Subunit structure of a cortical granule lectin involved in the block to polyspermy in Xenopus laevis eggs

Biochemical and ligand binding properties of recombinant Xenopus laevis cortical granule lectin-1

Intelectins are putative innate immune lectins that are found throughout chordates. The first intelectin reported was Xenopus laevis cortical granule lectin-1 (XCGL-1 or XL-35). XCGL-1 is critical in fertilization membrane development in Xenopus. Here, we explored the biochemical properties of XCGL-1. The cysteines responsible for forming intermolecular disulfide bonds were identified. XCGL-1 adopted a four-lobed structure as observed by electron microscopy. The full-length XCGL-1 and the carbohydrate recognition domain (CRD) bind galactose-containing carbohydrates at nanomolar to micromolar affinities. Molecular modeling suggested that galactoside ligands coordinated the binding site calcium ion and interacted with residues around the groove made available by the non-conserved substitution compared to human intelectin-1. Folding conditions for production of recombinant XCGL-1 CRD were also investigated. Our results not only provide new biochemical insights into the function of XCGL-1, but may also provide foundation for further applications of XCGL-1 as glycobiology tools.

Mammalian egg coat modifications and the block to polyspermy

Fertilization by more than one sperm causes polyploidy, a condition that is generally lethal to the embryo in the majority of animal species. To prevent this occurrence, eggs have developed a series of mechanisms that block polyspermy at the level of the plasma membrane or their extracellular coat. In this review, we first introduce the mammalian egg coat, the zona pellucida (ZP), and summarize what is currently known about its composition, structure, and biological functions. We then describe how this specialized extracellular matrix is modified by the contents of cortical granules (CG), secretory organelles that are exocytosed by the egg after gamete fusion. This process releases proteases, glycosidases, lectins and zinc onto the ZP, resulting in a series of changes in the properties of the egg coat that are collectively referred to as hardening. By drawing parallels with comparable modifications of the vitelline envelope of nonmammalian eggs, we discuss how CG‐dependent modifications of the ZP are thought to contribute to the block to polyspermy. Moreover, we argue for the importance of obtaining more information on the architecture of the ZP, as well as systematically investigating the many facets of ZP hardening.

Molecular cloning, expression and characterization of secreted ferritin in the silkworm, Bombyx mori

Ferritin is a ubiquitous iron storage protein which plays key role in regulating iron homeostasis and metabolism. In this paper, the ferritin heavy chain homologs (HCH) and light chain homologs (LCH) from Bombyx mori (BmFerHCH and BmFerLCH) were amplified through PCR and cloned into the expression vector pET-30a(+). The recombinant BmFerHCH and BmFerLCH expressed in Escherichia coli were in the form of insoluble inclusion bodies, indicating that the two proteins were not in their natural structural conformation. In order to obtain refolded ferritin in vitro, the inclusion bodies (BmFerHCH and/or BmFerLCH) were dissolved in denaturing buffer (100 mM Tris, 50 mM Glycine, 8 M urea, 5 mM DTT, pH 8.0) and then refolded in refolding buffer (100 mM Tris, 400 mM L-arginine, 0.2 mM PMSF, 0.5 mM DTT). The result showed that it was only when both BmFerHCH and BmFerLCH were present together in the denaturing buffer that refolding was successful and resulted in the formation of heteropolymers (H-L chain dimers) over homopolymers (H-H chain or L-L chain dimers). Moreover, the molecules (NaCl, Triton and glycerol) were found to enhance protein refolding. The optimum temperature, pH and ratios of BmFerHCH/BmFerLCH required for refolding were found to be 10 °C, pH 7, 1:1 or 1:2, respectively. Finally, the refolded ferritin had the ability to store iron, exhibited ferroxidase activity, and could withstand high temperatures and pH treatment, which is consistent with ferritin in other species.

Bacterial lipopolysaccharides stimulate production of XCL1, a calcium-dependent lipopolysaccharide-binding serum lectin, in Xenopus laevis

Xenopus laevis serum lectin XCL1 is a newly identified molecule of the XCGL (or X-lectin) family, a unique group of Ca(2+)-dependent lectins that have a fibrinogen-like domain. The XCL1 protein was purified from lipopolysaccharide (LPS)-stimulated frog sera by sequential affinity chromatography on heparin-acrylic beads and galactose-Sepharose. XCL1 comprises multiple oligomeric proteins consisting of 37-kDa subunit polypeptides, as revealed by sodium dodecyl sulfate-polyacrylamide electrophoresis (SDS-PAGE) and Western blot analyses using the monoclonal antibody (mAb) produced against the recombinant XCL1 polypeptide. In the presence of Ca(2+), the protein bound to Escherichia coli, Staphylococcus aureus, LPS and galactose and the bound XCL1 was competitively eluted using ribose and xylose, and the elution was as efficient as that using EDTA, whereas elution using hexoses, GalNAc or GlcNAc was less effective. In reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analyses, XCL1 expression was ubiquitously detected in frog tissues, with relatively high levels in hematopoietic tissues including the spleen, liver and kidney. Intraperitoneal injection of E. coli, S. aureus or 100-300μg S-type LPS from various bacteria induced several-fold increases in serum XCL1 concentrations on day 3, and the elevated levels retained up to day 12. It also caused a remarkable increase of the splenic XCL1 expression on day 3, followed by a rapid decline to nearly nonstimulated control levels by day 7. The R-type LPS with shortened polysaccharide chains was less effective in inducing the serum XCL1 response, indicating that the sugar chains of LPS were important, if not essential, for the stimulation of XCL1 production. These results suggest that XCL1 is a pathogen recognition molecule involved in antimicrobial innate immunity in Xenopus.

Regulation of glycan structures in murine embryonic stem cells: combined transcript profiling of glycan-related genes and glycan structural analysis

The abundance and structural diversity of glycans on glycoproteins and glycolipids are highly regulated and play important roles during vertebrate development. Because of the challenges associated with studying glycan regulation in vertebrate embryos, we have chosen to study mouse embryonic stem (ES) cells as they differentiate into embryoid bodies (EBs) or into extraembryonic endodermal (ExE) cells as a model for cellular differentiation. We profiled N- and O-glycan structures isolated from these cell populations and examined transcripts encoding the corresponding enzymatic machinery for glycan biosynthesis in an effort to probe the mechanisms that drive the regulation of glycan diversity. During differentiation from mouse ES cells to either EBs or ExE cells, general trends were detected. The predominance of high mannose N-glycans in ES cells shifted to an equal abundance of complex and high mannose structures, increased sialylation, and increased α-Gal termination in the differentiated cell populations. Whereas core 1 O-glycan structures predominated in all three cell populations, increased sialylation and increased core diversity characterized the O-glycans of both differentiated cell types. Increased polysialylation was also found in both differentiated cell types. Differences between the two differentiated cell types included greater sialylation of N-glycans in EBs, whereas α-Gal-capped structures were more prevalent in ExE cells. Changes in glycan structures generally, but not uniformly, correlated with alterations in transcript abundance for the corresponding biosynthetic enzymes, suggesting that transcriptional regulation contributes significantly to the regulation of glycan expression. Knowledge of glycan structural diversity and transcript regulation should provide greater understanding of the roles of protein glycosylation in vertebrate development.

Differential structure and activity between human and mouse intelectin-1: human intelectin-1 is a disulfide-linked trimer, whereas mouse homologue is a monomer

Human intelectin-1 (hITLN-1) is a 120-kDa lectin recognizing galactofuranosyl residues found in cell walls of various microorganisms but not in mammalian tissues. Although mouse intelectin-1 (mITLN-1) has been identified previously, its biochemical properties and functional characteristics have not been studied. Therefore, we have compared structures and saccharide-binding specificities of hITLN-1 and mITLN-1 using recombinant proteins produced by mammalian cells. Recombinant hITLN-1 is a trimer, disulfide-linked through Cys-31 and Cys-48, and N-glycosylated at Asn-163. Despite 84.9% amino acid identity to hITLN-1, recombinant and intestinal mITLN-1 are unglycosylated 30-kDa monomers. Recombinant hITLN-1, as well as recombinant and intestinal mITLN-1 were purified by Ca(2+)-dependent adsorption to galactose-Sepharose. In competitive binding studies, hITLN-1 was eluted from galactose-Sepharose by 100 mM 2-deoxygalactose, a galactofuranosyl disaccharide, d-xylose, and both d- and l-ribose. In contrast, mITLN-1 was partially eluted by the galactofuranosyl disaccharide, and only minimally by the other saccharides indicating that the two intelectins have different saccharide-binding specificities. When the N- and C-terminal regions of hITLN-1 were replaced, respectively, with those of mITLN-1, galactose-Sepharose binding was associated with the C-terminal regions. Finally, hITLN-1 binding to galactose-Sepharose was not affected by the substitution of the Cys residues in the N-terminal region that are necessary for oligomer formation, nor was it affected by the removal of the N-linked oligosaccharide at Asn-163. Although both hITLN-1 and mITLN-1 recognize galactofuranosyl residues, our comparative studies, taken together, demonstrate that these intelectins have different quaternary structures and saccharide-binding specificities.

The disulfide bond pattern of salmon egg lectin 24K from the Chinook salmon Oncorhynchus tshawytscha

The disulfide bonds in the galactose-specific lectin SEL 24K from the egg of the Chinook salmon Oncorhynchus tshawytscha were determined by mass spectrometry. Four predictive in silico tools were used to determine the oxidation state of cysteines in the sequence and possible location of the disulfide bonds. A combination of tryptic digestion, HPLC separation, and chemical modifications were used to establish the location of seven disulfide bonds and one pair of free cysteines. After proteolysis, peptides containing one or two disulfide bonds were identified by reduction and mass spectral comparison. MALDI mass spectrometry was supported by chemical modification (iodoacetamide) and in silico digestion. The assignments of disulfide bonds were further confirmed by mass spectral fragmentation studies including in-source dissociation (ISD) and collision-induced dissociation (CID). The experimentally determined disulfide bonds and free Cys residues were only partially consistent with those generated by several automated public-domain algorithms.

Xenopus galectin-VIIa binds N-glycans of members of the cortical granule lectin family (xCGL and xCGL2)

We have identified members of the Xenopus cortical granule lectin (xCGL) family as candidate target glycoproteins of Xenopus galectin-VIIa (xgalectin-VIIa) in Xenopus embryos. In addition to the original xCGL, we also identified a novel member of the xCGL family, xCGL2. Expression of the mRNAs of xCGL and xCGL2, as well as that of xgalectin-VIIa, was observed throughout early embryogenesis. Two and three potential N-glycosylation sites were deduced from the amino acid sequences of xCGL and xCGL2, respectively, and xgalectin-VIIa recognizes N-glycans linked to a common site in xCGL and xCGL2 and also recognizes N-glycans linked to a site specific to xCGL2. However, interaction between xgalectin-Ia and xCGLs was not detectable. We also obtained consistent results on surface plasmon resonance analysis involving xCGLs as ligands and xgalectins as analytes. The Kd value of the interaction between xgalectin-VIIa and xCGLs was calculated to be 35.9 nM. The structures of the N-glycans of xCGLs, which were recognized by xgalectin-VIIa, were analyzed by the two-dimensional sugar map method, and three kinds of N-acetyllactosamine type, biantennary N-glycans were identified as the major neutral N-glycans. The binding specificity of oligosaccharides for xgalectin-VIIa was analyzed by frontal affinity chromatography (FAC). The oligosaccharide specificity pattern of xgalectin-VIIa was similar to that of the human homolog galectin-3, and it was also shown that the N-acetyllactosamine type, biantennary N-glycans exhibit high affinity for xgalectin-VIIa (Kd = 11 microM). These results suggest that xgalectin-VIIa interacts with xCGLs through binding to N-acetyllactosamine type N-glycans and that this interaction might make it possible to organize a lectin network involving members of different lectin families.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

The Xenopus laevis cortical granule lectin: cDNA cloning, developmental expression, and identification of the eglectin family of lectins

A Xenopus laevis egg cortical granule, calcium-dependent, galactosyl-specific lectin participates in forming the fertilization layer of the egg envelope and functions in establishing a block to polyspermy. We report the cDNA cloning of the lectin, expression of the cortical granule lectin gene during oogenesis and early development, and identification of a new family of lectins. The translated cDNA for the cortical granule lectin had a signal peptide, a structural sequence of 298 amino acids, a molecular weight of 32.7 K, contained consensus sequence sites for N-glycosylation and a fibrinogen domain. The lectin cDNA was expressed during early stages of oogenesis. Lectin glycoprotein levels were constant during development with 2/3 of the lectin associated with the extracellular perivitelline space and the egg/embryo fertilization envelope. Lectin mRNA levels were from 100- to 1000-fold greater in ovary than in other adult tissues. The lectin had no sequence homology to the previously identified lectin families. The lectin had 41-88% amino acid identity with nine translated cDNA sequences from an ascidian, lamprey, frog, mouse, and human. Based on the conserved carbohydrate binding and structural properties of these glycoproteins, we propose a new family of lectins, the eglectin family.

High-avidity, low-affinity multivalent interactions and the block to polyspermy in Xenopus laevis

The interaction of the lectin XL35 with the jelly coat protein (JCP) surrounding oocytes in Xenopus laevis is essential for the block to polyspermy. The molecular details of this event are poorly understood, and the present study has been undertaken with a view to delineating the mechanism of formation of the fertilization envelope. A range of JCP-derived oligosaccharides were synthesized, and all were installed with an artificial aminopropyl arm. This arm allowed the preparation of monovalent derivatives by acetylation of the amino group or the synthesis of polyvalent compounds by attachment to an activated polyacrylamide polymer. A number of analytical techniques, including enzyme-linked lectin assays and surface plasmon resonance, have been developed and utilized to study the interactions of the mono- and polyvalent compounds with XL35. The results reveal that the lectin XL35 has remarkably broad specificity for galactose-containing saccharides and the affinities are only slightly modulated by secondary features, such as anomeric configuration of the terminal sugar or the identity and linkage pattern of branching sugars. Broad specificity was also observed when the saccharides were presented in a polyvalent fashion. The glycopolymers displayed 10-20-fold increases in valency-corrected affinities compared to the corresponding monovalent counterparts. Although the synthetic polymers are not as potent as the JCP, the kinetics of their interactions mirror closely those of the native ligand, and in each case extremely long-lived interactions were observed. The results of this study indicate that, in X. laevis, the true biological function of multivalency is not to create an extremely tightly binding complex between XL35 and its natural ligand but, instead, to create a very stable protective layer that will not dissociate and is yet flexible enough to encapsulate the developing embryo. It is postulated that, even if these partners are unable to attain true equilibrium on the time scale of the biological event, their mode of interaction would, nevertheless, be expected to guarantee an insurmountable physical block to polyspermy. This study has also highlighted that multivalent interactions require a very long time to achieve equilibrium, and this feature may well be the origin of several of the ambiguities reported in the literature when multivalent ligands have been evaluated.

Activation of protein kinase Cη triggers cortical granule exocytosis in Xenopus oocytes

Previous work has shown that phorbol esters or diacylglycerol trigger cortical granule exocytosis in Xenopus oocytes. We sought to identify the isoform(s) of protein kinase C (PKC) that mediate(s) this regulated secretory event. Because this process is initiated by lipid activators of PKC but is independent of calcium ions, we focused on the family of novel(calcium-independent) PKCs. Pharmacological investigations using Gö6976 and Gö6983 tended to exclude PKCδ, ϵ and μ as secretory triggers. Subcellular fractionation and immunoblot data revealed that these oocytes expressed all five members of the novel PKC family, but it was only PKCη that colocalized with cortical granules. Finally, expression of wild type or constitutively active forms of PKCδ and η strongly supported the conclusion that it is PKCη that initiates cortical granule exocytosis in these cells. These observations represent an important step in identifying the mechanism of secretory triggering in this system.

Molecular cloning and functional expression of a human intestinal lactoferrin receptor

Lactoferrin (Lf), a major iron-binding protein in human milk, has been suggested to have multiple biological roles such as facilitating iron absorption, modulating the immune system, embryonic development, and cell proliferation. Our previous binding studies suggested the presence of a specific receptor for Lf (LfR) in the small intestine of newborn infants, which may facilitate iron absorption. We here report the cloning and the functional expression of the human intestinal LfR and the evidence of its involvement in iron metabolism. The entire coding region of the LfR cDNA was cloned by PCR based on amino acid sequences of the purified native LfR (nLfR). The recombinant LfR (rLfR) was then expressed in a baculovirus-insect cell system and purified by immobilized human Lf (hLf) affinity chromatography where binding of hLf to the rLfR was partially Ca(2+) dependent. The apparent molecular mass was 136 kDa under nonreducing conditions and 34 kDa under reducing conditions. 125I-hLf bound to the rLfR with an apparent K(d) of approximately 360 nM. These biochemical properties of the rLfR are similar to those of the nLfR. RT-PCR revealed that the gene was expressed at high levels in fetal small intestine and in adult heart and at lower levels in Caco-2 cells. PI-PLC treatment of Caco-2 cells indicated that the LfR is GPI anchored. In Caco-2 cells transfected with the LfR gene, 125I-hLf binding and 59Fe-hLf uptake were increased by 1.7 and 3.4 times, respectively, compared to those in mock-transfected cells. Our findings demonstrate the presence of a unique receptor-mediated mechanism for nutrient uptake by the newborn.

Human homologs of the Xenopus oocyte cortical granule lectin XL35

The cDNAs encoding two human homologs of the Xenopus oocyte lectin, XL35, were isolated from a small intestine cDNA library and termed HL-1 and HL-2. The deduced amino acid sequence of each homolog is about 60% identical and 80% similar to that of XL35, and none of these sequences contains the C-type lectin motif, although it is known that XL35 requires calcium for ligand binding. By Northern analysis, HL-1 transcripts are present at relatively high levels in heart, small intestine, colon, thymus, ovary, and testis. HL-2 transcripts, by contrast, are expressed only in small intestine. Immunocytochemistry using a polyclonal antibody produced against XL35 shows HL-1 protein to be localized exclusively in endothelial cells in colon, thymus, liver, and other tissues. Primary cultures of human aortic endothelial cells are positive for HL-1 expression by immunoblotting and by PCR analysis, but several other human cell types are not. HL-1 and -2 are both encoded at chromosome 1q23, the same locus that encodes the selectins. XL35, HL-1 and -2, and another mouse homolog are members of a new family of proteins whose members most likely perform diverse functions.

Structure and function of the extracellular matrix of anuran eggs

The extracellular matrix (ECM) surrounding the anuran egg is composed of jelly coat layers, an envelope, and the perivitelline space, which separates the envelope from the egg plasma membrane. Both the jelly coat layers and egg envelopes are required for fertilization in anurans. This paper reviews the current understanding of the structure-function relations of the ECM, with emphasis on the egg envelope. The fibrous egg envelope exists in four related forms. The envelope forms differ in their ultrastructures, macromolecular compositions, and cellular functions. After the oocyte is released from the ovary, conversion of one envelope form to another is brought about by factors secreted by the oviduct prior to fertilization and by factors released from the egg in the sperm-triggered cortical reaction. An additional extracellular matrix structure, located in the perivitelline space, has recently been identified in Xenopus laevis, as well as a previously undescribed reorganization of envelope fibers occurring at fertilization. The molecular changes in the ECM glycoproteins (limited proteolysis, lectin-ligand binding, and conformational changes) and the oviductal and egg macromolecules responsible for the conversion of envelope forms are discussed. New experimental evidence that supports the lectin-ligand hypothesis for the formation of the fertilization layer is presented. It is proposed that the molecular changes in the ECM are responsible for the ultrastructural alterations of the ECM and for modifications of the fertilization and developmental functions of the anuran egg ECM.

Ultrastructural aspects of oogenesis and oocyte growth in fish and amphibians

Oogenesis, the early events of primary oocyte growth (meiotic arrest, synapsis, ribosomal gene duplication), and folliculogenesis can be seen to particular advantage in the germinal ridge of the syngnathan ovary. After budding off the germinal ridge (a compartment of the luminal epithelium), nascent follicles then enter into a linear array of developing follicles within which temporal and stage-specific events can be correlated with spatial distribution. Prominent features of the later phase of primary oocyte growth include intense transcriptional activity and the formation and subsequent dispersal of the Balbiani vitelline body (mitochondrial cloud) concomitant with an increase in cytoplasmic organelles and volume. Further oocyte growth is characterized by a period of cortical alveolus (in teleosts) or cortical granule (in anurans) formation, in which Golgi elements play a predominant role, and finally vitellogenesis. The latter process, which is responsible for the preponderance of oocyte growth, includes the hepatic synthesis and secretion of vitellogenin (VTG), the uptake of VTG from the bloodstream into the oocyte by receptor-mediated endocytosis, and the transport of VTG via endosomes and multivesicular bodies to forming yolk platelets. In the process, VTG is proteolytically cleaved into the yolk proteins, which assume either a monoclinic (in cyclostomes) or orthorhombic (in teleosts and amphibians) crystalline array. Other structures associated with the growing oocyte are also briefly discussed, including nuage, the vitelline envelope, intercellular junctions between the oocyte and overlying follicle cells, pigment, intramitochondrial crystals in ranidae, and annulate lamellae.

A micromethod for the estimation of oligosaccharides containing glycosidically linked sialic acid or hexoses, or both, in glycoproteins

The peeling reaction, the process by which oligosaccharides are degraded in alkali, was used as the basis for an assay to provide structural information about glycosidically linked oligosaccharides in glycoproteins. Glycoproteins were treated with 0.05 M NaOH at 50 degrees to induce release, and subsequent degradation ("peeling"), of glycosidically linked, but not of N-glycosydically linked, oligosaccharides. Among the degradation products generated from O-linked chains were three 3-deoxy sugar acids whose formation was correlated with certain structural features of the oligosaccharides. N-Acetylneuraminic acid was released from terminal positions in the oligosaccharides, and iso- and meta-saccharinic acids were derived from the degradation of 4-O- and 3-O-substituted hexoses, respectively. All of these sugar acids were detected colorimetrically by periodate oxidation and reaction of the product with 2-thiobarbituric acid. The ability of the method to generate 3-deoxy sugar acids was tested in 8 alkali-treated glycoproteins. 3-Deoxy sugar acids were detected only in those glycoproteins whose glycosidically linked carbohydrates contained N-acetylneuraminic acid, or 3-O- or 4-O-substituted hexoses, or both. As little as 0.12 microgram of 3-deoxy sugar acid produced from 5 micrograms of human chorionic gonadotropin was sufficient for detection. This method is novel in its ability to distinguish sialylation of glycosidically linked carbohydrates. Furthermore, it combines the specificity of beta-elimination with the sensitivity of the 2-thiobarbituric acid assay in targeting degradation products of the peeling reaction as candidates for an assay method.