View Program - 63rd Annual Drosophila Research Conference

The conserved Notch signaling pathway controls a plethora of cellular processes during development and tissue homeostasis across animal species, and abnormal signaling activity of Notch has been linked to several developmental disorders and pathologies. The Notch protein is a transmembrane receptor and cleavage of Notch to release its intracellular domain (NICD) allows the latter to enter the nucleus and enforce the transcriptional output of signaling. NICD cleavage is best studied in response to receptor-ligand interactions but can also occur in the absence of ligand. As cells continuously traffic Notch receptor from the cell surface into endocytic compartments regardless of ligand presence, release of the NICD at different steps of the endocytic pathway is possible. Thus, Notch endocytic trafficking offers a unique opportunity to finely-tune signaling output. In this context, a key regulator of the endosomal passage of Drosophila Notch is the Nedd4-family ubiquitin ligase Suppressor of Deltex (Su(dx)), which recognizes a PPxY motif within the NICD to promote Notch internalization from the membrane of late endosomes/multivesicular bodies (MVBs) into their luminal space. This prevents signaling activity by both topologically restricting the release of the NICD into the cytoplasm if cleaved and promoting the subsequent lysosomal degradation of Notch.
In this study we have used the pupal wing and S2 cultured cells to uncover a role for the non-receptor tyrosine kinase Abelson (Abl) in the late endosomal passage of Notch. We find that loss of abl in both models results in an aberrant accumulation of Notch at the membrane of late endosomes due to compromised internalization into the lumen of these compartments, and this de-represses signaling. Moreover, Abl performs this role in a kinase-dependent manner that requires the PPxY motif of the NICD, the same motif used by Su(dx) to promote internalization of Notch into the late endosomal lumen. As endocytic trafficking is a substantial contributor to the extraordinary variety of routes for Notch activation, careful and robust regulation of Notch at each step of this transport is critical for preventing non-physiological signaling activity. Our study offers important new mechanistic insight into how two post-translational modifiers, a kinase (Abl) and a ubiquitin ligase (Su(dx)), converge upon the same molecular motif (NICD PPxY) to confer robustness to the modulation of Notch activity during late steps of its endosomal passage.

The Abelson tyrosine kinase cooperates with the Nedd4-family ubiquitin ligase Suppressor of Deltex to regulate the late endosomal passage of Notch and modulate signaling activation