Rapid decrease in cardiac telomerase activity and CM telomere length after birth. (A) Telomerase activity in the postnatal heart measured by fluorescent TRAP. iPS cells and embryonic heart (embryonic day 11.5 [E11.5]) are positive controls; lysis buffer and inactivated iPS cells (Inac. iPS) are negative controls. In total, three litters were analyzed. The gel shows representative results from one litter. (B) Relative telomerase activity quantification. n indicates the number of animals analyzed. (C) Relative Tert gene expression levels in P1 and P8 hearts measured by quantitative PCR. n indicates the number of animals analyzed. (D) Detail of a confocal maximum projection image of telomere Q-FISH and TnT immunofluorescence. CM nuclei were manually selected using the TnT immunofluorescence image. Only unambiguously identified CMs were considered for the analysis. Arrowheads indicate CMs. Dashed lines indicate telomere signal in those CMs. Bars, 25 µm. (E) Mean CM telomere length during postnatal maturation. (F) CM telomere length distribution and percentage of relatively short (<4,000 auf) and long telomeres (>4,000 auf). Gray lines at 4,000 auf facilitate comparisons of telomere size distribution between conditions. Red lines indicate mean telomere length. n indicates the total number of CMs analyzed per group. The number of animals is shown in brackets. Data are mean ± SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001; NS, nonsignificant.