Deficiency of TNF receptors suppresses microglial activation and alters the susceptibility of brain regions to MPTP-induced neurotoxicity: role of TNF-alpha

Regulator of G-protein signaling 10 promotes dopaminergic neuron survival via regulation of the microglial inflammatory response

Epidemiological studies suggest that chronic use of nonsteroidal anti-inflammatory drugs lowers the incidence of Parkinson's disease (PD) in humans and implicate neuroinflammatory processes in the death of dopamine (DA) neurons. Here, we demonstrate that regulator of G-protein signaling 10 (RGS10), a microglia-enriched GAP (GTPase accelerating protein) for Galpha subunits, is an important regulator of microglia activation. Flow-cytometric and immunohistochemical analyses indicated that RGS10-deficient mice displayed increased microglial burden in the CNS, and exposure to chronic systemic inflammation induced nigral DA neuron loss measured by unbiased stereology. Primary microglia isolated from brains of RGS10-deficient mice displayed dysregulated inflammation-related gene expression profiles under basal and stimulated conditions in vitro compared with that of primary microglia isolated from wild-type littermates. Similarly, knockdown of RGS10 in the BV2 microglia cell line resulted in dysregulated inflammation-related gene expression, overproduction of tumor necrosis factor (TNF), and enhanced neurotoxic effects of BV2 microglia on the MN9D dopaminergic cell line that could be blocked by addition of the TNF decoy receptor etanercept. Importantly, ablation of RGS10 in MN9D dopaminergic cells further enhanced their vulnerability to microglial-derived death-inducing inflammatory mediators, suggesting a role for RGS10 in modulating the sensitivity of dopaminergic neurons against inflammation-mediated cell death. Together, our findings indicate that RGS10 limits microglial-derived TNF secretion and regulates the functional outcome of inflammatory stimuli in the ventral midbrain. RGS10 emerges as a novel drug target for prevention of nigrostriatal pathway degeneration, the neuropathological hallmark of PD.

Distinct mechanisms of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine resistance revealed by transcriptome mapping in mouse striatum

The etiology of idiopathic Parkinson's disease is thought to involve interplay between environmental factors and predisposing genetic traits, although the identification of genetic risk factors remain elusive. The neurotoxicant, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine (MPTP) produces parkinsonian-like symptoms and pathology in mice and humans. As sensitivity to MPTP is genetically determined in mice this provides an opportunity to identify genes and biological mechanisms that modify the response to an exogenous agent that produces a Parkinson's disease-like condition. MPTP primarily targets dopaminergic nerve terminals in the striatum and elicits changes in striatal gene expression. Therefore, we used Affymetrix and qRT-PCR technology to characterize temporal mRNA changes in striatum in response to MPTP in genetically MPTP-sensitive, C57BL/6J, and MPTP-resistant Swiss Webster and BCL2-associated X protein (Bax)-/- mice. We identified three phases of mRNA expression changes composed of largely distinct gene sets. An early response (5 h) occurred in all strains of mice and multiple brain regions. In contrast, intermediate (24 h) and late (72 h) phases were striatum specific and much reduced in Swiss Webster, indicating these genes contribute and/or are responsive to MPTP-induced pathology. However, Bax-/- mice have robust intermediate responses. We propose a model in which the acute entry of MPP+ into dopaminergic nerve terminals damages them but is insufficient per se to kill the neurons. Rather, we suggest that the compromised nerve terminals elicit longer lasting transcriptional responses in surrounding cells involving production of molecules that feedback on the terminals to cause additional damage that results in cell death. In Swiss Webster, resistance lies upstream in the cascade of events triggered by MPTP and uncouples the acute events elicited by MPTP from the damaging secondary responses. In contrast, in Bax-/- mice resistance lies downstream in the cascade and suggests enhanced tolerance to the secondary insult rather than its attenuation.

Human interleukin-10 gene transfer is protective in a rat model of Parkinson's disease

In Parkinson's disease (PD) chronic inflammation occurs in the substantia nigra (SNc) concurrently with dopaminergic neurodegeneration. In models of PD, microglial activation precedes neurodegeneration in the SNc, suggesting that the underlying pathogenesis involves a complex response in the nigrostriatal pathway, and that the innate immune system plays a significant role. We have investigated the neuroprotective effect of an adeno-associated viral type-2 (AAV2) vector containing the complementary DNA (cDNA) for human interleukin-10 (hIL-10) in the unilateral 6-hydroxydopamine (6-OHDA) rat model of PD. AAV2-hIL-10 reduced the 6-OHDA-induced loss of tyrosine hydroxylase (TH)-positive neurons in the SNc, and also reduced loss of striatal dopamine (DA). Pretreatment with AAV2-hIL-10 reduced glial activation in the SNc but did not attenuate striatal release of the inflammatory cytokine IL-1beta. Assessment of rotational behavior in response to apomorphine challenge showed absence of asymmetry, confirming protection of dopaminergic innervation of the lesioned striatum. At baseline, 6-OHDA-lesioned animals displayed a deficit in contralateral forelimb use, but pretreatment with AAV2-hIL-10 reduced this forelimb akinesia. Transcriptional analyses revealed alteration of a few genes by AAV2-hIL-10; these alterations may contribute to neuroprotection. This study supports the need for further investigations relating to gene therapies aimed at reducing neuroinflammation in early PD.

Tumor necrosis factor p55 and p75 receptors are involved in chemical-induced apoptosis of dentate granule neurons

Localized tumor necrosis factor-alpha (TNFalpha) elevation has diverse effects in brain injury often attributed to signaling via TNFp55 or TNFp75 receptors. Both dentate granule cells and CA pyramidal cells express TNF receptors (TNFR) at low levels in a punctate pattern. Using a model to induce selective death of dentate granule cells (trimethyltin; 2 mg/kg, i.p.), neuronal apoptosis [terminal deoxynucleotidyl transferase-mediated dUTP-biotin in situ end labeling, active caspase 3 (AC3)] was accompanied by amoeboid microglia and elevated TNFalpha mRNA levels. TNFp55R (55 kDa type-1 TNFR) and TNFp75R (75 kDa type-2 TNFR) immunoreactivity in AC3(+) neurons displayed a pattern suggestive of receptor internalization and a temporal sequence of expression of TNFp55R followed by TNFp75R associated with the progression of apoptosis. A distinct ramified microglia response occurred around CA1 neurons and healthy dentate neurons that displayed an increase in the normal punctate pattern of TNFRs. Neuronal damage was decreased with i.c.v. injection of TNFalpha antibody and in TNFp55R-/-p75R-/- mice that showed higher constitutive mRNA levels for interleukin (IL-1alpha), macrophage inflammatory protein 1-alpha (MIP-1alpha), TNFalpha, transforming growth factor beta1, Fas, and TNFRSF6-assoicated via death domain (FADD). TNFp75R-/- mice showed exacerbated injury and elevated mRNA levels for IL-1alpha, MIP-1alpha, and TNFalpha. In TNFp55R-/- mice, constitutive mRNA levels for TNFalpha, IL-6, caspase 8, FADD, and Fas-associated phosphatase were higher; IL-1alpha, MIP-1alpha, and transforming growth factor beta1 lower. The mice displayed exacerbated neuronal death, delayed microglia response, increased FADD and TNFp75R mRNA levels, and co-expression of TNFp75R in AC3(+) neurons. The data demonstrate TNFR-mediated apoptotic death of dentate granule neurons utilizing both TNFRs and suggest a TNFp75R-mediated apoptosis in the absence of normal TNFp55R activity.

Growth-compromised HSV-2 vector Delta RR protects from N-methyl-D-aspartate-induced neuronal degeneration through redundant activation of the MEK/ERK and PI3-K/Akt survival pathways, either one of which overrides apoptotic cascades

We have previously shown that intrastriatal injection of Delta RR, the growth-compromised herpes simplex virus type 2 (HSV-2) vector for the antiapoptotic protein ICP10PK, prevents apoptosis caused by the excitotoxin N-methyl-D-aspartate (NMDA) in a mouse model of glutamatergic neuronal cell death (Golembewski et al. [2007] Exp. Neurol. 203:381-393). Because apoptosis regulation is stimulus and cell type specific, our studies were designed to examine the mechanism of Delta RR-mediated neuroprotection in striatal neurons. Organotypic striatal cultures (OSC) that retain much of the synaptic circuitry of the intact striatum were infected with Delta RR or a growth-compromised HSV-2 vector that lacks ICP10PK (Delta PK) and examined for neuroprotection-associated signaling. The mutated ICP10 proteins (p175 and p95) were expressed in 70-80% of neurons from Delta RR- and Delta PK-infected cultures, respectively, as determined by double-immunofluorescent staining with antibodies to ICP10 and NeuN or GAD65. Delta RR- but not Delta PK-treated OSC were protected from NMDA-induced apoptosis, as verified by ethidium homodimer staining, TUNEL, caspase-3 activation, and poly(AD-ribose) polymerase (PARP) cleavage. Neuroprotection was through ICP10PK-mediated activation of the survival pathways MEK/ERK and PI3-K/Akt, up-regulation of the antiapoptotic proteins Bag-1 and Bcl-2, and phosphorylation (inactivation) of the proapoptotic protein Bad. It was blocked by the MEK inhibitor U0126 or the PI3-K inhibitor LY294002, suggesting that either pathway can prevent NMDA-induced apoptosis. The data indicate that Delta RR-delivered ICP10PK stimulates redundant survival pathways that override proapoptotic cascades. Delta RR is a promising gene therapy platform against glutamatergic cell death.

Mechanisms of microglia-mediated neurotoxicity in a new model of the stroke penumbra

After an ischemic stroke, neurons in the core are rapidly committed to die, whereas neuron death in the slowly developing penumbra is more amenable to therapeutic intervention. Microglia activation contributes to delayed inflammation, but because neurotoxic mechanisms in the penumbra are not well understood, we developed an in vitro model of microglia activation and propagated neuron killing. To recapitulate inflammatory triggers in the core, microglia were exposed to oxygen glucose-deprived neurons and astrocytes. To model the developing penumbra, the microglia were washed and allowed to interact with healthy naive neurons and astrocytes. We found that oxygen-glucose deprivation (OGD)-stressed neurons released glutamate, which activated microglia through their group II metabotropic glutamate receptors (mGluRs). Microglia activation involved nuclear factor kappaB (NF-kappaB), a transcription factor that promotes their proinflammatory functions. The activated microglia became neurotoxic, killing naive neurons through an apoptotic mechanism that was mediated by tumor necrosis factor-alpha (TNF-alpha), and involved activation of both caspase-8 and caspase-3. In contrast to some earlier models (e.g., microglia activation by lipopolysaccharide), neurotoxicity was not decreased by an inducible nitric oxide synthase (iNOS) inhibitor (S-methylisothiourea) or a peroxynitrite scavenger [5,10,15,20-tetrakis(N-methyl-4'-pyridyl)porphinato iron (III) chloride], and did not require p38 mitogen-activated protein kinase (MAPK) activation. The same microglia neurotoxic behavior was evoked without exposure to OGD-stressed neurons, by directly activating microglial group II mGluRs with (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl) glycine or glutamate, which stimulated production of TNF-alpha (not nitric oxide) and mediated TNF-alpha-dependent neurotoxicity through activation of NF-kappaB (not p38 MAPK). Together, these results support potential therapeutic strategies that target microglial group II mGluRs, TNFalpha overproduction, and NF-kappaB activation to reduce neuron death in the ischemic penumbra.

Predominant release of lysosomal enzymes by newborn rat microglia after LPS treatment revealed by proteomic studies

Growing evidence suggest that microglia may play an important role in the pathogenesis of neurodegenerative disease including Parkinson's disease, Alzheimer's disease, and so forth. The activation of microglia may cause neuronal damage through the release of reactive oxygen species and proinflammatory cytokines. However, the early response of microglial cells remains unclear before cells can secrete the proinflammatory cytokines. Here, a time course analysis showed the earliest expression of inducible nitric oxide synthase and cyclooxygenase-2 at 3 and 24 h following lipopolysaccharide (LPS) treatment. To further define initial response proteins of microglia after LPS treatment, we utilized a novel mass spectrometry-based quantitative proteomic technique termed SILAC (for stable isotope labeling by amino acids in cell culture) to compare the protein profiles of the cell culture-conditioned media of 1 h LPS-treated microglia as compared with controls. The proteomic analysis identified 77 secreted proteins using SignalP; of these, 28 proteins were associated with lysosome of cells and 13 lysosome-related proteins displayed significant changes in the relative abundance after 1 h LPS treatment. Four proteins were further evaluated with Western blot, demonstrating good agreement with quantitative proteomic data. These results suggested that microglia first released some lysosomal enzymes which may be involved in neuronal damage process. Furthermore, ammonium chloride, which inhibits microglia lysosomal enzyme activity, could prevent microglia from causing neuronal injury. Hence, in addition to the numerous novel proteins that are potentially important in microglial activation-mediated neurodegeneration revealed by the search, the study has indicated that the early release of lysosomal enzymes in microglial cells would contribute to LPS-activated inflammatory response.

Interactions of HIV and methamphetamine: cellular and molecular mechanisms of toxicity potentiation

Methamphetamine (METH) is a highly addictive psychostimulant drug, whose abuse has reached epidemic proportions worldwide. METH use is disproportionally represented among populations at high risks for developing HIV infection or who are already infected with the virus. Psychostimulant abuse has been reported to exacerbate the cognitive deficits and neurodegenerative abnormalities observed in HIV-positive patients. Thus, the purpose of the present paper is to review the clinical and basic observations that METH potentiates the adverse effects of HIV infection. An additional purpose is to provide a synthesis of the cellular and molecular mechanisms that might be responsible for the increased toxicity observed in co-morbid patients. The reviewed data indicate that METH and HIV proteins, including gp120, gp41, Tat, Vpr and Nef, converge on various caspase-dependent death pathways to cause neuronal apoptosis. The role of reactive microgliosis in METH- and in HIV-induced toxicity is also discussed.

Chronic microglial activation and progressive dopaminergic neurotoxicity

PD (Parkinson's disease) is characterized by the selective and progressive loss of DA neurons (dopaminergic neurons) in the substantia nigra. Inflammation and activation of microglia, the resident innate immune cell in the brain, have been strongly linked to neurodegenerative diseases, such as PD. Microglia can respond to immunological stimuli and neuronal death to produce a host of toxic factors, including cytokines and ROS (reactive oxygen species). Microglia can also become persistently activated after a single stimulus and maintain the elevated production of both cytokines and ROS, long after the instigating stimulus is gone. Current reports suggest that this chronic microglial activation may be fuelled by either dying/damaged neurons or autocrine and paracrine signals from local glial cells, such as cytokines. Here, we review proposed mechanisms responsible for chronic neuroinflammation and explain the interconnected relationship between deleterious microglial activation, DA neuron damage and neurodegenerative disease.

Microglia: active sensor and versatile effector cells in the normal and pathologic brain

Microglial cells constitute the resident macrophage population of the CNS. Recent in vivo studies have shown that microglia carry out active tissue scanning, which challenges the traditional notion of 'resting' microglia in the normal brain. Transformation of microglia to reactive states in response to pathology has been known for decades as microglial activation, but seems to be more diverse and dynamic than ever anticipated--in both transcriptional and nontranscriptional features and functional consequences. This may help to explain why engagement of microglia can be either neuroprotective or neurotoxic, resulting in containment or aggravation of disease progression. Moreover, little is known about the heterogeneity of microglial responses in different pathologic contexts that results from regional adaptations or from the progression of a disease. In this review, we focus on several key observations that illustrate the multi-faceted activities of microglia in the normal and pathologic brain.

Inflammatory priming of the substantia nigra influences the impact of later paraquat exposure: Neuroimmune sensitization of neurodegeneration

Activation of microglia along with the release of inflammatory cytokines and oxidative factors often accompanies toxin-induced degeneration of substantia nigra pars compacta (SNc) dopamine (DA) neurons. Multiple toxin exposure may synergistically influence microglial-dependent DA neuronal loss and, in fact, pre-treatment with one toxin may sensitize DA neurons to the impact of subsequent insults. Thus, we assessed whether priming SNc neurons with the inflammatory agent, lipopolysaccharide (LPS), influenced the impact of later exposure to the pesticide, paraquat, which has been reported to provoke DA loss. Indeed, LPS infusion into the SNc sensitized DA neurons to the neurodegenerative effects of a series of paraquat injections commencing 2 days later. In contrast, LPS pre-treatment actually protected against some of neurodegenerative effects of paraquat when the pesticide was administered 7 days after the endotoxin. These sensitization and de-sensitization effects were associated with altered expression of reactive microglia expressing inducible immunoproteasome subunits, as well as variations of fibroblast growth factor and a time-dependent infiltration of peripheral immune cells. Circulating levels of the inflammatory cytokines, interleukin (IL)-6, IL-2, tumor necrosis factor-alpha and interferon-gamma were also time-dependently elevated following intra-SNc LPS infusion. These data suggest that inflammatory priming may influence DA neuronal sensitivity to subsequent environmental toxins by modulating the state of glial and immune factors, and these findings may be important for neurodegenerative conditions, such as Parkinson's disease (PD).

Neuroinflammatory mechanisms in Parkinson's disease: potential environmental triggers, pathways, and targets for early therapeutic intervention

Most acute and chronic neurodegenerative conditions are accompanied by neuroinflammation; yet the exact nature of the inflammatory processes and whether they modify disease progression is not well understood. In this review, we discuss the key epidemiological, clinical, and experimental evidence implicating inflammatory processes in the progressive degeneration of the dopaminergic (DA) nigrostriatal pathway and their potential contribution to the pathophysiology of Parkinson's disease (PD). Given that interplay between genetics and environment are likely to contribute to risk for development of idiopathic PD, recent data showing interactions between products of genes linked to heritable PD that function to protect DA neurons against oxidative or proteolytic stress and inflammation pathways will be discussed. Cellular mechanisms activated or enhanced by inflammatory processes that may contribute to mitochondrial dysfunction, oxidative stress, or apoptosis of dopaminergic (DA) neurons will be reviewed, with special emphasis on tumor necrosis factor (TNF) and interleukin-1-beta (IL-1beta) signaling pathways. Epigenetic factors which have the potential to trigger neuroinflammation, including environmental exposures and age-associated chronic inflammatory conditions, will be discussed as possible 'second-hit' triggers that may affect disease onset or progression of idiopathic PD. If inflammatory processes have an active role in nigrostriatal pathway degeneration, then evidence should exist to indicate that such processes begin in the early stages of disease and that they contribute to neuronal dysfunction and/or hasten neurodegeneration of the nigrostriatal pathway. Therapeutically, if anti-inflammatory interventions can be shown to rescue nigral DA neurons from degeneration and lower PD risk, then timely use of anti-inflammatory therapies should be investigated further in well-designed clinical trials for their ability to prevent or delay the progressive loss of nigral DA neurons in genetically susceptible populations.

The complex etiology of multiple sclerosis

Multiple sclerosis is a demyelinating disease which is presumed to be a consequence of infiltrating lymphocytes autoreactive to myelin proteins. This is substantiated by several lines of clinical evidence and supported by correlative studies in preclinical models. The development of new therapeutics for MS has been guided by this perspective; however, the pathogenesis of MS has proven to be quite complex as observations exist which question the role of autoreactive lymphocytes in the etiology of MS. In addition the current immunomodulatory therapeutics do not prevent most patients from progressing into more serious forms of the disease. The development of truly transformational therapeutics for MS will likely require a broad assault that expands beyond the concept of MS being an autoimmune disease.

Neuroprotective activities of CD4+CD25+ regulatory T cells in an animal model of Parkinson's disease

Progressive loss of dopaminergic neurons in the substantia nigra pars compacta and their terminal connections in the striatum are central features in Parkinson's disease (PD). Emerging evidence supports the notion that microglia neuroinflammatory responses speed neurodegenerative events. We demonstrated previously that this can be slowed by adoptive transfer of T cells from Copolymer-1-immunized mice administered to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) recipients. The cellular basis for this neuroprotective response was the CD4+ T cell population, suggesting involvement of CD4+CD25+ regulatory T cells (Tregs), cells known to suppress immune activation and maintain immune homeostasis and tolerance. We show for the first time that adoptive transfer of CD3-activated Tregs to MPTP-intoxicated mice provides greater than 90% protection of the nigrostriatal system. The response was dose-dependent and paralleled modulation of microglial responses and up-regulation of glial cell-derived neurotrophic factor (CDNF) and TGF-beta. Interestingly, that adoptive transfer of effector T cells showed no significant neuroprotective activities. Tregs were found to mediate neuroprotection through suppression of microglial responses to stimuli, including aggregated, nitrated alpha-synuclein. Moreover, Treg-mediated suppression was also operative following removal of Tregs from culture prior to stimulation. This neuroprotection was achieved through modulation of microglial oxidative stress and inflammation. As Tregs can be modulated in vivo, these data strongly support the use of such immunomodulatory strategies to treat PD.

Molecular and cellular characterization of the age-related neuroinflammatory processes occurring in normal rat hippocampus: potential relation with the loss of somatostatin GABAergic neurons

Increased neuroinflammatory reaction is frequently observed during normal brain aging. However, a direct link between neuroinflammation and neurodegeneration during aging has not yet been clearly shown. Here, we have characterized the age-related hippocampal inflammatory processes and the potential relation with hippocampal neurodegeneration. The mRNA expression of the pro-inflammatory cytokines IL-1beta and tumor necrosis factor-alpha (TNF-alpha), and the iNOs enzyme was significantly increased in aged hippocampus. Accordingly, numerous activated microglial cells were observed in aged rats. These cells were differentially distributed along the hippocampus, being more frequently located in the hilus and the CA3 area. The mRNA expression of somatostatin, a neuropeptide expressed by some GABAergic interneurons, and the number of somatostatin-immunopositive cells decreased in aged rats. However, the number of hippocampal parvalbumin-containing GABAergic interneurons was preserved. Interestingly, in aged rats, the mRNA expression of somatostatin and IL-1beta was inversely correlated and, the decrease in the number of somatostatin-immunopositive cells was higher in the hilus of dentate gyrus than in the CA1 region. Finally, intraperitoneal chronic lipopolysaccharide (LPS) injection in young animals mimicked the age-related hippocampal inflammation as well as the decrease of somatostatin mRNA expression. Present results strongly support the neuroinflammation as a potential factor involved in the age-related degeneration of somatostatin GABAergic cells.

Divergent roles for tumor necrosis factor-alpha in the brain

Proinflammatory cytokines and chemokines have been implicated in the pathogenesis of several neurological and neurodegenerative disorders. Prominent among such factors is the pleiotropic cytokine, tumor necrosis factor (TNF)-alpha. Under normal physiological conditions, TNF-alpha orchestrates a diverse array of functions involved in immune surveillance and defense, cellular homeostasis, and protection against certain neurological insults. However, paradoxical effects of this cytokine have been observed. TNF-alpha is elicited in the brain following injury (ischemia, trauma), infection (HIV, meningitis), neurodegeneration (Alzheimer's, Parkinson's), and chemically induced neurotoxicity. The multifarious identity for this cytokine appears to be influenced by several mechanisms. Among the most prominent are the regulation of TNFalpha-induced NF-kappaB activation by adapter proteins such as TRADD and TRAF, and second, the heterogeneity of microglia and their distribution pattern across brain regions. Here, we review the differential role of TNF-alpha in response to brain injury, with emphasis on neurodegeneration, and discuss the possible mechanisms for such diverse and region-specific effects.

Systemic LPS causes chronic neuroinflammation and progressive neurodegeneration

Inflammation is implicated in the progressive nature of neurodegenerative diseases, such as Parkinson's disease, but the mechanisms are poorly understood. A single systemic lipopolysaccharide (LPS, 5 mg/kg, i.p.) or tumor necrosis factor alpha (TNFalpha, 0.25 mg/kg, i.p.) injection was administered in adult wild-type mice and in mice lacking TNFalpha receptors (TNF R1/R2(-/-)) to discern the mechanisms of inflammation transfer from the periphery to the brain and the neurodegenerative consequences. Systemic LPS administration resulted in rapid brain TNFalpha increase that remained elevated for 10 months, while peripheral TNFalpha (serum and liver) had subsided by 9 h (serum) and 1 week (liver). Systemic TNFalpha and LPS administration activated microglia and increased expression of brain pro-inflammatory factors (i.e., TNFalpha, MCP-1, IL-1beta, and NF-kappaB p65) in wild-type mice, but not in TNF R1/R2(-/-) mice. Further, LPS reduced the number of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra (SN) by 23% at 7-months post-treatment, which progressed to 47% at 10 months. Together, these data demonstrate that through TNFalpha, peripheral inflammation in adult animals can: (1) activate brain microglia to produce chronically elevated pro-inflammatory factors; (2) induce delayed and progressive loss of DA neurons in the SN. These findings provide valuable insight into the potential pathogenesis and self-propelling nature of Parkinson's disease.

The emergence of proteinase-activated receptor-2 as a novel target for the treatment of inflammation-related CNS disorders

The signalling molecules that are involved in inflammatory pathways are now thought to play a part in many disorders of the central nervous system (CNS). In common with peripheral chronic inflammatory diseases such a rheumatoid arthritis and ulcerative colitis, evidence now exists for the involvement of inflammatory cytokines, for example tumour necrosis factor (TNF) and interleukins (IL), in neurological disorders. A common factor observed with the up-regulation of these cytokines in peripheral inflammatory diseases, is the increased expression of the proteinase-activated receptor (PAR) subtype PAR-2. Indeed, recent evidence suggests that targeting PAR-2 helps reduce joint swelling observed in animal models of arthritis. So could targeting this receptor prove to be useful in treating those CNS disorders where inflammatory processes are thought to play an intrinsic role? The aim of this review is to summarize the emerging data regarding the role of PAR-2 in neuroinflammation and ischaemic injury and discuss its potential as an exciting new target for the prevention and/or treatment of CNS disorders.

Microglia-mediated neurotoxicity: uncovering the molecular mechanisms

Mounting evidence indicates that microglial activation contributes to neuronal damage in neurodegenerative diseases. Recent studies show that in response to certain environmental toxins and endogenous proteins, microglia can enter an overactivated state and release reactive oxygen species (ROS) that cause neurotoxicity. Pattern recognition receptors expressed on the microglial surface seem to be one of the primary, common pathways by which diverse toxin signals are transduced into ROS production. Overactivated microglia can be detected using imaging techniques and therefore this knowledge offers an opportunity not only for early diagnosis but, importantly, for the development of targeted anti-inflammatory therapies that might slow or halt the progression of neurodegenerative disease.

Norepinephrine: The redheaded stepchild of Parkinson's disease

Parkinson's disease (PD) affects approximately 1% of the world's aging population. Despite its prevalence and rigorous research in both humans and animal models, the etiology remains unknown. PD is most often characterized by the degeneration of dopamine (DA) neurons in the substantia nigra pars compacta (SNc), and models of PD generally attempt to mimic this deficit. However, PD is a true multisystem disorder marked by a profound but less appreciated loss of cells in the locus coeruleus (LC), which contains the major group of noradrenergic neurons in the brain. Historic and more recent experiments exploring the role of norepinephrine (NE) in PD will be analyzed in this review. First, we examine the evidence that NE is neuroprotective and that LC degeneration sensitizes DA neurons to damage. The second part of this review focuses on the potential contribution of NE loss to the behavioral symptoms associated with PD. We propose that LC loss represents a crucial turning point in PD progression and that pharmacotherapies aimed at restoring NE have important therapeutic potential.

Recapitulation of cell signaling events associated with astrogliosis using the brain slice preparation

Astroglial activation constitutes a dominant response to all types of injuries of the CNS. Despite the ubiquitous nature of this cellular reaction to neural injury, a little is known concerning the signaling mechanisms that initiate it. Recently, we demonstrated that astrocytic hypertrophy and enhanced expression of glial fibrillary acidic protein resulting from toxicant-induced neurodegeneration are linked to activation of the janus kinase (JAK)-signal transducer and activator of transcription-3 (STAT3) pathway. These observations implicate ligands at the gp130 receptor as potential upstream effectors of astrogliosis. Here we used the brain slice preparation to examine potential activators of the JAK-STAT3 pathway. Following incubation of freshly cut striatal slices in phosphate-free oxygenated buffer for up to 75 min, we found that slicing the striatum itself was a sufficient stimulus to initiate a rapid activation of the JAK-STAT3 pathway as assessed with immunoblots of pSTAT3((tyr705)) using phospho-state specific antibodies. The mRNA for the gp130 cytokines, leukemia inhibitory factor, interleukin-6 and oncostatin M or the beta-chemokine, monocyte chemoattractive protein (CCl2) also were up-regulated in the slice. Moreover, we could enhance the activation of STAT3((tyr705)) by adding exogenous cytokines to the slice and we could inhibit phosphorylation of STAT3((tyr705)) by addition of tyrosine kinase inhibitors (Lav A and AG490) or neutralizing antibodies directed against leukemia inhibitory factor or oncostatin M. These data suggest that STAT3 activation is an early event in slice-induced glial activation and establishes the brain slice preparation method as a reliable model to examine the signaling mechanisms that underlie glial activation.

Temporal mRNA profiles of inflammatory mediators in the murine 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). With the exception of a few rare familial forms of the disease, the precise molecular mechanisms underlying PD are unknown. Inflammation is a common finding in the PD brain, but due to the limitation of postmortem analysis its relationship to disease progression cannot be established. However, studies using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD have also identified inflammatory responses in the nigrostriatal pathway that precede neuronal degeneration in the SNpc. To assess the pathological relevance of these inflammatory responses and to identify candidate genes that might contribute to neuronal vulnerability, we used quantitative reverse-transcription polymerase chain reaction (qRT-PCR) to measure mRNA levels of 11 cytokine and chemokine encoding genes in the striatum of MPTP-sensitive (C57BL/6J) and MPTP-insensitive (Swiss Webster, SWR) mice following administration of MPTP. The mRNA levels of all 11 genes changed following MPTP treatment, indicating the presence of inflammatory responses in both strains. Furthermore, of the 11 genes examined only 3, interleukin 6 (Il-6), macrophage inflammatory protein 1 alpha/CC chemokine ligand 3 (Mip-1alpha/Ccl3) and macrophage inflammatory protein 1 beta/CC chemokine ligand 4 (Mip-1beta/Ccl4), were differentially regulated between C57BL/6J and SWR mice. In both mouse strains, the level of monocyte chemoattractant protein 1/CC chemokine ligand 2 (Mcp-1/Ccl2) mRNA was the first to increase following MPTP administration, and might represent a key initiating component of the inflammatory response. Using Mcp-1/Ccl2 knockout mice backcrossed onto a C57BL/6J background we found that MPTP-stimulated Mip-1alpha/Ccl3 and Mip-1beta/Ccl4 mRNA expression was significantly lower in the knockout mice; suggesting that Mcp-1/Ccl2 contributes to MPTP-enhanced expression of Mip-1alpha/Ccl3 and Mip-1beta/Ccl4. However, stereological analysis of SNpc neuronal loss in Mcp-1/Ccl2 knockout and wild-type mice showed no differences. These findings suggest that it is the ability of dopaminergic SNpc neurons to survive an inflammatory insult, rather than genetically determined differences in the inflammatory response itself, that underlie the molecular basis of MPTP resistance.

Influence of age and gender on cytokine expression in a murine model of Parkinson's disease

OBJECTIVE

The neuroinflammatory reaction has been linked with Parkinson's disease. One of the hypotheses to explain the significance of age and gender (male predominance) effects on neurodegeneration in Parkinson's disease may result from a link between these risk factors and the inflammatory processes. Here, we investigated the expression of inflammatory mediators in relation to 1-methyl-4-phenyl-1,2,3,6-tetrahydropiridine (MPTP)-induced neurodegenerative processes in nigrostriatal pathway in young and aged male and female mice.

METHODS AND RESULTS

We simultaneously assessed striatal tyrosine hydroxylase (TH) protein concentrations (Western blotting) and cytokine (TNFalpha, IFNgamma, IL-1beta, IL-6 and TGFbeta(1)) mRNA levels (RT-PCR) in young and aged (2- and 12-month-old) C57BL/6 male and female mice after 6 h, 1, 3, 7, 14, 21 days after MPTP intoxication. Western blotting analysis showed that at the early time points, males showed a greater reduction in striatal TH versus females. Additionally, in contrast to the aged mice, in young males and females the TH concentration gradually increased between the 7th and the 21st day after intoxication. The increases in TNFalpha, IL-1beta and IFNgamma after intoxication were faster in both young and aged males than females. In males (both ages), we observed an increase in TGFbeta(1) at the early time points. In contrast, in females (both ages) TGFbeta(1) was elevated at later time points. MPTP caused an increase in IL-6 in males and females, but this increase was significantly higher in females.

CONCLUSIONS

A gender and age skewing of the cytokine gene expression in the striatum after intoxication may be related to the greater susceptibility in males as well as older animals to the detrimental effects of MPTP.

Blocking soluble tumor necrosis factor signaling with dominant-negative tumor necrosis factor inhibitor attenuates loss of dopaminergic neurons in models of Parkinson's disease

The mechanisms that trigger or contribute to loss of dopaminergic (DA) neurons in Parkinson's disease (PD) remain unclear and controversial. Elevated levels of tumor necrosis factor (TNF) in CSF and postmortem brains of PD patients and animal models of PD implicate this proinflammatory cytokine in the pathophysiology of the disease; but a role for TNF in mediating loss of DA neurons in PD has not been clearly demonstrated. Here, we report that neutralization of soluble TNF (solTNF) in vivo with the engineered dominant-negative TNF compound XENP345 (a PEGylated version of the TNF variant A145R/I97T) reduced by 50% the retrograde nigral degeneration induced by a striatal injection of the oxidative neurotoxin 6-hydroxydopamine (6-OHDA). XENP345 was neuroprotective only when infused into the nigra, not the striatum. XENP345/6-OHDA rats displayed attenuated amphetamine-induced rotational behavior, indicating preservation of striatal dopamine levels. Similar protective effects were observed with chronic in vivo coinfusion of XENP345 with bacterial lipopolysaccharide (LPS) into the substantia nigra, confirming a role for solTNF-dependent neuroinflammation in nigral degeneration. In embryonic rat midbrain neuron/glia cell cultures exposed to LPS, even delayed administration of XENP345 prevented selective degeneration of DA neurons despite sustained microglia activation and secretion of solTNF. XENP345 also attenuated 6-OHDA-induced DA neuron toxicity in vitro. Collectively, our data demonstrate a role for TNF in vitro and in vivo in two models of PD, and raise the possibility that delaying the progressive degeneration of the nigrostriatal pathway in humans is therapeutically feasible with agents capable of blocking solTNF in early stages of PD.

Modulation of microglial pro-inflammatory and neurotoxic activity for the treatment of Parkinson's disease

Parkinson's disease (PD) is a debilitating movement disorder resulting from a progressive degeneration of the nigrostriatal dopaminergic pathway and depletion of neurotransmitter dopamine in the striatum. Molecular cloning studies have identified nearly a dozen genes or loci that are associated with small clusters of mostly early onset and genetic forms of PD. The etiology of the vast majority of PD cases remains unknown, and the precise molecular and biochemical processes governing the selective and progressive degeneration of the nigrostriatal dopaminergic pathway are poorly understood. Current drug therapies for PD are symptomatic and appear to bear little effect on the progressive neurodegenerative process. Studies of postmortem PD brains and various cellular and animal models of PD in the last 2 decades strongly suggest that the generation of pro-inflammatory and neurotoxic factors by the resident brain immune cells, microglia, plays a prominent role in mediating the progressive neurodegenerative process. This review discusses literature supporting the possibility of modulating the activity of microglia as a neuroprotective strategy for the treatment of PD.

Neurotoxins and neurotoxicity mechanisms. an overview

Neurotoxins represent unique chemical tools, providing a means to 1) gain insight into cellular mechanisms of apopotosis and necrosis, 2) achieve a morphological template for studies otherwise unattainable, 3) specifically produce a singular phenotype of denervation, and 4) provide the starting point to delve into processes and mechanisms of nerve regeneration and sprouting. There are many other notable uses of neurotoxins in neuroscience research, and ever more being discovered each year. The objective of this review paper is to highlight the broad areas of neuroscience in which neurotoxins and neurotoxicity mechanism come into play. This shifts the focus away from neurotoxins per se, and onto the major problems under study today. Neurotoxins broadly defined are used to explore neurodegenerative disorders, psychiatric disorders and substance use disorders. Neurotoxic mechanisms relating to protein aggregates are indigenous to Alzheimer disease, Parkinson's disease. NeuroAIDS is a disorder in which microglia and macrophages have enormous import. The gap between the immune system and nervous system has been bridged, as neuroinflammation is now considered to be part of the neurodegenerative process. Related mechanisms now arise in the process of neurogenesis. Accordingly, the entire spectrum of neuroscience is within the purview of neurotoxins and neurotoxicity mechanisms. Highlights on discoveries in the areas noted, and on selective neurotoxins, are included, mainly from the past 2 to 3 years.