TERT over-expression affects the growth of myocardial tissue derived from mouse embryonic stem cells

For heart failure, new therapeutic strategies based on augmentation of the regenerative resources of the heart muscle are under evaluation. More knowledge about the mechanisms regulating growth of the embryonic or adult heart muscle will help to improve the results. The present over-expression study provides further insight into the role of telomerase reverse transcriptase (TERT) in growth regulation of myocardial tissue derived from embryonic stem (ES) cells. Mouse ES cells (D3) exhibiting ectopic expression of TERT under the regulation of the beta-actin promoter were generated and allowed to differentiate over a period of up to 18 days. In contrast to the controls, the TRAP assay did not reveal any decrease of telomerase activity during differentiation of TERT transgenic ES cells. Following cell dissociation and staining for sarcomeric alpha-actinin, singular myocardial precursors could be identified and analyzed using fluorescence microscopy: compared with the controls, the outgrowths of TERT transgenic ES cells showed a significant enlargement of the cellular fraction formed by cardiomyocyte precursors, while BrdU-(double) staining did not reveal a change of its proliferation rate. In addition, the average physical dimensions of the precursors appeared to be enlarged. The myocardial precursors exhibited three different morphologies: spindle-like or round or tri-/multi-angular. While, compared with the controls, in TERT transgenic ES cell outgrowths the overall number of myocardial cells was enhanced, the formation of spindle-like or round-shaped precursors was suppressed. On the molecular level, RT-PCR analysis showed the mRNA-expression level of alpha-MHC, a gene whose expression is specific for pacemaker-like or atrial-like precursors (Kolossov et al., 2005), to be reduced. Furthermore, TERT transgenic outgrowths displayed a reduced beating frequency. It can be concluded that TERT over-expression promotes the differentiation of mouse ES cell-derived cardiomyocytes in a phenotype-specific manner.