An RNA interference screen uncovers a new molecule in stem cell self-renewal and long-term regeneration

Abstract

The transcription factor TBX1 has a role in stem cell activation and self-renewal during long-term tissue regeneration. Little is known about the mechanisms involved in stem-cell activation and self-renewal in the course of tissue regeneration. Using an elegant large-scale functional RNA interference screen for factors that may regulate long-term self-renewal of epidermal stem cells, Elaine Fuchs and colleagues identify a new function for the transcription factor TBX1 in long-term regeneration. The techniques used in the study might also have broad appeal in many other developmental systems. Adult stem cells sustain tissue maintenance and regeneration throughout the lifetime of an animal1,2. These cells often reside in specific signalling niches that orchestrate the stem cell’s balancing act between quiescence and cell-cycle re-entry based on the demand for tissue regeneration2,3,4. How stem cells maintain their capacity to replenish themselves after tissue regeneration is poorly understood. Here we use RNA-interference-based loss-of-function screening as a powerful approach to uncover transcriptional regulators that govern the self-renewal capacity and regenerative potential of stem cells. Hair follicle stem cells provide an ideal model. These cells have been purified and characterized from their native niche in vivo and, in contrast to their rapidly dividing progeny, they can be maintained and passaged long-term in vitro5,6,7. Focusing on the nuclear proteins and/or transcription factors that are enriched in stem cells compared with their progeny5,6, we screened ∼2,000 short hairpin RNAs for their effect on long-term, but not short-term, stem cell self-renewal in vitro. To address the physiological relevance of our findings, we selected one candidate that was uncovered in the screen: TBX1. This transcription factor is expressed in many tissues but has not been studied in the context of stem cell biology. By conditionally ablating Tbx1 in vivo, we showed that during homeostasis, tissue regeneration occurs normally but is markedly delayed. We then devised an in vivo assay for stem cell replenishment and found that when challenged with repetitive rounds of regeneration, the Tbx1-deficient stem cell niche becomes progressively depleted. Addressing the mechanism of TBX1 action, we discovered that TBX1 acts as an intrinsic rheostat of BMP signalling: it is a gatekeeper that governs the transition between stem cell quiescence and proliferation in hair follicles. Our results validate the RNA interference screen and underscore its power in unearthing new molecules that govern stem cell self-renewal and tissue-regenerative potential.