The appearance of phagocytic microglia in the postnatal brain of Niemann Pick type C mice is developmentally regulated and underscores shortfalls in fine odor discrimination

Abstract

The loss of NPC1 or NPC2 function results in cholesterol and sphingolipid dyshomeostasis that impairs developmental trajectories, predisposing the postnatal brain to the appearance of pathological signs, including progressive and stereotyped Purkinje cell loss and microgliosis. Despite increasing evidence reporting the activation of pro‐inflammatory microglia as a cardinal event of NPC1 disease progression at symptomatic stages both in patients and preclinical models, how microglia cells respond to altered neurodevelopmental dynamics remains not completely understood. To gain an insight on this issue, we have characterized patterns of microglia activation in the early postnatal cerebellum and young adult olfactory bulb of the hypomorphic Npc1nmf164 mouse model. Previous evidence has shown that both these areas display a number of anomalies affecting neuron and glial cell proliferation and differentiation, which largely anticipate cellular changes and clinical signs, raising our interest on how microglia interplay to these changes. Even so, to separate the contribution of cues provided by the dysfunctional microenvironment we have also studied microglia isolated from mice of increasing ages and cultured in vitro for 1 week. Our findings show that microglia of both cerebellum and olfactory bulb of Npc1nmf164 mice adopt an activated phenotype, characterized by increased cell proliferation, enlarged soma size and de‐ramified processes, as well as a robust phagocytic activity, in a time‐ and space‐specific manner. Enhanced phagocytosis associates with a profound remodeling of gene expression signatures towards gene products involved in chemotaxis, cell recognition and engulfment, including Cd68 and Trem2. These early changes in microglia morphology and activities are induced by region‐specific developmental anomalies that likely anticipate alterations in neuronal connectivity. As a proof of concept, we show that microglia activation within the granule cell layer and glomerular layer of the olfactory bulb of Npc1nmf164 mice is associated with shortfalls in fine odor discrimination.

Disruptive lysosomal-metabolic signaling and neurodevelopmental deficits that precede Purkinje cell loss in a mouse model of Niemann-Pick Type-C disease

“…PCs are particularly susceptible to NPC1 deficiency by degenerating earlier and to a greater severity than other neurons in the brain 4 . Significant changes in cerebellar synapses and glial phenotypes along with behavioral and motor deficits are found during postnatal development and before PC degeneration [5][6][7][8] . However, little is known about how Npc1 deficiency affects PC development.…”

Shadow imaging for panoptical visualization of brain tissue in vivo

“…Phagocytosis is the engulfment of large substrates (>0.5 µm 18 ) into cupshaped cellular structures and thus is amenable to confocal imaging. The phagocytic cups of microglia are positioned at the end of their fine processes and are on average ~6 µm in diameter 19 . While they are known to engulf both dead or damaged as well as intact structures [20][21][22] , how they triage interactions with these distinct targets in living brain tissue has remained difficult to assess.…”