Role of Arabidopsis RAB5 GEF vps9a in maintaining potassium levels under sodium chloride stress.

Salt stress is one of the major factors impacting crop productivity worldwide. Through a variety of effector and signaling pathways, plants achieve survival under salinity stress by maintaining high cytosolic potassium/sodium ion (K+/Na+) ratios, preventing Na+ cytotoxicity, and retaining osmotic balance. Ras-related protein 5 (Rab5) members are involved in the trafficking of endosomes to the vacuole or plasma membrane (PM). The vacuolar protein sorting- associated protein 9 (vps9a) encodes the single guanine nucleotide exchange factor (GEF) that activates all three known Rab5 proteins in Arabidopsis thaliana. Previous work from our group has reported the critical function of vps9a for the operation of salt-induced endocytic pathway, as well as the expansion of endomembrane compartments under saline stress conditions. Here we show an additional role of vps9a in plant response to salt stress via maintenance of K+ status of the cell rather than Na+ homeostasis. Our results show that roots from vps9a-2 mutant, subjected to 100 mM NaCl, display alterations in transcript levels of genes involved in the K+ homeostasis pathway. Concurrent with the observed sensitivity of vps9a-2 mutant under NaCl stress, exposure to low K+ environments resulted in growth retardation, and reduced rate of endocytosis. Furthermore, vps9a-2 mutant displays reduced expression of auxin reporter, Direct Repeat-5 (DR5), and alterations in polarity and abundance of auxin efflux carrier PIN- FORMED2 (PIN2). Imposition of NaCl stress was found to be restrictive to the elongation capacity of cells in the root elongation zone of vps9a-2 mutant. Together our results indicate that alterations in K+ homeostasis and associated cellular changes causing increased cell wall pH, contribute to diminished root growth and compromised survival of vps9a-2 mutant under salt stress conditions.

Pathological and immunological protection induced by inactivated reverse genetics-based H3N8 equine influenza vaccine candidate in murine model.

Equine influenza (EI) is an important viral respiratory disease of equines caused by influenza A virus (IAV). The antigenic drift in IAVs necessitates regular updating and harmonization of vaccine strain with the circulating virus. The reverse genetics-based recombinant viruses could be easy instrument in generating vaccine against circulating virus in a quick and effective manner. Present study has been envisaged to evaluate the immunogenicity and protective efficacy of inactivated recombinant equine influenza virus (rgEIV) vaccine candidate having six segments from H1N1 virus (A/WSN/33/H1N1) and HA (hemaglutinin) and NA (neuraminidase) segments from H3N8 equine influenza virus [(A/eq/Jammu-Katra/06/08) of clade 2 of Florida sublineage] generated through reverse genetic engineering. BALB/c mice were immunized with inactivated rgEIV adjuvanted with aluminium hydroxide gel and challenged with H3N8 virus (A/eq/Jammu-Katra/06/08). The protective efficacy was evaluated through serology, cytokine profiling, clinical signs, gross and histopathological changes, immunohistochemistry and residual virus quantification. Immunizations induced robust humoral immune response as estimated through hemagglutination inhibition assay (HAI). The antibodies were isotyped and the predominant subclass was IgG1. The vaccine candidate produced mixed Th1 and Th2 responses through stimulation of IFN-γ, IL-2, IL-4 and IL-6 expression. Immunization protected mice against challenge as reflected through reduction in clinical signs and body weight loss, early recovery, mild pathological changes (gross and histopathological lesions) as evident through scoring of lesions, low residual virus in nasopharynx and lungs quantified through egg titration and quantitative reverse transcriptase PCR (qRT-PCR). The study demonstrates that inactivated recombinant EIV generated through reverse genetic approach provides equivalent protection to that observed with inactivated whole H3N8 EIV vaccine. Keywords: equine influenza; reverse genetics; vaccine; pathology; murine model.

The Ca2+ Channel CNGC19 Regulates Arabidopsis Defense Against Spodoptera Herbivory.

Cellular calcium elevation is an important signal used by plants for recognition and signaling of environmental stress. Perception of the generalist insect, Spodoptera litura, by Arabidopsis (Arabidopsis thaliana) activates cytosolic Ca2+ elevation, which triggers downstream defense. However, not all the Ca2+ channels generating the signal have been identified, nor are their modes of action known. We report on a rapidly activated, leaf vasculature- and plasma membrane-localized, CYCLIC NUCLEOTIDE GATED CHANNEL19 (CNGC19), which activates herbivory-induced Ca2+ flux and plant defense. Loss of CNGC19 function results in decreased herbivory defense. The cngc19 mutant shows aberrant and attenuated intravascular Ca2+ fluxes. CNGC19 is a Ca2+-permeable channel, as hyperpolarization of CNGC19-expressing Xenopus oocytes in the presence of both cyclic adenosine monophosphate and Ca2+ results in Ca2+ influx. Breakdown of Ca2+-based defense in cngc19 mutants leads to a decrease in herbivory-induced jasmonoyl-l-isoleucine biosynthesis and expression of JA responsive genes. The cngc19 mutants are deficient in aliphatic glucosinolate accumulation and hyperaccumulate its precursor, methionine. CNGC19 modulates aliphatic glucosinolate biosynthesis in tandem with BRANCHED-CHAIN AMINO ACID TRANSAMINASE4, which is involved in the chain elongation pathway of Met-derived glucosinolates. Furthermore, CNGC19 interacts with herbivory-induced CALMODULIN2 in planta. Together, our work reveals a key mechanistic role for the Ca2+ channel CNGC19 in the recognition of herbivory and the activation of defense signaling.

Mechanism of aggregation and membrane interactions of mammalian prion protein.

The cellular prion protein (PrPC), which is present ubiquitously in all mammalian neurons, is normally found to be linked to the cell membrane through a glycosylphosphatidylinositol (GPI) anchor. The conformational conversion of PrPC into misfolded and aggregated forms is associated with transmissible neurodegenerative diseases known as prion diseases. The importance of different misfolded conformations in prion diseases, and the mechanism by which prion aggregates induce neurotoxicity remain poorly understood. Multiple studies have been shown that the toxicity of misfolded prion protein is directly correlated with its ability to interact with and perturb membranes. This review describes the current progress toward understanding prion protein misfolding and aggregation, as well as the interaction of prion protein aggregates with lipid membrane.

Fluid flow modulates electrical activity in cardiac hERG potassium channels.

Fluid movement within the heart generates substantial shear forces, but the effect of this mechanical stress on the electrical activity of the human heart has not been examined. The fast component of the delayed rectifier potassium currents responsible for repolarization of the cardiac action potential, Ikr, is encoded by the human ether-a-go-go related gene (hERG) channel. Here, we exposed hERG1a channel-expressing HEK293T cells to laminar shear stress (LSS) and observed that this mechanical stress increased the whole-cell current by 30-40%. LSS shifted the voltage dependence of steady-state activation of the hERG channel to the hyperpolarizing direction, accelerated the time course of activation and recovery from inactivation, slowed down deactivation, and shifted the steady-state inactivation to the positive direction, all of which favored the hERG open state. In contrast, the time course of inactivation was faster, favoring the closed state. Using specific inhibitors of focal adhesion kinase, a regulator of mechano-transduction via the integrin pathway, we also found that the LSS-induced modulation of the whole-cell current depended on the integrin pathway. The hERG1b channel variant, which lacks the Per-Arnt-Sim (PAS) domain, and long QT syndrome-associated variants having point mutations in the PAS domain were unaffected by LSS, suggesting that the PAS domain in hERG1a channel may be involved in sensing mechanical shear stress. We conclude that a mechano-electric feedback pathway modulates hERG channel activity through the integrin pathway, indicating that mechanical forces in the heart influence its electrical activity.

Regulation of VDAC trafficking modulates cell death.

The voltage-dependent anion channel (VDAC) and mitochondria-associated hexokinase (HxK) have crucial roles in both cell survival and death. Both the individual abundances and their ratio seem to influence the balance of survival and death and are thus critical in scenarios, such as neurodegeneration and cancer. Elevated levels of both VDAC and HxK have been reported in cancerous cells. Physical interaction is surmised and specific residues or regions involved have been identified, but details of the interaction and the mechanism by which it modulates survival are yet to be elucidated. We and others have shown that heterologous expression of VDAC can induce cell death, which can be mitigated by concomitant overexpression of HxK. We have also observed that upon overexpression, fluorescently tagged VDAC is distributed between the cytosol and mitochondria. In this study, we show that cell death ensues only when the protein, which is synthesized on cytoplasmic ribosomes, migrates to the mitochondrion. Further, coexpression of rat HxK II (rHxKII) can delay the translocation of human VDAC1 (hVDAC1) protein to mitochondria and thereby inhibit VDAC-induced cell death. Variation in the level of HxK protein as seen endogenously in different cell lines, or as experimentally manipulated by silencing and overexpression, can lead to differential VDAC translocation kinetics and related cell death. The N-terminal region of HxK and the Glu73 residue of hVDAC1, which have previously been implicated in a physical interaction, are required for cytosolic retention of VDAC. Finally, we show that, in otherwise unperturbed cells in culture, there is a small but significant amount of soluble VDAC in the cytosol present in a complex with HxK. This complex could well determine how a cell is poised with respect to incoming thanatopic signals, thereby tilting the survival/death balance in pharmacologically interesting situations, such as neurodegeneration and cancer.

The Pathogenic A116V Mutation Enhances Ion-Selective Channel Formation by Prion Protein in Membranes.

Prion diseases are a group of fatal neurodegenerative disorders that afflict mammals. Misfolded and aggregated forms of the prion protein (PrP(Sc)) have been associated with many prion diseases. A transmembrane form of PrP favored by the pathogenic mutation A116V is associated with Gerstmann-Sträussler-Scheinker syndrome, but no accumulation of PrP(Sc) is detected. However, the role of the transmembrane form of PrP in pathological processes leading to neuronal death remains unclear. This study reports that the full-length mouse PrP (moPrP) significantly increases the permeability of living cells to K(+), and forms K(+)- and Ca(2+)-selective channels in lipid membranes. Importantly, the pathogenic mutation A116V greatly increases the channel-forming capability of moPrP. The channels thus formed are impermeable to sodium and chloride ions, and are blocked by blockers of voltage-gated ion channels. Hydrogen-deuterium exchange studies coupled with mass spectrometry (HDX-MS) show that upon interaction with lipid, the central hydrophobic region (109-132) of the protein is protected against exchange, making it a good candidate for inserting into the membrane and lining the channel. HDX-MS also shows a dramatic increase in the protein-lipid stoichiometry for A116V moPrP, providing a rationale for its increased channel-forming capability. The results suggest that ion channel formation may be a possible mechanism of PrP-mediated neurodegeneration by the transmembrane forms of PrP.

Functional characterization of a transition metal ion transporter, OsZIP6 from rice (Oryza sativa L.).

Micronutrients are important for the growth and development of plants, which deploy families of transporters for their uptake and distribution. We have functionally characterized a novel transition metal ion transporter from rice, OsZIP6 (Oryza sativa zinc regulated transporter, iron regulated transporter-like protein 6). The transporter was found to be transcriptionally activated in shoot and root tissues in response to deficiency in Fe(2+), Zn(2+) and Mn(2+). OsZIP6 was expressed in Xenopus laevis oocytes, where currents were observed on addition of Co(2+), Fe(2+) and Cd(2+) but not Zn(2+), Mn(2+) and Ni(2+). This substrate range for OsZIP6, identified using two-electrode voltage clamp electrophysiology was confirmed by atomic absorption spectroscopy. Ion transport by OsZIP6 was found to be pH dependent and enhanced transport was observed at acidic pH. Radioisotope uptake suggested that Co(2+) competitively inhibits Fe(2+) uptake by OsZIP6. Identification and characterization of ZIP family members from crop plants will contribute to an understanding of nutrient mineral homeostasis in these plants.

Vesicular trafficking and salinity responses in plants.

Research spanning three decades has demonstrated that vesicles pinch off from the plasma membrane and traffic through the cytoplasm of plant cells, much as previously reported in animal cells. Although the well-conserved clathrin-mediated mechanism of endocytosis has been well characterized, relatively little is known about clathrin-independent pathways in plants. Modulation of endocytosis by both physical stimuli and chemical ligands has been reported in plants. Here, we review the effect of salinity-one of the most deleterious environmental assaults-on endocytosis and intracellular trafficking.

Salt-induced remodeling of spatially restricted clathrin-independent endocytic pathways in Arabidopsis root.

Endocytosis is a ubiquitous cellular process that is characterized well in animal cells in culture but poorly across intact, functioning tissue. Here, we analyze endocytosis throughout the Arabidopsis thaliana root using three classes of probes: a lipophilic dye, tagged transmembrane proteins, and a lipid-anchored protein. We observe a stratified distribution of endocytic processes. A clathrin-dependent endocytic pathway that internalizes transmembrane proteins functions in all cell layers, while a sterol-dependent, clathrin-independent pathway that takes up lipid and lipid-anchored proteins but not transmembrane proteins is restricted to the epidermal layer. Saline stress induces a third pathway that is clathrin-independent, nondiscriminatory in its choice of cargo, and operates across all layers of the root. Concomitantly, small acidic compartments in inner cell layers expand to form larger vacuole-like structures. Plants lacking function of the Rab-GEF (guanine nucleotide exchange factor) VPS9a (vacuolar protein sorting 9A) neither induce the third endocytic pathway nor expand the vacuolar system in response to salt stress. The plants are also hypersensitive to salt. Thus, saline stress reconfigures clathrin-independent endocytosis and remodels endomembrane systems, forming large vacuoles in the inner cell layers, both processes correlated by the requirement of VPS9a activity.

Expression and purification of OsVDAC4.

The voltage-dependent anion channel (VDAC), a major component of the mitochondrial outer membrane, has emerged as an important player in cell function, survival, and death signaling. VDAC function is modulated by its interaction with proteins such as hexokinase, adenine nucleotide translocator, and apoptotic proteins like Bax. Monitoring the activity of VDAC and its modulation in the complex cellular milieu is fraught with complications. Minimizing the number of components in the study is one approach to teasing apart various aspects of its function. In this chapter, we have described detailed protocols for the purification of a rice VDAC isoform, OsVDAC4 after overexpression in a bacterial system. The protein is solubilized with LDAO and then reconstituted into liposomes or planar bilayers to verify its competence to fold into a functionally active form.

Measurements of cytosolic ion concentrations in live cells.

Here, we describe a series of methods suitable for the measurement of cytosolic ion concentrations in living plant cells using ion selective dyes. We describe procedures for the use of SBFI for the measurement of Na(+) in live cells. The resulting material is suitable for most standard cell biology procedures.

Development of the structural core and of conformational heterogeneity during the conversion of oligomers of the mouse prion protein to worm-like amyloid fibrils.

Understanding how structure develops during the course of amyloid fibril formation by the prion protein is important for understanding prion diseases. Determining how conformational heterogeneity manifests itself in the fibrillar and pre-fibrillar amyloid aggregates is critical for understanding prion strain phenotypes. In this study, the formation of worm-like amyloid fibrils by the mouse prion protein has been characterized structurally by hydrogen-deuterium exchange coupled to mass spectrometry. The structural cores of these fibrils and of the oligomer on the direct pathway of amyloid fibril formation have been defined, showing how structure develops during fibril formation. The structural core of the oligomer not on the direct pathway has also been defined, allowing the delineation of the structural features that make this off-pathway oligomer incompetent to directly form fibrils. Sequence segments that exhibit multiple local conformations in the three amyloid aggregates have been identified, and the development of structural heterogeneity during fibril formation has been characterized. It is shown that conformational heterogeneity is not restricted to only the C-terminal domain region, which forms the structural core of the aggregates; it manifests itself in the N-terminal domain of the protein as well. Importantly, all three amyloid aggregates are shown to be capable of disrupting lipid membrane structure, pointing to a mechanism by which they may be toxic.

Rice cultivars with differing salt tolerance contain similar cation channels in their root cells.

Salinity poses a major threat for agriculture worldwide. Rice is one of the major crops where most of the high-yielding cultivars are highly sensitive to salinity. Several studies on the genetic variability across rice cultivars suggest that the activity and composition of root plasma membrane transporters could underlie the observed cultivar-specific salinity tolerance in rice. In the current study, it was found that the salt-tolerant cultivar Pokkali maintains a higher K+/Na+ ratio compared with the salt-sensitive IR20 in roots as well as in shoots. Using Na+ reporter dyes, IR20 root protoplasts showed a much faster Na+ accumulation than Pokkali protoplasts. Membrane potential measurements showed that root cells exposed to Na+ in IR20 depolarized considerably further than those of Pokkali. These results suggest that IR20 has a larger plasma membrane Na+ conductance. To assess whether this could be due to different ion channel properties, root protoplasts from both Pokkali and IR20 rice cultivars were patch-clamped. Voltage-dependent K+ inward rectifiers, K+ outward rectifiers, and voltage-independent, non-selective channels with unitary conductances of around 35, 40, and 10 pS, respectively, were identified. Only the non-selective channel showed significant Na+ permeability. Intriguingly, in both cultivars, the activity of the K+ inward rectifier was drastically down-regulated after plant growth in salt but gating, conductance, and activity of all channel types were very similar for the two cultivars.

Bacterial expression, purification and characterization of a rice voltage-dependent, anion-selective channel isoform, OsVDAC4.

The voltage-dependent anion-selective channel (VDAC) is the most abundant protein in the mitochondrial outer membrane and forms the major conduit for metabolite transport across this membrane. VDACs from different sources show varied primary sequence but conserved functional properties. Here, we report on the characterization of a rice channel, OsVDAC4, which complements a VDAC1 deficiency in yeast. We present a consensus secondary structure prediction of an N-terminal α-helix and 19 ß-strands. Bacterially expressed OsVDAC4 was purified from inclusion bodies into detergent-containing solution, where it is largely helical. Detergent-solubilized OsVDAC4 inserts spontaneously into artificial membranes of two topologies-spherical liposomes and planar bilayers. Insertion into liposomes results in an increase in ß-structure. Transport of polyethylene glycols was used to estimate a pore diameter of ~2.6 nm in liposomes. Channels formed in planar bilayers exhibit large conductance (4.6 ± 0.3 nS in 1 M KCl), strong voltage dependence and weak anion selectivity. The open state of the channel is shown to be permeable to ATP. These data are consistent with a large ß-barrel pore formed by OsVDAC4 on inserting into membranes. This study forms a platform to carry out studies of the interaction of OsVDAC4 with putative modulators.

Root apoplastic barriers block Na+ transport to shoots in rice (Oryza sativa L.).

Rice is an important crop that is very sensitive to salinity. However, some varieties differ greatly in this feature, making investigations of salinity tolerance mechanisms possible. The cultivar Pokkali is salinity tolerant and is known to have more extensive hydrophobic barriers in its roots than does IR20, a more sensitive cultivar. These barriers located in the root endodermis and exodermis prevent the direct entry of external fluid into the stele. However, it is known that in the case of rice, these barriers are bypassed by most of the Na(+) that enters the shoot. Exposing plants to a moderate stress of 100 mM NaCl resulted in deposition of additional hydrophobic aliphatic suberin in both cultivars. The present study demonstrated that Pokkali roots have a lower permeability to water (measured using a pressure chamber) than those of IR20. Conditioning plants with 100 mM NaCl effectively reduced Na(+) accumulation in the shoot and improved survival of the plants when they were subsequently subjected to a lethal stress of 200 mM NaCl. The Na(+) accumulated during the conditioning period was rapidly released when the plants were returned to the control medium. It has been suggested that the location of the bypass flow is around young lateral roots, the early development of which disrupts the continuity of the endodermal and exodermal Casparian bands. However, in the present study, the observed increase in lateral root densities during stress in both cultivars did not correlate with bypass flow. Overall the data suggest that in rice roots Na(+) bypass flow is reduced by the deposition of apoplastic barriers, leading to improved plant survival under salt stress.

Repression of the glucose-inducible outer-membrane protein OprB during utilization of aromatic compounds and organic acids in Pseudomonas putida CSV86.

Pseudomonas putida CSV86 shows preferential utilization of aromatic compounds over glucose. Protein analysis and [¹4C]glucose-binding studies of the outer membrane fraction of cells grown on different carbon sources revealed a 40 kDa protein that was transcriptionally induced by glucose and repressed by aromatics and succinate. Based on 2D gel electrophoresis and liquid chromatography-tandem mass spectrometry analysis, the 40 kDa protein closely resembled the porin B of P. putida KT2440 and carbohydrate-selective porin OprB of various Pseudomonas strains. The purified native protein (i) was estimated to be a homotrimer of 125 kDa with a subunit molecular mass of 40 kDa, (ii) displayed heat modifiability of electrophoretic mobility, (iii) showed channel conductance of 166 pS in 1 M KCl, (iv) permeated various sugars (mono-, di- and tri-saccharides), organic acids, amino acids and aromatic compounds, and (v) harboured a glucose-specific and saturable binding site with a dissociation constant of 1.3 µM. These results identify the glucose-inducible outer-membrane protein of P. putida CSV86 as a carbohydrate-selective protein OprB. Besides modulation of intracellular glucose-metabolizing enzymes and specific glucose-binding periplasmic space protein, the repression of OprB by aromatics and organic acids, even in the presence of glucose, also contributes significantly to the strain's ability to utilize aromatics and organic acids over glucose.

Solution structure of BTK-2, a novel hK(v)1.1 inhibiting scorpion toxin, from the eastern Indian scorpion Mesobuthus tamulus.

The three dimensional structure of a 32 residue three disulfide scorpion toxin, BTK-2, from the Indian red scorpion Mesobuthus tamulus has been determined using isotope edited solution NMR methods. Samples for structural and electrophysiological studies were prepared using recombinant DNA methods. Electrophysiological studies show that the peptide is active against hK(v)1.1 channels. The structure of BTK-2 was determined using 373 distance restraints from NOE data, 66 dihedral angle restraints from NOE, chemical shift and scalar coupling data, 6 constraints based on disulfide linkages and 8 constraints based on hydrogen bonds. The root mean square deviation (r.m.s.d) about the averaged co-ordinates of the backbone (N, C(α), C') and all heavy atoms are 0.81 ± 0.23Å and 1.51 ± 0.29Å respectively. The backbone dihedral angles (ϕ and ψ) for all residues occupy the favorable and allowed regions of the Ramachandran map. The three dimensional structure of BTK-2 is composed of three well defined secondary structural regions that constitute the α-ß-ß structural motif. Comparisons between the structure of BTK-2 and other closely related scorpion toxins pointed towards distinct differences in surface properties that provide insights into the structure-function relationships among this important class of voltage-gated potassium channel inhibiting peptides.

Potassium channel regulator KCNRG regulates surface expression of Shaker-type potassium channels.

Besides their role in the generation of action potentials, voltage-gated potassium channels are implicated in cellular processes ranging from cell division to cell death. The K(+) channel regulator protein (KCNRG), identified as a putative tumor suppressor, reduces K(+) currents through human K(+) channels hKv1.1 and hKv1.4 expressed in Xenopus oocytes. Current attenuation requires the presence of the N-terminal T1 Domain and immunoprecipitation experiments suggest association of KCNRG with the N-terminus of the channel. Our data indicates that KCNRG is an ER-associated protein, which we propose regulates Kv1 family channel proteins by retaining a fraction of channels in endomembranes.