Sperm Flagellar 2 (SPEF2) is Essential for Sperm Flagellar Assembly in Humans

Spermiogenesis is a complex and tightly regulated process, consisting of acrosomal biogenesis, condensation of chromatin, flagellar assembly, and disposal of extra cytoplasm. Previous studies have reported that sperm flagellar 2 (SPEF2) deficiency causes severe asthenoteratozoospermia owing to spermiogenesis failure, but the underlying molecular mechanism in humans remains unclear. Here, we performed proteomic analysis on spermatozoa from three SPEF2 mutant patients to study the functional role of SPEF2 during sperm tail development. A total of 1262 differentially expressed proteins were detected, including 486 upregulated and 776 downregulated. The constructed heat map of the differentially expressed proteins showed similar trends. Among these, the expression of proteins related to flagellar assembly, including SPEF2, sperm associated antigen 6 (SPAG6), dynein light chain tctex-type 1 (DYNLT1), radial spoke head component 1 (RSPH1), translocase of outer mitochondrial membrane 20 (TOM20), EF-hand domain containing 1 (EFHC1), meiosis-specific nuclear structural 1 (MNS1) and intraflagellar transport 20 (IFT20), was verified by western blot. Functional clustering analysis indicated that these differentially expressed proteins were specifically enriched for terms such as spermatid development and flagellar assembly. Furthermore, we showed that SPEF2 interacts with radial spoke head component 9 (RSPH9) and IFT20 in vitro, which are well-studied components of radial spokes or intra-flagellar transport and are essential for flagellar assembly. These results provide a rich resource for further investigation into the molecular mechanism underlying the role that SPEF2 plays in sperm tail development and could provide a theoretical basis for gene therapy in SPEF2 mutant patients in the future.