Figure 5.

Model for roles and interactions of Ca²⁺ stores in mammalian sperm. Zona binding (pathway shown in blue) induces generation of InsP₃, mobilisation of the acrosomal store and activation of store operated Ca²⁺ influx, leading to elevated [Ca²⁺]_i in the peri-acrosomal space and acrosome reaction. Progesterone (and probably other stimuli which cause Ca²⁺ influx; pathway shown in green) mobilise Ca²⁺ stored at the sperm neck/midpiece by Ca²⁺-induced Ca²⁺ release (CICR) and (probably) activation of store operated Ca²⁺ influx, causing elevation of Ca²⁺ in the midpiece/proximal flagellum leading to regulation of flagellar activity and hyperactivation. CatSper channels in the principal piece of the flagellum are activated by increased pH_i, causing influx of Ca²⁺, elevation of [Ca²⁺]_i and hyperactivation (shown in red). Dashed arrows (numbered) show potential crosstalk between these pathways. 1: InsP₃ generated downstream of zona binding may activate InsP₃Rs at the sperm neck, leading to Ca²⁺ mobilisation and hyperactivation. 2: Ca²⁺ influx through CatSpers in the principal piece may affect [Ca²⁺]_i at the sperm neck/midpiece, mobilising stores Ca²⁺ by CICR. 3: SOCs in the principal piece may be activated downstream of store mobilisation. 4: Ca²⁺ mobilisation in the neck/midpiece may spread forward into the head, potentially mobilising acrosomal Ca²⁺ by CICR.