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We present a high throughput traction force assay fabricated with silicone rubber (PDMS). This novel assay is suitable for studying physical changes in cell contractility during various biological and biomedical processes and diseases. We demonstrate this method's utility by measuring a TGF-β dependent increase in contractility during the epithelial-to-mesenchymal transition.
Cellular contractility is essential in diverse aspects of biology, driving processes that range from motility and division, to tissue contraction and mechanical stability, and represents a core element of multi-cellular animal life. In adherent cells, acto-myosin contraction is seen in traction forces that cells exert on their substrate. Dysregulation of cellular contractility appears in a myriad of pathologies, making contractility a promising target in diverse diagnostic approaches using biophysics as a metric. Moreover, novel therapeutic strategies can be based on correcting the apparent malfunction of cell contractility. These applications, however, require direct quantification of these forces.
We have developed silicone elastomer-based traction force microscopy (TFM) in a parallelized multi-well format. Our use of a silicone rubber, specifically polydimethylsiloxane (PDMS), rather than the commonly employed hydrogel polyacrylamide (PAA) enables us to make robust and inert substrates with indefinite shelf-lives requiring no specialized storage conditions. Unlike pillar-PDMS based approaches that have a modulus in the GPa range, the PDMS used here is very compliant, ranging from approximately 0.4 kPa to 100 kPa. We create a high-throughput platform for TFM by partitioning these large monolithic substrates spatially into biochemically independent wells, creating a multi-well platform for traction force screening that is compatible with existing multi-well systems.
In this manuscript, we use this multi-well traction force system to examine the Epithelial to Mesenchymal Transition (EMT); we induce EMT in NMuMG cells by exposing them to TGF-β, and to quantify the biophysical changes during EMT. We measure the contractility as a function of concentration and duration of TGF-β exposure. Our findings here demonstrate the utility of parallelized TFM in the context of disease biophysics.
Acto-myosin contractility is an essential element of active cell mechanics, impacting cell behaviors from motility and proliferation to stem cell differentiation. In tissues, contractility drives activity from polar separation in embryogenesis, to airway constriction and cardiac activity. Critically, to generate tension, cells must first adhere to their extracellular environment. In doing so, this contractility generates traction forces on their surroundings. Traction Force Microscopy (TFM) has emerged in a multitude of forms as a way to quantify these forces from diverse cells under different conditions.
The field of TFM has seen an exceptional breadth of innovation and application, and the results have paved the way for new perspectives in biology, which incorporate mechanics and physical forces. Starting with wrinkling silicone substrates1, researchers have applied various techniques to measure cell traction forces. These approaches have been continuously improved and have now reached a level of resolution on the order of several microns2. However, one principal problem has emerged, which is the difficulty in creating substrates of suitably low moduli using the available silicones. To circumvent this problem, polyacrylamide was adopted as a replacement due to the ease of creating substrates on the order of 1-20 kPa3. We recently implemented very compliant silicones in TFM4, allowing us to fabricate the same range of moduli as polyacrylamide, but with the advantages of inert and robust silicone.
TFM approaches have enabled valuable mechano-biological discoveries, however, a persistent shortcoming is their complexity, often restricting their use to researchers in the engineering or physical sciences disciplines. This is due in large part to the detailed calibrations and challenging calculations that are required to quantify contractility. Another significant challenge is that TFM methods are largely low-throughput and therefore ill-suited to study many different conditions or populations simultaneously5. This has presented a bottleneck, which has hampered transfer of TFM from a specialist biophysics setting into broader biological and pharmacological applications.
We have recently developed a multi-well format TFM plate, which allows researchers to parallelize their TFM measurements for faster quantification of contractility metrics, while exploring the impact of different compounds and also using less reagents4. This methodology has broad utility in diverse mechanobiology studies, from evaluating the effects of compounds on cellular activity, to quantifying the contractile changes in differentiation or disease.
One area of biomedical research that will benefit greatly from TFM is the study of how physical cues impact the malignant phenotypes of cancer cells. Metastasis, responsible for 90% of cancer-related deaths, is characterized by cancerous cells leaving their original tumor site and colonizing a secondary site. For cells to migrate through tissue and pass in and out of the vascular system, they must radically change their shapes to squeeze through these physical barriers while generating substantial forces to pull their way along extracellular matrix or move between other cells. These forces are transmitted to the substrate through focal adhesion interactions2,3, and can be quantified using TFM. While cancers are biochemically exceptionally diverse, with an expanding repertoire of known mutations and protein changes, some common physical changes have been observed; in a variety of cancers, including breast, prostate, and lung cancers, metastatic cells have been shown to exert 2-3 times the traction forces of non-metastatic cells6,7,8. These results suggest that there may be a strong correlation between metastatic progression and the traction forces exerted by cells; however, the detailed time-dependent changes in contractility are difficult to examine.
The epithelial-to-mesenchymal transition (EMT) is a process whereby cells reduce adherens- and tight-junction mediated cell-cell adhesion, becoming more migratory and invasive. In addition to physiological functions that include wound healing and developmental processes, EMT is also a process exploited during metastasis, making it a useful model system to study this process. Using TGF-β, we can induce the EMT in murine mammary epithelial cells (NMuMG)9 to directly quantify the physical changes during this transformation, and characterize the time and dose-dependent effects of TGF-β on EMT and cell contractility. In this article, we demonstrate the utility of this approach by measuring the changes in contractility during an induced EMT.
NOTE: The following protocol will guide researchers in fabricating and using the multi-well TFM dish shown in Figure 1.
1. Preparation of PDMS silicone substrates
2. Surface functionalization
3. UV sterilization
4. Cell culture
5. Data acquisition
6. Image analysis
7. Bead synthesis
NOTE: The following protocol is based on the synthesis method described by Klein et al.10.
8. Rheology measurement protocol
NOTE: Rheology is not required for every researcher or experiment, but is necessary to quantify the moduli for new formulations of PDMS. In this protocol, we employ a shear rheometer to measure the effects of crosslinker, frequency, and strain on moduli of PDMS samples. Depending on the available tools and expertise, moduli may also be measured using many other mechanical analysis approaches. Additionally, researchers using this protocol may elect to use our published moduli presented in Table 1, Figure 3 and Figure 4.
Before addition of TGF-β, a confluent monolayer of cells has a cobblestone like shape and is tightly packed. Upon TGF-β treatment, cells become more elongated in morphology, enlarging the cell area and acquiring a more mesenchymal phenotype. Utilizing the multi-well device fabricated with soft PDMS elastomers, the physical properties of cells in a total 17 different conditions were studied. The cells were treated with four different TGF-β concentrations (0.5, 1, 2, and 4 ng...
For the success of this method, it is critical to have a uniformly coated sample with a constant thickness of approximately 100 µm. The modulus should be carefully chosen to examine the physical significance of the biological system of interest. When fabricating a top layer, the concentration of the fiducial fluorescent particles should be optimized for accurate analysis of displacement and traction stress. Analyzing isolated single cells requires a denser fiduciary layer than measuring confluent monolayers. Additio...
AJE and RK have interest in Live Cell Technologies, a company which fabricates materials described in this article.
The authors thank Tom Kodger, Michael Landry, and Christopher J. Barrett for assistance with bead synthesis. A.J.E. acknowledges Natural Sciences and Engineering Research Council grants RGPIN/05843-2014 and EQPEQ/472339-2015, Canadian Institutes of Health Research grant no. 143327, Canadian Cancer Society grant no. 703930, and Canadian Foundation for Innovation Project #32749. R. Krishnan acknowledges National Institutes of Health grant no. R21HL123522 and R01HL136209. H.Y. was supported by Fonds de recherche Santé Québec, and Fonds de recherche Nature et Technologies Québec. The authors thank Johanan Idicula for assistance with the video and manuscript and Zixin He for assistance in preparing the video.
Name | Company | Catalog Number | Comments |
Plate | |||
GEL-8100 | Nusil Technology | GEL-8100 | High Purity Dielectric, Soft Silicone Gel kit |
Dow Corning Sylgard 184 Silicone Encapsulant Clear 0.5 kg Kit | Ellsworth Adhesives | 184 SIL ELAST KIT 0.5KG | curing agent |
Custom Cut Glass | Hausser Scientific Company | 109.6mm± x 72.8mm± x 1mm thickness | |
Target 2TM Nylon Syringe Filter | ThermoFisher Scientific | F2513-4 | |
96-well Stripwell Egg Crate Strip Holder | Corning | 2572 | |
Polystyrene Universal Microplate Lid With Corner Notch | Corning | 3099 | |
Ethyl alcohol | Greenfield Global | P016EA95 | 0.95 |
2-Propanol | Sigma-Aldrich | 190764 | ACS reagent, ≥99.5% |
Surface Coating | |||
Sulfo-SANPAH Crosslinker | Proteochem | c1111-100mg | |
Fibronectin bovine plasma | Sigma-Aldrich | F1141-1MG | solution, sterile-filtered, BioReagent, suitable for cell culture |
PBS, 1X | Wisent | 319-005-CL | pH 7.4, without calcium and magnesium |
DMSO | Sigma-Aldrich | 472301 | |
Cell Culture | |||
DMEM, 1X | Wisent | 319-005-CL | 4.5g/L glucose, with L-glutamine, sodium pyruvate and phenol red |
FBS (Fetal Bovine Serum) | Wisent | 080-150 | Premium Quality, Endotoxin <1, Hemoglobin <25 |
HEPES | Wisent | 330-050-EL | 1M, free acid |
Human Insulin Recombinant | Wisent | 511-016-CM | USP grade |
Penicillin-Streptomycin Solution | Wisent | 450-201-EL | 100 X, sterile filtered for cell culture |
L-Glutamine solution | Wisent | 609-065-EL | 200mM solution, sterile filtered for cell culture |
Amphotericine B | Wisent | 450-105-QL | 250μg/ml, sterile filtered for cell culture |
Recombinant Human TGF-β1 | Peprotech | 100-21 | HEK293 Derived |
Acetic acid | Sigma-Aldrich | 537020 | Glacial, ≥99.85% |
Cictric acid | Sigma-Aldrich | 251275 | ACS reagent, ≥99.5% |
NMuMG | ATCC | CRL-1636 | Mouse Mammary Gland Cell Line |
Sodium azide | Fisher Schientific | AC190385000 | 99%, extra pure, ACROS Organics |
Potassium hydroxide | Sigma-Aldrich | 221473 | ACS reagent, ≥85%, pellets |
TritonX-100 | Sigma-Aldrich | X100 | laboratory grade |
Bead Synthesis | |||
1,1′-Dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI) | Sigma-Aldrich | 468495-100MG | 97% |
Methyl methacrylate | Sigma-Aldrich | M55909-500ML | contains ≤30 ppm MEHQ as inhibitor, 99% |
Inhibitor Remover | Sigma-Aldrich | 306312-1EA | Prepacked column for removing hydroquinone and monomethyl ether hydroquinone |
Methacryloxylpropyl Terminated Polydimethylsiloxane | Gelest | DMS-R31 (25,000g/mol) | Polydimethylsiloxane stabilizer, 25,000g/mol, 1,000 cSt |
2,2′-Azobis(2-methylpropionitrile) (AIBN) | Sigma-Aldrich | 441090-25G | 98% |
Hexane | Sigma-Aldrich | 296090-2L | anhydrous, 95% |
Hexane, mixture of isomers | Sigma-Aldrich | 227064-1L | anhydrous, ≥99% |
Whatman qualitative filter paper, Grade 1 | Sigma-Aldrich | WHA1001055 | circles, diam. 55 mm, |
Equipment | |||
Laurell WS-650Mz-23NPPB | Laurell Technologies | ||
UVP Handheld UV Lamp Model UVGL-58 | VWR | 21474-622 | |
Rheometer | Anton Paar | MCR 302 WESP |
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