Notch signaling from the endosome requires a conserved dileucine motif

Abstract

Notch signaling is reliant on γ-secretase–mediated processing, although the subcellular location where it cleaves Notch to initiate signaling remains unresolved. Findings here support a model in which Notch signaling in mammalian systems is initiated from either the plasma membrane or lysosome, but not the early endosome.

Ligand-Independent Mechanisms of Notch Activity

“…Localization of Notch to the late endosomal limiting membrane is required for Dx-dependent Notch activation, as blocking Rab5, Rab7, HOPS complex, or AP-3 complex components attenuates Notch signaling in dx- expressing cells [27,28] (Figure 1). This requirement for the localization of Notch on the late endosomal limiting membrane is also observed in Notch activation occurring in lgd mutants (Table 1, Figure 1), and localization to the limiting membrane is directed by a conserved dileucine motif in mammals [29]. In addition, the E3 ubiquitin protein ligase family of Nedd4 proteins, dNedd4 and Su(dx), antagonize Notch signaling through ubiquitination [30–34].…”

Regulation of Notch Signaling Through Intracellular Transport

“…A negative regulatory role for DTX1 in Notch signaling does not appear to be cell-type specific given that liver stem cells genetically null for DTX1 and DTX2 reveal increased levels of Notch signaling (Lehar and Bevan, 2006). Moreover, DTX1 depletion in mammalian cell culture not only elevates signaling, but enhances Notch cell surface levels (Zheng et al, 2013). This later finding reinforces the idea that mammalian DTX1, like that in Drosophila, controls Notch transport decisions within cells.…”

“…Notch accumulates at the plasma membrane in cells lacking the E3 ubiquitin ligase in Drosophila (Yamada et al, 2011) or following DTX-1 depletion in mammalian cell culture (Zheng et al, 2013). However, evidence supporting a direct role for Deltex in Notch internalization is currently lacking.…”

Combining genetic and biophysical approaches to probe the structure and function relationships of the notch receptor

“…It is necessary therefore to look beyond the core signal activation mechanism to understand the different regulatory roles fulfilled by regions outside the ligand-binding interface. In this regard it is useful to consider that Notch can also be activated independently of DSL-domain ligands following its endocytosis (Hori et al, 2011;Mukherjee et al, 2011;Palmer et al, 2014;Sakata et al, 2004;Schneider et al, 2013;Shimizu et al, 2014;Wilkin et al, 2004Wilkin et al, , 2008, (Figure 4), by a mechanism best characterised in Drosophila models but which may also be relevant to human Notch in some contexts (Choy et al, 2017;G omez-del Arco et al, 2010;Zheng et al, 2013). Signal activation by this mechanism critically depends on Notch being retained in the endosomal membrane and not being transferred off the endosomal surface by incorporation into the intra-luminal vesicles (ILVs) (Hori et al, 2011;Shimizu et al, 2014;Wilkin et al, 2008).…”