Skeletal muscle is central to the development of metabolic dysfunction during type 2 diabetes (T2D) and obesity. In addition, these conditions are often accompanied by accelerated muscle loss despite the presence of nutrient excess. This suggests uncoupling of nutrient sensing mechanisms with the molecular pathways that control muscle plasticity. For instance, depletion of intramuscular nicotinamide adenine dinucleotide (NAD) is linked to skeletal muscle loss and dysfunction, while strategies that restore or increase its levels can reverse this pathogenesis. Particularly, genetic or pharmacological inhibition of poly(ADP-ribose) polymerases 1 (PARP1), a major NAD consumer, improves muscle fitness through increases in NAD availability and the activation of NAD-dependent deacetylase sirtuin-1 (SIRT1). Thus, identifying physiological pathways that link energy metabolism to the regulation of PARP1 activity can lead to the development of innovative therapies for the prevention or treatment of muscle degeneration and metabolic dysfunction. Preliminary data suggest that direct sensing of circulating glucose by sweet taste receptors (STRs) regulates PARP1 activity to control the adaptive potential of skeletal muscle. Specifically, whole body or skeletal muscle-specific deletion of T1r2 gene of STRs (T1R2-KO) enhances mitochondrial function, oxidative capacity, exercise tolerance, and induces mild increases in myofiber size. These improvements are linked to attenuated PARP1 activity, increased NAD pool, and enhanced glucose utilization towards nucleotide biosynthesis. Consequently, T1R2-KO mice are protected from metabolic derangements associated with diet-induced obesity, including muscle mass loss. Thus, it was hypothesized that the T1R2 receptor is a constitutive sensor of glucose availability to adjust intracellular pathways that control the metabolic basis of skeletal muscle plasticity. This hypothesis is tested through comprehensive studies using mice with constitutive or inducible muscle-specific deletion of the T1r2 gene to: 1) Elucidate the role of T1R2 signaling network in the regulation of muscle bioenergetics and function. Specifically, a) probe signaling pathway leading to PARP1 regulation and NAD bioavailability, b) identify downstream effectors of NAD-dependent activation of SIRT1 and 2, c) assess contributions of STRs in the regulation of substrate utilization, and d) determine interactions between STR signaling and established intracellular energy sensors (i.e. AMPK, mTORC1, Akt). 2) Investigate contributions of T1R2-mediated glucose sensing in the regulation of muscle mass. Specifically, a) assess physiological effects of inducible deletion of STR signaling in adult skeletal muscle to mimic longitudinal effects of pharmacological treatments targeting STRs, b) define contributions of STR signaling to muscle mass adaptations in response to treatments that induce muscle hypertrophy or atrophy, c) spatiotemporal expression of T1r2 gene during muscle development and growth using muscle-specific reporter mice, and d) contributions of STR signaling during postnatal muscle growth through the assessment of morphological, signaling and functional muscle adaptations. .
Skeletal muscle is central to the development of metabolic dysfunction during type 2 diabetes and obesity. Thus, it is critical to identify and target molecular networks that preserve or restore muscle function. Our proposed studies have identified a novel mechanism for the sensing of glucose that controls muscle fitness and growth.
