Title: Time keeping in the making of an animal -How circadian rhythm orthologs drive pulsatile miRNA transcription in C. elegans.

Abstract:

Events in a developing organism have occur in a particular sequence and at a particular rate to ensure successful outcomes. To establish proper developmental timing, gene regulatory networks can encode and read information controlling the tempo and order of developmental events. Prominent embryonic examples for this are molecular timers, such as the sequential expression of Hox genes around the time of gastrulation, or molecular oscillators such as the vertebrate segmentation clock.  

Here, I will discuss our recent advances to dissect a post-embryonic molecular timing mechanism that combines genetic oscillations with the sequential repression of temporal identity factors in C. elegans. C. elegans post-embryonic maturation is compartmentalized into four larval stages with distinct patterns of cell division, cell differentiation, and cuticle formation that are separated by molts. The transition from one stage-specific pattern of cell division to the next is mediated by the accumulation of heterochronic microRNAs (miRNAs), such as lin-4 and let-7 that post-transcriptionally downregulate temporal identity target genes that define stage-specific gene expression patterns.

We have developed a new approach to monitor miRNA transcription in developing C. elegans larvae using MS2/MCP-GFP based RNA-localization combined with high-resolution long-term imaging with microfluidics. We uncover that hypodermal transcription of the temporal patterning miRNA is highly pulsatile and exquisitely coordinated within each larval stage and that transcriptional pulses share quantitative features across cell types and larval stages. We show that transcriptional pulses of the lin-4 are generated by cooperative binding between the C. elegans orthologs of circadian regulators NHR-85/Rev-Erb and NHR-23/ROR, respectively to elements upstream of the lin-4 gene. Remarkably, both the precise timing and length of lin-4 transcriptional pulses are dictated by the phased overlap of NHR-85Rev-Erb and NHR-23ROR temporal expression patterns. Thus, the gene regulatory network controlling lin-4 transcriptional pulses shares integral components with the human circadian clock but exhibits essential differences in its regulatory architecture. These results suggest that an evolutionary rewiring of the circadian clock machinery is co-opted in nematodes to implement a molecular timer that generates periodic transcriptional patterns mediating post-embryonic cell-fate progression.

Finally, we have uncovered intriguing systematic spatiotemporal heterogeneity in transcription of lin-4 in hypodermal cells within larval stages. Through live imaging of endogenously tagged direct targets of lin-4, we establish that this translates into a spatiotemporal pattern of temporal identity gene downregulation.

About the speaker:

He is a theoretical physicist by training and did his PhD work in Theoretical Neurophysics studying visual information processing and the formation of neural circuits during mammalian brain development and evolution. For his postdoc, he transitioned to experimental work in developmental biology, focusing on the model organism Caenorhabditis elegans (C. elegans). In his current work, he combines bioengineering, microfluidics, live imaging and computational modeling to study cell-fate decisions, cell-lineage variability and genetic circuits that encode developmental time and timing.

2019 –         CNRS researcher and Junior Group Leader at UMR168, Institut Curie
2013 – 2018  Postdoctoral researcher Center for Studies in Physics and Biology, The Rockefeller University, New York; USA, Supervisors: Eric D. Siggia, Shai Shaham
2007 – 2013  PhD in Theoretical Physics, Max Planck Institute (MPI) for Dynamics and Self-Organization;  Supervisor: Fred Wolf

https://institut-curie.org/personne/wolfgang-keil