Hamster spermatozoa incorporate hypotaurine via TauT for self-protection.

In brief: Mammalian spermatozoa actively generate reactive oxygen species (ROS) during capacitation, a maturational process necessary for fertilization in vivo. This study shows that hypotaurine, a precursor of taurine present in the oviduct, is incorporated and concentrated in hamster sperm cells via the taurine transporter, TauT, for cytoprotection against self-produced ROS. Abstract: To achieve fertilization competence, mammalian spermatozoa undergo capacitation, during which they actively generate reactive oxygen species (ROS). Therefore, mammalian spermatozoa must protect themselves from these self-generated ROS. The mammalian oviductal fluid is rich in hypotaurine, a taurine precursor, which reportedly protects mammalian spermatozoa, including those of hamsters, from ROS; however, its precise mechanism remains unknown. This study aimed to elucidate the mechanism underlying hypotaurine-mediated protection of spermatozoa from ROS using hamsters, particularly focusing on the taurine/hypotaurine transporter TauT. The effect of hypotaurine on sperm motility and ROS levels was tested using sperm motility analysis and the CellROX dye and luminol assays. RNA sequencing analysis was performed to verify TauT expression. We found that hypotaurine was necessary for maintaining sperm motility and hyperactivated motility. Hypotaurine did not scavenge extracellular ROS but lowered intracellular ROS levels and was incorporated and concentrated in hamster spermatozoa. TauT was detected at both mRNA and protein levels. ß-Alanine blocked hypotaurine transport, increased intracellular ROS levels, and inhibited hyperactivation. Elimination of Na+ or Cl- ions inhibited hypotaurine transport and increased intracellular ROS levels. Thus, these results indicated that hamster spermatozoa incorporated and concentrated hypotaurine in sperm cells via TauT to protect themselves from self-generated ROS.

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.

Identification and characterization of sperm motility-initiating substance-2 gene in internally fertilizing Cynops species.

Sperm motility-initiating substance (SMIS) is an oviductal protein critical for internal fertilization in urodeles. It contributes to the establishment of various reproductive modes in amphibians and is thus a unique research model for the gene evolution of gamete-recognizing ligands that have diversified among animal species. In this study, a paralogous SMIS gene, smis2, was identified via the RNA sequencing of the oviduct of the newt, Cynops pyrrhogaster. The base sequence of the smis2 gene was homologous (˃90%) to that of the original smis gene (smis1), and deduced amino acid sequences of both genes conserved six cysteine residues essential for the cysteine knot motif. Furthermore, smis2 complementary DNA was identified in the oviduct of Cynops ensicauda, and the base substitution patterns also suggested that the smis gene was duplicated in the Salamandridae. Nonsynonymous/synonymous substitution ratios of smis1 and smis2 genes were 0.79 and 2.6, respectively, suggesting that smis2 gene evolution was independently driven by positive selection. Amino acid substitutions were concentrated in the cysteine knot motif of SMIS2. The smis2 gene was expressed in some organs in addition to the oviduct; in contrast, SMIS1 was only expressed in the oviduct. The SMIS2 protein was suggested to be produced and secreted at least in the oviduct and redundantly act in sperm. These results suggest that smis1 plays the original role in the oviduct, whereas smis2 may undergo neofunctionalization, which rarely occurs in gene evolution.

Hamster Sperm Possess Functional Na+/Ca2+-Exchanger 1: Its Implication in Hyperactivation.

Previous studies demonstrated that hamster sperm hyperactivation is suppressed by extracellular Na+ by lowering intracellular Ca2+ levels, and Na+/Ca2+-exchanger (NCX) specific inhibitors canceled the suppressive effects of extracellular Na+. These results suggest the involvement of NCX in the regulation of hyperactivation. However, direct evidence of the presence and functionality of NCX in hamster spermatozoa is still lacking. This study aimed to reveal that NCX is present and is functional in hamster spermatozoa. First, NCX1 and NCX2 transcripts were detected via RNA-seq analyses of hamster testis mRNAs, but only the NCX1 protein was detected. Next, NCX activity was determined by measuring the Na+-dependent Ca2+ influx using the Ca2+ indicator Fura-2. The Na+-dependent Ca2+ influx was detected in hamster spermatozoa, notably in the tail region. The Na+-dependent Ca2+ influx was inhibited by the NCX inhibitor SEA0400 at NCX1-specific concentrations. NCX1 activity was reduced after 3 h of incubation in capacitating conditions. These results, together with authors' previous study, showed that hamster spermatozoa possesses functional NCX1 and that its activity was downregulated upon capacitation to trigger hyperactivation. This is the first study to successfully reveal the presence of NCX1 and its physiological function as a hyperactivation brake.

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.

Sperm motility initiating substance may be insufficient to induce forward motility of Cynops ensicauda sperm.

Sperm motility-initiating substance (SMIS) is a key protein for internal fertilization of the newt, Cynops pyrrhogaster, and commonly enhances forward sperm motility in some amphibian species, including external fertilizers. SMIS action varies among different species in correlation with a species-specific reproductive environment. In the present study, we identified the gene of C. ensicauda SMIS (CeSMIS) and examined the mechanism of SMIS action with reference to that of the closely related Cynops species. The CeSMIS was identified by a 176-amino acid sequence including seven amino acids critical for the initiation of sperm motility. The amino acid sequence showed 91% homology to the whole sequence of C. pyrrhogaster SMIS (CpSMIS). By immunostaining with an anti-CpSMIS antibody, CeSMIS was shown to be localized in the outer layer of the egg jelly. A peptide presenting the active site of SMIS was observed to bind to the axial rod of the midpiece in C. ensicauda sperm. The localization and binding patterns of CeSMIS were fundamentally similar to those of CpSMIS. However, the SMIS peptide did not induce forward motility of C. ensicauda sperm, although it induced a fast wave of the undulating membrane. Forward sperm motility was induced in the egg jelly extract containing CeSMIS. These results suggest that the mechanism of initiation of sperm motility is differentiated between C. ensicauda and C. pyrrhogaster.

Sperm storage influences the potential for spontaneous acrosome reaction of the sperm in the newt Cynops pyrrhogaster.

Sperm storage is supposed to influence sperm quality, although the details remain unclear. In the present study, we found that sperm stored in a sperm storage site, the vas deferens of Cynops pyrrhogaster, spontaneously undergo acrosome reaction following incubation in Steinberg's salt solution (ST). Percentages of acrosome-reacted sperm increased time-dependently to about 60% in 24 hr. The concentration of cyclic adenosine monophosphate (cAMP) was elevated after incubating sperm in ST, while dibutylyl cAMP induced an acrosome reaction. Chelating of extracellular Ca2+ suppressed the dibutylyl cAMP-induced acrosome reaction as well as spontaneous acrosome reaction in ST. These results suggest that cAMP elevation driven by Ca2+ influx can be a cue for spontaneous acrosome reaction. Relatively low Ca2+ concentration and pH in the vas deferens were sufficient to suppress spontaneous acrosome reaction within 1 hr. In addition, the cysteine rich secretory protein 2 gene was expressed in the vas deferens, indicating that it may be involved in the continuous suppression of spontaneous acrosome reaction. Sperm that underwent spontaneous acrosome reaction in ST was significantly increased when stored in the vas deferens for longer periods, or by males experiencing temperatures in excess of 12°C during hibernation conditions. Percentages of the spontaneously acrosome-reacted sperm were found to differ among males even though they were of identical genetic background. Taken together, C. pyrrhogaster sperm possess the potential for spontaneous acrosome reaction that does not become obvious in the vas deferens, unless promoted in correlation with sperm storage.

Expression of Two Classes of Pax6 Transcripts in Reprogramming Retinal Pigment Epithelium Cells of the Adult Newt.

The adult newt has the remarkable ability to regenerate a functional retina from retinal pigment epithelium (RPE) cells, even when the neural retina (NR) is completely lost from the eye. In this system, RPE cells are reprogrammed into a unique state of multipotent cells, named RPESCs, in an early phase of retinal regeneration. However, the signals that trigger reprogramming remain unknown. Here, to approach this issue we focused on Pax6, a transcription factor known to be expressed in RPESCs. We first identified four classes (v1, v2, v3 and v4) of Pax6 variants in the eye of adult newt, Cynops pyrrhogaster. These variants were expressed in most tissues of the intact eye in different combinations but not in the RPE, choroid or sclera. On the basis of this information, we investigated the expression of Pax6 in RPE cells after the NR was removed from the eye by surgery (retinectomy), and found that two classes (v1 and v2) of Pax6 variants were newly expressed in RPE cells 10 days after retinectomy, both in vivo and in vitro (RLEC system). In the RLEC system, we found that Pax6 expression is mediated through a pathway separate from the MEK-ERK pathway, which is required for cell cycle re-entry of RPE cells. These results predict the existence of a pathway that may be of fundamental importance to a better understanding of the reprogramming of RPE cells in vivo.

Skin Wound Healing of the Adult Newt, Cynops pyrrhogaster: A Unique Re-Epithelialization and Scarless Model.

In surgical and cosmetic studies, scarless regeneration is an ideal method to heal skin wounds. To study the technologies that enable scarless skin wound healing in medicine, animal models are useful. However, four-limbed vertebrates, including humans, generally lose their competency of scarless regeneration as they transit to their terrestrial life-stages through metamorphosis, hatching or birth. Therefore, animals that serve as a model for postnatal humans must be an exception to this rule, such as the newt. Here, we evaluated the adult newt in detail for the first time. Using a Japanese fire-bellied newt, Cynops pyrrhogaster, we excised the full-thickness skin at various locations on the body, and surveyed their re-epithelialization, granulation or dermal fibrosis, and recovery of texture and appendages as well as color (hue, tone and pattern) for more than two years. We found that the skin of adult newts eventually regenerated exceptionally well through unique processes of re-epithelialization and the absence of fibrotic scar formation, except for the dorsal-lateral to ventral skin whose unique color patterns never recovered. Color pattern is species-specific. Consequently, the adult C. pyrrhogaster provides an ideal model system for studies aimed at perfect skin wound healing and regeneration in postnatal humans.

Acrosome reaction-inducing substance triggers two different pathways of sperm intracellular signaling in newt fertilization.

The acrosome reaction is induced in the sperm of Cynops pyrrhogaster immediately in response to a ligand protein called acrosome reaction-inducing substance (ARIS) in the egg jelly at fertilization, whereas a spontaneous acrosome reaction occurs time-dependently in correlation with the decline of sperm quality for fertilization. The ARIS-induced acrosome reaction was recently found to be mediated by TRPV4 in association with the NMDA type glutamate receptor, although the intracellular mediators for the acrosome reaction are largely unclear. In the present study, spontaneous acrosome reaction was significantly inhibited by Ni2+, RN1734, and diltiazem, which blocks Cav3.2, TRPV4 or TRPM8, and the cyclic nucleotide-gated channel, respectively. In contrast, expression of Ca2+-activated transmembrane and soluble adenylyl cyclases was detected in the sperm of C. pyrrhogaster by reverse transcription-polymerase chain reaction. Activator of transmembrane or soluble adenylyl cyclases (forskolin or HCO 3-) independently promoted spontaneous acrosome reaction, while an inhibitor of each enzyme (MD12330A or KH7) inhibited it only in the sperm with high potential for spontaneous acrosome reaction. An inhibitor of protein kinase A (H89) inhibited spontaneous acrosome reaction in a manner independent of sperm potential for spontaneous acrosome reaction. Surprisingly, KH7 significantly inhibited ARIS-induced acrosome reaction, but its effect was seen in a small percentage of sperm. H89 had no effect on ARIS-induced acrosome reaction. These results suggest that C. pyrrhogaster sperm possess multiple intracellular pathways for acrosome reaction, involving Ca2+ permeable channels, adenylyl cyclases and PKA, and that two pathways having distinct dependencies on adenylyl cyclases may contribute to ARIS-induced acrosome reaction at fertilization.

Novel erythrocyte clumps revealed by an orphan gene Newtic1 in circulating blood and regenerating limbs of the adult newt.

The newt, a group of urodele amphibians, has outstanding ability to repeatedly regenerate various body parts, even in the terrestrial life-stage. In this animal, when the limb is amputated, a cell mass named the blastema appears on the stump and eventually gives rise to a new functional limb. Erythrocytes (red blood cells) in most non-mammalian vertebrates, including the newt, preserve their nucleus throughout their life-span, although physiological roles of such nucleated erythrocytes, other than oxygen delivery, are not known. Here we report novel behavior of erythrocytes in the newt. We identified an orphan gene Newtic1, whose transcripts significantly increased in the blastema. Newtic1 was expressed in a subset of erythrocytes that formed a novel clump (EryC). EryC formed a complex with monocytes and was circulating throughout the body. When the limb was amputated, EryCs were newly generated in the stump and accumulated into a distal portion of the growing blastema. Our data suggested that the newt erythrocytes carried multiple secretory molecules including growth factors and matrix metalloproteases, and were capable of delivering these molecules into the blastema as a form of EryCs. This study provides insight into regulations and roles of nucleated erythrocytes, that are independent of oxygen delivery.

Implications of a Multi-Step Trigger of Retinal Regeneration in the Adult Newt.

The newt is an amazing four-limbed vertebrate that can regenerate various body parts including the retina. In this animal, when the neural retina (NR) is removed from the eye by surgery (retinectomy), both the NR and the retinal pigment epithelium (RPE) eventually regenerate through the process of reprogramming and proliferation of RPE cells. Thus far, we have pursued the onset mechanism of adult newt retinal regeneration. In this study, using an in vitro system, we found that both mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK)-ERK and ß-catenin were involved in cell cycle re-entry of RPE cells. MEK-ERK signaling activity in RPE cells was strengthened by retinectomy, and nuclear translocation of ß-catenin in RPE cells was induced by attenuation of cell-cell contact, which was promoted by incision of the RPE or its treatment with ethylene glycol tetraacetic acid (EGTA). EGTA is a Ca2+ chelator that disrupts cadherin-mediated cell-cell adhesion. Reinforcement of MEK-ERK signaling activity was a prerequisite for nuclear translocation of ß-catenin. These results suggest that retinectomy followed by attenuation of cell-cell contact may trigger cell cycle re-entry of RPE cells. This study, together with our previous findings concerning the proliferation and multipotency of adult newt RPE cells, provides insight into the mechanism of the multi-step trigger in which the onset of retinal regeneration in the adult newt is rigorously controlled.

Turning the fate of reprogramming cells from retinal disorder to regeneration by Pax6 in newts.

The newt, a urodele amphibian, has an outstanding ability- even as an adult -to regenerate a functional retina through reprogramming and proliferation of the retinal pigment epithelium (RPE) cells, even though the neural retina is completely removed from the eye by surgery. It remains unknown how the newt invented such a superior mechanism. Here we show that disability of RPE cells to regenerate the retina brings about a symptom of proliferative vitreoretinopathy (PVR), even in the newt. When Pax6, a transcription factor that is re-expressed in reprogramming RPE cells, is knocked down in transgenic juvenile newts, these cells proliferate but eventually give rise to cell aggregates that uniformly express alpha smooth muscle actin, Vimentin and N-cadherin, the markers of myofibroblasts which are a major component of the sub-/epi-retinal membranes in PVR. Our current study demonstrates that Pax6 is an essential factor that directs the fate of reprogramming RPE cells toward the retinal regeneration. The newt may have evolved the ability of retinal regeneration by modifying a mechanism that underlies the RPE-mediated retinal disorders.

A robust transcriptional program in newts undergoing multiple events of lens regeneration throughout their lifespan.

Newts have the ability to repeatedly regenerate their lens even during ageing. However, it is unclear whether this regeneration reflects an undisturbed genetic activity. To answer this question, we compared the transcriptomes of lenses, irises and tails from aged newts that had undergone lens regeneration 19 times with the equivalent tissues from young newts that had never experienced lens regeneration. Our analysis indicates that repeatedly regenerated lenses showed a robust transcriptional program comparable to young never-regenerated lenses. In contrast, the tail, which was never regenerated, showed gene expression signatures of ageing. Our analysis strongly suggests that, with respect to gene expression, the regenerated lenses have not deviated from a robust transcriptional program even after multiple events of regeneration throughout the life of the newt. In addition, our study provides a new paradigm in biology, and establishes the newt as a key model for the study of regeneration in relation to ageing.

A transcriptome for the study of early processes of retinal regeneration in the adult newt, Cynops pyrrhogaster.

Retinal regeneration in the adult newt is a useful system to uncover essential mechanisms underlying the regeneration of body parts of this animal as well as to find clues to treat retinal disorders such as proliferative vitreoretinopathy. Here, to facilitate the study of early processes of retinal regeneration, we provide a de novo assembly transcriptome and inferred proteome of the Japanese fire bellied newt (Cynops pyrrhogaster), which was obtained from eyeball samples of day 0-14 after surgical removal of the lens and neural retina. This transcriptome (237,120 in silico transcripts) contains most information of cDNAs/ESTs which has been reported in newts (C. pyrrhogaster, Pleurodeles waltl and Notophthalmus viridescence) thus far. On the other hand, de novo assembly transcriptomes reported lately for N. viridescence only covered 16-31% of this transcriptome, suggesting that most constituents of this transcriptome are specific to the regenerating eye tissues of C. pyrrhogaster. A total of 87,102 in silico transcripts of this transcriptome were functionally annotated. Coding sequence prediction in combination with functional annotation revealed that 76,968 in silico transcripts encode protein/peptides recorded in public databases so far, whereas 17,316 might be unique. qPCR and Sanger sequencing demonstrated that this transcriptome contains much information pertaining to genes that are regulated in association with cell reprogramming, cell-cycle re-entry/proliferation, and tissue patterning in an early phase of retinal regeneration. This data also provides important insight for further investigations addressing cellular mechanisms and molecular networks underlying retinal regeneration as well as differences between retinal regeneration and disorders. This transcriptome can be applied to ensuing comprehensive gene screening steps, providing candidate genes, regardless of whether annotated or unique, to uncover essential mechanisms underlying early processes of retinal regeneration.

The newt reprograms mature RPE cells into a unique multipotent state for retinal regeneration.

The reprogramming of retinal pigment epithelium (RPE) cells in the adult newt immediately after retinal injury is an area of active research for the study of retinal disorders and regeneration. We demonstrate here that unlike embryonic/larval retinal regeneration, adult newt RPE cells are not directly reprogrammed into retinal stem/progenitor cells; instead, they are programmed into a unique state of multipotency that is similar to the early optic vesicle (embryo) but preserves certain adult characteristics. These cells then differentiate into two populations from which the prospective-neural retina and -RPE layers are formed with the correct polarity. Furthermore, our findings provide insight into the similarity between these unique multipotent cells in newts and those implicated in retinal disorders, such as proliferative vitreoretinopathy, in humans. These findings provide a foundation for biomedical approaches that aim to induce retinal self-regeneration for the treatment of RPE-mediated retinal disorders.