ZIPT-7.1 and zinc signaling in sperm activation of Caenorhabditis elegans

Zinc is an essential trace metal. It is a cofactor of many enzymes, and it serves as a structural ion of many transcription factors. Zinc has also been shown to function as a signaling molecule. However, the signaling role of zinc has not been well characterized, and the mechanisms remain unclear. In the nematode Caenorhabditis elegans, sperm activation is a rapid process that results in the transformation of a round, immotile spermatid into a spermatozoon that crawls with a pseudopod. It is a cell differentiation process with no transcription involved as the nucleus is inaccessible at this stage. Zinc has been identified as one of the molecules that can activate C. elegans sperm in vitro.We found that deletion of the gene zipt-7.1 which encodes a ZIP (Zrt/ Irt-like protein) family protein results in a partially penetrant sterile phenotype. Biochemical studies indicate that ZIPT-7.1 is zinc transporter, and it is required for sperm activation in C. elegans. Immunostaining of both C. elegans expressing a tagged ZIPT-7.1 transgene and a human cell line expressing a wild-type ZIPT-7.1 suggest that it is expressed in the germline and likely localized to intracellular membranes. Tissue-specific RNAi down regulation experiments suggest that ZIPT-7.1 functions in the germline. Genetic analysis suggests zipt-7.1 is likely to function at a downstream position in the spe-8 sperm activation pathway. Based on these findings, we hypothesize that during sperm activation zinc is released from intracellular vesicles through ZIPT-7.1. This release increases the cytoplasmic zinc concentration, which serves as a signal for further differentiation. To test this hypothesis, we used live imaging and small molecular probes to show that there is a detectable change in cellular zinc level during sperm activation. A zinc ionophore- pyrithione- can activate sperm in vitro, bypassing the requirement of zipt-7.1 as well as other members of the spe-8 pathway. The results support our model, suggesting that zipt-7.1 and the spe-8 pathway activate sperm through zinc signaling