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| Haematopoetic Stem Cells

The fountain of youth; Choose your partner wisely.

The extent to which ageing affects the stem cell compartment and tissue function reflects how much the tissue relies on resident stem cells for normal tissue homeostasis. Tissues with high turnover (for example blood, skin and gut) contain a prominent stem cell compartment and demonstrate a high regenerative capacity, whilst those with low cell turnover but high regenerative capacity (E.g. pancreas, skeletal muscle and liver) contain fewer stem cells and use different mechanisms for tissue homeostasis and repair. Thus, each tissue contains its own specialized environment, or niche, to preserve stem cell potential. There are many hypotheses as to why various changes occur in the stem cell compartment with ageing. Discrete cell-intrinsic or population changes are implicated, with concomitant alterations in differentiation potential and hence tissue restoration capability. The stem cell niche provides a tight regulatory and supportive microenvironment for resident stem cells and it has been demonstrated that alterations in extrinsic signals can modulate the niche and may even coordinate the ageing process across various host tissues. The aged hematopoietic system exhibits reduced immune function and increased malignancy, particularly that of the myeloid subtype. There is an increase in the hematopoietic stem cell (HSC) population in aged bone marrow (Sudo et al., 2000), however these usually demonstrate impaired hematopoietic engraftment, reduced reconstitution of peripheral blood leukocytes and differentiation capacity (favouring myeloid over other phenotypes) when compared with young HSCs. Profiling studies demonstrate altered stem cell regulatory gene expression in aged ‘dysfunctional’ HSCs alongside the suppression of lymphoid and increased expression of myeloid specification genes (Rossi et al, 2005).

Using an in vivo parabiotic system coupled with the in vitro study of regulatory HSC niche cells, Mayack et al. studied the role of local microenvironmental and systemic factors on hematopoietic stem and progenitor cell (HSPC) ageing. The authors generated heterochronic pairs (where ‘young’ mice were surgically joined to ‘aged’ mice) and compared these with isochronic parabionts (age matched surgically joined pairs). Parabiosis allows a common blood circulation between the pair and thus assessment of the effects of age-regulated circulating cells or factors on tissue function (Harrison et al., 1977; Hotta et al., 1980; Carlson & Faulkner, 1989; Conboy et al., 2005). In isochronic parabionts and in young-heterochronic partners exposed to an aged circulatory system, the frequency and number of primitive and long-term reconstituting HSCs (LT-HSCs) was unaltered, as shown by long-term multi-lineage reconstruction of irradiated recipients. However in aged-heterochronic partners, the authors reveal that exposure of aged bone marrow to young systemic factors restored the engraftment and lineage potential of donor LT-HSCs to that of youthful levels.

The authors have demonstrated previously that bone forming osteoblasts, a component of the HSC niche, convey physiologically appropriate signals to modulate HSC activity (Mayack & Wagers, 2008). Like HSCs, the number and frequency of osteoblastic niche cells are increased in aged mice. Their current results show that short exposure of young bone marrow cells to isolated osteoblastic niche cells from aged mice is sufficient to significantly increase the HSC population, akin to levels observed in aged mice. To test whether the ‘age-reversal’ effects observed in heterochronic pairs result from a reversion of age-related changes in the osteoblastic niche by a young systemic environment, the authors determined osteoblast frequency and number in aged-heterochronic parabionts and found not only that the osteoblast niche was restored to youthful levels, but that there was a significant reduction in HSPC accumulation as a result of the ‘age-reversed’ niche cells. Although the number and frequency of niche cells was not affected in young mice that were heterochronically joined to aged partners, isolated niche cells from these animals induced increased expansion of HSPCs when compared with young isochronic pairs, which the authors attributed either to circulating ‘ageing’ factors from the elder, or a dilution of ‘youthful’ factors in the young-heterochronic parabiont.

Furthermore, young HSCs exposed in vitro to aged osteoblastic niche cells subsequently acted like aged HSCs, showing reduced capacity for hematopoietic reconstruction and myeloid-skewed differentiation potential, indicating that local niche cells regulate HSC function and specify their physiological ‘age’. However, the reconstituting activity of young HSCs exposed to osteoblastic niche cells from aged-heterochronically paired animals showed a youthful profile of hematopoietic engraftment, demonstrating that age-associated alterations in the HSC niche can be reversed by a young circulation. Similar results were demonstrated in vitro using short term co-culture assays with young HSPCs; increased accumulation of HSPCs was observed following co-culture with niche cells from young mice exposed ex vivo to serum from old mice or aged human donors, whilst aged niche cells exposed to young serum displayed reduced capacity to induce LT-HSC accumulation.

In order to determine the mechanism by which this occurs, Mayack et al. studied the survival and transcription profiles of HSCs exposed to young or aged osteoblasts. Whilst no change in LT-HSC apoptosis was observed, real-time PCR analysis revealed differential expression of stem cell regulatory genes (Sox4, Notch1 and Notch2) and significant upregulation of age-regulated myeloid markers in young LT-HSCs exposed to aged-isochronic niche cells, alongside decreased expression of lymphoid markers – a result which was not observed when young LT-HSCs were exposed to niche cells from aged-heterochronic parabionts.

The authors then sought to determine the role of insulin-like growth factor-1 (IGF-1), implicated in the regulation of ageing and longevity across multiple tissue types. Neutralisation of IGF-1 in aged osteoblastic niche cells or young HSCs with an anti-IGF-1 antibody ameliorated the ‘ageing’ effects of an old niche on young HSCs and the response of young niche cells to aged serum, however had no effect on isochronically-matched HSCs and niche cells, suggesting that IGF-1 signalling in niche cells impairs HSC regulation. Further, they went on to demonstrate in vivo that this effect occurs locally via changes in IGF-1 signalling within the niche. Whilst direct delivery of anti-IGF-1 antibody into the bone marrow of aged mice markedly decreased the subsequent HSC accumulation capacity of treated niche cells in vitro, systemic IGF-1 neutralisation had no effect on the osteoblastic niche or HSC population dynamics.

This study demonstrates that age-specific changes within the regulatory niche initiate age-related stem cell dysfunction and that this can be reversed by youthful systemic factors. Their results implicate a novel and important role for IGF-1 signalling in the regulation of HSPCs within the ageing hematopoietic niche. Although the systemic mechanism by which IGF-1 is regulated during ageing remains unclear, the involvement of Wnt signalling in other age-associated pathology highlights this pathway as a probable candidate, as both Wnt and IGF-1 signalling are implicated in age-related changes across various tissues in a tissue-specific manner. However this assumption cannot be transferred across all tissue types. Skeletal muscle for example, relies solely on resident satellite cells for tissue repair (Morgan & Partridge, 2003), which remain quiescent unless responding to injury or disease. In line with the current study, heterochronic parabiosis also elicits age-reversal effects on aged muscle (Conboy et al., 2005), however in this system the age-reversal effects of a youthful circulation are mediated by enhanced Notch signalling (Conboy et al., 2003). Notably, in contrast to its effects on the hematopoietic stem cell compartment, local expression of IGF-1 maintains regenerative capacity in aged muscle (Musarò et al., 2001).

Although the role of stem cells in overall longevity remains controversial, these impressive results indicate that the age-related decline in stem cell function is modulated by local and systemic cues and that therapeutic strategies utilising the systemic milieu to rejuvenate the niche, at least in the case of the hematopoietic system, might significantly extend the period of normal stem cell function and promote hematopoietic longevity. The observations of Mayack et al. could also support the idea that cellular ageing may have an epigenetic component. The relative contributions of genetic and epigenetic factors (either inherited or acquired) to the ageing process is a fascinating subject and if changes in epigenetic gene regulation are significant this offers the possibility of reversing at least some of the age related changes. Elucidation of the stem cell-niche interaction dynamics in other contexts will allow us to determine whether similar results can be achieved to promote the prolonged health of other tissue types. Given that aged human serum elicited similar effects to an aged mouse circulation on the HSC niche, it will be interesting to determine whether this effect also works in humans.



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