Spatial Patterning from an Integrated Wnt/β-catenin and Notch/Delta Gene Circuit

Abstract

Classically, the Wnt/β-catenin and Notch /Delta signaling pathways were thought to operate through separate mechanisms, performing distinct roles in tissue patterning. However, it has been shown that b-catenin activates transcription of Hesl, a signaling intermediate in the Notch /Delta pathway that controls its lateral inhibition mechanism. To investigate this non-canonical crosstalk mechanism, a new gene circuit, integrating the two pathways, is proposed and simulated in two-cell and multi-cell environments. This model also captures both Paneth cell- mediated and mesenchymal Wnt production. The simulations verify that the gene circuit is temporally bistable and capable of forming a pattern on a multi-cell grid. Last, the model exhibits a bifurcation based on the steady state concentration of Wnt and the relative amount of control b-catenin has over the Hesl promoter, providing a possible mechanism to explain why a homogeneous population of transit amplifying cells is observed directly above the more diverse stem niche.

Figure 1

Stem cells and Paneth cells form checkerboard patterns in the intestinal crypts. A) Transverse cross-sections of intestinal crypts. stem cells are green (LGR5-EGFP), and Paneth cells are red (RFP-anti-Lysozyme). B) Intestinal crypt cross-section schematic. C) A longitudinal cross-section of a crypt-villus structure (villus not to scale) depicting the various cell types.

Figure 2

a) Gene circuit diagrams for three modes of Wnt secretion. Arrows denote activation, and dots denote inhibition. Red and blue represent distinct cells. i) Cells experience constant Wnt (C). ii) Both cells experience total Wnt (P). iii) Cell experience neighbor’s Wnt (J) b) Concentration profiles of all species (nM) as a function of time (hrs, log) along with polarization. Bifurcation of cell fates is observed for all species except ß-catenin for r_w = r_N = 0.1, r_D = 0, θ₄ = 0.75, W = 1 (C), θ₈ = 0.7 nM/min (P), θ₈ = 1.2 nM/min (J)

Figure 3

Bifurcation analysis of the constant/mesenchymal Wnt model where the Wnt concentration (W) and the relative control of Hes1 (H) production by NICD/ß-catenin (I) compared to ß-catenin alone (B). Ideally, all the colormaps should be bounded from [0, 1] with numbers under 0.4 indicating bifurcation since the ratios are inverted.

Figure 4

a) Spatial simulation of the model (nodes colored by stemness) where green indicates stem cells and red indicates Paneth cells. All three models generate a checkerboard, but juxtacrine produces fewer Paneth cells. b) Time courses for the same simulation. r_w = r_N = 0.1, θ₄ = 0.75, W = 2 (C), W₀ = 0 (P/J), θ₈ = 3 (J), and θ₈ = 0.1. Parameters tuned to give final average Wnt concentration of 2.