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

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.

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.

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.