103 Telomeres, cell senescence and human ageing Thomas von Zglinicki, Carmen M. Martin-Ruiz, Gabriele Saretzki Henry Wellcome Laboratory for Biogerontology Research, University of Newcastle, UK Telomeres in most human cell types shorten during DNA replication in vitro because of various factors including the inability of DNA polymerases to fully copy the lagging strand, DNA end pro- cessing and random damage, often caused by oxidative stress. Short, uncapped telomeres activate replicative senescence, an irreversible cell cycle arrest and are thus a major cause of cell ageing in vitro. We will review how uncapped telomeres initiate a signalling cascade toward senescence, and why oxidative stress is a major cause of telomere shortening. Telomeres in most human cells shorten during ageing in vivo as well, suggesting two distinct possibilities. (1) Telomere shortening could be among the causes for ageing in vivo: Short telom- eres might lead to senescence of (stem) cells in a tissue-specific fashion, and this might contribute to age-related functional attenuation in this tissue and even to systemic effects. Evidence for this is mostly indirect. (2) Telomere length could be a biomarker of ageing and age-related morbidity: Short telomeres might indicate a history of high stress and damage in the individual and could thus act as risk markers for age-related disease residing in a completely different tissue. There is evi- dence to support this possibility, although it is mostly correlative and is often derived from un- derpowered studies. Keywords: Telomeres / senescence / oxidative stress / ageing. Short telomeres initiate signal transduction towards replicative senescence Telomeres and senescence Telomeres are nucleoprotein structures at the end of linear chromosomes. Their role is to prevent chromosomal ends to be recognised as double strand breaks and to protect chro- mosomes from end-to-end fusion and degradation. Telom- eres in eukaryotes form a higher order loop structure [1]. In mammals, its DNA component consists of regular repeats of hexanucleotides TTAGGG orientated towards the 3'-end, which is protruding over the 5'-end. The so formed single- stranded overhang is associated with single strand binding proteins and involved in a displacement loop [1] protecting the telomere end from degradation. Two major telomere binding proteins, TRF1 and TRF2, play vital roles for loop formation and function of telomeres [2]. In addition, proteins DOI: 10.1002/sita.200400049 Correspondence: Thomas von Zglinicki, University of New- castle, School of Clinical Medical Sciences - Gerontology, Henry Wellcome Laboratory for Biogerontology Research, Newcastle General Hospital, Newcastle upon Tyne, NE4 6BE United Kingdom. Fax: +44 191 256-3445, Phone: +44 191 256-3310, email: t.vonzglinicki@ncl.ac.uk Signal Transduction 2005, 3, 103-114 www.signaltrans.de  2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim interacting with TRF1 such as TIN2 [3], modifying enzymes like tankyrases [4] and various proteins involved in DNA damage repair such as Ku, DNA-PK and the complex Rad50/Mre11/NBS1 associate with telomeres, and most of these components have been shown to be involved in telo- mere length regulation and the maintenance of functional integrity of telomeric structures [5]. Telomeres in somatic human cells shorten with each round of replication due to the so called “end-replication” i.e. the inability of DNA polymerases to fully replicate the end of the lagging strand [6] and an increased sensitivity to oxidative stress-mediated DNA damage [7]. This shortening increases the probability of “uncapping”. This is a state in which loop stability is compromised [8] and a signal towards cell cycle arrest is triggered. A specialised reverse transcriptase- telomerase- is able to counteract telomere shortening by elongating the 3' over- hang of telomeres using its own RNA component as a tem- plate for synthesising telomeric sequences de novo. How- ever, in most human somatic cells telomerase activity is ab- sent or tightly regulated at a very low level. Only germ line cells, stem cells and cancer cells express sufficient telomer- ase to maintain their telomeres in a functional state for un- limited amounts of time. Most primary mammalian cells perform only a limited num- ber of cell divisions in culture. A well established model for the analysis of ageing at the cellular level is the in vitro culti-