Microglia <i>TREM2<sup>R47H</sup></i> Alzheimer-linked variant enhances excitatory transmission and reduces LTP via increased TNF-α levels.
To study the mechanisms by which the p.R47H variant of the microglia gene and Alzheimer's disease (AD) risk factor TREM2 increases dementia risk, we created <i>Trem2<sup>R47H</sup></i> KI rats. <i>Trem2<sup>R47H</sup></i> rats were engineered to produce human Aβ to define human-Aβ-dependent and -independent pathogenic mechanisms triggered by this variant. Interestingly, pre- and peri-adolescent <i>Trem2<sup>R47H</sup></i> rats present increased brain concentrations of TNF-α, augmented glutamatergic transmission, suppression of Long-term-Potentiation (LTP), an electrophysiological surrogate of learning and memory, but normal Aβ levels. Acute reduction of TNF-α activity with a neutralizing anti-TNF-α antibody occludes the boost in amplitude of glutamatergic transmission and LTP suppression observed in young <i>Trem2<sup>R47H/R47H</sup></i> rats. Thus, the microglia-specific pathogenic <i>Trem2</i> variant boosts glutamatergic neuronal transmission and suppresses LTP by increasing brain TNF-α concentrations, directly linking microglia to neuronal dysfunction. Future studies will determine whether this phenomenon represents an early, Aβ-independent pathway that facilitates dementia pathogenesis in humans.