Focusing of gene expression as the basis of stem cell differentiation.

In a prior report (Stem Cells Dev 14(4):354-366, 2005), we employed two-dimensional gel electrophoresis followed by advanced proteomics and the Database for Annotation, Visualization and Integrated Discovery (DAVID) to compare the protein expression profiles of mesenchymal stem cells to that of fully differentiated osteoblasts. These data were reported to advance technical approaches to define the basis of differentiation, but also led us to suggest that osteogenic differentiation of stem cells may result from the focusing of gene expression in functional clusters (e.g., calcium-regulated signaling proteins or adherence proteins) rather than simply from the induced expression of new genes, as many have assumed. Here, we have employed these analytical techniques to compare protein expression by mesenchymal stem cells directly with that of cells derived from them after induced osteogenic differentiation. Our results support the concept of gene focusing as the basis of differentiation. Specifically, induced differentiation results in a decrease in the number of mesenchymal cell markers and calcium-mediated signaling molecules expressed by their differentiated progeny. This effect was seen in parallel to increased expression of specific extracellular matrix (ECM) molecules and their receptors. These results strongly imply that changes in the ECM have a direct impact on stem cell differentiation, and that osteogenic differentiation of stem cells directed by matrix clues results from focusing of the expression of genes involved in Ca2+-dependent signaling pathways.