LRRC23 loss-of-function impairs radial spoke 3 head assembly and causes defective sperm motility underlying male infertility

Radial spokes (RSs) are T-shaped multiprotein complexes in the 9+2 axoneme of motile cilia and flagella that couple the central pair to the peripheral doublet microtubules. RS1, RS2, and RS3 are repeated along the outer microtubule of the axoneme and modulate the activity of dynein, thus ciliary and flagella movement. RS substructures are distinct in spermatozoa from other cells harboring motile cilia in mammals. However, the molecular components of the cell-type specific RS substructures remain largely unknown. Here, we report a leucine-rich repeat-containing protein, LRRC23, is a RS head component indispensable for the RS3 head assembly and flagellar movement in human and mouse sperm. From a Pakistani consanguineous family with infertile males due to reduced sperm motility, we identified a splice site variant of LRRC23 that leads to truncate LRRC23 at the C-terminus. In mutant mouse model mimicking the identified variant, the truncated LRRC23 protein is produced in testis but fails to localize in the mature sperm tail, causing severe sperm motility defects and male infertility. Purified recombinant human LRRC23 does not interacts with RS stalk proteins, but with a head protein, RSPH9, which is abolished by the C-terminus truncation of LRRC23. Cryo-electron tomography and sub-tomogram averaging unambiguously visualized that the RS3 head and sperm-specific RS2-RS3 bridge structure is missing in the LRRC23 mutant sperm. Our study provides new insights into RS3 structure and function in mammalian sperm flagella as well as molecular pathogenicity of LRRC23 underlying reduced sperm motility in infertile human males. ### Competing Interest Statement The authors have declared no competing interest.