Zebrafish organizer development and germ-layer formation require nodal-related signals

Abstract

The vertebrate body plan is established during gastrulation, when cells move inwards to form the mesodermal and endodermal germ layers. Signals from a region of dorsal mesoderm, which is termed the organizer, pattern the body axis by specifying the fates of neighbouring cells. The organizer is itself induced by earlier signals. Although members of the transforming growth factor-beta (TGF-beta) and Wnt families have been implicated in the formation of the organizer, no endogenous signalling molecule is known to be required for this process. Here we report that the zebrafish squint (sqt) and cyclops (cyc) genes have essential, although partly redundant, functions in organizer development and also in the formation of mesoderm and endoderm. We show that the sqt gene encodes a member of the TGF-beta superfamily that is related to mouse nodal. cyc encodes another nodal-related proteins, which is consistent with our genetic evidence that sqt and cyc have overlapping functions. The sqt gene is expressed in a dorsal region of the blastula that includes the extraembryonic yolk syncytial layer (YSL). The YSL has been implicated as a source of signals that induce organizer development and mesendoderm formation. Misexpression of sqt RNA within the embryo or specifically in the YSL induces expanded or ectopic dorsal mesoderm. These results establish an essential role for nodal-related signals in organizer development and mesendoderm formation.

Expression and functional analysis of Tgif during mouse midline development

“…The TGF-␤ signaling pathway is essential for the formation and patterning of functional axial mesendoderm and definitive endoderm. In mice and zebrafish, loss of function mutations in Nodal, Smad2, and Smad3 cause defects in the formation of the prechordal plate and anterior definitive endoderm, which subsequently lead to the malformation of the ventral forebrain (Dunn et al, 2004;Feldman et al, 1998;Norris et al, 2002;Sampath et al, 1998). In addition to the instructive signals from the axial mesendoderm, the regulation of the activity of bone morphogenetic proteins (BMPs) in the neural tissue is important for correct dorsal-ventral patterning.…”

Loss of procollagen IIA from the anterior mesendoderm disrupts the development of mouse embryonic forebrain

“…In the zebrafish, NODAL-related protein activity in the prechordal mesoderm is implicated in direct regulation of Shh expression in the ventral forebrain (Sampath et al, 1998;Feldman et al, 1998;Gritsman et al, 1999). Although our results of the compound mutant study indicate that IIA and NODAL do not interact genetically in head development, there is striking resemblance of the pattern of gene expression in the forebrain of Smad2 CA/CA (the epiblast conditional mutant of Smad2), Nodal DPEE/413.d (a hypomorphic Nodal mutant; Vincent et al, 2003) and IIA À/À embryos.…”

Identification of nodal signaling targets by array analysis of induced complex probes

“…Mice and zebrafish with mutations in nodal-related genes fail to form mesendoderm and the organizer (Conlon et al, 1994;Feldman et al, 1998a;Rebagliati et al, 1998;Sampath et al, 1998;Gritsman et al, 2000). Two closely related Nodal factors, Squint and Cyclops, have been isolated in zebrafish (Feldman et al, 1998a;Rebagliati et al, 1998;Sampath et al, 1998).…”

“…Mice and zebrafish with mutations in nodal-related genes fail to form mesendoderm and the organizer (Conlon et al, 1994;Feldman et al, 1998a;Rebagliati et al, 1998;Sampath et al, 1998;Gritsman et al, 2000). Two closely related Nodal factors, Squint and Cyclops, have been isolated in zebrafish (Feldman et al, 1998a;Rebagliati et al, 1998;Sampath et al, 1998). These Nodals, which are expressed in marginal cells of the zebrafish gastrula (germ ring, embryonic shield) and in the case of squint also dorsally in the underlying yolk syncytial layer, act as short to medium-range signals within the germ ring and the organizer (Feldman et al, 2000;Chen and Schier, 2001).…”

“…Marginal cells involute to form the inner mesendodermal germ layer of the gastrula and endodermal cells tend to involute before mesodermal cells (Warga and Kimmel, 1990;Warga and Nusslein-Volhard, 1999). In Nodaldeficient mutant zebrafish embryos, marginal cells fail to express mesendoderm-specific genes and contribute instead to the ectodermal germ layer (Feldman et al, 1998b(Feldman et al, , 2000.…”