Functions Of MSTN And FNDC5 On Fat And Insulin Sensitivity In Meishan Pigs

Myostatin, also known as GDF8, is a member of the TGFP superfamily. The main function of Myostatin is to negatively regulate the growth and development of skeletal muscle.Mstn-/- mice had a significant increase in skeletal muscle mass,leading to a significant double muscle phenotype.Compared with wild-type mice, the fat was significantly decreased and the insulin sensitivity of body was significantly enhanced in Mstn-/- mice fed with a high-fat diet. The current research on the relationship of Myostatin, fat deposition and insulin sensitivity is almost concentrated in mice and has not been reported in large animals. The physiological structure of pigs is similar to that of humans , so the pig is a very suitable model animal for the study of human diseases. The Meishan pig is a Chinese local pig breed, with very high fat mass, which becomes the best expereimental model animal in the study of human obesity and insulin resistance.Recently, our lab had successfully produced MSTN-/- Meishan pigs by ZFN technology, and their reproductive capacity is normal. In this paper, we studied the changes of skeletal muscle, fat and insulin sensitivity in MSTN-/- Meishan pigs at 4 months old (childhood) and 16 months old (adult), and the molecular mechanism of Myostatin on fat deposition and insulin Sensitivity. At the age of 4 months and 16 months, the skeletal muscle mass of MSTN-/- Meishan pigs increased significantly, and the number of muscle fibers in the longissimus dorsi also increased significantly. At the age of 16 months, the percentage of subcutaneous fat to body weight in MSTN-/- Meishan pigs is 5.32%, significantly lower than that in wild-type pigs, whose value is 8.57%, a 3.25% decrease. And the percentage of leaf fat also decreased significantly in MSTN-/- Meishan pigs (1.32% in Mstn-/- pigs vs 2.7% in wild-type pigs), a 1.38% drop. The content of triglyceride and leptin in serum of MSTN-/- Meishan pigs was also significantly decreased at 16 months old. The number of single-chamber lipid droplets in the subcutaneous fat of the MSTN-/- Meishan pigs was decreased and the number of multi-chamber lipid droplets was increased. The expression of brown fat marker genes PGC-la, PRDM16, Cidea and beige fat marker genes CD 137, Tmem26 increased and the browning of subcutaneous fat was significantly enhanced in MSTN-/- Meishan pigs at 16 months old. Moreover, the content of insulin was reduced in the serum of MSTN-/- Meishan pigs. The glucose tolerance was improved, and the insulin sensitivity was also significantly enhanced in MSTN-/- Meishan pigs. The signal pathway of insulin was significantly activated in the skeletal muscle of MSTN-/- Meishan pigs.Fibronectin type III domain containing 5 (FNDC5) is the precursor protein of Irisin, mainly expressed in skeletal muscle. Irisin is a secreted protein, which can enter into adipose tissue by the blood circulation and promote the browning of white fat, resuiting in the decrease of fat deposition. Our results showed that the expression of FNDC5 in skeletal muscle of MSTN-/- Meishan pigs wassignificantly increased, and the content of Irisin in serum was also significantly increased. We first examined the expression pattern of FNDC5 in Meishan pigs. The FNDC5 of Meishan pigs contains five transcripts, all of which can synthesize the complete Irisin protein. FNDC5 was mainly expressed in skeletal muscle, and the expression in embryonic stage was significantly higher than that in adulthood.Then we examined the relationship between Myostatin and FNDC5 in regulating the adipogenic differentiation of preadipocytes and the insulin sensitivity of myoblasts in Meishan pigs. FNDC5 could significantly promote the adipogenic differentiation of primary preadipocytes of Meishan pigs, and the adipogenic differentiation ability of primary preadipocytes of MSTN-/- Meishan pigs was significantly higher than that of wild-type Meishan pigs. We further studied that the primary preadipocytes of MSTNMeishan pigs can improve the ability of adipogenic differentiation by promoting the expression of FNDC5, but this is not the only way for Myostatin to regulate the adipogenic differentiation of preadipocytes. FNDC5 could also activate the insulin signaling pathway of primary myoblasts in Meishan pigs, and the activation of insulin signal pathway in primary myoblasts of MSTN-/- Meishan pigs was also significantly higher than that of wild-type Meishan pigs. We further studied that the primary myoblasts of MSTN-/- Meishan pigs can activate the insulin signaling pathway by promoting the expression of FNDC5, but this is not the only way for Myostatin to regulate the insulin signaling pathway of myoblasts.In this paper, we first used Meishan pigs as model animals to study the effects of Myostatin on fat deposition and insulin sensitivity. This paper further explored the function and association of Myostatin and FNDC5 in regulating the adipogenic differentiation of preadipocytes and the insulin sensitivity of myoblasts in Meishan pigs. This article can provide basic data for the study of obesity and insulin sensitivity, and provide new ideas for the treatment of human diseases such as obesity and insulin resistance.