L-Fucose for mammalian glycosylation contains an anomeric carbon atom generating alpha- and beta-L-fucoses. Based on sequence comparison of mouse and human homologs with the prokaryotic fucose mutarotases (FucU) characterized previously, we investigated their function in mammalian cells. By nuclear magnetic resonance (NMR) measurement with saturation difference analysis, the purified mammalian mutarotases were demonstrated to be involved in an interconversion between the two anomeric forms with comparable efficiency as that of the Escherichia coli FucU. The mouse gene was widely expressed in various tissues and cell lines, including kidney, liver, and pancreas, although expression was marginal in muscle and testis. By generating stably expressed cell lines for mutarotase genes in HepG2, it was shown that fucose incorporations into cellular proteins were increased as demonstrated by an incorporation of radiolabeled fucose into the cells. Furthermore, intracellular levels of GDP-L-fucose, measured with high performance liquid chromatography (HPLC), were enhanced by an overproduction of cellular mutarotase, which was reversed by gene silencing of mutarotase based on RNA interference. The results suggest that the mammalian mutarotase is functional in facilitated incorporation of fucose through the salvage pathway.
BackgroundMutarotases are recently characterized family of enzymes that are involved in the anomeric conversions of monosaccharides. The mammalian fucose mutarotase (FucM) was reported in cultured cells to facilitate fucose utilization and incorporation into protein by glycosylation. However, the role of this enzyme in animal has not been elucidated.ResultsWe generated a mutant mouse specifically lacking the fucose mutarotase (FucM) gene. The FucM knockout mice displayed an abnormal sexual receptivity with a drastic reduction in lordosis score, although the animals were fertile due to a rare and forced intromission by a typical male. We examined the anteroventral periventricular nucleus (AVPv) of the preoptic region in brain and found that the mutant females showed a reduction in tyrosine hydoxylase positive neurons compared to that of a normal female. Furthermore, the mutant females exhibited a masculine behavior, such as mounting to a normal female partner as well as showing a preference to female urine. We found a reduction of fucosylated serum alpha-fetoprotein (AFP) in a mutant embryo relative to that of a wild-type embryo.ConclusionsThe observation that FucM-/- female mouse exhibits a phenotypic similarity to a wild-type male in terms of its sexual behavior appears to be due to the neurodevelopmental changes in preoptic area of mutant brain resembling a wild-type male. Since the previous studies indicate that AFP plays a role in titrating estradiol that are required to consolidate sexual preference of female mice, we speculate that the reduced level of AFP in FucM-/- mouse, presumably resulting from the reduced fucosylation, is responsible for the male-like sexual behavior observed in the FucM knock-out mouse.
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