Differential effects of l-DOPA on monoamine metabolism, cell survival and glutathione production in midbrain neuronal-enriched cultures from parkin knockout and wild-type mice

Parkin-deficient rats are resistant to neurotoxicity of chronic high-dose methamphetamine

Methamphetamine (METH) is a highly addictive and powerful central nervous system psychostimulant with no FDA-approved pharmacotherapy. Parkin is a neuroprotective protein and its loss of function contributes to Parkinson's disease. This study used 3-month-old homozygous parkin knockout (PKO) rats to determine whether loss of parkin protein potentiates neurotoxicity of chronic METH to the nigrostriatal dopamine pathway. PKO rats were chronically treated with 10 mg/kg METH for 10 consecutive days and assessed for neurotoxicity markers in the striatum on the 5th and 10th day of withdrawal from METH. The PKO rats showed higher METH-induced hyperthermia; however, they did not display augmented deficits in dopaminergic and serotonergic neurotoxicity markers, astrocyte activation or decreased mitochondrial enzyme levels as compared to wild-type (WT) rats. Interestingly, saline-treated PKO rats had lower levels of dopamine (DA) as well as mitochondrial complex I and II levels while having increased basal levels of glial fibrillary acidic protein (GFAP), a marker of gliosis. These results indicate PKO display a certain resistance to METH neurotoxicity, possibly mediated by lowered DA levels and downregulated mitochondria.

Structure-Based Specific Detection and Inhibition of Monoamine Oxidases and Their Applications in Central Nervous System Diseases

Monoamine oxidases (MAOs) are the enzymes that catalyze the oxidation of monoamines, such as dopamine, norepinephrine, and serotonin, which serve as key neurotransmitters in the central nervous system (CNS). MAOs play important roles in maintaining the homeostasis of monoamines, and the aberrant expression or activation of MAOs underlies the pathogenesis of monoamine neurotransmitter disorders, including neuropsychiatric and neurodegenerative diseases. Clearly, detecting and inhibiting the activities of MAOs is of great value for the diagnosis and therapeutics of these diseases. Accordingly, many specific detection probes and inhibitors have been developed and substantially contributed to basic and clinical studies of these diseases. In this review, progress in the detecting and inhibiting of MAOs and their applications in mechanism exploration and treatment of neurotransmitter-related disorders is summarized. Notably, how the detection probes and inhibitors of MAOs were developed has been specifically addressed. It is hoped that this review will benefit the design of more effective and sensitive probes and inhibitors for MAOs, and eventually the treatment of monoamine neurotransmitter disorders.

Characterization of Dopaminergic System in the Striatum of Young Adult Park2-/- Knockout Rats

Mutations in parkin gene (Park2) are linked to early-onset autosomal recessive Parkinson's disease (PD) and young-onset sporadic PD. Park2 knockout (PKO) rodents; however, do not display neurodegeneration of the nigrostriatal pathway, suggesting age-dependent compensatory changes. Our goal was to examine dopaminergic (DAergic) system in the striatum of 2 month-old PKO rats in order to characterize compensatory mechanisms that may have occurred within the system. The striata form wild type (WT) and PKO Long Evans male rats were assessed for the levels of DAergic markers, for monoamine oxidase (MAO) A and B activities and levels, and for the levels of their respective preferred substrates, serotonin (5-HT) and ß-phenylethylamine (ß-PEA). The PKO rats displayed lower activities of MAOs and higher levels of ß-PEA in the striatum than their WT counterparts. Decreased levels of ß-PEA receptor, trace amine-associated receptor 1 (TAAR-1), and postsynaptic DA D2 (D2L) receptor accompanied these alterations. Drug-naive PKO rats displayed normal locomotor activity; however, they displayed decreased locomotor response to a low dose of psychostimulant methamphetamine, suggesting altered DAergic neurotransmission in the striatum when challenged with an indirect agonist. Altogether, our findings suggest that 2 month-old PKO male rats have altered DAergic and trace aminergic signaling.

Modulation of monoamine oxidase (MAO) expression in neuropsychiatric disorders: genetic and environmental factors involved in type A MAO expression

Monoamine oxidase types A and B (MAO-A, MAO-B) regulate the levels of monoamine neurotransmitters in the brain, and their dysfunction may be involved in the pathogenesis and influence the clinical phenotypes of neuropsychiatric disorders. Reversible MAO-A inhibitors, such as moclobemide and befloxatone, are currently employed in the treatment of emotional disorders by inhibiting the enzymatic degradation of dopamine, serotonin and norepinephrine in the central nervous system (CNS). It has been suggested that the irreversible MAO-B inhibitors selegiline and rasagiline exert a neuroprotective effect in Parkinson's and Alzheimer's diseases. This effect, however, is not related to their inhibition of MAO activity; in animal and cellular models, selegiline and rasagiline protect neuronal cells through their anti-apoptotic activity and induction of pro-survival genes. There is increasing evidence that MAO-A activity, but not that of MAO-B, is implicated in the pathophysiology of neurodegenerative disorders, but also in gene induction by MAO-B inhibitors; on the other hand, selegiline and rasagiline increase MAO-A mRNA, protein, and enzyme activity levels. Taken together, these results suggest that each MAO subtype exerts effects that modulate the expression and activity of the other isoenzyme. The roles of MAO-A and -B in the CNS should therefore be re-evaluated with respect to the "type-specificity" of their inhibitors, which may not be unconditional during chronic treatment. Mao-a expression, in particular, may be implicated in pathogenesis and phenotypes in neuropsychiatric disorders. MAO-A expression is modified by mao polymorphisms affecting its transcriptional efficiency, as well as by mutations and polymorphism of parkin, Sirt1, FOXO, microRNA, presenilin-1, and other regulatory proteins. In addition, childhood maltreatment has been shown to have an impact upon adolescent social behavior in children with mao-a polymorphisms of low transcriptional activity. Low MAO-A activity may increase the levels of serotonin and norepinephrine, resulting in disturbed neurotransmitter system development and behavior. This review discusses genetic and environmental factors involved in the regulation of MAO-A expression, in the contexts of neuropsychiatric function and of the regulation of neuronal survival and death.

Catecholamine autotoxicity. Implications for pharmacology and therapeutics of Parkinson disease and related disorders

Several neurodegenerative diseases involve loss of catecholamine neurons-Parkinson disease is a prototypical example. Catecholamine neurons are rare in the nervous system, and why they are vulnerable in PD and related disorders has been mysterious. Accumulating evidence supports the concept of "autotoxicity"-inherent cytotoxicity of catecholamines and their metabolites in the cells in which they are produced. According to the "catecholaldehyde hypothesis" for the pathogenesis of Parkinson disease, long-term increased build-up of 3,4-dihydroxyphenylacetaldehyde (DOPAL), the catecholaldehyde metabolite of dopamine, causes or contributes to the eventual death of dopaminergic neurons. Lewy bodies, a neuropathologic hallmark of PD, contain precipitated alpha-synuclein. Bases for the tendency of alpha-synuclein to precipitate in the cytoplasm of catecholaminergic neurons have also been mysterious. Since DOPAL potently oligomerizes and aggregates alpha-synuclein, the catecholaldehyde hypothesis provides a link between alpha-synucleinopathy and catecholamine neuron loss in Lewy body diseases. The concept developed here is that DOPAL and alpha-synuclein are nodes in a complex nexus of interacting homeostatic systems. Dysfunctions of several processes, including decreased vesicular sequestration of cytoplasmic catecholamines, decreased aldehyde dehydrogenase activity, and oligomerization of alpha-synuclein, lead to conversion from the stability afforded by negative feedback regulation to the instability, degeneration, and system failure caused by induction of positive feedback loops. These dysfunctions result from diverse combinations of genetic predispositions, environmental exposures, stress, and time. The notion of catecholamine autotoxicity has several implications for treatment, disease modification, and prevention. Conversely, disease modification clinical trials would provide key tests of the catecholaldehyde hypothesis.

Trehalose improves human fibroblast deficits in a new CHIP-mutation related ataxia

In this work we investigate the role of CHIP in a new CHIP-mutation related ataxia and the therapeutic potential of trehalose. The patient's fibroblasts with a new form of hereditary ataxia, related to STUB1 gene (CHIP) mutations, and three age and sex-matched controls were treated with epoxomicin and trehalose. The effects on cell death, protein misfolding and proteostasis were evaluated. Recent studies have revealed that mutations in STUB-1 gene lead to a growing list of molecular defects as deregulation of protein quality, inhibition of proteasome, cell death, decreased autophagy and alteration in CHIP and HSP70 levels. In this CHIP-mutant patient fibroblasts the inhibition of proteasome with epoxomicin induced severe pathophysiological age-associated changes, cell death and protein ubiquitination. Additionally, treatment with epoxomicin produced a dose-dependent increase in the number of cleaved caspase-3 positive cells. However, co-treatment with trehalose, a disaccharide of glucose present in a wide variety of organisms and known as a autophagy enhancer, reduced these pathological events. Trehalose application also increased CHIP and HSP70 expression and GSH free radical levels. Furthermore, trehalose augmented macro and chaperone mediated autophagy (CMA), rising the levels of LC3, LAMP2, CD63 and increasing the expression of Beclin-1 and Atg5-Atg12. Trehalose treatment in addition increased the percentage of immunoreactive cells to HSC70 and LAMP2 and reduced the autophagic substrate, p62. Although this is an individual case based on only one patient and the statistical comparisons are not valid between controls and patient, the low variability among controls and the obvious differences with this patient allow us to conclude that trehalose, through its autophagy activation capacity, anti-aggregation properties, anti-oxidative effects and lack of toxicity, could be very promising for the treatment of CHIP-mutation related ataxia, and possibly a wide spectrum of neurodegenerative disorders related to protein disconformation.

Trehalose reverses cell malfunction in fibroblasts from normal and Huntington's disease patients caused by proteosome inhibition

Huntington's disease (HD) is a neurodegenerative disorder characterized by progressive motor, cognitive and psychiatric deficits, associated with predominant loss of striatal neurons and is caused by polyglutamine expansion in the huntingtin protein. Mutant huntingtin protein and its fragments are resistant to protein degradation and produce a blockade of the ubiquitin proteasome system (UPS). In HD models, the proteasome inhibitor epoxomicin aggravates protein accumulation and the inductor of autophagy, trehalose, diminishes it. We have investigated the effects of epoxomicin and trehalose in skin fibroblasts of control and HD patients. Untreated HD fibroblasts have increased the levels of ubiquitinized proteins and higher levels of reactive oxygen species (ROS), huntingtin and the autophagy marker LAMP2A. Baseline replication rates were higher in HD than in controls fibroblasts but that was reverted after 12 passages. Epoxomicin increases the activated caspase-3, HSP70, huntingtin, ubiquitinated proteins and ROS levels in both HD and controls. Treatment with trehalose counteracts the increase in ROS, ubiquitinated proteins, huntingtin and activated caspase-3 levels induced by epoxomicin, and also increases the LC3 levels more in HD fibroblast than controls. These results suggest that trehalose could revert protein processing abnormalities in patients with Huntington's Disease.

Striatal infusion of glial conditioned medium diminishes huntingtin pathology in r6/1 mice

Huntington's disease is a neurodegenerative disorder caused by an expansion of CAG repeats in the huntingtin gene which produces widespread neuronal and glial pathology. We here investigated the possible therapeutic role of glia or glial products in Huntington's disease using striatal glial conditioned medium (GCM) from fetus mice (E16) continuously infused for 15 and 30 days with osmotic minipumps into the left striatum of R6/1 mice. Animals infused with GCM had significantly less huntingtin inclusions in the ipsilateral cerebral cortex and in the ipsilateral and contralateral striata than mice infused with cerebrospinal fluid. The numbers of DARPP-32 and TH positive neurons were also greater in the ipsilateral but not contralateral striata and substantia nigra, respectively, suggesting a neuroprotective effect of GCM on efferent striatal and nigro-striatal dopamine neurons. GCM increases activity of the autophagic pathway, as shown by the reduction of autophagic substrate, p-62, and the augmentation of LC3 II, Beclin-1 and LAMP-2 protein levels, direct markers of autophagy, in GCM infused mice. GCM also increases BDNF levels. These results suggest that CGM should be further explored as a putative neuroprotective agent in Huntington's disease.

Epigallocatechin gallate attenuates L-DOPA-induced apoptosis in rat PC12 cells

In this study, the protective effects of EGCG on L-3,4-dihydroxyphenylalanine (L-DOPA)-induced oxidative cell death in catecholaminergic PC12 cells, the in vitro model of Parkinson's disease, were investigated. Treatment with L-DOPA at concentrations higher than 150 µM caused cytotoxicity in PC12 cells, as determined using the 3-(4,5-dimetylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and flow cytometry detection. The apoptotic ratio was similar in cells treated with 100 µM EGCG plus 150 µM L-DOPA (5.02%) and the control (0.96%) (P > 0.05), and was lower than that of cells treated with L-DOPA only (32.24%, P < 0.05). The generation level of ROS (% of control) in cells treated with EGCG plus L-DOPA was lower than that in cells treated with L-DOPA only (123.90% vs 272.32%, P < 0.05). The optical density in production of TBARS in cells treated with L-DOPA only was higher than that in the control (0.27 ± 0.05 vs 0.08 ± 0.04, P < 0.05), and in cells treated with EGCG only (0.14 ± 0.02, P < 0.05), and EGCG plus L-DOPA (0.13 ± 0.02, P < 0.05). The intracellular level of GSH in cells treated with EGCG plus L-DOPA was higher than that in cells treated with L-DOPA only (233.25 ± 16.44 vs 119.23 ± 10.25, P < 0.05). These results suggest that EGCG protects against L-DOPA-induced oxidative apoptosis in PC12 cells, and might be a potent neuroprotective agent.

Protection by glia-conditioned medium in a cell model of Huntington disease

The physiological role of huntingtin and the pathogenic mechanisms that produce the disease are unknown. Mutant huntingtin changes its normal localization and produces cytoplasmic and intranuclear inclusions, changes gene transcription, alters synaptic transmission, impairs mitochondrial activity and activates caspases and other pro-apoptotic molecules, promotes excitotoxicity, energy deficits, synthesis and release reduction of neurotrophic factors and oxidative stress. Previous studies confirm that the mutant huntingtin difficult neurotrophic function of astrocytes leading to neuronal dysfunction in Huntington’s disease. Our objective was to study the neuroprotective potential role of glia-conditioned medium (GCM) in an in vitro model of Huntington’s disease. We used conditionally-immortalized striatal neuronal progenitor cell lines (STHdhQ7/Q7 and STHdhQ111/Q111) expressing endogenous levels of normal and mutant huntingtin with 7 and 111 glutamines, respectively. We studied the protection of fetal and postnatal glia conditioned medium (GCM) on H2O2 (2 µM), glutamate (5 mM) and 3-nitropropionic acid (2.5 mM) related toxicity. We also compared the neuroprotective effects of GCM versus that of the growth factors bFGF, BDNF and GDNF. Fetal GCM protects from every toxin, reducing the cell death and increasing the cell survival. Fetal GCM reduces the caspases fragmentation of the protein PARP, the expression of chaperone Hsp70 and the accumulation of ROS and polyubiquitinated proteins. In addition, in Q111 striatal cells treated with H2O2 (2 µM) for 24 hours, the intracellular GSH levels are higher in the presence of GCM. Notably, the 13-day and 2-month postnatal GCM, totally protects from H2O2 induced cell death in mutant striatal cells. GCM neuroprotective effects are more potent than those of the already identified neurotrophic factors. We conclude that GCM protects Q111 cells from neuronal neurotoxins and the effects of GCM are more potent than those of any known neurotrophic factor. GCM may contain new and more potent, as yet unidentified, neurotrophic molecules, potentially useful in patients with Huntington’s disease.

What genetics tells us about the causes and mechanisms of Parkinson's disease

Parkinson's disease (PD) is a common motor disorder of mysterious etiology. It is due to the progressive degeneration of the dopaminergic neurons of the substantia nigra and is accompanied by the appearance of intraneuronal inclusions enriched in α-synuclein, the Lewy bodies. It is becoming increasingly clear that genetic factors contribute to its complex pathogenesis. Over the past decade, the genetic basis of rare PD forms with Mendelian inheritance, representing no more than 10% of the cases, has been investigated. More than 16 loci and 11 associated genes have been identified so far; genome-wide association studies have provided convincing evidence that polymorphic variants in these genes contribute to sporadic PD. The knowledge acquired of the functions of their protein products has revealed pathways of neurodegeneration that may be shared between inherited and sporadic PD. An impressive set of data in different model systems strongly suggest that mitochondrial dysfunction plays a central role in clinically similar, early-onset autosomal recessive PD forms caused by parkin and PINK1, and possibly DJ-1 gene mutations. In contrast, α-synuclein accumulation in Lewy bodies defines a spectrum of disorders ranging from typical late-onset PD to PD dementia and including sporadic and autosomal dominant PD forms due to mutations in SCNA and LRRK2. However, the pathological role of Lewy bodies remains uncertain, as they may or may not be present in PD forms with one and the same LRRK2 mutation. Impairment of autophagy-based protein/organelle degradation pathways is emerging as a possible unifying but still fragile pathogenic scenario in PD. Strengthening these discoveries and finding other convergence points by identifying new genes responsible for Mendelian forms of PD and exploring their functions and relationships are the main challenges of the next decade. It is also the way to follow to open new promising avenues of neuroprotective treatment for this devastating disorder.

L-DOPA: a scapegoat for accelerated neurodegeneration in Parkinson's disease?

There is consensus that amelioration of the motor symptoms of Parkinson's disease is most effective with L-DOPA (levodopa). However, this necessary therapeutic step is biased by an enduring belief that L-DOPA is toxic to the remaining substantia nigra dopaminergic neurons by itself, or by specific metabolites such as dopamine. The concept of L-DOPA toxicity originated from pre-clinical studies conducted mainly in cell culture, demonstrating that L-DOPA or its derivatives damage dopaminergic neurons due to oxidative stress and other mechanisms. However, the in vitro data remain controversial as some studies showed neuroprotective, rather than toxic action of the drug. The relevance of this debate needs to be considered in the context of the studies conducted on animals and in clinical trials that do not provide convincing evidence for L-DOPA toxicity in vivo. This review presents the current views on the pathophysiology of Parkinson's disease, focusing on mitochondrial dysfunction and oxidative/proteolytic stress, the factors that can be affected by L-DOPA or its metabolites. We then critically discuss the evidence supporting the two opposing views on the effects of L-DOPA in vitro, as well as the animal and human data. We also address the problem of inadequate experimental models used in these studies. L-DOPA remains the symptomatic 'hero' of Parkinson's disease. Whether it contributes to degeneration of nigral dopaminergic neurons, or is a 'scapegoat' for explaining undesirable or unexpected effects of the treatment, remains a hotly debated topic.

Parkin degrades estrogen-related receptors to limit the expression of monoamine oxidases

Parkin, whose mutations cause Parkinson disease (PD), controls oxidative stress by limiting the expression of monoamine oxidases (MAO)--mitochondrial enzymes responsible for the oxidative de-amination of dopamine. Here, we show that parkin performed this function by increasing the ubiquitination and degradation of estrogen-related receptors (ERR), orphan nuclear receptors that play critical roles in the transcription regulation of many nuclear-encoded mitochondrial proteins. All three ERRs (α, β and γ) increased the transcription of MAOs A and B; the effects were abolished by parkin, but not by its PD-linked mutants. Parkin bound to ERRs and increased their ubiquitination and degradation. In fibroblasts from PD patients with parkin mutations or brain slices from parkin knockout mice, degradation of ERRs was significantly attenuated. The results reveal the molecular mechanism by which parkin suppresses the transcription of MAOs to control oxidative stress induced by dopamine oxidation.

Trehalose ameliorates dopaminergic and tau pathology in parkin deleted/tau overexpressing mice through autophagy activation

Tauopathies are neurodegenerative diseases, sporadic or familial, mainly characterized by dementia and parkinsonism associated to atrophy of the frontotemporal cortex and the basal ganglia, with deposition of abnormal tau in brain. Hereditary tauopathies are related with mutations of the tau gene. Up to the present, these diseases have not been helped by any disease-modifying treatment, and patients die a few years after the onset of symptoms. We have developed and characterized a mouse model of tauopathy with parkinsonism, overexpressing human mutated tau protein with deletion of parkin (PK(-/-)/Tau(VLW)). At 3 months of age, these mice present abnormal dopamine-related behavior, severe dropout of dopamine neurons in the ventral midbrain, reduced dopamine levels in the striatum and abundant phosphorylated tau-positive neuritic plaques, neurofibrillary tangles, astrogliosis, and, at 12 months old, plaques of murine beta-amyloid in the hippocampus. Trehalose is a natural disaccharide that increases the removal of abnormal proteins through enhancement of autophagy. In this work, we tested if 1% trehalose in the drinking water reverts the PK(-/-)/Tau(VLW) phenotype. The treatment with trehalose of 3-month-old PK(-/-)/Tau(VLW) mice for 2.5 months reverted the dropout of dopamine neurons, which takes place in the ventral midbrain of vehicle treated PK(-/-)/Tau(VLW) and the reduced dopamine-related proteins levels in the midbrain and striatum. The number of phosphorylated tau-positive neuritic plaques and the levels of phosphorylated tau decreased, as well as astrogliosis in brain regions. The autophagy markers in the brain, the autophagic vacuoles isolated from the liver, and the electron microscopy data indicate that these effects of trehalose are mediated by autophagy. The treatment with trehalose for 4 months of 3-month-old PK(-/-)/Tau(VLW) mice maintained the amelioration of the tau pathology and astrogliosis but failed to revert DA-related pathology in the striatum. Furthermore, the 3-week treatment with trehalose of 14-month-old PK(-/-)/Tau(VLW) mice, at the limit of their life expectancy, improved the motor behavior and anxiety of these animals, and reduced their levels of phosphorylated tau and the number of murine beta-amyloid plaques. Trehalose is neuroprotective in this model of tauopathy. Since trehalose is free of toxic effects at high concentrations, this study opens the way for clinical studies of the effects of trehalose in human tauopathies.

Parkin deficiency increases the resistance of midbrain neurons and glia to mild proteasome inhibition: the role of autophagy and glutathione homeostasis

Parkin mutations in humans produce parkinsonism whose pathogenesis is related to impaired protein degradation, increased free radicals and abnormal neurotransmitter release. In this study, we have investigated whether partial proteasomal inhibition by epoxomicin, an ubiquitin proteasomal system (UPS) irreversible inhibitor, further aggravates the cellular effects of parkin suppression in midbrain neurons and glia. We observed that parkin null (PK-KO) midbrain neuronal cultures are resistant to epoxomicin-induced cell death. This resistance is due to increased GSH and DJ-1 protein levels in PK-KO mice. The treatment with epoxomicin increases, in wild type (WT) cultures, the pro-apoptotic Bax/Bcl-2 ratio, the phosphorylation of tau, and the levels of chaperones heat-shock protein 70 and C-terminal Hsc-interacting protein, but none of these effects took place in epoxomicin-treated PK-KO cultures. Poly-ubiquitinated proteins increased more in WT than in PK-KO-treated neuronal cultures. Parkin accumulated in WT neuronal cultures treated with epoxomicin. Markers of autophagy, such as LC3II/I, were increased in naïve PK-KO cultures, and further increased after treatment with epoxomicin, implying that the blockade of the proteasome in PK-KO neurons triggers the enhancement of autophagy. The treatment with l-buthionine-S,R-sulfoximine and the inhibition of autophagy, however, reverted the increase resistance to epoxomicin of the PK-KO cultures. We also found that PK-KO glial cells, stressed by growth in defined medium and depleted of GSH, were more susceptible to epoxomicin induced cell death than WT glia treated similarly. This susceptibility was linked to reduced GSH levels and less heat-shock protein 70 response, and to activation of p-serine/threonine kinase protein signaling pathway as well as to increased poly-ubiquitinated proteins. These data suggest that mild UPS inhibition is compensated by other mechanisms in PK-KO midbrain neurons. However the depletion of GSH, as happens in stressed glia, suppresses the protection against UPS inhibition-induced cell death. Furthermore, GSH inhibition regulated differentially UPS activity and in old PK-KO mice, which have depletion of GSH, UPS activity is decreased in comparison with that of old-WT.

NP7 protects from cell death induced by oxidative stress in neuronal and glial midbrain cultures from parkin null mice

Parkin mutations produce Parkinson's disease (PD) in humans and nigrostriatal dopamine lesions related to increased free radicals in mice. We examined the effects of NP7, a synthetic, marine derived, free radical scavenger which enters the brain, on H(2)O(2) toxicity in cultured neurons and glia from wild-type (WT) and parkin null mice (PK-KO). NP7, 5-10 microM, prevented the H(2)O(2) induced apoptosis and necrosis of midbrain neuronal and glial cultures from WT and PK-KO mice. NP7 suppressed microglial activation and the H(2)O(2) induced drop-out of dopamine neurons(.) Furthermore, NP7 prevented the increased phosphorylation of ERK and AKT induced by H(2)O(2). NP7 may be a promising neuroprotector against oxidative stress in PD.

Glial Cells as Players in Parkinsonism: The “Good,” the “Bad,” and the “Mysterious” Glia

The role of glia in Parkinson's disease (PD) is very interesting because it may open new therapeutic strategies in this disease. Traditionally it has been considered that astrocytes and microglia play different roles in PD: Astroglia are considered the “good” glia and have traditionally been supposed to be neuroprotective due to their capacity to quench free radicals and secrete neurotrophic factors, whereas microglia, considered the “bad” glia, are thought to play a critical role in neuroinflammation. The proportion of astrocytes surrounding dopamine (DA) neurons in the substantia nigra, the target nucleus for neurodegeneration in PD, is the lowest for any brain area, suggesting that DA neurons are more vulnerable in terms of glial support than any neuron in other brain areas. Astrocytes are critical in the modulation of the neurotoxic effects of many toxins that induce experimental parkinsonism and they produce substances in vitro that could modify the effects of L-DOPA from neurotoxic to neurotrophic. There is a great interest in the role of inflammation in PD, and in the brains of these patients there is evidence for microglial production of cytokines and other substances that could be harmful to neurons, suggesting that microglia of the substantia nigra could be actively involved, primarily or secondarily, in the neurodegeneration process. There is, however, evidence in favor of the role of neurotoxic diffusible signals from microglia to DA neurons. More recently a third glial player, oligodendroglia, has been implicated in the pathogenesis of PD. Oligodendroglia play a key role in myelination of the nervous system. Recent neuropathological studies suggested that the nigrostriatal dopamine neurons, which were considered classically as the primary target for neurodegeneration in PD, degenerate at later stages than other neurons with poor myelination. Therefore, the role of oligodendroglia, which also secrete neurotrophic factors, has entered the center of interest of neuroscientists.

Gender differences and estrogen effects in parkin null mice

Estrogens are considered neurotrophic for dopamine neurons. Parkinson's disease is more frequent in males than in females, and more prevalent in females with short reproductive life. Estrogens are neuroprotective against neurotoxic agents for dopamine neurons in vivo and in vitro. Here, we have investigated the role of estrogens in wild-type (WT) and parkin null mice (PK-/-). WT mice present sexual dimorphisms in neuroprotective mechanisms (Bcl-2/Bax, chaperones, and GSH), but some of these inter-sex differences disappear in PK-/-. Tyrosine hydroxylase (TH) protein and TH+ cells decreased earlier and more severely in female than in male PK-/- mice. Neuronal cultures from midbrain of WT and PK-/- mice were treated with estradiol from 10 min to 48 h. Short-term treatments activated the mitogen-activated protein kinase pathway of WT and PK-/- neurons and the phosphatidylinositol 3'-kinase/AKT/glycogen synthase kinase-3 pathway of WT but not of PK-/- cultures. Long-term treatments with estradiol increased the number of TH+ neurons, the TH expression, and the extension of neurites, and decreased the level of apoptosis, the expression of glial fibrillary acidic protein, and the number of microglial cells in WT but not in PK-/- cultures. The levels of estrogen receptor-alpha were elevated in midbrain cultures and in the striatum of adult PK-/- male mice, suggesting that suppression of parkin changes the estrogen receptor-alpha turnover. From our data, it appears that parkin participates in the cellular estrogen response which could be of interest in the management of parkin-related Parkinson's disease patients.

Glial dysfunction in parkin null mice: effects of aging

Parkin mutations in humans produce parkinsonism whose pathogenesis is related to impaired protein degradation, increased free radicals, and abnormal neurotransmitter release. The role of glia in parkin deficiency is little known. We cultured midbrain glia from wild-type (WT) and parkin knock-out (PK-KO) mice. After 18-20 d in vitro, PK-KO glial cultures had less astrocytes, more microglia, reduced proliferation, and increased proapoptotic protein expression. PK-KO glia had greater levels of intracellular glutathione (GSH), increased mRNA expression of the GSH-synthesizing enzyme gamma-glutamylcysteine synthetase, and greater glutathione S-transferase and lower glutathione peroxidase activities than WT. The reverse happened in glia cultured in serum-free defined medium (EF12) or in old cultures. PK-KO glia was more susceptible than WT to transference to EF12 or neurotoxins (1-methyl-4-phenylpyridinium, blockers of GSH synthesis or catalase, inhibitors of extracellular signal-regulated kinase 1/2 and phosphatidylinositol 3 kinases), aging of the culture, or combination of these insults. PK-KO glia was less susceptible than WT to Fe2+ plus H2O2 and less responsive to protection by deferoxamine. Old WT glia increased the expression of heat shock protein 70, but PK-KO did not. Glia conditioned medium (GCM) from PK-KO was less neuroprotective and had lower levels of GSH than WT. GCM from WT increased the levels of dopamine markers in midbrain neuronal cultures transferred to EF12 more efficiently than GCM from PK-KO, and the difference was corrected by supplementation with GSH. PK-KO-GCM was a less powerful suppressor of apoptosis and microglia in neuronal cultures. Our data prove that abnormal glial function is critical in parkin mutations, and its role increases with aging.

Preferential vulnerability of mesencephalic dopamine neurons to glutamate transporter dysfunction

Nigral depletion of the main brain antioxidant GSH is the earliest biochemical event involved in Parkinson's disease pathogenesis. Its causes are completely unknown but increasing number of evidence suggests that glutamate transporters [excitatory amino acid transporters (EAATs)] are the main route by which GSH precursors may enter the cell. In this study, we report that dopamine (DA) neurons, which express the excitatory amino acid carrier 1, are preferentially affected by EAAT dysfunction when compared with non-DA neurons. In rat embryonic mesencephalic cultures, l-trans-pyrrolidine-2,4-dicarboxylate, a substrate inhibitor of EAATs, is directly and preferentially toxic for DA neurons by decreasing the availability of GSH precursors and lowering their resistance threshold to glutamate excitotoxicity through NMDA-receptors. In adult rat, acute intranigral injection of l-trans-pyrrolidine-2,4-dicarboxylate induces a large regionally selective and dose-dependent loss of DA neurons and alpha-synuclein aggregate formation. These data highlight for the first time the importance of excitatory amino acid carrier 1 function for the maintenance of antioxidant defense in DA neurons and suggest its dysfunction as a candidate mechanism for the selective death of DA neurons such as occurring in Parkinson's disease.

Colocalization of somatostatin receptor subtypes (SSTR1-5) with somatostatin, NADPH-diaphorase (NADPH-d), and tyrosine hydroxylase in the rat hypothalamus

The hypothalamus is a major site of somatostatin (SST) production and action. SST is synthesized in several hypothalamic nuclei and involved in a variety of functions. Using SST receptor (SSTR)-specific antibodies, we localized SSTR subtypes in the rat hypothalamus. In addition, we also demonstrated SSTRs colocalization with SST, NADPH-diaphorase (NADPH-d), and tyrosine hydroxylase (TH). SSTR1 is strongly localized in neurons in all major hypothalamic nuclei as well as in nerve fibers in the zona externa of the median eminence and the ependyma of the third ventricle. SSTR2 is also well expressed in most regions but with a relatively lower abundance in comparison to SSTR1. In contrast, SSTR3 is localized primarily in the paraventricular nucleus, dorsomedial hypothalamic nucleus, arcuate nucleus, and median eminence. SSTR4-like immunoreactivity is mainly confined to the arcuate nucleus, ventromedial hypothalamic nucleus, median eminence, and ependymal cells of third ventricle, with the rare SSTR4-positive neuron in the paraventricular nucleus. SSTR5 is the least expressed subtype occurring only in few cells in the inner layer of the median eminence. Overall, SSTR1 is the predominant subtype, followed by SSTR2, 4, 3, and 5. Combined immunofluorescence, immunocytochemistry, and histochemistry were used to demonstrate SSTRs colocalization with SST, TH, and NADPH-d. SSTRs colocalization with SST, TH, and NADPH-d displays in a region and receptor specificity. Colocalization of SST and NADPH-d with SSTRs in hypothalamic regions was similar, suggesting that SST and NADPH-d producing cells are same. In contrast, TH was selectively coexpressed with SSTRs in the hypothalamus in a receptor-specific manner. Taken together, these data suggest that SSTRs may interact with NADPH-d and TH to exert a physiological role in concert within the hypothalamus.

Mortality, oxidative stress and tau accumulation during ageing in parkin null mice

Young parkin null (pk-/-) mice have subtle abnormalities of behaviour, dopamine (DA) neurotransmission and free radical production, but no massive loss of DA neurons. We investigated whether these findings are maintained while ageing. Pk-/- mice have reduced life span and age-related reduced exploratory behaviour, abnormal walking and posture, and behaviours similar to those of early Parkinson's disease (PD), reduced number of nigrostriatal DA neurons and proapoptotic shifts in the survival/death proteins in midbrain and striatum. Contrary to young pk-/- animals 24-month-old pk-/- mice do not have compensatory elevation of GSH in striatum, glutathione reductase (GR) and glutathione peroxidase (GPx) activities are increased and catalase unchanged. Aged pk-/- mice accumulate high levels of tau and fail to up-regulate CHIP and HSP70. Our results suggest that aged pk-/- mice lack of the compensatory mechanisms that maintain a relatively normal DA function in early adulthood. This study could help to explain the effects of ageing in patients with genetic risks for Parkinson's disease.

MPTP and DSP-4 susceptibility of substantia nigra and locus coeruleus catecholaminergic neurons in mice is independent of parkin activity

Mutations in the parkin gene cause autosomal recessive familial Parkinson's disease (PD). Parkin-deficient mouse models fail to recapitulate nigrostriatal dopaminergic neurodegeneration as seen in PD, but produce deficits in dopaminergic neurotransmission and noradrenergic-dependent behavior. Since sporadic PD is thought to be caused by a combination of genetic susceptibilities and environmental factors, we hypothesized that neurotoxic insults from catecholaminergic toxins would render parkin knockout mice more vulnerable to neurodegeneration. Accordingly, we investigated the susceptibility of catecholaminergic neurons in parkin knockout mice to the potent dopaminergic and noradrenergic neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) respectively. We report that nigrostriatal dopaminergic neurons in parkin knockout mice do not show increased susceptibility to the parkinsonian neurotoxin, MPTP, in acute, subacute and chronic dose regimens of the neurotoxin. Additionally, parkin knockout mice do not show increased vulnerability to the noradrenergic neurotoxin, DSP-4, regarding levels of norepinephrine in cortex, brain stem and spinal cord. These findings suggest that absence of parkin in mice does not increase susceptibility to the loss of catecholaminergic neurons upon exposure to both dopaminergic and noradrenergic neurotoxins.

Midbrain neuronal cultures from parkin mutant mice are resistant to nitric oxide-induced toxicity

Nitric oxide (NO) is a modulator of differentiation and survival of dopamine (DA) neurons. NO may play a role in the pathogenesis of Parkinson's disease (PD) since its levels are increased in parkinsonian brains and it can nitrosylate and alter the function of key proteins involved in the pathogenesis of PD. NO producing neurons are spared in parkinsonian brains suggesting that toxicity by NO can be compensated. Furthermore, the neurotoxic or neurotrophic effects of NO on DA neurons depend on the balance between NO levels and the intracellular levels of glutathione (GSH). We have investigated the effects of NO-donating agents on midbrain neuronal cultures from parkin-deficient mice. Parkin mutations are the most common genetic deficit observed in hereditary parkinsonism. These mice have abnormal DA release and metabolism, increased production of free radicals and a compensatory elevation of GSH. Cultures from parkin knockout (PK-KO) mice were more resistant than those of wild type (WT) to the neurotoxicity by NO, and the difference of susceptibility applied equally to DA, GABA and total number of neurons, and to astrocytes. NO-induced cell death was mainly apoptotic and could be reduced by caspase inhibitors. Cultures from PK-KO had greater levels of GSH than WT and, after treatment with NO, greater levels of S-nitrosoglutathione. The differences in susceptibility disappear when the synthesis of GSH is inhibited or the GSH chelated with diethyl maleate. Our data show that, contrary to the expectations, and related to the enhanced production of GSH in parkin knockout mice, parkin-deficient dopamine neurons are less susceptible to toxicity by NO.

Suppression of Parkin enhances nigrostriatal and motor neuron lesion in mice over-expressing human-mutated tau protein

Abnormal deposition of protein tau takes place in the brain of patients with several neurodegenerative diseases. Few of these patients present frontotemporal dementia with parkinsonism and amyotrophy (FTDPA-17), an autosomal dominant tauopathy related to mutations of the gene that codes for protein tau, localized in chromosome 17. The great majority of patients with tauopathies such as Alzheimer's disease, sporadic frontotemporal dementia or progressive supranuclear palsy do not show a Mendelian pattern of inheritance. We have occasionally seen tauopathies in patients with parkin mutations and, therefore, hypothesized that the protein tau interacts with parkin. We have tested that hypothesis in mice with combined genetic modifications of tau (over-expression of human tau with three mutations known to produce FTDPA-17) and parkin (deleted) proteins. Homozygote parkin null or over-expressing mutated-human tau mice have subtle behavioral and molecular abnormalities but do not express a clinical phenotype of neurodegenerative disease. Mice with combined homozygous mutations of these two genes show progressively abnormal walking already noticeable at 3 months of age, loss of dopamine and dopamine markers in striatum, nuclear tau immunoreactive deposits in motor neurons of the spinal cord, abnormal expression of glial markers and enhanced levels of pro-apoptotic proteins; findings that were absent or less pronounced in homozygote animals with deletions of parkin or over-expression of tau. The double transgenic mice do not express normal mechanisms of adaptation to stress such as increased levels of GSH and Hsp-70. In addition, they have reduced levels of CHIP-Hsc70, a complex known to attenuate aggregation of tau and to enhance ubiquitination of phosphorylated tau. We have found high levels of phosphorylated tau in parkin-/-+tau(VLW) mice and a relative decrease of the inactivated pSer9 to total GSK-3 levels. Our data reveal that there are interactions between tau and parkin that could be relevant for the pathogenesis and treatment of tauopathies. Similarly, we hope that the double transgenic parkin-/-+tau(VLW) mice could be useful for testing of compounds with putative therapeutic value in human tauopathies.

Susceptibility to rotenone is increased in neurons from parkin null mice and is reduced by minocycline

Parkinson's disease is a neurodegenerative disorder which is in most cases of unknown etiology. Mutations of the Park-2 gene are the most frequent cause of familial parkinsonism and parkin knockout (PK-KO) mice have abnormalities that resemble the clinical syndrome. We investigated the interaction of genetic and environmental factors, treating midbrain neuronal cultures from PK-KO and wild-type (WT) mice with rotenone (ROT). ROT (0.025-0.1 microm) produced a dose-dependent selective reduction of tyrosine hydroxylase-immunoreactive cells and of other neurons, as shown by the immunoreactivity to microtubule-associated protein 2 in PK-KO cultures, suggesting that the toxic effect of ROT involved dopamine and other types of neurons. Neuronal death was mainly apoptotic and suppressible by the caspase inhibitor t-butoxycarbonyl-Asp(OMe)-fluoromethyl ketone (Boc-D-FMK). PK-KO cultures were more susceptible to apoptosis induced by low doses of ROT than those from WT. ROT increased the proportion of astroglia and microglia more in PK-KO than in WT cultures. Indomethacin, a cyclo-oxygenase inhibitor, worsened the effects of ROT on tyrosine hydroxylase cells, apoptosis and astroglial (glial fibrillary acidic protein) cells. N-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase, increased ROT-induced apoptosis but did not change tyrosine hydroxylase-immunoreactive or glial fibrillary acidic protein area. Neither indomethacin nor N-nitro-L-arginine methyl ester had any effect on the reduction by ROT of the mitochondrial potential as measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. Microglial NADPH oxidase inhibition, however, protected against ROT. The roles of p38 MAPK and extracellular signal-regulated kinase signaling pathways were tested by treatment with SB20358 and PD98059, respectively. These compounds were inactive in ROT-naive cultures but PD98059 slightly increased cellular necrosis, as measured by lactate dehydrogenase levels, caused by ROT, without changing mitochondrial activity. SB20358 increased the mitochondrial failure and lactate dehydrogenase elevation induced by ROT. Minocycline, an inhibitor of microglia, prevented the dropout of tyrosine hydroxylase and apoptosis by ROT; the addition of microglia from PK-KO to WT neuronal cultures increased the sensitivity of dopaminergic neurons to ROT. PK-KO mice were more susceptible than WT to ROT and the combined effects of Park-2 suppression and ROT reproduced the cellular events observed in Parkinson's disease. These events were prevented by minocycline.

Parkin suppresses the expression of monoamine oxidases

Mutations of parkin are linked to early onset Parkinson disease. Here we show that stable transfection of parkin in the human dopaminergic neuroblastoma cell line SH-SY5Y markedly reduced the activities of both monoamine oxidase (MAO) A and B. The amount of 3,4-dihydroxyphenylacetic acid, which is produced during dopamine oxidation by MAO, was greatly reduced by parkin overexpression. Radioligand binding assays showed that MAO binding sites were decreased accordingly. Consistent with these, MAO-B protein level was much lower, whereas the amount of MAO-A protein was not determined due to the lack of a suitable antibody. Co-transfection of either MAO with parkin in HEK293 cells did not significantly alter ubiquitination and degradation of each MAO. When we measured MAO expression by real-time quantitative reverse transcription-PCR, marked reductions were seen in SH-SY5Y cells stably expressing parkin compared with the parental cells or a control line stably transfected with luciferase. In addition, parkin mutants defective in E3 ligase activity exhibited different effects on MAO expression. We found that parkin also significantly decreased mRNA levels of both MAOs in the mouse fibroblast cell line NIH3T3. Furthermore, MAO expression was significantly increased in human B lymphocyte cell lines derived from Parkinson disease patients with homozygous but not heterozygous deletion of exon 4 of parkin. Together these results suggest that parkin suppresses MAO expression. This function may limit the production of reactive oxygen species generated by MAO in dopamine oxidation and would, thus, be beneficial to the survival of dopaminergic neurons.