Differential expression of Na+/D-glucose cotransport in isolated cells of Marsupenaeus japonicus hepatopancreas

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.

Application of D-Crustacean Hyperglycemic Hormone Induces Peptidases Transcription and Suppresses Glycolysis-Related Transcripts in the Hepatopancreas of the Crayfish Pontastacus leptodactylus - Results of a Transcriptomic Study

The crustacean Hyperglycemic Hormone (cHH) is a neuropeptide present in many decapods. Two different chiral isomers are simultaneously present in Astacid crayfish and their specific biological functions are still poorly understood. The present study is aimed at better understanding the potentially different effect of each of the isomers on the hepatopancreatic gene expression profile in the crayfish Pontastacus leptodactylus, in the context of short term hyperglycemia. Hence, two different chemically synthesized cHH enantiomers, containing either L- or D-Phe(3), were injected to the circulation of intermolt females following removal of their X organ-Sinus gland complex. The effects triggered by the injection of the two alternate isomers were detected after one hour through measurement of circulating glucose levels. Triggered changes of the transcriptome expression profile in the hepatopancreas were analyzed by RNA-seq. A whole transcriptome shotgun sequence assembly provided the assumedly complete transcriptome of P. leptodactylus hepatopancreas, followed by RNA-seq analysis of changes in the expression level of many genes caused by the application of each of the hormone isomers. Circulating glucose levels were much higher in response to the D-isoform than to the L-isoform injection, one hour from injection. Similarly, the RNA-seq analysis confirmed a stronger effect on gene expression following the administration of D-cHH, while just limited alterations were caused by the L-isomer. These findings demonstrated a more prominent short term effect of the D-cHH on the transcription profile and shed light on the effect of the D-isomer on specific functional gene groups. Another contribution of the study is the construction of a de novo assembly of the hepatopancreas transcriptome, consisting of 39,935 contigs, that dramatically increases the molecular information available for this species and for crustaceans in general, providing an efficient tool for studying gene expression patterns in this organ.

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.

Carbon assimilation and digestive toxicity in naïve grass shrimp (Palaemonetes pugio) exposed to dietary cadmium

Naïve grass shrimp Palaemonetes pugio were pulse-fed cadmium-contaminated meals containing carbon-14, fluorescent or near-infrared markers and analyzed for carbon assimilation efficiency, gut residence time, feces elimination rate, extracellular digestive protease activity or gut pH. Carbon assimilation efficiency (~83%), minimum gut residence time (~435 min) and proventriculus pH (~5.29 to ~6.01) were not impacted significantly by cadmium ingestion. A dose-dependent decrease in feces elimination rate (from ~14.4 to ~6.4 mm h(-1)) was observed for shrimp for 2 h following minimum gut residence time. Protease activities increased ~2.4-fold over the range of dietary cadmium exposures, however, this variation was not dose-dependent. Differential impacts of cadmium exposure on carbon and cadmium assimilation reported previously are consistent with work involving shrimp subjected to chronic field exposure. The influence of ingested cadmium on feces elimination rate may be related to pre-assimilatory impacts on packaging, intestinal transport or release of feces. Protease activities may have been influenced by pre-assimilatory interactions between available cadmium ions in gut fluid and enzyme-secreting cells of the hepatopancreatic epithelium or direct impacts on active enzymes.

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

Sugars are critical substrates for insect metabolism, but little is known about the transporters and epithelial routes that ensure their constant supply from dietary resources. We have characterized glucose and fructose uptakes across the apical and basolateral membranes of the isolated larval midgut of the aphid parasitoid Aphidius ervi. The uptake of radiolabeled glucose at the basal side of the epithelium was almost suppressed by 200 μM cytochalasin B, uninhibited by phlorizin, and showed the following decreasing rank of specificity for the tested substrates: glucose > glucosamine > fructose, with no recognition of galactose. These functional properties well agree with the expression of GLUT2-like transporters in this membrane. When the apical surface of the epithelium was also exposed to the labeled medium, a cation-dependent glucose uptake, inhibited by 10 μM phlorizin and by an excess of galactose, was detected suggesting the presence in the apical membrane of a cation-dependent cotransporter. Radiolabeled fructose uptakes were only partially inhibited by cytochalasin B. SGLT1-like and GLUT5-like transporters were detected in the apical membranes of the epithelial cell by immunocytochemical experiments. These results, along with the presence of GLUT2-like transporters both in the apical and basolateral cell membranes of the midgut, as we recently demonstrated, allow us to conclude that the model for sugar transepithelial transport in A. ervi midgut appears to be unexpectedly similar to that recently proposed for sugar intestinal absorption in mammals.

L-proline transport by purified cell types of lobster hepatopancreas

The hepatopancreas of the American lobster, Homarus americanus, has four epithelial cell types that are anatomically distinguishable and can be separated for in vitro investigation of their individual biological roles in the intact organ using centrifugal elutriation. Previous studies employing this separation method have produced hepatopancreatic cell suspensions that have been used to examine the nature of copper transport, 2 Na+/1 H+ exchange, and D-glucose absorption by each cell type in isolation from the other cells comprising the tubular epithelium. The present investigation used this method to study amino acid transport by E-, F-, R-, and B-cells of the lobster hepatopancreas in order to characterize the absorption processes for protein digestion products by this organ and to identify which cell type was most likely the responsible agent for net transcellular transfer of these organic molecules from lumen to blood. Results indicated that heptopancreatic E- and F-cell types were the only cells exhibiting Na+-dependent 3H-L-proline transport. Further examination of 3H-L-proline influx by F-cell suspensions indicated that this cell type possessed plasma membrane Na+-dependent IMINO-like and B0-like transport mechanisms and Na+-independent L-like transport mechanisms. Using selective inhibitors of these separate transport systems (e.g., L-pipecolate, L-alanine, and L-leucine), the IMINO-like transporter appeared to predominate in L-proline influx into F-cells, while lesser amounts of amino acid transport took place by the B0-like and L-like systems. The results of this study suggest that the hepatopancreatic F-cell is the epithelial cell type responsible for the bulk of amino acid absorption by this organ and that the IMINO-like transporter is responsible for most of the L-proline transfer through this agent. It is further suggested that as digestion and absorption proceeds in the hepatopancreas and concentrations of luminal amino acids and sodium fall, Na+-dependent transport systems, like the IMINO-like and B0-like, increase their binding affinities for their substrates to maximize nutrient transfer across the epithelium.