PROBING CELLULAR ADHESION AT THE SINGLE-MOLECULE LEVEL - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 237

constant pulling speed will be fairly linear given the mechanical properties of
the cytoskeletal cortex. This will lead to constant loading rates. In contrast, if
the link with the cytoskeleton ruptures or if there is not link at all, the force
response will be nonlinear and membrane tethers may eventually form ( Fig.
11.4d ) . 40,41 A force plateau preceding bond rupture has been interpreted as
a signature of tether formation and elegantly conirmed recently by lateral
inspection of the cell detachment process. 59 Thus, for some receptors, the
application of different loading rates might be dificult on living cells, given the
mechanical properties of the membrane and the underlying cytoskeleton. The
formation of membrane tethers is not ideal to carry out DFS measurements.
However, it is possible to take advantage of this a priori inconvenience.

11.3.2.2 Force clamp measurements

A more direct approach to determine the effect of force in the kinetics of
receptor-ligand interactions is the application of what has been called
force clamp technique, in analogy with patch clamp electrophysiology

measurements. In force clamp measurements, a constant force is applied to
the bond and the time until it disrupts is measured. This measured lifetime
at a certain clamping force will not be unique but will follow an exponential
distribution with an average value. Applying different levels of force and
determining the corresponding average lifetime will directly lead to lifetime
versus force plots. On the so-called

bonds, in which an applied force will
decrease the lifetime, itting the Bell model (described in the next section)
to the lifetime versus force data will allow us to determine the intrinsic
parameters of the interaction. Force clamp measurements, however, are
particularly useful to detect possible

slip

, in which moderate forces
counterintuitively increase the lifetime of the bond. 22,23
As mentioned before, living cells have the capacity of forming membrane
tethers. The physiological relevance of tethers is found, for example, in the
rolling of leukocytes on the vascular wall, which reduces their speed at the
early stages of the leukocyte adhesion cascade. 3 During AFM force
measurements at constant pulling speed tethers may form, giving rise to
characteristic force-distance proiles in which a force plateau precedes a
jump in force ( Fig. 11.4d ) . It has been suggested that tethers form when
the receptor detaches from the cytoskeleton. 40 In that case, a membrane
tube is pulled from the cell surface exerting a friction force. This force,
being dissipative in nature, will depend on the pulling speed but also on
the mechanical properties of the membrane and its interaction with the
cytoskeleton. It has been suggested that the main responsible to this force
is indeed the friction between the membrane, which lows toward the

catch bonds