Transplantation of cytoplasm subjected to oxidative stress as a model for study of mitochondrial disease inheritance

Pathologies caused by mutations in mitochondrial DNA (mtDNA) represent an important group of genetic diseases in humans. Nonetheless, due to our limited understanding of the molecular mechanisms of mitochondrial inheritance there are no efficient methods to predict or intervene in the inheritance of these diseases. Recent studies indicate that mutations in mtDNA are selectively eliminated in the germline. This project investigated the ability of the embryo to target and eliminate dysfunctional mitochondria during early development. To test that, mouse zygotes were treated with chloromethyl-X-rosamina (MitoTracker Red CMXRos) and photosensitized for 0, 2.5, 5, 10, 20 and 60 s. There was a decrease in the rate of blastocyst development and a developmental arrest when the photosensitization was performed for a period equal to or greater than 20 s. Photosensitization also resulted in mitochondrial dysfunction, as indicated by a decreased of mitochondrial membrane potential. However, cytoplasmic transfer from NZB/BINJ (NZB) zygotes photosensitized for 20 s resulted in no effect on development of C57BL/6 (B6) embryos. The amount of NZB mtDNA introduced also did not differ between B6 zygotes, regardless of whether they received or not photosensitized cytoplasm (30.6% ± 1.73 vs. 30.8 ± 1.73%). On the other hand, the amount of NZB mtDNA was lower (P = 0.008) in the blastocysts receiving photosensitized cytoplasm (31.4% ± 24.7% ± 1.43 vs. 1.43). Since the total amount of mtDNA was not different between the groups, these results suggest that dysfunctional mitochondria introduced by cytoplasmic transfer were destroyed. Analysis of autophagosomes indicated, however, that the NZB mitochondria were not eliminated by mitophagy. Different than expected, culture in the presence of rapamycin reversed the effect caused by introduction of photosensitized cytoplasm, resulting in similar levels of NZB mtDNA compared to blastocysts receiving cytoplasm not photosensitized. It was concluded that the mouse embryo may destroy dysfunctional mitochondria during development into blastocysts. Further studies should provide additional evidence and elucidate the molecular mechanisms underlying these findings. (AU)