FIGURE 6.

Signaling pathway model showing the role of NAADP in the acrosome reaction in sea urchin sperm. The fucose sulfate polymer (FSP) in the egg jelly activates the receptor of egg jelly (REJ, 1), which stimulates a plasma membrane channel resulting in a transient Ca²⁺ increase (2). The initial Ca²⁺ increase stimulates NAADP synthesis possibly by ADP-ribosyl cyclase (ARC, 3). NAADP binds to its receptor (Two-Pore Channel) on the acrosome and releases Ca²⁺ (4). These steps parallel the traditional pathway in which the Ca²⁺ increase stimulates phospholipase Cδ (PLCδ, 5) to produce inositol 1,4,5-trisphosphate that activates its receptor (6). Ca²⁺ release from the acrosome might trigger exocytosis directly or indirectly via store-activated Ca²⁺ entry (7).