Two-directional trafficking of the IFT25 protein in the developing mouse sperm flagella

Intraflagellar transport 25 is a component of the intraflagellar transport 25-B complex. In mice, even though this intraflagellar transport component is not required for cilia formation in somatic cells, it is essential for sperm formation. However, the intracellular localization of this protein in male germ cells is not known given no reliable antibodies are available for histologic studies, and the dynamic trafficking in the developing sperm flagella is not clear. To examine localization of the protein in male germ cells and further investigate the mechanism of intraflagellar transport in sperm formation, particularly to look into the dynamic trafficking of the protein, we generated a mouse intraflagellar transport 25-green fluorescent protein knock-in mouse model using the clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats associated protein 9 system, with the mouse intraflagellar transport 25 protein fused with a green fluorescent protein tag in the C-terminus. Three independent lines were analyzed. Western blotting using both anti-intraflagellar transport 25 and anti-green fluorescent protein antibodies showed that the intraflagellar transport 25-green fluorescent protein fusion protein was highly abundant only in the testis, which is consistent with the endogenous intraflagellar transport 25 protein. Examination of localization of the intraflagellar transport 25-green fluorescent protein in isolated germ cells revealed that the fusion protein was present in the cytoplasm of spermatocytes and round spermatids and a strong signal was present in the developing sperm flagellar. The homozygous knock-in mice had normal spermatogenesis, fertility and sperm parameters. Diffusion analysis of intraflagellar transport 25 within the developing flagellar revealed the presence of both mobile and immobile fractions as revealed by fluorescence recovery after photobleaching. Kymograph and fluorescence recovery after photobleaching analyses demonstrate the transport of intraflagellar transport 25-green fluorescent protein within the developing tail demonstrate no apparent preference for trafficking toward and away from the cell body. The speed of trafficking depends on the stage of sperm development, ranging from highly mobile unrestricted diffusion initially, mobile punctate structures in developing sperm, and immobile punctate structures in mature sperm. Our studies demonstrate that mouse intraflagellar transport 25 travels along the developing sperm flagella in two directions that might be essential for functional sperm formation.