-
Biophysical Journal 01/2009; 96:133. · 3.65 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Rheological properties of adherent cells are essential for their physiological functions, and microrheological measurements on living cells have shown that their viscoelastic responses follow a weak power law over a wide range of time scales. This power law is also influenced by mechanical prestress borne by the cytoskeleton, suggesting that cytoskeletal prestress determines the cell's viscoelasticity, but the biophysical origins of this behavior are largely unknown. We have recently developed a stochastic two-dimensional model of an elastically joined chain that links the power-law rheology to the prestress. Here we use a similar approach to study the creep response of a prestressed three-dimensional elastically jointed chain as a viscoelastic model of semiflexible polymers that comprise the prestressed cytoskeletal lattice. Using a Monte Carlo based algorithm, we show that numerical simulations of the chain's creep behavior closely correspond to the behavior observed experimentally in living cells. The power-law creep behavior results from a finite-speed propagation of free energy from the chain's end points toward the center of the chain in response to an externally applied stretching force. The property that links the power law to the prestress is the chain's stiffening with increasing prestress, which originates from entropic and enthalpic contributions. These results indicate that the essential features of cellular rheology can be explained by the viscoelastic behaviors of individual semiflexible polymers of the cytoskeleton.
Physical Review E 11/2008; 78(4 Pt 1):041922. · 2.26 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The dynamic mechanical behavior of living cells has been proposed to result from timescale-invariant processes governed by the soft glass rheology theory derived from soft matter physics. But this theory is based on experimental measurements over timescales that are shorter than those most relevant for cell growth and function. Here we report results measured over a wider range of timescales which demonstrate that rheological behaviors of living cells are not timescale-invariant. These findings demonstrate that although soft glass rheology appears to accurately predict certain cell mechanical behaviors, it is not a unified model of cell rheology under biologically relevant conditions and thus, alternative mechanisms need to be considered.
Biophysical Journal 11/2007; 93(8):L39-41. · 3.65 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Airway smooth muscle cells exhibit stiffening during contractile activation. This stiffening may be interpreted as a result of the stabilizing influence of the mechanical prestress stored within the cytoskeleton (CSK). However, in vivo, airway smooth muscle cells contract while simultaneously experiencing breathing-induced stretching. Excessive stretching of cells could cause actin-myosin crosslinks, and possibly other cytoskeletal filaments, to break, thereby leading to dissipation of the prestress and inhibition of further cell stiffening. The aim of this study is to investigate the stiffening behavior of individual human airway smooth muscle (HASM) cells exposed to a combination of substrate stretching, contractile activation and relaxation. We treated cultured HASM cells with either contractile (histamine) or relaxing (DBcAMP) pharmacological agonists and used magnetic cytometry technique to investigate the stiffening behavior of these cells during uniform substrate stretching (0-30%). Cells that were not treated, as well as those treated with histamine, exhibited increasing stiffening during stretching up to 20% of substrate strain, with additional stiffening becoming inhibited for substrate strains of 20-30%. In contrast, in cells treated with DBcAMP, stretching produced moderate but continuous stiffening with increasing substrate strain. These results indicate that both active and passive components of the prestress contribute to cell stiffening. We also observed that cells permeabilized with saponin exhibited stiffening at low levels (<10%) of substrate stretching, similar to non-permeabilized cells, but not at high levels (10-30%) of stretching, where stiffening was inhibited. These data suggest that at low levels of substrate strains the relative contributions of ion channel activation as well as actin and focal adhesion remodeling are less important for stiffening than passive distension of the CSK. Taken together, our results suggest that both the active and passive components of the cytoskeletal prestress contribute to the stiffening behavior of HASM cells under physiological conditions, but that at high levels of cellular distensions there is a possible tradeoff between these two components with the contribution from the passive component becoming increasingly more important.
Annals of Biomedical Engineering 02/2007; 35(2):224-34. · 2.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The current study investigated the repeatability (test-retest reliability) of ground reaction force parameters recorded during a voluntary step execution under single (motor task) and dual task (motor and cognitive task) conditions for healthy adults and elderly individuals as well as the number of trials required to produce repeatable results.
Twenty-four healthy adults (21-63 years old) and 16 elderly adults (66-87 years) performed a voluntary rapid step execution following a tap on their heel while standing on a force platform under single and dual task conditions on three separate occasions. The first two tests were performed 30-60 minutes apart and the third test was performed a week later. Variables analyzed from the ground reaction force data included onset latency of step initiation (initiation phase), preparation and swing phases, foot-off and foot-contact times.
Intraclass correlation coefficients (ICC(2,1)) were good to excellent across all parameters and test conditions for the pooled population and for elderly (0.74-0.92 and 0.62-0.88, respectively) except for the swing phase duration where lower values were seen (0.54-0.60 and 0.32-0.64 respectively). Values were similar under single and dual task conditions.
A voluntary step execution test, performed under single and dual task conditions especially foot-off and foot-contact times, is a reliable outcome measure that may be a useful tool to asses dynamic balance function for diagnostic purposes as well as clinical intervention trials.
Journal of NeuroEngineering and Rehabilitation 02/2007; 4:16. · 3.26 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Abstract
Background
The current study investigated the repeatability (test-retest reliability) of ground reaction force parameters recorded during a voluntary step execution under single (motor task) and dual task (motor and cognitive task) conditions for healthy adults and elderly individuals as well as the number of trials required to produce repeatable results.
Methods
Twenty-four healthy adults (21–63 years old) and 16 elderly adults (66–87 years) performed a voluntary rapid step execution following a tap on their heel while standing on a force platform under single and dual task conditions on three separate occasions. The first two tests were performed 30–60 minutes apart and the third test was performed a week later. Variables analyzed from the ground reaction force data included onset latency of step initiation (initiation phase), preparation and swing phases, foot-off and foot-contact times.
Results
Intraclass correlation coefficients (ICC(2,1)) were good to excellent across all parameters and test conditions for the pooled population and for elderly (0.74–0.92 and 0.62–0.88, respectively) except for the swing phase duration where lower values were seen (0.54–0.60 and 0.32–0.64 respectively). Values were similar under single and dual task conditions.
Conclusion
A voluntary step execution test, performed under single and dual task conditions especially foot-off and foot-contact times, is a reliable outcome measure that may be a useful tool to asses dynamic balance function for diagnostic purposes as well as clinical intervention trials.
Journal of NeuroEngineering and Rehabilitation (JNER). 01/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: We report on a model of a prestressed nonlinear semiflexible polymer chain that links thermally driven dynamics to the creep behavior of living cells. Numerical simulations show that the chain's creep follows a power law with an exponent that decreases with increasing prestress. This is related to the propagation of free energy through the chain in response to stretching, where the propagation speed is regulated by the prestress via the chain's nonlinear elasticity. These results indicate that the main aspects of cell rheology are consistent with the dynamics of single polymer chains under tension.
Physical Review Letters 11/2006; 97(16):168101. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: One fundamental question in cell biology is what determines rheological properties of living cells. If the cytoskeletal distending stress is a key determinant of cell rheology, then modulating this stress by cell stretching should have a major effect on cell rheological properties. If not, then other mechanisms must play a major role. We developed a stretchable cell culture device that could rapidly stretch cells and thus generate passive mechanical stress within the cytoskeleton. This device was placed inside a magnetic cytometry system to measure the effect of stretching on rheological properties of cultured human airway smooth muscle cells. A gradual increase in cell distension caused a systematic increase in cell dynamic stiffness in a manner which was consistent with earlier observations where the active component of the distending stress was modulated pharmacologically. These findings provide strong evidence that the cytoskeletal distending stress is a key determinant of cell rheological properties.
Biochemical and Biophysical Research Communications 09/2004; 321(3):617-22. · 2.48 Impact Factor
