Rho/ROCK and myosin II control the polarized distribution of endocytic clathrin structures at the uropod of moving T lymphocytes.

We have examined the spatio-temporal dynamics of clathrin-mediated endocytosis (CME) during T lymphocyte polarization and migration. Near the plasma membrane, we detected heterogeneous arrangements of GFP-clathrin that were clustered predominantly at the uropod; some diffraction limited spots ( approximately 200 nm) and a major population of larger clathrin structures (CSs) (300-800 nm). Membrane CSs fully co-localized with the endocytic adaptor complex AP-2, which was also polarized towards the rear membrane. During the direct incorporation of the endocytic cargo transferrin, large and relatively stable clathrin/AP-2 structures at the uropod membrane transiently co-localized with spots of transferrin, which suggests that they are endocytic competent platforms. The highly polarized distribution of membrane CSs towards the uropod and their endocytic ability support the existence of a preferential region of endocytosis located at or near the rear pole of T lymphocytes. Inactivation of Rho by dominant negative RhoA or C3 exoenzyme, and inhibition of Rho-kinase (ROCK) with Y-27632, or myosin II with blebbistatin, all resulted in suppression of CS polarization, which indicates that the posterior distribution of CSs relies on Rho/ROCK signaling and myosin II contractility. In addition, blocking CME with dominant negative mutants or by clathrin RNA interference, results in a remarkable inhibition of both basal and CXCL12-promoted migration, which suggests that CME is required for successful T-cell migration. We hypothesize that enhanced endocytic rates at the cell rear could provide a mechanism to remove leftover surface to accommodate cell retraction, and/or to spatially resolve signaling for guided cell migration.