Sperm motility-initiating substance in newt egg-jelly induces differential initiation of sperm motility based on sperm intracellular calcium levels

A CatSper-Uninvolved Mechanism to Induce Forward Sperm Motility in the Internal Fertilization

Sperm-specific cation channel (CatSper), sperm-specific Na + /H + exchanger (sNHE), and soluble adenylyl cyclase (sAC) are necessary in the signaling pathways to control sperm motility in many animals, whereas some animals have lost some or all of them. In the present study, we examined CatSper-uninvolved signaling for vigorous undulation of the undulating membrane that is attached to the sperm tail and gives thrust for forward motility in the internally fertilizing newt Cynops pyrrhogaster. Reverse-transcription PCR failed to detect sNHE in the newt sperm. However, the pH of sperm cytoplasm was raised under a high extracellular pH equivalent to that of egg jelly, where sperm motility is initiated by sperm motility-initiating substance (SMIS). Carbonic anhydrase XII/ XVI and SLC4A4/8 were suggested to be present in the sperm, and transported bicarbonates raised the intracellular pH. In egg jelly extract that contained SMIS, the anion transporter inhibitor DIDS weakened the undulation of the undulating membrane, while bicarbonates enhanced it. The cyclic AMP concentration was found to increase in sperm cytoplasm in the egg-jelly extract. An inhibitor of sAC (KH7) weakened the undulation of the undulating membrane, and dibutyryl cyclic AMP blocked the inhibitory effect. Inhibitor of transmembrane AC (DDA) limitedly affected the undulation. The undulation was weakened by an inhibitor of protein kinase A (H89), and by an inhibitor of transient receptor potential (TRP) channels (RN1747). Our results support the conclusions that the high pH of the egg jelly triggers a signaling pathway through sAC, PKA, and TRP channels, and coacts with SMIS to induce forward sperm motility.

Cryopreservation Cooling Rate Impacts Post-Thaw Sperm Motility and Survival in Litoria booroolongensis

The cryopreservation and storage of gametes (biobanking) can provide a long-term, low-cost option for the preservation of population genetic diversity and is particularly impactful when applied to manage selective breeding within conservation breeding programs (CBPs). This study aimed to develop a sperm cryopreservation protocol for the critically endangered Booroolong frog (Litoria booroolongensis) to capture founder genetics within the recently established (est. 2019) CBP for this species. Hormone-induced sperm release was achieved using established protocols, and spermic urine samples were collected over a 6-h period. Pooled spermic urine samples (n = 3 males) were divided equally between two cryoprotectant (CPA) treatments and diluted by 1:5 (sperm:CPA) with either 15% (v/v) dimethyl sulfoxide + 1% (w/v) sucrose in simplified amphibian Ringer's (SAR; CPAA) or 10% (v/v) dimethylformamide + 10% (w/v) trehalose dihydrate in SAR (CPAB). The samples were cryopreserved in 0.25 mL straws using either a programmable freezer (FrA) or an adapted dry shipper method (FrB). The thawed samples were activated via dilution in water and assessed for viability and motility using both manual assessment and computer-assisted sperm analysis (CASA; 0 h, 0.5 h post-thaw). Upon activation, the survival and recovery of motility (total motility, forward progression and velocity) of cryopreserved sperm suspensions were higher for sperm preserved using FrB than FrA, regardless of CPA composition. This work supports our long-term goal to pioneer the integration of biobanked cryopreserved sperm with population genetic management to maximize restoration program outcomes for Australian amphibian species.

Sperm Motility Regulation in Male and Female Bird Genital Tracts

Sperm drastically change their flagellar movement in response to the surrounding physical and chemical environment. Testicular sperm are immotile; however, they gain the competence to initiate motility during passage through the male reproductive tract. Once ejaculated, the sperm are activated and promptly initiate motility. Unlike mammals, ejaculated sperm in birds are stored in specialized tubular invaginations referred to as sperm storage tubules (SSTs), located between the vagina and uterus, before fertilization. The resident sperm in the SSTs are in a quiescent state and then re-activated after release from the SSTs. It is thought that avian sperm can undergo motility change from quiescent to active state twice; however, the molecular mechanism underlying sperm motility regulation is poorly understood. In this short review, we summarize the current understanding of sperm motility regulation in male and female bird reproductive tracts. We also describe signal transduction, which regulates sperm motility, mainly derived from in vitro studies.

Structure and Function of Ion Channels Regulating Sperm Motility-An Overview

Sperm motility is linked to the activation of signaling pathways that trigger movement. These pathways are mainly dependent on Ca²⁺, which acts as a secondary messenger. The maintenance of adequate Ca²⁺ concentrations is possible thanks to proper concentrations of other ions, such as K⁺ and Na⁺, among others, that modulate plasma membrane potential and the intracellular pH. Like in every cell, ion homeostasis in spermatozoa is ensured by a vast spectrum of ion channels supported by the work of ion pumps and transporters. To achieve success in fertilization, sperm ion channels have to be sensitive to various external and internal factors. This sensitivity is provided by specific channel structures. In addition, novel sperm-specific channels or isoforms have been found with compositions that increase the chance of fertilization. Notably, the most significant sperm ion channel is the cation channel of sperm (CatSper), which is a sperm-specific Ca²⁺ channel required for the hyperactivation of sperm motility. The role of other ion channels in the spermatozoa, such as voltage-gated Ca²⁺ channels (VGCCs), Ca²⁺-activated Cl-channels (CaCCs), SLO K⁺ channels or voltage-gated H⁺ channels (VGHCs), is to ensure the activation and modulation of CatSper. As the activation of sperm motility differs among metazoa, different ion channels may participate; however, knowledge regarding these channels is still scarce. In the present review, the roles and structures of the most important known ion channels are described in regard to regulation of sperm motility in animals.

NMDA-type glutamate receptors mediate the acrosome reaction and motility initiation in newt sperm

The N-methyl d-aspartate type glutamate receptor (NMDAR) is a ligand-gated cation channel that causes Ca²⁺ influx in nerve cells. An NMDAR agonist is effective to the sperm motility in fowls, although the actual role of NMDAR in sperm function is unknown. In the present study, RNA-seq of the spermatogenic testes suggested the presence of NMDAR in the sperm of the newt Cynops pyrrhogaster. Glutamate of at least 0.7 ± 0.5 mM was detected in the egg-jelly substances along with acrosome reaction-inducing substance (ARIS) and sperm motility-initiating substance (SMIS). In the egg-jelly extract (JE) that included the ARIS and SMIS, the acrosome reaction was inhibited by a NMDAR antagonists, memantine and MK801. MK801 also inhibited the spontaneous acrosome reaction in Steinberg's salt solution (ST). Furthermore, memantine and MK801 suppressed the progressive motility of the sperm in JE and spontaneous waving of the undulating membrane, which is the tail structure giving thrust for forward motility, in ST. The spontaneous waving of the undulating membrane was promoted when Mg²⁺ , which blocks Ca²⁺ influx through gated NMDARs, was removed from the ST. In addition, the ARIS-induced acrosome reaction was inhibited by a selective antagonist of the transient receptor potential vanilloid 4, whose activation might result in the membrane depolarization to release Mg²⁺ from the NMDAR. These results suggest that NMDAR acts together with other cation channels in the induction of the acrosome reaction and motility of the sperm during the fertilization process of C. pyrrhogaster.

Culex pipiens sperm motility is initiated by a trypsin-like protease from male accessory glands

In most animals, sperm are stored in a quiescent state in the male reproductive tract and only initiate motility when released into either the female reproductive tract, or, in the case of broadcast spawners, the external environment. Male accessory gland secretions transferred into the female reproductive tract may provide factors that modulate sperm viability and storage, or aid in sperm competition, as well as activate sperm motility. In several insects, serine proteases have been implicated in activating sperm motility. Our previous studies have shown that, in Culex quinquefasciatus, either a male accessory gland extract or purified trypsin is sufficient to initiate sperm motility in vitro. The objective of this study was to identify and characterize trypsin-like enzymes produced in the Culex male accessory glands. Mass spectrometry was used to analyze accessory gland proteins and this preliminary proteomic analysis identified 4 trypsin-like proteases (trypsin, trypsin4, and two trypsin7 isoforms). When measured with the chromogenic trypsin substrate N_a -benzoyl-L-arginine-ethyl-ester-hydrochloride (BAEE), trypsin-like protease activity in the accessory glands was robust, with a pH optimum of 8. The pH range for the Culex trypsin activity was substantially narrower than a mammalian homologue (porcine pancreatic trypsin). A soybean trypsin inhibitor (SBTI) -agarose affinity column was used to independently identify trypsin-like accessory gland proteins. Several proteins were enriched in the eluate, as detected by silver staining of SDS-PAGE gels. Taken together, these data demonstrate the presence of trypsin-like activity and several trypsin-like proteins in the Culex male accessory glands.

Contribution of different Ca2+ channels to the acrosome reaction-mediated initiation of sperm motility in the newt Cynops pyrrhogaster

Initiation of sperm motility in urodeles, which is induced by a sperm motility-initiating substance (SMIS) in the sequestered granules on the surface of egg jelly, is mediated by the acrosome reaction (AR), which is triggered by an AR-inducing substance (ARIS) on a sheet-like structure. Details of the unique process of the interaction between egg jelly and sperm in these species is still unclear. The current study showed the fine structure of egg jelly in the newt Cynops pyrrhogaster, a urodele species, revealing that its outer surface was covered by a sheet-like structure of approximately 0.29 μm in thickness. Granules of approximately 2 μm in diameter with small particles of approximately 54 nm were attached to its surface and distributed inhomogeneously just beneath the sheet-like structure. Emission spectrometry revealed that the Ca2+ concentration was maintained at a high level compared with that of the blood plasma and the vas deferens fluid, suggesting that egg jelly is a reliable source of Ca2+ for the sperm–egg interaction. Blockers of the T-type voltage-dependent Ca2+ channel (VDCC), but not the L-type VDCC, inhibited both AR and initiation of sperm motility. Conversely, Ni+, which affects the α1 H subunit of T-type VDCC, only inhibited the initiation of sperm motility. These data suggest that, in response to ARIS and SMIS, sequential gating of distinct Ca2+ channels occurs in the AR, followed by the initiation of sperm motility on the surface of the egg jelly in C. pyrrhogaster at fertilization.

Waveform generation is controlled by phosphorylation and swimming direction is controlled by Ca2+ in sperm from the mosquito Culex quinquefasciatus

Most animal sperm are quiescent in the male reproductive tract and become activated after mixing with accessory secretions from the male and/or female reproductive tract. Sperm from the mosquito Culex quinquefasciatus initiate flagellar motility after mixing with male accessory gland components, and the sperm flagellum displays three distinct motility patterns over time: a low amplitude, a long wavelength form (Wave A), a double waveform consisting of two superimposed waveforms over the length of the flagellum (Wave B), and finally, a single helical waveform that propels the sperm at high velocity (Wave C). This flagellar behavior is replicated by treating quiescent sperm with trypsin. When exposed to either broad spectrum or tyrosine kinase inhibitors, sperm activated by accessory gland secretions exhibited motility through Wave B but were unable to progress to Wave C. The MEK1/2 inhibitor UO126 and the ERK1/2 inhibitor FR180204 each blocked the transition from Wave B to Wave C, indicating a role for MAPK activity in the control of waveform and, accordingly, progressive movement. Furthermore, a MAPK substrate antibody stained the flagellum of activated sperm. In the absence of extracellular Ca(2+), a small fraction of sperm swam backwards, whereas most could not be activated by either accessory glands or trypsin and were immotile. However, the phosphatase inhibitor okadaic acid in the absence of extracellular Ca(2+) induced all sperm to swim backwards with a flagellar waveform similar to Wave A. These results indicate that flagellar waveform generation and direction of motility are controlled by protein phosphorylation and Ca(2+) levels, respectively.

Molecular genetic system for regenerative studies using newts

Urodele newts have the remarkable capability of organ regeneration, and have been used as a unique experimental model for more than a century. However, the mechanisms underlying regulation of the regeneration are not well understood, and gene functions in particular remain largely unknown. To elucidate gene function in regeneration, molecular genetic analyses are very powerful. In particular, it is important to establish transgenic or knockout (mutant) lines, and systematically cross these lines to study the functions of the genes. In fact, such systems have been developed for other vertebrate models. However, there is currently no experimental model system using molecular genetics for newt regenerative research due to difficulties with respect to breeding newts in the laboratory. Here, we show that the Iberian ribbed newt (Pleurodeles waltl) has outstanding properties as a laboratory newt. We developed conditions under which we can obtain a sufficient number and quality of eggs throughout the year, and shortened the period required for sexual maturation from 18 months to 6 months. In addition, P. waltl newts are known for their ability, like other newts, to regenerate various tissues. We revealed that their ability to regenerate various organs is equivalent to that of Japanese common newts. We also developed a method for efficient transgenesis. These studies demonstrate that P. waltl newts are a suitable model animal for analysis of regeneration using molecular genetics. Establishment of this experimental model will enable us to perform comparable studies using these newts and other vertebrate models.