Acclimations to Cold and Warm Conditions Differently Affect the Energy Metabolism of Diapausing Larvae of the European Corn Borer Ostrinia nubilalis (Hbn.)

The European corn borer Ostrinia nubilalis is a pest species, whose fifth instar larvae gradually develop cold hardiness during diapause. The physiological changes underlying diapause progression and cold hardiness development are still insufficiently understood in insects. Here, we follow a complex of changes related to energy metabolism during cold acclimation (5°C) of diapausing larvae and compare this to warm-acclimated (22°C) and non-diapause controls. Capillary electrophoresis of nucleotides and coenzymes has shown that in gradually cold-acclimated groups concentrations of ATP/ADP and, consequently, energy charge slowly decrease during diapause, while the concentration of AMP increases, especially in the first months of diapause. Also, the activity of cytochrome c oxidase (COX), as well as the concentrations of NAD+ and GMP, decline in cold-acclimated groups, until the latter part of diapause, when they recover. Relative expression of NADH dehydrogenase (nd1), coenzyme Q-cytochrome c reductase (uqcr), COX, ATP synthase (atp), ADP/ATP translocase (ant), and prohibitin 2 (phb2) is supressed in cold-acclimated larvae during the first months of diapause and gradually increases toward the termination of diapause. Contrary to this, NADP+ and UMP levels significantly increased in the first few months of diapause, after gradual cold acclimation, which is in connection with the biosynthesis of cryoprotective molecules, as well as regeneration of small antioxidants. Our findings evidence the existence of a cold-induced energy-saving program that facilitates long-term maintenance of larval diapause, as well as gradual development of cold hardiness. In contrast, warm acclimation induced faster depletion of ATP, ADP, UMP, NAD+, and NADP+, as well as higher activity of COX and generally higher expression of all energy-related genes in comparison to cold-acclimated larvae. Moreover, such unusually high metabolic activity, driven by high temperatures, lead to premature mortality in the warm-acclimated group after 2 months of diapause. Thus, our findings strongly support the importance of low temperature exposure in early diapause for gradual cold hardiness acquisition, successful maintenance of the resting state and return to active development. Moreover, they demonstrate potentially adverse effects of global climate changes and subsequent increase in winter temperatures on cold-adapted terrestrial organisms in temperate and subpolar regions.

Characteristics, dynamics and mechanisms of actions of some major stress-induced biomacromolecules; addressing Artemia as an excellent biological model

Stress tolerance is one of the most prominent and interesting topics in biology since many macro- and micro-adaptations have evolved in resistant organisms that are worth studying. When it comes to confronting various environmental stressors, the extremophile Artemia is unrivaled in the animal kingdom. In the present review, the evolved molecular and cellular basis of stress tolerance in resistant biological systems are described, focusing on Artemia cyst as an excellent biological model. The main purpose of the review is to discuss how the structure and physicochemical characteristics of protective factors such as late embryogenesis abundant proteins (LEAPs), small heat shock proteins (sHSPs) and trehalose are related to their functions and by which mechanisms, they exert their functions. In addition, some metabolic depressors in Artemia encysted embryos are also mentioned, indirectly playing important roles in stress tolerance. Importantly, a great deal of attention is given to the LEAPs, exhibiting distinctive folding behaviors and mechanisms of actions. For instance, molecular shield function, chaperone-like activity, moonlighting property, sponging and snorkeling capabilities of the LEAPs are delineated here. Moreover, the molecular interplay between some of these factors is mentioned, leading to their synergistic effects. Interestingly, Artemia life cycle adapts to environmental conditions. Diapause is the defense mode of this life cycle, safeguarding Artemia encysted embryos against various environmental stressors. Communicated by Ramaswamy H. Sarma.

Stress tolerance during diapause and quiescence of the brine shrimp, Artemia

Oviparously developing embryos of the brine shrimp, Artemia, arrest at gastrulation and are released from females as cysts before entering diapause, a state of dormancy and stress tolerance. Diapause is terminated by an external signal, and growth resumes if conditions are permissible. However, if circumstances are unfavorable, cysts enter quiescence, a dormant stage that continues as long as adverse conditions persist. Artemia embryos in diapause and quiescence are remarkably resistant to environmental and physiological stressors, withstanding desiccation, cold, heat, oxidation, ultraviolet radiation, and years of anoxia at ambient temperature when fully hydrated. Cysts have adapted to stress in several ways; they are surrounded by a rigid cell wall impermeable to most chemical compounds and which functions as a shield against ultraviolet radiation. Artemia cysts contain large amounts of trehalose, a non-reducing sugar thought to preserve membranes and proteins during desiccation by replacing water molecules and/or contributing to vitrification. Late embryogenesis abundant proteins similar to those in seeds and other anhydrobiotic organisms are found in cysts, and they safeguard cell organelles and proteins during desiccation. Artemia cysts contain abundant amounts of p26, a small heat shock protein, and artemin, a ferritin homologue, both ATP-independent molecular chaperones important in stress tolerance. The evidence provided in this review supports the conclusion that it is the interplay of these protective elements that make Artemia one of the most stress tolerant of all metazoan organisms.

Intracellular localization of group 3 LEA proteins in embryos of Artemia franciscana

Late embryogenesis abundant (LEA) proteins are accumulated by anhydrobiotic organisms in response to desiccation and improve survivorship during water stress. In this study we provide the first direct evidence for the subcellular localizations of AfrLEA2 and AfrLEA3m (and its subforms) in anhydrobiotic embryos of Artemia franciscana. Immunohistochemistry shows AfrLEA2 to reside in the cytoplasm and nucleus, and the four AfrLEA3m proteins to be localized to the mitochondrion. Cellular locations are supported by Western blots of mitochondrial, nuclear and cytoplasmic fractions. The presence of LEA proteins in multiple subcellular compartments of A. franciscana embryos suggests the need to protect biological structures in many areas of a cell in order for an organism to survive desiccation stress, and may explain in part why a multitude of different LEA proteins are expressed by a single organism.

Prawn lipocalin: characterization of a color shift induced by gene knockdown and ligand binding assay

The lipocalin family of proteins functions in the transport of steroids, carotenoids, retinoids, and other small hydrophobic molecules. Recently, a lipocalin (MrLC) was isolated from the prawn Macrobrachium rosenbergii and its expression varied with the molting cycle. In this study, knockdown of the MrLC gene by RNA interference (RNAi) was performed and resulted in a shift in body color from blue to orangish red over the entire carapace. By immune-gold electron microscopy, MrLC was found to co-localize with the lipid droplets in subepidermal adiose tissue that were found to be decreased dramatically in MrLC knockdown prawns, in which a reduction in relative fat content was also quantified. Furthermore, MrLC was found to specifically bind astaxanthin and molt hormone (20-hydroxyecdysone) in both in vitro ligand binding assay and in vivo native ligand detection. These results suggested that MrLC plays roles in the regulation of coloration through its association with astaxanthin and may also be involved in the regulation of molting in crustacean.

Extracellular matrix peptides of Artemia cyst shell participate in protecting encysted embryos from extreme environments

Background

Many species of the brine shrimp Artemia are found in various severe environments in many parts of the world where extreme salinity, high UV radiation levels, high pH, anoxia, large temperature fluctuations, and intermittent dry conditions are often recorded. To withstand adverse environments, Artemia undergoes an oviparous developmental pathway to release cysts whereas, under favorable conditions, swimming nauplius larvae are formed directly via an ovoviviparous pathway. In the former case these cysts have an extraordinary ability to keep the embryos protected from the harsh environment for long periods. This is achieved through the protection by a complex out-wrapping cyst shell. However, the formation and function of the cyst shell is complex; the details remain largely unclear.

Principal Finding

A shell gland-specific gene (SGEG2) was cloned and identified from a suppression subtractive hybridization library. Western blot analysis showed that SGEG2 presumably requires post-translational proteolysis in order to be processed into two mature peptides (SGEG2a and 2b). The three matrix peptides (SGEG1 reported previously, 2a, and 2b) were found to distribute throughout the cyst shell. The results of gene knockdown by RNAi and subsequent resistance to environmental stresses assays indicated that these matrix peptides are required for cyst shell formation and are involved in protecting the encysted embryos from environmental stress.

Conclusions/Significance

This study revealed that extracellular matrix peptides participate in protecting embryos from extreme salinity, UV radiation, large temperature fluctuations and dry environments, thereby facilitating their survival. The cyst shell provides an excellent opportunity to link the ecological setting of an organism to the underlying physiological and biochemical processes enabling its survival. The cyst shell material has also a high potential to become an excellent new biomaterial with a high number of prospective uses due, specifically, to such biological characteristics.

Formation of diapause cyst shell in brine shrimp, Artemia parthenogenetica, and its resistance role in environmental stresses

Artemia has attracted much attention for its ability to produce encysted embryos wrapped in a protective shell when subject to extremely harsh environmental conditions. However, what the cyst shell is synthesized from and how the formative process is performed remains, as yet, largely unknown. Over 20 oviparous specifically expressed genes were identified through screening the subtracted cDNA library enriched between oviparous and ovoviviparous Artemia ovisacs. Among them, a shell gland-specifically expressed gene (SGEG) has been found to be involved in the cyst shell formation. Lacking SGEG protein (by RNA interference) caused the cyst shell to become translucent and the chorion layer of the shell to become less compact and pultaceous and to show a marked decrease of iron composition within the shell. The RNA interference induced defective diapause cysts with a totally compromised resistibility to UV irradiation, extremely large temperature differences, osmotic pressure, dryness, and organic solvent stresses. In contrast, the natural cyst would provide adequate protection from all such factors. SGEG contains a 345-bp open reading frame, and its consequentially translated peptide consists of a 33-amino acid residue putative signal peptide and an 81-amino acid residue mature peptide. The results of Northern blotting and in situ hybridization indicate that the gene is specifically expressed in the cells of shell glands during the period of diapause cyst formation of oviparous Artemia. This investigation adds strong insight into the mechanism of cyst shell formation of Artemia and may be applicable to other areas of research in extremophile biology.

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.

Ultrastructure of cyst shell and underlying membranes of three strains of the brine shrimpArtemia (Branchiopoda: Anostraca) from South India

The cyst of Artemia has shell and membranous coverings over the embryo. The membranous coverings have special adaptive features to allow the physical changes accompanying repeated hydration and dehydration cycles that might occur and adversely influence postembryonic development. Whole and slices of cryptobiotic cysts were processed for electron microscopy to study the internal details and to compare the morphological architecture of three Artemia strains of South India. Surface topography of scanning electron microscopic (SEM) studies revealed distinct button shaped structures on the cyst of Puthalam strain. Transmission electron microscopic (TEM) studies of the cysts displayed the conventional pattern of anostracan crustaceans with outer cortex and alveolar layer, cuticular membranes, and the cytoplasmic inclusions namely nucleus, yolk droplets, lipoid bodies, and mitochondria. The prominent wavy outer cortex layer of Puthalam cysts corroborates the results of SEM studies.

Differences in Isolated Mitochondria Are Insufficient to Account for Respiratory Depression during Diapause inArtemia franciscanaEmbryos

In response to cues signifying the approach of winter, adult Artemia franciscana produce encysted embryos that enter diapause. We show that respiration rates of diapause embryos collected from the field (Great Salt Lake, Utah) are reduced up to 92% compared with postdiapause embryos when measured under conditions of normoxia and full hydration. However, mitochondria isolated from diapause embryos exhibit rates of state 3 and state 4 respiration on pyruvate that are equivalent to those from postdiapause embryos with active metabolism; a reduction in these rates (15%-27%) is measured with succinate for two of three collection years. Respiratory control ratios for diapause mitochondria are comparable to or higher than those from postdiapause embryos. The P : O flux ratios are statistically identical. Our calculations suggest that respiration of intact, postdiapause embryos is operating close to the state 3 oxygen fluxes measured for isolated mitochondria. Cytochrome c oxidase (COX) activity is 53% lower in diapause mitochondria during one collection year; the minimal impact of this COX reduction on mitochondrial respiration appears to be due to the 31% excess COX capacity in A. franciscana mitochondria. Transmission electron micrographs of embryos reveal mitochondria that are well differentiated and structurally similar in both states. As inferred from the similar amounts of mitochondrial protein extractable, tissue contents of mitochondria in diapause and postdiapause embryos are equivalent. Thus, metabolic depression during diapause cannot be fully explained by altered properties of isolated mitochondria. Rather, mechanisms for active inhibition or substrate limitation of mitochondrial metabolism in vivo may be operative.

Yolk platelets in artemia embryos: are they really storage sites of immature mitochondria?

We have used semi-quantitative polymerase chain reaction (PCR) technology to determine the mitochondrial DNA (mtDNA) content of yolk platelets isolated from embryos of the brine shrimp, Artemia franciscana, and ultrastructural analysis of yolk platelet formation to determine whether these organelles contain mitochondria as reported previously. Using six different isolation and purification protocols, we found one yolk platelet preparation to be devoid of mtDNA, while four yolk platelet preparations contained mtDNA ranging from 16.4 to 85 pg/10(6) yolk platelets. One preparation contained 600 pg mtDNA per 10(6) yolk platelets. Based on our PCR analyses, the mtDNA component of Artemia yolk platelets represented 0.16-4.5% of the total DNA isolated from the platelets. We calculated that Artemia yolk platelets contain, on average, approximately 1.78 molecules of mtDNA/platelet. Direct analysis of mtDNA in "free" mitochondria isolated from yolk platelet-free preparations of Artemia embryos and newly hatched larvae yielded 0.76-0.80 ng/animal. Based on these values, the mtDNA content of yolk platelets was approximately 0.2% of total mtDNA in Artemia embryos. Microscopic analysis of yolk platelet formation during oogenesis in Artemia failed to show the inclusion of mitochondria during the assemblage of yolk platelets. The "mitochondria-like" structures that appear in yolk platelets during their utilization lack the well defined inner and outer membranes characteristic of mitochondria making it unlikely that the yolk platelet inclusions are mitochondria. Our results from PCR technology and ultrastructure analysis demonstrate that mtDNA in yolk platelets of Artemia franciscana embryos is a minor component of the total mtDNA in the embryo, and they fail to support the notion that yolk platelets in Artemia are a major source of immature mitochondria for development.

Protein kinases in mitochondria of the invertebrate Artemia franciscana

The information concerning protein kinases in animal mitochondria is scarce and related only to mammals. No data are available for invertebrates. We demonstrate here the presence of casein kinase II (CK II) and cAMP-dependent protein kinase (PKA) in the purified mitochondria of the crustacean Artemia franciscana. Whereas the mitochondrial CK II showed the same characteristics of the cytosolic enzyme, mitochondrial PKA had an apparent Km for its substrate Kemptide 1 order of magnitude lower than that of the cytosolic enzyme. CK II and PKA phosphorylate different sets of proteins in Artemia mitochondria in vitro. The use of an activity gel assay has allowed the detection of additional protein kinases, as yet unidentified, in Artemia mitochondria.

Embryogenesis, hatching and larval development of Artemia during orbital spaceflight

Developmental biology studies, using gastrula-arrested cysts of the brine shrimp Artemia franciscana, were conducted during two flights of the space shuttle Atlantis (missions STS-37 and STS-43) in 1991. Dehydrated cysts were activated, on orbit, by addition of salt water to the cysts, and then development was terminated by the addition of fixative. Development took place in 5 ml syringes, connected by tubing to activation syringes, containing salt water, and termination syringes, containing fixative. Comparison of space results with simultaneous ground control experiments showed that equivalent percentages of naupliar larvae hatched in the syringes (40%). Thus, reactivation of development, completion of embryogenesis, emergence and hatching took place, during spaceflight, without recognizable alteration in numbers of larvae produced. Post-hatching larval development was studied in experiments where development was terminated, by introduction of fixative, 2 days, 4 days, and 8 days after reinitiation of development. During spaceflight, successive larval instars or stages, interrupted by molts, occurred, generating brine shrimp at appropriate larval instars. Naupliar larvae possessed the single naupliar eye, and development of the lateral pair of adult eyes also took place in space. Transmission electron microscopy revealed extensive differentiation, including skeletal muscle and gut endoderm, as well as the eye tissues. These studies demonstrate the potential value of Artemia for developmental biology studies during spaceflight, and show that extensive degrees of development can take place in this microgravity environment.