Sarcospan Regulates Cardiac Isoproterenol Response and Prevents Duchenne Muscular Dystrophy-Associated Cardiomyopathy

Abstract

Background: Duchenne muscular dystrophy is a fatal cardiac and skeletal muscle disease resulting from mutations in the dystrophin gene. We have previously demonstrated that a dystrophin-associated protein, sarcospan (SSPN), ameliorated Duchenne muscular dystrophy skeletal muscle degeneration by activating compensatory pathways that regulate muscle cell adhesion (laminin-binding) to the extracellular matrix. Conversely, loss of SSPN destabilized skeletal muscle adhesion, hampered muscle regeneration, and reduced force properties. Given the importance of SSPN to skeletal muscle, we investigated the consequences of SSPN ablation in cardiac muscle and determined whether overexpression of SSPN into mdx mice ameliorates cardiac disease symptoms associated with Duchenne muscular dystrophy cardiomyopathy.

Methods and results: SSPN-null mice exhibited cardiac enlargement, exacerbated cardiomyocyte hypertrophy, and increased fibrosis in response to β-adrenergic challenge (isoproterenol; 0.8 mg/day per 2 weeks). Biochemical analysis of SSPN-null cardiac muscle revealed reduced sarcolemma localization of many proteins with a known role in cardiomyopathy pathogenesis: dystrophin, the sarcoglycans (α-, δ-, and γ-subunits), and β1D integrin. Transgenic overexpression of SSPN in Duchenne muscular dystrophy mice (mdx(TG)) improved cardiomyofiber cell adhesion, sarcolemma integrity, cardiac functional parameters, as well as increased expression of compensatory transmembrane proteins that mediate attachment to the extracellular matrix.

Conclusions: SSPN regulates sarcolemmal expression of laminin-binding complexes that are critical to cardiac muscle function and protects against transient and chronic injury, including inherited cardiomyopathy.

Keywords: Duchenne muscular dystrophy; cardiac hypertrophy; cell adhesion molecules; gene therapy.

Figure 1

Cardiac morphology is altered in isoproterenol‐treated SSPN‐null mice. Mice in this study were designated as saline‐treated (Sal) or isoproterenol‐treated (Iso) for both SSPN‐null (SSPN ^−/−) and WT mice. All mice were 12 months of age. A, BWts of WT and SSPN ^−/− mice after saline‐ or isoproterenol‐treatment and the HWt/BWt ratio (mg/g) were compared for saline‐ and isoproterenol‐treated mice. Data are shown as mean and error expressed as SEM. B, Representative wet mount images of hearts from treated and untreated WT and SSPN ^−/− mice used in this study. In (A) BWt (g) (left panel) and HWt/BWt (right panel) measurements were plotted for saline (Sal, WT, n=4 and SSPN ^−/− mice, n=3) and isoproterenol (Iso, both WT and SSPN ^−/− mice, n=7) treatment for each genotype after completion of the 2‐week study. P‐values <0.05 were considered significant and statistics were calculated using 1‐way ANOVA with Bonferroni post hoc test. For BWt (g) left panel, *P=0.022 SSPN ^−/− (Sal) vs SSPN ^−/− (Iso), P=0.115 SSPN ^−/− (Iso) vs WT (Iso), P=0.408 SSPN ^−/− (Iso) vs WT (Sal). For HWt/BWt (mg/g) right panel, *P<0.001 SSPN ^−/− (Iso) vs WT (Sal), **P=0.016 WT (Iso) vs WT (Sal), P=0.363 SSPN ^−/− (Iso) vs SSPN ^−/− (Sal), P=0.149 SSPN ^−/− (Iso) vs WT (Iso). HWt/BWt indicates heart weight/body weight; SSPN, sarcospan; WT, wild‐type.

Figure 2

Cardiac dysfunction in SSPN‐null mice in response to isoproterenol treatment. Echocardiographic measurements change in mice as they age, (A) LV mass for 12‐ and 18‐month‐old WT and SSPN ^−/− mice. All mice used in (B through F) were 12 months of age. B, Echocardiographic measurements of LV mass, mg were obtained after saline‐ and isoproterenol‐administration to WT and SSPN‐null (SSPN ^−/−) mice. C, Increased left ventricle dimension in millimeters (mm) in SSPN‐null (SSPN ^−/−) compared to WT mice during diastole (LVEDD) and (D) systole (LVESD) measured by echocardiography after isoproterenol challenge. E, Echocardiographic measurements of LV EF% and (F) LV FS% are shown for WT and SSPN‐null (SSPN ^−/−) mice after saline‐ and isoproterenol treatment. For (A), 12‐month‐old WT and SSPN‐null (SSPN ^−/−) mice (each, n=11) and 18‐month‐old WT and SSPN‐null (SSPN ^−/−) mice (each, n=5) were compared. For (B through F) samples are compared for saline (Sal, n=4) and isoproterenol (Iso, n=6) treatment for each genotype. The data demonstrate the range of values, the black line symbol indicates the mean of each data set. ^* P‐values <0.05 were considered significant, and statistics were calculated using 1‐way ANOVA with Bonferroni post hoc test unless otherwise noted. For (A) *P<0.001 SSPN ^−/− (18) vs WT (12), **P<0.001 SSPN ^−/− (18) vs SSPN ^−/− (12), ^# P=0.016 SSPN ^−/− (18) vs WT (18), P=0.340 WT (18) vs WT (12); (B) *P=0.006 SSPN ^−/− (Sal) vs SSPN ^−/− (Iso), the Kruskal–Wallis test was utilized for **P=0.009 WT (Sal) vs SSPN ^−/− (Iso), P=0.086 SSPN ^−/− (Iso) vs WT (Iso); (C) *P<0.001 SSPN ^−/− (Iso) vs WT (Iso), **P=0.006 SSPN ^−/− (Iso) vs WT (Sal), P=0.076 SSPN ^−/− (Sal) vs SSPN ^−/− (Iso); (D) *P=0.006 SSPN ^−/− (Iso) vs WT (Iso); (E) *P=0.002 SSPN ^−/− (Iso) vs WT (Iso), **P=0.010 WT (Iso) vs WT (Sal), P=0.077 WT (Iso) vs SSPN ^−/− (Sal) and (F) *P=0.001 SSPN ^−/− (Iso) vs WT (Iso), **P=0.008 WT (Iso) vs WT (Sal), P=0.057 SSPN ^−/− (Sal) vs WT (Iso). LV indicates left ventricular; LVEDD, left ventricular end diastolic dimension; LV EF, left ventricular ejection fraction; LVESD, left ventricular end systolic dimension; LV FS, left ventricular fractional shortening; SSPN, sarcospan; WT, wild‐type.

Figure 3

Isoproterenol treatment alters cardiac architecture of SSPN‐null mice. A, Transverse cardiac sections from 12‐month‐old WT and SSPN‐null (SSPN ^−/−) mice were stained with H&E, revealing myocyte disarray, hypertrophy, and fibrosis in SSPN‐nulls. White asterisks indicate focal fibrotic regions, Bar, 50 μm. B, Masson's trichrome staining identifies regions of collagen deposition (dark blue) in cardiac cross‐sections (axial, short axis). White asterisks indicate focal fibrotic regions. Bar, 900 μm. C, Fibrosis was quantified by measuring the area of collagen staining in H&E‐stained whole heart cross‐sections after saline (Sal) and isoproterenol (Iso) administration. Data are represented as percentage of fibrosis per total area. D, Enlarged H&E cardiac images reveal myofiber hypertrophy in SSPN‐nulls after isoproterenol treatment. Bar, 50 μm. E, Cardiac myofiber cross‐sectional area was quantified after saline and isoproterenol challenge. H&E stained panels highlight fiber size differences in WT and SSPN ^−/− mice. For (C and E), data are presented as an average from 12‐month‐old, WT mice and SSPN‐null (SSPN ^−/−) that were saline‐ (Sal) (n=3) and isoproterenol‐treated (Iso) (n=3). Statistical analysis was performed using 1‐way ANOVA with Bonferroni post hoc test and *P‐values <0.05 were considered significant. For (C) *P<0.001 SSPN ^−/− (Iso) vs WT (Sal), **P<0.001 SSPN ^−/− (Iso) vs SSPN ^−/− (Sal), ^# P=0.002 SSPN ^−/− (Iso) vs WT (Iso), P=0.093 WT (Iso) vs WT (Sal) and (E) *P<0.001 SSPN ^−/− (Iso) vs WT (Sal), **P<0.001 SSPN ^−/− (Iso) vs SSPN ^−/− (Sal), ^# P=0.034 SSPN ^−/− (Iso) vs WT (Iso), ^§ P=0.013 WT (Iso) vs SSPN ^−/− (Sal), P=0.091 WT (Iso) vs WT (Sal). H&E indicates hematoxylin and eosin; SSPN, sarcospan; WT, wild‐type.

Figure 4

SSPN‐null mice have a blunted response to β‐adrenergic stimulation. Relative changes in gene expression changes of β‐adrenergic receptors (β1, β2, and β3) upon isoproterenol challenge were assessed by qRT‐PCR in cardiac muscle obtained from untreated (Untr) and isoproterenol‐treated (Iso) 12‐month‐old WT and SSPN‐null (SSPN ^−/−) mice. A, Relative gene expression values of the ADRB1 gene encoding (β‐AR 1). (B) Relative gene expression values for the ADRB2 gene encoding (β‐ΑR 2). (C) Relative gene expression values for the ADRB3 gene encoding (β‐AR 3) receptors in untreated (Untr) and isoproterenol‐treated (Iso) WT and SSPN ^−/− hearts. Data were normalized to mL32 (a ribosomal protein) as an internal control and are presented as an average. All mice were 12 months of age. Data are presented as an average from WT and SSPN‐null (SSPN ^−/−) mice, either saline (Sal) (n=6) or isoproterenol‐treated (Iso) (n=6) and error bars represent SEM. The Kruskal–Wallis test was applied since data did not meet assumptions of normality and *P‐values <0.05 were considered significant. For (A) *P=0.044 SSPN ^−/− (Iso) vs WT (Sal), P=0.081 SSPN ^−/− (Iso) vs SSPN ^−/− (Sal), P=0.177 WT (Iso) vs WT (Sal); (B) *P=0.005 SSPN ^−/− (Iso) vs SSPN ^−/− (Sal), **P=0.020 SSPN ^−/− (Iso) vs WT (Sal), P=0.091 WT (Iso) vs WT (Sal) and (C) *P=0.014 SSPN ^−/− (Iso) vs SSPN ^−/− (Sal), **P=0.018 SSPN ^−/− (Iso) vs WT (Sal), ^# P=0.049 WT (Iso) vs SSPN ^−/− (Sal), P=0.064 WT (Iso) vs WT (Sal). qRT‐PCR indicates quantitative reverse transcription–polymerase chain reaction; SSPN sarcospan; WT, wild‐type.

Figure 5

Loss of SSPN influences expression of cardiomyopathy genes. (A) Transverse cardiac cryosections from WT and SSPN‐null mice (SSPN ^−/−) stained with antibodies against dystrophin (Dys), utrophin (Utr), dystroglycans (α‐ and β‐DG), β1D‐integrin (Itgb) and sarcoglycans (α‐, β‐, δ‐ and γ‐SGs), and SSPN. Bar, 50 μm. B, Total protein lysates obtained from WT and SSPN‐null (SSPN^−/−) mice were immunoblotted for DGC and associated proteins. Equal protein samples (60 μg) were resolved by SDS‐PAGE and immunoblotted with the indicated antibodies. DG pulldown assay, whole heart lysates were enriched by sWGA lectin chromatography, which binds DG and allows biochemical determination of associations between members of DGC‐associated complexes with DG, core components, in the presence or absence of SSPN. In (C) immunoblots of DG void samples are shown and in (D) immunoblots of eluates containing DG associated proteins are shown. CB staining is shown as a loading control. Densitometry was used to quantify immunoblots shown in (D) to determine relative changes in DGC and associated proteins in SSPN‐null (SSPN^−/−) mice compared to WT and reported as normalized arbitrary units. All mice were 4 months old and similar results were obtained for 12‐month‐old mice (data not shown). Immunoblot data are presented as an average (n=5) for WT and SSPN‐null (SSPN ^−/−), except for δ‐SG (n=3) for WT and SSPN‐null (SSPN ^−/−), and error bars represent SEM. ^* P‐values <0.05 were considered significant, and statistics were calculated using 1‐way ANOVA. For comparisons of protein expression levels in WT vs SSPN ^−/− mice in (E) *P=0.003 Dys, *P=0.04 β‐DG, *P=0.009 SSPN, P=0.09 Utr, P=0.08 α‐DG, P=0.120 Itgb, P=0.227 α‐SG, P=0.065 β‐SG, P=0.326 δ‐SG, P=0.208 γ‐SG. CB indicates Coomassie blue; DGC, dystrophin‐glycoprotein complex; Itgb, β1D‐integrin; SDS‐PAGE, sodium dodecyl sulfate–polyacrylamide gel electrophoresis; SSPN, sarcospan; sWGA, succinylated wheat germ agglutinin; WT, wild‐type.

Figure 6

Schematic summarizing effects of SSPN loss in cardiac muscle. A schematic diagram illustrates the changes that occur at the sarcolemma due to alterations in SSPN levels in WT and SSPN‐null (SSPN ^−/−) mice. Protein levels of the entire DGC as well as β1D‐integrin (light purple) are depicted by changes in color intensity. The sarcoglycans (α‐, β‐, γ‐, δ‐SGs; light orange) are depicted as transmembrane oval shapes. Dystrophin (Dys; white) is shown on the cytoplasmic face of the sarcolemma membrane. Dystroglycan (α‐, β‐ DGs; light blue) levels do not change with SSPN levels. Changes to cardiac muscle fiber cross‐sectional area are shown by increased size. Alterations in left ventricular dimension are demonstrated for SSPN ^−/− hearts, shown as increased LVEDD and LVESD. Glycosylation is depicted as black branches on both the SGs and DGs. The extracellular matrix containing laminin to which DG associates is depicted as black lines. This schematic summarizes the major consequences of SSPN loss including: (1) reduction in dystrophin levels, (2) almost complete loss of δ‐ and γ‐SG from the sarcolemma, (3) reduction in laminin binding in the extracellular matrix reducing cell‐to‐cell adhesion, (4) blunted functional responsiveness to isoproterenol, and (5) increased hypertrophic response to isoproterenol administration and aging. DGC indicates dystrophin–glycoprotein complex; LVEDD, left ventricular end diastolic dimension; LVESD, left ventricular end systolic dimension; SSPN, sarcospan; WT, wild‐type.

Figure 7

SSPN attenuates membrane damage in DMD hearts. A, Mice were injected with EBD to detect regions of membrane instability (visualized by red fluorescence). Transverse cryosections of heart muscle from 12‐month‐old WT, SSPN‐null (SSPN ^−/−), DMD model (mdx), and mdx:SSPN‐Tg (mdx ^TG) mice were stained with laminin antibodies (green) to visualize individual myofibers. Bar, 900 μm. B, Magnified (×40) images of representative sections from (A). Bar, 50 μm. C, Quantification of EBD‐positive fibers (n=3 hearts per genotype). D, Cardiac muscle from 4‐month‐old WT, mdx and mdx ^TG mice were stained with H&E, MT, and Oil Red (n=2). Bar, 50 μm. E, Relative changes in gene expression changes of ANP were assessed by qRT‐PCR in 4‐month‐old WT, mdx, and mdx ^TG hearts and number of mice utilized was (WT (n=4), mdx (n=5), mdx ^TG (n=5). F, Serum CK‐MB levels were detected in WT; mdx and mdx ^TG groups of mice (WT (n=5), mdx (n=6), mdx ^TG (n=5). G, Immunoblots of sWGA cardiac eluates show changes in key proteins in WT, mdx and mdx ^TG hearts (n=3 experiments, samples from 3 different mice of each genotype) whereas, in (H) Immunoblots show changes in laminin binding in a laminin overlay experiment (Laminin O/L) upon overexpression of SSPN in WT, mdx and mdx ^TG hearts (n=3 experiments, samples from 3 different mice of each genotype). Equal sWGA protein samples (30 μg) were resolved by SDS‐PAGE and immunoblotted with the indicated antibodies. CB staining is shown as a loading control. All data are presented as averages and error bars are expressed as SEM. Statistics were performed by 1‐way ANOVA with Bonferroni correction for individual groups in (C through F) and *P‐values ≤0.05 are indicated. For (C) *P=0.018 mdx vs WT, **P=0.027 mdx vs mdx ^TG, ^# P<0.001 SSPN ^−/− vs WT; (E) *P=0.003 WT vs mdx, **P=0.036 mdx vs mdx ^TG and (F) ^* P<0.001 mdx vs WT and **P=0.009 mdx vs mdx ^TG. ANP indicates atrial natriuretic peptide; CB, Coomassie blue; CK‐MB, cardiac creatine kinase; DMD, Duchenne muscular dystrophy; EBD, Evans blue dye; H&E, hematoxylin & eosin; mdx, Duchenne muscular dystrophy mouse model; mdx ^TG, mdx mice overexpressing human sarcospan; MT, Masson's trichrome; qRT‐PCR indicates quantitative reverse transcription–polymerase chain reaction; SDS‐PAGE, sodium dodecyl sulfate–polyacrylamide gel electrophoresis; SSPN, sarcospan; sWGA, succinylated wheat germ agglutin; WT, wild‐type.

Figure 8

Physiological evaluation of DMD mice overexpressing SSPN. To determine the effects of SSPN on mice exhibiting mdx cardiac pathology we evaluated the following morphological and functional parameters. A, HWt/BWt; mg/g values obtained from 11‐ to 12 month‐old mice: WT (n=8), mdx (n=10), mdx ^TG (n=6). Echocardiographic data presented was obtained from mice ≈10 to 11 months of age: WT (n=12), mdx (n=5), mdx ^TG (n=5) Echocardiographic assessment of contractility shows differences in (B) LV EF (%) and (C) LV FS (%) of WT, mdx and mdx ^TG mice. D, LVmass (mg) measurements obtained by echocardiography indicate differences in ventricular mass in WT, mdx and mdx ^TG mice. E, Echocardiographic measurements of LVEDD and LVESD in mm of WT, mdx and mdx ^TG mice. F, M‐mode images obtained by echocardiography for WT, mdx and mdx ^TG mice. Statistical analysis was performed using 1‐way ANOVA with Bonferroni post hoc test and *P‐values <0.05 are indicated on the plots, all data presented as averages and error bars represent SEM. For (A) *P=0.003 mdx vs WT, **P=0.01 mdx vs mdx ^TG; (B) *P=0.01 mdx vs WT, **P=0.01 mdx vs mdx ^TG; (C) *P=0.03 mdx vs WT, **P=0.01 mdx vs mdx ^TG; (D) P=0.061 mdx vs WT; and (E) ESD *P=0.03 mdx vs mdx ^TG, P=0.112 mdx vs WT. DMD indicates Duchenne muscular dystrophy; HWt/BWt, heart weight/body weight; LV EF, left ventricular ejection fraction; LV FS, left ventricular fractional shortening; LVEDD, left ventricular end diastolic dimension; LVESD, left ventricular end systolic dimension; mdx, Duchenne muscular dystrophy mouse model; mdx ^TG, mdx mice overexpressing human sarcospan; SSPN, sarcospan; WT, wild‐type.

Figure 9

Influence of SSPN on adhesion complexes in cardiac muscle. A schematic diagram illustrates the changes that occur at the sarcolemma due to overexpression of human SSPN in mdx and mdx ^TG mice. Protein levels of the entire DGC as well as the α7/β1D‐integrin complexes (light purple) are depicted by changes in color intensity. SSPN (green) is represented as a 4‐transmembrane‐spanning protein, where dark green represents its 3‐fold overexpression. The sarcoglycans (α‐, β‐, γ‐, δ‐ SGs; light orange) are depicted as transmembrane oval shapes. Dystrophin (Dys; white) is shown on the cytoplasmic face of the sarcolemma membrane. Dystroglycan (α‐, β‐DGs; light blue) levels do not change with SSPN levels. Glycosylation is depicted as black branches on both the SGs and DGs. CK‐MB is shown as a green sphere and EBD is depicted as a red square. Black lines indicate laminin in the extracellular matrix to which α‐DG and the DGC associate.This schematic summarizes the major findings when SSPN is overexpressed including: (1) increased association of utrophin and Itgb to the DGC, (2) increased laminin binding, (3) improved membrane stability, (4) improved tissue histology and cardiac function in DMD mice, and (5) reduced cardiac hypertrophy. CK‐MB indicates cardiac creatine kinase; DGC, dystrophin–glycoprotein complex; DMD, Duchenne muscular dystrophy; EBD, Evans blue dye; mdx, Duchenne muscular dystrophy mouse model; Itgb, β1D‐integrin; mdx ^TG, mdx mice overexpressing human sarcospan; SSPN, sarcospan; SSPN, sarcospan.