Cholesterol esters of oligodendrocytes in developmental and ageing brain

Our world now has a big ageing population with more than 900 million people aged 60 and over living on the planet, and this number is set to double reaching 2 billion by 2050. This is one of the biggest challenges facing our time since cases of age-related diseases will be rising, placing a heavy burden on the health care systems of all affected countries including the UK. Ageing can cause a gradual decline in cognitive capacity, often manifested as forgetfulness and decreased problem solving ability in people of old age, and progressive cognitive decline is an ominous sign of developing dementia and Alzheimer's disease (AD) - the most common neurodegenerative disorder. To find the cause of age-related cognitive decline, we first need to understand what happens in our brain in ageing.

Brain is composed of different types of neurons and glial cells. Neurons are the function units in the brain; they communicate with each other through synapses and network together into circuits that process specific information and control particular biological functions. Glial cells provide support and protection for neurons and, importantly, form "myelin", the fatty insulating sheath enwrapping nerve fibres. Myelin enables extremely rapid propagation of nerve impulses, substantiating rapid and fine control of movement and high-speed brain computation. Age-related alterations to myelin structure have been seen in the brain of old primates, resulting in myelin breakdown.

Lipids account for 80% of myelin dry weight and among these lipids, cholesterol is thought able to drive myelin synthesis and provide stability for myelin sheaths. 70-90% of the brain cholesterol resides in myelin. Brain needs to synthesize cholesterol locally because there exists a barrier between our blood circulation and brain, which prevents materials including free cholesterol in the bloodstream from entering the brain. Excess brain cholesterol can be converted into cholesterol esters (CEs) by an enzyme, ACAT1, and stored as lipid droplets inside cells. Increasing research findings have linked CE accumulation to age-related neurodegenerative diseases such as AD.

In the brain, myelin is supplied by oligodendrocytes (OLs). OLs are generated from OL precursor cells (OPCs) through cell differentiation during OL development. Although OLs show remarkable longevity, OPCs can continue to proliferate and differentiate into OLs in adulthood, but these abilities decline with age. Our preliminary data show that ACAT1 is expressed in mouse OLs and OPCs, and we also discovered that ACAT1 expression along with the levels of certain CEs is markedly increased in old age in mouse brain tissues that are rich in myelin. Taken together, we hypothesize that CE accumulation in OLs may occur in ageing, impacting on OL function, myelin integrity and consequently on brain function.

In this proposed study, we will analyse lipid composition of mouse myelin collected from the brain at different ages to identify myelin CEs and reveal how their levels change with age. We will also delete Acat1 specifically in mouse OPCs and/or OLs by genetic engineering to block cholesterol conversion into CEs in those cells; we will then collect brain tissues from these mutant mice at different OL development stages or at different old ages to examine the effect of lacking CEs on OL development or on OPC function, OL generation and myelin maintenance in ageing. In addition, we will feed mice of old age on a high cholesterol diet and find out the impact of high cholesterol intake on myelin cholesterol/ CE levels, OPCs, OLs and myelin structure in ageing.

Brain cholesterol homeostasis is believed critical to brain function. Excess brain cholesterol can be enzymatically converted into cholesterol esters (CEs) by ACAT1 for intracellular storage as lipid droplets, and CE accumulation has been linked to age-related neurodegenerative diseases. 70-90% of brain cholesterol resides in myelin, which is supplied by oligodendrocytes (OLs) in the brain. Cholesterol is thought able to drive myelination and provide stability for myelin sheaths. Our preliminary data indicate that ACAT1 is expressed in both OL precursor cells (OPCs) and OLs in mice. Interestingly, we also found that ACAT1 expression and the levels of certain molecular species of CEs are markedly increased in old age in mouse brain white matter tissues that are rich in myelin. We hypothesize that CE accumulation in OLs may occur in ageing, impacting on OL function, myelin integrity and consequently on brain function. Currently little is known about the role of CEs in OLs. In this proposed study, we will purify myelin for lipidomics analysis to reveal how myelin CE levels change with age. We will also find out the role of CEs in OL development. Moreover, we will explore if CEs play a role in myelin maintenance in ageing as well as in age-related decline in OPCs' ability to proliferate and differentiate into OLs. We plan to block CE production specifically in OL lineage cells, OPCs or OLs by breeding Acat1 conditional knockout mice (Acat1fl/fl) to OL lineage specific Cre, OPC- or OL-specific inducible Cre mice; in the brain of the progeny, we will examine the effect of lacking CEs on OPC proliferation/ differentiation, OL generation, OL morphology and myelin structure during OL development and in ageing with histological methods. In addition, we will feed ageing mice on a high cholesterol diet and evaluate the impact on myelin cholesterol/CE levels, OPCs, OLs and myelin structure in the ageing brain.

Our world now has a big ageing population with more than 900 million people aged 60 and over living on the planet, and this number is set to double reaching 2 billion by 2050. This is one of the biggest challenges facing our time since cases of age-related diseases will be rising, placing a heavy burden on the health care system of all affected countries including the UK.

Most of the brain cholesterol resides in myelin, which is synthesized by oligodendrocytes (OLs) in the brain. Excess brain cholesterol can be stored as esterified cholesterol in the form of intracellular lipid droplets and accumulation of cholesterol esters has been linked to age-related neurodegeneration. Currently, little is known about the role of cholesterol esters in OLs. In this project, we will explore this new topic and try to find out if cholesterol ester levels play a role in OL development and in age-related decline in OL function. Our research will have immediate impact on the academic circle and may have social and economic impact in the long term.

Impact on academic community. At the moment, information on OL cholesterol esters is very limited. Our proposed study will reveal how the levels of cholesterol esters in myelin change with age, and find out if changing the balance between cholesterol and cholesterol esters in OL lineage cells can have an effect on OL development as well as on OL generation and myelin structure in ageing. Therefore, our work will contribute to building the knowledge base in academic community and inspire other researchers to study brain dysfunction from a new angle. In addition, the research assistant employed on this project will benefit from expanded knowledge of lipidomics and neuroscience as well as excellent training in histological techniques.

Impact on business/industry. This study is designed to discover how OL cholesterol ester levels affect brain function. Accumulation of cholesterol esters has been implicated in age-related neurodegenerative diseases. In ageing brain, age-related decline in adult OL generation and alterations to myelin structure may occur, causing myelin degeneration and consequently brain malfunction. In this project, we will try to find out the mechanisms behind ageing from the perspective of OL cholesterol balance. Our findings will shed new light on how brain cholesterol metabolism affects the ageing process. Therefore, this study has the potential to uncover new drug targets and attract the interest of pharmaceutical industry. In addition, our data will reveal the effect of high cholesterol diet on myelin maintenance in ageing, which could be of interest to the food industry.

Impact on funding bodies and government policy makers. OLs may play an important role in ageing and age-related brain dysfunction in more pro-active ways than we think. Our study will illustrate a new aspect of OL function and a new mechanism of age-related brain malfunction. Our findings will help raise funding bodies' and government policy makers' awareness of the newly discovered roles for OLs in brain function, ageing and age-related neurodegenerative diseases and hopefully help attract more funding for OL research.

Impact on the general public. Understanding how our brain works and what happens in our brain in ageing is of interest to the general public. Our research will show how fascinating our brain is and how precisely all kinds of cells and processes coordinate to make our brain function properly. Hopefully our work can inspire more people in the younger generation to take up neuroscience research.