666V Poster - 10. Cell biology: Cytoskeleton, organelles and trafficking
Wednesday April 06, 4:00 PM - 7:00 PM

Authors:
Zhe Chen ¹; Christian A Combs ¹; Yong Chen ¹; Annie Lee ²; Hong Xu ¹

Affiliations:
1) National Institutes of Health; 2) Walter Reed National Military Medical Center

Keywords:
h. mitochondria; a. spermatogenesis

Mitochondrial genome is exclusively transmitted through maternal linage in animals. The uniparental inheritance was once regarded as a passive outcome of distinct cytoplasmic contents of eggs and sperms. Recent studies demonstrated active mechanisms to remove mitochondrial DNA (mtDNA) during spermatogenesis in various species. However, the physiological significance of mtDNA clearance, or mitochondria uniparental inheritance in general remains a mystery and the factors involved in this process are largely unknown. Using proximity-labeling based proteomic screen, we recovered a putative mitochondrial nucleoid protein MDIS (mitochondrial DNA in sperm), encoded by CG12162 locus. MDIS is highly enriched in Drosophila testis, and specifically in late spermatogenesis stages, when the mtDNA clearance takes place. MDIS null flies are male semi-sterile, otherwise completely healthy. Spermatogenesis progresses normally in MDIS flies but produces immotile mature sperms that often contain multiple copies of mtDNA nucleoids, indicating that MDIS is required for mtDNA clearance. In vitro assay suggests that MDIS is an exonuclease that degrades both single-stranded DNA and double-stranded DNA with overhangs. Importantly, ectopic expression of a mitochondrially targeted E.coli exonuclease III in MDIS flies, effectively removes mtDNA remnants and largely restores male fertility, demonstrating that failure to eliminate mtDNA causes immotile sperms and male infertility. Surprisingly, we observed pronounced nuclear DNA fragmentations in mature sperms of MDIS flies, suggesting a potential detrimental consequence of persisting mtDNA in mature sperms. Our results demonstrate that MDIS play essential roles in preventing the transmission of paternal mtDNA to the offspring, and in safeguarding the nuclear genome integrity. MDIS, to our knowledge, represents the first-ever identified factor that specially enforces maternal inheritance of mitochondrial genomes. This will allow future studies to understand physiological significances and underlying mechanisms of this highly conserved, yet mysterious phenomenon.