Figure 2.

The structure and protein distribution of mouse NMJ AZs. (A) A mouse NMJ stained with Alexa-594 α-bungarotoxin (BTX; red) to demonstrate the shape of the NMJ and an Alexa-488 conjugated antibody to identify the bassoon protein the AZs (BSN; green). Inset shows an enlargement of one part of the NMJ to make it easier to visualize the distribution of AZs (green spots). Image adapted from [5]. (B) A freeze-fracture replica of an AZ from a mouse NMJ. The hypothesized locations of synaptic vesicles are superimposed as white circles. Scale bar = 50 nm. Adapted from [10,36]. (C) Diagram of a single AZ from a mouse NMJ based on electron microscope tomography data [36]. Diagram shows docked synaptic vesicles (gray spheres), along with AZ structures termed “pegs” (orange), “beams” (purple), and “ribs” (green). (D) The distribution of the AZ proteins bassoon (green) and P/Q-type VGCCs (magenta) at the mouse NMJ as revealed by STED super-resolution microscopy. (E) The distribution of the AZ proteins bassoon (green) and piccolo (magenta) at the mouse NMJ as revealed by STED super resolution microscopy. (F) A combined proposed overlay of all three AZ proteins (Bassoon, Piccolo, and P/Q-type VGCCs) onto the AZ fine structure based on STED super-resolution imaging. These data lead to the hypothesis that the “pegs” identified in electron microscopy tomography models from panel C represent P/Q-type VGCCs (orange), the “ribs” represent Bassoon (green), and the “beams” represent Piccolo (purple). C-F are adapted from [39] (G) The distribution of quantal content values determined from a population of mouse NMJs. Inset shows a sample miniature endplate current (mEPC; left) and a sample AP-triggered endplate current (EPC; right). (H) The distribution of AZ numbers counted from a population of mouse NMJs. Adapted from [5].