Phenotyping of post-fertilization sperm mitophagy determinants discovered in a mammalian gamete-based cell-free system.
The targeted, substrate-specific degradation of paternal mitochondria inside the zygote, known as post-fertilization sperm mitophagy, is a crucial and evolutionarily conserved early embryonic event. It ensures the exclusive maternal inheritance of the mitochondrial genome. Post-fertilization sperm mitophagy was initially thought to only be achieved via the ubiquitin-proteasome system. Until pro-autophagic receptor proteins such as SQSTM1, GABARAP, as well as the proteasome-interacting ubiquitinated protein dislocase VCP, were identified as contributors to the degradation of the sperm mitochondria early after mammalian fertilization. This synergy of proteasomal and autophagic pathways ensures a timely degradation of sperm mitochondria shortly after fertilization. The discovery of these autophagic receptors lead researchers to believe there might be other autophagic receptors and determinants necessary for proper post-fertilization sperm mitophagy. Based on the established inventory of proteins from mass spectrometry trials of boar spermatozoa exposed to porcine oocyte extracts in an intra-specific porcine cell-free system (CFS), five candidate mitophagy determinants were further investigated in this study, namely LACTB, PRDX3, PSMA8, TOMM34, and FUNDC1. These proteins of interest were studied and validated by using in vitro fertilization (IVF) protocols, cell imaging of spermatids, spermatozoa, oocytes and zygotes, protein interactome analysis, and the porcine CFS. The proteins PSMA8 and TOMM34 behaved in accordance with our proteomic study predictions. The PSMA8 labeling increased after exposure to CFS; in agreement with the classification PSMA8 was given from the mass spectrometry findings. TOMM34 underwent a visible decrease in labeling after exposure to CFS, which also agreed with its proteomic classification; this labeling persisted in IVF zygotes. Except for LACTB, the examined proteins showed mutual interactions as well as interactions with previously identified sperm mitophagy factors in the STRING interactome analysis. Results from this study validate the novel porcine CFS as a valuable tool for the exploration of early fertilization events at a molecular level. Future phenotyping and functional studies using porcine CFS will advance the understanding of mitochondrial inheritance and zygotic development and potentially shed light on the origins of certain mitochondrial diseases arising from the failure of post-fertilization sperm mitophagy.