Osmo- and mechanosensitivity of the transient outward K+ current in a mammalian neuronal cell line

Bioinspired Supramolecular Hydrogel from Design to Applications

Nature offers a wealth of opportunities to solve scientific and technological issues based on its unique structures and function. The dynamic non-covalent interaction is considered to be the main base of living functions of creatures including humans, animals, and plants. Supramolecular hydrogels formed by non-covalent bonding interactions has become a unique platform for constructing promising materials for medicine, energy, electronic, and biological substitute. In this review, the self-assemble principle of supramolecular hydrogels is summarized. Next, the stimulation of external environment that triggers the assembly or disassembly of supramolecular hydrogels are recapitulated, including temperature, mechanics, light, pH, ions, etc. The main applications of bioinspired supramolecular hydrogels in terms of bionic objects including humans, animals, and plants are also described. Although so many efforts are done for revealing the synergized mechanism of the function and non-covalent interactions on the supramolecular hydrogel, the complexity and variability between stimulus and non-covalent bonding in the supramolecular system still require impeccable theories. As an outlook, the bioinspired supramolecular hydrogel is just beginning to exhibit its great potential in human life, offering significant opportunities in drug delivery and screening, implantable devices and substitutions, tissue engineering, micro-fluidic devices, and biosensors.

Glutathione depletion activates the yeast vacuolar transient receptor potential channel, Yvc1p, by reversible glutathionylation of specific cysteines

Glutathione depletion leads to calcium influx in yeast cells via plasma membrane Cch1p and the vacuolar Yvc1p channels. Yvc1p, a yeast vacuolar transient receptor potential channel, is activated by glutathionylation carried out by the glutathione S-transferase Gtt1p, and this mechanism is reversible with deglutathionylation being mediated by the thioredoxin Trx2p.

Glutathione depletion and calcium influx into the cytoplasm are two hallmarks of apoptosis. We have been investigating how glutathione depletion leads to apoptosis in yeast. We show here that glutathione depletion in yeast leads to the activation of two cytoplasmically inward-facing channels: the plasma membrane, Cch1p, and the vacuolar calcium channel, Yvc1p. Deletion of these channels partially rescues cells from glutathione depletion–induced cell death. Subsequent investigations on the Yvc1p channel, a homologue of the mammalian TRP channels, revealed that the channel is activated by glutathionylation. Yvc1p has nine cysteine residues, of which eight are located in the cytoplasmic regions and one on the transmembrane domain. We show that three of these cysteines, Cys-17, Cys-79, and Cys-191, are specifically glutathionylated. Mutation of these cysteines to alanine leads to a loss in glutathionylation and a concomitant loss in calcium channel activity. We further investigated the mechanism of glutathionylation and demonstrate a role for the yeast glutathione S-transferase Gtt1p in glutathionylation. Yvc1p is also deglutathionylated, and this was found to be mediated by the yeast thioredoxin, Trx2p. A model for redox activation and deactivation of the yeast Yvc1p channel is presented.

Membrane tension accelerates rate-limiting voltage-dependent activation and slow inactivation steps in a Shaker channel

A classical voltage-sensitive channel is tension sensitive—the kinetics of Shaker and S3–S4 linker deletion mutants change with membrane stretch (Tabarean, I.V., and C.E. Morris. 2002. Biophys. J. 82:2982–2994.). Does stretch distort the channel protein, producing novel channel states, or, more interestingly, are existing transitions inherently tension sensitive? We examined stretch and voltage dependence of mutant 5aa, whose ultra-simple activation (Gonzalez, C., E. Rosenman, F. Bezanilla, O. Alvarez, and R. Latorre. 2000. J. Gen. Physiol. 115:193–208.) and temporally matched activation and slow inactivation were ideal for these studies. We focused on macroscopic patch current parameters related to elementary channel transitions: maximum slope and delay of current rise, and time constant of current decline. Stretch altered the magnitude of these parameters, but not, or minimally, their voltage dependence. Maximum slope and delay versus voltage with and without stretch as well as current rising phases were well described by expressions derived for an irreversible four-step activation model, indicating there is no separate stretch-activated opening pathway. This model, with slow inactivation added, explains most of our data. From this we infer that the voltage-dependent activation path is inherently stretch sensitive. Simulated currents for schemes with additional activation steps were compared against datasets; this showed that generally, additional complexity was not called for. Because the voltage sensitivities of activation and inactivation differ, it was not possible to substitute depolarization for stretch so as to produce the same overall P_O time course. What we found, however, was that at a given voltage, stretch-accelerated current rise and decline almost identically—normalized current traces with and without stretch could be matched by a rescaling of time. Rate-limitation of the current falling phase by activation was ruled out. We hypothesize, therefore, that stretch-induced bilayer decompression facilitates an in-plane expansion of the protein in both activation and inactivation. Dynamic structural models of this class of channels will need to take into account the inherent mechanosensitivity of voltage-dependent gating.

IK channels are involved in the regulatory volume decrease in human epithelial cells

Parallel activation of Ca(2+)-dependent K(+) channels and volume-sensitive Cl(-) channels is known to be responsible for KCl efflux during regulatory volume decrease (RVD) in human epithelial Intestine 407 cells. The present study was performed to identify the K(+) channel type. RT-PCR demonstrated mRNA expression of Ca(2+)-activated, intermediate conductance K(+) (IK), but not small conductance K(+) (SK1) or large conductance K(+) (BK) channels in this cell line. Whole cell recordings showed that ionomycin or hypotonic stress activated inwardly rectifying K(+) currents that were reversibly blocked by IK channel blockers [clotrimazole (CLT) and charybdotoxin] but not by SK and BK channel blockers (apamin and iberiotoxin). Inside-out recordings revealed the existence of CLT-sensitive single K(+)-channel activity, which exhibited an intermediate unitary conductance (30 pS at -100 mV). The channel was activated by cytosolic Ca(2+) in inside-out patches and by a hypotonic challenge in cell-attached patches. The RVD was suppressed by CLT, but not by apamin or iberiotoxin. Thus we conclude that the IK channel is involved in the RVD process in these human epithelial cells.

Signalling drought in guard cells

A number of environmental conditions including drought, low humidity, cold and salinity subject plants to osmotic stress. A rapid plant response to such stress conditions is stomatal closure to reduce water loss from plants. From an external stress signal to stomatal closure, many molecular components constitute a signal transduction network that couples the stimulus to the response. Numerous studies have been directed to resolving the framework and molecular details of stress signalling pathways in plants. In guard cells, studies focus on the regulation of ion channels by abscisic acid (ABA), a chemical messenger for osmotic stress. Calcium, protein kinases and phosphatases, and membrane trafficking components have been shown to play a role in ABA signalling process in guard cells. Studies also implicate ABA-independent regulation of ion channels by osmotic stress. In particular, a direct osmosensing pathway for ion channel regulation in guard cells has been identified. These pathways form a complex signalling web that monitors water status in the environment and initiates responses in stomatal movements.

Voltage-dependent K+ channels as targets of osmosensing in guard cells

Guard cell turgor responds to the osmogradient across the plasma membrane and controls the stomatal aperture. Here, we report that guard cells utilize voltage-dependent K+ channels as targets of the osmosensing pathway, providing a positive feedback mechanism for stomatal regulation. When exposed to a hypotonic condition, the inward K+ current (IKin) was highly activated, whereas the outward K+ current (IKout) was inactivated. In contrast, hypertonic conditions inactivated the IKin while activating IKout. Single-channel recording analyses indicated that an alteration in channel opening frequency was responsible for regulating IKin and IKout under different osmotic conditions. Further studies correlate osmoregulation of IKin with the pattern of organization of actin filaments, which may be a critical component in the osmosensing pathway in plant cells.

Characterisation of the effects of depolarising toxins on nerve terminal action potentials: apparent block of presynaptic potassium currents

Previous studies showed that toxic phospholipases A2 (Pa-8 and Pa-10F) from the venom of Pseudechis australis, the Australian king brown snake, reduced acetylcholine release at mouse neuromuscular junctions and depressed motor nerve terminal action potentials [Fatehi et al. (1994a), Toxicon 32, 1559-1572], and it was postulated that these toxins induced their effect on the action potential waveforms through nerve terminal depolarisation. To test this hypothesis, the effects of Pa-11 (another phospholipase A2 from the venom of Pseudechis australis), and the known depolarising agents. myotoxin a, from the venom of the rattlesnake. Crotalus viridis viridis, and ouabain on these waveforms were compared with the changes induced in the nerve terminal action potentials by Pa-8 and Pa-10F. The experiments were performed on the isolated mouse triangularis sterni preparation, using extracellular recordings. Pa-11 (0.1 microM) decreased the component of nerve terminal action potential related to Na+ and K+ currents to about 80% and 40% of control, respectively, after 60 min. Myotoxin alpha (5 microM) and ouabain (50 microM) produced similar, time-dependent changes in the nerve terminal action potential. These effects are similar to those produced by Pa-8 and Pa-10F, and are consistent with a slow but partial loss of membrane potential at the nerve terminal. In addition, whole-cell patch-clamp recording was employed to investigate possible direct actions of Pa-8. Pa10F and Pa-11 on Na+ and K+ currents in NG108 and PC12 cells in culture. None of these toxins (0.8 microM) reduced the Na+ and K+ currents in these cells. There was also no displacement of [125I]alpha-dendrotoxin bound to voltage-sensitive potassium channels on rat synaptosomal membranes induced by Pa-8, Pa-10F and Pa-11 (up to 100 microM). These results support the hypothesis that the alteration of nerve terminal waveforms by these toxic phospholipases A2 is mediated by nerve terminal depolarisation.

Osmolarity modulates K+ channel function on rat hippocampal interneurons but not CA1 pyramidal neurons

1. Whole-cell and single-channel recording methods were used in conjunction with infrared video microscopy techniques to examine the properties of voltage-activated potassium channels in hippocampal neurons during the application of hyposmolar solutions to hippocampal slices from rats. 2. Hyposmolar external solutions (osmolarity reduced by 10% to 267 mosmol l-1) produced a significant potentiation of voltage-activated K+ current on lacunosum/moleculare (L/M) hippocampal interneurons, but not on CA1 and subiculum pyramidal neurons. Hyperpolarization-activated (IH) and leak currents were not altered during the application of hyposmolar solutions in all cell types. 3. Mean channel open time and the probability of channel opening were dramatically increased under hyposmolar recording conditions for outside-out patches from L/M interneurons; no changes were observed for patches from CA1 pyramidal neurons. Mean current amplitude and the threshold for channel activation were not affected by hyposmotic challenge. 4. Hyposmolar external solutions produced a significant reduction in the firing frequency of L/M interneurons recorded in current-clamp mode. Hyposmolar solutions had no effect on resting membrane potential, action potential amplitude or duration, and spike after-hyperpolarization amplitude. 5. These results indicate that selective modulation of interneuron ion channel activity may be a critical mechanism by which osmolarity can regulate excitability in the central nervous system.