HDAC6 Negatively Regulates miR-155-5p Expression to Elicit Proliferation by Targeting RHEB in Microvascular Endothelial Cells under Mechanical Unloading

Mechanical unloading contributes to significant cardiovascular deconditioning. Endothelial dysfunction in the sites of microcirculation may be one of the causes of the cardiovascular degeneration induced by unloading, but the detailed mechanism is still unclear. Here, we first demonstrated that mechanical unloading inhibited brain microvascular endothelial cell proliferation and downregulated histone deacetylase 6 (HDAC6) expression. Furthermore, HDAC6 promoted microvascular endothelial cell proliferation and attenuated the inhibition of proliferation caused by clinorotation unloading. To comprehensively identify microRNAs (miRNAs) that are regulated by HDAC6, we analyzed differential miRNA expression in microvascular endothelial cells after transfection with HDAC6 siRNA and selected miR-155-5p, which was the miRNA with the most significantly increased expression. The ectopic expression of miR-155-5p inhibited microvascular endothelial cell proliferation and directly downregulated Ras homolog enriched in brain (RHEB) expression. Moreover, RHEB expression was downregulated under mechanical unloading and was essential for the miR-155-5p-mediated promotion of microvascular endothelial cell proliferation. Taken together, these results are the first to elucidate the role of HDAC6 in unloading-induced cell growth inhibition through the miR-155-5p/RHEB axis, suggesting that the HDAC6/miR-155-5p/RHEB pathway is a specific target for the preventative treatment of cardiovascular deconditioning.

Abstract 15153: Functional Cardiac Adaptation to 60 Days Strict Head-Down-Tilt Bedrest

Background: Reduced physical activity increases the risk for heart failure. Myocardial strain measurements capture subtle abnormalities in myocardial function. We tested the hypothesis that bedrest deconditioning produces cardiac dysfunction in healthy persons. Methods: In the AGBRESA study, which assessed artificial gravity through centrifugation as potential countermeasure for space travel, 24 healthy persons (8 women) were submitted to 60 days strict -6°-head-down-tilt bedrest. 8 subjects each were included in a control group or groups subjected to continuous artificial gravity training on a short-arm centrifuge [30 minutes/day] or intermittent centrifugation [6x5 min/day]. We assessed cardiac morphology, function, strain and hemodynamics by cardiac-MRI (baseline, end of bedrest, recovery) and echocardiography (baseline, end of bedrest). Before and after bedrest, we assessed orthostatic heart rate responses as measure of cardiovascular deconditioning. Results: We conducted a pooled analysis because cardiovascular responses to bedrest did not differ between groups. Supine heart rate (baseline: 64±9.6bpm; bedrest: 72.3±10.5bpm; recovery: 69.6±10.5bpm, p <0.0001) and diastolic blood-pressure (69.6±7.3mmHg; bedrest: 78.5±6.9mmHg; recovery: 70.3±6.3mmHg, p <0.0001) increased with bedrest. With head-up tilt, heart rate increased 22.8±10.5bpm before and 45.9±213bpm at the end of bedrest ( p <0.0001) consistent with cardiovascular deconditioning. Cardiac-output decreased from 6.6±0.9l/min to 6±1l/min at end of bedrest (recovery: 6.8±1.2l/min, p =0.0096). Left ventricular ejection fraction and mass-index did not change. Echocardiographic global longitudinal strain (baseline: -19.90±2.1%; bedrest: -18.1±2.1%, p <0.0001) decreased, whereas global circumferential strain in MRI tended to increase (baseline: -18.6±1.7%; bedrest: -19.1±1.6%, p =0.0843). After four days of recovery MRI measurements had returned to baseline. Conclusion: In healthy persons, cardiovascular deconditioning through 60 days head-down-tilt bedrest does not lead to cardiac atrophy or sustained reductions in cardiac performance. The transient nature of cardiac strain changes suggests functional rather than structural changes in the heart.

Cardiovascular adaptation to simulated microgravity and countermeasure efficacy assessed by ballistocardiography and seismocardiography

Head-down bed rest (HDBR) reproduces the cardiovascular effects of microgravity. We tested the hypothesis that regular high-intensity physical exercise (JUMP) could prevent this cardiovascular deconditioning, which could be detected using seismocardiography (SCG) and ballistocardiography (BCG). 23 healthy males were exposed to 60-day HDBR: 12 in a physical exercise group (JUMP), the others in a control group (CTRL). SCG and BCG were measured during supine controlled breathing protocols. From the linear and rotational SCG/BCG signals, the integral of kinetic energy ($$iK$$ iK ) was computed on each dimension over the cardiac cycle. At the end of HDBR, BCG rotational $$iK$$ iK and SCG transversal $$iK$$ iK decreased similarly for all participants (− 40% and − 44%, respectively, p < 0.05), and so did orthostatic tolerance (− 58%, p < 0.01). Resting heart rate decreased in JUMP (− 10%, p < 0.01), but not in CTRL. BCG linear $$iK$$ iK decreased in CTRL (− 50%, p < 0.05), but not in JUMP. The changes in the systolic component of BCG linear iK were correlated to those in stroke volume and VO2 max (R = 0.44 and 0.47, respectively, p < 0.05). JUMP was less affected by cardiovascular deconditioning, which could be detected by BCG in agreement with standard markers of the cardiovascular condition. This shows the potential of BCG to easily monitor cardiac deconditioning.

Cardiovascular deconditioning during long-term spaceflight through multiscale modeling

Human spaceflight has been fascinating man for centuries, representing the intangible need to explore the unknown, challenge new frontiers, advance technology, and push scientific boundaries further. A key area of importance is cardiovascular deconditioning, that is, the collection of hemodynamic changes—from blood volume shift and reduction to altered cardiac function—induced by sustained presence in microgravity. A thorough grasp of the 0G adjustment point per se is important from a physiological viewpoint and fundamental for astronauts’ safety and physical capability on long spaceflights. However, hemodynamic details of cardiovascular deconditioning are incomplete, inconsistent, and poorly measured to date; thus a computational approach can be quite valuable. We present a validated 1D–0D multiscale model to study the cardiovascular response to long-term 0G spaceflight in comparison to the 1G supine reference condition. Cardiac work, oxygen consumption, and contractility indexes, as well as central mean and pulse pressures were reduced, augmenting the cardiac deconditioning scenario. Exercise tolerance of a spaceflight traveler was found to be comparable to an untrained person with a sedentary lifestyle. At the capillary–venous level significant waveform alterations were observed which can modify the regular perfusion and average nutrient supply at the cellular level. The present study suggests special attention should be paid to future long spaceflights which demand prompt physical capacity at the time of restoration of partial gravity (e.g., Moon/Mars landing). Since spaceflight deconditioning has features similar to accelerated aging understanding deconditioning mechanisms in microgravity are also relevant to the understanding of aging physiology on the Earth.