The Details Matter for Reactive Oxygen Species in Aging
Here, researchers use genetic engineering try to pin down specific sources of oxidizing molecules within a cell and identify their different effects on health and longevity. Cells are liquid bags of chemical machinery, and the presence of too many reactive oxygen species (ROS) - also known as free radicals - can produce a level of damage to that machinery that significantly impacts cell function, or even kills cells. This is known as oxidative stress, and levels of oxidative stress are seen to rise alongside the other manifestations of degenerative aging. However, ROS molecules are also used as signals: produced by mitochondria and triggering increased cell maintenance, among other activities. They are critical in the beneficial response to exercise, for example. Many methods of modestly extending life span in laboratory species involve reductions in levels of ROS, and many others involve increases - either can under the right circumstances extend healthy life by reducing net levels of cell damage or triggering other mechanisms relevant to health. A cell is a complex, reactive system and few aspects of cell state have straightforward relationships with one another. Oxidative stress features prominently in many theories of aging, but many of these theories are older and too simplified to be useful given the present state of knowledge and explorations of oxidative stress and ROS signaling.
Historically, mitochondrial ROS (mtROS) production and oxidative damage have been associated with aging and age-related diseases such as Parkinson's disease. In fact, the age-related increase in ROS has been viewed as a cause of the aging process while mitochondrial dysfunction is considered a hallmark of aging, as a consequence of ROS accumulation. However, pioneering work in Caenorhabditis elegans has shown that mutations in genes encoding subunits of the electron transport chain (ETC) or genes required for biosynthesis of ubiquinone extend lifespan despite reducing mitochondrial function. The lifespan extension conferred by many of these alterations is ROS dependent, as reduction of ROS abolishes this effect. Various studies have shown that ROS act as secondary messengers in many cellular pathways, including those which protect against or repair damage. ROS-dependent activation of these protective pathways may explain their positive effect on lifespan. The confusion over the apparent dual nature of ROS may, in part, be due to a lack of resolution as without focused genetic or biochemical models it is impossible to determine the site from which ROS originate.
A promising path to resolving ROS production in vivo is the use of alternative respiratory enzymes, absent from mammals and flies, to modulate ROS generation at specific sites of the ETC. The alternative oxidase (AOX) of Ciona intestinalis is a cyanide-resistant terminal oxidase able to reduce oxygen to water with electrons from reduced ubiquinone (CoQ), thus bypassing complex III and complex IV. NDI1 is a rotenone-insensitive alternative NADH dehydrogenase found in plants and fungi, which is present on the matrix-face of the mitochondrial inner membrane where it is able to oxidize NADH and reduce ubiquinone, effectively bypassing complex I. Our group and others have demonstrated that allotopic expression of NDI1 in Drosophila melanogaster can extend lifespan under a variety of conditions and rescue developmental lethality in flies with an RNAi-mediated decrease in complex I levels.
To determine the role of increased ROS production in regulating longevity, we utilized allotopic expression of NDI1 and AOX, along with Drosophila genetic tools to regulate ROS production from specific sites in the ETC. We report that ROS increase with age as mitochondrial function deteriorates. However, we also demonstrate that increasing ROS production specifically through respiratory complex I reverse electron transport extends Drosophila lifespan. We show that NDI1 over-reduces the CoQ pool and increases ROS via reverse electron transport (RET) through complex I. Importantly, restoration of CoQ redox state via NDI1 expression rescued mitochondrial function and longevity in two distinct models of mitochondrial dysfunction. If the mechanism we describe here is conserved in mammals, manipulation of the redox state of CoQ may be a strategy for the extension of both mean and maximum lifespan and the road to new therapeutic interventions for aging and age-related diseases.
Hi ! Cool.
''Mitochondrial ultrastructure from dissected dorsal flight muscle was unaffected by SOD2 knockdown (Figures S3E and S3F), showing that high levels of superoxide recapitulate some (e.g., increased mortality and, decreased CI activity), but not all features of aging (e.g., alteration in mitochondrial morphology). Although high levels of superoxide in the absence of SOD2 are detrimental, this situation is unlikely to occur in vivo in aged flies since SOD2 levels (Figures S1G and S1H) and superoxide dismutase activity (Sohal et al., 1990) are reported to increase with age.''
''Loss of CoQ-Mediated ROS Signaling Accelerates Aging''
I think this is why you have to take ROS signaling increasing lifespan with a grain of salt, especially in small animals who live barely two months, they show 'dramatic' mitohormesis effect where increasing ROS increases their lifespan as survival signal, which activâtes the all too important Nrf2 (Nuclear Response Factor 2; which, also, alters ARE/ErPE Antioxidant/Electrophile Response Element, affecting thousands of enzymes of redox, which reinstate the oxidized-to-reduced balance - despite increasing ROS production; which itself is the 'triggering' survival signal to the system 'to compensate (feedback mechanism) to respond to rising oxidative stress by ROS increase - and rebalance it towards a reduced satisfactory state)...
In longevous mammals, ROS is tightly regulated and is a reason we age because it allows 'as a starter' to create the cascade for the dysfunction (such a lipoperoxidation and hydroperoxidation of the mitochondrial and plasma membranes biggest ones; since they act like 'magnifiers' catalyzing ever-growing cascades of oxidative end products).
Exercise does help by increasing ROS, increasing the 'survival signal' (Nrf2, ARE/EpRE) but it's no magic bullet either and provides temporary 'hormetic effect'...The fact that evolution - had - to alter the membrane composition (to slow damage accrual from mitoPUFA peroxidation) proves that ROS signaling increase is not enough to make a very very long lifespan of years (not months like flies).
And this :
''***Although high levels of superoxide in the absence of SOD2 are detrimental***, this situation is unlikely to occur in vivo in aged flies since SOD2 levels (Figures S1G and S1H) and superoxide dismutase activity (Sohal et al., 1990) are reported to increase with age.''''
.
This too, in the study they KO SOD2 and the fly live as long in parallel to the correlating amount of SOD2 (which quenches superoxide, the starter ROS of this oxidization).
What's more is they maintain CoQ in a Reduced state, not oxidized state; oxidizing (rusting) is bad so obviously oxidized CoQ would be detrimental, but maintaining reduced state CoQ signals longevity and stress resistance; ROS contribute to oxidizing shift; but in doing so activate the survival signal that enzymes 'get' (such as Nrf2, as a Compensating Response to the Stress - Stress Resistance Response, who's translocated to the nucleus and activâtes,
DAF-16, ARE and Phase II xenobiotics detoxyfying enzymes like SOD).
I'll take the word of a 500-Years living icelandic clam who maintains
bare-levels of ROS over a 3 month and a half old fly using ROS mitohormesis bag of tricks...
PS: if you compare a :
90-days living fly
60-days living worker bee
3-Years living Queen bee
30-YEARS living Queen ANT (maximum lifespan)
Which would you use as a model of aging ? Queen ants and queen bees are marvels, they are destroy all of the theories; for their size they should live no longer than the fly, mitohormesis or not; ..why do they live so long ? Simple (well not so simple, a major reason), evolution altered the membranes unsaturation of queen ants and bees to reduce oxidative damage to their mitochondrial DNA - JUST like us or the Naked Mole Rat or the little Brown Bats too (who live 30 to 40 years, yet the Queen ants beats them all by her tiny size and extreme lifespan). PLus, what'S more is both Queens show maintenance of their redox systems (including SOD2 in that fly) over the years; same as us, humans and same as a Clam, who lives 500 years.
Pps:
''Ubiquinone (CoQ10) is a 50-carbon polyisoprene with a terminal quinone domain.''
CoQ acts at the membrane lipid level by reducing lipoperoxidation and affecting OXPHOS ETC efficiency; basically it emulates evolution's solution of membrane lipid composition reordering towards ROS peroxidizing-unsusceptible one. Also, just like GSH, Ascorbate or other redox electron donors, CoQ/ubiquinol is a electron donor/exchanger in the ETC; ETC reverse/forward flow electron leakage is also an element of why we age; this makes the system imperfect/flawed and creates opportunities for ROS to form and go on damaging (and signaling, since evolution made the system adapt/circumvent that problem).
Wikip.:
''The natural ubiquinol form of coenzyme Q10 is 2,3-dimethoxy-5-methyl-6-poly prenyl-1,4-benzoquinol, where the polyprenylated side-chain is 9-10 units long in mammals.
Coenzyme Q10 (CoQ10) exists in three redox states,
fully oxidized (ubiquinone), partially reduced (semiquinone or ubisemiquinone), and
fully reduced (ubiquinol).
The redox functions of ubiquinol in cellular energy production and antioxidant protection are based on the ability to exchange two electrons in a redox cycle between ubiquinol (reduced) and the ubiquinone (oxidized) form''
This is an extremely interesting research path for people like me (mitochondrial myopathy patient). However, I wonder whether this will have some practical clinical application in the near (10-15 years) future.
I also found weird that the authors did not mention some previous works by Perry CN [1] and Yagi lab [2]
[1] http://www.ncbi.nlm.nih.gov/pubmed/21339825
[2] http://www.scripps.edu/yagi/resNdi1.shtml