An hypothesis: Phosphorylation fields as the source of positional information and cell differentiation—(cAMP, ATP) as the universal morphogenetic turing couple

Abstract

It is hypothesized that (cAMP, ATP) is the elusive, universal Turing morphogenetic couple, which defies the second law of thermodynamics, i.e. the inexorable march towards homogeneity. cAMP and ATP can be distributed nonhomogeneously because the whole of the intermediary metabolism is so organized that they mutually satisfy the Turing bifurcation conditions upon nonlocalized application of an extracellular ligand, in particular a soluble peptide growth factor, which is nature's distinguished universal bifurcation parameter, acting homogenously in space and removing the substrate inhibition from adenylate cyclase and thus triggering embryonic induction by triggering the (cAMP, ATP) Turing system. The hypothesis predicts that although the extracellular signal, the growth factor, is applied homogenously, an organized “dissipative structure” will emerge spontaneously in the responding tissue; this “symmetry breaking” in a reaction-diffusion system occurs precisely in the manner envisaged by Turing, where (cAMP, ATP) constitutes the “reaction-diffusion system”. This Turing bifurcation explicates the recent experiments where a differentiated embryoid emerges from the mere immersion of frog animal caps in an homogeneous growth factor solution, and similar experiments on chicks. The “metabolic” patterns found by Child and colleagues also reflect dissipative structures arising in a (cAMP, ATP) reaction-diffusion system when interpreted in the light of modern biochemistry: in particular, the localized glycogen depletion reflects localized cAMP; localized redox, respiratory or susceptibility activity reflects localized ATP. The dramatic collapse of organized structure found by Child and colleagues, for example, when Planaria or a section of it is exposed to an homogeneous environment of a narcotic solution, and the re-emergence structure upon return to water, are explained on the basis of the violation or satisfaction of the Turing bifurcation conditions with respect to (cAMP, ATP), respectively. cAMP is the “activator”, ATP is the “inhibitor”, and together they mutually satisfy the four activator-inhibitor inequalities, including the all-important autocatalytic cAMP production, as well as the lateral inhibition condition. The functional significance of gap junctions is to generate a multicellular purely reaction-diffusion system for (cAMP, ATP) as envisaged by Turing. It is emphasized that localization and pattern formation occur intracellularly in gap junction-coupled cells and not, as often suggested, extracellularly the latter localization being too fragile to be maintained for long enough, and soon succumbing to the mixing effect of convection and movement. The activator-inhibitor property of (cAMP, ATP) means that the spatial distribution of cAMP and ATP could be not only nonhomogeneous but also of the same shape. This colocalization is useful since cAMP will activate the kinase and ATP will donate the phosphoryl group required for the kinase phosphorylation action. This will generate positional information via a spatially nonuniform phosphorylation potential (=phosphorylation field) that can act on target proteins in a spatially differential manner. According to the bifurcation theory of reaction-diffusion systems, the growing (cAMP, ATP) Turing system (embryo) should manifest a succession of spatial patterns, uniform→polar→bipolar→etc., with an increasing number of nodes in each succesive pattern. Child and colleagues indeed find such a succession for the growing embryo. Each (cAMP, ATP) spatial pattern reversibly phosphorylates the transcription factors and can also trigger an hysteretic production of transcription factors before it disappears. This results in each region of the embryo having a different set of transcription factors and this leads to region-specific gene expression. The succession of “metabolic” patterns provides a mechanism for an hierarchical subdivision of the embryo into different multicellular regions with a progressive reduction of potency. One Turing couple, (cAMP, ATP), is sufficient to generate the progressively increasing spatial diversity of cell types, since the action of (cAMP, ATP) on the genes is contextual and depends on the stage of development. Oncogenic gene products can interfere with either the induction of positional information or its interpretation, and hence the increasing spatial differentiation as described above will fail with these defective products. The successive Turing phosphorylation fields are responsible not only for cell-specific gene expression, but also for cell-specific control of cell cycle, as well as cell-specific metabolic activities such as glycogen metabolism, redox activity or membrane permeability. In fact, they correlate in space and coordinate in time all nuclear and cytoplasmic events. The localized ATP will also constitute a localized source of free energy, and the chemical prepattern dictates the localized mechanical deformation. The theory can explain various classical experiments in embryology, e.g. the formation of a second axis when the dorsal lip is grafted is explained by the autocatalytic property of cAMP production which initiates differentiation in the host region.