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

The Extremophiles: Adaptation Mechanisms and Biotechnological Applications

This review provides an overview of terrestrial extremophiles, highlighting their adaptive strengths and strategies for coping with environmental challenges through the use of specialized proteins. It also explores why their unique lifestyle and ability to adapt to extreme conditions have become a major focus of research, as well as the main benefits and advancements in the study of these organisms in recent decades. This review aims to present an objective summary of the knowledge acquired and its translation into applied science and biotechnological applications.

Physiological adaptations of red blood cells during aestivation in the south American lungfish Lepidosiren paradoxa

The South American lungfish Lepidosiren paradoxa inhabits areas with variable pluvial regimes. During aestivation (dormancy state observed in some species during dry seasons), the prolonged period of dryness imposes osmotic stress. We aimed to investigate the physiological and morphological adaptations of RBCs in Lepidosiren paradoxa during aestivation. Here, the lungfish were subjected to aestivation for 20 and 40 days and compared to a control group in an active period. The osmotic fragility, blood osmolality, and pH were measured. Blood smears were performed to assess morphological changes in the RBCs. Lepidosiren paradoxa presented lower hemolysis when compared with a teleost fish and tegu lizard. Hemolysis increased when the lungfish was subjected to 40 days of aestivation (from 6.04 % to 16.51 %; control vs 40-day aestivation). Hematocrit rose in both aestivation groups compared to the control (26.36 %, 41.36 % and 41 %, control, 20 and 40 days, respectively; p < 0.05) indicating changes in RBC volume and hydration status. Moreover, the results revealed increased osmolality in the 40-day aestivation group (244.4 vs 372.1 mmol/Kg; control vs 40-day aestivation). 40 days of aestivation led to a decrease in blood pH when compared to the control and 20-day aestivation groups. Both aestivation durations resulted in a reduction in the perimeter and cell diameters in at least one direction of the RBCs (24 % mean reduction in size from control to 40 days aestivation). These findings suggest that South American lungfish possess remarkable physiological and morphological adaptations in their RBCs during aestivation.

Comparative transcriptomic evidence of physiological changes and potential relationships in vertebrates under different dormancy states

Dormancy represents a fascinating adaptive strategy for organisms to survive in unforgiving environments. After a period of dormancy, organisms often exhibit exceptional resilience. This period is typically divided into hibernation and aestivation based on seasonal patterns. However, the mechanisms by which organisms adapt to their environments during dormancy, as well as the potential relationships between different states of dormancy, deserve further exploration. Here, we selected Perccottus glenii and Protopterus annectens as the primary subjects to study hibernation and aestivation, respectively. Based on histological and transcriptomic analysis of multiple organs, we discovered that dormancy involved a coordinated functional response across organs. Enrichment analyses revealed noteworthy disparities between the two dormant species in their responses to extreme temperatures. Notably, similarities in gene expression patterns pertaining to energy metabolism, neural activity, and biosynthesis were noted during hibernation, suggesting a potential correlation between hibernation and aestivation. To further explore the relationship between these two phenomena, we analyzed other dormancy-capable species using data from publicly available databases. This comparative analysis revealed that most orthologous genes involved in metabolism, cell proliferation, and neural function exhibited consistent expression patterns during dormancy, indicating that the observed similarity between hibernation and aestivation may be attributable to convergent evolution. In conclusion, this study enhances our comprehension of the dormancy phenomenon and offers new insights into the molecular mechanisms underpinning vertebrate dormancy.

A single-cell atlas of West African lungfish respiratory system reveals evolutionary adaptations to terrestrialization

The six species of lungfish possess both lungs and gills and are the closest extant relatives of tetrapods. Here, we report a single-cell transcriptome atlas of the West African lungfish (Protopterus annectens). This species manifests the most extreme form of terrestrialization, a life history strategy to survive dry periods that can last for years, characterized by dormancy and reversible adaptive changes of the gills and lungs. Our atlas highlights the cell type diversity of the West African lungfish, including gene expression consistent with phenotype changes of terrestrialization. Comparison with terrestrial tetrapods and ray-finned fishes reveals broad homology between the swim bladder and lung cell types as well as shared and idiosyncratic changes of the external gills of the West African lungfish and the internal gills of Atlantic salmon. The single-cell atlas presented here provides a valuable resource for further exploration of the respiratory system evolution in vertebrates and the diversity of lungfish terrestrialization.

Aestivation induces widespread transcriptional changes in the African lungfish

Aestivation is a special ability possessed by some animals to cope with hot and dry environments utilizing dormancy. At a macroscopic level, dormant animals stop moving and eating. At the microscopic level, the expression of a large number of genes in these animals is strictly controlled. However, little is known about what changes occur during aestivation, especially in fish. In this study, we used transcriptome analysis to examine what changes occur in the gills and lungs of the African lungfish (Protopterus annectens) during the maintenance phase of aestivation and speculated on their causes. We found that aestivating transcriptomes were highly similar between gills and lungs. We also found that some genes showed differential expression or alternative splicing, which may be associated with different organs. In addition, differential expression analysis revealed that the lungs maintained significantly higher bioactivity during aestivation, which suggests that the main respiratory organ in aestivating lungfish can transform. Our study provides a reference point for studying the relationship between aestivation and hibernation and further increases understanding of aestivation.

Evidence for the involvement of branchial Vacuolar-type H+-ATPase in the acidification of the external medium by the West African lungfish, Protopterus annectens, exposed to ammonia-loading conditions

African lungfishes are obligatory air-breathers with exceptionally high environmental ammonia tolerance. They can lower the pH of the external medium during exposure to ammonia-loading conditions. This study aimed to demonstrate the possible involvement of branchial vacuolar-type H⁺-ATPase (Vha) in the ammonia-induced acidification of the external medium by the West African lungfish, Protopterus annectens, and to examine whether its capacity to acidify the medium could be augmented after exposure to 100 mmol l^-1 NH₄Cl for six days. Two full coding cDNA sequences of Vha subunit B (atp6v1b), atp6v1b1 and atp6v1b2, were obtained from the internal gills of P. annectens. The sequence of atp6v1b1 comprised 1548 bp, encoding 515 amino acids (57.4 kDa), while that of atp6v1b2 comprised 1536 bp, encoding 511 amino acids (56.6 kDa). Using a custom-made antibody reactive to both isoforms, immunofluorescence microscopy revealed the collective localization of Atp6v1b (atp6v1b1 and atp6v1b2) at the apical or the basolateral membrane of two different types of branchial Na⁺/K⁺-ATPase-immunoreactive ionocyte. The ionocytes labelled apically with Atp6v1b presumably expressed Atp6v1b1 containing a PDZ-binding domain, indicating that the apical Vha was positioned to transport H⁺ to the external medium. The expression of Atp6v1b was regulated post-transcriptionally, as the protein abundance of Atp6v1b and Vha activity increased significantly in the gills of fish exposed to 100 mmol l^-1 NH₄Cl for six days. Correspondingly, the fish exposed to ammonia had a greater capacity to acidify the external medium, presumably to decrease the ratio of [NH₃] to [NH₄⁺] in order to reduce the influx of exogenous NH₃.

Effects of tris (2-chloroethyl) phosphate (TCEP) on survival, growth, histological changes and gene expressions in juvenile yellow catfish Pelteobagrus fulvidraco

Tris (2-chloroethyl) phosphate (TCEP) is an emerging aquatic environmental pollutant. In the present study, juvenile yellow catfish (Pelteobagrus fulvidraco) were exposed to environmentally relevant concentrations of TCEP for 30 days. The results showed that TCEP exposure decreased the survival rate (100 μg/L), body weight (10 and 100 μg/L) and specific growth rate (10 and 100 μg/L) of juvenile yellow catfish. Exposure to TCEP resulted in pronounced damages of gill structures. Gene transcription analysis showed that the antioxidant capacity of the liver and gills was affected; CYP1A1 might contribute to phase I metabolism of TCEP in the liver rather than CYP1B1; TCEP stress might increase the demand of ion transport in fish gill; TCEP could stimulate the immune response and might induce apoptosis via a p53-Bax pathway and caspase-dependent pathway in gills. Collectively, these findings provide new insights into the toxic effects of TCEP on fish.

Partial Amino Acid Metabolism and Glutamine Synthesis as the Ammonia Defensive Strategies During Aerial Exposure in Chinese Loach Paramisgurnus dabryanus

The Paramisgurnus dabryanus was exposed to air to assess the changes in plasma, liver and muscle free amino acid (FAA) contents. The FAA concentrations in plasma, liver and muscle of P. dabryanus were significantly affected by aerial exposure (P < 0.05). After 12 h of aerial exposure, the plasma glutamate contents increased significantly (P < 0.05) and reached peak value at 24 h of air exposure. With increasing air exposure time, the plasma alanine contents increased significantly and more dramatically than the control values (P < 0.05). From 24 to 48 h of aerial exposure, the liver free glutamate contents increased significantly and reached the peak value at 48 h of air exposure (P < 0.05). The liver free alanine contents in air exposure group were markedly higher than these values in the control group (P < 0.05). After 72 h of air exposure, the muscle free glutamate contents increased markedly (P < 0.05) and were significantly higher than the control values (P < 0.05). The muscle free alanine contents remained at constant values during the first 12 h of aerial exposure (P > 0.05), thereafter, these concentrations increased significantly until the end of experiment (P < 0.05). Our results showed that glutamate and NH₄⁺ could be used to synthesize glutamine via glutamine synthetase to convert internal ammonia into non-toxic glutamine in P. dabryanus during air exposure. Furthermore, the P. dabryanus could catabolize several certain amino acids, leading alanine form to reduce endogenous ammonia production. The decrease in tissue free glutamate, arginine and proline in P. dabryanus indicated that these certain amino acids should be the starting substrate to be converted to alanine and energy.

Ionoregulatory and oxidative stress issues associated with the evolution of air-breathing

Aquatic areas frequently face hypoxic conditions. In order to get sufficient oxygen to support aerobic metabolism, a number of freshwater fish resort to aerial respiration to supplement gill respiration especially in situations with reduced oxygen availability in the water. In many species a concomitant reduction in gill surface area or in gill perfusion reduces possible loss of aerially acquired oxygen to the water at the gills, but it also compromises the ion regulatory capacity of gill tissue. In consequence, the reduced gill contact area with water requires appropriate compensation to maintain ion and acid-base homeostasis, often with important ramifications for other organs. Associated modifications in the structure and function of the gills themselves, the skin, the gut, the kidney, and the physiology of water exchange and ion-linked acid-base regulation are discussed. In air-breathing fish, the gut may gain particular importance for the uptake of ions. In addition, tissues frequently exposed to environmental air encounter much higher oxygen partial pressures than typically observed in fish tissues. Physostomous fish using the swimbladder for aerial respiration, for example, will encounter aerial oxygen partial pressure at the swimbladder epithelium when frequently gulping air in hypoxic water. Hyperoxic conditions or rapid changes in oxygen partial pressures result in an increase in the production of reactive oxygen species (ROS). Accordingly, in air-breathing fish, strategies of ionoregulation may be greatly modified, and the ROS defense capacity of air-exposed tissues is improved.

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.

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.

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.