Lloyd H. Michael

Regeneration of ischemic cardiac muscle and vascular endothelium by adult stem cells

Myocyte loss in the ischemically injured mammalian heart often leads to irreversible deficits in cardiac function. To identify a source of stem cells capable of restoring damaged cardiac tissue, we transplanted highly enriched hematopoietic stem cells, the so-called side population (SP) cells, into lethally irradiated mice subsequently rendered ischemic by coronary artery occlusion for 60 minutes followed by reperfusion. The engrafted SP cells (CD34(-)/low, c-Kit(+), Sca-1(+)) or their progeny migrated into ischemic cardiac muscle and blood vessels, differentiated to cardiomyocytes and endothelial cells, and contributed to the formation of functional tissue. SP cells were purified from Rosa26 transgenic mice, which express lacZ widely. Donor-derived cardiomyocytes were found primarily in the peri-infarct region at a prevalence of around 0.02% and were identified by expression of lacZ and alpha-actinin, and lack of expression of CD45. Donor-derived endothelial cells were identified by expression of lacZ and Flt-1, an endothelial marker shown to be absent on SP cells. Endothelial engraftment was found at a prevalence of around 3.3%, primarily in small vessels adjacent to the infarct. Our results demonstrate the cardiomyogenic potential of hematopoietic stem cells and suggest a therapeutic strategy that eventually could benefit patients with myocardial infarction.

Cardiac progenitor cells from adult myocardium: Homing, differentiation, and fusion after infarction

Potential repair by cell grafting or mobilizing endogenous cells holds particular attraction in heart disease, where the meager capacity for cardiomyocyte proliferation likely contributes to the irreversibility of heart failure. Whether cardiac progenitors exist in adult myocardium itself is unanswered, as is the question whether undifferentiated cardiac precursor cells merely fuse with preexisting myocytes. Here we report the existence of adult heart-derived cardiac progenitor cells expressing stem cell antigen-1. Initially, the cells express neither cardiac structural genes nor Nkx2.5 but differentiate in vitro in response to 5′-azacytidine, in part depending on Bmpr1a, a receptor for bone morphogenetic proteins. Given intravenously after ischemia/reperfusion, cardiac stem cell antigen 1 cells home to injured myocardium. By using a Cre/Lox donor/recipient pair (αMHC-Cre/R26R), differentiation was shown to occur roughly equally, with and without fusion to host cells.

Pathophysiologically Relevant Concentrations of Tumor Necrosis Factor-α Promote Progressive Left Ventricular Dysfunction and Remodeling in Rats

BACKGROUND Although patients with heart failure express elevated circulating levels of tumor necrosis factor-alpha (TNF-alpha) in their peripheral circulation, the structural and functional effects of circulating levels of pathophysiologically relevant concentrations of TNF-alpha on the heart are not known. METHODS AND RESULTS Osmotic infusion pumps containing either diluent or TNF-alpha were implanted into the peritoneal cavity of rats. The rate of TNF-alpha infusion was titrated to obtain systemic levels of biologically active TNF-alpha comparable to those reported in patients with heart failure (approximately 80 to 100 U/mL), and the animals were examined serially for 15 days. Two-dimensional echocardiography was used to assess changes in left ventricular (LV) structure (remodeling) and LV function. Video edge detection was used to assess isolated cell mechanics, and standard histological techniques were used to assess changes in the volume composition of LV cardiac myocytes and the extracellular matrix. The reversibility of cytokine-induced effects was determined either by removal of the osmotic infusion pumps on day 15 or by treatment of the animals with a soluble TNF-alpha antagonist (TNFR:Fc). The results of this study show that a continuous infusion of TNF-alpha led to a time-dependent depression in LV function, cardiac myocyte shortening, and LV dilation that were at least partially reversible by removal of the osmotic infusion pumps or treatment of the animals with TNFR:Fc. CONCLUSIONS These studies suggest that pathophysiologically relevant concentrations of TNF-alpha are sufficient to mimic certain aspects of the phenotype observed in experimental and clinical models of heart failure.

Resident Cardiac Mast Cells Degranulate and Release Preformed TNF-α, Initiating the Cytokine Cascade in Experimental Canine Myocardial Ischemia/Reperfusion

BACKGROUND Neutrophil-induced cardiomyocyte injury requires the expression of myocyte intercellular adhesion molecule (ICAM)-1 and ICAM-1-CD11b/CD18 adhesion. We have previously demonstrated interleukin (IL)-6 activity in postischemic cardiac lymph; IL-6 is the primary stimulus for myocyte ICAM- 1 induction. Furthermore, we found that induction of IL-6 mRNA occurred very early on reperfusion of the infarcted myocardium. We hypothesized that the release of a preformed upstream cytokine induced IL-6 in leukocytes infiltrating on reperfusion. METHODS AND RESULTS Constitutive expression of TNF-alpha and not IL-1beta was demonstrated in the normal canine myocardium and was localized predominantly in cardiac mast cells. Mast cell degranulation in the ischemic myocardium was documented by demonstration of a rapid release of histamine and TNF-alpha in the cardiac lymph after myocardial ischemia. Histochemical studies with FITC-labeled avidin demonstrated degranulating mast cells only in ischemic samples of canine myocardium. Immunohistochemistry suggested that degranulating mast cells were the primary source of TNF-alpha in the ischemic myocardium. In situ hybridization studies of reperfused myocardium localized IL-6 mRNA in infiltrating mononuclear cells and in mononuclear cells appearing in the postischemic cardiac lymph within the first 15 minutes of reperfusion. Furthermore, isolated canine mononuclear cells incubated with postischemic cardiac lymph demonstrated significant induction of IL-6 mRNA, which was partially blocked with a neutralizing antibody to TNF-alpha. CONCLUSIONS Cardiac mast cells degranulate after myocardial ischemia, releasing preformed mediators, such as histamine and TNF-alpha. We suggest that mast cell-derived TNF-alpha may be a crucial factor in upregulating IL-6 in infiltrating leukocytes and initiating the cytokine cascade responsible for myocyte ICAM-1 induction and subsequent neutrophil-induced injury.

Gene recombination in postmitotic cells. Targeted expression of Cre recombinase provokes cardiac-restricted, site-specific rearrangement in adult ventricular muscle in vivo.

Mouse models of human disease can be generated by homologous recombination for germline loss-of-function mutations. However, embryonic-lethal phenotypes and systemic, indirect dysfunction can confound the use of knock-outs to elucidate adult pathophysiology. Site-specific recombination using Cre recombinase can circumvent these pitfalls, in principle, enabling temporal and spatial control of gene recombination. However, direct evidence is lacking for the feasibility of Cre-mediated recombination in postmitotic cells. Here, we exploited transgenic mouse technology plus adenoviral gene transfer to achieve Cre-mediated recombination in cardiac muscle. In vitro, Cre driven by cardiac-specific alpha-myosin heavy chain (alphaMyHC) sequences elicited recombination selectively at loxP sites in purified cardiac myocytes, but not cardiac fibroblasts. In vivo, this alphaMyHC-Cre transgene elicited recombination in cardiac muscle, but not other organs, as ascertained by PCR analysis and localization of a recombination-dependent reporter protein. Adenoviral delivery of Cre in vivo provoked recombination in postmitotic, adult ventricular myocytes. Recombination between loxP sites was not detected in the absence of Cre. These studies demonstrate the feasibility of using Cre-mediated recombination to regulate gene expression in myocardium, with efficient induction of recombination even in terminally differentiated, postmitotic muscle cells. Moreover, delivery of Cre by viral infection provides a simple strategy to control the timing of recombination in myocardium.

TAK1 is activated in the myocardium after pressure overload and is sufficient to provoke heart failure in transgenic mice.

The transforming-growth-factor-β-activated kinase TAK1 is a member of the mitogen-activated protein kinase kinase kinase family, which couples extracellular stimuli to gene transcription. The in vivo function of TAK1 is not understood. Here, we investigated the potential involvement of TAK1 in cardiac hypertrophy. In adult mouse myocardium, TAK1 kinase activity was upregulated 7 days after aortic banding, a mechanical load that induces hypertrophy and expression of transforming growth factor β. An activating mutation of TAK1 expressed in myocardium of transgenic mice was sufficient to produce p38 mitogen-activated protein kinase phosphorylation in vivo, cardiac hypertrophy, interstitial fibrosis, severe myocardial dysfunction, ‘fetal’ gene induction, apoptosis and early lethality. Thus, TAK1 activity is induced as a delayed response to mechanical stress, and can suffice to elicit myocardial hypertrophy and fulminant heart failure.

Cardiac Myocytes Produce Interleukin-6 in Culture and in Viable Border Zone of Reperfused Infarctions

BACKGROUND Previous work from our laboratory demonstrated that interleukin (IL)-6 plays a potentially critical role in postreperfusion myocardial injury and is the major cytokine responsible for induction of intracellular adhesion molecule (ICAM)-1 on cardiac myocytes during reperfusion. Myocyte ICAM-1 induction is necessary for neutrophil-associated myocyte injury. We have previously demonstrated the induction of IL-6 in the ischemic myocardium, and the current study addresses the cells of origin of IL-6. METHODS AND RESULTS In the present study, we combined Northern blot analysis and in situ hybridization to demonstrate IL-6 gene expression in cardiac myocytes. Isolated ventricular myocytes were stimulated with tumor necrosis factor-alpha, IL-1beta, lipopolysaccharide, preischemic lymph, and postischemic lymph. Unstimulated myocytes showed no significant IL-6 mRNA expression. Myocytes stimulated with preischemic lymph showed minimal or no IL-6 mRNA expression, whereas myocytes stimulated with tumor necrosis factor-alpha, IL-1beta, lipopolysaccharide, or postischemic lymph showed a strong IL-6 mRNA induction. Northern blot with ICAM-1 probe revealed ICAM-1 expression under every condition that demonstrated IL-6 induction. We then investigated the expression of IL-6 mRNA in our canine model of ischemia and reperfusion. Cardiac myocytes in the viable border zone of a myocardial infarction exhibited reperfusion-dependent expression of IL-6 mRNA within 1 hour after reperfusion. Mononuclear cells infiltrate the border zone and express IL-6 mRNA. CONCLUSIONS Isolated cardiac myocytes produce IL-6 mRNA in response to several cytokines as well as postischemic cardiac lymph. In addition to its production by inflammatory cells, we demonstrate that IL-6 mRNA is induced in myocytes in the viable border zone of a myocardial infarct. The potential roles of IL-6 in cardiac myocytes in an infarct border are discussed.

Induction of Interleukin-6 Synthesis in the Myocardium Potential Role in Postreperfusion Inflammatory Injury

BACKGROUND Neutrophil-induced injury of myocardial cells requires the expression of intercellular adhesion molecule-1 (ICAM-1) on the myocyte surface and is mediated by ICAM-1-CD11b/CD18 adhesion. We have previously shown that interleukin-6 (IL-6) cytokine activity, present in cardiac lymph, induces ICAM-1 on isolated cardiac myocytes. Furthermore, in previous in vivo studies, we have also shown ICAM-1 mRNA induction in the myocardium within the first hour of reperfusion in the previously ischemic viable zone. We hypothesized that induction of IL-6 synthesis in the myocardium was an integral part of the reaction to injury resulting from ischemia and reperfusion and was associated with induction of ICAM-1 on myocardial cells. METHODS AND RESULTS In this study, cloned canine IL-6 cDNA was used as a molecular probe to study the regulation of IL-6 in an awake canine model of myocardial ischemia and reperfusion. IL-6 mRNA was induced in ischemic and reperfused segments of myocardium preferentially in segments previously exposed to severe ischemia. Peak levels of IL-6 mRNA were reached within 3 hours of reperfusion. At the same time, IL-6 mRNA and ICAM-1 mRNA were found in the same myocardial segments. In contrast to hearts that were ischemic for 1 hour and reperfused for 3 hours, nonreperfused hearts after 4 hours of persistent ischemia demonstrated minimal induction of ICAM-1 or IL-6 despite similar degrees of injury and blood flow reductions during ischemia. After 24 hours of persistent ischemia, levels of IL-6 mRNA were comparable to those observed in hearts that were ischemic for 1 hour and subsequently reperfused for 24 hours. CONCLUSIONS Our results demonstrate induction of IL-6 mRNA in the myocardium and that this synthesis is accelerated by reperfusion. Evidence is also provided to show that peak IL-6 mRNA precedes that of ICAM-1 mRNA. These findings are compatible with our hypothesis that IL-6 is important in the induction of ICAM-1 in the area of ischemia. In addition, these studies suggest that the necessary factors to promote adhesive interactions between transmigrated neutrophils and cardiac myocytes are present in reperfused myocardium.

Telomerase reverse transcriptase promotes cardiac muscle cell proliferation, hypertrophy, and survival

Cardiac muscle regeneration after injury is limited by “irreversible” cell cycle exit. Telomere shortening is one postulated basis for replicative senescence, via down-regulation of telomerase reverse transcriptase (TERT); telomere dysfunction also is associated with greater sensitivity to apoptosis. Forced expression of TERT in cardiac muscle in mice was sufficient to rescue telomerase activity and telomere length. Initially, the ventricle was hypercellular, with increased myocyte density and DNA synthesis. By 12 wk, cell cycling subsided; instead, cell enlargement (hypertrophy) was seen, without fibrosis or impaired function. Likewise, viral delivery of TERT was sufficient for hypertrophy in cultured cardiac myocytes. The TERT virus and transgene also conferred protection from apoptosis, in vitro and in vivo. Hyperplasia, hypertrophy, and survival all required active TERT and were not seen with a catalytically inactive mutation. Thus, TERT can delay cell cycle exit in cardiac muscle, induce hypertrophy in postmitotic cells, and promote cardiac myocyte survival.

Of mice and dogs: species-specific differences in the inflammatory response following myocardial infarction.

Large animal models have provided much of the descriptive data regarding the cellular and molecular events in myocardial infarction and repair. The availability of genetically altered mice may provide a valuable tool for specific cellular and molecular dissection of these processes. In this report we compare closed chest models of canine and mouse infarction/reperfusion qualitatively and quantitatively for temporal, cellular, and spatial differences. Much like the canine model, reperfused mouse hearts are associated with marked induction of endothelial adhesion molecules, cytokines, and chemokines. Reperfused mouse infarcts show accelerated replacement of cardiomyocytes by granulation tissue leading to a thin mature scar at 14 days, when the canine infarction is still cellular and evolving. Infarcted mouse hearts demonstrate a robust but transient postreperfusion inflammatory reaction, associated with a rapid up-regulation of interleukin-10 and transforming growth factor-beta. Unlike canine infarcts, infarcted mouse hearts show only transient macrophage infiltration and no significant mast cell accumulation. In correlation, the growth factor for macrophages, M-CSF, shows modest and transient up-regulation in the early days of reperfusion; and the obligate growth factor for mast cells, stem cell factor, SCF, is not induced. In summary, the postinfarction inflammatory response and resultant repair in the mouse heart shares many common characteristics with large mammalian species, but has distinct temporal and qualitative features. These important species-specific differences should be considered when interpreting findings derived from studies using genetically altered mice.