Neuromelanin biosynthesis is driven by excess cytosolic catecholamines notaccumulated by synaptic vesicles

Melanin, the pigment in hair, skin, eyes, and feathers, protects external t issue from damage by UV light. In contrast, neuromelanin (NM) is found in d eep brain regions, specifically in loci that degenerate in Parkinson's dise ase. Although this distribution suggests a role for NM in Parkinson's disea se neurodegeneration, the biosynthesis and function of NM have eluded chara cterization because of lack of an experimental system. We induced NM in rat substantia nigra and PC12 cell cultures by exposure to L-dihydroxy-phenyla lanine, which is rapidly converted to dopamine (DA) in the cytosol. This pi gment was identical to human NM as assessed by paramagnetic resonance and w as localized in double membrane autophagic vacuoles identical to NM granule s of human substantia nigra. NM synthesis was abolished by adenoviral-media ted overexpression of the synaptic vesicle catecholamine transporter VMAT2, which decreases cytosolic DA by increasing vesicular accumulation of neuro transmitter. The NM is in a stable complex with ferric iron, and NM synthes is was inhibited by the iron chelator desferrioxamine, indicating that cyto solic DA and dihydroxyphenylalanine are oxidized by iron-mediated catalysis to membrane-impermeant quinones and semiquinones. NM synthesis thus result s from excess cytosolic catecholamines not accumulated into synaptic vesicl es. The permanent accumulation of excess catechols, quinones, and catechol adducts into a membrane-impermeant substance trapped in organelles may prov ide an antioxidant mechanism for catecholamine neurons. However, NM in orga nelles associated with secretory pathways may interfere with signaling, as it delays stimulated neurite outgrowth in PC12 cells.