Myostatin does not regulate cardiac hypertrophy or fibrosis

Abstract

Myostatin is a negative regulator of muscle growth. Loss of myostatin has been shown to cause increase in skeletal muscle size and improve skeletal muscle function and fibrosis in the dystrophin-deficient mdx muscular dystrophy mouse model. We evaluated whether lack of myostatin has an impact on cardiac muscle growth and fibrosis in vivo. Using genetically modified mice we assessed whether myostatin absence induces similar beneficial effects on cardiac function and fibrosis. Cardiac mass and ejection fraction were measured in wild-type, myostatin-null, mdx and double mutant mdx/myostatin-null mice by high resolution echocardiography. Heart mass, myocyte area and extent of cardiac fibrosis were determined post mortem. Myostatin-null mice do not demonstrate ventricular hypertrophy when compared to wild-type mice as shown by echocardiography (ventricular mass 0.69+/-0.01 vs. 0.69+/-0.018 g) and morphometric analyses including heart/body weight ratio (5.39+/-0.45 vs. 5.62+/-0.58 mg/g) and cardiomyocyte area 113.67+/-1.5, 116.85+/-1.9 microm(2)). Moreover, absence of myostatin does not attenuate cardiac fibrosis in the dystrophin-deficient mdx mouse (12.2% vs. 12%). The physiological role of myostatin in cardiac muscle appears significantly different than that in skeletal muscle as it does not induce cardiac hypertrophy and does not modulate cardiac fibrosis in mdx mice.

Figure 1

Histopathological characterization of cardiomyopathy in mdx mice During the first 6 months only minimal morphological alterations presenting as single diffuse necrotic cells and fibrotic areas can be observed in cardiac muscle as shown by H&E staining in 6-week and 16-week old mice. In contrast, severe fibrosis can be detected in older mice as shown by Masson’s trichrome staining of cardiac muscles from 12 and 24 month old mice. Bar 75μm.

Figure 2

Quantification of cardiomyocyte cross section area from 2-year old wild-type, myostatin-null, mdx and mdx/myostatin-null mice (n=3 each) were analyzed. There was no statistical significance in cardiac cell size between either of the genotypes; mean±SEM. P = 1.02 for wild-type versus myostatin-null and P = 0.19 for mdx versus mdx/myostatin-null mice respectively.

Figure 3

Echocardiography data. Representative M mode tracing at the mid ventricular level demonstrating measurement of end-systolic (solid line) and end-diastolic (dashed line) dimensions (A). There were no significant differences between groups for LVEDD (B), LVESD (C), LV mass (D) and LV ejection fraction (E).

Figure 3

Echocardiography data. Representative M mode tracing at the mid ventricular level demonstrating measurement of end-systolic (solid line) and end-diastolic (dashed line) dimensions (A). There were no significant differences between groups for LVEDD (B), LVESD (C), LV mass (D) and LV ejection fraction (E).

Figure 3

Echocardiography data. Representative M mode tracing at the mid ventricular level demonstrating measurement of end-systolic (solid line) and end-diastolic (dashed line) dimensions (A). There were no significant differences between groups for LVEDD (B), LVESD (C), LV mass (D) and LV ejection fraction (E).

Figure 4

A Trichrome staining of 2-year old wild-type, myostatin-null, mdx and mdx/myostatin-null mice demonstrates no visible difference between wild-type and myostatin-null mice. In contrast mdx mice exhibit patchy areas of fibrosis. A similar amount of fibrosis can be detected in mdx/myostatin null mice. Figure 4 B+C Quantification of fibrotic areas in 1-year and 2-year old wild-type, myostatin-null, mdx and mdx/myostatin-null mice confirms the histological observations. There is only very minimal amount of cardiac fibrosis in wild-type and myostatin-null mice. In contrast, there areas of fibrosis in mdx and mdx/myostatin-null mice at both 1 and 2-years of age are significantly increased; mean±SEM. P < 0.003 for wild-type versus mdx and mdx/myostatin-null at 1 year of age, P < 0.002 for wild-type versus mdx and mdx/myostatin-null at 2 years of age. However, loss of myostatin does not prevent the development of cardiac fibrosis on mdx mice.

Figure 4

A Trichrome staining of 2-year old wild-type, myostatin-null, mdx and mdx/myostatin-null mice demonstrates no visible difference between wild-type and myostatin-null mice. In contrast mdx mice exhibit patchy areas of fibrosis. A similar amount of fibrosis can be detected in mdx/myostatin null mice. Figure 4 B+C Quantification of fibrotic areas in 1-year and 2-year old wild-type, myostatin-null, mdx and mdx/myostatin-null mice confirms the histological observations. There is only very minimal amount of cardiac fibrosis in wild-type and myostatin-null mice. In contrast, there areas of fibrosis in mdx and mdx/myostatin-null mice at both 1 and 2-years of age are significantly increased; mean±SEM. P < 0.003 for wild-type versus mdx and mdx/myostatin-null at 1 year of age, P < 0.002 for wild-type versus mdx and mdx/myostatin-null at 2 years of age. However, loss of myostatin does not prevent the development of cardiac fibrosis on mdx mice.

Figure 4

A Trichrome staining of 2-year old wild-type, myostatin-null, mdx and mdx/myostatin-null mice demonstrates no visible difference between wild-type and myostatin-null mice. In contrast mdx mice exhibit patchy areas of fibrosis. A similar amount of fibrosis can be detected in mdx/myostatin null mice. Figure 4 B+C Quantification of fibrotic areas in 1-year and 2-year old wild-type, myostatin-null, mdx and mdx/myostatin-null mice confirms the histological observations. There is only very minimal amount of cardiac fibrosis in wild-type and myostatin-null mice. In contrast, there areas of fibrosis in mdx and mdx/myostatin-null mice at both 1 and 2-years of age are significantly increased; mean±SEM. P < 0.003 for wild-type versus mdx and mdx/myostatin-null at 1 year of age, P < 0.002 for wild-type versus mdx and mdx/myostatin-null at 2 years of age. However, loss of myostatin does not prevent the development of cardiac fibrosis on mdx mice.