Evidence for skeletal muscle fiber type-specific expressions of mechanosensors
Mechanosensors govern muscle tissue integrity and constitute a subcellular structure known as costameres. Costameres physically link the muscle extracellular matrix to contractile and signaling ‘hubs’ inside muscle fibers mainly via integrins and are localized beneath sarcolemmas of muscle fibers. Costameres are the main mechanosensors converting mechanical cues into biological events. However, the fiber type-specific costamere architecture in muscles is unexplored. We hypothesized that fiber types differ in the expression of genes coding for costamere components. By coupling laser microdissection to a multiplex tandem qPCR approach, we demonstrate that type 1 and type 2 fibers indeed show substantial differences in their mechanosensor complexes. We confirmed these data by fiber type population-specific protein analysis and confocal microscopy-based localization studies. We further show that knockdown of the costamere gene integrin-linked kinase (Ilk) in muscle precursor cells results in significantly increased slow-myosin-coding Myh7 gene, while the fast-myosin-coding genes Myh1, Myh2, and Myh4 are downregulated. In parallel, protein synthesis-enhancing signaling molecules (p-mTORSer2448, p < 0.05; p-P70S6KThr389, tendency with p < 0.1) were reduced upon Ilk knockdown. However, overexpression of slow type-inducing NFATc1 in muscle precursor cells did not change Ilk or other costamere gene expressions. In addition, we demonstrate fiber type-specific costamere gene regulation upon mechanical loading and unloading conditions. Our data imply that costamere genes, such as Ilk, are involved in the control of muscle fiber characteristics. Further, they identify costameres as muscle fiber type-specific loading management ‘hubs’ and may explain adaptation differences of muscle fiber types to mechanical (un)loading.
KeywordsSkeletal muscle Fiber types Costamere Integrin-linked kinase Laser microdissection Multiplex PCR
This study was supported by the German Sport University (920122 and 920119), the Federal Institute of Sports Sciences (IIA1-070114/13), and a KU Leuven Grant (ZKD2412) to F.S. The authors thank Bianca Collins and Mojgan Ghilav (both Department of Molecular Cellular Sport Medicine, German Sport University Cologne) for their excellent technical assistance. Professor Rik Lories (Skeletal Biology and Engineering Research Center, KU Leuven) is highly acknowledged for critically reading the manuscript.
FS designed the study. SM, MV and FS carried out experiments, performed statistics and interpreted the data. WB contributed to data interpretation and reagents. SM and FS prepared the figures. FS wrote the manuscript. All authors approved the final version of the manuscript.
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Conflict of interest
The authors declare that they have no competing interests.
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