In chemotaxing fibroblasts, both high-fidelity and weakly biased cell movements track the localization of PI3K signaling

Abstract

Cell movement biased by a chemical gradient, or chemotaxis, coordinates the recruitment of cells and collective migration of cell populations. During wound healing, chemotaxis of fibroblasts is stimulated by platelet-derived growth factor (PDGF) and certain other chemoattractants. Whereas the immediate PDGF gradient sensing response has been characterized previously at the level of phosphoinositide 3-kinase (PI3K) signaling, the sensitivity of the response at the level of cell migration bias has not yet been studied quantitatively. In this work, we used live-cell total internal reflection fluorescence microscopy to monitor PI3K signaling dynamics and cell movements for extended periods. We show that persistent and properly aligned (i.e., high-fidelity) fibroblast migration does indeed correlate with polarized PI3K signaling; accordingly, this behavior is seen only under conditions of high gradient steepness (>10% across a typical cell length of 50 μm) and a certain range of PDGF concentrations. Under suboptimal conditions, cells execute a random or biased random walk, but nonetheless move in a predictable fashion according to the changing pattern of PI3K signaling. Inhibition of PI3K during chemotaxis is accompanied by loss of both cell-substratum contact and morphological polarity, but after a recovery period, PI3K-inhibited fibroblasts often regain the ability to orient toward the PDGF gradient.

Figure 1

Generation of chemotactic PDGF gradients by slow release from alginate microspheres. (a) The two plots show maps of the calculated PDGF concentration (left panel, normalized by the maximum value) and PDGF gradient vector (right panel, magnitude indicated by arrow length) evaluated at the surface of the coverslip (z = 0). PDGF-loaded microsphere locations and diameters are indicated by open circles. (b) Montage of a GFP-AktPH-expressing NIH 3T3 mouse fibroblast migrating chemotactically toward the PDGF gradient field depicted in A. The bright-field image in the top left corner shows the cell in relation to the PDGF-loaded microspheres; scale bar = 70 μm. The other panels show the time course of cell contact area translocation and relative PI3K signaling gradient, monitored by TIRF microscopy and displayed using a pseudo-color intensity scale (see also Movie S1). (c) Metrics of chemotactic fidelity for the cell in b. Cell movement and PI3K signaling vector orientations, expressed as angles relative to the PDGF gradient vector (left panel), and the normalized PDGF concentration and relative gradient (RG) evaluated at the cell centroid (right panel), are plotted as a function of time.

Figure 2

Morphological characteristics of fibroblasts exhibiting conflicted versus persistent chemotaxis behaviors. (a) The bright-field image in the top left corner shows the cell in relation to the PDGF-loaded microspheres; scale bar = 70 μm. The other panels show the time course of cell contact area translocation and relative PI3K signaling gradient, monitored by TIRF microscopy and displayed using a pseudo-color intensity scale (see also Movie S2). (b) Metrics of chemotactic fidelity for the cell in a. Cell movement and PI3K signaling vector orientations, expressed as angles relative to the PDGF gradient vector (left panel), and the normalized PDGF concentration and relative gradient (RG) evaluated at the cell centroid (right panel), are plotted as a function of time.

Figure 3

Characterization of chemotactic fidelity and relation to PDGF gradient properties. (a) The absolute value of the cell movement vector orientation angle, relative to the PDGF gradient vector, is color-coded for each cell and time interval: red, 0–60°; white, 60–120°; blue, 120–180°. The 54 cells are sorted according to the number of red intervals minus the number of blue intervals and grouped by that score into high, intermediate, and low subpopulations using the k-means algorithm. (b) Cell centroid translocation paths for the three cell subpopulations grouped in a, with the initial centroid positions located at the origin and the initial PDGF gradient vector aligned along the positive x axis. (c) For each of the 54 cells, the mean values of the relative gradient steepness (RG) and normalized ligand concentration ([L]_norm) are indicated, as is the membership of each cell in the high- (orange), intermediate- (green), or low- (black) fidelity subpopulation.

Figure 4

The fidelity of chemotactic cell movement correlates with the orientation of PI3K signaling. (a) Density map of the PI3K signaling vector-PDGF gradient angle versus the cell movement vector-PDGF gradient angle, both expressed in degrees. An angle of zero indicates perfect alignment with the PDGF gradient. All time intervals (for the same 54 cells as in Fig. 3) are pooled. Density is given as the absolute number of instances in each square. (b) Dot plot of SI versus CI for each of the 54 cells.

Figure 5

PI3K inhibition depolarizes chemotaxing fibroblasts, but they are capable of reorienting thereafter. GFP-AktPH-expressing NIH 3T3 cells were monitored by TIRF microscopy as they migrated in the vicinity of PDGF-loaded microspheres (positions and sizes as indicated). After ∼4 h, one of two PI3K inhibitors was added, and the cells were observed for an additional 3 h. After 17 out of 30 cells experienced a marked cringe response to inhibitor addition, they exhibited partial recovery of adhesion by respreading and were analyzed further. (a) Pseudo-color montage of a representative cell treated with LY294002 after 240 min; scale bar = 70 μm (see also Movie S3). (b) Time course of cell contact area, normalized by each cell's time-averaged contact area before inhibition, showing the dramatic loss and partial recovery of adhesion by these cells (mean ± SD, n = 17). (c) Comparison of CI values post- and pre-inhibition for each cell in the population.