Narine Sarvazyan Laboratory

ONGOING AND RECENTLY COMPLETED PROJECTS

ORIGINS OF ECTOPIC BEATS. Our lab is investigating how macroscopic ectopic beats are formed. Our in vitro experiments suggest that these beats occur during a so-called Ectopic Nexus state. The latter refers to a functional state of an injured cardiac tissue in which multiple poorly-coupled ectopic sources form a transient "wave breeding" microenvironment. In this microenvironment, ectopic activity develops from the firing of individual cells to slowly propagating local intercellular waves. These local waves are confined to the area of injury, but can spread to normal tissue upon change in external conditions. We have characterized the parameter space in which generation of local waves takes place and described, both numerically and experimentally, how the system behaves under dynamic conditions. Currently, we are investigating if ectopic beats in vivo are generated by a similar mechanism.

N-CADHERIN-MEDIATED STEM CELL ADHESION. We developed a new mouse embryonic stem cell line that constitutively overexpresses N-cadherin. The latter is a major adhesion protein found in the intercalated discs. We have shown that overexpression of N-cadherin leads to marked changes in the phenotype and adhesion of these cells, both during embryoid body formation and cell differentiation stages. We hypothesize that overexpressing N-cadherin would allow transplanted stem cells to integrate more efficiently to a host myocardium, and that increased adhesion would facilitate gap junctional connections between the host and graft cells. Indeed, in vitro, N-cadherin overexpressing cells showed elevated levels of p120, a protein involved in N-cadherin trafficking and stabilization. They also markedly overexpressed connexin-43, a gap junction protein that facilitates intercellular communication. Our next step is to conduct in vivo testing, in order to determine if N-cadherin overexpression improves the ability of stem cell derived-cardiomyocytes to engraft.

IN VITRO MODEL OF CARDIAC FIBERS.. Using Matrigel, a commercially available basement membrane preparation (BD Biosciences), we developed a simple in vitro model that mimics the three-dimensional environment and mechanical load conditions of cardiac muscle. A semisolid pillow from concentrated Matrigel can be overlaid with a suspension of neonatal rat cardiomyocytes in diluted Matrigel. This model provides an environment in which spontaneously formed multicellular fibers continuously contract against a mechanical load. Additionally, the Matrigel-cell mixture could be cut to form larger cardiomyocyte fibers. The fibers (20-300uM wide) can easily be monitored both functionally, using calcium or voltage sensitive dyes, and structurally using a variety of available immunomarkers.