Proposal:0907212/0907470 PI Name:Levine, Alexander/Dennin, Michael Proposal Title: Collaborative Research: Mechanics and Microrheology of Biomimetic Materials Institution: University of California-Los Angeles/University of California-Irvine
This award by the Biomaterials program in the Division of Materials Research in support of the collaborative efforts by University of California Los Angeles and University of California Irvine is to study the interaction between the nonequilibrium dynamics of molecular motors and the elastic nonlinearities of filamentous actin (F-actin) and determine the collective mechanical properties of the network, with coordinated experimental/theoretical approaches to address each of these challenges. The cytoskeleton of living cells is built primarily from cross-linked F-actin that, in living cells, is generically tensed by molecular motors such as myosin. The mechanical properties of this filament network have been shown to have a complex dependence on the state of activity of these molecular motors, and depend on a combination of the mechanics of the individual filaments, their network structure, and the non-equilibrium steady-state of the network. Understanding in detail how the mechanics of this network can be controlled by its internal stress state (imposed by the endogenous molecular motors) will enable us to better understand how cells control their mechanics and morphology, develop an understanding how cells sense and exert forces on their surroundings. To date, this field has focused on relating the equilibrium collective (non-)linear response properties of a material to the molecular structure of its constituents. With this award, F-actin networks associated with the air/water interface of a Langmuir monolayer will be studied. These 2D networks will then be tensed by molecular motors, and studied using both macro-and microrheology to elucidate the underlying relationship of network architecture (observed through fluorescent labeling of some of the filaments) and non-equilibrium stress state to its collective mechanics. The (quasi-) two-dimensional nature of the network allows for the direct observation of the local network structure, strain state, and provides a way to rapid in situ chemical modification of the system. Experiments on these biopolymer networks could provide insight about the active control of the nonequilibrium steady-state of biopolymer networks that allow the creation of a gel having reversibly tunable mechanical properties. Understanding this prototypical cytoskeletal biopolymer network may allow the development of novel biomimetic active materials with addressable mechanics. Teaching and training of graduate and undergraduate students in experimental and theoretical aspects of biophysics of soft materials, and developing a web site for the interpretation of microrheology are other parts of this award. Human cells are pervaded by a stiff biopolymer network that acts, much like the skeleton of our bodies, to maintain cellular shape and to allow the cell to exert forces on its environment through the action of molecular motors acting on this cytoskeleton. Recent advances have made it possible to deconstruct and then rebuild the principal structural elements of the cytoskeleton in the laboratory. With this award, the mechanical properties of this biopolymer network are measured, and will explore the relationship between network structure, molecular motor activity and large scale mechanics in these protein filament networks. The principal importance of this work is that these studies would provide better understanding how cells use molecular motors to exert forces on their environment and how the activity of these motors can modify the stiffness of the network in a reversible way. This understanding will help to elucidate fundamental design principles by which one may build artificial active materials that use nanomachines (i.e. molecular motors) to actively control their mechanical properties. Students, both graduate and undergraduate, will be trained in research activities that are related to biopolymer networks and their reversibly tunable mechanical properties.
One of the critical functions of cellular membranes is the generation of, response to, and measuring of forces. Membranes play a key role in maintaining cellular structure by balancing rigidity and flexibility. The detection of external forces by the membrane is important to cellular signaling, and finally, the generation of forces is necessary for cellular mobility. Given this, the ability to measure the mechanical properties of membranes and their response to forces plays an important role in our understanding of biological processes, including diseases such as cancer. However, standard methods for measuring mechanical properties of materials can not be applied to cellular membranes. A major achievement of this project was the development of a non-destructive, non-contact method of measuring the mechanical properties of cellular membranes. The basic idea is to trap a small particle in a focused laser beam, and use the interaction of the particle with the membrane through the intra-cellular medium to quantify the mechanical properties and response of the membrane. This requires a close collaboration between theory (to model the interactions) and experiment (to design and develop the apparatus). These results are part of both the intellectual merit and broader impact of the work, as the work involved both a creative collaboration between theory and experiment to solve one of the grand challenges in biological systems and will lead to an instrument that is used broadly throughout the biological community. Another aspect of the intellectual merit of the project is our studies of the transport of particles across the air-water barrier in the lung. This is specific example of how understanding the mechanical properties of a biological membrane are relevant to understanding the biological function. In traditional studies of transport across the lung barrier, static systems are used. However, the lung is fundamentally a dynamic system, as you expand and contract the interfaces during breathing. Our research in this area is transformative in that we developed a dynamic model system. We used a single layer of molecules on an air-water interface that could be compressed and expanded to mimic breathing, and then added particles to the system. We studied the impact of particle size on the structure and dynamics of the membrane system and found a specific size-window of particles that disrupted the normal function of the membrane. Understanding this type of behavior has a broad impact, as it will be able to provide insight into drug design and disease prevention. In addition to the broad impact of the specific scientific results, this project supported two other main areas of broad impact. First, the training of students took place in both an inter-disciplinary environment (they were primarily students with a physics background working on biological problems) and involved close interactions between theory and experiment. Most students only get to experience either theoretical or experimental work as part of their training. Second, both PIs were very active in public outreach that was made possible in part due to this grant.
