Calcium-induced exocytosis from actomyosin-driven, motile varicosities formed by dynamic clusters of organelles
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Varicosities are ubiquitous neuronal structures that appear as local swellings along neurites of invertebrate and vertebrate neurons. Surprisingly little is known about their cell biology. We use here cultured Aplysia neurons and demonstrate that varicosities are motile compartments that contain large clusters of organelles. The content of varicosities propagate along neurites within the plasma membrane “sleeve”, split and merge, or wobble in place. Confocal imaging, retrospective immunolabeling, electron microscopy and pharmacological perturbations reveal that the motility of the varicosities’ organelle content occurs in concert with an actin scaffold and is generated by actomyosin motors. Despite the motility of these organelle clusters within the cytoplasm along the neurites, elevation of the free intracellular calcium concentration within varicosities by trains of action potentials induces exocytosis followed by membrane retrieval. Our observations demonstrate that varicosities formed in the absence of postsynaptic cells behave as “ready to go” prefabricated presynaptic terminals. We suggest that the varicosities’ motility serves to increase the probability of encountering a postsynaptic cell and to rapidly form a functional synapse.
KeywordsSynaptic Vesicle Growth Cone Actin Network Dense Core Vesicle Blebbistatin
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(Fig. 1A). The motile behavior of VRs formed by a cultured metacerebral neuron. Frames taken every 4 minutes for a period of 40 minutes. Display rate: 6 frames per second (f.p.s).
A translocation of a VR containing RH237 labeled organelles enwrapped by actin. Left- transmitted image. Red- RH237 labeled organelles. Green- EGFP-actin. rightmerged image. Frames taken every 60 seconds. Frame rate: 5 f.p.s.
(Fig. 6). A cultured neuron expressing kaede-actin. A single VR was photo switched. Following the switch, the VR split, contributing a part of its actin to the new formed VR. Left- transmitted image. Green -actin kaede before switch. Red- actin kaede after switch. Right- merged image. Frames taken every 180 seconds. Frame rate: 4 f.p.s.
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