Inhibition of Mitochondrial Complex I Extends Life in Killifish and Zebrafish

In the open access paper linked below, researchers demonstrate modest life extension in the short-lived killifish and zebrafish species by inhibiting a specific portion of the protein machinery inside mitochondria, the power plants of the cell responsible for - among many other things - producing a supply of the chemical energy store molecule adenosine triphosphate (ATP). Mitochondria swarm in animal cells by the hundred. They are the evolved remnants of symbiotic bacteria, contain their own mitochondrial DNA, separate from the chromosomal DNA in the cell nucleus, and still replicate like bacteria even though they are tightly integrated into the cellular processes of monitoring and damage control. The cell culls the herd on a continual basis, destroying mitochondria that show signs of damage.

Mitochondria are known to be important in aging, but there are a number of different mechanisms involved. For one, there is a robust association between the details of mitochondrial biochemistry and longevity across species. Species with more resilient mitochondria, made up of a mix of lipids that is on average more resistant to oxidative damage, tend to be longer lived. Secondly, if mitochondria become dysfunctional or limited in number due to any sort of damage or change in environment - such as the sweeping changes of aging - then tissues with high energy requirements begin to suffer. The brain is particularly vulnerable from this perspective, and loss of mitochondrial function over time is associated with the progression of neurodegenerative conditions. Thirdly, mitochondrial signaling is involved in all sorts of processes known to be associated with aging and longevity, such as programmed cell death and triggering of cellular recycling and maintenance mechanisms. Many of the long-lived mutant lineages created over the past two decades in the lab are characterized by altered mitochondrial function and greater cellular repair activity. Lastly, and probably most importantly, rare forms of mitochondrial damage, such as large deletions in mitochondrial DNA, can evade quality control mechanisms, causing cells to be taken over by mutant mitochondria and fall into a harmful state. These cells grow in number with age, and export large quantities of reactive molecules out into tissues, contributing to many forms of age-related damage. For example, this increases the presence of the oxidized lipids that are the seed for the development of atherosclerosis in blood vessel walls.

The SENS rejuvenation research approach to mitochondrial damage is genetic engineering to create a backup copy of mitochondrial DNA in the cell nucleus. Thus there is always a supply of the necessary proteins, and mitochondria can't fall into a state in which they are malfunctioning due to DNA damage. Nuclear DNA is much more robustly protected and repaired than mitochondrial DNA. The challenge lies in the changes and additions needed to route the generated proteins from the nucleus back to the mitochondria. So far this has been achieved for only a few of the necessary genes, and it is a time-consuming process. Gensight is trialing this technology for a gene involved in an inherited mitochondrial disorder, for example, but everything they come up with as a technology platform is applicable to the end goal of carrying out this backup gene therapy for all mitochondrial genes, so as to remove this contribution to aging.

Longitudinal RNA-Seq Analysis of Vertebrate Aging Identifies Mitochondrial Complex I as a Small-Molecule-Sensitive Modifier of Lifespan

Here, we have used the short-lived killifish N. furzeri to perform a longitudinal study of gene expression during adult life. N. furzeri is the shortest-lived vertebrate that can be cultured in captivity and replicates many of the typical hallmarks of aging. The recent sequencing of its genome, and the establishment of genome-editing techniques makes it a convenient model species for experimental investigations on aging in vertebrates. Here, we report the observation that individual N. furzeri of different lifespans differ in their transcript levels at an early adult age. Further, we observed that genome-wide the rate of age-dependent gene modulation was lowest in the longest-lived individuals, suggesting that they are characterized globally by a slower aging rate.

Intuitively, differences in gene expression between individuals that differ in their aging rate should become larger as age progresses. However, we do not observe this consistently as differences between the longevity groups were larger at 10 weeks than at 20 weeks, and numbers of differentially expressed genes between adjacent age steps showed a U-shape. Our observations in N. furzeri are rather consistent with the results of a large-scale study of human aging in the prefrontal cortex: rates of age-dependent changes in gene expression are high during childhood, decline until age 20 years, rise again after 40 years, and, by the age of 60, exceed those observed during teenage years. The main result of this paper is that conditions favoring longevity are laid out during early adult life when inter-individual differences in gene expression are larger, and this result is consistent with observations in C. elegans where knock down of complex I genes or mitochondrial ribosomal proteins during development is necessary and sufficient for life extension.

Reduced mitochondrial mass and function is among the most conserved hallmarks of aging and is specifically observed also in N. furzeri at the levels of gene expression, mitochondrial mass, and mitochondrial functional parameters. Mitochondrial biogenesis is intimately connected to conserved longevity pathways such as the mTOR- and IGF1-pathways. Improved mitochondrial function is currently considered as a crucial component for the health-promoting action of physical exercise and calorie restriction. However, knock down of complex I genes expression induces life-extension in worms and flies. This contradiction between physiological age-dependent regulation and effects observed after genetic manipulations is also observed for another major longevity pathway: the IGF-I pathway. Genetic dampening of IGF-I signaling is life-extending in several models, yet growth hormone and IGF-I concentrations in blood decline during aging. Also, expression of mitochondrial ribosomal proteins declines during aging, but knock down of these proteins induces life-extension.

Complex I of the respiratory chain can be potently inhibited by small molecules, such as rotenone (ROT). The effects of ROT may also be explained by the mitohormesis hypothesis postulating that life-extending interventions act via a transient burst of free radical oxygen species that induce adaptive stress responses. In C. elegans, life-extending effects of calorie restriction or RNAi of the insulin signaling pathway are blocked by antioxidants, and partial inhibition of complex I by ROT prolongs lifespan, generates a burst of ROS, and antioxidants block the life-extending effects of ROT. Increasing the dosage of ROT, however, is life-shortening in N. furzeri, as it is expected by a hormetic effect. Life-extending effects of metformin on mice may also be mediated by mitohormesis, since this drug can inhibit complex I, and effects of metformin in C. elegans were directly linked to mitohormesis via induction of peroxiredoxin.

We observed that treatment with a dose of ROT three orders of magnitude below the median lethal concentration can revert the transcriptional profile of brain, liver, and skin to patterns characteristic of younger animals. This effect was seen not only in N. furzeri, but was replicated in the zebrafish D. rerio, showing that ROT effects are not linked to the peculiar physiology of this short-lived species. In D. rerio, effects of ROT were dependent of the length of treatment: treatment for 3 weeks had a smaller effect than a treatment of 8 weeks. The median lifespan of D. rerio is in the order of 3 years, therefore 8 weeks represent ∼5% of median lifespan indicating that a relatively short treatment can cause rejuvenation of the transcriptome. In summary, our data suggest complex I as a new potential target for prevention of age-related dysfunctions.