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. 2015 May;7(5):569-79.
doi: 10.1039/c5ib00060b. Epub 2015 Apr 24.

Microfluidic analysis of extracellular matrix-bFGF crosstalk on primary human myoblast chemoproliferation, chemokinesis, and chemotaxis

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Microfluidic analysis of extracellular matrix-bFGF crosstalk on primary human myoblast chemoproliferation, chemokinesis, and chemotaxis

Meghaan M Ferreira et al. Integr Biol (Camb). 2015 May.

Abstract

Exposing myoblasts to basic fibroblast growth factor (bFGF), which is released after muscle injury, results in receptor phosphorylation, faster migration, and increased proliferation. These effects occur on time scales that extend across three orders of magnitude (10(0)-10(3) minutes). Finite element modeling of Transwell assays, which are traditionally used to assess chemotaxis, revealed that the bFGF gradient formed across the membrane pore is short-lived and diminishes 45% within the first minute. Thus, to evaluate bFGF-induced migration over 10(2) minutes, we employed a microfluidic assay capable of producing a stable, linear concentration gradient to perform single-cell analyses of chemokinesis and chemotaxis. We hypothesized that the composition of the underlying extracellular matrix (ECM) may affect the behavioral response of myoblasts to soluble bFGF, as previous work with other cell types has suggested crosstalk between integrin and fibroblast growth factor (FGF) receptors. Consistent with this notion, we found that bFGF significantly reduced the doubling time of myoblasts cultured on laminin but not fibronectin or collagen. Laminin also promoted significantly faster migration speeds (13.4 μm h(-1)) than either fibronectin (10.6 μm h(-1)) or collagen (7.6 μm h(-1)) without bFGF stimulation. Chemokinesis driven by bFGF further increased migration speed in a strictly additive manner, resulting in an average increase of 2.3 μm h(-1) across all ECMs tested. We observed relatively mild chemoattraction (∼67% of myoblast population) in response to bFGF gradients of 3.2 ng mL(-1) mm(-1) regardless of ECM identity. Thus, while ECM-bFGF crosstalk did impact chemoproliferation, it did not have a significant effect on chemokinesis or chemotaxis. These data suggest that the main physiological effect of bFGF on myoblast migration is chemokinesis and that changes in the surrounding ECM, resulting from aging and/or disease may impact muscle regeneration by altering myoblast migration and proliferation.

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Figures

Fig. 1
Fig. 1
Primary human myoblast exposure to bFGF results in cellular responses that occur over a range of time scales from 100 – 103 min. (A) Phosphorylation of FGFR1 (pFGFR1) in primary myoblasts peaks 5 min after bFGF stimulation. (B) Stimulation with 5 ng/mL bFGF promoted the greatest amount of migration in myoblasts as measured in Transwell assays over 6 h. Error bars represent the standard deviation between three independent studies. (C) The doubling time for myoblasts with or without bFGF stimulation was determined on laminin, fibronectin, and collagen. Error bars represent standard deviation; * p < 0.05 and ns = not significant (p > 0.05).
Fig. 2
Fig. 2
A microfluidic device for chemokinesis and chemotaxis assays. (A) The microfluidic device viewed from the top-down. The outermost channels (500 μm wide) are separated from the cell-culture chamber (CCC, 1 mm wide) by a micro-capillary array with capillaries (250 μm long and 10 μm wide) spaced 15 μm apart. For chemokinesis assays, the same medium is perfused into both outer channels to create a uniform concentration across the device and replenish growth factors. (B) A cross-sectional view of the microfluidic device. The medium-carrying channels and the CCC are 120 μm high. The difference in height between CCC and the micro-capillaries (10 μm) increases fluidic resistance and, consequently, minimizes flow within the CCC. (C) For chemotaxis assays, the outermost channels (“source” and “sink”) are perfused with media containing different bFGF concentrations to create a stable, linear concentration gradient. Computational transport models (black line) of the predicted concentration profile were validated with experimental studies (red line).
Fig. 3
Fig. 3
An engineering analysis of diffusion reveals significant instability of the concentration gradient within a Transwell assay. (A) Schematic diagram of a Transwell assay with the sink above the membrane and the growth factor source below the membrane. (B) Concentration gradient profiles across a membrane pore are displayed as a percentage of the initial source concentration for times between 1 – 21600 s (i.e., 1 s – 6 h). (C) Finite element simulations of the concentration gradient across a membrane pore as a function of time. Red is 100% of the initial source concentration and dark blue is 0%.
Fig. 4
Fig. 4
Migration speed of primary human myoblasts is mediated through interactions with the extracellular matrix and soluble bFGF . (A) Migration speeds for myoblasts cultured on fibronectin were quantified for media with (5 ng/mL) and without bFGF supplementation using time-lapse microscopy. The bFGF-supplemented medium was delivered using three different methods: 1) without any replenishment, 2) replenishment immediately before imaging, and 3) continuous delivery of fresh supplemented medium using a microfluidic device. (B) Differences in myoblast migration speed as a result of substrate composition and bFGF stimulation with continuous medium replenishment. (C) The increased migration speed resulting from bFGF stimulation is not affected by the underlying substrate, indicating that the effect is strictly additive. Error bars represent 95% confidence intervals; * p 0.05).
Fig. 5
Fig. 5
Myoblasts subjected to a bFGF gradient exhibit mild chemoattraction above a threshold concentration regardless of the underlying substrate. (A) The chemotactic index for individual cells (black dots) and population averages (red lines) are plotted for each condition tested (-, NG, G1, G2, G3, and G4). Red asterisks are used to indicate population averages with a chemotactic index significantly different from zero (* p
Fig. 6
Fig. 6
Increasing the concentration range, while maintaining the gradient slope, reveals similar chemotactic behavior in myoblasts above a threshold concentration. The top panel of angular histograms illustrates the distribution of myoblasts after exposure to the corresponding bFGF gradient for 14 h when the device source and sink are perfused with 11 and 0 ng/mL bFGF, respectively. The bottom panel illustrates the distribution of myoblasts after 14 h when the device source and sink are perfused with 16 and 5 ng/mL bFGF, respectively. The gradient slopes are identical between the top and bottom panels. The number of individual cells tracked (n), and the p-value calculated using the Rayleigh Test for Vector Data (p) are shown. Red labels denote histograms with statistically significant asymmetry (p
Fig. 7
Fig. 7
Myoblasts establish a biased directional response towards the bFGF source after 6 h. Myoblast migration in the G4 bFGF gradient on laminin was observed for 14 h. The top panels show the paths taken by individual myoblasts, where all initial positions (t = 0) are plotted at the origin. Red indicates paths with a positive net displacement towards the bFGF source, and black indicates paths moving away from the source. The percent of cells exhibiting movement towards and away from the bFGF source are shown in red and black fonts, respectively. The bottom panels show the corresponding angular histograms, where n is the number of individual cells tracked, and p is the p-value calculated using the Rayleigh Test for Vector Data. Red labels denote statistically significant asymmetry of histograms.

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