Many bilaterally symmetrical animals develop genetically programmed left-right asymmetries. In vertebrates, this process is under the control of Nodal signaling, which is restricted to the left side by Nodal antagonists Cerberus and Lefty. Amphioxus, the earliest diverging chordate lineage, has profound left-right asymmetry as a larva. We show that Cerberus, Nodal, Lefty, and their target transcription factor Pitx are sequentially activated in amphioxus embryos. We then address their function by transcription activator-like effector nucleases (TALEN)-based knockout and heat-shock promoter (HSP)-driven overexpression. Knockout of Cerberus leads to ectopic right-sided expression of Nodal, Lefty, and Pitx, whereas overexpression of Cerberus represses their left-sided expression. Overexpression of Nodal in turn represses Cerberus and activates Lefty and Pitx ectopically on the right side. We also show Lefty represses Nodal, whereas Pitx activates Nodal. These data combine in a model in which Cerberus determines whether the left-sided gene expression cassette is activated or repressed. These regulatory steps are essential for normal left-right asymmetry to develop, as when they are disrupted embryos may instead form two phenotypic left sides or two phenotypic right sides. Our study shows the regulatory cassette controlling left-right asymmetry was in place in the ancestor of amphioxus and vertebrates. This includes the Nodal inhibitors Cerberus and Lefty, both of which operate in feedback loops with Nodal and combine to establish asymmetric Pitx expression. Cerberus and Lefty are missing from most invertebrate lineages, marking this mechanism as an innovation in the lineage leading to modern chordates.B ilaterians share three primary developmental axes. The anterior-posterior (AP) and dorsal-ventral (DV) axes define bilaterally symmetrical organization. The third axis, orthogonal to these, is known as the medial-lateral or left-right (LR) axis and displays mirror-image symmetry. However, many bilaterian species deviate consistently from true symmetry, raising fundamental questions of how symmetry is broken and how different developmental programs can unfold on the left and right sides of an organism (1). In vertebrates, this includes asymmetric development of the heart and viscera, disruption of which during embryogenesis causes a range of human disorders (2).Correct LR organization in vertebrates is regulated by a gene cassette in which right-sided Cerberus (Cer) and left-sided Nodal and Lefty regulate left-sided expression of the Pitx family gene Pitx2 and hence morphological LR asymmetry (3). Cer expression on the right of the embryonic node is required to repress Nodal signaling, which happens by Cer protein binding directly to Nodal protein. This restricts the ability of Nodal protein to activate the expression of the Nodal gene, leading to an up-regulation of Nodal on the left of the embryo. Lefty expression is also up-regulated by Nodal and, like Cer, acts as an extracellular inhibitor of Nodal. Coexpression o...

In vertebrate embryos Hedgehog (Hh) is expressed by notochord and floorplate cells, and ventral neural cells are patterned by the activities of Hh-regulated transcription factors. Hh signalling is antagonised by signals from the dorsal neural tube, and loss of Hh leads to loss of ventral patterning in the neural tube as the dorsal pattern expands. These mechanisms are critical for producing the neurons that implement motor responses to sensory inputs, but understanding how they evolved has been hindered by lack of insight from commonly-studied invertebrates where nervous system morphology and genetic mechanisms are not conserved with those of vertebrates. The invertebrate chordate amphioxus, which expresses Hh in its notochord and floorplate, provides a window into the pre-vertebrate condition. We have examined amphioxus neural development by manipulating function of Hh and downstream genes involved in neural pattern and cell identity. We show that Hh signalling regulates the differentiation of some neurons in amphioxus, including a subset of motor neurons. This demonstrates some conservation of mechanism between vertebrates and amphioxus. However other aspects of neural patterning differ between the two lineages, with amphioxus lacking aspects important in vertebrates. We suggest the complexity of Hhdependent neural patterning in vertebrates evolved in a step-wise manner. Initial recruitment of Hh occurred in an ancestor to the chordates to regulate the differentiation of a subset of neurons. This was followed, in the vertebrate lineage, by additional changes to the gene regulatory network downstream of Hh, which gave Hh a broader role in dorsal-ventral neural patterning.

No abstract

No abstract

No abstract