Absorption of horseradish peroxidase in Bombyx mori larval midgut

Regionality and Temporal Dynamics of Sequestration and Relocation of Cardenolides in the Monarch Butterfly, Danaus plexippus

The adaptation of monarch butterflies (Danaus plexippus) to milkweed plants and their ability to sequester toxic cardenolides is a model system for plant-herbivore coevolution. However, the physiological mechanisms underlying cardenolide sequestration and its temporal dynamics remain largely unknown. Here, we show that the polar cardenolide ouabain passes through the isolated midgut epithelium of D. plexippus in vitro and is also absorbed into the body cavity of monarch caterpillars. Remarkably, the same pattern was observed in caterpillars of the related, but non-sequestering milkweed butterfly Euploea core, and even in the non-adapted Solanaceae specialist Manduca sexta, although uptake across gut epithelia occurred at a lower rate. Furthermore, we demonstrated that cardenolides begin to cross the epithelium in the anterior part of the intestine and can be detected in body tissues as soon as one minute after ingestion. Finally, we show that not all cardenolides are translocated into butterfly tissues during metamorphosis, and that the most apolar cardenolides are removed with the last caterpillar exuviae. As a result, adult butterflies contain no cardenolides less polar than the milkweed cardenolide calactin. We conclude that uptake by the intestinal epithelium is a very rapid process and that quantitative differences in cardenolide sequestration among lepidopteran caterpillars are only partially mediated by the gut epithelium, likely involving additional mechanisms such as metabolism or excretion. In addition, the translocation of cardenolides from the caterpillar is a selective process which may be due to the limited mobility of highly apolar cardenolides.

Clathrin-mediated endocytosis is involved in Tomato yellow leaf curl virus transport across the midgut barrier of its whitefly vector

Tomato yellow leaf curl virus (TYLCV) is a begomovirus transmitted by the whitefly Bemisia tabaci. The circulative translocation of the virus in the insect is known in its broad line. However, transit of TYLCV from the digestive tract into the haemolymph is poorly understood. We studied the involvement of clathrin in this process by disrupting the clathrin-mediated endocytosis and the endosome network using inhibitor feeding, antibody blocking and dsRNA silencing. We monitored the quantities of TYLCV in the whitefly and virus transmission efficiency. Following endocytosis and endosome network disruption, the quantity of virus was higher in the midgut relative to that of the whole insect body, and the quantity of virus in the haemolymph was reduced. The transmission efficiency of TYLCV by the treated insects was also reduced. These findings indicate that clathrin-mediated endocytosis and endosomes play an important role in the transport of TYLCV across the whitefly midgut.

Midgut epithelium in molting silkworm: A fine balance among cell growth, differentiation, and survival

The midgut of insects has attracted great attention as a system for studying intestinal stem cells (ISCs) as well as cell death-related processes, such as apoptosis and autophagy. Among insects, Lepidoptera represent a good model to analyze these cells and processes. In particular, larva-larva molting is an interesting developmental phase since the larva must deal with nutrient starvation and its organs are subjected to rearrangements due to proliferation and differentiation events. Several studies have analyzed ISCs in vitro and characterized key factors involved in their division and differentiation during molt. However, in vivo studies performed during larva-larva transition on these cells, and on the whole midgut epithelium, are fragmentary. In the present study, we analyzed the larval midgut epithelium of the silkworm, Bombyx mori, during larva-larva molting, focusing our attention on ISCs. Moreover, we investigated the metabolic changes that occur in the epithelium and evaluated the intervention of autophagy. Our data on ISCs proliferation and differentiation, autophagy activation, and metabolic and functional activities of the midgut cells shed light on the complexity of this organ during the molting phase.

Mechanism of entomotoxicity of the plant lectin from Hippeastrum hybrid (Amaryllis) in Spodoptera littoralis larvae

Plant lectins have received a lot of attention because of their insecticidal properties. When orally administered in artificial diet or in transgenic plants, lectins provoke a wide range of detrimental effects, including alteration of the digestive enzyme machinery, fecundity drop, reduced feeding, changes in oviposition behavior, growth and development inhibition and mortality. Although many studies reported the entomotoxicity of lectins, only a few of them investigated the mode of action by which lectins exert toxicity. In the present paper we have studied for the first time the insecticidal potential of the plant lectin from Hippeastrum hybrid (Amaryllis) (HHA) bulbs against the larvae of the cotton leafworm (Spodoptera littoralis). Bioassays on neonate larvae showed that this mannose-specific lectin affected larval growth, causing a development retardation and larval weight decrease. Using primary cell cultures from S. littoralis midguts and confocal microscopy we have elucidated FITC-HHA binding and internalization mechanisms. We found that HHA did not exert a toxic effect on S. littoralis midgut cells, but HHA interaction with the brush border of midgut cells interfered with normal nutrient absorption in the S. littoralis midgut, thereby affecting normal larval growth in vivo. This study thus confirms the potential of mannose-specific lectins as pest control agents and sheds light on the mechanism underlying lectin entomotoxicity.

Uptake and incorporation of sialic acid by the tick Ixodes ricinus

We describe the detection of sialylated N-linked glycans in partially fed Ixodes ricinus tick females using matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry. Sialylated glycans were detected in salivary glands as well as in tick guts and we propose the host origin of these structures. In addition, we mapped the transport of sialylated structures from the blood meal through the gut to the salivary glands using electron microscopy. Specific localization of sialylated glycans to basement membranes of salivary glands was observed. Finally, the influence of the sample preparation methods for electron microscopy on ultrastructure and immunogold labeling was evaluated.

The CPP Tat enhances eGFP cell internalization and transepithelial transport by the larval midgut of Bombyx mori (Lepidoptera, Bombycidae)

Cell-Penetrating Peptides (CPPs) are short peptides that are able to translocate across the cell membrane a wide range of cargoes. In the past decade, different mammalian cell lines have been used to clarify the mechanism of CPPs penetration and to characterize the internalization process, which has been described either as an energy-independent direct penetration through the plasma membrane, or as endocytic uptake. Whatever the mechanism involved, the cell penetration properties of these peptides make their use very attractive as vector for promoting the cellular uptake of coupled bioactive macromolecules, such as peptides, proteins and oligonucleotides. Here we demonstrate, for the first time in insect, that cultured columnar cells from the larval midgut of Bombyx mori more readily internalize eGFP (enhanced Green Fluorescent Protein) when fused to CPP Tat. Tat-eGFP translocates across the plasma membrane of absorptive cells in an energy-independent and non-endocytic manner, since no inhibition of the fusion protein uptake is exerted by metabolic inhibitors and by drugs that interfere with the endocytic uptake. Moreover, the CPP Tat enhances the internalization of eGFP in the columnar cells of intact midgut tissue, mounted in a suitable perfusion apparatus, and the transepithelial flux of the protein. These results open new perspectives for effective delivery of insecticidal macromolecules targeting receptors located both within the insect gut epithelium and behind the gut barrier, in the hemocoel compartment.

The fate of vicilins, 7S storage globulins, in larvae and adult Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae)

The fate of vicilins ingested by Callosobruchus maculatus and the physiological importance of these proteins in larvae and adults were investigated. Vicilins were quantified by ELISA in the haemolymph and fat body during larval development (2nd to 4th instars), in pupae and adults, as well as in ovaries and eggs. Western blot analysis demonstrated that the majority of absorbed vicilins were degraded in the fat body. Tracing the fate of vicilins using FITC revealed that the FITC-vicilin complex was present inside cells of the fat body of the larvae and in the fat bodies of both male and female adult C. maculatus. Labelled vicilin was also detected in ovocytes and eggs. Based on the results presented here, we propose that following absorption, vicilins accumulate in the fat body, where they are partially degraded. These peptides are retained throughout the development of the insects and eventually are sequestered by the eggs. It is possible that accumulation in the eggs is a defensive strategy against pathogen attack as these peptides are known to have antimicrobial activity. Quantifications performed on internal organs from larvae of C. maculatus exposed to extremely dry seeds demonstrated that the vicilin concentration in the haemolymph and fat body was significantly higher when compared to larvae fed on control seeds. These results suggest that absorbed vicilins may also be involved in the survival of larvae in dry environments.

The intestinal barrier in lepidopteran larvae: permeability of the peritrophic membrane and of the midgut epithelium to two biologically active peptides

Endogenous peptide regulators of insect physiology and development are presently being considered as potential biopesticides, but their efficacy by oral delivery cannot be easily anticipated because of the limited information on how the insect gut barrier handles these kind of molecules. We investigated, in Bombyx mori larvae, the permeability properties of the two components of the intestinal barrier, the peritrophic membrane (PM) and the midgut epithelium, separately isolated and perfused in conventional Ussing chambers. The PM discriminated compounds of different dimensions but was easily crossed by two small peptides recently proposed as bioinsecticides, the neuropeptide proctolin and Aedes aegypti Trypsin Modulating Oostatic Factor (Aea-TMOF), although their flux values indicated that the permeability was highly affected by their steric conformation. To date, there is very little functional data available on how peptides cross the insect intestinal epithelium, but it has been speculated that peptides could reach the haemocoel through the paracellular pathway. We characterized the permeability properties of this route to a number of organic molecules, showing that B. mori septate junction was highly selective to both the dimension and the charge of the permeant compound. Confocal images of whole-mount midguts incubated with rhodamine(rh)-proctolin or fluorescein isothiocyanate (FITC)-Aea-TMOF added to the mucosal side of the epithelium, revealed that rh-proctolin did not enter the cell and crossed the midgut only by the paracellular pathway, while FITC-Aea-TMOF did cross the cell apical membrane, permeating also through the transcellular route.

A megalin-like receptor is involved in protein endocytosis in the midgut of an insect (Bombyx mori, Lepidoptera)

The mechanism responsible for fluorescein isothiocyanate (FITC)-albumin internalization by columnar cells in culture obtained from the midgut of Bombyx mori larvae was examined by confocal laser scanning microscopy. Protein uptake changed over time, and it appeared to be energy dependent, since it was strongly reduced by both low temperatures and metabolic inhibitors. Labeled albumin uptake as a function of increasing protein concentration showed a saturation kinetics with a Michaelis constant value of 2.0 +/- 0.6 microM. These data are compatible with the occurrence of receptor-mediated endocytosis. RT-PCR analysis and colocalization experiments with an anti-megalin primary antibody indicated that the receptor involved was a putative homolog of megalin, the multiligand endocytic receptor belonging to the low-density lipoprotein receptor family, responsible for the uptake of various molecules, albumin included, in many epithelial cells of mammals. This insect receptor, like the mammalian counterpart, required Ca(2+) for albumin internalization and was inhibited by gentamicin. FITC-albumin internalization was clathrin mediated, since two inhibitors of this process caused a significant reduction of the uptake, and clathrin and albumin colocalized in the intermicrovillar areas of the apical plasma membrane. The integrity of actin and microtubule organization was essential for the correct functioning of the endocytic machinery.