Modeling the bistable transition between cell phenotypes during limb precartilage condensation

Abstract Previous work showed that Gal-8 and Gal-1A, two proteins belonging to the galactoside-binding galectin family, are the earliest determinants of the patterning of the skeletal elements of embryonic chicken limbs, and further, that their experimentally determined interactions in the embryonic limb bud can be interpreted through a reaction-diffusion-adhesion framework. Here, we use an ordinary differential equation-based approach to analyze the intrinsic switching modality of the galectin reaction network and characterize the states of the network independent of the diffusive and adhesive arms of the patterning mechanism. We identify two steady states: where the concentrations of both the galectins are respectively, negligible, and very high. We provide an explicit Lyapunov function, which shows that there are no periodic solutions. In an extension of the model with sigmoidal galectin production terms, we show that an analogous bistable switch-like system arises via saddle-node bifurcation from a monostable one. Our model therefore predicts that the galectin network may exist in low expression and high expression states separated in space or time without any intermediate states. We verify these predictions in experiments performed with high density micromass cultures of chick limb mesenchymal cells and observe that cells inside and outside the precartilage protocondensations exhibit distinct behaviors with respect to galectin expression, motility, and spreading behavior on their substratum. The interactional complexity of the Gal-1 and −8-based patterning network is therefore sufficient to partition the mesenchymal cell population into two discrete cell types, which can be spatially patterned when incorporated into an adhesion and diffusion-enabled system.