ER71 directs mesodermal fate decisions during embryogenesis.

Er71 mutant embryos are nonviable and lack hematopoietic and endothelial lineages. To further define the functional role for ER71 in cell lineage decisions, we generated genetically modified mouse models. We engineered an Er71-EYFP transgenic mouse model by fusing the 3.9 kb Er71 promoter to the EYFP reporter gene. Using FACS and transcriptional profiling, we examined the EYFP(+) population of cells in Er71 mutant and wild-type littermates. In the absence of ER71, we observed an increase in the number of EYFP-expressing cells, increased expression of the cardiac molecular program and decreased expression of the hemato-endothelial program, as compared with wild-type littermate controls. We also generated a novel Er71-Cre transgenic mouse model using the same 3.9 kb Er71 promoter. Genetic fate-mapping studies revealed that the ER71-expressing cells give rise to the hematopoietic and endothelial lineages in the wild-type background. In the absence of ER71, these cell populations contributed to alternative mesodermal lineages, including the cardiac lineage. To extend these analyses, we used an inducible embryonic stem/embryoid body system and observed that ER71 overexpression repressed cardiogenesis. Together, these studies identify ER71 as a critical regulator of mesodermal fate decisions that acts to specify the hematopoietic and endothelial lineages at the expense of cardiac lineages. This enhances our understanding of the mechanisms that govern mesodermal fate decisions early during embryogenesis.

Efficient homing of multipotent adult mesenchymal stem cells depends on FROUNT-mediated clustering of CCR2.

Circulating stem cells of different origin have been demonstrated to improve repair of various organs both after systemic and local application, although the mechanisms that cause these effects are still not fully understood. We have used a combination of DNA microarray analysis and in vitro migration assays to screen for molecules that mediate homing of long-term renewing adult bone marrow-derived multipotent mesenchymal stem cells (BM-MASCs). We show that the cytokine receptor CCR2 is necessary for organ-specific homing of bone marrow-derived MASCs to the heart in a transgenic mouse model and into hearts damaged by ischemia/reperfusion. Homing and migration of stem cells was dependent on the intracellular adaptor molecule FROUNT, which interacts with CCR2. FROUNT was required for polarization of MASCs, resulting in clustering of CCR2 and reorganization of the cytoskeleton. Recruited MASCs summoned by the CCR2 ligand MCP-1/CCL2 expressed SDF1, which might trap additional bone marrow-derived circulating cells to contribute to the complex process of homing and retention of circulating stem and progenitor cells to remodel diseased organs.

Molecular cardiology in translation: gene, cell and chemical-based experimental therapeutics for the failing heart.

Acquired and inherited diseases of the heart represent a major health care issue in this country and throughout the World. Clinical medicine has made important advancements in the past quarter century to enable several effective treatment regimes for cardiac patients. Nevertheless, it is apparent that even with the best care, current treatment strategies and therapeutics are inadequate for treating heart disease, leaving it arguably the most pressing health issue today. In this context it is important to seek new approaches to redress the functional deficits in failing myocardium. This review focuses on several recent gene, cell and chemical-based experimental therapeutics currently being developed in the laboratory for potential translation to patient care. For example, new advances in bio-sensing inducible gene expression systems offer the potential for designer cardio-protective proteins to be expressed only during hypoxia/ischemia in the heart. Stem cells continue to offer the promise of cardiac repair, and some recent advances are discussed here. In addition, discovery and applications of synthetic polymers are presented as a chemical-based strategy for acute and chronic treatment of diseased and failing cardiac tissue. Collectively, these approaches serve as the front lines in basic biomedical research, with an eye toward translation of these findings to clinically meaningful applications in cardiac disease.

Activation of myogenic differentiation pathways in adult bone marrow-derived stem cells.

During embryogenesis, various cell types can be programmed by potent inducers to follow distinct differentiation paths. In adult life, this ability seems to be restricted to specific multipotent cells. We have identified two cell populations from adult murine bone marrow which express various "stemness" genes. Treatment with Wnt molecules induced transcription of different skeletal muscle marker genes and evoked expression of cardiomyocyte markers. Further characterization of Wnt-induced intracellular signaling cascades revealed that the skeletal muscle program depended on canonical Wnt signaling, while the induction of cardiomyocyte markers seems to require a protein kinase C-dependent pathway. CDO, another component of the machinery directing skeletal muscle induction and expansion, selectively activated skeletal muscle- but not cardiomyocyte-specific genes. Although we were able to turn on various cell-type-specific markers by different induction regimens, we never obtained fully differentiated, functional cells. We conclude that the differentiation of adult stem cells is incomplete and lacks certain cues necessary to acquire a truly functional status.

Mesenchymal stem cells are recruited to striated muscle by NFAT/IL-4-mediated cell fusion.

Mesenchymal stem cells (MSCs) or mesenchymal adult stem cells (MASCs) that are present in the stroma of several organs have been proposed to contribute to the regeneration of different tissues including liver, blood, heart, and skeletal muscle. Yet, it remains unclear whether MSCs can be programmed to differentiate cell-autonomously into fully functional cells or whether they are recruited by surrounding cells via fusion and thereby acquire specialized cellular functions. Here, we demonstrate that Wnt signaling molecules activate the expression of distinct sets of genes characteristic for cardiac and skeletal muscle cells in MASCs. However, such cells lack morphological criteria characteristic for functional muscle cells and do not show contractile activity. In contrast, MASCs fuse efficiently with native myotubes in an IL-4-dependent manner to form functional hybrid myotubes. Injection of genetically labeled MSCs into wild-type mouse blastocysts revealed a contribution to skeletal but not cardiac muscle development. Disruption of IL-4 and NFATc2/c3 reduced or prevented a contribution of adult stem cells to the development of Il-4 and NFATc2/c3 mutant embryos, further emphasizing the apparent inability of adult stem cells to differentiate fully into striated muscle in a cell-autonomous manner.