NFAT isoforms regulate muscle fiber type transition without altering CaN during aerobic training

Area of Science:

• Exercise Physiology
• Molecular Biology
• Muscle Adaptation

Background:

• Skeletal muscle exhibits plasticity in fiber type composition.
• Aerobic training is known to induce specific fiber type transitions.
• The molecular mechanisms, particularly involving transcription factors like NFAT, are not fully elucidated.

Purpose of the Study:

• To investigate the association between aerobic training-induced muscle fiber type transitions and NFAT isoforms gene expression.
• To test the hypothesis that NFATc1-c3 isoforms mediate fiber type switching during aerobic training.
• To determine if calcineurin (CaN) expression is altered during this process.

Main Methods:

• An 8-week swimming endurance training program was conducted on male Wistar rats.
• Soleus (SOL) and plantaris (PL) muscles were analyzed for morphometrical, histochemical, and molecular changes.
• Gene expression levels of NFAT isoforms and calcineurin (CaN) were quantified.

Main Results:

• Aerobic training induced a type I-to-IIA transition in SOL muscle and a type IIB-to-IIA transition in PL muscle.
• NFATc1-c3 gene expression significantly increased in both SOL and PL muscles post-training.
• Calcineurin (CaN) and NFATc4 expression levels remained unchanged.

Conclusions:

• Aerobic training-induced muscle fiber type switching is mediated by NFATc1-c3 isoforms.
• The observed fiber type transitions occur independently of changes in calcineurin (CaN) expression.