Fibroblasts secrete fibronectin under lamellipodia in a microtubule- and myosin II-dependent fashion.

Fibronectin (FN) is an essential structural and regulatory component of the extracellular matrix (ECM), and its binding to integrin receptors supports cell adhesion, migration, and signaling. Here, using live-cell microscopy of fibroblasts expressing FN tagged with a pH-sensitive fluorophore, we show that FN is secreted predominantly at the ventral surface of cells in an integrin-independent manner. Locally secreted FN then undergoes β1 integrin-dependent fibrillogenesis. We find that the site of FN secretion is regulated by cell polarization, which occurs in bursts under stabilized lamellipodia at the leading edge. Moreover, analysis of FN secretion and focal adhesion dynamics suggest that focal adhesion formation precedes FN deposition and that deposition continues during focal adhesion disassembly. Lastly, we show that the polarized FN deposition in spreading and migrating cells requires both intact microtubules and myosin II-mediated contractility. Thus, while FN secretion does not require integrin binding, the site of exocytosis is regulated by membrane and cytoskeletal dynamics with secretion occurring after new adhesion formation.