Date of Award

10-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Biomedical Sciences

Track

Molecular and Translational Physiology

Research Advisor

Stephen Alway, PhD

Committee

Stephen Alway, PhD; James A. Carson, PhD; Andy V. Khamoui, PhD; Salvatore Mancarella, PhD; Junaith S. Mohamed, PhD; Zhongjie Sun, MD, PhD

Keywords

muscle; mitochondria; aging; obesity; metabolism; NOR-1; nuclear orphan receptors

Abstract

Skeletal muscle mass and function are critical components of overall health and quality of life. Advanced age is associated with skeletal muscle disease characterized by a loss of muscle mass, function, and quality. Indeed, low muscle mass and function is associated with poor disease outcomes and greater mortality, greater incidence of falls, and a high fiscal and societal cost. Aging contributes to a decline in skeletal muscle health through several intrinsic and extrinsic mechanisms including impaired nutrient sensing, mitochondrial dysfunction, inflammation, physical inactivity, and proteostatic imbalance. Obesity, an increasingly prominent public health concern, is becoming more common in the elderly as the obesity epidemic intersects with an aging population demographic. Additionally, preclinical research on skeletal muscle aging often uses aged rodents which exhibit elevated fat mass when compared to young lean control mice. Therefore, how aging impacts skeletal muscle health in the absence or presence of obesity is currently a gap in our understanding and represents an obstacle to the interpretation of preclinical skeletal muscle research on aging. In contrast to aging and obesity, exercise is among the most potent interventions to improve skeletal muscle mass and health. Exercise effectively prevents and reverses the disease sequalae of skeletal muscle aging by mitigating the development of sarcopenia and preserving skeletal muscle metabolic health, in part through orchestrating transcriptional changes that confer improved muscle health. Indeed, the molecular transducers of exercise exist as attractive therapeutic targets for a plethora of muscle-centric pathologies. The identification of which exercise-responsive factors can be used to ameliorate the detrimental effects of aging on skeletal muscle mass and function is a critical step in the development of novel and innovative therapeutic targets. The nuclear receptor NR4A3 (NOR-1) is among the most responsive genes to exercise and is downregulated in aged muscle; whether NOR-1 expression can improve function in aged muscle is unknown.

The purpose of this dissertation was to explore the interaction of obesity and aging on skeletal muscle mass, mitochondrial function, and gene expression. Additionally, we sought to investigate the role of NOR-1 in skeletal muscle and examine whether NOR-1 overexpression can improve muscle function in aged animals. Our hypothesis was that aged skeletal muscle would mount a unique transcriptional response to obesity and that aging muscle would present with greater muscle lipid accumulation during obesity. Furthermore, we hypothesized that deficiencies in NOR-1 would perturb metabolic and contractile gene expression in vitro and that NOR-1 overexpression would improve skeletal muscle function in vivo and enhance the expression of genes associated with muscle oxidative capacity and endurance.

Chapter 3 of this dissertation examined how the coincidence of aging and obesity impacts skeletal muscle mitochondrial function, whole body metabolism, and transcriptomic changes in skeletal muscle. We observed that obesity and aging had divergent effects on skeletal muscle mass and size as well as mitochondrial function. Furthermore, we discovered that aged muscle produced a unique skeletal muscle transcriptomic signature in response to obesity, indicating that the presence of obesity alters the genetic landscape of aged skeletal muscle when compared to young and lean aged counterparts. We also discovered that aging is associated with reduced expression of NOR-1, a nuclear receptor that is among the most responsive genes to exercise in skeletal muscle. Therefore, in chapter 4 we investigated the regulation of NOR-1 by exercise and characterized the role of NOR-1 in C2C12 myotubes, an in vitro model of skeletal muscle myofibers. We observed that NOR-1 was critical for mitochondrial health and that loss of NOR 1 impaired metabolic and contractile gene expression as well as mTORC1 signaling. We also 17 identified key genes that are regulated by NOR-1 and therefore gained crucial insight into the role of NOR-1 in exercise-associated benefits. Lastly, we also explored the potential for NOR-1 to improve muscle function in aged mice. We observed that Adeno-associated virus (AAV)- mediated expression of NOR-1 resulted in improved muscle fatigability and mitochondrial function in aged mice. Indicating that NOR-1 can enhance function in aged muscle. These changes were concomitant with enhanced expression of a subset of genes associated with muscle oxidative capacity that we identified through in vitro loss-of-function experiments.

In summary, aging-associated alterations to skeletal muscle gene expression, mitochondrial function, and mass are altered with the presence of obesity. Expression of NOR-1, which is reduced with aging but enhanced by exercise in an intensity and time-dependent manner, is crucial for metabolic and contractile gene expression. Furthermore, NOR-1 overexpression in aged muscle is a viable intervention to improve muscle fatigability; however, chronic overexpression of NOR-1 resulted in a small but statistically significant loss of muscle mass. These findings fill a critical conceptual gap in our understanding of preclinical aging studies and suggest that the body composition of aged animals is a necessary factor to consider when investigating aging skeletal muscle biology. Additionally, these studies point towards a crucial role for the nuclear receptor NOR-1 in skeletal muscle health and suggest exercise may improve muscle function, in part through the actions of NOR-1. By enhancing our understanding of the factors that can influence skeletal muscle mass, health, and function we can develop interventions to improve quality of life and health during aging

Declaration of Authorship

Declaration of Authorship is included in the supplemental files.

DOI

10.21007/aetd.cghs.2024.0010

Additional Files
2024-010-Paez AETD DOA.pdf (323 kB)
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