Multiple molecular pathways are involved in the neuroprotection of GDNF against proteasome inhibitor induced dopamine neuron degeneration in vivo

Increased Physiological GDNF Levels Have No Effect on Dopamine Neuron Protection and Restoration in a Proteasome Inhibition Mouse Model of Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disease that comprises a range of motor and nonmotor symptoms. Glial cell line-derived neurotrophic factor (GDNF) promotes the survival of dopamine neurons in vitro and in vivo, and intracranial delivery of GDNF has been tested in six clinical trials for treating PD. However, clinical trials with ectopic GDNF have yielded variable results, which could in part result from abnormal expression site and levels caused by ectopic overexpression. Therefore, an important open question is whether an increase in endogenous GDNF expression could be potent in reversing PD progression. Here, we tested the therapeutic potential of endogenous GDNF using mice in which endogenous GDNF can be conditionally upregulated specifically in cells that express GDNF naturally (conditional GDNF hypermorphic mice; Gdnf^cHyper ). We analyzed the impact of endogenous GDNF upregulation in both neuroprotection and neurorestoration procedures, and for both motor and nonmotor symptoms in the proteasome inhibitor lactacystin (LC) model of PD. Our results showed that upregulation of endogenous GDNF in the adult striatum is not protective in LC-induced PD model in mice. Since age is the largest risk factor for PD, we also analyzed the effect of deletion of endogenous GDNF in aged Gdnf conditional knock-out mice. We found that GDNF deletion does not increase susceptibility to LC-induced damage. We conclude that endogenous GDNF does not impact the outcome in the LC-induced proteasome inhibition mouse model of Parkinson's disease.

HDAC6-mediated Hsp90 deacetylation reduces aggregation and toxicity of the protein alpha-synuclein by regulating chaperone-mediated autophagy

Histone deacetylase 6 (HDAC6) has been shown to control major cell response pathways to the cytotoxic ubiquitinated aggregates in some protein aggregation diseases. However, it is not well known whether HDAC6 affects the aggregation process of α-synuclein (α-syn) in Parkinson's disease (PD). Previously, we demonstrated that HDAC6 inhibition exacerbated the nigrostriatal dopamine neurodegeneration and up-regulated α-syn oligomers in a heat shock protein 90 (Hsp90)-dependent manner in PD mouse model. Here, we further showed that HDAC6 overexpression partly improved the behavior deficits of the PD model and alleviated the nigrostriatal dopamine (DA) neurons injury. Furthermore, HDAC6 was found to regulate α-syn oligomers levels through activation of chaperone-mediated autophagy (CMA). During this process, Hsp90 deacetylation mediated the crosstalk between HDAC6 and lysosome-associated membrane protein type 2A. Liquid chromatography-tandem mass spectrometry and mutational analysis showed that acetylation status Hsp90 at the K489 site was a strong determinant for HDAC6-induced CMA activation, α-syn oligomers levels, and cell survival in the cell model of PD. Therefore, our findings uncovered the mechanism of HDAC6 in the PD model that HDAC6 regulated α-syn oligomers levels and DA neurons survival partly through modulating CMA, and Hsp90 deacetylation at the K489 site mediated the crosstalk between HDAC6 and CMA. HDAC6 and its downstream effectors appear as key modulators of the cytotoxic α-syn aggregates, which deserve further investigations to evaluate their values as potential therapeutic targets in PD.

Glial cell line-derived neurotrophic factor receptor Rearranged during transfection agonist supports dopamine neurons in Vitro and enhances dopamine release In Vivo

BACKGROUND

Motor symptoms of Parkinson's disease (PD) are caused by degeneration and progressive loss of nigrostriatal dopamine neurons. Currently, no cure for this disease is available. Existing drugs alleviate PD symptoms but fail to halt neurodegeneration. Glial cell line-derived neurotrophic factor (GDNF) is able to protect and repair dopamine neurons in vitro and in animal models of PD, but the clinical use of GDNF is complicated by its pharmacokinetic properties. The present study aimed to evaluate the neuronal effects of a blood-brain-barrier penetrating small molecule GDNF receptor Rearranged in Transfection agonist, BT13, in the dopamine system.

METHODS

We characterized the ability of BT13 to activate RET in immortalized cells, to support the survival of cultured dopamine neurons, to protect cultured dopamine neurons against neurotoxin-induced cell death, to activate intracellular signaling pathways both in vitro and in vivo, and to regulate dopamine release in the mouse striatum as well as BT13's distribution in the brain.

RESULTS

BT13 potently activates RET and downstream signaling cascades such as Extracellular Signal Regulated Kinase and AKT in immortalized cells. It supports the survival of cultured dopamine neurons from wild-type but not from RET-knockout mice. BT13 protects cultured dopamine neurons from 6-Hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpyridinium (MPP+ )-induced cell death only if they express RET. In addition, BT13 is absorbed in the brain, activates intracellular signaling cascades in dopamine neurons both in vitro and in vivo, and also stimulates the release of dopamine in the mouse striatum.

CONCLUSION

The GDNF receptor RET agonist BT13 demonstrates the potential for further development of novel disease-modifying treatments against PD. © 2019 International Parkinson and Movement Disorder Society.

Regulatable Lentiviral Hematopoietic Stem Cell Gene Therapy in a Mouse Model of Parkinson's Disease

Glial cell line-derived neurotrophic factor (GDNF) exhibits potent neuroprotective properties in preclinical models of Parkinson's disease (PD), but challenges in GDNF delivery have been reported from clinical trials. To address this barrier, we developed a hematopoietic stem cell transplantation-based macrophage-mediated GDNF therapy platform. Here, we introduced a regulatable lentiviral vector (LV-MSP-Tet-Off-hGDNF) to allow the expression of human GDNF (hGDNF) to be adjusted or stopped by oral administration of doxycycline (Dox). C57BL/6J mice were lethally irradiated with head protection and then transplanted with syngeneic bone marrow cells transduced with either the hGDNF-expressing vector or a corresponding GFP-expressing vector, LV-MSP-Tet-Off-GFP. Suppression of vector gene expression was achieved through administration of Dox in drinking water. To create a toxin-induced Parkinsonian model, mice were injected in two cycles with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) to yield nigral cell/striatal dopamine loss and behavioral deficits. During the presence of Dox in the drinking water, plasma GDNF was at a basal level, whereas during the absence of Dox, plasma GDNF was significantly elevated, indicating reliable regulation of therapeutic gene expression. Midbrain GDNF levels were altered in parallel, although these did not return completely to basal levels during the periods of Dox withdrawal. Motor activities of the MPTP-Tet-off-hGDNF group were comparable to those of the Tet-off-GFP (subject to no MPTP treatment) group, but substantially better than those of the MPTP-Tet-off-GFP group. Interestingly, the improvement in motor activities was sustained during the Dox-withdrawn periods in MPTP-Tet-off-hGDNF animals. Neuroprotection by therapeutic GDNF expression was further evidenced by significant amelioration of nigral tyrosine hydroxylase loss after both the first and second MPTP treatment cycles. These data suggest that neurotrophic factor expression can be upregulated to achieve efficacy or downregulated in case of off-target effects or adverse events, a feature that may eventually increase the acceptance of this potentially neuroprotective/disease-modifying PD therapy.

The Proteasome Inhibition Model of Parkinson's Disease

The pathological hallmarks of Parkinson's disease are the progressive loss of nigral dopaminergic neurons and the formation of intracellular inclusion bodies, termed Lewy bodies, in surviving neurons. Accumulation of proteins in large insoluble cytoplasmic aggregates has been proposed to result, partly, from a failure in the function of intracellular protein degradation pathways. Evidence in support for such a hypothesis emerged in the beginning of the years 2000 with studies demonstrating structural and functional deficits in the ubiquitin-proteasome pathway in post-mortem nigral tissue of patients with Parkinson's disease. These fundamental findings have inspired the development of a new generation of animal models based on the use of proteasome inhibitors to disturb protein homeostasis and trigger nigral dopaminergic neurodegeneration. In this review, we provide an updated overview of the current approaches in employing proteasome inhibitors to model Parkinson's disease, with particular emphasis on rodent studies. In addition, the mechanisms underlying proteasome inhibition-induced cell death and the validity criteria (construct, face and predictive validity) of the model will be critically discussed. Due to its distinct, but highly relevant mechanism of inducing neuronal death, the proteasome inhibition model represents a useful addition to the repertoire of toxin-based models of Parkinson's disease that might provide novel clues to unravel the complex pathogenesis of this disorder.

A Central Role for Phosphorylated p38α in Linking Proteasome Inhibition-Induced Apoptosis and Autophagy

Autophagy and the ubiquitin proteasome system (UPS), as two major protein degradation pathways, coordinate with each other in regulating programmed cell death. Autophagy can compensate for the UPS impairment-induced cell dysfunction and apoptosis. However, it is not clear how cells maintain the delicate balance between UPS-related apoptosis and autophagy. Here, we showed that proteasome inhibition-mediated UPS impairment can activate the phosphorylated p38α (p-p38α)-dependent apoptotic pathway and autophagy pathway in both neuroblastoma cell line N2a and primary cortical neuronal cells. Multiple indices were utilized for the autophagy detection including LC3II transition, acidic vesicle formation, lysosomal accumulation, and p62 reduction. Blockade of autophagy flux with autophagy inhibitor 3-methyladenine or bafilomycin A1 resulted in further phosphorylation of p38α, polyubiquitinated protein aggregation, and greater apoptotic cell death. On the contrary, enhancement of autophagy by rapamycin attenuated the cell loss by lowering p-p38α level and degrading protein aggregates, indicating a protective role of autophagy in cell stress and apoptosis. Moreover, de-activation of p38α with pharmaceutical p38α inhibitor BIRB796 greatly increased autophagy activation, reduced protein aggregates, and attenuated cell loss, suggesting a bidirectional regulation between p-p38α and autophagy. In addition, manipulation of p-p38α by BIRB796 or p38α knockdown decreased the phosphorylation of key components of the mammalian target of rapamycin (mTOR)-dependent pathway, indicating that the mTOR pathway mediates the p-p38α regulation on autophagy. Overall, our data emphasize p-p38α as a key mediator in the antagonistic interaction between apoptosis and autophagy in response to UPS impairment. Centering p-p38α as a potential regulatory target may provide a dual advantage of proteostasis maintenance and cell survival for simultaneous inhibition of apoptosis and activation of autophagy.

Impact of the Secretome of Human Mesenchymal Stem Cells on Brain Structure and Animal Behavior in a Rat Model of Parkinson's Disease

Research in the last decade strongly suggests that mesenchymal stem cell (MSC)‐mediated therapeutic benefits are mainly due to their secretome, which has been proposed as a possible therapeutic tool for the treatment of Parkinson's disease (PD). Indeed, it has been shown that the MSC secretome increases neurogenesis and cell survival, and has numerous neuroprotective actions under different conditions. Additionally, using dynamic culturing conditions (through computer‐controlled bioreactors) can further modulate the MSC secretome, thereby generating a more potent neurotrophic factor cocktail (i.e., conditioned medium). In this study, we have characterized the MSC secretome by proteomic‐based analysis, investigating its therapeutic effects on the physiological recovery of a 6‐hydroxidopamine (6‐OHDA) PD rat model. For this purpose, we injected MSC secretome into the substantia nigra (SNc) and striatum (STR), characterizing the behavioral performance and determining histological parameters for injected animals versus untreated groups. We observed that the secretome potentiated the increase of dopaminergic neurons (i.e., tyrosine hydroxylase‐positive cells) and neuronal terminals in the SNc and STR, respectively, thereby supporting the recovery observed in the Parkinsonian rats’ motor performance outcomes (assessed by rotarod and staircase tests). Finally, proteomic characterization of the MSC secretome (through combined mass spectrometry analysis and Bioplex assays) revealed the presence of important neuroregulatory molecules, namely cystatin C, glia‐derived nexin, galectin‐1, pigment epithelium‐derived factor, vascular endothelial growth factor, brain‐derived neurotrophic factor, interleukin‐6, and glial cell line‐derived neurotrophic factor. Overall, we concluded that the use of human MSC secretome alone was able to partially revert the motor phenotype and the neuronal structure of 6‐OHDA PD animals. This indicates that the human MSC secretome could represent a novel therapeutic for the treatment of PD. Stem Cells Translational Medicine2017;6:634–646

Ret is essential to mediate GDNF's neuroprotective and neuroregenerative effect in a Parkinson disease mouse model

Glial cell line-derived neurotrophic factor (GDNF) is a potent survival and regeneration-promoting factor for dopaminergic neurons in cell and animal models of Parkinson disease (PD). GDNF is currently tested in clinical trials on PD patients with so far inconclusive results. The receptor tyrosine kinase Ret is the canonical GDNF receptor, but several alternative GDNF receptors have been proposed, raising the question of which signaling receptor mediates here the beneficial GDNF effects. To address this question we overexpressed GDNF in the striatum of mice deficient for Ret in dopaminergic neurons and subsequently challenged these mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Strikingly, in this established PD mouse model, the absence of Ret completely abolished GDNF's neuroprotective and regenerative effect on the midbrain dopaminergic system. This establishes Ret signaling as absolutely required for GDNF's effects to prevent and compensate dopaminergic system degeneration and suggests Ret activation as the primary target of GDNF therapy in PD.

The Ubiquitin-Proteasome System: Potential Therapeutic Targets for Alzheimer's Disease and Spinal Cord Injury

The ubiquitin-proteasome system (UPS) is a crucial protein degradation system in eukaryotes. Herein, we will review advances in the understanding of the role of several proteins of the UPS in Alzheimer’s disease (AD) and functional recovery after spinal cord injury (SCI). The UPS consists of many factors that include E3 ubiquitin ligases, ubiquitin hydrolases, ubiquitin and ubiquitin-like molecules, and the proteasome itself. An extensive body of work links UPS dysfunction with AD pathogenesis and progression. More recently, the UPS has been shown to have vital roles in recovery of function after SCI. The ubiquitin hydrolase (Uch-L1) has been proposed to increase cellular levels of mono-ubiquitin and hence to increase rates of protein turnover by the UPS. A low Uch-L1 level has been linked with Aβ accumulation in AD and reduced neuroregeneration after SCI. One likely mechanism for these beneficial effects of Uch-L1 is reduced turnover of the PKA regulatory subunit and consequently, reduced signaling via CREB. The neuron-specific F-box protein Fbx2 ubiquitinates β-secretase thus targeting it for proteasomal degradation and reducing generation of Aβ. Both Uch-L1 and Fbx2 improve synaptic plasticity and cognitive function in mouse AD models. The role of Fbx2 after SCI has not been examined, but abolishing ß-secretase reduces neuronal recovery after SCI, associated with reduced myelination. UBB+1, which arises through a frame-shift mutation in the ubiquitin gene that adds 19 amino acids to the C-terminus of ubiquitin, inhibits proteasomal function and is associated with increased neurofibrillary tangles in patients with AD, Pick’s disease and Down’s syndrome. These advances in understanding of the roles of the UPS in AD and SCI raise new questions but, also, identify attractive and exciting targets for potential, future therapeutic interventions.

A New VMAT-2 Inhibitor NBI-641449 in the Treatment of Huntington Disease

AIMS

To evaluate the effectiveness of a new VMAT-2 inhibitor NBI-641449 in controlling hyperkinetic movements of Huntington disease (HD) and to investigate its possible therapeutic effects.

METHODS

We applied three different doses of NBI-641449 (1, 10, 100 mg/kg/day) for 2 weeks in 4-month-old YAC128 mice and wild-type (WT) mice. Rotarod performance and locomotive activities were tested during the administration of the drug. The concentration of dopamine (DA) and its metabolites was quantified in the striatal tissues by high-performance liquid chromatography (HPLC). Neuron survival in striatum and huntingtin protein aggregates were assessed with immunostaining. Expression levels of endoplasmic reticulum (ER) stress proteins were detected by immunoblotting.

RESULTS

Rotarod performance was significantly improved after treatment with low or middle dose of NBI-641449 in YAC128 mice. Open field test showed that NBI-641449 treatment could attenuate the increased horizontal activity (HACTV), total vertical movement, moving time, and moving distance in YAC128 mice. High dose of NBI-641449 might cause sedative effects in WT and YAC128 mice. HPLC showed that NBI-641449 caused a dose-dependent decrease of DA, 3,4-dihydroxyphenylacetic acid, and homovanillic acid levels in the striatum. NeuN and DARPP-32 immunostaining revealed that NBI-641449 had no significant effect on the neuron survival in the striatum. However, NBI-641449 treatment reduced the huntingtin protein aggregates in the cortex of YAC128 mice. In addition, the levels of ER stress proteins were increased in YAC128 mice, which can be suppressed by NBI-641449.

CONCLUSIONS

These findings suggest that this new VMAT-2 inhibitor NBI-641449 may have therapeutic potential for the treatment of HD.

GDNF-based therapies, GDNF-producing interneurons, and trophic support of the dopaminergic nigrostriatal pathway. Implications for Parkinson's disease

The glial cell line-derived neurotrophic factor (GDNF) is a well-established trophic agent for dopaminergic (DA) neurons in vitro and in vivo. GDNF is necessary for maintenance of neuronal morphological and neurochemical phenotype and protects DA neurons from toxic damage. Numerous studies on animal models of Parkinson’s disease (PD) have reported beneficial effects of GDNF on nigrostriatal DA neuron survival. However, translation of these observations to the clinical setting has been hampered so far by side effects associated with the chronic continuous intra-striatal infusion of recombinant GDNF. In addition, double blind and placebo-controlled clinical trials have not reported any clinically relevant effect of GDNF on PD patients. In the past few years, experiments with conditional Gdnf knockout mice have suggested that GDNF is necessary for maintenance of DA neurons in adulthood. In parallel, new methodologies for exogenous GDNF delivery have been developed. Recently, it has been shown that a small population of scattered, electrically interconnected, parvalbumin positive (PV+) GABAergic interneurons is responsible for most of the GDNF produced in the rodent striatum. In addition, cholinergic striatal interneurons appear to be also involved in the modulation of striatal GDNF. In this review, we summarize current knowledge on brain GDNF delivery, homeostasis, and its effects on nigrostriatal DA neurons. Special attention is paid to the therapeutic potential of endogenous GDNF stimulation in PD.

The potential neuroprotection mechanism of GDNF in the 6-OHDA-induced cellular models of Parkinson's Disease

The glial cell line-derived neurotrophic factor (GDNF) potential as a therapeutic agent for the treatment of Parkinson's Disease (PD) has been extensively explored. However, the mechanism of the GDNF neuroprotective effects is still unclear. In this study, the neuroprotective mechanism of the GDNF in the PD cellular models, which was obtained by the 6-hydroxydopamine (6-OHDA)-induced dopaminergic (DA) cell line MN9D damage was investigated by microarray. Interestingly, 54 constitutively increased or decreased genes were detected, 17 of which have not been reported previously. The expression of 5 up-regulated and 5 down-regulated genes which displayed the most obvious changes compared to the no GDNF treatment cells and was previously proven to be related to cell survival was validated by real-time PCR and western blot. Moreover, the up-regulated gene Ager and down-regulated gene Ccnl2 which were related to the PI-3K/Akt signaling pathway, but not researched in the neuron-cells, were investigated by overexpression and RNA interference. Overexpression of Ager or knockdown the expression of Ccnl2 decreased the damage to MN9D cells caused by 6-OHDA and reduced their apoptosis. All these results suggested that the protective effects of the GDNF on the 6-OHDA damaged MN9D cells could be understood by enhancing the expression of the apoptosis inhibiting genes and decreasing the expression of the apoptosis promoting genes. Thus, this study might provide a number of specific candidates and potential targets to investigate the protective mechanism of GDNF in DA neurons.

Nicotinamide N-methyltransferase expression in SH-SY5Y neuroblastoma and N27 mesencephalic neurones induces changes in cell morphology via ephrin-B2 and Akt signalling

Nicotinamide N-methyltransferase (NNMT, E.C. 2.1.1.1) N-methylates nicotinamide to produce 1-methylnicotinamide (MeN). We have previously shown that NNMT expression protected against neurotoxin-mediated cell death by increasing Complex I (CxI) activity, resulting in increased ATP synthesis. This was mediated via protection of the NDUFS3 subunit of CxI from degradation by increased MeN production. In the present study, we have investigated the effects of NNMT expression on neurone morphology and differentiation. Expression of NNMT in SH-SY5Y human neuroblastoma and N27 rat mesencephalic dopaminergic neurones increased neurite branching, synaptophysin expression and dopamine accumulation and release. siRNA gene silencing of ephrin B2 (EFNB2), and inhibition of Akt phosphorylation using LY294002, demonstrated that their sequential activation was responsible for the increases observed. Incubation of SH-SY5Y with increasing concentrations of MeN also increased neurite branching, suggesting that the effects of NNMT may be mediated by MeN. NNMT had no significant effect on the expression of phenotypic and post-mitotic markers, suggesting that NNMT is not involved in determining phenotypic fate or differentiation status. These results demonstrate that NNMT expression regulates neurone morphology in vitro via the sequential activation of the EFNB2 and Akt cellular signalling pathways.

Cell based therapy in Parkinsonism

Parkinson's disease (PD) is a synucleinopathy-induced chronic progressive neurodegenerative disorder, worldwide affecting about 5 million humans. As of yet, actual therapies are symptomatic, and neuroprotective strategies are an unmet need. Due to their capability to transdifferentiate, to immune modulate and to increase neuroplasticity by producing neurotrophic factors, adult stem cells (ASC) might fill this gap. Preclinical research in 6-hydroxydopamine (6-OHDA) and/or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesioned animals established persistent improvements of motor behavior after ASC-treatment. Histological/histochemical measurements in these animals evidenced an intracerebral applied ASC-induced increase of Tyrosine Hydroxylase-positive (TH+) cells with increased striatal dopamine levels, suggesting cell rescue. Likewise, clinical experience with subventricular applied ASCs in PD patients, although limited, is encouraging, evidencing neurorescue especially during the early phase of the disease. In multiple system atrophy (MSA) or progressive supranuclear palsy (PSP) patients, though, only marginal reduced progression of natural progression could be established after subventricular or intravasal ASC implantations.

Adeno-associated virus type 2 vector-mediated glial cell line-derived neurotrophic factor gene transfer induces neuroprotection and neuroregeneration in a ubiquitin-proteasome system impairment animal model of Parkinson's disease

BACKGROUND

The impairment of the ubiquitin-proteasome system (UPS) is a cellular mechanism underlying the neurodegenerative process in Parkinson's disease (PD). A mouse model induced by the selective proteasome inhibitor lactacystin targeting on substantia nigra has been demonstrated to be valuable in investigating etiopathogenesis and neuroprotection for PD.

OBJECTIVE

In the present study, we used adeno-associated virus type 2 vector (AAV2) encoding glial cell line-derived neurotrophic factor (GDNF) injected into the striatum of this animal model to test the effectiveness and possible mechanisms of GDNF gene therapy.

RESULTS

Our results showed that AAV2-mediated GDNF gene therapy significantly attenuated lactacystin-induced loss of nigral dopamine (DA) neurons and striatal DA levels. Furthermore, we found that GDNF protein is mostly expressed in astrocytes in the subventricular zone (SVZ) and dentate gyrus (DG). AAV2-mediated GDNF therapy can induce neurogenesis in the SVZ and DG, and increase the number of nigral newborn DA neurons.

CONCLUSION

These data indicate that AAV2-mediated GDNF gene therapy can protect the nigral DA neurons from the UPS impairment-induced degeneration, which may partly result from the nigral DA neuron regeneration in the brain, and such experimental results may have implications for the treatment of PD.

Nurr1 regulates Top IIβ and functions in axon genesis of mesencephalic dopaminergic neurons

Background

NURR1 (also named as NR4A2) is a member of the steroid/thyroid hormone receptor family, which can bind to DNA and modulate expression of target genes. Previous studies have shown that NURR1 is essential for the nigral dopaminergic neuron phenotype and function maintenance, and the defects of the gene are possibly associated with Parkinson's disease (PD).

Results

In this study, we used new born Nurr1 knock-out mice combined with Affymetrix genechip technology and real time polymerase chain reaction (PCR) to identify Nurr1 regulated genes, which led to the discovery of several transcripts differentially expressed in the nigro-striatal pathway of Nurr1 knock-out mice. We found that an axon genesis gene called Topoisomerase IIβ (Top IIβ) was down-regulated in Nurr1 knock-out mice and we identified two functional NURR1 binding sites in the proximal Top IIβ promoter. While in Top IIβ null mice, we saw a significant loss of dopaminergic neurons in the substantial nigra and lack of neurites along the nigro-striatal pathway. Using specific TOP II antagonist ICRF-193 or Top IIβ siRNA in the primary cultures of ventral mesencephalic (VM) neurons, we documented that suppression of TOP IIβ expression resulted in VM neurites shortening and growth cones collapsing. Furthermore, microinjection of ICRF-193 into the mouse medial forebrain bundle (MFB) led to the loss of nigro-striatal projection.

Conclusion

Taken together, our findings suggest that Top IIβ might be a down-stream target of Nurr1, which might influence the processes of axon genesis in dopaminergic neurons via the regulation of TOP IIβ expression. The Nurr1-Top IIβ interaction may shed light on the pathologic role of Nurr1 defect in the nigro-striatal pathway deficiency associated with PD.

Neuroprotective effects of MAPK/ERK1/2 and calpain inhibitors on lactacystin-induced cell damage in primary cortical neurons

The dysfunction of the proteasome system is implicated in the pathomechanism of several chronic neurodegenerative diseases. Lactacystin (LC), an irreversible proteasome inhibitor, induces cell death in primary cortical neurons, however, the molecular mechanisms of its neurotoxic action has been only partially unraveled. In this study we aimed to elucidate an involvement of the key enzymatic pathways responsible for LC-induced neuronal cell death. Incubation of primary cortical neurons with LC (0.25-50 μg/ml) evoked neuronal cell death in concentration- and time-dependent manner. Lactacystin (2.5 μg/ml; 6.6μM) enhanced caspase-3 activity, but caspase-3 inhibitor, Ac-DEVD-CHO did not attenuate the LC-evoked cell damage. Western blot analysis showed a time-dependent, prolonged activation of MAPK/ERK1/2 pathway after LC exposure. Moreover, inhibitors of MAPK/ERK1/2 signaling, U0126 and PD98052 attenuated the LC-evoked cell death. We also found that LC-treatment resulted in the induction of calpains and calpain inhibitors (MDL28170 and calpeptin) protected neurons against the LC-induced cell damage. Neuroprotective action of MAPK/ERK1/2 and calpain inhibitors were connected with attenuation of LC-induced DNA fragmentation measured by Hoechst 33342 staining and TUNEL assay. However, only MAPK/ERK1/2 but not calpain inhibitors, attenuated the LC-induced AIF (apoptosis inducing factor) release. Further studies showed no synergy between neuroprotective effects of MAPK/ERK1/2 and calpain inhibitors given in combination when compared to their effects alone. The obtained data provided evidence for neuroprotective potency of MAPK/ERK1/2 and calpain, but not caspase-3 inhibition against the neurotoxic effects of LC in primary cortical neurons and give rationale for using these inhibitors in the treatment of neurodegenerative diseases connected with proteasome dysfunction.

The TrkB-positive dopaminergic neurons are less sensitive to MPTP insult in the substantia nigra of adult C57/BL mice

Tyrosine kinase receptors TrkB and TrkC mediate neuroprotective effects of the brain-derived neurotrophic factor (BDNF) and neurotrophins in the dopaminergic nigro-striatal system, but it is obscure about their responses or expression changes in the injured substantia nigra under Parkinson's disease. In present study, immunofluorescence, Fluoro-Jade staining and laser scanning confocal microscopy were applied to investigate distribution and changes of TrkB and TrkC in the dopamine neurons of the substantia nigra by comparison of control and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model. It revealed that TrkB and TrkC-immunoreactivities were substantially localized in cytoplasm and cell membrane of the substantia nigra neurons of control adults. While neurons double-labeled with tyrosine hydroxylase (TH)/TrkB, or TH/TrkC were distributed in a large numbers in the substantia nigra of controls, they apparently went down at 36.2-65.7% of normal level, respectively following MPTP insult. In MPTP model, cell apoptosis or degeneration of nigral neurons were confirmed by caspase-3 and Fluoro-Jade staining. More interestingly, TH/TrkB-positive neurons survived more in cell numbers in comparison with that of TH/TrkC-positive ones in the MPTP model. This study has indicated that TrkB-containing dopamine neurons are less sensitive in the substantia nigra of MPTP mouse model, suggesting that specific organization of Trks may be involved in neuronal vulnerability to MPTP insult, and BDNF-TrkB signaling may play more important role in protecting dopamine neurons and exhibit therapeutic potential for Parkinson's disease.

Effects of intrastriatal GDNF on the response of dopamine neurons to 6-hydroxydopamine: time course of protection and neurorestoration

Glial cell line-derived neurotrophic factor (GDNF) protects dopamine (DA) neurons from 6-hydroxydopamine (6-OHDA) toxicity. We have now explored this protection over 8 weeks following toxin administration. Infusion of Fluoro-Gold (FG) into the striatum was followed 1 week later by GDNF (9μg) or its vehicle. Six hours later, animals received 6-OHDA (4 μg) into the same site. 6-OHDA caused a loss of cells in the substantia nigra that expressed both FG and tyrosine hydroxylase (TH) and striatal terminals expressing TH, the high affinity dopamine transporter (DAT), and the vesicular monoamine transporter 2 (VMAT2) as assessed 2-8 weeks later. Loss of FG(+) cells, and striatal DA was completely blocked by GDNF by 2 weeks. In contrast, GDNF only slightly attenuated the loss of TH, DAT, or VMAT2 in the striatum at 2 weeks, but had restored these markers by 4-8 weeks. Thus, GDNF prevents DA cell death and loss of striatal DA content, but several weeks are required to fully restore the dopaminergic phenotype. These results provide insight into the mechanism of GDNF protection of DA neurons, and may help avoid incorrect interpretations of temporary phenotypic changes.

Mesenchymal stem cell-secreted superoxide dismutase promotes cerebellar neuronal survival

It has been postulated that bone marrow-derived mesenchymal stem cells (MSCs) might be effective treatments for neurodegenerative disorders either by replacement of lost cells by differentiation into functional neural tissue; modulation of the immune system to prevent further neurodegeneration; and/or provision of trophic support for the diseased nervous system. Here we have performed a series of experiments showing that human bone marrow-derived MSCs are able to protect cultured rodent cerebellar neurons, and specifically cells expressing Purkinje cell markers, against either nitric oxide exposure or withdrawal of trophic support via cell-cell contact and/or secretion of soluble factors, or through secretion of soluble factors alone. We have demonstrated that MSCs protect cerebellar neurons against toxic insults via modulation of both the phosphatidylinositol 3-kinase/Akt and MAPK pathways and defined superoxide dismutase 3 as a secreted active antioxidant biomolecule by which MSCs modulate, at least in part, their neuroprotective effect on cerebellar cells in vitro. Together, the results demonstrate new and specific mechanisms by which MSCs promote cerebellar neuronal survival and add further evidence to the concept that MSCs may be potential therapeutic agents for neurological disorders involving the cerebellum.

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

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

Akt signal transduction dysfunction in Parkinson's disease

Significant attention has been drawn to the potential role of defective PI3-kinase-Akt (PKB) signalling in Parkinson's disease (PD) neurodegeneration and to the possibility that activation of Akt may provide neuroprotection in PD. However, little knowledge exists on the integrity of the Akt system in PD. Results of the present study show diminished levels of both total and active phospho(Ser473)-Akt in the brain in PD. This was evident by western blot analysis of midbrain fractions from PD compared to non-PD control brain, but more specifically by immunofluorescence microscopy of the substantia nigra pars compacta (SNpc). Here, double immunofluorescence microscopy found Akt and phospho(Ser473)-Akt to be expressed at high levels in tyrosine hydroxylase (TH) immunopositive dopaminergic neurons in control human brain. Selective loss of these neurons was accompanied by a marked decrease of Akt and phospho(Ser473)-Akt expression in the PD brain, however Akt and active phospho(Ser473)-Akt are still evident in degenerating dopaminergic neurons in the disease. This suggests that it may be possible to target neuronal Akt in advanced PD. Converse to the marked loss of neuronal Akt in PD, increased Akt and phospho(Ser473)-Akt levels were observed in small non-TH positive cells in PD SNpc, whose increased number and small nuclear size indicate they are glia. These findings implicate defective Akt as a putative signalling pathway linked to loss of dopaminergic neurons in PD.

Phosphoproteome study reveals Hsp27 as a novel signaling molecule involved in GDNF-induced neurite outgrowth

Glial-cell-line-derived neurotrophic factor (GDNF) is a most potent survival factor for dopaminergic neurons. In addition, GDNF was also found to promote neurite outgrowth in dopaminergic neurons. However, despite the potential clinical and physiological importance of GDNF, its mechanism of action is unclear. Therefore, we employed a state-of-the-art proteomic technique, DIGE (Difference in two-dimensional gel electrophoresis), to quantitatively compare profiles of phosphoproteins of PC12-GFRalpha1-RET cells (that stably overexpress GDNF receptor alpha1 and RET) 0.5 and 10 h after GDNF challenge with control. A total of 92 differentially expressed proteins were successfully identified by mass spectrometry. Among them, the relative levels of phosphorylated Hsp27 increased significantly both in 0.5 and 10 h GDNF-treated PC12-GFRalpha1-RET cells. Confocal microscopy and Western blot results showed that the phosphorylation of Hsp27 after GDNF treatment was accompanied by its nuclear translocation. After the mRNA of Hsp27 was interfered, neurite outgrowth of PC12-GFRalpha1-RET cells induced by GDNF was significantly blocked. Furthermore, the percentage of neurite outgrowth induced by GDNF was also reduced by the expression of dominant-negative mutants of Hsp27, in which specific serine phosphorylation residues (Ser15, Ser78 and Ser82) were substituted with alanine. Our data also revealed that p38 MAPK and ERK are the upstream regulators of Hsp27 phosphorylation. Hence, in addition to the numerous novel proteins that are potentially important in GDNF mediated differentiation of dopaminergic cells revealed by our study, our data has indicated that Hsp27 is a novel signaling molecule involved in GDNF-induced neurite outgrowth of dopaminergic neurons.

Identification of glial-cell-line-derived neurotrophic factor-regulated proteins of striatum in mouse model of Parkinson disease

Glial-cell-line-derived neurotrophic factor (GDNF) is a potent survival factor for dopaminergic neurons, and hence serves as a therapeutic candidate for the treatment of Parkinson's disease. However, despite the potential clinical and physiological importance of GDNF, its mechanism of action is unclear. Therefore, we employed a state-of-the-art proteomic technique, DIGE, along with MS and a bioinformatics tool called Database for Annotation, Visualization and Integrated Discovery (DAVID), to profile proteome changes in the parkinsonian mouse striatum after GDNF challenge. Forty-six unique differentially expressed proteins were successfully identified, which were found either up-regulated and/or down-regulated at the two time points 4 and 72 h compared with the control. Proteins involved in cell differentiation and system development formed the largest part of the proteins regulated under GDNF. Furthermore, the aberrant expression of HSPs and mitochondria-associated proteins were noticeable. Moreover, mitochondrial stress 70 protein and heat shock cognate 71 kDa protein, whose relative levels increased significantly in GDNF-treated striatum, were further evaluated with Western blot and RT-PCR, demonstrating a good agreement with quantitative proteomic data. These data will provide some clues for understanding the mechanisms by which GDNF promotes the survival of dopaminergic neurons.

Disruption of the ubiquitin proteasome system following axonal stretch injury accelerates progression to secondary axotomy

The ubiquitin proteasome system (UPS) plays a vital role in the regulation of protein degradation. Ubiquitination of proteins has been implicated in the pathological cascade associated with neuronal degeneration in both neurodegenerative disease and following acquired central nervous system (CNS) injury. In the present study, we have investigated the role of the UPS following mild to moderate in vitro axonal stretch injury to mature primary cortical neurons, a model of the evolving axonal pathology characteristic of diffuse axonal injury following brain trauma. Transient axonal stretch injury in this model does not involve primary axotomy. However, delayed accumulation of ubiquitin in neuritic swellings at 48 h post-injury (PI) was present in axonal bundles, followed by approximately 60% of axonal bundles progressing to secondary axotomy at 72 h PI. This delayed accumulation of ubiquitin was temporally and spatially associated with cytoskeletal damage. Pharmacological inhibition of the UPS with both MG132 and lactacystin prior to axonal injury resulted in a significant (p < 0.05) increase in the number of axonal bundles progressing to secondary axotomy at 48 and 72 h PI. These results demonstrate that, following mild to moderate transient axonal stretch injury, UPS activity may assist structural reorganization within axons, potentially impeding secondary axotomy. Protein ubiquitination in the axon may therefore have a protective role relative to the diffuse axonal changes that follow traumatic brain injury.

Role of ventral tegmental area glial cell line-derived neurotrophic factor in incubation of cocaine craving

BACKGROUND

Ventral tegmental area (VTA) brain-derived neurotrophic factor (BDNF) contributes to time-dependent increases in cue-induced cocaine seeking after withdrawal (incubation of cocaine craving). Here, we studied the role of glial cell line-derived neurotrophic factor (GDNF) in incubation of cocaine craving because, like BDNF, GDNF provides trophic support to midbrain dopamine neurons.

METHODS

We first trained rats to self-administer intravenous cocaine for 10 days (6 hours/d, cocaine injections were paired with a tone-light cue). We then manipulated VTA GDNF function and assessed cue-induced cocaine seeking in extinction tests after withdrawal from cocaine.

RESULTS

VTA injections of an adeno-associated virus (AAV) vector containing rat GDNF cDNA (5 x 10(8) viral genomes) on withdrawal Day 1 increased cue-induced cocaine seeking on withdrawal days 11 and 31; this effect was not observed after VTA injections of an AAV viral vector containing red fluorescent protein (RFP). Additionally, VTA, but not substantial nigra (SN), GDNF injections (1.25 microg or 12.5 microg/side) immediately after the last cocaine self-administration session increased cue-induced drug seeking on withdrawal days 3 and 10; this effect was reversed by VTA injections of U0126, which inhibits the activity of extracellular signal-regulated kinases (ERK). Finally, interfering with VTA GDNF function by chronic delivery of anti-GDNF monoclonal neutralizing antibodies via minipumps (600 ng/side/d) during withdrawal Days 1-14 prevented the time-dependent increases in cue-induced cocaine seeking on withdrawal days 11 and 31.

CONCLUSIONS

Our results indicate that during the first weeks of withdrawal from cocaine self-administration, GDNF-dependent neuroadaptations in midbrain VTA neurons play an important role in the development of incubation of cocaine craving.

Cell death pathways in Parkinson's disease: proximal triggers, distal effectors, and final steps

Parkinson's disease (PD) is a common neurodegenerative disorder. Neuronal cell death in PD is still poorly understood, despite a wealth of potential pathogenic mechanisms and pathways. Defects in several cellular systems have been implicated as early triggers that start cells down the road toward neuronal death. These include abnormal protein accumulation, particularly of alpha-synuclein; altered protein degradation via multiple pathways; mitochondrial dysfunction; oxidative stress; neuroinflammation; and dysregulated kinase signaling. As dysfunction in these systems mounts, pathways that are more explicitly involved in cell death become recruited. These include JNK signaling, p53 activation, cell cycle re-activation, and signaling through bcl-2 family proteins. Eventually, neurons become overwhelmed and degenerate; however, even the mechanism of final cell death in PD is still unsettled. In this review, we will discuss cell death triggers and effectors that are relevant to PD, highlighting important unresolved issues and implications for the development of neuroprotective therapies.

Differential effects of the dopamine neurotoxin MPTP in animals with a partial deletion of the GDNF receptor, GFR alpha1, gene

Glial cell line-derived neurotrophic factor (GDNF), a member of the transforming growth factor beta (TGFbeta) superfamily, is a potent neurotrophic protein promoting the survival and maintenance of dopaminergic (DA) neurons in the substantia nigra during development and adulthood. DA neurons that project to the striatum in the nigrostriatal pathway express GDNF receptors, GFR alpha1. The purpose of this study was to determine whether these neurons are especially sensitive to neurotoxic insults. Therefore, we examined effects of the dopaminergic toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on locomotion and DA neurons in 26-month-old male GFR alpha1 heterozygous (GFR alpha1(+/-)) mice compared to aged-matched wild-type (WT) littermates. MPTP gave rise to increased locomotion, regardless of genotype, while GFR alpha1(+/-) mice treated with saline exhibited lower spontaneous locomotion, compared to WT mice. Moreover, GFR alpha1(+/-) saline mice had fewer TH-positive neurons, greater expression of inflammatory markers (CD45 immunostaining and phosphorylated p38 MAPK) in the nigra, and reduced striatal TH staining. MPTP exacerbated these effects, with the lowest density of striatal TH and highest density of nigral CD45 and phospho-p38 MAPK immunoreactivity observed in GFR alpha1(+/-) mice. The findings point to increased sensitivity of the DAergic system with age and neurotoxic exposure as a result of a genetic reduction of GFR alpha1.