ABSTRACT
Pluripotent stem cells offer an abundant and malleable source for the generation of differentiated cells for transplantation as well as for in vitro screens. Patterning and differentiation protocols have been developed to generate neural progeny from human embryonic or induced pluripotent stem cells. However, continued refinement is required to enhance efficiency and to prevent the generation of unwanted cell types. We summarize and interpret insights gained from studies of embryonic neuroepithelium. A multitude of factors including soluble molecules, interactions with the extracellular matrix and neighboring cells cooperate to control neural stem cell self-renewal versus differentiation. Applying these findings and concepts to human stem cell systems in vitro may yield more appropriately patterned cell types for biomedical applications.
Subject(s)
Cell Differentiation , Embryonic Stem Cells/physiology , Neural Stem Cells/physiology , Pluripotent Stem Cells/physiology , Stem Cell Niche , Animals , Cell Communication , Cell Polarity , Embryonic Stem Cells/metabolism , Extracellular Matrix/metabolism , Extracellular Matrix/physiology , Extracellular Matrix Proteins/physiology , Humans , Intercellular Signaling Peptides and Proteins/physiology , Membrane Proteins/metabolism , Neural Stem Cells/metabolism , Pluripotent Stem Cells/metabolism , Signal TransductionABSTRACT
p53 is well known as a "guardian of the genome" for differentiated cells, in which it induces cell cycle arrest and cell death after DNA damage and thus contributes to the maintenance of genomic stability. In addition to this tumor suppressor function for differentiated cells, p53 also plays an important role in stem cells. In this cell type, p53 not only ensures genomic integrity after genotoxic insults but also controls their proliferation and differentiation. Additionally, p53 provides an effective barrier for the generation of pluripotent stem cell-like cells from terminally differentiated cells. In this review, we summarize our current knowledge about p53 activities in embryonic, adult and induced pluripotent stem cells.
ABSTRACT
Despite an increasing interest in the role of the p53 tumour suppressor protein in embryonic stem cells, not much is known about its regulation in this cell type. We show that the relatively high amount of p53 protein correlates with a higher amount of p53 RNA in ES cells compared to differentiated cells. Moreover, p53 RNA is more stable in embryonic stem cells and the p53 protein is more often transcribed. This is at least partly due to decreased expression of miRNA-125a and 125b in embryonic stem cells. Despite its cytoplasmic localisation, p53 is degraded in 26S proteasomes in embryonic stem cells. This process is controlled by Mdm2, the deubiquitinating enzyme Hausp and Ubc13. In contrast, the E3 ligase PirH2 appears to be less important for the control of p53 in embryonic stem cells. During differentiation, p53 protein and RNA levels are decreased which corresponds to increased expression of miRNA-125a and miRNA-125b.
Subject(s)
Embryonic Stem Cells/metabolism , Genes, p53 , Animals , Cell Differentiation/drug effects , Cell Differentiation/genetics , Cells, Cultured , Cycloheximide/pharmacology , Gene Expression Regulation, Developmental , Mice , MicroRNAs/genetics , MicroRNAs/metabolism , MicroRNAs/physiology , NIH 3T3 Cells , Proteasome Endopeptidase Complex/metabolism , Proteasome Endopeptidase Complex/physiology , Protein Processing, Post-Translational/drug effects , Protein Synthesis Inhibitors/pharmacology , RNA Stability/drug effects , Tretinoin/pharmacology , Tumor Suppressor Protein p53/metabolismABSTRACT
BACKGROUND: P53 is a key tumor suppressor protein. In response to DNA damage, p53 accumulates to high levels in differentiated cells and activates target genes that initiate cell cycle arrest and apoptosis. Since stem cells provide the proliferative cell pool within organisms, an efficient DNA damage response is crucial. RESULTS: In proliferating embryonic stem cells, p53 is localized predominantly in the cytoplasm. DNA damage-induced nuclear accumulation of p53 in embryonic stem cells activates transcription of the target genes mdm2, p21, puma and noxa. We observed bi-phasic kinetics for nuclear accumulation of p53 after ionizing radiation. During the first wave of nuclear accumulation, p53 levels were increased and the p53 target genes mdm2, p21 and puma were transcribed. Transcription of noxa correlated with the second wave of nuclear accumulation. Transcriptional activation of p53 target genes resulted in an increased amount of proteins with the exception of p21. While p21 transcripts were efficiently translated in 3T3 cells, we failed to see an increase in p21 protein levels after IR in embryonal stem cells. CONCLUSION: In embryonic stem cells where (anti-proliferative) p53 activity is not necessary, or even unfavorable, p53 is retained in the cytoplasm and prevented from activating its target genes. However, if its activity is beneficial or required, p53 is allowed to accumulate in the nucleus and activates its target genes, even in embryonic stem cells.
