Skeletal muscle and adipose tissue omics and phenotypes : a comparison of highly trained and untrained males and females

Regular exercise training or physical activity improves athletic performance and reduces the risk of developing noncommunicable diseases such as diabetes, hypertension and cancer. This thesis aimed to examine the physiological and molecular adaptations to long-term exercise training, with a particular focus on the role of sex differences. To do so, three groups of males and females with distinctly different exercise training backgrounds were recruited, 1) highly endurance-trained individuals with at least 15 years of experience of regular endurance exercise, 2) highly strength-trained individuals with at least 15 years of experience of regular resistance exercise, and 3) a healthy untrained age- matched group of individuals not participating in regular exercise training.

Whole-body magnetic resonance imaging revealed that both endurance- and strength-trained individuals, compared with untrained, displayed a greater relative whole-body muscle mass, larger muscle cross-sectional areas, lower adipose tissue volumes and lower fatty infiltration in skeletal muscles of both the upper and lower body. However, this finding was more pronounced in males than in females.

To better understand the underlying molecular mechanisms of the skeletal muscle phenotypic differences, skeletal muscle transcriptomic and proteomic analyses were performed. The transcriptomic analysis showed large differences in the resting baseline muscle transcriptome between endurance trained and untrained individuals. The proteomic analyses further unraveled the molecular adaptations to exercise and largely confirmed the skeletal muscle phenotype observed in the transcriptomic analysis. Over 650 proteins were identified as differentially expressed in both endurance-trained men and women compared with their respective control groups. Of these, a striking 92% were mitochondrial proteins. While large differences were detected between the endurance-trained and untrained, few transcriptomic or proteomic differences were identified in the strength-trained group compared with the untrained, despite the distinct phenotypic and physiological performance differences between these groups. Furthermore, prominent sex differences between male and female controls were identified on the transcriptomic level, however these differences were reduced in the endurance trained state. The proteomic analysis confirmed the transcriptomic data suggesting the skeletal muscle sex differences identified in the untrained individuals are almost completely diminished in endurance-trained.

Next, exercise training influences whole-body metabolism and thus, other peripheral tissues than skeletal muscle are affected. As such, the adipose tissue has recently gained interest in the field of exercise physiology, however, it is less well studied in an exercise physiology context than skeletal muscle. So, subcutaneous abdominal white adipose tissue (scWAT) showed distinct gene expression patterns related to aerobic metabolism and immune response, which were notably influenced by the training status of the individuals. The study also identified significant sex differences in the transcriptomic profiles of long-term trained and untrained individuals, with female scWAT related to enhanced aerobic respiration while the male scWAT was enriched for inflammatory pathways. However, the inflammatory profile of male scWAT was significantly attenuated in endurance-trained vs untrained males. These findings underscore the importance of regular exercise training for improving or maintaining metabolic health and reducing inflammation, particularly in males, where endurance training attenuated the inflammatory profile seen in untrained individuals.

To further contextualize the data, the current data were cross-referenced to publicly available data from patients with various metabolic disorders, such as the metabolic syndrome, type 2 diabetes or obesity. Endurance-trained individuals display many genes (muscle and scWAT) and proteins (muscle) with an opposite expression compared to patients with various metabolic disorders. A majority of these were mitochondria-related, with many being directly involved in energy generating processes or in mitochondrial structure. After 6 to 12 months of exercise training in patients with the metabolic syndrome or type 2 diabetes, the number of oppositely regulated genes was reduced. In male patients with type 2 diabetes, a substantial number of genes became reciprocally expressed relative to endurance-trained males.

Taken together, based on a highly controlled study design comparing three groups with distinctly different training backgrounds, this thesis addressed tissue- specific differences in skeletal muscle and adipose tissue transcriptome and proteome. The results showed large transcriptomic and proteomic differences between the endurance trained group and the other two groups as well as distinct sex differences. These results provide a deepened understanding of how humans respond to training, which may help to impact future optimization of training regimens for performance and health.

List of scientific papers

I. Emanuelsson EB, Berry DB, Reitzner SM, Arif M, Mardinoglu A, Gustafsson T, Ward SR, Sundberg CJ, Chapman MA. MRI characterization of skeletal muscle size and fatty infiltration in long- term trained and untrained individuals. Physiological reports 2022 10;14 e15398-
https://doi.org/10.14814/phy2.15398

II. Chapman MA, Arif M, Emanuelsson EB, Reitzner SM, Lindholm ME, Mardinoglu A, Sundberg CJ. Skeletal Muscle Transcriptomic Comparison between Long-Term Trained and Untrained Men and Women. Cell reports 2020 31;12 107808-
https://doi.org/10.1016/j.celrep.2020.107808

III. Emanuelsson EB, Arif M, Reitzner SM, Perez S, Lindholm ME, Mardinoglu A, Daub C, Sundberg CJ, Chapman MA. Remodeling of the human skeletal muscle proteome found after long-term endurance training but not after strength training. iScience 2024 27;1108638-
https://doi.org/10.1016/j.isci.2023.108638

IV. Eric B. Emanuelsson*, Stefan M. Reitzner*, Allister Quizon, Joshua Burrows, Merve Elmastas, Jutta Jalkanen, Sean Perez, Hampus Gilljam, Malene E Lindholm, Jessica Norrbom, Bernhard O. Palsson, Daniel Zielinski, Mikael Rydén, Mark A. Chapman, Carl Johan Sundberg. Transcriptomic profiling of adipose tissue in highly trained and untrained individuals - a cross-sectional analysis. [Manuscript]

* indicates equal contribution