Ca²⁺ signalling through CatSper and Ca²⁺ stores regulate different behaviours in human sperm

Background: Ca²⁺ signals, elicited by cues from the oocyte and female tract, regulate human sperm behavior.

Results: CatSper channel activation (flagellum) and Ca²⁺ store mobilization (neck) caused similar [Ca²⁺]_i elevation but induced functionally different behaviors.

Conclusion: Sperm motility pattern is determined by the site of Ca²⁺ mobilization.

Significance: Selection of Ca²⁺ signaling components and/or regulation of their availability for activation controls human sperm behavior.

[Ca²⁺]_i signaling regulates sperm motility, enabling switching between functionally different behaviors that the sperm must employ as it ascends the female tract and fertilizes the oocyte. We report that different behaviors in human sperm are recruited according to the Ca²⁺ signaling pathway used. Activation of CatSper (by raising pHi or stimulating with progesterone) caused sustained [Ca²⁺]_i elevation but did not induce hyperactivation, the whiplash-like behavior required for progression along the oviduct and penetration of the zona pellucida. In contrast, penetration into methylcellulose (mimicking penetration into cervical mucus or cumulus matrix) was enhanced by activation of CatSper. NNC55-0396, which abolishes CatSper currents in human sperm, inhibited this effect. Treatment with 5 μm thimerosal to mobilize stored Ca²⁺ caused sustained [Ca²⁺]_i elevation and induced strong, sustained hyperactivation that was completely insensitive to NNC55-0396. Thimerosal had no effect on penetration into methylcellulose. 4-Aminopyridine, a powerful modulator of sperm motility, both raised pHi and mobilized Ca²⁺ stored in sperm (and from microsomal membrane preparations). 4-Aminopyridine-induced hyperactivation even in cells suspended in Ca²⁺-depleted medium and also potentiated penetration into methylcellulose. The latter effect was sensitive to NNC55-039, but induction of hyperactivation was not. We conclude that these two components of the [Ca²⁺]_i signaling apparatus have strikingly different effects on sperm motility. Furthermore, since stored Ca²⁺ at the sperm neck can be mobilized by Ca²⁺-induced Ca²⁺ release, we propose that CatSper activation can elicit functionally different behaviors according to the sensitivity of the Ca²⁺ store, which may be regulated by capacitation and NO from the cumulus.