Calcium transport mechanisms of crustacean hepatopancreatic mitochondria

Adaptive mitochondrial response of the whiteleg shrimp Litopenaeus vannamei to environmental challenges and pathogens

In most eukaryotic organisms, mitochondrial uncoupling mechanisms control ATP synthesis and reactive oxygen species production. One such mechanism is the permeability transition of the mitochondrial inner membrane. In mammals, ischemia-reperfusion events or viral diseases may induce ionic disturbances, such as calcium overload; this cation enters the mitochondria, thereby triggering the permeability transition. This phenomenon increases inner membrane permeability, affects transmembrane potential, promotes mitochondrial swelling, and induces apoptosis. Previous studies have found that the mitochondria of some crustaceans do not exhibit a calcium-regulated permeability transition. However, in the whiteleg shrimp Litopenaeus vannamei, contradictory evidence has prevented this phenomenon from being confirmed or rejected. Both the ability of L. vannamei mitochondria to take up large quantities of calcium through a putative mitochondrial calcium uniporter with conserved characteristics and permeability transition were investigated in this study by determining mitochondrial responses to cations overload. By measuring mitochondrial swelling and transmembrane potential, we investigated whether shrimp exposure to hypoxia-reoxygenation events or viral diseases may induce mitochondrial permeability transition. The results of this study demonstrate that shrimp mitochondria take up large quantities of calcium through a canonical mitochondrial calcium uniporter. Neither calcium nor other ions were observed to promote permeability transition. This phenomenon does not depend on the life cycle stage of shrimp, and it is not induced during hypoxia/reoxygenation events or in the presence of viral diseases. The absence of the permeability transition phenomenon and its adaptive meaning are discussed as a loss with biological advantages, possibly enabling organisms to survive under harsh environmental conditions.

Pyruvate transport in isolated cardiac mitochondria from two species of amphibian exhibiting dissimilar aerobic scope: Bufo marinus and Rana catesbeiana

Cardiac mitochondria were isolated from Bufo marinus and Rana catesbeiana, two species of amphibian whose cardiovascular systems are adapted to either predominantly aerobic or glycolytic modes of locomotion. Mitochondrial oxidative capacity was compared using VO2 max and respiratory control ratios in the presence of a variety of substrates including pyruvate, lactate, oxaloacetate, beta-hydroxybutyrate, and octanoyl-carnitine. B. marinus cardiac mitochondria exhibited VO2 max values twice that of R. catesbeiana cardiac mitochondria when oxidizing carbohydrate substrates. Pyruvate transport was measured via a radiolabeled-tracer assay in isolated B. marinus and R. catesbeiana cardiac mitochondria. Time-course experiments described both alpha-cyano-4-hydroxycinnamate-sensitive (MCT-like) and phenylsuccinate-sensitive pyruvate uptake mechanisms in both species. Pyruvate uptake by the MCT-like transporter was enhanced in the presence of a pH gradient, whereas the phenylsuccinate-sensitive transporter was inhibited. Notably, anuran cardiac mitochondria exhibited activities of lactate dehydrogenase and pyruvate carboxylase. The presence of both transporters on the inner mitochondrial membrane affords the net uptake of monocarboxylates including pyruvate, beta-hydroxybutyrate, and lactate; the latter potentially indicating the presence of a lactate/pyruvate shuttle allowing oxidation of extramitochondrial NADH. Intramitochondrial lactate dehydrogenase and pyruvate carboxylase enables lactate to be oxidized to pyruvate or converted to anaplerotic oxaloacetate. Kinetics of the MCT-like transporter differed significantly between the two species, suggesting differences in aerobic scope may be in part attributable to differences in mitochondrial carbohydrate utilization.

Analysis of calcium concentration fluctuations in hepatopancreatic R cells of Marsupenaeus japonicus during the molting cycle

In this study we examined the fluctuations of the intracellular calcium concentration in isolated hepatopancreatic R cells during the four molting stages of the prawn Marsupenaeus japonicus. In addition, we used the Fura-2-AM fluorescence technique to investigate the release of calcium from mitochondria and ATP-sensitive calcium stores (endoplasmic reticulum (ER), Golgi, and nucleus) into cytoplasm during the molting cycle. Results demonstrate that both the cytosolic free calcium concentration and the total cell calcium (free, bound to calcium-binding proteins, and stored in amorphous form) in the R cells strictly depend upon the molting cycle. Interestingly, the total cell calcium was higher (approximately 10 mmol l(-1)) in postmolt than in premolt (approximately 1 mmol l(-1)) and intermolt (approximately 0.3 mmol l(-1)). The calcium released from mitochondria was higher during premolt than during postmolt and intermolt, but the amount of calcium released from ATP-sensitive calcium stores was similar during all four stages. All together, our results suggest that the mitochondria-ATP-sensitive calcium stores system does not play a key role in calcium storage during the molting cycle but that it is involved in transcellular calcium flux. We hypothesize that lysosome or membrane-clad concretion vacuoles could represent the main site of calcium storage in hepatopancreatic R cells.

Physiological characterization of 45Ca2+ and 65Zn2+ transport by lobster hepatopancreatic endoplasmic reticulum

The crustacean hepatopancreas is an epithelial-lined, multifunctional organ that, among other activities, regulates the flow of calcium into and out of the animal's body throughout the life cycle. Transepithelial calcium flow across this epithelial cell layer occurs by the combination of calcium channels and cation exchangers at the apical pole of the cell and by an ATP-dependent, calcium ATPase in conjunction with a calcium channel and an Na+/Ca2+ antiporter in the basolateral cell region. The roles of intracellular organelles such as mitochondria, lysosomes, and endoplasmic reticulum (ER) in transepithelial calcium transport or in transient calcium sequestration are unclear, but may be involved in transferring cytosolic calcium from one cell pole to the other. The ER membrane has a complement of ATP-dependent calcium ATPases (SERCA) and calcium channels that regulate the uptake and possible transfer of calcium through this organelle during periods of intense calcium fluxes across the epithelium as a whole. This investigation characterized the mechanisms of calcium transport by lobster hepatopancreatic ER vesicles and the effects of drugs and heavy metals on them. Kinetic constants for 45Ca2+ influx under control conditions were K(n) (m)=10.38+/-1.01 microM, J(max)=14.75+/-1.27 pmol/mg protein x sec, and n=2.53+/-0.46. The Hill coefficient for 45Ca2+ influx under control conditions, approximating 2, suggests that approximately two calcium ions were transported for each transport cycle in the absence of ATP or the inhibitors. Addition of 1 mM ATP to the incubation medium significantly (P<0.01) elevated the rate of 45Ca2+ influx at all calcium activities used and retained the sigmoidal nature of the transport relationship. The kinetic constants for 45Ca2+ influx in the presence of 1 mM ATP were K(n) (m)=12.76+/-0.91 microM, J(max)=25.46+/-1.45 pmol/mg protein x sec, and n=1.95+/-0.15. Kinetic analyses of ER 65Zn2+ influx resulted in a sigmoidal relationship between transport rate and zinc activity under control conditions (K(n) (m)=38.63+/-0.52 microM, J(max)=19.35+/-0.17 pmol/mg protein x sec, n=1.81+/-0.03). The Addition of 1 mM ATP enhanced 65Zn2+ influx at each zinc activity, but maintained the overall sigmoidal nature of the kinetic relationship. The kinetic constants for zinc influx in the presence of 1 mM ATP were K(n) (m)=34.59+/-2.31 microM, J(max)=26.09+/-1.17 pmol/mg protein x sec, and n=1.96+/-0.17. Both sigmoidal and ATP-dependent calcium and zinc influxes by ER vesicles were reduced in the presence of thapsigargin and vanadate. This investigation found that lobster hepatopancreatic ER exhibited a thapsigargin- and vanadate-inhibited, SERCA-like, calcium ATPase. This transporter displayed cooperative calcium transport kinetics (Hill coefficient, n approximately 2.0) and was inhibited by the heavy metals zinc and copper, suggesting that the metals may reduce the binding and transport of calcium when they are present in the cytosol.

Microscopical and functional aspects of calcium-transport and deposition in terrestrial isopods

Terrestrial isopods (Crustacea) are excellent model organisms to study epithelial calcium-transport and the regulation of biomineralization processes. They molt frequently and resorb cuticular CaCO(3) before the molt to prevent excessive loss of Ca(2+) ions when the old cuticle is shed. The resorbed mineral is stored in CaCO(3) deposits within the ecdysial gap of the first four anterior sternites. After the molt, the deposits are quickly resorbed to mineralise the posterior part of the new cuticle. The deposits contain numerous small spherules composed of an organic matrix and amorphous CaCO(3), which has a high solubility and, therefore, facilitates quick mobilization of Ca(2+) and HCO(3)(-) ions. During the formation and resorption of the deposits large amounts of Ca(2+), HCO(3)(-) and H(+) are transported across the anterior sternal epithelial cells. Within the last years, various light and electron microscopical techniques have been used to characterize the CaCO(3) deposits and the cellular mechanisms involved in biomineralization. The work on the CaCO(3) deposits includes studies on the ultrastructure of the deposits, the sequence of events during deposit formation and dissolution, and the mineral composition of the sternal deposits. The differentiation of the anterior sternal epithelial cells and the mechanisms of epithelial ion transport required for the mineralization and demineralisation of the deposits was studied using various analytical light and electron microscopical techniques including polarized light microscopy, immunocytochemistry, electron microprobe analysis, electron energy loss spectroscopy and electron spectroscopic imaging. Comparative analysis of deposit morphology and the differentiation of the sternal epithelia provide information on the evolution of CaCO(3) deposit formation in relation to the degree of adaptation to terrestrial environments.

65Zn2+ transport by lobster hepato-pancreatic baso-lateral membrane vesicles

The lobster (Homarus americanus) hepato-pancreatic epithelial baso-lateral cell membrane possesses three transport proteins that transfer calcium between the cytoplasm and hemolymph: an ATP-dependent calcium ATPase, a sodium-calcium exchanger, and a verapamil-sensitive cation channel. We used standard centrifugation methods to prepare purified hepato-pancreatic baso-lateral membrane vesicles and a rapid filtration procedure to investigate whether (65)Zn(2+) transfer across this epithelial cell border occurs by any of these previously described transporters for calcium. Baso-lateral membrane vesicles were osmotically reactive and exhibited a time course of uptake that was linear for 10-15 s and approached equilibrium by 120 s. In the absence of sodium, (65)Zn(2+) influx was a hyperbolic function of external zinc concentration and followed the Michaelis-Menten equation for carrier transport. This carrier transport was stimulated by the addition of 150 microM ATP (increase in K(m) and J(max)) and inhibited by the simultaneous presence of 150 micromol l(-1) ATP+250 micromol l(-1) vanadate (decrease in both K(m) and J(max)). In the absence of ATP, (65)Zn(2+) influx was a sigmoidal function of preloaded vesicular sodium concentration (0, 5, 10, 20, 30, 45, and 75 mmol l(-1)) and exhibited a Hill Coefficient of 4.03+/-1.14, consistent with the exchange of 3 Na(+)/1Zn(2+). Using Dixon analysis, calcium was shown to be a competitive inhibitor of baso-lateral membrane vesicle (65)Zn(2+) influx by both the ATP-dependent (K(i)=205 nmol l(-1) Ca(2+)) and sodium-dependent (K(i)=2.47 micromol l(-1) Ca(2+)) transport processes. These results suggest that zinc transport across the lobster hepato-pancreatic baso-lateral membrane largely occurred by the ATP-dependent calcium ATPase and sodium-calcium exchanger carrier proteins.

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.

X-ray microprobe analysis of epithelial calcium transport

The sternal epithelium of Porcellio scaber was used as a novel model to study the subcellular elemental distribution in control and Ca(2+)-transporting stages in situ. The anterior sternal epithelium (ASE) is specialized for transport of cuticular Ca to sternal CaCO(3) deposits during premolt, and from these deposits during intramolt. The less specialized posterior sternal epithelium transports Ca(2+) to and from the cuticle. In the ASE cells basal [Na], [Cl], and [Mg] are higher than in the apical side. The basal [Na] increases from 105 to 173 mmol/kg dry mass between control and Ca(2+)-transporting stages, accompanied by a decrease in [Cl] and [K]. The [Mg] increases, suggesting transepithelial Mg(2+)-transport. Cytosolic [Ca] varied insignificantly between 4.5 and 5.7 mmol/kg dry mass, however, the number of Ca hot-spots with concentrations between 15 and 50 mmol/kg dry mass increased during transport. Mitochondrial [Ca] decreased in the ASE from 3.3 in the control to 1.0 in the late premolt and to 2.0 mmol/kg dry mass in the intramolt stage. The results suggest Na(+)-dependent mechanisms for transcellular Ca(2+)-transport and the presence of Ca(2+)-binding proteins. Organelles, probably the smooth endoplasmic reticulum, sequester Ca(2+) during intracellular Ca(2+)-transport. A role of mitochondria as a storage site for cuticular Ca is excluded.

Calcium homeostasis in crustaceans: subcellular Ca dynamics

The molting cycle of crustaceans, associated with renewal and remineralization of the cuticle, has emerged as a model system to study regulation of genes that code for Ca(2+)-transporting proteins, common to all eukaryotic cells. This article reviews state-of-the-art knowledge about how crustacean transporting epithelia (gills, hepatopancreas and antennal gland) effect mass transcellular movement of Ca(2+) while preventing cytotoxicity. The current model proposed is based on in vitro research on the intermolt stage with extrapolation to other molting stages. Plasma membrane proteins involved in apical and basolateral Ca(2+) movement (NCX, PMCA) are contrasted between aquatic species of different osmotic origin and among transporting epithelia of an individual species. Their roles are assessed in the context of epithelial Ca(2+) flux derived from organismic approaches. Exchange with extracellular environments is integrated with Ca(2+) sequestration mechanisms across endomembranes of the ER/SR and mitochondria. Finally, the review postulates how new Ca(2+) imaging techniques will allow spatial and temporal resolution of Ca(2+) concentration in subcellular domains.

Copper transport by lobster (Homarus americanus) hepatopancreatic mitochondria

Mechanisms of copper transport into purified mitochondrial suspensions prepared from the hepatopancreas of the Atlantic lobster Homarus americanus were investigated. Mitochondria were purified by combining methods of differential and Percoll-gradient centrifugation, and copper transport was studied using the copper-sensitive fluorescent dye Phen Green. Copper transport by this mitochondrial preparation was kinetically the sum of saturable and non-saturable transfer components. Addition of 500 μmol l–1 Ca2+ or 500 nmol l–1 Ruthenium Red abolished the non-saturable copper transport component, significantly (P&lt;0.01) reduced the apparent binding affinity of the saturable transport component, but was without effect (P&gt;0.05) on the apparent maximal transport velocity of the saturable transfer process. The antiport inhibitor diltiazem (500 μmol l–1) acted as a mixed inhibitor of the saturable transport mechanism, but had no effect on the non-saturable component of transfer. These results suggest that the non-saturable copper influx process was probably by way of the well-known Ruthenium-Red-sensitive Ca2+ uniporter and that the saturable transport component was probably due to a combination of both the Na+-dependent, diltiazem-sensitive 1Ca2+/2Na+ antiporter and the Na+-independent, diltiazem-insensitive 1Ca2+/2H+ antiporter. A model is discussed relating these mitochondrial copper uptake processes to the transfer of metal ions across the epithelial brush-border membrane.

Novel subcellular and molecular tools to study Ca(2+) transport mechanisms during the elusive moulting stages of crustaceans: flow cytometry and polyclonal antibodies

Our understanding of calcium homeostasis during the crustacean moulting cycle derives from research on intermoult animals that has been extrapolated to other stages. In terms of transepithelial Ca(2+) flux, the more interesting stages are those surrounding ecdysis since crustaceans experience a sizeable negative calcium balance in immediate premoult and a significant positive calcium balance in immediate postmoult. These stages are elusive in the sense that larger species such as lobsters are rarely captured at this time, and smaller species such as blue crabs and crayfish are seldom synchronized in their moulting cycle. The reductionist approaches employed in cellular physiology, such as vesicle techniques, employ pooling of fresh tissues from many organisms. Examination of the elusive moulting stages requires more sensitive approaches that can utilize tissue from an individual crustacean to characterize Ca(2+) pumps (Sarco/Endoplasmic Reticulum Ca(2+)-ATPase, SERCA; Plasma Membrane Ca(2+)-ATPase, PMCA) and the Na(+)/Ca(2+) eXchanger (NCX). An emerging subcellular approach described in this paper is to use flow cytometry as a technique to monitor Ca(2+) uptake into Fluo-3-loaded membrane vesicles. This paper illustrates the utility of this technique for measuring ATP-dependent Ca(2+) uptake into hepatopancreatic basolateral membrane vesicles. Obstacles to progress in molecular studies have not been limited by synchronization of moulting since tissue can be snap-frozen and collected from many animals over time. Here, the problem has been the lack of specific antibodies that hybridize with the Ca(2+) transporters of interest so that they can be localized within epithelia. In this paper, we introduce polyclonal antibodies raised in rabbits against crayfish SERCA, PMCA and NCX. Immunocytochemistry of SERCA in muscle, PMCA in antennal gland and NCX in heart confirms the specificity of the antibodies.