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Differentially expressed fibroblast growth factors regulate skeletal muscle development through autocrine and
K Hannon1, A J Kudla, M J McAvoy
1Department of Biochemistry, Purdue University, West Lafayette, Indiana 47907, USA.
Abstract:
Several FGF family members are expressed in skeletal muscle; however, the roles of these factors in skeletal muscle development are unclear. We examined the RNA expression, protein levels, and biological activities of the FGF family in the MM14 mouse skeletal muscle cell line. Proliferating skeletal muscle cells express FGF-1, FGF-2, FGF-6, and FGF-7 mRNA. Differentiated myofibers express FGF-5, FGF-7, and reduced levels of FGF-6 mRNA. FGF-3, FGF-4, and FGF-8 were not detectable by RT-PCR in either proliferating or differentiated skeletal muscle cells. FGF-I and FGF-2 proteins were present in proliferating skeletal muscle cells, but undetectable after terminal differentiation. We show that transfection of expression constructs encoding FGF-1 or FGF-2 mimics the effects of exogenously applied FGFs, inhibiting skeletal muscle cell differentiation and stimulating DNA synthesis. These effects require activation of an FGF tyrosine kinase receptor as they are blocked by transfection of a dominant negative mutant FGF receptor. Transient transfection of cells with FGF-1 or FGF-2 expression constructs exerted a global effect on myoblast DNA synthesis, as greater than 50% of the nontransfected cells responded by initiating DNA synthesis. The global effect of cultures transfected with FGF-2 expression vectors was blocked by an anti-FGF-2 monoclonal antibody, suggesting that FGF-2 was exported from the transfected cells. Despite the fact that both FGF-l and FGF-2 lack secretory signal sequences, when expressed intracellularly, they regulate skeletal muscle development. Thus, production of FGF-1 and FGF-2 by skeletal muscle cells may act as a paracrine and autocrine regulator of skeletal muscle development in vivo.
Insights
Fibroblast Growth Factors (FGFs), particularly FGF-1 and FGF-2, are crucial for skeletal muscle development. Intracellular expression of these FGFs regulates muscle cell differentiation and DNA synthesis, acting as paracrine and autocrine factors.
Area of Science:
- Cell Biology
- Molecular Biology
- Developmental Biology
Background:
- The roles of Fibroblast Growth Factors (FGFs) in skeletal muscle development are not fully understood.
- Several FGF family members are known to be expressed in skeletal muscle tissue.
Purpose of the Study:
- To investigate the expression, protein levels, and biological activities of FGF family members in a mouse skeletal muscle cell line (MM14).
- To elucidate the specific roles of FGF-1 and FGF-2 in regulating skeletal muscle cell differentiation and proliferation.
Main Methods:
- RT-PCR was used to detect FGF mRNA expression in proliferating and differentiated skeletal muscle cells.
- Western blotting or similar techniques were used to assess FGF protein levels.
- Transfection of expression constructs for FGF-1 and FGF-2, as well as dominant-negative FGF receptor mutants, was performed.
- Monoclonal antibodies against FGF-2 were used to investigate protein export.
Main Results:
- FGF-1, FGF-2, FGF-6, and FGF-7 mRNA were detected in proliferating cells; FGF-5, FGF-7, and reduced FGF-6 in differentiated cells.
- FGF-1 and FGF-2 proteins were present in proliferating cells but absent after differentiation.
- Intracellular expression of FGF-1 or FGF-2 inhibited differentiation and stimulated DNA synthesis, dependent on FGF receptor activation.
- FGF-2 expression stimulated DNA synthesis in non-transfected cells, indicating export and paracrine/autocrine activity.
Conclusions:
- FGF-1 and FGF-2, despite lacking secretory signals, are produced by skeletal muscle cells and regulate muscle development.
- These FGFs function as paracrine and autocrine regulators of skeletal muscle development in vivo.
- FGF signaling is critical for controlling skeletal muscle cell proliferation and differentiation.