Awakening adult neural stem cells: NOX signalling as a positive regulator of quiescence to proliferation transition in the Xenopus retina

Reactive oxygen species (ROS) are both harmful molecules sustaining the pathogenesis of several diseases and essential modulators of cell behaviours. In particular, a growing wealth of data suggest that ROS-dependent signalling pathways might be critical in conferring embryonic or adult stem cells their specific properties. However, how stem cells control ROS production and scavenging, and how ROS in turn contribute to stemness remain poorly understood. Using the Xenopus retina as a model system, we first investigated the redox status of retinal stem cells (RSCs). We discovered that they exhibit higher ROS levels compared to progenitors and retinal neurons and express a set of specific antioxidant genes. We next addressed the question of ROS functional involvement in these cells. Using pharmacological or genetic tools, we demonstrate that inhibition of NADPH oxidase (NOX)-dependent ROS production increases the proportion of quiescent RSCs. This is surprisingly accompanied by an apparent acceleration of the mean division speed within the remaining proliferating pool. Our data further unveil that such impact on RSC cell cycling is achieved by modulation of the Wnt/Hedgehog signalling balance. Altogether, we highlight that RSCs exhibit distinctive redox characteristics and exploit NOX signalling to limit quiescence and fine-tune their proliferation rate.