Parkinson's disease-derived α-synuclein assemblies combined with chronic-type inflammatory cues promote a neurotoxic microglial phenotype.

Parkinson's disease (PD) is a common age-related neurodegenerative disorder characterized by the aggregation of α-Synuclein (αSYN) building up intraneuronal inclusions termed Lewy pathology. Mounting evidence suggests that neuron-released αSYN aggregates could be central to microglial activation, which in turn mounts and orchestrates neuroinflammatory processes potentially harmful to neurons. Therefore, understanding the mechanisms that drive microglial cell activation, polarization and function in PD might have important therapeutic implications. Here, using primary microglia, we investigated the inflammatory potential of pure αSYN fibrils derived from PD patients. We further explored and characterized microglial cell responses to a chronic-type inflammatory stimulation combining PD patient-derived αSYN fibrils (FPD), Tumor necrosis factor-α (TNFα) and prostaglandin E2 (PGE2) (TPFPD). We showed that FPD hold stronger inflammatory potency than pure αSYN fibrils generated de novo. When combined with TNFα and PGE2, FPD polarizes microglia toward a particular functional phenotype departing from FPD-treated cells and featuring lower inflammatory cytokine and higher glutamate release. Whereas metabolomic studies showed that TPFPD-exposed microglia were closely related to classically activated M1 proinflammatory cells, notably with similar tricarboxylic acid cycle disruption, transcriptomic analysis revealed that TPFPD-activated microglia assume a unique molecular signature highlighting upregulation of genes involved in glutathione and iron metabolisms. In particular, TPFPD-specific upregulation of Slc7a11 (which encodes the cystine-glutamate antiporter xCT) was consistent with the increased glutamate response and cytotoxic activity of these cells toward midbrain dopaminergic neurons in vitro. Together, these data further extend the structure-pathological relationship of αSYN fibrillar polymorphs to their innate immune properties and demonstrate that PD-derived αSYN fibrils, TNFα and PGE2 act in concert to drive microglial cell activation toward a specific and highly neurotoxic chronic-type inflammatory phenotype characterized by robust glutamate release and iron retention.

Hippocampal T cell infiltration promotes neuroinflammation and cognitive decline in a mouse model of tauopathy.

Alzheimer's disease is characterized by the combined presence of amyloid plaques and tau pathology, the latter being correlated with the progression of clinical symptoms. Neuroinflammatory changes are thought to be major contributors to Alzheimer's disease pathophysiology, even if their precise role still remains largely debated. Notably, to what extent immune responses contribute to cognitive impairments promoted by tau pathology remains poorly understood. To address this question, we took advantage of the THY-Tau22 mouse model that progressively develops hippocampal tau pathology paralleling cognitive deficits and reappraised the interrelationship between tau pathology and brain immune responses. In addition to conventional astroglial and microglial responses, we identified a CD8-positive T cell infiltration in the hippocampus of tau transgenic mice associated with an early chemokine response, notably involving CCL3. Interestingly, CD8-positive lymphocyte infiltration was also observed in the cortex of patients exhibiting frontemporal dementia with P301L tau mutation. To gain insights into the functional involvement of T cell infiltration in the pathophysiological development of tauopathy in THY-Tau22 mice, we chronically depleted T cells using anti-CD3 antibody. Such anti-CD3 treatment prevented hippocampal T cell infiltration in tau transgenic animals and reverted spatial memory deficits, in absence of tau pathology modulation. Altogether, these data support an instrumental role of hippocampal T cell infiltration in tau-driven pathophysiology and cognitive impairments in Alzheimer's disease and other tauopathies.

Analysis of monocyte infiltration in MPTP mice reveals that microglial CX3CR1 protects against neurotoxic over-induction of monocyte-attracting CCL2 by astrocytes.

BACKGROUND: Evidence from mice suggests that brain infiltrating immune cells contribute to neurodegeneration, and we previously identified a deleterious lymphocyte infiltration in Parkinson's disease mice. However, this remains controversial for monocytes, due to artifact-prone techniques used to distinguish them from microglia. Our aim was to reassess this open question, by taking advantage of the recent recognition that chemokine receptors CCR2 and CX3CR1 can differentiate between inflammatory monocytes and microglia, enabling to test whether CCR2+ monocytes infiltrate the brain during dopaminergic (DA) neurodegeneration and whether they contribute to neuronal death. This revealed unexpected insights into possible regulation of monocyte-attracting CCL2 induction. METHODS: We used acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mice and assessed monocyte infiltration by combining laser microdissection-guided chemokine RNA profiling of the substantia nigra (SN) with immunohistochemistry and CCR2-GFP reporter mice. To determine contribution to neuronal loss, we used CCR2-deletion and CCL2-overexpression, to reduce and increase CCR2+ monocyte infiltration, and CX3CR1-deletion to assess a potential implication in CCL2 regulation. RESULTS: Nigral chemokine profiling revealed early CCL2/7/12-CCR2 axis induction, suggesting monocyte infiltration in MPTP mice. CCL2 protein showed early peak induction in nigral astrocytes, while CCR2-GFP mice revealed early but limited nigral monocyte infiltration. However, blocking infiltration by CCR2 deletion did not influence DA neuronal loss. In contrast, transgenic astrocytic CCL2 over-induction increased CCR2+ monocyte infiltration and DA neuronal loss in MPTP mice. Surprisingly, CCL2 over-induction was also detected in MPTP intoxicated CX3CR1-deleted mice, which are known to present increased DA neuronal loss. Importantly, CX3CR1/CCL2 double-deletion suggested that increased neurotoxicity was driven by astrocytic CCL2 over-induction. CONCLUSIONS: We show that CCR2+ monocytes infiltrate the affected CNS, but at the level observed in acute MPTP mice, this does not contribute to DA neuronal loss. In contrast, the underlying astrocytic CCL2 induction seemed to be tightly controled, as already moderate CCL2 over-induction led to increased neurotoxicity in MPTP mice, likely due to the increased CCR2+ monocyte infiltration. Importantly, we found evidence suggesting that during DA neurodegeneration, this control was mediated by microglial CX3CR1 signaling, which protects against such neurotoxic CCL2 over-induction by astrocytes, thus hinting at an endogenous mechanism to limit neurotoxic effects of the CCL2-CCR2 axis.

Neutral Sphingomyelinase Behaviour in Hippocampus Neuroinflammation of MPTP-Induced Mouse Model of Parkinson's Disease and in Embryonic Hippocampal Cells.

Neutral sphingomyelinase is known to be implicated in growth arrest, differentiation, proliferation, and apoptosis. Although previous studies have reported the involvement of neutral sphingomyelinase in hippocampus physiopathology, its behavior in the hippocampus during Parkinson's disease remains undetected. In this study, we show an upregulation of inducible nitric oxide synthase and a downregulation of neutral sphingomyelinase in the hippocampus of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine- (MPTP-) induced mouse model of Parkinson's disease. Moreover, the stimulation of neutral sphingomyelinase activity with vitamin 1,25-dihydroxyvitamin D3 reduces specifically saturated fatty acid sphingomyelin by making sphingomyelin a less rigid molecule that might influence neurite plasticity. The possible biological relevance of the increase of neutral sphingomyelinase in Parkinson's disease is discussed.

e-Cadherin in 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-Induced Parkinson Disease.

Today a large number of studies are focused on clarifying the complexity and diversity of the pathogenetic mechanisms inducing Parkinson disease. We used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxin that induces Parkinson disease, to evaluate the change of midbrain structure and the behavior of the anti-inflammatory factor e-cadherin, interleukin-6, tyrosine hydroxylase, phosphatase and tensin homolog, and caveolin-1. The results showed a strong expression of e-cadherin, variation of length and thickness of the heavy neurofilaments, increase of interleukin-6, and reduction of tyrosine hydroxylase known to be expression of dopamine cell loss, reduction of phosphatase and tensin homolog described to impair responses to dopamine, and reduction of caveolin-1 known to be expression of epithelial-mesenchymal transition and fibrosis. The possibility that the overexpression of the e-cadherin might be implicated in the anti-inflammatory reaction to MPTP treatment by influencing the behavior of the other analyzed molecules is discussed.

Heat shock protein 60: an endogenous inducer of dopaminergic cell death in Parkinson disease.

BACKGROUND: Increasing evidence suggests that inflammation associated with microglial cell activation in the substantia nigra (SN) of patients with Parkinson disease (PD) is not only a consequence of neuronal degeneration, but may actively sustain dopaminergic (DA) cell loss over time. We aimed to study whether the intracellular chaperone heat shock protein 60 (Hsp60) could serve as a signal of CNS injury for activation of microglial cells. METHODS: Hsp60 mRNA expression in the mesencephalon and the striatum of C57/BL6 mice treated with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and the Hsp60/TH mRNA ratios in the SN of PD patients and aged-matched subjects were measured. To further investigate a possible link between the neuronal Hsp60 response and PD-related cellular stress, Hsp60 immunoblot analysis and quantification in cell lysates from SH-SY5Y after treatment with 100 µM MPP+ (1-methyl-4-phenylpyridinium) at different time points (6, 12, 24 and 48 hours) compared to control cells were performed. Additional MTT and LDH assay were used. We next addressed the question as to whether Hsp60 influences the survival of TH+ neurons in mesencephalic neuron-glia cultures treated either with MPP+ (1 µM), hHsp60 (10 µg/ml) or a combination of both. Finally, we measured IL-1ß, IL-6, TNF-α and NO-release by ELISA in primary microglial cell cultures following treatment with different hHsp60 preparations. Control cultures were exposed to LPS. RESULTS: In the mesencephalon and striatum of mice treated with MPTP and also in the SN of PD patients, we found that Hsp60 mRNA was up-regulated. MPP+, the active metabolite of MPTP, also caused an increased expression and release of Hsp60 in the human dopaminergic cell line SH-SY5Y. Interestingly, in addition to being toxic to DA neurons in primary mesencephalic cultures, exogenous Hsp60 aggravated the effects of MPP+. Yet, although we demonstrated that Hsp60 specifically binds to microglial cells, it failed to stimulate the production of pro-inflammatory cytokines or NO by these cells. CONCLUSIONS: Overall, our data suggest that Hsp60 is likely to participate in DA cell death in PD but via a mechanism unrelated to cytokine release.

Microglial glucocorticoid receptors play a pivotal role in regulating dopaminergic neurodegeneration in parkinsonism.

Among the pathogenic processes contributing to dopaminergic neuron (DN) death in Parkinson disease (PD), evidence points to non-cell-autonomous mechanisms, particularly chronic inflammation mounted by activated microglia. Yet little is known about endogenous regulatory processes that determine microglial actions in pathological states. We examined the role of glucocorticoid receptors (GRs), activated by glucocorticoids released in response to stress and known to regulate inflammation, in DN survival. Overall GR level was decreased in substantia nigra of PD patients and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated mice. GR changes, specifically in the microglia after MPTP treatment, revealed a rapid augmentation in the number of microglia displaying nuclear localization of GR. Mice with selective inactivation of the GR gene in macrophages/microglia (GR(LysMCre)) but not in DNs (GR(DATCre)) showed increased loss of DNs after MPTP intoxication. This DN loss in GR(LysMCre) mice was not prevented by corticosterone treatment, in contrast to the protection observed in control littermates. Moreover, absence of microglial GRs augmented microglial reactivity and led to their persistent activation. Analysis of inflammatory genes revealed an up-regulation of Toll-like receptors (TLRs) by MPTP treatment, particularly TLR9, the level of which was high in postmortem parkinsonian brains. The regulatory control of GR was reflected by higher expression of proinflammatory genes (e.g., TNF-α) with a concomitant decrease in anti-inflammatory genes (e.g., IL-1R2) in GR(LysMCre) mice. Indeed, in GR(LysMCre) mice, alterations in phosphorylated NF-κB levels indicated its protracted activation. Together, our data indicate that GR is important in curtailing microglial reactivity, and its deregulation in PD could lead to sustained inflammation-mediated DN injury.

DRD2 activation inhibits choroidal neovascularization in patients with Parkinson's disease and age-related macular degeneration.

Neovascular age-related macular degeneration (nAMD) remains a major cause of visual impairment and puts considerable burden on patients and health care systems. L-DOPA-treated Parkinson Disease (PD) patients have been shown to be partially protected from nAMD, but the mechanism remains unknown. Using murine models, combining 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD and laser-induced nAMD, standard PD treatment of L-DOPA/DOPA-decarboxylase inhibitor, or specific dopamine receptor inhibitors, we here demonstrate that L-DOPA treatment-induced increase of dopamine mediated dopamine receptor D2 (DRD2) signaling inhibits choroidal neovascularization independently of MPTP-associated nigrostriatal pathway lesion. Analyzing a retrospective cohort of more than two hundred thousand nAMD patients receiving anti-VEGF treatment from the French nationwide insurance database, we show that DRD2-agonist treated (PD) patients have a significantly delayed age of onset for nAMD (81.4 (±7.0) vs 79.4 (±8.1) years old, respectively, p<0.0001) and reduced need for anti-VEGF therapies (-0.6 injections per 100 mg/day daily dose of DRD2 agonists the second year of treatment), similar to the L-DOPA treatment. While providing a mechanistic explanation for an intriguing epidemiological observation, our findings suggest that systemic DRD2 agonists might constitute an adjuvant therapy to delay and reduce the need for anti-VEGF therapy in nAMD patients.

MFGE8 does not orchestrate clearance of apoptotic neurons in a mouse model of Parkinson's disease.

Parkinson's disease (PD) is an age-related neurodegenerative disorder characterized by a loss of dopaminergic neurons (DN) in the substantia nigra (SN). Several lines of evidence suggest that apoptotic cell death of DN is driven in part by non-cell autonomous mechanisms implicating microglial cells and inflammatory processes. Yet, how apoptotic DNs get removed by professional phagocytes and how this process modulates inflammatory processes are still unresolved issues. In this study, we investigated the role of MFGE8, a soluble factor involved in phagocytic recognition, in apoptotic DN clearance and neuroinflammation in PD. We report that glial expression of MFGE8 is enhanced in post-mortem PD brains compared to control individuals. Then, in vivo functional analysis of Mfge8 was assessed in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated mouse model of PD using wild-type (WT) and Mfge8-deficient mice. Neuropathological analysis consisted in evaluating (i) the loss of nigral DN and striatal DN terminals, (ii) the extent of glial cell activation and (iii) the number of apoptotic profiles. In vivo microglial phagocytic activity was further assessed by measuring the engulfment of apoptotic DN preloaded with fluorescent latex beads. Here we show that Mfge8 deficiency neither impact the phagocytic clearance of apoptotic bodies nor change the overall neuropathological parameters (DN cell loss and glial cell activation). In summary, our data argue that MFGE8 is not likely involved in the phagocytic clearance of neuronal debris associated with nigrostriatal pathway injury.

DAP12 and CD11b contribute to the microglial-induced death of dopaminergic neurons in vitro but not in vivo in the MPTP mouse model of Parkinson's disease.

BACKGROUND: Parkinson's disease (PD) is a neurodegenerative disorder characterized by a loss of dopaminergic neurons (DN) in the substantia nigra (SN). Several lines of evidence suggest that apoptotic cell death of DN is driven in part by non-cell autonomous mechanisms orchestrated by microglial cell-mediated inflammatory processes. Although the mechanisms and molecular network underlying this deleterious cross-talk between DN and microglial cells remain largely unknown, previous work indicates that, upon DN injury, activation of the ß2 integrin subunit CD11b is required for microglia-mediated DN cell death. Interestingly, during brain development, the CD11b integrin is also involved in microglial induction of neuronal apoptosis and has been shown to act in concert with the DAP12 immunoreceptor. Whether such a developmental CD11b/DAP12 pathway could be reactivated in a pathological context such as PD and play a role in microglia-induced DN cell death is a tantalizing hypothesis that we wished to test in this study. METHODS: To test the possibility that DAP12 could be involved in microglia-associated DN injury, we used both in vitro and in vivo toxin-based experimental models of PD recapitulating microglial-mediated non-cell autonomous mechanisms of DN cell death. In vitro, enriched mesencephalic neuronal/microglial co-cultures were exposed to the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+) whereas in vivo, mice were administrated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) according to acute or subchronic mode. Mice deficient for DAP12 or CD11b were used to determine the pathological function of the CD11b/DAP12 pathway in our disease models. RESULTS: Our results show that DAP12 and CD11b partially contribute to microglia-induced DN cell death in vitro. Yet, in vivo, mice deficient for either of these factors develop similar neuropathological alterations as their wild-type counterparts in two different MPTP mouse models of PD. CONCLUSION: Overall, our data suggest that DAP12 and CD11b contribute to microglial-induced DN cell death in vitro but not in vivo in the MPTP mouse model of PD. Therefore, the CD11b/DAP12 pathway may not be considered as a promising therapeutic target for PD.

The Pesticide Chlordecone Promotes Parkinsonism-like Neurodegeneration with Tau Lesions in Midbrain Cultures and C. elegans Worms.

Chlordecone (CLD) is an organochlorine pesticide (OCP) that is currently banned but still contaminates ecosystems in the French Caribbean. Because OCPs are known to increase the risk of Parkinson's disease (PD), we tested whether chronic low-level intoxication with CLD could reproduce certain key characteristics of Parkinsonism-like neurodegeneration. For that, we used culture systems of mouse midbrain dopamine (DA) neurons and glial cells, together with the nematode C. elegans as an in vivo model organism. We established that CLD kills cultured DA neurons in a concentration- and time-dependent manner while exerting no direct proinflammatory effects on glial cells. DA cell loss was not impacted by the degree of maturation of the culture. The use of fluorogenic probes revealed that CLD neurotoxicity was the consequence of oxidative stress-mediated insults and mitochondrial disturbances. In C. elegans worms, CLD exposure caused a progressive loss of DA neurons associated with locomotor deficits secondary to alterations in food perception. L-DOPA, a molecule used for PD treatment, corrected these deficits. Cholinergic and serotoninergic neuronal cells were also affected by CLD in C. elegans, although to a lesser extent than DA neurons. Noticeably, CLD also promoted the phosphorylation of the aggregation-prone protein tau (but not of α-synuclein) both in midbrain cell cultures and in a transgenic C. elegans strain expressing a human form of tau in neurons. In summary, our data suggest that CLD is more likely to promote atypical forms of Parkinsonism characterized by tau pathology than classical synucleinopathy-associated PD.

Tumor necrosis factor-like weak inducer of apoptosis induces astrocyte proliferation through the activation of transforming-growth factor-α/epidermal growth factor receptor signaling pathway.

Reactive astrogliosis is beneficial in many aspects; however, it is also detrimental in some pathological states such as the development of lethal brain tumors. It is therefore crucial to understand the mechanisms regulating astrocyte proliferation. Tumor necrosis factor-like weak inducer of apoptosis (TWEAK), a member of the tumor necrosis factor family, was shown to stimulate astrocyte proliferation in vitro. Herein, we further characterize the mitogenic potential of TWEAK on central nervous system cells. Among these cells, astrocytes express the highest level of TWEAK and Fn14 transcripts, suggesting that they are particularly sensitive to TWEAK stimulation. Using in vitro model systems, we found that TWEAK was as potent as epidermal growth factor (EGF) (a prototypical astrocyte mitogen) in mediating astrocyte proliferation. However, its mitogenic activity was delayed compared with that of EGF, suggesting distinct mechanisms of action. Using cell signaling pathway inhibitors, neutralizing antibodies, and protein assays, we further show that the mitogenic activity of TWEAK on primary astrocytes requires stimulation of the transforming growth factor-α (TGF-α) and of the epidermal growth factor receptor (EGFR) signaling pathway through extracellular signal-regulated kinase and p38 mitogen-activated protein kinase activation. In aggregates, our data demonstrate that TWEAK acts as a potent astrocyte mitogen through the induction of a TGF-α/EGFR signaling pathway. We anticipate that description of such a mechanism may allow novel approaches to human pathologies associated with astrocyte proliferation.

Infiltration of CD4+ lymphocytes into the brain contributes to neurodegeneration in a mouse model of Parkinson disease.

Parkinson disease (PD) is a neurodegenerative disorder characterized by a loss of dopamine-containing neurons. Mounting evidence suggests that dopaminergic cell death is influenced by the innate immune system. However, the pathogenic role of the adaptive immune system in PD remains enigmatic. Here we showed that CD8+ and CD4+ T cells but not B cells had invaded the brain in both postmortem human PD specimens and in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD during the course of neuronal degeneration. We further demonstrated that MPTP-induced dopaminergic cell death was markedly attenuated in the absence of mature T lymphocytes in 2 different immunodeficient mouse strains (Rag1-/- and Tcrb-/- mice). Importantly, similar attenuation of MPTP-induced dopaminergic cell death was seen in mice lacking CD4 as well as in Rag1-/- mice reconstituted with FasL-deficient splenocytes. However, mice lacking CD8 and Rag1-/- mice reconstituted with IFN-gamma-deficient splenocytes were not protected. These data indicate that T cell-mediated dopaminergic toxicity is almost exclusively arbitrated by CD4+ T cells and requires the expression of FasL but not IFNgamma. Further, our data may provide a rationale for targeting the adaptive arm of the immune system as a therapeutic strategy in PD.

Modelling α-Synuclein Aggregation and Neurodegeneration with Fibril Seeds in Primary Cultures of Mouse Dopaminergic Neurons.

To model α-Synuclein (αS) aggregation and neurodegeneration in Parkinson's disease (PD), we established cultures of mouse midbrain dopamine (DA) neurons and chronically exposed them to fibrils 91 (F91) generated from recombinant human αS. We found that F91 have an exquisite propensity to seed the aggregation of endogenous αS in DA neurons when compared to other neurons in midbrain cultures. Until two weeks post-exposure, somal aggregation in DA neurons increased with F91 concentrations (0.01-0.75 µM) and the time elapsed since the initiation of seeding, with, however, no evidence of DA cell loss within this time interval. Neither toxin-induced mitochondrial deficits nor genetically induced loss of mitochondrial quality control mechanisms promoted F91-mediated αS aggregation or neurodegeneration under these conditions. Yet, a significant loss of DA neurons (~30%) was detectable three weeks after exposure to F91 (0.5 µM), i.e., at a time point where somal aggregation reached a plateau. This loss was preceded by early deficits in DA uptake. Unlike αS aggregation, the loss of DA neurons was prevented by treatment with GDNF, suggesting that αS aggregation in DA neurons may induce a form of cell death mimicking a state of trophic factor deprivation. Overall, our model system may be useful for exploring PD-related pathomechanisms and for testing molecules of therapeutic interest for this disorder.

Neurochemokines: a menage a trois providing new insights on the functions of chemokines in the central nervous system.

Recent observations suggest that besides their role in the immune system, chemokines have important functions in the brain. There is a great line of evidence to suggest that chemokines are a unique class of neurotransmitters/neuromodulators, which regulate many biological aspects as diverse as neurodevelopment, neuroinflammation and synaptic transmission. In physiopathological conditions, many chemokines are synthesized in activated astrocytes and microglial cells, suggesting their involvement in brain defense mechanisms. However, when evoking chemokine functions in the nervous system, it is important to make a distinction between resting conditions and various pathological states including inflammatory diseases, autoimmune or neurodegenerative disorders in which chemokine functions have been extensively studied. We illustrate here the emergent concept of the neuromodulatory/neurotransmitter activities of neurochemokines and their potential role as a regulatory alarm system and as a group of messenger molecules for the crosstalk between neurons and cells from their surrounding microenvironment. In this deliberately challenging review, we provide novel hypotheses on the role of these subtle messenger molecules in brain functions leading to the evidence that previous dogmas concerning chemokines should be reconsidered.

Divalent metal transporter 1 (DMT1) contributes to neurodegeneration in animal models of Parkinson's disease.

Dopaminergic cell death in the substantia nigra (SN) is central to Parkinson's disease (PD), but the neurodegenerative mechanisms have not been completely elucidated. Iron accumulation in dopaminergic and glial cells in the SN of PD patients may contribute to the generation of oxidative stress, protein aggregation, and neuronal death. The mechanisms involved in iron accumulation also remain unclear. Here, we describe an increase in the expression of an isoform of the divalent metal transporter 1 (DMT1/Nramp2/Slc11a2) in the SN of PD patients. Using the PD animal model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication in mice, we showed that DMT1 expression increases in the ventral mesencephalon of intoxicated animals, concomitant with iron accumulation, oxidative stress, and dopaminergic cell loss. In addition, we report that a mutation in DMT1 that impairs iron transport protects rodents against parkinsonism-inducing neurotoxins MPTP and 6-hydroxydopamine. This study supports a critical role for DMT1 in iron-mediated neurodegeneration in PD.

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.

Glucocorticoid receptor in astrocytes regulates midbrain dopamine neurodegeneration through connexin hemichannel activity.

The precise contribution of astrocytes in neuroinflammatory process occurring in Parkinson's disease (PD) is not well characterized. In this study, using GRCx30CreERT2 mice that are conditionally inactivated for glucocorticoid receptor (GR) in astrocytes, we have examined the actions of astrocytic GR during dopamine neuron (DN) degeneration triggered by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The results show significantly augmented DN loss in GRCx30CreERT2 mutant mice in substantia nigra (SN) compared to controls. Hypertrophy of microglia but not of astrocytes was greatly enhanced in SN of these astrocytic GR mutants intoxicated with MPTP, indicating heightened microglial reactivity compared to similarly-treated control mice. In the SN of GR astrocyte mutants, specific inflammation-associated transcripts ICAM-1, TNF-α and Il-1ß as well as TNF-α protein levels were significantly elevated after MPTP neurotoxicity compared to controls. Interestingly, this paralleled increased connexin hemichannel activity and elevated intracellular calcium levels in astrocytes examined in acute midbrain slices from control and mutant mice treated with MPP+ . The increased connexin-43 hemichannel activity was found in vivo in MPTP-intoxicated mice. Importantly, treatment of MPTP-injected GRCx30CreERT2 mutant mice with TAT-Gap19 peptide, a specific connexin-43 hemichannel blocker, reverted both DN loss and microglial activation; in wild-type mice there was partial but significant survival effect. In the SN of post-mortem PD patients, a significant decrease in the number of astrocytes expressing nuclear GR was observed, suggesting the participation of astrocytic GR deregulation of inflammatory process in PD. Overall, these data provide mechanistic insights into GR-modulated processes in vivo, specifically in astrocytes, that contribute to a pro-inflammatory state and dopamine neurodegeneration in PD pathology.

Pleiotrophin receptor RPTP-zeta/beta expression is up-regulated by L-DOPA in striatal medium spiny neurons of parkinsonian rats.

L-DOPA is still the drug of choice to treat Parkinson's disease although adverse side effects appear after several years of treatment. These are thought to be the consequence of plastic re-arrangements of the nigrostriatal connections, such as sprouting of the dopaminergic terminals or post-synaptic changes. Pleiotrophin, a trophic factor that we have shown to be up-regulated in the striatum of parkinsonian rats after long-term L-DOPA treatment may play a role in these plastic changes. To determine whether one of the three known pleiotrophin receptors [N-syndecan, receptor protein tyrosine phosphatase type zeta beta (RPTP-zeta/beta) and anaplastic lymphoma kinase] might be implicated in these putative plastic effects, we quantified their expression levels by real-time RT-PCR in the striatum and mesencephalon of rats with partial lesions of the nigrostriatal pathway undergoing L-DOPA treatment. Both pleiotrophin and RPTP-zeta/beta expression was up-regulated in the striatum but not in the mesencephalon of lesioned rats and RPTP-zeta/beta expression was even further increased by L-DOPA. The levels of the RPTP-zeta/beta protein were also increased in the striatum of L-DOPA-treated lesioned rats. Immunofluorescence labeling showed the protein to be constitutively expressed in striatal medium spiny neurons, which are innervated by both the corticostriatal glutamatergic and nigrostriatal dopaminergic systems. RPTP-zeta/beta might therefore be implicated in the plastic changes triggered by L-DOPA treatment and might merit further study as a potential candidate for Parkinon's disease therapy.

Modelling Parkinson-like neurodegeneration via osmotic minipump delivery of MPTP and probenecid.

Mouse models of MPTP intoxication have been used extensively to explore the molecular mechanisms of Parkinson's disease. However, these models present some limitations since; (i) Dopaminergic (DA) cell death occurs rapidly in contrast to the presumably slow evolution of the disease process. (ii) Some of the key histological features of the disease such as Lewy body like inclusions and long-term inflammatory changes are lacking. Fornai et al. [Proc. Natl Acad. Sci. USA 102 (2005), 3413] suggested that continuous delivery of MPTP with Alzet osmotic minipumps may possibly circumvent these problems. Our results show, however, that MPTP infusion via Alzet osmotic minipumps (40 mg/kg/day) produces only a transient depletion in striatal dopamine (DA) without causing dopaminergic cell loss in the substantia nigra. Neuronal cell loss occurred, however, if MPTP was infused concomitantly with probenecid, an uricosuric agent which potentiates the effects of the toxin injected via the i.p. route. Even under these conditions, dopaminergic cell loss was moderate (-25%) and other neurodegenerative changes characteristic of Parkinson's disease remained undetectable.