The time to measure positional information: maternal Hunchback is required for the synchrony of the Bicoid transcriptional response at the onset of zygotic transcription

Abstract

SUMMARYIt is widely accepted that morphogenetic gradients determine cell identity by concentration-dependent activation of target genes. How precise is each step in the gene expression process that acts downstream of morphogens, however, remains unclear. The Bicoid morphogen is a transcription factor directly activating its target genes and provides thus a simple system to address this issue in a quantitative manner. Recent studies indicate that the Bicoid gradient is precisely established in Drosophila embryos after eight nuclear divisions (cycle 9) and that target protein expression is specified five divisions later (cycle 14), with a precision that corresponds to a relative difference of Bicoid concentration of 10%. To understand how such precision was achieved, we directly analyzed nascent transcripts of the hunchback target gene at their site of synthesis. Most anterior nuclei in cycle 11 interphasic embryos exhibit efficient biallelic transcription of hunchback and this synchronous expression is specified within a 10% difference of Bicoid concentration. The fast diffusion of Bcd-EGFP (7.7 m 2 /s) that we captured by fluorescent correlation spectroscopy in the nucleus is consistent with this robust expression at cycle 11. However, given the interruption of transcription during mitosis, it remains too slow to be consistent with precise de novo reading of Bicoid concentration at each interphase, suggesting the existence of a memorization process that recalls this information from earlier cycles. The two anterior maternal morphogens, Bicoid and Hunchback, contribute differently to this early response: whereas Bicoid provides dose-dependent positional information along the axis, maternal Hunchback is required for the synchrony of the response and is therefore likely to be involved in this memorization process.

Dense Bicoid hubs accentuate binding along the morphogen gradient

“…The rapid dynamics of BCD indicated by the FRAP data are consistent with previous measurements by others using fluorescence correlation spectroscopy (FCS) (Porcher et al 2010). Although the FCS measurements on nuclear BCD dynamics (Porcher et al 2010) were previously analyzed only to estimate the diffusion coefficients of fastmoving and slow-moving populations, when these data are reanalyzed using a "stick and diffuse" model (Yeung et al 2007), which takes into account binding events between diffusion, the RT for a short-lived binding population of 122 msec is found, consistent with our measurements (Fradin 2017). These results confirm the transient nature of BCD binding; however, due to the limitations on the dynamic range of FCS measurements and the kinetic modeling used, the less prevalent longer-lived binding population that we quantified with single-molecule imaging cannot be detected.…”

Scaling of the Bicoid morphogen gradient by a volume-dependent production rate

“…How a Bcd concentration gradient is formed remains highly controversial (Coppey et al, 2007;Deng et al, 2010;Gregor et al, 2008;Gregor et al, 2007b;Grimm and Wieschaus, 2010;Little et al, 2011;Porcher et al, 2010;Spirov et al, 2009), even whether the positional information provided by Bcd is decoded before the gradient reaches its steady state (Bergmann et al, 2007;Bergmann et al, 2008;Bialek et al, 2008;de Lachapelle and Bergmann, 2010a;de Lachapelle and Bergmann, 2010b;He et al, 2011;Jaeger, 2010;. Although we have not specifically investigated the process of Bcd gradient formation, the observed scaling properties of the Bcd gradient may be readily explained by a diffusion model if the Bcd production rate is scaled with embryo volume (Deng et al, 2010).…”

Modulation of temporal dynamics of gene transcription by activator potency in theDrosophilaembryo

“…We generally took 8-10 z-sections (covering 4-5 µm along the z-axis) to capture all the hb intron dots in each nucleus . Threshold settings for hb intron dot detection were as follows (Porcher et al, 2010;Liu and Ma, 2013a,b). The pixel number threshold was set at ≥3 and the intensity threshold for an individual embryo was set to be the lower limit at which no nuclei in the broad anterior hb expression domain had more than two intron dots.…”