Periodic Reporting for period 4 - PlastiCell (Using a natural cellular plasticity event to decypher the cellular requirements and molecular circuitry promoting transdifferentiation at the single cell level.)
Reporting period: 2020-01-01 to 2021-06-30
To address these questions, we have taken advantage of a natural event of direct cellular reprogramming, that occurs during the development of the worm C. elegans, whereby a rectal cell, named "Y", retracts from the rectum during mid development of the worm, and transforms into a motoneuron, named "PDA" (Fig 1A). This model has been first characterised in the laboratory, and occurs with 100% efficiency in all wild type animals. Because this event is accessible at the single cell level in live animals and is predictable, giving access to the early steps of the process, we have been able to obtain a number of mechanistic insights that represent common themes with cellular plasticity events in vertebrates, as described below.
During the entire lifetime of this project, we also have made advances in our understanding of the mechanisms at work. We identified essential and dichotomic effects of Notch activity on natural Td; in particular, Notch activity confers competence to change cellular identity in permissive cellular backgrounds, and this during a defined window of time (in preparation). In addition, we have determined that at least another natural Td event occurs in the worm, where the only other rectal cell to change its identity, named "K", does so through a cell division (K-to-DVB, Riva et al, 2021; Fig 1B). We have found that both common principles and factors exist, as well as events specific mechanisms when comparing the Y-to-PDA and K-to-DVB Tds. In the latter, the Wnt signalling pathway act in parallel of a plasticity cassette to both allow one specific K daughter to change identity and control the timing of re-differentiation (Riva et al, 2021). Thus here, both intrinsic cellular context and an extracellular signal combine to allow a cell to change its identity. Our data further suggest a model where the dynamic interplay between these factors controls the timing of re-differentiation (Fig 1C). We have further described two other natural Td in hermaphrodites, that occur though either a symmetric or asymmetric cell division (unpublished). Finally, we have found that the ability of a rectal cell to be naturally reprogrammed depends on at least two types of activities: the drivers, transcription factors which act as effectors and which activity is absolutely needed; and the licencers, nuclear factors which activity is needed to counteracts brakes to direct reprogramming (aka transdifferentiation, Becker et al, in prep). Several articles with our results have been published and several more are being written up. In addition, oral and written presentations have been made at numerous national and international meetings, both by team members or myself.