CAX3 (cation/proton exchanger) mediates a Cd tolerance by decreasing ROS through Ca elevation in Arabidopsis

KEY MESSAGE

Over-expression of CAX3 encoding a cation/proton exchanger enhances Cd tolerance by decreasing ROS (Reactive Oxygen Species) through activating anti-oxidative enzymes via elevation of Ca level in Arabidopsis CAXs (cation/proton exchangers) are involved in the sequestration of cations such as Mn, Li, and Cd, as well as Ca, from cytosol into the vacuole using proton gradients. In addition, it has been reported that CAX1, 2 and 4 are involved in Cd tolerance. Interestingly, it has been reported that CAX3 expressions were enhanced by Cd in Cd-tolerant transgenic plants expressing Hb1 (hemoglobin 1) or UBC1 (Ub-conjugating enzyme 1). Therefore, to investigate whether CAX3 plays a role in increasing Cd tolerance, CAX3 of Arabidopsis and tobacco were over-expressed in Arabidopsis thaliana. Compared to control plants, both transgenic plants displayed an increase in Cd tolerance, no change in Cd accumulation, and enhanced Ca levels. In support of these, AtCAX3-Arabidopsis showed no change in expressions of Cd transporters, but reduced expressions of Ca exporters and lower rate of Ca efflux. By contrast, atcax3 knockout Arabidopsis exhibited a reduced Cd tolerance, while the Cd level was not altered. The expression of Δ90-AtCAX3 (deletion of autoinhibitory domain) increased Cd and Ca tolerance in yeast, while AtCAX3 expression did not. Interestingly, less accumulation of ROS (H2O2 and O2-) was observed in CAX3-expressing transgenic plants and was accompanied with higher antioxidant enzyme activities (SOD, CAT, GR). Taken together, CAX3 over-expression may enhance Cd tolerance by decreasing Cd-induced ROS production by activating antioxidant enzymes and by intervening the positive feedback circuit between ROS generation and Cd-induced spikes of cytoplasmic Ca.

Genome-Wide Identification and Functional Characterization of the Cation Proton Antiporter (CPA) Family Related to Salt Stress Response in Radish (Raphanus sativus L.)

The CPA (cation proton antiporter) family plays an essential role during plant stress tolerance by regulating ionic and pH homeostasis of the cell. Radish fleshy roots are susceptible to abiotic stress during growth and development, especially salt stress. To date, CPA family genes have not yet been identified in radish and the biological functions remain unclear. In this study, 60 CPA candidate genes in radish were identified on the whole genome level, which were divided into three subfamilies including the Na+/H+ exchanger (NHX), K+ efflux antiporter (KEA), and cation/H+ exchanger (CHX) families. In total, 58 of the 60 RsCPA genes were localized to the nine chromosomes. RNA-seq. data showed that 60 RsCPA genes had various expression levels in the leaves, roots, cortex, cambium, and xylem at different development stages, as well as under different abiotic stresses. RT-qPCR analysis indicated that all nine RsNHXs genes showed up regulated trends after 250 mM NaCl exposure at 3, 6, 12, and 24h. The RsCPA31 (RsNHX1) gene, which might be the most important members of the RsNHX subfamily, exhibited obvious increased expression levels during 24h salt stress treatment. Heterologous over-and inhibited-expression of RsNHX1 in Arabidopsis showed that RsNHX1 had a positive function in salt tolerance. Furthermore, a turnip yellow mosaic virus (TYMV)-induced gene silence (VIGS) system was firstly used to functionally characterize the candidate gene in radish, which showed that plant with the silence of endogenous RsNHX1 was more susceptible to the salt stress. According to our results we provide insights into the complexity of the RsCPA gene family and a valuable resource to explore the potential functions of RsCPA genes in radish.

Interstitial ions: A key regulator of state-dependent neural activity?

Throughout the nervous system, ion gradients drive fundamental processes. Yet, the roles of interstitial ions in brain functioning is largely forgotten. Emerging literature is now revitalizing this area of neuroscience by showing that interstitial cations (K+, Ca2+ and Mg2+) are not static quantities but change dynamically across states such as sleep and locomotion. In turn, these state-dependent changes are capable of sculpting neuronal activity; for example, changing the local interstitial ion composition in the cortex is sufficient for modulating the prevalence of slow-frequency neuronal oscillations, or potentiating the gain of visually evoked responses. Disturbances in interstitial ionic homeostasis may also play a central role in the pathogenesis of central nervous system diseases. For example, impairments in K+ buffering occur in a number of neurodegenerative diseases, and abnormalities in neuronal activity in disease models disappear when interstitial K+ is normalized. Here we provide an overview of the roles of interstitial ions in physiology and pathology. We propose the brain uses interstitial ion signaling as a global mechanism to coordinate its complex activity patterns, and ion homeostasis failure contributes to central nervous system diseases affecting cognitive functions and behavior.

Cooperative Binding of the Cationic Porphyrin Tris-T4 Enhances Catalytic Activity of 20S Proteasome Unveiling a Complex Distribution of Functional States

The present study provides new evidence that cationic porphyrins may be considered as tunable platforms to interfere with the structural “key code” present on the 20S proteasome α-rings and, by consequence, with its catalytic activity. Here, we describe the functional and conformational effects on the 20S proteasome induced by the cooperative binding of the tri-cationic 5-(phenyl)-10,15,20-(tri N-methyl-4-pyridyl) porphyrin (Tris-T4). Our integrated kinetic, NMR, and in silico analysis allowed us to disclose a complex effect on the 20S catalytic activity depending on substrate/porphyrin concentration. The analysis of the kinetic data shows that Tris-T4 shifts the relative populations of the multiple interconverting 20S proteasome conformations leading to an increase in substrate hydrolysis by an allosteric pathway. Based on our Tris-T4/h20S interaction model, Tris-T4 is able to affect gating dynamics and substrate hydrolysis by binding to an array of negatively charged and hydrophobic residues present on the protein surface involved in the 20S molecular activation by the regulatory proteins (RPs). Accordingly, despite the fact that Tris-T4 also binds to the α3ΔN mutant, allosteric modulation is not observed since the molecular mechanism connecting gate dynamics with substrate hydrolysis is impaired. We envisage that the dynamic view of the 20S conformational equilibria, activated through cooperative Tris-T4 binding, may work as a simplified model for a better understanding of the intricate network of 20S conformational/functional states that may be mobilized by exogenous ligands, paving the way for the development of a new generation of proteasome allosteric modulators.

A Survey of Barley PIP Aquaporin Ionic Conductance Reveals Ca2+-Sensitive HvPIP2;8 Na+ and K+ Conductance

Some plasma membrane intrinsic protein (PIP) aquaporins can facilitate ion transport. Here we report that one of the 12 barley PIPs (PIP1 and PIP2) tested, HvPIP2;8, facilitated cation transport when expressed in Xenopus laevis oocytes. HvPIP2;8-associated ion currents were detected with Na+ and K+, but not Cs+, Rb+, or Li+, and was inhibited by Ba2+, Ca2+, and Cd2+ and to a lesser extent Mg2+, which also interacted with Ca2+. Currents were reduced in the presence of K+, Cs+, Rb+, or Li+ relative to Na+ alone. Five HvPIP1 isoforms co-expressed with HvPIP2;8 inhibited the ion conductance relative to HvPIP2;8 alone but HvPIP1;3 and HvPIP1;4 with HvPIP2;8 maintained the ion conductance at a lower level. HvPIP2;8 water permeability was similar to that of a C-terminal phosphorylation mimic mutant HvPIP2;8 S285D, but HvPIP2;8 S285D showed a negative linear correlation between water permeability and ion conductance that was modified by a kinase inhibitor treatment. HvPIP2;8 transcript abundance increased in barley shoot tissues following salt treatments in a salt-tolerant cultivar Haruna-Nijo, but not in salt-sensitive I743. There is potential for HvPIP2;8 to be involved in barley salt-stress responses, and HvPIP2;8 could facilitate both water and Na+/K+ transport activity, depending on the phosphorylation status.

Identification and Characterization of a Novel Thermostable and Salt-Tolerant β-1,3 Xylanase from Flammeovirga pacifica Strain WPAGA1

β-1,3 xylanase is an important enzyme in the biorefinery process for some algae. The discovery and characterization of new β-1,3 xylanase is a hot research topic. In this paper, a novel β-1,3 xylanase (Xyl88) is revealed from the annotated genome of Flammeovirga pacifica strain WPAGA1. Bioinformatic analysis shows that Xyl88 belongs to the glycoside hydrolase 26 (GH26) with a suspected CBM (carbohydrate-binding module) sequence. The activity of rXyl88 is 75% of the highest enzyme activity (1.5 mol/L NaCl) in 3 mol/L NaCl buffer, which suggests good salt tolerance of rXy188. The optimum reaction temperature in the buffer without NaCl and with 1.5 mol/L NaCl is 45 °C and 55 °C, respectively. Notably, the catalytic efficiency of rXyl88 (k_cat/K_m) is approximately 20 higher than that of the thermophilic β-1,3 xylanase that has the highest catalytic efficiency. Xyl88 in this study becomes the most efficient enzyme ever found, and it is also the first reported moderately thermophilic and salt-tolerant β-1,3 xylanase. Results of molecular dynamics simulation further prove the excellent thermal stability of Xyl88. Moreover, according to the predicted 3D structure of the Xyl88, the surface of the enzyme is distributed with more negative charges, which is related to its salt tolerance, and significantly more hydrogen bonds and Van der Waals force between the intramolecular residues, which is related to its thermal stability.

The metal binding site composition of the cation diffusion facilitator protein MamM cytoplasmic domain impacts its metal responsivity

The cation diffusion facilitator (CDF) is a conserved family of divalent d-block metal cation transporters that extrude these cations selectively from the cytoplasm. CDF proteins are composed of two domains: the transmembrane domain, through which the cations are transported, and a regulatory cytoplasmic C-terminal domain (CTD). It was recently shown that the CTD of the CDF protein MamM from magnetotactic bacteria has a role in metal selectivity, as binding of different metal cations exhibits distinctive affinities and conformations. It is yet unclear whether the composition of the CTD binding sites can impact metal selectivity and if we can manipulate the CTD to response to other non-native metals in CDF proteins. Here we performed a mutational study of the model protein MamM CTD, where we exchanged the native metal binding residues with different metal-binding amino acids. Using X-ray crystallography and Trp-fluorescence spectrometry, we studied the impact of these mutations on the CTD conformation in the presence of non-native metals. Our results reveal that the incorporation of such mutations alters the domain response to metals in vitro, as mutant forms of the CTD bind metals differently in terms of the composition of the binding sites and the CTD conformation. Therefore, the results demonstrate the direct influence of the CTD binding site composition on CDF proteins structure and hence, function, and constitute a first step for rational design of MamM for transporting different metals in vivo.

Multivalent ions and biomolecules: Attempting a comprehensive perspective

Ions are ubiquitous in nature. They play a key role for many biological processes on the molecular scale, from molecular interactions, to mechanical properties, to folding, to self-organisation and assembly, to reaction equilibria, to signalling, to energy and material transport, to recognition etc. Going beyond monovalent ions to multivalent ions, the effects of the ions are frequently not only stronger (due to the obviously higher charge), but qualitatively different. A typical example is the process of binding of multivalent ions, such as Ca2+ , to a macromolecule and the consequences of this ion binding such as compaction, collapse, potential charge inversion and precipitation of the macromolecule. Here we review these effects and phenomena induced by multivalent ions for biological (macro)molecules, from the "atomistic/molecular" local picture of (potentially specific) interactions to the more global picture of phase behaviour including, e. g., crystallisation, phase separation, oligomerisation etc. Rather than attempting an encyclopedic list of systems, we rather aim for an embracing discussion using typical case studies. We try to cover predominantly three main classes: proteins, nucleic acids, and amphiphilic molecules including interface effects. We do not cover in detail, but make some comparisons to, ion channels, colloidal systems, and synthetic polymers. While there are obvious differences in the behaviour of, and the relevance of multivalent ions for, the three main classes of systems, we also point out analogies. Our attempt of a comprehensive discussion is guided by the idea that there are not only important differences and specific phenomena with regard to the effects of multivalent ions on the main systems, but also important similarities. We hope to bridge physico-chemical mechanisms, concepts of soft matter, and biological observations and connect the different communities further.

Light-triggered switching of liposome surface charge directs delivery of membrane impermeable payloads in vivo

Surface charge plays a fundamental role in determining the fate of a nanoparticle, and any encapsulated contents, in vivo. Herein, we describe, and visualise in real time, light-triggered switching of liposome surface charge, from neutral to cationic, in situ and in vivo (embryonic zebrafish). Prior to light activation, intravenously administered liposomes, composed of just two lipid reagents, freely circulate and successfully evade innate immune cells present in the fish. Upon in situ irradiation and surface charge switching, however, liposomes rapidly adsorb to, and are taken up by, endothelial cells and/or are phagocytosed by blood resident macrophages. Coupling complete external control of nanoparticle targeting together with the intracellular delivery of encapsulated (and membrane impermeable) cargos, these compositionally simple liposomes are proof that advanced nanoparticle function in vivo does not require increased design complexity but rather a thorough understanding of the fundamental nano-bio interactions involved.

Topical nanoparticles interfering with the DNA-LL37 complex to alleviate psoriatic inflammation in mice and monkeys

Cell-free DNA (cfDNA) released from damaged or dead cells combines with LL37 and is converted into an immune response activator to exacerbate psoriasis. Here, we show that cationic nanoparticles (cNPs) efficiently compete for DNA from the DNA-LL37 immunocomplex and inhibit DNA-LL37-induced cell activation. Using phenotypical images, psoriasis area and severity index scoring, histology, and immunohistochemical analysis, we demonstrate that topical application of cNPs on psoriasiform skin of a mouse model relieves psoriatic symptoms. It is noteworthy that the results were confirmed in a cynomolgus monkey model. Moreover, topically administrated cNPs showed low in vivo toxicity because of their retention in skin. Mechanistic analyses of cytokine expression in the psoriatic site, cfDNA levels in circulation and inflamed skin, skin permeation, and biodistribution of cNPs also matched the therapeutic outcomes. Therefore, we present a previously unidentified strategy of nanomedicine to treat skin inflammatory diseases, which demonstrates great potential for clinical application.

Fe2+ Ions Alleviate the Symptom of Citrus Greening Disease

Citrus greening (CG) is among the most devastating citrus diseases worldwide. CG-infected trees exhibit interveinal chlorotic leaves due to iron (Fe) deficiency derived from CG; thus, Fe content is lower in infected leaves than in healthy leaves. In this study, we demonstrated that the foliar application of Fe2+ relieves the symptom of CG infection in citrus trees. We applied Fe2+ and citrate to the leaves of infected rough lemon plants. Following this treatment, a reduction in the number of yellow symptomatic leaves was observed, and their growth was restored. Using chlorophyll content as an index, we screened for effective Fe complexes and found that a high ratio of citrate to Fe2+ in the applied solution led to effects against CG in Shikuwasa trees. A high proportion of Fe2+ to total Fe was another key factor explaining the effectiveness of the solution in CG infection, indicating the importance of Fe2+ absorption into plant cells. We confirmed the proportion of Fe2+ to total Fe through the high correlation of reflectometry data via a triazine reaction and X-ray absorption fine structure analysis. These results demonstrate that the foliar application of a high-Fe2+ citrate solution can restore the growth of CG diseased trees.

The TRPV3 channel of the bovine rumen: localization and functional characterization of a protein relevant for ruminal ammonia transport

Large quantities of ammonia (NH3 or NH4+) are absorbed from the gut, associated with encephalitis in hepatic disease, poor protein efficiency in livestock, and emissions of nitrogenous climate gasses. Identifying the transport mechanisms appears urgent. Recent functional and mRNA data suggest that absorption of ammonia from the forestomach of cattle may involve TRPV3 channels. The purpose of the present study was to sequence the bovine homologue of TRPV3 (bTRPV3), localize the protein in ruminal tissue, and confirm transport of NH4+. After sequencing, bTRPV3 was overexpressed in HEK-293 cells and Xenopus oocytes. An antibody was selected via epitope screening and used to detect the protein in immunoblots of overexpressing cells and bovine rumen, revealing a signal of the predicted ~ 90 kDa. In rumen only, an additional ~ 60 kDa band appeared, which may represent a previously described bTRPV3 splice variant of equal length. Immunohistochemistry revealed staining from the ruminal stratum basale to stratum granulosum. Measurements with pH-sensitive microelectrodes showed that NH4+ acidifies Xenopus oocytes, with overexpression of bTRPV3 enhancing permeability to NH4+. Single-channel measurements revealed that Xenopus oocytes endogenously expressed small cation channels in addition to fourfold-larger channels only observed after expression of bTRPV3. Both endogenous and bTRPV3 channels conducted NH4+, Na+, and K+. We conclude that bTRPV3 is expressed by the ruminal epithelium on the protein level. In conjunction with data from previous studies, a role in the transport of Na+, Ca2+, and NH4+ emerges. Consequences for calcium homeostasis, ruminal pH, and nitrogen efficiency in cattle are discussed.

Conductance Mechanisms of Rapidly Desensitizing Cation Channelrhodopsins from Cryptophyte Algae

Channelrhodopsins guide algal phototaxis and are widely used as optogenetic probes for control of membrane potential with light. "Bacteriorhodopsin-like" cation channelrhodopsins (BCCRs) from cryptophytes differ in primary structure from other CCRs, lacking usual residues important for their cation conductance. Instead, the sequences of BCCR match more closely those of rhodopsin proton pumps, containing residues responsible for critical proton transfer reactions. We report 19 new BCCRs which, together with the earlier 6 known members of this family, form three branches (subfamilies) of a phylogenetic tree. Here, we show that the conductance mechanisms in two subfamilies differ with respect to involvement of the homolog of the proton donor in rhodopsin pumps. Two BCCRs from the genus Rhodomonas generate photocurrents that rapidly desensitize under continuous illumination. Using a combination of patch clamp electrophysiology, absorption, Raman spectroscopy, and flash photolysis, we found that the desensitization is due to rapid accumulation of a long-lived nonconducting intermediate of the photocycle with unusually blue-shifted absorption with a maximum at 330 nm. These observations reveal diversity within the BCCR family and contribute to deeper understanding of their independently evolved cation channel function.IMPORTANCE Cation channelrhodopsins, light-gated channels from flagellate green algae, are extensively used as optogenetic photoactivators of neurons in research and recently have progressed to clinical trials for vision restoration. However, the molecular mechanisms of their photoactivation remain poorly understood. We recently identified cryptophyte cation channelrhodopsins, structurally different from those of green algae, which have separately evolved to converge on light-gated cation conductance. This study reveals diversity within this new protein family and describes a subclade with unusually rapid desensitization that results in short transient photocurrents in continuous light. Such transient currents have not been observed in the green algae channelrhodopsins and are potentially useful in optogenetic protocols. Kinetic UV-visible (UV-vis) spectroscopy and photoelectrophysiology reveal that the desensitization is caused by rapid accumulation of a nonconductive photointermediate in the photochemical reaction cycle. The absorption maximum of the intermediate is 330 nm, the shortest wavelength reported in any rhodopsin, indicating a novel chromophore structure.

From conservation to structure, studies of magnetosome associated cation diffusion facilitators (CDF) proteins in Proteobacteria

Magnetotactic bacteria (MTB) are prokaryotes that sense the geomagnetic field lines to geolocate and navigate in aquatic sediments. They are polyphyletically distributed in several bacterial divisions but are mainly represented in the Proteobacteria. In this phylum, magnetotactic Deltaproteobacteria represent the most ancestral class of MTB. Like all MTB, they synthesize membrane-enclosed magnetic nanoparticles, called magnetosomes, for magnetic sensing. Magnetosome biogenesis is a complex process involving a specific set of genes that are conserved across MTB. Two of the most conserved genes are mamB and mamM, that encode for the magnetosome-associated proteins and are homologous to the cation diffusion facilitator (CDF) protein family. In magnetotactic Alphaproteobacteria MTB species, MamB and MamM proteins have been well characterized and play a central role in iron-transport required for biomineralization. However, their structural conservation and their role in more ancestral groups of MTB like the Deltaproteobacteria have not been established. Here we studied magnetite cluster MamB and MamM cytosolic C-terminal domain (CTD) structures from a phylogenetically distant magnetotactic Deltaproteobacteria species represented by BW-1 strain, which has the unique ability to biomineralize magnetite and greigite. We characterized them in solution, analyzed their crystal structures and compared them to those characterized in Alphaproteobacteria MTB species. We showed that despite the high phylogenetic distance, MamB_BW-1 and MamM_BW-1 CTDs share high structural similarity with known CDF-CTDs and will probably share a common function with the Alphaproteobacteria MamB and MamM.

GC ends control topology of DNA G-quadruplexes and their cation-dependent assembly

GCn and GCnCG, where n = (G2AG4AG2), fold into well-defined, dimeric G-quadruplexes with unprecedented folding topologies in the presence of Na+ ions as revealed by nuclear magnetic resonance spectroscopy. Both G-quadruplexes exhibit unique combination of structural elements among which are two G-quartets, A(GGGG)A hexad and GCGC-quartet. Detailed structural characterization uncovered the crucial role of 5'-GC ends in formation of GCn and GCnCG G-quadruplexes. Folding in the presence of 15NH4+ and K+ ions leads to 3'-3' stacking of terminal G-quartets of GCn G-quadruplexes, while 3'-GC overhangs in GCnCG prevent dimerization. Results of the present study expand repertoire of possible G-quadruplex structures. This knowledge will be useful in DNA sequence design for nanotechnological applications that may require specific folding topology and multimerization properties.

Preparation and Utilization of Freshly Isolated Human Detrusor Smooth Muscle Cells for Characterization of 9-Phenanthrol-Sensitive Cation Currents

Detrusor smooth muscle (DSM) cells present within the urinary bladder wall ultimately facilitate urine storage and voiding. Preparation of the viable, fresh, and isolated DSM cells presents an important technical challenge whose achievement provides optimal cells for subsequent functional and molecular studies. The method developed and elaborated herein, successfully used by our group for over a decade, describes dissection of human urinary bladder specimens obtained from open bladder surgeries followed by an enzymatic two-step treatment of DSM pieces and mechanical trituration to obtain freshly isolated DSM cells. The initial step involves dissection to separate the DSM layer (also known as muscularis propria) from mucosa (urothelium, lamina propria, and muscularis mucosa) and the adjacent connective, vascular, and adipose tissues present. The DSM is then cut into pieces (2-3 mm x 4-6 mm) in nominal Ca2+-containing dissection/digestion solution (DS). DSM pieces are next transferred to and sequentially treated separately with DS containing papain and collagenase at ~37 °C for 30-45 min per step. Following washes with DS containing enzyme-free bovine serum and trituration with a fire-polished pipette, the pieces release single DSM cells. Freshly isolated DSM cells are ideally suited for patch-clamp electrophysiological and pharmacological characterizations of ion channels. Specifically, we show that the TRPM4 channel blocker 9-phenanthrol reduces voltage-step evoked cation currents recorded with the amphotericin-B perforated patch-clamp approach. DSM cells can also be studied by other techniques such as single cell RT-PCR, microarray analysis, immunocytochemistry, in situ proximity ligation assay, and Ca2+ imaging. The main advantage of utilizing single DSM cells is that the observations made relate directly to single cell characteristics revealed. Studies of freshly isolated human DSM cells have provided important insights characterizing the properties of various ion channels including cation-permeable in the urinary bladder and will continue as a gold standard in elucidating DSM cellular properties and regulatory mechanisms.

The Evaluation of Mitochondrial Membrane Potential Using Fluorescent Dyes or a Membrane-Permeable Cation (TPP+) Electrode in Isolated Mitochondria and Intact Cells

The proton electrochemical gradient generated by respiratory chain activity accounts for over 90% of all available ATP and, as such, its evaluation and accurate measurements regarding its total values and fluctuations is an invaluable component in the understanding of mitochondrial functions. Consequently, alterations in electric potential across the inner mitochondrial membrane generated by differential protonic accumulations and transport are known as the mitochondrial membrane potential, or Δψ, and are reflective of the functional metabolic status of mitochondria. There are several experimental approaches to measure Δψ, ranging from fluorometric evaluations to electrochemical probes. Here we discuss the advantages and disadvantages of several of these methods, ranging from one that is dependent on the movement of a particular ion (tetraphenylphosphonium (TPP⁺) with a selective electrode) to the selection of a fluorescent dye from various types to achieve the same goal. The evaluation of the accumulation and movements of TPP⁺ across the inner mitochondrial membrane, or the fluorescence of accumulated dye particles, is a sensitive and accurate method of evaluating the Δψ in respiring mitochondria (either isolated or still inside the cell).

Sevoflurane-Induced Dysregulation of Cation-Chloride Cotransporters NKCC1 and KCC2 in Neonatal Mouse Brain

The cation-chloride cotransporters Na+-K+-2Cl--1 (NKCC1) and K+-2Cl--2 (KCC2) critically regulate neuronal responses to gamma-aminobutyric acid (GABA). NKCC1 renders GABA excitatory in immature neurons while expression of KCC2 signals GABA maturation to its inhibitory role. Imbalances in NKCC1/KCC2 alter GABA neurotransmission, which may contribute to hyperexcitability and blunted inhibition in neurocircuitry after neonatal exposure to anesthesia. Thus, we hypothesized that anesthetics may dysregulate NKCC1 and/or KCC2 in developing brain. We exposed postnatal day (PND) 7 mice to sevoflurane or carrier gases and assessed NKCC1 and KCC2 expression across three brain regions 6 h and 24 h after initial exposure. To test differences in behavior, we challenged pups receiving sevoflurane or carrier gases on PND7 with propofol on PND8 and recorded parameters of anesthesia induction and maintenance. Sevoflurane exposure increased cortical NKCC1 at 6 h (p = 0.03) and decreased cortical and hippocampal KCC2 at 24 h (p = 0.009 and p = 0.007, respectively). NKCC1/KCC2 ratio was significantly increased at both 6 h (p = 0.02) and 24 h (p = 0.03) in cortex and at 24 h (p = 0.02) in hippocampus. After propofol challenge on PND8, pups previously exposed to sevoflurane on PND7 regained righting reflex significantly faster than their non-exposed cohort (p < 0.001). Disturbing NKCC1/KCC2 balance may underlie circuit hyperexcitability and contribute to neurodevelopmental impairments we have observed in previous studies of neonatal anesthesia exposure. Human infants previously exposed to anesthesia may require higher concentrations of anesthetic drugs, potentially compounding their susceptibility for neurodevelopmental sequalae.

Mechanistic Understanding of the Interactions of Cationic Conjugated Oligo- and Polyelectrolytes with Wild-type and Ampicillin-resistant Escherichia coli

An in-depth understanding of cell-drug binding modes and action mechanisms can potentially guide the future design of novel drugs and antimicrobial materials and help to combat antibiotic resistance. Light-harvesting π-conjugated molecules have been demonstrated for their antimicrobial effects, but their impact on bacterial outer cell envelope needs to be studied in detail. Here, we synthesized poly(phenylene) based model cationic conjugated oligo- (2QA-CCOE, 4QA-CCOE) and polyelectrolytes (CCPE), and systematically explored their interactions with the outer cell membrane of wild-type and ampicillin (amp)-resistant Gram-negative bacteria, Escherichia coli (E. coli). Incubation of the E. coli cells in CCOE/CCPE solution inhibited the subsequent bacterial growth in LB media. About 99% growth inhibition was achieved if amp-resistant E. coli was treated for ~3-5 min, 1 h and 6 h with 100 μM of CCPE, 4QA-CCOE, and 2QA-CCOE solutions, respectively. Interestingly, these CCPE and CCOEs inhibited the growth of both wild-type and amp-resistant E. coli to a similar extent. A large surface charge reversal of bacteria upon treatment with CCPE suggested the formation of a coating of CCPE on the outer surface of bacteria; while a low reversal of bacterial surface charge suggested intercalation of CCOEs within the lipid bilayer of bacteria.

K+ Channel-SEC11 Binding Exchange Regulates SNARE Assembly for Secretory Traffic

Cell expansion requires that ion transport and secretory membrane traffic operate in concert. Evidence from Arabidopsis (Arabidopsis thaliana) indicates that such coordination is mediated by physical interactions between subsets of so-called SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, which drive the final stages of vesicle fusion, and K⁺ channels, which facilitate uptake of the cation to maintain cell turgor pressure as the cell expands. However, the sequence of SNARE binding with the K⁺ channels and its interweaving within the events of SNARE complex assembly for exocytosis remains unclear. We have combined protein-protein interaction and electrophysiological analyses to resolve the binding interactions of the hetero-oligomeric associations. We find that the RYxxWE motif, located within the voltage sensor of the K⁺ channels, is a nexus for multiple SNARE interactions. Of these, K⁺ channel binding and its displacement of the regulatory protein SEC11 is critical to prime the Qa-SNARE SYP121. Our results indicate a stabilizing role for the Qbc-SNARE SNAP33 in the Qa-SNARE transition to SNARE complex assembly with the R-SNARE VAMP721. They also suggest that, on its own, the R-SNARE enters an anomalous binding mode with the channels, possibly as a fail-safe measure to ensure a correct binding sequence. Thus, we suggest that SYP121 binding to the K⁺ channels serves the role of a primary trigger to initiate assembly of the secretory machinery for exocytosis.

The effect of cations (Na+, Mg2+, and Ca2+) on the activity and structure of nitrifying and denitrifying bacterial communities

Wastewaters generated in regions with water scarcity usually have high alkalinity, hardness, and elevated osmotic pressure (OP). Those characteristics should be considered when using biological systems for wastewater treatment along with the salinity heterogeneity. The interaction of different salts in mixed electrolyte solutions may cause inhibition, antagonism, synergism, and stimulation effects on microbial communities. Little is known about those effects on microbial activity and community structure of nitrifying and denitrifying bacteria. In this work, factorial design was used to evaluate the effects of NaCl, MgCl₂ and CaCl₂ on nitrifying and denitrifying communities. Antagonistic relationships between all salts were observed and they had greater magnitude on the nitrifying community. Stimulus and synernism were more evident on the nitrifying and denitrifying experiments, respectively. For this reason, the highest nitrification and denitrification specific rates were 1.1 × 10^-1 mgN-NH₄⁺ gSSV^-1 min^-1 for condition 01 and 6.5 × 10^-2 mgN-NO₃^- gSSV^-1 min^-1 for control condition, respectively. The toxicity of the salts followed the order of NaCl > MgCl₂ > CaCl₂ and the antagonism between MgCl₂ and NaCl was the most significant. PCR/DGGE analyses showed that Mg²⁺ may be the element that expresses the least influence in the differentiation of microbial structure even though it significantly affects the activity of the autotrophic microorganisms. The same behavior was observed for Ca²⁺ on denitrifying microorganism. In addition, microbial diversity and richness was not negatively affected by different salinities. Genetic sequencing suggested that the genus Aeromonas, Alishewanella, Azospirillum, Pseudoalteromonas, and Thioalkalivibrio were outstanding on ammonium and nitrate removal under saline conditions. The specific toxicity of each salt and the interactions among them are the major effects on microbial activity in biological wastewater treatments rather than the osmotic pressure caused by the final salinity.

The protosteryl and dammarenyl cation dichotomy in polycyclic triterpene biosynthesis revisited: has this 'rule' finally been broken?

Covering: 1948 up to the end of 2018. The triterpene alcohols represent an important and diverse class of natural products. This diversity is believed to originate from the differential enzymatically controlled cyclisation of 2,3-oxidosqualene. It is now a well-established presumption that all naturally occurring tetra- and penta-cyclic triterpene alcohols can be rationalised by the resolution of one of two intermediary tetracyclic cations, termed the protosteryl and dammarenyl cations. Here, a discussion of typical key triterpene structures and their proposed derivation from either of these progenitors is followed by comparison with a recently reported novel pentacyclic triterpene orysatinol which appears to correspond to an unprecedented divergence from this dichotomous protosteryl/dammarenyl view of triterpene biogenesis. Not only does this discovery widen the potential scope of triterpene scaffolds that could exist in nature, it could call into question the reliability of stereochemical assignments of some existing triterpene structures that are supported by only limited spectroscopic evidence. The discovery of orysatinol provides direct experimental evidence to support considering more flexibility in the stereochemical interpretation of the biogenic isoprene rule.

Dynamic change of electrostatic field in TMEM16F permeation pathway shifts its ion selectivity

TMEM16F is activated by elevated intracellular Ca2+, and functions as a small-conductance ion channel and as a phospholipid scramblase. In contrast to its paralogs, the TMEM16A/B calcium-activated chloride channels, mouse TMEM16F has been reported as a cation-, anion-, or non-selective ion channel, without a definite conclusion. Starting with the Q559K mutant that shows no current rundown and less outward rectification in excised patch, we found that the channel shifted its ion selectivity in response to the change of intracellular Ca2+ concentration, with an increased permeability ratio of Cl- to Na+ (PCl-/PNa+) at a higher Ca2+ level. The gradual shift of relative ion permeability did not correlate with the channel activation state. Instead, it was indicative of an alteration of electrostatic field in the permeation pathway. The dynamic change of ion selectivity suggests a charge-screening mechanism for TMEM16F ion conduction, and it provides hints to further studies of TMEM16F physiological functions.

An extracellular cation coordination site influences ion conduction of OsHKT2;2

BACKGROUND

HKT channels mediate sodium uniport or sodium and potassium symport in plants. Monocotyledons express a higher number of HKT proteins than dicotyledons, and it is only within this clade of HKT channels that cation symport mechanisms are found. The prevailing ion composition in the extracellular medium affects the transport abilities of various HKT channels by changing their selectivity or ion transport rates. How this mutual effect is achieved at the molecular level is still unknown. Here, we built a homology model of the monocotyledonous OsHKT2;2, which shows sodium and potassium symport activity. We performed molecular dynamics simulations in the presence of sodium and potassium ions to investigate the mutual effect of cation species.

RESULTS

By analyzing ion-protein interactions, we identified a cation coordination site on the extracellular protein surface, which is formed by residues P71, D75, D501 and K504. Proline and the two aspartate residues coordinate cations, while K504 forms salt bridges with D75 and D501 and may be involved in the forwarding of cations towards the pore entrance. Functional validation via electrophysiological experiments confirmed the biological relevance of the predicted ion coordination site and identified K504 as a central key residue. Mutation of the cation coordinating residues affected the functionality of HKT only slightly. Additional in silico mutants and simulations of K504 supported experimental results.

CONCLUSION

We identified an extracellular cation coordination site, which is involved in ion coordination and influences the conduction of OsHKT2;2. This finding proposes a new viewpoint in the discussion of how the mutual effect of variable ion species may be achieved in HKT channels.

A repertoire of cationic and anionic conductances at the plasma membrane of Medicago truncatula root hairs

Root hairs, as lateral extensions of epidermal cells, provide large absorptive surfaces to the root and are major actors in plant hydromineral nutrition. In contact with the soil they also constitute a site of interactions between the plant and rhizospheric microorganisms. In legumes, initiation of symbiotic interactions with N₂ -fixing rhizobia is often triggered at the root hair cell membrane in response to nodulation factors secreted by rhizobia, and involves early signaling events with changes in H⁺ , Ca²⁺ , K⁺ and Cl^- fluxes inducing transient depolarization of the cell membrane. Here, we aimed to build a functional repertoire of the major root hair conductances to cations and anions in the sequenced legume model Medicago truncatula. Five root hair conductances were characterized through patch-clamp experiments on enzymatically recovered root hair protoplasts. These conductances displayed varying properties of voltage dependence, kinetics and ion selectivity. They consisted of hyperpolarization- and depolarization-activated conductances for K⁺ , cations or Cl^- . Among these, one weakly outwardly rectifying cationic conductance and one hyperpolarization-activated slowly inactivating anionic conductance were not known as active in root hairs. All five conductances were detected in apical regions of young growing root hairs using membrane spheroplasts obtained by laser-assisted cell-wall microdissection. Combined with recent root hair transcriptomes of M. truncatula, this functional repertoire of conductances is expected to help the identification of candidate genes for reverse genetics studies to investigate the possible role of each conductance in root hair growth and interaction with the biotic and abiotic environment.

Cationic porphyrins with large side arm substituents as resonance light scattering ratiometric probes for specific recognition of nucleic acid G-quadruplexes

Specific G-quadruplex-probing is crucial for both biological sciences and biosensing applications. Most reported probes are focused on fluorescent or colorimetric recognition of G-quadruplexes. Herein, for the first time, we reported a new specific G-quadruplex-probing technique-resonance light scattering (RLS)-based ratiometric recognition. To achieve the RLS probing of G-quadruplexes in the important physiological pH range of 7.4-6.0, four water soluble cationic porphyrin derivatives, including an unreported octa-cationic porphyrin, with large side arm substituents were synthesized and developed as RLS probes. These RLS probes were demonstrated to work well for ratiometric recognition of G-quadruplexes with high specificity against single- and double-stranded DNAs, including long double-stranded ones. The working mechanism was speculated to be based on the RLS signal changes caused by porphyrin protonation that was promoted by the end-stacking of porphyrins on G-quadruplexes. This work adds an important member in G-quadruplex probe family, thus providing a useful tool for studies on G-quadruplex-related events concerning G-quadruplex formation, destruction and changes in size, shape and aggregation. As a proof-of-concept example of applications, the RLS probes were demonstrated to work well for label-free and sequence-specific sensing of microRNA. This work also provides a simple and useful way for the preparation of cationic porphyrins with high charges.

Identification and characterization of multifunctional cationic peptides from traditional Japanese fermented soybean Natto extracts

In this study, we investigated the lipopolysaccharide (LPS)-neutralizing and angiogenic activities of cationic peptides derived from the traditional Japanese fermented product Natto, which is made by fermenting cooked soybeans using Bacillus subtilis. Initially, we prepared 20 fractions of Natto extracts with various isoelectric points (pI's) using ampholyte-free isoelectric focusing (autofocusing). Cationic peptides were then purified from fractions 19 and 20, whose pH values were greater than 12, using reversed-phase high-performance liquid chromatography, and were identified using matrix-assisted laser/desorption ionization-time-of-flight mass spectroscopy. Among the 13 identified cationic peptides, seven (KFNKYGR, FPFPRPPHQK, GQSSRPQDRHQK, QRFDQRSPQ, ERQFPFPRPPHQK, GEIPRPRPRPQHPE, and EQPRPIPFPRPQPR) had pI's greater than 9.5, positive net charges, and differing molecular weights. These peptides were then chemically synthesized and applied to chromogenic LPS-neutralizing assays using Limulus amebocyte lysates, and 50% effective (neutralizing) concentrations of 2.6-5.5 μM were demonstrated. In addition, tube formation assays in human umbilical vein endothelial cells revealed angiogenic activities for all but one (GEIPRPRPRPQHPE) of these seven cationic peptides, with increases in relative tube lengths of 23-31% in the presence of peptides at 10 μM. Subsequent experiments showed negligible hemolytic activity of these peptides at concentrations of up to 500 μM in mammalian red blood cells. Collectively, these data demonstrate that six cationic peptides from Natto extracts, with the exception of GEIPRPRPRPQHPE, have LPS-neutralizing and angiogenic activities but do not induce hemolysis.

Growth inhibition and oxidative stress caused by four ionic liquids in Scenedesmus obliquus: Role of cations and anions

Ionic liquids (ILs) are widely used in various industrial applications. However, they are considered potential toxins in aquatic environments because of their physical stability and solubility. The growth inhibition and oxidative stress induced by four ionic liquids with different cations and anions on the green algae Scenedesmus obliquus was investigated in this study. The order of growth inhibition was 1‑hexyl‑3‑methylimidazolium nitrate ([HMIM]NO₃) > 1‑hexyl‑3‑methylimidazolium chloride ([HMIM]Cl) > N‑hexyl‑3‑metylpyridinium bromide ([HMPy]Br) > N‑hexyl‑3‑metylpyridinium chloride ([HMPy]Cl). Imidazolium IL had a higher growth inhibition effect than pyridinium IL, nitrate IL and bromide IL had a higher effect than chloride IL. Reactive oxygen species (ROS) level in S. obliquus increased with increasing IL concentrations. Green fluorescence in [HMIM]Cl treated algae showed increased brightness compared to the [HMPy]Cl treatment, and [HMIM]NO₃ treatment produced increased brightness compared to the [HMPy]Br treatment, suggesting that higher ROS levels were induced by [HMIM]Cl and [HMIM]NO₃. Soluble protein, catalase (CAT), and superoxide dismutase (SOD) activities were stimulated at lower concentrations but were inhibited at higher concentrations. Regression analysis suggested that ROS level is the main index responsible for oxidative stress induced by the four ILs. The ILs induced oxidative damage on S. obliquus, and ROS in high concentration treatments could not be effectively removed by the antioxidant system, leading to oxidative damage and ultimately resulting in growth inhibition and cell death.

Electrophysiological evidence for light-activated cation transport in calcifying corals

Light has been demonstrated to enhance calcification rates in hermatypic coral species. To date, it remains unresolved whether calcifying epithelia change their ion transport activity during illumination, and whether such a process is mediated by the endosymbiotic algae or can be controlled by the coral host itself. Using a modified Ussing chamber in combination with H+ sensitive microelectrode measurements, the present work demonstrates that light triggers the generation of a skeleton positive potential of up to 0.9 mV in the hermatypic coral Stylophora pistillata. This potential is generated by a net flux of cations towards the skeleton and reaches its maximum at blue (450 nm) light. The effects of pharmacological inhibitors targeting photosynthesis 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and anion transport 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) were investigated by pH microelectrode measurements in coral tissues demonstrating a rapid decrease in tissue pH under illumination. However, these inhibitors showed no effect on the electrophysiological light response of the coral host. By contrast, metabolic inhibition by cyanide and deoxyglucose reversibly inhibited the light-induced cation flux towards the skeleton. These results suggest that ion transport across coral epithelia is directly triggered by blue light, independent of photosynthetic activity of algal endosymbionts. Measurements of this very specific and quantifiable physiological response can provide parameters to identify photoreception mechanisms and will help to broaden our understanding of the mechanistic link between light stimulation and epithelial ion transport, potentially relevant for calcification in hermatypic corals.

Accessibility of Cations to the Selectivity Filter of KcsA in the Inactivated State: An Equilibrium Binding Study

Cation binding under equilibrium conditions has been used as a tool to explore the accessibility of permeant and nonpermeant cations to the selectivity filter in three different inactivated models of the potassium channel KcsA. The results show that the stack of ion binding sites (S1 to S4) in the inactivated filter models remain accessible to cations as they are in the resting channel state. The inactivated state of the selectivity filter is therefore “resting-like” under such equilibrium conditions. Nonetheless, quantitative differences in the apparent K_D’s of the binding processes reveal that the affinity for the binding of permeant cations to the inactivated channel models, mainly K⁺, decreases considerably with respect to the resting channel. This is likely to cause a loss of K⁺ from the inactivated filter and consequently, to promote nonconductive conformations. The most affected site by the affinity loss seems to be S4, which is interesting because S4 is the first site to accommodate K⁺ coming from the channel vestibule when K⁺ exits the cell. Moreover, binding of the nonpermeant species, Na⁺, is not substantially affected by inactivation, meaning that the inactivated channels are also less selective for permeant versus nonpermeant cations under equilibrium conditions.

Cationic peptides from enzymatic hydrolysates of soybean proteins exhibit LPS-neutralizing and angiogenic activities

In this study, we prepared fractions containing multifunctional cationic peptides by separating the commercial soybean protein hydrolysate Hinute-AM into 20 fractions. These fractions contained peptides with various isoelectric points (pI), as indicated by ampholyte-free isoelectric focusing (autofocusing). Thus, we purified and identified the cationic peptides from fractions 19 and 20, which had pH values greater than 10, using reversed-phase high-performance liquid chromatography and matrix-assisted laser/desorption ionization-time-of-flight mass spectroscopy. Among 19 identified cationic peptides, NKNAKPPSPR, PGKKNAIV, KSGPGMSPR, NVSKPPRVV, RKVGAGGRKPLG, and LPCVIGGVPKRV had high pI values and were included as chemically synthesized peptides in assays of various functions, including lipopolysaccharide (LPS)-neutralizing and angiogenic activities. Chromogenic LPS-neutralizing assays using Limulus amebocyte lysates showed that 50% effective concentrations of these six peptides were between 1.63 and 2.65 μM, and were higher than that (0.12 μM) of polymyxin B. Moreover, in tube-formation assays in human umbilical vein endothelial cells, all of the six cationic peptides except LPCVIGGVPKRV exhibited angiogenic activities similar to those of the positive control LL-37. In addition, the six identified cationic peptides had no hemolytic activity at concentrations up to 500 μM in mammalian red blood cells. Our results demonstrate that five of the identified cationic peptides, excluding LPCVIGGVPKRV, have multiple functions and little or no hemolytic activity. These data indicate that fractions containing cationic peptides from Hinute-AM have the potential to be used as dietary supplements and functional ingredients in food products.

Identification of zebrafish steroid sulfatase and comparative analysis of the enzymatic properties with human steroid sulfatase

Steroid sulfatase (STS) plays an important role in the regulation of steroid hormones. Metabolism of steroid hormones in zebrafish has been investigated, but the action of steroid sulfatase remains unknown. In this study, a zebrafish sts was cloned, expressed, purified, and characterized in comparison with the orthologous human enzyme. Enzymatic assays demonstrated that similar to human STS, zebrafish Sts was most active in catalyzing the hydrolysis of estrone-sulfate and estradiol-sulfate, among five steroid sulfates tested as substrates. Kinetic analyses revealed that the Km values of zebrafish Sts and human STS differed with respective substrates, but the catalytic efficiency as reflected by the Vmax/Km appeared comparable, except for DHEA-sulfate with which zebrafish Sts appeared less efficient. While zebrafish Sts was catalytically active at 28 °C, the enzyme appeared more active at 37 °C and with similar Km values to those determined at 28 °C. Assays performed in the presence of different divalent cations showed that the activities of both zebrafish and human STSs were stimulated by Ca2+, Mg2+, and Mn2+, and inhibited by Zn+2 and Fe2+. EMATE and STX64, two known mammalian steroid sulafatase inhibitors, were shown to be capable of inhibiting the activity of zebrafish Sts. Collectively, the results obtained indicated that zebrafish Sts exhibited enzymatic characteristics comparable to the human STS, suggesting that the physiological function of STS may be conserved between zebrafish and humans.

Evolution of cation binding in the active sites of P-loop nucleoside triphosphatases in relation to the basic catalytic mechanism

The ubiquitous P-loop fold nucleoside triphosphatases (NTPases) are typically activated by an arginine or lysine 'finger'. Some of the apparently ancestral NTPases are, instead, activated by potassium ions. To clarify the activation mechanism, we combined comparative structure analysis with molecular dynamics (MD) simulations of Mg-ATP and Mg-GTP complexes in water and in the presence of potassium, sodium, or ammonium ions. In all analyzed structures of diverse P-loop NTPases, the conserved P-loop motif keeps the triphosphate chain of bound NTPs (or their analogs) in an extended, catalytically prone conformation, similar to that imposed on NTPs in water by potassium or ammonium ions. MD simulations of potassium-dependent GTPase MnmE showed that linking of alpha- and gamma phosphates by the activating potassium ion led to the rotation of the gamma-phosphate group yielding an almost eclipsed, catalytically productive conformation of the triphosphate chain, which could represent the basic mechanism of hydrolysis by P-loop NTPases.

A response surface optimization approach to adjust ionic current conductances of cardiac electrophysiological models. Application to the study of potassium level changes

Cardiac electrophysiological computational models are often developed from previously published models. The new models may incorporate additional features to adapt the model to a different species or may upgrade a specific ionic formulation based on newly available experimental data. A relevant challenge in the development of a new model is the estimation of certain ionic current conductances that cannot be reliably identified from experiments. A common strategy to estimate those conductances is by means of constrained non-linear least-squares optimization. In this work, a novel methodology is proposed for estimation of ionic current conductances of cardiac electrophysiological models by using a response surface approximation-based constrained optimization with trust region management. Polynomial response surfaces of a number of electrophysiological markers were built using statistical sampling methods. These markers included action potential duration (APD), triangulation, diastolic and systolic intracellular calcium concentration, and time constants of APD rate adaptation. The proposed methodology was applied to update the Carro et al. human ventricular action potential model after incorporation of intracellular potassium ([K⁺]_i) dynamics. While the Carro et al. model was well suited for investigation of arrhythmogenesis, it did not allow simulation of [K⁺]_i changes. With the methodology proposed in this study, the updated Carro et al. human ventricular model could be used to simulate [K⁺]_i changes in response to varying extracellular potassium ([K⁺]_o) levels. Additionally, it rendered values of evaluated electrophysiological markers within physiologically plausible ranges. The optimal values of ionic current conductances in the updated model were found in a notably shorter time than with previously proposed methodologies. As a conclusion, the response surface optimization-based approach proposed in this study allows estimating ionic current conductances of cardiac electrophysiological computational models while guaranteeing replication of key electrophysiological features and with an important reduction in computational cost with respect to previously published approaches. The updated Carro et al. model developed in this study is thus suitable for the investigation of arrhythmic risk-related conditions, including those involving large changes in potassium concentration.

ATP competes with PIP2 for binding to gelsolin

Gelsolin is a severing and capping protein that targets filamentous actin and regulates filament lengths near plasma membranes, contributing to cell movement and plasma membrane morphology. Gelsolin binds to the plasma membrane via phosphatidylinositol 4,5-bisphosphate (PIP2) in a state that cannot cap F-actin, and gelsolin-capped actin filaments are uncapped by PIP2 leading to filament elongation. The process by which gelsolin is removed from PIP2 at the plasma membrane is currently unknown. Gelsolin also binds ATP with unknown function. Here we characterize the role of ATP on PIP2-gelsolin complex dynamics. Fluorophore-labeled PIP2 and ATP were used to study their interactions with gelsolin using steady-state fluorescence anisotropy, and Alexa488-labeled gelsolin was utilized to reconstitute the regulation of gelsolin binding to PIP2-containing phospholipid vesicles by ATP. Under physiological salt conditions ATP competes with PIP2 for binding to gelsolin, while calcium causes the release of ATP from gelsolin. These data suggest a cycle for gelsolin activity. Firstly, calcium activates ATP-bound gelsolin allowing it to sever and cap F-actin. Secondly, PIP2-binding removes the gelsolin cap from F-actin at low calcium levels, leading to filament elongation. Finally, ATP competes with PIP2 to release the calcium-free ATP-bound gelsolin, allowing it to undergo a further round of severing.

The permeation mechanism of organic cations through a CNG mimic channel

Several channels, ranging from TRP receptors to Gap junctions, allow the exchange of small organic solute across cell membrane. However, very little is known about the molecular mechanism of their permeation. Cyclic Nucleotide Gated (CNG) channels, despite their homology with K+ channels and in contrast with them, allow the passage of larger methylated and ethylated ammonium ions like dimethylammonium (DMA) and ethylammonium (EA). We combined electrophysiology and molecular dynamics simulations to examine how DMA interacts with the pore and permeates through it. Due to the presence of hydrophobic groups, DMA enters easily in the channel and, unlike the alkali cations, does not need to cross any barrier. We also show that while the crystal structure is consistent with the presence of a single DMA ion at full occupancy, the channel is able to conduct a sizable current of DMA ions only when two ions are present inside the channel. Moreover, the second DMA ion dramatically changes the free energy landscape, destabilizing the crystallographic binding site and lowering by almost 25 kJ/mol the binding affinity between DMA and the channel. Based on the results of the simulation the experimental electron density maps can be re-interpreted with the presence of a second ion at lower occupancy. In this mechanism the flexibility of the channel plays a key role, extending the classical multi-ion permeation paradigm in which conductance is enhanced by the plain interaction between the ions.

Microbial Mineralization of Montmorillonite in Low-Permeability Oil Reservoirs for Microbial Enhanced Oil Recovery

Microbial mineralization (corrosion, decomposition, and weathering) has been investigated for its role in the extraction and recovery of metals from ores. Here we report our application of biomineralization for the microbial enhanced oil recovery in low-permeability oil reservoirs. It aimed to reveal the etching mechanism of the four Fe(III)-reducing microbial strains under anaerobic growth conditions on Ca-montmorillonite. The mineralogical characterization of Ca-montmorillonite was performed by Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, and energy-dispersive spectrometry. Results showed that the microbial strains could efficiently reduce Fe(III) at an optimal rate of 71%, alter the crystal lattice structure of the lamella to promote interlayer cation exchange, and efficiently inhibit Ca-montmorillonite swelling at a rate of 48.9%.IMPORTANCE Microbial mineralization is ubiquitous in the natural environment. Microbes in low-permeability reservoirs are able to facilitate alteration of the structure and phase of the Fe-poor minerals by reducing Fe(III) and inhibiting clay swelling, which is still poorly studied. This study aimed to reveal the interaction mechanism between Fe(III)-reducing bacterial strains and Ca-montmorillonite under anaerobic conditions and to investigate the extent and rates of Fe(III) reduction and phase changes with their activities. Application of Fe(III)-reducing bacteria will provide a new way to inhibit clay swelling, to elevate reservoir permeability, and to reduce pore throat resistance after water flooding for enhanced oil recovery in low-permeability reservoirs.

Mitochondrial membrane potential

The mitochondrial membrane potential (ΔΨm) generated by proton pumps (Complexes I, III and IV) is an essential component in the process of energy storage during oxidative phosphorylation. Together with the proton gradient (ΔpH), ΔΨm forms the transmembrane potential of hydrogen ions which is harnessed to make ATP. The levels of ΔΨm and ATP in the cell are kept relatively stable although there are limited fluctuations of both these factors that can occur reflecting normal physiological activity. However, sustained changes in both factors may be deleterious. A long-lasting drop or rise of ΔΨm vs normal levels may induce unwanted loss of cell viability and be a cause of various pathologies. Among other factors, ΔΨm plays a key role in mitochondrial homeostasis through selective elimination of dysfunctional mitochondria. It is also a driving force for transport of ions (other than H+) and proteins which are necessary for healthy mitochondrial functioning. We propose additional potential mechanisms for which ΔΨm is essential for maintenance of cellular health and viability and provide recommendations how to accurately measure ΔΨm in a cell and discuss potential sources of artifacts.

Characterization of a non-nudix pyrophosphatase points to interplay between flavin and NAD(H) homeostasis in Saccharomyces cerevisiae

The flavin cofactors FMN and FAD are required for a wide variety of biological processes, however, little is known about their metabolism. Here, we report the cloning and biochemical characterization of the Saccharomyces cerevisiae pyrophosphatase Fpy1p. Genetic and functional studies suggest that Fpy1p may play a key role in flavin metabolism and is the first-reported non-Nudix superfamily enzyme to display FAD pyrophosphatase activity. Characterization of mutant yeast strains found that deletion of fpy1 counteracts the adverse effects that are caused by deletion of flx1, a known mitochondrial FAD transporter. We show that Fpy1p is capable of hydrolyzing FAD, NAD(H), and ADP-ribose. The enzymatic activity of Fpy1p is dependent upon the presence of K+ and divalent metal cations, with similar kinetic parameters to those that have been reported for Nudix FAD pyrophosphatases. In addition, we report that the deletion of fpy1 intensifies the FMN-dependence of null mutants of the riboflavin kinase Fmn1p, demonstrate that fpy1 mutation abolishes the decreased fitness resulting from the deletion of the flx1 ORF, and offer a possible mechanism for the genetic interplay between fpy1, flx1 and fmn1.

Diverse antimalarials from whole-cell phenotypic screens disrupt malaria parasite ion and volume homeostasis

Four hundred structurally diverse drug-like compounds comprising the Medicines for Malaria Venture's 'Pathogen Box' were screened for their effect on a range of physiological parameters in asexual blood-stage malaria (Plasmodium falciparum) parasites. Eleven of these compounds were found to perturb parasite Na+, pH and volume in a manner consistent with inhibition of the putative Na+ efflux P-type ATPase PfATP4. All eleven compounds fell within the subset of 125 compounds included in the Pathogen Box on the basis of their having been identified as potent inhibitors of the growth of asexual blood-stage P. falciparum parasites. All eleven compounds inhibited the Na+-dependent ATPase activity of parasite membranes and showed reduced efficacy against parasites carrying mutations in PfATP4. This study increases the number of chemically diverse structures known to show a 'PfATP4-associated' phenotype, and adds to emerging evidence that a high proportion (7-9%) of the structurally diverse antimalarial compounds identified in whole cell phenotypic screens share the same mechanism of action, exerting their antimalarial effect via an interaction with PfATP4.

Influence of Na+ and Mg2+ ions on RNA structures studied with molecular dynamics simulations

The structure of ribonucleic acid (RNA) polymers is strongly dependent on the presence of, in particular Mg2+ cations to stabilize structural features. Only in high-resolution X-ray crystallography structures can ions be identified reliably. Here, we perform molecular dynamics simulations of 24 RNA structures with varying ion concentrations. Twelve of the structures were helical and the others complex folded. The aim of the study is to predict ion positions but also to evaluate the impact of different types of ions (Na+ or Mg2+) and the ionic strength on structural stability and variations of RNA. As a general conclusion Mg2+ is found to conserve the experimental structure better than Na+ and, where experimental ion positions are available, they can be reproduced with reasonable accuracy. If a large surplus of ions is present the added electrostatic screening makes prediction of binding-sites less reproducible. Distinct differences in ion-binding between helical and complex folded structures are found. The strength of binding (ΔG‡ for breaking RNA atom-ion interactions) is found to differ between roughly 10 and 26 kJ/mol for the different RNA atoms. Differences in stability between helical and complex folded structures and of the influence of metal ions on either are discussed.

Reversible Cation-Selective Attachment and Self-Assembly of Human Tau on Supported Brain Lipid Membranes

Misfolding and aggregation of the neuronal, microtubule-associated protein tau is involved in the pathogenesis of Alzheimer's disease and tauopathies. It has been proposed that neuronal membranes could play a role in tau release, internalization, and aggregation and that tau aggregates could exert toxicity via membrane permeabilization. Whether and how tau interacts with lipid membranes remains a matter of discussion. Here, we characterize the interaction of full-length human tau (htau40) with supported lipid membranes (SLMs) made from brain total lipid extract by time-lapse high-resolution atomic force microscopy (AFM). We observe that tau attaches to brain lipid membranes where it self-assembles in a cation-dependent manner. Sodium triggers the attachment, self-assembly, and growth, whereas potassium inhibits these processes. Moreover, tau assemblies are stable in the presence of sodium and lithium but disassemble in the presence of potassium and rubidium. Whereas the pseudorepeat domains (R1-R4) of htau40 promote the sodium-dependent attachment to the membrane and stabilize the tau assemblies, the N-terminal region promotes tau self-assembly and growth.

Cationic nucleopeptides as novel non-covalent carriers for the delivery of peptide nucleic acid (PNA) and RNA oligomers

Cationic nucleopeptides belong to a family of synthetic oligomers composed by amino acids and nucleobases. Their capability to recognize nucleic acid targets and to cross cellular membranes provided the basis for considering them as novel non-covalent delivery agents for nucleic acid pharmaceuticals. Herein, starting from a 12-mer nucleopeptide model, the number of cationic residues was modulated in order to obtain new nucleopeptides endowed with high solubility in acqueous medium, acceptable bio-stability, low cytotoxicity and good capability to bind nucleic acid. Two candidates were selected to further investigate their potential as nucleic acid carriers, showing higher efficiency to deliver PNA in comparison with RNA. Noteworthy, this study encourages the development of nucleopeptides as new carriers to extend the known strategies for those nucleic acid analogues, especially PNA, that still remain difficult to drive into the cells.

Single-molecule compaction of megabase-long chromatin molecules by multivalent cations

To gain insight into the conformational properties and compaction of megabase-long chromatin molecules, we reconstituted chromatin from T4 phage DNA (165 kb) and recombinant human histone octamers (HO). The unimolecular compaction, induced by divalent Mg2+ or tetravalent spermine4+ cations, studied by single-molecule fluorescence microscopy (FM) and dynamic light scattering (DLS) techniques, resulted in the formation of 250-400 nm chromatin condensates. The compaction on this scale of DNA size is comparable to that of chromatin topologically associated domains (TAD) in vivo. Variation of HO loading revealed a number of unique features related to the efficiency of chromatin compaction by multivalent cations, the mechanism of compaction, and the character of partly compact chromatin structures. The observations may be relevant for how DNA accessibility in chromatin is maintained. Compaction of saturated chromatin, in turn, is accompanied by an intra-chain segregation at the level of single chromatin molecules, suggesting an intriguing scenario of selective activation/deactivation of DNA as a result of chromatin fiber heterogeneity due to the nucleosome positioning. We suggest that this chromatin, reconstituted on megabase-long DNA because of its large size, is a useful model of eukaryotic chromatin.

Physical Chemical and Biomolecular Methods for the Optimization of Cationic Lipid-Based Lipoplexes In Vitro for the Gene Therapy Applications

Preparation and application protocols play a very important role while optimizing the cationic lipid-based lipoplexes in vitro. These protocols serve as the basis for the betterment of the lipoplexes with regard to their successful application in animals and eventually human subjects. Starting from the chemical structures of used cationic lipids (CLs), optimization of the additive inclusions, methods of nanoparticle (lipoplex) formation, presence of blood serum, time intervals of lipoplex incubation, and type of efficiency read-outs in various conditions play important roles in reaching insightful conclusions. Such steps of summarizing protocols and requirements of the pertinent events focus on getting improved lipoplexes for achieving optimal effects in terms of post transfection gene and protein expression. The progression of optimization and efficiency evaluation lead to predictable structure-method-activity relationship with involvement of various feedback principles including physical chemical and biomolecular evaluations before and after the use of lipoplexes in biological systems. This chapter discusses some of the focused strategies for the establishment of lipoplexes for a better post transfection activity with reduced risk of failure.

The cationic (calcium and lead) and enzyme conundrum

The environmental toxicant lead (Pb) and the essential element calcium (Ca) play an interactive role in extracellular and intracellular regulatory functions that affect health. Lead's usurping calcium binding sites, as well as its interactions with thiols and phosphates have been suggested to be the basis for adverse effects on many organ systems especially the nervous system. Among regulatory processes controlled by Ca are calmodulin-dependent phosphodiesterase, calmodulin-dependent protein kinases, calmodulin inhibitor sensitive potassium channels, and calmodulin-independent protein kinase C (PKC) activation. This review focused on Pb studies describing the modulation of PKC, which is also regulated by steroids. Steroid hormone regulation may relate to a focal point for the sex differences of Pb and cellular signaling events. Picomolar concentrations of Pb may stimulate partially purified PKC, but higher concentrations inhibit activity. Although knowledge exists regarding Pb and PKC isoforms, especially interaction of Pb with the purified enzyme, there are conflicting reports concerning metal-mediated activation or inhibition of PKC and downstream signaling events. The effect of Pb on PKC in vivo remains elusive. Most reports of Pb and PKC in whole animal and human studies indicated that Pb either inhibits PKC or exerts no significant effect. However, most of the animal studies were performed with males. Recent studies performed with females and males separately revealed that females and males respond to Pb quite differently, and for this reason, it is suggested that future Pb studies of PKC and other biomedical investigations be performed with females and males.

The grapevine VvCAX3 is a cation/H+ exchanger involved in vacuolar Ca2+ homeostasis

The grapevine VvCAX3 mediates calcium transport in the vacuole and is mostly expressed in green grape berries and upregulated by Ca ²⁺ , Na ⁺ and methyl jasmonate. Calcium is an essential plant nutrient with important regulatory and structural roles in the berries of grapevine (Vitis vinifera L.). On the other hand, the proton-cation exchanger CAX proteins have been shown to impact Ca²⁺ homeostasis with important consequences for fruit integrity and resistance to biotic/abiotic stress. Here, the CAX gene found in transcriptomic databases as having one of the highest expressions in grapevine tissues, VvCAX3, was cloned and functionally characterized. Heterologous expression in yeast showed that a truncated version of VvCAX3 lacking its NNR autoinhibitory domain (sCAX3) restored the ability of the yeast strain to grow in 100-200 mM Ca²⁺, demonstrating a role in Ca²⁺ transport. The truncated VvCAX3 was further shown to be involved in the transport of Na⁺, Li⁺, Mn²⁺ and Cu²⁺ in yeast cells. Subcellular localization studies using fluorescently tagged proteins confirmed VvCAX3 as a tonoplast transporter. VvCAX3 is expressed in grapevine stems, leaves, roots, and berries, especially at pea size, decreasing gradually throughout development, in parallel with the pattern of calcium accumulation in the fruit. The transcript abundance of VvCAX3 was shown to be regulated by methyl jasmonate (MeJA), Ca²⁺, and Na⁺ in grape cell suspensions, and the VvCAX3 promotor contains several predicted cis-acting elements related to developmental and stress response processes. As a whole, the results obtained add new insights on the mechanisms involved in calcium homeostasis and intracellular compartmentation in grapevine, and indicate that VvCAX3 may be an interesting target towards the development of strategies for enhancement of grape berry properties.

Control of basal jasmonate signalling and defence through modulation of intracellular cation flux capacity

Unknown mechanisms tightly regulate the basal activity of the wound-inducible defence mediator jasmonate (JA) in undamaged tissues. However, the Arabidopsis fatty acid oxygenation upregulated2 (fou2) mutant in vacuolar two-pore channel 1 (TPC1^D454N ) displays high JA pathway activity in undamaged leaves. This mutant was used to explore mechanisms controlling basal JA pathway regulation. fou2 was re-mutated to generate novel 'ouf' suppressor mutants. Patch-clamping was used to examine TPC1 cation channel characteristics in the ouf suppressor mutants and in fou2. Calcium (Ca²⁺ ) imaging was used to study the effects fou2 on cytosolic Ca²⁺ concentrations. Six intragenic ouf suppressors with near wild-type (WT) JA pathway activity were recovered and one mutant, ouf8, affected the channel pore. At low luminal calcium concentrations, ouf8 had little detectable effect on fou2. However, increased vacuolar Ca²⁺ concentrations caused channel occlusion, selectively blocking K⁺ fluxes towards the cytoplasm. Cytosolic Ca²⁺ concentrations in unwounded fou2 were found to be lower than in the unwounded WT, but they increased in a similar manner in both genotypes following wounding. Basal JA pathway activity can be controlled solely by manipulating endomembrane cation flux capacities. We suggest that changes in endomembrane potential affect JA pathway activity.

Transition metal binding selectivity in proteins and its correlation with the phylogenomic classification of the cation diffusion facilitator protein family

Divalent d-block metal cations (DDMCs), such as Fe, Zn and Mn, participate in many biological processes. Understanding how specific DDMCs are transported to and within the cell and what controls their binding selectivity to different proteins is crucial for defining the mechanisms of metalloproteins. To better understand such processes, we scanned the RCSB Protein Data Bank, performed a de novo structural-based comprehensive analysis of seven DDMCs and found their amino acid binding and coordination geometry propensities. We then utilized these results to characterize the correlation between metal selectivity, specific binding site composition and phylogenetic classification of the cation diffusion facilitator (CDF) protein family, a family of DDMC transporters found throughout evolution and sharing a conserved structure, yet with different members displaying distinct metal selectivity. Our analysis shows that DDMCs differ, at times significantly, in terms of their binding propensities, and that in each CDF clade, the metal selectivity-related binding site has a unique and conserved sequence signature. However, only limited correlation exists between the composition of the DDMC binding site in each clade and the metal selectivity shown by its proteins.

Input of different kinds of soluble pectin to cation binding properties of roots cell walls

It is widely believed that pectin are responsible for the vast majority of cation binding positions in the root cell walls. To estimate the role of particular kinds of pectin, we studied the cell wall material isolated from the roots of monocots (wheat and rye) and dicots (clover and lupine) before and after removal of different fractions of soluble pectin. Simultaneously PME activity and degree of pectin methylation were determined. From potentiometric titration curves cation exchange capacity, total surface charge and acidic strength of surface functional groups responsible for surface charging were determined. Monocots had smaller cation exchange capacity and lower pectin content than dicots. Removal of pectin induced up to 50% reduction in the cell walls surface charge. Pectin seem to have more acidic character than the other roots components that is seen from an increase in very weakly acidic groups fraction and significant decrease in the average dissociation constant of the cell walls material after pectin removal. Water soluble pectin and non-pectic soluble compounds had the dominant role in surface charging, while chelator and diluted alkali soluble pectin contributed to surface charge only at high pH's.