The mammalian brain is composed of a large number of various types of neurons and glial cells. All cells are generated from neural stem/progenitor cells (NPCs), which first proliferate to expand their numbers, then start to differentiate into neurons, and finally give rise to glial cells1,2,3,4,5. Once cells have differentiated into neurons, they cannot proliferate or increase their numbers, and, therefore, the maintenance of NPCs until later stages is important. Notch signaling via cell-cell interactions plays an important role in maintaining NPCs6,7. Notch ligands interact with the membrane protein, Notch, on the surface of neighboring cells and activates the Notch protein. After activation, proteolysis of Notch protein occurs, thereby releasing the intracellular domain of Notch (NICD) from the cell membrane into the nucleus8,9,10. In the nucleus, NICD binds to the promoter regions of Hes1 and Hes5 (Hes1/5) and activates the expression of these genes. Hes1/5 repress the expression of the proneural genes Ascl1 and Neurogenin1/2 (Neurog1/2)11,12,13,14. Because proneural genes induce neuronal differentiation, Hes1/5 play essential roles in maintaining NPCs. Furthermore, as proneural genes can activate the expression of the Notch ligand Delta-like1 (Dll1), Hes1/5 also repress the expression of Dll1. Therefore, the expression of Dll1 leads to neighboring cells being negative for Dll1 via Notch signaling. In this way, cells inhibit adjacent cells from following their same fate, a phenomenon known as the lateral inhibition8. In the developing brain, lateral inhibition plays a role in generating various different cell types.
Real-time imaging at the single cell level reveals dynamic expressions of the components of Notch signaling in NPCs15,16,17. Notch signaling activates the expression of Hes1, but Hes1 protein binds to its own promoter and represses its own expression. Furthermore, Hes1 is an extremely unstable protein, that is degraded by the ubiquitin-proteasome pathway; therefore, the repression of its own promoter is only short lived and then the transcription starts again. In this way, the expression of Hes1 oscillates at both the transcription and translational levels in a 2 h cycle18. The oscillatory expression of Hes1, in turn, induces the oscillatory expression of the downstream target genes, such as Ascl1, Neurog2, and Dll1, via periodic repression15,16,17,19. While proneural genes can induce neuronal differentiation, their oscillatory expression is not sufficient for neuronal differentiation; rather their sustained expression is essential for the neuronal differentiation. The oscillatory expression of proneural genes is important for maintaining NPCs rather than for inducing neuronal differentiation14,15,16. The expression of Dll1 oscillates at both the transcription and translational levels during various morphogenesis, such as neurogenesis and somitogenesis. The dynamic expression of Dll1 is important for the normal morphogenesis and steady expression of Dll1 induces defects in neurogenesis and somitogenesis17. These findings demonstrate the important function that the dynamics of gene expression and protein kinetics have on the regulation of various developmental events (i.e., different expression dynamics produce different outputs in cellular behaviors).
To analyze the dynamics of Notch signaling, the static analysis of tissues and cells are insufficient because they are constantly changing. Real-time imaging of single cells is a powerful tool to reveal the dynamics in gene expression. The dynamic expression of Notch signaling molecules undergo rapid cyclic responses in the period of 2-3 h. This rapid periodic expression presents two difficult problems for the real-time monitoring: (1) the expression of the molecules is suppressed to low levels, and (2) rapid turnover requires fast-response reporters. To overcome these problems, we previously developed a bioluminescence real-time imaging method20. Because the bioluminescence reporter has a higher sensitivity and shorter maturation time than fluorescent reporters, this strategy enables us to monitor the rapid dynamics in living cells. Using real-time visualization, we found that more genes exhibited dynamic expression than we had previously thought. In addition, the number of reports showing expression and protein dynamics in living cells and the significance of these dynamics in various biological events has increased, suggesting a fundamental role of the dynamics in gene expressions21,22.
In this report, we describe a way to visualize the expression of the Notch ligand Dll1 in NPCs in both dissociated cultures and in cortical slice cultures. To monitor the dynamics of Dll1 transcription at single cell levels, we generated dissociated cultures of NPCs derived from the embryonic telencephalon of transgenic mice carrying pDll1-Ub-Fluc reporter, a Dll1 promoter-driven destabilized luciferase reporter. To monitor Dll1 protein dynamics in vivo, we introduced the Dll1-Fluc fusion reporter into NPCs in the cortex and visualized the expression of the reporter in NPCs in cortical slice cultures. Real-time imaging enabled us to capture the various features of gene expression and protein dynamics in living cells at high temporal resolution.