Lamin A/C and emerin regulate MKL1–SRF activity by modulating actin dynamics

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Abstract

Laminopathies, caused by mutations in the LMNA gene encoding the nuclear envelope proteins lamins A and C, represent a diverse group of diseases that include Emery–Dreifuss muscular dystrophy (EDMD), dilated cardiomyopathy (DCM), limb-girdle muscular dystrophy, and Hutchison–Gilford progeria syndrome1. Most LMNA mutations affect skeletal and cardiac muscle by mechanisms that remain incompletely understood. Loss of structural function and altered interaction of mutant lamins with (tissue-specific) transcription factors have been proposed to explain the tissue-specific phenotypes1. Here we report in mice that lamin-A/C-deficient (Lmna−/−) and LmnaN195K/N195K mutant cells have impaired nuclear translocation and downstream signalling of the mechanosensitive transcription factor megakaryoblastic leukaemia 1 (MKL1), a myocardin family member that is pivotal in cardiac development and function2. Altered nucleo-cytoplasmic shuttling of MKL1 was caused by altered actin dynamics in Lmna−/− and LmnaN195K/N195K mutant cells. Ectopic expression of the nuclear envelope protein emerin, which is mislocalized in Lmna mutant cells and also linked to EDMD and DCM, restored MKL1 nuclear translocation and rescued actin dynamics in mutant cells. These findings present a novel mechanism that could provide insight into the disease aetiology for the cardiac phenotype in many laminopathies, whereby lamin A/C and emerin regulate gene expression through modulation of nuclear and cytoskeletal actin polymerization.

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Figure 1: Impaired nuclear translocation of MKL1 in lamin-A/C-deficient and Lmna N195K mutant cells.
Figure 2: Changes in nuclear import and export are specific to MKL1 and are caused by altered actin dynamics in Lmna−/− and Lmna N195K cells.
Figure 3: Lmna−/− and Lmna N195K cells have altered actin dynamics and polymerization kinetics.
Figure 4: Emerin expression rescues actin dynamics and restores MKL1 nuclear translocation in Lmna−/− and Lmna N195K cells.

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Acknowledgements

We thank C. Stewart for the mouse models and cell lines. We thank J. Gannon for TAC surgeries and M. Cupesi for collecting the cardiac samples from the pressure-overload model. This work was supported by National Institutes of Health awards (R01 NS059348 and R01 HL082792); the Department of Defense Breast Cancer Idea Award (BC102152); an award from the Progeria Research Foundation (PRF 2011-035); and a postdoctoral fellowship from the American Heart Association to D.E.J. (AHA award 09POST2320042). The work in the laboratory of M.K.V. is funded by the Academy of Finland and the Sigrid Juselius Foundation.

Author information

C.Y.H., D.E.J. and J.L. conceived and designed the overall project, with valuable help from M.K.V. C.Y.H. and D.E.J. performed the experiments. C.Y.H., D.E.J. and J.L. analysed data. C.Y.H. and J.L. wrote the paper.

Correspondence to Jan Lammerding.

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Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Figures 1-10 and Supplementary References. (PDF 2022 kb)

Nuclear translocation of MKL1-GFP in Lmna+/+ MEFs

This video shows an Lmna+/+ mouse embryonic fibroblast expressing MKL1-GFP imaged before (frame 1) and immediately after serum stimulation (frames 2 and onwards). This time lapse covers a period of about 20 minutes. MKL1-GFP accumulated in the nucleus during the course of the video. (MOV 169 kb)

Nuclear translocation of MKL1-GFP in Lmna–/– MEFs

This video shows an Lmna–/– mouse embryonic fibroblast expressing MKL1-GFP imaged before (frame 1) and immediately after serum stimulation (frames 2 and onwards). This time lapse covers a period of about 20 minutes. Nuclear accumulation of MKL1-GFP is not evident during the course of the video. (MOV 428 kb)

Nuclear translocation of MKL1-GFP in Lmna N195K MEFs

This video shows an Lmna N195K mouse embryonic fibroblast expressing MKL1-GFP imaged before (frame 1) and immediately after serum stimulation (frames 2 and onwards). This time lapse covers a period of about 20 minutes. Nuclear accumulation of MKL1-GFP is not evident during the course of the video. (MOV 488 kb)

Nuclear translocation of MKL1(1-204)- 2×GFP in Lmna+/+ MEFs

This video shows an Lmna+/+ mouse embryonic fibroblast expressing MKL1(1-204)-2×GFP imaged before (frame 1) and immediately after serum stimulation (frames 2 and onwards). This time lapse covers a period of about 15 minutes. MKL1(1-204)-2×GFP accumulated rapidly in the nucleus during the course of the video. (MOV 64 kb)

Nuclear translocation of MKL1(1-204)- 2×GFP in Lmna–/– MEFs

This video shows an Lmna–/– mouse embryonic fibroblast expressing MKL1(1-204)-2×GFP imaged before (frame 1) and immediately after serum stimulation (frames 2 and onwards). This time lapse covers a period of about 15 minutes. Little or very low levels of MKL1(1-204)-2×GFP accumulated in the nucleus during the course of the video. (MOV 101 kb)

Nuclear translocation of MKL1(1-204)- 2×GFP in Lmna N195K

This video shows an Lmna N195K mouse embryonic fibroblast expressing MKL1(1-204)-2×GFP imaged before (frame 1) and immediately after serum stimulation (frames 2 and onwards). This time lapse covers a period of about 15 minutes. Little or very low levels of MKL1(1-204)-2×GFP accumulated in the nucleus during the course of the video. (MOV 191 kb)

Photoactivation and nuclear translocation of MKL1-PAGFP in Lmna+/+ MEFs

This video shows an Lmna+/+ mouse embryonic fibroblast (outlined in red) expressing MKL1-PAGFP after serum stimulation. Cytoplasmic MKL1-PAGFP was stimulated with a 405 nm laser and entry of the activated pool of MKL1-PAGFP is monitored for 1 minute. Frame 1 was captured before photoactivation. MKL1-PAGFP accumulated in the nucleus during the course of the video. (MOV 528 kb)

Photoactivation and nuclear translocation of MKL1-PAGFP in Lmna–/– MEFs

This video shows an Lmna–/– mouse embryonic fibroblast (outlined in red) expressing MKL1-PAGFP after serum stimulation. Cytoplasmic MKL1-PAGFP was stimulated with a 405 nm laser and entry of the activated pool of MKL1-PAGFP is monitored for 1 minute. Frame 1 was captured before photoactivation. MKL1-PAGFP did not accumulate in the nucleus during the course of the video. (MOV 387 kb)

Photoactivation and nuclear translocation of MKL1-PAGFP in Lmna N195K MEFs

This video shows an Lmna N195K mouse embryonic fibroblast (outlined in red) expressing MKL1-PAGFP after serum stimulation. Cytoplasmic MKL1-PAGFP was stimulated with a 405 nm laser and entry of the activated pool of MKL1-PAGFP is monitored for 1 minute. Frame 1 was captured before photoactivation. MKL1-PAGFP did not accumulate in the nucleus during the course of the video. (MOV 490 kb)

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Ho, C., Jaalouk, D., Vartiainen, M. et al. Lamin A/C and emerin regulate MKL1–SRF activity by modulating actin dynamics. Nature 497, 507–511 (2013) doi:10.1038/nature12105

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