Unexpected similarity of intestinal sugar absorption by SGLT1 and apical GLUT2 in an insect (Aphidius ervi,Hymenoptera) and mammals

Diet Impacts the Reproductive System's Maturation in the Male Moth Agrotis ipsilon (Noctuidae, Lepidoptera)

In male moth Agrotis ipsilon, sexual maturation occurs between the third and the fifth day of adult life and is characterized by the development of the reproductive organs such as testes and accessory sex glands. Since sexual maturation requires considerable energy investment, we hypothesized that diet would be an essential regulatory factor in this developmental process. Indeed, the links between the male diet and reproductive physiology have not been described as in females. To test the previous hypothesis, we offered male moths diets corresponding to different flower nectars found in nature, and measured morphological and functional changes in the testes and accessory sex glands. In comparison to a diet composed of sucrose only, males fed with a diet composed of diverse sugars, including glucose, supplemented with sodium led to an earlier increase in the length and the protein content of accessory sex glands, as well as a reduction of the testicular volume accompanied by an acceleration of the sperm bundle transfer from the testes to the duplex. These results show that these specific diets accelerate the maturation of the reproductive system in male moth Agrotis ipsilon.

Evolution and developmental expression of the sodium-iodide symporter (NIS, slc5a5) gene family: Implications for perchlorate toxicology

The vertebrate sodium-iodide symporter (NIS or SLC5A5) transports iodide into the thyroid follicular cells that synthesize thyroid hormone. The SLC5A protein family includes transporters of vitamins, minerals, and nutrients. Disruption of SLC5A5 function by perchlorate, a pervasive environmental contaminant, leads to human pathologies, especially hypothyroidism. Perchlorate also disrupts the sexual development of model animals, including threespine stickleback (Gasterosteus aculeatus) and zebrafish (Danio rerio), but the mechanism of action is unknown. To test the hypothesis that SLC5A5 paralogs are expressed in tissues necessary for the development of reproductive organs, and therefore are plausible candidates to mediate the effects of perchlorate on sexual development, we first investigated the evolutionary history of Slc5a paralogs to better understand potential functional trajectories of the gene family. We identified two clades of slc5a paralogs with respect to an outgroup of sodium/choline cotransporters (slc5a7); these clades are the NIS clade of sodium/iodide and lactate cotransporters (slc5a5, slc5a6, slc5a8, slc5a8, and slc5a12) and the SGLT clade of sodium/glucose cotransporters (slc5a1, slc5a2, slc5a3, slc5a4, slc5a10, and slc5a11). We also characterized expression patterns of slc5a genes during development. Stickleback embryos and early larvae expressed NIS clade genes in connective tissue, cartilage, teeth, and thyroid. Stickleback males and females expressed slc5a5 and its paralogs in gonads. Single-cell transcriptomics (scRNA-seq) on zebrafish sex-genotyped gonads revealed that NIS clade-expressing cells included germ cells (slc5a5, slc5a6a, and slc5a6b) and gonadal soma cells (slc5a8l). These results are consistent with the hypothesis that perchlorate exerts its effects on sexual development by interacting with slc5a5 or its paralogs in reproductive tissues. These findings show novel expression domains of slc5 genes in stickleback and zebrafish, which suggest similar functions across vertebrates including humans, and provide candidates to mediate the effects of perchlorate on sexual development.

Products of Sericulture and Their Hypoglycemic Action Evaluated by Using the Silkworm, Bombyx mori (Lepidoptera: Bombycidae), as a Model

Simple Summary

The use of invertebrates as animal models is gaining attention within the scientific community due to numerous advantages during the development of the experiments, low cost of rearing, and fewer ethical problems. The well-documented biology of the silkworm (Bombyx mori) makes this insect an ideal candidate to be used in different fields of research. In this study, we demonstrated the feasibility of using the silkworm to evaluate the hypoglycemic action of various products of sericulture included in the diet after promoting glucose or sucrose-induced hyperglycemia in silkworms. The postprandial antihyperglycemic activity of fibroin, sericin, and powder made from pupae of silkworms is confirmed. These natural products are therefore ideal candidates for the prevention and treatment of diabetes, obesity, and other lifestyle-related diseases.

Abstract

Sericulture generates different natural products with potential medical applications. Silk peptides, worms, or even pupae are commonly employed in traditional Asian medicine with a wide variety of purposes, and some scientific work has been focused on their antidiabetic properties. This work evaluates the postprandial antihyperglycemic activity of fibroin, sericin, and powder made from either larvae or pupae of silkworms, and Bombyx mori L. (Lepidoptera: Bombycidae), employing the silkworm itself as an animal model. The results indicate a reduction in the glucose levels in hemolymph after sucrose or glucose-induced hyperglycemia when these products are included in the diet of the worms.

Drosophila Solute Carrier 5A5 Regulates Systemic Glucose Homeostasis by Mediating Glucose Absorption in the Midgut

The small intestine is the initial site of glucose absorption and thus represents the first of a continuum of events that modulate normal systemic glucose homeostasis. A better understanding of the regulation of intestinal glucose transporters is therefore pertinent to our efforts in curbing metabolic disorders. Using molecular genetic approaches, we investigated the role of Drosophila Solute Carrier 5A5 (dSLC5A5) in regulating glucose homeostasis by mediating glucose uptake in the fly midgut. By genetically knocking down dSLC5A5 in flies, we found that systemic and circulating glucose and trehalose levels are significantly decreased, which correlates with an attenuation in glucose uptake in the enterocytes. Reciprocally, overexpression of dSLC5A5 significantly increases systemic and circulating glucose and trehalose levels and promotes glucose uptake in the enterocytes. We showed that dSLC5A5 undergoes apical endocytosis in a dynamin-dependent manner, which is essential for glucose uptake in the enterocytes. Furthermore, we showed that the dSLC5A5 level in the midgut is upregulated by glucose and that dSLC5A5 critically directs systemic glucose homeostasis on a high-sugar diet. Together, our studies have uncovered the first Drosophila glucose transporter in the midgut and revealed new mechanisms that regulate glucose transporter levels and activity in the enterocyte apical membrane.

Mechanisms underlying gut microbiota-host interactions in insects

Insects are the most diverse group of animals and colonize almost all environments on our planet. This diversity is reflected in the structure and function of the microbial communities inhabiting the insect digestive system. As in mammals, the gut microbiota of insects can have important symbiotic functions, complementing host nutrition, facilitating dietary breakdown or providing protection against pathogens. There is an increasing number of insect models that are experimentally tractable, facilitating mechanistic studies of gut microbiota-host interactions. In this Review, we will summarize recent findings that have advanced our understanding of the molecular mechanisms underlying the symbiosis between insects and their gut microbiota. We will open the article with a general introduction to the insect gut microbiota and then turn towards the discussion of particular mechanisms and molecular processes governing the colonization of the insect gut environment as well as the diverse beneficial roles mediated by the gut microbiota. The Review highlights that, although the gut microbiota of insects is an active field of research with implications for fundamental and applied science, we are still in an early stage of understanding molecular mechanisms. However, the expanding capability to culture microbiomes and to manipulate microbe-host interactions in insects promises new molecular insights from diverse symbioses.

Absorption of glucosamine is improved by considering circadian rhythm and feeding time in rats

Although many basic and clinical studies have shown that glucosamine (GlcN) improves osteoarthritis, it has not been widely used in the clinic because its bioavailability is only 6%. We investigated the influence of dosing-time factors, which influence pharmacokinetics and food intake in rats to improve its bioavailability. When GlcN was orally administered to rats housed under conditions of free access to food for 12 h or fasting conditions, no significant differences in GlcN concentration were observed in the rat plasma between the two groups. There were no significant differences in the plasma GlcN concentrations among the dosing-time groups when GlcN was orally administered at 4:00, 10:00, 16:00, or 22:00 h to rats. However, the plasma concentration in the fasted group was significantly higher than that in the fed group after GlcN was orally administered at 22:00 h in rats and the AUC of the fasted group was 1.7-fold higher than that of the fed group. In conclusion, the pharmacokinetics of GlcN was improved by considering not only food intake but also the circadian rhythm of its transporter, which is a major factor influencing pharmacokinetic changes.

Fructose and irritable bowel syndrome

Irritable bowel syndrome (IBS) is a chronic disorder characterised by recurrent abdominal pain or discomfort and transit disturbances with heterogeneous pathophysiological mechanisms. The link between food and gastrointestinal (GI) symptoms is often reported by patients with IBS and the role of fructose has recently been highlighted. Fructose malabsorption can easily be assessed by hydrogen and/or methane breath test in response to 25 g fructose; and its prevalence is about 22 % in patients with IBS. The mechanism of fructose-related symptoms is incompletely understood. Osmotic load, fermentation and visceral hypersensitivity are likely to participate in GI symptoms in the IBS population and may be triggered or worsened by fructose. A low-fructose diet could be integrated in the overall treatment strategy, but its role and implication in the improvement of IBS symptoms should be evaluated. In the present review, we discuss fructose malabsorption in adult patients with IBS and the interest of a low-fructose diet in order to underline the important role of fructose in IBS.

Anatomy and Physiology of the Digestive Tract of Drosophila melanogaster

The gastrointestinal tract has recently come to the forefront of multiple research fields. It is now recognized as a major source of signals modulating food intake, insulin secretion and energy balance. It is also a key player in immunity and, through its interaction with microbiota, can shape our physiology and behavior in complex and sometimes unexpected ways. The insect intestine had remained, by comparison, relatively unexplored until the identification of adult somatic stem cells in the Drosophila intestine over a decade ago. Since then, a growing scientific community has exploited the genetic amenability of this insect organ in powerful and creative ways. By doing so, we have shed light on a broad range of biological questions revolving around stem cells and their niches, interorgan signaling and immunity. Despite their relatively recent discovery, some of the mechanisms active in the intestine of flies have already been shown to be more widely applicable to other gastrointestinal systems, and may therefore become relevant in the context of human pathologies such as gastrointestinal cancers, aging, or obesity. This review summarizes our current knowledge of both the formation and function of the Drosophila melanogaster digestive tract, with a major focus on its main digestive/absorptive portion: the strikingly adaptable adult midgut.

High-throughput mass spectrometry analysis revealed a role for glucosamine in potentiating recovery following desiccation stress in Chironomus

Desiccation tolerance is an essential survival trait, especially in tropical aquatic organisms that are vulnerable to severe challenges posed by hydroperiodicity patterns in their habitats, characterized by dehydration-rehydration cycles. Here, we report a novel role for glucosamine as a desiccation stress-responsive metabolite in the underexplored tropical aquatic midge, Chironomus ramosus. Using high- throughput liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QToF-MS) analysis, biochemical assays and gene expression studies, we confirmed that glucosamine was essential during the recovery phase in C. ramosus larvae. Additionally, we demonstrated that trehalose, a known stress-protectant was crucial during desiccation but did not offer any advantage to the larvae during recovery. Based on our findings, we emphasise on the collaborative interplay of glucosamine and trehalose in conferring overall resilience to desiccation stress and propose the involvement of the trehalose-chitin metabolic interface in insects as one of the stress-management strategies to potentiate recovery post desiccation through recruitment of glucosamine.

Association between changes in reproductive activity and D-glucose metabolism in the tephritid fruit fly, Bactrocera dorsalis (Hendel)

Reproduction is an important life process in insects; however, few studies have attempted to demonstrate the association between reproductive activity and energy metabolism. To address this problem, we focused on the reproductive changes in Bactrocera dorsalis males. We analyzed B. dorsalis male gene expression profiles during mating (DM), 3 h after mating (A3HM) and 12 h after mating (A12HM). Gene annotation and pathway analyses of differentially expressed genes show that galactose metabolism and the starch and sucrose metabolism pathway activities were significantly higher in A12HM group. Moreover, the maltase D gene was the most strongly up-regulated gene. The D-glucose levels were significantly higher in A12HM group. Maltase D expression level was significantly higher in males reared with sucrose. Body weights of the males reared with D-glucose and sucrose were significantly higher than those of the males reared with yeast extract. We observed more mated males from the groups fed sucrose and D-glucose than from those fed yeast extract. The D-glucose levels in individual males were highest at 18:00 h, when flies exhibit the most active mating behavior. This study shows that the maltase D gene and D-glucose are the critical gene and substrate, respectively, in male B. dorsalis mating process.

Comparative cation dependency of sugar transport by crustacean hepatopancreas and intestine

Glucose is transported in crustacean hepatopancreas and intestine by Na(+)-dependent co-transport, while Na(+)-dependent D-fructose influx has only been described for the hepatopancreas. It is still unclear if the two sugars are independently transported by two distinct cation-dependent co-transporter carrier systems. In this study, lobster (Homarus americanus) hepatopancreas brush border membrane vesicles (BBMV) were used to characterize, in detail, the cation-dependency of both D-[(3)H]-glucose and D-[(3)H]-fructose influxes, while in vitro perfused intestines were employed to determine the nature of cation-dependent sugar transport across this organ. Over the sodium concentration range of 0-100 mM, both [(3)H]-glucose and [(3)H]-fructose influxes (0.1 mM; 1 min uptakes) by hepatopancreatic BBMV were hyperbolic functions of [Na(+)]. [(3)H]-glucose and [(3)H]-fructose influxes by hepatopancreatic BBMV over a potassium concentration range of 15-100 mM were hyperbolic functions of [K(+)]. Both sugars displayed significant (p<0.01) Na(+)/K(+)-dependent and cation-independent uptake processes. Transepithelial 25 µM [(3)H]-glucose and [(3)H]-fructose fluxes across lobster intestine over luminal sodium and potassium concentration ranges of 0-50 mM and 5-100 mM, respectively, were hyperbolic functions of luminal [Na(+)] and [K(+)]. As with hepatopancreatic sugar transport, transepithelial intestinal sugar transport exhibited both significant (p<0.01) Na(+)/K(+)-dependent and cation-independent processes. Results suggest that both D-glucose and D-fructose are transported by a single SGLT-type carrier in each organ with sodium being the "preferred", high affinity, cation for both sugars in the hepatopancreas, and potassium being the "preferred", high affinity, cation for both sugars in the intestine.

Putative sugar transporters of the mustard leaf beetle Phaedon cochleariae: their phylogeny and role for nutrient supply in larval defensive glands

Background

Phytophagous insects have emerged successfully on the planet also because of the development of diverse and often astonishing defensive strategies against their enemies. The larvae of the mustard leaf beetle Phaedon cochleariae, for example, secrete deterrents from specialized defensive glands on their back. The secretion process involves ATP-binding cassette transporters. Therefore, sugar as one of the major energy sources to fuel the ATP synthesis for the cellular metabolism and transport processes, has to be present in the defensive glands. However, the role of sugar transporters for the production of defensive secretions was not addressed until now.

Results

To identify sugar transporters in P. cochleariae, a transcript catalogue was created by Illumina sequencing of cDNA libraries. A total of 68,667 transcripts were identified and 68 proteins were annotated as either members of the solute carrier 2 (SLC2) family or trehalose transporters. Phylogenetic analyses revealed an extension of the mammalian GLUT6/8 class in insects as well as one group of transporters exhibiting distinctive conserved motifs only present in the insect order Coleoptera. RNA-seq data of samples derived from the defensive glands revealed six transcripts encoding sugar transporters with more than 3,000 counts. Two of them are exclusively expressed in the glandular tissue. Reduction in secretions production was accomplished by silencing two of four selected transporters. RNA-seq experiments of transporter-silenced larvae showed the down-regulation of the silenced transporter but concurrently the up-regulation of other SLC2 transporters suggesting an adaptive system to maintain sugar homeostasis in the defensive glands.

Conclusion

We provide the first comprehensive phylogenetic study of the SLC2 family in a phytophagous beetle species. RNAi and RNA-seq experiments underline the importance of SLC2 transporters in defensive glands to achieve a chemical defense for successful competitive interaction in natural ecosystems.

Comparative digestive physiology

In vertebrates and invertebrates, morphological and functional features of gastrointestinal (GI) tracts generally reflect food chemistry, such as content of carbohydrates, proteins, fats, and material(s) refractory to rapid digestion (e.g., cellulose). The expression of digestive enzymes and nutrient transporters approximately matches the dietary load of their respective substrates, with relatively modest excess capacity. Mechanisms explaining differences in hydrolase activity between populations and species include gene copy number variations and single-nucleotide polymorphisms. Transcriptional and posttranscriptional adjustments mediate phenotypic changes in the expression of hydrolases and transporters in response to dietary signals. Many species respond to higher food intake by flexibly increasing digestive compartment size. Fermentative processes by symbiotic microorganisms are important for cellulose degradation but are relatively slow, so animals that rely on those processes typically possess special enlarged compartment(s) to maintain a microbiota and other GI structures that slow digesta flow. The taxon richness of the gut microbiota, usually identified by 16S rRNA gene sequencing, is typically an order of magnitude greater in vertebrates than invertebrates, and the interspecific variation in microbial composition is strongly influenced by diet. Many of the nutrient transporters are orthologous across different animal phyla, though functional details may vary (e.g., glucose and amino acid transport with K+ rather than Na+ as a counter ion). Paracellular absorption is important in many birds. Natural toxins are ubiquitous in foods and may influence key features such as digesta transit, enzymatic breakdown, microbial fermentation, and absorption.

K⁺-dependent ³H-D-glucose transport by hepatopancreatic brush border membrane vesicles of a marine shrimp

The effects of sodium, potassium, sugar inhibitors, and membrane potential on ³H-D-glucose uptake by hepatopancreatic epithelial brush border membrane vesicles (BBMV) of the Atlantic marine shrimp, Litopenaeus setiferus, were investigated. Brush border membrane vesicles were prepared using a MgCl₂/EGTA precipitation method and uptake experiments were conducted using a high speed filtration technique. ³H-D-Glucose uptake was stimulated by both sodium and potassium and these transport rates were almost doubled in the presence of an inside-negative-induced membrane potential. Kinetics of ³H-D-glucose influx were hyperbolic functions of both external Na⁺ or K⁺, and an induced membrane potential increased influx J(max) and lowered K(m) in both salts. ³H-D-Glucose influx versus [glucose] in both Na⁺ or K⁺ media also displayed Michaelis-Menten properties that were only slightly affected by induced membrane potential. Phloridzin was a poor inhibitor of 0.5 mM ³H-D-glucose influx, requiring at least 5 mM in NaCl and 10 mM in KCl to significantly reduce hexose transport. Several sugars (D-galactose, α-methyl-D-gluco-pyranoside, unlabeled D-glucose, D-fructose, and D-mannose) were used at 75 mM as potential inhibitors of 0.1 mM ³H-D-glucose influx. Only unlabeled D-glucose, D-fructose, and D-mannose significantly (p < 0.05) reduced labeled glucose transport. An additional experiment using increasing concentrations of D-mannose (0, 10, 25, 75, and 100 mM) showed this hexose to be an effective inhibitor of 0.1 mM ³H-D-glucose uptake at concentrations of 75 mM and higher. As a whole these results suggest that ³H-D-glucose transport by hepatopancreatic BBMV occurs by a carrier system that is able to use both Na⁺ and K⁺ as drivers, is enhanced by membrane potential, is relatively refractory to phloridzin, and is only inhibited by itself, D-fructose, and D-mannose. These properties are similar to those exhibited by the mammalian SLC5A9/SGLT4 transporter, suggesting that an invertebrate analogue of this protein may occur in shrimp.

Functional analysis of a fatty acid binding protein produced by Aphidius ervi teratocytes

Aphidius ervi (Hymenoptera, Braconidae) is an endophagous parasitoid of various aphid species, including Acyrthosiphon pisum (Homoptera, Aphididae), the model host used in the present study. Parasitized hosts show a marked increase of their nutritional suitability for the developing parasitoid larvae. This alteration of the biochemical and metabolic profile is due to a castration process mediated by the combined action of the venom, injected at the oviposition, and of the teratocytes, cells deriving from the dissociation of the embryonic membrane. Teratocytes produce and release in the host haemocoel two parasitism-specific proteins, which are of crucial importance for the development of their sister larvae. One of the proteins is a fatty acid binding protein (Ae-FABP), which shows a high affinity for C14-C18 saturated fatty acids (FAs) and for oleic and arachidonic acids. To better define the possible nutritional role of this protein, we have studied its immunolocalization profile in vivo and the impact on FA uptake by the epidermal and midgut epithelia of A. ervi larvae. During the exponential growth of A. ervi larvae, Ae-FABP is distributed around discrete lipid particles, which are abundantly present in the haemocoel of parasitized host aphids and in the midgut lumen of parasitoid larvae. Moreover, a strong immunodetection signal is evident on the surface of the two larval epithelia involved in nutrient absorption: the parasitoid midgut epithelium and the external epidermal layer. These two epithelia can effectively absorb radiolabelled myristic acid, but the FA transport rates are not affected by the presence in the medium of Ae-FABP. The protein appears to act essentially as a vector in the host haemolymph, transferring FAs from the digestion sites of host lipids to the growing parasitoid larvae. These data indicate that the proteins produced by A. ervi teratocytes may play complementary roles in the nutritional exploitation of the host.

Nerveless and gutsy: intestinal nutrient sensing from invertebrates to humans

The increasingly recognized role of gastrointestinal signals in the regulation of food intake, insulin production and peripheral nutrient storage has prompted a surge of interest in studying how the gastrointestinal tract senses and responds to nutritional information. Identification of metabolically important intestinal nutrient sensors could provide potential new drug targets for the treatment of diabetes, obesity and gastrointestinal disorders. From a more fundamental perspective, the study of intestinal chemosensation is revealing novel, non-neuronal modes of communication involving differentiated epithelial cells. It is also identifying signalling mechanisms downstream of not only canonical receptors but also nutrient transporters, thereby supporting a chemosensory role for “transceptors” in the intestine. This review describes known and proposed mechanisms of intestinal carbohydrate, protein and lipid sensing, best characterized in mammalian systems. It also highlights the potential of invertebrate model systems such as C. elegans and Drosophila melanogaster by summarizing known examples of molecular evolutionary conservation. Recently developed genetic tools in Drosophila, an emerging model system for the study of physiology and metabolism, allow the temporal, spatial and high-throughput manipulation of putative intestinal sensors. Hence, fruit flies may prove particularly suited to the study of the link between intestinal nutrient sensing and metabolic homeostasis.

Glucose and fructose uptake by Limulus polyphemus hepatopancreatic brush border and basolateral membrane vesicles: evidence for Na+-dependent sugar transport activity

[(3)H]-fructose and [(3)H]-glucose transport activities were determined in brush border membrane vesicles (BBMV) and basolateral membrane vesicles (BLMV) from Limulus polyphemus (horseshoe crab) hepatopancreas. Glucose transport was equilibrative in the absence of sodium and sodium dependent in the presence of sodium in BBMV, suggesting GLUT-like and SGLT-like transport activity. Glucose transport by BLMV was equilibrative and sodium independent. Fructose uptake by BBMV and BLMV was equilibrative in the absence of sodium and sodium dependent in the presence of sodium. Western blot analysis using a rabbit anti-mouse SGLT-1 polyclonal antibody indicated the presence of a cross-reacting horseshoe crab BBMV protein of similar molecular weight to the mammalian SGLT1. Sequence alignment of the mouse SGLT-4 and SGLT1 with a translated, horseshoe crab-expressed sequence tag also indicated significant identity between species. Fructose and glucose uptake in the absence and presence of sodium by hepatopancreas BBMV and BLMV indicated the presence of sodium-dependent transport activity for each sugar that may result from the presence of transporters similar to those described for other species.

Identification of a novel sodium-dependent fructose transport activity in the hepatopancreas of the Atlantic lobster Homarus americanus

[(3)H]Fructose and [(3)H]glucose transport were determined in brush-border membrane vesicles (BBMV), basolateral membrane vesicles (BLMV) and isolated cells (E, R, F, B) of H. americanus (Atlantic lobster) hepatopancreas. Glucose transport in BBMV was equilibrative in the absence of sodium and concentrative in the presence of sodium. Sodium-dependent glucose transport by BBMV was not inhibited by a tenfold molar excess of fructose. Glucose transport by BLMV was equilibrative and sodium independent. Fructose uptake by BBMV and BLMV was equilibrative in the absence of sodium and concentrative in the presence of sodium. This enhancement was not affected by a tenfold molar excess of glucose in the presence of sodium. E-, F- and B-cells showed sodium-dependent uptake of fructose, while R-cells did not. Sodium-dependent fructose uptake by E-cells was not inhibited by a tenfold molar excess of glucose or mannose. Western blot analysis of BBMV, BLMV and E-, R-, F- and B-cells using rabbit polyclonal antibodies directed against epitopes of mammalian GLUT2, GLUT5, SGLT1 and SGLT4 indicated the presence of cross-reacting lobster proteins. Sequence alignment of the mammalian proteins with translated, lobster expressed sequence tags also indicated significant identity between species. Comparison of fructose and glucose uptake in the absence and presence of sodium by BBMV, BLMV and isolated cells indicated the presence of a distinct sodium-dependent transport activity for each sugar in the Atlantic lobster.

Notes on midgut ultrastructure of cimex hemipterus (Hemiptera: Cimicidae)

This work studied the ultrastructure of the midgut cells of Cimex hemipterus Fabricius (Hemiptera: Cimicidae). The midgut of adult insects was analyzed on different days after a bloodmeal, and three anatomical regions with different digestive functions were apparent. In the anterior midgut, the digestive cells had many spherocrystals, lipid inclusions, and glycogen deposits, suggesting a role in water absorption, ion regulation, digestion, and storage of lipids and sugars. The digestive cells in the middle midgut contained secretory granules in the apical cytoplasm, lysosomes, and large amounts of rough endoplasmic reticulum, suggesting that this midgut region was active in digestive processes. The posterior midgut contained digestive cells with secretory vesicles, lysosomes, rough endoplasmic reticulum, and spherocrystals, suggesting digestion and ion/water absorption. Also, there was strong evidence that the posterior midgut may be the major site of nutrient absorption. The hematophagous heteropteran groups share many of these blood digestion mechanisms.

Sugar absorption in the intestine: the role of GLUT2

Intestinal glucose absorption comprises two components. One is classical active absorption mediated by the Na+/glucose cotransporter. The other is a diffusive component, formerly attributed to paracellular flow. Recent evidence, however, indicates that the diffusive component is mediated by the transient insertion of glucose transporter type 2 (GLUT2) into the apical membrane. This apical GLUT2 pathway of intestinal sugar absorption is present in species from insect to human, providing a major route at high sugar concentrations. The pathway is regulated by rapid trafficking of GLUT2 to the apical membrane induced by glucose during assimilation of a meal. Apical GLUT2 is therefore a target for multiple short-term and long-term nutrient-sensing mechanisms. These include regulation by a newly recognized pathway of calcium absorption through the nonclassical neuroendocrine l-type channel Cav1.3 operating during digestion, activation of intestinal sweet taste receptors by natural sugars and artificial sweeteners, paracrine and endocrine hormones, especially insulin and GLP-2, and stress. Permanent apical GLUT2, resulting in increased sugar absorption, is a characteristic of experimental diabetes and of insulin-resistant states induced by fructose and fat. The nutritional consequences of apical and basolateral GLUT2 regulation are discussed in the context of Western diet, processed foods containing artificial sweeteners, obesity, and diabetes.

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.

Sweet taste receptors in rat small intestine stimulate glucose absorption through apical GLUT2

Natural sugars and artificial sweeteners are sensed by receptors in taste buds. T2R bitter and T1R sweet taste receptors are coupled through G-proteins, alpha-gustducin and transducin, to activate phospholipase C beta2 and increase intracellular calcium concentration. Intestinal brush cells or solitary chemosensory cells (SCCs) have a structure similar to lingual taste cells and strongly express alpha-gustducin. It has therefore been suggested over the last decade that brush cells may participate in sugar sensing by a mechanism analogous to that in taste buds. We provide here functional evidence for an intestinal sensing system based on lingual taste receptors. Western blotting and immunocytochemistry revealed that all T1R members are expressed in rat jejunum at strategic locations including Paneth cells, SCCs or the apical membrane of enterocytes; T1Rs are colocalized with each other and with alpha-gustducin, transducin or phospholipase C beta2 to different extents. Intestinal glucose absorption consists of two components: one is classical active Na+-glucose cotransport, the other is the diffusive apical GLUT2 pathway. Artificial sweeteners increase glucose absorption in the order acesulfame potassium approximately sucralose > saccharin, in parallel with their ability to increase intracellular calcium concentration. Stimulation occurs within minutes by an increase in apical GLUT2, which correlates with reciprocal regulation of T1R2, T1R3 and alpha-gustducin versus T1R1, transducin and phospholipase C beta2. Our observation that artificial sweeteners are nutritionally active, because they can signal to a functional taste reception system to increase sugar absorption during a meal, has wide implications for nutrient sensing and nutrition in the treatment of obesity and diabetes.