Midbrain degeneration triggers astrocyte reactivity and tau pathology in experimental Alzheimer’s Disease

Smaller midbrain volumes have been identified as predictors of Alzheimer’s disease (AD) progression and of faster conversion from Mild Cognitive Impairment (MCI) to dementia. Consistently, several midbrain-target regions exhibit neuroinflammation as early as the MCI stage. The concomitant presence of neuroinflammation, amyloid-β (Aβ) and tau pathology appears to be a strong predictor of conversion to dementia. However, the mechanisms through which midbrain degeneration contributes to disease progression − particularly how it triggers neuroinflammation in midbrain-target areas such as the hippocampus − remain unclear. To address this gap, using adult C57BL/6N mice, we generated a new mouse model bearing lesions in three midbrain nuclei: the dopaminergic Ventral Tegmental Area (VTA) and Substantia Nigra pars compacta (SNpc), and the serotonergic Interpeduncular Nucleus (IPN). This new model allowed us to evaluate the consequences of dopamine (DA) and serotonin (5-HT) depletion in downstream target regions. We characterized the model through stereological cell counts, analyses of monoaminergic fibres and monoamine levels, electrophysiological recordings, and behavioural testing. Hippocampal neuroinflammation was determined by quantifying glial cells, assessing morphological changes, evaluating NLRP3 inflammasome activation and cytokine levels, and performing microglial transcriptional profiling. In parallel, we induced analogous midbrain lesions in Tg2576 transgenic mice, which overexpress the Swedish mutant of amyloid precursor protein, to examine how monoamine depletion interacts with Aβ accumulation. While lesions in C57BL/6N mice enabled isolation of the effects of monoaminergic dysfunction, lesions in Tg2576 mice provided insight into the interplay between impaired monoaminergic signalling and Aβ-driven pathology. In C57BL/6N mice, combined DA and 5-HT depletion within the hippocampus elicited pronounced microglial activation via NLRP3 inflammasome signalling resulting in elevated IL-1β expression. Pharmacological treatment with dopaminergic agents (L-DOPA or A68930) or the serotonergic agent (fluoxetine) fully prevented this inflammatory response. In Tg2576 mice, superimposed midbrain degeneration markedly exacerbated AD-like pathology, including intensified microglial and astrocytic reactivity, accelerated Aβ plaque deposition, and induction of pathological tau hyperphosphorylation. Notably, L-DOPA or fluoxetine treatment significantly reduced astrocytic activation and tau hyperphosphorylation in lesioned Tg2576 mice. These findings underscore the pivotal role of midbrain integrity in modulating neuroinflammatory cascades and AD progression. They provide mechanistic insight into the accelerated cognitive decline observed in patients with midbrain deficits and support the development of precision medicine strategies targeting monoaminergic dysfunction in neurodegenerative disease.