Interestingness: 7
tl;dr 20% lifespan increase from mitochondrially hyperexpressed catalase
By Samuel E Schriner, Nancy J Linford, George M Martin,Piper Treuting, Charles E Ogburn, Mary Emond, Pinar E Coskun, Warren Ladiges, Norman Wolf, Holly Van Remmen, Douglas C Wallace and Peter S Rabinovitch in Science 308, 1909 (2005). doi: 10.1126/science.1106653
This was not published in Rejuvenation Research. It was published in Science. It was commented about by Richard Cutler in issue 3 of 2005 of Rejuvenation Research, but I couldn't find access to the comments. I did find this one though, and since it is a pretty famous result, I read it.
They overexpressed human catalase in mice in six separate lineages, two in their peroxisome, two in their nucleus and two in their mitochondria. That is, there are four different mice lineages per experiment, two for controls, and two with the intervention, one corresponding to each of the controls. They show results per lineage with respect to their corresponding control, and I'll write them down like that too since I can't think of a better way. In all cases, the amount of catalase expressed is very high compared to wild type.
Going by the graphs, control median lifespan was about 26-27 months, and maximum lifespan (age at 10% survival, not average lifespan of top 10% like they measure in the paper (because I don't have the raw data)) at around 33 months. Expression in the nucleus extended median lifespan by 1 and 3 months (p > 0.05) with no increase in maximum lifespan. Expression in the peroxisome increased median by 3 (p > 0.05) and 3.5 months (p < 0.02) with no increase in maximum. Expression in the mitochondria increased median by 4.5 months (p < 0.0001) and 5.5 months (p < 0.0002) with maximum lifespan increases of 4.5 months (p < 0.001 combined lineages)
Most of the paper focuses on the mitochondrial branch since it's the most impressive. In the mitochondrial branch, there's equivalent lifespan increases for males and females. They also observe lots of good shit happening to the heart (less heart disease in general).
In what seems like a side-experiment they cross the peroxisome-expressing mice with a superoxide-dismutase expressing mice, and they get a 18.5% (p < 0.0001) median life extension with respect to wild type and 7% (p=0.036) compared to the peroxisome mice, but no maximum life extension. They note that the mitochondrially expressing mice would be a better one to try.
By the way, superoxide anion O2- goes to hydrogen peroxide H2O2 helped by superoxide dismutase. Hydrogen peroxide goes hammertime unless defused by catalase (or glutathione peroxidase).
Showing posts with label mitochondria. Show all posts
Showing posts with label mitochondria. Show all posts
Tuesday, February 5, 2013
Sunday, February 3, 2013
Reactive oxygen species production in the mitochondrial matrix: implications for the mechanism of mitochondrial mutation accumulation
Interestingness: 3
By Aubrey D.N.J. De Grey. Rejuvenation Research. Spring 2005, 8(1): 13-17. doi:10.1089/rej.2005.8.13.
Comparison between three different theories of mitochondrial DNA mutation, and the how to interpret new findings that alpha-ketoglutarate dehydrogenase (AKDH), an enzyme in the mitochondrial matrix, makes hydrogen peroxide and possibly superoxide when exposed to high concentrations of NADH.
One of the theories is the vicious cycle hypothesis, in which mutations in the mtDNA trigger creation of superoxides which then trigger more mtDNA mutations, etc. Another is de Grey's own survival of the slowest (SOS) hypothesis described earlier in the blog. The third is the crippled mitochondrion (CM) (underspecified) hypothesis in which mutant mitochondria are stimulated to replicate by some mechanism internal to the mitochondria.
de Grey claims that the vicious cycle theory is refuted by the commonality of mutations that would get rid of the possibility to make superoxide (mtDNA deletions that get rid of the genes encoding for Complex I and III, and also get rid of at least one tRNA, for which there are no redundancies in the mtDNA), and also by the observation that the mutations in all the mitochondria's DNA tend to be the same within any one individual cell.
Differences in predictions by the other two theories:
That last prediction by the SOS hypothesis, lack of ROS production, seems to run counter with findings that oxidation-damaged DNA and RNA are found in respiration-deficient muscle fiber segments. This is the reason for bringing up the new findings of generation of ROS by AKDH when under high concentration of NADH, a state de Grey claims would be more common in mitochondria with broken respiratory chains, giving the oxidised DNA an alternate cause.
By Aubrey D.N.J. De Grey. Rejuvenation Research. Spring 2005, 8(1): 13-17. doi:10.1089/rej.2005.8.13.
Comparison between three different theories of mitochondrial DNA mutation, and the how to interpret new findings that alpha-ketoglutarate dehydrogenase (AKDH), an enzyme in the mitochondrial matrix, makes hydrogen peroxide and possibly superoxide when exposed to high concentrations of NADH.
One of the theories is the vicious cycle hypothesis, in which mutations in the mtDNA trigger creation of superoxides which then trigger more mtDNA mutations, etc. Another is de Grey's own survival of the slowest (SOS) hypothesis described earlier in the blog. The third is the crippled mitochondrion (CM) (underspecified) hypothesis in which mutant mitochondria are stimulated to replicate by some mechanism internal to the mitochondria.
de Grey claims that the vicious cycle theory is refuted by the commonality of mutations that would get rid of the possibility to make superoxide (mtDNA deletions that get rid of the genes encoding for Complex I and III, and also get rid of at least one tRNA, for which there are no redundancies in the mtDNA), and also by the observation that the mutations in all the mitochondria's DNA tend to be the same within any one individual cell.
Differences in predictions by the other two theories:
| SOS | CM |
|---|---|
| Autophagy of mitochondria selects mitochondria with damaged membranes | No prediction |
| Loss of ATP-synthase does not preferentially replicate | All functional losses replicate |
| No prediction | Cell overpopulated with mitochondria comes before or at the same time as loss of respiratory function |
| ROS production by mutant mitochondria eliminated | No prediction |
That last prediction by the SOS hypothesis, lack of ROS production, seems to run counter with findings that oxidation-damaged DNA and RNA are found in respiration-deficient muscle fiber segments. This is the reason for bringing up the new findings of generation of ROS by AKDH when under high concentration of NADH, a state de Grey claims would be more common in mitochondria with broken respiratory chains, giving the oxidised DNA an alternate cause.
Thursday, January 31, 2013
Selective Mitochondrial Autophagy, or Mitophagy, as a Targeted Defense Against Oxidative Stress, Mitochondrial Dysfunction, and Aging
Interestingness: 6
By Dr. John J. Lemasters. Rejuvenation Research. Spring 2005, 8(1): 3-5. doi:10.1089/rej.2005.8.3.
Short speculative piece about the importance of a mitochondrial outer membrane protein, Uth1p, in aging. Yeast cells with mutant versions of the gene have lower levels of mitochondrial autophagy when induced by rapamycin and starvation. If this means that the particular mitochondria that don't express Uth1p are not recycled, then it would give an "evolutionary" advantage for mitochondria to have mutant versions of this gene, as long as they are picked as often as others for mitochondrial replication. If this is the case, then other mutations in the same mitochondrion as ones with Uth1p mutations would proliferate throughout the cell.
As a downer, it seems there isn't (wasn't?) much information about the human analogue/s of the gene.
By Dr. John J. Lemasters. Rejuvenation Research. Spring 2005, 8(1): 3-5. doi:10.1089/rej.2005.8.3.
Short speculative piece about the importance of a mitochondrial outer membrane protein, Uth1p, in aging. Yeast cells with mutant versions of the gene have lower levels of mitochondrial autophagy when induced by rapamycin and starvation. If this means that the particular mitochondria that don't express Uth1p are not recycled, then it would give an "evolutionary" advantage for mitochondria to have mutant versions of this gene, as long as they are picked as often as others for mitochondrial replication. If this is the case, then other mutations in the same mitochondrion as ones with Uth1p mutations would proliferate throughout the cell.
As a downer, it seems there isn't (wasn't?) much information about the human analogue/s of the gene.
Monday, August 27, 2012
Mitochondrial Mutations in Mammalian Aging: An Over-Hasty About-Turn?
Interestingness: 7
By Aubrey DNJ De Grey, in Rejuvenation Research, Northern Autumn 2004, 7(3): 171-174. doi:10.1089/rej.2004.7.171.
A short piece of commentary trying to raise the profile of, I think, this paper: "Construction of transgenic mice with tissue-specific acceleration of mitochondrial DNA mutagenesis", by Zhang D et al (http://www.ncbi.nlm.nih.gov/pubmed/11031098), but mainly about lowering the importance of "Premature ageing in mice expressing defective mitochondrial DNA polymerase" by Trifunovic A et al (http://www.ncbi.nlm.nih.gov/pubmed/15164064 ) . I haven't read the former, and I've only read the latter just now (separate post).
Zhang's paper is about mice with a mutant mtDNA polymerase which produced more errors, but is only activated in the heart, and only after birth. Trifunovic's paper is a similar concept but they made it active in all tissue. Also, while in Trifunovic's experiment they modified the built-in version of the mtDNA polymerase, I am not sure if that is what was done in Zhang's version, or if they just added a second copy.
The results of the modifications are that in Zhang's, the mice get cardiomyopathy, while Trifunovic's mice get fucked up all over and die at around the year mark.
de Grey's objection to the importance of Trifunovic's paper is that the effects seem to hit mainly tissue undergoing rapid replication (the spleen, the skin, testes and blood), and that the heart issues were a compensation mechanism for the anaemia. His claim is that this failure of rapidly replicating tissue cannot cause a shortening of lifespan since the failing cells can just be replaced by the replication of the still-good cells, unless the cells are failing faster than they can be replaced, which is what he thinks is happening during the experiment.
It would be good if he'd have addressed specifically the coincidence that in both cases the mice suffered from enlarged hearts, but according to him due to separate causes (ie is it a coincidence?). I don't know the timing of the cardiomyopathy in the Zhang experiment though, so it might have come around much later and there might not be anything to address.
By Aubrey DNJ De Grey, in Rejuvenation Research, Northern Autumn 2004, 7(3): 171-174. doi:10.1089/rej.2004.7.171.
A short piece of commentary trying to raise the profile of, I think, this paper: "Construction of transgenic mice with tissue-specific acceleration of mitochondrial DNA mutagenesis", by Zhang D et al (http://www.ncbi.nlm.nih.gov/pubmed/11031098), but mainly about lowering the importance of "Premature ageing in mice expressing defective mitochondrial DNA polymerase" by Trifunovic A et al (http://www.ncbi.nlm.nih.gov/pubmed/15164064 ) . I haven't read the former, and I've only read the latter just now (separate post).
Zhang's paper is about mice with a mutant mtDNA polymerase which produced more errors, but is only activated in the heart, and only after birth. Trifunovic's paper is a similar concept but they made it active in all tissue. Also, while in Trifunovic's experiment they modified the built-in version of the mtDNA polymerase, I am not sure if that is what was done in Zhang's version, or if they just added a second copy.
The results of the modifications are that in Zhang's, the mice get cardiomyopathy, while Trifunovic's mice get fucked up all over and die at around the year mark.
de Grey's objection to the importance of Trifunovic's paper is that the effects seem to hit mainly tissue undergoing rapid replication (the spleen, the skin, testes and blood), and that the heart issues were a compensation mechanism for the anaemia. His claim is that this failure of rapidly replicating tissue cannot cause a shortening of lifespan since the failing cells can just be replaced by the replication of the still-good cells, unless the cells are failing faster than they can be replaced, which is what he thinks is happening during the experiment.
It would be good if he'd have addressed specifically the coincidence that in both cases the mice suffered from enlarged hearts, but according to him due to separate causes (ie is it a coincidence?). I don't know the timing of the cardiomyopathy in the Zhang experiment though, so it might have come around much later and there might not be anything to address.
Wednesday, March 16, 2011
Noncorrelation Between Maximum Life Span and Antioxidant Enzyme Levels Among Homeotherms: Implications for Retarding Human Aging
Summary: de Grey thinks that we need superoxide dismutase in our mitochondrial intermembrane spaces
Interestingness: 6
Paper by Aubrey DNJ de Grey in the Journal of Anti-Aging Medicine, Volume 3, Issue 1, Spring 2000.
(((
This is another theory/speculation paper by de Grey. He's trying to explain why there is no correlation between life span and antioxydant enzyme levels in warm blooded animals. His explanation sticks to the oxidative damage is bad, m'kay, trend and tries to fit the model to the data.
He partitions the rate of damage created by oxidation into four:
He notes that under this scheme, levels of antioxidant vitamins affect the OT and not the SP, since they act mainly to stop the chain of oxidation. SP is the rate factor that is not correlated with lifespan that is being explained in this paper.
SMR in warm blooded animals (homeotherms) is mostly determined by body size, and lifespan does correlate with body size. Lifespan depends not only on size though but seems to be well correlated with lifespan restrictions imposed by external causes. There isn't much evolutionary pressure to raise the aging-based lifespan of the animal if it is likely to die from other causes (eg getting eaten). Animals of similar weight but different chances of dying due to external causes have different lifespans (eg birds vs mammals).
In a study in primates, superoxide dismutase (SOD) did correlate with lifespan if the SOD levels were divided by the SMR when doing the calculation. Catalase, glutathione peroxidase (GP) and glutathione (G) didn't though. A less dodgy comparison, in that it didn't need the division by SMR factor, was one between rats and pigeons. Pigeons live about 8 times longer than rats even though they weight about the same. SOD levels in the pigeon were slightly higher, catalase much lower, and G and GP levels varied depending on the tissue. Another study showed similar results when looking at the canary (very low mass, very high lifespan), with not even SOD showing higher levels.
In the same studies, they showed a correlation between leakiness and rate of aging, and maybe one between OT and rate of aging. Lending support to this second correlation, de Grey mentions that fatty acids saturation in the membranes of the mitochondria and levels of non-enzymatic anti-oxidants (eg vitamins C and E) are higher in longer lived animals and these lower OT. This supposedly leaves SP as the only one out of the four factors that doesn't correlate in the predicted way with longevity.
de Grey's hypothesis to explain this is that there is no easy way for evolution to lower the SP because there are no SOD enzymes in the mitochondrial intermembrane space (MIMS) to mop up the superoxides. The selection for longer lifespan instead pushes the concentrations of non-enzymatic antioxidants (vitamins C and E) up all over the cell to get levels up in the MIMS, and the leakiness of the MIMS down which is the same mechanism that controls leakiness elsewhere in the chain. This then means that the concentrations of antioxidant enzymes in the non-MIMS regions become too high for the resulting lower radicals due to the improved leakiness and non-enzymatic antioxidant profiles, and these enzymatic antioxidant levels drift down to save resources until they match the levels that would lead to the same rate of damage as the other parts of the oxidation chain.
Since that paragraph contained the whole hypothesis I will write it again, but in expanded form. Homeotherms supposedly don't have any SODs in their MIMS but we do produce superoxides there (the evidence for that second part is probably not great). The damage caused by this, somehow (more on this later), limits our lifespan. For intelligent or otherwise flighty animals, where the external causes of dying are lower, there is a selective pressure, apparently, to raise our lifespan due to aging to match the lower external causes. Since it seems to be troublesome to introduce a SOD into our MIMS (and this supposed trouble to evolve a MIMS-SOD is the bit that to me seems weakest out of the chain of reasoning), homeotherms instead reduce the leakiness of the ATP-making mechanism, the leakiness factor, and raise the levels of non-enzymatic anti-oxidants, lowering the OT factor, to lower the total rate of aging. Now, lowering the leakiness of the process lowered the production of oxidants everywhere, not just in the MIMS, and raising the level of non-enzymatic anti-oxidants did the same everywhere, not just in the MIMS, so now, if we kept the same level of enzymatic anti-oxidants as before these last two improvements, the level of oxidants everywhere non-MIMS becomes too low for the available enzymatic anti-oxidants. By too low, he means that the bottleneck will be the MIMS oxidants, and everywhere else the oxidant damage will always be too low to matter. Since now the organism can get away with lowering the enzymatic oxidant levels in the non-MIMS sections, it does so, since it saves energy doing so.
That this non-correlation between enzymatic antioxidant levels and lifespan does not occur in flies and worms, (ie, in those species, the correlation does exist and is positive), means that the lifespan-limiting mechanism in flies and worms is different from homeotherms. de Grey suggests that this mechanism is the mutation of mitochondrial DNA (mtDNA) which tends to be attached to the inner surface of the inner membrane of the mitochondria. The mtDNA would somehow be damaged by the higher unquenched superoxide concentration across the inner membrane, in the MIMS. Old mammals have been shown to have high levels of mtDNA mutations, while this doesn't happen in flies and worms, maybe because they do not live long enough for the process of mtDNA amplification to take place. He's trying to tie it all back to his other paper (http://readingrejuvenationresearch.blogspot.com/2010/01/proposed-refinement-of-mitochondrial.html)
The suggested methods for testing the hypothesis: retarget MnSOD and CuZnSOD to the MIMS and check that they are useless there. If they are not useless, then it should have been easy to evolve those. Afterwards, retarget E Coli's iron-dependent SOD to the MIMS of mice, as has supposedly been done before in yeast, and see if that affects lifespan. That last check doesn't make sense to me. If the enzymatic anti-oxidants in the non-MIMS sections have drifted down until they are causing as much trouble as the MIMS oxidation, then lowering MIMS oxidation damage shouldn't affect the lifespan of the beasts. Doing this while raising enzymatic anti-oxidants throughout the cell might though.
In conclusion, another very interesting chain of causation hypothesis, but probably too long to have much of a chance of being correct.
)))
Abstract follows:
Interestingness: 6
Paper by Aubrey DNJ de Grey in the Journal of Anti-Aging Medicine, Volume 3, Issue 1, Spring 2000.
(((
This is another theory/speculation paper by de Grey. He's trying to explain why there is no correlation between life span and antioxydant enzyme levels in warm blooded animals. His explanation sticks to the oxidative damage is bad, m'kay, trend and tries to fit the model to the data.
He partitions the rate of damage created by oxidation into four:
- Specific metabolic rate (SMR): rate of consumption of oxygen per gram of body.
- Leakiness: Proportion of oxygen that becomes superoxide.
- Superoxide potency (SP): Proportion of superoxide converted into other oxide radicals (eg hydrogen peroxide) instead of being pacified by antioxidant enzymes.
- Oxidisability of the tissue (OT): How easily the relevant tissue gets oxidised.
He notes that under this scheme, levels of antioxidant vitamins affect the OT and not the SP, since they act mainly to stop the chain of oxidation. SP is the rate factor that is not correlated with lifespan that is being explained in this paper.
SMR in warm blooded animals (homeotherms) is mostly determined by body size, and lifespan does correlate with body size. Lifespan depends not only on size though but seems to be well correlated with lifespan restrictions imposed by external causes. There isn't much evolutionary pressure to raise the aging-based lifespan of the animal if it is likely to die from other causes (eg getting eaten). Animals of similar weight but different chances of dying due to external causes have different lifespans (eg birds vs mammals).
In a study in primates, superoxide dismutase (SOD) did correlate with lifespan if the SOD levels were divided by the SMR when doing the calculation. Catalase, glutathione peroxidase (GP) and glutathione (G) didn't though. A less dodgy comparison, in that it didn't need the division by SMR factor, was one between rats and pigeons. Pigeons live about 8 times longer than rats even though they weight about the same. SOD levels in the pigeon were slightly higher, catalase much lower, and G and GP levels varied depending on the tissue. Another study showed similar results when looking at the canary (very low mass, very high lifespan), with not even SOD showing higher levels.
In the same studies, they showed a correlation between leakiness and rate of aging, and maybe one between OT and rate of aging. Lending support to this second correlation, de Grey mentions that fatty acids saturation in the membranes of the mitochondria and levels of non-enzymatic anti-oxidants (eg vitamins C and E) are higher in longer lived animals and these lower OT. This supposedly leaves SP as the only one out of the four factors that doesn't correlate in the predicted way with longevity.
de Grey's hypothesis to explain this is that there is no easy way for evolution to lower the SP because there are no SOD enzymes in the mitochondrial intermembrane space (MIMS) to mop up the superoxides. The selection for longer lifespan instead pushes the concentrations of non-enzymatic antioxidants (vitamins C and E) up all over the cell to get levels up in the MIMS, and the leakiness of the MIMS down which is the same mechanism that controls leakiness elsewhere in the chain. This then means that the concentrations of antioxidant enzymes in the non-MIMS regions become too high for the resulting lower radicals due to the improved leakiness and non-enzymatic antioxidant profiles, and these enzymatic antioxidant levels drift down to save resources until they match the levels that would lead to the same rate of damage as the other parts of the oxidation chain.
Since that paragraph contained the whole hypothesis I will write it again, but in expanded form. Homeotherms supposedly don't have any SODs in their MIMS but we do produce superoxides there (the evidence for that second part is probably not great). The damage caused by this, somehow (more on this later), limits our lifespan. For intelligent or otherwise flighty animals, where the external causes of dying are lower, there is a selective pressure, apparently, to raise our lifespan due to aging to match the lower external causes. Since it seems to be troublesome to introduce a SOD into our MIMS (and this supposed trouble to evolve a MIMS-SOD is the bit that to me seems weakest out of the chain of reasoning), homeotherms instead reduce the leakiness of the ATP-making mechanism, the leakiness factor, and raise the levels of non-enzymatic anti-oxidants, lowering the OT factor, to lower the total rate of aging. Now, lowering the leakiness of the process lowered the production of oxidants everywhere, not just in the MIMS, and raising the level of non-enzymatic anti-oxidants did the same everywhere, not just in the MIMS, so now, if we kept the same level of enzymatic anti-oxidants as before these last two improvements, the level of oxidants everywhere non-MIMS becomes too low for the available enzymatic anti-oxidants. By too low, he means that the bottleneck will be the MIMS oxidants, and everywhere else the oxidant damage will always be too low to matter. Since now the organism can get away with lowering the enzymatic oxidant levels in the non-MIMS sections, it does so, since it saves energy doing so.
That this non-correlation between enzymatic antioxidant levels and lifespan does not occur in flies and worms, (ie, in those species, the correlation does exist and is positive), means that the lifespan-limiting mechanism in flies and worms is different from homeotherms. de Grey suggests that this mechanism is the mutation of mitochondrial DNA (mtDNA) which tends to be attached to the inner surface of the inner membrane of the mitochondria. The mtDNA would somehow be damaged by the higher unquenched superoxide concentration across the inner membrane, in the MIMS. Old mammals have been shown to have high levels of mtDNA mutations, while this doesn't happen in flies and worms, maybe because they do not live long enough for the process of mtDNA amplification to take place. He's trying to tie it all back to his other paper (http://readingrejuvenationresearch.blogspot.com/2010/01/proposed-refinement-of-mitochondrial.html)
The suggested methods for testing the hypothesis: retarget MnSOD and CuZnSOD to the MIMS and check that they are useless there. If they are not useless, then it should have been easy to evolve those. Afterwards, retarget E Coli's iron-dependent SOD to the MIMS of mice, as has supposedly been done before in yeast, and see if that affects lifespan. That last check doesn't make sense to me. If the enzymatic anti-oxidants in the non-MIMS sections have drifted down until they are causing as much trouble as the MIMS oxidation, then lowering MIMS oxidation damage shouldn't affect the lifespan of the beasts. Doing this while raising enzymatic anti-oxidants throughout the cell might though.
In conclusion, another very interesting chain of causation hypothesis, but probably too long to have much of a chance of being correct.
)))
Abstract follows:
A series of studies over many years has conclusively disproved the hypothesis that longevity in warm-blooded animals (homeotherms) correlates with high levels of antioxidant enzymes: in fact, these variables generally exhibit a strong negative cross-species correlation. In flies and nematodes, however, substantial extension of maximum life span has resulted from genetic manipulations that increase antioxidant enzyme levels; these manipulations have always been associated with increased resistance to oxidative challenge, indicating that the life span extension is directly caused by the raised antioxidant capacity. This stark contrast merits careful analysis because it casts doubt on the value of short-lived invertebrates as models for the investigation of mammalian aging. Here is it shown that these results imply the existence, in homeotherms but not in flies or worms, of life span-limiting pathways of oxidative damage that are impervious to enzymatic antioxidants. This is shown to lend weight to the currently controversial theory that somatic mitochondrial DNA mutations contribute significantly to determining the rate of aging of homeotherms, and thereby suggests a feasible intervention to retard human aging.
Tuesday, December 14, 2010
Segmental Nature of Age-Associated, Skeletal Muscle Mitochondrial Abnormalities Necessitates Three-Dimensional Analyses
Summary: Mitochondria with abnormal electron transport chain activity are grouped along the fibre in muscle tissue
Interestingness: 4
Paper by Nathan L Van Zeeland, Jonathan Wanagat, Marisol E Lopez and Judd M Aiken in the Journal of Anti-Aging Medicine, Volume 2, Issue 3, Fall 1999.
(((
They looked at low cytochrome c oxidase (COX, complex IV) activity and high succinate dehydrogenase (SDH, complex II) activity in muscle tissue, which are supposedly common markers for age-related mitochondrial abnormalities. They are colocated with mitochondrial DNA (mtDNA) deletion (mtDNA4977). Also, muscle fibres with these abnormal mitochondrial activity are more commonly atrophied/have much lower cross-sections in rhesus monkeys. Part of the COX enzyme is encoded in the mtDNA, while all of the SDH enzyme is encoded in the nuclear DNA. (That explains why COX activity goes down, but why does the SDH activity go up?)
They measured COX and SDH activity in muscles of old (3-year old) rat and old (33 year old) rhesus monkey, making 200 slices across the muscle fibre so that they got a cross-section of the muscle at each slice. Each slice was about 10 microns thick, and they followed the muscle for about 1.6 millimetres in the monkey and 2 in the rat.
They found that the mutations were grouped along each muscle fibre. They found that in their sample, 3% of the rat's fibers had abnormal activity at some point along its length, and 0.31% of the monkey's (a 25-year old monkey though, not sure what happened to the other monkey), and contrasted these with the much lower values they would have gotten if they would have just sliced at one point (about six times lower). Through some dodgy extrapolation, they claim that 50% of the muscles fibers in the rat's case would be abnormal at some point if they had followed it through the whole length of the muscle, although they say that further studies by them point the number to be closer to 25%
)))
Abstract follows:
Interestingness: 4
Paper by Nathan L Van Zeeland, Jonathan Wanagat, Marisol E Lopez and Judd M Aiken in the Journal of Anti-Aging Medicine, Volume 2, Issue 3, Fall 1999.
(((
They looked at low cytochrome c oxidase (COX, complex IV) activity and high succinate dehydrogenase (SDH, complex II) activity in muscle tissue, which are supposedly common markers for age-related mitochondrial abnormalities. They are colocated with mitochondrial DNA (mtDNA) deletion (mtDNA4977). Also, muscle fibres with these abnormal mitochondrial activity are more commonly atrophied/have much lower cross-sections in rhesus monkeys. Part of the COX enzyme is encoded in the mtDNA, while all of the SDH enzyme is encoded in the nuclear DNA. (That explains why COX activity goes down, but why does the SDH activity go up?)
They measured COX and SDH activity in muscles of old (3-year old) rat and old (33 year old) rhesus monkey, making 200 slices across the muscle fibre so that they got a cross-section of the muscle at each slice. Each slice was about 10 microns thick, and they followed the muscle for about 1.6 millimetres in the monkey and 2 in the rat.
They found that the mutations were grouped along each muscle fibre. They found that in their sample, 3% of the rat's fibers had abnormal activity at some point along its length, and 0.31% of the monkey's (a 25-year old monkey though, not sure what happened to the other monkey), and contrasted these with the much lower values they would have gotten if they would have just sliced at one point (about six times lower). Through some dodgy extrapolation, they claim that 50% of the muscles fibers in the rat's case would be abnormal at some point if they had followed it through the whole length of the muscle, although they say that further studies by them point the number to be closer to 25%
)))
Abstract follows:
Age-associated electron transport system (ETS) abnormalities in skeletal muscle are distributed in a mosaic and segmental fashion; thus, histological techniques examining a single cross-section of tissue underestimate the number of fibers harboring such mitochondrial abnormalities. Analyses of consecutive cross-sections along the length of a muscle are necessary to determine the absolute number of ETS abnormal fibers within a given skeletal muscle. Two hundred serial cross-sections of old rat and rhesus monkey skeletal muscle were obtained by cryostat sectioning. Sections were stained and examined for cytochrome c oxidase and succinate dehydrogenase activity at regular intervals spanning a 1,600-micrometre region of muscle. All fibers staining negative for cytochrome c oxidase activity or hyperreactive for succinate dehydrogenase activity were then followed along their lengths to determine the extent of the ETS abnormal regions. ETS abnormalities in both animal models were found to be distributed in localized regions of individual muscle fibers (i.e., segmental). Examination of fibers along their length lead to a fourfold increase in detection of rat muscle fibers bearing mitochondrial abnormalities. In situ histological techniques that examine numerous sections at multiple positions along the length of skeletal muscles are particularly well suited for determining numbers and assessing the cellular impact of skeletal muscle fibers harboring age-related mitochondrial abnormalities.
Monday, December 6, 2010
RNA Oxidation in Alzheimer and Parkinson Diseases
Summary: RNA is oxidised in some of Alzheimer's, Parkinson's and Down syndrome patients' neurons
Interestingness: 2
Paper by Akihiko Nunomura, George Perry, Jing Zhang, Thomas J Montine, Atsushi Takeda, Shigeru Chiba and Mark A Smith in the Journal of Anti-Aging Medicine, Volume 2, Issue 3, Fall 1999.
(((
They measured 8-hydroxydeoxyguanosine (8-OHdG) and 8-hydroxyguanosine (8-OHG) as markers for DNA and RNA oxidation respectively in an unknown number of brains of postmortem Alzheimer's (AD), Parkinson's (PD) and Down syndrome (DS) patients. They found more 8-OHG in some parts of the brains of some types of disease, and less in others, but the parts of the brain still don't mean much to me. In any case, here they are:
)))
Abstract follows:
Interestingness: 2
Paper by Akihiko Nunomura, George Perry, Jing Zhang, Thomas J Montine, Atsushi Takeda, Shigeru Chiba and Mark A Smith in the Journal of Anti-Aging Medicine, Volume 2, Issue 3, Fall 1999.
(((
They measured 8-hydroxydeoxyguanosine (8-OHdG) and 8-hydroxyguanosine (8-OHG) as markers for DNA and RNA oxidation respectively in an unknown number of brains of postmortem Alzheimer's (AD), Parkinson's (PD) and Down syndrome (DS) patients. They found more 8-OHG in some parts of the brains of some types of disease, and less in others, but the parts of the brain still don't mean much to me. In any case, here they are:
- More oxidation in the cytoplasm than in the nucleolus and nuclear envelope in the neurons of AD and DS, clean in controls
- No difference in cerebellum between AD, DS and controls
- RNA oxidation was the main thing being detected in AD and DS
- Less oxidation with increased amyloid beta (AB) and neurofibrillary tangles (NFT)
- Increased oxidation in substantia negra in PD, dementia with Lewy bodies (DLB), and multiple system atrophy-Parkinsonian type (MSA-P). More in PD than other two
- Both RNA and DNA oxidation in PD, DLB and MSA-P
- No increase in RNA oxidation in PD in cerebellum or cerebral cortex, but increase in cerebral cortex for DLB
)))
Abstract follows:
In Alzheimer and Parkinson diseases, oxidative alterations, affecting lipids, proteins, and DNA, have been described. Using an in situ approach to identify 8-hydroxyguanosine, an oxidized nucleoside, we recently identified RNA as a major target of oxidation in Alzheimer and Parkinson diseases as well as Down syndrome, where premature Alzheimer-like neuropathology is invariably found. RNA oxidation is localized to the neuronal populations potentially affected in these diseases. Together with the known mitochondrial dysfunction in Alzheimer and Parkinson diseases, the cytoplasmic predominance of neuronal 8-hydroxyguanosine supports mitochondria as the most likely source of reactive oxygen responsible for RNA oxidation. The consequence of oxidatively damaged RNA is not fully understood; however, the potential of oxidized RNA to cause errors in translation indicates a metabolic abnormality in neurodegenerative diseases.
Mitochondrial DNA Oxidation
Summary: Most of the oxidising damage in mitochondrial DNA (mtDNA) is in bits/fractions of mtDNA, not in the circular form. And iron relaxes mtDNA loop and increases its replication.
Interestingness: 5
Paper by Christoph Richter in the Journal of Anti-Aging Medicine, Volume 2, Issue 3, Fall 1999.
(((
This paper starts by describing how mtDNA gets oxidised: superoxide radicals (O2-) are formed "when cytochrome oxidase is blocked, when cytochrome c is detached from the inner mitochondrial membrane, " ... and " when mitochondrial oxidative phosphorylation is inhibited". The superoxide radical then gives the electron to a water molecule, which forms hydrogen peroxide (H2O2), which then forms hydroxyl radical (OH.) in the presence of iron or copper (Fenton reaction). The hydroxyl radical is the bastard that then goes and reacts with everything.
It then mentions radical nitrogen species, usual description of mtDNA (16.3 kb pair coding for 13 peptides, 22 tRNAs and 2 rRNAs), how people started thinking of mtDNA damage as important for diseases, measurement of mtDNA damage (usually measuring 8-hydroxyguanine and strand breaks), sidetrack into azidothymidine (AZT, the anti-AIDS drug) causing problems in mitochondria, and Friedreich's ataxia (FA) probably being a problem with oxidation damage in mitochondria.
Now, interesting bit, measurements of amount of oxidative damage in mtDNA differ depending on methodology. Detection of 8-hydroxydeoxyguanosine (8-OHdG) gives big numbers (4 modifications per mtDNA molecule) while numbers from repair enzymes (dunno how it works) give much lower numbers. High number doubted also from seemingly high number of working mitochondria. They do analysis of mtDNA from rat's livers, detecting 8-OHdG. They get 0.051 picomole per microgram of DNA for circular mtDNA, which they say is about one 8-OHdG mutation every two mtDNA molecules, 0.014 picomole per microgram of DNA for nDNA, which is contamination in the sample, but 0.741 picomole per microgram in low molecular mtDNA, ie fractions of floating mtDNA. They don't know what the fractions of mtDNA are doing or why they are so highly oxidised. It could be that they are being actively degraded, or they could be new chunks being made. Having found these fragments, he then hypothesises that these fragments integrate with nDNA, and that this is the main mechanism of aging of mtDNA oxidation damage.
The part that follows is also interesting. Experimenting with iron overload into the mtDNA of rat's livers in vitro they find that it (iron, in the form of Fe3+ gluconate), relaxes mtDNA from the standard supercoiled form to the open circular form. Anti-oxidants prevent some of the change but not all. The iron forms colloids that bind to mtDNA, and there may be a purely physical mechanism of relaxation. They then repeat the experiment in vivo also observing more relaxed circular DNA compared to controls, as well as increased mitochondrial surface and volume density, increased intracellular ferritin and hemosiderin, and higher replication of mtDNA.
It then switches to mtDNA damage prevention, mentions caloric restriction as reducing 8-OHdG counts, AZT leading to higher urinary 8-OHdG but vitamins C and E reducing those levels in AZT-taking people (I thought vitamins C and E didn't enter the mitochondria). Finishes by looking at future studies, evidence that mtDNA inserts in nDNA are more common in tumours, Drosophila overexpressing superoxide dismutase and catalase having increased lifespan, and some wacky suggestion of using bacteria to transfect genes into mitochondria.
)))
Abstract follows:
Interestingness: 5
Paper by Christoph Richter in the Journal of Anti-Aging Medicine, Volume 2, Issue 3, Fall 1999.
(((
This paper starts by describing how mtDNA gets oxidised: superoxide radicals (O2-) are formed "when cytochrome oxidase is blocked, when cytochrome c is detached from the inner mitochondrial membrane, " ... and " when mitochondrial oxidative phosphorylation is inhibited". The superoxide radical then gives the electron to a water molecule, which forms hydrogen peroxide (H2O2), which then forms hydroxyl radical (OH.) in the presence of iron or copper (Fenton reaction). The hydroxyl radical is the bastard that then goes and reacts with everything.
It then mentions radical nitrogen species, usual description of mtDNA (16.3 kb pair coding for 13 peptides, 22 tRNAs and 2 rRNAs), how people started thinking of mtDNA damage as important for diseases, measurement of mtDNA damage (usually measuring 8-hydroxyguanine and strand breaks), sidetrack into azidothymidine (AZT, the anti-AIDS drug) causing problems in mitochondria, and Friedreich's ataxia (FA) probably being a problem with oxidation damage in mitochondria.
Now, interesting bit, measurements of amount of oxidative damage in mtDNA differ depending on methodology. Detection of 8-hydroxydeoxyguanosine (8-OHdG) gives big numbers (4 modifications per mtDNA molecule) while numbers from repair enzymes (dunno how it works) give much lower numbers. High number doubted also from seemingly high number of working mitochondria. They do analysis of mtDNA from rat's livers, detecting 8-OHdG. They get 0.051 picomole per microgram of DNA for circular mtDNA, which they say is about one 8-OHdG mutation every two mtDNA molecules, 0.014 picomole per microgram of DNA for nDNA, which is contamination in the sample, but 0.741 picomole per microgram in low molecular mtDNA, ie fractions of floating mtDNA. They don't know what the fractions of mtDNA are doing or why they are so highly oxidised. It could be that they are being actively degraded, or they could be new chunks being made. Having found these fragments, he then hypothesises that these fragments integrate with nDNA, and that this is the main mechanism of aging of mtDNA oxidation damage.
The part that follows is also interesting. Experimenting with iron overload into the mtDNA of rat's livers in vitro they find that it (iron, in the form of Fe3+ gluconate), relaxes mtDNA from the standard supercoiled form to the open circular form. Anti-oxidants prevent some of the change but not all. The iron forms colloids that bind to mtDNA, and there may be a purely physical mechanism of relaxation. They then repeat the experiment in vivo also observing more relaxed circular DNA compared to controls, as well as increased mitochondrial surface and volume density, increased intracellular ferritin and hemosiderin, and higher replication of mtDNA.
It then switches to mtDNA damage prevention, mentions caloric restriction as reducing 8-OHdG counts, AZT leading to higher urinary 8-OHdG but vitamins C and E reducing those levels in AZT-taking people (I thought vitamins C and E didn't enter the mitochondria). Finishes by looking at future studies, evidence that mtDNA inserts in nDNA are more common in tumours, Drosophila overexpressing superoxide dismutase and catalase having increased lifespan, and some wacky suggestion of using bacteria to transfect genes into mitochondria.
)))
Abstract follows:
Mitochondrial diseases have been known for more than three decades. A refinement of the free radical theory of aging states that oxidative damage to mitochondria, particularly to mitochondrial DNA (mtDNA), is responsible for aging. About 10 years ago, oxidative damage to mtDNA was first reported, and human diseases were related to mutations of mtDNA. Subsequent reports suggested that oxidative mtDNA damage is more pronounced in old individuals and during certain diseases. Studies of animal models indicated that oxidative mtDNA damage can be ameliorated by dietary antioxidants and caloric restriction, an established method to increase life span. More recent data indicate that fragmented mtDNA is the predominant carrier of oxidized mtDNA bases and that fragments constitute a substantial amount of the total mtDNA. This article discusses the emerging relationship among mtDNA oxidation, diseases, and aging, and suggests experiments by which such a relationship can be further substantiated.
Sunday, January 24, 2010
A Mechanism Proposed to Explain the Rise in Oxidative Stress During Aging
Summary: Longish speculative chain on how cells dominated by mutant mitochondria, even though they are rare, can cause system wide oxidation damage. The speculation sounds speculative.
Interestingness factor: 6ish
Paper by Aubrey de Grey, published in Journal of Anti-Aging Medicine, Volume 1, Issue 1, Spring 1998. (((can be gotten from http://www.sens.org/files/sens/AdGpubs.htm)))
(((A long theoretical piece. I'm not going to do it justice. Reading about glycolysis and the Krebs cycle helps)))
The previous blog reviewed the article that forms the basis for this paper. Assuming that mutant mitochondria take over individual cells, how can they affect the rest of the body, if, as this paper points out, they seem to only make up about one percent of tissue cells?
The hypothesis starts with an explanation of how these cells survive at all. It uses data gathered from experiments with cells that lack mitochondrial DNA altogether (p0 cells), that can survive if supplemented with pyruvate or a whole bunch of other molecules.
The problem to solve for them, aside from the lower amounts of energy available, is how to restore levels of NAD+ that get converted to NADH during glycolysis. The proposed solution is that they do this by reduction of extracellular molecules through an enzyme that sits on the membrane called plasma membrane oxidoreductase (PMOR) that exist in every cell. That is, they do this by exporting electrons. Evidence presented is that succinate dehydrogenase is upregulated in these p0 cells, and that since it is part of the Krebs cycle that consumes pyruvate, it shows that the main alternative method of exporting those electrons (by reducing pyruvate to lactate with NADH and then exporting the lactate) is probably not being used. (((Which doesn't show me how some other third method is not what is really going on)))
Those exported electrons primarily go to vitamin C in extracellular fluid, but once you run out of that, they'd go to oxygen, creating superoxide radicals. Most of this would be cleaned up by superoxide dismutase, but some would escape and react with the iron in haemin (((non-protein bit of haemoglobin))) (other iron options are well protected) which would then oxidase LDL particles. This last part, the oxidation of LDL particles by haemin, seems to have some evidence behind it.
The oxidation of some LDL would raise the intake of somewhat oxidised LDL by all cells in the body, which would raise the amount of oxidation damage that all cells have to deal with. Since this mechanism would be going on constantly, ie those mutant mtDNA cells would be constantly spewing electrons, quite a lot of oxidised LDL would be created.
The paper then moves onto methods of testing the suggested chain of events:
And then onto methods for checking that it affects aging:
(((Conclusion: The chain of events suggested here sounds much more dubious to me than the one suggested in the previous article. The electrons from NADH might be used some other way inside the cells, or the electrons might be quenched in some benign way outside the cell, or oxidation of LDL might not have any major effect on aging (outside of the effects on cardiovascular disease). The tests seem simple enough though, and they'll pop up regardless, if they haven't already. It would be good if this was correct since killing the mutant cells doesn't sound insanely hard to me. Easier than curing cancer since these cells don't reproduce, as determined by the method that they come into existence)))
(((This is the last paper that I'll write about in the first issue. The rest consists of some futurist speculation, a meeting report and literary review. While interesting, they are already in summary form)))
Abstract follows:
Interestingness factor: 6ish
Paper by Aubrey de Grey, published in Journal of Anti-Aging Medicine, Volume 1, Issue 1, Spring 1998. (((can be gotten from http://www.sens.org/files/sens/AdGpubs.htm)))
(((A long theoretical piece. I'm not going to do it justice. Reading about glycolysis and the Krebs cycle helps)))
The previous blog reviewed the article that forms the basis for this paper. Assuming that mutant mitochondria take over individual cells, how can they affect the rest of the body, if, as this paper points out, they seem to only make up about one percent of tissue cells?
The hypothesis starts with an explanation of how these cells survive at all. It uses data gathered from experiments with cells that lack mitochondrial DNA altogether (p0 cells), that can survive if supplemented with pyruvate or a whole bunch of other molecules.
The problem to solve for them, aside from the lower amounts of energy available, is how to restore levels of NAD+ that get converted to NADH during glycolysis. The proposed solution is that they do this by reduction of extracellular molecules through an enzyme that sits on the membrane called plasma membrane oxidoreductase (PMOR) that exist in every cell. That is, they do this by exporting electrons. Evidence presented is that succinate dehydrogenase is upregulated in these p0 cells, and that since it is part of the Krebs cycle that consumes pyruvate, it shows that the main alternative method of exporting those electrons (by reducing pyruvate to lactate with NADH and then exporting the lactate) is probably not being used. (((Which doesn't show me how some other third method is not what is really going on)))
Those exported electrons primarily go to vitamin C in extracellular fluid, but once you run out of that, they'd go to oxygen, creating superoxide radicals. Most of this would be cleaned up by superoxide dismutase, but some would escape and react with the iron in haemin (((non-protein bit of haemoglobin))) (other iron options are well protected) which would then oxidase LDL particles. This last part, the oxidation of LDL particles by haemin, seems to have some evidence behind it.
The oxidation of some LDL would raise the intake of somewhat oxidised LDL by all cells in the body, which would raise the amount of oxidation damage that all cells have to deal with. Since this mechanism would be going on constantly, ie those mutant mtDNA cells would be constantly spewing electrons, quite a lot of oxidised LDL would be created.
The paper then moves onto methods of testing the suggested chain of events:
- Seeing if cells that do not have a functioning electron transport chain (by assessing cytochrome c oxidase activity) have high PMOR activity.
- Seeing if there's high levels of superoxide near cells with busted mitochondria
- Checking if LDL is highly oxidised near mutant cells
- Checking if oxidised LDL particles stress normal cells anti-oxidant system.
And then onto methods for checking that it affects aging:
- Restoring the function of the mutant mitochondria by importing the proteins encoded by the mitochondrial DNA and seeing what happens (((hard project)))
- Targeting those zombie cells controlled by mutant mitochondria and destroying them, then seeing what happens. (((I like it)))
(((Conclusion: The chain of events suggested here sounds much more dubious to me than the one suggested in the previous article. The electrons from NADH might be used some other way inside the cells, or the electrons might be quenched in some benign way outside the cell, or oxidation of LDL might not have any major effect on aging (outside of the effects on cardiovascular disease). The tests seem simple enough though, and they'll pop up regardless, if they haven't already. It would be good if this was correct since killing the mutant cells doesn't sound insanely hard to me. Easier than curing cancer since these cells don't reproduce, as determined by the method that they come into existence)))
(((This is the last paper that I'll write about in the first issue. The rest consists of some futurist speculation, a meeting report and literary review. While interesting, they are already in summary form)))
Abstract follows:
Most phenotypes of aging in vertebrates may be caused by a progressive decline in the ability of antioxidant defences to maintain cellular and systemic homeostasis. This is due both to a diminished efficacy of those defences and to an enhanced level of pro-oxidant toxicity; the imbalance between the two has been termed oxidative stress. However, the cause of this increasing imbalance remains obscure. This article proposes a mechanism by which spontaneously mutant mitochondrial DNA (mtDNA), despite being present only in very small quantities in the body, may be the main generator of oxidative stress. Mutant mtDNA is distributed very unevenly within a tissue: some cells apparently contain no wild-type mtDNA whatever. Those cells must rely on glycolysis for ATP production; furthermore, they require a system to stabilize their NAD+/NADH ratio. This can only be achieved by an efflux of electrons from the cell, most probably mediated by the plasma membrane oxidoreductase (PMOR). It is proposed that the required rate of electron efflux from these anaerobic cells exceeds the local electron-accepting capacity of "safe" acceptors in plasma such as dehydroascorbate, with the result that reactive species, such as Superoxide, are formed. This leads to increased oxidation of lipids in the plasma, notably of low-density lipoprotein (LDL) particles, which are subsequently imported into mitochondrially healthy cells. This oxidized lipoprotein must be destroyed by the recipient cells' antioxidant defences. That task diverts the cell from the degradation of pro-oxidants that it is itself generating; thus, it imposes oxidative stress on the cell. As the number of anaerobic cells in the body rises, so does oxidative stress in all cells. The consistency of this hypothesis with known facts is discussed, and technically feasible tests are suggested both of the proposed mechanism and of its overall contribution to mammalian aging, including plausible interventions to retard the process.
Sunday, January 17, 2010
A proposed refinement of the mitochondrial free radical theory of aging
(((This isn't really part of the series. I was reading "A Mechanism Proposed to Explain the Rise in Oxidative Stress During Aging" in the series but it assumes that the reader is familiar with this paper. Since I found it interesting, I'm writing a summary of this one as well)))
Summary: Hypothesis is that cells get filled up with mutant mitochondria. This is because their membrane gets destroyed at a slower rate due to them being slower in producing free radicals. This leads to them not being destroyed by the cell recycling mechanism.
Interestingness factor: 7ish
Paper by Aubrey de Grey, published in BioEssays, 1997, volume 19, issue 2. (((can be gotten from http://www.sens.org/files/sens/AdGpubs.htm)))
(((Harder to summarise theoretical papers, since most of it tends to be important for the theory to hold)))
The paper makes the case that the replication of mutant mitochondria fills up cells with mostly useless mitochondria that do not feed the cell enough ATP, and that this is important for mammalian aging. I'll focus on the description of the mechanism of how mutant mitochondria supposedly get selectively replicated and come to represent most/all of the mitochondria in a cell, since that's the bit that's relevant to the paper in the Anti-Aging journal. I'll mostly ignore the importance of this to aging.
(((Reading up on mitochondria, and http://en.wikipedia.org/wiki/Electron_transport_chain#Electron_transport_chains_in_mitochondria helps)))
Mitochondria reproduce more frequently that the cells that contain them, especially if those cells don't replicate at all
(senescent). Their DNA (mtDNA) is not as well protected as nuclear DNA and is close to the reactive molecules that they produce. There are 13 genes in mtDNA that are not duplicated in the nuclear DNA and so are essential to the functioning of the mitochondria. They include proteins that form part of the respiration chain and the ATP generation. Therefore point mutations in those parts of their DNA would interfere with those functions. If there was a selective process which would preferentially replicate these mitochondria over the non-mutant ones, then mutants would dominate the cell and all/most mitochondria in the cell would have non-working or slow ATP production.
(((The most speculative part is the following))) Mitochondria damage their cell membrane in the production of the proton gradient. The process creates radical molecules that attack the lipids in their inner membrane. If the membrane is damaged enough small molecules from the interior of the mitochondrion will leak into the cytoplasm. The cell uses these as markers of damaged mitochondria and destroys it (by lysosomal degradation). Because the mutant mitochondria have faulty electron transport chains, their membrane degrades slower. This means they are left alive and reproduced when the cell thinks it needs more ATP (cell has to pick from the mitochondria that are alive). (((Tada!)))
Evidence offered for this theory: 1) Mitochondria in cells tend to share the same mutations, and these are different from the mutations in the cell next door. 2) Mutations that affect the ATP synthetising enzymes do not become popular among cells, because their transport chain is intact and therefore their membranes are just as damaged as non-mutant mitochondria (((this is offered as a prediction in the paper but he claims the result is provisionally known to be true)))
Refutations of potential counter-arguments. (((I'm restricting to only the ones that refer to the spread of the mitochondria again)))
1) Objection: Mitochondria have many copies of mtDNA, single mutation in one copy won't make much of an impact. Counter-counter: Some mutations will hang around and become homozyguous in all copies of a mitochondrion by genetic drift. Also, even arecessive mutation would have some small effect on electron transport and the mitochondrion only has to survive a little bit longer than the rest to be replicated.
2) Objections: Membranes get repaired. Counter-counter: Not all damage can be fixed.
3) Objection/question: Does a mitochondria with a damaged electron transport chain mechanism actually cause less damage on their inner membrane? Counter-counter: Yes. Three paths of lipid peroxidation: perhydroxy levels lower on the outside of an inner membrane of a slowly respiring mitochondrion, so will levels of ubisemiquinone. Metal-catalysed pathway still just as active probably (((I'm just parroting this bit. No idea if it's complete nonsense)))
Abstract follows:
Summary: Hypothesis is that cells get filled up with mutant mitochondria. This is because their membrane gets destroyed at a slower rate due to them being slower in producing free radicals. This leads to them not being destroyed by the cell recycling mechanism.
Interestingness factor: 7ish
Paper by Aubrey de Grey, published in BioEssays, 1997, volume 19, issue 2. (((can be gotten from http://www.sens.org/files/sens/AdGpubs.htm)))
(((Harder to summarise theoretical papers, since most of it tends to be important for the theory to hold)))
The paper makes the case that the replication of mutant mitochondria fills up cells with mostly useless mitochondria that do not feed the cell enough ATP, and that this is important for mammalian aging. I'll focus on the description of the mechanism of how mutant mitochondria supposedly get selectively replicated and come to represent most/all of the mitochondria in a cell, since that's the bit that's relevant to the paper in the Anti-Aging journal. I'll mostly ignore the importance of this to aging.
(((Reading up on mitochondria, and http://en.wikipedia.org/wiki/Electron_transport_chain#Electron_transport_chains_in_mitochondria helps)))
Mitochondria reproduce more frequently that the cells that contain them, especially if those cells don't replicate at all
(senescent). Their DNA (mtDNA) is not as well protected as nuclear DNA and is close to the reactive molecules that they produce. There are 13 genes in mtDNA that are not duplicated in the nuclear DNA and so are essential to the functioning of the mitochondria. They include proteins that form part of the respiration chain and the ATP generation. Therefore point mutations in those parts of their DNA would interfere with those functions. If there was a selective process which would preferentially replicate these mitochondria over the non-mutant ones, then mutants would dominate the cell and all/most mitochondria in the cell would have non-working or slow ATP production.
(((The most speculative part is the following))) Mitochondria damage their cell membrane in the production of the proton gradient. The process creates radical molecules that attack the lipids in their inner membrane. If the membrane is damaged enough small molecules from the interior of the mitochondrion will leak into the cytoplasm. The cell uses these as markers of damaged mitochondria and destroys it (by lysosomal degradation). Because the mutant mitochondria have faulty electron transport chains, their membrane degrades slower. This means they are left alive and reproduced when the cell thinks it needs more ATP (cell has to pick from the mitochondria that are alive). (((Tada!)))
Evidence offered for this theory: 1) Mitochondria in cells tend to share the same mutations, and these are different from the mutations in the cell next door. 2) Mutations that affect the ATP synthetising enzymes do not become popular among cells, because their transport chain is intact and therefore their membranes are just as damaged as non-mutant mitochondria (((this is offered as a prediction in the paper but he claims the result is provisionally known to be true)))
Refutations of potential counter-arguments. (((I'm restricting to only the ones that refer to the spread of the mitochondria again)))
1) Objection: Mitochondria have many copies of mtDNA, single mutation in one copy won't make much of an impact. Counter-counter: Some mutations will hang around and become homozyguous in all copies of a mitochondrion by genetic drift. Also, even arecessive mutation would have some small effect on electron transport and the mitochondrion only has to survive a little bit longer than the rest to be replicated.
2) Objections: Membranes get repaired. Counter-counter: Not all damage can be fixed.
3) Objection/question: Does a mitochondria with a damaged electron transport chain mechanism actually cause less damage on their inner membrane? Counter-counter: Yes. Three paths of lipid peroxidation: perhydroxy levels lower on the outside of an inner membrane of a slowly respiring mitochondrion, so will levels of ubisemiquinone. Metal-catalysed pathway still just as active probably (((I'm just parroting this bit. No idea if it's complete nonsense)))
Abstract follows:
Over recent years, evidence has been accumulating in favour of the free radical theory of aging, first proposed by Harman. Despite this, an understanding of the mechanism by which cells might succumb to the effects of free radicals has proved elusive. This paper proposes such a mechanism, based on a previously unexplored hypothesis for the proliferation of mutant mitochondrial DNA: that mitochondria with reduced respiratory function, due to a mutation or deletion affecting the respiratory chain, suffer less frequent lysosomal degradation, because they inflict free radical damage more slowly on their own membranes. Once such a mutation occurs in a mitochondrion of a non-dividing cell, therefore, mitochondria carrying it will rapidly populate that cell, thereby destroying the cell's respiratory capability. The accumulation of cells that have undergone this transition results in aging at the organismal level. The consistency of the hypothesis with known facts is discussed, and technically feasible tests are suggested, of both the proposed mechanism and its overall contribution to mammalian aging.
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