A research team at the University of Michigan in Ann Arbor has discovered a new mutated protein called TMEM175 that acts as a risk factor in about 20 percent of Parkinson’s disease cases.
In Parkinson’s, nerve cells in the part of the brain that control movement begin to fail and die. According to the National Institute of Aging, researchers think Parkinson’s is a result of a combination of genetic and environmental factors.
In about a fifth of the cases of Parkinson’s disease, U-M researchers looked to a small, malfunctioning protein in the lysosome as a risk factor. Lysosomes are organelles responsible for breaking down the unwanted proteins and worn-out organelles.
When lysosomes malfunction and cause cellular debris build up, it can lead to various degenerative disorders such as Alzheimer’s disease, Duchenne muscular dystrophy, and Tay-Sachs disease.
The U-M researchers found that if mutated, TMEM 175 does not properly regulate the acidity of the environment within the lysosome, resulting in a malfunction. As a result, organelles fail to perform their roles correctly.
“The lysosome actually needs an optimal pH. Here is an analogy: body temperature has to be 37 degrees Celsius. Thirty-eight is too high and 36 is too low,” says lead author Haoxing Xu, professor of molecular, cellular, and developmental biology.
“The enzymes in lysosomes require a pH optimum of roughly 4.6. Anything outside of 4.6 pH may cause metabolic dysfunction, and (lead to the) accumulation of cellular garbage, which will eventually cause neurodegeneration and metabolic diseases.”
With the help of Richard Hume, a U-M professor of molecular, cellular, and development biology, Xu first determined the exact acidity of the lysosome’s lumen. From there, the researchers combed through a list of proteins associated with the lysosome but had not yet been well described.
According to Hume, for protons to get into lysosomes, there’s a specific membrane transporter called the V-ATPase that loads protons, however, it was unclear how to get the pH to 4.6 and not let it become more acidic.
“We were interested that if there was the V-ATPase to pump a proton in, lysosomes must have an ion channel protein to release the proton when the proton level was too high inside the lysosome,” says Xu.
The Xu lab previously developed a specialized technique called the lysosome patch-clamp, to allow researchers to selectively “turn on” certain ion channels to better understand their function. The researchers have used this technique to find channels for other ions: calcium, sodium, potassium, chloride, iron, and now protons.
U-M postdoctoral researchers and co-authors Meiqin Hu and Ping Li made a list of all the genes that encode membrane proteins known to be present in lysosomes, and followed up with an overexpression screening assay to see if lysosome’s membrane becomes permeable to certain protons. This suggests that one of the screened candidates was responsible for proton flow in the lysosome.
The researchers then zeroed in on the trans-membrane TMEM 175, which gave dramatically enhanced proton permeability in the screening assay, Hume says.
“That got Hu, Xu, and the rest of the lab really excited, because that protein was not totally unknown,” Hume says. “Importantly, it is one of the highest vulnerability genes for mutations that cause Parkinson’s disease.”
Previous theories about how the protein was a risk factor for Parkinson’s centered around examining its function as a potassium channel.
“Quite honestly, those explanations didn’t make sense because it is hard to rationalize (how) a mechanism such as changing potassium flux across the lysosome should have led to Parkinson’s-like deficits,” says Hume. “But as soon as one realized that TMEM175 was probably a proton channel, then the rationale for how a mutation of that protein could cause Parkinson’s seemed pretty obvious.”
The researchers cross-checked their work by Hu and doctoral student Ce Wang using proton imaging with protein fluorescent indicators to measure the acidity of the lysosomes when they knocked out TMEM175.
They also used mice whose ability to produce the protein TMEM175 was eliminated, meaning they weren’t making that transporter protein channel in their lysosomes. The team showed alpha-synuclein accumulation in the cells of the research mice and alpha-synuclein accumulation is known to be toxic in patients with Parkinson’s disease.
In both cases, cells showed a decrease in the enzymatic activity that breaks down cellular debris, including alpha-synuclein aggregates, indicating that TMEM175 was responsible for regulating cellular acidity and degradation.
“In the end, we are very confident that this is the protein that’s controlling the proton leak in the lysosome,” says Xu. “This paper is exciting because mutations in this protein happen to be high risk for Parkinson’s disease.”