By Brendan Hussey
In my previous post, The Nuclear DNA Damage/Mutation Theory of Aging, I presented evidence that inequities in cell maintenance between germ and somatic cells are a major causal factor for the deterioration of somatic cells resulting in ageing phenotypes. To briefly reiterate: “The disposable soma is an evolutionary theory that seeks to explain the disparity in cellular maintenance between germ and somatic cells by means of limited resource allocation between the two energetically intensive processes of reproduction/growth and repair/maintenance.” Essentially, the organism is programmed (in an evolutionary sense) to live only as long is necessary for it to have a good chance of reproducing, and energy is allocated toward the ratio of growth/reproduction over repair/maintenance in inverse proportion to how long it takes to reach this stage. That is, the longer an organism must live in order to have a good chance of successfully reproducing (as well as intrinsic features that help it live longer like wings, shells and big brains), the more the organism invests cell maintenance/repair. On the other hand organisms that quickly reach a point where they can have a good chance of successfully reproducing and/or are heavily predated, or lack intrinsic protective features invest most their energy growth/reproduction.
Germ cells however, due to their obvious importance in reproduction, are essentially immortalized through highly efficacious repair/maintenance mechanisms despite their high rate of cell division. While high fidelity repair and maintenance occurs in other tissues, none compares to the carriers of DNA that has been passed down for billions of years. This feature of germ cells demonstrates the existence of cellular mechanisms capable of maintaining a high level of DNA integrity and cell functionality for not only matured cells but perhaps more importantly the stem cell populations that sustain them. All cells harbor the same DNA (with the exclusion of some mature white and red blood cells) and therefore, epigenetic complications aside, it may be possible to turn on specific germline genetic programs in somatic cells in order to extend their lifespan. Indeed, preliminary work conducted with the ever faithful model of science, yeast, has demonstrated such an effect and is supported by earlier observations made in C. elegans mutants.
Recently (See 1, Gametogenesis Eliminates Age-Induced Cellular Damage and Resets Life Span in Yeast) it was demonstrated in two species of yeast that inducing sporulation (meiosis: gamete formation) in aged yeast resets their replicative life span. That is, activating the germ cell program effectively reset their age. While yeast are relatively far removed from humans phylogenetically, useful parallels can be drawn from them, testament in their high regard as a model species for eukaryotic molecular and cell biology. For example, yeast mother cells and somatic cells in the human body both age through mitotic replication in similar ways. Yeast, as do somatic cells, accumulate forms of chromosome/DNA damage and protein aggregates with replicative aging. Likewise, both will eventually die when these aberration accumulate to a critical amount, defining their replicative lifespan. So it is important to note that these aberrations in DNA/chromosome stability as well as protein aggregation were returned to youthful levels after sporulation.
One problem so far for human application is that meiosis is required for this resetting to occur, an event neither possible nor desirable in somatic cells. Interestingly though, it was discovered that meiosis itself, including the first nuclear division was dispensable for age resetting1. Furthermore, just transient expression of a single protein, NDT80, reset replicative age of yeast cells to young cell levels1. Interestingly, protein aggregation was not affected when measured, although measures of DNA/chromosomal stability were returned to youthful states. These results suggest that just transient events in aged cells can reset their lifespans without major changes in cellular morphology and dynamics. It also suggests particular importance of DNA/chromosomal stability, likely involving DNA repair, in age resetting.
Issues remain, however, before results of this type can inspire expectations of similar outcomes in higher eukaryotes. First, NDT80 has no known homologue and is thought to be involved in sporulation regulation, a process unknown to the likes of higher eukaryotes. See NDT80, a meiosis-specific gene required for exit from pachytene in Saccharomyces cerevisiae. Although this certainly does not disqualify the possibility of age resetting in higher eukaryotic somatic cells through gametic programs, it does not aid in the search for protein candidates. Second, the measures of DNA/chromosomal integrity in the aforementioned study were focused on the formation of large scale nucleolar structures and other large scale DNA elements as opposed to discrete measures of DNA damage, for example telomere length. This makes the association with the DNA damage/mutation theory of ageing less robust. More thorough investigations into a wider range of ageing symptoms, as well as DNA damage, would be valuable. Third, while replicative aging was assessed, no measurement of chronological aging of non-dividing cells was made. Considering they can inversely affect each other in yeast (see Chronological aging-independent replicative life span regulation by Msn2/Msn4 and Sod2 in Saccharomyces cerevisiae) and the fact that chronological lifespan is important in many human tissues composed of terminally differentiated cells, this measure is important to inform future studies in higher eukaryotes. Finally, although transient NDT80 expression returned the replicative age to youthful levels, long term over expression of NDT80 was not conducted and so the full potential of mitotic cell rejuvenation was not assessed. It would be very interesting to see how long the replicative lifespan of yeast could be extended in this way.
Nevertheless, this proof of concept study demonstrates that the replicative lifespan of a eukaryotic organism can be reset by inducing germline genes, involving restoration of DNA/genome integrity. While no study the author is aware of has attempted to identify and induce germline programs in higher eukaryotes a study in the flat worm C. elegans found that in some long lived mutants deficient in insulin signaling, a specific set of genes normally restricted to the germline were active in somatic cells. See A soma-to-germline transformation in long-lived Caenorhabditis elegans mutants. These programs were causative in the long lived phenotype, and maintenance of DNA/chromosome integrity was correlated with longevity. Measures of DNA damage were more discrete than in the yeast experiments with authors measuring DNA mutations resulting in for example DNA frame shifting. Interestingly these germline genes were expressed only in a subset of tissues, suggesting that they may not even need to be expressed in all tissues to extend lifespan of higher eukaryotes, owing perhaps to endocrine like signaling. In addition this finding brings to question whether expressing such germline genes in all tissues could further extend lifespan. Another consideration is whether expressing these germline genes in other long lived mutants which do not normally express them as well as dietary restricted worms would serve to further increase longevity. Since these genes were selected for based on their known exclusive expression in germ cells and not by genetic screen or a known role in cell maintenance pathways, it would be informative to know whether the results of a thorough genetic screen of germ cells would elucidate other candidates for longevity with more direct relation to anti-aging mechanisms such as DNA repair, proteolysis, etc.
Many questions arise from these recent preliminary studies, but they do provide proof of concept for the tantalizing prospect of tapping into the highly efficacious potential of germline cell maintenance to increase the lifespan of somatic cells as well as supporting the importance of DNA maintenance in longevity. Follow up experiments in C. elegans utilizing targeted overexpression of germline genes identified through unbiased genetic screens combined with discrete measures of cell and DNA maintenance as they correlate to lifespan increases could provide the evidence needed to move to systematic testing of more complex organisms such as drosophila, mice and non-human primates. As aforementioned regarding yeast, it may be the case that only a few factors actually need to be active to confer longevity benefits. Due to the highly conserved nature of growth and repair pathways associated with longevity, it is possible that protein homologues could be targeted via small molecules in humans and/or homologous genes could be induced via epigenetic modulators such as non-coding RNA’s. Given the success of reprogramming tissue specific differentiated cells into all manner of other cell types using just a handful of factors, inducing germline cell maintenance systems in somatic cells is not out of the question. Depending on the nature of aging, it may primarily be necessary to rejuvenate stem cells (see the blog entry Longevity of stem cells and the roles of stem cells in aging), significantly simplifying the problem. Furthermore, it may be possible that inducing germline maintenance genes in iPSC’s would ameliorate their chromosomal instability (outlined in the blog entry Update on induced pluripotent stem cells), thus amending them for use in regenerative therapies.
1. Elçin Ünal, Benyam Kinde, and Angelika Amon. Gametogenesis Eliminates Age-Induced Cellular Damage and Resets Life Span in Yeast. Science 24 June 2011: 332 (6037), 1554-1557.
2. Xu, L., Ajimura, M., Padmore, R., Klein, C., Kleckner, N. NDT80, a meiosis-specific gene required for exit from pachytene in Saccharomyces cerevisiae. Mol. Cell. Biol. (1995) Dec;15(12):6572-81
3. Fabrizio P, Pletcher SD, Minois N, Vaupel JW, Longo VD. Chronological aging-independent replicative life span regulation by Msn2/Msn4 and Sod2 in Saccharomyces cerevisiae. FEBS Lett. 2004 Jan 16;557(1-3):136-42.
4. Curran SP, Wu X, Riedel CG, Ruvkun G. A soma-to-germline transformation in long-lived Caenorhabditis elegans mutants. Nature. 2009 Jun 25;459(7250):1079-84.
