The vacuolar-type H(+)-ATPase (VHA) is a multi-subunit enzyme that uses the energy from ATP hydrolysis to transport H(+) across biological membranes. VHA plays a universal role in essential cellular ...functions, such as the acidification of lysosomes and endosomes. In addition, the VHA-generated H(+)-motive force can drive the transport of diverse molecules across cell membranes and epithelia for specialized physiological functions. Here, I discuss diverse physiological functions of VHA in marine animals, focusing on recent discoveries about base secretion in shark gills, potential bone dissolution by Osedax bone-eating worms and its participation in a carbon-concentrating mechanism that promotes coral photosynthesis. Because VHA is evolutionarily conserved among eukaryotes, it is likely to play many other essential physiological roles in diverse marine organisms. Elucidating and characterizing basic VHA-dependent mechanisms could help to determine species responses to environmental stress, including (but not limited to) that resulting from climate change.
ABSTRACT
Experimental exposure to ocean and freshwater acidification affects the behaviour of multiple aquatic organisms in laboratory tests. One proposed cause involves an imbalance in plasma ...chloride and bicarbonate ion concentrations as a result of acid–base regulation, causing the reversal of ionic fluxes through GABAA receptors, which leads to altered neuronal function. This model is exclusively based on differential effects of the GABAA receptor antagonist gabazine on control animals and those exposed to elevated CO2. However, direct measurements of actual chloride and bicarbonate concentrations in neurons and their extracellular fluids and of GABAA receptor properties in aquatic organisms are largely lacking. Similarly, very little is known about potential compensatory mechanisms, and about alternative mechanisms that might lead to ocean acidification-induced behavioural changes. This article reviews the current knowledge on acid–base physiology, neurobiology, pharmacology and behaviour in relation to marine CO2-induced acidification, and identifies important topics for future research that will help us to understand the potential effects of predicted levels of aquatic acidification on organisms.
An ongoing loss of experts in marine cellular biochemistry and physiology (CBP) is stagnating the generation of knowledge upon which rapidly growing "omics" approaches rely, ultimately hampering our ...ability to predict organismal responses to climate change.
Identification of a molecular pH sensor in coral Barott, Katie L.; Barron, Megan E.; Tresguerres, Martin
Proceedings - Royal Society. Biological sciences/Proceedings - Royal Society. Biological Sciences,
11/2017, Volume:
284, Issue:
1866
Journal Article
Peer reviewed
Open access
Maintaining stable intracellular pH (pHi) is essential for homeostasis, and requires the ability to both sense pH changes that may result from internal and external sources, and to regulate ...downstream compensatory pH pathways. Here we identified the cAMP-producing enzyme soluble adenylyl cyclase (sAC) as the first molecular pH sensor in corals. sAC protein was detected throughout coral tissues, including those involved in symbiosis and calcification. Application of a sAC-specific inhibitor caused significant and reversible pHi acidosis in isolated coral cells under both dark and light conditions, indicating sAC is essential for sensing and regulating pHi perturbations caused by respiration and photosynthesis. Furthermore, pHi regulation during external acidification was also dependent on sAC activity. Thus, sAC is a sensor and regulator of pH disturbances from both metabolic and external origin in corals. Since sAC is present in all coral cell types, and the cAMP pathway can regulate virtually every aspect of cell physiology through post-translational modifications of proteins, sAC is likely to trigger multiple homeostatic mechanisms in response to pH disturbances. This is also the first evidence that sAC modulates pHi in any non-mammalian animal. Since corals are basal metazoans, our results indicate this function is evolutionarily conserved across animals.
The inner ear is essential for maintaining balance and hearing predator and prey in the environment. Each inner ear contains three CaCO
3
otolith polycrystals, which are calcified within an alkaline, ...K
+
-rich endolymph secreted by the surrounding epithelium. However, the underlying cellular mechanisms are poorly understood, especially in marine fish. Here, we investigated the presence and cellular localization of several ion-transporting proteins within the saccular epithelium of the Pacific Chub Mackerel (
Scomber japonicus
). Western blotting revealed the presence of Na
+
/K
+
-ATPase (NKA), carbonic anhydrase (CA), Na
+
-K
+
-2Cl
−
-co-transporter (NKCC), vacuolar-type H
+
-ATPase (VHA), plasma membrane Ca
2+
ATPase (PMCA), and soluble adenylyl cyclase (sAC). Immunohistochemistry analysis identified two distinct ionocytes types in the saccular epithelium: Type-I ionocytes were mitochondrion-rich and abundantly expressed NKA and NKCC in their basolateral membrane, indicating a role in secreting K
+
into the endolymph. On the other hand, Type-II ionocytes were enriched in cytoplasmic CA and VHA, suggesting they help transport HCO
3
−
into the endolymph and remove H
+
. In addition, both types of ionocytes expressed cytoplasmic PMCA, which is likely involved in Ca
2+
transport and homeostasis, as well as sAC, an evolutionary conserved acid–base sensing enzyme that regulates epithelial ion transport. Furthermore, CA, VHA, and sAC were also expressed within the capillaries that supply blood to the meshwork area, suggesting additional mechanisms that contribute to otolith calcification. This information improves our knowledge about the cellular mechanisms responsible for endolymph ion regulation and otolith formation, and can help understand responses to environmental stressors such as ocean acidification.
Previously, we showed that the evolution of high acuity vision in fishes was directly associated with their unique pH-sensitive hemoglobins that allow O
to be delivered to the retina at PO
s more ...than ten-fold that of arterial blood (Damsgaard et al., 2019). Here, we show strong evidence that vacuolar-type H
-ATPase and plasma-accessible carbonic anhydrase in the vascular structure supplying the retina act together to acidify the red blood cell leading to O
secretion. In vivo data indicate that this pathway primarily affects the oxygenation of the inner retina involved in signal processing and transduction, and that the evolution of this pathway was tightly associated with the morphological expansion of the inner retina. We conclude that this mechanism for retinal oxygenation played a vital role in the adaptive evolution of vision in teleost fishes.
Coral reefs are naturally exposed to daily and seasonal variations in environmental oxygen levels, which can be exacerbated in intensity and duration by anthropogenic activities. However, coral's ...diel oxygen dynamics and fermentative pathways remain poorly understood. Here, continuous oxygen microelectrode recordings in the coral diffusive boundary layer revealed hyperoxia during daytime and hypoxia at nighttime resulting from net photosynthesis and net respiration, respectively. The activities of the metabolic enzymes citrate synthase (CS), malate dehydrogenase, and strombine dehydrogenase remained constant throughout the day/night cycle, suggesting that energy metabolism was regulated through adjustments in metabolite fluxes and not through changes in enzyme abundance. Liquid chromatography-mass spectrometry analyses identified strombine as coral's main fermentative end product. Strombine levels peaked as oxygen became depleted at dusk, indicating increased fermentation rates at the onset of nightly hypoxia, and again at dawn as photosynthesis restored oxygen and photosynthate supply. When these peaks were excluded from the analyses, average strombine levels during the day were nearly double those at night, indicating sifnificant fermentation rates even during aerobic conditions. These results highlight the dynamic changes in oxygen levels in the coral diffusive boundary layer, and the importance of fermentative metabolism for coral biology.
Symbiotic dinoflagellate algae residing inside coral tissues supply the host with the majority of their energy requirements through the translocation of photosynthetically fixed carbon. The algae, in ...turn, rely on the host for the supply of inorganic carbon. Carbon must be concentrated as CO ₂ in order for photosynthesis to proceed, and here we show that the coral host plays an active role in this process. The host-derived symbiosome membrane surrounding the algae abundantly expresses vacuolar H ⁺-ATPase (VHA), which acidifies the symbiosome space down to pH ∼4. Inhibition of VHA results in a significant decrease in average H ⁺ activity in the symbiosome of up to 75% and a significant reduction in O ₂ production rate, a measure of photosynthetic activity. These results suggest that host VHA is part of a previously unidentified carbon concentrating mechanism for algal photosynthesis and provide mechanistic evidence that coral host cells can actively modulate the physiology of their symbionts.
Significance Coral growth and calcification is supported by sugars acquired from symbiotic algae, allowing corals to thrive in otherwise nutrient-poor environments. This symbiosis depends on the coordinated exchange of compounds between partners, the mechanisms of which are poorly understood. Here we found that coral host cells acidify the microenvironment where the symbiotic algae reside using a proton pump, the V-type H ⁺-ATPase (VHA), which is present in the host membrane surrounding the algae. Acidification of the algal microenvironment by VHA promotes photosynthesis, demonstrating that the coral host can actively regulate symbiont physiology. This work is an important step toward understanding how animal symbioses function and provides mechanistic models that can help understand the capacity of corals to adapt to global climate change.
Scopolamine (hyoscine) is a muscarinic acetylcholine receptor antagonist that has traditionally been used to treat motion sickness in humans. However, studies investigating depressed and bipolar ...populations have found that scopolamine is also effective at reducing depression and anxiety symptoms. The potential anxiety-reducing (anxiolytic) effects of scopolamine could have great clinical implications for humans; however, rats and mice administered scopolamine showed increased anxiety in standard behavioural tests. This is in direct contrast to findings in humans, and complicates studies to elucidate the specific mechanisms of scopolamine action. The aim of this study was to assess the suitability of zebrafish as a model system to test anxiety-like compounds using scopolamine. Similar to humans, scopolamine acted as an anxiolytic in individual behavioural tests (novel approach test and novel tank diving test). The anxiolytic effect of scopolamine was dose dependent and biphasic, reaching maximum effect at 800 µM. Scopolamine (800 µM) also had an anxiolytic effect in a group behavioural test, as it significantly decreased their tendency to shoal. These results establish zebrafish as a model organism for studying the anxiolytic effects of scopolamine, its mechanisms of action and side effects.
Ion transport is fundamental for multiple physiological processes, including but not limited to pH regulation, calcification, and photosynthesis. Here, we investigated ion-transporting processes in ...tissues from the corals Acropora yongei and Stylophora pistillata, representatives of the complex and robust clades that diverged over 250 million years ago. Antibodies against complex IV revealed that mitochondria, an essential source of ATP for energetically costly ion transporters, were abundant throughout the tissues of A. yongei. Additionally, transmission electron microscopy revealed septate junctions in all cell layers of A. yongei, as previously reported for S. pistillata, as well as evidence for transcellular vesicular transport in calicoblastic cells. Antibodies against the alpha subunit of Na(+)/K(+)-ATPase (NKA) and plasma membrane Ca(2+)-ATPase (PMCA) immunolabeled cells in the calicoblastic epithelium of both species, suggesting conserved roles in calcification. However, NKA was abundant in the apical membrane of the oral epithelium in A. yongei but not S. pistillata, while PMCA was abundant in the gastroderm of S. pistillata but not A. yongei. These differences indicate that these two coral species utilize distinct pathways to deliver ions to the sites of calcification and photosynthesis. Finally, antibodies against mammalian sodium bicarbonate cotransporters (NBC; SLC4 family) resulted in strong immunostaining in the apical membrane of oral epithelial cells and in calicoblastic cells in A. yongei, a pattern identical to NKA. Characterization of ion transport mechanisms is an essential step toward understanding the cellular mechanisms of coral physiology and will help predict how different coral species respond to environmental stress.
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