Heart


Telomere length is regarded as a potential biomarker of aging; there is a growing body of evidence indicating that shorter telomeres are associated with various diseases, including cancer, infectious disease, psychological stress, and cardiovascular disease. In cardiovascular disease in particular this had led to the hypothesis that premature, or accelerated, aging of the vasculature is a major contributing factor.

While a handful of papers have estimated telomere length in specific vascular tissues, the numbers and sizes of samples in these studies are usually small, due to the limitations in obtaining the tissues and cells of interest. Thus, telomere length in genomic DNA obtained from circulating leucocytes is routinely used as a proxy for telomere length in other tissues. However, the inflammatory processes involved in disorders such as cardiovascular disease can result in increased white cell turnover, raising the possibility that the shorter telomeres measured simply reflect recent replicative activity. As many telomere researchers (myself included) believe that shorter telomere length is a primary abnormality that contributes to increased risk of cardiovascular disease, evidence demonstrating a correlation in telomere length between matched samples (leucocyte and vascular DNA from the same individual) is always welcome. A recent paper presented at the European Society of Cardiology and published in European Heart Journal by Wilson et al. provides just such evidence:

Blood leucocyte telomere DNA content predicts vascular telomere DNA content in humans with and without vascular disease

Blood leucocyte telomere DNA content predicts vascular telomere DNA content in humans with and without vascular disease. Aims. Previous studies have suggested that reduced telomere length in circulating leucocytes in humans is associated with premature vascular disease and by implication, accelerated vascular ageing. Importantly, a link between telomere length in circulating leucocytes and the blood vessel wall has never been established. We, thus, investigated the relationship between vascular wall and circulating leucocyte telomere length in humans with and without overt vascular disease. Methods and results. Aortic biopsies and paired blood leucocytes were obtained from 20 patients with asymptomatic abdominal aortic aneurysms (AAAs), undergoing elective open repair, and 12 morphologically normal aortas from a group of cadaveric organ donors of similar mean age. Telomere content was compared by quantitative PCR and expressed as telomere:genomic DNA ratio. The telomere:genomic DNA content was significantly reduced in wall biopsies of AAA vs. normal aorta, and this difference remained after adjusting for age and gender. There were strong correlations between leucocyte and vascular telomere content when the AAA and control groups were analysed either separately or grouped irrespective of the presence of vascular disease (r = 0.62, P < 0.001). Conclusion. The findings demonstrate that leucocyte DNA content is predictive of vascular telomere content and is an accurate surrogate for human vascular age.

The authors showed that subjects with abdominal aortic aneurysms (AAA) had decreased telomere content in their vascular tissues, relative to tissues from subjects free of aortic disease, supporting the link between dysfunctional telomeres and dysfunctional vasculature. However, the difference in telomere content between healthy and diseased vascular tissue appears to be smaller than the difference between leucocytes from healthy and diseased subjects. Shorter telomeres in the vasculature may predispose to developing cardiovascular disease, but has increased white cell turnover in the systemic circulation of subjects with aortic disease led to the larger observed difference? If so, perhaps the differences in leucocyte telomere length between cases and controls in previous association studies of cardiovascular disease have resulted in a slight over-estimate of the differences in telomere length within the vasculature. Nevertheless, after adjusting for age there was a strong and highly significant positive correlation between tissue and leucocyte telomere length. This is the first study of its kind to demonstrate that “telomere attrition in blood leucocytes is indicative of similar changes in the vascular wall”.

So, is leucocyte telomere length a suitable proxy? The issue of increased white cell turnover in subjects with cardiovascular disease remains critical; adjustments for markers of inflammation (something that was not performed in the present study) are crucial in downstream analysis and interpretation of results if the impact of cell turnover (and the associated telomere attrition) is to be assessed. However, the data presented certainly indicate that differences in telomere length observed in circulating blood cells are reflected in the vasculature, providing support for the validity of previous association studies.

One major barrier to the therapeutic use of pluripotent and totipotent cells is that by the time a patient needs them, their body has become less able to use them. The stem cell niche (i.e., those factors in the tissue microenvironment that stem cells require in order to function normally) changes with age, and not for the better: for example, embryonic stem cells lose proliferative capacity when confronted with aged niches.

This appears to be a general problem in metazoans, and is conserved between humans and relations as distant as arthropods — fortunately for us, because it means that the tools and genius of the Drosophila community can be brought to bear on the problem. In the fruit fly, age-related changes in the stem cell niche are well-documented, especially in the reproductive system, and the molecular players are starting to be individually identified (see our previous post on Dpp, this one on BMP, unpaired and cadherins, and this nice review of the whole story). There are one or two tissues in which stem cells actually become more numerous with age, but the consensus seems to be that the aged microenvironment is generally not beneficial for stem cells. At least in the fly.

But what about species nearer and dearer to us? Fortunately, the human case is starting to be fleshed out in equally fine detail, and the state of the art has been thoroughly and artfully reviewed by Stefanie Dimmeler and Annarosa Leri. (The article is open access, so the full text is available to everyone.) The authors focus on the heart but also address more general (and less tissue-specific) issues along the way:

Aging and Disease as Modifiers of Efficacy of Cell Therapy

Cell therapy is a promising option for treating ischemic diseases and heart failure. Adult stem and progenitor cells from various sources have experimentally been shown to augment the functional recovery after ischemia, and clinical trials have confirmed that autologous cell therapy using bone marrow—derived or circulating blood–derived progenitor cells is safe and provides beneficial effects. However, aging and risk factors for coronary artery disease affect the functional activity of the endogenous stem/progenitor cell pools, thereby at least partially limiting the therapeutic potential of the applied cells. In addition, age and disease affect the tissue environment, in which the cells are infused or injected. The present review article will summarize current evidence for cell impairment during aging and disease but also discuss novel approaches how to reverse the dysfunction of cells or to refresh the target tissue. Pretreatment of cells or the target tissue by small molecules, polymers, growth factors, or a combination thereof may provide useful approaches for enhancement of cell therapy for cardiovascular diseases.

The bad news about the impact of aging and disease factors on the prospects for cell therapy is tempered with good news — the final quarter of the piece is devoted to therapeutic strategies for overcoming the negative effects of the aged niche, with the ultimate goal of using stem cells even in suboptimal tissue microenvironments — i.e., in patients who need them.

Do DNA double-strand breaks (DSBs) have anything to do with aging?

We have some reason to believe that they do. in yeast, longevity-enhancing mutations suppress genomic instability. Conversely, in the rodent heart, aging animals show evidence of sporadic large-scale genomic rearrangements, which could be caused by misrepair of DSBs. These rearrangements are thought to contribute to transcriptional “noise,” which in turn could explain some age-related decline in tissue function.

On the other hand, genomic rearrangements do not always occur as a concomitant of aging, even within the same organism: in mouse, the rearrangements are visible in cardiomyocytes, but are absent in actively dividing tissues like the bone marrow.

Paul Hasty has written two recent reviews, critically evaluating the role of DNA DSBs in the aging process. In the first (written with colleagues Han Li and James Mitchell), the authors argue from genetic evidence that DSB repair pathways are intimately connected with aging, but that the relationship is distinct from the well-documented connection between aging and repair of UV-type damage. (Mutants the nucleotide- and base-excision pathways involved in repair of single strand lesions are well documented to exhibit segmental progerias, in some cases entering a state somewhat resembling the response to caloric restriction in a last-ditch attempt to defend the body against accumulating DNA damage.) (link):

DNA double-strand breaks: A potential causative factor for mammalian aging?

… There are many forms of DNA damage with double-strand breaks (DSBs) being the most toxic. Here we discuss DNA DSBs as a potential causative factor for aging including factors that generate DNA DSBs, pathways that repair DNA DSBs, consequences of faulty or failed DSB repair and how these consequences may lead to age-dependent decline in fitness. At the end we compare mouse models of premature aging that are defective for repairing either DSBs or UV light-induced lesions. Based on these comparisons we believe the basic mechanisms responsible for their aging phenotypes are fundamentally different demonstrating the complex and pleiotropic nature of this process.

In the second review, Hasty (this time writing alone) argues that non-homologous end-joining NHEJ), a major pathway of DSB repair, evolved primarily as a means to slow aging — rather than to prevent cancer, as is likely the case for other DNA repair pathways. One corollary of this is that mutants in NHEJ either display no increase in cancer or a very small one (which Hasty argues is either incidental to accelerated aging or altogether artifactual). (link):

Is NHEJ a tumor suppressor or an aging suppressor?

… Nonhomologous end joining (NHEJ) is considered a caretaker since it repairs DNA double-strand breaks that would otherwise lead to gross chromosomal rearrangements (GCRs). NHEJ-mutant mice display increased GCRs, but without increased cancer. Instead these mice show early aging. This commentary focuses on the role NHEJ has on aging and cancer. I propose that NHEJ evolved to reduce GCRs and moderate gatekeeper responses that would otherwise cause early aging. Furthermore, NHEJ did not evolve to suppress tumors and any observed tumor suppression is merely circumstantial to unnatural laboratory conditions coupled with human bias that favors defining all DNA repair pathways as caretakers.

If it does turn out that NHEJ mutants have increased genomic rearrangements and also increasing transcriptional noise, it could go a long way toward bolstering the theory of aging described above.

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.

In the absence of any original blogging from me today, I’m going to beseech the readers to check out the ongoing series on cellular senescence over at Ageing Research. Dominick has now turned to the relationship between senescence and human disease states, focusing first on atherosclerosis and vascular calcification (it’s a four-part series: 1 2 3 4).

For those of us working on senescence, Dominick’s ambitious and thus far unflagging efforts to review the entire field are sure to generate a gold mine, and perhaps the gold standard online reference on this subject. As I read about the disease connections, some of which I hadn’t known about prior to now, I found myself salivating, thinking, I will never again have to do a literature search to write a “medical significance” section of a grant…

What are you still doing here?

Gestational protein restriction (i.e., limiting the intake of a pregnant female) has a powerful influence on a number of aging-related biomarkers, including DNA damage, antioxidant defenses and telomere length. These findings have important implications for our understanding of the relationship between development and calorie restriction (CR), and major ramifications for anyone considering both CR and pregnancy. Caveat: the study was performed in rats, but then again, most CR studies rely on rodent models, so this is at least as germane as other studies describing the other side of the CR coin.

Insulin-like Growth Factor-I (IGF-I) and insulin itself have been convincingly implicated in the genetic control of lifespan. Mutants in the worm gene DAF-2 (a homolog of IGF-I receptor) are long-lived, and disruptions in insuling signaling also boost longevity in mammals. It’s not yet clear whether IGF-I mutations extend lifespan via the same mechanisms as calorie restriction (CR); the evidence is piling up in favor of independent mechanisms with some degree of convergence (q.v. this excellent recent paper from Iser and Wolkow, a careful genetic dissection of the phenotypic interactions between daf-2 and CR in the worm). Regardless of mode of action, biogerontologists widely accept that loss of function in insulin and insulin-like pathways increases lifespan in a wide range of organisms.

It is therefore troubling that the outcome of decreased IGF-I levels on specific organ systems is so uniformly negative: low IGF-I seems to slow down brain activity; high IGF-I paradoxically seems to delay cardiac aging even as it shortens the lifespan of a particular heart’s owner. To this list of perplexities add a recent report that IGF-I supplementation in elderly rodents cures a whole host of metabolic ills. From García-Fernández et al.:

Low Doses of Insulin-Like Growth Factor I improve insulin resistance, lipid metabolism, and oxidative damage in Aging Rats

Growth Hormone (GH) and Insulin-like Growth Factor I (IGF-I) concentrations decline with age. Age-related changes appear to be linked to decreases in the anabolic hormones, GH and IGF-I.

The aim of this study was to investigate the antioxidant, anabolic and metabolic effects of the IGF-I replacement therapy, at low doses, in aging rats. …

Compared with young controls, untreated aging rats showed a reduction of IGF-I and testosterone levels and a decrease of serum total antioxidant status, which were corrected by IGF-I therapy. In addition, untreated old rats presented increased levels of serum glucose with hyperinsulinemia, cholesterol and triglycerides and a reduction of free fatty acid concentrations. IGF-I therapy was able to revert insulin resistance and to reduce cholesterol and triglycerides levels increasing significantly free fatty acid concentrations.

Old rats showed higher oxidative damage in brain and liver tissues associated with alterations in antioxidant enzyme activities. IGF-I therapy reduced oxidative damage in brain and liver, normalizing antioxidant enzyme activities and mitochondrial dysfunction.

In conclusion, low doses of IGF-I restore circulating IGF-I, improve glucose and lipid metabolism, increase testosterone levels and serum total antioxidant capability and reduce oxidative damage in brain and liver associated with a normalization of antioxidant enzyme activities and mitochondrial function.

This is exasperating: Why should a hormone that is neuroprotective and cardioprotective, and that prevents oxidative damage, insulin resistance and mitochondrial dysfunction, shorten the overall lifespan?

I’m tempted to suspect that the answer lies in developmental timing. The IGF-I mutants studied thus far in lifespan studies have been non-conditional hypomorphs, hemizygotes or full knockouts — in other words, the mutations were present over the entire lifespan of the organism. (Note that the brain study cited above used an adult-onset model of IGF-I deficiency, confounded somewhat by a simultaneous adult-onset loss of growth hormone, GH.)

If IGF-I has some effect in early life that sets the body on a course for a short lifespan, but is beneficial at all other times, then it might be possible to reconcile the seemingly paradoxical observations that IGF-I both accelerates aging and mitigates age-related disease. This is not as much of a stretch as it might first appear: we already know that that early-life gene expression can control late-life outcomes (q.v. McCarroll et al., who demonstrated that most of the evolutionarily conserved transcriptional program of aging is implemented in early adulthood).

Both for the fundamental science of biogerontology and for the human health implications, it seems essential to parse the contributions of IGF-I to programming short lifespans on the one hand, and protecting against age-related decline on the other.

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