Lipid content determines aggregation of neuromelanin granules in vitro

The neuromelanin pigment of the substantia nigra of the human brain is closely associated with lipids and other non-melanogenic compounds which appear to contribute to the unique and complex morphology of neuromelanin pigment granules. In this work we show that insoluble granules isolated from the human substantia nigra associate in vitro to form pigment aggregates similar to those present in the human brain. Extraction of neuromelanin-associated polar lipids by methanol and/or hexane significantly enhanced melanin aggregate size. A marked (10-fold) increase in granule size was seen after methanol treatment, whereas the application of hexane after methanol reduced this pro-aggregation effect. We have previously reported that hexane and methanol remove the neuromelanin-associated polyisoprenoids dolichol and cholesterol respectively. Thus, the current data suggests that pigment-associated lipids may be a factor regulating pigment aggregation and neuromelanin granule size in vivo.