Synucleinopathies, the accumulation of α-Syn, are key pathological traits of PD[10]. Our current study shows that ectopic α-Syn within the intestine ultimately produces PD-like pathology in Drosophila, including a shorter life span, loss of DA neurons, and progressive motor defects (Fig. 1). Consistently, the PPM1/2 cluster that accounts for locomotor deficits and lifespan impairment has a propensity to suffer from greater DA neuron loss upon genetic mutation and environmental toxin[32]. Our findings highlight that intestinal α-Syn is capable of triggering the progression of PD in Drosophila, supporting the Braak hypothesis that this disease spreads from the digestive tract to the central nervous system[11, 12]. Ectopic intestinal α-Syn triggers the pathology of PD by disrupting intestinal homeostasis and metabolic profile (Figs. 2 and 3). These results corroborate observations of gastrointestinal dysfunction in PD patients, including drooling, gastroparesis, decreased bowel movement frequency, constipation, and anorectal dysfunction[46–48].
These results are consistent with observations that gut injection of α-Syn fibrils disrupts intestine function and initiates inflammatory responses[13, 14]. More importantly, intestinal α-Syn is sufficient to directly cause progressive PD-like pathology in Drosophila, differing from the hypothesis that gut injection of α-Syn fibrils has to convert endogenous α-Syn in brains to a pathologic species[13, 14]. However, the propagation of α-Syn from the gastrointestinal tract to the brain still needs to be elucidated. Interestingly, other studies have provided experimental evidence that α-Syn aggregates in a prion-like manner[9], supporting the propagation of α-Syn from the intestines to the brain in Drosophila. In mammals, α-Syn is expressed in enteroendocrine cells that directly connect to α-Syn–containing nerves[49]. Far from being a passive tube, the gut of adult Drosophila provides the first line of defense against pathogens and maintains energy homeostasis by exchanging neuronal and endocrine signals, with other organs[25], partially explaining the potential role of α-Syn in PD pathogenesis. The digestive tract of Drosophila adults is innervated in three distinct regions, including the esophagus-crop-anterior midgut, the midgut-hindgut junction, and the posterior hindgut[50]. Some neuronal fibers terminate the underlying epithelium, so epithelial innervation provides an access to the brains, plausibly making α-Syn spread from cell-to-cell in a prion-like manner. Moreover, the midgut of adults is maintained by ISCs that directly differentiate into absorptive enterocytes and secretory enteroendocrine cells. These secretory enteroendocrine cells are neural-like cells and secrete sets of hormone peptides reminiscent of neuroendocrine cells in the Drosophila brain[51, 52]. These enteroendocrine cells display characteristics that are typical of neurons, being best known for their membrane excitability and hormone secretion. They reside within the mucosa of the intestine in a particular anatomical orientation, allowing them to respond to signals in the lumen of the intestine. This newly recognized EC-neural circuit raises many interesting possibilities. First, ECs may be a portal for the entry of pathogens into the nervous system[53], which aggravates the pathology of PD induced by brain α-Syn. Second, ECs can receive bacterial stimuli from the intestine lumen, and send messages to enteric nerves, and eventually the brain, by regulating hormones or neurotransmitters[54, 55]. These findings suggest that ECs may be susceptible to pathogen or toxin exposure that could affect the misfolding of α-Syn, which could be the first step in a prion-like cascade leading to PD. A recent study reported that enteropathogen infection in Drosophila modulates olfaction through metabolic reprogramming of ensheathing glia in brains[56]. Due to age-related intestinal inflammation, metabolic reprogramming of ensheathing glia induced by gut-derived inflammatory cytokines causes lasting changes in a sensory system in ageing or disease flies. Given the potent role in neurodegeneration[57], it is proposed that dysbiosis aggravates the progression of this disease through inter-organ communications. In addition, intestinal dysplasia frequently accompanies cell death, including apoptosis and necrosis[58, 59]. It is postulated that intestinal α-Syn-induced dysplasia is coupled with cell death that possibly contributes to PD-like phenotypes and pathology in Drosophila. Altogether, these studies provide an underlying mechanism for initiation and progression of α-Syn pathology in the ENS consistent with Braak’s hypothesis.
Interactions between genetic and environmental factors, such as toxins, likely trigger pathogenesis by initiating α-Syn oligomerization, aggregation, and propagation[60]. A growing number of studies indicate that the microbiome affects a range of animal physiology and diseases[61, 62]. Pathological alterations in microbial community composition, termed dysbiosis, result in microbial community dysfunction that is linked to human diseases, including inflammatory bowel disease, obesity, pathogen infection, and colon cancer[63, 64]. Indeed, the Drosophila model allows us to elucidate that ectopic intestinal α-Syn triggers the pathology of PD by disrupting the intestinal microbiome (Figs. 4 and 5), suggesting that intestinal α-Syn contributes to the burden of PD pathologies through the DUOX-ROS-JNK pathway. The expression of DUOX was significantly higher in the midguts of esgTS> Syn flies (Fig. 6), suggesting that intestinal α-Syn triggers the DUOX pathway. We have shown that it dampened oxidative innate immunity by knocking down DUOX in flies[29], suggesting that DUOX silencing could ameliorate the progression of PD in esgTS> Syn flies. Long-term exposure to pathogens can cause deleterious health outcomes, including metabolic alterations, immunological changes, and neurotoxicity, because α-Syn misfolding is the key pathological mechanism underlying synucleinopathies, including Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy[2, 65]. Individual heterogeneity in intestine microbiota may modulate resilience to synucleinopathy-associated disorders[66]. Animal models possessing microbiomes with lower taxonomic diversity are required to elucidate the underlying mechanisms by which microbes influence host traits. Recent studies on the intestine microbiome suggested that short-chain fatty acids and extracellular fibers, such as curli, generated from microbes in the gut could affect α-Syn aggregation[22]. Indeed, α-Syn overexpressing mice colonized with curli-producing bacteria displayed increased α-Syn fibril reactivity and detergent-insoluble α-Syn in the midbrain[13]. Interestingly, increased colonization and mucosal association with Enterobacteriaceae, such as Escherichia coli, have been reported in individuals with PD compared to healthy controls, as well as a positive association between Enterobacteriaceae abundance and disease severity[18, 19]. Studies have found synergy between dysbiosis and genetic predisposition in intestinal dysplasia[58, 65]. It is conceived that bacterial infection gives rise to blood cell infiltration and immunity activation, which consequently aggravates PD pathologies. Moreover, previous studies found that ageing is associated with innate immune activation in Drosophila brains[67, 68]. Given that intestinal α-Syn accelerated the ageing of flies, intestine-derived α-Syn might activate a systemic innate immune response that contribute to PD-pathogenesis. Future studies to determine if bacterial metabolites can contribute to the efficacy in modulating the α-Syn-mediated pathology of PD will be interesting. Commensal bacteria frequently antagonize pathogens through chemical inhibition and nutritional competitive exclusion. Advances in microbiome studies are contributing to therapies for chronic human diseases, especially metabolic, immunological, and mental disorders[69]. Probiotics that are effective in inhibiting α-Syn aggregation could be effective in halting the misfolding of other proteins[70, 71]. Hence studies that are able to mechanistically disentangle the influence of microbiological communities on α-Syn fibril reactivity will be warranted. Identifying new specific cellular and molecular pathways that can explain the vulnerabilities of intestines to pathologic α-Syn may lead to potential therapeutic interventions for PD.