Annette Denker

Publications

• 9.43

  Impact points

  The reserve pool of synaptic vesicles acts as a buffer for proteins involved in synaptic vesicle recycling.

  Annette Denker, Katharina Kröhnert, Johanna Bückers, Erwin Neher, Silvio O Rizzoli

  Proceedings of the National Academy of Sciences of the United States of America. 09/2011; 108(41):17183-8.

  Presynaptic nerve terminals contain between several hundred vesicles (for example in small CNS synapses) and several tens of thousands (as in neuromuscular junctions). Although it has long been assumed that such high numbers of vesicles are required to sustain neurotransmission during conditions of ... [more] Presynaptic nerve terminals contain between several hundred vesicles (for example in small CNS synapses) and several tens of thousands (as in neuromuscular junctions). Although it has long been assumed that such high numbers of vesicles are required to sustain neurotransmission during conditions of high demand, we found that activity in vivo requires the recycling of only a few percent of the vesicles. However, the maintenance of large amounts of reserve vesicles in many evolutionarily distinct species suggests that they are relevant for synaptic function. We suggest here that these vesicles constitute buffers for soluble accessory proteins involved in vesicle recycling, preventing their loss into the axon. Supporting this hypothesis, we found that vesicle clusters contain a large variety of proteins needed for vesicle recycling, but without an obvious function within the clusters. Disrupting the clusters by application of black widow spider venom resulted in the diffusion of numerous soluble proteins into the axons. Prolonged stimulation and ionomycin application had a similar effect, suggesting that calcium influx causes the unbinding of soluble proteins from vesicles. Confirming this hypothesis, we found that isolated synaptic vesicles in vitro sequestered soluble proteins from the cytosol in a process that was inhibited by calcium addition. We conclude that the reserve vesicles support neurotransmission indirectly, ensuring that soluble recycling proteins are delivered upon demand during synaptic activity.
• 9.43

  Impact points

  A small pool of vesicles maintains synaptic activity in vivo.

  Annette Denker, Ioanna Bethani, Katharina Kröhnert, Christoph Körber, Heinz Horstmann, Benjamin G Wilhelm, Sina V Barysch, Thomas Kuner, Erwin Neher, Silvio O Rizzoli

  Proceedings of the National Academy of Sciences of the United States of America. 09/2011; 108(41):17177-82.

  Chemical synapses contain substantial numbers of neurotransmitter-filled synaptic vesicles, ranging from approximately 100 to many thousands. The vesicles fuse with the plasma membrane to release neurotransmitter and are subsequently reformed and recycled. Stimulation of synapses in vitro generally ... [more] Chemical synapses contain substantial numbers of neurotransmitter-filled synaptic vesicles, ranging from approximately 100 to many thousands. The vesicles fuse with the plasma membrane to release neurotransmitter and are subsequently reformed and recycled. Stimulation of synapses in vitro generally causes the majority of the synaptic vesicles to release neurotransmitter, leading to the assumption that synapses contain numerous vesicles to sustain transmission during high activity. We tested this assumption by an approach we termed cellular ethology, monitoring vesicle function in behaving animals (10 animal models, nematodes to mammals). Using FM dye photooxidation, pHluorin imaging, and HRP uptake we found that only approximately 1-5% of the vesicles recycled over several hours, in both CNS synapses and neuromuscular junctions. These vesicles recycle repeatedly, intermixing slowly (over hours) with the reserve vesicles. The latter can eventually release when recycling is inhibited in vivo but do not seem to participate under normal activity. Vesicle recycling increased only to ≈ 5% in animals subjected to an extreme stress situation (frog predation on locusts). Synapsin, a molecule binding both vesicles and the cytoskeleton, may be a marker for the reserve vesicles: the proportion of vesicles recycling in vivo increased to 30% in synapsin-null Drosophila. We conclude that synapses do not require numerous reserve vesicles to sustain neurotransmitter release and thus may use them for other purposes, examined in the accompanying paper.

• Synaptic vesicle pools: an update.

  Annette Denker, Silvio O Rizzoli

  Frontiers in synaptic neuroscience. 01/2010; 2:135.

  During the last few decades synaptic vesicles have been assigned to a variety of functional and morphological classes or "pools". We have argued in the past (Rizzoli and Betz, 2005) that synaptic activity in several preparations is accounted for by the function of three vesicle pools: the ... [more] During the last few decades synaptic vesicles have been assigned to a variety of functional and morphological classes or "pools". We have argued in the past (Rizzoli and Betz, 2005) that synaptic activity in several preparations is accounted for by the function of three vesicle pools: the readily releasable pool (docked at active zones and ready to go upon stimulation), the recycling pool (scattered throughout the nerve terminals and recycling upon moderate stimulation), and finally the reserve pool (occupying most of the vesicle clusters and only recycling upon strong stimulation). We discuss here the advancements in the vesicle pool field which took place in the ensuing years, focusing on the behavior of different pools under both strong stimulation and physiological activity. Several new findings have enhanced the three-pool model, with, for example, the disparity between recycling and reserve vesicles being underlined by the observation that the former are mobile, while the latter are "fixed". Finally, a number of altogether new concepts have also evolved such as the current controversy on the identity of the spontaneously recycling vesicle pool.
• 4.76

  Impact points

  Revisiting synaptic vesicle pool localization in the Drosophila neuromuscular junction.

  Annette Denker, Katharina Kröhnert, Silvio O Rizzoli

  The Journal of physiology. 05/2009;

  It has been recognized for a few decades that the synaptic vesicles are organized in distinct populations (or 'pools') with different functional properties. In most preparations investigated to date, the vesicles segregate in the recycling pool, which maintains synaptic release upon moderate... [more] It has been recognized for a few decades that the synaptic vesicles are organized in distinct populations (or 'pools') with different functional properties. In most preparations investigated to date, the vesicles segregate in the recycling pool, which maintains synaptic release upon moderate activity, and the reserve pool, which is called into action only upon harsh (often unphysiological) stimulation. A major question in the field is whether the pools consist of biochemically different vesicles, with different intrinsic properties, or whether the pool tag is simply a spatial one. The second hypothesis predicts that the recycling vesicles are those found close to the release sites (active zones), with the reserve pool being the population of vesicles found far from the active zones. In many preparations (frog neuromuscular junction (NMJ), hippocampal synapses, the calyx of Held, or bipolar goldfish synapses) the recycling vesicles mix thoroughly with the reserve vesicles. The only major preparation for which the described spatial segregation is still proposed is the larval Drosophila NMJ - albeit based only on light microscopy experiments. We have tested this hypothesis using the photoconversion technique, which allowed us to transform the fluorescent dye used previously to investigate this preparation into a dense precipitate, visible in electron microscopy. The higher resolution of electron microscopy allowed us to demonstrate that the two pools are intermixed, with no clear spatial segregation. This finding completes a picture of synaptic recycling in which it is biochemical tags, rather than spatial location, which determine the functional properties of the vesicle.