Characterisation of newt neural stem cells during development and regeneration

Abstract

The adult newt brain has a unique potential to regenerate neurons after injury. Ependymoglial cells that line the ventricular system of the newt brain are critical for neuronal regeneration, since they reenter the cell cycle upon injury and differentiate into neurons. Ependymoglial cells share key features with radial glial cells in mammals. In contrast to mammals, where the majority of radial glial cells disappear during development, the adult newts retain ependymoglial cells. This thesis aimed to characterise ependymoglial cells under homeostatic conditions and after injury, both during development as well as in the adult animal.

In Paper I we characterised ependymoglial cells during development in two newt species, Notophthalmus viridescens and Pleurodeles waltl. Here we describe the ependymoglia maturation as regards to their proliferation pattern and gene expression profile. Moreover, we correlate the cell cycle length, and exit from the proliferative state to brain maturation and to the acquisition of complex behaviours. The findings also suggest that early cell cycle exit is essential for the persistent presence of ependymoglial cells in adulthood.

In Paper II we evaluated adult newt ependymoglial cells in normal homeostasis and during regeneration following ablation of cholinergic neurons in the forebrain. We find that ependymoglial cells are not a homogenous cell population. Despite their morphological homogeneity, gene expression profile identifies subpopulations among ependymoglial cells. The majority of ependymoglial cells are fast-dividing cells in homeostatically proliferating hotspots, whereas, proliferating ependymoglia in quiescent areas are slowly cycling cells with stem cell features. Neuronal ablation altered the fate of ependymoglial cells, and neurogenic niches with neuroblasts in normally non-germinal regions were created. This study identifies processes of both homeostatic as well as injury-induced neurogenesis in the adult newt brain.

In Paper III we assessed how the production of reactive oxygen species impacts the brain during normal and regenerative neurogenesis. By manipulating environmental oxygen availability, we find that newts could cope with hypoxia and subsequent re-oxygenation. The shifts in environmental oxygen concentration causes initial neuronal loss and subsequent increase in neurogenesis, which is dependent on the production of reactive oxygen species. Also, we find that neuronal regeneration in the homeostatically quiescent midbrain is dependent on the production of reactive oxygen species during constant normoxia. Altogether the data assign a key role to reactive oxygen species in adult neurogenesis in newts and suggests that naturally occurring environmental changes in oxygen concentration might be an evolutionary driving force to replace lost neurons in newts.