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Regulation of the growth of multinucleated muscle cells by an NFATC2-dependent pathway.

Horsley V, Friday BB, Matteson S, Kegley KM, Gephart J, Pavlath GK - J. Cell Biol. (2001)

Bottom Line: The growth defect is intrinsic to muscle cells, since the lack of NFATC2 in primary muscle cultures results in reduced cell size and myonuclear number in myotubes.Taken together, these results implicate a novel role for NFATC2 in skeletal muscle growth.We demonstrate that during growth of multinucleated muscle cells, myoblasts initially fuse to form myotubes with a limited number of nuclei and that subsequent nuclear addition and increases in myotube size are controlled by a molecular pathway regulated by NFATC2.

View Article: PubMed Central - PubMed

Affiliation: Department of Pharmacology, Emory University, Atlanta, Georgia 30322, USA.

ABSTRACT
The nuclear factor of activated T cells (NFAT) family of transcription factors regulates the development and differentiation of several tissue types. Here, we examine the role of NFATC2 in skeletal muscle by analyzing adult NFATC2(-/)- mice. These mice exhibit reduced muscle size due to a decrease in myofiber cross-sectional area, suggesting that growth is blunted. Muscle growth was examined during regeneration after injury, wherein NFATC2- myofibers form normally but display impaired growth. The growth defect is intrinsic to muscle cells, since the lack of NFATC2 in primary muscle cultures results in reduced cell size and myonuclear number in myotubes. Retroviral-mediated expression of NFATC2 in the mutant cells rescues this cellular phenotype. Myonuclear number is similarly decreased in NFATC2(-/)- mice. Taken together, these results implicate a novel role for NFATC2 in skeletal muscle growth. We demonstrate that during growth of multinucleated muscle cells, myoblasts initially fuse to form myotubes with a limited number of nuclei and that subsequent nuclear addition and increases in myotube size are controlled by a molecular pathway regulated by NFATC2.

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Reduced muscle size in NFATC2−/− soleus muscles is due to a decrease in myofiber CSA. (A) A comparison of hematoxylin and eosin–stained sections of wild-type and NFATC2−/− soleus muscles clearly demonstrates the smaller size of the mutant muscle (top). At higher magnification, CSA of individual NFATC2−/− myofibers is also smaller (bottom). (B) The CSA of both type I and type II MyHC-expressing myofibers is reduced in the NFATC2−/− muscles. (C) No difference exists in the total number of myofibers in NFATC2−/− soleus muscles compared with wild-type. (D) Representative sections immunostained with antibodies against type I and type II MyHC are shown (left). A small decrease in the percentage of myofibers expressing type I MyHC occurs, but no difference exists in the percentage of type II myofibers (right). Data are mean ± standard error; n = 5–6 for wild-type and n = 5–6 for NFATC2−/− (*P < 0.05). Bars: (A, top) 200 μm; (A, bottom and D) 60 μm.
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Figure 1: Reduced muscle size in NFATC2−/− soleus muscles is due to a decrease in myofiber CSA. (A) A comparison of hematoxylin and eosin–stained sections of wild-type and NFATC2−/− soleus muscles clearly demonstrates the smaller size of the mutant muscle (top). At higher magnification, CSA of individual NFATC2−/− myofibers is also smaller (bottom). (B) The CSA of both type I and type II MyHC-expressing myofibers is reduced in the NFATC2−/− muscles. (C) No difference exists in the total number of myofibers in NFATC2−/− soleus muscles compared with wild-type. (D) Representative sections immunostained with antibodies against type I and type II MyHC are shown (left). A small decrease in the percentage of myofibers expressing type I MyHC occurs, but no difference exists in the percentage of type II myofibers (right). Data are mean ± standard error; n = 5–6 for wild-type and n = 5–6 for NFATC2−/− (*P < 0.05). Bars: (A, top) 200 μm; (A, bottom and D) 60 μm.

Mentions: Our previous studies in cultured muscle cells indicated that individual NFAT isoforms are specifically induced to undergo nuclear translocation in response to changes in intracellular calcium at different stages in myogenesis (Abbott et al. 1998). Such activation of NFATC2 occurred only in small nascent myotubes, suggesting that NFATC2 may play a role in regulating growth of differentiated muscle cells. To test this hypothesis in vivo, we analyzed the soleus muscles from adult wild-type and NFATC2−/− mice. Soleus muscles were examined because in BALB/c mice this muscle is composed of approximately equal amounts of slow and fast myofibers (Kegley et al. 2001), allowing us to determine if NFATC2 has similar roles in different fiber types. As seen in the top of Fig. 1 A, the CSA of soleus muscles from NFATC2−/− mice is smaller than wild-type muscles. This size difference correlates with a decrease in the CSA of individual myofibers (Fig. 1 A, bottom). To determine if type I or type II myofibers are differentially affected by the absence of NFATC2, the CSA of myofibers expressing type I and type II MyHC was measured. The CSA of both type I and type II myofibers is decreased significantly in NFATC2−/− mice (Fig. 1 B) with a 44 and 33% decrease, respectively. These results suggest that NFATC2 plays a role in regulating myofiber size.


Regulation of the growth of multinucleated muscle cells by an NFATC2-dependent pathway.

Horsley V, Friday BB, Matteson S, Kegley KM, Gephart J, Pavlath GK - J. Cell Biol. (2001)

Reduced muscle size in NFATC2−/− soleus muscles is due to a decrease in myofiber CSA. (A) A comparison of hematoxylin and eosin–stained sections of wild-type and NFATC2−/− soleus muscles clearly demonstrates the smaller size of the mutant muscle (top). At higher magnification, CSA of individual NFATC2−/− myofibers is also smaller (bottom). (B) The CSA of both type I and type II MyHC-expressing myofibers is reduced in the NFATC2−/− muscles. (C) No difference exists in the total number of myofibers in NFATC2−/− soleus muscles compared with wild-type. (D) Representative sections immunostained with antibodies against type I and type II MyHC are shown (left). A small decrease in the percentage of myofibers expressing type I MyHC occurs, but no difference exists in the percentage of type II myofibers (right). Data are mean ± standard error; n = 5–6 for wild-type and n = 5–6 for NFATC2−/− (*P < 0.05). Bars: (A, top) 200 μm; (A, bottom and D) 60 μm.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2169453&req=5

Figure 1: Reduced muscle size in NFATC2−/− soleus muscles is due to a decrease in myofiber CSA. (A) A comparison of hematoxylin and eosin–stained sections of wild-type and NFATC2−/− soleus muscles clearly demonstrates the smaller size of the mutant muscle (top). At higher magnification, CSA of individual NFATC2−/− myofibers is also smaller (bottom). (B) The CSA of both type I and type II MyHC-expressing myofibers is reduced in the NFATC2−/− muscles. (C) No difference exists in the total number of myofibers in NFATC2−/− soleus muscles compared with wild-type. (D) Representative sections immunostained with antibodies against type I and type II MyHC are shown (left). A small decrease in the percentage of myofibers expressing type I MyHC occurs, but no difference exists in the percentage of type II myofibers (right). Data are mean ± standard error; n = 5–6 for wild-type and n = 5–6 for NFATC2−/− (*P < 0.05). Bars: (A, top) 200 μm; (A, bottom and D) 60 μm.
Mentions: Our previous studies in cultured muscle cells indicated that individual NFAT isoforms are specifically induced to undergo nuclear translocation in response to changes in intracellular calcium at different stages in myogenesis (Abbott et al. 1998). Such activation of NFATC2 occurred only in small nascent myotubes, suggesting that NFATC2 may play a role in regulating growth of differentiated muscle cells. To test this hypothesis in vivo, we analyzed the soleus muscles from adult wild-type and NFATC2−/− mice. Soleus muscles were examined because in BALB/c mice this muscle is composed of approximately equal amounts of slow and fast myofibers (Kegley et al. 2001), allowing us to determine if NFATC2 has similar roles in different fiber types. As seen in the top of Fig. 1 A, the CSA of soleus muscles from NFATC2−/− mice is smaller than wild-type muscles. This size difference correlates with a decrease in the CSA of individual myofibers (Fig. 1 A, bottom). To determine if type I or type II myofibers are differentially affected by the absence of NFATC2, the CSA of myofibers expressing type I and type II MyHC was measured. The CSA of both type I and type II myofibers is decreased significantly in NFATC2−/− mice (Fig. 1 B) with a 44 and 33% decrease, respectively. These results suggest that NFATC2 plays a role in regulating myofiber size.

Bottom Line: The growth defect is intrinsic to muscle cells, since the lack of NFATC2 in primary muscle cultures results in reduced cell size and myonuclear number in myotubes.Taken together, these results implicate a novel role for NFATC2 in skeletal muscle growth.We demonstrate that during growth of multinucleated muscle cells, myoblasts initially fuse to form myotubes with a limited number of nuclei and that subsequent nuclear addition and increases in myotube size are controlled by a molecular pathway regulated by NFATC2.

View Article: PubMed Central - PubMed

Affiliation: Department of Pharmacology, Emory University, Atlanta, Georgia 30322, USA.

ABSTRACT
The nuclear factor of activated T cells (NFAT) family of transcription factors regulates the development and differentiation of several tissue types. Here, we examine the role of NFATC2 in skeletal muscle by analyzing adult NFATC2(-/)- mice. These mice exhibit reduced muscle size due to a decrease in myofiber cross-sectional area, suggesting that growth is blunted. Muscle growth was examined during regeneration after injury, wherein NFATC2- myofibers form normally but display impaired growth. The growth defect is intrinsic to muscle cells, since the lack of NFATC2 in primary muscle cultures results in reduced cell size and myonuclear number in myotubes. Retroviral-mediated expression of NFATC2 in the mutant cells rescues this cellular phenotype. Myonuclear number is similarly decreased in NFATC2(-/)- mice. Taken together, these results implicate a novel role for NFATC2 in skeletal muscle growth. We demonstrate that during growth of multinucleated muscle cells, myoblasts initially fuse to form myotubes with a limited number of nuclei and that subsequent nuclear addition and increases in myotube size are controlled by a molecular pathway regulated by NFATC2.

Show MeSH
Related in: MedlinePlus