Haematopoietic stem cells (HSC) and their progeny from exhibit a range of functional declines during biological aging. Most research probing the reasons for these declines have focused on aging damage accumulating in the HSCs themselves, such as the rising burden of oxidative stress and DNA damage (and, as a result, senescent cells) in the compartment. But there has been comparatively little exploration of the possibility of outside causes for age-related HSC dysfunctions, such as the role of age-related shifts in the systemic and local environment and the aging of the bone marrow HSC niche. In a recent report, Dr. Amy Wagers’ stem cell group at Harvard and affiliated institutes1 have demonstrated the reality — and the reversibility — of both of these influences on age-related HSC dysfunction, using an intriguing model of systemic rejuvenation.
The investigators used the heterochronic rodent parabiosis model, in which the circulatory systems of young (2 mo old) and biologically aged (>20 mo) mice are surgically conjoined, allowing their circulatory systems to commingle and equilibrate; as controls, these young-to-old pairs are compared with age-matched pairs of each age group. In an earlier collaboration with Dr. Irina Conboy, Wagers had previously used this captivating experimental system to strikingly demonstrate the rejuvenating effects of exposure to a more youthful systemic environment on skeletal muscle satellite cell mobilization, proliferation, and regenerative capacity, and on the proliferation of hepatic progenitor cells.7For these new studies in HSC niche function, all parabiotic pairs were composed of partners that differed at the CD45 locus (CD45.1 and CD45.2), allowing haematopoietic cells to be traced to their originating hosts.
Youthful Circulation Restores Youthful Regulation of HSCs in Aged Animals
Comparing isochronic pairs from the 2 age groups, old mice exhibited superfluous accumulations of primitive long-term reconstituting HSCs relative to young comparitors. These cells were characterized by impaired haematopoietic engraftment, manifested in a less efficient reconstitution of peripheral blood leukocytes, and a bias toward the myeloid lineage at the expense of B-cell development. But all of these changes in old-derived HSC numbers and function were substantially normalized following exposure to the more youthful heterochronic environment (see Figure 1). The frequency and number of bone-forming osteoblasts in aged mice — a constituent of the HSC niche with a central role in regulating HSC number and activity — were also inflated (up to fourfold) in aged mice. Consistent with the findings in the parabiosis model, these old cells induced the development of more HSCs from cocultured lineage-negative bone marrow cells in vitro than did young-derived bone-forming osteoblasts, possibly explaining the age-related increase in long-term reconstituting HSCs formed in aged animals in vivo.1
Figure 1. Exposure of aged mice to youthful circulatory system restores long-term reconstituting HSC number and function and osteoblastic niche cell number. a: Frequency of endogenous LT-HSCs. b: Frequency of osteoblastic niche cells. c: Reconstitution of haematopoietic system in irradiated recipients by donor bone marrow cells 12 wk posttransplant. From (1).
Rejuvenating Effects of Young Circulation is Mediated by the HSC Niche
The fact that aged animals’ osteoblastic HSC niche cells recapitulate the bias toward excessive accumulation of long-term reconstituting HSC observed in vivo in aged animals suggested that that the effects of a ‘younger’ heterochronic parabiotic systemic environment on old animals’ HSC were the result of an indirect effect, mediated by a rejuvenated HSC niche. Indeed, exposure to a more youthful systemic environment through heterochronic parabiosis normalized the old animals’ frequency and number of bone-forming osteoblasts, and their rate of formation of long-term reconstituting HSCs from lineage-negative bone marrow cells ex vivo.1
More dramatically, lineage-neutral HSCs cocultured with old-derived osteoblastic niche cells demonstrated impaired engraftment and differentiation when transplanted into old or even young irradiated hosts, recapitulating the defects seen in old animals and reinforcing the importance of the aging of the niche in those defects in the aged host. In contrast, there was no effect of osteoblastic niche cells from aged animals on the engraftment, lineage bias, or reconstituting ability of HSCs from young animals.1
Role of Excess IGF-1 in Impaired HSC Niche Regulatory Function
Exposure of young-derived HSCs to serum derived from aged murine or human donors again led to superfluous long-term reconstituting HSC accumulation, and exposure of aged osteoblastic niche cells to young serum blunted their dysregulatory influence on young-derived cells. Exposure of such cells to aged niche cells also increased their expression of several age-related myeloid-biasing markers, and decreased their expression of lymphoid markers whose expression is known to be reduced in aged organisms, consistent with the observed effects of such cells on lineage in vivo. Notably, these shifts were not observed following coculture with niche cells derived from aged animals that had benefited from the rejuvenating systemic environment of heterochronic parabiosis.1
Further studies to probe the mechanistic basis of the systemic influence on the HSC niche led to the surprising conclusion that a significant mediator of the impaired HSC regulation of the aged osteoblastic HSC niche is an excess of local IGF-1 signaling. Prior exposure of old-derived haematopoietic osteoblastic niche cells or serum to anti-IGF-1antibodies abolished the abnormal accumulation of long-term reconstituting HSCs during coculture with young-derived HSCs. By contrast, anti-IGF-1 Abs had no effect on the influence of young niche on HSCs, nor on old-derived HSCs in isolation. Similar effects were observed on a dose-dependent basis in vivo following neutralizing antibody injection into aged animals’ bone marrows, but not following systemic injection via the peritoneum, showing that the disrupting effect of excessive IGF-1 on HSC function occurs in the haematopoietic niche microenvironment, rather than in the circulation at large, narrowing the effects observed in circulatory parabiosis.1
This counterintuitive finding is superficially opposite to the effects of local IGF-1 expression observed in aging muscle.2 It is instructive, if only by analogy, to compare the contrasting pro- and anti-aging local effects of IGF-1 in aging organisms to the paradoxical findings in other systems, such as the restoration of declining IGF-1 in normally-aging organisms vs. the many models of retarded biological aging in mice and other species characterized by reduced IGF-1 signaling.3 Notably, the effects of these experimental systems on local IGF-1 signaling is in at least some cases tissue-specific: local brain IGF-1 levels and signaling are preserved or enhanced in several models of retarded aging characterized by low systemic IGF-1 levels.4-6 And while systemic IGF-1 levels are low throughout the lifespan in these models, they remain stable at advanced ages when they have declined substantially in normally-aging animals; moreover, Calorie restriction preserves the regulated, pulsatile release of growth hormone with aging (Fig. 2) and selectively retains celllular IGF-1 signaling, even as these capacities are progressively lost in animals fed ad libitum.17-19
Figure 2. Preservation of growth hormone secretory dynamics in by CR in aging Brown Norway rats. From (18)
Implications for Rejuvenation Biotechnology and WILT
Whole-body Interdiction of Lengthening of Telomeres (WILT, or OncoSENS) is proposed by de Grey et al(14,15) as an impregnable defense against cancer, as part of a comprehensive panel of rejuvenation biotechnology to repair the damage and diseases of aging. At its core, it entails the ablation of gene(s) essential to the telomere maintenance machinery (TMM), accompanied by periodic re-seeding of somatic stem-cell pools with autologous cells rendered equally defective for telomere elongation but whose telomeres have been lengthened ex vivo to allow for ongoing tissue repair and maintenance. The strongest challenge to this approach has been the possible existence of functions of TMM other than the lengthening of telomeres itself,8-13 and that even if TMM were dispensible in telomere-elongated stem cells, it might be essential to the functioning of the niche.
SENS Foundation is now funding research by Dr. Zhenyu Ju (formerly a telomerase researcher in Dr. K. Lenhard Rudolph’s laboratory and now at the Max Planck Partner Group on Stem Cell Aging at the Chinese Academy of Medical Sciences) to help resolve the latter question, by monitoring the effects of transplanting telomerase-deficient but ex vivo telomere-extended bone marrow into first normal and then TMM-deficient mice. The finding that the age-related loss of HSC function is substantially attributable to derangement of HSC regulation by the aging niche, much of which is secondary to shifts in systemic factors in the aging niche microenvironment rather than to cell-autonomous defects, provides some very preliminary reassurance on this issue.
More confidently, this latest example of the rejuvenating effect of youthful systemic environment (cf. (7), and Conboy’s later reports in rejuvenation of muscle satellite cell function, as well as loosely-related studies in rejuvenation of the aging female reproductive system) reinforces the expectation that the effects of regenerative engineering therapies will not be narrowly confined to restoring the function of their specific target tissues. As we remove, repair, replace, or render harmless the cellular and molecular damage of aging, the progressive restoration of normal cell and tissue function can be expected to result in a concomitant, progressive normalization of the systemic milieu, as oxidative stress, inflammation, endocrine and paracrine signaling, and other systemic responses to — and sequelae of– the damage of aging are obviated and the body’s inherent maintenance capacities are engaged. With this normalization, these studies suggest, the deranging effects of an aged systemic environment will gradually be alleviated, and remote tissues will begin to return to more youthful function; in turn, the renewal of those remote tissues’ function then further contribute to the re-establishment of youthful homeostasis in the system as a whole. As the regenerative process feeds back upon itself, accelerated with each new therapy applied and each additional form of damage repaired, the function of tissues, organs, we may hypothesize that the organism as a whole will re-emerge in unexpected ways and with unanticipated inflection points, until we stand restored to the full health, vigor, and capacity of youth.
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