14-3-3 mitigates alpha-synuclein aggregation and toxicity in the in vivo preformed fibril model

Alpha-synuclein (alpha syn) is the key component of proteinaceous aggregates termed Lewy Bodies that pathologically define a group of disorders known as synucleinopathies, including Parkinson's Disease (PD) and Dementia with Lewy Bodies. alpha Syn is hypothesized to misfold and spread throughout the brain in a prion-like fashion. Transmission of alpha syn necessitates the release of misfolded alpha syn from one cell and the uptake of that alpha syn by another, in which it can template the misfolding of endogenous alpha syn upon cell internalization. 14-3-3 proteins are a family of highly expressed brain proteins that are neuroprotective in multiple PD models. We have previously shown that 14-3-3 theta acts as a chaperone to reduce alpha syn aggregation, cell-to-cell transmission, and neurotoxicity in the in vitro pre-formed fibril (PFF) model. In this study, we expanded our studies to test the impact of 14-3-3s on alpha syn toxicity in the in vivo alpha syn PFF model. We used both transgenic expression models and adenovirus associated virus (AAV)-mediated expression to examine whether 14-3-3 manipulation impacts behavioral deficits, alpha syn aggregation, and neuronal counts in the PFF model. 14-3-3 theta transgene overexpression in cortical and amygdala regions rescued social dominance deficits induced by PFFs at 6 months post injection, whereas 14-3-3 inhibition by transgene expression of the competitive 14-3-3 peptide inhibitor difopein in the cortex and amygdala accelerated social dominance deficits. The behavioral rescue by 14-3-3 theta overexpression was associated with delayed alpha syn aggregation induced by PFFs in these brain regions. Conversely, 14-3-3 inhibition by difopein in the cortex and amygdala accelerated alpha syn aggregation and reduction in NECAB1-positive neuron counts induced by PFFs. 14-3-3 theta overexpression by AAV in the substantia nigra (SN) also delayed alpha syn aggregation in the SN and partially rescued PFF-induced reduction in tyrosine hydroxylase (TH)-positive dopaminergic cells in the SN. 14-3-3 inhibition in the SN accelerated nigral alpha syn aggregation and enhanced PFF-induced reduction in TH-positive dopaminergic cells. These data indicate a neuroprotective role for 14-3-3 theta against alpha syn toxicity in vivo.