Stem Cell Epigenetics


Alternative Routes to Induced Pluripotent Stem Cells Revealed by Reprogramming of the Neural Lineage, Jackson, Steven A; Olufs, Zachariah PG; Tran, Khoa A; Zaidan, Nur Zafirah; Sridharan, Rupa

The main scientific focus of the lab is in defining how the epigenome controls pluripotency. Pluripotent stem cells can self renew indefinitely and possess the remarkable property of differentiating into any cell type when presented with the appropriate stimuli. Gene regulation in pluripotent stem cells has to be plastic to maintain the cellular identity of a stem cell but also to be responsive to differentiation cues.  This is controlled by the unique epigenetics of pluripotent stem cells: At the genic level in the form of poised chromatin states and at the global level in the form of lower levels of repressive histone modifications. Pluripotent stem cells can be obtained by culturing the inner cell mass of the blastocyst, giving rise to embryonic stem cells (ES cells). Differentiated somatic cells can also be reprogrammed into induced pluripotent stem cells (iPS cells) by overexpression of various factors. Multiple molecular and functional studies have shown that iPS cells are highly similar to ES cells. Human somatic cells can also be reprogrammed, providing iPS cells both as tools for translational research such as for in vitro drug screens and for cell replacement therapy.

Specifically, we want to answer the following questions:

How do the reprogramming factors activate pluripotency loci?

Only a small percentage of cells complete the reprogramming process, suggesting that multiple barriers have to be overcome for this dramatic change in cell fate to occur. We have found that a combination of small molecules can convert reprogramming intermediates into iPS cells at 80% efficiency.

We are identifying the molecular mechanism of the process of activating pluripotency loci.

Is the reprogramming process the same from different cell types?

Given that reprogramming represents a complete change in cell fate we wondered whether the stages in the process be identical. Would multipotent progenitors reprogram with better efficiency or kinetics that differentiated cells. Surprisingly we found that cells of the neural lineage follow a different route to pluripotency than fibroblasts. Thus given the same input = overexpression of reprogramming factors and the same output = iPS cells, the route taken is different

How does the chromatin state of pluripotent cells maintained; and how is it disrupted when lineage commitment occurs?