The Number of Very Small Embryonic Like Stem Cells (VSELs) Decreases During Aging In An IGF-1-Dependent Manner - a Novel Link Between Aging, Caloric Restriction, and the Size of the Stem Cell Pool

Abstract 4796

We have previously demonstrated the presence of very small (smaller than erythrocytes) Oct-4⁺SSEA-1⁺Sca-1⁺Lin^-CD45^- VSELs in bone marrow (BM) and in several murine adult organs (Leukemia 2006;20:857 and Cytometry 2009;73:1116). These small cells i) have large nuclei that contain primitive open chromatin, ii) express Oct-4 gene (as confirmed by our recent promoter methylation and chromatin structure analysis studies), and iii) posses bivalent domain-marked promoter regions of homeodomain-containing developmental master transcription factors, such as Dlx-, Irx-, Lhx-, Pou-, Pax-, and Six-family proteins. Furthermore, the epigenetic changes in selected somatic-imprinted genes (e.g., Igf2-H19 and RasGrf1) involved in insulin-factor signaling (Igf-1, Igf-2, and Insulin) govern their quiescent state, thus preventing them from unleashing proliferation and spontaneous growth of teratomas (Leukemia 2009;23:2042). O other hand it is well known that i) Igf-1 signaling negatively regulates lifespan in worms, flies, and mammals (Cell 2005;120:449) and that ii) Igf-1 and insulin levels in blood are positively regulated by caloric uptake. Indeed, we found that the pool size and pluripotentiality of VSELs decreases during aging. Accordingly, in our studies performed on young (4-week-old) and old (2-year-old) mice we found that i) the number of VSELs and their pluripotentiality decreases with age, ii) VSELs from old mice show lower expression of Oct-4, Nanog, Sox2, Klf4, and cMyc, iii) the Oct-4 promoter becomes hypermethylated with age and has a closed chromatin structure, iv) VSELs from old mice show somatic methylation in both Igf2-H19 and Rasgrf1 loci, and v), as a result, VSELs from these mice have increased sensitivity to insulin/insulin factors signaling. This suggests that chronic insulin receptor/Igf-1 receptor signaling in VSELs may contribute to age-related depletion of these cells. To explain better the role of insulin signaling in VSELs, we measured by FACS the number of VSELs in murine Laron dwarfs, which exhibit chronic Igf-1 deficiency and, as a result, live 30–40% longer than their normal littermates. We report here, for the first time, that the number of VSELs in the BM of Igf-1–deficient Laron dwarfs is 3–4 fold higher and is maintained at a higher level during aging compared to normal wild type (wt) littermates. Molecular analysis studies will confirm whether or not the molecular signature of VSELs in aging Laron dwarfs is somehow protected from age-related changes (e.g., by the methylation status of the Oct-4 promoter and/or genomic imprinted genes). Based on our data, we postulate novel linkages between Igf-1 level, aging, and the stem cell compartment. According to our hypothesis, early in development a population of VSELs would be deposited in developing organs as a backup for tissue-committed stem cells and play a role in rejuvenation of tissues and organ regeneration after damage. These cells would be protected from uncontrolled proliferation and age-related depletion by changes in imprinted genes that regulate insulin signaling. We further hypothesize that in the adult body the pool of VSEL cells is regulated by the circulating Igf-1 level. An increase in Igf-1 level (e.g., resulting from a chronically high calorie diet) would accelerate an age-dependent decrease in VSELs and their potential to rejuvenate tissues. By contrast, a low Igf-1 level (e.g., as seen in Laron dwarf mutants or due to caloric restriction) would have a protective effect on the overall pool of these cells. Thus, we present for the first time a hypothesis that reconciles aging, longevity, Igf-1 signaling, and caloric uptake and negative effect of Igf-1 and high calorie uptake on number/function of pluripotent VSELs deposited in adult tissues.