Multiple signalling pathways redundantly control glucose transporter GLUT4 gene transcription in skeletal muscle

Area of Science:

• Muscle physiology
• Molecular biology
• Metabolic regulation

Background:

• Regular exercise increases skeletal muscle glucose transporter GLUT4 (solute carrier family 2, member 4), enhancing insulin sensitivity and glucose tolerance.
• Pharmacological studies suggest Ca(2+)-calmodulin-dependent kinase II (CaMKII), calcineurin, and AMP-activated protein kinase (AMPK) pathways regulate GLUT4 gene expression.

Purpose of the Study:

• To genetically determine the specific roles of CaMKII, calcineurin, and AMPK in regulating GLUT4 expression in distinct muscle types (fast vs. slow).
• To elucidate the interplay between these signaling pathways in controlling GLUT4 gene transcription.

Main Methods:

• In vivo transfection of plasmids encoding protein inhibitors of CaMKII or calcineurin into mouse skeletal muscle.
• Co-transfection with a GLUT4 enhancer-reporter construct in normal mice and mice expressing a kinase-dead (KD) AMPK mutant.
• Analysis of GLUT4 reporter activity in slow soleus and fast tibialis anterior (TA) muscles.

Main Results:

• In slow soleus muscle, blocking CaMKII or calcineurin alone did not affect GLUT4 reporter activity, but combined inhibition did.
• In KD-AMPK mice soleus, GLUT4 reporter activity was unaffected by AMPK mutation but reduced by CaMKII or calcineurin inhibition.
• In fast tibialis anterior (TA) muscle, calcineurin significantly controlled GLUT4 reporter activity independently of CaMKII and AMPK.

Conclusions:

• GLUT4 gene regulation in skeletal muscle is muscle-type specific and involves redundant signaling pathways.
• CaMKII, calcineurin, and AMPK pathways converge on MEF2 transcriptional activity to regulate GLUT4 expression.
• Understanding these pathways offers insights into exercise benefits and metabolic disease interventions.