Integration of Soluble and Adhesive Signals During Fibroblast Migration

Abstract

Cell migration is found in everyday biological processes including embryogenesis, cancer metastasis, and wound healing. During wound healing, fibroblasts must detect and transform soluble (platelet-derived growth factor) and adhesive (fibronectin) stimuli into directed movement in order to rapidly invade the wound to facilitate repair.

In fibroblasts and many other cells, activation of the phosphoinositide 3-kinase (PI3K) pathway is required for certain motility processes. PI3Ks generate specific 3' PI lipid products which act as membrane second messengers, and the spatial pattern of 3' PI density in the membrane is thought to control the directionality of membrane protrusion and cell migration. Using a specific 3' PI lipid fluorescent-biosensor, PI3K signaling and changes in the contact area of spreading and randomly migrating fibroblasts were monitored using total internal reflection fluorescence (TIRF) microscopy.

We report that PI3K is spontaneously and locally activated during cell spreading in a manner that is uncoupled from classical integrin-mediated pathways and feedback from the actin cytoskeleton. Inhibition of PI3K impairs efficient cell spreading, while disruption of microtubules causes contact area contraction giving rise to distinct and dynamic protrusion events that are PI3K dependent. We speculate that the interplay between motile forces, such as contraction, and PI3K signaling might constitute a positive feedback loop.

During random migration, we observe that fibroblasts exhibit either periods of persistence in the direction migration or zig-zag migration paths. PI3K signaling appears more localized and sustained in the former, while in the latter patterns of PI3K signaling is more transient. However, in both instances PI3K frequently localized to areas of active, membrane protrusion. Using a novel image analysis framework, we find that PI3K signaling quantitatively correlate with the velocity of migration. We also report that random migration is mediated by retraction events and PI3K-coupled protrusions. These results suggest that PI3K might influence a cell's intrinsic polarity before extracellular stimulation.

Finally, we present the development of two TIRF-compatible techniques designed to probe the effects of PDGF gradients and substrate compliance on PI3K signaling dynamics during fibroblast migration.