Notch activity acts as a sensor for extracellular calcium during vertebrate left–right determination

Abstract

During vertebrate embryo development, the breaking of the initial bilateral symmetry is translated into asymmetric gene expression around the node and/or in the lateral plate mesoderm. The earliest conserved feature of this asymmetric gene expression cascade is the left-sided expression of Nodal, which depends on the activity of the Notch signalling pathway. Here we present a mathematical model describing the dynamics of the Notch signalling pathway during chick embryo gastrulation, which reveals a complex and highly robust genetic network that locally activates Notch on the left side of Hensen's node. We identify the source of the asymmetric activation of Notch as a transient accumulation of extracellular calcium, which in turn depends on left-right differences in H+/K+-ATPase activity. Our results uncover a mechanism by which the Notch signalling pathway translates asymmetry in epigenetic factors into asymmetric gene expression around the node.

Far from solved: A perspective on what we know about early mechanisms of left–right asymmetry

“…1). The specifics (e.g., spatial distribution, timing) differ among model species with divergent early embryonic architectures, these and the other biophysical components elucidated in Xenopus have now also been shown to be required for asymmetry in sea urchin (Duboc et al, 2005;Hibino et al, 2006), Ciona (Shimeld and Levin, 2006), chick (Levin et al, 2002;Raya et al, 2004;Adams et al, 2006), zebrafish (Adams et al, 2006), C. elegans (Chuang et al, 2007), and snails (Shibazaki et al, 2004).…”

“…Three non-mutually exclusive models have been proposed for how this may occur in vertebrates. The first involves control of calcium signaling which activates Notch (Raya et al, 2004;Raya and Belmonte, 2006). This appears to occur in chick, but not Xenopus (Levin et al, 2002;Adams et al, 2006).…”

“…Indeed, chemical genetic strategies (Smukste and Stockwell, 2005;Adams and Levin, 2006a,b;Wheeler and Brandli, 2009) are a rapid and inexpensive method for initially implicating targets for further characterization, as well as allowing dissection of the timing of different mechanisms (which is often impossible with genetic manipulations). However, it is crucial to note that the drug screens that first suggested the involvement of physiological early signals and helped narrow their timing of activity were accompanied by experiments using molecular loss-of-function (and sometimes also gain-of-function) gene-specific reagents Levin and Mercola, 1999;Levin et al, 2002;Bunney et al, 2003;Raya et al, 2004;Fukumoto et al, 2005a,b;Adams et al, 2006;Morokuma et al, 2008) in every major study, rendering concerns about pharmacological specificity irrelevant and firmly implicating several ion transport and neurotransmitter pathways in LR patterning.…”

“…(Levin and Mercola, 1998b;Levin and Mercola, 1999;Levin et al, 2002;Fukumoto et al, 2005b). Yet, the chick expresses these targets and drives asymmetric ion flows at much later stages than does the frog, when tens of thousands of cells are present (Levin et al, 2002;Raya et al, 2004). Moreover, while serotonin and associated signaling pathways have been shown to play a role in both model systems, no asymmetric distribution of serotonin has been observed in chick as it has in frog; this suggests that while similar components are used in amphibia and avians, the pathway has undergone significant modifications in adaptation to a very different bodyplan architecture.…”

Laterality defects are influenced by timing of treatments and animal model

“…In the second sub-model, the flow generated by cilia at the node generates a gradient of calcium ions (Shimeld, 2004). This extracellular increase in calcium on the left side of the embryo is sensed by a second group of cilia, which are not motile (Tabin and Vogan, 2003), and stimulates activation of genes such as Notch on the left side (Raya et al, 2004). In both of these sub-models, the original symmetry breaking event is due to biases produced by the motion of cilia at the node.…”