Identification and characterization of a novel gene differentially expressed in zebrafish cross-subfamily cloned embryos
© Pei et al; licensee BioMed Central Ltd. 2008
Received: 29 June 2007
Accepted: 18 March 2008
Published: 18 March 2008
Cross-species nuclear transfer has been shown to be a potent approach to retain the genetic viability of a certain species near extinction. However, most embryos produced by cross-species nuclear transfer were compromised because that they were unable to develop to later stages. Gene expression analysis of cross-species cloned embryos will yield new insights into the regulatory mechanisms involved in cross-species nuclear transfer and embryonic development.
A novel gene, K31, was identified as an up-regulated gene in fish cross-subfamily cloned embryos using SSH approach and RACE method. K31 complete cDNA sequence is 1106 base pairs (bp) in length, with a 342 bp open reading frame (ORF) encoding a putative protein of 113 amino acids (aa). Comparative analysis revealed no homologous known gene in zebrafish and other species database. K31 protein contains a putative transmembrane helix and five putative phosphorylation sites but without a signal peptide. Expression pattern analysis by real time RT-PCR and whole-mount in situ hybridization (WISH) shows that it has the characteristics of constitutively expressed gene. Sub-cellular localization assay shows that K31 protein can not penetrate the nuclei. Interestingly, over-expression of K31 gene can cause lethality in the epithelioma papulosum cyprinid (EPC) cells in cell culture, which gave hint to the inefficient reprogramming events occurred in cloned embryos.
Taken together, our findings indicated that K31 gene is a novel gene differentially expressed in fish cross-subfamily cloned embryos and over-expression of K31 gene can cause lethality of cultured fish cells. To our knowledge, this is the first report on the determination of novel genes involved in nucleo-cytoplasmic interaction of fish cross-subfamily cloned embryos.
Nuclear reprogramming is used to describe that the transferred nucleus from partially or fully differentiated cell has the potential to direct the reconstructed embryo to develop like a normal embryo. Although successful production of animal clones from somatic cells has been achieved in various species, many problems in offspring could not be hurdled due to incomplete nuclear reprogramming . Cross-species nuclear transfer involves transferring cell nuclei of one species into enucleated oocytes of another species, which has been shown to be a potent approach to retain the genetic viability of a certain species near extinction . However, most embryos produced by cross-species nuclear transfer were compromised because they were unable to develop to later developmental stages. To study inefficient reprogramming of the donor nuclei in the recipient cytoplasm from another species, nuclear transfer (NT) between two fish species was used as a model in the present study. A pioneering study on fish NT was carried out by Tung et al  and extensive studies on fish cross-species NT were mainly conducted in Cyprinid . Recently, cross-genus cloned fish derived from transgenic common carp nuclei and goldfish enucleated eggs were generated and the somitogenesis and vertebral number of the cloned fish were consistent to the egg-providing species, goldfish (Carassius auratus), instead of the donor cell species, common carp (Cyprinus carpio) . Gene expression analysis of cross-species cloned embryos will shed light on the regulatory mechanisms involved in cross-species nuclear transfer and embryonic development.
Nuclear transfer between two laboratory fish species, rare minnow (Gobiocypris rarus) and zebrafish (Danio rerio), provides an ideal model for the study of cross-species nuclear transfer. Rare minnow and zebrafish belong to different subfamily – the Gobioninae and the Danioninae [7, 8]. Zebrafish is a notable model for developmental and genetic studies for its short sex-maturity cycle, high reproductive capacity, and transparent eggs, etc [9, 10]. Rare minnow, a special local species in China, not only shares aforementioned advantages with zebrafish, but also has many unique traits for laboratory study such as typical eurytherm and high adaptation , and sensitivity to toxicity and virus [12, 13]. Such advantages enable rare minnow to be an excellent type of experimental fish .
In the present study, we performed cross-subfamily nuclear transfer between zebrafish and rare minnow and obtained nuclear transfer embryos derived from zebrafish nuclei and rare minnow enucleated eggs. Using a suppression subtractive hybridization (SSH) approach, we found a novel gene – K31 over-expressed in cloned embryos, potentially participating in the improper reprogramming of transferred nuclei.
Identification of K31 as an up-regulated gene
Cloning and characterization of K31 gene
Expression profile of K31 during development
Sub-cellular localization and ectopic over-expression of K31 gene
Nuclear transfer in fish has been successfully manipulated for over 40 years . Cross-genus cloned fish derived from common carp nuclei and goldfish enucleated eggs was given birth as reported by Sun et al . In our recent study, we not only demonstrated the success of cloned embryos derived from zebrafish nuclei and rare minnow enucleated eggs, but also the success of cloned embryos derived from rare minnow nuclei and zebrafish enucleated eggs by SCAR approaches . Samples used in the present study were strictly chosen at sphere stage of embryogenesis as described by Kimmel et al  to ensure the accuracy. In single cloned embryo, SCAR PCR was performed with primers distinguished rare minnow from zebrafish, revealing the success of nuclear transfer . Consequently, the novel gene, K31, is certainly resulted from the nucleo-cytoplasmic interaction between zebrafish and rare minnow, excluding any artificial interfere.
Previous studies demonstrated that the majority of embryos produced by nuclear transfer were compromised because they were unable to develop past the early development stages . A common hypothesis is that inefficient reprogramming of the donor nucleus results in inappropriate expression of genes required for embryonic development. Gene expression analysis of individual embryos will undoubtedly yield new insights into the regulatory mechanisms involved in nuclear transfer inducing reprogramming and embryonic development. In the present study, a novel gene, K31 was found to be over-expressed in cross-subfamily cloned embryos by SSH approach. To our knowledge, this is the first report of gene expression analysis in cross-subfamily cloned embryos. Interestingly, K31 transcripts were found in all embryonic development stages of zebrafish and K31 was also expressed maternally in ovum, but its ectopic over-expression caused the EPC cells to die during the cell culture. Result of sub-cellular localization indicates that K31 protein can not penetrate the nuclei, and it may be participate in signal transduction by its five putative phosphorylation sites. Previous studies indicated that the reprogramming efficiency of cloned embryos was influenced by many factors, such as epigenetic changes like DNA methylation and XCI patterns[20, 21], failure to suppress previously active gene transcription as well as failure to activate previously inactive genes [22–24], failure of chromatin remodeling [21, 25–27] and mitochondria effects of proper reprogramming , etc. Yet, we can conclude that nucleo-cytoplasmic interaction in such cross-subfamily cloned embryos caused the over-expression of K31 gene. Further reliable studies in a comprehensive way should provide solid evidences to unveil whether K31 gene can affect the nuclear reprogramming in cloned embryos.
Cross-species nuclear transfer can be used to maintain limited populations of highly endangered species, especially when the oocytes of these species are difficult to obtain. However, most embryos produced by cross-species nuclear transfer were compromised because they were unable to develop to later developmental stages. Therefore, gene expression analysis of cross-species cloned embryos is necessary. Here we used two laboratory fish species, rare minnow and zebrafish as cross-subfamily nuclear transfer model and report K31 gene as an up-regulated gene in fish cross-subfamily cloned embryos. Importantly, ectopic over-expression of K31 gene can cause lethality of EPC cells in the cell culture, which gave hint that why most of the cloned embryos were developmentally arrested in between the stages of sphere and 50% epiboly.
Preparation of cross-subfamily cloned embryos and non-cloned embryos
The cross-subfamily cloned embryos were generated by nuclear transfer as described by Sun et al. , with nuclei derived from zebrafish at blastula stage and the enucleated unfertilized eggs of rare minnow. Meanwhile, batches of non-cloned zebrafish embryos were produced by in vitro fertilization. All embryos were incubated in Holtfreter's solution at 28°C, and collected at sphere stage . The manipulations in the experiment adhered to Guidelines for Animal Use in Biomedical Research Laboratories (ILAR, 1996).
SMART cDNA synthesis and construction of SSH cDNA libraries
Total RNA was extracted from cloned and zebrafish embryos by SV™ total RNA kit (Promega, WI, USA). Poly(A)+ RNA was purified with Poly(A)Tract mRNA Isolation system (Promega WI, USA) and then used to synthesize SMART cDNA according to the instructions of BD SMART cDNA Library Construction Kit (Clontech, CA, USA). The forward subtracted cDNA library was obtained using tester dscDNAs from cloned embryos and driver dscDNA from zebrafish embryos. At the same time, the reverse subtracted cDNA library was obtained using tester dscDNAs from zebrafish embryos and driver dscDNA from cloned embryos. To evaluate the efficiency of the cDNA subtraction, reverse transcriptase PCR was performed with GAPDH (glyceraldehyde-3-phosphate dehydrogenase) primers in forward subtracted and unsubtracted cDNA. After the secondary PCR, the PCR products generated by SSH were cloned into the pGEM-T Easy vector (Promega, WI, U.S.A). PCR and dot blots were applied to screen improper reprogramming genes from the subtracted cDNA library as described by Sung et al .
RACE-PCR and real-time PCR analysis
Primers used in the present study
The relative quantification with real-time RT-PCR was done as described by the manufacturer (Applied Biosystems, USA) with slight modification. In brief, the samples were placed in 96 well plates and amplified in an automated fluorometer (ABI PRISM 7000 Sequence Detection System, Applied Biosystems). Each PCR proceeded in 30 μl SYBR Green PCR buffer (Applied Biosystems) containing 400 nM K31F and K31R primers, 1 U AmpliTaq Gold DNA polymerase, 2.5 mM dNTPs, 0.5 U AmpErase UNG, 3 mM MgCl2 and 50 ng cloned embryos or non-cloned embryos cDNA template. Amplification conditions were 2 min at 50°C, 10 min at 95°C, 40 cycles of 30 s at 95°C and 60 s at 60°C. All samples were analyzed in triplet and the results were expressed as relative fold of the expression of the GAPDH gene with 2 [-deltadeltaCT] method with correction for different amplification efficiencies (the amplification efficiency of cloned embryos or zebrafish embryos is 0.995 or 0.990, respectively) .
Data mining and bio-information analyses
Homology search of K31 gene was performed on the sequences listed in EMBL/GenBank/DDBJ databases using PHI- and PSI-BLAST, EST-BLAST and Protein-protein BLAST (blastp) at the web site of the National Center of Biotechnology Information (NCBI) . Secondary structure analysis of K31 protein sequence was performed by DNAstar software (Lasergene, Madison, Wis.). Transmembrane helices, phosphorylation sites and signal peptide were predicted by TMHMM2.0 software , NetPhos 2.0 software  and SignalP 3.0 software , respectively.
Analysis of expression pattern by real time RT-PCR and whole-mount in situ hybridization (WISH)
Zebrafish embryos were obtained by in vitro fertilization and raised in Holtfreter's solution (0.35% NaCl, 0.01% KCl, and 0.01% CaCl2) at 28°C. Embryos were staged according to Kimmel et al . Total RNA of 8 samples (ovum, zygote, 256-cell stage, sphere stage, 50%-epiboly stage, 90%-epiboly stage, 15-somite stage, pharyngula period) was separately isolated using SV™ total RNA kit (Promega, WI, USA). Then, 3 μg total RNA was reverse transcribed (RT) for each sample and 2 μl of the RT product was amplified to quantify K31 transcripts by real-time RT-PCR. The manipulation of real-time RT-PCR was all the same as described above (to see materials and methods 2.3).
For WISH, embryos were fixed in MEMPFA (100 mM Mops (Sigma), pH 7.4; 2 mM EGTA (Sigma); 1 mM MgSO4 (Merck); 4% (w/v) paraformaldehyde (Sigma) at different developmental stages. For generation of full-length K31 antisense probes, pBluescripts KS II-K31 plasmids were linearized and used as templates for synthesis of DIG-labeled antisense RNA (Roche). RNA probe was purified using RNeasy columns (QIAGEN). In situ hybridization essentially following the protocol described by Thisse et al. . WISH was performed with a probe concentration of 100 ng/mL at 65°C. As a control, WISH with similarly produced sense probes was performed. Images of zebrafish embryos were recorded using an Olympus SZX12 microscope and a digital camera.
Cell culture and sub-cellular localization and ectopic over-expression of K31
EPC cells from carp (Cyprinus carpio) were cultured in medium 199 supplemented with 10% fetal calf serum (FCS) and antibiotics (100 U/ml penicillin and 100 μg/ml streptomycin). Cultures were maintained at 28°C in an atmosphere of 5% CO2 in air.
For fluorescence microscopy, the coding region of K31 gene was amplified using K31DW primers (Table 1) and cloned into pEGFP-N3 (BD Biosciences, PaloAlto, CA, USA) using PstI and BamHI sites. After sequencing validation, the p K31-EGFP construct was transfected into EPC cells using Lipofectamine 2000 reagent (Invitrogen). After 24, 28 and 32 h culture, cells were removed by trypsin/EDTA and analysed for sub-cellular localization and ectopic over-expression with 5 mg/L Hochest 33342 (Calbiochem) to stain the nuclei of cells as described  and 0.4% trypan blue dye (sigma) to stain the dead cells as described , respectively.
open reading frame
suppression subtractive hybridization
whole-mount in situ hybridization
rapid amplification cDNA ends
switch mechanism at the 5' end of RNA templates.
This work was supported by the State Key Fundamental Research of China (Grant No 2004CB117406) and the National Natural Science Foundation of China (Grant No. 30771100 and 30700607). In addition, the authors are grateful to Ming Li for supplying experimental materials and Qiya Zhang for providing EPC cells.
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