Cerebral dopamine neurotrophic factor reduces α-synuclein aggregation and propagation and alleviates behavioral alterations in vivo

GDNF reduces fibril-induced early-stage alpha-synuclein pathology after delivery of 20S proteasome inhibitor lactacystin

Failures in protein homeostasis are linked to Parkinson's disease (PD) and other neurodegenerative diseases. Lewy bodies, proteinaceous inclusions rich in phosphorylated alpha-synuclein are a hallmark of PD. Glial cell line-derived neurotrophic factor (GDNF) can eliminate Lewy body-like inclusions in mouse dopamine neurons. This study explores whether GDNF has protective effects against alpha-synuclein protofibril toxicity under proteasome inhibition by lactacystin, both in vitro and in vivo. GDNF did not shield midbrain dopamine neurons from lactacystin-induced neurodegeneration, but still prevented phosphorylated alpha-synuclein accumulation. In vivo experiment with control or GDNF-expressing viral vectors assessed alpha-synuclein pathology spread in the nigrostriatal pathway and lactacystin-caused damage in the midbrain. GDNF overexpression reduced phosphorylated alpha-synuclein inclusions. Lactacystin-treated mice showed motor asymmetry and decreased spontaneous activity, exacerbated without AAV-GDNF pre-treatment. However, GDNF's neuroprotective effect could not be confirmed in vivo, due to side-effects from overexpression in the midbrain. Importantly, these findings show that GDNF continues to eliminate alpha-synuclein aggregation despite lactacystin-induced proteasome inhibition. Activating neurotrophic signaling pathways may protect against alpha-synuclein pathology in PD, even with impaired protein degradation mechanisms.

Unraveling Dysregulated Cell Signaling Pathways, Genetic and Epigenetic Mysteries of Parkinson's Disease

Parkinson's disease (PD) is a prevalent and burdensome neurodegenerative disorder that has been extensively researched to understand its complex etiology, diagnosis, and treatment. The interplay between genetic and environmental factors in PD makes its pathophysiology difficult to comprehend, emphasizing the need for further investigation into genetic and epigenetic markers involved in the disease. Early diagnosis is crucial for optimal management of the disease, and the development of novel diagnostic biomarkers is ongoing. Although many efforts have been made in the field of recognition and interpretation of the mechanisms involved in the pathophysiology of the disease, the current knowledge about PD is just the tip of the iceberg. By scrutinizing genetic and epigenetic patterns underlying PD, new avenues can be opened for dissecting the pathology of the disorder, leading to more precise and efficient diagnostic and therapeutic approaches. This review emphasizes the importance of studying dysregulated cell signaling pathways and molecular processes associated with genes and epigenetic alterations in understanding PD, paving the way for the development of novel therapeutic strategies to combat this devastating disease.

Neurotrophic factors for Parkinson's disease: Current status, progress, and remaining questions. Conclusions from a 2023 workshop

In 2023, a workshop was organized by the UK charity Cure Parkinson's with The Michael J Fox Foundation for Parkinson's Research and Parkinson's UK to review the field of growth factors (GFs) for Parkinson's disease (PD). This was a follow up to a previous meeting held in 2019.1 This 2023 workshop reviewed new relevant data that has emerged in the intervening 4 years around the development of new GFs and better models for studying them including the merit of combining treatments as well as therapies that can be modulated. We also discussed new insights into GF delivery and trial design that have emerged from the analyses of completed GDNF trials, including the patient voice, as well as the recently completed CDNF trial.2 We then concluded with our recommendations on how GF studies in PD should develop going forward.

CDNF Exerts Anxiolytic, Antidepressant-like, and Procognitive Effects and Modulates Serotonin Turnover and Neuroplasticity-Related Genes

Cerebral dopamine neurotrophic factor (CDNF) is an unconventional neurotrophic factor because it does not bind to a known specific receptor on the plasma membrane and functions primarily as an unfolded protein response (UPR) regulator in the endoplasmic reticulum. Data on the effects of CDNF on nonmotor behavior and monoamine metabolism are limited. Here, we performed the intracerebroventricular injection of a recombinant CDNF protein at doses of 3, 10, and 30 μg in C57BL/6 mice. No adverse effects of the CDNF injection on feed and water consumption or locomotor activity were observed for 3 days afterwards. Decreases in body weight and sleep duration were transient. CDNF-treated animals demonstrated improved performance on the operant learning task and a substantial decrease in anxiety and behavioral despair. CDNF in all the doses enhanced serotonin (5-HT) turnover in the murine frontal cortex, hippocampus, and midbrain. This alteration was accompanied by changes in the mRNA levels of the 5-HT1A and 5-HT7 receptors and in monoamine oxidase A mRNA and protein levels. We found that CDNF dramatically increased c-Fos mRNA levels in all investigated brain areas but elevated the phosphorylated-c-Fos level only in the midbrain. Similarly, enhanced CREB phosphorylation was found in the midbrain in experimental animals. Additionally, the upregulation of a spliced transcript of XBP1 (UPR regulator) was detected in the midbrain and frontal cortex. Thus, we can hypothesize that exogenous CDNF modulates the UPR pathway and overall neuronal activation and enhances 5-HT turnover, thereby affecting learning and emotion-related behavior.

Structural basis of CDNF interaction with the UPR regulator GRP78

Cerebral dopamine neurotrophic factor (CDNF) is an unconventional neurotrophic factor that is a disease-modifying drug candidate for Parkinson's disease. CDNF has pleiotropic protective effects on stressed cells, but its mechanism of action remains incompletely understood. Here, we use state-of-the-art advanced structural techniques to resolve the structural basis of CDNF interaction with GRP78, the master regulator of the unfolded protein response (UPR) pathway. Subsequent binding studies confirm the obtained structural model of the complex, eventually revealing the interaction site of CDNF and GRP78. Finally, mutating the key residues of CDNF mediating its interaction with GRP78 not only results in impaired binding of CDNF but also abolishes the neuroprotective activity of CDNF-derived peptides in mesencephalic neuron cultures. These results suggest that the molecular interaction with GRP78 mediates the neuroprotective actions of CDNF and provide a structural basis for development of next generation CDNF-based therapeutic compounds against neurodegenerative diseases.

Unfolded protein response markers Grp78 and eIF2alpha are upregulated with increasing alpha-synuclein levels in Lewy body disease

AIMS

Endoplasmic reticulum stress followed by the unfolded protein response is one of the cellular mechanisms contributing to the progression of α-synuclein pathology in Parkinson's disease and other Lewy body diseases. We aimed to investigate the activation of endoplasmic reticulum stress and its correlation with α-synuclein pathology in human post-mortem brain tissue.

METHODS

We analysed brain tissue from 45 subjects-14 symptomatic patients with Lewy body disease, 19 subjects with incidental Lewy body disease, and 12 healthy controls. The analysed brain regions included the medulla, pons, midbrain, striatum, amygdala and entorhinal, temporal, frontal and occipital cortex. We analysed activation of endoplasmic reticulum stress via levels of the unfolded protein response-related proteins (Grp78, eIF2α) and endoplasmic reticulum stress-regulating neurotrophic factors (MANF, CDNF).

RESULTS

We showed that regional levels of two endoplasmic reticulum-localised neurotrophic factors, MANF and CDNF, did not change in response to accumulating α-synuclein pathology. The concentration of MANF negatively correlated with age in specific regions. eIF2α was upregulated in the striatum of Lewy body disease patients and correlated with increased α-synuclein levels. We found the upregulation of chaperone Grp78 in the amygdala and nigral dopaminergic neurons of Lewy body disease patients. Grp78 levels in the amygdala strongly correlated with soluble α-synuclein levels.

CONCLUSIONS

Our data suggest a strong but regionally specific change in Grp78 and eIF2α levels, which positively correlates with soluble α-synuclein levels. Additionally, MANF levels decreased in dopaminergic neurons in the substantia nigra. Our research suggests that endoplasmic reticulum stress activation is not associated with Lewy pathology but rather with soluble α-synuclein concentration and disease progression.

Delivery of CDNF by AAV-mediated gene transfer protects dopamine neurons and regulates ER stress and inflammation in an acute MPTP mouse model of Parkinson's disease

Cerebral dopamine neurotrophic factor (CDNF) and its close structural relative, mesencephalic astrocyte-derived neurotrophic factor (MANF), are proteins with neurotrophic properties. CDNF protects and restores the function of dopamine (DA) neurons in rodent and non-human primate (NHP) toxin models of Parkinson's disease (PD) and therefore shows promise as a drug candidate for disease-modifying treatment of PD. Moreover, CDNF was found to be safe and to have some therapeutic effects on PD patients in phase 1/2 clinical trials. However, the mechanism underlying the neurotrophic activity of CDNF is unknown. In this study, we delivered human CDNF (hCDNF) to the brain using an adeno-associated viral (AAV) vector and demonstrated the neurotrophic effect of AAV-hCDNF in an acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. AAV-hCDNF resulted in the expression of hCDNF in the striatum (STR) and substantia nigra (SN), and no toxic effects on the nigrostriatal pathway were observed. Intrastriatal injection of AAV-hCDNF reduced motor impairment and partially alleviated gait dysfunction in the acute MPTP mouse model. In addition, gene therapy with AAV-hCDNF had significant neuroprotective effects on the nigrostriatal pathway and decreased the levels of interleukin 1beta (IL-1β) and complement 3 (C3) in glial cells in the acute MPTP mouse model. Moreover, AAV-hCDNF reduced C/EBP homologous protein (CHOP) and glucose regulatory protein 78 (GRP78) expression in astroglia. These results suggest that the neuroprotective effects of CDNF may be mediated at least in part through the regulation of neuroinflammation and the UPR pathway in a mouse MPTP model of PD in vivo.

CDNF and ER stress: Pharmacology and therapeutic possibilities

Cerebral dopamine neurotrophic factor (CDNF) is an endogenous protein in humans and other vertebrates, and it has been shown to have protective and restorative effects on cells in various disease models. Although it is named as a neurotrophic factor, its actions are drastically different from classical neurotrophic factors such as neurotrophins or the glial cell line-derived neurotrophic family of proteins. Like all secreted proteins, CDNF has a signal sequence at the N-terminus, but unlike common growth factors it has a KDEL-receptor retrieval sequence at the C-terminus. Thus, CDNF is mainly located in the ER. In response to adverse effects, such as ER stress, the expression of CDNF is upregulated and can alleviate ER stress. Also different from other neurotrophic factors, CDNF reduces protein aggregation and inflammation in disease models. Although it is an ER luminal protein, it can surprisingly directly interact with alpha-synuclein, a protein involved in the pathogenesis of synucleinopathies e.g., Parkinson's disease. Pleiotropic CDNF has therapeutic potential and has been tested as a recombinant human protein and gene therapy. The neuroprotective and neurorestorative effects have been described in a number of preclinical studies of Parkinson's disease, stroke and amyotrophic lateral sclerosis. Currently, it was successfully evaluated for safety in a phase 1/2 clinical trial for Parkinson's disease. Collectively, based on recent findings on the mode of action and therapeutic potential of CDNF, its use as a drug could be expanded to other ER stress-related diseases.

Viral and nonviral approaches

Neurodegenerative diseases pose a substantial unmet medical need, and no disease-modifying treatments exist. Neurotrophic factors have been studied for decades as a therapy to slow down or stop the progression of these diseases. In this chapter, we focus on Parkinson disease, the second most common neurodegenerative disorder, and on studies carried out with neurotrophic factors. We explore the routes of administration, how the invasive intracranial administration is the challenge, and different ways to deliver the therapeutic proteins, for example, gene therapy and protein therapy. This therapy concept has been developed to mostly work on the restoration of the lost nigrostriatal dopaminergic neuronal connectivity in the brain. However, in recent years, the center of attention of neurotrophic factors has been on maintaining proteostasis and dissolving and preventing protein inclusions called Lewy bodies. We describe the most studied neurotrophic factor families and compare different preclinical experiments that have been carried out. We also analyze several clinical trials and describe their challenges and breakthroughs and discuss the prospects and challenges of neurotrophic support as a therapy for neurodegenerative diseases. In this chapter, we discuss why they still do and why it is essential to continue to work with this area of neurorestorative research around neurotrophic factors.

Combining fibril-induced alpha-synuclein aggregation and 6-hydroxydopamine in a mouse model of Parkinson's disease and the effect of cerebral dopamine neurotrophic factor on the induced neurodegeneration

The existent pre-clinical models of Parkinson's disease do not simultaneously recapitulate severe degeneration of dopamine neurons and the occurrence of alpha-synuclein (aSyn) aggregation in one study system. In this study, we injected aSyn pre-formed fibrils (PFF) and 6-hydroxydopamine (6-OHDA) unilaterally into the striatum of C57BL/6 wild-type male mice at an interval of 2 weeks to induce aggregation of aSyn protein and trigger the loss of dopamine neurons simultaneously in one model and studied the behavioural effects of the combination in these mice. 6-OHDA was tested at three different doses, and 2 μg of 6-OHDA combined with PFF-induced aSyn aggregation was found to produce the most optimal disease phenotype. At 14 weeks timepoint, mice injected with a combination of PFF and 6-OHDA sustained significant damage to the nigrostriatal pathway and exhibited aSyn-positive aggregation. Our data suggest that the neurons that formed large aSyn aggregates were particularly vulnerable to 6-OHDA-induced degeneration. We also demonstrate the manifestation of a relatively aggressive pathology in 2- to 4-month-old mice, as compared to younger 7- to 9-week-old ones. Furthermore, cerebral dopamine neurotrophic factor (CDNF) administered intrastriatally rescued dopamine neurons and motor behaviour of the animals to some extent from 6-OHDA toxicity. However, no such effect could be seen in the novel 6-OHDA + PFFs combination model. For the first time, we demonstrate the combined effect of PFF and 6-OHDA simultaneously in one model. We further discuss the scope for further optimizing this combination model to develop it as a promising pre-clinical platform for drug screening and development.

Neuroprotective effects of hepatoma-derived growth factor in models of Huntington's disease

Huntington's disease (HD) is a movement disorder caused by a mutation in the Huntingtin gene that leads to severe neurodegeneration. Molecular mechanisms of HD are not sufficiently understood, and no cure is currently available. Here, we demonstrate neuroprotective effects of hepatoma-derived growth factor (HDGF) in cellular and mouse HD models. We show that HD-vulnerable neurons in the striatum and cortex express lower levels of HDGF than resistant ones. Moreover, lack of endogenous HDGF exacerbated motor impairments and reduced the life span of R6/2 Huntington's disease mice. AAV-mediated delivery of HDGF into the brain reduced mutant Huntingtin inclusion load, but had no significant effect on motor behavior or life span. Interestingly, both nuclear and cytoplasmic versions of HDGF were efficient in rescuing mutant Huntingtin toxicity in cellular HD models. Moreover, extracellular application of recombinant HDGF improved viability of mutant Huntingtin-expressing primary neurons and reduced mutant Huntingtin aggregation in neural progenitor cells differentiated from human patient-derived induced pluripotent stem cells. Our findings provide new insights into the pathomechanisms of HD and demonstrate neuroprotective potential of HDGF in neurodegeneration.

Key Subdomains of Cerebral Dopamine Neurotrophic Factor Regulate Its Protective Function in 6-Hydroxydopamine-Lesioned PC12 Cells

Cerebral dopamine neurotrophic factor (CDNF) is a unique neurotrophic factor (NTF) that has shown significant neuroprotective and neurorestorative functions on midbrain dopaminergic neurons. The secondary structure of human CDNF protein contains eight α-helices. We previously found that two key helices, α1 and α7, regulated the intracellular trafficking and secretion of CDNF protein in different manners. The α1 mutation (M1) induced most CDNF proteins to reside in the endoplasmic reticulum and little be secreted extracellularly, while the α7 mutation (M7) caused the majority of CDNF proteins to be secreted out of the cells and little reside in the cells. However, the regulation of the two mutants on the function of CDNF remains unclear. In this study, we investigated the effects of M1 and M7 on the protective activity of CDNF in PC12 cells, which were treated with 6-hydroxydopamine (6-OHDA) to mimic Parkinson's disease. We found that both M1 and M7 could promote survival and inhibit apoptosis more effectively than Wt in 6-OHDA-lesioned PC12 cells. Therefore, these findings will advance our understanding of the important regulation of subdomains on the function of NTFs.

Feasibility of combining alpha-synuclein aggregation and 6-OHDA in embryonic midbrain culture for modeling dopamine neuron degeneration

Parkinson's disease (PD) is characterized by the loss of nigrostriatal dopamine (DA) neurons and the presence of alpha-synuclein (αSyn)-positive Lewy body (LB) pathology. In this study, we attempted to recapitulate both these features in a novel in vitro model for PD. To achieve this, we combined the αSyn pre-formed fibril (PFF)-seeded LB-like pathology with 6-hydroxydopamine (6-OHDA)-induced mitochondrial toxicity in mouse embryonic midbrain cultures. To pilot the model for therapeutics testing, we assessed the effects of cerebral dopamine neurotrophic factor (CDNF) on αSyn aggregation and neuron survival. PFF-seeded pathology did not lead to DA neuron loss even with the highest dose of PFFs. The combination of PFFs and 6-OHDA did not trigger additional neurodegeneration or LB-like pathology and instead presented DA neuron loss to a similar extent as with 6-OHDA only. CDNF did not affect the PFF-seeded αSyn pathology or the DA neuron survival in the combination model but showed a trend toward neuroprotection in the 6-OHDA-only cultures.

Inhibition of Protein Aggregation and Endoplasmic Reticulum Stress as a Targeted Therapy for α-Synucleinopathy

α-synuclein (α-syn) is an intrinsically disordered protein abundant in the central nervous system. Physiologically, the protein regulates vesicle trafficking and neurotransmitter release in the presynaptic terminals. Pathologies related to misfolding and aggregation of α-syn are referred to as α-synucleinopathies, and they constitute a frequent cause of neurodegeneration. The most common α-synucleinopathy, Parkinson's disease (PD), is caused by abnormal accumulation of α-syn in the dopaminergic neurons of the midbrain. This results in protein overload, activation of endoplasmic reticulum (ER) stress, and, ultimately, neural cell apoptosis and neurodegeneration. To date, the available treatment options for PD are only symptomatic and rely on dopamine replacement therapy or palliative surgery. As the prevalence of PD has skyrocketed in recent years, there is a pending issue for development of new disease-modifying strategies. These include anti-aggregative agents that target α-syn directly (gene therapy, small molecules and immunization), indirectly (modulators of ER stress, oxidative stress and clearance pathways) or combine both actions (natural compounds). Herein, we provide an overview on the characteristic features of the structure and pathogenic mechanisms of α-syn that could be targeted with novel molecular-based therapies.

CDNF and MANF in the brain dopamine system and their potential as treatment for Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by gradual loss of midbrain dopamine neurons, leading to impaired motor function. Preclinical studies have indicated cerebral dopamine neurotrophic factor (CDNF) and mesencephalic astrocyte-derived neurotrophic factor (MANF) to be potential therapeutic molecules for the treatment of PD. CDNF was proven to be safe and well tolerated when tested in Phase I-II clinical trials in PD patients. Neuroprotective and neurorestorative effects of CDNF and MANF were demonstrated in animal models of PD, where they promoted the survival of dopamine neurons and improved motor function. However, biological roles of endogenous CDNF and MANF proteins in the midbrain dopamine system have been less clear. In addition to extracellular trophic activities, CDNF/MANF proteins function intracellularly in the endoplasmic reticulum (ER), where they modulate protein homeostasis and protect cells against ER stress by regulating the unfolded protein response (UPR). Here, our aim is to give an overview of the biology of endogenous CDNF and MANF in the brain dopamine system. We will discuss recent studies on CDNF and MANF knockout animal models, and effects of CDNF and MANF in preclinical models of PD. To elucidate possible roles of CDNF and MANF in human biology, we will review CDNF and MANF tissue expression patterns and regulation of CDNF/MANF levels in human diseases. Finally, we will discuss novel findings related to the molecular mechanism of CDNF and MANF action in ER stress, UPR, and inflammation, all of which are mechanisms potentially involved in the pathophysiology of PD.

Intraputamenal Cerebral Dopamine Neurotrophic Factor in Parkinson's Disease: A Randomized, Double-Blind, Multicenter Phase 1 Trial

BACKGROUND

Cerebral dopamine neurotrophic factor (CDNF) is an unconventional neurotrophic factor that protects dopamine neurons and improves motor function in animal models of Parkinson's disease (PD).

OBJECTIVE

The primary objectives of this study were to assess the safety and tolerability of both CDNF and the drug delivery system (DDS) in patients with PD of moderate severity.

METHODS

We assessed the safety and tolerability of monthly intraputamenal CDNF infusions in patients with PD using an investigational DDS, a bone-anchored transcutaneous port connected to four catheters. This phase 1 trial was divided into a placebo-controlled, double-blind, 6-month main study followed by an active-treatment 6-month extension. Eligible patients, aged 35 to 75 years, had moderate idiopathic PD for 5 to 15 years and Hoehn and Yahr score ≤ 3 (off state). Seventeen patients were randomized to placebo (n = 6), 0.4 mg CDNF (n = 6), or 1.2 mg CDNF (n = 5). The primary endpoints were safety and tolerability of CDNF and DDS and catheter implantation accuracy. Secondary endpoints were measures of PD symptoms, including Unified Parkinson's Disease Rating Scale, and DDS patency and port stability. Exploratory endpoints included motor symptom assessment (PKG, Global Kinetics Pty Ltd, Melbourne, Australia) and positron emission tomography using dopamine transporter radioligand [18 F]FE-PE2I.

RESULTS

Drug-related adverse events were mild to moderate with no difference between placebo and treatment groups. No severe adverse events were associated with the drug, and device delivery accuracy met specification. The severe adverse events recorded were associated with the infusion procedure and did not reoccur after procedural modification. There were no significant changes between placebo and CDNF treatment groups in secondary endpoints between baseline and the end of the main and extension studies.

CONCLUSIONS

Intraputamenally administered CDNF was safe and well tolerated, and possible signs of biological response to the drug were observed in individual patients. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Glia-Neurotrophic Factor Relationships: Possible Role in Pathobiology of Neuroinflammation-Related Brain Disorders

Neurotrophic factors (NTFs) play an important role in maintaining homeostasis of the central nervous system (CNS) by regulating the survival, differentiation, maturation, and development of neurons and by participating in the regeneration of damaged tissues. Disturbances in the level and functioning of NTFs can lead to many diseases of the nervous system, including degenerative diseases, mental diseases, and neurodevelopmental disorders. Each CNS disease is characterized by a unique pathomechanism, however, the involvement of certain processes in its etiology is common, such as neuroinflammation, dysregulation of NTFs levels, or mitochondrial dysfunction. It has been shown that NTFs can control the activation of glial cells by directing them toward a neuroprotective and anti-inflammatory phenotype and activating signaling pathways responsible for neuronal survival. In this review, our goal is to outline the current state of knowledge about the processes affected by NTFs, the crosstalk between NTFs, mitochondria, and the nervous and immune systems, leading to the inhibition of neuroinflammation and oxidative stress, and thus the inhibition of the development and progression of CNS disorders.

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.

CDNF Interacts with ER Chaperones and Requires UPR Sensors to Promote Neuronal Survival

Cerebral dopamine neurotrophic factor (CDNF) is a neurotrophic factor that has beneficial effects on dopamine neurons in both in vitro and in vivo models of Parkinson's disease (PD). CDNF was recently tested in phase I-II clinical trials for the treatment of PD, but the mechanisms underlying its neuroprotective properties are still poorly understood, although studies have suggested its role in the regulation of endoplasmic reticulum (ER) homeostasis and the unfolded protein response (UPR). The aim of this study was to investigate the mechanism of action of CDNF through analyzing the involvement of UPR signaling in its anti-apoptotic function. We used tunicamycin to induce ER stress in mice in vivo and used cultured primary neurons and found that CDNF expression is regulated by ER stress in vivo and that the involvement of UPR pathways is important for the neuroprotective function of CDNF. Moreover, we used AP-MS and BiFC to perform the first interactome screening for CDNF and report novel binding partners of CDNF. These findings allowed us to hypothesize that CDNF protects neurons from ER-stress-inducing agents by modulating UPR signaling towards cell survival outcomes.

GSK-3β-mediated regulation of Nrf2/HO-1 signaling as a new therapeutic approach in the treatment of movement disorders

Movement disorders are neurological conditions characterized by involuntary motor movements, such as dystonia, ataxia, chorea myoclonus, tremors, Huntington's disease (HD), and Parkinson's disease (PD). It is classified into two categories: hypokinetic and hyperkinetic movements. Globally, movement disorders are a major cause of death. The pathophysiological process is initiated by excessive ROS generation, mitochondrial dysfunction, neuroinflammation, and neurotransmitters imbalance that lead to motor dysfunction in PD and HD patients. Several endogenous targets including Nrf2 maintain oxidative balance in the body. Activation of Nrf2 signaling is regulated by the enzyme glycogen synthase kinase (GSK-3β). In the cytoplasm, inhibition of GSK-3β regulates cellular proliferation, homeostasis, and apoptotic process by stimulating the nuclear factor erythroid 2 (Nrf2) pathway which is involved in the elevation of the cellular antioxidant enzymes which controls the ROS generation. The activation of Nrf2 increases the expression of antioxidant response elements (ARE), such as (Hemeoxygenase-1) HO-1, which decreases excessive cellular stress, mitochondrial dysfunction, apoptosis, and neuronal degeneration, which is the major cause of motor dysfunction. The present review explores the GSK-3β-mediated neuroprotection in various movement disorders through the Nrf2/HO-1 antioxidant pathway. This review provides a link between GSK-3β and the Nrf2/HO-1 signaling pathway in the treatment of PD and HD. In addition to that it highlights various GSK-3β inhibitors and the Nrf2/HO-1 activators, which exert robust neuroprotection against motor disorders. Therefore, the present review will help in the discovery of new therapy for PD and HD patients.

Endoplasmic Reticulum in Metaplasticity: From Information Processing to Synaptic Proteostasis

The ER (endoplasmic reticulum) is a Ca²⁺ reservoir and the unique protein-synthesizing machinery which is distributed throughout the neuron and composed of multiple different structural domains. One such domain is called EMC (endoplasmic reticulum membrane protein complex), pleiotropic nature in cellular functions. The ER/EMC position inside the neurons unmasks its contribution to synaptic plasticity via regulating various cellular processes from protein synthesis to Ca²⁺ signaling. Since presynaptic Ca²⁺ channels and postsynaptic ionotropic receptors are organized into the nanodomains, thus ER can be a crucial player in establishing TMNCs (transsynaptic molecular nanocolumns) to shape efficient neural communications. This review hypothesized that ER is not only involved in stress-mediated neurodegeneration but also axon regrowth, remyelination, neurotransmitter switching, information processing, and regulation of pre- and post-synaptic functions. Thus ER might not only be a protein-synthesizing and quality control machinery but also orchestrates plasticity of plasticity (metaplasticity) within the neuron to execute higher-order brain functions and neural repair.

Modulating Microglia/Macrophage Activation by CDNF Promotes Transplantation of Fetal Ventral Mesencephalic Graft Survival and Function in a Hemiparkinsonian Rat Model

Parkinson’s disease (PD) is characterized by the loss of dopaminergic neurons in substantia nigra pars compacta, which leads to the motor control deficits. Recently, cell transplantation is a cutting-edge technique for the therapy of PD. Nevertheless, one key bottleneck to realizing such potential is allogenic immune reaction of tissue grafts by recipients. Cerebral dopamine neurotrophic factor (CDNF) was shown to possess immune-modulatory properties that benefit neurodegenerative diseases. We hypothesized that co-administration of CDNF with fetal ventral mesencephalic (VM) tissue can improve the success of VM replacement therapies by attenuating immune responses. Hemiparkinsonian rats were generated by injecting 6-hydroxydopamine (6-OHDA) into the right medial forebrain bundle of Sprague Dawley (SD) rats. The rats were then intrastriatally transplanted with VM tissue from rats, with/without CDNF administration. Recovery of dopaminergic function and survival of the grafts were evaluated using the apomorphine-induced rotation test and small-animal positron emission tomography (PET) coupled with [¹⁸F] DOPA or [¹⁸F] FE-PE2I, respectively. In addition, transplantation-related inflammatory response was determined by uptake of [¹⁸F] FEPPA in the grafted side of striatum. Immunohistochemistry (IHC) examination was used to determine the survival of the grated dopaminergic neurons in the striatum and to investigate immune-modulatory effects of CDNF. The modulation of inflammatory responses caused by CDNF might involve enhancing M2 subset polarization and increasing fractal dimensions of 6-OHDA-treated BV2 microglial cell line. Analysis of CDNF-induced changes to gene expressions of 6-OHDA-stimulated BV2 cells implies that these alternations of the biomarkers and microglial morphology are implicated in the upregulation of protein kinase B signaling as well as regulation of catalytic, transferase, and protein serine/threonine kinase activity. The effects of CDNF on 6-OHDA-induced alternation of the canonical pathway in BV2 microglial cells is highly associated with PI3K-mediated phagosome formation. Our results are the first to show that CDNF administration enhances the survival of the grafted dopaminergic neurons and improves functional recovery in PD animal model. Modulation of the polarization, morphological characteristics, and transcriptional profiles of 6-OHDA-stimualted microglia by CDNF may possess these properties in transplantation-based regenerative therapies.

Similarities and differences between nigral and enteric dopaminergic neurons unravel distinctive involvement in Parkinson's disease

In addition to the well-known degeneration of midbrain dopaminergic neurons, enteric neurons can also be affected in neurodegenerative disorders such as Parkinson's disease (PD). Dopaminergic neurons have recently been identified in the enteric nervous system (ENS). While ENS dopaminergic neurons have been shown to degenerate in genetic mouse models of PD, analyses of their survival in enteric biopsies of PD patients have provided inconsistent results to date. In this context, this review seeks to highlight the distinctive and shared factors and properties that control the evolution of these two sets of dopaminergic neurons from neuronal precursors to aging neurons. Although their cellular sources and developmental times of origin differ, midbrain and ENS dopaminergic neurons express many transcription factors in common and their respective environments express similar neurotrophic molecules. For example, Foxa2 and Sox6 are expressed by both populations to promote the specification, differentiation, and long-term maintenance of the dopaminergic phenotype. Both populations exhibit sustained patterns of excitability that drive intrinsic vulnerability over time. In disorders such as PD, colon biopsies have revealed aggregation of alpha-synuclein in the submucosal plexus where dopaminergic neurons reside and lack blood barrier protection. Thus, these enteric neurons may be more susceptible to neurotoxic insults and aggregation of α-synuclein that spreads from gut to midbrain. Under sustained stress, inefficient autophagy leads to neurodegeneration, GI motility dysfunction, and PD symptoms. Recent findings suggest that novel neurotrophic factors such as CDNF have the potential to be used as neuroprotective agents to prevent and treat ENS symptoms of PD.

Cerebral dopamine neurotrophic factor protects and repairs dopamine neurons by novel mechanism

Midbrain dopamine neurons deteriorate in Parkinson's disease (PD) that is a progressive neurodegenerative movement disorder. No cure is available that would stop the dopaminergic decline or restore function of injured neurons in PD. Neurotrophic factors (NTFs), e.g., glial cell line-derived neurotrophic factor (GDNF) are small, secreted proteins that promote neuron survival during mammalian development and regulate adult neuronal plasticity, and they are studied as potential therapeutic agents for the treatment of neurodegenerative diseases. However, results from clinical trials of GDNF and related NTF neurturin (NRTN) in PD have been modest so far. In this review, we focus on cerebral dopamine neurotrophic factor (CDNF), an unconventional neurotrophic protein. CDNF delivered to the brain parenchyma protects and restores dopamine neurons in animal models of PD. In a recent Phase I-II clinical trial CDNF was found safe and well tolerated. CDNF deletion in mice led to age-dependent functional changes in the brain dopaminergic system and loss of enteric neurons resulting in slower gastrointestinal motility. These defects in Cdnf^-/- mice intriguingly resemble deficiencies observed in early stage PD. Different from classical NTFs, CDNF can function both as an extracellular trophic factor and as an intracellular, endoplasmic reticulum (ER) luminal protein that protects neurons and other cell types against ER stress. Similarly to the homologous mesencephalic astrocyte-derived neurotrophic factor (MANF), CDNF is able to regulate ER stress-induced unfolded protein response (UPR) signaling and promote protein homeostasis in the ER. Since ER stress is thought to be one of the pathophysiological mechanisms contributing to the dopaminergic degeneration in PD, CDNF, and its small-molecule derivatives that are under development may provide useful tools for experimental medicine and future therapies for the treatment of PD and other neurodegenerative protein-misfolding diseases.

Cerebral Dopamine Neurotrophic Factor (CDNF): Structure, Functions, and Therapeutic Potential

The cerebral dopamine neurotrophic factor (CDNF) together with the mesencephalic astrocyte-derived neurotrophic factor (MANF) form a unique family of neurotrophic factors (NTFs) structurally and functionally different from other proteins with neurotrophic activity. CDNF has no receptors on the cell membrane, is localized mainly in the cavity of endoplasmic reticulum (ER), and its primary function is to regulate ER stress. In addition, CDNF is able to suppress inflammation and apoptosis. Due to its functions, CDNF has demonstrated outstanding protective and restorative properties in various models of neuropathology associated with ER stress, including Parkinson's disease (PD). That is why CDNF already passed clinical trials in patients with PD. However, despite the name, CDNF functions extend far beyond the dopamine system in the brain. In particular, there are data on participation of CDNF in the maturation and maintenance of other neurotransmitter systems, regulation of the processes of neuroplasticity and non-motor behavior. In the present review, we discuss the features of CDNF structure and functions, its protective and regenerative properties.

Endoplasmic Reticulum Stress Regulators: New Drug Targets for Parkinson's Disease

Parkinson's disease (PD) pathology involves progressive degeneration and death of vulnerable dopamine neurons in the substantia nigra. Extensive axonal arborization and distinct functions make this type of neurons particularly sensitive to homeostatic perturbations, such as protein misfolding and Ca2+ dysregulation. Endoplasmic reticulum (ER) is a cell compartment orchestrating protein synthesis and folding, as well as synthesis of lipids and maintenance of Ca2+ homeostasis in eukaryotic cells. When misfolded proteins start to accumulate in ER lumen the unfolded protein response (UPR) is activated. UPR is an adaptive signaling machinery aimed at relieving of protein folding load in the ER. When UPR is chronic, it can either boost neurodegeneration and apoptosis or cause neuronal dysfunctions. We have recently discovered that mesencephalic astrocyte-derived neurotrophic factor (MANF) exerts its prosurvival action in dopamine neurons and in an animal model of PD through the direct binding to UPR sensor inositol-requiring protein 1 alpha (IRE1α) and attenuation of UPR. In line with this, UPR targeting resulted in neuroprotection and neurorestoration in various preclinical animal models of PD. Therefore, growth factors (GFs), possessing both neurorestorative activity and restoration of protein folding capacity are attractive as drug candidates for PD treatment especially their blood-brain barrier penetrating analogs and small molecule mimetics. In this review, we discuss ER stress as a therapeutic target to treat PD; we summarize the existing preclinical data on the regulation of ER stress for PD treatment. In addition, we point out the crucial aspects for successful clinical translation of UPR-regulating GFs and new prospective in GFs-based treatments of PD, focusing on ER stress regulation.