The Resilient Brain. Imaging Biomarkers of Brain Metabolic Reserve

This proposal aims to validate two innovative biomarkers of brain resilience to ageing. The biomarkers use two well-known and widely available imaging technologies, magnetic resonance imaging (MRI) and positron emission tomography (PET) that are presently used both in animals and humans.
The quest for biomarkers of ageing is a difficult one as their necessary requirement is prediction power - ideally, when studying an intervention aimed at modulating the ageing process, we would like to know its effect on the ageing trajectory as soon as possible without having to wait for the ageing process to eventually happen. In the case of brain, this can be achieved if we can measure its resilience - and the one relevant biological parameter determining the ability to withhold cellular pathology is its metabolic reserve.
Why metabolism? In the average adult human, the brain represents about 2% of the total body weight yet it accounts for 20% of all the energy consumed, 10 times that predicted by its weight alone. Note that this very high rate of energy consumption is required only by its resting state while the additional energy associated with any mental activity is remarkably small, often less than 5% of the baseline.
In this highly energetic organ, metabolic reserve is the amount of energy reserve that brain cells can still use to fight all those pathological phenomena that we call ageing. Big metabolic reserve will then mean great resilience to ageing, poor or no reserve means little or no resilience.
When measuring energy reserve we wish to target the functioning of small cellular organelles called mitochondria. There are two key observations about mitochondria.
1)They are the fundamental elements of the cellular machine performing aerobic respiration e.g. using oxygen to oxidise the products of glucose and convert them into energy.
2) The process of mitochondria maintenance is highly dynamic with a constant turnover of these components particularly the older and damaged ones.
We have focused on these two key aspects of mitochondrial function and devised two measures.
1) MEASURE 1 - We wish to measure mitochondrial respiratory reserve capacity by measuring oxygen cellular metabolism with MRI before and after a challenge with Methylene Blue (MB). MB is a compound, already authorized for human use, that induces substantial increases (>40%) in cell respiration. Using MB we can expand mitochondrial capacity to its limit, far more than using any mental task that requires very little incremental energy and is far less controllable in an experimental or clinical setting.
2) MEASURE 2 - We wish to measure the level of expression of one particular mitochondrial protein, called the 18Kd translocator protein or TSPO. TSPO regulates mitochondrial turnover and acts as a quality controller driving the process that removes defective mitochondria following damage or stress. Importantly TSPO levels can be measured in-vivo by PET in both animals and humans.
We will validate these measures by following through time two groups of rats. One group of rats will be submitted to an intervention which is known to benefit the brain ageing process and mitochondria function in particular. The treatment will consist in environmental enrichment and dietary restriction by which we will mimic human healthy lifestyle. During their lifetime, we will continuously monitor their cognitive ability using appropriate tests and we will verify whether our measures, taken early on, predict their mental decline.
Once validated, these measures will be extremely powerful tools for ageing research. They will shorten timings in experimental settings and will allow an optimal use of animals as they do not require animal sacrifice. They will be immediately translated to humans as both imaging technologies and the pharmacological challenge used have been separately validated and authorized for human use.

The great challenge of ageing research stays in its inherent time-scale where the ideal biomarkers are those that can qualify treatments at baseline without having to wait for the degenerative process to take place. Here we are concerned with brain ageing and focusing on metabolism as the brain has, by far, the highest energy demands of any organ based on its size. Importantly, the brain works at the border of its energy envelope as the majority of this energy is used to support neuronal activity and functional processes and is little affected by task performance. The metabolic reserve theory indeed proposes that a brain that is metabolic efficient is more resilient to declining cognition. It is now established that lifestyle determinants of metabolic reserve promote neuroprotection by increasing brain metabolic efficiency and creating an energetic buffer that can be used by the cell to fight pathology.
Mitochondria are fundamental to the maintenance of cellular health but with age they lose capacity for efficient ATP generation. Damaged mitochondria are generally cleared by mitophagy and protects from ageing-associated degeneration. This leads us to propose two in-vivo measures of brain metabolic reserve:
1) Cerebral Oxygen Metabolic Reserve obtained by measuring oxygen metabolism with quantitative BOLD MRI before and after administration of methylene blue (MB), an inducer of metabolic respiration already approved for use in man. The use of MB will allow us to tap into cellular energetic buffers that would not be otherwise accessed by experimental tasks (e.g. sensory stimulation).
2) Density expression of the 18K Translocator Protein (TSPO). TSPO is a mitochondrial protein with a fundamental role in mitochondrial quality control that in man is visualized using Positron Emission Tomography.
Measures will be validated in a longitudinal setting to determine their potential to predict cognitive resilience in well-characterized models of healthy ageing.

This proposal focuses on the validation of two novel measurements of brain metabolic reserve to be used in the study of ageing and neurodegenerative disorders in animal and, in the future, human models. This project, if successful, will have substantial and lasting impact.
The increasingly large ageing population places great pressure on the HEALTH AND SOCIAL CARE SYSTEMS. Currently 16% of the European population is over 65, with this figure to reach 25% by 2030. Effective and predictive biomarkers are key to develop therapies and products aimed at improving lifelong health and wellbeing. Effective biomarkers will allow INDUSTRIES to effectively accelerate the development of products aimed at the ageing population market and enable REGULATORY AGENCIES to test the anti-ageing treatments that are now flooding the market. These include, for example, some poorly validated interventions such as improving antioxidant status and hormone replacement therapies, including growth hormone, testosterone, dehydroepiandrosterone, and melatonin (Butler et al, Journal of Gerontology: Biological Sciences, 2004, Vol. 59A, No. 6, 560-567).
It is easy to predict the immediate translation of these methodologies also to the study of neurodegenerative disorders. These are conditions that result in the progressive degeneration or death of nerve cells. They include Alzheimer's disease and other dementias, Parkinson's disease, Huntington's disease, motor neuron disease and multiple sclerosis. In the UK it has been estimated that dementia alone costs the ECONOMY £17 billion pounds a year, hence any effective development in this area will impact not only patients but also CARERS, HEALTH PROFESSIONALS and SOCIETY AT LARGE.
The methodologies proposed are destined to be used on MRI and PET scanners for animal and human use. If effective, they will expand their number and their use of these existing technologies in research, industry and in the clinic making them more economically viable. Hence they will have direct impact on MEDICAL DIAGNOSTICS INDUSTRIES (General Electric, Siemens, Phillips, Bruker etc.).
Importantly, by targeting in-vivo biomarkers of mitochondrial function, we are working in a largely un-explored area of the neurosciences. In the recent past, concerns have existed about the growing number of companies disinvesting from neuroscience research in the UK. This has negatively impacted JOBS in this area and the UK ECONOMY at large. The tide has been turning recently with companies with long heritage in brain research such as Lily, Johnson and Johnson, Lundbeck and GSK supporting innovative programmes such as One Mind for research (1mind4research.org) to create a united effort to advance translational research for mental health between GOVERNMENT, ACADEMIA and INDUSTRY. We are already part of one Wellcome supported consortium that unites the organizations above for a strategic award in the area of dementias and mood disorders, all related to ageing, where the need for effective markers of brain bioenergetics is strongly felt.
Finally, it is worth remarking the unique nature of the proposing team with members working in an original setting that stretches from cellular biology to pharmacology, brain energetics, physics, engineering, nuclear imaging and psychology. The scientists and RAs working in this project will have a unique opportunity to expand their KNOWLEDGE BASE from cell biology to cognition, increasing their SKILL-SET in unprecedented fashion. As academics we will be able to communicate the hopefully successful return of this investment to our student population and to the PUBLIC.