Initiation of the human sperm acrosome reaction by thapsigargin

Abstract

Previous studies have established that the mammalian sperm acrosome reaction (AR) is dependent upon an influx of extracellular Ca²⁺, but the involvement of a mobilizable store of intracellular Ca²⁺ has not been shown. In many other cells, the endoplasmic reticulum is the site of such a Ca²⁺ -store. Here, we show that thapsigargin, a highly specific inhibitor of the endoplasmic reticulum Ca²⁺ -ATPase Ca²⁺ -pump (and thus a mobilizer of intracellular Ca²⁺) in other cells, can initiate the AR in capacitated human sperm. Thapsigargin at concentrations from 50–500 nM significantly increased the AR to the same extent when incubated with capacitated sperm for 1 min (assayed by indirect immunofluorescence). Transmission electron microscopy confirmed the occurrence of normal morphologyin the AR initiated by thapsigargin. Thapsigargin (200 nM) did not initiate the AR in noncapacitated sperm. Inititation of the AR by thapsigargin apparently requires an influx of Ca²⁺ since 1 min preincubation with the calcium channel blockers La³⁺ (250 μM) or Ni²⁺ (250 μM) prior to addition of thapsigargin completely inhibits AR-initiation. Mobilization of an intracellular Ca²⁺ -store by thapsigargin in capacitated human sperm may lead to an influx of extracellular Ca²⁺ and subsequently the AR. Putative sites for thapsigargin-sensitive intracellular Ca²⁺ -stores in human sperm include the cytoplasmic droplet, the sperm nucleus and the acrosome.