Calcium-induced exocytosis from actomyosin-driven, motile varicosities formed by dynamic clusters of organelles

Abstract

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.

Tau‐Induced Traffic Jams Reflect Organelles Accumulation at Points of Microtubule Polar Mismatching

“…The ratio of endogenous Ap‐MAP and the exogenous human tau expressed by the neurons was estimated as follows: mRNA encoding human tau was microinjected into the cell body of a neuron (experiment), while a second neuron cultured in the same dish served as the a control. Forty‐eight hours later, both neurons were fixed and prepared for immunolabeling as previously described (54,55). The neurons were then labeled with tau‐5 antibody (1:500), followed by Cy‐2‐conjugated α‐mouse antibody (1:50; Jackson Immunoresearch).…”

Autaptic Excitation Elicits Persistent Activity and a Plateau Potential in a Neuron of Known Behavioral Function

“…After the initial segment, B31/B32 axons give off relatively few neurites as they innervate the I2 muscle [4]. Neurites close to the soma have many varicosities (Supplemental Data, Movie S1 and Figure S2), which are generally regarded as presynaptic release sites [13][14][15]. In many cases neurites loop back and contact themselves.…”

Synaptic Functions of Invertebrate Varicosities: What Molecular Mechanisms Lie Beneath

“…Moreover, cultured Xenopus spinal neurons, rat hippocampal neurons, and Drosophila neurons show some ability for evoked synaptic vesicle recycling along entire axonal segments, even in the absence of their targets [29, 33, 35–37, 39–41]. Morphological studies performed on Aplysia sensory neurons cultured in contact with postsynaptic neurons as well as in isolated configuration suggest that varicosities are formed either at the tips of advancing growth cones, or along neurites after their advancement, or by splitting of pre-existing varicosities [23, 24, 53, 54]. …”

“…Finally, the newly formed varicosity is further supplemented with organelles delivered along the axons by anterograde transport. Varicosities host a heterogeneous population of subcellular organelles that include clear and dense core vesicles, mitochondria, and endoplasmic reticulum [54]. Electron microscope studies revealed that the content of varicosities formed by neurons grown in the absence of postsynaptic partners ranges from organelle high-density varicosity to those that are almost free of organelles [57, 58].…”