The role of the focal adhesion following varying types of muscle loading

Abstract

Introduction: Integrins are membrane-spanning heterodimers that connect the extracellular matrix (ECM) with the interior of the cell. They are capable of detecting signals from the ECM and initiating changes within the cytosol and vice versa. The most prominent integrin in skeletal muscle is the α7β1 integrin. It connects the muscle fiber to laminin and can direct signaling through focal adhesion kinase (FAK), a prominent tyrosine kinase that responds to sarcolemmal loading. In principle, FAK can initiate the activation of multiple muscle hypertrophy pathways. FAK expression and activation can be modulated by external stretch from massage and exercise and blunted with unloading. Methods: Three distinct experiments were conducted to understand the role of the α7β1 integrin pathway and its response to differing muscle loads. The first project investigated the role of instrument-assisted soft tissue manipulation (IASTM) and the α7β1 integrin response. Eleven healthy males participated in the study. Each received approximately 9 min of IASTM over their gastrocnemius muscle. Muscle biopsies were taken pre-treatment and 24, 48 and 72 h post-treatment. The second project studied the role of acetaminophen (APAP) supplementation and aerobic exercise on the integrin pathway in both the soleus and gastrocnemius. Twenty-four male Wistar rats were randomly assigned to four groups: sedentary +placebo (SED+PLAC), sedentary+APAP (SED+APAP), exercise+placebo (EX+PLAC) or exercise+APAP (EX+APAP). Rats in the APAP groups were given 200 mg/kg body weight with PLAC receiving equal volumes of saline solution. Rats in the exercise group exercised 5 d/wk for 8 wk at 20 m/min at an 8 ° grade. Exercise duration was gradually increased to 60 min/ session. The third project investigated the role of spinal cord injury (SCI) and its affect on the α7β1integrin pathway. Twenty male Wistar rats were used in this study. They were randomly placed into two groups, an SCI or sham injury (SHAM) group. SCI rats received complete transection of the spinal cord at T4 while SHAM had only the spinous process removed from the vertebrae. The gastrocnemius was removed from the rats 56 d post-surgery. Results: For the first project there were no significant interactions for time and IASTM or differences in individual time points. For the APAP project, there were no significant interaction effects for drugs and exercise. APAP supplementation increased expression of the 70 kDa α7 integrin, total and phosphorylated FAK and cSrc, and total p70s6k in the soleus independent of exercise. There were no exercise effects in the soleus. In the gastrocnemius there were no interaction effects for any of the proteins investigated. APAP supplementation decreased phosphorylated p70s6k. There were no significant exercise effects, however there were large mean increases in FAK phosphorylation and ERK1/2 levels. In rats with SCI, a large but not significant increase in the expression of the α7 integrin was seen. There was a significant decrease in total and phosphorylated FAK and phosphorylated p70s6k. Conclusion: External manipulation of skeletal muscle via IASTM has no affect on the α7β1 integrin pathway up to 72 h post-treatment. APAP supplementation can induce hypertrophic signaling in a fiber-type dependent manner, possibly through the focal adhesion and independent of exercise. SCI induces decreases in mechanotransduction signaling in the cell through FAK although there is an strong but not significant indication for an increase in α7 integrin expression.