Utrophin deficiency worsens cardiac contractile dysfunction present in dystrophin-deficient mdx mice
ABSTRACT The loss of dystrophin in patients with Duchenne muscular dystrophy (DMD) causes devastating skeletal muscle degeneration and cardiomyopathy. Dystrophin-deficient (mdx) mice have a much milder phenotype, whereas double knockout (DKO) mice lacking both dystrophin and its homolog, utrophin, exhibit the clinical signs observed in DMD patients. We have previously shown that DKO and mdx mice have similar severities of histological features of cardiomyopathy, but no contractile functional measurements of DKO heart have ever been carried out. To investigate whether DKO mice display cardiac dysfunction at the tissue level, contractile response of the myocardium was tested in small, unbranched, ultrathin, right ventricular muscles. Under near physiological conditions, peak isometric active developed tension (F(dev), in mN/mm2) at a stimulation frequency of 4 Hz was depressed in DKO mice (15.3 +/- 3.7, n = 8) compared with mdx mice (24.2 +/- 5.4, n = 7), which in turn were depressed compared with wild-type (WT) control mice (33.2 +/- 4.5, n = 7). This reduced Fdev was also observed at frequencies within the murine physiological range; at 12 Hz, Fdev was (in mN/mm2) 11.4 +/- 1.8 in DKO, 14.5 +/- 4.2 in mdx, and 28.8 +/- 5.4 in WT mice. The depression of Fdev was observed over the entire frequency range of 4-14 Hz and was significant between DKO versus mdx mice, as well as between DKO or mdx mice versus WT mice. Under beta-adrenergic stimulation (1 micromol/l isoproterenol), Fdev in DKO preparations was only (in mN/mm2) 14.7 +/- 5.1 compared with 30.9 +/- 8.9 in mdx and 41.0 +/- 4.9 in WT mice. These data show that cardiac contractile dysfunction of mdx mice is generally worsened in mice also lacking utrophin.
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ABSTRACT: The mdx mouse model of Duchenne muscular dystrophy (DMD) is used to study disease mechanisms and potential treatments, but its pathology is less severe than DMD patients. Other mouse models were developed to more closely mimic the human disease based on knowledge that upregulation of utrophin has a protective effect in mdx muscle. An mdx:utrophin(-/-) (dko) mouse was created, which had a severe disease phenotype and a shortened life span. An mdx:utrophin(+/-) mouse was also created, which had an intermediate disease phenotype compared to the mdx and dko mice. To determine the usefulness of mdx:utrophin(+/-) mice for long-term DMD studies, limb muscle pathology and function were assessed across the life span of wild-type, mdx, mdx:utrophin(+/-), and dko mice. Muscle function assessment, specifically grip duration and rotarod performance, demonstrated that mdx:utrophin(+/-) mice were weaker for a longer time than mdx mice. Mean myofiber area was smaller in mdx:utrophin(+/-) mice compared to mdx mice at 12 months. Mdx:utrophin(+/-) mice had a higher percentage of centrally nucleated myofibers compared to mdx mice at 6 and 12 months. Collagen I and IV density was significantly higher in mdx:utrophin(+/-) muscle compared to mdx at most ages examined. Generally, mdx:utrophin(+/-) mice showed an intermediate disease phenotype over a longer time course compared to the mdx and dko mice. While they do not genetically mirror human DMD, mdx:utrophin(+/-) mice may be a more useful animal model than mdx or dko mice for investigating long-term efficacy of potential treatments when fibrosis or muscle function is the focus. © 2015 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.04/2015; 3(4). DOI:10.14814/phy2.12391
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ABSTRACT: Thyroid hormones are key regulators of basal metabolic state and oxidative metabolism. Hyperthyroidism has been reported to cause significant alterations in hemodynamics, and in cardiac and diaphragm muscle function, all which have been linked to increased oxidative stress. However, the definite source of increased reactive oxygen species (ROS) in each of these phenotypes is still unknown. The goal of the current study was to test the hypothesis that T4 may produce distinct hemodynamic, cardiac and diaphragm muscle abnormalities by differentially affecting various sources of ROS. Wild-type and T4 mice with and without 2-week treatments with allopurinol (xanthine oxidase inhibitor), apocynin (NADPH-oxidase inhibitor), L-NIO (nitric oxide synthase inhibitor), or MitoTEMPO (mitochondria-targeted antioxidant) were studied. Blood pressure and echocardiography were non-invasively evaluated, followed by ex-vivo assessments of isolated heart and diaphragm muscle functions. Treatment with L-NIO attenuated the T4-induced hypertension in mice. However, apocynin improved the left ventricular (LV) dysfunction without preventing the cardiac hypertrophy in these mice. Both allopurinol and MitoTEMPO reduced the T4-induced fatigability of the diaphragm muscles. In conclusion, we show here for the first time that T4 exerts differential effects on various sources of ROS to induce distinct cardiovascular and skeletal muscle phenotypes. Additionally, we find that T4-induced LV dysfunction is independent of cardiac hypertrophy, while NADPH-oxidase is a key player in this process. Furthermore, we prove the significance of both xanthine-oxidase and mitochondrial ROS pathways in T4-induced fatigability of diaphragm muscles. Finally, we confirm the importance of the nitric oxide (NO) pathway in T4-induced hypertension. Copyright © 2015. Published by Elsevier Inc.Free Radical Biology and Medicine 03/2015; 83(2). DOI:10.1016/j.freeradbiomed.2015.02.035 · 5.71 Impact Factor
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ABSTRACT: Although it has been speculated that stem cell depletion plays a role in the rapid progression of the muscle histopathology associated with Duchenne Muscular Dystrophy (DMD), the molecular and cellular mechanisms responsible for stem cell depletion remain poorly understood. The rapid depletion of muscle stem cells has not been observed in the dystrophin-deficient model of DMD (mdx mouse), which may explain the relatively mild dystrophic phenotype observed in this animal model. In contrast, we have observed a rapid occurrence of stem cell depletion in the dystrophin/utrophin double knockout (dKO) mouse model, which exhibits histopathological features that more closely recapitulate the phenotype observed in DMD patients compared to the mdx mouse. Notch signaling has been found to be a key regulator of stem cell self-renewal and myogenesis in normal skeletal muscle; however, little is known about the role that Notch plays in the development of the dystrophic histopathology associated with DMD. Our results revealed an over-activation of Notch in the skeletal muscles of dKO mice, which correlated with sustained inflammation, impaired muscle regeneration and the rapid depletion and senescence of the muscle progenitor cells (MPCs, i.e., Pax7+ cells). Consequently, the repression of Notch in the skeletal muscle of dKO mice delayed/reduced the depletion and senescence of MPCs, and restored the myogenesis capacity while reducing inflammation and fibrosis. We suggest that the down-regulation of Notch could represent a viable approach to reduce the dystrophic histopathologies associated with DMD. © The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: firstname.lastname@example.org.Human Molecular Genetics 02/2015; 24(10). DOI:10.1093/hmg/ddv055 · 6.68 Impact Factor