Translational motion of neurotransmitter receptors is key for determining receptor number at the synapse and hence, synaptic efficacy. We combine live-cell STORM superresolution microscopy of nicotinic acetylcholine receptor (nAChR) with single-particle tracking, mean-squared displacement (MSD), turning angle, ergodicity, and clustering analyses to characterize the lateral motion of individual molecules and their collective behaviour. nAChR diffusion is highly heterogeneous: subdiffusive, Brownian and, less frequently, superdiffusive. At the single-track level, free walks are transiently interrupted by ms-long confinement sojourns occurring in nanodomains of ~36 nm radius. Cholesterol modulates the time and the area spent in confinement. Turning angle analysis reveals anticorrelated steps with time-lag dependence, in good agreement with the permeable fence model. At the ensemble level, nanocluster assembly occurs in second-long bursts separated by periods of cluster disassembly. Thus, millisecond-long confinement sojourns and second-long reversible nanoclustering with similar cholesterol sensitivities affect all trajectories; the proportion of the two regimes determines the resulting macroscopic motional mode and breadth of heterogeneity in the ensemble population.

Synaptic strength depends on the number of cell‐surface neurotransmitter receptors in dynamic equilibrium with intracellular pools. Dysregulation of this homeostatic balance occurs, for example in myasthenia gravis, an autoimmune disease characterized by a decrease in the number of postsynaptic nicotinic acetylcholine receptors (nAChRs). Monoclonal antibody mAb35 mimics this effect. Here we use STORM nanoscopy to characterize the individual and ensemble dynamics of monoclonal antibody‐crosslinked receptors in the clonal cell line CHO‐K1/A5, which robustly expresses adult muscle‐type nAChRs. Antibody labeling of live cells results in 80% receptor immobilization. The remaining mobile fraction exhibits a heterogeneous combination of Brownian and anomalous diffusion. Single‐molecule trajectories exhibit a two‐state switching behavior between free Brownian walks and anticorrelated walks within confinement areas. The latter act as permeable fences (~34 nm radius, ~400 ms lifetime). Dynamic clustering, trapping, and immobilization also occur in larger nanocluster zones (120–180 nm radius) with longer lifetimes (11 ± 1 s), in a strongly cholesterol‐sensitive manner. Cholesterol depletion increases the size of the clustering phenomenon; cholesterol enrichment has the opposite effect. The disclosed high proportion of monoclonal antibody‐crosslinked immobile receptors, together with their anomalous, cholesterol‐sensitive diffusion and clustering, provides new insights into the antibody‐enhanced antigenic modulation that leads to physiopathological internalization and degradation of receptors in myasthenia.

Short title: Antibody-acetylcholine receptor diffusion and myastheniaThe abbreviations used are: BTX, α-bungarotoxin; mAb, monoclonal antibody; nAChR, nicotinic acetylcholine receptor; CDx, methyl-β-cyclodextrin; CDx-Chol, cholesterol-methyl-β-cyclodextrin complex; MSD, mean-squared displacement; SPT, single-particle tracking. Abstract Synaptic strength depends on the number of cell-surface neurotransmitter receptors in dynamic equilibrium with intracellular pools. Dysregulation of this homeostatic balance occurs e.g. in myasthenia gravis, an autoimmune disease characterized by a decrease in the number of postsynaptic nicotinic acetylcholine receptors (nAChRs). Monoclonal antibody mAb35 mimics this effect. Here we use STORM nanoscopy to characterize the individual and ensemble dynamics of mAb35-crosslinked receptors in the clonal cell line CHO-K1/A5, which robustly expresses adult muscle-type nAChRs. Antibody labeling of live cells results in 80% receptor immobilization. The remaining mobile fraction exhibits a heterogeneous combination of Brownian and anomalous diffusion. Single-molecule trajectories exhibit a two-state switching behavior between free Brownian walks and anticorrelated walks within confinement areas. The latter act as permeable fences (~34 nm radius, ~400 ms lifetime). Dynamic clustering, trapping and immobilization also occur in larger nanocluster zones (120-180 nm radius) with longer lifetimes (11 ± 1 s), in a strongly cholesterol-sensitive manner. Cholesterol depletion increases the size and average duration of the clustering phenomenon; cholesterol enrichment has the opposite effect. The disclosed high proportion of mAb35-crosslinked immobile receptors, together with their anomalous, cholesterol-sensitive diffusion and clustering, provides new insights into the antibody-enhanced antigenic modulation that leads to physiopathological internalization and degradation of receptors in myasthenia.A preliminary version of this work has appeared in the biorXiv repository: https://www.biorxiv.org/content/10.1101/744664v1. The study was not pre-registered.

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