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Stem Cell Niches in Adult Heart (A Murine Model)

Urbanek et al, Proc Natl Acad Sci USA 2006;103:9226
Summary by Bobby Reddy and Babak Baseri

LAY SUMMARY

Although cardiac stem cells (CSCs) have previously been identified in the adult cardiac cells, the microenvironment which supports their growth and survival has yet to be determined. Urbanek et al. suggest that the clusters of undifferentiated CSCs and lineage-committed cells (LCCs) in the adult mice’s myocardium are connected structurally to two types of supporting cells (myocytes and fibroblasts) by junctional and adhesion proteins. The CSCs are distinguished from myocytes, endothelial cells (ECs), and smooth muscle cells by expressing specific stem cell antigens and by not expressing transcription factors and membrane proteins of the other cells.

The lack of protein expression observed between CSCs-LCCs clusters and endothelial cells (ECs) or smooth muscle cells indicates that myocytes and fibroblasts are the only supporting cells in the cardiac niche. Analogous to this observation is the presence of stromal cells and osteoblasts as supporting cells for the stem cells in the bone marrow (Figure 1).

The researchers also report the mediation of cardiac microenvironment homeostasis by symmetric and asymmetric division of cardiac stem cells. Symmetric and asymmetric divisions were defined by uniform and nonuniform intracellular segregation of two proteins at the time of cell division, respectively. The asymmetric division predominates in the CSCs and results in an “invariant” mechanism of renewal and growth, meaning that the number of CSCs-LCCs within the microenvironment remains constant over time (Figure 2).

In addition, cardiac stem cell homeostasis involves plasma membrane adhesion receptors that interact with specific extracellular proteins to bring binding and co-localization. Since these receptor-ligand interactions are restricted to CSCs, this suggests a critical role for the adhesion receptors in the preservation of the undifferentiated state of cardiac stem cells (Figure 3).

Further studies need to address the specific mechanisms of action through which structural constituents in the microenvironment of the myocardium support and sustain the cardiac stem cells.

SCIENTIFIC SUMMARY

Cardiac stem cells (CSCs) and early lineage-committed cells (LCCs) exist in niches in the adult heart; however, a comprehensive understanding of the microenvironment in which these cells survive has yet to be attained.  Urbanek et al. (2006) investigated structures in the interstitium of the adult mouse heart that facilitate the survival and growth of undifferentiated CSCs.  In this experiment, the hearts of 3 month old FVB/N mice were studied. The CSCs are distinguished from other cardiac cells because they possess stem cell antigens c-kit, MDR1, and Sca-1, and do not express the transcription factors and membrane and cytoplasmic proteins of other cardiac cells such as myocytes, endothelial cells (ECs) and smooth muscle cells.

Since CSCs cannot exist without supporting cells, which assist them through harboring them to the niche and modulating growth signals, the researchers performed a functional assay to investigate the role of connexins 43 and 45 in the generation of gap junctions between c-kit+ CSCs-LCCs and other cardiac cells.  After loading and incubating CSCs-LCCs with the green fluorescent dye calcein, it was found that calcein translocated from the CSCs-LCCs to myocytes and fibroblasts but not ECs; this finding suggests that myocytes and fibroblasts function as supporting cells (Figure 1).

The researchers also found that fibronectin adjoining α4-integrin and α2-chain of laminin were expressed only in CSCs, unlike β1-integrin which was found on both CSCs and LCCs.  This co-localization of the fibronectin with α4-integrin and the α2-chain of laminin suggest that the ligand-receptor relationship is essential in maintaining the undifferentiated state of CSCs (Figure 3).

In addition, the researchers performed a long-term label-retaining assay with BrdU on CSCs and found that over a period of ten weeks, the aggregate number of CSCs remained the same within the niches; this confirms the preservation of CSCs in the adult heart. Also, since the number of CSCs remains constant, it can be inferred that asymmetric division of CSCs is more prevalent than symmetric in the myocardium.  This finding was further supported by the researchers’ observations of individual CSCs undergoing mitosis in the heart (Figure 2).

Based on this study, the researchers are able to conclude that cardiac niches function to facilitate the long-term residence, survival, and growth of CSCs.  However, further research is needed in order to discover the precise mechanisms through which cardiac niches sustain CSCs.