A Sperms Tale: Calcium gets the Turn On! Wonder how a sperm finds the egg?
• The original guided missile: A chemoattractant released by the egg binds to its receptor on the sperm surface, and sets off a signaling cascade. First, cyclic GMP is made, which opens a set of K+ channels. These alter the voltage across the membrane to make it more negatively charged inside, which in turn opens voltage sensing Ca2+ channels. Calcium ions rush in, and direct the movement of the sperm by controlling the beating of tail (flagellum).
• As the sperm turns: To understand how calcium controls the sperm, researchers used a “caged” form of cGMP that could be light-activated,and a fluorescent dye to measure Ca2+ ions. Then they tracked sperm movement under a fluorescence microscope. The left panel tracks a single sperm (thick lines drawn over it), first in slow motion and then in real time. In both graphs on the right, the pink lines track the path, with turns shown as peaks. The sperm alternately makes straight runs and turns. In the upper graph, calcium levels (F) are tracked in green. Although, there is some general overlap with sperm movement, the two are not perfectly correlated. But in the lower graph, the rate of change of calcium (dF/dT, mathematically, the time derivative) matches the turns perfectly. In other words, sperms respond to change, not absolute levels of signal. The fastness of the calcium surge controls the sharpness of the turn! The path of the subsequent run depends on the steepness of the decline .
• The tail that wags the dog: How does the sperm “compute” the time derivative of calcium and how is this information translated into steering the tail? The authors describe a chemical differentiator model, in which calcium binds with differing strengths to proteins with opposing control of the tail. It’s all about chemistry!
Original Paper: Alavarez et al., 2012 J Cell Biol. http://jcb.rupress.org/content/196/5/653.full
☼ SPECIAL THANKS to Kevin Staff for making this animated Gif and for being the tail that wags my #ScienceSunday posts! Konstantin Makov is hereby challenged to come up with mood music for the sperm turns 😉