Identification of an amino-terminus determinant critical for ryanodine receptor/Ca2+ release channel function.

AIMS: The cardiac ryanodine receptor (RyR2), which mediates intracellular Ca2+ release to trigger cardiomyocyte contraction, participates in development of acquired and inherited arrhythmogenic cardiac disease. This study was undertaken to characterize the network of inter- and intra-subunit interactions regulating the activity of the RyR2 homotetramer. METHODS AND RESULTS: We use mutational investigations combined with biochemical assays to identify the peptide sequence bridging the ß8 with ß9 strand as the primary determinant mediating RyR2 N-terminus self-association. The negatively charged side chains of two aspartate residues (D179 and D180) within the ß8-ß9 loop are crucial for the N-terminal inter-subunit interaction. We also show that the RyR2 N-terminus domain interacts with the C-terminal channel pore region in a Ca2+-independent manner. The ß8-ß9 loop is required for efficient RyR2 subunit oligomerization but it is dispensable for N-terminus interaction with C-terminus. Deletion of the ß8-ß9 sequence produces unstable tetrameric channels with subdued intracellular Ca2+ mobilization implicating a role for this domain in channel opening. The arrhythmia-linked R176Q mutation within the ß8-ß9 loop decreases N-terminus tetramerization but does not affect RyR2 subunit tetramerization or the N-terminus interaction with C-terminus. RyR2R176Q is a characteristic hypersensitive channel displaying enhanced intracellular Ca2+ mobilization suggesting an additional role for the ß8-ß9 domain in channel closing. CONCLUSION: These results suggest that efficient N-terminus inter-subunit communication mediated by the ß8-ß9 loop may constitute a primary regulatory mechanism for both RyR2 channel activation and suppression.

Association of cardiac myosin-binding protein-C with the ryanodine receptor channel - putative retrograde regulation?

The cardiac muscle ryanodine receptor-Ca2+ release channel (RyR2) constitutes the sarcoplasmic reticulum (SR) Ca2+ efflux mechanism that initiates myocyte contraction, while cardiac myosin-binding protein-C (cMyBP-C; also known as MYBPC3) mediates regulation of acto-myosin cross-bridge cycling. In this paper, we provide the first evidence for the presence of direct interaction between these two proteins, forming a RyR2-cMyBP-C complex. The C-terminus of cMyBP-C binds with the RyR2 N-terminus in mammalian cells and the interaction is not mediated by a fibronectin-like domain. Notably, we detected complex formation between both recombinant cMyBP-C and RyR2, as well as between the native proteins in cardiac tissue. Cellular Ca2+ dynamics in HEK293 cells is altered upon co-expression of cMyBP-C and RyR2, with lowered frequency of RyR2-mediated spontaneous Ca2+ oscillations, suggesting that cMyBP-C exerts a potential inhibitory effect on RyR2-dependent Ca2+ release. Discovery of a functional RyR2 association with cMyBP-C provides direct evidence for a putative mechanistic link between cytosolic soluble cMyBP-C and SR-mediated Ca2+ release, via RyR2. Importantly, this interaction may have clinical relevance to the observed cMyBP-C and RyR2 dysfunction in cardiac pathologies, such as hypertrophic cardiomyopathy.

Structural and functional interactions within ryanodine receptor.

The ryanodine receptor/Ca2+ release channel plays a pivotal role in skeletal and cardiac muscle excitation-contraction coupling. Defective regulation leads to neuromuscular disorders and arrhythmogenic cardiac disease. This mini-review focuses on channel regulation through structural intra- and inter-subunit interactions and their implications in ryanodine receptor pathophysiology.

Dantrolene rescues aberrant N-terminus intersubunit interactions in mutant pro-arrhythmic cardiac ryanodine receptors.

AIMS: The ryanodine receptor (RyR2) is an intracellular Ca(2+) release channel essential for cardiac excitation-contraction coupling. Abnormal RyR2 channel function results in the generation of arrhythmias and sudden cardiac death. The present study was undertaken to investigate the mechanistic basis of RyR2 dysfunction in inherited arrhythmogenic cardiac disease. METHODS AND RESULTS: We present several lines of complementary evidence, indicating that the arrhythmia-associated L433P mutation disrupts RyR2 N-terminus self-association. A combination of yeast two-hybrid, co-immunoprecipitation, and chemical cross-linking assays collectively demonstrate that a RyR2 N-terminal fragment carrying the L433P mutation displays substantially reduced self-interaction compared with wild type. Moreover, sucrose density gradient centrifugation reveals that the L433P mutation impairs tetramerization of the full-length channel. [(3)H]Ryanodine-binding assays demonstrate that disrupted N-terminal intersubunit interactions within RyR2(L433P) confer an altered sensitivity to Ca(2+) activation. Calcium imaging of RyR2(L433P)-expressing cells reveals substantially prolonged Ca(2+) transients and reduced Ca(2+) store content indicating defective channel closure. Importantly, dantrolene treatment reverses the L433P mutation-induced impairment and restores channel function. CONCLUSION: The N-terminus domain constitutes an important structural determinant for the functional oligomerization of RyR2. Our findings are consistent with defective N-terminus self-association as a molecular mechanism underlying RyR2 channel deregulation in inherited arrhythmogenic cardiac disease. Significantly, the therapeutic action of dantrolene may occur via the restoration of normal RyR2 N-terminal intersubunit interactions.

N-terminus oligomerization is conserved in intracellular calcium release channels.

Oligomerization of all three mammalian ryanodine receptor isoforms, a structural requirement for normal intracellular Ca2+ release channel function, is displayed by the discrete N-terminal domain which assembles into homo- and hetero-tetramers. This is demonstrated in yeast, mammalian cells and native tissue by complementary yeast two-hybrid, chemical cross-linking and co-immunoprecipitation assays. The IP3 (inositol 1,4,5-trisphosphate) receptor N-terminus (residues 1-667) similarly exhibits tetrameric association as indicated by chemical cross-linking and co-immunoprecipitation assays. The presence of either a 15-residue splice insertion or of the cognate ligand IP3 did not affect tetramerization of the IP3 receptor N-terminus. Thus N-terminus tetramerization appears to be an essential intrinsic property that is conserved in both the ryanodine receptor and IP3 receptor families of mammalian intracellular Ca2+ release channels.

N-terminus oligomerization regulates the function of cardiac ryanodine receptors.

The ryanodine receptor (RyR) is an ion channel composed of four identical subunits mediating calcium efflux from the endo/sarcoplasmic reticulum of excitable and non-excitable cells. We present several lines of evidence indicating that the RyR2 N-terminus is capable of self-association. A combination of yeast two-hybrid screens, co-immunoprecipitation analysis, chemical crosslinking and gel filtration assays collectively demonstrate that a RyR2 N-terminal fragment possesses the intrinsic ability to oligomerize, enabling apparent tetramer formation. Interestingly, N-terminus tetramerization mediated by endogenous disulfide bond formation occurs in native RyR2, but notably not in RyR1. Disruption of N-terminal inter-subunit interactions within RyR2 results in dysregulation of channel activation at diastolic Ca(2+) concentrations from ryanodine binding and single channel measurements. Our findings suggest that the N-terminus interactions mediating tetramer assembly are involved in RyR channel closure, identifying a crucial role for this structural association in the dynamic regulation of intracellular Ca(2+) release.

Evaluation of activity and combination strategies with the microtubule-targeting drug sagopilone in breast cancer cell lines.

Sagopilone, a fully synthetic epothilone, is a microtubule-stabilizing agent optimized for high in vitro and in vivo activity against a broad range of tumor models, including those resistant to paclitaxel and other systemic treatments. Sagopilone development is accompanied by translational research studies to evaluate the molecular mode of action, to recognize mechanisms leading to resistance, to identify predictive response biomarkers, and to establish a rationale for combination with different therapies. Here, we profiled sagopilone activity in breast cancer cell lines. To analyze the mechanisms of mitotic arrest and apoptosis and to identify additional targets and biomarkers, an siRNA-based RNAi drug modifier screen interrogating 300 genes was performed in four cancer cell lines. Defects of the spindle assembly checkpoint (SAC) were identified to cause resistance against sagopilone-induced mitotic arrest and apoptosis. Potential biomarkers for resistance could therefore be functional defects like polymorphisms or mutations in the SAC, particularly in the central SAC kinase BUB1B. Moreover, chromosomal heterogeneity and polyploidy are also potential biomarkers of sagopilone resistance since they imply an increased tolerance for aberrant mitosis. RNAi screening further demonstrated that the sagopilone-induced mitotic arrest can be enhanced by concomitant inhibition of mitotic kinesins, thus suggesting a potential combination therapy of sagopilone with a KIF2C (MCAK) kinesin inhibitor. However, the combination of sagopilone and inhibition of the prophase kinesin KIF11 (EG5) is antagonistic, indicating that the kinesin inhibitor has to be highly specific to bring about the required therapeutic benefit.

Cell-based therapy for heart failure: skeletal myoblasts.

Satellite cells are committed precursor cells residing in the skeletal muscle. These cells provide an almost unlimited regeneration potential to the muscle, contrary to the heart, which, although proved to contain cardiac stem cells, possesses a very limited ability for self-renewal. The idea that myoblasts (satellite cell progenies) may repopulate postinfarction scar occurred around the mid-1990s. Encouraging results of preclinical studies triggered extensive research, which led to the onset of clinical trials. These trials have shown that autologous skeletal myoblast transplantation to cure heart failure is feasible and relatively safe (observed incidences of arrhythmia). Because most of the initial studies on myoblast application into postischemic heart have been carried out as an adjunct to routine surgical procedures, the true clinical outcome of such therapy in regard to cell implantation is blurred and requires to be elucidated. The mechanism by which implantation of skeletal myoblast may improve heart function is not clear, especially in the light of inability of these cells to couple electromechanically with a host myocardium. Successful myoblast therapy depends on a number of factors, including: delivery to the target tissue, long-term survival, efficacious engraftment, differentiation into cardiomyocytes, and integration into the new, unique microenvironment. All these steps constitute a potential goal for cell manipulation aiming to improve the overall outcome of such therapy. Precise understanding of the mechanism by which cells improve cardiac function is essential in giving the sensible direction of further research.

Regulation of eNOS expression in HCAEC cell line treated with opioids and proinflammatory cytokines.

BACKGROUND: Nitric oxide (NO) generated by endothelial nitric oxide synthase (eNOS) plays a crucial role in vascular function and homeostasis. eNOS activity maintains normal vascular tone, regulates leukocyte-endothelial cell interactions, inhibits platelet aggregation, limits smooth muscle cell proliferation and influences cardiac myocyte contractility. The loss of endothelium-derived NO has been shown to result in serious cardiovascular abnormalities, which may indicate eNOS involvement in the origin/development of cardiovascular disorders. AIM: Evaluation of eNOS mRNA level in the endothelium of human coronary arteries upon opioids treatment (mediators of ischaemic preconditioning) and after incubation with proinflammatory cytokines (stress stimuli). METHODS: Different concentrations of beta-endorphin, endomorphin-1 and endomorphin-2 (alone or in combination with the opioid receptor blocker naloxone) as well as different concentrations of cytokines alone (IL-1beta, TNF-alpha) or in combination were applied to in vitro cultured human coronary artery endothelial cells (HCAEC). After 24 hrs incubation, the cells were harvested, mRNA extracted and relative quantification of eNOS mRNA was conducted using real-time PCR. RESULTS: Opioid treatment altered eNOS expression; however, none of the changes reached a statistically significant level. As for proinflammatory cytokines, both TNF-alpha and IL-1beta substantially down-regulated the eNOS mRNA level (p <0.05). When applied in combination, these cytokines lowered eNOS mRNA even further (p <0.05). The effect was independent of the cytokine combination used. CONCLUSIONS: It was demonstrated that proinflammatory cytokines exert a substantial and statistically significant negative effect on eNOS mRNA level in human coronary artery endothelial cells in in vitro culture. Unfortunately, we were unable to demonstrate significant changes within the eNOS mRNA pool upon opioid treatment. These results indicate that opioids (basal release) do not affect eNOS expression in normal in vitro culture conditions but might do so upon stress stimuli.