Samstag, 13. Dezember 2014

Expanded Deletions vs Mutations: an unsolved Mystery

Based on a study I read, or rather skimmed. The problem of aging (stem) cells explained (1):
A significant increase in the frequency of respiratory chain deficient colonic crypts with age correlates with the frequency of mutations detected by [next generation sequencing]
But to this day we do not know why deletions accumulate in muscle and point mutations in the colonic epithelium.

Mechanism of Action (1): Speculation
Mathematical modelling studies have suggested that clonal expansion of mtDNA mutations within an individual cell is likely to be due to random genetic drift and predict that it can take at least 20 years for an mtDNA mutation to clonally expand to high levels sufficient to cause COX deficiency [37]–[39]....Previously we examined colonic epithelial tissue from a similar ageing mouse colony and showed that clonal expansion of mtDNA mutations was a very rare event in these animals compared with aged humans [41]. This may explain the species differences in these data, consistent with modelling studies that emphasise the difficulty of generating clonal expansion through random drift in short-lived animals [39].
The question is if COX negative mutations are additionally selected for, like in muscle? (2) If not, then why? Induction of mitochondrial biogenesis could definitely answer this question in a mouse model. If I had to speculate, I guess, the aetiopathogenesis might really be completely different between muscle and colonic mitochondrial dysfunction. Perhaps, the reason is that substantia nigra and muscle rely on OXPHOS (aerobic) whereas stem cells do not, thus deletions in muscle would not trigger a vicious cycle as in (2)?
The pattern of somatic mtDNA mutations detected in the buccal epithelium was similar to those in the colonic epithelium. The mtDNA mutations detected were base transitions and were randomly located throughout the genome.
If there is no preference for coding regions, I do think this favours clonal expanions by genetic drift.

1. PLoS Genet. 2014 Sep 18;10(9):e1004620. doi: 10.1371/journal.pgen.1004620. eCollection 2014.
Clonal expansion of early to mid-life mitochondrial DNA point mutations drives mitochondrial dysfunction during human ageing.
Greaves LC1, Nooteboom M2, Elson JL3, Tuppen HA2, Taylor GA2, Commane DM4, Arasaradnam RP4, Khrapko K5, Taylor RW2, Kirkwood TB6, Mathers JC7, Turnbull DM1.

2.  PLoS One. 2013;8(3):e59006. doi: 10.1371/journal.pone.0059006. Epub 2013 Mar 13. Mitochondrial biogenesis drives a vicious cycle of metabolic insufficiency and mitochondrial DNA deletion mutation accumulation in aged rat skeletal muscle fibers. Herbst A, Johnson CJ, Hynes K, McKenzie D, Aiken JM.

Freitag, 5. Dezember 2014

Parabiosis enabled mouse rejuvenation - will the data transfer to humans?

Two recent papers make you wonder, will the mouse aging data ever translate to humans? So far we have found out that mouse cancer data rarely applies to humans, but anti-aging interventions have never been tested in humans before.

Katsimpardi et al. 2014 (1a) (Wagers and Rubin labs)
GDF-11 leads to "Vascular and neurogenic rejuvenation of the aging mouse brain". Interestingly, the protein, which has been indentified in parabiosis experiments, is related to myostatin (!) and a "circulating transforming growth factor–β (TGF-β) family member". It also "reverses cardiac hypertrophy in aged mice" apparently mimicking many effects of actual parabiosis.

And as a side-note: "Systemic factors in old blood can have detrimental effects on hippocampal neurogenesis in young animals..."

Donnerstag, 4. Dezember 2014

The death of a Hypothesis: Cholesterol and Heart Disease (Ezetimibe edition)

Recent events have cast doubt on the well established "lipid hypothesis" of heart disease. In particular, the failure of Vytorin, aka ezetimibe, to lower intima media thickening. Instead the cholesterol absorption inhibitor appeared to worsen thickening, if it had any effect at all, in this rather small study. Of course, the lipid hypothesis is backed by other lines of evidence (1) but the most reliable evidence always comes from interventional studies.

So the key point of this controversy has been that no lipid lowering drug has ever reduced CVD (usually measured as a composite endpoint) in a large study, with the exception of statins. This means it would be conceivable that some other effect of statins is responsible for their protective effects, the so called pleiotropic effects of statins.

The controversy can be put to rest given the results of the  IMPROVE-IT study with n ~ 18 000. I will spare you the details and just link to two other reviews instead. (Note, that the arseholes from medscape may require you to register before you can read the summary)

Dienstag, 18. November 2014

Short notes: massively parallel in vivo screening; thoughts on senescence and telomeres

Recently, I attended two very useful seminars/talks. There are two kinds of talks. Those seminars that are helpful, but boring. And those that leave you amazed at every step. The boring ones can be useful when they give your mind time to wander, and ponder some minor detail of the talk, or think about your own research. These talks had a little of both worlds:

1. FunSel: Functional in vivo selection using adeno associated viruses [AAV].
AAV lead to efficient infection of post-mitotic tissues. Selection is based on a simple principle: if AAVs express protective proteins, they will be enriched in surviving cells. The main assumption is that there is some selection on the cell level, e.g. some cells die, others survive. If this works, you could imagine applying it to most diseases e.g. neurodegeneration, beta-cell death, muscle cell loss (sarcopenia), etc.
I am still searching for publications on this topic, as it appears that most of the research is still ongoing.

How could we adapt this technique to aging research if it works as promised? For example:
A. Using FunSel with a model of mitochondrial aging (the interested reader can figure out which one I mean)
B. More speculative: Using it for in vivo optimization of amino acid sequence and function, e.g. vector optimization

Montag, 3. November 2014

Age-related calcification: how prevalent is it in the animal kingdom?

It has been some time since I read about calcification. I can't access the below paper, but I find it fascinating just how prevalent this phenomenon seems to be across different species. Unfortunately, the wording of the abstract does not distinguish between idiopathic and age-related calcification. Perhaps I missed these papers, but I don't think age-related calcification has been studied (and shown to occur) in all these species. In fact, judging by the references most refer to the idiopathic type, but I have no time to read it in more depth. Edit: A brief e-mail exchange with the authors more or less confirmed my suspicion, their secondary/tertiary sources mostly deal with "regular" pathologic calcification.

Comp Med. 2014 Jun;64(3):224-9.
Extensive vascular mineralization in the brain of a chimpanzee (Pan troglodytes).
Connor-Stroud et al.

Spontaneous vascular mineralization (deposition of iron or calcium salts) has been observed in marble brain syndrome, mineralizing microangiopathy, hypothyroidism, Fahr syndrome, Sturge-Weber syndrome, cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, and calciphylaxis in humans and as an aging or idiopathic lesion in the brains of horses, cats, nonhuman primates, mice, rats, cattle, white-tailed deer, and dogs. Here we present a 27-y-old, adult male chimpanzee (Pan troglodytes) with spontaneous, extensive vascular mineralization localized solely to the brain. The chimpanzee exhibited tremors and weakness of the limbs, which progressed to paralysis before euthanasia. Magnetic resonance brain imaging in 2002 and 2010 (immediately before euthanasia) revealed multiple hypointense foci, suggestive of iron- and calcium-rich deposits. At necropsy, the brain parenchyma had occasional petechial hemorrhage, and microscopically, the cerebral, cerebellar and brain stem, gray and white matter had moderate to severe mural aggregates of a granular, basophilic material (mineral) in the blood vessels. In addition, these regions often had moderate to severe medial to transmural deposition of mature collagen in the blood vessels. We ruled out common causes of brain mineralization in humans and animals, but an etiology for the mineralization could not be determined. To our knowledge, mineralization in brain has been reported only once to occur in a chimpanzee, but its chronicity in our case makes it particularly interesting.

Mittwoch, 15. Oktober 2014

Short Notes: Methionine-mediated lifespan extension solely dependent on GH signalling?

Aging Cell. 2014 Sep 19. doi: 10.1111/acel.12269.
[Epub ahead of print]Growth hormone signaling is necessary for lifespan extension by dietary methionine.
Brown-Borg HM1, Rakoczy SG, Wonderlich JA, Rojanathammanee L, Kopchick JJ, Armstrong V, Raasakka D.

...Methionine intake affects also lifespan, and thus, GH mutant mice and respective wild-type littermates were fed 0.16%, 0.43%, or 1.3% methionine to evaluate the interaction between hormone status and methionine. All wild-type and GH transgenic mice lived longer when fed 0.16% methionine but not when fed higher levels. In contrast, animals without growth hormone signaling due to hormone deficiency or resistance did not respond to altered levels of methionine in terms of lifespan, body weight, or food consumption. Taken together, our results suggest that the presence of growth hormone is necessary to sense dietary methionine changes, thus strongly linking growth and lifespan to amino acid availability.

Methodological note: some of the control mice in this paper are somewhat short-lived, especially dwarf control and some GHRKO controls.

This reminds me of the Bartke papers showing how difficult (almost impossible) it is to further extend the lifespan of GHRKO mice by restricting them. Taken together these papers have the obvious implication that most life extending interventions (CR, MR possibly essential AA restriction) act through suppressed GH signalling or converge on the same effectors.

Overall, this means that we should seek to find interventions that extend LS on top of CR/GHRKO, if we want to identify novel pathwys.

Methionine moderation
Met moderation has been proposed as a feasible strategy to promote health span and lifespan (e.g. by Michael Rae, or Richie JP). In contrast, methionine moderation (0.43%) had no effect whatsoever in this study.