Cyclic Nucleotide-Specific Optogenetics Highlights Compartmentalization of the Sperm Flagellum into cAMP Microdomains.

Inside the female genital tract, mammalian sperm undergo a maturation process called capacitation, which primes the sperm to navigate across the oviduct and fertilize the egg. Sperm capacitation and motility are controlled by 3′,5′-cyclic adenosine monophosphate (cAMP). Here, we show that optogenetics, the control of cellular signaling by genetically encoded light-activated proteins, allows to manipulate cAMP dynamics in sperm flagella and, thereby, sperm capacitation and motility by light. To this end, we used sperm that express the light-activated phosphodiesterase LAPD or the photo-activated adenylate cyclase bPAC. The control of cAMP by LAPD or bPAC combined with pharmacological interventions provides spatiotemporal precision and allows to probe the physiological function of cAMP compartmentalization in mammalian sperm

Rotational motion and rheotaxis of human sperm do not require functional CatSper channels and transmembrane Ca2+ signaling.

Navigation of sperm in fluid flow, called rheotaxis, provides long-range guidance in the mammalian oviduct. The rotation of sperm around their longitudinal axis (rolling) promotes rheotaxis. Whether sperm rolling and rheotaxis require calcium (Ca2+ ) influx via the sperm-specific Ca2+ channel CatSper, or rather represent passive biomechanical and hydrodynamic processes, has remained controversial. Here, we study the swimming behavior of sperm from healthy donors and from infertile patients that lack functional CatSper channels, using dark-field microscopy, optical tweezers, and microfluidics. We demonstrate that rolling and rheotaxis persist in CatSper-deficient human sperm. Furthermore, human sperm undergo rolling and rheotaxis even when Ca2+ influx is prevented. Finally, we show that rolling and rheotaxis also persist in mouse sperm deficient in both CatSper and flagellar Ca2+ -signaling domains. Our results strongly support the concept that passive biomechanical and hydrodynamic processes enable sperm rolling and rheotaxis, rather than calcium signaling mediated by CatSper or other mechanisms controlling transmembrane Ca2+ flux

Involvement of Complexin 2 in Docking, Locking and Unlocking of Different SNARE Complexes during Sperm Capacitation and Induced Acrosomal Exocytosis

Acrosomal exocytosis (AE) is an intracellular multipoint fusion reaction of the sperm plasma membrane (PM) with the outer acrosomal membrane (OAM). This unique exocytotic event enables the penetration of the sperm through the zona pellucida of the oocyte. We previously observed a stable docking of OAM to the PM brought about by the formation of the trans-SNARE complex (syntaxin 1B, SNAP 23 and VAMP 3). By using electron microscopy, immunochemistry and immunofluorescence techniques in combination with functional studies and proteomic approaches, we here demonstrate that calcium ionophore-induced AE results in the formation of unilamellar hybrid membrane vesicles containing a mixture of components originating from the two fused membranes. These mixed vesicles (MV) do not contain the earlier reported trimeric SNARE complex but instead possess a novel trimeric SNARE complex that contained syntaxin 3, SNAP 23 and VAMP 2, with an additional SNARE interacting protein, complexin 2. Our data indicate that the earlier reported raft and capacitation-dependent docking phenomenon between the PM and OAM allows a specific rearrangement of molecules between the two docked membranes and is involved in (1) recruiting SNAREs and complexin 2 in the newly formed lipid-ordered microdomains, (2) the assembly of a fusion-driving SNARE complex which executes Ca2+-dependent AE, (3) the disassembly of the earlier reported docking SNARE complex, (4) the recruitment of secondary zona binding proteins at the zona interacting sperm surface. The possibility to study separate and dynamic interactions between SNARE proteins, complexin and Ca2+ which are all involved in AE make sperm an ideal model for studying exocytosis

Behavioral Mechanism during Human Sperm Chemotaxis: Involvement of Hyperactivation

When mammalian spermatozoa become capacitated they acquire, among other activities, chemotactic responsiveness and the ability to exhibit occasional events of hyperactivated motility—a vigorous motility type with large amplitudes of head displacement. Although a number of roles have been proposed for this type of motility, its function is still obscure. Here we provide evidence suggesting that hyperactivation is part of the chemotactic response. By analyzing tracks of spermatozoa swimming in a spatial chemoattractant gradient we demonstrate that, in such a gradient, the level of hyperactivation events is significantly lower than in proper controls. This suggests that upon sensing an increase in the chemoattractant concentration capacitated cells repress their hyperactivation events and thus maintain their course of swimming toward the chemoattractant. Furthermore, in response to a temporal concentration jump achieved by photorelease of the chemoattractant progesterone from its caged form, the responsive cells exhibited a delayed turn, often accompanied by hyperactivation events or an even more intense response in the form of flagellar arrest. This study suggests that the function of hyperactivation is to cause a rather sharp turn during the chemotactic response of capacitated cells so as to assist them to reorient according to the chemoattractant gradient. On the basis of these results a model for the behavior of spermatozoa responding to a spatial chemoattractant gradient is proposed

Larry Cohen—50 ways to DYE your science

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.13,1