Skeletal Muscle-specific G Protein-coupled Receptor Kinase 2 Ablation Alters Isolated Skeletal Muscle Mechanics and Enhances Clenbuterol-stimulated Hypertrophy

Abstract

GRK2, a G protein-coupled receptor kinase, plays a critical role in cardiac physiology. 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 after cardiac insult exacerbates injury and speeds progression to heart failure. Despite the importance of this kinase 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 this study we generated a novel skeletal muscle-specific GRK2 knock-out (KO) mouse (MLC-Cre:GRK2 fl/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:GRK2 fl/fl mice compared with wild-type controls. Skeletal muscle hypertrophy stimulated by clenbuterol, a ␤ 2 -adrenergic receptor (␤ 2 AR) agonist, was significantly enhanced in MLC-Cre:GRK2 fl/fl mice; mechanistically, this seems to be due to increased clenbuterol-stimulated pro-hypertrophic Akt signaling in the GRK2 KO skeletal muscle. In summary, our 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 ␤ 2 AR-induced hypertrophy. G protein-coupled receptors (GPCRs)2 comprise the largest family of membrane proteins in the genome. GPCRs regulate diverse processes throughout the body by responding to a vast array of extracellular stimuli including hormones, neurotransmitters, and photons of light (1). Equally important to the stimulation and activation of GPCRs is the desensitization and "shutting-off" of the receptor. This task is primarily conducted by the GPCR kinase (GRK) family of proteins (2, 3). GRK2 is ubiquitously expressed throughout the tissues of the body, perhaps most notably in the heart where its regulation of adrenergic receptors (ARs) is critical to physiological heart function. Cardiac overexpression of GRK2 in mice suppresses contractility, whereas cardiac overexpression of the GRK2 inhibitor ␤ARKct (a C-terminal peptide that competes with GRK2 binding to G ␤␥ ) enhances contractile function (4). Catecholamine overdrive during heart failure drives increased GRK2 expression in the cardiomyocyte, ultimately leading to excessive desensitization of ␤ARs, loss of receptor density, and a drop in inotropic reserve (5-7). Indeed, myocardial inhibition or deletion of GRK2 can prevent and even reverse heart failure in numerous animal models (4, 8 -13).Although we have a firm understanding of the role of GRK2 in the physiology and pathophysiology of t...

Molecular Mechanisms of Skeletal Muscle Hypertrophy

“…Both GRKs and β-arrestin initiate signaling cascades, which are G protein-dependent and receptor independent, that may affect the pro-hypertrophic pathway. For example, muscle-specific GRK2 knockout enhances clenbuterol-stimulated hypertrophy [ 22 ] whereas the hypertrophic response was abrogated in mice lacking β-arrestin 1, which is the predominant β-arrestin isoform in skeletal muscle [ 23 ].…”

Dysregulation of GPCR Signaling in Cardiovascular Diseases: A Potential Role for Exercise Training?

“…GRK2 has been shown to terminate GPCR signaling by desensitizing the receptor [59]. Recent studies have demonstrated that skeletal muscle-specific GRK2 knockout substantially attenuates the force of contraction of the extensor digitorum longus muscle [60]. This genetic animal model enhanced β2-adrenergic receptor (β2-AR)-mediated muscle hypertrophy by augmenting β2-AR/Akt-mediated pro-hypertrophic signaling, thereby suggesting that GRK2-dependent GPCR regulation is significant for skeletal muscle function and diseases [60].…”

“…Recent studies have demonstrated that skeletal muscle-specific GRK2 knockout substantially attenuates the force of contraction of the extensor digitorum longus muscle [60]. This genetic animal model enhanced β2-adrenergic receptor (β2-AR)-mediated muscle hypertrophy by augmenting β2-AR/Akt-mediated pro-hypertrophic signaling, thereby suggesting that GRK2-dependent GPCR regulation is significant for skeletal muscle function and diseases [60]. Further, desensitization of β-AR by elevated GRK2 expression has been reported in hypertension or congestive heart failure [61,62].…”

Metabolic Pathways and Ion Channels Involved in Skeletal Muscle Atrophy: A Starting Point for Potential Therapeutic Strategies

“…The mechanism seems to involve the stimulation of protein synthesis through the enhancement of TORC1 activation [ 61 , 62 ]. However, hypertrophy is not always functional since a reduced normalized strength was observed [ 63 ]. In addition, nitric oxide (NO) was suggested to play a role in muscle hypertrophy via the activation of the transient receptor potential cation channel, subfamily V, member 1 (TRPV1), causing an increase in intracellular Ca 2+ levels and an increase in protein synthesis through TORC1 activation by an unknown mechanism [ 64 ].…”