Nitrogen Metabolism and Excretion During Aestivation

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.

Decreased heat sensitivity of lungfish TRPV1 revealed by the heterologous expression system

In this study, we focused on TRPV1 of African lungfish, Protopterus annectens. During drought at high temperature, African lungfish can survive by undergoing into aestivation in mud cocoons. Therefore, lungfish is considered to have some specialized heat-sensor, TRPV1, for heat tolerance. Further, lungfish which shares similarities with fishes and amphibians, is one of important species for investigating the fish-tetrapod transition. Since fish TRPV1 and tetrapod TRPV1 have some differences, character of lungfish TRPV1 attracts attention. Here, we first cloned TRPV1 paralogue from lungfish, Protopterus annectens (lfTRPV1) and determined the chemical and thermal sensitivities of lfTRPV1 by two-electrode voltage clamp method using frog oocytes. We detected activation of lfTRPV1 by acid and 2-APB, but capsaicin-induced activation was not observed. The sensitivity to acid of lfTRPV1 was similar to that of rat TRPV1 (rTRPV1), but the 2-APB sensitivity of lfTRPV1 was relatively weaker than rTRPV1. Heat stimulation up to 44 °C did not activate lfTRPV1 and the heat-activation was not detected even on acid condition of pH6. This dramatically decreased heat-sensitivity of TRPV1 may contribute the heat tolerance of African lungfish. Moreover, this might be the property of ancient tetrapod-type TRPV1 gene.

Adaptive physiological water conservation explains hypertension and muscle catabolism in experimental chronic renal failure

Aim

We have reported earlier that a high salt intake triggered an aestivation‐like natriuretic‐ureotelic body water conservation response that lowered muscle mass and increased blood pressure. Here, we tested the hypothesis that a similar adaptive water conservation response occurs in experimental chronic renal failure.

Methods

In four subsequent experiments in Sprague Dawley rats, we used surgical 5/6 renal mass reduction (5/6 Nx) to induce chronic renal failure. We studied solute and water excretion in 24‐hour metabolic cage experiments, chronic blood pressure by radiotelemetry, chronic metabolic adjustment in liver and skeletal muscle by metabolomics and selected enzyme activity measurements, body Na⁺, K⁺ and water by dry ashing, and acute transepidermal water loss in conjunction with skin blood flow and intra‐arterial blood pressure.

Results

5/6 Nx rats were polyuric, because their kidneys could not sufficiently concentrate the urine. Physiological adaptation to this renal water loss included mobilization of nitrogen and energy from muscle for organic osmolyte production, elevated norepinephrine and copeptin levels with reduced skin blood flow, which by means of compensation reduced their transepidermal water loss. This complex physiologic‐metabolic adjustment across multiple organs allowed the rats to stabilize their body water content despite persisting renal water loss, albeit at the expense of hypertension and catabolic mobilization of muscle protein.

Conclusion

Physiological adaptation to body water loss, termed aestivation, is an evolutionary conserved survival strategy and an under‐studied research area in medical physiology, which besides hypertension and muscle mass loss in chronic renal failure may explain many otherwise unexplainable phenomena in medicine.

Giant African snail genomes provide insights into molluscan whole-genome duplication and aquatic-terrestrial transition

Whole-genome duplication (WGD), contributing to evolutionary diversity and environmental adaptability, has been observed across a wide variety of eukaryotic groups, but not in molluscs. Molluscs are the second largest animal phylum in terms of species numbers, and among the organisms that have successfully adapted to the nonmarine realm through aquatic-terrestrial (A-T) transition. We assembled a chromosome-level reference genome for Achatina immaculata, a globally invasive species, and compared the genomes of two giant African snails (A. immaculata and Achatina fulica) to other available mollusc genomes. Macrosynteny, colinearity blocks, Ks peak and Hox gene clusters collectively suggested a WGD event in the two snails. The estimated WGD timing (~70 million years ago) was close to the speciation age of the Sigmurethra-Orthurethra (within Stylommatophora) lineage and the Cretaceous-Tertiary (K-T) mass extinction, indicating that the WGD may have been a common event shared by all Sigmurethra-Orthurethra species and conferred ecological adaptability allowing survival after the K-T extinction event. Furthermore, the adaptive mechanism of WGD in terrestrial ecosystems was confirmed by the presence of gene families related to the respiration, aestivation and immune defence. Several mucus-related gene families expanded early in the Stylommatophora lineage, and the haemocyanin and phosphoenolpyruvate carboxykinase families doubled during WGD, and zinc metalloproteinase genes were highly tandemly duplicated after WGD. This evidence suggests that although WGD may not have been the direct driver of the A-T transition, it played an important part in the terrestrial adaptation of giant African snails.

Organ protection by SGLT2 inhibitors: role of metabolic energy and water conservation

Therapeutic inhibition of the sodium-glucose co-transporter 2 (SGLT2) leads to substantial loss of energy (in the form of glucose) and additional solutes (in the form of Na⁺ and its accompanying anions) in urine. However, despite the continuously elevated solute excretion, long-term osmotic diuresis does not occur in humans with SGLT2 inhibition. Rather, patients on SGLT2 inhibitor therapy adjust to the reduction in energy availability and conserve water. The metabolic adaptations that are induced by SGLT2 inhibition are similar to those observed in aestivation - an evolutionarily conserved survival strategy that enables physiological adaptation to energy and water shortage. Aestivators exploit amino acids from muscle to produce glucose and fatty acid fuels. This endogenous energy supply chain is coupled with nitrogen transfer for organic osmolyte production, which allows parallel water conservation. Moreover, this process is often accompanied by a reduction in metabolic rate. By comparing aestivation metabolism with the fuel switches that occur during therapeutic SGLT2 inhibition, we suggest that SGLT2 inhibitors induce aestivation-like metabolic patterns, which may contribute to the improvements in cardiac and renal function observed with this class of therapeutics.

The NOS/NO system in an example of extreme adaptation: The African lungfish

African dipnoi (lungfish) are aestivating fish and obligate air breathers that, throughout their complex life cycle, undergo remarkable morpho-functional organ readjustment from biochemical to morphological level. In the present review we summarize the changes of the NOS/NO (Nitric Oxide Synthase/Nitric Oxide) system occurring in lungs, gills, kidney, heart, and myotomal muscle of African lungfish of the genus Protopterus (P. dolloi and P. annectens), in relation to the switch from freshwater to aestivation, and vice-versa. In particular, the expression and localization patterns of NOS, and its protein partners Akt, Hsp-90 and HIF-1α, have been discussed, together with the apoptosis rate, evaluated by TUNEL technique. We hypothesize that all these molecular components are crucial in signalling transduction/integration networks induced by environmental challenges (temperature, dehydration, inactivity)experienced at the beginning, during, and at the end of the dry season.

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.

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.

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.

Physiological Responses to Fasting and Estivation for the Three-Toed Amphiuma (Amphiuma tridactylum)

Species of Amphiuma enter a state of subterranean estivation with the drying of their aquatic habitat. Characteristic of amphibian fasting and estivation is an initial depression of metabolism and tissue mass and function with fasting, followed by a more pronounced adaptive decrease in metabolism and tissue function with estivation. We hypothesized that Amphiuma likewise experiences a two-stage set of responses to estivation. Therefore, we examined the physiological responses of the three-toed amphiuma (Amphiuma tridactylum) to fasting and estivation treatments. Recently fed A. tridactylum served as controls for fasting treatments of 1, 3, and 6 mo (in water) and estivation treatments of 3 and 6 mo (buried in dried substrate). After a 1-mo fast, A. tridactylum experienced no further depression of metabolic rate following 3 or 6 mo of fasting or estivation. For all fasting and estivation trials, A. tridactylum maintained blood chemistry homeostasis, with the exception of an increase in blood urea following 6 mo of estivation. Compared with fed controls, the mass of most organs did not vary even after 6 mo of fasting and estivation. Only the small intestine (decreasing) and the full gall bladder (increasing) experienced significant changes in mass with fasting or estivation. The fasting decrease in small intestinal mass was in part due to enterocyte atrophy, which resulted in a decrease in mucosa/submucosa thickness. In contrast to many estivating anurans and the ecologically convergent sirens, A. tridactylum does not surround itself in a cocoon of dried skin or mucus during estivation. The thickness and architecture of their skin remains unchanged even after 6 mo of estivation. Following months of fasting or estivation, individuals still maintain gastric acid production, pancreatic enzyme activity, and intestinal enzyme and transporter activities. Contrary to our hypothesis that A. tridactylum experiences two stages of metabolic depression and tissue downregulation, first with fasting and second with estivation, we observed a relatively modest single-stage response to both. Rather than becoming dormant and engaging in mechanisms to depress metabolism and tissue performance with estivation, A. tridactylum employs an alternative strategy of remaining alert and possibly eating to survive extended periods when their aquatic habitats become dry.

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.

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.

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.

Aestivation: signaling and hypometabolism

Aestivation is a survival strategy used by many vertebrates and invertebrates to endure arid environmental conditions. Key features of aestivation include strong metabolic rate suppression, strategies to retain body water, conservation of energy and body fuel reserves, altered nitrogen metabolism, and mechanisms to preserve and stabilize organs, cells and macromolecules over many weeks or months of dormancy. Cell signaling is crucial to achieving both a hypometabolic state and reorganizing multiple metabolic pathways to optimize long-term viability during aestivation. This commentary examines the current knowledge about cell signaling pathways that participate in regulating aestivation, including signaling cascades mediated by the AMP-activated kinase, Akt, ERK, and FoxO1.

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.

Ammonia production, excretion, toxicity, and defense in fish: a review

Many fishes are ammonotelic but some species can detoxify ammonia to glutamine or urea. Certain fish species can accumulate high levels of ammonia in the brain or defense against ammonia toxicity by enhancing the effectiveness of ammonia excretion through active NH4+transport, manipulation of ambient pH, or reduction in ammonia permeability through the branchial and cutaneous epithelia. Recent reports on ammonia toxicity in mammalian brain reveal the importance of permeation of ammonia through the blood-brain barrier and passages of ammonia and water through transporters in the plasmalemma of brain cells. Additionally, brain ammonia toxicity could be related to the passage of glutamine through the mitochondrial membranes into the mitochondrial matrix. On the other hand, recent reports on ammonia excretion in fish confirm the involvement of Rhesus glycoproteins in the branchial and cutaneous epithelia. Therefore, this review focuses on both the earlier literature and the up-to-date information on the problems and mechanisms concerning the permeation of ammonia, as NH(3), NH4+ or proton-neutral nitrogenous compounds, across mitochondrial membranes, the blood-brain barrier, the plasmalemma of neurons, and the branchial and cutaneous epithelia of fish. It also addresses how certain fishes with high ammonia tolerance defend against ammonia toxicity through the regulation of the permeation of ammonia and related nitrogenous compounds through various types of membranes. It is hoped that this review would revive the interests in investigations on the passage of ammonia through the mitochondrial membranes and the blood-brain barrier of ammonotelic fishes and fishes with high brain ammonia tolerance, respectively.