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

Assessment of the cyst wall and surface microbiota in dormant embryos of the Antarctic calanoid copepod, Boeckella poppei

Embryos of zooplankton from inland waters and estuaries can remain viable for years in an extreme state of metabolic suppression. How these embryos resist microbial attack with limited metabolic capacity for immune defence or repair is unknown. As a first step in evaluating resistance to microbial attack in dormant zooplankton, surface colonization of the Antarctic freshwater copepod, Boeckella poppei, was evaluated. Scanning electron micrographs demonstrate the outer two layers of a five-layered cyst wall in B. poppei fragment and create a complex environment for microbial colonization. By contrast, the third layer remains undamaged during years of embryo storage in native sediment. The absence of damage to the third layer indicates that it is resistant to degradation by microbial enzymes. Scanning electron microscopy and microbiome analysis using the 16S ribosomal subunit gene and internal transcribed spacer (ITS) region demonstrate the presence of a diverse microbial community on the embryo surface. Coverage of the embryos with microbial life varies from a sparse population with individual microbes to complete coverage by a thick biofilm. Extracellular polymeric substance binds debris and provides a structural element for the microbial community. Frequent observation of bacterial fission indicates that the biofilm is viable in stored sediments.

The pre- and postembryonic development of Artemia franciscana (Anostraca: Artemiidae)

Artemia franciscana is a universal live feed in aquaculture, and it has been reported as an invasive species in many Asian hypersaline ecosystems. The present observations illustrated the pre- and postembryonic development stages of the A. franciscana population confined to the Indian saltern of Kelambakkam. We observed their growth patterns during various hydration periods with specific time intervals. Results showed differences in the development stages with respect to unique identity. Interestingly, a period of hydration showed notable cellular movement toward clockwise positions in the hydrating cysts. After 10 h of hydration, blastocoel appeared, accelerating the dynamic route of nuclei movement. At the end of the invagination, the embryo burst out of the cyst, and a sequence of emerging stages was noted. With reference to light microscopic observations, a series of developmental stages were observed, and each instar was documented by developing limb buds of nauplii. Excitingly, the 10th and 11th instar stages reveal sexual differentiation between male and female individuals. Thus, the laboratory culture study clearly documented the different developmental stages with their specific characteristic features. However, further molecular study would provide a cellular basis for understanding the early development of A. franciscana.

Lack of dormancy to protect diversity: Decrease in diversity of active zooplankton community observed in lake with depauperate egg bank

The study of zooplankton communities in freshwater resources under anthropogenic pressures rarely includes the simultaneous assessment of dormant embryos in bottom sediments and active life-stages in the water column. A coastal lake with a history of coal-ash contamination and disruption by hurricanes provided an ideal opportunity to demonstrate the power of examining both dormant and active zooplankton. The primary objective of this study was to evaluate changes in structure of a multicellular zooplankton community that is under simultaneous pressure from anthropogenic pollution and hurricane-induced flooding. To evaluate change in community structure, the active zooplankton community in 2015 was compared to that observed in 1985. Shannon-Wiener and Simpson indices demonstrate that diversity of the active zooplankton community decreased during this 30-year span. In total, 31% of zooplankton species were lost, and new colonization accounts for 27% of species richness. Dominant species of all major taxonomic groupings changed. Because most zooplankton in freshwater lakes depend on dormant embryos to reestablish active populations after major disruptions, dormant embryos in the sediment "egg bank" were also quantified. Dormant cladoceran ephippia are present in bottom sediments, but dormant copepods and rotifers are missing. The existence of a dormant egg bank that is less diverse than the active community in a freshwater lake is unprecedented, and a depauperate "egg bank" would certainly impair community recovery after severe flooding from hurricanes. It is argued that a paradigm shift is needed in the ecological assessment of inland lakes in order to account for the critical role that dormant embryos (egg banks) play in freshwater zooplankton communities. Two challenges to achieving this are that 1. long-term monitoring is expensive and 2. data on dormant zooplankton are rarely available. This study provides an example of how to conduct such studies by leveraging historic data when long-term monitoring is not possible.

Embryos of an Antarctic zooplankton require anoxia for dormancy, are permeable to lipophilic chemicals, and reside in sediments containing PCBs

Zooplankton in Antarctic maritime lakes face challenges imposed by anthropogenic chemicals. Studies on temperate species suggest that lipophilic chemicals will accumulate in dormant embryos of Antarctic zooplankton and decrease hatching success, thereby threatening centuries of accumulated genetic diversity that would increase population resilience in the face of climate change. We evaluated the potential for lakes to act as sinks for legacy pollutants in the maritime Antarctic by testing sediments for polychlorinated biphenyls (PCBs) previously identified in soil, flora and fauna of lake catchments. Direct tests of embryo permeability to chemicals are confounded by potential adhesion of chemicals to the embryo surface and limited biomass available. Therefore, in order to assess the potential for lipophilic chemicals to penetrate and passively accumulate in dormant embryos of Antarctic lacustrine zooplankton, we evaluated the effect of anoxia on post-diapause development in the calanoid copepod, Boeckella poppei, and then used chemical anoxia induced by rotenone as a reporter for permeability of these embryos to moderately lipophilic chemicals. The data presented demonstrate that embryos of B. poppei from Antarctic lake sediments will passively accumulate moderately lipophilic chemicals while lying dormant in anoxic sediments. Implications for legacy POPs in sediments of Antarctic maritime lakes are discussed.

Rotenone Decreases Hatching Success in Brine Shrimp Embryos by Blocking Development: Implications for Zooplankton Egg Banks

While many zooplankton species recover quickly after the treatment of water resources with the piscicide, rotenone, some fail to reach pretreatment population density or, in rare cases, do not reappear at all. The variable impact of rotenone on zooplankton populations could stem from differences in the capacity of species to switch entirely to anaerobic catabolic pathways in the presence of rotenone, which blocks mitochondrial electron transport. Alternatively, variable responses among species could originate from differences in permeability of dormant life-stages to lipophilic chemicals like rotenone. The purpose of the present study was to determine the effects of rotenone on development, emergence and hatching of zooplankton embryos that lack both the anaerobic capacity to develop in the presence of rotenone and a permeability barrier to prevent the entry of rotenone during dormancy. Post-diapause embryos of the brine shrimp, Artemia franciscana, were employed as a model system, because they are permeable to lipophilic compounds when dechorionated and require aerobic conditions to support development. Early development in this species is also well characterized in the literature. Brine shrimp embryos were exposed to rotenone while development was either slowed by chilling or suspended by anoxia. Development, emergence and hatching were then observed in rotenone-free artificial seawater. The data presented demonstrate that rotenone freely diffuses across the embryonic cuticle in a matter of hours, and prevents development and emergence after brief exposures to ecologically relevant concentrations (0.025–0.5 mg L^-1) of the piscicide. Neither the removal of rotenone from the environment, nor the removal of embryonic water with a hypertonic solution, are sufficient to reverse this block on development and emergence. These data indicate that rotenone could impair recruitment from egg banks for species of zooplankton that lack both an embryonic barrier to the entry of lipophilic compounds and the anaerobic capacity to develop when NADH:ubiquinone oxidoreductase activity is inhibited by rotenone.

Analysis, characterisation and expression of gill-expressed carbonic anhydrase genes in the freshwater crayfish Cherax quadricarinatus

Changes in water quality parameters such as pH and salinity can have a significant effect on productivity of aquaculture species. Similarly, relative osmotic pressure influences various physiological processes and regulates expression of a number of osmoregulatory genes. Among those, carbonic anhydrase (CA) plays a key role in systemic acid-base balance and ion regulation. Redclaw crayfish (Cherax quadricarinatus) are unique in their ability to thrive in environments with naturally varied pH levels, suggesting unique adaptation to pH stress. To date, however, no studies have focused on identification and characterisation of CA or other osmoregulatory genes in C. quadricarinatus. Here, we analysed the redclaw gill transcriptome and characterized CA genes along with a number of other key osmoregulatory genes that were identified in the transcriptome. We also examined patterns of gene expression of these CA genes when exposed to three pH treatments. In total, 72,382,710 paired end Illumina reads were assembled into 36,128 contigs with an average length of 800bp. Approximately 37% of contigs received significant BLAST hits and 22% were assigned gene ontology terms. Three full length CA isoforms; cytoplasmic CA (ChqCAc), glycosyl-phosphatidylinositol-linked CA (ChqCAg), and β-CA (ChqCA-beta) as well as two partial CA gene sequences were identified. Both partial CA genes showed high similarity to ChqCAg and appeared to be duplicated from the ChqCAg. Full length coding sequences of Na(+)/K(+)-ATPase, V-type H(+)-ATPase, sarcoplasmic Ca(+)-ATPase, arginine kinase, calreticulin and Cl(-) channel protein 2 were also identified. Only the ChqCAc gene showed significant differences in expression across the three pH treatments. These data provide valuable information on the gill expressed CA genes and their expression patterns in freshwater crayfish. Overall our data suggest an important role for the ChqCAc gene in response to changes in pH and in systemic acid-base balance in freshwater crayfish.

Differential distribution of V-type H(+)-ATPase and Na (+)/K (+)-ATPase in the branchial chamber of the palaemonid shrimp Macrobrachium amazonicum

V-H(+)-ATPase and Na(+)/K(+)-ATPase were localized in the gills and branchiostegites of M. amazonicum and the effects of salinity on the branchial chamber ultrastructure and on the localization of transporters were investigated. Gills present septal and pillar cells. In freshwater (FW), the apical surface of pillar cells is amplified by extensive evaginations associated with mitochondria. V-H(+)-ATPase immunofluorescence was localized in the membranes of the apical evaginations and in clustered subapical areas of pillar cells, suggesting labeling of intracellular vesicle membranes. Na(+)/K(+)-ATPase labeling was restricted to the septal cells. No difference in immunostaining was recorded for both proteins according to salinity (FW vs. 25 PSU). In the branchiostegite, both V-H(+)-ATPase and Na(+)/K(+)-ATPase immunofluorescence were localized in the same cells of the internal epithelium. Immunogold revealed that V-H(+)-ATPase was localized in apical evaginations and in electron-dense areas throughout the inner epithelium, while Na(+)/K(+)-ATPase occurred densely along the basal infoldings of the cytoplasmic membrane. Our results suggest that morphologically different cell types within the gill lamellae may also be functionally specialized. We propose that, in FW, pillar cells expressing V-H(+)-ATPase absorb ions (Cl(-), Na(+)) that are transported either directly to the hemolymph space or through a junctional complex to the septal cells, which may be responsible for active Na(+) delivery to the hemolymph through Na(+)/K(+)-ATPase. This suggests a functional link between septal and pillar cells in osmoregulation. When shrimps are transferred to FW, gill and branchiostegite epithelia undergo ultrastructural changes, most probably resulting from their involvement in osmoregulatory processes.

Metabolic restructuring during energy-limited states: insights from Artemia franciscana embryos and other animals

Many life history stages of animals that experience environmental insults enter developmental arrested states that are characterized by reduced cellular proliferation, with or without a concurrent reduction in overall metabolism. In the case of the most profound metabolic arrest reported in invertebrates, i.e., anaerobic quiescence in Artemia franciscana embryos, acidification of the intracellular milieu is a major factor governing catabolic and anabolic downregulation. Release of ions from intracellular compartments is the source for approximately 50% of the proton equivalents needed for the 1.5 unit acidification that is observed. Recovery from the metabolic arrest requires re-sequestration of the protons with a vacuolar-type ATPase (V-ATPase). The remarkable facet of this mechanism is the ability of embryonic cells to survive the dissipation of intracellular ion gradients. Across many diapause-like states, the metabolic reduction and subsequent matching of energy demand is accomplished by shifting energy metabolism from oxidative phosphorylation to aerobic glycolysis. Molecular pathways that are activated to induce these resilient hypometabolic states include stimulation of the AMP-activated protein kinase (AMPK) and insulin signaling via suite of daf (dauer formation) genes for diapause-like states in nematodes and insects. Contributing factors for other metabolically depressed states involve hypoxia-inducible factor-1 and downregulation of the pyruvate dehydrogenase complex. Metabolic similarities between natural states of stasis and some cancer phenotypes are noteworthy. Reduction of flux through oxidative phosphorylation helps prevent cell death in certain cancer types, similar to the way it increases viability of dauer stages in Caenorhabditis elegans. Mechanisms that underlie natural stasis are being used to pre-condition mammalian cells prior to cell biostabilization and storage.

Gene expression, metabolic regulation and stress tolerance during diapause

Diapause entails molecular, physiological and morphological remodeling of living animals, culminating in a dormant state characterized by enhanced stress tolerance. Molecular mechanisms driving diapause resemble those responsible for biochemical processes in proliferating cells and include transcriptional, post-transcriptional and post-translational processes. The results are directed gene expression, differential mRNA and protein accumulation and protein modifications, including those that occur in response to changes in cellular redox potential. Biochemical pathways switch, metabolic products change and energy production is adjusted. Changes to biosynthetic activities result for example in the synthesis of molecular chaperones, late embryogenesis abundant (LEA) proteins and protective coverings, all contributing to stress tolerance. The purpose of this review is to consider regulatory and mechanistic strategies that are potentially key to metabolic control and stress tolerance during diapause, while remembering that organisms undergoing diapause are as diverse as the processes itself. Some of the parameters described have well-established roles in diapause, whereas the evidence for others is cursory.

Metabolic Depression is Delayed and Mitochondrial Impairment Averted during Prolonged Anoxia in the ghost shrimp, Lepidophthalmus louisianensis (Schmitt, 1935)

Lepidophthalmus louisianensis burrows deeply into oxygen-limited estuarine sediments and is subjected to extended anoxia at low tides. Large specimens (>2 g) have a lethal time for 50% mortality (LT(50)) of 64 h under anoxia at 25º C. Small specimens (<1 g) have a significantly higher LT(50) of 113 h, which is the longest ever reported for a crustacean. Whole body lactate levels rise dramatically under anoxia and exceed 120 µmol g.f.w.(-1) by 72 h. ATP, ADP, and AMP do not change during 48 h of anoxia, but arginine phosphate declines by over 50%. Thus arginine phosphate may help stabilize the ATP pool. Surprisingly, when compared to the aerobic resting rate, ATP production under anoxia is unchanged during the first 12 h, and drops to only about 50% between 12 and 48 h. Finally, after 48 h of anoxia, a major metabolic depression to less than 5% occurs. Downregulation of metabolism is delayed in L. louisianensis compared to many invertebrates that exhibit facultative anaerobiosis. Bioenergetic constraints as a result of eventual metabolic depression led to ionic disturbances like calcium overload and compromised membrane potential of mitochondria. Because these phenomena trigger apoptosis in mammalian species, we evaluated the susceptibility of ghost shrimp mitochondria to opening of the mitochondrial permeability transition pore (MPTP) and associated damage. Energized mitochondria isolated from hepatopancreas possess a pronounced capacity for calcium uptake. Exogenous calcium does not stimulate opening of the MPTP, which potentially could reduce cell death during prolonged anoxia.

Occurrence of mitochondria-targeted Late Embryogenesis Abundant (LEA) gene in animals increases organelle resistance to water stress

Anhydrobiotic animals survive virtually complete loss of cellular water. The mechanisms that explain this phenomenon are not fully understood but often include the accumulation of low molecular weight solutes such as trehalose and macromolecules like Late Embryogenesis Abundant (LEA) proteins. Here we report for the first time the occurrence of a mitochondria-targeted LEA gene (Afrlea3m) product in an animal species. The deduced molecular mass of the 307-amino acid polypeptide from the brine shrimp Artemia franciscana is 34 kDa. Bioinformatic analyses reveal features typical of a Group 3 LEA protein, and subcellular localization programs predict targeting of the mature peptide to the mitochondrial matrix, based on an N-terminal, amphipathic presequence. Real-time quantitative PCR shows that Afralea3m mRNA is expressed manyfold higher in desiccation-tolerant embryonic stages when compared with intolerant nauplius larvae. Mitochondrial localization of the protein was confirmed by transfection of human hepatoma cells (HepG2/C3A) with a nucleotide construct encoding the first 70 N-terminal amino acids of AfrLEA3m in-frame with the nucleotide sequence for green fluorescence protein. The chimeric protein was readily incorporated into mitochondria of these cells. Successful targeting of a protein to human mitochondria by use of an arthropod signaling sequence clearly reveals the highly conserved nature of such presequences, as well as of the import machinery. Finally, mitochondria isolated from A. franciscana embryos, which naturally contain AfrLEA3m and trehalose, exhibit resistance to water stress (freezing) as evidenced by an unchanged capacity for oxidative phosphorylation on succinate + rotenone, a resistance that is absent in mammalian mitochondria lacking AfrLEA3m.

Mitochondria in energy-limited states: mechanisms that blunt the signaling of cell death

Cellular conditions experienced during energy-limited states--elevated calcium, shifts in cellular adenylate status, compromised mitochondrial membrane potential--are precisely those that trigger, at least in mammals, the mitochondrion to initiate opening of the permeability transition pore, to assemble additional protein release channels, and to release pro-apoptotic factors. These pro-apototic factors in turn activate initiator and executer caspases. How is activation of mitochondria-based pathways for the signaling of apoptotic and necrotic cell death avoided under conditions of hypoxia, anoxia, diapause, estivation and anhydrobiosis? Functional trade-offs in environmental tolerance may have occurred in parallel with the evolution of diversified pathways for the signaling of cell death in eukaryotic organisms. Embryos of the brine shrimp, Artemia franciscana, survive extended periods of anoxia and diapause, and evidence indicates that opening of the mitochondrial permeability transition pore and release of cytochrome c (cyt-c) do not occur. Further, caspase activation in this crustacean is not dependent on cyt-c. Its caspases display regulation by nucleotides that is consistent with ;applying the brakes' to cell death during energy limitation. Unraveling the mechanisms by which organisms in extreme environments avoid cell death may suggest possible interventions during disease states and biostabilization of mammalian cells.

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.

Life without water: expression of plant LEA genes by an anhydrobiotic arthropod

Anhydrobiotic animals protect cellular architecture and metabolic machinery in the dry state, yet the molecular repertoire supporting this profound dehydration tolerance is not fully understood. For the desiccation-tolerant crustacean, Artemia franciscana, we report differential expression of two distinct mRNAs encoding for proteins that share sequence similarities and structural features with late-embryogenesis abundant (LEA) proteins originally discovered in plants. Bioinformatic analyses support assignment of the LEA proteins from A. franciscana to group 3. This eucoelomate species is the most highly evolved animal for which LEA gene expression has been reported. It is becoming clear that an ensemble of micromolecules and macromolecules is important for establishing the physical conditions required for cellular stabilization during drying in nature.

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.

Protein stability in Artemia embryos during prolonged anoxia

Encysted embryos (cysts) of the brine shrimp, Artemia franciscana, are arguably the most stress-resistant of all animal life-history stages. One of their many adaptations is the ability to tolerate anoxia for periods of years, while fully hydrated and at physiological temperatures. Previous work indicated that the overall metabolism of anoxic embryos is brought to a reversible standstill, including the transduction of free energy and the turnover of macromolecules. But the issue of protein stability at the level of tertiary and quaternary structure was not examined. Here I provide evidence that the great majority of proteins do not irreversibly lose their native conformation during years of anoxia, despite the absence of detectable protein turnover. Although a modest degree of protein denaturation and aggregation occurs, that is quickly reversed by a brief post-anoxic aerobic incubation. I consider how such extraordinary stability is achieved and suggest that at least part of the answer involves massive amounts of a small heat shock protein (p26) that acts as a molecular chaperone, the function of which does not appear to require ribonucleoside di- or tri-phosphates.

V-ATPase inhibition prevents recovery from anoxia in Artemia franciscana embryos: quiescence signaling through dissipation of proton gradients

The metabolic downregulation critical for long-term survival of Artemia franciscana embryos under anoxia is mediated, in part, by a progressive intracellular acidification. However, very little is known about the mechanisms responsible for the pH transitions associated with exposure to, and recovery from, oxygen deprivation. In the present study, we demonstrate with 31P-NMR that incubation of intact embryos with the V-ATPase inhibitor bafilomycin A1 severely limits intracellular alkalinization during recovery from anoxia without affecting the restoration of cellular nucleotide triphosphate levels. Based on these data, it appears that oxidative phosphorylation and ATP resynthesis can only account for the first 0.3 pH unit alkalinization observed during aerobic recovery from the 1 pH unit acidification produced during 1 h of anoxia. The additional 0.7 pH unit increase requires proton pumping by the V-ATPase. Aerobic incubation with bafilomycin also suggests that V-ATPase inhibition alone is not enough to induce an acute dissipation of proton gradients under anoxia. In intact embryos, the dissipation of proton gradients and uncoupling of oxidative phosphorylation with carbonyl cyanide 3-chlorophenylhydrazone (CCCP) leads to an intracellular acidification similar to that seen after 1 h of anoxia. Subsequent exposure to anoxia, in the continued presence of CCCP, yields little additional acidification, suggesting that proton gradients are normally dissipated under anoxia. When combined with protons generated from net ATP hydrolysis, these data show that the dissipation of proton chemical gradients is sufficient to account for the reversible acidification associated with quiescence in these embryos.