SUMMARYThe basic helix-loop-helix (bHLH) family of transcription factors orchestrates cell-fate specification, commitment and differentiation in multiple cell lineages during development. Here, we describe the role of a bHLH transcription factor, Tcf21 (epicardin/Pod1/capsulin), in specification of the cardiac fibroblast lineage. In the developing heart, the epicardium constitutes the primary source of progenitor cells that form two cell lineages: coronary vascular smooth muscle cells (cVSMCs) and cardiac fibroblasts. Currently, there is a debate regarding whether the specification of these lineages occurs early in the formation of the epicardium or later after the cells have entered the myocardium. Lineage tracing using a tamoxifen-inducible Cre expressed from the Tcf21 locus demonstrated that the majority of Tcf21-expressing epicardial cells are committed to the cardiac fibroblast lineage prior to initiation of epicardial epithelial-to-mesenchymal transition (EMT). Furthermore, Tcf21 null hearts fail to form cardiac fibroblasts, and lineage tracing of the null cells showed their inability to undergo EMT. This is the first report of a transcription factor essential for the development of cardiac fibroblasts. We demonstrate a unique role for Tcf21 in multipotent epicardial progenitors, prior to the process of EMT that is essential for cardiac fibroblast development.

Stem cells are important in the maintenance and repair of adult tissues. A population of cells, termed side population (SP) cells, has stem cell characteristics as they have been shown to contribute to diverse lineages. In this study, we confirm that Abcg2 is a determinant of the SP cell phenotype. Therefore, we examined Abcg2 expression during murine embryogenesis and observed robust expression in the blood islands of the E8.5 yolk sac and in developing tissues including the heart. During the latter stages of embryogenesis, Abcg2 identifies a rare cell population in the developing organs. We further establish that the adult heart contains an Abcg2 expressing SP cell population and these progenitor cells are capable of proliferation and differentiation. We define the molecular signature of cardiac SP cells and compare it to embryonic stem cells and adult cardiomyocytes using emerging technologies. We propose that the cardiac SP cell population functions as a progenitor cell population for the development, maintenance, and repair of the heart.

Muscle regeneration is a complex process requiring the coordinated interaction between the myogenic progenitor cells or satellite cells, growth factors, cytokines, inflammatory components, vascular components and the extracellular matrix (ECM). Previous studies have elegantly described the physiological modulation of the regenerative process in response to muscle injury, but the molecular response that characterizes stages of the repair process remains ill-defined. The recent completion of the Human and Mouse Genome Projects and the advent of technologies such as high-density oligonucleotide array analysis facilitate an expanded analysis of complex processes such as muscle regeneration. In the present study, we define cellular and molecular events that characterize stages of muscle injury and regeneration. Utilization of transcriptional profiling strategies revealed coordinated expression of growth factors [i.e., Tgfb1, Igf1, Egf, chemokine (C-C motif) ligand 6 and 7], the fetal myogenic program (Myod1, Myf5, Myf6), and the biomatrix (procollagen genes, Mmp3, Mmp9, biglycan, periostin) during muscle regeneration. Corroboration of the transcriptional profiling analysis included quantitative real-time RT-PCR and in situ hybridization analyses of selected candidate genes. In situ hybridization studies for periostin [osteoblast-specific factor 2 (fasciclin I-like)] and biglycan revealed that these genes are restricted to mesenchymal derivatives during embryogenesis and are significantly regulated during regeneration of the injured hindlimb skeletal muscle. We conclude that muscle regeneration is a complex process that requires the coordinated modulation of the inflammatory response, myogenic progenitor cells, growth factors, and ECM for complete restoration of muscle architecture.

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Objective: We asked whether, as in other tissues of this mouse, EGFP localized and functionally tagged adult cardiac tissue progenitors, and, if so, whether this cell-based signal could serve as a quantitative and qualitative biosensor of the injury repair response of the heart. Key Words: Notch Ⅲ epicardium Ⅲ myocardial infarction Ⅲ adult progenitors Ⅲ repair C ardiovascular disease leading to heart failure is the most common and costly cause of death and disability in the modern world. The adult mammalian heart responds to biomechanical stress and injury with fibrosis. Cardiac fibrosis could have several cellular inputs: (1) preexisting interstitial fibroblasts, (2) circulating fibrocytes, (3) fibroblast progenitors arising by endothelial-mesenchymal transition of endocardial or microvascular coronary endothelial cells, or (4) fibroblast progenitors arising by epithelial-mesenchymal transition (EMT) of epicardial mesothelial cells. [1][2][3] Recently, there has been keen focus on the epicardium as a candidate source of adult heart repair fibroblasts and other cells. Methods and Results: In addition to scattered endothelial and interstitial cells, Notch-activated (EGFP؉The origin of the epicardium from the proepicardial organ and its essential role in cardiovascular development have been elegantly elucidated. However, until recently, the biology of the adult epicardium has been largely ignored. Traditionally viewed as a fibrous mesothelial covering, mechanically insulating and lubricating the outer surface of the heart muscle, the adult epicardium is now believed to have a more complex and active role in myocardial homeostasis and repair. The epicardium is a common residence for advanced metastatic cancers and infectious, inflammatory, and rheumatologic diseases; a host for (and possibly source of) unique epicardial adipose tissue; and, most importantly, a potential cardiac stem/progenitor cell niche. 4 Interestingly, recent electron and immunofluorescence microscopy studies identified at least 10 distinct cell types, including putative early cardiomyocyte precursors, in specialized niche-like structures in adult epicardium. 5,6 When "activated" by injury, the epicardium develops organ-wide thickening, with increased cellularity and extracellular matrix, and complex regional topography. New investigative tools and approaches are needed to Original received April 20, 2010; resubmission received September 22, 2010; revised resubmission received November 8, 2010; accepted November 11, 2010. In October 2010, the average time from submission to first decision for all original research papers submitted to Circulation Research was 13.9 days.From One of the key unanswered questions in the field is whether adult epicardium is a birthplace of newly born cardiomyocytes. 5,7-10 Recent fate-mapping studies have provided genetic evidence that new cardiomyocytes are produced in the adult mammalian heart following myocardial injury. 11 The origin of these cells remains unknown. The regenerative capacity of the adult mammali...