Src triggers circular ruffling and macropinocytosis at the apical surface of polarized MDCK cells

We addressed the role of Src on cortical actin dynamics and polarized endocytosis in MDCK cells harboring a thermosensitive v-src mutant. Shifting monolayers established at 40 °C (non-permissive temperature) to 34 °C (permissive temperature) rapidly reactivated v-Src kinase, but tight junctions and cell polarity resisted for >6 h. At this interval, activated v-src was recruited on apical vesicles, induced cortactin-associated apical circular ruffles productive of macropinosomes, thereby accelerating apical pinocytosis by approximately fivefold. Ruffling and macropinosome formation were selectively abrogated by inhibitors of actin polymerization, phosphoinositide 3-kinase, phospholipase C, and phospholipase D, which all returned apical pinocytosis to the level observed at 40 °C, underscoring the distinct control of apical micropinocytosis and macropinocytosis. Src promoted microtubule-dependent fusion of macropinosomes to the apical recycling endosome (ARE), causing its strong vacuolation. However, preservation of tubulation and apical polarity indicated that its function was not affected. The ARE was labeled for v-src, Rab11, and rabankyrin-5 but not early endosome antigen 1, thus distinguishing two separate Rab5-dependent apical pathways. The mechanisms of Src-induced apical ruffling and macropinocytosis could shed light on the triggered apical enteroinvasive pathogens entry and on the apical differentiation of osteoclasts.