Telomeres


(previous session)

Matt Hirschey (Verdin Lab, UCSF-Gladstone): Lack of SIRT3 results in the metabolic syndrome. SIRT3 is a mitochondrial sirtuin (NAD+-dependent deacetylase) that is upregulated in liver upon fasting; knockout mice (SIRT3KO) are grossly normal but have trouble with lipid metabolism (specifically, beta-oxidation). Hershey identified several mitochondrial proteins involved in lipid oxidation that are deacetylated in response to fasting, in wildtype but not SIRT3KO. The knockouts are prone to developing obesity and metabolic syndrome with age.

Kate Brown (Chen lab, UC-Berkeley): Calorie restriction reduces oxidative stress by inducing SIRT3. Beginning with an invocation of the free radical theory of aging, and the observation that calorie restriction (CR) reduces oxidative stress, Brown asked whether the mitochondrial sirtuin SIRT3 could be involved in resistance to reactive oxygen species. She showed that CR induces SIRT3 expression, and that the SIRT3 protein deacetylates the mitochondrial antioxidant enzyme SOD2. Furthermore, consistent with Subhash Katewa’s talk in the first session, she demonstrated that CR reduces oxidative stress by switching from glucose to fatty acid oxidation, and that this switch requires SIRT3 activity.

(We’ve discussed SIRT3 before, most recently regarding its role as a tumor suppressor and also with respect to its relationship with exercise).

Ruth Tennen (Chua lab, Stanford): Insight into SIRT6 function at telomeres and beyond. Another member of the sirtuin family, SIRT6, is not localized to mitochondria but rather to telomeres, where it maintains telomeric chromatin in a healthy state and regulates the activity of the senescence-associated transcription factor NF-κB – for more background, see this previous post.) Tennen has shown that SIRT6 is involved in regulating the telomere position effect (TPE) – the silencing of gene expression caused by proximity to a telomere. The TPE has been implicated in age-related changes in gene expression: as telomeres shorten over time, telomere-proximal genes are aberrantly expressed — meanwhile, silencing factors are liberated to wander throughout the genome, repressing genes that should be turned on; similar logic has been applied to the relationship between DNA damage and transcriptional dysregulation.

Jue Lin (Blackburn Lab, UCSF): Telomere length maintenance and aging-related diseases. This talk described work that builds on significant progress, from this lab and others, demonstrating relationships between telomere length and stress, psychological outlook, and lifespan. Lin reviewed evidence that perceived stress is correlated with telomere length in white blood cells (consistent with previous results showing a relationship with intrusive thoughts). New-to-me data included a demonstration that people who increased omega-3 levels or made favorable lifestyle changes exhibited a slower rate of telomere shortening.

(next session)

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.

ResearchBlogging.orgAtzmon, 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:

Telomere length shortening occurs with age in most cells types, and shorter telomeres are associated with increased risk of age-related diseases, including cardiovascular disease. The majority of studies to-date analyze mean telomere length in peripheral leukocytes, as material is plentiful and circumvents the practical and ethical problems associated with obtaining vascular tissue. Telomere length is strongly correlated between leukocytes and vascular tissues, validating the use of leukocyte telomere length as a proxy for length in vascular cells. However, leukocytes comprise a number of cell types; so do they all “age” at the same rate?

In a step towards answering this question Hoffmann et al. examined telomere length in lymphocytes and granulocytes separately:

Telomere length-heterogeneity among myeloid cells is a predictor for chronological ageing.

We have previously shown that mean telomere length (TL) of granulocytes reflects TL of myeloid bone marrow cells extremely well. Here we analysed the distribution of TL from peripheral blood granulocytes and lymphocytes in 61 female and 68 age-matched male healthy volunteers. We show that the age-dependent decline in TL occurs more rapid in peripheral blood lymphocytes (53 bp/year) than in granulocytes (39 bp/year; p<0.001), while women having longer telomeres than men. We also observed the best correlation for age and telomere length to be present in lymphocytes. The coefficient variation (CV) of TL distribution in granulocytes and monocytes was significantly higher in older compared to younger individuals, indicating an increase in telomere length heterogeneity of myeloid cells and their bone marrow residing precursors during ageing. In a multivariate statistical model, CV and lymphocyte TL were able to explain 50% of chronological ageing.

Telomere attrition is significantly faster in lymphocytes than in granulocytes, with lymphocyte telomere length correlating most strongly with age. Furthermore, the coefficient of variation (CV; essentially an indication of the distribution of telomere lengths) increased significantly with age in both cell types. As the authors have previously reported that telomere length in granulocytes reflects that of myeloid bone marrow precursors, the present study implies that telomere length heterogeneity in bone marrow residing precursors also increases with age.

These findings imply that if two subjects have the same mean telomere length, the subject with increased heterogeneity (i.e., the older one) is likely to have a greater proportion of shorter telomeres in the stem cell compartment. The presence of just one critically shortened telomere may be sufficient to induce senescence (though this hypothesis is controversial; hence, telomere length heterogeneity may provide useful information that mean telomere length alone cannot.

The impact of sex on telomere length is the subject of some debate, with some studies reporting longer telomeres in women, while others show no difference. The present study may shed a little light on the subject: women were found to have longer lymphocyte telomere lengths, while granulocyte length did not differ. Thus, the relative number of each cell type in prior studies may skew interpretation of the data.

All subjects in this study were physically fit, walking or exercising “at least 20 km/week”; the authors stress this point in the methods, but fail to mention it in the discussion. This is disappointing in light of a recent high-impact study demonstrating that exercise improves the cardiovascular risk profile, increasing telomerase activity in peripheral blood mononuclear cells. The study may have aimed to examine the “healthiest” possible subjects, but as a consequence the results may not be representative of the general population.

Overall, this study highlights the importance of analyzing cell types individually to avoid the impact of differences in the ratios of cell type on interpreting telomere length data from peripheral blood. Furthermore, it adds telomere length heterogeneity to the list of important factors requiring further study.

Since the first report of induced pluripotent stem cells in 2006, the field of regenerative medicine has been buzzing about the potential for such cells to provide a source of cells that avoid the ethical minefield that plagues the use of embryonic stem cells.

The original paper demonstrated that simply over-expressing four “reprogramming” factors (4F) allowed reprogramming of differentiated mouse and human cells into iPS cells, but a thorough characterization of the resulting cells is still underway. iPS cells exhibit telomerase activity (as do ES cells), but whether this is sufficient to restore telomere length, or if telomeric chromatin acquires ES-like characteristics, remains unclear.

To address these issues, a team led by Maria Blasco generated iPS cells using either the standard 4 factors (4F), or omitting cMyc (3F), from wild-type and telomerase-deficient mice (both young and old), and investigated various aspects of telomere dynamics:

Telomeres Acquire Embryonic Stem Cell Characteristics in Induced Pluripotent Stem Cells

Telomere shortening is associated with organismal aging. iPS cells have been recently derived from old patients; however, it is not known whether telomere chromatin acquires the same characteristics as in ES cells. We show here that telomeres are elongated in iPS cells compared to the parental differentiated cells both when using four (Oct3/4, Sox2, Klf4, cMyc) or three (Oct3/4, Sox2, Klf4) reprogramming factors and both from young and aged individuals. We demonstrate genetically that, during reprogramming, telomere elongation is usually mediated by telomerase and that iPS telomeres acquire the epigenetic marks of ES cells, including a low density of trimethylated histones H3K9 and H4K20 and increased abundance of telomere transcripts. Finally, reprogramming efficiency of cells derived from increasing generations of telomerase-deficient mice shows a dramatic decrease in iPS cell efficiency, a defect that is restored by telomerase reintroduction. Together, these results highlight the importance of telomere biology for iPS cell generation and functionality.

The key findings were (1) in telomerase-competent cells, telomere lengthening occurs via telomerase extension (rather than via a recombination), and (2) telomeric chromatin acquires ES-like characteristics. Furthermore, cMyc (one of the original 4 transcription factors used to generate iPS cells ) is dispensible for telomerase activation in mouse iPS cells (telomerase activity was only marginally lower in the absence of cMyc).

The authors also derived iPS cells from telomerase-deficient G1 (first generation) mice. It became clear that while telomerase activity is not limiting for in vitro iPS cell proliferation when telomeres are long (as is the case in G1 mice), these cells were nonetheless severely impaired in their ability to generate viable mice. Furthermore, the efficiency of iPS cell generation from telomerase-deficient G2 and G3 mice dropped significantly, indicating that telomere shortening is a critical barrier to iPS cell generation. Consistent with this, the cells exhibited an increased number of signal-free ends and chromosome end-to-end fusions, both events that are common in cells with very short telomeres. Crucially, re-introduction of telomerase into G3 telomerase-deficient mice restored iPS cell production efficiency, despite the cell inheriting short telomeres from their G2 deficient parents.

Overall, these findings highlight the role that telomere/telomerase dynamics play in successful iPS cell generation and provide evidence that cells from older donors are suitable. The caveat is that telomerase activity is vital if iPS cells are to be generated from cells with short telomeres. (Obviously!)

Two special journal issues of note to biogerontologists:

I included the ISBNs because publishers have an annoying way of using very temporary, dynamic links to journal issues (so far as I know, there’s no DOI-equivalent yet for specific issues of a given journal), so I am betting those links will be broken in a month or so.

Just reminding you that the abstract deadline for the CSHL meeting on “Telomeres and telomerase” is tomorrow, February 6th. The conference itself starts on April 28th.

Here’s the latest in our series of review roundups — links, without extensive further comment, to the reviews I found most intriguing over the past few weeks. For the previous foray into the secondary literature, see here.

Cancer:

Endocrinology:

IGF-1:

Inflammation:

Mitochondria:

Sarcopenia:

Stem cells:

Telomeres:

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