Roles of O-fucose glycans in notch signaling revealed by mutant mice

Unravelling the Pathogenetic Mechanisms in Congenital Aortopathies: Need for an Integrative Translational Approach

Congenital heart disease (CHD)-associated aortopathy is a very heterogeneous entity with a wide spectrum of clinical presentations. The pathogenesis of aortopathy is still incompletely understood, and, therefore, the best prevention and management strategy is currently unknown. The most common entity of CHD-associated aortopathies is bicuspid aortic valve (BAV)-associated aortic disease (so called bicuspid aortopathy) that is found in 50%–60% of BAV individuals. BAV aortopathy has been reported in association with an increased risk of aortic events, especially aortic dissection and sudden cardiac death. Risk stratification of adverse aortic events is still very rudimentary and considers only the maximal aortic diameter, which makes it unsuitable for an individual risk prediction. This introductory Editorial highlights the unmet clinical need for more integrative and translational research to unravel pathogenetic pathways in the development of CHD-associated aortopathies, integrating recently identified genetic lesions and knowledge on circulating biomarkers and microstructural changes in the diseased aorta.

Glycans and glycosaminoglycans in neurobiology: key regulators of neuronal cell function and fate

The aim of the present study was to examine the roles of l-fucose and the glycosaminoglycans (GAGs) keratan sulfate (KS) and chondroitin sulfate/dermatan sulfate (CS/DS) with selected functional molecules in neural tissues. Cell surface glycans and GAGs have evolved over millions of years to become cellular mediators which regulate fundamental aspects of cellular survival. The glycocalyx, which surrounds all cells, actuates responses to growth factors, cytokines and morphogens at the cellular boundary, silencing or activating downstream signaling pathways and gene expression. In this review, we have focused on interactions mediated by l-fucose, KS and CS/DS in the central and peripheral nervous systems. Fucose makes critical contributions in the area of molecular recognition and information transfer in the blood group substances, cytotoxic immunoglobulins, cell fate-mediated Notch-1 interactions, regulation of selectin-mediated neutrophil extravasation in innate immunity and CD-34-mediated new blood vessel development, and the targeting of neuroprogenitor cells to damaged neural tissue. Fucosylated glycoproteins regulate delivery of synaptic neurotransmitters and neural function. Neural KS proteoglycans (PGs) were examined in terms of cellular regulation and their interactive properties with neuroregulatory molecules. The paradoxical properties of CS/DS isomers decorating matrix and transmembrane PGs and the positive and negative regulatory cues they provide to neurons are also discussed.

MiR-145 expression and rare NOTCH1 variants in bicuspid aortic valve-associated aortopathy

MicroRNAs (miRNAs) may serve as elegant tool to improve risk stratification in bicuspid aortic valve (BAV)-associated aortopathy. However, the exact pathogenetic pathway by which miRNAs impact aortopathy progression is unknown. Herewith, we aimed to analyze the association between circulating miRNAs and rare variants of aortopathy-related genes. 63 BAV patients (mean age 47.3±11.3 years, 92% male) with a root dilatation phenotype, who underwent aortic valve+/-proximal aortic surgery at a single institution (mean post-AVR follow-up 10.3±6.9 years) were analyzed. A custom-made HaloPlex HS panel including 20 aortopathy-related genes was used for the genetic testing. miRNAs were extracted from whole blood and miRNA analysis was performed using miRNA-specific assay. Study endpoint was the association between circulating miRNAs and rare genetic variants in the aortopathy gene panel. The study cohort was divided into a subgroup with rare variants vs. a subgroup without rare variants based on the presence of rare variants in the respective genes (i.e., at least one variant present). The genetic analysis yielded n = 6 potentially and likely pathogenic rare variants within the NOTCH1 gene as being the most common finding. Univariate analysis between blood miRNAs and NOTCH1 variants revealed a significantly lower expression of miR-145 in the subgroup of patients with NOTCH1 variants vs. those without NOTCH1 variants (i.e., delta Ct 4.95±0.74 vs. delta Ct 5.57±0.78, p = 0.04). Our preliminary data demonstrate a significant association between blood miR-145 expression and the presence of rare NOTCH1 variants. This association may be indicative of a specific pathogenetic pathway in the development of genetically-triggered bicuspid aortopathy.

Fucosylation Deficiency in Mice Leads to Colitis and Adenocarcinoma

BACKGROUND & AIMS

De novo synthesis of guanosine diphosphate (GDP)-fucose, a substrate for fucosylglycans, requires sequential reactions mediated by GDP-mannose 4,6-dehydratase (GMDS) and GDP-4-keto-6-deoxymannose 3,5-epimerase-4-reductase (FX or tissue specific transplantation antigen P35B [TSTA3]). GMDS deletions and mutations are found in 6%-13% of colorectal cancers; these mostly affect the ascending and transverse colon. We investigated whether a lack of fucosylation consequent to loss of GDP-fucose synthesis contributes to colon carcinogenesis.

METHODS

FX deficiency and GMDS deletion produce the same biochemical phenotype of GDP-fucose deficiency. We studied a mouse model of fucosylation deficiency (Fx-/- mice) and mice with the full-length Fx gene (controls). Mice were placed on standard chow or fucose-containing diet (equivalent to a control fucosylglycan phenotype). Colon tissues were collected and analyzed histologically or by enzyme-linked immunosorbent assays to measure cytokine levels; T cells also were collected and analyzed. Fecal samples were analyzed by 16s ribosomal RNA sequencing. Mucosal barrier function was measured by uptake of fluorescent dextran. We transplanted bone marrow cells from Fx-/- or control mice (Ly5.2) into irradiated 8-week-old Fx-/- or control mice (Ly5.1). We performed immunohistochemical analyses for expression of Notch and the hes family bHLH transcription factor (HES1) in colon tissues from mice and a panel of 60 human colorectal cancer specimens (27 left-sided, 33 right-sided).

RESULTS

Fx-/- mice developed colitis and serrated-like lesions. The intestinal pathology of Fx-/- mice was reversed by addition of fucose to the diet, which restored fucosylation via a salvage pathway. In the absence of fucosylation, dysplasia appeared and progressed to adenocarcinoma in up to 40% of mice, affecting mainly the right colon and cecum. Notch was not activated in Fx-/- mice fed standard chow, leading to decreased expression of its target Hes1. Fucosylation deficiency altered the composition of the fecal microbiota, reduced mucosal barrier function, and altered epithelial proliferation marked by Ki67. Fx-/- mice receiving control bone marrow cells had intestinal inflammation and dysplasia, and reduced expression of cytokines produced by cytotoxic T cells. Human sessile serrated adenomas and right-sided colorectal tumors with epigenetic loss of MutL homolog 1 (MLH1) had lost or had lower levels of HES1 than other colorectal tumor types or nontumor tissues.

CONCLUSIONS

In mice, fucosylation deficiency leads to colitis and adenocarcinoma, loss of Notch activation, and down-regulation of Hes1. HES1 loss correlates with the development of human right-sided colorectal tumors with epigenetic loss of MLH1. These findings indicate that carcinogenesis in a subset of colon cancer is consequent to a molecular mechanism driven by fucosylation deficiency and/or HES1-loss.

Haploinsufficiency of the NOTCH1 Receptor as a Cause of Adams-Oliver Syndrome With Variable Cardiac Anomalies

BACKGROUND

Adams-Oliver syndrome (AOS) is a rare disorder characterized by congenital limb defects and scalp cutis aplasia. In a proportion of cases, notable cardiac involvement is also apparent. Despite recent advances in the understanding of the genetic basis of AOS, for the majority of affected subjects, the underlying molecular defect remains unresolved. This study aimed to identify novel genetic determinants of AOS.

METHODS AND RESULTS

Whole-exome sequencing was performed for 12 probands, each with a clinical diagnosis of AOS. Analyses led to the identification of novel heterozygous truncating NOTCH1 mutations (c.1649dupA and c.6049_6050delTC) in 2 kindreds in which AOS was segregating as an autosomal dominant trait. Screening a cohort of 52 unrelated AOS subjects, we detected 8 additional unique NOTCH1 mutations, including 3 de novo amino acid substitutions, all within the ligand-binding domain. Congenital heart anomalies were noted in 47% (8/17) of NOTCH1-positive probands and affected family members. In leukocyte-derived RNA from subjects harboring NOTCH1 extracellular domain mutations, we observed significant reduction of NOTCH1 expression, suggesting instability and degradation of mutant mRNA transcripts by the cellular machinery. Transient transfection of mutagenized NOTCH1 missense constructs also revealed significant reduction in gene expression. Mutant NOTCH1 expression was associated with downregulation of the Notch target genes HEY1 and HES1, indicating that NOTCH1-related AOS arises through dysregulation of the Notch signaling pathway.

CONCLUSIONS

These findings highlight a key role for NOTCH1 across a range of developmental anomalies that include cardiac defects and implicate NOTCH1 haploinsufficiency as a likely molecular mechanism for this group of disorders.

Glycans in regeneration

Glycans participate in many key cellular processes during development and in physiology and disease. In this review, the functional role of various glycans in the regeneration of neurons and body parts in adult metazoans is discussed. Understanding glycosylation may facilitate research in the field of stem cell biology and regenerative medicine.

Aberrant fucosylation of glycosphingolipids in human hepatocellular carcinoma tissues

BACKGROUNDS & AIMS

Glycosylation promoting or inhibiting tumour cell invasion and metastasis is of crucial importance in current cancer research. Tumour-associated carbohydrate antigens are predominantly expressed on the tumour cell surface. Glycosphingolipids (GSLs) are members of the family. To perform glycosphingolipidomic assays on neutral GSLs obtained from solid hepatocellular carcinoma (HCC) tissues and paired peritumoural tissues by linear ion trap quadrupole-electrospray ionization mass spectrometry.

METHODS

Qualitative and quantitative analysis of fucosylated neutral GSLs was performed in the positive ion mode on the LTQ-XL mass spectrometer and MALDI-TOF-MS.

RESULTS

A group of fucosylated neutral GSLs in HCC was found to be expressed higher in the tumour tissues, as their proportion in total cellular GSLs was 3.3-fold higher in the tumour tissues than in the peritumoural tissues (P < 0.01). Moreover, qualitative analysis of the aberrant fucosylated GSLs were completed, and seven types of fucosylated GSLs that contained terminal Fuca2Gal- structure were identified by mass spectrometry.

CONCLUSIONS

Our results may lead to improved immunotherapy of HCC and contribute to understanding the role of aberrant fucosylated GSLs in the development and progress of HCC in following studies.

Imaging the glycome in living systems

The glycome, the full complement of glycans that cells produce, is an attractive target for molecular imaging. Imaging of the glycome in living systems has recently been enabled via bioorthogonal chemical reporter-based approaches. In this chapter, we describe two approaches to introduce bioorthogonal chemical reporters (tags) onto cell surface fucosylated glycans and glycans bearing LacNAc disaccharides, respectively. The tagged glycans can then be conjugated to imaging probes via bioorthogonal click chemistry. Similar approaches can be extended to image other sectors of the glycome in living systems.

Protein O-fucosyltransferase 1 (Pofut1) regulates lymphoid and myeloid homeostasis through modulation of Notch receptor ligand interactions

Notch signaling is essential for lymphocyte development and is also implicated in myelopoiesis. Notch receptors are modified by O-fucosylation catalyzed by protein O-fucosyltransferase 1 (Pofut1). Fringe enzymes add N-acetylglucosamine to O-fucose and modify Notch signaling by altering the sensitivity of Notch receptors to Notch ligands. To address physiologic functions in hematopoiesis of Notch modified by O-fucose glycans, we examined mice with inducible inactivation of Pofut1 using Mx-Cre. These mice exhibited a reduction in T lymphopoiesis and in the production of marginal-zone B cells, in addition to myeloid hyperplasia. Restoration of Notch1 signaling rescued T lymphopoiesis and the marrow myeloid hyperplasia. After marrow transfer, both cell-autonomous and environmental cues were found to contribute to lymphoid developmental defects and myeloid hyperplasia in Pofut1-deleted mice. Although Pofut1 deficiency slightly decreased cell surface expression of Notch1 and Notch2, it completely abrogated the binding of Notch receptors with Delta-like Notch ligands and suppressed downstream Notch target activation, indicating that O-fucose glycans are critical for efficient Notch-ligand binding that transduce Notch signals. The combined data support a key role for the O-fucose glycans generated by Pofut1 in Notch regulation of hematopoietic homeostasis through modulation of Notch-ligand interactions.

Secretory cell hyperplasia and defects in Notch activity in a mouse model of leukocyte adhesion deficiency type II

BACKGROUND & AIMS

Leukocyte adhesion deficiency II (LAD II) is a rare condition caused by defective protein fucosylation, causing decreased leukocyte rolling, psychomotor retardation, and poor growth. The ligand-binding activity of Notch, a gastrointestinal signaling protein, depends on O-fucosylation. We investigated Notch signaling and intestinal epithelial architecture in a mouse model of LAD II.

METHODS

Mice lacking 3,5-epimerase/4-reductase (FX) or FX(-/-) bone marrow chimeras (with either wild-type or FX(-/-) bone marrow) were maintained on a fucose-free diet. Intestinal secretory epithelial cells were quantified by histology and immunohistochemistry. Reverse transcription-polymerase chain reaction and immunoblot analyses were used to detect Notch-regulated genes in isolated crypt epithelium. Intestinal leukocyte-endothelial interaction was quantified by intravital microscopy. The intestinal epithelium of 2-week-old FX(-/-) mice was transfected with an adenoviral vector expressing a constitutively active form of Notch.

RESULTS

FX(-/-) mice rapidly exhibited secretory epithelial cell hyperplasia, reduced cell proliferation, and altered epithelial gene expression patterns consistent with reduced Notch signaling. These effects were reversed when mice were given dietary fucose or by adenoviral transfection of the intestinal epithelium with the Notch intracellular domain.

CONCLUSIONS

In a mouse model of LAD II, secretory cell hyperplasia occurs in the small intestine and colon; these effects depend on Notch signaling. Defects in Notch signaling might therefore be involved in the pathogenesis of this rare pediatric condition.

Roles of glycosylation in Notch signaling

Notch and the DSL Notch ligands Delta and Serrate/Jagged are glycoproteins with a single transmembrane domain. The extracellular domain (ECD) of both Notch receptors and Notch ligands contains numerous epidermal growth factor (EGF)-like repeats which are post-translationally modified by a variety of glycans. Inactivation of a subset of genes that encode glycosyltransferases which initiate and elongate these glycans inhibits Notch signaling. In the formation of developmental boundaries in Drosophila and mammals, in mouse T-cell and marginal zone B-cell development, and in co-culture Notch signaling assays, the regulation of Notch signaling by glycans is to date a cell-autonomous effect of the Notch-expressing cell. The regulation of Notch signaling by glycans represents a new paradigm of signal transduction. O-fucose glycans modulate the strength of Notch binding to DSL Notch ligands, while O-glucose glycans facilitate juxta-membrane cleavage of Notch, generating the substrate for intramembrane cleavage and Notch activation. Identifying precisely how the addition of particular sugars at specific locations on Notch modifies Notch signaling is a challenge for the future.

Glycan antagonists and inhibitors: a fount for drug discovery

Glycans, the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids, represent a relatively unexploited area for drug development compared with other macromolecules. This review describes the major classes of glycans synthesized by animal cells, their mode of assembly, and available inhibitors for blocking their biosynthesis and function. Many of these agents have proven useful for studying the biological activities of glycans in isolated cells, during embryological development, and in physiology. Some are being used to develop drugs for treating metabolic disorders, cancer, and infection, suggesting that glycans are excellent targets for future drug development.

Mouse fertility is enhanced by oocyte-specific loss of core 1-derived O-glycans

Regulation of the number of eggs ovulated by different mammalian species remains poorly understood. Here we show that oocyte-specific deletion at the primary follicle stage of core 1 beta1,3-galactosyltransferase (T-synthase; generates core 1-derived O-glycans), leads to a sustained increase in fertility. T-syn mutant females ovulated 30-50% more eggs and had a sustained increase in litter size compared to controls. Ovarian weights and follicle numbers were greater in mutants, but follicular apoptosis was not decreased. The number of follicles entering the growing pool was unaltered, but 3-wk mutants ovulated fewer eggs, suggesting that increased fertility results from prolonged follicle development. T-syn mutant ovaries also contained numerous multiple-oocyte follicles (MOFs) that appeared to form by adjacent, predominantly preantral, follicles joining--a new mechanism for MOF generation. Ovulation of multiple eggs from MOFs was not the reason for increased fertility based on ovulated egg and corpora lutea numbers. Thus, the absence of T-synthase caused modified follicular development, leading to the maturation and ovulation of more follicles, to MOF formation at late stages of folliculogenesis, and to increased fertility. These results identify novel roles for glycoproteins from the oocyte as suppressors of fertility and regulators of follicular integrity in the mouse.

Regulation of Notch signaling by glycosylation

Notch receptors are approximately 300 kDa cell surface glycoproteins whose activation by Notch ligands regulates cell fate decisions in the metazoa. The extracellular domain of Notch receptors has many epidermal growth factor like repeats that are glycosylated with O-fucose and O-glucose glycans as well as N-glycans. Disruption of O-fucose glycan synthesis leads to severe Notch signaling defects in Drosophila and mammals. Removal or addition of O-fucose glycan consensus sites on Notch receptors also leads to Notch signaling defects. Ligand binding and ligand-induced Notch signaling assays have provided insights into how changes in the O-fucose glycans of Notch receptors alter Notch signaling.