Myofibril breakdown during atrophy is a delayed response requiring the transcription factor PAX4 and desmin...

Area of Science:

• Muscle Biology
• Cellular Degradation Pathways
• Biochemistry

Background:

• Muscle atrophy is characterized by excessive myofibrillar protein breakdown via the ubiquitin proteasome system.
• The ubiquitin ligase tripartite motif-containing protein 32 (Trim32) degrades desmin and Z-band-bound filaments during fasting-induced atrophy.

Purpose of the Study:

• To elucidate the sequential events in myofibril destruction during denervation-induced muscle atrophy.
• To investigate the roles of Trim32 and paired box 4 (PAX4) in the two-phase process of myofibril breakdown.

Main Methods:

• Investigated muscle atrophy in mouse tibialis anterior muscles following denervation.
• Analyzed desmin phosphorylation, ubiquitination, and depolymerization.
• Assessed the impact of Trim32 and PAX4 down-regulation on myofibril integrity and protein degradation.
• Examined the role of the p97/VCP ATPase in myofibril disassembly.

Main Results:

• Denervation induced rapid desmin phosphorylation and Trim32-dependent ubiquitination, leading to gradual desmin filament depolymerization.
• Trim32 down-regulation reduced desmin loss and overall myofibrillar protein degradation, mitigating atrophy.
• Myofibril breakdown at later stages (14 days) required both desmin filament dissociation and PAX4-mediated gene induction.
• Down-regulation of PAX4 or p97/VCP attenuated myofibril disassembly and degradation during both denervation and fasting-induced atrophy.

Conclusions:

• Muscle atrophy proceeds in a two-phase manner, initiated by Trim32-mediated desmin disassembly.
• PAX4 activation is crucial for the subsequent enzymatic degradation of myofibrils, particularly over time.
• The initial loss of desmin is a critical prerequisite for subsequent myofibril destruction during atrophy.