Na+/H+ and Na+/NH+4 activities of zebrafish NHE3b expressed in Xenopus oocytes

Ion uptake in naturally acidic water

The first studies on ion regulation in fish exposed to low pH, which were inspired by the Acid Rain environmental crisis, seemed to indicate that ion transport at the gills was completely and irreversibly inhibited at pH 4.0-4.5 and below. However, work on characid fish native to the Rio Negro, a naturally acidic, blackwater tributary of the Amazon River, found that they possess ion transport mechanisms that are completely insensitive to pHs as low as 3.25. As more species were examined it appeared that pH-insensitive transport was a trait shared by many, if not most, species in the Order Characiformes. Subsequently, a few other species of fish have been shown to be able to transport ions at low pH, in particular zebrafish (Danio rerio), which show rapid recovery of Na+ uptake at pH 4.0 after initial inhibition. Measurements of rates of Na+ transport during exposure to pharmacological agents that inhibit various transport proteins suggested that characiform fish do not utilize the generally accepted mechanisms for Na+ transport that rely on some form of H+ extrusion. Examination of zebrafish transport at low pH suggest the rapid recovery may be due to a novel Na+/K+ exchanger, but after longer term exposure they may rely on a coupling of Na+/H+ exchangers and NH3 excretion. Further work is needed to clarify these mechanisms of transport and to find other acid-tolerant species to fully gain an appreciation of the diversity of physiological mechansisms involved.

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.

Electroneutral Na+/Cl- cotransport activity of zebrafish Slc12a10.1 expressed in Xenopus oocytes

Na+/Cl- cotransporter 2 (Ncc2 or Slc12a10) is a membrane transport protein that belongs to the electroneutral cation-chloride cotransporter family. The Slc12a10 gene (slc12a10) is widely present in bony vertebrates but is deleted or pseudogenized in birds, some bony fishes, and most mammals. Slc12a10 is highly homologous to Ncc (Slc12a3 or Ncc1); however, there are only a few reports measuring the activity of Slc12a10. In this study, we focused on zebrafish Slc12a10.1 (zSlc12a10.1) and analyzed its activity using Xenopus oocyte electrophysiology. Analysis using Na+-selective microelectrodes showed that intracellular sodium activity (aNai) in zSlc12a10.1 oocytes was significantly decreased in Na+- or Cl--free medium and recovered when Na+ or Cl- was readded to the medium. Similar analysis using a Cl--selective microelectrode showed that intracellular chloride activity (aCli) in zSlc12a10.1 oocytes significantly decreased in Na+- or Cl--free medium and recovered when Na+ or Cl- was readded to the medium. When a similar experiment was performed with a voltage clamp, the membrane current did not change when aNai of zSlc12a10.1 oocytes was decreased in Na+-free medium. Molecular phylogenetic and synteny analyses suggest that gene duplication between slc12a10.2 and slc12a10.3 in zebrafish is a relatively recent event, whereas gene duplication between slc12a10.1 and the ancestral gene of slc12a10.2/slc12a10.3 occurred at least about 2 million years ago. slc12a10 deficiency was observed in species belonging to Ictaluridae, Salmoniformes, Osmeriformes, Batrachoididae, Syngnathiformes, Gobiesociformes, Labriformes, and Tetraodontiformes. These results indicate that zebrafish Slc12a10.1 is an electroneutral Na+/Cl-cotransporter and establish its evolutionary position among various teleost slc12a10 paralogs.NEW & NOTEWORTHY Na+/Cl- cotransporter 2 (Slc12a10; Ncc2) is a protein highly homologous to Ncc (Slc12a3; Ncc1); however, there are only a few reports measuring the activity of Slc12a10. Electrophysiological analysis of Xenopus oocytes expressing zebrafish Slc12a10.1 showed that Slc12a10.1 acts as an electroneutral Na+/Cl-cotransporter. This is the third report on the activity of Slc12a10, following previous reports on Slc12a10 in eels.

Branchial CO2 and ammonia excretion in crustaceans: Involvement of an apical Rhesus-like glycoprotein

AIM

To determine whether the crustacean Rh1 protein functions as a dual CO2 /ammonia transporter and investigate its role in branchial ammonia excretion and acid-base regulation.

METHODS

Sequence analysis of decapod Rh1 proteins was used to determine the conservation of amino acid residues putatively involved in ammonia transport and CO2 binding in human and bacterial Rh proteins. Using the Carcinus maenas Rh1 protein (CmRh1) as a representative of decapod Rh1 proteins, we test the ammonia and CO2 transport capabilities of CmRh1 through heterologous expression in yeast and Xenopus oocytes coupled with site-directed mutagenesis. Quantitative PCR was used to assess the distribution of CmRh1 mRNA in various tissues. Western blotting was used to assess CmRh1 protein expression changes in response to high environmental ammonia and CO2 . Further, immunohistochemistry was used to assess sub-cellular localization of CmRh1 and a membrane-bound carbonic anhydrase (CmCAg).

RESULTS

Sequence analysis of decapod Rh proteins revealed high conservation of several amino acid residues putatively involved in conducting ammonia transport and CO2 binding. Expression of CmRh1 in Xenopus oocytes enhanced both ammonia and CO2 transport which was nullified in CmRh1 D180N mutant oocytes. Transport of the ammonia analog methylamine by CmRh1 is dependent on both ionized and un-ionized ammonia/methylamine species. CmRh1 was co-localized with CmCAg to the apical membrane of the crustacean gill and only experienced decreased protein expression in the anterior gills when exposed to high environmental ammonia.

CONCLUSION

CmRh1 is the first identified apical transporter-mediated route for ammonia and CO2 excretion in the crustacean gill. Our findings shed further light on the potential universality of dual ammonia and CO2 transport capacity of Rhesus glycoproteins in both vertebrates and invertebrates.

Molecular Physiological Evidence for the Role of Na+-Cl− Co-Transporter in Branchial Na+ Uptake in Freshwater Teleosts

The gills are the major organ for Na+ uptake in teleosts. It was proposed that freshwater (FW) teleosts adopt Na+/H+ exchanger 3 (Nhe3) as the primary transporter for Na+ uptake and Na+-Cl− co-transporter (Ncc) as the backup transporter. However, convincing molecular physiological evidence to support the role of Ncc in branchial Na+ uptake is still lacking due to the limitations of functional assays in the gills. Thus, this study aimed to reveal the role of branchial Ncc in Na+ uptake with an in vivo detection platform (scanning ion-selective electrode technique, SIET) that has been recently established in fish gills. First, we identified that Ncc2-expressing cells in zebrafish gills are a specific subtype of ionocyte (NCC ionocytes) by using single-cell transcriptome analysis and immunofluorescence. After a long-term low-Na+ FW exposure, zebrafish increased branchial Ncc2 expression and the number of NCC ionocytes and enhanced gill Na+ uptake capacity. Pharmacological treatments further suggested that Na+ is indeed taken up by Ncc, in addition to Nhe, in the gills. These findings reveal the uptake roles of both branchial Ncc and Nhe under FW and shed light on osmoregulatory physiology in adult fish.

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.

Teleostean fishes may have developed an efficient Na+ uptake for adaptation to the freshwater system

Understanding Na+ uptake mechanisms in vertebrates has been a research priority since vertebrate ancestors were thought to originate from hyperosmotic marine habitats to the hypoosmotic freshwater system. Given the evolutionary success of osmoregulator teleosts, these freshwater conquerors from the marine habitats are reasonably considered to develop the traits of absorbing Na+ from the Na+-poor circumstances for ionic homeostasis. However, in teleosts, the loss of epithelial Na+ channel (ENaC) has long been a mystery and an issue under debate in the evolution of vertebrates. In this study, we evaluate the idea that energetic efficiency in teleosts may have been improved by selection for ENaC loss and an evolved energy-saving alternative, the Na+/H+ exchangers (NHE3)-mediated Na+ uptake/NH4 + excretion machinery. The present study approaches this question from the lamprey, a pioneer invader of freshwater habitats, initially developed ENaC-mediated Na+ uptake driven by energy-consuming apical H+-ATPase (VHA) in the gills, similar to amphibian skin and external gills. Later, teleosts may have intensified ammonotelism to generate larger NH4 + outward gradients that facilitate NHE3-mediated Na+ uptake against an unfavorable Na+ gradient in freshwater without consuming additional ATP. Therefore, this study provides a fresh starting point for expanding our understanding of vertebrate ion regulation and environmental adaptation within the framework of the energy constraint concept.

Acute Stress in Lesser-Spotted Catshark (Scyliorhinus canicula Linnaeus, 1758) Promotes Amino Acid Catabolism and Osmoregulatory Imbalances

Simple Summary

In catsharks (Scyliorhinus canicula), air exposure induces amino acid catabolism altogether with osmoregulatory imbalances. This study describes a novel NHE isoform being expressed in gills that may be involved in ammonium excretion.

Abstract

Acute-stress situations in vertebrates induce a series of physiological responses to cope with the event. While common secondary stress responses include increased catabolism and osmoregulatory imbalances, specific processes depend on the taxa. In this sense, these processes are still largely unknown in ancient vertebrates such as marine elasmobranchs. Thus, we challenged the lesser spotted catshark (Scyliorhinus canicula) to 18 min of air exposure, and monitored their recovery after 0, 5, and 24 h. This study describes amino acid turnover in the liver, white muscle, gills, and rectal gland, and plasma parameters related to energy metabolism and osmoregulatory imbalances. Catsharks rely on white muscle amino acid catabolism to face the energy demand imposed by the stressor, producing NH₄⁺. While some plasma ions (K⁺, Cl⁻ and Ca²⁺) increased in concentration after 18 min of air exposure, returning to basal values after 5 h of recovery, Na⁺ increased after just 5 h of recovery, coinciding with a decrease in plasma NH₄⁺. These changes were accompanied by increased activity of a branchial amiloride-sensitive ATPase. Therefore, we hypothesize that this enzyme may be a Na⁺/H⁺ exchanger (NHE) related to NH₄⁺ excretion. The action of an omeprazole-sensitive ATPase, putatively associated to a H⁺/K⁺-ATPase (HKA), is also affected by these allostatic processes. Some complementary experiments were carried out to delve a little deeper into the possible branchial enzymes sensitive to amiloride, including in vivo and ex vivo approaches, and partial sequencing of a nhe1 in the gills. This study describes the possible presence of an HKA enzyme in the rectal gland, as well as a NHE in the gills, highlighting the importance of understanding the relationship between acute stress and osmoregulation in elasmobranchs.

Cortisol and glucocorticoid receptor 2 regulate acid secretion in medaka (Oryzias latipes) larvae

Freshwater fish live in environments where pH levels fluctuate more than those in seawater. During acidic stress, the acid-base balance in these fish is regulated by ionocytes in the gills, which directly contact water and function as an external kidney. In ionocytes, apical acid secretion is largely mediated by H⁺-ATPase and the sodium/hydrogen exchanger (NHE). Control of this system was previously proposed to depend on the hormone, cortisol, mostly based on studies of zebrafish, a stenohaline fish, which utilize H⁺-ATPase as the main route for apical acid secretion. However, the role of cortisol is poorly understood in euryhaline fish species that preferentially use NHE as the main transporter. In the present study, we explored the role of cortisol in NHE-mediated acid secretion in medaka larvae. mRNA expression levels of transporters related to acid secretion and cortisol-synthesis enzyme were enhanced by acidic FW treatment (pH 4.5, 2 days) in medaka larvae. Moreover, exogenous cortisol treatment (25 mg/L, 2 days) resulted in upregulation of nhe3 and rhcg1 expression, as well as acid secretion in 7 dpf medaka larvae. In loss-of-function experiments, microinjection of glucocorticoid receptor (GR)2 morpholino (MO) caused reductions in nhe3 and rhcg1 expression and diminished acid secretion, but microinjection of mineralocorticoid receptor (MR) and GR1 MOs did not. Together, these results suggest a conserved action of cortisol and GR2 on fish body fluid acid-base regulation.

Rainbow Trout (Oncorhynchus mykiss) Na+/H+ Exchangers tNhe3a and tNhe3b Display Unique Inhibitory Profiles Dissimilar from Mammalian NHE Isoforms

Freshwater fishes maintain an internal osmolality of ~300 mOsm, while living in dilute environments ranging from 0 to 50 mOsm. This osmotic challenge is met at least partially, by Na⁺/H⁺ exchangers (NHE) of fish gill and kidney. In this study, we cloned, expressed, and pharmacologically characterized fish-specific Nhes of the commercially important species Oncorhynchus mykiss. Trout (t) Nhe3a and Nhe3b isoforms from gill and kidney were expressed and characterized in an NHE-deficient cell line. Western blotting and immunocytochemistry confirmed stable expression of the tagged trout tNhe proteins. To measure NHE activity, a transient acid load was induced in trout tNhe expressing cells and intracellular pH was measured. Both isoforms demonstrated significant activity and recovered from an acute acid load. The effect of the NHE transport inhibitors amiloride, EIPA (5-(N-ethyl-N-isopropyl)-amiloride), phenamil, and DAPI was examined. tNhe3a was inhibited in a dose-dependent manner by amiloride and EIPA and tNhe3a was more sensitive to amiloride than EIPA, unlike mammalian NHE1. tNhe3b was inhibited by high concentrations of amiloride, while even in the presence of high concentrations of EIPA (500 µM), some activity of tNhe3b remained. Phenamil and DAPI were ineffective at inhibiting tNhe activity of either isoform. The current study aids in understanding the pharmacology of fish ion transporters. Both isoforms display inhibitory profiles uniquely different from mammalian NHEs and show resistance to inhibition. Our study allows for more direct interpretation of past, present, and future fish-specific sodium transport studies, with less reliance on mammalian NHE data for interpretation.

ATP-mediated increase in H+ flux from retinal Müller cells: a role for Na+/H+ exchange

Small alterations in extracellular H⁺ can profoundly alter neurotransmitter release by neurons. We examined mechanisms by which extracellular ATP induces an extracellular H⁺ flux from Müller glial cells, which surround synaptic connections throughout the vertebrate retina. Müller glia were isolated from tiger salamander retinae and H⁺ fluxes examined using self-referencing H⁺-selective microelectrodes. Experiments were performed in 1 mM HEPES with no bicarbonate present. Replacement of extracellular sodium by choline decreased H⁺ efflux induced by 10 µM ATP by 75%. ATP-induced H⁺ efflux was also reduced by Na⁺/H⁺ exchange inhibitors. Amiloride reduced H⁺ efflux initiated by 10 µM ATP by 60%, while 10 µM cariporide decreased H⁺ flux by 37%, and 25 µM zoniporide reduced H⁺ flux by 32%. ATP-induced H⁺ fluxes were not significantly altered by the K⁺/H⁺ pump blockers SCH28080 or TAK438, and replacement of all extracellular chloride with gluconate was without effect on H⁺ fluxes. Recordings of ATP-induced H⁺ efflux from cells that were simultaneously whole cell voltage clamped revealed no effect of membrane potential from -70 mV to 0 mV. Restoration of extracellular potassium after cells were bathed in 0 mM potassium produced a transient alteration in ATP-dependent H⁺ efflux. The transient response to extracellular potassium occurred only when extracellular sodium was present and was abolished by 1 mM ouabain, suggesting that alterations in sodium gradients were mediated by Na⁺/K⁺-ATPase activity. Our data indicate that the majority of H⁺ efflux elicited by extracellular ATP from isolated Müller cells is mediated by Na⁺/H⁺ exchange.NEW & NOTEWORTHY Glial cells are known to regulate neuronal activity, but the exact mechanism(s) whereby these "support" cells modulate synaptic transmission remains unclear. Small changes in extracellular levels of acidity are known to be particularly powerful regulators of neurotransmitter release. Here, we show that extracellular ATP, known to be a potent activator of glial cells, induces H⁺ efflux from retinal Müller (glial) cells and that the bulk of the H⁺ efflux is mediated by Na⁺/H⁺ exchange.

Expression of ion transport genes in ionocytes isolated from larval zebrafish (Danio rerio) exposed to acidic or Na+-deficient water

In zebrafish (Danio rerio), a specific ionocyte subtype, the H⁺-ATPase-rich (HR) cell, is presumed to be a significant site of transepithelial Na⁺ uptake/acid secretion. During acclimation to environments differing in ionic composition or pH, ionic and acid-base regulations are achieved by adjustments to the activity level of HR cell ion transport proteins. In previous studies, the quantitative assessment of mRNA levels for genes involved in ionic and acid-base regulations relied on measurements using homogenates derived from the whole body (larvae) or the gill (adult). Such studies cannot distinguish whether any differences in gene expression arise from adjustments of ionocyte subtype numbers or transcriptional regulation specifically within individual ionocytes. The goal of the present study was to use fluorescence-activated cell sorting to separate the HR cells from other cellular subpopulations to facilitate the measurement of gene expression of HR cell-specific transporters and enzymes from larvae exposed to low pH (pH 4.0) or low Na⁺ (5 μM) conditions. The data demonstrate that treatment of larvae with acidic water for 4 days postfertilization caused cell-specific increases in H⁺-ATPase (atp6v1aa), ca17a, ca15a, nhe3b, and rhcgb mRNA in addition to increases in mRNA linked to cell proliferation. In fish exposed to low Na⁺, expression of nhe3b and rhcgb was increased owing to HR cell-specific regulation and elevated numbers of HR cells. Thus, the results of this study demonstrate that acclimation to low pH or low Na⁺ environmental conditions is facilitated by HR cell-specific transcriptional control and by HR cell proliferation.

Extra-gastric expression of the proton pump H+/K+-ATPase in the gills and kidney of the teleost Oreochromis niloticus

Potassium regulation is essential for the proper functioning of excitable tissues in vertebrates. The H+/K+-ATPase (HKA), which is composed of the HKα1 (gene: atp4a) and HKβ (gene: atp4b) subunits, has an established role in potassium and acid-base regulation in mammals and is well known for its role in gastric acidification. However, the role of HKA in extra-gastric organs such as the gill and kidney is less clear, especially in fishes. In the present study in Nile tilapia, Oreochromis niloticus, uptake of the K+ surrogate flux marker rubidium (Rb+) was demonstrated in vivo; however, this uptake was not inhibited with omeprazole, a potent inhibitor of the gastric HKA. This contrasts with gill and kidney ex vivo preparations, where tissue Rb+ uptake was significantly inhibited by omeprazole and SCH28080, another gastric HKA inhibitor. The cellular localization of this pump in both the gill and kidney was demonstrated using immunohistochemical techniques with custom-made antibodies specific for Atp4a and Atp4b. Antibodies against the two subunits showed the same apical ionocyte distribution pattern in the gill and collecting tubules/ducts in the kidney. Atp4a antibody specificity was confirmed by western blotting. RT-PCT was used to confirm the expression of both subunits in the gill and kidney. Taken together, these results indicate for the first time K+ (Rb+) uptake in O. niloticus and that HKA is implicated, as shown through the ex vivo uptake inhibition by omeprazole and SCH28080, verifying a role for HKA in K+ absorption in the gill's ionocytes and collecting tubule/duct segments of the kidney.

The Rhesus glycoprotein Rhcgb is expendable for ammonia excretion and Na+ uptake in zebrafish (Danio rerio)

In zebrafish (Danio rerio), the ammonia-transporting Rhesus glycoprotein Rhcgb is implicated in mechanisms of ammonia excretion and Na⁺ uptake. In particular, Rhcgb is thought to play an important role in maintaining ammonia excretion in response to alkaline conditions and high external ammonia (HEA) exposure, in addition to facilitating Na⁺ uptake via a functional metabolon with the Na⁺/H⁺-exchanger Nhe3b, specifically under low Na⁺ conditions. In the present study, we hypothesized that CRISPR/Cas9 knockout of rhcgb would reduce ammonia excretion and Na⁺ uptake capacity, particularly under the conditions listed above that have elicited increases in Rhcgb-mediated ammonia excretion and/or Na⁺ uptake. Contrary to this hypothesis, however, larval and juvenile rhcgb knockout (KO) mutants showed no reductions in ammonia excretion or Na⁺ uptake under any of the conditions tested in our study. In fact, under control conditions, rhcgb KO mutants generally displayed an increase in ammonia excretion, potentially due to increased transcript abundance of another rh gene, rhbg. Under alkaline conditions, rhcgb KO mutants were also able to maintain ammonia excretion, similar to wild-type fish, and stimulation of ammonia excretion after HEA exposure also was not affected by rhcgb KO. Surprisingly, ammonia excretion and Na⁺ uptake were unaffected by rhcgb or nhe3b KO in juvenile zebrafish acclimated to normal (800 μmol/L) or low (10 μmol/L) Na⁺ conditions. These results demonstrate that Rhcgb is expendable for ammonia excretion and Na⁺ uptake in zebrafish, highlighting the plasticity and flexibility of these physiological systems in this species.

Did Acidic Stress Resistance in Vertebrates Evolve as Na+ /H+ Exchanger-Mediated Ammonia Excretion in Fish?

How vertebrates evolved different traits for acid excretion to maintain body fluid pH homeostasis is largely unknown. The evolution of Na⁺ /H⁺  exchanger (NHE)-mediated NH₄ ⁺ excretion in fishes is reported, and the coevolution with increased ammoniagenesis and accompanying gluconeogenesis is speculated to benefit vertebrates in terms of both internal homeostasis and energy metabolism response to acidic stress. The findings provide new insights into our understanding of the possible adaptation of fishes to progressing global environmental acidification. In human kidney, titratable H⁺ and NH₄ ⁺ comprise the two main components of net acid excretion. V-type H⁺ -ATPase-mediated H⁺ excretion may have developed in stenohaline lampreys when they initially invaded freshwater from marine habitats, but this trait is lost in most fishes. Instead, increased reliance on NHE-mediated NH₄ ⁺ excretion is gradually developed and intensified during fish evolution. Further investigations on more species will be needed to support the hypothesis. Also see the video abstract here https://youtu.be/vZuObtfm-34.

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.

(Uncommon) Mechanisms of Branchial Ammonia Excretion in the Common Carp (Cyprinus carpio) in Response to Environmentally Induced Metabolic Acidosis

Freshwater fishes generally increase ammonia excretion in acidic waters. The new model of ammonia transport in freshwater fish involves an association between the Rhesus (Rh) protein Rhcg-b, the Na(+)/H(+) exchanger (NHE), and a suite of other membrane transporters. We tested the hypothesis that Rhcg-b and NHE3 together play a critical role in branchial ammonia excretion in common carp (Cyprinus carpio) chronically exposed to a low-pH environment. Carp were exposed to three sequential environmental treatments-control pH 7.6 water (24 h), pH 4.0 water (72 h), and recovery pH 7.6 water (24 h)-or in a separate series were simply exposed to either control (72 h) or pH 4.0 (72 h) water. Branchial ammonia excretion was increased by ∼2.5-fold in the acid compared with the control period, despite the absence of an increase in the plasma-to-water partial pressure NH3 gradient. Alanine aminotransferase activity was higher in the gills of fish exposed to pH 4 versus control water, suggesting that ammonia may be generated in gill tissue. Gill Rhcg-b and NHE3b messenger RNA levels were significantly elevated in acid-treated relative to control fish, but at the protein level Rhcg-b decreased (30%) and NHE3b increased (2-fold) in response to water of pH 4.0. Using immunofluorescence microscopy, NHE3b and Rhcg-b were found to be colocalized to ionocytes along the interlamellar space of the filament of control fish. After 72 h of acid exposure, Rhcg-b staining almost disappeared from this region, and NHE3b was more prominent along the lamellae. We propose that ammoniagenesis within the gill tissue itself is responsible for the higher rates of branchial ammonia excretion during chronic metabolic acidosis. Unexpectedly, gill Rhcg-b does not appear to be important in gill ammonia transport in low-pH water, but the strong induction of NHE3b suggests that some NH4(+) may be eliminated directly in exchange for Na(+). These findings contrast with previous studies in larval zebrafish (Danio rerio) and medaka (Oryzias latipes), underlining the importance of species comparisons.

Na+ /H+ exchange via the Drosophila vesicular glutamate transporter mediates activity-induced acid efflux from presynaptic terminals

KEY POINTS

Intracellular pH regulation is vital to neurons as nerve activity produces large and rapid acid loads in presynaptic terminals. Rapid clearance of acid loads is necessary to maintain control of neurotransmission, but neuronal acid clearance mechanisms remain poorly understood. Glutamate is loaded into synaptic vesicles via the vesicular glutamate transporter (VGLUT), a mechanism conserved across phyla, and this study reports a previously unknown role for VGLUT as an acid-extruding protein when deposited in the plasmamembrane during exocytosis. The finding was made in Drosophila (fruit fly) larval motor neurons through a combined pharamacological and genetic dissection of presynaptic pH homeostatic mechanisms. A dual role for VGLUT serves to integrate neuronal activity and pH regulation in presynaptic nerve terminals.

ABSTRACT

Neuronal activity can result in transient acidification of presynaptic terminals, and such shifts in cytosolic pH (pH_cyto ) probably influence mechanisms underlying forms of synaptic plasticity with a presynaptic locus. As neuronal activity drives acid loading in presynaptic terminals, we hypothesized that the same activity might drive acid efflux mechanisms to maintain pH_cyto homeostasis. To better understand the integration of neuronal activity and pH_cyto regulation we investigated the acid extrusion mechanisms at Drosophila glutamatergic motorneuron terminals. Expression of a fluorescent genetically encoded pH indicator, named 'pHerry', in the presynaptic cytosol revealed acid efflux following nerve activity to be greater than that predicted from measurements of the intrinsic rate of acid efflux. Analysis of activity-induced acid transients in terminals deficient in either endocytosis or exocytosis revealed an acid efflux mechanism reliant upon synaptic vesicle exocytosis. Pharmacological and genetic dissection in situ and in a heterologous expression system indicate that this acid efflux is mediated by conventional plasmamembrane acid transporters, and also by previously unrecognized intrinsic H⁺ /Na⁺ exchange via the Drosophila vesicular glutamate transporter (DVGLUT). DVGLUT functions not only as a vesicular glutamate transporter but also serves as an acid-extruding protein when deposited on the plasmamembrane.

A role for sodium-chloride cotransporters in the rapid regulation of ion uptake following acute environmental acidosis: new insights from the zebrafish model

The effects of acute exposure to acidic water on Na⁺ and Cl^- homeostasis, and the mechanisms underlying their compensatory regulation, were investigated in the larval zebrafish Danio rerio Exposure to acidic water (pH 4.0; control pH 7.6) for 2 h significantly reduced Na⁺ uptake and whole body Na⁺ content. Nevertheless, the capacity for Na⁺ uptake was substantially increased in fish preexposed to acidic water but measured in control water. Based on the accumulation of the Na⁺-selective dye, Sodium Green, two ionocyte subtypes exhibited intracellular Na⁺ enrichment after preexposure to acidic water: H⁺-ATPase rich (HR) cells, which coexpress the Na⁺/H⁺ exchanger isoform 3b (NHE3b), and a non-HR cell population. In fish experiencing Na⁺-Cl^- cotransporter (NCC) knockdown, we observed no Sodium Green accumulation in the latter cell type, suggesting the non-HR cells were NCC cells. Elimination of NHE3b-expressing HR cells did not prevent the increased Na⁺ uptake following acid exposure. On the other hand, the increased Na⁺ uptake was abolished when the acidic water was enriched with Na⁺ and Cl^-, but not with Na⁺ only, indicating that the elevated Na⁺ uptake after acid exposure was associated with the compensatory regulation of Cl^- Further examinations demonstrated that acute acid exposure also reduced whole body Cl^- levels and increased the capacity for Cl^- uptake. Moreover, knockdown of NCC prevented the increased uptake of both Na⁺ and Cl^- after exposure to acidic water. Together, the results of the present study revealed a novel role of NCC in the compensatory regulation of Na⁺ and Cl^- uptake following acute acidosis.

Transport proteins NHA1 and NHA2 are essential for survival, but have distinct transport modalities

The cation/proton antiporter (CPA) family includes the well-known sodium/proton exchanger (NHE; SLC9A) family of Na(+)/H(+) exchangers, and the more recently discovered and less well understood CPA2s (SLC9B), found widely in living organisms. In Drosophila, as in humans, they are represented by two genes, Nha1 (Slc9b1) and Nha2 (Slc9b2), which are enriched and functionally significant in renal tubules. The importance of their role in organismal survival has not been investigated in animals, however. Here we show that single RNAi knockdowns of either Nha1 or Nha2 reduce survival and in combination are lethal. Knockdown of either gene alone results in up-regulation of the other, suggesting functional complementation of the two genes. Under salt stress, knockdown of either gene decreases survival, demonstrating a key role for the CPA2 family in ion homeostasis. This is specific to Na(+) stress; survival on K(+) intoxication is not affected by sodium/hydrogen antiporter (NHA) knockdown. A direct functional assay in Xenopus oocytes shows that Nha2 acts as a Na(+)/H(+) exchanger. In contrast, Nha1 expressed in Xenopus oocytes shows strong Cl(-) conductance and acts as a H(+)-Cl(-) cotransporter. The activity of Nha1 is inhibited by chloride-binding competitors 4,4'-diiso-thiocyano-2,2'-disulfonic acid stilbene and 4,4'-dibenzamido-2,2'-stilbenedisulphonate. Salt stress induces a massive up-regulation of NHA gene expression not in the major osmoregulatory tissues of the alimentary canal, but in the crop, cuticle, and associated tissues. Thus, it is necessary to revise the classical view of the coordination of different tissues in the coordination of the response to osmoregulatory stress.

Cloning and characterization of Na(+)/H(+) Exchanger isoforms NHE2 and NHE3 from the gill of Pacific dogfish Squalus suckleyi

Na(+)/H(+) Exchanger (NHE) proteins mediate cellular and systemic homeostasis of sodium and acid and may be the major sodium uptake method for fishes. We cloned and sequenced NHE2 and NHE3 from the gill of the North Pacific Spiny Dogfish shark Squalus suckleyi and expressed them in functional form in NHE-deficient (AP-1) cell lines. Estimated IC50 for inhibition of NHE activity by amiloride and EIPA were 55 μmol l(-1) and 4.8 μmol l(-1), respectively, for NHE2 and 9 μmol l(-1) and 24 μmol l(-1), respectively, for NHE3. Phenamil at 100 μmol l(-1) caused less than 16% inhibition of activity for each isoform. Although the IC50 are similar for the two isoforms, dfNHE2 is less sensitive than human NHE2 to inhibition by amiloride and EIPA, while dfNHE3 is more sensitive than human NHE3. These IC50 estimates should be considered when selecting inhibitor doses for fishes and for reinterpretation of previous studies that use these pharmacological agents.

Physiological and molecular responses of the goldfish (Carassius auratus) kidney to metabolic acidosis, and potential mechanisms of renal ammonia transport

Relative to the gills, the mechanisms by which the kidney contributes to ammonia and acid-base homeostasis in fish are poorly understood. Goldfish were exposed to a low pH environment (pH 4.0, 48 h), which induced a characteristic metabolic acidosis and an increase in total plasma [ammonia] but reduced plasma ammonia partial pressure (PNH3). In the kidney tissue, total ammonia, lactate and intracellular pH remained unchanged. The urinary excretion rate of net base under control conditions changed to net acid excretion under low pH, with contributions from both the NH4 (+) (∼30%) and titratable acidity minus bicarbonate (∼70%; TA-HCO3 (-)) components. Inorganic phosphate (Pi), urea and Na(+) excretion rates were also elevated while Cl(-) excretion rates were unchanged. Renal alanine aminotransferase activity increased under acidosis. The increase in renal ammonia excretion was due to significant increases in both the glomerular filtration and the tubular secretion rates of ammonia, with the latter accounting for ∼75% of the increase. There was also a 3.5-fold increase in the mRNA expression of renal Rhcg-b (Rhcg1) mRNA. There was no relationship between ammonia secretion and Na(+) reabsorption. These data indicate that increased renal ammonia secretion during acidosis is probably mediated through Rhesus (Rh) glycoproteins and occurs independently of Na(+) transport, in contrast to branchial and epidermal models of Na(+)-dependent ammonia transport in freshwater fish. Rather, we propose a model of parallel H(+)/NH3 transport as the primary mechanism of renal tubular ammonia secretion that is dependent on renal amino acid catabolism.

The role of acid-sensing ion channels in epithelial Na+ uptake in adult zebrafish (Danio rerio)

Acid-sensing ion channels (ASICs) are epithelial Na(+) channels gated by external H(+). Recently, it has been demonstrated that ASICs play a role in Na(+) uptake in freshwater rainbow trout. Here, we investigate the potential involvement of ASICs in Na(+) transport in another freshwater fish species, the zebrafish (Danio rerio). Using molecular and histological techniques we found that asic genes and the ASIC4.2 protein are expressed in the gill of adult zebrafish. Immunohistochemistry revealed that mitochondrion-rich cells positive for ASIC4.2 do not co-localize with Na(+)/K(+)-ATPase-rich cells, but co-localize with cells expressing vacuolar-type H(+)-ATPase. Furthermore, pharmacological inhibitors of ASIC and Na(+)/H(+)-exchanger significantly reduced uptake of Na(+) in adult zebrafish exposed to low-Na(+) media, but did not cause the same response in individuals exposed to ultra-low-Na(+) water. Our results suggest that in adult zebrafish ASICs play a role in branchial Na(+) uptake in media with low Na(+) concentrations and that mechanisms used for Na(+) uptake by zebrafish may depend on the Na(+) concentration in the acclimation medium.

Mudskipper genomes provide insights into the terrestrial adaptation of amphibious fishes

Mudskippers are amphibious fishes that have developed morphological and physiological adaptations to match their unique lifestyles. Here we perform whole-genome sequencing of four representative mudskippers to elucidate the molecular mechanisms underlying these adaptations. We discover an expansion of innate immune system genes in the mudskippers that may provide defence against terrestrial pathogens. Several genes of the ammonia excretion pathway in the gills have experienced positive selection, suggesting their important roles in mudskippers’ tolerance to environmental ammonia. Some vision-related genes are differentially lost or mutated, illustrating genomic changes associated with aerial vision. Transcriptomic analyses of mudskippers exposed to air highlight regulatory pathways that are up- or down-regulated in response to hypoxia. The present study provides a valuable resource for understanding the molecular mechanisms underlying water-to-land transition of vertebrates.

Mudskippers are amphibious fishes that have adapted to live on mudflats. Here, the authors sequence the genomes of four different mudskipper species and highlight genetic changes that may have had an evolutionary role in the water-to-land transition of vertebrates.

Hydrogen sulfide inhibits Na+ uptake in larval zebrafish, Danio rerio

The present study investigated the role of hydrogen sulfide (H2S) in regulating Na(+) uptake in larval zebrafish, Danio rerio. Waterborne treatment of larvae at 4 days post-fertilization (dpf) with Na2S or GYY-4137 (chemicals known to generate H2S) significantly reduced Na(+) uptake. Exposure of larvae to water enriched with NaCl (1 mM NaCl) caused a pronounced reduction in Na(+) uptake which was prevented by pharmacological inhibition of cystathionine β-synthase (CBS) or cystathionine γ-lyase (CSE), two key enzymes involved in the endogenous synthesis of H2S. Furthermore, translational gene knockdown of CSE and CBSb significantly increased the basal rate of Na(+) uptake. Waterborne treatment with Na2S significantly decreased whole-body acid excretion and reduced Na(+) uptake in larval zebrafish preexposed to acidic (pH 4.0) water (a condition shown to promote Na(+) uptake via Na(+)-H(+)-exchanger 3b, NHE3b). However, Na2S did not affect Na(+) uptake in larvae depleted of NHE3b-containing ionocytes (HR cells) after knockdown of transcription factor glial cell missing 2 (gcm2) in which Na(+) uptake occurs predominantly via Na(+)-Cl(-) co-transporter (NCC)-containing cells. These observations suggest that Na(+) uptake via NHE3b, but not NCC, is regulated by H2S. Whole-mount immunohistochemistry demonstrated that ionocytes expressing NHE3b also express CSE. These data suggests a physiologically relevant role of H2S as a mechanism to lower Na(+) uptake in zebrafish larvae, probably through its inhibitory action on NHE3b.