Abstract
Nodal class TGF-β signalling molecules play essential roles in establishing the vertebrate body plan. In all vertebrates, nodal family members have specific waves of expression required for tissue specification and axis formation. In Xenopus laevis, six nodal genes are expressed before gastrulation, raising the question of whether they have specific roles or act redundantly with each other. Here, we examine the role of Xnr5. We find it acts at the late blastula stage as a mesoderm inducer and repressor of ectodermal gene expression, a role it shares with Vg1. However, unlike Vg1, Xnr5 depletion reduces the expression of the nodal family member xnr1 at the gastrula stage. It is also required for left/right laterality by controlling the expression of the laterality genes xnr1, antivin (lefty) and pitx2 at the tailbud stage. In Xnr5-depleted embryos, the heart field is established normally, but symmetrical reduction in Xnr5 levels causes a severely stunted midline heart, first evidenced by a reduction in cardiac troponin mRNA levels, while left-sided reduction leads to randomization of the left/right axis. This work identifies Xnr5 as the earliest step in the signalling pathway establishing normal heart laterality in Xenopus.
Highlights
Nodal class TGF-b signalling molecules play essential roles in establishing the vertebrate body plan
In the experiments reported here we show that Xnr5 has roles at three time points in early development
At the early tailbud stage it is required for the left-sided expression of xnr1, antivin and pitx2, and we show that the expression of xnr1 and antivin is partially rescued by injection of xnr5 mRNA
Summary
Nodal class TGF-b signalling molecules play essential roles in establishing the vertebrate body plan. The earliest dorsal–ventral asymmetry in zebrafish is the dorsal localization of squint mRNA at the 4–8 cell stage [3], while in Xenopus, all six nodal-related mRNAs (Xnr1–6) are dorsally enriched at the blastula stage [4,5,6,7,8]. As development proceeds in chicks and mice, new waves of nodal expression occur asymmetrically in the left/right axis of the organizer and lateral plate mesoderm (LPM) [9,10,11,12], while in zebrafish and Xenopus, cyclops, southpaw and xnr (the same as nodal homolog 1), mRNAs are enriched in the left LPM [12,13,14,15,16]. Many aspects of the functions of nodal type TGF-bs are known in these locations, many questions remain, the individual roles of Xnrs in Xenopus. We re-examined the function of Xnr using two independent and verifiable loss-of-function strategies
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