JAK-STAT Signaling in Stem Cells

Abstract

The specialized cell types of tissues and organs are generated during development and are replenished over lifetime though the process of differentiation. During differentiation the characteristics and identity of cells are changed to meet their functional requirements. Differentiated cells then faithfully maintain their characteristic gene expression patterns. On the molecular level transcription factors have a key role in instructing speci fi c gene expression programs. They act together with chromatin regulators which stabilize expression patterns. Current evidence indicates that epigenetic mechanisms are essential for maintaining stable cell identi- ties. Conversely, the disruption of chromatin regulators is associated with disease and cellular transformation. In mammals, a large number of chro- matin regulators have been identi fi ed. The Polycomb group complexes and the DNA methylation system have been widely studied in develop- ment. Other chromatin regulators remain to be explored. This chapter focuses on recent advances in understanding epigenetic regulation in embryonic and adult stem cells in mammals. The available data illustrate that several chromatin regulators control key lineage speci fi c genes. Different epigenetic systems potentially could provide stability and guard against loss or mutation of individual components. Recent experiments also suggest intervals in cell differentiation and development when new epigenetic patterns are established. Epigenetic patterns have been observed to change at a progenitor state after stem cells commit to differentiation. This fi nding is consistent with a role of epigenetic regulation in stabilizing expression patterns after their establishment by transcription factors. However, the available data also suggest that additional, presently unidenti fi ed, chromatin regulatory mechanisms exist. Identification of these mechanisms is an important aim for future research to obtain a more complete framework for understanding stem cell differentiation during tissue homeostasis.