Scaling of embryonic patterning based on phase-gradient encoding.
Lauschke, V.M., Tsiairis, C.D., Francois, P. & Aulehla, A.
Nature. 2013 Jan 3;493(7430):101-5. doi: 10.1038/nature11804. Epub 2012 Dec 19.
A fundamental feature of embryonic patterning is the ability to scale and maintain stable proportions despite changes in overall size, for instance during growth. A notable example occurs during vertebrate segment formation: after experimental reduction of embryo size, segments form proportionally smaller, and consequently, a normal number of segments is formed. Despite decades of experimental and theoretical work, the underlying mechanism remains unknown. More recently, ultradian oscillations in gene activity have been linked to the temporal control of segmentation; however, their implication in scaling remains elusive. Here we show that scaling of gene oscillation dynamics underlies segment scaling. To this end, we develop a new experimental model, an ex vivo primary cell culture assay that recapitulates mouse mesoderm patterning and segment scaling, in a quasi-monolayer of presomitic mesoderm cells (hereafter termed monolayer PSM or mPSM). Combined with real-time imaging of gene activity, this enabled us to quantify the gradual shift in the oscillation phase and thus determine the resulting phase gradient across the mPSM. Crucially, we show that this phase gradient scales by maintaining a fixed amplitude across mPSM of different lengths. We identify the slope of this phase gradient as a single predictive parameter for segment size, which functions in a size- and temperature-independent manner, revealing a hitherto unrecognized mechanism for scaling. Notably, in contrast to molecular gradients, a phase gradient describes the distribution of a dynamical cellular state. Thus, our phase-gradient scaling findings reveal a new level of dynamic information-processing, and provide evidence for the concept of phase-gradient encoding during embryonic patterning and scaling.
A beta-catenin gradient links the clock and wavefront systems in mouse embryo segmentation.
Aulehla, A., Wiegraebe, W., Baubet, V., Wahl, M.B., Deng, C., Taketo, M., Lewandoski, M. & Pourquie, O.
Nat Cell Biol. 2008 Feb;10(2):186-93. Epub 2007 Dec 23.
Rhythmic production of vertebral precursors, the somites, causes bilateral columns of embryonic segments to form. This process involves a molecular oscillator--the segmentation clock--whose signal is translated into a spatial, periodic pattern by a complex signalling gradient system within the presomitic mesoderm (PSM). In mouse embryos, Wnt signalling has been implicated in both the clock and gradient mechanisms, but how the Wnt pathway can perform these two functions simultaneously remains unclear. Here, we use a yellow fluorescent protein (YFP)-based, real-time imaging system in mouse embryos to demonstrate that clock oscillations are independent of beta-catenin protein levels. In contrast, we show that the Wnt-signalling gradient is established through a nuclear beta-catenin protein gradient in the posterior PSM. This gradient of nuclear beta-catenin defines the size of the oscillatory field and controls key aspects of PSM maturation and segment formation, emphasizing the central role of Wnt signalling in this process.
Wnt3a plays a major role in the segmentation clock controlling somitogenesis.
Aulehla, A., Wehrle, C., Brand-Saberi, B., Kemler, R., Gossler, A., Kanzler, B. & Herrmann, B.G.
Dev Cell. 2003 Mar;4(3):395-406.
The vertebral column derives from somites generated by segmentation of presomitic mesoderm (PSM). Somitogenesis involves a molecular oscillator, the segmentation clock, controlling periodic Notch signaling in the PSM. Here, we establish a novel link between Wnt/beta-catenin signaling and the segmentation clock. Axin2, a negative regulator of the Wnt pathway, is directly controlled by Wnt/beta-catenin and shows oscillating expression in the PSM, even when Notch signaling is impaired, alternating with Lfng expression. Moreover, Wnt3a is required for oscillating Notch signaling activity in the PSM. We propose that the segmentation clock is established by Wnt/beta-catenin signaling via a negative-feedback mechanism and that Wnt3a controls the segmentation process in vertebrates.