Neuronal overexpression of Nrf2 reduces dystrophic neurites in 5XFAD Alzheimer's disease model mice.

Background: The hallmark lesions of the Alzheimer's disease (AD) brain are amyloid plaques consisting of the β-amyloid protein and neurofibrillary tangles comprised of hyperphosphorylated, aggregated tau protein, which both cause neuronal dysfunction and loss. One goal of neuroprotective therapies is to maintain normal neuronal function and survival in the presence of toxic pathologies such as plaques and tangles. A potential neuroprotective target is nuclear factor erythroid 2-related factor 2 (Nrf2) transcription factor, which regulates the expression of many antioxidant and detoxification genes. Nrf2 mRNA is decreased in AD brains, and deletion of the Nrf2 gene causes increased BACE1 and Aβ production and worsened cognitive deficits in amyloid pathology mouse models. Overexpression of Nrf2 in astrocytes has been shown to be protective against neurodegeneration, but the role of Nrf2 is neurons is unclear.

Methods: We overexpressed Nrf2 from birth in neurons of 5XFAD amyloid pathology model mice using AAV8, hypothesizing that neuronal Nrf2 overexpression decreases cortical neuron loss and reduces plaque load by decreasing BACE1 levels. We quantified protein levels by immunoblot and neuropathology by immunofluorescent staining, using two-way ANOVA to measure differences between genotypes and AAV treatments. To assess genetic changes, we performed bulk mRNA seq.

Results: While neuronal overexpression of Nrf2 in 5XFAD mice did not prevent neuronal loss as measured by NeuN labeling, decrease neuroinflammation by Iba1 or GFAP labeling, or reduce amyloid load by Aβ antibody or methoxy-XO4 staining, we show that increased Nrf2 expression reduces BACE1 protein levels, especially in swollen axonal dystrophic neurites around amyloid plaques. Other proteins that accumulate in dystrophic neurites were also reduced, indicating decreased dystrophic neurites overall. Immunoblot analysis suggested increased autophagy was unlikely to play a role, while bulk mRNA sequencing indicated changes in lipid metabolism and microtubule stability may have contributed to reduced dystrophic neurite formation.

Conclusions: Dystrophic neurites impair action potential conductance and contribute to tau seeding and spreading. Their reduction by neuronal Nrf2 overexpression may protect neurons against these pathologic changes. Further study of the mechanisms by which Nrf2 reduces dystrophic neurites may lead to therapeutic strategies that can limit neuritic damage caused by cerebral amyloid accumulation.