A fragile X mental retardation-like gene in a cnidarian
Introduction
The fragile X syndrome is the most common form of inherited mental retardation in humans. The disease is caused by a loss of function mutation in the fragile X mental retardation gene, FMR1, mostly due to the expansion of a CGG repeat in the 5′ untranslated region of the gene. Repeat expansion is followed by hypermethylation resulting in the transcriptional silencing of the FMR1 gene (Pieretti et al., 1991, Verkerk et al., 1991).
FMR1 (also termed fragile X mental retardation protein [FMRP]) is an RNA binding protein that contains at least three types of RNA binding motifs: a ribonucleoprotein K homology domain (KH domain; FMR1 has two such domains), an arginine and glycine rich domain (RGG box) and an N-terminal NDF domain shown to have RNA and protein binding properties (Siomi et al., 1994, Adinolfi et al., 2003, Ramos et al., 2003a). The so-called FMR1/FXR interaction domain located near the amino terminus of the protein is responsible for the dimerization and interaction with the two other members of the same protein family (Zhang et al., 1995, Siomi et al., 1996). A ribosomal interaction domain mediates interactions with the 60 S ribosomal subunit (Khandjian et al., 1996, Siomi et al., 1996, Tamanini et al., 1996, Feng et al., 1997). Due to the presence of a nuclear export signal sequence (NES), it has been proposed that FMR1 may shuttle between the nucleus and cytoplasm while carrying its target RNAs (Eberhart et al., 1996, Bardoni et al., 1997). Functional studies have revealed a role of FMR1 in dendritic and synapse development (Laggerbauer et al., 2001, Zhang et al., 2001, Morales et al., 2002, Sung et al., 2003). Recently, FMR1 has also been proposed to function in the RNAi machinery as it was shown to be present in the RISC, the RNA-induced silencing complex (Caudy et al., 2002, Jin et al., 2004).
Vertebrate genomes contain one copy of the FMR1 gene and two autosomal paralogues termed fraxile X related 1 (FXR1) and FXR2 (Zhang et al., 1995). The evolutionary history of the three genes is unclear. The first invertebrate orthologue of FMR1 was isolated from Drosophila (Wan et al., 2000) and was termed dFMR1. Its gene product exhibits a very high sequence similarity with the human and other vertebrate FXR proteins and is the only invertebrate FXR characterized so far. Database searches of available complete genome sequences of Caenorhabditis elegans and Saccharomyces cerevisiae have not revealed any FMR1/FXR homologue (Wan et al., 2000).
We have isolated and analyzed the cDNA of a member of the FXR gene family from the marine hydroid Hydractinia echinata (for a description of the animal, see Frank et al., 2001). This animal belongs to the most ancient extant eumetazoan phylum, the Cnidaria. The fact that cnidarians are the most basal, living metazoans possessing a nervous system is intriguing in this regard. It raises the possibility that the function of FMR1 has been conserved during the evolution of the Eumetazoa, and that it has been primarily related to the development of the nervous system.
Section snippets
Isolation of HyFMR1 full-length cDNA
A cDNA fragment of 600 bp with a predicted amino acid sequence showing high similarity to FXR proteins was isolated during a differential gene expression analysis from an arrayed, nonamplified cDNA library. The full-length cDNA was obtained by rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR) according to the SMART RACE protocol (Clontech) from polyp and larvae cDNA. The oligonucleotides 5′-ACAAGGGGCCAATATCCAAG-3′ and 5′-TCTGGCTCAGGGTCAATCTTTA-3′ were used for the 3′ and
Full-length cDNA and predicted amino acid sequence
Using RACE PCR, we have isolated a cDNA clone of 1992 bp. The cDNA contained an open reading frame of 1863 bp, encoding a putative protein of 621 amino acid residues with a predicted molecular mass of 70.6 kDa. The nucleotide sequence is deposited at the EMBL database under accession number AJ829441. The predicted amino acid sequence showed the highest similarity with the human FMR1 in the first 2/3 of the protein. Similarity decreased towards the C-terminus known to be less conserved among all
Discussion
The FXR protein family was originally described in vertebrates, where its members fulfill a role in neuronal development and synapse plasticity, mostly through translational repression of specific neuronal mRNAs (Laggerbauer et al., 2001, Zhang et al., 2001, Sung et al., 2003). The first report of an FMR1 homologue outside the Vertebrata came from a study on Drosophila. Functional studies in this animal have revealed that dFMR1 is involved in neurite extension, guidance and branching (Zhang et
Acknowledgements
We thank Günter Plickert (University of Cologne) for anti-RFamide antibodies. Werner A. Müller and Regina Teo commented upon an early version of the manuscript. Funds were provided by the DFG, the German Research Foundation (grant number FR 1346/5-1 to U.F.). J.G.F. was a doctoral fellow of the DFG's Graduiertenkolleg 484 and the Landes Graduiertenförderungsprogramm Baden-Württemberg.
References (34)
- et al.
Analysis of domains affecting intracellular localization of the FMRP protein
Neurobiol. Dis.
(1997) - et al.
Drosophila lacking dfmr1 activity show defects in circadian output and fail to maintain courtship interest
Neuron
(2002) - et al.
A role for the Drosophila fragile X-related gene in circadian output
Curr. Biol.
(2002) - et al.
Comparative genomic sequence analysis of the FXR gene family: FMR1, FXR1, and FXR2
Genomics
(2001) - et al.
Drosophila fragile X protein, DFXR, regulates neuronal morphology and function in the brain
Neuron
(2002) - et al.
Totipotent migratory stem cells in a hydroid
Dev. Biol.
(2004) - et al.
Absence of expression of the FMR-1 gene in fragile X syndrome
Cell
(1991) - et al.
Essential role for KH domains in RNA binding: impaired RNA binding by a mutation in the KH domain of FMR1 that causes fragile X syndrome
Cell
(1994) - et al.
The fragile X mental retardation protein FMRP binds elongation factor 1A mRNA and negatively regulates its translation in vivo
J. Biol. Chem.
(2003) - et al.
Identification of a gene (FMR-1) containing a CGG repeat coincident with a breakpoint cluster region exhibiting length variation in fragile X syndrome
Cell
(1991)
Drosophila fragile X-related gene regulates the MAP1B homolog futsch to control synaptic structure and function
Cell
The N-terminus of the fragile X mental retardation protein contains a novel domain involved in dimerization and RNA binding
Biochemistry
Expression of FMR1, FXR1, and FXR2 genes in human prenatal tissues
J. Neuropathol. Exp. Neurol.
Fragile X-related protein and VIG associate with the RNA interference machinery
Genes Dev.
The FMR-1 protein is cytoplasmic, most abundant in neurons and appears normal in carriers of a fragile X premutation
Nat. Genet.
The fragile X mental retardation protein is a ribonucleoprotein containing both nuclear localization and nuclear export signals
Hum. Mol. Genet.
Fragile X mental retardation protein: nucleocytoplasmic shuttling and association with somatodendritic ribosomes
J. Neurosci.
Cited by (13)
Oxidative stress reveals heterogeneity of FMRP granules in PC12 cell neurites
2006, Brain ResearchCitation Excerpt :Thus, the TIA-1-containing granules induced by arsenite treatment mark mRNAs that normally would go on to be translated absent the stress. Since it is thought that mRNAs traffic down dendrites and neurites as translationally repressed particles, some of which associate with FMRP, we asked whether FMRP was always associated with TIA-1 in neurites following arsenite treatment (De Diego Otero et al., 2002; Guduric-Fuchs et al., 2004). To address this question, we measured the percentage of co-localizing TIA-1 and FMRP granules in the soma, neurites and growth cones of PC12 cells treated with NGF and arsenite.
Structural but not functional conservation of an immune molecule: A tachylectin-like gene in Hydractinia
2006, Developmental and Comparative ImmunologyThe fragile X syndrome: Exploring its molecular basis and seeking a treatment
2006, Expert Reviews in Molecular MedicineShared cell biological functions may underlie pleiotropy of molecular interactions in the germ lines and nervous systems of animals
2020, Frontiers in Ecology and EvolutionIncreasing our understanding of human cognition through the study of fragile X syndrome
2014, Developmental NeurobiologyManipulating the fragile X mental retardation proteins in the frog
2012, Results and Problems in Cell Differentiation