Neurodegeneration


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Craig Skinner (Lin Lab, UC Davis): Identification of potential calorie restriction mimics in yeast using a nitric oxide-based screen. Yeast are an important model system in biogerontology, useful not only for genetic studies of longevity control but also for discovery of bioactive compounds. Calorie restriction (CR) in yeast causes increased levels of nitric oxide (NO) — somewhat surprising in that yeast cells lack a homolog of nitric oxide synthase — and elevated NO is sufficient to extend yeast lifespan. These observations led Skinner to screen a yeast deletion library for elevated NO levels, yielding several genes that extend lifespan.

Mark Lucanic (Lithgow Lab, Buck): Endocannabinoid signaling mediates the effect of diet on lifespan in C. elegans. Mutants in the dauer pathway in C. elegans often influence longevity; the daf-2 mutation, which causes constitutive dauer formation at elevated temperatures, extends lifespan by several fold. Lucanic discovered that endocannabinoids are involved in the regulation of the dauer pathway — and therefore, of longevity — either independently of or far downstream of daf-2 and daf-16. Endocannabinoids are upregulated under well-fed conditions, and shorten lifespan.

Delia David (Kenyon Lab, UCSF): Widespread protein aggregation is an inherent part of aging in C. elegans. Protein aggregates are a hallmark of many age-related neurodegenerative diseases, leading to the hypotheses that the cellular mileu changes with age in a manner that causes native, aggregation-prone proteins to form aggregates. David used mass spectrometry to identify a subset of normal worm proteins aggregate as a function of age. As with the proteins associated with neurodegeneration, specific proteins aggregate in specific cell types. Mutations that extend lifespan (such as daf-2) decrease aggregation, and tend to downregulate the expression of genes encoding aggregation-prone proteins. Curiously, regulators of protein homeostasis tend to aggregate themselves, leading to a destructive positive feedback loop in which the very factors that protect the cell from proteotoxicity disappear into aggregates, leading to further aggregation.

Cherry Tang (Zhong Lab, Berkeley): The Clearance of Ubiquitinated Protein Aggregates Via Autophagy. Autophagic protein degradation has been implicated in control of lifespan: autophagy slows cell and tissue aging. Tang has identified a protein that participates in degradation of ubiquitinated proteins and co-localizes with autophagosomes; when the protein is knocked down, protein aggregates become more toxic.

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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:

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.)

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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:

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:

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.

The Alzheimer’s Project, produced by HBO and the National Institute on Aging, is a new website on Alzheimer’s disease. The site contains information on Alzheimer’s for the general public as well as resources for those suffering from the disease.

The center of the collaboration is a four part documentary, which is now streaming on the project’s website. Although you may feel the need to jump to the film about the science of Alzheimer’s (and its supplemental videos), the real power of this series lies in the other sections.

Grandpa, do you know who I am? features interviews with the grandchildren of Alzheimer’s patients, and Caregivers provides a look at lives of those who care for Alzheimer’s patients. Both of these films are worth watching, especially if someone you know has Alzheimer’s.

For scientists, though, The Memory Loss Tapes is a must see. This section profiles seven people with Alzheimer’s, each at different stages of the disease. As basic scientists, we are often too far removed from those actually suffering from the diseases we study. This film gives us an up-close view of what it is like living with Alzheimer’s, and is a wonderful reminder of why we do what we do.

Protein degradation is an essential longevity assurance pathway. Maintaining high levels of autophagy can delay age-related decline in liver function. Obstacles to protein degradation tend to shorten the lifespan: blocking autophagy causes hypersensitivity to stress, and inhibiting the ubiquitin/proteasome pathway damages the mitochondria; both of these treatments kill neurons.

Conversely, longevity enhancement tends to enhance disposal of cellular garbage: In a worm model of Alzheimer’s disease, long-lived daf-2 mutants exhibit slower protein aggregation and decreased proteotoxicity, probably as a result of higher rates of protein degradation.

Despite the overall importance of protein degradation in delaying aging, the destruction of individual proteins is not always a good thing. During a screen of worm E3 ubiquitin ligases, Mehta et al. discovered that blocking the degradation of the HIF-1 protein dramatically increases lifespan and blocked the toxicity of pathogenic, aggregation-prone proteins.

Proteasomal Regulation of the Hypoxic Response Modulates Aging in C. elegans

The Caenorhabditis elegans von Hippel-Lindau tumor suppressor homolog VHL-1 is a cullin E3 ubiquitin ligase that negatively regulates the hypoxic response by promoting ubiquitination and degradation of the hypoxic response transcription factor HIF-1. Here, we report that loss of VHL-1 significantly increased life span and enhanced resistance to polyglutamine and amyloid beta toxicity. Deletion of HIF-1 was epistatic to VHL-1, indicating that HIF-1 acts downstream of VHL-1 to modulate aging and proteotoxicity. VHL-1 and HIF-1 control longevity by a mechanism distinct from both dietary restriction and insulin/IGF-1-like signaling. These findings define VHL-1 and the hypoxic response as an alternative longevity and protein homeostasis pathway.

The initial finding was that knockdowns of the E3 ligase VHL-1 were long-lived. VHL-1 is known to degrade HIF-1, the transcription factor involved in the hypoxic response. To rule out the possibility that another substrate of VHL-1 was important in the longevity enhancement, the authors used fairly straightforward genetic analysis: Mutating EGL-9, another gene required for HIF-1 degradation, also confers the lifespan extension, but neither vhl-1 not egl- mutants could live long in the absence of HIF-1.

The VHL-1/EGL-9/HIF-1 pathway is distinct from other means of lifespan extension: both daf-2 mutants and calorie restricted animals could extend the lifespan of hif-1 mutants, and conversely vhl-1 mutations could further extend the lifespan of daf-2 animals. This distinction may exist only at the level of the more upstream signaling events, however: DAF-16, the longevity assurance transcription factor that is disinhibited by daf-2 mutation, shares many target genes with HIF-1, so it is possible that the longevity enhancements rely on the same stable of stress-resistance and repair genes.

Will boosting HIF-1 levels also influence lifespan in mammals? Probably not, at least not in any simple way: the proteins involved are highly conserved — but VHL-1 is a tumor suppressor in humans, so targeting it with a drug is almost definitely a bad idea. Since suppression of the hypoxic response (especially angiogenesis) is likely to be important to the mechanism of tumor suppression by VHL-1, the same goes for HIF-1. It wouldn’t be incredibly surprising if this particular mechanism of lifespan regulation weren’t conserved between worms and mammals: worms don’t like oxygen as much as we do, so even if the machinery is conserved, the physiological consequences of activating that machinery might not be.

Still, as the authors point out, there might be some value in exploring manipulations of the hypoxic response in post-mitotic tissue – like brain — where the risk of tumorigenesis would presumably be smaller.

ResearchBlogging.orgMehta, R., Steinkraus, K., Sutphin, G., Ramos, F., Shamieh, L., Huh, A., Davis, C., Chandler-Brown, D., & Kaeberlein, M. (2009). Proteasomal Regulation of the Hypoxic Response Modulates Aging in C. elegans Science DOI: 10.1126/science.1173507

Most Alzheimer’s disease (AD) is sporadic, i.e., not the result of inheritance. Familial AD is relatively quite rare, but by studying heritable AD we’ve learned a disproportionately large amount about the genetic risk factors that predispose an individual to contract the disease.

One of the major risk factors for AD is mutation in the amyloid precursor protein (APP) gene. Mutant APP is more likely to be proteolytically cleaved into the β-amyloid (Aβ) form, which generates the amyloid fibrils and plaques that characterize AD pathology. Production of the toxic protein form is, genetically, speaking, a gain of function — so these APP mutations cause dominant inheritance of familial AD, i.e., a patient only needs one copy of the gene in order to have a very high risk of early-onset AD, and they’re 50% likely to pass this phenotype on to their children.

A newly discovered mutation, however, turns that inheritance pattern on its head. The A673V mutation in APP is associated with AD, but the inheritance pattern is recessive, i.e., a patient needs two mutant alleles in order to acquire the disease risk. In combination with wildtype allele, A673V doesn’t cause AD. Furthermore, the presence of the mutant protein prevents the wildtype protein from forming amyloid fibrils, even under very favorable in vitro conditions. Di Fede et al.:

A Recessive Mutation in the APP Gene with Dominant-Negative Effect on Amyloidogenesis

β-Amyloid precursor protein (APP) mutations cause familial Alzheimer’s disease with nearly complete penetrance. We found an APP mutation [alanine-673valine-673 (A673V)] that causes disease only in the homozygous state, whereas heterozygous carriers were unaffected, consistent with a recessive Mendelian trait of inheritance. The A673V mutation affected APP processing, resulting in enhanced β-amyloid (Aβ) production and formation of amyloid fibrils in vitro. Co-incubation of mutated and wild-type peptides conferred instability on Aβ aggregates and inhibited amyloidogenesis and neurotoxicity. The highly amyloidogenic effect of the A673V mutation in the homozygous state and its anti-amyloidogenic effect in the heterozygous state account for the autosomal recessive pattern of inheritance and have implications for genetic screening and the potential treatment of Alzheimer’s disease.

The basis of this effect is unknown — why would an amyloidogenic peptide fail to form fibrils simply because a non-amyloidogenic peptide is present? It’s tempting to speculate that is has to do with some aspect of the way in which Aβ proteins assemble into oligomers. Crystals form only from assemblies of like objects; by analogy, perhaps there are flavors of APP that can only form oligomers with their own kind — oligomers that are disrupted by the presence of other similar, but non-identical, monomers. At present we’ve got very little information to inform hypothesis building: amyloid fibrils are poor subjects for the standard techniques of structural biology, so their molecular details — and any clue about how this unusual mutant behaves — remain a mystery.

One experiment I’m dying to see: Do the A673V mutant proteins prevent other APP mutant proteins (the ones associated with the dominant form of familial AD) from forming fibrils?

(Weird: Found this one buried in my “Drafts” folder. I think I originally wrote it in early January. Better late than never, I suppose.)

The December issue of Psychology and Aging is devoted to the relationship between cognitive plasticity and aging. The pieces range from hardcore cellular neuroscience (“Stem-cell-associated structural and functional plasticity in the aging hippocampus“) to the consequences of video game training. Many of the articles address an important and often ignored issue: Is there anything we can do about age-related cognitive decline?

Here’s the abstract of the introductory article, by Ulrich Mayr:

Decades of cognitive aging research have led to a picture of the aging mind that is primarily characterized by gradual, though relatively broad, cognitive decline across the life span. Until recently, relatively little attention has been devoted to the question of whether there are ways to slow down, if not stop, this decline. With the special section on cognitive plasticity in the aging mind, we respond to what seems to be a beginning of the reversal of this trend (e.g., Kramer & Willis, 2002). In this short introduction, the author provides some context and a preview of the articles that appear in the special section.

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