The Bruchpilot cytomatrix determines the size of the readily releasable pool of synaptic vesicles

Abstract

Two Bruchpilot isoforms create a stereotypic arrangement of the cytomatrix that defines the size of the readily releasable pool of synaptic vesicles.

Numbers of presynaptic Ca 2+ channel clusters match those of functionally defined vesicular docking sites in single central synapses

“…Based on the present finding that N D and N C are equal, we propose that, at PF-MLI synapses, either 0 or 1 fusion-competent SV is docked per VGCC cluster. This model is similar to the proposed association of precisely defined docking sites with a cluster of cacophony channels at the fly neuromuscular junction (23,24) with the important difference that, in the fly, several docking sites are associated with one VGCC cluster, whereas we propose here a one-to-one stoichiometry.…”

“…Another remaining source of uncertainty concerns SV supply. Docking-site replenishment is a Ca 2+ -driven process (3) that depends on filament-like links between SVs and plasma membrane (20,24,56,57), possibly involving actin and myosin II (58)(59)(60)(61). In PF-MLI synapses, recent results indicate that an empty docking site is resupplied by a replacement SV that moves toward the docking site by a cytoskeleton-driven transition (twostep model) (29).…”

“…Interestingly, in the mouse neuromuscular junction, estimates for the number of docking sites located inside the two rows of VGCCs and for the size of the readily releasable pool of SVs coincide at two per active zone (22). In the fly neuromuscular junction, superresolution optical imaging studies indicate a circular organization with a cluster of VGCCs in the center, several SVs docked next to the VGCCs, and additional SVs linked to VGCCs by bruchpilot proteins, presumably waiting for docking (23,24). However, precise release statistics are difficult to obtain in giant synapses such as neuromuscular junction preparations, making estimates of functional docking-site numbers uncertain.…”

Transmission, Development, and Plasticity of Synapses

“…The Brp C terminus is important for SV tethering, as an allele that lacks the C-terminal 17 amino acids (brp nude ) shows an impairment in SV tethering near T-bars (Hallermann et al 2010). Two isoforms of Brp appear to be arranged in an alternating circular pattern that is important for normal T-bar structure and SV tethering (Matkovic et al 2013). Furthermore, quantitative inferences about CAZ substructure and stoichiometry were made using data from stochastic optical reconstruction microscopy (dSTORM), which can provide information on the distribution of single molecules (Ehmann et al 2014).…”

Recent applications of superresolution microscopy in neurobiology

“…Another presynaptic protein extensively studied with STED microscopy is the Drosophila melanogaster ELKS family protein Bruchpilot (BRP), which forms the electron-dense active zone cytomatrix. Following an early study where the doughnut-like structure was shown for the first time [10], two-color STED microscopy was applied, making apparent a substructure of two different isoforms appearing in an alternating array of clusters [11]. After studying isoform-specific mutants these authors proposed that the size of the Bruchpilot cytomatrix determines the size of the readily releasable pool of synaptic vesicles.…”