Molecular characterization of the mosquito vitellogenin receptor reveals unexpected high homology to the Drosophila yolk protein receptor

The mosquito (Aedes aegypti) vitellogenin receptor (AaVgR) is a large membrane-bound protein (214 kDa when linearized) that mediates internalization of vitellogenin, the major yolk-protein precursor, by oocytes during egg development. We have cloned and sequenced two cDNA fragments encompassing the entire coding region of AaVgR mRNA, to our knowledge the first insect VgR sequence to be reported. The 7.3-kb AaVgR mRNA is present only in female germ-line cells and is abundant in previtellogenic oocytes, suggesting that the AaVgR gene is expressed early in oocyte differentiation. The deduced amino acid sequence predicts a 202.7-kDa protein before posttranslational processing. The AaVgR is a member of the low density lipoprotein receptor superfamily, sharing significant homology with the chicken (Gallus gallus) VgR and particularly the Drosophila melanogaster yolk protein receptor, in spite of a very different ligand for the latter. Distance-based phylogenetic analyses suggest that the insect VgR/yolk protein receptor lineage and the vertebrate VgR/low density lipoprotein receptor lineage diverged before the bifurcation of nematode and deuterostome lines.

The mosquito ultraspiracle homologue, a partner of ecdysteroid receptor heterodimer: cloning and characterization of isoforms expressed during vitellogenesis

We report the cloning and characterization of two isoforms of the Ultraspiracle homologue (AaUSP) from the mosquito, Aedes aegypti. The 2.33-kb AaUSPa cDNA has an open reading frame (ORF) of 484 amino acids encoding a polypeptide of 54 kDa, whereas the 2.14-kb AaUSPb ORF of 459 amino acids encodes a 51.3 kDa polypeptide. The AaUSPa and AaUSPb proteins differ only in the N-terminal portion of the variable A/B domain. The AaUSP DNA-binding domain shares 92% and 97% identities with the respective domains of the Drosophila (DmUSP) and Bombyx (BmUSP) Ultraspiracles. However, the AaUSP ligand-binding domain is only 57% and 52% identical to those of DmUSP and BmUSP, respectively. In spite of the relatively low level of sequence conservation, electrophoretic mobility shift assay (EMSA) and hormone-binding assay clearly demonstrated that the products of the AaUSPa and AaUSPb cDNAs are functional heterodimeric partners of the mosquito ecdysteroid receptor. In vitellogenic tissues, each of the two AaUSP isoforms is expressed differently: the AaUSPa is predominant in the fat body and the AaUSPb in the ovary. The kinetics of ovarian AaUSP mRNA coincide with those of the ecdysteroid receptor, being elevated during the previtellogenic period and shortly after the onset of vitellogenesis. In contrast, the level of the AaUSP in the fat body remains relatively constant throughout most of the vitellogenic cycle.

Subunit cleavage of mosquito pro-vitellogenin by a subtilisin-like convertase

The eukaryotic convertase family plays an important role in posttranslational proteolytic processing and activation of many pro- and polypeptides that have at their cleavage sites the paired basic motif, RX(K/R)R. Recent studies have revealed that the cleavage site of insect pro-vitellogenins (pro-Vg) also contains this motif. To identify and characterize the insect pro-Vg processing enzyme, Vg convertase (VC), its cDNA was cloned from a vitellogenic female fat body cDNA library of the mosquito, Aedes aegypti. The 3735-bp-long VC cDNA has an open reading frame encoding a 115-kDa protein. In vitro transcription/translation of VC cDNA revealed that this 115-kDa protein becomes 140 kDa after co- and posttranslational modifications. The VC deduced amino acid sequence has high similarity to and a domain structure characteristic of furin-like convertases. Northern blot analysis showed that a single 4.2-kb transcript was expressed in the fat body during the first 18 hr of the Vg synthetic period. Coexpression of VC cDNA with mosquito Vg cDNA resulted in correct cleavage of pro-Vg. Thus, this newly identified convertase is, indeed, a functional fat body-specific enzyme for pro-Vg cleavage.

Mosquito vitellogenin receptor: purification, developmental and biochemical characterization

Vitellogenin receptors (VgRs) play a critical role in egg development of oviparous animals by mediating endocytosis of the major yolk protein precursor, vitellogenin. A modification of the method for extracting the mosquito (Aedes aegypti) VgR from ovary membranes resulted in an 11-fold higher yield and 56-fold increase in relative purity of the VgR, in turn permitting purification, antibody production, and microsequencing. A Kd of 15 nM was estimated from binding assays for the enriched VgR, indicating a very high affinity for its ligand. Immunoprecipitation of [14C]VgR using anti-VgR polyclonal antibodies followed by SDS-PAGE under reducing conditions and fluorography demonstrated that the 205 kDa VgR does not consist of subunits held together with disulfide bonds. However, an immunoblot of the native VgR suggests that it exists as an approximately 390 kDa noncovalent homodimer in its native state. Immunoblot assays confirmed that the VgR is present only in ovarian tissue. A quantitative immunoassay of VgR extracts showed that VgR was present in previtellogenic ovaries on the day of emergence, increasing from 2 ng to more than 10 ng per ovary by day 5. After initiation of vitellogenesis and onset of Vg uptake, VgR quantity increased rapidly between 8 and 24 h after a blood meal, then began to decline between 24 and 36 h. Immunocytochemistry confirmed the presence of substantial amounts of the VgR in 4-day-old previtellogenic oocytes. In both previtellogenic and vitellogenic ovaries, the VgR was present only in the oocyte, primarily in the cortex.

Indirect control of yolk protein genes by 20-hydroxyecdysone in the fat body of the mosquito, Aedes aegypti

In response to a blood meal, the fat body of the female mosquito, Aedes aegypti, begins massive production of several yolk proteins which are subsequently stored in the developing oocytes. Although 20-hydroxyecdysone (20E) is important for initiation and maintenance of expression of the yolk protein genes encoding vitellogenin (Vg) and vitellogenic carboxypeptidase (VCP), the exact nature of 20E action has not been clearly defined. A primary question is whether this hormone directly stimulates expression of the genes for Vg and VCP or if it acts indirectly through a hormone response cascade. We have demonstrated that 10(-4) M cycloheximide (Chx) reversibly inhibits > 98% of protein synthesis in in vitro fat body culture. 10(-5) M 20E stimulates high levels of the mRNAs for Vg and VCP in previtellogenic fat bodies cultured in vitro, but initiation of this expression is eliminated by Chx. Thus, our results indicate that protein synthesis is required in response to 20E before increased levels of yolk protein mRNAs can be measured. We therefore conclude that the action of 20E is indirect.

Mosquito ecdysteroid receptor: analysis of the cDNA and expression during vitellogenesis

An insect steroid hormone, 20-hydroxyecdysone (20E), plays an important role in regulating egg maturation in mosquitoes. To better understand its role, we cloned the cDNA coding for the putative ecdysteroid receptor from the mosquito, Aedes aegypti (AaEcR). The 4158 bp AaEcR cDNA has an open reading frame of 675 amino acids with 10 potential glycosylation sites and a putative phosphorylation polyserine domain. The AaEcR has a DNA binding domain with two zinc fingers and a ligand binding domain characteristic of members of the steroid hormone receptor superfamily. These AaEcR domains share 97 and 87% identities with the respective domains of the Drosophila ecdysteroid receptor (DmEcR). However, the A/B region of the AaEcR shares 35% identity with that of DmEcR-B1 isoform. The F region, located at the carboxyl-terminal of the AaEcR, has only 9% identity with the corresponding region of DmEcR. Potential nuclear targeting and dimerization signals are also present in the AaEcR sequence. There are three AaEcR transcripts of 4.2 kb, 6 kb and 11 kb in adult mosquitoes. 4.2 kb mRNA is predominantly expressed in female mosquitoes during vitellogenesis. In both the fat body and ovaries of the female mosquito, the level of AaEcR mRNA is high at the previtellogenic period and after the onset of vitellogenesis (6 h post blood meal, PBM).

Analysis of mosquito vitellogenin cDNA. Similarity with vertebrate phosvitins and arthropod serum proteins

The cDNA coding for vitellogenin of the mosquito Aedes aegypti was cloned and sequenced. An immunological analysis of expressed deletions from the 5'-end of the vitellogenin cDNA clones using vitellogenin subunit-specific antibodies showed that the small vitellogenin subunit is located at the N terminus and the large one at the carboxy-portion of the pre-provitellogenin. The position of the cleavage between the vitellogenin subunits in the pre-provitellogenin was identified by locating the N terminus of the large subunit. The cleavage site has a consensus RXRR for the subtilisin-processing endoprotease. Mosquito vitellogenin is highly hydrophilic with 17 putative N-linked glycosylation sites and 13 potential tyrosine sulfation sites. In contrast to known invertebrate vitellogenins, mosquito vitellogenin contains three polyserine domains that are similar to those of phosvitins in vertebrate vitellogenins. These polyserine domains, originally presumed to be vertebrate-specific, have several phosphorylation consensus sites in their sequences. Unlike other known vitellogenins, mosquito vitellogenin is rich in aromatic amino acid residues, tyrosine and phenylalanine, and in this respect is similar to insect serum proteins, arylphorins. This similarity suggests that mosquito vitellogenin may supply aromatic amino acids to the cuticle of rapidly developing embryos.

Cloning and analysis of the locus for mosquito vitellogenic carboxypeptidase

Vitellogenic carboxypeptidase is a 53 kDa yolk protein produced by the fat body of the female mosquito, Aedes aegypti, in response to a blood meal. Its expression is sex-, stage- and tissue-specific and is identical to that of the major yolk protein, vitellogenin. The gene is intronless and two alleles have been cloned and sequenced, including more than 1.5 kb on both sides of the coding region. A capsite consensus recently identified as an arthropod initiator is present at the start site of transcription. Upstream of this capsite is a 16 bp imperfect palindrome repeated four times showing strong homology to defined hormone-response elements. In addition, a region that closely resembles the fat body enhancer and double sex binding site from the Drosophila yolk protein genes and several potential fat body-specific regulatory protein binding sites were found.

Cloning of cDNA for mosquito lysosomal aspartic protease. Sequence analysis of an insect lysosomal enzyme similar to cathepsins D and E

A cDNA coding for the lysosomal aspartic protease from the mosquito (mLAP) was cloned and sequenced. The mLAP cDNA is 1420 base pairs long with an open reading frame of 387 amino acids. The deduced amino acid sequence contains a signal pre-propeptide sequence of 18 amino acids followed by 369 amino acids with a 35-amino acid putative pro-enzyme domain in the NH2-terminal. The amino acid sequence of mLAP is 92 and 81% similar to human cathepsin D and cathepsin E, respectively. Typical cleavage sites for cathepsin D processing into light and heavy chains are lacking in mLAP. A single glycosylation site occurs in the mLAP sequence at a position corresponding to the first glycosylation site of cathepsins D. The mLAP sequence shares putative phosphorylation determinants, which in cathepsins D are linked to the formation of mannose 6-phosphate. In the mosquito fat body, lysosomal enzymes specifically degrade organelles involved in the biosynthesis and secretion of vitellogenin. The mLAP mRNA accumulates to its highest level 24 h after initiation of vitellogenin synthesis and 12 h before the peak of mLAP protein accumulation and its enzymatic activity. Translational regulation of mLAP mRNA may occur. The 5'-untranslated region of mLAP mRNA is similar to elements conferring negative translational control by steroids.

An extraovarian protein accumulated in mosquito oocytes is a carboxypeptidase activated in embryos

We report a phenomenon previously unknown for oviparous animals; in Aedes aegypti mosquitoes a serine carboxypeptidase is synthesized extraovarially and then internalized by oocytes. The cDNA encoding mosquito vitellogenic carboxypeptidase (VCP) was cloned and sequenced. The VCP cDNA hybridizes to a 1.5-kilobase mRNA present only in the fat body of vitellogenic females. The deduced amino acid sequence of VCP shares significant homology with members of the serine carboxypeptidase family. Binding assays using a serine protease inhibitor, [3H]diisopropyl fluorophosphate, showed that VCP is activated in eggs at the onset of embryonic development. Activation of VCP is associated with the reduction in its size from 53 kDa (inactive proenzyme) to 48 kDa (active enzyme). The active, 48-kDa, form of VCP is maximally present at the middle of embryonic development and disappears by the end.

Control of follicular epithelium development and vitelline envelope formation in the mosquito; role of juvenile hormone and 20-hydroxyecdysone

Using microsurgical manipulations, hormone applications, and transmission electron microscopy we have investigated the regulation of differentiation of the follicular epithelium and formation of the vitelline envelope (VE) in primary follicles in the ovary of the mosquito, Aedes aegypti. During the first 3 days after eclosion, the primary follicle grows, and cells of the follicular epithelium differentiate, their content of mitochondria, rough endoplasmic reticulum, and Golgi complexes increases significantly. Growth and differentiation of the follicular epithelium appear to be under the control of juvenile hormone (JH), because they are blocked by removal of corpora allata in newly closed adult females and can be restored by either implantation of corpora allata or application of JH III. In insects, including mosquitoes, VE is the first layer of the eggshell to be deposited. It is formed from the secretory products of the follicle cells and its deposition coincides with yolk accumulation by developing oocytes. Only follicle cells adjacent to the oocyte deposit VE. In decapitated females, given a blood meal by enema and injected with picogram doses of 20-hydroxyecdysone (20-HE), follicle cells synthesize the VE precursors and deposit morphologically normal VE, in contrast to saline injected controls which deposit no VE. We conclude that 20-HE, as well as factors originating from the blood meal and the oocyte, are required for the normal formation of VE in the mosquito follicles.

Biosynthesis of mosquito vitellogenin

Vitellogenin (Vg), the hemolymph precursor to the major yolk protein in mosquitoes, is synthesized in the fat body of blood-fed females. Mosquito Vg consists of two subunits with Mr = 200,000 and 66,000. Here, we demonstrate that both the Vg subunits are first synthesized as a single precursor. The identity of this Vg precursor was confirmed by immunoprecipitation with subunit-specific monoclonal antibodies. In cell-free translation of fat body poly (A)+ RNA, the Vg precursor had Mr = 224,000 which increased to 240,000 in the presence of canine pancreatic microsomal membranes. A precursor with Mr = 250,000 was immunoprecipitated in microsomal fractions isolated from rat bodies. With in vitro pulse labeling, the 250-kDa precursor could be detected in homogenates of fat bodies from blood-fed mosquitoes only during the first few hours accumulation of the Vg precursor was achieved by an in vitro stimulation of Vg synthesis in previtellogenic fat bodies cultured with an insect hormone, 20-hydroxyecdysone. The 250-kDa precursor was glycosylated and to a much lesser degree phosphorylated. Treatment of fat bodies with tunicamycin yielded the precursor with Mr = 226,000 which was neither glycosylated nor phosphorylated. The reduction in molecular mass of the 250-kDa Vg precursor and of both mature Vg subunits combined was similar after digestion with endoglycosidase H, indicating that glycosylation is completed prior to cleavage of the Vg precursor. In vitro pulse-chase experiments revealed rapid proteolytic cleavage of the 250-kDa precursor to two polypeptides with Mr = 190,000 and 62,000 which transformed into mature Vg subunits of 200- and 66-kDa as the last step prior to Vg secretion. This last step in Vg processing was inhibited by an ionophore, monensin, and therefore occurred in the Golgi complex. Sulfation as an additional, previously unknown, modification of mosquito Vg was revealed by the incorporation of sodium [35S]sulfate into both Vg subunits. Since sulfation of Vg was predominantly blocked by monensin, the final maturation of Vg subunits in the Golgi complex is, at least in part, due to this modification.

Juvenile hormone controls previtellogenic proliferation of ribosomal RNA in the mosquito fat body

During the previtellogenic development of mosquito fat body cells, the nucleolus, the organelle responsible for producing ribosomes, enlarges threefold, reaching maximal size between 2 and 3 days after eclosion. The granular component of the nucleolus containing ribosomal precursors increases considerably as well. These signs of nucleolar activation correlate with the synthetic rate and accumulation of poly(A)- RNA (predominantly ribosomal RNA) in the fat body cells. The amount of poly(A)- RNA in fat body cells increases during the first 2 days after eclosion and then declines gradually. The rate of RNA synthesis exhibits similar kinetics, but both the rise and the decline are sharper than for the accumulation of RNA. All the characteristics of nucleolar activation, its enlargement, accumulation of poly(A)- RNA, and the increased rate of RNA synthesis, are blocked by removal of the corpora allata (CA) in newly eclosed adult females but could be restored by either implantation of CA or topical application of juvenile hormone III or its analog, 7-S-methoprene, to allatectomized females. Thus, previtellogenic activation of fat body nucleoli for ribosomal RNA production is controlled by juvenile hormone from the corpora allata.

Mosquito vitellogenin subunits originate from a common precursor

Using a cell-free translation system, we demonstrated that the two subunits of mosquito vitellogenin (VG), 200 kDa and 65 kDa, originate from a common precursor. The precursor polypeptide of 220 kDa is a translation product specific to mRNA from vitellogenic mosquitoes. In immunoprecipitation analysis, the 220-kDa polypeptide was recognized by monoclonal antibodies directed either to the large or the small VG subunit. Peptide mapping showed homology between the 220-kDa polypeptide and both subunits, thus providing further proof that the 220-kDa product of translation is the precursor for both VG subunits. In the presence of microsomal membranes, the molecular size of the VG precursor increased to 235 kDa suggesting this as a first step in co-translational modifications of VG.

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.

The cell biology of mosquito vitellogenesis

Insect vitellogenesis involves coordinated activities of the fat body and oocytes. We have studied these activities at the cellular level in the mosquito. During each vitellogenic cycle, the fat body undergoes three successive stages: 1) proliferation of biosynthetic organelles, 2) vitellogenin synthesis, 3) termination of vitellogenin synthesis and degradation of biosynthetic organelles by lysosomes. Analysis with monoclonal antibodies and radiolabelling demonstrated that the mosquito yolk protein consists of two subunits (200-kDa and 65-kDa). Both subunits are glycosylated, their carbohydrate moieties are composed of high-mannose oligosaccharides. The yolk protein subunits are derived from a single 220 kDa precursor detected by an in vitro translation. Oocytes become competent to internalize proteins as a result of juvenile hormone-mediated biogenesis of endocytotic organelles. The yolk protein is then accumulated by receptor-mediated endocytosis. A pathway of the yold protein and factors determining its routing in the oocyte have been studied.

Mosquito trypsin: immunocytochemical localization in the midgut of blood-fed Aedes aegypti (L.)

A polyclonal antibody was raised against trypsin purified from the midgut of blood-fed Aedes aegypti. Using this antibody and our modification of the peroxidase-antiperoxidase immunocytochemical reaction, strong activity was found in the lumen of the midgut at the light-microscopical level. The activity was localized mainly in the posterior part of the distensible, abdominal midgut, along the periphery of the blood bolus and within the peritrophic membrane. Immunoreactivity appeared 8 h after the blood meal and was most prominent around 24 h, coinciding with our previous spectrophotometric determinations of trypsin. At the electron-microscopical level, secretory granules, immunocytochemically labelled with anti-trypsin antibody and protein A-colloidal gold, were first detected about 12 h after the blood meal. At 18 h, the secretory pathway could be followed immunocytochemically from the formation of granules in the Golgi complex until their release by exocytosis in the midgut lumen. By 24 h, there was a reduction in secretory granules, and large lysosomes appeared. The process of secretion described for this mosquito is comparable to similar events in vertebrate secretory systems and the presence of an intracellular trypsinogen is suggested.

Lysosomes in the cessation of vitellogenin secretion by the mosquito fat body; selective degradation of Golgi complexes and secretory granules

A massive and selective degradation of Golgi complexes, secretory granules, and RER is the mechanism responsible for the rapid termination of Vg secretion by trophocytes of the mosquito fat body. These cells are involved in an intensive synthesis of a glycoprotein, vitellogenin (Vg), which is accumulated by developing oocytes as yolk protein. Previously, assays for lysosomal enzymes have demonstrated that the cessation of Vg synthesis is characterized by a sharp increase in lysosomal activity; and fluorescent microscopy has shown that, during this intense lysosomal activity, Vg concentrates in lysosomes. In this report, electron microscopy combined with cytochemistry for lysosomal enzymes and localization of Vg with colloidal gold immunocytochemistry has shown that this lysosomal activity is directed towards selective degradation of Vg and organelles associated with its synthesis and secretion. Three organelles undergo lysosomal breakdown: the Golgi complex, Vg-containing secretory granules, and RER. The degradation of Golgi complexes occurs in two steps similar to that for RER: first, the organelle is sequestered by double isolation membranes, and the resulting pre-lysosome then fuses with a primary or secondary lysosome. In contrast, mature Vg-containing secretory granules fuse with lysosomes directly. This combination of crino- and autophagy is a specific, highly intense, and precisely timed event.

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

We have investigated the internalization pathways for a specific protein, vitellogenin, and a non-specific protein, horseradish peroxidase, in the mosquito oocyte in vivo. The internalized proteins were localized by electron microscopical immunocytochemistry or autoradiography; the relationship of their destination compartments with lysosomes was monitored by visualization of acid phosphatase. Proteins internalized by the oocyte follow either a specific accumulative route or a lysosomal degradative route. Via coated vesicles, both proteins enter the same compartment, the endosome, where they dissociate from membrane-binding sites. The route to their final destination depends on the presence of the specific ligand. In its absence, the degradative route is followed, and the endosome with non-specific protein fuses with lysosomes. In the presence of the specific ligand, the accumulative route is followed, and both specific and non-specific proteins are delivered into an accumulative compartment, the transitional yolk body. During the transformation of the transitional yolk body into the final storage compartment, a mature yolk body, vitellogenin undergoes crystallization, whereas the non-specific protein is concentrated in small vesicular extensions of the compartmental membrane. These vesicles are separated from the yolk bodies and apparently deliver the non-specific protein into the lysosomal system. We concluded that any protein bound to the membrane would be internalized by the oocyte, but only binding of the specific ligand to its receptor serves as a transmembrane signal stimulating the formation of accumulative compartments.

Hormone-mediated formation of the endocytic complex in mosquito oocytes

The developmental events leading to oocyte competence to internalize proteins, and the hormonal control of these events in the mosquito Aedes aegypti have been studied. The oocytes of newly eclosed females have an undifferentiated cortex. During previtellogenic development, a highly specialized endocytic complex, consisting of numerous coated vesicles and uncoated endosomes, microvilli, and, presumably, vitellogenin receptors, forms in the oocyte cortex. Morphometric analysis and probes with a protein tracer, horseradish peroxidase, have shown that only the oocytes with developed endocytic complexes are competent for protein uptake. In vivo experiments have demonstrated that the formation of the endocytic complex is controlled by juvenile hormone from the corpora allata. This developmental event was blocked by ablation of corpora allata at eclosion, but it was restored by either implantation of corpora allata into allatectomized female or the application of juvenile hormone III.

Ultrastructure of midgut endocrine cells in the adult mosquito, Aedes aegypti

The ultrastructure of endocrine cells in the midgut of the adult mosquito, Aedes aegypti, resembled that of endocrine cells in the vertebrate gastro-intestinal tract. Midgut endocrine cells, positioned basally in the epithelium as single cells, were cone-shaped and smaller than the columnar digestive cells. The most distinctive characteristic of endocrine cells was numerous round secretory granules along the lateral and basal plasma membranes where contents of the granules were released by exocytosis. Secretory granules in each individual cell were exclusively of one type, either solid or 'haloed', and for all cells observed, the range in granule diameter was 60-120 nm. The cytoplasm varied in density from clear to dark. Lamellar bodies were prominent in the apical and lateral cellular regions and did not exhibit acid phosphatase activity. The basal plasma membrane was smooth adjacent to the basal lamina, whereas in digestive cells the membrane formed a labyrinth. Some endocrine cells reached the midgut lumen and were capped by microvilli; a system of vesicles and tubules extended from beneath the microvilli to the cell body. An estimated 500 endocrine cells were distributed in both the thoracic and abdominal regions of the adult midgut. In one midgut, we classified a sample of endocrine cells according to cytoplasmic density and granule type and size; endocrine cells with certain types of granules had specific distributions within the midgut.

Accumulations of membrane-free clathrin-like lattices in the mosquito oocyte

Mosquito oocytes assemble their protein yolk by internalizing extraovarian vitellogenin and therefore are highly specialized for receptor-mediated endocytosis. Large aggregates of filamentous material appear in the ooplasm during the previtellogenic development of endocytic organelles and consist of a meshwork of filamentous subunits and numerous membrane-free clathrin-like cages. The cages, 50 to 60 nm in diameter, are arranged in polyhedral lattices. The morphological characteristics of the aggregates strongly suggest that they represent accumulations of clathrin and possibly other proteins of the coat that are synthesized in large amounts prior to endocytic activity.

Previtellogenic development and vitellogenin synthesis in the fat body of a mosquito: an ultrastructural and immunocytochemical study

We describe two phases, previtellogenic and vitellogenic, in the activity of the trophocytes in the fat body of the mosquito Aedes aegypti. The previtellogenic phase, leading to trophocyte competence to synthesize vitellogenin (Vg), occurred during the first 3 days after eclosion. This phase was characterized by enlargement and activation of the nucleoli, proliferation of ribosomes and rough endoplasmic reticulum (RER), development of Golgi complexes, and extensive invaginations of the plasma membrane. During the vitellogenic phase, initiated by a blood meal, Vg was first detected, by immunofluorescence, 1 hr after feeding. The intensity of the immunoreaction increased for the next 24 hr, was declining at 30 hr, and had disappeared by 48 hr. Vg synthesis was characterized ultrastructurally by the enlargement of the RER and the formation of dense secretion granules in Golgi complexes. These secretion granules were two to three times larger at the peak of Vg synthesis than at the beginning. The granules discharged their contents by exocytosis. Two electron microscopical immunocytochemical methods, immunoferritin and peroxidase-antiperoxidase, confirmed this pathway of Vg processing. For the first 12 hr after feeding. Vg synthetic organelles proliferated and the active nucleoli were multilobed; thereafter, while Vg synthesis continued, the nucleoli began to regress into compact bodies. Termination of Vg synthesis was marked by autophagical degradation of Vg synthetic and processing organelles.