Actin filament organization in the AIS. (A and B) Immunogold PREM of actin filaments in DIV10 neurons. (A) The AIS coat along the main axon. Gold particles (18 nm; yellow shade) form linear arrangements (box a, enlarged in panel a) or appear as individual particles (red arrows) or small clusters. (B, inset) Axonal branch (arrowhead) emerging from the main axon (arrow) at the AIS. (B) Magnified branch junction with the AIS coat. Phalloidin immunogold is highlighted in yellow. (b1 and b2) Non-stretched (b1) and stretched (b2) regions of the AIS coat boxed in B. Selected fibrils (blue), globules (magenta), MTs (orange), neurofilaments (red), and phalloidin immunogold (yellow) are highlighted. (C) Decoration of actin filaments with myosin S1. Yellow arrows indicate filament lengths and direction of the pointed end. The boxed area is enlarged in panel c to show an S1-decorated filament with individual polar features highlighted in blue and yellow. (D) Quantification reveals unbiased orientation of actin filaments in the AIS. χ2 (1, n = 181 from 10 AISs) = 0.786; P = 0.3752. Bars: (A and C) 200 nm; (B) 0.5 µm; (a, b1, and b2) 50 nm; (c) 20 nm.
