The Majority of Multipotent Epidermal Stem Cells Do Not Protect Their Genome by Asymmetrical Chromosome Segregation

Abstract

The maintenance of genome integrity in stem cells (SCs) is critical for preventing cancer formation and cellular senescence. The immortal strand hypothesis postulates that SCs protect their genome by keeping the same DNA strand throughout life by asymmetrical cell divisions, thus avoiding accumulation of mutations that can arise during DNA replication. The in vivo relevance of this model remains to date a matter of intense debate. In this study, we revisited this long-standing hypothesis, by analyzing how multipotent hair follicle (HF) SCs segregate their DNA strands during morphogenesis, skin homeostasis, and SC activation. We used three different in vivo approaches to determine how HF SCs segregate their DNA strand during cell divisions. Double-labeling studies using pulse-chase experiments during morphogenesis and the first adult hair cycle showed that HF SCs incorporate two different nucleotide analogs, contradictory to the immortal strand hypothesis. The co-segregation of DNA and chromatin labeling during pulse-chase experiments demonstrated that label retention in HF SCs is rather a mark of relative quiescence. Moreover, DNA labeling of adult SCs, similar to labeling during morphogenesis, also resulted in label retention in HF SCs, indicating that chromosome segregation occurs randomly in most of these cells. Altogether, our results demonstrate that DNA strand segregation occurs randomly in the majority of HF SCs during development, tissue homeostasis, and following SC activation. STEM CELLS 2008

Microenvironmental modulation of asymmetric cell division in human lung cancer cells

“…Pulse-chase experiments involving the halogenated thymidine analog 5-bromo-2-deoxyuridine (BrdU) have been used to label DNA and track label-retaining cells that are growing slowly or asymmetrically dividing (9,(12)(13)(14)(15). Using a strategy similar to that reported for single mitotic cells (Fig.…”

“…Asymmetric division of template DNA has been shown in fibroblasts and epithelial, neural, and muscle satellite cells (1,(5)(6)(7)(8)(9)(10). Other studies have demonstrated that hematopoietic stem cells (HSCs), epidermal basal cells, hair follicle bulge cells, and intestinal epithelial stem cells segregate their chromosomes randomly (11)(12)(13)(14)(15). Perhaps asymmetric division of template DNA is cell type-specific, or perhaps only a very limited number of stem cells retain their template DNA strands in some tissues, which may go undetected by the methods or mathematical models used.…”

Development and Homeostasis of the Skin Epidermis

“…It has been proposed that SCs may undergo asymmetric divisions during which the SC retains the older ("immortal") DNA strand and the committed daughter cell inherits the newest synthesized DNA strand (Cairns 1975). Double-labeling pulse-chase experiments (Sotiropoulou et al 2008) and chromatin labeling with single uridine labeling provided evidence that in vivo HF SCs segregate their chromosomes randomly. Consistent with the skin barrier function, epidermal SCs are continuously assaulted by UV irradiation.…”

“…Bromodeoxyuridine pulse during the early stage of HF regeneration indicates that the first cells to incorporate bromodeoxyuridine are the cells of the hair germ (HG), followed by bulge SCs, suggesting a two-step mechanism of HF regeneration in which the first cells to be activated are the HG cells, followed by bulge SCs (Greco et al 2009). After an initial burst of proliferation that accompanies the early stage of HF regeneration, bulge SCs continue to divide at a lower rate during all of the growing stages of the hair cycle and then stop dividing during catagen (Sotiropoulou et al 2008;Waghmare et al 2008). Lineage tracing of single bulge SCs shows their ability to migrate from the bulge to the HG and their multipotency at a clonal level (Zhang et al 2009b).…”

Asymmetric Division in the Immuno-Hematopoietic System

“…This seems to be true for intestinal epithelium (Potten et al 1978), muscle (Shinin et al 2006), and neural stem cells (Karpowicz et al 2005) but not for hair follicle (Sotiropoulou, Candi, and Blanpain 2008) or hematopoietic (Kiel et al 2007) stem cells. However, it is not clear whether these disparities arise from looking at cells with different specifics in terms of lag between divisions.…”