In skeletal muscle fibers, tropomodulin 1 (Tmod1) can be compensated for, structurally but not functionally, by Tmod3 and -4.

The short actin filaments in the red blood cell (RBC) membrane skeleton are capped at their pointed ends by tropomodulin 1 (Tmod1) and coated with tropomyosin (TM) along their length. Tmod1-TM control of actin filament length is hypothesized to regulate spectrin-actin lattice organization and membrane stability. We used a Tmod1 knockout mouse to investigate the in vivo role of Tmod1 in the RBC membrane skeleton. Western blots of Tmod1-null RBCs confirm the absence of Tmod1 and show the presence of Tmod3, which is normally not present in RBCs. Tmod3 is present at only one-fifth levels of Tmod1 present on wild-type membranes, but levels of actin, TMs, adducins, and other membrane skeleton proteins remain unchanged. Electron microscopy shows that actin filament lengths are more variable with spectrin-actin lattices displaying abnormally large and more variable pore sizes. Tmod1-null mice display a mild anemia with features resembling hereditary spherocytic elliptocytosis, including decreased RBC mean corpuscular volume, cellular dehydration, increased osmotic fragility, reduced deformability, and heterogeneity in osmotic ektacytometry. Insufficient capping of actin filaments by Tmod3 may allow greater actin dynamics at pointed ends, resulting in filament length redistribution, leading to irregular and attenuated spectrin-actin lattice connectivity, and concomitant RBC membrane instability.(Blood. 2010;116(14): 2590-2599) IntroductionThe membrane skeleton is composed of a highly cross-linked network of spectrin, actin filaments, and accessory proteins that underlies the plasma membrane of differentiated cells. This network creates membrane domains by anchoring and restricting the long-range distribution of membrane proteins and plays an important role in determining cell shapes, membrane contours, and mechanical properties. 1,2 The organization of the membrane skeleton is best known in red blood cells (RBCs), where it is organized as a quasi-hexagonal network with connecting strands formed by long, flexible spectrin molecules and vertices formed by short actin filaments. 3-5 Each short actin filament forms the core of a junctional complex with 2 rod-shaped tropomyosin (TM) molecules along the filament, 2 tropomodulin 1 (Tmod1) molecules capping the pointed filament end and an ␣/␤-adducin heterodimer capping the barbed filament end. 6,7 Dematin (protein 4.9) is also associated with the short actin filaments as are ␤1-spectrin protein 4.1R complexes that extend from the sides of the filaments to form the extended spectrin-actin network. The network is connected to membrane macromolecular complexes by multiple linkages: from ␤-spectrin by ankryin to band 3, and from the junctional complex by protein 4.1, ␣/␤-adducin and dematin to band 3, glycophorin C, the glucose transporter, and other components. 8 Disruptions either of attachments of the spectrin-actin lattice to membrane macromolecular complexes ("vertical" connections), or linkages within the plane of the spectrin-actin lattice ("horizontal" connections) lead to ...

The basis for mammalian lens fiber cell organization, transparency, and biomechanical properties has contributions from two specialized cytoskeletal systems: the spectrin-actin membrane skeleton and beaded filament cytoskeleton. The spectrin-actin membrane skeleton predominantly consists of α2β2-spectrin strands interconnecting short, tropomyosin-coated actin filaments, which are stabilized by pointed-end capping by tropomodulin 1 (Tmod1) and structurally disrupted in the absence of Tmod1. The beaded filament cytoskeleton consists of the intermediate filament proteins CP49 and filensin, which require CP49 for assembly and contribute to lens transparency and biomechanics. To assess the simultaneous physiological contributions of these cytoskeletal networks and uncover potential functional synergy between them, we subjected lenses from mice lacking Tmod1, CP49, or both to a battery of structural and physiological assays to analyze fiber cell disorder, light scattering, and compressive biomechanical properties. Findings show that deletion of Tmod1 and/or CP49 increases lens fiber cell disorder and light scattering while impairing compressive load-bearing, with the double mutant exhibiting a distinct phenotype compared to either single mutant. Moreover, Tmod1 is in a protein complex with CP49 and filensin, indicating that the spectrin-actin network and beaded filament cytoskeleton are biochemically linked. These experiments reveal that the spectrin-actin membrane skeleton and beaded filament cytoskeleton establish a novel functional synergy critical for regulating lens fiber cell geometry, transparency, and mechanical stiffness.

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Key Points Tmod3 deletion leads to reduced erythroid progenitors and impaired erythroblast survival, cell-cycle exit, and enucleation. Erythroblast-macrophage islands are reduced in the absence of Tmod3, which is required in both cell types for island formation.