The viscoelastic properties of microvilli are dependent upon the cell-surface molecule

Abstract

We studied at nanometer resolution the viscoelastic properties of microvilli and tethers pulled from myelogenous cells via P-selectin glycoprotein ligand 1 (PSGL-1) and found that in contrast to pure membrane tethers, the viscoelastic properties of microvillus deformations are dependent upon the cell-surface molecule through which load is applied. A laser trap and polymer bead coated with anti-PSGL-1 (KPL-1) were used to apply step loads to microvilli. The lengthening of the microvillus in response to the induced step loads was fitted with a viscoelastic model. The quasi-steady state force on the microvillus at any given length was approximately fourfold lower in cells treated with cytochalasin D or when pulled with concanavalin A-coated rather than KPL-1-coated beads. These data suggest that associations between PSGL-1 and the underlying actin cytoskeleton significantly affect the early stages of leukocyte deformation under flow.

Figure 1

Experimental protocol. A: A microsphere coated with either concanavalin A or KPL-1 (anti-PSGL-1) is trapped and brought into contact with an adherent cell. B: The laser trap is displaced away from the cell ~ ¼ μm. C: The microvillus deforms under the new imposed load and the microsphere creeps back toward cell center. D: Output from the quadrant detector reporting the position of the microsphere relative to trap center. Phases a–c are as above, while d indicates where we measure quasi-steady state force on the microvillus.

Figure 2

Quasi-steady state force versus microvillus length for control (KPL-1/PSGL-1, ●○), cytochalasin D treated (▲) and microvilli pulled via conanavalin A (■). Each data marker represents the mean of all similarly sized steps for that condition. Error bars represent s.e.m. Microvillus elongations that yield forces above 30 pN are shown as open symbols (○) and without error bars because trap properties become non-linear (see text); those values should be treated as approximate.

Figure 3

Ensemble averaged data from a single step, fitted by equation 4 (solid line). Inset: schematic showing a standard linear solid in the form used here.

Figure 4

Viscoelastic parameters as a function of microvillus length for control cells pulled via KPL-1/PSGL-1 (●,+), cytochalasin-D treated cells pulled via KPL-1/PSGL-1 (○) and cells pulled via concanavalin-A (■). Crosshairs (+) specifically indicate values for KPL-1/PSGL-1 that are above the limit for linear trap behavior; the exact values have low confidence.