Sperm Chemotaxis: Egg Peptides Control Cytosolic Calcium to Regulate Flagellar Responses

Fragmentary evidence indicates that intracellular [Ca2+] (Cai) mediates sperm chemotaxis. However, neither correlations of swimming responses to chemoattractant-induced alterations of Cai nor explanations of how chemoattractant gradients control Cai exist. Here Cai increases produced by the egg peptide speract—not previously known to cause flagellar responses—were prolonged by treatment with 3-isobultyl-1-methylxanthine (IBMX). Flagellar waveform asymmetry then increased 40% and swimming paths became tightly circular. Moreover, both responses required external Ca2+ (as does sperm chemotaxis to eggs and egg products). Cai increases by the established chemotactic peptide resact also required external Ca2+ and were enhanced by IBMX. Therefore, diverse egg peptides may use fundamentally similar mechanisms to control Cai and thereby swimming behavior in chemotaxis. Repetitive increasing additions of speract produced adaptive membrane potential and Cai responses indicating that sperm can detect increasing gradients of egg peptide over a broad concentration range. We offer a model in which shallow or decreasing gradients elevate Cai and redirect swimming paths but sufficiently steep gradients keep Cai low and swimming linear until the egg is reached. A negative-feedback loop, initiated by cGMP-mediated activation of sperm K+ channels and terminated by subsequent inactivation of guanylyl cyclase, may coordinate gradient detection with control of Cai. Continued stimulation of more receptors by steeply increasing gradients of egg peptide thus maintains membrane hyperpolarization and suppresses Ca2+ entry and Cai elevation. The molecular basis for chemotaxis therefore is explained as translation of the spatial gradient of peptide concentration into changes in K+ channel activity in the time domain.