NMNAT suppresses Tau-induced neurodegeneration by promoting clearance of hyperphosphorylated Tau oligomers in a Drosophila model of tauopathy

Tauopathies, including Alzheimer's disease, are a group of neurodegenerative diseases characterized by abnormal tau hyperphosphorylation that leads to formation of neurofibrillary tangles. Drosophila models of tauopathy display prominent features of the human disease including compromised lifespan, impairments of learning, memory and locomotor functions and age-dependent neurodegeneration visible as vacuolization. Here, we use a Drosophila model of frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), in order to study the neuroprotective capacity of a recently identified neuronal maintenance factor, nicotinamide mononucleotide (NAD) adenylyl transferase (NMNAT), a protein that has both NAD synthase and chaperone function. NMNAT is essential for maintaining neuronal integrity under normal conditions and has been shown to protect against several neurodegenerative conditions. However, its protective role in tauopathy has not been examined. Here, we show that overexpression of NMNAT significantly suppresses both behavioral and morphological deficits associated with tauopathy by means of reducing the levels of hyperphosphorylated tau oligomers. Importantly, the protective activity of NMNAT protein is independent of its NAD synthesis activity, indicating a role for direct protein-protein interaction. Next, we show that NMNAT interacts with phosphorylated tau in vivo and promotes the ubiquitination and clearance of toxic tau species. Consequently, apoptosis activation was significantly reduced in brains overexpressing NMNAT, and neurodegeneration was suppressed. Our report on the molecular basis of NMNAT-mediated neuroprotection in tauopathies opens future investigation of this factor in other protein foldopathies.