What are the important steps in the progression of neurodegenerative diseases characterized by the presence of protein aggregates? These aggregates are misfolded or otherwise altered proteins that precipitate to form solid deposits. This means α-synuclein in the case of Parkinson's disease, or amyloid-β and tau in the cause of Alzheimer's disease, to pick the best known examples. A growing body of evidence is pointing to dysfunction and inflammation in the immune cells known as microglia, a type of macrophage resident in the central nervous system. Like macrophages elsewhere in the body, microglia are responsible for chasing down pathogens and clearing up debris. They also participate in a range of other supporting activities that assist the function of neurons.
In Alzheimer's disease, there is compelling evidence for microglia to be driven into an inflammatory state by the presence of amyloid-β. They act as the bridge between the mild earlier stage of the condition, in which amyloid-β accumulates, and the later stage in which tau aggregates form and neurons die. It is the chronic inflammation and dysfunction of microglia in brain tissue that drives this more severe tau pathology. Inflammatory behavior in microglia appears to involve significant numbers of senescent microglia, and researchers have shown that removing these senescent cells can turn back tau pathology in mouse models and reduce levels of neuroinflammation. Lingering senescent cells of any cell type cause harm through secreting inflammatory and other signals, the senescence-associated secretory phenotype (SASP). This actively maintains a disordered tissue environment, and we'd all benefit from its removal in old age.
Given that microglia have this role in Alzheimer's disease, are they also causing similar issues in other neurodegenerative disease processes? Most likely yes. The article here examines the role of microglia in α-synuclein aggregation, an important part of the progression of Parkinson's disease. This continues to add support for the idea that senolytic therapies, capable of removing senescent cells and dampening the inflammation that they cause, will prove to be a useful treatment for neurodegenerative conditions. Indeed, they should be a useful preventative treatment prior to the advent of neurodegenerative disease. Chronic inflammation drives many of the common diseases of aging, and to the extent that the causes of that inflammation can be prevented, then age-related disease - and aging itself - will be pushed back.
How does α-synuclein pathology spread? Researchers say immune cells bear some of the blame. 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. Other research implicates brain immune cells in propagation. 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.
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. This strengthened the idea that Parkinson's disease might start in the intestine and travel from there to the brain. People who suffer from inflammatory bowel disorders are at elevated risk of Parkinson's disease, and genetic studies have found shared risk between the two. While the links are suggestive, no one had yet shown directly that gut inflammation triggered brain pathology.
Researchers 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. 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. These mice had but half as many dopaminergic neurons as controls, suggestive of neurodegeneration.
Researchers are also interested in how α-synuclein aggregates propagate within the brain. 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, researchers 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.
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.
Based on these findings, researchers 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.