In many species a loss of proliferative capacity of somatic cells can be observed during cellular ageing. The shortening of telomeres is discussed as one responsible factor for the replicative senescence of somatic cells, and is proposed to be one of the reasons why animals age and show an age-related increase of mortality (Wright and Shay, 2005). The telomere structure, consisting of DNA repeat sequences (5′-TTAGGG-3′), is highly conserved among vertebrates (Meyne et al., 1989) as well as among invertebrates (Traut et al., 2007). Once telomere length reaches a certain threshold cells seem to approach senescence, the so-called Hayflick limit (Bolzán and Bianchi, 2006; Hayflick, 1965).
Activation of the enzyme telomerase (Aubert and Lansdorp, 2008) or other telomere lengthening pathways (Bryan et al., 1997) can maintain telomere lengths, leading to theoretically unlimited proliferative potential. Thus, telomerase is expressed primarily in germ cells (Zalenskaya and Zalensky, 2002), stem cells (Mason, 2003) and in actively proliferating transit cells (Buchkovich and Greider, 1996). In regular human tissue telomerase is activated in early embryogenesis and whilst the first trimester, after that it is repressed in adult tissues, supposedly resulting from the relationship of active telomerase linked to cancer (Forsyth et al., 2002; Shay and Wright, 2011).
Progressive telomere shortening is very often linked to tissue and organismal ageing (Campisi, 1996; Proctor and Kirkwood, 2002) or to stressful environments (Metcalfe and Monaghan, 2003), furthermore telomere maintenance has been shown to play a key role in organismal longevity (Haussmann et al., 2005; Joeng et al., 2004). In contrast, an increase of telomere length with age has been observed in the extreme long-lived bird (Leach‘s storm petrel) which reaches a maximum life span of 36 years (Haussmann et al., 2003). Similar results with such a positive relationship of telomere length and or telomerase activity with age are observed in long-lived trees (Pinus longaeva, (Flanary and Kletetschka, 2005)), the water python (Liasis fuscus, (Ujvari and Madsen, 2009)) and the sand lizard, (Lacerta agilis, (Olsson et al., 2010)). Even more fascinating examples are planarian flatworms or colonial ascidians, showing significantly different patterns in telomere maintenance and telomerase activities (Sköld et al., 2011; Tan et al., 2012).
Aubert, G., Lansdorp, P.M., 2008. Telomeres and aging. Physiol. Rev. 88, 557–579.
Bryan, T.M., Englezou, A., Dalla-Pozza, L., Dunham, M.A., Reddel, R.R., 1997. Evidence for an alternative mechanism for maintaining telomere length in human tumors and tumor-derived cell lines. Nat. Med. 3, 1271–1274.
Bolzán, A.D., Bianchi, M.S., 2006. Telomeres, interstitial telomeric repeat sequences, and chromosomal aberrations. Mutat. Res. Rev. Mutat. Res. 612, 189–214.
Buchkovich, K.J., Greider, C.W., 1996. Telomerase regulation during entry into the cell cycle in normal human T cells. Mol. Biol. Cell 7, 1443–1454.
Campisi, J., 1996. Replicative Senescence: An Old Lives' Tale? Cell Press, Cambridge, MA, ETATS-UNIS.
Flanary, B.E., Kletetschka, G., 2005. Analysis of telomere length and telomerase activity in tree species of various life-spans, and with age in the bristlecone pine Pinus longaeva. Biogerontology 6, 101–111.
Forsyth, N.R., Wright, W.E., Shay, J.W., 2002. Telomerase and differentiation in multicellular organisms: turn it off, turn it on, and turn it off again. Differentiation 69, 188–197.
Joeng, K.S.S., Joo, Eun, Lee, Kong-Joo, Lee, Junho, 2004. Long lifespan in worms with long telomeric DNA. Nat. Genet. 36, 607–611.
Haussmann, M.F., Winkler, D.W., Vleck, C.M., 2005. Longer telomeres associated with higher survival in birds. Biol. Lett. 1, 212–214.
Haussmann, M.F.,Winkler, D.W., O'Reilly, K.M., Huntington, C.E., Nisbet, I.C.T., Vleck, C.M., 2003. Telomeres shorten more slowly in long-lived birds and mammals than in short–lived ones. Proceedings of the Royal Society of London. Ser. B: Biol. Sci. 270,
Mason, P.J., 2003. Stem cells, telomerase and dyskeratosis congenita. BioEssays 25, 126–133.
Metcalfe, N.B., Monaghan, P., 2003. Growth versus lifespan: perspectives from evolutionary ecology. Exp. Gerontol. 38, 935–940.
Meyne, J., Ratliff, R.L., Moyzis, R.K., 1989. Conservation of the human telomere sequence (TTAGGG)n among vertebrates. Proc. Natl. Acad. Sci. 86, 7049–7053.
Olsson, M., Pauliny, A., Wapstra, E., Blomqvist, D., 2010. Proximate determinants of telomere length in sand lizards (Lacerta agilis). Biol. Lett. 6, 651–653.
Shay, J.W.,Wright,W.E., 2011. Role of telomeres and telomerase in cancer, Semin. Cancer Biol.Elsevier 349–353.
Sköld, H.N., Asplund, M.E.,Wood, C.A., Bishop, J.D., 2011. Telomerase deficiency in a colonial ascidian after prolonged asexual propagation. J. Exp. Zool. 316, 276–283.
Tan, T.C., Rahman, R., Jaber-Hijazi, F., Felix, D.A., Chen, C., Louis, E.J., Aboobaker, A., 2012. Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms. Proc. Natl. Acad. Sci. U. S. A. 109, 4209–4214.
Traut, W., Szczepanowski, M., Vítkova, M., Opitz, C., Marec, F., Zrzavy, J., 2007. The telomere repeat motif of basal metazoa. Chromosom. Res. 15, 371–382.
Ujvari, B., Madsen, T., 2009. Short telomeres in hatchling snakes: erythrocyte telomere
dynamics and longevity in tropical pythons. PloS ONE 4, e7493.
Wright, W.E., Shay, J.W., 2005. Telomere biology in aging and cancer. J. Am. Geriatr. Soc. 53, S292–S294.
Zalenskaya, I.A., Zalensky, A.O., 2002. Telomeres in mammalian male germline cells. Int. Rev. Cytol. 218, 37–67.