Phosphate Fluxes Research Articles

Increase in phosphaturia by inhibition of renal sodium-dependent phosphate cotransporter NPT2a

Numerous mechanisms are involved in maintaining serum phosphate concentration in the normal range, mainly by regulating phosphate absorption in the gut and tubular phosphate reabsorption in the kidney. In the gut, phosphate is absorbed by passive paracellular diffusion and active transcellular transport, mainly via NPT2b, the major sodium phosphate cotransporter in the small intestine.1 Active phosphate absorption is enhanced by 1,25(OH)2 vitamin D. Intestinal phosphate absorption can be reduced by several approaches,1 including reduction of dietary phosphate intake, chelation of phosphate in the gut lumen,2 specific inhibition of NPT2b activity by small-molecule inhibitors,3 nonspecific inhibition of NPT2b by niacin or its derivative nicotinamide,2 and reduction of passive paracellular phosphate flux via inhibition of the sodium/hydrogen exchanger 3.

Thallium diphosphates

AbstractThree thallium(I) diphosphates with compositions Tl4P2O7, Tl2H2P2O7, Tl2H2P2O7(H2O)0.5 and the mixed-valent thallium(I,III) diphosphate Tl2P2O7 (= TlITlIIIP2O7) were obtained from aqueous solutions using an ion-exchange resin (Tl4P2O7), through thermal treatment of TlH2PO4 at 190 °C and subsequent crystallization from aqueous solutions (Tl2H2P2O7, Tl2H2P2O7(H2O)0.5), and from a phosphate flux at 220 °C (Tl2P2O7). The crystal structures of monoclinic Tl4P2O7 (C2/c, Z = 4) and orthorhombic Tl2H2P2O7 (Pbca, Z = 8) are unique, whereas monoclinic Tl2H2P2O7(H2O)0.5 (C2/c, Z = 8) is isotypic with its potassium analogue, and triclinic Tl2P2O7 (P$‾{1}$, Z = 4) crystallizes in the TlInAs2O7 structure type. The crystal structure of Tl2P2O7 is related to that of In2P2O7 (= InIInIIIP2O7; P21/c, Z = 4) by a translationengleiche group-subgroup relationship (t2). IR spectra of Tl4P2O7, Tl2H2P2O7 and Tl2P2O7 are reported and discussed.

Thallium diphosphates

Abstract Three thallium(I) diphosphates with compositions Tl4P2O7, Tl2H2P2O7, Tl2H2P2O7(H2O)0.5 and the mixed-valent thallium(I,III) diphosphate Tl2P2O7 (= TlITlIIIP2O7) were obtained from aqueous solutions using an ion-exchange resin (Tl4P2O7), through thermal treatment of TlH2PO4 at 190 °C and subsequent crystallization from aqueous solutions (Tl2H2P2O7, Tl2H2P2O7(H2O)0.5), and from a phosphate flux at 220 °C (Tl2P2O7). The crystal structures of monoclinic Tl4P2O7 (C2/c, Z = 4) and orthorhombic Tl2H2P2O7 (Pbca, Z = 8) are unique, whereas monoclinic Tl2H2P2O7(H2O)0.5 (C2/c, Z = 8) is isotypic with its potassium analogue, and triclinic Tl2P2O7 (P 1 ‾ $‾{1}$ , Z = 4) crystallizes in the TlInAs2O7 structure type. The crystal structure of Tl2P2O7 is related to that of In2P2O7 (= InIInIIIP2O7; P21/c, Z = 4) by a translationengleiche group-subgroup relationship (t2). IR spectra of Tl4P2O7, Tl2H2P2O7 and Tl2P2O7 are reported and discussed.

Fueling of a marine-terrestrial ecosystem by a major seabird colony

Seabirds redistribute nutrients between different ecosystem compartments and over vast geographical areas. This nutrient transfer may impact both local ecosystems on seabird breeding islands and regional biogeochemical cycling, but these processes are seldom considered in local conservation plans or biogeochemical models. The island of Stora Karlsö in the Baltic Sea hosts the largest concentration of piscivorous seabirds in the region, and also hosts a large colony of insectivorous House martins Delichon urbicum adjacent to the breeding seabirds. We show that a previously reported unusually high insectivore abundance was explained by large amounts of chironomids—highly enriched in δ15N—that feed on seabird residues as larvae along rocky shores to eventually emerge as flying adults. Benthic ammonium and phosphate fluxes were up to 163% and 153% higher close to the colony (1,300 m distance) than further away (2,700 m) and the estimated nutrient release from the seabirds at were in the same order of magnitude as the loads from the largest waste-water treatment plants in the region. The trophic cascade impacting insectivorous passerines and the substantial redistribution of nutrients suggest that seabird nutrient transfer should be increasingly considered in local conservation plans and regional nutrient cycling models.

Benthic-pelagic coupling in the Barents Sea: an integrated data-model framework.

The Barents Sea is experiencing long-term climate-driven changes, e.g. modification in oceanographic conditions and extensive sea ice loss, which can lead to large, yet unquantified disruptions to ecosystem functioning. This key region hosts a large fraction of Arctic primary productivity. However, processes governing benthic and pelagic coupling are not mechanistically understood, limiting our ability to predict the impacts of future perturbations. We combine field observations with a reaction-transport model approach to quantify organic matter (OM) processing and disentangle its drivers. Sedimentary OM reactivity patterns show no gradients relative to sea ice extent, being mostly driven by seafloor spatial heterogeneity. Burial of high reactivity, marine-derived OM is evident at sites influenced by Atlantic Water (AW), whereas low reactivity material is linked to terrestrial inputs on the central shelf. Degradation rates are mainly driven by aerobic respiration (40–75%), being greater at sites where highly reactive material is buried. Similarly, ammonium and phosphate fluxes are greater at those sites. The present-day AW-dominated shelf might represent the future scenario for the entire Barents Sea. Our results represent a baseline systematic understanding of seafloor geochemistry, allowing us to anticipate changes that could be imposed on the pan-Arctic in the future if climate-driven perturbations persist.This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning’.

Interplay between primary familial brain calcification-associated SLC20A2 and XPR1 phosphate transporters requires inositol polyphosphates for control of cellular phosphate homeostasis

Solute carrier family 20 member 2 (SLC20A2) and xenotropic and polytropic retrovirus receptor 1 (XPR1) are transporters with phosphate uptake and efflux functions, respectively. Both are associated with primary familial brain calcification (PFBC), a genetic disease characterized by cerebral calcium-phosphate deposition and associated with neuropsychiatric symptoms. The association of the two transporters with the same disease suggests that they jointly regulate phosphate fluxes and cellular homeostasis, but direct evidence is missing. Here, we found that cross-talk between SLC20A2 and XPR1 regulates phosphate homeostasis, and we identified XPR1 as a key inositol polyphosphate (IP)-dependent regulator of this process. We found that overexpression of WT SLC20A2 increased phosphate uptake, as expected, but also unexpectedly increased phosphate efflux, whereas PFBC-associated SLC20A2 variants did not. Conversely, SLC20A2 depletion decreased phosphate uptake only slightly, most likely compensated for by the related SLC20A1 transporter, but strongly decreased XPR1-mediated phosphate efflux. The SLC20A2-XPR1 axis maintained constant intracellular phosphate and ATP levels, which both increased in XPR1 KO cells. Elevated ATP levels are a hallmark of altered inositol pyrophosphate (PP-IP) synthesis, and basal ATP levels were restored after phosphate efflux rescue with WT XPR1 but not with XPR1 harboring a mutated PP-IP-binding pocket. Accordingly, inositol hexakisphosphate kinase 1-2 (IP6K1-2) gene inactivation or IP6K inhibitor treatment abolished XPR1-mediated phosphate efflux regulation and homeostasis. Our findings unveil an SLC20A2-XPR1 interplay that depends on IPs such as PP-IPs and controls cellular phosphate homeostasis via the efflux route, and alteration of this interplay likely contributes to PFBC.

A Sediment Diagenesis Model of Seasonal Nitrate and Ammonium Flux Spatial Variation Contributing to Eutrophication at Taihu, China.

Algal blooms have thrived on the third-largest shallow lake in China, Taihu over the past decade. Due to the recycling of nutrients such as nitrate and ammonium, this problem has been difficult to eradicate. Sediment flux, a product of diagenesis, explains the recycling of nutrients. The objective was to simulate the seasonal spatial variations of nitrate and ammonium flux. In this paper, sediment diagenesis modeling was applied to Taihu with Environmental Fluid Dynamics Code (EFDC). Latin hypercube sampling was used to create an input file from twelve (12) nitrogen related parameters of sediment diagenesis and incorporated into the EFDC. The results were analyzed under four seasons: summer, autumn, winter, and spring. The concentration of NH4–N in the sediment–water column increased from 2.744903 to 22.38613 (g/m3). In summer, there was an accumulation of ammonium in the water column. In autumn and winter, the sediment was progressively oxidized. In spring, low-oxygen conditions intensify denitrification. This allows algal blooms to continue to thrive, creating a threat to water quality sustainability. The sediment diagenesis model, coupled with water quality measured data, showed an average relative error for Total Nitrogen (TN) of 38.137%, making the model suitable. Future studies should simulate phosphate flux and measure sediment fluxes on the lake.

The Role of Glyceraldehyde-3-Phosphate Dehydrogenases in NADPH Supply in the Oleaginous Filamentous Fungus Mortierella alpina.

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a highly conserved enzyme within the glycolytic pathway. GAPDH catalyzes the transformation of glyceraldehyde 3-phosphate to glycerate-1, 3-biphosphate, a process accompanied by the production of NADH. Its role in the NADPH production system of the oleaginous filamentous fungus Mortierella alpina was explored. Two copies of genes encoding GAPDH were characterized, then endogenously overexpressed and silenced through Agrobacterium tumefaciens-mediated transformation methods. The results showed that the lipid content of the overexpression strain, MA-GAPDH1, increased by around 13%. RNA interference of GAPDH1 and GAPDH2 (MA-RGAPDH1 and MA-RGAPDH2) greatly reduced the biomass of the fungus. The lipid content of MA-RGAPDH2 was found to be about 23% higher than that of the control. Both of the lipid-increasing transformants showed a higher NADPH/NADP ratio. Analysis of metabolite and enzyme expression levels revealed that the increased lipid content of MA-GAPDH1 was due to enhanced flux of glyceraldehyde-3-phosphate to glycerate-1, 3-biphosphate. MA-RGAPDH2 was found to strengthen the metabolic flux of dihydroxyacetone phosphate to glycerol-3-phosphate. Thus, GAPDH1 contributes to NADPH supply and lipid accumulation in M. alpina, and has a distinct role from GAPDH2.

Control of XPR1-dependent cellular phosphate efflux by InsP8 is an exemplar for functionally-exclusive inositol pyrophosphate signaling

Homeostasis of cellular fluxes of inorganic phosphate (Pi) supervises its structural roles in bones and teeth, its pervasive regulation of cellular metabolism, and its functionalization of numerous organic compounds. Cellular Pi efflux is heavily reliant on Xenotropic and Polytropic Retrovirus Receptor 1 (XPR1), regulation of which is largely unknown. We demonstrate specificity of XPR1 regulation by a comparatively uncharacterized member of the inositol pyrophosphate (PP-InsP) signaling family: 1,5-bis-diphosphoinositol 2,3,4,6-tetrakisphosphate (InsP8). XPR1-mediated Pi efflux was inhibited by reducing cellular InsP8 synthesis, either genetically (knockout [KO] of diphosphoinositol pentakisphosphate kinases [PPIP5Ks] that synthesize InsP8) or pharmacologically [cell treatment with 2.5 µM dietary flavonoid or 10 µM N2-(m-trifluorobenzyl), N6-(p-nitrobenzyl) purine], to inhibit inositol hexakisphosphate kinases upstream of PPIP5Ks. Attenuated Pi efflux from PPIP5K KO cells was quantitatively phenocopied by KO of XPR1 itself. Moreover, Pi efflux from PPIP5K KO cells was rescued by restoration of InsP8 levels through transfection of wild-type PPIP5K1; transfection of kinase-dead PPIP5K1 was ineffective. Pi efflux was also rescued in a dose-dependent manner by liposomal delivery of a metabolically resistant methylene bisphosphonate (PCP) analog of InsP8; PCP analogs of other PP-InsP signaling molecules were ineffective. High-affinity binding of InsP8 to the XPR1 N-terminus (Kd = 180 nM) was demonstrated by isothermal titration calorimetry. To derive a cellular biology perspective, we studied biomineralization in the Soas-2 osteosarcoma cell line. KO of PPIP5Ks or XPR1 strongly reduced Pi efflux and accelerated differentiation to the mineralization end point. We propose that catalytically compromising PPIP5K mutations might extend an epistatic repertoire for XPR1 dysregulation, with pathological consequences for bone maintenance and ectopic calcification.

Acanthamoeba castellanii phosphate transporter (AcPHS) is important to maintain inorganic phosphate influx and is related to trophozoite metabolic processes.

Acanthamoeba castellanii is a free-living amoeba and the etiological agent of granulomatous amoebic encephalitis and amoebic keratitis. A. castellanii can be present as trophozoites or cysts. The trophozoite is the vegetative form of the cell and has great infective capacity compared to the cysts, which are the dormant form that protect the cell from environmental changes. Phosphate transporters are a group of proteins that are able to internalize inorganic phosphate from the extracellular to intracellular medium. Plasma membrane phosphate transporters are responsible for maintaining phosphate homeostasis, and in some organisms, regulating cellular growth. The aim of this work was to biochemically characterize the plasma membrane phosphate transporter in A. castellanii and its role in cellular growth and metabolism. To measure inorganic phosphate (Pi) uptake, trophozoites were grown in liquid PYG medium at 28°C for 2days. The phosphate uptake was measured by the rapid filtration of intact cells incubated with 0.5μCi of 32Pi for 1h. The Pi transport was linear as a function of time and exhibited Michaelis-Menten kinetics with a Km = 88.78 ± 6.86μM Pi and Vmax = 547.5 ± 16.9 Pi × h-1 × 10-6 cells. A. castellanii presented linear phosphate uptake up to 1h with a cell density ranging from 1 × 105 to 2 × 106 amoeba × ml-1. The Pi uptake was higher in the acidic pH range than in the alkaline range. The oxygen consumption of living trophozoites increased according to Pi addition to the extracellular medium. When the cells were treated with FCCP, no effect from Pi on the oxygen flow was observed. The addition of increasing Pi concentrations not only increased oxygen consumption but also increased the intracellular ATP pool. These phenomena were abolished when the cells were treated with FCCP or exposed to hypoxia. Together, these results reinforce the hypothesis that Pi is a key nutrient for Acanthamoeba castellanii metabolism.

Seasonal patterns of benthic-pelagic coupling in oyster habitats

Oysters enhance benthic-pelagic coupling in coastal systems by moving large quantities of suspended particulates to the sediments, stimulating biogeochemical processes. Recent research efforts have focused on quantifying the impact of oysters on coastal biogeochemical cycling, yet there is little consensus on how oysters influence processes across systems. A potential driver of this variance is availability of organic material suspended in the water column and subsequent loading to sediment by oysters. Here, we measured fluxes of sediment di-nitrogen (N2-N), ammonium (NH4+), combined nitrate-nitrite (NOx), and phosphate (PO43-) in spring, summer, and fall at 2 oyster reefs and 1 farm in a temperate estuary (Narragansett Bay, Rhode Island). We then linked these fluxes with patterns of water column primary production. Nitrogen removal from the system was highest in spring, when we detected net sediment denitrification (48.8 µmol N2-N m-2 h-1) following a winter-spring diatom bloom. In contrast, we measured sediment N2 fixation in fall (-44.8 µmol N2-N m-2 h-1) at rates nearly equivalent to spring denitrification. In the summer, we measured a nearly net zero sediment N2-N flux (-2.7 µmol N2-N m-2 h-1). Recycling of nitrogen to the water column was consistent across seasons, composed almost exclusively of NH4+. These results demonstrate that sediment nitrogen cycling in oyster habitats is dynamic and can change rapidly based on seasonal patterns of productivity. At carrying capacity, the impact of oysters on nitrogen cycling is large and should be considered during efforts to increase oyster populations through aquaculture or reef restoration.

Distribution of nitrogen and phosphorus and estimation of nutrient fluxes in the water and sediments of Liangzi Lake, China.

Water samples and sediments from Liangzi Lake were taken and used to study the vertical distribution characteristics of nitrogen (N) and phosphorus (P) in both the overlying and pore water. Fluxes of ammonia (NH4+-N) and phosphate (PO43--P) were calculated using a one-dimensional transport-reaction model based on Fick's First Law. The results showed that the mean NH4+-N and NO3--N concentrations in the overlying water of Liangzi Lake were 2.59 and 0.46mgL-1, respectively. The mean PO43--P concentrations were lower than the detection limit. Both N and P displayed peaks at the sediment-water interface. For example, the mean concentration of ammonia in pore water in the surficial layer (0-5cm) was 4.29 ± 2.74mg·L-1, which was twice than that of the overlying water. Two PO43--P vertical profile regimes were identified; one had a gradually increasing trend, while the other first increased and then decreased. The mean orthophosphate concentration in the pore water of the surface layer (0-5cm) was 0.01 ± 0.01mg·L-1. The spatial distribution of ammonia flux values was highly heterogeneous. Using these data, the annual load contribution of autochthonous ammonia was calculated to be 481t a-1. Studying N and P pollution and fluxes in the lakes of urban drinking water sources facilitates the provision of protection measures.

NosP Signaling Modulates the NO/H-NOX-Mediated Multicomponent c-Di-GMP Network and Biofilm Formation in Shewanella oneidensis.

Biofilms form when bacteria aggregate in a self-secreted exopolysaccharide matrix; they are resistant to antibiotics and implicated in disease. Nitric oxide (NO) is known to mediate biofilm formation in many bacteria via ligation to H-NOX (heme-NO/oxygen binding) domains. Most NO-responsive bacteria, however, lack H-NOX domain-containing proteins. We have identified another NO-sensing protein (NosP), which is predicted to be involved in two-component signaling and biofilm regulation in many species. Here, we demonstrate that NosP participates in the previously described H-NOX/NO-responsive multicomponent c-di-GMP signaling network in Shewanella oneidensis. Strains lacking either nosP or its co-cistronic kinase nahK (previously hnoS) produce immature biofilms, while hnoX and hnoK (kinase responsive to NO/H-NOX) mutants result in wild-type biofilm architecture. We demonstrate that NosP regulates the autophosphorylation activity of NahK as well as HnoK. HnoK and NahK have been shown to regulate three response regulators (HnoB, HnoC, and HnoD) that together comprise a NO-responsive multicomponent c-di-GMP signaling network. Here, we propose that NosP/NahK adds regulation on top of H-NOX/HnoK to modulate this c-di-GMP signaling network, and ultimately biofilm formation, by governing the flux of phosphate through both HnoK and NahK. In addition, it appears that NosP and H-NOX act to counter each other in a push-pull mechanism; NosP/NahK promotes biofilm formation through inhibition of H-NOX/HnoK signaling, which itself reduces the extent of biofilm formation. Addition of NO results in a reduction of c-di-GMP and biofilm formation, primarily through disinhibition of HnoK activity.

Contrasting behavior of nitrate and phosphate flux from high flow events on small agricultural and urban watersheds

Soluble nutrient flux is increasingly implicated in the degradation of receiving water quality. With high-frequency sensors, river discharge along with nitrate and orthophosphate concentrations were collected over 2 years. We examined how storms (16 events) affected the dissolved nutrient flux in two watersheds with contrasting landcover—urban and agricultural. The adjacent watersheds, both < 7 km2, had very similar soil, slope, relief, and physiography. Wastewater is conveyed outside of the watersheds minimizing this nutrient source. Cumulative nitrate and orthophosphate fluxes in the agricultural watershed were substantially higher than the urban watershed. In both watersheds, a disproportionate amount of discharge, higher orthophosphate concentrations and flux occurred during the highest flow events—approximately 50–70% of flux occurring in the highest 5% of daily discharge. The nitrate flux was dampened compared to discharge during storm events. Baseflow accounted for 66% of the nitrate flux and had higher or comparable concentrations than storm events. The pattern of flux and concentration within storm events also differed for the two solutes. Nitrate exhibited positive hysteresis (higher concentrations on the rising limb of the hydrograph) and magnification of flux during the first flush of storm runoff. In contrast, orthophosphate concentrations were lower on the rising limb and flux during the first flush of storm runoff was dampened. In addition to targeting source reductions, orthophosphate flux may require runoff reducing strategies (i.e., enhanced infiltration) from large runoff events. First flush stormwater management practices combined with enhancement/protection of groundwater nitrate sinks (e.g., riparian wetlands) may help reduce nitrate fluxes.

Microbial metabolite fluxes in a model marine anoxic ecosystem.

Permanently anoxic regions in the ocean are widespread and exhibit unique microbial metabolic activity exerting substantial influence on global elemental cycles and climate. Reconstructing microbial metabolic activity rates in these regions has been challenging, due to the technical difficulty of direct rate measurements. In Cariaco Basin, which is the largest permanently anoxic marine basin and an important model system for geobiology, long-term monitoring has yielded time series for the concentrations of biologically important compounds; however, the underlying metabolite fluxes remain poorly quantified. Here, we present a computational approach for reconstructing vertical fluxes and in situ net production/consumption rates from chemical concentration data, based on a 1-dimensional time-dependent diffusive transport model that includes adaptive penalization of overfitting. We use this approach to estimate spatiotemporally resolved fluxes of oxygen, nitrate, hydrogen sulfide, ammonium, methane, and phosphate within the sub-euphotic Cariaco Basin water column (depths 150-900m, years 2001-2014) and to identify hotspots of microbial chemolithotrophic activity. Predictions of the fitted models are in excellent agreement with the data and substantially expand our knowledge of the geobiology in Cariaco Basin. In particular, we find that the diffusivity, and consequently fluxes of major reductants such as hydrogen sulfide, and methane, is about two orders of magnitude greater than previously estimated, thus resolving a long-standing apparent conundrum between electron donor fluxes and measured dark carbon assimilation rates.

Nutrient absorption by Ulva prolifera and the growth mechanism leading to green-tides

Nutrient pollution and algal blooms in coastal waters have long been a major concern, and understanding the response of algae to nutrient dynamics is thus essential. The minimum concentration under which Ulva prolifera will not grow, estimated from a nutrient-alga coupling model developed in this study, was 6.5 μmol L−1 for nitrate and 0.27 μmol L−1 for phosphate. The model was parameterized with cultural experiments to examine the effects of nitrate and phosphate on the growth of Ulva prolifera and the development of green tide. The cultural experiments showed that flux of nitrate absorbed by Ulva prolifera increased significantly with the concentration of nitrate or phosphate, which followed the Michaelis-Menten equation, while the flux of phosphate linearly increased with its concentrations. Nitrate concentrations at >25 μmol L−1 had little influence on the absorption of phosphate, but compensating absorption of phosphate occurred when the nitrate concentration was below 15 μmol L−1. Phosphate concentrations had a remarkable enhancement to the absorption of nitrate when phosphate concentration was <1.4 μmol L−1. The absorption rates of nitrate and phosphate by Ulva prolifera followed the same variation trend as nutrient absorption fluxes. The increase in nitrate absorption rates has a limited impact on the relative growth rate of Ulva prolifera, while the increase in phosphate absorption rates has a marked impact on the relative growth rate. Our results provide new insights into the growth mechanism of Ulva prolifera and the development and evolution of green tides in coastal marine environments. Applications of our model should help governmental agencies in environmental remediation and policy making.

The inositol hexakisphosphate kinases IP6K1 and -2 regulate human cellular phosphate homeostasis, including XPR1-mediated phosphate export

Phosphate's central role in most biochemical reactions in a living organism requires carefully maintained homeostasis. Although phosphate homeostasis in mammals has long been studied at the organismal level, the intracellular mechanisms controlling phosphate metabolism are not well-understood. Inositol pyrophosphates have emerged as important regulatory elements controlling yeast phosphate homeostasis. To verify whether inositol pyrophosphates also regulate mammalian cellular phosphate homeostasis, here we knocked out inositol hexakisphosphate kinase (IP6K) 1 and IP6K2 to generate human HCT116 cells devoid of any inositol pyrophosphates. Using PAGE and HPLC analysis, we observed that the IP6K1/2-knockout cells have nondetectable levels of the IP6-derived IP7 and IP8 and also exhibit reduced synthesis of the IP5-derived PP-IP4. Nucleotide analysis showed that the knockout cells contain increased amounts of ATP, whereas the Malachite green assay found elevated levels of free intracellular phosphate. Furthermore, [32Pi] pulse labeling experiments uncovered alterations in phosphate flux, with both import and export of phosphate being decreased in the knockout cells. Functional analysis of the phosphate exporter xenotropic and polytropic retrovirus receptor 1 (XPR1) revealed that it is regulated by inositol pyrophosphates, which can bind to its SPX domain. We conclude that IP6K1 and -2 together control inositol pyrophosphate metabolism and thereby physiologically regulate phosphate export and other aspects of mammalian cellular phosphate homeostasis.

Paracellular transport of phosphate along the intestine.

Inorganic phosphate (Pi) is crucial for many biological functions, such as energy metabolism, signal transduction, and pH buffering. Efficient systems must exist to ensure sufficient supply for the body of Pi from diet. Previous experiments in humans and rodents suggest that two pathways for the absorption of Pi exist, an active transcellular Pi transport and a second paracellular pathway. Whereas the identity, role, and regulation of active Pi transport have been extensively studied, much less is known about the properties of the paracellular pathway. In Ussing chamber experiments, we characterized paracellular intestinal Pi permeabilities and fluxes. Dilution potential measurements in intestinal cell culture models demonstrated that the tight junction is permeable to Pi, with monovalent Pi having a higher permeability than divalent Pi. These findings were confirmed in rat and mouse intestinal segments by use of Ussing chambers and a combination of dilution potential measurements and fluxes of radiolabeled 32Pi. Both techniques yielded very similar results, showing that paracellular Pi fluxes were bidirectional and that Pi permeability was ~50% of the permeability for Na+ or Cl-. Pi fluxes were a function of the concentration gradient and Pi species (mono- vs. divalent Pi). In mice lacking the active transcellular Pi transport component sodium-dependent Pi transporter NaPi-IIb, the paracellular pathway was not upregulated. In summary, the small and large intestines have a very high paracellular Pi permeability, which may favor monovalent Pi fluxes and allow efficient uptake of Pi even in the absence of active transcellular Pi uptake.NEW & NOTEWORTHY The paracellular permeability for phosphate is high along the entire axis of the small and large intestine. There is a slight preference for monovalent phosphate. Paracellular phosphate fluxes do not increase when transcellular phosphate transport is genetically abolished. Paracellular phosphate transport may be an important target for therapies aiming to reduce intestinal phosphate absorption.

285-LB: Short-Term Effect of Saturated and Monounsaturated Fatty Acids on Hepatic Energy Metabolism

Nonalcoholic fatty liver disease (NAFLD) is the most common form of liver disease affecting about 30% of the U.S. population. The prognosis for simple liver steatosis is relatively benign; however, chronic inflammation and progressive fibrosis may lead to nonalcoholic steatohepatitis (NASH) and later to cirrhosis and hepatocellular carcinoma. There is no established treatment to stop progression from NAFLD to NASH to date. Therefore, we developed a protocol to investigate the acute impact of saturated (SFA) and monounsaturated (MUFA) fatty acids on liver energy metabolism and their potential lipotoxic effects. Conscious, unrestrained, overnight fasted 12 weeks old male C57Bl/6J mice were given continuous, intravenous infusions of either saline, 20% lard oil or 20% olive oil emulsions (as described in Stein DT et al. J Clin Invest. 1997; 100 (2): 398-403) for 5 hrs. Lipid infusion was accompanied by primed, continuous infusion of [6,6-2H2] glucose (16mg/kg/min for 5 min and 0.8 mg/kg/min continuously) and sodium [13C3]lactate (200 μM/kg/min 5 min and 40 μM/kg/min continuously) intravenously for 4 and 2 hrs, respectively. Both plasma and liver free fatty acids were upregulated in SFA and MUFA groups. However, liver triglycerides, alanine aminotransferase (ALT) and aspartate transaminase (AST) were significantly increased, whereas plasma insulin and glucose levels significantly decreased, only in the MUFA group. 13C and 2H-enrichment of plasma glucose and liver alanine, glutamate, lactate and urea obtained at the end of the study were measured by GC-MS and used to regress liver metabolic fluxes using a flexible flux modeling platform (INCA). We observed that both SFA and MUFA increased the absolute rate of dihydroxyacetone phosphate (DHAP) flux from glycerol. Absolute flux of acetyl-CoA production was shifted from synthesis through pyruvate dehydrogenase toward increased contribution from fatty acid oxidation. On the other hand, only MUFA decreased the gluconeogenesis rate. Disclosure T. Bednarski: None. M. Rahim: None. J. Young: Consultant; Self; Pfizer Inc. Stock/Shareholder; Self; Metalytics. Funding National Institutes of Health (R01DK106348)

Benthic Respiration and Nutrient Release Associated with Net Cage Fish and Longline Oyster Aquaculture in the Geoje-Tongyeong Coastal Waters in Korea

Sediment oxygen demand (SOD) and benthic nutrient fluxes were measured at five sites using an in situ benthic chamber: fish farm (FF), oyster farm (OF), Tongyeong Marine Science Station (TMSS), and two controls (FF-C and OF-C) June 27–July 1, 2017, to assess the impact of aquaculture on coastal waters of southern Korea. The net oxygen production rate of the epiphyte community attached to farming facilities was also measured using an oxygen microsensor to quantify the potential contribution of organic matter to bottom sediment. The SODs at the FF and OF were − 155.2 ± 0.2 m−2 day−1 and − 88.6 ± 0.1 m−2 day−1 respectively, more than twice those of the control sites (FF-C and OF-C). The benthic fluxes of ammonium and phosphate at the fish farm were 12.7 mmol m−2 day−1 and 3.3 mmol m−2 day−1, respectively, which were ~ 10 times higher than those of the control sites and ~ 1.5 times higher than at the OF site. The benthic N and P fluxes accounted for 128% and 501%, respectively, at FF and 89% and 70% of N and P requirements, respectively, at OF for primary production. This suggests that aquaculture activities result in an excess supply of nutrients to the water column, thereby inducing eutrophication in the coastal ecosystem. Potentially, the net production of organic carbon by the epiphyte community yielded about 65 mmol C m−2 day−1, which suggests that organic matter produced by epiphytic communities substantially contributed to benthic respiration. Thus, the commercial finfish and oyster farms increased SOD and benthic nutrient flux, but at a conservatively managed aquaculture (TMSS), there were minimal benthic impacts.