Scientific merit. The detailed visualization of the earliest stages of heart formation in the Ciona embryo has the potential to reveal insights that have been missed in the more complex vertebrate systems. Dr. Levin has recently found that the definitive heart arises from a single pair of cells, the B7.5 blastomeres, at the 110-cell stage of embryogenesis, just before the onset of gastrulation. These cells specifically express two regulatory genes, Tbx6 and Mesp, which encode T-box and bHLH transcription factors, respectively. B7.6 divides to produce two daughter cells. The anterior daughter cell expresses only the Mesp regulatory gene and thereby forms the heart. In contrast, the posterior daughter cell expresses both Mesp and Tbx6, and consequently, this cell forms the anterior-most tail muscles. These "A-cells" have a number of properties related to the ventricular regions of vertebrate hearts, although they do not form components of the definitive heart. The anterior daughter cell migrates into the head and undergoes an asymmetric division before embarking on the differentiation pathway leading to the heart. In contrast, the posterior daughter cell remains in the tail and forms A-type muscle cells. He will perform a detailed analysis of the genes required for the anterior migration and asymmetric division of the heart progenitors. He has recently produced mutant tadpoles that should facilitate the identification of genes required for heart migration and division. For example, the overexpression of Tbx6 causes the anterior daughter cells of the B7.5 blastomeres to arrest and form supernumerary A-cells, while overexpression of Mesp causes the opposite phenotype: ectopic migration and division of the presumptive A-cells, which form a second heart. There is every indication that the initial stages of heart formation in vertebrates depends on the same type of directed cell migration and asymmetric division seen in the Ciona tadpole.
Broader impact. Heart disease is a major cause of death and suffering in human populations. To gain insights into the basis of heart disease this study will investigate the earliest phases of heart development in a simple organism, the sea squirt or Ascidian, Ciona intestinalis. Ascidians represent the simplest chordates--the animal phylum that includes vertebrates such as ourselves. The Ascidian tadpole represents a simplified version of vertebrate tadpoles. The early tadpole is composed of just 1,000 cells. The ascidian heart is simple, but nonetheless represents the evolutionary forerunner of multi-chambered vertebrate hearts. Despite its simplicity, the ascidian heart possesses an intriguing property not seen in vertebrates: it has a reversible beat. The single-chambered heart exhibits rhythmic beating in one direction for several minutes, then stops and reverses direction for a few minutes before once again changing direction. Many millions of dollars have been invested in understanding various aspects of heart development during the embryonic development of different vertebrates, including zebrafish, frogs, chickens, and mice. However, the complexity of these organisms (the developing heart is composed of many hundreds-even thousands-of cells) precludes a detailed understanding of the earliest stages of heart formation. The simplicity of the Ciona tadpole permits the visualization of the earliest events. There is no other relative of vertebrates that possesses such a simple heart. It is initially composed of just 8 cells that are easily visualized using simple microscopy.
