Cloning of non-human primates: the road "less traveled by"

Reprogramming mechanism dissection and trophoblast replacement application in monkey somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) successfully clones cynomolgus monkeys, but the efficiency remains low due to a limited understanding of the reprogramming mechanism. Notably, no rhesus monkey has been cloned through SCNT so far. Our study conducts a comparative analysis of multi-omics datasets, comparing embryos resulting from intracytoplasmic sperm injection (ICSI) with those from SCNT. Our findings reveal a widespread decrease in DNA methylation and the loss of imprinting in maternally imprinted genes within SCNT monkey blastocysts. This loss of imprinting persists in SCNT embryos cultured in-vitro until E17 and in full-term SCNT placentas. Additionally, histological examination of SCNT placentas shows noticeable hyperplasia and calcification. To address these defects, we develop a trophoblast replacement method, ultimately leading to the successful cloning of a healthy male rhesus monkey. These discoveries provide valuable insights into the reprogramming mechanism of monkey SCNT and introduce a promising strategy for primate cloning.

Strategies to Improve the Efficiency of Somatic Cell Nuclear Transfer

Mammalian oocytes can reprogram differentiated somatic cells into a totipotent state through somatic cell nuclear transfer (SCNT), which is known as cloning. Although many mammalian species have been successfully cloned, the majority of cloned embryos failed to develop to term, resulting in the overall cloning efficiency being still low. There are many factors contributing to the cloning success. Aberrant epigenetic reprogramming is a major cause for the developmental failure of cloned embryos and abnormalities in the cloned offspring. Numerous research groups attempted multiple strategies to technically improve each step of the SCNT procedure and rescue abnormal epigenetic reprogramming by modulating DNA methylation and histone modifications, overexpression or repression of embryonic-related genes, etc. Here, we review the recent approaches for technical SCNT improvement and ameliorating epigenetic modifications in donor cells, oocytes, and cloned embryos in order to enhance cloning efficiency.

Current status of assisted reproductive technologies in buffaloes

Buffaloes are raised by small farm holders primarily as source of draft power owing to its resistance to hot climate, disease, and stress conditions. Over the years, transformation of these animals from draft to dairy was deliberately carried out through genetic improvement program leading to the development of buffalo-based enterprises. Buffalo production is now getting more attention and interest from buffalo raisers due to its socioeconomic impact as well as its contribution to propelling the livestock industry in many developing countries. Reproduction of buffaloes, however, is confronted with huge challenge and concern as being generally less efficient to reproduce compared with cattle due to both intrinsic and extrinsic factors such as poor estrus manifestation, silent heat, marked seasonal infertility, postpartum anestrus, long calving interval, delayed puberty, inherently low number of primordial follicles in their ovaries, high incidence of atresia, and apoptosis. Assisted reproductive technologies (ARTs) are major interventions for the efficient utilization of follicle reserve in buffaloes. The present review focuses on estrus and ovulation synchronization for fixed time artificial insemination, in vitro embryo production, intracytoplasmic sperm injection, cryopreservation of oocytes and embryos, somatic cell nuclear transfer, the factors affecting utilization in various ARTs, and future perspectives in buffaloes.

Assisted Reproductive Techniques and Genetic Manipulation in the Common Marmoset

Genetic modification of nonhuman primate (NHP) zygotes is a useful method for the development of NHP models of human diseases. This review summarizes the recent advances in the development of assisted reproductive and genetic manipulation techniques in NHP, providing the basis for the generation of genetically modified NHP disease models. In this study, we review assisted reproductive techniques, including ovarian stimulation, in vitro maturation of oocytes, in vitro fertilization, embryo culture, embryo transfer, and intracytoplasmic sperm injection protocols in marmosets. Furthermore, we review genetic manipulation techniques, including transgenic strategies, target gene knock-out and knock-in using gene editing protocols, and newly developed gene-editing approaches that may potentially impact the production of genetically manipulated NHP models. We further discuss the progress of assisted reproductive and genetic manipulation techniques in NHP; future prospects on genetically modified NHP models for biomedical research are also highlighted.

Targeting epigenetic nuclear reprogramming in aggregated cloned equine embryos

Epigenetic perturbations during the reprogramming process have been described as the primary cause of the low efficiency of somatic cell nuclear transfer (SCNT). In this study, we tested three strategies targeting nuclear reprogramming to investigate effects on equine SCNT. First, we evaluated the effect of treating somatic cells with chetomin, a fungal secondary metabolite reported to inhibit the trimethylation on histone 3 lysine 9 (H3K9 me3). Second, caffeine was added to the culture medium during the enucleation of oocytes and before activation of reconstructed embryos as a protein phosphatase inhibitor to improve nuclear reprogramming. Third, we tested the effects of the histone deacetylase inhibitor trichostatin A (TSA) added during both activation and early embryo culture. Although none of these treatments significantly improved the developmental rates of the invitro aggregated cloned equine embryos, the first equine cloned foal born in Australia was produced with somatic cells treated with chetomin. The present study describes the use of chetomin, caffeine and TSA for the first time in horses, serving as a starting point for the establishment of future protocols to target epigenetic reprogramming for improving the efficiency of equine cloning. Cloning is an expensive and inefficient process, but has gained particular interest in the equine industry. In this study we explored different strategies to improve cloning efficiency and produced the first cloned foal born in Australia. Our data serve as a starting point for the establishment of future protocols for improving equine cloning efficiency.

Lessons Learned from Somatic Cell Nuclear Transfer

Somatic cell nuclear transfer (SCNT) has been an area of interest in the field of stem cell research and regenerative medicine for the past 20 years. The main biological goal of SCNT is to reverse the differentiated state of a somatic cell, for the purpose of creating blastocysts from which embryonic stem cells (ESCs) can be derived for therapeutic cloning, or for the purpose of reproductive cloning. However, the consensus is that the low efficiency in creating normal viable offspring in animals by SCNT (1-5%) and the high number of abnormalities seen in these cloned animals is due to epigenetic reprogramming failure. In this review we provide an overview of the current literature on SCNT, focusing on protocol development, which includes early SCNT protocol deficiencies and optimizations along with donor cell type and cell cycle synchrony; epigenetic reprogramming in SCNT; current protocol optimizations such as nuclear reprogramming strategies that can be applied to improve epigenetic reprogramming by SCNT; applications of SCNT; the ethical and legal implications of SCNT in humans; and specific lessons learned for establishing an optimized SCNT protocol using a mouse model.

Potential role of intraspecific and interspecific cloning in the conservation of wild mammals

Intraspecific and interspecific cloning via somatic cell nuclear transfer (iSCNT) is a biotechnique with great possibilities for wild mammals because it allows the maintenance of biodiversity by recovering species, nuclear reprogramming for the production of pluripotency-induced cells, and studies related to embryonic development. Nevertheless, many areas in cloning, especially those associated with wild mammals, are still in question because of the difficulty in obtaining cytoplasmic donor cells (or cytoplasts). Conversely, donor cell nuclei (or karyoplasts) are widely obtained from the skin of living or post-mortem individuals and often maintained in somatic cell banks. Moreover, the creation of karyoplast–cytoplast complexes by fusion followed by activation and embryo development is one of the most difficult steps that requires further clarification to avoid genetic failures. Although difficult, cloning different species, such as wild carnivores and ungulates, can be successful via iSCNT with embryo development and the birth of offspring. Thus, novel research in the area that contributes to the conservation of biodiversity and knowledge of the physiology of species continues. The present review presents the failures and successes that occurred with the application of the technique in wild mammals, with the goal of helping future work on cloning via iSCNT.

Generation of genetically engineered non-human primate models of brain function and neurological disorders

Research with non-human primates (NHP) has been essential and effective in increasing our ability to find cures for a large number of diseases that cause human suffering and death. Extending the availability and use of genetic engineering techniques to NHP will allow the creation and study of NHP models of human disease, as well as broaden our understanding of neural circuits in the primate brain. With the recent development of efficient genetic engineering techniques that can be used for NHP, there's increased hope that NHP will significantly accelerate our understanding of the etiology of human neurological and neuropsychiatric disorders. In this article, we review the present state of genetic engineering tools used in NHP, from the early efforts to induce exogeneous gene expression in macaques and marmosets, to the latest results in producing germline transmission of different transgenes and the establishment of knockout lines of specific genes. We conclude with future perspectives on the further development and employment of these tools to generate genetically engineered NHP.

Cloning of Macaque Monkeys by Somatic Cell Nuclear Transfer

Generation of genetically uniform non-human primates may help to establish animal models for primate biology and biomedical research. In this study, we have successfully cloned cynomolgus monkeys (Macaca fascicularis) by somatic cell nuclear transfer (SCNT). We found that injection of H3K9me3 demethylase Kdm4d mRNA and treatment with histone deacetylase inhibitor trichostatin A at one-cell stage following SCNT greatly improved blastocyst development and pregnancy rate of transplanted SCNT embryos in surrogate monkeys. For SCNT using fetal monkey fibroblasts, 6 pregnancies were confirmed in 21 surrogates and yielded 2 healthy babies. For SCNT using adult monkey cumulus cells, 22 pregnancies were confirmed in 42 surrogates and yielded 2 babies that were short-lived. In both cases, genetic analyses confirmed that the nuclear DNA and mitochondria DNA of the monkey offspring originated from the nucleus donor cell and the oocyte donor monkey, respectively. Thus, cloning macaque monkeys by SCNT is feasible using fetal fibroblasts.

Human embryonic stem cells derived by somatic cell nuclear transfer

Reprogramming somatic cells into pluripotent embryonic stem cells (ESCs) by somatic cell nuclear transfer (SCNT) has been envisioned as an approach for generating patient-matched nuclear transfer (NT)-ESCs for studies of disease mechanisms and for developing specific therapies. Past attempts to produce human NT-ESCs have failed secondary to early embryonic arrest of SCNT embryos. Here, we identified premature exit from meiosis in human oocytes and suboptimal activation as key factors that are responsible for these outcomes. Optimized SCNT approaches designed to circumvent these limitations allowed derivation of human NT-ESCs. When applied to premium quality human oocytes, NT-ESC lines were derived from as few as two oocytes. NT-ESCs displayed normal diploid karyotypes and inherited their nuclear genome exclusively from parental somatic cells. Gene expression and differentiation profiles in human NT-ESCs were similar to embryo-derived ESCs, suggesting efficient reprogramming of somatic cells to a pluripotent state.

Human germline genetic modification: scientific and bioethical perspectives

The latest mammalian genetic modification technology offers efficient and reliable targeting of genomic sequences, in the guise of designer genetic recombination tools. These and other improvements in genetic engineering technology suggest that human germline genetic modification (HGGM) will become a safe and effective prospect in the relatively near future. Several substantive ethical objections have been raised against HGGM including claims of unacceptably high levels of risk, damage to the status of future persons, and violations of justice and autonomy. This paper critically reviews the latest GM science and discusses the key ethical objections to HGGM. We conclude that major benefits are likely to accrue through the use of safe and effective HGGM and that it would thus be unethical to take a precautionary stance against HGGM.

Reprogramming and development in nuclear transfer embryos and in interspecific systems

Nuclear transfer (NT) remains the most effective method to reprogram somatic cells to totipotency. Somatic cell nuclear transfer (SCNT) efficiency however remains low, but recurrent problems occurring in partially reprogrammed cloned embryos have recently been identified and some remedied. In particular, the trophectoderm has been identified as a lineage whose reprogramming success has a large influence on SCNT embryo development. Several interspecific hybrid and cybrid reprogramming systems have been developed as they offer various technical advantages and potential applications, and together with SCNT, they have led to the identification of a series of reprogramming events and responsible reprogramming factors. Interspecific incompatibilities hinder full exploitation of cross-species reprogramming systems, yet recent findings suggest that these may not constitute insurmountable obstacles.

Genome resource banking of biomedically important laboratory animals

Genome resource banking is the systematic collection, storage, and redistribution of biomaterials in an organized, logistical, and secure manner. Genome cryobanks usually contain biomaterials and associated genomic information essential for progression of biomedicine, human health, and research. In that regard, appropriate genome cryobanks could provide essential biomaterials for both current and future research projects in the form of various cell types and tissues, including sperm, oocytes, embryos, embryonic or adult stem cells, induced pluripotent stem cells, and gonadal tissues. In addition to cryobanked germplasm, cryobanking of DNA, serum, blood products, and tissues from scientifically, economically, and ecologically important species has become a common practice. For revitalization of the whole organism, cryopreserved germplasm in conjunction with assisted reproductive technologies, offer a powerful approach for research model management, as well as assisting in animal production for agriculture, conservation, and human reproductive medicine. Recently, many developed and developing countries have allocated substantial resources to establish genome resources banks which are responsible for safeguarding scientifically, economically, and ecologically important wild type, mutant, and transgenic plants, fish, and local livestock breeds, as well as wildlife species. This review is dedicated to the memory of Dr. John K. Critser, who has made profound contributions to the science of cryobiology and establishment of genome research and resources centers for mice, rats, and swine. Emphasis will be given to application of genome resource banks to species with substantial contributions to the advancement of biomedicine and human health.

Generation of chimeric rhesus monkeys

Totipotent cells in early embryos are progenitors of all stem cells and are capable of developing into a whole organism, including extraembryonic tissues such as placenta. Pluripotent cells in the inner cell mass (ICM) are the descendants of totipotent cells and can differentiate into any cell type of a body except extraembryonic tissues. The ability to contribute to chimeric animals upon reintroduction into host embryos is the key feature of murine totipotent and pluripotent cells. Here, we demonstrate that rhesus monkey embryonic stem cells (ESCs) and isolated ICMs fail to incorporate into host embryos and develop into chimeras. However, chimeric offspring were produced following aggregation of totipotent cells of the four-cell embryos. These results provide insights into the species-specific nature of primate embryos and suggest that a chimera assay using pluripotent cells may not be feasible.

Reprogramming of somatic cells

Reprogramming of adult somatic cells into pluripotent stem cells may provide an attractive source of stem cells for regenerative medicine. It has emerged as an invaluable method for generating patient-specific stem cells of any cell lineage without the use of embryonic stem cells. A revolutionary study in 2006 showed that it is possible to convert adult somatic cells directly into pluripotent stem cells by using a limited number of pluripotent transcription factors and is called as iPS cells. Currently, both genomic integrating viral and nonintegrating nonviral methods are used to generate iPS cells. However, the viral-based technology poses increased risk of safety, and more studies are now focused on nonviral-based technology to obtain autologous stem cells for clinical therapy. In this review, the pros and cons of the present iPS cell technology and the future direction for the successful translation of this technology into the clinic are discussed.