A Slowed Classical Pathway Rather Than Kiss-and-Run Mediates Endocytosis at Synapses Lacking Synaptojanin and Endophilin

Abstract

The extent to which a "kiss-and-run" mode of endocytosis contributes to synaptic-vesicle recycling remains controversial. The only genetic evidence for kiss-and-run at the synapse comes from mutations in the genes encoding synaptojanin and endophilin, proteins that together function to uncoat vesicles in classical clathrin-mediated endocytosis. Here we have characterized the endocytosis that persists in null alleles of Drosophila synaptojanin and endophilin. In response to high-frequency stimulation, the synaptic-vesicle pool can be reversibly depleted in these mutants. Recovery from this depletion is slow and indicates the persistence of an impaired form of classical endocytosis. Steady-state exocytosis rates reveal that endocytosis saturates in mutant neuromuscular terminals at approximately 80 vesicles/s, 10%-20% of the wild-type rate. Analyses of quantal size, FM1-43 loading, and dynamin function further demonstrate that, even in the absence of synaptojanin or endophilin, vesicles undergo full fusion and re-formation. Therefore, no genetic evidence remains to indicate that synaptic vesicles undergo kiss-and-run.

Stoned B mediates sorting of integral synaptic vesicle proteins

“…Simplistically, defective synaptic release might be entirely a secondary effect of a primary impairment in vesicle pool maintenance, due to disrupted vesicle endocytosis. However, recent studies of endophilin and synaptojanin endocytic mutants suggest that only a surprisingly small number of synaptic vesicles is actually required to support normal synaptic function at basal stimulation frequencies: In both endophilin and synaptojanin mutants, basal synaptic transmission is completely normal despite the near elimination of the presynaptic vesicle population, and the loss of synaptic uptake of FM1-43 (Verstreken et al, 2002(Verstreken et al, , 2003Dickman et al, 2005). This raises the question that STNB may be involved in other processes that affect exocytosis, apart from limiting the availability of vesicles.…”

“…Similar paradigms have recently been used to unveil functional deficits in endophilin and synaptojanin mutants (Verstreken et al, 2002(Verstreken et al, , 2003Dickman et al, 2005). The applied stimulation paradigm consisted of a 1 minute baseline interval with stimulation at 0.1Hz, a 10 minute interval with high-frequency stimulation (HFS) at 10Hz to strongly challenge synapses, and a subsequent recovery interval with 0.1Hz stimulation (Fig.…”

Protein scaffolds in the coupling of synaptic exocytosis and endocytosis

“…A crucial part in synaptic vesicle endocytosis is played by the GTPase dynamin (FIG. 2), a mechanochemical enzyme recruited to endocytic sites by interaction with SH3 domain proteins 58,59 , including intersectin [60][61][62] , amphiphysin 63 , endophilin 64,65 and syndapin 66 , suggesting a tight interplay between BAR-SH3 protein-mediated membrane deformation (FIG. 2) and dynamin-catalyzed fission 58,67 .…”

Elucidation of intracellular recycling pathways leading to exocytosis of the Fc receptor, FcRn, by using multifocal plane microscopy

“…However, the illumination of only a thin section of the cell, including the plasma membrane, has the limitation that events cannot be visualized that precede exocytosis at sites deeper within the cell (11). As a consequence, there is uncertainty concerning the nature of the processes that lead to exocytic release (12)(13)(14)(15). For example, the origin of the exocytosing vesicles or tubules is typically difficult to determine, and it therefore remains a matter of controversy as to whether recycling from sorting endosomes involves direct fusion of endosomes or some vesicular/tubular intermediates (16).…”