NOVEL ROLES FOR GRK2 IN METABOLIC HOMEOSTASIS AND SKELETAL MUSCLE PHYSIOLOGY

Abstract

Over the past two decades, a vast body of research has demonstrated the importance of G protein-coupled receptor kinase 2 (GRK2) in the physiology and pathophysiology of the heart. Adrenergic receptors are the primary target for GRK2 activity in the heart; phosphorylation by GRK2 leads to desensitization of these receptors. As such, levels of GRK2 activity in the heart directly correlate with cardiac contractile function. Furthermore, increased expression of GRK2 following cardiac insult exacerbates injury and speeds progression to heart failure. In this dissertation we turned our attention towards two novel aspects of GRK2 biology. Firstly, despite the importance of this GRK2 activity in both the physiology and pathophysiology of the heart, relatively little is known about the role of GRK2 in skeletal muscle function and disease. In the first study of this dissertation, we generated a novel skeletal muscle specific GRK2 knockout (KO) mouse (MLC-Cre:GRK2fl/fl) to gain a better understanding of the role of GRK2 in skeletal muscle physiology. In isolated muscle mechanics testing, GRK2 ablation caused a significant decrease in the specific force of contraction of the fast-twitch extensor digitorum longus muscle, yet had no effect on the slow-twitch soleus muscle. Despite these effects in isolated muscle, exercise capacity was not altered in MLC-Cre:GRK2fl/fl mice compared to wild-type controls. Skeletal muscle hypertrophy stimulated by clenbuterol, a β2-adrenergic receptor agonist, was significantly enhanced in MLC-Cre:GRK2fl/fl mice; mechanistically, this seems to be due to increased clenbuterol-stimulated pro-hypertrophic Akt signaling in the GRK2 KO skeletal muscle. In summary, this study provides the first insights into the role of GRK2 in skeletal muscle physiology, and points to a role for GRK2 as a modulator of contractile properties in skeletal muscle as well as β2-adrenergic receptor-induced hypertrophy. In the second part of this dissertation, we report surprising novel metabolic phenotypes that arise from modulating GRK2 activity exclusively in the heart. We show that transgenic βARKct (TgβARKct) mice (cardiac specific expression of a GRK2 inhibitory peptide) are more susceptible to high-fat diet (HFD) induced obesity. TgβARKct mice exhibit marked increase in adiposity on HFD relative to control animals. Conversely transgenic GRK2 mice (TgGRK2) mice (cardiac specific overexpression of GRK2) show resistance to weight gain on a HFD and decrease in adipose tissue mass relative to control animals. Furthermore, conditioned media from βARKct expressing neonatal rat ventricular myocytes enhances adipocyte differentiation in vitro. These results suggest that the heart produces a secreted factor to control whole body metabolism, and that GRK2 is a regulator of this mechanism.