Phorbol ester activation of an NHE-like electroneutral Na+/H+ antiporter in isolated E-cells of lobster (Homarus americanus) hepatopancreas

Comparative transcriptome reveals the response of oriental river prawn (Macrobrachium nipponense) to sulfide toxicity at molecular level

Aquatic environmental pollutants have various impacts on aquaculture. Specifically, sulfide has been established as being toxic to aquatic animals including the oriental river prawn Macrobrachium nipponense. In response, the hepatopancreas has been broadly studied, as it plays a pivotal role in arthropod nutrient digestion and absorption, energy supply, and organ development as well as in crustacean immunity. However, the underlying molecular mechanisms of hepatopancreas's response to sulfide toxicity are still poorly understand. Herein, we used Nova-seq 6000 platform to conduct a comparative transcriptome analysis of gene expression profiles in the hepatopancreas of M. nipponense, while it was under the influence of a semi-lethal sulfide concentration (3.20 mg/L at 48 h). A total of 139 million raw reads were obtained, in which 67,602 transcripts were clustered into 37,041 unigenes for further analysis. After constant sulfide exposure for 48 h, 235 differentially expressed genes, i.e., DEGs (151 up-regulated and 84 down-regulated) were identified in the sulfide treatment group (TGHP) compared with the control group (CGHP). We used GO and KEGG databases to annotate all the DEGs into 1180 functions and 123 pathways, respectively. The metabolic pathways included proximal tubule bicarbonate reclamation, sulfur metabolism, glycolysis and gluconeogenesis, and the TCA cycle; while immune-related pathways contained Ras, Rap1, focal adhesion and platelet activation. Additionally, apoptosis-involved pathways e.g., lysosome, also exhibited remarkable alteration in the presence of sulfide stress. Notably, responses to external stimuli and detoxification genes- such as GSKIP, CRT2, APOD, TRET1, CYP4C3 and HR39- were significantly altered by the sulfide stress, indicating that significant toxicity was transferred through energy metabolism, growth, osmoregulatory processes and immunity. Finally, we demonstrated that in the present of sulfide stress, M. nipponense altered the expression of detoxification- and extracellular stimulation-related genes, and displayed positive resistance via tight junction activation and lysosome pathways. The results of these novel experiments shed light on the hepatopancreas's molecular response to sulfide stress resistance and the corresponding adaptation mechanism; and enable us to identify several potential biomarkers for further studies.

Na+/H+ exchanger (NHE) in Pacific white shrimp (Litopenaeus vannamei): Molecular cloning, transcriptional response to acidity stress, and physiological roles in pH homeostasis

Na⁺/H⁺ exchangers are the most common membrane proteins involved in the regulation of intracellular pH that concurrently transport Na⁺ into the cells and H⁺ out of the cells. In this study, the full-length cDNA of the Na⁺/H⁺ exchanger (NHE) from the Pacific white shrimp (Litopenaeus vannamei) was cloned. The LvNHE cDNA is 3167 bp long, contains a 5’-untranslated region (UTR) of 74 bp and a 3’-UTR of 456 bp and an open reading frame (ORF) of 2637 bp, coding for a protein of 878 amino acids with 11 putative transmembrane domains and a long cytoplasmic tail. LvNHE shows high sequence homology with mud crab NHE at the amino acid level. LvNHE mRNA was detected in the hepatopancreas, gill, eyestalk, skin, heart, intestine, muscle, brain and stomach, with the highest abundance in the intestine. In the shrimp intestinal fragment cultures exposed to gradually declining pH medium (from pH 8.0 to pH 6.4), the LvNHE mRNA expression was significantly stimulated, with the highest response when incubated in pH 7.0 medium for 6 h. To investigate the functional roles of LvNHE in pH regulation at the physiological and cellular levels, the LvNHE mRNA expression was silenced by siRNA knockdown. Upon low-pH challenge, the hemolymph pH was significantly reduced in the LvNHE mRNA knockdown shrimp. In addition, knockdown of LvNHE mRNA reduced the recovery capacity of intracellular pH in intestinal fragment cultures after acidification. Altogether, this study demonstrates the role of NHE in shrimp response to low pH stress and provides new insights into the acid/base homeostasis mechanisms of crustaceans.

Lobster hepatopancreatic epithelial single cell suspensions as models for electrogenic sodium–proton exchange

Sodium-proton antiporters, also called Na+/H+ exchangers (NHE), are vital transmembrane proteins involved in multiple cellular functions including transepithelial ion transport and Na+ homeostasis of cells throughout the biological kingdom. Na+/H+ exchange is accelerated by cytosolic acidification and also by osmotically induced cell shrinking, thereby promoting recovery of the physiological pHi and volume. Eight isoforms of Na+/H+ exchangers have been cloned and characterized to date and share the same overall structure, but exhibit differences with respect to cellular localization, kinetic variables and plasma membrane targeting, in polarized epithelial cells. The electrogenic Na+ absorption across tight epithelia from invertebrates follow significantly different principles from the electroneutral Na+/H+ antiporter found in vertebrates. In all invertebrate cells examined, the antiporter displayed a 2Na+/1H+ transport stoichiometry and this transport was markedly inhibited by exogenous calcium and zinc. Na+/H+ exchangers (NHE) are present in crustacean hepatopancreatic cell type suspensions and are believed to function in acid-base regulation by driving the extrusion of protons across the hepatopancreatic epithelium in exchange for Na+ in the sea water. A brief review of current knowledge about Na+/H+ exchangers has been presented. In addition, understanding of hepatopancreatic Na+/H+ exchange is described as obtained after isolation of purified E-, R-, F- and B-cell suspensions from the whole organ by centrifugal elutriation.

Cellular localization of calcium, heavy metals, and metallothionein in lobster (Homarus americanus) hepatopancreas

This investigation combines confocal microscopy with the cation-specific fluorescent dyes Fluo-3 and BTC-5N to localize calcium and heavy metals along the length of intact lobster (Homarus americanus) hepatopancreatic tubules and isolated cells. A metallothionein-specific antibody, developed in mollusks with cross-reactivity in crustaceans, showed the tissue-specific occurrence of this metal-binding protein in several organ systems in lobster and in single cell types isolated from lobster hepatopancreas. Individual lobster hepatopancreatic epithelial cell types were separated into pure single cell type suspensions for confocal and antibody experiments. Intact hepatopancreatic tubules showed high concentrations of both calcium and heavy metals at the distal tips of tubules where mitotic stem cells (E-cells) are localized. In addition, a concentrated distribution of calcium signal within isolated single premolt E-cells in solution was disclosed that might suggest an endoplasmic reticulum compartmentation of this cation within these stem cells. Both E- and R-cells showed significantly (P < 0.05) greater intracellular calcium concentrations in premolt than intermolt, suggesting the accumulation of this cation in these cells prior to the molt. Antibody studies with lobster tissues indicated that the hepatopancreas possessed 5-10 times the metallothionein concentration as other lobster organ systems and that isolated E-cells from the hepatopancreas displayed more than twice the binding protein concentrations of other cells of this organ or those of blood cells. These results suggest that crustacean hepatopancreatic stem cells (E-cells) and R-cells play significant roles in calcium and heavy metal homeostasis in this tissue. Interactions between the four hepatopancreatic cell types in this regulatory activity remain to be elucidated.

Differential physiological expression of the invertebrate 2Na+/1H+ antiporter in single epithelial cell type suspensions of lobster hepatopancreas

Lobster (Homarus americanus) hepatopancreas is a complex, heterogeneous tissue composed of four epithelial cell types that individually contribute to the overall functional properties of digestion, absorption, secretion, and detoxification. Previous studies, using purified hepatopancreatic brush border membrane vesicles, have described the properties of an electrogenic, 2Na+/1H+ antiporter in this tissue that regulates the absorption and secretion of these cations. These studies were not able to localize this cation exchange phenomenon to specific epithelial cell types. In the present study, sodium/proton exchange by purified, single cell, suspensions of lobster (Homarus americanus) hepatopancreatic epithelium was investigated using a centrifugal elutriation method to cleanly separate the four individual cell types for subsequent physiological characterization. Results indicate that all four hepatopancreatic epithelial cell types possessed the 2Na+/1H+ antiporter as a result of its unique sigmoidal influx properties. Hill Coefficients, measures of transport sigmodicity obtained from kinetic analyses of 22Na+ influx by single cell type suspensions, varied from 1.56 +/- 0.30 (R-cell suspensions) to 2.79 +/- 0.41 (F-cell suspensions), suggesting that different numbers of sodium ions may be accommodated by each cell type. Both calcium and zinc were competitive inhibitors of 22Na+ influx in E-cells (calcium Ki = 105.1+/-5.2 microM; zinc Ki = 46.2 +/- 7.8 microM), but the extent to which these divalent cations inhibited monovalent cation transport by each cell type varied. It is concluded that different isoforms of the electrogenic 2Na+/1H+ antiporter may be present in each hepatopancreatic cell type and thereby contribute in differing degrees to the cation regulatory functions performed by the overall organ.

D-glucose transport in decapod crustacean hepatopancreas

Physiological mechanisms of gastrointestinal absorption of organic solutes among crustaceans remain severely underinvestigated, in spite of the considerable relevance of characterizing the routes of nutrient absorption for both nutritional purposes and formulation of balanced diets in aquaculture. Several lines of evidence attribute a primary absorptive role to the digestive gland (hepatopancreas) and a secondary role to the midgut (intestine). Among absorbed organic solutes, the importance of D-glucose in crustacean metabolism is paramount. Its plasma levels are finely tuned by hormones (crustacean hyperglycemic hormone, insulin-like peptides and insulin-like growth factors) and the function of certain organs (i.e. brain and muscle) largely depends on a balanced D-glucose supply. In the last few decades, D-glucose absorptive processes of the gastrointestinal tract of crustaceans have been described and transport mechanisms investigated, but not fully disclosed. We briefly review our present knowledge of D-glucose transport processes in the crustacean hepatopancreas. A discussion of previous results from experiments with hepatopancreatic epithelial brush-border membrane vesicles is presented. In addition, recent advances in our understandings of hepatopancreatic D-glucose transport are shown, as obtained (1) after isolation of purified R-, F-, B- and E-cell suspensions from the whole organ by centrifugal elutriation, and (2) by protein expression in hepatopancreatic mRNA-injected Xenopus laevis oocytes. In a perspective, the applicability of these novel methods to the study of hepatopancreatic absorptive function will certainly improve our knowledge of this structurally complex organ.

Copper transport by lobster hepatopancreatic epithelial cells separated by centrifugal elutriation: measurements with the fluorescent dye Phen Green

The hepatopancreas of the American lobster (Homarus americanus) possesses four types of epithelial cells arranged along blind-ended tubules. At the distal tips of these tubules, stem cells termed E-cells differentiate into three other cell types, R-cells, F-cells and B-cells, each of which have different absorptive and secretory roles in the biology of the overall organ. This investigation uses centrifugal elutriation to separate the individual hepatopancreatic epithelial cell types of Homarus americanus and to investigate their plasma membrane copper transport properties using the copper-sensitive fluorescent dye Phen Green. Results show highly dissimilar endogenous concentrations of copper in each cell type and within the vacuoles (vesicles) released from these cells during the centrifugation process ([copper] in vacuoles&gt;E-cells&gt;R-cells&gt;F-cells approximately B-cells). All four cell types were able to absorb copper from external concentrations ranging from 0.01 to 8 micromol l(−1), but considerable differences in transport rates occurred between the cell types. External calcium (0--10 mmol l(−1)) stimulated the uptake of external copper in a saturable fashion, suggesting the occurrence of carrier-mediated metal uptake. Addition of the Ca(2+) channel blocker verapamil (30 micromol l(−1)) to the external medium reduced the uptake rate of copper by all four cell types, but to different extents in each type of cell. External zinc (0--1000 nmol l(−1)) was a competitive inhibitor of copper influx in E- and R-cells, suggesting that the two metals shared the same binding and transport mechanism. A model is proposed which suggests that copper may enter all hepatopancreatic epithelial cell types by a divalent cation antiport process that exchanges intracellular Ca(2+) (or other cations) with either external copper or zinc. Verapamil-sensitive Ca(2+) channels may allow access of external calcium to cytoplasmic exchange sites on the antiporter or to activator sites on the same transport protein. The results suggest that elutriation is an excellent technique for the separation of complex invertebrate organ systems into their separate cell types and for analyzing the physiological properties of each cell type in isolation.

Ion transport processes of crustacean epithelial cells

Epithelial cells of the gut, antennal glands, integument, and gills of crustaceans regulate the movements of ions into and across these structures and thereby influence the concentrations of ions in the hemolymph. Specific transport proteins serving cations and anions are found on apical and basolateral cell membranes of epithelia in these tissues. In recent years, a considerable research effort has been directed at elucidating their physiological and molecular properties and relating these characteristics to the overall biology of the organisms. Efforts to describe ion transport in crustaceans have focused on the membrane transfer properties of Na+/H+ exchange, calcium uptake as it relates to the molt cycle, heavy metal sequestration and detoxification, and anion movements into and across epithelial cells. In addition to defining the properties and mechanisms of cation movements across specific cell borders, work over the past 5 yr has also centered on defining the molecular nature of certain transport proteins such as the Na+/H+ exchanger in gill and gut tissues. Monovalent anion transport proteins of the gills and gut have received attention as they relate to osmotic and ionic balance in euryhaline species. Divalent anion secretion events of the gut have been defined relative to potential roles they may have in hyporegulation of the blood and in hepatopancreatic detoxification events involving complexation with cationic metals.