In mammals, most cardiomyocytes (CMs) become polyploid (they have more than two complete sets of chromosomes). The purpose of this review is to evaluate assumptions about CM ploidy that are commonly ...discussed, even if not experimentally demonstrated, and to highlight key issues that are still to be resolved. Topics discussed here include (
a
) technical and conceptual difficulties in defining a polyploid CM, (
b
) the candidate role of reactive oxygen as a proximal trigger for the onset of polyploidy, (
c
) the relationship between polyploidization and other aspects of CM maturation, (
d
) recent insights related to the regenerative role of the subpopulation of CMs that are not polyploid, and (
e
) speculations as to why CMs become polyploid at all. New approaches to experimentally manipulate CM ploidy may resolve some of these long-standing and fundamental questions.
Adult mammalian cardiomyocyte regeneration after injury is thought to be minimal. Mononuclear diploid cardiomyocytes (MNDCMs), a relatively small subpopulation in the adult heart, may account for the ...observed degree of regeneration, but this has not been tested. We surveyed 120 inbred mouse strains and found that the frequency of adult mononuclear cardiomyocytes was surprisingly variable (>7-fold). Cardiomyocyte proliferation and heart functional recovery after coronary artery ligation both correlated with pre-injury MNDCM content. Using genome-wide association, we identified Tnni3k as one gene that influences variation in this composition and demonstrated that Tnni3k knockout resulted in elevated MNDCM content and increased cardiomyocyte proliferation after injury. Reciprocally, overexpression of Tnni3k in zebrafish promoted cardiomyocyte polyploidization and compromised heart regeneration. Our results corroborate the relevance of MNDCMs in heart regeneration. Moreover, they imply that intrinsic heart regeneration is not limited nor uniform in all individuals, but rather is a variable trait influenced by multiple genes.
Maturation of a vascular plexus is a critical and yet incompletely understood process in organ development, and known maturation factors act universally in all vascular beds. In this study, we show ...that CXCL12 is an organ-specific maturation factor of particular relevance in coronary arterial vasculature. In vitro, CXCL12 does not influence nascent vessel formation, but promotes higher-order complexity of preinitiated vessels. In the heart, CXCL12 is expressed principally by the epicardium, and its receptor CXCR4 is expressed by coronary endothelial cells. CXCL12 is not a chemotactic signal for endothelial cell migration, but rather acts in a paracrine manner to influence the maturation of the coronary vascular plexus. Mutants in CXCL12 signaling show an excess of immature capillary chains and a selective failure in arterial maturation, and become leaky with the onset of coronary perfusion. Failed maturation of the coronary system explains the late-gestation lethality of these mutants.
•CXCL12 is a vascular maturation factor important in coronary artery development•CXCL12 acts in an organ-specific and paracrine manner•Defective coronary formation explains late-gestation lethality of CXCL12 mutants•The sinus venosus is the primary origin of embryonic coronary endothelium
Cavallero et al. show that CXCL12 is an organ-specific vascular maturation factor that promotes higher-order complexity of preinitiated vessels. In the heart, deficiency in CXCL12/CXCR4 signaling results in a poorly formed coronary system and insufficient myocardial perfusion, and accounts for late-gestation lethality seen in these mutants.
Unlike its mammalian counterpart, the adult zebrafish heart is able to fully regenerate after severe injury. One of the most important events during the regeneration process is cardiomyocyte ...proliferation, which results in the replacement of lost myocardium. Growth factors that induce cardiomyocyte proliferation during zebrafish heart regeneration remain to be identified. Signaling pathways important for heart development might be reutilized during heart regeneration. IGF2 was recently shown to be important for cardiomyocyte proliferation and heart growth during mid-gestation heart development in mice, although its role in heart regeneration is unknown. We found that expression of igf2b was upregulated during zebrafish heart regeneration. Following resection of the ventricle apex, igf2b expression was detected in the wound, endocardium and epicardium at a time that coincides with cardiomyocyte proliferation. Transgenic zebrafish embryos expressing a dominant negative form of Igf1 receptor (dn-Igf1r) had fewer cardiomyocytes and impaired heart development, as did embryos treated with an Igf1r inhibitor. Moreover, inhibition of Igf1r signaling blocked cardiomyocyte proliferation during heart development and regeneration. We found that Igf signaling is required for a subpopulation of cardiomyocytes marked by gata4:EGFP to contribute to the regenerating area. Our findings suggest that Igf signaling is important for heart development and myocardial regeneration in zebrafish.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
RATIONALE:There is persistent uncertainty regarding the developmental origins of coronary vessels, with 2 principal sources suggested as ventricular endocardium or sinus venosus (SV). These 2 ...proposed origins implicate fundamentally distinct mechanisms of vessel formation. Resolution of this controversy is critical for deciphering the programs that result in the formation of coronary vessels and has implications for research on therapeutic angiogenesis.
OBJECTIVE:To resolve the controversy over the developmental origin of coronary vessels.
METHODS AND RESULTS:We first generated nuclear factor of activated T cells (Nfatc1)-Cre and Nfatc1-Dre lineage tracers for endocardium labeling. We found that Nfatc1 recombinases also label a significant portion of SV endothelial cells in addition to endocardium. Therefore, restricted endocardial lineage tracing requires a specific marker that distinguishes endocardium from SV. By single-cell gene expression analysis, we identified a novel endocardial gene natriuretic peptide receptor 3 (Npr3). Npr3 is expressed in the entirety of the endocardium but not in the SV. Genetic lineage tracing based on Npr3-CreER showed that endocardium contributes to a minority of coronary vessels in the free walls of embryonic heart. Intersectional genetic lineage tracing experiments demonstrated that endocardium minimally contributes to coronary endothelium in the embryonic ventricular free walls.
CONCLUSIONS:Our study suggested that SV, but not endocardium, is the major origin for coronary endothelium in the embryonic ventricular free walls. This work thus resolves the recent controversy over the developmental origin of coronary endothelium, providing the basis for studying coronary vessel formation and regeneration after injury.
Significance A layer of fat surrounds the heart in most mammals, including humans. The biology of this tissue has been speculated for centuries, but never subjected to experimental analysis because ...common experimental model species are thought to not have this tissue. In this study, we show that rodents have cardiac fat, albeit in a very specific location in the heart. We implicate the origin of this tissue from the epicardium (the outer epithelium of the heart) and the underlying mechanisms that account for its derivation. By comparing human and mouse epicardial cells, we provide an explanation for the prominent species differences in the presence and amount of cardiac adipose tissue.
The hearts of many mammalian species are surrounded by an extensive layer of fat called epicardial adipose tissue (EAT). The lineage origins and determinative mechanisms of EAT development are unclear, in part because mice and other experimentally tractable model organisms are thought to not have this tissue. In this study, we show that mouse hearts have EAT, localized to a specific region in the atrial–ventricular groove. Lineage analysis indicates that this adipose tissue originates from the epicardium, a multipotent epithelium that until now is only established to normally generate cardiac fibroblasts and coronary smooth muscle cells. We show that adoption of the adipocyte fate in vivo requires activation of the peroxisome proliferator activated receptor gamma (PPARγ) pathway, and that this fate can be ectopically induced in mouse ventricular epicardium, either in embryonic or adult stages, by expression and activation of PPARγ at times of epicardium–mesenchymal transformation. Human embryonic ventricular epicardial cells natively express PPARγ, which explains the abundant presence of fat seen in human hearts at birth and throughout life.
Interruption of the coronary blood supply severely impairs heart function with often fatal consequences for patients. However, the formation and maturation of these coronary vessels is not fully ...understood. Here we provide a detailed analysis of coronary vessel development in zebrafish. We observe that coronary vessels form in zebrafish by angiogenic sprouting of arterial cells derived from the endocardium at the atrioventricular canal. Endothelial cells express the CXC-motif chemokine receptor Cxcr4a and migrate to vascularize the ventricle under the guidance of the myocardium-expressed ligand Cxcl12b. cxcr4a mutant zebrafish fail to form a vascular network, whereas ectopic expression of Cxcl12b ligand induces coronary vessel formation. Importantly, cxcr4a mutant zebrafish fail to undergo heart regeneration following injury. Our results suggest that chemokine signaling has an essential role in coronary vessel formation by directing migration of endocardium-derived endothelial cells. Poorly developed vasculature in cxcr4a mutants likely underlies decreased regenerative potential in adults.
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•Zebrafish develop coronary vessels during late juvenile development•Coronary vessel endothelial cells are derived from the endocardium•Endothelial cells form coronary vessels under the guidance of CXC signaling•cxcr4a mutant zebrafish hearts lack coronary vessels and are unable to regenerate
Harrison et al. describe coronary vessel formation in juvenile zebrafish by endocardial cells that migrate onto the ventricle. These endothelial cells sprout to create the vascular network under the guidance of chemokine signaling. Without these signals, the coronary vessels fail to form, resulting in reduced regenerative potential in adult zebrafish.
Recent evidence implicates mononuclear diploid cardiomyocytes as a proliferative and regenerative subpopulation of the postnatal heart. The number of these cardiomyocytes is a complex trait showing ...substantial natural variation among inbred mouse strains based on the combined influences of multiple polymorphic genes. One gene confirmed to influence this parameter is the cardiomyocyte-specific kinase Tnni3k. Here, we have studied Tnni3k alleles across a number of species. Using a newly-generated kinase-dead allele in mice, we show that Tnni3k function is dependent on its kinase activity. In an in vitro kinase assay, we show that several common human TNNI3K kinase domain variants substantially compromise kinase activity, suggesting that TNNI3K may influence human heart regenerative capacity and potentially also other aspects of human heart disease. We show that two kinase domain frameshift mutations in mice cause loss-of-function consequences by nonsense-mediated decay. We further show that the Tnni3k gene in two species of mole-rat has independently devolved into a pseudogene, presumably associated with the transition of these species to a low metabolism and hypoxic subterranean life. This may be explained by the observation that Tnni3k function in mice converges with oxidative stress to regulate mononuclear diploid cardiomyocyte frequency. Unlike other studied rodents, naked mole-rats have a surprisingly high (30%) mononuclear cardiomyocyte level but most of their mononuclear cardiomyocytes are polyploid; their mononuclear diploid cardiomyocyte level (7%) is within the known range (2-10%) of inbred mouse strains. Naked mole-rats provide further insight on a recent proposal that cardiomyocyte polyploidy is associated with evolutionary acquisition of endothermy.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract Mammalian cardiomyocytes (CMs) mostly become polyploid shortly after birth. Because this feature may relate to several aspects of heart biology, including regeneration after injury, the ...mechanisms that cause polyploidy are of interest. BALB/cJ and BALB/cByJ mice are highly related sister strains that diverge substantially in CM ploidy. We identified a large deletion in the Cyth1 gene that arose uniquely in BALB/cByJ mice that creates a null allele. The deletion also results in ectopic transcription of the downstream gene Dnah17, although this transcript is unlikely to encode a protein. By evaluating the natural null allele from BALB/cByJ and an engineered knockout allele in the C57BL/6J background, we determined that absence of Cyth1 does not by itself influence CM ploidy. The ready availability of BALB/cByJ mice may be helpful to other investigations of Cyth1 in other biological processes.
A complex regulatory network of morphogens and transcription factors is essential for normal cardiac development. Nkx2-5 is among the earliest known markers of cardiac mesoderm that is central to the ...regulatory pathways mediating second heart field (SHF) development. Here, we have examined the specific requirements for Nkx2-5 in the SHF progenitors. We show that Nkx2-5 potentiates Wnt signaling by regulating the expression of the R-spondin3 (Rspo3) gene during cardiogenesis. R-spondins are secreted factors and potent Wnt agonists that in part regulate stem cell proliferation. Our data show that Rspo3 is markedly downregulated in Nkx2-5 mutants and that Rspo3 expression is regulated by Nkx2-5. Conditional inactivation of Rspo3 in the Isl1 lineage resulted in embryonic lethality secondary to impaired development of SHF. More importantly, we find that Wnt signaling is significantly attenuated in Nkx2-5 mutants and that enhancing Wnt/β-catenin signaling by pharmacological treatment or by transgenic expression of Rspo3 rescues the SHF defects in the conditional Nkx2-5(+/-) mutants. We have identified a previously unrecognized genetic link between Nkx2-5 and Wnt signaling that supports continued cardiac growth and proliferation during development. Identification of Rspo3 in cardiac development provides a new paradigm in temporal regulation of Wnt signaling by cardiac-specific transcription factors.
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