Iron and neuromelanin in Parkinson's disease

Introduction: The pigmented dopaminergic neurons in the substantia nigra pars compacta are the most vulnerable in Parkinson's disease (PD). Due to the high iron content of neuromelanin (NM), its potential cytotoxity as a free radical generator and the observed depletion of NM in the course of the disease, the characterization of NM provides a clue to the etiopathology of PD. The structure of the iron-NM assembly in controls (CO) and PD patients was investigated by I3C-NMR spectroscopy, 'HNMR relaxometry (NMRD), EPR spectroscopy and magnetometry. Methods: CO and PD samples were obtained from pooled tissues. The extraction procedure included a lipid extraction followed by a drastic proteolytic treatment. I3C NMR spectroscopy measurements were performed under CPMAS conditions. NMRD allows the measurements of the longitudinal relaxation times of water at different magnetic field strengths, yielding a plot which reports about the mobility of water molecules in the tissue, and its functional form is usually related to the organization of proteins in the tissue. EPR spectroscopy in the Q-band was chosen to study the binding of Fe(III) species to NM. Magnetometric data were obtained on a superconducting quantum interference device and report on the magnetic behavior of iron domains in the sample. Results: C CPMAS NMR of NM from CO shows a pattern close to other natural melanins, plus a characteristic glycolipidic matrix, whereas NM from PD shows typical protein absorptions. From NMRD data we found small differences between NM of PD and CO, meaning that there is no evidence of massive release of paramagnetic species during the progress of the disease. The absence of iron ions chelated by the melanin matrix is assessed by both EPR and magnetometric measurements, although a very small contribution is observed in NM from CO. Both techniques point out the presence of extended iron-oxygen clusters. Conclusions: NM from PD patients contains a proteic component, which is insoluble and protease resistant, not evident in the NM extracted from CO. Our data suggest that in PD a deposition of insoluble, protease resistant material covalently linked to the melanin backbone is occurring. The difference observed in the NMRD profiles of SN from PD patients and CO is consistent with a difference in the concentration and organization of immobilized proteins in the SN tissue. It is definitely demonstrated that iron is organized in a 3D network. The magnetic behavior suggests the occurrence of different iron-oxygen clusters.