Studies on ion channel antagonist-binding sites in sunflower protoplasts

Pharmacological Strategies for Manipulating Plant Ca2+ Signalling

Calcium is one of the most pleiotropic second messengers in all living organisms. However, signalling specificity is encoded via spatio-temporally regulated signatures that act with surgical precision to elicit highly specific cellular responses. How this is brought about remains a big challenge in the plant field, in part due to a lack of specific tools to manipulate/interrogate the plant Ca2+ toolkit. In many cases, researchers resort to tools that were optimized in animal cells. However, the obviously large evolutionary distance between plants and animals implies that there is a good chance observed effects may not be specific to the intended plant target. Here, we provide an overview of pharmacological strategies that are commonly used to activate or inhibit plant Ca2+ signalling. We focus on highlighting modes of action where possible, and warn for potential pitfalls. Together, this review aims at guiding plant researchers through the Ca2+ pharmacology swamp.

Polarized and persistent Ca²⁺ plumes define loci for formation of wall ingrowth papillae in transfer cells

Transfer cell morphology is characterized by a polarized ingrowth wall comprising a uniform wall upon which wall ingrowth papillae develop at right angles into the cytoplasm. The hypothesis that positional information directing construction of wall ingrowth papillae is mediated by Ca(2+) signals generated by spatiotemporal alterations in cytosolic Ca(2+) ([Ca(2+)]cyt) of cells trans-differentiating to a transfer cell morphology was tested. This hypothesis was examined using Vicia faba cotyledons. On transferring cotyledons to culture, their adaxial epidermal cells synchronously trans-differentiate to epidermal transfer cells. A polarized and persistent Ca(2+) signal, generated during epidermal cell trans-differentiation, was found to co-localize with the site of ingrowth wall formation. Dampening Ca(2+) signal intensity, by withdrawing extracellular Ca(2+) or blocking Ca(2+) channel activity, inhibited formation of wall ingrowth papillae. Maintenance of Ca(2+) signal polarity and persistence depended upon a rapid turnover (minutes) of cytosolic Ca(2+) by co-operative functioning of plasma membrane Ca(2+)-permeable channels and Ca(2+)-ATPases. Viewed paradermally, and proximal to the cytosol-plasma membrane interface, the Ca(2+) signal was organized into discrete patches that aligned spatially with clusters of Ca(2+)-permeable channels. Mathematical modelling demonstrated that these patches of cytosolic Ca(2+) were consistent with inward-directed plumes of elevated [Ca(2+)]cyt. Plume formation depended upon an alternating distribution of Ca(2+)-permeable channels and Ca(2+)-ATPase clusters. On further inward diffusion, the Ca(2+) plumes coalesced into a uniform Ca(2+) signal. Blocking or dispersing the Ca(2+) plumes inhibited deposition of wall ingrowth papillae, while uniform wall formation remained unaltered. A working model envisages that cytosolic Ca(2+) plumes define the loci at which wall ingrowth papillae are deposited.

Changes in gravity rapidly alter the magnitude and direction of a cellular calcium current

In single-celled spores of the fern Ceratopteris richardii, gravity directs polarity of development and induces a directional, trans-cellular calcium (Ca(2+)) current. To clarify how gravity polarizes this electrophysiological process, we measured the kinetics of the cellular response to changes in the gravity vector, which we initially estimated using the self-referencing calcium microsensor. In order to generate more precise and detailed data, we developed a silicon microfabricated sensor array which facilitated a lab-on-a-chip approach to simultaneously measure calcium currents from multiple cells in real time. These experiments revealed that the direction of the gravity-dependent polar calcium current is reversed in less than 25 s when the cells are inverted, and that changes in the magnitude of the calcium current parallel rapidly changing g-forces during parabolic flight on the NASA C-9 aircraft. The data also revealed a hysteresis in the response of cells in the transition from 2g to micro-g in comparison to cells in the micro-g to 2-g transition, a result consistent with a role for mechanosensitive ion channels in the gravity response. The calcium current is suppressed by either nifedipine (calcium-channel blocker) or eosin yellow (plasma membrane calcium pump inhibitor). Nifedipine disrupts gravity-directed cell polarity, but not spore germination. These results indicate that gravity perception in single plant cells may be mediated by mechanosensitive calcium channels, an idea consistent with some previously proposed models of plant gravity perception.

Co-localization of putative calcium channels (phenylalkylamine-binding sites) on oil bodies in protoplasts from dark-grown sunflower seedling cotyledons

Oil bodies are spherical entities containing a triacylglycerol (TAG) matrix encased by a phospholipid monolayer, which is stabilized by oil body-specific proteins, principally oleosins. Biochemical investigations in the recent past have also demonstrated the expression of calcium-binding proteins, called caleosins, as a component of oil body membranes during seed germination. Using DM-Bodipy-phenylalkylamine (PAA; a fluorescent derivative of phenylalkylamine)-a fluorescent probe known to bind L-type calcium channel proteins, present investigations provide the first report on the localization and preferential accumulation of putative calcium channel proteins on/around oil bodies during peak lipolytic phase in protoplasts derived from dark-grown sunflower (Helianthus annuus L. cv Morden) seedling cotyledons. Specificity of DM-Bodipy-PAA labeling was confirmed by using bepridil, a non-fluorescent competitor of PAA while non-specific dye accumulation has been ruled out by using Bodipy-FL as control. Co-localization of fluorescence from DM-Bodipy-PAA binding sites (ex: 504 nm; em: 511 nm) and nile red fluorescing oil bodies (ex: 552 nm; em: 636 nm) has been undertaken by epifluorescence and confocal laser scanning microscopy (CLSM). It revealed the affinity of PAA-sensitive ion channels for the oil body surface. Findings from the current investigations highlight the significance of calcium and calcium channel proteins during oil body mobilization in sunflower.

Sieve element Ca2+ channels as relay stations between remote stimuli and sieve tube occlusion in Vicia faba

Damage induces remote occlusion of sieve tubes in Vicia faba by forisome dispersion, triggered during the passage of an electropotential wave (EPW). This study addresses the role of Ca2+ channels and cytosolic Ca2+ elevation as a link between EPWs and forisome dispersion. Ca2+ channel antagonists affect the initial phase of the EPW as well as the prolonged plateau phase. Resting levels of sieve tube Ca2+ of approximately 50 nM were independently estimated using Ca2+-selective electrodes and a Ca2+-sensitive dye. Transient changes in cytosolic Ca2+ were observed in phloem tissue in response to remote stimuli and showed profiles similar to those of EPWs. The measured elevation of Ca2+ in sieve tubes was below the threshold necessary for forisome dispersion. Therefore, forisomes need to be associated with Ca2+ release sites. We found an association between forisomes and endoplasmic reticulum (ER) at sieve plates and pore-plasmodesma units where high-affinity binding of a fluorescent Ca2+ channel blocker mapped an increased density of Ca2+ channels. In conclusion, propagation of EPWs in response to remote stimuli is linked to forisome dispersion through transiently high levels of parietal Ca2+, release of which depends on both plasma membrane and ER Ca2+ channels.

Ion channel clustering enhances weak electric field detection by neutrophils: apparent roles of SKF96365-sensitive cation channels and myeloperoxidase trafficking in cellular responses

We have tested Galvanovskis and Sandblom's prediction that ion channel clustering enhances weak electric field detection by cells as well as how the elicited signals couple to metabolic alterations. Electric field application was timed to coincide with certain known intracellular chemical oscillators (phase-matched conditions). Polarized, but not spherical, neutrophils labeled with anti-K(v)1.3, FL-DHP, and anti-TRP1, but not anti-T-type Ca(2+) channels, displayed clusters at the lamellipodium. Resonance energy transfer experiments showed that these channel pairs were in close proximity. Dose-field sensitivity studies of channel blockers suggested that K(+) and Ca(2+) channels participate in field detection, as judged by enhanced oscillatory NAD(P)H amplitudes. Further studies suggested that K(+) channel blockers act by reducing the neutrophil's membrane potential. Mibefradil and SKF93635, which block T-type Ca(2+) channels and SOCs, respectively, affected field detection at appropriate doses. Microfluorometry and high-speed imaging of indo-1-labeled neutrophils was used to examine Ca(2+) signaling. Electric fields enhanced Ca(2+) spike amplitude and triggered formation of a second traveling Ca(2+) wave. Mibefradil blocked Ca(2+) spikes and waves. Although 10 microM SKF96365 mimicked mibefradil, 7 microM SKF96365 specifically inhibited electric field-induced Ca(2+) signals, suggesting that one SKF96365-senstive site is influenced by electric fields. Although cells remained morphologically polarized, ion channel clusters at the lamellipodium and electric field sensitivity were inhibited by methyl-beta-cyclodextrin. As a result of phase-matched electric field application in the presence of ion channel clusters, myeloperoxidase (MPO) was found to traffic to the cell surface. As MPO participates in high amplitude metabolic oscillations, this suggests a link between the signaling apparatus and metabolic changes. Furthermore, electric field effects could be blocked by MPO inhibition or removal while certain electric field effects were mimicked by the addition of MPO to untreated cells. Therefore, channel clustering plays an important role in electric field detection and downstream responses of morphologically polarized neutrophils. In addition to providing new mechanistic insights concerning electric field interactions with cells, our work suggests novel methods to remotely manipulate physiological pathways.

Calcium entry through L-type calcium channels is essential for neurite regeneration in cultured sympathetic neurons

Previous work showed that a post-neuritotomy rise in [Ca2+]i is required for regeneration. We tested the following hypotheses in cultured sympathetic neurons: (1) blocking L-type channels at the time of injury inhibits regeneration; (2) enhancing Ca2+ entry through L-type Ca2+ channels enhances regeneration; (3) L-type Ca2+ channel distribution is predominantly on the soma and proximal neurites of uninjured and injured neurons. To visualize L-type Ca2+ channels and block Ca2+ influx, the fluorescent dihydropyridine antagonist, DM-BODIPY, was used. Our results show that regeneration is markedly inhibited by the antagonist when administered 20 min. prior to injury, in the presence or absence of nerve growth factor (NGF) (p < 0.0001). Severe degeneration of proximal and distal neurites was seen 48 h after injury. Regeneration was minimally inhibited by the antagonist when administered 5 min after injury (p < 0.05), but not inhibited when administered 2 or 24 h after injury (p > 0.05). We found that L-type channels are distributed ubiquitously on the soma and neurites of uninjured and injured cells, and on regenerating neurites. The addition of the L-type channel agonist, BayK8644, (1 microM) 20 min prior to injury enhanced neurite length at 24 h post-injury (p = 0.002). Blocking L-type channels did not affect the viability of uninjured or injured cells. For the first time, it has been shown that Ca2+ entry through L-type Ca2+ channels is essential for post-neuritotomy sympathetic neurite regeneration, and that this effect shows a strict temporal dependency. We also demonstrated that regeneration can be enhanced by increasing Ca2+ influx through L-type channels.