How a Hidden Proton Channel “TMEM175” Sabotages Brain Cells in Parkinson’s

TL;DR: A newly decoded proton channel called TMEM175 lies dormant in lysosomes until acid arrives—then it opens wide and floods the cell with hydrogen ions, disrupting the delicate pH balance linked to Parkinson’s disease and other neurodegeneration.

Your cells run a 24/7 recycling system inside tiny acid baths called lysosomes — and when those baths lose their pH balance, the brain pays the price. A new study reveals that the ion channel TMEM175, long linked to Parkinson’s disease risk, has a hidden proton-gating mechanism that could be the key to understanding why.

Source: PNAS (2026) | Schulze et al.

The pH Collapse Problem

The research team, led by Tobias Schulze and colleagues at multiple European institutions, used patch-clamp electrophysiology—a gold-standard technique for measuring single-channel currents—to directly observe what TMEM175 does when lysosomes turn acidic. The finding was stark: the channel doesn’t just passively let protons through; it actively pumps them in, and does so with such ferocity that it erodes the very pH gradient the lysosome needs to survive.

This is a functional paradox. The lysosome must be acid to activate TMEM175. But once activated, TMEM175 shuts down the acidity that activated it. The researchers quantified this: in less than 90 seconds, the reversal potential (a measure of the transmembrane ion gradient) drifted toward zero, indicating the proton concentration difference had nearly vanished.

Finding the Molecular Culprit

To understand *how* TMEM175 achieved this feat, the team turned to molecular dynamics simulations—computational models that show how amino acids move and interact inside the channel pore. They tested which residues might control the pH-dependence and ion selectivity of TMEM175.

Histidine at position 57 (H57) emerged as the key. This amino acid sits on the luminal-facing side of the open channel, and its chemical behavior changes with pH. At neutral pH, H57 is uncharged; at acidic pH, it gains a positive charge. The simulations showed that protonated H57 forms salt bridges—electrostatic attractions—with nearby aspartate (D279) and glutamate (E282) residues, stabilizing the channel in a state that favors proton influx.

To test this prediction, the researchers created a mutant: H57Y (replacing histidine with tyrosine). Tyrosine cannot change its charge with pH, so it should break the pH-sensing mechanism.

The Mutation Proves the Point

The H57Y mutant delivered compelling evidence. In whole-cell recordings, H57Y cut proton permeability by a factor of four compared to wild-type TMEM175. Equally striking: potassium permeability plummeted by 15-fold. This wasn’t a minor tweak—it was a near-complete loss of function.

The researchers then moved to whole-lysosome patch-clamp recordings, a more physiologically relevant setup. Lysosomes carrying the H57Y mutation showed only slightly higher currents even during acidification, far below the sustained increases seen in wild-type lysosomes. The pH-sensing behavior was gone.

Why This Matters for Parkinson’s Disease

TMEM175 dysfunction is linked to Parkinson’s disease. Genetic variants in TMEM175 show up in large genome-wide association studies (GWAS) as risk factors for the disease. The mechanism is still being pieced together, but the broad logic is clear: if TMEM175 fails to regulate lysosomal pH properly, waste products and damaged proteins accumulate. In neurons, this junk piles up until the cell dies.

Now researchers understand one side of that story. A normal TMEM175 channel can be too efficient—it admits so many protons that it collapses the pH gradient and sabotages lysosomal function. A mutant TMEM175, especially one affecting H57, loses this destructive capacity but also loses protective capacity. The problem is balance.

The finding also clarifies what makes TMEM175 unusual. Most proton channels attempt to maintain electrochemical gradients. TMEM175 appears designed to *collapse* them—perhaps as a pressure-relief valve when lysosomes become too acidic, or to enable pH-dependent recycling pathways. But if this collapse mechanism misfires, the lysosome becomes a ghost organelle: unable to digest, unable to maintain itself, and unable to protect the cell from neurotoxic aggregates.

A Cautionary Tale in Channel Design

This study reveals something deeper about how ion channels evolved. TMEM175 is elegant: a single amino acid (H57) acts as both a pH-sensor and a gating control. When acid arrives, H57 gains a proton charge, locks into place via salt bridges, and opens the floodgates. But elegance carries risk. A tiny mutation, or a genetic variant that slightly shifts H57’s pKa (the pH at which it changes charge), could turn a protective valve into a broken dam.

For the millions of people at risk for Parkinson’s disease, these discoveries hint at new therapeutic angles. Instead of trying to block TMEM175 outright—which might cripple lysosomal function further—a more surgical approach might target H57 or the salt-bridge network around it. Drugs that stabilize the closed state, or that prevent excessive pH-gradient collapse, could restore balance.

The Bottom Line

TMEM175 is not a simple K+ channel with a hint of proton permeability. It is a sophisticated acid-sensor that, when triggered, unleashes a torrent of H+ influx potent enough to obliterate the lysosome’s acid gradient in seconds. The H57 residue acts as the molecular switch, and a single mutation can disable the entire system.

For neuroscience, this study cracks open a mechanism long suspected but never clearly shown: how a lysosomal channel linked to Parkinson’s disease actually behaves at the biophysical level. The next challenge is to translate these channel mechanics into cell biology—to understand how H57 variants lead to neurodegeneration in living brains—and ultimately into drugs that can restore balance without breaking the lysosome altogether.