The mammalian heart has a very limited regenerative capacity and, hence, heals by scar formation. Recent reports suggest that haematopoietic stem cells can transdifferentiate into unexpected phenotypes such as skeletal muscle, hepatocytes, epithelial cells, neurons, endothelial cells and cardiomyocytes, in response to tissue injury or placement in a new environment. Furthermore, transplanted human hearts contain myocytes derived from extra-cardiac progenitor cells, which may have originated from bone marrow. Although most studies suggest that transdifferentiation is extremely rare under physiological conditions, extensive regeneration of myocardial infarcts was reported recently after direct stem cell injection, prompting several clinical trials. Here, we used both cardiomyocyte-restricted and ubiquitously expressed reporter transgenes to track the fate of haematopoietic stem cells after 145 transplants into normal and injured adult mouse hearts. No transdifferentiation into cardiomyocytes was detectable when using these genetic techniques to follow cell fate, and stem-cell-engrafted hearts showed no overt increase in cardiomyocytes compared to sham-engrafted hearts. These results indicate that haematopoietic stem cells do not readily acquire a cardiac phenotype, and raise a cautionary note for clinical studies of infarct repair.

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Axl is a receptor tyrosine kinase originally identified as a transforming gene product in human myeloid leukemia cells. Cultured rat vascular smooth muscle cells also express Axl, where it has been proposed that Axl may play a role in cell proliferation. In the current study, we tested the hypotheses that Axl expression would parallel neointima formation in balloon-injured rat carotid, and that Axl expression would be regulated by growth factors present at sites of vascular injury. Ribonuclease protection assay showed dynamic increases in Axl mRNA in vessels, with peak expression 7 and 14 days after injury. Immunohistochemical analysis confirmed these results and demonstrated that Axl protein expression was localized primarily to cells of the neointima after injury. Northern blot analysis indicated increased mRNA expression for the secreted Axl ligand, Gas6, in injured carotids, with a time course paralleling that of Axl upregulation. Axl and Gas6 expression were temporally correlated with neointima formation, suggesting a role for Axl signaling in this process. Other studies, performed in cultured rat vascular smooth muscle cells, revealed positive regulation of Axl mRNA expression by thrombin or angiotensin II but not by basic fibroblast growth factor, platelet-derived growth factor-BB, or transforming growth factor-ss1. Western blot analysis confirmed these results, showing that Axl protein expression was specifically increased by thrombin or angiotensin II. Our results implicate Axl as a potential mediator of vascular smooth muscle migration and proliferation caused by vascular injury and G protein-coupled receptor agonists.

Abstract-Synthesis of nitric oxide (NO) by endothelial nitric oxide synthase (eNOS) is critical for normal vascular homeostasis. eNOS function is rapidly regulated by agonists and blood flow and chronically by factors that regulate mRNA stability and gene transcription. Recently, localization of eNOS to specialized plasma membrane invaginations termed caveolae has been proposed to be required for maximal eNOS activity. Because caveolae are highly enriched in cholesterol, and hypercholesterolemia is associated with increased NO production, we first studied the effects of cholesterol loading on eNOS localization and NO production in cultured bovine aortic endothelial cells (BAECs). Caveolae-enriched fractions were prepared by OptiPrep gradient density centrifugation. Treatment of BAECs with 30 g/mL cholesterol for 24 hours stimulated significant increases in total eNOS protein expression (1.50-fold), eNOS associated with caveolae-enriched membranes (2.23-fold), and calcium ionophore-stimulated NO production (1.56-fold). Because reactive oxygen species (ROS) contribute to endothelial dysfunction in hypercholesterolemia, we next studied the effects of ROS on eNOS localization and caveolae number. Treatment of BAECs for 24 hours with 1 mol/L LY83583, a superoxide-generating napthoquinolinedione, decreased caveolae number measured by electron microscopy and prevented the cholesterol-mediated increases in eNOS expression. In vitro exposure of caveolae-enriched membranes to ROS (xanthine plus xanthine oxidase) dissociated caveolin more readily than eNOS from the membranes. These results show that cholesterol treatment increases eNOS expression, whereas ROS treatment decreases eNOS expression and the association of eNOS with caveolin in caveolae-enriched membranes. Our data suggest that oxidative stress modulates endothelial function by regulating caveolae formation, eNOS expression, and eNOS-caveolin interactions. (Circ Res. 1999;85:29-37.)

Vascular remodeling is regulated by a combination of hemodynamic, environmental, and genetic factors and may be influenced by age. To evaluate age-dependent remodeling in rats, we developed and used a quantitative highly reproducible model of carotid flow alteration. Fourteen juvenile (99+/-3 g) and 9 adult (199+/-5 g) male inbred Fischer rats underwent ligation of the left internal and external carotid arteries under anesthesia. Left common carotid blood flow immediately decreased by approximately 93%, whereas flow in the contralateral carotid increased by approximately 46%. After 4 weeks, the left carotid outer diameter (OD) significantly decreased in both juvenile and adult rats (as measured in vivo and by histological morphometry) compared with sham-operated rats. Changes in shear stress acutely mirrored the changes in blood flow. OD increased and shear stress returned to initial values after chronic exposure to increased flow in juvenile but not adult rats. To develop a simple quantitative index of remodeling that would not require killing the animals, we measured the OD in vivo and compared the ratio of right to left OD (OD ratio [ODR]) between groups. The initial ODR for all groups was approximately 1.0. After 4 weeks of altered flow, the ODR was significantly greater in juvenile than in adult rats (1.48+/-0.05 versus 1.29+/-0.04, respectively; P=.030), indicating that juvenile rats experienced more extensive remodeling than did the adult rats. We also found that unilateral carotid ligation caused a left versus right difference in endothelial NO synthase protein levels after 4 weeks that was not present in the sham-operated animals. Thus, the model described here shows that flow-induced vascular remodeling is dependent on age and supports the hypothesis that the driving force for remodeling involves shear stress and possibly NO. Because the model is quantitative, it allows dissection of the genetic factors that regulate remodeling in inbred rat strains.

Abstract-Cardiomyoplasty with skeletal myoblasts may benefit cardiac function after infarction. Recent reports indicate that adult stem cells can fuse with other cell types. Because myoblasts are "fusigenic" cells by nature, we hypothesized they might be particularly likely to fuse with cardiomyocytes. To test this, neonatal rat cardiomyocytes labeled with LacZ and green fluorescent protein (GFP) were cocultured with unlabeled C2C12 myoblasts. After 3 days, we observed a small population of skeletal myotubes that expressed LacZ and GFP, indicating cell fusion. To test whether such fusion occurred in vivo, LacZ-expressing C2C12 myoblasts were grafted into normal nude mouse hearts. At 2 weeks after grafting, cells at the graft-host interface expressed both LacZ and cardiac-specific myosin light chain 2v (MLC2v). To test more definitively whether fusion between skeletal and cardiac muscle could occur, we used a Cre/lox reporter system that activated LacZ only upon cell fusion. When neonatal cardiomyocytes from ␣-myosin heavy chain promoter (␣-MHC)-Cre mice were cocultured with myoblasts from floxed-lacZ reporter mice, LacZ was activated in a subset of cells, indicating cell fusion occurred in vitro. Finally, we grafted the floxed-lacZ myoblasts into normal hearts of ␣-MHC-Cre ϩ and ␣-MHC-Cre Ϫ mice (nϭ5 each). Hearts analyzed at 4 days and 1 week after transplantation demonstrated activation of LacZ when the skeletal muscle cells were implanted into hearts of ␣-MHC-Cre ϩ mice, but not after implantation into ␣-MHC-Cre Ϫ mice. These data indicate that skeletal muscle cell grafting gives rise to a subpopulation of skeletal-cardiac hybrid cells with a currently unknown phenotype. The full text of this article is available online at http://circres.ahajournals.org. Key Words: cardiomyoplasty Ⅲ skeletal myoblast Ⅲ cell transplantation Ⅲ cell fusion Ⅲ Cre/lox C ellular cardiomyoplasty has emerged as a promising therapy for myocardial infarct repair. Skeletal myoblasts and bone marrow-derived stem cells have been tried in various animal models and are now undergoing clinical trials. [1][2][3][4] A recent study has shown that circulating cells derived from transplanted bone marrow can fuse with cardiomyocytes, Purkinje cells and hepatocytes. 5 Because skeletal myoblasts are "fusigenic" cells by nature, we tested if myoblasts can fuse with cardiomyocytes. Our study shows that skeletal myoblast grafting into the heart results in fusion with host cardiomyocytes, giving rise to a subpopulation of hybrid cells. Materials and Methods Cell CultureMouse neonatal cardiomyocytes and primary neonatal skeletal myoblasts were isolated as described previously. 6,7 Spontaneous cell fusion (coculture) experiments were set up in gelatin-coated 6-well plates at a total density of 2ϫ10 4 cells/cm 2 (1:1 ratio) in DMEM/ M199 (4:1) supplemented with 10% horse serum, 5% FBS, and 6 g/mL insulin. These conditions were optimized for cardiomyocytes while allowing terminal differentiation of skeletal myoblasts into multinucleated myotubes.Forced fusi...

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