Characterization of lipophorin binding to the fat body of Rhodnius prolixus

Silencing of the 20S proteasomal subunit-α6 triggers full oogenesis arrest and increased mRNA levels of the selective autophagy adaptor protein p62/SQSTM1 in the ovary of the vector Rhodnius prolixus

The high reproductive rates of insects contribute significantly to their ability to act as vectors of a variety of vector-borne diseases. Therefore, it is strategically critical to find molecular targets with biotechnological potential through the functional study of genes essential for insect reproduction. The ubiquitin-proteasome system is a vital degradative pathway that contributes to the maintenance of regular eukaryotic cell proteostasis. This mechanism involves the action of enzymes to covalently link ubiquitin to proteins that are meant to be delivered to the 26S proteasome and broken down. The 26S proteasome is a large protease complex (including the 20S and 19S subcomplexes) that binds, deubiquitylates, unfolds, and degrades its substrates. Here, we used bioinformatics to identify the genes that encode the seven α and β subunits of the 20S proteasome in the genome of R. prolixus and learned that those transcripts are accumulated into mature oocytes. To access proteasome function during oogenesis, we conducted RNAi functional tests employing one of the 20S proteasome subunits (Prosα6) as a tool to suppress 20S proteasomal activity. We found that Prosα6 silencing resulted in no changes in TAG buildup in the fat body and unaffected availability of yolk proteins in the hemolymph of vitellogenic females. Despite this, the silencing of Prosα6 culminated in the impairment of oocyte maturation at the early stages of oogenesis. Overall, we discovered that proteasome activity is especially important for the signals that initiate oogenesis in R. prolixus and discuss in what manner further investigations on the regulation of proteasome assembly and activity might contribute to the unraveling of oogenesis molecular mechanisms and oocyte maturation in this vector.

ATP synthase affects lipid metabolism in the kissing bug Rhodnius prolixus beyond its role in energy metabolism

ATP synthase plays an essential role in mitochondrial metabolism, being responsible for the production of ATP in oxidative phosphorylation. However, recent results have shown that it may also be present in the cell membrane, involved in lipophorin binding to its receptors. Here, we used a functional genetics approach to investigate the roles of ATP synthase in lipid metabolism in the kissing bug Rhodnius prolixus. The genome of R. prolixus encodes five nucleotide-binding domain genes of the ATP synthase alpha and beta family, including the alpha and beta subunits of ATP synthase (RpATPSynA and RpATPSynB), and the catalytic and non-catalytic subunits of the vacuolar ATPase (RpVha68 and RpVha55). These genes were expressed in all analyzed organs, being their expression highest in the ovaries, fat body and flight muscle. Feeding did not regulate the expression of ATP synthases in the posterior midgut or fat body. Furthermore, ATP synthase is present in the fat body's mitochondrial and membrane fractions. RpATPSynB knockdown by RNAi impaired ovarian development and reduced egg-laying by approximately 85%. Furthermore, the lack of RpATPSynB increased the amount of triacylglycerol in the fat body due to increased de novo fatty acid synthesis and reduced transfer of lipids to lipophorin. RpATPSynA knockdown had similar effects, with altered ovarian development, reduced oviposition, and triacylglycerol accumulation in the fat body. However, ATP synthases knockdown had only a slight effect on the amount of ATP in the fat body. These results support the hypothesis that ATP synthase has a direct role in lipid metabolism and lipophorin physiology, which are not directly due to changes in energy metabolism.

Knockdown of E1- and E2-ubiquitin enzymes triggers defective chorion biogenesis and modulation of autophagy-related genes in the follicle cells of the vector Rhodnius prolixus

In insects, the last stage of oogenesis is the process where the chorion layers (eggshell) are synthesized and deposited on the surface of the oocytes by the follicle cells. Protein homeostasis is determined by the fine-tuning of translation and degradation pathways, and the ubiquitin-proteasome system is one of the major degradative routes in eukaryotic cells. The conjugation of ubiquitin to targeted substrates is mediated by the ordered action of E1-activating, E2-conjugating, and E3-ligase enzymes, which covalently link ubiquitin to degradation-targeted proteins delivering them to the proteolytic complex proteasome. Here, we found that the mRNAs encoding polyubiquitin (pUbq), E1, and E2 enzymes are highly expressed in the ovaries of the insect vector of Chagas Disease Rhodnius prolixus. RNAi silencing of pUbq was lethal whereas the silencing of E1 and E2 enzymes resulted in drastic decreases in oviposition and embryo viability. Eggs produced by the E1- and E2-silenced insects presented particular phenotypes of altered chorion ultrastructure observed by high-resolution scanning electron microscopy as well as readings for dityrosine cross-linking and X-ray elemental microanalysis, suggesting a disruption in the secretory routes responsible for the chorion biogenesis. In addition, the ovaries from silenced insects presented altered levels of autophagy-related genes as well as a tendency of upregulation in ER chaperones, indicating a disturbance in the general biosynthetic-secretory pathway. Altogether, we found that E1 and E2 enzymes are essential for chorion biogenesis and that their silencing triggers the modulation of autophagy genes suggesting a coordinated function of both pathways for the progression of choriogenesis.

The metabolism and role of free fatty acids in key physiological processes in insects of medical, veterinary and forensic importance

Insects are the most widespread group of organisms and more than one million species have been described. These animals have significant ecological functions, for example they are pollinators of many types of plants. However, they also have direct influence on human life in different manners. They have high medical and veterinary significance, stemming from their role as vectors of disease and infection of wounds and necrotic tissue; they are also plant pests, parasitoids and predators whose activities can influence agriculture. In addition, their use in medical treatments, such as maggot therapy of gangrene and wounds, has grown considerably. They also have many uses in forensic science to determine the minimum post-mortem interval and provide valuable information about the movement of the body, cause of the death, drug use, or poisoning. It has also been proposed that they may be used as model organisms to replace mammal systems in research. The present review describes the role of free fatty acids (FFAs) in key physiological processes in insects. By focusing on insects of medical, veterinary significance, we have limited our description of the physiological processes to those most important from the point of view of insect control; the study examines their effects on insect reproduction and resistance to the adverse effects of abiotic (low temperature) and biotic (pathogens) factors.

The Fate of Dietary Cholesterol in the Kissing Bug Rhodnius prolixus

Insects are unable to synthesize cholesterol and depend on the presence of sterols in the diet for cell membrane composition and hormone production. Thus, cholesterol absorption, transport, and metabolism are potential targets for vector and pest control strategies. Here, we investigate the dietary cholesterol absorption and tissue distribution in the kissing bug Rhodnius prolixus using radiolabeled cholesterol. Both the anterior and posterior midguts absorbed cholesterol from the ingested blood, although the anterior midgut absorbed more. We also observed esterified cholesterol labeling in the epithelium, indicating that midgut cells can metabolize and store cholesterol. Only a small amount of labeled cholesterol was found in the hemolymph, where it was mainly in the free form and associated with lipophorin (Lp). The fat body transiently accumulated cholesterol, showing a labeled cholesterol peak on the fifth day after the blood meal. The ovaries also incorporated cholesterol, but cumulatively. The insects did not absorb almost half of the ingested labeled cholesterol, and radioactivity was present in the feces. After injection of 3H-cholesterol-labeled Lp into females, a half-life of 5.5 ± 0.7 h in the hemolymph was determined. Both the fat body and ovaries incorporated Lp-associated cholesterol, which was inhibited at low temperature, indicating the participation of active cholesterol transport. These results help describe an unexplored part of R. prolixus lipid metabolism.

Insulin receptor deficiency reduces lipid synthesis and reproductive function in the insect Rhodnius prolixus

Rhodnius prolixus, a vector of Chagas disease, is a hematophagous insect that feeds exclusively on blood. Each blood meal is digested within the first fourteen days after feeding, providing substrates for lipid synthesis for storage and egg production. These events are precisely regulated and emerging evidence points to a key function of insulin-like peptides (ILPs) in this control. Here we investigated the role of insulin receptor in the regulation of nutrient metabolism in fed adult females. The expression of insulin receptor (RhoprIR) gene was determined in adult organs, and it was highest in ovaries and previtellogenic follicles. We generated insects with RNAi-mediated knockdown of RhoprIR to address the physiological role of this receptor. RhoprIR deficiency improved longevity and reduced triacylglycerol storage in the fat body, whereas blood digestion remained unchanged for seven days after blood meal. The lower lipid content was attributable to decreased de novo lipogenesis as well as reduced incorporation of hemolymph-derived fatty acids into newly synthesized lipids within this organ. Consistent with that, fat bodies from RhoprIR-deficient insects exhibited decreased gene expression levels of lipophorin receptor (RhoprLpR), glycerol-3-phosphate acyltransferase 1 and 4 (RhoprGpat1 and RhoprGpat4), and carnitine palmitoyltransferase 1 (RhoprCpt1). Although hemolymph lipid profile was not affected by RhoprIR disruption, the concentration of circulating vitellogenin was increased. In line with these changes, RhoprIR-deficient females exhibited smaller ovaries and a marked reduction in oviposition. Taken together, these findings support a key role of insulin receptor in nutrient homeostasis, lipid synthesis and egg production following a blood meal.

Lipid metabolism in insect disease vectors

More than a third of the world population is at constant risk of contracting some insect-transmitted disease, such as Dengue fever, Zika virus disease, malaria, Chagas' disease, African trypanosomiasis, and others. Independent of the life cycle of the pathogen causing the disease, the insect vector hematophagous habit is a common and crucial trait for the transmission of all these diseases. This lifestyle is unique, as hematophagous insects feed on blood, a diet that is rich in protein but relatively poor in lipids and carbohydrates, in huge amounts and low frequency. Another unique feature of these insects is that blood meal triggers essential metabolic processes, as molting and oogenesis and, in this way, regulates the expression of various genes that are involved in these events. In this paper, we review current knowledge of the physiology and biochemistry of lipid metabolism in insect disease vectors, comparing with classical models whenever possible. We address lipid digestion and absorption, hemolymphatic transport, and lipid storage by the fat body and ovary. In this context, both de novo fatty acid and triacylglycerol synthesis are discussed, including the related fatty acid activation process and the intracellular lipid binding proteins. As lipids are stored in order to be mobilized later on, e.g. for flight activity or survivorship, lipolysis and β-oxidation are also considered. All these events need to be finely regulated, and the role of hormones in this control is summarized. Finally, we also review information about infection, when vector insect physiology is affected, and there is a crosstalk between its immune system and lipid metabolism. There is not abundant information about lipid metabolism in vector insects, and significant current gaps in the field are indicated, as well as questions to be answered in the future.

Lipid metabolism in Rhodnius prolixus: Lessons from the genome

The kissing bug Rhodnius prolixus is both an important vector of Chagas' disease and an interesting model for investigation into the field of physiology, including lipid metabolism. The publication of this insect genome will bring a huge amount of new molecular biology data to be used in future experiments. Although this work represents a promising scenario, a preliminary analysis of the sequence data is necessary to identify and annotate the genes involved in lipid metabolism. Here, we used bioinformatics tools and gene expression analysis to explore genes from different genes families and pathways, including genes for fat breakdown, as lipases and phospholipases, and enzymes from β-oxidation, fatty acid metabolism, and acyl-CoA and glycerolipid synthesis. The R. prolixus genome encodes 31 putative lipase genes, including 21 neutral lipases and 5 acid lipases. The expression profiles of some of these genes were analyzed. We were able to identify nine phospholipase A2 genes. A variety of gene families that participate in fatty acid synthesis and modification were studied, including fatty acid synthase, elongase, desaturase and reductase. Concerning the synthesis of glycerolipids, we found a second isoform of glycerol-3-phosphate acyltransferase that was ubiquitously expressed throughout the organs. Finally, all genes involved in fatty acid β-oxidation were identified, but not a long-chain acyl-CoA dehydrogenase. These results provide fundamental data to be used in future research on insect lipid metabolism and its possible relevance to Chagas' disease transmission.

Interaction of lipophorin with Rhodnius prolixus oocytes: biochemical properties and the importance of blood feeding

Lipophorin (Lp) is the main haemolymphatic lipoprotein in insects and transports lipids between different organs. In adult females, lipophorin delivers lipids to growing oocytes. In this study, the interaction of this lipoprotein with the ovaries of Rhodnius prolixus was characterised using an oocyte membrane preparation and purified radiolabelled Lp (125I-Lp). Lp-specific binding to the oocyte membrane reached equilibrium after 40-60 min and when 125I-Lp was incubated with increasing amounts of membrane protein, corresponding increases in Lp binding were observed. The specific binding of Lp to the membrane preparation was a saturable process, with a K(d) of 7.1 ± 0.9 x 10-8M and a maximal binding capacity of 430 ± 40 ng 125I-Lp/µg of membrane protein. The binding was calcium independent and pH sensitive, reaching its maximum at pH 5.2-5.7. Suramin inhibited the binding interaction between Lp and the oocyte membranes, which was completely abolished at 0.5 mM suramin. The oocyte membrane preparation from R. prolixus also showed binding to Lp from Manduca sexta. When Lp was fluorescently labelled and injected into vitellogenic females, the level of Lp-oocyte binding was much higher in females that were fed whole blood than in those fed blood plasma.

Lipophorin acts as a shuttle of lipids to the milk gland during tsetse fly pregnancy

During pregnancy in the viviparous tsetse fly, lipid mobilization is essential for the production of milk to feed the developing intrauterine larva. Lipophorin (Lp) functions as the major lipid transport protein in insects and closely-related arthropods. In this study, we assessed the role of Lp and the lipophorin receptor (LpR) in the lipid mobilization process during tsetse reproduction. We identified single gene sequences for GmmLp and GmmLpR from the genome of Glossinamorsitansmorsitans, and measured spatial and temporal expression of gmmlp and gmmlpr during the female reproductive cycle. Our results show that expression of gmmlp is specific to the adult fat body and larvae. In the adult female, gmmlp expression is constitutive. However transcript levels increase in the larva as it matures within the mother's uterus, reaching peak expression just prior to parturition. GmmLp was detected in the hemolymph of pregnant females and larvae, but not in the uterine fluid or larval gut contents ruling out the possibility of direct transfer of GmmLp from mother to offspring. Transcripts for gmmlpr were detected in the head, ovaries, midgut, milk gland/fat body, ovaries and developing larva. Levels of gmmlpr remain stable throughout the first and second gonotrophic cycles with a slight dip observed during the first gonotrophic cycle. GmmLpR was detected in multiple tissues, including the midgut, fat body, milk gland, spermatheca and head. Knockdown of gmmlp by RNA interference resulted in reduced hemolymph lipid levels, delayed oocyte development and extended larval gestation. Similar suppresion of gmmlpr did not significantly reduce hemolymph lipid levels or oogenesis duration, but did extend the duration of larval development. Thus, GmmLp function as the primary shuttle for lipids originating from the midgut and fat body to the ovaries and milk gland to supply resources for developing oocytes and larval nourishment, respectively. Once in the milk gland however, lipids are apparently transferred into the developing larva not by lipophorin but by another carrier lipoprotein.

The storage of nutritional resources during vitellogenesis of Panstrongylus megistus (Hemiptera: Reduviidae): the pathways of lipophorin in lipid delivery to developing oocytes

In this work, we have analyzed the pathways by which lipophorin (Lp) delivers its lipid cargo to developing oocytes of Panstrongylus megistus, a hematophagous vector of Chagas' disease. Lp, vitellin, total lipids and proteins were measured in ovarian tissues at different stages of the reproductive cycle. Localization of Lp in developing oocytes, mainly at their cortical area, was demonstrated by immunofluorescence assays using an anti-Lp antibody labeled with FITC. In vivo approaches injecting fluorescently labeled Lp to follow the course of the entire particle (Lp-DiI or Lp-Oregon Green) or its lipid cargo (Lp-Bodipy-FA) were monitored by laser scanning confocal microscopy. Significant increases in the amounts of lipids, proteins and vitellin were observed in ovarian tissue with the progress of vitellogenesis. Unexpectedly, an increase in the amount of Lp was also observed. The experiments in vivo demonstrated that the uptake of fluorescent Lp labeled on its protein or lipid moiety by developing oocytes occurred very fast, being impaired at low temperatures. The co-injection of fluorescent Lp and vitellogenin (Vg) showed that both particles co-localized inside yolk bodies, confirming the endocytic pathway for Lp. When the fate of lipids transferred to oocytes was evaluated in vitellogenic females by co-injecting Lp-Bodipy-FA and Lp-DiI, the signal for Bodipy-FA was found in both lipid droplets and yolk bodies. In contrast, in injected females kept at 4°C the fluorescence was reduced, being observed exclusively in lipid droplets, implying that lipid transfer to the oocyte was diminished but not abolished. Taken together, the results demonstrate that in the hematophagous P. megistus, the storage of lipid resources by developing oocytes occurs by the convergence of different pathways by which Lp maximizes the delivery of its lipid cargo. In addition, it was also shown that, to some extent, lipids stored in the oocyte lipid droplets can also originate from endocytosed Vg. The relevance of these events in the context of the physiology of reproduction in P. megistus is discussed.

A standardizable protocol for infection of Rhodnius prolixus with Trypanosoma rangeli, which mimics natural infections and reveals physiological effects of infection upon the insect

Trypanosoma rangeli is a protozoan parasite that shares hosts - mammals and triatomines - with Trypanosoma cruzi, the etiological agent of Chagas disease. Although T. rangeli is customarily considered to be non-pathogenic to human hosts, it is able to produce pathologies in its invertebrate hosts. However, advances are hindered by a lack of standardization of infection procedures and these pathologies need documentation. To establish a suitable, and standardizable, infection protocol, the duration of the fourth instar was evaluated in nymphs infected by injection into the thorax with different concentrations of parasites, and compared with nymphs infected naturally (i.e. orally). We demonstrate that delays in moult were attributable to the presence of the parasite in the haemolymph (vs. the gut) and propose that the protocol presented here simulates closely natural infections. This methodology was then used for the evaluation of physiological parameters and several hitherto unreported effects of T. rangeli infection on Rhodnius prolixus were revealed. Haemolymph volume was greater in infected than uninfected nymphs but this alteration could not be attributed to water retention, since infected insects lost the same amount of water as controls. However, we found that lipid content and fat body weight were both increased in insects infected by T. rangeli. We propose that this is due to the parasite's sequestration of host blood lipids and carrier proteins. With these findings, we have taken a few first steps to unravelling physiological details of the host-parasite interaction. We suggest future directions towards a fuller understanding of mechanistic and adaptive aspects of triatomine-trypanosomatid interactions.

Biochemical and cellular characterization of lipophorin-midgut interaction in the hematophagous Panstrongylus megistus (Hemiptera: Reduviidae)

In order to better understand the metabolism of dietary lipids in hematophagous insects, we have performed a biochemical and cellular characterization of lipophorin (Lp)-midgut interaction in Panstrongylus megistus, a vector of Chagas' disease. The study was accomplished by solid-phase binding assays or with iodinated Lp ((125)I-Lp), using midgut membranes from fifth instar nymphs after ecdysis and after insects received a blood meal. Results obtained from both physiological conditions indicated that Lp interacted specifically with the midgut, implying the participation of receptors. Binding capacity of lipophorin to membranes was dependent on the amount of membranes added in the system, reaching saturation at 0.1 microg/ml. However, membranes obtained after a blood meal exhibited higher binding activity. Saturation kinetics results using (125)I-Lp suggested a single binding site with high affinity for Lp in the midgut membranes (K(d) = 5.1 +/- 3.6 x 10(-8) M). The unrelated lipoprotein, human LDL, did not compete with Lp for its binding site in the midgut. The binding was dependent on pH and the treatment of membranes with trypsin or heat causes a significant inhibition of the binding. Midgut-Lp interaction was affected by changes in ionic strength and by suramin, but showed no requirement of calcium. Ligand blotting assays revealed two membrane proteins that specifically bound Lp (61 and 45 kDa). At cellular level, Lp binding sites were located mainly at the basal plasma membrane of isolated enterocytes. Labeled Lp with fluorescent probes directed to its proteins or its phospholipids fraction co-localized mainly at the basement membrane of the midgut. In addition, no intracellular Lp was observed at any condition. The lack of an endocytic pathway for Lp in the midgut of P. megistus is analyzed in the context of insect physiology.

Dynamics of lipid accumulation by the fat body of Rhodnius prolixus: the involvement of lipophorin binding sites

In insects, lipids are stored in the fat body, mainly as triacylglycerol (TAG). In Rhodnius prolixus, a hematophagous hemipteran, lipids are accumulated after blood meal to be used later on. In adult females, at the second day after feeding, the amount of TAG was 57+/-17 microg/fat body, it increased almost five times and at fourth day it was 244+/-35 microg/fat body. TAG content remained constant until day 13, but it then decreased and, at day 20th it was very low (31+/-4.9 microg/fat body). Radiolabeled free fatty acid was used to follow lipid accumulation by the fat body, as it was previously shown that, in R. prolixus, injected free fatty acids associate with lipophorin, a major hemolymphatic lipoprotein. (3)H-palmitic acid was injected into the hemocoel of R. prolixus females. It disappeared from the hemolymph very rapidly, and radioactivity was incorporated by the fat body. Sixty minutes after injection, radioactivity in the fat body was found mainly in TAGs. The capacity of the fat body to incorporate fatty acids from the hemolymph varied according to the days after blood meal, and it was maximal around the fourth day. Lipophorin binding to specific sites in fat body membrane preparations also showed variation at different days. When membranes obtained from insects at the second, fifth and tenth days were compared, binding was highest at fifth day after feeding.

Lipid metabolism in Rhodnius prolixus (Hemiptera: Reduviidae): role of a midgut triacylglycerol-lipase

The utilization of dietary lipids was studied in adult females of Rhodnius prolixus with the use of radiolabeled triacylglycerol (TAG). It was shown that (3)H-triolein, when added to blood meal, was hydrolyzed to free fatty acids in the posterior midgut lumen. Subsequently, free fatty acids were absorbed by posterior midgut epithelium and used in the synthesis of phospholipids, diacylglycerol (DAG) and TAG. Phospholipids, DAG and free fatty acids were then found in hemolymph, from where they were rapidly cleared, and label was found in the fat body, mainly associated with TAG. Radioactive lipids, especially TAG and phospholipids, also accumulated in the ovaries. The TAG-lipase activities of posterior midgut luminal content and tissue were characterized by incubation of these samples with (3)H-triolein in the presence of the detergent Triton X-100 and determination of the amounts of released radioactive free fatty acids. Under the conditions employed here, the release of free fatty acids was proportional to the incubation time and to the amount of sample obtained from insect midgut (enzyme source) that was added. TAG-lipase activities were affected by pH and posterior midgut tissue showed optimum activity around pH 7.0-7.5, but the luminal content had the highest activities as pH decreased. Differences in activities were observed according to calcium concentration in the medium. TAG-lipase activities were also affected by the concentration of NaCl and were activated in the presence of increasing salt concentrations. These activities were inhibited by phenylmethylsulphonyl fluoride (PMSF). On the second day after blood meal, when digestion is very intense, TAG-lipase activities were maximal and then gradually decreased.

Oogenesis and egg development in triatomines: a biochemical approach

In triatomines, as well as in other insects, accumulation of yolk is a process in which an extra-ovarian tissue, the fat body, produces yolk proteins that are packed in the egg. The main protein, synthesized by the fat body, which is accumulated inside the oocyte, is vitellogenin. This process is also known as vitellogenesis. There are growing evidences in triatomines that besides fat body the ovary also produces yolk proteins. The way these yolk proteins enter the oocyte will be discussed. Yolk is a complex material composed of proteins, lipids, carbohydrates and other minor components which are packed inside the oocyte in an organized manner. Fertilization triggers embryogenesis, a process where an embryo will develop. During embryogenesis the yolk will be used for the construction of a new individual, the first instar nymph. The challenge for the next decade is to understand how and where these egg proteins are used up together with their non-protein components, in pace with the genetic program of the embryo, which enables cell differentiation (early phase of embryogenesis) and embryo differentiation (late phase) inside the egg.

Lipophorin interaction with the midgut of Rhodnius prolixus: characterization and changes in binding capacity

Several classes of lipids are transported in insect hemolymph by lipophorin, a major hemolymphatic lipoprotein. The binding of lipophorin to the midgut of the hematophagous insect Rhodnius prolixus was characterized in a midgut membrane preparation, using purified lipophorin radiolabelled in protein moiety ((125)I-HDLp). Lipophorin specific binding to membranes achieved equilibrium after 30-40 min, was sensitive to pH, and was maximal at pH 7.0. In the presence of increasing concentrations of membrane protein, corresponding increases in lipophorin binding were observed. The specific binding of lipophorin to the membrane preparation was a saturable process, with K(d)=0.9+/-0.06 x 10(-7) M and a maximal binding capacity of 70+/-11 ng lipophorin/microg of membrane protein. Lipophorin binding did not depend on calcium, but it was affected by ionic strength and was inhibited in the presence of increasing salt concentrations. Suramin interfered with lipophorin binding to the midgut receptor, and it was abolished in the presence of 2 mM suramin, but at concentrations between 0.05 and 0.2 mM it was slightly increased. Condroitin 4-sulfate also affected lipophorin binding, which was reduced to 56% of control. Pre-incubation of the midgut membrane preparation with trypsin or at high temperature inhibited binding. Midgut capacity to bind lipophorin varied at different days after blood meal. It was highest at second day after feeding, and then gradually decreased.