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

Abstract

Mutations in the mitocondrial DNA (mtDNA) are a major cause of disease in humans. Nonetheless, due to the unique pattern of mtDNA segregation, it is not yet possible to predict or intervene in the inheritance of these pathologies. This is mainly due to our limited understanding of the molecular basis of mitochondrial inheritance in mammals. Recent studies suggest that mutant mtDNAs causing severe mitochondrial dysfunctions are selected against from colonizing female germline. The mechanisms involved in this process are still unclear, but there are evidences supporting that mutant mtDNAs are selected and excluded based on their effect on organelle function. In somatic cells mitochondrial fusion and fission can segregate mutant and wild type mtDNAs into distinct organelles. In this case, when the mutation leads to mitochondrial dysfunction, the cell is able to identify and destroy it by autophagy of the organelle harboring mutant molecules. Similarly, mitochondria introduced by sperm into the oocyte are destroyed by autophagy soon after fertilization. Based on these reports, our hypothesis is that autophagy could also target and destroy mutant mtDNAs inherited from the oocyte. Hence, this project aims to investigate whether the mouse embryo is able to target and eliminate dysfunctional mitochondria during early development. To test that, zygotes with mtDNA from NZB/BINJ mouse line will be photosensitized with methilene blue, which generates singlet oxygen, and part of their cytoplasm will be transplanted to zygotes from the C57BL/6 line. The embryos generated by cytoplasm transplantation will be cultured in vitro for analysis of developmental rates, mtDNA copy numer and mtDNA heteroplasmy. This study should contribute significantly to the knowledge of the molecular basis of mitochondrial inheritance which has implications to the prevention and intervention upon inheritance of severe human pathologies caused by mutations in mtDNA.