Title

The role of a Xenopus DAZ-like gene (Xdazl) in the development of primordial germ cells

Date of Award

1999

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Molecular Cell and Developmental Biology

First Committee Member

Mary Lou King, Committee Chair

Abstract

In the frog Xenopus laevis, a region of cytoplasm at the vegetal pole of oocytes and eggs, the germ plasm, is critical for the formation of primordial germ cells (PGCs). Only blastomeres that inherit the germ plasm can differentiate as PGCs. The germ plasm contains unique "germinal granules," and localized RNAs, as well as mitochondria. In order to study the nature and role of germ plasm components in PGC development, I have identified and characterized a localized RNA component of Xenopus germ plasm. This RNA, Xdazl (Xenopus DAZ -like), encodes a protein homologous to human DAZ (Deleted in Azoospermia), vertebrate DAZL and Drosophila Boule proteins. Human males deficient in DAZ have few or no sperm, mice deficient in Dazl have defects in gametogenesis and boule mutant flies exhibit complete azoospermia and male sterility.Xdazl RNA co-localizes with the germ plasm in the mitochondrial cloud and vegetal cortex of oocytes. In early embryos, the RNA was also localized exclusively in the germ plasm until the gastrula stage, when the transcript became undetectable. Consistent with other organisms, Xdazl RNA was also expressed in the spermatogonia and spermatocytes of frog testis. Proteins in the DAZ-family contain a conserved RNP domain implying an RNA-binding function. I have shown that Xdazl can function in vitro as an RNA-binding protein. To determine if the function of Xdazl in spermatogenesis was conserved, the Xdazl cDNA was introduced into boule flies. This resulted in rescue of the boule meiotic entry phenotype, including formation of spindles, phosphorylation of histone H3 and completion of meiotic cell division.Immunostaining with antisera to Xdazl showed that the protein was expressed in the cytoplasm of premeiotic testis germ cells, as well as in early PGCs during embryogenesis. In PGCs, Xdazl protein is expressed specifically in the germ plasm within the PGCs from blastula to early tailbud stages. These results demonstrate that Xdazl is expressed at the correct place and time to function in meiotic regulation in spermatogenesis as well as maternally in PGC development.This work also shows that Xdazl is critically involved in PGC development in Xenopus, as suggested from its germ plasm localization. Specific depletion of maternal Xdazl RNA results in tadpoles lacking, or severely deficient in, PGCs. In the absence of Xdazl, PGCs do not successfully migrate from the ventral to the dorsal endoderm and do not reach the dorsal mesentery. Germ plasm aggregation and intracellular movements are normal indicating that the defect occurs after PGC formation. I propose that Xdazl is required for early PGC differentiation and is indirectly necessary for the migration of PGCs through the endoderm. As an RNA-binding protein, Xdazl may regulate translation or expression of factors that mediate migration of PGCs.

Keywords

Biology, Molecular; Biology, Cell

Link to Full Text

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