Critical developmental stages for the efficiency of somatic cell nuclear transfer in zebrafish

Exogenous Expression of pou5f3 Facilitates the Development of Somatic Cell Nuclear Transfer Embryos in Zebrafish at the Early Stage

Enucleated oocytes can reprogram differentiated nuclei to totipotency after somatic cell nuclear transfer (SCNT), which is valuable in understanding nuclear reprogramming and generating genetically modified animals. To date, reprogramming efficiency is low and the development of SCNT embryos is not going as well as anticipated. To further disclose the reprogramming mechanisms during SCNT zebrafish embryo development, we examined the expression patterns of transcription regulation factors and regulated them by mRNA and morpholino microinjection. In this study, we show that stem cell-related transcription factors are downregulated in zebrafish SCNT embryos at the blastula stage. Exogenous expression of pou5f3 at the single-cell stage improves SCNT embryo development from the blastula to the gastrula stage. We also found that exogenous expression of klf4 or sox2 decreases SCNT embryo development from the blastula to the gastrula stage, while expression of nanog is necessary for the development of SCNT embryos. Our results conclude that zebrafish pou5f3 facilitates the development of SCNT embryos from the blastula to gastrula stage.

Embryonic fate after somatic cell nuclear transfer in non-enucleated goldfish oocytes is determined by first cleavages and DNA methylation patterns

Reducing the variability in nuclear transfer outcome requires a better understanding of its cellular and epigenetic determinants, in order to ensure safer fish regeneration from cryobanked somatic material. In this work, clones from goldfish were obtained using cryopreserved fin cells as donor and non-enucleated oocytes as recipients. We showed that the high variability of clones survival was not correlated to spawn quality. Clones were then characterized for their first cleavages pattern in relation to their developmental fate up to hatching. The first cell cycle duration was increased in clones with abnormal first cleavage, and symmetric first two cleavages increased clone probability to reach later on 24 h- and hatching-stages. At 24 h-stage, 24% of the clones were diploids and from donor genetic origin only. However, ploidy and genetic origin did not determine clones morphological quality. DNA methylation reprogramming in the promoter region of pou2, nanog, and notail marker genes was highly variable, but clones with the nicest morphologies displayed the best DNA methylation reprogramming. To conclude, non-enucleated oocytes did allow authentic clones production. The first two cell cycles were a critical determinant of the clone ability to reach hatching-stage, and DNA methylation reprogramming significantly influenced clones morphological quality.

Overview of the betta fish genome regarding species radiation, parental care, behavioral aggression, and pigmentation model relevant to humans

BACKGROUND

The Siamese fighting fish (Betta splendens, also known as the betta) is well known in aquarium markets, and also presents an exciting new research model for studying parental care, aggressive behavior, and cryptically diverse pigmentation. However, concentrated efforts are required, both in the context of conservation biology and in its genetics, to address the problems of ongoing outbreeding depression, loss of biodiversity, and lack of scientific biological information.

OBJECTIVE

The evolutionary dynamics of the betta must be better understood at the genomic scale in order to resolve the phylogenetic status of unrecognized species, develop molecular markers to study variation in traits, and identify interesting sets of genes encoding various bioresource functions.

METHODS

The recent revolution in multi-omics approaches such as genomics, transcriptomics, epigenomics, and proteomics has uncovered genetic diversity and gained insights into many aspects of betta bioresources.

RESULTS

Here, we present current research and future plans in an ongoing megaproject to characterize the betta genome as de novo assemblies, genes and repeat annotations, generating data to study diverse biological phenomena. We highlight key questions that require answers and propose new directions and recommendations to develop bioresource management to protect and enhance the betta genus.

CONCLUSION

Successful accomplishment of these plans will allow the creation of a reference annotated genome and provide valuable information at the molecular level that can be utilized to sustain biodiversity and eco-management of the betta to improve breeding programs for future biomedical research.

Somatic cell nuclear transfer in non-enucleated goldfish oocytes: understanding DNA fate during oocyte activation and first cellular division

Nuclear transfer consists in injecting a somatic nucleus carrying valuable genetic information into a recipient oocyte to sire a diploid offspring which bears the genome of interest. It requires that the oocyte (maternal) DNA is removed. In fish, because enucleation is difficult to achieve, non-enucleated oocytes are often used and disappearance of the maternal DNA was reported in some clones. The present work explores which cellular events explain spontaneous erasure of maternal DNA, as mastering this phenomenon would circumvent the painstaking procedure of fish oocyte enucleation. The fate of the somatic and maternal DNA during oocyte activation and first cell cycle was studied using DNA labeling and immunofluorescence in goldfish clones. Maternal DNA was always found as an intact metaphase within the oocyte, and polar body extrusion was minimally affected after oocyte activation. During the first cell cycle, only 40% of the clones displayed symmetric cleavage, and these symmetric clones contributed to 80% of those surviving at hatching. Maternal DNA was often fragmented and located under the cleavage furrow. The somatic DNA was organized either into a normal mitotic spindle or abnormal multinuclear spindle. Scenarios matching the DNA behavior and the embryo fate are proposed.

A newly developed cloning technique in sturgeons; an important step towards recovering endangered species

Several steps of sturgeon somatic cell nuclear transfer (SCNT) have been recently established, but improvements are needed to make it a feasible tool to preserve the natural populations of this group of endangered species. The donor cell position inside the recipient egg seems to be crucial for its reprogramming; therefore by injecting multiple donor somatic cells instead of a single cell with a single manipulation, we increased the potential for embryo development. Using the Russian sturgeon Acipenser gueldenstaedtii as a multiple cell donor and sterlet Acipenser ruthenus as the non-enucleated egg recipient, we obtained higher proportion of eggs developing into embryos than previously reported with single-SCNT. Molecular data showed the production of a specimen (0.8%) contained only the donor genome with no contribution from the recipient, while two specimens (1.6%) showed both recipient and donor genome. These findings are the first report of donor DNA integration into a sturgeon embryo after interspecific cloning. In all, we provide evidence that cloning with the multiple donor somatic cells can be feasible in the future. Despite the fact that the sturgeon cloning faces limitations, to date it is the most promising technique for their preservation.

Nuclear import of Xenopus egg extract components into cultured cells for reprogramming purposes: a case study on goldfish fin cells

Reprogramming of cultured cells using Xenopus egg extract involves controlling four major steps: plasma membrane permeabilization, egg factors import into the nucleus, membrane resealing, and cell proliferation. Using propidium iodide to assess plasma membrane permeability, we established that 90% of the cultured fin cells were permeabilized by digitonin without any cell losses. We showed that egg extract at metaphase II stage was essential to maintain nuclear import function in the permeabilized cells, as assessed with a fusion GFP protein carrying the nuclear import signal NLS. Moreover, the Xenopus-egg-specific Lamin B3 was detected in 87% of the cell nuclei, suggesting that other egg extract reprogramming factors of similar size could successfully enter the nucleus. Lamin B3 labelling was maintained in most cells recovered 24 h after membrane resealing with calcium, and cells successfully resumed cell cycle in culture. In contrast, permeabilized cells that were not treated with egg extract failed to proliferate in culture and died, implying that egg extract provided factor essential to the survival of those cells. To conclude, fish fin cells were successfully primed for treatment with reprogramming factors, and egg extract was shown to play a major role in their survival and recovery after permeabilization.

The Role of Reproductive Sciences in the Preservation and Breeding of Commercial and Threatened Teleost Fishes

The teleost fishes are the largest and most diverse vertebrate group, accounting for nearly half of all known vertebrate species. Teleost fish exhibit greater species diversity than any other group of vertebrates and this is reflected in the unique variety of different reproductive strategies displayed by fish. Fish have always been an important resource for humans worldwide, especially as food. While wild capture fisheries have historically been the main source of fish, the farming of fish (aquaculture) is increasingly becoming the more dominant source of food fish, and is predicted to account for 60% of total global fish production by 2030.Fishes are increasingly threatened by a wide range of anthropogenic impacts, including loss of habitat, pollution, invasive species and over-exploitation. In addition, climate change, especially the consequences of global warming, can impact fish at all levels of biological organization from the individual to the population level, influencing both physiological and ecological processes in a variety of direct and indirect ways. As such, there is an urgent need to protect and conserve the huge genetic diversity offered by this diverse vertebrate group, not just as a source of genes for contemporary breeding and for protection against the consequences of climate change and disease, but also as part of our national heritage. While the cryopreservation of reproductive cells is a means of achieving these objectives, currently only fish sperm can be successfully frozen. Due to their large size, large yolk compartment, low membrane permeability and high chilling sensitivity, successful and reproducible protocols for the cryopreservation of fish oocytes and embryos still remains elusive. However, significant advances have been made in the cryopreservation of primordial germ cells as an alternative means of conserving both paternal and maternal genomes. Although more research needs to be carried out on how these cells can be optimally applied to emerging reproductive technologies, including transplantation techniques and surrogate broodstock technologies, the successful cryopreservation of fish germ cells, and the establishment of genetic resource banks, offers the possibility of both conserving and restoring threatened species. Further, current and future conservation efforts need to consider the impact of climate change in both in situ conservation and reintroduction efforts.In conclusion, it is anticipated that the successful cryopreservation of fish germplasm will result in a range of economic, ecological and societal benefits. In partnership with emerging assisted reproductive technologies, the successful cryopreservation of fish germplasm will lead to more efficient reproduction in aquaculture, assist selective breeding programmes, and be of crucial importance to future species conservation actions.

Long non-coding RNAs involved in the regulatory network during porcine pre-implantation embryonic development and iPSC induction

Long non-coding RNAs (lncRNA) play a key role in the orchestration of transcriptional regulation during development and many other cellular processes. The importance of the regulatory co-expression network was highlighted in the identification of the mechanism of these processes in humans and mice. However, elucidation of the properties of porcine lncRNAs involved in the regulatory network during pre-implantation embryonic development and fibroblast reprogramming to induced pluripotent stem cell (iPSC) has been limited to date. Using a weighted gene co-expression network analysis, we constructed the regulatory network and determined that the novel lncRNAs were functionally involved in key events of embryonic development during the pre-implantation period; moreover, reprogramming could be delineated by a small number of potentially functional modules of co-expressed genes. These findings indicate that lncRNAs may be involved in the transcriptional regulation of zygotic genome activation, first lineage segregation and somatic reprogramming to pluripotency. Furthermore, we performed a conservation and synteny analysis with the significant lncRNAs involved in these vital events and validated the results via experimental assays. In summary, the current findings provide a valuable resource to dissect the protein coding gene and lncRNA regulatory networks that underlie the progressive development of embryos and somatic reprogramming.

Application of interspecific Somatic Cell Nuclear Transfer (iSCNT) in sturgeons and an unexpectedly produced gynogenetic sterlet with homozygous quadruple haploid

Somatic cell nuclear transfer (SCNT) is a very promising cloning technique for reconstruction of endangered animals. The aim of the present research is to implement the interspecific SCNT (iSCNT) technique to sturgeon; one fish family bearing some of the most critically endangered species. We transplanted single cells enzymatically isolated from a dissociated fin-fragment of the Russian sturgeon (Acipenser gueldenstaedtii) into non-enucleated eggs of the sterlet (Acipenser ruthenus), two species bearing different ploidy (4n and 2n, respectively). Up to 6.7% of the transplanted eggs underwent early development, and one feeding larva (0.5%) was successfully produced. Interestingly, although this transplant displayed tetraploidism (4n) as the donor species, the microsatellite and species-specific analysis showed recipient-exclusive homozygosis without any donor markers. Namely, with regards to this viable larva, host genome duplication occurred twice to form tetraploidism during its early development, probably due to iSCNT manipulation. The importance of this first attempt is to apply iSCNT in sturgeon species, establishing the crucial first steps by adjusting the cloning-methodology in sturgeon's biology. Future improvements in sturgeon's cloning are necessary for providing with great hope in sturgeon's reproduction.

Fam60al as a novel factor involved in reprogramming of somatic cell nuclear transfer in zebrafish (Danio rerio)

The main reason for abnormal development of cloned animals or embryos, and inefficient animal cloning, is a poor understanding of the reprogramming mechanism. To better comprehend reprogramming and subsequent generation of pluripotent stem cells, we must investigate factors related to reprogramming of somatic cells as nuclear donors. As we know, fam60al (family with sequence similarity 60, member A, like) is a coding gene only found in zebrafish and frog (Xenopus laevis) among vertebrates. However, until now, its functions have remained unknown. Here, we generated a zebrafish fam60al^-/- mutant line using transcription activator-like effector nucleases (TALENs), and found that both nanog and klf4b expression significantly decreased while myca expression significantly increased in fam60al^-/- mutant embryos. Concurrently, we also uncovered that in developmentally arrested embryos of somatic cell nuclear transfer, nanog, klf4b and myca expression was down-regulated, accompanying a decrease of fam60al expression. Interestingly, we identified a long noncoding RNA (lncRNA) of fam60al, named fam60al-AS, which negatively regulated fam60al by forming double-stranded RNA (dsRNA). RNase protection assay and real-time PCR confirmed these findings. Taken together, these results suggest that fam60al is a novel factor related to the reprogramming of somatic cell nuclear transfer in zebrafish, which is regulated by its reverse lncRNA.

Isolation, characterization, and expression of proto-oncogene cMyc in large yellow croaker Larimichthys crocea

cMyc is a vital transcription factor that involves in the regulation of cell proliferation, growth, differentiation, and apoptosis. In the present study, cMyc in Larimichthys crocea (Lc-cMyc) was cloned and analyzed for investigating its function. The full-length cDNA of Lc-cMyc was 2089 bp encoding a 440-amino-acid protein (Lc-cMyc). Lc-cMyc had the characteristic helix-loop-helix-leucine-zipper (HLH-LZ) DNA-binding domain and highly conservative in evolution. The expression of Lc-cMyc was detected by quantitative real-time PCR (qRT-PCR) and in situ hybridization, respectively. In tissues, the gender differences of Lc-cMyc expression existed only in gonad and Lc-cMyc was extremely significantly expressed in ovary with the highest level in 635-dph ovary, especially in stages II (late) and III (early) oocytes. A certain degree of expression was examined in head kidney of both sexes and testis with high expression in spermatocyte. In embryos, Lc-cMyc was expressed at all embryonic stages. In early embryogenesis (from two-cell stage to mutiple-cell stage), Lc-cMyc was expressed very highly with a peak at two-cell stage. In late embryogenesis (from blastula stage to 1-day-post-hatching stage), the high expression of Lc-cMyc was detected as the following order: 1-day-post-hatching stage > pre-hatching stage > the appearance-of-optic-vesicles stage = mutiple-cell stage > beginning-of-heart-pulsation stage. The distribution of Lc-cMyc concentrated gradually in the back of embryos until in the head. In conclusion, the spatio-temporal expression patterns of Lc-cMyc indicated an essential role in oogenesis and embryogenesis and contributed to insight into the molecular mechanisms of regulating pluripotency in large yellow croaker.

Klf4 is required for germ-layer differentiation and body axis patterning during Xenopus embryogenesis

Klf4 is a transcription factor of the family of Kruppel-like factors and plays important roles in stem cell biology; however, its function during embryogenesis is unknown. Here, we report the characterization of a Klf4 homologue in Xenopus laevis during embryogenesis. Klf4 is transcribed both maternally and zygotically and the transcript is ubiquitous in embryos during germ-layer formation. Klf4 promotes endoderm differentiation in both Nodal/Activin-dependent and -independent manners. Moreover, Klf4 regulates anteroposterior body axis patterning via activation of a subset of genes in the Spemann organizer, such as Noggin, Dkk1 and Cerberus, which encode Nodal, Wnt and BMP antagonists. Loss of Klf4 function leads to the failure of germ-layer differentiation, the loss of responsiveness of early embryonic cells to inducing signals, e.g. Nodal/Activin, and the loss of transcription of genes involved in axis patterning. We conclude that Klf4 is required for germ-layer differentiation and body axis patterning by means of rendering early embryonic cells competent to differentiation signals.

Evolutionary history of c-myc in teleosts and characterization of the duplicated c-myca genes in goldfish embryos

c-Myc plays an important role during embryogenesis in mammals, but little is known about its function during embryonic development in teleosts. In addition, the evolutionary history of c-myc gene in teleosts remains unclear, and depending on the species, a variable number of gene duplicates exist in teleosts. To gain new insight into c-myc genes in teleosts, the present study was designed to clarify the evolutionary history of c-myc gene(s) in teleosts and to subsequently characterize DNA methylation and early embryonic expression patterns in a cyprinid fish. Our results show that a duplication of c-myc gene occurred before or around the teleost radiation, as a result of the teleost-specific whole genome duplication giving rise to c-myca and c-mycb in teleosts and was followed by a loss of the c-mycb gene in the Gasterosteiforms and Tetraodontiforms. Our data also demonstrate that both c-myc genes previously identified in carp and goldfish are co-orthologs of the zebrafish c-myca. These results indicate the presence of additional c-myca duplication in Cyprininae. We were able to identify differences between the expression patterns of the two goldfish c-myca genes in oocytes and early embryos. These differences suggest a partial sub-functionalization of c-myca genes after duplication. Despite differences in transcription patterns, both of the c-myca genes displayed similar DNA methylation patterns during early development and in gametes. Together, our results clarify the evolutionary history of the c-myc gene in teleosts and provide new insight into the involvement of c-myc in early embryonic development in cyprinids.

Massive production of all-female diploids and triploids in the crucian carp

In many species of aquaculture importance, all-female and sterile populations possess superior productivity due to faster growth and a relatively homogenous size of individuals. However, the production of all-female and sterile fish in a large scale for aquaculture is a challenge in practice, because treatments necessary for gynogenesis induction usually cause massive embryonic and larval mortality, and the number of induced gynogens is too small for their direct use in aquaculture. Here we report the massive production of all-female triploid crucian carp by combining artificial gynogenesis, sex reversal and diploid-tetraploid hybridization. Previously, we have obtained an allotetraploid carp population (4n = 200) by hybridization between red crucian carp (Carassius auratus red var; ♀) and common carp (Cyprinus carpio; ♂). We induced all-female diploid gynogens of the Japanese crucian carp (Carassius cuvieri; 2n = 100). We also generated male diploid gynogens of the same species treated gynogenetic fry with 17-α-methyltestosterone, leading to the production of sex-revered gynogenetic males. Finally, these males were used to cross with the female diploid Japanese crucian carp gynogens and the allotetraploid females, resulting in the production of fertile all-female diploid Japanese crucian carp (2n=100) and sterile all-female triploid hybrids (3n = 150), respectively. Therefore, diploid crucian carp gynogenetic females and sex-reversed male together with an allotetraploid line provide an opportunity to produce all-female triploid populations in a large scale to meet demands in aquaculture industry.

Identification of pluripotency genes in the fish medaka

Stem cell cultures can be derived directly from early developing embryos and indirectly from differentiated cells by forced expression of pluripotency transcription factors. Pluripotency genes are routinely used to characterize mammalian stem cell cultures at the molecular level. However, such genes have remained unknown in lower vertebrates. In this regard, the laboratory fish medaka is uniquely suited because it has embryonic stem (ES) cells and genome sequence data. We identified seven medaka pluripotency genes by homology search and expression in vivo and in vitro. By RT-PCR analysis, the seven genes fall into three groups of expression pattern. Group I includes nanog and oct4 showing gonad-specific expression; Group II contains sall4 and zfp281 displaying gonad-preferential expression; Group III has klf4, ronin and tcf3 exhibiting expression also in several somatic tissues apart from the gonads. The transcripts of the seven genes are maternally supplied and persist at a high level during early embryogenesis. We made use of early embryos and adult gonads to examine expression in stem cells and differentiated derivatives by in situ hybridization. Strikingly, nanog and oct4 are highly expressed in pluripotent blastomeres of 16-cell embryos. In the adult testis, nanog expression was specific to spermatogonia, the germ stem cells, whereas tcf3 expression occurred in spermatogonia and differentiated cells. Most importantly, all the seven genes are pluripotency markers in vitro, because they have high expression in undifferentiated ES cells but dramatic down-regulation upon differentiation. Therefore, these genes have conserved their pluripotency-specific expression in vitro from mammals to lower vertebrates.

Fishing fish stem cells and nuclear transplants

Fish has been the subject of various research fields, ranging from ecology, evolution, physiology and toxicology to aquaculture. In the past decades fish has attracted considerable attention for functional genomics, cancer biology and developmental genetics, in particular nuclear transfer for understanding of cytoplasmic-nuclear relationship. This special issue reports on recent progress made in fish stem cells and nuclear transfer.