Maternal Dead-End1 is required for vegetal cortical microtubule assembly during Xenopus axis specification

Abstract

SUMMARYVertebrate axis specification is an evolutionarily conserved developmental process that relies on asymmetric activation of Wnt signaling and subsequent organizer formation on the future dorsal side of the embryo. Although roles of Wnt signaling during organizer formation have been studied extensively, it is unclear how the Wnt pathway is asymmetrically activated. In Xenopus and zebrafish, the Wnt pathway is triggered by dorsal determinants, which are translocated from the vegetal pole to the future dorsal side of the embryo shortly after fertilization. The transport of dorsal determinants requires a unique microtubule network formed in the vegetal cortex shortly after fertilization. However, molecular mechanisms governing the formation of vegetal cortical microtubule arrays are not fully understood. Here we report that Dead-End 1 (Dnd1), an RNA-binding protein required for primordial germ cell development during later stages of embryogenesis, is essential for Xenopus axis specification. We show that knockdown of maternal Dnd1 specifically interferes with the formation of vegetal cortical microtubules. This, in turn, impairs translocation of dorsal determinants, the initiation of Wnt signaling, organizer formation, and ultimately results in ventralized embryos. Furthermore, we found that Dnd1 binds to a uridine-rich sequence in the 3Ј-UTR of trim36, a vegetally localized maternal RNA essential for vegetal cortical microtubule assembly. Dnd1 anchors trim36 to the vegetal cortex in the egg, promoting high concentrations of Trim36 protein there. Our work thus demonstrates a novel and surprising function for Dnd1 during early development and provides an important link between Dnd1, mRNA localization, the microtubule cytoskeleton and axis specification.

Maternal syntabulin is required for dorsal axis formation and is a germ plasm component in Xenopus

“…Increasing lines of evidence show that some germ plasm-associated mRNAs, such as Fatvg (a lipid droplet-associated protein likely involved in intracellular vesicle trafficking) and dead end have dual activities, being required for both germ cell differentiation and embryonic D-V axial development (Chan et al, 2001(Chan et al, , 2007Horvay et al, 2006;Mei et al, 2013). Although not investigated in this study, our expression data suggest that Syntabulin may also have a role in germ cell specification, an intriguing possibility that should be tested in future.…”

“…One of these mRNAs, dead end, is required for D-V patterning (Mei et al, 2013). Dead end is thought to function by activating the translation of Trim36, a vegetally localized RING finger ubiquitin ligase that controls microtubule polymerization and, like dead end, is required for dorsal axis formation (Cuykendall and Houston, 2009).…”

Germ cell-specific expression of dead end (dnd) in rare minnow (Gobiocypris rarus)

“…Dnd proteins are conserved in the vertebrate means they have similar structure and function. Dnd proteins are RNA-binding proteins with the typical RRM motifs, which had been proved binding the RNA of cell cycle inhibitor P27 and cell cycle regulator and tumor suppressor LATS2 in human cells (Kedde et al 2007), geminin in zebrafish (Chen et al 2010), and trim36 in frog (Mei et al 2013). Rare minnow dnd is expressed exclusively in the germ cells of both ovary and testis.…”

DND1 (DND microRNA-mediated repression inhibitor 1)

“…Also expressed in other cell types such as skin and pancreas (Basu et al, 2011;Bhandari et al, 2012). Is expressed in the early embryo of Xenopus (Bauermeister et al, 2015;Mei et al, 2013).…”

“…The microtubules translocate dorsal determinants. Lack of Dnd causes ventralization of frog embryos (Mei et al, 2013). In turn, Xenopus Dnd mRNA is localized vegetally to the RNP complex by Celf, a component of the vegetal localization RNP complex (Bauermeister et al, 2015).…”