Biophysics

We use our expertise and our experimental tools to investigate the properties of biological materials and to study the behavior of cells. Much of our focus is on developing an understanding of the mechanical properties of biopolymer networks, formed by reconstituting proteins into gelled networks. These include networks of actin, microtubules, intermediate filaments, fibrin and collagen. We combine visualization of the network structure with probes of their mechanical properties to understand the nature of these properties. We also add molecular motors to the networks to investigate the properties of active gels. In addition, we extend our investigations to study the mechanical properties of cells and collections of cells. We also have an effort in understanding the growth and physical properties of biofilms.

Here are some current projects from our group:

JING XIA SEM

Decoupling the effects of nanopore size and surface roughness on the attachment, spreading and differentiation of bone marrow-derived stem cells: The nanopore size and roughness of nanoporous surface are two critical variables in determining stem cell fate, but little is known about the contribution from each cue individually. To address this gap, we use two-dimensional nanoporous membranes with controlled nanopore size and roughness to culture bone marrow-derived mesenchymal stem cells (BMSCs), and study their behaviors such as attachment, spreading and differentiation, as shown in Figure 1. We find that increasing the roughness of nanoporous surface has no noticeable effect on cell attachment, and only slightly decreases cell spreading areas and inhibits osteogenic differentiation. However, BMSCs cultured on membranes with larger nanopores have significantly fewer attached cells and larger spreading areas. Moreover, these cells cultured on larger nanopores undergo enhanced osteogenic differentiation by expressing more alkaline phosphatase, osteocalcin, osteopontin, and secreting more collagen type I. These results suggest that although both nanopore size and roughness can affect BMSCs, nanopore size plays a more significant role than roughness in controlling BMSC behavior. Jing Xia

Mechanical properties of composite cytoskeleton networks: The cytoskeletal network is known to be responsible for maintaining cell mechanical integrity and determining cellular functions. We develop a method that enables us to reconstruct a three-component in vitro network composed of F-actin, intermediate filaments (vimentin filaments), and microtubules, which are the three fundamental cytoskeletal components. This composite is more physiologically relevant compared with any of the previously reported reconstituted cytoskeletal networks, which are composed of one or two components only. We investigate the structure and mechanical properties of this multi-component cytoskeletal network using several microscopy techniques as well as one- and two-point microrheology. We show that vimentin filaments hardly increase the network elastic modulus; however, they slow down the longitudinal motion of the polymers and significantly prolong the network relaxation time, by imposing configurational constraints within the network. These findings deepen our understanding of the mechanical role that vimentin filaments play in regulating cellular activities. Yinan Shen 

Fluorescent image of MEF cell on microcontact pattern

In-vivo study of Yielding and Post-yielding behavior of Cytoplasm and its linkage with the cytoskeleton: Cytoplasm, as an omnipresent component of the cells, is known to have finite Young’s modulus and resists deformation, while intracellular cargo transports as fast as micron per second is observed in cytoplasm, thus it must exists a transition from solid to liquid, so called yielding and post yielding behavior of the cytoplasm. We find that cytoplasm yields at 10^1 Pascal scale. The post yielding behavior may be modelled as non-Newton fluid. Further control experiment illustrates that both heterogeneity and microtubules significantly contribute to the yielding behavior: The resistance of cytoplasm has multiple resources, and the dynamic assembling and dissembling of the microtubules are essential for yielding. Also, the cytoplasm close to microtubule-organizing centre (MTOC) has higher resistance towards yielding. Our experiment can potentially enhance people’s understanding towards cargo transport and cell mechanics. Sijie Sun

The interplay of phase behavior, membrane tension and membrane-membrane interaction in lipid vesicles: Lipid vesicles are aqueous volumes enclosed by lipid bilayers, which resemble the membrane backbone of cells and many intracellular organelles. Lipid vesicles have been used as an experimental model system to study membrane biophysics. However, traditional methods for vesicle generation depend on the self-assembly of lipid bilayers, which have shown poor control over the membrane composition, size distribution and structure of lipid vesicles, limiting the collection of quantitative, statistically sound data. To circumvent these problems, we have developed a microfluidics method for vesicle generation, which templates the formation of lipid vesicles on well-controlled double emulsions. The lipid vesicles made in this method have very narrow size distribution and uniform membrane composition. We use 3D reconstructed confocal microscopy to record the phase behavior and shape of lipid vesicles, and measure their membrane tension using micropipette aspiration. Combining these data, we are now investigating the rich interplay between membrane tension, phase behavior and membrane-membrane interaction. In the meanwhile, we are exploring robust methods to incorporate membrane proteins into the vesicles consistently. We hope this study can provide statistically sound data to further our understanding of lipid membrane biophysics. Anqi Chen

jing_xia_pulling_a_paramagnetic_bead_with_electromagnetic_tweezer

Design of electromagnetic tweezer for cellular mechanics studies: The mechanical properties of cell are very important for maintaining cellular integrity and fulfill their functions. However, there are only limited tools to investigate the mechanics of cells at submicron scale, such as atomic force microscope and optical tweezer, partially due to the limited force range available in each tool. Here, we develop an electromagnetic tweezer that can apply forces spanning several orders of magnitude, as shown in Figure 2. By controlling the electric current in the solenoid, the force can be varied from a few piconewton to hundreds nanonewton. This tool is further used to investigate the cell mechanics. By applied the magnetic force on the paramagnetic beads that’s attached on the cells, we can probe the mechanical properties of cells, as shown in Figure 3.Jing Xia

Here are some previous projects from our group:

Microrheology in cells and networks: Cells are mechanical objects, with mechanics largely determined by their biopolymer cytoskeleton. We characterize local mechanics in cells and reconstituted biopolymer networks using active optical tweezer microrheology. We aim to determine the mechanical role of the biopolymers, as well as the effects of molecular motors and enzymes on the mechanics of the networks. Helen Wu, Mikkel Jensen, Ming Guo

Mechanics of vimentin intermediate filament networks: Intermediate filaments (IF) make up a component of the cytoskeleton that has received much less attention than actin and microtubules. I am studying the mechanics of vimentin, a Type III IF, using bulk and microrheological techniques as well as traction force microscopy on single cells. I am interested in its contribution to cell stiffness and its role in cell mechanosensitivity. Helen Wu