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After introducing the stages of male germ cell development, we present the testicular somatic cell types, jointly providing indispensable structural and nutritional support as well as regulatory input during spermatogenesis. We will examine how somatic and germ cells interact to form the germinal epithelium that produces spermatozoa in the cystic mode of spermatogenesis typically found in fish (and amphibia). We discuss how endocrine signals from the pituitary and paracrine signals produced in testis tissue, often in response to endocrine signals, coordinate the cellular and molecular processes that constitute spermatogenesis. In this regard, we first discuss the mitotic phase, when proliferating spermatogonia generate the high number of germ cells required for male fertility. These cells enter meiosis, before completing final differentiation, i.e., the spermiogenesis phase and the acquisition of the spermatozoon fertility potential

The term peroxiporin has been created to define a transmembrane water channel protein (aquaporin) that also conducts hydrogen peroxide (H2O2). Such channels facilitate cell signaling and redox balance. Recent studies have shown that multiple differentially localized aquaporins play important roles in the activation, detoxification, maintenance of motility and swimming performance of piscine spermatozoa. For the first time, the peroxiporin function of one of these channels has been identified as a critical player in the mitochondrial elimination of H2O2, with important implications for sperm competition in fishes. This chapter provides an up-to-date overview of the phylogenetic interrelationships of the gnathostome aquaporin superfamily and the duplicates in teleosts, together with information on which of these channels are expressed in spermatozoa and which retain the biophysical property of H2O2 conductance. We discuss the site-specific functions of the aquaporins for piscine sperm cell physiology and summarize how the signaling pathways governing the intracellular trafficking and function of mitochondrial peroxiporins evolved in their spermatozoa

The dual aquaporin (Aqp1ab1/Aqp1ab2)-mediated hydration of marine teleost eggs, which occurs during oocyte meiosis resumption (maturation), is considered a key adaptation underpinning their evolutionary success in the oceans. However, the endocrine signals controlling this mechanism are almost unknown. Here, we investigated whether the nonapeptides arginine vasopressin (Avp, formerly vasotocin) and oxytocin (Oxt, formerly isotocin) are involved in marine teleost oocyte hydration using the gilthead seabream (Sparus aurata) as a model. We show that concomitant with an increased systemic production of Avp and Oxt, the nonapeptides are also produced and accumulated locally in the ovarian follicles during oocyte maturation and hydration. Functional characterization of representative Avp and Oxt receptor subtypes indicates that Avpr1aa and Oxtrb, expressed in the postvitellogenic oocyte, activate phospholipase C and protein kinase C pathways, while Avpr2aa, which is highly expressed in the oocyte and in the follicular theca and granulosa cells, activates the cAMP-protein kinase A (PKA) cascade. Using ex vivo, in vitro and mutagenesis approaches, we determined that Avpr2aa plays a major role in the PKA-mediated phosphorylation of the aquaporin subdomains driving membrane insertion of Aqp1ab2 in the theca and granulosa cells, and of Aqp1ab1 and Aqp1ab2 in the distal and proximal regions of the oocyte microvilli, respectively. The data further indicate that luteinizing hormone, which surges during oocyte maturation, induces the synthesis of Avp in the granulosa cells via progestin production and the nuclear progestin receptor. Collectively, our data suggest that both the neurohypophysial and paracrine vasopressinergic systems integrate to differentially regulate the trafficking of the Aqp1ab-type paralogs via a common Avp-Avpr2aa-PKA pathway to avoid competitive occupancy of the same plasma membrane space and maximize water influx during oocyte hydration

(A-C) Changes in Pf of X. laevis oocytes injected with water or expressing SaAqp1ab1-WT (A), HhAqp1ab1-WT (B) or SsAqp1ab2-WT (C) alone or in combination with different amounts of the isoforms of each paralog. For SsAqp1ab2-WT and splice forms, oocytes were co-injected with halibut YwhazLb and exposed to FSK. (D) Effect of the inhibition of the oocyte Pf by the different Aqp1ab1 splice forms, at a concentrations that produced approximately half-maximal reduction (as observed in A-C), in SaAqp1ab1 or HhAqp1ab1 expressing oocytes in the presence or absence of YwhazLa, and exposed to FSK. The percentage of Pf inhibition elicited by each isoform under the two conditions is indicated in each plot. In all panels, data are the mean ± SEM (n indicated above each bar) and were statistically analyzed by an unpaired Student’s t-test (*, P < 0.05: **, P < 0.01; ***, P < 0.001; with respect to oocytes injected with the WT form alone).

(A-C) Schematic diagram of the seabream and halibut aqp1ab1 and sole aqp1ab2 genes, and the different emerging transcript variants identified in each species. The numbers represent the exons, and the initiation and stop codons in each mRNA are indicated. (D-F) Integral membrane views (cartoon renders) of the wild type and variant channels. Canonical transmembrane domains are shown in cyan, loops in magenta and the two conserved Asn-Pro-Ala (NPA) motifs in dark blue. Variant regions are shown in red. In sole Aqp1ab2_v2 (C and F), the splicing of the distal region of intron 2 causes out-of-frame readthrough of exon 3 and the spliced exon 4 resulting in a premature stop codon and the truncated protein.

(A-C) Genomic organization of tandemly arranged aqp1ab2 and aqp1ab1 genes in seabream (A) and Atlantic halibut (B), and of the single aqp1ab2 gene in Senegalese sole and Common sole (C). The four exons (1–4) of both genes are represented by blue and red boxes, and the start codons are indicated by upper arrows. Lower arrows indicate the position of the oligonucleotide primers used for RT-PCR. (D-E) Representative RT-PCR analysis of aqp1ab1 and aqp1ab2 expression in different adult tissues from seabream (D) and halibut (E). A single aqp1ab2 mRNA species is expressed in seabream and halibut, whereas two or one splice variants (aqp1ab1_v1 and aqp1ab1_v2) in addition to aqp1ab1 are detected, respectively, as indicated. (F-G) RT-PCR of aqp1ab2 expression in Senegalese sole (F) and Common sole (G) where two splice forms (aqp1ab2_v1 and aqp1ab2 _v2) in addition to aqp1ab2 are amplified in the ovary and testis. In D-G, the sizes (kb) of molecular markers are indicated on the left.

Oligonucleotide primers employed for the RT-PCR screening of splice forms expression of aqp1ab-type genes in teleosts.

In oviparous marine fishes, the hyperosmotic induction of sperm motility in seawater (SW) is well established, however, the potential function of ion channels in the maintenance of post activated spermatozoon swimming performance remains largely unknown. Here, we investigated the influence of ion channels on the spermatozoon swimming parameters using the gilthead seabream (Sparus aurata) as a model for modern marine teleosts. Our data show that the SW-induced activation of seabream sperm motility requires three concomitant processes, the hyperosmotic shock, an ion-flux independent increase of the intracellular concentration of Ca2+ ([Ca2+]i), but not of [K+]i or [Na+]i, and the alkalization of the cytosol. The combination of all three processes is obligatory to trigger flagellar beating. However, the time-course monitoring of sperm motion kinetics and changes in the [Ca2+]i, [K+]i and [Na+]i in SW or in non-ionic activation media, showed that the post activated maintenance of spermatozoa motility is dependent on extracellular Ca2+ and K+. A meta-analysis of a seabream sperm transcriptome uncovered the expression of multiple ion channels, some of which were immunolocalized in the head and/or tail of the spermatozoon. Selective pharmacological inhibition of these ion channel families impaired the long-term motility, progressivity, and velocity of SW-activated spermatozoa. The data further revealed that some antagonists of K+-selective or Ca2+-selective channels, as well as of stretch-activated and mechanosensitive channels, altered the trajectory of spermatozoa, suggesting that these ion channels are likely involved in the control of the swimming pattern of the post activated spermatozoon. These combined findings provide new insight into the signaling pathways regulating spermatozoon activation and swimming performance in marine fishes

In non-mammalian vertebrates, the molecular mechanisms involved in the transformation of haploid germ cells (HGCs) into spermatozoa (spermiogenesis) are largely unknown. Here, we investigated this process in the marine teleost gilthead seabream (Sparus aurata) through the examination of the changes in the transcriptome between cell-sorted HGCs and ejaculated sperm (SPZEJ). Samples were collected under strict quality controls employing immunofluorescence microscopy as well as by determining the sperm motion kinematic parameters by computer-assisted sperm analysis. Deep sequencing by RNA-seq identified a total of 7286 differentially expressed genes (DEGs) (p-value < 0.01) between both cell types, of which nearly half were upregulated in SPZEJ compared to HCGs. In addition, approximately 9000 long non-coding RNAs (lncRNAs) were found, of which 56% were accumulated or emerged de novo in SPZEJ. The upregulated transcripts are involved in transcriptional and translational regulation, chromatin and cytoskeleton organization, metabolic processes such as glycolysis and oxidative phosphorylation, and also include a number of ion and water channels, exchangers, transporters and receptors. Pathway analysis conducted on DEGs identified 37 different signaling pathways enriched in SPZEJ, including 13 receptor pathways, from which the most predominant correspond to the chemokine and cytokine, gonadotropin-releasing hormone receptor and platelet derived growth factor signaling pathways. Our data provide new insight into the mRNA and lncRNA cargos of teleost spermatozoa and uncover the possible involvement of novel endocrine mechanisms during the differentiation and maturation of spermatozoa

The primary task of a spermatozoon is to deliver its nuclear payload to the egg to form the next-generation zygote. With polyandry repeatedly evolving in the animal kingdom, however, sperm competition has become widespread, with the highest known intensities occurring in fish. Yet, the molecular controls regulating spermatozoon swimming performance in these organisms are largely unknown. Here, we show that the kinematic properties of postactivated piscine spermatozoa are regulated through a conserved trafficking mechanism whereby a peroxiporin ortholog of mammalian aquaporin-8 (Aqp8bb) is inserted into the inner mitochondrial membrane to facilitate HO efflux in order to maintain ATP production. In teleosts from more ancestral lineages, such as the zebrafish (Danio rerio) and the Atlantic salmon (Salmo salar), in which spermatozoa are activated in freshwater, an intracellular Ca-signaling directly regulates this mechanism through monophosphorylation of the Aqp8bb N terminus. In contrast, in more recently evolved marine teleosts, such the gilthead seabream (Sparus aurata), in which spermatozoa activation occurs in seawater, a cross-talk between Ca- and oxidative stress-activated pathways generate a multiplier regulation of channel trafficking via dual N-terminal phosphorylation. These findings reveal that teleost spermatozoa evolved increasingly sophisticated detoxification pathways to maintain swimming performance under a high osmotic stress, and provide insight into molecular traits that are advantageous for postcopulatory sexual selection.

Kisspeptin regulates puberty and reproduction onset, acting upstream of the brain–pituitary–gonad (HPG) axis. This study aimed to test a kisspeptin-based hormonal therapy on cultured Senegalese sole (G1) breeders, known to have reproductive dysfunctions. A single intramuscular injection of KISS2-10 decapeptide (250 µg/kg) was tested in females and males during the reproductive season, and gonad maturation, sperm motility, plasma levels of gonadotropins (Fsh and Lh) and sex steroids (11-ketotestosterone, testosterone and estradiol), as well as changes in small non-coding RNAs (sncRNAs) in plasma, were investigated. Fsh, Lh, and testosterone levels increased after kisspeptin injection in both sexes, while sperm analysis did not show differences between groups. Let7e, miR-199a-3p and miR-100-5p were differentially expressed in females, while miR-1-3p miRNA was up-regulated in kisspeptin-treated males. In silico prediction of mRNAs targeted by miRNAs revealed that kisspeptin treatment might affect paracellular transporters, regulate structural and functional polarity of cells, neural networks and intracellular trafficking in Senegalese sole females; also, DNA methylation and sphingolipid metabolism might be altered in kisspeptin-treated males. Results demonstrated that kisspeptin stimulated gonadotropin and testosterone secretion in both sexes and induced an unanticipated alteration of plasma miRNAs, opening new research venues to understand how this neuropeptide impacts in fish HPG axis

Water channels (aquaporins) were originally discovered in mammals with fourteen subfamilies now identified (AQP0-13). Here we show that a functional Aqp14 subfamily phylogenetically related to AQP4-type channels exists in all vertebrate lineages except hagfishes and eutherian mammals. In contrast to the water-selective classical aquaporins, which have four aromatic-arginine constriction residues, Aqp14 proteins present five non-aromatic constriction residues and facilitate the permeation of water, urea, ammonia, H2O2 and glycerol. Immunocytochemical assays suggest that Aqp14 channels play important osmoregulatory roles in piscine seawater adaptation. Our data indicate that Aqp14 intracellular trafficking is tightly regulated by the vasotocinergic/isotocinergic neuropeptide and receptor systems, whereby protein kinase C and A transduction pathways phosphorylate highly conserved C-terminal residues to control channel plasma membrane insertion. The neuropeptide regulation of Aqp14 channels thus predates the vasotocin/vasopressin regulation of AQP2-5-6 orthologs observed in tetrapods. These findings demonstrate that vertebrate Aqp14 channels represent an ancient subfamily of neuropeptide-regulated polytransporters