Exposure to the Functional Bacterial Amyloid Protein Curli Enhances Alpha-Synuclein Aggregation in Aged Fischer 344 Rats and Caenorhabditis elegans. Sci Rep. 2016;6:34477. [PMID: 27708338 PMCID: PMC5052651 DOI: 10.1038/srep34477]

Prion-like Domains in Eukaryotic Viruses

Prions are proteins that can self-propagate, leading to the misfolding of proteins. In addition to the previously demonstrated pathogenic roles of prions during the development of different mammalian diseases, including neurodegenerative diseases, they have recently been shown to represent an important functional component in many prokaryotic and eukaryotic organisms and bacteriophages, confirming the previously unexplored important regulatory and functional roles. However, an in-depth analysis of these domains in eukaryotic viruses has not been performed. Here, we examined the presence of prion-like proteins in eukaryotic viruses that play a primary role in different ecosystems and that are associated with emerging diseases in humans. We identified relevant functional associations in different viral processes and regularities in their presence at different taxonomic levels. Using the prion-like amino-acid composition computational algorithm, we detected 2679 unique putative prion-like domains within 2,742,160 publicly available viral protein sequences. Our findings indicate that viral prion-like proteins can be found in different viruses of insects, plants, mammals, and humans. The analysis performed here demonstrated common patterns in the distribution of prion-like domains across viral orders and families, and revealed probable functional associations with different steps of viral replication and interaction with host cells. These data allow the identification of the viral prion-like proteins as potential novel regulators of viral infections.

Neuroinflammation, Gut Microbiome, and Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease that develops insidiously and causes dementia finally. There are also clinical complications in advanced dementia, such as eating problems, infections, which will lead to the decline of patients' life quality, and the rising cost of care for AD to our society. AD will be important public health challenge. Early detection of AD may be a key issue to prevent, delay, and stop the disease. Gut microbiome and neuroinflammation are closely related with nervous system diseases, although the specific mechanism is not clear. This review introduces the relationship between neuroinflammation, gut microbiome, and AD.

The Gut Microbiota and Alzheimer's Disease

The gut microbiota comprises a complex community of microorganism species that resides in our gastrointestinal ecosystem and whose alterations influence not only various gut disorders but also central nervous system disorders such as Alzheimer's disease (AD). AD, the most common form of dementia, is a neurodegenerative disorder associated with impaired cognition and cerebral accumulation of amyloid-β peptides (Aβ). Most notably, the microbiota-gut-brain axis is a bidirectional communication system that is not fully understood, but includes neural, immune, endocrine, and metabolic pathways. Studies in germ-free animals and in animals exposed to pathogenic microbial infections, antibiotics, probiotics, or fecal microbiota transplantation suggest a role for the gut microbiota in host cognition or AD-related pathogenesis. The increased permeability of the gut and blood-brain barrier induced by microbiota dysbiosis may mediate or affect AD pathogenesis and other neurodegenerative disorders, especially those associated with aging. In addition, bacteria populating the gut microbiota can secrete large amounts of amyloids and lipopolysaccharides, which might contribute to the modulation of signaling pathways and the production of proinflammatory cytokines associated with the pathogenesis of AD. Moreover, imbalances in the gut microbiota can induce inflammation that is associated with the pathogenesis of obesity, type 2 diabetes mellitus, and AD. The purpose of this review is to summarize and discuss the current findings that may elucidate the role of the gut microbiota in the development of AD. Understanding the underlying mechanisms may provide new insights into novel therapeutic strategies for AD.

Somatostatin in Alzheimer's disease: A new Role for an Old Player

The amyloid beta (Aβ) peptide is central to the pathogenesis of Alzheimer's disease (AD). Insights into Aβ-interacting proteins are critical for understanding the molecular mechanisms underlying Aβ-mediated toxicity. We recently undertook an in-depth in vitro interrogation of the Aβ1–42 interactome using human frontal lobes as the biological source material and taking advantage of advances in mass spectrometry performance characteristics. These analyses uncovered the small cyclic neuropeptide somatostatin (SST) to be the most selectively enriched binder to oligomeric Aβ1–42. Subsequent validation experiments revealed that SST interferes with Aβ fibrillization and promotes the formation of Aβ assemblies characterized by a 50–60 kDa SDS-resistant core. The distributions of SST and Aβ overlap in the brain and SST has been linked to AD by several additional observations. This perspective summarizes this body of literature and draws attention to the fact that SST is one of several neuropeptide hormones that acquire amyloid properties before their synaptic release. The latter places the interaction between SST and Aβ among an increasing number of observations that attest to the ability of amyloidogenic proteins to influence each other. A model is presented which attempts to reconcile existing data on the involvement of SST in the AD etiology.

Oral administration of Proteus mirabilis damages dopaminergic neurons and motor functions in mice

Recently, studies on the relationship between gut dysbiosis and Parkinson's disease (PD) have increased, but whether a specific gut bacterium may cause PD remains unexplored. Here, we report, for the first time, that a specific gut bacterium directly induces PD symptoms and dopaminergic neuronal damage in the mouse brain. We found that the number of Enterobacteriaceae, particularly Proteus mirabilis, markedly and commonly increased in PD mouse models. Administration of P. mirabilis isolated from PD mice significantly induced motor deficits, selectively caused dopaminergic neuronal damage and inflammation in substantia nigra and striatum, and stimulated α-synuclein aggregation in the brain as well as in the colon. We found that lipopolysaccharides, a virulence factor of P. mirabilis, may be associated in these pathological changes via gut leakage and inflammatory actions. Our results suggest a role of P. mirabilis on PD pathogenesis in the brain.

Does Parkinson's disease start in the gut?

Parkinson's disease (PD) is pathologically characterized by the presence of intraneuronal inclusions, termed Lewy bodies and Lewy neurites, whose main component is alpha-synuclein. Based on the topographic distribution of Lewy bodies and neurites established after autopsy from PD patients, Braak and coworkers hypothesized that PD pathology may start in the gastrointestinal tract then spread through the vagus nerve to the brain. This hypothesis has been reinforced by the discovery that alpha-synuclein may be capable of spreading transcellularly, thereby providing a mechanistic basis for Braak's hypothesis. This 'gut to brain' scenario has ignited heated debates within the movement disorders community and prompted a large number of studies in both humans and animals. Here, we review the arguments for and against the gut as the origin of PD. We conclude that the human autopsy evidence does not support the hypothesis and that it is too early to draw any definitive conclusions. We discuss how this issue might be further addressed in future research.

The role of microbial amyloid in neurodegeneration

It has become apparent that the intestinal microbiota orchestrates important aspects of our metabolism, immunity, and development. Recent work has demonstrated that the microbiota also influences brain function in healthy and diseased individuals. Of great interest are reports that intestinal bacteria play a role in the pathogenic cascade of both Parkinson and Alzheimer diseases. These neurodegenerative disorders both involve misfolding of endogenous proteins that spreads from one region of the body to another in a manner analogous to prions. The mechanisms of how the microbiota influences or is correlated with disease require elaboration. Microbial proteins or metabolites may influence neurodegeneration through the promotion of amyloid formation by human proteins or by enhancing inflammatory responses to endogenous neuronal amyloids. We review the current knowledge concerning bacterial amyloids and their potential to influence cerebral amyloid aggregation and neuroinflammation. We propose the term “mapranosis” to describe the process of microbiota-associated proteopathy and neuroinflammation. The study of amyloid proteins made by the microbiota and their influence on health and disease is in its infancy. This is a promising area for therapeutic intervention because there are many ways to alter our microbial partners and their products, including amyloid proteins.

Modeling Host-Microbiome Interactions in Caenorhabditis elegans

The microbiome influences host processes including nutritional availability, development, immunity, and behavioral responses. Caenorhabditis elegans is a powerful model to study molecular mechanisms of host-microbial interactions. Recent efforts have been made to profile the natural microbiome of C. elegans, laying a foundation for mechanistic studies of host-microbiome interactions in this genetically tractable model system. Studies using single-species microbes, multi-microbial systems, and humanized worm-microbiome interaction studies reveal metabolic and microbial-microbial interactions relevant in higher organisms. This article discusses recent developments in modeling the effects of host-microbiome interactions in C. elegans.

Amyotrophic lateral sclerosis as a protein level, non-genomic disease: Therapy with S2RM exosome released molecules

Amyotrophic lateral sclerosis (ALS) is a rapidly progressing neurodegenerative disease that leads to death. No effective treatments are currently available. Based on data from epidemiological, etiological, laboratory, and clinical studies, I offer a new way of thinking about ALS and its treatment. This paper describes a host of extrinsic factors, including the exposome, that disrupt the extracellular matrix and protein function such that a spreading, prion-like disease leads to neurodegeneration in the motor tracts. A treatment regimen is described using the stem cell released molecules from a number of types of adult stem cells to provide tissue dependent molecules that restore homeostasis, including proteostasis, in the ALS patient. Because stem cells themselves as a therapeutic are cumbersome and expensive, and when implanted in a host cause aging of the host tissue and often fail to engraft or remain viable, only the S2RM molecules are used. Rebuilding of the extracellular matrix and repair of the dysfunctional proteins in the ALS patient ensues.

Vertebrate food products as a potential source of prion-like α-synuclein

The aberrant aggregation of the protein α-synuclein is thought to be involved in Parkinson’s disease (PD). However, the factors that lead to initiation and propagation of α-synuclein aggregation are not clearly understood. Recently, the hypothesis that α-synuclein aggregation spreads via a prion-like mechanism originating in the gut has gained much scientific attention. If α-synuclein spreads via a prion-like mechanism, then an important question becomes, what are the origins of this prion-like species? Here we review the possibility that α-synuclein aggregation could be seeded via the ingestion of a prion-like α-synuclein species contained within food products originating from vertebrates. To do this, we highlight current evidence for the gut-to-brain hypothesis of PD, and put this in context of available routes of α-synuclein prion infectivity via the gastrointestinal (GI) tract. We then discuss meat as a ready exogenous source of α-synuclein and how certain risk factors, including inflammation, may allow for dietary α-synuclein to pass from the GI lumen into the host to induce pathology. Lastly, we review epidemiological evidence that dietary factors may be involved in PD. Overall, research to date has yet to directly test the contribution of dietary α-synuclein to the mechanism of initiation and progression of the disease. However, numerous experimental findings, including the potent seeding and spreading behavior of α-synuclein fibrils, seem to support, at least in part, the feasibility of an infection with a prion α-synuclein particle via the GI tract. Further studies are required to determine whether dietary α-synuclein contributes to seeding pathology in the gut.

Prion-Like Domains in Phagobiota

Prions are molecules characterized by self-propagation, which can undergo a conformational switch leading to the creation of new prions. Prion proteins have originally been associated with the development of mammalian pathologies; however, recently they have been shown to contribute to the environmental adaptation in a variety of prokaryotic and eukaryotic organisms. Bacteriophages are widespread and represent the important regulators of microbiota homeostasis and have been shown to be diverse across various bacterial families. Here, we examined whether bacteriophages contain prion-like proteins and whether these prion-like protein domains are involved in the regulation of homeostasis. We used a computational algorithm, prion-like amino acid composition, to detect prion-like domains in 370,617 publicly available bacteriophage protein sequences, which resulted in the identification of 5040 putative prions. We analyzed a set of these prion-like proteins, and observed regularities in their distribution across different phage families, associated with their interactions with the bacterial host cells. We found that prion-like domains could be found across all phages of various groups of bacteria and archaea. The results obtained in this study indicate that bacteriophage prion-like proteins are predominantly involved in the interactions between bacteriophages and bacterial cell, such as those associated with the attachment and penetration of bacteriophage in the cell, and the release of the phage progeny. These data allow the identification of phage prion-like proteins as novel regulators of the interactions between bacteriophages and bacterial cells.

Prying into the Prion Hypothesis for Parkinson's Disease

In Parkinson's disease, intracellular α-synuclein inclusions form in neurons. We suggest that prion-like behavior of α-synuclein is a key component in Parkinson's disease pathogenesis. Although multiple molecular changes are involved in the triggering of the disease process, we propose that neuron-to-neuron transfer is a crucial event that is essential for Lewy pathology to spread from one brain region to another. In this review, we describe key findings in human postmortem brains, cultured cells, and animal models of disease that support the idea that α-synuclein can act as a prion. We consider potential triggers of the α-synuclein misfolding and why the aggregates escape cellular degradation under disease conditions. We also discuss whether different strains of α-synuclein fibrils can underlie differences in cellular and regional distribution of aggregates in different synucleinopathies. Our conclusion is that α-synuclein probably acts as a prion in human diseases, and a deeper understanding of this step in the pathogenesis of Parkinson's disease can facilitate the development of disease-modifying therapies in the future.Dual Perspectives Companion Paper: Parkinson's Disease Is Not Simply a Prion Disorder, by D. James Surmeier, José A. Obeso, and Glenda M. Halliday.

Microbiota-Brain-Gut Axis and Neurodegenerative Diseases

PURPOSE OF REVIEW

The purposes of this review were as follows: first, to provide an overview of the gut microbiota and its interactions with the gut and the central nervous system (the microbiota-gut-brain axis) in health, second, to review the relevance of this axis to the pathogenesis of neurodegenerative diseases, such as Parkinson's disease, and, finally, to assess the potential for microbiota-targeted therapies.

RECENT FINDINGS

Work on animal models has established the microbiota-gut-brain axis as a real phenomenon; to date, the evidence for its operation in man has been limited and has been confronted by considerable logistical challenges. Animal and translational models have incriminated a disturbed gut microbiota in a number of CNS disorders, including Parkinson's disease; data from human studies is scanty. While a theoretical basis can be developed for the use of microbiota-directed therapies in neurodegenerative disorders, support is yet to come from high-quality clinical trials. In theory, a role for the microbiota-gut-brain axis is highly plausible; clinical confirmation is awaited.

The Gut Microbiome Feelings of the Brain: A Perspective for Non-Microbiologists

Objectives: To comprehensively review the scientific knowledge on the gut-brain axis. Methods: Various publications on the gut-brain axis, until 31 July 2017, were screened using the Medline, Google, and Cochrane Library databases. The search was performed using the following keywords: "gut-brain axis", "gut-microbiota-brain axis", "nutrition microbiome/microbiota", "enteric nervous system", "enteric glial cells/network", "gut-brain pathways", "microbiome immune system", "microbiome neuroendocrine system" and "intestinal/gut/enteric neuropeptides". Relevant articles were selected and reviewed. Results: Tremendous progress has been made in exploring the interactions between nutrients, the microbiome, and the intestinal, epithelium-enteric nervous, endocrine and immune systems and the brain. The basis of the gut-brain axis comprises of an array of multichannel sensing and trafficking pathways that are suggested to convey the enteric signals to the brain. These are mediated by neuroanatomy (represented by the vagal and spinal afferent neurons), the neuroendocrine-hypothalamic-pituitary-adrenal (HPA) axis (represented by the gut hormones), immune routes (represented by multiple cytokines), microbially-derived neurotransmitters, and finally the gate keepers of the intestinal and brain barriers. Their mutual and harmonious but intricate interaction is essential for human life and brain performance. However, a failure in the interaction leads to a number of inflammatory-, autoimmune-, neurodegenerative-, metabolic-, mood-, behavioral-, cognitive-, autism-spectrum-, stress- and pain-related disorders. The limited availability of information on the mechanisms, pathways and cause-and-effect relationships hinders us from translating and implementing the knowledge from the bench to the clinic. Implications: Further understanding of this intricate field might potentially shed light on novel preventive and therapeutic strategies to combat these disorders. Nutritional approaches, microbiome manipulations, enteric and brain barrier reinforcement and sensing and trafficking modulation might improve physical and mental health outcomes.

Alpha-Synuclein to the Rescue: Immune Cell Recruitment by Alpha-Synuclein during Gastrointestinal Infection

Intraneuronal accumulation of misfolded alpha-synuclein in the central and peripheral nervous systems is strongly linked to Parkinson disease (PD) and other related synucleinopathies. In rare inherited forms of PD, point mutations or gene multiplications mediate the formation of alpha-synuclein protein aggregates. However, in most PD cases it is presumed that the combined effects of ageing and environmental factors drive the formation of alpha-synuclein aggregates. Despite advances regarding alpha-synuclein pathobiology, the normal functions of this protein and factors that regulate its expression are not well understood. We discuss a recent study reporting that viral infection induces alpha-synuclein expression in neurons of the gastrointestinal tract. Alpha-synuclein levels increased during norovirus infection in the duodenum of children. In an in vitro paradigm, monomeric and oligomeric alpha-synuclein acted as chemoattractants for neutrophils and monocytes, and promoted the maturation of dendritic cells. This suggests that alpha-synuclein facilitates immune responses to infection. We explore the possibility that intestinal infections, and associated inflammation, place individuals at increased risk of PD by increasing alpha-synuclein levels and promoting the formation of alpha-synuclein aggregates that propagate in a prion-like fashion via the vagal nerve to the brainstem.

The nasal and gut microbiome in Parkinson's disease and idiopathic rapid eye movement sleep behavior disorder

Background

Increasing evidence connects the gut microbiota and the onset and/or phenotype of Parkinson's disease (PD). Differences in the abundances of specific bacterial taxa have been reported in PD patients. It is, however, unknown whether these differences can be observed in individuals at high risk, for example, with idiopathic rapid eye movement sleep behavior disorder, a prodromal condition of α‐synuclein aggregation disorders including PD.

Objectives

To compare microbiota in carefully preserved nasal wash and stool samples of subjects with idiopathic rapid eye movement sleep behavior disorder, manifest PD, and healthy individuals.

Methods

Microbiota of flash‐frozen stool and nasal wash samples from 76 PD patients, 21 idiopathic rapid eye movement sleep behavior disorder patients, and 78 healthy controls were assessed by 16S and 18S ribosomal RNA amplicon sequencing. Seventy variables, related to demographics, clinical parameters including nonmotor symptoms, and sample processing, were analyzed in relation to microbiome variability and controlled differential analyses were performed.

Results

Differentially abundant gut microbes, such as Akkermansia, were observed in PD, but no strong differences in nasal microbiota. Eighty percent of the differential gut microbes in PD versus healthy controls showed similar trends in idiopathic rapid eye movement sleep behavior disorder, for example, Anaerotruncus and several Bacteroides spp., and correlated with nonmotor symptoms. Metagenomic sequencing of select samples enabled the reconstruction of genomes of so far uncharacterized differentially abundant organisms.

Conclusion

Our study reveals differential abundances of gut microbial taxa in PD and its prodrome idiopathic rapid eye movement sleep behavior disorder in comparison to the healthy controls, and highlights the potential of metagenomics to identify and characterize microbial taxa, which are enriched or depleted in PD and/or idiopathic rapid eye movement sleep behavior disorder. © 2017 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

A Role for Neuronal Alpha-Synuclein in Gastrointestinal Immunity

BACKGROUND

Alpha-synuclein (αS) is a nerve cell protein associated with Parkinson disease (PD). Accumulation of αS within the enteric nervous system (ENS) and its traffic from the gut to the brain are implicated in the pathogenesis and progression of PD. αS has no known function in humans and the reason for its accumulation within the ENS is unknown. Several recent studies conducted in rodents have linked αS to immune cell activation in the central nervous system. We hypothesized that αS in the ENS might play a role in the innate immune defenses of the human gastrointestinal (GI) tract.

METHODS

We immunostained endoscopic biopsies for αS from children with documented gastric and duodenal inflammation and intestinal allograft recipients who contracted norovirus. To determine whether αS exhibited immune-modulatory activity, we examined whether human αS induced leukocyte migration and dendritic cell maturation.

FINDINGS

We showed that the expression of αS in the enteric neurites of the upper GI tract of pediatric patients positively correlated with the degree of acute and chronic inflammation in the intestinal wall. In intestinal allograft subjects who were closely monitored for infection, expression of αS was induced during norovirus infection. We also demonstrated that both monomeric and oligomeric αS have potent chemoattractant activity, causing the migration of neutrophils and monocytes dependent on the presence of the integrin subunit, CD11b, and that both forms of αS stimulate dendritic cell maturation.

INTERPRETATION

These findings strongly suggest that αS is expressed within the human ENS to direct intestinal inflammation and implicates common GI infections in the pathogenesis of PD.

16S rRNA Next Generation Sequencing Analysis Shows Bacteria in Alzheimer's Post-Mortem Brain

The neurological deterioration associated with Alzheimer's disease (AD), involving accumulation of amyloid-beta peptides and neurofibrillary tangles, is associated with evident neuroinflammation. This is now seen to be a significant contributor to pathology. Recently the tenet of the privileged status of the brain, regarding microbial compromise, has been questioned, particularly in terms of neurodegenerative diseases. It is now being considered that microbiological incursion into the central nervous system could be either an initiator or significant contributor to these. This is a novel study using 16S ribosomal gene-specific Next generation sequencing (NGS) of extracted brain tissue. A comparison was made of the bacterial species content of both frozen and formaldehyde fixed sections of a small cohort of Alzheimer-affected cases with those of cognitively unimpaired (normal). Our findings suggest an increase in bacterial populations in Alzheimer brain tissue compared with normal.

Synthetic biology engineering of biofilms as nanomaterials factories

Bottom-up fabrication of nanoscale materials has been a significant focus in materials science for expanding our technological frontiers. This assembly concept, however, is old news to biology — all living organisms fabricate themselves using bottom-up principles through a vast self-organizing system of incredibly complex biomolecules, a marvelous dynamic that we are still attempting to unravel. Can we use what we have gleaned from biology thus far to illuminate alternative strategies for designer nanomaterial manufacturing? In the present review article, new synthetic biology efforts toward using bacterial biofilms as platforms for the synthesis and secretion of programmable nanomaterials are described. Particular focus is given to self-assembling functional amyloids found in bacterial biofilms as re-engineerable modular nanomolecular components. Potential applications and existing challenges for this technology are also explored. This novel approach for repurposing biofilm systems will enable future technologies for using engineered living systems to grow artificial nanomaterials.

The Presence of Alpha-Synuclein in Skin from Melanoma and Patients with Parkinson's Disease

Background

The misfolding and prion-like propagation of the protein α-synuclein (α-syn) is the leading molecular signature in Parkinson's disease (PD). There is a significant coincidence of PD and melanoma that may suggest a shared pathophysiology. This study compared the presence of α-syn in neural crest-derived tissues, such as nevi, melanoma, skin tags, and skin biopsies from patients with PD and healthy controls.

Methods

Biopsies from participants with PD were obtained from patients from a tertiary referral center for dermatology and neurology in Mexico and a private dermatopathology center in Florida between January 2015 and March 2016. Biopsies from 7 patients with melanoma, 15 with nevi, 9 with skin tags, 8 with PD, and 9 skin biopsies from healthy volunteers were analyzed for immunohistochemical determination of α-syn and tyrosinase. All analyses were performed by pathologists who were blinded with respect to the clinical diagnosis.

Results

In healthy controls, positive α-syn status was restricted to scattered cells in the basal layer of the epidermis and accounted for 1 ± 0.8% of the analyzed area. In patients with PD, there was increased staining for α-syn PD (3.3 ± 2.3%), with a higher percentage of positive cells in nevi (7.7 ± 5.5%) and melanoma (13.6 ± 3.5%). There was no increased staining in skin tags compared with healthy controls.

Conclusion

Patients with PD and melanoma have increased staining for α-syn in their skin. The authors propose that neurons and melanocytes, both derived from neuroectodermal cells, may share protein synthesis and regulation pathways that become dysfunctional in PD and melanoma.

Holocranohistochemistry enables the visualization of α-synuclein expression in the murine olfactory system and discovery of its systemic anti-microbial effects

Braak and Del Tredici have proposed that typical Parkinson disease (PD) has its origins in the olfactory bulb and gastrointestinal tract. However, the role of the olfactory system has insufficiently been explored in the pathogeneses of PD and Alzheimer disease (AD) in laboratory models. Here, we demonstrate applications of a new method to process mouse heads for microscopy by sectioning, mounting, and staining whole skulls (‘holocranohistochemistry’). This technique permits the visualization of the olfactory system from the nasal cavity to mitral cells and dopamine-producing interneurons of glomeruli in the olfactory bulb. We applied this method to two specific goals: first, to visualize PD- and AD-linked gene expression in the olfactory system, where we detected abundant, endogenous α-synuclein and tau expression in the olfactory epithelium. Furthermore, we observed amyloid-β plaques and proteinase-K-resistant α-synuclein species, respectively, in cranial nerve-I of APP- and human SNCA-over-expressing mice. The second application of the technique was to the modeling of gene–environment interactions in the nasal cavity of mice. We tracked the infection of a neurotropic respiratory-enteric-orphan virus from the nose pad into cranial nerves-I (and -V) and monitored the ensuing brain infection. Given its abundance in the olfactory epithelia, we questioned whether α-synuclein played a role in innate host defenses to modify the outcome of infections. Indeed, Snca-null mice were more likely to succumb to viral encephalitis versus their wild-type littermates. Moreover, using a bacterial sepsis model, Snca-null mice were less able to control infection after intravenous inoculation with Salmonella typhimurium. Together, holocranohistochemistry enabled new discoveries related to α-synuclein expression and its function in mice. Future studies will address: the role of Mapt and mutant SNCA alleles in infection paradigms; the contribution of xenobiotics in the initiation of idiopathic PD; and the safety to the host when systemically targeting α-synuclein by immunotherapy.

Interactions between the microbiota, immune and nervous systems in health and disease

The diverse collection of microorganisms that inhabit the gastrointestinal tract, collectively called the gut microbiota, profoundly influences many aspects of host physiology, including nutrient metabolism, resistance to infection and immune system development. Studies investigating the gut-brain axis demonstrate a critical role for the gut microbiota in orchestrating brain development and behavior, and the immune system is emerging as an important regulator of these interactions. Intestinal microbes modulate the maturation and function of tissue-resident immune cells in the CNS. Microbes also influence the activation of peripheral immune cells, which regulate responses to neuroinflammation, brain injury, autoimmunity and neurogenesis. Accordingly, both the gut microbiota and immune system are implicated in the etiopathogenesis or manifestation of neurodevelopmental, psychiatric and neurodegenerative diseases, such as autism spectrum disorder, depression and Alzheimer's disease. In this review, we discuss the role of CNS-resident and peripheral immune pathways in microbiota-gut-brain communication during health and neurological disease.