Roadmap for the multiscale coupling of biochemical and mechanical signals during development.

The way in which interactions between mechanics and biochemistry lead to the emergence of complex cell and tissue organization is an old question that has recently attracted renewed interest from biologists, physicists, mathematicians and computer scientists. Rapid advances in optical physics, microscopy and computational image analysis have greatly enhanced our ability to observe and quantify spatiotemporal patterns of signalling, force generation, deformation, and flow in living cells and tissues. Powerful new tools for genetic, biophysical and optogenetic manipulation are allowing us to perturb the underlying machinery that generates these patterns in increasingly sophisticated ways. Rapid advances in theory and computing have made it possible to construct predictive models that describe how cell and tissue organization and dynamics emerge from the local coupling of biochemistry and mechanics. Together, these advances have opened up a wealth of new opportunities to explore how mechanochemical patterning shapes organismal development. In this roadmap, we present a series of forward-looking case studies on mechanochemical patterning in development, written by scientists working at the interface between the physical and biological sciences, and covering a wide range of spatial and temporal scales, organisms, and modes of development. Together, these contributions highlight the many ways in which the dynamic coupling of mechanics and biochemistry shapes biological dynamics: from mechanoenzymes that sense force to tune their activity and motor output, to collectives of cells in tissues that flow and redistribute biochemical signals during development.

Functional Comparison between Endogenous and Synthetic Notch Systems.

The Notch pathway converts receptor-ligand interactions at the cell surface into a transcriptional response in the receiver cell. In recent years, synthetic Notch systems (synNotch) that respond to different inputs and transduce different transcriptional responses have been engineered. One class of synNotch systems uses antibody-antigen interactions at the cell surface to induce the proteolytic cleavage cascade of the endogenous Notch autoregulatory core and the consequent release of a synNotch intracellular domain (ICD), converting surface antigen detection into a cellular response. While the activation of endogenous Notch requires ubiquitylation and subsequent endocytosis of the ligand ICD, these synNotch systems do not seem to have such a requirement because the synNotch ligands completely lack an ICD. This observation raises questions about existing models for the synNotch activation mechanism. Here, we test how different structural and biochemical factors affect the dependence of endogenous and synthetic Notch activation on ligand ICD. We compare the behavior of antibody-antigen synNotch (aa-synNotch) to that of endogenous Notch, and to a synNotch system that uses rapamycin induced dimerization of FK506 binding protein (FKBP) and FKBP rapamycin binding (FRB) domaindimerization domains (ff-synNotch), which still requires a ligand ICD. We found that differences in receptor-ligand affinity, in the identity of the transmembrane domain, or in the presence or absence of extracellular epidermal growth factor repeats cannot explain the differences in ligand ICD requirement that distinguishes aa-synNotch from endogenous Notch or ff-synNotch. We also found that unlike endogenous Notch and ff-synNotch, the aa-synNotch system does not exhibit trans-endocytosis of the receptor extracellular domain into the sender cell. These findings suggest that the aa-synNotch systems bypass the ligand ICD requirement because antigen-antibody pairs are able to promote other adhesive cell-cell interactions that provide the mechanical tension needed for ligand activation.

Cell-Cell Contact Area Affects Notch Signaling and Notch-Dependent Patterning.

During development, cells undergo dramatic changes in their morphology. By affecting contact geometry, these morphological changes could influence cellular communication. However, it has remained unclear whether and how signaling depends on contact geometry. This question is particularly relevant for Notch signaling, which coordinates neighboring cell fates through direct cell-cell signaling. Using micropatterning with a receptor trans-endocytosis assay, we show that signaling between pairs of cells correlates with their contact area. This relationship extends across contact diameters ranging from micrometers to tens of micrometers. Mathematical modeling predicts that dependence of signaling on contact area can bias cellular differentiation in Notch-mediated lateral inhibition processes, such that smaller cells are more likely to differentiate into signal-producing cells. Consistent with this prediction, analysis of developing chick inner ear revealed that ligand-producing hair cell precursors have smaller apical footprints than non-hair cells. Together, these results highlight the influence of cell morphology on fate determination processes.