Final Report Summary - BIOSELFORGANIZATION (Biophysical aspects of self-organization in actin-based cell motility)
Actin-based cell movement as one of the prime examples of biological self-organization. Actin-based motility is crucial for a wide range of biological phenomena such as defense against injury or infection, embryogenesis and cancer metastasis. From the physical point of view, the actin machinery is a far-from-equilibrium system of semi-flexible filaments and auxiliary proteins which self-organizes into a myriad of dynamic structures. The molecular players involved in actin-based movement and the basic biochemical mechanisms are largely known. However, the principles governing their self-organization, which involve an intricate interplay between biophysical and biochemical processes, are still poorly understood. Our research project focused on understanding this self-organization.
One line of research centered on utilizing the simplest available natural model system for actin-based movement, namely fish keratocytes cells and fragments, to explore the basic mechanisms underlying cell movement. Important biophysical aspects of the motility process, including the intracellular fluid flow and the tension in the cell membrane, were characterized providing insight into their influence on cell movement. We found that the overall shape and movement of keratocyte cells and fragments can be recapitulated by a relatively simple model of a treadmilling actin network within an inextensible membrane “bag”, highlighting the role of membrane tension as a mechanical regulator of cell boundary dynamics, coupling protrusion at the front and retraction at the rear.
The second research direction involved the development of artificial cells in which we reconstitute actin dynamics in a controlled environment, detached from the inherent complexity of the living cell. Our initial efforts were directed at reconstituting cortical actin networks. The actin cortex is a thin shell of actin filaments underneath the cell membrane which provides mechanical integrity to cells and plays a pivotal role in many cellular process including cell division, the generation and maintenance of cell polarity and motility. In all these contexts, the cortical network has to break symmetry to generate polar cytoskeletal dynamics. Despite extensive research, the mechanisms responsible for regulating cortical dynamics in vivo and inducing symmetry breaking are still unclear. We have developed a reconstituted system that self-organizes into dynamic actin cortices at the inner interface of water-in-oil emulsions. We have shown that this artificial system undergoes spontaneous symmetry breaking, driven by myosin-induced cortical actin flows, which appears remarkably similar to the initial polarization of the embryo in many species. The contractile behavior of the reconstituted cortices exhibits a sharp temperature-dependent transition, facilitating the use of temperature as an external parameter to control the onset of symmetry breaking. The artificial cortices recapitulate the rich dynamics of actin cortices in vivo, revealing the basic biophysical and biochemical requirements for cortex formation and symmetry breaking. In particular, we showed that symmetry breaking requires the energy-consuming activity of myosin motors and sufficient crosslinking, but does not depend upon pre-patterned localization of actin nucleators, the involvement of microtubules, or any local changes in the properties of the interface.
One line of research centered on utilizing the simplest available natural model system for actin-based movement, namely fish keratocytes cells and fragments, to explore the basic mechanisms underlying cell movement. Important biophysical aspects of the motility process, including the intracellular fluid flow and the tension in the cell membrane, were characterized providing insight into their influence on cell movement. We found that the overall shape and movement of keratocyte cells and fragments can be recapitulated by a relatively simple model of a treadmilling actin network within an inextensible membrane “bag”, highlighting the role of membrane tension as a mechanical regulator of cell boundary dynamics, coupling protrusion at the front and retraction at the rear.
The second research direction involved the development of artificial cells in which we reconstitute actin dynamics in a controlled environment, detached from the inherent complexity of the living cell. Our initial efforts were directed at reconstituting cortical actin networks. The actin cortex is a thin shell of actin filaments underneath the cell membrane which provides mechanical integrity to cells and plays a pivotal role in many cellular process including cell division, the generation and maintenance of cell polarity and motility. In all these contexts, the cortical network has to break symmetry to generate polar cytoskeletal dynamics. Despite extensive research, the mechanisms responsible for regulating cortical dynamics in vivo and inducing symmetry breaking are still unclear. We have developed a reconstituted system that self-organizes into dynamic actin cortices at the inner interface of water-in-oil emulsions. We have shown that this artificial system undergoes spontaneous symmetry breaking, driven by myosin-induced cortical actin flows, which appears remarkably similar to the initial polarization of the embryo in many species. The contractile behavior of the reconstituted cortices exhibits a sharp temperature-dependent transition, facilitating the use of temperature as an external parameter to control the onset of symmetry breaking. The artificial cortices recapitulate the rich dynamics of actin cortices in vivo, revealing the basic biophysical and biochemical requirements for cortex formation and symmetry breaking. In particular, we showed that symmetry breaking requires the energy-consuming activity of myosin motors and sufficient crosslinking, but does not depend upon pre-patterned localization of actin nucleators, the involvement of microtubules, or any local changes in the properties of the interface.