Follow the Embryonic Stem Cell Road to Cardiac Cell Progenitors

Even the Wizard of Oz couldn’t give the Tin Man a heart but the Johns Hopkins Medical Institution laboratory of John Gearhart has taken another small step on the road toward replenishing damaged cardiac tissue with pre-cursor cardiac cells grown from human embryonic stem cells (ES cells). Gearhart and his colleague, Nicolas Christoforou, here reveal preliminary data demonstrating what they say is a highly reproducible system for deriving cardiac progenitor cells from ES cells through controlled differentiation.

Differentiation is cell fate. The fate of any embryonic stem cell in a human blastocyst is virtually unlimited since any ES cell can differentiate into any of the three primary germ layers in the embryo— the ectoderm, mesoderm and endoderm. These germ cells can, in turn, differentiate into any of the 200 cell types in the human body. The ability to control differentiation in the laboratory is vital if ES cells are to be used as research tools into fundamental cell development and as a possible source for cell-based transplantation therapies.

The Gearhart lab reports that grown in culture, differentiating ES cells form three-dimensional structures termed embryoid bodies (EBs). Molecular signals and proteins expressed in EBs are similar to those present in early embryogenesis. These markers can be used as indicators of the cell types present in the differentiating EBs. The cells fated to become heart muscle, cardiomyocytes, differentiate in vivo through a conserved signaling pathway that contains both activating and inhibitory signals. These signaling pathways allow an area of the embryonic mesoderm to differentiate first into cardiac progenitor cells and then into the many cell types that constitute a functional heart. The presence of cardiac progenitor cells and cardiomyocytes in the differentiating ES cells is directly correlated to the expression of the same activating and inhibitory signals normally found in heart.

Expression of the appropriate molecular markers (genes with names like Nkx2.5, Tbx5, Tbx20) pinpoints the initial formation of cardiac progenitor cells in differentiating EBs followed by their further differentiation into functional cardiomyocytes (with yet another set of marker genes expressed–aMHC, TNNT2). The level of gene expression is also directly correlated to the percentage of cardiac progenitor cells in the EBs. Introduction into ES cells of a transgene that results in expression of a fluorescent protein under the control of the DNA sequences that control Nkx2.5 expression enables the researchers to observe the emergence of these cells in vitro in real time.

Preliminary data show that the emergence of cardiac progenitor cells and the pattern of marker expression in EBs closely resemble cardiogenesis in vivo, and make this a highly reproducible system of deriving cardiac progenitor cells from ES cells. Enhancement of cardiogenesis is achieved through monitoring of the level of cardiac gene expression while altering the differentiation conditions. Says Christoforou, “The information gained from this system allows the identification and efficient isolation of cardiac progenitors from ES cells. These cells are ideal candidates not only for cell-based transplantation therapies, but also for gene expression assays.”

Cardiac progenitor cells from embryonic stem cells, N. Christoforou, J. Gearhart; ICE, Johns Hopkins Medical Institution, Baltimore, MD.

At the meeting: Session 238 Embryonic Stem Cells, Poster Presentation 1208, Halls D/E. Author presents: Monday, Dec. 6, Noon—1:30 PM.