Cardiac failure has a principal underlying aetiology of ischaemic damage arising from vascular insufficiency. Molecules that regulate collateral growth in the ischaemic heart also regulate coronary vasculature formation during embryogenesis. Here we identify thymosin beta4 (Tbeta4) as essential for all aspects of coronary vessel development in mice, and demonstrate that Tbeta4 stimulates significant outgrowth from quiescent adult epicardial explants, restoring pluripotency and triggering differentiation of fibroblasts, smooth muscle cells and endothelial cells. Tbeta4 knockdown in the heart is accompanied by significant reduction in the pro-angiogenic cleavage product N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP). Although injection of AcSDKP was unable to rescue Tbeta4 mutant hearts, it significantly enhanced endothelial cell differentiation from adult epicardially derived precursor cells. This study identifies Tbeta4 and AcSDKP as potent stimulators of coronary vasculogenesis and angiogenesis, and reveals Tbeta4-induced adult epicardial cells as a viable source of vascular progenitors for continued renewal of regressed vessels at low basal level or sustained neovascularization following cardiac injury.

A significant bottleneck in cardiovascular regenerative medicine is the identification of a viable source of stem/progenitor cells that could contribute new muscle after ischaemic heart disease and acute myocardial infarction1. A therapeutic ideal—relative to cell transplantation—would be to stimulate a resident source, thus avoiding the caveats of limited graft survival, restricted homing to the site of injury and host immune rejection. Here we demonstrate in mice that the adult heart contains a resident stem or progenitor cell population, which has the potential to contribute bona fide terminally differentiated cardiomyocytes after myocardial infarction. We reveal a novel genetic label of the activated adult progenitors via re-expression of a key embryonic epicardial gene, Wilm’s tumour 1 (Wt1), through priming by thymosin β4, a peptide previously shown to restore vascular potential to adult epicardium-derived progenitor cells2 with injury. Cumulative evidence indicates an epicardial origin of the progenitor population, and embryonic reprogramming results in the mobilization of this population and concomitant differentiation to give rise to de novo cardiomyocytes. Cell transplantation confirmed a progenitor source and chromosome painting of labelled donor cells revealed transdifferentiation to a myocyte fate in the absence of cell fusion. Derived cardiomyocytes are shown here to structurally and functionally integrate with resident muscle; as such, stimulation of this adult progenitor pool represents a significant step towards residentcell-based therapy in human ischaemic heart disease.

The lymphatic vasculature is a blind-ended network crucial for tissue fluid homeostasis, immune surveillance and lipid absorption from the gut. Recent evidence has proposed an entirely venous-derived mammalian lymphatic system. In contrast, we reveal here that cardiac lymphatic vessels have a heterogeneous cellular origin, whereby formation of at least part of the cardiac lymphatic network is independent of sprouting from veins. Multiple cre-lox based lineage tracing revealed a potential contribution from the hemogenic endothelium during development and discrete lymphatic endothelial progenitor populations were confirmed by conditional knockout of Prox1 in Tie2+ and Vav1+ compartments. In the adult heart, myocardial infarction (MI) promoted a significant lymphangiogenic response, which was augmented by treatment with VEGF-C resulting in improved cardiac function. These data prompt the re-evaluation of a century-long debate on the origin of lymphatic vessels and suggest that lymphangiogenesis may represent a therapeutic target to promote cardiac repair following injury.

The placenta and cardiovascular system are the first organ systems to form during mammalian embryogenesis. We show here that a single gene is critical for development of both. The Hand1 gene, previously called Hxt, eHAND and Thing1, encodes a basic helix-loop-helix (bHLH) transcription factor that starts to be expressed during pre-implantation development. After implantation, Hand1 expression is restricted to placental trophoblast cells and later to embryonic cardiac and neural crest cells. We generated Hand1-null mutant mice by gene targetting. Homozygous mutant embryos arrested by embryonic day (E) 7.5 of gestation with defects in trophoblast giant cell differentiation. This early mortality could be rescued by aggregation of mutant embryos with wild-type tetraploid embryos, which contribute wild-type cells to the trophoblast, but not the embryo. By E10.5, however, the Hand1-null fetuses derived from tetraploid chimaeras died due to cardiac failure. Their heart tubes showed abnormal looping and ventricular myocardial differentiation. Therefore, Hand1 is essential for differentiation of both trophoblast and cardiomyocytes, which are embryologically distinct cell lineages.

Myocardial infarction (MI) arising from obstruction of the coronary circulation engenders massive cardiomyocyte loss and replacement by non-contractile scar tissue, leading to pathological remodeling, dysfunction, and ultimately heart failure. This is presently a global health problem for which there is no effective cure. Following MI, the innate immune system directs the phagocytosis of dead cell debris in an effort to stimulate cell repopulation and tissue renewal. In the mammalian adult heart, however, the persistent influx of immune cells, coupled with the lack of an inherent regenerative capacity, results in cardiac fibrosis. Here, we reveal that stimulation of cardiac lymphangiogenesis with VEGF-C improves clearance of the acute inflammatory response after MI by trafficking immune cells to draining mediastinal lymph nodes (MLNs) in a process dependent on lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1). Deletion of Lyve1 in mice, preventing docking and transit of leukocytes through the lymphatic endothelium, results in exacerbation of chronic inflammation and long-term deterioration of cardiac function. Our findings support targeting of the lymphatic/immune cell axis as a therapeutic paradigm to promote immune modulation and heart repair.

Canonical roles for macrophages in mediating the fibrotic response after a heart attack include extracellular matrix turnover and activation of cardiac fibroblasts to initiate collagen deposition. Here we reveal that macrophages directly contribute collagen to the forming post-injury scar. Unbiased transcriptomics shows an upregulation of collagens in both zebrafish and mouse macrophages following heart injury. Adoptive transfer of macrophages, from either collagen-tagged zebrafish or adult mouse GFPtpz-collagen donors, enhances scar formation via cell autonomous production of collagen. In zebrafish, the majority of tagged collagen localises proximal to the injury, within the overlying epicardial region, suggesting a possible distinction between macrophage-deposited collagen and that predominantly laiddown by myofibroblasts. Macrophage-specific targeting of col4a3bpa and cognate col4a1 in zebrafish significantly reduces scarring in cryoinjured hosts. Our findings contrast with the current model of scarring, whereby collagen deposition is exclusively attributed to myofibroblasts, and implicate macrophages as direct contributors to fibrosis during heart repair.

The basic helix-loop-helix (bHLH) transcription factor genes Hand1 and Mash2 are essential for placental development in mice. Hand1 promotes differentiation of trophoblast giant cells, whereas Mash2 is required for the maintenance of giant cell precursors, and its overexpression prevents giant cell differentiation. We found that Hand1 expression and Mash2 expression overlap in the ectoplacental cone and spongiotrophoblast, layers of the placenta that contain the giant cell precursors, indicating that the antagonistic activities of Hand1 and Mash2 must be coordinated. MASH2 and HAND1 both heterodimerize with E factors, bHLH proteins that are the DNA-binding partners for most class B bHLH factors and which are also expressed in the ectoplacental cone and spongiotrophoblast. In vitro, HAND1 could antagonize MASH2 function by competing for E-factor binding. However, the Hand1 mutant phenotype cannot be solely explained by ectopic activity of MASH2, as the Hand1 mutant phenotype was not altered by further mutation of Mash2. Interestingly, expression of E-factor genes (ITF2 and ALF1) was down-regulated in the trophoblast lineage prior to giant cell differentiation. Therefore, suppression of MASH2 function, required to allow giant cell differentiation, may occur in vivo by loss of its E-factor partner due to loss of its expression and/or competition from HAND1. In giant cells, where E-factor expression was not detected, HAND1 presumably associates with a different bHLH partner. This may account for the distinct functions of HAND1 in giant cells and their precursors. We conclude that development of the trophoblast lineage is regulated by the interacting functions of HAND1, MASH2, and their cofactors.The placenta is critical for the intrauterine survival of mammalian embryos. In mice, mutations that severely disrupt placentation or establishment of the chorioallantoic circulation result in embryonic lethality by day 10.5 of gestation (E10.5). Defects in placentation also contribute to diseases of human pregnancy, including spontaneous abortion and preeclampsia (11). However, surprisingly little is known regarding the molecular events that regulate development of the trophoblast cell lineage, the epithelial component of the placenta. At the blastocyst stage, trophoblast cells in contact with the inner cell mass (polar trophectoderm) continue to proliferate and later contribute to the chorion and ectoplacental cone (24). In contrast, trophoblast cells distal to the inner cell mass (mural trophectoderm) terminally differentiate to form primary trophoblast giant cells. While mitotically arrested, these cells undergo continued rounds of DNA synthesis (endocycles), thereby acquiring their characteristic giant polyploid nuclei (54). Secondary giant cells subsequently arise due to differentiation of precursor cells present in the ectoplacental cone and, later in gestation, the spongiotrophoblast (17). Trophoblast giant cells participate in a number of processes critical to a successful pregnancy, including blastocyst implantatio...

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