Reactive oxygen species (ROS) influence stem cell behaviour (such as renewal, proliferation and differentiation) but the underlying mechanisms have been unclear. Morgane Locker and colleagues investigated this question, taking advantage of the Xenopus retina, which harbours active retinal stem cells (RSCs). Using a fluorescent sensor, they show that ROS levels are dynamic, temporally and spatially, with post-embryonic neural stem cells exhibiting higher levels of ROS than neural progenitors or differentiated cells. Examining the cellular redox machinery, which removes or produces ROS, the researchers show that RSCs express a specific antioxidant repertoire and several NADPH oxidases (NOX). EdU incorporation and cell-death labelling approaches revealed that chemical or morpholino-mediated knockdown of NOX-dependent redox signalling reduces RSC proliferation, but not maintenance, increasing the number of quiescent RSCs. In addition, detailed studies of the cell cycle show that RSCs that remain proliferative divide faster. Finally, using qPCR or a transgenic reporter, the authors show that NOX inhibition reduces Wnt and increases Sonic hedgehog (Shh) signalling activity. Furthermore, experimental activation of Wnt or Shh inhibition rescues NOX-inhibition phenotypes. Together, these data highlight that NOX affects Wnt/Shh signalling to regulate RSC quiescence-to-proliferation transition.
