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Spatio-temporal regulation of Notch signaling |
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Notch signaling plays important roles in many biological systems. Especially, 'lateral inhibition', discussed in the previous section, is essential to establish a precise spatial pattern of neurogenesis. On the other hand, temporal dynamics of Notch sigaling is thought to be improtant as well. However, molecular mechanisms that regulate the temporal dynamics of Notch signaling remain largely elusive.
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| Notch is activated twice nearby the proneural wave |
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The previous mathematical model of the proneural wave reproduced the single activity peak of Notch at the wave front (Green, peak 1; see the previous section). However, Notch is activated again behind the proneural wave in vivo (peak 2). When the Delta expression at the wave front was eliminated, both the first and second peaks of Notch activity dissapeared, suggesting that the two peaks of Notch activity are tightly related.
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| Non-linear cis-inhibition induces the twin peaks of Notch activity |
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When Delta expressed in the neighboring cell binds Notch, the intracellular domain of Notch is celaved to activate the down stream target genes (trans-activation, left). Onthe other hand, when Delta and Notch are expressed in the same cell, Delta suppresses Notch activity (cis-inhibition).
We modified the mathematical model and found that strong non-linearity in cis-inhibition causes the twin peaks of Notch activity. When Delta expression is below the threshold, cis-inhibition does not take place. However, cis-inhibition suddenly exceeds tran-activation when Delta expression exceeds the threshold, and split the single Notch activation peak into the twin peaks (right).
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Delta induces Notch degradation through intracellular trafficking
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The non-linear cis-inhibition can be explained if the membrane localized Notch is quickly degraded by Delta. Indeed, the membrane localized Notch is temporally down regulated behind the proneural wave progression (left). In Delta mutant clones, the down-regulation of Notch was restored showing upregulation of Notch (middle). These resutls suggest that Delta triggers quick degradation of Notch.
In general, membrane proteins are transported to late endosomes and degraded (right). We demonstrated that Delta-Notch complex is transported to late endosomes. While Delta is returned to the cell membrane through recycling endosomes, Notch is degraded through late endosomes.
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The second Notch activity regulates the temporal change of the neuroblasts
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The second Notch peak is found in the neuroblasts behind the proneural wave. The neuroblasts sequentially express the temporal transcription factors such as Hth, Klu and Ey, and produce diverse types of neurons (left). Surprisingly, the second Notch activation and Klu expression occur simultaneously. Indeed, loss of Notch activity eliminates Klu expression, suggesting that the second Notch activity regulates Klu expression in neuroblasts.
Partial reduction of Delta expression causes the fusion of twin peaks of Notch activity into a single peak. In this case, Klu is expressed precociously and Run-positive neurons are precociously produced .
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Biological significance of cis-inhibition and its molecular mechanism were largely unknown. However, we demonstrated that 1. Delta triggers cis-inhibition by Notch degradation through intracellular trafficking and 2. non-linear cis-inhibition causes the twin activation peaks of Notch and regulates the temporal changes of neuroblasts. Similar mechanisms may play important roles in a wide variety of biological systems.
Wang, M., Han, X., Liu, C., Takayama, R., Yasugi, T., Ei, S., Nagayama, M., Tanaka, Y. and Sato, M.
Intracellular trafficking of Notch orchestrates temporal dynamics of Notch activity in the fly brain.
Nature Communications 12, 2083 (2021). |
