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Mechanical strain in actin networks regulates FilGAP and integrin binding to filamin A

Abstract

Mechanical stresses elicit cellular reactions mediated by chemical signals. Defective responses to forces underlie human medical disorders1,2,3,4 such as cardiac failure5 and pulmonary injury6. The actin cytoskeleton’s connectivity enables it to transmit forces rapidly over large distances7, implicating it in these physiological and pathological responses. Despite detailed knowledge of the cytoskeletal structure, the specific molecular switches that convert mechanical stimuli into chemical signals have remained elusive. Here we identify the actin-binding protein filamin A (FLNA)8,9 as a central mechanotransduction element of the cytoskeleton. We reconstituted a minimal system consisting of actin filaments, FLNA and two FLNA-binding partners: the cytoplasmic tail of β-integrin, and FilGAP. Integrins form an essential mechanical linkage between extracellular and intracellular environments, with β-integrin tails connecting to the actin cytoskeleton by binding directly to filamin4. FilGAP is an FLNA-binding GTPase-activating protein specific for RAC, which in vivo regulates cell spreading and bleb formation10. Using fluorescence loss after photoconversion, a novel, high-speed alternative to fluorescence recovery after photobleaching11, we demonstrate that both externally imposed bulk shear and myosin-II-driven forces differentially regulate the binding of these partners to FLNA. Consistent with structural predictions, strain increases β-integrin binding to FLNA, whereas it causes FilGAP to dissociate from FLNA, providing a direct and specific molecular basis for cellular mechanotransduction. These results identify a molecular mechanotransduction element within the actin cytoskeleton, revealing that mechanical strain of key proteins regulates the binding of signalling molecules.

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Figure 1: Differential mechanotransduction in FLNA occurs through spatial separation of binding sites and opening cryptic sites.
Figure 2: External bulk shear of F-actin/FLNA networks alters FLNA’s binding affinity for β 7 -integrin and FilGAP.
Figure 3: Myosin II forces applied to F-actin/FLNA networks change FLNA’s binding affinity to β 7 -integrin and FilGAP.

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Acknowledgements

The authors acknowledge the Harvard Materials Research and Engineering Center (DMR-0820484) for confocal imaging, and M. Ginsberg, J. Lippincott-Schwartz and V. Verkhusha for providing complementary DNA for PA-GFP and PA-mCherry constructs. We thank T. Collins for technical assistance, L. Jawerth and V. Zaburdaev for discussions, and J. Wilking and K. Guenthner for help with the manuscript. This work was supported by grants NIH R01 HL19429 (T.P.S.) and NIH T32 HL07680 (A.J.E.) and by the Harvard University Science and Engineering Committee Seed Fund for Interdisciplinary Science (D.A.W., T.P.S., F.N.).

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Contributions

The project was initiated by T.P.S., F.N. and D.A.W. Experiments were designed by A.J.E., T.P.S., J.H.H. and F.N. Proteins and materials were synthesized and purified by F.N. and A.J.E. FLAC experiments were performed by A.J.E. and binding assays were performed by F.N. Data was analysed by A.J.E. All authors discussed data and aided in preparing the manuscript. A.J.E. and F.N. contributed equally to this project.

Corresponding authors

Correspondence to A. J. Ehrlicher or F. Nakamura.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Figures

The file contains Supplementary Figures 1-9 with legends. (PDF 3491 kb)

Supplementary Movie 1

The movie shows PA-GFP β7 Integrin binds longer to del41 mutant FLNa than wild type FLNa. (MOV 6767 kb)

Supplementary Movie 2

The movie shows PA-GFP FilGAP binds longer to wild-type FLNa than to M2474E mutant FLNa. (MOV 9969 kb)

Supplementary Movie 3

The movie shows Myosin II deforms actin networks when ATP is available. (MOV 4196 kb)

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Ehrlicher, A., Nakamura, F., Hartwig, J. et al. Mechanical strain in actin networks regulates FilGAP and integrin binding to filamin A. Nature 478, 260–263 (2011). https://doi.org/10.1038/nature10430

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