TISSUE ENGINEERING: Part A Volume 16, Number 9, 2010 ª Mary Ann Liebert, Inc. DOI: 10.1089/ten.tea.2010.0096
Biodegradable, Photocrosslinked Alginate Hydrogels with Independently Tailorable Physical Properties and Cell Adhesivity Oju Jeon, Ph.D.,1 Caitlin Powell,1 Shaoly M. Ahmed,1 and Eben Alsberg, Ph.D.1,2
Biocompatible polymers capable of photopolymerization are of immense interest for tissue engineering applications as they can be injected in a minimally invasive manner into a defect site and, then upon application of ultraviolet light, rapidly form hydrogels in situ. Cell adhesion interactions with a biomaterial are known to be important in regulating cell behaviors such as proliferation and differentiation. Therefore, we have covalently modified photocrosslinkable alginate with cell adhesion ligands containing the Arg-Gly-Asp amino acid sequence to form biodegradable, photocrosslinked alginate hydrogels with controlled cell adhesivity. This unique polymer system allows for independent modulation of the physical and biochemical signaling environment presented to cells. The physical properties of the hydrogels such as elastic moduli, swelling ratios, and degradation profiles were similar at the same crosslinking density regardless of the presence of adhesion ligands. Chondrocytes seeded on the surface of the adhesion ligand-modified hydrogels were able to attach and spread, whereas those seeded on unmodified hydrogels exhibited minimal adherence. Importantly, the adhesion-ligandmodified hydrogels enhanced the proliferation and chondrogenic differentiated function of encapsulated chondrocytes as demonstrated by increased DNA content and production of glycosaminoglycans compared to unmodified control hydrogels. This new photocrosslinkable, biodegradable biomaterial system in which the soluble (e.g., growth factors) and insoluble (e.g., cell adhesion ligands) biochemical signaling environment and the biomaterial physical properties (e.g., the elastic moduli) can be independently controlled may be a powerful tool for elucidating the individual and combined effects of these parameters on cell function for cartilage tissue engineering and other regenerative medicine applications.
Introduction
H
ydrogels, which are used extensively in biomedical applications as drug and gene delivery vehicles and tissue engineering scaffolds, are water-insoluble threedimensional (3D) networks formed via the crosslinking of water-soluble polymers. Hydrogels made from natural polymers such as alginate, chitosan, collagen, hyaluronate, and dextran are frequently used in tissue regeneration strategies because they are either components of or have similar macromolecular structure to constituents of natural tissue extracellular matrix (ECM). For example, ionically crosslinked alginate has great potential as a biomaterial for tissue engineering because it can form highly hydrated hydrogels that present a hospitable environment for transplanted cells and cellular infiltration. Unfortunately, due to the limited control over the loss of crosslinking ions,1 it is difficult to manipulate many of the physical properties of these gels, such as their mechanical properties and degra-
dation rates, which have been shown to be important parameters in regulating cell behavior and new tissue formation.2,3 To overcome these limitations, we recently reported on the development of new methacrylated alginate macromers that can be photopolymerized to form hydrogels with controllable mechanical properties, swelling ratios, and degradation rates.4 The photocrosslinked alginate hydrogels exhibit excellent cytocompatibility, as encapsulated cells within the material remain highly viable, and are promising as cell carriers for tissue engineering applications since macromer solutions containing cells can be injected minimally invasively into the target tissue defect and, then upon application of light, rapidly transformed into hydrogels in situ. However, due to the hydrophilic nature of alginate and resultant low protein adsorption, cells are unable to interact with the alginate via cell surface receptors.5 Since cell adhesion interactions with a biomaterial are important in directing cell behaviors (e.g., proliferation, migration, differentiation) and the development of new tissue,6,7
Departments of 1Biomedical Engineering and 2Orthopaedic Surgery, Case Western Reserve University, Cleveland, Ohio.
2915
2916 cell adhesion ligand-modified photopolymerized biomaterials have been widely used for regulating cell function in tissue engineering applications. A variety of synthetic [e.g., poly(ethylene glycol), poly(vinyl alcohol)], natural (e.g., alginate, hyaluronic acid, and chitosan), and hybrid photopolymerizable biomaterials have been modified with ArgGly-Asp (RGD)-containing peptide sequences and others to regulate cell–biomaterial interactions.8–19 There is a paucity of photopolymerizable biomaterial systems, however, that permit independent regulation of biopolymer physical and biochemical properties. In this study, photocrosslinkable alginate was covalently modified with a peptide containing the RGD sequence using standard carbodiimide chemistry to influence chondrocyte adhesion, spreading, proliferation, and differentiated function. To our knowledge this is the first report of a photocrosslinkable alginate hydrogel system with tunable biodegradation and mechanical properties that offers the additional capacity to indepently regulate its cell adhesive properties. The ultimate goal of this work was to engineer a photocrosslinkable, biodegradable biomaterial system in which the soluble (e.g., growth factors) and insoluble (e.g., cell adhesion ligands) biochemical signaling environment and the biomaterial physical properties (e.g., the elastic moduli) can be separately controlled so that their individual and combined effects on cell function may be elucidated. Materials and Methods Preparation of RGD-modified methacrylated alginate RGD-modified methacrylated alginate was synthesized in a two-step reaction utilizing standard carbodiimide chemistry (Fig. 1). Low-molecular-weight sodium alginate (37,000 g/mol) was prepared by irradiating Protanal LF 20/ 40 (196,000 g/mol; FMC Biopolymer, Philadelphia, PA) at a gamma dose of 5 Mrad. Twenty-five percent actual methacrylation of alginate carboxylic acid groups was performed as described previously.4 Methacrylated alginate solutions (1%, w/v) were prepared with 50 mM of 2-(N-morpholino)ethanesulfonic acid hydrate (Sigma, St. Louis, MO) buffer solution containing 0.5 M NaCl (Sigma) at pH 6.5, and sequentially mixed with N-hydroxysuccinimide (Sigma) and 1-ethyl-3-[3-(dimethylamino)propyl] carbodiimide (EDC; Sigma). The molar ratio of N-hydroxysuccinimide to EDC was 0.5:1.0, and the weight ratio of EDC to methacrylated alginate was 1.0:20.7. The Gly-Arg-Gly-Asp-Ser-Pro (Commonwealth Biotechnologies, Richmond, VA) amino acid peptide sequence was added to the methacrylated alginate solution at a weight ratio of 10 mg/g methacrylated alginate. After reacting for 24 h at 48C, the reaction was stopped by addition of hydroxylamine (0.18 mg/mL; Sigma), and the solution was purified by dialysis against ultrapure deionized water (diH2O) (MWCO 3500; Spectrum Laboratories, Rancho Dominguez, CA) for 3 days, treated with activated charcoal (0.5 mg/100 mL, 50–200 mesh; Fisher, Pittsburgh, PA) for 30 min, filtered (0.22 mm filter), and lyophilized. Control methacrylated alginate was prepared in the same manner but without the presence of peptide. Characterization of RGD-modified methacrylated alginate To verify the RGD modification of the methacrylated alginate, an 1H-nuclear magnetic resonance (1H-NMR)
JEON ET AL. spectra of RGD-modified methacrylated alginate was recorded. RGD-modified methacrylated alginate was dissolved in deuterium oxide (D2O Sigma) and placed in an NMR tube. The 1H-NMR spectra of the RGD-modified methacrylated alginate was recorded on a Varian Unity-300 (300 MHz) NMR spectrometer (Varian, Palo Alto, CA) using tetramethylsilane as an internal standard. To analyze the degree of peptide modification, a ninhydrin assay was performed. Briefly, 100 mg of RGD-modified methacrylated alginate was dissolved in 5 mL of 1 M sodium acetate buffer (pH 5), and 5 mg of ninhydrin reagent was added. The mixture was kept in boiling water for 20 min. After incubation, 75 mL of a diH2O/absolute ethanol mixture (1/1, v/ v) was added, and the reaction mixture was cooled to room temperature for 2 h in complete darkness. Ninhydrin reacted with free amino groups and created a water-soluble blue compound. The amount of free amino groups in the RGD-modified methacrylated alginate was determined by measuring the ultraviolet (UV) absorbance of the supernatant at 570 nm. Methacrylated alginate and glycine (Fisher) were used as the control and the standard, respectively. Photocrosslinking To fabricate RGD-modified or unmodified photocrosslinked alginate hydrogels, RGD-modified methacrylated alginate (0.2 g) or unmodified methacrylated alginate (0.2 g) was dissolved in Dulbecco’s modified Eagle’s medium with high glucose (DMEM; Sigma) or diH2O (10 mL) with 0.05% (w/v) photoinitiator (Irgacure D-2959; Sigma) for ultimate placement in DMEM or diH2O, respectively. The alginate solutions were injected between two glass plates separated by 0.75 mm spacers and photocrosslinked with 365 nm UV light (Model ENF-260C; Spectroline, Westbury, NY) at *1 mW/cm2 for 10 min to form the hydrogels. Photocrosslinked hydrogel disks were created using a 6-mmdiameter biopsy punch and placed in DMEM or diH2O for swelling and degradation studies, mechanical testing, and culture of cells on the hydrogel surfaces. Swelling and degradation of hydrogels The RGD-modified or unmodified photocrosslinked alginate hydrogels were lyophilized, and dry weights (Wi) were measured. Dried hydrogel samples were immersed in 50 mL of DMEM or diH20 and incubated at 378C to reach an equilibrium swelling state. The DMEM or diH20 was replaced every 3 days. Over the course of 8 weeks, samples were removed from the DMEM or diH20, and the swollen (Ws) hydrogel sample weights were measured. The swelling ratio (Q) was calculated as follows: Q ¼ Ws/Wi (N ¼ 3 for each time point). After weighing the swollen hydrogels, the samples were lyophilized and weighed (Wd). The percent mass loss was calculated as follows: [(Wi Wd)/Wi]100 (N ¼ 3 for each time point). Mechanical testing The elastic moduli of the RGD-modified and unmodified photocrosslinked alginate hydrogels were determined by performing constant strain rate compression tests using a Rheometrics Solid Analyzer (RSAII; Rheometrics, Piscataway, NJ) equipped with a 10-N load cell. The RGD-
PHOTOCROSSLINKED ALGINATE HYDROGEL WITH CONTROLLED CELL ADHESIVITY
2917
FIG. 1. (A) Schematic illustration for the preparation and photocrosslinking of RGD-modified methacrylated alginate. (B) 1 H-NMR spectra of RGD-modified methacrylated alginate in D2O. Proton peaks of the methacrylate and coupled peptide are labeled a, b, and c and identified in the text. Morphology of RGD-modified and unmodified photocrosslinked alginate hydrogels after 24-h equilibration in (C) DMEM and (D) deionized water. RGD, Arg-Gly-Asp; DMEM, Dulbecco’s modified Eagle’s medium; NHS, N-hydroxysuccinimide; EDC, 1-ethyl-3-[3-(dimethylamino)propyl] carbodiimide; UV, ultraviolet; GRGDSP, Gly-Arg-Gly-Asp-Ser-Pro. Color images available online at www.liebertonline.com/ten. modified and unmodified photocrosslinked alginate hydrogel disks were prepared as described in the photocrosslinking section and maintained in DMEM or diH2O at 378C. After 24-h incubation, swollen alginate hydrogel disks were punched once again to form 6mm diameter disks, their thickness was measured using calipers, and uniaxial, unconfined compression tests were performed on the hydrogel disks at room temperature using a constant crosshead speed of 5%/s. Elastic moduli of photocrosslinked alginate hydrogels were determined from the slope of stress versus strain plots, and limited to the first 5% of strain (N ¼ 3).
Cell culture on the alginate hydrogels Chondrocytes (passage number 2) isolated from bovine articular cartilage as previously reported20 were seeded on RGD-modified or unmodified photocrosslinked alginate hydrogel disks in DMEM (1 mL) containing 10% fetal bovine serum (FBS) at a seeding density of 1104 cells/cm2 in 24-well tissue culture plates and allowed to adhere for 4 h in a humidified incubator at 378C with 5% CO2. The RGDmodified or unmodified photocrosslinked alginate disks were then transferred to new plates containing fresh media (1 mL), and cultured. The viability and morphology of
2918
JEON ET AL.
FIG. 2. (A) Representative stress–strain curves and (B) elastic moduli in compression of RGD-modified and unmodified photocrosslinked alginate hydrogels after 24-h equilibration in DMEM. (C) Swelling ratios and (D) in vitro degradation of RGD-modified and unmodified photocrosslinked alginate hydrogels in DMEM. (E) Swelling ratios and (F) in vitro degradation of RGD-modified and unmodified photocrosslinked alginate hydrogels in deionized water. *p < 0.05 compared to unmodified control. adhered cells on the alginate disks were examined using a live/dead assay comprised of fluorescein diacetate (FDA; Sigma) and ethidium bromide (EB; Sigma). FDA stains the cytoplasm of viable cells green, whereas EB stains the nuclei of nonviable cells orange-red. The staining solution was freshly prepared by mixing 1 mL of FDA solution (1.5 mg/ mL of FDA in dimethyl sulfoxide; Research Organics, Cle-
veland, OH) and 0.5 mL of EB solution (1 mg/mL of EB in phosphate-buffered saline [PBS]) with 0.3 mL of PBS (pH 8). At predetermined time points, 20 mL of staining solution was added into each well and incubated for 3–5 min at room temperature, and then stained hydrogel–cell constructs were imaged using a fluorescence microscope (ECLIPSE TE 300; Nikon, Tokyo, Japan) equipped with
PHOTOCROSSLINKED ALGINATE HYDROGEL WITH CONTROLLED CELL ADHESIVITY
2919
FIG. 3. (A) Fluorescence photomicrographs of live (FDA, green) and dead (EB, orange-red) chondrocytes cultured in vitro for 2, 5, and 7 days on the surface of RGD-modified and unmodified photocrosslinked alginate hydrogels. (B) Fluorescence photomicrographs of live (FDA, green) and dead (EB, orange-red) bovine chondrocytes encapsulated and cultured in RGDmodified and unmodified photocrosslinked alginate hydrogels in vitro for 2, 4, and 6 weeks, and (C) quantification of DNA/ dried hydrogel weight and (D) GAG/DNA content in the constructs. The scale bar indicates 200 mm and all photographs were taken at the same magnification. *p < 0.05. FDA, fluorescein diacetate; EB, ethidium bromide; GAG, glycosaminoglycan. Color images available online at www.liebertonline.com/ten.
2920 a digital camera (Retiga-SRV; Qimaging, Burnaby, BC, Canada). Encapsulation of chondrocytes Chondrocytes (passage number 2) were photoencapsulated in RGD-modified or unmodified alginate hydrogels by suspension in RGD-modified or unmodified methacrylated alginate solution (2% [w/v] in DMEM) with 0.05% (w/ v) photoinitiator. The cell/macromer solutions (300 mL, 1107 cells/mL) were pipetted into 96-well tissue culture plates) and photocrosslinked with UV light for 10 min. The resulting hydrogel–cell constructs were removed from the wells, placed in new 24-well tissue culture plates with 1 mL of fresh DMEM containing 10% FBS, and cultured in a humidified incubator at 378C with 5% CO2 for 6 weeks. The viability of encapsulated chondrocytes in the photocrosslinked RGD-modified alginate hydrogels was investigated using the live/dead assay (N ¼ 3 for each time point). Images were obtained at a depth halfway into the hydrogels using a fluorescence microscope. Biochemical assays for DNA content and glycosaminoglycan production At each time point, hydrogel–cell constructs were removed from media, homogenized, and digested in 1 mL papain buffer solution (25 mg/mL papain [Sigma], 2 mM cysteine [Sigma], 50 mM sodium phosphate [Fisher], and 2 mM ethylenediaminetetraacetic acid [Fisher], pH 6.5, in nuclease-free water) at 658C for 3 h. Hoechst 33258 dye (0.1 mg/mL in nuclease-free water; Acros Organics, Morris Plains, NJ) was used for the DNA assay as previously described.21 Calf thymus DNA standards (Rockland Immunochemicals, Gilbertsville, PA) were prepared with 0– 4 mg/mL DNA in nuclease-free water. After the centrifugation of papain-digested samples, 100 mL of supernatant was mixed with 100 mL of the prepared dye solution. Fluorescence intensity of the dye-conjugated DNA solution was measured in 96-well plates on a plate reader (358 nm excitation and 452 nm emission; Safire, Tecan, Austria), and the DNA content was calculated from a standard curve generated with the calf thymus DNA. Glycosaminoglycan (GAG) content was measured using the standard dimethylmethylene blue (Sigma) assay in 96-well plates.22 In each well, 50 mL of supernatant was mixed with 250 mL of dye containing 16 mg/L dimethylmethylene blue and 3.04 g/L glycine (pH 1.5). The absorbance was read at 595 nm using the plate reader. Chondroitin-6-sulfate (Sigma) from shark cartilage was used to construct the standard curve. Encapsulation of chondrocytes and transforming growth factor-beta 1 An in vitro transforming growth factor-beta 1 (TGF-b1) release study was performed to examine the time course of
JEON ET AL. TGF-b1 release from photocrosslinked RGD-modified alginate hydrogels. TGF-b1 (0.75 mg; PeproTech, Rochy Hill, NJ) was added to RGD-modified methacrylated alginate solution (1.5 mL, 2% [w/v] in diH2O) with 0.05% (w/v) photoinitiator. After gently mixing for 5 min, aliquots (300 mL) of solution were placed in 96-well tissue culture plates and photocrosslinked as before for 10 min. Each photocrosslinked hydrogel (150 ng TGF-b1/hydrogel) was immerged in a 15mL centrifuge tube containing 10 mL PBS and incubated at 378C (N ¼ 5). At predetermined time points, the supernatant was withdrawn and fresh buffer was replenished. The amount of TGF-b1 in the supernatants was determined using an enzyme-linked immunosorption assay kit (Human TGFb1 Duoset; R&D Systems, Minnepolis, MN). TGF-b1-loaded unmodified photocrosslinked alginate hydrogels were used as a comparative control (N ¼ 5). RGD-modified and unmodified photocrosslinked alginate hydrogel–cell constructs containing TGF-b1 (100 ng/ hydrogel) were prepared in 96-well tissue culture plates as described above (300 mL, 1107 cells/mL), removed from the wells, placed in new 24-well tissue culture plates with 1 mL of fresh DMEM containing 10% FBS, and cultured in a humidified incubator at 378C with 5% CO2 for 6 weeks (N ¼ 3). As a control, hydrogel–cell constructs without TGF-b1 were cultured in DMEM containing 10% FBS and 10 ng/mL TGFb1 (N ¼ 3). The medium was changed every 3 days. At predetermined time points, live/dead, GAG, and DNA assays were performed as described above. Statistical analysis All quantitative data are expressed as mean standard deviation. Statistical analysis was performed with one-way analysis of variance with Tukey honestly significant difference post hoc test using Origin software (OriginLab, Northampton, MA). A value of p < 0.05 was considered statistically significant. Results and Discussion Preparation and characterization of cell adhesion peptide-modified methacrylated alginate While alginate hydrogels provide space and mechanical support for tissue regeneration, in their native form they do not provide a mechanism for encapsulated cells to interact and receive important signaling information via adhesion. To partially mimic the cell adhesion capacity of native ECM, specific cell adhesion ligands such as the ubiquitous cell adhesion peptide sequence RGD,5,23,24 which is present in numerous ECM molecules such as fibronectin, collagen, and laminin,25,26 may be chemically incorporated onto substrate surfaces and into biomaterial systems to promote cell– biomaterial interactions. In fact, ionically crosslinked alginate hydrogels covalently modified with RGD-containing peptides have been shown to promote proliferation of cells such
‰ FIG. 4. (A) Release profiles of TGF-b1 from RGD-modified and unmodified photocrosslinked alginate hydrogels over 3 weeks. (B) Fluorescence photomicrographs of live (FDA, green) and dead (EB, orange-red) cells and (C) GAG/DNA content of bovine chondrocytes encapsulated with TGF-b1 in RGD-modified and unmodified photocrosslinked alginate hydrogels cultured in DMEM containing 10% fetal bovine serum and encapsulated bovine chondrocytes without TGF-b1 in RGD-modified and unmodified photocrosslinked alginate hydrogels cultured in DMEM containing 10% fetal bovine serum and TGF-b1 for 2, 4, and 6 weeks. The scale bar indicates 200 mm and all photographs were taken at the same magnification. *p < 0.05 compared to 6 weeks in same group. TGF-b1, transforming growth factor-beta 1. Color images available online at www.liebertonline.com/ten.
2921
2922 as osteoblasts and chondrocytes and the formation of new bone,7,27 cartilage,27 and blood vessels.28 In this study, peptides containing the RGD sequence were covalently coupled onto the methacrylated alginate main chains to prepare the first reported photocrosslinkable and biodegradable alginate hydrogels with controlled cell adhesion as shown in Figure 1A. After alginate methacrylation, the remaining carboxylic acid functional groups along the alginate polymer backbone offered the potential for covalent modification with RGDcontaining cell adhesion ligands. 1H-NMR spectra of RGDmodified methacrylated alginate macromers (Fig. 1B) exhibit proton peaks that were newly formed by the reaction with the peptide Gly-Arg-Gly-Asp-Ser-Pro, which are located at d2.75 (from aspartic acid) and 1.7 (from arginine)29 (Fig. 1Bc). The proton peaks of vinyl methylene (Fig. 1Ba) and methyl (Fig. 1Bb) protons of 2-aminoethyl methacrylate (AEMA) are located at d6.2 and d5.7, and d1.9, respectively. The conjugated RGD concentration was 3.76 0.24 mg/g methacrylated alginate as measured by ninhydrin assay, which makes the peptide coupling reaction efficiency 37.6%. The gross morphologies of DMEM-equilibrated RGD-modified and unmodified photocrosslinked alginate hydrogel disks are exhibited in Figure 1C, and their mean diameters are 6.6 0.2 mm and 6.6 0.1 mm, respectively. There are no significant differences in gross morphology or size between the groups after 24 h equilibration. The gross morphologies of the diH2O-equilibrated RGD-modified and unmodified photocrosslinked alginate hydrogel disks are shown in Figure 1D, and their mean diameters are 10.1 0.6 mm and 8.9 0.5 mm, respectively. While there are no significant differences in gross morphology or size between the diH2Oequilibrated groups after 24 h equilibration, these hydrogels were significantly larger than the DMEM-equilibrated groups. This difference in size may be attributable to increased osmosis driven swelling in diH2O. Elastic moduli, swelling kinetics, and degradation of the RGD-modified and unmodified photocrosslinked alginate hydrogels To use this system to study the effects of controlled biomaterial cell adhesion on cell behavior as an independent variable, it is important to demonstrate that the adhesion ligand coupling modification minimally alters the biomaterial physical properties. To examine whether peptide modification has an effect on photocrosslinked hydrogel mechanical properties, constant strain-rate compression tests were performed after 24 h equilibration in DMEM. Representative stress–strain curves of the RGD-modified and unmodified photocrosslinked alginate hydrogels (Fig. 2A) are similar in shape. There was no significant difference in compressive modulus between the two groups (Fig. 2B). These results provide evidence that adhesion peptide modification of methacrylated alginate does not substantially affect the crosslinked structure of photocrosslinked alginate hydrogels because the compressive mechanical response of the hydrogels was unaltered. In addition, the swelling ratio changes and degradation profiles in DMEM and diH2O were measured to further evaluate whether RGD modification has an effect on the crosslinked structure of the alginate hydrogels. In DMEM, both RGD-modified and unmodified photocrosslinked alginate hydrogels showed rapid swelling for
JEON ET AL. the first day (Fig. 2C). The swelling of both groups then gradually increased over the course of 8 weeks. The hydrogels exhibited similar swelling kinetics for 4 weeks, and the swelling ratio of the RGD-modified photocrosslinked alginate hydrogels was only slightly higher than that of unmodified alginate hydrogels at 8 weeks. These results also indicate that RGD modification of methacrylated alginate does not substantially affect the macromolecular structure of photocrosslinked alginate hydrogel over time. The mass loss (%) of alginate hydrogels in DMEM over time was determined as a measure of degradation (Fig. 2D). The degradation of photocrosslinked RGD-modified alginate hydrogels was slightly faster than that of unmodified alginate hydrogels at 2 and 4 weeks, but there was no difference between the two groups at 8 weeks. In diH2O, both RGD-modified and unmodified photocrosslinked alginate hydrogels exhibited rapid swelling for the first day (Fig. 2E), increased up to 1 week, and then gradually decreased. Compared to the unmodified hydrogels, RGD-modified hydrogels exhibited greater swelling at 1 day and 1 week. The unmodified hydrogels swelled to a greater extent at 2 and 4 weeks. The RGD-modified hydrogels exhibited greater degradation than the unmodified hydrogels after 2 and 4 weeks in diH2O (Fig. 2F). However, both RGD-modified and unmodified alginate hydrogels were completely degraded by 6 weeks. Characterization of 2D and 3D chondrocyte cultures Bovine chondrocytes were seeded onto RGD-modified and unmodified photocrosslinked alginate hydrogels to evaluate if the peptide modification would enhance chondrocyte adhesion and spreading. Chondrocytes adhered to the surfaces of RGD-modified hydrogels by 4 h (data not shown) and exhibited substantial spreading at 5 and 7 days (Fig. 3A). Few chondrocytes seeded on the surfaces of unmodified hydrogels were able to adhere, and those that did remained rounded through 7 days. These results indicate that the chondrocyte cell adhesion and spreading were promoted by the immobilized adhesion ligands. Chondrocytes were then photoencapsulated within RGDmodified or unmodified photocrosslinked alginate hydrogels to provide a 3D culture environment that more closely resembles native cartilage tissue. To examine cell survival during the photocrosslinking process and culture, the viability of the photoencapsulated chondrocytes in the alginate hydrogels was evaluated using a live/dead assay. High cell viability was observed throughout both alginate hydrogel compositions for 6 weeks (Fig. 3B). Chondrocytes in native cartilage are located in lacunae surrounded by ECM.30 ECM–cell interactions promote chondrocyte aggregation,31 reduce the level of chondrocyte apoptosis,32 and are essential for chondrocyte proliferation, differentiation and survival.33,34 The DNA content of RGD-modified hydrogels was signficantly greater than that of unmodified hydrogels at 2 and 6 weeks, indicating that RGD modification promoted chondrocyte proliferation in alginate hydrogels (Fig. 3C). DNA content also significantly increased over time in both groups. The capacity of the new biomaterial system to enhance specific chondrogenic differentiated function, such as production of cartilage ECM, was then investigated. After 2 and 4 weeks of culture, chondrocytes that were photoencapsulated in the RGD-modified alginate hydrogels pro-
PHOTOCROSSLINKED ALGINATE HYDROGEL WITH CONTROLLED CELL ADHESIVITY duced significantly more GAG, one of the major constituents of cartilage ECM,35 normalized to DNA content than cells in the unmodified alginate hydrogels (Fig. 3D). No difference was present at 6 weeks. This result demonstrates that regulating chondrocyte–ECM interactions through controlled integrin adhesion ligand signaling promotes and accelerates the chondrogenic activity of chondrocytes encapsulated in the photocrosslinked alginate hydrogels. Importantly, this positive effect on chondrogenesis occurred in the absence of any specific soluble chondrogenic factors other than those present in the serum used. This finding corroborates other reports where hydrogels modified with RGD-containing ligands enhanced the chondrogenic activity of encapsulated chondrocytes without the exogenous addition of specific chondrogenic growth factors.27,36
Encapsulation of chondrocytes and TGF-b1 Growth factors, such as TGF-b1, a member of the TGF-b superfamily, are an important part of the soluble biochemical signaling environment that regulates chondrogenesis during development and promotes chondrocyte-specific cellular function and cartilaginous ECM production.37 When chondrocytes are cultured in a 3D environment (e.g., aggregate culture or in a hydrogel), TGF-b1 can stimulate synthesis of cartilaginous ECM components such as GAG and collagen type II.38,39 Therefore, the effect of adhesion ligand modification on the responsiveness of photoencapsulated chondrocytes to TGF-b1 delivered either exogenously in the cell culture media or released from the alginate hydrogel itself was investigated. TGF-b1 release profiles over 21 days were similar from both peptide-modified and unmodified hydrogels with most TGF-b1 released within the first 4 days (Fig. 4A). This demonstrates that the peptide modification did not affect TGF-b1 release. The photoencapsulated chondrocytes exposed to TGF-b1 in the alginate hydrogels exhibited high cell viability as observed throughout all construct compositions for 6 weeks using the live/dead assay (Fig. 4B). Chondrogenic activity of the chondrocytes as measured by GAG/DNA content revealed that TGF-b1, delivered in the media or from the hydrogels, promoted GAG production per cell (Fig. 4C). No significant difference in chondrogenic activity was revealed between cells encapsulated in unmodified or RGD-modified alginate hydrogels that were exposed to TGF-b1 either in the media or from the hydrogels at all time points. These results indicate that for the specific conditions examined in this study (i.e., peptide type and concentration, and TGF-b1 concentration), chondrogenic activity of the chondrocytes in the hydrogels was more strongly influenced by the presence of TGF-b1 than controlled ECM–cell interactions by RGD modification. Importantly, however, similar levels of GAG production were measured when growth factor was delivered via either manner at all time points, even though release of TGF-b1 from the alginate hydrogels is relatively rapid and the total amount released was less than that presented exogenously. This biomaterial system will permit future studies aimed at elucidating the relative roles of these insoluble and soluble biochemical signaling parameters individually and together on promoting chondrogenic specific differentiated function.
2923
Conclusions Herein we have engineered biodegradable, photocrosslinked alginate hydrogels with controlled cell adhesivity properties. Cell adhesive and nonadhesive photocrosslinked alginate hydrogels exhibited similar elastic moduli and swelling ratios over time, whereas the peptide-modified hydrogels degraded slightly faster than nonmodified hydrogels. Since peptides vary greatly in their chemical properties (e.g., hydrophilicity, charge, and secondary structure), future studies will determine if the hydrogel physical characterization results in this current report may be generalizable to other peptides. RGD modification promoted chondrocyte adhesion and spreading on the surface of the hydrogels, whereas minimal adhesion was supported on the unmodified hydrogels. In addition, chondrocytes encapsulated in these hydrogels retained high viability with RGDmodified alginate, significantly enhancing production of GAG for 2 and 4 weeks in the absence of soluble chondrogenic growth factors. Delivery of TGF-b1 from within the hydrogels or the surrounding media to chondrocytes promoted GAG production to similar levels irrespective of growth factor delivery route or presence of peptide modification. While in this study only a single type and concentration of adhesion peptide and growth factor were examined, the novel biomaterial system described is highly modular, and will allow for future studies using high throughput systems to examine the effects of multiple adhesion peptide types and concentrations, growth factor types and concentrations, and the degree of alginate methacrylation both separately and in combination on encapsulated cell behavior. This new photocrosslinkable biomaterial with controlled degradation and cell adhesive properties may find great utility in cartilage tissue engineering and other regenerative medicine applications. Acknowledgments The authors thank Rachel Manthe for technical assistance and acknowledge funding support from a New Scholar in Aging Award from the Ellison Medical Foundation, the Muskuloskeletal Transplant Foundation, and Biomedical Research and Technology Transfer Grant 08-081 from the Ohio Department of Development. Disclosure Statement No competing financial interests exist. References 1. Malafaya, P.B., Silva, G.A., and Reis, R.L. Natural-origin polymers as carriers and scaffolds for biomolecules and cell delivery in tissue engineering applications. Adv Drug Deliv Rev 59, 207, 2007. 2. Alsberg, E., Kong, H.J., Hirano, Y., Smith, M.K., Albeiruti, A., and Mooney, D.J. Regulating bone formation via controlled scaffold degradation. J Dent Res 82, 903, 2003. 3. Engler, A.J., Sen, S., Sweeney, H.L., and Discher, D.E. Matrix elasticity directs stem cell lineage specification. Cell 126, 677, 2006. 4. Jeon, O., Bouhadir, K.H., Mansour, J.M., and Alsberg, E. Photocrosslinked alginate hydrogels with tunable biodegradation rates and mechanical properties. Biomaterials 30, 2724, 2009.
2924 5. Rowley, J.A., Madlambayan, G., and Mooney, D.J. Alginate hydrogels as synthetic extracellular matrix materials. Biomaterials 20, 45, 1999. 6. Hsiong, S.X., Carampin, P., Kong, H.J., Lee, K.Y., and Mooney, D.J. Differentiation stage alters matrix control of stem cells. J Biomed Mater Res A 85A, 145, 2008. 7. Alsberg, E., Anderson, K.W., Albeiruti, A., Franceschi, R.T., and Mooney, D.J. Cell-interactive alginate hydrogels for bone tissue engineering. J Dent Res 80, 2025, 2001. 8. Akdemir, Z.S., Akcakaya, H., Kahraman, M.V., Ceyhan, T., Kayaman-Apohan, N., and Gungor, A. Photopolymerized injectable RGD-modified fumarated poly(ethylene glycol) diglycidyl ether hydrogels for cell growth. Macromol Biosci 8, 852, 2008. 9. Bryant, S.J., Nicodemus, G.D., and Villanueva, I. Designing 3D photopolymer hydrogels to regulate biomechanical cues and tissue growth for cartilage tissue engineering. Pharm Res 25, 2379, 2008. 10. Hern, D.L., and Hubbell, J.A. Incorporation of adhesion peptides into nonadhesive hydrogels useful for tissue resurfacing. J Biomed Mater Res 39, 266, 1998. 11. Mann, B.K., Gobin, A.S., Tsai, A.T., Schmedlen, R.H., and West, J.L. Smooth muscle cell growth in photopolymerized hydrogels with cell adhesive and proteolytically degradable domains: synthetic ECM analogs for tissue engineering. Biomaterials 22, 3045, 2001. 12. Mapili, G., Lu, Y., Chen, S.C., and Roy, K. Laser-layered microfabrication of spatially patterned functionalized tissueengineering scaffolds. J Biomed Mater Res B Appl Biomater 75B, 414, 2005. 13. Nuttelman, C.R., Tripodi, M.C., and Anseth, K.S. Synthetic hydrogel niches that promote hMSC viability. Matrix Biol 24, 208, 2005. 14. Poon, Y.F., Cao, Y., Zhu, Y.B., Judeh, Z.M.A., and Chan-Park, M.B. Addition of beta-malic acid-containing poly(ethylene glycol) dimethacrylate to form biodegradable and biocompatible hydrogels. Biomacromolecules 10, 2043, 2009. 15. Schmedlen, K.H., Masters, K.S., and West, J.L. Photocrosslinkable polyvinyl alcohol hydrogels that can be modified with cell adhesion peptides for use in tissue engineering. Biomaterials 23, 4325, 2002. 16. Schmidt, D.R., and Kao, W.J. Monocyte activation in response to polyethylene glycol hydrogels grafted with RGD and PHSRN separated by interpositional spacers of various lengths. J Biomed Mater Res A 83A, 617, 2007. 17. Weber, L.M., Hayda, K.N., Haskins, K., and Anseth, K.S. The effects of cell-matrix interactions on encapsulated betacell function within hydrogels functionalized with matrixderived adhesive peptides. Biomaterials 28, 3004, 2007. 18. Yeo, Y., Geng, W.L., Ito, T., Kohane, D.S., Burdick, J.A., and Radisic, M. Photocrosslinkable hydrogel for myocyte cell culture and injection. J Biomed Mater Res B Appl Biomater 81B, 312, 2007. 19. Leach, J.B., and Schmidt, C.E. Characterization of protein release from photocrosslinkable hyaluronic acid-polyethylene glycol hydrogel tissue engineering scaffolds. Biomaterials 26, 125, 2005. 20. Paige, K.T., Cima, L.G., Yaremchuk, M.J., Vacanti, J.P., and Vacanti, C.A. Injectable cartilage. Plast Reconstr Surg 96, 1390, 1995. 21. Kim, Y.J., Sah, R.L., Doong, J.Y., and Grodzinsky, A.J. Fluorometric assay of DNA in cartilage explants using Hoechst 33258. Anal Biochem 174, 168, 1988.
JEON ET AL. 22. Enobakhare, B.O., Bader, D.L., and Lee, D.A. Quantification of sulfated glycosaminoglycans in chondrocyte/alginate cultures, by use of 1,9-dimethylmethylene blue. Anal Biochem 243, 189, 1996. 23. Burdick, J.A., and Anseth, K.S. Photoencapsulation of osteoblasts in injectable RGD-modified PEG hydrogels for bone tissue engineering. Biomaterials 23, 4315, 2002. 24. Massia, S.P., and Hubbell, J.A. Covalent surface immobilization of Arg-Gly-Asp- and Tyr-Ile-Gly-Ser-Arg-containing peptides to obtain well-defined cell-adhesive substrates. Anal Biochem 187, 292, 1990. 25. Bernard, M.P., Myers, J.C., Chu, M.L., Ramirez, F., Eikenberry, E.F., and Prockop, D.J. Structure of a cDNA for the pro-alpha2 chain of human type-I procollagen—comparison with chick cDNA for proalpha2(I) identifies structurally conserved features of the protein and the gene. Biochemistry 22, 1139, 1983. 26. Pierschbacher, M.D., Ruoslahti, E., Sundelin, J., Lind, P., and Peterson, P.A. The cell attachment domain of fibronectin— determination of the primary structure. J Biol Chem 257, 9593, 1982. 27. Alsberg, E., Anderson, K.W., Albeiruti, A., Rowley, J.A., and Mooney, D.J. Engineering growing tissues. Proc Natl Acad Sci USA 99, 12025, 2002. 28. Yu, J., Gu, Y., Du, K.T., Mihardja, S., Sievers, R.E., and Lee, R.J. The effect of injected RGD modified alginate on angiogenesis and left ventricular function in a chronic rat infarct model. Biomaterials 30, 751, 2009. 29. Mayo, K.H., Fan, F., Beavers, M.P., Eckardt, A., Keane, P., Hoekstra, W.J., and AndradeGordon, P. NOE-derived conformation of GRGDSP cell adhesion recognition site in the presence of SDS micelles and integrin receptor GPIIB/IIIA. Biochim Biophys Acta 1296, 95, 1996. 30. Chiang, H., Kuo, T.F., Tsai, C.C., Lin, M.C., She, B.R., Huang, Y.Y., Lee, H.S., Shieh, C.S., Chen, M.H., Ramshaw, J.A., Werkmeister, J.A., Tuan, R.S., and Jiang, C.C. Repair of porcine articular cartilage defect with autologous chondrocyte transplantation. J Orthop Res 23, 584, 2005. 31. Cao, L., Lee, V., Adams, M.E., Kiani, C., Zhang, Y.O., Hu, W., and Yang, B.B. Beta(1)-integrin-collagen interaction reduces chondrocyte apoptosis. Matrix Biol 18, 343, 1999. 32. Shakibaei, M., Schulze-Tanzil, G., de Souza, P., John, T., Rahmanzadeh, M., Rahmanzadeh, R., and Merker, H.J. Inhibition of mitogen-activated protein kinase kinase induces apoptosis of human chondrocytes. J Biol Chem 276, 13289, 2001. 33. Svoboda, K.K.H. Chondrocyte-matrix attachment complexes mediate survival and differentiation. Microsc Res Tech 43, 111, 1998. 34. Grashoff, C., Aszodi, A., Sakai, T., Hunziker, E.B., and Fassler, R. Integrin-linked kinase regulates chondrocyte shape and proliferation. EMBO Rep 4, 432, 2003. 35. Comper, W.D., and Laurent, T.C. Physiological function of connective-tissue polysaccharides. Physiol Rev 58, 255, 1978. 36. Park, K.M., Joung, Y.K., Park, K.D., Lee, S.Y., and Lee, M.C. RGD-conjugated chitosan-pluronic hydrogels as a cell supported scaffold for articular cartilage regeneration. Macromol Res 16, 517, 2008. 37. Goldring, M.B., Tsuchimochi, K., and Ijiri, K. The control of chondrogenesis. J Cell Biochem 97, 33, 2006.
PHOTOCROSSLINKED ALGINATE HYDROGEL WITH CONTROLLED CELL ADHESIVITY 38. Goldberg, A.J., Lee, D.A., Bader, D.L., and Bentley, G. Autologous chondrocyte implantation—culture in a TGFbeta-containing medium enhances the reexpression of a chondrocytic phenotype in passaged human chondrocytes in pellet culture. J Bone Joint Surg Br 87B, 128, 2005. 39. Lee, J.E., Kim, K.E., Kwon, I.C., Ahn, H.J., Lee, S.H., Cho, H.C., Kim, H.J., Seong, S.C., and Lee, M.C. Effects of the controlled-released TGF-beta 1 from chitosan microspheres on chondrocytes cultured in a collagen/ chitosan/glycosaminoglycan scaffold. Biomaterials 25, 4163, 2004.
2925
Address correspondence to: Eben Alsberg, Ph.D. Departments of Biomedical Engineering and Orthopaedic Surgery Case Western Reserve University 10900 Euclid Ave. Cleveland, OH 44106 E-mail:
[email protected] Received: February 12, 2010 Accepted: April 23, 2010 Online Publication Date: June 21, 2010
