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at focal adhesion sites, regulates MT turnover, and silences spindle assembly check-
points to enable cell cycle control (
Kaverina et al., 2002; Musacchio & Salmon,
2007; Yvon, Gross, &Wadsworth, 2001
). There are increasing efforts to characterize
the mechanical properties of the cytoskeleton, and numerous
in vitro
studies have
measured the viscoelastic responses of filamentous actin (
Broedersz et al., 2010;
Chaudhuri, Parekh, & Fletcher, 2007; Gardel, Valentine, Crocker, Bausch, &
Weitz, 2003; Gardel et al., 2006; Janmey, Euteneuer, Traub, & Schliwa, 1991;
Lee, Ferrer, Lang, & Kamm, 2010; Liu, Koenderink, Kasza, MacKintosh, &
Weitz, 2007; Tharmann, Claessens, & Bausch, 2007; Uhde, Ter-Oganessian,
Pink, Sackmann, & Boulbitch, 2005
). By contrast, relatively little is known of the
physical properties of MT networks and how morphology and mechanical interac-
tions give rise to elasticity and force transmission across cellular distances
(
Janmey et al., 1991; Lin, Koenderink, MacKintosh, & Weitz, 2007; Pelletier,
Gal, Fournier, & Kilfoil, 2009; Sato, Schwartz, Selden, & Pollard, 1988
). Single
MTs are extremely rigid, so the molecular origins of stiffness and stress dissipation
are fundamentally different than those of semiflexible actin or flexible polymer
systems (
Gittes, Mickey, Nettleton, & Howard, 1993; Hawkins, Mirigian, Selcuk
Yasar, & Ross, 2010; Head, Levine, & MacKintosh, 2003, 2005; Mickey & Howard,
1995; Taute, Pampaloni, & Florin, 2010; Taute, Pampaloni, Frey, & Florin, 2008;
Valdman, Atzberger, Yu, Kuei, & Valentine, 2012; Yang, Bai, Klug, Levine, &
Valentine, 2013; Yang, Lin, Kaytanli, Saleh, & Valentine, 2012
). This motivates the
development and use of new characterization tools that enable direct measurement of
structure-property relationships in MT networks.
Here, we describe our development and implementation of a broad set of imaging
and microscale manipulation tools to study the properties of reconstituted MT net-
works. Using custom-built magnetic tweezers devices, we apply calibrated step forces
to MT networks using microscale magnetic beads and measure simultaneously the re-
sultant displacement as a function of time (
Yang et al., 2012, 2013
). The resultant non-
uniformdeformation field has a characteristic bending radius dictated by the size of the
embedded magnetic particle, typically several microns. This length scale is relevant to
cargo transport and cellular remodeling. Yet, in this regime, mean-field models that
require uniform stretching or bending of filaments no longer apply, limiting the use
of traditional continuummechanics approaches for modeling. Therefore, direct exper-
imental tests are necessary to provide much-needed insight into how the cytoskeleton
generates forces, transmits mechanical signals, and maintains cell strength.
The broad class of experimental tools used for microscale manipulation and char-
acterization of polymer materials is collectively known as active microrheology
methods. These are distinguished from passive microrheology methods, in which
the thermal motions of embedded particles are analyzed. Both forms of microrheology
are useful in determining the spatial distributions of stiffness and/or viscosity in het-
erogeneous materials, or in determining the moduli of precious samples that cannot be
obtained in large quantities (
Crocker et al., 2000; Gardel, Valentine, & Weitz, 2005;
Gardel et al., 2003; Mason & Weitz, 1995; Squires & Mason, 2010
). Experimental
platforms for active microrheology commonly incorporate an optical microscope to
visualize samples and direct the application of force using micron-scale probes.
