Won-Hee Songa, Young-Joo Yia,1, Miriam Sutovskya, Stuart Meyersb, and Peter Sutovskya,c,2
aDivision of Animal Sciences, University of Missouri, Columbia, MO 65211-5300;
bDepartment of Anatomy, Physiology, and Cell Biology, School of Veterinary Medicine, University of California, Davis, CA 95616;
cDepartment of Obstetrics, Gynecology and Women’s Health, University of Missouri, Columbia, MO 65211-5300
Maternal inheritance of mitochondria and mtDNA is a universal principle in human and animal development, guided by selective ubiquitin-dependent degradation of the sperm-borne mitochondria after fertilization. However, it is not clear how the 26S proteasome, the ubiquitin-dependent protease that is only capable of degrading one protein molecule at a time, can dispose of a whole sperm mitochondrial sheath. We hypothesized that the canonical ubiquitin-like autophagy receptors [sequestosome 1 (SQSTM1), microtubule-associated protein 1 light chain 3 (LC3), gamma-aminobutyric acid receptor-associated protein (GABARAP)] and the nontraditional mitophagy pathways involving ubiquitin-proteasome system and the ubiquitin-binding protein dislocase, valosin-containing protein (VCP), may act in concert during mammalian sperm mitophagy. We found that the SQSTM1, but not GABARAP or LC3, associated with sperm mitochondria after fertilization in pig and rhesus monkey zygotes. Three sperm mitochondrial proteins copurified with the recombinant, ubiquitin-associated domain of SQSTM1. The accumulation of GABARAP-containing protein aggregates was observed in the vicinity of sperm mitochondrial sheaths in the zygotes and increased in the embryos treated with proteasomal inhibitor MG132, in which intact sperm mitochondrial sheaths were observed. Pharmacological inhibition of VCP significantly delayed the process of sperm mitophagy and completely prevented it when combined with microinjection of autophagy-targeting antibodies specific to SQSTM1 and/or GABARAP. Sperm mitophagy in higher mammals thus relies on a combined action of SQSTM1-dependent autophagy and VCP-mediated dislocation and presentation of ubiquitinated sperm mitochondrial proteins to the 26S proteasome, explaining how the whole sperm mitochondria are degraded inside the fertilized mammalian oocytes by a protein recycling system involved in degradation of single protein molecules.
mitochondria, mtDNA, ubiquitin, autophagy, mitophagy
1Present address: College of Environmental & Bioresource Sciences, Division of Biotechnology, Chonbuk National University, Iksan 54596, Jeonbuk, Korea.
2To whom correspondence should be addressed.
Author contributions: W.-H.S., S.M., and P.S. designed research; W.-H.S., Y.-J.Y., M.S., S.M., and P.S. performed research; W.-H.S., Y.-J.Y., S.M., and P.S. analyzed data; and W.-H.S., S.M., and P.S. wrote the paper.