Xoom: a novel oocyte membrane protein maternally expressed and involved in the gastrulation movement of Xenopus embryos.

During a process of differential display screening for lithium-responsive mRNA in Xenopus embryos, we found a maternal transcript which is remarkably reduced by lithium. The isolated cDNA, designated Xoom, encoded a novel oocyte membrane protein with a signal sequence in the N-terminus and a single transmembrane domain. The extracellular domain contained a cysteine-rich region and a serine/threonine-rich region, which suggests an extracellular association with some proteins. Expression of Xoom maternally occurred in the whole oocyte at the early stage of oogenesis and the transcript was gradually localized in the animal hemisphere of full-grown oocytes. The zygotic expression was detected at first in the dorsoanterior region of the neural fold stage embryo. Thereafter, localized expression of Xoom was observed in neural crest cells of the neural tube stage embryo and in optic and otic vesicles of tadpole. Xoom has been expressed ubiquitously in adult organs, especially with a high level in the eye, heart, liver and kidney. In examining a relation between Xoom and the dorsoventral patterning, lithium-treatment at 32-cell stage embryo decreased Xoom mRNA level within an hour, but coinjection of lithium with myo-inositol reversed the decreasing Xoom mRNA to normal level. UV-irradiation had no effect on the maternal mRNA level of Xoom. Overexpression of Xoom showed no effect on development, but antisense Xoom RNA causes interference with normal gastrulation movement. These results suggest that maternally expressed and membrane-associated Xoom is closely involved in the gastrulation movement through a lithium-inducible signal pathway.