Alpha4beta1-dependent adhesion strengthening under mechanical strain is regulated by paxillin association with the alpha4-cytoplasmic domain

The capacity of integrins to mediate adhesiveness is modulated by their cytoplasmic associations. In this study, we describe a novel mechanism by which alpha4-integrin adhesiveness is regulated by the cytoskeletal adaptor paxillin. A mutation of the alpha4 tail that disrupts paxillin binding, alpha4(Y991A), reduced talin association to the alpha4beta1 heterodimer, impaired integrin anchorage to the cytoskeleton, and suppressed alpha4beta1-dependent capture and adhesion strengthening of Jurkat T cells to VCAM-1 under shear stress. The mutant retained intrinsic avidity to soluble or bead-immobilized VCAM-1, supported normal cell spreading at short-lived contacts, had normal alpha4-microvillar distribution, and responded to inside-out signals. This is the first demonstration that cytoskeletal anchorage of an integrin enhances the mechanical stability of its adhesive bonds under strain and, thereby, promotes its ability to mediate leukocyte adhesion under physiological shear stress conditions.     

A common biomechanical model for the formation of stationary cell domains and propagating waves in the developing organisms

Many important morphogenetic processes that take place in the development of an animal start from the segregation of a homogeneous layer of cells into a different number of the domains of columnar and flattened cells. In many cases, waves of cell shape transformation travel throughout embryonic tissues. A biomechanical model is presented which embraces both kinds of event. The model is based on the idea of interplay between short- and long-range factors. While the former promote the spreading of a given cell state along a cell row in the recalculation direction, long-range factors are associated with self-generated tensions which, after exceeding a certain threshold, induce active cell extension and hence the rise of tangential pressure. Different kinds of biologically realistic stationary structures, as well as various kinds of the running waves, can be modelled under different parameter values. Moreover, the current model can be coupled with the previous one (Beloussov and Grabovsky, Comput. Methods Biomech. Biomed. Eng., 6: 53–63 (2003)) permitting a common causal chain to be created, moving from the state of an initial homogeneous cell layer towards the complicated shapes of embryonic rudiments.     

Assembly of the sarcoplasmic reticulum proteins in differentiating rat skeletal muscle cell cultures: localization by immunofluorescence of sarcoplasmic reticulum proteins in differentiating rat skeletal muscle cell cultures

Immunofluorescent staining techniques were used to study the distribution of the Ca(2) + Mg(2+)-dependent ATPase and calsequestrin in primary cultures of differentiating rat skeletal muscle cells, grown for different periods of time under various culture conditions. In mononucleated myoblasts calsequestrin was detected after 45 h in culture whereas the ATPase was not detected until 60 h. After cell fusion began, both proteins could be identified in all multinucleated cells. Myoblasts grown for longer than 60 h in low Ca(2+) medium contained calsequestrin and the ATPase, even though they were unable to fuse. These studies at the cellular level confirm biochemical findings on the biosynthesis of calsequestrin and the ATPase. Immunofluorescent staining of myoblasts showed that calsequestrin first appears in a well-defined region of the cell near one end of the nucleus. At later times, the staining occupied progressively larger regions adjacent to the nucleus and took on a fibrous appearance. This suggests that calsequestrin first accumulates in the Golgi region and then gradually spreads throughout the cell. In contrast, the ATPase appeared to be concentrated in many small patches or foci throughout the cytoplasm and was never confined to one particular region, although some parts of the cell often stained more intensely than others. In multinucleated cells, alternating dark and fluorescent strands parallel to the longitudinal axis of the cells were evident.