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Selective and continuous elimination of mitochondria microinjected into mouse eggs from spermatids, but not from
1Department of Laboratory Animal Science, Tokyo Metropolitan Institute of Medical Science, Tokyo 113-8613, Japan. jih45@sakura.cc.tsukuba.ac.jp
Abstract:
Exclusion of paternal mitochondria in fertilized mammalian eggs is very stringent and ensures strictly maternal mtDNA inheritance. In this study, to examine whether elimination was specific to sperm mitochondria, we microinjected spermatid or liver mitochondria into mouse embryos. Congenic B6-mt(spr) strain mice, which are different from C57BL/6J (B6) strain mice (Mus musculus domesticus) only in possessing M. spretus mtDNA, were used as mitochondrial donors. B6-mt(spr) mice and a quantitative PCR method enabled selective estimation of the amount of M. spretus mtDNA introduced even in the presence of host M. m. domesticus mtDNA and monitoring subsequent changes of its amount during embryogenesis. Results showed that M. spretus mtDNA in spermatid mitochondria was not eliminated by the blastocyst stage, probably due to the introduction of a larger amount of spermatid mtDNA than of sperm mtDNA into embryos on fertilization. However, spermatid-derived M. spretus mtDNA was eliminated by the time of birth, whereas liver-derived M. spretus mtDNA was still present in most newborn mice, even though its amount introduced was significantly less than that of spermatid mtDNA. These observations suggest that mitochondria from spermatids but not from liver have specific factors that ensure their selective elimination and resultant elimination of mtDNA in them, and that the occurrence of elimination is not limited to early stage embryos, but continues throughout embryogenesis.
Insights
Paternal mitochondria are typically excluded in mammalian eggs, ensuring maternal inheritance. This study found that while sperm mitochondria are eliminated early, spermatid mitochondria persist longer, suggesting specific elimination factors beyond early embryogenesis.
Area of Science:
- Developmental Biology
- Mitochondrial Genetics
Background:
- Mammalian fertilization strictly enforces maternal inheritance of mitochondrial DNA (mtDNA).
- The precise mechanisms and timing of paternal mitochondrial exclusion remain incompletely understood.
Purpose of the Study:
- To investigate whether the elimination of paternal mitochondria is specific to sperm mitochondria.
- To determine if mitochondria from other cell types, like spermatids or liver cells, are also eliminated during embryogenesis.
Main Methods:
- Microinjection of spermatid or liver mitochondria from congenic B6-mt(spr) mice into C57BL/6J (B6) embryos.
- Quantitative PCR to selectively quantify M. spretus mtDNA in the presence of M. m. domesticus mtDNA.
- Monitoring mtDNA levels throughout embryogenesis and post-birth.
Main Results:
- M. spretus mtDNA from spermatid mitochondria was not eliminated by the blastocyst stage.
- Spermatid-derived mtDNA was eliminated by birth, while liver-derived mtDNA persisted in some newborns.
- Elimination of mtDNA was observed to continue throughout embryogenesis, not just in early stages.
Conclusions:
- Mitochondria from spermatids, unlike liver mitochondria, possess specific factors promoting their selective elimination.
- The process of paternal mtDNA elimination is not restricted to early embryonic stages but extends throughout development.