Ageing is accompanied by deterioration in the haematopoietic stem cells that are responsible for regenerating the blood system. Cellular stress in the aged stem cells could be a cause of this decline. See Letter p.198
Tissue renewal is a fundamental process that relies on the regenerative capacity of long-lived, self-renewing stem cells. But during ageing, stem-cell function deteriorates. The haematopoietic stem cells (HSCs) that maintain all blood-cell lineages are, like other long-lived stem cells, prone to accumulating DNA damage as they age. In the case of HSCs, the damage can reduce the cells' ability to regenerate blood-cell lineages, and can increase the risk of diseases such as leukaemia. But little is known about what causes the damage and how it contributes to the decline of old HSCs. On page 198 of this issue, Flach et al.1 report that damage is caused mostly by cellular stress that arises as a result of inefficient DNA replication, and they point to the probable molecular defects involved.
DNA damage occurs when cells cannot repair genetic inaccuracies, which frequently arise while DNA is being replicated during cell proliferation. The idea that DNA damage is a major driver of the deterioration of stem cells in general, and old HSCs in particular, is supported by the fact that both mice and people with deficiencies in DNA repair age more quickly than those without such deficiencies2,3,4,5. But debate over the potential causes of DNA damage in HSCs has been lively and multifaceted, because factors both intrinsic to the cell itself (for example, loss of cell polarity) and extrinsic (such as secreted proteins or changes in the types of cell surrounding the HSCs) can affect the environment in which old HSCs reside6.
To investigate the origin and impact of DNA damage in aged HSCs, Flach and colleagues compared HSCs isolated from the bone marrow of young and old mice. Compared with young cells, old HSCs showed a functional decline, together with signalling indicative of DNA damage, which the authors gauged by presence of the γH2AX protein. γH2AX was accompanied by an increased abundance of proteins associated with inefficient DNA replication (known as DNA replication stress)7. These proteins promote signalling by the enzyme ATR, which modifies many cellular functions3,7.
Following up on this unexpected result, the authors found that ATR signalling was activated in old HSCs, another indication that they were subject to replication stress. The cells also showed delayed entry into and progression through S phase, the period of the cell cycle in which the genome is replicated. Furthermore, DNA replication frequently stalled in old HSCs, and the number of 53BP1 bodies — structures that mark chromosomal breaks in the nuclei of cells that have experienced replication stress8 — rose.
To look at what molecular defects could be responsible for enhanced replication stress in aged HSCs, Flach et al. compared gene-expression profiles in young and old HSCs. Genes encoding the proteins MCM4 and MCM6 (two components of an MCM protein complex that is essential for proper replication) showed lower expression in old than young HSCs, as did a variety of other factors.
The authors found that experimental depletion of MCM4 and MCM6 in young HSCs impaired the cells' function. Like old HSCs, the altered cells had a poor capacity to regenerate the blood system when transplanted into mice, suggesting that low levels of MCM4 and MCM6 are linked with replication stress, and thereby with functional deterioration. In agreement with this, young HSCs were also impaired if replication stress was caused by chemical compounds.
Finally, Flach et al. investigated why γH2AX was present in HSCs that had stopped proliferating and therefore could not be experiencing replication stress. They found signs of long-term damage signals in genes within ribosomal DNA (rDNA), which includes many genes that encode components involved in assembly of the ribosome (the cellular machinery responsible for producing protein from messenger RNA). This makes sense, because rDNA is difficult to replicate and is therefore prone to replication stress. The authors showed that persistent damage was linked to lowered expression of rDNA genes. Consequently, the cells made fewer ribosomes, and could not produce enough protein to sustain cellular function — a state known as ribosomal biogenesis stress9.
Overall, Flach and colleagues' work shows that old HSCs experience both replication stress and ribosome biogenesis stress. The former probably triggers the latter, and is clearly at least partly responsible for impaired blood regeneration in advanced age (Fig. 1). The results have broad implications for medicine, and raise many questions. For example, is replication stress involved in the deterioration of ageing stem cells in other tissues? Is the authors' mechanism relevant to human HSCs?
Because replication stress underlies many tumours10, it is possible that stress in HSCs contributes to the progressive accrual of gene mutations that cause ageing-related cancers of the blood. It will be interesting to determine how ribosome biogenesis stress influences HSC decline, and to investigate whether the p53 tumour-suppressor protein — a known sensor of both replication and ribosomal stress3,9,10 — is involved.
Finally, could restoration of MCM4 and MCM6 levels avert replication stress or even functional decline in old HSCs? If it could, understanding how MCM genes are inhibited in old age might be a good starting point for defining strategies to postpone, prevent or even reverse the deterioration of the ageing blood-regeneration system.
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Cellular and molecular basis of the imbalance between vascular damage and repair in ageing and age-related diseases: As biomarkers and targets for new treatments
Mechanisms of Ageing and Development (2016)