Internalization of macromolecules and membrane into cells through endocytosis is critical for cellular growth, signaling and plasma membrane (PM) tension homeostasis. Although endocytosis is responsive to both biochemical and physical stimuli, how physical cues modulate endocytic pathways is less understood. Contrary to the accumulating discoveries on the effects of increased PM tension on endocytosis, less is known about how a decrease of PM tension impacts on membrane trafficking. Here, we reveal that an acute decrease of PM tension results in phosphatidic acid (PA) production, F-actin and phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P 2 ]enriched dorsal membrane ruffling and subsequent macropinocytosis in myoblasts. The PA production induced by decreased PM tension depends on phospholipase D2 (PLD2) activation via PLD2 nanodomain disintegration. Furthermore, the 'decreased PM tension-PLD2-macropinocytosis' pathway is prominent in myotubes, reflecting a potential mechanism of PM tension homeostasis upon intensive muscle stretching and relaxation. Together, we identify a new mechanotransduction pathway that converts an acute decrease in PM tension into PA production and then initiates macropinocytosis via actin and PI(4,5)P 2 -mediated processes.

Insulin-stimulated translocation of glucose transporter 4 (GLUT4) to plasma membrane of skeletal muscle is critical for postprandial glucose uptake; however, whether the internalization of GLUT4 is also regulated by insulin signaling remains unclear. Here, we discover that the activity of dynamin-2 (Dyn2) in catalyzing GLUT4 endocytosis is negatively regulated by insulin signaling in muscle cells. Mechanistically, the fission activity of Dyn2 is inhibited by binding with the SH3 domain of Bin1. In the absence of insulin, GSK3α phosphorylates Dyn2 to relieve the inhibition of Bin1 and promotes endocytosis. Conversely, insulin signaling inactivates GSK3α and leads to attenuated GLUT4 internalization. Furthermore, the isoform-specific pharmacological inhibition of GSK3α significantly improves insulin sensitivity and glucose tolerance in diet-induced insulin-resistant mice. Together, we identify a new role of GSK3α in insulin-stimulated glucose disposal by regulating Dyn2-mediated GLUT4 endocytosis in muscle cells. These results highlight the isoform-specific function of GSK3α on membrane trafficking and its potential as a therapeutic target for metabolic disorders.

Tight regulation of endocytosis ensures accurate control of cellular signaling and membrane dynamics, which are crucial for tissue morphogenesis and functions. Mutations of Bin1 and dynamin-2 (Dyn2), proteins that generate membrane curvature and sever endocytic invaginations, respectively, cause progressive hereditary myopathy. Here, we show that Bin1 inhibits Dyn2 via direct interaction of its SRC Homology 3 (SH3) domain with the proline-rich domain (PRD) of Dyn2. Phosphorylation of S848 of Dyn2 by GSK3α, a kinase downstream of insulin signaling, relieves Dyn2 from the inhibition of Bin1 and promotes endocytosis in muscle. Mutations of Bin1 associated with centronuclear myopathy disrupt its inhibition of Dyn2, thereby exaggerating Dyn2 fission activity and causing excessive fragmentation of T-tubules in the muscle cells. Our work reveals how Bin1-Dyn2 interaction fine-tunes membrane remodeling at the molecular level, and lay the foundation for future exploration of endocytic regulation and hereditary muscle diseases.

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Internalization of macromolecules and membrane into cells through endocytosis is critical for cellular growth, signaling, and membrane tension homeostasis. Although endocytosis is responsive to both biochemical and physical stimuli, how physical cues modulate endocytic pathways is less understood. In contrary to the accumulating discoveries on effects of increased membrane tension on endocytosis, little is known about how a drop of tension impacts membrane trafficking. Here we reveal that acute reduction of plasma membrane tension results in phosphatidic acid, F-actin and dynamin 2-enriched dorsal membrane ruffling and subsequent macropinocytosis in myoblast. The membrane flaccidity-induced local phosphatidic acid production depends on phospholipase D2 (PLD2) that is activated via lipid raft disruption. Furthermore, the “membrane flaccidity-PLD2-macropinocytosis” pathway is dominant in myotube, reflecting a potential mechanism of membrane tension homeostasis upon intensive muscle stretching and relaxation. Together, we identify a new mechanotransduction pathway which converts acute tension drop into PA production and subsequently initiates macropinocytosis via actin and dynamin activities. Summary We reveal a mechanical induction of macropinocytosis that is elicited by acute decrease of plasma membrane tension, followed by lipid raft destabilization, PLD2 activation and PA production.