Time-course of nigrostriatal neurodegeneration and neuroinflammation in the 6-hydroxydopamine-induced axonal and terminal lesion models of Parkinson's disease in the rat

Microglia Response During Parkinson's Disease: Alpha-Synuclein Intervention

The discovery of the central role played by the protein alpha-synuclein in Parkinson's disease and other Lewy body brain disorders has had a great relevance in the understanding of the degenerative process occurring in these diseases. In addition, during the last two decades, the evidence suggesting an immune response in Parkinson's disease patients have multiplied. The role of the immune system in the disease is supported by data from genetic studies and patients, as well as from laboratory animal models and cell cultures. In the immune response, the microglia, the immune cell of the brain, will have a determinant role. Interestingly, alpha-synuclein is suggested to have a central function not only in the neuronal events occurring in Parkinson's disease, but also in the immune response during the disease. Numerous studies have shown that alpha-synuclein can act directly on immune cells, such as microglia in brain, initiating a sterile response that will have consequences for the neuronal health and that could also translate in a peripheral immune response. In parallel, microglia should also act clearing alpha-synuclein thus avoiding an overabundance of the protein, which is crucial to the disease progression. Therefore, the microglia response in each moment will have significant consequences for the neuronal fate. Here we will review the literature addressing the microglia response in Parkinson's disease with an especial focus on the protein alpha-synuclein. We will also reflect upon the limitations of the studies carried so far and in the therapeutic possibilities opened based on these recent findings.

Evidence for cross-hemispheric preconditioning in experimental Parkinson's disease

Dopamine loss and motor deficits in Parkinson's disease typically commence unilaterally and remain asymmetric for many years, raising the possibility that endogenous defenses slow the cross-hemispheric transmission of pathology. It is well-established that the biological response to subtoxic stress prepares cells to survive subsequent toxic challenges, a phenomenon known as preconditioning, tolerance, or stress adaptation. Here we demonstrate that unilateral striatal infusions of the oxidative toxicant 6-hydroxydopamine (6-OHDA) precondition the contralateral nigrostriatal pathway against the toxicity of a second 6-OHDA infusion in the opposite hemisphere. 6-OHDA-induced loss of dopaminergic terminals in the contralateral striatum was ablated by cross-hemispheric preconditioning, as shown by two independent markers of the dopaminergic phenotype, each measured by two blinded observers. Similarly, loss of dopaminergic somata in the contralateral substantia nigra was also abolished, according to two blinded measurements. Motor asymmetries in floor landings, forelimb contacts with a wall, and spontaneous turning behavior were consistent with these histological observations. Unilateral 6-OHDA infusions increased phosphorylation of the kinase ERK2 and expression of the antioxidant enzyme CuZn superoxide dismutase in both striata, consistent with our previous mechanistic work showing that these two proteins mediate preconditioning in dopaminergic cells. These findings support the existence of cross-hemispheric preconditioning in Parkinson's disease and suggest that dopaminergic neurons mount impressive natural defenses, despite their reputation as being vulnerable to oxidative injury. If these results generalize to humans, Parkinson's pathology may progress slowly and asymmetrically because exposure to a disease-precipitating insult induces bilateral upregulation of endogenous defenses and elicits cross-hemispheric preconditioning.

Time-course behavioral features are correlated with Parkinson's disease‑associated pathology in a 6-hydroxydopamine hemiparkinsonian rat model

Parkinson's disease (PD) is one of the most common neurodegenerative diseases. For decades, the unilateral 6‑hydroxydopamine (6‑OHDA) rat model has been employed to investigate the pathogenesis and therapy of PD. However, the behavior and associated pathological features of the model long term have not previously been described dynamically. In the present study, the unilateral model was established by 6‑OHDA injection in the striatum. The PD rat model was determined 2 weeks following surgery, according to the apomorphine (APO)‑induced rotations, cylinder, rotarod and open field tests. TH‑positive neurons and fibers in the substantia nigra pars compacta (SNpc) and striatum, respectively, and glial activation in the SNpc, determined by glial fibrillary acidic protein (GFAP) expression for astrocytes and CD11b (Mac1) expression for microglia, were detected by immunohistological staining. Correlation analysis was performed to understand the association between PD‑associated behavior and pathology. The behavioral impairment progressively deteriorated during the process of experiment. In addition, the decrease in TH‑positive neurons was associated with an increase in GFAP‑ and Mac1‑positive cells in the SNpc. Linear regression analysis indicated the association between behavioral and pathological changes. The results of the present study indicate that the APO‑induced rotation, cylinder and rotarod tests are all sensitive and reliable strategies to predict the loss of TH+ neurons. These results provide a potential intervention time‑point and a comprehensive evaluation index system for assessment of PD therapeutic strategies using the hemiparkinsonian rat.

Application of the Physical Disector Principle for Quantification of Dopaminergic Neuronal Loss in a Rat 6-Hydroxydopamine Nigral Lesion Model of Parkinson's Disease

Stereological analysis is the optimal tool for quantitative assessment of brain morphological and cellular changes induced by neurotoxic lesions or treatment interventions. Stereological methods based on random sampling techniques yield unbiased estimates of particle counts within a defined volume, thereby providing a true quantitative estimate of the target cell population. Neurodegenerative diseases involve loss of specific neuron types, such as the midbrain tyrosine hydroxylase-positive dopamine neurons in Parkinson's disease and in animal models of nigrostriatal degeneration. Therefore, we applied an established automated physical disector principle in a fractionator design for efficient stereological quantitative analysis of tyrosine hydroxylase (TH)-positive dopamine neurons in the substantia nigra pars compacta of hemiparkinsonian rats with unilateral 6-hydroxydopamine (6-OHDA) lesions. We obtained reliable estimates of dopamine neuron numbers, and established the relationship between behavioral asymmetry and dopamine neuron loss on the lesioned side. In conclusion, the automated physical disector principle provided a useful and efficient tool for unbiased estimation of TH-positive neurons in rat midbrain, and should prove valuable for investigating neuroprotective strategies in 6-OHDA model of parkinsonism, while generalizing to other immunohistochemically-defined cell populations.

Striatal astrocytes engulf dopaminergic debris in Parkinson's disease: A study in an animal model

The role of astrocytes in Parkinson’s disease is still not well understood. This work studied the astrocytic response to the dopaminergic denervation. Rats were injected in the lateral ventricles with 6-hydroxydopamine (25μg), inducing a dopaminergic denervation of the striatum not accompanied by non-selective tissue damage. The dopaminergic debris were found within spheroids (free-spheroids) which retained some proteins of dopaminergic neurons (e.g., tyrosine hydroxylase, the dopamine transporter protein, and APP) but not others (e.g., α-synuclein). Free-spheroids showed the initial (LC3-autophagosomes) but not the late (Lamp1/Lamp2-lysosomes) components of autophagy (incomplete autophagy), preparing their autophagosomes for an external phagocytosis (accumulation of phosphatidylserine). Free-spheroids were penetrated by astrocyte processes (fenestrated-spheroids) which made them immunoreactive for GFAP and S100β, and which had some elements needed to continue the debris degradation (Lamp1/Lamp2). Finally, proteins normally found in neurons (TH, DAT and α-synuclein) were observed within astrocytes 2–5 days after the dopaminergic degeneration, suggesting that the intracellular contents of degenerated cells had been transferred to astrocytes. Taken together, present data suggest phagocytosis as a physiological role of striatal astrocytes, a role which could be critical for cleaning striatal debris during the initial stages of Parkinson’s disease.

Direct Reprogramming of Resident NG2 Glia into Neurons with Properties of Fast-Spiking Parvalbumin-Containing Interneurons

Converting resident glia into functional and subtype-specific neurons in vivo by delivering reprogramming genes directly to the brain provides a step forward toward the possibility of treating brain injuries or diseases. To date, it has been possible to obtain GABAergic and glutamatergic neurons via in vivo conversion, but the precise phenotype of these cells has not yet been analyzed in detail. Here, we show that neurons reprogrammed using Ascl1, Lmx1a, and Nurr1 functionally mature and integrate into existing brain circuitry and that the majority of the reprogrammed neurons have properties of fast-spiking, parvalbumin-containing interneurons. When testing different combinations of genes for neural conversion with a focus on pro-neural genes and dopamine fate determinants, we found that functional neurons can be generated using different gene combinations and in different brain regions and that most of the reprogrammed neurons become interneurons, independently of the combination of reprogramming factors used.

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Reprogramed striatal NG2 glia yield neurons similar to fast-spiking interneurons

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Reprogrammed neurons reach functional maturation after 12 weeks

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Dopamine (DA) denervation leads to TH expression but no reprogramming into DA neurons

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Variation in pro-neural genes or fate specifiers does not affect neuronal phenotype

Microglial phenotypes in Parkinson's disease and animal models of the disease

Over the last decade the important concept has emerged that microglia, similar to other tissue macrophages, assume different phenotypes and serve several effector functions, generating the theory that activated microglia can be organized by their pro-inflammatory or anti-inflammatory and repairing functions. Importantly, microglia exist in a heterogenous population and their phenotypes are not permanently polarized into two categories; they exist along a continuum where they acquire different profiles based on their local environment. In Parkinson's disease (PD), neuroinflammation and microglia activation are considered neuropathological hallmarks, however their precise role in relation to disease progression is not clear, yet represent a critical challenge in the search of disease-modifying strategies. This review will critically address current knowledge on the activation states of microglia as well as microglial phenotypes found in PD and in animal models of PD, focusing on the expression of surface molecules as well as pro-inflammatory and anti-inflammatory cytokine production during the disease process. While human studies have reported an elevation of both pro- or anti-inflammatory markers in the serum and CSF of PD patients, animal models have provided insights on dynamic changes of microglia phenotypes in relation to disease progression especially prior to the development of motor deficits. We also review recent evidence of malfunction at multiple steps of NFκB signaling that may have a causal interrelationship with pathological microglia activation in animal models of PD. Finally, we discuss the immune-modifying strategies that have been explored regarding mechanisms of chronic microglial activation.

Vitamin D Treatment Attenuates Neuroinflammation and Dopaminergic Neurodegeneration in an Animal Model of Parkinson's Disease, Shifting M1 to M2 Microglia Responses

Microglia-mediated neuroinflammation has been described as a common hallmark of Parkinson's disease (PD) and is believed to further exacerbate the progressive degeneration of dopaminergic neurons. Current therapies are unable to prevent the disease progression. A significant association has been demonstrated between PD and low levels of vitamin D in patients serum, and vitamin D supplement appears to have a beneficial clinical effect. Herein, we investigated whether vitamin D administered orally in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced preclinical animal model of PD protects against glia-mediated inflammation and nigrostriatal neurodegeneration. Vitamin D significantly attenuated the MPTP-induced loss of tyrosine hydrlase (TH)-positive neuronal cells, microglial cell activation (Iba1-immunoreactive), inducible nitric oxide synthase (iNOS) and TLR-4 expression, typical hallmarks of the pro-inflammatory (M1) activation of microglia. Additionally, Vitamin D was able to decrease pro-inflammatory cytokines mRNA expression in distinct brain areas of the MPTP mouse. Importantly, we also assessed the anti-inflammatory property of vitamin D in the MPTP mouse, in which it upregulated the anti-inflammatory cytokines (IL-10, IL-4 and TGF-β) mRNA expression as well as increasing the expression of CD163, CD206 and CD204, typical hallmarks of alternative activation of microglia for anti-inflammatory signalling (M2). Collectively, these results demonstrate that vitamin D exhibits substantial neuroprotective effects in this PD animal model, by attenuating pro-inflammatory and up-regulating anti-inflammatory processes.

Anti-Inflammatory Modulation of Microglia via CD163-Targeted Glucocorticoids Protects Dopaminergic Neurons in the 6-OHDA Parkinson's Disease Model

Increasing evidence supports a decisive role for inflammation in the neurodegenerative process of Parkinson's disease (PD). The immune response in PD seems to involve, not only microglia, but also other immune cells infiltrated into the brain. Indeed, we observed here the infiltration of macrophages, specifically CD163+ macrophages, into the area of neurodegeneration in the 6-hydroxydopamine (6-OHDA) PD model. Therefore, we investigated the therapeutic potential of the infiltrated CD163+ macrophages to modulate local microglia in the brain to achieve neuroprotection. To do so, we designed liposomes targeted for the CD163 receptor to deliver dexamethasone (Dexa) into the CD163+ macrophages in the 6-OHDA PD model. Our data show that a fraction of the CD163-targeted liposomes were carried into the brain after peripheral intravenous injection. The 6-OHDA-lesioned rats that received repeated intravenous CD163-targeted liposomes with Dexa for 3 weeks exhibited better motor performance than the control groups and had minimal glucocorticoid-driven side effects. Furthermore, these animals showed better survival of dopaminergic neurons in substantia nigra and an increased number of microglia expressing major histocompatibility complex II. Therefore, rats receiving CD163-targeted liposomes with Dexa were partially protected against 6-OHDA-induced dopaminergic neurodegeneration, which correlated with a distinctive microglia response. Altogether, our data support the use of macrophages for the modulation of brain neurodegeneration and specifically highlight the potential of CD163-targeted liposomes as a therapeutic tool in PD.

SIGNIFICANCE STATEMENT

The immune response now evident in the progression of Parkinson's disease comprises both local microglia and other immune cells. We provide evidence that CD163+ macrophages can be a target to modulate brain immune response to achieve neuroprotection in the 6-hydroxydopamine model. To do so, we targeted the CD163+ population, which to a low but significant extent infiltrated in the neurodegenerating area of the brain. Specially designed liposomes targeted for the CD163 receptor were loaded with glucocorticoids and injected peripherally to modify the infiltrated CD163 cells toward an anti-inflammatory profile. This modification of the CD163 population resulted in a distinctive microglial response that correlated with decreased dopaminergic cell death and better motor performance.

Interaction between subclinical doses of the Parkinson's disease associated gene, α-synuclein, and the pesticide, rotenone, precipitates motor dysfunction and nigrostriatal neurodegeneration in rats

In most patients, Parkinson's disease is thought to emerge after a lifetime of exposure to, and interaction between, various genetic and environmental risk factors. One of the key genetic factors linked to this condition is α-synuclein, and the α-synuclein protein is pathologically associated with idiopathic cases. However, α-synuclein pathology is also present in presymptomatic, clinically "normal" individuals suggesting that environmental factors, such as Parkinson's disease-linked agricultural pesticides, may be required to precipitate Parkinson's disease in these individuals. In this context, the aim of this study was to assess the behavioural and neuropathological impact of exposing rats with a subclinical load of α-synuclein to subclinical doses of the organic pesticide, rotenone. Rats were randomly assigned to two groups for intra-nigral infusion of AAV_2/5-GFP or AAV_2/5-α-synuclein. Post viral motor function was assessed at 8, 10 and 12 weeks in the Corridor, Stepping and Whisker tests of lateralised motor function. At week 12, animals were performance-matched to receive a subsequent intra-striatal challenge of the organic pesticide rotenone (or its vehicle) to yield four final groups (Control, Rotenone, AAV_2/5-α-synuclein and Combined). Behavioural testing resumed one week after rotenone surgery and continued for 5 weeks. We found that, when administered alone, neither intra-nigral AAV-α-synuclein nor intra-striatal rotenone caused sufficient nigrostriatal neurodegeneration to induce a significant motor impairment in their own right. However, when these were administered sequentially to the same rats, the interaction between the two Parkinsonian challenges significantly exacerbated nigrostriatal neurodegeneration which precipitated a pronounced impairment in motor function. These results indicate that exposing rats with a subclinical α-synuclein-induced pathology to the pesticide, rotenone, profoundly exacerbates their Parkinsonian neuropathology and dysfunction, and highlights the potential importance of this interaction in the etiology of, and in driving the pathogenesis of Parkinson's disease.

Progressive loss of nigrostriatal dopaminergic neurons induced by inflammatory responses to fipronil

Inflammatory responses are involved in mechanisms of neuronal cell damage in the pathogenesis of neurodegenerative diseases such as Parkinson's disease (PD). We investigated the mechanisms whereby inflammatory responses contribute to loss of dopaminergic neurons in fipronil (FPN)-treated rats. After stereotaxic injection of FPN in the substantia nigra (SN), the number of tyrosine hydroxylase (TH)-positive neurons and the levels of TH expression in the SN decreased at 7days, and a significant decrease was observed at 14days with a subsequent reduction in striatal TH expression. Decreases in dopamine (DA) levels, however, began at 3days post-injection, preceding the changes in TH expression. In contrast, glial fibrillary acidic protein (GFAP) expression was significantly increased at 3days and persisted for up to 14days post-lesion; these changes in GFAP expression appeared to be inversely correlated with TH expression. Furthermore, we found that FPN administration induced an inflammatory response characterized by increased levels of inducible NO synthase (iNOS), cyclooxygenase-2 (COX-2), and tumor necrosis factor-α (TNF-α), which was mediated by activated microglia following infusion of FPN unilaterally into the SN. Intranigral injection of FPN underwent an inflammatory response with a resultant ongoing loss of dopaminergic neurons, indicating that pesticides may have important implication for the study of PD.

6-hydroxydopamine-induced Parkinson's disease-like degeneration generates acute microgliosis and astrogliosis in the nigrostriatal system but no bioluminescence imaging-detectable alteration in adult neurogenesis

Parkinson's disease is a slowly progressing neurodegenerative disorder caused by loss of dopaminergic neurons in the substantia nigra (SN), leading to severe impairment in motor and non-motor functions. Endogenous subventricular zone (SVZ) neural stem cells constantly give birth to new cells that might serve as a possible source for regeneration in the adult brain. However, neurodegeneration is accompanied by neuroinflammation and dopamine depletion, potentially compromising regeneration. We therefore employed in vivo imaging methods to study striatal deafferentation (N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-[(123) I]iodophenyl)nortropane single photon emission computed tomography, DaTscan(™) ) and neuroinflammation in the SN and striatum (N,N-diethyl-2-(2-(4-(2-[(18) F]fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide positron emission tomography, [(18) F]DPA-714 PET) in the intranigral 6-hydroxydopamine Parkinson's disease mouse model. Additionally, we transduced cells in the SVZ with a lentivirus encoding firefly luciferase and followed migration of progenitor cells in the SVZ-olfactory bulb axis via bioluminescence imaging under disease and control conditions. We found that activation of microglia in the SN is an acute process accompanying the degeneration of dopaminergic cell bodies in the SN. Dopaminergic deafferentation of the striatum does not influence the generation of doublecortin-positive neuroblasts in the SVZ, but generates chronic astrogliosis in the nigrostriatal system.

Expression of Tgfβ1 and Inflammatory Markers in the 6-hydroxydopamine Mouse Model of Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder that is characterized by loss of midbrain dopaminergic (mDA) neurons in the substantia nigra (SN). Microglia-mediated neuroinflammation has been described as a common hallmark of PD and is believed to further trigger the progression of neurodegenerative events. Injections of 6-hydroxydopamine (6-OHDA) are widely used to induce degeneration of mDA neurons in rodents as an attempt to mimic PD and to study neurodegeneration, neuroinflammation as well as potential therapeutic approaches. In the present study, we addressed microglia and astroglia reactivity in the SN and the caudatoputamen (CPu) after 6-OHDA injections into the medial forebrain bundle (MFB), and further analyzed the temporal and spatial expression patterns of pro-inflammatory and anti-inflammatory markers in this mouse model of PD. We provide evidence that activated microglia as well as neurons in the lesioned SN and CPu express Transforming growth factor β1 (Tgfβ1), which overlaps with the downregulation of pro-inflammatory markers Tnfα, and iNos, and upregulation of anti-inflammatory markers Ym1 and Arg1. Taken together, the data presented in this study suggest an important role for Tgfβ1 as a lesion-associated factor that might be involved in regulating microglia activation states in the 6-OHDA mouse model of PD in order to prevent degeneration of uninjured neurons by microglia-mediated release of neurotoxic factors such as Tnfα and nitric oxide (NO).

Growth/differentiation factor-15 deficiency compromises dopaminergic neuron survival and microglial response in the 6-hydroxydopamine mouse model of Parkinson's disease

Growth/differentiation factor-15 (Gdf-15) is a member of the TGF-β superfamily and a pleiotropic, widely distributed cytokine, which has been shown to play roles in various pathologies, including inflammation. Analysis of Gdf-15(-/-) mice has revealed that it serves the postnatal maintenance of spinal cord motor neurons and sensory neurons. In a previous study, exogenous Gdf-15 rescued 6-hydroxydopamine (6-OHDA) lesioned Gdf-15(+/+) nigrostriatal dopaminergic (DAergic) neurons in vitro and in vivo. Whether endogenous Gdf-15 serves the physiological maintenance of nigrostriatal DAergic neurons in health and disease is not known and was addressed in the present study. Stereotactic injection of 6-OHDA into the medial forebrain bundle (MFB) led to a significant decline in the numbers of DAergic neurons in both Gdf-15(+/+) and Gdf-15(-/-) mice over a time-period of 14days. However, this decrease was exacerbated in the Gdf-15(-/-) mice, with only 5.5% surviving neurons as compared to 24% in the Gdf-15(+/+) mice. Furthermore, the microglial response to the 6-OHDA lesion was reduced in Gdf-15(-/-) mice, with significantly lower numbers of total and activated microglia and a differential cytokine expression as compared to the Gdf-15(+/+) mice. Using in vitro models, we could demonstrate the importance of endogenous Gdf-15 in promoting DAergic neuron survival thus highlighting its relevance in a direct neurotrophic supportive role. Taken together, these results indicate the importance of Gdf-15 in promoting survival of DAergic neurons and regulating the inflammatory response post 6-OHDA lesion.

Friends or Foes: Matrix Metalloproteinases and Their Multifaceted Roles in Neurodegenerative Diseases

Neurodegeneration is a chronic progressive loss of neuronal cells leading to deterioration of central nervous system (CNS) functionality. It has been shown that neuroinflammation precedes neurodegeneration in various neurodegenerative diseases. Matrix metalloproteinases (MMPs), a protein family of zinc-containing endopeptidases, are essential in (neuro)inflammation and might be involved in neurodegeneration. Although MMPs are indispensable for physiological development and functioning of the organism, they are often referred to as double-edged swords due to their ability to also inflict substantial damage in various pathological conditions. MMP activity is strictly controlled, and its dysregulation leads to a variety of pathologies. Investigation of their potential use as therapeutic targets requires a better understanding of their contributions to the development of neurodegenerative diseases. Here, we review MMPs and their roles in neurodegenerative diseases: Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and multiple sclerosis (MS). We also discuss MMP inhibition as a possible therapeutic strategy to treat neurodegenerative diseases.

Differential pattern of motor impairments in neurotoxic, environmental and inflammation-driven rat models of Parkinson's disease

One of the reasons proposed for the paucity of drug discovery for Parkinson's disease is the lack of relevant animal models of the condition. Parkinson's disease has been modelled extensively using the selective neurotoxin, 6-hydroxydopamine (6-OHDA). However, as this model bears little etiological resemblance to the human condition, there has been a drive to develop models with improved etiological validity. Two such models are those induced by the pesticide, rotenone, and the inflammagen, lipopolysaccharide (LPS). However, to date, these models have been poorly characterised in terms of their motor profiles and have never been directly compared to the more established models. Thus, the aim of this study was to characterise the behavioural profile of the rotenone and LPS models, and to compare them with the 6-OHDA model. Animals underwent baseline testing on the Stepping, Whisker, Corridor and Cylinder Tests of motor function. They were then grouped for unilateral intra-striatal infusion of 6-OHDA, rotenone or LPS. Motor testing continued for ten weeks after which the rats were processed for immunohistochemical analysis of nigrostriatal integrity. We found that, although all neurotoxins induced a similar level of nigrostriatal neurodegeneration, neither the rotenone nor LPS models were associated with amphetamine-induced rotation, and they were associated with significantly less pronounced and stable impairments in the spontaneous tasks than the 6-OHDA model. In conclusion, this study demonstrates key differences in the pattern of motor dysfunction induced by Parkinsonian neurotoxins which should be taken into consideration when selecting the most appropriate model for Parkinson's disease preclinical studies.

The 6-hydroxydopamine model and parkinsonian pathophysiology: Novel findings in an older model

The neurotoxin 6-hydroxydopamine (6-OHDA) is widely used to induce models of Parkinson's disease (PD). We now know that the model induced by 6-OHDA does not include all PD symptoms, although it does reproduce the main cellular processes involved in PD, such as oxidative stress, neurodegeneration, neuroinflammation, and neuronal death by apoptosis. In this review we analyse the factors affecting the vulnerability of dopaminergic neurons as well as the close relationships between neuroinflammation, neurodegeneration, and apoptosis in the 6-OHDA model. Knowledge of the mechanisms involved in neurodegeneration and cell death in this model is the key to identifying potential therapeutic targets for PD.

Peripheral administration of the selective inhibitor of soluble tumor necrosis factor (TNF) XPro®1595 attenuates nigral cell loss and glial activation in 6-OHDA hemiparkinsonian rats

BACKGROUND

Parkinson's disease (PD) is a complex multi-system age-related neurodegenerative disorder. Targeting the ongoing neuroinflammation in PD patients is one strategy postulated to slow down or halt disease progression. Proof-of-concept studies from our group demonstrated that selective inhibition of soluble Tumor Necrosis Factor (solTNF) by intranigral delivery of dominant negative TNF (DN-TNF) inhibitors reduced neuroinflammation and nigral dopamine (DA) neuron loss in endotoxin and neurotoxin rat models of nigral degeneration.

OBJECTIVE

As a next step toward human clinical trials, we aimed to determine the extent to which peripherally administered DN-TNF inhibitor XPro®1595 could: i) cross the blood-brain-barrier in therapeutically relevant concentrations, ii) attenuate neuroinflammation (microglia and astrocyte), and iii) mitigate loss of nigral DA neurons in rats receiving a unilateral 6-hydroxydopamine (6-OHDA) striatal lesion.

METHODS

Rats received unilateral 6-OHDA (20 μg into the right striatum). Three or 14 days after lesion, rats were dosed with XPro®1595 (10 mg/kg in saline, subcutaneous) every third day for 35 days. Forelimb asymmetry was used to assess motor deficits after the lesion; brains were harvested 35 days after the lesion for analysis of XPro®1595 levels, glial activation and nigral DA neuron number.

RESULTS

Peripheral subcutaneous dosing of XPro®1595 achieved plasma levels of 1-8 microgram/mL and CSF levels of 1-6 ng/mL depending on the time the rats were killed after final XPro®1595 injection. Irrespective of start date, XPro®1595 significantly reduced microglia and astrocyte number in SNpc whereas loss of nigral DA neurons was attenuated when drug was started 3, but not 14 days after the 6-OHDA lesion.

CONCLUSIONS

Our data suggest that systemically administered XPro®1595 may have disease-modifying potential in PD patients where inflammation is part of their pathology.

The degeneration of dopaminergic synapses in Parkinson's disease: A selective animal model

Available evidence increasingly suggests that the degeneration of dopamine neurons in Parkinson's disease starts in the striatal axons and synaptic terminals. A selective procedure is described here to study the mechanisms involved in the striatal denervation of dopaminergic terminals. This procedure can also be used to analyze mechanisms involved in the dopaminergic re-innervation of the striatum, and the role of astrocytes and microglia in both processes. Adult Sprague-Dawley rats were injected in the lateral ventricles with increasing doses of 6-hydroxydopamine (12-50 μg), which generated a dose-dependent loss of dopaminergic synapses and axons in the striatum, followed by an axonal sprouting (weeks later) and by a progressive recovery of striatal dopaminergic synapses (months later). Both the degeneration and regeneration of the dopaminergic terminals were accompanied by astrogliosis. Because the experimental manipulations did not induce unspecific damage in the striatal tissue, this method could be particularly suitable to study the basic mechanisms involved in the distal degeneration and regeneration of dopaminergic nigrostriatal neurons, and the possible role of astrocytes and microglia in the dynamics of both processes.

Differential upregulation of the cannabinoid CB₂ receptor in neurotoxic and inflammation-driven rat models of Parkinson's disease

The cannabinoid CB2 receptor has recently emerged as a potential anti-inflammatory target to break the self-sustaining cycle of neuroinflammation and neurodegeneration that is associated with neurodegenerative diseases. However, in order to facilitate the development of cannabinoid drugs for neurodegenerative disease, the changes that occur in the endocannabinoid system in response to different neurodegenerative triggers needs to be elucidated. Therefore, the aim of this study was to investigate and compare the changes that occur in the endocannabinoid system in neurotoxic and inflammation-driven models of Parkinson's disease. To do so, male Sprague Dawley rats were given unilateral, intra-striatal injections of the dopaminergic neurotoxin, 6-hydroxydopamine, or the bacterial inflammagen, lipopolysaccharide (LPS). Animals underwent behavioural testing for motor dysfunction on Days 7, 14 and 28 post-surgery, and were sacrificed on Days 1, 4, 14 and 28. Changes in the endocannabinoid system were investigated by qRT-PCR, liquid chromatography-mass spectrometry and immunohistochemistry. After injection of 6-hydroxydopamine or LPS into the rat striatum, we found that expression of the CB2 receptor was significantly elevated in both models, and that this increase correlated significantly with an increase in microglial activation. Interestingly, the increase in CB2 receptor expression in the inflammation-driven model was significantly more pronounced than that in the neurotoxic model. Moreover, endocannabinoid levels were also elevated in the LPS model but not the 6-hydroxydopamine model. Thus, this study has shown that the endocannabinoid system is dysregulated in animal models of Parkinson's disease, and has also revealed significant differences in the level of dysregulation between the models themselves. This study indicates that targeting the CB2 receptor may represent a viable target for anti-inflammatory disease modification in Parkinson's disease.

Alterations of BDNF and trkB mRNA expression in the 6-hydroxydopamine-induced model of preclinical stages of Parkinson's disease: an influence of chronic pramipexole in rats

Our recent study has indicated that a moderate lesion of the mesostriatal and mesolimbic pathways in rats, modelling preclinical stages of Parkinson’s disease, induces a depressive-like behaviour which is reversed by chronic treatment with pramipexole. The purpose of the present study was to examine the role of brain derived neurotrophic factor (BDNF) signalling in the aforementioned model of depression. Therefore, we investigated the influence of 6-hydoxydopamine (6-OHDA) administration into the ventral region of the caudate-putamen on mRNA levels of BDNF and tropomyosin-related kinase B (trkB) receptor. The BDNF and trkB mRNA levels were determined in the nigrostriatal and limbic structures by in situ hybridization 2 weeks after the operation. Pramipexole (1 mg/kg sc twice a day) and imipramine (10 mg/kg ip once a day) were injected for 2 weeks. The lesion lowered the BDNF and trkB mRNA levels in the hippocampus [CA1, CA3 and dentate gyrus (DG)] and amygdala (basolateral/lateral) as well as the BDNF mRNA content in the habenula (medial/lateral). The lesion did not influence BDNF and trkB expression in the caudate-putamen, substantia nigra, nucleus accumbens (shell and core) and ventral tegmental area (VTA). Chronic imipramine reversed the lesion-induced decreases in BDNF mRNA in the DG. Chronic pramipexole increased BDNF mRNA, but decreased trkB mRNA in the VTA in lesioned rats. Furthermore, it reduced BDNF and trkB mRNA expression in the shell and core of the nucleus accumbens, BDNF mRNA in the amygdala and trkB mRNA in the caudate-putamen in these animals. The present study indicates that both the 6-OHDA-induced dopaminergic lesion and chronic pramipexole influence BDNF signalling in limbic structures, which may be related to their pro-depressive and antidepressant activity in rats, respectively.

Neurotoxicity effects of atrazine-induced SH-SY5Y human dopaminergic neuroblastoma cells via microglial activation

Atrazine (2-chloro-4-ethytlamino-6-isopropylamine-1,3,5-triazine; ATR) is a broad-spectrum herbicide with a wide range of applications worldwide.

Transient transfection of human CDNF gene reduces the 6-hydroxydopamine-induced neuroinflammation in the rat substantia nigra

BACKGROUND

The anti-inflammatory effect of the cerebral dopamine neurotrophic factor (CDNF) was shown recently in primary glial cell cultures, yet such effect remains unknown both in vivo and in 6-hydroxydopamine (6-OHDA) models of Parkinson's disease (PD). We addressed this issue by performing an intranigral transfection of the human CDNF (hCDNF) gene in the critical period of inflammation after a single intrastriatal 6-OHDA injection in the rat.

METHODS

At day 15 after lesion, the plasmids p3xNBRE-hCDNF or p3xNBRE-EGFP, coding for enhanced green florescent protein (EGFP), were transfected into the rat substantia nigra (SN) using neurotensin (NTS)-polyplex. At day 15 post-transfection, we measured nitrite and lipoperoxide levels in the SN. We used ELISA to quantify the levels of TNF-α, IL-1β, IL-6, endogenous rat CDNF (rCDNF) and hCDNF. We also used qRT-PCR to measure rCDNF and hCDNF transcripts, and immunofluorescence assays to evaluate iNOS, CDNF and glial cells (microglia, astrocytes and Neuron/Glial type 2 (NG2) cells). Intact SNs were additional controls.

RESULTS

In the SN, 6-OHDA triggered nitrosative stress, increased inflammatory cytokines levels, and activated the multipotent progenitor NG2 cells, which convert into astrocytes to produce rCDNF. In comparison with the hemiparkinsonian rats that were transfected with the EGFP gene or without transfection, 6-OHDA treatment and p3xNBRE-hCDNF transfection increased the conversion of NG2 cells into astrocytes resulting in 4-fold increase in the rCDNF protein levels. The overexpressed CDNF reduced nitrosative stress, glial markers and IL-6 levels in the SN, but not TNF-α and IL-1β levels.

CONCLUSION

Our results show the anti-inflammatory effect of CDNF in a 6-OHDA rat of Parkinson's disease. Our results also suggest the possible participation of TNF-α, IL-1β and IL-6 in rCDNF production by astrocytes, supporting their anti-inflammatory role.

Expression of endogenous Mkp1 in 6-OHDA rat models of Parkinson's disease

We have previously demonstrated that mitogen-activated protein kinase phosphatase 1, Mkp1, is expressed in the developing and rat adult substantia nigra and striatum, where it promotes the growth of nigral dopaminergic neurons. Mkp1 may therefore have therapeutic potential for Parkinson’s disease. In the present study, we have assessed the expression of Mkp1 and TH in the substantia nigra and striatum of parkinsonian rat models. Expression was measured at 4 and 10 days post-lesion in the 6-hydroxydopamine (6-OHDA) medial forebrain bundle lesion model and after 4, 10 and 28 days in the 6-OHDA striatal lesion model. Our results show that Mkp1 expression was transiently up-regulated in the substantia nigra at 4 days post-6-OHDA administration in the two models while TH expression was decreased at the later time-points examined. These data suggest that Mkp1 may play a role in counteracting the neurotoxic effects of 6-OHDA in nigral dopaminergic neurons.

Knockdown of interleukin-1 receptor 1 is not neuroprotective in the 6-hydroxydopamine striatal lesion rat model of Parkinson's disease

It is well established that neuroinflammation is associated with the progression of many neurodegenerative diseases, including Parkinson's disease (PD). Activated microglia and elevated levels of pro-inflammatory cytokines such as interleukin-1β (IL-1β) have been found in the brain and cerebrospinal fluid of PD patients, suggesting that IL-1β may be involved in the pathogenesis of this disease. This study aimed to knock down the expression of the interleukin-1 type 1 receptor (IL-1R1) to evaluate any potential therapeutic effect of limiting the action of IL-1β in the substantia nigra following a unilateral intrastriatal 6-hydroxydopamine (6-OHDA) lesion in rats. Adult Sprague-Dawley rats received intranigral injections of shRNA specific for IL-1R1, followed 2 weeks later by intrastriatal 6-OHDA. Injection of IL-1R1 shRNA did not prevent 6-OHDA-induced loss of motor function or loss of nigral dopamine neurons. IL-1R1 expression was increased in the midbrain following 6-OHDA injection; this effect was attenuated in 6-OHDA-treated animals that had received IL-1R1 shRNA. These data suggest that while IL-1R1 was increased in 6-OHDA-treated animals and reduced following shRNA injection, the neurodegeneration induced by 6-OHDA was not mediated through IL-1R1.

Microglial dynamics and role in the healthy and diseased brain: a paradigm of functional plasticity

The study of the dynamics and functions of microglia in the healthy and diseased brain is a matter of intense scientific activity. The application of new techniques and new experimental approaches has allowed the identification of novel microglial functions and the redefinition of classic ones. In this review, we propose the study of microglial functions, rather than their molecular profiles, to better understand and define the roles of these cells in the brain. We review current knowledge on the role of surveillant microglia, proliferating microglia, pruning/neuromodulatory microglia, phagocytic microglia, and inflammatory microglia and the molecular profiles that are associated with these functions. In the remodeling scenario of microglial biology, the analysis of microglial functional states will inform about the roles in health and disease and will guide us to a more precise understanding of the multifaceted roles of this never-resting cells.

Time course of dopamine neuron loss and glial response in the 6-OHDA striatal mouse model of Parkinson's disease

The 6-hydroxydopamine (6-OHDA) neurotoxic lesion of the midbrain dopamine (DA) system is one of the most widely used techniques for modelling Parkinson's disease in rodents. The majority of studies using this approach, however, largely limit their analysis to lesioning acutely, and looking at behavioural deficits and the number of surviving tyrosine hydroxylase (TH)-stained cells in the midbrain. Here we have analysed additional characteristics that occur following intrastriatal delivery of 6-OHDA, providing better understanding of the neurodegenerative process. Female C57/Black mice were given lesions at 10 weeks old, and killed at several different time points postoperatively (3 and 6 h, 1, 3, 6, 9 and 12 days). While the detrimental effect of the toxin on the TH+ fibres in the striatum was immediate, we found that the loss of TH+ dendritic fibres, reduction in cell size and intensity of TH expression, and eventual reduction in the number of TH+ neurons in the substantia nigra was delayed for several days post-surgery. We also investigated the expression of various transcription factors and proteins expressed by midbrain DA neurons following lesioning, and observed changes in the expression of Aldh1a1 (aldehyde dehydrogenase 1 family, member A1) as the neurodegenerative process evolved. Extracellularly, we looked at microglia and astrocytes in reaction to the 6-OHDA striatal lesion, and found a delay in their response and proliferation in the substantia nigra. In summary, this work highlights aspects of the neurodegenerative process in the 6-OHDA mouse model that can be applied to future studies looking at therapeutic interventions.

Metalloproteinase-9 contributes to inflammatory glia activation and nigro-striatal pathway degeneration in both mouse and monkey models of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinsonism

Inflammation is a predominant aspect of neurodegenerative diseases, manifested by glia activation and expression of pro-inflammatory mediators. Studies on animal models of Parkinson's disease (PD) suggest that sustained neuroinflammation exacerbates degeneration of the dopaminergic (DA) nigro-striatal pathway. Therefore, insights into the inflammatory mechanisms of PD may help the development of novel therapeutic strategies against this disease. As extracellular matrix metalloproteinases (MMPs) could be major players in the progression of Parkinsonism, we investigated, in the substantia nigra and striatum of mice acutely injected with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), changes in mRNA expression, protein levels, and cell localization of MMP-9. This protease is mainly neuronal, but early after MPTP injection its mRNA and protein levels, as well as the number of MMP-9-expressing microglia and astrocytes, increase concomitantly to a prominent inflammation. Neuroinflammation and MMP-9(+) glia begin to decline within 2 weeks, although protein levels remain higher than control, in association with a partial recovery of DA nigro-striatal circuit. Comparable quantitative studies on MMP-9 knock-out mice, show a significant decrease in both glia activation and loss of DA neurons and fibers, with respect to wild-type. Moreover, in a parallel study on chronically MPTP-injected macaques, we observed that perpetuation of inflammation and high levels of MMP-9 are associated to DA neuron loss. Our data suggest that MMP-9 released by injured neurons favors glia activation; glial cells in turn reinforce their reactive state via autocrine MMP-9 release, contributing to nigro-striatal pathway degeneration. Specific modulation of MMP-9 activity may, therefore, be a strategy to ameliorate harmful inflammatory outcomes in Parkinsonism.

Integrating pathways of Parkinson's disease in a molecular interaction map

Parkinson's disease (PD) is a major neurodegenerative chronic disease, most likely caused by a complex interplay of genetic and environmental factors. Information on various aspects of PD pathogenesis is rapidly increasing and needs to be efficiently organized, so that the resulting data is available for exploration and analysis. Here we introduce a computationally tractable, comprehensive molecular interaction map of PD. This map integrates pathways implicated in PD pathogenesis such as synaptic and mitochondrial dysfunction, impaired protein degradation, alpha-synuclein pathobiology and neuroinflammation. We also present bioinformatics tools for the analysis, enrichment and annotation of the map, allowing the research community to open new avenues in PD research. The PD map is accessible at http://minerva.uni.lu/pd_map .

Activation of GSK-3β and caspase-3 occurs in Nigral dopamine neurons during the development of apoptosis activated by a striatal injection of 6-hydroxydopamine

The 6-Hydroxydopamine (6-OHDA) rat model of Parkinson's disease is essential for a better understanding of the pathological processes underlying the human disease and for the evaluation of promising therapeutic interventions. This work evaluated whether a single striatal injection of 6-OHDA causes progressive apoptosis of dopamine (DA) neurons and activation of glycogen synthase kinase 3β (GSK-3β) and caspase-3 in the substantia nigra compacta (SNc). The loss of DA neurons was shown by three neuron markers; tyrosine hydroxylase (TH), NeuN, and β-III tubulin. Apoptosis activation was determined using Apostain and immunostaining against cleaved caspase-3 and GSK-3β pY216. We also explored the possibility that cleaved caspase-3 is produced by microglia and astrocytes. Our results showed that the 6-OHDA caused loss of nigral TH(+) cells, progressing mainly in rostrocaudal and lateromedial directions. In the neostriatum, a severe loss of TH(+) terminals occurred from day 3 after lesion. The disappearance of TH(+) cells was associated with a decrease in NeuN and β-III tubulin immunoreactivity and an increase in Apostain, cleaved caspase-3, and GSK-3β pY216 in the SNc. Apostain immunoreactivity was observed from days 3 to 21 postlesion. Increased levels of caspase-3 immunoreactivity in TH(+) cells were detected from days 1 to 15, and the levels then decreased to day 30 postlesion. The cleaved caspase-3 also collocated with microglia and astrocytes indicating its participation in glial activation. Our results suggest that caspase-3 and GSK-3β pY216 activation might participate in the DA cell death and that the active caspase-3 might also participate in the neuroinflammation caused by the striatal 6-OHDA injection.

Cellular and molecular mediators of neuroinflammation in the pathogenesis of Parkinson's disease

Neuroinflammation is a host-defense mechanism associated with restoration of normal structure and function of the brain and neutralization of an insult. Increasing neuropathological and biochemical evidence from the brains of individuals with Parkinson's disease (PD) provides strong evidence for activation of neuroinflammatory pathways. Microglia, the resident innate immune cells, may play a major role in the inflammatory process of the diseased brain of patients with PD. Although microglia forms the first line of defense for the neural parenchyma, uncontrolled activation of microglia may directly affect neurons by releasing various molecular mediators such as inflammatory cytokines (tumor necrosis factor-α, interleukin [IL]-6, and IL-1β), nitric oxide, prostaglandin E2, and reactive oxygen and nitrogen species. Moreover, recent studies have reported that activated microglia phagocytose not only damaged cell debris but also intact neighboring cells. This phenomenon further supports their active participation in self-enduring neuronal damage cycles. As the relationship between PD and neuroinflammation is being studied, there is a realization that both cellular and molecular mediators are most likely assisting pathological processes leading to disease progression. Here, we discuss mediators of neuroinflammation, which are known activators released from damaged parenchyma of the brain and result in neuronal degeneration in patients with PD.

Enhanced neurotrophic distribution, cell signaling and neuroprotection following substantia nigral versus striatal delivery of AAV2-NRTN (CERE-120)

This paper reassesses the currently accepted viewpoint that targeting the terminal fields (i.e. striatum) of degenerating nigrostriatal dopamine neurons with neurotrophic factors in Parkinson's disease (PD) is sufficient for achieving an optimal neurotrophic response. Recent insight indicating that PD is an axonopathy characterized by axonal transport deficits prompted this effort. We tested whether a significantly greater neurotrophic response might be induced in SN neurons when the neurotrophic factor neurturin (NRTN) is also targeted to the substantia nigra (SN), compared to the more conventional, striatum-only target. While recognizing the importance of maintaining the integrity of nigrostriatal fibers and terminals (especially for achieving optimal function), we refocused attention to the fate of SN neurons. Under conditions of axonal degeneration and neuronal transport deficits, this component of the nigrostriatal system is most vulnerable to the lack of neurotrophic exposure following striatal-only delivery. Given the location of repair genes induced by neurotrophic factors, achieving adequate neurotrophic exposure to the SN neurons is essential for an optimal neurotrophic response, while the survival of these neurons is essential to the very survival of the fibers. Two separate studies were performed using the 6-OHDA model of nigrostriatal degeneration, in conjunction with delivery of the viral vector AAV2-NRTN (CERE-120) to continuously express NRTN to either striatum or nigra alone or combined striatal/nigral exposure, including conditions of ongoing axonopathy. These studies provide additional insight for reinterpreting past animal neurotrophic/6-OHDA studies conducted under conditions where axon transport deficiencies were generally not accounted for, which suggested that targeting the striatum was both necessary and sufficient. The current data demonstrate that delivering NRTN directly to the SN produces 1) expanded NRTN distribution within the terminal field and cell bodies of targeted nigrostriatal neurons, 2) enhanced intracellular neurotrophic factor signaling in the nigrostriatal neurons, and 3) produced greater numbers of surviving dopamine neurons against 6-OHDA-induced toxicity, particularly under the conditions of active axonopathy. Thus, these data provide empirical support that targeting the SN with neurotrophic factors (in addition to striatum) may help enhance the neurotrophic response in midbrain neurons, particularly under conditions of active neurodegeneration which occurs in PD patients.

Doxycycline restrains glia and confers neuroprotection in a 6-OHDA Parkinson model

Neuron-glia interactions play a key role in maintaining and regulating the central nervous system. Glial cells are implicated in the function of dopamine neurons and regulate their survival and resistance to injury. Parkinson's disease is characterized by the loss of dopamine neurons in the substantia nigra pars compacta, decreased striatal dopamine levels and consequent onset of extrapyramidal motor dysfunction. Parkinson's disease is a common chronic, neurodegenerative disorder with no effective protective treatment. In the 6-OHDA mouse model of Parkinson's disease, doxycycline administered at a dose that both induces/represses conditional transgene expression in the tetracycline system, mitigates the loss of dopaminergic neurons in the substantia nigra compacta and nerve terminals in the striatum. This protective effect was associated with: (1) a reduction of microglia in normal mice as a result of doxycycline administration per se; (2) a decrease in the astrocyte and microglia response to the neurotoxin 6-OHDA in the globus pallidus and substantia nigra compacta, and (3) the astrocyte reaction in the striatum. Our results suggest that doxycycline blocks 6-OHDA neurotoxicity in vivo by inhibiting microglial and astrocyte expression. This action of doxycycline in nigrostriatal dopaminergic neuron protection is consistent with a role of glial cells in Parkinson's disease neurodegeneration. The neuroprotective effect of doxycycline may be useful in preventing or slowing the progression of Parkinson's disease and other neurodegenerative diseases linked to glia function.

Mitogen-activated protein kinase phosphatase (MKP)-1 as a neuroprotective agent: promotion of the morphological development of midbrain dopaminergic neurons

A greater understanding of the mechanisms that promote the survival and growth of dopaminergic neurons is essential for the advancement of cell replacement therapies for Parkinson's disease (PD). Evidence supports a role for the mitogen-activated protein kinase p38 in the demise of dopaminergic neurons, while mitogen-activated protein kinase phosphatase-1 (MKP-1), which negatively regulates p38 activity, has not yet been investigated in this context. Here, we show that MKP-1 is expressed in dopaminergic neurons cultured from E14 rat ventral mesencephalon (VM). When dopaminergic neurons were transfected to overexpress MKP-1, they displayed a more complex morphology than their control counterparts in vitro. Specifically, MKP-1-transfection induced significant increases in neurite length and branching with a maximum increase observed in primary branches. We demonstrate that inhibition of dopaminergic neurite growth induced by treatment of rat VM neurons with the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA) in vitro is mediated by p38 and is concomitant with a significant and selective decrease in MKP-1 expression in these neurons. We further show that overexpression of MKP-1 in dopaminergic neurons contributes to neuroprotection against the effects of 6-OHDA. Collectively, we report that MKP-1 can promote the growth and elaboration of dopaminergic neuronal processes and can help protect them from the neurotoxic effects of 6-OHDA. Thus, we propose that strategies aimed at augmenting MKP-1 expression or activity may be beneficial in protecting dopaminergic neurons and may provide potential therapeutic approaches for PD.

Further characterisation of the LPS model of Parkinson's disease: a comparison of intra-nigral and intra-striatal lipopolysaccharide administration on motor function, microgliosis and nigrostriatal neurodegeneration in the rat

Chronic neuroinflammation has been established as one of the many processes involved in the pathogenesis of Parkinson's disease (PD). Because of this, researchers have attempted to replicate this pathogenic feature in animal models using the potent inflammagen, lipopolysaccharide (LPS), in order to gain better understanding of immune-mediated events in PD. However, although the effect of intra-cerebral LPS on neuroinflammation and neurodegeneration has been relatively well characterised, its impact on motor function has been less well studied. Therefore, the aim of this study was to further characterise the neuropathological and behavioural impact of intra-nigral and intra-striatal administration of LPS. To do, LPS (10 μg) or vehicle (sterile saline) were stereotaxically injected into the adult rat substantia nigra or striatum on one side only. The effect of LPS administration on lateralised motor function was assessed using the Corridor, Stepping and Whisker tests for two weeks post-injection, after which, amphetamine-induced rotational asymmetry was completed. Post-mortem, the impact of LPS on nigrostriatal degeneration and microgliosis was assessed using quantitative tyrosine hydroxylase and OX-42 immunohistochemistry respectively. We found that intra-nigral administration of LPS led to localised microgliosis in the substantia nigra and this was accompanied by nigrostriatal neurodegeneration and stable spontaneous motor deficits. In contrast, intra-striatal administration of LPS led to localised microgliosis in the striatum but this did not lead to any nigrostriatal neurodegeneration and only induced transient motor dysfunction. In conclusion, this study reveals the impact of intra-cerebral LPS administration on PD-related neuropathology and motor function, and it indicates that the intra-nigral model may be a highly relevant model as it is associated with stable motor decline underpinned by nigral microgliosis and nigrostriatal neurodegeneration.

Antioxidant-Enriched Diet Affects Early Microglia Accumulation and Promotes Regeneration of the Striatal Dopamine System after a 6-Hydroxidopamine-Induced Lesion in a Rat

Neuroinflammation is found both in the brain of humans suffering from Parkinson's disease and in animal models of disease. It is suggested to be involved in the pathogenesis of the disease. In the present study, in order to study the effects of antioxidants on neuroinflammation, microglial phenotypes were evaluated in rats fed with diets containing bilberries, blueberries, or crowberries at 1 and 4 weeks following striatal injection of 6-hydroxydopamine. The dopamine innervation was visualized using antibodies raised against tyrosine hydroxlase (TH) in the striatum and in the globus pallidus. One week post-lesion, the expression of Iba1-positive cells, a general microglial marker, was significantly increased in the striatum of all animals fed with antioxidant-enriched diets compared to control-diet fed animals, while the diameter of the TH-negative zone was similar in all animals. At four weeks post-lesion, the Iba1-positive microglia was significantly reduced in animals fed with antioxidant-enriched diets. The diameter of the TH-negative zone was significantly reduced in animals fed bilberry and crowberry. The expression and distribution of ED1-positive cells was similar to that of Iba1-positive cells found in the lesioned areas. A cell division marker Ki67 revealed that few microglia were proliferating in crowberry-treated animals. Otherwise dividing cells were associated with blood capillary cells. Although the antioxidant level should be equal in the entire brain, no regeneration was found in globus pallidus, suggesting the mechanism promoting regeneration in the striatum is not effective in the globus pallidus. In conclusion, diets rich in bilberries and crowberries and with high contents of antioxidants stimulate an early phase of accumulation of reactive migroglia that fades at longer time points i.e. promotes regeneration of the striatal dopamine system.