Cytosolic crowding by age-dependent neuromelanin accumulation: relevance to Parkinson's disease and brain aging.

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder and the fastest growing neurological disorder in the world. Despite the availability of symptomatic treatments, PD remains to date an incurable disease as existing treatments do not modify the neurodegenerative process nor the disease course. In PD there is a preferential degeneration of neurons containing high levels of neuromelanin, a dark-brown cytoplasmic pigment that accumulates with age in the human brain. While neuromelanin does not appear spontaneously in most animal species, including rodents, we recently developed the first rodent models of human-like neuromelanin production based on the overexpression of human melanin-producing enzyme tyrosinase. This has revealed that increased neuromelanin accumulation above a pathogenic threshold is sufficient to trigger neuronal dysfunction/degeneration and replicate many features of PD. Relevant to humans, intracellular neuromelanin levels reach this pathogenic threshold in PD patients and pre-PD subjects. However, the molecular mechanisms driving neuromelanin-linked PD pathology remain uncertain. Here, by taking advantage of a unique neuromelanin-producing cellular model, we propose to assess the three-dimensional (3D) spatial ultrastructure of these cells by cryosoft X-ray tomography (cryo-SXT) as they progressively accumulate neuromelanin, as in humans. We hypothesize that age-dependent accumulation of neuromelanin, until occupying most of the neuronal cytoplasm, leads to an overcrowded cytoplasmic environment filled with vesicular structures and dysmorphic organelles that contribute to neuronal dysfunction and degeneration associated with PD and, in a broader sense, brain aging. The results from this proposal could offer new insights into the pathogenesis of PD and establish a foundation for developing novel therapeutic strategies for this currently incurable condition.