10 May 2019

How does α-synuclein pathology spread? At the 14th International Conference on Alzheimer’s and Parkinson’s Diseases, held March 27–31 in Lisbon, Portugal, speakers said immune cells bear some of the blame. Markus Britschgi of Roche in Basel, Switzerland, said certain types of inflammation in the intestine modulate α-synuclein accumulation there. In mice, experimental colitis at a young age accelerated α-synuclein pathology in the brain 18 months later, consistent with the idea that misfolded protein can travel from gut to brain. Others implicated brain immune cells in propagation. Seung-Jae Lee of Seoul National University College of Medicine, South Korea, found that mutant α-synuclein oligomers that were incapable of forming fibrils still stimulated aggregation in brain. They appeared to work their mischief by firing up inflammation, suggesting that microglia somehow mediate α-synuclein spread. Together, the findings highlight the role of the immune system in Parkinson’s disease.

In mice, gut inflammation triggers α-synuclein accumulation in enteric neurons.

Eighteen months later, these mice have PD-like pathology in the brain.

In a different model, propagation within the brain seems to ride on microglia, not templated seeding.

First, peripheral immunity. Scientists know that intestinal infections or inflammation can pump up α-synuclein production in the gut, perhaps as part of an antimicrobial defense (Jul 2017 news; Breen et al., 2019; Prigent et al., 2019). This strengthened the idea that Parkinson’s disease might start in the intestine and travel from there to the brain (Jul 2011 news series; Dec 2016 news). People who suffer from inflammatory bowel disorders are at elevated risk of PD, and genetic studies have found shared risk between the two (Jan 2018 news; Apr 2018 news).

While the links are suggestive, no one had yet shown directly that gut inflammation triggered brain pathology. Britschgi and colleagues provoked colitis in 3-month-old transgenic α-synuclein mice by adding dextran sulfate sodium (DSS) to their water. This irritant caused macrophages to invade the lining of the gut wall. In response, enteric neurons lying just below the mucosa, in the submucosal plexus, began to accumulate α-synuclein, although expression of the protein remained unchanged. Britschgi previously reported that such excess α-synuclein persisted for months (Apr 2015 conference news).

Promoting Pathology. More α-synuclein aggregates (brown) accumulate in two brainstem regions of aged transgenic mice that had colitis when young (bottom panels), compared with aged healthy transgenics (top). [Courtesy of Emmanuel Quansah, Patrik Brundin, and Markus Britschgi.]

In Lisbon, Britschgi connected these local effects to brain pathology. In collaboration with Patrik Brundin’s group at the Van Andel Research Institute in Grand Rapids, Michigan, the researchers aged the mice to 12 or 21 months. At 12 months, they saw no difference between the brains of control transgenics and those that had colitis as youngsters. By 21 months, however, the colitis group had six times more α-synuclein aggregates in brainstem regions than controls did (see image at right). These mice had but half as many nigral dopaminergic neurons as controls, suggestive of neurodegeneration.

The finding supports the idea that α-synuclein pathology can propagate from gut to brain. Other studies have shown directly that α-synuclein aggregates injected into the gut travel through the vagus nerve and reach the brain (Holmqvist et al., 2014; Uemura et al., 2018). However, Britschgi said he cannot prove this was the mode of transmission in the Roche study, since they did not cut the vagus nerve to see if that prevented brain pathology.

How do infiltrating immune cells contribute to α-synuclein accumulation in the gut? Previously, Britschgi reported that more α-synuclein built up in mice that lacked fractalkine signaling, a key activator of macrophages and microglia. In Lisbon, he noted that the type of peripheral inflammation also matters. When the researchers irritated the guts of transgenic mice with lipopolysaccharide, α-synuclein did not accumulate, even though macrophages invaded the gut mucosa. The researchers compared the cytokine response in the two types of inflammation, and found that colitis caused IL-6 to spike, while LPS triggered IL-10. Injecting IL-10 into transgenic mice at the same time as DSS dampened the resulting colitis and macrophage infiltration, and prevented α-synuclein buildup. The results suggest that the way macrophages respond to intestinal irritation determines the degree of damage.

Are these mouse data applicable to Parkinson’s? To see if the data were an artifact of α-synuclein overexpression, Britschgi and colleagues repeated the experiments in wild-type mice. As in transgenics, DSS induced α-synuclein deposits in the colon that persisted long after the inflammation had subsided.

Turning to people, the researchers examined intestinal biopsies from 11 people with colitis, 11 with Crohn’s disease, and eight healthy controls. Eight of the colitis patients had macrophages containing α-synuclein in the walls of their guts, and also enteric neurites that stained for α-synuclein accumulation (see image below). Among the Crohn’s patients, four had α-synuclein-positive macrophages in their mucosa but no α-synuclein-positive neurites. Only one of the controls had any α-synuclein reactivity in gut, in a handful of cells. Britschgi noted that they could not tell if the α-synuclein in enteric neurons was aggregated. These data are available online in preprint format (Grathwohl et al., 2019). The data suggest that what macrophages are up to in Parkinson’s pathogenesis should be explored further, Britschgi said.

Synuclein Buildup. α-Synuclein (red) accumulates in enteric neurites (arrowheads) and infiltrating macrophages (arrows) in the intestinal wall of a person with active colitis. [Courtesy of Markus Britschgi.]

Lee made a similar argument, though he approached the question of immune cell involvement from a different angle. Lee is interested in how α-synuclein aggregates propagate within the brain. He noted that when researchers injected aggregated material into mouse brain, it was quickly cleared to undetectable levels. Then, after an incubation period, aggregates appeared and spread through brain. The leading theory holds that this occurs through templated seeding of endogenous α-synuclein by the injected aggregates.

To test this idea, Lee used a mutant form of α-synuclein, V40G, that forms unstructured oligomers but is incapable of forming fibrils. In a test tube, V40G blocks fibrillization of wild-type α-synuclein as well. Thus, this form should prevent templated seeding in vivo, Lee reasoned. The researchers injected either V40G or wild-type α-synuclein into the striata of wild-type mice. To their surprise, V40G seeded aggregates even better than wild-type α-synuclein did. Four weeks after injection, mice that had received V40G had far more α-synuclein pathology in the rhinal cortex than did mice treated with wild-type protein.

Why might this be? The researchers analyzed gene expression in injected brains to glean clues. They found heightened inflammatory and innate immune responses in V40G-treated animals relative to those treated with wild-type α-synuclein. Supporting this, levels of the inflammatory cytokine IL-1β shot up in numerous brain regions after V40G administration, and this spike preceded the spread of α-synuclein aggregates to these regions. Treating mice with the anti-inflammatory drug lenalidomide along with V40G prevented this spike in IL-1β, Lee said. In preliminary experiments, lenalidomide treatment also appeared to ameliorate behavioral deficits in the open field, Y-maze, and wire-hanging tasks; these studies are ongoing.

Based on these findings, Lee proposed a new model of α-synuclein propagation. Perhaps α-synuclein oligomers kick off microglial activation and cytokine release, and this inflammatory microenvironment then aggravates nearby neurons, causing α-synuclein to clump up in their cell bodies. By this logic, rather than α-synuclein aggregates passing directly from neuron to neuron, microglia would be essentially the conveyor belt for α-synuclein pathology.

This new model remains to be tested, but even so, the findings further reinforce the central role inflammation appears to play in neurodegenerative disease, scientists agreed.—Madolyn Bowman Rogers