Basal cytoplasmic calcium levels regulate C. elegans germ stem cell proliferation

The Caenorhabditis elegans germline is one of the foremost models for the study of germ cell biology. We created an integrated ratiometric calcium reporter to assess calcium levels in the C. elegans germ line. The reporter reveals changes in basal cytoplasmic calcium levels during hermaphrodite sexual development. Oogenic cells have elevated calcium levels, while spermatogenic cells have significantly reduced levels of calcium. There are a small number of germ cells in the bend region of mid-to-late L4 hermaphrodite germ lines with elevated calcium levels that are correlated with the spermatogenesis-to-oogenesis transition. We identified GON-2 as a calcium channel that is required for the maintenance of normal calcium levels in the germline. Significantly, basal calcium levels correlate with germ cell proliferation. Starvation and inactivation of pro-proliferation signaling pathways reduce basal calcium levels, while proliferating germline tumor cells have elevated basal calcium levels. Experimentally altering calcium levels leads to a concomitant increase or decrease in the proliferation rate of germ stem cells. These results imply that basal cytoplasmic calcium acts as a rheostat to regulate germ stem cell proliferation.