Mammalian organs vary widely in regenerative capacity. Poorly regenerative organs, such as the heart are particularly vulnerable to organ failure. Once established, heart failure (HF) commonly results in mortality1. The Hippo pathway, a kinase cascade that prevents adult cardiomyocyte proliferation and regeneration2, is upregulated in human HF. We show that deletion of the Hippo pathway component Salvador (Salv) in mouse hearts with established ischemic HF after myocardial infarction (MI) induced a reparative genetic program with increased scar border vascularity, reduced fibrosis, and recovery of pumping function compared to controls. Using TRAP (translating ribosomal affinity purification), we isolated cardiomyocyte specific translating mRNA. Hippo deficient cardiomyocytes had increased expression of proliferative genes and stress response genes, such as the mitochondrial quality control (MQC) gene, Park2. Genetic studies indicated that Park2 was essential for heart repair suggesting a requirement for MQC in regenerating myocardium. Gene therapy with a virus encoding Salv shRNA improved heart function when delivered at the time of infarct or after ischemic HF post-MI was established. Our findings indicate that the failing heart has a previously unrecognized reparative capacity involving more than cardiomyocyte renewal.

Summary Myocardial infarction results in compromised myocardial function with heart failure due to insufficient cardiomyocyte self-renewal1. Unlike lower vertebrates, mammalian hearts only have a transient neonatal renewal capacity2. Reactivating primitive reparative ability in the mature heart requires knowledge of the mechanisms promoting early heart repair. By testing an established Hippo-deficient heart regeneration model for renewal promoting factors, we found that Pitx2 expression was induced in injured, Hippo-deficient ventricles. Pitx2-deficient neonatal hearts failed to repair after apex resection while Pitx2-gain-of-function in adult cardiomyocytes conferred reparative ability after myocardial infarction. Genomic analyses indicated that Pitx2 activated genes encoding electron transport chain components and reactive oxygen species scavengers. A subset of Pitx2 target genes was cooperatively regulated with the Hippo effector, Yap. Furthermore, Nrf2, a regulator of antioxidant response3, directly regulated Pitx2 expression and subcellular localization. Pitx2 mutant myocardium had elevated reactive oxygen species levels while antioxidant supplementation suppressed the Pitx2-loss-of-function phenotype. These findings reveal a genetic pathway, activated by tissue damage that is essential for cardiac repair.

Mucus is a protective gel that lines respiratory tract surfaces. To identify potential roles for secreted gel--forming mucins in lung development, we isolated murine lungs on embryonic days (E) 12.5-18.5, and postnatal days (PN) days 5, 14, and 28. We measured the mucin gene expression by quantitative RT-PCR, and localization by histochemical and immunohistochemical labeling. Alcian blue/periodic acid--Schiff--positive cells are present from E15.5 through PN28. Muc5b transcripts were abundant at all time points from E14.5 to PN28. By contrast, transcript levels of Muc5ac and Muc2 were approximately 300 and 85,000 times lower, respectively. These data are supported by immunohistochemical studies demonstrating the production and localization of Muc5ac and Muc5b protein. This study indicates that mucin production is prominent in developing murine lungs and that Muc5b is an early, abundant, and persistent marker of bronchial airway secretory cells, thereby implicating it as an intrinsic component of homeostatic mucosal defense in the lungs.

Mucus is a protective gel that lines respiratory tract surfaces. To identify potential roles for secreted gel-forming mucins in lung development, we isolated murine lungs on embryonic days (E) 12.5-18.5, and postnatal days (PN) days 5, 14, and 28. We measured the mucin gene expression by quantitative RT-PCR, and localization by histochemical and immunohistochemical labeling. Alcian blue/periodic acid-Schiff-positive cells are present from E15.5 through PN28. Muc5b transcripts were abundant at all time points from E14.5 to PN28. By contrast, transcript levels of Muc5ac and Muc2 were approximately 300 and 85,000 times lower, respectively. These data are supported by immunohistochemical studies demonstrating the production and localization of Muc5ac and Muc5b protein. This study indicates that mucin production is prominent in developing murine lungs and that Muc5b is an early, abundant, and persistent marker of bronchial airway secretory cells, thereby implicating it as an intrinsic component of homeostatic mucosal defense in the lungs. At approximately 4 weeks of gestation in humans (embryonic days 9.0-9.5 in mice), tracheal and lung-bud formation initiates as a pocket of endoderm evaginates from the ventral floor of the embryonic foregut and invades the underlying mesoderm. The developing lungs undergo repeated branching steps, giving rise to the complex of tubes that form the conducting airways and alveolar saccules. During this process of branching and morphogenesis, the endoderm changes from an undifferentiated mass to a pseudostratified squamous layer, and then to a differentiated layer comprised of heterogeneous progenitor and mature cells (1).Mature respiratory epithelial cells possess many unique functions that can be attributed to their structural features or their secretory products. Structural features such as the long, apical projections of tracheobronchial ciliated cells and the thin sheet-like squamous morphologies of alveolar Type I cells are examples of obvious morphological traits that are specifically linked to cellular function (ciliary clearance and gas exchange, respectively). Secreted products also demarcate the functions of individual respiratory epithelial subtypes. The tracheobronchial epithelium produces secreted products, such as mucins, that serve defensive functions by preventing the accumulation of particles and pathogens in the distal lung (2-4), whereas Type II pneumocytes produce surfactant lipids that reduce surface tension and prevent alveolar collapse, thereby permitting effective gas exchange (5). Additional anatomically selective markers include the secretaglobin (Scgb) family, with members such as Scgb1a1 (or Clara cell secretory protein) in the airways, and surfactant proteins A, B, and C in the alveoli. The expression of many of these markers is low early during lung development, when the bronchial epithelium is poorly differentiated, and becomes more abundant as epithelial maturation occurs late in embryonic gestation and throughout postnatal life (6).The importance of muc...

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