Fluorescence fluctuations and equivalence classes of Ca²⁺ imaging experiments

High rates of calcium-free diffusion in the cytosol of living cells

Calcium (Ca²⁺) is a universal second messenger that participates in the regulation of innumerous physiological processes. The way in which local elevations of the cytosolic Ca²⁺ concentration spread in space and time is key for the versatility of the signals. Ca²⁺ diffusion in the cytosol is hindered by its interaction with proteins that act as buffers. Depending on the concentrations and the kinetics of the interactions, there is a large range of values at which Ca²⁺ diffusion can proceed. Having reliable estimates of this range, particularly of its highest end, which corresponds to the ions free diffusion, is key to understand how the signals propagate. In this work, we present the first experimental results with which the Ca²⁺-free diffusion coefficient is directly quantified in the cytosol of living cells. By means of fluorescence correlation spectroscopy experiments performed in Xenopus laevis oocytes and in cells of Saccharomyces cerevisiae, we show that the ions can freely diffuse in the cytosol at a higher rate than previously thought.

Using two dyes to observe the competition of Ca2+ trapping mechanisms and their effect on intracellular Ca2+ signals

The specificity and universality of intracellular [Formula: see text] signals rely on the variety of spatio-temporal patterns that the [Formula: see text] concentration can display. [Formula: see text] liberation through inositol 1,4,5-trisphosphate receptors ([Formula: see text]) is key for this variety. In this paper, we study how the competition between buffers of different kinetics affects [Formula: see text] signals that involve [Formula: see text] release through [Formula: see text]. The study also provides insight into the underlying spatial distribution of the channels that participate in the signals. Previous works on the effects of [Formula: see text] buffers have drawn conclusions 'indirectly' by observing the [Formula: see text]-bound dye distributions in the presence of varying concentrations of exogenous buffers and using simulations to interpret the results. In this paper, we make visible the invisible by observing the signals simultaneously with two dyes, [Formula: see text] and [Formula: see text], each of which plays the role of a slow or fast [Formula: see text] buffer, respectively. Our observations obtained for different concentrations of [Formula: see text] highlight the dual role that fast buffers exert on the dynamics, either reducing the intracluster channel coupling or preventing channel inhibition and allowing the occurrence of relatively long cycles of [Formula: see text] release. Our experiments also show that signals with relatively high [Formula: see text] release rates remain localized in the presence of large [Formula: see text] concentrations, while the mean speed of the elicited waves increases. We interpret this as a consequence of the more effective uncoupling between [Formula: see text] clusters as the slow dye concentration increases. Combining the analysis of the experiments with numerical simulations, we also conclude that [Formula: see text] release not only occurs within the close vicinity of the centers of the clearly identifiable release sites ([Formula: see text] clusters) but there are also functional [Formula: see text] in between them.

Fluorescence correlation spectroscopy experiments to quantify free diffusion coefficients in reaction-diffusion systems: The case of Ca^{2+} and its dyes

Many cell signaling pathways involve the diffusion of messengers that bind and unbind to and from intracellular components. Quantifying their net transport rate under different conditions then requires having separate estimates of their free diffusion coefficient and binding or unbinding rates. In this paper, we show how performing sets of fluorescence correlation spectroscopy (FCS) experiments under different conditions, it is possible to quantify free diffusion coefficients and on and off rates of reaction-diffusion systems. We develop the theory and present a practical implementation for the case of the universal second messenger, calcium (Ca^{2+}) and single-wavelength dyes that increase their fluorescence upon Ca^{2+} binding. We validate the approach with experiments performed in aqueous solutions containing Ca^{2+} and Fluo4 dextran (both in its high and low affinity versions). Performing FCS experiments with tetramethylrhodamine-dextran in Xenopus laevis oocytes, we infer the corresponding free diffusion coefficients in the cytosol of these cells. Our approach can be extended to other physiologically relevant reaction-diffusion systems to quantify biophysical parameters that determine the dynamics of various variables of interest.

Luminal Ca(2+) dynamics during IP3R mediated signals

The role of cytosolic Ca(2+) on the kinetics of Inositol 1,4,5-triphosphate receptors (IP3Rs) and on the dynamics of IP3R-mediated Ca(2+) signals has been studied at large both experimentally and by modeling. The role of luminal Ca(2+) has not been investigated with that much detail although it has been found that it is relevant for signal termination in the case of Ca(2+) release through ryanodine receptors. In this work we present the results of observing the dynamics of luminal and cytosolic Ca(2+) simultaneously in Xenopus laevis oocytes. Combining observations and modeling we conclude that there is a rapid mechanism that guarantees the availability of free Ca(2+) in the lumen even when a relatively large Ca(2+) release is evoked. Comparing the dynamics of cytosolic and luminal Ca(2+) during a release, we estimate that they are consistent with a 80% of luminal Ca(2+) being buffered. The rapid availability of free luminal Ca(2+) correlates with the observation that the lumen occupies a considerable volume in several regions across the images.

Oral dosing of chemical indicators for in vivo monitoring of Ca2+ dynamics in insect muscle

This paper proposes a remarkably facile staining protocol to visually investigate dynamic physiological events in insect tissues. We attempted to monitor Ca²⁺ dynamics during contraction of electrically stimulated living muscle. Advances in circuit miniaturization and insect neuromuscular physiology have enabled the hybridization of living insects and man-made electronic components, such as microcomputers, the result of which has been often referred as a Living Machine, Biohybrid, or Cyborg Insect. In order for Cyborg Insects to be of practical use, electrical stimulation parameters need to be optimized to induce desired muscle response (motor action) and minimize the damage in the muscle due to the electrical stimuli. Staining tissues and organs as well as measuring the dynamics of chemicals of interest in muscle should be conducted to quantitatively and systematically evaluate the effect of various stimulation parameters on the muscle response. However, existing staining processes require invasive surgery and/or arduous procedures using genetically encoded sensors. In this study, we developed a non-invasive and remarkably facile method for staining, in which chemical indicators can be orally administered (oral dosing). A chemical Ca²⁺ indicator was orally introduced into an insect of interest via food containing the chemical indicator and the indicator diffused from the insect digestion system to the target muscle tissue. We found that there was a positive relationship between the fluorescence intensity of the indicator and the frequency of electrical stimulation which indicates the orally dosed indicator successfully monitored Ca²⁺ dynamics in the muscle tissue. This oral dosing method has a potential to globally stain tissues including neurons, and investigating various physiological events in insects.

Ca²⁺ images obtained in different experimental conditions shed light on the spatial distribution of IP₃ receptors that underlie Ca²⁺ puffs

Many intracellular Ca(2+) signals involve Ca(2+) release from the endoplasmic reticulum through inositol 1,4,5-trisphosphate receptors (IP3Rs). The open probability of IP3Rs depends on cytosolic Ca(2+) so that these signals involve Ca(2+)-induced Ca(2+)-release (CICR). IP3Rs are organized in clusters. The signals they mediate are observed using single-wavelength dyes and, often, a slow Ca(2+) buffer (EGTA) is added to disrupt CICR between clusters and keep the signals spatially restricted. It is assumed that the presence of the dye or of EGTA does not alter the intra-cluster Ca(2+) dynamics. In this paper we analyze this issue combining experiments and numerical simulations. We compare the properties of local signals known as puffs observed with different dyes and EGTA concentrations. We determine that although the dye or EGTA does not alter the intra-cluster dynamics, the set of observable events is different depending on the degree of inter-cluster uncoupling of the experiment. An analysis of the observations shows that the events that are missed for insufficient inter-cluster uncoupling are those of fastest amplitude growth rate. This agrees with a spatial organization in which the largest amplitude events correspond to clusters with densely packed active IP3Rs.