Quantitative Analysis of Gene Expression Patterning in Gastruloids

Sammanfattning

Gastruloids are embryoid structures derived from mouse embryonic stemcells, extensively studied in quantitative developmental biology. Without thechallenges of in vivo investigation, they recapitulate a key feature ofmammalian development, namely the progressive organization of adevelopmental gene expression pattern along an anterior–posterior (AP) bodyaxis. In this work, we focus on characterizing the expression pattern of one suchregulated gene involved in development, Brachyury. Using smFISHtechnology, which allows the localization of individual mRNA transcripts infixed gastruloids, we characterize the expression profile of this gene as agradient along the anterior-posterior axis. At the same time, we show that thevariability in expression observed within each region is in partial agreementwith the predictions of a classical mathematical model of transcription—thetwo-state model. A comparison is made with the housekeeping gene Tbp,whose stochasticity is limited only by Poisson noise, in order to highlight thechallenges inherent to this type of analysis. In addition, a complementarylive-imaging analysis is initiated to quantify the evolution of morphogenesisand the polarization of Brachyury expression by following individualgastruloids over time. These results underscore the ability to quantitatively investigate a complexbiological phenomenon, which is the organization of regulated geneexpression in a developmental context. They also illustrate the challengesposed by a system that, although more reproducible than many biologicalobjects, still exhibits a degree of variability intrinsic to biological processes.