Establishment of a normal medakafish spermatogonial cell line capable of sperm production in vitro

A novel modified peptide derived from tilapia piscidin 4 with improved cytotoxicity, stability and antibacterial activity against fish pathogens and its underlying antibacterial mechanism

Tilapia piscidin 4 (TP4) is an amphiphilic cationic antimicrobial peptide derived from Nile tilapia (Oreochromis niloticus), known for its broad-spectrum antimicrobial activity, potent anti-tumor effects, and immunomodulatory property. However, its significant toxicity and poor stability pose major challenges for practical applications. In this study, the TP4 sequence was modified by deleting nine amino acids from the N-terminal region and substituting glycine at the 13th position with cysteine, resulting in a modified peptide designated TP4-16G4C (FSACKAIHRLIRRRRR). The dimer of TP4-16G4C (bis-TP4-16G4C) was obtained by facilitating the formation of disulfide bonds through the oxidation of cysteine. Subsequently, their antibacterial activity, cytotoxicity, stability, and underlying mechanisms were investigated. TP4-16G4C and its dimer exhibited excellent antibacterial activity against a range of fish pathogens, particularly the dimer in vivo. Further study indicated that bis-TP4-16G4C exhibited significantly reduced toxicity toward fish red blood cells and other cell lines, alongside improved stability against proteases and serum, compared to the parental peptide TP4. Mechanistically, bis-TP4-16G4C disrupted the integrity of the bacterial membrane, leading to the leakage of cellular contents; additionally, it interacted with lipopolysaccharides, bound to bacterial genomic DNA, and effectively inhibited bacterial biofilm formation, similar to the action of TP4. In summary, the modified and dimerized antimicrobial peptide bis-TP4-16G4C exhibits reduced toxicity, enhanced stability, and superior antimicrobial activity in vivo, suggesting its greater suitability for practical applications in aquaculture and other fields compared to its parental peptide.

Establishment of mud crab (Scylla paramamosain) spermatogonial stem cell line: A potential tool for immunological research

Spermatogonial stem cells (SSCs) can differentiate into sperm and are important for studying on genetic information transmission of animals. However, the establishment of the SSC line in crustaceans is still in its infancy. This study aimed to establish a method for the isolation, culture, and identification of SSCs derived from the gonad of a marine crustacean (mud crab, Scylla paramamosain), and evaluate their differentiation ability and potential application in immunological research, in vitro. SSCs showed robust growth, proliferation, and passaging ability (up to 35 passages) in germ cell culture medium. Proteomic analysis showed that the protein expression profile of SSC was closely related to the gonadal tissue. SSCs were found to be able to express male-specific and pluripotent markers, such as CD9, PIWI, DDX4, DAZL, NANOG, SOX2, and EPHA1. Furthermore, SSCs were differentiated into osteoblasts and adipocytes under in vitro induction. Green fluorescent protein (GFP), packaged by lentivirus, was able to be overexpressed in SSCs after infection. In addition, the infection of white spot syndrome virus (WSSV) simulated the expression of inflammation-associated factors, including TRAF6, TNF-α, MyD88, Dorsal, and Relish, and apoptosis-related genes (BAX and Bcl2) in SSCs. Thus, SSCs were initially isolated and characterized from mud crabs for the first time. Our results proved that SSCs can be used in reproduction technology, germplasm conservation, and immunological studies in crustaceans.

Establishment and Characteristics of the Spermatogonial Stem Cell Line from the Yellow River Carp (Cyprinus carpio haematopterus)

To address the growing consumer demands for improved fish meat quality, desirable morphological traits, and sustainable production practices, researchers have intensified efforts in the selective breeding and genetic improvement of carp (Cyprinus carpio) varieties. However, traditional breeding methods are often time-consuming and inefficient, which poses challenges to the sustainable development of the carp aquaculture industry. The establishment of germ stem cell lines offers a crucial tool for the study of germ cells, genetic improvement, and species conservation. In this study, we successfully established a spermatogonial stem cell line (YRSSCs) from Yellow River carp (Cyprinus carpio haematopterus) that can be cultured in vitro for the long term. We optimized the culture conditions to maintain their self-renewal and differentiation capabilities. The results demonstrated that YRSSCs have a diploid karyotype and can stably proliferate for over a year in L-15 medium supplemented with 5 mmol/L HEPES, 50 μmol/L β-mercaptoethanol, 15% FBS, 2 ng/mL bFGF, 2 ng/mL LIF, 1% carp serum, 800 IU/mL penicillin, 0.8 mg/mL streptomycin, 2 μg/mL amphotericin B, 1% zebrafish embryo extract, and 1% glutamine at 30 °C in the absence of CO2. The cells exhibited a typical germ stem cell gene expression profile, with strong expression of the vasa, plzf-a, and Oct4-a genes. Additionally, this study found that YRSSCs possess the ability to differentiate in vitro and functionally colonize in vivo within recipient bodies. This research explored the establishment of YRSSCs and their differentiation potential both in vitro and in vivo, providing a novel strategy for the genetic improvement of aquaculture fish species through germ stem cell-based gene editing and transplantation technologies.

Establishment, identification, and transcriptome analysis of a Sertoli cell line from ovoviviparous black rockfish Sebastes schlegelii

Black rockfish (Sebastes schlegelii) is an economically important species with a unique ovoviviparous reproductive mode, in which reproduction is limited by incomplete fertilization. In order to understand the mechanism of spermatogenesis of black rockfish, a cell line derived from the testis, named SSTC, was successfully established and cultured in L-15 medium at 25 °C, and it was passaged to the 50th generation. The SSTC exhibited fibroblast-like and epithelial-like morphology during in vitro culture, and 62% of the SSTC retained the diploid karyotype with 48 chromosomes by the 30th passage (P30). To evaluate the ability of SSTC to express exogenous genes, lipofection and electrotransfection were performed, achieving transfection efficiencies of 9% and 12%, respectively. Transcriptome analysis showed that SSTC at P15 and P30 scarcely expressed germ cell and Leydig cell marker genes, while only expressing the Sertoli cell marker genes sox9a, amh, krt18 and fasl, indicating that SSTC mainly consists of Sertoli cells. GO and KEGG enrichment analyses revealed that, compared to the primary cells, the MAPK, TGF-β, and Wnt signaling pathways, which are crucial for spermatogenesis in Sertoli cells, were significantly upregulated in SSTC after passaging. Additionally, cell cycle-related pathways were upregulated, while pathways associated with cell adhesion, extracellular matrix, cell communication and membrane signal transduction were significantly downregulated. This study demonstrated that SSTC can be used as a tool for exploring the molecular mechanisms of gonadal differentiation and development in black rockfish, providing an effective platform for research on reproduction and endocrinology in this species.

Establishment and Characterization of OFT and OFO Cell Lines from Olive Flounder (Paralichthys olivaceus) for Use as Feeder Cells

Olive flounder (Paralichthys olivaceus) is a commercially important fish species in Japan, China, and the Republic of Korea. Despite numerous attempts to improve productivity, there have been no studies of in vitro germline stem cell (GSC) culture in this species. Here, olive flounder testicular and ovarian cell lines (OFT and OFO, respectively) were established and characterized. RT-PCR demonstrated that OFT and OFO expressed several gonadal somatic cell markers, including wt1 and fgf2, but lacked expression of germ cell markers, such as vasa, nanos2, and scp3. In addition, SNP analysis revealed that OFT originated from XY male P. olivaceus and OFO originated from XX female P. olivaceus. These results suggest that OFT was composed of Sertoli cells and OFO was composed of granulosa cells and theca cells. Finally, coculture of OFT or OFO with enriched male P. olivaceus GSCs isolated from the top 20% and 20-30% Percoll density gradient layers showed that GSCs were attached on both cell lines. In conclusion, we established P. olivaceus testicular and ovarian cell lines, which were expected to use for development of an in vitro GSC culture system.

Transcriptome analysis of the spermatogonial stem cell line of Opsariichthys bidens after infection with three strains of ranaviruses

The long-term-cultured Opsariichthys bidens spermatogonial stem cell line (ObSSC) was established in our previous study, which could be induced to differentiate into sperm in vitro. The genus Ranavirus is a large double-stranded DNA virus with a high degree of pathogenicity, which can be widely transmitted in fish, amphibians and reptiles. It is highly representative and possesses significant research value. The present study aimed to investigate the morphological changes and transcriptome analyses of ObSSC following infection by the three strains of ranaviruses (Andrias davidianus ranavirus, ADRV; Rana grylio virus, RGV; Siniperca chuatsi ranavirus, SCRaV). Microscopic observation and polymerase chain reaction (PCR) detection techniques were employed to ascertain that ObSSC could be invaded by ranaviruses, resulting in the formation of cell shed plaques and other lesions. Transcriptome analysis revealed the high degree of similarity among different infections of ranaviruses. The genes Egr1 (early growth response protein 1) and Dusp2 (dual specificity protein phosphatase 2) were downregulated after infection. The downregulation of PI3K-AKT and MAPK signaling pathways was observed in all infected groups, which may be related to the interference of viral infection on cell growth, biological process and apoptosis. At the same time, specific responses of the host cells were observed by infections of different types of ranaviruses. The expression of phagosome was upregulated in the ADRV-infected group, whereas the Junctional Adhesion Molecule (JAM2) and the leukocyte transendothelial migration pathway was upregulated in the RGV-infected group, and the Fc gamma R-mediated phagocytosis was upregulated in the SCRaV-infected group. These findings provided important clues for further understanding of the mechanism of ranaviruses infection and host cell response.

Genomic Analysis and Functional Validation of Bidirectional Promoters in Medaka (Oryzias latipes)

Bidirectional promoters (BDPs) regulate the transcription of two adjacent, oppositely oriented genes, offering a compact structure with significant potential for multigene expression systems. Although BDPs are evolutionarily conserved, their regulatory roles and sequence characteristics vary across species, with limited studies in fish. Here, we systematically analyzed the distribution, sequence features, and expression patterns of BDPs in the medaka (Oryzias latipes) genome. A total of 1737 divergent gene pairs, representing 13% of medaka genes, were identified as potentially regulated by BDPs. These genes are enriched in essential biological processes, including organelle function, RNA processing, and ribosome biogenesis. Transcriptomic analysis revealed that co-regulation (co-expression and co-silencing) is a prominent feature of these gene pairs, with variability influenced by tissue and sex. Sequence analysis showed that medaka BDPs are compact, with most fragments under 400 bp and an average GC content of 42.06%. Validation experiments confirmed the bidirectional transcriptional activity of three histone-related BDPs in both medaka SG3 cells and embryos, demonstrating effective and robust regulatory efficiency. This study enhances our understanding of the genomic organization and transcriptional regulation in fish and provides a valuable reference for developing species-specific multigene expression systems in fish genetic engineering.

Cryopreservation and transplantation of spermatogonia stem cells in piracanjuba Brycon orbignyanus (Characiformes: Characidae), an endangered fish species

Piracanjuba (Brycon orbignyanus) is an endangered fish species from the Neotropical region. The establishment of a cryobank using spermatogonial stem cells (SSCs) and subsequent production of a germline chimera is thus a promising strategy for such species. In the present work, procedures for the isolation and cryopreservation of piracanjuba SSCs and subsequent transplantation into sterile recipients were established. The piracanjuba SSCs were obtained by Percoll density gradient centrifugation and differential plating. SSC fractions were evaluated by relative ddx4 expression, alkaline phosphatase activity, and light microscopy. SSC cryopreservation was performed using five cryoprotectants at three different concentrations. The mix of the cells from the 20% and 30% Percoll density gradients showed 58.35 ± 0.03% purity of SSCs. The purity of SSCs increased to 66.00 ± 0.01% after differential plating. The relative ddx4 expression was 3.5 times higher in cells from the Percoll density gradient centrifugation than in the gonad and cells after differential plating. Propanediol (1 M) was the most effective cryoprotector evaluated (P = 1.000), showing 90.75 ± 1.85% cell viability. Freshly isolated and cryopreserved cells from the Percoll density gradient centrifugation were transplanted into a sterile male adult triploid hybrid with germ cell-less gonads. SSCs were observed in the germinal epithelium of the testes of recipients 20 days after transplantation. The results are promising for obtaining functional germline chimeras in Neotropical fish. Consequently, although the number of males used for the experiment was borderline, the procedures established here can be applied in future actions for the conservation and reconstitution of the piracanjuba in case of extinction.

Characterization and enrichment of spermatogonial stem cells of common carp (Cyprinus carpio)

Spermatogenesis is a systematically organized process that ensures uninterrupted sperm production in which the spermatogonial stem cells (SSCs) play a crucial role. However, the existing absence of teleost-specific molecular markers for SSCs presents a notable challenge. Herein we characterized phenotypically the spermatogonial stem cells using specific molecular markers and transmission electron microscopy. Moreover, we also describe a simple method to suppress common carp spermatogenesis using the combination of Busulfan and thermo-chemical treatment, and finally, we isolate and enrich the undifferentiated spermatogonial fraction. Our results showed that C-kit, GFRα1, and POU2 proteins were expressed by germ cells, meanwhile, undifferentiated spermatogonial populations preferentially expressed GFRα1 and POU2. Moreover, the combination of high temperature (35 °C) and Busulfan (40 mg/kg/BW) effectively suppressed the spermatogenesis of common carp males. Additionally, the amh expression analysis showed differences between the control (26 °C) when compared to 35 °C with a single or two Busulfan doses, confirming that the testes were depleted by the association of Busulfan at high temperatures. In an attempt to isolate the undifferentiated spermatogonial fraction, we used the Percoll discontinuous density gradient. Thus, we successfully dissociated the carp whole testes in different cellular fractions; subsequently, we isolated and enriched the undifferentiated spermatogonial population. Therefore, our results suggest that probably both GFRα-1 and POU2 are highly conserved factors expressed in common carp germinative epithelium and that these molecules were well conserved along the evolutionary process. Furthermore, the enriched undifferentiated spermatogonial population developed here can be used in further germ cell transplantation experiments to preserve and propagate valued and endangered fish species.

Profiling the Physiological Roles in Fish Primary Cell Culture

Fish primary cell culture has emerged as a valuable tool for investigating the physiological roles and responses of various cell types found in fish species. This review aims to provide an overview of the advancements and applications of fish primary cell culture techniques, focusing on the profiling of physiological roles exhibited by fish cells in vitro. Fish primary cell culture involves the isolation and cultivation of cells directly derived from fish tissues, maintaining their functional characteristics and enabling researchers to study their behavior and responses under controlled conditions. Over the years, significant progress has been made in optimizing the culture conditions, establishing standardized protocols, and improving the characterization techniques for fish primary cell cultures. The review highlights the diverse cell types that have been successfully cultured from different fish species, including gonad cells, pituitary cells, muscle cells, hepatocytes, kidney and immune cells, adipocyte cells and myeloid cells, brain cells, primary fin cells, gill cells, and other cells. Each cell type exhibits distinct physiological functions, contributing to vital processes such as metabolism, tissue regeneration, immune response, and toxin metabolism. Furthermore, this paper explores the pivotal role of fish primary cell culture in elucidating the mechanisms underlying various physiological processes. Researchers have utilized fish primary cell cultures to study the effects of environmental factors, toxins, pathogens, and pharmaceutical compounds on cellular functions, providing valuable insights into fish health, disease pathogenesis, and drug development. The paper also discusses the application of fish primary cell cultures in aquaculture research, particularly in investigating fish growth, nutrition, reproduction, and stress responses. By mimicking the in vivo conditions in vitro, primary cell culture has proven instrumental in identifying key factors influencing fish health and performance, thereby contributing to the development of sustainable aquaculture practices.

Smoothened mediates medaka spermatogonia proliferation via Gli1-Rgcc-Cdk1 axis†

The proliferation of spermatogonia directly affects spermatogenesis and male fertility, but its underlying molecular mechanisms are poorly understood. In this study, Smoothened (Smo), the central transducer of Hedgehog signaling pathway, was characterized in medaka (Oryzias latipes), and its role and underlying mechanisms in the proliferation of spermatogonia were investigated. Smo was highly expressed in spermatogonia. In ex vivo testicular organ culture and a spermatogonial cell line (SG3) derived from medaka mature testis, Smo activation promoted spermatogonia proliferation, while its inhibition induced apoptosis. The expression of glioma-associated oncogene homolog 1 (gli1) and regulator of cell cycle (rgcc) was significantly upregulated in SG3 after Smo activation. Furthermore, Gli1 transcriptionally upregulated the expression of rgcc, and Rgcc overexpression rescued cell apoptosis caused by Smo or Gli1 inhibition. Co-immunoprecipitation assay indicated that Rgcc could interact with cyclin-dependent kinase 1 (Cdk1) to regulate the cell cycle of spermatogonia. Collectively, our study firstly reveals that Smo mediates the proliferation of spermatogonia through Gli1-Rgcc-Cdk1 axis. In addition, Smo and Gli1 are necessary of the survival of spermatogonia. This study deepens our understanding of spermatogonia proliferation and survival at the molecular level, and provides insights into male fertility control and reproductive disease treatment.

Establishment and Characterization of a Spermatogonial Stem Cell Line from Tiger Puffer Fish (Takifugu rubripes)

Tiger puffer fish (Takifugu rubripes) has become the main fish species cultured in China since the last century because of its high economic value. Male and female tiger puffer fish need 2 and 3 years each to reach sexual maturity, which limits the development of breeding research for this species. In recent years, in vitro culture of fish spermatogonial stem cells (SSCs) have shown potential in aquaculture. In the present study, we established a spermatogenic stem cell line from T. rubripes (TrSSCs). TrSSCs were characterized by polygonal morphology, predominantly retained 44 chromosomes, and grew rapidly at 26 °C and in L-15. TrSSCs were still able to grow stably after more than one year of in vitro culture. TrSSCs showed positive alkaline phosphatase staining. TrSSCs expressed germ cell-associated genes, including dnd, ddx4, piwil, gfra1b, sox2, myca, nanog, ly75, and dazl, as determined by semiquantitative assays, and almost all cells were found to express the germ cell genes ddx4 and gfra1b in a fluorescence in situ hybridization assay. In vitro, induction experiments demonstrated the TrSSCs possessed the ability to differentiate into other types of cells. Our research has enriched the fish spermatogonial stem cell resource bank, which will provide an efficient research model for sex determination and sex control breeding in fish, establishing a foundation for subsequent breeding research.

Nesfatin-1 in a reptile: its role and hormonal regulation in wall lizard testis

Nesfatin-1 is a pleiotropic hormone implicated in various physiological functions including reproduction. Studies though limited, have established an important role of the peptide in regulation of testicular functions in mammals and fishes. However, role of nesfatin-1 in regulation of spermatogenesis and testicular steroidogenesis remains completely unexplored in reptiles. Therefore, present study aimed to develop an insight into reproductive phase-dependent testicular expression, function and regulation of nucb2/nesfatin-1 in a reptile, Hemidactylus flaviviridis. Expression of nucb2/nesfatin-1 in testis of wall lizard varied significantly depending upon reproductive phase, being highest in the active phase while lowest during regressed phase. Further, in vitro treatment of wall lizard testis with nesfatin-1 showed a concentration- and time-dependent stimulatory effect of the peptide on expression of cell proliferation and differentiation markers like scf, c-kit and pcna suggesting a spermatogenic role of nesfatin-1 in wall lizard. Also, nesfatin-1 stimulated the anti-apoptotic marker, bcl-2 while inhibited the apoptotic marker, caspase-3, suggesting its role as an inhibitor of apoptosis of testicular cells. Further, treatment with nesfatin-1 resulted in significantly higher expression of star along with a concomitant increase in testosterone production by the lizard testis. The present study also demonstrates hormonal regulation of testicular nucb2/nesfatin-1 wherein follicle-stimulating hormone (FSH) inhibited while sex steroids like dihydrotestosterone (DHT) and 17β-estradiol-3-benzoate (E2) stimulated the mRNA expression of nesfatin-1. Observations from the current study for the first time provide comprehensive evidence of spermatogenic and steroidogenic role of nesfatin-1 as well as its hormonal regulation in the testis of a reptile, H. flaviviridis.

Establishment of a Coilia nasus Spermatogonial Stem Cell Line Capable of Spermatogenesis In Vitro

The process by which spermatogonial stem cells (SSCs) continuously go through mitosis, meiosis, and differentiation to produce gametes that transmit genetic information is known as spermatogenesis. Recapitulation of spermatogenesis in vitro is hindered by the challenge of collecting spermatogonial stem cells under long-term in vitro culture conditions. Coilia nasus is a commercially valuable anadromous migrant fish found in the Yangtze River in China. In the past few decades, exploitation and a deteriorating ecological environment have nearly caused the extinction of C. nasus's natural resources. In the present study, we established a stable spermatogonial stem cell line (CnSSC) from the gonadal tissue of the endangered species C. nasus. The cell line continued to proliferate and maintain stable cell morphology, a normal diploid karyotype, and gene expression patterns after more than one year of cell culture (>80 passages). Additionally, CnSSC cells could successfully differentiate into sperm cells through a coculture system. Therefore, the establishment of endangered species spermatogonial stem cell lines is a model for studying spermatogenesis in vitro and a feasible way to preserve germplasm resources.

Medaka (Oryzias latipes) Olpax6.2 acquires maternal inheritance and germ cells expression, but was functionally degenerated in the eye

Gene duplication provides raw material for the evolution of genetic and phenotypic complexity. It has remained a long-standing mystery how duplicated genes evolve into new genes by neofunctionalization via the acquisition of new expression and/or activity and simultaneous loss of the old expression and activity. Fishes have many gene duplicates from whole genome duplication, making them excellent for studying the evolution of gene duplicates. In the fish medaka (Oryzias latipes), an ancestral pax6 gene has given rise to Olpax6.1 and Olpax6.2. Here we report that medaka Olpax6.2 is evolving towards neofunctionalization. A chromosomal syntenic analysis indicated that Olpax6.1 and Olpax6.2 are structurally co-homologous to the single pax6 in other organisms. Interestingly, Olpax6.2 maintains all conserved coding exons but loses the non-coding exons of Olpax6.1, and has 4 promoters versus 8 in Olpax6.1. RT-PCR revealed that Olpax6.2 maintained expression in the brain eye, pancreas as Olpax6.1. Surprisingly, Olpax6.2 also exhibited maternal inheritance and gonadal expression by RT-PCR, in situ hybridization and RNA transcriptome analysis. The expression and distribution of Olpax6.2 is not different from Olpax6.1 in the adult brain, eye and pancreas, but exhibited overlapping and distinct expression in early embryogenesis. We show that ovarian Olpax6.2 expression occurs in female germ cells. Olpax6.2 knockout showed no obvious defect in eye development, while Olpax6.1 F0 mutant have severe defects in eye development. Thus, Olpax6.2 has acquired maternal inheritance and germ cell expression, but was functionally degenerated in the eye, making this gene as an excellent model to study the neofunctionalization of duplicated genes.

Nonylphenol reduced the number of haploids in in vitro spermatogenesis of the endangered cyprinid Gnathopogon caerulescens

Nonylphenol (NP), an endocrine disrupting chemical, is widely used in industrial and agricultural processes, causing NP influx into aquatic environments. NP induces hormonal imbalance, and male feminization, and reduces germ cell production during spermatogenesis; however, the mechanism by which it affects spermatogenesis remains unknown. Here, we investigated the effect of NP on spermatogenesis in honmoroko (Gnathopogon caerulescens), an endangered fish endemic to Lake Biwa, Japan, using an in vitro differentiation system. We collected spermatogonia from the testes of non-spawning G. caerulescens and subjected them to suspension culture. The spermatogonia differentiated into flagellated spermatozoa in 3 weeks, regardless of the presence of NP. NP concentrations as low as 1 nM caused a decrease in the number of germ cells in a dose-dependent manner, whereas the number of somatic cells decreased only at a high concentration of 1 μM. Flow cytometric analysis revealed that the decrease in germ cell number was attributed to haploids (spermatids and spermatozoa); the number of spermatogonia and spermatocytes was not affected by NP treatment. This result is consistent with the hypothesis that NP might repress the second meiosis or induce apoptosis in haploids. This study demonstrated that the combination of in vitro germ cell differentiation and flow cytometric analysis is useful for evaluating the direct effects of NP on germ cell differentiation in endangered endemic fish.

Transcriptomes of Testes at Different Developmental Stages in the Opsariichthys bidens Predict Key Genes for Testis Development and Spermatogenesis

Testis development is a complex process involving multiple genes, and the molecular mechanisms underlying testis development in Opsariichthys bidens remain unclear. We performed transcriptome sequencing analysis on a total of 12 samples of testes from stages II, III, IV, and V of O. bidens and obtained a total of 79.52 Gb clean data, as well as 288,573 transcripts and 116,215 unigenes. Differential expression analysis showed that 22,857 differentially expressed genes (DEGs) were screened in six comparison groups (III vs. II, IV vs. II, V vs. II, IV vs. III, V vs. III, and V vs. IV). Kyoto Encyclopedia of Genes and Genomes enrichment analysis of DEGs showed that six comparison groups were significantly enriched for a total of 20 significantly up- or down-regulated pathways, including six pathways related to signal transduction, three pathways related to energy metabolism, five pathways related to disease, and two pathways related to ribosomes. Furthermore, our investigation revealed that DEGs were enriched in several important functional pathways, such as Huntington's disease signaling pathway, TGF-β signaling pathway, and ribosome signaling pathway. Protein-protein interaction network analysis of DEGs identified 63 up-regulated hub genes, including 9 kinesin genes and 2 cytoplasmic dynein genes, and 39 down-regulated hub genes, including 13 ribosomal protein genes. This result contributes to the knowledge of spermatogenesis and testis development in O. bidens.

The Role of Promyelocytic Leukemia Zinc Finger (PLZF) and Glial-Derived Neurotrophic Factor Family Receptor Alpha 1 (GFRα1) in the Cryopreservation of Spermatogonia Stem Cells

The cryopreservation of spermatogonia stem cells (SSCs) has been widely used as an alternative treatment for infertility. However, cryopreservation itself induces cryoinjury due to oxidative and osmotic stress, leading to reduction in the survival rate and functionality of SSCs. Glial-derived neurotrophic factor family receptor alpha 1 (GFRα1) and promyelocytic leukemia zinc finger (PLZF) are expressed during the self-renewal and differentiation of SSCs, making them key tools for identifying the functionality of SSCs. To the best of our knowledge, the involvement of GFRα1 and PLZF in determining the functionality of SSCs after cryopreservation with therapeutic intervention is limited. Therefore, the purpose of this review is to determine the role of GFRα1 and PLZF as biomarkers for evaluating the functionality of SSCs in cryopreservation with therapeutic intervention. Therapeutic intervention, such as the use of antioxidants, and enhancement in cryopreservation protocols, such as cell encapsulation, cryoprotectant agents (CPA), and equilibrium of time and temperature increase the expression of GFRα1 and PLZF, resulting in maintaining the functionality of SSCs. In conclusion, GFRα1 and PLZF have the potential as biomarkers in cryopreservation with therapeutic intervention of SSCs to ensure the functionality of the stem cells.

Optimization of in vitro culture conditions of common carp germ cells for purpose of surrogate production

Common carp (Cyprinus carpio) is the fourth most-produced fish species in aquaculture and frequently used model species with significant effort invested in development of biotechnological applications. In present study, we attempted to establish an in vitro germ cell culture condition for short term cell culture, which could facilitate further applications such as surrogacy or gene manipulation. Basal media and different types of feeder cells were investigated to optimize carp germ cell culture condition to favor maintenance of mitotic proliferation. Results indicated that germ cells cultured with hESC media and RTG2 cell line as feeder possessed significantly higher proliferation and survival rate compared to that cultured with StemPro media and Sertoli cell line as feeder. In addition, we compared two dissection strategies to compare risk of cell culture contamination and body cavity was open from dorsal part or from ventral part. As a result, carp open from the dorsal side can minimize the risk of contamination. In summary, this is the first study to optimize the cultivation of germ cells in common carp. This opens up new opportunities for the application of specific techniques in the breeding of those species with high commercial value and frequent use as a model fish. Results obtained in this study are important for implementation of new strategies in common carp breeding, conservation of genetic resources, restoration of lines or development of clonal and isogenic carp lines.

Establishment of a Spermatogonial Stem Cell Line with Potential of Meiosis in a Hermaphroditic Fish, Epinephelus coioides

Spermatogonial stem cells (SSCs) are unique adult stem cells capable of self-renewal and differentiation into sperm. Grouper is a protogynous hermaphroditic fish farmed widely in the tropical and subtropical seas. In this study, we established an SSC line derived from adult testis of orange-spotted grouper, Epinephelus coioides. In the presence of basic fibroblast growth factor (bFGF) and leukemia inhibitory factor (LIF), the cells could be maintained with proliferation and self-renewal over 20 months and 120 passages under in vitro culture conditions. The cells exhibited strong alkaline phosphatase activity and the characteristics of SSCs with the expression of germ cell markers, including Vasa, Dazl, and Plzf, as well as the stem cell markers Nanog, Oct4, and Ssea1. Furthermore, the cultured cells could be induced by 11-ketotestosterone treatment to highly express the meiotic markers Rec8, Sycp3, and Dmc1, and produce some spherical cells, and even sperm-like cells with a tail. The findings of this study suggested that the cultured grouper SSC line would serve as an excellent tool to study the molecular mechanisms behind SSCs self-renewal and differentiation, meiosis during spermatogenesis, and sex reversal in hermaphroditic vertebrates. Moreover, this SSC line has great application value in grouper fish aquaculture, such as germ cell transplantation, genetic manipulation, and disease research.

Generation of a Normal Long-Term-Cultured Chinese Hook Snout Carp Spermatogonial Stem Cell Line Capable of Sperm Production In Vitro

Opsariichthys bidens belongs to the family Cyprinidae and is a small freshwater economic fish widely distributed in China. In recent years, the natural resources of O. bidens have been drastically reduced due to overfishing and the destruction of the water environment. The in vitro culture and long-term preservation of germ stem cells are the key technologies to keep genetic resources from degeneration. However, except for the establishment of the first long-term cultured medaka spermatogonia cell line (SSC) capable of producing sperm in vitro in 2004, no other long-term cultured SSC line has been found in other fish species. In this study, we successfully established another long-term-cultured spermatogonial stem cell line from Opsariichthys bidens (ObSSC). After more than 2 years of culture, ObSSC had a diploid karyotype and stable growth, with the typical gene expression patterns of SSC. Under in vitro culture, ObSSC could be induced to differentiate into sperm and other different types of somatic cells. In vivo, ObSSC could differentiate into different cells of three germ layers upon being transplanted into zebrafish embryos. Our research helps to explore the potential and regulation mechanism of fish SSC differentiation and spermatogenesis in vitro, provides a new way for solving the problem of fish genetic resource degradation and lays a foundation for further research on fish germ cell transplantation.

Establishment of a Coilia nasus Gonadal Somatic Cell Line Capable of Sperm Induction In Vitro

Coilia nasus is an important economic anadromous migratory fish of the Yangtze River in China. In recent years, overfishing and the deterioration of the ecological environment almost led to the extinction of the wild resources of C.nasus. Thus, there is an urgent need to protect this endangered fish. Recently, cell lines derived from fish have proven a promising tool for studying important aspects of aquaculture. In this study, a stable C. nasus gonadal somatic cell line (CnCSC) was established and characterized. After over one year of cell culture (>80 passages), this cell line kept stable growth. RT-PCR results revealed that the CnGSC expressed some somatic cell markers such as clu, fshr, hsd3β, and sox9b instead of germ cell markers like dazl, piwi, and vasa. The strong phagocytic activity of CnGSC suggested that it contained a large number of Sertoli cells. Interestingly, CnGSC could induce medaka spermatogonial cells (SG3) to differentiate into elongated spermatids while co-cultured together. In conclusion, we established a C. nasus gonadal somatic cell line capable of sperm induction in vitro. This research provides scientific evidence for the long-term culture of a gonadal cell line from farmed fish, which would lay the foundation for exploring the regulatory mechanisms between germ cells and somatic cells in fish.

Differential Expression of Duplicate Insulin-like Growth Factor-1 Receptors (igf1rs) in Medaka Gonads

Insulin-like growth factor-1 receptors (igf1rs) play important roles in regulating development, differentiation, and proliferation in diverse organisms. In the present study, subtypes of medaka igf1r, igf1ra, and igf1rb were isolated and characterized. RT-PCR results showed that igf1ra and igf1rb mRNA were expressed in all tissues and throughout embryogenesis. Using real-time PCR, the differential expression of igf1ra and igf1rb mRNA during folliculogenesis was observed. The results of in situ hybridization (ISH) revealed that both of them were expressed in ovarian follicles at different stages, and igf1rb was also expressed in theca cells and granulosa cells. In the testis, both igf1ra and igf1rb mRNA were highly expressed in sperm, while igf1rb mRNA was also obviously detected in spermatogonia. In addition, igf1ra mRNA was also present in Leydig cells in contrast to the distribution of igf1rb mRNA in Sertoli cells. Collectively, we demonstrated that differential igf1rs RNA expression identifies medaka meiotic germ cells and somatic cells of both sexes. These findings highlight the importance of the igf system in the development of fish gonads.

Models and Molecular Markers of Spermatogonial Stem Cells in Vertebrates: To Find Models in Nonmammals

Spermatogonial stem cells (SSCs) are the germline stem cells that are essential for the maintenance of spermatogenesis in the testis. However, it has not been sufficiently understood in amphibians, reptiles, and fish because numerous studies have been focused mainly on mammals. The aim of this review is to discuss scientific ways to elucidate SSC models of nonmammals in the context of the evolution of testicular organization since rodent SSC models. To further understand the SSC models in nonmammals, we point out common markers of an SSC pool (undifferentiated spermatogonia) in various types of testes where the kinetics of the SSC pool appears. This review includes the knowledge of (1) common molecular markers of vertebrate type A spermatogonia including putative SSC markers, (2) localization of the markers on the spermatogonia that have been reported in previous studies, (3) highlighting the most common markers in vertebrates, and (4) suggesting ways of finding SSC models in nonmammals.

Isolation and Characterization of Germline Stem Cells in Protogynous Hermaphroditic Monopterus albus

Germline stem cells (GSCs) are a group of unique adult stem cells in gonads that act as important transmitters for genetic information. Donor GSCs have been used to produce offspring by transplantation in fisheries. In this study, we successfully isolated and enriched GSCs from the ovary, ovotestis, and testis of Monopterus albus, one of the most important breeding freshwater fishes in China. Transcriptome comparison assay suggests that a distinct molecular signature exists in each type of GSC, and that different signaling activities are required for the maintenance of distinct GSCs. Functional analysis shows that fGSCs can successfully colonize and contribute to the germline cell lineage of a host zebrafish gonad after transplantation. Finally, we describe a simple feeder-free method for the isolation and enrichment of GSCs that can contribute to the germline cell lineage of zebrafish embryos and generate the germline chimeras after transplantation.

Restoring Genetic Resource through In Vitro Culturing Testicular Cells from the Cryo-Preserved Tissue of the American Shad (Alosa sapidissima)

Simple Summary

Cryopreservation and in vitro culture of germ cells are key techniques for the genetic resource preservation of the declining population of American shad. Two types of cryopreserved samples, namely testis pieces and testicular cells of American shad, were comparatively analyzed for cell viability. The results showed that the cell viability of the cryopreserved testis pieces was much higher than that of the cryopreserved testicular cells. The viability of the cells from the cryopreserved testis pieces ranged from 65.2 ± 2.2 (%) to 93.8 ± 0.6 (%), whereas the viability of the dissociated cells after cryopreservation was 38.5 ± 0.8 (%) to 87.1 ± 2.6 (%). Moreover, the testicular cells isolated from the post-thaw testicular tissue could be cultured and propagated in vitro. Our findings would benefit further investigations on genetic resource preservation and other manipulations of germ cells in a commercially and ecologically important fish species.

Abstract

Germ cells, as opposed to somatic cells, can transmit heredity information between generations. Cryopreservation and in vitro culture of germ cells are key techniques for genetic resource preservation and cellular engineering breeding. In this study, two types of cryopreserved samples, namely testis pieces and testicular cells of American shad, were comparatively analyzed for cell viability. The results showed that the cell viability of the cryopreserved testis pieces was much higher than that of the cryopreserved testicular cells. The viability of cells from the cryopreserved testis pieces ranged from 65.2 ± 2.2 (%) to 93.8 ± 0.6 (%), whereas the viability of the dissociated cells after cryopreservation was 38.5 ± 0.8 (%) to 87.1 ± 2.6 (%). Intriguingly, the testicular cells from the post-thaw testicular tissue could be cultured in vitro. Likewise, most of the cultured cells exhibited germ cell properties and highly expressed Vasa and PCNA protein. This study is the first attempt to effectively preserve and culture the male germ cells through freezing tissues in the American shad. The findings of this study would benefit further investigations on genetic resource preservation and other manipulations of germ cells in a commercially and ecologically important fish species.

Fish Pluripotent Stem-Like Cell Line Induced by Small-Molecule Compounds From Caudal Fin and its Developmental Potentiality

The technique of induced pluripotent stem cells has significant application value in breeding and preserving the genetic integrity of fish species. However, it is still unclear whether the chemically induced pluripotent stem cells can be induced from non-mammalian cells or not. In this article, we first verify that fibroblasts of fish can be chemically reprogrammed into pluripotent stem cells. These induced pluripotent stem-like cells possess features of colony morphology, expression of pluripotent marker genes, formation of embryoid bodies, teratoma formation, and the potential to differentiate into germ cell-like cells in vitro. Our findings will offer a new way to generate induced pluripotent stem cells in teleost fish and a unique opportunity to breed commercial fish and even save endangered fish species.

Efficient gene editing in a medaka (Oryzias latipes) cell line and embryos by SpCas9/tRNA-gRNA

Generation of mutants with clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) is commonly carried out in fish species by co-injecting a mixture of Cas9 messenger RNA (mRNA) or protein and transcribed guide RNA (gRNA). However, the appropriate expression system to produce functional gRNAs in fish embryos and cells is rarely present. In this study, we employed a poly-transfer RNA (tRNA)-gRNA (PTG) system driven by cytomegalovirus (CMV) promoter to target the medaka (Oryzias latipes) endogenous gene tyrosinase(tyr) or paired box 6.1 (pax6.1) and illustrated its function in a medaka cell line and embryos. The PTG system was combined with the CRISPR/Cas9 system under high levels of promoter to successfully induce gene editing in medaka. This is a valuable step forward in potential application of the CRISPR/Cas9 system in medaka and other teleosts.

Role and relevance of fish cell lines in advanced in vitro research

Introduction

Cell line derived from fish has been established as a promising tool for studying many key issues of aquaculture covering fish growth, disease, reproduction, genetics, and biotechnology. In addition, fish cell lines are very useful in vitro models for toxicological, pathological, and immunological studies. The easier maintenance of fish cell lines in flexible temperature regimes and hypoxic conditions make them preferable in vitro tools over mammalian cell lines. Great excitement has been observed in establishing and characterizing new fish cell lines representing diverse fish species and tissue types. The well-characterized and authenticated cell lines are of utmost essential as these represent cellular functions very similar to in vivo state of an organism otherwise it would affect the reproducibility of scientific research.

Conclusion

The fish cell lines have exhibited encouraging results in several key aspects of in vitro research in aquaculture including virology, nutrition and metabolism, production of vaccines, and transgenic fish production. The review paper reports the cell lines developed from fish, their characterization, and biobanking along with their potential applications and challenges in in vitro research.

Establishment of a stem Leydig cell line capable of 11-ketotestosterone production

The deficiency or insufficiency of androgen can trigger a range of reproductive diseases as well as other symptoms. Stem Leydig cells (SLCs) are critical for the formation and maintenance of a functional androgen-producing cell (Leydig cell, LC) population throughout adult male life. However, to date, our knowledge about SLCs is poor. Here we report the derivation and characterisation of a clonal stem LC line (designated as TSL) capable of 11- ketotestosterone (11-KT) production from a 3-month-old Nile tilapia (Oreochromis niloticus) testis. The cells retained stable proliferation after 77 generations with normal karyotype and growth factor dependency. They expressed platelet-derived growth factor receptor-α (pdgfrα), nestin and chicken ovalbumin upstream promoter transcription factor II (coup-tfIIa), which are characteristic of SLCs. Upon induction in defined medium, TSLs could undergo differentiation into steroidogenically active LCs and produce 11-KT. When implanted into recipient Nile tilapia testes from which endogenous LCs had been eliminated by ethane dimethanesulphonate (EDS) treatment, the PKH26-labelled TSLs could colonise the interstitium, subsequently express steroidogenic genes and restore 11-KT production. Taken together, our data suggest that TSLs possess the ability of continuous proliferation and potential of differentiation into functional LCs invitro and invivo. To the best of our knowledge TSL might represent the first stem LC line capable of 11-KT production to date. Our study may offer new opportunities for investigating the self-renewal of SLCs and steroidogenesis invitro, and provide an invaluable invitro model for investigating endocrine disruptors.

Allelic diversification after transposable element exaptation promoted gsdf as the master sex determining gene of sablefish

Concepts of evolutionary biology suggest that morphological change may occur by rare punctual but rather large changes, or by more steady and gradual transformations. It can therefore be asked whether genetic changes underlying morphological, physiological, and/or behavioral innovations during evolution occur in a punctual manner, whereby a single mutational event has prominent phenotypic consequences, or if many consecutive alterations in the DNA over longer time periods lead to phenotypic divergence. In the marine teleost, sablefish (Anoplopoma fimbria), complementary genomic and genetic studies led to the identification of a sex locus on the Y Chromosome. Further characterization of this locus resulted in identification of the transforming growth factor, beta receptor 1a (tgfbr1a) gene, gonadal somatic cell derived factor (gsdf), as the main candidate for fulfilling the master sex determining (MSD) function. The presence of different X and Y Chromosome copies of this gene indicated that the male heterogametic (XY) system of sex determination in sablefish arose by allelic diversification. The gsdfY gene has a spatio-temporal expression profile characteristic of a male MSD gene. We provide experimental evidence demonstrating a pivotal role of a transposable element (TE) for the divergent function of gsdfY. By insertion within the gsdfY promoter region, this TE generated allelic diversification by bringing cis-regulatory modules that led to transcriptional rewiring and thus creation of a new MSD gene. This points out, for the first time in the scenario of MSD gene evolution by allelic diversification, a single, punctual molecular event in the appearance of a new trigger for male development.

CircRNA profiling reveals circ880 functions as miR-375-3p sponge in medaka gonads

Circular RNAs (circRNAs) have been regarded as regulators in the biological processes of various species. However, there is no report about circRNAs in the gonads of model fish medaka (Oryzias latipes). In this study, 1157 and 1570 circRNAs were obtained in the ovary and testis by RNA-sequencing. The characteristics of circRNAs were explored in sequence length, exon composition, and chromosome position. 24 circRNAs were significantly up or down-regulated in the testis compared to the ovary, 9 of which were verified by qRT-PCR. Interestingly, circ452 was highly expressed in the testis while circ880 expression exhibited sexual dimorphism. In situ hybridization (ISH) revealed that circ452 and circ880 were expressed in meiotic germ cells, and circ880 was also abundant in spermatogonia. In addition, dual-luciferase reporter assay manifested that circ880 and Oldnd can combine with miR-375-3p. Overall, these results provide emerging circRNA libraries and open new avenues for future investigation of circRNAs in medaka.

Reconstruction of the birth of a male sex chromosome present in Atlantic herring

Understanding the evolution of sex determination mechanisms and sex chromosomes is of fundamental importance in biology. Here we have reconstructed the evolution of the sex-determining region in the Atlantic herring. The region is small and contains only three intact genes. The candidate sex-determining factor BMPR1BBY is an evolutionary innovation in the herring lineage. It encodes a truncated form of a BMP type I receptor, which originated by gene duplication and underwent rapid protein evolution. The receptor has maintained its kinase activity and has the potential to induce development of testis. The other two genes in the sex-determining region, CATSPERG and CATSPER3Y, are male beneficial genes because they encode proteins predicted to be essential for sperm to fertilize the egg.

The mechanisms underlying sex determination are astonishingly plastic. Particularly the triggers for the molecular machinery, which recalls either the male or female developmental program, are highly variable and have evolved independently and repeatedly. Fish show a huge variety of sex determination systems, including both genetic and environmental triggers. The advent of sex chromosomes is assumed to stabilize genetic sex determination. However, because sex chromosomes are notoriously cluttered with repetitive DNA and pseudogenes, the study of their evolution is hampered. Here we reconstruct the birth of a Y chromosome present in the Atlantic herring. The region is tiny (230 kb) and contains only three intact genes. The candidate male-determining gene BMPR1BBY encodes a truncated form of a BMP1B receptor, which originated by gene duplication and translocation and underwent rapid protein evolution. BMPR1BBY phosphorylates SMADs in the absence of ligand and thus has the potential to induce testis formation. The Y region also contains two genes encoding subunits of the sperm-specific Ca²⁺ channel CatSper required for male fertility. The herring Y chromosome conforms with a characteristic feature of many sex chromosomes, namely, suppressed recombination between a sex-determining factor and genes that are beneficial for the given sex. However, the herring Y differs from other sex chromosomes in that suppression of recombination is restricted to an ∼500-kb region harboring the male-specific and sex-associated regions. As a consequence, any degeneration on the herring Y chromosome is restricted to those genes located in the small region affected by suppressed recombination.

Isolation, in vitro study, and stem cell markers for type A spermatogonia in a Characiformes species

The objective of this study was to establish a protocol for the characterization, isolation, and culture of type A spermatogonia using specific molecular markers for these cells in fish. To this end, adult Prochilodus lineatus testes were collected and digested enzymatically and the resulting testicular suspension was separated using a discontinuous Percoll gradient, followed by differential plating. The cell cultures obtained were monitored for 15 days and analyzed using the immunofluorescence method with anti-Vasa, anti-GFRα1, and anti-OCT4 antibodies. Spermatogonial enrichment was also performed using flow cytometry. Although discontinuous Percoll gradient centrifugation followed by differential plating enabled the removal of differentiated germ cells and somatic cells, enriching the pool of type A spermatogonia, the enrichment of type A spermatogonia through flow cytometry of samples without Percoll proved to be more efficient. Prominent cell agglomerates that were characterized according to different stem cell markers as type A spermatogonia were observed during the 15 days of the cell culture. The use of immunoperoxidase and western blot analysis methods confirmed the specificity of the markers for type A spermatogonia of P. lineatus. When combined with specific cell culture conditions, the positive characterization of these molecular markers clarified certain aspects of spermatogonial regulation, such as survival and proliferation. Finally, understanding the regulation of the in vitro germ cell maintenance process may contribute to the enhancement of in vivo and in vitro reproduction techniques of endangered or aquaculture fish species.

Production of functional eggs and sperm from in vitro-expanded type A spermatogonia in rainbow trout

Combining cryopreservation of germline stem cells (GSCs) with their subsequent transplantation into recipient fish is a powerful tool for long-term preservation of genetic resources of endangered fishes. However, application of this technique has been limited because endangered species sometimes have small gonads and do not supply enough GSCs to be used for transplantation. This limitation could be overcome by expanding GSCs in vitro, though this has been difficult due to the complexity of reconstructing the gonadal microenvironment that surrounds GSCs. Here, we describe a novel method of in vitro expansion of rainbow trout GSCs using a feeder layer derived from Sertoli cells and a culture medium containing trout plasma. A transplantation assay demonstrated that the in vitro-expanded GSCs exhibited stem cell activity and potency to produce functional eggs, sperm, and eventually healthy offspring. In vitro expansion of GSCs can aid in rescuing fishes that are on the verge of extinction.

Spermatogonial Stem Cells in Fish: Characterization, Isolation, Enrichment, and Recent Advances of In Vitro Culture Systems

Spermatogenesis is a continuous and dynamic developmental process, in which a single diploid spermatogonial stem cell (SSC) proliferates and differentiates to form a mature spermatozoon. Herein, we summarize the accumulated knowledge of SSCs and their distribution in the testes of teleosts. We also reviewed the primary endocrine and paracrine influence on spermatogonium self-renewal vs. differentiation in fish. To provide insight into techniques and research related to SSCs, we review available protocols and advances in enriching undifferentiated spermatogonia based on their unique physiochemical and biochemical properties, such as size, density, and differential expression of specific surface markers. We summarize in vitro germ cell culture conditions developed to maintain proliferation and survival of spermatogonia in selected fish species. In traditional culture systems, sera and feeder cells were considered to be essential for SSC self-renewal, in contrast to recently developed systems with well-defined media and growth factors to induce either SSC self-renewal or differentiation in long-term cultures. The establishment of a germ cell culture contributes to efficient SSC propagation in rare, endangered, or commercially cultured fish species for use in biotechnological manipulation, such as cryopreservation and transplantation. Finally, we discuss organ culture and three-dimensional models for in vitro investigation of fish spermatogenesis.

Isolation, characterization, in vitro expansion and transplantation of Caspian trout (Salmo caspius) type a spermatogonia

Sprmatogonial stem cells (SSCs) are valuable for preservation of endangered fish species, biological experimentation, as well as biotechnological applications. However, the rarity of SSCs in the testes has been a great obstacle in their application. Thus, establishment of an efficient in-vitro culture system to support continuous proliferation of SSCs is essential. The present study aimed to establish an efficient and simple method for in vitro culture of Caspian trout undifferentiated spermatogonial cells. Using a two-step enzymatic digestion, testicular cells were isolated from immature testes composed of mainly undifferentiated spermatogonial cells with gonadosomatic indices of <0.05%. The spermatogonial cells were purified by differential plating through serial passaging. The purified cells indicated high expression of type A spermatogonia-related genes (Ly75, Gfrα1, Nanos2, Plzf and Vasa). Proliferation of purified cells was confirmed by BrdU incorporation. Co-culture of purified cells with testicular somatic cells as a feeder layer, resulted in continuous proliferation of type A spermatogonia. The cultured cells continued to express type A spermatogonia-specific markers after one month culture. The cultured spermatogonia were successfully incorporated into the germline after being intraperitoneally transplanted into sterile triploid rainbow trout hatchlings. These results, for the first time, demonstrated that the somatic microenvironment of the rainbow trout gonad can support the colonization and survival of intraperitoneally transplanted cells derived from a fish species belonging to a different genus. Therefore, the combination of in vitro culture system and xenotransplantation can be considered as a promising strategy for conservation of Caspian trout genetic resources.

Function of leukaemia inhibitory factor in spermatogenesis of a teleost fish, the medaka Oryzias latipes

In response to gonadotropins and androgens, testicular cells produce various molecules that control proper proliferation and differentiation of spermatogenic cells through their paracrine and autocrine actions. However, molecules functioning downstream of the hormonal stimulation are poorly understood. Leukaemia inhibitory factor (Lif) is known to maintain the pluripotency of stem cells including embryonic stem cells and primordial germ cells at least in vitro, but its actual roles in vivo remain to be elucidated. To clarify the function of Lif in teleost (medaka) testes, we examined the effects of Lif on spermatogenesis in a newly established cell culture system using a cell line (named Mtp1) derived from medaka testicular somatic cells as feeder cells. We found that addition of baculovirus-produced recombinant medaka Lif to the culture medium or co-culture with Lif-overexpressing Mtp1 cells increased the number of spermatogonia. In situ hybridization and immunohistochemical analyses of the medaka testes showed that mRNAs and proteins of Lif are expressed in spermatogonia and the surrounding Sertoli cells, with higher expression levels in type A (undifferentiated) spermatogonia than in type B (differentiated) spermatogonia. Our findings suggest that Lif regulates spermatogonial cell proliferation in the medaka.

Optimization of In Vitro Culture Conditions of Sturgeon Germ Cells for Purpose of Surrogate Production

Simple Summary

The sturgeon is among the most ancient of actinopterygian fishes. Most species of sturgeon are listed as critically endangered due to habitat alteration caused by damming of rivers, pollution and overharvesting. Germ cell transplant is a useful tool to save these endangered species. To expand germ cell populations and sustain the supply for long periods for transplant, we established basal culture conditions for sturgeon germ cells. Germ cell mitotic activity has been enhanced by eliminating gonad somatic cells, supplementing with growth factor and using an alternative to fetal bovine serum. The optimal condition identified was purified germ cells cultured in serum-free medium supplemented with leukemia inhibitory factor (LIF) and glial cell line-derived neurotrophic factor (GDNF) at 21 °C. Cultured sterlet germ cells showed development after transplant into Russian sturgeon. The study provided useful information for sturgeon germ cell culture.

Abstract

To expand germ cell populations and provide a consistent supply for transplantation, we established basal culture conditions for sturgeon germ cells and subsequently increased their mitotic activity by eliminating gonad somatic cells, supplementing with growth factor, and replacing fetal bovine serum (FBS). The initial basal culture conditions were Leibovitz’s L-15 medium (pH 8.0) supplemented with 5% FBS (p < 0.001) at 21 °C. Proliferation of germ cells was significantly enhanced and maintained for longer periods by elimination of gonad somatic cells and culture under feeder-cell free conditions, with addition of leukemia inhibitory factor and glial-cell-derived neurotrophic factor (p < 0.001). A serum-free culture medium improved germ cell proliferation compared to the L-15 with FBS (p < 0.05). Morphology remained similar to that of fresh germ cells for at least 40 d culture. Germline-specific gene expression analysis revealed no significant changes to germ cells before and after culture. Sterlet Acipenser ruthenus germ cells cultured more than 40 days showed development after transplant into Russian sturgeon Acipenser gueldenstaedtii. Polymerase chain reaction showed 33.3% of recipient gonads to contain sterlet cells after four months. This study developed optimal culture condition for sturgeon germ cells. Germ cells after 40 d culture developed in recipient gonads. This study provided useful information for culture of sturgeon germ cells.

Enhanced Adhesion of Fish Ovarian Germline Stem Cells on Solid Surfaces by Mussel-Inspired Polymer Coating

Development of advanced cell culture methods has gained increasing attention because it allows for efficient genetic engineering and precise regulation of animal reproduction on a cellular basis. Numerous studies have attempted to develop an advanced cell culture method. Previous studies have altered cell culture media and pretreated culture plates with functional molecules. Among them, a mussel-inspired polymer coating has been extensively utilized owing to its wide applicability. For instance, adhesion of human embryonic stem cells and neuronal cells on solid surfaces has been improved. Despite the excellent capability of the mussel-inspired polymer coating, most studies have primarily focused on mammalian cells. However, the efficacy of these coatings on the adhesion of other cell lines is yet unclear. This study aimed to assess the potential of the mussel-inspired polymer coating in the regulation of the adhesion of fish ovarian germline stem cells on solid surfaces. Solid surfaces were coated by polydopamine and poly-L-lysine, and the effect of the coatings on cellular behaviors was investigated.

Efficient and Stable Delivery of Multiple Genes to Fish Cells by a Modified Recombinant Baculovirus System

The recombinant baculovirus has been widely used as an efficient tool to mediate gene delivery into mammalian cells but has barely been used in fish cells. In the present study, we constructed a recombinant baculovirus containing the dual-promoter cytomegalovirus (CMV) and white spot syndrome virus (WSSV) immediate-early gene 1 (ie1) (WSSV ie1), followed by a puromycin⁻green fluorescent protein (Puro-GFP, pf) or puromycin⁻red fluorescent protein (Puro-RFP, pr) cassette, which simultaneously allowed for easy observation, rapid titer determination, drug selection, and exogenous gene expression. This recombinant baculovirus was successfully transduced into fish cells, including Mylopharyngodon piceus bladder (MPB), fin (MPF), and kidney (MPK); Oryzias latipes spermatogonia (SG3); and Danio rerio embryonic fibroblast (ZF4) cells. Stable transgenic cell lines were generated after drug selection, which was further verified by Western blot. A cell monoclonal formation assay proved the stable heredity of transgenic MPB cells. In addition, a recombinant baculovirus containing a pr cassette and four transcription factors for induced pluripotent stem cells (iPSC) was constructed and transduced into ZF4 cells, and these exogenous genes were simultaneously delivered and transcribed efficiently in drug-selected ZF4 cells, proving the practicability of this modified recombinant baculovirus system. We also proved that the WSSV ie1 promoter had robust activity in fish cells in vitro and in vivo. Taken together, this modified recombinant baculovirus can be a favorable transgenic tool to obtain transient or stable transgenic fish cells.

Both Gfrα1a and Gfrα1b Are Involved in the Self-renewal and Maintenance of Spermatogonial Stem Cells in Medaka

Glial cell-derived neurotrophic factor family receptor alpha-1 (GFRα1) plays a crucial role in the self-renewal and maintenance of spermatogonial stem cells (SSCs) from mammals. However, to date, our knowledge about its role in fish SSCs is limited. In the present study, the medaka (Oryzias latipes) gfrα1 duplicate genes, Olgfrα1a and Olgfrα1b, were cloned and characterized. Furthermore, their expression profile and biological activity were investigated. OlGfrα1a and OlGfrα1b predict 524 and 466 amino acid residues, respectively. Both are orthologous to mammalian Gfrα1 by sequence analyses and appear high in spermatogonia by in situ hybridization assay. The knockdown of OlGfrα1a and/or OlGfrα1b via Vivo-Morpholino oligos significantly inhibited the self-renewal and maintenance of SSCs, as evidenced by the decreased proliferation activity of SG3 cells (a spermatogonial stem cell line derived from adult medaka testis) as well as spermatogonia in the testicular organ culture and by the decreased survival rate and expression levels of pluripotency-related genes (klf4, lin28b, bcl6b, and etv5) in SG3 cells. Additionally, our study indicates that OlGfrα1a might function by binding either Gdnfa or Gdnfb (the two medaka Gdnf homologs), whereas OlGfrα1b function by binding Gdnfa not Gdnfb. Taken together, our study indicates that both OlGfrα1a and OlGfrα1b are involved in the self-renewal and maintenance of SSCs by binding Gdnfa and/or Gdnfb, respectively. These findings suggest that the GDNF/GFRα1 signaling pathway might be conserved from mammals to fish species.

Expression profiling of c-kit and its impact after esiRNA silencing during gonadal development in catfish

Receptor, c-Kit is a member of a family of growth factor receptors that have tyrosine kinase activity, and are involved in the transduction of growth regulatory signals across plasma membrane by activation of its ligand, kitl/scf. The present study analyzed mRNA and protein expression profiles of c-kit in the gonads of catfish, Clarias gariepinus, using real time PCR, in situ hybridization and immunohistochemistry. Tissue distribution analysis revealed higher expression mainly in the catfish gonads. Ontogeny studies showed minimal expression during early developmental stages and highest during 50-75 days post hatch, and the dimorphic expression in gonads decreased gradually till adulthood, which might suggest an important role for this gene around later stages of sex differentiation and gonadal development. Expression of c-kit was analyzed at various phases of gonadal cycle in both male and female, which showed minimal expression during the resting phase, and higher expression during the pre-spawning phase in male compared to females. In vitro and in vivo induction using human chorionic gonadotropin elevated the expression of c-kit indicating the regulatory influence of hypothalamo-hypophyseal axis. In vivo transient gene silencing using c-kit-esiRNA in adult catfish during gonadal recrudescence showed a decrease in c-kit expression, which affected the expression levels of germ cell meiotic marker sycp3, as well as several factors and steroidogenic enzyme genes that are involved in germ cell development. Decrease in the levels of 11-ketotestosterone and testosterone in serum were also observed after esiRNA silencing. The findings suggests that c-kit has an important role in the process of germ cell proliferation, development and maturation during gonadal development and recrudescence in catfish.

Efficient genome editing using CRISPR/Cas9 ribonucleoprotein approach in cultured Medaka fish cells

Gene editing with CRISPR/Cas9 is a powerful tool to study the function of target genes. Although this technology has demonstrated wide efficiency in many species, including fertilized zebrafish and medaka fish embryos when microinjected, its application to achieve efficient gene editing in cultured fish cells have met some difficulty. Here, we report an efficient and reliable approach to edit genes in cultured medaka (Oryzias latipes) fish cells using pre-formed gRNA-Cas9 ribonucleoprotein (RNP) complex. Both medaka fish haploid and diploid cells were transfected with the RNP complex by electroporation. Efficient gene editing was demonstrated by polymerase chain reaction (PCR) amplification of the target gene from genomic DNA and heteroduplex mobility assay carried out with polyacrylamide gel electrophoresis (PAGE). The heteroduplex bands caused by RNP cleavage and non-homologous end joining could be readily detected by PAGE. DNA sequencing confirmed that these heteroduplex bands contains the mutated target gene sequence. The average gene editing efficiency in haploid cells reached 50%, enabling us to generate a clonal cell line with ntrk3b gene mutation for further study. This RNP transfection method also works efficiently in diploid medaka cells, with the highest mutation efficiency of 61.5%. The specificity of this synthetic RNP CRISPR/Cas9 approach was verified by candidate off-target gene sequencing. Our result indicated that transfection of pre-formed gRNA-Cas9 RNP into fish cells is efficient and reliable to edit target genes in cultured medaka fish cells. This method will be very useful for gene function studies using cultured fish cells.

Isolation and in vitro culture of ovarian stem cells in Chinese soft-shell turtle (Pelodiscus sinensis)

Gonadal cell lines provide valuable tools for studying gametogenesis, sex differentiation, and manipulating germ cells in reproductive biology. Female germline stem cells have been characterized and isolated from ovaries of mammalian species, including mice and human, but there has been very few studies on female germline stem cells in reptiles. Here, we described an ovarian stem cell-like line isolated and cultured from the Chinese soft-shell turtle (Pelodiscus sinensis), designated as PSO1. The cells showed high alkaline phosphatase activity with a normal diploid karyotype. As shown by reverse transcription-polymerase chain reaction, the cells were positive for the expression of germ cell-specific genes, vasa and dazl, as well as a stem cell marker, nanog, but negative for the expression of the folliculogenesis-specific gene, figla. Likewise, through fluorescent immunostaining analyses, both the Dazl and Vasa proteins were detected abundantly in the cytoplasm of perinuclear region, whereas Nanog and PCNA were dominantly observed in the nuclei in PSO1 cells. Moreover, PSO1 cells transfected with pCS2:h2b-egfp could properly express the fusion protein in the nuclei. Taken together, the findings suggested that the germline stem cells exist in the ovary of juvenile Chinese soft-shell turtle and these cells can be isolated for a long-term in vitro culture under experimental conditions. This study has provided a valuable basis for further investigations on the molecular mechanisms whereby the germline stem cells develop and differentiate into gametes in turtles. Also, it has paved the way for studies on oogenesis in turtles, even in the other reptiles.