Mechanical tension contributes to clustering of neurotransmitter vesicles at presynaptic terminals

Abstract

Memory and learning in animals are mediated by neurotransmitters that are released from vesicles clustered at the synapse. As a synapse is used more frequently, its neurotransmission efficiency increases, partly because of increased vesicle clustering in the presynaptic neuron. Vesicle clustering has been believed to result primarily from biochemical signaling processes that require the connectivity of the presynaptic terminal with the cell body, the central nervous system, and the postsynaptic cell. Our in vivo experiments on the embryonic Drosophila nervous system show that vesicle clustering at the neuromuscular presynaptic terminal depends on mechanical tension within the axons. Vesicle clustering vanishes upon severing the axon from the cell body, but is restored when mechanical tension is applied to the severed end of the axon. Clustering increases when intact axons are stretched mechanically by pulling the postsynaptic muscle. Using micro mechanical force sensors, we find that embryonic axons that have formed neuromuscular junctions maintain a rest tension of Ϸ1 nanonewton. If the rest tension is perturbed mechanically, axons restore the rest tension either by relaxing or by contracting over a period of Ϸ15 min. Our results suggest that neuromuscular synapses employ mechanical tension as a signal to modulate vesicle accumulation and synaptic plasticity.T he accumulation of neurotransmitter containing vesicles at the presynaptic terminal is essential for neural communication. On the arrival of an action potential at the terminal, neurotransmitters are released through exocytosis of the vesicles. The transmitters excite the postsynaptic terminal in a millisecond time frame (1). The amount of neurotransmitter release for a given action potential depends on several factors including how frequently the synapse has been used. This usage dependent plasticity is believed to be the basis of memory and learning (2). Despite a wealth of knowledge on the molecular components of the synapse (3, 4) and the modulation of the postsynaptic machinery during long-term potentiation and depression (5-7), the mechanism of vesicle accumulation at the presynaptic terminal and regulation of neurotransmission in a usage-dependent manner remains unclear (8). There is increasing experimental evidence suggesting that the mechanical microenvironment has a significant influence on a variety of cell functions including gene expression, cell growth and morphology, cytoskeletal organization, and apoptosis (9-13). Our in vivo experiments reveal that mechanical tension in axons plays a key role in vesicle accumulation.We examine the neuromuscular synapse within live embryos of Drosophila melanogaster (14) (Fig. 1A). In Drosophila, an aCC (anterior corner cell) pioneer motoneuron extends a single axon and invariably innervates its target, muscle1. A pair of aCC motoneurons occurs in every segment of the embryo, and their development is highly stereotyped. The first contact between the aCC axon and muscle1 occurs at hour 14 of embryogenesis, ...

Axon Stretch Growth: The Mechanotransduction of Neuronal Growth

“…In the second approach, forces are applied directly to the axon by pulling on the growth cone. Using glass needles, this latter approach has been used to model formation of new axon (Bernal, Pullarkat et al 2007; O'Toole, Lamoureux et al 2008;Lamoureux, Heidemann et al 2010), to identify force thresholds for elongation and retraction (Dennerll, Lamoureux et al 1989;Zheng, Lamoureux et al 1991;Lamoureux, Zheng et al 1992), and to analyze neurotransmitter clustering in axon terminals (Siechen, Yang et al 2009). A unique tissue engineering extension of this method was developed to produce large nerve constructs using an automated Axon Stretch Growth (ASG) bioreactor system (Iwata, Browne et al 2006;Pfister, Iwata et al 2006).…”

Acute and Chronic Neural Stimulation via Mechano-Sensitive Ion Channels

“…84 Siechen and co-workers found that vesicle clustering at the neuromuscular presynaptic terminals of embryonic Drosphila depended on the mechanical tension in the axons. 6 In severed axons which are slack, vesicle clustering was absent; however, vesicle clustering was restored with the application of mechanical tension to the severed end of the axon. Vesicle clustering was also observably increased when mechanical tension was applied to axons indirectly via the post-synaptic muscle.…”

“…The partial derivative of is shown in Equation 6 by knowing that during magnetic ratcheting, is kept constant and using the substitution = ̅ . The can thus be numerically represented in Equation 7.…”

“…Our next step was then to cultivate the AMB-1 (10 µL suspension) for an additional [5][6][7][8][9][10][11][12] days to allow the desired AMB-1 (higher MNPs number and metabolism) to out-compete the nondesired ones. Note that the AMB-1 were not grown in a single tube (2 mL Eppendorf) for the entire duration but were transferred to a new tube for expansion every 3 days.…”

Relax? Don't do it!—Linking presynaptic vesicle clustering with mechanical tension

“…Recent experiments by Siechen et al (2009) introduced a novel, unexpected jigsaw piece to this scene. In an innovative study, the authors demonstrated that the mechanical tension within axons is essential for effective vesicle clustering in the presynaptic terminal.…”