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Sorting of a nonmuscle tropomyosin to a novel cytoskeletal compartment in skeletal muscle results in muscular dystrophy.

Kee AJ, Schevzov G, Nair-Shalliker V, Robinson CS, Vrhovski B, Ghoddusi M, Qiu MR, Lin JJ, Weinberger R, Gunning PW, Hardeman EC - J. Cell Biol. (2004)

Bottom Line: Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals.These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure.Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

View Article: PubMed Central - PubMed

Affiliation: Muscle Development Unit, Children's Medical Research Institute, Locked Bag 23, Wentworthville, New South Wales 2145, Australia.

ABSTRACT
Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals. In addition to the isoforms in the sarcomere, we now report the existence of two nonsarcomeric (NS) isoforms in skeletal muscle. These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure. Immunostained cross sections indicate that one Tm defines a Z-line adjacent structure common to all myofibers, whereas the second Tm defines a spatially distinct structure unique to muscles that undergo chronic or repetitive contractions. When a Tm (Tm3) that is normally absent from muscle was expressed in mice it became associated with the Z-line adjacent structure. These mice display a muscular dystrophy and ragged-red fiber phenotype, suggestive of disruption of the membrane-associated cytoskeletal network. Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

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Expression of high levels of Tm3 in muscle has little effect on expression of sarcomeric and NS Tms. Western blots of 6-mo-old Tm3 skeletal muscle showing the influence of high levels of Tm3 expression on (A) NS or (B) sarcomeric Tm isoforms. Results for both wild-type (WT) and high-expressing transgenic lines (3/70 and 3/66) are shown. NS Tms containing exon 9d from the α- and β-TM genes are recognized by α9d. Products containing exon 1b and exon 9d from the γ-TM gene are recognized by CG3 and γ9d, respectively. A Tm3 band is detected by the γ9d antibody in Tm3 mice (3/70 and 3/66) due to cross-reactivity of the antibody with exon 9d products from the α-TM gene. The 311 antibody recognizes Tms containing exon 1a (Fig. 1), which includes Tm3 and the three sarcomeric Tms (αTmfast, αTmslow, and βTm). Under the electrophoretic conditions used the αTmfast and αTmslow isoforms are seen as a single band. See Fig. 2 legend for muscle abbreviations.
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fig7: Expression of high levels of Tm3 in muscle has little effect on expression of sarcomeric and NS Tms. Western blots of 6-mo-old Tm3 skeletal muscle showing the influence of high levels of Tm3 expression on (A) NS or (B) sarcomeric Tm isoforms. Results for both wild-type (WT) and high-expressing transgenic lines (3/70 and 3/66) are shown. NS Tms containing exon 9d from the α- and β-TM genes are recognized by α9d. Products containing exon 1b and exon 9d from the γ-TM gene are recognized by CG3 and γ9d, respectively. A Tm3 band is detected by the γ9d antibody in Tm3 mice (3/70 and 3/66) due to cross-reactivity of the antibody with exon 9d products from the α-TM gene. The 311 antibody recognizes Tms containing exon 1a (Fig. 1), which includes Tm3 and the three sarcomeric Tms (αTmfast, αTmslow, and βTm). Under the electrophoretic conditions used the αTmfast and αTmslow isoforms are seen as a single band. See Fig. 2 legend for muscle abbreviations.

Mentions: As the NS Tms appear to be part of different cytoskeletal structures in muscle we were interested to examine what effect the expression of an inappropriate cytoskeletal Tm would have on the Tms defining these structures. Therefore, we have studied transgenic mice that express an NS Tm, Tm3, that is not normally expressed in skeletal muscle (Fig. 7 A). Western blots with the α9d antibody, show that transgenic lines 3/66 and 3/70 accumulate very high levels of Tm3 in a wide range of muscles (Fig. 7 A). Tm3 is also detected by γ9d in the transgenic mice (Fig. 7 A), as there is some cross-reactivity of the antibody to exon 9d from the α- and β-TM genes.


Sorting of a nonmuscle tropomyosin to a novel cytoskeletal compartment in skeletal muscle results in muscular dystrophy.

Kee AJ, Schevzov G, Nair-Shalliker V, Robinson CS, Vrhovski B, Ghoddusi M, Qiu MR, Lin JJ, Weinberger R, Gunning PW, Hardeman EC - J. Cell Biol. (2004)

Expression of high levels of Tm3 in muscle has little effect on expression of sarcomeric and NS Tms. Western blots of 6-mo-old Tm3 skeletal muscle showing the influence of high levels of Tm3 expression on (A) NS or (B) sarcomeric Tm isoforms. Results for both wild-type (WT) and high-expressing transgenic lines (3/70 and 3/66) are shown. NS Tms containing exon 9d from the α- and β-TM genes are recognized by α9d. Products containing exon 1b and exon 9d from the γ-TM gene are recognized by CG3 and γ9d, respectively. A Tm3 band is detected by the γ9d antibody in Tm3 mice (3/70 and 3/66) due to cross-reactivity of the antibody with exon 9d products from the α-TM gene. The 311 antibody recognizes Tms containing exon 1a (Fig. 1), which includes Tm3 and the three sarcomeric Tms (αTmfast, αTmslow, and βTm). Under the electrophoretic conditions used the αTmfast and αTmslow isoforms are seen as a single band. See Fig. 2 legend for muscle abbreviations.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2172434&req=5

fig7: Expression of high levels of Tm3 in muscle has little effect on expression of sarcomeric and NS Tms. Western blots of 6-mo-old Tm3 skeletal muscle showing the influence of high levels of Tm3 expression on (A) NS or (B) sarcomeric Tm isoforms. Results for both wild-type (WT) and high-expressing transgenic lines (3/70 and 3/66) are shown. NS Tms containing exon 9d from the α- and β-TM genes are recognized by α9d. Products containing exon 1b and exon 9d from the γ-TM gene are recognized by CG3 and γ9d, respectively. A Tm3 band is detected by the γ9d antibody in Tm3 mice (3/70 and 3/66) due to cross-reactivity of the antibody with exon 9d products from the α-TM gene. The 311 antibody recognizes Tms containing exon 1a (Fig. 1), which includes Tm3 and the three sarcomeric Tms (αTmfast, αTmslow, and βTm). Under the electrophoretic conditions used the αTmfast and αTmslow isoforms are seen as a single band. See Fig. 2 legend for muscle abbreviations.
Mentions: As the NS Tms appear to be part of different cytoskeletal structures in muscle we were interested to examine what effect the expression of an inappropriate cytoskeletal Tm would have on the Tms defining these structures. Therefore, we have studied transgenic mice that express an NS Tm, Tm3, that is not normally expressed in skeletal muscle (Fig. 7 A). Western blots with the α9d antibody, show that transgenic lines 3/66 and 3/70 accumulate very high levels of Tm3 in a wide range of muscles (Fig. 7 A). Tm3 is also detected by γ9d in the transgenic mice (Fig. 7 A), as there is some cross-reactivity of the antibody to exon 9d from the α- and β-TM genes.

Bottom Line: Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals.These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure.Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

View Article: PubMed Central - PubMed

Affiliation: Muscle Development Unit, Children's Medical Research Institute, Locked Bag 23, Wentworthville, New South Wales 2145, Australia.

ABSTRACT
Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals. In addition to the isoforms in the sarcomere, we now report the existence of two nonsarcomeric (NS) isoforms in skeletal muscle. These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure. Immunostained cross sections indicate that one Tm defines a Z-line adjacent structure common to all myofibers, whereas the second Tm defines a spatially distinct structure unique to muscles that undergo chronic or repetitive contractions. When a Tm (Tm3) that is normally absent from muscle was expressed in mice it became associated with the Z-line adjacent structure. These mice display a muscular dystrophy and ragged-red fiber phenotype, suggestive of disruption of the membrane-associated cytoskeletal network. Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

Show MeSH
Related in: MedlinePlus