Studies reported by scientists at Johns Hopkins University School of Medicine support a more than decade-old theory that Parkinson’s disease (PD) can start in the gut and spread via the nervous system up to the brain. The research, carried out in a new mouse model that recapitulates the motor and non-motor deficits, and early- and late-stage features of PD, could help scientists develop new therapeutic strategies against the neurodegenerative disorder, or identify biomarkers of early, presymptomatic disease.

“These findings provide further proof of the gut’s role in Parkinson’s disease, and give us a model to study the disease’s progression from the start,” stated research lead Ted Dawson, MD, PhD, who is director of the Johns Hopkins Institute for Cell Engineering and professor of neurology at the Johns Hopkins University School of Medicine. “Since this model starts in the gut, one can use it [to] study the full spectrum and time course of the pathogenesis of Parkinson’s disease. For instance, one could test preventive therapies at early pre-symptomatic stages of Parkinson’s disease all the way to full-blown Parkinson’s disease in one animal model.”

The investigators report their findings in Neuron, in a paper titled, “Transneuronal Propagation of Pathologic α-Synuclein from the Gut to the Brain Models Parkinson’s Disease.”

Parkinson’s disease is a neurodegenerative disorder that leads to progressively worsening motor symptoms including tremors and rigidity, together with speech and coordination difficulties, and cognitive decline. The disorder is characterized by the accumulation of misfolded α-synuclein (α-syn) protein in nerve cells in the brain, and the death of dopamine-producing neurons, particularly in the substantia nigra pars compacta (SNc). Deposits of α-synuclein are the main component of the telltale Lewy body structures that form in the brains of Parkinson’s disease patients and individuals with other synucleinopathies, which together form the second most common form of neurodegenerative dementias, the authors wrote.

Although PD is a disease of the brain, post-mortem studies in humans by German neuroanatomist Heiko Braak to propose, back in 2003, that α-synuclein pathology may, in fact, start in the gut and spread to the brain via the nerves that control the gastrointestinal system. “The Braak hypothesis posits that α-syn pathology can spread in a stereotyped fashion from the gastrointestinal tract via the vagus nerve to the ventral midbrain, where it selectively kills dopamine (DA) neurons of the substantia nigra pars compacta,” the researchers explained.

Accumulating evidence does support a gut-brain connection, but the spread of abnormal α-synuclein protein via the vagus nerve from gut to the SNc at the base of the brain hasn’t yet been demonstrated in animal models. For their studies, Dawson and co-lead author Han Seok Ko, PhD, an associate professor of neurology at Johns Hopkins University School of Medicine, injected synthetic, preformed α-synuclein fibrils (preformed fibrils; PFFs) into specific gastrointestinal muscles that connect with the vagus nerve in live mice. They studied the animals for several months to track the movement of abnormal α-synuclein protein (the majority of α-synuclein in PFFs is phosphorylated at a serine site; pSer129-α-syn) and monitor associated signs of Parkinson’s-like disease. The key was optimizing the injection site, the amount and size of α-synuclein PFFs injected. “When the initial experiments started to work, we were utterly amazed,” Dawson said. “Now it is fairly routine for our research team.”

The researchers analyzed brain tissue from the treated animals at one, three, seven, and 10 months post injection, and saw evidence of α-synuclein build-up, initially where the vagus nerve connected to the gut, but then spreading to the brain. At one month post-injection pathologic α-synuclein had spread to the dorsal motor nucleus of the vagus, which is located in the lowest part of the brainstem. This corresponds to Braak’s stage 1. Within three months, pathologic α-synuclein had spread up the brainstem to the locus coeruleus and higher to the SNc, and had penetrated past the brainstem into the amygdala, hypothalamus, and prefrontal cortex. This corresponded to later Braak stages. By seven months, pathologic α-synuclein had spread to additional brain regions, including the hippocampus, striatum, and olfactory bulb. By these later time points, there was also significant loss of dopamine-producing neurons in the SNc and striatum.

The images are of DAT scans of mice injected in the gut with pathologic alpha-synuclein on the right and control injected animals on the left. Note the loss of dopamine in the striatum of mice injected with pathologic alpha-synuclein. [Ted Dawson et al. / Neuron, 2019]
Mice receiving gastrointestinal injections of the PFFs also developed to motor deficits, and a battery of behavioral and other tests showed that the animals exhibited olfactory dysfunction, and cognitive deficits that affected their spatial learning and memory, as well as novel object recognition, fear memory, and working memory. “In addition to the motor deficits, injection of α-syn PFF in the muscle layers of the pylorus and duodenum causes cognitive impairments, including memory and social deficits, anxiety, depression, and olfactory and gastrointestinal dysfunction,” the team wrote.

Interestingly surgically severing the vagus nerve before giving animals the PFF injections stopped the transmission of pathologic α-synuclein from the gut to the brain, and these animals didn’t exhibit either nerve cell death or develop disease-related symptoms, even by seven months. The presence of native, normal α-synuclein was also a requisite for the spread of pathologic α-synuclein following PFF injections. Mice that were genetically engineered to lack normal α-synuclein didn’t progress to exhibit nerve cell death or symptoms of Parkinson’s disease after receiving the synthetic misfolded α-synuclein.

The authors say their results support the Braak hypothesis, and offer up a model system for PD development that avoids having to inject pathologic proteins directly into the brain. “The major findings of this study provide support for the concept that pathologic α-synuclein is capable of spreading from the gastrointestinal tract in a stereotypic manner via the vagus nerve into the brain … Taken together, this study supports the Braak hypothesis of temporal and stereotypical spreading of LB pathology from gut to brain, leading to clinical features of idiopathic PD, including both motor and non-motor symptoms. This new model implicates the gut-brain axis in the etiology of idiopathic PD and will serve to accelerate the study of specific cellular and molecular pathways associated with the centripetal trafficking of pathologic α-syn from the gut to brain in PD and related α-synucleinopathies and will help to test potential therapeutic interventions to mitigate risk of developing sporadic PD.”

Route of Parkinson’s disease-causing protein propagation in mice. [Ted Dawson]
Dawson lists three primary implications of the results. “One: it is likely to galvanize future studies exploring the gut-brain connection. Two: it will spur investigations focused on the factors, molecules, or infections that might start the misfolding and propagation of α-synuclein. And three: it suggests that treatments could be aimed at preventing the spread of pathologic α-synuclein from the gut to the brain.”

Interestingly, it has already been reported that truncal vagotomy, a surgical procedure on the vagus nerve that is typically carried out to treat ulcers, may reduce the risk of Parkinson’s disease. However, Dawson asserted, “Patients should not pursue vagotomies as a preventive therapy for Parkinson’s disease … But if one could find the major initiating factor in the gut, then one could target this in a future study and show that it prevents Parkinson’s disease. Patients with pathologic α-synuclein in the gastrointestinal tract would be ideal candidates for future neuroprotective studies.”

The researchers aim to extend their work into nonhuman primates and to further investigate the mechanisms that underlie the spread of pathologic α-synuclein from gut to brain. They also hope to identify potential gastrointestinal markers of early presymptomatic PD, and test potential preventive strategies that might halt the spread of α-synuclein. “This is an exciting discovery for the field and presents a target for early intervention in the disease,” Dawson concluded.

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