Investigating the Morphology and Behaviour of Quiescent Neural Stem Cells in Drosophila Melanogaster
Understanding stem cell behaviour and the mechanisms that govern stem cell proliferation is a key question in developmental biology. Stem cell divisions must be carefully regulated as too little proliferation can cause developmental defects or reduce the response to injury, whilst too much proliferation can induce overgrowth. In adult mammalian tissues many stem cell populations reside in a quiescent state, in which the cell cycle is arrested and divisions cease. When needed, these stem cell can resume divisions in response to signals from the environment, a process known as reactivation. In this way stem cells can balance the amount of proliferation in order to maintain tissue homeostasis.
During the larval development of Drosophila melanogaster, neural stem cells, also known as neuroblasts, undergo a similar period of quiescence. Neuroblasts divide during embryogenesis to generate neurons and glia for the larval nervous system. Just before hatching neuroblasts enter quiescence and cease diving. They also adopt a specific morphology in which a long primary process is extended centrifugally into the centre of the brain. As the larvae eat, the intake of amino acids signals to glia at the surface of the brain to release insulin like peptides that are sensed by neuroblasts and signal for reactivation to occur.Neuroblasts then retract their primary processes, round up and resume divisions to generate a second wave of neurons and glia which make up the adult central nervous system.
Whilst insulin signalling is known to be involved in this reactivation switch, it remains unclear as to whether this is the only signalling pathway involved. Moreover, the primary process that is extended during quiescence is directed away from the glial insulin signal, and towards the synapse-dense neuropil in the centre of the brain. In this thesis, I investigate the role of these neuroblast primary processes and their effect on stem cell behaviour, including quiescence and reactivation.
I have found that quiescent neuroblasts can be subdivided into four distinct groups based on the locations in which their primary processes terminate and the cell types that they contact. Distinct neuroblast lineages consistently target their processes to specific regions of the neuropil and this targeting is maintained throughout quiescence. Neuroblasts were found to frequently contact each other via their processes and direct cell-cell contacts were found between processes and both neurons and glia in the neuropil. This work identified several novel interactions between neuroblasts and their niche, which occur via their primary processes.
In order to assess the function of the primary process several methods of disruption were tested, and multiphoton ablation was identified as a promising technique to physically disrupt process maintenance. Furthermore, the functional significance of neuroblast-neuroblast, neuroblast-neuron and neuroblast-glia interactions was investigated, and I have identified neuropil glia as important regulators of neuroblast reactivation. These data offer an insight into the role of the neuroblast primary process, a distinct morphological feature whose function was previously unknown.