14 replies to this topic

[manofsan]

So awhile ago we had this interesting discussion here about Mitochondrial DNA and aging.
It was said that one of the problems in the repain of Mitochondrial DNA was that healthy/repaired mitochondria have been shown to not replicate as fast as defective ones, because defective ones don't generate as much damaging oxidative activity as healthy ones.

So I was just thinking -- why not insert backup genes into Mitochondria as part of the repair process? With more genetic redundancy in place, the repaired/healthy mitochondria would be able to withstand more damaging oxidative activity, as compared to the defective mitochondria.

Also, as part of the genetic modification, why not program the repaired mitochondria to replicate slightly faster?

And what advantages do the mitochondria of avian species have over humans?

[olaf.larsson]

I would rather go for eliminating mito genes and putting them in many copies in other chromosomes or a new autosome in the cell nuclear. There are organisms without mito genes Plasmodesma is one if I remember right.
I sounds easy when we are speaking about it here but the only mito genetic manipulation I have heard about yet took four years to do.

[John Schloendorn]

Either way should be tried if only someone had the resources to do it... The big minus I see in relocation of the genes to the nucleus is the strain on intracellular traffic, and perhaps other reasons why evolution left those few of all genes to the mito. What works in some creature need not work in us. But these things sure should be tried.

I was thinking of a nuclear encoded, mitochondrial targeted retrotransposon to do recombination repair. Recombination strands would be sent as mito-targeted mRNAs. But sure the details of this have yet to be worked out!

Manofsan, I would not go for enhanced mito-proliferation because, repaired mitos that mutate again might replicate even faster and you're getting nowhere.

[manofsan]

Gosh, you're right, mutated defectives could indeed replicate faster once again. Perhaps we need to think of both "carrot and stick" -- ie. selectively attack the defectives while providing better repair functions for the healthies.

[olaf.larsson]

"I see in relocation of the genes to the nucleus is the strain on intracellular traffic"

Since nobody have yet tried we dont know if there would be any traficjam in the transport to mitos. But its an interesting observation that the genes that have not moved to nucleus are those for the proteins which have to be replaced often.

[manofsan]

Interesting, so what if you put the duplicate copies of those mito genes in the nucleus, instead of putting those duplicates mito side?

You could keep their expression low, so as not to strain the intracellular traffic. Then in the even of gene failure on the mito side, the duplicates in the nucleus would still be functioning.

But I'm curious -- alterations to nuclear DNA are considered to be dangerous due to potential for disruption of the myriad of nuclear genes. But the mito DNA is much smaller, and you guys told me it's free-floating, right? (ie. there are no chromosomes or nuclear envelope around the mito genes)

So in principle, shouldn't gene insertion into the mitochondria be easier than putting modifications into the nucleus?

Also, are there any known downsides to increasing the expression of repair enzymes? Any negative consequences? Maybe someone should just try to increase the repair enzyme expression rate.

When it comes to mitochondrial gene defects, are there any genes which are known to break down more than others? I know DNA is DNA, and so perhaps free radical damage is just the luck of the draw, but then are there any mito genes which are more critical to mitochondrial function than others? Or are all mito genes of equal importance?

[olaf.larsson]

I would say all mito genes are i equal importance their products make up the respiratory complexes and must be frequently replaced due to oxidational damage.
Maybee one could somehow devide the mitochondria in compartment one for dna and one for respiration. But I dont see how something like this could be performed with todays technology.

[manofsan]

Aha! Why doesn't the mitochondrion have its own internal nuclear envelope? Just the lack of evolution on the part of mitos, before having been incorporated into cells? Are there any independent bacteria which are evolutionary cousins of our mitos and which do have more advance features than our mitos?

Why do mito genes have to be free-floating? Is there any reason why they can't be contained in a mito nuclear envelope? Couldn't one be engineered, using genes that made the nuclear envelope for the cell's own nucleus?
Would mito genes be less effective if they were compartmentalized into a mito nuclear envelope?

[John Schloendorn]

So in principle, shouldn't gene insertion into the mitochondria be easier than putting modifications into the nucleus?

Nah, just the other way round. In the nucleus we have only a few % coding sequences, so by random insertion you're likely to disrupt something dispensable. In the mito, there is much more coding sequence, so you really don't want any random insertion there. The point is, even in the nucleus, random insertion is way too dangerous to be useful to us. A chance of a few % becomes a hell lot when you try it in billions of body cells. If we want to do anything like this, anywhere, we will need a VERY specific mechanism and that remains science fiction for now.

Why doesn't the mitochondrion have its own internal nuclear envelope?

Short answer: Because it's evolutionarily stable without one.
Slightly longer answer: I do not want to revive the 'programmed aging vs. wear and tear' debate all over again, but if a decent case can be made that aging in humans severs to some degree the propagation of the genes that cause it (You could search on PubMed for Kirkwood's original papers or on Amazon for Clark's "sex and the evolution of death".) then we should expect to find mechanisms that ensure a genetically optimal rate of aging.
If mitochondrial mutations have to do with human aging at all, the given rate of mutations may be part of the mechanism that generates an evolutionarily optimal life span and evolution would move no finger to slow it down. That's where we come in.

Also, are there any known downsides to increasing the expression of repair enzymes? Any negative consequences? Maybe someone should just try to increase the repair enzyme expression rate.

I am currently not aware of any experiments involving mitochondrial DNA repair enzyme overexpression, but there are numerous examples of mitochondrial overexpression of radical detoxifying enzymes such as SOD, catalase and GSH peroxidase. In these latter cases, there was little or negative impact on the life span of already long lived strains and the underlying mechanisms are unclear. It may have to do with the involvement of certain radicals in cellular signaling processes. If this is a major factor, then relocalization of the mitochondrial to the nucleus might prove a solution.

In summary, the best shot we currently have is gene relocalization to the nucleus. It is already being tried in tissue culture and we should undertake every effort to move it to the transgenic mouse model soon.

[manofsan]

But I thought you guys told me that in the mitochondrion, there's no chromosome-style DNA to insert into. Aren't the mito genes supposed to be free-floating? In which case, I thought you just insert the new genes into the mito itself, and they can express themselves like the others.

Instead of gene relocation, why not just duplicate them in the nucleus? You'd still need the original copies where they are, because they're probably necessary for the near-site maintenance of the mitochondrion.

If regular messenger RNA can pass out of the nucleus to the main cellular ribosomes, can't another type of messenger RNA be created to pass out of the nucleus and penetrate the mitochondria?

I wonder if in early organisms that were integrating with their newly-acquired mitochondria, perhaps they might have had mRNA passing between the nucleus and the mitos, before eventually evolving to the current gene distribution. You never know, it's possible.

[John Schloendorn]

But I thought you guys told me that in the mitochondrion, there's no chromosome-style DNA to insert into. Aren't the mito genes supposed to be free-floating?

Mitochondrial genes are actually arranged on a circular DNA molecule that is sometimes called the mitochondrial chromosome. Free floating in this context probably means that it is not associated to histones, nor condensed into chromatin like the nuclear chromosomes. If you are interested in these things, you might want to read through a cell biology text book, such as those by Alberts, or Lodish.
If anything, it would be this circular structure that is required for mitochondrial maintenance and proliferation, rather than the actual sequence of the genes.

Instead of gene relocation, why not just duplicate them in the nucleus?

A straightforward thing to do would be three things:

1) Transcribe mitochondrial genes in the nucleus, translate in the cytoplasm in to proteins targeted to the mitochondria. (That would be what you call duplication)
2) Do 1) and delete the original mitochondrial chromosome from the mitos
3) Do 2) and add a mitochondrial chromosome containing only noncoding sequences

See which goes best in tissue culture and, dependent on these results, move some of them to the mouse.

1) Might have the problem that proteins are still being made from mitochondrial genes. Eventually, they might generate defective electron carrier proteins that leak radicals and do unspecific damage. Eventually, however, this damage might scramble the mitochondrial genes so much that they become inactive.
2) Might have the maintenance and fission problems you mentioned.

1) and 2) are easier in that order and we might get away with one of them.

1) Is by far the most preferable because it involves no tampering with the mitos themselves. There are multiple mitos per cell and for 2) or 3) we'd have to modify/replace most or all of them.

If regular messenger RNA can pass out of the nucleus to the main cellular ribosomes, can't another type of messenger RNA be created to pass out of the nucleus and penetrate the mitochondria?

There are sporadic reports of artificially targeting RNA to the mitochondria, but to my knowledge this has not been used for actual gene expression. I would prefer having to use only the relatively established methods required for protein targeting, but we may have to try mRNA targeting if that fails for some reason. Remember we do not need a perfectly elegant solution right now, but rather one that is quick and cheap enough to be developed in our life time and effective enough to buy us enough years to solve all the other problems.

Best wishes, John.

[olaf.larsson]

According to Great Guru mr. de Grey there have been at least one experiment of successfull transfer of a gene from mito to nucleus. In oct 2005 a experiment will be finished in which all genes are moved to nucleus of a cell I see forward to the results.

[Mind]

If regular messenger RNA can pass out of the nucleus to the main cellular ribosomes, can't another type of messenger RNA be created to pass out of the nucleus and penetrate the mitochondria?

There are sporadic reports of artificially targeting RNA to the mitochondria, but to my knowledge this has not been used for actual gene expression. I would prefer having to use only the relatively established methods required for protein targeting, but we may have to try mRNA targeting if that fails for some reason. Remember we do not need a perfectly elegant solution right now, but rather one that is quick and cheap enough to be developed in our life time and effective enough to buy us enough years to solve all the other problems.

Best wishes, John.

Reading this fun blog post about Moderna, made me recall this past article, and this thread as well. Are designer mRNA a huge deal? What side effects are possible with this type of treatment?

LongeCity video for background.

Edited by Mind, 14 April 2013 - 05:15 PM.

[olaf.larsson]

Here I am 9 years later with the same avatar, different but similar username.

Does anyone know how the experiment with movement of mito genes to nucleus went? Do you have some references to any papers?

Edited by olaf.larsson, 06 December 2013 - 05:43 PM.

[abhishek0990]

Very nice information. Thank you for sharing it.
Thanks
pmd