Optimizing Western Blots for the Detection of Endogenous α-Synuclein in the Enteric Nervous System

Revisiting the specificity and ability of phospho-S129 antibodies to capture alpha-synuclein biochemical and pathological diversity

Antibodies against phosphorylated alpha-synuclein (aSyn) at S129 have emerged as the primary tools to investigate, monitor, and quantify aSyn pathology in the brain and peripheral tissues of patients with Parkinson's disease and other neurodegenerative diseases. Herein, we demonstrate that the co-occurrence of multiple pathology-associated C-terminal post-translational modifications (PTMs) (e.g., phosphorylation at Tyrosine 125 or truncation at residue 133 or 135) differentially influences the detection of pS129-aSyn species by pS129-aSyn antibodies. These observations prompted us to systematically reassess the specificity of the most commonly used pS129 antibodies against monomeric and aggregated forms of pS129-aSyn in mouse brain slices, primary neurons, mammalian cells and seeding models of aSyn pathology formation. We identified two antibodies that are insensitive to pS129 neighboring PTMs. Although most pS129 antibodies showed good performance in detecting aSyn aggregates in cells, neurons and mouse brain tissue containing abundant aSyn pathology, they also showed cross-reactivity towards other proteins and often detected non-specific low and high molecular weight bands in aSyn knock-out samples that could be easily mistaken for monomeric or high molecular weight aSyn species. Our observations suggest that not all pS129 antibodies capture the biochemical and morphological diversity of aSyn pathology, and all should be used with the appropriate protein standards and controls when investigating aSyn under physiological conditions. Finally, our work underscores the need for more pS129 antibodies that are not sensitive to neighboring PTMs and more thorough characterization and validation of existing and new antibodies.

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.

Systemic α-synuclein injection triggers selective neuronal pathology as seen in patients with Parkinson's disease

Parkinson's disease (PD) is an α-synucleinopathy characterized by the progressive loss of specific neuronal populations. Here, we develop a novel approach to transvascularly deliver proteins of complex quaternary structures, including α-synuclein preformed fibrils (pff). We show that a single systemic administration of α-synuclein pff triggers pathological transformation of endogenous α-synuclein in non-transgenic rats, which leads to neurodegeneration in discrete brain regions. Specifically, pff-exposed animals displayed a progressive deterioration in gastrointestinal and olfactory functions, which corresponded with the presence of cellular pathology in the central and enteric nervous systems. The α-synuclein pathology generated was both time dependent and region specific. Interestingly, the most significant neuropathological changes were observed in those brain regions affected in the early stages of PD. Our data therefore demonstrate for the first time that a single, transvascular administration of α-synuclein pff can lead to selective regional neuropathology resembling the premotor stage of idiopathic PD. Furthermore, this novel delivery approach could also be used to deliver a range of other pathogenic, as well as therapeutic, protein cargos transvascularly to the brain.

Tau accumulates in Crohn's disease gut

A sizeable body of evidence has recently emerged to suggest that gastrointestinal (GI) inflammation might be involved in the development of Parkinson's disease (PD). There is now strong epidemiological and genetical evidence linking PD to inflammatory bowel diseases and we recently demonstrated that the neuronal protein alpha-synuclein, which is critically involved in PD pathophysiology, is upregulated in inflamed segments of Crohn's colon. The microtubule associated protein tau is another neuronal protein critically involved in neurodegenerative disorders but, in contrast to alpha-synuclein, no data are available about its expression and phosphorylation patterns in inflammatory bowel diseases. Here, we examined the expression levels of tau isoforms, their phosphorylation profile and truncation in colon biopsy specimens from 16 Crohn's disease (CD) and 6 ulcerative colitis (UC) patients and compared them to samples from 16 controls. Additional experiments were performed in full thickness segments of colon of five CD and five control subjects, in primary cultures of rat enteric neurons and in nuclear factor erythroid 2-related factor (Nrf2) knockout mice. Our results show the upregulation of two main human tau isoforms in the enteric nervous system (ENS) in CD but not in UC. This upregulation was not transcriptionally regulated but instead likely resulted from a decrease in protein clearance via an Nrf2 pathway. Our findings, which provide the first detailed characterization of tau in CD, suggest that the key proteins involved in neurodegenerative disorders such as alpha-synuclein and tau, might also play a role in CD.

Characterization of lysosomal proteins Progranulin and Prosaposin and their interactions in Alzheimer's disease and aged brains: increased levels correlate with neuropathology

Progranulin (PGRN) is a protein encoded by the GRN gene with multiple identified functions including as a neurotrophic factor, tumorigenic growth factor, anti-inflammatory cytokine and regulator of lysosomal function. A single mutation in the human GRN gene resulting in reduced PGRN expression causes types of frontotemporal lobar degeneration resulting in frontotemporal dementia. Prosaposin (PSAP) is also a multifunctional neuroprotective secreted protein and regulator of lysosomal function. Interactions of PGRN and PSAP affect their functional properties. Their roles in Alzheimer's disease (AD), the leading cause of dementia, have not been defined. In this report, we examined in detail the cellular expression of PGRN in middle temporal gyrus samples of a series of human brain cases (n = 45) staged for increasing plaque pathology. Immunohistochemistry showed PGRN expression in cortical neurons, microglia, cerebral vessels and amyloid beta (Aβ) plaques, while PSAP expression was mainly detected in neurons and Aβ plaques, and to a limited extent in astrocytes. We showed that there were increased levels of PGRN protein in AD cases and corresponding increased levels of PSAP. Levels of PGRN and PSAP protein positively correlated with amyloid beta (Aβ), with PGRN levels correlating with phosphorylated tau (serine 205) levels in these samples. Although PGRN colocalized with lysosomal-associated membrane protein-1 in neurons, most PGRN associated with Aβ plaques did not. Aβ plaques with PGRN and PSAP deposits were identified in the low plaque non-demented cases suggesting this was an early event in plaque formation. We did not observe PGRN-positive neurofibrillary tangles. Co-immunoprecipitation studies of PGRN from brain samples identified only PSAP associated with PGRN, not sortilin or other known PGRN-binding proteins, under conditions used. Most PGRN associated with Aβ plaques were immunoreactive for PSAP showing a high degree of colocalization of these proteins that did not change between disease groups. As PGRN supplementation has been considered as a therapeutic approach for AD, the possible involvement of PGRN and PSAP interactions in AD pathology needs to be further considered.

A Novel Antiserum Against a Predicted Human Peripheral Choline Acetyltransferase (hpChAT) for Labeling Neuronal Structures in Human Colon

Choline acetyltransferase (ChAT), the enzyme synthesizing acetylcholine (ACh), has an exon-skipping splice variant which is expressed preferentially in the peripheral nervous system (PNS) and thus termed peripheral ChAT (pChAT). A rabbit antiserum previously produced against rat pChAT (rpChAT) has been used for immunohistochemistry (IHC) to study peripheral cholinergic structures in various animals. The present study was undertaken to develop a specific antiserum against a predicted human pChAT (hpChAT) protein. A novel mouse antiserum has been successfully raised against a unique 14-amino acid sequence of hpChAT protein. Our Western blot using this antiserum (termed here anti-hpChAT serum) on human colon extracts revealed only a single band of 47 kDa, matching the deduced size of hpChAT protein. By IHC, the antiserum gave intense staining in many neuronal cells and fibers of human colon but not brain, and such a pattern of staining seemed identical with that reported in colon of various animals using anti-rpChAT serum. In the antibody-absorption test, hpChAT-immunoreactive staining in human colon was completely blocked by using the antiserum pre-absorbed with the antigen peptide. Double immunofluorescence in human colon moreover indicated that structures stained with anti-hpChAT were also stained with anti-rpChAT, and vice versa. hpChAT antiserum allowed the identification of cell types, as Dogiel type cells in intramural plexuses, and fiber innervation of colon muscles and mucosae. The present results demonstrate the specificity and reliability of the hpChAT antiserum as a novel tool for immunohistochemical studies in human colon, opening venues to map cholinergic innervation in other human PNS tissues.

Acute inflammation down-regulates alpha-synuclein expression in enteric neurons

The protein alpha-synuclein whose expression is strongly implicated in Parkinson's disease (PD) is not only expressed in the CNS but also in the enteric nervous system (ENS). The growing body of evidence suggesting that gastrointestinal inflammation is involved in the development of PD led us to investigate the effects of inflammation on alpha-synuclein expression in primary culture of rat ENS and in mice with dextran sulfate sodium-induced colitis. Using western blot and qPCR, we found that both lipopolysaccharide and a combination of tumor necrosis factor-α and interleukin 1-β decreased the expression levels of alpha-synuclein in primary culture of rat ENS, an effect that was prevented in the presence of the p38 inhibitors SB203580 and BIRB 796. Lipopolysaccharide and tumor necrosis factor-α/interleukin 1-β had no effect on alpha-synuclein expression in primary culture of rat CNS and in human erythroid leukemia cells. In mice, acute but not chronic dextran sulfate sodium-induced colitis was associated with a decreased expression of colonic alpha-synuclein. As a whole, our findings indicate that acute inflammatory insults down-regulate alpha-synuclein expression in the ENS via a p38 pathway. They provide new insights into the widely discussed concepts of alpha-synuclein expression and aggregation in the ENS in PD and raise issues about the possible role of gastrointestinal inflammation in the development of PD. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Biochemical analysis of α-synuclein extracted from control and Parkinson's disease colonic biopsies

Lewy bodies and neurites, the pathological hallmarks found in the brain of Parkinson's disease (PD) patients, are primarily composed of aggregated and hyperphosphorylated alpha-synuclein. The observation that alpha-synuclein inclusions are also found in the gut of the vast majority of parkinsonian patients has led to an increasing number of studies aimed at developing diagnostic procedures based on the detection of pathological alpha-synuclein in gastrointestinal biopsies. The previous studies, which have all used immunohistochemistry for the detection of alpha-synuclein, have provided conflicting results. In the current survey, we used a different approach by analyzing the immunoreactivity pattern of alpha-synuclein separated by one- and two-dimensional electrophoresis, in colonic biopsies from PD subjects and healthy individuals. We did not observe any differences between controls and PD in the expression levels, phosphorylation or aggregation status of alpha-synuclein. Overall, our study suggests that the two biochemical methods tested here are not adequate for the prediction of PD using gastrointestinal biopsies. Further studies, using other biochemical approaches, are warranted to test whether there exists specific forms of pathological alpha-synuclein that distinguish PD from control subjects.

Cross-linking for the analysis of α-synuclein in the enteric nervous system

Since the observation that aggregated α-synuclein, the pathological hallmark of Parkinson's disease (PD), is found in the gut in almost all patients, it has been suggested that the enteric nervous system (ENS) could be a starting point for α-synuclein pathology. α-synuclein has long been thought to occur as a monomer in living cells, but recent studies reported that it instead exists as a tetramer in non-neuronal cells and in neurons. Given the possible key role of the ENS in PD pathophysiology, we undertook the current research to characterize the native state of α-synuclein in rat primary culture of ENS and in adult human healthy ENS. Using amine-reactive cross-linking, we showed that, by contrast to cell lines and brain neurons, α-synuclein exists primarily as a monomer in intact enteric neurons, suggesting that the native state of α-synuclein is different between the ENS and the brain. Our results provide new insights into the widely discussed concepts of α-synuclein aggregation and misfolding in PD and raise issue about the possible transmission of α-synuclein from the ENS to the brain.

The bowel and beyond: the enteric nervous system in neurological disorders

The enteric nervous system (ENS) is large, complex and uniquely able to orchestrate gastrointestinal behaviour independently of the central nervous system (CNS). An intact ENS is essential for life and ENS dysfunction is often linked to digestive disorders. The part the ENS plays in neurological disorders, as a portal or participant, has also become increasingly evident. ENS structure and neurochemistry resemble that of the CNS, therefore pathogenic mechanisms that give rise to CNS disorders might also lead to ENS dysfunction, and nerves that interconnect the ENS and CNS can be conduits for disease spread. We review evidence for ENS dysfunction in the aetiopathogenesis of autism spectrum disorder, amyotrophic lateral sclerosis, transmissible spongiform encephalopathies, Parkinson disease and Alzheimer disease. Animal models suggest that common pathophysiological mechanisms account for the frequency of gastrointestinal comorbidity in these conditions. Moreover, the neurotropic pathogen, varicella zoster virus (VZV), unexpectedly establishes latency in enteric and other autonomic neurons that do not innervate skin. VZV reactivation in these neurons produces no rash and is therefore a clandestine cause of gastrointestinal disease, meningitis and strokes. The gut-brain alliance has raised consciousness as a contributor to health, but a gut-brain axis that contributes to disease merits equal attention.