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).