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Overexpression of SMPX in adult skeletal muscle does not change skeletal muscle fiber type or size.

Eftestøl E, Alver TN, Gundersen K, Bruusgaard JC - PLoS ONE (2014)

Bottom Line: The fusion protein was localized predominantly in repetitive double stripes flanking the Z-disc, and was excluded from all nuclei.This localization would be consistent with SMPX being a mechanoreceptor, but not with SMPX playing a role as a transcription factor.In vivo overexpression of ectopic SMPX in skeletal muscle of adult mice gave no significant changes in fiber type distribution or cross sectional area, thus a role of SMPX in regulating muscle phenotype remains unclear.

View Article: PubMed Central - PubMed

Affiliation: Department of Biosciences, University of Oslo, Oslo, Norway.

ABSTRACT
Mechanical factors such as stretch are thought to be important in the regulation of muscle phenotype. Small muscle protein X-linked (SMPX) is upregulated by stretch in skeletal muscle and has been suggested to serve both as a transcription factor and a mechanosensor, possibly giving rise to changes in both fiber size and fiber type. We have used in vivo confocal imaging to study the subcellular localization of SMPX in skeletal muscle fibers of adult rats using a SMPX-EGFP fusion protein. The fusion protein was localized predominantly in repetitive double stripes flanking the Z-disc, and was excluded from all nuclei. This localization would be consistent with SMPX being a mechanoreceptor, but not with SMPX playing a role as a transcription factor. In vivo overexpression of ectopic SMPX in skeletal muscle of adult mice gave no significant changes in fiber type distribution or cross sectional area, thus a role of SMPX in regulating muscle phenotype remains unclear.

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SMPX-EGFP was excluded from all myonuclei.A) C2C12 myoblasts expressing either EGFP or SMPX-EGFP stained with Hoechst 33342 to visualize nuclei. B) In vitro confocal image of dissected single rat EDL muscle fibers expressing EGFP or SMPX-EGFP stained with DAPI to visualize nuclei. C) Confocal images of EDL muscle fibers in situ, after no treatment (CON) or functional overload for 18 hours (LOAD), expressing SMPX-EGFP stained with Hoechst 33342 to visualize nuclei. Myonuclei are labeled with arrowheads. Scale bar is 10 microns.
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pone-0099232-g002: SMPX-EGFP was excluded from all myonuclei.A) C2C12 myoblasts expressing either EGFP or SMPX-EGFP stained with Hoechst 33342 to visualize nuclei. B) In vitro confocal image of dissected single rat EDL muscle fibers expressing EGFP or SMPX-EGFP stained with DAPI to visualize nuclei. C) Confocal images of EDL muscle fibers in situ, after no treatment (CON) or functional overload for 18 hours (LOAD), expressing SMPX-EGFP stained with Hoechst 33342 to visualize nuclei. Myonuclei are labeled with arrowheads. Scale bar is 10 microns.

Mentions: Our results show that SMPX-EGFP did not accumulate in the nuclei in cell culture (Figure 2A), levels were below those in the cytosol. In adult skeletal muscle fibers, SMPX-EGFP was not seen above background levels in any investigated nuclei (Figure 2B). In the control experiments with EGFP alone, the signal was localized to the nuclei in much the same intensity as in the cytosol, i.e. no accumulation of protein in the nucleus. The fusion protein was also excluded from the nuclei after 18 hours of overload (2C). As has been described in earlier reports we noticed an increased level of SMPX in the perinuclear area. This was observed in fibers from both normal and overloaded muscles (Figure 2B and 2C) (see also [12], [13]).


Overexpression of SMPX in adult skeletal muscle does not change skeletal muscle fiber type or size.

Eftestøl E, Alver TN, Gundersen K, Bruusgaard JC - PLoS ONE (2014)

SMPX-EGFP was excluded from all myonuclei.A) C2C12 myoblasts expressing either EGFP or SMPX-EGFP stained with Hoechst 33342 to visualize nuclei. B) In vitro confocal image of dissected single rat EDL muscle fibers expressing EGFP or SMPX-EGFP stained with DAPI to visualize nuclei. C) Confocal images of EDL muscle fibers in situ, after no treatment (CON) or functional overload for 18 hours (LOAD), expressing SMPX-EGFP stained with Hoechst 33342 to visualize nuclei. Myonuclei are labeled with arrowheads. Scale bar is 10 microns.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4060999&req=5

pone-0099232-g002: SMPX-EGFP was excluded from all myonuclei.A) C2C12 myoblasts expressing either EGFP or SMPX-EGFP stained with Hoechst 33342 to visualize nuclei. B) In vitro confocal image of dissected single rat EDL muscle fibers expressing EGFP or SMPX-EGFP stained with DAPI to visualize nuclei. C) Confocal images of EDL muscle fibers in situ, after no treatment (CON) or functional overload for 18 hours (LOAD), expressing SMPX-EGFP stained with Hoechst 33342 to visualize nuclei. Myonuclei are labeled with arrowheads. Scale bar is 10 microns.
Mentions: Our results show that SMPX-EGFP did not accumulate in the nuclei in cell culture (Figure 2A), levels were below those in the cytosol. In adult skeletal muscle fibers, SMPX-EGFP was not seen above background levels in any investigated nuclei (Figure 2B). In the control experiments with EGFP alone, the signal was localized to the nuclei in much the same intensity as in the cytosol, i.e. no accumulation of protein in the nucleus. The fusion protein was also excluded from the nuclei after 18 hours of overload (2C). As has been described in earlier reports we noticed an increased level of SMPX in the perinuclear area. This was observed in fibers from both normal and overloaded muscles (Figure 2B and 2C) (see also [12], [13]).

Bottom Line: The fusion protein was localized predominantly in repetitive double stripes flanking the Z-disc, and was excluded from all nuclei.This localization would be consistent with SMPX being a mechanoreceptor, but not with SMPX playing a role as a transcription factor.In vivo overexpression of ectopic SMPX in skeletal muscle of adult mice gave no significant changes in fiber type distribution or cross sectional area, thus a role of SMPX in regulating muscle phenotype remains unclear.

View Article: PubMed Central - PubMed

Affiliation: Department of Biosciences, University of Oslo, Oslo, Norway.

ABSTRACT
Mechanical factors such as stretch are thought to be important in the regulation of muscle phenotype. Small muscle protein X-linked (SMPX) is upregulated by stretch in skeletal muscle and has been suggested to serve both as a transcription factor and a mechanosensor, possibly giving rise to changes in both fiber size and fiber type. We have used in vivo confocal imaging to study the subcellular localization of SMPX in skeletal muscle fibers of adult rats using a SMPX-EGFP fusion protein. The fusion protein was localized predominantly in repetitive double stripes flanking the Z-disc, and was excluded from all nuclei. This localization would be consistent with SMPX being a mechanoreceptor, but not with SMPX playing a role as a transcription factor. In vivo overexpression of ectopic SMPX in skeletal muscle of adult mice gave no significant changes in fiber type distribution or cross sectional area, thus a role of SMPX in regulating muscle phenotype remains unclear.

Show MeSH
Related in: MedlinePlus