Telomeres as a component of organismal aging
Accumulating evidence exists indicating that stem cell function, regeneration, and organ maintenance, all of which largely contribute to the aging process, are connected to telomere biology. Telomeres are nucleoprotein structures located at chromosome ends that consist of short DNA repeats with well-defined sequence composition and telomere-specific protein complexes. Through a multiprotein structure called a telomere cap, telomeres allow cells to distinguish natural chromosome ends from chromosome breaks, and the formation of telomere caps requires a satisfactory length of telomeric DNA (Blackburn 1991; Capkova Frydrychova et al. 2009; Mason et al. 2011). Disruption of telomere cap function that can occur either by loss of telomeric DNA or telomere-binding proteins activates the DNA damage response, and this response, in turn, induces a permanent proliferation arrest known as cell senescence (Greenberg 2005).
Due to the limitations of semiconservative DNA replication and the inability of conventional DNA polymerase to fully replicate the end of linear DNA strands, telomeres are gradually shortened with each round of cell division. This shortening can be circumvented by the extension of telomeric DNA via special telomere maintenance mechanisms, and the most common mechanism of telomere elongation involves telomerase activity. Telomerase is a specialized reverse transcriptase that uses its RNA template to repeatedly synthesize a short telomeric sequence onto the chromosome ends (Blackburn 2005; Mason et al. 2015). Telomerase activity is strictly controlled. In humans, the highest telomerase activity is observed during embryogenesis, and telomerase activity is downregulated in most somatic cells during later development. This suggests an important role for telomerase in fetal tissue differentiation and development (Wright et al. 1996; Ulaner and Giudice 1997). In adult humans, the majority of somatic cell types are telomerase-negative, and telomerase activity is primarily present in germ, stem, and cancer cells. In contrast to germ and cancer cells, the level of telomerase in most stem cells of human adults is low and insufficient to prevent replicative senescence (Hiyama and Hiyama 2007; Choudhary et al. 2012). Telomerase in adult humans is also upregulated in cells with high reproducible activity such as hematopoietic progenitor cells, endometrial and intestinal cells, activated lymphocytes, or keratinocytes (Wright et al. 1996; Razgonova et al. 2020). In contrast to other cell types, embryonic stem cells and cancer stem cells are, due to their high telomerase activity, considered immortal having the capacity of indefinite self-renewal and proliferation (Hiyama and Hiyama 2007).
The absence of or low activity of telomerase in somatic cells results in significant telomere shortening throughout the lifespan, and this limits the replicative capacity of the cells and acts not only as a major determinant of organismal development but also aging and age-related diseases (Ulaner and Giudice 1997; Jiang et al. 2007; Razgonova et al. 2020). Also, as documented in zebra finches, there is a positive correlation between telomere length early in life and the length of realized lifespan, thus indicating that the length of telomeres may act even as a predictor of lifespan predisposition (Heidinger et al. 2011).
Telomerase is upregulated in the long-lived eusocial reproductives
The common presumption that telomerase activity is a marker of aging and advancing organismal development even in insects is supported by observations in hemimetabolous insects such as cockroaches and termites (Korandová et al. 2014; Koubová and Čapková Frydrychová 2021). Hemimetabolous insects exhibit incomplete metamorphosis, where during development the insects lack larval and pupal stages and instead undergo several nymphal stages before their final molt into adults. Recent phylogenetic studies indicate that termites evolved from cockroaches, and along with cockroaches, they form the order Blattodea. But, in contrast to cockroaches, termites are eusocial insects. Both cockroaches and termites exhibit upregulated telomerase activity in young instars, and this activity gradually declines in later development. However, there were two exceptions for the decline: the germline cells in both insects and somatic tissues in the long-lived reproductives, exhibiting high telomerase activity throughout the duration of their lives (Korandová et al. 2014; Koubová and Čapková Frydrychová 2021). A similar finding, which contradicts the common scheme of telomerase decline in adult somatic tissues, was obtained in the long-lived honeybee and ant queens (Korandová and Frydrychová 2016; our unpublished data), representing holometabolous insects (adult growth in holometabolous insects is largely determined and ended by metamorphosis). These observations suggest that the upregulation of telomerase activity may be a key factor in the caste differentiation process in eusocial insects and in the extended longevity of their reproductive individuals.
It is well established that if telomerase is required for the maintenance of telomeres, it must be active during the DNA replication stage (S-phase), and while the highest levels of telomerase activity are found in the S-phase, telomerase activity is virtually absent in the G2/M or G0 phases (Zhu et al. 1996; Holt et al. 1997). Surprisingly, no DNA synthesis was detected in any of the telomerase-positive somatic tissues of eusocial reproductive insects (Koubová and Čapková Frydrychová 2021), and no differences were observed in telomere length between the long-lived and short-lived castes (Jemielity et al. 2005; Korandová and Frydrychová 2016; Koubová et al. 2021a).
To explain the role of telomerase in the caste system of eusocial insects and to identify its possible engagement in the disparity between fertility and life expectancy, research was further conducted examining the bumblebee Bombus terrestris . Bumblebees are members of the group of insects possessing a primitive social organization, and there are significant differences in the life expectancy of their female castes. Workers of the bumblebee species B. terrestris typically live for 2-3 months; however, the queens can live up to one year. Nevertheless, the lifespan comparison is not unbiased, as the bumblebee queens spend the majority of their lives (approximately 6-9 months) in a diapause in which most biological processes take place at only low-cost levels. Thus, the bumblebee queens cannot provide an example of a full-bodied extension of life expectancy, or at least they cannot provide it in the way that exists in advanced eusocial species. In contrast to eusocial reproductive individuals, the only adult somatic tissue of B. terrestris showing upregulated telomerase was the fat body of very young and pre-diapause queens (Koubová et al. 2019). Additionally, telomerase activity was co-localized with the DNA endoreduplication cycles that were followed by a massive increase in fat body mass and nutrient content, which suggests that the upregulation of telomerase activity in the fat body mass in B. terrestris is tightly linked to the ability of queens to survive upcoming diapause (Koubová et al. 2019). A similar observation was obtained in honeybee workers, where telomerase activity, DNA synthesis, and nutrient content were reinforced in the fat body cells of winter-generation workers (Koubová et al. 2021b).
Collectively, these findings suggest that the caste-related differences in telomerase activity in eusocial insects such as honeybees or termites are not linked to telomere maintenance mechanisms, and instead, they are associated with some non-canonical telomerase roles without the typical telomerase catalytic activity that directly serves to elongate telomeres.