Regulation of Na,K-ATPase biosynthesis in developing Artemia salina

Toxic effects of harmful benthic dinoflagellate Ostreopsis ovata on invertebrate and vertebrate marine organisms

Harmful benthic microalgae blooms are an emerging phenomenon causing health and economic concern, especially in tourist areas. This is the case of the Mediterranean Sea, where Ostreopsis ovata blooms occur in summer, with increasing regularity. Ostreopsis species produce palytoxin (PTX) and analogues, and a number of deaths directly associated with the ingestion of PTX contaminated seafood have been reported. PTX is considered one of the most toxic molecules occurring in nature and can provoke severe and sometimes lethal intoxications in humans. So far in temperate areas, O. ovata blooms were reported to cause intoxications of humans by inhalation and irritations by contact. In addition, invertebrate mass mortalities have been reported, possibly linked to O. ovata blooms, although other causes cannot be ruled out, such as oxygen depletion or high seawater temperature. In order to improve our knowledge about the direct toxicity of this species on invertebrate and vertebrate marine organisms, we performed an ecotoxicological screening to investigate the toxic effects of different concentrations of O. ovata (cultured in the laboratory and sampled in the field during blooms) on crustaceans and fish as model organisms. Artemia salina, Tigriopus fulvus, and Amphibalanus amphitrite larvae and juveniles of the sea bass Dicentrarchus labrax were used as model species. Toxic effects associated with cultured O. ovata cells were investigated using a crossed design: testing two different temperatures (20 and 25 °C), four different cell concentrations, and four treatments (untreated O. ovata culture, filtered and resuspended algal cells, growth medium devoid of algal cells, and sonicated algal cells). The results indicate that the toxicity of cultured O. ovata is related to the presence of living O. ovata cells, and that this effect is amplified by temperature. Furthermore, both tests with laboratory cultured algae and field sampled cells pointed out that A. salina is the most sensitive species even at concentrations below the Environmental Alarm Threshold set by the Italian Ministry of Health. Some possible explanations of such sensitivity are discussed, taking into account evidence of O. ovata cells ingestion and the activity of its toxins on the Na⁺/K⁺-ATPase.

Molecular domains in epithelial salt cellNaCl of crustacean salt gland (Artemia)

The salt secretory cell has two distinct patterns of plasma membrane development. First, the basolateral surface forms a tubular labyrinth. It contains the subunit alpha-2 of the Na(+)-K(+)-ATPase bound together with a beta subunit for structural attachment within the lipid bilayer. Second, the apical plasma membranes form a multiple array of extending tufts. These tufts contain the subunit alpha-1 of the Na(+)-K(+)-ATPase bound together with a beta subunit for structural integrity within the lipid bilayer. The presence of an active transporter for chloride remains as an open question. It has been taken as preliminary evidence from brine shrimp cystic fibrosis toxicity that a cystic fibrosis transmembrane conductance regulator chloride channel could be present in the apical region. The presence of cytoskeletal elements being involved in the construction of a hypo-osmoregulatory apparatus is supported by the homeobox gene products derived from APH-1 m RNA found in the salt gland.

Energizing an Invertebrate Embryo: Bafilomycin‐Dependent Respiration and the Metabolic Cost of Proton Pumping by the V‐ATPase

We examine herein the contribution of V-ATPase activity to the energy budget of aerobically developing embryos of Artemia franciscana and discuss the results in the context of quiescence under anoxia. (31)P-NMR analysis indicates that intracellular pH and NTP levels are unaffected by acute incubation of dechorionated embryos with the V-ATPase inhibitor, bafilomycin A(1). Bafilomycin A(1) also has no significant effect on oxygen consumption by isolated mitochondria. Taken together, these data indicate that bafilomycin does not affect energy-producing pathways in the developing embryo. However, the V-ATPase inhibitor exhibits a concentration-dependent inhibition of oxygen consumption in aerobic embryos. A conservative analysis of respirometric data indicates that proton pumping by the V-ATPase, and processes immediately dependent on this activity, constitutes approximately 31% of the aerobic energy budget of the preemergent embryo. Given the complete absence of detectable Na(+)K(+)-ATPase activity during the first hours of aerobic development, it is plausible that the V-ATPase is performing a role in both the acidification of intracellular compartments and the energization of plasma membranes. Importantly, the high metabolic cost associated with maintaining these diverse proton gradients requires that V-ATPase activity be downregulated under anoxia in order to attain the almost complete metabolic depression observed in the quiescent embryo.

V-ATPase expression during development of Artemia franciscana embryos: potential role for proton gradients in anoxia signaling

Under anoxia, Artemia franciscana embryos downregulate metabolic processes and approach an ametabolic state. Entrance into this quiescent state is accompanied by a profound acidification of the intracellular space, and more than two decades of research now clearly demonstrates that this acidification is critical to metabolic downregulation in anoxic embryos. However, the proximal mechanisms responsible for the pH shift remain largely unidentified. Here, we report evidence demonstrating expression of the V-ATPase in encysted embryos and present an argument for its involvement in the intracellular acidification induced by anoxia. We identified a single B-subunit cDNA sharing the greatest degree of sequence similarity with ;generalist-type' homologues from mammals (brain-type) and invertebrates. Quantitative analysis of B-subunit mRNA demonstrates differential expression throughout early development, and western blot analyses confirm the expression of at least six V-ATPase subunits in both heavy membranes and microsomal vesicles. The critical need for proton pumping during the anoxia-tolerant stage of development is demonstrated by incubation with the V-ATPase inhibitor bafilomycin A1, which halts embryonic development. Importantly, net proton flux from V-ATPase-acidified compartments to the surrounding cytoplasm is likely under anoxia and may significantly contribute to the enigmatic acidification critical to quiescence.

Molecular cloning of the beta-subunit of the Na,K-ATPase in the brine shrimp, Artemia. The cDNA-derived amino acid sequence shows low homology with the beta-subunits of vertebrates except in the single transmembrane and the carboxy-terminal domains

A cDNA encoding the beta-subunit of the Na,K-ATPase of brine shrimp (Artemia) has been cloned. Its nucleotide sequence and predicted amino acid sequence have been determined. The amino acid sequence shows considerable divergence from that of chicken, dog, human, pig, rat, sheep, Torpedo, and Xenopus. This is not entirely unexpected since brine shrimp is a 'fast clock' organism which diverged from the precursor of the vertebrates 0.5-1.0 billion years ago. However, a highly hydrophobic putative transmembrane domain and the carboxy-terminal domain show considerable conservation. The relatively small degree of conservation in the beta-subunit of Artemia should provide information about the functional significance of this protein.

The search for the endogenous digitalis: an alternative hypothesis

The universal presence of a binding site for cardiac glycosides on Na+-K+-ATPase has engendered speculation as to whether it also serves as a receptor for an endogenous digitalis-like hormone or autacoid. If such a hormone were to exist, it could play a role in sodium homeostasis and in the pathophysiology of primary hypertension and uremia. However, we believe that this hypothesis rests on unproven assumptions. Although typical of many toxins and drugs, binding to a single protein that acts as both its receptor and effector mechanism at the cell membrane, thereby directly affecting transmembrane ion flux, would be unusual for a hormone or autacoid. As an alternative hypothesis for the evolutionary conservation of the cardiac glycoside binding site, we suggest that its endogenous ligand may exist within the cell. After cotranslational insertion of the alpha- and beta-subunits into the membrane of the rough endoplasmic reticulum, Na+-K+-ATPase, like most integral membrane proteins, 1) must be targeted through a complex network of intracellular organelles to the correct plasmalemmal domain, 2) must be monitored for appropriate protein conformation and subunit assembly, and perhaps 3) could have its catalytic function regulated before insertion in the cell membrane. Because the lumina of the endoplasmic reticulum, Golgi, and other organelles and vesicles are topologically equivalent to the outside of the cell, all three functions could be subserved by an intraorganellar ligand for the cardiac glycoside binding site.

Isoforms of Na,K-ATPase in Artemia saline: I. Detection by FITC binding and time course

Partially purified Na,K-ATPase from whole nauplii at various stages of development, analyzed by SDS-PAGE, reveals a polydisperse beta and two alpha subunits (denoted alpha 1 and alpha 2). In the absence of Ca2+, ATP-inhibitable fluorescein isothiocyanate (FITC) labeling is restricted to the alpha subunit of this enzyme, even in crude naupliar homogenates. The intensity of the alpha-specific fluorescent signal (i.e., the sum of the yield from both alpha isoforms) is proportional to Na,K-ATPase activity during development. FITC-labeled subunits were detected at 8 hr of development prior to the detection of measurable Na,K-ATPase activity. The alpha 2/alpha 1 ratio changed from an initial value of 1.25 to a peak of 1.75 at 32 hr of development, then reverted to a ratio of 1.25 by 42 hr, and remained constant thereafter. Pulse chase studies with 35S-methionine indicated that the developmental increase in enzyme activity is coincident with amino acid incorporation into the alpha subunits, implying that enzyme synthesis is active during enzyme accumulation.

Sodium-calcium exchange in membrane vesicles from Artemia

Membrane vesicles were prepared from Artemia nauplii (San Francisco Bay variety) 45 h after hydration of the dry cysts. Na+-loaded vesicles accumulated up to 10 nmol Ca2+/mg protein when diluted 50-fold into 160 mM KCl containing 15 microM CaCl2. Practically no accumulation of Ca2+ was observed if the vesicles were diluted into 160 mM NaCl instead of KCl, or if they were treated with monensin, a Na+ ionophore, for 30 s prior to addition of CaCl2 to the KCl medium. These observations indicate that the Artemia vesicles exhibit Na-Ca exchange activity. The velocity of Ca2+ accumulation by the vesicles in KCl was stimulated 2.6-fold by the K+ ionophore valinomycin, suggesting that the exchange system is electrogenic, with a stoichiometry greater than 2Na+ per Ca2+. Km,Ca and Vmax values were 15 microM and 7.5 nmol/mg protein.s, respectively. Exchange activity in the Artemia vesicles was inhibited by benzamil (IC50 approximately equal to 100 microM) and by quinacrine (IC50 approximately equal to 250 microM), agents that also inhibit exchange activity in cardiac sarcolemmal vesicles. Unlike cardiac vesicles, however, exchange activity in Artemia was not stimulated by limited proteolysis, redox reagents, or intravesicular Ca2+. This indicates that the two exchange systems are regulated by different mechanisms. Vesicles were prepared from Artemia at various times after hydration of the dry cysts and examined for exchange activity. Activity was first observed at approximately 10 h after hydration and increased to a maximal value by 30-40 h; hatching of the free swimming nauplii occurred at 18-24 h. The results suggest that hatching Artemia nauplii might be a particularly rich source of mRNA coding for the Na+-Ca2+ exchange carrier.