Subplasma membrane Ca2+ signals

Computational modeling of stretch induced calcium signaling at the apical membrane domain in umbrella cells

The urinary bladder epithelium comprises a specialised population of superficially placed cells called the umbrella cells. The apical membrane domain of umbrella cells has several intriguing morphological properties and is the site for various signaling activities. A key function of umbrella cells is to sense mechanical stimuli as the bladder stretches in response to filling. More specifically, the mechanotransduction of stretch into subcellular signals is brought about by the activation of Piezo1 channels that mediate calcium into the cell interior. The incoming calcium is critical to several aspects of umbrella cell signaling, including regulation of exocytosis, ATP release and downstream purinergic signaling. We report here a computational framework that models stretch-induced mechanotransduction via Piezo1 channels and the resulting calcium signaling in umbrella cells factoring in morphological details of subcellular compartment volumes. Our results show the following: (i) activation of Piezo1 conductance in response to stretch; (ii) development of varying Piezo1 mediated [Ca2+] profiles in subcellular compartments, namely, the apical sub-plasma membrane space, cytosol and mitochondria. The varying calcium amplitudes and temporal profiles in the subcellular compartments indicate highly specialised roles for stretch-mediated calcium in umbrella cells, including its potential effect on the energetics of mitochondria and the regulation of exocytosis.

Myography of isolated blood vessels: Considerations for experimental design and combination with supplementary techniques

The study of the mechanisms of regulation of vascular tone is an urgent task of modern science, since diseases of the cardiovascular system remain the main cause of reduction in the quality of life and mortality of the population. Myography (isometric and isobaric) of isolated blood vessels is one of the most physiologically relevant approaches to study the function of cells in the vessel wall. On the one hand, cell-cell interactions as well as mechanical stretch of the vessel wall remain preserved in myography studies, in contrast to studies on isolated cells, e.g., cell culture. On the other hand, in vitro studies in isolated vessels allow control of numerous parameters that are difficult to control in vivo. The aim of this review was to 1) discuss the specifics of experimental design and interpretation of data obtained by myography and 2) highlight the importance of the combined use of myography with various complementary techniques necessary for a deep understanding of vascular physiology.

Managing activity and Ca2+ dependence through mutation in the Junction of the AtCPK1 coordinates the salt tolerance in transgenic tobacco plants

Calcium-dependent protein kinases (CDPKs) are Ca2+ decoders in plants. AtCPK1 is a positive regulator in the plant response to biotic and abiotic stress. Inactivation of the autoinhibitory domain of AtCPK1 in the mutated form KJM23 provides constitutive activity of the kinase. In the present study, we investigated the effect of overexpressed native and mutant KJM23 forms on salinity tolerance in Nicotiana tabacum. Overexpression of native AtCPK1 provided tobacco resistance to 120 mM NaCl during germination and 180 mM NaCl during long-term growth, while the resistance of plants increased to 240 mM NaCl during both phases of plant development when transformed with KJM23. Mutation in the junction KJM4, which disrupted Ca2+ induced activation, completely nullified the acquired salt tolerance up to levels of normal plants. Analysis by confocal microscopy showed that under high salinity conditions, overexpression of AtCPK1 and KJM23 inhibited reactive oxygen species (ROS) accumulation to levels observed in untreated plants. Quantitative real-time PCR analysis showed that overexpression of AtCPK1 and KJM23 was associated with changes in expression of genes encoding heat shock factors. In all cases, the KJM23 mutation enhanced the effect of AtCPK1, while the KJM4 mutation reduced it to the control level. We suggest that the autoinhibitory domains in CDPKs could be promising targets for manipulation in engineering salt-tolerant plants.

Co-localization and confinement of ecto-nucleotidases modulate extracellular adenosine nucleotide distributions

Nucleotides comprise small molecules that perform critical signaling roles in biological systems. Adenosine-based nucleotides, including adenosine tri-, di-, and mono-phosphate, are controlled through their rapid degradation by diphosphohydrolases and ecto-nucleotidases (NDAs). The interplay between nucleotide signaling and degradation is especially important in synapses formed between cells, which create signaling 'nanodomains'. Within these 'nanodomains', charged nucleotides interact with densely-packed membranes and biomolecules. While the contributions of electrostatic and steric interactions within such nanodomains are known to shape diffusion-limited reaction rates, less is understood about how these factors control the kinetics of nucleotidase activity. To quantify these factors, we utilized reaction-diffusion numerical simulations of 1) adenosine triphosphate (ATP) hydrolysis into adenosine monophosphate (AMP) and 2) AMP into adenosine (Ado) via two representative nucleotidases, CD39 and CD73. We evaluate these sequentially-coupled reactions in nanodomain geometries representative of extracellular synapses, within which we localize the nucleotidases. With this model, we find that 1) nucleotidase confinement reduces reaction rates relative to an open (bulk) system, 2) the rates of AMP and ADO formation are accelerated by restricting the diffusion of substrates away from the enzymes, and 3) nucleotidase co-localization and the presence of complementary (positive) charges to ATP enhance reaction rates, though the impact of these contributions on nucleotide pools depends on the degree to which the membrane competes for substrates. As a result, these contributions integratively control the relative concentrations and distributions of ATP and its metabolites within the junctional space. Altogether, our studies suggest that CD39 and CD73 nucleotidase activity within junctional spaces can exploit their confinement and favorable electrostatic interactions to finely control nucleotide signaling.

Revisiting paradigms of Ca2+ signaling protein kinase regulation in plants

Calcium (Ca2+) serves as a universal second messenger in eukaryotic signal transduction. Understanding the Ca2+ activation kinetics of Ca2+ sensors is critical to understanding the cellular signaling mechanisms involved. In this review, we discuss the regulatory properties of two sensor classes: the Ca2+-dependent protein kinases (CPKs/CDPKs) and the calcineurin B-like (CBL) proteins that control the activity of CBL-interacting protein kinases (CIPKs) and identify emerging topics and some foundational points that are not well established experimentally. Most plant CPKs are activated by physiologically relevant Ca2+ concentrations except for those with degenerate EF hands, and new results suggest that the Ca2+-dependence of kinase activation may be modulated by both protein–protein interactions and CPK autophosphorylation. Early results indicated that activation of plant CPKs by Ca2+ occurred by relief of autoinhibition. However, recent studies of protist CDPKs suggest that intramolecular interactions between CDPK domains contribute allosteric control to CDPK activation. Further studies are required to elucidate the mechanisms regulating plant CPKs. With CBL–CIPKs, the two major activation mechanisms are thought to be (i) binding of Ca2+-bound CBL to the CIPK and (ii) phosphorylation of residues in the CIPK activation loop. However, the relative importance of these two mechanisms in regulating CIPK activity is unclear. Furthermore, information detailing activation by physiologically relevant [Ca2+] is lacking, such that the paradigm of CBLs as Ca2+ sensors still requires critical, experimental validation. Developing models of CPK and CIPK regulation is essential to understand how these kinases mediate Ca2+ signaling and to the design of experiments to test function in vivo.

Dynamic Ca2+ imaging with a simplified lattice light-sheet microscope: A sideways view of subcellular Ca2+ puffs

We describe the construction of a simplified, inexpensive lattice light-sheet microscope, and illustrate its use for imaging subcellular Ca²⁺ puffs evoked by photoreleased i-IP₃ in cultured SH-SY5Y neuroblastoma cells loaded with the Ca²⁺ probe Cal520. The microscope provides sub-micron spatial resolution and enables recording of local Ca²⁺ transients in single-slice mode with a signal-to-noise ratio and temporal resolution (2ms) at least as good as confocal or total internal reflection microscopy. Signals arising from openings of individual IP₃R channels are clearly resolved, as are stepwise changes in fluorescence reflecting openings and closings of individual channels during puffs. Moreover, by stepping the specimen through the light-sheet, the entire volume of a cell can be scanned within a few hundred ms. The ability to directly visualize a sideways (axial) section through cells directly reveals that IP₃-evoked Ca²⁺ puffs originate at sites in very close (≤a few hundred nm) to the plasma membrane, suggesting they play a specific role in signaling to the membrane.

Kinetic regulation of multi-ligand binding proteins

BACKGROUND

Second messengers, such as calcium, regulate the activity of multisite binding proteins in a concentration-dependent manner. For example, calcium binding has been shown to induce conformational transitions in the calcium-dependent protein calmodulin, under steady state conditions. However, intracellular concentrations of these second messengers are often subject to rapid change. The mechanisms underlying dynamic ligand-dependent regulation of multisite proteins require further elucidation.

RESULTS

In this study, a computational analysis of multisite protein kinetics in response to rapid changes in ligand concentrations is presented. Two major physiological scenarios are investigated: i) Ligand concentration is abundant and the ligand-multisite protein binding does not affect free ligand concentration, ii) Ligand concentration is of the same order of magnitude as the interacting multisite protein concentration and does not change. Therefore, buffering effects significantly influence the amounts of free ligands. For each of these scenarios the influence of the number of binding sites, the temporal effects on intermediate apo- and fully saturated conformations and the multisite regulatory effects on target proteins are investigated.

CONCLUSIONS

The developed models allow for a novel and accurate interpretation of concentration and pressure jump-dependent kinetic experiments. The presented model makes predictions for the temporal distribution of multisite protein conformations in complex with variable numbers of ligands. Furthermore, it derives the characteristic time and the dynamics for the kinetic responses elicited by a ligand concentration change as a function of ligand concentration and the number of ligand binding sites. Effector proteins regulated by multisite ligand binding are shown to depend on ligand concentration in a highly nonlinear fashion.

Evoked centripetal Ca(2+) mobilization in cardiac Purkinje cells: insight from a model of three Ca(2+) release regions

Despite strong suspicion that abnormal Ca(2+) handling in Purkinje cells (P-cells) is implicated in life-threatening forms of ventricular tachycardias, the mechanism underlying the Ca(2+) cycling of these cells under normal conditions is still unclear. There is mounting evidence that P-cells have a unique Ca(2+) handling system. Notably complex spontaneous Ca(2+) activity was previously recorded in canine P-cells and was explained by a mechanistic hypothesis involving a triple layered system of Ca(2+) release channels. Here we examined the validity of this hypothesis for the electrically evoked Ca(2+) transient which was shown, in the dog and rabbit, to occur progressively from the periphery to the interior of the cell. To do so, the hypothesis was incorporated in a model of intracellular Ca(2+) dynamics which was then used to reproduce numerically the Ca(2+) activity of P-cells under stimulated conditions. The modelling was thus performed through a 2D computational array that encompassed three distinct Ca(2+) release nodes arranged, respectively, into three consecutive adjacent regions. A system of partial differential equations (PDEs) expressed numerically the principal cellular functions that modulate the local cytosolic Ca(2+) concentration (Cai). The apparent node-to-node progression of elevated Cai was obtained by combining Ca(2+) diffusion and 'Ca(2+)-induced Ca(2+) release'. To provide the modelling with a reliable experimental reference, we first re-examined the Ca(2+) mobilization in swine stimulated P-cells by 2D confocal microscopy. As reported earlier for the dog and rabbit, a centripetal Ca(2+) transient was readily visible in 22 stimulated P-cells from six adult Yucatan swine hearts (pacing rate: 0.1 Hz; pulse duration: 25 ms, pulse amplitude: 10% above threshold; 1 mm Ca(2+); 35°C; pH 7.3). An accurate replication of the observed centripetal Ca(2+) propagation was generated by the model for four representative cell examples and confirmed by statistical comparisons of simulations against cell data. Selective inactivation of Ca(2+) release regions of the computational array showed that an intermediate layer of Ca(2+) release nodes with an ~30-40% lower Ca(2+) activation threshold was required to reproduce the phenomenon. Our computational analysis was therefore fully consistent with the activation of a triple layered system of Ca(2+) release channels as a mechanism of centripetal Ca(2+) signalling in P-cells. Moreover, the model clearly indicated that the intermediate Ca(2+) release layer with increased sensitivity for Ca(2+) plays an important role in the specific intracellular Ca(2+) mobilization of Purkinje fibres and could therefore be a relevant determinant of cardiac conduction.