O-linked glycans mediate apical sorting of human intestinal sucrase-isomaltase through association with lipid rafts

Impaired digestive function of sucrase-isomaltase in a complex with the Greenlandic sucrase-isomaltase variant

Sucrase isomaltase (SI) is the most prominent disaccharidase in the small intestine. Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive disorder caused by variants in the SI gene. A homozygous frameshift mutation, c.273_274delAG (p.Gly92Leufs*8), has been identified in CSID in the Greenlandic population. This variant eliminates the luminal domain of SI and results in loss of its digestive function. Surprisingly, the truncated mutant is transport-competent and localized at the cell surface; it interacts avidly with wild type SI and negatively impacts its enzymatic function. The data propose that heterozygote carriers of p.Gly92Leufs*8 may also present with CSID symptoms.

Hexosaminidase A (HEXA) regulates hepatic sphingolipid and lipoprotein metabolism in mice

Hexosaminidase A (HexA), a heterodimer consisting of HEXA and HEXB, converts the ganglioside sphingolipid GM2 to GM3 by removing a terminal N-acetyl-d-galactosamine. HexA enzyme deficiency in humans leads to GM2 accumulation in cells, particularly in neurons, and is associated with neurodegeneration. While HexA and sphingolipid metabolism have been extensively investigated in the context of neuronal lipid metabolism, little is known about the metabolic impact of HexA and ganglioside degradation in other tissues. Here, we focussed on the role of HexA in the liver, which is a major regulator of systemic lipid metabolism. We find that hepatic Hexa expression is induced by lipid availability and increased in the presence of hepatic steatosis, which is associated with increased hepatic GM3 content. To assess the impact of HEXA on hepatic lipid metabolism, we used an adeno-associated virus to overexpress HEXA in the livers of high-fat diet fed mice. HEXA overexpression was associated with increased hepatic GM3 content and increased expression of enzymes involved in the degradation of glycated sphingolipids, ultimately driving sphingomyelin accumulation in the liver. In addition, HEXA overexpression led to substantial proteome remodeling in cell surface lipid rafts, which was associated with increased VLDL processing and secretion, hypertriglyceridemia and ectopic lipid accumulation in peripheral tissues. This study established an important role of HEXA in modulating hepatic sphingolipid and lipoprotein metabolism.

Proliferation and Differentiation of Intestinal Caco-2 Cells Are Maintained in Culture with Human Platelet Lysate Instead of Fetal Calf Serum

Cell lines are widely used as in vitro model systems and substitute for animal experiments. The frequently used Caco-2 cell line is considered to reflect characteristics of differentiated intestinal epithelium. However, the need to culture the cells with fetal calf serum (FCS) induces a high variability, risk of contamination and is ethically disputed. We tested the culture of Caco-2 cells with human platelet lysate (PL) instead of FCS. We compared cell viability and differentiation by measuring ATP levels, gene and protein expression of specific markers in total cell extracts, brush border membrane vesicles (BBM) and lipid rafts (LR). Cell viability was slightly enhanced in cells grown with PL compared to FCS. The cells differentiated to an intestinal phenotype like the cells cultured in FCS, as indicated by the similar gene expression levels of hexose and protein transport proteins and the structural protein VILLIN. BBM showed a comparable distribution of the intestinal hydrolases, indicating a maintained cell membrane polarity. The distribution of the marker protein FLOTILLIN-2 in LR was also similar. We conclude that PL is an exquisite and suitable replacement for FCS in the culture of Caco-2 cells that can eliminate many disadvantages incurred due to the use of FCS.

Staphylococcus aureus Infection Influences the Function of Intestinal Cells by Altering the Lipid Raft-Dependent Sorting of Sucrase-Isomaltase

Staphylococcus aureus is an important nosocomial and community-acquired facultative intracellular pathogen. Many studies have reported that S. aureus infections are associated with intestinal symptoms, but little is known about the molecular mechanisms implicated in S. aureus-induced alterations of intestinal functions. In this study, we investigated the implication of lipid rafts in the interaction of S. aureus with Caco-2 cells. To assess potential alterations in the lipid raft structure and effects on the hydrolytic function, we utilized sucrase–isomaltase (SI) as the major intestinal α-glucosidase that is associated with and sorted to the apical membrane via lipid rafts. Seven days post-confluent, Caco-2 cells were infected with S. aureus Newman and further incubated for an additional 2 days. After 48 h, the levels of SI expression as well as the enzymatic function of this protein were assessed in the infected versus non-infected cells. Analysis of the sorting behavior of SI to the apical membrane constituted another crucial aspect in studying the effects of S. aureus on Caco-2 cells. For this purpose, the apical membranes or brush border membranes (BBMs; referred to as P2 fraction) were separated in both infected and non-infected cells from the basolateral and intracellular membranes (referred to as P1 fraction) by employing a cationic-based procedure using CaCl₂. The data show that there is no significant change in the overall expression levels of SI in the infected versus non-infected cells as assessed by Western blotting analysis using monoclonal anti-SI antibodies. By contrast, a significant decrease in the localization as well as the specific hydrolytic activities of SI toward sucrose and isomaltose (Palatinose) was observed in the BBM (P2 fraction) in Caco-2 cells 48 h post-infection. Concomitantly, the specific SI activities increased in the basolateral membrane/intracellular fraction (P1). Noteworthy, the specific activity of SI in the BBM of infected cells was markedly reduced as compared with that of the non-infected counterparts. The data accumulated from this study strongly suggest that infections with S. aureus influence the final step in the lipid raft-associated trafficking of human SI and thereby may trigger secondary functional gastrointestinal disorders.

The glucose-regulated protein GRP94 interacts avidly in the endoplasmic reticulum with sucrase-isomaltase isoforms that are associated with congenital sucrase-isomaltase deficiency

The glucose-regulated protein GRP94 is a molecular chaperone that is located in the endoplasmic reticulum (ER). Here, we demonstrate in pull down experiments an interaction between GRP94 and sucrase-isomaltase (SI), the most prominent disaccharidase of the small intestine. GRP94 binds to SI exclusively via its mannose-rich form compatible with an interaction occurring in the ER. We have also examined the interaction GRP94 to a panel of SI mutants that are associated with congenital sucrase-isomaltase deficiency (CSID). These mutants exhibited more efficient binding to GRP94 than wild type SI underlining a specific role of this chaperone in the quality control in the ER. In view of the hypoxic milieu of the intestine, we probed the interaction of GRP94 to SI and its mutants in cell culture under hypoxic conditions and observed a substantial increase in the binding of GRP94 to the SI mutants. The interaction of GRP94 to the major carbohydrate digesting enzyme and regulating its folding as well as retaining SI mutants in the ER points to a potential role of GRP94 in maintenance of intestinal homeostasis by chaperoning and stabilizing SI.

Sucrase-Isomaltase Deficiency Causing Persistent Bloating and Diarrhea in an Adult Female

Congenital sucrase isomaltase deficiency (CSID) is an autosomal recessive disorder which leads to chronic intestinal malabsorption of nutrients from ingested starch and sucrose. Symptoms usually present after consumption of fruits, juices, grains, and starches, leading to failure to thrive and malnutrition. Diagnosis is suspected on detailed patient history and confirmed by a disaccharidase assay using small intestinal biopsies or sucrose hydrogen breath test. Treatment of CSID consists of limiting sucrose in diet and replacement therapy with sacrosidase. Due to its nonspecific symptoms, CSID may be undiagnosed in many patients for several years. We present a case of a 50-year-old woman with persistent symptoms of bloating in spite of extensive evaluation and treatment.

Efficient inhibition of O-glycan biosynthesis using the hexosamine analog Ac5GalNTGc

There is a critical need to develop small-molecule inhibitors of mucin-type O-linked glycosylation. The best-known reagent currently is benzyl-GalNAc, but it is effective only at millimolar concentrations. This article demonstrates that Ac₅GalNTGc, a peracetylated C-2 sulfhydryl-substituted GalNAc, fulfills this unmet need. When added to cultured leukocytes, breast cells, and prostate cells, Ac₅GalNTGc increased cell-surface VVA binding by ∼10-fold, indicating truncation of O-glycan biosynthesis. Cytometry, mass spectrometry, and western blot analysis of HL-60 promyelocytes demonstrated that 50-80 μM Ac₅GalNTGc prevented elaboration of 30%-60% of the O-glycans beyond the Tn-antigen (GalNAcα1-Ser/Thr) stage. The effect of the compound on N-glycans and glycosphingolipids was small. Glycan inhibition induced by Ac₅GalNTGc resulted in 50%-80% reduction in leukocyte sialyl-Lewis X expression and L-/P-selectin-mediated rolling under flow conditions. Ac₅GalNTGc was pharmacologically active in mouse. It reduced neutrophil infiltration to sites of inflammation by ∼60%. Overall, Ac₅GalNTGc may find diverse applications as a potent inhibitor of O-glycosylation.

Rosa canina L. Can Restore Endoplasmic Reticulum Alterations, Protein Trafficking and Membrane Integrity in a Dextran Sulfate Sodium-Induced Inflammatory Bowel Disease Phenotype

Rosa canina L. is a natural polyphenol-rich medicinal plant that exhibits antioxidant and anti-inflammatory activities. Recent in vivo studies have demonstrated that a methanol extract of Rosa canina L. (RCME) has reversed an inflammatory bowel disease (IBD)-like phenotype that has been triggered by dextran sulfate sodium (DSS) in mice. In the current study, we investigated the effects of RCME on perturbations of cellular mechanisms induced by DSS-treatment of intestinal Caco-2 cells, including stress response in the endoplasmic reticulum (ER), protein trafficking and sorting as well as lipid rafts integrity and functional capacities of an intestinal enzyme. 6 days post-confluent cells were treated for 24 h with DSS (3%) or simultaneously with DSS (3%) and RCME (100 µg/mL) or exclusively with RCME (100 µg/mL) or not treated. The results obtained demonstrate the ability of RCME to counteract the substantial increase in the expression levels of several ER stress markers in DSS-treated cells. Concomitantly, the delayed trafficking of intestinal membrane glycoproteins sucrase-isomaltase (SI) and dipeptidyl peptidase 4 (DPP4) induced by DSS between the ER and the Golgi has been compromised by RCME. Furthermore, RCME restored the partially impaired polarized sorting of SI and DPP4 to the brush border membrane. An efficient sorting mechanism of SI and DPP4 is tightly associated with intact lipid rafts structures in the trans-Golgi network (TGN), which have been distorted by DSS and normalized by RCME. Finally, the enzymatic activities of SI are enhanced in the presence of RCME. Altogether, DSS treatment has triggered ER stress, impaired trafficking and function of membrane glycoproteins and distorted lipid rafts, all of which can be compromised by RCME. These findings indicate that the antioxidants in RCME act at two major sites in Caco-2 cells, the ER and the TGN and are thus capable of maintaining the membrane integrity by correcting the sorting of membrane-associated proteins.

Distinct roles for luminal acidification in apical protein sorting and trafficking in zebrafish

Epithelial cell physiology critically depends on the asymmetric distribution of channels and transporters. However, the mechanisms targeting membrane proteins to the apical surface are still poorly understood. Here, we performed a visual forward genetic screen in the zebrafish intestine and identified mutants with defective apical targeting of membrane proteins. One of these mutants, affecting the vacuolar H⁺-ATPase gene atp6ap1b, revealed specific requirements for luminal acidification in apical, but not basolateral, membrane protein sorting and transport. Using a low temperature block assay combined with genetic and pharmacologic perturbation of luminal pH, we monitored transport of newly synthesized membrane proteins from the TGN to apical membrane in live zebrafish. We show that vacuolar H⁺-ATPase activity regulates sorting of O-glycosylated proteins at the TGN, as well as Rab8-dependent post-Golgi trafficking of different classes of apical membrane proteins. Thus, luminal acidification plays distinct and specific roles in apical membrane biogenesis.

Galectins in Intra- and Extracellular Vesicles

Carbohydrate-binding galectins are expressed in various tissues of multicellular organisms. They are involved in autophagy, cell migration, immune response, inflammation, intracellular transport, and signaling. In recent years, novel roles of galectin-interaction with membrane components have been characterized, which lead to the formation of vesicles with diverse functions. These vesicles are part of intracellular transport pathways, belong to the cellular degradation machinery, or can be released for cell-to-cell communication. Several characteristics of galectins in the lumen or at the membrane of newly formed vesicular structures are discussed in this review and illustrate the need to fully elucidate their contributions at the molecular and structural level.

Secretory proteostasis of the retinal pigmented epithelium: Impairment links to age-related macular degeneration

Secretory proteostasis integrates protein synthesis, processing, folding and trafficking pathways that are essential for efficient cellular secretion. For the retinal pigment epithelium (RPE), secretory proteostasis is of vital importance for the maintenance of the structural and functional integrity of apical (photoreceptors) and basal (Bruch's membrane/choroidal blood supply) sides of the environment it resides in. This integrity is achieved through functions governed by RPE secreted proteins, which include extracellular matrix modelling/remodelling, angiogenesis and immune response modulation. Impaired RPE secretory proteostasis affects not only the extracellular environment, but leads to intracellular protein aggregation and ER-stress with subsequent cell death. Ample recent evidence implicates dysregulated proteostasis as a key factor in the development of age-related macular degeneration (AMD), the leading cause of blindness in the developed world, and research aiming to characterise the roles of various proteins implicated in AMD-associated dysregulated proteostasis unveiled unexpected facets of the mechanisms involved in degenerative pathogenesis. This review analyses cellular processes unveiled by the study of the top 200 transcripts most abundantly expressed by the RPE/choroid in the light of the specialised secretory nature of the RPE. Functional roles of these proteins and the mechanisms of their impaired secretion, due to age and genetic-related causes, are analysed in relation to AMD development. Understanding the importance of RPE secretory proteostasis in relation to maintaining retinal health and how it becomes impaired in disease is of paramount importance for the development and assessment of future therapeutic advancements involving gene and cell therapies.

Heterozygotes Are a Potential New Entity among Homozygotes and Compound Heterozygotes in Congenital Sucrase-Isomaltase Deficiency

Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive disorder of carbohydrate maldigestion and malabsorption caused by mutations in the sucrase-isomaltase (SI) gene. SI, together with maltase-glucoamylase (MGAM), belongs to the enzyme family of disaccharidases required for breakdown of α-glycosidic linkages in the small intestine. The effects of homozygote and compound heterozygote inheritance trait of SI mutations in CSID patients have been well described in former studies. Here we propose the inclusion of heterozygote mutation carriers as a new entity in CSID, possibly presenting with milder symptoms. The hypothesis is supported by recent observations of heterozygote mutation carriers among patients suffering from CSID or patients diagnosed with functional gastrointestinal disorders. Recent studies implicate significant phenotypic heterogeneity depending on the character of the mutation and call for more research regarding the correlation of genetics, function at the cellular and molecular level and clinical presentation. The increased importance of SI gene variants in irritable bowel syndrome (IBS) or other functional gastrointestinal disorders FGIDs and their available symptom relief diets like fermentable oligo-, di-, mono-saccharides and polyols FODMAPs suggest that the heterozygote mutants may affect the disease development and treatment.

Role of the cytosolic domain of occludin in trafficking and hepatitis C virus infection

The role of the tight-junction (TJ) protein occludin (OCLN) in hepatitis C virus (HCV) entry remains elusive. Here, we investigated the OCLN C-terminal cytosolic domain in HCV infection. We expressed a series of C-terminal deletion mutants in Huh-7 cells KO for OCLN and characterized their functionality in HCV infection and trafficking. Deleting the OCLN cytosolic domain led to protein instability and intracellular retention. The first 15 residues (OCLN-C15 mutant) of the cytosolic domain were sufficient for OCLN stability, but led to its accumulation in the trans-Golgi network (TGN) due to a deficient cell surface export after synthesis. In contrast, the OCLN-C18 mutant, containing the first 18 residues of the cytosolic domain, was expressed at the cell surface and could mediate HCV infection. Point mutations in the context of C18 showed that I279 and W281 are crucial residues for cell surface expression of OCLN-C18. However, in the context of full-length OCLN, mutation of these residues only partially affected infection and cell surface localization. Importantly, the characterization of OCLN-C18 in human-polarized hepatocytes revealed a defect in its TJ localization without affecting HCV infection. These data suggest that TJ localization of OCLN is not a prerequisite for HCV infection in polarized hepatocytes.

Dietary vitamin E affects small intestinal histomorphology, digestive enzyme activity, and the expression of nutrient transporters by inhibiting proliferation of intestinal epithelial cells within jejunum in weaned piglets1

Vitamin E (VE) is an indispensable vitamin in piglet feed formula. Among other things, it affects tissues including small intestine tissues and in particular its major unit intestinal epithelial cells. Previously, limited in vivo experiments have focused on the effect of VE on the intestine, particularly digestion and absorption. VE has been shown to inhibit proliferation of some types of cells. This experiment was conducted to test the hypothesis that VE affects intestinal functions by influencing the intestinal epithelial cell proliferation. Thirty 21-d old weaned [(Yorkshire × Landrace) × Duroc] piglets with BWs of 6.36 ± 0.55 kg were randomly divided into five VE-containing feeding formula groups. The treatments were (i) 0 IU (control), (ii) 16 IU, (iii) 32 IU, (iv) 4. 80 IU, and (v) 5. 160 IU. The treatments lasted 14 d. At the end of the experiment, all subjects were sacrificed to obtain blood and tissue samples. The results suggest that VE did not affect the growth performance. VE did tend to decrease jejunal crypt depth (linear, P = 0.056) and villus width (linear, P < 0.05). Sucrase activity significantly decreased in the adding 80 IU VE compared with the control (P < 0.05). Jejunal crypt, cell proliferation in 80 IU group significantly decreased compared with the control group (P < 0.05). This study suggests that dietary VE may affect intestinal morphology and functions by inhibiting weaned piglet jejunal epithelial cell proliferation.

Thioglycosides Are Efficient Metabolic Decoys of Glycosylation that Reduce Selectin Dependent Leukocyte Adhesion

Metabolic decoys are synthetic analogs of naturally occurring biosynthetic acceptors. These compounds divert cellular biosynthetic pathways by acting as artificial substrates that usurp the activity of natural enzymes. While O-linked glycosides are common, they are only partially effective even at millimolar concentrations. In contrast, we report that N-acetylglucosamine (GlcNAc) incorporated into various thioglycosides robustly truncate cell surface N- and O-linked glycan biosynthesis at 10-100 μM concentrations. The >10-fold greater inhibition is in part due to the resistance of thioglycosides to hydrolysis by intracellular hexosaminidases. The thioglycosides reduce β-galactose incorporation into lactosamine chains, cell surface sialyl Lewis-X expression, and leukocyte rolling on selectin substrates including inflamed endothelial cells under fluid shear. Treatment of granulocytes with thioglycosides prior to infusion into mouse inhibited neutrophil homing to sites of acute inflammation and bone marrow by ∼80%-90%. Overall, thioglycosides represent an easy to synthesize class of efficient metabolic inhibitors or decoys. They reduce N-/O-linked glycan biosynthesis and inflammatory leukocyte accumulation.

Galectin-3 modulates the polarized surface delivery of β1-integrin in epithelial cells

Epithelial cells require a precise intracellular transport and sorting machinery to establish and maintain their polarized architecture. This machinery includes β-galactoside-binding galectins for targeting of glycoprotein to the apical membrane. Galectin-3 sorts cargo destined for the apical plasma membrane into vesicular carriers. After delivery of cargo to the apical milieu, galectin-3 recycles back into sorting organelles. We analysed the role of galectin-3 in the polarized distribution of β1-integrin in MDCK cells. Integrins are located primarily at the basolateral domain of epithelial cells. We demonstrate that a minor pool of β1-integrin interacts with galectin-3 at the apical plasma membrane. Knockdown of galectin-3 decreases apical delivery of β1-integrin. This loss is restored by supplementation with recombinant galectin-3 and galectin-3 overexpression. Our data suggest that galectin-3 targets newly synthesized β1-integrin to the apical membrane and promotes apical delivery of β1-integrin internalized from the basolateral membrane. In parallel, knockout of galectin-3 results in a reduction in cell proliferation and an impairment in proper cyst development. Our results suggest that galectin-3 modulates the surface distribution of β1-integrin and affects the morphogenesis of polarized cells.

Posttranslational Processing and Function of Mucosal Maltases

The final step of carbohydrate digestion in the intestine is performed by 2 major α-glucosidases of the intestinal mucosa, sucrase-isomaltase (SI) and maltase-glucoamylase. Both of these enzymes are type II membrane glycoproteins, which share a significant level of homology in gene and protein structures and yet have differences in the posttranslational processing, substrate specificity and functional capacity. Insufficient activity of these disaccharidases particularly SI as a result of genetic mutations or secondary intestinal pathologies is associated with carbohydrate maldigestion and gastrointestinal intolerances. This review will discuss the maturation profiles of SI and maltase-glucoamylase relative to their functional capacities and deficiencies.

Trafficking Ion Transporters to the Apical Membrane of Polarized Intestinal Enterocytes

Epithelial cells lining the gastrointestinal tract require distinct apical and basolateral domains to function properly. Trafficking and insertion of enzymes and transporters into the apical brush border of intestinal epithelial cells is essential for effective digestion and absorption of nutrients. Specific critical ion transporters are delivered to the apical brush border to facilitate fluid and electrolyte uptake. Maintenance of these apical transporters requires both targeted delivery and regulated membrane recycling. Examination of altered apical trafficking in patients with Microvillus Inclusion disease caused by inactivating mutations in MYO5B has led to insights into the regulation of apical trafficking by elements of the apical recycling system. Modeling of MYO5B loss in cell culture and animal models has led to recognition of Rab11a and Rab8a as critical regulators of apical brush border function. All of these studies show the importance of apical membrane trafficking dynamics in maintenance of polarized epithelial cell function.

Distribution of Glycan Motifs at the Surface of Midgut Cells in the Cotton Leafworm (Spodoptera littoralis) Demonstrated by Lectin Binding

Glycans are involved in many biological phenomena, including signal transduction, cell adhesion, immune response or differentiation. Although a few papers have reported on the role of glycans in the development and proper functioning of the insect midgut, no data are available regarding the localization of the glycan structures on the surface of the cells in the gut of insects. In this paper, we analyzed the spatial distribution of glycans present on the surface of the midgut cells in larvae of the cotton leafworm Spodoptera littoralis, an important agricultural pest insect worldwide. For this purpose, we established primary midgut cell cultures, probed these individual cells that are freely suspended in liquid medium with a selection of seven fluorescently labeled lectins covering a range of different carbohydrate binding specificities [mannose oligomers (GNA and HHA), GalNAc/Gal (RSA and SSA), GlcNAc (WGA and Nictaba) and Neu5Ac(α-2,6)Gal/GalNAc (SNA-I)], and visualized the interaction of these lectins with the different zones of the midgut cells using confocal microscopy. Our analysis focused on the typical differentiated columnar cells with a microvillar brush border at their apical side, which are dominantly present in the Lepidopteran midgut and function in food digestion and absorption, and as well as on the undifferentiated stem cells that are important for midgut development and repair. Confocal microscopy analyses showed that the GalNAc/Gal-binding lectins SSA and RSA and the terminal GlcNAc-recognizing WGA bound preferentially to the apical microvillar zone of the differentiated columnar cells as compared to the basolateral pole. The reverse result was observed for the mannose-binding lectins GNA and HHA, as well as Nictaba that binds preferentially to GlcNAc oligomers. Furthermore, differences in lectin binding to the basal and lateral zones of the cell membranes of the columnar cells were apparent. In the midgut stem cells, GNA and Nictaba bound more strongly to the membrane of these undifferentiated cells compared to the microvillar pole of the columnar cells, while SSA, HHA, WGA, and SNA-I showed stronger binding to the microvilli. Our results indicated that polarization of the midgut cells is also reflected by a specific distribution of glycans, especially between the basal and microvillar pole. The data are discussed in relation to the functioning and development of the insect midgut.

Characterization of Mucosal Disaccharidases from Human Intestine

In this study, we used a brush border membrane (BBM) preparation from human small intestine to analyze the proportion and the activity of major intestinal disaccharidases, including sucrase-isomaltase (SI), maltase-glucoamylase (MGAM) and lactase-phlorizin hydrolase (LPH). SI, MGAM and LPH respectively constituted 8.2%, 2.7% and 1.4% of total BBM protein. The activity of SI and LPH decreased threefold after purification from the brush border membrane, which highlights the effect of membrane microdomains on the functional capacity of these enzymes. All of the disaccharidases showed optimal activity at pH 6, over 50% residual activity between pH 5 to pH 7, and increasing activity with rising temperatures up to 45 °C, along with a stable functional structure. Therefore the enzymes can withstand mild intraluminal pH alterations with adequate function, and are able to increase their activity with elevated core body temperature. Our data provide a functional measure for characterization of intestinal disaccharidases under different physiological and pathological conditions.

Characterizing the O-glycosylation landscape of human plasma, platelets, and endothelial cells

The hemostatic system comprises platelet aggregation, coagulation, and fibrinolysis, and is critical to the maintenance of vascular integrity. Multiple studies indicate that glycans play important roles in the hemostatic system; however, most investigations have focused on N-glycans because of the complexity of O-glycan analysis. Here we performed the first systematic analysis of native-O-glycosylation using lectin affinity chromatography coupled to liquid chromatography mass spectrometry (LC-MS)/MS to determine the precise location of O-glycans in human plasma, platelets, and endothelial cells, which coordinately regulate hemostasis. We identified the hitherto largest O-glycoproteome from native tissue with a total of 649 glycoproteins and 1123 nonambiguous O-glycosites, demonstrating that O-glycosylation is a ubiquitous modification of extracellular proteins. Investigation of the general properties of O-glycosylation established that it is a heterogeneous modification, frequently occurring at low density within disordered regions in a cell-dependent manner. Using an unbiased screen to identify associations between O-glycosites and protein annotations we found that O-glycans were over-represented close (± 15 amino acids) to tandem repeat regions, protease cleavage sites, within propeptides, and located on a select group of protein domains. The importance of O-glycosites in proximity to proteolytic cleavage sites was further supported by in vitro peptide assays demonstrating that proteolysis of key hemostatic proteins can be inhibited by the presence of O-glycans. Collectively, these data illustrate the global properties of native O-glycosylation and provide the requisite roadmap for future biomarker and structure-function studies.

Molecular pathogenicity of novel sucrase-isomaltase mutations found in congenital sucrase-isomaltase deficiency patients

BACKGROUND & AIMS

Congenital sucrase-isomaltase deficiency (CSID) is a genetic disorder associated with mutations in the sucrase-isomaltase (SI) gene. The diagnosis of congenital diarrheal disorders like CSID is difficult due to unspecific symptoms and usually requires invasive biopsy sampling of the intestine. Sequencing of the SI gene and molecular analysis of the resulting potentially pathogenic SI protein variants may facilitate a diagnosis in the future. This study aimed to categorize SI mutations based on their functional consequences.

METHODS

cDNAs encoding 13 SI mutants were expressed in COS-1 cells. The molecular pathogenicity of the resulting SI mutants was defined by analyzing their biosynthesis, cellular localization, structure and enzymatic functions.

RESULTS

Three biosynthetic phenotypes for the novel SI mutations were identified. The first biosynthetic phenotype was defined by mutants that are intracellularly transported in a fashion similar to wild type SI and with normal, but varying, levels of enzymatic activity. The second biosynthetic phenotype was defined by mutants with delayed maturation and trafficking kinetics and reduced activity. The third group of mutants is entirely transport incompetent and functionally inactive.

CONCLUSIONS

The current study unraveled CSID as a multifaceted malabsorption disorder that comprises three major classes of functional and trafficking mutants of SI and established a gradient of mild to severe functional deficits in the enzymatic functions of the enzyme.

GENERAL SIGNIFICANCE

This novel concept and the existence of mild consequences in a number of SI mutants strongly propose that CSID is an underdiagnosed and a more common intestinal disease than currently known.

The multiple roles of sucrase-isomaltase in the intestinal physiology

Osmotic diarrhea and abdominal pain in humans are oftentimes associated with carbohydrate malabsorption in the small intestine due to loss of function of microvillar disaccharidases. Disaccharidases are crucial for the digestion and the subsequent absorption of carbohydrates. This review focuses on sucrase-isomaltase as the most abundant intestinal disaccharidase and the primary or induced pathological conditions that affect its physiological function. Congenital defects are primary factors which directly influence the transport and function of sucrase-isomaltase in a healthy epithelium. Based on the mutation type and the pattern of inheritance, a mutation in the sucrase-isomaltase gene may exert a variety of symptoms ranging from mild to severe. However, structure and function of wild type sucrase-isomaltase can be also affected by secondary factors which influence its structure and function either specifically via certain inhibitors and therapeutic agents or generally as a part of intestinal pathogenesis, for example in the inflammatory responses. Diagnosis of sucrase-isomaltase deficiency and discriminating it from other gastrointestinal intolerances can be latent in the patients because of common symptoms observed in all of these cases.

Here, we summarize the disorders that implicate the digestive function of sucrase-isomaltase as well as the diagnostic and therapeutic strategies utilized to restore normal intestinal function.

Fucosylation and protein glycosylation create functional receptors for cholera toxin

Cholera toxin (CT) enters and intoxicates host cells after binding cell surface receptors using its B subunit (CTB). The ganglioside (glycolipid) GM1 is thought to be the sole CT receptor; however, the mechanism by which CTB binding to GM1 mediates internalization of CT remains enigmatic. Here we report that CTB binds cell surface glycoproteins. Relative contributions of gangliosides and glycoproteins to CTB binding depend on cell type, and CTB binds primarily to glycoproteins in colonic epithelial cell lines. Using a metabolically incorporated photocrosslinking sugar, we identified one CTB-binding glycoprotein and demonstrated that the glycan portion of the molecule, not the protein, provides the CTB interaction motif. We further show that fucosylated structures promote CTB entry into a colonic epithelial cell line and subsequent host cell intoxication. CTB-binding fucosylated glycoproteins are present in normal human intestinal epithelia and could play a role in cholera.

DOI:http://dx.doi.org/10.7554/eLife.09545.001

Cholera is a serious diarrheal disease that can be deadly if left untreated. It is caused by eating food, or drinking water, contaminated by the bacterium Vibrio cholerae. This bacterium can survive passage through the acidic conditions of the stomach. Inside the small intestine, V. cholerae attaches to the intestinal wall and starts producing cholera toxin. The toxin enters intestinal cells, causing them to release water and ions, including sodium and chloride ions. The salt-water environment created inside the intestine can, by osmosis, draw up to a further six liters of water into the intestine each day. This results in the copious production of watery diarrhea and severe dehydration.

Cholera toxin is composed of six protein subunits, including five copies of cholera toxin subunit B (CTB). CTB subunits help the uptake of the toxin by intestinal cells, and it has long been reported that CTB subunits attach to intestinal cells by binding to a cell surface molecule called GM1. CTB subunits have a high affinity for GM1, yet recent work suggests CTB may not bind exclusively to GM1; one or more additional cell surface molecules may be directly involved in cholera toxin uptake.

Wands et al. now reveal that numerous cell surface molecules are recognized by CTB, and that these molecules can assist cholera toxin uptake by host cells. Glycoproteins, proteins that are marked with sugar molecules, were shown to be the primary CTB binding sites on human colon cells, and it was the glycoprotein’s sugar component, not the protein itself, that interacted with CTB. Wands et al. discovered that in particular glycoproteins containing a sugar called fucose were largely responsible for CTB binding and toxin uptake. Together these findings reveal a previously unrecognized mechanism for cholera toxin entry into host cells, and suggest that fucose-containing or fucose-mimicking molecules could be developed as new treatments for cholera.

DOI:http://dx.doi.org/10.7554/eLife.09545.002

The Diverse Forms of Lactose Intolerance and the Putative Linkage to Several Cancers

Lactase-phlorizin hydrolase (LPH) is a membrane glycoprotein and the only β-galactosidase of the brush border membrane of the intestinal epithelium. Besides active transcription, expression of the active LPH requires different maturation steps of the polypeptide through the secretory pathway, including N- and O-glycosylation, dimerization and proteolytic cleavage steps. The inability to digest lactose due to insufficient lactase activity results in gastrointestinal symptoms known as lactose intolerance. In this review, we will concentrate on the structural and functional features of LPH protein and summarize the cellular and molecular mechanism required for its maturation and trafficking. Then, different types of lactose intolerance are discussed, and the molecular aspects of lactase persistence/non-persistence phenotypes are investigated. Finally, we will review the literature focusing on the lactase persistence/non-persistence populations as a comparative model in order to determine the protective or adverse effects of milk and dairy foods on the incidence of colorectal, ovarian and prostate cancers.

O-glycans direct selectin ligands to lipid rafts on leukocytes

Palmitoylated cysteines typically target transmembrane proteins to domains enriched in cholesterol and sphingolipids (lipid rafts). P-selectin glycoprotein ligand-1 (PSGL-1), CD43, and CD44 are O-glycosylated proteins on leukocytes that associate with lipid rafts. During inflammation, they transduce signals by engaging selectins as leukocytes roll in venules, and they move to the raft-enriched uropods of polarized cells upon chemokine stimulation. It is not known how these glycoproteins associate with lipid rafts or whether this association is required for signaling or for translocation to uropods. Here, we found that loss of core 1-derived O-glycans in murine C1galt1(-/-) neutrophils blocked raft targeting of PSGL-1, CD43, and CD44, but not of other glycosylated proteins, as measured by resistance to solubilization in nonionic detergent and by copatching with a raft-resident sphingolipid on intact cells. Neuraminidase removal of sialic acids from wild-type neutrophils also blocked raft targeting. C1galt1(-/-) neutrophils or neuraminidase-treated neutrophils failed to activate tyrosine kinases when plated on immobilized anti-PSGL-1 or anti-CD44 F(ab')2. Furthermore, C1galt1(-/-) neutrophils incubated with anti-PSGL-1 F(ab')2 did not generate microparticles. In marked contrast, PSGL-1, CD43, and CD44 moved normally to the uropods of chemokine-stimulated C1galt1(-/-) neutrophils. These data define a role for core 1-derived O-glycans and terminal sialic acids in targeting glycoprotein ligands for selectins to lipid rafts of leukocytes. Preassociation of these glycoproteins with rafts is required for signaling but not for movement to uropods.

Recycling of galectin-3 in epithelial cells

Galectins, a family of β-galactoside binding proteins, do not possess a signalling sequence to enter the endoplasmic reticulum as a starting point for the classical secretory pathway. They use a so-called unconventional secretion mechanism for translocation across the plasma membrane and/or into the lumen of transport vesicles. The β-galactoside binding protein galectin-3 is highly expressed in a variety of epithelial cell lines. Polarized MDCK cells secrete this lectin predominantly into the apical medium. The lectin re-enters the cell by non-clathrin mediated endocytosis and passages through endosomal organelles. This internalized galectin-3 plays an important role in apical protein trafficking by directing the subcellular targeting of apical glycoproteins via oligomerization into high molecular weight clusters, a process that can be fine-tuned by changes in the environmental pH. Following release at the apical plasma membrane, the lectin can reenter the cell for another round of recycling and apical protein sorting. This review will briefly address galectin-3-functions in epithelia and focus on distinct phases in apical recycling of the lectin.

Long term differential consequences of miglustat therapy on intestinal disaccharidases

Miglustat is an oral medication for treatment of lysosomal storage diseases such as Gaucher disease type I and Niemann Pick disease type C. In many cases application of Miglustat is associated with symptoms similar to those observed in intestinal carbohydrate malabsorption. Previously, we have demonstrated that intestinal disaccharidases are inhibited immediately by Miglustat in the intestinal lumen. Nevertheless, the multiple functions of Miglustat hypothesize long term effects of Miglustat on intracellular mechanisms, including glycosylation, maturation and trafficking of the intestinal disaccharidases. Our data show that a major long term effect of Miglustat is its interference with N-glycosylation of the proteins in the ER leading to a delay in the trafficking of sucrase-isomaltase. Also association with lipid rafts and plausibly apical targeting of this protein is partly affected in the presence of Miglustat. More drastic is the effect of Miglustat on lactase-phlorizin hydrolase which is partially blocked intracellularly. The de novo synthesized SI and LPH in the presence of Miglustat show reduced functional efficiencies according to altered posttranslational processing of these proteins. However, at physiological concentrations of Miglustat (≤50 μM) a major part of the activity of these disaccharidases is found to be still preserved, which puts the charge of the observed carbohydrate maldigestion mostly on the direct inhibition of disaccharidases in the intestinal lumen by Miglustat as the immediate side effect.

Podoplanin requires sialylated O-glycans for stable expression on lymphatic endothelial cells and for interaction with platelets

O-glycosylation of podoplanin (PDPN) on lymphatic endothelial cells is critical for the separation of blood and lymphatic systems by interacting with platelet C-type lectin-like receptor 2 during development. However, how O-glycosylation controls endothelial PDPN function and expression remains unclear. In this study, we report that core 1 O-glycan-deficient or desialylated PDPN was highly susceptible to proteolytic degradation by various proteases, including metalloproteinases (MMP)-2/9. We found that the lymph contained activated MMP-2/9 and incubation of the lymph reduced surface levels of PDPN on core 1 O-glycan-deficient endothelial cells, but not on wild-type ECs. The lymph from mice with sepsis induced by cecal ligation and puncture, which contained bacteria-derived sialidase, reduced PDPN levels on wild-type ECs. The MMP inhibitor, GM6001, rescued these reductions. Additionally, GM6001 treatment rescued the reduction of PDPN level on lymphatic endothelial cells in mice lacking endothelial core 1 O-glycan or cecal ligation and puncture-treated mice. Furthermore, core 1 O-glycan-deficient or desialylated PDPN impaired platelet interaction under physiological flow. These data indicate that sialylated O-glycans of PDPN are essential for platelet adhesion and prevent PDPN from proteolytic degradation primarily mediated by MMPs in the lymph.

Sorting of GPI-anchored proteins from yeast to mammals--common pathways at different sites?

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are luminal secretory cargos that are attached by a post-translational glycolipid modification, the GPI anchor, to the external leaflet of the plasma membrane. GPI-APs are conserved among eukaryotes and possess many diverse and vital functions for which the GPI membrane attachment appears to be essential. The presence of the GPI anchor and its subsequent modifications along the secretory pathway confer to the anchored proteins unique trafficking properties that make GPI-APs an exceptional system to study mechanisms of sorting. In this Commentary, we discuss the recent advances in the field of GPI-AP sorting focusing on the mechanisms operating at the level of the exit from the ER and from the trans-Golgi network (TGN), which take place, respectively, in yeast and in polarized mammalian cells. By considering the similarities and differences between these two sorting events, we present unifying principles that appear to work at different sorting stations and in different organisms.

A computational and experimental study of O-glycosylation. Catalysis by human UDP-GalNAc polypeptide:GalNAc transferase-T2

It is estimated that >50% of proteins are glycosylated with sugar tags that can modulate protein activity through what has been called the sugar code. Here we present the first QM/MM calculations of human GalNAc-T2, a retaining glycosyltransferase, which initiates the biosynthesis of mucin-type O-glycans. Importantly, we have characterized a hydrogen bond between the β-phosphate of UDP and the backbone amide group from the Thr7 of the sugar acceptor (EA2 peptide) that promotes catalysis and that we propose could be a general catalytic strategy used in peptide O-glycosylation by retaining glycosyltransferases. Additional important substrate-substrate interactions have been identified, for example, between the β-phosphate of UDP with the attacking hydroxyl group from the acceptor substrate and with the substituent at the C2' position of the transferred sugar. Our results support a front-side attack mechanism for this enzyme, with a barrier height of ~20 kcal mol(-1) at the QM(M05-2X/TZVP//BP86/SVP)/CHARMM22 level, in reasonable agreement with the experimental kinetic data. Experimental and in silico mutations show that transferase activity is very sensitive to changes in residues Glu334, Asn335 and Arg362. Additionally, our calculations for different donor substrates suggest that human GalNAc-T2 would be inactive if 2'-deoxy-Gal or 2'-oxymethyl-Gal were used, while UDP-Gal is confirmed as a valid sugar donor. Finally, the analysis herein presented highlights that both the substrate-substrate and the enzyme-substrate interactions are mainly concentrated on stabilizing the negative charge developing at the UDP leaving group as the transition state is approached, identifying this as a key aspect of retaining glycosyltransferases catalysis.

Trafficking to the apical and basolateral membranes in polarized epithelial cells

Renal epithelial cells must maintain distinct protein compositions in their apical and basolateral membranes in order to perform their transport functions. The creation of these polarized protein distributions depends on sorting signals that designate the trafficking route and site of ultimate functional residence for each protein. Segregation of newly synthesized apical and basolateral proteins into distinct carrier vesicles can occur at the trans-Golgi network, recycling endosomes, or a growing assortment of stations along the cellular trafficking pathway. The nature of the specific sorting signal and the mechanism through which it is interpreted can influence the route a protein takes through the cell. Cell type-specific variations in the targeting motifs of a protein, as are evident for Na,K-ATPase, demonstrate a remarkable capacity to adapt sorting pathways to different developmental states or physiologic requirements. This review summarizes our current understanding of apical and basolateral trafficking routes in polarized epithelial cells.

Study of the biological functions of mucin type core 3 O-glycans

Core 3 O-glycan is very short glycan structure which is composed of one N-acetylglucosamine and one N-acetylgalactosamine. The core 3 O-glycan structure is synthesized by core3 synthase (beta 1, 3-N-acetylglucosaminyltransferase 6) using UDP-N-acetylglucosamine as substrate. We revealed that the core 3 O-glycan structure modulates prostate cancer formation and gastrointestinal cell differentiation through regulating the heterodimerization of α2β1 integrin and cell surface expression of differentiation marker proteins respectively. This chapter describes the way to determine the functions of core 3 O-glycan in tumor formation and gastrointestinal cell differentiation.

The lectin domain of the polypeptide GalNAc transferase family of glycosyltransferases (ppGalNAc Ts) acts as a switch directing glycopeptide substrate glycosylation in an N- or C-terminal direction, further controlling mucin type O-glycosylation

Mucin type O-glycosylation is initiated by a large family of polypeptide GalNAc transferases (ppGalNAc Ts) that add α-GalNAc to the Ser and Thr residues of peptides. Of the 20 human isoforms, all but one are composed of two globular domains linked by a short flexible linker: a catalytic domain and a ricin-like lectin carbohydrate binding domain. Presently, the roles of the catalytic and lectin domains in peptide and glycopeptide recognition and specificity remain unclear. To systematically study the role of the lectin domain in ppGalNAc T glycopeptide substrate utilization, we have developed a series of novel random glycopeptide substrates containing a single GalNAc-O-Thr residue placed near either the N or C terminus of the glycopeptide substrate. Our results reveal that the presence and N- or C-terminal placement of the GalNAc-O-Thr can be important determinants of overall catalytic activity and specificity that differ between transferase isoforms. For example, ppGalNAc T1, T2, and T14 prefer C-terminally placed GalNAc-O-Thr, whereas ppGalNAc T3 and T6 prefer N-terminally placed GalNAc-O-Thr. Several transferase isoforms, ppGalNAc T5, T13, and T16, display equally enhanced N- or C-terminal activities relative to the nonglycosylated control peptides. This N- and/or C-terminal selectivity is presumably due to weak glycopeptide binding to the lectin domain, whose orientation relative to the catalytic domain is dynamic and isoform-dependent. Such N- or C-terminal glycopeptide selectivity provides an additional level of control or fidelity for the O-glycosylation of biologically significant sites and suggests that O-glycosylation may in some instances be exquisitely controlled.

Embryologie und Physiologie

Die Entwicklung der für die Digestion verantwortlichen Enzymsysteme ist eng mit der Entwicklung des fetalen Pankreas und des Dünndarms sowie der Speicheldrüsen korreliert. Morphologisch beginnt die Entwicklung der fetalen Speicheldrüsen und des Pankreas in der 12. Schwangerschaftswoche. Bis zur 20. Woche ist diese Entwicklung morphologisch so weit abgeschlossen, dass die exokrinen Drüsen beginnen, ihre Enzyme freizusetzen.

Miglustat-induced intestinal carbohydrate malabsorption is due to the inhibition of α-glucosidases, but not β-galactosidases

Miglustat is an oral medication that has approved indication for type I Gaucher disease and Niemann pick disease type C. Usually treatment with Miglustat is associated with occurrence of gastrointestinal side effects similar to carbohydrate maldigestion symptoms. Here, we studied the direct influence of Miglustat on the enzymatic function of the major disaccharidases of the intestinal epithelium. Our findings show that an immediate effect of Miglustat is its interference with carbohydrate digestion in the intestinal lumen via reversible inhibition of disaccharidases that cleave α-glycosidically linked carbohydrates. Higher non physiological concentrations of Miglustat can partly affect lactase activity. We further show that the inhibition of the disaccharidases function by Miglustat is mainly competitive and does not occur via alteration of the enzyme folding.

Intestinal peptidases form functional complexes with the neutral amino acid transporter B(0)AT1

The brush-border membrane of the small intestine and kidney proximal tubule are the major sites for the absorption and re-absorption of nutrients in the body respectively. Transport of amino acids is mediated through the action of numerous secondary active transporters. In the mouse, neutral amino acids are transported by B(0)AT1 [broad neutral ((0)) amino acid transporter 1; SLC6A19 (solute carrier family 6 member 19)] in the intestine and by B(0)AT1 and B(0)AT3 (SLC6A18) in the kidney. Immunoprecipitation and Blue native electrophoresis of intestinal brush-border membrane proteins revealed that B(0)AT1 forms complexes with two peptidases, APN (aminopeptidase N/CD13) and ACE2 (angiotensin-converting enzyme 2). Physiological characterization of B(0)AT1 expressed together with these peptidases in Xenopus laevis oocytes revealed that APN increased the substrate affinity of the transporter up to 2.5-fold and also increased its surface expression (V(max)). Peptide competition experiments, in silico modelling and site-directed mutagenesis of APN suggest that the catalytic site of the peptidase is involved in the observed changes of B(0)AT1 apparent substrate affinity, possibly by increasing the local substrate concentration. These results provide evidence for the existence of B(0)AT1-containing digestive complexes in the brush-border membrane, interacting differentially with various peptidases, and responding to the dynamic needs of nutrient absorption in the intestine and kidney.

Association between tetrodotoxin resistant channels and lipid rafts regulates sensory neuron excitability

Voltage-gated sodium channels (VGSCs) play a key role in the initiation and propagation of action potentials in neurons. Na_V1.8 is a tetrodotoxin (TTX) resistant VGSC expressed in nociceptors, peripheral small-diameter neurons able to detect noxious stimuli. Na_V1.8 underlies the vast majority of sodium currents during action potentials. Many studies have highlighted a key role for Na_V1.8 in inflammatory and chronic pain models. Lipid rafts are microdomains of the plasma membrane highly enriched in cholesterol and sphingolipids. Lipid rafts tune the spatial and temporal organisation of proteins and lipids on the plasma membrane. They are thought to act as platforms on the membrane where proteins and lipids can be trafficked, compartmentalised and functionally clustered. In the present study we investigated Na_V1.8 sub-cellular localisation and explored the idea that it is associated with lipid rafts in nociceptors. We found that Na_V1.8 is distributed in clusters along the axons of DRG neurons in vitro and ex vivo. We also demonstrated, by biochemical and imaging studies, that Na_V1.8 is associated with lipid rafts along the sciatic nerve ex vivo and in DRG neurons in vitro. Moreover, treatments with methyl-β-cyclodextrin (MβCD) and 7-ketocholesterol (7KC) led to the dissociation between rafts and Na_V1.8. By calcium imaging we demonstrated that the lack of association between rafts and Na_V1.8 correlated with impaired neuronal excitability, highlighted by a reduction in the number of neurons able to conduct mechanically- and chemically-evoked depolarisations. These findings reveal the sub-cellular localisation of Na_V1.8 in nociceptors and highlight the importance of the association between Na_V1.8 and lipid rafts in the control of nociceptor excitability.

N-Glycosylation instead of cholesterol mediates oligomerization and apical sorting of GPI-APs in FRT cells

In contrast to MDCK cells, in FRT cells oligomerization and apical sorting of GPI-APs are mediated by N-glycosylation independent of cholesterol and raft association.

Sorting of glycosylphosphatidyl-inositol–anchored proteins (GPI-APs) in polarized epithelial cells is not fully understood. Oligomerization in the Golgi complex has emerged as the crucial event driving apical segregation of GPI-APs in two different kind of epithelial cells, Madin–Darby canine kidney (MDCK) and Fisher rat thyroid (FRT) cells, but whether the mechanism is conserved is unknown. In MDCK cells cholesterol promotes GPI-AP oligomerization, as well as apical sorting of GPI-APs. Here we show that FRT cells lack this cholesterol-driven oligomerization as apical sorting mechanism. In these cells both apical and basolateral GPI-APs display restricted diffusion in the Golgi likely due to a cholesterol-enriched membrane environment. It is striking that N-glycosylation is the critical event for oligomerization and apical sorting of GPI-APs in FRT cells but not in MDCK cells. Our data indicate that at least two mechanisms exist to determine oligomerization in the Golgi leading to apical sorting of GPI-APs. One depends on cholesterol, and the other depends on N-glycosylation and is insensitive to cholesterol addition or depletion.

Characterization of mouse sperm TMEM190, a small transmembrane protein with the trefoil domain: evidence for co-localization with IZUMO1 and complex formation with other sperm proteins

TMEM190, a small transmembrane protein containing the trefoil domain, was previously identified by our proteomic analysis of mouse sperm. Two structural features of TMEM190, ‘trefoil domain’ and ‘small transmembrane protein’, led us to hypothesize that this protein forms a protein–protein complex required during fertilization, and we characterized TMEM190 by biochemical, cytological, and genetic approaches. We showed in this study that the mouse Tmem190 gene exhibits testis-specific mRNA expression and that the encoded RNA is translated into a 19-kDa protein found in both testicular germ cells and cauda epididymal sperm. Treatment of the cell surface with proteinase K, subcellular fractionation, and immunofluorescence assay all revealed that mouse TMEM190 is an inner-acrosomal membrane protein of cauda epididymal sperm. During the acrosome reaction, TMEM190 partly relocated onto the surface of the equatorial segment, on which sperm–oocyte fusion occurs. Moreover, TMEM190 and IZUMO1, which is an immunoglobulin-like protein required for gamete fusion, co-localized in mouse sperm both before and after the acrosome reaction. However, immunoprecipitates of TMEM190 contained several sperm proteins, but did not include IZUMO1. These findings suggest that a mouse sperm protein complex(es) including TMEM190 plays an indirect role(s) in sperm–oocyte fusion. The role(s), if any, is probably dispensable since Tmem190-null male mice were normally fertile.

Core2 O-glycan structure is essential for the cell surface expression of sucrase isomaltase and dipeptidyl peptidase-IV during intestinal cell differentiation

Alterations in glycosylation play an important role during intestinal cell differentiation. Here, we compared expression of mucin-type O-glycan synthases from proliferating and differentiated HT-29 and Caco-2 cells. Mucin-type O-glycan structures were analyzed at both stages by mass spectrometry. Core2 β1,6-N-acetylglucosaminyltransferase-2 (C2GnT-2) was markedly increased in differentiated HT-29 and Caco-2 cells, but the core3 structure was hardly detectable. To determine whether such differential expression of mucin-type O-glycan structures has physiological significance in intestinal cell differentiation, expression of sucrase isomaltase (SI) and dipeptidyl-peptidase IV (DPP-IV), two well known intestinal differentiation markers, was examined. Interestingly, the fully glycosylated mature form of SI was decreased in C2GnT-2 knock-out mice but not in core2 N-acetylglucosaminyltransferase-3 (C2GnT-3) nulls. In addition, expression of SI and DPP-IV was dramatically reduced in C2GnT-1-3 triple knock-out mice. These patterns were confirmed by RNAi analysis; C2GnT-2 knockdown significantly reduced cell surface expression of SI and DPP-IV in Caco-2 cells. Similarly, overexpression of the core3 structure in HT-29 cells attenuated cell surface expression of both enzymes. These findings indicate that core3 O-glycan structure regulates cell surface expression of SI and DPP-IV and that core2 O-glycan is presumably an essential mucin-type O-glycan structure found in both molecules in vivo. Finally, goblet cells in the upper part of the crypt showed impaired maturation in the core2 O-glycan-deficient mice. These studies are the first to clearly identify functional mucin-type O-glycan structures modulating cell surface expression of SI and DPP-IV during the intestinal cell differentiation.

Protein trafficking in immune cells

The majority of cells of the immune system are specialized secretory cells, whose function depends on regulated exocytosis. The latter is mediated by vesicular transport involving the sorting of specialized cargo into the secretory granules (SGs), thereby generating the transport vesicles; their transport along the microtubules and eventually their signal-dependent fusion with the plasma membrane. Each of these steps is tightly controlled by mechanisms, which involve the participation of specific sorting signals on the cargo proteins and their recognition by cognate adaptor proteins, posttranslational modifications of the cargo proteins and multiple GTPases and SNARE proteins. In some of the cells (i.e. mast cells, T killer cells) an intimate connection exists between the secretory system and the endocytic one, whereby the SGs are lysosome related organelles (LROs) also referred to as secretory lysosomes. Herein, we discuss these mechanisms in health and disease states.