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Stem Cell Prevention but Not Stem Cell Repair?

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Original article from STEM CELLS

"Injection of Vessel-Derived Stem Cells Prevents Dilated Cardiomyopathy and Promotes Angiogenesis and Endogenous Cardiac Stem Cell Proliferation in mdx/utrn−/− but Not Aged mdx Mouse Models for Duchenne Muscular Dystrophy"

Duchenne muscular dystrophy (DMD) is an X-linked muscle wasting disease affecting approximately 1/3,500 male live births (Emery) and results from mutations in the dystrophin gene.   While advances in care have alleviated some aspects of the disease, dilated cardiomyopathy (DCM) incidence has increased.   Several pharmacological agents are used to target the symptoms but they do not address the underlying absence of dystrophin or the loss of cardiomyocytes.   One potentially exciting avenue of exploration is the transplantation of exogenous stem cells, which can restore dystrophin expression (Berry et al and Sampaolesi et al). Fetal cardiomyocytes (Koh et al) and skeletal muscle-derived stem cells (MDSCs) (Payne et al) have both been investigated after injection directly into the heart.    Now, in a study in Stem Cells Translational Medicine, researchers from the group of Suzanne E. Berry at the University of Illinois, USA have studied a role for adult-derived aorta-derived mesoangioblasts (ADMs) in the restoration of dystrophin expression and prevention/alleviation of cardiomyopathy in dystrophin-deficient mdx mice.   In this study they show that ADMs induce cardiac marker expression and delayed the onset of DCM in young mice, but could not reverse symptoms in older mice (Chun et al).

ADMs changed their proliferative spindle morphology to resemble control rat cardiomyocytes (98% at day 14) when cultured in cardiomyocyte medium and the resultant cells made tight contacts with each other at which Connexin 43 (Cx43) was present, important for electrical coupling.   Additionally they expressed cardiac specific markers (Nkx2.5, cardiac troponin I (cTnI), and cardiac tropomyosin (cTm) and α-actinin), together suggesting that they have the ability to differentiate properly into cardiomyocytes.   Transplantation of undifferentiated ADMs labelled with the lipophilic dye Dil into the cardiac muscle of 5 week old mdx/utrn-/- mice, deficient in both dystrophin and utrophin, a homolog of dystrophin that functionally compensates for dystrophin in mice (Tinsley et al), led to the detection of dystrophin expression but not control saline injected mdx/utrn-/- mice.   Importantly, ADM-injection led to the thickening of the left ventricular wall, an increase in the diameter of the left ventricle at both diastole and systole and a large increase in end diastolic volume (EDV) as compared to controls.   Levels of angiogenesis, as measured solely by CD31 levels were also increased in the ADM-injected mdx/utrn-/- mice.   Analysis of Ki67-positive proliferating cells in the heart found numerous positive cells in the ventricles of the wild type heart, which was decreased 5-fold in the saline injected mdx/utrn-/- mice, but recovered by the injection of ADMs.   Although the Ki67-positive cells did not express the resident cardiac stem cell marker Sca-1, they did co-express Nestin, which is present  in stem cells in normal and infarcted myocardium where they contribute to neuronal innervation, remodeling, and angiogenesis and give rise to Nkx2.5-positive cardiomyocytes after ischemic injury (Beguin et al, El-Helou  et al, 2005, El-Helou et al, 2008 and Scobioala et al).   Additionally some Nestin-positive cells were striated and expressed cardiac troponin I.   At 5 weeks, Dil-labelled cells were not observed in the myocardial layer of the heart, even though dystrophin-positive cardiomyocytes were present, but were found in the epicardium where around half expressed the cardiac fibroblast marker DDR2 and 20% expressed cardiac troponin I.

ADM cells were then injected into the hearts of aged mdx mice to assess their ability to alleviate existing cardiomyopathy, but this did not show the expected for positive results.   Upon stem cell injection, large changes consistent with the development of dilated cardiomyopathy where observed, as compared to saline-injected mice.   Ventricular dilation and wall thickening were observed after ADM-injection and exhibited a significant decline in heart function (as measured by ejection fraction (EF), fractional shortening (FS), end diastolic volume (EDV) and end systolic volume (ESV)), all consistent with the later stages of ventricular remodeling following damage and with poorer survival following recovery from damage (Konstam et al, St John Sutton and Sharpe and White et al).   Additionally ADM-injected mdx hearts in aged mice did not express Dystrophin, did not show any signs of increased angiogenesis as measured by CD31 levels and did not show increased levels of proliferating cells, as observed in the young mice.

This first report of stem cell therapy yielding functional benefit in the dystrophin-deficient heart suggests that injected ADMs could mediate the prevention of dilated cardiomyopathy but cannot recover this in the aged heart where pathology is present.   Injection into young mdx/utrn-/- mouse hearts prevented wall thinning and increased heart function, however this was not observed in the aged mice.   The authors suggest that this may be due to the presence of Utrophin in the mdx mice but is more likely to be due to the altered cardiac microenvironment due to age and the presence of pathology.   Mechanisms require to be understood for this prevention or delay the onset of dilated cardiomyopathy in young mice but further research in this area should outline the role of ADMs in this therapeutic context.

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Study originally appeared in Stem Cells Translational Medicine.

Stem CellCorrespondent Stuart P Atkinson reports on those studies appearing in current journals that are destined to make an impact on stem cell research and clinical studies.