Final Report Summary - STOCHCELLFATE (Quantitative network-level analysis of stochastic cell fate decisions)
The aim of this grant was to understand how cells in developing organisms can use random fluctuations on the molecular level to drive random cell fate decision. We focused on two such stochastic cell fate decisions that occur during development of the reproductive organ in the nematode worm C. elegans: the AC/VU decision and VPC specification. To understand how small molecular fluctuations are amplified into one cell fate or another, it is essential to follow these decision-making processes in time using time-lapse microscopy. However, because these decisions take in place in C. elegans worms that move, feed and grow, time-lapse microscopy was so far impossible. As the first step, we established the first technique to perform time-lapse microscopy at the single-cell level in live and moving C. elegans larvae, for the full ~40 hours of their development. In combination with image and data analysis techniques we developed, we are now able to track any cell and analyze its division dynamics, movement and gene expression dynamics. Using this approach, we identified the different noise sources controlling stochastic cell fate decisions: stochastic variability in birth order and bursty gene expression (AC/VU decision, project 1) and variability in timing of a Wnt signaling pulse (stochastic VPC specification, project 2). In both cases, we combined quantitative experimental data with mathematical modeling to establish the key feedback mechanisms that amplify stochastic variability into cell fate. Surprisingly, we found that the two fate decisions used very different mechanisms. For the AC/VU decision, positive feedback via lateral Notch signaling was identified as the key amplifying mechanisms. However, for stochastic VPC specification, no positive or negative feedback loops were identified, but instead we arrived at a mechanism where variability in timing and amplitude of a cell signaling pulse determined whether the decision to assume hypodermal instead of VPC fate was triggered. These results provide a new dynamical picture of how these stochastic cell fate decisions are regulated and will form an ideal starting point to study similar stochastic decision in higher organisms.