Network model predicts that CatSper is the main Ca2+ channel in the regulation of sea urchin sperm motility

Abstract

Spermatozoa sea urchin swimming behaviour is regulated by small peptides from the egg outer envelope. Speract, such a peptide, after binding to its receptor in Strongylocentrotus purpuratus sperm flagella, triggers a signaling pathway that culminates with a train of intracellular calcium oscillations, correlated with changes in sperm swimming pattern. This pathway has been widely studied but not fully characterized. Recent work on Arbacia punctulata sea urchin spermatozoa has documented the presence of the Ca2+ CatSper channel in their flagella and its involvement in chemotaxis. However, if other calcium channels participate in chemotaxis remains unclear. Here, based on an experimentally-backed logical network model, we conclude that CatSper is fundamental in the S. purpuratus speract-activated sea urchin sperm signaling cascade, although other Ca2+ channels could still be relevant. We also present for the first time experimental corroboration of its active presence in S. purpuratus sperm flagella. We argue, prompted by in silico knock-out calculations, that CatSper is the main generator of calcium oscillations in the signaling pathway and that other calcium channels, if present, have a complementary role. The approach adopted here allows us to unveil processes, which are hard to detect exclusively by experimental procedures.

Absolute proteomic quantification reveals design principles of sperm flagellar chemosensation

“…The ensuing hyperpolarization activates a voltage‐gated Na + /H + exchanger (sNHE) (Lee, ; Windler et al , ) and a hyperpolarization‐activated, cyclic nucleotide‐gated (HCN) pacemaker channel (Gauss et al , ; Galindo et al , ). Na + /H + exchange increases pH i , which primes CatSper Ca 2+ channels to open during the recovery from hyperpolarization probably driven by HCN channels that carry an inward current (Seifert et al , ; Espinal‐Enriquez et al , ). Changes in free Ca 2+ concentration ([Ca 2+ ] i ) control the swimming path.…”

“…For signal termination, a cGMP-specific phosphodiesterase (PDE5) (Su & Vacquier, 2006) breaks down cGMP and, finally, a Na + /Ca 2+ -K + exchanger (NCKX) (Su & Vacquier, 2002) and/or a plasma membrane Ca 2+ -ATPase (PMCA) (Gunaratne et al, 2006) restore resting Ca 2+ levels. Mass spectrometric (MS) analysis of purified flagella from A. punctulata sperm identified all previously known signaling components, but also additional proteins that might be involved in the signaling pathway (Appendix Table S1): (i) the Ca 2+ -activated Cl À channel TMEM16 of olfactory cilia, which has been predicted to exist in sperm from pharmacological and modeling studies (Guerrero et al, 2013;Espinal-Enriquez et al, 2017); (ii) a dual-specific PDE10 that, unlike PDE5, can also hydrolyze cAMP, and, together with sAC, may control cAMP metabolism; (iii) K + /Cl À co-transporters KCC1 (SLC12A4) and/or KCC3 (SLC12A5); (iv) a Na + /HCO 3 À co-transporter (SLC4A11); (v) a sperm-specific Na + /K + -ATPase a subunit; and (vi) two members (TMC 5 and 7) of a mechano-sensitive channel family (Zhao & Müller, 2015;Pan et al, 2018). Sperm respond to mechanical stimulation with a Ca 2+ response (Kambara et al, 2011).…”

Modular mathematical analysis of the control of flagellar Ca2+-spike trains produced by CatSper and CaVchannels in sea urchin sperm

“…b Antibody raised against a polypeptide of the α subunit in rat, which also recognises the homologous version in mouse and human sperm. c Antibody raised against pore-forming subunit polypeptides of A. punctulata [14] and S. purpuratus [15]. d It is of note that direct electrophysiological recordings for this ion channel have not been achieved in mature sea urchin spermatozoa compared with the homologous complex in mouse, human and macaque spermatozoa [16].…”

“…Furthermore, the expression of these subunits has been demonstrated in A. punctulata 952 sperm flagella [14] and most of them in S. purpuratus [15]. In order to model…”

“…The ion current density through these channels is modelled as: The sea urchin genome contains the gene set necessary for the expression of the four 942 core and the three auxiliary subunits that constitute the CatSper channel [72]. 943 Furthermore, the expression of these subunits has been demonstrated in A. punctulata 944 sperm flagella [14] and most of them in S. purpuratus [15]. In order to model…”