Tyrosinase is not detected in human catecholaminergic neurons by immunohistochemistry and Western blot analysis

Catecholaminergic neurons of the primate substantia nigra (SN) pars compacta (SNc) and the locus coeruleus contain neuromelanin (NM) granules as characteristic structures underlying the pigmentation of these brain areas. Due to a phylogenetic appearance NM granules are absent in the rodent brain, but gradually become present in primates until they reach a maximal expression in humans. Although a possible mechanism of pigment formation may be autoxidation of the NM precursors dopamine or noradrenalin, several groups have suggested an enzymatic formation of NM mediated by tyrosinase or a related enzyme. Since tyrosinase mRNA is suggested to be expressed in the SN of mice and humans, we reinvestigated the expression of tyrosinase in the human SNc and the locus coeruleus at the protein level by immunohistochemistry and Western blot analysis, but could not detect tyrosinase in these brain regions.

Proteomics of the human brain: sub-proteomes might hold the key to handle brain complexity

Proteomics is a promising approach, which provides information about the expression of proteins and increasingly finds application in life science and disease research. Meanwhile, proteomics has proven to be applicable even on post mortem human brain tissue and has opened a new area in neuroproteomics. Thereby, neuroproteomics is usually employed to generate large protein profiles of brain tissue, which mostly reflect the expression of highly abundant proteins. As a complementary approach, the focus on sub-proteomes would enhance more specific insight into brain function. Sub-proteomes are accessible via several strategies, including affinity pull-down approaches, immunoprecipitation or subcellular fractionation. The extraordinary potential of subcellular proteomics to reveal even minute differences in the protein constitution of related cellular organelles is exemplified by a recent global description of neuromelanin granules from the human brain, which could be identified as pigmented lysosome-related organelles.

Subcellular proteomics reveals neuromelanin granules to be a lysosome-related organelle

The powerful combination of subcellular fractionation and protein identification by electrospray ionization tandem mass spectrometry (ESI-MS/MS) pioneered the molecular elucidation of neuromelanin (NM) granules. We recently isolated NM granules from the human brain and succeeded in the establishment of the first protein profile of this compartment. NM granules are pigmented organelles, which are mainly found in the catecholaminergic neurons of the human substantia nigra (SN) pars compacta and the locus coeruleus. These granules contain the insoluble pigment NM, which is regarded as the most important iron storage system in these neurons. A global examination of NM granules, however, has so far been hampered due to the lack of a pigmented brain stem in rodents, the absence of an appropriate experimental system and their scarcity in the human brain. 'Subcellular proteomics', which increasingly emerges as the method of choice to characterize cellular compartments and to elucidate their biogenesis, has recently been shown to be an adequate approach to tackle a thorough description of NM granules. Thereby, NM granules could be described as a 'lysosome-related organelle'. This indicates a genetic program underlying a biogenesis of NM rather than its autoxidative formation.

Neuromelanin induces oxidative stress in mitochondria through release of iron: mechanism behind the inhibition of 26S proteasome

Parkinson's disease is characterized by the selective depletion of dopamine neurons in the substantia nigra, particular those containing neuromelanin. Involvement of neuromelanin in the pathogenesis may be either cytotoxic or protective. Recently we found that neuromelanin reduces the activity of 26S proteasome. In this paper, the detailed mechanisms behind the reduced activity were studied using neuromelanin isolated from the human brain. Neuromelanin increased the oxidative stress, but synthetic melanin did not. Superoxide dismutase and deferoxamine completely suppressed the increase, indicating that superoxide produced by an iron-mediated reaction plays a central role. Iron was shown to reduce in situ 26S proteasome activity in SH-SY5Y cells and the reduction was protected by antioxidants. These results suggest that iron released from neuromelanin increases oxidative stress in mitochondria, and then causes mitochondrial dysfunction and reduces proteasome function. The role of neuromelanin is discussed in relation to the selective vulnerability of dopamine neurons in Parkinson's disease.

Neuromelanin in human dopamine neurons: comparison with peripheral melanins and relevance to Parkinson's disease

Neuromelanin (NM) is a dark polymer pigment produced in specific populations of catecholaminergic neurons in the brain. It appears in greatest quantities in the human brain, in lesser amounts in some other non-human primates, but is absent from the brain in many lower species. Interest in this pigment has seen a resurgence in recent years because of a hypothesised link between neuromelanin and the especial vulnerability of neuromelanin-containing neurons to cell death in Parkinson's disease (PD). Little is known regarding the biology of neuromelanin. As neuromelanin appears to have characteristics in common with the better studied peripheral melanin pigments this review compares what is known about neuromelanin with melanins found in other body tissues. Unlike peripheral melanins, which are produced in specialised cells called melanocytes and may be transferred to other cell types, neuromelanin granules are believed to be stored in the cell in which they are produced. Neuromelanin granules display a unique, more heterogeneous appearance compared with peripheral melanins. Unlike melanin, neuromelanin is traditionally thought to result from a non-enzymatic synthesis pathway with no known pathway for neuromelanin catabolism. More recent data, however, is indicative of some regulation of neuromelanin synthesis and turnover. By analogy with peripheral melanins, neuromelanin may function in vivo to attenuate the effects of damaging stimuli. Among several possible mechanisms suggested, the ability of neuromelanin to interact with transition metals, especially iron, and to mediate intracellular oxidative mechanisms has received particular attention. Recent data from neuromelanin in the Parkinson's disease brain suggests that this proposed function may be compromised, thus rendering pigmented neurons vulnerable to oxidative damage in this disorder.

Neuromelanin inhibits enzymatic activity of 26S proteasome in human dopaminergic SH-SY5Y cells

Recently, impairment of the ubiquitin-proteasome system is suggested to be responsible for the neuronal death in ageing and Parkinson's disease. The specific degeneration of dopamine neurons containing neuromelanin (NM) suggests that NM itself may be involved in the cellular dysfunction and death, even though the direct link has never been reported. We examined the effects of NM isolated from the human substantia nigra on the proteasome activity in human dopaminergic SH-SY5Y cells. NM reduced the activities of 26S proteasome, as shown in situ using a green fluorescent protein homologue targeted to 26S proteasome and also in vitro using ubiquitinated lysozyme as a substrate. However, NM did not affect 20S proteasome activity in vitro. NM reduced the amount of PA700 regulatory subunit of 26S proteasome, but did not affect that of alpha- and beta-subunits of 20S proteasome. These results suggest that NM may inhibit the ubiquitin-26S proteasome system, and determine the selective vulnerability of dopamine neurons in ageing and related disorders.

Degeneration of neuronal cells due to oxidative stress--microglial contribution

Various neurodegenerative disorders including Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis have been causally linked to the generation of free radicals and oxidative stress. In this review, we discuss the implication of oxidative stress in neuronal death and point out the role of intracellular signaling pathways leading to activation of transcription factors associated with cell death and repair. In particular, the impact of microglia as contributors in promoting oxidative stress in neurodegeneration is highlighted. Finally, pivotal molecular targets for drug therapies of brain disorders are reported.