Reprogramming of Cell Fates How do you determine your fate? The fate of a human is sometimes determined by small events such as an encounter with a teacher, a sense of frustration --- and perhaps an expression of a single gene? The topic of this session is "reprogramming of cell fates". Our body is made up of 60 trillion cells and these cells can be classified into more than 100 distinct cell types that include epithelial cells (skin), neurons, blood cells, muscle cells, fat cells, germ cells, etc. Each of these cell types can be regarded to have a specific "fate" as a particular cell type. Here comes a question: how are various cell types generated from a single fertilized egg or dividing cells? We now know that there are genes and molecules that specify cell fates. Spemann organizer induces mesoderm in the Xenopus embryo by secreting growth factors. Homeotic genes, initially discovered in Drosophila, specify anterior-posterior positional identities in many animals. The paper that we will be discussing on November 19th is a classic paper published in 1987 on the cloning of the MyoD gene, which can specify the muscle cell fate. Many developmental biologists were shocked by the finding that a single gene can reprogram cell fates, i.e. convert fibroblasts into muscle cells. The Introduction of this paper will be an interesting reading on how such remarkable discoveries are made. The cell line that the authors use (C3H10T1/2 cells) is a typical fibroblast cell line used in cell culture experiments. It had been shown that an application of 5-azacytidine, which is an analogue of cytidine and thus inhibits DNA methyltransferase, can transform C3H10T1/2 cells into differentiated multinucleated muscle cells (Taylor and Jones, 1979). Because DNA methylation often causes repression of gene expression, this observation suggested that there is a gene (or genes) that is repressed in fibroblasts but its activation via demethylation leads to conversion to muscle cells. In 1980s it was unknown whether certain gene expression can be a "master regulator" for muscle differentiation, nor how many genes are involved in such a process. Nonetheless, Davis, Weintraub and Lassar believed that there is a master gene (or master genes) that is responsible for the reprogramming of cell fates mediated by 5-azacytidine. At that time Microarray analysis or whole transcripts sequencing had not yet been invented. They thus screened a phage cDNA library to identify genes that were expressed in 5-azacytidine-treated C3H10T1/2 but not in non-treated cells, and isolated the MyoD gene. Imagine that you were in the authors' situation. How would you screen the library and what kind of experiments would you do to show that MyoD is the cell fate determination gene? Recall that a similar idea was applied to identify four genes to create iPS cells. |