Nuclear trapping shapes the terminal gradient in the Drosophila embryo

Abstract

Patterning of the terminal regions of the Drosophila embryo relies on the gradient of phosphorylated ERK/MAPK (dpERK), which is controlled by the localized activation of the Torso receptor tyrosine kinase [1-4]. This model is supported by a large amount of data, but the gradient itself has never been quantified. We present the first measurements of the dpERK gradient and establish a new intracellular layer of its regulation. Based on the quantitative analysis of the spatial pattern of dpERK in mutants with different levels of Torso as well as the dynamics of the wild-type dpERK pattern, we propose that the terminal-patterning gradient is controlled by a cascade of diffusion-trapping modules. A ligand-trapping mechanism establishes a sharply localized pattern of the Torso receptor occupancy on the surface of the embryo. Inside the syncytial embryo, nuclei play the role of traps that localize diffusible dpERK. We argue that the length scale of the terminal-patterning gradient is determined mainly by the intracellular module.

Figure 1. Quantifying the dpERK Gradients in Syncytial Drosophila Embryos

We use an antibody against the double phosphorylated MAPK (dpERK) to visualize the spatial pattern of MAPK phosphorylation along the embryo. The timings of nuclear divisions were characterized in detail, so we use the nuclear density as a marker of time. Thus, a double staining of the dpERK and nuclei provides simultaneous information about the age of the embryo and the spatial pattern of MAPK phosphorylation (B). Details of the assays and gradient quantification procedure are described in Supplemental Data. (A) Top: spatial pattern of MAPK phosphorylation, detected with the dpERK antibody (red), in a representative cycle 14 embryo. Bottom: quantified dpERK gradient. A, anterior pole; P, posterior pole; EL/2, half of the egg length. (B) Simultaneous visualization of nuclei (green, top view) and MAPK phosphorylation (red, mid-sagital view) in the representative cycle 10 and 14 embryos (top and bottom, respectively).

Figure 2. The dpERK Gradients in Embryos with Different Levels of Receptor Expression

(A) Summary of the experimental procedure for comparing the gradients across genetic backgrounds: wild-type and mutant embryos are taken through the same fixation and staining procedure and imaged on the same microscope slide. In these experiments, embryos from a given mutant background are mixed and processed together with progeny of wild-type flies carrying the histone-GFP fusion transgene. (B and C) Comparison of the dpERK gradients in embryos with different copy numbers of Torso. Numbers indicate the numbers of embryos quantified in each of the genetic backgrounds. Note that the y axes in (B) and (C) are not normalized to 100. After the normalization procedure, the amplitudes of the average profiles were rescaled to the mean peak value of the raw profiles independently for each genetic background. The differences in the amplitudes between the two wild-type profiles in (B) and (C) are due to variations in fixation and staining conditions; the need for a pairwise comparison of gradients in different backgrounds is described in (A).

Figure 3. Dynamics of the Wild-Type dpERK Gradients

(A) Superposition of the unnormalized anterior dpERK gradients from a collection of 63 embryos. Even though the fixation, staining, and imaging conditions for all of these embryos were identical, the raw intensity profiles varied considerably. The spatially averaged variability was ∼80%, almost an order of magnitude greater than the variability associated with imaging of a single embryo (see Supplemental Data for details). (B) Cumulative distribution function (c.d.f.) of nuclear densities from the same collection of embryos. Clear breaks in the c.d.f. correspond to the consecutive doublings in the number of syncytial nuclei. Different colors denote five temporal classes of embryos, corresponding to the nuclear cycles 10, 11, 12, 13, and 14. See Supplemental Data for details of clustering of embryos according to their nuclear densities.The absolute value of nuclei count correspond to the number of nuclei in a fixed window size (see Figure S3 for details). (C) A significant fraction of high variability is explained by the dynamic changes of the dpERK profiles over consecutive nuclear divisions. The heights and widths of the unnormalized gradients are anticorrelated; in addition, both of these parameters are correlated with the nuclear density. In this image, each embryo is represented by a circle, colored according to its nuclear cycle. (D) A gradient normalization procedure, applied to the embryos that have been grouped according to their age, reveals that the anterior gradients get progressively “taller” and “thinner” as the nuclear density increases. The dpERK profiles from the same temporal classes are strikingly similar (mean spatial variability of 6.9% for nuclear cycle 14). For example, the profiles from cycle 14 embryos are as similar as the ones obtained by repetitive imaging of the same embryo (mean spatial variability of 6%). See Supplemental Data for the details of normalization procedure.

Figure 4. Spatial Regulation of the dpERK Gradient by Syncytial Nuclei

(A) dpERK is a diffusible molecule that shuttles in and out of the nuclei and can be dephosphorylated in either of these compartments. (B) As a result of a spatially uniform increase in the nuclear density, dpERK is trapped close to the poles of the embryo, where it is generated by the locally activated Torso receptors. This limits the extent of its diffusion and lowers the dpERK level in the middle of the embryo. (C) A mathematical model, described in the Supplemental Data, can predict gradient sharpening in response to a uniform change in nuclear density. C_tot(x)is the sum of the nuclear and cytoplasmic levels of dpERK: C_tot(x) = C_c(x) + C_n(x). The amplitude and decay length of the signal are increased and decreased, respectively, by a uniform increase in the nuclear density. (D) Splitting the dpERK gradients into the nuclear and cytoplasmic parts. Nuclear gradients are extracted from the histone-GFP embryos; see the Supplemental Data for the definition and processing of nuclear and cytoplasmic gradients. (E) The N/C ratio is constant throughout the embryo and increases between cycles 13 and 14. For details of the statistical analysis, see the Supplemental Data. (F) Defects in the nuclear density are correlated with the defects in the dpERK pattern in shkl embryos. The two images correspond to the two different focal planes of the same embryo. (G) The average cycle 14 wild-type gradient was subtracted from the quantified posterior dpERK gradient in the shkl embryo, revealing the correlation between the nuclear density and dpERK.