The Notch receptor mediates a short-range signal that regulates many cell fate decisions. The misregulation of Notch has been linked to cancer and to developmental disorders. Upon binding to its ligands, Delta (Dl) or Serrate (Ser), the Notch ectodomain is shed by the action of an ADAM protease. The Notch intracellular domain is subsequently released proteolytically from the membrane by Presenilin and translocates to the nucleus to activate the transcription factor, Suppressor of Hairless. We show in Drosophila that Notch signaling is limited by the activity of two Nedd4 family HECT domain proteins, Suppressor of deltex [Su(dx)] and DNedd4. We rule out models by which Su(dx) downregulates Notch through modulating Deltex or by limiting the adherens junction accumulation of Notch. Instead, we show that Su(dx) regulates the postendocytic sorting of Notch within the early endosome to an Hrs- and ubiquitin-enriched subdomain en route to the late endosome. We propose a model in which endocytic sorting of Notch mediates a decision between its activation and downregulation. Such intersections between trafficking routes may provide key points at which other signals can modulate Notch activity in both normal development and in the pathological misactivation of Notch.

Tracheal and nervous system development are two model systems for the study of organogenesis in Drosophila. In two independent screens, we identified three alleles of a gene involved in tracheal, cuticle and CNS development. Here, we show that these alleles, and the previously identified cystic and mummy, all belong to the same complementation group. These are mutants of a gene encoding the UDP-N-acetylglucosamine diphosphorylase, an enzyme responsible for the production of UDP-N-acetylglucosamine, an important intermediate in chitin and glycan biosynthesis. cyst was originally singled out as a gene required for the regulation of tracheal tube diameter. We characterized the cyst/mmy tracheal phenotype and upon histological examination concluded that mmy mutant embryos lack chitin-containing structures, such as the procuticle at the epidermis and the taenidial folds in the tracheal lumen. While most of their tracheal morphogenesis defects can be attributed to the lack of chitin, when compared to krotzkopf verkehrt (kkv) chitin-synthase mutants, mmy mutants showed a stronger phenotype, suggesting that some of the mmy phenotypes, like the axon guidance defects, are chitin-independent. We discuss the implications of these new data in the mechanism of size control in the Drosophila trachea.

Notch is a vitally important signalling receptor controlling cell fate determination and pattern formation in numerous ways during development of both invertebrate and vertebrate species. An intriguing pathway for the Notch signal has emerged where, after ligand-dependent proteolysis, an intracellular fragment of the receptor itself translocates to the nucleus to regulate gene expression. The nuclear activity of the Notch intracellular domain is linked to complexes regulating chromatin organization through histone deacetylation and acetylation. To allow the Notch signal to be deployed in numerous contexts, many different mechanisms have evolved to regulate the level, duration and spatial distribution of Notch activity. Regulation occurs at multiple levels including patterns of ligand and receptor expression, Notch-ligand interactions, trafficking of the receptor and ligands, and covalent modifications including glycosylation, phosphorylation and ubiquitination. Several Notch regulatory proteins have conserved domains that link them to the ubiquitination pathway, and ubiquitination of the Notch intracellular domain has recently been linked to its degradation. Different proteolytically derived isoforms of Notch have also been identified that may be involved in alternative Notch-dependent signals or regulatory mechanisms, and differences between the four mammalian Notch homologues are beginning to be appreciated.

Type 1 serine/threonine protein phosphatases (PP1) are important regulators of many cellular and developmental processes, including glycogen metabolism, muscle contraction, and the cell cycle [1] [2] [3] [4] [5]. Drosophila and humans both have multiple genes encoding PP1 isoforms [3] [6] [7]; each has one beta and several alpha isoform genes (alpha(1), alpha(2), alpha(3) in flies, alpha and gamma in humans; mammalian PP1beta is also known as PP1delta). The alpha/beta subtype differences are highly conserved between flies and mammals [6]. Though all these proteins are >85% identical to each other and have indistinguishable activities in vitro, we show here that the Drosophila beta isoform has a distinct biological role. We show that PP1beta9C corresponds to flapwing (flw), previously identified mutants of which are viable but flightless because of defects in indirect flight muscles (IFMs) [8]. We have isolated a new, semi-lethal flw allele that shows a range of defects, especially in muscles, which break away from their attachment sites and degenerate.

In Drosophila, Suppressor of deltex (Su(dx)) mutations display a wing vein gap phenotype resembling that of Notch gain of function alleles. The Su(dx) protein may therefore act as a negative regulator of Notch but its activity on actual Notch signalling levels has not been demonstrated. Here we show that Su(dx) does regulate the level of Notch signalling in vivo, upstream of Notch target genes and in different developmental contexts, including a previously unknown role in leg joint formation. Overexpression of Su(dx) was capable of blocking both the endogenous activity of Notch and the ectopic Notch signalling induced by the overexpression of Deltex, an intracellular Notch binding protein. In addition, using the conditional phenotype of the Su(dx)(sp) allele, we show that loss of Su(dx) activity is rapidly followed by an up-regulation of E(spl)mbeta expression, the immediate target of Notch signal activation during wing vein development. While Su(dx) adult wing vein phenotypes are quite mild, only affecting the distal tips of the veins, we show that the initial consequence of loss of Su(dx) activity is more severe than previously thought. Using a time-course experiment we show that the phenotype is buffered by feedback regulation illustrating how signalling networks can make development robust to perturbation.