Fate mapping studies show that proepicardial cells are the forerunners for the endothelium, smooth muscle and adventitial cells of the coronary vasculature. Formation of coronary vessels occurs by an epithelial to mesenchymal transformation (EMT) of proepicardial cells at the surface of the heart, followed by vasculogenesis in the subepicardial layer, assembly and remodeling of a primitive coronary plexus, and investment by coronary smooth muscle cells (CoSMCs).

We apply molecular biological and developmental genetic approaches to identify pathways for CoSMC commitment and differentiation during heart development. We use both chick and mouse models to explore mechanisms that couple proepicardial cell EMT and transcriptional activation of CoSMC target genes. Current projects in the lab employ transgenic and knockout mice to examine the role of sonic hedgehog signaling, rhoGTPase activation, and cytoskeletal remodeling in the stimulation of SRF and Tbx-dependent transcription that mediates proepicardial cell differentiation to CoSMCs.

Role of Tbx18 in Murine Development
T-box-containing proteins play critical roles as transcription factors that regulate cell fate and differentiation in development. Tbx18 is highly expressed in septum transversum and proepicardium. We used gene targeting in ES cells to produce Tbx18 knockout mice, which die shortly after birth with skeletal, renal and cardiac defects. Current projects aim to characterize these defects in detail, identify binding partners and gene targets for Tbx18, and prepare a conditional null allele for analysis of Tbx18 function in adult mice. Tbx18 is highly expressed in mesothelial membranes that line the pleural, pericardial and peritoneal cavities. These membranes are prone to fibrotic reactions to injury. Fibrosis limits regeneration and repair, restricts end organ function, and is an important cause failure in renal dialysis, and reproductive function after pelvic surgery. Adult models of mesothelial wound repair will allow us to genetically dissect key steps leading to pathological fibrosis.

Another goal of this project is to identify enhancer elements that direct Tbx18 expression to the proepicardium. Using rVISTA phylogenetic sequence comparisons, several conserved intronic elements have been identified as candidate proepicardial enhancers. These elements have been multimerized, ligated into minimal promoter vectors, and are being tested in transgenic frogs and mice.

Vascular Fate of Adult Stem Cells
Blood vessels are continuously undergoing formation, regression, remodeling and repair throughout life. New endothelial and smooth muscle cells are produced by proliferation, and by recruitment from multipotential stem cells. Using genetically-tagged donor cells, we can monitor engraftment of progenitor cells into newly formed or remodeled vessels at sites of angiogenesis in vivo. Our current interests are to explore the roles of sonic hedgehog, notch and BMP signaling in the artery wall during stem cell maintenance, recruitment and differentiation in animal models of vascular injury, remodeling and disease.