Mechanisms regulating Drosophila neural stem cell proliferation

Abstract

Background Neural stem cells are cells that can self-renew and differentiate to give rise to specialized cell types, such as neurons. The appropriate regulation of neural stem cell activities during development guarantees that the correct number of functional neurons are generated to form the adult nervous system. Dysregulation of this process can result in a number of neurological diseases. Too few neural stem cells (and their neuronal progeny) can lead to microcephaly, and too many neural stem cells (and their neuronal progeny) can lead to brain tumour formation. Therefore, understanding the mechanisms regulating neural stem cell activities may shed light on novel therapeutics for neurological diseases.

Drosophila neural stem cells, the neuroblasts (NBs), have been an advantageous model for the study of neural stem cell biology during development and in disease contexts. In the developing larval Central Nervous System (CNS), NB activities are maintained by intrinsic regulatory machinery as well as extrinsic cues from glial cells, which form a microenvironment enwrapping NBs. Dysregulation of either mechanism can have severe consequences. Deregulation of NB intrinsic regulators such as a homeobox transcription factor called Prospero (Pros) can result in decreased differentiation and an expansion of NBs, resulting in increased brain size. The disruption of glial niche or glial derived signals can lead to an alteration of NB proliferation rate, thus affecting the number of neurons in the adult CNS. While multiple intrinsic factors regulating NB activities during development have been intensively studied, it remained poorly understood how NB intrinsic metabolic pathways are important for NB proliferation under physiological and disease context. Furthermore, the mechanisms of how NBs respond to extrinsic cues and how the glial niche relay extrinsic signals to control NB activities is still not well understood.

Hence, the aim of my study is to expand our knowledge on how intrinsic metabolic adaptations and extrinsic cues regulate the progenitor pool of NBs and the rate at which neuronal progeny is produced.

Aims, methods and results: In this work, I have addressed two key questions: 1) what metabolic rewiring events occur as a result of NB tumour formation caused by dedifferentiation? 2) How do glial niche and glial derived signals non-autonomously regulate NB proliferative activities?

1) Metabolic rewiring is known to accompany aberrant proliferation and has recently been shown to drive tumour growth. Utilising the Drosophila NB tumour model caused by deregulation of pros (a homeobox transcription factor), we conducted transcriptomics and metabolomics studies to examine metabolic changes that occur as a result of defective NB differentiation. I found that glutathione metabolism is altered as a result of pros loss-of-function (LOF) tumour formation. It is known that the Pros homologue PROX1 is deregulated in neuroblastoma. Therefore, findings from our work may inform the search of novel metabolite-based targeted therapies for the treatment of neuroblastoma.

2) In the larval CNS, NBs re-enter into cell cycle from a quiescent state in response to extrinsic signals from their microenvironment made up of glial cells. In my study, I have uncovered several new regulators of glia-NB interaction using genetic analyses.

I found that lipid metabolism and Hedgehog (Hh) in the glial niche are involved in regulating NB proliferation. Glial Hh is autonomously required for the growth of glial niche, which is key in maintaining NB proliferation during development. Upon glial overgrowth mediated by FGF activation, excessive Hh from the glial niche triggers a delay in NB cell cycle progression. In this glial overgrowth context, Hh function is modulated by two lipid regulators, Fatty acid synthase 1 (Fasn1) and Lipid droplet storage 2 (Lsd-2). I demonstrated that Fasn1 can modulate Hh palmitoylation. However, it remains unclear how Lsd-2 modulates Hh function, despite the observation that Lsd-2 and Hh both localize to the surface of lipid droplets localized to glial cells.

I also found that glial derived chitinase like proteins, called imaginal disc growth factors (IDGFs) are important non-autonomous regulators of NB proliferation. I showed that IDGF2 is necessary for NB exit from quiescence and is required to sustain NB proliferation at later larval stages.

Collectively, these findings revealed novel mechanism by which Drosophila neural stem cell activities are regulated during development and during tumorigenesis.