Internalized proteins directed into accumulative compartments of mosquito oocytes by the specific ligand, vitellogenin

Maternal lipid mobilization is essential for embryonic development in the malaria vector Anopheles gambiae

Lipid metabolism is an essential component in reproductive physiology. While lipid mobilization has been implicated in the growth of Plasmodium falciparum malaria parasites in their Anopheles vectors, the role of this process in the reproductive biology of these mosquitoes remains elusive. Here, we show that impairing lipolysis in Anopheles gambiae, the major malaria vector, leads to embryonic lethality. Embryos derived from females in which we silenced the triglyceride lipase AgTL2 or the lipid storage droplet AgLSD1 develop normally during early embryogenesis but fail to hatch due to severely impaired metabolism. Embryonic lethality is efficiently recapitulated by exposing adult females to broad-spectrum lipase inhibitors prior to blood feeding, unveiling lipolysis as a potential target for inducing mosquito sterility. Our findings provide mechanistic insights into the importance of maternal lipid mobilization in embryonic health that may inform studies on human reproduction.

Uptake and transfer of a Bt toxin by a Lepidoptera to its eggs and effects on its offspring

Research on non-target effects of transgenic crop plants has focused primarily on bitrophic, tritrophic and indirect effects of entomotoxins from Bacillus thuringiensis, but little work has considered intergenerational transfer of Cry proteins. This work reports a lepidopteran (Chlosyne lacinia) taking up a Bt entomotoxin when exposed to sublethal or low concentrations, transferring the entomotoxin to eggs, and having adverse effects on the first filial generation (F1) offspring. Two bioassays were conducted using a sublethal concentration of toxin (100.0 ng/µl Cry1Ac) for adults and a concentration equal to the LC10 (2.0 ng/µl Cry1Ac) for larvae. Cry1Ac is the most common entomotoxin expressed in Bt cotton in Brazil. In the adult diet bioassay there was no adverse effect on the parental generation (P0) adults, but the F1 larvae had higher mortality and longer development time compared to F1 larvae of parents that did not ingest Cry1Ac. For the 3rd instar larvae, there was no measurable effect on the P0 larvae, pupae and adults, but the F1 larvae had higher mortality and longer development time. Using chemiluminescent Western Blot, Cry1Ac was detected in F₁ eggs laid by P₀ butterflies from both bioassays. Our study indicates that, at least for this species and these experimental conditions, a ∼65 kDa insecticidal protein can be taken up and transferred to descendants where it can increase mortality and development time.

Characterization of a tyrosine phosphatase activity in the oogenesis of Periplaneta americana

In this work, phosphatase activity was characterized in the ovary and the haemolymph of Periplaneta americana. The optimum pH for these activities was 4.0, and a temperature of 44 degrees C was ideal for the maximal enzyme activity. The phosphatase activities were inhibited by NaF, sodium tartrate, Pi, sodium orthovanadate, and ammonium molybdate. The ovarian phosphatase activity at pH 4.0 was almost exclusive against phosphotyrosine, with little or no effect on the residues of phosphoserine or phosphothreonine. These results indicate that this phosphatase activity is due to the presence of an acid tyrosine phosphatase. The phosphatase activities of acid extracts from P. americana ovaries (OEX) and an acid extract from P. americana haemolymph (HEX) were analyzed in non-denaturant gel electrophoresis using an analog substrate beta-naphtyl phosphate. The gel revealed two bands with phosphatase activity in the ovary and one band in the haemolymph; these bands were excised and submitted to a 10% SDS-PAGE showing a single 70-kDa polypeptide in both samples. Histochemistry of the ovary with alpha-naphtyl phosphate for localization of acid phosphatase activity showed mainly labeling associated to the oocyte peripheral vesicles, basal lamina, and between follicle cells. Electron microscopy analysis showed that acid phosphatase was localized in small peripheral vesicles in the oocyte, but not inside yolk granules. The possible role of this phosphatase during oogenesis and embryogenesis is also discussed in this article.

Cytochemical characterization of the endomembranous system during the oocyte primary growth in Serrasalmus spilopleura (Teleostei, Characiformes, Characidae)

The morphophysiological changes that occur during oocyte primary growth in Serrasalmus spilopleura were studied using ultrastructural cytochemical techniques. In the previtellogenic oocytes endoplasmic reticulum components, Golgi complex cisternae and vesicles, lysosomes, multivesicular bodies and some electron-dense vesicles react to acid phosphatase (AcPase) detection. The endoplasmic reticulum components, Golgi complex cisternae and vesicles also react to osmium tetroxide and potassium iodide impregnation (KI). These structures, except for the Golgi complex cisternae, are strongly contrasted by osmium tetroxide and zinc iodide impregnation (ZIO). Some electron-dense vesicles are ZIO-stained, while microvesicles in the multivesicular bodies and other large isolated cytoplasmic vesicles are contrasted by KI. At primary oocyte growth, the activity of the endomembranous system and the proliferation of membranous organelles are intense. The biosynthetic pathway of the lysosomal proteins such as acid phosphatase, involves the endoplasmic reticulum, Golgi complex, vesicles with inactive hydrolytic enzymes and, finally, the lysosomes. The oocyte endomembranous system have reduction capacity and are involved in the metabolism of rich in SH groups.

Receptor-mediated endocytosis and intracellular trafficking of lipoproteins and transferrin in insect cells

While the intracellular pathways of ligands after receptor-mediated endocytosis have been studied extensively in mammalian cells, in insect cells these pathways are largely unknown. We transfected Drosophila Schneider line 2 (S2) cells with the human low-density lipoprotein (LDL) receptor (LDLR) and transferrin (Tf) receptor (TfR), and used endocytosis of LDL and Tf as markers. After endocytosis in mammalian cells, LDL is degraded in lysosomes, whereas Tf is recycled. Fluorescence microscopy analysis revealed that LDL and Tf are internalized by S2 cells transfected with LDLR or TfR, respectively. In transfectants simultaneously expressing LDLR and TfR, both ligands colocalize in endosomes immediately after endocytic uptake, and their location remained unchanged after a chase. Similar results were obtained with Spodoptera frugiperda Sf9 cells that were transfected with TfR, suggesting that Tf is retained intracellularly by both cell lines. The insect lipoprotein, lipophorin, is recycled upon lipophorin receptor (LpR)-mediated endocytosis by mammalian cells, however, not after endocytosis by LpR-expressing S2 transfectants, suggesting that this recycling mechanism is cell-type specific. LpR is endogenously expressed by fat body tissue of Locusta migratoria for a limited period after an ecdysis. A chase following endocytosis of labeled lipophorin by isolated fat body tissue at this developmental stage resulted in a significant decrease of lipophorin-containing vesicles, indicative of recycling of the ligand.

Gene expression patterns associated with blood-feeding in the malaria mosquito Anopheles gambiae

Background

Blood feeding, or hematophagy, is a behavior exhibited by female mosquitoes required both for reproduction and for transmission of pathogens. We determined the expression patterns of 3,068 ESTs, representing ~2,000 unique gene transcripts using cDNA microarrays in adult female Anopheles gambiae at selected times during the first two days following blood ingestion, at 5 and 30 min during a 40 minute blood meal and at 0, 1, 3, 5, 12, 16, 24 and 48 hours after completion of the blood meal and compared their expression to transcript levels in mosquitoes with access only to a sugar solution.

Results

In blood-fed mosquitoes, 413 unique transcripts, approximately 25% of the total, were expressed at least two-fold above or below their levels in the sugar-fed mosquitoes, at one or more time points. These differentially expressed gene products were clustered using k-means clustering into Early Genes, Middle Genes, and Late Genes, containing 144, 130, and 139 unique transcripts, respectively. Several genes from each group were analyzed by quantitative real-time PCR in order to validate the microarray results.

Conclusion

The expression patterns and annotation of the genes in these three groups (Early, Middle, and Late genes) are discussed in the context of female mosquitoes' physiological responses to blood feeding, including blood digestion, peritrophic matrix formation, egg development, and immunity.

Sequential deposition of yolk components during oogenesis in an insect, Aedes aegypti (Diptera: Culicidae)

Vitellogenesis in Aedes aegypti of uniform body size was followed at 27 degrees C in narrow time intervals throughout their first reproductive cycle by measuring the length, diameter, and volume of follicles and oocytes, the latter as an expression of the yolk mass (vitellus). Independent of all experimental conditions, a two-step process of elongation was recognized for both follicle length and yolk length, so that growth curves were consistently composed of two linear regressions with different slopes against time. Follicle lengths started to increase immediately after the blood meal, while oocytes took up to 6 h to show a measurable increase in yolk length. The first linear phase continued until 30 h, when yolk length reached 268+/-22 micro m. At this point, a transition occurred where the linearity shifted sharply for the next 6 h to 2-4-times higher slopes for both regressions. This second growth phase represented a 40% elongation of oocytes and follicles. Then, both curves leveled off at their final size, characteristic of mature ovaries: 462+/-10 micro m for oocytes, 489+/-11 micro m for follicles. These values remained constant until oviposition. The first linear growth phase was associated with an equicaloric and synchronous protein and lipid incorporation into the oocytes; levels of these substances reached their maximum by the end of this first phase and remained constant until oviposition. The second linear growth phase was characterized by rapid glycogen incorporation into oocytes from 20 to 100% of the maximum. Subsequently, the surface pattern of the exochorion became visible, marking the end of yolk incorporation. Since eggs are always laid on moist substrates, within 2-3 h of oviposition they double in volume and fresh weight, driven by more than tripling of their water content. When blood-fed females were exposed to five different temperatures between 17 and 37 degrees C, the distinction between the two linear growth phases persisted, but the slopes of the respective regressions, and therefore their durations, were affected. Eggs still matured at 37 degrees C but never hatched and at 12 degrees C only 18% hatched, whereas at all the intermittent temperatures hatching was 80-90%. Oogenesis appears to be limited to the range between 12 and about 32 degrees C. The effects of age, maternal body size and the source of the blood on vitellogenesis were also examined. These parameters affected the onset and/or extent of oogenesis in various ways.

Oocyte fertilization triggers acid phosphatase activity during Rhodnius prolixus embryogenesis

Acid phosphatase activity, previously identified in Rhodnius prolixus oocytes, was studied during egg development. Fertilized eggs exhibited a five fold increase of total acid phosphatase activity during the first days of development. In contrast non-fertilized oviposited eggs showed no activation of this enzyme. An optimum pH of 4.0 for pNPP hydrolysis in a saturable linear reaction and a strong inhibition by lysosomal acid phosphatase inhibitors such as NaF (10 mM) and Na(+)/K(+) tartrate (0.5 mM) are the major biochemical properties of this enzyme. Fractionation of egg homogenates through gel filtration chromatography revealed a single peak of activity with a molecular mass of 94 kDa. The role of this enzyme in VT dephosphorylation was next evaluated. Western blots probed with anti-phosphoserine polyclonal antibody demonstrated that VT phosphoaminoacid content decreases during egg development. In vivo dephosphorylation during egg development was confirmed by following the removal of (32)P from (32)P-VT in metabolically labeled eggs. Vitellin was the only phosphorylated molecule able to inhibit pNPPase activity of partially purified acid phosphatase. These data indicate that acid phosphatase activation follows oocyte fertilization and this enzyme seems to be involved in VT dephosphorylation.

Yolk platelets in artemia embryos: are they really storage sites of immature mitochondria?

We have used semi-quantitative polymerase chain reaction (PCR) technology to determine the mitochondrial DNA (mtDNA) content of yolk platelets isolated from embryos of the brine shrimp, Artemia franciscana, and ultrastructural analysis of yolk platelet formation to determine whether these organelles contain mitochondria as reported previously. Using six different isolation and purification protocols, we found one yolk platelet preparation to be devoid of mtDNA, while four yolk platelet preparations contained mtDNA ranging from 16.4 to 85 pg/10(6) yolk platelets. One preparation contained 600 pg mtDNA per 10(6) yolk platelets. Based on our PCR analyses, the mtDNA component of Artemia yolk platelets represented 0.16-4.5% of the total DNA isolated from the platelets. We calculated that Artemia yolk platelets contain, on average, approximately 1.78 molecules of mtDNA/platelet. Direct analysis of mtDNA in "free" mitochondria isolated from yolk platelet-free preparations of Artemia embryos and newly hatched larvae yielded 0.76-0.80 ng/animal. Based on these values, the mtDNA content of yolk platelets was approximately 0.2% of total mtDNA in Artemia embryos. Microscopic analysis of yolk platelet formation during oogenesis in Artemia failed to show the inclusion of mitochondria during the assemblage of yolk platelets. The "mitochondria-like" structures that appear in yolk platelets during their utilization lack the well defined inner and outer membranes characteristic of mitochondria making it unlikely that the yolk platelet inclusions are mitochondria. Our results from PCR technology and ultrastructure analysis demonstrate that mtDNA in yolk platelets of Artemia franciscana embryos is a minor component of the total mtDNA in the embryo, and they fail to support the notion that yolk platelets in Artemia are a major source of immature mitochondria for development.

Regulation of the vitellogenin receptor during Drosophila melanogaster oogenesis

In many insects, development of the oocyte arrests temporarily just before vitellogenesis, the period when vitellogenins (yolk proteins) accumulate in the oocyte. Following hormonal and environmental cues, development of the oocyte resumes, and endocytosis of vitellogenins begins. An essential component of yolk uptake is the vitellogenin receptor. In this report, we describe the ovarian expression pattern and subcellular localization of the mRNA and protein encoded by the Drosophila melanogaster vitellogenin receptor gene yolkless (yl). yl RNA and protein are both expressed very early during the development of the oocyte, long before vitellogenesis begins. RNA in situ hybridization and lacZ reporter analyses show that yl RNA is synthesized by the germ line nurse cells and then transported to the oocyte. Yl protein is evenly distributed throughout the oocyte during the previtellogenic stages of oogenesis, demonstrating that the failure to take up yolk in these early stage oocyte is not due to the absence of the receptor. The transition to the vitellogenic stages is marked by the accumulation of yolk via clathrin-coated vesicles. After this transition, yolk protein receptor levels increase markedly at the cortex of the egg. Consistent with its role in yolk uptake, immunogold labeling of the receptor reveals Yl in endocytic structures at the cortex of wild-type vitellogenic oocytes. In addition, shortly after the inception of yolk uptake, we find multivesicular bodies where the yolk and receptor are distinctly partitioned. By the end of vitellogenesis, the receptor localizes predominantly to the cortex of the oocyte. However, during oogenesis in yl mutants that express full-length protein yet fail to incorporate yolk proteins, the receptor remains evenly distributed throughout the oocyte.

Vitellin processing and protein synthesis during cricket embryogenesis

At the start of insect embryogenesis most of the protein mass of the egg cytoplasm exists as vitellin (Vt) obtained endocytically during vitellogenesis. Of the new embryo polypeptides (EP) appearing in the egg during embryogenesis, many are synthesized de novo, while, in some species, others derive from developmentally programmed partial proteolysis of Vt. Earlier we showed that by the end of vitellogenesis the two native Vts in Acheta domesticus exist in opposing gradients along the longitudinal axis of the egg. Here we hypothesize that this ooplasmic Vt distribution presents a milieu for Vt processing out of which region-specific regulatory molecules could arise. The metabolic origin and stage-specific patterns of seven predominant EPs (EP 1-7) identified by SDS-PAGE were examined and the results correlated with developmental morphology during the 14 days of embryogenesis. Based on antibody reactivity, peptide mapping and in vitro radiolabeling, we determined that EPs 1-3, 6 and 7 are Vt-derived, while EPs 4 and 5 are produced de novo by the embryo. The five Vt-derived EPs appear during the first 24 h of embryogenesis when migrating cleavage nuclei and associated cytoplasm form the cellular blastoderm, and levels of EPs 4 and 5 increase during days 4-6 of embryogenesis when katatrepsis and yolk mass contraction occur. Positive periodic acid-Schiff staining indicated that EPs 1-3 and their Vt-precursor polypeptides are glycoproteins. This work shows that developmental stage-specific Vt processing occurs during A. domesticus embryogenesis and points next to investigation of the functional significance of Vt cleavage products during development.

Distribution and accumulation of storage protein-1 in ovary of Hyphantria cunea Drury

Storage protein-1 (SP-1) is a major storage protein found in the hemolymph and fat body of Hyphantria cunea. In this study, the uptake and accumulation of SP-1 into the ovary of H. cunea was investigated using biochemical and immunocytochemical methods. SP-1 in H. cunea has a high methionine content (4.6%) but is not female-specific, like other high methionine storage proteins. In the 6-day-old pupal ovary, SP-1 was detectable in trace amounts but accumulated to significant levels toward the end of the pupal stage. After adult emergence, SP-1 rapidly decreased in the ovarian follicles and remained low in the egg. This suggest that SP-1 is either extensively modified or degraded, causing a loss of its antigenic property in the ovary after adult emergence. During vitellogenesis, SP-1 is present in the hemolymph and penetrates through the tunica propria to reach the perioocytic space. From there, SP-1 is incorporated into yolk bodies. These results clearly show that SP-1 is taken up by the developing oocyte. Its disappearance suggests that SP-1 might be an amino acid reservoir for providing precursors for egg formation, in contrast to yolk proteins, which are utilized during postembryonic development.

Cell membrane receptors and regulation of cell function in ticks and blood-sucking insects

Immunoglobulins cross the midgut epithelium and enter the haemolymph of many blood-feeding arthropods without losing their immunological properties. Antigens essential to the survival of the blood-sucking arthropods which may be affected by the small amounts of specific antibody that cross the gut epithelium include membrane receptors or other factors which regulate cell function. Membrane receptors implicated in transmembrane signalling in response to specific neural and endocrine factors fall into three major classes: (1) gated ion channels, (2) agonist-stimulated tyrosine kinases and (3) receptors that interact with GTP-binding (G) proteins. Examples of all three types have been found in insects and ticks. A dopamine receptor interacts with a G-protein essential for controlling fluid secretion by the salivary glands of ixodid ticks. Another receptor in the ixodid tick salivary gland binds a neuropeptide from the tick synganglion and stimulates turnover of plasma membrane phosphoinositides, but its mechanisms of transmembrane signalling and function remain elusive. Another large class of membrane receptors are those concerned with endocytosis. Examples of receptor-mediated endocytosis include incorporation of vitellogenin by developing oocytes in mosquitoes and ticks and uptake of lysed blood-meal components by digest cells of the tick gut. Many cell membrane receptors and possibly hormones could serve as targets for vaccines in blood-feeding insects and ticks. The major challenge is to identify and characterize essential internal receptors and cellular components that are accessible to and affected by specific antibodies that are introduced into the body of blood-feeding arthropods.

Protein and lipoprotein uptake by developing oocytes of the hawkmoth Manduca sexta. An ultrastructural and immunocytochemical study

The ultrastructure of developing Manduca sexta oocytes is described with respect to the endocytic pathway for protein incorporation. Three major (lipo) protein components of mature M. sexta eggs, lipophorin, vitellogenin and microvitellogenin, were localized along this pathway by immuno-fluorescence and immuno-gold labeling techniques. Labeling of the antigens was observed in the extracellular spaces of the follicle. In those cases where fixation and en bloc staining procedures did not destroy antigenicity, antigens were detected in coated pits and coated vesicles near the plasma membrane of the oocyte. All three antigens were demonstrated to be present in endosomes in the cortex of the oocyte. Both the morphology and the labeling pattern of the endosomes indicate that this organelle is a compartment of uncoupling of receptor and ligand. Tubular elements at the surface of the endosome, interpreted to be involved in the recycling of receptors and membrane to the oocyte surface, were not labeled. Strong labeling of lipophorin, vitellogenin and microvitellogenin was observed in the developing yolk bodies, the main protein storage compartment of the oocyte. The uptake and storage of hemolymph proteins and lipoproteins by M. sexta oocytes is discussed in comparison with other insect and vertebrate endocytic systems.

Ultrastructural analysis of Drosophila ovarian follicles differing in yolk polypeptide (yps) composition

Drosophila ovarian follicles were examined ultrastructurally to study the vesicular traffic in the cortical ooplasm. The endocytic pathway leading to the production of yolk spheres was visualized following in vivo or in vitro exposure to peroxidase. The Golgi apparatus and the yolk spheres of wild-type ovarian follicles were preferentially labelled by fixation with osmium zinc iodide (OZI). Labelling of wild-type ovarian follicles was compared to that of several mutant follicles--L186/Basc, fs(2)A17 and ap4--which are defective in vitellogenesis. In these mutants, the Golgi apparatus and the vesicles nearby were either scantly labelled or not labelled at all. In oocytes from flies homozygous for the gene fs(1)1163, the Golgi apparatus was labelled as in the controls, but no yolk spheres appeared to be labelled with OZI at any of the developmental stages. In several Drosophila strains, the pattern of OZI label in the cortical ooplasm was seen to vary in relation to the number of yp structural genes. In starved Drosophila females, OZI labelling of the cortical ooplasm appeared restricted to the Golgi apparatus and to an extended tubular network. A similar labelling pattern was also detected in in vitro cultured vitellogenic follicles. Refeeding, topical application of juvenile hormone analogue to starved females or hormone addition to the culture medium, all caused the yolk spheres to become labelled with OZI and to incorporate peroxidase. These observations prove that impairing endocytic uptake by either mutation or lack of juvenile hormone prevents fusion of coated vesicles and tubules with the yolk spheres and leads them instead to form an intermediate cell compartment with Golgi-derived vesicles.

The specificity of yolk protein uptake in cyclorrhaphan diptera is conserved through evolution

Yolk proteins are transported from the hemolymph into the oocytes of insects during vitellogenesis by receptor-mediated endocytosis. Since other hemolymph proteins, both native and foreign, are not accumulated in the oocyte, the process of uptake is selective for yolk proteins. Peptide domains within the yolk proteins must therefore be involved in receptor recognition. With the long-term aim of identifying these domains and to open the possibility of understanding the molecular basis of receptor-mediated endocytosis of yolk proteins, we began investigating how well this mechanism has been conserved in evolution. We studied the uptake of yolk proteins from 13 different Drosophila species and five other dipteran species, namely, Calliphora erythrocephala, Sarcophaga argyrostoma, Musca domestica, Lucilia servicata, and Protophormia terrae-novae, into the ovaries of Drosophila melanogaster and Drosophila funebris. The results from these experiments showed that in all cases the foreign yolk proteins were taken up by the host ovaries, indicating that the mechanism and peptide domains of the yolk proteins involved in recognition of the receptor have been well conserved in dipteran evolution.

Ultrastructural immunocytochemical localization of secretory proteins in autophagic vacuoles of parotid acinar cells of starved rats

Previous studies have shown that reduction of mastication has marked effects on the structure and biochemistry of the rat parotid gland. Acute starvation results in the formation in the acinar cells of large autophagic vacuoles which contain lysosomal hydrolases and within which secretory granules appear to undergo degradation. In this study we used electron microscopic immunocytochemistry and antibodies to two secretory proteins, alpha-amylase and B1-immunoreactive protein, to determine whether secretory proteins are present in autophagic vacuoles of parotid acinar cells of starved rats. Small vacuoles were observed after 24-h starvation; they increased in size and number up to 72-h starvation. Both secretory proteins were present in the secretory granules and in the dense content of the autophagic vacuoles, as shown by immunogold labelling. The lighter matrix of the vacuoles was unlabelled. These findings confirm that secretory granules may fuse with lysosomal structures, where their content of secretory proteins is presumably degraded. Thus, the rat parotid appears to be similar to other secretory cells in which cellular levels of stored secretory proteins may be regulated by the process of crinophagy.

Monoclonal antibodies as probes for processing of the mosquito yolk protein; a high-resolution immunolocalization of secretory and accumulative pathways

A library of monoclonal antibodies (mAB) directed against yolk polypeptides of the mosquito Aedes aegypti was utilized to visualize the secretory pathway of these polypeptides in the fat body and their accumulative pathway in developing oocytes. Single and double immunolabelling using mABs and colloidal gold of different sizes confirmed biochemical observation that 200 +/- 5 and 65 +/- 3 kDa polypeptides represent subunits of the yolk protein. This immunocytochemical analysis showed that, in trophocytes of the fat body, both the subunits of the yolk protein were routed simultaneously through the Golgi complex into secretory granules and were subsequently secreted. The yolk protein subunits were also directed together through all the steps of the accumulative pathway in the oocyte. Double immunogold labelling revealed that the subunits were present together during their binding to the oocyte membrane, transportation into and accumulation in the transitional yolk body, and, finally, crystallization in the mature yolk body. Electron microscopical immunocytochemistry also confirmed immunofluorescent data and showed that mABs directed against different steps in the biosynthetic processing of the yolk protein in the fat body, as well as in its accumulative pathway in oocytes.