Protein O-Glucosyltransferase 1 (POGLUT1) Promotes Mouse Gastrulation through Modification of the Apical Polarity Protein CRUMBS2

Disease modeling and gene correction of LGMDR21 iPSCs elucidates the role of POGLUT1 in skeletal muscle maintenance, regeneration, and the satellite cell niche

Autosomal recessive limb-girdle muscular dystrophy 21 (LGMDR21) is caused by pathogenic variants in protein O-glucosyltransferase 1 (POGLUT1), which is responsible for O-glucosylation of specific epidermal growth factor (EGF) repeats found in ∼50 mammalian proteins, including Notch receptors. Previous data from patient biopsies indicated that impaired Notch signaling, reduction of muscle stem cells, and accelerated differentiation are probably involved in disease etiopathology. Using patient induced pluripotent stem cells (iPSCs), their corrected isotypes, and control iPSCs, gene expression profiling indicated dysregulation of POGLUT1, NOTCH, muscle development, extracellular matrix (ECM), cell adhesion, and migration as involved pathways. They also exhibited reduced in vitro POGLUT1 enzymatic activity and NOTCH signaling as well as defective myogenesis, proliferation, migration and differentiation. Furthermore, in vivo studies demonstrated significant reductions in engraftment, muscle stem cell formation, PAX7 expression, and maintenance, along with an increased percentage of mislocalized PAX7+ cells in the interstitial space. Gene correction in patient iPSCs using CRISPR-Cas9 nickase led to the rescue of the main in vitro and in vivo phenotypes. These results demonstrate the efficacy of iPSCs and gene correction in disease modeling and rescue of the phenotypes and provide evidence of the involvement of muscle stem cell niche localization, PAX7 expression, and cell migration as possible mechanisms in LGMDR21.

Other Types of Glycosylation

O-Linked glycosylation such as O-fucose, O-glucose, and O-N-acetylglucosamine are considered to be unusual. As suggested by the high levels of evolutional conservation, these O-glycans are fundamentally important for life. In the last two decades, our understanding of the importance of these glycans has greatly advanced. In particular, identification of the glycosyltransferases responsible for the biosynthesis of these glycans has accelerated basic research on the functional significance and molecular mechanisms by which these O-glycans regulate protein functions as well as clinical research on human diseases due to changes in these types of O-glycosylation. Notably, Notch receptor signaling is modified with and regulated by these types of O-glycans. Here, we summarize the current view of the structures and the significance of these O-glycans mainly in the context of Notch signaling regulation and human diseases.

Crumbs2 Is an Essential Slit Diaphragm Protein of the Renal Filtration Barrier

BACKGROUND

Crumbs2 is expressed at embryonic stages as well as in the retina, brain, and glomerular podocytes. Recent studies identified CRB2 mutations as a novel cause of steroid-resistant nephrotic syndrome (SRNS).

METHODS

To study the function of Crb2 at the renal filtration barrier, mice lacking Crb2 exclusively in podocytes were generated. Gene expression and histologic studies as well as transmission and scanning electron microscopy were used to analyze these Crb2^podKO knockout mice and their littermate controls. Furthermore, high-resolution expansion microscopy was used to investigate Crb2 distribution in murine glomeruli. For pull-down experiments, live cell imaging, and transcriptome analyses, cell lines were applied that inducibly express fluorescent protein-tagged CRB2 wild type and mutants.

RESULTS

Crb2^podKO mice developed proteinuria directly after birth that preceded a prominent development of disordered and effaced foot processes, upregulation of renal injury and inflammatory markers, and glomerulosclerosis. Pull-down assays revealed an interaction of CRB2 with Nephrin, mediated by their extracellular domains. Expansion microscopy showed that in mice glomeruli, Crb2 and Nephrin are organized in adjacent clusters. SRNS-associated CRB2 protein variants and a mutant that lacks a putative conserved O-glycosylation site were not transported to the cell surface. Instead, mutants accumulated in the ER, showed altered glycosylation pattern, and triggered an ER stress response.

CONCLUSIONS

Crb2 is an essential component of the podocyte's slit diaphragm, interacting with Nephrin. Loss of slit diaphragm targeting and increasing ER stress are pivotal factors for onset and progression of CRB2-related SRNS.

Structure, function, and pathology of protein O-glucosyltransferases

Protein O-glucosylation is a crucial form of O-glycosylation, which involves glucose (Glc) addition to a serine residue within a consensus sequence of epidermal growth factor epidermal growth factor (EGF)-like repeats found in several proteins, including Notch. Glc provides stability to EGF-like repeats, is required for S2 cleavage of Notch, and serves to regulate the trafficking of Notch, crumbs2, and Eyes shut proteins to the cell surface. Genetic and biochemical studies have shown a link between aberrant protein O-glucosylation and human diseases. The main players of protein O-glucosylation, protein O-glucosyltransferases (POGLUTs), use uridine diphosphate (UDP)-Glc as a substrate to modify EGF repeats and reside in the endoplasmic reticulum via C-terminal KDEL-like signals. In addition to O-glucosylation activity, POGLUTs can also perform protein O-xylosylation function, i.e., adding xylose (Xyl) from UDP-Xyl; however, both activities rely on residues of EGF repeats, active-site conformations of POGLUTs and sugar substrate concentrations in the ER. Impaired expression of POGLUTs has been associated with initiation and progression of human diseases such as limb-girdle muscular dystrophy, Dowling-Degos disease 4, acute myeloid leukemia, and hepatocytes and pancreatic dysfunction. POGLUTs have been found to alter the expression of cyclin-dependent kinase inhibitors (CDKIs), by affecting Notch or transforming growth factor-β1 signaling, and cause cell proliferation inhibition or induction depending on the particular cell types, which characterizes POGLUT's cell-dependent dual role. Except for a few downstream elements, the precise mechanisms whereby aberrant protein O-glucosylation causes diseases are largely unknown, leaving behind many questions that need to be addressed. This systemic review comprehensively covers literature to understand the O-glucosyltransferases with a focus on POGLUT1 structure and function, and their role in health and diseases. Moreover, this study also raises unanswered issues for future research in cancer biology, cell communications, muscular diseases, etc.

Diseases related to Notch glycosylation

The Notch receptors are a family of transmembrane proteins that mediate direct cell-cell interactions and control numerous cell-fate specifications in humans. The extracellular domains of mammalian Notch proteins contain 29-36 tandem epidermal growth factor-like (EGF) repeats, most of which have O-linked glycan modifications: O-glucose added by POGLUT1, O-fucose added by POFUT1 and elongated by Fringe enzymes, and O-GlcNAc added by EOGT. The extracellular domain is also N-glycosylated. Mutations in the glycosyltransferases modifying Notch have been identified in several diseases, including Dowling-Degos Disease (haploinsufficiency of POFUT1 or POGLUT1), a form of limb-girdle muscular dystrophy (autosomal recessive mutations in POGLUT1), Spondylocostal Dysostosis 3 (autosomal recessive mutations in LFNG), Adams-Oliver syndrome (autosomal recessive mutations in EOGT), and some cancers (amplification, gain or loss-of-function of POFUT1, Fringe enzymes, POGLUT1, MGAT3). Here we review the characteristics of these diseases and potential molecular mechanisms.

Asymmetry in the frequency and position of mitosis in the mouse embryo epiblast at gastrulation

At gastrulation, a subpopulation of epiblast cells constitutes a transient posteriorly located structure called the primitive streak, where cells that undergo epithelial-mesenchymal transition make up the mesoderm and endoderm lineages. Mouse embryo epiblast cells were labelled ubiquitously or in a mosaic fashion. Cell shape, packing, organization and division were recorded through live imaging during primitive streak formation. Posterior epiblast displays a higher frequency of rosettes, some of which associate with a central cell undergoing mitosis. Cells at the primitive streak, in particular delaminating cells, undergo mitosis more frequently than other epiblast cells. In pseudostratified epithelia, mitosis takes place at the apical side of the epithelium. However, mitosis is not restricted to the apical side of the epiblast, particularly on its posterior side. Non-apical mitosis occurs specifically in the streak even when ectopically located. Posterior non-apical mitosis results in one or two daughter cells leaving the epiblast layer. Cell rearrangement associated with mitotic cell rounding in posterior epiblast, in particular when non-apical, might thus facilitate cell ingression and transition to a mesenchymal phenotype.

The single EGF-like domain of mouse PAMR1 is modified by O-Glucose, O-Fucose and O-GlcNAc

Epidermal growth factor-like domains (EGF-LDs) of membrane and secreted proteins can be modified by N-glycans and/or potentially elongated O-linked monosaccharides such as O-glucose (O-Glc) found at two positions (O-Glc 1 and O-Glc2), O-fucose (O-Fuc) and O-N-acetylglucosamine (O-GlcNAc). The presence of three O-linked sugars within the same EGF-LD, such as in EGF-LD 20 of NOTCH1, has rarely been evidenced. We searched in KEGG GENES database to list mouse and human proteins with an EGF-LD sequence including one, two, three or four potential O-glycosylation consensus sites. Among the 129 murine retrieved proteins, most had predicted O-fucosylation and/or O-GlcNAcylation sites. Around 68% of EGF-LDs were subjected to only one O-linked sugar modification and near 5% to three modifications. Among these latter, we focused on the peptidase domain-containing protein associated with muscle regeneration 1 (PAMR1), having only one EGF-LD. To test the ability of this domain to be glycosylated, a correctly folded EGF-LD was produced in Escherichia coli periplasm, purified and subjected to in vitro incubations with the recombinant O-glycosyltransferases POGLUT1, POFUT1 and EOGT, adding O-Glc1, O-Fuc and O-GlcNAc, respectively. Using click chemistry and mass spectrometry, isolated PAMR1 EGF-LD was demonstrated to be modified by the three O-linked sugars. Their presence was individually confirmed on EGF-LD of full-length mouse recombinant PAMR1, with at least some molecules modified by both O-Glc1 and O-Fuc. Overall, these results are consistent with the presence of a triple O-glycosylated EGF-LD in mouse PAMR1.

Multifaceted regulation of Notch signaling by glycosylation

To build a complex body composed of various cell types and tissues and to maintain tissue homeostasis in the postembryonic period, animals use a small number of highly conserved intercellular communication pathways. Among these is the Notch signaling pathway, which is mediated via the interaction of transmembrane Notch receptors and ligands usually expressed by neighboring cells. Maintaining optimal Notch pathway activity is essential for normal development, as evidenced by various human diseases caused by decreased and increased Notch signaling. It is therefore not surprising that multiple mechanisms are used to control the activation of this pathway in time and space. Over the last 20 years, protein glycosylation has been recognized as a major regulatory mechanism for Notch signaling. In this review, we will provide a summary of the various types of glycan that have been shown to modulate Notch signaling. Building on recent advances in the biochemistry, structural biology, cell biology and genetics of Notch receptors and the glycosyltransferases that modify them, we will provide a detailed discussion on how various steps during Notch activation are regulated by glycans. Our hope is that the current review article will stimulate additional research in the field of Notch glycobiology and will potentially be of benefit to investigators examining the contribution of glycosylation to other developmental processes.

Crumbs2 mediates ventricular layer remodelling to form the spinal cord central canal

In the spinal cord, the central canal forms through a poorly understood process termed dorsal collapse that involves attrition and remodelling of pseudostratified ventricular layer (VL) cells. Here, we use mouse and chick models to show that dorsal ventricular layer (dVL) cells adjacent to dorsal midline Nestin(+) radial glia (dmNes+RG) down-regulate apical polarity proteins, including Crumbs2 (CRB2) and delaminate in a stepwise manner; live imaging shows that as one cell delaminates, the next cell ratchets up, the dmNes+RG endfoot ratchets down, and the process repeats. We show that dmNes+RG secrete a factor that promotes loss of cell polarity and delamination. This activity is mimicked by a secreted variant of Crumbs2 (CRB2S) which is specifically expressed by dmNes+RG. In cultured MDCK cells, CRB2S associates with apical membranes and decreases cell cohesion. Analysis of Crb2F/F/Nestin-Cre+/- mice, and targeted reduction of Crb2/CRB2S in slice cultures reveal essential roles for transmembrane CRB2 (CRB2TM) and CRB2S on VL cells and dmNes+RG, respectively. We propose a model in which a CRB2S-CRB2TM interaction promotes the progressive attrition of the dVL without loss of overall VL integrity. This novel mechanism may operate more widely to promote orderly progenitor delamination.

Effects of Notch glycosylation on health and diseases

Notch signaling is an evolutionarily conserved signaling pathway and is essential for cell-fate specification in metazoans. Dysregulation of Notch signaling results in various human diseases, including cardiovascular defects and cancer. In 2000, Fringe, a known regulator of Notch signaling, was discovered as a Notch-modifying glycosyltransferase. Since then, glycosylation-a post-translational modification involving literal sugars-on the Notch extracellular domain has been noted as a critical mechanism for the regulation of Notch signaling. Additionally, the presence of diverse O-glycans decorating Notch receptors has been revealed in the extracellular domain epidermal growth factor-like (EGF) repeats. Here, we concisely summarize the recent studies in the human diseases associated with aberrant Notch glycosylation.

The Epithelial-to-Mesenchymal Transition (EMT) in Development and Cancer

Epithelial-to-mesenchymal transitions (EMTs) are complex cellular processes where cells undergo dramatic changes in signaling, transcriptional programming, and cell shape, while directing the exit of cells from the epithelium and promoting migratory properties of the resulting mesenchyme. EMTs are essential for morphogenesis during development and are also a critical step in cancer progression and metastasis formation. Here we provide an overview of the molecular regulation of the EMT process during embryo development, focusing on chick and mouse gastrulation and neural crest development. We go on to describe how EMT regulators participate in the progression of pancreatic and breast cancer in mouse models, and discuss the parallels with developmental EMTs and how these help to understand cancer EMTs. We also highlight the differences between EMTs in tumor and in development to arrive at a broader view of cancer EMT. We conclude by discussing how further advances in the field will rely on in vivo dynamic imaging of the cellular events of EMT.

Decoding the PTM-switchboard of Notch

The developmentally indispensable Notch pathway exhibits a high grade of pleiotropism in its biological output. Emerging evidence supports the notion of post-translational modifications (PTMs) as a modus operandi controlling dynamic fine-tuning of Notch activity. Although, the intricacy of Notch post-translational regulation, as well as how these modifications lead to multiples of divergent Notch phenotypes is still largely unknown, numerous studies show a correlation between the site of modification and the output. These include glycosylation of the extracellular domain of Notch modulating ligand binding, and phosphorylation of the PEST domain controlling half-life of the intracellular domain of Notch. Furthermore, several reports show that multiple PTMs can act in concert, or compete for the same sites to drive opposite outputs. However, further investigation of the complex PTM crosstalk is required for a complete understanding of the PTM-mediated Notch switchboard. In this review, we aim to provide a consistent and up-to-date summary of the currently known PTMs acting on the Notch signaling pathway, their functions in different contexts, as well as explore their implications in physiology and disease. Furthermore, we give an overview of the present state of PTM research methodology, and allude to a future with PTM-targeted Notch therapeutics.

Multiple roles for O-glycans in Notch signalling

Notch signalling regulates a plethora of developmental processes and is also essential for the maintenance of tissue homeostasis in adults. Therefore, fine-tuning of Notch signalling strength needs to be tightly regulated. Of key importance for the regulation of Notch signalling are O-fucose, O-GlcNAc and O-glucose glycans attached to the extracellular domain of Notch receptors. The EGF repeats of the Notch receptor extracellular domain harbour consensus sites for addition of the different types of O-glycan to Ser or Thr, which takes place in the endoplasmic reticulum. Studies from Drosophila to mammals have demonstrated the multifaceted roles of O-glycosylation in regulating Notch signalling. O-glycosylation modulates different aspects of Notch signalling including recognition by Notch ligands, the strength of ligand binding, Notch receptor trafficking, stability and activation at the cell surface. Defects in O-glycosylation of Notch receptors give rise to pathologies in humans. This Review summarizes the nature of the O-glycans on Notch receptors and their differential effects on Notch signalling.

Sensitized genetic backgrounds reveal differential roles for EGF repeat xylosyltransferases in Drosophila Notch signaling

In multicellular organisms, glycosylation regulates various developmental signaling pathways including the Notch pathway. One of the O-linked glycans added to epidermal growth factor-like (EGF) repeats in animal proteins including the Notch receptors is the xylose-xylose-glucose-O oligosaccharide. Drosophila glucoside xylosyltransferase (Gxylt) Shams negatively regulates Notch signaling in specific contexts. Since Shams adds the first xylose residue to O-glucose, its loss-of-function phenotype could be due to the loss of the first xylose, the second xylose or both. To examine the contribution of the second xylose residues to Drosophila Notch signaling, we have performed biochemical and genetic analysis on CG11388, which is the Drosophila homolog of human xyloside xylosyltransferase 1 (XXYLT1). Experiments in S2 cells indicated that similar to human XXYLT1, CG11388 can add the second xylose to xylose-glucose-O glycans. Flies lacking both copies of CG11388 (Xxylt) are viable and fertile and do not show gross phenotypes indicative of altered Notch signaling. However, genetic interaction experiments show that in sensitized genetic backgrounds with decreased or increased Notch pathway components, loss of Xxylt promotes Delta-mediated activation of Notch. Unexpectedly, we find that in such sensitized backgrounds, even loss of one copy of the fly Gxylt shams enhances Delta-mediated Notch activation. Taken together, these data indicate that while the first xylose plays a key role in tuning the Delta-mediated Notch signaling in Drosophila, the second xylose has a fine-tuning role only revealed in sensitized genetic backgrounds.

Identification of an apiosyltransferase in the plant pathogen Xanthomonas pisi

The rare branched-chain sugar apiose, once thought to only be present in the plant kingdom, was found in two bacterial species: Geminicoccus roseus and Xanthomonas pisi. Glycans with apiose residues were detected in aqueous methanol-soluble fractions as well as in the insoluble pellet fraction of X. pisi. Genes encoding bacterial uridine diphosphate apiose (UDP-apiose) synthases (bUASs) were characterized in these bacterial species, but the enzyme(s) involved in the incorporation of the apiose into glycans remained unknown. In the X. pisi genome two genes flanking the XpUAS were annotated as hypothetical glycosyltransferase (GT) proteins. The first GT (here on named XpApiT) belongs to GT family 90 and has a Leloir type B fold and a putative lipopolysaccharide-modifying (LPS) domain. The second GT (here on XpXylT) belongs to GT family 2 and has a type A fold. The XpXylT and XpApiT genes were cloned and heterologously expressed in E. coli. Analysis of nucleotide sugar extracts from E. coli expressing XpXylT or XpApiT with UAS showed that recombinant XpApiT utilized UDP-apiose and XpXylT utilized UDP-xylose as substrate. Indirect activity assay (UDP-Glo) revealed that XpApiT is an apiosyltransferase (ApiT) able to specifically use UDP-apiose. Further support for the apiosyltransferase activity was demonstrated by in microbe co-expression of UAS and XpApiT in E. coli showing the utilization of UDP-apiose to generate an apioside detectable in the pellet fraction. This work provides evidence that X. pisi developed the ability to synthesize an apioside of indeterminate function; however, the evolution of the bacterial ApiT remains to be determined. From genetic and evolutionary perspectives, the apiose operon may provide a unique opportunity to examine how genomic changes reflect ecological adaptation during the divergence of a bacterial group.

Expression and localization of the polarity protein CRB2 in adult mouse brain: a comparison with the CRB1rd8 mutant mouse model

Acquisition of cell polarization is essential for the performance of crucial functions, like a successful secretion and appropriate cell signaling in many tissues, and it depends on the correct functioning of polarity proteins, including the Crumbs complex. The CRB proteins, CRB1, CRB2 and CRB3, identified in mammals, are expressed in epithelial-derived tissues like brain, kidney and retina. CRB2 has a ubiquitous expression and has been detected in embryonic brain tissue; but currently there is no data regarding its localization in the adult brain. In our study, we characterized the presence of CRB2 in adult mice brain, where it is particularly enriched in cortex, hippocampus, hypothalamus and cerebellum. Double immunofluorescence analysis confirmed that CRB2 is a neuron-specific protein, present in both soma and projections where colocalizes with certain populations of exocytic and endocytic vesicles and with other members of the Crumbs complex. Finally, in the cortex of CRB1^rd8 mutant mice that contain a mutation in the Crb1 gene generating a truncated CRB1 protein, there is an abnormal increase in the expression levels of the CRB2 protein which suggests a possible compensatory mechanism for the malfunction of CRB1 in this mutant background.

A dual role of the extracellular domain of Drosophila Crumbs for morphogenesis of the embryonic neuroectoderm

Epithelia are highly polarised tissues and several highly conserved polarity protein complexes serve to establish and maintain polarity. The transmembrane protein Crumbs (Crb), the central component of the Crb protein complex, is required, among others, for the maintenance of polarity in most epithelia in the Drosophila embryo. However, different epithelia exhibit different phenotypic severity upon loss of crb. Using a transgenomic approach allowed us to more accurately define the role of crb in different epithelia. In particular, we provide evidence that the loss of epithelial tissue integrity in the ventral epidermis of crb mutant embryos is due to impaired actomyosin activity and an excess number of neuroblasts. We demonstrate that the intracellular domain of Crb could only partially rescue this phenotype, while it is able to completely restore tissue integrity in other epithelia. Based on these results we suggest a dual role of the extracellular domain of Crb in the ventral neuroectoderm. First, it is required for apical enrichment of the Crb protein, which in turn regulates actomyosin activity and thereby ensures tissue integrity; and second, the extracellular domain of Crb stabilises the Notch receptor and thereby ensures proper Notch signalling and specification of the correct number of neuroblasts.

Summary: Using a transgenomic approach we determine specific roles of the intra- and extracellular domain of the Crumbs protein for the maintenance of apico-basal epithelial polarity and epithelial morphogenesis in Drosophila embryos.

Regulation of Notch Function by O-Glycosylation

The Notch receptor initiates a unique intercellular signaling pathway that is evolutionarily conserved across all metazoans and contributes to the development and maintenance of numerous tissues. Consequently, many diseases result from aberrant Notch signaling. Emerging roles for Notch in disease are being uncovered as studies reveal new information regarding various components of this signaling pathway. Notch activity is regulated at several levels, but O-linked glycosylation of Epidermal Growth Factor (EGF) repeats in the Notch extracellular domain has emerged as a major regulator that, depending on context, can increase or decrease Notch activity. Three types of O-linked glycosylation occur at consensus sequences found within the EGF repeats of Notch: O-fucosylation, O-glucosylation, and O-GlcNAcylation. Recent studies have investigated the site occupancy of these types of glycosylation and also defined specific roles for these glycans on Notch structure and function. Nevertheless, there are many functional aspects to each type of O-glycosylation that remain unclear. Here, we will discuss molecular mechanisms of how O-glycosylation regulates Notch signaling and describe disorders associated with defects in Notch O-glycosylation.

STRIP1, a core component of STRIPAK complexes, is essential for normal mesoderm migration in the mouse embryo

Striatin-interacting phosphatases and kinases (STRIPAK) complexes can regulate the cytoskeleton and cell migration in cell lines, but their roles in vivo in mammals are not known. Here, we show that mouse embryos that lack striatin-interacting protein 1 (STRIP1), a core component of STRIPAK complexes, arrest at midgestation with striking morphological defects. Strip1 mutants lack a trunk, and both paraxial and axial mesoderm fail to elongate along the anterior–posterior body axis. Mesodermal cells from Strip1 mutants have defects in actin organization, focal adhesions, and cell migration that can account for the failure of normal mesoderm migration. The findings demonstrate that STRIPAK is a critical regulator of mammalian cell migration and is likely to have important roles in tumor progression as well as development.

Regulated mesoderm migration is necessary for the proper morphogenesis and organ formation during embryonic development. Cell migration and its dependence on the cytoskeleton and signaling machines have been studied extensively in cultured cells; in contrast, remarkably little is known about the mechanisms that regulate mesoderm cell migration in vivo. Here, we report the identification and characterization of a mouse mutation in striatin-interacting protein 1 (Strip1) that disrupts migration of the mesoderm after the gastrulation epithelial-to-mesenchymal transition (EMT). STRIP1 is a core component of the biochemically defined mammalian striatin-interacting phosphatases and kinase (STRIPAK) complexes that appear to act through regulation of protein phosphatase 2A (PP2A), but their functions in mammals in vivo have not been examined. Strip1-null mutants arrest development at midgestation with profound disruptions in the organization of the mesoderm and its derivatives, including a complete failure of the anterior extension of axial mesoderm. Analysis of cultured mesoderm explants and mouse embryonic fibroblasts from null mutants shows that the mesoderm migration defect is correlated with decreased cell spreading, abnormal focal adhesions, changes in the organization of the actin cytoskeleton, and decreased velocity of cell migration. The results show that STRIPAK complexes are essential for cell migration and tissue morphogenesis in vivo.

Structural basis of Notch O-glucosylation and O-xylosylation by mammalian protein-O-glucosyltransferase 1 (POGLUT1)

Protein O-glucosyltransferase 1/Rumi-mediated glucosylation of Notch epidermal growth factor-like (EGF-like) domains plays an important role in Notch signaling. Protein O-glucosyltransferase 1 shows specificity for folded EGF-like domains, it can only glycosylate serine residues in the C¹XSXPC² motif, and it possesses an uncommon dual donor substrate specificity. Using several EGF-like domains and donor substrate analogs, we have determined the structures of human Protein O-glucosyltransferase 1 substrate/product complexes that provide mechanistic insight into the basis for these properties. Notably, we show that Protein O-glucosyltransferase 1’s requirement for folded EGF-like domains also leads to its serine specificity and that two distinct local conformational states are likely responsible for its ability to transfer both glucose and xylose. We also show that Protein O-glucosyltransferase 1 possesses the potential to xylosylate a much broader range of EGF-like domain substrates than was previously thought. Finally, we show that Protein O-glucosyltransferase 1 has co-evolved with EGF-like domains of the type found in Notch.

POGLUT1 is a protein-O-glucosyltransferase that transfers glucose and xylose to the EGF-like domains of Notch and other signaling receptors. Here the authors report the structure of human POGLUT1 in complexes with 3 different EGF-like domains and donor substrates and shed light on the enzyme’s substrate specificity and catalytic mechanism

The CRB1 Complex: Following the Trail of Crumbs to a Feasible Gene Therapy Strategy

Once considered science fiction, gene therapy is rapidly becoming scientific reality, targeting a growing number of the approximately 250 genes linked to hereditary retinal disorders such as retinitis pigmentosa and Leber's congenital amaurosis. Powerful new technologies have emerged, leading to the development of humanized models for testing and screening these therapies, bringing us closer to the goal of personalized medicine. These tools include the ability to differentiate human induced pluripotent stem cells (iPSCs) to create a “retina-in-a-dish” model and the self-formed ectodermal autonomous multi-zone, which can mimic whole eye development. In addition, highly specific gene-editing tools are now available, including the CRISPR/Cas9 system and the recently developed homology-independent targeted integration approach, which allows gene editing in non-dividing cells. Variants in the CRB1 gene have long been associated with retinopathies, and more recently the CRB2 gene has also been shown to have possible clinical relevance with respect to retinopathies. In this review, we discuss the role of the CRB protein complex in patients with retinopathy. In addition, we discuss new opportunities provided by stem cells and gene-editing tools, and we provide insight into how the retinal therapeutic pipeline can be improved. Finally, we discuss the current state of adeno-associated virus-mediated gene therapy and how it can be applied to treat retinopathies associated with mutations in CRB1.

The Crumbs_C isoform of Drosophila shows tissue- and stage-specific expression and prevents light-dependent retinal degeneration

Drosophila Crumbs (Crb) is a key regulator of epithelial polarity and fulfils a plethora of other functions, such as growth regulation, morphogenesis of photoreceptor cells and prevention of retinal degeneration. This raises the question how a single gene regulates such diverse functions, which in mammals are controlled by three different paralogs. Here, we show that in Drosophila different Crb protein isoforms are differentially expressed as a result of alternative splicing. All isoforms are transmembrane proteins that differ by just one EGF-like repeat in their extracellular portion. Unlike Crb_A, which is expressed in most embryonic epithelia from early stages onward, Crb_C is expressed later and only in a subset of embryonic epithelia. Flies specifically lacking Crb_C are homozygous viable and fertile. Strikingly, these flies undergo light-dependent photoreceptor degeneration despite the fact that the other isoforms are expressed and properly localised at the stalk membrane. This allele now provides an ideal possibility to further unravel the molecular mechanisms by which Drosophila crb protects photoreceptor cells from the detrimental consequences of light-induced cell stress.

Summary: Loss of Crb_C, one protein isoform encoded by Drosophila crumbs, results in light-dependent retinal degeneration, but does not affect any of the other crumbs-specific functions.

Crumbs2 promotes cell ingression during the epithelial-to-mesenchymal transition at gastrulation

During gastrulation of the mouse embryo, individual cells ingress in an apparently stochastic pattern during the epithelial-to-mesenchymal transition (EMT). Here we define a critical role of the apical protein Crumbs2 (CRB2) in the gastrulation EMT. Static and live imaging show that ingressing cells in Crumbs2 mutant embryos become trapped at the primitive streak, where they continue to express the epiblast transcription factor SOX2 and retain thin E-cadherin-containing connections to the epiblast surface that trap them at the streak. CRB2 is distributed in a complex anisotropic pattern on apical cell edges, and the level of CRB2 on a cell edge is inversely correlated with the level of myosin IIB. The data suggest that the distributions of CRB2 and myosin IIB define which cells will ingress, and we propose that cells with high apical CRB2 are basally extruded from the epiblast by neighbouring cells with high levels of apical myosin.

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 POGLUT1 (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 PAX7⁺ 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.

Structural analysis of Notch-regulating Rumi reveals basis for pathogenic mutations

Rumi O-glucosylates the EGF repeats of a growing list of proteins essential in metazoan development, including Notch. Rumi is essential for Notch signaling, and Rumi dysregulation is linked to several human diseases. Despite Rumi's critical roles, it is unknown how Rumi glucosylates a serine of many but not all EGF repeats. Here we report crystal structures of Drosophila Rumi as binary and ternary complexes with a folded EGF repeat and/or donor substrates. These structures provide insights into the catalytic mechanism and show that Rumi recognizes structural signatures of the EGF motif, the U-shaped consensus sequence, C-X-S-X-(P/A)-C and a conserved hydrophobic region. We found that five Rumi mutations identified in cancers and Dowling-Degos disease are clustered around the enzyme active site and adversely affect its activity. Our study suggests that loss of Rumi activity may underlie these diseases, and the mechanistic insights may facilitate the development of modulators of Notch signaling.

Mapping Sites of O-Glycosylation and Fringe Elongation on Drosophila Notch

Glycosylation of the Notch receptor is essential for its activity and serves as an important modulator of signaling. Three major forms of O-glycosylation are predicted to occur at consensus sites within the epidermal growth factor-like repeats in the extracellular domain of the receptor: O-fucosylation, O-glucosylation, and O-GlcNAcylation. We have performed comprehensive mass spectral analyses of these three types of O-glycosylation on Drosophila Notch produced in S2 cells and identified peptides containing all 22 predicted O-fucose sites, all 18 predicted O-glucose sites, and all 18 putative O-GlcNAc sites. Using semiquantitative mass spectral methods, we have evaluated the occupancy and relative amounts of glycans at each site. The majority of the O-fucose sites were modified to high stoichiometries. Upon expression of the β3-N-acetylglucosaminyltransferase Fringe with Notch, we observed varying degrees of elongation beyond O-fucose monosaccharide, indicating that Fringe preferentially modifies certain sites more than others. Rumi modified O-glucose sites to high stoichiometries, although elongation of the O-glucose was site-specific. Although the current putative consensus sequence for O-GlcNAcylation predicts 18 O-GlcNAc sites on Notch, we only observed apparent O-GlcNAc modification at five sites. In addition, we performed mass spectral analysis on endogenous Notch purified from Drosophila embryos and found that the glycosylation states were similar to those found on Notch from S2 cells. These data provide foundational information for future studies investigating the mechanisms of how O-glycosylation regulates Notch activity.

Membrane dynamics in mammalian embryogenesis: Implication in signal regulation

Eukaryotes have evolved an array of membrane compartments constituting secretory and endocytic pathways that allow the flow of materials. Both pathways perform important regulatory roles. The secretory pathway is essential for the production of extracellular, secreted signal molecules, but its function is not restricted to a mere route connecting intra- and extracellular compartments. Post-translational modifications also play an integral function in the secretory pathway and are implicated in developmental regulation. The endocytic pathway serves as a platform for relaying signals from the extracellular stimuli to intracellular mediators, and then ultimately inducing signal termination. Here, we discuss recent studies showing that dysfunction in membrane dynamics causes patterning defects in embryogenesis and tissue morphogenesis in mammals.