Composition and function of AP-1 transcription complexes during muscle cell differentiation

Area of Science:

• Molecular Biology
• Cellular Biology
• Muscle Development

Background:

• The role of activating protein-1 (AP-1) in myogenesis is controversial, with studies suggesting both inhibitory and positive functions.
• This ambiguity may stem from variations in AP-1 subunit composition during muscle cell differentiation.

Purpose of the Study:

• To investigate how AP-1 subunit composition influences its function in muscle differentiation.
• To determine the specific AP-1 subunits involved and their interactions during myogenesis.

Main Methods:

• Western analysis to detect AP-1 subunit expression (c-Jun, Fra-2, JunD) during differentiation.
• Phosphatase assays to assess JunD and Fra-2 phosphorylation.
• Electrophoretic mobility shift assays (EMSA) with antibody supershifts to identify AP-1 DNA-binding complexes.
• Reporter gene assays to study the transcriptional activity of c-jun and MyoD enhancers.

Main Results:

• c-Jun, Fra-2, and JunD are expressed throughout muscle differentiation.
• Fra-2 becomes a major component of AP-1 DNA-binding complexes, heterodimerizing with c-Jun and JunD.
• AP-1 complexes containing Fra-2 can activate the MyoD enhancer/promoter, and AP-1 binding sites show enhanced activity during differentiation.
• Muscle transcription factors MEF2A and MyoD activate the c-jun enhancer, with MEF2A being the predominant isoform.

Conclusions:

• AP-1's function in myogenesis is dependent on its specific subunit composition.
• AP-1 complexes containing Fra-2, c-Jun, or JunD are associated with promoting muscle differentiation.