The role of maternal syntabulin in Xenopus primordial germ cell development and germ cell specification

In anuran amphibian and teleost embryos, cytoplasmic localizations in the egg are essential for establishing initial differences in cell fates. The cytoplasmic inheritance of germ plasm, which consists of mitochondria, germ line granules, and various RNAs, specifies primordial germ cell (PGC) fate. In Xenopus, the germ plasm originates within the mitochondrial cloud. The roles of several of these RNAs in PGC formation have been well characterized; recently however, it has become apparent that a subset of germ plasm RNAs in the vegetal cortex may have roles in dorsal axis formation. These include trim36 and dnd1 in Xenopus, which are predicted to facilitate microtubule assembly during cortical rotation, and syntabulin (sybu), which encodes a kinesin motor protein linker. Sybu is defective in ventralized tokkeabi mutant zebrafish and mRNA knockdown of sybu in Xenopus results in ventralized embryos. Thus, Sybu is thought to function in the transport of dorsal determinants in the zygote. Here I further examine the germline functions of Sybu in early Xenopus development. In contrast to the case in zebrafish, Xenopus sybu mRNA remains localized to the germ plasm of early embryos through the gastrula stage. Maternal antisense inhibition of sybu results in embryos lacking PGCs by the tailbud stage, and analysis of sybu-depleted embryos revealed altered germ plasm dynamics, reducing germ plasm aggregation. In addition, during initial cell divisions, a novel accelerated wave of germ plasm was discovered to mediate trafficking toward the cleavage furrows, which was found to be dependent on Syntabulin. As Syntabulin is a motor linker protein I also detected interactions with the kinesins Kif3b and Kif4a to mediate germ plasm aggregation. Depletion of any of these factors resulted in a reduction of PGC formation and altered migration patterns within gastrulating embryos and early tadpoles. I also found that Syntabulin is required for recruitment of the germ plasm to a perinuclear position in gastrula stage blastomeres, indicating that Syntabulin is required at multiple steps for proper organization and localization of the germ plasm.