Permeability and the mechanism of transport of boric acid across the plasma membrane of Xenopus laevis oocytes

Drosophila melanogaster as a Model Organism to Study Lithium and Boron Bioactivity

The fruit fly Drosophila melanogaster has become a valuable model organism in nutritional science, which can be applied to elucidate the physiology and the biological function of nutrients, including trace elements. Importantly, the application of chemically defined diets enables the supply of trace elements for nutritional studies under highly standardized dietary conditions. Thus, the bioavailability and bioactivity of trace elements can be systematically monitored in D. melanogaster. Numerous studies have already revealed that central aspects of trace element homeostasis are evolutionary conserved among the fruit fly and mammalian species. While there is sufficient evidence of vital functions of boron (B) in plants, there is also evidence regarding its bioactivity in animals and humans. Lithium (Li) is well known for its role in the therapy of bipolar disorder. Furthermore, recent findings suggest beneficial effects of Li regarding neuroprotection as well as healthy ageing and longevity in D. melanogaster. However, no specific essential function in the animal kingdom has been found for either of the two elements so far. Here, we summarize the current knowledge of Li and B bioactivity in D. melanogaster in the context of health and disease prevention.

Involvement of nitric oxide (NO) in plant responses to metalloids

Plants respond to the limited or excess supply of metalloids, boron (B), silicon (Si), selenium (Se), arsenic (As), and antimony (Sb) via complex signaling pathways that are mainly regulated by nitric oxide (NO). The absorption of metalloids from the soil is facilitated by pathways that involve aquaporins, aquaglyceroporins, phosphate, and sulfate transporters; however, their regulation by NO is poorly understood. Using in silico software, we predicted the S-nitrosation of known metalloid transporters, proposing NO-dependent regulation of metalloid transport systems at the posttranslational level. NO intensifies the stress-mitigating effect of Si, whereas in the case of Se, As, and Sb, the accumulation of NO or reactive nitrogen species contributes to toxicity. NO promotes the beneficial effect of low Se concentrations and mitigates the damage caused by B deficiency. In addition, the exogenous application of NO donor, sodium nitroprusside, reduces B, Se, and As toxicity. The primary role of NO in metalloid stress response is to mitigate oxidative stress by activating antioxidant defense at the level of protein activity and gene expression. This review discusses the role of NO in plant responses to metalloids and suggests future research directions.

Radiobiological response of U251MG, CHO-K1 and V79 cell lines to accelerator-based boron neutron capture therapy

In the current article, we provide in vitro efficacy evaluation of a unique accelerator-based neutron source, constructed at the Budker Institute of Nuclear Physics (Novosibirsk, Russian Federation), for boron neutron capture therapy (BNCT), which is particularly effective in the case of invasive cancers. U251MG, CHO-K1 and V79 cells were incubated and irradiated in various concentrations of boric acid with epithermal neutrons for 2-3 h in a plexiglass phantom, using 2.0 MeV proton energy and 1.5-3.0 mA proton current, resulting in a neutron fluence of 2.16 × 1012 cm-2. The survival curves of cells loaded with boron were normalized to those irradiated without boron (to exclude the influence of the fast neutron and gamma dose components) and fit to the linear-quadratic (LQ) model. Colony formation assays showed the following cell survival rates (means ± SDs): CHO-K1: 0.348 ± 0.069 (10 ppm), 0.058 ± 0.017 (20 ppm), 0.018 ± 0.005 (40 ppm); V79: 0.476 ± 0.160 (10 ppm), 0.346 ± 0.053 (20 ppm), 0.078 ± 0.015 (40 ppm); and U251MG: 0.311 ± 0.061 (10 ppm), 0.131 ± 0.022 (20 ppm), 0.020 ± 0.010 (40 ppm). The difference between treated cells and controls was significant in all cases (P < 0.01) and confirmed that the neutron source and irradiation regimen were sufficient for control over cell colony formation. We believe our study will serve as a model for ongoing in vitro experiments on neutron capture therapy to advance in this area for further development of accelerator-based BNCT into the clinical phase.

Heterotetramerization of Plant PIP1 and PIP2 Aquaporins Is an Evolutionary Ancient Feature to Guide PIP1 Plasma Membrane Localization and Function

Aquaporins (AQPs) are tetrameric channel proteins regulating the transmembrane flux of small uncharged solutes and in particular water in living organisms. In plants, members of the plasma membrane intrinsic protein (PIP) AQP subfamily are important for the maintenance of the plant water status through the control of cell and tissue hydraulics. The PIP subfamily is subdivided into two groups: PIP1 and PIP2 that exhibit different water-channel activities when expressed in Xenopus oocytes or yeast cells. Most PIP1 and PIP2 isoforms physically interact and assemble in heterotetramers to modulate their subcellular localization and channel activity when they are co-expressed in oocytes, yeasts, and plants. Whether the interaction between different PIPs is stochastic or controlled by cell regulatory processes is still unknown. Here, we analyzed the water transport activity and the subcellular localization behavior of the complete PIP subfamily (SmPIP1;1, SmPIP2;1, and SmPIP2;2) of the lycophyte Selaginella moellendorffii upon (co-)expression in yeast and Xenopus oocytes. As observed for most of the PIP1 and PIP2 isoforms in other species, SmPIP1;1 was retained in the ER while SmPIP2;1 was found in the plasma membrane but, upon co-expression, both isoforms were found in the plasma membrane, leading to a synergistic effect on the water membrane permeability. SmPIP2;2 behaves as a PIP1, being retained in the endoplasmic reticulum when expressed alone in oocytes or in yeasts. Interestingly, in contrast to the oocyte system, in yeasts no synergistic effect on the membrane permeability was observed upon SmPIP1;1/SmPIP2;1 co-expression. We also demonstrated that SmPIP2;1 is permeable to water and the signaling molecule hydrogen peroxide. Moreover, growth- and complementation assays in the yeast system showed that heteromerization in all possible SmPIP combinations did not modify the substrate specificity of the channels. These results suggest that the characteristics known for angiosperm PIP1 and PIP2 isoforms in terms of their water transport activity, trafficking, and interaction emerged already as early as in non-seed vascular plants. The existence and conservation of these characteristics may argue for the fact that PIP2s are indeed involved in the delivery of PIP1s to the plasma membrane and that the formation of functional heterotetramers is of biological relevance.

The Combined Action of Duplicated Boron Transporters Is Required for Maize Growth in Boron-Deficient Conditions

The micronutrient boron is essential in maintaining the structure of plant cell walls and is critical for high yields in crop species. Boron can move into plants by diffusion or by active and facilitated transport mechanisms. We recently showed that mutations in the maize boron efflux transporter ROTTEN EAR (RTE) cause severe developmental defects and sterility. RTE is part of a small gene family containing five additional members (RTE2-RTE6) that show tissue-specific expression. The close paralogous gene RTE2 encodes a protein with 95% amino acid identity with RTE and is similarly expressed in shoot and root cells surrounding the vasculature. Despite sharing a similar function with RTE, mutations in the RTE2 gene do not cause growth defects in the shoot, even in boron-deficient conditions. However, rte2 mutants strongly enhance the rte phenotype in soils with low boron content, producing shorter plants that fail to form all reproductive structures. The joint action of RTE and RTE2 is also required in root development. These defects can be fully complemented by supplying boric acid, suggesting that diffusion or additional transport mechanisms overcome active boron transport deficiencies in the presence of an excess of boron. Overall, these results suggest that RTE2 and RTE function are essential for maize shoot and root growth in boron-deficient conditions.

Corals concentrate dissolved inorganic carbon to facilitate calcification

The sources of dissolved inorganic carbon (DIC) used to produce scleractinian coral skeletons are not understood. Yet this knowledge is essential for understanding coral biomineralization and assessing the potential impacts of ocean acidification on coral reefs. Here we use skeletal boron geochemistry to reconstruct the DIC chemistry of the fluid used for coral calcification. We show that corals concentrate DIC at the calcification site substantially above seawater values and that bicarbonate contributes a significant amount of the DIC pool used to build the skeleton. Corals actively increase the pH of the calcification fluid, decreasing the proportion of DIC present as CO2 and creating a diffusion gradient favouring the transport of molecular CO2 from the overlying coral tissue into the calcification site. Coupling the increases in calcification fluid pH and [DIC] yields high calcification fluid [CO3(2-)] and induces high aragonite saturation states, favourable to the precipitation of the skeleton.

SLC4A11 prevents osmotic imbalance leading to corneal endothelial dystrophy, deafness, and polyuria

Maintenance of ion concentration gradients is essential for the function of many organs, including the kidney, the cornea, and the inner ear. Ion concentrations and fluid content in the cornea are regulated by endothelial cells that separate the collagenous avascular corneal stroma from the anterior eye chamber. Failure to maintain correct ion concentrations leads to swelling and destruction of the cornea. In the inner ear, the stria vascularis is responsible for generating proper ion concentrations in the endolymph, which is essential for hearing. Mutations of SLC4A11 in humans lead to syndromes associated with corneal dystrophy and perceptive deafness. The molecular mechanisms underlying these symptoms are poorly understood, impeding therapeutic interventions. The ion transporter SLC4A11 mediates sodium-dependent transport of borate as well as flux of sodium and hydroxyl ions in vitro. Here, we show that SLC4A11 is expressed in the endothelial cells of the cornea where it prevents severe morphological changes of the cornea caused by increased sodium chloride concentrations in the stroma. In the inner ear, SLC4A11 is located in fibrocytes underlying the stria vascularis. Loss of SLC4A11 leads to morphological changes in the fibrocytes and deafness. We demonstrate that SLC4A11 is essential for the generation of the endocochlear potential but not for regulation of potassium concentrations in the endolymph. In the kidney, SLC4A11 is expressed in the thin descending limb of Henle loop. SLC4A11 is essential for urinary concentration, suggesting that SLC4A11 participates in the countercurrent multiplication that concentrates urine in the kidney medulla.

Boric acid inhibits stored Ca2+ release in DU-145 prostate cancer cells

Boron (B) is a developmental and reproductive toxin. It is also essential for some organisms. Plants use uptake and efflux transport proteins to maintain homeostasis, and in humans, boron has been reported to reduce prostate cancer. Ca2+ signaling is one of the primary mechanisms used by cells to respond to their environment. In this paper, we report that boric acid (BA) inhibits NAD+ and NADP+ as well as mechanically induced release of stored Ca2+ in growing DU-145 prostate cancer cells. Cell proliferation was inhibited by 30% at 100 microM, 60% at 250 microM, and 97% at 1,000 microM BA. NAD+-induced Ca2+ transients were partly inhibited at 250 microM BA and completely at 1,000 microM BA, whereas both NADP+ and mechanically induced transients were inhibited by 1,000 microM BA. Expression of CD38 protein increased in proportion to BA exposure (0-1,000 microM). In vitro mass spectrometry analysis showed that BA formed adducts with the CD38 products and Ca2+ channel agonists cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). Vesicles positive for the Ca2+ fluorophore fluo-3 acetoxymethyl ester accumulated in cells exposed to 250 and 1,000 microM BA. The BA analog, methylboronic acid (MBA; 250 and 1,000 microM), did not inhibit cell proliferation or NAD+, NADP+, or mechanically stimulated Ca2+ store release. Nor did MBA increase CD38 expression or cause the formation of intracellular vesicles. Thus, mammalian cells can distinguish between BA and its synthetic analog MBA and exhibit graded concentration-dependent responses. Based on these observations, we hypothesize that toxicity of BA stems from the ability of high concentrations to impair Ca2+ signaling.

Arabidopsis boron transporter for xylem loading

Boron deficiency hampers the productivity of 132 crops in more than 80 countries. Boron is essential in higher plants primarily for maintaining the integrity of cell walls and is also beneficial and might be essential in animals and in yeast. Understanding the molecular mechanism(s) of boron transport is crucial for alleviating boron deficiency. Here we describe the molecular identification of boron transporters in biological systems. The Arabidopsis thaliana mutant bor1-1 is sensitive to boron deficiency. Uptake studies indicated that xylem loading is the key step for boron accumulation in shoots with a low external boron supply and that the bor1-1 mutant is defective in this process. Positional cloning identified BOR1 as a membrane protein with homology to bicarbonate transporters in animals. Moreover, a fusion protein of BOR1 and green fluorescent protein (GFP) localized to the plasma membrane in transformed cells. The promoter of BOR1 drove GFP expression in root pericycle cells. When expressed in yeast, BOR1 decreased boron concentrations in cells. We show here that BOR1 is an efflux-type boron transporter for xylem loading and is essential for protecting shoots from boron deficiency.