The acrosomal vesicle of mouse sperm is a calcium store

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Abstract
Subsequent to binding to the zona pellucida, mammalian sperm undergo a regulated sequence of events that ultimately lead to acrosomal exocytosis. Like most regulated exocytotic processes, a rise in intracellular calcium is sufficient to trigger this event although the precise mechanism of how this is achieved is still unclear. Numerous studies on mouse sperm have indicated that a voltage‐operated Ca2+ channel plays some immediate role following sperm binding to the zona pellucida glycoprotein ZP3. However, there is also evidence that the mammalian sperm acrosome contains a high density of IP3 receptors, suggesting that the exocytotic event involves the release of Ca2+ from the acrosome. The release of Ca2+ from the acrosome may directly trigger exocytosis or may activate store‐operated Ca2+ channels in the plasma membrane. To test the hypothesis that the acrosome is an intracellular store we loaded mammalian sperm with the membrane permeant forms of three Ca2+‐sensitive fluorescent indicator dyes: fura‐2, indo‐1, and Calcium Green‐5N. Fluorescence microscopy revealed that the sperm were labeled in all intracellular compartments. When fura‐2 labeled sperm were treated with 150 μM MnCl2 to quench all fluorescence in the cytosol, or when the sperm were labeled with the low affinity dye Calcium Green‐5N, there was a large Ca2+ signal in the acrosome. Consistent with the acrosome serving as an intracellular Ca2+ reservoir, the addition of 20 μM thapsigargin, a potent inhibitor of the smooth endoplasmic reticular Ca2+‐ATPase (SERCA), to populations of capacitated sperm resulted in nearly 100% acrosomal exocytosis within 60 min (τ1/2 ∼ 10 min), in the absence of extracellular Ca2+. Additionally, treatment of sperm with 100 μM thimerosal, an IP3 receptor agonist, also resulted in acrosomal exocytosis. Taken together, these data suggest that the mouse sperm acrosome is a Ca2+ store that regulates its own exocytosis through an IP3 Ca2+ mobilization pathway.