Propagation of paternal sperm-contributed mitochondrial genes, resulting in heteroplasmy, is seldom observed in mammals due to post-fertilization degradation of sperm mitochondria, referred to as sperm mitophagy. Whole organelle sperm mitochondrion degradation is thought to be mediated by the interplay between the ubiquitin-proteasome system (UPS) and the autophagic pathway (Song et al., Proc. Natl. Acad. Sci. USA, 2016). Both porcine and primate post-fertilization sperm mitophagy rely on the ubiquitin-binding autophagy receptor, sequestosome 1 (SQSTM1), and the proteasome-interacting ubiquitinated protein dislocase, valosin-containing protein (VCP). Consequently, we anticipated that sperm mitophagy could be reconstituted in a cell-free system consisting of permeabilized mammalian spermatozoa co-incubated with porcine oocyte extracts. We found that SQSTM1 was detected in the midpiece/mitochondrial sheath of the sperm tail after, but not before, co-incubation with oocyte extracts. VCP was prominent in the sperm mitochondrial sheath both before and after the extract co-incubation and was also detected in the acrosome and postacrosomal sheath and the subacrosomal layer of the spermatozoa co-incubated with extraction buffer as control. Such patterns are consistent with our previous observation of SQSTM1 and VCP associating with sperm mitochondria inside the porcine zygote. In addition, it was observed that sperm head expansion mimicked the early stages of paternal pronucleus development in a zygote during prolonged sperm-oocyte extract co-incubation. Treatment with anti-SQSTM1 antibody during extract co-incubation prevented ooplasmic SQSTM1 binding to sperm mitochondria. Even in an interspecific cellular environment encompassing bull spermatozoa and porcine oocyte extract, ooplasmic SQSTM1 was recruited to heterospecific sperm mitochondria. Complementary with the binding of SQSTM1 and VCP to sperm mitochondria, two sperm-borne pro-mitophagy proteins, parkin co-regulated gene product (PACRG) and spermatogenesis associated 18 (SPATA18), underwent localization changes after extract coincubation, which were consistent with their degradation observed inside fertilized porcine oocytes. These results demonstrate that the early developmental events of post-fertilization sperm mitophagy observed in porcine zygote can be reconstituted in a cell-free system, which could become a useful tool for identifying additional molecules that regulate mitochondrial inheritance in mammals.
Keywords: PACRG; SPATA18; SQSTM1; VCP; autophagy; cell-free system; mitochondria; mtDNA; ubiquitin-proteasome system.