RT Journal Article
SR Electronic
T1 3 minutes to precisely measure morphogen concentration
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 305516
DO 10.1101/305516
A1 Tanguy Lucas
A1 Huy Tran
A1 Carmina Angelica Perez Romero
A1 Aurélien Guillou
A1 Cécile Fradin
A1 Mathieu Coppey
A1 Aleksandra M. Walczak
A1 Nathalie Dostatni
YR 2018
UL http://biorxiv.org/content/early/2018/04/20/305516.abstract
AB Morphogen gradients provide concentration-dependent positional information along polarity axes. Although the dynamics of establishment of these gradients is well described, precision and noise in the downstream activation processes remain elusive. A simple paradigm to address these questions is the Bicoid morphogen gradient that elicits a rapid step-like transcriptional response in young fruit fly embryos. Focusing on the expression of the main Bicoid target, hunchback (hb), at the onset of zygotic transcription, we used the MS2-MCP approach which combines fluorescent labeling of nascent mRNA with live imaging at high spatial and temporal resolution. Removing 36 putative Zelda binding sites unexpectedly present in the original MS2 reporter, we show that the 750 bp of the hb promoter are sufficient to recapitulate endogenous expression at the onset of zygotic transcription. After each mitosis, in the anterior, expression is turned on to rapidly reach a plateau with all nuclei expressing the reporter. Consistent with a Bicoid dose-dependent activation process, the time period required to reach the plateau increases with the distance to the anterior pole. Remarkably, despite the challenge imposed by frequent mitoses and high nuclei-to-nuclei variability in transcription kinetics, it only takes 3 minutes at each interphase for the MS2 reporter loci to measure subtle differences in Bicoid concentration and establish a steadily positioned and steep (Hill coefficient ~ 7) expression boundary. Modeling based on cooperativity between the 6 known Bicoid binding sites in the hb promoter region and assuming rate limiting concentrations of the Bicoid transcription factor at the boundary is able to capture the observed dynamics of pattern establishment but not the steepness of the boundary. This suggests that additional mechanisms are involved in the steepness of the response.