Wt1 and epicardial fate mapping.
BACKGROUND
The embryonic epicardium is a crucial cell source of the cardiac fibrous skeleton as well as of the coronary system. Genetic lineage tracing systems based on Wt1 regulatory sequences provided evidence that epicardium-derived cells also adopt a myocardial fate in the mouse.


OBJECTIVE
To define the adequacy of Wt1-based lineage tracing systems for epicardial fate mapping.


RESULTS
Using in situ hybridization analysis and immunofluorescence on tissue sections, we detected endogenous expression of Wt1 mRNA and Wt1 protein in the proepicardium and epicardium and also in endothelial cells throughout cardiogenesis. Expression analysis of a sensitive GFP reporter showed that recombination mediated by cre recombinase in the Wt1(creEGFP) line occurs randomly and sporadically in all cells of the embryo. Recombination in cardiomyocytes was found in the linear heart tube before establishment of a (pro)epicardium. In contrast, the tamoxifen-inducible Wt1(creERT2) mouse line mediated poor and variable recombination in the epicardium. Recombination in cardiomyocytes was not detected in this case.


CONCLUSIONS
Frequently used Wt1 based cre-mediated lineage tracing systems are not suitable for epicardial fate mapping because of endogenous endothelial expression of Wt1, ectopic recombination (Wt1(creEGFP)), and poor recombination efficiency (Wt1(creERT2)) in the developing heart. We conclude that claims of a cardiomyocyte fate of epicardial cells in the mouse are not substantiated.
Institut für Molekularbiologie, Medizinische Hochschule Hannover, Hannover, Germany.
Carsten
C
Rudat
Andreas
A
Kispert
D000818:Animals; D019070:Cell Lineage / Q000235:genetics; D002874:Chromosome Mapping / Q000379:methods*; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000235:genetics*; D055879:Gene Knock-In Techniques; D019426:Integrases / Q000235:genetics; D051379:Mice; D008822:Mice, Transgenic; D032383:Myocytes, Cardiac / Q000166:cytology / Q000378:metabolism; D038081:Organogenesis / Q000235:genetics; D010496:Pericardium / Q000166:cytology* / Q000196:embryology* / Q000378:metabolism; D011247:Pregnancy; D011897:Random Allocation; D025721:WT1 Proteins / Q000096:biosynthesis / Q000235:genetics* / Q000502:physiology
D025721:WT1 Proteins; C045073:Cre recombinase; D019426:Integrases