Bay Area Aging Club – Session I: Model organisms & model systems

Subhash Katewa (Kapahi lab, Buck Institute) talked about the metabolic adaptations that occur in flies whose lifespan is being extended by dietary restriction (DR). Katewa is studying translational control in DR using a method called translational profiling, which uses the number of ribosomes bound to each mRNA as an index of translational activity (more ribosomes = more translation). He found that DR increases translation of messages that encode a variety of mitochondrial functions; this observation led to some interesting findings about the differential turnover of triglycerides in DR vs ad libitum flies.

Adam Freund (Campisi lab, Buck Institute) spoke about the sources of age-related inflammation, focusing on the senescence-associated secretory phenotype (SASP). Freund has elucidated mechanisms of SASP control that intermediate between the most upstream events in senescence (DNA damage) and its downstream effects (secretion of inflammatory factors). I have it on good authority that he has a completed manuscript on the subject, hopefully to be publshed soon, so I won’t say more about his story here. (Mr. Freund happens to be my baymate.)

Dario Valenzano (Brunet lab, Stanford University) is studying the genetic architecture of longevity in a short-lived fish Nothobranchius furzeri, the shortest-living vertebrate that can be reared in captivity. As a graduate student, Valenzano developed a system of biomarkers for tracking the progress of aging in skin, brain and other tissues – not only physical markers like the senescence-associated beta-galactosidase but also behavioral markers that change over the lifespan. He is now proceeding to map the longevity-associated genes in N. furzeri and testing the sufficiency of the genes he finds. Early results indicate that short-lived and long-lived fish are dying from different causes, as evidenced by a bimodal distribution of death rate vs. age.

Adolfo Sánchez-Blanco (Kim lab, Stanford University Medical School) described the “molecular odometer” for aging in the worm C. elegans. He began with the observation that lifespan is variable, even among clonally identical individuals kept under identical conditions. With genetics and environment taken out of the picture, what makes some individuals live longer than others? In order to address this question, SB had to develop a molecular marker (e.g., promoter activity of some gene) that measures physiological age (as opposed to chronological age), and then determine whether the expression level of that marker in individual worms is predictive of lifespan. SB has identified several such genes whose expression at middle age strongly predicts remaining lifespan. He is now actively looking for interventions that abolish the correlation between marker expression and longevity: if the marker gene’s activity is serving to overcome the life-shortening effect of some stress, then removing that stress will not necessarily abolish the variability in the marker, but will eliminate the correlation between marker levels and lifespan. (This is a subtle but important logical issue; I would have thought that one should look for interventions that drove the population distribution of marker levels toward the favorable side of the distribution. It was clear from questions that a lot of audience members had trouble with this logic, and I’m still not sure I understand it myself.)

(next session)

 

Nature Insight: Ageing

Nature’s most recent “Insight” supplement is devoted to a topic near and dear to our hearts, even when spelled with that superfluous UK “e”: Ageing. From the introductory editorial:

Ageing, the accumulation of damage to molecules, cells and tissues over a lifetime, often leads to frailty and malfunction. Old age is the biggest risk factor for many diseases, including cancer and cardiovascular and neurodegenerative diseases. … Ageing research is clearly gaining momentum, as the reviews in this Insight testify, bringing hope that at some time in the future we will be able to keep age-related diseases at bay by suppressing ageing itself.

The five reviews are all by prominent scholars — many of whose work we’ve discussed here — and cover a wide range of subjects within gerontology and biogerontology:

• The genetics of ageing, Cynthia J. Kenyon
• Lessons on longevity from budding yeast, Matt Kaeberlein
• Linking functional decline of telomeres, mitochondria and stem cells during ageing, Ergün Sahin & Ronald A. DePinho
• Neural mechanisms of ageing and cognitive decline,
  Nicholas A. Bishop, Tao Lu & Bruce A. Yankner
• Biodemography of human ageing, James W. Vaupel

As always, Nature Insight supplements are free-access, so even if you don’t have access to a university subscription, you can still read these articles.

(For a previous aging-related Nature Insight on DNA repair, see here.)

 

Review Roundup #5

Here are the biogerontological reviews from the last month or so that I’ve found interesting and noteworthy. The field as a whole continues to massively overproduce review papers; by my totally unscientific estimate, these represent less than ten percent of the review abstracts that crossed my desk since Thanksgiving.

The last installment of review roundup can be found here. As always, each Review Roundup is guaranteed to contain at least one link to a review you will find highly educational, or your money back.

Comparative biogerontology:

A while back I attended a NAKFI meeting about aging. Along with a few others, I applied for (and got) a seed grant to use comparative zoology to study aging — in a nutshell, to study the various ways that nature has solved various problems that arise during aging, and see whether we might learn something that could be applied to enhancing human healthspan or lifespan.

The initial small grant funded a series of meetings, culminating in a large-scale gathering of scientist with wide expertise not only in biogerontology but also zoology, evolutionary biology, metabolomics, and other disparate fields. While this conference didn’t end up leading to the creation a single comprehensive Comparative Biogerontology Initiative, as some of my fellow applicants had hoped, it did provoke a great deal of excellent discussion. There are a few smaller-scale efforts currently underway, initiated by people who came together to talk about the original idea.

Two of the attendees of the big meeting have published reviews recently. I haven’t asked them personally but I am assuming that they’re discussing ideas that germinated at the CBI conferences.

• Insights from comparative analyses of aging in birds and mammals, Robert Ricklefs
• Methusaleh’s Zoo: How Nature provides us with Clues for Extending Human Health Span, Steven Austad

Gene regulation:

• Genetic, epigenetic and posttranslational mechanisms of aging, Robert et al.

Inflammation:

• Evolution of the human lifespan and diseases of aging: Roles of infection, inflammation, and nutrition, Caleb Finch
• Inflammaging (inflammation + aging): A driving force for human aging based on an evolutionarily antagonistic pleiotropy theory?, M. Goto

Mitochondria:

One of the authors of the first paper is Thomas Nyström, whose lab recently described the role of cell polarity in sorting protein aggregates preferentially into the mother cell during cell division. That story lacked a significant mitochondrial component, so this review is a nice complement to the primary study published earlier this year.

• Perspectives on the mitochondrial etiology of replicative aging in yeast, Ugidos et al.
• The mtDNA mutator mouse: Dissecting mitochondrial involvement in aging, Edgar & Trifunovic

Nuclear organization:

• Nuclear and Chromatin Reorganization during Cell Senescence and Aging – A Mini-Review, Shin et al.

Stem cells:

Leanne Jones, the senior author on this review, is one of the folks writing the proverbial book on the critical interactions between stem cells and the tissue microenvironment. Her lab uses the Drosophila gonad as a model system.

• Stem Cells and the Niche: A Dynamic Duo, Voog & Jones

 

FOXO, centenarians, and other longevity coverage at Singularity Hub

As I was wandering the net today I found a very nice writeup about the 2009 report of an association between the FOXO3A gene and human aging. I found the article at the apparently quite popular but new-to-me blog Singularity Hub.

We mentioned this work in a brief post last year. The overall conclusion is that natural variants in this gene that are associated with extreme longevity. (The FOXO3A gene is a homolog of DAF-16, a longevity determinant in worms.) The 2009 paper describes a study of German centenarians, and is consistent with similar results in Japanese-Americans, published in 2008. Other genetic variants associated with lifespan include the hTERT and hTERC loci, recently described in a study of Ashkenazi Jewish centenarians.

Mostly I’m writing this post to introduce our readers to an interesting site: Singularity Hub contains a lot of excellent biogerontology coverage (in their longevity category). Much of the writing on that topic is by senior editor Aaron Saenz, who does a great job of critically addressing the newest findings in a very reader-friendly and accessible style. I’m going to subscribe to their feed and start reading regularly. Overall it’s a very professional and well-written site, and I’d recommend it to Ouroboros readers.

 

A snippet about pharmacogenetic variation and the future of anti-aging medicine

SNPedia is a wiki-style index of genetic variants that have interesting phenotypic associations in human beings. The name comes from the acronym for “single nucleotide polymorphisms,” i.e., one-letter variations among different individuals’ genomes.

In honor of the new year, one of the proprietors has posted SNPedia’s Top 10 SNPs of the Year, based on an admittedly “subjective combination of medical importance, statistical believability, and overall general interest.” The variants that made the list are associated with a wide range of phenotypes, but they fall into a few categories:

• benefits of major drugs (e.g., effect of Plavix on heart disease risk);
• likelihood of drug side effects (e.g., myopathy in response to statins);
• risk for specific diseases (CVD, periodontitis, cancers)

The list, especially the items regarding drug efficacy and adverse reactions, got me thinking about anti-aging medicine.

Any hypothetical longevity-enhancing therapies will be more or less effective, and be subject to more or less severe side effects, as a function of individual genetic variation. One consequence of pharmacogenetic variability is that small or insufficiently diverse trial populations (in which specific genetic variants might be underrepresented) can result in misleading results about a therapy’s potential efficacy in the general population. And it’s hard to know, in advance of preliminary results, what the relevant variants might be.

This logic is general to a wide variety of therapies. Drugs are just molecules of varying shapes and sizes, and molecules of all shapes and sizes mediate cell-cell interactions, so it’s likely that pharmacogenetics will influence cellular therapies as well as more conventional pharmaceutical approaches. I suspect that cellular therapies might even be more vulnerable to genetic variation, since cell-cell interactions rely on proteins and other molecules produced by multiple genetic loci – e.g., not just a receptor or a ligand but both the receptor and the ligand acting together – and these pairwise interactions will be even more difficult to tease out than phenotypes that rely on a single locus.

It’s already going to be hard to determine over short intervals whether a given anti-aging therapeutic is effective, since we don’t (yet) have biomarkers that allow us to measure the rate of aging. Most of the best biomarkers are most convincing at the population level, and it’s hard to use them to compare the rate of physiological vs. chronological aging in a single individual. Therefore, proof of efficacy of longevity-enhancing treatments will rely on long studies and sizable populations of subjects – and the existence of unresponsive genotypes in the population will further confound that analysis.

Granted, we already know that building an anti-aging pharmacopeia will be challenging, and I’m not suggesting that this line of reasoning means we should pack up and go home. I mention it mostly because genetic variations will almost certainly play an important role in determining the efficacy of any given therapy, and we had best be prepared for that.

 

Mazel tov! You should have such long telomeres

A study of Ashkenazi Jewish centenarians by Atzmon et al. has revealed that telomere length is correlated with longer lifespan and slower biological aging (reflected in measurements of several biomarkers of aging). Both lifespan and telomere length are, in turn, correlated with polymorphisms at the hTERT and hTERC loci, two genes that respectively encode the major protein and RNA component of telomerase.

Recently we learned that telomere length is a biomarker of chronological age – in other words, that younger people have longer telomeres in general. This correlation is imperfect, unsurprisingly, and probably for lots of reasons, including individual variations in lifestyle, outlook, stress levels, and other factors. This new study demonstrates that there some of the difference between individuals in the rate of telomere shortening over time is under genetic control.

Atzmon, G., Cho, M., Cawthon, R., Budagov, T., Katz, M., Yang, X., Siegel, G., Bergman, A., Huffman, D., Schechter, C., Wright, W., Shay, J., Barzilai, N., Govindaraju, D., & Suh, Y. (2009). Evolution in Health and Medicine Sackler Colloquium: Genetic variation in human telomerase is associated with telomere length in Ashkenazi centenarians Proceedings of the National Academy of Sciences, 107 (suppl_1), 1710-1717 DOI: 10.1073/pnas.0906191106

Related articles elsewhere:

• TERC, telomerase, and rate of aging (The Longevity Meme)
• Common variants near TERC are associated with mean telomere length (Nature Genetics)

 

Hourglass X

Welcome to the tenth edition of Hourglass, our blog carnival about the biology of aging. This month, the carnival has returned home to Ouroboros. In this issue, we have submissions from six bloggers, including a nice mix of veterans and new participants. Several of the posts are united by common themes: we have heavy representation from the neuroscience community, and multiple discussions of the clinical and social payoffs that are likely to result from progress in lifespan extension.

At psique (which hosted Hourglass IX), Laura Kilarski describes an important, evolving online tool for biogerontologists: the Human Aging Genomics Resources:

As I was reading a paper earlier about chromosomal region 11.5p and its putative association with aging (Lescai et al, 2009) I came across an interesting sounding url, namely http://genomics.senescence.info. Turns out that the website is home to HAGR, an interdisciplinary project devoted to the genetic study of aging … GenAge constitutes a major part of the site, and is a manually curated database of genes which could possibly be associated with human aging, largely based on studies done on the usual suspects: Mr. Mouse, Drosophila, C. elegans, and yeast. … The AnAge database on the other hand contains entries for over 4000 animals and some basic life-span-related facts. … And then there’s the ‘Δ Project’, the aim of which is to figure out transcriptional differences between young and old organisms.

Laura describes HAGR in depth and also provides some of her own analysis of the available resources.

On another age-related subject, neurodegeneration, Laura discusses the potential value of regular brain scans for early ascertainment of diseases such as Parkinson’s. Free brain scans for all! It’s a moving piece, which underscores the human cost of neurodegenerative illness and describes the author’s personal reactions on the subject, while also addressing important clinical and scientific issues.

As we age, we all suffer from some level of neurodegeneration, though in most cases this falls below the threshold of a clinical pathology. Slow chronic change isn’t the only form of age-related brain damage: let’s not forget about strokes, which can wipe out otherwise healthy neurons in macroscopic regions of the brain. While the risk factors for stroke and neurodegeneration are distinct, therapies might ultimately be quite similar — since in both cases, the goal is to regrow neurons to replace those that have been lost. At Brain Stimulant, Mike tell us about a clinical trial that will use stem cells to treat stroke:

The company Reneuron has just recently gotten the go ahead to commence a new trial that will use stem cells to treat patients with stroke damage. The trial will use stem cells to replace missing brain matter in those who have had stroke brain trauma. They are injecting doses of approximately 20 million stem cells into the stroke patients brain. Interestingly these ReN001 stem cells will not require a patient to have immunosuppression therapy.

He goes on to discuss the future challenges posed by the prospect for brain engineering: precise cell delivery, control of axon sprouting and pathfinding, and the possibility of using non-invasive methods to encourage the growth of new cells.

Also coming from a neuroscience perspective, Christopher Harris of Best Before Yesterday writes about What we need to accelerate biomedical research and fight aging.

A few hundred years ago I could not have been born. I was massive – 5.5kg – and the birth eventually turned caesarean and took many long hours. I owe my life to medical science. One day, 11 years later, I was out biking and realized for the first time that the annihilation following my death would be infinite. Now, 25 years after my complicated birth, I think a lot about whether medical science, rejuvenation research of the SENS variety in particular, will save me a second time.

What do we need? According to Harris: (1) Safe and inexpensive brain surgery (to install devices that can manipulate the reward circuitry of the brain); (2) Widespread use of enhanced motivation through deep brain stimulation (specifically to encourage exercise and healthy living); and (3) Rewarding brain stimulation for research centers (to accelerate scientific progress).

One of my favorite new sites, the Science of Aging Timeline, has a new entry about the Sinclair lab’s discovery of sirtuin-activating compounds:

Working off a model of calorie restriction via sirtuins David Sinclair et al. worked to find molecules which could modulate sitruins activity, and thus longevity.

They accomplished this by screening a number of small molecule libraries, which included analogues of epsilon-acetyl lysine, NAD+, NAD+ precursors, nucleotides and purinergic ligands. Results from the screening where assayed against human SIRT1 to identify potential inhibitors, and the following molecules where found: Resveratrol, Butein, Piceatannol, Isoliquiritigenin, Fisetin, and Quercetin. Of all of these, resveratrol proved to be the most potent …

In the copious spare time left when he’s not working on the comprehensive history of biogerontology, timeline curator Paul House has started another ambitious project: a catalog of all the labs working on aging. It’s early days yet, and only a few labs are listed, but I’ve already seen Paul take one great idea (the timeline) from seed to oak, so I have every confidence that this page will grow substantially in the weeks and months to come. Those who are interested in having their labs listed on the page can send Paul an email.

Over at Fight Aging!, Reason continues excellent coverage of recent papers in biogerontology; I daresay that the detail of coverage on primary scientific literature has improved even further in the past month or so, concomitant with the site’s participation in the ResearchBlogging tracking system for blog posts about journal articles. For this edition of Hourglass, Reason has submitted two excellent analyses of recent papers, and a third piece of a more philosophical bent:

• Autophagy and cellular senescence
• Accumulating Mitochondrial DNA Damage: More Harm or Less Repair?
• Lazy immortality

It is from the last piece that I’ve chosen an excerpt:

Wouldn’t it be nice to wake up and find that we were all immortal? That would save a whole lot of work, uncertainty, and existential angst – and we humans are nothing if not motivated to do less work. The best of us toil endlessly in search of saving a few minutes here and a few minutes there. So it happens that there exist a range of metaphysical lines of thought – outside the bounds of theology – that suggest we humans are immortal. We should cast a suspicious eye upon any line of philosophy that would be extraordinarily convenient if true, human nature being what it is.

Moving on from a philosophical post written by a scientifically minded life-extension advocate, our next posts are scientific posts written about life extension from a political philosopher. Colin Farrelly of In Search of Enlightenment has submitted two long, thoughtful articles, the first about the clinical and social importance of tackling aging, the second about the cognitive biases that affect the way we think about risk and the significance of aging as a cause of mortality:

• Idealism Meets Realism: Tackling Chronic Disease Via Age Retardation
• The Availability Heuristic and the Inborn Aging Process

The “availability heuristic” was a new one on me. Here’s an operational definition as it applies to our thinking about aging:

In a rational world, aging research would be at the forefront of a global collaborative initiative to improve the health and economic prospects of today’s aging populations (and all future generations).

But humans are not rational. We suffer many cognitive biases. One prominent bias is the availability heuristic. Risks that are easily brought to mind are given a higher probability; and conversely, the less vivid a risk, the more likely we are to underestimate the probability of their occurring.

The two tests above reveal how prominent this heuristic is in your own comprehension of the risks facing yourself, your loved ones and humanity. Because death by aging is not something that is vivid is most people’s minds (though it is in the minds of the scientists who study the biology of aging and thus know all too well how it affects a species functional capacities), odds are you probably underestimated it as a risk of mortality.

The benefits of lifespan extension, both with regard to human health and society as a whole is sometimes called the Longevity Dividend. Alvaro Fernandez from SharpBrains sent in a long piece about the Longevity Dividend (written by a contributor from the Kronos Longevity Research Institute). Ever heard of the Longevity Dividend? Perhaps Gray is the New Gold:

The Longevity Dividend is a theory that says we hope to intervene scientifically to slow the aging process, which will also delay the onset of age-related diseases. Delaying aging just seven years would slash rates of conditions like cancer, diabetes, Alzheimer’s disease and heart disease in half. That’s the longevity part. … The dividend comes from the social, economic, and health bonuses that would then be available to spend on schools, energy, jobs, infrastructure—trillions of dollars that today we spend on healthcare services. In fact, at the rate we’re going, by the year 2020 one out of every $5 spent in this country will be spent on healthcare. Obviously, something has to change.

Alvaro, the editor of SharpBrains and founder of the parent website, has recently published a book, The SharpBrains Guide to Brain Fitness, which is the subject of this recent (and quite favoriable) review. If you’re interested in learning more, here’s list of cognitive fitness references, based on the authors’ research for the book.

That’s all for now. If you’d like to host a future installation of Hourglass, please email me.

 

Giving DAF-16 the upper hand: a transcriptional switch in the IGF pathway?

The insulin-like growth factor (IGF) pathway is one of the longest-known and well-studied regulators of longevity. Extracellular signals (insulin-like peptides) activate insulin-receptor homologs (in worm, DAF-2) which in turn recruit and activate phosphoinositol 3-kinases (AGE-1). PI3Ks convert PIP2 into PIP3, which tethers and recruits other kinases such as AKT-1. Eventually, activation of these upstream kinases results in phosphorylation and inactivation of the longevity assurance gene DAF-16, which encodes a transcription factor that activates (among many other things) stress resistance genes.

To recap: High DAF-2 and AGE-1 activity => low DAF-16 activity => shorter lifespan. Lower DAF-2 or AGE-1 => high DAF-16 activity => longer lifespan. The lifespan extension of daf-2 or age-1 mutants absolutely requires wildtype DAF-16.

From this simple model, it would seem that the levels of DAF-2 agonists would run the show; DAF-16 activity would simply be a readout of signaling upstream of the insulin-like growth factor receptor. As is so often the case, however, this simple model turns out to be simplistic: DAF-16 plays an active role in determining the signaling through this pathway, as revealed by Tazearslan et al.:

Positive Feedback between Transcriptional and Kinase Suppression in Nematodes with Extraordinary Longevity and Stress Resistance

Insulin/IGF-1 signaling (IIS) regulates development and metabolism, and modulates aging, of Caenorhabditis elegans. In nematodes, as in mammals, IIS is understood to operate through a kinase-phosphorylation cascade that inactivates the DAF-16/FOXO transcription factor. Situated at the center of this pathway, phosphatidylinositol 3-kinase (PI3K) phosphorylates PIP2 to form PIP3, a phospholipid required for membrane tethering and activation of many signaling molecules. Nonsense mutants of age-1, the nematode gene encoding the class-I catalytic subunit of PI3K, produce only a truncated protein lacking the kinase domain, and yet confer 10-fold greater longevity on second-generation (F2) homozygotes, and comparable gains in stress resistance. Their F1 parents, like weaker age-1 mutants, are far less robust—implying that maternally contributed trace amounts of PI3K activity or of PIP3 block the extreme age-1 phenotypes. We find that F2-mutant adults have <10% of wild-type kinase activity in vitro and <60% of normal phosphoprotein levels in vivo. Inactivation of PI3K not only disrupts PIP3-dependent kinase signaling, but surprisingly also attenuates transcripts of numerous IIS components, even upstream of PI3K, and those of signaling molecules that cross-talk with IIS. The age-1(mg44) nonsense mutation results, in F2 adults, in changes to kinase profiles and to expression levels of multiple transcripts that distinguish this mutant from F1 age-1 homozygotes, a weaker age-1 mutant, or wild-type adults. Most but not all of those changes are reversed by a second mutation to daf-16, implicating both DAF-16/ FOXO–dependent and –independent mechanisms. RNAi, silencing genes that are downregulated in long-lived worms, improves oxidative-stress resistance of wild-type adults. It is therefore plausible that attenuation of those genes in age-1(mg44)-F2 adults contributes to their exceptional survival. IIS in nematodes (and presumably in other species) thus involves transcriptional as well as kinase regulation in a positive-feedback circuit, favoring either survival or reproduction. Hyperlongevity of strong age-1(mg44) mutants may result from their inability to reset this molecular switch to the reproductive mode.

PIP3 is necessary for the membrane tethering and activation of a great many kinases; thus, profound defects in PI3K activity would be predicted to result in profound defects in phosphoprotein signaling. Indeed, the authors see dramatic effects on in vitro kinase activity and steady-state phosphoprotein levels in their age-1 mutants — not just for known downstream targets of the IGF pathway but for bulk protein.

Note the importance of using F2 age-1 mutants, i.e., animals that are themselves the offspring of homozygous age-1 mutants: Wildtype AGE-1 activity in the parent, even from a single copy of the gene, is sufficient to maternally rescue PI3K activity in the offspring to some extent. This results in a weaker phenotype in the F1s than in their progeny, the F2s, who completely lack PI3K activity and consequently enjoy much longer lifespan and higher resistance to stress. There’s no way to find out about F3s, since the F2s are completely sterile.

In the absence of upstream IGF signaling, downstream effector kinases would not be activated by phosphorylation. Here’s where the story throws us a curve-ball: As predicted, in the profoundly long-lived age-1 F2′s, the effector kinases are inactive — but the transcripts encoding them are also downregulated. In the absence of upstream signaling, there’s no longer a kinase cascade bearing down on DAF-16, which therefore remains unphosphorylated and active. And what does DAF-16 do? It heads to the nucleus and transcriptionally silences the genes encoding the upstream kinases DAF-2, AGE-1 and others — in other words, DAF-16 turns off the genes that could turn off DAF-16.

It’s a feedback loop! Disinhibition of DAF-16 by lowering PIP3 levels is self-sustaining, because disinhibited DAF-16 lowers transcription of PI3Ks, thereby further lowering PIP3 levels. The authors argue that this arrangement represents a biological switch between a short-lived “reproductive state” and a non-reproducing “longevity state”, characterized by DAF-16 activation of stress-resistance and other types of longevity assurance genes.

They have a point, but I think they might be overstating the “switchiness” of this switch. One of the main features of a switch (of the sort that motivates the analogy) is that it can be on or off but not halfway between the two — there are disequilibrating forces at work that push it away from the middle and toward either pole. Unfortunately for the authors’ interpretation, most of the work done in this field to date has been done in the equivalent of the F1 age-1 mutants, where a combination of partial gene function and maternal factors have placed worms somewhere between “all reproduction” and “all longevity”. The existence of these animals does somewhat mitigate the argument that this system represents a binary switch in the strictest sense of the word.

To be fair to the authors, they acknowledge this, most clearly in Figure 6, a model that allows for three states — reproductive (low DAF-16, e.g., wildtype), longevity (high DAF-16, e.g. daf-2 or age-1 F1), and “hyperlongevity” (extremely high DAF-16 that completely shuts off PI3K activity, e.g. age-1 F2). The distinction between the two long-lived cases is somewhat elided in the Discussion, where the authors emphasize the feedback loop and consider the longevity states as though they were the same.

Tazearslan, C., Ayyadevara, S., Bharill, P., & Shmookler Reis, R. (2009). Positive Feedback between Transcriptional and Kinase Suppression in Nematodes with Extraordinary Longevity and Stress Resistance PLoS Genetics, 5 (4) DOI: 10.1371/journal.pgen.1000452

 

Inhibiting mitochondrial respiration delays aging: CLK-1 and the retrograde response

The CLK-1 gene is a longevity regulator that has been conserved across evolution: loss of function in both C. elegans (where the gene was first described) and hemizygosity in mouse (where the gene is called mCLK1) results inincreased lifespan (though at a cost to evolutionary fitness). Consistent with this, at least one anti-neurodegeneration drug inhibits mCLK1, raising the possibility that the drug acts by delaying aging and thereby postponing age-related neurological disease. Genetic crosses and other evidence suggest that the longevity extension is independent of the action of the insulin-like growth factor (IGF)/DAF-2 pathway, another conserved regulator of longevity.

The CLK-1 protein catalyzes a late step in the biosynthesis of ubiquinone (aka coenzyme Q or coQ), an essential cofactor in mitochondrial electron transport, but it’s not completely clear whether this enzymatic function has to do with the life-extension phenotype. In worm, even among clk-1 mutants that completely lack detectable coQ and accumulate the same levels of the metabolic precursor DMQ, null mutants (which make no CLK-1 protein) exhibit more severe phenotypes than missense mutants (which express normal levels of a defective protein). Furthermore, tRNA suppressor mutations appear to differentially affect the lifespan, developmental and ubiquinone synthesis phenotypes. Finally, dietary supplementation with coQ fails to rescue the developmental and lifespan phenotypes (though this is confounded by the fact that these worms never accumulate cellular levels of coQ that are comparable to wildtype). (For a more detailed treatment of CLK-1 biology in both worm and mouse, see this excellent review by Stepanyan et al.).

How, then, do clk-1 mutants extend longevity? To address this question, Cristina et al. compared the the gene expression profiles of wildtype and clk-1 worms:

A regulated response to impaired respiration slows behavioral rates and increases lifespan in Caenorhabditis elegans

When mitochondrial respiration or ubiquinone production is inhibited in Caenorhabditis elegans, behavioral rates are slowed and lifespan is extended. Here, we show that these perturbations increase the expression of cell-protective and metabolic genes and the abundance of mitochondrial DNA. This response is similar to the response triggered by inhibiting respiration in yeast and mammalian cells, termed the “retrograde response”. As in yeast, genes switched on in C. elegans mitochondrial mutants extend lifespan, suggesting an underlying evolutionary conservation of mechanism. Inhibition of fstr-1, a potential signaling gene that is up-regulated in clk-1 (ubiquinone-defective) mutants, and its close homolog fstr-2 prevents the expression of many retrograde-response genes and accelerates clk-1 behavioral and aging rates. Thus, clk-1 mutants live in “slow motion” because of a fstr-1/2-dependent pathway that responds to ubiquinone. Loss of fstr-1/2 does not suppress the phenotypes of all long-lived mitochondrial mutants. Thus, although different mitochondrial perturbations activate similar transcriptional and physiological responses, they do so in different ways.

The transcriptional phenotype of clk-1 mutant animals is similar to the phenotype observed when mitochondrial respiration is inhibited; this is the “retrograde response” mentioned in the abstract (so called because the signal travels from the mitochondria to the nucleus rather than in the other direction). Importantly, blocking the retrograde response by knocking down two effector molecules (FSTR-1 and -2) both prevents many of these transcriptional changes and rescues the lifespan phenotype — strongly implying that the retrograde response is inducing genes that contribute to the enhanced longevity of the mutants.

Taken together, I think these findings make a strong argument that coQ synthesis and lifespan phenotypes are closely related. The observation that clk-1 mutants phenocopy inhibition of respiration is not surprising when one recalls that coQ is a key mitochondrial cofactor. If knocking down respiration causes life-extending transcriptional changes, and reduction in coQ levels knocks down respiration, then it follows that the reduction in coQ levels is the ultimate cause of lifespan extension in clk-1 mutants.

There is a danger that we’re seeing the forest but missing a specific tree: The transcriptional responses to clk-1 mutation and inhibited respiration are not identical. It remains a formal possibility that wildtype CLK-1 mediates lifespan effects via one of the genes that is regulated (in a FSTR-1/2-dependent manner) in clk-1 but not during the retrograde response, and that the similarity to the retrograde response is a red herring. This is a tough sell, however, since one must still account for the growing body of evidence (cited by Cristina et al.) that inhibition of respiration by other means can also increase longevity.

Cristina, D., Cary, M., Lunceford, A., Clarke, C., & Kenyon, C. (2009). A Regulated Response to Impaired Respiration Slows Behavioral Rates and Increases Lifespan in Caenorhabditis elegans PLoS Genetics, 5 (4) DOI: 10.1371/journal.pgen.1000450

 

FOXO3A polymorphisms linked to human longevity – again!

In my first contribution to Ouroboros, I reviewed a paper in which the authors found an allele of FOXO3A associated with long lived Japanese men. So it seemed highly appropriate that I wrote the review for a follow-up paper: Flachsbart et al. analyzed 16 polymorphisms in the FOXO3A allele in 1,031 long-lived individuals (95-110 years old, 388 centenarians included) and 731 younger controls. They found that polymorphisms in the FOXO3A gene were significantly associated with German centenarians and human longevity.

Association of FOXO3A variation with human longevity confirmed in German centenarians
Here, we have investigated 16 known FOXO3A SNPs in an extensive collection of 1,762 German centenarians/nonagenarians and younger controls and provide evidence that polymorphisms in this gene were indeed associated with the ability to attain exceptional old age. The FOXO3A association was considerably stronger in centenarians than in nonagenarians, highlighting the importance of centenarians for genetic longevity research. Our study extended the initial finding observed in Japanese men to women and indicates that both genders were likely to be equally affected by variation in FOXO3A. Replication in a French centenarian sample generated a trend that supported the previous results. Our findings confirmed the initial discovery in the Japanese sample and indicate FOXO3A as a susceptibility gene for prolonged survival in humans.

The next exciting advances in our knowledge of the molecular function of the FOXO3A protein will address how these polymorphisms affect protein function to impart this long-lived phenotype. FOXO3A (a human homo of the worm gene daf-16, whose function is necessary for lifespan extension conferred by daf-2/IGF-I pathway mutations) has been shown to play a role in insulin sensitivity, coronary heart disease, and diabetes which implicates FOXO3A as a “master regulator” that can regulate a wide variety of downstream targets which can have large effects on extending lifespan.

(Editor’s note: Liz will be taking a hiatus from blogging while she finishes her thesis. Let’s all wish her luck!)

Flachsbart, F., Caliebe, A., Kleindorp, R., Blanche, H., von Eller-Eberstein, H., Nikolaus, S., Schreiber, S., & Nebel, A. (2009). Association of FOXO3A variation with human longevity confirmed in German centenarians Proceedings of the National Academy of Sciences, 106 (8), 2700-2705 DOI: 10.1073/pnas.0809594106