Early upregulation of 18-kDa translocator protein in response to acute neurodegenerative damage in TREM2-deficient mice.

Mutations in the TREM2 gene confer risk for Alzheimer's disease and susceptibility for Parkinson's disease (PD). We evaluated the effect of TREM2 deletion in a 1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine (MPTP)-induced PD mouse model, measuring neurodegeneration and microglia activation using a combined in vivo imaging and postmortem molecular approach. In wild-type mice, MPTP administration induced a progressive decrease of [11C]FECIT uptake, culminating at day 7. Neuronal loss was accompanied by an increase of TREM2, IL-1β, and translocator protein (TSPO) transcript levels, [11C]PK11195 binding and GFAP staining (from day 2), and an early and transient increase of TNF-α, Galectin-3, and Iba-1 (from day 1). In TREM2 null (TREM2-/-) mice, MPTP similarly affected neuron viability and microglial cells, as shown by the lower level of Iba-1 staining in basal condition, and reduced increment of Iba-1, TNF-α, and IL-1β in response to MPTP. Likely to compensate for TREM2 absence, TREM2-/- mice showed an earlier increment of [11C]PK11195 binding and a significant increase of IL-4. Taken together, our data demonstrate a central role of TREM2 in the regulation of microglia response to acute neurotoxic insults and suggest a potential modulatory role of TSPO in response to immune system deficit.