Name: imaging integrin tension and cellular force at submicron resolution with an integrative tension sensor
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Description: Watch this Scientific Journal Video about Imaging Integrin Tension and Cellular Force at Submicron Resolution with an Integrative Tension Sensor at JoVE.com

This work was supported by the startup fund provided by Iowa State University and by the National Institute of General Medical Sciences (R35GM128747).

All methods described here have been approved by the Institutional Animal Care and Use Committee (IACUC, 8-16-8333-I) of Iowa State University.
1. Synthesis of the integrative tension sensor
<ol>
	<li>Customize and order ssDNAs (see the Table of Materials). 
	NOTE: The ssDNA sequences are as follows. The upper strand is /5ThioMC6-D/GGG AGG ACG CAG CGG GCC/3BHQ_2/. The lower strands are as follows. 
	12 pN ITS: /5Cy3/GGC CCG CTG CGT CCT CCC /3Bio/ 
	23 pN ITS: ...

<ol>
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The ITS is a highly accessible yet powerful technique for cellular force mapping in terms of both synthesis and application. With all materials ready, the ITS can be synthesized within 1 day. During experiments, only three steps of surface coating are needed prior to cell plating. Recently, we further simplified the coating procedure to one step by directly linking the ITS to bovine serum albumin, which enables direct physical adsorption of the ITS to glass or polystyrene surfaces33. The ITS bring...

Cells rely on integrins to adhere and exert cellular forces to extracellular matrix. Integrin-mediated cell adhesion and force transmission are crucial for cell spreading1,2, migration3,4, and survival5,6,7. In the long term, integrin biomechanical signaling also influences cell proliferation8,9,10 and differentiation11,12. Researchers have developed various methods to measure and map integrin-transmitted cellular forces at the cell-matrix interface. These methods are based on elastic substratum13, array of micropost14, or atomic force microscopy (AFM)15,16. Elastic substratum and micropost methods rely on the deformation of substrates to report the cellular stress and have limitations in terms of spatial resolution and force sensitivity. AFM has high force sensitivity, but it cannot detect force at multiple spots simultaneously, making it difficult to map cellular force transmitted by integrins.
In recent years, several techniques were developed to study cellular force at the molecular level. A collection of molecular tension sensors based on polyethylene glycol17,18, spider silk peptide19, and DNA20,21,22,23 were developed to visualize and monitor tension transmitted by molecular proteins. Among these techniques, DNA was first adopted as the synthesis material in the tension gauge tether (TGT), a rupturable linker that modulates the upper limit of integrin tensions in live cells22,24. Later, DNA and fluorescence resonance transfer technique were combined to create hairpin DNA-based fluorescent tension sensors first by Chen&#8217;s group23 and Salaita&#8217;s group20. The hairpin DNA-based tension sensor reports integrin tension in real-time and has been successfully applied to the study of a series of cellular functions21. Afterward, Wang&#8217;s lab combined a TGT with the fluorophore-quencher pair to report integrin tension. This sensor is named an ITS25,26. The ITS is based on double-stranded DNA (dsDNA) and has a broader dynamic range (10-60 pN) for integrin tension calibration. In contrast to hairpin DNA-based sensors, the ITS does not report cellular force in real-time but records all historic integrin events as the footprint of cellular force; this signal accumulation process improves the sensitivity for cellular force imaging, making it feasible to image cellular force even with a low-end fluorescence microscope. The synthesis of ITS is relatively more convenient as it is created by hybridizing two single-stranded DNAs (ssDNA).
The ITS is an 18-base-paired dsDNA conjugated with biotin, a fluorophore, a quencher (Black Hole Quencher 2 [BHQ2])27, and a cyclic arginylglycylaspartic acid (RGD) peptide28 as an integrin peptide ligand (Figure 1). The lower strand is conjugated with the fluorophore (Cy3 is used in this manuscript, while other dyes, such as Cy5 or Alexa series, have also been proven feasible in our lab) and the biotin tag, with which the ITS is immobilized on a substrate by biotin-avidin bond. The upper strand is conjugated with the RGD peptide and the Black Hole Quencher, which quenches Cy3 with approximately 98% quenching efficiency26,27. With the protocol presented in this paper, the coating density of the ITS on a substrate is around 1,100/&#181;m2. This is the density we previously calibrated for 18 bp biotinylated dsDNA coated on the neutrAvidin-functionalized substrate by following the same coating protocol29. When cells adhere to the substrate coated with the ITS, integrin binds the ITS through RGD and transmits tension to the ITS. The ITS has a specific tension tolerance (Ttol) which is defined as the tension threshold that mechanically separates the dsDNA of the ITS within 2 s22. ITS rupture by integrin tension leads to the separation of the quencher from the dye that subsequently emits fluorescence. As a result, the invisible integrin tension is converted to a fluorescence signal and the cellular force can be mapped by fluorescence imaging.
To demonstrate the application of the ITS, we use fish keratocyte here, a widely used cell model for cell migration study30,31,32, CHO-K1 cell, a commonly used nonmotile cell line, and NIH 3T3 fibroblast. Coimaging of integrin tension and cell structures is also conducted.

<table>
	<tbody>
		<tr>
			<td>BSA-biotin</td>
			<td>Sigma-Aldrich</td>
			<td>A8549</td>
			<td></td>
		</tr>
		<tr>
			<td>Neutravidin</td>
			<td>Thermo Fisher Scientific</td>
			<td>31000</td>
			<td></td>
		</tr>
		<tr>
			<td>Streptavidin</td>
			<td>Thermo Fisher Scientific</td>
			<td>434301</td>
			<td></td>
		</tr>
		<tr>
			<td>upper strand DNA</td>
			<td>Integrated DNA Technologies</td>
			<td>N/A</td>
			<td>Customer designed. DNA sequence is shown in PROTOCOL section</td>
		</tr>
		<tr>
			<td>lower strand DNA</td>
			<td>Integrated DNA Technologies</td>
			<td>N/A</td>
			<td>Customer designed. DNA sequences are shown in PROTOCOL section.</td>
		</tr>
		<tr>
			<td>sulfo-SMCC</td>
			<td>Thermo Fisher Scientific</td>
			<td>A39268</td>
			<td></td>
		</tr>
		<tr>
			<td>Cyclic peptide RGD with an amine group</td>
			<td>Peptides International</td>
			<td>PCI-3696-PI</td>
			<td></td>
		</tr>
		<tr>
			<td>IMDM</td>
			<td>ATCC</td>
			<td>&#8206;62996227</td>
			<td></td>
		</tr>
		<tr>
			<td>FBS</td>
			<td>ATCC</td>
			<td>302020</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin</td>
			<td>gibco</td>
			<td>15140122</td>
			<td></td>
		</tr>
		<tr>
			<td>TCEP</td>
			<td>Sigma-Aldrich</td>
			<td>C4706</td>
			<td></td>
		</tr>
		<tr>
			<td>200 uL petri dish</td>
			<td>Cellvis</td>
			<td>D29-14-1.5-N</td>
			<td></td>
		</tr>
		<tr>
			<td>NanoDrop 2000</td>
			<td>Thermo Scientific</td>
			<td>N/A</td>
			<td>spectrometer</td>
		</tr>
		<tr>
			<td>SE410 Tall Air-Cooled Vertical Protein Electrophoresis Unit</td>
			<td>Hoefer</td>
			<td>SE410-15-1.5</td>
			<td>Device for electroporesis</td>
		</tr>
		<tr>
			<td>CHO-K1 cell line</td>
			<td>ATCC</td>
			<td>CCL-61</td>
			<td></td>
		</tr>
		<tr>
			<td>NIH/3T3 cell line</td>
			<td>ATCC</td>
			<td>CRL-1658</td>
			<td></td>
		</tr>
		<tr>
			<td>Anti-Vinculin Antibody</td>
			<td>EMD Millipore</td>
			<td>90227</td>
			<td>Primary antibody for vinculin immunostaining</td>
		</tr>
		<tr>
			<td>Goat anti-Mouse IgG (H+L) Superclonal Secondary Antibody, Alexa Fluor 488</td>
			<td>Invitrogen</td>
			<td>A28175</td>
			<td>Secondary antibody for vinculin immunostaining</td>
		</tr>
		<tr>
			<td>Alexa Fluor 647 Phalloidin</td>
			<td>Invitrogen</td>
			<td>A22287</td>
			<td></td>
		</tr>
		<tr>
			<td>Eclipse Ti</td>
			<td>Nikon</td>
			<td>N/A</td>
			<td>microscope</td>
		</tr>
	</tbody>
</table>

imaging integrin tension and cellular force at submicron resolution with an integrative tension sensor

Molecular tension transmitted by integrin-ligand bonds is the fundamental mechanical signal in the integrin pathway that plays significant roles in many cell functions and behaviors. To calibrate and image integrin tension with high force sensitivity and spatial resolution, we developed an integrative tension sensor (ITS), a DNA-based fluorescent tension sensor. The ITS is activated to fluoresce if sustaining a molecular tension, thus converting force to fluorescent signal at the molecular level. The tension threshold for ITS activation is tunable in the range of 10&#8211;60 pN that well covers the dynamic range of integrin tension in cells. On a substrate grafted with an ITS, the integrin tension of adherent cells is visualized by fluorescence and imaged at submicron resolution. The ITS is also compatible with cell structural imaging in both live cells and fixed cells. The ITS has been successfully applied to the study of platelet contraction and cell migration. This paper details the procedure for the synthesis and application of the ITS in the study of integrin-transmitted cellular force.

With the ITS, the integrin tension map of fish keratocytes was captured. It shows that a keratocyte migrates and generates integrin tension at two force tracks (Figure 2A). The resolution of the force map was calibrated to be 0.4 &#181;m (Figure 2B). High integrin tension concentrates at the rear margin (Figure 3A). The ITS also shows different specific patterns of different cells. A nonmotile cell, ...

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Imaging Integrin Tension and Cellular Force at Submicron Resolution with an Integrative Tension Sensor

Integrin tension plays important roles in various cell functions. With an integrative tension sensor, integrin tension is calibrated with picoNewton (pN) sensitivity and imaged at submicron resolution.

Molecular tension transmitted by integrin-ligand bonds is the fundamental mechanical signal in the integrin pathway that plays significant roles in many ...

This DNA-based integrative tension sensor, or ITS, converting force signal to fluorescence locally enables the direct imaging of cellular force with fluorescence microscopy. ITS has a high spatial resolution and a high sensitivity. The activation threshold is tunable, allowing selective image of integrin tension at different levels.After receiving the customized single-stranded DNA, or ssDNA, of interest, to conjugate RGD peptide to the thiolated ssDNA quencher, first add 100 microliters of 0.5-molar EDTA in 400 microliters of water to about 450 microliters of freshly prepared TCEP solution. Then, add 10 microliters of the TCEP-EDTA solution to 20 microliters of one-millimolar thiol DNA quencher in PBS for a 30-minute incubation at room temperature. To conjugate RGD amine with sulfo-SMCC, add 40 microliters of freshly prepared 23-millimolar sulfo-SMCC to 100 microliters of freshly prepared 11-millimolar RGD amine solution for a 20-minute incubation at room temperature.To conjugate RGD amine SMCC with the thiolated ssDNA quencher, mix the entire volume of the thiolated ssDNA quencher solution with the entire volume of the RGD amine SMCC solution for a one-hour incubation at room temperature. At the end of the incubation, transfer the solution to four-degree Celsius storage overnight. The next morning, mix three-molar sodium chloride with the thiol-conjugated DNA solution and minus 20-degree Celsius, chilled, 100%ethanol at a one-to-10-to-25 ratio, and place the reaction at minus 20 degrees Celsius for about 30 minutes.When the DNA has precipitated, collect the DNA by centrifugation, and resuspend the pellet in PBS for measurement of the DNA concentration on a spectrometer. For integrative tension sensor, or ITS, synthesis, mix the upper and lower strand DNA solutions at a 1.1-to-one-molar ratio, and incubate the reaction overnight at four degrees Celsius before placing aliquots of the mixture at minus 20 degrees Celsius for long-term storage. For ITS immobilization, coat a 29-millimeter-diameter, glass-bottom Petri dish with 200 microliters of 0.1-milligram-per-milliliter biotinylated bovine serum albumin, or BSA-biotin, for 30 minutes at four degrees Celsius.When the BSA-biotin has physically adsorbed on the glass surface, wash the dish three times with 200 microliters of fresh PBS per wash, followed by incubation with 200 microliters of 50 micrograms per milliliter of avidin protein for 30 minutes at four degrees Celsius. At the end of the incubation, wash the dish three times with PBS as demonstrated, and add 200 microliters of 0.1-micromolar ITS for another 30-minute incubation at four degrees Celsius. At the end of the incubation, wash the dish three times with PBS, leaving the last wash in the dish until the cell plating.To initiate a fish keratocyte culture, use a flat-tipped tweezer to pluck a piece of scale from a fish, and gently press the scale onto the well of an unmodified glass-bottom Petri dish, with the inner side of the scale contacting the glass. Wait for about 30 seconds for the fish scale to adhere to the glass surface of the dish before covering the scale with one millimeter of complete Iscove's Modified Dulbecco's Medium. Then, incubate the scale in a dark and humid box for six to 48 hours at room temperature.To plate the keratocytes onto the ITS surface, replace the medium from the keratocyte culture dish with 1X cell detaching solution for a three-minute incubation at room temperature. At the end of the incubation, replace the detaching solution with fresh complete medium, and dissociate the cells with gentle pipetting. Then, adjust the cells to the appropriate concentration for plating, and seed them onto the ITS coated dish for 30 minutes at room temperature before imaging.For quasi-real-time integrin tension imaging in live cells, transfer the cells to the stage of a fluorescence microscope, and select the 40 or 100 times objective. In the imaging software, set the exposure time to one second and the frame interval of 20 seconds to obtain a video of the ITS images. Then, use an appropriate image analysis software program to subtract a previous frame of the ITS video from a current frame to compute the ITS signal newly produced in the latest frame interval.The newly produced ITS signal represents the integrin tension generated in the latest frame interval, thereby reporting the cellular force in quasi-real-time manner. Here, integrin tension mapping demonstrates the induction of integrin tension at two force tracks during keratocyte migration. The resolution of the force map could then be calibrated to be 0.4 micrometers.High integrin tension concentrates at the rear margin in motile fish keratocytes. In nonmotile cells, a specific integrin tension pattern that is quite different from that observed in the fast migrating keratocyte is formed. Focal adhesions and the integrin tension of fish keratocytes can be co-imaged to evaluate the relationship between integrin tension and cell structure in cells of interest.With ITS, cell adhesion force becomes visible with a resolution and a sensitivity comparable to cell structural imaging. This technique paves the road for the study of a force-structure interplay in cells.

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