Nitrogen metabolism in the African lungfish (Protopterus dolloi) aestivating in a mucus cocoon on land

The West African lungfish secretes a living cocoon during aestivation with uncertain antimicrobial function

One of the most exceptional adaptations to extreme drought is found in the sister group to tetrapods, the lungfishes (Dipnoi), which can aestivate inside a mucus cocoon for multiple years at reduced metabolic rates with complete cessation of ingestion and excretion. However, the function of the cocoon tissue is not fully understood. Here we developed a new more natural laboratory protocol for inducing aestivation in the West African lungfish, Protopterus annectens, and investigated the structure and function of the cocoon. We used electron microscopy and imaging of live tissue-stains to confirm that the inner and outer layers of the paper-thin cocoon are composed primarily of living cells. However, we also repeatedly observed extensive bacterial and fungal growth covering the cocoon and found no evidence of anti-microbial activity in vitro against E. coli for the cocoon tissue in this species. This classroom discovery-based research, performed during a course-based undergraduate research experience course (CURE), provides a robust laboratory protocol for investigating aestivation and calls into the question the function of this bizarre vertebrate adaptation.

Ascorbic acid biosynthesis in Pacific abalone Haliotis discus hannai Ino and L-gulonolactone oxidase gene loss as an independent event

Up to now, it has been believed that invertebrates are unable to synthesize ascorbic acid (AA) in vivo. However, in the present study, the full-length CDs (Coding sequence) of L-gulonolactone oxidase (GLO) from Pacific abalone (Haliotis discus hannai Ino) were obtained through molecular cloning. The Pacific abalone GLO contained a FAD-binding domain in the N-termination, and ALO domain and conserved HWAK motif in the C-termination. The GLO gene possesses 12 exons and 11 introns. The Pacific abalone GLO was expressed in various tissues, including the kidney, digestive gland, gill, intestine, muscle and mantle. The GLO activity assay revealed that GLO activity was only detected in the kidney of Pacific abalone. After a 100-day feeding trial, dietary AA levels did not significantly affect the survival, weight gain, daily increment in shell length, and feed conversion ratio of Pacific abalone. The expression of GLO in the kidney was downregulated by dietary AA. These results implied that the ability to synthesize AA in abalone had not been lost. From the evolutionary perspective, the loss of GLO occurred independently as an independent event by matching with the genomes of various species. The positive selection analysis revealed that the GLO gene underwent purifying selective pressure during its evolution. In conclusion, the present study provided direct evidence to prove that the GLO activity and the ability to synthesize AA exist in abalone. The AA synthesis ability in vertebrates might have originated from invertebrates dating back 930.31 million years.

Early onset of urea synthesis and ammonia detoxification pathways in three terrestrially developing frogs

Frogs evolved terrestrial development multiple times, necessitating mechanisms to avoid ammonia toxicity at early stages. Urea synthesis from ammonia is a key adaptation that reduces water dependence after metamorphosis. We tested for early expression and plasticity of enzymatic mechanisms of ammonia detoxification in three terrestrial-breeding frogs: foam-nest-dwelling larvae of Leptodactylus fragilis (Lf) and arboreal embryos of Hyalinobatrachium fleischmanni (Hf) and Agalychnis callidryas (Ac). Activity of two ornithine-urea cycle (OUC) enzymes, arginase and CPSase, and levels of their products urea and CP in tissues were high in Lf regardless of nest hydration, but reduced in experimental low- vs. high-ammonia environments. High OUC activity in wet and dry nests, comparable to that under experimental high ammonia, suggests terrestrial Lf larvae maintain high capacity for urea excretion regardless of their immediate risk of ammonia toxicity. This may aid survival through unpredictably long waiting periods before rain enables their transition to water. Moderate levels of urea and CP were present in Hf and Ac tissues and enzymatic activities were lower than in Lf. In both species, embryos in drying clutches can hatch and enter the water early, behaviorally avoiding ammonia toxicity. Moreover, glutamine synthetase was active in early stages of all three species, condensing ammonia and glutamate to glutamine as another mechanism of detoxification. Enzyme activity appeared highest in Lf, although substrate and product levels were higher in Ac and Lf. Our results reveal that multiple biochemical mechanisms of ammonia detoxification occur in early life stages of anuran lineages that evolved terrestrial development.

Aestivation in Nature: Physiological Strategies and Evolutionary Adaptations in Hypometabolic States

Aestivation is considered to be one of the "purest" hypometabolic states in nature, as it involves aerobic dormancy that can be induced and sustained without complex factors. Animals that undergo aestivation to protect themselves from environmental stressors such as high temperatures, droughts, and food shortages. However, this shift in body metabolism presents new challenges for survival, including oxidative stress upon awakening from aestivation, accumulation of toxic metabolites, changes in energy sources, adjustments to immune status, muscle atrophy due to prolonged immobility, and degeneration of internal organs due to prolonged food deprivation. In this review, we summarize the physiological and metabolic strategies, key regulatory factors, and networks utilized by aestivating animals to address the aforementioned components of aestivation. Furthermore, we present a comprehensive overview of the advancements made in aestivation research across major species, including amphibians, fish, reptiles, annelids, mollusks, and echinoderms, categorized according to their respective evolutionary positions. This approach offers a distinct perspective for comparative analysis, facilitating an understanding of the shared traits and unique features of aestivation across different groups of organisms.

Morpho-functional changes of lungfish Protopterus dolloi skin in the shift from freshwater to aestivating conditions

African dipnoi (Protopterus sp.) are obligate air-breathing fish that, during dry season, may experience a period of dormancy named aestivation. Aestivation is characterized by complete reliance on pulmonary breathing, general decrease of metabolism and down-regulation of respiratory and cardiovascular functions. To date, little is known about morpho-functional rearrangements induced by aestivation in the skin of African lungfishes. Our study aims to identify, in the skin of P. dolloi, structural modifications and stress-induced molecules in response to short-term (6 days) and long-term (40 days) aestivation. Light microscopy showed that short-term aestivation induces major reorganization, with narrowing of epidermal layers and decrease of mucous cells; prolonged aestivation is characterized by regenerative processes and re-thickening of epidermal layers. Immunofluorescence reveals that aestivation correlates with an increased oxidative stress and changes of Heat Shock Proteins expression, suggesting a protective role for these chaperons. Our findings revealed that lungfish skin undergoes remarkable morphological and biochemical readjustments in response to stressful conditions associated with aestivation.

Reproductive colonization of land by frogs: Embryos and larvae excrete urea to avoid ammonia toxicity

Vertebrate colonization of land has occurred multiple times, including over 50 origins of terrestrial eggs in frogs. Some environmental factors and phenotypic responses that facilitated these transitions are known, but responses to water constraints and risk of ammonia toxicity during early development are poorly understood. We tested if ammonia accumulation and dehydration risk induce a shift from ammonia to urea excretion during early stages of four anurans, from three origins of terrestrial development. We quantified ammonia and urea concentrations during early development on land, under well‐hydrated and dry conditions. Where we found urea excretion, we tested for a plastic increase under dry conditions and with ammonia accumulation in developmental environments. We assessed the potential adaptive role of urea excretion by comparing ammonia tolerance measured in 96h‐LC₅₀ tests with ammonia levels in developmental environments. Ammonia accumulated in foam nests and perivitelline fluid, increasing over development and reaching higher concentrations under dry conditions. All four species showed high ammonia tolerance, compared to fishes and aquatic‐breeding frogs. Both nest‐dwelling larvae of Leptodactylus fragilis and late embryos of Hyalinobatrachium fleischmanni excreted urea, showing a plastic increase under dry conditions. These two species can develop the longest on land and urea excretion appears adaptive, preventing their exposure to potentially lethal levels of ammonia. Neither late embryos of Agalychnis callidryas nor nest‐dwelling larvae of Engystomops pustulosus experienced toxic ammonia levels under dry conditions, and neither excreted urea. Our results suggest that an early onset of urea excretion, its increase under dry conditions, and elevated ammonia tolerance can all help prevent ammonia toxicity during terrestrial development. High ammonia represents a general risk for development which may be exacerbated as climate change increases dehydration risk for terrestrial‐breeding frogs. It may also be a cue that elicits adaptive physiological responses during early development.

Habitat choice promotes and constrains phenotypic plasticity

Habitat choice can either speed up or slow rates of phenotypic evolution, depending on which trait is measured. We suggest that habitat choice plays an analogous, and generally overlooked, role in shaping patterns of phenotypic plasticity. Using our work with an amphibious fish, we discuss two case studies that demonstrate how habitat choice can both promote and constrain expression of plasticity. First, habitat choice during the dry season accentuates adaptive metabolic plasticity and minimizes maladaptive changes to muscle, ultimately increasing survival time out of water. Second, a trade-off between water- and air-breathing drives matching habitat choice, resulting in positive feedback that reinforces respiratory specialization and environmental preference. Overall, these case studies demonstrate that we must consider the interactions between plasticity and habitat choice to fully understand how animals survive in the face of environmental change. Without considering both processes simultaneously, the performance of animals in challenging conditions can be either under- or over-estimated. Finally, because habitat choice shapes the frequency and predictability of environmental changes that animals experience, feedback between habitat choice and expressions of phenotypic plasticity may be an important factor that influences how plasticity evolves.

Acute hypoxia elevates arginase 2 and induces polyamine stress response in zebrafish via evolutionarily conserved mechanism

Living organisms repeatedly encounter stressful events and apply various strategies to survive. Polyamines are omnipresent bioactive molecules with multiple functions. Their transient synthesis, inducible by numerous stressful stimuli, is termed the polyamine stress response. Animals developed evolutionarily conserved strategies to cope with stresses. The urea cycle is an ancient attribute that deals with ammonia excess in terrestrial species. Remarkably, most fish retain the urea cycle genes fully expressed during the early stages of development and silenced in adult animals. Environmental challenges instigate urea synthesis in fish despite substantial energetic costs, which poses the question of the urea cycle's evolutionary significance. Arginase plays a critical role in oxidative stress-dependent reactions being the final urea cycle enzyme. Its unique subcellular localization, high inducibility, and several regulation levels provide a supreme ability to control the polyamine synthesis rate. Notably, oxidative stress instigates the arginase-1 activity in mammals. Arginase is also dysregulated in aging organisms' brain and muscle tissues, indicating its role in the pathogenesis of age-associated diseases. We designed a study to investigate the levels of the urea cycle and polyamine synthesis-related enzymes in a fish model of acute hypoxia. We evidence synchronized elevation of arginase-2 and ornithine decarboxylase following oxidative stress in adult fish and aging animals signifying the specific function of arginase-2 in fish. Moreover, we demonstrate oxidative stress-associated polyamine synthesis' induction and urea cycle' arrest in adult fish. The subcellular arginase localization found in the fish seems to correspond to its possible evolutionary roles.

Sodium Handling and Interaction in Numerous Organs

Salt (NaCl) is a prerequisite for life. Excessive intake of salt, however, is said to increase disease risk, including hypertension, arteriosclerosis, heart failure, renal disease, stroke, and cancer. Therefore, considerable research has been expended on the mechanism of sodium handling based on the current concepts of sodium balance. The studies have necessarily relied on relatively short-term experiments and focused on extremes of salt intake in humans. Ultra-long-term salt balance has received far less attention. We performed long-term salt balance studies at intakes of 6, 9, and 12 g/day and found that although the kidney remains the long-term excretory gate, tissue and plasma sodium concentrations are not necessarily the same and that urinary salt excretion does not necessarily reflect total-body salt content. We found that to excrete salt, the body makes a great effort to conserve water, resulting in a natriuretic-ureotelic principle of salt excretion. Of note, renal sodium handling is characterized by osmolyte excretion with anti-parallel water reabsorption, a state-of-affairs that is achieved through the interaction of multiple organs. In this review, we discuss novel sodium and water balance concepts in reference to our ultra-long-term study. An important key to understanding body sodium metabolism is to focus on water conservation, a biological principle to protect from dehydration, since excess dietary salt excretion into the urine predisposes to renal water loss because of natriuresis. We believe that our research direction is relevant not only to salt balance but also to cardiovascular regulatory mechanisms.

The Non-ureogenic Stinging Catfish, Heteropneustes fossilis, Actively Excretes Ammonia With the Help of Na+/K+-ATPase When Exposed to Environmental Ammonia

The stinging catfish, Heteropneustes fossilis, can tolerate high concentrations of environmental ammonia. Previously, it was regarded as ureogenic, having a functional ornithine-urea cycle (OUC) that could be up-regulated during ammonia-loading. However, contradictory results indicated that increased urea synthesis and switching to ureotelism could not explain its high ammonia tolerance. Hence, we re-examined the effects of exposure to 30 mmol l^–1 NH₄Cl on its ammonia and urea excretion rates, and its tissue ammonia and urea concentrations. Our results confirmed that H. fossilis did not increase urea excretion or accumulation during 6 days of ammonia exposure, and lacked detectable carbamoyl phosphate synthetase I or III activity in its liver. However, we discovered that it could actively excrete ammonia during exposure to 8 mmol l^–1 NH₄Cl. As active ammonia excretion is known to involve Na⁺/K⁺-ATPase (Nka) indirectly in several ammonia-tolerant fishes, we also cloned various nkaα-subunit isoforms from the gills of H. fossilis, and determined the effects of ammonia exposure on their branchial transcripts levels and protein abundances. Results obtained revealed the presence of five nkaα-subunit isoforms, with nkaα1b having the highest transcript level. Exposure to 30 mmol l^–1 NH₄Cl led to significant increases in the transcript levels of nkaα1b (on day 6) and nkaα1c1 (on day 1 and 3) as compared with the control. In addition, the protein abundances of Nkaα1c1, Nkaα1c2, and total NKAα increased significantly on day 6. Therefore, the high environmental ammonia tolerance of H. fossilis is attributable partly to its ability to actively excrete ammonia with the aid of Nka.

Frogs seek hypoxic microhabitats that accentuate metabolic depression during dormancy

Many animals occupy microhabitats during dormancy where they may encounter hypoxic conditions (e.g. subterranean burrows). We used the green-striped burrowing frog (Cyclorana alboguttata) to test the hypothesis that animals seek hypoxic microhabitats that accentuate metabolic depression during dormancy. We first measured the partial pressure of oxygen (P _O2 ) within artificial cavities excavated in wet clay soil, which simulated C. alboguttata underground aestivation chambers, and recorded hypoxic conditions (P _O2  as low as 8.9 kPa). Using custom-built tunnels that maintained a longitudinal P _O2  gradient (hypoxic to normoxic), we then examined the P _O2  preference of C. alboguttata in response to drying habitat conditions. In support of our hypothesis, we found that C. alboguttata chose to spend a greater proportion of time at the hypoxic end of the P _O2  gradient compared with the normoxic end. To determine whether hypoxia accentuates metabolic depression in C. alboguttata, we exposed frogs to normoxia (21.0 kPa) or hypoxia (10.5 kPa) for 7 weeks during the transition from an active to an aestivating state. We found that hypoxia exposure accelerated the onset of metabolic depression in C. alboguttata by 2 weeks. Furthermore, we found that frogs exposed to hypoxia exhibited a 66% reduction in O₂ consumption after 7 weeks compared with active frogs in normoxia, whereas frogs exposed to normoxia reduced O₂ consumption by only 51%. Overall, our findings indicate that some animals may seek microhabitats to maximally depress metabolic rate during dormancy, and that microhabitat O₂ availability can have significant implications for energy metabolism.

Hypoxia-seeking behavior, metabolic depression and skeletal muscle function in an amphibious fish out of water

Several animals enter a state of dormancy to survive harsh environmental conditions. During dormancy, metabolic depression can be critical for economizing on limited endogenous energy reserves. We used two isogenic strains (strain 1 and strain 2) of a self-fertilizing amphibious fish (Kryptolebias marmoratus) to test the hypothesis that animals seek hypoxic microhabitats that, in turn, accentuate metabolic depression during dormancy. Using custom-built tunnels that maintained a longitudinal O₂ gradient (hypoxic to normoxic), we assessed the O₂ preference of K. marmoratus during prolonged air exposure. In support of our hypothesis, we found that one isogenic strain (strain 2) spent more time in hypoxia compared with normoxia after 21 days in air. Prolonged air exposure in both strains resulted in lower O₂ consumption rates compared with active fish (35% depression), which was accentuated (51% depression) when fish were exposed to aerial hypoxia acutely. We then tested the hypothesis that chronic aerial hypoxia acclimation would protect endogenous energy reserves and skeletal muscle integrity, thereby maintaining locomotor performance, possibly owing to hypoxic hypometabolism. We found that air-acclimated fish from both strains were in poorer body condition relative to fish acclimated to aerial hypoxia. Furthermore, aerial hypoxia acclimation minimized glycogen usage (strain 1), lipid catabolism (strain 2) and white muscle atrophy (strain 2), as well as preserved terrestrial locomotor performance compared with fish in air (strain 2). Overall, our findings suggest that some K. marmoratus strains seek microhabitats that accentuate metabolic depression during dormancy, and that microhabitat O₂ availability may have significant implications for energy metabolism, and the structure and function of skeletal muscle. Furthermore, the differential responses between isogenic strains suggests that genetic factors also contribute to phenotypic differences in the emersion behavior and physiology of this species.

Genetic and functional diversity of the multiple lungfish myoglobins

It is known that the West African lungfish (Protopterus annectens) harbours multiple myoglobin (Mb) genes that are differentially expressed in various tissues and that the Mbs differ in their abilities to confer tolerance towards hypoxia. Here, we show that other lungfish species (Protopterus dolloi, Protopterus aethiopicus and Lepidosiren paradoxa) display a similar diversity of Mb genes and have orthologous Mb genes. To investigate the functional diversification of these genes, we studied the structures, O₂ binding properties and nitrite reductase enzymatic activities of recombinantly expressed P. annectens Mbs (PanMbs). CD spectroscopy and small-angle X-ray scattering revealed the typical globin-fold in all investigated recombinant Mbs, indicating a conserved structure. The highest O₂ affinity was measured for PanMb2 (P₅₀  = 0.88 Torr at 20 °C), which is mainly expressed in the brain, whereas the muscle-specific PanMb1 has the lowest O₂ affinity (P₅₀  = 3.78 Torr at 20 °C), suggesting that tissue-specific O₂ requirements have resulted in the emergence of distinct Mb types. Two of the mainly neuronally expressed Mbs (PanMb3 and PanMb4b) have the highest nitrite reductase rates. These data show different O₂ binding and enzymatic properties of lungfish Mbs, reflecting multiple subfunctionalisation and neofunctionalisation events that occurred early in the evolution of lungfish. Some Mbs may have also taken over the functions of neuroglobin and cytoglobin, which are widely expressed in vertebrates but appear to be missing in lungfish.

The morphological and functional significance of the NOS/NO system in the respiratory, osmoregulatory, and contractile organs of the African lungfish

This review aims to summarize the changes of the NOS/NO system which occur in the lungs, gills, kidney, heart, and myotomal muscle of air breathing fish of the genus Protopterus, i.e. P. dolloi and P. annectens, in relation to the switch from freshwater to aestivation, and vice-versa. The modifications of NOS and its partners Akt and Hsp-90, and HIF-1α, detected by immunohistochemical and molecular biology methods, are discussed together with the apoptosis rate, evaluated by TUNEL. We hypothesize that these molecular components are key elements of the stress-induced signal transduction/integration networks which allow the lungfish to overcome the dramatic environmental challenges experienced at the beginning, during, and at the end of the dry season.

Air-breathing and excretory nitrogen metabolism in fishes

During water-land transition, ancient fishes acquired the ability to breathe air, but air-breathing engendered problems in nitrogenous waste excretion. Nitrogen is a fundamental component of amino acids, proteins, and nucleic acids, and the degradation of these nitrogen-containing compounds releases ammonia. Ammonia is toxic and must be removed. Fishes in water excrete ammonia as the major nitrogenous waste through gills, but gills of air-breathing fishes are modified for air-breathing or largely replaced by air-breathing organs. Notably, fishes emerged from water can no longer excrete ammonia effectively because of a lack of water to flush the gills. Hence, ancient fishes that participated in water-land transition must have developed means to deal with ammonia toxicity. Extant air-breathing fishes, particularly amphibious ones, can serve as models to examine adaptations which might have facilitated the emergence of ancient fishes from water. Some of these fishes can actively emerge from water and display complex behaviors on land, while a few can burrow into mud and survive for years during drought. Many of them are equipped with mechanisms to ameliorate ammonia toxicity during emersion. In this review, the mechanisms adopted by air-breathing fishes to deal with ammonia toxicity during emersion were organized into seven disparate strategies. In addition, eight extant air-breathing fishes with distinctive terrestrial behaviors and peculiar natural habitats were selected to describe in detail how these seven strategies could be adopted in disparate combinations to ameliorate ammonia toxicity during emersion.

Effects of Experimental Terrestrialization on the Skin Mucus Proteome of African Lungfish (Protopterus dolloi)

Animal mucosal barriers constantly interact with the external environment, and this interaction is markedly different in aquatic and terrestrial environments. Transitioning from water to land was a critical step in vertebrate evolution, but the immune adaptations that mucosal barriers such as the skin underwent during that process are essentially unknown. Vertebrate animals such as the African lungfish have a bimodal life, switching from freshwater to terrestrial habitats when environmental conditions are not favorable. African lungfish skin mucus secretions contribute to the terrestrialization process by forming a cocoon that surrounds and protects the lungfish body. The goal of this study was to characterize the skin mucus immunoproteome of African lungfish, Protopterus dolloi, before and during the induction phase of terrestrialization as well as the immunoproteome of the gill mucus during the terrestrialization induction phase. Using LC-MS/MS, we identified a total of 974 proteins using a lungfish Illumina RNA-seq database, 1,256 proteins from previously published lungfish sequence read archive and 880 proteins using a lungfish 454 RNA-seq database for annotation in the three samples analyzed (free-swimming skin mucus, terrestrialized skin mucus, and terrestrialized gill mucus). The terrestrialized skin mucus proteome was enriched in proteins with known antimicrobial functions such as histones and S100 proteins compared to free-swimming skin mucus. In support, gene ontology analyses showed that the terrestrialized skin mucus proteome has predicted functions in processes such as viral process, defense response to Gram-negative bacterium, and tumor necrosis factor-mediated signaling. Importantly, we observed a switch in immunoglobulin heavy chain secretion upon terrestrialization, with IgW1 long form (IgW1L) and IgM1 present in free-swimming skin mucus and IgW1L, IgM1, and IgM2 in terrestrialized skin mucus. Combined, these results indicate an increase in investment in the production of unique immune molecules in P. dolloi skin mucus in response to terrestrialization that likely better protects lungfish against external aggressors found in land.

Metabolism drives distribution and abundance in extremophile fish

Differences in population density between species of varying size are frequently attributed to metabolic rates which are assumed to scale with body size with a slope of 0.75. This assumption is often criticised on the grounds that 0.75 scaling of metabolic rate with body size is not universal and can vary significantly depending on species and life-history. However, few studies have investigated how interspecific variation in metabolic scaling relationships affects population density in different sized species. Here we predict inter-specific differences in metabolism from niche requirements, thereby allowing metabolic predictions of species distribution and abundance at fine spatial scales. Due to the differences in energetic efficiency required along harsh-benign gradients, an extremophile fish (brown mudfish, Neochanna apoda) living in harsh environments had slower metabolism, and thus higher population densities, compared to a fish species (banded kōkopu, Galaxias fasciatus) in physiologically more benign habitats. Interspecific differences in the intercepts for the relationship between body and density disappeared when species mass-specific metabolic rates, rather than body sizes, were used to predict density, implying population energy use was equivalent between mudfish and kōkopu. Nevertheless, despite significant interspecific differences in the slope of the metabolic scaling relationships, mudfish and kōkopu had a common slope for the relationship between body size and population density. These results support underlying logic of energetic equivalence between different size species implicit in metabolic theory. However, the precise slope of metabolic scaling relationships, which is the subject of much debate, may not be a reliable indicator of population density as expected under metabolic theory.

Molecular characterization of three Rhesus glycoproteins from the gills of the African lungfish, Protopterus annectens, and effects of aestivation on their mRNA expression levels and protein abundance

African lungfishes are ammonotelic in water. They can aestivate for long periods on land during drought. During aestivation, the gills are covered with dried mucus and ammonia excretion ceases. In fishes, ammonia excretion through the gills involves Rhesus glycoproteins (RhGP/Rhgp). This study aimed to obtain the complete cDNA coding sequences of rhgp from the gills of Protopterus annectens, and to determine their branchial mRNA and protein expression levels during the induction, maintenance and arousal phases of aestivation. Three isoforms of rhgp (rhag, rhbg and rhcg) were obtained in the gills of P. annectens. Their complete cDNA coding sequences ranged between 1311 and 1398 bp, coding for 436 to 465 amino acids with estimated molecular masses between 46.8 and 50.9 kDa. Dendrogramic analyses indicated that Rhag was grouped closer to fishes, while Rhbg and Rhcg were grouped closer to tetrapods. During the induction phase, the protein abundance of Rhag, but not its transcript level, was down-regulated in the gills, suggesting that there could be a decrease in the release of ammonia from the erythrocytes to the plasma. Furthermore, the branchial transcript levels of rhbg and rhcg decreased significantly, in preparation for the subsequent shutdown of gill functions. During the maintenance phase, the branchial expression levels of rhag/Rhag, rhbg/Rhbg and rhcg/Rhcg decreased significantly, indicating that their transcription and translation were down-regulated. This could be part of an overall mechanism to shut down branchial functions and save metabolic energy used for transcription and translation. It could also be regarded as an adaptive response to stop ammonia excretion. During the arousal phase, it is essential for the lungfish to regain the ability to excrete ammonia. Indeed, the protein abundance of Rhag, Rhbg and Rhcg recovered to the corresponding control levels after 1 day or 3 days of recovery from 6 months of aestivation.

Amphibious fishes: evolution and phenotypic plasticity

Amphibious fishes spend part of their life in terrestrial habitats. The ability to tolerate life on land has evolved independently many times, with more than 200 extant species of amphibious fishes spanning 17 orders now reported. Many adaptations for life out of water have been described in the literature, and adaptive phenotypic plasticity may play an equally important role in promoting favourable matches between the terrestrial habitat and behavioural, physiological, biochemical and morphological characteristics. Amphibious fishes living at the interface of two very different environments must respond to issues relating to buoyancy/gravity, hydration/desiccation, low/high O2 availability, low/high CO2 accumulation and high/low NH3 solubility each time they traverse the air-water interface. Here, we review the literature for examples of plastic traits associated with the response to each of these challenges. Because there is evidence that phenotypic plasticity can facilitate the evolution of fixed traits in general, we summarize the types of investigations needed to more fully determine whether plasticity in extant amphibious fishes can provide indications of the strategies used during the evolution of terrestriality in tetrapods.

Changes of ammonia, urea contents and transaminase activity in the body during aerial exposure and ammonia loading in Chinese loach Paramisgurnus dabryanus

The Paramisgurnus dabryanus was exposed to 30 mmol L^-1 NH₄Cl solution and air to assessing the change of body ammonia and urea contents and the activities of alanine aminotransferase (ALT) and aspartate transaminase (AST). After 48 h of ammonia exposure, ammonia concentration in the plasma, brain, liver and muscle were 3.3-fold, 5.6-fold, 3.5-fold and 4.2-fold, respectively, those of the control values. Plasma, brain, liver and muscle ammonia concentrations increased to 2.2-fold, 3.3-fold, 2.5-fold and 2.9-fold, respectively, those of control values in response to 48 h of aerial exposure. Within the given treatment (ammonia or aerial exposure), there was no change in plasma, brain and liver urea concentrations between exposure durations. The plasma ALT activity was significantly affected by exposure time during aerial exposure, while the liver ALT activity was not affected by ammonia or aerial exposure. Exposure to NH₄Cl or air had no effect on either plasma or liver AST activity. Our results suggested that P. dabryanus could accumulate quite high level of internal ammonia because of the high ammonia tolerance in its cells and tissues, and NH₃ volatilization would be a possible ammonia detoxification strategy in P. dabryanus. Urea synthesis was not an effective mechanism to deal with environmental or internal ammonia problem. The significant increase of ALT activity in plasma during aerial exposure, indicating that alanine synthesis through certain amino acid catabolism may be subsistent in P. dabryanus.

Aestivation Induces Changes in the mRNA Expression Levels and Protein Abundance of Two Isoforms of Urea Transporters in the Gills of the African Lungfish, Protopterus annectens

The African lungfish, Protopterus annectens, is ammonotelic in water despite being ureogenic. When it aestivates in mucus cocoon on land, ammonia is detoxified to urea. During the maintenance phase of aestivation, urea accumulates in the body, which is subsequently excreted upon arousal. Urea excretion involves urea transporters (UT/Ut). This study aimed to clone and sequence the ut isoforms from the gills of P. annectens, and to test the hypothesis that the mRNA and/or protein expression levels of ut/Ut isoforms could vary in the gills of P. annectens during the induction, maintenance, and arousal phases of aestivation. Two isoforms of ut, ut-a2a and ut-a2b, were obtained from the gills of P. annectens. ut-a2a consisted of 1227 bp and coded for 408 amino acids with an estimated molecular mass of 44.7 kDa, while ut-a2b consisted of 1392 bp and coded for 464 amino acids with an estimated molecular mass of 51.2 kDa. Ut-a2a and Ut-a2b of P. annectens had a closer phylogenetic relationship with Ut/UT of tetrapods than Ut of fishes. While the mRNA expression pattern of ut-a2a and ut-a2b across various tissues of P. annectens differed, the transcript levels of ut-a2a and ut-a2b in the gills were comparable, indicating that they might be equally important for branchial urea excretion during the initial arousal phase of aestivation. During the maintenance phase of aestivation, the transcript level of ut-a2a increased significantly, but the protein abundance of Ut-a2a remained unchanged in the gills of P. annectens. This could be an adaptive feature to prepare for an increase in the production of Ut-a2a upon arousal. Indeed, arousal led to a significant increase in the branchial Ut-a2a protein abundance. Although the transcript level of ut-a2b remained unchanged, there were significant increases in the protein abundance of Ut-a2b in the gills of P. annectens throughout the three phases of aestivation. The increase in the protein abundance of Ut-a2b during the maintenance phase could also be an adaptive feature to prepare for efficient urea excretion when water becomes available.

Tolerance of hyperammonemia in brain of Heteropneustes fossilis is supported by glutamate-glutamine cycle

This report presents analysis of molecular switches associated with tolerance to hyperammonemia in Heteropneustes fossilis because it tolerates about 100-fold more ammonia than mammals. Brains of Heteropneustes fossilis exposed to 100mM ammonium chloride were dissected after Zero hour as control, 16h and 20h exposure. The status of neuron and glia were analysed by Golgi staining, Luxol Fast Blue, and Nissl's staining. The expression patterns of genes associated to homeostasis of neuron and glia, management of oxidative stress and inflammation, ammonia metabolism and brain derived neurotrophic factor were analysed through reverse-transcriptase-polymerase chain reaction. After 20h of hyperammonemia glia were more degenerated than neurons. The expression of mRNA of lactate dehydrogenase (Ldh), super oxide dismutase (Sod), catalase (Catalase), arginase-I (Arg I), inducible nitric oxide (iNos), glutaminase (GA), and brain derived neurotrophic factor (Bdnf) was up-regulated than the control. The levels of mRNA of Arg II, glutamate dehydrogenase (Gdh), glutamine synthetase (GS), glial fibrillary acidic protein (Gfap), proliferating cell nuclear antigen (Pcna) and S100β were down-regulated than control due to hyperammonemia. It appears first observation on impact of hyperammonemia on the status of neurons, myelination and glial cells in brain of Heteropneustes fossilis by Golgi staining, Nissl's and Luxol Fast Blue staining. The distribution of ammonia and glutamate metabolising enzymes in brain supports multi-centric mechanism (s) of regulation. The expression of Arg I and Arg II gets inversely regulated and glutamate-glutamine cycle also operates in Heteropneustes fossilis against hyperammonemia in brain.

Molecular Characterization of Aquaporin 1 and Aquaporin 3 from the Gills of the African Lungfish, Protopterus annectens, and Changes in Their Branchial mRNA Expression Levels and Protein Abundance during Three Phases of Aestivation

African lungfishes can undergo long periods of aestivation on land during drought. During aestivation, lungfishes are confronted with desiccation and dehydration, and their gills become non-functional and covered with a thick layer of dried mucus. Aquaporins (Aqps) are a superfamily of integral membrane proteins which generally facilitate the permeation of water through plasma membranes. This study aimed to obtain the complete cDNA coding sequences of aqp1 and aqp3 from the gills of Protopterus annectens, and to determine their branchial mRNA and protein expression levels during the induction, maintenance and arousal phases of aestivation. Dendrogramic analyses of the deduced Aqp1 and Aqp3 amino acid sequences of P. annectens revealed their close relationships with those of Latimeria chalumnae and tetrapods. During the induction phase, there were significant decreases in the transcript levels of aqp1 and aqp3 in the gills of P. annectens, but the branchial Aqp1 and Aqp3 protein abundance remained unchanged. As changes in transcription might precede changes in translation, this could be regarded as an adaptive response to decrease the protein abundance of Aqp1 and Aqp3 in the subsequent maintenance phase of aestivation. As expected, the branchial transcript levels and protein abundance of aqp1/Aqp1 and aqp3/Aqp3 were significantly down-regulated during the maintenance phase, probably attributable to the shutdown of branchial functions and the cessation of volume regulation of branchial epithelial cells. Additionally, these changes could reduce the loss of water through branchial epithelial surfaces, supplementing the anti-desiccating property of the dried mucus. Upon arousal, it was essential for the lungfish to restore branchial functions. Indeed, the protein abundance of Aqp1 recovered partially, with complete recovery of mRNA expression level and protein abundance of Aqp3, in the gills of P. annectens after 3 days of arousal. These results provide insights into how P. annectens regulates branchial Aqp expression to cope with desiccation and rehydration during different phases of aestivation.

Differential peptide expression in the central nervous system of the land snail Theba pisana, between active and aestivated

Hypometabolism is a physiological state of dormancy entered by many animals in times of environmental stress. There are gaps in our understanding of the molecular components used by animals to achieve this metabolic state. The availability of genomic and transcriptome data can be useful to study the process of hypometabolism at the molecular level. In this study, we use the land snail Theba pisana to identify peptides that may be involved in the hypometabolic state known as aestivation. We found a total of 22 neuropeptides in the central nervous system (CNS) that were differentially produced during activity and aestivation based on mass spectral-based neuropeptidome analysis. Of these, 4 were upregulated in active animals and 18 were upregulated in aestivation. A neuropeptide known to regulate muscle contractions in a variety of molluscs, the small cardioactive peptide A (sCAPA), and a peptide of yet unknown function (termed Aestivation Associated Peptide 12) were chosen for further investigation using temporal and spatial expression analysis of the precursor gene and peptide. Both peptides share expression within regions of the CNS cerebral ganglia and suboesophageal ganglia. Relative transcript abundance suggests that regulation of peptide synthesis and secretion is post-transcriptional. In summary, we provide new insights into the molecular basis of the regulation of aestivation in land snails through CNS peptide control.

Integrative Physiology of Fasting

Extended bouts of fasting are ingrained in the ecology of many organisms, characterizing aspects of reproduction, development, hibernation, estivation, migration, and infrequent feeding habits. The challenge of long fasting episodes is the need to maintain physiological homeostasis while relying solely on endogenous resources. To meet that challenge, animals utilize an integrated repertoire of behavioral, physiological, and biochemical responses that reduce metabolic rates, maintain tissue structure and function, and thus enhance survival. We have synthesized in this review the integrative physiological, morphological, and biochemical responses, and their stages, that characterize natural fasting bouts. Underlying the capacity to survive extended fasts are behaviors and mechanisms that reduce metabolic expenditure and shift the dependency to lipid utilization. Hormonal regulation and immune capacity are altered by fasting; hormones that trigger digestion, elevate metabolism, and support immune performance become depressed, whereas hormones that enhance the utilization of endogenous substrates are elevated. The negative energy budget that accompanies fasting leads to the loss of body mass as fat stores are depleted and tissues undergo atrophy (i.e., loss of mass). Absolute rates of body mass loss scale allometrically among vertebrates. Tissues and organs vary in the degree of atrophy and downregulation of function, depending on the degree to which they are used during the fast. Fasting affects the population dynamics and activities of the gut microbiota, an interplay that impacts the host's fasting biology. Fasting-induced gene expression programs underlie the broad spectrum of integrated physiological mechanisms responsible for an animal's ability to survive long episodes of natural fasting.

Differential gene expression in the liver of the African lungfish, Protopterus annectens, after 6 months of aestivation in air or 1 day of arousal from 6 months of aestivation

The African lungfish, Protopterus annectens, can undergo aestivation during drought. Aestivation has three phases: induction, maintenance and arousal. The objective of this study was to examine the differential gene expression in the liver of P. annectens after 6 months (the maintenance phase) of aestivation as compared with the freshwater control, or after 1 day of arousal from 6 months aestivation as compared with 6 months of aestivation using suppression subtractive hybridization. During the maintenance phase of aestivation, the mRNA expression of argininosuccinate synthetase 1 and carbamoyl phosphate synthetase III were up-regulated, indicating an increase in the ornithine-urea cycle capacity to detoxify ammonia to urea. There was also an increase in the expression of betaine homocysteine-S-transferase 1 which could reduce and prevent the accumulation of hepatic homocysteine. On the other hand, the down-regulation of superoxide dismutase 1 expression could signify a decrease in ROS production during the maintenance phase of aestivation. In addition, the maintenance phase was marked by decreases in expressions of genes related to blood coagulation, complement fixation and iron and copper metabolism, which could be strategies used to prevent thrombosis and to conserve energy. Unlike the maintenance phase of aestivation, there were increases in expressions of genes related to nitrogen, carbohydrate and lipid metabolism and fatty acid transport after 1 day of arousal from 6 months aestivation. There were also up-regulation in expressions of genes that were involved in the electron transport system and ATP synthesis, indicating a greater demand for metabolic energy during arousal. Overall, our results signify the importance of sustaining a low rate of waste production and conservation of energy store during the maintenance phase, and the dependence on internal energy store for repair and structural modification during the arousal phase, of aestivation in the liver of P. annectens.

Fluorescence microscopy visualization of halomucin, a secreted 927 kDa protein surrounding Haloquadratum walsbyi cells

At the time of its first publication, halomucin from Haloquadratum walsbyi strain HBSQ001 was the largest archaeal protein known (9159 aa). It has a predicted signal sequence, making it likely to be an extracellular or secreted protein. Best BLAST matches were found to be mammalian mucins that protect tissues to dehydration and chemical stress. It was hypothesized that halomucin participates in protection against desiccation by retaining water in a hull around the halophilic organisms that live at the limits of water activity. We visualized Haloquadratum cells by staining their intracellular polyhydroxybutyrate granules using Nile Blue. Halomucin was stained by immunofluorescence with antibodies generated against synthetic peptides derived from the halomucin amino acid sequence. Polyhydroxybutyrate stained cells were reconstructed in 3D which highlights not only the highly regular square shape but also the extreme flatness of Haloquadratum. Double-staining proves halomucin to be extracellular but to be only loosely associated to cells in agreement with its hypothesized function.

Molecular characterization of betaine-homocysteine methyltransferase 1 from the liver, and effects of aestivation on its expressions and homocysteine concentrations in the liver, kidney and muscle, of the African lungfish, Protopterus annectens

Homocysteine accumulation has numerous deleterious effects, and betaine-homocysteine S-methyltransferase (BHMT) catalyses the synthesis of methionine from homocysteine and betaine. This study aimed to determine homocysteine concentrations, and mRNA expression levels and protein abundances of bhmt1/Bhmt1 in the liver, kidney and muscle of the African lungfish, Protopterus annectens, during the induction (6 days), maintenance (6 months) or arousal (3 days after arousal) phase of aestivation. The homocysteine concentration decreased significantly in the liver of P. annectens after 6 days or 6 months of aestivation, but it returned to the control level upon arousal. By contrast, homocysteine concentrations in the kidney and muscle remained unchanged during the three phases of aestivation. The complete coding cDNA sequence of bhmt1 from P. annectens consisted of 1236 bp, coding for 412 amino acids. The Bhmt1 from P. annectens had a close phylogenetic relationship with those from tetrapods and Callorhinchus milii. The expression of bhmt1 was detected in multiple organs/tissues of P. annectens, and this is the first report on the expression of bhmt1/Bhmt1 in animal skeletal muscle. The mRNA and protein expression levels of bhmt1/Bhmt1 were up-regulated in the liver of P. annectens during the induction and maintenance phases of aestivation, possibly to regulate the hepatic homocysteine concentration. The significant increase in hepatic Bhmt1 protein abundance during the arousal phase could be a response to increased cellular methylation for the purpose of tissue reconstruction. Unlike the liver, Bhmt1 expression in the kidney and muscle of P. annectens was regulated translationally, and its up-regulation could be crucial to prevent homocysteine accumulation.

Aestivation induces changes in transcription and translation of coagulation factor II and fibrinogen gamma chain in the liver of the African lungfish, Protopterus annectens

This study aimed to sequence and characterize two pro-coagulant genes, coagulation factor II (f2) and fibrinogen gamma chain (fgg), from the liver of the African lungfish Protopterus annectens, and to determine their hepatic mRNA expression levels during three phases of aestivation. The protein abundances of F2 and Fgg in the liver and plasma were determined by immunoblotting. Results indicated that F2 and Fgg of P. annectens were phylogenetically closer to those of amphibians than those of teleosts. Three days of aestivation resulted in an up-regulation in the hepatic fgg mRNA expression level, while 6 days of aestivation led to a significant increase (3-fold) in the protein abundance of Fgg in the plasma. Hence, there could be an increase in the blood clotting ability in P. annectens during the induction phase of aestivation. By contrast, the blood clotting ability in P. annectens might be reduced in response to decreased blood flow and increased possibility of thrombosis during the maintenance phase of aestivation, as 6 months of aestivation led to significant decreases in mRNA expression levels of f2 and fgg in the liver. There could also be a decrease in the export of F2 and Fgg from the liver to the plasma so as to avert thrombosis. Upon 3-6 days of arousal from 6 months of aestivation, the protein abundances of F2 and Fgg recovered partially in the plasma of P. annectens, and a complete recovery of the transcription and translation of f2/F2 in the liver might occur only after refeeding.

Cloning and expression of the epithelial sodium channel and its role in osmoregulation of aquatic and estivating African lungfish Protopterus annectens

The epithelial sodium channel (ENaC) is a sodium (Na(+))-selective aldosterone-stimulated ion channel involved in Na(+) transport homeostasis of tetrapods. We examined full-length cDNA sequences and tissue distributions of ENaCα, ENaCβ, and ENaCγ subunits in the African lungfish Protopterus annectens. Protopterus ENaC (pENaC) comprises 3 subunits: pENaCα, pENaCβ, and pENaCγ. pENaCα, pENaCβ, and pENaCγ subunits are closely related to α, β, and γ subunits of the Australian lungfish Neoceratodus forsteri ENaC (nENaC), respectively. Three ENaC subunit mRNAs were highly expressed in the gills and moderately expressed in the kidney and rectum of P. annectens. During estivation for 2-4weeks and 2-3months, plasma Na(+) concentration was relatively stable, but plasma urea concentration significantly increased in comparison with the control fish kept in a freshwater environment. Plasma aldosterone concentration and mRNA expression of the ENaCα subunit gradually and significantly decreased in the gills and kidney after 2months of estivation. Thus, aldosterone-dependent Na(+) absorption via ENaC probably exists in the epithelial cells of osmoregulatory organs of lungfish kept in fresh water, whereas plasma Na(+) concentration may be maintained by a mechanism independent of aldosterone-ENaC axis during estivation in lungfish.

Molecular characterization of argininosuccinate synthase and argininosuccinate lyase from the liver of the African lungfish Protopterus annectens, and their mRNA expression levels in the liver, kidney, brain and skeletal muscle during aestivation

Argininosuccinate synthase (Ass) and argininosuccinate lyase (Asl) are involved in arginine synthesis for various purposes. The complete cDNA coding sequences of ass and asl from the liver of Protopterus annectens consisted of 1,296 and 1,398 bp, respectively. Phylogenetic analyses revealed that the deduced Ass and Asl of P. annectens had close relationship with that of the cartilaginous fish Callorhinchus milii. Besides being strongly expressed in the liver, ass and asl expression were detectable in many tissues/organs. In the liver, mRNA expression levels of ass and asl increased significantly during the induction phase of aestivation, probably to increase arginine production to support increased urea synthesis. The increases in ass and asl mRNA expression levels during the prolonged maintenance phase and early arousal phase of aestivation could reflect increased demand on arginine for nitric oxide (NO) production in the liver. In the kidney, there was a significant decrease in ass mRNA expression level after 6 months of aestivation, indicating possible decreases in the synthesis and supply of arginine to other tissues/organs. In the brain, changes in ass and asl mRNA expression levels during the three phases of aestivation could be related to the supply of arginine for NO synthesis in response to conditions that resemble ischaemia and ischaemia-reperfusion during the maintenance and arousal phase of aestivation, respectively. The decrease in ass mRNA expression level, accompanied with decreases in the concentrations of arginine and NO, in the skeletal muscle of aestivating P. annectens might ameliorate the potential of disuse muscle atrophy.

Aestivation and brain of the African lungfish Protopterus annectens

Scientists have long been fascinated by animals undergoing aestivation, a state of torpor at high temperature, due to its great potential in fields ranging from medicine to space travel. The brain of the African lungfish is able to coordinate a whole-body response to induce aestivation and to arouse from aestivation.

Brain Na+/K+-ATPase α-subunit isoforms and aestivation in the African lungfish, Protopterus annectens

This study aimed to clone and sequence Na (+) / K (+)-ATPase (nka) α-subunit isoforms from, and to determine their mRNA expression levels and protein abundance in the brain of the African lungfish, Protopterus annectens during the induction, maintenance and arousal phases of aestivation in air. We obtained the full cDNA sequences of nkaα1, nkaα2 and nkaα3 from the brain of P. annectens. Phylogenetic analysis of their deduced amino acid sequences revealed that they are closer to the corresponding NKA α-subunits of tetrapods than to those of fishes. The mRNA expression of these three nkaα isoforms showed differential changes in the brain of P. annectens during the three phases of aestivation. After 12 days of aestivation, there was a significant increase in the protein abundance of Nkaα1 in the brain of P. annectens. This could be an important response to maintain cellular Na(+) and K(+) concentrations and regulate cell volume during the early maintenance phase of aestivation. On the other hand, the mRNA expression of nkaα2 decreased significantly in the brain of P. annectens after 6 months of aestivation, which could be a result of a suppression of transcriptional activities to reduce energy expenditure. The down-regulation of mRNA expression of nkaα1, nkaα2 and nkaα3 and the overall protein abundance of Nka α-subunit isoforms in the brain of P. annectens after 1 day of arousal from 6 months of aestivation were novel observations, and it could be an adaptive response to restore blood pressure and/or to prevent brain oedema.

Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes

With the development of air-breathing capabilities, some fishes can emerge from water, make excursions onto land or even burrow into mud during droughts. Air-breathing fishes have modified gill morphology and morphometry and accessory breathing organs, which would tend to reduce branchial ammonia excretion. As ammonia is toxic, air-breathing fishes, especially amphibious ones, are equipped with various strategies to ameliorate ammonia toxicity during emersion or ammonia exposure. These strategies can be categorized into (1) enhancement of ammonia excretion and reduction of ammonia entry, (2) conversion of ammonia to a less toxic product for accumulation and subsequent excretion, (3) reduction of ammonia production and avoidance of ammonia accumulation and (4) tolerance of ammonia at cellular and tissue levels. Active ammonia excretion, operating in conjunction with lowering of ambient pH and reduction in branchial and cutaneous NH₃ permeability, is theoretically the most effective strategy to maintain low internal ammonia concentrations. NH₃ volatilization involves the alkalization of certain epithelial surfaces and requires mechanisms to prevent NH₃ back flux. Urea synthesis is an energy-intensive process and hence uncommon among air-breathing teleosts. Aestivating African lungfishes detoxify ammonia to urea and the accumulated urea is excreted following arousal. Reduction in ammonia production is achieved in some air-breathing fishes through suppression of amino acid catabolism and proteolysis, or through partial amino acid catabolism leading to alanine formation. Others can slow down ammonia accumulation through increased glutamine synthesis in the liver and muscle. Yet, some others develop high tolerance of ammonia at cellular and tissue levels, including tissues in the brain. In summary, the responses of air-breathing fishes to ameliorate ammonia toxicity are many and varied, determined by the behaviour of the species and the nature of the environment in which it lives.

The importance of cutaneous gas exchange during aerial and aquatic respiration in galaxiids

The Canterbury mudfish Neochanna burrowsius was found to be a pseudo-aestivating galaxiid with a low metabolic rate and significant cutaneous oxygen uptake (c. 43%) in both air and water. Another galaxiid, inanga Galaxias maculatus, had a higher metabolic rate in both media but the proportion of oxygen uptake met by cutaneous respiration rose significantly from 38 to 63% when the fish were exposed to air. Besides its important role in oxygen uptake, the skin of both species also contributed significantly to excretion of carbon dioxide in air, indicating the critical role of the integument as a respiratory tissue. In air, G. maculatus may increase cutaneous gas exchange to meet metabolic demands owing to the reduced utility of the gills, but as emersed G. maculatus were only able to maintain metabolic rates at c. 67% of that measured in water, this strategy probably only permits short-term survival. By contrast, the low and unchanging metabolic rate in water and air in N. burrowsius is a feature that may facilitate tolerance of long periods of emersion in the desiccating environments they inhabit.

Expression pattern and biochemical properties of zebrafish N-acetylglutamate synthase

The urea cycle converts ammonia, a waste product of protein catabolism, into urea. Because fish dispose ammonia directly into water, the role of the urea cycle in fish remains unknown. Six enzymes, N-acetylglutamate synthase (NAGS), carbamylphosphate synthetase III, ornithine transcarbamylase, argininosuccinate synthase, argininosuccinate lyase and arginase 1, and two membrane transporters, ornithine transporter and aralar, comprise the urea cycle. The genes for all six enzymes and both transporters are present in the zebrafish genome. NAGS (EC 2.3.1.1) catalyzes the formation of N-acetylglutamate from glutamate and acetyl coenzyme A and in zebrafish is partially inhibited by L-arginine. NAGS and other urea cycle genes are highly expressed during the first four days of zebrafish development. Sequence alignment of NAGS proteins from six fish species revealed three regions of sequence conservation: the mitochondrial targeting signal (MTS) at the N-terminus, followed by the variable and conserved segments. Removal of the MTS yields mature zebrafish NAGS (zfNAGS-M) while removal of the variable segment from zfNAGS-M results in conserved NAGS (zfNAGS-C). Both zfNAGS-M and zfNAGS-C are tetramers in the absence of L-arginine; addition of L-arginine decreased partition coefficients of both proteins. The zfNAGS-C unfolds over a broader temperature range and has higher specific activity than zfNAGS-M. In the presence of L-arginine the apparent Vmax of zfNAGS-M and zfNAGS-C decreased, their Km(app) for acetyl coenzyme A increased while the Km(app) for glutamate remained unchanged. The expression pattern of NAGS and other urea cycle genes in developing zebrafish suggests that they may have a role in citrulline and/or arginine biosynthesis during the first day of development and in ammonia detoxification thereafter. Biophysical and biochemical properties of zebrafish NAGS suggest that the variable segment may stabilize a tetrameric state of zfNAGS-M and that under physiological conditions zebrafish NAGS catalyzes formation of N-acetylglutamate at the maximal rate.

Lympho-granulocytic tissue associated with the wall of the spiral valve in the African lungfish Protopterus annectens

We describe the structure of the lympho-granulocytic tissue associated with the wall of the spiral valve of the African lungfish Protopterus annectens. The study was performed under freshwater conditions and after 6 months of aestivation. The lympho-granulocytic tissue consists of nodes surrounded by reticular tissue. The nodes are formed by an outer and an inner component separated by a thin collagenous layer. The outer component is a reticular-like tissue that contains two types of granulocytes, developing and mature plasma cells and melanomacrophage centres (MMCs). The inner component, the parenchyma, contains a meshwork of trabeculae and vascular sinusoids and shows dark and pale areas. The dark areas contain diffuse lymphoid tissue, with a large number of mitoses and plasma cell clusters. The pale areas contain a small number of macrophages and lymphocytes. Macrophages and sinus endothelial cells are filled with haemosiderin granules and appear to form part of the reticuloendothelial system of the lungfish. The reticular tissue houses granulocytes, plasma cells and MMCs and might serve for the housing and maturation of cells of the white series. After aestivation, the nodes undergo lymphocyte depletion, the suppression of mitosis, granulocyte invasion and the occurrence of cell death. By contrast, few histological changes occur in the reticular tissue. Whereas the nodes appear to be involved in lymphocyte proliferation and plasma cell maturation, the function of the reticular tissue remains obscure.

Differential gene expression in the brain of the African lungfish, Protopterus annectens, after six days or six months of aestivation in air

The African lungfish, Protopterus annectens, can undergo aestivation during drought. Aestivation has three phases: induction, maintenance and arousal. The objective of this study was to examine the differential gene expression in the brain of P. annectens during the induction (6 days) and maintenance (6 months) phases of aestivation as compared with the freshwater control using suppression subtractive hybridization. During the induction phase of aestivation, the mRNA expression of prolactin (prl) and growth hormone were up-regulated in the brain of P. annectens, which indicate for the first time the possible induction role of these two hormones in aestivation. Also, the up-regulation of mRNA expression of tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein γ polypeptide and the down-regulation of phosphatidylethanolamine binding protein, suggest that there could be a reduction in biological and neuronal activities in the brain. The mRNA expression of cold inducible RNA-binding protein and glucose regulated protein 58 were also up-regulated in the brain, probably to enhance their cytoprotective effects. Furthermore, the down-regulation of prothymosin α expression suggests that there could be a suppression of transcription and cell proliferation in preparation for the maintenance phase. In general, the induction phase appeared to be characterized by reduction in glycolytic capacity and metabolic activity, suppression of protein synthesis and degradation, and an increase in defense against ammonia toxicity. In contrast, there was a down-regulation in the mRNA expression of prl in the brain of P. annectens during the maintenance phase of aestivation. In addition, there could be an increase in oxidative defense capacity, and up-regulation of transcription, translation, and glycolytic capacities in preparation for arousal. Overall, our results signify the importance of reconstruction of protein structures and regulation of energy expenditure during the induction phase, and the needs to suppress protein degradation and conserve metabolic fuel stores during the maintenance phase of aestivation.

Nitric oxide synthase-dependent "on/off" switch and apoptosis in freshwater and aestivating lungfish, Protopterus annectens: skeletal muscle versus cardiac muscle

African lungfishes (Protopterus spp.) are obligate air breathers which enter in a prolonged torpor (aestivation) in association with metabolic depression, and biochemical and morpho-functional readjustments during the dry season. During aestivation, the lungfish heart continues to pump, while the skeletal muscle stops to function but can immediately contract during arousal. Currently, nothing is known regarding the orchestration of the multilevel rearrangements occurring in myotomal and myocardial muscles during aestivation and arousal. Because of its universal role in cardio-circulatory and muscle homeostasis, nitric oxide (NO) could be involved in coordinating these stress-induced adaptations. Western blotting and immunofluorescence microscopy on cardiac and skeletal muscles of Protopterus annectens (freshwater, 6months of aestivation and 6days after arousal) showed that expression, localization and activity of the endothelial-like nitric oxide synthase (eNOS) isoform and its partners Akt and Hsp-90 are tissue-specifically modulated. During aestivation, phospho-eNOS/eNOS and phospho-Akt/Akt ratios increased in the heart but decreased in the skeletal muscle. By contrast, Hsp-90 increased in both muscle types during aestivation. TUNEL assay revealed that increased apoptosis occurred in the skeletal muscle of aestivating lungfish, but the myocardial apoptotic rate of the aestivating lungfish remained unchanged as compared with the freshwater control. Consistent with the preserved cardiac activity during aestivation, the expression of apoptosis repressor (ARC) also remained unchanged in the heart of aestivating and aroused fish as compared with the freshwater control. Contrarily, ARC expression was strongly reduced in the skeletal muscle of aestivating lungfish. On the whole, our data indicate that changes in the eNOS/NO system and cell turnover are implicated in the morpho-functional readjustments occurring in lungfish cardiac and skeletal muscle during the switch from freshwater to aestivation, and between the maintenance and arousal phases of aestivation.

The Chinese soft-shelled turtle, Pelodiscus sinensis, decreases nitrogenous excretion, reduces urea synthesis and suppresses ammonia production during emersion

The objective of this study was to examine the effects of 6 days of emersion on nitrogen metabolism and excretion in the Chinese soft-shelled turtle, Pelodiscus sinensis. Despite having a soft shell with a cutaneous surface that is known to be water permeable, P. sinensis lost only ~2% of body mass and was able to maintain its hematocrit and plasma osmolality, [Na(+)] and [Cl(-)] during 6 days of emersion. During emersion, it ameliorated water loss by reducing urine output, which led to a reduction (by 29-76%) in ammonia excretion. In comparison, there was a more prominent reduction (by 82-99%) in urea excretion during emersion due to a lack of water to flush the buccopharyngeal epithelium, which is known to be the major route of urea excretion. Consequently, emersion resulted in an apparent shift from ureotely to ammonotely in P. sinensis. Although urea concentration increased in several tissues, the excess urea accumulated could only account for 13-22% of the deficit in urea excretion. Hence, it can be concluded that a decrease (~80%) in urea synthesis occurred in P. sinensis during the 6 days of emersion. Indeed, emersion led to significant decreases in the activity of some ornithine-urea cycle enzymes (argininosuccinate synthetase/argininosuccinate lyase and arginase) from the liver of P. sinensis. As a decrease in urea synthesis occurred without the accumulation of ammonia and total free amino acids, it can be deduced that ammonia production through amino acid catabolism was suppressed with a proportional reduction in proteolysis in P. sinensis during emersion. Indeed, calculated results revealed that there could be a prominent decrease (~88%) in ammonia production in turtles after 6 days of emersion. In summary, despite being ureogenic and ureotelic in water, P. sinensis adopted a reduction in ammonia production, instead of increased urea synthesis, as the major strategy to ameliorate ammonia toxicity and problems associated with dehydration during terrestrial exposure.

Hepatic carbamoyl phosphate synthetase (CPS) I and urea contents in the hylid tree frog, Litoria caerulea: transition from CPS III to CPS I

The complete cDNA sequence of CPS I obtained from the liver of the hylid tree frog, Litoria caerulea, consisted of 4,485 bp which coded for 1,495 amino acids with an estimated molecular mass of 163.7 kDa. The deduced CPS I consisted of a mitochondrial targeting sequence of 33 amino acid residues, a glutaminase amidotransferase component spanning from tyrosine 95 to leucine 425, and a methylglyoxal synthetase-like component spanning from valine 441 to lysine 1566. It also comprised two cysteine residues (cysteine 1360 and cysteine 1370) that are characteristic of N-acetyl-L-glutamate dependency. Similar to the CPS I of Rana catesbeiana and Cps III of lungfishes and teleosts, it contained the Cys-His-Glu catalytic triad (cysteine 304, histidine 388 and glutamate 390). All Cps III contain methionine 305 and glutamine 308, which are essential for the Cys-His-Glu triad to react with glutamine, but the CPS I of R. catesbeiana contains lysine 305 and glutamate 308, and therefore cannot effectively utilize glutamine as a substrate. However, the CPS I of L. caerulea, unlike that of R. catesbeiana, contained besides glutamate 308, methionine 305 instead of lysine 305, and thus represented a transitional form between Cps III and CPS I. Indeed, CPS I of L. caerulea could utilize glutamine or NH₄⁺ as a substrate in vitro, but the activity obtained with glutamine + NH₄⁺ reflected that obtained with NH₄⁺ alone. Furthermore, only <5 % of the glutamine synthetase activity was present in the hepatic mitochondria, indicating that CPS I of L. caerulea did not have an effective supply of glutamine in vivo. Hence, our results confirmed that the evolution of CPS I from Cps III occurred in amphibians. Since L. caerulea contained high levels of urea in its muscle and liver, which increased significantly in response to desiccation, its CPS I had the dual functions of detoxifying ammonia to urea and producing urea to reduce evaporative water loss.

Aestivation and hypoxia-related events share common silent neuron trafficking processes

Background

The availability of oxygen is a limiting factor for neuronal survival since low levels account not only for the impairment of physiological activities such as sleep-wake cycle, but above all for ischemic-like neurodegenerative disorders. In an attempt to improve our knowledge concerning the type of molecular mechanisms operating during stressful states like those of hypoxic conditions, attention was focused on eventual transcriptional alterations of some key AMPAergic silent neuronal receptor subtypes (GluR1 and GluR2) along with HSPs and HIF-1α during either a normoxic or a hypoxic aestivation of a typical aquatic aestivator, i.e. the lungfish (Protopterus annectens).

Results

The identification of partial nucleotide fragments codifying for both AMPA receptor subtypes in Protopterus annectens displayed a putative high degree of similarity to that of not only fish but also to those of amphibians, birds and mammals. qPCR and in situ hybridization supplied a very high (p < 0.001) reduction of GluR1 mRNA expression in diencephalic areas after 6 months of aerial normoxic aestivation (6mAE). Concomitantly, high (p < 0.01) levels of HSP70 mRNAs in hypothalamic, mesencephalic and cerebellar areas of both 6mAE and after 6 months of mud hypoxic aestivation (6mMUD) were detected together with evident apoptotic signals. Surprisingly, very high levels of GluR2 mRNAs were instead detected in thalamic along with mesencephalic areas after 6 days of normoxic (6dAE) and hypoxic (6dMUD) aestivation. Moreover, even short- and long-term hypoxic states featured high levels of HIF-1α and HSP27 transcripts in the different brain regions of the lungfish.

Conclusions

The distinct transcriptional variations of silent neurons expressing GluR1/2 and HSPs tend to corroborate these factors as determining elements for the physiological success of normoxic and hypoxic aestivation. A distinct switching among these AMPA receptor subtypes during aestivation highlights new potential adaptive strategies operating in key brain regions of the lungfish in relation to oxygen availability. This functional relationship might have therapeutic bearings for hypoxia-related dysfunctions, above all in view of recently identified silent neuron-dependent motor activity ameliorations in mammals.

Molecular characterization and mRNA expression of carbamoyl phosphate synthetase III in the liver of the African lungfish, Protopterus annectens, during aestivation or exposure to ammonia

This study aimed to obtain the full sequence of carbamoyl phosphate synthetase III (cps III) from, and to determine the mRNA expression of cps III in, the liver of P. annectens during aestivation in air, hypoxia or mud, or exposure to environmental ammonia (100 mmol l(-1) NH(4)Cl). The complete coding cDNA sequence of cps III from the liver of P. annectens consisted of 4530 bp, which coded for 1,510 amino acids with an estimated molecular mass of 166.1 kDa. The Cps III of P. annectens consisted of a mitochondrial targeting sequence of 44 amino acid residues, a GAT domain spanning from tyrosine 45 to isoleucine 414, and a methylglyoxal synthase-like domain spanning from valine 433 to arginine 1513. Two cysteine residues (cysteine 1337 and cysteine 1347) that are characteristic of N-acetylglutamate dependency were also present. The critical Cys-His-Glu catalytic triad (cysteine 301, histidine 385 and glutamate 387) together with methionine 302 and glutamine 305 affirmed that P. annectens expressed Cps III and not Cps I. A comparison of the translated amino acid sequence of Cps III from P. annectens with CPS sequences from other animals revealed that it shared the highest similarity with elasmobranch Cps III. A phylogenetic analysis indicates that P. annectens CPS III could have evolved from Cps III of elasmobranchs. Indeed, Cps III from P. annectens used mainly glutamine as the substrate, and its activity decreased significantly when glutamine and ammonia were included together in the assay system. There were significant increases (9- to 12-fold) in the mRNA expression of cps III in the liver of fish during the induction phase (days 3 and 6) of aestivation in air. Aestivation in hypoxia or in mud had a delayed effect on the increase in the mRNA expression of cps III, which extended beyond the induction phase of aestivation, reiterating the importance of differentiating effects that are intrinsic to aestivation from those intrinsic to hypoxia. Furthermore, results from this study confirmed that environmental ammonia exposure led to a significant increase in the mRNA expression of cps III in the liver of P. annectens, alluding to the important functional role of urea not only as a product of ammonia detoxification but also as a putative internal cue for aestivation.

Differential gene expression in the liver of the African lungfish, Protopterus annectens, after 6 days of estivation in air

This study aimed to identify estivation-specific gene clusters through the determination of differential gene expressions in the liver of Protopterus annectens after 6 days of estivation in a mucus cocoon in air (normoxia) using suppression subtractive hybridization polymerase chain reaction. Our results demonstrated that 6 days of estivation in normoxia led to up-regulation of mRNA expressions of several genes related to urea synthesis, including carbamoyl phosphate synthetase (Cps), argininosuccinate synthetase and glutamine synthetase. They indicate that increased urea synthesis, despite being energy-intensive, is an important adaptive response of estivation. They also offer indirect support to the proposition that urea synthesis in this lungfish involved a Cps that uses glutamine as a substrate. In addition, up- or down-regulation of several gene clusters occurred in the liver of P. annectens after 6 days of estivation in normoxia. These estivation-specific genes were involved in the prevention of clot formation, activation of the lectin pathway for complement activation, conservation of minerals (e.g. iron and copper) and increased production of hemoglobin beta. Since there were up- and down-regulation of mRNA expressions of genes related to ribosomal proteins and translational elongation factors, there could be simultaneous increases in protein degradation and protein synthesis during the first 6 days (the induction phase) of estivation, confirming the importance of reconstruction of protein structures in preparation for the maintenance phase of estivation.