Unilateral rNurr1-V5 transgene expression in nigral dopaminergic neurons mitigates bilateral neuropathology and behavioral deficits in parkinsonian rats with α-synucleinopathy

JOURNAL/nrgr/04.03/01300535-202409000-00039/figure1/v/2024-01-16T170235Z/r/image-tiff Parkinsonism by unilateral, intranigral β-sitosterol β-D-glucoside administration in rats is distinguished in that the α-synuclein insult begins unilaterally but spreads bilaterally and increases in severity over time, thus replicating several clinical features of Parkinson's disease, a typical α-synucleinopathy. As Nurr1 represses α-synuclein, we evaluated whether unilateral transfected of rNurr1-V5 transgene via neurotensin-polyplex to the substantia nigra on day 30 after unilateral β-sitosterol β-D-glucoside lesion could affect bilateral neuropathology and sensorimotor deficits on day 30 post-transfection. This study found that rNurr1-V5 expression but not that of the green fluorescent protein (the negative control) reduced β-sitosterol β-D-glucoside-induced neuropathology. Accordingly, a bilateral increase in tyrosine hydroxylase-positive cells and arborization occurred in the substantia nigra and increased tyrosine hydroxylase-positive ramifications in the striatum. In addition, tyrosine hydroxylase-positive cells displayed less senescence marker β-galactosidase and more neuron-cytoskeleton marker βIII-tubulin and brain-derived neurotrophic factor. A significant decrease in activated microglia (positive to ionized calcium-binding adaptor molecule 1) and neurotoxic astrocytes (positive to glial fibrillary acidic protein and complement component 3) and increased neurotrophic astrocytes (positive to glial fibrillary acidic protein and S100 calcium-binding protein A10) also occurred in the substantia nigra. These effects followed the bilateral reduction in α-synuclein aggregates in the nigrostriatal system, improving sensorimotor behavior. Our results show that unilateral rNurr1-V5 transgene expression in nigral dopaminergic neurons mitigates bilateral neurodegeneration (senescence and loss of neuron-cytoskeleton and tyrosine hydroxylase-positive cells), neuroinflammation (activated microglia, neurotoxic astrocytes), α-synuclein aggregation, and sensorimotor deficits. Increased neurotrophic astrocytes and brain-derived neurotrophic factor can mediate the rNurr1-V5 effect, supporting its potential clinical use in the treatment of Parkinson's disease.

Nurr1 overexpression in the primary motor cortex alleviates motor dysfunction induced by intracerebral hemorrhage in the striatum in mice

Hemorrhage-induced injury of the corticospinal tract (CST) in the internal capsule (IC) causes severe neurological dysfunction in both human patients and rodent models of intracerebral hemorrhage (ICH). A nuclear receptor Nurr1 (NR4A2) is known to exert anti-inflammatory and neuroprotective effects in several neurological disorders. Previously we showed that Nurr1 ligands prevented CST injury and alleviated neurological deficits after ICH in mice. To prove direct effect of Nurr1 on CST integrity, we examined the effect of Nurr1 overexpression in neurons of the primary motor cortex on pathological consequences of ICH in mice. ICH was induced by intrastriatal injection of collagenase type VII, where hematoma invaded into IC. Neuron-specific overexpression of Nurr1 was induced by microinjection of synapsin I promoter-driven adeno-associated virus (AAV) vector into the primary motor cortex. Nurr1 overexpression significantly alleviated motor dysfunction but showed only modest effect on sensorimotor dysfunction after ICH. Nurr1 overexpression also preserved axonal structures in IC, while having no effect on hematoma-associated inflammatory events, oxidative stress, and neuronal death in the striatum after ICH. Immunostaining revealed that Nurr1 overexpression increased the expression of Ret tyrosine kinase and phosphorylation of Akt and ERK1/2 in neurons in the motor cortex. Moreover, administration of Nurr1 ligands 1,1-bis(3'-indolyl)-1-(p-chlorophenyl)methane or amodiaquine increased phosphorylation levels of Akt and ERK1/2 as well as expression of glial cell line-derived neurotrophic factor and Ret genes in the cerebral cortex. These results suggest that the therapeutic effect of Nurr1 on striatal ICH is attributable to the preservation of CST by acting on cortical neurons.

Preparation of human astrocytes with potent therapeutic functions from human pluripotent stem cells using ventral midbrain patterning

INTRODUCTION

Astrocytes are glial-type cells that protect neurons from toxic insults and support neuronal functions and metabolism in a healthy brain. Leveraging these physiological functions, transplantation of astrocytes or their derivatives has emerged as a potential therapeutic approach for neurodegenerative disorders.

METHODS

To substantiate the clinical application of astrocyte-based therapy, we aimed to prepare human astrocytes with potent therapeutic capacities from human pluripotent stem cells (hPSCs). To that end, we used ventral midbrain patterning during the differentiation of hPSCs into astrocytes, based on the roles of midbrain-specific factors in potentiating glial neurotrophic/anti-inflammatory activity. To assess the therapeutic effects of human midbrain-type astrocytes, we transplanted them into mouse models of Parkinson's disease (PD) and Alzheimer's disease (AD).

RESULTS

Through a comprehensive series of in-vitro and in-vivo experiments, we were able to establish that the midbrain-type astrocytes exhibited the abilities to effectively combat oxidative stress, counter excitotoxic glutamate, and manage pathological protein aggregates. Our strategy for preparing midbrain-type astrocytes yielded promising results, demonstrating the strong therapeutic potential of these cells in various neurotoxic contexts. Particularly noteworthy is their efficacy in PD and AD-specific proteopathic conditions, in which the midbrain-type astrocytes outperformed forebrain-type astrocytes derived by the same organoid-based method.

CONCLUSION

The enhanced functions of the midbrain-type astrocytes extended to their ability to release signaling molecules that inhibited neuronal deterioration and senescence while steering microglial cells away from a pro-inflammatory state. This success was evident in both in-vitro studies using human cells and in-vivo experiments conducted in mouse models of PD and AD. In the end, our human midbrain-type astrocytes demonstrated remarkable effectiveness in alleviating neurodegeneration, neuroinflammation, and the pathologies associated with the accumulation of α-synuclein and Amyloid β proteins.

Investigating the Effect of an Anti-Inflammatory Drug in Determining NURR1 Expression and Thus Exploring the Progression of Parkinson's Disease

Nonsteroidal anti-inflammatory drugs are the most widely used drugs for Parkinson's disease (PD), of which ibuprofen shows positive effects in suppressing symptoms; however, the associated risk needs to be addressed in different pathological stages. Initially, we developed an initial and advanced stage of the Parkinson disease mouse model by intraperitoneal injection of MPTP (20 mg/kg; 1-methyl-4-phenyl-1,2,3,6-tetrahydro-pyridine) for 10 and 20 days, respectively. Subsequently, ibuprofen treatment was administered for 2 months, and a pole test, rotarod test, histology, immunohistochemistry, and western blotting were performed to determine neuronal motor function. Histological analysis for 10 days after mice were injected with MPTP showed the onset of neurodegeneration and cell aggregation, indicating the initial stages of Parkinson's disease. Advanced Parkinson's disease was marked by Lewy body formation after another 10 days of MPTP injection. Neurodegeneration reverted after ibuprofen therapy in initial Parkinson's disease but not in advanced Parkinson's disease. The pole and rotarod tests confirmed that motor activity in the initial Parkinson disease with ibuprofen treatment recovered (p<0.01). However, no improvement was observed in the ibuprofen-treated mice with advanced disease mice. Interestingly, ibuprofen treatment resulted in a significant improvement (p<0.01) in NURR1 (Nuclear receptor-related 1) expression in mice with early PD, but no substantial improvement was observed in its expression in mice with advanced PD. Our findings indicate that NURR1 exerts anti-inflammatory and neuroprotective effects. Overall, NURR1 contributed to the effects of ibuprofen on PD at different pathological stages.

The Synergistic Effect Study of Lipopolysaccharide (LPS) and A53T-α-Synuclein: Intranasal LPS Exposure on the A53T-α-Synuclein Transgenic Mouse Model of Parkinson's Disease

Aging and interactions between genetic and environmental factors are believed to be involved the chronic development of Parkinson's disease (PD). Among PD patients, abnormally aggregated α-synuclein is a major component of the Lewy body. Generally, the intranasal route is believed to be a gate way to the brain, and it assists environmental neurotoxins in entering the brain and is related to anosmia during early PD. The current study applies the chronic intranasal application of lipopolysaccharides (LPS) in 4-, 8-, 12- and 16-month-old A53T-α-synuclein (A53T-α-Syn) transgenic C57BL/6 mice at 2-day intervals for a 2-month period, for evaluating the behavioral, pathological, and biochemical changes and microglial activation in these animals. According to our results, after intranasal administration of LPS, A53T-α-Syn mice showed severe progressive anosmia, hypokinesia, selective dopaminergic (DAergic) neuronal losses, decreased striatal dopamine (DA) level, and enhanced α-synuclein accumulation within the substantia nigra (SN) in an age-dependent way. In addition, we found obvious NF-кB activation, Nurr1 inhibition, IL-1β, and TNF-α generation within the microglia of the SN. Conversely, the wild-type (WT) mice showed mild, whereas A53T-α-Syn mice had moderate PD-like changes among the old mice. This study demonstrated the synergistic effect of intranasal LPS and α-synuclein burden on PD development. Its underlying mechanism may be associated with Nurr1 inhibition within microglia and the amplification of CNS neuroinflammation. The mice with multiple factors, including aging, neuroinflammation, and α-synuclein mutation, have played a significant role in enhancing our understanding of how inflammation and α-synuclein mutation contribute to the neurodegeneration observed in PD.

Structural Perspective of NR4A Nuclear Receptor Family and Their Potential Endogenous Ligands

There are 48 nuclear receptors in the human genome, and many members of this superfamily have been implicated in human diseases. The NR4A nuclear receptor family consisting of three members, NR4A1, NR4A2, and NR4A3 (formerly annotated as Nur77, Nurr1, and NOR1, respectively), are still orphan receptors but exert pathological effects on immune-related and neurological diseases. We previously reported that prostaglandin A1 (PGA1) and prostaglandin A2 (PGA2) are potent activators of NR4A3, which bind directly to the ligand-binding domain (LBD) of the receptor. Recently, the co-crystallographic structures of NR4A2-LBD bound to PGA1 and PGA2 were reported, followed by reports of the neuroprotective effects of these possible endogenous ligands in mouse models of Parkinson's disease. Based on these structures, we modeled the binding structures of the other two members (NR4A1 and NR4A3) with these potential endogenous ligands using a template-based modeling method, and reviewed the similarity and diversity of ligand-binding mechanisms in the nuclear receptor family.

Adeno-associated virus (AAV) 9-mediated gene delivery of Nurr1 and Foxa2 ameliorates symptoms and pathologies of Alzheimer disease model mice by suppressing neuro-inflammation and glial pathology

There is a compelling need to develop disease-modifying therapies for Alzheimer's disease (AD), the most common neuro-degenerative disorder. Together with recent progress in vector development for efficiently targeting the central nervous system, gene therapy has been suggested as a potential therapeutic modality to overcome the limited delivery of conventional types of drugs to and within the damaged brain. In addition, given increasing evidence of the strong link between glia and AD pathophysiology, therapeutic targets have been moving toward those addressing glial cell pathology. Nurr1 and Foxa2 are transcription/epigenetic regulators that have been reported to cooperatively regulate inflammatory and neurotrophic response in glial cells. In this study, we tested the therapeutic potential of Nurr1 and Foxa2 gene delivery to treat AD symptoms and pathologies. A series of functional, histologic, and transcriptome analyses revealed that the combined expression of Nurr1 and Foxa2 substantially ameliorated AD-associated amyloid β and Tau proteinopathy, cell senescence, synaptic loss, and neuro-inflammation in multiple in vitro and in vivo AD models. Intra-cranial delivery of Nurr1 and Foxa2 genes using adeno-associated virus (AAV) serotype 9 improved the memory and cognitive function of AD model mice. The therapeutic benefits of gene delivery were attained mainly by correcting pathologic glial function. These findings collectively indicate that AAV9-mediated Nurr1 and Foxa2 gene transfer could be an effective disease-modifying therapy for AD.

Structure-Guided Design of Nurr1 Agonists Derived from the Natural Ligand Dihydroxyindole

The neuroprotective transcription factor Nurr1 was recently found to bind the dopamine metabolite 5,6-dihydroxyindole (DHI) providing access to Nurr1 ligand design from a natural template. We screened a custom set of 14 k extended DHI analogues in silico for optimized descendants to select 24 candidates for microscale synthesis and in vitro testing. Three out of six primary hits were validated as novel Nurr1 agonists with up to sub-micromolar binding affinity, highlighting the druggability of the Nurr1 surface region lining helix 12. In vitro profiling confirmed cellular target engagement of DHI descendants and demonstrated remarkable additive effects of combined Nurr1 agonist treatment, indicating diverse binding sites mediating Nurr1 activation, which may open new avenues in Nurr1 modulation.

The relationship between depletion of brain GM1 ganglioside and Parkinson's disease

GM1 is one of the main gangliosides of the nervous system, and it exerts neurotrophic and neuroprotective properties in neurons. It is involved in many processes necessary for the correct physiology of neuronal cells. In particular, it is necessary for the activity of neuronal receptors that control processes such as differentiation, survival, and mitochondrial activity. A shortage of GM1 in the substantia nigra is potentially responsible for the neurodegeneration present in Parkinson's disease patients. In this review, I report on the role played by GM1 in neurons and how its genetic shortage may be responsible for the onset of Parkinson's disease.

Post-Mortem Studies of Neurturin Gene Therapy for Parkinson's Disease: Two Subjects with 10 Years CERE120 Delivery

BACKGROUND

Neurturin is a member of the glial cell line-derived neurotrophic factor family of neurotrophic factors and has the potential to protectdegenerating dopaminergic neurons.

OBJECTIVE

Here, we performed post-mortem studies on two patients with advanced Parkinson's disease that survived 10 years following AAV-neurturin gene (Cere120) delivery to verify long-term effects of trophic factor neurturin.

METHODS

Cere120 was delivered to the putamen bilaterally in one case and to the putamen plus substantia nigra bilaterally in the second. Immunohistochemistry was used to examine neurturin, Rearranged during transfection(RET), phosphor-S6, and tyrosine hydroxylase expressions, inflammatory reactions, and α-synuclein accumulation.

RESULTS

In both patients there was persistent, albeit limited, neurturin expression in the putamen covering 1.31% to 5.92% of the putamen. Dense staining of tyrosine hydroxylase-positive fibers was observed in areas that contained detectable neurturin expression. In substantia nigra, neurturin expression was detected in 11% of remaining melanin-containing neurons in the patient with combined putamenal and nigral gene delivery, but not in the patient with putamenal gene delivery alone. Tyrosine hydroxylase positive neurons were 66% to 84% of remaining neuromelanin neurons in substantia nigra with Cere120 delivery and 23% to 24% in substantia nigra without gene delivery. More RET and phosphor-S6 positive neurons were observed in substantia nigra following nigral Cere120. Inflammatory and Lewy pathologies were similar in substantia nigra with or without Cere120 delivery.

CONCLUSIONS

This study provides evidence of long-term persistent transgene expression and bioactivity following gene delivery to the nigrostriatal system. Therefore, future efforts using gene therapy for neurodegenerative diseases should consider means to enhance remaining dopamine neuron function and stop pathological propagation. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

An optimized Nurr1 agonist provides disease-modifying effects in Parkinson's disease models

The nuclear receptor, Nurr1, is critical for both the development and maintenance of midbrain dopamine neurons, representing a promising molecular target for Parkinson's disease (PD). We previously identified three Nurr1 agonists (amodiaquine, chloroquine and glafenine) that share an identical chemical scaffold, 4-amino-7-chloroquinoline (4A7C), suggesting a structure-activity relationship. Herein we report a systematic medicinal chemistry search in which over 570 4A7C-derivatives were generated and characterized. Multiple compounds enhance Nurr1's transcriptional activity, leading to identification of an optimized, brain-penetrant agonist, 4A7C-301, that exhibits robust neuroprotective effects in vitro. In addition, 4A7C-301 protects midbrain dopamine neurons in the MPTP-induced male mouse model of PD and improves both motor and non-motor olfactory deficits without dyskinesia-like behaviors. Furthermore, 4A7C-301 significantly ameliorates neuropathological abnormalities and improves motor and olfactory dysfunctions in AAV2-mediated α-synuclein-overexpressing male mouse models. These disease-modifying properties of 4A7C-301 may warrant clinical evaluation of this or analogous compounds for the treatment of patients with PD.

Lithium's effects on therapeutic targets and MRI biomarkers in Parkinson's disease: A pilot clinical trial

Background

Lithium has a wide range of neuroprotective actions, has been effective in Parkinson's disease (PD) animal models and may account for the decreased risk of PD in smokers.

Methods

This open-label pilot clinical trial randomized 16 PD patients to "high-dose" (n = 5, lithium carbonate titrated to achieve serum level of 0.4-0.5 mmol/L), "medium-dose" (n = 6, 45 mg/day lithium aspartate) or "low-dose" (n = 5, 15 mg/day lithium aspartate) lithium therapy for 24-weeks. Peripheral blood mononuclear cell (PBMC) mRNA expression of nuclear receptor-related-1 (Nurr1) and superoxide dismutase-1 (SOD1) were assessed by qPCR in addition to other PD therapeutic targets. Two patients from each group received multi-shell diffusion MRI scans to assess for free water (FW) changes in the dorsomedial nucleus of the thalamus and nucleus basalis of Meynert, which reflect cognitive decline in PD, and the posterior substantia nigra, which reflects motor decline in PD.

Results

Two of the six patients receiving medium-dose lithium therapy withdrew due to side effects. Medium-dose lithium therapy was associated with the greatest numerical increases in PBMC Nurr1 and SOD1 expression (679% and 127%, respectively). Also, medium-dose lithium therapy was the only dosage associated with mean numerical decreases in brain FW in all three regions of interest, which is the opposite of the known longitudinal FW changes in PD.

Conclusion

Medium-dose lithium aspartate therapy was associated with engagement of blood-based therapeutic targets and improvements in MRI disease-progression biomarkers but was poorly tolerated in 33% of patients. Further PD clinical research is merited examining lithium's tolerability, effects on biomarkers and potential disease-modifying effects.

Want of Wnt in Parkinson's disease: Could sFRP disrupt interplay between Nurr1 and Wnt signaling?

Nuclear receptor related 1 (Nurr1) is a transcription factor known to regulate the development and maintenance of midbrain dopaminergic (mDA) neurons. Reports have confirmed that defect or obliteration of Nurr1 results in neurodegeneration and motor function impairment leading to Parkinson's disease (PD). Studies have also indicated that Nurr1 regulates the expression of alpha-synuclein (α-SYN) and mutations in Nurr1 cause α-SYN overexpression, thereby increasing the risk of PD. Nurr1 is modulated via various pathways including Wnt signaling pathway which is known to play an important role in neurogenesis and deregulation of it contributes to PD pathogenesis. Both Wnt/β-catenin dependent and independent pathways are implicated in the activation of Nurr1 and subsequent downregulation of α-SYN. This review highlights the interaction between Nurr1 and Wnt signaling pathways in mDA neuronal development. We further hypothesize how modulation of Wnt signaling pathway by its antagonist, secreted frizzled related proteins (sFRPs) could be a potential route to treat PD.

Genetic modifiers of synucleinopathies-lessons from experimental models

α-Synuclein is a pleiotropic protein underlying a group of progressive neurodegenerative diseases, including Parkinson's disease and dementia with Lewy bodies. Together, these are known as synucleinopathies. Like all neurological diseases, understanding of disease mechanisms is hampered by the lack of access to biopsy tissues, precluding a real-time view of disease progression in the human body. This has driven researchers to devise various experimental models ranging from yeast to flies to human brain organoids, aiming to recapitulate aspects of synucleinopathies. Studies of these models have uncovered numerous genetic modifiers of α-synuclein, most of which are evolutionarily conserved. This review discusses what we have learned about disease mechanisms from these modifiers, and ways in which the study of modifiers have supported ongoing efforts to engineer disease-modifying interventions for synucleinopathies.

Protective mechanisms by glial cell line-derived neurotrophic factor and cerebral dopamine neurotrophic factor against the α-synuclein accumulation in Parkinson's disease

Synucleinopathies constitute a disease family named after alpha-synuclein protein, which is a significant component of the intracellular inclusions called Lewy bodies. Accompanying the progressive neurodegeneration, Lewy bodies and neurites are the main histopathologies of synucleinopathies. The complicated role of alpha-synuclein in the disease pathology makes it an attractive therapeutic target for disease-modifying treatments. GDNF is one of the most potent neurotrophic factors for dopamine neurons, whereas CDNF is protective and neurorestorative with entirely different mechanisms of action. Both have been in the clinical trials for the most common synucleinopathy, Parkinson's disease. With the AAV-GDNF clinical trials ongoing and the CDNF trial being finalized, their effects on abnormal alpha-synuclein accumulation are of great interest. Previous animal studies with an alpha-synuclein overexpression model have shown that GDNF was ineffective against alpha-synuclein accumulation. However, a recent study with cell culture and animal models of alpha-synuclein fibril inoculation has demonstrated the opposite by revealing that the GDNF/RET signaling cascade is required for the protective effect of GDNF on alpha-synuclein aggregation. CDNF, an ER resident protein, was shown to bind alpha-synuclein directly. CDNF reduced the uptake of alpha-synuclein fibrils by the neurons and alleviated the behavioral deficits induced by fibrils injected into the mouse brain. Thus, GDNF and CDNF can modulate different symptoms and pathologies of Parkinson's disease, and perhaps, similarly for other synucleinopathies. Their unique mechanisms for preventing alpha-synuclein-related pathology should be studied more carefully to develop disease-modifying therapies.

Restorative cell and gene therapies for Parkinson's disease

One of the core pathological features of Parkinson's disease (PD) is the loss of the dopaminergic nigrostriatal pathway which lies at the heart of many of the motor features of this condition as well as some of the cognitive problems. The importance of this pathological event is evident through the clinical benefits that are seen when patients with PD are treated with dopaminergic agents, at least in early-stage disease. However, these agents create problems of their own through stimulation of more intact dopaminergic networks within the central nervous system causing major neuropsychiatric problems including dopamine dysregulation. In addition, over time the nonphysiological stimulation of striatal dopamine receptors by l-dopa containing drugs leads to the genesis of l-dopa-induced dyskinesias that can become very disabling in many cases. As such, there has been much interest in trying to better reconstitute the dopaminergic nigrostriatal pathway using either factors to regrow it, cells to replace it, or gene therapies to restore dopamine transmission in the striatum. In this chapter, we lay out the rationale, history and current status of these different therapies as well as highlighting where the field is heading and what new interventions might come to clinic in the coming years.

From 2D to 3D: Development of Monolayer Dopaminergic Neuronal and Midbrain Organoid Cultures for Parkinson's Disease Modeling and Regenerative Therapy

Parkinson's Disease (PD) is a prevalent neurodegenerative disorder that is characterized pathologically by the loss of A9-specific dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) of the midbrain. Despite intensive research, the etiology of PD is currently unresolved, and the disease remains incurable. This, in part, is due to the lack of an experimental disease model that could faithfully recapitulate the features of human PD. However, the recent advent of induced pluripotent stem cell (iPSC) technology has allowed PD models to be created from patient-derived cells. Indeed, DA neurons from PD patients are now routinely established in many laboratories as monolayers as well as 3D organoid cultures that serve as useful toolboxes for understanding the mechanism underlying PD and also for drug discovery. At the same time, the iPSC technology also provides unprecedented opportunity for autologous cell-based therapy for the PD patient to be performed using the patient's own cells as starting materials. In this review, we provide an update on the molecular processes underpinning the development and differentiation of human pluripotent stem cells (PSCs) into midbrain DA neurons in both 2D and 3D cultures, as well as the latest advancements in using these cells for drug discovery and regenerative medicine. For the novice entering the field, the cornucopia of differentiation protocols reported for the generation of midbrain DA neurons may seem daunting. Here, we have distilled the essence of the different approaches and summarized the main factors driving DA neuronal differentiation, with the view to provide a useful guide to newcomers who are interested in developing iPSC-based models of PD.

Developmental pathways linked to the vulnerability of adult midbrain dopaminergic neurons to neurodegeneration

The degeneration of dopaminergic and other neurons in the aging brain is considered a process starting well beyond the infantile and juvenile period. In contrast to other dopamine-associated neuropsychiatric disorders, such as schizophrenia and drug addiction, typically diagnosed during adolescence or young adulthood and, thus, thought to be rooted in the developing brain, Parkinson's Disease (PD) is rarely viewed as such. However, evidences have accumulated suggesting that several factors might contribute to an increased vulnerability to death of the dopaminergic neurons at an already very early (developmental) phase in life. Despite the remarkable ability of the brain to compensate such dopamine deficits, the early loss or dysfunction of these neurons might predispose an individual to suffer from PD because the critical threshold of dopamine function will be reached much earlier in life, even if the time-course and strength of naturally occurring and age-dependent dopaminergic cell death is not markedly altered in this individual. Several signaling and transcriptional pathways required for the proper embryonic development of the midbrain dopaminergic neurons, which are the most affected in PD, either continue to be active in the adult mammalian midbrain or are reactivated at the transition to adulthood and under neurotoxic conditions. The persistent activity of these pathways often has neuroprotective functions in adult midbrain dopaminergic neurons, whereas the reactivation of silenced pathways under pathological conditions can promote the survival and even regeneration of these neurons in the lesioned or aging brain. This article summarizes our current knowledge about signaling and transcription factors involved in midbrain dopaminergic neuron development, whose reduced gene dosage or signaling activity are implicated in a lower survival rate of these neurons in the postnatal or aging brain. It also discusses the evidences supporting the neuroprotection of the midbrain dopaminergic system after the external supply or ectopic expression of some of these secreted and nuclear factors in the adult and aging brain. Altogether, the timely monitoring and/or correction of these signaling and transcriptional pathways might be a promising approach to a much earlier diagnosis and/or prevention of PD.

Advances in NURR1-Regulated Neuroinflammation Associated with Parkinson's Disease

Neuroinflammation plays a crucial role in the progression of neurodegenerative disorders, particularly Parkinson's disease (PD). Glial cell activation and subsequent adaptive immune involvement are neuroinflammatory features in familial and idiopathic PD, resulting in the death of dopaminergic neuron cells. An oxidative stress response, inflammatory mediator production, and immune cell recruitment and activation are all hallmarks of this activation, leading to chronic neuroinflammation and progressive neurodegeneration. Several studies in PD patients' cerebrospinal fluid and peripheral blood revealed alterations in inflammatory markers and immune cell populations that may lead to or exacerbate neuroinflammation and perpetuate the neurodegenerative process. Most of the genes causing PD are also expressed in astrocytes and microglia, converting their neuroprotective role into a pathogenic one and contributing to disease onset and progression. Nuclear receptor-related transcription factor 1 (NURR1) regulates gene expression linked to dopaminergic neuron genesis and functional maintenance. In addition to playing a key role in developing and maintaining neurotransmitter phenotypes in dopaminergic neurons, NURR1 agonists have been shown to reverse behavioral and histological abnormalities in animal PD models. NURR1 protects dopaminergic neurons from inflammation-induced degeneration, specifically attenuating neuronal death by suppressing the expression of inflammatory genes in microglia and astrocytes. This narrative review highlights the inflammatory changes in PD and the advances in NURR1-regulated neuroinflammation associated with PD. Further, we present new evidence that targeting this inflammation with a variety of potential NURR1 target therapy medications can effectively slow the progression of chronic neuroinflammation-induced PD.

A Combined α-Synuclein/Fibril (SynFib) Model of Parkinson-Like Synucleinopathy Targeting the Nigrostriatal Dopamine System

Injections of pre-formed α-synuclein fibrils (PFFs) or overexpression of α-synuclein using AAV vectors are commonly used as models of Parkinson-like synucleinopathy in rats and mice. In the modified method reviewed here, the "SynFib" model, the PFFs and the AAV vector are administered together unilaterally into the substantia nigra. This approach combines the key features of these two models, i.e., the generation of toxic α-synuclein aggregates and Lewy body-like inclusions, in combination with the increased vulnerability caused by increased cellular levels of α-synuclein. The combined AAV/PFF delivery offers several advantages over the standard PFF model due to the enhanced and accelerated α-synuclein pathology and microglial response induced by the PFF seeds in the presence of an elevated α-synuclein level. Injection of the AAV/PFF mixture into the substantia nigra makes it possible to target a larger proportion of the nigral dopamine neurons and obtain a level of dopamine cell loss (>60%) needed to induce significant impairments in drug-induced and spontaneous motor tests. The SynFib model shares attractive features of the standard 6-OHDA lesion model: a single unilateral stereotaxic intervention; pathology and cell loss developing over a short time span; and the possibility to monitor the degenerative changes using tests of motor behavior.

Functional Interaction Between α-Synuclein and Nurr1 in Dopaminergic Neurons

Increased expression of alpha-synuclein (ASYN) and decreased expression of Nurr1 are associated with Parkinson's disease (PD) pathogenesis. These two proteins interact functionally and ASYN overexpression suppresses Nurr1 levels. ASYN pan-neuronal overexpression coupled with Nurr1 hemizygosity followed by Nurr1 repression in aging mice results in the manifestation of a typical PD-related phenotype and pathology. Here we investigated in mice the effects of C-terminally truncated ASYN(120) overexpression in dopaminergic (DA-ergic) neurons compounded with Nurr1 hemizygosity ('2-hit-DA'). We report that '2-hit-DA' animals did not manifest a characteristic PD-related phenotype, despite further substantia nigra ASYN-overexpression-dependent and age dependent Nurr1 protein downregulation. However, they displayed increased energy expenditure, reduced striatal dopamine (DA) and prolonged hyperactivity to a novel environment indicating impaired habituation. This DA-ergic dysfunction was observed in young adult '2-hit-DA' mice, persisted throughout life and it was associated with ASYN and Nurr1 synergistic alterations of DAT levels and function. Our experiments indicate that the expression levels of ASYN and Nurr1 are critical in the dysregulation of the nigrostriatal DA system and may be involved in neuropsychiatric aspects of PD.

Pathogenesis of α-Synuclein in Parkinson's Disease: From a Neuron-Glia Crosstalk Perspective

Parkinson's disease (PD) is a progressive neurodegenerative disorder. The classical behavioral defects of PD patients involve motor symptoms such as bradykinesia, tremor, and rigidity, as well as non-motor symptoms such as anosmia, depression, and cognitive impairment. Pathologically, the progressive loss of dopaminergic (DA) neurons in the substantia nigra (SN) and the accumulation of α-synuclein (α-syn)-composed Lewy bodies (LBs) and Lewy neurites (LNs) are key hallmarks. Glia are more than mere bystanders that simply support neurons, they actively contribute to almost every aspect of neuronal development and function; glial dysregulation has been implicated in a series of neurodegenerative diseases including PD. Importantly, amounting evidence has added glial activation and neuroinflammation as new features of PD onset and progression. Thus, gaining a better understanding of glia, especially neuron-glia crosstalk, will not only provide insight into brain physiology events but also advance our knowledge of PD pathologies. This review addresses the current understanding of α-syn pathogenesis in PD, with a focus on neuron-glia crosstalk. Particularly, the transmission of α-syn between neurons and glia, α-syn-induced glial activation, and feedbacks of glial activation on DA neuron degeneration are thoroughly discussed. In addition, α-syn aggregation, iron deposition, and glial activation in regulating DA neuron ferroptosis in PD are covered. Lastly, we summarize the preclinical and clinical therapies, especially targeting glia, in PD treatments.

Transfer of neuron-derived α-synuclein to astrocytes induces neuroinflammation and blood-brain barrier damage after methamphetamine exposure: Involving the regulation of nuclear receptor-associated protein 1

The α-synuclein (α-syn) is involved in methamphetamine (METH)-induced neurotoxicity. Neurons can transfer excessive α-syn to neighboring neurons and glial cells. The effects of α-syn aggregation in astrocytes after METH exposure on the blood-brain barrier (BBB) remains unclear. Our previous study demonstrated that nuclear receptor-related protein 1 (Nurr1), a member of the nuclear receptor family widely expressed in the brain, was involved in the process of METH-induced α-syn accumulated in astrocytes to activate neuroinflammation. The role Nurr1 plays in astrocyte-mediated neuroinflammation, which results in BBB injury induced by METH, remains uncertain. This study found that METH up-regulated α-syn expression in neurons extended to astrocytes, thereby eliciting astrocyte activation, increasing and decreasing IL-1β, IL-6, TNF-α, and GDNF levels by down-regulating Nurr1 expression, and ultimately damaging the BBB. Specifically, the permeability of BBB to Evans blue and sodium fluorescein (NaF) increased; IgG deposits in the brain parenchyma increased; the Claudin5, Occludin, and PDGFRβ levels decreased. Several ultrastructural pathological changes occurred in the BBB, such as abnormal cerebral microvascular diameter, astrocyte end-foot swelling, decreased pericyte coverage, and loss of tight junctions. However, knockout or inhibition of α-syn or astrocyte-specific overexpression of Nurr1 partially alleviated these symptoms and BBB injury. Moreover, the in vitro experiments confirmed that METH increased α-syn level in the primary cultured neurons, which could be further transferred to primary cultured astrocytes, resulting in decreased Nurr1 levels. The decreased Nurr1 levels mediated the increase of IL-1β, IL-6, and TNF-α, and the decrease of GDNF, thereby changing the permeability to NaF, transendothelial electrical resistance, and Claudin5 and Occludin levels of primary cultured brain microvascular endothelial cells. Based on our findings, we proposed a new mechanism to elucidate METH-induced BBB injury and presented α-syn and Nurr1 as promising drug intervention targets to reduce BBB injury and resulting neurotoxicity in METH abusers.

Oxidative Stress and Mitochondrial Complex I Dysfunction Correlate with Neurodegeneration in an α-Synucleinopathy Animal Model

The α-synucleinopathies constitute a subset of neurodegenerative disorders, of which Parkinson’s disease (PD) is the most common worldwide, characterized by the accumulation of misfolded α-synuclein in the cytoplasm of neurons, which spreads in a prion-like manner to anatomically interconnected brain areas. However, it is not clear how α-synucleinopathy triggers neurodegeneration. We recently developed a rat model through a single intranigral administration of the neurotoxic β-sitosterol β-D-glucoside (BSSG), which produces α-synucleinopathy. In this model, we aimed to evaluate the temporal pattern of levels in oxidative and nitrosative stress and mitochondrial complex I (CI) dysfunction and how these biochemical parameters are associated with neurodegeneration in different brain areas with α-synucleinopathy (Substantia nigra pars compacta, the striatum, in the hippocampus and the olfactory bulb, where α-syn aggregation spreads). Interestingly, an increase in oxidative stress and mitochondrial CI dysfunction accompanied neurodegeneration in those brain regions. Furthermore, in silico analysis suggests a high-affinity binding site for BSSG with peroxisome proliferator-activated receptors (PPAR) alpha (PPAR-α) and gamma (PPAR-γ). These findings will contribute to elucidating the pathophysiological mechanisms associated with α-synucleinopathies and lead to the identification of new early biomarkers and therapeutic targets.

The neuroprotective effects of glucagon-like peptide 1 in Alzheimer’s and Parkinson’s disease: An in-depth review

Currently, there is no disease-modifying treatment available for Alzheimer’s and Parkinson’s disease (AD and PD) and that includes the highly controversial approval of the Aβ-targeting antibody aducanumab for the treatment of AD. Hence, there is still an unmet need for a neuroprotective drug treatment in both AD and PD. Type 2 diabetes is a risk factor for both AD and PD. Glucagon-like peptide 1 (GLP-1) is a peptide hormone and growth factor that has shown neuroprotective effects in preclinical studies, and the success of GLP-1 mimetics in phase II clinical trials in AD and PD has raised new hope. GLP-1 mimetics are currently on the market as treatments for type 2 diabetes. GLP-1 analogs are safe, well tolerated, resistant to desensitization and well characterized in the clinic. Herein, we review the existing evidence and illustrate the neuroprotective pathways that are induced following GLP-1R activation in neurons, microglia and astrocytes. The latter include synaptic protection, improvements in cognition, learning and motor function, amyloid pathology-ameliorating properties (Aβ, Tau, and α-synuclein), the suppression of Ca2+ deregulation and ER stress, potent anti-inflammatory effects, the blockage of oxidative stress, mitochondrial dysfunction and apoptosis pathways, enhancements in the neuronal insulin sensitivity and energy metabolism, functional improvements in autophagy and mitophagy, elevated BDNF and glial cell line-derived neurotrophic factor (GDNF) synthesis as well as neurogenesis. The many beneficial features of GLP-1R and GLP-1/GIPR dual agonists encourage the development of novel drug treatments for AD and PD.

Gene-Based Therapeutics for Parkinson’s Disease

Parkinson’s disease (PD) is a complex multifactorial disorder that is not yet fully surmised, and it is only when such a disease is tackled on multiple levels simultaneously that we should expect to see fruitful results. Gene therapy is a modern medical practice that theoretically and, so far, practically, has demonstrated its capability in joining the battle against PD and other complex disorders on most if not all fronts. This review discusses how gene therapy can efficiently replace current forms of therapy such as drugs, personalized medicine or invasive surgery. Furthermore, we discuss the importance of enhancing delivery techniques to increase the level of transduction and control of gene expression or tissue specificity. Importantly, the results of current trials establish the safety, efficacy and applicability of gene therapy for PD. Gene therapy’s variety of potential in interfering with PD’s pathology by improving basal ganglial circuitry, enhancing dopamine synthesis, delivering neuroprotection or preventing neurodegeneration may one day achieve symptomatic benefit, disease modification and eradication.

Medicinal Chemistry and Chemical Biology of Nurr1 Modulators: An Emerging Strategy in Neurodegeneration

Nuclear receptor related 1 (Nurr1) is a transcription factor with neuroprotective and antineuroinflammatory properties. Observations from genetic studies and human patients support potential of Nurr1 as a therapeutic target in neurodegeneration, but due to a lack of high-quality chemical tools for pharmacological control of Nurr1, its target validation is pending. Nevertheless, considerable progress has recently been made in elucidating structural and functional characteristics of Nurr1, and several ligand scaffolds have been discovered. Here, we analyze Nurr1's structure and mechanisms compared to other nuclear receptors, summarize the known small molecule Nurr1 ligands, and discuss the available evidence for the therapeutic potential of Nurr1 in neurodegeneration.

The promise of the TGF-β superfamily as a therapeutic target for Parkinson's disease

A large body of evidence underscore the regulatory role of TGF-β superfamily in the central nervous system. Components of the TGF-β superfamily modulate key events during embryonic brain development and adult brain tissue injury repair. With respect to Parkinson's disease (PD), TGF-ß signaling pathways are implicated in the differentiation, maintenance and synaptic function of the dopaminergic neurons, as well as in processes related to the activation state of astrocytes and microglia. In vitro and in vivo studies using toxin models, have interrogated on the dopaminotrophic and protective role of the TGF-β superfamily members. The evolution of genetic and animal models of PD that more closely recapitulate the disease condition has made possible the dissection of intracellular pathways in response to TGF-ß treatment. Although the first clinical trials using GDNF did not meet their primary endpoints, substantial work has been carried out to reappraise the TGF-β superfamily's clinical benefit.

CK2 alpha prime and alpha-synuclein pathogenic functional interaction mediates synaptic dysregulation in huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the HTT gene for which no therapies are available. HTT mutation causes protein misfolding and aggregation, preferentially affecting medium spiny neurons (MSNs) of the basal ganglia. Transcriptional perturbations in synaptic genes and neuroinflammation are key processes that precede MSN dysfunction and motor symptom onset. Understanding the interplay between these processes is crucial to develop effective therapeutic strategies to treat HD. We investigated the role of protein kinase CK2α', a kinase upregulated in MSNs in HD and previously associated with Parkinson's disease (PD), in the regulation of neuroinflammation and synaptic function in HD. We used the heterozygous knock-in zQ175 HD mouse model and compared that to zQ175 mice lacking one allele of CK2α' (zQ175:CK2α'(±)). CK2α' haploinsufficiency in zQ175 mice resulted in decreased levels of pro-inflammatory cytokines, HTT aggregation, astrogliosis and transcriptional alterations of synaptic genes related to glutamatergic signaling. zQ175:CK2α'(±) mice also presented increased frequency of striatal miniature excitatory postsynaptic currents (mEPSCs), an indicator of synaptic activity, and improved motor coordination compared to zQ175 mice. Neuropathological and phenotypic changes mediated by CK2α' were connected to alpha-synuclein (α-syn) dysregulation and correlated with differences in α-syn serine 129 phosphorylation (pS129-α-syn), a post-translational modification involved in α-synucleinopathy and shown to be regulated by CK2 in PD. pS129-α-syn was increased in the nuclei of MSNs in zQ175 mice and in the striatum of patients with HD, and it decreased in zQ175:CK2α'(±) mice. Collectively, our data established a novel connection between CK2α', neuroinflammation and synaptic gene dysregulation with synucleinopathy in HD and suggested common molecular mechanisms of neurodegeneration between HD and PD. Our results also support CK2α' inhibition as a potential therapeutic strategy to modulate neuronal function and neuroprotection in HD.

Induced Cognitive Impairments Reversed by Grafts of Neural Precursors: A Longitudinal Study in a Macaque Model of Parkinson's Disease

Parkinson's disease (PD) evolves over an extended and variable period in humans; years prior to the onset of classical motor symptoms, sleep and biological rhythm disorders develop, significantly impacting the quality-of-life of patients. Circadian-rhythm disorders are accompanied by mild cognitive deficits that progressively worsen with disease progression and can constitute a severe burden for patients at later stages. The gold-standard 6-methyl-1-methyl-4-phenyl-1,2,3,6-tetrahydropyridin (MPTP) macaque model of PD recapitulates the progression of motor and nonmotor symptoms over contracted periods of time. Here, this multidisciplinary/multiparametric study follows, in five animals, the steady progression of motor and nonmotor symptoms and describes their reversal following grafts of neural precursors in diverse functional domains of the basal ganglia. Results show unprecedented recovery from cognitive symptoms in addition to a strong clinical motor recuperation. Both motor and cognitive recovery and partial circadian rhythm recovery correlate with the degree of graft integration, and in a subset of animals, with in vivo levels of striatal dopaminergic innervation and function. The present study provides empirical evidence that integration of neural precursors following transplantation efficiently restores function at multiple levels in parkinsonian nonhuman primates and, given interindividuality of disease progression and recovery, underlines the importance of longitudinal multidisciplinary assessments in view of clinical translation.

Neurotrophins as Therapeutic Agents for Parkinson’s Disease; New Chances From Focused Ultrasound?

Magnetic Resonance–guided Focused Ultrasound (MRgFUS) represents an effective micro-lesioning approach to target pharmaco-resistant tremor, mostly in patients afflicted by essential tremor (ET) and/or Parkinson’s disease (PD). So far, experimental protocols are verifying the clinical extension to other facets of the movement disorder galaxy (i.e., internal pallidus for disabling dyskinesias). Aside from those neurosurgical options, one of the most intriguing opportunities of this technique relies on its capability to remedy the impermeability of blood–brain barrier (BBB). Temporary BBB opening through low-intensity focused ultrasound turned out to be safe and feasible in patients with PD, Alzheimer’s disease, and amyotrophic lateral sclerosis. As a mere consequence of the procedures, some groups described even reversible but significant mild cognitive amelioration, up to hippocampal neurogenesis partially associated to the increased of endogenous brain-derived neurotrophic factor (BDNF). A further development elevates MRgFUS to the status of therapeutic tool for drug delivery of putative neurorestorative therapies. Since 2012, FUS-assisted intravenous administration of BDNF or neurturin allowed hippocampal or striatal delivery. Experimental studies emphasized synergistic modalities. In a rodent model for Huntington’s disease, engineered liposomes can carry glial cell line–derived neurotrophic factor (GDNF) plasmid DNA (GDNFp) to form a GDNFp-liposome (GDNFp-LPs) complex through pulsed FUS exposures with microbubbles; in a subacute MPTP-PD model, the combination of intravenous administration of neurotrophic factors (either through protein or gene delivery) plus FUS did curb nigrostriatal degeneration. Here, we explore these arguments, focusing on the current, translational application of neurotrophins in neurodegenerative diseases.

Synergistic Effects of Combined Nurr1 Overexpression and Natural Inducers on the More Efficient Production of Dopaminergic Neuron-Like Cells From Stem Cells

The development of dopaminergic (DA) neurons is a very complex process, and a combination of extrinsic and intrinsic factors involves their differentiation. Transcription factor, Nurr1 plays an essential role in the differentiation and maintenance of midbrain DA neurons. Nurr1-based therapies may restore DA function in Parkinson's disease (PD) by replacing damaged cells with differentiated cells derived from stem cells. Providing tissue-specific microenvironments such as brain extract can effectively induce dopaminergic gene expression in stem cells. The present study aimed to investigate the combined effects of Nurr1 gene overexpression and a neonatal rat brain extract (NRBE) induction on dopaminergic differentiation of P19 stem cells. In order to neural differentiation induction, stably Nurr1-transfected cells were treated with 100 μg/ml of NRBE. The differentiation potential of the cells was then evaluated during a period of 1–3 weeks via various methods. The initial evaluation of the cells by direct observation under a light microscope and cresyl violet specific staining, confirmed neuron-like morphology in the differentiated cells. In addition, different molecular and cellular techniques, including real-time PCR, immunofluorescence, and flow cytometry, demonstrated that the treated cells expressed pan-neuronal and dopaminergic markers. In all experimental groups, neuronal phenotype with dopaminergic neuron-like cells characteristics mainly appeared in the second week of the differentiation protocol. Overall, the results of the present study revealed for the first time the synergistic effects of Nurr1 gene overexpression and possible soluble factors that existed in NRBE on the differentiation of P19 stem cells into dopaminergic neuron-like cells.

Growth differentiation factor 5: a neurotrophic factor with neuroprotective potential in Parkinson's disease

Parkinson's disease is the most common movement disorder worldwide, affecting over 6 million people. It is an age-related disease, occurring in 1% of people over the age of 60, and 3% of the population over 80 years. The disease is characterized by the progressive loss of midbrain dopaminergic neurons from the substantia nigra, and their axons, which innervate the striatum, resulting in the characteristic motor and non-motor symptoms of Parkinson's disease. This is paralleled by the intracellular accumulation of α-synuclein in several regions of the nervous system. Current therapies are solely symptomatic and do not stop or slow disease progression. One promising disease-modifying strategy to arrest the loss of dopaminergic neurons is the targeted delivery of neurotrophic factors to the substantia nigra or striatum, to protect the remaining dopaminergic neurons of the nigrostriatal pathway. However, clinical trials of two well-established neurotrophic factors, glial cell line-derived neurotrophic factor and neurturin, have failed to meet their primary end-points. This failure is thought to be at least partly due to the downregulation by α-synuclein of Ret, the common co-receptor of glial cell line-derived neurorophic factor and neurturin. Growth/differentiation factor 5 is a member of the bone morphogenetic protein family of neurotrophic factors, that signals through the Ret-independent canonical Smad signaling pathway. Here, we review the evidence for the neurotrophic potential of growth/differentiation factor 5 in in vitro and in vivo models of Parkinson's disease. We discuss new work on growth/differentiation factor 5's mechanisms of action, as well as data showing that viral delivery of growth/differentiation factor 5 to the substantia nigra is neuroprotective in the α-synuclein rat model of Parkinson's disease. These data highlight the potential for growth/differentiation factor 5 as a disease-modifying therapy for Parkinson's disease.

NME1 Protects Against Neurotoxin-, α-Synuclein- and LRRK2-Induced Neurite Degeneration in Cell Models of Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disease characterised by the progressive degeneration of midbrain dopaminergic neurons, coupled with the intracellular accumulation of α-synuclein. Axonal degeneration is a central part of the pathology of PD. While the majority of PD cases are sporadic, some are genetic; the G2019S mutation in leucine-rich repeat kinase 2 (LRRK2) is the most common genetic form. The application of neurotrophic factors to protect dopaminergic neurons is a proposed experimental therapy. One such neurotrophic factor is growth differentiation factor (GDF)5. GDF5 is a dopaminergic neurotrophic factor that has been shown to upregulate the expression of a protein called nucleoside diphosphate kinase A (NME1). However, whether NME1 is neuroprotective in cell models of axonal degeneration of relevance to PD is unknown. Here we show that treatment with NME1 can promote neurite growth in SH-SY5Y cells, and in cultured dopaminergic neurons treated with the neurotoxin 6-hydroxydopamine (6-OHDA). Similar effects of NME1 were found in SH-SY5Y cells and dopaminergic neurons overexpressing human wild-type α-synuclein, and in stable SH-SY5Y cell lines carrying the G2019S LRRK2 mutation. We found that the effects of NME1 require the RORα/ROR2 receptors. Furthermore, increased NF-κB-dependent transcription was partially required for the neurite growth-promoting effects of NME1. Finally, a combined bioinformatics and biochemical analysis of the mitochondrial oxygen consumption rate revealed that NME1 enhanced mitochondrial function, which is known to be impaired in PD. These data show that recombinant NME1 is worthy of further study as a potential therapeutic agent for axonal protection in PD.

Rapid Induction of Dopaminergic Neuron Loss Accompanied by Lewy Body-Like Inclusions in A53T BAC-SNCA Transgenic Mice

Parkinson's disease (PD), the most common neurodegenerative movement disorder, is characterized by dopaminergic neuron loss in the substantia nigra pars compacta (SNpc) and intraneuronal α-synuclein (α-syn) inclusions. It is highly needed to establish a rodent model that recapitulates the clinicopathological features of PD within a short period to efficiently investigate the pathological mechanisms and test disease-modifying therapies. To this end, we analyzed three mouse lines, i.e., wild-type mice, wild-type human α-syn bacterial artificial chromosome (BAC) transgenic (BAC-SNCA Tg) mice, and A53T human α-syn BAC transgenic (A53T BAC-SNCA Tg) mice, receiving dorsal striatum injections of human and mouse α-syn preformed fibrils (hPFFs and mPFFs, respectively). mPFF injections induced more severe α-syn pathology in most brain regions, including the ipsilateral SNpc, than hPFF injections in all genotypes at 1-month post-injection. Although these Tg mouse lines expressed a comparable amount of α-syn in the brains, the mPFF-injected A53T BAC-SNCA Tg mice exhibited the most severe α-syn pathology as early as 0.5-month post-injection. The mPFF-injected A53T BAC-SNCA Tg mice showed a 38% reduction in tyrosine hydroxylase (TH)-positive neurons in the ipsilateral SNpc, apomorphine-induced rotational behavior, and motor dysfunction at 2 months post-injection. These data indicate that the extent of α-syn pathology induced by α-syn PFF injection depends on the types of α-syn PFFs and exogenously expressed α-syn in Tg mice. The mPFF-injected A53T BAC-SNCA Tg mice recapitulate the key features of PD more rapidly than previously reported mouse models, suggesting their usefulness for testing disease-modifying therapies as well as analyzing the pathological mechanisms.

Is activation of GDNF/RET signaling the answer for successful treatment of Parkinson's disease? A discussion of data from the culture dish to the clinic

The neurotrophic signaling of glial cell line-derived neurotrophic factor (GDNF) with its canonical receptor, the receptor tyrosine kinase RET, coupled together with the GDNF family receptor alpha 1 is important for dopaminergic neuron survival and physiology in cell culture experiments and animal models. This prompted the idea to try GDNF/RET signaling as a therapeutic approach to treat Parkinson's disease with the hallmark of dopaminergic cell death in the substantia nigra of the midbrain. Despite several clinical trials with GDNF in Parkinson's disease patients, which mainly focused on optimizing the GDNF delivery technique, benefits were only seen in a few patients. In general, the endpoints did not show significant improvements. This suggests that it will be helpful to learn more about the basic biology of this fascinating but complicated GDNF/RET signaling system in the dopaminergic midbrain and about recent developments in the field to facilitate its use in the clinic. Here we will refer to the latest publications and point out important open questions in the field.

Sgt1 Regulates α-Synuclein Subcellular Localization and Expression of Parkinson's Disease Related Genes, PINK1 and PARK9

The SGT1 protein is highly expressed in the mammalian brain, particularly in neurons of the hippocampus and cortex, and in Purkinje cells of the cerebellum. There are literature data indicating that the protein may be involved in pathogenesis of neurodegenerative disorders such as Parkinson’s disease (PD). In the present work we have found that SGT1 protected cells from the toxicity of rotenone, an agent that evokes behavioral and histopathological symptoms of PD. To gain more insight into the possible mechanism underlying the protective action of SGT1 we looked at α-synuclein subcellular distribution in HEK293 cells with an altered SGT1 level. By immunofluorescent staining we have found that in HEK293 cells overexpressing SGT1 α-synuclein was mainly localized in the cytoplasm while in control cells it was present in the nucleus. Accordingly, when SGT1 expression was silenced, α-synuclein was predominantly present in the nucleus. These results were then confirmed by subcellular fractionation and Western blot analysis. Moreover, we have found that altered level of SGT1 in HEK293 cells influenced the expression of PD related genes, PINK1 and PARK9. Altogether, our results point to SGT1 as an important factor that might be involved in the pathogenesis of Parkinson’s disease (PD).

Mechanistic Insight from Preclinical Models of Parkinson's Disease Could Help Redirect Clinical Trial Efforts in GDNF Therapy

Parkinson’s disease (PD) is characterized by four pathognomonic hallmarks: (1) motor and non-motor deficits; (2) neuroinflammation and oxidative stress; (3) pathological aggregates of the α-synuclein (α-syn) protein; (4) neurodegeneration of the nigrostriatal system. Recent evidence sustains that the aggregation of pathological α-syn occurs in the early stages of the disease, becoming the first trigger of neuroinflammation and subsequent neurodegeneration. Thus, a therapeutic line aims at striking back α-synucleinopathy and neuroinflammation to impede neurodegeneration. Another therapeutic line is restoring the compromised dopaminergic system using neurotrophic factors, particularly the glial cell-derived neurotrophic factor (GDNF). Preclinical studies with GDNF have provided encouraging results but often lack evaluation of anti-α-syn and anti-inflammatory effects. In contrast, clinical trials have yielded imprecise results and have reported the emergence of severe side effects. Here, we analyze the discrepancy between preclinical and clinical outcomes, review the mechanisms of the aggregation of pathological α-syn, including neuroinflammation, and evaluate the neurorestorative properties of GDNF, emphasizing its anti-α-syn and anti-inflammatory effects in preclinical and clinical trials.

α-Synuclein Induces the GSK-3-Mediated Phosphorylation and Degradation of NURR1 and Loss of Dopaminergic Hallmarks

In Parkinson’s disease, the dysfunction of the dopaminergic nigrostriatal tract involves the loss of function of dopaminergic neurons of the substantia nigra pars compacta followed by death of these neurons. The functional recovery of these neurons requires a deep knowledge of the molecules that maintain the dopaminergic phenotype during adulthood and the mechanisms that subvert their activity. Previous studies have shown that transcription factor NURR1, involved in differentiation and maintenance of the dopaminergic phenotype, is downregulated by α-synuclein (α-SYN). In this study, we provide a mechanistic explanation to this finding by connecting α-SYN-induced activation of glycogen synthase kinase-3 (GSK-3) with NURR1 phosphorylation followed by proteasomal degradation. The use of sequential deletion mutants and single point mutants of NURR1 allowed the identification of a domain comprising amino acids 123-PSSPPTPSTPS-134 that is targeted by GSK-3 and leads to subsequent ubiquitination and proteasome degradation. This study provides a detailed analysis of the regulation of NURR1 stability by phosphorylation in synucleinopathies such as Parkinson’s disease.

Animal Models of Parkinson's Disease: Are They Useful or Not?

The use of animal models in Parkinson's disease research has been controversial in terms of how well they relate to the clinical condition and thus their utility for translating therapies from the lab to the clinic. In this article, two researchers debate this issue with Roger Barker taking the view that such models are not useful and may even be misleading, while Anders Björklund defends their use and highlights their value in better understanding and treating this condition.

Soluble endogenous oligomeric α-synuclein species in neurodegenerative diseases: Expression, spreading, and cross-talk

There is growing recognition in the field of neurodegenerative diseases that mixed proteinopathies are occurring at greater frequency than originally thought. This is particularly true for three amyloid proteins defining most of these neurological disorders, amyloid-beta (Aβ), tau, and alpha-synuclein (αSyn). The co-existence and often co-localization of aggregated forms of these proteins has led to the emergence of concepts positing molecular interactions and cross-seeding between Aβ, tau, and αSyn aggregates. Amongst this trio, αSyn has received particular attention in this context during recent years due to its ability to modulate Aβ and tau aggregation in vivo, to interact at a molecular level with Aβ and tau in vivo and to cross-seed tau in mice. Here we provide a comprehensive, critical, and accessible review about the expression, role and nature of endogenous soluble αSyn oligomers because of recent developments in the understanding of αSyn multimerization, misfolding, aggregation, cross-talk, spreading and cross-seeding in neurodegenerative disorders, including Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, Alzheimer's disease, and Huntington's disease. We will also discuss our current understanding about the relative toxicity of endogenous αSyn oligomers in vivo and in vitro, and introduce potential opportunities to counter their deleterious effects.

Gene Therapy in the Management of Parkinson's Disease: Potential of GDNF as a Promising Therapeutic Strategy

The limitations of conventional treatment therapies in Parkinson's disorder, a common neurodegenerative disorder, lead to the development of an alternative gene therapy approach. Multiple treatment options targeting dopaminergic neuronal regeneration, production of enzymes linked with dopamine synthesis, subthalamic nucleus neurons, regulation of astrocytes and microglial cells and potentiating neurotrophic factors, were established. Viral vector-based dopamine delivery, prodrug approaches, fetal ventral mesencephalon tissue transplantation and dopamine synthesizing enzyme encoding gene delivery are significant therapies evidently supported by numerous trials. The review primarily elaborates on the significant role of glial cell-line derived neurotrophic factor in alleviating motor symptoms and the loss of dopaminergic neurons in Parkinson's disease. Neuroprotective and neuroregenerative effects of GDNF were established via preclinical and clinical study outcomes. The binding of GDNF family ligands with associated receptors leads to the formation of a receptor-ligand complex activating Ret receptor of tyrosine kinase family, which is only expressed in dopaminergic neurons, playing an important role in Parkinson's disease, via its association with the essential protein encoded genes. Furthermore, the review establishes delivery aspects, like ventricular delivery of recombinant GDNF, intraparenchymal and intraputaminal delivery using infusion catheters. The review highlights problems and challenges of GDNF delivery, and essential measures to overcome them, like gene therapy combinations, optimization of delivery vectors, newer targeting devices, motor symptoms curbing focused ultrasound techniques, modifications in patient selection criteria and development of novel delivery strategies based on liposomes and encapsulated cells, to promote safe and effective delivery of neurotrophic factor and establishment of routine treatment therapy for patients.

GDNF Therapy: Can We Make It Work?

In two recent postmortem studies, Jeffrey Kordower and colleagues report new findings that open up for an interesting discussion on the status of GDNF/NRTN signaling in patients with Parkinson's disease (PD), adding an interesting perspective on the, admittedly very limited, signs of restorative effects previously seen in GDNF/NRTN-treated patients. Their new findings show that the level of the GDNF signaling receptor Ret is overall markedly reduced relative to the non-PD controls, and most severely, up to 80%, in nigral neurons containing α-synuclein inclusions, accompanied by impaired signaling downstream of the Ret receptor. Notably, however, the vast majority of the remaining nigral neurons retained a low level of Ret expression, and hence a threshold level of signaling. Further observations made in two patients who had received AAV-NRTN gene therapy 8-10 years earlier suggest the intriguing possibility that NRTN is able to restore Ret expression and upregulate its own signaling pathway. This "wind-up" mechanism, which is likely to depend on an interaction with dopaminergic transcription factor Nurr1, has therapeutic potential and should encourage renewed efforts to turn GDNF/NRTN therapy into success, once the recurring problem of under-dosing is resolved.

Targeting Nuclear Receptors in Neurodegeneration and Neuroinflammation

Nuclear receptors, also known as ligand-activated transcription factors, regulate gene expression upon ligand signals and present as attractive therapeutic targets especially in chronic diseases. Despite the therapeutic relevance of some nuclear receptors in various pathologies, their potential in neurodegeneration and neuroinflammation is insufficiently established. This perspective gathers preclinical and clinical data for a potential role of individual nuclear receptors as future targets in Alzheimer's disease, Parkinson's disease, and multiple sclerosis, and concomitantly evaluates the level of medicinal chemistry targeting these proteins. Considerable evidence suggests the high promise of ligand-activated transcription factors to counteract neurodegenerative diseases with a particularly high potential of several orphan nuclear receptors. However, potent tools are lacking for orphan receptors, and limited central nervous system exposure or insufficient selectivity also compromises the suitability of well-studied nuclear receptor ligands for functional studies. Medicinal chemistry efforts are needed to develop dedicated high-quality tool compounds for the therapeutic validation of nuclear receptors in neurodegenerative pathologies.

Nurr1 downregulation is caused by CREB inactivation in a Parkinson's disease mouse model

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra (SN). Nurr1 (NR4A2), a nuclear receptor essential for the maintenance of midbrain dopaminergic neurons, is transcriptionally downregulated in both patients with PD and animal models and has been considered as a promising therapeutic target for neuroprotection in PD. However, the mechanism underlying Nurr1 downregulation during dopaminergic degeneration has not been fully elucidated. Here, we report that the pro-survival transcription factor CREB is constitutively bound to the Nurr1 promoter in the mouse SN. CREB inactivation by dephosphorylation at Ser133 occurred in parallel with Nurr1 downregulation in the MPTP mouse model of PD. Forced expression of VP16-CREB, a constitutively active mutant, rescued Nurr1 expression and showed prominent neuroprotection in MPTP-intoxicated mice. Collectively, our results demonstrate that Nurr1 downregulation in the MPTP-induced PD mouse model is caused by CREB inactivation, which may provide a new target for neuroprotective therapy in PD.

Neural stem cells derived from human midbrain organoids as a stable source for treating Parkinson's disease: Midbrain organoid-NSCs (Og-NSC) as a stable source for PD treatment

Successful clinical translation of stem cell-based therapy largely relies on the scalable and reproducible preparation of donor cells with potent therapeutic capacities. In this study, midbrain organoids were yielded from human pluripotent stem cells (hPSCs) to prepare cells for Parkinson's disease (PD) therapy. Neural stem/precursor cells (NSCs) isolated from midbrain organoids (Og-NSCs) expanded stably and differentiated into midbrain-type dopamine(mDA) neurons, and an unprecedentedly high proportion expressed midbrain-specific factors, with relatively low cell line and batch-to-batch variations. Single cell transcriptome analysis followed by in vitro assays indicated that the majority of cells in the Og-NSC cultures are ventral midbrain (VM)-patterned with low levels of cellular senescence/aging and mitochondrial stress, compared to those derived from 2D-culture environments. Notably, in contrast to current methods yielding mDA neurons without astrocyte differentiation, mDA neurons that differentiated from Og-NSCs were interspersed with astrocytes as in the physiologic brain environment. Thus, the Og-NSC-derived mDA neurons exhibited improved synaptic maturity, functionality, resistance to toxic insults, and faithful expressions of the midbrain-specific factors, in vitro and in vivo long after transplantation. Consequently, Og-NSC transplantation yielded potent therapeutic outcomes that are reproducible in PD model animals. Collectively, our observations demonstrate that the organoid-based method may satisfy the demands needed in the clinical setting of PD cell therapy.

Nurr1 repression mediates cardinal features of Parkinson's disease in α-synuclein transgenic mice

Duplication/triplication mutations of the SNCA locus, encoding alpha-synuclein (ASYN), and loss of function mutations in Nurr1, a nuclear receptor guiding midbrain dopaminergic neuron development, are associated with familial Parkinson’s disease (PD). As we age, the expression levels of these two genes in midbrain dopaminergic neurons follow opposite directions and ASYN expression increases while the expression of Nurr1 decreases. We investigated the effect of ASYN and Nurr1 age-related expression alterations in the pathogenesis of PD by coupling Nurr1 hemizygous with ASYN(s) (heterozygote) or ASYN(d) (homozygote) transgenic mice. ASYN(d)/Nurr1+/− (2-hit) mice, contrary to the individual genetic traits, developed phenotypes consistent with dopaminergic dysfunction. Aging ‘2-hit’ mice manifested kyphosis, severe rigid paralysis, L-DOPA responsive movement impairment and cachexia and died prematurely. Pathological abnormalities of phenotypic mice included SN neuron degeneration, extensive neuroinflammation and enhanced ASYN aggregation. Mice with two wt Nurr1 alleles [ASYN(d)/Nurr1+/+] or with reduced ASYN load [ASYN(s)/Nurr1+/−] did not develop the phenotype or pathology. Critically, we found that aging ASYN(d), in contrast to ASYN(s), mice suppress Nurr1-protein levels in a brain region–specific manner, which in addition to Nurr1 hemizygosity is necessary to instigate PD pathogenesis. Our experiments demonstrate that ASYN-dependent PD-related pathophysiology is mediated at least in part by Nurr1 down-regulation.

Pramipexole attenuates 6-OHDA-induced Parkinson's disease by mediating the Nurr1/NF-κB pathway

Neuroinflammation is the key factor associated with the progression of Parkinson's disease (PD). Pramipexole (PPX) has anti-inflammatory and antioxidant properties. This study explored the effects of PPX on PD and its related mechanisms. A PD rat model was established using 6-hydroxydopamine (6-OHDA). Thirty rats were divided into the following three groups: control, PD, and PD + PPX. The rats in the PD and PD + PPX groups were first administered 6-OHDA and then respectively treated with saline and PPX. Afterward, rotational behavior tests were performed to evaluate the efficiency of PPX. The level of tyrosine hydroxylase (TH) was measured using immunohistochemical staining. Subsequently, real-time quantitative PCR (RT-qPCR) and western blot were used to determine the expression of α-synuclein (α-syn), nuclear receptor subfamily 4 group A member 2 (Nurr1), and nuclear factor kappa B (NF-κB). PPX improved the motor behavior of PD rats caused by 6-OHDA. The number of TH-positive neurons in the PD group was significantly lower than that in the control group (P < 0.05), while PPX could rescue 6-OHDA-induced TH loss. RT-qPCR and western blot showed that Nurr1 expression was significantly downregulated in the PD group compared to that of the control group (P < 0.05), while after PPX treatment, its expression was significantly upregulated (P < 0.05). For α-syn and NF-κB, 6-OHDA significantly upregulated their expressions (P < 0.05), whereas PPX reversed them. PPX improved the motor behavior of PD through mediating the inflammatory response and regulating the Nurr1/NF-κB signaling pathway.