A Drosophila single-cell resource for brain metabolism research

A healthy human fetus grows at an impressive rate as it develops inside the mother's uterus. In about 5% of pregnancies, however, nutrients and oxygen cannot be efficiently supplied from mother to fetus. When this metabolic stress occurs, it is not always possible for the fetal body to continue to grow at the normal rate. Nevertheless, in many of these cases, the growth of the fetal brain remains remarkably unperturbed. Our research aims to identify, as yet unknown, protective processes that spare the growth of the brain over that of the body. Towards this goal, our previous work has modelled some aspects of brain sparing in the fruit fly Drosophila, which shares many genes with humans - including about three-quarters of those linked to human diseases. Although the brain of Drosophila is much smaller and less complex than that of humans, they both contain very similar cell types - neural stem cells, neurons and glia. We therefore propose that Drosophila, with its relatively simple nervous system, can be used to replace a proportion of the brain sparing experiments that would otherwise be carried out in mice or other animals. Our research plan has three main aims. In the first aim, we will use a technology called single-cell sequencing to produce a map or atlas of the genes active in every one of the tens of thousands of cells in the normal versus the metabolically stressed Drosophila brain. In this way, we will identify which genes are most active during Drosophila brain sparing, anticipating that some of the equivalent genes will also be active during mammalian brain sparing. In the second aim, we will use the sophisticated genetics possible in Drosophila to test which of the highly active genes are the most essential for the growth processes that drive brain sparing. In the third and final aim, we will use the results of the sequencing and genetic approaches to pinpoint how the different cell types in the brain use genes to communicate with each during brain sparing. An enduring legacy of this project will be a Drosophila brain atlas, an open-access web resource that can be used by all scientists interested in brain sparing and brain research. This atlas may also help researchers to work out how metabolism in the mammalian brain is linked to health and disease.

In developing humans and other mammals, malnourishment and placental insufficiency alter fetal metabolism, leading to growth restriction. In many cases, however, the growth of the brain is less affected than that of the body. This brain sparing response helps to safeguard cognitive development but it is imperfect - in some cases leading to adult offspring that are prone to neural disorders. To understand fetal brain sparing at the molecular level, we need to determine how environmental stresses change metabolic gene expression and function at the level of single cells, especially neural stem cells. The gold standard for mechanistic studies of brain sparing are mouse models, where animals are bred with complex genetic backgrounds and then subjected to metabolic stresses. We previously established an alternative model for brain sparing that harnesses the powerful genetics of the invertebrate species Drosophila. We now propose to build upon this foundational work to generate the first Drosophila single-cell atlas of brain metabolism. Importantly, our open-access web resource (the Drosophila CNS Stressome Atlas) will combine gene expression data with functional information from an RNAi screen of evolutionarily conserved metabolic genes. It will be designed to be used by the mammalian neurobiology community, in conjunction with existing mouse brain atlases of "unstressed" gene expression. This in silico evolutionary conservation approach will not provide an alternative to all mice brain sparing experiments but it will replace some of those at the early "trial-and-error" phase of hypothesis generation. As a proof-of-principle, preliminary data from the Drosophila atlas was used to generate two novel hypotheses involving metabolic interactions between neural stem cells and their niche. The Drosophila atlas promises to shed new light on how metabolism in the mammalian CNS is linked to health and disease.