What is the real purpose of telomerase activity in somatic cells ?
To sum up, telomere shortening and lack of telomerase in most human somatic cells result in an irreversible cell cycle arrest termed replicative senescence, which is one of determinants of organismal development, an anticancer mechanism, and a marker of organismal aging. The presence of telomerase activity and telomere restoration in certain somatic cells of human adults maintains the proliferative capacity of these cells and contributes to their regenerative potential, and telomerase activity and telomere length in these cells both are commonly considered lifespan predictors (Blasco 2007; Tan et al. 2012).
However, from a broad perspective, the connection between the length of telomeres and the predicted lifespan or the reason of telomerase activity in somatic tissues of numerous animal species are not clear, which has been documented by series studies reviewed in (Gomes et al. 2010; Smith et al. 2021). For example, telomerase activity and telomere length have been studied in sea urchins in the context of the long-livedStrongylocentrotus franciscanus (live over 100 years) and the short-lived Lytechinus variegatus (with an estimated lifespan of only 3-4 years). While both species exhibit telomerase activity in their somatic tissues, L. variegatus possessed longer telomeres (average length of approximately 21 kb) compared to those of S. franciscanus (average telomere length of approximately 5.5 kb). Moreover, no differences in telomere length were observed between the young and old individuals. This suggests that sea urchins do not utilize telomerase repression as a mechanism to suppress neoplastic transformation (as a decline of telomerase is hypothesized to prevent tumor development), and the continuous telomerase activity in somatic tissues of the sea urchins is explained by the undetermined growth of these organisms throughout their lifespan (Francis et al. 2006). Similarly, the presence of telomerase activity throughout the lifespan or a lack of a positive correlation between the predicted lifespan, telomerase activity, and telomere length have been reported in numerous other invertebrate and vertebrate species (Gomes et al. 2010). Studies have provided no detailed information relating telomerase activity to the proliferation status of the tissues, and the presence of telomerase in the tissues was hypothetically explained by the existence of telomerase function that differs from that involved in the elongation of telomeres (Lau et al. 2008; Gomes et al. 2010).
Also, the existence of different patterns of telomerase regulation has been documented in mammals. While adult humans express telomerase almost exclusively in the germline and a few specialized cell types and possess short telomeres (<20 kb), most rodents express telomerase in both germline and a wide array of somatic tissues and possess long telomeres (25-150 kb) (Gomes et al. 2010). It indicates that rodents do not use telomere length to control their replicative aging. A similar lack of telomere-based replicative aging is predicted in lagomorphs (rabbits, pikas, and hares) as prolonged tissue cultures derived from different lagomorph species displayed no growth arrest or a decrease in doubling time but show either very long telomere arrays or detectable telomere activity (Forsyth et al. 2005). A comparative study examining telomerase activity and telomere length in mammals revealed that telomere length inversely correlates with lifespan (longer telomeres are present in short-lived mammal species), and telomerase activity in somatic cells inversely correlates with body mass (small mammal species such as small rodents exhibit telomerase activity in somatic cells) (Seluanov et al. 2007; Gomes et al. 2010). It has been suggested that larger and longer-lived mammals protectively repress telomerase activity in somatic cells, as their large body masses and longer lifespans contribute to an increased risk of cancer development (Seluanov et al. 2007).