Hyperactivated motility is prominent to terrestrial mammals that reproduce by internal fertilization. However, the CatSper1-4 genes appear first in ancient uniflagellates that do no manifest hyperactivated motility-like behaviors, suggesting that additional Ca2+ signaling machinery is required. Biochemical purification of native CatSper channel from mouse testes showed that CatSper1-4 subunits form a complex with 4 additional transmembrane proteins. CatSper channel is the most structurally diverse of all ion channels, suggesting evolutionary refinement of CatSper function according to reproductive requirements. 

Calcium signaling specificity is accomplished via the ion’s precise spatiotemporal localization in a cell. Mammalian sperm has elaborate cytoskeletal structures in the tail for motility regulation. As the sperm flagella is less than 1 um in diameter, the spatial information of the signaling molecules inside the flagella cannot be resolved by conventional light microscope due to diffraction limit of light. Thus, we have applied super-resolution stochastic optical reconstruction microscopy (STORM) to image CatSper and the potential downstream signaling molecules within the flagella. Our studies showed that the CatSper channel forms unique four linear calcium domains that organize calcium signaling proteins along the flagella, providing strong evidence for molecularly defined, structured calcium signaling domains.

Combined with mouse genetics/CRISPR-Cas9 genomoe editing, we are currently studying molecular and spatial organization of various sperm ion channels and membrane receptors at super-resolution levels to better understand signal transduction in these tiny but mighty cells. In particular, we study the accessory subunits of CatSper channel to understand the molecular mechanisms of the channel assembly. We are also interested in understanding how CatSper-mediated Ca2+ entry relays spatial information to the axoneme. Spatial control of axonemal proteins that regulate dynein activity could result in the asymmetry of hyperactivated motility. 

Photo credits: Sang-Hee Shim & Jean-Ju Chung