If the purifying filter takes effect only before the cease of PGC proliferation by 13

If the purifying filter takes effect only before the cease of PGC proliferation by 13.5 dpc, females carrying severe mtDNA mutations would produce progeny with a similar frequency of mutant variants in any given litter. oocyte maturation and not during embryonic oogenesis, despite a detected a reduction in mtDNA content in early PGCs. To clarify these contradictory results, we examined the mtDNA copy number in PGCs isolated from transgenic mice expressing fluorescent proteins specifically in PGCs as in the aforementioned two other studies. We provide clear evidence Thiamine pyrophosphate to confirm that no remarkable reduction in mtDNA content occurs in PGCs and reinforce that the bottleneck is generated without reduction of mtDNA content in germ cells. == Author Summary == Rabbit Polyclonal to AhR (phospho-Ser36) Mutations of mtDNA are responsible for many types of mitochondrial diseases in humans, including myopathy and neurological disorders. Females carrying a mixture of mutant and wild-type mtDNA variants transmit a variable amount of mutant mtDNA to each offspring. The proportion of mutated mtDNA inherited from the mother determines the onset and severity of diseases. Studies have suggested that the mtDNA genome is transmitted through a bottleneck, but the underlying mechanism remains controversial. By detecting mtDNA copy number in single cells, we previously showed that the bottleneck occurs without reduction of mtDNA content in germline cells. However, recently a study reported a marked decline of mtDNA copies in embryonic germ cells and attributed this reduction to the creation of the bottleneck. Yet another study concluded that the bottleneck occurs during Thiamine pyrophosphate postnatal oocyte maturation and not during embryonic oogenesis. To resolve these controversies, we examined mtDNA copies in embryonic germ cells identified using the same methodology as in the other two studies. We show solid evidence to confirm our previous findings. This confirmation is important because the understanding of mtDNA content in female germ cells will facilitate the development of therapeutic strategies preventing the transmission of mitochondrial diseases from mother to offspring. == Introduction == Mammalian mitochondrial genome shows a 5 to 10 times greater mutation rate than the nuclear genome[1],[2]. This elevated mutation rate coupled with clonal maternal transmission leads to the high mtDNA polymorphism in populations. However, despite the prevalence of genetic variance within a species, most individuals possess only a single mtDNA variant. Pedigree analyses of heteroplasmic individuals in cattle, mice and humans revealed that mtDNA genotypes shift rapidly among offspring and return to homoplasmy in some progeny within a few generations[3][8], suggesting that a mtDNA bottleneck accounts for the rapid segregation. Early studies have proposed that the bottleneck occurs in embryonic development in consequence of Thiamine pyrophosphate a drastic reduction of mtDNA content in PGCs[9],[10]. In mice, the size of the bottleneck is estimated as to be 200 mtDNA segregation units[11]. To test these hypotheses, three independent research groups have attempted to quantify mtDNA copy number in single germ cells at different developmental stages in mice. Cao et al.[12]made the first direct measurements of mtDNA copy number in single PGCs (identified by endogenous alkaline phosphatase (ALP) activity) in wild-type mice using quantitative real-time PCR (qrt-PCR) and found that PGCs contained consistent amounts of mtDNA with a mean of 13503600 copies per cell between 7.5 days post coitum (dpc) and 13.5 dpc, indicating that the bottleneck occurs without a marked reduction of mtDNA copies in PGCs. Recently usingStella-GFP transgenic mice to isolate PGCs, a study determined a mean of 450 mtDNA copies per PGC at 7.5 dpc (median 200) and a mean of 11002200 copies between 8.5 dpc and 14.5 dpc. The drastic reduction in PGC mtDNA content at 7.5 dpc was suggested to be the cause of the bottleneck[13]. Taking advantage of usingOct4PE-EGFP mice heteroplasmic for two mtDNA sequence variants, Wai et al.[14]measured both mtDNA copy number and heteroplasmy in single germ cells. They detected that PGCs possessed a mean of 280 mtDNA copies (median 145) per cell at 8.5 dpc, the earliest stage at which PGCs could be isolated in their mouse strain, and a mean of 20006000 copies per germ.