Muscle fibre-type specific autophagy responses following an overnight fast and mixed meal ingestion in human skeletal muscle

Aim: The aim of the present study was to investigate the fibre-type specific abundance of autophagy-related proteins after an overnight fast and following ingestion of a mixed meal in human skeletal muscle. Methods: Twelve overweight, healthy young male volunteers underwent a 3h mixed meal tolerance test following an overnight fast. Blood samples were collected at baseline and throughout the 180 min post meal period and skeletal muscle biopsies were collected at baseline, 30- and 90-min post meal ingestion. Protein content of key autophagy markers and upstream signalling responses were measured in whole muscle and pooled single fibres using immunoblotting. Results: In the fasted state, type I fibres displayed lower LC3B-I but higher LC3B-II abundance and higher LC3B-II/LC3B-I ratio compared to type II fibres (P < 0.05). However, there were no fibre type differences in p62/SQSTM1, ULK1, ATG5 or ATG12 (P > 0.05). Compared to the fasted state, there was a reduction in LC3B-II abundance, indicative of lower autophagosome content, in whole muscle and in both type I and type II fibres following meal ingestion (P < 0.05). This reduction in autophagosome content occurred alongside similar increases in p-AktS473 and p-mTORS2448 in both type I and type II muscle fibres (P < 0.05). Conclusions: In human skeletal muscle, type I fibres have a greater autophagosome content than type II fibres in the overnight fasted state despite comparable abundance of other key upstream autophagy proteins. Autophagy is rapidly inhibited in both fibre types following the ingestion of a mixed meal.