Transcription Factors Active in the Anterior Blastema of Schmidtea mediterranea.

Regeneration, the restoration of body parts after injury, is quite widespread in the animal kingdom. Species from virtually all Phyla possess regenerative abilities. Human beings, however, are poor regenerators. Yet, the progress of knowledge and technology in the fields of bioengineering, stem cells, and regenerative biology have fostered major advancements in regenerative medical treatments, which aim to regenerate tissues and organs and restore function. Human induced pluripotent stem cells can differentiate into any cell type of the body; however, the structural and cellular complexity of the human tissues, together with the inability of our adult body to control pluripotency, require a better mechanistic understanding. Planarians, with their capacity to regenerate lost body parts thanks to the presence of adult pluripotent stem cells could help providing such an understanding. In this paper, we used a top-down approach to shortlist blastema transcription factors (TFs) active during anterior regeneration. We found 44 TFs-31 of which are novel in planarian-that are expressed in the regenerating blastema. We analyzed the function of half of them and found that they play a role in the regeneration of anterior structures, like the anterior organizer, the positional instruction muscle cells, the brain, the photoreceptor, the intestine. Our findings revealed a glimpse of the complexity of the transcriptional network governing anterior regeneration in planarians, confirming that this animal model is the perfect playground to study in vivo how pluripotency copes with adulthood.

Establishment of totipotency does not depend on Oct4A.

Oct4A is a core component of the regulatory network of pluripotent cells, and by itself can reprogram neural stem cells into pluripotent cells in mice and humans. However, its role in defining totipotency and inducing pluripotency during embryonic development is still unclear. We genetically eliminated maternal Oct4A using a Cre/loxP approach in mouse and found that the establishment of totipotency was not affected, as shown by the generation of live pups. After complete inactivation of both maternal and zygotic Oct4A expression, the embryos still formed Oct4-GFP- and Nanog-expressing inner cell masses, albeit non-pluripotent, indicating that Oct4A is not a determinant for the pluripotent cell lineage separation. Interestingly, Oct4A-deficient oocytes were able to reprogram fibroblasts into pluripotent cells. Our results clearly demonstrate that, in contrast to its role in the maintenance of pluripotency, maternal Oct4A is not crucial for either the establishment of totipotency in embryos, or the induction of pluripotency in somatic cells using oocytes.

Somatic cell nuclear reprogramming of mouse oocytes endures beyond reproductive decline.

The mammalian oocyte has the unique feature of supporting fertilization and normal development, while capable of reprogramming nuclei of somatic cells toward pluripotency, and occasionally even totipotency. While oocyte quality is known to decay with somatic aging, it is not a given that different biological functions decay concurrently. In this study, we tested whether oocyte's reprogramming ability decreases with aging. We show that oocytes isolated from mice aged beyond the usual reproductive age (climacteric) yield ooplasts that retain reprogramming capacity after somatic nuclear transfer (SCNT), giving rise to higher blastocysts rates compared to young donors ooplasts. Despite the differences in transcriptome between climacteric and young ooplasts, gene expression profiles of SCNT blastocysts were very similar. Importantly, embryonic stem cell lines with capacity to differentiate into tissues from all germ layers were derived from SCNT blastocysts obtained from climacteric ooplasts. Although apoptosis-related genes were down-regulated in climacteric ooplasts, and reprogramming by transcription factors (direct-induced pluripotency) benefits from the inhibition of p53-mediated apoptosis, reprogramming capacity of young ooplasts was not improved by blocking p53. However, more outgrowths were derived from SCNT blastocysts developed in the presence of a p53 inhibitor, indicating a beneficial effect on trophectoderm function. Results strongly suggest that oocyte-induced reprogramming outcome is determined by the availability and balance of intrinsic pro- and anti-reprogramming factors tightly regulated and even improved throughout aging, leading to the proposal that oocytes can still be a resource for somatic reprogramming when they cease to be considered safe for sexual reproduction.

Neural induction intermediates exhibit distinct roles of Fgf signaling.

Formation of the neural plate is an intricate process in early mammalian embryonic development mediated by cells of the inner cell mass and involving a series of steps, including development of the epiblast. Here, we report on the creation of an embryonic stem (ES) cell-based system to isolate and identify neural induction intermediates with characteristics of epiblast cells and neural plate. We demonstrate that neural commitment requires prior differentiation of ES cells into epiblast cells that are indistinguishable from those derived from natural embryos. We also demonstrate that epiblast cells can be isolated and cultured as epiblast stem cell lines. Fgf signaling is shown to be required for the differentiation of ES cells into these epiblast cells. Fgf2, widely used for maintenance of both human ES cells and epiblast stem cells, inhibits formation of early neural cells by epiblast intermediates in a dose-dependent manner and is sufficient to promote transient self-renewal of epiblast stem cells. In contrast, Fgf8, the endogenous embryonic neural inducer, fails to promote epiblast self-renewal, but rather promotes more homogenous neural induction with transient self-renewal of early neural cells. Removal of Fgf signaling entirely from epiblast cells promotes rapid neural induction and subsequent neurogenesis. We conclude that Fgf signaling plays different roles during the differentiation of ES cells, with an initial requirement in epiblast formation and a subsequent role in self-renewal. Fgf2 and Fgf8 thus stimulate self-renewal in different cell types.