Raymond Runyan, PhD

In the heart, cells that form the valves are induced to develop by interaction between endothelial cells and adjacent muscle. As a result, cardiac endothelial cells transform into mesenchyme and become the constituents of the valves and walls of the heart. This process is known as an epithelial-mesenchymal transition or EMT. The objective of my research program is to understand the molecular mechanisms that mediate cell transformation in the heart.

Using a tissue culture assay, we showed that cardiac endothelia would only transform after stimulation by adjacent muscle. My laboratory has continued to examine the events which take place during this epithelial-mesenchymal cell transformation. Our studies focus upon three basic questions. 1) What is the nature of the signal produced by the muscle? 2) How do the target cells recognize the signal? 3) What events occur in the target cells in response to the stimulus? Answers to these questions will identify potential causes of congenital heart defects.

Work in the laboratory initially focused on the observation that the growth factor, Transforming Growth Factor Beta, is a component of the transformation process. As this growth factor is actually a member of a family of related molecules, we used RNase protection assays to show that two isoforms, TGFbeta2 and TGFbeta3, were found in the heart. Experiments using antisense DNA oligonucleotides demonstrated that TGFbeta3 is the critical component. More recently, our work shows that both isoforms are used in complementary roles.

Experiments show that this EMT in the heart is blocked by loss of several different classes of TGFbeta receptors, including Alk2, Alk5, TGFbetaType II, TGFbetaType III and Endoglin. As seen with the two different TGFbeta isoforms, each receptor component appears to mediate play a distinct role in the EMT process. Other experiments in this laboratory showed that EMT is sensitive to inhibitors of several classes of kinases, Gi proteins, and the phosphoinositide pathway. These pathways are being explored by cloning and targeted degradation of mRNAs of various components in order to track signals from the cell surface to the nucleus.

Another approach to the problem is towards the identification of regulatory genes expressed in response to the cardiac signal. Our focus is the immediate early genes which are likely to be transcriptional regulators of other genes required for valve formation. Our most recent efforts have focused on the genes Slug (Snail2), Meox-1, Runx2 and Paraxis. All of these genes appear to be critical transcription factors for the process of cell transformation.

A related research program in the lab is focused upon examining the relationship between events which take place during early valve development and the apparent cardio-teratogenicity of the environmental contaminant Trichloroethylene (TCE). In association with members of the Southwestern Environmental Health Sciences Center and the Superfund Program here at Arizona, we are trying to identify genes that are altered in developing hearts by exposure to this chemical.