Numb “Adapting” Notch for Endocytosis

AbstractDuring sensory organ precursor divisions in Drosophila, the numb gene product segregates asymmetrically into one of the two daughter cells, to which it confers a specific fate by inhibiting Notch signaling. In this issue of Developmental Cell, Berdnik et al. show that Numb recruits α-Adaptin and that this physical interaction plays a role in downregulating Notch, presumably by stimulating endocytosis of Notch

Fringe proteins modulate Notch-ligand cis and trans interactions to specify signaling states

The Notch signaling pathway consists of multiple types of receptors and ligands, whose interactions can be tuned by Fringe glycosyltransferases. A major challenge is to determine how these components control the specificity and directionality of Notch signaling in developmental contexts. Here, we analyzed same-cell (cis) Notch-ligand interactions for Notch1, Dll1, and Jag1, and their dependence on Fringe protein expression in mammalian cells. We found that Dll1 and Jag1 can cis-inhibit Notch1, and Fringe proteins modulate these interactions in a way that parallels their effects on trans interactions. Fringe similarly modulated Notch-ligand cis interactions during Drosophila development. Based on these and previously identified interactions, we show how the design of the Notch signaling pathway leads to a restricted repertoire of signaling states that promote heterotypic signaling between distinct cell types, providing insight into the design principles of the Notch signaling system, and the specific developmental process of Drosophila dorsal-ventral boundary formation

The AXH Domain of Ataxin-1 Mediates Neurodegeneration through Its Interaction with Gfi-1/Senseless Proteins

SummarySpinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disease caused by an expanded glutamine tract in human Ataxin-1 (hAtx-1). The expansion stabilizes hAtx-1, leading to its accumulation. To understand how stabilized hAtx-1 induces selective neuronal degeneration, we studied Drosophila Atx-1 (dAtx-1), which has a conserved AXH domain but lacks a polyglutamine tract. Overexpression of hAtx-1 in fruit flies produces phenotypes similar to those of dAtx-1 but different from the polyglutamine peptide alone. We show that the Drosophila and mammalian transcription factors Senseless/Gfi-1 interact with Atx-1’s AXH domain. In flies, overexpression of Atx-1 inhibits sensory-organ development by decreasing Senseless protein. Similarly, overexpression of wild-type and glutamine-expanded hAtx-1 reduces Gfi-1 levels in Purkinje cells. Deletion of the AXH domain abolishes the effects of glutamine-expanded hAtx-1 on Senseless/Gfi-1. Interestingly, loss of Gfi-1 mimics SCA1 phenotypes in Purkinje cells. These results indicate that the Atx-1/Gfi-1 interaction contributes to the selective Purkinje cell degeneration in SCA1

Regulation of BMP4/Dpp retrotranslocation and signaling by deglycosylation.

During endoplasmic reticulum-associated degradation (ERAD), the cytoplasmic enzyme N-glycanase 1 (NGLY1) is proposed to remove N-glycans from misfolded N-glycoproteins after their retrotranslocation from the ER to the cytosol. We previously reported that NGLY1 regulates Drosophila BMP signaling in a tissue-specific manner (Galeone et al., 2017). Here, we establish the Drosophila Dpp and its mouse ortholog BMP4 as biologically relevant targets of NGLY1 and find, unexpectedly, that NGLY1-mediated deglycosylation of misfolded BMP4 is required for its retrotranslocation. Accumulation of misfolded BMP4 in the ER results in ER stress and prompts the ER recruitment of NGLY1. The ER-associated NGLY1 then deglycosylates misfolded BMP4 molecules to promote their retrotranslocation and proteasomal degradation, thereby allowing properly-folded BMP4 molecules to proceed through the secretory pathway and activate signaling in other cells. Our study redefines the role of NGLY1 during ERAD and suggests that impaired BMP4 signaling might underlie some of the NGLY1 deficiency patient phenotypes

A POGLUT1 mutation causes a muscular dystrophy with reduced Notch signaling and satellite cell loss

Skeletal muscle regeneration by muscle satellite cells is a physiological mechanism activated upon muscle damage and regulated by Notch signaling. In a family with autosomal recessive limb-girdle muscular dystrophy, we identified a missense mutation in 1 (protein O -glucosyltransferase 1), an enzyme involved in Notch posttranslational modification and function. In vitro and in vivo experiments demonstrated that the mutation reduces O -glucosyltransferase activity on Notch and impairs muscle development. Muscles from patients revealed decreased Notch signaling, dramatic reduction in satellite cell pool and a muscle-specific α-dystroglycan hypoglycosylation not present in patients' fibroblasts. Primary myoblasts from patients showed slow proliferation, facilitated differentiation, and a decreased pool of quiescent 7 + cells. A robust rescue of the myogenesis was demonstrated by increasing Notch signaling. None of these alterations were found in muscles from secondary dystroglycanopathy patients. These data suggest that a key pathomechanism for this novel form of muscular dystrophy is Notch-dependent loss of satellite cells

AI is a viable alternative to high throughput screening: a 318-target study

: High throughput screening (HTS) is routinely used to identify bioactive small molecules. This requires physical compounds, which limits coverage of accessible chemical space. Computational approaches combined with vast on-demand chemical libraries can access far greater chemical space, provided that the predictive accuracy is sufficient to identify useful molecules. Through the largest and most diverse virtual HTS campaign reported to date, comprising 318 individual projects, we demonstrate that our AtomNet® convolutional neural network successfully finds novel hits across every major therapeutic area and protein class. We address historical limitations of computational screening by demonstrating success for target proteins without known binders, high-quality X-ray crystal structures, or manual cherry-picking of compounds. We show that the molecules selected by the AtomNet® model are novel drug-like scaffolds rather than minor modifications to known bioactive compounds. Our empirical results suggest that computational methods can substantially replace HTS as the first step of small-molecule drug discovery

The Roles of Notch Signaling in Liver Development and Disease

The Notch signaling pathway plays major roles in organ development across animal species. In the mammalian liver, Notch has been found critical in development, regeneration and disease. In this review, we highlight the major advances in our understanding of the role of Notch activity in proper liver development and function. Specifically, we discuss the latest discoveries on how Notch, in conjunction with other signaling pathways, aids in proper liver development, regeneration and repair. In addition, we review the latest in the role of Notch signaling in the pathogenesis of liver fibrosis and chronic liver disease. Finally, recent evidence has shed light on the emerging connection between Notch signaling and glucose and lipid metabolism. We hope that highlighting the major advances in the roles of Notch signaling in the liver will stimulate further research in this exciting field and generate additional ideas for therapeutic manipulation of the Notch pathway in liver diseases

Xylosylation of the Notch receptor preserves the balance between its activation by <i>trans</i>-Delta and inhibition by <i>cis</i>-ligands in <i>Drosophila</i>

<div><p>The <i>Drosophila</i> glucoside xylosyltransferase Shams xylosylates Notch and inhibits Notch signaling in specific contexts including wing vein development. However, the molecular mechanisms underlying context-specificity of the <i>shams</i> phenotype is not known. Considering the role of Delta-Notch signaling in wing vein formation, we hypothesized that Shams might affect Delta-mediated Notch signaling in <i>Drosophila</i>. Using genetic interaction studies, we find that altering the gene dosage of <i>Delta</i> affects the wing vein and head bristle phenotypes caused by loss of Shams or by mutations in the Notch xylosylation sites. Clonal analysis suggests that loss of <i>shams</i> promotes Delta-mediated Notch activation. Further, Notch <i>trans</i>-activation by ectopically overexpressed Delta shows a dramatic increase upon loss of <i>shams</i>. In agreement with the above <i>in vivo</i> observations, cell aggregation and ligand-receptor binding assays show that <i>shams</i> knock-down in Notch-expressing cells enhances the binding between Notch and <i>trans</i>-Delta without affecting the binding between Notch and <i>trans</i>-Serrate and cell surface levels of Notch. Loss of Shams does not impair the <i>cis</i>-inhibition of Notch by ectopic overexpression of ligands <i>in vivo</i> or the interaction of Notch and <i>cis</i>-ligands in S2 cells. Nevertheless, removing one copy of endogenous ligands mimics the effects of loss <i>shams</i> on Notch <i>trans</i>-activation by ectopic Delta. This favors the notion that <i>trans</i>-activation of Notch by Delta overcomes the <i>cis</i>-inhibition of Notch by endogenous ligands upon loss of <i>shams</i>. Taken together, our data suggest that xylosylation selectively impedes the binding of Notch with <i>trans</i>-Delta without affecting its binding with <i>cis</i>-ligands and thereby assists in determining the balance of Notch receptor’s response to <i>cis-</i>ligands vs. <i>trans-</i>Delta during <i>Drosophila</i> development.</p></div