Non-equivalence of cloned and clonal mice

Genome-Scale Analyses Reveal Roadblocks to Monkey Cloning

Cloning by somatic cell nuclear transfer (SCNT) remained challenging for Rhesus monkeys, mostly due to its low efficiency and neonatal death. Genome-scale analyses revealed that monkey SCNT embryos displayed widespread DNA methylation and transcriptional alterations, thus including loss of genomic imprinting that correlated with placental dysfunction. The transfer of inner cell masses (ICM) from cloned blastocysts into ICM-depleted fertilized embryos rescued placental insufficiency and gave rise to a cloned Rhesus monkey that reached adulthood without noticeable abnormalities.

Exposure of chimaeric embryos to exogenous FGF4 leads to the production of pure ESC-derived mice

Producing chimaeras constitutes the most reliable method of verifying the pluripotency of newly established cells. Moreover, forming chimaeras by injecting genetically modified embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) into the embryo is part of the procedure for generating transgenic mice, which are used for understanding gene function. Conventional methods for generating transgenic mice, including the breeding of chimaeras and tetraploid complementation, are time-consuming and cost-inefficient, with significant limitations that hinder their effectiveness and widespread applications. In the present study, we modified the traditional method of chimaera generation to significantly speed up this process by generating mice exclusively derived from ESCs. This study aimed to assess whether fully ESC-derived mice could be obtained by modulating fibroblast growth factor 4 (FGF4) levels in the culture medium and changing the direction of cell differentiation in the chimaeric embryo. We found that exogenous FGF4 directs all host blastomeres to the primitive endoderm fate, but does not affect the localisation of ESCs in the epiblast of the chimaeric embryos. Consequently, all FGF4-treated chimaeric embryos contained an epiblast composed exclusively of ESCs, and following transfer into recipient mice, these embryos developed into fully ESC-derived newborns. Collectively, this simple approach could accelerate the generation of ESC-derived animals and thus optimise ESC-mediated transgenesis and the verification of cell pluripotency. Compared to traditional methods, it could speed up functional studies by several weeks and significantly reduce costs related to maintaining and breeding chimaeras. Moreover, since the effect of stimulating the FGF signalling pathway is universal across different animal species, our approach can be applied not only to rodents but also to other animals, offering its utility beyond laboratory settings.

Completely ES cell-derived mice produced by tetraploid complementation using inner cell mass (ICM) deficient blastocysts

Tetraploid complementation is often used to produce mice from embryonic stem cells (ESCs) by injection of diploid (2n) ESCs into tetraploid (4n) blastocysts (ESC-derived mice). This method has also been adapted to mouse cloning and the derivation of mice from induced pluripotent stem (iPS) cells. However, the underlying mechanism(s) of the tetraploid complementation remains largely unclear. Whether this approach can give rise to completely ES cell-derived mice is an open question, and has not yet been unambiguously proven. Here, we show that mouse tetraploid blastocysts can be classified into two groups, according to the presence or absence of an inner cell mass (ICM). We designate these as type a (presence of ICM at blastocyst stage) or type b (absence of ICM). ESC lines were readily derived from type a blastocysts, suggesting that these embryos retain a pluripotent epiblast compartment; whereas the type b blastocysts possessed very low potential to give rise to ESC lines, suggesting that they had lost the pluripotent epiblast. When the type a blastocysts were used for tetraploid complementation, some of the resulting mice were found to be 2n/4n chimeric; whereas when type b blastocysts were used as hosts, the resulting mice are all completely ES cell-derived, with the newborn pups displaying a high frequency of abdominal hernias. Our results demonstrate that completely ES cell-derived mice can be produced using ICM-deficient 4n blastocysts, and provide evidence that the exclusion of tetraploid cells from the fetus in 2n/4n chimeras can largely be attributed to the formation of ICM-deficient blastocysts.

Genetic dissection of GABAergic neural circuits in mouse neocortex

Diverse and flexible cortical functions rely on the ability of neural circuits to perform multiple types of neuronal computations. GABAergic inhibitory interneurons significantly contribute to this task by regulating the balance of activity, synaptic integration, spiking, synchrony, and oscillation in a neural ensemble. GABAergic interneurons display a high degree of cellular diversity in morphology, physiology, connectivity, and gene expression. A considerable number of subtypes of GABAergic interneurons diversify modes of cortical inhibition, enabling various types of information processing in the cortex. Thus, comprehensively understanding fate specification, circuit assembly, and physiological function of GABAergic interneurons is a key to elucidate the principles of cortical wiring and function. Recent advances in genetically encoded molecular tools have made a breakthrough to systematically study cortical circuitry at the molecular, cellular, circuit, and whole animal levels. However, the biggest obstacle to fully applying the power of these to analysis of GABAergic circuits was that there were no efficient and reliable methods to express them in subtypes of GABAergic interneurons. Here, I first summarize cortical interneuron diversity and current understanding of mechanisms, by which distinct classes of GABAergic interneurons are generated. I then review recent development in genetically encoded molecular tools for neural circuit research, and genetic targeting of GABAergic interneuron subtypes, particularly focusing on our recent effort to develop and characterize Cre/CreER knockin lines. Finally, I highlight recent success in genetic targeting of chandelier cells, the most unique and distinct GABAergic interneuron subtype, and discuss what kind of questions need to be addressed to understand development and function of cortical inhibitory circuits.

Chromosomes in the porcine first polar body possess competence of second meiotic division within enucleated MII stage oocytes

To determine whether chromosomes in the porcine first polar body (PB1) can complete the second meiotic division and subsequently undergo normal pre-implantation embryonic development, we examined the developmental competence of PB1 chromosomes injected into enucleated MII stage oocytes by nuclear transfer method (chromosome replacement group, CR group). After parthenogenetic activation (PA) or in vitro fertilization (IVF), the cleavage rate of reconstructed oocytes in the IVF group (CR-IVF group, 36.4 ± 3.2%) and PA group (CR-PA group, 50.8 ± 4.2%) were significantly lower than that of control groups in which normal MII oocytes were subjected to IVF (MII-IVF group, 75.8 ± 1.5%) and PA (MII-PA group, 86.9 ± 3.7%). Unfertilized rates was significantly higher in the CR-IVF group (48.6 ± 3.3%) than in the MII-IVF group (13.1 ± 3.4%). The blastocyst formation rate was 8.3 ± 1.9% in the CR-PA group, whereas no blastocyst formation was observed in the CR-IVF group. To produce tetraploid parthenogenetic embryos, intact MII stage oocytes injected with PB1 chromosomes were electrically stimulated, treated with 7.5 μg/mL cytochalasin B for 3 h (MII oocyte + PB1 + CB group), and then cultured without cytochalasin B. The average cleavage rate of reconstructed oocytes was 72.5% (48 of 66), and the blastocyst formation rate was 18.7% (9 of 48). Chromosome analysis showed similar proportions of haploid and diploid cells in the control (normal MII oocytes) and CR groups after PA; overall, 23.6% of blastocysts were tetraploid in the MII oocyte + PB1 + CB group. These results demonstrate that chromosomes in PB1 can participate in normal pre-implantation embryonic development when injected into enucleated MII stage oocytes, and that tetraploid PA blastocysts are produced (although at a low proportion) when PB1 chromosomes are injected into intact MII stage oocytes.

Altered gene expression profiles in mouse tetraploid blastocysts

In this study, it was demonstrated that tetraploid-derived blastocyst embryos had very few Oct4-positive cells at the mid-blastocyst stage and that the inner cell mass at biomarkers Oct4, Sox2 and Klf4 was expressed at less than 10% of the level observed in diploid blastocysts. In contrast, trophectoderm-related gene transcripts showed an approximately 10 to 40% increase. Of 32,996 individual mouse genes evaluated by microarray, 50 genes were differentially expressed between tetraploid or diploid and parthenote embryos at the blastocyst stage (P<0.05). Of these 50 genes, 28 were more highly expressed in tetraploid-derived blastocysts, whereas 22 were more highly downregulated. However, some genes involved in receptor activity, cell adhesion molecule, calcium ion binding, protein biosynthesis, redox processes, transport, and transcription showed a significant decrease or increase in gene expression in the tetraploid-derived blastocyst embryos. Thus, microarray analysis can be used as a tool to screen for underlying defects responsible for the development of tetraploid-derived embryos.

Defects in trophoblast cell lineage account for the impaired in vivo development of cloned embryos generated by somatic nuclear transfer

The low success rate of somatic nuclear transfer (NT) is hypothesized to be mainly due to functional defects in the trophoblast cell lineage rather than the inner cell mass (ICM); this hypothesis, however, remains to be tested directly. Here we separated the ICMs from cloned blastocysts and aggregated the cloned ICM with two fertilization-derived (FD) tetraploid (4N) embryos. We found that the full-term development of cloned ICMs was dramatically improved after the trophoblast cells in the cloned blastocysts were replaced by cells from tetraploid embryos, thus providing direct evidence that defects in trophoblast cell lineage underlie the low success rate of somatic NT.

Functional full-term placentas formed from parthenogenetic embryos using serial nuclear transfer

Mammalian parthenogenetic embryos invariably die in mid-gestation from imprinted gene defects and placental hypoplasia. Based on chimera experiments, trophoblastic proliferation is supposed to be inhibited in the absence of a male genome. Here, we show that parthenogenetic mouse embryonic cell nuclei can be reprogrammed by serial rounds of nuclear transfer without using any genetic modification. The durations of survival in uteri of cloned foetuses derived from green fluorescent protein (GFP)-labelled parthenogenetic cell nuclei were extended with repeated nuclear transfers. After five repeats, live cloned foetuses were obtained up to day 14.5 of gestation; however, they did not survive longer even when we repeated nuclear transfer up to nine times. All foetuses showed intestinal herniation and possessed well-expanded large placentas. When embryonic stem (ES) cells derived from fertilised embryos were aggregated with the cloned embryos, full-term offspring with large placentas were obtained from the chimeric embryos. Those placentas were derived from parthenogenetic cell nuclei, judging from GFP expression. The patterns of imprinted gene expression and methylation status were similar to their parthenogenetic origin, except for Peg10, which showed the same level as in the normal placenta. These results suggest that there is a limitation for foetal development in the ability to reprogramme imprinted genes by repeated rounds of nuclear transfer. However, the placentas of parthenogenetic embryos can escape epigenetic regulation when developed using nuclear transfer techniques and can support foetal development to full gestation.

Acceptance of embryonic stem cells by a wide developmental range of mouse tetraploid embryos

Tetraploid (4N) complementation assay is regard as the most stringent characterization test for the pluripotency of embryonic stem (ES) cells. The technology can generate mice fully derived from the injected ES cell (ES-4N) with 4N placentas. However, it remains a very inefficient procedure owing to a lack of information on the optimal conditions for ES incorporation into the 4N embryos. In the present study, we injected ES cells from embryos of natural fertilization (fES) and somatic cell nuclear transfer (ntES) into 4N embryos at various stages of development to determine the optimal stage of ES cells integration by comparing the efficiency of full-term ES-4N mouse generation. Our results demonstrate that fES/ntES cells can be incorporated into 4N embryos at 2-cell, 4-cell and blastocyst stages and full-term mice can be generated. Interestingly, ntES cells injected into the 4-cell group resulted in the lowest efficiency (5.6%) compared to the 2-cell (13.8%, P > 0.05) and blastocyst (16.7%, P < 0.05) stages. Because 4N embryos start to form compacted morulae at the 4-cell stage, we investigated whether the lower efficiency at this stage was due to early compaction by injecting ntES cells into artificially de-compacted embryos treated with calcium free medium. Although the treatment changed the embryonic morphology, it did not increase the efficiency of ES-4N mice generation. Immunochemistry of the cytoskeleton displayed microtubule and microfilament polarization at the late 4-cell stage in 4N embryos, which suggests that de-compaction treatment cannot reverse the polarization process. Taken together, we show here that a wide developmental range of 4N embryos can be used for 4N complementation and embryo polarization and compaction may restrict incorporation of ES cells into 4N embryos.

Embryonic rather than extraembryonic tissues have more impact on the development of placental hyperplasia in cloned mice

Somatic cell cloning by nuclear transfer (NT) in mice is associated with hyperplastic placentas at term. To dissect the effects of embryonic and extraembryonic tissues on this clone-associated phenotype, we constructed diploid (2n) fused with (<-->) tetraploid (4n) chimeras from NT- and fertilization-derived (FD) embryos. Generally, the 4n cells contributed efficiently to all the extraembryonic tissues but not to the embryo itself. Embryos constructed by 2n NT<-->4n FD aggregation developed hyperplastic placentas (0.33+/-0.22 g) with a predominant contribution by NT-derived cells. Even when the population of FD-derived cells in placentas was increased using multiple FD embryos (up to four) for aggregation, most placentas remained hyperplastic (0.36+/-0.13 g). By contrast, placentas of the reciprocal combination, 2n FD<-->4n NT, were less hyperplastic (0.15+/-0.02 g). These nearly normal-looking placentas had a large proportion of NT-derived cells. Thus, embryonic rather than extraembryonic tissues had more impact on the onset of placental hyperplasia, and that the abnormal placentation in clones occurs in a noncell-autonomous manner. These findings suggest that for improvement of cloning efficiency we should understand the mechanisms regulating placentation, especially those of embryonic origin that might control the proliferation of trophoblastic lineage cells.

Production of healthy cloned mice from bodies frozen at -20 degrees C for 16 years

Cloning animals by nuclear transfer provides an opportunity to preserve endangered mammalian species. However, it has been suggested that the "resurrection" of frozen extinct species (such as the woolly mammoth) is impracticable, as no live cells are available, and the genomic material that remains is inevitably degraded. Here we report production of cloned mice from bodies kept frozen at -20 degrees C for up to 16 years without any cryoprotection. As all of the cells were ruptured after thawing, we used a modified cloning method and examined nuclei from several organs for use in nuclear transfer attempts. Using brain nuclei as nuclear donors, we established embryonic stem cell lines from the cloned embryos. Healthy cloned mice were then produced from these nuclear transferred embryonic stem cells by serial nuclear transfer. Thus, nuclear transfer techniques could be used to "resurrect" animals or maintain valuable genomic stocks from tissues frozen for prolonged periods without any cryopreservation.

Nuclear transfer-mediated rescue of the nuclear genome of nonviable mouse cells frozen without cryoprotectant

Nuclear transfer (NT) provides an opportunity for clonal amplification of a nuclear genome of interest. Here, we report NT-mediated reprogramming with frozen mouse cells that were nonviable because they were frozen at -80 degrees C for up to 342 days without a cryoprotectant. We derived eight embryonic stem (ES) cell lines from cloned blastocysts by conventional NT procedure and five ntES (nuclear transfer embryonic stem) cell lines by a modified NT procedure in which a whole cell instead of a nucleus was injected into an enucleated oocyte. Chromosome analysis revealed that 12 of 13 ntES cell lines have normal karyotypes. On injection of ntES cells into tetraploid blastocysts to generate clonal mice that are nearly completely ntES-cell derived, live pups were obtained; four clonal mice survived until adulthood. On injection of ntES cells into diploid blastocysts, chimeric mice with a high somatic ES cell contribution were generated; germ-line transmission was obtained. Our findings indicate that chromosome stability and genomic integrity can be maintained in mouse somatic cells after freezing without cryoprotection and that NT and ES cell techniques can rescue the genome of these cells.

Differentiated cells are more efficient than adult stem cells for cloning by somatic cell nuclear transfer

Since the creation of Dolly via somatic cell nuclear transfer (SCNT), more than a dozen species of mammals have been cloned using this technology. One hypothesis for the limited success of cloning via SCNT (1%-5%) is that the clones are likely to be derived from adult stem cells. Support for this hypothesis comes from the findings that the reproductive cloning efficiency for embryonic stem cells is five to ten times higher than that for somatic cells as donors and that cloned pups cannot be produced directly from cloned embryos derived from differentiated B and T cells or neuronal cells. The question remains as to whether SCNT-derived animal clones can be derived from truly differentiated somatic cells. We tested this hypothesis with mouse hematopoietic cells at different differentiation stages: hematopoietic stem cells, progenitor cells and granulocytes. We found that cloning efficiency increases over the differentiation hierarchy, and terminally differentiated postmitotic granulocytes yield cloned pups with the greatest cloning efficiency.

Equivalency of nuclear transfer-derived embryonic stem cells to those derived from fertilized mouse blastocysts

Therapeutic cloning, whereby nuclear transfer (NT) is used to generate embryonic stem cells (ESCs) from blastocysts, has been demonstrated successfully in mice and cattle. However, if NT-ESCs have abnormalities, such as those associated with the offspring produced by reproductive cloning, their scientific and medical utilities might prove limited. To evaluate the characteristics of NT-ESCs, we established more than 150 NT-ESC lines from adult somatic cells of several mouse strains. Here, we show that these NT-ESCs were able to differentiate into all functional embryonic tissues in vivo. Moreover, they were identical to blastocyst-derived ESCs in terms of their expression of pluripotency markers in the presence of tissue-dependent differentially DNA methylated regions, in DNA microarray profiles, and in high-coverage gene expression profiling. Importantly, the NT procedure did not cause irreversible damage to the nuclei. These similarities of NT-ESCs and ESCs indicate that murine therapeutic cloning by somatic cell NT can provide a reliable model for preclinical stem cell research.