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Troponin T nuclear localization and its role in aging skeletal muscle

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Abstract

Troponin T (TnT) is known to mediate the interaction between Tn complex and tropomyosin (Tm), which is essential for calcium-activated striated muscle contraction. This regulatory function takes place in the myoplasm, where TnT binds Tm. However, recent findings of troponin I and Tm nuclear translocation in Drosophila and mammalian cells imply other roles for the Tn–Tm complex. We hypothesized that TnT plays a nonclassical role through nuclear translocation. Immunoblotting with different antibodies targeting the NH2- or COOH-terminal region uncovered a pool of fast skeletal muscle TnT3 localized in the nuclear fraction of mouse skeletal muscle as either an intact or fragmented protein. Construction of TnT3–DsRed fusion proteins led to the further observation that TnT3 fragments are closely related to nucleolus and RNA polymerase activity, suggesting a role for TnT3 in regulating transcription. Functionally, overexpression of TnT3 fragments produced significant defects in nuclear shape and caused high levels of apoptosis. Interestingly, nuclear TnT3 and its fragments were highly regulated by aging, thus creating a possible link between the deleterious effects of TnT3 and sarcopenia. We propose that changes in nuclear TnT3 and its fragments cause the number of myonuclei to decrease with age, contributing to muscle damage and wasting.

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Abbreviations

TnT3:

Troponin T3

Tm:

Tropomyosin

ActD:

Actinomycin D

Pol:

Polymerase

PML:

Promyelocytic leukemia

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Acknowledgments

This study was supported by grants from the National Institutes of Health/National Institute on Aging (AG13934, AG033385, AG15820, and FIRCA-BB TW008091) and the Muscular Dystrophy Association (MDA #33149) to Osvaldo Delbono and the Wake Forest Claude D. Pepper Older Americans Independence Center (P30-AG21332).

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Authors and Affiliations

  1. Department of Internal Medicine-Gerontology and Geriatric Medicine, Wake Forest School of Medicine, 1 Medical Center Blvd., Winston-Salem, NC, 27157, USA

    Tan Zhang, Alexander Birbrair, Zhong-Min Wang, Jackson Taylor, María Laura Messi & Osvaldo Delbono

  2. Neuroscience Program, Wake Forest School of Medicine, 1 Medical Center Blvd., Winston-Salem, NC, 27157, USA

    Alexander Birbrair, Jackson Taylor & Osvaldo Delbono

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  1. Tan Zhang
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  2. Alexander Birbrair
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  3. Zhong-Min Wang
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  6. Osvaldo Delbono
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Correspondence to Osvaldo Delbono.

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Fig. S1

Construction of TnT3/DsRed cDNAs by PCR and subcloning. Primers targeting different regions (arrows) of the mouse TnT3 cDNA were designed with HindIII or SacII restriction enzyme cutting sequences overhanging at the 5′ end of each primer. PCR products were subcloned into pDsRed2-N1 multiple cloning sites (MCS) after HindIII and SacII digestion to finally construct TnFL/DsRed, TnNT/DsRed, TnM/DsRed, and TnCT/DsRed, respectively (TIFF 5576 kb)

Fig. S2

Flowchart representing sample preparation for whole cell lysis, cytosolic, myofibrillar, and nuclear fractions from mouse TA skeletal muscle. Fractions were subsequently analyzed by western blotting (Fig. 1a) to confirm the successful separation of nuclear fraction from myofibrillar fraction and the presence of endogenous nuclear TnT3. Trichloroacetic acid (TCA) was used to precipitate and concentrate the total protein from W, C, and N fractions (TIFF 1245 kb)

Fig. S3

Subcellular localization of N-terminally GFP-tagged TnT3 in C2C12 cells. When transiently overexpressed in the C2C12 myoblasts, GFP/TnFL shows similar nuclear localization to that of TnFL/DsRed. Scale bar, 20 μm (TIFF 1503 kb)

Fig. S4

TnFL/DsRed, but not TnCT/DsRed, showed a striated distribution pattern in addition to its myonuclear localization when transiently overexpressed in myofibers. FVB fibers transiently transfected with TnFL/DsRed or TnCT/DsRed were isolated and imaged under fluorescence microscope. By overexposure, with the nuclear DsRed saturated, only the TnFL/DsRed transfected fibers showed a striated fluorescent pattern. Scale bar, 50 μm (TIFF 6511 kb)

Fig. S5

DsRed overexpression in mouse FDB muscle fiber. Control DsRed protein diffuses throughout the muscle fiber. Scale bar, 50 μm (TIFF 4968 kb)

Fig. S6

TnCT/DsRed and TnFL/DsRed overexpression induces apoptosis in NIH3T3 fibroblasts. As with C2C12, NIH3T3 cells were analyzed with a FACSCalibur flow cytometer 48 h posttransfection. Data were collected on at least 100,000 freshly stained cells. Representative analyses of 7-AAD and Annexin V staining (B) followed pregating on DsRed (A). B Results were plotted as fluorescence intensity of Annexin V as a function of fluorescence intensity of 7-AAD. The numbers in each square are explained for Fig. 8. C The percent of total apoptotic cells was obtained by adding early and late apoptotic cells (TIFF 6362 kb)

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Zhang, T., Birbrair, A., Wang, ZM. et al. Troponin T nuclear localization and its role in aging skeletal muscle. AGE 35, 353–370 (2013). https://doi.org/10.1007/s11357-011-9368-4

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