Fat facets and Liquid facets promote Delta endocytosis and Delta signaling in the signaling cells

Abstract

Endocytosis modulates the Notch signaling pathway in both the signaling and receiving cells. One recent hypothesis is that endocytosis of the ligand Delta by the signaling cells is essential for Notch activation in the receiving cells. Here, we present evidence in strong support of this model. We show that in the developing Drosophila eye Fat facets (Faf), a deubiquitinating enzyme, and its substrate Liquid facets (Lqf), an endocytic epsin, promote Delta internalization and Delta signaling in the signaling cells. We demonstrate that while Lqf is necessary for three different Notch/Delta signaling events at the morphogenetic furrow, Faf is essential only for one:Delta signaling by photoreceptor precluster cells, which prevents recruitment of ectopic neurons. In addition, we show that the ubiquitin-ligase Neuralized(Neur), which ubiquitinates Delta, functions in the signaling cells with Faf and Lqf. The results presented bolster one model for Neur function in which Neur enhances Delta signaling by stimulating Delta internalization in the signaling cells. We propose that Faf plays a role similar to that of Neur in the Delta signaling cells. By deubiquitinating Lqf, which enhances the efficiency of Delta internalization, Faf stimulates Delta signaling.

The association of epsin with ubiquitinated cargo along the endocytic pathway is negatively regulated by its interaction with clathrin

“…A pool of epsin bound to a ubiquitinated cargo may be recruited to clathrincoated pits, for example, by EH domain endocytic proteins, but then dissociate from Ub, thus allowing other interactions of such cargo with coat components. Interestingly, a recent study demonstrated that epsin is critically needed for the function of the Notch receptor Delta (18,19), and that this function involves not only the ubiquitination of Delta and its endocytosis, but also the targeting of internalized Delta to a specific endosomal subcompartment (19). Perhaps, epsin functions to recruit to clathrincoated pits and a recycling endosomal compartment, a pool of ubiquitinated Delta that is otherwise internalized and targeted to lysosomes through a clathrin-independent pathway.…”

“…This reaction, which occurs constitutively and is further stimulated by growth factors (6), inhibits the property of epsin to interact with the clathrin coat and the membrane (15). The importance of Ub metabolism in epsin function is further supported by genetic studies in Drosophila (16)(17)(18)(19). The presence in epsin of binding sites for both Ub and core components of the clathrin coat, together with the localization of epsin at clathrin-coated pits, suggested that a function of this protein is to recruit ubiquitinated cargo to clathrin-coated pits, thus triggering its internalization (13).…”

Deubiquitylases From Genes to Organism

“…The Drosophila DUB Fat Facets (Faf) (USP9X in humans) interacts directly with the epsin homolog Liquid facets (Laf). Deubiquitylation of Laf by Faf is proposed to facilitate Notch signaling by promoting the internalization of the Notch ligand Delta during fly development (192).…”

Embryonic arrest at midgestation and disruption of Notch signaling produced by the absence of both epsin 1 and epsin 2 in mice

“…In Dictyostelium, epsin is dispensable for clathrin-mediated endocytosis (11). In Drosophila, the only epsin ortholog, liquid facets, was shown to have a critical role in the Notch signaling pathway most likely via a specific endocytic function in Notch ligands expressing cells (12)(13)(14)(15). Activation of Notch critically requires its proteolytic cleavage, which is triggered by the transendocytosis of Notch receptors by Notch ligands (16)(17)(18), an action that depends on ubiquitination of these latter proteins (19).…”