Neuromelanins in brain aging and Parkinson's disease: synthesis, structure, neuroinflammatory, and neurodegenerative role.

Neuromelanins are compounds accumulating in neurons of human and animal brain during aging, with neurons of substantia nigra and locus coeruleus having the highest levels of neuromelanins. These compounds have melanic, lipid, peptide, and inorganic components and are contained inside special autolysosomes. Neuromelanins can participate in neuroprotective or toxic processes occurring in Parkinson's disease according to cellular environment. Their synthesis depends on the concentration of cytosolic catechols and is a protective process since it prevents the toxic accumulation of catechols-derived reactive compounds. Neuromelanins can be neuroprotective also by binding reactive/toxic metals to produce stable and non-toxic complexes. Extraneuronal neuromelanin released by dying dopamine neurons in Parkinson's disease activates microglia which generate reactive oxygen species, reactive nitrogen species, and proinflammatory molecules, thus producing still neuroinflammation and neuronal death. Synthetic neuromelanins have been prepared with melanic, protein structure, and metal content closely mimicking the natural brain pigment, and these models are also able to activate microglia. Neuromelanins have different structure, synthesis, cellular/subcellular distribution, and role than melanins of hair, skin, and other tissues. The main common aspect between brain neuromelanin and peripheral melanin is the presence of eumelanin and/or pheomelanin moieties in their structure.

The role of tyrosine hydroxylase as a key player in neuromelanin synthesis and the association of neuromelanin with Parkinson's disease.

The dark pigment neuromelanin (NM) is abundant in cell bodies of dopamine (DA) neurons in the substantia nigra (SN) and norepinephrine (NE) neurons in the locus coeruleus (LC) in the human brain. During the progression of Parkinson's disease (PD), together with the degeneration of the respective catecholamine (CA) neurons, the NM levels in the SN and LC markedly decrease. However, questions remain among others on how NM is associated with PD and how it is synthesized. The biosynthesis pathway of NM in the human brain has been controversial because the presence of tyrosinase in CA neurons in the SN and LC has been elusive. We propose the following NM synthesis pathway in these CA neurons: (1) Tyrosine is converted by tyrosine hydroxylase (TH) to L-3,4-dihydroxyphenylalanine (L-DOPA), which is converted by aromatic L-amino acid decarboxylase to DA, which in LC neurons is converted by dopamine ß-hydroxylase to NE; (2) DA or NE is autoxidized to dopamine quinone (DAQ) or norepinephrine quinone (NEQ); and (3) DAQ or NEQ is converted to eumelanic NM (euNM) and pheomelanic NM (pheoNM) in the absence and presence of cysteine, respectively. This process involves proteins as cysteine source and iron. We also discuss whether the NM amounts per neuromelanin-positive (NM+) CA neuron are higher in PD brain, whether NM quantitatively correlates with neurodegeneration, and whether an active lifestyle may reduce NM formation.

Interactions of dopamine, iron, and alpha-synuclein linked to dopaminergic neuron vulnerability in Parkinson's disease and Neurodegeneration with Brain Iron Accumulation disorders.

Dopamine metabolism, alpha-synuclein pathology, and iron homeostasis have all been implicated as potential contributors to the unique vulnerability of substantia nigra dopaminergic neurons which preferentially decline in Parkinson's disease and some rare neurodegenerative disorders with shared pathological features. However, the mechanisms contributing to disease progression and resulting in dopaminergic neuron loss in the substantia nigra are still not completely understood. Increasing evidence demonstrates that disrupted dopamine, alpha-synuclein, and/or iron pathways, when combined with the unique morphological, physiological, and metabolic features of this neuron population, may culminate in weakened resilience to multiple stressors. This review analyzes the involvement of each of these pathways in dopamine neuron physiology and function, and discusses how disrupted interplay of dopamine, alpha-synuclein, and iron pathways may synergize to promote pathology and drive the unique vulnerability to disease states. We suggest that elucidating the interactions of dopamine with iron and alpha-synuclein, and the role of dopamine metabolism in driving pathogenic phenotypes will be critical for developing therapeutics to prevent progression in diseases that show degeneration of nigral dopamine neurons such as Parkinson's disease and the rare family of disorders known as Neurodegeneration with Brain Iron Accumulation.

Neuromelanin-sensitive MRI as a noninvasive proxy measure of dopamine function in the human brain.

Neuromelanin-sensitive MRI (NM-MRI) purports to detect the content of neuromelanin (NM), a product of dopamine metabolism that accumulates with age in dopamine neurons of the substantia nigra (SN). Interindividual variability in dopamine function may result in varying levels of NM accumulation in the SN; however, the ability of NM-MRI to measure dopamine function in nonneurodegenerative conditions has not been established. Here, we validated that NM-MRI signal intensity in postmortem midbrain specimens correlated with regional NM concentration even in the absence of neurodegeneration, a prerequisite for its use as a proxy for dopamine function. We then validated a voxelwise NM-MRI approach with sufficient anatomical sensitivity to resolve SN subregions. Using this approach and a multimodal dataset of molecular PET and fMRI data, we further showed the NM-MRI signal was related to both dopamine release in the dorsal striatum and resting blood flow within the SN. These results suggest that NM-MRI signal in the SN is a proxy for function of dopamine neurons in the nigrostriatal pathway. As a proof of concept for its clinical utility, we show that the NM-MRI signal correlated to severity of psychosis in schizophrenia and individuals at risk for schizophrenia, consistent with the well-established dysfunction of the nigrostriatal pathway in psychosis. Our results indicate that noninvasive NM-MRI is a promising tool that could have diverse research and clinical applications to investigate in vivo the role of dopamine in neuropsychiatric illness.

Water-Soluble Melanin-Protein-Fe/Cu Conjugates Derived from Norepinephrine as Reliable Models for Neuromelanin of Human Brain Locus Coeruleus.

Water-soluble melanin-protein-Fe/Cu conjugates derived from norepinephrine and fibrillar ß-lactoglobulin are reliable models for neuromelanin (NM) of human brain locus coeruleus. Both iron and copper promote catecholamine oxidation and exhibit strong tendency to remain coupled in oligonuclear aggregates. The Fe-Cu clusters are EPR silent and affect the 1 H NMR spectra of the conjugates through a specific sequence of signals. Derivatives containing only Fe or Cu exhibit different NMR patterns. The EPR spectra show weak signals of paramagnetic FeIII in conjugates containing Fe or mixed Fe-Cu sites due to small amounts of mononuclear centers. The latter derivatives exhibit EPR signals for isolated CuII centers. These features parallel the EPR behavior of NM from locus coeruleus. The spectral data indicate that FeIII is bound to the melanic fraction, whereas CuII is bound on the protein fibrils, suggesting that the Fe-Cu clusters occur at the interface between the two components of the synthetic NMs.

Highly Effective Protocol for Differentiation of Induced Pluripotent Stem Cells (iPS) into Melanin-Producing Cells.

Melanin is a black/brown pigment present in abundance in human skin. Its main function is photo-protection of underlying tissues from harmful UV light. Natural sources of isolated human melanin are limited; thus, in vitro cultures of human cells may be a promising source of human melanin. Here, we present an innovative in vitro differentiation protocol of induced pluripotent stem cells (iPS) into melanin-producing cells, delivering highly pigmented cells in quantity and quality incomparably higher than any other methods previously described. Pigmented cells constitute over 90% of a terminally differentiated population and exhibit features characteristic for melanocytes, i.e., expression of specific markers such as MITF-M (microphthalmia-associated transcription factor isoform M), TRP-1 (tyrosinase-related protein 1), and TYR (tyrosinase) and accumulation of black pigment in organelles closely resembling melanosomes. Black pigment is unambiguously identified as melanin with features corresponding to those of melanin produced by typical melanocytes. The advantage of our method is that it does not require any sophisticated procedures and can be conducted in standard laboratory conditions. Moreover, our protocol is highly reproducible and optimized to generate high-purity melanin-producing cells from iPS cells; thus, it can serve as an unlimited source of human melanin for modeling human skin diseases. We speculate that FGF-8 might play an important role during differentiation processes toward pigmented cells.

Locus coeruleus imaging as a biomarker for noradrenergic dysfunction in neurodegenerative diseases.

Pathological alterations to the locus coeruleus, the major source of noradrenaline in the brain, are histologically evident in early stages of neurodegenerative diseases. Novel MRI approaches now provide an opportunity to quantify structural features of the locus coeruleus in vivo during disease progression. In combination with neuropathological biomarkers, in vivo locus coeruleus imaging could help to understand the contribution of locus coeruleus neurodegeneration to clinical and pathological manifestations in Alzheimer's disease, atypical neurodegenerative dementias and Parkinson's disease. Moreover, as the functional sensitivity of the noradrenergic system is likely to change with disease progression, in vivo measures of locus coeruleus integrity could provide new pathophysiological insights into cognitive and behavioural symptoms. Locus coeruleus imaging also holds the promise to stratify patients into clinical trials according to noradrenergic dysfunction. In this article, we present a consensus on how non-invasive in vivo assessment of locus coeruleus integrity can be used for clinical research in neurodegenerative diseases. We outline the next steps for in vivo, post-mortem and clinical studies that can lay the groundwork to evaluate the potential of locus coeruleus imaging as a biomarker for neurodegenerative diseases.

A lysosome-plasma membrane-sphingolipid axis linking lysosomal storage to cell growth arrest.

Lysosomal accumulation of undegraded materials is a common feature of lysosomal storage diseases, neurodegenerative disorders, and the aging process. To better understand the role of lysosomal storage in the onset of cell damage, we used human fibroblasts loaded with sucrose as a model of lysosomal accumulation. Sucrose-loaded fibroblasts displayed increased lysosomal biogenesis followed by arrested cell proliferation. Notably, we found that reduced lysosomal catabolism and autophagy impairment led to an increase in sphingolipids ( i.e., sphingomyelin, glucosylceramide, ceramide, and the gangliosides GM3 and GD3), at both intracellular and plasma membrane (PM) levels. In addition, we observed an increase in the lysosomal membrane protein Lamp-1 on the PM of sucrose-loaded fibroblasts and a greater release of the soluble lysosomal protein cathepsin D in their extracellular medium compared with controls. These results indicate increased fusion between lysosomes and the PM, as also suggested by the increased activity of lysosomal glycosphingolipid hydrolases on the PM of sucrose-loaded fibroblasts. The inhibition of ß-glucocerebrosidase and nonlysosomal glucosylceramidase, both involved in ceramide production resulting from glycosphingolipid catabolism on the PM, partially restored cell proliferation. Our findings indicate the existence of a new molecular mechanism underlying cell damage triggered by lysosomal impairment.-Samarani, M., Loberto, N., Soldà, G., Straniero, L., Asselta, R., Duga, S., Lunghi, G., Zucca, F. A., Mauri, L., Ciampa, M. G., Schiumarini, D., Bassi, R., Giussani, P., Chiricozzi, E., Prinetti, A., Aureli, M., Sonnino, S. A lysosome-plasma membrane-sphingolipid axis linking lysosomal storage to cell growth arrest.

Dopamine, Oxidative Stress and Protein-Quinone Modifications in Parkinson's and Other Neurodegenerative Diseases.

Dopamine (DA) is the most important catecholamine in the brain, as it is the most abundant and the precursor of other neurotransmitters. Degeneration of nigrostriatal neurons of substantia nigra pars compacta in Parkinson's disease represents the best-studied link between DA neurotransmission and neuropathology. Catecholamines are reactive molecules that are handled through complex control and transport systems. Under normal conditions, small amounts of cytosolic DA are converted to neuromelanin in a stepwise process involving melanization of peptides and proteins. However, excessive cytosolic or extraneuronal DA can give rise to nonselective protein modifications. These reactions involve DA oxidation to quinone species and depend on the presence of redox-active transition metal ions such as iron and copper. Other oxidized DA metabolites likely participate in post-translational protein modification. Thus, protein-quinone modification is a heterogeneous process involving multiple DA-derived residues that produce structural and conformational changes of proteins and can lead to aggregation and inactivation of the modified proteins.

Contrast mechanisms associated with neuromelanin-MRI.

PURPOSE: To investigate the physical mechanisms associated with the contrast observed in neuromelanin MRI. METHODS: Phantoms having different concentrations of synthetic melanins with different degrees of iron loading were examined on a 3 Tesla scanner using relaxometry and quantitative magnetization transfer (MT). RESULTS: Concentration-dependent T1 and T2 shortening was most pronounced for the melanin pigment when combined with iron. Metal-free melanin had a negligible effect on the magnetization transfer spectra. On the contrary, the presence of iron-laden melanins resulted in a decreased magnetization transfer ratio. The presence of melanin or iron (or both) did not have a significant effect on the macromolecular content, represented by the pool size ratio. CONCLUSION: The primary mechanism underlying contrast in neuromelanin-MRI appears to be the T1 reduction associated with melanin-iron complexes. The macromolecular content is not significantly influenced by the presence of melanin with or without iron, and thus the MT is not directly affected. However, as T1 plays a role in determining the MT-weighted signal, the magnetization transfer ratio is reduced in the presence of melanin-iron complexes. Magn Reson Med 78:1790-1800, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Elemental mapping of Neuromelanin organelles of human Substantia Nigra: correlative ultrastructural and chemical analysis by analytical transmission electron microscopy and nano-secondary ion mass spectrometry.

Neuromelanin (NM) is a compound which highly accumulates mainly in catecholamine neurons of the substantia nigra (SN), and is contained in organelles (NM-containing organelles) with lipid bodies and proteins. These neurons selectively degenerate in Parkinson's disease and NM can play either a protective or toxic role. NM-containing organelles of SN were investigated by Analytical Electron Microscopy (AEM) and Nano-Secondary Ion Mass Spectrometry (NanoSIMS) within human tissue sections with respect to ultrastructure and elemental composition. Within the NM-containing organelle, the single NM granules and lipid bodies had sizes of about 200-600 nm. Energy-Dispersive X-ray microanalysis spectra of the NM granules and lipid bodies were acquired with 100 nm beam diameter in AEM, NanoSIMS yielded elemental maps with a lateral resolution of about 150 nm. AEM yielded the quantitative elemental composition of NM granules and bound metals, e.g., iron with a mole fraction of about 0.15 atomic percent. Chemical analyses by AEM and NanoSIMS were consistent at the subcellular level so that nanoSIMS measurements have been quantitated. In NM granules of SN from healthy subjects, a significant amount of S, Fe, and Cu was found. In lipid bodies an amount of P consistent with the presence of phospholipids was measured. The improved detection limits of nanoSIMS offer new possibilities for chemical mapping, high-sensitivity trace element detection, and reduced acquisition times. Variations between individual NM granules can now be investigated effectively and quantitatively by NanoSIMS mapping Cu and Fe. This should yield new insight into the changes in chemical composition of NM pigments during healthy aging and disease. Neuromelanin-containing organelles of dopamine neurons in normal human substantia nigra were investigated by analytical electron mircoscopy and secondary ion mass spectroscopy (NanoSIMS) yielding the ultrastructure and elemental composition. In neuromelanin granules a significant amount of S, Fe and Cu was found. In lipid bodies an amount of P consistent with the presence of phospholipids was measured. The improved sensitivity of NanoSIMS shows differences in chemical composition between individual neuromelanin granules and allows to study chemical changes of neuromelanin organelles during aging and disease.

Norepinephrine and its metabolites are involved in the synthesis of neuromelanin derived from the locus coeruleus.

In order to elucidate the chemical structure of black to brown pigments, neuromelanins (NMs), in the substantia nigra (SN) and the locus coeruleus (LC) in the central nervous system of humans and other mammalian species during aging, chemical degradative methods are powerful tools. HPLC analysis after hydroiodic acid hydrolysis detected aminohydroxyphenylethylamines, aminohydroxyphenylacetic acids, and aminohydroxyethylbenzenes, which confirmed that SN-NM and LC-NM contain melanin derived not only from dopamine and norepinephrine (NE) but also from several other catecholic metabolites, such as 3,4-dihydroxyphenylalanine, 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxymandelic acid, 3,4-dihydroxyphenylethanol, and 3,4-dihydroxyphenylethylene glycol, in addition to the corresponding Cys-derivatives in varying degrees. However, hydroiodic acid hydrolysis showed that LC-NM produced the same degradation products as were detected in SN-NM. Thus, we needed to develop a new chemical detection method to validate the existence of NE in LC-NM. In the present study, we report that HCl hydrolysis of LC-NM in the presence of thioglycolic acid yields new products arising from substitution of the hydroxyl group by thioglycolic acid at the benzyl position of NE and cysteinyl-NE. This is the first chemical evidence showing that NE and cysteinyl-NE are incorporated into LC-NM. Using the chemical degradation methods for the determination of catechols in neuromelanin (NM), we have shown that dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), 3,4-dihydroxyphenylethanol (DOPE), and 3,4-dihydroxyphenylalanine (DOPA) are mainly responsible for the structure of NM from substantia nigra (SN), while norepinephrine (NE), 3,4-dihydroxymandelic acid (DOMA), and 3,4-dihydroxyphenylethylene glycol (DOPEG) are additionally responsible for the structure of NM from locus coeruleus (LC).

Protective and toxic roles of dopamine in Parkinson's disease.

The molecular mechanisms causing the loss of dopaminergic neurons containing neuromelanin in the substantia nigra and responsible for motor symptoms of Parkinson's disease are still unknown. The discovery of genes associated with Parkinson's disease (such as alpha synuclein (SNCA), E3 ubiquitin protein ligase (parkin), DJ-1 (PARK7), ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCHL-1), serine/threonine-protein kinase (PINK-1), leucine-rich repeat kinase 2 (LRRK2), cation-transporting ATPase 13A1 (ATP13A), etc.) contributed enormously to basic research towards understanding the role of these proteins in the sporadic form of the disease. However, it is generally accepted by the scientific community that mitochondria dysfunction, alpha synuclein aggregation, dysfunction of protein degradation, oxidative stress and neuroinflammation are involved in neurodegeneration. Dopamine oxidation seems to be a complex pathway in which dopamine o-quinone, aminochrome and 5,6-indolequinone are formed. However, both dopamine o-quinone and 5,6-indolequinone are so unstable that is difficult to study and separate their roles in the degenerative process occurring in Parkinson's disease. Dopamine oxidation to dopamine o-quinone, aminochrome and 5,6-indolequinone seems to play an important role in the neurodegenerative processes of Parkinson's disease as aminochrome induces: (i) mitochondria dysfunction, (ii) formation and stabilization of neurotoxic protofibrils of alpha synuclein, (iii) protein degradation dysfunction of both proteasomal and lysosomal systems and (iv) oxidative stress. The neurotoxic effects of aminochrome in dopaminergic neurons can be inhibited by: (i) preventing dopamine oxidation of the transporter that takes up dopamine into monoaminergic vesicles with low pH and dopamine oxidative deamination catalyzed by monoamino oxidase (ii) dopamine o-quinone, aminochrome and 5,6-indolequinone polymerization to neuromelanin and (iii) two-electron reduction of aminochrome catalyzed by DT-diaphorase. Furthermore, dopamine conversion to NM seems to have a dual role, protective and toxic, depending mostly on the cellular context. Dopamine oxidation to dopamine o-quinone, aminochrome and 5,6-indolequinone plays an important role in neurodegeneration in Parkinson's disease since they induce mitochondria and protein degradation dysfunction; formation of neurotoxic alpha synuclein protofibrils and oxidative stress. However, the cells have a protective system against dopamine oxidation composed by dopamine uptake mediated by Vesicular monoaminergic transporter-2 (VMAT-2), neuromelanin formation, two-electron reduction and GSH-conjugation mediated by Glutathione S-transferase M2-2 (GSTM2).

Reduction of the nitro group to amine by hydroiodic acid to synthesize o-aminophenol derivatives as putative degradative markers of neuromelanin.

Neuromelanin (NM) is produced in dopaminergic neurons of the substantia nigra (SN) and in noradrenergic neurons of the locus coeruleus (LC). The synthesis of NM in those neurons is a component of brain aging and there is the evidence that this pigment can be involved in the pathogenesis of neurodegenerative diseases such as Parkinson's disease. NM is believed to derive from the oxidative polymerization of dopamine (DA) or norepinephrine (NE) with the participation of cysteine, dolichols and proteins. However, there are still unknown aspects in the chemical structure of NM from SN (SN-NM) and LC (LC-NM). In this study, we designed a new method to synthesize o-aminophenol compounds as putative degradation products of catecholamines and their metabolites which may be incorporated into NM. Those compounds are aminohydroxyphenylethylamine (AHPEA) isomers, aminohydroxyphenylacetic acid (AHPAA) isomers and aminohydroxyethylbenzene (AHEB) isomers, which are expected to arise from DA or NE, 3,4-dihydroxyphenylacetic acid (DOPAC) or 3,4-dihydroxyphenylmandelic acid (DOMA) and 3,4-dihydroxyphenylethanol (DOPE) or 3,4-dihydroxyphenylethyleneglycol (DOPEG), respectively. These o-aminophenol compounds were synthesized by the nitration of phenol derivatives followed by reduction with hydroiodic acid (HI), and they could be identified by HPLC in HI hydrolysates of SN-NM and LC-NM. This degradative approach by HI hydrolysis allows the identification of catecholic precursors unique to SN-NM and LC-NM, which are present in catecholaminergic neurons.

DOPA pheomelanin is increased in nigral neuromelanin of Parkinson's disease.

Neuromelanin (NM) in dopaminergic neurons of human substantia nigra (SN) has a melanic component that consists of pheomelanin and eumelanin moieties and has been proposed as a key factor contributing to dopaminergic neuron vulnerability in Parkinson's disease (PD). While eumelanin is considered as an antioxidant, pheomelanin and related oxidative stress are associated with compromised drug and metal ion binding and melanoma risk. Using postmortem SN from patients with PD or Alzheimer's disease (AD) and unaffected controls, we identified increased L-3,4-dihydroxyphenylalanine (DOPA) pheomelanin and increased ratios of dopamine (DA) pheomelanin markers to DA in PD SN compared to controls. Eumelanins derived from both DOPA and DA were reduced in PD group. In addition, we report an increase in DOPA pheomelanin relative to DA pheomelanin in PD SN. In AD SN, we observed unaltered melanin markers despite reduced DOPA compared to controls. Furthermore, synthetic DOPA pheomelanin induced neuronal cell death in vitro while synthetic DOPA eumelanin showed no significant effect on cell viability. Our findings provide insights into the different roles of pheomelanin and eumelanin in PD pathophysiology. We anticipate our study will lead to further investigations on pheomelanin and eumelanin individually as biomarkers and possibly therapeutic targets for PD.

New melanic pigments in the human brain that accumulate in aging and block environmental toxic metals.

Neuronal pigments of melanic type were identified in the putamen, cortex, cerebellum, and other major regions of human brain. These pigments consist of granules 30 nm in size, contained in organelles together with lipid droplets, and they accumulate in aging, reaching concentrations as high as 1.5-2.6 microg/mg tissue in major brain regions. These pigments, which we term neuromelanins, contain melanic, lipid, and peptide components. The melanic component is aromatic in structure, contains a stable free radical, and is synthesized from the precursor molecule cysteinyl-3,4-dihydroxyphenylalanine. This contrasts with neuromelanin of the substantia nigra, where the melanic precursor is cysteinyl-dopamine. These neuronal pigments have some structural similarities to the melanin found in skin. The precursors of lipid components of the neuromelanins are the polyunsaturated lipids present in the surrounding organelles. The synthesis of neuromelanins in the various regions of the human brain is an important protective process because the melanic component is generated through the removal of reactive/toxic quinones that would otherwise cause neurotoxicity. Furthermore, the resulting melanic component serves an additional protective role through its ability to chelate and accumulate metals, including environmentally toxic metals such as mercury and lead.

The neurobiology of human aggressive behavior: Neuroimaging, genetic, and neurochemical aspects.

In modern societies, there is a strive to improve the quality of life related to risk of crimes which inevitably requires a better understanding of brain determinants and mediators of aggression. Neurobiology provides powerful tools to achieve this end. Pre-clinical and clinical studies show that changes in regional volumes, metabolism-function and connectivity within specific neural networks are related to aggression. Subregions of prefrontal cortex, insula, amygdala, basal ganglia and hippocampus play a major role within these circuits and have been consistently implicated in biology of aggression. Genetic variations in proteins regulating the synthesis, degradation, and transport of serotonin and dopamine as well as their signal transduction have been found to mediate behavioral variability observed in aggression. Gene-gene and gene-environment interactions represent additional important risk factors for aggressiveness. Considering the social burden of pathological forms of aggression, more basic and translational studies should be conducted to accelerate applications to clinical practice, justice courts, and policy making.

Interaction of Neuromelanin with Xenobiotics and Consequences for Neurodegeneration; Promising Experimental Models.

Neuromelanin (NM) accumulates in catecholamine long-lived brain neurons that are lost in neurodegenerative diseases. NM is a complex substance made of melanic, peptide and lipid components. NM formation is a natural protective process since toxic endogenous metabolites are removed during its formation and as it binds excess metals and xenobiotics. However, disturbances of NM synthesis and function could be toxic. Here, we review recent knowledge on NM formation, toxic mechanisms involving NM, go over NM binding substances and suggest experimental models that can help identifying xenobiotic modulators of NM formation or function. Given the high likelihood of a central NM role in age-related human neurodegenerative diseases such as Parkinson's and Alzheimer's, resembling such diseases using animal models that do not form NM to a high degree, e.g., mice or rats, may not be optimal. Rather, use of animal models (i.e., sheep and goats) that better resemble human brain aging in terms of NM formation, as well as using human NM forming stem cellbased in vitro (e.g., mid-brain organoids) models can be more suitable. Toxicants could also be identified during chemical synthesis of NM in the test tube.

Neuromelanins in various regions of human brain are associated with native and oxidized isoprenoid lipids.

Neuromelanin (NM) isolated from seven regions of the human brain is found to contain series of natural and oxidized isoprenoid lipids. Specifically, dolichols (dol) and dolichoic acids (dol-CA) with 14-22 and 14-21 isoprene units are identified. Standards of nor-dolichol and nor-dolichoic acid were used to determine the relative amounts of dol and dol-CA compared to the total lipids present in NM for each region. The cerebellum, putamen, globus pallidus, and premotor cortex contained similar amounts of dol, comprising approximately 8-9.5% of the total lipid weight. Interestingly, the corpus callosum contains substantially lower quantities of both dol and dol-CA compared to the other regions-less than 4% dol relative to the total lipid weight. Oxidized and reduced dolichol-related species were identified and determined to be region-dependent.

Neuromelanins isolated from different regions of the human brain exhibit a common surface photoionization threshold.

Neuromelanin isolated from the premotor cortex, cerebellum, putamen, globus pallidus and corpus callosum of the human brain is studied by scanning probe and photoelectron emission microscopies and the results are compared with previously published work on neuromelanin from the substantia nigra. Scanning electron microscopy reveals common structure for all neuromelanins. All exhibit spherical entities of diameters between 200 and 400 nm, composed of smaller spherical substructures, approximately 30 nm in diameter. These features are similar to that observed for many melanin systems including Sepia cuttlefish, bovine eye, and human eye and hair melanosomes. Photoelectron microscopy images were collected for all neuromelanins at specific wavelengths of ultraviolet light between 248 and 413 nm, using the spontaneous emission output from the Duke free electron laser. Analysis of the data establishes a common threshold photoionization potential for neuromelanins of 4.7 +/- 0.2 eV, corresponding to an oxidation potential of -0.3 +/- 0.2 V vs the normal hydrogen electrode (NHE). These results are consistent with previously reported potentials for neuromelanin from the substantia nigra of 4.5 +/- 0.2 eV (-0.1 +/- 0.2 V vs NHE). All neuromelanins exhibit a common low surface oxidation potential, reflecting their eumelanic component and their inability to trigger redox processes with neurotoxic effect.