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Boric acid transport activity of human aquaporins expressed in Xenopus oocytes.

ABSTRACT: Boric acid is a vital micronutrient that is toxic at high concentrations in animals. However, the mechanisms underlying boric acid transport in animal cells remain unclear. To identify the plasma membrane boric acid channels in animals, we analyzed the function of human aquaporins (AQPs), which are homologous to the nodulin-like intrinsic protein family of plant boric acid channels. When human AQPs were expressed in Xenopus laevis oocytes, the results of the swelling assay showed that boric acid permeability significantly increased in oocytes expressing AQP3, 7, 8, 9, and 10, but not in those expressing AQP1, 2, 4, and 5. The boric acid influxes of these oocytes were also confirmed by elemental quantification. Electrophysiological analysis using a pH microelectrode showed that these AQPs transported boric acid (B(OH)₃ ) but not borate ions (B(OH)₄^- ). These results indicate that AQP3, 7, 8, 9, and 10 act as boric acid transport systems, likely as channels in humans.

SUBMITTER: Ushio K

PROVIDER: S-EPMC8749175 | biostudies-literature | 2022 Jan

REPOSITORIES: biostudies-literature

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Boric acid transport activity of human aquaporins expressed in Xenopus oocytes.

Ushio Kazutaka K Watanabe Erika E Kamiya Takehiro T Nagashima Ayumi A Furuta Tadaomi T Imaizumi Genki G Fujiwara Toru T Romero Michael F MF Kato Akira A

Physiological reports 20220101 1

Boric acid is a vital micronutrient that is toxic at high concentrations in animals. However, the mechanisms underlying boric acid transport in animal cells remain unclear. To identify the plasma membrane boric acid channels in animals, we analyzed the function of human aquaporins (AQPs), which are homologous to the nodulin-like intrinsic protein family of plant boric acid channels. When human AQPs were expressed in Xenopus laevis oocytes, the results of the swelling assay showed that boric acid ...[more]

PMID: 35014212

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Project description:The mammalian nervous system expresses proton-gated ion channels known as acid-sensing ion channels (ASICs). Depending on their location and specialization some neurons express more than one type of ASIC where they may form homo- or heteromeric channels. Macroscopic characteristics of the ASIC currents have been described, but little is known at the single channel level. Here, we have examined the properties of unitary currents of homomeric rat ASIC1alpha, ASIC2a, and ASIC3 expressed in Xenopus oocytes with the patch clamp technique. We describe and characterize properties unique to each of these channels that can be used to distinguish the various types of ASIC channels expressed in mammalian neurons. The amplitudes of the unitary currents in symmetrical Na(+) are similar for the three types of channels (23-18 pS) and are not voltage dependent. However, ASIC1alpha exhibits three subconductance states, ASIC2a exhibits only one, and ASIC3 none. The kinetics of the three types of channels are different: ASIC1alpha and ASIC2a shift between modes of activity, each mode has different open probability and kinetics. In contrast, the kinetics of ASIC3 are uniform throughout the burst of activity. ASIC1alpha, ASIC2a, and ASIC3 are activated by external protons with apparent pH(50) of 5.9, 5.0, and 5.4, respectively. Desensitization in the continual presence of protons is fast and complete in ASIC1alpha and ASIC3 (2.0 and 4.5 s(-1), respectively) but slow and only partial in ASIC2a (0.045 s(-1)). The response to external Ca(2+) also differs: micro M concentrations of extracellular Ca(2+) are necessary for proton gating of ASIC3 (EC(50) = 0.28 micro M), whereas ASIC1alpha and ASIC2a do not require Ca(2+). In addition, Ca(2+) inhibits ASIC1alpha (K(D) = 9.2 +/- 2 mM) by several mechanisms: decrease in the amplitude of unitary currents, shortening of the burst of activity, and decrease in the number of activated channels. Contrary to previous reports, our results indicate that the Ca(2+) permeability of ASIC1alpha is very small.

Project description:1. We have investigated the effect of diltiazem and its newly synthesized derivative (+,-)-trans-3-acetoxy-8-chloro-2,3-dihydro-5[2-diisopropylamine)ethyl]-2-(4-methoxyphenyl)-1,5-benzothiazepin-4-(5H)-ona hydrochloride (JAC-65) on several recombinant human neuronal nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes. 2. At 3 microM, both drugs have little effect on the maximal currents evoked by brief pulses of acetylcholine (ACh) in five subtypes of nAChRs (alpha7, alpha3beta2, alpha4beta2, alpha3beta4, and alpha4beta4), showing little selectivity among subtypes. 3. However, both drugs accelerate the decay of the ionic currents evoked upon continuous stimulation of ACh, being this effect larger with JAC-65, and in beta4*-nAChRs. Such an effect was dependent on the concentrations of both the drug and of the agonist used, and showed the characteristics of a non-competitive antagonism. 4. We have further investigated the effect of both drugs when combined with submicromolar concentrations of nicotine, such as those present in plasma of cigarette smokers, and found that JAC-65, but not diltiazem, is able to greatly enhance the desensitizing effect of these low concentrations of nicotine, specially in beta4*-nAChRs. 5. Experiments in alpha4beta4-nAChRs failed to show voltage dependence of the action of JAC-65. Moreover, recovery from desensitization followed the same time course regardless of the presence of the drug, suggesting that the main mechanism of action of JAC-65 does not involve open channel block. 6. In summary, both drugs, diltiazem and JAC-65, seem to act through a non-competitive mechanism, accelerating the decay of the ionic currents, being JAC-65 more effective than diltiazem at the concentrations used in beta4*-nAChRs. Thus, the differences between both benzothiazepines when measuring various parameters suggest that their mechanisms of action could be slightly different. This would require further investigation.

Dataset Information

Boric acid transport activity of human aquaporins expressed in Xenopus oocytes.

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Boric acid transport activity of human aquaporins expressed in Xenopus oocytes.

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