Sperm chemotaxis towards factors that are secreted by the egg and the egg-associated somatic cells is a pre-requisite for fertilization. In sea urchins, the egg peptides speract and resact are known to induce the synthesis of cGMP and trigger rapid transient changes in intracellular Ca²⁺ concentrations, which regulate the asymmetry of flagellar beat and thus, swimming trajectory. However, until now, little was known about the ion channels involved. Benjamin Kaupp and colleagues report in Nature Cell Biology that cGMP-gated K⁺ channels and low voltage-activated Ca_v channels are involved in the response of sea urchin sperm to resact.

By staining intact motile sperm with a fluorescent dye that shifts its emission spectrum in response to changes in voltage, the authors showed that resact evokes a brief hyperpolarization followed by a sustained depolarization. 0.25 pM resact, which delivers, on average, one bound resact molecule per cell, was able to change the voltage by about 2 mV. The hyperpolarization was found to be dependent on the concentration of both resact and external K⁺, indicating the involvement of a K⁺-selective ion channel. The instantaneous opening of the channel in response to either resact or the release of cGMP from caged compounds indicates that it is directly gated by cyclic nucleotides. Sequence comparison between mammalian CNG channels and a cloned cDNA fragment from A. punctulata revealed a flagellar protein that bears all the hallmarks of a K⁺-selective cyclic nucleotide-gated (CNG) channel. Furthermore, 20 peptides predicted from the cloned cDNA fragment were found in the membranes of purified flagella by mass spectrometry.

Next, the authors turned their attention to the channels that mediate the Ca²⁺ entry that follows hyperpolarization. Ca²⁺ signals rise at, or shortly after, the peak of hyperpolarization. The voltage threshold and kinetics at which these channels open indicate that low voltage-activated channels recovering from inactivation allow this influx of Ca²⁺. High voltage-gated Ca²⁺ channels are likely to mediate the second long-lasting increase in Ca²⁺, also known as the late Ca²⁺ response, that has been observed at saturating concentrations of resact.

The identification of CNG channels as mediators of sperm chemotaxis strengthens the similarities between two evolutionary distinct detection systems, photoreceptors and sperm. Not only are they able to detect single molecules and photons, respectively, they employ cGMP, CNG channels and Ca²⁺ to transduce, amplify, and adapt the signal. The biochemical and biophysical techniques of high-time-resolution that were used in this study may be applicable to other systems to improve our understanding of the motility response at the molecular level.

Author: Monica Hoyos-Flight