TNF-α-mediated reduction in inhibitory neurotransmission precedes sporadic Alzheimer’s disease pathology in young Trem2R47H rats

Alzheimer's disease is a neurodegenerative dementia associated with deposition in the central nervous system (CNS) of amyloid plaques and neurofibrillary tangles, formed by Aβ peptides and phosphor-tau, respectively. ~2% of AD cases are due to familial mutations (FAD); ~98% of cases are sporadic (SAD). FAD animal models are commonly used to study SAD pathogenesis. Because mechanisms leading to FAD and SAD may be distinct, to study SAD pathogenesis we generated Trem2R47H knock-in rats, which carry the SAD risk factor p.R47H variant of the microglia gene Triggering Receptor Expressed on Myeloid Cells 2 (TREM2).  Trem2R47H rats produce human-Aβ from a humanized-App rat allele because human-Aβ is more toxic than rodent-Aβ and the pathogenic role of the p.R47H TREM2 variant has been linked to human-Aβ-clearing deficits. Using peri-adolescent Trem2R47H rats, we previously demonstrated that supraphysiological TNF-α boosts glutamatergic transmission, which is excitatory, and suppresses long-term potentiation (LTP), a surrogate of learning and memory. Here, we tested the effect of the p.R47H variant on the inhibitory neurotransmitter GABA. We report that GABAergic transmission is decreased in Trem2R47H/R47H rats. This decrease is due to acute and reversible action of TNF-α and is not associated with increased human-Aβ levels and AD-pathology. Thus, the p.R47H variant changes the excitatory/inhibitory balance, favoring excitation. This imbalance could potentiate glutamate excitotoxicity and contribute to neuronal dysfunction, enhanced neuronal death and neurodegeneration. Future studies will determine whether this imbalance represents an early, Aβ-independent pathway leading to dementia and may reveal the AD-modifying therapeutic potential of TNF-α inhibition in the CNS.