Specific proteolysis regulates fusion between endocytic compartments in Xenopus oocytes

Vitellogenin-derived fragment in embryos of Japanese flounder Paralichthys olivaceus with binding and bactericidal activities against an infectious bacterium via an interaction with saccharides

Thirty- and 90-kDa proteins with binding ability to Edwardsiella tarda, a causative bacterium of Edwardsiellosis in fish, were purified from the embryo of Japanese flounder Paralichthys olivaceus. The proteins were isolated with affinity chromatography, in which the bacterium was used as a ligand and galactose, mannose, and ethylenediaminetetraacetic acid (EDTA) were used as elution agents, followed by gel filtration chromatography. N-terminal amino acid sequencing and liquid chromatography with quadrupole time-of-flight tandem mass spectrometry (LC/Q-TOF-MS) analysis revealed that the 90-kDa protein was lipovitellin heavy-chain (LvH), which is one of the proteolytically cleaved products of maternal vitellogenin (Vg) and represents the main precursor of the egg yolk in teleosts, and the 30-kDa protein was an N-terminal bit of LvH. On the other hand, Vg in the serum of the mother fish did not bind to E. tarda. While the 90-kDa protein did not show anti-bacterial activity, the 30-kDa protein strongly exhibited activity toward E. tarda, with a minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) below 0.06 μM, suggesting that the latter protein plays an important role during embryogenesis in the flounder. This is the first report showing that Vg-derived products have monosaccharides-binding activity and a fragment derived from LvH exhibits bactericidal activity.

Molecular Characterization of a Novel Cathepsin L in Macrobrachium nipponense and Its Function in Ovary Maturation

Cathepsin L genes, which belonged to cysteine proteases, were a series of multifunctional protease and played important roles in a lot of pathological and physiological processes. In this study, we analyzed the characteristics a cathepsin L (named Mn-CL2) in the female oriental river prawn, Macrobrachium nipponense which was involved in ovary maturation. The Mn-CL2 was1,582 bp in length, including a 978 bp open reading frame that encoded 326 amino acids. The Mn-CL2 was classified into the cathepsin L group by phylogenetic analysis. Real-time PCR (qPCR) analysis indicated that Mn-CL2 was highly expressed in the hepatopancreas and ovaries of female prawns. During the different ovarian stages, Mn-CL2 expression in the hepatopancreas and ovaries peaked before ovarian maturation. In situ hybridization studies revealed that Mn-CL2 was localized in the oocyte of the ovary. Injection of Mn-CL2 dsRNA significantly reduced the expression of vitellogenin. Changes in the gonad somatic index also confirmed the inhibitory effects of Mn-CL2 dsRNA on ovary maturation. These results suggest that Mn-CL2 has a key role in promoting ovary maturation.

A proteomics analysis of the ovarian development in females of Haemaphysalis longicornis

Haemaphysalis longicornis is an ixodid tick that can spread a wide variety of pathogens, affecting humans, livestock and wildlife health. The high reproductive capability of this species is initiated by the ingestion of a large amount of blood ingested by the engorged female tick. The degree of ovarian development is proportional to the number of eggs laid. Studying the regulatory mechanism of tick ovary development is relevant for the development of novel tick control methods. In this study, we used quantitative proteomics to study the dynamic changes in protein expression and protein phosphorylation during ovarian development of engorged female H. longicornis ticks. Synergistic action of many proteins (n = 3031) is required to achieve ovarian development and oocyte formation rapidly. Through bioinformatics analysis, changes in protein expressions and phosphorylation modifications in regulating the ovarian development of female ticks are described. Many proteins play an essential role during ovarian development. Also, protein phosphorylation appeared an important reproductive strategy to enable ticks to efficiently convert large amounts of blood in the ovaries into egg-producing components and ultimately produce many eggs.

Multiple vitellogenin genes (vtgs) in large yellow croaker (Larimichthys crocea): molecular characterization and expression pattern analysis during ovarian development

The large yellow croaker (Larimichthys crocea) is a marine fish that is economically important to Chinese fisheries, and its reproductive and developmental biology have been extensively investigated. However, the molecular mechanism of oogenesis in L. crocea is not clear. Here, we investigated the multiple vitellogenin (Vtg) system in large yellow croaker. Three different vtg cDNA sequences, including vtgAa, vtgAb and vtgC, were cloned, which indicate the existence of multiple Vtg proteins in large yellow croaker (Lc-Vtgs). Subsequently, the vtg cDNA sequences and predicted Vtg protein structures were analysed, and Vtg protein structures were found to be highly conserved. To research the expression of vtgs during the development of the ovaries, we examined ovarian development and oogenesis by histological analysis. Four stages of ovary development - stages II, III, IV and V - were observed and their boundaries were defined. Soon afterwards, the expression of vtgs in the liver (known as the main site of Vtg synthesis in teleosts) and ovary were analysed. The expression of vtgs was detected in the two tissues. Interestingly, in the early stages of development (stages II and III), there is little or no generation of yolk granules and the expression of vtgs in the liver is low. However, in the late stages (stages IV and V), yolk granules are generated rapidly and the expression of vtgs is significantly increased in the liver. These results support the hypothesis that the Vtgs were synthetized by the liver, and absorbed by the growing oocytes to promote oogenesis in large yellow croaker. We also detected the presence of vtg mRNA in the liver cells and oocytes by in situ hybridization, which indicated that vths were expressed both in the liver and ovaries. Importantly, we found that the distribution of vtgAa and vtgAb mRNA was close to the sites of yolk granule formation in oocytes.

Structures and developmental alterations of N-glycans of zebrafish embryos

Zebrafish is a model organism suitable for studying vertebrate development. We analyzed the N-glycan structures of zebrafish embryos and their alterations during zebrafish embryogenesis to obtain basic data for studying the roles of N-glycosylation. Multiple modes of high-performance liquid chromatography and multistage mass spectrometry were used for structural analysis of N-glycans. The N-glycans from deyolked embryos at 36 hours postfertilization, a mid-pharyngula stage, contained relatively higher amounts of complex- and hybrid-type glycans with LacNAc (Galβ1-4GlcNAc) and/or sialyl LacNAc without additional β1,4-Gal, which are commonly found in mammalian tissues, as well as abundant oligomannose-type glycans. Some of the complex- and hybrid-type glycans possessed various extended LacNAc structures, such as Galβ1-4LacNAc, LacNAc-repeat or unique (+/- dHex)-GalNAcα1-GlcNAcβ1-LacNAc. In contrast, the yolk of the embryo contains predominant oligomannose-type glycans and complex-type glycans with Galβ1-4(Siaα2-3)Galβ1-4(Fucα1-3)GlcNAc antennae. N-Glycan profiles obtained from deyolked embryos at different stages showed stage-dependent variation of complex- and hybrid-type glycans. At gastrula and early segmentation stages, complex- and hybrid-type glycans were minor components, and their antenna structures were mainly sialyl LacdiNAc (Siaα2-6GalNAcβ1-4GlcNAc). From the mid-segmentation to pharyngula stages, those with LacNAc and/or α2,6-sialyl LacNAc antenna structures increased remarkably, and those with α2,3-sialyl LacNAc antenna, core α1,6-Fuc and bisecting GlcNAc modifications increased gradually. These results suggest the presence of mechanisms for regulating the antenna structures of complex/hybrid N-glycan biosynthesis in the phylotypic stage of vertebrate development.

Identification of sea bass pIgR shows its interaction with vitellogenin inducing antibody-like activities in HEK 293T cells

Fish vitellogenin (Vg) has been shown to mediate the phagocytosis via interaction with a Fcγ-like phagocytic receptor on macrophages, but identification of such a receptor and its functional characterization remains lacking. In this study, we isolated a cDNA of polymeric immunoglobulin receptor (pIgR) from sea bass, which encoded a single-spanning transmembrane protein of 326 amino acids including a 21-amino acid signal peptide, an extracellular region, a transmembrane region and a 36-amino acid intracellular region included two Ig-like domains (ILDs), and was expressed in multiple lymphoid organs. We then showed that recombinant extracellular domain of sea bass pIgR was capable of binding to Vg as well as IgG and IgM. We also showed that Vg as well as IgG and IgM interacted with pIgR-expressing HEK 293T cells. Importantly, we demonstrated that Vg as well as IgG and IgM were all capable of enhancing phagocytosis by HEK 293T cells and inducing expression of tnf-α and il-1β, via interacting with pIgR. Collectively, these results suggest that fish Vg, analogous to IgG and IgM, can interact with pIgR and result in similar down-stream immune responses, providing an additional evidence that Vg plays an antibody-like role.

Immune-Relevant and Antioxidant Activities of Vitellogenin and Yolk Proteins in Fish

Vitellogenin (Vtg), the major egg yolk precursor protein, is traditionally thought to provide protein- and lipid-rich nutrients for developing embryos and larvae. However, the roles of Vtg as well as its derived yolk proteins lipovitellin (Lv) and phosvitin (Pv) extend beyond nutritional functions. Accumulating data have demonstrated that Vtg, Lv and Pv participate in host innate immune defense with multifaceted functions. They can all act as multivalent pattern recognition receptors capable of identifying invading microbes. Vtg and Pv can also act as immune effectors capable of killing bacteria and virus. Moreover, Vtg and Lv are shown to possess phagocytosis-promoting activity as opsonins. In addition to these immune-relevant functions, Vtg and Pv are found to have antioxidant activity, which is able to protect the host from oxidant stress. These non-nutritional functions clearly deepen our understanding of the physiological roles of the molecules, and at the same time, provide a sound basis for potential application of the molecules in human health.

Immune-Relevant and Antioxidant Activities of Vitellogenin and Yolk Proteins in Fish

Vitellogenin (Vtg), the major egg yolk precursor protein, is traditionally thought to provide protein- and lipid-rich nutrients for developing embryos and larvae. However, the roles of Vtg as well as its derived yolk proteins lipovitellin (Lv) and phosvitin (Pv) extend beyond nutritional functions. Accumulating data have demonstrated that Vtg, Lv and Pv participate in host innate immune defense with multifaceted functions. They can all act as multivalent pattern recognition receptors capable of identifying invading microbes. Vtg and Pv can also act as immune effectors capable of killing bacteria and virus. Moreover, Vtg and Lv are shown to possess phagocytosis-promoting activity as opsonins. In addition to these immune-relevant functions, Vtg and Pv are found to have antioxidant activity, which is able to protect the host from oxidant stress. These non-nutritional functions clearly deepen our understanding of the physiological roles of the molecules, and at the same time, provide a sound basis for potential application of the molecules in human health.

Vitellogenin is an immunocompetent molecule for mother and offspring in fish

Our understanding of the function of vitellogenin (Vg) in reproduction has undergone a transformation over the past decade in parallel with new insights into the role of Vg in immunity. Initially, Vg was regarded as a female-specific reproductive protein, which is cleaved into yolk proteins such as phosvitin (Pv) and lipovitellin (Lv), stored in egg, providing the nutrients for developing embryos. Recently, Vg is shown to be an immune-relevant molecule involved in the defense of the host against the microbes including bacterium and virus. Furthermore, Pv and Lv, that both are proteolytically cleaved products of Vg, play a defense role in developing embryos. Importantly, yolk protein-derived small peptides also display antimicrobial activity. These data together indicate that Vg, in addition to being involved in yolk protein formation, plays a non-reproductive role via functioning as an immune-relevant molecule in both parent fishes and their offspring. It also shows that yolk proteins and their degraded peptides are novel players in maternal immunity, opening a new avenue to study the functions of reproductive proteins.

Deep proteomics of the Xenopus laevis egg using an mRNA-derived reference database

BACKGROUND

Mass spectrometry-based proteomics enables the global identification and quantification of proteins and their posttranslational modifications in complex biological samples. However, proteomic analysis requires a complete and accurate reference set of proteins and is therefore largely restricted to model organisms with sequenced genomes.

RESULTS

Here, we demonstrate the feasibility of deep genome-free proteomics by using a reference proteome derived from heterogeneous mRNA data. We identify more than 11,000 proteins with 99% confidence from the unfertilized Xenopus laevis egg and estimate protein abundance with approximately 2-fold precision. Our reference database outperforms the provisional gene models based on genomic DNA sequencing and references generated by other methods. Surprisingly, we find that many proteins in the egg lack mRNA support and that many of these proteins are found in blood or liver, suggesting that they are taken up from the blood plasma, together with yolk, during oocyte growth and maturation, potentially contributing to early embryogenesis.

CONCLUSION

To facilitate proteomics in nonmodel organisms, we make our platform available as an online resource that converts heterogeneous mRNA data into a protein reference set. Thus, we demonstrate the feasibility and power of genome-free proteomics while shedding new light on embryogenesis in vertebrates.

Selective yolk deposition and mannose phosphorylation of lysosomal glycosidases in zebrafish

The regulation and function of lysosomal hydrolases during yolk consumption and embryogenesis in zebrafish are poorly understood. In an effort to better define the lysosomal biochemistry of this organism, we analyzed the developmental expression, biochemical properties, and function of several glycosidases in zebrafish eggs, embryos, and adult tissues. Our results demonstrated that the specific activity of most enzymes increases during embryogenesis, likely reflecting a greater need for turnover within the embryo as yolk-derived nutrients are depleted. Analysis of glycosidase activity in zebrafish and medaka eggs revealed selective deposition of enzymes required for the degradation of N-linked glycans, including an abundance of acidic mannosidases. Treatment of zebrafish embryos with the α-mannosidase inhibitor swainsonine resulted in the accumulation of glycosylated vitellogenin fragments and demonstrated a function for maternally deposited acid α-mannosidase in yolk consumption. Surprisingly, we also found that, unlike mammals, acid α-glucosidase from zebrafish and medaka does not appear to be modified with mannose 6-phosphate residues. We further showed these residues were not acquired on human acid α-glucosidase when expressed in zebrafish embryos, suggesting unique differences in the ability of the human and zebrafish N-acetylglucosamine-1-phosphotransferase to recognize and modify certain lysosomal glycosidases. Together, these results provide novel insight into the role of acidic glycosidases during yolk utilization and the evolution of the mannose 6-phosphate targeting system in vertebrates.

Accumulation of yolk in a caecilian (Gegeneophis ramaswamii) oocyte: a light and transmission electron microscopic study

There is a paucity of information on the female reproductive biology of the caecilian amphibians when compared with the other vertebrate groups. Hence, the accumulation of nutrient reserves in the form of yolk and formation of yolk platelets were studied in Gegeneophis ramaswamii, adopting light microscopic histological and transmission electron microscopy analysis. Previtellogenic as well as vitellogenic follicles were observed in appropriate preparations. On the basis of the source and the routes of entry, we identified four types of yolk precursor materials, precursors 1 to 4. The earliest material appearing in the oocyte consists of abundant lipid vesicles during the previtellogenic phase, i.e., much before the follicular epithelium is fully established. This is a contribution from the oocyte mitochondria, which we identified as yolk precursor material 1, and it is autosynthetic. Once the follicle cell-oocyte interface is fully established, there is an accumulation of the principal component of the heterosynthetic yolk by sequestration from the blood through the intercellular spaces between follicle cells in a pinocytic process. This we identified as yolk precursor material 2. There was also an indication of a lipidic yolk material synthesis in the follicle cells sequestered from maternal blood through the follicle cells in an endocytic process in which the macrovilli of follicle cells and the microvilli of the oocyte play a role. This we identified as yolk precursor material 3. Contribution to the yolk of peptidic, glycosidic, and/or lipidic material synthesized in the vitellogenic oocyte was also indicated. This we identified as yolk precursor material 4. The sequential development of intercellular associations and indications of synthesis/sequestration of the yolk have been traced. Thus, we report the mechanistic details of synthesis/sequestration of the yolk materials in a caecilian.

Changes in cathepsin gene expression and relative enzymatic activity during gilthead sea bream oogenesis

The aim of this study was to provide evidence on the modulation of lysosomal enzymes in terms of both gene expression and enzymatic activity during follicle maturation. For this purpose three lysosomal enzymes, cathepsins B, D, and L, were studied in relation to yolk formation and degradation, during the main phases of ovarian follicle growth in the pelagophil species, the sea bream Sparus aurata. Specific attention was focused on the gene expression quantification method, on the assay of enzymatic activities, and on the relationship between the proteolytic cleavage of yolk proteins (YPs), cathepsin gene expression and cathepsin activities. For the gene expression study, the cathepsins B-like and L-like mRNAs were isolated and partially or fully characterized, respectively; the sequences were used as design specific primers for the quantification of cathepsin gene expression by real-time PCR, in follicles at different stages of maturation. The enzymatic assays for cathepsins B, D, and L were optimized in terms of specificity, sensitivity and reliability, using specific substrates and inhibitors. In ovulated eggs, the lipovitellin I (LV I) was degraded and the changes in electrophoretic pattern were preceded by an increase in the activity of a cysteine proteinase, cathepsin L, and its mRNA. Cathepsin B did not appear to be involved in YP changes during the final maturation stage.

Phylogenetic relationships and gene expression pattern of three different cathepsin L (Ctsl) isoforms in zebrafish: Ctsla is the putative yolk processing enzyme

Certain cysteine proteases, such as cathepsin L (Ctsl), have been involved in yolk processing mechanisms in oocytes and embryos of lower vertebrates. In zebrafish (Danio rerio), three different ctsl genes, ctsla, ctslb and ctslc, have been found in the genome, but their pattern of expression, as well as information on which the encoded enzymes are potentially involved in yolk absorption during embryogenesis, is unknown. Here, phylogenetic and gene structure analysis revealed that zebrafish ctsla and ctslb genes are similar, showing a highly conserved structure in comparison with human ctsl, while ctslc presents different exon organization together with an earlier evolution. Thus, ctslc appears to be evolved from a common ancestral ctsl-like gene, possibly through an early duplication event, whereas ctsla and ctslb may be originated from a second duplication mechanism. Zebrafish ctsla, ctslb and ctslc also showed different patterns of mRNA expression during embryogenesis and in adult tissues. While Ctsla transcripts were accumulated in embryos throughout development and in the adult ovary, those encoding Ctslb were detected only in embryos around the time of hatching as previously reported, and those for Ctslc appeared only in larvae and in some adult tissues, but not in the ovary. In zebrafish and killifish (Fundulus heteroclitus) embryos, Ctsla mRNA was first detected in blastomers, and later in development it was localized in cells of the yolk syncytial layer, an embryonic structure involved in yolk absorption. These data therefore suggested that Ctsla is most likely the putative protease involved in yolk processing in fish embryos, while Ctslc seems not to be required during early embryogenesis in zebrafish.

Oogenesis: Single cell development and differentiation

Oocytes express a unique set of genes that are essential for their growth, for meiotic recombination and division, for storage of nutrients, and for fertilization. We have utilized the newly sequenced genome of Strongylocentrotus purpuratus to identify genes that help the oocyte accomplish each of these tasks. This study emphasizes four classes of genes that are specialized for oocyte function: (1) Transcription factors: many of these factors are not significantly expressed in embryos, but are shared by other adult tissues, namely the ovary, testis, and gut. (2) Meiosis: A full set of meiotic genes is present in the sea urchin, including those involved in cohesion, in synaptonemal complex formation, and in meiotic recombination. (3) Yolk uptake and storage: Nutrient storage for use during early embryogenesis is essential to oocyte function in most animals; the sea urchin accomplishes this task by using the major yolk protein and a family of accessory proteins called YP30. Comparison of the YP30 family members across their conserved, tandem fasciclin domains with their intervening introns reveals an incongruence in the evolution of its major clades. (4) Fertilization: This set of genes includes many of the cell surface proteins involved in sperm interaction and in the physical block to polyspermy. The majority of these genes are active only in oocytes, and in many cases, their anatomy reflects the tandem repeating interaction domains essential for the function of these proteins. Together, the expression profile of these four gene classes highlights the transitions of the oocyte from a stem cell precursor, through stages of development, to the clearing and re-programming of gene expression necessary to transition from oocyte, to egg, to embryo.

The Caenorhabditis elegans CPI-2a cystatin-like inhibitor has an essential regulatory role during oogenesis and fertilization

In the present study, we characterized a sterile cpi-2a(ok1256) deletion mutant in Caenorhabditis elegans and showed that CPI-2a has an essential regulatory role during oogenesis and fertilization. We have also shown that the CPI2a inhibitor and both Ce-CPL-1 and Ce-CPZ-1 enzymes are present in the myoepithelial sheath surrounding germ cells, oocytes, and embryos as well as in the yolk granules within normal oocytes. Staining of mutant worms with anti-yolk protein antibodies has indicted that the proteins are not present in the mature oocytes. Moreover, green fluorescent protein expression was absence or reduced in cpi-2a/yp170:gfp mutant oocytes, although it was expressed in one of the successfully developed embryos. Based on these results, we hypothesize that the sterility in cpi-2a(ok1256) mutant worms is potentially caused by two possible mechanisms: 1) defects in the uptake and/or processing of yolk proteins by the growing oocytes and 2) indirect induction of defects in cell-cell signaling that is critical for promoting germ line development, oocyte maturation, ovulation, and fertilization. A defect in any of these processes would have detrimental effects on the development of normal embryos and consequently normal production of progenies as we observed in cpi-2a mutant worms. This is the first study that demonstrates the expression of cysteine proteases and their endogenous inhibitor in the gonadal sheath cells surrounding germ cells and oocytes, which indirectly have established their potential involvement in proteolytic processing of molecules within the gonadal sheath cells, such as components of the extracellular matrix or the cytoskeletal proteins, which are essential for proper cell-cell signaling activities of the gonadal sheath cells during normal maturation and ovulation processes.

Role of cathepsins in ovarian follicle growth and maturation

Several complex processes are involved in the production of viable eggs. The aim of this review is to provide an overview on the role played by lysosomal enzymes, especially cathepsins B, D, and L, during ovarian follicle growth and maturation. Specific attention is focused on the relationship between the second proteolytic cleavage of yolk proteins (YP) and the resumption of the meiosis during germinal vesicle break down (GVBD). Maturation represents the final stage of oocytes development prior to ovulation. Oocytes in this phase appear translucent. In many teleosts GVBD is accompanied by water uptake and among marine teleosts with pelagic eggs, most of the final volume is reached by this process. The last phase of maturation in benthonic eggs also occurs concomitant to a second proteolytic cleavage and is related with a slight hydration process. In vitro maturation by 17alpha,20beta-dihydroxy-4-pregnen-3one in class III Danio rerio oocytes, induced 80% of GVBD. The maturation of these oocytes is known to be associated with proteolysis of their major yolk components. In the present study, we show that inhibition of specific enzymes (cathepsins) involved in the second YP processing, did not affect the occurrence of GVBD as the oocytes become translucent and display a slight increase in size. More specifically, in vitro incubation of the maturing oocytes with a cathepsin B inhibitor suppressed both cathepsin B and L activities and the proteolysis of YP. On the contrary, the addition of cathepsin L inhibitor, only affected cathepsin L activity, indicating that cathepsin B is probably involved in Cathepsin L activation, and this enzyme is probably responsible for the second YP processing. These results, together with previous studies, indicate that the GVBD process is independent of the occurrence of the second proteolytic process. It supports the hypothesis that the maturation process is under K+ ion flux control, while yolk proteolysis is related to the temporal and specific activation of cathepsins by acidification of yolk spheres.

Multiple vitellogenins (Vgs) in mosquitofish (Gambusia affinis): identification and characterization of three functional Vg genes and their circulating and yolk protein products

The objectives of this study were to characterize multiple forms of vitellogenin (Vg) in mosquitofish (Gambusia affinis) and to discover the fate of each Vg during its processing into product yolk proteins. Two Vg preparations, with apparent masses of 600 kDa (600 Vg) and 400 kDa (400 Vg), were isolated from the plasma of fish treated with estradiol-17beta (E(2)) by various chromatographic procedures. Immunological analyses verified the presence of two different Vg proteins (600 VgA and 600 VgB) in the 600 Vg preparation and of a single protein in the 400 Vg preparation. Three major yolk proteins (Yps) with apparent masses of 560, 400, and 28 kDa were observed in extracts of ovarian follicles from vitellogenic females. Immunological analyses demonstrated that the 400 Vg underwent no change in native mass after being incorporated into oocytes. The 600 Vgs gave rise to a 28 kDa beta'-component and a native 560 kDa Yp, which was heterodimeric in structure, consisting of two types of complexes between phosvitin (Pv) and lipovitellin (Lv) heavy- and light-chains. Full-length cDNAs encoding the 600 VgA, 600 VgB, and 400 Vg were isolated from a liver cDNA library of E(2) treated fish. Similar to the zebrafish vg3 gene, the 400 Vg cDNA lacked a Pv domain and was classified as an incomplete or phosvitinless (C-type) Vg. The deduced primary structures of 600 VgA and 600 VgB were complete, and these were categorized as type A and type B Vgs, respectively, according to our recent classification scheme. This is the first report on the characterization of three functional Vg genes and their circulating and yolk protein products in any vertebrate species.

Cathepsin L protease (CPL-1) is essential for yolk processing during embryogenesis in Caenorhabditis elegans

Cysteine proteases are involved in the degradation of intracellular and extracellular proteins, although their precise roles in vivo are not well understood. Here we characterise a genetic mutant of the Caenorhabditis elegans cathepsin L protease gene cpl-1. CPL-1 is provided maternally and is essential for C. elegans embryogenesis. Immunofluorescence and electron microscopy data show that yolk endocytosis and initial yolk platelet formation occur normally in cpl-1 mutant oocytes and embryos. However, at around the 8-12 cell stage of embryogenesis, yolk platelets begin to aggregate and these enlarged yolk platelets fill the cytoplasm of cpl-1 mutant embryos. Coincident with this aggregation is loss of fluorescence from a yolk green fluorescent protein (YP170::GFP). This suggests that loss of CPL-1 activity leads to aberrant processing and/or conformational changes in yolk proteins, resulting in abnormal platelet fusion. This study has relevance to the abnormal fusion and aggregation of lysosomes in cathepsin L-deficient mice and to other lysosomal disorders.

Ovarian cysteine proteinases in the teleost Fundulus heteroclitus: molecular cloning and gene expression during vitellogenesis and oocyte maturation

The cysteine proteinases cathepsins B and L are members of the multigene family of lysosomal proteases that have been implicated in the processing of yolk proteins (YPs) in teleost oocytes. However, the full identification of the type of cathepsins expressed in fish ovarian follicles and embryos, as well as their regulatory mechanisms and specific function(s), are not yet elucidated. In this study, cDNAs encoding cathepsins B, L, F, K, S, Z, C, and H have been isolated from the teleost Fundulus heteroclitus, and the analysis of their deduced amino acid sequences revealed highly similar structural features to vertebrate orthologs, and confirmed in this species the existence of cathepsin L-like, cathepsin B-like, and cathepsin F-like subfamilies of cysteine proteinases. While all identified cathepsins were expressed in ovarian follicles, the corresponding mRNAs showed different temporal expression patterns. Thus, similar mRNA levels of cathepsins L, F, S, B, C, and Z were found throughout the oocyte growth or vitellogenesis period, whereas those for cathepsin H and K appeared to decrease as vitellogenesis advanced. During oocyte maturation, a transient accumulation of cathepsins L, S, H, and F mRNAs, approximately a 3-, 1.5-, 1.6-, and 6-fold increase, respectively, was detected in ovarian follicles within the 20-25 hr after hormone stimulation, coincident with the maximum proteolysis of the oocyte major YPs. The specific temporal pattern of expression of these genes may indicate a potential role of cathepsin L-like and cathepsin F proteases in the YP processing events occurring during fish oocyte maturation and/or early embryogenesis.

Multihormonal control of vitellogenesis in lower vertebrates

The comparative approach on how and when vitellogenesis occurs in the diverse reproductive strategies displayed by aquatic and terrestrial lower vertebrates is presented in this chapter; moreover, attention has been paid to the multihormonal control of hepatic vitellogenin synthesis as it is related to seasonal changes and to vitellogenin use by growing oocytes. The hormonal mechanisms regulating vitellogenin synthesis are also considered, and the effects of environmental estrogens on the feminization process in wildlife and humans have been reported. It is then considered how fundamental nonmammalian models appear to be, for vitellogenesis research, addressed to clarifying the yolkless egg and the evolution of eutherian viviparity.

Bafilomycin A1 inhibits proteolytic cleavage and hydration but not yolk crystal disassembly or meiosis during maturation of sea bass oocytes

Oocytes of the black sea bass, Centropristes striata, were enlarged in volume more than three-fold over a 24-hr period during oocyte maturation, both in vivo and in vitro. At the same time, the opaque oocytes clarified while the crystalline yolk inclusions lost their ordered structure, fused with one another, and formed a continuous electron-lucent mass. The oocyte size increase was due almost entirely to water uptake, which was accompanied by the accumulation of Na+, K+, and free amino acids (FAAs). The absolute amounts of each of these small molecular weight osmotic effectors increased 2x, 4x, and over 10x, respectively, indicating that the generation of FAAs is the major cause of water uptake during maturation. Amino acid analyses indicated that the amounts of all amino acids except taurine increased, so that selective amino acids were not produced during maturation. The increase in FAAs was accompanied by the loss of certain high-molecular-weight yolk proteins and the generation of many smaller peptides. Oocytes stimulated to undergo maturation in the presence of bafilomycin A1, a specific inhibitor of the vacuolar ATPase-dependent proton pump, clarified and underwent maturation but did not increase significantly in size. Cytological examination revealed that yolk crystals fused and became homogeneous but maintained their electron density. No evidence of proteolysis was found in bafilomycin A1-treated oocytes and the generation of FAAs together with hydration was inhibited in a dose-dependent manner (I50 = 3 nM bafilomycin A1). Taken together, we postulate that the pronounced oocyte hydration in marine teleosts that spawn pelagic (floating) eggs is accomplished by a two-step process whereby (i) K+ influx promotes yolk crystal disassembly and yolk sphere fusion and (ii) acidification of the yolk spheres activates yolk proteolysis and concomitant hydration. Bafilomycin A1 inhibits only the second step so that many of the events of oocyte maturation, including germinal vesicle breakdown, occur in its presence but oocyte hydration is suppressed.

Degradation of yolk platelets in the early amphibian embryo is regulated by fusion with late endosomes

The eggs of many animal species contain a large store of yolk platelets, lipid droplets and glycogen granules; these are consumed during early embryogenesis. However, the mechanisms by which degradation of these stored materials occurs during early embryogenesis are not clearly understood. The mechanisms underlying yolk degradation in amphibian (newt) embryos were investigated. Electron microscopy using an anion marker, cationic ferritin, revealed that yolk platelets were degraded after fusion with late endosomes containing primary lysosomes. Electron microscopy and the results of experiments using a number of reagents with selective effects on intracellular transport suggested that yolk degradation activity in early amphibian embryos may be regulated at the point of fusion between late endosomes and yolk platelets.

Isolation, characterization and molecular cloning of cathepsin D from lizard ovary: changes in enzyme activity and mRNA expression throughout ovarian cycle

During vitellogenesis, the oocytes of oviparous species accumulate in the cytoplasm a large amount of proteic nutrients synthetized in the liver. Once incorporated into the oocytes, these nutrients, especially represented by vitellogenin (VTG) and very low-density lipoprotein (VLDL), are cleaved into a characteristic set of polypeptides forming yolk platelets. We have studied the molecular mechanisms involved in yolk formation in a reptilian species Podarcis sicula, a lizard characterized by a seasonal reproductive cycle. Our results demonstrate the existence in the lizard ovary of an aspartic proteinase having a maximal activity at acidic pH and a molecular mass of 40 kDa. The full-length aspartic proteinase cDNA produced from total RNA by RT-PCR is 1,442 base pairs long and encodes a protein of 403 amino acids. A comparison of the proteic sequence with aspartic proteinases from various sources demonstrates that the lizard enzyme is a cathepsin D. Lizard ovarian cathepsin D activity is maximal in June, in coincidence with vitellogenesis and ovulation, and is especially abundant in vitellogenic follicles and in eggs. Ovarian cathepsin D activity can be enhanced during the resting period by treatment with FSH in vivo. Northern blot analysis shows that cathepsin D mRNA is exceedingly abundant during the reproductive period, and accumulates preferentially in previtellogenic oocytes.

Yolk formation and degradation during oocyte maturation in seabream Sparus aurata: involvement of two lysosomal proteinases

Oocyte growth within the follicle is preponderantly due to the accumulation of hepatically derived yolk protein (vitellogenin, VTG) by receptor-mediated endocytosis; once in the oocyte, VTG is partially processed and stored in yolk globules. In some pelagic egg-laying marine teleosts, additional cleavages of yolk proteins followed by a pronounced water uptake occur concomitantly with final oocyte maturation. The aim of this study was to establish the lysosomal enzymes involved in these two proteolytic processes that characterize oocyte maturation of seabream Sparus aurata. The enzymatic activities of several cathepsins were assessed in the various classes of oocytes. Changes in cathepsin B, D, and L activity were found depending on the oocyte maturation stage; cathepsin B and D were found to be at maximum level in early-vitellogenesis oocytes, and cathepsin L in mid-vitellogenesis ones. Cathepsin D and L were purified from seabream ovary, and their roles in VTG and lipovitellin (LV) proteolysis, respectively, were analyzed. Here we demonstrate directly that one of the catalysts for the intraoocytic processing of VTG in yolk proteins is cathepsin D; however, we cannot exclude also a role of cathepsin B in the same process. On the other hand, cathepsin L is responsible for the second proteolytic cleavage of the LV components. We postulate that the acquisition of buoyancy by eggs through the hydration process may be regulated by enzymatic activation at the appropriate time of oocyte maturation, this process probably being the key event in the reproduction of this marine pelagic egg spawner.

Vitellogenin is glycosylated in the fat body of the stick insect Carausius morosus and not further modified upon transfer to the ovarian follicle

Synthesis and secretion of vitellogenin (Vg) polypeptides were studied in egg-laying females of the stick insect Carausius morosus following in vivo exposure to [35S]-methionine and acetyl-N-[3H]-glucosamine. The specificity of radioisotope incorporation was assessed by in vitro inhibition with tunicamycin and carbohydrate extraction with endo-glycosidase H. Vg polypeptides change in molecular weight during synthesis in the fat body and are not further modified upon transfer to the haemolymph or to the oocyte, suggesting that they are already fully glycosylated prior to secretion. Radioactivity in the fat body was initially distributed over cisternae of the rough endoplasmic reticulum and gradually transferred to the Golgi apparatus. Within an hour of exposure, electron-dense granules budding from the trans-Golgi network became preferentially labeled. Radioactivity in the ovarian follicle was restricted to the yolk granules of the cortical ooplasm and to the amorphous material lying within the intercellular channels of the follicular epithelium. This amorphous material was also shown to react positively when tested with a monoclonal antibody raised specifically against a Vg polypeptide.

Active nuclear transport of chicken lipovitellin-2

Chicken lipovitellin-2 is a small, approximately 30 kDa yolk protein, derived from the intracellular breakdown of the precursor protein vitellogenin. In principle, lipovitellin-2 is small enough to directly diffuse into the nucleus. Our results, however, demonstrate that its nuclear transport is an active process which can be inhibited by wheat germ agglutinin, chilling, and energy depletion. The N-terminal sequence analysis identifies chicken lipovitellin-2 beginning at Ala1544 in the C-terminal region of vitellogenin yielding of protein of 30,982 Da.

Rab7 regulates transport from early to late endocytic compartments in Xenopus oocytes

Rab7 has been shown to localize to late endosomes and to mediate transport from early to late endosome/lysosome in mammalian cells and in yeast. We developed a novel assay to quantify transport from early to late endosomes using the Xenopus oocyte. Oocytes were pulsed with avidin after which the oocytes were incubated to allow avidin transport to a late compartment. The oocytes were then allowed to internalize biotin-horseradish peroxidase (HRP). The oocytes were then injected with test proteins and incubated further to allow transport of biotin-HRP from early endosomes to late endosomal/lysosomal compartments. Transport was quantified by assessing the formation of HRP-biotin-avidin complexes. Injection of Rab7:wild-type (WT) and Rab7:Q67L, a GTPase defective mutant, stimulated transport. Rab5:WT had no effect. Rab7:WT-stimulated transport was inhibited by nocodazole, suggesting a role for intact microtubules. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, blocked Rab7:WT-stimulated transport, but Rab7:Q67L-stimulated transport was unaffected by the drug. Rab7:Q67L is constitutively activated and may not require phosphatidylinositol 3-kinase activity for activation. Rab7-stimulated transport requires N-ethylmaleimide-sensitive factor (NSF) activity as transport was blocked by N-ethylmaleimide and ATPase defective NSF mutants. Our results indicate that sequentially acting endocytic Rab GTPases utilize similar factors although their modes of action may be different.

Sequential actions of Rab5 and Rab7 regulate endocytosis in the Xenopus oocyte

To explore the role of GTPases in endocytosis, we developed an assay using Xenopus oocytes injected with recombinant proteins to follow the uptake of the fluid phase marker HRP. HRP uptake was inhibited in cells injected with GTPgammaS or incubated with aluminum fluoride, suggesting a general role for GTPases in endocytosis. Injection of Rab5 into oocytes, as well as Rab5:Q79L, a mutant with decreased GTPase activity, increased HRP uptake. Injection of Rab5:S34N, the dominant-negative mutant, inhibited HRP uptake. Injection of N-ethylmaleimide-sensitive factor (NSF) stimulated HRP uptake, and ATPase-defective NSF mutants inhibited HRP uptake when coinjected with Rab5:Q79L, confirming a requirement for NSF in endocytosis. Surprisingly, injection of Rab7:WT stimulated both uptake and degradation/activation of HRP. The latter appears to be due to enhanced transport to a late endosomal/prelysosomal degradative compartment that is monensin sensitive. Enhancement of uptake by Rab7 appears to function via an Rab5-sensitive pathway in oocytes since the stimulatory effect of Rab7 was blocked by coinjection of Rab5:S34N. Stimulation of uptake by Rab5 was blocked by Rab5:S34N but not by Rab7:T22N. Our results suggest that Rab7, while functioning downstream of Rab5, may be rate limiting for endocytosis in oocytes.

Native vitellins are modified during ovarian development in the stick insect Carausius morosus (Br.)

Vitellins from ovarian follicles and newly laid eggs of the stick insect Carausius morosus were examined by ion exchange chromatography on a HPLC Mono Q column. Under these conditions, vitellins from newly laid eggs resolved as two distinct peaks, referred to as VtA and VtB, that eluted at 8.5 and 12.0 min, respectively. On native gels, both VtA and VtB separated into two different variant forms (VtA' and VtA", VtB' and VtB"). By two-dimensional gel electrophoresis, VtA' and VtA" were shown to contain polypeptides A1, A2 and A3. On the other hand, VtB' and VtB" appeared to comprise polypeptides B1 and B2 and B1, A1, A2, B2 and A3*, respectively. A similar Vt polypeptide composition was also observed by size-exclusion chromatography of vitellins from newly laid eggs. Vitellins from early vitellogenic ovarian follicles resolved into a single chromatographic peak at 7.5 min that coeluted with a major peak from the hemolymph of egg-laying females. Ovarian follicles progressively more advanced in development exhibited a more complex chromatographic profile, consisting of three separate peaks. By two-dimensional gel immunoelectrophoresis, vitellins from ovarian follicles appeared to consist of two closely related, immunologically cross-reacting antigens that gradually shifted apart as ovarian development proceeded to completion. By size-exclusion chromatography, each Vt from ovarian follicles was shown to consist of a unique set of polypeptides different from those listed above. Single ovarian follicles were fractionated into yolk granules and yolk fluid ooplasm and tested by immunoblotting against Mab 12. Under these conditions, VtA variant forms in yolk granules and yolk fluid ooplasm reacted differently. Sections from ovarian follicles in different developmental stages were exposed to Mab 12 and stained with a peroxidase-conjugated, goat anti-mouse antibody. Regardless of the developmental stage attained, staining for peroxidase was restricted to free yolk granules, suggesting that native vitellins in stick insects are structurally modified upon fusion into the yolk fluid ooplasm.

Isolation and characterization of a vitellogenin cDNA from rainbow trout (Oncorhynchus mykiss) and the complete sequence of a phosvitin coding segment

Phosvitins are extensively phosphorylated serine-rich proteins that are derived from a large hepatic phosphoglycolipoprotein, vitellogenin, and are deposited, after suitable processing, in the eggs of oviparous vertebrates. Despite their widespread occurrence and apparent importance for early embryonic development, very few phosvitins have been sequenced thus far, including no teleost protein. We used a polymerase chain reaction (PCR)-assisted approach to isolate a vitellogenin cDNA clone, complementary to mRNA synthesized in the liver of estrogen-treated rainbow trout. This clone contains a sequence that corresponds to the composition of a phosvitin previously characterized in our laboratory. The amino terminus was identified by amino acid sequencing of the protein. The carboxyl terminus was inferred from homology with other phosvitin-specific sequences from chicken, Xenopus, and lamprey. Trout phosvitin appears to be a small, 53-residue-long protein displaying the long runs of serines that are characteristic of all phosvitins described so far.

Endosomal proteolysis of internalized proteins

Endosomal proteases have been implicated in the degradation of internalized regulatory peptides involved in the control of metabolic pathways and in the processing of intracellular antigens for cytolytic immune responses. Processing in the endocytic vesicles is regulated by changes in endosomal acidity due to the presence of an ATP-dependent proton pump which modulates protease activity, protein unfolding and receptor-ligand interactions. A limited number of proteases appear to reside in endosomes which do not contain the full complement of active proteases capable of completely degrading all internalized polypeptides. Retention of some acid hydrolases in endosomes is apparently related to their association with undefined endosomal membrane receptors. The limited number of proteases and the pH gradient from neutral to acidic (pH 7 to 5) within endosomes make possible a selective and controlled processing environment in comparison to lysosomes. The full set of endo- and exopeptidases that break down proteins to amino acids are active later in the pathway in lysosomes.

Protease activity appeared after trypsin treatment of the purified vitellogenin from eel Anguilla japonica

Density gradient ultracentrifugation and anion-exchange chromatography combination were effective for the purification of the eel vitellogenin from the plasma of estradiol-treated eels. The vitellogenin was very high density glycolipoprotein (P = 1.27 g/ml) and its apolipoprotein was M(r) 196 k in both reduced and non-reduced conditions by SDS-PAGE. The major lipid component was phospholipid. The N-terminal amino-acid sequence of the vitellogenin was as follows: (Ac)Thr-Pro-Ala-Leu/Ala-Asp-Tyr. Amino-acid composition of the eel vitellogenin was similar to those of other teleosts. The protease activity appeared in the trypsinized vitellogenin, but was not detected in the purified vitellogenin. The protease was separated from the used trypsin and the other cleaved vitellogenin by a dextran sulfate cellulose column. The molecular weight of the protease was determined by zymogram using SDS-polyacrylamide gel containing casein and was 50 k. It was concluded that the eel vitellogenin possesses the protease activity as a latent form.

Precursor-product relationship between chicken vitellogenin and the yolk proteins: the 40 kDa yolk plasma glycoprotein is derived from the C-terminal cysteine-rich domain of vitellogenin II

Chicken vitellogenin, a serum lipoprotein specific for laying hens, has been thought to be proteolytically cleaved into the heavy and light chain lipovitellins and phosvitin, the major yolk granule proteins, during or after transportation into oocyte. In this study, another proteolytic product of vitellogenin has newly been isolated from the 'beta-livetin' fraction of yolk plasma. It is a yolk glycoprotein of 40 kDa (YGP40) with asparagine-linked carbohydrate chain(s) recognized by Concanavalin A and castor bean lectin (RCA-I), and it is identified as a C-terminal cysteine-rich fragment of the major vitellogenin (vitellogenin II), the cysteine-rich domain homologous to D2 region of von Willebrand factor. Another yolk plasma glycoprotein of 42 kDa is suggested to be one of the proteolytic products of the minor vitellogenin (vitellogenin I). Both 40 kDa and 42 kDa glycoproteins were shown to be present in growing oocytes but absent in laying hen's serum. Limited proteolysis of vitellogenin II with cathepsin D produced a 40 kDa protein with reactivity to anti-YGP40 antibody. Gel filtration analysis of vitellogenin II digested with cathepsin D showed that YGP40 dissociated from lipovitellin-phosvitin complex after the proteolytic cleavage. These results suggest that after incorporation from serum via a specific receptor vitellogenin II is cleaved in the oocyte into four fragments, heavy and light chain lipovitellins, phosvitin and YGP40, and that YGP40 is released into the yolk plasma before or during compartmentation of lipovitellin-phosvitin complex into the yolk granule.

Physiological functions of endosomal proteolysis

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Changes in yolk platelet pH during Xenopus laevis development correlate with yolk utilization. A quantitative confocal microscopy study

The variations of the pH in Xenopus yolk platelets have been estimated by fluorescence confocal microscopy and computer image processing. For pH measurements in vitellogenic oocytes, the pH-sensitive fluorescent dye, DM-NERF, was coupled to vitellogenin, and the DM-NERF-vitellogenin was taken up by oocytes via receptor-mediated endocytosis. Dual emission ratio measurements of internalized DM-NERF-vitellogenin indicated that the mature yolk platelets are mildly acidic (pH 5.6). Their precursors, the primordial yolk platelets, have a similar pH. This pH is probably sufficiently low for the partial cleavage of vitellogenin to yolk proteins, but not for yolk degradation. The yolk platelet pH at various developmental stages was estimated by measuring the accumulation of Acridine Orange, both in isolated yolk platelets and in disaggregated embryonic cells. During oogenesis, the yolk platelets accumulated a constant amount of Acridine Orange, corresponding to a pH of around 5.7. During embryogenesis, however, yolk platelets became progressively much more acidic (pH < 5). Acidification correlated with yolk degradation in the various tissues examined, and yolk utilization was blocked when acidification was inhibited with bafilomycin, an inhibitor of vacuolar H+-ATPase. Bafilomycin also inhibited differentiation of cells isolated from stage 13–15 embryos. These data show that the yolk platelet pH is developmentally regulated and is involved in triggering yolk degradation. Also, yolk acidification and degradation appeared to be associated with cell differentiation and with the formation of the endosomal/lysosomal compartment, typical of adult cells, but absent in early embryos.

Chicken oocyte growth: receptor-mediated yolk deposition

During the rapid final stage of growth, chicken oocytes take up massive amounts of plasma components and convert them to yolk. The oocyte expresses a receptor that binds both major yolk lipoprotein precursors, vitellogenin (VTG) and very low density lipoprotein (VLDL). In the present study, in vivo transport tracing methodology, isolation of coated vesicles, ligand- and immuno-blotting, and ultrastructural immunocytochemistry were used for the analysis of receptor-mediated yolk formation. The VTG/VLDL receptor was identified in coated profiles in the oocyte periphery, in isolated coated vesicles, and within vesicular compartments both outside and inside membrane-bounded yolk storage organelles (yolk spheres). VLDL particles colocalized with the receptor, as demonstrated by ultrastructural visualization of VLDL-gold following intravenous administration, as well as by immunocytochemical analysis with antibodies to VLDL. Lipoprotein particles were shown to reach the oocyte surface by passage across the basement membrane, which possibly plays an active and selective role in yolk precursor accessibility to the oocyte surface, and through gaps between the follicular granulosa cells. Following delivery of ligands from the plasma membrane into yolk spheres, proteolytic processing of VTG and VLDL by cathepsin D appears to correlate with segregation of receptors and ligands which enter disparate sub-compartments within the yolk spheres. In small, quiescent oocytes, the VTG/VLDL receptor was localized to the central portion of the cell. At onset of the rapid growth phase, it appears that this pre-existing pool of receptors redistributes to the peripheral region, thereby initiating yolk formation. Such a redistribution mechanism would obliterate the need for de novo synthesis of receptors when the oocyte's energy expenditure is to be utilized for plasma membrane synthesis, establishment and maintenance of intracellular topography and yolk formation, and preparation for ovulation.

Molecular cloning and functional characterization of chicken cathepsin D, a key enzyme for yolk formation

Upon receptor-mediated endocytosis of very-low-density lipoprotein (VLDL) and vitellogenin into growing chicken oocytes, the protein moieties of these lipoproteins are proteolytically cleaved. Unlike the complete lysosomal degradation in somatic cells, enzymatic ligand breakdown in oocytes generates a characteristic set of polypeptides, which enter yolk storage compartments for subsequent utilization by the embryo. Here, we demonstrate directly that the catalyst for the intraoocytic processing of both apolipoprotein B and vitellogenin is cathepsin D. The enzyme was purified from oocytic yolk, preovulatory follicle homogenates, and liver by affinity chromatography. When plasma VLDL and vitellogenin were incubated with the purified enzyme, fragments indistinguishable from those found in yolk were generated from both precursors under identical, mildly acidic conditions. Amino-terminal sequencing of the pure enzyme demonstrated 88% identity with mammalian cathepsin Ds over 34 residues. On the basis of this information, a full-length clone specifying chicken preprocathepsin D was isolated from a chicken follicle cDNA library by screening with a human cathepsin D probe. Whereas previous studies have demonstrated that the receptors for lipoproteins in somatic cells and oocytes, respectively, of the chicken are the products of different genes, Southern and Northern blot hybridization experiments showed that the enzymes expressed in oocytes and liver are the product of a single gene, giving rise to a 3.3-kb transcript. The primary structure of the 335-residue mature protein suggests a high degree of conservation of known crucial features of aspartyl proteases; however, the absence of the so-called processing region and of an aromatic residue in a region thought to partake in catalysis raise questions with possible evolutionary implications.

Expression of the chicken hepatic glycoprotein receptor in Xenopus oocytes: conservation of ligand and receptor targeting signals

We have obtained expression of the beta-N-acetylglucosamine-binding receptor from chicken hepatocytes in Xenopus oocytes by injecting mRNA synthesized in vitro from a full length cDNA cloned into an expression vector (Mellow et al: J. Biol Chem 263: 5468-5473, 1988). Immunoprecipitation of the receptor after labeling of oocytes with [35S]-methionine for times ranging from 6 to 72 h revealed 4-5 closely spaced bands of 25-30 kDa after SDS-PAGE. Although these bands were largely resistant to endoglycosidase H cleavage, endoglycosidase F reduced the size of all bands to a single species at 23-24 kDa, indicating that they resulted from heterogeneity in glycosylation of a single polypeptide. Incubation of oocytes expressing this receptor with [125I]-GlcNAc-BSA resulted in 1.8 to 10 x higher levels of cell-associated ligand in mRNA-injected vs. water-injected control oocytes, 2-35% of cell-associated counts was removed by EGTA rinse at 20 degrees C, suggesting that most ligand was inaccessible (presumably intracellular). Immunoprecipitation of sucrose gradient fractions detected receptor molecules predominantly in a light organelle at 1.09-1.12 g/cc (the density of early endosomes and plasma membrane vesicles), with no evidence of the receptor in much heavier yolk platelet fractions even in the presence of ligand. In contrast, internalized [125I]-GlcNAc-BSA was found either at the top of the gradients or in organelles at 1.09-1.17 g/cc and in yolk platelets. TCA precipitation indicated that much intracellular ligand was degraded to acid-soluble fragments. Addition of vitellogenin (the yolk protein precursor) to the medium together with the [125I]-GlcNAc-BSA shifted much of the ligand into yolk platelets. These data indicate that the chicken glycoprotein receptor expressed in oocytes mediates binding and internalization of this ligand into an organelle in which ligand-receptor dissociation occurs, allowing for separation of these two molecules into different compartments. The behavior of ligand in Xenopus oocytes expressing the chicken receptor closely resembles its behavior in hepatocytes.

Role of lipid transfer particle in transformation of lipophorin in insect oocytes

Oocyte development in Manduca sexta involves the deposition of large amounts of lipoprotein-derived lipid. One source of this lipid is hemolymph high-density lipophorin-adult (HDLp-A) which, upon entry into the oocyte, is transformed into a lipid and apolipophorin III deficient product particle, egg very-high- density lipophorin (VHDLp-E; density = 1.24 g/ml; Kawooya et al. (1988) J. Biol. Chem. 263, 8740-8747). An in vitro model of this transformation has been established using human low-density lipoprotein (LDL) as acceptor of HDLp-A associated lipid in a reaction catalyzed by isolated M. sexta hemolymph lipid transfer particle (LTP). Facilitated vectorial net transfer of lipid from HDLp-A to LDL resulted in formation of a very-high-density lipophorin (VHDLp) product with a density and apolipoprotein content similar to that of VHDLp-E. Lipid was found to comprise 25% of the VHDLp particle mass, whereas over 50% of HDLp-A mass is lipid. Based on these observations it was hypothesized that a lipid transfer factor may be present in M. sexta oocytes and function in the transformation of HDLp-A to VHDLp-E in vivo. Transfer activity was present in the buffer soluble fraction of oocyte homogenates and purification of the active material revealed a catalyst with electrophoretic and immunological properties identical to hemolymph LTP. Incubation of 125I-HDLp-A with an M. sexta oocyte homogenate resulted in transformation of the radiolabeled lipoprotein to a density corresponding to that of VHDLp-E. When the incubation media was preincubated with anti-LTP IgG this conversion was inhibited to a large extent. Inhibition was relieved, however, by addition of exogenous LTP. The results provide the first demonstration of M. sexta LTP in a tissue other than hemolymph and support the concept that LTP-catalyzed lipid transfer plays an integral role in the conversion of HDLp-A to VHDLp-E in vivo.