no tail integrates two modes of mesoderm induction

Abstract

SUMMARYDuring early zebrafish development the nodal signalling pathway patterns the embryo into three germ layers, in part by inducing the expression of no tail (ntl), which is essential for correct mesoderm formation. When nodal signalling is inhibited ntl fails to be expressed in the dorsal margin, but ventral ntl expression is unaffected. These observations indicate that ntl transcription is under both nodal-dependent and nodal-independent regulation. Consistent with these observations and with a role for ntl in mesoderm formation, some somites form within the tail region of embryos lacking nodal signalling. In an effort to understand how ntl is regulated and thus how mesoderm forms, we have mapped the elements responsible for nodaldependent and nodal-independent expression of ntl in the margin of the embryo. Our work demonstrates that expression of ntl in the margin is the consequence of two separate enhancers, which act to mediate different mechanisms of mesoderm formation. One of these enhancers responds to nodal signalling, and the other to Wnt and BMP signalling. We demonstrate that the nodal-independent regulation of ntl is essential for tail formation. Misexpression of Wnt and BMP ligands can induce the formation of an ectopic tail, which contains somites, in embryos devoid of nodal signalling, and this tail formation is dependent on ntl function. Similarly, nodal-independent tail somite formation requires ntl. At later stages in development ntl is required for notochord formation, and our analysis has also led to the identification of the enhancer required for ntl expression in the developing notochord.

Biphasic wnt8a expression is achieved through interactions of multiple regulatory inputs

“…However, we have not tested the regulative ability of putative YSL enhancers from other species. One possible role for YSLexpressed wnt8a that would be consistent with published reports is participation in stimulating ntla expression through enhancer E1 (Harvey et al, 2010). Recently, Mxtx2 was found to be required in the YSL and non-autonomously synergizes with Nodal signaling to stimulate ntla expression in ventrolateral mesoderm progenitors (Hong et al, 2011).…”

“…As a control, we assayed induction of ntla by Taram-D. Consistent with the known regulation of ntla by Nodal (e.g., Harvey et al, 2010), Taram-D overexpression strongly activates ntla transcription ( Fig. 5G; 32/32 embryos affected).…”

“…Enhancer E2 drives Nodal-dependent expression while enhancer E1, located upstream of E2, drives expression in response to Wnt and BMP signaling (Harvey et al, 2010), suggesting that zebrafish ntla is a downstream target of Wnt signaling as has been shown for Xenopus Xbra (Vonica and Gumbiner, 2002). The presence of two ntla enhancers explains Nodal-independent ntla expression and the generation of tail mesoderm observed in Nodal mutants (Feldman et al, 1998;Harvey et al, 2010). Wnt8a is the most likely Wnt factor responsible for early ntla regulation (Harvey et al, 2010).…”

“…The presence of two ntla enhancers explains Nodal-independent ntla expression and the generation of tail mesoderm observed in Nodal mutants (Feldman et al, 1998;Harvey et al, 2010). Wnt8a is the most likely Wnt factor responsible for early ntla regulation (Harvey et al, 2010). Therefore, in the pre-and early gastrula, Nodal, BMP, and Wnt8a signaling converge on dual enhancers to generate the expression of ntl and bra.…”

A direct role for Wnt8 in ventrolateral mesoderm patterning

“…BMP mutants define the classic dorsalized phenotype characterized by expanded neurectoderm at the expense of epidermis as well as loss of posterior and ventral mesoderm cell types (Little and Mullins, 2006). In the mesoderm, high BMP levels during gastrulation direct progenitor cells to contribute to the tail (Agathon et al, 2003;Szeto and Kimelman, 2006;Tucker et al, 2008;Harvey et al, 2010), possibly by preventing their premature differentiation, and also influence cell convergence (Myers et al, 2002;von der Hardt et al, 2007). In zebrafish, Bmp2b co-regulates the Wnt8 target genes vent, vox, and ved during early gastrulation (Melby et al, 2000;Imai et al, 2001;Ramel et al, 2005), although the fact that BMP loss-of-function does not result in an expanded organizer (Mullins et al, 1996;Ramel and Lekven, 2004) or defective anteroposterior patterning in the gastrula (Barth et al, 1999) illustrates differences between their regulatory roles.…”