Role of Mitochondrial Reactive Oxygen Species in Stress Adaptation during Ageing

Nowadays, ageing is one of the main questions that modern biology needs to answer. We need to understand how and especially why we age to fully understand the process of evolution. In addition, a growing ageing population is one of the main problems in United Kingdom. The only way to alleviate the suffering caused by age-related degenerative disease (e.g. Alzheimer, Parkinson, cancer or diabetes) is to fully understand the underlying evolutionary forces, which drive ageing and design strategies to delay the ageing process. Mitochondria are the powerhouses of the cell generating most of the energy required for survival. These small cell factories deteriorate during ageing, failing to deliver the energy required for cellular maintenance. The reason why mitochondria fail is currently unknown, but it could be related with the way they produce energy. To operate, mitochondria use oxygen as final electron acceptor. Normally, this oxygen is safely managed by mitochondria being completely reduced to water with four electrons and two protons. However, in a minimal number of occasions oxygen is incompletely reduced (with less than four electrons) producing the so-called Reactive Oxygen Species (ROS) that can damage all cellular components.

The Mitochondrial Free Radical Theory of Ageing (MFRTA) was a popular theory to explain ageing in the past century. MFRTA proposes that ROS, produced as by-products of respiration, cause oxidative damage that accumulates and causes ageing. MFRTA is mainly supported by correlative data. Oxidative damage accumulates with age, and mtROS levels are altered in degenerative disease associated with ageing. However, direct experimental evidence fails to support MFRTA. Increasing mtROS does not shorten lifespan, and antioxidant supplementation has poor effects on health. It has been shown that mtROS are instrumental for cell differentiation, the immune response and stress adaptation. In conclusion, the contribution of mtROS to ageing is unclear. Because of the importance ROS have in pathological and non-pathological situations it is imperative to understand the physiological role they play in vivo.

In this proposal, we aim to understand in detail the role ROS play in normal physiology and in stress adaptation, particularly during ageing. Based on our preliminary results, we hypothesize that there are two different types of ROS populations. One population is good, and its generation is associated with the activation of mechanisms that clean up the cells. When these ROS are suppressed quality control mechanisms do not work properly and cellular homeostasis is lost. This would explain the negative consequences associated with supplementation or overexpression of antioxidants. The other population is deleterious, and it is produced only when mechanisms of mitochondrial quality control fail. These ROS are characterized by a very aggressive chemistry led by high levels of free iron and hydroxyl radicals.

Using the power of fruit fly genetics we will generate new transgenic models that will allow a precise manipulation of these two ROS populations in vivo. We will use this new technology to characterize the downstream physiological responses activated by ROS. We aim to find the exact pathways and genes that may be targeted by specific drugs or genetic interventions. These interventions should help to extend healthy lifespan. Since essential metabolic pathways are highly conserved during evolution, it is expected that similar strategies may be implemented in humans to delay ageing and prevent the onset of age-related diseases.

During ageing, mitochondrial function is severely reduced. However, it is unknown why mitochondrial function deteriorates, and if this is a cause or a consequence of ageing. In many cell types, mitochondria are the main generators of Reactive Oxygen Species (ROS) and accumulation of oxidative damage has been postulated as the main cause of ageing in the past. However, mitochondrial ROS (mtROS) has two faces. One is negative, and decreases survival when ROS overcomes antioxidant defences. The other is positive, and participates in normal cellular signalling, extending lifespan when properly induced.

Our main hypothesis is that there are two main populations of mtROS. The first ROS population is generated when the ubiquinone (CoQ) pool becomes over-reduced. We propose that these ROS are implicated in the activation of quality control mechanisms that remove damaged molecules and organelles. This is the most prevalent ROS population in young cells. During ageing the accumulation of different types of damage blocks the transfer of electrons within respiratory complex I (CI). Under these conditions, electrons accumulate within CI stimulating the generation of superoxide that attacks the iron-sulphur clusters within CI. This causes the release of ferrous iron and the generation of hydroxyl radicals. These ROS are highly deleterious and are responsible for the negative effects associated with oxidative stress.

The main objective of this proposal is to understand by which mechanism(s) mtROS are physiologically produced, to delineate the role of mtROS in health and disease, and to find ways to manipulate mtROS in order to extend healthy lifespan. This main objective will be achieved through three specific aims: (i) manipulating the redox state of the CoQ pool, (ii) blocking the transfer of electrons within CI, and (iii) describing the physiological consequences produced as a consequence of (i) and (ii).

Our main aim is to understand the role of mitochondrial Reactive Oxygen Species in cellular physiology. We will pay special attention to how ROS contribute to the decline in the capacity to confront stress that is observed during ageing. Three main social agents will benefit from our research: (i) academia, (ii) industry and (iii) society. Academia: ageing is one of the most fascinating questions in modern biology. Our project will increase knowledge at three fundamental levels molecular (mtROS), cellular (stress) and physiological (ageing). We will create new models to manipulate mtROS in vivo, we will describe specific mechanisms of stress adaptation during ageing and we will study new ways to extend lifespan manipulating evolutionary conserved signalling pathways. Scientists working on ageing and age-related disease will benefit from our research. Society: ageing is a priority of the research and social policies of both the United Kingdom and the European Union. It is imperative to find a way to extend healthy lifespan of the population and guarantee the long-term independence of senior citizens. Only basic research in ageing guarantees find new ways to delay and reverse ageing and prevent the onset of ageing-related diseases (Parkinson's and Alzheimer's disease, cancer, sarcopenia, etc). Therefore, our research will have a direct benefit improving health of the population. Additionally, publication of excellent research on ageing science catches mass media attention, and increases the public interest in healthier lifestyles. This alone could save millions in social security services. Industry: ageing is an emergent market for pharmaceutical companies. Our research will show new mechanisms to delay ageing and to identify specific targets in the form of signalling pathways and redox-regulated genes. Additionally, we will create new ageing and Parkinson's disease models to screen for new drugs and genetic interactions. This should be useful for pharmaceutical companies in order to design and test specific drugs against age-related diseases.