Hyperactivated motility is prominent in land mammals that reproduce by internal fertilization. However, the CatSper1-4 genes first appear in ancient uniflagellates that do no exhibit hyperactivated motility-like behaviors, suggesting that additional Ca2+ signaling machinery is required. Biochemical purification of native CatSper channel from mouse testis revealed that CatSper1-4 subunits form a complex with at least 6 additional transmembrane and 3 cytosolic ancillary proteins. The CatSper channel is the most structurally diverse of all ion channels, suggesting an evolutionary refinement of CatSper function according to reproductive requirements. 

Various cutting-edge imaging approach combined with mouse genetics/CRISPR-Cas9 genome editing, we have studied the molecular and spatial organization of various sperm ion channels and membrane receptors to better understand signal transduction in these tiny but powerful cells. For example, the specificity of calcium signaling is achieved by the precise spatiotemporal localization of the ion within a cell. Since sperm flagella are less than 1 um in diameter, we have applied super-resolution microcopy to extract the spatial information of the calcium signaling molecules inside the flagella. For example, our studies showed that the CatSper channel forms unique four linear calcium nanodomains that organize calcium signaling proteins along the flagella, providing strong evidence for molecularly defined, structured calcium signaling domains. Recently, we used cryoelectron tomography to visualize the higher-order organization and 3D in situ structure of the CatSper channels in intact spermatozoa: the CatSper channel complexes are zigzag arranged within the linear nanodomains, formed by the canopy of transmembrane anciliary subunits interconnected by their large extracellular domains over the pore-forming tetrameric channels.

Currently, we are studying the accessory subunits of the CatSper channel to understand the molecular mechanisms of the channel assembly and trafficking. We are also establishing tools and assays to taget CatSper function to identify CatSper modulators that can be used in the development of new contraceptives and fertility enhancers.

Photo credits: Jean-Ju Chung