Defining the role of matrix compliance and proteolysis in three-dimensional cell spreading and remodeling

Abstract

Recent studies have identified extracellular matrix (ECM) compliance as an influential factor in determining the fate of anchorage-dependent cells. We explore a method of examining the influence of ECM compliance on cell morphology and remodeling in three-dimensional culture. For this purpose, a biological ECM analog material was developed to pseudo-independently alter its biochemical and physical properties. A set of 18 material variants were prepared with shear modulus ranging from 10 to 700 Pa. Smooth muscle cells were encapsulated in these materials and time-lapse video microscopy was used to show a relationship between matrix modulus, proteolytic biodegradation, cell spreading, and cell compaction of the matrix. The proteolytic susceptibility of the matrix, the degree of matrix compaction, and the cell morphology were quantified for each of the material variants to correlate with the modulus data. The initial cell spreading into the hydrogel matrix was dependent on the proteolytic susceptibility of the materials, whereas the extent of cell compaction proved to be more correlated to the modulus of the material. Inhibition of matrix metalloproteinases profoundly affected initial cell spreading and remodeling even in the most compliant materials. We concluded that smooth muscle cells use proteolysis to form lamellipodia and tractional forces to contract and remodel their surrounding microenvironment. Matrix modulus can therefore be used to control the extent of cellular remodeling and compaction. This study further shows that the interconnection between matrix modulus and proteolytic resistance in the ECM may be partly uncoupled to provide insight into how cells interpret their physical three-dimensional microenvironment.

FIGURE 1

Schematic illustration of the biomimetic materials used for controlling matrix modulus and proteolytic resistance when studying 3D cellular remodeling. Preparation of the material precursor involved conjugating poly(ethylene glycol) (PEG) molecules (10- and 20-kDa) to denatured fibrinogen to form a protein–polymer macromer. The PF macromers were cross-linked by a light activated free-radical reaction (photopolymerization) with or without additional 4-arm star PEG-multiacrylate cross-linker (denoted by +) to form the 3D matrix. The fibrinogen backbone ensures biodegradability and biological activity whereas the PEG provides control of the physicochemical properties.

FIGURE 2

The modulus of the biomimetic PF matrix can be precisely controlled through its composition. (A) On the left, the shear modulus profiles (G′) during photopolymerization (30 s into the time-sweep) illustrates the rapid liquid-to-solid transition of the materials. The arrow indicates the location on the graph where G′ and G″ intersect (G″ not shown). The time-sweep curves correspond to four PF materials (8 mg/ml) made with different precursor formulations. The plateau shear modulus value (G′) was used to describe the stiffness of each material. On the right, a comparative plot of shear modulus as a function of precursor protein concentration for the four different formulations illustrates the composition dependence of the matrix mechanics. The shear modulus value (G′) is represented as the mean ± SD of at least three different batches of the PF material in each data point. (B) Frequency-dependent storage (G′) and loss (G″) modulus from oscillatory frequency-sweep rheological testing of PF, PEG-DA and fibrin only hydrogels. (C) Strain-dependent storage (G′) and loss (G″) modulus from strain sweep rheological testing of PF, PEG-DA and fibrin only hydrogels.

FIGURE 3

The proteolytic sensitivity of the PF matrix is composition dependent. Degradation profiles of the various PF hydrogels were recorded by quantifying the release kinetics of fluorescently labeled fibrinogen from the intact matrix over the course of an 8-h incubation in trypsin solution. The graph associates matrix degradation kinetics (percent degradation profile) with the three independent variables used to control the physicochemical properties of the PF matrix in the experimental design: PEG MW (10- and 20-kDa), precursor protein concentration (7 and 9 mg/ml) and additional PEG-multiacrylate cross-linker (+). Each material is represented in the legend by its composition, degradation half-life (t₅₀ in min) and shear modulus (G′ in Pa).

FIGURE 4

Encapsulated smooth muscle cells (SMCs) in PF hydrogels express different morphologies depending on the material composition. SMC morphology after 24 h in 3D culture was visualized by phase contrast microscopy (top and middle rows) and f-actin was visualized by fluorescent microscopy (bottom row). A differential spindled index scale (0-4) was used to characterize the typical SMC morphologies observed in the various materials. The spindled index was ranked as follows: 4, highly spindled with regular lamellipodia (A); 3, spindled with frayed lamellipodia (B); 2, nonspindled with frayed lamellipodia (C); 1, rounded with minor lamellipodia (D); 0, completely rounded (not shown). The materials shown include the following variations: PF10 having a modulus of 11.6 ± 6 Pa (A); PF10 having a modulus of 67 ± 25 Pa (B); PF10+ having a modulus of 147 ± 6 Pa (C); PF20+ having a modulus of 497 ± 45 Pa (D).

FIGURE 5

Correlation charts representing interdependence between modulus, proteolysis, and spindled index. (Top) The correlation between the shear modulus (G′) and degradation half-life (t₅₀) of the material is shown with a curve-fit linear regression (R² = 0.71). (Middle) The correlation between the shear modulus (G′) of the material and the SMCs spindled index is shown with a curve-fit logarithmic regression (R² = 0.87). (Bottom) The correlation between the degradation half-life (t₅₀) of the material and the SMCs spindled index is shown with a curve-fit linear regression (R² = 0.76).

FIGURE 6

The material composition and matrix stiffness influence macroscopic compaction of SMC-seeded PF hydrogel constructs. The compaction data is taken from the diameter measurements of the constructs after 3 and 7 days in culture and normalized to acellular controls. The extent of compaction in each material is dependent on its composition and can be linked to the matrix modulus (reported in Pa) and cells initial spindled index at 30 h (shown for each treatment as a numeric insert). *Statistically significant difference between day 3 and day 7 (p < 0.05, n ≥ 6). **Statistically significant difference between day 3 and no compaction (100%) (p < 0.05, n ≥ 6).

FIGURE 7

The SMCs remodel the PF hydrogel matrix in 3D culture through initial spreading followed by tractional force generation and contraction. The contraction by SMCs was quantified by imaging the cells together with 2-μm beads in sequential image frames and measuring the bead displacement vectors of individual beads surrounding the cells between the time increments (arrows). The interaction between cells and the PF matrix was characterized by the magnitude and direction of the bead displacement vectors. (A) During the initial 10 h of cells spreading, the cells first extend in the PF matrix without displacing the surrounding beads and then cause radially inward bead displacement (signifying matrix contraction) after 15 h (Supplementary Material, Video 2). (B) After 20 h, the cells caused extensive matrix contraction as indicated by the large inward radial bead displacements. (C) Eventually, the cells interconnected and produced macroscopic contraction of the PF matrix; when two cells disconnected spontaneously, the PF matrix exhibits viscoelastic recoil. The representative images were of constructs made from PF10 (6 mg/ml).

FIGURE 8

SMC remodeling is dependent on protein and protease expression. (A) SMCs produce type-I collagen after 3 days in culture (shown: PF10, 6 mg/ml). (B) Western blotting detects levels of MMP-2 but not MMP-9 and MMP-3 in the SMC-seeded PF hydrogels after 3 days in culture (s1, sample, c1, acellular controls). (C) Selective inhibition of matrix metalloproteinases (MMPs) in SMC-seeded hydrogel constructs alters the morphology of the cells. MMP inhibitors were used to investigate the significance of proteases in the 3D cell spreading process after 2 days in culture. SMCs were cultured in highly compliant materials made from PF10 (6 mg/ml) in the presence of various MMP inhibitors including: MMP 2/9 inhibitor (0.6 mM), MMP 3 inhibitor (0.3 mM), and MMP 8 inhibitor (0.1 mM). Three controls were used in the experiment including: SMCs encapsulated without inhibitors; SMCs encapsulated in 2% DMSO (similar to the DMSO content in the inhibitor solution); and SMCs cultured on tissue culture plastic (TCP) with MMP 2/9 inhibitor (0.6 mM). Scale bars = 50 μm.