Decoding of position in the developing neural tube from antiparallel morphogen gradients

Abstract

Like many developing tissues, the vertebrate neural tube is patterned by antiparallel morphogen gradients. To understand how these inputs are interpreted, we measured morphogen signaling and target gene expression in mouse embryos and chick ex vivo assays. From these data, we derived and validated a characteristic decoding map that relates morphogen input to the positional identity of neural progenitors. Analysis of the observed responses indicates that the underlying interpretation strategy minimizes patterning errors in response to the joint input of noisy opposing gradients. We reverse-engineered a transcriptional network that provides a mechanistic basis for the observed cell fate decisions and accounts for the precision and dynamics of pattern formation. Together, our data link opposing gradient dynamics in a growing tissue to precise pattern formation.

Figure 1

A) Brachial cross-section of mouse neural tube at 30h, immunostained for GBS-GFP and pSmad1/5/8. Scale bar, 50μm. B, C) Mean pSmad (B) and GBS-GFP (C) profiles at different developmental stages (sample sizes, Table S1). D) Mean pSmad and GBS-GFP profiles as a function of relative DV position. Time color-code in C. E) Positional error of pSmad and GBS-GFP gradients. Joint positional error of pSmad and GBS-GFP (last panel). F) Mean Pax3 and Nkx6.1 expression profiles. Time color-code in C.

Figure 2

A) ML decoding map obtained from wildtype mouse sections at 5h, using a parameter-free procedure (eq. 1, (10)). Colors correspond to predicted positional identities. Mean pSmad and GBS-GFP levels from exponential fits to the profiles, black line (wildtype), green (Shh^Hypo). Dots, 7 equidistant positions along DV axis. In Shh^Hypo the GBS-GFP profile decay length is half of WT. The probability distributions of DV identities are bimodal in the grey region. B) Exponential decay lengths of pSmad and GBS-GFP for Shh^Hypo (solid) and wildtype (dashed) datasets. Shaded regions, 95% CI from bootstrapping. C) Amplitudes of pSmad and GBS-GFP profiles, denoted as in B. D) Above: DV identities predicted from the decoding map in A. Below: measured Pax3 and Nkx6.1 boundary positions in Shh^Hypo (solid) are shifted ventrally compared to wildtype (dashed).

Figure 3

A) DV expression domains of indicated genes (see also (2)). B) Mean proportion of explant area positive for Pax7 (magenta), Olig2 (cyan) or Nkx2.2 (yellow) after culture in the indicated Shh and BMP7 concentrations for 24h (sample sizes, Table S2). Last panels are overlays. C) Representative images for the conditions outlined in grey in the last panel of B. D) Proportion of Pax7 and Olig2 positive pixels as a function of the calibrated Shh/BMP7 concentrations for a set of explants cultured in microfluidic devices. Color legend (right). Sample size, mean per bin n=4.7 (min. 2, max. 14). Hatched regions, not sampled.

Figure 4

A) Morphogen-driven transcriptional network in the neural tube. Shh and BMP signaling activate ventral (V), intermediate (I) and dorsal (D) target TFs. Uniform activating inputs (grey arrows). Degradation not depicted. B) Criteria of the computational screen. C) Principal component analysis of successful parameter sets. Contributions of the first two principal components to the total variation indicated. D) Distribution of parameter values for solutions that pass the screen. Uniform random distributions (dashed lines) are given for comparison.