Xenopus Bsx links daily cell cycle rhythms and pineal photoreceptor fate

In the developing central nervous system, the cell cycle clock plays a crucial role in determining cell fate specification. A second clock, the circadian oscillator, generates daily rhythms of cell cycle progression. Although these two clocks interact, the mechanisms linking circadian cell cycle progression and cell fate determination are still poorly understood. A convenient system to address this issue is the pineal organ of lower vertebrates, which contains only two neuronal types, photoreceptors and projection neurons. In particular, photoreceptors constitute the core of the pineal circadian system, being able to transduce daily light inputs into the rhythmical production of melatonin. However, the genetic program leading to photoreceptor fate largely remains to be deciphered. Here, we report a previously undescribed function for the homeobox gene Bsx in controlling pineal proliferation and photoreceptor fate in Xenopus. We show that Xenopus Bsx (Xbsx) is expressed rhythmically in postmitotic photoreceptor precursors, reaching a peak during the night, with a cycle that is complementary to the daily rhythms of S-phase entry displayed by pineal cells. Xbsx knockdown results in increased night levels of pineal proliferation, whereas activation of a GR-Xbsx protein flattens the daily rhythms of S-phase entry to the lowest level. Furthermore, evidence is presented that Xbsx is necessary and sufficient to promote a photoreceptor fate. Altogether, these data indicate that Xbsx plays a dual role in contributing to shape the profile of the circadian cell cycle progression and in the specification of pineal photoreceptors, thus acting as a unique link between these two events.