Article

Exocytosis and Endocytosis of Synaptic Vesicles and Functional Roles of Vesicle Pools: Lessons from the Drosophila Neuromuscular Junction

Institute for Behavioral Sciences, Gunma University School of Medicine, 3-39-22 Showamachi, Maebashi 371-8511, Japan.
The Neuroscientist (Impact Factor: 6.84). 05/2005; 11(2):138-47. DOI: 10.1177/1073858404271679
Source: PubMed

ABSTRACT

To maintain synaptic transmission during intense neuronal activities, the synaptic vesicle (SV) pool at release sites is effectively replenished by recruitment of SVs from the reserve pool and/or by endocytosis. The authors have studied dynamics of SVs using a fluorescence dye, FM1-43, which is incorporated into SVs during endocytosis and released by exocytosis. Drosophila is one of the most suitable preparations for genetic and pharmacological analyses, and this provides a useful model system. The authors found at the neuromuscular junctions of Drosophila that exocytosis and endocytosis of SVs are triggered by Ca(2+) influx through distinct routes and that selective inhibition of exocytosis or endocytosis resulted in depression of synaptic transmission with a distinct time course. They identified two SV pools in a single presynaptic bouton. The exo/endo cycling pool (ECP) is loaded with FM1-43 during low-frequency stimulation and locates close to release sites in the periphery of boutons, whereas the reserve pool (RP) is loaded and unloaded only during high-frequency stimulation and resides primarily in the center of boutons. The size of ECP closely correlates with the quantal content of evoked release, suggesting that SVs in the ECP are primarily involved in synaptic transmission. SVs in the RP are recruited to synaptic transmission by a process involving the cAMP/PKA cascade during high-frequency stimulation. Cytochalasin D blocked this recruitment process, suggesting involvement of filamentous actin. Endocytosed SVs replenish the ECP during stimulation and the RP after tetanic stimulation. Replenishment of the ECP depends on Ca(2+) influx from external solutions, and that of the RP is initiated by Ca(2+) release from internal stores. Thus, SV dynamics is closely involved in modulation of synaptic efficacy and influences synaptic plasticity.

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    • "It is well established that calcium influx through voltage-dependent calcium channels (VDCCs) serves as the trigger for evoked synaptic vesicle exocytosis and neurotransmitter release (Katz and Miledi 1967; Augustine 2001; Jahn and Fasshauer 2012; Sudhof 2012). In general , calcium positively regulates the initiation, speed, and amount of vesicle endocytosis in a range of central and peripheral synapses in vertebrates and invertebrates (von Gersdorff and Matthews 1994; Gad et al. 1998; Ales et al. 1999; Neves et al. 2001; Sankaranarayanan and Ryan 2001; Kuromi and Kidokoro 2005; Wu et al. 2005, 2009; Balaji et al. 2008; Yamashita 2012; Yao et al. 2012b). For a long time, it was assumed that the calcium influx needed for synaptic vesicle endocytosis is through VDCCs on the presynaptic plasma membrane. "
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    • "Upon stimulation, FM 1-43 dye is taken up by synaptic vesicles and labels newly endocytosed vesicles within the nerve terminal. Thus, defects in synaptic labeling with FM 1-43 dye are indicative of compromised vesicle cycling (Kuromi and Kidokoro, 2005; Verstreken et al., 2008). Stimulation with 90 mM KCl caused robust labeling of synaptic boutons in wild-type larvae (Fig. 5, A and D). "
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    • "Calcium also plays a critical role in the endocytosis of synaptic vesicles (Sudhof, 2004; Kuromi and Kidokoro, 2005; Balaji et al., 2008; Yamashita et al., 2010). To measure Ca 2+ levels and endocytosis of synaptic vesicles, we stimulated cells coexpressing SyGCaMP3 and VGLUT1-2XmOr2 in the absence of bafilomycin (Figures 4A,B). "
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