PubMed · 6894785
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H White. 1981-07-23. The actin-myosin interface.. https://doi.org/10.1038/292294a0
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Myoblast fusion is crucial for the formation, growth, maintenance and regeneration of healthy skeletal muscle. Unfortunately, the molecular machinery, cell behaviors, and membrane and cytoskeletal remodeling events that govern fusion and myofiber formation remain poorly understood. Using time-lapse imaging approaches on mouse C2C12 myoblasts, we identify discrete and specific molecular events at myoblast membranes during fusion and myotube formation. These events include rearrangement of cell shape from fibroblast to spindle-like morphologies, changes in lamellipodial and filopodial extensions during different periods of differentiation, and changes in membrane alignment and organization during fusion. We find that actin-cytoskeleton remodeling is crucial for these events: pharmacological inhibition of F-actin polymerization leads to decreased lamellipodial and filopodial extensions and to reduced myoblast fusion. Additionally, shRNA-mediated inhibition of Nap1, a member of the WAVE actin-remodeling complex, results in accumulations of F-actin structures at the plasma membrane that are concomitant with a decrease in myoblast fusion. Our data highlight distinct and essential roles for actin cytoskeleton remodeling during mammalian myoblast fusion, provide a platform for cellular and molecular dissection of the fusion process, and suggest a functional conservation of Nap1-regulated actin-cytoskeleton remodeling during myoblast fusion between mammals and Drosophila.
Dendritic spines differ considerably in their size, shape, and internal organization between brain regions. We examined the actin cytoskeleton in dendritic spines in hippocampus (areas CA1, CA3, and dentate gyrus), neostriatum, and cerebellum at both light and electron microscopic levels by using a novel high-resolution photoconversion method based in the high affinity of phalloidin for filamentous (F)-actin. In all brain regions, labeling was strongest in the heads of dendritic spines, diminishing in the spine neck. The number of labeled spines varied by region. Compared with the cerebellar molecular layer and area CA3, where nearly every dendritic spine was labeled, less than half the spines were labeled in CA1, dentate gyrus, and neostriatum. Serial section reconstructions of spines in these areas indicated that phalloidin labeling was restricted to the largest and most morphologically diverse dendritic spines. The resolution of the photoconversion technique allowed us to examine the localization and organization of actin filaments in the spine. The most intense staining for actin was found in the postsynaptic density and associated with the spines internal membrane system. In mushroom-shaped spines, F-actin staining was particularly strong between the lamellae of the spine apparatus. Three-dimensional reconstruction of labeled spines by using electron tomography showed that the labeled dense material was in continuity with the postsynaptic density. These results highlight differences in the actin cytoskeleton between different spine populations and provide novel information on the organization of the actin cytoskeleton in vivo.
The regulatory properties of naturally occurring tropomyosins (Tms) of differing lengths have been examined. These Tms span from 4 to 7 actin subunits. Native proteins have been used to study the common 7 actin-spanning skeletal and smooth muscle variants and expressed recombinant proteins to study the shorter fibroblast 5a, 5b, yeast Tm1 and yeast Tm2 Tms (6, 6, 5, and 4 actin-spanning variants, respectively). The yTm2 has been overexpressed in Escherichia coli with N-terminal constructs equivalent to those previously used for yTm1 [Maytum, R., et al. (2000) Biochemistry 39, 11913]. The regulation of myosin subfragment 1 (S1) binding to actin by Tm has been assessed using a sensitive S1 binding titration. The equilibrium between closed and open (C to M states, KT = 0.1-0.14) was similar for all vertebrate Tms. Apart from skTm where the apparent cooperative unit size (n) is the same as the structural size (n = 7 actin sites), the other vertebrate Tms that were studied exhibited large n values (n = 12-14). The yeast Tms also exhibited large values of n (6-9) in comparison to their structural sizes (4-5). The determined value of KT depended on the N-terminal sequence (KT = 0.15-1). These results are compared with the effect of S1 upon Tm's affinity for actin. The yeast Tms have regulatory parameters similar to those of skTm, but unlike skTm, S1 has little effect upon their actin affinity. This shows that an actin state with a high affinity for S1 and Tm is not necessary for regulation, and the higher affinity of S1 for actin in the presence of vertebrate Tms is probably the result of a direct interaction of S1 with Tm.