High Incidence of Non-Random Template Strand Segregation and Asymmetric Fate Determination In Dividing Stem Cells and their Progeny

Abstract

Decades ago, the “immortal strand hypothesis” was proposed as a means by which stem cells might limit acquiring mutations that could give rise to cancer, while continuing to proliferate for the life of an organism. Originally based on observations in embryonic cells, and later studied in terms of stem cell self-renewal, this hypothesis has remained largely unaccepted because of few additional reports, the rarity of the cells displaying template strand segregation, and alternative interpretations of experiments involving single labels or different types of labels to follow template strands. Using sequential pulses of halogenated thymidine analogs (bromodeoxyuridine [BrdU], chlorodeoxyuridine [CldU], and iododeoxyuridine [IdU]), and analyzing stem cell progeny during induced regeneration in vivo, we observed extraordinarily high frequencies of segregation of older and younger template strands during a period of proliferative expansion of muscle stem cells. Furthermore, template strand co-segregation was strongly associated with asymmetric cell divisions yielding daughters with divergent fates. Daughter cells inheriting the older templates retained the more immature phenotype, whereas daughters inheriting the newer templates acquired a more differentiated phenotype. These data provide compelling evidence of template strand co-segregation based on template age and associated with cell fate determination, suggest that template strand age is monitored during stem cell lineage progression, and raise important caveats for the interpretation of label-retaining cells.

Satellite cell heterogeneity and hierarchy in skeletal muscle

“…This phenomenon has also been observed in satellite cells 32,33) . Recently, using transgenic Tg:Pax7-nGFP mice, Rocheteau et al showed that a small cell population with a highly active Pax7 locus (Pax7-nGFP high ) has a lower metabolic status and can self-renew and generate cell populations with an active, but weak Pax7 locus (Pax7-nGFP low ) after serial engraftments 34) .…”

“…Cells derived from the slow-dividing cell population that show a highly expressed Id1 protein are capable of generating both slow-and fast-dividing cell populations 42) . It is unclear whether the slow-dividing cells correspond to other stem cell-like populations such as Pax7 + Myf5 -cells 29) , Pax7-nGFP high cells 34) , SP-satellite cells 27) , and older DNA-retaining cells 32,33) . …”

“…In the postnatal skeletal muscle tissue of growing rats, Schultz showed that approximately 80% of the satellite cell population is a highly proliferative population that is readily labelled by BrdU pulsing, whereas the remaining population divides slowly 40) . Subsequent reports described fastand slow-dividing cell populations among activated satellite cells in adult muscles 24,32,33,41) . However, there was no direct evidence regarding whether these two populations have such distinct roles during myogenic progression.…”

Hematopoietic stem cells: generation and self-renewal

“…A recent study provided evidence for divisional asymmetry in muscle stem cells by showing that the template strand of DNA was more often retained by cells fated to remain undifferentiated, while the newly synthesized DNA strand was more often found in cells that had acquired a more differentiated phenotype. 35 The concept of HSC niches was first proposed by Schofield. 36 However, it has been only recently that the HSC niches in adult BM were directly observed and their components partially revealed.…”

Selective tracking of template DNA strands after induction of mitosis with unreplicated genomes (MUGs) in Drosophila S2 cells

“…Support for and against asymmetric segregation of all or just a few DNA strands associated with specific chromosomes can be found in the literature in the most diversified systems, from yeast to man (Tajbakhsh 2008, Rocheteau et al 2012, Escobar et al 2011, Schepers et al 2011, Armakolas and Klar 2006, Conboy et al 2007, Armakolas et al 2010, making this a highly controversial topic, mostly due to current limitations in the molecular identification of truly stem cell populations and to selectively label and track template DNA strands throughout and upon cell division. The latter traditionally involves the administration of labelled nucleotides (e.g., 5-bromo-deoxyuridine or H 3 -thymidine) that will mark older or newer DNA strands, depending on the protocol used, but which cannot reliably distinguish stem cells from differentiated cells with a slow cell cycle.…”