Mechanisms Of Ion Homeostasis Research Articles

Optogenetic Determination of Dynamic and Cell-Type-Specific Inhibitory Reversal Potentials.

The reversal potential refers to the membrane potential at which the net current flow through a channel reverses direction. The reversal potential is determined by transmembrane ion gradients and, in turn, determines how the channel's activity will affect the membrane potential. Traditional investigation into the reversal potential of inhibitory ligand-gated ion channels (EInh) has relied upon the activation of endogenous receptors, such as the GABA-A receptor (GABAAR). There are, however, challenges associated with activating endogenous receptors, including agonist delivery, isolating channel responses, and the effects of receptor saturation and desensitization. Here, we demonstrate the utility of using a light-gated anion channel, stGtACR2, to probe EInh in the rodent brain. Using mice of both sexes, we demonstrate that the properties of this optically activated channel make it a suitable proxy for studying GABAAR receptor-mediated inhibition. We validate this agonist-independent optogenetic strategy in vitro and in vivo and further show how it can accurately capture differences in EInh dynamics following manipulations of endogenous ion fluxes. This allows us to explore distinct resting EInh differences across genetically defined neuronal subpopulations. Using this approach to challenge ion homeostasis mechanisms in neurons, we uncover cell-specific EInh dynamics that are supported by the differential expression of endogenous ion handling mechanisms. Our findings therefore establish an effective optical strategy for revealing novel aspects of inhibitory reversal potentials and thereby expand the repertoire of optogenetics.

High-affinity potassium transporter from a mangrove tree Avicennia officinalis increases salinity tolerance of Arabidopsis thaliana

Salinity reduces the growth and productivity of crop plants worldwide. Mangroves have evolved efficient ion homeostasis mechanisms to survive under their natural saline growth habitat. Information obtained from them may be utilized for increasing the salt tolerance of crop plants. We identified and characterized a high-affinity potassium transporter gene (AoHKT1) from Avicennia officinalis. The expression of AoHKT1 was induced by NaCl mainly in the leaves. Functional study by heterologous expression of AoHKT1 in Arabidopsis T-DNA insertional mutants athkt1–1 and athkt1–4 revealed that it could enhance the salt tolerance of the mutant plants. This was accompanied by an increase in K+ accumulation in the leaves. AoHKT1 was localized to the plasma membrane in Arabidopsis, and when expressed in yeast, it could complement the functions of both Na+ and K+ transporters. An attempt was made to identify the upstream regulator of AtHKT1, a close homolog of AoHKT1. Using chromatin immunoprecipitation, luciferase assay and yeast one-hybrid assays, WRKY9 was identified as the main transcription factor in the process. Furthermore, this was corroborated by the observation that AtHKT1 levels were significantly reduced in the atwrky9 seedlings. These findings revealed a part of the molecular regulatory mechanism of HKT1 induction in response to salt treatment in Arabidopsis. Our study suggests that AoHKT1 is a potential candidate for generating crop plants with increased salt tolerance.

Zinc nanoparticles: Mode of action and efficacy against boscalid-resistant Alternaria alternata isolates

The antifungal potential of ZnO-NPs against Alternaria alternata isolates with reduced sensitivity to the succinate dehydrogenase inhibitor (SDHI) boscalid, resulting from target site modifications, was evaluated in vitro and in vivo. ZnO-NPs could effectively inhibit mycelial growth in a dose-dependent way in both boscalid (BOSC) sensitive (BOSC-S) and resistant (BOSC-R) isolates. The fungitoxic effect of ZnO-NPs against the pathogen was significantly enhanced when combined with boscalid compared to the individual treatments in all phenotype cases (BOSC-S/R) both in vitro and in vivo. Fungitoxic effect of ZnO-NPs could be, at least partly, attributed to zinc ion release as indicated by the positive correlation between sensitivities to the nanoparticles and their ionic counterpart ZnSO4 and the alleviation of the ZnO-NPs fungitoxic action in the presence of the strong chelating agent EDTA. The superior effectiveness of ZnO-NPs against A. alternata, compared to ZnSO4, could be due to nanoparticle properties interfering with cellular ion homeostasis mechanisms. The observed additive action of the oxidative phosphorylation-uncoupler fluazinam (FM) against all phenotypes indicates a possible role of ATP-dependent ion efflux mechanism in the mode of action of ZnO-NPs. A potential role of ROS production in the fungitoxic action of ZnO-NPs was evident by the additive/synergistic action of salicylhydroxamate (SHAM), which blocks the alternative oxidase antioxidant action. Mixture of ZnO-NPs and boscalid, resulting in a “capping” effect for the nanoparticles and significantly reducing their mean size, probably accounted for the synergistic effect of the mixture against both sensitive and resistant A. alternata isolates. Summarizing, results indicated that ZnO-NPs can be effectively used against A. alternata both alone or in combination with boscalid, providing an effective tool for combating SDHI-resistance and reducing the environmental fingerprint of synthetic fungicides.

Identification and Characterization of Rice OsHKT1;3 Variants.

In rice, the high-affinity K+ transporter, OsHKT1;3, functions as a Na+-selective transporter. mRNA variants of OsHKT1;3 have been reported previously, but their functions remain unknown. In this study, five OsHKT1;3 variants (V1-V5) were identified from japonica rice (Nipponbare) in addition to OsHKT1;3_FL. Absolute quantification qPCR analyses revealed that the transcript level of OsHKT1;3_FL was significantly higher than other variants in both the roots and shoots. Expression levels of OsHKT1;3_FL, and some variants, increased after 24 h of salt stress. Two electrode voltage clamp experiments in a heterologous expression system using Xenopus laevis oocytes revealed that oocytes expressing OsHKT1;3_FL and all of its variants exhibited smaller Na+ currents. The presented data, together with previous data, provide insights to understanding how OsHKT family members are involved in the mechanisms of ion homeostasis and salt tolerance in rice.

Different mechanisms of ion homeostasis are dominant in the recretohalophyte Tamarix ramosissima under different soil salinity

Total ion (Na+, K+, Ca2+, SO4 2− and Cl−) accumulation by plants, ion contents in plant tissues and ion secretion by salt glands on the surface of shoots of Tamarix ramosissima adapted to different soil salinity, namely low (0.06 mmol Na+/g soil), moderate (3.14–4.85 mmol Na+/g soil) and strong (7.56 mmol Na+/g soil) were analyzed. There are two stages of interrelated and complementary regulation of ion homeostasis in whole T. ramosissima plants: (1) regulation of ion influx into the plant from the soil and (2) changing the secretion efficiency of salt glands on shoots. The secretion efficiency of salt glands was appraised by the ratio of ion secretion to tissue ion content. Independent of soil salinity, the accumulation of K+ and Ca2+ was higher than the contents of these ions in the soil. Furthermore, the accumulation of K+, Ca2+ and SO4 2− ions by plants was maintained within a narrow range of values. Under low soil salinity, Na+ was accumulated, whereas under moderate and strong salinity, the influxes of Na+ were limited. However, under strong salinity, the accumulation of Na+ was threefold higher than that under low soil salinity. This led to a change in the Na+/K+ ratio (tenfold), an increase in the activity of salt glands (tenfold) and a reduction in plant growth (fivefold). An apparently high Na+/K+ ratio was the main factor determining over-active functioning of salt glands under strong salinity. Principal component analysis showed that K+ ions played a key role in ion homeostasis at all levels of salinity. Ca2+ played a significant role at low salinity, whereas Cl− and interrelated regulatory components (K+ and proline) played a role under strong salinity. Proline, despite its low concentration under strong salinity, was involved in the regulation of secretion by salt glands. Different stages and mechanisms of ion homeostasis were dominant in T. ramosissima plants adapted to different levels of salinity. These mechanisms facilitated the accumulation of Na+ in plants under low soil salinity, the limitation of Na+ under moderate salinity and the over-activation of Na+ secretion by salt glands under strong salinity, which are all necessary for maintaining ion homeostasis and water potential in the whole plant.

Salt stress in maize: effects, resistance mechanisms, and management. A review

Maize is grown under a wide spectrum of soil and climatic conditions. Maize is moderately sensitive to salt stress; therefore, soil salinity is a serious threat to its production worldwide. Understanding maize response to salt stress and resistance mechanisms and overviewing management options may help to devise strategies for improved maize performance in saline environments. Here, we reviewed the effects, resistance mechanisms, and management of salt stress in maize. Our main conclusions are as follows: (1) germination and stand establishment are more sensitive to salt stress than later developmental stages. (2) High rhizosphere sodium and chloride decrease plant uptake of nitrogen, potassium, calcium, magnesium, and iron. (3) Reduced grain weight and number are responsible for low grain yield in maize under salt stress. Sink limitations and reduced acid invertase activity in developing grains is responsible for poor kernel setting under salt stress. (4) Exclusion of excessive sodium or its compartmentation into vacuoles is an important adaptive strategy for maize under salt stress. (5) Apoplastic acidification, required for cell wall extensibility, is an important indicator of salt resistance, but not essential for better maize growth under salt stress. (6) Upregulation of antioxidant defense genes and β-expansin proteins is important for salt resistance in maize. (7) Arbuscular mycorrhizal fungi improve salt resistance in maize due to better plant nutrient availability. (8) Seed priming is an effective approach for improving maize germination under salt stress. (9) Integration of screening, breeding and ion homeostasis mechanisms into a functional paradigm for the whole plant may help to enhance salt resistance in maize.

Dopamine alleviates salt-induced stress in Malus hupehensis.

Dopamine mediates many physiological processes in plants. We investigated its role in regulating growth, ion homeostasis and the response to salinity in Malus hupehensis Rehd. Both hydroponics and field-pot experiments were conducted under saline conditions. Salt-stressed plants had reduced growth and a marked decline in their net photosynthetic rates, values for Fv /Fm and chlorophyll contents. However, pretreatment with 100 or 200 μM dopamine significantly alleviated this inhibition and enabled plants to maintain their photosynthetic capacity. In addition to changing stomatal behavior, supplementation with dopamine positively influenced the uptake of K, N, P, S, Cu and Mn ions but had an inhibitory effect on Na and Cl uptake, the balance of which is responsible for managing the response to salinity by Malus plants. Dopamine pretreatment also controlled the burst of hydrogen peroxide, possibly through direct scavenging and by enhancing the activities of antioxidative enzymes and the capacity of the ascorbate-glutathione cycle. We also investigated whether dopamine might regulate salt overly sensitive pathway genes under salinity. Here, MdHKT1, MdNHX1 and MdSOS1 were greatly upregulated in roots and leaves, which possibly contributed to the maintenance of ion homeostasis and, thus, improved salinity resistance in plants exposed earlier to exogenous dopamine. These results support our conclusion that dopamine alleviates salt-induced stress not only at the level of antioxidant defense but also by regulating other mechanisms of ion homeostasis.

A glimpse of the yeast Saccharomyces cerevisiae responses to NaCl stress

The budding yeast Saccharomyces cerevisiae has been widely used in fermentation and brewing industries. In industrial fermentation processes, the raw material often attaches certain NaCl or Na+ into bioreactor. High salinity represents an osmotic stress and specific ion toxicity for yeast cell. Protective biochemical reactions range from the synthesis of osmolyte to altered ion transportation, signal transduction, transcriptional response and translational response. This review will focus on responses of the yeast S. cerevisiae and mechanisms of adaptation to NaCl stress and offer a glimpse at the mechanisms of ion homeostasis, regulation of HOG (High Osmosis Glycerol) signaling pathway and the genomic expression response of the cells against the stress. Key words: Genomic expression response, HOG (High Osmosis Glycerol) signaling pathway, Saccharomyces cerevisiae, saline stress.

A glimpse of the yeast Saccharomyces cerevisiae responses to NaCl stress

The budding yeast Saccharomyces cerevisiae has been widely used in fermentation and brewing industries. In industrial fermentation processes, the raw material often attaches certain NaCl or Na+ into bioreactor. High salinity represents an osmotic stress and specific ion toxicity for yeast cell. Protective biochemical reactions range from the synthesis of osmolyte to altered ion transportation, signal transduction, transcriptional response and translational response. This review will focus on responses of the yeast S. cerevisiae and mechanisms of adaptation to NaCl stress and offer a glimpse at the mechanisms of ion homeostasis, regulation of HOG (High Osmosis Glycerol) signaling pathway and the genomic expression response of the cells against the stress. Key words: Genomic expression response, HOG (High Osmosis Glycerol) signaling pathway, Saccharomyces cerevisiae, saline stress.

Palytoxin action on the Na +,K +-ATPase and the disruption of ion equilibria in biological systems

Palytoxin-group toxins (PlTX) exert their potent biological activity by altering mechanisms of ion homeostasis in excitable and non-excitable tissues. This review will describe major aspects that led to the relatively early identification of the Na +,K +-ATPase as the molecular target and receptor of the toxin in sensitive systems. The importance of this pump in the normal functioning of animal cells has driven extensive investigative efforts. The recognized molecular mechanism of action of PlTX involves its binding to the extracellular portion of alpha subunit of this plasma membrane protein, which converts an enzyme carrying ions against their concentration gradients at the expense of chemical energy (ATP) into a non-selective cation channel, allowing passive flow of ions following their concentration gradients. More recent findings have indicated that PlTX would interfere with the normal strict coupling between inner and outer gates of the pump controlling the ion access to the Na +,K +-ATPase, allowing the gates to be simultaneously open. The ability of PlTX to make internal portions of the Na +,K +-ATPase accessible to relatively large molecules has been exploited to characterize the structure–function relationship of the pump, leading to a better understanding of its ion translocation pathway. Thus, forty years from the isolation of this potent marine biotoxin, a considerable understanding of its mode of action and of its potential as a research tool have been achieved and are the basis for promising future advancement in the characterization of biological systems and their alteration by PlTX.

What molecular mechanism is adapted by plants during salt stress tolerance

Salt stress harmfully shocks agricultural yield throughout the world affecting production whether it is for subsistence or economic outcomes. The plant response to salinity consists of numerous processes that must function in coordination to alleviate both cellular hyper-osmolarity and ion disequilibrium. Salt tolerance and yield stability are complex genetic traits that are difficult to establish in crops since salt stress may occur as a catastrophic episode, be imposed continuously or intermittently and become gradually more severe at any stage during development. Molecular biology research has provided new insight into the plant response to salinity and identified genetic determinants that effect salt tolerance. Recent confirmation that many salt tolerance determinants are ubiquitous in plants has led to the use of genetic models, like Arabidopsis thaliana, to further dissect the plant salt stress response. Since many of the most fundamental salt tolerance determinants are those that mediate cellular ion homeostasis, this review will focus primarily on the functional essentiality of ion homeostasis mechanisms in plant salt tolerance. The transport systems that facilitate cellular capacity to utilize Na+ for osmotic adjustment and growth and the role of the Salt-Overly-Sensitive (SOS) signal transduction pathway in the regulation of ion homeostasis and salt tolerance will be particularly emphasized. The objective of the review is to know “What molecular mechanism is adopted by plants during salt stress tolerance?” A conclusion will be presented that integrates cellular based stress signaling and ion homeostasis mechanisms into a functional paradigm for whole plants and defines biotechnology strategies for enhancing salt tolerance of crops.

A Multicopper Oxidase (Cj1516) and a CopA Homologue (Cj1161) Are Major Components of the Copper Homeostasis System of Campylobacter jejuni

Metal ion homeostasis mechanisms in the food-borne human pathogen Campylobacter jejuni are poorly understood. The Cj1516 gene product is homologous to the multicopper oxidase CueO, which is known to contribute to copper tolerance in Escherichia coli. Here we show, by optical absorbance and electron paramagnetic resonance spectroscopy, that purified recombinant Cj1516 contains both T1 and trinuclear copper centers, which are characteristic of multicopper oxidases. Inductively coupled plasma mass spectrometry revealed that the protein contained approximately six copper atoms per polypeptide. The presence of an N-terminal "twin arginine" signal sequence suggested a periplasmic location for Cj1516, which was confirmed by the presence of p-phenylenediamine (p-PD) oxidase activity in periplasmic fractions of wild-type but not Cj1516 mutant cells. Kinetic studies showed that the pure protein exhibited p-PD, ferroxidase, and cuprous oxidase activities and was able to oxidize an analogue of the bacterial siderophore anthrachelin (3,4-dihydroxybenzoate), although no iron uptake impairment was observed in a Cj1516 mutant. However, this mutant was very sensitive to increased copper levels in minimal media, suggesting a role in copper tolerance. This was supported by increased expression of the Cj1516 gene in copper-rich media. A mutation in a second gene, the Cj1161c gene, encoding a putative CopA homologue, was also found to result in copper hypersensitivity, and a Cj1516 Cj1161c double mutant was found to be more copper sensitive than either single mutant. These observations and the apparent lack of alternative copper tolerance systems suggest that Cj1516 (CueO) and Cj1161 (CopA) are major proteins involved in copper homeostasis in C. jejuni.

Cloning and functional comparison of a high-affinity K+ transporter gene PhaHKT1 of salt-tolerant and salt-sensitive reed plants

To understand the mechanisms of ion homeostasis in salt-tolerant and salt-sensitive plants, cDNAs for a high-affinity K(+) transporter PhaHKT1 were isolated from salt-sensitive (Utsunomiya) and salt-tolerant (Nanpi, Enchi) reed plants. A cDNA of Utsunomiya (PhaHKT1-u) contained two insertions in the region corresponding to the first and second introns of the PhaHKT1 gene, which resulted in a sequence 141 amino acid residues shorter than that of Nanpi. Expression of PhaHKT1 mRNA was detected in the roots of Nanpi and Enchi plants under K(+) starvation conditions and also under Na(+) treatment conditions, whereas it was only slightly detected in the roots of Utsunomiya plants under each of these conditions. In the upper parts, PhaHKT1 expression was detected in the Utsunomiya plants, and two signals were obtained in the Nanpi and Enchi plants under all and K(+) starvation conditions, respectively. Yeasts expressing the PhaHKT1 of Nanpi (PhaHKT1-n) or the PhaHKT1 of Enchi (PhaHKT1-e) grew better in the presence of NaCl than yeast expressing PhaHKT1-u. Furthermore, yeast expressing a chimeric cDNA containing the 5' region of the Utsunomiya gene and the 3' region of the Nanpi gene had partial salt tolerance, and yeast expressing a chimeric cDNA containing the 5' region of the Nanpi gene and the 3' region of the Utsunomiya gene had a reduced ability to take up ions. These results suggest that PhaHKT1 plays an important role in the acquisition of K(+) and maintenance of ion balance under saline conditions.

In the beginning, there was the cell: cellular homeostasis

In the past 5 years, the biomedical, scientific community has sequenced the genomes of several organisms (including Homo sapiens), has cloned entire organisms and has determine the molecular structures for several membrane proteins. These advances combined with the advances in technology enabling high-throughput drug screening, gene expression readout using DNA chips and evolving proteomic techniques, make it imperative that physiologist and biomedical professionals understand the basis of cellular function and homeostasis. The Cellular Homeostasis Refresher Course at Experimental Biology 2004 in Washington, DC, was designed to fulfill this need. The specific topics covered were 1) generation of membrane potential, 2) an update on cellular mechanisms of ion homeostasis, channels and transporters, and 3) cellular volume homeostasis, and regulation of intracellular pH.

Maturational Aspects of Epilepsy Mechanisms and Consequences for the Immature Brain

Maturational Aspects of Epilepsy Mechanisms and Consequences for the Immature Brain

A glimpse of the mechanisms of ion homeostasis during salt stress

A glimpse of the mechanisms of ion homeostasis during salt stress

A glimpse of the mechanisms of ion homeostasis during salt stress

Cells respond to salt stress by osmolyte synthesis and by increase of K + uptake and Na + efflux at the plasma membrane and of Na + accumulation at the vacuole. This cellular level of response has been approached by the molecular genetics of Saccharomyces cerevisiae. Two plasma membrane proteins, Sln1 and Shol, operate as sensors for turgor loss under mild osmotic stress. They activate the Hog1 MAP kinase which induces several defence genes, including the ENA1 Na + -efflux pump. Hog1-mediated induction occurs by inactivation of the Skol repressor. High concentrations of Na + activate the calcium-dependent protein phosphatase calcineurin, which induces the ENA1 gene via the positive transcription factor Hal8/ Crz1/Tcn1. Vacuoles could operate as Na + sensors through a Na + -Ca 2+ exchange mechanism. The electrical potential of the plasma membrane is a major determinant of the uptake of toxic cations, including Na + , by non-specific leakage pathways. This biophysical parameter is modulated by the concerted activities of the proton pumping Pma1 ATPase and the Trk1,2 K + uptake system. Both calcineurin and the protein kinase Hal4,5 modulate Trk1,2 and, indirectly, the membrane potential and salt tolerance. The limiting role of vacuolar cation accumulation in salt tolerance has been demonstrated by expression in yeast vacuoles of the bacterial antiporter NhaA.