The development of a complete human being from a single cell is one of the great miracles
of life. A human egg cell contains about 30,000 genes that encode proteins, and of these,
about 3,000 of these genes encode transcription factors. Transcription factors determine
when genes will be turned on and turned off, orchestrating the many processes involved in
the development of an embryo and the many tasks performed by each cell after a child is
born. Amazingly, there is only about 1 transcription factor for every 10 genes, posing a
puzzle: how does this limited set of proteins control the many genes and processes that
must be regulated?
Combinatorial Control
One of the answers to this question may be discovered by looking at the binding sites for
transcription factors in the genome. Typical genes in our cells have extensive regulatory
regions before and after the genes, sometimes 100,000 base pairs away, and occasionally
even inside the genes. These regions act in many different ways, as enhancers, silencers,
insulators, and promotors of the gene. Each gene is controlled by a combination of many
transcription factors, which together form a consensus as to whether the gene will be
expressed or not at any given time.
Choosing a Path
Oct4 and its cofactor Sox2 are at the center of a collection of transcription factors that
control the first decisions in the development of an embryo. Oct4 is present in embryonic
stem cells, and its levels drop when the cell starts to divide and differentiate into different
types of cells. It has been called the "gatekeeper" of development, since it is necessary for
maintaining the stem cell state. The structure shown here, from PDB entry
1gt0,
shows the
DNA-binding portions of a similar protein, Oct1 (at the bottom in turquoise), and Sox2 (at
the top in blue) bound to a short piece of DNA (in orange and pink).
Reprogramming
Unfortunately, once stem cells make their choices and differentiate into nerve cells or skin
cells or other types of cells, they are normally unable to reverse their choices and become
stem cells once again. If this were possible, however, it would be very useful: for instance,
imagine taking a few skin cells from a patient with diabetes, and then changing these cells
into pancreatic cells that can make insulin. Researchers have recently used Oct4 and Sox2 to
make the first steps towards this amazing goal. By adding the genes for these proteins,
along with a few other transcription factors, to skin cells, they were able to reprogram the
cells into "pluripotent" stem cells that are able to form many other cell types.
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