Abstract
Irisin is a myokine encoded in its precursor fibronectin type III domain containing 5 (FNDC5). It is abundantly expressed in cardiac and skeletal muscle, and is secreted upon the activation of peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1 alpha). We aimed to study the role of irisin on cardiac function and muscle protein regulation in zebrafish. Western blot analyses detected the presence of irisin protein (23 kDa) in zebrafish heart and skeletal muscle, and irisin immunoreactivity was detected in both tissues. Irisin siRNA treated samples did not show bands corresponding to irisin in zebrafish. In vitro studies found that treatment with irisin (0.1 nM) downregulated the expression of PGC-1 alpha, myostatin a, and b, while upregulating troponin C mRNA expression in zebrafish heart and skeletal muscle. Exogenous irisin (0.1 and 1 ng/g B.W) increased diastolic volume, heart rate and cardiac output, while knockdown of irisin (10 ng/g B.W) showed opposing effects on cardiovascular function. Irisin (1 and 10 ng/g B.W) downregulated PGC-1 alpha, myostatin a and b, and upregulated troponin C and troponin T2D mRNA expression. Meanwhile, knockdown of irisin showed opposing effects on troponin C, troponin T2D and myostatin a and b mRNAs in zebrafish heart and skeletal muscle. Collectively, these results identified muscle proteins as novel targets of irisin, and added irisin to the list of peptide modulators of cardiovascular physiology in zebrafish.
Highlights
Skeletal muscle constitutes up to 40% of total body weight, and is considered an exercise dependent endocrine organ that constitutes approximately 75% of body proteins [1, 2]
fibronectin type III domain containing 5 (FNDC5) is regulated by PGC-1 alpha, which forms an integral part of the muscle post-exercise, and causes an increase in energy expenditure in mammals [9]
Irisin immunoreactivity was detected in zebrafish skeletal muscle (Fig 2C)
Summary
Skeletal muscle constitutes up to 40% of total body weight, and is considered an exercise dependent endocrine organ that constitutes approximately 75% of body proteins [1, 2]. Skeletal muscle regulates cytokines and myokines that exert autocrine and paracrine effects in humans [3,4,5,6]. Some of the skeletal muscle derived cytokines, including interleukin-6 have the ability to regulate glucose and lipid levels [7]. Irisin is a recently confirmed, exercise-induced, 23 kDa myokine abundantly expressed in rodent and human skeletal muscle [8]. FNDC5 is regulated by PGC-1 alpha, which forms an integral part of the muscle post-exercise, and causes an increase in energy expenditure in mammals [9]. Processing of FNDC5 by PGC-1 alpha triggers the release of irisin into circulation [9, 11].
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