Establishment of novel monoclonal antibodies for identification of type A spermatogonia in teleosts†

Advantages, Factors, Obstacles, Potential Solutions, and Recent Advances of Fish Germ Cell Transplantation for Aquaculture-A Practical Review

Simple Summary

This review aims to provide practical information and viewpoints regarding fish germ cell transplantation for enhancing its commercial applications. We reviewed and summarized the data from more than 70 important studies and described the advantages, obstacles, recent advances, and future perspectives of fish germ cell transplantation. We concluded and proposed the critical factors for achieving better success and various options for germ cell transplantation with their pros and cons. Additionally, we discussed why this technology has not actively been utilized for commercial purposes, what barriers need to be overcome, and what potential solutions can advance its applications in aquaculture.

Abstract

Germ cell transplantation technology enables surrogate offspring production in fish. This technology has been expected to mitigate reproductive barriers, such as long generation time, limited fecundity, and complex broodstock management, enhancing seed production and productivity in aquaculture. Many studies of germ cell transplantation in various fish species have been reported over a few decades. So far, surrogate offspring production has been achieved in many commercial species. In addition, the knowledge of fish germ cell biology and the related technologies that can enhance transplantation efficiency and productivity has been developed. Nevertheless, the commercial application of this technology still seems to lag behind, indicating that the established models are neither beneficial nor cost-effective enough to attract potential commercial users of this technology. Furthermore, there are existing bottlenecks in practical aspects such as impractical shortening of generation time, shortage of donor cells with limited resources, low efficiency, and unsuccessful surrogate offspring production in some fish species. These obstacles need to be overcome through further technology developments. Thus, we thoroughly reviewed the studies on fish germ cell transplantation reported to date, focusing on the practicality, and proposed potential solutions and future perspectives.

Characterization of a vasa homolog in Mekong giant catfish (Pangasianodon gigas): Potential use as a germ cell marker

For the long-term preservation of the genetic resources of endangered fish species, a combination of germ cell cryopreservation and transplantation can be an effective technique. To optimize these techniques, it is important to identify undifferentiated germ cells possessing transplantability, such as primordial germ cells, type A spermatogonia (ASGs), and oogonia. In this study, a homolog of vasa cDNA in Mekong giant catfish (MGC-vasa) (Pangasianodon gigas), which is an endangered species inhabiting the Mekong river, was cloned and characterized for use as a putative germ cell marker. Results indicate that MGC-Vasa contained all of the consensus motifs, including the arginine-glycine and arginine-glycine-glycine motifs, as well as the nine conserved motifs belonging to the DEAD-box family of proteins. Results from phylogenetic analysis indicated MGC-vasa also grouped with Vasa and was clearly distinguishable from Pl10 in other teleosts. Results from analysis of abundance of mRNA transcripts using reverse transcription-polymerase chain reaction and in situ hybridization performed on immature Mekong giant catfish testis indicated vasa was present specifically in germ cells, with large abundances of the relevant mRNA in spermatogonia and spermatocytes. Sequence similarity and the specific localization of MGC-vasa in these germ cells suggest that the sequence ascertained in this study was a vasa homolog in Mekong giant catfish. Furthermore, vasa-positive cells were detected in prepared smears of testicular cells, indicating that it may be a useful germ cell marker for enzymatically dissociated cells used for transplantation studies.

Early warming stress on rainbow trout juveniles impairs male reproduction but contrastingly elicits intergenerational thermotolerance

The exposure of adult fish to warm or high temperatures is known to impair reproduction, yet the long-term reproductive impacts for treatments at early life are not well clarified. This study aimed to evaluate the effects of warm temperature (WT) during juvenile stage on gonad maturation, gamete quality, and offspring thermotolerance in rainbow trout. While the comparison of basic reproductive parameters in WT females did not reveal any kind of impairment, many WT males showed an atrophied, undeveloped gonad, or a smaller testis with lower milt volume; sperm quality parameters in WT males and deformity rates in the respective progeny were also highly affected. However, despite of such negative effects, many of the remaining progeny presented better rates of survival and growth when exposed to the same conditions as those of parental fish (WT), suggesting that thermal stress in parr stage males elicited intergenerational thermotolerance after a single generation. The present results support that prolonged warming stress during early life stages can adversely affect key reproductive aspects, but contrastingly increase offspring performance at upper thermal ranges. These findings have implications on the capacity of fish to adapt and to cope with global warming.

Cryopreservation of Germ Cells of Banana Shrimp (Fenneropenaeus merguiensis) and Black Tiger Shrimp (Penaeus monodon)

Germ cell cryopreservation has been used to preserve many fish species. However, this method has not been established for crustaceans; thus, we attempted to do this herein. The efficiency of slow freezing was compared to vitrification methods for germ cell cryopreservation in two types of marine shrimp, Fenneropenaeus merguiensis and Penaeus monodon. In situ hybridization with a vasa probe was used to identify germ cells. The effects of three cryoprotectants, dimethyl sulfoxide (DMSO), glycerol (GLY), and magnesium chloride (MgCl₂), on germ cell viability and recovery rate were compared at three concentrations (5%, 10%, and 15%). The effects of thawing temperature, including 10 and 27 °C, were also investigated. We discovered that 10% DMSO with the vitrification is suitable for preserving the germ cells of F. merguiensis for a long time, whereas 10% GLY with vitrification is suitable for P. monodon. Moreover, the most suitable thawing temperature was 10 °C for both species. This is the first report of germ cell cryopreservation in crustaceans. Thus, we provide evidence that crustacean germ cells can be preserved long-term in liquid nitrogen; this is the first step in the sustainable preservation of crustaceans, especially shrimp.

Testicular subcutaneous allografting followed by immunosuppressive treatment promotes maintenance of spermatogonial cells in rainbow trout (Oncorhynchus mykiss)

Germ cell transplantation and testis graft represent promising biotechnologies that can be applied for the reproduction of commercial or endangered species. However, mechanisms of rejection from the host immune system might remove the transplanted donor cells/tissues and limit the surrogate production of gametes. In this work, we administered emulsion containing-immunosuppressants to verify whether they are capable to prevent immune rejection and promote survival of testis allografts in rainbow trout. In the first part of this study, we demonstrated in vitro that tacrolimus and cyclosporine were able to affect viability, inhibit leucocyte proliferation, and suppress il2 expression in vitro. In in vivo experiments, both doses of tacrolimus (0.5 and 1.5 mg/kg) and the lower dose of cyclosporine (20 mg/kg) significantly inhibited the expression of il2 in head kidney, three days post-injection. A higher dose of cyclosporine (40 mg/kg) was able to inhibit il2 expression for up to seven days post-injection. In the second part, testis allografts were conducted in fish treated weekly with emulsion containing-tacrolimus. Immunohistochemical, conventional histology, and qRT-PCR (vasa) analysis demonstrated the presence of spermatogonial cells by the fifth week, in animals treated with 0.5 mg/kg of tacrolimus similar as found in autografted group. In the group treated with the highest tacrolimus dose (1.5 mg/kg) and in the non-treated group (without immunosuppressant), no germ cells or their respective markers were detected. il2 expression in head kidney was also suppressed in grafted animals treated with tacrolimus compared to non-treated group. These results suggest that tacrolimus may be a promising immunosuppressant for testis allografts or germ cell transplantation in rainbow trout. Co-administration combining tacrolimus (at lower dose) with other immunosuppressive drugs for inhibiting other activation pathways of the immune system, as performed in human organ transplantation, could be an alternative approach to optimize the immunosuppressive effects in host organisms.

Surrogate broodstock to enhance biotechnology research and applications in aquaculture

Aquaculture is playing an increasingly important role in meeting global demands for seafood, particularly in low and middle income countries. Genetic improvement of aquaculture species has major untapped potential to help achieve this, with selective breeding and genome editing offering exciting avenues to expedite this process. However, limitations to these breeding and editing approaches include long generation intervals of many fish species, alongside both technical and regulatory barriers to the application of genome editing in commercial production. Surrogate broodstock technology facilitates the production of donor-derived gametes in surrogate parents, and comprises transplantation of germ cells of donors into sterilised recipients. There are many successful examples of intra- and inter-species germ cell transfer and production of viable offspring in finfish, and this leads to new opportunities to address the aforementioned limitations. Firstly, surrogate broodstock technology raises the opportunity to improve genome editing via the use of cultured germ cells, to reduce mosaicism and potentially enable in vivo CRISPR screens in the progeny of surrogate parents. Secondly, the technology has pertinent applications in preservation of aquatic genetic resources, and in facilitating breeding of high-value species which are otherwise difficult to rear in captivity. Thirdly, it holds potential to drastically reduce the effective generation interval in aquaculture breeding programmes, expediting the rate of genetic gain. Finally, it provides new opportunities for dissemination of tailored, potentially genome edited, production animals of high genetic merit for farming. This review focuses on the state-of-the-art of surrogate broodstock technology, and discusses the next steps for its applications in research and production. The integration and synergy of genomics, genome editing, and reproductive technologies have exceptional potential to expedite genetic gain in aquaculture species in the coming decades.

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Genetic improvement in aquaculture species has a major role in global food security. Advances in biotechnology provide new opportunities to support aquaculture breeding.

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Advances in biotechnology provide new opportunities to support aquaculture breeding.

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Donor-derived gametes can be produced from surrogate broodstock of several aquaculture species.

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Surrogate broodstock technology provides new opportunities for application of genome editing.

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Surrogate broodstock can accelerate genetic gain, and improve dissemination of elite germplasm.

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.

Enrichment of transplantable germ cells in salmonids using a novel monoclonal antibody by magnetic-activated cell sorting

In the fish germ cell transplantation system, only type A spermatogonia (ASGs) and oogonia are known to be incorporated into the recipient genital ridges, where they undergo gametogenesis. Therefore, high colonization efficiency can be achieved by enriching undifferentiated germ cells out of whole testicular cells. In this study, we used magnetic-activated cell sorting (MACS) for enriching undifferentiated germ cells of rainbow trout using a monoclonal antibody that recognizes a specific antigen located on the germ cell membrane. We screened the antibodies to be used for MACS by performing immunohistochemistry on rainbow trout gonads. Two antibodies, nos. 172 and 189, showed strong signals for ASGs and oogonia. Next, we performed MACS with antibody no. 172 using gonadal cells isolated from vasa-gfp rainbow trout showing GFP in undifferentiated germ cells. We found that GFP-positive cells are highly enriched in antibody no. 172-positive fractions. Finally, to examine the transplantability of MACS-enriched cells, we intraperitoneally transplanted sorted or unsorted cells into recipient larvae. We observed that transplantability of sorted cells, particularly ovarian cells, were significantly higher than that of unsorted cells. Therefore, MACS with antibody no. 172 could enrich ASGs and oogonia and become a powerful tool to improve transplantation efficiency in salmonids.