Spatio-Temporal Signaling Dynamics in Neuronal Cell Fate Decision and Patterning

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Tissue patterning during development and regeneration requires consistent cell fate decisions in space and time. Secreted, diffusible molecules, named morphogens, set up the spatial coordinates for cells to know where they are and what they should become. Recent studies have revealed that the spatial profile of morphogens and their intracellular signal activities are highly time-dependent, and these dynamic features are essential for fate decisions in individual cells and tissue patterning across a population of cells. This presents a central challenge in biology to understand how discrete cell fates and precise tissue patterning are achieved using the quantitative information encoded by the highly dynamic morphogen signals. The neural system provides a fascinating example of well-timed signal inputs from several morphogens generating diverse cell types and intricate patterns. Although the major morphogens involved in neural development have been discovered, their spatio-temporal dynamics and the mechanism of precise signal interpretation have been largely overlooked due to the lack of direct, dynamic analysis and manipulation of signaling at the level of individual cells in vivo. To overcome these limitations, I have developed a novel tissue engineering technique allowing morphogen gradient and patterns to form in vitro. I have also adapted new methods for embryonic stem cell (ESC) differentiation that enable the analysis of neural development closely resembling those occurring in vivo in a system amenable to high resolution quantitative analysis. In combination with our innovative single cell time-lapse imaging and gene expression analysis, genetic circuit engineering and mathematical modeling, I will reveal the mechanisms that control both the spatio-temporal dynamics of intracellular signals (temporal encoding) and their interpretation (temporal decoding) during neural progenitor cell specification. During K99 phase, I will focus on the morphogen Sonic Hedgehog (Shh), which is essential for specifying ventral neural types in all regions of the developing neural tube. I will systematically address the following questions: (1) How are dynamic Shh signaling gradients produced and transduced to form pattern? (2) How do architectural features of the Shh pathway affect signaling dynamics and patterning behavior? (3) What aspects of the design of downstream signal decoding mechanism determine the precision and fidelity of the cellular response and pattern formation? During R00 phase, I will expand the platform to examine how coordinating spatiotemporal dynamics of signals from multiple gradients, such as Shh and RA (retinoic acid), can diversify cell fates to allow complex pattern formation in the neural tube. Together, these results will provide a comprehensive understanding of the relationship between spatio-temporal dynamics of morphogen gradients and tissue patterning, as well as a quantitative framework for precisely controlling in vitro ES/iPS cell differentiation towards self-patterned tissue.