A novel reporter of notch signalling indicates regulated and random Notch activation during vertebrate neurogenesis.

BACKGROUND:Building the complex vertebrate nervous system involves the regulated production of neurons and glia while maintaining a progenitor cell population. Neurogenesis starts asynchronously in different regions of the embryo and occurs over a long period of time, allowing progenitor cells to be exposed to multiple extrinsic signals that regulate the production of different cell types. Notch-mediated cell-cell signalling is one of the mechanisms that maintain the progenitor pool, however, little is known about how the timing of Notch activation is related to the cell cycle and the distinct modes of cell division that generate neurons. An essential tool with which to investigate the role of Notch signalling on cell by cell basis is the development a faithful reporter of Notch activity. RESULTS:Here we present a novel reporter for Notch activity based on the promoter of the well characterised Notch target chick Hes5-1, coupled with multiple elements that confer instability, including a destabilized nuclear Venus fluorescent protein and the 3' untranslated region (UTR) of Hes5-1. We demonstrate that this reporter faithfully recapitulates the endogenous expression of Hes5-1 and that it robustly responds to Notch activation in the chick neural tube. Analysis of the patterns of Notch activity revealed by this reporter indicates that although Notch is most frequently activated prior to mitosis it can be activated at any time within the cell cycle. Notch active progenitors undergoing mitosis generate two daughters that both continue to experience Notch signalling. However, cells lacking Notch activity before and during mitosis generate daughters with dissimilar Notch activity profiles. CONCLUSIONS:A novel Notch reporter with multiple destabilisation elements provides a faithful read-out of endogenous Notch activity on a cell-by-cell basis, as neural progenitors progress through the cell cycle in the chick neural tube. Notch activity patterns in this cell population provide evidence for distinct Notch signalling dynamics underlying different cell division modes and for the involvement of random initiation of Notch signalling within the neuroepithelium. These findings highlight the importance of single-cell analysis in the study of the complexity of Notch activity and provide new insights into the mechanisms underlying cell fate decisions in neural progenitors.

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