On BC1 RNA and the fragile X mental retardation protein

Abstract

The fragile X mental retardation protein (FMRP), the functional absence of which causes fragile X syndrome, is an RNA-binding protein that has been implicated in the regulation of local protein synthesis at the synapse. The mechanism of FMRP's interaction with its target mRNAs, however, has remained controversial. In one model, it has been proposed that BC1 RNA, a small nonprotein-coding RNA that localizes to synaptodendritic domains, operates as a requisite adaptor by specifically binding to both FMRP and, via direct base-pairing, to FMRP target mRNAs. Other models posit that FMRP interacts with its target mRNAs directly, i.e., in a BC1-independent manner. Here five laboratories independently set out to test the BC1-FMRP model. We report that specific BC1-FMRP interactions could be documented neither in vitro nor in vivo. Interactions between BC1 RNA and FMRP target mRNAs were determined to be of a nonspecific nature. Significantly, the association of FMRP with bona fide target mRNAs was independent of the presence of BC1 RNA in vivo. The combined experimental evidence is discordant with a proposed scenario in which BC1 RNA acts as a bridge between FMRP and its target mRNAs and rather supports a model in which BC1 RNA and FMRP are translational repressors that operate independently.fragile X syndrome ͉ non-protein-coding RNAs ͉ translational control S mall non-protein-coding RNAs perform important functions in the regulation of eukaryotic gene expression (1). In the mammalian central nervous system, they have been implicated in promoting organism-environment interactions (2). Small untranslated BC1 RNA is a translational repressor that is thought to participate in the regulation of local protein synthesis at the synapse (2, 3). BC1 RNA represses translation by targeting assembly of 48S initiation complexes (4). Interacting with eukaryotic initiation factor 4A (eIF4A) and poly(A) binding protein (PABP) (4-6), BC1 RNA prevents recruitment of the 43S preinitiation complex to the mRNA. Targets of BC1-mediated repression are those mRNAs that depend on the eIF4 family of factors for efficient initiation (4).Fragile X syndrome is caused by the functional absence of fragile X mental retardation protein (FMRP) (7,8). Consensus has developed over recent years that FMRP is, like BC1 RNA, a translational repressor that is active in postsynaptic microdomains (7, 9, 10). However, in contrast to BC1 RNA, FMRP is associated with polysomes (11-15), indicating that BC1 RNA and FMRP operate at different levels in the translation pathway.In an alternative model, it has been proposed that BC1 RNA and FMRP interact directly with each other (16,17). In this scenario, BC1 RNA (i) physically binds to FMRP, (ii) directly interacts, by base-pairing of its 5Ј domain, with select mRNAs that are FMRP targets, and (iii) acts as a bridge between FMRP and such target mRNAs, thus serving as a requisite adaptor (16, 17).The above two models cannot be reconciled. We, five independent groups with a longstanding interest in the molecular biology of...

A metabolomic and systems biology perspective on the brain of the Fragile X syndrome mouse model

“…6A, lane 4), corroborating previous studies (Lu et al 2004;Iacoangeli et al 2008). Conversely, no amplicon corresponding to the unrelated mRNA encoding beta-Tubulin 3 (Tubb3) was detected in either immunoprecipitate.…”

“…The iMIM network derived from the metabolic signature of FXS highlights that among the 30 most pivotal proteins, 13 are either FMRP protein interactors, such as the protein FXR2P (PB = 8.23 3 10 À6 ) (Zhang et al 1995), the homolog of FMRP, or are encoded by mRNA targets of FMRP (Fig. 5), such as the mRNA encoding super oxide dismutase 1 (SOD1, PB = 1.35 3 10 À5 ) (Bechara et al 2009), amyloid beta precursor protein (APP, PB = 1.56 3 10 À5 ) (Westmark and Malter 2007), calcium/calmodulin-dependent protein kinase II alpha (CAMK2A, PB = 7.93 3 10 À6 ) (Iacoangeli et al 2008), eukaryotic translation elongation factor 1 alpha 1 (EEF1A1, PB = 6.06 3 10 À6 ) (Sung et al 2003), and microtubule-associated protein 1B (MAP1B, PB = 3.93 3 10 À6 ) Chen et al 2003;Lu et al 2004;Iacoangeli et al 2008). Interestingly, poorly characterized putative mRNA FMRP targets, such as the mRNA encoding the small GTP-ase and cytoskeleton-regulator RHOA (PB = 1.83 3 10 À5 ) (Chen et al 2003) and the calcium regulatory protein Calbindin1 (CALB1, PB = 1.35 3 10 À5 ) (Miyashiro et al 2003), appear as pivotal in our network ( Biological confirmation of the in vivo association of FMRP with mRNA encoding pivotal proteins of the iMIM network…”

DrosophilaTubulin-specific chaperone E functions at neuromuscular synapses and is required for microtubule network formation

“…Fragile X mental retardation protein (FMRP, encoded by FMR1) plays an important role at synapses (Penagarikano et al, 2007;Zhang and Broadie, 2005). At the molecular level, the MT-associated protein MAP1B is a target of FMRP and is upregulated in Fmr1 knockout mice and mutant flies 1577 RESEARCH ARTICLE TBCE functions at synapses and promotes microtubule formation (Iacoangeli et al, 2008;Lu et al, 2004;Zhang et al, 2001). Since defective MTs are implicated in both Fragile X syndrome and HRD, and both diseases show mental retardation, we sought to unravel the in vivo functions of TBCE with regard to synapse development and MT formation.…”

Cells Lacking the Fragile X Mental Retardation Protein (FMRP) have Normal RISC Activity but Exhibit Altered Stress Granule Assembly

“…FMRP was found to be associated specifically with distinct classes of mRNAs: mRNAs harboring G-quartet, poly (U) stretches, or a kissing complex motif (Schaeffer et al, 2003;Jin et al, 2004a;Darnell et al, 2005). An indirect interaction of FMRP with several mRNAs has also been proposed through its interaction with the noncoding BC1 RNA (Zalfa et al, 2003), but the biological importance of this later type of interaction appears unclear (Iacoangeli et al, 2008). Generally FMRP was found associated with several aspects of the RNA metabolism, notably translation repression (Laggerbauer et al, 2001;Castets et al, 2005), transport (Dictenberg et al, 2008), and degradation (Zalfa et al, 2007).…”