Lay Summary Understanding the genetic basis of disease involves ascribing gene mutations directly to a deleterious effect. Technological advances in whole genome sequencing now provide us with a plethora of information linking genetics to disease. An emerging bottleneck in translating that information to curing disease is a more complete understanding of how cells affected by disease interact in the body. Broadly, genetic disease can be caused at the level of the cell by gene mutation(s) preventing a certain cell-type from doing its job correctly. In the body, cells interact in 3 dimensions in a dynamic way within tissues and organs but also potentially send signals throughout the body. For example, neurons in the brain pass signals through complex local circuits that are initiated remotely and have consequences remotely. This circuitry can be damaged by neurological disease that can result in wide-ranging affects such as in Huntington's disease. However, the brain is not made up wholly of neurons and many neurological diseases are caused by dysfunction in other cells such as microglia, astrocytes and oligodendrocytes. This is exemplified by the now clear understanding that Multiple Sclerosis (MS) is caused by defects in oligodendrocyte function. Great strides have been made recently in the understanding of the cellular events that underlie MS resulting in novel drugs being trialed to treat the disease. We have developed new technologies and techniques to unravel the way that cells communicate with each other in living animals as disease develops. This study is focused on investigating type 2 Gaucher's disease (GD) that severely affects the brain but the tools and technologies involved would be broadly applicable to the study of virtually any type of cell in any tissue of the body. For the first time, we will combine the ability to generate neurons (but we can generate virtually any cell type) from mouse or human stem cells in the lab and genetically manipulate them so they emit quantifiable light when stimulated. Once these cells have been proven to respond to such stimuli we will engraft these normal reporter neurons into the brains of mice affected by GD before they are born. These experiments have been designed so as disease starts to affect the mice, the injected normal neurons will send signals by emitting light that will tell us how the cells are behaving and how they are interacting with cells affected by disease. Scientific Abstract We aim to combine for the first time induced pluripotent stem cell (iPSc) technologies with next generation light-emitting reporters to gain new insights into the cell:cell interactions that underlie disease progression in living animals. The use of luciferase luminescence for continued bioimaging of engrafted cells has already resulted in increased data output in substantially reduced cohorts of animals to track cell fate and distribution. This has so far been largely restricted to models of tumorigenesis and stem cell tracking. Here, we propose to use dual reporter cell lines generated from mouse iPSc to not only track cell fate but also transcription factor activity within these cells to provide new insights into how the engrafted cells interact with the local environment. For this focused study we will intracranially inject normal dual reporter neural stem cells (NSC) into fetal Gba1 knockout mice; a model of type 2 neuronopathic Gaucher's Disease and serially image for both reporters within the 14 day window after which most GD mice are sacrificed at a humane endpoint. We will also compare the reverse i.e. Gba1 knockout NSC into normal mice to determine if it is primarily the neural cells or resident activated microglia that are primarily responsible for neural degeneration. The technology being developed is not brain specific but is broadly applicable to many cell-types in many disease settings.