Nanoscale architecture of the actomyosin cortex: Investigating the structural basis of cell surface tension

Cell shape is intimately linked to function and is often altered in disease. A precise control of cell shape is fundamental to a wide array of physiological processes, including embryonic development, tissue homeostasis, wound healing, and immune response. Cell shape defects have been implicated in numerous pathologies, from developmental disorders to cancer. Yet, despite its importance, our current understanding of cell shape control remains limited. This is largely due to the challenge of connecting molecular-scale interactions to the cell-scale mechanical forces that ultimately determine shape. This project focuses on the actomyosin cortex, a cellular cytoskeletal network that is a key determinant of cell shape. The cellular cortex supports the plasma membrane and comprises a thin layer of actin filaments, myosin motors and associated proteins. Myosin motors generate contractile forces within the cortex, which put the cortical network under tension. Cortex tension helps support cellular shape against external constraints, and gradients in tension drive cellular deformations in processes like cell division, cell migration, and tissue contractions. The cortex has been in the spotlight as a key regulator of cell shape for over a decade. Yet, our understanding of the regulation of cortical tension is very superficial, greatly limiting studies aiming to understand and perturb cellular shape. This poor understanding is in great part due of the technical challenges in imaging the dense and thin cortical network, which is typically under the resolution of classical microscopy techniques. As a result, the nanoscale organisation of the cortex, which ultimately determines cortex tension, remains mostly a black box. The proposed project aims to address these challenges by leveraging recent advances in super-resolution imaging and cryo-electron tomography of cellular structures. We propose to develop innovative imaging pipelines for quantitative analysis of the structural organisation of cortical actin and the arrangement and dynamics of cortical myosin motors. We will then use these tools to investigate how changes in actomyosin network architecture regulate cortical tension, focusing on mitotic cell rounding, a mechanical process of key importance for the success of cell division. Our central hypothesis is that changes in the nanoscale architecture of the cortex can trigger a switch-like increase in cortex tension. We speculate that such a structurally-triggered tension switch would provide a more robust and responsive mechanism for the control of cortical tension compared to tension regulation through gradual changes in myosin activity. We will explore this hypothesis using a combination of experiments and theory. By providing insights into the nanoscale organization of the actomyosin cortex, our findings will have wide-ranging implications for our understanding of cellular mechanics, potentially informing new therapeutic strategies for diseases linked to cell shape abnormalities, such as cancer and developmental disorders. Moreover, the imaging and analysis tools we propose to develop will be of broad interest in cell biology and biophysics, enabling further studies connecting the architecture of cellular cytoskeletal networks to their function. By bridging the gap between nanoscale cytoskeletal architecture and cell-scale forces, our study will shed light on how molecular interactions translate into macroscopic cell behaviours, illuminating fundamental principles of cell morphogenesis.