Neuromelanin inhibits CXCL10 expression in human astroglial cells

Chemokines in neurodegenerative diseases

Neurodegeneration and neuroinflammation disorders are mainly the result of the deposition of various proteins, such as α-synuclein, amyloid-β and prions, which lead to the initiation and activation of inflammatory responses. Different chemokines are involved in the infiltration and movement of inflammatory leukocytes into the central nervous system (CNS) that express chemokine receptors. Dysregulation of several members of chemokines has been shown in the CNS, cerebrospinal fluid and peripheral blood of patients who have neurodegenerative disorders. Upon infiltration of various cells, they produce many inflammatory mediators such as cytokines. Besides them, some CNS-resident cells, such as neurons and astrocytes, are also involved in the pathogenesis of neurodegeneration by producing chemokines. In this review, we summarize the role of chemokines and their related receptors in the pathogenesis of neurodegeneration and neuroinflammation disorders, including multiple sclerosis, Parkinson's disease and Alzheimer's disease. Therapeutic strategies targeting chemokines or their related receptors are also discussed in this article.

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.

Potential neurotoxic activity of diverse molecules released by astrocytes

Astrocytes are the main support cells of the central nervous system. They also participate in neuroimmune reactions. In response to pathological and immune stimuli, astrocytes transform to reactive states characterized by increased release of inflammatory mediators. Some of these molecules are neuroprotective and inflammation resolving while others, including reactive oxygen species (ROS), nitric oxide (NO), matrix metalloproteinase (MMP)- 9, L-glutamate, and tumor necrosis factor α (TNF), are well-established toxins known to cause damage to surrounding cells and tissues. We hypothesized that similar to microglia, the brain immune cells, reactive astrocytes can release a broader set of diverse molecules that are potentially neurotoxic. A literature search was conducted to identify such molecules using the following two criteria: 1) evidence of their expression and secretion by astrocytes and 2) direct neurotoxic action. This review describes 14 structurally diverse molecules as less-established astrocyte neurotoxins, including C-X-C motif chemokine ligand (CXCL)10, CXCL12/CXCL12(5-67), FS-7-associated surface antigen ligand (FasL), macrophage inflammatory protein (MIP)- 2α, TNF-related apoptosis inducing ligand (TRAIL), pro-nerve growth factor (proNGF), pro-brain-derived neurotrophic factor (proBDNF), chondroitin sulfate proteoglycans (CSPGs), cathepsin (Cat)B, group IIA secretory phospholipase A₂ (sPLA₂-IIA), amyloid beta peptides (Aβ), high mobility group box (HMGB)1, ceramides, and lipocalin (LCN)2. For some of these molecules, further studies are required to establish either their direct neurotoxic effects or the full spectrum of stimuli that induce their release by astrocytes. Only limited studies with human-derived astrocytes and neurons are available for most of these potential neurotoxins, which is a knowledge gap that should be addressed in the future. We also summarize available evidence of the role these molecules play in select neuropathologies where reactive astrocytes are a key feature. A comprehensive understanding of the full spectrum of neurotoxins released by reactive astrocytes is key to understanding neuroinflammatory diseases characterized by the adverse activation of these cells and may guide the development of novel treatment strategies.

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.

The Neuromelanin Paradox and Its Dual Role in Oxidative Stress and Neurodegeneration

Aging is associated with an increasing dysfunction of key brain homeostasis mechanisms and represents the main risk factor across most neurodegenerative disorders. However, the degree of dysregulation and the affectation of specific pathways set apart normal aging from neurodegenerative disorders. In particular, the neuronal metabolism of catecholaminergic neurotransmitters appears to be a specifically sensitive pathway that is affected in different neurodegenerations. In humans, catecholaminergic neurons are characterized by an age-related accumulation of neuromelanin (NM), rendering the soma of the neurons black. This intracellular NM appears to serve as a very efficient quencher for toxic molecules. However, when a neuron degenerates, NM is released together with its load (many undegraded cellular components, transition metals, lipids, xenobiotics) contributing to initiate and worsen an eventual immune response, exacerbating the oxidative stress, ultimately leading to the neurodegenerative process. This review focuses on the analysis of the role of NM in normal aging and neurodegeneration related to its capabilities as an antioxidant and scavenging of harmful molecules, versus its involvement in oxidative stress and aberrant immune response, depending on NM saturation state and its extracellular release.

The opioid antagonist, β-funaltrexamine, inhibits NF-κB signaling and chemokine expression in human astrocytes and in mice

Opioid-immune crosstalk occurs when opioid drugs alter the activity of the immune system. In this study, the opioid antagonist β-funaltrexamine (β-FNA) decreases the expression and release of an inflammatory chemokine, interferon-γ inducible protein-10 (CXCL10) from normal human astrocytes stimulated by interleukin 1β (IL-1β). β-FNA decreased CXCL10 by an unknown action that did not involve the mu opioid receptor (MOR). As IL-1β acts through its receptor to activate NF-κB/MAPK signaling pathways which leads to CXCL10 expression and release, key steps in the IL-1β signaling pathways were examined following β-FNA treatment. IL-1β-induced activation of p38 mitogen-activated protein kinases (p38 MAPK) was inhibited by β-FNA as shown by decreased p38 MAPK phosphorylation in treated cells. β-FNA also decreased the levels of activated subunits of NF-κB (p50 and p65) in treated astrocytes. The impact of β-FNA was also observed in proteins that act to negatively regulate NF-κB signaling. IL-1β upregulated the expression of A20, a ubiquitin (Ub)-editing enzyme that dampens NF-κB signaling by altering ubiquination patterns on IL-1 receptor second messengers, and the increase in A20 was significantly inhibited by β-FNA treatment. Inhibition of the Ub-activating enzyme E1 by the inhibitor PYR41 also decreased CXCL10 release, like β-FNA, and concurrent treatment with both PYR41 and β-FNA inhibited CXCL10 more than did either agent alone. In mice, lipopolysaccharide-induced CXCL10 expression in the brain was inhibited by treatment with β-FNA. These findings suggest that β-FNA exerts an anti-inflammatory action in vitro and in vivo that is MOR-independent and possibly due to the alkylating ability of β-FNA.

Neuromelanin of the human substantia nigra: an update

Dopaminergic neurons of the substantia nigra selectively degenerate over the course of Parkinson's disease. These neurons are also the most heavily pigmented cells of the brain, accumulating the dark pigment neuromelanin over a lifetime. The massive presence of neuromelanin in these brain areas has long been suspected as a key factor involved in the selective vulnerability of neurons. The high concentration of neuromelanin in substantia nigra neurons seems to be linked to the presence of considerable amounts of cytosolic dopamine that have not been sequestered into synaptic vesicles. Over the past few years, studies have uncovered a dual nature of neuromelanin. Intraneuronal neuromelanin can be a protective factor, shielding the cells from toxic effects of redox active metals, toxins, and excess of cytosolic catecholamines. In contrast, neuromelanin released by dying neurons can contribute to the activation of neuroglia triggering the neuroinflammation that characterizes Parkinson's disease. This article reviews recent studies on the molecular aspects of neuromelanin of the human substantia nigra.

Β-funaltrexamine inhibits chemokine (CXCL10) expression in normal human astrocytes

Neuroinflammation is an integral component of neurodegenerative disorders, CNS infection and trauma. Astroglial chemokines, such as CXCL10, are instrumental in neuroinflammatory signaling as well as neurotoxicity. We have utilized proinflammatory-induced CXCL10 expression in normal human astrocytes (NHA) as a model in which to assess the anti-inflammatory actions of the selective, mu-opioid receptor (MOR) antagonist, β-funaltrexamine (β-FNA). Interferon (IFN)γ+HIV-1 Tat-induced CXCL10 expression (secreted protein and mRNA) was inhibited by co-treatment with β-FNA. Neither the MOR-selective antagonist, D-Phe-Cys-Tyr-D-Trp-Arg-Pen-Thr-NH2 (CTAP) nor the nonselective opioid receptor antagonist, naltrexone inhibited IFNγ+HIV-1 Tat-induced CXCL10 expression. Furthermore, co-treatment with excess CTAP or naltrexone did not prevent β-FNA mediated inhibition of IFNγ+HIV-1 Tat-induced CXCL10 expression. Additionally, we utilized an inhibitor of NF-κB activation (SN50) to demonstrate that IFNγ+HIV-1 Tat-induced CXCL10 expression is NF-κB-dependent in NHA. Subsequent experiments revealed that β-FNA did not significantly affect NF-κB activation. Interestingly, we discovered that β-FNA inhibited p38 activation as indicated by decreased expression of phospho-p38. Together, these findings suggest that the inhibitory actions of β-FNA are MOR-independent and mediated, in part, via a transcriptional mechanism. These findings add to our understanding of the mechanism by which chemokine expression is inhibited by β-FNA. In conjunction with future investigations, these novel findings are expected to provide insights into the development of safe and effective treatments for neuroinflammation.

α-Synuclein potentiates interleukin-1β-induced CXCL10 expression in human A172 astrocytoma cells

Neuroinflammation and neuronal degeneration observed in Parkinson's disease (PD) has been attributed in part to glial-mediated events. Increased expression of proinflammatory cytokines and abnormal accumulation of the neuronal protein, α-synuclein in the brain are also characteristic of PD. While increasing evidence suggests that astrocytes contribute to neuroinflammation and dopaminergic neuronal degeneration associated with PD, there remains much to learn about these astroglial-mediated events. Therefore, we investigated the in vitro effects of interleukin-1β (IL-1β) and α-synuclein on astroglial expression of interferon-γ inducible protein-10 (CXCL10), a proinflammatory and neurotoxic chemokine. IL-1β-induced CXCL10 protein expression was potentiated by co-exposure to α-synuclein. α-Synuclein did not significantly affect IL-1β-induced CXCL10 mRNA expression, but did mediate increased CXCL10 mRNA stability, which may explain, in part, the increased levels of secreted CXCL10 protein. Future investigations are warranted to more fully define the mechanism by which α-synuclein enhances IL-1β-induced astroglial CXCL10 expression. These findings highlight the importance of α-synuclein in modulating inflammatory events in astroglia. These events may be particularly relevant to the pathology of CNS disorders involving α-synuclein accumulation, including PD and HIV-1 associated dementia.