RET signaling does not modulate MPTP toxicity but is required for regeneration of dopaminergic axon terminals

Optogenetic dissection of RET signaling reveals robust activation of ERK and enhanced filopodia-like protrusions of regenerating axons

RET (REarranged during Transfection) is a receptor tyrosine kinase that transduces various external stimuli into biological functions, such as survival and differentiation, in neurons. In the current study, we developed an optogenetic tool for modulating RET signaling, termed optoRET, combining the cytosolic region of human RET with a blue-light-inducible homo-oligomerizing protein. By varying the duration of photoactivation, we were able to dynamically modulate RET signaling. Activation of optoRET recruited Grb2 (growth factor receptor-bound protein 2) and stimulated AKT and ERK (extracellular signal-regulated kinase) in cultured neurons, evoking robust and efficient ERK activation. By locally activating the distal part of the neuron, we were able to retrogradely transduce the AKT and ERK signal to the soma and trigger formation of filopodia-like F-actin structures at stimulated regions through Cdc42 (cell division control 42) activation. Importantly, we successfully modulated RET signaling in dopaminergic neurons of the substantia nigra in the mouse brain. Collectively, optoRET has the potential to be developed as a future therapeutic intervention, modulating RET downstream signaling with light.

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.

The Role of NEDD4 E3 Ubiquitin–Protein Ligases in Parkinson’s Disease

Parkinson’s disease (PD) is a debilitating neurodegenerative disease that causes a great clinical burden. However, its exact molecular pathologies are not fully understood. Whilst there are a number of avenues for research into slowing, halting, or reversing PD, one central idea is to enhance the clearance of the proposed aetiological protein, oligomeric α-synuclein. Oligomeric α-synuclein is the main constituent protein in Lewy bodies and neurites and is considered neurotoxic. Multiple E3 ubiquitin-protein ligases, including the NEDD4 (neural precursor cell expressed developmentally downregulated protein 4) family, parkin, SIAH (mammalian homologues of Drosophila seven in absentia), CHIP (carboxy-terminus of Hsc70 interacting protein), and SCFFXBL5 SCF ubiquitin ligase assembled by the S-phase kinase-associated protein (SKP1), cullin-1 (Cul1), a zinc-binding RING finger protein, and the F-box domain/Leucine-rich repeat protein 5-containing protein FBXL5), have been shown to be able to ubiquitinate α-synuclein, influencing its subsequent degradation via the proteasome or lysosome. Here, we explore the link between NEDD4 ligases and PD, which is not only via α-synuclein but further strengthened by several additional substrates and interaction partners. Some members of the NEDD4 family of ligases are thought to crosstalk even with PD-related genes and proteins found to be mutated in familial forms of PD. Mutations in NEDD4 family genes have not been observed in PD patients, most likely because of their essential survival function during development. Following further in vivo studies, it has been thought that NEDD4 ligases may be viable therapeutic targets in PD. NEDD4 family members could clear toxic proteins, enhancing cell survival and slowing disease progression, or might diminish beneficial proteins, reducing cell survival and accelerating disease progression. Here, we review studies to date on the expression and function of NEDD4 ubiquitin ligases in the brain and their possible impact on PD pathology.

Rab7 reduces α-synuclein toxicity in rats and primary neurons

During the pathogenesis of Parkinson's disease (PD), aggregation of alpha-synuclein (αSyn) induces a vicious cycle of cellular impairments that lead to neurodegeneration. Consequently, removing toxic αSyn aggregates constitutes a plausible strategy against PD. In this work, we tested whether stimulating the autolysosomal degradation of αSyn aggregates through the Ras-related in brain 7 (Rab7) pathway can reverse αSyn-induced cellular impairment and prevent neurodegeneration in vivo. The disease-related A53T mutant of αSyn was expressed in primary neurons and in dopaminergic neurons of the rat brain simultaneously with wild type (WT) Rab7 or the T22N mutant as negative control. The cellular integrity was quantified by morphological and biochemical analyses. In primary neurons, WT Rab7 rescued the αSyn-induced loss of neurons and neurites. Furthermore, Rab7 decreased the amount of reactive oxygen species and the amount of Triton X-100 insoluble αSyn. In rat brain, WT Rab7 reduced αSyn-induced loss of dopaminergic axon terminals in the striatum and the loss of dopaminergic dendrites in the substantia nigra pars reticulata. Further, WT Rab7 lowered αSyn pathology as quantified by phosphorylated αSyn staining. Finally, WT Rab7 attenuated αSyn-induced DNA damage in primary neurons and rat brain. In brief, Rab7 reduced αSyn-induced pathology, ameliorated αSyn-induced neuronal degeneration, oxidative stress and DNA damage. These findings indicate that Rab7 is able to disrupt the vicious cycle of cellular impairment, αSyn pathology and neurodegeneration present in PD. Stimulation of Rab7 and the autolysosomal degradation pathway could therefore constitute a beneficial strategy for PD.

Failure of Glial Cell-Line Derived Neurotrophic Factor (GDNF) in Clinical Trials Orchestrated By Reduced NR4A2 (NURR1) Transcription Factor in Parkinson's Disease. A Systematic Review

Parkinson’s disease (PD) is one of the most common neurodegenerative maladies with unforeseen complex pathologies. While this neurodegenerative disorder’s neuropathology is reasonably well known, its etiology remains a mystery, making it challenging to aim therapy. Glial cell-line derived neurotrophic factor (GDNF) remains an auspicious therapeutic molecule for treating PD. Neurotrophic factor derived from glial cell lines is effective in rodents and nonhuman primates, but clinical findings have been equivocal. Laborious exertions have been made over the past few decades to improve and assess GDNF in treating PD (clinical studies). Definitive clinical trials have, however, failed to demonstrate a survival advantage. Consequently, there seemed to be a doubt as to whether GDNF has merit in the potential treatment of PD. The purpose of this cutting edge review is to speculate as to why the clinical trials have failed to meet the primary endpoint. We introduce a hypothesis, “Failure of GDNF in clinical trials succumbed by nuclear receptor-related factor 1 (Nurr1) shortfall.” We demonstrate how Nurr1 binds to GDNF to induce dopaminergic neuron synthesis. Due to its undisputable neuro-protection aptitude, we display Nurr1 (also called Nr4a2) as a promising therapeutic target for PD.

Changes in Striatal Medium Spiny Neuron Morphology Resulting from Dopamine Depletion Are Reversible

The classical motor symptoms of Parkinson’s disease (PD) are caused by degeneration of dopaminergic neurons in the substantia nigra, which is followed by secondary dendritic pruning and spine loss at striatal medium spiny neurons (MSN). We hypothesize that these morphological changes at MSN underlie at least in part long-term motor complications in PD patients. In order to define the potential benefits and limitations of dopamine substitution, we tested in a mouse model whether dendritic pruning and spine loss can be reversible when dopaminergic axon terminals regenerate. In order to induce degeneration of nigrostriatal dopaminergic neurons we used the toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in C57BL/6J mice; 30 mg/kg MPTP was applied i.p. on five consecutive days. In order to assess the consequences of dopamine depletion, mice were analyzed 21 days after the last injection. In order to test reversibility of MSN changes we exploited the property of this model that striatal axon terminals regenerate by sprouting within 90 days and analyzed a second cohort 90 days after MPTP. Degeneration of dopaminergic neurons was confirmed by counting TH-positive neurons in the substantia nigra and by analyzing striatal catecholamines. Striatal catecholamine recovered 90 days after MPTP. MSN morphology was visualized by Golgi staining and quantified as total dendritic length, number of dendritic branch points, and density of dendritic spines. All morphological parameters of striatal MSN were reduced 21 days after MPTP. Statistical analysis indicated that dendritic pruning and the reduction of spine density represent two distinct responses to dopamine depletion. Ninety days after MPTP, all morphological changes recovered. Our findings demonstrate that morphological changes in striatal MSN resulting from dopamine depletion are reversible. They suggest that under optimal conditions, symptomatic dopaminergic therapy might be able to prevent maladaptive plasticity and long-term motor complications in PD patients.

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.

Neuroregeneration in Parkinson's Disease: From Proteins to Small Molecules

Background:

Parkinson’s disease (PD) is the second most common neurodegenerative disorder worldwide, the lifetime risk of developing this disease is 1.5%. Motor diagnostic symptoms of PD are caused by degeneration of nigrostria-tal dopamine neurons. There is no cure for PD and current therapy is limited to supportive care that partially alleviates dis-ease signs and symptoms. As diagnostic symptoms of PD result from progressive degeneration of dopamine neurons, drugs restoring these neurons may significantly improve treatment of PD.

Method:

A literature search was performed using the PubMed, Web of Science and Scopus databases to discuss the pro-gress achieved in the development of neuroregenerative agents for PD. Papers published before early 2018 were taken into account.

Results:

Here, we review several groups of potential agents capable of protecting and restoring dopamine neurons in cul-tures or animal models of PD including neurotrophic factors and small molecular weight compounds.

Conclusion:

Despite the promising results of in vitro and in vivo experiments, none of the found agents have yet shown conclusive neurorestorative properties in PD patients. Meanwhile, a few promising biologicals and small molecules have been identified. Their further clinical development can eventually give rise to disease-modifying drugs for PD. Thus, inten-sive research in the field is justified.

Fingolimod (FTY720) is not protective in the subacute MPTP mouse model of Parkinson's disease and does not lead to a sustainable increase of brain-derived neurotrophic factor

Parkinson's disease (PD) is characterized by the loss of midbrain dopaminergic neurons and aggregates of α-synuclein termed Lewy bodies. Fingolimod (FTY720) is an agonist of sphingosine-1 phosphate receptors and an approved oral treatment for multiple sclerosis. Fingolimod elevates brain-derived neurotrophic factor (BDNF), an important neurotrophic factor for dopaminergic neurons. BDNF and fingolimod are beneficial in several animal models of PD. In order to validate the therapeutic potential of fingolimod for the treatment of PD, we tested its effect in the subacute MPTP mouse model of PD. MPTP or vehicle was applied i.p. in doses of 30 mg/kg MPTP on five consecutive days. In order to recapitulate the combination of dopamine loss and α-synuclein aggregates found in PD, MPTP was first administered in Thy1-A30P-α-synuclein transgenic mice. Fingolimod was administered i.p. at a dose of 0.1 mg/kg every second day. Nigrostriatal degeneration was assayed by stereologically counting the number of dopaminergic neurons in the substantia nigra pars compacta, by analysing the concentration of catecholamines and the density of dopaminergic fibres in the striatum. MPTP administration produced a robust nigrostriatal degeneration, comparable to previous studies. Unexpectedly, we found no difference between mice with and without fingolimod treatment, neither at baseline, nor at 14 or 90 days after MPTP. Also, we found no effect of fingolimod in the subacute MPTP mouse model when we used wildtype mice instead of α-synuclein transgenic mice, and no effect with an increased dose of 1 mg/kg fingolimod administered every day. In order to explain these findings, we analysed BDNF regulation by fingolimod. We did find an increase of BDNF protein after a single injection of fingolimod 0.1 or 1.0 mg/kg, but not after multiple injections, indicating that the BDNF response to fingolimod is unsustainable over time. Taken together we did not observe a neuroprotective effect of fingolimod in the subacute MPTP mouse model of PD. We discuss possible explanations for this discrepancy with previous findings and conclude fingolimod might be beneficial for the nonmotor symptoms of PD. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* and *Open Data* because it provided all relevant information to reproduce the study in the manuscript and because it made the data publicly available. The data can be accessed at https://osf.io/6xgfn/. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Vitamin D regulation of GDNF/Ret signaling in dopaminergic neurons

1,25(OH)₂D₃ (vitamin D) appears essential for the normal development of dopaminergic neurons. Vitamin D affects dopamine synthesis and metabolism as well as expression of glial cell line-derived neurotrophic factor (GDNF), which is crucial for the survival of dopaminergic neurons. We investigated the role of vitamin D on GDNF and its receptors protooncogene tyrosine-protein kinase receptor Ret (C-Ret) and GDNF family receptor alpha 1 (GFRα1) signaling. To this end, we used a developmental vitamin D-deficient rat model and SH-SY5Y cells transfected with vitamin D receptor (VDR). The absence of vitamin D ligand in gestation reduces C-Ret expression, but not GDNF and GFRα1, in embryo forebrains. Overexpression of VDR in SH-SY5Y in the absence of ligand (mimicking in vivo developmental vitamin D deficiency) also suppressed C-Ret mRNA levels. In the presence of vitamin D, C-Ret mRNA and protein expression were increased. The chromatin immunoprecipitation results suggested that C-Ret is directly regulated by vitamin D via VDR. GDNF was also increased by vitamin D in these cells. Our small interfering RNA studies showed that knocking down VDR leads to an increase in C-Ret in the absence of ligand. Finally, we confirmed the inverse relationship between GFRα1 and C-Ret, as knocking down C-Ret led to increases in GFRα1 expression. These data extend our knowledge of the diverse and important roles played by vitamin D in dopamine physiology.-Pertile, R. A. N., Cui, X., Hammond, L., Eyles, D. W. Vitamin D regulation of GDNF/Ret signaling in dopaminergic neurons.

ASIC1a Deficient Mice Show Unaltered Neurodegeneration in the Subacute MPTP Model of Parkinson Disease

Inflammation contributes to the death of dopaminergic neurons in Parkinson disease and can be accompanied by acidification of extracellular pH, which may activate acid-sensing ion channels (ASIC). Accordingly, amiloride, a non-selective inhibitor of ASIC, was protective in an acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson disease. To complement these findings we determined MPTP toxicity in mice deficient for ASIC1a, the most common ASIC isoform in neurons. MPTP was applied i.p. in doses of 30 mg per kg on five consecutive days. We determined the number of dopaminergic neurons in the substantia nigra, assayed by stereological counting 14 days after the last MPTP injection, the number of Nissl positive neurons in the substantia nigra, and the concentration of catecholamines in the striatum. There was no difference between ASIC1a-deficient mice and wildtype controls. We are therefore not able to confirm that ASIC1a are involved in MPTP toxicity. The difference might relate to the subacute MPTP model we used, which more closely resembles the pathogenesis of Parkinson disease, or to further targets of amiloride.

Mice lacking Faim2 show increased cell death in the MPTP mouse model of Parkinson disease

The death receptor Fas/CD95 mediates apoptotic cell death in response to external stimuli. In neurons, Fas-induced apoptosis is prevented by Fas-apoptotic inhibitory molecule 2 (Faim2). Mice lacking Faim2 showed increased neurodegeneration in animal models of stroke and bacterial meningitis. We therefore tested the relevance of Faim2 in a classical animal model of Parkinson disease and determined the toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in Faim2-deficient mice. Without MPTP treatment, there was no difference in the dopaminergic system between Faim2-deficient mice and control mice. MPTP was applied i.p. in doses of 30 mg per kg on five consecutive days. Fourteen days after the last MPTP injection, the number of dopaminergic neurons in the lateral substantia nigra, assayed by stereological counting, was reduced by 39% in control mice and 53% in Faim2-deficient mice. The density of dopaminergic fibers in the dorsal striatum was reduced by 36% in control mice and 69% in Faim2-deficient mice, in the ventral striatum 44% in control mice and 76% in Faim2-deficient mice. Fiber density recovered at 90 days after MPTP with similar density in both groups. Striatal catecholamine levels were reduced by 81-84% in both groups and recovered at 90 days. Faim2 expression was documented in mouse midbrain using quantitative reverse transcription-PCR (qRT-PCR) and found decreased after MPTP administration. Taken together, our findings demonstrate increased degeneration of dopaminergic neurons with Faim2 deficiency, indicating that Fas-induced apoptosis contributes to cell death in the MPTP mouse model. Along with the decreased expression of Faim2 after MPTP, this finding indicates that boosting Faim2 function might represent a therapeutic strategy for Parkinson disease.

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.

Neuroprotective Effect of Coptis chinensis in MPP+ and MPTP-Induced Parkinson’s Disease Models

The rhizome of Coptis chinensis is commonly used in traditional Chinese medicine alone or in combination with other herbs to treat diseases characterized by causing oxidative stress including inflammatory diseases, diabetes mellitus and neurodegenerative diseases. In particular, there is emerging evidence that Coptis chinensis is effective in the treatment of neurodegenerative diseases associated with oxidative stress. Hence, the aim of this study was to investigate the neuroprotective effect of Coptis chinensis in vitro and in vivo using MPP[Formula: see text] and MPTP models of Parkinson’s disease. MPP[Formula: see text] treated human SH-SY5Y neuroblastoma cells were used as a cell model of Parkinson’s disease. A 24[Formula: see text]h pre-treatment of the cells with the watery extract of Coptis chinensis significantly increased cell viability, as well as the intracellular ATP concentration and attenuated apoptosis compared to the MPP[Formula: see text] control. Further experiments with the main alkaloids of Coptidis chinensis, berberine, coptisine, jaterorrhizine and palmatine revealed that berberine and coptisine were the main active compounds responsible for the observed neuroprotective effect. However, the full extract of Coptis chinensis was more effective than the tested single alkaloids. In the MPTP-induced animal model of Parkinson’s disease, Coptis chinensis dose-dependently improved motor functions and increased tyrosine hydroxylase-positive neurons in the substantia nigra compared to the MPTP control. Based on the results of this work, Coptis chinensis and its main alkaloids could be considered potential candidates for the development of new treatment options for Parkinson’s disease.

Gdf-15 deficiency does not alter vulnerability of nigrostriatal dopaminergic system in MPTP-intoxicated mice

Growth/differentiation factor-15 (Gdf-15) is a member of the transforming growth factor-β (Tgf-β) superfamily and has been shown to be a potent neurotrophic factor for midbrain dopaminergic (DAergic) neurons both in vitro and in vivo. Gdf-15 has also been shown to be involved in inflammatory processes. The aim of this study was to identify the role of endogenous Gdf-15 in the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) mouse model of Parkinson's disease (PD) by comparing Gdf-15 (+/+) and Gdf-15 (-/-) mice. At 4 days and 14 days post-MPTP administration, both Gdf-15 (+/+) and Gdf-15 (-/-) mice showed a similar decline in DAergic neuron numbers and in striatal dopamine (DA) levels. This was followed by a comparable restorative phase at 90 days and 120 days, indicating that the absence of Gdf-15 does not affect the susceptibility or the recovery capacity of the nigrostriatal system after MPTP administration. The MPTP-induced microglial and astrocytic response was not significantly altered between the two genotypes. However, pro-inflammatory and anti-inflammatory cytokine profiling revealed the differential expression of markers in Gdf-15 (+/+) and Gdf-15 (-/-) mice after MPTP administration. Thus, the MPTP mouse model fails to uncover a major role of endogenous Gdf-15 in the protection of MPTP-lesioned nigrostriatal DAergic neurons, in contrast to its capacity to protect the 6-hydroxydopamine-intoxicated nigrostriatal system.

Microglia-Mediated Neuroinflammation and Neurotrophic Factor-Induced Protection in the MPTP Mouse Model of Parkinson's Disease-Lessons from Transgenic Mice

Parkinson's disease (PD) is a neurodegenerative disease characterised by histopathological and biochemical manifestations such as loss of midbrain dopaminergic (DA) neurons and decrease in dopamine levels accompanied by a concomitant neuroinflammatory response in the affected brain regions. Over the past decades, the use of toxin-based animal models has been crucial to elucidate disease pathophysiology, and to develop therapeutic approaches aimed to alleviate its motor symptoms. Analyses of transgenic mice deficient for cytokines, chemokine as well as neurotrophic factors and their respective receptors in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD have broadened the current knowledge of neuroinflammation and neurotrophic support. Here, we provide a comprehensive review that summarises the contribution of microglia-mediated neuroinflammation in MPTP-induced neurodegeneration. Moreover, we highlight the contribution of neurotrophic factors as endogenous and/or exogenous molecules to slow the progression of midbrain dopaminergic (mDA) neurons and further discuss the potential of combined therapeutic approaches employing neuroinflammation modifying agents and neurotrophic factors.

GDNF Overexpression from the Native Locus Reveals its Role in the Nigrostriatal Dopaminergic System Function

Degeneration of nigrostriatal dopaminergic system is the principal lesion in Parkinson’s disease. Because glial cell line-derived neurotrophic factor (GDNF) promotes survival of dopamine neurons in vitro and in vivo, intracranial delivery of GDNF has been attempted for Parkinson’s disease treatment but with variable success. For improving GDNF-based therapies, knowledge on physiological role of endogenous GDNF at the sites of its expression is important. However, due to limitations of existing genetic model systems, such knowledge is scarce. Here, we report that prevention of transcription of Gdnf 3’UTR in Gdnf endogenous locus yields GDNF hypermorphic mice with increased, but spatially unchanged GDNF expression, enabling analysis of postnatal GDNF function. We found that increased level of GDNF in the central nervous system increases the number of adult dopamine neurons in the substantia nigra pars compacta and the number of dopaminergic terminals in the dorsal striatum. At the functional level, GDNF levels increased striatal tissue dopamine levels and augmented striatal dopamine release and re-uptake. In a proteasome inhibitor lactacystin-induced model of Parkinson’s disease GDNF hypermorphic mice were protected from the reduction in striatal dopamine and failure of dopaminergic system function. Importantly, adverse phenotypic effects associated with spatially unregulated GDNF applications were not observed. Enhanced GDNF levels up-regulated striatal dopamine transporter activity by at least five fold resulting in enhanced susceptibility to 6-OHDA, a toxin transported into dopamine neurons by DAT. Further, we report how GDNF levels regulate kidney development and identify microRNAs miR-9, miR-96, miR-133, and miR-146a as negative regulators of GDNF expression via interaction with Gdnf 3’UTR in vitro. Our results reveal the role of GDNF in nigrostriatal dopamine system postnatal development and adult function, and highlight the importance of correct spatial expression of GDNF. Furthermore, our results suggest that 3’UTR targeting may constitute a useful tool in analyzing gene function.

Intracranial delivery of GDNF has been attempted for Parkinson’s disease (PD) treatment but with variable success. For improving GDNF-based therapies, knowledge on physiological role of endogenous GDNF at the sites of its expression is important. However, due to limitations of existing genetic model systems, such knowledge is scarce. Here, we utilize an innovative genetic approach by targeting the 3’UTR regulation of Gdnf in mice. Such animals express elevated levels of Gdnf exclusively in natively Gdnf-expressing cells, enabling dissection of endogenous GDNF functions in vivo. We show that endogenous GDNF regulates dopamine system development and function and protects mice in a rodent PD model without side effects associated with ectopic GDNF applications. Further, we report how GDNF levels regulate kidney development and identify microRNAs which control GDNF expression. Our study highlights the importance of correct spatial expression of GDNF and opens a novel approach to study gene function in mice.

Parkin cooperates with GDNF/RET signaling to prevent dopaminergic neuron degeneration

Parkin and the glial cell line-derived neurotrophic factor (GDNF) receptor RET have both been independently linked to the dopaminergic neuron degeneration that underlies Parkinson's disease (PD). In the present study, we demonstrate that there is genetic crosstalk between parkin and the receptor tyrosine kinase RET in two different mouse models of PD. Mice lacking both parkin and RET exhibited accelerated dopaminergic cell and axonal loss compared with parkin-deficient animals, which showed none, and RET-deficient mice, in which we found moderate degeneration. Transgenic expression of parkin protected the dopaminergic systems of aged RET-deficient mice. Downregulation of either parkin or RET in neuronal cells impaired mitochondrial function and morphology. Parkin expression restored mitochondrial function in GDNF/RET-deficient cells, while GDNF stimulation rescued mitochondrial defects in parkin-deficient cells. In both cases, improved mitochondrial function was the result of activation of the prosurvival NF-κB pathway, which was mediated by RET through the phosphoinositide-3-kinase (PI3K) pathway. Taken together, these observations indicate that parkin and the RET signaling cascade converge to control mitochondrial integrity and thereby properly maintain substantia nigra pars compacta dopaminergic neurons and their innervation in the striatum. The demonstration of crosstalk between parkin and RET highlights the interplay in the protein network that is altered in PD and suggests potential therapeutic targets and strategies to treat PD.

The Ret receptor regulates sensory neuron dendrite growth and integrin mediated adhesion

Neurons develop highly stereotyped receptive fields by coordinated growth of their dendrites. Although cell surface cues play a major role in this process, few dendrite specific signals have been identified to date. We conducted an in vivo RNAi screen in Drosophila class IV dendritic arborization (C4da) neurons and identified the conserved Ret receptor, known to play a role in axon guidance, as an important regulator of dendrite development. The loss of Ret results in severe dendrite defects due to loss of extracellular matrix adhesion, thus impairing growth within a 2D plane. We provide evidence that Ret interacts with integrins to regulate dendrite adhesion via rac1. In addition, Ret is required for dendrite stability and normal F-actin distribution suggesting it has an essential role in dendrite maintenance. We propose novel functions for Ret as a regulator in dendrite patterning and adhesion distinct from its role in axon guidance.

Heat shock alters the expression of schizophrenia and autism candidate genes in an induced pluripotent stem cell model of the human telencephalon

Schizophrenia (SZ) and autism spectrum disorders (ASD) are highly heritable neuropsychiatric disorders, although environmental factors, such as maternal immune activation (MIA), play a role as well. Cytokines mediate the effects of MIA on neurogenesis and behavior in animal models. However, MIA stimulators can also induce a febrile reaction, which could have independent effects on neurogenesis through heat shock (HS)-regulated cellular stress pathways. However, this has not been well-studied. To help understand the role of fever in MIA, we used a recently described model of human brain development in which induced pluripotent stem cells (iPSCs) differentiate into 3-dimensional neuronal aggregates that resemble a first trimester telencephalon. RNA-seq was carried out on aggregates that were heat shocked at 39°C for 24 hours, along with their control partners maintained at 37°C. 186 genes showed significant differences in expression following HS (p<0.05), including known HS-inducible genes, as expected, as well as those coding for NGFR and a number of SZ and ASD candidates, including SMARCA2, DPP10, ARNT2, AHI1 and ZNF804A. The degree to which the expression of these genes decrease or increase during HS is similar to that found in copy loss and copy gain copy number variants (CNVs), although the effects of HS are likely to be transient. The dramatic effect on the expression of some SZ and ASD genes places HS, and perhaps other cellular stressors, into a common conceptual framework with disease-causing genetic variants. The findings also suggest that some candidate genes that are assumed to have a relatively limited impact on SZ and ASD pathogenesis based on a small number of positive genetic findings, such as SMARCA2 and ARNT2, may in fact have a much more substantial role in these disorders - as targets of common environmental stressors.

Glial cell line-derived neurotrophic factor partially ameliorates motor symptoms without slowing neurodegeneration in mice with respiratory chain-deficient dopamine neurons

Degeneration of midbrain dopamine neurons causes the striatal dopamine deficiency responsible for the hallmark motor symptoms of Parkinson's disease (PD). Intraparenchymal delivery of neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF), is a possible future therapeutic approach. In animal PD models, GDNF can both ameliorate neurodegeneration and promote recovery of the dopamine system following a toxic insult. However, clinical studies have generated mixed results, and GDNF has not been efficacious in genetic animal models based on α-synuclein overexpression. We have tested the response to GDNF in a genetic mouse PD model with progressive degeneration of dopamine neurons caused by mitochondrial impairment. We find that GDNF, delivered to the striatum by either an adeno-associated virus or via miniosmotic pumps, partially alleviates the progressive motor symptoms without modifying the rate of neurodegeneration. These behavioral changes are accompanied by increased levels of dopamine in the midbrain, but not in striatum. At high levels, GDNF may instead reduce striatal dopamine levels. These results demonstrate the therapeutic potential of GDNF in a progressively impaired dopamine system.

Neuroprotection by GDNF in the ischemic brain

The glial cell line-derived neurotrophic factor (GDNF) was first identified as a survival factor for midbrain dopaminergic neurons, but additional studies provided evidences for a role as a trophic factor for other neurons of the central and peripheral nervous systems. GDNF regulates cellular activity through interaction with glycosyl-phosphatidylinositol-anchored cell surface receptors, GDNF family receptor-α1, which might signal through the transmembrane Ret tyrosine receptors or the neural cell adhesion molecule, to promote cell survival, neurite outgrowth, and synaptogenesis. The neuroprotective effect of exogenous GDNF has been shown in different experimental models of focal and global brain ischemia, by local administration of the trophic factor, using viral vectors carrying the GDNF gene and by transplantation of GDNF-expressing cells. These different strategies and the mechanisms contributing to neuroprotection by GDNF are discussed in this review. Importantly, neuroprotection by GDNF was observed even when administered after the ischemic injury.

Impairment of the septal cholinergic neurons in MPTP-treated A30P α-synuclein mice

Dementia in Parkinson's disease (PDD) and dementia with Lewy bodies (DLB) are characterized by loss of acetylcholine (ACh) from cortical areas. Clinical studies report positive effects of acetylcholine esterase (AChE) inhibitors in PDD and dementia with Lewy bodies. We here report that the number of neurons expressing a cholinergic marker in the medial septum-diagonal band of Broca complex decreases in A30P α-synuclein-expressing mice during aging, paralleled by a lower AChE fiber density in the dentate gyrus and in the hippocampal CA1 field. After inducing dopamine depletion by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP), no acute but a delayed loss of cholinergic neurons and AChE-positive fibers was observed, which was attenuated by L-3,4-dihydroxyphenylalanine (DOPA) treatment. Expression of nerve growth factor (NGF) and tyrosine receptor kinase A (TrkA) genes was upregulated in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride-treated wild type mice, but not in A30P α-synuclein expressing animals. In contrast, upregulation of sortilin and p75(NTR) genes was found in the A30P α-synuclein-expressing mice. These results suggest that dopamine deficiency may contribute to the impairment of the septohippocampal system in patients with PDD and that L-3,4-dihydroxyphenylalanine may not only result in symptomatic treatment of the akinetic-rigid syndrome but may also alleviate the degeneration of basal forebrain cholinergic system and the cognitive decline.

Pegylated granulocyte colony-stimulating factor conveys long-term neuroprotection and improves functional outcome in a model of Parkinson's disease

Recent proof-of-principle data showed that the haematopoietic growth factor granulocyte colony-stimulating factor (filgrastim) mediates neuroprotection in rodent models of Parkinson's disease. In preparation for future clinical trials, we performed a preclinical characterization of a pegylated derivative of granulocyte colony-stimulating factor (pegfilgrastim) in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinson's disease. We determined serum and cerebrospinal fluid drug levels after subcutaneous injection. A single injection of pegfilgrastim was shown to achieve stable levels of granulocyte colony-stimulating factor in both serum and cerebrospinal fluid with substantially higher levels compared to repetitive filgrastim injections. Leucocyte blood counts were only transiently increased after repeated injections. We demonstrated substantial dose-dependent long-term neuroprotection by pegfilgrastim in both young and aged mice, using bodyweight-adjusted doses that are applicable in clinical settings. Importantly, we found evidence for the functionally relevant preservation of nigrostriatal projections by pegfilgrastim in our model of Parkinson's disease, which resulted in improved motor performance. The more stable levels of pegylated neuroprotective proteins in serum and cerebrospinal fluid may represent a general advantage in the treatment of chronic neurodegenerative diseases and the resulting longer injection intervals are likely to improve patient compliance. In summary, we found that pegylation of a neuroprotective growth factor improved its pharmacokinetic profile over its non-modified counterpart in an in vivo model of Parkinson's disease. As the clinical safety profile of pegfilgrastim is already established, these data suggest that evaluation of pegfilgrastim in further Parkinson's disease models and ultimately clinical feasibility studies are warranted.

Idebenone and resveratrol extend lifespan and improve motor function of HtrA2 knockout mice

Heterozygous loss-of-function mutation of the human gene for the mitochondrial protease HtrA2 has been associated with increased risk to develop mitochondrial dysfunction, a process known to contribute to neurodegenerative disorders such as Huntington's disease (HD) and Parkinson's disease (PD). Knockout of HtrA2 in mice also leads to mitochondrial dysfunction and to phenotypes that resemble those found in neurodegenerative disorders and, ultimately, lead to death of animals around postnatal day 30. Here, we show that Idebenone, a synthetic antioxidant of the coenzyme Q family, and Resveratrol, a bioactive compound extracted from grapes, are both able to ameliorate this phenotype. Feeding HtrA2 knockout mice with either compound extends lifespan and delays worsening of the motor phenotype. Experiments conducted in cell culture and on brain tissue of mice revealed that each compound has a different mechanism of action. While Idebenone acts by downregulating the integrated stress response, Resveratrol acts by attenuating apoptosis at the level of Bax. These activities can account for the delay in neuronal degeneration in the striata of these mice and illustrate the potential of these compounds as effective therapeutic approaches against neurodegenerative disorders such as HD or PD.

Efficient gene therapy for Parkinson's disease using astrocytes as hosts for localized neurotrophic factor delivery

Current gene therapy approaches for Parkinson's disease (PD) deliver neurotrophic factors like glial cell line-derived neurotrophic factor (GDNF) or neurturin via neuronal transgene expression. Since these potent signaling-inducing neurotrophic factors can be distributed through long-distance neuronal projections to unaffected brain sites, this mode of delivery may eventually cause side effects. To explore a localized and thus potentially safer alternative for gene therapy of PD, we expressed GDNF exclusively in astrocytes and evaluated the efficacy of this approach in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and rat 6-hydroxy-dopamine (6-OHDA) models of PD. In terms of protection of dopaminergic cell bodies and projections, dopamine (DA) synthesis and behaviour, astrocyte-derived GDNF demonstrated the same efficacy as neuron-derived GDNF. In terms of safety, unilateral striatal GDNF expression in astrocytes did not result in delivery of bio-active GDNF to the contralateral hemispheres (potential off-target sites) as happened when GDNF was expressed in neurons. Thus, astrocytic GDNF expression represents a localized but efficient alternative to current gene therapeutic strategies for the treatment of PD, especially if viral vectors with enhanced tissue penetration are considered. Astrocytic neurotrophic factor expression may open new venues for neurotrophic factor-based gene therapy targeting severe diseases of the brain.

A30P α-synuclein impairs dopaminergic fiber regeneration and interacts with L-DOPA replacement in MPTP-treated mice

Parkinson's disease (PD) is the most common neurodegenerative movement disorder and is characterized by the loss of dopaminergic neurons from the substantia nigra pars compacta (SNpc). α-synuclein (αsyn) has been linked to the pathophysiology of PD, because of its mutations causing familial PD and its accumulation in brains of patients with familial and sporadic PD. Dopamine (DA) replacement is the most effective therapy for ameliorating the motor symptoms of PD; however, it remains controversial whether DA-replacement boosts regeneration in the dopaminergic system or accelerates disease progression and enhances neuronal loss. Here, we studied the effect of chronic L-DOPA treatment on dopaminergic neurons in wild-type (WT) and A30P αsyn transgenic mice after MPTP treatment. Acute MPTP intoxication induced degeneration of dopaminergic neurons in both WT and A30P αsyn transgenic mice. A strong regeneration of dopaminergic fibers at 90 days after MPTP was observed in WT mice. In contrast, regeneration was less pronounced in A30P αsyn mice. Chronic L-DOPA treatment after MPTP intoxication did not only reduce the regeneration of nigrostriatal fibers but also led to an increased apoptotic gene-expression profile in the SNpc and to a decline of TH-positive neurons in A30P αsyn. Our findings reveal that the presence of A30P αsyn inhibits the regeneration of nigrostriatal dopaminergic fibers, and that L-DOPA treatment might interact with the pathogenesis in PD.

CDNF protects the nigrostriatal dopamine system and promotes recovery after MPTP treatment in mice

Cerebral dopamine neurotrophic factor (CDNF) is a recently discovered protein, which belongs to the evolutionarily conserved CDNF/MANF family of neurotrophic factors. The degeneration of dopamine neurons following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment is well characterized, and efficacy in this model is considered a standard criterion for development of parkinsonian therapies. MPTP is a neurotoxin, which produces parkinsonian symptoms in humans and in C57/Bl6 mice. To date, there are no reports about the effects of CDNF on dopamine neuron survival or function in the MPTP rodent model, a critical gap. Therefore, we studied whether CDNF has neuroprotective and neurorestorative properties for the nigrostriatal dopamine system after MPTP injections in C57/Bl6 mice. We found that bilateral striatal CDNF injections, given 20 h before MPTP, improved horizontal and vertical motor behavior. CDNF pretreatment increased tyrosine hydroxylase (TH) immunoreactivity in the striatum and in the substantia nigra pars reticulata (SNpr), as well as the number of TH-positive cells in substantia nigra pars compacta (SNpc). Posttreatment with CDNF, given 1 week after MPTP injections, increased horizontal and vertical motor behavior of mice, as well as dopamine fiber densities in the striatum and the number of TH-positive cells in SNpc. CDNF did not alter any of the analyzed dopaminergic biomarkers or locomotor behavior in MPTP-untreated animals. We conclude that striatal CDNF administration is both neuroprotective and neurorestorative for the TH-positive cells in the nigrostriatal dopamine system in the MPTP model, which supports the development of CDNF-based treatment for Parkinson's disease.

Heat shock protein 70 kDa over-expression and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced nigrostriatal degeneration in mice

Oxidative damage in the dopaminergic neurons of substantia nigra pars compacta (SNpc) plays an important role in the pathogenesis of Parkinson's disease (PD). Heat shock proteins 70 kDa (HSP70s) are a sub-family of molecular chaperones involved in not only protein folding and degradation but also antioxidant defense and anti-apoptotic pathways. Here, a transgenic mice over-expressing an inducible form of Hsp70 was used to determine whether HSP70 affects 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced nigrostriatal degeneration, an experimental model of PD. The Hsp70 transgenic animals exhibited a high level of expression of HSP70 protein in ventral mesencephalon. Dopaminergic cell death in the SNpc was similar between wild-type and Hsp70 transgenic mice with either acute (40 mg/kg, single dose) or chronic (20 mg/kg, three times/week during 1 month) MPTP treatment. In addition, striatal dopamine loss was not different between wild-type and transgenic animals. Three months after the acute MPTP treatment, dopamine loss was partially recovered into a similar level between wild-type and transgenic groups. In conclusion, over-expression of Hsp70 does not suppress dopaminergic neuronal damage at either the somata or the axon terminals of dopaminergic neurons. Hsp70 over-expression does not help axon terminal regeneration either. These results indicate that HSP70 alone is not sufficient to reduce MPTP-induced dopaminergic neuronal damage.

Dopamine-depletion and increased α-synuclein load induce degeneration of cortical cholinergic fibers in mice

Cognitive dysfunction can be common among Parkinson's disease (PD) patients, and multiplication of the gene α-synuclein (αsyn) increases the risk of dementia. Here, we studied the role of dopamine-depletion and increased αsyn load and aggregation on cholinergic structures in vivo. Wild-type (WT) and mice with A30P αsyn overexpression were treated subacutely with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and the number of cholinergic cells in their nucleus basalis magnocellularis-substantia innominata (NBM-SI), their cortical fiber density and their expression of different genes 1day or 90 days after the last MPTP-injection were measured. Long-term dopamine depletion decreased the expression of choline acetyl transferase (ChAT) in the NBM-SI of WT mice, but no neuron loss was observed. In contrast, cortical cholinergic fiber density was decreased three months after MPTP-injection. Increased brain-derived neurotrophic factor expression could maintain cholinergic functions under these conditions. Expression of A30P αsyn in six-months-old transgenic mice resulted in decreased tyrosine receptor kinase B expression, and lower cortical cholinergic fiber density. Dopamine-depletion by MPTP induced cholinergic cell loss in the NBM-SI and increased cortical fiber loss. Our findings may explain why cholinergic cells are more vulnerable in PD, leading to an increased probability of dementia.

Motor neuron trophic factors: therapeutic use in ALS?

The modest effects of neurotrophic factor (NTF) treatment on lifespan in both animal models and clinical studies of Amyotropic Lateral Sclerosis (ALS) may result from any one or combination of the four following explanations: 1.) NTFs block cell death in some physiological contexts but not in ALS; 2.) NTFs do not rescue motoneurons (MNs) from death in any physiological context; 3.) NTFs block cell death in ALS but to no avail; and 4.) NTFs are physiologically effective but limited by pharmacokinetic constraints. The object of this review is to critically evaluate the role of both NTFs and the intracellular cell death pathway itself in regulating the survival of spinal and cranial (lower) MNs during development, after injury and in response to disease. Because the role of molecules mediating MN survival has been most clearly resolved by the in vivo analysis of genetically engineered mice, this review will focus on studies of such mice expressing reporter, null or other mutant alleles of NTFs, NTF receptors, cell death or ALS-associated genes.

Glutathione depletion and overproduction both initiate degeneration of nigral dopaminergic neurons

Parkinson's disease is a neurodegenerative disorder characterized by severe motor deficits mainly due to degeneration of dopaminergic neurons in the substantia nigra. Decreased levels of the cell's most important anti-oxidant, glutathione, have been detected in nigral neurons of Parkinson patients, but it is unknown if they are the cause or merely the consequence of the disease. To elucidate if glutathione depletion causes selective degeneration of nigral dopaminergic neurons, we down-regulated glutathione synthesis in different brain areas of adult rats by a viral vector-based RNAi approach. Decreased glutathione synthesis resulted in progressive degeneration of nigral dopaminergic neurons, while extra-nigral and striatal neurons were significantly less vulnerable. Degeneration of dopaminergic neurons was accompanied by progressive protein aggregate formation and functional motor deficits and was partially rescued by α-synuclein. That the survival of nigral dopaminergic neurons depends on the precise control of glutathione levels was further demonstrated by significant degeneration induced through moderate overproduction of glutathione. Over-expression of either of the two subunits of glutamate-cysteine ligase induced aberrant glutathiolation of cellular proteins and significant degeneration of dopaminergic neurons. Thus, while glutathione depletion was demonstrated to be a selective trigger for dopaminergic neuron degeneration, a glutathione replacement approach as a potential treatment option for Parkinson's patients must be considered with great care. In conclusion, our data demonstrate that survival of nigral dopaminergic neurons crucially depends on a tight regulation of their glutathione levels and that the depleted glutathione content detected in the brains of Parkinson's disease patients can be a causative insult for neuronal degeneration.

Nurr1 regulates RET expression in dopamine neurons of adult rat midbrain

Genesis of midbrain dopamine (DA) neurons depends on Nurr1, a nuclear receptor expressed during development and adulthood in these neurons. Nurr1 is required for the expression of genes of dopaminergic phenotype such as tyrosine hydroxylase and DA transporter. The expression of the tyrosine kinase receptor RET also depends on Nurr1 during development. However, it is unknown whether RET expression is regulated by Nurr1 during adulthood, and the mechanism by which Nurr1 regulates RET expression. Using an adeno-associated vector-delivered anti-Nurr1 ribozyme, we knocked-down Nurr1 expression unilaterally in the substantia nigra (SN) of adult rats. Animals injected with the ribozyme displayed a 57.3% decrease in Nurr1 mRNA in the SN accompanied by decreased DA extracellular levels in the striatum. RET mRNA in the injected SN and RET protein in the ipsilateral striatum decreased 76.9% and 47%, respectively. Tyrosine hydroxylase and DA transporter mRNA did not change in Nurr1 knocked-down SN. Nurr1 induced the transcription of the human RET promoter in cell type and concentration-dependent manner. Nurr1 induction of RET promoter is independent of NBRE elements. These results show that the expression of RET in rat adult SN is regulated by Nurr1 and suggest that RET is a transcriptional target of this nuclear receptor.

Aged Tgfbeta2/Gdnf double-heterozygous mice show no morphological and functional alterations in the nigrostriatal system

Loss of dopaminergic neurons in the substantia nigra pars compacta and the resulting decrease in striatal dopamine levels are the hallmarks of Parkinson’s disease. Tgfβ and Gdnf have been identified as neurotrophic factors for dopaminergic midbrain neurons in vivo and in vitro. Haploinsufficiency for either Tgfβ or Gdnf led to dopaminergic deficits. In this study we therefore analyzed the nigrostriatal system of aged Tgfβ2^+/−/Gdnf^+/− double-heterozygous mice. Unexpectedly, we found no morphological changes in the nigrostriatal system as compared with age-matched wild-type mice. There were no significant differences in the number of TH-positive midbrain neurons and no changes in the optical density of TH immunoreactivity in striata of Tgfβ2^+/−/Gdnf^+/− double-heterozygous mice. Moreover, we found no significant differences in the striatal levels of dopamine and its metabolites dihydroxyphenylacetic acid and homovanillic acid. Our results indicate that a combined haploinsufficiency for Tgfβ2 and Gdnf has no impact on the function and the survival of midbrain DA neurons under normal aging conditions.

Pro-survival role for Parkinson's associated gene DJ-1 revealed in trophically impaired dopaminergic neurons

A mouse genetic study reveals a novel cell-survival role for the Parkinson's disease-associated gene DJ-1 in dopaminergic neurons that have reduced support from endogenous survival factors.

The mechanisms underlying the selective death of substantia nigra (SN) neurons in Parkinson disease (PD) remain elusive. While inactivation of DJ-1, an oxidative stress suppressor, causes PD, animal models lacking DJ-1 show no overt dopaminergic (DA) neuron degeneration in the SN. Here, we show that aging mice lacking DJ-1 and the GDNF-receptor Ret in the DA system display an accelerated loss of SN cell bodies, but not axons, compared to mice that only lack Ret signaling. The survival requirement for DJ-1 is specific for the GIRK2-positive subpopulation in the SN which projects exclusively to the striatum and is more vulnerable in PD. Using Drosophila genetics, we show that constitutively active Ret and associated Ras/ERK, but not PI3K/Akt, signaling components interact genetically with DJ-1. Double loss-of-function experiments indicate that DJ-1 interacts with ERK signaling to control eye and wing development. Our study uncovers a conserved interaction between DJ-1 and Ret-mediated signaling and a novel cell survival role for DJ-1 in the mouse. A better understanding of the molecular connections between trophic signaling, cellular stress and aging could uncover new targets for drug development in PD.

The major pathological event in Parkinson disease is the loss of dopaminergic neurons in a midbrain structure, the substantia nigra. The study of familial Parkinson disease has uncovered several disease-associated genes, including DJ-1. Subsequent studies have suggested that the DJ-1 protein is a suppressor of oxidative stress that might modify signaling pathways that regulate cell survival. However, because animal models lacking DJ-1 function do not show dopaminergic neurodegeneration, the function(s) of DJ-1 in vivo remain unclear. Using mouse genetics, we found that DJ-1 is required for survival of neurons of the substantia nigra only in aging conditions and only in neurons that are partially impaired in receiving trophic signals. Aging mice that lack DJ-1 and Ret, a receptor for a neuronal survival factor, lose more dopaminergic neurons in the substantia nigra as compared with aging mice that lack only Ret. Using the fruit fly Drosophila, we determined that DJ-1 interacts with constitutively active Ret and with its associated downstream signaling pathways. Therefore, understanding the molecular connections between trophic signaling, cellular stress and aging could facilitate the identification of new targets for drug development in Parkinson Disease.

Constitutive Ret signaling is protective for dopaminergic cell bodies but not for axonal terminals

Ret is the canonical signaling receptor for glial cell line-derived neurotrophic factor (GDNF), which has been shown to have neuroprotective effects when administered prior to neurotoxic challenge. A missense Meth918Thr mutation causes the constitutive activation of Ret, resulting in multiple endocrine neoplasia type 2 B (MEN2B). To clarify the role of Ret signaling in neuroprotection, we studied the effects of the neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) on the dopaminergic system of mice carrying the MEN2B mutation. We found that MEN2B mice were significantly more resistant to nigral tyrosine hydroxylase (TH)-positive cell loss induced by unilateral striatal 6-OHDA than Wt mice. However, 6-OHDA caused profound dopamine (DA) depletion in the striatum of both MEN2B and Wt mice. Systemic MPTP caused similar DA depletion and a decrease in TH-immunostaining in the striatum of MEN2B and Wt mice. Neither neurotoxin induced a compensatory increase in striatal metabolite/DA ratios in the MEN2B mice, possibly contributing to an increased amphetamine-induced turning behavior observed in behavioral assessments of these mice. Thus, our data suggest that activated Ret protects DA cell bodies in the substantia nigra pars compacta, but does not protect DA axons in the striatum.

Signalling mechanisms underlying development and maintenance of dopamine neurons

Majority of the brain dopamine (DA) neurons reside in a distinct area in the midbrain and project axons into the striatum and frontal cortex to control central nervous system (CNS) functions such as movement, motivation and mood. Age-associated specific loss of DA neurons particularly in the midbrain region substantia nigra pars compacta (SNpc) causes Parkinson disease (PD), an incurable condition characterized by rigidity, involuntary and slowed movement affecting about 1% of people over the age of 60 years. Dopamine neurons appear to be one of the most sensitive types of neurons to both intrinsic and extrinsic stressors in the brain. Here we summarize how transcription factors, growth factors and in particular neurotrophic factors are used to make and maintain DA neurons. We also discuss mechanisms that underlie their specific vulnerability and highlight current state of art in drug development.

Aggregate formation and toxicity by wild-type and R621C synphilin-1 in the nigrostriatal system of mice using adenoviral vectors

Synphilin-1 was described as a protein interacting with alpha-synuclein and is commonly found in Lewy bodies, the pathological hallmark of Parkinson's disease (PD). Our group has previously described and characterized in vitro a mutation in the synphilin-1 gene (R621C) in PD patients. Providing the first characterization of synphilin-1 expression in an animal model, we here used adenoviral gene transfer to study the effects of wild-type (WT) and R621C synphilin-1 in dopaminergic neurons in mouse brain. As synphilin-1 is commonly used to trigger aggregation of alpha-synuclein in cell culture, we investigated not only non-transgenic C57Bl/6 mice but also A30P-alpha-synuclein transgenic animals. Both WT synphilin-1 and R621C synphilin-1 led to the formation of Thioflavine-S positive inclusions in C57Bl/6 mice and degeneration of dopaminergic neurons in the substantia nigra. R621C synphilin-1 induced more aggregate formation than WT synphilin-1 in A30P-alpha-synuclein transgenic mice, consistent with the role of the R621C mutation as a susceptibility factor for PD. Synphilin-1 expression may be used to improve current mouse models of PD, as it induced both the formation of aggregates and degeneration of dopaminergic neurons, two core characteristics of PD that have not been well reproduced with expression of alpha-synuclein.

GDNF is an endogenous negative regulator of ethanol-mediated reward and of ethanol consumption after a period of abstinence

BACKGROUND

We previously found that activation of the glial cell line-derived neurotrophic factor (GDNF) pathway in the ventral tegmental area (VTA) reduces ethanol-drinking behaviors. In this study, we set out to assess the contribution of endogenous GDNF or its receptor GFRalpha1 to the regulation of ethanol-related behaviors.

METHODS

GDNF and GFRalpha1 heterozygote mice (HET) and their wild-type littermate controls (WT) were used for the studies. Ethanol-induced hyperlocomotion, sensitization, and conditioned place preference (CPP), as well as ethanol consumption before and after a period of abstinence were evaluated. Blood ethanol concentration (BEC) was also measured.

RESULTS

We observed no differences between the GDNF HET and WT mice in the level of locomotor activity or in sensitization to ethanol-induced hyperlocomotion after systemic injection of a nonhypnotic dose of ethanol and in BEC. However, GDNF and GFRalpha1 mice exhibited increased place preference to ethanol as compared with their WT littermates. The levels of voluntary ethanol or quinine consumption were similar in the GDNF HET and WT mice, however, a small but significant increase in saccharin intake was observed in the GDNF HET mice. No changes were detected in voluntary ethanol, saccharin or quinine consumption of GFRalpha1 HET mice as compared with their WT littermates. Interestingly, however, both the GDNF and GFRalpha1 HET mice consumed much larger quantities of ethanol after a period of abstinence from ethanol as compared with their WT littermates. Furthermore, the increase in ethanol consumption after abstinence was found to be specific for ethanol as similar levels of saccharin intake were measured in the GDNF and GFRalpha1 HET and WT mice after abstinence.

CONCLUSIONS

Our results suggest that endogenous GDNF negatively regulates the rewarding effect of ethanol and ethanol-drinking behaviors after a period of abstinence.

Excessive alcohol consumption is blocked by glial cell line-derived neurotrophic factor

We previously found that activation of the glial cell line-derived neurotrophic factor (GDNF) pathway in the ventral tegmental area (VTA) reduces moderate alcohol (ethanol) intake in a rat operant self-administration paradigm. Here, we set out to assess the effect of GDNF in the VTA on excessive voluntary consumption of ethanol. Long-Evans rats were trained to drink large quantities of a 20% ethanol solution in an intermittent-access two-bottle choice drinking paradigm. The rats were given three 24-h sessions per week, and GDNF's actions were measured when rats achieved a baseline of ethanol consumption of 5.5g/kg/24h. We found that microinjection of GDNF into the VTA 10min before the beginning of an ethanol-drinking session significantly reduced ethanol intake and preference, but did not affect total fluid intake. We further show that GDNF greatly decreased both the first bout of excessive ethanol intake at the beginning of the session, and the later consummatory activity occurring during the dark cycle. These data suggest that GDNF is a rapid and long-lasting inhibitor of "binge-like" ethanol consumption.

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.

GDNF and GFRalpha: a versatile molecular complex for developing neurons

The GDNF family ligands (GFLs) signal through the canonical signaling receptor Ret and a glycosyl-phosphatidylinositol-anchored co-receptor, GFRalpha. In recent years, signaling by GFLs has been shown to be more complex than originally assumed. The discrepant expression between GFRalphas and Ret has suggested the existence of additional signal-transducing GDNF receptors, such as NCAM. Here we summarize novel functions and Ret-independent signaling mechanisms for GDNF and GFRalpha, focusing on developing neurons. Emerging evidence indicates a prominent role of GDNF and GFRalpha in the control of neuroblast migration and chemoattraction and in the formation of neuronal synapses by a new mechanism of ligand-induced cell adhesion. Therefore, these data highlight the importance of this versatile molecular complex for nervous system development, function and regeneration.

GDNF is a fast-acting potent inhibitor of alcohol consumption and relapse

Previously, we demonstrated that the action of the natural alkaloid, ibogaine, to reduce alcohol (ethanol) consumption is mediated by the glial cell line-derived neurotrophic factor (GDNF) in the ventral tegmental area (VTA). Here we set out to test the actions of GDNF in the VTA on ethanol-drinking behaviors. We found that GDNF infusion very rapidly and dose-dependently reduced rat ethanol, but not sucrose, operant self-administration. A GDNF-mediated decrease in ethanol consumption was also observed in rats with a history of high voluntary ethanol intake. We found that the action of GDNF on ethanol consumption was specific to the VTA as infusion of the growth factor into the neighboring substantia nigra did not affect operant responses for ethanol. We further show that intra-VTA GDNF administration rapidly activated the MAPK signaling pathway in the VTA and that inhibition of the MAPK pathway in the VTA blocked the reduction of ethanol self-administration by GDNF. Importantly, we demonstrate that GDNF infused into the VTA alters rats' responses in a model of relapse. Specifically, GDNF application blocked reacquisition of ethanol self-administration after extinction. Together, these results suggest that GDNF, via activation of the MAPK pathway, is a fast-acting selective agent to reduce the motivation to consume and seek alcohol.