Mitochondrial Ubiquitination In Spermatid Elongation

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Cellular remodelling is critical for the production of functional sperm. While there is extensive descriptive information available on how germ cells and Sertoli cells cooperate to dispose of excess germ cell cytoplasm and sculpt the mature sperm cell shape, little is thus far understood about the underpinning biochemistry and the pathways involved. We have identified a novel mouse that specifically perturbs the final stages of sperm differentiation. Male mice homozygous for a hypomorphic mutation in Fbxo7 are sterile, and developing spermatids are phagocytosed at the point they would normally start to remodel their cytoplasm. Fbxo7 is the substrate recognition component of an SCF-type E3 ligase complex, implicating ubiquitination as a key factor in late stages of sperm development. Fbxo7 interacts with PI31, which we have recently described as an adaptor for the SCF-Fbxo7 ubiquitin ligase, and which has also been described as a proteasome transporter in other cell types. Evidence from related fruit fly and mouse models, together with Fbxo7 and PI31's known roles in mitophagy and proteasome regulation, highlight these processes as being of particular interest for investigation. This project will build on our previous work by combining cutting edge super-resolution and electron microscopic investigation of sperm ultrastructure with a proteomics-based investigation of the disrupted ubiquitination pathways and proteasome interactiomes in Fbxo7 and PI31 mutant testes. These studies will bridge the gap from genotype to phenotype and trace how defective Fbxo7 ubiquitination affects mitochondrial remodelling, proteasome localisation and autophagic / phagocytic activity during sperm development. Concurrent with this, we will explore dosage effects and the contribution of testes cell lineages by comparing a targeted deletions of Fbxo7 and PI31 to the existing Fbxo7 hypomorphic mutant.