Identification of the molecular substrate for L-lactate-mediated catecholamine signalling in the brain

It is becoming evident that the function of the brain under normal and pathological conditions cannot be understood without taking into account the role of non-neuronal elements of this complex organ, numerous star-shaped cells known as astrocytes. The roles which astrocytes play in the brain are only currently emerging. It is established that these cells have a unique ability to store glucose in the form of glycogen and convert it into lactic acid (lactate) which they then release. Lactate for many years was thought to be used as an intermediate in the energy generating machinery of the body. However we have evidence that it has also another role in the brain where it acts as a transmitter or messenger between astrocytes and neurones.
Using cutting edge technologies we have discovered that lactate can act on specific subsets of brain neurones which release a critical neurotransmitter and modulator, called noradrenaline. Action of lactate on these neurones excites them and makes them release more noradrenaline. This mechanism could be involved in many functions of the brain, including regulation of sleep-wake cycle, appetite, emotions, blood pressure, attention and others. This needs to be further investigated. Our research implies that there is a dedicated excitatory receptor for lactate in the brain and we aim to discover it in this project. We are therefore proposing a programme of research which will look for the candidate genes which could encode for this new receptor, verify its sensitivity to lactate and prove that it is able to confer sensitivity to lactate to neurones which are naturally not sensitive to lactate. These steps will form the basis for more in depth studies aimed at detailed characterisation of this novel mechanism and ultimately to the understanding of its role in health and disease. Without knowing the molecular identity of the lactate receptor further progress in this direction is essentially impossible. However, when we find the protein responsible for lactate effects we will be in a position to design drugs which act on this receptor and via this route potentially develop new therapies.

We have recently found that L-lactate (LL) acts as a powerful excitatory stimulus on central noradrenergic neurones, depolarising them and triggering release of noradrenaline. Interestingly, LL is not excitatory to non-noradrenergic neurones in the hippocampus. Our observations allow us to propose the existence of a yet unknown G-protein coupled receptor for LL which is coupled via Gs proteins to cAMP production. Importantly, the EC50 for this new receptor is ~0.5 mM which is within the physiological range of LL concentrations in the brain. Given the potency of this pathway, ubiquitous presence and prominent activity-dependent fluctuations of LL on the one hand, and the paramount role of noradrenergic transmission for the normal functioning of the brain on the other, this new signalling mechanism could be of major biological importance. We have performed an initial transcriptomic analysis of an LL-sensitive area and found a substantial number of currently orphan G-protein coupled receptors expressed. We hypothesise that the new LL receptor is one of them. The project involves a detailed and selective transcriptomic analysis of cells which are sensitive to LL in comparison with cells which are not; wide-scale screens of orphan GPCRs using a beta-arrestin assay; identification of the LL-sensitive receptors using a cAMP assay in cell lines; detailed characterisation of the pharmacological profile of the responses in the areas which we know to be LL sensitive in search of initial lead agonist and antagonist compounds; pharmacological analysis of any candidate receptors; and finally, an experiment where we will attempt to confer LL sensitivity to naturally LL insensitive neurones by expressing in them the newly identified putative target receptor. These studies will form the basis for further analysis of the physiology and pathophysiology associated with this signalling mechanism and for design of small molecule modulators which could have novel therapeutic properties.

A novel receptor involved in fundamental adaptation of brain functions towards active states and autonomic regulation is likely to incite considerable interest in the public, research communities and pharmaceutical industry. In order to ensure the maximum impact of our findings, we will ensure dissemination of information to a range of relevant audiences:

Scientific audiences
A new receptor with intriguing functional properties will answer some and open up a host of new scientific questions relevant to various fields of neuroscience and neuro-degeneration. We will address researchers by the well-established pathways of
- Presentation of our work at conferences such as IUPS, Society of Neuroscience, Physiological Society, FENS, Gordon Conferences
- Publication in high profile scientific media
- Engagement with young research audiences, including under- and postgraduate students in UK and internationally, for example Young Physiologists (UK) and Neuronus (international scientific youth conference).
- Training in multidisciplinary research skills such as use of viral vectors and optogenetics, in combination with confocal imaging and physiological experiments.

Public audiences
Since lactate is a generally well-known metabolite, findings that explain its central roles will attract interest of the public, creating opportunities to publically highlight issues surrounding healthy brain functions and age-dependent decline. In conjunction with scientific publication, we will
- Prepare press releases in collaboration with the University of Bristol press office
- Publicize findings through websites such as the Bristol Neuroscience platform lectures

Pharmaceutical industry
The receptor which we are seeking is a potentially promising drug target to address health problems associated with a number of diseases with high potential for drug therapy. Drug industry will be informed of our work through
- Our current collaborations with industrial partners at Novartis and GlaxoSmithKline with whom we communicate regularly
- Development of links with clinical research once the relevant protein has been identified
- Pathways that impact on the scientific community (see above) which may attract further interest from pharmaceutical industry. New contacts established in this way will be expanded and any potential patentable issues negotiated via the University of Bristol Research and Development department.