BALANCING COMPETING EFFECTS OF TISSUE GROWTH AND CYTOSKELETAL REGULATION DURING ORGANOGENESIS

Introduction: How a developing organ robustly coordinates the cellular mechanics and growth to reach a final size and shape remains poorly understood. Through iterations between experiments and new model simulations that include a mechanistic description of interkinetic nuclear migration, we show that the local curvature, height, and nuclear positioning of cells in the Drosophila wing imaginal disc, a model epithelial organ used as a test case, are defined by the concurrent patterning of actomyosin contractility, cell-ECM adhesion, ECM stiffness, and interfacial membrane tension. We show that increasing cell proliferation via different growth-promoting pathways results in two distinct phenotypes. Triggering proliferation through insulin signaling increases basal curvature, but an increase in growth through Bone morphogenetic protein (BMP)/Decapentaplegic (Dpp) signaling and Myc causes tissue flattening. These distinct phenotypic outcomes arise from differences in how each growth pathway regulates the cellular cytoskeleton, including contractility and cell-ECM adhesion. The coupled regulation of proliferation and cytoskeletal regulators is a general strategy to meet the multiple context-dependent criteria defining tissue morphogenesis.

Materials and

Methods: A multi-scale Subcellular Element (SCE) modeling approach was employed to simulate cell mechanics and division dynamics. This involved computational simulations validated against experimental data to predict the impact of various mechanical factors on tissue morphology. In parallel, we utilized the GAL4-UAS system for tissue-specific gene expression in an accessible and powerful genetic system for studying organ growth control mechanisms, the Drosophila wing disc.

The general SCE modeling approach was used as the basis for the model presented here. Our novel SCE-type multi-scale model in detail represents cell deformation and proliferation and integrates mechanical properties, including cell membrane elasticity, cell pressure, polarized actomyosin contractility with nuclear movement, and cell-cell and cell-ECM adhesivity. Another key advantage of this novel SCE modeling approach is that most of the cell mechanical properties considered can be directly or indirectly calibrated based on experimental data. This computational modeling framework is used to simulate a cross-sectional profile of the Drosophila wing imaginal disc along the anterior-posterior axis adjacent to the DV compartment boundary. Using a detailed 2D model allows for simulating large numbers of cells with high resolution and with particular attention to mechanical cell properties and small changes in tissue structure and shape. Given that the cross-section comprises boundary, squamous, and columnar cells, the model includes a separate description for each cell type. Important distinctions of this model include detailed representations of cell division and actomyosin contractility in different parts of a cell.

Results, Conclusions, and Discussions: In this study, we iterated between quantitative analysis of wing disc morphogenesis unders specific genetic perturbations and computational simulations of wing disc growth, which provides a mechanism for hypothesis generation.

Our results demonstrated that the ratio of apical to basal contractility, ECM stiffness, and cell-ECM adhesion are key determinants of basal curvature. We categorized growth regulators into two classes based on their impact on tissue curvature: one class increased basal curvature, while the other led to tissue flattening with growth. The simulations confirmed that these outcomes are tied to their differential regulation of the cytoskeleton.

The morphogenetic processes observed in the Drosophila wing disc are similar to those in other developmental systems, such as the formation of the mammalian optic cup, suggesting a commonality in the underlying mechanisms across species. Our findings underscore the importance of a balanced regulation between proliferation and cytoskeletal dynamics in shaping organs. The study's insights broaden our understanding of developmental biology and provide a foundation for future explorations into organogenesis. Overall, the principal findings identified in our work can be generalized and incorporated into our understanding of development across multiple organ systems in animals due to the underlying biochemical and biomechanical similarities that drive morphogenesis.

Acknowledgements (Optional): This work was supported by the National Science Foundation grant NSF2029814 to all co-authors and by a pilot grant from the Northwestern University NSF-Simons Center for Quantitative Biology to JZ. The Zartman group was also supported in part by NSF DBI- 2120200 and NIH Grant R35GM124935. The authors would also like to thank members of the Zartman lab for helpful discussions. Model simulations were run on UC Riverside HPCC.