BEYOND COLOR: THE HIDDEN IMPACT OF NEUROMELANIN IN THE BRAIN

Pigmented neurons and selective vulnerability It has long been established that neurons with the highest neuromelanin levels preferentially degenerate in Parkinson’s disease, including dopaminergic neurons from the substantia nigra pars compacta and noradrenergic neurons from the locus coeruleus, whose degeneration leads to the characteristic motor and non-motor symptoms of the disease. This pigment—termed neuromelanin because of its resemblance to peripheral melanin—appears in neurons that synthesize catecholamines, like dopamine and noradrenaline, as a byproduct of its metabolism. Because neurons do not have the capacity to degrade or eliminate this pigment, neuromelanin progressively accumulates within autolysosomal vesicles over time until occupying most of the neuronal cytoplasm. Although similar pigments have been reported in some other animal species as varied as monkeys, dolphins, and frogs, the highly abundant amount of pigment is unique to humans, which in some brain areas is even visible with the naked eye. This distinctive trait has long sparked interest as a potential vulnerability factor for neurons that are lost in Parkinson’s disease. Yet, the exact relationship between neuromelanin accumulation and neuronal vulnerability has remained speculative for decades, as most of what is known is inferred from human postmortem analyses, making it difficult to establish causal relationships. Its unique presence in the human brain and the still elusive understanding of its precise synthesis mechanisms have hindered in vivo modeling and subsequent comprehensive analyses of its accumulation for a long time. Novel in vivo models for neuromelanin accumulationTo address this significant constraint, our research team pioneered the development of an in vivo rodent model for the accumulation of neuromelanin. We achieved this by performing stereotactic injection of adenoassociated viral vectors to overexpress the human melanogenic enzyme tyrosinase within the rodent substantia nigra. This groundbreaking model marked a significant advancement in neuromelanin research, as it represented the inaugural experimental model capable of replicating age-dependent production and accumulation of neuromelanin akin to what is observed in elderly humans (https://www.nature.com/articles/s41467-019-08858-y). Through this study, we demonstrated that the progressive intracellular accumulation of neuromelanin in rodent nigral neurons triggers nigrostriatal neurodegeneration, Lewy body-like pathology and motor deficits. Beyond the substantia nigra: unraveling the full impact In recent years, research has unveiled an essential aspect of Parkinson's disease: it is not confined solely to the nigral dopaminergic system, nor is it exclusively a motor disorder. Instead, it affects multiple neuronal systems and manifests a wide array of non-motor and peripheral symptoms. Furthermore, pigmentation within the human brain is not exclusive to the substantia nigra but is present also in the catecholaminergic neurons of the locus coeruleus and dorsal vagal complex, which are also affected in the disease. To address this broader complexity, we have now generated a new transgenic animal model to reproduce the exact pigmentation distribution of the human brain. This new animal model, termed tgNM, constitutively expresses tyrosinase in catecholaminergic neurons, resulting in progressive neuromelanin accumulation within all catecholaminergic regions, mirroring the age-dependent distribution of pigmentation observed in the human brain (https://www.biorxiv.org/content/10.1101/2023.08.08.552400v1). The novel neuromelanin-producing mice manifest a wide array of symptoms, covering both motor and non-motor domains, including peripheral changes such as alterations in motor balance, olfaction, sleep patterns, emotional memory, vocalization, cardiovascular function, and respiratory function. In parallel to neuromelanin accumulation, the main pigmented catecholaminergic systems in the brain reveal dysfunctional phenotypes. Dopaminergic dysfunction becomes evident with a decrease in tyrosine hydroxylase expression and decreased release of dopamine in the striatum. Noradrenergic neurodegeneration in the pigmented locus coeruleus is detected early in tgNM mice, mirroring the known noradrenergic system disruptions seen in the disease. Remarkably, the alterations in neuromelanin-producing mice extend beyond the pigmented systems. The cholinergic systems in the nucleus basalis of Meynert and the pedunculopontine nucleus are also affected possibly due to its connections to the pigmented regions undergoing degeneration. Of significance, while absent in wild-type mice, tgNM mice exhibit intracytoplasmic Lewy body-like inclusions, stained positive for synuclein and p62, in all the main catecholaminergic regions and exclusively in pigmented neurons. Interestingly, increased neuroinflammatory markers (microglia and astroglia) are detected early in tgNM, prior to overt neuronal dysfunction, possibly associated with an incipient release of extraneuronal neuromelanin granules. Lastly, transcriptomic profiles in pigmented regions, when compared to age-matched non-pigmented regions, reveal neuromelanin-specific transcriptomic alterations. These changes correlate with previously published human postmortem Parkinson’s disease transcriptomic profiles and include changes in neuroinflammation, transcription, translation, and proteasomal and mitochondrial functions. In conclusion, this study demonstrates that brain-wide pigmentation, as observed in humans, is sufficient to ultimately trigger a progressive neurodegenerative phenotype. This process affects numerous neurotransmission systems in the brain (dopaminergic, noradrenergic, and cholinergic) and tissues in the body (vagal-innervated peripheral organs), resulting in a myriad of motor and non-motor symptoms, including peripheral alterations. These symptoms are all reminiscent of what is observed in the prodromal and early stages of Parkinson’s disease. Overall, the novel tgNM mouse model, now available to the scientific community, provides the opportunity to experimentally test potential therapeutic strategies in a human-relevant context. The failure, to date, of many rodent model-based findings to translate to clinical utility in the context of age-related neurodegenerative disease strengthens the importance of introducing into in vivo research a human factor so intimately linked to brain aging and neurodegeneration such as neuromelanin.