Contribution of METRO pathway localized molecules to the organization of the germ cell lineage

To elucidate the potential role of localized components in the specification of the germ cell lineage we analyzed the composition of the germ plasm in Xenopus laevis oocytes and early embryos with respect to the vegetally-localized RNAs. We focused on Xlsirts, Xcat2, and Xwnt11 transcripts that are localized to the vegetal cortex through a region of the mitochondrial cloud called the messenger transport organizer (METRO) that also contains the nuage or germ plasm. At the ultrastructural level Xcat2 mRNA was detected on germinal granules while Xlsirts and Xwnt11 were associated with a fibrillar network of the germ plasm in stage-1 and stage-4 oocytes. In embryos, we found that all three RNAs remained associated with the germ plasm. Vg1 mRNA, a transcript localized through the late pathway, was excluded from the germ plasm in oocytes and embryos. Additionally, we detected the protein spectrin within 16 cell nests of germ cells, in a structure reminiscent of the Drosophila spectrosome. Spectrin was detected in the mitochondrial cloud and was found in the germ plasm during embryogenesis. These data indicate that the various RNAs found within METRO and the protein spectrin are integral components of the Xenopus germ plasm with the RNAs being associated with different subcellular structures. They also suggest that the pathway through which RNAs are localized during oogenesis may be an important factor in biasing their distribution into specific cell lineages. The presence of Xwnt11 in the germ cell lineage suggests that a wnt-directed signaling pathway may be involved in germ cell specification, differentiation or migration.