A Conserved Requirement for Fbxo7 During Male Germ Cell Cytoplasmic Remodeling

Abstract

Fbxo7 is the substrate-recognition subunit of an SCF-type ubiquitin E3 ligase complex. It has physiologically important functions in regulating mitophagy, proteasome activity and the cell cycle in multiple cell types, like neurons, lymphocytes and erythrocytes. Here, we show that in addition to the previously known Parkinsonian and hematopoietic phenotypes, male mice with reduced Fbxo7 expression are sterile. In these males, despite successful meiosis, nuclear elongation and eviction of histones from chromatin, the developing spermatids are phagocytosed by Sertoli cells during late spermiogenesis, as the spermatids undergo cytoplasmic remodeling. Surprisingly, despite the loss of all germ cells, there was no evidence of the symplast formation and cell sloughing that is typically associated with spermatid death in other mouse sterility models, suggesting that novel cell death and/or cell disposal mechanisms may be engaged in Fbxo7 mutant males. Mutation of the Drosophila Fbxo7 ortholog, nutcracker (ntc) also leads to sterility with germ cell death during cytoplasmic remodeling, indicating that the requirement for Fbxo7 at this stage is conserved. The ntc phenotype was attributed to decreased levels of the proteasome regulator, DmPI31 and reduced proteasome activity. Consistent with the fly model, we observe a reduction in PI31 levels in mutant mice; however, there is no alteration in proteasome activity in whole mouse testes. Our results are consistent with findings that Fbxo7 regulates PI31 protein levels, and indicates that a defect at the late stages of spermiogenesis, possibly due to faulty spatial dynamics of proteasomes during cytoplasmic remodeling, may underlie the fertility phenotype in mice.

Fis1 ablation in the male germline disrupts mitochondrial morphology and mitophagy, and arrests spermatid maturation

“…Although key roles for mitochondrial fusion and fission have been demonstrated in spermatogenesis ( Varuzhanyan and Chan, 2020 ), the role of mitophagy – the autophagic degradation of mitochondria ( Pickles et al, 2018 ) – is largely unknown ( Lv et al, 2020 ; Rathje et al, 2019 ). Because mitophagy counterbalances mitochondrial biogenesis and can remove dysfunctional mitochondria, it can control mitochondrial abundance and quality.…”

PI31 expression prevents neuronal degeneration in a mouse Parkinson Disease model

“…Like Fbxo7 mutant mice, PI31 CKO mice have a dramatic decrease in testis and thymus size with similar histopathology (Fig. 1B, S1) 25 . Moreover, inactivation of Fbxo7 in mouse motor neurons produced phenotypes strikingly similar to PI31 CKO ( Fig.…”

Proteasome complexes experience profound structural and functional rearrangements throughout mammalian spermatogenesis

“…We observed that PI31, previously described as a 20S proteasome inhibitor (Li et al ., 2014) and a 26S proteasome activator (Bader et al ., 2011), and its binding partner Fbxo7 (Kirk et al ., 2008) were both enriched in the s20S samples. This agrees with earlier observations in Drosophila and mice spermatogenesis (Bader et al ., 2011; Rathje et al ., 2019). Furthermore, PI31 was shown to mediate proteasome transport in axons and dendrites in mice, by regulating the loading of proteasomes onto microtubule-dependent molecular motors (Liu et al ., 2019).…”