Local sleep homeostasis and single cell rest

Sleep and wake are strictly regulated processes. The need for sleep ("sleep pressure") increases gradually during the periods that we are awake, as reflected by us feeling tired. However, this sleep need dissipates when we sleep, reaching the lowest levels just before we wake up fresh and rested. It has therefore been proposed that sleep is necessary for various restorative processes, which include metabolic recovery and renormalisation of brain functions. Suppressed immune function, reduced mood, and increased risk of obesity and cardiovascular disease are only a few of many detrimental effects resulting from chronic sleep deprivation or disrupted and mistimed sleep. Most direct evidence for the immediate consequences of sleep deprivation comes from psychological experiments. Impairments range from the slowing of responsiveness, reduced attention and increased variability in task performance, to the reduction of higher order brain functions such as executive functions, memory or emotional control. This indicates that the brain is among the first targets that are impacted by sleep deprivation. However, despite this extensive knowledge of the importance of sleep, much controversy remains about the biological mechanisms that convey the numerous benefits of sleep to the brain and body.

The predominant idea that sleep plays a "restorative" role fits well with our subjective experience. However, the question remains: what precisely needs to be restored after a period of wakefulness and how do restorative changes occurring at the level of individual cells benefit from a global shut down occurring during sleep? We have recently advanced a new hypothesis that the biological function of sleep is to allow for vital "repair and maintenance" of the neurons in our brains. We have also proposed that these repair functions can only occur if the rest periods of individual neurons are aligned precisely at a time scale of seconds or less. The reasoning for this is that we have billions of neurons in our brain, and each of them is connected with thousands of other neurons, all of whom are constantly talking to one another. Therefore, it appears that our neurons cannot rest and repair themselves independently, but must shut down at the same time so that they do not disturb one another and allow each individual cell to obtain the rest it needs. The flip side of this is that when areas of our brain are unable to "fall asleep", they remain in a state of "local wakefulness" that leads to us experiencing a bad night's sleep. Indeed, there are intriguing parallels between the repair processes in brain cells after waking and those observed in muscle cells after exercise. However, whilst you can rest your muscles while being awake, to rest the brain during waking is much more difficult. If neurons attempt to obtain rest while we are awake, it is not only much less efficient but also has serious negative effects on our performance. Similar phenomena can be found outside of biology. The London Underground system, for example, can only function properly during the day because it has extensive maintenance every night, during which all trains stop running between the interconnected stations. We suggest that sleep allows a similar period of maintenance for the brain.

In our project we will combine expertise in cutting edge techniques, including electrophysiology, molecular-genetics and pharmacology, and will perform research at several distinct scales - from single cells, to local networks of cells, to the behaviour of the organism. Our project will thus make a major advance in the field of sleep neuroscience and the knowledge obtained will benefit numerous clinical applications that are concerned with the prevention and treatment of sleep disturbances, including improving the management of sleep in shift workers and the prognosis of patients suffering from neurodegenerative disorders, such as Alzheimer's disease.

Sleep is of vital importance for health, and insufficient or disrupted sleep has been linked to a broad range of neurological disorders. Crucially, despite decades of research many fundamental questions remain, such as why sleep deprivation impacts brain function and what are the specific mechanisms that convey the benefits of sleep to the brain. We propose that the most important role of sleep is to provide individual neurons with a period in which to perform processes of cellular maintenance that are required after periods of elevated synaptic activity. In particular, we hypothesise that sleep enables neurons to deal with cellular stress associated with the accumulation of misfolded proteins in the lumen of the endoplasmic reticulum - so called "ER stress". Whilst most cells in the body can presumably undergo rest relatively independently, we propose that neurons must rest together due to their extensive interconnectivity. Thus, rest at the single neuron level requires both synchronised network activity and withdrawal from the environment.

In this project we will test this hypothesis for the first time using a combination of electrophysiological, molecular and pharmacological techniques. First, we will investigate whether elevated synaptic activity during waking is associated with both an increased local sleep drive and naturally induced cellular stress. Second, we will induce cellular stress directly using local application of drugs, or by manipulating proteins, which lead to enhanced protein misfolding and accumulation. Finally, we will investigate whether sleep is necessary for the local recovery from ER stress and elucidate the time scale of this recovery. This project will address the function of sleep from the molecular to the behavioural level, it will develop novel tools and concepts for understanding sleep regulation and it will inform future studies that aim to target sleep as a means of preventing cognitive decline and neurodegenerative disorders.

This research is of strategic relevance in terms of the extent to which the proposal meets the priorities identified by the MRC Neurosciences and Mental Health Board, as the project proposes research relevant for sleep and neurodegenerative disorders. Understanding the basic physiology of sleep is most essential for developing proper therapeutic strategies to improve our sleep. This project will be of high value not only for the academic community, but will also have high impact for society, the UK economy and the population's wellbeing.

1. Societal impact. Poor sleep is among the most prevalent complaints observed in epidemiological studies, and the second most common overall complaint reported in primary care settings after pain. A recent survey revealed that in the general population, insomnia prevalence and hypnotics use has shown a steady increase over a 15-year period. Sleep problems bring significant challenges to society, including in such key areas as social care, workforce and transport. Our project will investigate the link between sleep need and cellular stress, which has been implicated in age-related neurodegenerative disorders, sleep disruptions and dementia.

2. Economic impact. This project has the potential to make a substantial economic contribution. The amount of sleep and its quality deteriorate with increasing age, and neurodegeneration has been implicated in this relationship. About 20% of all people aged 65 and over (i.e. ~ 2 million in the UK alone) report chronic insomnia symptoms, which may worsen neurodegenerative processes. As a result, elderly people are the main consumers of hypnotics, which on one hand improve the quality of life, but on the other, often have unwanted side effects. Furthermore, according to the UK Health and Safety Executive (HSE), sleep-deprivation related fatigue can lead to errors and accidents, ill-health and injury, and reduced productivity and has also been implicated in 20% of accidents on major UK roads, and is said to cost the UK £115-£240 million per year in terms of work accidents alone. The proposed research has a high potential to provide a much needed stimulus for the pharmaceutical industry to invest more in sleep therapeutics and clinical trials, as this work may furnish them with new tools and improved assays for testing the efficacy of their products. Thus, this project will have an impact on the pharmaceutical industry and thereby on the economy. Moreover, improved diagnostics of sleep disorders will reduce the costs associated with out-patient visits to the clinics and thereby decrease burden on tax-payers.

3. Well-being impact. Insufficient and disrupted sleep loss affects subjective well-being and health in general. Surprisingly, it is still unclear why sleep is so critical for our brain and bodily functions, and our project will address a specific and testable hypothesis. The proposed project will contribute to changing the attitude towards sleep and has the potential to inform clinical advice that is provided by physicians.

4. Biomedical research impact. The UK is a world leader in pioneering biomedical research, which receives multi-billion investments from the government, charities and other funding bodies. While most funding is allocated to individual laboratories, it is increasingly recognised that collaborative initiatives are necessary to achieve important breakthroughs. This interdisciplinary project brings together co-PIs and collaborators from Oxford, UCL, Cambridge and The University of Pennsylvania to tackle a fundamentally important biological question relevant for health. The project involves both basic and translational/clinical researchers and capitalises upon expertise in large-scale multi-centre collaborative research, data sharing and standardisation of experimental designs. This project will establish new standards and improve the quality of collaborative biomedical research in the UK.