Genetic lineage tracing identifies in situ Kit-expressing cardiomyocytes

Abstract

Cardiac cells marked by c-Kit or Kit, dubbed cardiac stem cells (CSCs), are in clinical trials to investigate their ability to stimulate cardiac regeneration and repair. These studies were initially motivated by the purported cardiogenic activity of these cells. Recent lineage tracing studies using Kit promoter to drive expression of the inducible Cre recombinase showed that these CSCs had highly limited cardiogenic activity, inadequate to support efficient cardiac repair. Here we reassess the lineage tracing data by investigating the identity of cells immediately after Cre labeling. Our instant lineage tracing approach identifies Kit-expressing cardiomyocytes, which are labeled immediately after tamoxifen induction. In combination with long-term lineage tracing experiments, these data reveal that the large majority of long-term labeled cardiomyocytes are pre-existing Kit-expressing cardiomyocytes rather than cardiomyocytes formed de novo from CSCs. This study presents a new interpretation for the contribution of Kit+ cells to cardiomyocytes and shows that Kit genetic lineage tracing over-estimates the cardiogenic activity of Kit+ CSCs.

Developmental origin and lineage plasticity of endogenous cardiac stem cells

“…Attempting to shed light on these issues, Cai and Zhou and colleagues recently reported the generation of more sensitive knock-in alleles inserting conditional CRE and/or marker cassettes directly into the ATG start codon of the first Kit coding exon (Liu et al, 2016;Sultana et al, 2015). With these tools, in contrast to previous studies, KIT + cells were found to be abundant in fetal hearts and throughout postnatal life into adulthood.…”

“…They maintained their endothelial identity after myocardial infarction and, consistent with the van Berlo et al study discussed above, only very rare cells were identified in healthy or injured hearts that coexpressed KIT and a marker of cardiomyocyte precursors (NKX2-5) or differentiated cardiomyocytes (troponin T). With these new lineage tools, some cardiomyocytes could be labeled immediately after lineage tracing, suggesting that those traced in the adult heart did not derive from myocardial stem cells (Liu et al, 2016). These studies claim to have revealed the identity and behavior of the majority population of KIT + cells within the heart, most of which were not detected previously owing to the insensitivity of antibodies and lineage-tracing strategies.…”

Preexisting endothelial cells mediate cardiac neovascularization after injury

“…One may ask whether cardiac stem cells such as Kit + cells, Sca1 + cells, or epicardial progenitors contribute to new coronary endothelial cells after cardiac injury. As for Kit + cells, genetic-lineage-tracing studies showed that Kit-Cre labels a substantial number of endothelial cells in the normal heart and that those cells expand to generate more coronary endothelial cells after injury (40)(41)(42) (43), and lineage-tracing studies show that Sca1-derived cells are most endothelial cells in the normal heart (44). Our endothelial cell-tracing study suggests that these Sca1 + endothelial cells might expand after injury and that Sca1 + nonendothelial cells minimally contributed to coronary endothelial cells.…”

Pathologic Stimulus Determines Lineage Commitment of Cardiac C-kit + Cells

“…These results suggest that under homeostatic conditions, CD45 − c-kit + cells are unlikely to give rise to cardiomyocytes, without any detectable differences between wild type or Kit +/MCM c-kit + cells, in line with published genetic lineage-tracing studies. 10, 20, 21 …”