Clathrin-mediated endocytosis is independent of actin dynamics in many circumstances but requires actin polymerization in others. We show that membrane tension determines the actin dependence of clathrin-coat assembly. As found previously, clathrin assembly supports formation of mature coated pits in the absence of actin polymerization on both dorsal and ventral surfaces of non-polarized mammalian cells, and also on basolateral surfaces of polarized cells. Actin engagement is necessary, however, to complete membrane deformation into a coated pit on apical surfaces of polarized cells and, more generally, on the surface of any cell in which the plasma membrane is under tension from osmotic swelling or mechanical stretching. We use these observations to alter actin dependence experimentally and show that resistance of the membrane to propagation of the clathrin lattice determines the distinction between "actin-dependent" and "actin-independent". We also find that light-chain bound Hip1R mediates actin engagement. These data thus provide a unifying explanation for the role of actin dynamics in coated-pit budding.

The mechanisms driving pathological beta-amyloid (Ab) generation in late-onset Alzheimer's disease (AD) are unclear. Two late-onset AD risk factors, Bin1 and CD2AP, are regulators of endocytic trafficking, but it is unclear how their endocytic function regulates Ab generation in neurons. We identify a novel neuron-specific polarisation of Ab generation controlled by Bin1 and CD2AP. We discover that Bin1 and CD2AP control Ab generation in axonal and dendritic early endosomes, respectively. Both Bin1 loss of function and CD2AP loss of function raise Ab generation by increasing APP and BACE1 convergence in early endosomes, however via distinct sorting events. When Bin1 levels are reduced, BACE1 is trapped in tubules of early endosomes and fails to recycle in axons. When CD2AP levels are reduced, APP is trapped at the limiting membrane of early endosomes and fails to be sorted for degradation in dendrites. Hence, Bin1 and CD2AP keep APP and BACE1 apart in early endosomes by distinct mechanisms in axon and dendrites. Individuals carrying variants of either factor would slowly accumulate Ab in neurons increasing the risk for late-onset AD.

The bundled architecture of actin filaments is not needed for intestinal microvillar morphogenesis, as shown in knockout mice devoid of microvillar actin-bundling proteins. This architecture is essential for the apical anchorage of digestive proteins, probably via the recruitment of key players in apical retention, such as myosin-1a, and, as a result, for intestinal physiology.

Significance Intestinal epithelium damage is common but becomes recurrent in chronic intestinal disorders. Healing implies cell migration, which necessitates extensive cellular reorganization. We demonstrate that intestinal epithelial cells completely disassemble their apical actin-based microvilli upon migration, and we identify the protein villin and its actin-severing function as responsible for this physiological process. We show that this apical pole effacement is required for the acquisition of a motile phenotype and efficient wound healing. These findings demonstrate how intestinal epithelial cells acquired a mechanism at the level of the actin cytoskeleton to convert efficiently from a highly differentiated to a motile polarity.