Cellular Aging: Telomeres

As discussed above, cells have a finite division potential, often called the Hayflick limit. Interestingly, the number of divisions a cell is capable of undergoing in culture is inversely proportional to the age of the donor. That is, cells derived from younger individuals will undergo mote divisions than those from older individuals. Thus, the limitation on division potential is hypothesized to play a role in aspects of human aging. Cells that have reached their division limit undergo a process called replicative senescence, which is accompanied by morphological changes and changes in gene expression patterns. Interestingly, the Hayflick limit is ordained by the total number of divisions experienced by a cell and not by elapsed time. Senescent cells are alive (metabolically active), but can no longer be induced to divide. For many human cell types, the onset of replicative senescence has been linked to the length of the telomere.

The first clue that telomeres might play a role in capping the total number of divisions any given cell may undergo came from observations in the late 1980s made by Howard Cooke and coworkers. These investigators noted that telomeres in the germ line were longer than telomeres present in somatic tissue (i.e., blood) from the same individual. Over the next several years, a number of laboratories demonstrated that telomeres are shorter in older individuals than in younger individuals and that telomeres become shorter with increased numbers of cell divisions in culture. In addition, it was noted that chromosome instability of a type that would be predicted to accompany loss of telomere function, such as fusion of chromosome ends, is increased in older individuals. This was also observed in cultured cells as they approached senescence. There are two essential points to these observations. First, telomeres only shorten if cells divide. Metabolically active cells that are quiescent (those that do not divide) do not lose telomeric DNA. Secondly, telomeric DNA is only lost in somatic (body) cells, which do not, as a rule, contain telomerase activity. Based on these observations, it was proposed that the telomere might act as the elusive intracellular clock that triggered senescence. This became known as the telomere hypothesis of cellular aging. This hypothesis suggests that attrition of telomeric DNA eventually compromises telomere function. This would result in a signal being generated that causes the cell to undergo replicative senescence Figure 2 Conventional DNA synthesis requires RNA primers to initiate replication. The primers are then removed and the gaps filled in. Upon removal of the most terminal primer, a region of unreplicated DNA remains. This results in loss of DNA from the end of the chromosome each time the cell divides. SOURCE: Reddel, R. (see Bibliography). and cease dividing. Although these observations were suggestive of a causal relationship between functioning telomeres and the ability to divide, the link between telomere loss and replicative senescence remained correlative.

Following the identification and cloning of the RNA and catalytic components of telomerase, it became possible to force expression of this enzyme in primary human cell cultures. Primary human cell cultures have a finite division capacity and do not contain telomerase activity. In the late 1990s it was finally directly demonstrated that, for specific cell types, expression of telomerase and the concomitant extension of telomeric DNA is sufficient to impart cellular immortality—the potential for an infinite number of divisions. The inverse is true as well. Thus, inhibition of telomerase activity in immortal cells, such as tumor-derived cell lines, results in telomere loss and culture senescence. These experiments directly demonstrated a causal link between maintenance of telomeric DNA and a cells ability to divide. In a series of experiments, de Lange and coworkers demonstrated that disruption of telomeric structure by removing the telomeric protein TRF2 resulted in loss of protective function and a senescent-like growth arrest. These experiments identified the first type of signal that might emanate from telomeres to elicit cellular responses. According to the t-loop model described above, loss of TRF2 would open the end of the chromosome by disrupting the t-loop. This, in turn, alarms the cell because the telomere now resembles a broken DNA molecule and results in activation of the ATM-dependent and p53-dependent DNA damage response pathway. Activation of p53 has been linked to both senescence and apoptosis. Thus, cells that are unable to sequester chromosome ends through maintenance of the t-loop structure, either because the telomere is too short or due to absence of essential proteins, are prevented from dividing further by activating the senescence or apoptotic pathways. The question of whether complete loss of function is required to evoke senescence, or whether the cell has some means of identifying a short but still functional telomere, has yet to be answered.