Age-dependent brain pigmentation drives early neuroinflammatory molecular signatures linked to neurodegeneration

Abstract

BackgroundNeuromelanin (NM) is a pigment that progressively accumulates with age in catecholaminergic neurons, particularly in the substantia nigra, ventral tegmental area, and locus coeruleus. These neuronal populations are especially vulnerable to degeneration in Parkinsons disease (PD). Elevated intracellular NM levels have been linked to neurodegeneration and PD-like phenotypes in experimental models. However, the molecular mechanisms underlying NM-induced pathology remain poorly understood, as human studies cannot disentangle the specific effects of NM accumulation from those of normal aging. MethodsWe performed transcriptomic microarray analysis on laser-captured catecholaminergic neurons and regions (substantia nigra, ventral tegmental area, locus coeruleus) from NM-producing transgenic mice (tgNM) and NM-free wild-type controls across different ages, and compared them to data from postmortem human brain tissue. One of the molecular targets identified, GPNMB, was validated in mouse and human tissue, and functionally tested in vivo. ResultsWe identified region- and age-dependent transcriptional changes associated with progressive NM accumulation. NM consistently upregulated neuroinflammatory pathways with enrichment of disease-associated microglial genes, while downregulating transcription, translation, and mitochondrial functions. Locus coeruleus exhibited the earliest and strongest transcriptional alterations, whereas substantia nigra and ventral tegmental area showed a later-onset, age-progressive transcriptional dysfunction. Neuron-specific analyses revealed that many changes originated within NM-containing neurons rather than being solely glial-driven. NM-driven transcriptional profiles in mice strongly correlated with postmortem data from PD patients, underscoring their translational relevance. Among molecular targets, the glycoprotein GPNMB was consistently upregulated in NM-containing neurons and validated at RNA and protein levels in both NM-producing transgenic mice and human PD brains. Functional experiments demonstrated that GPNMB overexpression attenuated NM-linked dopaminergic neurodegeneration and improved motor performance in mice. ConclusionThis study provides a comprehensive in vivo characterization of NM-specific transcriptomic changes in catecholaminergic neurons, showing that NM accumulation drives neuroinflammatory and neurodegenerative programs. Our results support that the neuroinflammatory changes observed in tgNM mice and in human PD represent early pathological events that precede overt neurodegeneration. The disease-associated gene GPNMB emerged as a conserved NM-induced factor with protective properties, highlighting its potential as a therapeutic target in PD and aging-related neurodegeneration.