Protocol for neuromelanin accumulation drives endogenous synucleinopathy in non-human primates

This study was aimed to develop and characterize a non-human primate (NHP) model of Parkinson’s disease mimicking the known neuropathological hallmarks of Parkinson’s disease to the best possible extent. Accordingly, we sought to determine whether AAV-mediated enhanced expression of human tyrosinase (hTyr) in the substantia nigra (SNpc) of non-human primates (NHPs) is able to induce a time-dependent accumulation of neuromelanin (NMel) in dopaminergic neurons, further triggering and endogenous synucleinopathy, progressive cell death and a pro-inflammatory scenario, in keeping with what was formerly reported in rats by taking advantage of a similar strategy (Carballo-Carbajal et al., 2019). Furthermore, the potential prionoid spread of endogenous alpha-synuclein (a-Syn) species towards the prefrontal cortex was analyzed, in an attempt to evaluate to what extent there is a propagation of endogenous a-Syn by permissive trans-synaptic templating (e.g. the so-called Braak hypothesis). Adult juvenile NHPs (Macaca fascicularis) were injected with adeno-associated viral vectors (AAVs) encoding either the hTyr gene (AAV-hTyr; delivered into the left SNpc) or a null construct for control purposes (AAV-null; injected into the right SNpc). In order to delineate a timeline for the underlying processes, one group of NHPs was sacrificed four months post-AAV deliveries (animals M308F4 and M310M4), whereby the follow-up timing for second experimental group was settled at eight months post-AAVs surgeries (animals M307F8 and M309M8). Neuroimage studies (MRI and MicroPET) were conducted in vivo at different time points. Upon animal sacrifices, brain tissue samples were processed for histological analysis comprising intracellular NMel levels, intracellular aggregates, nigrostriatal degeneration and neuroinflammation.