The Notch signaling pathway regulates left-right determination by detecting a transient build-up of extracellular calcium around the left side of the node, which is caused by an asymmetrical reduction in H+/K+-ATPase activity.
Vertebrates convert from bilateral symmetry to left-right (LR) asymmetry during the early stages of embryonic development. One of the earliest markers of LR asymmetry is expression of the Nodal gene on the left side of Hensen's node. While Nodal expression is known to be regulated by Notch signaling, it remains unclear what initiates asymmetric signaling. Raya et al. now show that an asymmetrical reduction in activity of H+/K+-ATPase results in a transient build-up of extracellular calcium around the left side of the node. The resulting asymmetric Notch signaling translates into left-sided Nodal expression.
Raya et al. analyzed the expression pattern of Notch signaling components in the chick embryo, and compared them with those of Nodal. The Notch ligands Delta-like 1 (Dll1) and Serrate1, and the Notch modulator Lunatic fringe (Lfng), exhibited left-side perinodal expression similar to the subsequent Nodal expression. The authors used these findings to mathematically model Notch activation dynamics. The model suggested that, during LR asymmetry establishment, Notch hyperactivation coincides with Dll1 and Serrate1 expression, is augmented through lfng expression and works within a pre-existing transient LR asymmetry. The model accounts for the very specific temporal and spatial control of Nodal expression, and its predicted robustness is in agreement with experimental perturbation analysis.
A reduction in the activity of H+/K+-ATPase ion pumps on the left side of the node is known to cause localized membrane depolarization. Inhibiting H+/K+-ATPase in the chick embryo resulted in bilateral expression of Nodal in the lateral plate mesoderm and a loss of Dll1 and Lfng asymmetric expression around the node. This indicates that H+/K+-ATPase activity may be responsible for left-sided Notch activation. The theoretical model indicated the need for an external LR modulator, prompting the researchers to visualize transient, local accumulations of extracellular Ca2+ on the left side of the node. The Ca2+ gradient was shown to depend on differential H+/K+-ATPase activity. Furthermore, manipulation of extracellular Ca2+ concentrations altered LR patterning and this was rescued by left-sided expression of active Notch. The authors confirmed that extracellular Ca2+ concentration is detected directly by the Notch-signaling pathway and thus regulates localized Nodal expression and consequently LR asymmetry.
Brenda Riley, Assistant Editor Signaling Gateway
References
Raya, Ángel Kawakami, Yasuhiko Rodríguez-Esteban, Concepción Ibañes, Marta Rasskin–Gutman, Diego Rodríguez–León, Joaquín Büscher, Dirk Feihó, José A. Izpisúa Belmonte, Juan Carlos Notch activity acts as a sensor for extracellular calcium during vertebrate left-right determination.
Nature427 121–128
(08 January 2004) 10.1038/427111a
Monk, Nick Developmental biology: Asymmetric fixation.
Nature427 111–112
(08 January 2004) 10.1038/427111a
