Calcium-permeable cation channels are involved in uranium uptake in Arabidopsis thaliana

Uranium (U) is a non-essential and toxic element that is taken up by plants from the environment. The assimilation pathway of U is still unknown in plants and any other organism. In this study, we provide several evidences that U is taken up by the roots of Arabidopsis thaliana through Ca2+-permeable cation channels. First, we showed that deprivation of Arabidopsis plants with calcium induced a 1.5-fold increase in the capacity of roots to accumulate U, suggesting that calcium deficiency promoted the radionuclide import pathway. Second, we showed that external calcium inhibits U accumulation in roots, suggesting a common route for the uptake of both cations. Third, we found that gadolinium, nifedipine and verapamil inhibit the absorption of U, suggesting that different types of Ca2+-permeable channels serve as a route for U uptake. Last, we showed that U bioaccumulation in Arabidopsis mutants deficient for the Ca2+-permeable channels MCA1 and ANN1 was decreased by 40%. This suggests that MCA1 and ANN1 contribute to the absorption of U in different zones and cell layers of the root. Together, our results describe for the first time the involvement of Ca2+-permeable cation channels in the cellular uptake of U.

Quantitative Correlation between the Degree of Reaction and Compressive Strength of Metakaolin-Based Geopolymers

For geopolymers (usually composed of unreacted precursor and gel), the compressive strength is controlled by two factors. The first is the degree of reaction, or, equivalently, the amount of gel formed, including any calcium silicate hydrate gel in calcium-containing mixtures. The second factor is the gel composition, generally given by the Si/Al ratio. These two parameters are interrelated for typical silicate-activated metakaolin geopolymers. By separating out effects of Si/Al ratio and degree of reaction, this study quantitatively correlates the degree of reaction with the compressive strength of metakaolin-based geopolymers with and without calcium. Solid-state 29Si nuclear magnetic resonance (NMR) aided with chemical extractions was used to determine gel amounts and composition for several geopolymer mixtures. The compressive strength was also measured for each mixture at 7 days. Both the increase of Na/Al ratio in mixtures without calcium and addition of external calcium increased the degree of reaction, and compressive strength correlated linearly (R2 > 0.88) with the degree of reaction.

Incomplete vesicular docking limits synaptic strength under high release probability conditions

Central mammalian synapses release synaptic vesicles in dedicated structures called docking/release sites. It has been assumed that when voltage-dependent calcium entry is sufficiently large, synaptic output attains a maximum value of one synaptic vesicle per action potential and per site. Here we use deconvolution to count synaptic vesicle output at single sites (mean site number per synapse: 3.6). When increasing calcium entry with tetraethylammonium in 1.5 mM external calcium concentration, we find that synaptic output saturates at 0.22 vesicle per site, not at 1 vesicle per site. Fitting the results with current models of calcium-dependent exocytosis indicates that the 0.22 vesicle limit reflects the probability of docking sites to be occupied by synaptic vesicles at rest, as only docked vesicles can be released. With 3 mM external calcium, the maximum output per site increases to 0.47, indicating an increase in docking site occupancy as a function of external calcium concentration.

Building a synthetic mechanosensitive signaling pathway in compartmentalized artificial cells

To date, reconstitution of one of the fundamental methods of cell communication, the signaling pathway, has been unaddressed in the bottom-up construction of artificial cells (ACs). Such developments are needed to increase the functionality and biomimicry of ACs, accelerating their translation and application in biotechnology. Here, we report the construction of a de novo synthetic signaling pathway in microscale nested vesicles. Vesicle-cell models respond to external calcium signals through activation of an intracellular interaction between phospholipase A2 and a mechanosensitive channel present in the internal membranes, triggering content mixing between compartments and controlling cell fluorescence. Emulsion-based approaches to AC construction are therefore shown to be ideal for the quick design and testing of new signaling networks and can readily include synthetic molecules difficult to introduce to biological cells. This work represents a foundation for the engineering of multicompartment-spanning designer pathways that can be utilized to control downstream events inside an AC, leading to the assembly of micromachines capable of sensing and responding to changes in their local environment.

An analog to digital converter creates nuclear localization pulses in yeast calcium signaling

Several examples of transcription factors that show stochastic, unsynchronized pulses of nuclear localization have been described. Here we show that under constant calcium stress, nuclear localization pulses of the transcription factor Crz1 follow stochastic variations in cytoplasmic calcium concentration. We find that the size of the stochastic calcium pulses is positively correlated with the number of subsequent Crz1 pulses. Based on our observations, we propose a simple stochastic model of how the signaling pathway converts a constant external calcium concentration into a digital number of Crz1 pulses in the nucleus, due to the time delay from nuclear transport and the stochastic decoherence of individual Crz1 molecule dynamics. We find support for several additional predictions of the model and conclude that stochastic input to nuclear transport may produce digital responses to analog signals in other signaling systems.

Ergosterol induces mobilization of internal calcium in tobacco cells

As for natural sterols, only ergosterol is recognized very specifically and sensitively (nM) by plants cells. Ergosterol interacts with tobacco suspension cells and trigger pH changes of extracellular medium, oxidative burst and synthesis of phytoalexins. Compared with the responses induced by cryptogein, a proteinaceous elicitor from Phytophthora sp., oxidative burst, DpH and phytoalexin accumulation were weaker with ergosterol. Cryptogein stimulated an apparent continuous uptake of external calcium within 40 min, whereas no net uptake of external calcium occurred upon the addition of ergosterol. However, the elicitation with either cryptogein or ergosterol resulted in an increase of the fluorescence of calcium green 1 in cytosol. The use of several inhibitors of calcium channels (La³⁺, TMB-8, verapamil, ruthenium red, nifedipine) and a protein-kinase inhibitors (staurosporin, NPC-15437, H-89) suggests that the elicitation with ergosterol includes the mobilization of internal calcium stores in vacuoles mediated by IP3 and some protein kinases.

Developmental changes in contractile responses to cholinergic stimuli: role of calcium sensitization and related pathways

This study was performed to analyze the developmental changes in bladder response to cholinergic stimulation in detail, highlighting calcium sensitization (CS) and its related pathways. Rats were divided into three groups in accordance with reported time of developmental milestones (newborns, days 1–4; youngsters, days 5–14; and grown-ups, days 15–28). Following cholinergic stimulation (carbachol, 5 µM), the contractile response to detrusor was analyzed with respect to three phases (initial phasic, tonic, and superimposed phasic contractions). Contractile responses were analyzed by their dynamic and kinetic aspects. The responses were further compared in varying external calcium concentrations and in the presence of inhibitors of protein kinase C (PKC) and Rho kinase (ROCK), which are involved in CS. The responses of newborns contrasted with the others by their short and brisk initial phasic contractions, prominent tonic contractions, and delayed participation of irregular superimposed phasic contractions. With development, phasic contractions became prominent, and tonic contractions diminished. These developmental changes in phasic contractions were reproduced when exposed to increasing calcium concentrations. Application of specific inhibitors and molecular phasic analysis revealed that PKC was functional in tonic contractions of the newborns, whereas ROCK took over its role with development. Within a few days of birth, rats’ bladders experienced drastic changes in contractile mechanisms. This included dominance of phasic contractions over tonic contractions due to increased calcium dependence and the maturational shift of the calcium sensitivity mechanism from PKC to ROCK.