Evaluation of the effect of age of the younger mice on the rejuvenation of the older mice by heterochronic parabiosis.

Heterochronic parabiosis is used to study the systemic effects of aging and involves surgically connecting two animals of different ages such that they have common blood circulation. Although this technique has been prevalent for a long time, there is no scientific consensus on the age of the animals that should be used. We hypothesized that the younger the animal, the greater would be its rejuvenating effect. Hence, to test this hypothesis, we created parabiosis of 67-week-old mice with younger mice of different ages (4-week-old and 8-week-old). We evaluated the changes in appearance and the expression IL-1A, IL-6, and Cdkn2a (p16) in the liver, kidney, brain, and skin. These cytokines belong to the senescence-associated secretory phenotype (SASP) factors, and are indicators of aging. Although we did not find any significant changes in the appearance of the mice, we found statistically significant differences in some SASP factors between the liver of the 4-week-old and 8-week-old pairs. However, overall, compared to the 8-week-old mice, the 4-week-old does not exert a significantly higher rejuvenation effect on the older mice. Hence, we concluded that the rejuvenation of older mice during heterochronic parabiosis might not be affected by the exact age of the younger mice.