Figure 7(A) Single Particle Tracking using PALM (SPT-PALM) was performed on α3-NKA-Dendra expressing primary neurons. A representative image shows an epifluorescence image and trajectories (strands of different colors) of minimum 5 points in length. Scale bars: 1 µm in panel (A) (B, C) Extraction of diffusion coefficient using mean squared displacement (MSD) approach using minimum 5-point trajectory length. Spine and shaft were visually identified. An upward shift of MSD curve, indicating reduced confinement was observed following exposure to Fib-Tau (0.36nM, 60 min) (B). As expected, an increased diffusion coefficient (i.e. increased mobility) (C) of α3-NKA is observed in dendritic shaft following exposure to Fib-Tau. Mann-Whitney-test, n=21 cells, obtained from 4 independent experiments; Each dot represents average diffusion coefficient of all trajectories value per cell. Data information: *p<0.05; ns= not significant. Extraction of the number of diffusive states of α3-NKA and the transition rates between these diffusive states using thousands of short (minimum 2-point) trajectories using Hidden Markov Model Two diffusive states could be derived for α3-NKA with confidence (termed Bound / slow-diffusing or Free / fast-diffusing) (D) between which α3-NKA can transition. The characteristic parameters for neurons exposed or not to Fib-Tau- are shown (D). The proportion of single molecules transitioning to bound state decreased (from 71.4% to 57.1%) following neuron exposure to Fib-Tau. Extraction of the number of diffusive states of α3-NKA and the transition rates between these diffusive states using thousands of short (minimum 2-point) trajectories using Hidden Markov Model The occupancy time of α3-NKA in bound-state also decreased whereas that in free-state increased upon neuron exposure to Fib-Tau (E). Box-plot represents: median, interquartile range and entire distribution, Mann-Whitney-test on averaged value per movie from 4 experiments (n=21 cells). Data information: *p<0.05; ns= not significant. Extraction of the number of diffusive states of α3-NKA and the transition rates between these diffusive states using thousands of short (minimum 2-point) trajectories using Hidden Markov Model Dwell-time of α3-NKA is also reduced in bound state indicating destabilization of native α3-NKA-complexes following interaction with Fib-Tau (F). Box-plot represents: median, interquartile range and entire distribution, Mann-Whitney-test on averaged value per movie from 4 experiments (n=21 cells). Data information: *p<0.05; ns= not significant. (G-H) Cell-surface biotinylation on primary neurons exposed or not to Fib-Tau (1.8nM, 60 min). The amount of α-NKA at cell surface decreased upon exposure of neurons to Fib-Tau (G). Quantification was performed by normalizing the western blot band intensity with input. Paired-t-test, 3 independent experiments. Data information: *p<0.05; ns= not significant. (I) Paraformaldehyde fixation α3-NKA-pHluorin (green, top row) reveals endocytic spots otherwise not visible in live neurons (green, bottom row). Many Fib-Tau-ATTO647 (red) clusters overlapped (arrow) with α3-NKA-pHluorin spots (exposure, 0.36nM, 60 min). Scale bars 2µm (J) The majority of the overlapping Fib-Tau-ATTO647 clusters (red) and α3-NKA-pHluorin spots (green) localized on top of Clathrin-mRFP (blue) puncta. Scale bars: 2µm