1. “Molecular Tension Sensors Report Forces
Paper Introduction
“Molecular Tension Sensors Report Forces
Generated by Single Integrin Molecules
in Living Cells”
Masatoshi Morimatsu, Armen H. Mekhdjian,
Arjun S. Adhikari, and Alexander R. Dunn*
Nano Lett. 2013, 13, 3985.
Yoshimitsu Sagara

2. Significance of Evaluation of Mechanical Force
Generated by Integrines in Living Cells
Disruption in the ability of cells to sense the mechanical properties of their surroundings represents a causes of many diseases.
One of the most prominent is through integrins, heterodimeric transmembrane proteins that cluster into micrometer-sized assemblies called focal adhesions (FAs). FAs link the cell cytoskeleton to the surrounding extracellular matrix (ex. fibronectin ).
Force transmission through integrins is essential for cell migration and adhesion, while force sensing at FAs regulates numerous cellular processes including proliferation and differentiation.
The mechanisms by which integrins and their associated proteins both transmit and sense mechanical tension is the subject of intense interest.
However, it is very difficult to evaluate the values of the forces.

3. Overview of the Molecular Tension Sensor
Figure 1. MTS overview. (a) Sensors are site-specifically labeled with biotin (Avitag), the FRET donor Alexa 546 (KC), and FRET acceptor CoA 647 (ACP tag) and present the RGD sequence from fibronectin (TVYAVTGRGDSPASSAA). The (GPGGA)8 sequence acts as an entropic spring that is stretched upon application of force. (b) Sensor molecules are attached to a coverslip via biotin and Neutravidin; the biotinylated PEG brush prevents nonspecific cell and sensor attachment. Integrin heterodimers attach to the RGD domain and apply load generated by the cell cytoskeleton. (c) Immunofluorescence image of fixed human foreskin fibroblast cells seeded on a MTS-functionalized surface; note the prominent actin stress
fibers and FAs (blue: nucleus; red: actin; green: paxillin). Scale bar: 25 μm.

4. Effect of the Substrates
Supplementary Figure 2. Brightfield images of HFFs on various functionalized surfaces after 60 min incubation. HFFs spread well on surfaces coated with collagen or the MTS, but do not spread on PEG passivated coverslips or on coverslips functionalized with MTS probes lacking the RGD sequence. Red arrows indicate spread cells. Scale bar in all images is 30 μm.

5. Ensemble FRET Imaging
Figure 2. Ensemble FRET maps reveal regions of low FRET that colocalize with FAs. (a) Bright-field image of a spreading HFF on a MTSfunctionalized surface. (b) Corresponding FRET map showing areas of high force localized at the cell periphery. Color bar represents FRET index (blue: low FRET/high force; red: high FRET/low force). (c) Live-cell image of a HFF expressing eGFP-paxillin with adhesions outlined in red. (d)
Low FRET colocalizes with paxillin fluorescence. Scale bar: 10 μm.

6. Control Experiment Using Cytochalasin D
Supplemental Figure 8. Addition of 1 μM cytochalasin D, an actin depolymerizing agent, disrupts force at integrin complexes within 5 minutes. The black outline traces the low FRET regions prior to the addition of cytochalasin D. Scale bar in all images is 10 μm.

7. Mechanical Force Generated by Living Cells
Flipped Sensor
(Original)

8. Ensemble FRET Imaging
Figure 2. Ensemble FRET maps reveal regions of low FRET that colocalize with FAs. (a) Bright-field image of a spreading HFF on a MTSfunctionalized surface. (b) Corresponding FRET map showing areas of high force localized at the cell periphery. Color bar represents FRET index (blue: low FRET/high force; red: high FRET/low force). (c) Live-cell image of a HFF expressing eGFP-paxillin with adhesions outlined in red. (d)
Low FRET colocalizes with paxillin fluorescence. Scale bar: 10 μm.

9. Single Molecular FRET Imaging Single Molecualr FRET Imaging
Ensemble FRET Imaging 1
1000

10. Single Molecular FRET Imaging
Figure 3. Force exerted by single integrins. (a) HFFs spreading on a MTS-coated coverslip visualized in brightfield; individual molecules are clearly visible in the donor and acceptor channels. Scale bar: 10 μm. The outlined regions (i) and (ii) correspond to molecules exhibiting low and high FRET, and residing underneath the cell and outside the cell boundary, respectively. (b) Raw single-molecule FRET traces from molecules (i) and (ii). Two-step photobleaching indicates the presence of a doubly labeled sensor. (c) Cumulative probability distributions of FRET values measured for individual MTS molecules in the presence (green; 398 molecules, 13 cells) and absence of cells (black; 266 molecules).

11. Calculated Force Distribution (Conclusion)
Supplementary Figure 13. FRET versus force calibration curves calculated with Förster radii ranging from 5.0 to 8.0 nm show that even large errors in R0 lead to modest, single-pN changes in the calculated force.
Supplementary Figure 14. Force distribution for MTS molecules underneath the cell indicates that a population of molecules experiences tensions in the 1-5 pN range. Forces are calculated using the FRET-force calibration curve shown in Figure S13 with an R0 of 6.5 nm. FRET efficiencies greater than 47% are counted as being under no force (0 pN).
C. Grashoff et al. Nature 2010, 466, 263.
MTS experiences tensions of approximately 1−5 pN.

12. Thank you for your kind attention!