176J.L. McGrath and C.F. Dewey, Jr.

of membrane adhesive receptors including β1 and β7 integrin and GP1bα (Andrews and Fox, 1991; Andrews and Fox, 1992; Fox, 1985; Sprandio et al., 1988; Takafuta et al., 1998).

With many binding partners now described, FLNa participates in signaling cascades by spatially collecting and concentrating signaling proteins at the plasma membrane– cytoskeletal junction and may possibly function as an organizing center for actin network rearrangements (see Fig. 9-3). Important partner interactions that may be dependent on filamin include GTPase targeting and charging and linkage of the actin cytoskeleton to membrane glycoproteins such as GP1bα and β-integrins. FLNa is part of a larger family of proteins that include FLNb and FLNc, whose genes are on chromosomes 3 and 7, respectively (Brocker¨ and al, 1999; Krakow et al., 2004; Sheen et al., 2002; Thompson et al., 2000).

FLNa is an elongated homodimer (Hartwig and Stossel, 1981). Each subunit has an N-terminal actin-binding site joined to twenty-four repeat motifs, each 100 residues in length. Repeats are ββ-barrel structures that are believed to interconnect like beads on a string. Subunits self-associate into dimers using only the most C-T repeat motif. The location of known binding partner proteins along each FLNa subunit is indicated in Fig. 9-3. Molecules are 160 nm in length in the electron microscope (Fig. 9-3, bottom right) but can organize actin filaments into branching networks (Fig. 9-3, bottom left).

The FLNa concentrations in endothelial and other cells is normally such that there are many times more FLNa molecules than junctions in the cell cytoskeleton. This can be ascertained by measuring the amount of FLNa in the soluble portion of the cell, computing the molecular concentration per unit cell volume, and then comparing that to the concentration of filament junctions per unit volume of cytoskeleton observable in electron microscopy (see Fig. 9-1).

The role of cytoskeletal structure

The internal structure of the cell has several functions. One is to provide a sufficient amount of rigidity so that the cell can withstand external forces. Figure 9-4 illustrates the functions that the cytoskeleton performs when the cell is subjected to fluid shear stress. A balance of forces requires that the cytoskeleton transmit the entire applied force to the substrate.

The second function to be served is that the cytoskeleton must be malleable enough to allow the cell to accommodate new environmental parameters such as imposed mechanical forces from fluid shear stress and mechanical deformation of the artery. There are two separate time scales to be considered; the first is short time behavior, where fluctuations such as systolic and diastolic changes in flow must be accommodated, and longer times, where the actin matrix can completely disassociate and reform. This latter scale is typically on the order of tens of minutes. More discussion of these mechanisms is given later in this chapter.

The following section will draw upon the ultrastructure presented above and describe how simple mechanical models of the lattice can be used to predict the mechanical properties of the cell.