Ammonia excretion by the fish gill: discoveries and ideas that shaped our current understanding

The fish gill serves many physiological functions, among which is the excretion of ammonia, the primary nitrogenous waste in most fishes. Although it is the end-product of nitrogen metabolism, ammonia serves many physiological functions including acting as an acid equivalent and as a counter-ion in mechanisms of ion regulation. Our current understanding of the mechanisms of ammonia excretion have been influenced by classic experimental work, clever mechanistic approaches, and modern molecular and genetic techniques. In this review, I will overview the history of the study of ammonia excretion by the gills of fishes, highlighting the important advancements that have shaped this field with a nearly 100-year history. The developmental and evolutionary implications of an ammonia and gill-dominated nitrogen regulation strategy in most fishes will also be discussed. Throughout the review, I point to areas in which more work is needed to push forward this field of research that continues to produce novel insights and discoveries that will undoubtedly shape our overall understanding of fish physiology.

Cellular mechanisms of ion and acid-base regulation in teleost gill ionocytes

The mechanism(s) of sodium, chloride and pH regulation in teleost fishes has been the subject of intense interest for researchers over the past 100 years. The primary organ responsible for ionoregulatory homeostasis is the gill, and more specifically, gill ionocytes. Building on the theoretical and experimental research of the past, recent advances in molecular and cellular techniques in the past two decades have allowed for substantial advances in our understanding of mechanisms involved. With an increased diversity of teleost species and environmental conditions being investigated, it has become apparent that there are multiple strategies and mechanisms employed to achieve ion and acid-base homeostasis. This review will cover the historical developments in our understanding of the teleost fish gill, highlight some of the recent advances and conflicting information in our understanding of ionocyte function, and serve to identify areas that require further investigation to improve our understanding of complex cellular and molecular machineries involved in iono- and acid-base regulation.

Mechanisms of Na+ uptake from freshwater habitats in animals

Life in fresh water is osmotically and energetically challenging for living organisms, requiring increases in ion uptake from dilute environments. However, mechanisms of ion uptake from freshwater environments are still poorly understood and controversial, especially in arthropods, for which several hypothetical models have been proposed based on incomplete data. One compelling model involves the proton pump V-type H⁺ ATPase (VHA), which energizes the apical membrane, enabling the uptake of Na⁺ (and other cations) via an unknown Na⁺ transporter (referred to as the "Wieczorek Exchanger" in insects). What evidence exists for this model of ion uptake and what is this mystery exchanger or channel that cooperates with VHA? We present results from studies that explore this question in crustaceans, insects, and teleost fish. We argue that the Na⁺/H⁺ antiporter (NHA) is a likely candidate for the Wieczorek Exchanger in many crustaceans and insects; although, there is no evidence that this is the case for fish. NHA was discovered relatively recently in animals and its functions have not been well characterized. Teleost fish exhibit redundancy of Na⁺ uptake pathways at the gill level, performed by different ion transporter paralogs in diverse cell types, apparently enabling tolerance of low environmental salinity and various pH levels. We argue that much more research is needed on overall mechanisms of ion uptake from freshwater habitats, especially on NHA and other potential Wieczorek Exchangers. Such insights gained would contribute greatly to our general understanding of ionic regulation in diverse species across habitats.

Genome-wide identification of the NHE gene family in Coilia nasus and its response to salinity challenge and ammonia stress

Background

In aquatic environments, pH, salinity, and ammonia concentration are extremely important for aquatic animals. NHE is a two-way ion exchange carrier protein, which can transport Na⁺ into cells and exchange out H⁺, and also plays key roles in regulating intracellular pH, osmotic pressure, and ammonia concentration.

Results

In the present study, ten NHEs, the entire NHE gene family, were identified from Coilia nasus genome and systemically analyzed via phylogenetic, structural, and synteny analysis. Different expression patterns of C. nasus NHEs in multiple tissues indicated that expression profiles of NHE genes displayed tissue-specific. Expression patterns of C. nasus NHEs were related to ammonia excretion during multiple embryonic development stages. To explore the potential functions on salinity challenge and ammonia stress, expression levels of ten NHEs were detected in C. nasus gills under hypotonic stress, hypertonic stress, and ammonia stress. Expression levels of all NHEs were upregulated during hypotonic stress, while they were downregulated during hypertonic stress. NHE2 and NHE3 displayed higher expression levels in C. nasus larvae and juvenile gills under ammonia stress.

Conclusions

Our study revealed that NHE genes played distinct roles in embryonic development, salinity stress, and ammonia exposure. Syntenic analysis showed significant difference between stenohaline fish and euryhaline fishes. Our findings will provide insight into effects of C. nasus NHE gene family on ion transport and ammonia tolerance and be beneficial for healthy aquaculture of C. nasus.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08761-9.

Reassessing the contribution of the Na+/H+ exchanger Nhe3b to Na+ uptake in zebrafish (Danio rerio) using CRISPR/Cas9 gene editing

Freshwater fishes absorb Na+ from their dilute environment using ion-transporting cells. In larval zebrafish (Danio rerio), Na+ uptake is coordinated by (1) Na+/H+-exchanger 3b (Nhe3b) and (2) H+-ATPase-powered electrogenic uptake in H+-ATPase-rich (HR) cells and by (3) Na+-Cl−­­-cotransporter (Ncc) expressed in NCC cells. The present study aimed to better understand the roles of these 3 proteins in Na+ uptake by larval zebrafish under ‘normal’ (800 µmol/L) and ‘low’ (10 µmol/L) Na+ conditions. We hypothesized that Na+ uptake would be reduced by CRISPR/Cas9 knockout (KO) of slc9a3.2 (encoding Nhe3b), particularly in low Na+ where Nhe3b is believed to play a dominant role. Contrary to this hypothesis, Na+ uptake was sustained in nhe3b KO larvae under both Na+ conditions, which led to the exploration of whether compensatory regulation of H+-ATPase or Ncc was responsible for maintaining Na+ uptake in nhe3b KO larvae. mRNA expression of the genes encoding H+-ATPase and Ncc were not altered in nhe3b KO. Moreover, morpholino knockdown of H+-ATPase, which significantly reduced H+ flux by HR cells, did not reduce Na+ uptake in nhe3b KO larvae, nor did rearing larvae in chloride-free conditions, thereby eliminating any driving force for Na+-Cl−-cotransport via Ncc. Finally, simultaneously treating nhe3b KO larvae with H+-ATPase morpholino and chloride-free conditions did not reduce Na+ uptake under normal or low Na+. These findings highlight the flexibility of the Na+ uptake system and demonstrate that Nhe3b is expendable to Na+ uptake in zebrafish and that our understanding of Na+ uptake mechanisms in this species is incomplete.

Widespread use of emersion and cutaneous ammonia excretion in Aplocheiloid killifishes

The invasion of land required amphibious fishes to evolve new strategies to avoid toxic ammonia accumulation in the absence of water flow over the gills. We investigated amphibious behaviour and nitrogen excretion strategies in six phylogenetically diverse Aplocheiloid killifishes (Anablepsoides hartii, Cynodonichthys hildebrandi, Rivulus cylindraceus, Kryptolebias marmoratus, Fundulopanchax gardneri, and Aplocheilus lineatus) in order to determine if a common strategy evolved. All species voluntarily emersed (left water) over several days, and also in response to environmental stressors (low O₂, high temperature). All species were ammoniotelic in water and released gaseous ammonia (NH₃ volatilization) during air exposure as the primary route for nitrogen excretion. Metabolic depression, urea synthesis, and/or ammonia accumulation during air exposure were not common strategies used by these species. Immunostaining revealed the presence of ammonia-transporting Rhesus proteins (Rhcg1 and Rhcg2) in the skin of all six species, indicating a shared mechanism for ammonia volatilization. We also found Rhcg in the skin of several other fully aquatic fishes, implying that cutaneous ammonia excretion is not exclusive to amphibious fishes. Overall, our results demonstrate that similar nitrogen excretion strategies while out of water were used by all killifish species tested; possibly the result of shared ancestral amphibious traits, phenotypic convergence, or a combination of both.

Section-specific expression of acid-base and ammonia transporters in the kidney tubules of the goldfish Carassius auratus and their responses to feeding

In teleost fishes, renal contributions to acid-base and ammonia regulation are often neglected compared with the gills. In goldfish, increased renal acid excretion in response to feeding was indicated by increased urine ammonia and inorganic phosphate concentrations and decreased urine pH. By microdissecting the kidney tubules and performing quantitative real-time PCR and/or immunohistochemistry, we profiled the section-specific expression of glutamate dehydrogenase (GDH), glutamine synthetase (GS), Na⁺/H⁺-exchanger 3 (NHE3), carbonic anhydrase II (CAIIa), V-H⁺-ATPase subunit 1b, Cl^-/ HCO3- -exchanger 1 (AE1), Na⁺/ HCO3- -cotransporter 1 (NBC1), Na⁺/K⁺-ATPase subunit 1α, and Rhesus-proteins Rhbg, Rhcg1a, and Rhcg1b. Here, we show for the first time that 1) the proximal tubule appears to be the major site for ammoniagenesis, 2) epithelial transporters are differentially expressed along the renal tubule, and 3) a potential feeding-related "acidic tide" results in the differential regulation of epithelial transporters, resembling the mammalian renal response to a metabolic acidosis. Specifically, GDH and NHE3 mRNAs were upregulated and GS downregulated in the proximal tubule upon feeding, suggesting this section as a major site for ammoniagenesis and acid secretion. The distal tubule may play a major role in renal ammonia secretion, with feeding-induced upregulation of mRNA and protein for apical NHE3, cytoplasmic CAIIa, universal Rhcg1a and apical Rhcg1b, and downregulation of basolateral Rhbg and AE1. Changes in mRNA expression of the Wolffian ducts and bladder suggest supporting roles in fine-tuning urine composition. The present study verifies an important renal contribution to acid-base balance and emphasizes that studies looking at the whole kidney may overlook key section-specific responses.

Ammonia and urea handling by early life stages of fishes

Nitrogen metabolism in fishes has been a focus of comparative physiologists for nearly a century. In this Review, we focus specifically on early life stages of fishes, which have received considerable attention in more recent work. Nitrogen metabolism and excretion in early life differs fundamentally from that of juvenile and adult fishes because of (1) the presence of a chorion capsule in embryos that imposes a limitation on effective ammonia excretion, (2) an amino acid-based metabolism that generates a substantial ammonia load, and (3) the lack of a functional gill, which is the primary site of nitrogen excretion in juvenile and adult fishes. Recent findings have shed considerable light on the mechanisms by which these constraints are overcome in early life. Perhaps most importantly, the discovery of Rhesus (Rh) glycoproteins as ammonia transporters and their expression in ion-transporting cells on the skin of larval fishes has transformed our understanding of ammonia excretion by fishes in general. The emergence of larval zebrafish as a model species, together with genetic knockdown techniques, has similarly advanced our understanding of ammonia and urea metabolism and excretion by larval fishes. It has also now been demonstrated that ammonia excretion is one of the primary functions of the developing gill in rainbow trout larvae, leading to new hypotheses regarding the physiological demands driving gill development in larval fishes. Here, we highlight and discuss the dramatic changes in nitrogen handling that occur over early life development in fishes.