Efficient derivation of embryonic stem cells from nuclear transfer and parthenogenetic embryos derived from cryopreserved oocytes

Cryopreservation of equine oocytes: looking into the crystal ball

Invitro embryo production has evolved rapidly in the horse over the past decade, but blastocyst rates from vitrified equine oocytes remain quite poor and further research is needed to warrant application. Oocyte vitrification is affected by several technical and biological factors. In the horse, short exposure of immature oocytes to the combination of permeating and non-permeating cryoprotective agents has been associated with the best results so far. High cooling and warming rates are also crucial and can be obtained by using minimal volumes and open cryodevices. Vitrification of invivo-matured oocytes has yielded better results, but is less practical. The presence of the corona radiata seems to partially protect those factors that are necessary for the construction of the normal spindle and for chromosome alignment, but multiple layers of cumulus cells may impair permeation of cryoprotective agents. In addition to the spindle, the oolemma and mitochondria are also particularly sensitive to vitrification damage, which should be minimised in future vitrification procedures. This review presents promising protocols and novel strategies in equine oocyte vitrification, with a focus on blastocyst development and foal production as most reliable outcome parameters.

Compromised Chondrocyte Differentiation Capacity in TERC Knockout Mouse Embryonic Stem Cells Derived by Somatic Cell Nuclear Transfer

Mammalian telomere lengths are primarily regulated by telomerase, consisting of a reverse transcriptase protein (TERT) and an RNA subunit (TERC). We previously reported the generation of mouse Terc^+/- and Terc^-/- embryonic stem cells (ntESCs) by somatic cell nuclear transfer. In the present work, we investigated the germ layer development competence of Terc^-/-, Terc^+/- and wild-type (Terc^+/+) ntESCs. The telomere lengths are longest in wild-type but shortest in Terc^-/- ntESCs, and correlate reversely with the population doubling time. Interestingly, while in vitro embryoid body (EB) differentiation assay reveals EB size difference among ntESCs of different genotypes, the more stringent in vivo teratoma assay demonstrates that Terc^-/- ntESCs are severely defective in differentiating into the mesodermal lineage cartilage. Consistently, in a directed in vitro chondrocyte differentiation assay, the Terc^-/- cells failed in forming Collagen II expressing cells. These findings underscore the significance in maintaining proper telomere lengths in stem cells and their derivatives for regenerative medicine.

Parthenogenetic Activation of Oocytes

Numerous studies have been initiated to investigate the influence of maternal and paternal genomes on early mammalian development. For this type of study, parthenogenetic embryos provide a unique source of preimplantation and early postimplantation embryos that (by definition) develop in the absence of any contribution from a male gamete. Parthenogenetic activation is used for biochemical and morphological studies of oocytes during fertilization and early development and is a critical component of the cloning procedure. This protocol describes the activation of oocytes using ethanol. Parthenogenesis can also be induced by exposure of unfertilized oocytes to strontium-containing medium.

Production of Live Offspring from Vitrified-Warmed Oocytes Collected at Metaphase I Stage

Vitrification of matured oocytes is widely adopted in human clinics and animal research laboratories. Cryopreservation of immature oocytes, particularly those at metaphase I (MI), remains a challenge. In the present work, mouse MI oocytes denuded of cumulus cells were vitrified and warmed (V/W) either prior to (V/W-BEFORE-IVM, n = 562) or after (V/W-AFTER-IVM, n = 664) in vitro maturation (IVM). Derivative metaphase II (MII) oocytes were then used for intracytoplasmic sperm injection (ICSI). In the control groups, in vivo matured MII oocytes were used freshly (FRESH-MII, n = 517) or after V/W (MII-V/W, n = 617). In vitro and in vivo developmental competencies were compared among groups. Satisfactory blastocyst rates were achieved in V/W-BEFORE-IVM (27.5%) and V/W-AFTER-IVM (32.4%) groups, albeit as expected still lower than those from fresh-MII (56.1%) or MII-V/W (45.6%) oocytes. Similarly, the term development rates from V/W-BEFORE-IVM and V/W-AFTER-IVM were 12.4% and 16.7% respectively, acceptable but lower than those of the fresh-MII (41.2%) and MII-V/W (23.3%) groups. These data demonstrate that oocytes collected at MI stage are amenable to V/W, which can be performed before or after IVM with acceptable development rates including production of healthy pups. These findings provide useful knowledge to researchers and clinical practitioners for preservation and use of the otherwise discarded MI oocytes.

Dnmt3l-knockout donor cells improve somatic cell nuclear transfer reprogramming efficiency

Nuclear transfer (NT) is a technique used to investigate the development and reprogramming potential of a single cell. DNA methyltransferase-3-like, which has been characterized as a repressive transcriptional regulator, is expressed in naturally fertilized egg and morula/blastocyst at pre-implantation stages. In this study, we demonstrate that the use of Dnmt3l-knockout (Dnmt3l-KO) donor cells in combination with Trichostatin A treatment improved the developmental efficiency and quality of the cloned embryos. Compared with the WT group, Dnmt3l-KO donor cell-derived cloned embryos exhibited increased cell numbers as well as restricted OCT4 expression in the inner cell mass (ICM) and silencing of transposable elements at the blastocyst stage. In addition, our results indicate that zygotic Dnmt3l is dispensable for cloned embryo development at pre-implantation stages. In Dnmt3l-KO mouse embryonic fibroblasts, we observed reduced nuclear localization of HDAC1, increased levels of the active histone mark H3K27ac and decreased accumulation of the repressive histone marks H3K27me3 and H3K9me3, suggesting that Dnmt3l-KO donor cells may offer a more permissive epigenetic state that is beneficial for NT reprogramming.

Telomere elongation and naive pluripotent stem cells achieved from telomerase haplo-insufficient cells by somatic cell nuclear transfer

Haplo-insufficiency of telomerase genes in humans leads to telomere syndromes such as dyskeratosis congenital and idiopathic pulmonary fibrosis. Generation of pluripotent stem cells from telomerase haplo-insufficient donor cells would provide unique opportunities toward the realization of patient-specific stem cell therapies. Recently, pluripotent human embryonic stem cells (ntESCs) have been efficiently achieved by somatic cell nuclear transfer (SCNT). We tested the hypothesis that SCNT could effectively elongate shortening telomeres of telomerase haplo-insufficient cells in the ntESCs with relevant mouse models. Indeed, telomeres of telomerase haplo-insufficient (Terc(+/-)) mouse cells are elongated in ntESCs. Moreover, ntESCs derived from Terc(+/-) cells exhibit naive pluripotency as evidenced by generation of Terc(+/-) ntESC clone pups by tetraploid embryo complementation, the most stringent test of naive pluripotency. These data suggest that SCNT could offer a powerful tool to reprogram telomeres and to discover the factors for robust restoration of telomeres and pluripotency of telomerase haplo-insufficient somatic cells.

Mesenchymal-like stem cells derived from human parthenogenetic embryonic stem cells

Human parthenogenetic embryonic stem cells (hpESCs) established from artificially activated oocytes have a wider immune-matching ability because of their homozygosity in the major histocompatibility complex alleles. Whether these cells possess the differentiation capacity similar to regular human embryonic stem cells (hESCs) derived from fertilized eggs is unclear. The aims of this study were to determine whether hpESCs could be differentiated into multipotent mesenchymal stem cell (MSC)-like cells in vitro and then compare these cells with those derived from hESCs. MSC-like cells were obtained from both hpESCs and hESCs, which exhibited similar cell surface marker expression profiles. Further analyses revealed that cells derived from hpESCs possessed stronger osteogenic but weaker adipogenic potentials compared with cells derived from hESCs. This is the first work that demonstrates the differentiation of hpESCs into multipotent MSC-like cells. These hpESCs could be a potential source for cell-based therapies.

Cryopreservation of oocytes in experimental models

Until recently, success in oocyte cryopreservation has been very limited mainly due to poor understanding of the complex physiological processes that lead to cell damage during cryopreservation. In the past three decades, however, a wealth of information has been collected using various different animal models, which has led to development of new technologies and optimization of existing ones. The use of these models has provided the opportunity for research that may not have been possible with human material. Today, results of these studies still continue to form the basis of oocyte cryobiology. This review discusses these studies, especially the physiological impacts of cryopreservation on oocyte biology. It will also focus on the role that animal models have played in improvement strategies, validation before translating new techniques into the human model and the advances made in the human in IVF because of these animal models. Finally, existing investigations and their potential impact in other areas of research will be discussed. Until recently, success in oocyte cryopreservation has been very limited mainly due to poor understanding of the complex physiological processes that lead to cell damage during cryopreservation. In the past three decades, however, a wealth of information has been collected using various different animal models, which has led to development of new technologies and optimization of existing ones. The use of these models provided the opportunity for research that may not have been possible with human material. Today, animal models still continuously provide imperative data that facilitate further advancements in oocyte cryobiology. This review will focus on the physiological impacts, current improvement strategies and future applications of oocyte cryopreservation using animal models as they benefit not only human oocyte cryopreservation procedures, but also the human species through their usefulness in agriculture, medicine and conservation.