Cytoplasmic versus intra-SR: the battle of the Ca2+ diffusion coefficients in cardiac muscle

Mechanistic insights into store-operated Ca2+ entry during excitation-contraction coupling in skeletal muscle

Skeletal muscle fibres support store-operated Ca²⁺-entry (SOCE) across the t-tubular membrane upon exhaustive depletion of Ca²⁺ from the sarcoplasmic reticulum (SR). Recently we demonstrated the presence of a novel mode of SOCE activated under conditions of maintained [Ca²⁺]_SR. This phasic SOCE manifested in a fast and transient manner in synchrony with excitation contraction (EC)-coupling mediated SR Ca²⁺-release (Communications Biology 1:31, doi: https://doi.org/10.1038/s42003-018-0033-7). Stromal interaction molecule 1 (STIM1) and calcium release-activated calcium channel 1 (ORAI1), positioned at the SR and t-system membranes, respectively, are the considered molecular correlate of SOCE. The evidence suggests that at the triads, where the terminal cisternae of the SR sandwich a t-tubule, STIM1 and ORAI1 proteins pre-position to allow for enhanced SOCE transduction. Here we show that phasic SOCE is not only shaped by global [Ca²⁺]_SR but provide evidence for a local activation within nanodomains at the terminal cisternae of the SR. This feature may allow SOCE to modulate [Ca²⁺]_SR during EC coupling. We define SOCE to occur on the same timescale as EC coupling and determine the temporal coherence of SOCE activation to SR Ca²⁺ release. We derive a delay of 0.3 ms reflecting diffusive Ca²⁺-equilibration at the luminal ryanodine receptor 1 (RyR1) channel mouth upon SR Ca²⁺-release. Numerical simulations of Ca²⁺-calsequestrin binding estimates a characteristic diffusion length and confines an upper limit for the spatial distance between STIM1 and RyR1. Experimental evidence for a 4- fold change in t-system Ca²⁺-permeability upon prolonged electrical stimulation in conjunction with numerical simulations of Ca²⁺-STIM1 binding suggests a Ca²⁺ dissociation constant of STIM1 below 0.35 mM. Our results show that phasic SOCE is intimately linked with RyR opening and closing, with only μs delays, because [Ca²⁺] in the terminal cisternae is just above the threshold for Ca²⁺ dissociation from STIM1 under physiological resting conditions. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

A polarized Ca2+, diacylglycerol and STIM1 signalling system regulates directed cell migration

Ca(2+) signals control cell migration by regulating forward movement and cell adhesion. However, it is not well understood how Ca(2+)-regulatory proteins and second messengers are spatially organized in migrating cells. Here we show that receptor tyrosine kinase and phospholipase C signalling are restricted to the front of migrating endothelial leader cells, triggering local Ca(2+) pulses, local depletion of Ca(2+) in the endoplasmic reticulum and local activation of STIM1, supporting pulsatile front retraction and adhesion. At the same time, the mediator of store-operated Ca(2+) influx, STIM1, is transported by microtubule plus ends to the front. Furthermore, higher Ca(2+) pump rates in the front relative to the back of the plasma membrane enable effective local Ca(2+) signalling by locally decreasing basal Ca(2+). Finally, polarized phospholipase C signalling generates a diacylglycerol gradient towards the front that promotes persistent forward migration. Thus, cells employ an integrated Ca(2+) control system with polarized Ca(2+) signalling proteins and second messengers to synergistically promote directed cell migration.

Intracellular dyssynchrony of diastolic cytosolic [Ca²⁺] decay in ventricular cardiomyocytes in cardiac remodeling and human heart failure

RATIONALE

Synchronized release of Ca²⁺ into the cytosol during each cardiac cycle determines cardiomyocyte contraction.

OBJECTIVE

We investigated synchrony of cytosolic [Ca²⁺] decay during diastole and the impact of cardiac remodeling.

METHODS AND RESULTS

Local cytosolic [Ca²⁺] transients (1-µm intervals) were recorded in murine, porcine, and human ventricular single cardiomyocytes. We identified intracellular regions of slow (slowCaR) and fast (fastCaR) [Ca²⁺] decay based on the local time constants of decay (TAUlocal). The SD of TAUlocal as a measure of dyssynchrony was not related to the amplitude or the timing of local Ca²⁺ release. Stimulation of sarcoplasmic reticulum Ca²⁺ ATPase with forskolin or istaroxime accelerated and its inhibition with cyclopiazonic acid slowed TAUlocal significantly more in slowCaR, thus altering the relationship between SD of TAUlocal and global [Ca²⁺] decay (TAUglobal). Na⁺/Ca²⁺ exchanger inhibitor SEA0400 prolonged TAUlocal similarly in slowCaR and fastCaR. FastCaR were associated with increased mitochondrial density and were more sensitive to the mitochondrial Ca²⁺ uniporter blocker Ru360. Variation in TAUlocal was higher in pig and human cardiomyocytes and higher with increased stimulation frequency (2 Hz). TAUlocal correlated with local sarcomere relengthening. In mice with myocardial hypertrophy after transverse aortic constriction, in pigs with chronic myocardial ischemia, and in end-stage human heart failure, variation in TAUlocal was increased and related to cardiomyocyte hypertrophy and increased mitochondrial density.

CONCLUSIONS

In cardiomyocytes, cytosolic [Ca²⁺] decay is regulated locally and related to local sarcomere relengthening. Dyssynchronous intracellular [Ca²⁺] decay in cardiac remodeling and end-stage heart failure suggests a novel mechanism of cellular contractile dysfunction.

Modeling calcium waves in cardiac myocytes: importance of calcium diffusion

Under certain conditions, cardiac myocytes engage in a mode of calcium signaling in which calcium release from the sarcoplasmic reticulum (SR) to myoplasm occurs in self-propagating succession along the length of the cell. This event is called a calcium wave and is fundamentally a diffusion-reaction phenomenon. We present a simple, continuum mathematical model that simulates calcium waves. The framework features calcium diffusion within the SR and myoplasm, and dual modulation of ryanodine receptor (RyR) release channels by myoplasmic and SR calcium. The model is used to illustrate the effect of varying RyR permeability, sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) activity and calcium ion mobility in myoplasm and SR on wave velocity. The model successfully reproduces calcium waves using experimentally-derived variables. It also supports the proposal for wave propagation driven by the diffusive spread of myoplasmic calcium, and highlights the importance of SR calcium load on wave propagation.