Like most neurodegenerative conditions, Parkinson's disease is driven in large part by the pathological aggregation of misfolded proteins, in this case α-synuclein. These solid deposits of protein spread from cell to cell, and are accompanied by a surrounding halo of harmful biochemical interactions. There is evidence for the protein aggregation of Parkinson's disease to start in the gut and then spread to the brain. You might look at a recent paper that discusses whether or not we should consider Parkinson's to be two diseases with a similar outcome, one in which the α-synuclein aggregation originates in the gut, and the other in which it originates in the brain. In the research noted here, scientists are following the gut origin hypothesis and targeting α-synuclein there in order to slow or reverse the progression of Parkinson's disease.
Aggregates of the protein alpha-synuclein arising in the gut may play a key role in the development of Parkinson's disease (PD). Investigators are testing the hypothesis that by targeting the enteric nervous system with a compound that can inhibit the intracellular aggregation of alpha-synuclein, they can restore enteric functioning in the short term, and possibly slow the progressive deterioration of the central nervous system in the long term. "The concept is that aggregates of the protein alpha-synuclein, thought to play a key role in the disease, arise within the enteric nervous system (ENS) and travel up the peripheral nerves to the central nervous system (CNS) where they ultimately cause inflammation and destruction of parts of the brain. Targeting the formation of alpha-synuclein aggregates in the ENS may therefore slow the progression of the disease."
Alpha-synuclein is one of the defensive proteins produced by enteric nerves when they encounter infections. In children with acute bacterial gastrointestinal (GI) infections, for example, intestinal nerves produce alpha-synuclein. In children who have undergone intestinal transplants and who are prone to GI infections, investigators have shown that enteric neurons start making alpha-synuclein at the time of acute viral infections, and this outlasts the infection by many months, protecting nerve cells for prolonged periods of time. Within a nerve cell, alpha-synuclein could envelop invading viruses and disrupt their replication. It could also attach itself to small vesicles containing neurotransmitters and be released from the nerve cell hitching a ride with them. Once on the outside, it can attract protective immune cells from surrounding tissues.
To determine whether targeting alpha-synuclein within enteric neurons might help patients with PD, researchers are currently conducting clinical trials with a compound called ENT-01, a synthetic derivative of squalamine, a compound originally isolated from dogfish bile. It displaces alpha-synuclein from nerve cell membranes and restores the normal electrical activity of enteric neurons. Investigators completed a 50-patient Phase 2a study (RASMET) in patients with PD in 2018, which corrected constipation, a common symptom of PD, in more than 80% of participants, with the dose titrated up for each patient until a response was obtained. "The RASMET study demonstrated that the ENS is not irreversibly damaged in patients with PD. We believe that this is the first demonstration of the reversal of a neurodegenerative process in humans." Possible benefits were also observed in motor and non-motor symptoms such as hallucinations, depression, and cognitive function. A 110-patient double-blind, placebo-controlled Phase 2b trial (KARMET) evaluating the effect of oral ENT-01 tablets on constipation and neurologic symptoms is currently being conducted.