Volume 167, 2022, Pages 81-99Author links open overlay panelAbstract

Emerging data suggest that disrupted intestinal microbiota, or dysbiosis, may be responsible for multiple features of Parkinson's disease (PD), from initiation, to progression, to therapeutic response. We have progressed greatly in our understanding of microbial signatures associated with PD, and have gained important insights into how dysbiosis and intestinal permeability promote neurodegeneration through neuroinflammation and Lewy body formation. These insights underscore the potential of microbiota-directed therapies, which include dietary, pharmacologic, and lifestyle interventions.

Section snippetsIntroduction: Intestinal microbiota in health and disease

Human gut microbiota comprise a vast ecosystem of bacteria, archaea, viruses, and fungi that reside in the digestive tract. Although these 100 trillion microbes represent more than 500 different species, 99% of bacterial abundance is derived from less than 50 species (Lozupone, Stombaugh, Gordon, Jansson, & Knight, 2012). The sum of genetic material from these microbes, known as the microbiome, contains more than three million unique genes, greatly outnumbering the full set of human genes (

Microbial signatures and their significance in Parkinson's disease

The reported prevalence of PD is rising in both the developed and developing world. More than 1% of adults over the age of 60 in the United States are affected (Marras et al., 2018). As the second most common neurodegenerative disease, PD is a synucleinopathy, characterized by aggregates of misfolded α-synuclein protein called Lewy bodies that accumulate in the substantia nigra and disable the dopaminergic neurons responsible for coordinated motor activity (Tysnes & Storstein, 2017). In their

Neuroprotection by short-chain fatty acids

Several enteric bacterial species can ferment complex carbohydrate fibers to produce the short-chain fatty acids (SCFAs) acetate, propionate, and butyrate. Strains of Lactobacillus and Bifidobacterium are able to produce acetate, while Firmicutes such as Clostridium and Roseburia can produce butyrate. The SCFAs, in particular butyrate, are the preferred energy substrates for colonic epithelial cells (Makki, Deehan, Walter, & Bäckhed, 2018). SCFAs are vital for maintaining a healthy epithelium,

Microbial effects on neuroinflammation

The intestinal barrier in PD is characterized by higher-than-normal permeability, which enables translocation of PAMPs and subsequent neuroinflammation (Tang et al., 2015). Mucosal bacteria, accessible by biopsy, are separated from those in the lumen exposed to feces, creating two distinct populations. In the setting of mucosal irritation or a pathologic state, these communities are more likely to mix, and mucosal contents may leak through a disrupted epithelial barrier. In PD, LPS-producing

Environmental and dietary influences

Environmental factors seem to play greater roles in PD pathogenesis than genetic factors as suggested by studies of twins (Turnbaugh et al., 2009). These environmental factors include not only dietary elements, but also chemicals, herbicides, and other xenobiotics. Experimental evidence suggests that environmental factors that are associated with increased risk of PD and parkinsonism, which include xenobiotics and high fat/low fiber diets, are also associated with dysbiosis and altered

Common medications and microbiota

Although most evidence to this point has favored dysbiosis as a trigger and/or promoter of PD rather than a consequence of PD, it should be noted that PD patients often experience worsening gastrointestinal symptoms and alter their diet and lifestyle accordingly in ways that may exacerbate dysbiosis. Constipation, a low-fiber diet, and reversal of sleep/wake cycles are features of PD that are independently associated with dysbiosis (Voigt, Forsyth, Green, Engen, & Keshavarzian, 2016). Despite

Therapeutic strategies

Given the mounting evidence for dysbiosis in triggering or promoting the PD phenotype, it follows that there may be a useful role for gut microbiota-directed therapeutic strategies that prevent, delay, or modify PD onset or disease course. In particular, dietary intervention may be a useful strategy for stopping or improving dysbiosis; this may be achieved by implementing diets known to be associated with putative anti-inflammatory microbiota and increased SCFA-production such as the

Conclusions

Recent animal and human studies suggest that a microbiota-mediated gut-brain axis may play a principal role in PD pathogenesis and progression. Microbial community structure is central to a healthy gut-brain axis; disrupted homeostasis can potentiate enteric and systemic inflammation in a susceptible host. Intestinal dysbiosis is a key feature of PD, which among other neurodegenerative diseases is characterized by alterations in gut microbiota that favor pro-inflammatory over anti-inflammatory

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