PORTEFEUILLE

Stem cells are multipotent immortal cells with unlimited proliferation capacity, and the source of tissue regeneration. During aging, cellular damage accumulates in all cells, but intriguingly stem cells, can obviously escape this intrinsic damage. Stem cells display unique cellular properties, including antioxidant defense and repair mechanisms, for maintenance and repair of oxidative DNA and cellular damage. The goal of this project was to identify the specific factors that protect stem cells from oxidative damage. Stem cells are very sparse in most tissues and therefore most studies are performed on stem cells maintained in culture. However, in vitro culturing could mask the characteristics of stem cell identity in vivo, which is also determined by the environment of stem cells, the stem cell niche. Our approach was to identify stem cell-specific factors by comparing the respective expression profiles of male gonadal stem cells (GSC) and spermatogonia (G), which look like GSC, but are already committed and represent the differentiated mortal cell type emerged from GSC. GSC and G can be purified in relatively large amounts from rats before puberty (to obtain GSC) and at postpuberty (to obtain mostly G). The comparison of mRNA and protein profiles from GSC and G was proposed in order to identify new stem cell factors that contribute to the maintenance, protection, and repair of cellular functions, in particular the protection from and repair of oxidative damage.

The Gebert Rüf Stiftung supports this innovative project in the important field of stem cells. The project will open the way to a variety of different projects that will be very close to application.

We found 1405 genes specifically up-regulated in GSC, of a total of 31099. Of this group, 573 were genes with known function and 832 were unknown ESTs. Data mining resulted in annotation of function for additional 71 genes by homology search for rat genes within the mouse genome.
The most important result of this study was the large number of genes that were specifically up-or downregulated in during the differentiation from GSC to G. Bioinformatics analysis is still ongoing to attribute functions to unknown ESTs by homology searches. Since we confirmed our data in various ways, this finding indicates that stem cell maintenance is complex and requires the regulation of a multitude of molecular pathways.
Amongst the known GSC genes, 19 were encoding anti-oxidant factors and an equal number might be hidden in the ESTs of unknown function. To validate the importance of anti-oxidant components in GSC maintenance and differentiation we developed GSC in vitro culture supported by co-culture of Sertoli cells.
We compared not only the expression in GSC and G but also the expression pattern of their respective “niche” cells, the Sertoli cells. This way a cluster of genes could be identified in the stem cells and in the stem cell niche cells that belonged to known signaling pathways. Different pathways were lighted up in GSC and their niche than in G and their niche. Typically these molecular pathways comprise growth factors secreted by the niche cells and receptors on GSC of G cells. Our data thus show that the signals send by the niche change during development, and that niche cells supporting only GSC have a different potential than niche cells supporting mostly G at a later stage. Some of these signaling pathways are well known in other systems/tissues but have not been described in stem cells and their niche so far.
Since stem cells also exist also in cancers and cancer stem cells are the cells that might drive tumor re-growth and resistance to therapy, genes that are specifically expressed in GSC or in the GSC niche might play a role in cancer stem cells maintenance. Therefore the GSC specific genes will be screened for cancer stem cell markers.

The described results were presented at international meetings such as European Cell Biology (ELSO) Meeting in Dresden 2005 and American Association of Cancer Research (AACR) Meeting in San Francisco 2005.

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