Drosophila Trpm mediates calcium influx during egg activation

Abstract

Egg activation is the process in which mature oocytes are released from developmental arrest and gain competency for embryonic development. In Drosophila and other arthropods, eggs are activated by mechanical pressure in the female reproductive tract, whereas in most other species, eggs are activated by fertilization. Despite the difference in the trigger, Drosophila shares many conserved features with higher vertebrates in egg activation, including a rise of intracellular calcium in response to the trigger. In Drosophila, this calcium rise is initiated by entry of extracellular calcium due to opening of mechanosensitive ion channels and initiates a wave that passes across the egg prior to initiation of downstream activation events. Here, we combined inhibitor tests, germline specific RNAi knockdown, and germline specific CRISPR/Cas9 knockout to identify the Transient receptor potential (TRP) channel subfamily M (Trpm) as a critical channel that mediates the calcium influx and initiates the calcium wave during Drosophila egg activation. We observed reduced egg hatchability in trpm germline knockout mutant females, although eggs were able to complete some egg activation events including cell cycle resumption. Since the mouse Trpm ortholog was recently reported also to be involved in calcium influx during egg activation and in further embryonic development our results suggest that calcium uptake from the environment via TRPM channels is a deeply conserved aspect of egg activation.\n\nSignificanceA rise in intracellular free calcium is a conserved feature of the egg-to-embryo transition in almost all animals. In Drosophila, as in vertebrates, the rise starts at one end of the egg, and then travels across the egg in a wave. The Drosophila calcium rise is mediated by an influx of calcium, due to the action of mechanically-gated ion channels. Here we identify the ion channel that is critical for the calcium entry as TRPM. TRPM is the ortholog of the channel recently shown to mediate the post-fertilization calcium influxes needed to sustain calcium oscillations in fertilized mouse eggs, suggesting a deep homology - despite species differences in the trigger for egg activation.