Bionanoscience Seminar, 08.05.2015
Speaker: Pia Cosma (Center for Genomic Regulation, Barcelona (Spain))
Author: Edgar Schönfeld
Pia Cosma’s research is focused on cell reprogramming. Her talk was divided into two major subjects that deal with cellular reprogramming: Wnt/ß-Catenin signaling and chromatin organization. Reprogramming can be achieved via nuclear transfer or direct reprogramming, which involves the introduction of the 4 pluripotency factors Oct4, Sox2, cMyc, and Klf4 into a somatic cell. A third option is reprogramming via cell fusion. Cell fusion itself is a natural process, which occurs for instance during the differentiation of muscle cells. However, cells can also be fused artificially in the lab, with the help of chemicals or electricity. If you fuse a stem cell with a somatic cell they give rise to pluripotent hybrid cell, because the pluripotent genome is dominant over the somatic one. (This process also happens in vivo and is involved in wound healing.) After such a fusion the pluripotency factor Oct4 is reactivated. In an experiment Pia tagged Oct4 with GFP to observe spontaneous cell fusions. She found out that reprogramming only occurs when the right amount of ß-Catenin is present, which is activated by the Wnt pathway.
In the last years she discovered a lot more details. She discovered that the ß-Catenin concentration fluctuates in embryonic stem cells (ESCs), which is absolutely important for the reprogramming of somatic cells via fusion with ESCs. Different levels of activity of ß-Catenin can regulate either pluripotency or differentiation. Furthermore she found out that repression of TSF3, a regulator of the Wnt pathway, can increase the reprogramming frequency several hundred times.
With that in mind, she wanted to know what happens to chromatin organization during reprogramming. As a result the textbook model of chromatin packing needs an important update! To be precise, the 3rd panel from the left in figure 1 is inaccurate. Pia discovered (using STORM) that the nucleosomes are not distributed uniformly over the genome. Instead nucleosome aggregate in small or big groups, which Pia calls ‘clutches’ (Fig.2).
These clutches are separated by stretches of nucleosome-free DNA. Thereby the larger clutches form the silenced heterochromatin, because they contain more of the linker histone H1. RNA polymerases can still associate with the small clutches. In stem cells and during reprogramming the DNA is mostly covered with small clutches, which facilitates the docking of transcription factors and RNA polymerase. In somatic cells however, DNA is mostly packed around big silencing clutches. In an reprogramming event the cell has to switch from big dense to small loose clutches, but how is this achieved? Pia’s team hypothesized that this might occur through nucleosome sliding and removal, which is in accordance with their simulations. To summarize their findings the team created this video: