Repeated electrical and mechanical stimulation causes Foxo1-GFP efflux from the nucleus of isolated muscle fibers.

Abstract

Skeletal muscle is one of the main tissues in the human body in normal-weight subjects. The integrity of this tissue depends on the balance between protein synthesis and degradation. Foxo proteins correspond to a family of transcription factors that participate as a negative regulator promoting muscle atrophy. Flexor digitorum brevis (FDB) skeletal muscle fibers were isolated from adult (4–6 weeks old) female CD1 mice. Foxo-GFP trafficking was determined as a function of time and nucleus-to-cytoplasm (N/C) ratio. Our laboratory has shown that trophic factors such as IGF-1 or insulin promote Foxo efflux from the nucleus, preventing atrogen expression. Foxo activation in this pathway is Akt-dependent. On the other hand, the use of trains of 5 s duration of repeated electrical stimulation at low frequency (10 Hz) decreases Foxo entry into the nucleus in isolated fibers compared to a basal condition without electrical stimulation. Other studies have shown that mechanical stress in muscle without electrical stimulation can phosphorylate Foxo through an Akt-dependent pathway, allowing it to efflux from the nucleus. To study the effect of this type of stress on muscle, we will use a passive mechanical deformation system on individual fibers to determine Foxo trafficking. Our findings suggest that Foxo may be a key regulator in skeletal muscle adaptation to mechanical stress and stimulation frequency. Studying these mechanisms could be relevant to better understand how skeletal muscle fibers respond to different environmental stimuli, which could have therapeutic implications in diseases such as muscular dystrophy and sarcopenia.