The present work offers a proof of concept supporting the validity of the model that the difference between muscle TL and TL of proliferative tissues (that is leukocytes or skin) provides information over and above that of cross-sectional analysis of age-dependent TL shortening in proliferating tissues. That said, the present work shows no evidence that CR in this study of rhesus monkeys influences age-dependent TL shortening in leukocytes (or skin cells). Although the mechanism underlying CR benefits is still not fully understood, these results do not support a link between TL dynamics in leukocytes and CR in rhesus monkeys. With the recent publication from the nonhuman primate CR study being conducted at the University of Wisconsin that demonstrated mortality reduction with CR (3
), it will be interesting to determine whether National Institute on Aging colony monkeys subjected to CR, from which the samples were acquired, also display reduced mortality risk.
Results of the current study confirm earlier findings of synchrony in TL within cells and tissues of humans and macaques at birth and later in life (12
). Moreover, they underscore the considerably longer telomeres in the rhesus monkey than in humans (12
). With such long telomeres and a comparatively shorter life span, which amounts to ~25 years on average and ~40 years for maximum life span in captivity (3
), it is uncertain if telomere-mediated replicative aging would be a determinant in interindividual variations in longevity and age-related changes among macaques. Similarly, TL in most established mouse strains is considerably longer than that of humans, although in two wild-derived strains, CAST Ei
and Mus speretus
, TL is approximately 19 and 8 kb, respectively (29
). However, even in these strains, TL is relatively long in relation to their life span, which is ~30 times shorter than that of humans (32
). Although TL in the mouse is evidently not calibrated to play a role in its life span, the genetically engineered telomerase-deficient mice indicates that short TL can impose a limit on the life span of a mammal (33
). We note, however, that given that the shortest TL is the determinant of replicative senescence in cultured cells (34
), in principle, even in short-lived mammalian species the shortest telomeres rather than the mean length of all telomeres might impact the animal's life span.
TL in leukocytes at birth and its shortening afterwards corresponds to the respective variables in hematopoietic stem cells (10
). Therefore, leukocyte TL dynamics (birth leukocyte TL and its age-dependent shortening) would mirror variations with age in the rate of replication and the amount of telomeric DNA repeats lost with each replication of hematopoietic stem cells. Chronic inflammation increases replicative demand on hematopoietic stem cells to accommodate the inflammatory response, whereas an increase in oxidative stress augments the loss of telomere repeats per replication (36
). The associations of shortened leukocyte TL with age-related diseases in humans have been attributed in part to these processes (10
), with exceptions (39
). We anticipated, therefore, that CR animals would display attenuation of leukocyte TL shortening if CR diminished inflammation and oxidative stress burdens. However, there was no apparent evidence of attenuation of leukocyte (and skin) telomere shortening by CR.
One of the limitations of this study is that the youngest monkeys were all controls, resulting in the CR animals grouping around ~15 years of age or older. This is due to the longevity study design for the primary outcome measures but prevents a broad-scale age group in the CR animals. Similarly, only a limited sample size of CR animals is available. Although all monkeys were routinely monitored for nutritional and health status, with no difference between the diet groups, we cannot exclude the possibility that the limited numbers have reduced the sensitivity of detection for any CR-related difference. However, the fact that both sex- and age-related differences were observed in the TL measures provides confidence in the null findings for diet.
Leukocyte TL attrition is very rapid during early life (41
). This was shown not only in humans (41
) but also in a longitudinal study in baboons (43
). More TL measurements from younger and older CR monkeys, along with controls, would have provided more precise information of TL dynamics. Additionally, although the “standardized” age-dependent TL shortening can be estimated from the ΔTL, longitudinal measures from the same animal would be preferred to reduce potential interindividual variation.
We have focused the results and discussion on the mean TL but have also tested the median, mode, and 50th or 25th percentiles and observed similar outcomes, namely that although age and sex significantly affect TL shortening in leukocytes and skin after controlling for muscle TL, diet was not significantly related to TL dynamics.
Although mortality may be considered the ultimate outcome related to TL, other related outcomes of health (eg, insulin resistance, inflammatory indices, etc.) and disease patterns of specific organs might be of interest in relationship to TL dynamics in CR macaques and their controls.
In conclusion, TL is synchronized among different tissues/cells in rhesus monkeys. Controlling for a surrogate early development TL with skeletal muscle, both skin and leukocytes showed age- and sex-related effects on TL attrition. However, CR did not significantly attenuate leukocyte and skin TL attrition in CR macaques involved in this long-term study.