Targeted mutations in beta1,4-galactosyltransferase I reveal its multiple cellular functions

Molecular Mechanisms Underlying the Loop-Closing Dynamics of β-1,4 Galactosyltransferase 1

The β-1,4 galactosylation catalyzed by β-1,4 galactosyltransferases (β4Gal-Ts) is not only closely associated with diverse physiological and pathological processes in humans but also widely applied in the N-glycan modification of protein glycoengineering. The loop-closing process of β4Gal-Ts is an essential intermediate step intervening in the binding events of donor substrate (UDP-Gal/Mn2+) and acceptor substrate during its catalytic cycle, with a significant impact on the galactosylation activities. However, the molecular mechanisms in regulating loop-closing dynamics are not entirely clear. Here, we construct Markov state models (MSMs) based on approximately 20 μs of all-atom molecular dynamics simulations to explore the loop-closing dynamics for β-1,4 galactosyltransferase 1 (β4Gal-T1). Our MSM reveals five key metastable states of β4Gal-T1 upon substrate binding, indicating that the entire conformational transition occurs on a time scale of ∼10 μs. Moreover, a regulatory mechanism involving six conserved residues (R187, H190, F222, W310, I341, and D346) among β4Gal-Ts is validated to account for the loop-closing dynamics of the C-loop and W-loop by site-directed mutagenesis and enzymatic activity assays, exhibiting high consistency with our computational predictions. Overall, our research proposes detailed atomic-level insight into the loop-closing dynamics of the C-loop and W-loop on β4Gal-T1, contributing to a deeper understanding of catalytic mechanisms of β-1,4 galactosylation.

B4GALT5 high expression associated with poor prognosis of hepatocellular carcinoma

BACKGROUND

B4GALT5 is postulated to be an important protein in sugar metabolism that catalyzes the synthesis of lactosylceramide (LacCer). However, its role in hepatocellular carcinoma (HCC) remains unknown.

METHOD

We characterized the expression of B4GALT5 in HCC tissue compared to normal tissue, and explored its function of B4GALT5 in HCC by enrichment analysis based on its co-expressed gene set. Next, we checked whether B4GALT5 expression is correlated to immune infiltration level and clinical prognosis in hepatocellular carcinoma. Finally, we verified the expression of B4GALT5 using clinical samples evaluated by RT-PCR, and conducted in vitro experiments with B4GALT5-knockdown HCC cells to investigate the function of B4GALT5 in the HCC cell proliferation, migration and invasion.

RESULTS

We found B4GALT5 mRNA and protein expression levels were significantly high in HCC tissue compared to normal tissue. The enrichment analysis of the gene sets that co-expressed with B4GALT5 showed specificity in HCC-related pathways and functions. Also, the expression pattern of B4GALT5 was significantly related to the immune infiltration level, especially CD4+ T cell and macrophage cells. B4GALT5 higher mRNA expression was associated with poor overall survival (OS) in HCC patients. Furthermore, In vitro experiments showed that depletion of B4GALT5 significantly inhibited HCC cell proliferation, migration and invasion. This study revealed the function and its mediated pathways of B4GALT5 in HCC, indicating that B4GALT5 may serve as a prognostic biomarker of HCC.

β1,4-Galactosyltransferase V Modulates Breast Cancer Stem Cells through Wnt/β-catenin Signaling Pathway

PURPOSE

Breast cancer stem cells (BCSCs) contribute to the initiation, development, and recurrence of breast carcinomas. β1,4-Galactosyltransferase V (B4GalT5), which catalyzes the addition of galactose to GlcNAcβ1-4Man of N-glycans, is involved in embryogenesis. However, its role in the modulation of BCSCs remains unknown.

Materials and Methods

The relationship between B4GalT5 and breast cancer stemness was investigated by online clinical databases and immunohistochemistry analysis. Mammosphere formation, fluorescence-activated cell sorting (FACS), and in-vivo assays were used to evaluate B4GalT5 expression in BCSCs and its effect on BCSCs. B4GalT5 regulation of Wnt/β-catenin signaling was examined by immunofluorescence and Ricinus communis agglutinin I pull-down assays. Cell surface biotinylation and FACS assays were performed to assess the association of cell surface B4GalT5 and BCSCs.

RESULTS

B4GalT5, but not other B4GalTs, was highly correlated with BCSC markers and poor prognosis. B4GalT5 significantly increased the stem cell marker aldehyde dehydrogenase 1A1 (ALDH1A1) and promoted the production of CD44+CD24-/low cells and the formation of mammospheres. Furthermore, B4GalT5 overexpression resulted in dramatic tumor growth in vivo. Mechanistically, B4GalT5 modified and protected Frizzled-1 from degradation via the lysosomal pathway, promoting Wnt/β-catenin signaling which was hyperactivated in BCSCs. B4GalT5, located on the surface of a small subset of breast carcinoma cells, was not responsible for the stemness of BCSCs.

CONCLUSION

B4GalT5 modulates the stemness of breast cancer through glycosylation modification to stabilize Frizzled-1 and activate Wnt/β-catenin signaling independent of its cell surface location. Our studies highlight a previously unknown role of B4GalT5 in regulating the stemness of breast cancer and provide a potential drug target for anticancer drug development.

Phenotype and response to growth hormone therapy in siblings with B4GALT7 deficiency

B4GALT7 encodes beta-1,4-galactosyltransferase which links glycosaminoglycans to proteoglycans in connective tissues. Rare, biallelic variants in B4GALT7 have been associated with spondylodysplastic Ehlers-Danlos and Larsen of Reunion Island syndromes. Thirty patients with B4GALT7-related disorders have been reported to date with phenotypic variability. Using whole exome sequencing, we identified male and female siblings with biallelic, pathogenic B4GALT7 variants and phenotypic features of spondylodysplastic Ehlers-Danlos syndrome as well as previously unreported skeletal characteristics. We also provide detailed radiological characterization and describe the siblings' responses to growth hormone treatment. Our report extends the phenotypic spectrum of B4GALT7-associated spondylodysplastic Ehlers-Danlos syndrome and reports results of growth hormone treatment for patients with this rare disorder.

The Immunological Regulation Roles of Porcine β-1, 4 Galactosyltransferase V (B4GALT5) in PRRSV Infection

B4GALT5, also known as β-1, 4 galactosyltransferase V, is one of the members of β-1, 4 galactosyltransferase gene (B4GALT) family, which was concerned with embryonic development, tumor generation, other malignant diseases. In this study, we firstly cloned porcine B4GALT (pB4GALT5) from porcine alveolar macrophages, and predicted the structural domain and function of seven porcine β-1, 4 galactosyltransferase (I–VII) based on transcriptome analysis of PRRSV infected cells. Additionally, the upregulated porcine B4GALT5 expression was detected from PRRSV infected porcine alveolar macrophage (PAM) cells. The PRRSV proliferation were slightly inhibited in overexpression of pB4GALT5 transfected cells, the interaction of B4GALT5 and GP5 of PRRSV was firstly be detected by Co-IP, and the co-location between B4GALT5 and GP5 were also observed in golgi membranes by confocal microscopy. A significant increasing mRNA transcription, including inflammatory cytokines (IFN-α, IL-6, IL-18, IL-1β, TNF-α) and some cell surface glycosylated protein involved in antigen present (MHC-I/II), cell adhesion and migration (chemokine MCP-1 and receptor CCR2; LFA-1, ICAM-1) were upregulated in B4GALT5 overexpressed PRRSV infected cells. Our results demonstrated that the regulation of pB4GALT5 plays an important roles in PRRSV proliferation and modification function in viral infection cells. And these results will make achievements by supporting the research of latent mechanisms of β-1, 4 galactosyltransferase V in antiviral immunity.

Microfluidics Cell Loading-Dock System: Ordered Cellular Array for Dynamic Lymphocyte-Communication Study

It remains a great challenge to establish a high-throughput platform that can explore the interactions among multiple lymphocytes (>2 cells) and retrieve the interested cells for downstream analysis. This study demonstrates a microfluidics cell loading-dock system (Cell-Dock) to enclose multiple cells in 1D, 2D, and 3D chambers with high throughput and efficiency and single-cell accuracy. The loading efficiencies of 95%, 85%, and 74% for one-, three-, and five-cell systems are achieved, respectively. The Cell-Dock system provides precise and dynamic cell packing models to facilitate lymphocyte-interaction studies. The results demonstrate that individual natural killer (NK) cells may function independently rather than cooperate to lyse target cells in the defined microenvironment. Furthermore, the strong/weak NK cells are retrieved based on their on-chip cytotoxicity and mRNA sequencing is conducted to find the possible mechanisms for "serial killing," an important but unsolved issue. This study finds that the stronger NK cells overexpress multiple genes involved in cytotoxicity and adhesion molecules (including the well-known ICAM1 and seldom reported B4GALT1) might play important roles in the regulation of NK cytolysis.

Mouse fibroblasts null for the long isoform of β1,4-galactosyltransferase-I show defective cell-matrix interactions

β1,4 Galactosyltransferase-I (GalT-I) is expressed as two nearly identical polypeptides that differ only in the length of their cytoplasmic domains. The longer isoform has been implicated as a cell surface receptor for extracellular glycoside ligands, such as laminin. To more stringently test the function of the long GalT-I isoform during cell interactions with laminin, we created multiple independent fibroblastic cell lines that fail to express the long isoform, but which express the short GalT-I isoform normally and appear to have normal intracellular galactosylation. Cells devoid of the long GalT-I isoform are unable to adhere and spread on laminin substrates as well as control cells, but retain near normal interactions with fibronectin, which do not rely upon surface GalT-I function. The loss of the long GalT-I isoform also leads to a loss of actin stress fibers, focal adhesions and rac GTPase activation.

What Have We Learned from Glycosyltransferase Knockouts in Mice?

There are five major classes of glycan including N- and O-glycans, glycosaminoglycans, glycosphingolipids, and glycophosphatidylinositol anchors, all expressed at the molecular frontier of each mammalian cell. Numerous biological consequences of altering the expression of mammalian glycans are understood at a mechanistic level, but many more remain to be characterized. Mouse mutants with deleted, defective, or misexpressed genes that encode activities necessary for glycosylation have led the way to identifying key functions of glycans in biology. However, with the advent of exome sequencing, humans with mutations in genes involved in glycosylation are also revealing specific requirements for glycans in mammalian development. The aim of this review is to summarize glycosylation genes that are necessary for mouse embryonic development, pathway-specific glycosylation genes whose deletion leads to postnatal morbidity, and glycosylation genes for which effects are mild, but perturbation of the organism may reveal functional consequences. General strategies for generating and interpreting the phenotype of mice with glycosylation defects are discussed in relation to human congenital disorders of glycosylation (CDG).

Characterization of plasma membrane associated type II α-D-mannosidase and β-N-acetylglucosaminidase of Aquarius remigis sperm

For successful fertilization to occur, molecules on the surface of male and female gametes must recognize each other in a complementary manner. In some organisms, sperm possess a glycosidase on the plasma membrane overlying the head while eggs have glycoproteins that are recognized by those glycosidases resulting in sperm-egg recognition. In this study, two glycosidases, mannosidase and β-N-acetylglucosaminidase, were identified and biochemically characterized in Aquarius remigis sperm. The mannosidase had a Km of 2.36 ± 0.19 mM, a Vmax of 27.49 ± 0.88 pmol/min and a Hill coefficient of 0.94 ± 0.18 at its optimal pH of 7.0. The mannosidase was extracted most efficiently with CHAPSO but was also efficiently extracted with sodium chloride. Mannosidase activity was effectively inhibited by swainsonine, but not by kifunesine, and was significantly reduced in the presence of Mn(2+) and Mg(2+), but not Zn(2+). N-acetylglucosaminidase had a Km of 0.093 ± 0.01 mM, a Vmax of 153.80 ± 2.97 pmol/min and a Hill coefficient of 0.96 ± 0.63 at its optimal pH of 7.0. N-acetylglucosaminidase was extracted most efficiently with potassium iodide but was also efficiently extracted with Triton X-100 and Zn(2+), but not Ca(2+), Co(2+), Mn(2+) or Mg(2+), significantly inhibited its activity. Taken together, these results indicate that the A. remigis sperm surface contains at least two glycosidases that may recognize complementary glycoconjugates on the surface of water strider eggs.

Arachidonic acid induces an increase of β-1,4-galactosyltransferase I expression in MDA-MB-231 breast cancer cells

Arachidonic acid (AA) is a common dietary n-6 cis polyunsaturated fatty acid that under physiological conditions is present in an esterified form in cell membrane phospholipids, and it might be present in the extracellular microenvironment. AA and its metabolites are implicated in FAK activation and cell migration in MDA-MB-231 breast cancer cells, and an epithelial-to-mesenchymal-like transition process in mammary non-tumorigenic epithelial cells MCF10A. During malignant transformation is present an altered expression of glycosiltransferases, which promote changes on the glycosilation of cell-surface proteins. The β-1,4-galactosyltransferase I (GalT I) is an enzyme that participates in a variety of biological functions including cell growth, migration, and spreading. However, the participation of AA in the regulation of GalT I expression and the role of this enzyme in the cell adhesion process in breast cancer cells remains to be investigated. In the present study, we demonstrate that AA induces an increase of GalT I expression through a PLA2α, Src, ERK1/2, and LOXs activities-dependent pathway in MDA-MB-231 breast cancer cells. Moreover, MDA-MB-231 cells adhere to laminin via GalT I expression and pretreatment of cells with AA induces an increase of cell adhesion to laminin. In conclusion, our findings demonstrate, for the first time, that AA promotes an increase of GalT I expression through an AA metabolism, Src and ERK1/2 activities-dependent pathway, and that GalT I plays a pivotal role in cell adhesion to laminin in MDA-MB-231 breast cancer cells.

Osteopontin increases the expression of β1, 4-galactosyltransferase-I and promotes adhesion in human RL95-2 cells

Beta1, 4-Galactosyltransferase-I (β1, 4-GalT-I), which transfers galactose from UDP-Gal to N-acetylglucosamine and N-acetylglucosamine-terminated oligosaccharides of N- and O-linked glycans in a β(1-4) linkage, plays a critical role in cell adhesion, sperm-egg recognition, neurite growth, and tumor cell migration and invasion. Our previously experiments also show that β1, 4-GalT-I was up-regulated by estrogens and some important cytokines of embryo implantation especially Interleukin-1 (IL-1), TGF-α and Leukemia Inhibitory Factor (LIF) in endometrial cells. In the receptive phase human uterus, osteopontin (OPN) is the most highly up-regulated extracellular matrix/adhesion molecule/cytokine. In this study, we demonstrated the correlated expression of OPN and β1, 4-GalT-I in endometrium during early pregnancy, and recombinant human OPN (rhOPN) protein induced the β1, 4-GalT-I up-regulation in RL95-2 cells. Inhibition of MEK/ERK, PI3K/AKT and NF-κB suppressed rhOPN-induced β1, 4-GalT-I expression. In addition, rhOPN promoted the adhesion of blastocysts cells in vitro in β1, 4-GalT-I-dependent manner. Moreover, the adhesion is greatly inhibited when β1, 4-GalT-I was blocked with the specific antibody. Taken together, our data suggest that β1, 4-GalT-I provides a mechanism to bridge embryo to endometrium during implantation.

A testis-specific regulator of complex and hybrid N-glycan synthesis

GnT1IP inhibits GlcNAcT-1 to change N-glycosylation patterns on secretory proteins, potentially regulating germ cell adhesion to Sertoli cells during spermatogenesis.

Database analyses identified 4933434I20Rik as a glycosyltransferase-like gene expressed mainly in testicular germ cells and regulated during spermatogenesis. Expression of a membrane-bound form of the protein resulted in a marked and specific reduction in N-acetylglucosaminyltransferase I (GlcNAcT-I) activity and complex and hybrid N-glycan synthesis. Thus, the novel activity was termed GlcNAcT-I inhibitory protein (GnT1IP). Membrane-bound GnT1IP localizes to the ER, the ER-Golgi intermediate compartment (ERGIC), and the cis-Golgi. Coexpression of membrane-anchored GnT1IP with GlcNAcT-I causes association of the two proteins, inactivation of GlcNAcT-I, and mislocalization of GlcNAcT-I from the medial-Golgi to earlier compartments. Therefore, GnT1IP is a regulator of GlcNAcT-I and complex and hybrid N-glycan production. Importantly, the formation of high mannose N-glycans resulting from inhibition of GlcNAcT-I by GnT1IP markedly increases the adhesion of CHO cells to TM4 Sertoli cells. Testicular germ cells might use GnT1IP to induce the expression of high mannose N-glycans on glycoproteins, thereby facilitating Sertoli–germ cell attachment at a particular stage of spermatogenesis.

Expression of genes encoding for proteins involved in heparan sulphate and chondroitin sulphate chain synthesis and modification in normal and malignant plasma cells

Syndecan-1 is a proteoglycan that concentrates heparin-binding factors on the surface of multiple myeloma cells, and probably plays a major role in multiple myeloma biology. As heparan sulphate and chondroitin sulphate are the bioactive components of syndecan-1, we analysed the signature of genes encoding 100 proteins involved in synthesis of these chains, i.e. from precursor uptake to post-translational modifications, using Affymetrix microarrays. The expression of enzymes required for heparan sulphate and chondroitin sulphate biosynthesis was shown to increase in parallel with syndecan-1 expression, throughout the differentiation of memory B cells into plasmablasts and normal bone marrow plasma cells. Sixteen genes were significantly different between normal and malignant plasma cells, nine of these genes -EXT2, CHSY3, CSGALNACT1, HS3ST2, HS2ST1, CHST11, CSGALNACT2, HPSE, SULF2 - encode proteins involved in glycosaminoglycan chain synthesis or modifications. Kaplan-Meier analysis was performed in two independent series of patients: B4GALT7, CSGALNACT1, HS2ST1 were associated with a good prognosis whereas EXT1 was linked to a bad prognosis. This study provides an overall picture of the major genes encoding for proteins involved in heparan sulphate and chondroitin sulphate synthesis and modifications that can be implicated in normal and malignant plasma cells.

A kinetic model for chain length modulation of Streptococcus pneumoniae cellubiuronan capsular polysaccharide by nucleotide sugar donor concentrations

The chain length of Streptococcus pneumoniae type 3 capsular polysaccharide (cellubiuronic acid) is tightly regulated by the cellubiuronic acid synthase through an assembly process involving a catalytic motif that is potentially conserved over a wide range of related processive beta-glucan synthases. Cellubiuronic acid is initiated on a lipid and is composed of alternating beta-1,3-Glc and beta-1,4-glucuronic acid (GlcUA) linkages. The entire assembly process is carried out by a polypeptide synthase thought to contain a single active site, suggesting that the donor specificity is controlled by the terminal nonreducing sugar in the acceptor subsite. Shortly after initiation, the synthase undergoes an allosteric transition accompanied by the tight binding of the nascent chain via its nonreducing oligosaccharide terminal segment to the carbohydrate acceptor recognition site. The chain length of polysaccharide assembled by recombinant synthase in Escherichia coli membranes was determined by an ejection mechanism that appeared to be a reversal of the allosteric transition of the synthase from the transitory to the fully processive state. The rates of both ejection and transition were shown to be highly sensitive to the concentration of UDP-GlcUA. As the concentration of UDP-GlcUA was increased, both the rate of synthesis and the processive turnover time increased. The product of the processive turnover time and the rate of synthesis predicted a marked increase in polysaccharide chain size (from 50 to 1150 kDa) over a relatively narrow concentration range of 1-11.5 microm UDP-GlcUA. The kinetic model chain length predictions were in close agreement with chemically determined sizes of polysaccharides synthesized at the same UDP-sugar concentrations. The model indicates that translocation occurs following the addition of GlcUA to the chain terminus, whereas UDP-Glc drives chain termination when inadequate levels of UDP-GlcUA are present. In sum, type 3 synthase appears to modulate polysaccharide chain length by functioning as a concentration-dependent kinetic timing device.

Reassessing the role of protein-carbohydrate complementarity during sperm-egg interactions in the mouse

Despite years of intense study by many investigators, it may appear that we have made little progress towards a molecular understanding of mammalian sperm binding to the egg zona pellucida. An abundance of evidence derived from in vitro assays suggests that sperm-zona pellucida binding is dependent upon sperm recognition of specific glycan moieties on the zona pellucida glycoproteins. However, there is considerable disagreement regarding the identity of the zona pellucida sugars thought to mediate sperm binding, as well as disagreement over the identity of the sperm receptors themselves. Moreover, results from in vivo gene-targeting strategies fail to support a role for many, if not all, of the sperm receptors and their zona pellucida ligands implicated from in vitro assays. Nevertheless, a retrospective view of the literature suggests that some common principles are emerging regarding the molecular basis of mammalian sperm-zona binding, both with respect to the nature of the components that mediate binding, as well as the involvement of distinct receptor-ligand interactions, that involve both protein- and carbohydrate-dependent mechanisms of binding.

Structure and function of beta -1,4-galactosyltransferase

Beta-1,4-galactosylransferase (beta4Gal-T1) participates in the synthesis of Galbeta1-4-GlcNAc-disaccharide unit of glycoconjugates. It is a trans-Golgi glycosyltransferase (Glyco-T) with a type II membrane protein topology, a short N-terminal cytoplasmic domain, a membrane-spanning region, as well as a stem and a C-terminal catalytic domain facing the trans-Golgi-lumen. Its hydrophobic membrane-spanning region, like that of other Glyco-T, has a shorter length compared to plasma membrane proteins, an important feature for its retention in the trans-Golgi. The catalytic domain has two flexible loops, a long and a small one. The primary metal binding site is located at the N-terminal hinge region of the long flexible loop. Upon binding of metal ion and sugar-nucleotide, the flexible loops undergo a marked conformational change, from an open to a closed conformation. Conformational change simultaneously creates at the C-terminal region of the flexible loop an oligosaccharide acceptor binding site that did not exist before. The loop acts as a lid covering the bound donor substrate. After completion of the transfer of the glycosyl unit to the acceptor, the saccharide product is ejected; the loop reverts to its native conformation to release the remaining nucleotide moiety. The conformational change in beta4Gal-T1 also creates the binding site for a mammary gland-specific protein, alpha-lactalbumin (LA), which changes the acceptor specificity of the enzyme toward glucose to synthesize lactose during lactation. The specificity of the sugar donor is generally determined by a few residues in the sugar-nucleotide binding pocket of Glyco-T, conserved among the family members from different species. Mutation of these residues has allowed us to design new and novel glycosyltransferases, with broader or requisite donor and acceptor specificities, and to synthesize specific complex carbohydrates as well as specific inhibitors for these enzymes.

Expression change of beta-1,4 galactosyltransferase I, V mRNAs and Galbeta1,4GlcNAc group in rat sciatic nerve after crush

Glycosylation is one of the most important post-translational modifications. It is clear that the single step of beta-1,4-galactosylation is performed by a family of beta-1,4-galactosyltransferases (beta-1,4-GalTs), and that each member of this family may play a distinct role in different tissues and cells. beta-1,4-GalT I and V are involved in the biosynthesis of N-linked oligosaccharides. In the present study, Real-time PCR revealed that the beta-1,4-GalT I and V mRNAs reached peaks at 2 w after sciatic nerve crush. In situ hybridization showed that at 1 d after sciatic nerve crush, the expression levels of beta-1,4-GalT I and V mRNAs were strong at the crush site, and decreased gradually from crush site to the distal segments. In addition, combined in situ hybridization for beta1,4-GalT I and V mRNAs and immunohistochemistry for S100 showed that beta1,4-GalT I and V mRNAs were mainly located in Schwann cells. Lectin blot showed that the expression of Galbeta1,4GlcNAc group increased at 6 h immediately, reached a peak at 12 h and remained elevated up to 4 w after sciatic nerve crush. In conclusion, beta1,4-GalT I and V might play important roles in the regeneration of the injured sciatic nerve, and upregulation of Galbeta1,4GlcNAc group might be correlated with the process of the sciatic nerve injury.

ABO blood group and related antigens, natural antibodies and transplantation

The current success rate of transplant surgery and immunosuppression has led to a demand for organs that has outstripped the supply. This has required investigation of alternate strategies. Therefore, allotransplantation across the ABO blood group barrier has commenced, and pig-to-human xenotransplantation is under consideration. The first immunological barrier to both these types of transplantation is the prevention of the antibody-mediated rejection. This rejection is a result of natural preformed antibodies circulating in the serum of the recipient binding to either ABO (for allo) or alpha-galactose (alpha-Gal) (for xeno) antigens expressed on the donor tissue. These antibodies recognise antigens that are, in both cases, carbohydrate molecules with the characteristic feature that the nonreducing terminal carbohydrate is either a Gal or N-acetlygalactosamine residue in an alpha1,3 linkage. These epitopes are synthesised by closely related members of a single family of glycosyltransferases. This review discusses the carbohydrate antigens, the enzymes involved in their synthesis and the consequences of natural antibodies binding these antigens.

Identification of novel gamete receptors that mediate sperm adhesion to the egg coat

Mammalian fertilization is initiated by the species-specific binding of sperm to the zona pellucida, or egg coat. Earlier studies suggested that sperm-egg adhesion in mouse is mediated by the binding of beta1,4-galactosyltransferase-I (GalT) on the sperm surface to specific glycoside ligands on the egg coat glycoprotein, ZP3. Binding of multiple ZP3 oligosaccharides induces GalT aggregation, triggering a pertussis toxin-sensitive G-protein cascade leading to induction of the acrosome reaction. Consistent with this, sperm bearing targeted deletions in GalT are unable to bind ZP3 nor undergo ZP3-dependent acrosomal exocytosis; however, GalT-null sperm are still able to bind to the egg coat. This indicates that sperm-egg binding requires at least two independent binding mechanisms: a GalT-ZP3-independent event that mediates initial adhesion, followed by a GalT-ZP3 interaction that facilitates acrosomal exocytosis. During the past few years, novel GalT-ZP3-independent gamete receptors have been identified that appear to participate in initial gamete adhesion. On such receptor is SED1, an EGF repeat and discoidin domain protein that coats sperm as they traverse through the epididymis, and which is required for sperm to bind the egg coat. Similarly, a novel egg coat ligand is present on ovulated oocytes, but not on ovarian eggs, and which also appears to function in initial sperm binding. The identification of novel gamete receptors that are required for sperm-egg binding opens up new avenues for the development of specific contraceptive strategies.

Mutant glycosyltransferases assist in the development of a targeted drug delivery system and contrast agents for MRI

The availability of structural information on glycosyltransferases is beginning to make structure-based reengineering of these enzymes possible. Mutant glycosyltransferases have been generated that can transfer a sugar residue with a chemically reactive unique functional group to a sugar moiety of glycoproteins, glycolipids, and proteoglycans (glycoconjugates). The presence of modified sugar moiety on a glycoprotein makes it possible to link bioactive molecules via modified glycan chains, thereby assisting in the assembly of bionanoparticles that are useful for developing the targeted drug delivery system and contrast agents for magnetic resonance imaging. The reengineered recombinant glycosyltransferases also make it possible to (1) remodel the oligosaccharide chains of glycoprotein drugs, and (2) synthesize oligosaccharides for vaccine development.

An α-L-fucosidase potentially involved in fertilization is present onDrosophila spermatozoa surface

Drosophila is emerging as a model organism to investigate egg fertilization in insects and the possible conservation of molecular mechanisms of gamete interactions demonstrated in higher organisms. This study shows that the spermatozoa of several species of Drosophila belonging to the melanogaster group have a plasma membrane associated alpha-L-fucosidase with features in common with alpha-L-fucosidases from sperm of other animals, including mammals. The enzyme has been purified and completely characterized in D. ananassae, because of its stability in this species. The sperm alpha-L-fucosidase is an integral protein terminally mannosylated, with the catalytic site oriented toward the extracellular space. It has a M(r) of 256 kDa and a multimeric structure made up by subunits of 48 and 55 kDa. Enzyme characterization included kinetic properties, pI, optimal pH, and thermal stability. A soluble form of the enzyme similar to the sperm associated alpha-L-fucosidase is secreted by the seminal vesicles. Synthetic peptides designed from the deduced product of the D. melanogaster gene encoding an alpha-L-fucosidase were used to raise a specific polyclonal antibody. Immunofluorescence labeling of spermatozoa showed that the enzyme is present in the sperm plasma membrane overlying the acrosome and the tail. Lectin cytochemistry analysis of the egg surface indicated that alpha-L-fucose terminal residues are present on the chorion with a strongly polarized localization on the micropyle. The alpha-L-fucosidase of Drosophila sperm plasma membrane appears to be potentially involved in gamete recognition by interacting with its glycoside ligands present on the egg surface at the site of sperm entry.

Deletion of the cytoplasmic domain of human α3/4 fucosyltransferase III causes the shift of the enzyme to early Golgi compartments

The transmembrane domain (TM) and flanking regions of glycosyltransferases (GTs) have been implicated in the localization of these proteins in the Golgi apparatus (GA). alpha3/4 Fucosyltransferase III (FT3wt) (EC 2.4.1.65) is localized in the trans-Golgi and trans-Golgi network (TGN) of baby hamster kidney (BHK) cells and synthesizes Lewis determinants associated with cell adhesion events. We have evaluated the effect of removing the cytosolic domain on the localization of the enzyme and its capacity for synthesizing the Lewis A (Le A) determinant. The mutant where the cytoplasmic domain (Asp-2 to Trp-13) of FT3wt has been deleted (FT3dc) was localized in the Golgi but it was shifted to earlier compartments than FT3wt. The mutant was not detected on the plasma membrane (PM) and glycosylation analysis indicated that FT3dc was transported beyond the endoplasmic reticulum (ER) since complex type glycosylation was observed. Cells expressing FT3dc showed a significantly lower efficiency to synthesize Le A when compared with cells expressing FT3wt, in vivo. This reduction was not due to lower specific activity because both enzyme forms had a similar specific activity in vitro. Therefore, removal of FT3 cytosolic tail caused a shift in enzyme distribution to earlier Golgi compartments concomitant to the decrease of its biosynthetic capacity.

Structure and catalytic cycle of β-1,4-galactosyltransferase

Beta-1,4-galactosyltransferase-1, a housekeeping enzyme that functions in the synthesis of glycoconjugates, has two flexible loops, one short and one long. Upon binding a metal ion and UDP-galactose, the loops change from an open to a closed conformation, repositioning residues to lock the ligands in place. Residues at the N-terminal region of the long loop form the metal-binding site and those at the C-terminal region form a helix, which becomes part of the binding site for the oligosaccharide acceptor; the remaining residues cover the bound sugar-nucleotide. After binding of the oligosaccharide acceptor and transfer of the galactose moiety, the product disaccharide unit is ejected and the enzyme returns to the open conformation, repeating the catalytic cycle.

A novel missense mutation in the galactosyltransferase-I (B4GALT7) gene in a family exhibiting facioskeletal anomalies and Ehlers-Danlos syndrome resembling the progeroid type

The Ehlers-Danlos syndrome (EDS) is a heterogeneous group of heritable connective tissue disorders characterized by skin hyperextensibility, joint hypermobility, and tissue fragility. Several genes have been implicated to result in EDS phenotypes. The progeroid type of EDS is characterized by wrinkled, loose skin on the face, curly fine hair, scanty eyebrows and eyelashes, in addition to the classical features of EDS. Here we describe two similarly affected individuals in two sibships of a large consanguineous family from Qatar. DNA samples from affected and unaffected members of the family were analyzed for homozygosity of polymorphic markers associated with genes that have been implicated in EDS. Among 28 markers analyzed, homozygosity was only observed for D5S469 and D5S2111, which were markers for galactosyltransferase-I (B4GALT7) located on chromosome 5q35.2, where the previously reported progeroid-like variant of EDS has been mapped. Exons harboring the coding regions and exon-intron junctions of B4GALT7 were amplified by PCR and examined for mutations. A homozygous misssense C to T substitution at nucleotide 808 in the coding region was discovered in both affected individuals. The carrier parents were heterozygous for this mutation, which was not found among 76 DNA samples from control individuals of the same ethnicity. Segregation of this novel mutation in the family further confirmed the allelic variant and its recessive mode of inheritance in this type of EDS. The C to T substitution results in an arginine to cysteine change at amino acid residue 270 that is located in the catalytically active extracellular C-terminal domain. This change could result in abnormal protein folding and/or aberrant interactions of mutated galactosyltransferase-I with other extracellular matrix proteins leading to the development of a progeroid-like phenotype in affected individuals.

Sperm from beta1,4-galactosyltransferase I-null mice exhibit precocious capacitation

Mammalian sperm must undergo a physiological maturation, termed capacitation, before they are able to fertilize eggs. Despite its importance, the molecular mechanisms underlying capacitation are poorly understood. In this paper, we describe the capacitation phenotype of sperm lacking the long isoform of beta1,4-galactosyltransferase I (GalT I), a sperm surface protein that functions as a receptor for the zona pellucida glycoprotein, ZP3, and as an inducer of the acrosome reaction following ZP3-dependent aggregation. As expected, wild-type sperm must undergo capacitation in order to bind the zona pellucida and undergo a Ca(2+) ionophore-induced acrosome reaction. By contrast, GalT I-null sperm behave as though they are precociously capacitated, in that they demonstrate maximal binding to the zona pellucida and greatly increased sensitivity to ionophore-induced acrosome reactions without undergoing capacitation in vitro. The loss of GalT I from sperm results in an inability to bind epididymal glycoconjugates that normally maintain sperm in an 'uncapacitated' state; removing these decapacitating factors from wild-type sperm phenocopies the capacitation behavior of GalT I-null sperm. Interestingly, capacitation of GalT I-null sperm is independent of the presence of albumin, Ca(2+) and HCO(3)(-); three co-factors normally required by wild-type sperm to achieve capacitation. This implies that intracellular targets of albumin, Ca(2+) and/or HCO(3)(-) may be constitutively active in GalT I-null sperm. Consistent with this, GalT I-null sperm have increased levels of cAMP that correlate closely with both the accelerated kinetics and co-factor-independence of GalT I-null sperm capacitation. By contrast, the kinetics of protein tyrosine phosphorylation and sperm motility are unaltered in mutant sperm relative to wild-type. These data suggest that GalT I may function as a negative regulator of capacitation in the sperm head by suppressing intracellular signaling pathways that promote this process.

Effect of a null mutation of the oviduct-specific glycoprotein gene on mouse fertilization

The mammalian fertilization process takes place in a complex microenvironment within the female genital tract. A member of the chitinase protein family, oviduct-specific glycoprotein (OGP), has been identified in oviductal fluid from various mammalian species, including humans. Although OGP is widely believed to be involved in the process of mammalian fertilization, including spermatozoon function and gamete interactions, based on experimental results obtained in vitro, its physiological significance remains controversial. The present study established OGP gene-null ( ogp (-/-)) mice, and primarily characterized their reproductive properties to study the physiological function(s) of OGP. Results obtained from studies using an in vivo or in vitro system showed that the fertility of ogp (-/-) females was within normal limits. These results indicate that OGP is not essential for the process of in vivo fertilization, at least in mice.