Michael D. Schneider, M.D.
Baylor College of Medicine
- The requisite physiological and molecular methods were implemented to assess cardiac function in mouse and rat hearts in response to cardiac loading and unloading, including M-mode and Doppler echocardiographic assessment of cardiac structure and function in mice, and in vivo left ventricular hemodynamic recordings.
- Programmed cell death (apoptosis) in cardiac myocytes was demonstrated in response to altered load. Multiple complementary approaches to monitor and alleviate apoptosis were investigated.
- Contractility is impaired in mouse myocytes with down-regulation of SERCA2, a gene whose expression is defective, following decreased loading.
- Heterotopic transplantation, the best-established model of cardiac unloading, was implented successfully. Unloading by this means was shown to induce cardiac atrophy, down-regulation of SERCA2, and up-regulation of nominal markers of hypertrophy (despite the opposite effect on growth).
- Contractile reserve was depressed in myocytes from unloaded hearts. Impaired contractile reserve in myocytes from unloaded hearts is related to the inability to augment intracellular systolic Ca2+ during this challenge.
- Growth hormone is a potential countermeasure, which partials rescues SERCA2 expression during cardiac atrophy, but higher doses than tested will be required to correct the defects in contractile reserve and cardiac mass.
- Biochemical studies to identify novel load-regulated proteins, for mechanistic insights and as potential sites for intervention, led to two fundamental discoveries. Load regulates activation of RNA polymerase II, which controls global rates of RNA synthesis per cell, via the protein kinase, Cdk7. Load also regulates the mitogen-activated protein kinase, TAK1, which mediates (in part) the effects of load on cell survival and gene expression.
- Genetic studies to identify load-regulated genes, for mechanistic insights and as potential sites for intervention, led to the discovery of more than 50 differentially expressed genes, beyond those that have been reported previously to be targets of load, including many signaling proteins: Rap1B, protein phosphatase 1γ PP1γ, inhibitor protein phosphatase 2A IPP2A, mss4, dynamin-like protein 1 DLP-1, and the putative mechanosensor, ILK.
The cardiovascular system undergoes multiple changes during prolonged space-flight as adaptation to the microgravity environment. During spaceflight, the cardiovascular system is not subjected to the biomechanical stresses associated with changes in posture in a gravitational field. Space-flight is associated with a modest reduction in intravascular volume and red blood cell mass, a decrease in arterial blood pressure, and a relative shift of intravascular volume from the lower body to the thorax and head; importantly, these adaptations occur even in the presence of regular exercise regimens and hydration during space-flight missions. It is recognized that the integrated cardiovascular adaptation to prolonged spaceflight may be in the net beneficial in the microgravity environment, but may be maladaptive when the cardiovascular system is subjected to severe abrupt stresses such as reentry into a higher gravitational field or the requirement to perform sustained and near-maximal exercise. In the present grant period, the NSBRI Cardiovascular Alterations team addressed three potential critical risks that may be imposed during long duration spaceflight. Each of these potential critical risks requires the elucidation of mechanisms in order to develop rational and effective countermeasures that can be applied in human long-duration spaceflight. The critical risks include: (1) the development of orthostatic hypotension and risk of syncope upon reentry into the earth (and potentially Mars) gravitational field; (2) the susceptibility to rhythm disturbances; and (3) the reduction in cardiac mass.
Our team focused specifically on the problem of changes in cardiac remodeling and gene expression which occur in response to cardiac unloading (cardiac atrophy) in rodent models, and compared these observations with changes that occur in response to excess load (cardiac hypertrophy). The observations in this grant period support our index hypothesis that the plasticity of the heart to adapt to perturbations in load is limited, and that cardiac unloading stimulates changes in gene expression which are phenotypic of cardiac hypertrophy. In short, directionally similar changes in gene expression occur during BOTH increases and decreases in cardiac muscle cell size, indicating these are reflective of cell remodeling per se.
This paradigm shift has important predictive implications for future hypothesis-testing and for specific counter-measures, as a number of adverse pathways affecting contractility, cardiac compliance, and even cell survival are activated as part of the known hypertrophic gene program.
The significance of this work, directed at cardiac atrophy, was highlighted by preliminary human data, made known during the study period by Dr. J. Yelle, Head of the Cardiovascular Laboratory of the Johnson Space Center. Whereas no reduction in LV mass was found using echocardiography, after short-duration NASA missions (n=13), a significant, reproducible, 10 percent loss of LV mass was seen in long-duration MIR missions (n=4). This compels greater diligence, in monitoring the long-term effects of microgravity on cardiac mass in astronauts, and also reinforces the scientific need to understand the mechanisms and molecular details, underlying this gross change.
Thus, the overall hypotheses posed by the investigators have gained substantial reinforcement from both animal and human studies.
Implications for risk reduction related to the Critical Research Path and Earth medical problems:
The whole-animal and isolated-cell studies together demonstrate, unambiguously, loss of cardiac mass, alterations of normal gene expression, and impaired contractile reserve. Thus, further studies of cardiac atrophy during unloading are clearly warranted.
Growth hormone can partially rescue impaired expression of SERCA2, encoding the calcium "pump," in this model of cardiac atrophy. Thus, further studies of growth hormone are clearly warranted, at higher dosages and in concert with other countermeasures.
Mechanical load affects numerous targets that had never previously been identified but were disclosed by novel biochemical and genetic methods. Thus, future work is needed to explore the functional contribution of these candidate effectors, to develop countermeasures for those, like TAK1, whose consequences are adverse, and, conversely, to develop therapies based on those, like Cdk7, whose effects on cardiac mass or function is shown to be beneficial.