Myosin IIA interacts with the spectrin-actin membrane skeleton to control red blood cell membrane curvature and deformability

Abstract

The biconcave disc shape and deformability of mammalian red blood cells (RBCs) relies upon the membrane skeleton, a viscoelastic network of short, membrane-associated actin filaments (F-actin) cross-linked by long, flexible spectrin tetramers. Nonmuscle myosin II (NMII) motors exert force on diverse F-actin networks to control cell shapes, but a function for NMII contractility in the 2D spectrin-F-actin network in RBCs has not been tested. Here, we show that RBCs contain membrane skeleton-associated NMIIA puncta, identified as bipolar filaments by super-resolution fluorescence microscopy. NMIIA association with the membrane skeleton is ATP-dependent, consistent with NMIIA motor domains binding to membrane skeleton F-actin and contributing to membrane mechanical stability. In addition, the NMIIA heavy and light chains are phosphorylated in vivo in RBCs, indicating active regulation of NMIIA motor activity and filament assembly, while reduced heavy chain phosphorylation of membrane skeleton-associated NMIIA indicates assembly of stable filaments at the membrane. Treatment of RBCs with blebbistatin, an inhibitor of NMII motor activity, decreases the number of NMIIA filaments associated with the membrane and enhances local, nanoscale membrane oscillations, suggesting decreased membrane tension. Blebbistatin-treated RBCs also exhibit elongated shapes, loss of membrane curvature, and enhanced deformability, indicating a role for NMIIA contractility in promoting membrane stiffness and maintaining RBC biconcave disc cell shape. As structures similar to the RBC membrane skeleton are conserved in many metazoan cell types, these data demonstrate a general function for NMII in controlling specialized membrane morphology and mechanical properties through contractile interactions with short F-actin in spectrin-F-actin networks.\n\nSignificance statementThe biconcave disc shape and deformability of the mammalian RBC is vital to its circulatory function, relying upon a 2D viscoelastic spectrin-F-actin network attached to the membrane. A role for myosin II (NMII) contractility in generating tension in this network and controlling RBC shape has never been tested. We show that NMIIA forms phosphorylated bipolar filaments in RBCs, which associate with F-actin at the membrane. NMIIA motor activity is required for interactions with the spectrin-F-actin network, and regulates RBC biconcave shape and deformability. These results provide a novel mechanism for actomyosin force generation at the plasma membrane, and may be applicable to other cell types such as neurons and polarized epithelial cells with a spectrin-F-actin-based membrane skeleton.