Ancestral telomere shortening: a countdown that will increase mean life span?

A shorter telomere is the key factor in preventing cultured human mesenchymal stem cells from senescence escape

Mesenchymal stem cells (MSCs) from various animals undergo spontaneous transformation in vitro,establishing some malignant characteristics. However,this phenomenon seems seldom appearing in human (h)MSCs. To address the question whether the hMSCs really do not undergo the spontaneous transformation and why,the present study compared MSCs from two species under the same conditions, the commercialized primary hMSCs whose in vitro life span is very uniform, and the rat (r)MSCs whose spontaneous transformation in vitro is well defined.It was demonstrated that in rMSCs, there were small numbers of re-proliferating cells appearing after a substantial senescent period. These “senescence-escaped”rMSCs were highly proliferative and did not show any sign of growth arrest during the following subcultures upto observed passage 32. Whereas after entering senescence, hMSCs no longer re-proliferated and finally died from apoptosis. Compared with rMSCs, the hMSCs possessed a much shorter telomere, and lacked both telomerase reverse transcriptase expression and telomerase activity. When proliferating from pre-senescent to senescent stages,the hMSCs had a greater loss of relative telomere length(51 % in hMSC vs. 15 % in rMSC), but both cells displayed a similar average telomere shortening per population doubling (0.50 ± 0.06 kb in rMSC vs. 0.49 ± 0.06 kbin hMSC; p > 0.05), indicating that the greater relative shortening of the hMSC telomeres was due to their original shorter length, rather than lack of telomere maintenance mechanisms. In conclusion, the hMSCs do not spontaneously initiate transformation, because they cannot escape senescence. This is particularly due to their much shorter telomere.

Human Mesenchymal Stem Cells from Chorionic Villi and Amniotic Fluid are not Susceptible to Transformation after Extensive in Vitro Expansion

Mesenchymal stem cells (MSCs) are promising candidates for cell therapy and tissue engineering. Increasing evidence suggests that MSCs isolated from fetal tissues are more plastic and grow faster than adult MSCs. In this study, we characterized human mesenchymal progenitor cells from chorionic villi (CV) and amniotic fluid (AF) isolated during the first and second trimesters, respectively, and compared them with adult bone marrow-derived MSCs (BM). We evaluated 10 CV, 10 AF, and 6 BM samples expanded until the MSCs reached senescence. We used discarded cells from prenatal analyses for all the experiments. To evaluate the replicative stability of these cells, we studied the telomerase activity, hTERT gene transcription, and telomere length in these cells. Spontaneous chromosomal alterations were excluded by cytogenetic analysis. We studied the expression of c-myc and p53, tumor-associated genes, at different passage in culture and the capacity of these cells to grow in an anchorage-independent manner by using soft agar assay. We isolated homogeneous populations of spindle-shaped CV, AF, and BM cells expressing mesenchymal immunophenotypic markers throughout the period of expansion. CV cells achieved 14 ± 0.9 logs of expansion in 118 days and AF cells achieved 21 ± 0.9 logs in 118 days, while BM cells achieved 11 × 0.4 logs in 84 days. Despite their high proliferation capacity, fetal MSCs showed no telomerase activity, no hTERT and c-myc transcriptions, and maintained long, stable telomeres. A constant expression level of p53 and a normal karyotype were preserved throughout long-term expansion, suggesting the safety of fetal MSCs. In conclusion, our results indicate that fetal MSCs could be an alternative, more accessible resource for cell therapy and regenerative medicine.

Telomere and telomerase in stem cells

Telomeres, guanine-rich tandem DNA repeats of the chromosomal end, provide chromosomal stability, and cellular replication causes their loss. In somatic cells, the activity of telomerase, a reverse transcriptase that can elongate telomeric repeats, is usually diminished after birth so that the telomere length is gradually shortened with cell divisions, and triggers cellular senescence. In embryonic stem cells, telomerase is activated and maintains telomere length and cellular immortality; however, the level of telomerase activity is low or absent in the majority of stem cells regardless of their proliferative capacity. Thus, even in stem cells, except for embryonal stem cells and cancer stem cells, telomere shortening occurs during replicative ageing, possibly at a slower rate than that in normal somatic cells. Recently, the importance of telomere maintenance in human stem cells has been highlighted by studies on dyskeratosis congenital, which is a genetic disorder in the human telomerase component. The regulation of telomere length and telomerase activity is a complex and dynamic process that is tightly linked to cell cycle regulation in human stem cells. Here we review the role of telomeres and telomerase in the function and capacity of the human stem cells.