live-imaging-synaptic-vesicle-recycling-neuromuscular-junction

Live Imaging of Synaptic Vesicle Recycling in the Neuromuscular Junction of Dissected Larval Zebrafish

Neuronal communication is mediated by synaptic transmission, which depends primarily on the release of neurotransmitters stored in synaptic vesicles (SVs) ...

Osaka Medical and Pharmaceutical University

Newly generated synaptic vesicles (SVs) are re-acidified by the activity of the vacuolar-type H-ATPases. Since H gradient across SV membrane drives neurotransmitter uptake into SVs, precise measurements of steady-state vesicular pH and dynamics of re-acidification process will provide important information concerning the H-driven neurotransmitter uptake. Indeed, we recently demonstrated distinct features of steady state and dynamics of vesicular pH between glutamatergic vesicles and GABAergic vesicles in cultured hippocampal neurons. In this article, we focus on an experimental protocol and setup required to determine steady-state luminal pH of SVs in living neurons. This protocol is composed of efficient expression of a pH-sensitive fluorescent protein in the lumen of SVs in cultured neurons, and recordings of its fluorescence changes under a conventional fluorescent microscope during local applications of acidic buffer and ionophores-containing solution at a given pH. The method described here can be easily applied for measuring luminal pH of different types of secretory organelles and other acidic organelles such as lysosomes and endosomes in cultured cell preparations.

Quantitative Analysis of Presynaptic Vesicle Luminal pH in Cultured Neurons.

Emerging evidence shows that spontaneous synaptic transmission plays crucial roles on neuronal functions through presynaptic molecular mechanisms distinct from that of action potential (AP)-evoked transmission. However, whether the synaptic vesicle (SV) population undergoing the two forms of transmission is segregated remains controversial due in part to the conflicting results observed in cultured neurons. Here we address this issue in intact neuromuscular synapses using transgenic zebrafish larvae expressing two different indicators targeted in the SVs: a pH-sensitive fluorescent protein, pHluorin, and a tag protein, HaloTag. By establishing a quantitative measure of recycled SV fractions, we found that ∼85% of SVs were mobilized by high-frequency AP firings. In contrast, spontaneously recycling SVs were mobilized only from <8% of SVs with a time constant of 45 min at 25°C, although prolonged AP inhibition mobilized an additional population with a delayed onset. The mobilization of the early-onset population was less temperature-sensitive and resistant to tetanus toxin, whereas that of the late-onset population was more sensitive to temperature and was inhibited by tetanus toxin, indicating that prolonged AP inhibition activated a distinct molecular machinery for spontaneous SV fusion. Therefore, the early-onset population limited to <8% was likely the only source of spontaneous release that occurred physiologically. We further showed that this limited population was independent from those reluctant to fuse during AP firing and was used in both the hypertonic stimulation and the immediate phase of AP-evoked releases, thereby matching the characteristics of the readily releasable pool. Synaptic vesicles (SVs) are divided into functionally distinct pools depending on how they respond to action potential (AP) firing. The origin of SVs used for spontaneous fusion remains enigmatic despite intensive studies in cultured preparations. We addressed this question in intact neuromuscular synapses and provided two findings. First, prolonged AP inhibition activated a distinct population of fusion, which needs to be distinguished from genuine spontaneous fusion arising from a highly limited fraction. Second, the limited fraction observed early in the AP inhibition period exhibited the characteristics of readily releasable pool in the subsequent round of stimulation. Our study revealed that the origin of spontaneous SV fusion is restricted to the readily releasable pool among the SV pools involved in AP-evoked fusion.

Spontaneously Recycling Synaptic Vesicles Constitute Readily Releasable Vesicles in Intact Neuromuscular Synapses.

A line of studies in the 1960s-1980s suggested that muscle relaxants do not work uniformly on all skeletal muscles, though its mechanism has not been clarified. We showed here that a classical non-depolarizing muscle relaxant pancuronium inhibits fast muscle fibers at lower concentration compared to slow muscle fibers in zebrafish. The difference of effective concentration was observed in locomotion caused by tactile stimulation as well as in synaptic currents of the neuromuscular junction induced by motor neuron excitation. We further showed that this difference arises from the different composition of acetylcholine receptors between slow and fast muscle fibers in the neuromuscular junction of zebrafish. It will be interesting to examine the difference of subunit composition and sensitivity to muscle relaxants in other species.

Article	Year
Quantitative Analysis of Presynaptic Vesicle Luminal pH in Cultured Neurons. Methods in molecular biology (Clifton, N.J.)\| PubMed ID: 35099790	2022
Spontaneously Recycling Synaptic Vesicles Constitute Readily Releasable Vesicles in Intact Neuromuscular Synapses. The Journal of neuroscience : the official journal of the Society for Neuroscience\| PubMed ID: 35332083	2022
Receptor subunit compositions underly distinct potencies of a muscle relaxant in fast and slow muscle fibers. Frontiers in physiology\| PubMed ID: 36304584	2022

Yoshihiro Egashira

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