The major goal of my research is to investigate novel pathophysiological mechanisms of heart failure at whole animal, organ, cellular and molecular levels, with applications to improve therapy and survival. Studies are focused on heart failure following myocardial infarction (MI), cardiac dysfunction during endotoxemia, and embryonic heart development during maternal diabetes. Genetically altered mice are employed to elucidate the contribution of specific molecules in the disease process.
1. Nitric Oxide and Heart Development
Endothelial nitric oxide synthase (eNOS) is expressed early during heart development (E9.5) in mice. However, its role in cardiac development is not clear. We were the first to show that eNOS is pivotal to normal heart development. Mice deficient in the eNOS gene develop congenital septal defects and heart failure, leading to a high mortality after birth. This article was published in Circulation and featured on the journalís cover on August 13, 2002. Further to this work, we showed that eNOS is important in myocardial angiogenesis. Additionally, fetal myocardial eNOS expression promotes cardiomyocyte proliferation and maturation during heart development. Importantly, we demonstrated that the effects of eNOS on cardiomyocyte proliferation are through inhibition of TIMP-3 (tissue inhibitor of metalloproteinase-3), which decreases cardiomyocyte proliferation. These studies have increased our understanding on the important role of eNOS in normal heart. Ongoing research is to study the role of eNOS in cardiac malformation induced by maternal diabetes.
2. Role of Nitric Oxide in Heart Failure
A major research focus of my research program has been the role of nitric oxide in heart failure post-MI. When nitric oxide was discovered in the vascular endothelium about 30 years ago, it was considered solely as a vasodilator that relaxes the vascular smooth muscle. We now know that nitric oxide is also a signaling molecule that regulates functions of almost every organ during health and disease. An important contribution from my lab is the demonstration that myocardial inducible nitric oxide synthase (iNOS) expression is increased after MI and increased iNOS expression plays an important role in the development of heart failure post-MI. Our study showed for the first time that iNOS may represent a therapeutic target for the treatment of heart failure post-MI. In addition, our recent studies demonstrated that neuronal nitric oxide synthase (nNOS) protects the heart from ventricular arrhythmia post-MI via regulation of several key Ca2+ handling proteins.
3. Molecular Mechanisms of Cardioprotection by Erythropoietin
We are also interested in the cardioprotective effects of erythropoietin (EPO), a glycoprotein essential for red blood cell production. While previous studies have shown cardioprotection by EPO, its molecular mechanisms responsible for the beneficial effects are not fully understood. We have studied mechanisms by which EPO protects the heart from ischemic injuries. We demonstrated that EPO increases expression, phosphorylation and activity of eNOS, which is critical to the cardioprotective effects of EPO. This paper was published in Cardiovascular Research and was one of most downloaded papers of that journal in 2006. Importantly, we have also shown that EPO increases nNOS expression and that upregulation of nNOS contributes to the anti-arrhythmic effects of EPO following myocardial ischemia and reperfusion (I/R). These studies elucidated a pivotal role of nitric oxide in the cardioprotective effects of EPO and may help to further assess the potential of EPO as a therapeutic agent in heart disease.
4. Stem Cell Migration and Cardiac Repair
Adult stem cells have the potential of tissue repair/regeneration following organ injuries. This is particularly important to the heart post-MI as adult cardiomyocytes have very limited ability to regenerate. Adult bone marrow stromal cells (MSCs) are multi-potential non-hematopoietic stem cells. Accumulating evidence suggests that MSCs are an attractive candidate for cardiovascular therapy, due to their capacity to facilitate myocardial repair and neovascularization in models of cardiac injury. Although the ability of MSCs to differentiate into cardiomyocytes in vivo is still debatable, studies have consistently shown that MSCs are able to migrate into the injured myocardium from the circulation and contribute to cardiac repair via paracrine effects post-MI. However, the molecular mechanisms governing MSC migration into the ischemic myocardium are not fully understood. Our recent study demonstrated that eNOS expression in the host myocardium is critical in MSC migration to the heart following myocardial I/R. Cardiac specific overexpression of eNOS increases MSC migration to the ischemic myocardium via upregulation of stromal cell derived factor (SDF-1) and increased SDF-1/CXCR4 coupling leading to improvement in cardiac function after I/R. Another strategy that we are using to improve stem cell migration to the ischemic myocardium is the conditional and cardiac specific overexpression of membrane associated human stem cell factor (hSCF), an important chemoattractant for stem cell migration. Our data showed that conditional and cardiac specific overexpression of hSCF using a Tet-Off system enhances endothelial progenitor cell (EPC) migration to the infarct myocardium, increases myocardial production of growth factors, promotes angiogenesis, and improves cardiac function post-MI. These studies may open up new therapeutic avenues for enhancing stem cell migration and cardiac repair post-MI.
5. Signal Transduction and Cardiac dysfunction in Sepsis
Another area of my research is the signal transduction mechanisms that lead to inflammatory cytokine expression and cardiac dysfunction during sepsis. Our work is focused on tumor necrosis factor-alpha (TNF-α) in the heart as TNF-α is considered a major cytokine that causes cardiac depression during sepsis. We demonstrated that eNOS promotes while nNOS inhibits TNF-α expression in the heart during sepsis. We showed for the first time an important role of Nox2-containing NADPH oxidase in cytokine expression and cardiac dysfunction during sepsis. Genetic deletion of Nox2 or pharmacological inhibition of NADPH oxidase activity decreases myocardial TNF-α expression and improves cardiac function during sepsis. In addition, we demonstrated an important cross-talk between MAPKs in sepsis. Specifically, we showed that JNK1/c-fos inhibits ERK1/2 and p38 MAPK signalling, leading to decreased cardiomyocyte TNF-α expression and improvements in cardiac function during sepsis. These studies have provided novel insights into the pathogenesis of cardiac dysfunction during sepsis. Furthermore, our studies suggest that modulation of different NOS isoforms and inhibition of Nox2-containing NADPH oxidase may serve as potential strategies for the treatment of sepsis.