A novel aquaporin 3 in killifish (Fundulus heteroclitus) is not an arsenic channel

Endocrine control of gill ionocyte function in euryhaline fishes

The endocrine system is an essential regulator of the osmoregulatory organs that enable euryhaline fishes to maintain hydromineral balance in a broad range of environmental salinities. Because branchial ionocytes are the primary site for the active exchange of Na+, Cl-, and Ca2+ with the external environment, their functional regulation is inextricably linked with adaptive responses to changes in salinity. Here, we review the molecular-level processes that connect osmoregulatory hormones with branchial ion transport. We focus on how factors such as prolactin, growth hormone, cortisol, and insulin-like growth-factors operate through their cognate receptors to direct the expression of specific ion transporters/channels, Na+/K+-ATPases, tight-junction proteins, and aquaporins in ion-absorptive (freshwater-type) and ion-secretory (seawater-type) ionocytes. While these connections have historically been deduced in teleost models, more recently, increased attention has been given to understanding the nature of these connections in basal lineages. We conclude our review by proposing areas for future investigation that aim to fill gaps in the collective understanding of how hormonal signaling underlies ionocyte-based processes.

The dynamic changes of arsenic bioaccumulation and antioxidant responses in the marine medaka Oryzias melastigma during chronic exposure

Arsenic (As) is a highly toxic metalloid to aquatic organisms, but the effects of low-dose chronic inorganic As exposure on marine fish are still unclear. A 28-day study was conducted on chronic exposure of 100 μg/L inorganic As [As(III) and As(V)] in the marine medaka Oryzias melastigma to quantify the effects of chronic inorganic As exposure on its bioaccumulation, biotransformation, oxidative stress, and antioxidant enzymes response. During the exposure period, chronic inorganic As exposure had no significant effect on the total As bioaccumulation except at 7 d for As(V) treatment. Based on the toxicokinetic data, the low As bioaccumulation was caused by low dissolved uptake (k_u), internal transfer (k₁₂, k₂₁) and high efflux (k_e1). The organic As were the predominated species (77 - 80%) and remained constant, while the inorganic As increased 35% (from 0.26 μg/g to 0.35 μg/g) during the initial 4-d exposure and then recovered. The increased inorganic As could be covered by the high contribution of organic As especially arsenobetaine (AsB) to the total As accumulation. Coincidently, the malondialdehyde (MDA) levels followed similar trends as the inorganic As concentrations, indicating that inorganic As bioaccumulation induced oxidative stress at the initial stage. Both the contents of the nonenzymatic antioxidant (glutathione, GSH) and the activities of the enzymatic antioxidants (superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and glutathione-S-transferase (GST)) increased initially and then decreased as the inorganic As concentrations, thus lowering the hydrogen peroxide (H₂O₂) levels and displaying a typical antioxidant defense mechanism. In summary, this study elucidated that although the marine medaka had a limited ability to accumulate waterborne As, the increase in the inorganic As at the early stage of exposure still caused toxic effects, which could be ignored by constant total As concentrations. Therefore, the toxicity of As could be underestimated if only the total As instead of the inorganic As is monitored in marine fish.

Arsenic Reduces Gene Expression Response to Changing Salinity in Killifish

Toxicogenomic approaches can detect and classify adverse interactions between environmental toxicants and other environmental stressors but require more complex experimental designs and analytical approaches. Here we use novel toxicogenomic techniques to analyze the effect of arsenic exposure in wild killifish populations acclimating to changing salinity. Fish from three populations were acclimated to full strength seawater and transferred to fresh water for 1 or 24 h. Linear models of gene expression in gill tissue identified 31 genes that responded to osmotic shock at 1 h and 178 genes that responded at 24 h. Arsenic exposure (100 μg/L) diminished the responses (reaction norms) of these genes by 22% at 1 h ( p = 1.0 × 10^-6) and by 10% at 24 h ( p = 3.0 × 10^-10). Arsenic also significantly reduced gene coregulation in gene regulatory networks ( p = 0.002, paired Levene's test), and interactions between arsenic and salinity acclimation were uniformly antagonistic at the biological pathway level ( p < 0.05, binomial test). Arsenic's systematic interference with gene expression reaction norms was validated in a mouse multistressor experiment, demonstrating the ability of these toxicogenomic approaches to identify biologically relevant adverse interactions between environmental toxicants and other environmental stressors.

Arsenic-phosphorus interactions in the soil-plant-microbe system: Dynamics of uptake, suppression and toxicity to plants

High arsenic (As) concentrations in the soil, water and plant systems can pose a direct health risk to humans and ecosystems. Phosphate (Pi) ions strongly influence As availability in soil, its uptake and toxicity to plants. Better understanding of As(V)-Pi interactions in soils and plants will facilitate a potential remediation strategy for As contaminated soils, reducing As uptake by crop plants and toxicity to human populations via manipulation of soil Pi content. However, the As(V)-Pi interactions in soil-plant systems are complex, leading to contradictory findings among different studies. Therefore, this review investigates the role of soil type, soil properties, minerals, Pi levels in soil and plant, Pi transporters, mycorrhizal association and microbial activities on As-Pi interactions in soils and hydroponics, and uptake by plants, elucidate the key mechanisms, identify key knowledge gaps and recommend new research directions. Although Pi suppresses As uptake by plants in hydroponic systems, in soils it could either increase or decrease As availability and toxicity to plants depending on the soil types, properties and charge characteristics. In soil, As(V) availability is typically increased by the addition of Pi. At the root surface, the Pi transport system has high affinity for Pi over As(V). However, Pi concentration in plant influences the As transport from roots to shoots. Mycorrhizal association may reduce As uptake via a physiological shift to the mycorrhizal uptake pathway, which has a greater affinity for Pi over As(V) than the root epidermal uptake pathway.

New Molecular Mechanisms to Reduce Arsenic in Crops

Arsenic is toxic to all life forms and is a potent carcinogen. Its accumulation in crop plants and subsequent consumption poses a serious threat to public health worldwide. Recent developments have enhanced our understanding of the molecular mechanisms governing arsenic uptake, detoxification, and accumulation in plants. In particular, the identification of plant arsenate reductase enzymes and emerging details of the processes underlying arsenic distribution and deposition in the seed will prove invaluable in the development of new strategies to mitigate this threat. Here we provide an outline of these recent developments and suggest new molecular mechanisms that could be employed to reduce arsenic in crops.

Uptake of antimonite and antimonate by arsenic hyperaccumulator Pteris vittata: Effects of chemical analogs and transporter inhibitor

Antimonite (SbIII) is transported into plants via aquaglyceroporin channels but it is unknown in As-hyperaccumulator Ptreis vittata (PV). We tested the effects of SbIII analogs (arsenite-AsIII, glycerol, silicic acid-Si, and, glucose), antimonate (SbV) analog (phosphate-P), and aquaglyceroporin transporter inhibitor (silver, Ag) on the uptake of SbIII or SbV by PV gametophytes. PV gametophytes were grown in 20% Hoagland solution containing 65 μM SbIII or SbV and increasing concentrations of analogs at 65-6500 μM for 2 h or 4 h under sterile condition. After exposing to 65 μM Sb for 2 h, PV accumulated 767 mg/kg Sb in SbIII treatment and 419 mg/kg in SbV treatment. SbIII uptake by PV gametophytes was not impacted by glycerol or AsIII nor aquaglyceroporin inhibitor Ag during 2 h exposure. While Si increased SbIII uptake and glucose decreased SbIII uptake by PV gametophytes, the impact disappeared during 4 h exposure. Under P-sufficient condition, P increased SbIII uptake and decreased SbV uptake during 2 h exposure, but the effect again disappeared after 4 h. After being P-starved for 2 weeks, P decreased SbIII with no effect on SbV uptake during 2 h exposure. Our results indicated that: 1) PV gametophytes could serve as an efficient model to study Sb uptake, and 2) unique SbIII uptake by PV may be related to its trait of As hyperaccumulation.

Aquaporin 1 Is Involved in Acid Secretion by Ionocytes of Zebrafish Embryos through Facilitating CO2 Transport

Mammalian aquaporin 1 (AQP1) is well known to function as a membrane channel for H₂O and CO₂ transport. Zebrafish AQP1a.1 (the homologue of mammalian AQP1) was recently identified in ionocytes of embryos; however its role in ionocytes is still unclear. In this study, we hypothesized that zebrafish AQP1a.1 is involved in the acid secretion by ionocytes through facilitating H₂O and CO₂ diffusion. A real-time PCR showed that mRNA levels of AQP1a.1 in embryos were induced by exposure to 1% CO₂ hypercapnia for 3 days. In situ hybridization and immunohistochemistry showed that the AQP1a.1 transcript was highly expressed by acid-secreting ionocytes, i.e., H⁺-ATPase-rich (HR) cells. A scanning ion-selective electrode technique (SIET) was applied to analyze CO₂-induced H⁺ secretion by individual ionocytes in embryos. H⁺ secretion by HR cells remarkably increased after a transient loading of CO₂ (1% for 10 min). AQP1a.1 knockdown with morpholino oligonucleotides decreased the H⁺ secretion of HR cells by about half and limited the CO₂ stimulated increase. In addition, exposure to an AQP inhibitor (PCMB) for 10 min also suppressed CO₂-induced H⁺ secretion. Results from this study support our hypothesis and provide in vivo evidence of the physiological role of AQP1 in CO₂ transport.

A novel variant of aquaporin 3 is expressed in killifish (Fundulus heteroclitus) intestine

Killifish (Fundulus heteroclitus) are euryhaline teleosts that are widely used in environmental and toxicological studies, and they are tolerant to arsenic, in part due to very low assimilation of arsenic from the environment. The mechanism of arsenic uptake by the intestine, a major route of arsenic uptake in humans is unknown. Thus, the goal of this study was to determine if aquaglyceroporins (AQPs), which transport water and other small molecules including arsenite across cell membranes, are expressed in the killifish intestine, and whether AQP expression is affected by osmotic stress. Through RT-PCR and sequence analysis of PCR amplicons, we demonstrated that the intestine expresses kfAQP3a and kfAQP3b, two previously identified variants, and also identified a novel variant of killifish AQP3 (kfAQP3c) in the intestine. The variants likely represent alternate splice forms. A BLAST search of the F. heteroclitus reference genome revealed that the AQP3 gene resides on a single locus, while an alignment of the AQP3 sequence among 384 individuals from eight population ranging from Rhode Island to North Carolina revealed that its coding sequence was remarkably conserved with no fixed polymorphism residing in the region that distinguishes these variants. We further demonstrate that the novel variant transports arsenite into HEK293T cells. Whereas kfAQP3a, which does not transport arsenite, was expressed in both freshwater (FW) and saltwater (SW) acclimated fish, kfAQP3b, an arsenic transporter, was expressed only in FW acclimated fish, and kfAQP3c was expressed only in SW acclimated fish. Thus, we have identified a novel, putative splice variant of kfAQP3, kfAQP3c, which transports arsenic and is expressed only in SW acclimated fish.