Grazia Iaffaldano

Bone marrow cells adopt the cardiomyogenic fate in vivo

The possibility that adult bone marrow cells (BMCs) retain a remarkable degree of developmental plasticity and acquire the cardiomyocyte lineage after infarction has been challenged, and the notion of BMC transdifferentiation has been questioned. The center of the controversy is the lack of unequivocal evidence in favor of myocardial regeneration by the injection of BMCs in the infarcted heart. Because of the interest in cell-based therapy for heart failure, several approaches including gene reporter assay, genetic tagging, cell genotyping, PCR-based detection of donor genes, and direct immunofluorescence with quantum dots were used to prove or disprove BMC transdifferentiation. Our results indicate that BMCs engraft, survive, and grow within the spared myocardium after infarction by forming junctional complexes with resident myocytes. BMCs and myocytes express at their interface connexin 43 and N-cadherin, and this interaction may be critical for BMCs to adopt the cardiomyogenic fate. With time, a large number of myocytes and coronary vessels are generated. Myocytes show a diploid DNA content and carry, at most, two sex chromosomes. Old and new myocytes show synchronicity in calcium transients, providing strong evidence in favor of the functional coupling of these two cell populations. Thus, BMCs transdifferentiate and acquire the cardiomyogenic and vascular phenotypes restoring the infarcted heart. Together, our studies reveal that locally delivered BMCs generate de novo myocardium composed of integrated cardiomyocytes and coronary vessels. This process occurs independently of cell fusion and ameliorates structurally and functionally the outcome of the heart after infarction.

Cardiomyogenesis in the Developing Heart Is Regulated by C-Kit–Positive Cardiac Stem Cells

Rationale: Embryonic and fetal myocardial growth is characterized by a dramatic increase in myocyte number, but whether the expansion of the myocyte compartment is dictated by activation and commitment of resident cardiac stem cells (CSCs), division of immature myocytes or both is currently unknown. Objective: In this study, we tested whether prenatal cardiac development is controlled by activation and differentiation of CSCs and whether division of c-kit–positive CSCs in the mouse heart is triggered by spontaneous Ca2+ oscillations. Methods and Results: We report that embryonic-fetal c-kit–positive CSCs are self-renewing, clonogenic and multipotent in vitro and in vivo. The growth and commitment of c-kit–positive CSCs is responsible for the generation of the myocyte progeny of the developing heart. The close correspondence between values computed by mathematical modeling and direct measurements of myocyte number at E9, E14, E19 and 1 day after birth strongly suggests that the organogenesis of the embryonic heart is dependent on a hierarchical model of cell differentiation regulated by resident CSCs. The growth promoting effects of c-kit–positive CSCs are triggered by spontaneous oscillations in intracellular Ca2+, mediated by IP3 receptor activation, which condition asymmetrical stem cell division and myocyte lineage specification. Conclusions: Myocyte formation derived from CSC differentiation is the major determinant of cardiac growth during development. Division of c-kit–positive CSCs in the mouse is promoted by spontaneous Ca2+ spikes, which dictate the pattern of stem cell replication and the generation of a myocyte progeny at all phases of prenatal life and up to one day after birth.

Endothelial Fate and Angiogenic Properties of Human CD34+ Progenitor Cells in Zebrafish

Objective—The vascular competence of human-derived hematopoietic progenitors for postnatal vascularization is still poorly characterized. It is unclear whether, in the absence of ischemia, hematopoietic progenitors participate in neovascularization and whether they play a role in new blood vessel formation by incorporating into developing vessels or by a paracrine action. Methods and Results—In the present study, human cord blood–derived CD34+ (hCD34+) cells were transplanted into pre- and postgastrulation zebrafish embryos and in an adult vascular regeneration model induced by caudal fin amputation. When injected before gastrulation, hCD34+ cells cosegregated with the presumptive zebrafish hemangioblasts, characterized by Scl and Gata2 expression, in the anterior and posterior lateral mesoderm and were involved in early development of the embryonic vasculature. These morphogenetic events occurred without apparent lineage reprogramming, as shown by CD45 expression. When transplanted postgastrulation, hCD34+ cells were recruited into developing vessels, where they exhibited a potent paracrine proangiogenic action. Finally, hCD34+ cells rescued vascular defects induced by Vegf-c in vivo targeting and enhanced vascular repair in the zebrafish fin amputation model. Conclusion—These results indicate an unexpected developmental ability of human-derived hematopoietic progenitors and support the hypothesis of an evolutionary conservation of molecular pathways involved in endothelial progenitor differentiation in vivo.