mechanical behavior in living cells consistent with the tensegrity model by simpson
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Mechanical Behavior in Living Cells Consistent with the Tensegrity Model
Ning Wang et al. PNAS 2001; 98:7765-7770
Presented by: David L. Simpson
ARTNews- February 1981
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Outline
• Background and Introduction
• Methods
• Results
• Strengths and Limitations
• Discussion Topics
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Current Models of Cell Mechanics
• Simple Mechanical Continuum
• Porous Filament Gel
• Tensed Cortical Membrane
• Tensegrity Model
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Tensegrity Model
• Continuous and intricate network of tensed and compressed filaments
• Deformability Depends on level of prestress
• Prestress dependent on active (actomyosin contractile apparatus) and passive (distension by cell adhesions to the ECM) elements
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Intent of Selected Paper
• Demonstrate that the cytoskeleton (CSK) behaves as a discrete network of filaments
• Demonstrate that microtubules behave as compression struts
• Demonstrate that CSK prestress is a major determinant of cell deformability
• Experimental confirmation of the a priori predictions of model
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Motivation for Study
• In vitro experimentation inadequate
• Limitations of other models suggest the need for a more accurate model which links mechanics to microstructure and molecular biochemistry
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Oscillatory Magnetic Twisting Cytometry
• Bead Rotation and oscillatory pattern resisted by intrinsic stresses developed by the cell
• Ratio of specific torque and angular rotation of bead define Complex Elastic Modulus a. G(f) = T(f)/φ(f) b. G(f) = G’(f) + G”(f) Real and imaginary parts
J. Appl. Physiol. Vol. 89, Issue 4, 1619-1632, October 2000
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Traction Force Microscopy
• Cells cultured on flexible ECM-coated, PA gels with fluorescent beads inside
• Traction at the cell-gel interfaced obtained by solving the problem of Boussinesq using a Fourier transform
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MLC Phosphorylation and Intracellular Calcium
• Immunoblotting with MLC mAbs
• Microfluormetric quantitation of intracelluar calcium
Fig 3. PNAS 2001; 98:7765-7770
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Internal Cell Deformation Depends on Molecular Connectivity
Closed circles: RGD Beads
Open circle: AcLDL Beads
Fig 1. PNAS 2001; 98:7765-7770
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Microtubules Bear Compression in Living Cells
Fig 2. PNAS 2001; 98:7765-7770
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Microtubules Bear Compression in Living Cells
Prestress is transferred to external adhesions which increases traction.
Fig 5. PNAS 2001; 98:7765-7770
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Quantitative Measurements Support the Tensegrity Model
Prestress as a function of histamine and colchicine concentration
Shear Modulus as a function of histamine concentration
Shear modulus as a function of prestress. Notice the linear relationship over specified range
Fig 4. PNAS 2001; 98:7765-7770
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An a priori Prediction
• Tensegrity predicts that the cells dynamic shear modulus can be decomposed into the product of a prestress-dependent component and a frequency-dependent component
• Models confirms prediction in that both the storage and loss moduli increase by 70% while hysteresivity coefficient remained constant with increased prestress (J. of Appl. Physiol. 2000; 89:1619-1632)
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Strengths and Limitations
• Model is able to accurately model a priori prediction offered by the authors
• Models and defines prestress as a major player in cell mechanics
• Tensegrity does not consider viscoelastic forces, thermal motions and polymer. Dynamics
• Authors find ways to test there predictions buy offer no alternatives to observations– For example, the mitochondria moves away from and toward
induced forced– Is it possible that signaling plays a role as well as polymerization
dynamics• Focus of this paper is mainly surrounded around
microtubules. Not much on other the CSK elements
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Let’s Discuss
• Let’s look at the most obvious: Is tensegrity a reasonable model?– Prestress is a major part of this model, is it accounted
for in other models?– Are there alternatives for the predictions verified by
this model?– Adam Smith may describe such a situation by the
distribution of labor• How would divide the tensegrity model• Could more than one model be combined to achieve a final
product
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• Critics to tensegrity indicate that only local responses occur w/ an induced forced– If an advocate, why might these responses be
occurring– If a critic why does this hurt the concept of tensegrity
• Referring to the famous engineer Rene Descartes, he found the concept of “seeking certainty in an uncertain world.” In other words always ask questions, so…ANY QUESTIONS?
Let’s Discuss
AND I’M SPENT…THANKS FOR YOUR ATTENTION