Role of Methoprene-tolerant in the regulation of oogenesis in Dipetalogaster maxima

Juvenile hormone (JH) signalling, via its receptor Methoprene-tolerant (Met), controls metamorphosis and reproduction in insects. Met belongs to a superfamily of transcription factors containing the basic Helix Loop Helix (bHLH) and Per Arnt Sim (PAS) domains. Since its discovery in 1986, Met has been characterized in several insect species. However, in spite of the importance as vectors of Chagas disease, our knowledge on the role of Met in JH signalling in Triatominae is limited. In this study, we cloned and sequenced the Dipetalogaster maxima Met transcript (DmaxMet). Molecular modelling was used to build the structure of Met and identify the JH binding site. To further understand the role of the JH receptor during oogenesis, transcript levels were evaluated in two main target organs of JH, fat body and ovary. Functional studies using Met RNAi revealed significant decreases of transcripts for vitellogenin (Vg) and lipophorin (Lp), as well as their receptors. Lp and Vg protein amounts in fat body, as well as Vg in hemolymph were also decreased, and ovarian development was impaired. Overall, these studies provide additional molecular insights on the roles of JH signalling in oogenesis in Triatominae; and therefore are relevant for the epidemiology of Chagas´ disease.

Juvenile hormone mediates lipid storage in the oocytes of Dipetalogaster maxima

Triatomines are vectors of Chagas disease and important model organisms in insect physiology. "Kissing bugs" are obligatory hematophagous insects. A blood meal is required to successfully complete oogenesis, a process primarily controlled by juvenile hormone (JH). We used Dipetalogaster maxima as an experimental model to further understand the roles of JH in the regulation of vitellogenesis and oogenesis. A particular focus was set on the role of JH controlling lipid and protein recruitment by the oocytes. The hemolymph titer of JH III skipped bisepoxide increased after a blood meal. Following a blood meal there were increased levels of mRNAs in the fat body for the yolk protein precursors, vitellogenin (Vg) and lipophorin (Lp), as well as of their protein products in the hemolymph; mRNAs of the Vg and Lp receptors (VgR and LpR) were concomitantly up-regulated in the ovaries. Topical administration of JH induced the expression of Lp/LpR and Vg/VgR genes, and prompted the uptake of Lp and Vg in pre-vitellogenic females. Knockdown of the expression of LpR by RNA interference in fed females did not impair the Lp-mediated lipid transfer to oocytes, suggesting that the bulk of lipid acquisition by oocytes occurred by other pathways rather than by the endocytic Lp/LpR pathway. In conclusion, our results strongly suggest that JH signaling is critical for lipid storage in oocytes, by regulating Vg and Lp gene expression in the fat body as well as by modulating the expression of LpR and VgR genes in ovaries.

The role of extracellular nucleic acids in the immune system modulation of Rhodnius prolixus (Hemiptera: Reduviidae)

Extracellular traps (ETs) are extracellular nucleic acids associated with cytoplasmic proteins that may aid in the capture and killing of pathogens. To date, only a few insects were shown to display this kind of immune response. Jaburetox, a peptide derived from jack bean urease, showed toxic effects in Rhodnius prolixus, affecting its immune response. The present study aims to evaluate the role of extracellular nucleic acids in R. prolixus' immune response, using Jaburetox as a model entomotoxin. The insects were treated with extracellular nucleic acids and/or Jaburetox, and the cellular and humoral responses were assessed. We also evaluated the release of extracellular nucleic acids induced by toxins, and performed immunocompetence assays using pathogenic bacteria. Our results demonstrated that extracellular nucleic acids can modulate the insect immune responses, either alone or associated with the toxin. Although RNA and DNA induced a cellular immune response, only DNA was able to neutralize the Jaburetox-induced aggregation of hemocytes. Likewise, the activation of the humoral response was different for RNA and DNA. Nevertheless, it was observed that both, extracellular DNA and RNA, immunocompensated the Jaburetox effects on insect defenses upon the challenge of a pathogenic bacterium. The toxin was not able to alter cellular viability, in spite of inducing an increase in the reactive species of oxygen formation. In conclusion, we have demonstrated a protective role for extracellular nucleic acids in R. prolixus´ immune response to toxins and pathogenic bacteria.

Lipid transfer particle mediates the delivery of diacylglycerol from lipophorin to fat body in larval Manduca sexta

This work analyzed the process of lipid storage in fat body of larval Manduca sexta, focusing on the role of lipid transfer particle (LTP). Incubation of fat bodies with [(3)H]diacylglycerol-labeled lipophorin resulted in a significant accumulation of diacylglycerol (DAG) and triacylglycerol (TAG) in the tissue. Transfer of DAG to fat body and its storage as TAG was significantly inhibited (60%) by preincubating the tissue with anti-LTP antibody. Lipid transfer was restored to control values by adding LTP to fat body. Incubation of fat body with dual-labeled DAG lipophorin or its treatment with ammonium chloride showed that neither a membrane-bound lipoprotein lipase nor lipophorin endocytosis is a relevant pathway to transfer or to storage lipids into fat body, respectively. Treatment of fat body with suramin caused a 50% inhibition in [(3)H]DAG transfer from lipophorin. Treatment of [(3)H]DAG-labeled fat body with lipase significantly reduced the amount of [(3)H]DAG associated with the tissue, suggesting that the lipid is still on the external surface of the membrane. Whether this lipid represents irreversibly adsorbed lipophorin or a DAG lipase-sensitive pool is unknown. Nevertheless, these results indicate that the main pathway for DAG transfer from lipophorin to fat body is via LTP and receptor-mediated processes.

FATMETABOLISM ININSECTS

The study of fat metabolism in insects has received considerable attention over the years. Although by no means complete, there is a growing body of information about dietary lipid requirements, and the absolute requirement for sterol is of particular note. In this review we (a) summarize the state of understanding of the dietary requirements for the major lipids and (b) describe in detail the insect lipid transport system. Insects digest and absorb lipids similarly to vertebrates, but with some important differences. The hallmark of fat metabolism in insects centers on the lipid transport system. The major lipid transported is diacylglycerol, and it is carried by a high-density lipoprotein called lipophorin. Lipophorin is a reusable shuttle that picks up lipid from the gut and delivers it to tissues for storage or utilization without using the endocytic processes common to vertebrate cells. The mechanisms by which this occurs are not completely understood and offer fruitful areas for future research.

Role of lipid transfer particle in delivery of diacylglycerol from midgut to lipophorin in larval Manduca sexta

The present work analyzed the function of lipid transfer particle (LTP) in the process of exporting diacylglycerol from larval Manduca sexta midgut cells to lipophorin. When midgut sacs, which had been prelabeled in vivo with [(3)H]oleic acid, were incubated in vitro with a lipophorin-containing medium, a significant amount of radiolabeled diacylglycerol was transferred to lipophorin. Negligible amounts of diacylglycerol were released into lipophorin-free medium. In contrast, lipid-labeled lipophorin did not transfer diacylglycerol to the midgut sacs. The transfer of diacylglycerol from the midgut sac to lipophorin was blocked by preincubation of midgut sacs with antibody against LTP. Diacylglycerol transfer was restored to control values by the addition of purified LTP to midgut sacs that had been treated with antibody against LTP. Under these conditions the amount of diacylglycerol transferred was a function of the LTP concentration. These are the first results showing that LTP is required to export diacylglycerol from the midgut to lipophorin.

Lipid storage and mobilization in insects: current status and future directions

In this paper we review the current status of research on fatty acid absorption and conversion to diacylglycerol in the midgut. We further discuss how diacylglycerol may leave the midgut and associate with lipophorin in hemolymph. We review the present understanding of the role of the lipid transfer particle and lipophorin receptors in lipid delivery between lipophorin and tissues. Finally, we discuss recent studies on the mobilization of diacylglycerol from the fat body in response to adipokinetic hormone. Several suggestions for exciting areas of future research are described.

Metabolic pathways for diacylglycerol biosynthesis and release in the midgut of larval Manduca sexta

The pathway for the synthesis of diacylglycerol in larval Manduca sexta midgut was studied. Fifth instar larvae were fed with [9, 10-(3)H]-oleic acid-labeled triolein and the incorporation of the label into lipid intermediates was analyzed as a function of time. The results showed that the triacylglycerol was hydrolyzed to fatty acids and glycerol in the midgut lumen. In midgut tissue, the labeled fatty acids were rapidly incorporated into phosphatidic acid, diacylglycerol and triacylglycerol, but no significant labeling of monoacylglycerol was observed. Dual-labeling experiments were performed in order to characterize the kinetics of diacylglycerol biosynthesis in the midgut, its incorporation into hemolymph lipophorin and its clearance from hemolymph. The results were best described by a model in which the rate-limiting step in diacylglycerol biosynthesis was the uptake of fatty acid from the lumen of the midgut. Once in the cell the fatty acid was rapidly incorporated in phosphatidic acid and diacylglycerol. Diacylglycerol was converted to triacylglycerol or exported into hemolymph. The interconversion of diacylglycerol and triacylglycerol was fairly rapid, suggesting that triacylglycerol serves as a reservoir from which diacylglycerol can be produced. This mechanism permits the cell to maintain a low steady-state concentration of diacylglycerol and yet efficiently absorb fatty acids from the lumen of the midgut.

Effect of fasting on the composition of the fat body lipid of Dipetalogaster maximus, Triatoma infestans and Panstrongylus megistus (Hemiptera: Reduviidae)

Modifications in content and lipid composition induced by fasting were examined in fat bodies from adults of Triatominae, Dipetalogaster maximus, Triatoma infestans and Panstrongylus megistus. With fasting, total lipid stores dropped approximately 50% for T. infestans and more than 70% for P. megistus. Total lipids analyzed by thin layer chromatography and fractionated by column chromatography on Unisil showed triacylglycerols as the main component in the three species, although P. megistus showed high levels of diacylglycerols (31-46%). Cholesterol amounted to 8-15%. In diacylglycerol fractions, C16:0, C18:1 and C18:0 fatty acids were detected; their ratio varied with species but it was not dependent on nutritional status. In triacylglycerol fractions C18:1 fatty acid was the major component at different times (48-68%); the ratio of monounsaturated to saturated in this fraction was 1.3, 2.6 and 1.2 for D. maximus, T. infestans and P. megistus respectively. The remarkable drop in lipid stores without noticeable changes in their relative composition would suggest that all types of lipid are used at similar rates. The higher content of diacylglycerols in P. megistus may be associated with the better flight performance of this species.

Metabolic post-feeding changes in fat body and hemolymph of Dipetalogaster maximus (Hemiptera: Reduviidae)

Lipids and glycogen in fat body as well as the modifications in the wet weight of this organ were evaluated in an unfed insect, Dipetalogaster maximus, on day 5 after adult ecdysis (time 0) and during a 30-day period after ingestion of blood meal, total lipids, high density lipophorin (HDLp), carbohydrates, total proteins and uric acid were determined in the hemolymph during the same period. Fat body wet weight was maximum on day 10 post-feeding and represented on day only 42% of the maximum weight. Lipids stored in the fat body increased up to day 15 reach 24% of the total weight of tissue. Glycogen was maximum on day 20, representing approximately 3% of the fat body weight. HDLp represented at all times between 17-24% of the total proteins, whose levels ranged between 35 and 47 mg/ml uric acid showed at 20, 25 and 30 days similar levels and significantly higher than the one shown at days 10 and 15. Hemolymphatic lipids fluctuated during starvation between 3-4 mg/ml and carbohydrates showed a maximum on day 15 after a blood meal, decreasing up to 0.26 mg/ml on day 25. The above results suggest that during physiological events such as starvation, the availability of nutrients is affected, involving principally the fat body reserves.

Hemolymphatic components in vectors of Trypanosoma cruzi: study in several species of the subfamily Triatominae (Hemiptera: Reduviidae)

The members of the subfamily Triatominae (Hemiptera: Reduviidae) comprise a great number of species of medical importance in the transmission of the T. cruzi (American trypanosomiasis). The aim of this study was to contribute to the knowledge about the chemical composition in proteins, lipids, lipoproteins, and carbohydrates of vectors of Chagas' disease corresponding to twelve members of the subfamily Triatominae. This study was carried out in ninphs of the fifth instar and adult males of the species: T. delpontei, T. dimidiata, T. guasayana, T. infestans, T. mazzotti, T. pallidipennis, T. patagonica, T. platensis, T. rubrovaria, T. sordida of the Triatoma genus, and D. maximus and P. megistus of the Dipatalogaster and Panstrongylus genera respectively. The results show on one hand, qualitative differences in the protein composition, and on the other hand, similarity in the lipoprotein profiles. Lipids, proteins, and carbohydrates did not show significant differences between species or/and stages.