Brain tyrosinase overexpression implicates age-dependent neuromelanin production in Parkinson’s disease pathogenesis

Abstract

In Parkinson’s disease (PD) there is a selective degeneration of neuromelanin-containing neurons, especially substantia nigra dopaminergic neurons. In humans, neuromelanin accumulates with age, the latter being the main risk factor for PD. The contribution of neuromelanin to PD pathogenesis remains unknown because, unlike humans, common laboratory animals lack neuromelanin. Synthesis of peripheral melanins is mediated by tyrosinase, an enzyme also present at low levels in the brain. Here we report that overexpression of human tyrosinase in rat substantia nigra results in age-dependent production of human-like neuromelanin within nigral dopaminergic neurons, up to levels reached in elderly humans. In these animals, intracellular neuromelanin accumulation above a specific threshold is associated to an age-dependent PD phenotype, including hypokinesia, Lewy body-like formation and nigrostriatal neurodegeneration. Enhancing lysosomal proteostasis reduces intracellular neuromelanin and prevents neurodegeneration in tyrosinase-overexpressing animals. Our results suggest that intracellular neuromelanin levels may set the threshold for the initiation of PD.

Neuromelanin, aging, and neuronal vulnerability in Parkinson's disease

“…). Relevant to humans, intracellular neuromelanin levels reach this threshold in both PD patients and subjects with incidental LB disease (ie, clinically healthy individuals exhibiting LB pathology at autopsy who are considered to represent early, presymptomatic stages of PD) . In contrast, in healthy elderly individuals, intracellular neuromelanin levels remain below this threshold .…”

“…The identification of specific phases and changes in the rate of neuromelanin production over time in humans suggests the regulation of neuromelanin production and turnover, possibly through enzymatic processes, as is the case for all other melanin pigments . Interestingly, tyrosinase expression might not be restricted to melanocytes but could also be present, although at very low levels, in the brain, including the human SNpc . For instance, brain tyrosinase messenger ribonucleic acid (mRNA) expression has been reported in human SN by reverse transcription polymerase chain reaction/real‐time polymerase chain reaction, although at barely detectable levels .…”

“…To assess the potential role of neuromelanin in PD pathogenesis, it would first be necessary to develop an in vivo model of human‐like neuromelanin production, up to the abundant levels reached in elder humans, as no such animal model currently exists, experimentally or naturally. Considering the failure by previous studies to produce neuromelanin in vivo solely by increasing the levels of dopamine and oxidized dopamine, we recently opted as a potential alternative strategy for overexpressing human tyrosinase (hTyr) via a viral vector in the SNpc of both rats and mice, which normally lack neuromelanin . Strikingly, hTyr‐overexpressing rodents exhibited an age‐dependent production and accumulation of a neuromelanin‐like pigment within nigral dopaminergic neurons that was virtually analogous to human neuromelanin (henceforth referred to as neuromelanin; see Fig.…”

“…Indeed, it was found in hTyr‐overexpressing rodents that the progressive intracellular build‐up of neuromelanin ultimately compromised neuronal function when allowed to accumulate above a specific threshold, eventually triggering in an age‐dependent manner the main pathological features of PD, including hypokinesia, LB‐like inclusion formation and nigrostriatal neurodegeneration . By ~4 months, nigrostriatal denervation in hTyr‐overexpressing rodents was equivalent to that seen in early PD patients (<10 years’ evolution), and by ~2 years the extent of denervation reached levels comparable to those seen in advanced PD (>20 years’ evolution) . In parallel to dopaminergic cell death, these animals exhibited other neuropathological features typical of aged and PD human brains, including (1) extracellular neuromelanin, released from dying neurons; (2) neuronophagia (ie, extracellular neuromelanin surrounded by, or within, activated microglia), indicative of an active, ongoing neurodegenerative process; and (3) perivascular neuromelanin, resulting from the migration of neuromelanin‐filled microglia to blood vessels to remove extracellular neuromelanin from the brain …”

“…Please note that in these studies intracellular neuromelanin levels were measured by optical densitometry within individual neuromelanin‐containing neurons, as opposed to regional quantifications of total SNpc neuromelanin, the latter being dramatically decreased in PD brains because of the massive loss of neuromelanin‐containing dopaminergic neurons . Supporting a pathogenic role for intracellular neuromelanin accumulation when allowed to accumulate above a specific threshold within individual neurons, the lowering of intracellular neuromelanin to levels below this pathogenic threshold in hTyr‐overexpressing rodents, in part by promoting the release of intracellular neuromelanin to the outside of the cell (see the next section for details), diminished PD‐linked inclusion formation, attenuated nigrostriatal neurodegeneration, and reversed hypokinesia in these animals . These results demonstrate the feasibility and therapeutic potential of modulating intracellular neuromelanin levels in vivo and indicate that strategies to maintain or decrease intracellular neuromelanin to levels below its pathogenic threshold may provide unprecedented therapeutic opportunities to prevent, halt, or delay neuronal dysfunction and degeneration linked to PD and brain aging (Fig.…”