Mechano-Electric Coupling and Arrhythmogenic Current Generation in a Computational Model of Coupled Myocytes

A wide range of arrhythmogenic phenotypes have been associated with heterogeneous mechanical dyskinesis. Pro-arrhythmic effects are often associated with dysregulated intra-cellular calcium handling, especially via the development of intra- and inter-cellular calcium waves. Experimental evidence suggests that mechanical strain can contribute to the generation and maintenance of these calcium waves via a variety of mechano-electric coupling mechanisms. Most model studies of mechano-electric coupling mechanisms have been focused on mechano-sensitive ion channels, even though experimental studies have shown that intra- and inter-cellular calcium waves triggered by mechanical perturbations are likely to be more prevalent pro-arrhythmic mechanisms in the diseased heart. A one-dimensional strongly coupled computational model of electromechanics in rabbit ventricular cardiomyocytes showed that specific myocyte stretch sequences can modulate the susceptibility threshold for delayed after-depolarizations. In simulations of mechanically-triggered calcium waves in cardiomyocytes coupled to fibroblasts, susceptibility to calcium wave propagation was reduced as the current through the gap junction caused current drain from the myocytes. In 1D multi-cellular arrays coupled via gap junctions, mechanically-induced waves may contribute to synchronizing arrhythmogenic calcium waves and after-depolarizations.

Hypotonic Stress Induces Fast, Reversible Degradation of the Vimentin Cytoskeleton via Intracellular Calcium Release

The dynamic response of the cell to osmotic changes is critical to its physiology and is widely exploited for cell manipulation. Here, using three-dimensional stochastic optical reconstruction microscopy (3D-STORM), a super-resolution technique, the hypotonic stress-induced ultrastructural changes of the cytoskeleton of a common fibroblast cell type are examined. Unexpectedly, these efforts lead to the discovery of a fast, yet reversible dissolution of the vimentin intermediate filament system that precedes ultrastructural changes of the supposedly more dynamic actin and tubulin cytoskeletal systems as well as changes in cell morphology. In combination with calcium imaging and biochemical analysis, it is shown that the vimentin-specific fast cytoskeletal degradation under hypotonic stress is due to proteolysis by the calcium-dependent protease calpain. The process is found to be activated by the hypotonic stress-induced calcium release from intracellular stores, and is therefore efficiently suppressed by inhibiting any part of the IP3-Ca2+-calpain pathway established in this study. Together, these findings highlight an unexpected, fast degradation mechanism for the vimentin cytoskeleton in response to external stimuli, and point to the significant, yet previously overlooked physiological impacts of hypotonic stress-induced intracellular calcium release on cell ultrastructure and function.

Absinthin, an agonist of the bitter taste receptor hTAS2R46, uncovers an ER-to-mitochondria Ca2+-shuttling event

Type 2 taste receptors (TAS2R) are G protein-coupled receptors first described in the gustatory system, but have also been shown to have extraoral localizations, including airway smooth muscle (ASM) cells, in which TAS2R have been reported to induce relaxation. TAS2R46 is an unexplored subtype that responds to its highly specific agonist absinthin. Here, we first demonstrate that, unlike other bitter-taste receptor agonists, absinthin alone (1 μm) in ASM cells does not induce Ca²⁺ signals but reduces histamine-induced cytosolic Ca²⁺ increases. To investigate this mechanism, we introduced into ASM cells aequorin-based Ca²⁺ probes targeted to the cytosol, subplasma membrane domain, or the mitochondrial matrix. We show that absinthin reduces cytosolic histamine-induced Ca²⁺ rises and simultaneously increases Ca²⁺ influx into mitochondria. We found that this effect is inhibited by the potent human TAS2R46 (hTAS2R46) antagonist 3β-hydroxydihydrocostunolide and is no longer evident in hTAS2R46-silenced ASM cells, indicating that it is hTAS2R46-dependent. Furthermore, these changes were sensitive to the mitochondrial uncoupler carbonyl cyanide p-(trifluoromethoxy)phenyl-hydrazone (FCCP); the mitochondrial calcium uniporter inhibitor KB-R7943 (carbamimidothioic acid); the cytoskeletal disrupter latrunculin; and an inhibitor of the exchange protein directly activated by cAMP (EPAC), ESI-09. Similarly, the β2 agonist salbutamol also could induce Ca²⁺ shuttling from cytoplasm to mitochondria, suggesting that this new mechanism might be generalizable. Moreover, forskolin and an EPAC activator mimicked this effect in HeLa cells. Our findings support the hypothesis that plasma membrane receptors can positively regulate mitochondrial Ca²⁺ uptake, adding a further facet to the ability of cells to encode complex Ca²⁺ signals.

Intracellular Calcium Mobilization Is Required for Sonic Hedgehog Signaling

Graded Shh signaling across fields of precursor cells coordinates patterns of gene expression, differentiation, and morphogenetic behavior as precursors form complex structures, such as the nervous system, the limbs, and craniofacial skeleton. Here we discover that intracellular calcium mobilization, a process tightly controlled and readily modulated, regulates the level of Shh-dependent gene expression in responding cells and affects the development of all Shh-dependent cell types in the zebrafish embryo. Reduced expression or modified activity of ryanodine receptor (RyR) intracellular calcium release channels shifted the allocation of Shh-dependent cell fates in the somitic muscle and neural tube. Mosaic analysis revealed that RyR-mediated calcium mobilization is required specifically in Shh ligand-receiving cells. This work reveals that RyR channels participate in intercellular signal transduction events. As modulation of RyR activity modifies tissue patterning, we hypothesize that alterations in intracellular calcium mobilization contribute to both birth defects and evolutionary modifications of morphology.

Cardiac Ryanodine Receptor (Ryr2)-mediated Calcium Signals Specifically Promote Glucose Oxidation via Pyruvate Dehydrogenase

Cardiac ryanodine receptor (Ryr2) Ca²⁺ release channels and cellular metabolism are both disrupted in heart disease. Recently, we demonstrated that total loss of Ryr2 leads to cardiomyocyte contractile dysfunction, arrhythmia, and reduced heart rate. Acute total Ryr2 ablation also impaired metabolism, but it was not clear whether this was a cause or consequence of heart failure. Previous in vitro studies revealed that Ca²⁺ flux into the mitochondria helps pace oxidative metabolism, but there is limited in vivo evidence supporting this concept. Here, we studied heart-specific, inducible Ryr2 haploinsufficient (cRyr2Δ50) mice with a stable 50% reduction in Ryr2 protein. This manipulation decreased the amplitude and frequency of cytosolic and mitochondrial Ca²⁺ signals in isolated cardiomyocytes, without changes in cardiomyocyte contraction. Remarkably, in the context of well preserved contractile function in perfused hearts, we observed decreased glucose oxidation, but not fat oxidation, with increased glycolysis. cRyr2Δ50 hearts exhibited hyperphosphorylation and inhibition of pyruvate dehydrogenase, the key Ca²⁺-sensitive gatekeeper to glucose oxidation. Metabolomic, proteomic, and transcriptomic analyses revealed additional functional networks associated with altered metabolism in this model. These results demonstrate that Ryr2 controls mitochondrial Ca²⁺ dynamics and plays a specific, critical role in promoting glucose oxidation in cardiomyocytes. Our findings indicate that partial RYR2 loss is sufficient to cause metabolic abnormalities seen in heart disease.

Tyr-167/Trp-168 in type 1/3 inositol 1,4,5-trisphosphate receptor mediates functional coupling between ligand binding and channel opening

The N-terminal ∼220-amino acid region of the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)/Ca(2+) release channel has been referred to as the suppressor/coupling domain because it is required for both IP(3) binding suppression and IP(3)-induced channel gating. Measurements of IP(3)-induced Ca(2+) fluxes of mutagenized mouse type 1 IP(3)R (IP(3)R1) showed that the residues responsible for IP(3) binding suppression in this domain were not essential for channel opening. On the other hand, a single amino acid substitution of Tyr-167 to alanine completely impaired IP(3)-induced Ca(2+) release without reducing the IP(3) binding activity. The corresponding residue in type 3 IP(3)R (IP(3)R3), Trp-168, was also critical for channel opening. Limited trypsin digestion experiments showed that the trypsin sensitivities of the C-terminal gatekeeper domain differed markedly between the wild-type channel and the Tyr-167 mutant under the optimal conditions for channel opening. These results strongly suggest that the Tyr/Trp residue (Tyr-167 in IP(3)R1 and Trp-168 in IP(3)R3) is critical for the functional coupling between IP(3) binding and channel gating by maintaining the structural integrity of the C-terminal gatekeeper domain at least under activation gating.

Maturation and long-term hypoxia alters Ca2+-induced Ca2+ release in sheep cerebrovascular sympathetic neurons

The contribution of sympathetic nerves arising from the superior cervical ganglia (SCG) toward the growth and function of cerebral blood vessels is pertinent throughout maturation as well as in response to cardiovascular stress imposed by high-altitude long-term hypoxia (LTH). The function of SCG sympathetic neurons is dependent on intracellular Ca2+ concentration ([Ca2+]i) signaling, which is strongly influenced by a process known as Ca(2+)-induced Ca2+ release (CICR) from the smooth endoplasmic reticulum (SER). In this study, we used the sheep SCG neuronal model to test the hypotheses that maturation decreases CICR and high-altitude LTH depresses CICR in fetal SCG neurons but not in those of the adult. We found that the contribution of CICR to electric field stimulation (EFS)-evoked [Ca2+]i transients was greatest in SCG cells from normoxic fetuses and was abolished by LTH. The decline in CICR was associated with a reduction in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) function in fetal SCG cells during LTH, reducing SER Ca2+ levels below the threshold needed for the coupling of Ca2+ influx and CICR. With respect to the maturation from the fetus to adult, the decrease in CICR may reflect both a reduction in the levels of ryanodine receptor isoforms 2 and 3 and SERCA function. In response to LTH and in contrast to the fetus, CICR function in adult SCG cells is maintained and may reflect alterations in other mechanisms that modulate the CICR process. As CICR is instrumental in the function of sympathetic neurons within the cerebrovasculature, the loss of this signaling mechanism in the fetus may have consequences for the adaptation to LTH in terms of fetal susceptibility to vascular insults.