Photopatterned Hydrogels to Investigate the Endothelial Cell

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Photopatterned Hydrogels to Investigate Endothelial Cell Response to Matrix Stiffness Heterogeneity Marsha C. Lampi, Murat Guvendiren, Jason A. Burdick, and Cynthia A. Reinhart-King ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.6b00633 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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ACS Biomaterials Science & Engineering

Photopatterned Hydrogels to Investigate the Endothelial Cell Response to Matrix Stiffness Heterogeneity Marsha C. Lampi,# Murat Guvendiren,+ Jason A. Burdick,§ and Cynthia A. Reinhart-King¶* #

Meinig School of Biomedical Engineering, Cornell University, 101 Weill Hall, 526 Campus

Road, Ithaca, New York 14850, United States +

Otto H. York Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of

Technology, 240 York Center, Newark, New Jersey 07102, United States §

Department of Bioengineering, University of Pennsylvania, 240 Skirkanich Hall, 210 S. 33rd

Street, Philadelphia, Pennsylvania 19104, United States ¶

Department of Biomedical Engineering, Vanderbilt University, 2414 Highland Avenue, 440

Engineering Sciences Building, Nashville, TN, 37240, United States

ABSTRACT

Age-related intimal stiffening is associated with increased endothelium permeability, an initiating step in atherosclerosis. Notably, in addition to a bulk increase in matrix stiffness, the aged intima also exhibits increased spatial stiffness heterogeneity. We investigate the effect of

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heterogeneous matrix stiffness on endothelial cells. Methacrylated hyaluronic acid hydrogels are fabricated and photopatterned to create substrates with 50 and 100 µm squares containing soft and stiff matrix regions of 2.7 ± 0.7 and 10.3 ± 3.9 kPa. On the patterned matrices, endothelial cells integrate subcellular matrix stiffness cues at stiffness interfaces and focal adhesions are increased in the cell body adhered to stiff matrix regions. Increased matrix stiffness heterogeneity disrupts cell-cell junctions in confluent endothelial monolayers. Together, this work indicates the spatial presentation of matrix mechanical cues, in addition to bulk substrate compliance play a role in governing endothelial single cell and monolayer behaviors.

KEYWORDS: photopatterning, matrix stiffness, atherosclerosis, endothelium, VE-cadherin

1. INTRODUCTION Extracellular matrix stiffening is associated with a diverse array of pathologies including cancer, diabetes, and cardiovascular diseases, and matrix mechanical cues can alter cellular behaviors to contribute to disease progression.1–5 Increased bulk artery stiffness is a hallmark of both aging and atherosclerosis, and is used as a clinical marker to assess cardiovascular health.6,7 Notably, macro-scale measurements of arterial stiffness do not reflect the cell-scale matrix mechanics of the microenvironment where endothelial cells reside lining the arterial intima lumen and act in part, as a semi-permeable barrier that maintains vascular homeostasis.8 Compromised endothelial adherens junctions that contribute to increased endothelial permeability and accumulation of LDL cholesterol in the intima, are initiating steps in atherosclerosis.9,10 Endothelial junction integrity relies on a critical balance between tensional forces and Rho family GTPase activity to promote the formation of stable junctional complexes without leading to their disassembly.11–16


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Evidence that intimal stiffening precedes cardiovascular disease, combined with prior reports that increased matrix stiffness elevates cell contractility and stress fiber formation, disrupts endothelial cell-cell junctions, and inhibits endothelium barrier recovery suggest that extracellular matrix mechanics may play a critical role in atherogenesis.12,17–23 In addition to increased average intima stiffness with age, ex vivo mechanical characterization of murine aortas revealed that aged vessels also exhibit increased mechanical spatial heterogeneity within the intima over an area where a single endothelial cell would reside.24,25 Within a 100 by 100 µm area, stiffness ‘hotspots’ were detected and intima stiffness increased as much as 50-fold from 2 to 100 kPa. Because focal adhesions individually probe the extracellular environment and in vitro models have shown that cells can discern subcellular differences in matrix compliance that alter the cell polarization, local traction stresses, and durotactic behaviors, we hypothesize that endothelial cells in vivo are integrating distinct matrix mechanical cues from their extracellular environment that alter junction integrity.26–29 Although increased bulk matrix stiffness is recognized to compromise endothelium integrity, the role of spatially heterogeneous matrix mechanics on endothelial cells remains unknown and is important to understanding how the altered mechanical properties of the intima associated with aging contribute to a disrupted endothelium. Here, we used methacrylated hyaluronic acid hydrogels that undergo sequential polymerization to fabricate and characterize substrates with regions of patterned crosslinking.30,31 Motivated by the increased spatial heterogeneity and abrupt increases in extracellular matrix stiffness within the aged arterial intima, these stiffness patterned substrates were then used to study the endothelial cell response to step-wise spatial changes in matrix stiffness. Based on the location of focal adhesion formation, within an individual endothelial cell, there was a local response to


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multiple matrix stiffness cues. Furthermore, matrix stiffness heterogeneity affected monolayer formation and disrupted cell-cell junction integrity. Average intercellular gap size increased with increased matrix stiffness heterogeneity, and the greatest cell-cell junction disruption occurred at stiffness interfaces. These findings suggest the spatial presentation of extracellular matrix mechanical cues, in addition to the average bulk modulus contribute to atherogenesis caused by age-related intimal stiffening.are important to understand how age-related intima stiffening contributes to atherogenesis.

2. MATERIALS AND METHODS 2.1 Methacrylated Hyaluronic Acid Synthesis Methacylated hyaluronic acid (MeHA) was synthesized as described previously.31,32 Sodium hyaluronate (51 kDa, Lifecore Biomedical, Chaska, MN) was dissolved in DI water at 1 wt% and reacted with 2.4 mL methacrylic anhydride (Sigma-Aldrich, St. Louis, MO) per gram HA on ice at pH 8.0 for 8 hours. The reaction mixture was incubated overnight at 4 °C followed by the introduction of an additional 1.2 mL methacrylic anhydride per gram HA on ice at pH 8.0 for 4 hours. The reaction product was dialyzed against DI water (6-8 kDa MWCO, SpectraPor, Rancho Dominguez, CA) and lyophilized. MeHA modification was determined to be ~100% by 1

H NMR (600 MHz Varian).

2.2 Photopatterned MeHA Gel Fabrication MeHA hydrogels were synthesized as described previously.30,31 Briefly, substrates were fabricated by dissolving the synthesized MeHA polymer in 0.2 M trioethanolamine (Sigma-


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Aldrich, St. Louis, MO) in PBS, pH 10.25 at 3 wt%. A custom synthesized RGD cell-binding peptide, GCGRGDSPG (Genscript, Piscataway, NJ) was added to the polymer solution at a final concentration of 1 mM and reacted for 45 minutes. Gels were then polymerized between two glass coverslips by Michael-type addition using dithiothreitol (DTT) (Sigma-Aldrich, St. Louis, MO) to consume 13% of the available methacrylates. Polymerized hydrogels were incubated overnight at room temperature in PBS prior to use or characterization. These hydrogels were used as uniform soft controls. To fabricate substrates with stiffness patterns, polymerized hydrogels were incubated in solutions of 0.05 wt% Irgacure 2959 (BASF, Overland Park, KS) photoinitiator and 200 µM methacrylated rhodamine (Polysciences, Inc., Warrington, PA ) in PBS for 30 minutes at 37 °C. Custom 24,500 DPI photomasks (CAD/Art Services, Bandon, OR) were then overlaid on the gel surface followed by immediate exposure to 320-500 nm collimated UV light at an intensity of 10 mW/cm2 for 4 minutes using an Omnicure S1500 (Excelitas Technologies, Waltham, MA). Gels were thoroughly rinsed with PBS prior to cell studies to remove excess photoinitiator and unreacted methacrylated rhodamine. Uniform stiff control hydrogels were fabricated by substituting the patterned photomask with a clear film of the same material. 2.3 Atomic Force Microscopy Hydrogel elastic moduli of uniform gels were measured using an Asylum MFP-3D (Asylum Research, Santa Barbara, CA) AFM in contact mode at locations distributed throughout the gel surface. Elastic moduli of 50 and 100 µm patterned gels were measured in 10 µm increments over 100 or 200 µm distances respectively, perpendicular to stiffness interfaces. The gel surface was indented with a 1.0 µm diameter spherical silicon dioxide particle on a silicon nitride


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cantilever with a manufacturer pre-calibrated spring constant of 0.06 N/m (Novascan Technologies, Ames, IA).33 The resulting force vs. indentation curves were fit to the Hertz model assuming a Poisson’s ratio of 0.5 to extract the Young’s modulus using the Asylum software.34 Measurements taken at the interface point were excluded from the mean stiffness values calculated for the soft and stiff patterned regions. 2.4 Patterned Hydrogel Characterization Stiffness pattern fidelity was assessed using a Zeiss LSM700 microscope using a 10x objective and the 568 nm excitation laser line to visualize the covalently bound rhodamine fluorophore in the stiffened gel regions. Color intensity profiles to distinguish soft and stiffened gel regions were obtained using ImageJ software (National Institutes of Health, Bethesda, MD). To visualize gel topographical features, phase contrast images of patterned hydrogels were acquired using a Zeiss Axio Observer.Z1m microscope using a 10x objective. For gel topography measurements, gels were incubated overnight at room temperature in 10 µM, 40 kDa FITC-dextran (Sigma-Aldrich, St. Louis, MO) in PBS and XZ confocal z-slices were acquired using a Zeiss LSM700 microscope using a 40x objective and the 488 nm excitation laser line. The distance between the highest and lowest z-positions at the gel surface within each field of view were measured using ImageJ. Because the width of the stripe pattern exceeded a single field of view, tile scans combining 5 images were used. 2.5 Cell Culture Bovine aortic endothelial cells (VEC Technologies, Rensselaer, NY) from passages 7-12 were used. Cells were maintained in Medium 199 (Invitrogen, Carlsbad, CA) supplemented with 10%


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Fetal Clone III serum (HyClone, Logan, UT), 1% MEM amino acids (Invitrogen), 1% MEM vitamins (MedTech, Manassas, VA), and 1% penicillin streptomycin (Invitrogen) at 37 °C and 5% CO2. 2.6 Immunofluorescence Endothelial cells were fixed with 3.2% paraformaldehyde (Electron Microscopy Sciences, Hatfield, PA) and permeabilized with 1% Triton (VWR, Radnor, PA). For focal adhesion studies, a mouse monoclonal vinculin primary antibody (Santa Cruz Biotechnology, Dallas, TX) and an Alexa Fluor 488 donkey anti-mouse secondary (Invitrogen) were used. Confocal z-stack images were acquired using a LSM700 microscope equipped with a 40x objective and the 488 nm and 568 excitation laser lines to visualize vinculin and the rhodamine marked stiffness patterns, respectively. For subconfluent monolayer studies, actin was visualized with Alexa Fluor 488 conjugated phalloidin (Invitrogen). Images were captured using a LSM700 microscope equipped with a 20x objective and the 488 nm and 568 excitation laser lines to visualize actin and the rhodamine marked stiffness patterns, respectively. For confluent monolayer studies, a goat polyclonal VE-cadherin primary antibody (Santa Cruz Biotechnology) was used with an Alexa Fluor 488 donkey anti-goat secondary antibody (Invitrogen). Actin was visualized using Alexa Fluor 350 phalloidin (Invitrogen). Fluorescent images were captured using a Zeiss Axio Observer.Z1m microscope with a 20x objective. 2.7 Quantification of Focal Adhesion Length and Area Focal adhesion length and area were quantified as described previously.13 Briefly, confocal zstack images of endothelial cells stained for vinculin and the rhodamine labeled stiffness patterns were captured. Vinculin images were converted to maximum intensity z-projections and


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subjected to an adaptive Wiener filter, a top-hat filter, and a median filter to extract focal adhesion structures using Matlab. Maximum intensity z-projections of the stiffness pattern images were converted to binary images to create a mask of the stiff hydrogel areas. The pattern mask was multiplied by the extracted focal adhesion image to quantify structures on the stiff substrate regions. To quantify focal adhesions on the soft substrate regions, a soft pattern mask was obtained from the complement image of the stiff pattern mask, and multiplied by the extracted focal adhesion image. 2.8 Quantification of Monolayer Intercellular Gap Size Intercellular gap size was quantified using ImageJ software and corresponding fluorescent images of VE-cadherin, actin, and the substrate stiffness pattern. Areas of endothelial cell-cell junction disruption were identified using the VE-cadherin images and the corresponding actin image was used to trace and measure the gap area. The corresponding stiffness patterned image was used to identify the location of the monolayer disruption.

2.8 2.9 Statistics Statistical analysis was conducted using GraphPad Prism Version 6.0d. All data in graphical form is presented as the mean ± standard error of the mean, and all numerical values in the text are presented as the mean ± standard deviation. Statistical significance was evaluated using Analysis of variance (ANOVA) with a Tukey’s Honestly Significant Difference (HSD) test for AFM and monolayer data. A two-tailed Student’s t-tests was used for focal adhesion data.


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Statistical significance was considered with a and p-value

Photopatterned Hydrogels to Investigate the Endothelial Cell

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