<p class="first" id="d5692289e167">Mechanical stimulation is recognized as a potent
modulator of cellular behaviors such
as proliferation, differentiation, and extracellular matrix assembly. However, the
study of how cell-generated traction force changes in response to stretch is generally
limited to short-term stimulation. The goal of this work is to determine how cells
actively alter their traction force in response to long-term physiological cyclic
stretch as a function of cell pre-stress. We have developed, to our knowledge, a novel
method to assess traction force after long-term (24 h) uniaxial or biaxial cyclic
stretch under conditions of high cell pre-stress with culture on stiff (7.5 kPa) polyacrylamide
gels (with or without transforming growth factor
<i>β</i>1 (TGF-
<i>β</i>1)) and low pre-stress by treating with blebbistatin or culture on soft gels
(0.6 kPa).
In response to equibiaxial stretch, valvular interstitial cells on stiff substrates
decreased their traction force (from 300 nN to 100 nN) and spread area (from 3000
to 2100
<i>μ</i>m
<sup>2</sup>). With uniaxial stretch, the cells had similar decreases in traction
force and area
and reoriented perpendicular to the stretch. TGF-
<i>β</i>1-treated valvular interstitial cells had higher pre-stress (1100 nN) and
exhibited
a larger drop in traction force with uniaxial stretch, but the percentage changes
in force and area with stretch were similar to the non-TGF-
<i>β</i>1-treated group. Cells with inhibited myosin II motors increased traction
force (from
41 nN to 63 nN) and slightly reoriented toward the stretch direction. In contrast,
cells cultured on soft gels increased their traction force significantly, from 15
nN to 45 nN, doubled their spread area, elongated from an initially rounded morphology,
and reoriented perpendicular to the uniaxial stretch. Contractile-moment measurements
provided results consistent with total traction force measurements. The combined results
indicate that the change in traction force in response to external cyclic stretch
is dependent upon the initial cell pre-stress. This finding is consistent with depolymerization
of initially high-tension actin stress fibers, and reinforcement of an initially low-tension
actin cytoskeleton.
</p>