There is a common assumption that adult stem cells in our body actively divide only in case of regeneration of damaged tissues, at least in the hematopoietic system. In steady-state conditions hematopoietic stem cells (HSC) divide infrequently and usually don’t enter the cell cycle, giving this role to multipotent progenitors which mostly maintain blood turnover. The ability to maintain this non-dividing condition (stay in the G-0 phase of cell cycle) is called quiescence. According to this modern hypothesis, quiescence prevents stem cell pool exhaustion and protects HSC from acquiring mutations leading to malignant transformation.
This concept was questioned in some studies, including the most recent work of Sean Morrison’s group, which challenged the existence of long-term quiescent HSC.
Moreover, these studies suggested that HSCs are a rather homogenous infrequently dividing pool of cells that regularly but asynchronously enter the cell cycle, thus excluding the existence of a dormant HSC population.
A new study, lead by Andreas Trumpp, demonstrates the existence of 2 distinct subsets of HSC in terms of quiescence – dormant population and activated slowly dividing population.
… two populations with different division kinetics: a d-HSC (red) subset (15%) dividing approximately every 145 days and a rarely dividing “activated” (a-HSC, green) population (85%) dividing around every 36 days.
This work amazed me by its technical aspects. I’ll indicate some points of this study solving tough questions in the field below.
1. Identification of highly dormant population and mapping of HSC quiescence.
(picture credit: Anne Wilson and Andreas Trumpp, 2008)
In my knowledge they drew the first quiescence map, combined SLAM markers with LSK CD34-
2. Clarification of confusions about detection of non-dividing cells by label-retaining assays.
A technique often used to detect nondividing cells in various tissues is the label-retaining assay in which DNA is labeled in vivo using BrdU followed by a long “chase” period that reveals long-lived dormant “label-retaining cells”.
In contrast to Mark Kiel’s work, authors postulate that BrdU could be use for identification of quiescent fraction of HSC. Clarification of the confusion was following:
Wilson et al. found that the presence of BrdU, like some other pyrimidine analogs, induces cells to cycle, presumably through low levels of toxicity. The estimates of BrdU uptake are therefore not necessarily those of basal level entry into cell cycle, but rather reflect cellular response to injury. The uptake rate alone may therefore lead to artificially high estimates of cell cycling.
That’s why Trumpp’s group got 10-fold more quiescent HSC then Morrison’s group.
Thus, previous studies using long-term BrdU uptake kinetics have missed the d-HSC population since all HSCs are induced to proliferate due to BrdU toxicity on differentiated blood cells, and, therefore, no dormant HSCs remain in this setting.
… by modeling BrdU decay in LRC-HSCs, over time we can reveal the presence of a dormant population that comprises about one-seventh of the CD34negLSKCD150+48− subset dividing once every 145 days, which is equivalent to about five divisions per C57/BL6 lifetime.
3. Introduction of new techniques for identification of quiescent population of stem cells.
For the first time authors used in vivo method of quiescence measuring. All studies before were done on dead (fixed) cells. Probably this cell division tracing technique could be applied for study of adult stem cells in other tissues.
The combined results from the two labeling methods revealed that the previously defined uniform and highly stringent immunophenotypic stem cell pool is not homogeneous and has a dormant, label-retaining cell (LRC) pool represented by only about 600 cells per mouse. This population contained the majority of the long-term multilineage reconstitution potential.
This technique allows to sort that highly quiescent population of HSC and test their self-renewal and differentiation potential. And indeed vast majority of self-renewal and long-term multilineage reconstitution potential was found within the dormant HSC subset.
4. Researchers described physiological role of quiescent HSC population in detail.
Now it seems like the function of HSC in development looks amazingly logical:
In embryogenesis HSC are actively dividing, which supports extensive fetal growth. After birth some HSC exit cell cycle, switch off the “expansion program” and become dormant.
Thus, adult HSCs are not simply resting but are in a deeply inactive state involving reduced metabolism, ribosomal biogenesis, and DNA replication, all consistent with dormancy. Since despite their dormancy, these cells exhibit the highest self-renewal potential of all blood cells, the organism must protect them from any mutations that may unleash their potential in an uncontrolled manner that may, in turn, promote their stepwise transformation into leukemic stem cells.
So if highly quiescent (dormant) HSC population resting in bone marrow single niches, what do they do? And their guess – exit of quiescence and start to divide after injury – find confirmation in a series of experiments that introduce stresses on the hematopoietic system. Moreover, they were able to show that the dormant population of HSC can come back to a quiescent state after stress-factor is removed. So they do their job and get back to sleep.
(picture credit: Anne Wilson and Andreas Trumpp, 2008)
5. Techniques used by authors seem to be very useful for other purposes and reproducible.
Oh god, I love the reproducibility! And it’s kind of magic but exactly in the same time yet another group published quite similar findings using the same in vivo model.
Excellent study! Must read, must cite.