Two members of the Fxr gene family, Fmr1 and Fxr1, are differentially expressed in Xenopus tropicalis

Abstract

The Fxr gene family is composed of three members, FMR1, FXR1 and FXR2. The FMR1 gene is involved in the fragile X syndrome, whereas for the other two members, no human disorder has been identified yet. An appropriate animal model to study in vivo gene function is essential to unravel the cellular function of the gene products FMRP, FXR1P and FXR2P, respectively. In Xenopus tropicalis both Fmr1 and Fxr1 were identified; however, unexpectedly Fxr2 was not. Here we describe the characterization of both Fmrp and Fxr1p in Xenopus tropicalis. Fmrp is expressed ubiquitously throughout the embryo during embryonic development, whereas Fxr1p shows a more tissue-specific expression particularly during late embryonic development. In adult frogs both proteins are highly expressed in most neurons of the central nervous system and in all spermatogenic cells in the testis. In addition, Fxr1p is also highly expressed in striated muscle tissue. Western blotting experiments revealed only one prominent isoform for both proteins using different tissue homogenates from adult frogs. Thus, for in vivo gene function studies, this relative simple animal model may serve as a highly advantageous and complementary model. KEY WORDS: FMR1, FXR1, fragile X syndrome, costameres, mental retardationFMRP, FXR1P and FXR2P belong to a small family of fragile Xrelated proteins (FXR family). Absence of FMRP in neurons of the central nervous system is the cause of the fragile X syndrome, the most common inherited form of mental retardation in humans (Bardoni and Mandel, 2002). The fragile X syndrome is a neurodevelopmental disorder characterized by immature dendritic spines and altered synaptic strength . The absence or dysfunction of both FXR1P and FXR2P has not yet been defined to a human disease; however, a function of FXR1P in striated muscle development has been suggested (Dube et al., 2000, Mientjes et al., 2004. The cellular function of FXR2P, like FMRP, seems to be related to learning processes (Bontekoe et al., 2002). For all three genes mouse models have been generated and specific phenotypes have been reported (Bakker et al., 1994, Bontekoe et al., 2002, Mientjes et al., 2004. Unfortunately, in vivo gene function studies in mammalians, including mice, focusing on the period of embryonic development are difficult. As a first step to generate a more suitable vertebrate animal model to study the physiological function of the three Fxr genes during embryonic development an initial characterization of the Fxr gene family has been performed in Xenopus tropicalis. Unlike the slow-growing, tetraploid Xenopus laevis, however, Int. J. Dev. Biol. 49: 437-441 (2005) doi: 10.1387/ijdb.051974lb Xenopus tropicalis is diploid and has a relatively short (<5 months) life cycle. A high level of sequence conservation of the FXR genes throughout various vertebrates (mouse, rat, zebrafish) has been demonstrated (Engels et al., 2004). This was used to screen the Sanger Institute X. tropicalis EST database with sequences from the human FXR family memb...

Fragile X Mental Retardation Protein FMRP Binds mRNAs in the Nucleus

“…One explanation for the relative weakness of FMRP's NLS might be that it requires additional factors to be activated. Perhaps cell-type or cell-cycle-specific proteins facilitate the nuclear import of FMRP under specific conditions, as in the early stages of Xenopus and zebrafish development when FMRP is primarily nuclear (9,81). In contrast, the removal of FMRP's NLS led to an improperly functioning protein that could not be evaluated for its ability to traffic in cells.…”

“…To determine whether endogenous Xenopus FMRP is also present on LBCs, we stained nuclear spreads of uninjected oocytes with an antibody directed against Xenopus FMRP (9). Figure 10B and D show that anti-FMRP labeled the chromosomal loops, strongly suggesting that endogenous FMRP associates with nascent transcripts.…”

“…In addition, the presence of FMRP in the nucleus is regulated temporally, such that at specific times during development, FMRP is predominantly nuclear. Studies in Xenopus tropicalis embryos showed that FMRP was largely nuclear 2 h postfertilization (stage 6), suggesting a special nuclear function during this developmental period (9). Zebrafish embryos also demonstrated predominantly nuclear FMRP staining very early in development, 3 h postfertilization (81).…”

A Chromatin-Dependent Role of the Fragile X Mental Retardation Protein FMRP in the DNA Damage Response

“…However, several reports implicated a potential role for FMRP in the nucleus. Studies in Xenopus and zebrafish showed that at 2–3 hours post-fertilization, Fmrp is predominantly nuclear (Blonden et al, 2005; Kim et al, 2009; van’t Padje et al, 2005). In addition, Fmrp was found to decorate lampbrush chromosomes in Xenopus oocytes (Kim et al, 2009).…”

Variabilidade do domínio KH-2 da proteína do retardo mental do X frágil (FMRP)

“…As Fmr1 exon 12 is alternatively spliced Eichler et al, 1993;, its inclusion in Fmr1 transcripts leads to an in-frame extension of the variable loop of FMRP KH-2 domain. Interestingly, the segment encoded by Fmr1 exons 11 and 12 is absent in Fmr1 invertebrate orthologs and in the vertebrate paralogous genes, Fragile X-related 1 (FXR1) and 2 (FXR2) genes, potentially implicating FMRP in differential, specialized roles, as yet unknown (Wan et al, 2000;Tucker et al, 2004;Blonden et al, 2005).…”