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

Pre-Embryonic Period Observation Shows a Unique Reproductive Strategy of the Critically Endangered Anji Salamander (Hynobius amjiensis)

Hynobius amjiensis, also known as the Anji salamander, is an amphibian species currently categorized as endangered due to its limited geographical distribution, primarily in China. To address the critical conservation status of this species, artificial breeding is essential for population expansion. However, progress in artificial breeding efforts has been hindered by the scarcity of research on the reproductive biology of the Anji salamander. In this study, we identified 25 distinct early stages of embryo development. Additionally, we observed that Anji salamander embryos contain a lesser amount of yolk compared to other salamanders or frogs. We further discovered that the Anji salamander employs a highly competitive reproductive strategy, producing a smaller number of high-quality offspring. This strategy aims to generate adaptive individuals through intense intraspecific competition, driven by three factors: extremely confined breeding habitats, a substantial number of eggs, and a reduced yolk content. We introduce the term "mass escape" to describe this strategy, which provides a novel perspective on cannibalization, focusing on the consumption of specific body parts rather than a single-individual activity. This study offers valuable insights into artificial breeding techniques designed to mitigate inherent intraspecific competitive pressure, thereby improving metamorphosis and survival rates. Additionally, it provides a comprehensive table detailing the pre-embryonic developmental stages of the Anji salamander.

Cadmium induces physiological and behavioral changes associated with 180 kDa NCAM lower expression and higher polysialic acid, in the African clawed Xenopus laevis tadpoles

Heavy metals are released into the environment in increasing amounts from different natural and anthropogenic sources. Among them, cadmium contaminates aquatic habitats and represents a threat to Amphibians. To assess the risks of exposure to cadmium in the aquatic environment, we studied the survival rate of early tadpoles of Xenopus laevis under exposure to CdCl2 for 6 days in the concentration range between 0.15 and 150 µM of Cd2+. Tadpoles survived and reached stage 45 before feeding at all concentrations tested except 150 µM Cd2+, which significantly induced death. With an exposure of 15 µM Cd2+, tadpoles' mean body length decreased, heart rate increased, fastest swimming speed decreased, and distance traveled was greater compared to unexposed controls. Additionally, a witness of neuronal normal development, the neural cell adhesion molecules (NCAM) expression, was decreased. Moreover, this cell-surface glycoprotein exhibited higher polysialylation, a post-translational modification capable to reduce cell adhesion properties and to affect organ development. Our study highlights the effects of Cd2+ on a series of parameters including morphology, physiology, and behavior. They emphasize the deregulation of molecular NCAM suggesting this effector is an interesting biomarker to detect cadmic toxicity in early tadpoles.

Open questions on the functional biology of the yolk granules during embryo development

Biogenesis and consumption of the yolk are well-conserved aspects of the reproductive biology in oviparous species. Most egg-laying animals accumulate yolk proteins within the oocytes thus creating the source of nutrients and energy that will feed embryo development. Yolk accumulation drives the generation of a highly specialized oocyte cytoplasm with maternal mRNAs, ribosomes, mitochondria, and, mainly, a set of organelles collectively referred to as yolk granules (Ygs). Following fertilization, the Ygs are involved in regulated mechanisms of yolk degradation to fuel the anabolic metabolism of the growing embryo. Thus, yolk accumulation and degradation are essential processes that allow successful development in many species. Nevertheless, the molecular machinery and mechanisms dedicated to the programmed yolk mobilization throughout development are still enigmatic and remain mostly unexplored. Moreover, while the Ygs functional biology as a nutritional source for the embryo has been acknowledged, several reports have suggested that Ygs cargoes and functions go far beyond yolk storage. Evidence of the role of Ygs in gene expression, microbiota harboring, and paracrine signaling has been proposed. In this study, we summarize the current knowledge of the Ygs functional biology pointing to open questions and where further investigation is needed.

Protein crystallization in living cells

Protein crystallization in living cells has been observed surprisingly often as a native assembly process during the past decades, and emerging evidence indicates that this phenomenon is also accessible for recombinant proteins. But only recently the advent of high-brilliance synchrotron sources, X-ray free-electron lasers, and improved serial data collection strategies has allowed the use of these micrometer-sized crystals for structural biology. Thus, in cellulo crystallization could offer exciting new possibilities for proteins that do not crystallize applying conventional approaches. In this review, we comprehensively summarize the current knowledge of intracellular protein crystallization. This includes an overview of the cellular functions, the physical properties, and, if known, the mode of regulation of native in cellulo crystal formation, complemented with a discussion of the reported crystallization events of recombinant proteins and the current method developments to successfully collect X-ray diffraction data from in cellulo crystals. Although the intracellular protein self-assembly mechanisms are still poorly understood, regulatory differences between native in cellulo crystallization linked to a specific function and accidently crystallizing proteins, either disease associated or recombinantly introduced, become evident. These insights are important to systematically exploit living cells as protein crystallization chambers in the future.

Activity and intracellular localization of senescence-associated β-galactosidase in aging Xenopus oocytes and eggs

Senescence-associated β-galactosidase (SA-β-gal) serves as a marker of senescence in aging somatic cells. However, little is known about SA-β-gal dynamics in aging gamete cells. To address this issue, here we investigated activity and intracellular localization of SA-β-gal in freshly obtained and aging oocytes and eggs of the African clawed frog Xenopus laevis. Data base mining revealed the presence of several homologous β-galactosidase sequences in the annotated Xenopus genome. Some of them were predicted to contain an N-terminal signal peptide sequence, suggesting enzyme translocation to cellular organelles. Biochemical and microscopic analyses confirmed SA-β-gal localization in the particulate and cytosolic fractions of oocytes and eggs. SA-β-gal activity was found to reside predominantly within a fraction of dense cytoplasmic granules that were extensively stained with the lysosome-specific dye LysoTracker Green DND-26 and had an average size of 8.9 ± 5.6 μm. These features identify the SA-β-gal-containing granules as a subpopulation of yolk platelets, specialized late endosomes or lysosomes that accumulate and store processed protein in frog oocytes. Further analysis revealed an increase of SA-β-gal activity in Xenopus eggs, but not in oocytes, aged in vitro over 48 h. Our data suggest that endosomal acidification during egg aging may be responsible for this increase.

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.

The Atg1-Tor pathway regulates yolk catabolism in Drosophila embryos

Yolk provides an important source of nutrients during the early development of oviparous organisms. It is composed mainly of vitellogenin proteins packed into membrane-bound compartments called yolk platelets. Catabolism of yolk is initiated by acidification of the yolk platelet, leading to the activation of Cathepsin-like proteinases, but it is unknown how this process is triggered. Yolk catabolism initiates at cellularization in Drosophila melanogaster embryos. Using maternal shRNA technology we found that yolk catabolism depends on the Tor pathway and on the autophagy-initiating kinase Atg1. Whereas Atg1 was required for a burst of spatially regulated autophagy during late cellularization, autophagy was not required for initiating yolk catabolism. We propose that the conserved Tor metabolic sensing pathway regulates yolk catabolism, similar to Tor-dependent metabolic regulation on the lysosome.

Regulation of G-protein signaling via Gnas is required to regulate proximal tubular growth in the Xenopus pronephros

In the kidney, proximal tubules are very important for the reabsorption of water, ions and organic solutes from the primary urine. They are composed of highly specialized epithelial cells that are characterized by an elaborate apical brush border to increase transport efficiency. Using the pronephric kidney of Xenopus laevis we discovered that the G-protein modulator cholera toxin resulted in a dramatic reduction of the proximal tubular size. This phenotype was accompanied by changes in the cytoarchitecture characterized by ectopic expression of the distal tubular marker 4A6 and an impairment of yolk platelet degradation. In addition, cholera toxin caused edema formation. However, this phenotype was not due to kidney defects, but rather due to impaired vasculature development. Based on experiments with antisense morpholino oligomers as well as pharmacological agonists and antagonists, we could show that the complex phenotype of cholera toxin in the pronephric kidney was caused by the hyperactivation of a single G-protein alpha subunit, Gnas. This-in turn-caused elevated cAMP levels, triggered a Rapgef4-dependent signaling cassette and perturbed exo- and endocytosis. This perturbation of the secretory pathway by Ctx was not only observed in Xenopus embryos. Also, in a human proximal tubular cell line, cholera toxin or a Rapgef4-specific agonist increased uptake and decreased secretion of FITC-labeled Albumin. Based on these data we propose that the Gnas/cAMP/Rapgef4 pathway regulates the signals inducing the proliferation of proximal tubules to acquire their final organ size.

V-ATPase-dependent ectodermal voltage and pH regionalization are required for craniofacial morphogenesis

Using voltage and pH reporter dyes, we have discovered a never-before-seen regionalization of the Xenopus ectoderm, with cell subpopulations delimited by different membrane voltage and pH. We distinguished three courses of bioelectrical activity. Course I is a wave of hyperpolarization that travels across the gastrula. Course II comprises the appearance of patterns that match shape changes and gene expression domains of the developing face; hyperpolarization marks folding epithelium and both hyperpolarized and depolarized regions overlap domains of head patterning genes. In Course III, localized regions of hyperpolarization form at various positions, expand, and disappear. Inhibiting H(+) -transport by the H(+) -V-ATPase causes abnormalities in: (1) the morphology of craniofacial structures; (2) Course II voltage patterns; and (3) patterns of sox9, pax8, slug, mitf, xfz3, otx2, and pax6. We conclude that this bioelectric signal has a role in development of the face. Thus, it exemplifies an important, under-studied mechanism of developmental regulation.

Novel regulation of yolk utilization by thyroid hormone in embryos of the direct developing frog Eleutherodactylus coqui

Thyroid hormone (TH) is required for metamorphosis of the long, coiled tadpole gut into the short frog gut. Eleutherodactylus coqui, a direct developing frog, lacks a tadpole. Its embryonic gut is a miniature adult form with a mass of yolky cells, called nutritional endoderm, attached to the small intestine. We tested the TH requirement for gut development in E. coqui. Inhibition of TH synthesis with methimazole arrested gut development in its embryonic form. Embryos treated with methimazole failed to utilize the yolk in their nutritional endoderm, and survived for weeks without further development. Conversely, methimazole and 3,3',5-tri-iodo-l-thyronine, the active form of TH, stimulated gut development and utilization and disappearance of the nutritional endoderm. In Xenopus laevis, the receptor for TH, TRβ, is upregulated in response to TH. Similarly, EcTRβ, the E. coqui ortholog, was upregulated by TH in the gut. EcTRβ expression was high in the nutritional endoderm, suggesting a direct role for TH in yolk utilization by these cells. An initial step in the breakdown of yolk in X. laevis is acidification of the yolk platelet. E. coqui embryos in methimazole failed to acidify their yolk platelets, but acidification was stimulated by TH indicating its role in an early step of yolk utilization. In addition to a conserved TH role in gut development, a novel regulatory role for TH in yolk utilization has evolved in these direct developers.

Inorganic polyphosphate inhibits an aspartic protease-like activity in the eggs of Rhodnius prolixus (Stahl) and impairs yolk mobilization in vitro

Inorganic polyphosphate (poly P) is a polymer of phosphate residues that has been shown to act as modulator of some vertebrate cathepsins. In the egg yolk granules of Rhodnius prolixus, a cathepsin D is the main protease involved in yolk mobilization and is dependent on an activation by acid phosphatases. In this study, we showed a possible role of poly P stored inside yolk granules on the inhibition of cathepsin D and arrest of yolk mobilization during early embryogenesis of these insects. Enzymatic assays detected poly P stores inside the eggs of R. prolixus. We observed that micromolar poly P concentrations inhibited cathepsin D proteolytic activity using both synthetic peptides and homogenates of egg yolk as substrates. Poly P was a substrate for Rhodnius acid phosphatase and also a strong competitive inhibitor of a pNPPase activity. Fusion events have been suggested as important steps towards acid phosphatase transport to yolk granules. We observed that poly P levels in those compartments were reduced after in vitro fusion assays and that the remaining poly P did not have the same cathepsin D inhibition activity after fusion. Our results are consistent with the hypothesis that poly P is a cathepsin D inhibitor and a substrate for acid phosphatase inside yolk granules. It is possible that, once activated, acid phosphatase might degrade poly P, allowing cathepsin D to initiate yolk proteolysis. We, therefore, suggest that degradation of poly P might represent a new step toward yolk mobilization during embryogenesis of R. prolixus.

Proton-pyrophosphatase and polyphosphate in acidocalcisome-like vesicles from oocytes and eggs of Periplaneta americana

Acidocalcisomes are acidic organelles containing large amounts of polyphosphate (poly P), a number of cations, and a variety of cation pumps in their limiting membrane. The vacuolar proton-pyrophosphatase (V-H(+)-PPase), a unique electrogenic proton-pump that couples pyrophosphate (PPi) hydrolysis to the active transport of protons across membranes, is commonly present in membranes of acidocalcisomes. In the course of insect oogenesis, a large amount of yolk protein is incorporated by the oocytes and stored in organelles called yolk granules (YGs). During embryogenesis, the content of these granules is degraded by acid hydrolases. These enzymes are activated by the acidification of the YG by a mechanism that is mediated by proton-pumps present in their membranes. In this work, we describe an H(+)-PPase activity in membrane fractions of oocytes and eggs of the domestic cockroach Periplaneta americana. The enzyme activity was optimum at pH around 7.0, and was dependent on Mg(2+) and inhibited by NaF, as well as by IDP and Ca(2+). Immunolocalization of the yolk preparation using antibodies against a conserved sequence of V-H(+)-PPases showed labeling of small vesicles, which also showed the presence of high concentrations of phosphorus, calcium and other elements, as revealed by electron probe X-ray microanalysis. In addition, poly P content was detected in ovaries and eggs and localized inside the yolk granules and the small vesicles. Altogether, our results provide evidence that numerous small vesicles of the eggs of P. americana present acidocalcisome-like characteristics. In addition, the possible role of these organelles during embryogenesis of this insect is discussed.

Concentration-dependent effects on intracellular and surface pH of exposing Xenopus oocytes to solutions containing NH3/NH4(+)

Others have shown that exposing oocytes to high levels of NH(3)/NH(4)(+) (10-20 mM) causes a paradoxical fall in intracellular pH (pH(i)), whereas low levels (e.g., 0.5 mM) cause little pH(i) change. Here we monitored pH(i) and extracellular surface pH (pH(S)) while exposing oocytes to 5 or 0.5 mM NH(3)/NH(4)(+). We confirm that 5 mM NH(3)/NH(4)(+) causes a paradoxical pH(i) fall (-DeltapH(i) approximately equal 0.2), but also observe an abrupt pH(S) fall (-DeltapH(S) approximately equal 0.2)-indicative of NH(3) influx-followed by a slow decay. Reducing [NH(3)/NH(4)(+)] to 0.5 mM minimizes pH(i) changes but maintains pH(S) changes at a reduced magnitude. Expressing AmtB (bacterial Rh homologue) exaggerates -DeltapH(S) at both NH(3)/NH(4)(+) levels. During removal of 0.5 or 5 mM NH(3)/NH(4)(+), failure of pH(S) to markedly overshoot bulk extracellular pH implies little NH(3) efflux and, thus, little free cytosolic NH(3)/NH(4)(+). A new analysis of the effects of NH(3) vs. NH(4)(+) fluxes on pH(S) and pH(i) indicates that (a) NH(3) rather than NH(4)(+) fluxes dominate pH(i) and pH(S) changes and (b) oocytes dispose of most incoming NH(3). NMR studies of oocytes exposed to (15)N-labeled NH(3)/NH(4)(+) show no significant formation of glutamine but substantial NH(3)/NH(4)(+) accumulation in what is likely an acid intracellular compartment. In conclusion, parallel measurements of pH(i) and pH(S) demonstrate that NH(3) flows across the plasma membrane and provide new insights into how a protein molecule in the plasma membrane-AmtB-enhances the flux of a gas across a biological membrane.

Peroxisome biogenesis occurs in late dorsal-anterior structures in the development of Xenopus laevis

Metabolism and development are two important processes not often examined in the same context. The focus of the present study is the expression of specific peroxisomal genes, the subsequent biogenesis of peroxisomes, and their potential role in the metabolism associated with the development of Xenopus laevis embryos. The temporal and expression patterns of six peroxisomal genes (PEX5, ACO, PEX19, PMP70, PEX16, and catalase) were elucidated using RT-PCR. Functionally related peroxisomal genes exhibited similar expression patterns with their RNA levels elevated relatively late during embryogenesis. Using immunohistochemistry PMP70 and catalase protein was localized largely to dorsal-anterior structures. Peroxisomal function was assayed with peroxisomal targeted-GFP, which when microinjected, revealed peroxisomes in dorsal-anterior structures at stage 45. A requirement for peroxisomal function appears to be present only late in development as organogenesis is finishing, yolk stores are depleted, and ingestion commences.

Fertilization and nicotinic acid adenine dinucleotide phosphate induce pH changes in acidic Ca(2+) stores in sea urchin eggs

The second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) releases Ca(2+) from the acidic Ca(2+) stores of many organisms, including those of the sea urchin egg. We investigated whether the pH within the lumen of these acidic organelles changes in response to stimuli. Fertilization activates the egg by Ca(2+) release dependent upon NAADP, and accordingly, we report that fertilization also alters organellar pH in a spatio-temporally complex manner. Upon sperm fusion, vesicles deep in the egg center slowly acidify, whereas cortical vesicles undergo a rapid alkalinization. The cortical vesicle alkalinization is independent of exocytosis and cytosolic pH but coincides with the NAADP-dependent fertilization Ca(2+) wave. Microinjection of NAADP mimicked the fertilization cortical response, suggesting that it occurred within NAADP-sensitive acidic Ca(2+) stores. Our data show that NAADP and physiological stimuli alter the pH within intracellular organelles and suggest that NAADP signals through pH as well as Ca(2+).

Energizing an Invertebrate Embryo: Bafilomycin‐Dependent Respiration and the Metabolic Cost of Proton Pumping by the V‐ATPase

We examine herein the contribution of V-ATPase activity to the energy budget of aerobically developing embryos of Artemia franciscana and discuss the results in the context of quiescence under anoxia. (31)P-NMR analysis indicates that intracellular pH and NTP levels are unaffected by acute incubation of dechorionated embryos with the V-ATPase inhibitor, bafilomycin A(1). Bafilomycin A(1) also has no significant effect on oxygen consumption by isolated mitochondria. Taken together, these data indicate that bafilomycin does not affect energy-producing pathways in the developing embryo. However, the V-ATPase inhibitor exhibits a concentration-dependent inhibition of oxygen consumption in aerobic embryos. A conservative analysis of respirometric data indicates that proton pumping by the V-ATPase, and processes immediately dependent on this activity, constitutes approximately 31% of the aerobic energy budget of the preemergent embryo. Given the complete absence of detectable Na(+)K(+)-ATPase activity during the first hours of aerobic development, it is plausible that the V-ATPase is performing a role in both the acidification of intracellular compartments and the energization of plasma membranes. Importantly, the high metabolic cost associated with maintaining these diverse proton gradients requires that V-ATPase activity be downregulated under anoxia in order to attain the almost complete metabolic depression observed in the quiescent embryo.

The Maturational Disassembly and Differential Proteolysis of Paralogous Vitellogenins in a Marine Pelagophil Teleost: A Conserved Mechanism of Oocyte Hydration1

A structural analysis of the differential proteolysis of vitellogenin (Vtg)-derived yolk proteins in the maturing oocytes of a marine teleost that spawns very large pelagic eggs is presented. Two full-length hepatic cDNAs (hhvtgAa and hhvtgAb) encoding paralogous vitellogenins (HhvtgAa and HhvtgAb) were cloned from nonestrogenized Atlantic halibut, and the N-termini of their subdomain structures were mapped to the oocyte and egg yolk proteins (Yps). The maturational oocyte Yp degradation products were further mapped to the free amino acid (FAA) pool in the ovulated egg. The deduced amino acid sequences conformed to the linear NH(2)-(LvH-Pv-LvL-beta'-CT)-COO(-) structure of complete teleost Vtgs. However, the Yps did not match the expected cleavage products of complete Vtgs. Specifically, the phosvitin subdomain of the HhvtgAa paralogue remains covalently attached to the lipovitellin light chain, while the phosvitin subdomain of the HhvtgAb paralogue remains covalently attached to a C-terminal fragment of the lipovitellin heavy chain (LvH). During oocyte hydration, the LvH of the HhvtgAa paralogue is disassembled and extensively degraded to FAA. In the HhvtgAb paralogue, the LvH is nicked in the C-sheet in a manner similar to that seen in lamprey and other teleosts. A small part of the C-terminal end of the LvH-Ab undergoes proteolysis to FAA, together with the phosvitin, beta' component, and much ( approximately 65%) of the lipovitellin light chain (LvL-Ab). The independently measured FAA pool in the ovulated egg corroborates that calculated from differential proteolysis of the Yps. Based on the 3:1 (HhvtgAb:HhvtgAa) Yp expression ratio, each paralogue contributes approximately equal amounts of FAA to the organic osmolyte pool of the hydrating oocyte during maturation.

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.

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.

Tick vitellin is dephosphorylated by a protein tyrosine phosphatase during egg development: effect of dephosphorylation on VT proteolysis

Vitellin (VT) is a phospholipoglycoprotein that is the main component of arthropod egg yolk. Phosphorylation is a recurrent feature of every VT molecule described so far. However, the role played by such post-translational modification during egg development is not yet clear. In the eggs of the hard tick Boophilus microplus, VT is a phosphotyrosine-containing protein. VT-phosphotyrosine residues are gradually removed during tick embryogenesis due to the action of a 45 kDa egg tyrosine phosphatase. This enzyme is strongly inhibited by ammonium molybdate, sodium vanadate and cupric ion. The role of phosphotyrosine residues in VT proteolytic degradation was evaluated. Western blots probed with a monoclonal anti-phosphotyrosine antibody demonstrated that the high molecular mass VT subunits (VT 1 and VT 2) are the main targets of dephosphorylation during egg development. Both dephosphorylation and proteolysis of VT 1 and VT 2 are blocked by ammonium molybdate in total egg homogenates. When purified VT was dephosphorylated in vitro with lambda phosphatase and then incubated in the presence of bovine cathepsin D, VT proteolysis increased dramatically. Altogether, these data are the first to show that phosphotyrosine residues are present in a yolk protein, and that such residues might be involved in the regulation of VT breakdown during egg development.

V-ATPase expression during development of Artemia franciscana embryos: potential role for proton gradients in anoxia signaling

Under anoxia, Artemia franciscana embryos downregulate metabolic processes and approach an ametabolic state. Entrance into this quiescent state is accompanied by a profound acidification of the intracellular space, and more than two decades of research now clearly demonstrates that this acidification is critical to metabolic downregulation in anoxic embryos. However, the proximal mechanisms responsible for the pH shift remain largely unidentified. Here, we report evidence demonstrating expression of the V-ATPase in encysted embryos and present an argument for its involvement in the intracellular acidification induced by anoxia. We identified a single B-subunit cDNA sharing the greatest degree of sequence similarity with ;generalist-type' homologues from mammals (brain-type) and invertebrates. Quantitative analysis of B-subunit mRNA demonstrates differential expression throughout early development, and western blot analyses confirm the expression of at least six V-ATPase subunits in both heavy membranes and microsomal vesicles. The critical need for proton pumping during the anoxia-tolerant stage of development is demonstrated by incubation with the V-ATPase inhibitor bafilomycin A1, which halts embryonic development. Importantly, net proton flux from V-ATPase-acidified compartments to the surrounding cytoplasm is likely under anoxia and may significantly contribute to the enigmatic acidification critical to quiescence.

Cathepsin B-mediated yolk protein degradation during killifish oocyte maturation is blocked by an H+-ATPase inhibitor: effects on the hydration mechanism

In teleost oocytes, yolk proteins (YPs) derived from the yolk precursors vitellogenins are partially cleaved into free amino acids and small peptides during meiotic maturation before ovulation. This process increases the osmotic pressure of the oocyte that drives its hydration, which is essential for the production of buoyant eggs by marine teleosts (pelagophil species). However, this mechanism also occurs in marine species that produce benthic eggs (benthophil), such as the killifish (Fundulus heteroclitus), in which oocyte hydration is driven by K+. Both in pelagophil and benthophil teleosts, the enzymatic machinery underlying the maturation-associated proteolysis of YPs is poorly understood. In this study, lysosomal cysteine proteinases potentially involved in YP processing, cathepsins L, B, and F (CatL, CatB, and CatF, respectively), were immunolocalized in acidic yolk globules of vitellogenic oocytes from the killifish. During oocyte maturation in vitro induced with the maturation-inducing steroid (MIS), CatF disappeared from yolk organelles and CatL became inactivated, whereas CatB proenzyme was processed into active enzyme. Consequently, CatB enzyme activity and hydrolysis of major YPs were enhanced. Follicle-enclosed oocytes incubated with the MIS in the presence of bafilomycin A1, a specific inhibitor of vacuolar-type H+-ATPase, underwent maturation in vitro, but acidification of yolk globules, activation of CatB, and proteolysis of YPs were prevented. In addition, MIS plus bafilomycin A1-treated oocytes accumulated less K+ than those stimulated with MIS alone; hence, oocyte hydration was reduced. These results suggest that CatB is the major protease involved in yolk processing during the maturation of killifish oocytes, whose activation requires acidic conditions maintained by a vacuolar-type H+-ATPase. Also, the data indicate a link between ion translocation and YP proteolysis, suggesting that both events may be equally important physiological mechanisms for oocyte hydration in benthophil teleosts.

CELL BIOLOGY OF ANTIGEN PROCESSING IN VITRO AND IN VIVO

The conversion of exogenous and endogenous proteins into immunogenic peptides recognized by T lymphocytes involves a series of proteolytic and other enzymatic events culminating in the formation of peptides bound to MHC class I or class II molecules. Although the biochemistry of these events has been studied in detail, only in the past few years has similar information begun to emerge describing the cellular context in which these events take place. This review thus concentrates on the properties of antigen-presenting cells, especially those aspects of their overall organization, regulation, and intracellular transport that both facilitate and modulate the processing of protein antigens. Emphasis is placed on dendritic cells and the specializations that help account for their marked efficiency at antigen processing and presentation both in vitro and, importantly, in vivo. How dendritic cells handle antigens is likely to be as important a determinant of immunogenicity and tolerance as is the nature of the antigens themselves.

Cathepsin D-mediated yolk protein degradation is blocked by acid phosphatase inhibitors

Vitellin (VT) is a lipoglycophosphoprotein stored inside the eggs of every oviparous organism during oogenesis. In the blood-sucking bug Rhodnius prolixus, VT is deposited inside growing oocytes together with two acid hydrolases: acid phosphatase (AP) and cathepsin D (CD). Egg fertilization triggers AP activity and VT proteolysis in vivo [Insect Biochem. Mol. Biol. 2002 (32) 847]. Here, we show that CD is the main protease targeting VT proteolysis during egg development. CD activity in total egg homogenates is blocked by the classical aspartyl protease inhibitor, pepstatin A. Surprisingly, AP inhibitors such as NaF, Na+/K+ tartrate, and inorganic phosphate also block VT proteolysis, whereas this effect is not observed when tyrosine phosphatase inhibitors such as vanadate and phenylarsine oxide or an inhibitor of alkaline phosphatases such as levamisole are used in a VT proteolysis assay. NaF concentrations that block isolated AP activity do not affect the activity of partially purified CD. Therefore, a specific repressor of VT proteolysis must be dephosphorylated by AP in vivo. In conclusion, these results demonstrate for the first time that acid hydrolases act cooperatively to promote yolk degradation during egg development in arthropods.

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.

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.

Four-dimensional imaging of cytoskeletal dynamics in Xenopus oocytes and eggs

The Xenopus laevis (African clawed frog) system has long been popular for studies of both developmental and cell biology, based on a variety of its intrinsic features including the large size of Xenopus oocytes, eggs, and embryos, and the relative ease of manipulation. Unfortunately, the large size has also been considered a serious impediment for high-resolution light microscopy, as has the heavy pigmentation. However, the recent development and exploitation of 4D imaging approaches, and the fact that much of what is of most interest to cell and developmental biologists takes place near the cell surface, indicates that such concerns are no longer valid. Consequently, the Xenopus system in many respects is now as good as other model systems considered to be ideal for microscopy-based studies. Here, 4D imaging and its recent applications to cytoskeletal imaging in Xenopus oocytes and eggs are discussed.

Two-step consumption of yolk granules during the development of quail embryos

The mechanism of yolk consumption was studied morphologically and biochemically in Japanese quail Coturnix japonica. The amount of yolk granules in the yolk (or 'yolk cell') decreased in two steps during embryonic development. In the first step, during days 0-4 of incubation, the yolk-granule weight decreased at a rate of 13 mg/day. This decrease was due to segregation by endodermal cells that were newly formed in the developing yolk sac. In the second step after day 6, the decrease was drastic at a rate of 29.8 mg/day during days 6-12 and very slow thereafter. The decrease at the second step was due to the enzymatic digestion of yolk granules by cathepsin D that coexisted in yolk spheres. This digesting reaction was triggered by the solubilization of the granules with high concentrations of salts that were supplied after disruption of the limiting membrane of yolk spheres. The 'yolk cell' seemed to die around day 5 of incubation. Thus the digestion products might be taken up together with yolk lipids by endocytosis into the endodermal cells and transported to blood vessels.

Proteolytic activity of Boophilus microplus Yolk pro-Cathepsin D (BYC) is coincident with cortical acidification during embryogenesis

In a previous report (Parasitology 116 (1998) 525) we isolated and characterized Boophilus Yolk pro-Cathepsin (BYC), an aspartic proteinase precursor from the eggs of the hard tick. The present study was designed to characterize the function of BYC in the consumption of vitellin (VT), the major yolk protein, during embryogenesis. Both purified BYC and total egg homogenate proteolytic activity showed a similar pH dependence profile with an acidic optimum. Purified BYC presented higher activity against VT as a substrate when compared to other proteins. The VT degradation pattern observed in vitro also showed a similar profile to that observed in vivo. Co-localization of BYC and acidic cortical yolk granules was performed by immunocytochemistry and confocal microscopy. Proton-pumping activity of yolk granules in vitro was higher in eggs collected 4 day after oviposition than in newly laid eggs. Taken together, our data suggest that BYC plays a major role in the degradation of VT and that its activity is controlled by acidification of yolk platelets localized at the cortical cytoplasm of the developing Boophilus microplus egg.

Ratio imaging instrumentation

Using ratio imaging to obtain quantitative information from microscope images is a powerful tool that has been used successfully in numerous studies. Although ratio imaging reduces the effects of many parameters that can interfere with accurate measurements, it is not a panacea. In designing a ratio imaging experiment, all of the potential problems discussed in this chapter must be considered. Undoubtedly, other problems that were not discussed can also interfere with accurate and meaningful measurements. Many of the problems discussed here were observed in the authors' laboratories. In our experience there are no standard routines or methods that can foresee every problem before it has been encountered. Good experimental design can minimize problems, but the investigator must continue to be alert. Progress in instrumentation continues to overcome some of the difficulties encountered in ratio imaging. CCD cameras with 12- to 14-bit pixel depth are being used more frequently, and several confocal microscope manufacturers are now also using 12-bit digitization. The dramatic increase in the use of confocal microscopes over the past decade is now causing microscope manufacturers to more critically evaluate the effect of axial chromatic aberration in objectives, and recent designs to minimize this problem are being implemented. Other developments such as the use of AOTFs to attenuate laser lines extend the applicability of ratio imaging. Ratio imaging is clearly applicable to a wide range of cell biological problems beyond its widespread use for measuring ion concentrations. Imaginative but careful use of this technique should continue to provide novel insights into the properties of cells.

A new model for proton pumping in animal cells: the role of pyrophosphate

The H+-PPase activity was characterized in membrane fractions of ovary and eggs of Rhodnius prolixus. This activity is totally dependent on Mg2+, independent of K+ and strongly inhibited by NaF, IDP and Ca2+. The membrane proteins of eggs were analyzed by western blot using antibodies to the H+-PPase from Arabidopsis thaliana. The immunostain was associated with a single 65-kDa polypeptide. This polypeptide was immunolocalized in yolk granule membranes by optical and transmission electron microscopy. We describe the acidification of yolk granules in the presence of PPi and ATP. This acidification is inhibited in the presence of NAF, Ca2+ and antibodies against H+-PPase. These data show for the first time in animal cells that acidification of yolk granules involves an H+-PPase as well as H+-ATPase.

Xenopus autosomal recessive hypercholesterolemia protein couples lipoprotein receptors with the AP-2 complex in oocytes and embryos and is required for vitellogenesis

ARH is required for normal endocytosis of the low density lipoprotein (LDL) receptor in liver and mutations within this gene cause autosomal recessive hypercholesterolemia in humans. xARH is a localized maternal RNA in Xenopus with an unknown function in oogenesis and embryogenesis. Like ARH, xARH contains a highly conserved phosphotyrosine binding domain and a clathrin box. To address the function of xARH, we examined its expression pattern in development and used pull-down experiments to assess interactions between xARH, lipoprotein receptors and proteins in embryo extracts. xARH was detected concentrated at the cell periphery as well as in the perinuclear region of oocytes and embryos. In pull-down experiments, the xARH phosphotyrosine binding domain interacted with the LDL and vitellogenin receptors found in Xenopus oocytes and embryos. Mutations within the receptor internalization signal specifically abolished this interaction. The xARH C-terminal region pulled-down several proteins from embryo extracts including alpha- and beta-adaptins, subunits of the AP-2 endocytic complex. Mutations within either of the two Dvarphi(F/W) motifs found in xARH abolished binding to alpha- and beta-adaptins. Expression of a dominant negative mutant of xARH missing the clathrin box and one functional Dvarphi(F/W) motif severely inhibited endocytosis of vitellogenin in cultured oocytes. The data indicate that xARH acts as an adaptor protein linking LDL and vitellogenin receptors directly with the AP-2 complex. In oocytes, we propose that xARH mediates the uptake of lipoproteins from the blood for storage in endosomes and later use in the embryo. Our findings point to an evolutionarily conserved function for ARH in lipoprotein uptake.

Activation of lysosomal function during dendritic cell maturation

In response to a variety of stimuli, dendritic cells (DCs) transform from immature cells specialized for antigen capture into mature cells specialized for T cell stimulation. During maturation, the DCs acquire an enhanced capacity to form and accumulate peptide-MHC (major histocompatibility complex) class II complexes. Here we show that a key mechanism responsible for this alteration was the generalized activation of lysosomal function. In immature DCs, internalized antigens were slowly degraded and inefficiently used for peptide loading. Maturation induced activation of the vacuolar proton pump that enhanced lysosomal acidification and antigen proteolysis, facilitating efficient formation of peptide-MHC class II complexes. Lysosomal function in DCs thus appears to be specialized for the developmentally regulated processing of internalized antigens.

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.

Absorption, transportation and digestion of egg white in quail embryos

The present study was done to reveal how egg white is taken up by embryonic tissues, the pathway through which egg white is transported, and the location where it is digested during the development of the quail Coturnix japonica. Antiserum against quail ovalbumin was raised in rabbit and used as a probe. By immunoelectron microscopy, the uptake of ovalbumin on a small scale by receptor-mediated endocytosis was observed in the ectodermal cells of the yolk sac on days four to seven of incubation. The uptake of egg white on a large scale by fluid-phase endocytosis took place in the cells generally referred to collectively as the 'albumen sac'. The ovalbumin was transported through the albumen sac into the extraembryonic cavity during days eight to 10, and then into the amniotic cavity through the amnion approximately on day 10. Ovalbumin was present in the intestinal lumen on days 11 and 14, but it was not digested in the intestinal epithelial cells. The ovalbumin was detected in the yolk of embryos after day 10. Immunoblot testing, as well as a fluoroimmunoassay, revealed that the location where the amount of ovalbumin was highest changed chronologically from the extraembryonic cavity on day 10 to the amniotic cavity on day 11, the intestinal lumen on day 12 and then to the yolk on day 13. Several low molecular proteins which cross-reacted with the antiserum were observed in the extracts of the yolk. The reaction producing these proteins depended on low pH (approximately 3.0) and was inhibited by pepstatin A. The ovotransferrin was similarly digested. These results indicate that egg white is, for the most part, transported through the albumen sac to the yolk via the extraembryonic cavity, the amniotic cavity, and the intestinal lumen, and is digested in the yolk by aspartic proteinases.

In vivo oocyte hydration in Atlantic halibut (Hippoglossus hippoglossus); proteolytic liberation of free amino acids, and ion transport, are driving forces for osmotic water influx

The in vivo swelling and hydration of maturing oocytes of Atlantic halibut Hippoglossus hippoglossus were studied in order to characterise the osmotic mechanism underlying oocyte hydration in oviparous marine teleosts that spawn pelagic eggs. Sequential biopsies from two females, spanning four hydration cycles, were examined by osmometry, solute analysis and electrophoresis of dissected hydrating oocytes and ovulated eggs. The hydration cycle of the biopsied halibuts lasted 33–54 h. The majority of ovarian oocytes existed in a pre-hydrated condition (individual wet mass approx. 3.7 mg, diameter approx. 1.87 mm, 63 % H2O) with easily visible, non-coalesced, yolk platelets. Group-synchronous batches of the pre-hydrated oocytes increased in individual wet mass, diameter and water content to reach the ovulated egg stage of approximately 15 mg, 3.0 mm and 90 % H2O, respectively. The yolk osmolality of the hydrating oocytes was transiently hyperosmotic to the ovarian fluid (range 305–350 mOsmol l–1) with a peak osmolality of about 450 mOsmol l–1 in oocytes of 6–8 mg individual wet mass. The transient hyperosmolality was well accounted for by the increase in oocyte content of free amino acids (FAAs; approx. 2300 nmol oocyte–1), K+ (approx. 750 nmol oocyte–1), Cl– (approx. 900 nmol oocyte–1), total ammonium (approx. 300 nmol oocyte–1) and inorganic phosphate (Pi; approx. 200 nmol oocyte–1) when relating to the increase in cellular water. The oocyte content of Na+ did not increase during the hydration phase. Extensive proteolysis of yolk proteins, in particular a 110 kDa protein, correlated with the increase in the FAA pool, although the latter increased by approx. 20 % more than could be accounted for by the decrease in the oocyte protein content. Both indispensable and dispensable amino acids increased in the FAA pool, and particularly serine, alanine, leucine, lysine, glutamine and glutamate. Taurine content remained stable at approx. 70 nmol oocyte–1 during oocyte hydration. The results show that final hydration of Atlantic halibut oocytes is caused by an osmotic water uptake in which FAAs, derived mainly from the hydrolysis of a 110 kDa yolk protein, contribute approximately 50 % of the yolk osmolality and ions (Cl–, K+, Pi, NH4+) make up the balance.

Deciphering endocytosis in Caenorhabditis elegans

We discuss in this review recent studies using the worm Caenorhabditis elegans to decipher endocytic trafficking in a multicellular organism. Recent advances, including in vivo assay systems, new genetic screens, comparative functional analysis of conserved proteins, and RNA-mediated interference (RNAi) in C. elegans, are being used to study the functions of known membrane trafficking factors and to identify new ones. The ability to monitor endocytosis in vivo in worms allows one to test current endocytosis models and to demonstrate the physiological significance of factors identified by genetic and biochemical methods. The available human genome sequence facilitates comparative studies where human homologs of new factors identified in C. elegans can be quickly assayed for similar function using traditional cell biological methods in mammalian cell systems. New studies in C. elegans have used a combination of these techniques to reveal novel metazoan-specific trafficking factors required for endocytosis. Many more metazoan-specific trafficking factors and insights into the mechanisms of endocytosis are likely to be uncovered by analysis in C. elegans.

Serosa membrane plays a key role in transferring vitellin polypeptides to the perivitelline fluid in insect embryos

In mid-embryogenesis, the stick insect Carausius morosus comes to be comprised of three distinct districts: the embryo proper, the yolk sac and the perivitelline fluid. A monolayered epithelium, the so-called serosa membrane, encloses the yolk sac and its content of vitellophages and large yolk granules. During embryonic development, the yolk sac declines gradually in protein concentration due to Vt polypeptides undergoing limited proteolysis to yield a number of Vt cleavage products of lower molecular weights. mAbs 1D1 and 5H11 are monoclonal antibodies raised against some of the Vt cleavage products generated by this process in the yolk sac. At the confocal microscope, antibody fluorescence is initially associated with a few yolk granules, while it is gradually displaced in the cytosolic spaces of the vitellophages. With the proceeding of embryonic development, label appears also in the serosa membrane in the form of clustered dots. At the ultrastructural level, gold particles are initially associated with the vitellophages that are labeled on a few yolk granules and in the cytosolic space flanking the yolk granules. Subsequently, the serosa cells become labeled on vesicles close to the yolk granules or just underneath the plasma membrane. Inside the serosa cells, label is also associated with granules budding from the Golgi apparatus, but never with the intercellular channels percolating the serosa membrane. These observations are interpreted as indicating that Vt cleavage products leak out from the yolk granules into the cytosolic spaces of the vitellophages and are eventually transferred to the perivitelline fluid via transcytosis through the serosa cells.

Preoviposition activation of cathepsin-like proteinases in degenerating ovarian follicles of the mosquito Culex pipiens pallens

Within developing ovaries of many insects, some developing follicles or oocytes usually degenerate (follicular atresia or oosorption), while the others may continue to grow to maturity, thus maintaining the balance between the number of eggs and reproductive circumstances such as available nutrients. To help clarify the phenomenon of follicular atresia during ovarian development, we examined cysteine proteinases stored in mosquito Culex pipiens pallens ovaries. First, analysis using synthesized substrates showed that cathepsin B- and L-like proteinases gradually accumulated in the developing ovaries after a blood meal, which required more than 10 min of preincubation under acidic conditions to reach their maximum activities. However, homogenates of degenerating follicles 3 days after feeding showed proteolytic activities without acid treatment, suggesting that the proteinases had already been activated, while the extract of normally developing follicles collected from the same ovaries required more than 10 min of acid preincubation to reach the optimum activities, suggesting that the enzymes remained as inactive forms. Chemical and immunohistochemical analyses showed that more proteinases are located in the cytoplasm, rather than being associated with yolk granules. Ovarian proteinases, which are believed to become activated at the onset of embryogenesis, should also be activated during oogenesis, presumably to enhance oosorption.

The HPV16 E5 oncogene inhibits endocytic trafficking

The small hydrophobic E5 protein of Human Papillomavirus type 16 (HPV16) binds to the 16-kDa subunit of the V-H+-ATPase. This binding has been suggested to interfere with acidification of late endocytic structures. We here used video microscopy, ratio imaging and confocal microscopy of living C127 fibroblasts to study the effects of E5. Various endocytic markers including the pH-sensitive probe DM-NERF coupled to dextran, TransFluoSpheres and TRITC-concanavalin A, were applied. In E5-transfected cells, none of these markers colocalized with the membrane permeable probe LysoTracker Red, which accumulates in acidic, late endocytic structures, or with a green fluorescent version of the small GTPase Rab7 labeling late endocytic structures. Importantly, however, late endocytic structures accumulating LysoTracker were still present in the E5-transfected cells. It is therefore concluded that HPV16 E5 perturbs trafficking from early to late endocytic structures rather than acidification.

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.

Cysteine proteinase plays a key role for the initiation of yolk digestion during development of Xenopus laevis

In electrophoretic analyses, extracts of Xenopus laevis neurulae exhibited activities digesting yolk proteins maximally at pH 4.8. These activities were completely inhibited by a mixture of pepstatin A and Z-Phe-Phe-CHN2, thus being identifiable as cathepsin D and cysteine proteinase. The electrophoretic profiles of yolk proteins cleaved by embryonic extracts changed at gastrula stages; the profile before stage 13 was the same as that given by cathepsin D treatment and the profile at stage 13 was a combination of the profile given by cathepsin D treatment and that given by cysteine proteinase treatment. Quantitative measurement of enzyme activities showed that the cathepsin D activity that was preserved from the beginning of development increased from stages 13 to 25 and decreased thereafter, whereas the cysteine proteinase activity appeared at stage 13, gradually increased until stage 35 and strongly increased thereafter. Immunoblot analyses showed that the 43 kDa form of cathepsin D was processed to its 36 kDa form, presumably by cysteine proteinase. This change can explain the increase of cathepsin D activity at stage 13 and thereafter. Immunofluorescent staining with the antibody against cysteine proteinase occurred in mesodermal and ectodermal cells other than neural ones at stages 13-24, and in the endodermal cells at stages 24-36. Faint staining in the neural ectoderm persisted from stages 18 to 36. Immunoelectron microscope observation showed that what stained was the superficial layer of yolk platelets. All these results indicate that cysteine proteinase plays a key role in the initiation of yolk digestion during embryonic development.

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.

Defective acidification in human breast tumor cells and implications for chemotherapy

Multidrug resistance (MDR) is a significant problem in the treatment of cancer. Chemotherapeutic drugs distribute through the cyto- and nucleoplasm of drug-sensitive cells but are excluded from the nucleus in drug-resistant cells, concentrating in cytoplasmic organelles. Weak base chemotherapeutic drugs (e.g., anthracyclines and vinca alkaloids) should concentrate in acidic organelles. This report presents a quantification of the pH for identified compartments of the MCF-7 human breast tumor cell line and demonstrates that (a) the chemotherapeutic Adriamycin concentrates in acidified organelles of drug-resistant but not drug-sensitive cells; (b) the lysosomes and recycling endosomes are not acidified in drug-sensitive cells; (c) the cytosol of drug-sensitive cells is 0.4 pH units more acidic than the cytosol of resistant cells; and (d) disrupting the acidification of the organelles of resistant cells with monensin, bafilomycin A1, or concanamycin A is sufficient to change the Adriamycin distribution to that found in drug-sensitive cells, rendering the cell vulnerable once again to chemotherapy. These results suggest that acidification of organelles is causally related to drug resistance and is consistent with the hypothesis that sequestration of drugs in acidic organelles and subsequent extrusion from the cell through the secretory pathways contribute to chemotherapeutic resistance.

Purification and properties of embryonic cysteine proteinase which participates in yolk-lysis of Xenopus laevis

The study reported here aimed to purify a cysteine proteinase from neurula embryos of Xenopus laevis, since this enzyme was thought to be involved in yolk-lysis in developing embryos. The purification procedure consisted of fractionation of an embryonic extract by means of 30-90% ammonium sulfate, chromatography on diethylaminoethyl cellulose and carboxymethyl cellulose, gel filtration on Sephadex G-75 and affinity chromatography on concanavalin A-agarose. The purified enzyme had a molecular weight of 30 kDa according to both SDS-PAGE and Sephadex G-75 gel-filtration and an optimum pH of 5.5, and it preferentially cleaved the synthetic substrate, Z-Phe-Arg-MCA. Its activity was inhibited by Z-Phe-Phe-CHN2, a specific cathepsin L inhibitor, as well as by leupeptin and E-64. The NH2-terminal amino acid sequence of the enzyme was similar to that of chicken cathepsin B. These characteristics indicate that the purified enzyme is a member of the cysteine proteinase family. The antibody raised against the purified enzyme specifically stained a 30 kDa protein of neurula embryo extracts on immunoblot tests. The enzyme effectively digested Xenopus yolk proteins when the NaCl concentration in test solutions was 0.2 M. It was also confirmed that cysteine proteinase inhibitors inhibited yolk-lysis by the enzyme.

alpha-Crystallin protein cognates in eggs of the moth, Plodia interpunctella: possible chaperones for the follicular epithelium yolk protein

alpha-Crystallin protein cognates were found in germ cells of the Indianmeal moth, Plodia interpunctella (Shirk and Zimowska, 1997). A cDNA clone of 674 bp with a single open reading frame was isolated for a 25,000 molecular weight polypeptide member of this family, alpha CP25, and a single transcript of approximately 700 bp was found in the ovary of vitellogenic females. Both the DNA sequence and predicted amino acid sequence showed considerable homology with the embryonic lethal gene, l(2)efl, in Drosophila melanogaster. In addition to the sequence for l(2)efl, the predicted amino acid sequence for acp25 also showed significant sequence similarly with the alpha-crystallin A chain polypeptides from the lenses of vertebrae eyes. An N-terminal hydrophobic aggregation site and a C-terminal protective binding site common to alpha-crystallin proteins were present in the predicted acp25 and l(2)efl amino acid sequences, while only the C-terminal protective binding site was present in the small heat shock protein sequences from D. melanogaster. This evidence suggests that although the alpha-crystallin protein cognates in P. interpunctella evolved from a gene common with small heat shock protein genes, the amino acid sequence has converged on a structure similar to that of alpha-crystallin proteins. Native immunoblot analysis showed that the alpha-crystallin proteins formed high molecular weight complexes with the follicular epithelium yolk protein (FEYP) but not vitellin in yolk. An electroblot binding assay was used to show that the germ-cell alpha-crystallins of P. interpunctella bind specifically with the FEYP and that the binding was reversible in the presence of ATP or low pH. This evidence in conjunction with the evidence that the alpha-crystallins and FEYP form a stable complex that co-purifies from native egg proteins suggests that the alpha-cystallin cognates function as chaperones for the follicular epithelium yolk proteins in the embryos of P. interpunctella.

Localization of the proenzyme form of the vitellin-processing protease in Blattella germanica by affinity-purified antibodies

During Blattella germanica embryo development, the nutritive yolk protein vitellin is processed by a cysteine protease, which is activated proteolytically from a proprotease during acidification of yolk granules. A murine polyclonal antiserum was generated with the purified proprotease as the immunogen. The antiserum was made monospecific to proprotease by subtractive affinity chromatography using proprotease-free yolk proteins as ligand. The purified antibodies were employed to investigate the temporal and spatial expression of the proprotease during vitellogenesis and embryo development. Anti-proprotease-reactive peptides appeared in extracts of fat bodies and ovarian follicles of post-mating females, but not in fat bodies of males or the fat bodies or follicles of unmated females, suggesting that the proprotease is synthesized extraovarially. Use of the antibodies was extended to monitor the kinetics of proprotease disappearance during early embryo development.