Single Cell Durotaxis Assay for Assessing Mechanical Control of Cellular Movement and Related Signaling Events

Summary

Mechanical forces are important for controlling cell migration. This protocol demonstrates the use of elastic hydrogels that can be deformed using a glass micropipette and a micromanipulator to stimulate cells with a local stiffness gradient to elicit changes in cell structure and migration.

Abstract

Durotaxis is the process by which cells sense and respond to gradients of tension. In order to study this process in vitro, the stiffness of the substrate underlying a cell must be manipulated. While hydrogels with graded stiffness and long-term migration assays have proven useful in durotaxis studies, immediate, acute responses to local changes in substrate tension allow focused study of individual cell movements and subcellular signaling events. To repeatably test the ability of cells to sense and respond to the underlying substrate stiffness, a modified method for application of acute gradients of increased tension to individual cells cultured on deformable hydrogels is used which allows for real time manipulation of the strength and direction of stiffness gradients imparted upon cells in question. Additionally, by fine tuning the details and parameters of the assay, such as the shape and dimensions of the micropipette or the relative position, placement, and direction of the applied gradient, the assay can be optimized for the study of any mechanically sensitive cell type and system. These parameters can be altered to reliably change the applied stimulus and expand the functionality and versatility of the assay. This method allows examination of both long term durotactic movement as well as more immediate changes in cellular signaling and morphological dynamics in response to changing stiffness.

Introduction

Over the past few decades, the importance of the mechanical properties of a cell’s environment has garnered increasing recognition in cell biology. Different tissues and extracellular matrices have different relative stiffnesses and, as cells migrate throughout the body, they navigate these changes, using these mechanical properties to guide them^{1,2,3,4,5,6,7}. Cells use the stiffness of a given tissue to inform their motile behavior....

Protocol

1. Fabrication of Deformable Polyacrylamide Hydrogels with Embedded Fluorescent Microspheres

NOTE: Directions describe polymerization of a 25 kPa hydrogel that is 22 μm in diameter and approximately 66 μm thick. Each or all of these parameters can be modified and directions to do so can be found in Table 1 and in the notes¹⁷.

1. Activation of glass-bottom dishes or coverslips
   1. Prepare the bind silane working solution for activation of a glass-bottom imaging dish or a coverslip that fits into a live-cell imaging chamber. Mix 950 μL o....

Results

By preparing micropipettes (Figure 1) and normalizing the force generation of the pulls (Figure 2 and Figure 3) as described above, optimal durotactic conditions have been identified for multiple cell lines. Using this technique, as outlined in Figure 4, both SKOV-3 ovarian cancer cells¹⁷ and Ref52 rat embryonic fibroblasts (Figure 5) move towar.......

Discussion

Demonstrated here is a repeatable, single-cell durotaxis assay that allows assessment of a cell’s ability to alter its migration behavior in response to acute mechanical cues. This technique can also be used in combination with fluorescence microscopy and appropriate fusion proteins or biosensors to examine subcellular signaling and cytoskeletal events within seconds of mechanical stimulation or over a longer timescale during durotactic movement. Understanding a cell’s relationship to its environment involves.......

Materials

Name	Company	Catalog Number	Comments
Acrylamide 40 %	National Diagnostic	EC-810
Ammonium Persulfate	Fisher	BP179-25
BD20A High frequency generator	Electro Technic Products	12011A	115 V - Handheld Corona Wand
Bind Silane (y-methacryloxypropyltrimethoxysilane) (	Sigma Aldrich	M6514
Bis-acrylamide 2%	National Diagnostic	EC-820
Borosilicate glass capillaries	World Precision Instruments	1B100-4
Branson 2510 Ultrasonic Cleaner	Bransonic		40 kHz frequency
Coarse Manipulator	Narshige	MC35A
DMEM	Corning	10-013-CV
DMEM without phenol red	Sigma Aldrich	D5030
Dual-Stage Glass Micropipette Puller	Narshige	PC-10
Epidermal Growth Factor	Peprotech	AF-100-15
Ethanol	Pharmco-aaper	111000200
Fetal Bovine Serum (Qualified One Shot)	Gibco	A31606-02
Fibronectin	EMD Millipore	FC010
Fluospheres Carboxylate 0.2 um	Invitrogen	F8810, F8807, F8811
Fugene 6	Roche	1815091	1.5 ug DNA / 6uL fugene 6 per 35mm dish
Glacial Acetic Acid	Fisher Chemical	A38SI-212
Glass Bottom Dish	CellVis	D60-60-1.5-N
Glass Coverslip	Electron Microscopy Sciences	72224-01	22 mm, #1.5
HCl	JT Baker	9535-03
Hellmanex III Special cleaning concentrate	Sigma Aldrich	Z805939	Used at 2% in ddH2O for cleaning coverslips
HEPES powder	Sigma Aldrich	H3375	Make 50mM HEPES buffer, pH 8.5
Intelli-Ray 400 Shuttered UV Flood Light	Uviton International	UV0338
Isopropanol	Fisher Chemical	A417-4
Microforge	Narshige	MF900
Micromanipulator	Narshige	MHW3
Mineral Oil	Sigma Aldrich	M5904
Nanopure Life Science UV/UF System	Barnstead	D11931	ddH2O
Nikon Eclipse Ti	Nikon
OptiMEM	Invitrogen	31985062
Parafilm M	Bemis Company, Inc	PM-992
PBS			139 mM NaCl, 2.5 mM KCl, 28.6 mM Na2HPO4, 1.6 mM KH2PO4, pH 7.4
Platelet Derived Growth Factor-BB (PDGF-BB)	Sigma Aldrich	P4056
Ref52			Rat embryonic fibroblast cell line; Culture in DMEM + 10% FBS
Ringer's Buffer			134 mM NaCl, 5.4 mM KCl, 1 mM MgSO4, 2.4 mM CaCl2, 20 mM HEPES, 5 mM D-Glucose, pH 7.4
SKOV-3	American Type Culture Collection		Culture in DMEM + 10% FBS
Sulfo-SANPAH	Covachem	12414-1
Tabletop Plasma Cleaner	Harrick Plasma	PDC-32G
TEMED	Sigma Aldrich	T9281-50