Spastin is a microtubule-severing enzyme responsible for the regulation of microtubule functions and mutations in Spast are the leading cause of hereditary spastic paraplegia. Similar microtubule structures and dynamics as those disrupted in HSP underpin many of the processes occurring during spermatogenesis. Recently, it was shown that homozygous Spast loss-of-function males are sterile, but how spastin causes sterility is still unknown1. Herein we sought to elucidate the precise roles of spastin in spermatogenesis.
Using a Spast KO mouse model, we reveal that spastin is essential for multiple aspects of germ cell development, and its loss resulted in an almost complete absence of elongated spermatids in the testes and epididymides. The most pronounced defect was in the assembly and function of the meiotic spindle where we observed the misalignment of chromosomes and stalled anaphase. In the most severe instances, this led to apoptosis. Further, the nuclei of cells that entered into spermiogenesis were larger than normal suggesting a failure of chromosomal segregation, and/or cytokinesis, consistent with the role of spastin in the microtubule-mediated poleward movement of chromosomes and cytokinesis2.
During spermiogenesis, spastin is localised to the manchette – a microtubule-based structure with roles in sperm nuclear shaping and protein transport. Consistent with this localization we observed extreme abnormalities in manchette structure, including increased microtubule density and detachment, leading to abnormal sperm head morphology. Additionally, acrosome defects were visualised via PAS staining and electron microscopy, including ectopic formation and fragmentation. The acrosome defects and the presence of additional vesicular abnormalities indicate disruption of lysosomal function and is consistent with previous observations in human cell culture3.
Our results demonstrate an essential role for spastin in the regulation of microtubule dynamics and lysosomal function during spermatogenesis. This data will inform the counselling and assisted reproductive technology treatment of patients with spasticity disorders.