Parabiosis is a surgical union of two organisms allowing sharing of the blood circulation. Attaching the skin of two animals promotes formation of microvasculature at the site of inflammation. Parabiotic partners share their circulating antigens and thus are free of adverse immune reaction. First described by Paul Bert in 1864(1), the parabiosis surgery was refined by Bunster and Meyer in 1933 to improve animal survival(2). In the current protocol, two mice are surgically joined following a modification of the Bunster and Meyer technique. Animals are connected through the elbow and knee joints followed by attachment of the skin allowing firm support that prevents strain on the sutured skin. Herein, we describe in detail the parabiotic joining of a ubiquitous GFP expressing mouse to a wild type (WT) mouse. Two weeks after the procedure, the pair is separated and GFP positive cells can be detected by flow cytometric analysis in the blood circulation of the WT mouse. The blood chimerism allows one to examine the contribution of the circulating cells from one animal in the other.

Light Sheet Fluorescence Microscopy (LSFM) enables multi-dimensional and multi-scale imaging via illuminating specimens with a separate thin sheet of laser. It allows rapid plane illumination for reduced photo-damage and superior axial resolution and contrast. We hereby demonstrate cardiac LSFM (c-LSFM) imaging to assess the functional architecture of zebrafish embryos with a retrospective cardiac synchronization algorithm for four-dimensional reconstruction (3-D space + time). By combining our approach with tissue clearing techniques, we reveal the entire cardiac structures and hypertrabeculation of adult zebrafish hearts in response to doxorubicin treatment. By integrating the resolution enhancement technique with c-LSFM to increase the resolving power under a large field-of-view, we demonstrate the use of low power objective to resolve the entire architecture of large-scale neonatal mouse hearts, revealing the helical orientation of individual myocardial fibers. Therefore, our c-LSFM imaging approach provides multi-scale visualization of architecture and function to drive cardiovascular research with translational implication in congenital heart diseases.

The cellular mechanisms driving cardiac tissue formation remain poorly understood, largely due to the structural and functional complexity of the heart. It is unclear whether newly generated myocytes originate from cardiac stem/progenitor cells or from pre-existing cardiomyocytes that re-enter the cell cycle. Here, we identify the source of new cardiomyocytes during mouse development and after injury. Our findings suggest that cardiac progenitors maintain proliferative potential and are the main source of cardiomyocytes during development; however, the onset of αMHC expression leads to reduced cycling capacity. Single-cell RNA sequencing reveals a proliferative, “progenitor-like” population abundant in early embryonic stages that decreases to minimal levels postnatally. Furthermore, cardiac injury by ligation of the left anterior descending artery was found to activate cardiomyocyte proliferation in neonatal but not adult mice. Our data suggest that clonal dominance of differentiating progenitors mediates cardiac development, while a distinct subpopulation of cardiomyocytes may have the potential for limited proliferation during late embryonic development and shortly after birth.

Rationale Recent work in animal models and humans has demonstrated the presence of organ-specific progenitor cells required for the regenerative capacity of the adult heart. In response to tissue injury, progenitor cells differentiate into specialized cells, while their numbers are maintained through mechanisms of self-renewal. The molecular cues that dictate the self-renewal of adult progenitor cells in the heart, however, remain unclear. Objective Herein, we investigate the role of canonical Wnt signaling on adult cardiac side population (CSP) cells under physiological and disease conditions. Methods and Results CSP cells isolated from C57BL/6J mice were utilized to study the effects of canonical Wnt signaling on their proliferative capacity. The proliferative capacity of CSP cells was also tested following injection of recombinant Wnt3a protein (r-Wnt3a) in the left ventricular free wall. Wnt signaling was found to decrease the proliferation of adult CSP cells, both in vitro and in vivo, through suppression of cell cycle progression. Wnt stimulation exerted its anti-proliferative effects through a previously unappreciated activation of insulin-like growth factor binding protein 3 (IGFBP3), which requires intact IGF binding site for its action. Moreover, injection of r-Wnt3a following myocardial infarction in mice showed that Wnt signaling limits CSP cell renewal, blocks endogenous cardiac regeneration and impairs cardiac performance, highlighting the importance of progenitor cells in maintaining tissue function after injury. Conclusions Our study identifies canonical Wnt signaling and the novel downstream mediator, IGFBP3, as key regulators of adult cardiac progenitor self-renewal in physiological and pathological states.

Accumulating evidence supports limited regenerative potential of the mammalian heart. However, this endogenous regenerative capacity significantly decreases with age and it is not sufficient to replenish the lost myocardium following injury in the adult. Both resident cardiac stem/progenitor cells and mature cardiomyocytes have been proposed to contribute to cardiac tissue generation. Understanding the molecular and cellular mechanisms governing cardiac cell formation is imperative towards the development of novel therapeutic strategies for cardiac regeneration. Hypothesis: Cardiac growth occurs primarily through progenitor cells and to a lesser extent through cardiomyocyte proliferation. Nkx2.5+ cells are the predominant population driving cardiac growth during development and may represent candidate progenitors for cardiac regeneration. Moreover, a subset of “proliferating” αMHC+ cardiomyocytes may also contribute to cardiac growth during early embryonic development and the first week of life. Materials and methods: We performed clonal analysis using a multicolor reporter system (Rainbow) that allows labeling of single cells with one of three fluorescent proteins and retrospective analysis of their progeny. Rainbow mice were crossed to βactinCreER, αMHCCreER, Nkx2.5CreER and Rosa26CreER mice. Tamoxifen was administered at E9.5 or E12.5 and analysis was performed at P1, P7, P15 and P30. Results: We observed significant clonal expansion in βactinCreER;Rainbow and Nkx2.5CreER;Rainbow hearts while αMHCCreER;Rainbow hearts exhibited clones comprising of smaller cell number. We also found that αMHC positive cells lose their proliferative capacity from E9.5 to E12.5, whereas cells expressing Nkx2.5 continue to clonally expand during that same time period. Finally, we demonstrated that αMHC+ cardiomyocyte proliferation is reactivated following myocardial injury soon after birth. Conclusion: Our data support that clonal dominance of progenitor cells promotes cardiac development, while cardiomyocyte proliferation contributes to a lesser extent early in development and postnatally.

Background: Increased hemodynamic stress induces left ventricular hypertrophy followed by heart failure. Accumulating evidence suggests that the adult mammalian heart possess regeneration ability through the action of resident stem/progenitor cell. However, the effects of hemodynamic fluctuation on cardiac progenitor cell proliferation and cardiac repair remain unknown. Results: Proliferation of Cardiac Side Population (CSP) cells and cardiomyocytes was evaluated in experimental models of Ascending Aortic Constriction (AAC)-induced left ventricular hypertrophy (LVH) in mice, over a period of seven weeks. Continuous AAC caused LVH while increasing the number of BrdU + CSP cells (one week post-AAC) and the amount of BrdU + cardiomyocytes (seven weeks post-AAC). To evaluate the effects of pressure fluctuation on CSP cell and cardiomyocyte proliferation we generated another group (de-AAC) in which the constriction was removed one week post-AAC. Interestingly, restoration of cardiac pressure further increased the proliferative capacity of CSP cells and cardiomyocytes. Conclusion: Our data show that restoration of hemodynamic stress confers favorable effects on cardiac repair through increase proliferation of CSP cells and cardiomyocytes. Thus, the hemodynamic state of the heart might be related to CSP cell proliferation and cardiomyogenesis.

It has been proposed that cardiac development in lower vertebrates is driven by the proliferation of cardiomyocytes. Similarly, cycling myocytes have been suggested to direct cardiac regeneration in neonatal mice after injury. Although, the role of cardiomyocyte proliferation in cardiac tissue generation during development has been well documented, the extent of this contribution as well as the role of other cell types, such as progenitor cells, still remains controversial. Here we used a novel stochastic four-color Cre-dependent reporter system (Rainbow) that allows labeling at a single cell level and retrospective analysis of the progeny. Cardiac progenitors expressing Mesp1 or Nkx2.5 were shown to be a source of cardiomyocytes during embryonic development while the onset of αMHC expression marked the developmental stage where the capacity of cardiac cells to proliferate diminishes significantly. Through direct clonal analysis we provide strong evidence supporting that cardiac progenitors, as opposed to mature cardiomyocytes, are the main source of cardiomyocytes during cardiac development. Moreover, we have identified quadri-, tri-, bi, and uni-potent progenitors that at a single cell level can generate cardiomyocytes, fibroblasts, endothelial and smooth muscle cells. Although existing cardiomyocytes undergo limited proliferation, our data indicates that it is mainly the progenitors that contribute to heart development. Furthermore, we show that the limited proliferation capacity of cardiomyocytes observed during normal development was enhanced following neonatal cardiac injury allowing almost complete regeneration of the scared tissue. However, this ability was largely absent in adult injured hearts. Detailed characterization of dividing cardiomyocytes and proliferating progenitors would greatly benefit the development of novel therapeutic options for cardiovascular diseases.