Here is the draft paper, edited for clarity. I hope it is of interest and sparks some debate.
Paving the way for the 'speeding car': the evolutionary advantage of long telomeres in the context of high mTOR
Draft manuscript (not peer reviewed)
M B Williams
Aug 16 2022 Updated Aug 17
Abstract
Small animals have long telomeres but short lifespans, which is in direct contradiction to what is observed in cell culture, where long telomeres delay senescence. This paper resolves this paradox by explaining longer telomeres (and active telomerase) as a selective adaptation to greater growth signalling, as described by Hyperfunction theory. From an evolutionary perspective longer telomeres are selected to delay senescence (geroconversion), permitting faster development and potentially extended reproduction, rather than longer life. This may also be permissive for cancer, which is why longer telomeres are precluded in larger animals. But from an anti aging perspective, activating telomerase is likely to synergise with post-development mTOR inhibition.
The understanding presented in this paper extends the influence of Blagosklonny's proximal driver of aging, mTOR signalling, to other putative aging mechanisms, primarily the Hayflick limit, previously posited to limit life only after hyperfunction.
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Telomeres are a paradox in aging. Long telomeres and active telomerase are associated with faster aging species like mice, whereas humans have shorter telomeres and inactive telomerase, but live much longer [1]. This is in contrast to the benefits of longer telomeres in cell culture, where they prolong cellular division and delay the gradual conversion to senescence (geroconversion), as demonstrated in a recent rework of Hayflick's experiments [2], where it was proven that senescence sets in gradually and long before cells reach the Hayflick limit. This presents us with the possibility that longer telomeres could be an advantage during the growth phase of a young animal, and also, indirectly, delay aging.
In the light of hyperfunction theory [3], longer telomeres can be understood as an evolutionary adaptation to offset the geroconversion caused by faster growth.
Longer telomeres permit cells to divide rather than become blocked in the cell cycle and then become senescent. Active telomerase reduces or eliminates telomere shortening, and the consequent slowing of division and increase in geroconversion that occurs with continual growth stimulation. The greater the stimulation, the faster the division, and the greater the telomerase production required to offset it. This is a direct consequence of telomere length itself, where shorter telomeres mean slower cellular division, and greater cell size, as can be seen in cell culture experiments.
Therefore longer telomeres are an adaptation to permit faster growth. But despite their much longer telomeres and active telomerase enzyme, mice still lose telomere repeats faster than humans [4].
This parallels the situation with murine reactive oxygen species (ROS) defence, which regardless of its efficacy relative to humans, cannot compensate for their higher ROS production[5]. Telomere length (and possibly also ROS defence) are likely increased by evolution to protect against the senescence induced by faster growth. Without such adaptation, aging might occur before reproductive maturity in fast growing animals.
On the flip side, it has been postulated that long telomeres are a risk factor for cancer and so are selected against in larger species with a protracted growth phase, despite the deleterious effects of short telomeres in later life [6], which exist in a post reproductive evolutionary 'shadow'. This is as expected, given longer telomeres delay senescence, which is protective against cancer.
Despite the logic of this theory, and indirect support from Mendelian studies in humans [7], it is contradicted by some experiments - where mice created from embryonic stem cells with hyperlong telomeres were healthier and lived longer than controls, with a delay in diseases of aging, including cancer [8]. Longer lifespans were also achieved by providing middle and old aged mice with telomerase gene therapy, without increases in cancer [9].
It is unknown why increases in cancer were not seen in these studies, but mice born with hyperlong telomeres were metabolically healthier than controls[5], suggesting telomere length regulation of growth signalling (more on this below). Further stratification of the data in Mendelian studies[7] may discover that those humans that have longer telomeres and went on to develop cancer, also matured faster because of elevated growth signalling and that this is the real driver of cancer occurrence, with longer telomeres merely being permissive.
Returning to aging, if antioxidant defence is akin to 'catching bullets', then longer telomeres can be considered as 'paving the way' for Blagosklonny's 'Speeding Car without brakes' [10]. Paving more road can delay the crash (aging), which is what is required for reproductive success in fast growing animals.
In further support of this idea, it has been found that telomerase inhibits mTOR1 without inhibiting mTOR2 and proliferation [11]. Inhibition of mTOR1 upregulates autophagy (an adaptation to faster cellular protein production) and can be understood as negative feedback against excessive growth stimulation, allowing continued proliferation with reduced geroconversion (senescence). This is utilised by cancer, but equally it might also be used by other fast proliferating cells like human T lymphocytes (which do have active telomerase). So perhaps mTOR does have brakes, after all. This may explain why mTOR stimulation is so harmful later in life, as telomere length is reduced.
What other evidence do we have for the theory that longer telomeres are an adaptation to faster growth?
1.Similar to the body-size longevity relationship [12], where larger species live longer, but smaller individuals within those species live the longest, telomeres are shorter in large, slow growing creatures but longer in smaller, faster growing creatures. This matches the hypothesis that longer telomeres are required to offset geroconversion in species that grow faster. But within a species, longer telomeres (like smaller size), are associated with longer life. This is because telomere length is largely set in infancy [13] and those with most telomere repeats remaining after development is complete will live the longest.
This is exemplified by African killifish [14]. Killifish evolve their life cycle to match the pools in which they live. In areas with a shorter wet season, killifish must grow, reproduce and lay eggs before the pools dry out, and in this case the species has long telomeres, as is predicted by the requirement for faster growth, relative to the killifish species that live in longer lasting pools. But within a given killifish species, those individuals with longer telomeres will survive the longest, showing longer telomeres are beneficial later in life, even when the body is fully grown.
2. People of African ancestry have longer telomeres than Europeans [15]. People of African ancestry also grow up faster than Europeans, on average, as seen in studies of menarche onset [16]. Therefore even within the human species there is evidence for longer telomeres within faster maturing ethnic groups.
Furthermore, it is also expected that a smaller, slower developing individual, if they should happen to also inherit longer telomeres, would be very long lived indeed, benefiting from both lower growth stimulus, and greater compensating telomere length.
Conclusions and Consequences for Aging Research
Blagosklonny proposed hyperfunction driven aging is proximal [17], with other potential aging mechanisms like the Hayflick limit and ROS limiting life only if hyperfunction is prevented first.
In this short paper I propose that the Hayflick limit (and ROS) are not independent from growth signalling (mTOR), but argue that their regulation has evolved in direct response to the level of mTOR.
Evidence is presented here to show that longer telomeres are a selective adaptation that compensates for faster growth driven by mTOR. This compensation is sufficient to maintain health to reproductive age, and likely permits faster growth than would otherwise be possible.
Future developments in anti aging research should focus on telomerase activation as an adjuvant to mTOR inhibition. Such a combination is likely to be synergistic because whereas telomerase permits more cell division, mTOR inhibition (specifically inhibiting mTOR2) reduces proliferation. This preservation of telomere length will then lead to extended post development lifespan, so long as growth signalling remains repressed.
mTOR inhibition is regarded as deleterious before development is complete. Because of cancer concerns, telomerase activation might likewise also be started later in life, when telomeres have shortened relative to their length during development.
References:
[1] Rodrigo T. Calado et al.,Science Direct, 11 June 2013, Telomere dynamics in mice and humans,
https://doi.org/10.1...tol.2013.03.030
[2] Michelle Chan et al., Elifesciences, Feb 4 2022, Novel insights from a multiomics
dissection of the Hayflick limit
https://doi.org/10.7554/eLife.70283
[3] Mikhail V. Blagosklonny, Cell Cycle, Sept 15 2006, Aging and immortality: quasi-programmed senescence and its pharmacologic inhibition
https://doi.org/10.4161/cc.5.18.3288
[4] Kurt Whittemore et al., PNAS, July 8 2019, Telomere shortening rate predicts species life span
https://doi.org/10.1...pnas.1902452116
[5] Gustavo Barja, Science Direct, August 20 2014, The Mitochondrial Free Radical Theory of Aging
https://doi.org/10.1...94625-6.00001-5
[6] Peter M Lansdorp, Blood, Feb 10 2022, Telomeres, Aging and Cancer: The Big Picture,
https://doi.org/10.1...lood.2021014299
[7] The Telomeres Mendelian Randomization Collaboration, JAMA Oncol, May 2017, Association Between Telomere Length and Risk of Cancer and Non-Neoplastic Diseases
https://doi.org/10.1...oncol.2016.5945
[8] Miguel A. Muñoz-Lorente et al., Nature Communications, Oct 17 2019, Mice with hyper-long telomeres show less metabolic aging and longer lifespans
https://doi.org/10.1...467-019-12664-x
[9] Bruno Bernardes de Jesus et al., EMBO Mol Med, May 15 2012, Telomerase gene therapy in adult and old mice delays aging and increases longevity without increasing cancer
https://doi.org/10.1002/emmm.201200245
[10] Mikhail V. Blagosklonny, Cell Cycle, Dec 15 2009, TOR-driven aging: speeding car without brakes
https://doi.org/10.4161/cc.8.24.10310
[11] Muhammad Ali et al., Science Direct, Sept 23 2016, Telomerase reverse transcriptase induces basal and amino acid starvation-induced autophagy through mTORC1
http://dx.doi.org/10...brc.2016.08.094
[12] Mikhail V. Blagosklonny, Aging, April 20 2013, Big mice die young but large animals live longer
https://doi.org/10.18632/aging.100551
[13] Jacob B Hjelmborg et al., Medical genetics, Oct 14 2014, The heritability of leucocyte telomere length dynamics
http://dx.doi.org/10...net-2014-102736
[14] Martin Reichard et al., Wiley Online Library, Nov 26 2021, Lifespan and telomere length variation across populations of wild-derived African killifish
https://doi.org/10.1111/mec.16287
[15] Steven C Hunt, Wiley Online Library, July 10 2008, Leukocyte telomeres are longer in African Americans than in whites
https://doi.org/10.1...26.2008.00397.x
[16] Maria E. Bleil et al., Springer Link, Aug 7 2017, Race disparities in pubertal timing: Implications for cardiovascular disease risk among African American women
https://doi.org/10.1...1113-017-9441-5
[17] Mikhail V. Blagosklonny, Aging, Dec 30 2012, Answering the ultimate question "what is the proximal cause of aging?"
https://doi.org/10.18632/aging.100525
Edited by QuestforLife, 17 August 2022 - 06:15 PM.