Rotenone and elevated extracellular potassium concentration induce cell-specific fibrillation of α-synuclein in axons of cholinergic enteric neurons in the guinea-pig ileum

Beyond the Microbiota: Understanding the Role of the Enteric Nervous System in Parkinson's Disease from Mice to Human

The enteric nervous system (ENS) is a nerve network composed of neurons and glial cells that regulates the motor and secretory functions of the gastrointestinal (GI) tract. There is abundant evidence of mutual communication between the brain and the GI tract. Dysfunction of these connections appears to be involved in the pathophysiology of Parkinson's disease (PD). Alterations in the ENS have been shown to occur very early in PD, even before central nervous system (CNS) involvement. Post-mortem studies of PD patients have shown aggregation of α-synuclein (αS) in specific subtypes of neurons in the ENS. Subsequently, αS spreads retrogradely in the CNS through preganglionic vagal fibers to this nerve's dorsal motor nucleus (DMV) and other central nervous structures. Here, we highlight the role of the ENS in PD pathogenesis based on evidence observed in animal models and using a translational perspective. While acknowledging the putative role of the microbiome in the gut-brain axis (GBA), this review provides a comprehensive view of the ENS not only as a "second brain", but also as a window into the "first brain", a potentially crucial element in the search for new therapeutic approaches that can delay and even cure the disease.

Comparison of commercially available antibodies for the detection of phosphorylated alpha-synuclein in primary culture of ENS

BACKGROUND

It is now well established that phosphorylated alpha-synuclein histopathology, the pathologic hallmark of Parkinson's disease (PD) is not limited to the brain but also extends to the enteric nervous system (ENS). This observation led to the hypothesis that the ENS could play a pivotal role in the development of PD. Research on the enteric synucleinopathy has, however, been hampered by difficulties in detecting phosphorylated alpha-synuclein in the ENS by Western blotting, even when the transferred membrane is fixed with an optimized protocol. This suggests that the available antibodies used in previous studies lacked of sensitivity for the detection of phosphorylated alpha-synuclein at Ser129 in enteric neurons. Here, we evaluated three recent commercially available phospho-alpha-synuclein antibodies and compared them to two antibodies used in previous research.

METHODS

The specificity and sensitivity of the 5 antibodies were evaluated by Western blot performed with recombinant alpha-synuclein and with protein lysates from rat primary cultures of ENS. In primary culture of ENS, additional experiments were performed with the most specific antibody in order to modulate alpha-synuclein phosphorylation and to validate its utilization in immunofluorescence experiments.

RESULTS

The rabbit monoclonal antibody D1R1R uniquely and robustly detected endogenous phosphorylated alpha-synuclein at Ser129 in rat primary culture of ENS without any non-specific bands, allowing for a reliable analysis of phosphorylated alpha-synuclein regulation by pharmacologic means.

CONCLUSIONS AND INFERENCES

Using D1R1R antibody together with the optimized protocol for membrane fixation may help deciphering the signaling pathways involved in enteric alpha-synuclein post-translational regulation in PD.

Pathophysiological Changes in the Enteric Nervous System of Rotenone-Exposed Mice as Early Radiological Markers for Parkinson's Disease

Parkinson's disease (PD) is known to involve the peripheral nervous system (PNS) and the enteric nervous system (ENS). Functional changes in PNS and ENS appear early in the course of the disease and are responsible for some of the non-motor symptoms observed in PD patients like constipation, that can precede the appearance of motor symptoms by years. Here we analyzed the effect of the pesticide rotenone, a mitochondrial Complex I inhibitor, on the function and neuronal composition of the ENS by measuring intestinal contractility in a tissue bath and by analyzing related protein expression. Our results show that rotenone changes the normal physiological response of the intestine to carbachol, dopamine and electric field stimulation (EFS). Changes in the reaction to EFS seem to be related to the reduction in the cholinergic input but also related to the noradrenergic input, as suggested by the non-adrenergic non-cholinergic (NANC) reaction to the EFS in rotenone-exposed mice. The magnitude and direction of these alterations varies between intestinal regions and exposure times and is associated with an early up-regulation of dopaminergic, cholinergic and adrenergic receptors and an irregular reduction in the amount of enteric neurons in rotenone-exposed mice. The early appearance of these alterations, that start occurring before the substantia nigra is affected in this mouse model, suggests that these alterations could be also observed in patients before the onset of motor symptoms and makes them ideal potential candidates to be used as radiological markers for the detection of Parkinson's disease in its early stages.

POSCAbilities: The Application of the Prion Organotypic Slice Culture Assay to Neurodegenerative Disease Research

Prion diseases are fatal, transmissible neurodegenerative disorders whose pathogenesis is driven by the misfolding, self-templating and cell-to-cell spread of the prion protein. Other neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis and Huntington’s disease, share some of these prion-like features, with different aggregation-prone proteins. Consequently, researchers have begun to apply prion-specific techniques, like the prion organotypic slice culture assay (POSCA), to these disorders. In this review we explore the ways in which the prion phenomenon has been used in organotypic cultures to study neurodegenerative diseases from the perspective of protein aggregation and spreading, strain propagation, the role of glia in pathogenesis, and efficacy of drug treatments. We also present an overview of the advantages and disadvantages of this culture system compared to in vivo and in vitro models and provide suggestions for new directions.

Interactions Between Commensal Bacteria and Enteric Neurons, via FPR1 Induction of ROS, Increase Gastrointestinal Motility in Mice

BACKGROUND & AIMS

Reduced gastrointestinal (GI) motility is a feature of disorders associated with intestinal dysbiosis and loss of beneficial microbes. It is not clear how consumption of beneficial commensal microbes, marketed as probiotics, affects the enteric nervous system (ENS). We studied the effects of the widely used probiotic and the commensal Lactobacillus rhamnosus GG (LGG) on ENS and GI motility in mice.

METHODS

Conventional and germ free C57B6 mice were gavaged with LGG and intestinal tissues were collected; changes in the enteric neuronal subtypes were assessed by real-time polymerase chain reaction, immunoblots, and immunostaining. Production of reactive oxygen species (ROS) in the jejunal myenteric plexi and phosphorylation (p) of mitogen-activated protein kinase 1 (MAPK1) in the enteric ganglia were assessed by immunoblots and immunostaining. Fluorescence in situ hybridization was performed on jejunal cryosections with probes to detect formyl peptide receptor 1 (FPR1). GI motility in conventional mice was assessed after daily gavage of LGG for 1 week.

RESULTS

Feeding of LGG to mice stimulated myenteric production of ROS, increased levels of phosphorylated MAPK1, and increased expression of choline acetyl transferase by neurons (P < .001). These effects were not observed in mice given N-acetyl cysteine (a ROS inhibitor) or LGGΩSpaC (an adhesion-mutant strain of LGG) or FPR1-knockout mice. Gavage of mice with LGG for 1 week significantly increased stool frequency, reduced total GI transit time, and increased contractions of ileal circular muscle strips in ex vivo experiments (P < .05).

CONCLUSIONS

Using mouse models, we found that LGG-mediated signaling in the ENS requires bacterial adhesion, redox mechanisms, and FPR1. This pathway might be activated to increase GI motility in patients.

Antibodies against alpha-synuclein: tools and therapies

Synucleinopathies including Parkinson's disease, dementia with Lewy bodies and multiple system atrophy are characterized by the abnormal accumulation and propagation of α-synuclein (α-syn) pathology in the central and peripheral nervous system as Lewy bodies or glial cytoplasmic inclusions. Several antibodies against α-syn have been developed since it was first detected as the major component of Lewy bodies and glial cytoplasmic inclusions. Over the years, researchers have generated specific antibodies that alleviate the accumulation of intracellular aggregated α-syn and associated pathology in cellular and preclinical models of synucleinopathies. So far, antibodies have been the first choice as tools for research and diagnosis and currently, a wide variety of antibody fragments have been developed as an alternative to full-length antibodies for increasing its therapeutic usefulness. Recently, conformation specific antibody-based approaches have been found to be promising as therapeutic strategies, both to block α-syn aggregation and ameliorate the resultant cytotoxicity, and as diagnostic tools. In this review, we summarize different α-syn specific antibodies and provide their usefulness in tackling synucleinopathies. This article is part of the Special Issue "Synuclein".

Enteric Neurodegeneration is Mediated Through Independent Neuritic and Somal Mechanisms in Rotenone and MPP+ Toxicity

Gut motility malfunction and pathological changes in the enteric nervous system (ENS) are observed in the early stages of Parkinson's disease (PD). In many cases disturbances in the autonomous functions such as gut motility precedes the observed loss of central motor functions in PD. However, the mechanism by which ENS degeneration occurs in PD is unknown. We show that parkinsonian mimetics rotenone and MPP+ induce neurite degeneration that precedes cell death in primary enteric neurons cultured in vitro. If the neuronal death signals originate from degenerating neurites, neuronal death should be prevented by inhibiting neurite degeneration. Our data demonstrate that overexpression of cytNmnat1, an axon protector, maintains healthy neurites in enteric neurons treated with either of the parkinsonian mimetics, but cannot protect the soma. We also demonstrate that neurite protection via cytNmnat1 is independent of mitochondrial dynamics or ATP levels. Overexpression of Bcl-xl, an anti-apoptotic factor, protects both the neuronal cell body and the neurites in both rotenone and MPP+ treated enteric neurons. Our data reveals that Bcl-xl and cytNmnat1 act through separate mechanisms to protect enteric neurites. Our findings suggest that neurite protection alone is not sufficient to inhibit enteric neuronal degeneration in rotenone or MPP+ toxicity, and enteric neurodegeneration in PD may be occurring through independent somatic and neuritic mechanisms. Thus, therapies targeting both axonal and somal protection can be important in finding interventions for enteric symptoms in PD.

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.

The enteric nervous system in PD: gateway, bystander victim, or source of solutions

Apart from the characteristic and progressive motor- and movement-related problems, Parkinson's disease (PD) patients also suffer from several non-motor symptoms, including gastrointestinal dysfunction. The fact that the enteric nervous system (ENS) controls motility and that one of the typical PD hallmarks, α-synuclein-positive deposits, has also been found in the intestinal wall have rendered the ENS and the gut a popular subject of study in the context of PD. The possibility that these deposits could serve as an early biomarker is obviously of tremendous medical benefit but also the idea that the gut may possibly be a gateway via which the disease is initiated and progressively makes its way via the peripheral nerves to the central nervous system has increased the interest in the ENS-PD link. Furthermore, the fact that gastrointestinal symptoms are present in PD suggests that the ENS might be affected as well. However, despite a large body of literature on the topic, the actual role or the magnitude of involvement of the ENS in PD remains elusive. The multitudes of experimental approaches and animal models have complicated the interpretation of results and the outcome of different studies does not necessarily align well. In this review, we chose to highlight some elements of interest and some items of confusion, particularly those where research should be focusing. We also list a number of open questions in the field that could serve as a guideline for future, preferably concerted research.

Are Dopamine Oxidation Metabolites Involved in the Loss of Dopaminergic Neurons in the Nigrostriatal System in Parkinson's Disease?

In 1967, L-dopa was introduced as part of the pharmacological therapy of Parkinson's disease (PD) and, in spite of extensive research, no additional effective drugs have been discovered to treat PD. This brings forward the question: why have no new drugs been developed? We consider that one of the problems preventing the discovery of new drugs is that we still have no information on the pathophysiology of the neurodegeneration of the neuromelanin-containing nigrostriatal dopaminergic neurons. Currently, it is widely accepted that the degeneration of dopaminergic neurons, i.e., in the substantia nigra pars compacta, involves mitochondrial dysfunction, the formation of neurotoxic oligomers of alpha-synuclein, the dysfunction of protein degradation systems, neuroinflammation, and oxidative and endoplasmic reticulum stress. However, the initial trigger of these mechanisms in the nigrostriatal system is still unknown. It has been reported that aminochrome induces the majority of these mechanisms involved in the neurodegeneration process. Aminochrome is formed within the cytoplasm of neuromelanin-containing dopaminergic neurons during the oxidation of dopamine to neuromelanin. The oxidation of dopamine to neuromelanin is a normal and harmless process, because healthy individuals have intact neuromelanin-containing dopaminergic neurons. Interestingly, aminochrome-induced neurotoxicity is prevented by two enzymes: DT-diaphorase and glutathione transferase M2-2, which explains why melanin-containing dopaminergic neurons are intact in healthy human brains.

Neuropathological Staging of Brain Pathology in Sporadic Parkinson's disease: Separating the Wheat from the Chaff

A relatively small number of especially susceptible nerve cell types within multiple neurotransmitter systems of the human central, peripheral, and enteric nervous systems (CNS, PNS, ENS) become involved in the degenerative process underlying sporadic Parkinson's disease (sPD). The six-stage model we proposed for brain pathology related to sPD (Neurobiol Aging 2003) was a retrospective study of incidental and clinically diagnosed cases performed on unconventionally thick tissue sections (100 μm) from a large number of brain regions.The staging model emphasized what we perceived to be a sequential development of increasing degrees of Lewy pathology in anatomically interconnected regions together with the loss of aminergic projection neurons in, but not limited to, the locus coeruleus and substantia nigra. The same weight was assigned to axonal and somatodendritic Lewy pathology, and the olfactory bulb was included for the first time in a sPD staging system. After years of research, it now appears that the earliest lesions could develop at nonnigral (dopamine agonist nonresponsive) sites, where the surrounding environment is potentially hostile: the olfactory bulb and, possibly, the ENS. The current lack of knowledge regarding the development of Lewy pathology within the peripheral autonomic nervous system, however, means that alternative extra-CNS sites of origin cannot be disregarded as possible candidates. The PD staging system not only caused controversy but contributed a framework for (1) assessing pathology in the spinal cord, ENS, and PNS in relationship to that evolving in the brain, (2) defining prodromal disease and cohorts of at-risk individuals, (3) developing potential prognostic biomarkers for very early disease, (4) testing novel hypotheses and experimental models of α-synuclein propagation and disease progression, and (5) finding causally-oriented therapies that intervene before the substantia nigra becomes involved. The identification of new disease mechanisms at the molecular and cellular levels indicates that physical contacts (transsynaptic) and transneuronal transmission between vulnerable nerve cells are somehow crucial to the pathogenesis of sPD.