Proteome analysis of the culture environment supporting undifferentiated mouse embryonic stem and germ cell growth

The therapeutical interest of pluripotent cells and ethical issues related to the establishment of human embryonic stem cell (ESC) or embryonic germ cell (EGC) lines raise the understanding of the mechanism underlying pluripotency to a fundamental issue. Establishing a protein pluripotency signature for these cells can be complicated by the presence of unrelated proteins produced by the culture environment. Here, we have analyzed the environment supporting ESC and EGC growth, and established 2-D reference maps for each constituent present in this culture environment: mouse embryonic fibroblast feeder cells, culture medium (CM) and gelatin. The establishment of these reference maps is essential prior to the study of ESC and EGC specific proteomes. Indeed, these maps can be subtracted from ESC or EGC maps to allow focusing on spots specific for ESCs or EGCs. Our study led to the identification of 110 unique proteins from fibroblast feeder cells and 23 unique proteins from the CM, which represent major contaminants of ESC and EGC proteomes. For gelatin, no collagen-specific proteins were identified, most likely due to difficulties in resolution and low quantities. Furthermore, no differences were observed between naive and conditioned CM. Finally, we compared these reference maps to ESC 2-D gels and isolated 17 ESC specific spots. Among these spots, proteins that had already been identified in previous human and mouse ESC proteomes were identified but no apparent ESC-specific pluripotency marker could be identified. This work represents an essential step in furthering the knowledge of environmental factors supporting ESC and EGC growth.