Dynamics of Neural Stem Cell Fate Decision and Proliferation in a 3D Human Brain Model System

This PhD project involves a combination of state-of-the-art stem cell technologies and mathematical modelling to test a series of hypotheses regarding fate determination of stem cells. The cerebral organoid method is at the forefront of 3D tissue engineering and developmental neurobiology. Furthermore, the student will benefit from the uniquely rich environment of Cambridge which includes many other stem cell groups, and will seek to collaborate with groups such as the Livesey lab, where there are parallel but distinct activities using the rosette culture system. Thus, this position provides a unique opportunity to conduct cutting edge functional studies of human brain development in a manner not previously possible.

Human brain tumours are a devastating type of cancer with one of the lowest survival rates. Although numerous animal models exist for studying brain tumour formation, mouse models do not adequately recapitulate the genetic and molecular events leading to human brain cancer. This may be partly due to unique qualities of human brain development, not least of which is the ability to produce a dramatically expanded neural output and overall size compared with other mammals. Relatively little is known about how human neural stem cells undergo specific fate choices to generate the expanded number of neurons characteristic of the human brain. Furthermore, how these fate decisions go awry and lead to tumours, is still unclear.

The lack of clear insight into human neural stem cell biology has largely been due to the absence of an appropriate model system to examine human specific features. Recently, we established an in vitro model of human brain development, termed cerebral organoids, or mini-brains for short (Lancaster et al. 2013, Lancaster and Knoblich 2014). These brain organoids are 3D tissues generated from human pluripotent stem cells that allow modelling of human brain development in vitro. Through a process of directed differentiation and a supportive 3D microenvironment, neural precursor tissue can spontaneously self-organize to form the stereotypic organization of the early human embryonic brain.

This project will make use of this new technology to examine fate decisions and proliferative potential of human neural stem cells, and to investigate how this potential is kept in check under normal conditions versus overgrowth. Through a joint effort with Ben Simons, we will perform lineage tracing to test the developmental potential of human neural stem cells. Recently, in work involving Ben Simons, clonal lineage tracing analysis was carried out in the developing rodent brain (Gao et al. 2014), which fit a mathematical model consistent with deterministic unitary neuronal output generating 8-9 neurons per neural stem cell. This finding was a key discovery in the field of neuroscience, indicating that neural stem cells track their output over time and stop proliferating at a very specific point. We will take a similar clonal lineage tracing approach to examine the greater neuronal output and proliferative capacity of human neural stem cells, and whether they similarly track their output in a controlled deterministic fashion, or whether the process is more stochastic in humans.

With the characterization of the wild-type condition in hand, we will then perform studies in conditions of brain overgrowth. We will start with disorders displaying abnormal growth of all or part of the brain in an effort to investigate abnormal developmental regulation of neural stem cell dynamics. We will introduce mutations seen in the conditions macrocephaly and hemimegalencephaly, and examine stem cell output and fate determination to test how the genes involved may regulate neural stem cell proliferation. In addition, these disorders often exhibit mutations in common with those identified in paediatric and adult brain tumour types, such as mutations in the key tumour suppressor - PTEN. Therefore, we will also examine how mutations associated with these cancers lead to abnormal regulation of neural stem cell proliferation. By examining the cell and molecular defects associated with these disorders, this project will hopefully shed light on mechanisms of neural stem cell overproliferation associated with brain overgrowth and cancer.

Overall, the goal of this project will be to identify the mechanisms that regulate controlled human neural stem cell proliferation and how this process goes awry in conditions of abnormal proliferation seen in brain overgrowth and cancer.

For full project details please to go www.cambridgecancercentre.org.uk/studentships