Intracellular trafficking of Notch orchestrates temporal dynamics of Notch activity in the fly brain

Abstract

While Delta non-autonomously activates Notch in neighboring cells, it autonomously inactivates Notch through cis-inhibition, the molecular mechanism and biological roles of which remain elusive. The wave of differentiation in the Drosophila brain, the ‘proneural wave’, is an excellent model for studying Notch signaling in vivo. Here, we show that strong nonlinearity in cis-inhibition reproduces the second peak of Notch activity behind the proneural wave in silico. Based on this, we demonstrate that Delta expression induces a quick degradation of Notch in late endosomes and the formation of the twin peaks of Notch activity in vivo. Indeed, the amount of Notch is upregulated and the twin peaks are fused forming a single peak when the function of Delta or late endosomes is compromised. Additionally, we show that the second Notch peak behind the wavefront controls neurogenesis. Thus, intracellular trafficking of Notch orchestrates the temporal dynamics of Notch activity and the temporal patterning of neurogenesis.

Mathematical modeling of Notch dynamics in Drosophila neural development

“…Additionally, we showed that the onset of the second Notch activation in NBs regulates neuronal cell fate decisions. These findings demonstrate the molecular mechanism of cis -inhibition of Notch signalling and the biological importance of the complex activation mechanism of Notch signalling in neurogenesis [ 104 ].…”

“…In Su(H) or Delta mutant clones, the proneural wave progression is accelerated [ 98 ]. The second Notch activation in NBs regulates the expression of temporal transcription factors in NBs, which defines the neuronal subtype specification [ 104 ]. EGFR signalling is activated in the wavefront and induces NB differentiation ( Figure 3b , c) [ 98 ].…”

“…To elucidate the Notch dynamics in proneural wave progression and interactions among signalling pathways, we and others have developed mathematical models [ 37 , 104 , 108–111 ]. First, we focused on the interaction between Notch and EGFR signalling pathways because these pathways show transient activation in the transition zone from NEs to NBs and play central roles in proneural wave progression ( Figure 3d , e) [ 98 , 111 ].…”

“…In the mathematical model described above, Notch signalling shows a single peak at wavefront [ 111 ]. Recently, we reported that a modified mathematical model, formulated by introducing strong non-linearity in cis -inhibition of Notch signalling, reproduced two peaks of Notch activation in silico [ 104 ]. in vivo experiments showed that Delta expression induces rapid degradation of the Notch protein in late endosomes, which generates a gap between the two Notch activation peaks.…”

Cutting edge technologies expose the temporal regulation of neurogenesis in the Drosophila nervous system

“…Although the Notch signal is activated at the wave front of the proneural wave to repress its progression, it is activated again in NBs behind the proneural wave. Interestingly, the second peak of Notch activation coincides with Klu expression and upregulates its expression ( Figure 2b ) [ 62 ]. Similarly, Notch signalling controls the expression of Slp in medulla NBs [ 50 ].…”