The central goal of our research is to discern fundamental principles of cell shape control and then to apply this knowledge to a variety of disease states. Importantly, numerous diseases, including most cancers, lung diseases such as chronic obstructive pulmonary disease (COPD), and degenerative motor neuron disease, derive a significant portion of their etiology from defects in cell mechanics. Yet, cell mechanics has really not been explored as a source of novel therapeutic targets.
Among the earliest inputs that cells experienced, mechanical stress (forces) guide and direct behavior of cells, including when they are part of tissues, organs, and organ systems. These mechanical stresses are propagated through the cell’s skin (the cell cortex), which is a composite material of membrane and cytoskeleton. Key molecular machinery senses the forces, and through mechanotransduction, the mechanical signals may be converted into biochemical signals, which guide cell behavior. Different proteins sense and respond to different types of deformation.
Over many years, we have deciphered the mechanochemical system that governs cytokinesis cell-shape change. While the textbook view has the mitotic spindle as the principle driver, the system is really structured as a control system with many feedback loops. The long axis rule, which was described in the 19th Century and states that the mitotic spindle will elongate along the long axis of the cell, already predicts such a feedback system. Indeed, the mitotic spindle sends cues to the equatorial and polar cortices.
Chemical signal transduction cascades are often thought to be the principle drivers of protein localization. However, we have found that mechanical stress is just as important and in many cases over-rides chemical cues. Many cytoskeletal proteins, including myosin II, cortexillin I, IQGAP scaffolding proteins, and several others, lock in in the cytoskeletal network.
While much is understood about how myosin II is regulated by light chain and heavy chain phosphorylation, we still do not know how the protein is specifically localized. Mechanosensing through cooperative binding of myosin motors to allosteric actin filaments is one major mechanism.
For decades, pathologists have known that formation of cell-in-cell structures, where one cell engulfs another (entosis), could be frequently observed in human tumors. We found that these structures require cell-cell adhesion and a mechanical differential between the inner and outer cells. Specifically, the inner cell has ~2-fold greater elasticity than the outer cell. This mechanical differential comes from inactivation of myosin II in the outer cell and activation in the inner cell.
To decipher the relative contributions of the membrane and cortex to cells, we built a minimal cortex inside a giant unilamellar vesicle and compared the mechanics of the synthetic cell to those of wild type cells and cells where we progressively removed important cytoskeletal components. By comparing the reconstituted cell with the 'deconstituted' cell, we found that the membrane contributed no more than 2% to total cell mechanics. This maximum places important limitations on what membrane can contribute to cell mechanics.
Cytokinesis is inherently a mechanical process, driven by active expansive and contractile stresses and Laplace pressure differentials (cortical tension x local curvature), all of which act upon the cell's viscoelastic material. By developing a complete mechanical description of the cell, we could account quantitatively for cytokinesis shape change, including the kinetics of furrow ingression. Thus, cells divide by mechanics, not biochemistry. The relevant question then becomes how does the cell use biochemistry to generate these mechanics.
Using Dictyostelium, we identified a pathway in which the dimeric acidic protein 14-3-3 acts genetically between microtubules, racE and myosin II to control cell shape, cortical tension, and cytokinesis. In this context, 14-3-3 requires GTP and racE for cortical association, controls the extent of microtubule-cortex interactions, and binds myosin II heavy chain, controlling its assembly into bipolar thick filaments.