Transmission of Synucleinopathies in the Enteric Nervous System of A53T Alpha-Synuclein Transgenic Mice

Neuroinflammation, immune response and α-synuclein pathology: how animal models are helping us to connect dots

INTRODUCTION

A key pathological event occurring in Parkinson's disease (PD) is the transneuronal spreading of alpha-synuclein (α-syn). Other hallmarks of PD include neurodegeneration, glial activation, and immune cell infiltration in susceptible brain regions. Although preclinical models can mimic most of the key characteristics of PD, it is crucial to know the biological bases of individual differences between them when choosing one over another, to ensure proper interpretation of the results and to positively influence the outcome of the experiments.

AREAS COVERED

This review provides an overview of current preclinical models actively used to study the interplay between α-syn pathology, neuroinflammation and immune response in PD but also to explore new potential preclinical models or emerging therapeutic strategies intended to fulfill the unmet medical needs in this disease. Lastly, this review also considers the current state of the ongoing clinical trials of new drugs designed to target these processes and delay the initiation or progression of the disease.

EXPERT OPINION

Anti-inflammatory and immunomodulatory agents have been demonstrated to be very promising candidates for reducing disease progression; however, more efforts are needed to reduce the enormous gap between these and dopaminergic drugs, which have dominated the therapeutic market for the last sixty years.

Intranasal Rotenone Induces Alpha-Synuclein Accumulation, Neuroinflammation and Dopaminergic Neurodegeneration in Middle-Aged Mice

Accumulation of alpha-synuclein (α-syn) is central to the pathogenesis of Parkinson's disease (PD). Previous studies suggest that α-syn pathology may originate from the olfactory bulb (OB) or gut in response to an unknown pathogen and later progress to the different brain regions. Aging is viewed as the utmost threat to PD development. Therefore, studies depicting the role of age in α-syn accumulation and its progression in PD are important. In the present study, we gave intranasal rotenone microemulsion for 6 weeks in 12-month-old female BALB/c mice and found olfactory dysfunction after 4 and 6 weeks of rotenone administration. Interestingly, motor impairment was observed only after 6 weeks. The animals were sacrificed after 6 weeks to perform western blotting and immunohistochemical studies to detect α-syn pathology, neuroinflammation and neurodegeneration. We found α-syn accumulation in OB, striatum, substantia nigra (SN) and cortex. Importantly, we found significant glial cell activation and neurodegeneration in all the analysed regions which were absent in our previous published studies with 3 months old mice even after they were exposed to rotenone for 9 weeks indicating age is a crucial factor for α-syn induced neuroinflammation and neurodegeneration. We also observed increased iron accumulation in SN of rotenone-exposed aged mice. Moreover, inflammaging was observed in OB and striatum of 12-month-old BALB/c mice as compared to 3-month-old BALB/c mice. In conclusion, there is a difference in sensitivity between adult and aged mice in the development and progression of α-syn pathology and subsequent neurodegeneration, for which inflammaging might be the crucial probable mechanism.

Deciphering the prion-like behavior of pathogenic protein aggregates in neurodegenerative diseases

Neurodegenerative diseases are hitherto classified based on their core clinical features, the anatomical distribution of neurodegeneration, and the cell populations mainly affected. On the other hand, the wealth of neuropathological, genetic, molecular and biochemical studies have identified the existence of distinct insoluble protein aggregates in the affected brain regions. These findings have spread the use of a collective term, proteinopathy, for neurodegenerative disorders with particular type of structurally altered protein accumulation. Particularly, a recent breakthrough in this field came with the discovery that these protein aggregates can transfer from one cell to another, thereby converting normal proteins to potentially toxic, misfolded species in a prion-like manner. In this review, we focus specifically on the molecular and cellular basis that underlies the seeding activity and transcellular spreading phenomenon of neurodegeneration-related protein aggregates, and discuss how these events contribute to the disease progression.

Molecular Communication Between Neuronal Networks and Intestinal Epithelial Cells in Gut Inflammation and Parkinson's Disease

Intestinal symptoms, such as nausea, vomiting, and constipation, are common in Parkinson's disease patients. These clinical signs normally appear years before the diagnosis of the neurodegenerative disease, preceding the occurrence of motor manifestations. Moreover, it is postulated that Parkinson's disease might originate in the gut, due to a response against the intestinal microbiota leading to alterations in alpha-synuclein in the intestinal autonomic nervous system. Transmission of this protein to the central nervous system is mediated potentially via the vagus nerve. Thus, deposition of aggregated alpha-synuclein in the gastrointestinal tract has been suggested as a potential prodromal diagnostic marker for Parkinson's disease. Interestingly, hallmarks of chronic intestinal inflammation in inflammatory bowel disease, such as dysbiosis and increased intestinal permeability, are also observed in Parkinson's disease patients. Additionally, alpha-synuclein accumulations were detected in the gut of Crohn's disease patients. Despite a solid association between neurodegenerative diseases and gut inflammation, it is not clear whether intestinal alterations represent cause or consequence of neuroinflammation in the central nervous system. In this review, we summarize the bidirectional communication between the brain and the gut in the context of Parkinson's disease and intestinal dysfunction/inflammation as present in inflammatory bowel disease. Further, we focus on the contribution of intestinal epithelium, the communication between intestinal epithelial cells, microbiota, immune and neuronal cells, as well as mechanisms causing alterations of epithelial integrity.

Mechanisms of Neurodegeneration in Various Forms of Parkinsonism-Similarities and Differences

Parkinson’s disease (PD), dementia with Lewy body (DLB), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and multiple system atrophy (MSA) belong to a group of neurodegenerative diseases called parkinsonian syndromes. They share several clinical, neuropathological and genetic features. Neurodegenerative diseases are characterized by the progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra- and intracellular accumulation of misfolded proteins. The parkinsonian diseases affect distinct areas of the brain. PD and MSA belong to a group of synucleinopathies that are characterized by the presence of fibrillary aggregates of α-synuclein protein in the cytoplasm of selected populations of neurons and glial cells. PSP is a tauopathy associated with the pathological aggregation of the microtubule associated tau protein. Although PD is common in the world’s aging population and has been extensively studied, the exact mechanisms of the neurodegeneration are still not fully understood. Growing evidence indicates that parkinsonian disorders to some extent share a genetic background, with two key components identified so far: the microtubule associated tau protein gene (MAPT) and the α-synuclein gene (SNCA). The main pathways of parkinsonian neurodegeneration described in the literature are the protein and mitochondrial pathways. The factors that lead to neurodegeneration are primarily environmental toxins, inflammatory factors, oxidative stress and traumatic brain injury.

Parkinson's Disease and the Gut: Future Perspectives for Early Diagnosis

Parkinson's disease (PD) is a neurodegenerative disease characterized by progressive degeneration of dopaminergic neurons, and at the cellular level by the formation of Lewy bodies in the central nervous system (CNS). However, the onset of the disease is believed to be localized to peripheral organs, particularly the gastrointestinal tract (GIT) and the olfactory bulb sooner before neuropathological changes occur in the CNS. Patients already in the pre-motor stage of PD suffer from various digestive problems and/or due to significant changes in the composition of the intestinal microbiome in this early stage of the disease. Detailed analyses of patient biopsies and autopsies as well as animal models of neuropathological changes characteristic of PD provided important information on the pathology or treatment of PD symptoms. However, presently is not clarified (i) the specific tissue in the GIT where the pathological processes associated with PD is initiated; (ii) the mechanism by which these processes are disseminated to the CNS or other tissues within the GIT; and (iii) which neuropathological changes could also serve as a reliable diagnostic marker of the premotor stages of PD, or (iv) which type of GIT tissue would be the most appropriate choice for routine examination of patient biopsies.

What is the Evidence That Parkinson's Disease is a Prion Disorder, Which Originates in the Gut?

Parkinson’s disease (PD) is a neurodegenerative disorder resulting from degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc). PD is characterized by motor dysfunctions as well as gastrointestinal symptoms and mental impairment. The pathological hallmark of PD is an accumulation of misfolded α-synuclein aggregates within the brain. The etiology of PD and related synucleinopathy is poorly understood, but recently, the hypothesis that α-synuclein pathology spreads in a prion-like fashion originating in the gut has gained much scientific attention. A crucial clue was the appearance of constipation before the onset of motor symptoms, gut dysbiosis and synucleinopathy in PD patients. Another line of evidence, demonstrating accumulation of α-synuclein within the peripheral autonomic nervous system (PANS), including the enteric nervous system (ENS), and the dorsal motor nucleus of the vagus (DMV) support the concept that α-synuclein can spread from the ENS to the brain by the vagus nerve. The decreased risk of PD following truncal vagotomy supports this. The convincing evidence of the prion-like behavior of α-synuclein came from postmortem observations that pathological α-synuclein inclusions appeared in healthy grafted neurons. In this review, we summarize the available data from human subjects’ research and animal experiments, which seem to be the most suggestive for explaining the hypotheses.

Enteric nervous system manifestations of neurodegenerative disease

Neurological disorders cause gastrointestinal (GI) symptoms that are debilitating and markedly diminish quality of life in patients. The enteric nervous system (ENS), the intrinsic nervous system of the GI tract that is often referred to as "the second brain", shares many features with the central nervous system. The ENS plays an essential role in regulating many GI functions including motility and fluid secretion. Enteric neuronal degeneration could therefore be responsible for the GI symptoms commonly observed in neurological conditions. Here we describe the organization and functions of the ENS and then review the evidence for ENS involvement in two common neurodegenerative disorders, Parkinson's disease (PD) and Alzheimer's disease (AD). Data from patients as well as animal models suggest that PD affects distinct subsets of neurons and glia in the ENS, and that the ENS may participate in the pathogenesis of this disorder. While there has been great enthusiasm for the possibility of sampling the ENS for diagnosis or therapeutic monitoring of PD, further work is needed to determine which enteric neurons are most affected and how ENS function could be modulated to ameliorate GI symptoms in patients. Although AD is far more common than PD and AD patients also experience GI symptoms, understanding of ENS dysfunction in AD is in its infancy. Much work remains to be done in both of these fields to determine how the ENS contributes to and/or is altered by these disorders, and how to target the ENS for more effective treatment of GI comorbidities.

Prion-like propagation of α-synuclein in the gut-brain axis

Parkinson's disease (PD) is a progressive degenerative disease of the nervous system, which is characterized by movement disorders, such as static tremor, rigidity, and bradykinesia in advanced patients. Gastrointestinal (GI) dysfunction, such as gastric dysmotility, constipation, and anorectic dysfunction, is common non-motor symptom in the early stage of PD. The progression of PD includes the degenerative loss of dopaminergic (DA) neurons and aggregation of α-synuclein in the substantia nigra (SN). Interestingly, both of them are also present in the enteric nervous system (ENS) of PD patients. In this review, we describe the relationship between non-motor symptoms particularly GI dysfunction and the pathogenesis of PD, aiming to show the powerful evidences about the prion-like propagation of α-synuclein and support the hypothesis of gut-brain axis in PD. We then summarize the mechanism of the gut-brain axis and confirm α-synuclein as a potential target for drug design or new clinical treatment.

Possible Function of Molecular Chaperones in Diseases Caused by Propagating Amyloid Aggregates

The vast majority of neurodegenerative pathologies stem from the formation of toxic oligomers and aggregates composed of wrongly folded proteins. These protein complexes can be released from pathogenic cells and enthralled by other cells, causing the formation of new aggregates in a prion-like manner. By this mechanism, migrating complexes can transmit a disorder to distant regions of the brain and promote gradually transmitting degenerative processes. Molecular chaperones can counteract the toxicity of misfolded proteins. In this review, we discuss recent data on the possible cytoprotective functions of chaperones in horizontally transmitting neurological disorders.

Potential Pathways of Abnormal Tau and α-Synuclein Dissemination in Sporadic Alzheimer's and Parkinson's Diseases

Experimental data indicate that transneuronal propagation of abnormal protein aggregates in neurodegenerative proteinopathies, such as sporadic Alzheimer's disease (AD) and Parkinson's disease (PD), is capable of a self-propagating process that leads to a progression of neurodegeneration and accumulation of prion-like particles. The mechanisms by which misfolded tau and α-synuclein possibly spread from one involved nerve cell to the next in the neuronal chain to induce abnormal aggregation are still unknown. Based on findings from studies of human autopsy cases, we review potential pathways and mechanisms related to axonal and transneuronal dissemination of tau (sporadic AD) and α-synuclein (sporadic PD) aggregates between anatomically interconnected regions.

Total α-synuclein levels in human blood cells, CSF, and saliva determined by a lipid-ELISA

The validity of α-synuclein (α-Syn) as a biomarker for Parkinson's disease (PD) is still under investigation. Conventional methods for capture and quantitation of α-Syn protein in human samples are primarily based on anti-α-Syn antibodies. Specific and competent antibodies were raised against α-Syn. However, capture by anti-α-Syn antibodies may be limited to specific epitope recognition, attributed to protein structure or post-translational modifications. Hence, antibody-based methods for α-Syn capture raise a concern regarding their efficacy to detect the intracellular, unfolded α-Syn pool. An alternative is α-Syn capture by membrane lipids, i.e., to utilize the biochemical property of α-Syn to specifically bind membrane lipids and acquire a characteristic structure following binding. We determined α-Syn levels in human samples using immobilized lipids for α-Syn capture. The lipids used for α-Syn capture consist of phosphatidyl inositol (PI), phosphatidyl serine (PS), and phosphatidyl ethanolamine (PE). Addition of mono-sialoganglioside, GM1 ganglioside, to the immobilized lipids significantly improved α-Syn detection. Following capture, the lipid-bound α-Syn was detected using an anti-α-Syn antibody. Total α-Syn levels in whole blood cells (WBC), cerebrospinal fluid (CSF), and saliva were determined by the lipid-ELISA method.

Prion-like disorders and Transmissible Spongiform Encephalopathies: An overview of the mechanistic features that are shared by the various disease-related misfolded proteins

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are a group of fatal neurodegenerative disorders affecting several mammalian species. Its causative agent, disease-associated prion protein (PrP^d), is a self-propagating β-sheet rich aberrant conformation of the cellular prion protein (PrP^C) with neurotoxic and aggregation-prone properties, capable of inducing misfolding of PrP^C molecules. PrP^d is the major constituent of prions and, most importantly, is the first known example of a protein with infectious attributes. It has been suggested that similar molecular mechanisms could be shared by other proteins implicated in diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis or systemic amyloidoses. Accordingly, several terms have been proposed to collectively group all these disorders. Through the stringent evaluation of those aspects that characterise TSE-causing prions, in particular propagation and spread, strain variability or transmissibility, we will discuss whether terms such as "prion", "prion-like", "prionoid" or "propagon" can be used when referring to the aetiological agents of the above other disorders. Moreover, it will also be discussed whether the term "infectious", which defines a prion essential trait, is currently misused when referring to the other misfolded proteins.

Restoration of intestinal function in an MPTP model of Parkinson's Disease

Patients with Parkinson’s disease often experience non-motor symptoms including constipation, which manifest prior to the onset of debilitating motor signs. Understanding the causes of these non-motor deficits and developing disease modifying therapeutic strategies has the potential to prevent disease progression. Specific neuronal subpopulations were reduced within the myenteric plexus of mice 21 days after intoxication by the intraperitoneal administration of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and was associated with a reduction in stool frequency, indicative of intestinal dysfunction. Oral administration of the divalent copper complex, Cu^II(atsm), which has been shown to be neuroprotective and restore motor performance to MPTP lesioned mice, improved stool frequency and was correlated with restoration of neuronal subpopulations in the myenteric plexus of MPTP lesioned mice. Restoration of intestinal function was associated with reduced enteric glial cell reactivity and reduction of markers of inflammation. Therapeutics that have been shown to be neuroprotective in the central nervous system, such as Cu^II(atsm), therefore also provide symptom relief and are disease modifying in the intestinal tract, suggesting that there is a common cause of Parkinson’s disease pathogenesis in the enteric nervous system and central nervous system.

Antagonizing Neuronal Toll-like Receptor 2 Prevents Synucleinopathy by Activating Autophagy

Impaired autophagy has been implicated in many neurodegenerative diseases, such as Parkinson's disease (PD), and might be responsible for deposition of aggregated proteins in neurons. However, little is known about how neuronal autophagy and clearance of aggregated proteins are regulated. Here, we show a role for Toll-like receptor 2 (TLR2), a pathogen-recognizing receptor in innate immunity, in regulation of neuronal autophagy and clearance of α-synuclein, a protein aggregated in synucleinopathies, including in PD. Activation of TLR2 resulted in the accumulation of α-synuclein aggregates in neurons as a result of inhibition of autophagic activity through regulation of the AKT/mTOR pathway. In contrast, inactivation of TLR2 resulted in autophagy activation and increased clearance of neuronal α-synuclein, and hence reduced neurodegeneration, in transgenic mice and in in vitro models. These results uncover roles of TLR2 in regulating neuronal autophagy and suggest that the TLR2 pathway may be targeted for autophagy activation strategies in treating neurodegenerative disorders.

FcγRIIB mediates the inhibitory effect of aggregated α-synuclein on microglial phagocytosis

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease. Although the etiology of PD has not yet been fully understood, accumulating evidence indicates that neuroinflammation plays a critical role in the progression of PD. α-Synuclein (α-Syn) has been considered to be a key player of the pathogenesis of PD, and recent reports that prion-like propagation of misfolded α-syn released from neurons may play an important role in the progression of PD have led to increased attention to the studies elucidating the roles of extracellular α-syn in the CNS. Extracellular α-syn has also been reported to regulate microglial inflammatory response. In this study, we demonstrated that aggregated α-syn inhibited microglial phagocytosis by activating SHP-1. SHP-1 activation was also observed in A53T α-syn transgenic mice. In addition, aggregated α-syn bound to FcγRIIB on microglia, inducing SHP-1 activation, further inhibiting microglial phagocytosis. Aggregated α-syn upregulated FcγRIIB expression in microglia and upregulated FcγRIIB was also observed in A53T α-syn transgenic mice. These data suggest that aggregated α-syn released from neurons dysregulates microglial immune response through inhibiting microglial phagocytosis, further causing neurodegeneration observed in PD. The interaction of aggregated α-syn and FcγRIIB and further SHP-1 activation can be a new therapeutic target against PD.

Total and Proteinase K-Resistant α-Synuclein Levels in Erythrocytes, Determined by their Ability to Bind Phospholipids, Associate with Parkinson's Disease

A marker for diagnosis of Parkinson's disease (PD), which reflects on the occurrence of peripheral pathogenic mechanisms, would potentially improve therapy. The significance of α-Synuclein (α-Syn) expression in red blood cells (RBC) is currently unclear. Here we investigated whether RBC's-expressed α-Syn may associate with PD. To this aim, we determined the levels of total and proteinase K-resistant α-Syn in samples of packed red blood cells (PRBCs). Twenty-one individuals with PD at various disease stages and 15 healthy controls, with similar demographic features, were recruited to this study. α-Syn levels were determined by their biochemical property to bind phospholipids, using a phospholipid-ELISA assay. A significantly lower ratio of total-to-proteinase K-resistant α-Syn levels was detected in PD patients than in the healthy control group. However, there was considerable overlap between the two groups. Suggesting a need for additional markers to be tested in combination with α-Syn levels. To the best of our knowledge, this is the first evidence for an association between RBCs-expressed α-Syn and pathogenic mechanisms involved in PD.

Animal models for prion-like diseases

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are a group of fatal neurodegenerative disorders affecting several mammalian species being Creutzfeldt-Jacob Disease (CJD) the most representative in human beings, scrapie in ovine, Bovine Spongiform Encephalopathy (BSE) in bovine and Chronic Wasting Disease (CWD) in cervids. As stated by the "protein-only hypothesis", the causal agent of TSEs is a self-propagating aberrant form of the prion protein (PrP) that through a misfolding event acquires a β-sheet rich conformation known as PrP(Sc) (from scrapie). This isoform is neurotoxic, aggregation prone and induces misfolding of native cellular PrP. Compelling evidence indicates that disease-specific protein misfolding in amyloid deposits could be shared by other disorders showing aberrant protein aggregates such as Alzheimer's Disease (AD), Parkinson's Disease (PD), Amyotrophic lateral sclerosis (ALS) and systemic Amyloid A amyloidosis (AA amyloidosis). Evidences of shared mechanisms of the proteins related to each disease with prions will be reviewed through the available in vivo models. Taking prion research as reference, typical prion-like features such as seeding and propagation ability, neurotoxic species causing disease, infectivity, transmission barrier and strain evidences will be analyzed for other protein-related diseases. Thus, prion-like features of amyloid β peptide and tau present in AD, α-synuclein in PD, SOD-1, TDP-43 and others in ALS and serum α-amyloid (SAA) in systemic AA amyloidosis will be reviewed through models available for each disease.

The relation between α-synuclein and microglia in Parkinson's disease: Recent developments

Recent research suggests a complex role for microglia not only in Parkinson's disease but in other disorders involving alpha-synuclein aggregation, such as multiple system atrophy. In these neurodegenerative processes, the activation of microglia is a common pathological finding, which disturbs the homeostasis of the neuronal environment otherwise maintained, among others, by microglia. The term activation comprises any deviation from what otherwise is considered normal microglia status, including cellular abundance, morphology or protein expression. The microglial response during disease will sustain survival or otherwise promote cell degeneration. The novel concepts of alpha-synuclein being released and uptaken by neighboring cells, and their importance in disease progression, positions microglia as the main cell that can clear and handle alpha-synuclein efficiently. Microglia's behavior will therefore be a determinant on the disease's progression. For this reason we believe that the better understanding of microglia's response to alpha-synuclein pathological accumulation across brain areas and disease stages is essential to develop novel therapeutic tools for Parkinson's disease and other alpha-synucleinopathies. In this review we will revise the most recent findings and developments with regard to alpha-synuclein and microglia in Parkinson's disease.

Protein phosphatase 2A is involved in the tyrosine hydroxylase phosphorylation regulated by α-synuclein

α-Synuclein (α-Syn) plays a crucial role in the pathophysiology of Parkinson's disease (PD), the degeneration of dopaminergic neurons. Previous studies have shown that α-Syn regulates dopamine synthesis by binding to and inhibiting tyrosine hydroxylase (TH). In neurons, protein phosphatases (PPs) play a prominent role in directing signaling toward survival or degeneration. This study was to re-evaluate whether α-Syn could regulate the tyrosine hydroxylase phosphorylation by protein phosphatase-2A (PP2A) in dopaminergic MN9D cells and cortex neurons. Our data demonstrated for the first time that α-Syn stimulates PP2A activity and reduces phosphorylation of TH through regulating the methylation of PP2A in dopaminergic MN9D cells and primary cortex neurons. Increased PP2A activity and reduced phosphorylation of PP2A at Y307 (inactive form of PP2A) were observed in α-Syn overexpression dopaminergic cells (Syn) and primary cortex neurons, and the TH phosphorylation relieved by enhancing PP2A methylation in Syn group could be abated by using PP inhibitors, okadaic acid (OKA). OKA could reduce the cell damage and cell apoptosis induced by α-Syn. Thus our findings may provide an insight into the complicated pathogenesis of PD as well as some clues to the development of novel therapeutic strategies targeting at PP2A.

ATP13A2/PARK9 Deficiency Neither Cause Lysosomal Impairment Nor Alter α-Synuclein Metabolism in SH-SY5Y Cells

Parkinson's disease is a multifactorial disorder with several genes linked to the familial types of the disease. ATP13A2 is one of those genes and encode for a transmembrane protein localized in lysosomes and late endosomes. Previous studies suggested the roles of this protein in lysosomal functions and cellular ion homeostasis. Here, we set out to investigate the role of ATP13A2 in lysosomal function and in metabolism of α-synuclein, another PD-linked protein whose accumulation is implicated in the pathogenesis. We generated non-sense mutations in both copies of ATP13A2 gene in SH-SY5Y human neuroblastoma cells. We examined lysosomal function of ATP13A2-/- cells by measuring the accumulation of lysosomal substrate proteins, such as p62 and polyubiquitinated proteins, induction of acidic compartments, and degradation of ectopically introduced dextran. None of these measures were altered by ATP13A2 deficiency. The steady-state levels of α-synuclein in cells or secretion of this protein were unaltered either in ATP13A2-/- compared to the normal cells. Therefore, the proposed roles of ATP13A2 in lysosomal functions may not be generalized and may depend on the cellular context. The ATP13A2-/- cells generated in the current study may provide a useful control for studies on the roles of PD genes in lysosomal functions.

Direct evidence of Parkinson pathology spread from the gastrointestinal tract to the brain in rats

The cellular hallmarks of Parkinson's disease (PD) are the loss of nigral dopaminergic neurons and the formation of α-synuclein-enriched Lewy bodies and Lewy neurites in the remaining neurons. Based on the topographic distribution of Lewy bodies established after autopsy of brains from PD patients, Braak and coworkers hypothesized that Lewy pathology primes in the enteric nervous system and spreads to the brain, suggesting an active retrograde transport of α-synuclein (the key protein component in Lewy bodies), via the vagal nerve. This hypothesis, however, has not been tested experimentally thus far. Here, we use a human PD brain lysate containing different forms of α-synuclein (monomeric, oligomeric and fibrillar), and recombinant α-synuclein in an in vivo animal model to test this hypothesis. We demonstrate that α-synuclein present in the human PD brain lysate and distinct recombinant α-synuclein forms are transported via the vagal nerve and reach the dorsal motor nucleus of the vagus in the brainstem in a time-dependent manner after injection into the intestinal wall. Using live cell imaging in a differentiated neuroblastoma cell line, we determine that both slow and fast components of axonal transport are involved in the transport of aggregated α-synuclein. In conclusion, we here provide the first experimental evidence that different α-synuclein forms can propagate from the gut to the brain, and that microtubule-associated transport is involved in the translocation of aggregated α-synuclein in neurons.

The pallidopyramidal syndromes: nosology, aetiology and pathogenesis

Purpose of review

The aims of this review is to suggest a new nomenclature and classification system for the diseases currently categorized as neurodegeneration with brain iron accumulation (NBIA) or dystonia-parkinsonism, and to discuss the mechanisms implicated in the pathogenesis of these diseases.

Recent findings

NBIA is a disease category encompassing syndromes with iron accumulation and prominent dystonia–parkinsonism. However, as there are many diseases with similar clinical presentations but without iron accumulation and/or known genetic cause, the current classification system and nomenclature remain confusing. The pathogenetic mechanisms of these diseases and the causes of gross iron accumulation and significant burden of neuroaxonal spheroids are also elusive. Recent genetic and functional studies have identified surprising links between NBIA, Parkinson's disease and lysosomal storage disorders (LSD) with the common theme being a combined lysosomal–mitochondrial dysfunction. We hypothesize that mitochondria and lysosomes form a functional continuum with a predominance of mitochondrial and lysosomal pathways in NBIA and LSD, respectively, and with Parkinson's disease representing an intermediate form of disease.

Summary

During the past 18 months, important advances have been made towards understanding the genetic and pathological underpinnings of the pallidopyramidal syndromes with important implications for clinical practice and future treatment developments.

Progression of intestinal permeability changes and alpha-synuclein expression in a mouse model of Parkinson's disease

Parkinson's disease (PD) is a multifocal degenerative disorder for which there is no cure. The majority of cases are sporadic with unknown etiology. Recent data indicate that untreated patients with de novo PD have increased colonic permeability and that both de novo and premotor patients have pathological expression of α-synuclein (α-syn) in their colon. Both endpoints potentially can serve as disease biomarkers and even may initiate PD events through gut-derived, lipopolysaccharide (LPS)-induced neuronal injury. Animal models could be ideal for interrogating the potential role of the intestines in the pathogenesis of PD; however, few current animal models of PD encompass these nonmotor features. We sought to establish a progressive model of PD that includes the gastrointestinal (GI) dysfunction present in human patients. C57/BL6 mice were systemically administered one dose of either LPS (2.5 mg/kg) or saline and were sacrificed in monthly intervals (n = 5 mice for 5 months) to create a time-course. Small and large intestinal permeability was assessed by analyzing the urinary output of orally ingested sugar probes through capillary column gas chromatography. α-Syn expression was assessed by counting the number of mildly, moderately, and severely affected myenteric ganglia neurons throughout the GI tract, and the counts were validated by quantitative optical density measurements. Nigrostriatal integrity was assessed by tyrosine hydroxylase immunohistochemistry stereology and densitometry. LPS caused an immediate and progressive increase in α-syn expression in the large intestine but not in the small intestine. Intestinal permeability of the whole gut (large and small intestines) progressively increased between months 2 and 4 after LPS administration but returned to baseline levels at month 5. Selective measurements demonstrated that intestinal permeability in the small intestine remained largely intact, suggesting that gut leakiness was predominately in the large intestine. Phosphorylated serine 129-α-syn was identified in a subset of colonic myenteric neurons at months 4 and 5. Although these changes were observed in the absence of nigrostriatal degeneration, an abrupt but insignificant increase in brainstem α-syn was observed that paralleled the restoration of permeability. No changes were observed over time in controls. LPS, an endotoxin used to model PD, causes sequential increases in α-syn immunoreactivity, intestinal permeability, and pathological α-syn accumulation in the colon in a manner similar to that observed in patients with PD. These features are observed without nigrostriatal degeneration and incorporate PD features before the motor syndrome. This allows for the potential use of this model in testing neuroprotective and disease-modifying therapies, including intestinal-directed therapies to fortify intestinal barrier integrity.

Parkinson's disease and alpha synuclein: is Parkinson's disease a prion-like disorder?

Altered protein handling is thought to play a key role in the etiopathogenesis of Parkinson's disease (PD), as the disorder is characterized neuropathologically by the accumulation of intraneuronal protein aggregates (Lewy bodies and Lewy neurites). Attention has particularly focused on the α-synuclein protein, as it is the principal component of Lewy pathology. Moreover, point mutations in the α-synuclein gene cause rare familial forms of PD. Importantly, duplication/triplication of the wild type α-synuclein gene also cause a form of PD, indicating that increased levels of the normal α-synuclein protein is sufficient to cause the disease. Further, single nucleotide polymorphisms in the α-synuclein gene are associated with an increased risk of developing sporadic PD. Recent evidence now suggests the possibility that α-synuclein is a prion-like protein and that PD is a prion-like disease. Within cells, α-synuclein normally adopts an α-helical conformation. However, under certain circumstances, the protein can undergo a profound conformational transition to a β-sheet-rich structure that polymerizes to form toxic oligomers and amyloid plaques. Recent autopsy studies of patients with advanced PD who received transplantation of fetal nigral mesencephalic cells more than a decade earlier demonstrated that typical Lewy pathology had developed within grafted neurons. This suggests that α-synuclein in an aberrantly folded, β-sheet-rich form had migrated from affected to unaffected neurons. Laboratory studies confirm that α-synuclein can transfer from affected to unaffected nerve cells, where it appears that the misfolded protein can act as a template to promote misfolding of host α-synuclein. This leads to the formation of larger aggregates, neuronal dysfunction, and neurodegeneration. Indeed, recent reports demonstrate that a single intracerebral inoculation of misfolded α-synuclein can induce Lewy-like pathology in cells that can spread from affected to unaffected regions and can induce neurodegeneration with motor disturbances in both transgenic and normal mice. Further, inoculates derived from the brains of elderly α-synuclein-overexpressing transgenic mice have now been shown to accelerate the disease process when injected into the brains of young transgenic animals. Collectively, these findings support the hypothesis that α-synuclein is a prion-like protein that can adopt a self-propagating conformation that causes neurodegeneration. We propose that this mechanism plays an important role in the development of PD and provides novel targets for candidate neuroprotective therapies.

Oxidative and nitrative alpha-synuclein modifications and proteostatic stress: implications for disease mechanisms and interventions in synucleinopathies

Alpha-synuclein (ASYN) is a major constituent of the typical protein aggregates observed in several neurodegenerative diseases that are collectively referred to as synucleinopathies. A causal involvement of ASYN in the initiation and progression of neurological diseases is suggested by observations indicating that single-point (e.g., A30P, A53T) or multiplication mutations of the gene encoding for ASYN cause early onset forms of Parkinson's disease (PD). The relative regional specificity of ASYN pathology is still a riddle that cannot be simply explained by its expression pattern. Also, transgenic over-expression of ASYN in mice does not recapitulate the typical dopaminergic neuronal death observed in PD. Thus, additional factors must contribute to ASYN-related toxicity. For instance, synucleinopathies are usually associated with inflammation and elevated levels of oxidative stress in affected brain areas. In turn, these conditions favor oxidative modifications of ASYN. Among these modifications, nitration of tyrosine residues, formation of covalent ASYN dimers, as well as methionine sulfoxidations are prominent examples that are observed in post-mortem PD brain sections. Oxidative modifications can affect ASYN aggregation, as well as its binding to biological membranes. This would affect neurotransmitter recycling, mitochondrial function and dynamics (fission/fusion), ASYN's degradation within a cell and, possibly, the transfer of modified ASYN to adjacent cells. Here, we propose a model on how covalent modifications of ASYN link energy stress, altered proteostasis, and oxidative stress, three major pathogenic processes involved in PD progression. Moreover, we hypothesize that ASYN may act physiologically as a catalytically regenerated scavenger of oxidants in healthy cells, thus performing an important protective role prior to the onset of disease or during aging.

What's to like about the prion-like hypothesis for the spreading of aggregated α-synuclein in Parkinson disease?

α-Synuclein is a key protein in Parkinson disease. Not only is it the major protein component of Lewy bodies, but it is implicated in several cellular processes that are disrupted in Parkinson disease. Misfolded α-synuclein has also been shown to spread from cell-to-cell and, in a prion-like fashion, trigger aggregation of α-synuclein in the recipient cell. In this mini-review we explore the evidence that misfolded α-synuclein underlies the spread of pathology in Parkinson disease and discuss why it should be considered a prion-like protein.

Corruption and spread of pathogenic proteins in neurodegenerative diseases

With advancing age, the brain becomes increasingly susceptible to neurodegenerative diseases, most of which are characterized by the misfolding and errant aggregation of certain proteins. The induction of aggregation involves a crystallization-like seeding mechanism by which a specific protein is structurally corrupted by its misfolded conformer. The latest research indicates that, once formed, proteopathic seeds can spread from one locale to another via cellular uptake, transport, and release. Impeding this process could represent a unified therapeutic strategy for slowing the progression of a wide range of currently intractable disorders.

Prion pathogenesis and secondary lymphoid organs (SLO): tracking the SLO spread of prions to the brain

Prion diseases are subacute neurodegenerative diseases that affect humans and a range of domestic and free-ranging animal species. These diseases are characterized by the accumulation of PrP^Sc, an abnormally folded isoform of the cellular prion protein (PrP^C), in affected tissues. The pathology during prion disease appears to occur almost exclusively within the central nervous system. The extensive neurodegeneration which occurs ultimately leads to the death of the host. An intriguing feature of the prion diseases, when compared with other protein-misfolding diseases, is their transmissibility. Following peripheral exposure, some prion diseases accumulate to high levels within lymphoid tissues. The replication of prions within lymphoid tissue has been shown to be important for the efficient spread of disease to the brain. This article describes recent progress in our understanding of the cellular mechanisms that influence the propagation of prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. A thorough understanding of these events will lead to the identification of important targets for therapeutic intervention, or alternatively, reveal additional processes that influence disease susceptibility to peripherally-acquired prion diseases.