Atomic force microscopy demonstration of cytoskeleton instability in mouse erythrocytes with dematin-headpiece and β-adducin deficiency

Nanomechanical properties of composite protein networks of erythroid membranes at lipid surfaces

Erythrocyte membranes have been particularly useful as a model for studies of membrane structure and mechanics. Native erythroid membranes can be electroformed as giant unilamellar vesicles (eGUVs). In the presence of ATP, the erythroid membrane proteins of eGUVs rearrange into protein networks at the microscale. Here, we present a detailed nanomechanical study of individual protein microfilaments forming the protein networks of eGUVs when spread on supporting surfaces. Using Peak Force tapping Atomic Force Microscopy (PF-AFM) in liquid environment we have obtained the mechanical maps of the composite lipid-protein networks supported on solid surface. In the absence of ATP, the protein pool was characterized by a Young's Modulus E_pool≈5-15MPa whereas the complex filaments were found softer after protein supramolecular rearrangement; E_fil≈0.4MPa. The observed protein softening and reassembling could be relevant for understanding the mechanisms of cytoskeleton reorganization found in pathological erythrocytes or erythrocytes that are affected by biological agents.

Dynamic actin filaments control the mechanical behavior of the human red blood cell membrane

The short actin filaments in the spectrin-actin membrane skeleton of human red blood cells (RBCs) are capable of dynamic subunit exchange and mobility. Actin dynamics in RBCs regulates the biomechanical properties of the RBC membrane.

Short, uniform-length actin filaments function as structural nodes in the spectrin-actin membrane skeleton to optimize the biomechanical properties of red blood cells (RBCs). Despite the widespread assumption that RBC actin filaments are not dynamic (i.e., do not exchange subunits with G-actin in the cytosol), this assumption has never been rigorously tested. Here we show that a subpopulation of human RBC actin filaments is indeed dynamic, based on rhodamine-actin incorporation into filaments in resealed ghosts and fluorescence recovery after photobleaching (FRAP) analysis of actin filament mobility in intact RBCs (∼25–30% of total filaments). Cytochalasin-D inhibition of barbed-end exchange reduces rhodamine-actin incorporation and partially attenuates FRAP recovery, indicating functional interaction between actin subunit turnover at the single-filament level and mobility at the membrane-skeleton level. Moreover, perturbation of RBC actin filament assembly/disassembly with latrunculin-A or jasplakinolide induces an approximately twofold increase or ∼60% decrease, respectively, in soluble actin, resulting in altered membrane deformability, as determined by alterations in RBC transit time in a microfluidic channel assay, as well as by abnormalities in spontaneous membrane oscillations (flickering). These experiments identify a heretofore-unrecognized but functionally important subpopulation of RBC actin filaments, whose properties and architecture directly control the biomechanical properties of the RBC membrane.

Spectrins: a structural platform for stabilization and activation of membrane channels, receptors and transporters

This review focuses on structure and functions of spectrin as a major component of the membrane skeleton. Recent advances on spectrin function as an interface for signal transduction mediation and a number of data concerning interaction of spectrin with membrane channels, adhesion molecules, receptors and transporters draw a picture of multifaceted protein. Here, we attempted to show the current depiction of multitask role of spectrin in cell physiology. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Calpain-1 knockout reveals broad effects on erythrocyte deformability and physiology

Pharmacological inhibitors of cysteine proteases have provided useful insights into the regulation of calpain activity in erythrocytes. However, the precise biological function of calpain activity in erythrocytes remains poorly understood. Erythrocytes express calpain-1, an isoform regulated by calpastatin, the endogenous inhibitor of calpains. In the present study, we investigated the function of calpain-1 in mature erythrocytes using our calpain-1-null [KO (knockout)] mouse model. The calpain-1 gene deletion results in improved erythrocyte deformability without any measurable effect on erythrocyte lifespan in vivo. The calcium-induced sphero-echinocyte shape transition is compromised in the KO erythrocytes. Erythrocyte membrane proteins ankyrin, band 3, protein 4.1R, adducin and dematin are degraded in the calcium-loaded normal erythrocytes but not in the KO erythrocytes. In contrast, the integrity of spectrin and its state of phosphorylation are not affected in the calcium-loaded erythrocytes of either genotype. To assess the functional consequences of attenuated cytoskeletal remodelling in the KO erythrocytes, the activity of major membrane transporters was measured. The activity of the K+-Cl- co-transporter and the Gardos channel was significantly reduced in the KO erythrocytes. Similarly, the basal activity of the calcium pump was reduced in the absence of calmodulin in the KO erythrocyte membrane. Interestingly, the calmodulin-stimulated calcium pump activity was significantly elevated in the KO erythrocytes, implying a wider range of pump regulation by calcium and calmodulin. Taken together, and with the atomic force microscopy of the skeletal network, the results of the present study provide the first evidence for the physiological function of calpain-1 in erythrocytes with therapeutic implications for calcium imbalance pathologies such as sickle cell disease.

The human erythrocyte plasma membrane: a Rosetta Stone for decoding membrane-cytoskeleton structure

The mammalian erythrocyte, or red blood cell (RBC), is a unique experiment of nature: a cell with no intracellular organelles, nucleus or transcellular cytoskeleton, and a plasma membrane with uniform structure across its entire surface. By virtue of these specialized properties, the RBC membrane has provided a template for discovery of the fundamental actin filament network machine of the membrane skeleton, now known to confer mechanical resilience, anchor membrane proteins, and organize membrane domains in all cells. This chapter provides a historical perspective and critical analysis of the biochemistry, structure, and physiological functions of this actin filament network in RBCs. The core units of this network are nodes of ~35-37 nm-long actin filaments, interconnected by long strands of (α1β1)₂-spectrin tetramers, forming a 2D isotropic lattice with quasi-hexagonal symmetry. Actin filament length and stability is critical for network formation, relying upon filament capping at both ends: tropomodulin-1 at pointed ends and αβ-adducin at barbed ends. Tropomodulin-1 capping is essential for precise filament lengths, and is enhanced by tropomyosin, which binds along the short actin filaments. αβ-adducin capping recruits spectrins to sites near barbed ends, promoting network formation. Accessory proteins, 4.1R and dematin, also promote spectrin binding to actin and, with αβ-adducin, link to membrane proteins, targeting actin nodes to the membrane. Dissection of the molecular organization within the RBC membrane skeleton is one of the paramount achievements of cell biological research in the past century. Future studies will reveal the structure and dynamics of actin filament capping, mechanisms of precise length regulation, and spectrin-actin lattice symmetry.