A potential model for studying the plasticity and reprogramming of human epidermal stem cells through preimplantation blastocyst microinjection

Microinjection of adult stem cells (ASCs) into blastocysts provides a classic model for studying ASC plasticity. To explore the molecular mechanisms that govern the reprogramming of ASCs, we evaluated the experimental model through microinjection of human epidermal stem cells (hEpiSCs) into mouse blastocysts. Mouse blastocysts underwent regular embryogenesis after microinjection of allogeneic cells, confirmed by morphological observation and embryo cell counting. hEpiSCs survive and integrate into mouse embryos, by monitoring the migration of injected cells at 2, 4, 12, 16 and 24 h. In this xenogeneic system, hEpiSCs could be reprogrammed within 24 h, as evidenced by the silencing of CK15 and Integrin beta 1 gene expression, without activation of Oct4 and Nanog. Microinjection of hEpiSCs into mouse blastocysts provides an efficient model for studying the molecular mechanisms of their plasticity. Moreover, the possibility of inducing pluripotent stem cells without transgenes or viruses can be entertained.     

tiRNAs: A novel class of small noncoding RNAs that helps cells respond to stressors and plays roles in cancer progression

tRNA-derived stress-induced RNAs (tiRNAs), important components of tRNA-derived fragments, are gaining popularity for their functions as small noncoding RNAs involved in cancer progression. Under cellular stress, tiRNAs are generated when mature tRNA is specifically cleaved by angiogenin and suggested to act as transducers or effectors involved in cellular stress responses. tiRNAs facilitate cells to respond to stresses mainly via reprogramming translation, inhibiting apoptosis, degrading mRNA, and generating stress granules. This review introduces the cellular biogenesis, molecular mechanisms, and biological roles of tiRNAs in stress response and disease regulation. A better understanding of their roles in regulating cancer may provide novel biomarkers or therapeutic targets for diagnosis and treatment.     

High-dimensional switches and the modelling of cellular differentiation

Many genes have been identified as driving cellular differentiation, but because of their complex interactions, the understanding of their collective behaviour requires mathematical modelling. Intriguingly, it has been observed in numerous developmental contexts, and particularly haematopoiesis, that genes regulating differentiation are initially co-expressed in progenitors despite their antagonism, before one is upregulated and others downregulated. We characterise conditions under which three classes of generic "master regulatory networks", modelled at the molecular level after experimentally observed interactions (including bHLH protein dimerisation), and including an arbitrary number of antagonistic components, can behave as a "multi-switch", directing differentiation in an all-or-none fashion to a specific cell-type chosen among more than two possible outcomes. bHLH dimerisation networks can readily display coexistence of many antagonistic factors when competition is low (a simple characterisation is derived). Decision-making can be forced by a transient increase in competition, which could correspond to some unexplained experimental observations related to Id proteins; the speed of response varies with the initial conditions the network is subjected to, which could explain some aspects of cell behaviour upon reprogramming. The coexistence of antagonistic factors at low levels, early in the differentiation process or in pluripotent stem cells, could be an intrinsic property of the interaction between those factors, not requiring a specific regulatory system.     

Aberrant methylation patterns at the two-cell stage as an indicator of early developmental failure

The fertilized mouse egg actively demethylates the paternal genome within a few hours after fertilization, whereas the maternal genome is only passively demethylated by a replication-dependent mechanism after the two-cell stage. This evolutionarily conserved assymetry in the early diploid mammalian embryo may have a role in methylation reprogramming of the two very different sets of sperm and egg chromatin for somatic development and formation of totipotent cells. Immunofluorescence staining with an antibody against 5-methylcytosine (MeC) showed that the incidence of abnormal methylation patterns differs between mouse two-cell embryos from superovulated females, nonsuperovulated matings, and in vitro fertilization (IVF). It also depends on embryo culture conditions and genetic background. In general, there was a good correlation with the number of embryos (from the same experiment) which did not develop in vitro up to the blastocyst stage. Thus, aberrant genome-wide DNA methylation in early embryos may be an important mechanism contributing to the high incidence of developmental failure in mammals. Similar to the situation in abnormally methylated embryos from nuclear transfer, it may cause a high incidence of pregnancy loss and abnormal phenotypes.     

The plasticity of germ cell cancers and its dependence on the cellular microenvironment

So far, the understanding of germ cell cancer (GCC) pathogenesis is based on a model, where seminomas and non‐seminomas represent distinct entities although originating from a common precursor termed germ cell neoplasia in situ (GCNIS). Embryonal carcinomas (ECs), the stem cell population of the non‐seminomas, is pluri‐ to totipotent and able to differentiate into cells of all three germ layers, giving rise to teratomas or tumours mimicking extraembryonic tissues (yolk sac tumours, choriocarcinomas). With regard to gene expression, (epi)genetics and histology, seminomas are highly similar to GCNIS and primordial germ cells, but limited in development. It remains elusive, whether this block in differentiation is controlled by cell intrinsic mechanisms or by signals from the surrounding microenvironment. Here, we reviewed the recent literature emphasizing the plasticity of GCCs, especially of seminomas. We propose that this plasticity is controlled by the microenvironment, allowing seminomas to transit into an EC or mixed non‐seminoma and vice versa. We discuss several mechanisms and routes of reprogramming that might be responsible for this change in the cell fate. We finally integrate this plasticity into a new model of GCC pathogenesis, allowing for an alternative view on the dynamics of GCC development and progression.