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.

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.

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.

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.