Thesis title: Impact of cholesterol dyshomeostasis on the physiological functions of developing mouse brain microglia
Background and Aims. Niemann Pick type C disease is a fatal lysosomal storage disorder caused by mutations of NPC1 or NPC2 transporters, which cooperatively mediate the export of endocytosed cholesterol from lysosomes. This leads to the engorgement of lysosomes with cholesterol and other lipids affecting, among others, the autophagic flux. On the other hand, it disrupts the fraction of cholesterol at the level of intracellular compartments and to the plasma membrane, likely affecting a number of signalling mechanisms Although the overt manifestation of the disease is landmarked by neurodegeneration and microgliosis, our recent studies have disclosed significant alterations of development trajectories in the postnatal brain of Npc1-deficient mice, including reduced neurogenic activity, defective synapse connectivity and glial differentiation. Surprisingly, despite the vital role that microglia play in the maturation and maintenance of neuronal circuitry, the issue as to whether the loss of brain homeostasis affects microglia physiological function during postnatal developmental in the Npc1-deficient mice has been poorly explored. The purpose of thesis was to fill this relevant gap in the field.
Methods. Exploiting the Npc1nmf164 mouse model on BALB/cJ background, which exhibits a delayed onset and slower disease progression, I have characterized the microglia activation and phagocytic profiles in two areas of the early postnatal brain, the cerebellum (CB) and olfactory bulb (OB), by immunohistological and gene expression analysis. These two regions were selected because they display defective neuronal remodelling in Npc1-deficient mouse models. I have also investigated whether microglia changes associate with functional impairments, by analysing the olfactory performance in complex odour discrimination tasks.
Results. My findings demonstrate that depending on the brain homeostasis status at a given developmental stage Npc1-deficient microglia adopt an activated phenotype, characterized by an increased cell proliferation, enlarged soma size and de-ramified processes, as well as a higher phagocytic activity. These changes in microglia activities occur similarly in the CB and OB of Npc1nmf164 mice, despite they are likely induced by region-specific developmental anomalies. Gene expression pattern analyses fully matched microglia activation features. Finally, I found that early microglia activation in the OB is associated to significant defects in the fine odour discriminatory ability of Npc1nmf164 mice.
Conclusions: Our results identify microglia activation as an early sign of defective developmental processes in Npc1-deficient mice, further emphasizing that the derangement of developmental trajectories underlies the subsequent appearance of overt signs of the disease. Moreover, our data suggest the employ of challenging olfactory tasks as a valid non-invasive tool to investigate the onset and progression of neurodegenerative diseases, where chemosensory impairment largely precedes the onset of cognitive and motor decline.
Key words: Cholesterol, NPC1 transporter, Microglia activation, brain dyshomeostasis