The serine-threonine kinases Pim-1 and Akt regulate cellular proliferation and survival. Although Akt is known to be a crucial signaling protein in the myocardium, the role of Pim-1 has been overlooked. Pim-1 expression in the myocardium of mice decreased during postnatal development, re-emerged after acute pathological injury in mice and was increased in failing hearts of both mice and humans. Cardioprotective stimuli associated with Akt activation induced Pim-1 expression, but compensatory increases in Akt abundance and phosphorylation after pathological injury by infarction or pressure overload did not protect the myocardium in Pim-1-deficient mice. Transgenic expression of Pim-1 in the myocardium protected mice from infarction injury, and Pim-1 expression inhibited cardiomyocyte apoptosis with concomitant increases in Bcl-2 and Bcl-X(L) protein levels, as well as in Bad phosphorylation levels. Relative to nontransgenic controls, calcium dynamics were significantly enhanced in Pim-1-overexpressing transgenic hearts, associated with increased expression of SERCA2a, and were depressed in Pim-1-deficient hearts. Collectively, these data suggest that Pim-1 is a crucial facet of cardioprotection downstream of Akt.

The possibility that adult bone marrow cells (BMCs) retain a remarkable degree of developmental plasticity and acquire the cardiomyocyte lineage after infarction has been challenged, and the notion of BMC transdifferentiation has been questioned. The center of the controversy is the lack of unequivocal evidence in favor of myocardial regeneration by the injection of BMCs in the infarcted heart. Because of the interest in cell-based therapy for heart failure, several approaches including gene reporter assay, genetic tagging, cell genotyping, PCR-based detection of donor genes, and direct immunofluorescence with quantum dots were used to prove or disprove BMC transdifferentiation. Our results indicate that BMCs engraft, survive, and grow within the spared myocardium after infarction by forming junctional complexes with resident myocytes. BMCs and myocytes express at their interface connexin 43 and N-cadherin, and this interaction may be critical for BMCs to adopt the cardiomyogenic fate. With time, a large number of myocytes and coronary vessels are generated. Myocytes show a diploid DNA content and carry, at most, two sex chromosomes. Old and new myocytes show synchronicity in calcium transients, providing strong evidence in favor of the functional coupling of these two cell populations. Thus, BMCs transdifferentiate and acquire the cardiomyogenic and vascular phenotypes restoring the infarcted heart. Together, our studies reveal that locally delivered BMCs generate de novo myocardium composed of integrated cardiomyocytes and coronary vessels. This process occurs independently of cell fusion and ameliorates structurally and functionally the outcome of the heart after infarction. myocardial infarction ͉ myocardial regeneration ͉ stem cells ͉ transdifferentiation T o date, the hematopoietic stem cell appears to be the most versatile stem cell in crossing lineage boundaries and the most prone to break the law of tissue fidelity (1). Early studies on c-kit-positive bone marrow cell (BMC) differentiation into myocardium have generated great enthusiasm (2, 3), but other observations have rejected the initial results (4-6) and promoted a wave of skepticism about the therapeutic potential of BMCs for the injured heart. The major criticisms include: (i) lack of utilization of genetic markers for the recognition of donor BMCs and their progeny; (ii) inaccurate interpretation of the original data due to autofluorescence artifacts; and (iii) the possibility that myocyte regeneration is mediated by fusion of BMCs with resident myocytes rather than BMC transdifferentiation. To address these important questions and demonstrate reproducibility of results, four laboratories with complementary expertise have undertaken a series of joined experiments to acquire information on the plasticity of BMCs and their therapeutic potential for the infarcted heart.In this effort, BMCs for myocardial regeneration were obtained from three transgenic mice. In the first, EGFP was driven by the ubiquitous ␤-actin promoter; in the second, EGFP was ...

One of the greatest examples of integrated signal transduction is revealed by examination of effects mediated by AKT kinase in myocardial biology. Positioned at the intersection of multiple afferent and efferent signals, AKT exemplifies a molecular sensing node that coordinates dynamic responses of the cell in literally every aspect of biological responses. The balanced and nuanced nature of homeostatic signaling is particularly essential within the myocardial context, where regulation of survival, energy production, contractility, and response to pathological stress all flow through the nexus of AKT activation or repression. Equally important, the loss of regulated AKT activity is primarily the cause or consequence of pathological conditions leading to remodeling of the heart and eventual decompensation. This review presents an overview compendium of the complex world of myocardial AKT biology gleaned from more than a decade of research. Summarization of the widespread influence that AKT exerts upon myocardial responses leaves no doubt that the participation of AKT in molecular signaling will need to be reckoned with as a seemingly omnipresent regulator of myocardial molecular biological responses.

Abstract-The Notch network regulates multiple cellular processes, including cell fate determination, development, differentiation, proliferation, apoptosis, and regeneration. These processes are regulated via Notch-mediated activity that involves hepatocyte growth factor (HGF)/c-Met receptor and phosphatidylinositol 3-kinase/Akt signaling cascades.

STEM CELLS 2008;26:1315-1324 Disclosure of potential conflicts of interest is found at the end of this article.

Rationale Dual cell transplantation of cardiac progenitor cells (CPCs) and mesenchymal stem cells (MSCs) after infarction improves myocardial repair and performance in large animal models relative to delivery of either cell population. Objective To demonstrate that CardioChimeras (CCs) formed by fusion between CPCs and MSCs have enhanced reparative potential in a mouse model of myocardial infarction relative to individual stem cells or combined cell delivery. Methods and Results Two distinct and clonally derived CCs, CC1 and CC2 were utilized for this study. CCs improved left ventricular anterior wall thickness (AWT) at 4 weeks post injury, but only CC1 treatment preserved AWT at 18 weeks. Ejection fraction was enhanced at 6 weeks in CCs, and functional improvements were maintained in CCs and CPC + MSC groups at 18 weeks. Infarct size was decreased in CCs, whereas CPC + MSC and CPC parent groups remained unchanged at 12 weeks. CCs exhibited increased persistence, engraftment, and expression of early commitment markers within the border zone relative to combinatorial and individual cell population-injected groups. CCs increased capillary density and preserved cardiomyocyte size in the infarcted regions suggesting CCs role in protective paracrine secretion. Conclusions CCs merge the application of distinct cells into a single entity for cellular therapeutic intervention in the progression of heart failure. CCs are a novel cell therapy that improves upon combinatorial cell approaches to support myocardial regeneration.

Key Words: Pim-1 Ⅲ mitochondria Ⅲ cardiomyocyte Ⅲ apoptosis C ardiovascular disease is the leading cause of death among men and women and affects approximately 33% of the US population. 1 A direct correlation between the decline in heart function and loss of cardiomyocytes via apoptosis involving the mitochondria occurs in cardiomyopathy, myocardial ischemia/reperfusion (I/R), and congestive heart failure. 2-9 Specifically, myocardial I/R injury generates calcium overload and oxidative stress, which initiate the intrinsic apoptotic pathway through activation of the mitochondrial permeability transition pore (mPTP). The ensuing chain of events result in dramatic changes to mitochondrial morphology associated with uncoupling of the electron transport chain, depolarization of the inner membrane, matrix swelling, unfolding of the cristae, and ultimately outer membrane rupture, with release of proapoptotic cytochrome c. 10 -15 Release of cytochrome c into the cytosol consequently activates apoptotic protease-activating factor, which mediates caspase cascade programmed cell death. 16 Thus, preservation of mitochondrial integrity is essential in designing molecular strategies to enhance cardiomyocyte cell survival by blunting injury attributed to cardiomyopathic insult.Cardioprotection mediated by survival kinase signal transduction acts through multiple mechanisms including preservation of mitochondrial integrity. 17 Numerous studies have documented antiapoptotic actions of the serine/threonine kinase AKT, which acts in part through protecting mitochon-

Rationale Cardioprotective effects of Pim-1 kinase have been previously reported but the underlying mechanistic basis may involve a combination of cellular and molecular mechanisms that remain unresolved. The elucidation of the mechanistic basis for Pim-1 mediated cardioprotection provides important insights for designing therapeutic interventional strategies to treat heart disease. Objective Effects of cardiac-specific Pim-1 kinase expression on the cardiac progenitor cell (CPC) population were examined to determine whether Pim-1 mediates beneficial effects through augmenting CPC activity. Methods and Results Transgenic mice created with cardiac-specific Pim-1 overexpression (Pim-wt) exhibit enhanced Pim-1 expression in both cardiomyocytes and CPCs, both of which show increased proliferative activity assessed using 5-bromodeoxyuridine (BrdU), Ki-67, and c-Myc relative to nontransgenic controls. However, the total number of CPCs was not increased in the Pim-wt hearts during normal postnatal growth or after infarction challenge. These results suggest that Pim-1 overexpression leads to asymmetric division resulting in maintenance of the CPC population. Localization and quantitation of cell fate determinants Numb and α-adaptin by confocal microscopy were used to assess frequency of asymmetric division in the CPC population. Polarization of Numb in mitotic phospho-histone positive cells demonstrates asymmetric division in 65% of the CPC population in hearts of Pim-wt mice versus 26% in nontransgenic hearts after infarction challenge. Similarly, Pim-wt hearts had fewer cells with uniform α-adaptin staining indicative of symmetrically dividing CPCs, with 36% of the CPCs versus 73% in nontransgenic sections. Conclusions These findings define a mechanistic basis for enhanced myocardial regeneration in transgenic mice overexpressing Pim-1 kinase.