Nitric oxide synthase and neuronal vulnerability in Parkinson's disease

Thiamine and Parkinson's disease

Parkinson's disease (PD) is the second most common form of neurodegeneration in the elderly population. PD is clinically characterized by tremors, rigidity, slowness of movement and postural imbalance. A significant association has been demonstrated between PD and low levels of thiamine in the serum, which suggests that elevated thiamine levels might provide protection against PD. Genetic studies have helped identify a number of factors that link thiamine to PD pathology, including the DJ-1 gene, excitatory amino acid transporters (EAATs), the α-ketoglutarate dehydrogenase complex (KGDHC), coenzyme Q10 (CoQ10 or ubiquinone), lipoamide dehydrogenase (LAD), chromosome 7, transcription factor p53, the renin-angiotensin system (RAS), heme oxygenase-1 (HO-1), and poly(ADP-ribose) polymerase-1 gene (PARP-1). Thiamine has also been implicated in PD through its effects on L-type voltage-sensitive calcium channels (L-VSCC), matrix metalloproteinases (MMPs), prostaglandins (PGs), cyclooxygenase-2 (COX-2), reactive oxygen species (ROS), and nitric oxide synthase (NOS). Recent studies highlight a possible relationship between thiamine and PD. Genetic studies provide opportunities to determine which proteins may link thiamine to PD pathology. Thiamine can also act through a number of non-genomic mechanisms that include protein expression, oxidative stress, inflammation, and cellular metabolism. Further studies are needed to determine the benefits of using thiamine as a treatment for PD.

The nitric oxide-cyclic GMP pathway regulates FoxO and alters dopaminergic neuron survival in Drosophila

Activation of the forkhead box transcription factor FoxO is suggested to be involved in dopaminergic (DA) neurodegeneration in a Drosophila model of Parkinson's disease (PD), in which a PD gene product LRRK2 activates FoxO through phosphorylation. In the current study that combines Drosophila genetics and biochemical analysis, we show that cyclic guanosine monophosphate (cGMP)-dependent kinase II (cGKII) also phosphorylates FoxO at the same residue as LRRK2, and Drosophila orthologues of cGKII and LRRK2, DG2/For and dLRRK, respectively, enhance the neurotoxic activity of FoxO in an additive manner. Biochemical assays using mammalian cGKII and FoxO1 reveal that cGKII enhances the transcriptional activity of FoxO1 through phosphorylation of the FoxO1 S319 site in the same manner as LRRK2. A Drosophila FoxO mutant resistant to phosphorylation by DG2 and dLRRK (dFoxO S259A corresponding to human FoxO1 S319A) suppressed the neurotoxicity and improved motor dysfunction caused by co-expression of FoxO and DG2. Nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) also increased FoxO's activity, whereas the administration of a NOS inhibitor L-NAME suppressed the loss of DA neurons in aged flies co-expressing FoxO and DG2. These results strongly suggest that the NO-FoxO axis contributes to DA neurodegeneration in LRRK2-linked PD.

Contributions of central and systemic inflammation to the pathophysiology of Parkinson's disease

Idiopathic Parkinson's disease (PD) represents a complex interaction between the inherent vulnerability of the nigrostriatal dopaminergic system, a possible genetic predisposition, and exposure to environmental toxins including inflammatory triggers. Evidence now suggests that chronic neuroinflammation is consistently associated with the pathophysiology of PD. Activation of microglia and increased levels of pro-inflammatory mediators such as TNF-α, IL-1β and IL-6, reactive oxygen species and eicosanoids has been reported after post-mortem analysis of the substantia nigra from PD patients and in animal models of PD. It is hypothesised that chronically activated microglia secrete high levels of pro-inflammatory mediators which damage neurons and further activate microglia, resulting in a feed forward cycle promoting further inflammation and neurodegeneration. Moreover, nigrostriatal dopaminergic neurons are more vulnerable to pro-inflammatory and oxidative mediators than other cell types because of their low intracellular glutathione concentration. Systemic inflammation has also been suggested to contribute to neurodegeneration in PD, as lymphocyte infiltration has been observed in brains of PD patients and in animal models of PD, substantiating the current theory of a fundamental role of inflammation in neurodegeneration. We will examine the current evidence in the literature which offers insight into the premise that both central and systemic inflammation may contribute to neurodegeneration in PD. We will discuss the emerging possibility of the use of diagnostic tools such as imaging technologies for PD patients. Finally, we will present the immunomodulatory therapeutic strategies that are now under investigation and in clinical trials as potential neuroprotective drugs for PD.

The role of the MYD88-dependent pathway in MPTP-induced brain dopaminergic degeneration

Background

Mounting evidence supports a significant role of inflammation in Parkinson's disease (PD) pathophysiology, with several inflammatory pathways being suggested as playing a role in the dopaminergic degeneration seen in humans and animal models of the disease. These include tumor necrosis factor, prostaglandins and oxidative-related stress components. However, the role of innate immunity has not been established in PD.

Methods

Based on the fact that the myeloid differentiation primary response gene (88) (MyD88) is the most common adaptor protein implicated in toll-like receptor (TLR) signaling, critical in the innate immune response, we undertook a study to investigate the potential contribution of this specific pathway to MPTP-induced brain dopaminergic degeneration using MyD88 knock out mice (MyD88-/-), following our observations that the MyD88-dependent pathway was critical for MPTP dopaminergic toxicity in the enteric nervous system. Post-mortem analyses assessing nigrostriatal dopaminergic degeneration and inflammation were performed using HPLC, western blots, autoradiography and immunofluorescence.

Results

Our results demonstrate that MyD88-/- mice are as vulnerable to MPTP-induced dopamine and DOPAC striatal depletion as wild type mice. Furthermore, MyD88-/- mice show similar striatal dopamine transporter and tyrosine hydroxylase loss, as well as dopaminergic cell loss in the substantia nigra pars compacta in response to MPTP. To evaluate the extent of the inflammatory response created by the MPTP regimen utilized, we further performed bioluminescence imaging using TLR2-luc/gfp transgenic mice and microglial density analysis, which revealed a modest brain microglial response following MPTP. This was accompanied by a significant astrocytic reaction in the striatum, which was of similar magnitude both in wild type and MyD88-/- mice.

Conclusions

Our results suggest that subacute MPTP-induced dopaminergic degeneration observed in the central nervous system is MyD88-independent, in contrast to our recent observations that this pathway, in the same cohort of animals, is critical in the loss of dopaminergic neurons in the enteric nervous system.

Antioxidants, redox signaling, and pathophysiology in schizophrenia: an integrative view

Schizophrenia (SZ) is a brain disorder that has been intensively studied for over a century; yet, its etiology and multifactorial pathophysiology remain a puzzle. However, significant advances have been made in identifying numerous abnormalities in key biochemical systems. One among these is the antioxidant defense system (AODS) and redox signaling. This review summarizes the findings to date in human studies. The evidence can be broadly clustered into three major themes: perturbations in AODS, relationships between AODS alterations and other systems (i.e., membrane structure, immune function, and neurotransmission), and clinical implications. These domains of AODS have been examined in samples from both the central nervous system and peripheral tissues. Findings in patients with SZ include decreased nonenzymatic antioxidants, increased lipid peroxides and nitric oxides, and homeostatic imbalance of purine catabolism. Reductions of plasma antioxidant capacity are seen in patients with chronic illness as well as early in the course of SZ. Notably, these data indicate that many AODS alterations are independent of treatment effects. Moreover, there is burgeoning evidence indicating a link among oxidative stress, membrane defects, immune dysfunction, and multineurotransmitter pathologies in SZ. Finally, the body of evidence reviewed herein provides a theoretical rationale for the development of novel treatment approaches.

SEA0400, a specific Na+/Ca2+ exchange inhibitor, prevents dopaminergic neurotoxicity in an MPTP mouse model of Parkinson's disease

We have recently shown that the Na(+)/Ca(2+) exchanger (NCX) is involved in nitric oxide (NO)-induced cytotoxicity in cultured astrocytes and neurons. However, there is no in vivo evidence suggesting the role of NCX in neurodegenerative disorders associated with NO. NO is implicated in the pathogenesis of neurodegenerative disorders such as Parkinson's disease. This study examined the effect of SEA0400, the specific NCX inhibitor, on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic neurotoxicity, a model of Parkinson's disease, in C57BL/6J mice. MPTP treatment (10 mg/kg, four times at 2-h intervals) decreased dopamine levels in the midbrain and impaired motor coordination, and these effects were counteracted by S-methylthiocitrulline, a selective neuronal NO synthase inhibitor. SEA0400 protected against the dopaminergic neurotoxicity (determined by dopamine levels in the midbrain and striatum, tyrosine hydroxylase immunoreactivity in the substantia nigra and striatum, striatal dopamine release, and motor deficits) in MPTP-treated mice. SEA0400 had no radical-scavenging activity. SEA0400 did not affect MPTP metabolism and MPTP-induced NO production and microglial activation, while it attenuated MPTP-induced increases in extracellular signal-regulated kinase (ERK) phosphorylation and lipid peroxidation product, thiobarbituric acid reactive substance. These findings suggest that SEA0400 protects against MPTP-induced neurotoxicity probably by blocking ERK phosphorylation and lipid peroxidation which are downstream of NCX-mediated Ca(2+) influx.

Ethyl pyruvate rescues nigrostriatal dopaminergic neurons by regulating glial activation in a mouse model of Parkinson's disease

This study examined whether ethyl pyruvate (EP) promotes the survival of nigrostriatal dopaminergic (DA) neurons in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. MPTP induced degeneration of nigrostriatal DA neurons and glial activation as visualized by tyrosine hydroxylase, macrophage Ag complex-1, and/or glial fibrillary acidic protein immunoreactivity. Western blotting and immunohistochemistry showed activation of microglial NADPH oxidase and astroglial myeloperoxidase (MPO) and subsequent reactive oxygen species/reactive nitrogen species production and oxidative DNA damage in the MPTP-treated substantia nigra. Treatment with EP prevented degeneration of nigrostriatal DA neurons, increased striatal dopamine levels, and improved motor function. This neuroprotection afforded by EP was associated with the suppression of astroglial MPO expression, NADPH oxidase-, and/or inducible NO synthase-derived reactive oxygen species/reactive nitrogen species production by activated microglia. Interestingly, EP was found to protect DA neurons from 1-methyl-4-phenyl-pyridinium neurotoxicity in cocultures of mesencephalic neurons and microglia but not in neuron-enriched mesencephalic cultures devoid of microglia. The present findings show that EP may inhibit glial-mediated oxidative stress, suggesting that EP may have therapeutic value in the treatment of aspects of Parkinson's disease related to glia-derived oxidative damage.

The effect of nNOS inhibitors on toxin-induced cell death in dopaminergic cell lines depends on the extent of enzyme expression

Nitric oxide is linked with neurodegeneration in Parkinson's disease (PD) through the involvement of both inducible (iNOS) and neuronal nitric oxide synthase (nNOS). While non-selective NOS inhibitors are neuroprotective, the role of nNOS has not been determined using selective NOS inhibitors. The present study investigated the neuroprotective effect of selective iNOS and nNOS inhibitors on MPP(+)- and MG-132-induced cell death in cell lines with differing levels of nNOS expression. Inhibition of endogenously expressed nNOS by 7-NI and ARR17477 enhanced the toxicity of MPP(+) and MG-132 in N1E-115 cells, whereas in transfected SH-SY5Y cells overexpressing nNOS, ARR17477 and 7-NI protected against MPP(+)- and MG-132-induced cell death. In contrast, inhibition of iNOS by 1400W was ineffective in preventing MPP(+) and MG-132 toxicity in these cell lines. These results suggest a dual role for NOS in dopaminergic cell viability. nNOS is protective against toxic insult when produced endogenously. When nNOS is overexpressed, it becomes neurotoxic to cells suggesting that inhibition of nNOS may be a promising strategy to prevent cell death in PD.

Nitric Oxide Synthase Inhibitor Improves De Novo and Long-Term l-DOPA-Induced Dyskinesia in Hemiparkinsonian Rats

Inhibitors of neuronal and endothelial nitric oxide synthase decrease l-3,4-dihidroxifenilalanine (l-DOPA)-induced dyskinesias in rodents. The mechanism of nitric oxide inhibitor action is unknown. The aims of the present study were to investigate the decrease of l-DOPA-induced abnormal involuntary movements (AIMs) in 6-hydroxydopamine (6-OHDA)-lesioned rats by nitric oxide inhibitors following either acute or chronic treatment. The primary findings of this study were that NG-nitro-l-Arginine, an inhibitor of endothelial and neuronal nitric oxide synthase, attenuated AIMs induced by chronic and acute l-DOPA. In contrast, rotational behavior was attenuated only after chronic l-DOPA. The 6-OHDA lesion and the l-DOPA treatment induced a bilateral increase (1.5 times) in the neuronal nitric oxide synthase (nNOS) protein and nNOS mRNA in the striatum and in the frontal cortex. There was a parallel increase, bilaterally, of the FosB/ΔFosB, primarily in the ipsilateral striatum. The exception was in the contralateral striatum and the ipsilateral frontal cortex, where chronic l-DOPA treatment induced an increase of approximately 10 times the nNOS mRNA. Our results provided further evidence of an anti-dyskinetic effect of NOS inhibitor. The effect appeared under l-DOPA acute and chronic treatment. The l-DOPA treatment also revealed an over-expression of the neuronal NOS in the frontal cortex and striatum. Our results corroborated findings that l-DOPA-induced rotation differs between acute and chronic treatment. The effect of the NOS inhibitor conceivably relied on the l-DOPA structural modifications in the Parkinsonian brain. Taken together, these data provided a rationale for further evaluation of NOS inhibitors in the treatment of l-DOPA-induced dyskinesia.

Glia: initiators and progressors of pathology in Parkinson's disease

BACKGROUND

Glia are traditionally known as support cells for neurons, and their role in neurodegeneration has been largely considered secondary to neuronal dysfunction. We review newer concepts on glial function and assess glial changes in Parkinson's disease (PD) at the time of disease initiation when α-synuclein is accumulating in brain tissue but there is limited neuronal loss, and also as the disease progresses and neuronal loss is evident.

RESULTS

Of the two main types of astrocytes, only protoplasmic astrocytes are involved in PD, where they become nonreactive and accumulate α-synuclein. Experimental evidence has shown that astrocytic α-synuclein deposition initiates the noncell autonomous killing of neurons through microglial signaling. As the disease progresses, more protoplasmic astrocytes are affected by the disease with an increasing microglial response. Although there is still controversy on the role microglia play in neurodegeneration, there is evidence that microglia are activated early in PD and possibly assist with the clearance of extracellular α-synuclein at this time. Microglia transform to phagocytes and target neurons as the disease progresses but appear to become dysfunctional with increasing amounts of ingested debris. Only nonmyelinating oligodendroglial cells are affected in PD, and only late in the disease process.

CONCLUSIONS

Glial cells are responsible for the progression of PD and play an important role in initiating the early tissue response. In particular, early dysfunction and α-synuclein accumulation in astrocytes causes recruitment of phagocytic microglia that attack selected neurons in restricted brain regions causing the clinical symptoms of PD.

Peripheral inflammation increases the damage in animal models of nigrostriatal dopaminergic neurodegeneration: possible implication in Parkinson's disease incidence

Inflammatory processes described in Parkinson's disease (PD) and its animal models appear to be important in the progression of the pathogenesis, or even a triggering factor. Here we review that peripheral inflammation enhances the degeneration of the nigrostriatal dopaminergic system induced by different insults; different peripheral inflammations have been used, such as IL-1β and the ulcerative colitis model, as well as insults to the dopaminergic system such as 6-hydroxydopamine or lipopolysaccharide. In all cases, an increased loss of dopaminergic neurons was described; inflammation in the substantia nigra increased, displaying a great activation of microglia along with an increase in the production of cytokines such as IL-1β and TNF-α. Increased permeability or disruption of the BBB, with overexpression of the ICAM-1 adhesion molecule and infiltration of circulating monocytes into the substantia nigra, is also involved, since the depletion of circulating monocytes prevents the effects of peripheral inflammation. Data are reviewed in relation to epidemiological studies of PD.

Interferon-γ plays a role in paraquat-induced neurodegeneration involving oxidative and proinflammatory pathways

Exposure to environmental contaminants, particularly pesticides, may be an important etiological factor in Parkinson's disease (PD); and evidence suggests a role for microglia-dependent inflammatory and oxidative processes in nigrostriatal pathology induced by such toxins. Yet, the events mediating microglial activation and their effects are not fully known. To this end, we hypothesized that the proinflammatory cytokine, interferon-gamma (IFN-γ), may be a prime factor in the pathogenesis of PD, given its critical role in regulating microglial responses to pathogens. Indeed, the present investigation demonstrated that genetic deletion of IFN-γ protected substantia nigra pars compacta (SNc) dopamine (DA) neurons from the toxic effects of the pesticide, paraquat, and normalized changes in inflammatory and oxidative factors within this brain region. Specifically, IFN-γ knockout prevented the paraquat-induced morphological signs of microglial activation and expression of key nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunits, while also preventing time-dependent changes in proinflammatory enzymes (inducible nitric oxide synthase [iNOS], cyclooxygenase-2 [COX-2]), cytokines (interleukin-1β [IL-1β], tumor necrosis factor-α [TNF-α]), and signaling factors (c-Jun N-terminal kinase [JNK], p38 MAP kinase [p38], Signal transducer and activator of transcription-1 [STAT1], nuclear factor kappa B [NF-κB]). Moreover, paraquat transiently suppressed substantia nigra pars compacta expression of trophic and proneuroplastic factors (cyclic-AMP response element binding protein [CREB], brain-derived neurotrophic factor [BDNF]), and IFN-γ deficiency again reversed these effects. These data suggest that IFN-γ is important for paraquat-induced neurodegeneration and the accompanying oxidative, inflammatory, and trophic changes that characterize the response to the toxin. Targeting IFN-γ could thus have therapeutic implications for PD and other neurodegenerative conditions that involve multiple inflammatory pathways.

Transcriptional Factor NF-κB as a Target for Therapy in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative condition characterized by chronic inflammation. Nuclear factor κB (NF-κB) is a family of inducible transcription factors that are expressed in a wide variety of cells and tissues, including microglia, astrocytes, and neurons, and the classical NF-κB pathway plays a key role in the activation and regulation of inflammatory mediator production during inflammation. Activation of the classical NF-κB pathway is mediated through the activity of the IKK kinase complex, which consists of a heterotrimer of IKKα, IKKβ, and IKKγ subunits. Targeting NF-κB has been proposed as an approach to the treatment of acute and chronic inflammatory conditions, and the use of inhibitors specific for either IKKβ or IKKγ has now been found to inhibit neurodegeneration of TH+ DA-producing neurons in murine and primate models of Parkinson's disease. These studies suggest that targeting the classical pathway of NF-κB through the inhibition of the IKK complex can serve as a useful therapeutic approach to the treatment of PD.

Neuroprotection by the selective iNOS inhibitor GW274150 in a model of Parkinson disease

Neuroinflammation and the activation of inducible nitric oxide synthase (iNOS) have been proposed to play a role in the pathogenesis of Parkinson disease (PD). In this study we investigated the effects of the selective iNOS inhibitor GW274150 in the 6-OHDA model of PD. 6-OHDA administration was associated with increased numbers of cells expressing iNOS. Administration of the iNOS inhibitor twice daily for 7 days, beginning 2 days after the 6-OHDA lesioning, led to a significant neuroprotection as shown by assessment of the integrity of the nigrostriatal system by tyrosine hydroxylase immunocytochemistry and HPLC assessment of striatal dopamine content. However, GW274150 displayed a bell-shaped neuroprotective profile, being ineffective at high doses. 6-OHDA lesioning was associated with an increase in microglial activation as assessed by the MHC II antigen OX-6 and the number of matrix metalloproteinase 9 (MMP-9)-immunopositive cells. NO is a known modulator of MMP-9, and iNOS inhibition was associated with decreased numbers of MMP-9-immunopositive cells, culminating in a reduction in the numbers of reactive microglia. Withdrawal of GW274150 for a further 7 days negated any neuroprotective effects of iNOS inhibition, suggesting that the damaging effects of inflammation last beyond 7 days in this model and the continued administration of the drug may be required.

Prospects of statins in Parkinson disease

Parkinson disease (PD) is second only to Alzheimer disease as the most common neurodegenerative disorder in humans. Despite intense investigations, no effective therapy is available to halt the progression of PD. Although statins are widely used cholesterol-lowering drugs throughout the world, recent studies suggest that these drugs modulate neurodegeneration-related signaling processes and may be beneficial for PD. Simvastatin is the most potent statin in crossing the blood-brain barrier, and this particular statin drug negatively correlates with the incidence of PD and shows efficacy in animal models of PD. However, PD mainly occurs in the aging population, who are more vulnerable to cholesterol or lipid-related disorders, raising questions whether this possible beneficial effect of statins in PD patients is cholesterol dependent or cholesterol independent. This article presents data on the therapeutic efficacy of simvastatin in a chronic MPTP model of PD, reviews recent literature, and discusses the pros and cons of statin therapy in PD.

Nitric oxide signaling in brain function, dysfunction, and dementia

Nitric oxide (NO) is an important signaling molecule that is widely used in the nervous system. With recognition of its roles in synaptic plasticity (long-term potentiation, LTP; long-term depression, LTD) and elucidation of calcium-dependent, NMDAR-mediated activation of neuronal nitric oxide synthase (nNOS), numerous molecular and pharmacological tools have been used to explore the physiology and pathological consequences for nitrergic signaling. In this review, the authors summarize the current understanding of this subtle signaling pathway, discuss the evidence for nitrergic modulation of ion channels and homeostatic modulation of intrinsic excitability, and speculate about the pathological consequences of spillover between different nitrergic compartments in contributing to aberrant signaling in neurodegenerative disorders. Accumulating evidence points to various ion channels and particularly voltage-gated potassium channels as signaling targets, whereby NO mediates activity-dependent control of intrinsic neuronal excitability; such changes could underlie broader mechanisms of synaptic plasticity across neuronal networks. In addition, the inability to constrain NO diffusion suggests that spillover from endothelium (eNOS) and/or immune compartments (iNOS) into the nervous system provides potential pathological sources of NO and where control failure in these other systems could have broader neurological implications. Abnormal NO signaling could therefore contribute to a variety of neurodegenerative pathologies such as stroke/excitotoxicity, Alzheimer's disease, multiple sclerosis, and Parkinson's disease.

Peripheral vs. Central Sex Steroid Hormones in Experimental Parkinson's Disease

The nigrostriatal dopaminergic (NSDA) pathway degenerates in Parkinson's disease (PD), which occurs with approximately twice the incidence in men than women. Studies of the influence of systemic estrogens in females suggest sex hormones contribute to these differences. In this review we analyze the evidence revealing great complexity in the response of the healthy and injured NSDA system to hormonal influences, and emphasize the importance of centrally generated estrogens. At physiological levels, circulating estrogen (in females) or estrogen precursors (testosterone in males, aromatized to estrogen centrally) have negligible effects on dopaminergic neuron survival in experimental PD, but can modify striatal dopamine levels via actions on the activity or adaptive responses of surviving cells. However, these effects are sexually dimorphic. In females, estradiol promotes adaptive responses in the partially injured NSDA pathway, preserving striatal dopamine, whereas in males gonadal steroids and exogenous estradiol have a negligible or even suppressive effect, effectively exacerbating dopamine loss. On balance, the different effects of gonadal factors in males and females contribute to sex differences in experimental PD. Fundamental sex differences in brain organization, including the sexually dimorphic networks regulating NSDA activity are likely to underpin these responses. In contrast, estrogen generated locally appears to preserve striatal dopamine in both sexes. The available data therefore highlight the need to understand the biological basis of sex-specific responses of the NSDA system to peripheral hormones, so as to realize the potential for sex-specific, hormone-based therapies in PD. Furthermore, they suggest that targeting central steroid generation could be equally effective in preserving striatal dopamine in both sexes. Clarification of the relative roles of peripheral and central sex steroid hormones is thus an important challenge for future studies.

Inflammatory mechanisms of neurodegeneration in toxin-based models of Parkinson's disease

Parkinson's disease (PD) has been associated with exposure to a variety of environmental agents, including pesticides, heavy metals, and organic pollutants; and inflammatory processes appear to constitute a common mechanistic link among these insults. Indeed, toxin exposure has been repeatedly demonstrated to induce the release of oxidative and inflammatory factors from immunocompetent microglia, leading to damage and death of midbrain dopamine (DA) neurons. In particular, proinflammatory cytokines such as tumor necrosis factor-α and interferon-γ, which are produced locally within the brain by microglia, have been implicated in the loss of DA neurons in toxin-based models of PD; and mounting evidence suggests a contributory role of the inflammatory enzyme, cyclooxygenase-2. Likewise, immune-activating bacterial and viral agents were reported to have neurodegenerative effects themselves and to augment the deleterious impact of chemical toxins upon DA neurons. The present paper will focus upon the evidence linking microglia and their inflammatory processes to the death of DA neurons following toxin exposure. Particular attention will be devoted to the possibility that environmental toxins can activate microglia, resulting in these cells adopting a “sensitized” state that favors the production of proinflammatory cytokines and damaging oxidative radicals.

Toxin-induced and genetic animal models of Parkinson's disease

Parkinson's disease (PD) is a common progressive neurodegenerative disorder. The major pathological hallmarks of PD are the selective loss of nigrostriatal dopaminergic neurons and the presence of intraneuronal aggregates termed Lewy bodies (LBs), but the pathophysiological mechanisms are not fully understood. Epidemiologically, environmental neurotoxins such as pesticides are promising candidates for causative factors of PD. Oxidative stress and mitochondrial dysfunction induced by these toxins could contribute to the progression of PD. While most cases of PD are sporadic, specific mutations in genes that cause familial forms of PD have led to provide new insights into its pathogenesis. This paper focuses on animal models of both toxin-induced and genetically determined PD that have provided significant insight for understanding this disease. We also discuss the validity, benefits, and limitations of representative models.

[Biological roles of trace elements in the brain with special focus on Zn and Fe]

INTRODUCTION

Many metals like iron (Fe), copper (Cu) or zinc (Zn) fulfil various essential biological functions and are thus vital for all living organisms. For instance, they play important roles in nervous tissue, participating in a wide range of processes such as neurotransmitter synthesis, myelination or synaptic transmission.

STATE OF THE ART

As in other tissues, brain cells tightly control the concentration of metals but any excess or deficit can lead to deleterious responses and alter cognitive functions. Of note, certain metals such as Zn, Fe or Cu accumulate in specific brain structures over lifespan and several neurodegenerative diseases are associated with a dysregulation of the homeostatic mechanisms controlling the concentration of these cations.

CONCLUSION AND PERSPECTIVES

This review will address some of the cellular and molecular processes controlling the entry and distribution of selected metals (mainly Zn and Fe) in the brain, as well as their roles in synaptic transmission, in the pathogenesis of some neurologic diseases such as Parkinson's disease and Alzheimer's disease, and their impact on cognitive functions.

Vulnerability of mesostriatal dopaminergic neurons in Parkinson's disease

The term vulnerability was first associated with the midbrain dopaminergic neurons 85 years ago, before they were identified as monoaminergic neurons, when Foix and Nicolesco (1925) reported the loss of neuromelanin containing neurons in the midbrain of patients with post-encephalitic Parkinson's disease (PD). A few years later, Hassler (1938) showed that degeneration is more intense in the ventral tier of the substantia nigra compacta than in its dorsal tier and the ventral tegmental area (VTA), outlining the concept of differential vulnerability of midbrain dopaminergic (DA-) neurons. Nowadays, we know that other neuronal groups degenerate in PD, but the massive loss of nigral DA-cells is its pathological hallmark, having a pivotal position in the pathophysiology of the disease as it is responsible for the motor symptoms. Data from humans as well as cellular and animal models indicate that DA-cell degeneration is a complex process, probably precipitated by the convergence of different risk factors, mediated by oxidative stress, and involving pathogenic factors arising within the DA-neuron (intrinsic factors), and from its environment and distant interconnected brain regions (extrinsic factors). In light of current data, intrinsic factors seem to be preferentially involved in the first steps of the degenerative process, and extrinsic factors in its progression. A controversial issue is the relative weight of the impairment of common cell functions, such as energy metabolism and proteostasis, and specific dopaminergic functions, such as pacemaking activity and DA handling, in the pathogenesis of DA-cell degeneration. Here we will review the current knowledge about the relevance of these factors at the beginning and during the progression of PD, and in the differential vulnerability of midbrain DA-cells.

Fluoxetine prevents LPS-induced degeneration of nigral dopaminergic neurons by inhibiting microglia-mediated oxidative stress

Lipopolysaccharide (LPS)-induced microglial activation causes degeneration of nigral dopaminergic (DA) neurons. Here, we examined whether fluoxetine prevents LPS-induced degeneration of DA in the rat substantia nigra (SN) in vivo. Seven days after LPS injection into the SN, immunostaining for tyrosine hydroxylase (TH) revealed a significant loss of nigral DA neurons. Parallel activation of microglia (visualized by OX-42 and ED1 immunohistochemistry), production of reactive oxygen species (ROS) (assessed by hydroethidine histochemistry), and degeneration of nigral DA neurons were also observed in the SN. Western blot analyses and double-label immunohistochemistry showed an increase in the expression of inducible nitric oxide synthase (iNOS) within activated microglia. LPS also induced translocation of p67(phox), the cytosolic component of NADPH oxidase, to the membrane of SN microglia, indicating activation of NADPH oxidase. The LPS-induced loss of nigral DA neurons was partially inhibited by fluoxetine, and the observed neuroprotective effects were associated with fluoxetine-mediated suppression of microglial NADPH oxidase activation and iNOS upregulation, and decreased ROS generation and oxidative stress. These results suggest that fluoxetine and analogs thereof may be beneficial for the treatment of neurodegenerative diseases, such as PD, that are associated with microglia-derived oxidative damage.

Differing short-term neuroprotective effects of the fibrates fenofibrate and bezafibrate in MPTP and 6-OHDA experimental models of Parkinson's disease

Earlier study from our group indicated that the peroxisome proliferator-activated receptor-alpha agonist fenofibrate prevents 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic cell loss in C57Bl/6 mice. Our objective was to determine whether or not fibratescan improve motor activity in two experimental models of Parkinson's disease: the MPTP C57Bl/6 mouse and the 6-hydroxydopamine (6-OHDA) Wistar rat. Six groups of mice were set up: sham, sham-fenofibrate, sham-bezafibrate, MPTP, MPTP-fenofibrate and MPTP-bezafibrate. Mice were fed a diet containing 0.2% fenofibrate, 0.2% bezafibrate or no hypolipidaemic agent for 2 weeks. Four groups of rats were set up: sham, sham-fenofibrate, 6-OHDA and 6-OHDA-fenofibrate. Rats were fed a diet containing 0.2% fenofibrate or no hypolipidaemic agent for 4 weeks. In mice, motor activity was quantified using actimetry. Nine parameters were recorded. The results were analyzed with a mixed linear model. In rats, behavioural sensitization was studied with repetitive injections of apomorphine. All the actimetry parameters indicated a decrease of locomotion the day after MPTP injections and eight parameters improved in MPTP mice treated with fenofibrate or bezafibrate. The apomorphine-induced rotation behaviour mildly decreased in 6-OHDA rats treated with fenofibrate, but behavioural sensitization was unchanged. The 6-OHDA and MPTP compounds have different toxicity mechanisms, which could explain why we did not observe the same effect in 6-OHDA rats as in MPTP mice. These data suggest that the protective effect of fibrates can be carried through inhibition of inflammation rather than oxidative stress.

The role of neuroinflammation on the pathogenesis of Parkinson's disease

Parkinson's Disease (PD) is a common neurodegenerative disease characterized by the progressive degeneration of nigrostriatal dopaminergic (DA) neurons. Although the causative factors of PD remain elusive, many studies on PD animal models or humans suggest that glial activation along with neuroinflammatory processes contribute to the initiation or progression of PD. Additionally, several groups have proposed that dysfunction of the blood-brain barrier (BBB) combined with infiltration of peripheral immune cells play important roles in the degeneration of DA neurons. However, these neuroinflammatory events have only been investigated separately, and the issue of whether these phenomena are neuroprotective or neurotoxic remains controversial. We here review the current knowledge regarding the functions of these neuroinflammatory processes in the brain. Finally, we describe therapeutic strategies for the regulation of neuroinflammation with the goal of improving the symptoms of PD.

Mitochondrial dysfunction: a potential link between neuroinflammation and neurodegeneration?

Dysfunctional mitochondria are thought to play a cardinal role in the pathogenesis of various neurological disorders, such as multiple sclerosis, Alzheimer's disease, Parkinson's disease and stroke. In addition, neuroinflammation is a common denominator of these diseases. Both mitochondrial dysfunction and neuroinflammatory processes lead to increased production of reactive oxygen species (ROS) which are detrimental to neurons. Therefore, neuroinflammation is increasingly recognized to contribute to processes underlying neurodegeneration. Here we describe the involvement of mitochondrial (dys)function in various neurological disorders and discuss the putative link between mitochondrial function and neuroinflammation.

The protective role of D-glucose against 1-methyl-4-phenylpyridinium ion (MPP+): induced mitochondrial dysfunction in C6 astroglial cells

Impaired mitochondrial function in glial and neuronal cells in the substantia nigra is one of the most likely causes of Parkinson's disease. In this study, we investigated the protective role of glucose on early key events associated with MPP(+)-induced changes in rat C6 astroglial cells. Studies were carried out to examine alterations in mitochondrial respiratory status, membrane potential, glutathione levels, and cell cycle phase inhibition at 48 h in 2 and 10 mM glucose in media. The results obtained suggest that MPP(+) caused significant cell death in 2 mM glucose with LC(50) 0.14 +/- 0.005 mM, while 10 mM glucose showed highly significant protection against MPP(+) toxicity with LC(50) 0.835 +/- 0.03 mM. This protection was not observed with cocaine, demonstrating its compound specificity. MPP(+) in 2 mM glucose decreased significantly mitochondrial respiration, membrane potential and glutathione levels in a dose dependent manner, while 10 mM glucose significantly restored them. MPP(+) in 2 mM glucose arrested the cells at G0/G1 and G2/M phases, demonstrating its dual inhibitory effects. However, in 10 mM glucose, MPP(+) caused G0/G1 arrest only. In summary, the results suggest that loss of cell viability in 2 mM glucose group with MPP(+) treatments was due to mitochondrial dysfunction caused by multilevel mechanism, involving significant decrease in mitochondrial respiration, membrane potential, glutathione levels, and dual arrest of cell phases, while 10 mM glucose rescued astroglial cells from MPP(+) toxicity by significant maintenance of these factors.

Neuroprotective herbs and foods from different traditional medicines and diets

Plant secondary metabolites include an array of bioactive constituents form both medicinal and food plants able to improve human health. The exposure to these phytochemicals, including phenylpropanoids, isoprenoids and alkaloids, through correct dietary habits, may promote health benefits, protecting against the chronic degenerative disorders mainly seen in Western industrialized countries, such as cancer, cardiovascular and neurodegenerative diseases. In this review, we briefly deal with some plant foods and herbs of traditional medicines and diets, focusing on their neuroprotective active components. Because oxidative stress and neuroinflammation resulting from neuroglial activation, at the level of neurons, microglial cells and astrocytes, are key factors in the etiopathogenesis of both neurodegenerative and neurological diseases, emphasis will be placed on the antioxidant and anti-inflammatory activity exerted by specific molecules present in food plants or in remedies prescribed by herbal medicines.

Glial cell line-derived neurotrophic factor protects midbrain dopaminergic neurons against lipopolysaccharide neurotoxicity

Aberrant microglia activation causes dopaminergic neuronal loss and nitric oxide produced by microglia plays a critical role in dopaminergic neuronal degeneration. However, no study has determined if GDNF protects dopaminergic neurons via inhibiting nitric oxide generation in Parkinson's disease animal model. We report that GDNF not only reduces lipopolysaccharide-induced degeneration of dopaminergic neurons, suppresses microglia activation and nitric oxide generation, but also reverses the inhibition of phosphoinositide 3-kinase (PI3K) in dopaminergic neurons and microglia. It suggests that the neuroprotective effect of GDNF on dopaminergic neurons may be related to its suppression of microglia activation-mediated nitric oxide via releasing the inhibition of PI3K in both neurons and microglia.

Mechanisms underlying inflammation in neurodegeneration

Inflammation is associated with many neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis. In this Review, we discuss inducers, sensors, transducers, and effectors of neuroinflammation that contribute to neuronal dysfunction and death. Although inducers of inflammation may be generated in a disease-specific manner, there is evidence for a remarkable convergence in the mechanisms responsible for the sensing, transduction, and amplification of inflammatory processes that result in the production of neurotoxic mediators. A major unanswered question is whether pharmacological inhibition of inflammation pathways will be able to safely reverse or slow the course of disease.

Altered expression of neuronal nitric oxide synthase in weaver mutant mice

The weaver mouse represents the only genetic animal model of gradual nigrostriatal dopaminergic neurodegeneration which is proposed as a pathophysiological phenotype of Parkinson's disease. The aim of the present study was to analyze the nitric oxide and dopaminergic systems in selected brain regions of homozygous weaver mice at different postnatal ages corresponding to specific stages of the dopamine loss. Structural deficits were evaluated by quantification of tyrosine hydroxylase and neuronal nitric oxide synthase-immunostaining in the cortex, striatum, accumbens nuclei, subthalamic nuclei, ventral tegmental area, and substantia nigra compacta of 10-day, 1- and 2-month-old wild-type and weaver mutant mice. The results confirmed the progressive loss of dopamine during the postnatal development in the adult weaver mainly affecting the substantia nigra pars compacta, striatum, and subthalamic nucleus and slightly affecting the accumbens nuclei and ventral tegmental area. A general decrease in neuronal nitric oxide synthase-immunostaining with age was revealed in both the weaver and wild-type mice, with the decrease being most pronounced in the weaver. In contrast, there was an increase in the substantia nigra pars compacta nitric oxide synthase-immunostaining and a decrease mainly in the subthalamic and accumbens nuclei of the 2-month-old weaver mutant. The decrease in the expression of nNOS may bear functional significance related to the process of aging. DA neurons from the substantia nigra directly modulate the activity of subthalamic nucleus neurons, and their loss may contribute to the abnormal activity of subthalamic nucleus neurons. Although the functional significance of these changes is not clear, it may represent plastic compensating adjustments resulting from the loss of dopamine innervation, highlighting a possible role of nitric oxide in this process.

Inflexin attenuates proinflammatory responses and nuclear factor-kappaB activation in LPS-treated microglia

Activated microglia participate in neuroinflammation which contribute to neuronal damage. Suppression of microglial activation would have therapeutic benefits, which lead to alleviation of the progression of neurodegeneration. In this study, the inhibitory effects of inflexin, a putative antiinflammatory agent isolated from Isodon excisus (Max.) Kudo (Labiateae), on the production of proinflammatory mediators were investigated in the lipopolysaccharide (LPS)-stimulated microglia. Inflexin significantly inhibited the release of nitric oxide (NO). Consistently, both the mRNA and the protein levels for the inducible NO synthase were decreased by inflexin in a concentration-dependent manner. Inflexin also inhibited the expression of cyclooxygenase (COX)-2, but not the COX-1 and effectively reduced the LPS-induced expression of proinflammatory cytokines in a dose-dependent manner. Furthermore, inflexin inhibited the degradation of IkappaB-alpha and the activation of NF-kappaB, p65 and Akt, while the MAPKs signal pathway was not affected. Our data suggest that inflexin was able to suppress neuroinflammation via inhibition of NF-kappaB activation and Akt pathway indicating that inflexin may be developed as a potent therapeutic agent in treating neuroinflammatory diseases.

Simvastatin inhibits the activation of p21ras and prevents the loss of dopaminergic neurons in a mouse model of Parkinson's disease

Parkinson's disease (PD) is second only to Alzheimer's disease as the most common devastating human neurodegenerative disorder. Despite intense investigation, no interdictive therapy is available for PD. We investigated whether simvastatin, a Food and Drug Administration-approved cholesterol-lowering drug, could protect against nigrostriatal degeneration after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication to model PD in mice. First, MPP(+) induced the activation of p21(ras) and nuclear factor-kappaB (NF-kappaB) in mouse microglial cells. Inhibition of MPP(+)-induced activation of NF-kappaB by Deltap21(ras), a dominant-negative mutant of p21(ras), supported the involvement of p21(ras) in MPP(+)-induced microglial activation of NF-kappaB. Interestingly, simvastatin attenuated activation of both p21(ras) and NF-kappaB in MPP(+)-stimulated microglial cells. Consistently, we found a very rapid activation of p21(ras) in vivo in the substantia nigra pars compacta of MPTP-intoxicated mice. However, after oral administration, simvastatin entered into the nigra, reduced nigral activation of p21(ras), attenuated nigral activation of NF-kappaB, inhibited nigral expression of proinflammatory molecules, and suppressed nigral activation of glial cells. These findings paralleled dopaminergic neuronal protection, normalized striatal neurotransmitters, and improved motor functions in MPTP-intoxicated mice. Similarly, pravastatin, another cholesterol-lowering drug, suppressed microglial inflammatory responses and protected dopaminergic neurons in MPTP-intoxicated mice, but at levels less than simvastatin. Furthermore, both the statins administered 2 d after initiation of the disease were still capable of inhibiting the demise of dopaminergic neurons and concomitant loss of neurotransmitters, suggesting that statins are capable of slowing down the progression of neuronal loss in the MPTP mouse model. Therefore, we conclude that statins may be of therapeutic benefit for PD patients.

Neuroprotective effects of compounds with antioxidant and anti-inflammatory properties in a Drosophila model of Parkinson's disease

Background

Parkinson's disease (PD) is the most common movement disorder. Extrapyramidal motor symptoms stem from the degeneration of the dopaminergic pathways in patient brain. Current treatments for PD are symptomatic, alleviating disease symptoms without reversing or retarding disease progression. Although the cause of PD remains unknown, several pathogenic factors have been identified, which cause dopaminergic neuron (DN) death in the substantia nigra (SN). These include oxidative stress, mitochondrial dysfunction, inflammation and excitotoxicity. Manipulation of these factors may allow the development of disease-modifying treatment strategies to slow neuronal death. Inhibition of DJ-1A, the Drosophila homologue of the familial PD gene DJ-1, leads to oxidative stress, mitochondrial dysfunction, and DN loss, making fly DJ-1A model an excellent in vivo system to test for compounds with therapeutic potential.

Results

In the present study, a Drosophila DJ-1A model of PD was used to test potential neuroprotective drugs. The drugs applied are the Chinese herb celastrol, the antibiotic minocycline, the bioenergetic amine coenzyme Q10 (coQ10), and the glutamate antagonist 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo[f]-quinoxaline (NBQX). All of these drugs target pathogenic processes implicated in PD, thus constitute mechanism-based treatment strategies. We show that celastrol and minocycline, both having antioxidant and anti-inflammatory properties, confer potent dopaminergic neuroprotection in Drosophila DJ-1A model, while coQ10 shows no protective effect. NBQX exerts differential effects on cell survival and brain dopamine content: it protects against DN loss but fails to restore brain dopamine level.

Conclusion

The present study further validates Drosophila as a valuable model for preclinical testing of drugs with therapeutic potential for neurodegenerative diseases. The lower cost and amenability to high throughput testing make Drosophila PD models effective in vivo tools for screening novel therapeutic compounds. If our findings can be further validated in mammalian PD models, they would implicate drugs combining antioxidant and anti-inflammatory properties as strong therapeutic candidates for mechanism-based PD treatment.

The role of NADPH oxidase 1-derived reactive oxygen species in paraquat-mediated dopaminergic cell death

Oxidative stress is the common downstream effect of a variety of environmental neurotoxins that are strongly implicated in the pathogenesis of Parkinson's disease. We demonstrate here that the activation of NADPH oxidase 1 (Nox1), a specialized superoxide-generating enzyme complex, plays a key role in the oxidative stress and subsequent dopaminergic cell death elicited by paraquat. Paraquat increased the expression of Nox1 in a concentration-dependent manner in rat dopaminergic N27 cells. Rac1, a key component necessary for Nox1-mediated superoxide generation, also was activated by paraquat. Paraquat-induced reactive oxygen species generation and dopaminergic cell death were significantly reduced after pretreatment with apocynin, a putative NADPH oxidase inhibitor, and Nox1 knockdown with siRNA. Male C57BL/6 mice received intraperitoneal (IP) injections of paraquat (10 mg/kg) once every 3 days and showed increased Nox1 levels in the substantia nigra as well as a 35% reduction in tyrosine hydroxylase-positive dopaminergic neurons 5 days after the last injection. Preadministration of apocynin (200 mg/kg, IP) led to a significant decrease in dopaminergic neuronal loss. Our results suggest that Nox1-generated superoxide is implicated in the oxidative stress elicited by paraquat in DA cells, and it can serve as a novel target for pharmacologic intervention.

The effect of a promoter polymorphism on the transcription of nitric oxide synthase 1 and its relevance to Parkinson's disease

Transcriptional changes of the enzyme nitric oxide synthase I (NOS1) are believed to play a role in the development of many diseases. The gene for NOS1 has 12 alternative first exons (1A-1L). The 1F exon is one of the most highly utilized first exons in the brain and has a polymorphism ((TG)(m)TA(TG)(n)) located in its promoter region. The polymorphism's length has been suggested to affect NOS1 transcription and play a role in Parkinson's disease (PD); however, the actual influence of the polymorphism on NOS1 transcription has not been studied. To better characterize the links of the polymorphism with PD, a genotyping study was done comparing polymorphism length among 170 PD patients and 150 age-matched controls. The pattern of changes between the two group's allele frequencies shows statistical significance (P = 0.0359). The smallest polymorphism sizes are more predominant among PD patients than controls. To study the effects of this polymorphism on NOS1 gene transcription, reporter gene constructs were made by cloning the NOS1 1F promoter with polymorphism lengths of either 42, 54, or 62 bp in front of the luciferase gene and transfecting them into HeLa or Sk-N-MC cells. NOS1-directed reporter gene constructs with the 62-bp polymorphism increased transcription of luciferase 2.2-fold in HeLa and 1.8-fold in Sk-N-MC cells compared with reporter gene constructs with the 42-bp polymorphism. These data suggest that if smaller polymorphism size contributes to the higher NOS1 levels in PD patients, an as yet unknown transcriptional mechanism is required.

Intrastriatal lipopolysaccharide injection induces parkinsonism in C57/B6 mice

A role for inflammation has been hypothesized in the etiology and progression of Parkinson's disease (PD). In this study, we generated, characterized, and validated the first progressive PD-related mouse model (C57/B6) with intrastriatal injection of lipopolysaccharide (LPS). We showed progressive and specific dopaminergic neurodegeneration in the substantia nigra, which is accompanied by striatal dopamine depletion and progressive behavioral impairment, which was alleviated by the use of the PD drug L-Dopa. We focused on the role of nitric oxide (NO) in inflammation-promoted cell death and suggest that the expression of the inducible NO synthase plays a role in the progressive loss of dopaminergic neurons but not the initial loss induced by LPS. With this model, future research can be performed in gene knockout mice to study other potential mechanisms of inflammation-induced neurodegeneration. In addition, this model can be used to screen therapeutics for PD at a more clinically relevant time (i.e., after LPS injection but before manifestation of PD-related behavioral impairment), because most PD drugs are screened in animal models in which inhibitors are given predisease induction. Thus, this novel PD-related model should be further characterized and strongly considered as a tool for future drug studies.

Striatal neuroinflammation promotes Parkinsonism in rats

Background

Sporadic Parkinson's disease (PD) is a progressive neurodegenerative disorder with unknown cause, but it has been suggested that neuroinflammation may play a role in pathogenesis of the disease. Neuroinflammatory component in process of PD neurodegeneration was proposed by postmortem, epidemiological and animal model studies. However, it remains unclear how neuroinflammatory factors contribute to dopaminergic neuronal death in PD.

Findings

In this study, we analyzed the relationship among inducible nitric oxide synthase (iNOS)-derived NO, mitochondrial dysfunction and dopaminergic neurodegeneration to examine the possibility that microglial neuroinflammation may induce dopaminergic neuronal loss in the substantia nigra. Unilateral injection of lipopolysaccharide (LPS) into the striatum of rat was followed by immunocytochemical, histological, neurochemical and biochemical analyses. In addition, behavioral assessments including cylinder test and amphetamine-induced rotational behavior test were employed to validate ipsilateral damage to the dopamine nigrostriatal pathway. LPS injection caused progressive degeneration of the dopamine nigrostriatal system, which was accompanied by motor impairments including asymmetric usage of forelimbs and amphetamine-induced turning behavior in animals. Interestingly, some of the remaining nigral dopaminergic neurons had intracytoplasmic accumulation of α-synuclein and ubiquitin. Furthermore, defect in the mitochondrial respiratory chain, and extensive S-nitrosylation/nitration of mitochondrial complex I were detected prior to the dopaminergic neuronal loss. The mitochondrial injury was prevented by treatment with L-N⁶-(l-iminoethyl)-lysine, an iNOS inhibitor, suggesting that iNOS-derived NO is associated with the mitochondrial impairment.

Conclusions

These results implicate neuroinflammation-induced S-nitrosylation/nitration of mitochondrial complex I in mitochondrial malfunction and subsequent degeneration of the nigral dopamine neurons.

Neuroinflammation in Parkinson's disease: a target for neuroprotection?

Parkinson's disease is characterised by a slow and progressive degeneration of dopaminergic neurons in the substantia nigra. Despite intensive research, the cause of the neuronal loss in Parkinson's disease is poorly understood. Neuroinflammatory mechanisms might contribute to the cascade of events leading to neuronal degeneration. In this Review, we describe the evidence for neuroinflammatory processes from post-mortem and in vivo studies in Parkinson's disease. We further identify the cellular and molecular events associated with neuroinflammation that are involved in the degeneration of dopaminergic neurons in animal models of the disease. Overall, available data support the importance of non-cell-autonomous pathological mechanisms in Parkinson's disease, which are mostly mediated by activated glial and peripheral immune cells. This cellular response to neurodegeneration triggers deleterious events (eg, oxidative stress and cytokine-receptor-mediated apoptosis), which might eventually lead to dopaminergic cell death and hence disease progression. Finally, we highlight possible therapeutic strategies (including immunomodulatory drugs and therapeutic immunisation) aimed at downregulating these inflammatory processes that might be important to slow the progression of Parkinson's disease.

Microglial cells and Parkinson's disease

Chronic inflammation mediated by microglial cells is the fundamental process contributing to the death of dopamine (DA)-producing neurons in the brain. Production of inflammatory products by these microglial cells characterizes the slow destructive process in Parkinson's disease (PD). The activation of microglial cells and the generation of pro-inflammatory cytokines that characterize PD are mediated by several different signaling pathways, with the activation of the respiratory burst by microglial cells being a critical event in the ultimate toxicity of DA-neurons. The work on our lab is concerned with understanding the mechanisms of activation, response, and therapeutic targets of microglial cells, with the aim to provide more effective treatments for PD and other inflammatory diseases of the CNS.