Conformational changes and signaling in cell and matrix physics

Dynamics and regulation of contractile actin-myosin networks in morphogenesis

Contractile actin-myosin networks generate forces that drive cell shape changes and tissue remodeling during development. These forces can also actively regulate cell signaling and behavior. Novel features of actin-myosin network dynamics, such as pulsed contractile behaviors and the regulation of myosin localization by tension, have been uncovered in recent studies of Drosophila. In vitro studies of single molecules and reconstituted protein networks reveal intrinsic properties of motor proteins and actin-myosin networks, while in vivo studies have provided insight into the regulation of their dynamics and organization. Analysis of the complex behaviors of actin-myosin networks will be crucial for understanding force generation in actively remodeling cells and the coordination of cell shape and movement at the tissue level.

Correction of the viscous drag induced errors in macromolecular manipulation experiments using atomic force microscope

We describe a method to correct the errors induced by viscous drag on the cantilever in macromolecular manipulation experiments using the atomic force microscope. The cantilever experiences a viscous drag force in these experiments because of its motion relative to the surrounding liquid. This viscous force superimposes onto the force generated by the macromolecule under study, causing ambiguity in the experimental data. To remove this artifact, we analyzed the motions of the cantilever and the liquid in macromolecular manipulation experiments, and developed a novel model to treat the viscous drag on the cantilever as the superposition of the viscous force on a static cantilever in a moving liquid and that on a bending cantilever in a static liquid. The viscous force was measured under both conditions and the results were used to correct the viscous drag induced errors from the experimental data. The method will be useful for many other cantilever based techniques, especially when high viscosity and high cantilever speed are involved.

Mechanical control of tissue and organ development

Many genes and molecules that drive tissue patterning during organogenesis and tissue regeneration have been discovered. Yet, we still lack a full understanding of how these chemical cues induce the formation of living tissues with their unique shapes and material properties. Here, we review work based on the convergence of physics, engineering and biology that suggests that mechanical forces generated by living cells are as crucial as genes and chemical signals for the control of embryological development, morphogenesis and tissue patterning.

Self inhibition of phagocytosis: the affinity of 'marker of self' CD47 for SIRPalpha dictates potency of inhibition but only at low expression levels

Phagocytes engulf foreign cells but not 'self' in part because self cells express CD47 as a ligand for signal regulatory protein SIRPalpha, which inhibits phagocytosis. Motivated by reports of upregulation of CD47 on both normal and cancerous stem cells [1: Jaiswal et al., 2009] and also by polymorphisms in SIRPalpha [2: Takenaka et al., 2007], we show here that inhibition of engulfment correlates with affinity of CD47 for SIRPalpha - but only at low levels of CD47. One common human polymorph of SIRPalpha is studied and binds more strongly to human-CD47 than to mouse-CD47 (K(d) approximately 0.12 microM and 6.9 microM, respectively) and does not bind sheep red blood cells (RBCs) - which are well-established targets of human macrophages; in comparison, a common mouse polymorph of SIRPalpha binds with similar affinity to human and mouse CD47 (K(d) approximately 0.22 microM). Using immunoglobulin (IgG)-opsonized particles with varying levels of either human- or mouse-CD47, the effective inhibition constants K(i) for blocking phagocytosis are then determined with both human- and mouse-derived macrophages. Only human phagocytes show significant differences in man versus mouse K(i)'s and only at CD47 levels below normal densities for RBCs. While phospho-signaling through human-SIRPalpha shows similar trends, consistent again with the affinity differences, saturating levels of CD47 (>K(i)) can signal and inhibit phagocytosis regardless of man versus mouse. Quantitative analyses here prompt more complete characterizations of both CD47 levels and SIRPalpha polymorphisms when attempting to study in vivo effects of these key proteins in innate immunity.