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Abstract The eukaryotic flagellum propels sperm cells and simultaneously detects physical and chemical cues that modulate the waveform of the flagellar beat. Most previous studies have characterized the flagellar beat and swimming trajectories in two space dimensions (2D) at a water/glass interface. Here, using refined holographic imaging methods, we report high-quality recordings of three-dimensional (3D) flagellar bending waves. As predicted by theory, we observed that an asymmetric and planar flagellar beat results in a circular swimming path, whereas a symmetric and non-planar flagellar beat results in a twisted-ribbon swimming path. During swimming in 3D, human sperm flagella exhibit torsion waves characterized by maxima at the low curvature regions of the flagellar wave. We suggest that these torsion waves are common in nature and that they are an intrinsic property of beating axonemes. We discuss how 3D beat patterns result in twisted-ribbon swimming paths. This study provides new insight into the axoneme dynamics, the 3D flagellar beat, and the resulting swimming behavior. Graphic abstract

The reaction of CO2 with H2O to form HCO3- and H+ is one of the most important chemical equilibria in cells. In mammalian sperm, a soluble adenylyl cyclase (sAC) serves as cellular HCO3- sensor that conveys the equilibrium state via cAMP synthesis to cAMP-signaling molecules. The function of sAC and cAMP in non-mammalian sperm is largely unknown. Here, we identify sAC orthologs in sea urchin and salmon sperm that, surprisingly, are activated by alkaline pH rather than HCO3-. Two amino-acid residues required for HCO3- binding of mammalian sAC are lacking in pH-regulated sAC. Orthologs identified in ten other phyla are also lacking either one of these key residues, suggesting that pH control is widespread among non-mammalian metazoan. The pH-sensitive sAC controls several functions of sperm from external fertilizers. Upon spawning, alkalization triggers cAMP synthesis and, thereby, activates motility of quiescent sperm. Egg-derived chemoattractants also alkalize sperm and elevate cAMP, which then-modulates pacemaker HCN channels to trigger a chemotactic Ca2+ response. Finally, the sAC and the voltage- and cAMP-activated Na+/H+ exchanger sNHE mutually control each other. A picture of evolutionary significance is emerging: motility and sensory signaling of sperm from both internal and external fertilizers rely on cAMP, yet, their sAC is regulated by HCO3- or pHi, respectively. Acidification of aquatic habitats due to climate change may adversely affect pH-sensing by sAC and thereby sexual reproduction in the sea.Statement of significanceAdenylyl cyclases synthesize cAMP, a prominent cellular messenger. A bicarbonate-sensitive AC family member, soluble AC (sAC), is tied to the chemical equilibrium: H2O + CO2 ↔ HCO3- (bicarbonate) + H+. The sAC is required for fertilization: Mammals lacking sAC are infertile and sperm immotile. We now identify a new sAC form in sperm of non-mammalian animals that reproduce in the sea. This novel sAC is activated at alkaline pH rather than bicarbonate. It controls sperm motility and chemotaxis. The switch from HCO3- to pH rests on substitution of two amino-acids, which represents an adaptation to aquatic environments low in bicarbonate. Acidification of aquatic habitats due to climate change may adversely affect sAC activity and, thereby, fertilization.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Voltage-sensing (VSD) and cyclic nucleotide-binding domains (CNBD) gate ion channels for rapid electrical signaling. By contrast, solute carriers (SLCs) that passively redistribute substrates are gated by their substrates themselves. Here, we study the orphan sperm-specific solute carriers SLC9C1 that feature a unique tripartite structure: an exchanger domain, a VSD, and a CNBD. Voltage-clamp fluorimetry shows that SLC9C1 is a genuine Na+/H+ exchanger gated by voltage. The cellular messenger cAMP shifts the voltage range of activation. Mutations in the transport domain, the VSD, or the CNBD strongly affect Na+/H+ exchange, voltage gating, or cAMP sensitivity, respectively. Our results establish SLC9C1 as a phylogenetic chimaera that combines the ion-exchange mechanism of solute carriers with the gating mechanism of ion channels. Classic SLCs slowly readjust changes in the intra- and extracellular milieu, whereas voltage gating endows the Na+/H+ exchanger with the ability to produce a rapid pH response that enables downstream signaling events.

For a given sensory cell type, signaling motifs are rather uniform across phyla. By contrast, sperm from different species use diverse repertoires of sperm-specific signaling molecules and even closely related protein isoforms feature different properties and serve different functions. This surprising diversity has consequences for strategies in fertilization research and it will take some time to get the big picture. We discuss the function of receptors, ion channels, and exchangers embedded in cellular pathways from different sperm species.

Sperm are highly specialized cells, which have been subject to substantial evolutionary pressure. Whereas some sperm features are highly conserved, others have undergone major modifications. Some of these variations are driven by adaptation to mating behaviours or fitness at the organismic level. Others represent alternative solutions to the same task. Sperm must find the egg for fertilization. During this task, sperm rely on long slender appendages termed flagella that serve as sensory antennas, propellers and steering rudders. The beat of the flagellum is periodic. The resulting travelling wave generates the necessary thrust for propulsion in the fluid. Recent studies reveal that, for steering, different species rely on different fundamental features of the beat wave. Here, we discuss some examples of unity and diversity across sperm from different species with a particular emphasis on the steering mechanisms.This article is part of the Theo Murphy meeting issue ‘Unity and diversity of cilia in locomotion and transport’.

STUDY QUESTION Does the chemosensory activation of CatSper Ca2+ channels in human sperm give rise to additive, sub-additive or even synergistic actions among agonists? SUMMARY ANSWER We show that oviductal ligands and endocrine disrupting chemicals (EDCs) activate human CatSper highly synergistically. WHAT IS KNOWN ALREADY In human sperm, the sperm-specific CatSper channel controls the intracellular Ca2+ concentration and, thereby, several crucial stages toward fertilization. CatSper is activated by oviductal ligands and structurally diverse EDCs. The chemicals mimic the action of the physiological ligands, which might interfere with the precisely coordinated sequence of events underlying fertilization. STUDY DESIGN, SIZE, DURATION For both oviductal ligands and EDCs, we examined in quantitative terms whether stimulation of human sperm in vitro with mixtures results in additive, sub-additive or synergistic actions. PARTICIPANTS/MATERIALS, SETTING, METHODS We studied activation of CatSper in sperm of healthy volunteers, using kinetic Ca2+ fluorimetry and patch-clamp recordings. The combined action of progesterone and prostaglandins and of the EDCs benzylidene camphor sulfonic acid (BCSA) and α-Zearalenol was evaluated by curve-shift analysis, curvilinear isobolographic analysis and the combination-index method. MAIN RESULTS AND THE ROLE OF CHANCE Analysis of the action of progesterone/prostaglandin and BCSA/α-Zearalenol mixtures in human sperm by fluorimetry revealed that the oviductal ligands and EDCs both evoke Ca2+ influx via CatSper in a highly synergistic fashion. Patch-clamp recordings of CatSper currents in human sperm corroborated the synergistic ligand-activation of the channel. LIMITATIONS, REASONS FOR CAUTION This is an in vitro study. Future studies have to assess the physiological relevance in vivo. WIDER IMPLICATIONS OF THE FINDINGS These findings indicate that the fertilization process is orchestrated by multiple oviductal CatSper agonists that act in concert to control the behavior of sperm. Moreover, our results substantiate the concerns regarding the negative impact of EDCs on male reproductive health. So far, safety thresholds like the "No Observed Adverse Effect Level (NOAEL)" or "No Observed Effect Concentration (NOEC)" are set for individual EDCs. Our finding that EDCs act synergistically in human sperm challenges the validity of this procedure. STUDY FUNDING/COMPETING INTEREST(S) This work was supported by the German Research Foundation (SFB 645; CRU326), the Cells-in-Motion (CiM) Cluster of Excellence, Munster, (FF-2016-17), the 'Innovative Medical Research' of the University of Munster Medical School (BR121507), an EDMaRC research grant from the Kirsten and Freddy Johansen's Foundation, and the Innovation Fund Denmark (InnovationsFonden; 14-2013-4). The authors have no competing financial interests.

Hyperpolarization activated and cyclic nucleotide-gated (HCN) ion channels as well as cyclic nucleotide-gated (CNG) ion channels are essential for the regulation of cardiac cells, neuronal excitability, and signaling in sensory cells. Both classes are composed of four subunits. Each subunit comprises a transmembrane region, intracellular N- and C-termini, and a C-terminal cyclic nucleotide-binding domain (CNBD). Binding of cyclic nucleotides to the CNBD promotes opening of both CNG and HCN channels. In case of CNG channels, binding of cyclic nucleotides to the CNBD is sufficient to open the channel. In contrast, HCN channels open upon membrane hyperpolarization and their activity is modulated by binding of cyclic nucleotides shifting the activation potential to more positive values. Although several high-resolution structures of CNBDs from HCN and CNG channels are available, the gating mechanism for murine HCN2 channel, which leads to the opening of the channel pore, is still poorly understood. As part of a structural investigation, here, we report the complete backbone and side chain resonance assignments of the murine HCN2 CNBD with part of the C-linker.

The cellular messenger cAMP regulates multiple cellular functions, including signaling in cilia and flagella. The cAMP dynamics in these subcellular compartments are ill-defined. We introduce a novel FRET-based cAMP biosensor with nanomolar sensitivity that is out of reach for other sensors. To measure cAMP dynamics in the sperm flagellum, we generated transgenic mice and reveal that the hitherto methods determining total cAMP levels do not reflect changes in free cAMP levels. Moreover, cAMP dynamics in the midpiece and principal piece of the flagellum are distinctively different. The sole cAMP source in the flagellum is the soluble adenylate cyclase (SACY). Although bicarbonate-dependent SACY activity requires Ca2+, basal SACY activity is suppressed by Ca2+. Finally, we also applied the sensor to primary cilia. Our new cAMP biosensor features unique characteristics that allow gaining new insights into cAMP signaling and unravel the molecular mechanisms underlying ciliary function in vitro and in vivo. DOI: http://dx.doi.org/10.7554/eLife.14052.001, eLife digest Cells can change the way they grow, move or develop in response to information from their environment. This information is first detected at the surface of the cell and then the information is relayed around the interior of the cell by signaling molecules known as “second messengers”. A molecule called cAMP is a well-known second messenger that is involved in many different signaling pathways. Therefore, the levels of cAMP in specific areas of the cell need to be precisely regulated to enable different signaling pathways to be activated at specific times and locations. Some cells have hair-like structures called cilia or flagella on their surface. Cilia and flagella are able to move the fluid that surrounds the cells or even move the cells themselves. The second messenger cAMP plays an essential role in making cilia move, but it is challenging to analyze the dynamics of cAMP – that this, how the levels of this molecule change over time – in these structures. The levels of cAMP in live cells can only be measured using fluorescent biosensors. Introducing these biosensors into specific cell structures is difficult and they are not sensitive enough to respond to low levels of cAMP. Furthermore, it is difficult to measure cAMP activity inside such tiny structures using these biosensors. Mukherjee, Jansen, Jikeli et al. now address some of these challenges by creating a new cAMP biosensor that has several unique features. Most importantly, it can respond to very low levels of cAMP, making it more sensitive than previous biosensors. Mukherjee et al. test this new biosensor in the flagella of sperm cells from mice, which reveals how the production of cAMP is regulated in the flagellum. The new biosensor also shows that different parts of the flagellum can have different cAMP dynamics. In the future, this new biosensor could be used to study cAMP in other structures and compartments within cells. DOI: http://dx.doi.org/10.7554/eLife.14052.002

The sperm-specific CatSper channel controls the intracellular Ca2+ concentration ([Ca2+]i) and, thereby, the swimming behaviour of sperm. In humans, CatSper is directly activated by progesterone and prostaglandins—female factors that stimulate Ca2+ influx. Other factors including neurotransmitters, chemokines, and odorants also affect sperm function by changing [Ca2+]i. Several ligands, notably odorants, have been proposed to control Ca2+ entry and motility via G protein-coupled receptors (GPCRs) and cAMP-signalling pathways. Here, we show that odorants directly activate CatSper without involving GPCRs and cAMP. Moreover, membrane-permeable analogues of cyclic nucleotides that have been frequently used to study cAMP-mediated Ca2+ signalling also activate CatSper directly via an extracellular site. Thus, CatSper or associated protein(s) harbour promiscuous binding sites that can host various ligands. These results contest current concepts of Ca2+ signalling by GPCR and cAMP in mammalian sperm: ligands thought to activate metabotropic pathways, in fact, act via a common ionotropic mechanism. We propose that the CatSper channel complex serves as a polymodal sensor for multiple chemical cues that assist sperm during their voyage across the female genital tract.

Sperm use temporal sampling, resetting of intracellular calcium level, and adaptation of their sensitivity to respond to a wide range of chemoattractant concentrations during their voyage toward the egg., Sperm, navigating in a chemical gradient, are exposed to a periodic stream of chemoattractant molecules. The periodic stimulation entrains Ca2+ oscillations that control looping steering responses. It is not known how sperm sample chemoattractant molecules during periodic stimulation and adjust their sensitivity. We report that sea urchin sperm sampled molecules for 0.2–0.6 s before a Ca2+ response was produced. Additional molecules delivered during a Ca2+ response reset the cell by causing a pronounced Ca2+ drop that terminated the response; this reset was followed by a new Ca2+ rise. After stimulation, sperm adapted their sensitivity following the Weber–Fechner law. Taking into account the single-molecule sensitivity, we estimate that sperm can register a minimal gradient of 0.8 fM/µm and be attracted from as far away as 4.7 mm. Many microorganisms sense stimulus gradients along periodic paths to translate a spatial distribution of the stimulus into a temporal pattern of the cell response. Orchestration of temporal sampling, resetting, and adaptation might control gradient sensing in such organisms as well.

Cyclic nucleotide-sensitive ion channels, known as HCN and CNG channels, are activated by binding of ligands to a domain (CNBD) located on the cytoplasmic side of the channel. The underlying mechanisms are not well understood. To elucidate the gating mechanism, structures of both the ligand-free and -bound CNBD are required. Several crystal structures of the CNBD from HCN2 and a bacterial CNG channel (MloK1) have been solved. However, for HCN2, the cAMP-free and -bound state did not reveal substantial structural rearrangements. For MloK1, structural information for the cAMP-free state has only been gained from mutant CNBDs. Moreover, in the crystal, the CNBD molecules form an interface between dimers, proposed to be important for allosteric channel gating. Here, we have determined the solution structure by NMR spectroscopy of the cAMP-free wild-type CNBD of MloK1. A comparison of the solution structure of cAMP-free and -bound states reveals large conformational rearrangement on ligand binding. The two structures provide insights on a unique set of conformational events that accompany gating within the ligand-binding site.

Cyclic nucleotide-sensitive ion channels, known as HCN and CNG channels, are crucial in neuronal excitability and signal transduction of sensory cells. HCN and CNG channels are activated by binding of cyclic nucleotides to their intracellular cyclic nucleotide-binding domain (CNBD). However, the mechanism by which the binding of cyclic nucleotides opens these channels is not well understood. Here, we report the solution structure of the isolated CNBD of a cyclic nucleotide-sensitive K(+) channel from Mesorhizobium loti. The protein consists of a wide anti-parallel beta-roll topped by a helical bundle comprising five alpha-helices and a short 3(10)-helix. In contrast to the dimeric arrangement ('dimer-of-dimers') in the crystal structure, the solution structure clearly shows a monomeric fold. The monomeric structure of the CNBD supports the hypothesis that the CNBDs transmit the binding signal to the channel pore independently of each other.

Dendritic voltage-gated ion channels profoundly shape the integrative properties of neuronal dendrites. In epilepsy, numerous changes in dendritic ion channels have been described, all of them due to either their altered transcription or phosphorylation. In pilocarpine-treated chronically epileptic rats, we describe a novel mechanism that causes an increased proximal dendritic persistent Na+current (INaP). We demonstrate using a combination of electrophysiology and molecular approaches that the upregulation of dendriticINaPis due to a relief from polyamine-dependent inhibition. The polyamine deficit in hippocampal neurons is likely caused by an upregulation of the degrading enzyme spermidine/spermine acetyltransferase. Multiphoton glutamate uncaging experiments revealed that the increase in dendriticINaPcauses augmented dendritic summation of excitatory inputs. These results establish a novel post-transcriptional modification of ion channels in chronic epilepsy and may provide a novel avenue for treatment of temporal lobe epilepsy.SIGNIFICANCE STATEMENTIn this paper, we describe a novel mechanism that causes increased dendritic persistent Na+current. We demonstrate using a combination of electrophysiology and molecular approaches that the upregulation of persistent Na+currents is due to a relief from polyamine-dependent inhibition. The polyamine deficit in hippocampal neurons is likely caused by an upregulation of the degrading enzyme spermidine/spermine acetyltransferase. Multiphoton glutamate uncaging experiments revealed that the increase in dendritic persistent Na current causes augmented dendritic summation of excitatory inputs. We believe that these results establish a novel post-transcriptional modification of ion channels in chronic epilepsy.

Many cells probe their environment for chemical cues. Some cells respond to picomolar concentrations of neuropeptides, hormones, pheromones, or chemoattractants. At such low concentrations, cells encounter only a few molecules. The mechanistic underpinnings of single-molecule sensitivity are not known for any eukaryotic cell. Sea urchin sperm offer a unique model to unveil in quantitative terms the principles underlying chemosensation at the physical limit. Here, we discuss the mechanisms of such exquisite sensitivity and the computational operations performed by sperm during chemotactic steering. Moreover, we highlight commonalities and differences between signalling in sperm and photoreceptors and among sperm from different species.

SummaryThe visual pigment rhodopsin belongs to the family of G protein-coupled receptors that can form higher oligomers. It is controversial whether rhodopsin forms oligomers and whether oligomers are functionally relevant. Here, we study rhodopsin organization in cryosections of dark-adapted mouse rod photoreceptors by cryoelectron tomography. We identify four hierarchical levels of organization. Rhodopsin forms dimers; at least ten dimers form a row. Rows form pairs (tracks) that are aligned parallel to the disk incisures. Particle-based simulation shows that the combination of tracks with fast precomplex formation, i.e. rapid association and dissociation between inactive rhodopsin and the G protein transducin, leads to kinetic trapping: rhodopsin first activates transducin from its own track, whereas recruitment of transducin from other tracks proceeds more slowly. The trap mechanism could produce uniform single-photon responses independent of rhodopsin lifetime. In general, tracks might provide a platform that coordinates the spatiotemporal interaction of signaling molecules.

Receptor tyrosine kinases participate in several signaling pathways through small G proteins such as Ras (rat sarcoma). An important component in the activation of these G proteins is Son of sevenless (SOS), which catalyzes the nucleotide exchange on Ras. For optimal activity, a second Ras molecule acts as an allosteric activator by binding to a second Ras-binding site within SOS. This allosteric Ras-binding site is blocked by autoinhibitory domains of SOS. We have reported recently that Ras activation also requires the actin-binding proteins ezrin, radixin, and moesin. Here we report the mechanism by which ezrin modulates SOS activity and thereby Ras activation. Active ezrin enhances Ras/MAPK signaling and interacts with both SOS and Ras in vivo and in vitro. Moreover, in vitro kinetic assays with recombinant proteins show that ezrin also is important for the activity of SOS itself. Ezrin interacts with GDP-Ras and with the Dbl homology (DH)/pleckstrin homology (PH) domains of SOS, bringing GDP-Ras to the proximity of the allosteric site of SOS. These actions of ezrin are antagonized by the neurofibromatosis type 2 tumor-suppressor protein merlin. We propose an additional essential step in SOS/Ras control that is relevant for human cancer as well as all physiological processes involving Ras.

Ligand-protein binding processes play an essential role in biological systems, be it signalling immune response or enzymatic activity. An experimentally well-studied system is the binding of cyclic adenosine monophosphate (cAMP) at the cyclic nucleotide binding domain (CNBD) of the bacterial potassium channel MloK1. The channel's conductivity is modulated by cAMP binding and is prototypical for cyclic nucleotide gated ion channels.Two models were postulated to describe the binding process: The first is a two-step model consisting of bulk-surface diffusion and surfing-binding site rolling. The second consists of a diffusion process into a binding funnel combined with stochastic barrier crossing.Here we present a comprehensive molecular dynamics study of the primary cAMP binding events, i.e. the ligands path to the binding site in the CNBD, to identify substates, establish a model of the ligand pathways and to predict an effective on-rate for the binding process. Available experimental high precision measurements of the binding kinetics render the system suitable for comparison.We identified multiple surface substates that play a crucial role for the dynamics for high ligand concentrations. Using rate estimates from the MD simulations and extrapolation to low, experimentally accessible ligand concentrations we ruled out the two-step models. We quantified the conformational confinement and the probability of binding barrier crossing to obtain values for the funnel model. The overall estimate for the on-rate rate constant was estimated to be 30/(µs∗mol/l), which is in excellent agreement with the experimental measurements.

Larry Cohen has been a great inspiration for our work; the indicators Arsenazo III and di-8-ANNEPS, introduced by Larry and his colleagues to biology,1,2 were crucial for the elucidation of photo- and chemotransduction pathways in photoreceptors and sperm cells, respectively. During the heydays of research on signaling in retinal photoreceptors in the 1980s, central questions concerned (1) the identity of the cellular messenger transducing the absorption of light into an electrical signal and (2) the nature of the ion channels gated by that messenger. For good reasons, both Ca2+ and cGMP had been proposed to carry the signal from the visual pigment rhodopsin in the disk membrane across the cytosol to the ion channels in the plasma membrane. In 1985, in one fell swoop, a series of papers identified in rod photoreceptors of amphibians and cow a conductance, which is directly gated by cGMP without involving phosphorylation by protein kinase G.3,4,5 We and others discovered a cGMP-induced Ca2+ efflux from isolated Ca2+-filled disks,4,6 suggesting that the cGMP-gated channel is also localized in the disk membrane. The release of Ca2+ from isolated disks was detected by the metallochromic Ca2+-indicator dye Arsenazo III. This dye, for the first time, was used by Brown et al.1 to measure minute Ca2+ changes in the squid giant axon evoked by changes in membrane voltage. Recordings from excised membrane patches or truncated rod outer segments revealed that the cGMP-gated channel is, in fact, located in the plasma membrane.3,5 It turned out that robust cytoskeletal filaments connect plasma and disk membranes;7 therefore, during permeabilization of photoreceptors and purification of their membrane fractions, plasma and disk membranes partially fuse. This process is prevented by mild trypsinization of cytoskeletal elements.8 Although the presence of the cGMP-gated channel in disks was a consequence of the isolation procedure, it allowed for the channel’s molecular identification, purification, and cloning of the gene. We characterized the cGMP-gated channel by functional reconstitution into artificial liposomes.9,10 Membrane proteins of rod outer segments were solubilized and separated by affinity and size-exclusion chromatography; protein fractions eluting from the column were reconstituted into liposomes and tested for channel activity using the Arsenazo III-based Ca2+ flux assay.9 This assay also allowed identifying the channel’s accessory 240 kD β-subunit11 and the first Na+/Ca2+ exchanger protein.12 Finally, partial amino-acid information derived from the purified 63 kD pore-forming α-subunit and the β-subunit paved the way to clone the genes of the CNG channel subunits and their functional expression in heterologous cell systems.13,14 It came as a surprise that relatives of these CNG channels are essential components of chemotactic signaling in sperm.15,16,17 Stimulation of sea urchin sperm with chemoattractants, short peptides, causes a hyperpolarization followed by a transient Ca2+ response. Electrical recording from mouse and human sperm had been established,18,19,20 whereas direct probing of electrical events in sea urchin sperm by the patch-clamp technique was unsuccessful. Instead, fluorescent potentiometric probes had been employed to record changes in membrane voltage. Owing to their “Nernstian mechanism” of voltage sensing, the dyes initially used for sperm studies exhibit slow response times and, moreover, are prone to solvatochromic artefacts.21 We sought the advice of Larry Cohen, who recommended di-8-ANEPPS, a dye that exclusively intercalates into the plasma membrane and exhibits a rapid response at the millisecond time scale. Its electrochromic mechanism of voltage sensing allows ratiometric measurement of ΔVm and, thereby, overcomes potential artefacts resulting from unspecific solvatochromic effects.17 The necessity of ratiometric measurements is illustrated in Fig. 1. Di-8-ANEPPS faithfully and, most importantly, quantitatively reports ΔVm in intact, freely moving sperm; optoelectrical recordings from sperm revealed that stimulation with a single chemoattractant molecule evokes a ΔVm of about 2.5 mV,17 similar to the single-photon response of rod photoreceptors. The dye also allows using flash photolysis of caged compounds that exhibit absorption spectra around 400 nm.15,17 Competing dyes like the Annine collection of potentiometric probes require excitation wavelengths

In human sperm, CatSper (cation channel of sperm) Ca2+ channels control the intracellular Ca2+ concentration ([Ca2+]i) and, thereby, the swimming behaviour. By patch-clamp recordings from human sperm and kinetic Ca2+ fluorimetry, we have shown that CatSper is directly activated by the female sex hormone progesterone; cells surrounding the oocyte release progesterone into the oviduct to assist sperm for fertilization. The rapid Ca2+ influx evoked by progesterone has been implicated in sperm chemotaxis, hyperactivation, and acrosomal exocytosis. We studied progesterone-evoked voltage responses in human sperm with the kinetic stopped-flow technique, using electrochromic voltage-sensitive dyes. We show that progesterone evokes instantaneous changes in membrane voltage (Vm) governed by activation of CatSper and another ion channel present in human sperm. We elucidated the molecular identity of this channel by patch-clamp recordings from human sperm.

Many cells and microorganisms have evolved a motility apparatus to explore their surroundings. For guidance, these biological microswimmers rely on physical and chemical cues that are transduced by cellular pathways into directed movement – a process called taxis. Only few biological microswimmers have been studied as detailed as sperm from sea urchins. Sperm and eggs are released into the seawater. To enhance the chances of fertilization, eggs release chemical factors – called chemoattractants – that establish a chemical gradient and, thereby, guide sperm to the egg. Sea urchin sperm constitute a unique model system for understanding cell navigation at every level: from molecules to cell behaviours. We will outline the chemotactic signalling pathway of sperm from the sea urchin Arbacia punctulata and discuss how signalling controls navigation in a chemical gradient. Finally, we discuss recent insights into sperm chemotaxis in three dimensions (3D).