Year of Publication

2018

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Education

Department

Kinesiology and Health Promotion

First Advisor

Dr. Lance Bollinger

Second Advisor

Dr. Mark Abel

Abstract

Autophagy is a major degradation mechanism, responsible for clearing damaged and dysfunctional organelles, including the endoplasmic reticulum, a structure essential for protein synthesis and myocellular hypertrophy. Alterations in autophagy throughout various tissues of the body have been linked to various negative side effects such as decreased myocellular hypertrophy and insulin resistance. High fat diets lead to changes (both increases and decreases) in autophagy in various tissues throughout the body in a tissue-specific manner.

Skeletal muscle autophagy is decreased in myotubes cultured from obese women, however the mechanism by which this occurs is unknown. As the largest organ system in the human body, skeletal muscle serves an important role in overall metabolic health. Therefore, sufficient skeletal muscle autophagy is important for proper metabolic function. Moreover, a decrease in liver and pancreas autophagy has been found to lead to endoplasmic reticulum (ER) stress and the development of insulin resistance. Understanding the relationship between autophagy and ER stress in the skeletal muscle following a high fat diet may help elucidate a novel target for decreasing negative side effects.

Interestingly, both acute and chronic exercise have been shown to increase skeletal muscle autophagy. This points to a potential therapeutic treatment for those suffering with decreased skeletal muscle autophagy and may help improve ER stress.

The purpose of this study was to compare the in vivo and in vitro effects of high fat exposure on skeletal muscle autophagy. Additionally, the relationship of autophagy and ER stress in skeletal muscle was explored. Lastly, this project identified changes in skeletal muscle autophagy and ER stress following cyclic stretch, an in vitro model of exercise in C2C12 myotubes.

Eight-week-old C57BL/6J were fed a high fat diet for 16 weeks and tibialis anterior muscle examined for changes in autophagy markers. Gene expression (mRNA content) of autophagy markers Atg3 (p=0.011, fold change 1.37), Atg12 (p=0.026, 1.38), and Atg16L (p=0.004, 1.49) were increased in skeletal muscle of obese mice. Protein content was also measured, where increases in Atg3 (p = 0.04, 1.22), Atg12 (p = 0.027, 1.21), and Atg16L1(p = 0.021, 1.59) were found. However, there was no difference in LC3 II:I ration. No changes were seen in Atg5 or LC3.

Additionally, C2C12 myotubes were treated with equimolar palmitate and oleate for 24h then assessed for mRNA content of genes involved in autophagy and ER stress. Autophagy genes Atg5 (p = 0.007, fold change 1.78), Atg12 (p = 0.001, fold change 1.99), and LC3 (p = 0.01, fold change 2.02) were decreased with high fat treatment. Paradoxically, there was an increase in Atg16L (p = 0.005, fold change 1.90). There were no changes in protein content. ER stress was increased indicated by an increase of sXBP1 (p = 0.005, fold change 1.33). Furthermore, inhibition of autophagy lead to changes in ER morphology and ER stress.

To identify the impact of cyclic stretch on skeletal muscle autophagy and ER stress, C2C12 myotubes were subjected to 30 minutes of equibaxial stretch and examined for changes in autophagy and ER stress. Autophagy flux, measured by tyrosine release, increased by 34% (p = 0.04) following exercise and ER stress was decreased.

In conclusion, this study provides the novel finding that decreased skeletal muscle autophagy is sufficient for inducing ER stress. Additionally, cyclic stretch increases autophagy and improves ER homeostasis.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2018.299

Available for download on Monday, July 27, 2020

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