The DEAD-box RNA helicase Vasa functions in embryonic mitotic progression in the sea urchin

Abstract

SUMMARYVasa is a broadly conserved ATP-dependent RNA helicase that functions in the germ line of organisms from cnidarians to mammals. Curiously, Vasa is also present in the somatic cells of many animals and functions as a regulator of multipotent cells. Here, we report a mitotic function of Vasa revealed in the sea urchin embryo. We found that Vasa protein is present in all blastomeres of the early embryo and that its abundance oscillates with the cell cycle. Vasa associates with the spindle and the separating sister chromatids at metaphase, and then quickly disappears after telophase. Inhibition of Vasa protein synthesis interferes with proper chromosome segregation, arrests cells at M-phase, and delays overall cell cycle progression. Cdk activity is necessary for the proper localization of Vasa, implying that Vasa is involved in the cyclin-dependent cell cycle network, and Vasa is required for the efficient translation of cyclinB mRNA. Our results suggest an evolutionarily conserved role of Vasa that is independent of its function in germ line determination.

Essential elements for translation: the germline factor Vasa functions broadly in somatic cells

“…S1B, day 5, arrows). Previous studies documented a strict Vasa regulation by post-translational degradation -vasa mRNA is ubiquitously present until gastrula stage and is translated in all cells of the early embryo and larva, but is degraded rapidly in all cells except the germ cells (Gustafson et al, 2011;Yajima and Wessel, 2011b). Thus, the accumulation of Vasa in macromere descendants in the larva, which were previously Vasa-less cells, demonstrated a definitive somatic and posttranscriptional regulation of Vasa, consistent with the presence and dynamics of endogenous Vasa ( Fig.…”

“…Injection of Vasa-MO (0.5 mM stock) into one of the 2-cell stage blastomeres resulted in a delay in cell cycle progression in the lineage containing the Vasa-MO (Fig. 4B, Vasa-MO only, arrow) (see Yajima and Wessel, 2011b for the detailed phenotype), but not in the sibling lineage. However, when Vasa-GFP (lacking the MOsite) was first injected into the egg, and subsequently the Vasa-MO (recognizing the endogenous but not the reporter Vasa) was injected into one of the 2-cell stage blastomeres, the Vasa-MO-containing blastomere was significantly rescued in development.…”

Autonomy in specification of primordial germ cells and their passive translocation in the sea urchin

“…The sea urchin egg does not have localized maternal germ line components, and adults are fertile when Mics, the parent blastomeres of SMics, are removed at the fourth division (Ransick et al, 1996). Furthermore, recent findings have demonstrated that Vasa, a wellconserved germ line marker originally found in Drosophila pole cells, is expressed in every blastomere of the sea urchin embryo and even in somatic adult rudiment cells (Yajima and Wessel, 2011b;Yajima and Wessel, 2011c) (our unpublished results). This type of broad expression of Vasa protein has been found in many animals, including planarians (Shibata et al, 1999), hydra (Bosch and David, 1987;Mochizuki et al, 2001;Rebscher et al, 2008), annelids (Carre et al, 2002;Oyama and Shimizu, 2007;Rebscher et al, 2007;Rebscher et al, 2012) and urochordates (Rosner et al, 2009), and suggests that Vasa might have much broader functional roles than originally anticipated, possibly including compensatory activities for the germ line.…”

“…Vasa protein selectively accumulated asymmetrically into the SMics, just as it does in vivo (Fig. 1B,C, arrows) (Yajima and Wessel, 2011b), such that the skeletogenic LMics only have background levels of Vasa whereas the SMics possess strong Vasa signal. Vasa accumulation increased in the SMics following this unequal division and Vasa accumulated in its typical, perinuclear and granular-like structure (Fig.…”

“…These phenotypes were never observed in control embryos injected with dye and other genespecific MOs, suggesting a specific knockdown phenotype driven by G-cad MO (supplementary material Fig. S3, control embryos) Yajima et al, 2012;Yajima and Wessel, 2011b). Further, because the different Cadherins of S. purpuratus have unique 5ЈUTR and signal sequences, the knockdown is specific to G-cadherin.…”