Dynamic expression of notch signaling genes in neural stem/progenitor cells

Abstract

In neural stem/progenitor cells, expression of the Notch effector Hes1, a transcriptional repressor, oscillates with a period of 2-3 h by negative feedback, and Hes1 oscillations induce the oscillatory expression of the proneural gene Neurogenin2 (Ngn2) and the Notch ligand gene Delta-like1 (Dll1). Dll1 oscillation leads to the mutual activation of Notch signaling between neighboring cells, thereby maintaining a group of cells in the undifferentiated state. Not all cells express Hes1 in an oscillatory manner: cells in boundary regions such as the isthmus express Hes1 in a sustained manner, and these cells are rather dormant with regard to proliferation and differentiation. Thus, Hes1 allows cell proliferation and differentiation when its expression oscillates but induces dormancy when its expression is sustained. After Hes1 expression is repressed, Ngn2 is expressed in a sustained manner, promoting neuronal differentiation. Thus, Ngn2 leads to the maintenance of neural stem/progenitor cells by inducing Dll1 oscillation when its expression oscillates but to neuronal differentiation when its expression is sustained. These results indicate that the different dynamics of Hes1 and Ngn2 lead to different outcomes.

Keywords: OSVZ progenitor; basal progenitor; neuroepithelial cell; oscillatory expression; proneural gene; radial glia.

Figure 1

Neural stem/progenitor cells and their differentiation. Initially, neuroepithelial cells undergo repeated self-renewal by symmetric division (progenitor expansion phase). As development proceeds, neuroepithelial cells elongate to become radial glial cells, which have cell bodies on the inner side (called the ventricular zone) of the neural tube and radial fibers that reach the outer surface. Radial glial cells gives rise to neurons or basal progenitors (neurogenic phase). After the production of neurons, some radial glial cells give rise to oligodendrocytes and ependymal cells. Radial glial cells finally differentiate into astrocytes (gliogenic phase). Both neuroepithelial cells and radial glial cells are considered embryonic neural stem/progenitor cells.

Figure 2

The core pathway of Notch signaling. Proneural genes such as Mash1 and Ngn2 promote neuronal differentiation and induce the expression of Dll1, which in turn activates Notch in neighboring cells. Upon activation of Notch, the Notch intracellular domain (NICD) is released from the transmembrane portion and transferred to the nucleus, where it forms a complex with the DNA-binding protein Rbpj and the transcriptional co-activator Maml. The NICD–Rbpj–Maml complex induces the expression of transcriptional repressor genes such as Hes1 and Hes5. Hes1 and Hes5 then repress the expression of proneural genes and Dll1, thereby leading to the maintenance of neural stem/progenitor cells. Numb inhibits Notch signaling and induces neuronal differentiation.

Figure 3

Oscillatory expression of Hes1. Hes1 expression oscillates with a period of ∼2 h in many cell types such as neural stem/progenitor cells and fibroblasts. Hes1 represses its own expression by directly binding to its promoter. This negative feedback leads to the disappearance of Hes1 mRNA and protein, because they are extremely unstable, allowing the next round of its expression. In this way, Hes1 autonomously starts an oscillatory expression pattern.

Figure 4

Dynamic expression in neural stem/progenitor cells. Hes1 expression oscillates with a period of ∼2–3 h in neural progenitor cells. In these cells, Ngn2 and Dll1 expression also oscillates. Hes1 protein expression exhibits an inverse correlation with Ngn2 protein and Dll1 mRNA expression. In contrast, the expression of Ngn2 and Dll1 is sustained in post-mitotic neurons, which lose Hes1 expression. It is likely that Dll1 oscillation periodically activates Notch signaling between neighboring cells. This reciprocal regulation is particularly important for the maintenance of neural stem/progenitor cells before the formation of neurons.

Figure 5

Maintenance of neural stem/progenitor cells by the mutual activation of Notch signaling. Ngn2 and Dll1 oscillations are regulated by Hes1 oscillation in neural stem/progenitor cells. Ngn2 oscillation may be advantageous for the maintenance/proliferation of neural stem/progenitor cells during the early stages of development, because it induces Dll1 expression and activates Notch signaling without promoting neuronal differentiation. A salt-and-pepper pattern induced by lateral inhibition is just a snapshot of oscillatory expression. The precise interactive regulation among Dll1-positive and -negative cells remains to be determined.

Figure 6

Notch signaling in OSVZ/OVZ progenitors. OSVZ/OVZ progenitors have radial glia-like morphology and extend radial fibers to the pial surface, but lack apical processes. These cells undergo asymmetric cell division multiple times and generate a lot of neurons. These neurons seem to express Notch ligands and activate Notch signaling in their sibling OSVZ/OVZ progenitors.