Irisin ameliorates age-associated sarcopenia and metabolic dysfunction

Abstract

Background: Age-associated sarcopenia is characterized of progressed loss of skeletal muscle power, mass, and function, which affects human physical activity and life quality. Besides, accompanied with sarcopenia, aged population also faces a series of metabolic dysfunctions. Irisin, the cleaved form of fibronectin type III domain-containing protein 5 (FNDC5), is a myokine induced by exercise and has been shown to exert multiple beneficial effects on health. The goal of the study is to investigate the alterations of Fndc5/irisin in skeletal muscles during ageing and whether irisin administration could ameliorate age-associated sarcopenia and metabolic dysfunction.

Methods: The mRNA and protein levels of FNDC5/irisin in skeletal muscle and serum from 2- and 24-month-old mice or human subjects were analysed using qRT-PCR and western blot. FNDC5/irisin knockout mice were generated to investigate the consequences of FNDC5/irisin deletion on skeletal muscle mass, as well as morphological and molecular changes in muscle during ageing via histological and molecular analysis. To identify the therapeutic effects of chronic irisin treatment in mice during ageing, in vivo intraperitoneal administration of 2 mg/kg recombinant irisin was performed three times per week in ageing mice (14-month-old) for 4 months or in aged mice (22-month-old) for 1 month to systematically investigate irisin's effects on age-associated sarcopenia and metabolic performances, including grip strength, body weights, body composition, insulin sensitivity, energy expenditure, serum parameters and phenotypical and molecular changes in fat and liver.

Results: We showed that the expression levels of irisin, as well as its precursor Fndc5, were reduced at mRNA and protein expression levels in muscle during ageing. In addition, via phenotypic analysis of FNDC5/irisin knockout mice, we found that FNDC5/irisin deficiency in aged mice exhibited aggravated muscle atrophy including smaller grip strength (-3.23%, P < 0.05), muscle weights (quadriceps femoris [QU]: -20.05%; gastrocnemius [GAS]: -17.91%; tibialis anterior [TA]: -19.51%, all P < 0.05), fibre size (QU: P < 0.01) and worse molecular phenotypes compared with wild-type mice. We then delivered recombinant irisin protein intraperitoneally into ageing or aged mice and found that it could improve sarcopenia with grip strength (+18.42%, P < 0.01 or +13.88%, P < 0.01), muscle weights (QU: +9.02%, P < 0.01 or +16.39%, P < 0.05), fibre size (QU: both P < 0.05) and molecular phenotypes and alleviated age-associated fat tissues expansion, insulin resistance and hepatic steatosis (all P < 0.05), accompanied with altered gene signatures.

Conclusions: Together, this study revealed the importance of irisin in the maintenance of muscle physiology and systematic energy homeostasis during ageing and suggested a potent therapeutic strategy against age-associated metabolic diseases via irisin administration.

Keywords: FNDC5; ageing; irisin; metabolic dysfunction; sarcopenia.

Figure 1

The expression levels of FNDC5/irisin are reduced in muscles during ageing. (A–F) Phenotypical and molecular analysis of muscles from young (2‐month‐old) and aged mice (24‐month‐old). N = 6 per group. (A) The forelimb grip strength and weights of quadriceps femoris (QU), gastrocnemius (GAS) and tibialis anterior (TA); (B) haematoxylin and eosin histological analysis and average cross‐sectional area of fibre sizes and (C) gene expression profiles of atrophic and inflammatory genes from QU muscle. (D) Relative expression levels of Fndc5 in the hind limb muscle of young and aged mice from GSE75523 database. N = 8 per group. (E) Relative mRNA levels of Fndc5 in QU, GAS and TA muscle from young and aged mice (n = 6 per group), immunoblotting and quantification analysis for FNDC5 and irisin protein levels in the QU muscle from young and aged mice (n = 4 per group). (F) Immunoblotting analysis for irisin protein levels in the serum from young and aged mice (n = 3 per group) and Pearson correlation analysis for FNDC5 mRNA levels in the muscle biopsies from young and aged human (n = 25). Data are presented as mean ± SEM and *P < 0.05, **P < 0.01 compared with control group. Scale bar represents 100 μm.

Figure 2

FNDC5/irisin deficiency in aged mice exacerbates skeletal muscle wasting. (A–F) Phenotypical and molecular analysis of muscles from aged wild‐type (WT) and FNDC5 knockout (KO) mice at 22‐month‐old. N = 8 per group. Body weight; (B) lean mass; (C) forelimb grip strength and weights of QU, GAS and TA muscles; (D) representative haematoxylin and eosin staining, quantifications of muscle fibre sizes distribution and average cross‐sectional areas (CSAs); (E) mRNA levels of atrophic and inflammatory genes and (F) protein levels of MAFbx and MuRF‐1 in QU muscle. Data are presented as mean ± SEM and *P < 0.05, **P < 0.01 compared with control group. Scale bar represents 100 μm.

Figure 3

Chronic irisin administration alleviates sarcopenia in ageing mice. (A–F) Phenotypical and molecular analysis of muscles from middle‐aged mice (14‐month‐old) intraperitoneally administrated with recombinant His‐tag or irisin for 4 months. N = 5 per group. (A) Body weight; (B) lean mass; (C) forelimb grip strength and weights of QU, GAS and TA muscles; (D) representative haematoxylin and eosin staining, quantifications of muscle fibre sizes distribution and average cross‐sectional areas (CSAs); (E) mRNA expression levels of atrophic and inflammatory genes and protein levels of MAFbx and MuRF‐1; (F) mRNA expression levels of mitochondrial genes, protein levels of mitochondrial complex, citrate synthase activity and ATP levels in QU muscle. Data are presented as mean ± SEM and *P < 0.05, **P < 0.01 compared with control group. Scale bar represents 100 μm.

Figure 4

Chronic irisin administration ameliorates metabolic dysfunction in ageing mice. (A–F) Metabolic performances of middle‐aged mice (14‐month‐old) intraperitoneally administrated with recombinant His‐tag or irisin for 4 months. N = 5 per group. (A) Serum parameters analysis including total triglyceride (TG), total cholesterol (TC) and low‐density lipoprotein cholesterol (LDL‐C); (B) glucose tolerance test (GTT) and area under the curve (AUC); (C) insulin tolerance test (ITT) and AUC; (D) oxygen consumption and quantification; (E) carbon dioxide production and quantification; (F) total energy expenditure and quantification. Data are presented as mean ± SEM and *P < 0.05, **P < 0.01 compared with control group.

Figure 5

Chronic irisin administration improves beige fat functionality in ageing mice. (A–F) Analysis of adipose tissues from middle‐aged mice (14‐month‐old) intraperitoneally administrated with recombinant His‐tag or irisin for 4 months. N = 5 per group. (A) Weights of brown (BAT), inguinal (iWAT) and epididymal (eWAT) fat pads; (B) representative haematoxylin and eosin staining, quantification of adipocyte sizes and average adipocyte sizes; (C) mRNA levels of thermogenic and mitochondrial genes; (D) protein levels of PGC1α and UCP1; (E) mRNA levels of fibrotic genes and (F) mason, Sirius red staining in iWAT. Data are presented as mean ± SEM and *P < 0.05, **P < 0.01 compared with control group. Scale bar represents 100 μm.

Figure 6

Chronic irisin administration treats sarcopenia in aged mice. (A–F) Phenotypical and molecular analysis of muscles from aged mice (22‐month‐old) intraperitoneally administrated with recombinant His‐tag or irisin for 1 month. N = 8 per group. (A) Body weight; (B) lean mass; (C) forelimb grip strength and weights of QU, GAS and TA muscles; (D) representative haematoxylin and eosin staining, quantifications of muscle fibre sizes distribution and average cross‐sectional areas (CSAs); (E) protein levels of MAFbx and MuRF‐1; (F) mRNA expression levels of mitochondrial genes, citrate synthase activity and ATP levels in QU muscle. Data are presented as mean ± SEM and *P < 0.05, **P < 0.01 compared with control group. Scale bar represents 100 μm.

Figure 7

Chronic irisin administration treats metabolic dysfunction in aged mice. (A–F) Metabolic performances of aged mice (22‐month‐old) intraperitoneally administrated with recombinant His‐tag or irisin for 1 month. N = 8 per group. (A) Serum parameters analysis including total triglyceride (TG), total cholesterol (TC) and low‐density lipoprotein cholesterol (LDL‐C); (B) glucose tolerance test (GTT) and area under the curve (AUC); (C) insulin tolerance test (ITT) and AUC; (D) oxygen consumption and quantification; (E) carbon dioxide production and quantification; (F) total energy expenditure and quantification. Data are presented as mean ± SEM and *P < 0.05, **P < 0.01 compared with control group.

Figure 8

Chronic irisin administration improves beige fat functionality in aged mice. (A–F) Analysis of adipose tissues from aged mice (22‐month‐old) intraperitoneally administrated with recombinant His‐tag or irisin for 1 month. N = 8 per group. (A) Weights of brown (BAT), inguinal (iWAT) and epididymal (eWAT) fat pads; (B) representative haematoxylin and eosin staining, quantification of adipocyte sizes and average adipocyte sizes; (C) mRNA levels of thermogenic and mitochondrial genes; (D) protein levels of PGC1α and UCP1; (E) mRNA levels of fibrotic genes and (F) mason, Sirius red staining in iWAT. Data are presented as mean ± SEM and *P < 0.05, **P < 0.01 compared with control group. Scale bar represents 100 μm.