Cells tend to produce unwanted protein aggregates and other molecular refuse slightly faster than they can get rid of it, resulting in a time-dependent accumulation of potentially toxic cellular garbage. This, in turn, can cause an age-dependent loss of cellular viability, which is (in certain contexts) a fair operational definition of aging.

How can cells deal with their garbage? Protein aggregates are both sticky and insoluble, making it hard for cellular machinery to deal with them at an enzymatic level. If the gunk can’t be eliminated, however, it might still be possible to move it around in a useful way. Specifically, at mitosis, the cell could make sure that all the potentially toxic aggregates stay in one of the progeny. To illustrate the argument I’ll turn to the words of the estimable Alex Palazzo:

One approach is to distribute everything equally amongst your two offspring. …

A second approach is to give all the crap to one of the two new cells and keep the other one pristine. Lets call these two cells the crap cell and the pristine cell. What’s the result of this second strategy? Using our crap metric from above, the first cell accumulates 10 units of garbage over its lifetime and then gives it all to one offspring, the crap cell, and none to the other offspring, the pristine cell. Those cells then grow and by the time they divide each second generation cells have made 10 units of additional crap each. The crap cell has 20 units the pristine cell 10. The two cells divide and dump all their garbage on one of their offsprings. One cell starts with 20 units of crap, one cell with 10 units and two cells are again crap free. The end result of this strategy? Part of your descendents will become more and more decrepit as they fill up with crap, while others remain pristine.

The crap cell (I love this nomenclature) will become inviable sooner under this strategy, but the alternative would be a symmetric division strategy in which all descendants accumulate garbage, ultimately causing the extinction of the entire lineage. The idea here is that assuming certain values for adjustable parameters re: the rate of garbage accumulation and the effect of garbage level on reproductive fitness, this can be an advantageous strategy to ensure reproductive success. Both single-celled yeast and mammalian stem cells employ this asymmetric strategy in order to preserve the viability of an indefinitely dividing lineage.

In yeast, the crap cell is called the “mother”; the pristine cell is called the “daughter” — mom accumulates garbage of various kinds, both protein aggregates and rDNA circles. When the mother is ready to divide, a bud forms at a specific site on her cell wall, defined by a set of macromolecular complexes that determine cellular polarity. Liu et al. have demonstrated that the daughter cell is using some of the same polarity-determining machinery (the “polarisome”) to actively transport protein aggregates back into the mother:

The Polarisome Is Required for Segregation and Retrograde Transport of Protein Aggregates

The paradigm sirtuin, Sir2p, of budding yeast is required for establishing cellular age asymmetry, which includes the retention of damaged and aggregated proteins in mother cells. By establishing the global genetic interaction network of SIR2 we identified the polarisome, the formin Bni1p, and myosin motor protein Myo2p as essential components of the machinery segregating protein aggregates during mitotic cytokinesis. Moreover, we found that daughter cells can clear themselves of damage by a polarisome- and tropomyosin-dependent polarized flow of aggregates into the mother cell compartment. The role of Sir2p in cytoskeletal functions and polarity is linked to the CCT chaperonin in sir2Δ cells being compromised in folding actin. We discuss the findings in view of recent models hypothesizing that polarity may have evolved to avoid clonal senescence by establishing an aging (soma-like) and rejuvenated (germ-like) lineage.

Note the role for Sir2p, the founding member of the sirtuin family of longevity assurance genes: Sir2p is required, via another protein’s activity, for the normal folding of actin, the cytoskeletal protein from which the daughter-mother transport cable is built. It’s an indirect interaction, and more complex than I’m making it out to be here. Nonetheless, it is satisfying for those of us looking for unifying theories in aging that one of the most widely studied proteins in lifespan regulation is involved in the deep connection between polarity and aging.

I’ll close with a few questions:

  • Why can’t the mother cell export the aggregates? One of our initial premises was that aggregates are biochemically hard to handle, which is why they accumulate rather than being degraded. But now we know that cells can bundle aggregates onto actin cables and move them around — why not sort the aggregates into vesicles or membrane blebs and dispose of them? Granted, in order to export an aggregate out of the cell, it would have to cross a membrane, but this would be no more difficult topologically than mitophagy. The obvious (and trivial) answer to this question is “because it didn’t evolve that way,” but I’m curious to know whether there’s some compelling reason why it couldn’t have evolved that way.

  • How do symmetrically dividing cells overcome this problem? In order to exploit asymmetric division, one must first establish polarity. The argument above about the rate of garbage accumulation would seem to apply equally well to non-polarized cells like bacteria – why, then, do clonal lineages of symmetrically dividing cells not invariably go extinct? Maybe the cells that we think are symmetric are secretly asymmetric, with a crap/pristine segregation that has yet to be uncovered. Or maybe the symmetric cells know something about garbage disposal that we don’t. In either case, there’s something important to learn that might help us keep mammalian cells youthful.

ResearchBlogging.orgLiu, B., Larsson, L., Caballero, A., Hao, X., Öling, D., Grantham, J., & Nyström, T. (2010). The Polarisome Is Required for Segregation and Retrograde Transport of Protein Aggregates Cell, 140 (2), 257-267 DOI: 10.1016/j.cell.2009.12.031

Evolution generally rewards early reproduction, but there is some advantage to maintaining reproductive capacity into later life — e.g., if a young organism encounters conditions that would be adverse for offspring viability, it makes sense to delay reproduction or at least to have the capacity to try again when that organism is older. How best to maintain the viability of gametes over the course of the lifespan? The answer appears to be stringent quality control: Andux and Ellis report that in C. elegans, apoptosis in the germ line helps guarantee that high-viability oocytes keep getting the resources they need:

Apoptosis Maintains Oocyte Quality in Aging Caenorhabditis elegans Females

In women, oocytes arrest development at the end of prophase of meiosis I and remain quiescent for years. Over time, the quality and quantity of these oocytes decreases, resulting in fewer pregnancies and an increased occurrence of birth defects. We used the nematode Caenorhabditis elegans to study how oocyte quality is regulated during aging. To assay quality, we determine the fraction of oocytes that produce viable eggs after fertilization. Our results show that oocyte quality declines in aging nematodes, as in humans. This decline affects oocytes arrested in late prophase, waiting for a signal to mature, and also oocytes that develop later in life. Furthermore, mutations that block all cell deaths result in a severe, early decline in oocyte quality, and this effect increases with age. However, mutations that block only somatic cell deaths or DNA-damage–induced deaths do not lower oocyte quality. Two lines of evidence imply that most developmentally programmed germ cell deaths promote the proper allocation of resources among oocytes, rather than eliminate oocytes with damaged chromosomes. First, oocyte quality is lowered by mutations that do not prevent germ cell deaths but do block the engulfment and recycling of cell corpses. Second, the decrease in quality caused by apoptosis mutants is mirrored by a decrease in the size of many mature oocytes. We conclude that competition for resources is a serious problem in aging germ lines, and that apoptosis helps alleviate this problem.

Natural selection can modify the rate of aging. Often, the evolution of profoundly delayed (or negligible) senescence can be explained by thinking in reproductive terms: Organisms want to maximize production of descendants who are themselves well-situated to maximize their own reproductive success. Hence whales live long enough to help out their grandchildren, and the long lifespans of certain sessile species probably evolved because young organisms have to wait for older individuals to die before they can settle down to grow large and multiply (a similar phenomenon is likely operating on eusocial colonies).

In those examples, long life facilitates reproductive success. But what about the converse? Do species that evolve mechanisms to delay reproduction (e.g. under suboptimal conditions) achieve this goal by delaying aging, or do they let their biological clocks run on unhindered during reproductive arrest? At least in Drosophila, it appears that reproductive delay is also accompanied by a delay in the aging process. From Tatar et al.:

Negligible Senescence during Reproductive Dormancy in Drosophila melanogaster

Some endemic Drosophila overwinter in a state of adult reproductive diapause where egg maturation is arrested in previtellogenic stages. When maintained at cool temperatures, adult Drosophila melanogaster enter reproductive dormancy, that is, diapause or diapause-like quiescence. The ability to survive for extended periods is a typical feature of diapause syndromes. In adults this somatic persistence may involve reduced or slowed senescence. Here we assess whether reproductively dormant D. melanogaster age at slow rates. Adults were exposed to dormancy-inducing conditions for 3, 6, or 9 wk. After this period, demographic parameters were measured under normal conditions and compared to the demography of newly eclosed cohorts. The age-specific mortality rates of postdormancy adults were essentially identical to the mortality rates of newly eclosed, young flies. Postdormancy reproduction, in contrast, declined with the duration of the treatment; somatic survival during dormancy may tradeoff with later reproduction. Adults in reproductive dormancy were highly resistant to heat and to oxidative stress. Suppressed synthesis of juvenile hormone is known to regulate reproductive diapause of many insects. Treatment of dormant D. melanogaster with a juvenile hormone analog restored vitellogenesis, suppressed stress resistance, and increased demographic senescence. We conclude that D. melanogaster age at slow rates as part of their reproductive dormancy syndrome; the data do not agree with an alternative hypothesis based on heat-dependent “rate of living.” We suggest that low temperature reduces neuroendocrine function, which in turn slows senescence as a function of altered stress response, nutrient reallocation, and metabolism.

Postdormancy flies have the same mortality curve as young flies that never underwent the reproductive arrest — thus, they’ve delayed aging (in the sense of “the increased risk of dying per unit time as a function of chronological age”).

But not every aspect of the flies’ physiology is equally well preserved: Even though they’re surviving at the same rate, postdormancy flies are less fertile than young flies that have not experienced diapause — perhaps the endocrine systems that help preserve the somatic tissues are less efficient at maintaining the germ line. (The aging is of the fly germ line has been well studied in its own right, and is understood at sufficient molecular detail to allow very directed questions about how diapause affects the gonadal stem cell niche.)

That might seem to contradict the principle outlined above — that the purpose of delayed aging would be to increase reproductive success. If an organism’s fertility declines, who cares — in an evolutionary sense — how long it ultimately lives? The answer, I think, is to make the right comparison: The appropriate “control” for a postdormancy fly isn’t a young, well-fed compatriot that never encountered enviornmental conditions adverse enough to initiate diapause; rather, it’s the fly that died because it was dumping resources into reproduction when it should have been bolstering its stress responses and lining its body with fat in order to ride out the bad times. That fly’s fertility, obviously, is zero.

Last year we heard about the counterintuitive observation that DNA repair mutants (which exhibit premature aging and shortened lifespans) have significant phenotypic and transcriptional overlap with genetically dwarfed or calorie restricted (CR) animals (which exhibit delayed aging and extended longevity).

The interpretation of those surprising results: Both DNA repair deficiency and CR cause organisms to divert resources away from reproduction and growth, and toward maintenance and repair. (It just happens to be fruitless for the DNA repair mutants, since they’re dumping energy into a compromised pathway.) Chalk one up for the disposable soma theory of aging.

Now, a follow-up paper from (basically) the same research group compares age-related transcriptional changes between mice aging normally, prematurely or slowly. The study, which includes data from multiple organ systems across the entire lifespan, confirms and expands on the observation that progeria and extended lifespan share common phenotypic features. From Schumacher et al.:

Delayed and Accelerated Aging Share Common Longevity Assurance Mechanisms

Mutant dwarf and calorie-restricted mice benefit from healthy aging and unusually long lifespan. In contrast, mouse models for DNA repair-deficient progeroid syndromes age and die prematurely. To identify mechanisms that regulate mammalian longevity, we quantified the parallels between the genome-wide liver expression profiles of mice with those two extremes of lifespan. Contrary to expectation, we find significant, genome-wide expression associations between the progeroid and long-lived mice. Subsequent analysis of significantly over-represented biological processes revealed suppression of the endocrine and energy pathways with increased stress responses in both delayed and premature aging. To test the relevance of these processes in natural aging, we compared the transcriptomes of liver, lung, kidney, and spleen over the entire murine adult lifespan and subsequently confirmed these findings on an independent aging cohort. The majority of genes showed similar expression changes in all four organs, indicating a systemic transcriptional response with aging. This systemic response included the same biological processes that are triggered in progeroid and long-lived mice. However, on a genome-wide scale, transcriptomes of naturally aged mice showed a strong association to progeroid but not to long-lived mice. Thus, endocrine and metabolic changes are indicative of “survival” responses to genotoxic stress or starvation, whereas genome-wide associations in gene expression with natural aging are indicative of biological age, which may thus delineate pro- and anti-aging effects of treatments aimed at health-span extension.

Those last two sentences are very important, in that they address a critical issue in studies of transcription (indeed of any phenotype) as it changes with age. Given the observation that expression of gene X (or hormone Z) changes with age, one must next ask: How do we know whether this change reflects a causative feature of aging, a defensive response to another age-related change, a passive response of no great import, an epiphenomenon, or an artifact of the experimental system? (I’ve discussed this concern before, in the context of age-specific regulation of micro-RNAs.)

The authors would argue that the changes that are common to both progeroid and long-lived animals represent true protective/defensive responses to age-related stresses (according to the same logic that underlies the interpretation of the earlier work, discussed above). In contrast, those features shared between natural aging and progeria — of which there are far more — are signs of deterioration and decrepitude, and thus reflect age-related decline.

This logic is powerful: Having distinguished between these two classes of age-related transcriptional change, we’re far better equipped to start meaningfully measuring biological age.

Many insects live a long time as larvae and only briefly as sexually mature adults — extreme examples include the mayfly, the cicada, and some crane flies, though there are countless others. In most cases, the brevity of adult life is not because of rapid onset of decrepitude but rather because the adult morph lacks some essential tool (like a mouth).

Such life histories are vanishingly rare among vertebrates — though they do exist, as revealed by this fascinating (and, to my mind, somewhat poignant) tale from Karsten et al., in which the short-lived adult does appear to be undergoing accelerated senescence:

A unique life history among tetrapods: An annual chameleon living mostly as an egg

The ≈28,300 species of tetrapods (four-limbed vertebrates) almost exclusively have perennial life spans. Here, we report the discovery of a remarkable annual tetrapod from the arid southwest of Madagascar: the chameleon Furcifer labordi, with a posthatching life span of just 4–5 months. At the start of the active season (November), an age cohort of hatchlings emerges; larger juveniles or adults are not present. These hatchlings grow rapidly, reach sexual maturity in less than 2 months, and reproduce in January–February. After reproduction, senescence appears, and the active season concludes with population-wide adult death. Consequently, during the dry season, the entire population is represented by developing eggs that incubate for 8–9 months before synchronously hatching at the onset of the following rainy season. Remarkably, this chameleon spends more of its short annual life cycle inside the egg than outside of it. Our review of tetrapod longevity (>1,700 species) finds no others with such a short life span. These findings suggest that the notorious rapid death of chameleons in captivity may, for some species, actually represent the natural adult life span. Consequently, a new appraisal may be warranted concerning the viability of chameleon breeding programs, which could have special significance for species of conservation concern. Additionally, because F. labordi is closely related to other perennial species, this chameleon group may prove also to be especially well suited for comparative studies that focus on life history evolution and the ecological, genetic, and/or hormonal determinants of aging, longevity, and senescence.

If nothing else, an apt reminder of the crazy games evolution plays in determining the genetic control of lifespan.

But examples like this are more than curiosities — per the final sentence of the abstract (emphasis mine), the vast diversity of life histories generated over the course of evolution provide an ideal laboratory in which to investigate the determinants of lifespan. Specifically, what can we learn from organisms with similar body plans and overall metabolism but significantly different lifespans? At least one project that will explore and exploit the longevity differences between related species is already underway.

Ever since the discovery that loss-of-function daf-2 mutations extend lifespan in C. elegans (a phenotype for which the forkhead-like transcription factor daf-16 is required), biogerontologists have devoted a tremendous amount of attention to the pathway, both in worm and in mammal (where DAF-2 and DAF-16 have homologs: insulin-like growth factor receptor (IGF-I-R) and various FOXO proteins, respectively).

As I mentioned yesterday, this week I’m clearing the backlog of articles that has accumulated over the past couple of months. Lots has been happening on the IGF/FOXO front. As always, each of these papers probably deserves its own post, but time is not permitting. Quoted passages are excerpts from the abstracts.

Low IGF-I decreases cancer: Reduced Susceptibility to Two-Stage Skin Carcinogenesis in Mice with Low Circulating Insulin-Like Growth Factor I Levels, Moore et al.:

These data suggest a possible mechanism whereby reduced circulating IGF-I leads to attenuated activation of the Akt and mTOR signaling pathways, and thus, diminished epidermal response to tumor promotion, and ultimately, two-stage skin carcinogenesis. The current data also suggest that reduced circulating IGF-I levels which occur as a result of calorie restriction may lead to the inhibition of skin tumorigenesis, at least in part, by a similar mechanism.

Downregulating IGF-I enhances stress tolerance: Cellular conditioning with trichostatin A enhances the anti-stress response through up-regulation of HDAC4 and down-regulation of the IGF/Akt pathway, Chu et al.:

Interestingly, the insulin signaling pathway mediated by Akt was inhibited in the TSA-resistant cells, mirroring the effect of glucose deprivation on this pathway. … Together, these findings suggest that cellular conditioning with TSA may represent a useful approach to mimic the effects of caloric restriction.

Inflammation: Regulation of IGF-I function by proinflammatory cytokines: At the interface of immunology and endocrinology, O’Connor et al.:

Over the past decade, research in our laboratory has focused on the ability of the major proinflammatory cytokines, tumor necrosis factor (TNF) and interleukin (IL)-1β, to induce a state of IGF resistance. This review will highlight these and other new findings by explaining how proinflammatory cytokines induce resistance to the major growth factor, insulin-like growth factor-I (IGF-I).

Gonadal regulation: Drosophila germ-line modulation of insulin signaling and lifespan, Flatt et al.:

Here we report that eliminating germ cells (GCs) in Drosophila melanogaster increases lifespan and modulates insulin signaling. … These results suggest that signals from the gonad regulate lifespan and modulate insulin sensitivity in the fly and that the gonadal regulation of aging is evolutionarily conserved.

Target genes: Identification of Direct Target Genes Using Joint Sequence and Expression Likelihood with Application to DAF-16, Yu et al.:

We found that 189 genes were tightly regulated by DAF-16. In addition, DAF-16 has differential preference for motifs when acting as an activator or repressor, which awaits experimental verification.

Stem cells: FoxO Transcription Factors and Stem Cell Homeostasis: Insights from the Hematopoietic System, Tothova and Gilliland:

… FoxO-dependent signaling is required for long-term regenerative potential of the hematopoietic stem cell (HSC) compartment through regulation of HSC response to physiologic oxidative stress, quiescence, and survival. These observations link FoxO function in mammalian systems with the evolutionarily conserved role of FoxO in promotion of stress resistance and longevity in lower phylogenetic systems.

As therapeutic targets: OutFOXOing disease and disability: the therapeutic potential of targeting FoxO proteins, Malese et al.:

Forkhead transcription factors have a ‘winged helix’ domain and regulate processes that range from cell longevity to cell death. … Here we discuss recent advances that have elucidated the unique cellular pathways and clinical potential of targeting FoxO proteins to develop novel therapeutic strategies and avert potential pitfalls that might be closely intertwined with its benefits for patient care.

There’s plenty to chew on. Tomorrow: telomeres.

Once again the booming literature on calorie restriction (CR) has bested me, and I’ve fallen hopelessly behind. Therefore, without comment, I’ll just run through the last month’s abstracts, with a smattering of brief commentary here and there. Each paper deserves its own entry, but we’re just going to have to make do with this. Quoted passages are all abstract excerpts.

The Nrf2 pathway: Mechanisms Underlying Caloric Restriction and Lifespan Regulation: Implications for Vascular Aging, Ungvari et al.:

We propose that caloric restriction increases bioavailability of NO, decreases vascular reactive oxygen species generation, activates the Nrf2/antioxidant response element pathway, inducing reactive oxygen species detoxification systems, exerts antiinflammatory effects, and, thereby, suppresses initiation/progression of vascular disease that accompany aging.

More on Nrf2 and aging here and here.

Protein vs. sugar in insulin signaling: Opposing Effects of Dietary Protein and Sugar Regulate a Transcriptional Target of Drosophila Insulin-like Peptide Signaling, Buch et al.

Through microarray analysis of flies in which the insulin-producing cells (IPCs) were ablated, we identified a target gene, target of brain insulin (tobi), that encodes an evolutionarily conserved -glucosidase. Flies with lowered tobi levels are viable, whereas tobi overexpression causes severe growth defects and a decrease in body glycogen. Interestingly, tobi expression is increased by dietary protein and decreased by dietary sugar.

Inactivity and inflammation: Calorie restriction modulates inactivity-induced changes in the inflammatory markers CRP and PTX3, Busutti et al.:

Calorie restriction prevents the inflammatory response induced by 14 days of bed rest. We suggest an inverse regulation of CRP and PTX3 in response to changes in energy balance.

*** This was a human study.

“Nutritional emphysema”: Effect of Severe Calorie Restriction on the Lung in Two Strains of Mice, Bishai and Mitzner:

Although the baseline mechanics and alveolar size were quantitatively different in the two strains, both strains showed similar qualitative changes during the starvation and refeeding periods. Thus, in two strains of mice with genetically determined differences in alveolar size neither the mechanics nor the histology show any evidence of emphysema-like changes with this severe caloric insult.

SIRT1 stabilization: Regulation of SIRT1 protein levels by nutrient availability, Kanfi et al.:

We show here that levels of SIRT1 increased in response to nutrient deprivation in cultured cells, and in multiple tissues of mice after fasting. The increase in SIRT1 levels was due to stabilization of SIRT1 protein, and not an increase in SIRT1 mRNA. In addition, p53 negatively regulated SIRT1 levels under normal growth conditions and is also required for the elevation of SIRT1 under limited nutrient conditions.

Protein modification in the heart: Aging and dietary restriction effects on ubiquitination, sumoylation, and the proteasome in the heart, Li et al.:

Cumulatively, our data indicate that DR has many beneficial effects towards the UPP [ubiquitin-proteasome pathway] in the heart, and suggests that a preservation of the UPP may be a potential mechanism by which DR mediates beneficial effects on the cardiovascular system.

Males vs. females, round 1: The brain: Conserved and Differential Effects of Dietary Energy Intake on the Hippocampal Transcriptomes of Females and Males, Martin et al.:

Genes involved in energy metabolism, oxidative stress responses and cell death were affected by the HFG diet in both males and females. The gender-specific molecular genetic responses of hippocampal cells to variations in dietary energy intake identified in this study may mediate differential behavioral responses of males and females to differences in energy availability.

Males vs, females, round 2: The gonad: Effects of aging and calorie restriction on the global gene expression profiles of mouse testis and ovary, Sharov et al.:

CR-mediated reversal of age-associated gene expression changes, reported in somatic organs previously, was limited to a small number of genes in gonads. Instead, in both ovary and testis, CR caused small and mostly gonad-specific effects: suppression of ovulation in ovary and activation of testis-specific genes in testis.

Whew. OK, have a great weekend, everyone.

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