signalling pathways in neurons. mTORC1 phosphorylates eukaryotic initiation factor 4E (eIF4E) binding proteins (4E-BPs), including 4E-BP2, the predominant 4E-BP isoform in the mouse brain, which derepresses eIF4E to promote ‘cap’-dependent translation. mTORC1 also phosphorylates and activates p70 S6 kinase 1 (S6K1), which phosphorylates ribosomal protein S6 and eIF4B. Phosphorylation of eIF4B by S6K1 stimulates the helicase activity of eIF4A to promote cap-dependent translation. mTORC1-dependent translation is triggered upstream by a signalling pathway involving PI3K, 3-phosphoinositide-dependent protein kinase 1 (PDK1) and/or PDK2, AKT, tuberous sclerosis 1 (TSC1) and/ or TSC2 and RAS homologue enhanced in brain (RHEB). ERK phosphorylates and activates MAP kinase-interacting serine/threonine-protein kinases (MNKs), which phosphorylate eIF4E to promote translation. This ERK-dependent translation is triggered upstream by a pathway involving RAS, RAF and MAPK/ERK kinase 1 (MEK1). mGluR5 signals to the PI3K–AKT–mTORC1 pathway via HOMER1A, SH3 and multiple ankyrin repeat domains (SHANK) proteins and PI3K enhancer (PIKE), and NMDARs signal to ERK via postsynaptic density protein 95 (PSD95). The protein levels of several targets of the RNA-binding protein fragile X mental retardation protein (FMRP)^, are increased in fragile X syndrome (FXS). The increased expression of these proteins results in basally elevated PI3K, mTORC1 and ERK signalling and thus increased translation. m⁷G, 7-methyl-guanosine; PtdIns(4,5)P₂, phosphatidylinositol-4,5-bisphosphate; PtdIns(3,4,5)P₃, phosphatidylinositol-3,4,5-trisphosphate; UTR, untranslated region. Adapted from REF. , Nature Publishing Group.

Figure 2. FMRP may stall polyribosomes to reduce the rate of translation elongation

a | It is hypothesized that phosphorylated fragile X mental retardation protein (FMRP) associates with actively translating ribosomes and causes them to stall and accumulate on the mRNA molecule, slowing elongation (AUG and UAG are the initiation and termination codons, respectively). b | FMRP-regulated translation increases when FMRP is dephosphorylated^,, ubiquitylated^, and eventually destroyed via proteasomal degradation. c | In fragile X syndrome, the absence of FMRP results in faster translation of FMRP target mRNAs. Adapted from REF. , Nature Publishing Group.

Figure 3. Involvement of CPEB in mediating FMRP activity

Cytoplasmic polyadenylation element-binding protein (CPEB) associates with CPEs in the 3′ untranslated region (UTR) of target mRNAs and stimulates translation by promoting lengthening of the poly(A) tail. Neuroguidin is a eukaryotic initiation factor 4E (eIF4E)-binding protein (4E-BP) that binds to CPEB to prevent this CPE-dependent translation. It is known that excessive translation in fragile X syndrome (FXS) model mice is normalized by genetic reduction of CPEB, although the mechanisms through which the two proteins interact are currently unclear. In the left panel, CPEB is shown associated with fragile X mental retardation protein (FMRP) and neuroguidin, which in turn interacts with the ‘cap’-binding factor eIF4E. This configuration of factors would be hypothesized to silence mRNAs that are bound by both FMRP and CPEB. As shown in the right panel, group 1 metabotropic glutamate receptor (mGluR; not shown) activation could lead to poly(A)-tail elongation and dissociation of neuroguidin from eIF4E, thereby allowing for the assembly of the eIF4F complex (which consists of eIF4E, eIF4G and eIF4A) on the cap and the initiation of translation. In this scenario, FMRP remains bound to the mRNA and S6K1 phosphorylates the initiation factors and ribosomal protein S6 on the 40S subunit to stimulate translation. m⁷G, 7-methyl-guanosine.

Figure 4. Identification of dysregulated mRNAs that are rescued by pharmacological or genetic manipulations in FXS model mice

Venn diagram in which the circles represent populations of mRNAs (currently unknown) whose aberrant translation in fragile X mental retardation 1 (Fmr1)-knockout mice is rescued when the following are either inhibited with pharmacological agents or genetically ablated: HOMER1A; metabotropic glutamate receptor 5 (mGluR5); phosphoinositide 3-kinase (PI3K) and/ or PI3K enhancer (PIKE); p70 S6 kinase 1 (S6K1); cytoplasmic polyadenylation element-binding protein (CPEB); and MAP kinase-interacting serine/threonine-protein kinase (MNK) and/or eukaryotic initiation factor 4E (eIF4E). It is proposed that there is a subset of mRNAs that are rescued under all conditions. These mRNAs are likely to be essential for normal neuronal function and have the potential to be novel therapeutic targets. FXS, fragile X syndrome.