More than 30 mutations in the muscle acetylcholine (ACh) receptor are known to cause congenital myasthenic syndromes (CMS), rare genetic disorders that impair communication between nerves and muscles, leading to muscle weakness, fatigue, and in severe cases, respiratory failure. But the structural diversity of these mutations has made it difficult to develop effective treatments. In a study published July 1 in Nature, researchers at UC San Diego and the University of Minnesota determined 11 cryo-electron microscopy structures of mutant ACh receptors, revealing two fundamentally different disease mechanisms and identifying drugs that could correct each type.
The work represents the most comprehensive structural analysis of the muscle ACh receptor in disease states to date, and it points toward a potential repurposing of the antidepressant reboxetine for one class of CMS.
Two Classes, Two Mechanisms
Congenital myasthenic syndromes are broadly divided into two types based on how the mutations affect the receptor’s channel-opening behavior. Fast-channel CMS mutations impair the receptor’s ability to open in response to acetylcholine, reducing the strength of the signal from nerve to muscle. Slow-channel CMS mutations do the opposite, they keep the channel open too long, causing excitotoxicity and progressive muscle damage.
The researchers solved cryo-EM structures of wild-type and mutant receptors in multiple functional states, capturing both fast-channel mutants (εP121L, αV285I, αV132L) and slow-channel mutants (εL269F, εT264P, βV266M, αV249F) with and without bound drugs. The differences between the two classes were stark.
In fast-channel mutants, the coupling between the receptor’s extracellular acetylcholine-binding domain and its transmembrane pore is broken. The agonist binds, but the signal that the binding site is occupied fails to propagate to the channel gate. “The mutation disrupts key interactions that normally link extracellular domain movement to M2 helix opening,” the authors note, describing a decoupling that leaves the pore stubbornly closed even in the presence of acetylcholine.
Slow-channel mutants, by contrast, show a pore that is constitutively widened, stuck open. The εL269F mutation, for example, physically pushes the M2 helices outward. Others like εT264P twist the helix, while αV249F induces a contractive movement in the opposite direction. The net effect is the same: a channel that stays open too long, letting a flood of cations into the muscle cell.
Correcting the Fast Channel
The team identified two positive allosteric modulators, compounds XG-590 and EC-216, that restore gating in certain fast-channel mutants. Both compounds bind to a previously unknown cryptic allosteric pocket in the receptor’s transmembrane domain, detectable only in the mutant structures. The binding site sits at the interface between β and ε subunits, a region that is structurally silent in the wild-type receptor.
The correction is mutation-specific: not every fast-channel mutant responds to these modulators. The εP121L mutant, for example, shows significant recovery of channel opening probability and burst duration in single-channel recordings, while other mutants respond poorly or not at all. This mutation-specificity means that any future treatment for fast-channel CMS will require genotyping and tailored pharmacology, a precision-medicine approach for a disease that affects perhaps a few thousand people worldwide.
Reboxetine for the Slow Channel
For slow-channel CMS, the researchers tested three drugs: quinidine, fluoxetine, and reboxetine. All three are pore blockers, but reboxetine stood out. The antidepressant, which is already approved for major depressive disorder in several countries, selectively blocks desensitized receptors, those that have entered a long-lived inactive state, in a mutation-independent fashion.
Reboxetine’s mechanism is unusual. The cryo-EM structures show the drug bound within the channel pore at multiple positions simultaneously, an upper binding site, a lower site, and what the authors describe as a “lower 2” site, suggesting that reboxetine molecules stack within the pore like plugs in a pipe. This multi-site occupancy may explain why the drug works regardless of which specific slow-channel mutation is present: rather than correcting a specific structural defect, it simply blocks the overactive pore.
“The discovery that reboxetine works across multiple slow-channel mutations suggests repurposing potential for this drug,” the researchers write. Quinidine and fluoxetine also block the pore, but with different binding positions and less selectivity for the desensitized state.
From Structure to Therapy
The study’s most immediate clinical implication is the reboxetine finding. Because slow-channel CMS mutations are rare, the cumulative prevalence of all CMS forms is estimated at 1 in 200,000 births, developing a new drug specifically for these patients is commercially unviable. Repurposing an existing, off-patent antidepressant dramatically lowers the barrier to clinical use.
For fast-channel CMS, the path is longer. The mutation-specific nature of the allosteric correctors means that each genetic variant needs its own structural characterization and drug optimization. However, the discovery of the cryptic allosteric pocket itself, invisible in wild-type structures and only revealed by cryo-EM of the mutants, opens a new drug target that was previously unknown in the muscle ACh receptor.
The researchers note that all structures and associated cryo-EM maps have been deposited in the Protein Data Bank and Electron Microscopy Data Bank, with accession numbers (PDB 9YE6–9YEI and associated EMDB entries) provided for open access by the scientific community.
Disclosure: Based on a peer-reviewed paper in Nature, published July 1, 2026. DOI: 10.1038/s41586-026-10706-1. Senior author Ryan E. Hibbs, UC San Diego.