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Actin dynamics at pointed ends regulates thin filament length in striated muscle

Nature Cell Biology volume 3, pages 544–551 (2001)Cite this article

Abstract

Regulation of actin dynamics at filament ends determines the organization and turnover of actin cytoskeletal structures. In striated muscle, it is believed that tight capping of the fast-growing (barbed) ends by CapZ and of the slow-growing (pointed) ends by tropomodulin (Tmod) stabilizes the uniform lengths of actin (thin) filaments in myofibrils. Here we demonstrate for the first time that both CapZ and Tmod are dynamic on the basis of the rapid incorporation of microinjected rhodamine-labelled actin (rho-actin) at both barbed and pointed ends and from the photobleaching of green fluorescent protein (GFP)-labelled Tmod. Unexpectedly, the inhibition of actin dynamics at pointed ends by GFP–Tmod overexpression results in shorter thin filaments, whereas the inhibition of actin dynamics at barbed ends by cytochalasin D has no effect on length. These data demonstrate that the actin filaments in myofibrils are relatively dynamic despite the presence of capping proteins, and that regulated actin assembly at pointed ends determines the length of thin filaments.

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Figure 1: Rho-actin is incorporated at pointed ends and Z-lines in cardiac myocytes.
Figure 2: Cytochalasin D inhibits rho-actin incorporation at the Z-line.
Figure 3: Overexpression of GFP–Tmod prevents rho-actin incorporation at pointed ends and results in shorter thin filaments.
Figure 4: Direct observation of rapid GFP–Tmod exchange at pointed ends by photobleaching.
Figure 5: Model for rho-actin incorporation into thin filament ends in the presence of endogenous (a) and excess (b) levels of Tmod.

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Acknowledgements

We thank K. Hahn for use of his microscope and for advice, K. Sullivan for his instruction in live-cell imaging, K. Monier for advice on photobleaching, C. Waterman-Storer for editorial comments, and C. Subauste and J. Montebella for help in preparing the rhodamine-actin. We also thank past and present members of the Fowler laboratory for suggestions and help. R.L. was the recipient of a research fellowship from the American Heart Association, Western States Affiliate. This work was supported by grants to V.M.F. from the N.I.H. (GM 34225) and from the Human Frontiers in Science Program.

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  1. Department of Cell Biology, The Scripps Research Institute, 10596 North Torrey Pines Road, La Jolla, 92037, California, USA

    Ryan Littlefield, Angels Almenar-Queralt & Velia M. Fowler

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  1. Ryan Littlefield
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  3. Velia M. Fowler
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Correspondence to Velia M. Fowler.

Supplementary information

Supplementary figures

Figure S1 Overview of distributed deconvolution method. (PDF 96 kb)

Figure S2 Rho-actin and phallacidin distribution functions (a, c) and profiles (b, d).

Figure S3 Comparison of 3-peak and 1-peak rho-actin profiles reveals that incorporation sites are uniformly organized within myofibrils.

Figure S4 Rho-actin distribution parameters determined by distributed deconvolution are uniform within cells but exhibit variation between cells. for all 20 images.

Supplementary movie

Movie 1 A living, beating cardiac myocyte imaged 1 h after microinjection with rho-actin. There are 20 frames, each ~1 s apart. Myofibrils do not seem to contract smoothly when imaged at this frame rate, and only occasional snapshots of myofibril stretching and contraction are visible. The myofibril shown in Fig. 1b is in the bottom half of this movie. (MOV 659 kb)

Supplementary movie

Movie 2 A different region from the same cell as in movie 1. There are several myofibrils that show instances of myofibril stretching and contraction. (MOV 1513 kb)

Introduction to distributed deconvolution and application to analysis of rho-actin incorporation patterns. (PDF 34 kb)

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Littlefield, R., Almenar-Queralt, A. & Fowler, V. Actin dynamics at pointed ends regulates thin filament length in striated muscle. Nat Cell Biol 3, 544–551 (2001). https://doi.org/10.1038/35078517

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