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
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.
Available from: Victor Faundez
- "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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ABSTRACT: The release and uptake of neurotransmitters by synaptic vesicles is a tightly controlled process that occurs in response to diverse stimuli at morphologically disparate synapses. To meet these architectural and functional synaptic demands, it follows that there should be diversity in the mechanisms that control their secretion and retrieval and possibly in the composition of synaptic vesicles within the same terminal. Here we pay particular attention to areas where such diversity is generated, such as the variance in exocytosis/endocytosis coupling, SNAREs defining functionally diverse synaptic vesicle populations and the adaptor-dependent sorting machineries capable of generating vesicle diversity. We argue that there are various synaptic vesicle recycling pathways at any given synapse and discuss several lines of evidence that support the role of the endosome in synaptic vesicle recycling.
Available from: PubMed Central
- "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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ABSTRACT: Transcription factors are essential for regulating neuronal microtubules (MTs) during development and after axon damage. In this paper, we identify a novel neuronal function for Drosophila melanogaster FoxO in limiting MT stability at the neuromuscular junction (NMJ). foxO loss-of-function NMJs displayed augmented MT stability. In contrast, motor neuronal overexpression of wild-type FoxO moderately destabilized MTs, whereas overexpression of constitutively nuclear FoxO severely destabilized MTs. Thus, FoxO negatively regulates synaptic MT stability. FoxO family members are well-established components of stress-activated feedback loops. We hypothesized that FoxO might also be regulated by cytoskeletal stress because it was well situated to shape neuronal MT organization after cytoskeletal damage. Indeed, levels of neuronal FoxO were strongly reduced after acute pharmacological MT disruption as well as sustained genetic disruption of the neuronal cytoskeleton. This decrease was independent of the dual leucine zipper kinase-Wallenda pathway and required function of Akt kinase. We present a model wherein FoxO degradation is a component of a stabilizing, protective response to cytoskeletal insult.
Available from: Samuel Andrew Hires
- "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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ABSTRACT: Synaptic transmission involves the calcium dependent release of neurotransmitter from synaptic vesicles. Genetically encoded optical probes emitting different wavelengths of fluorescent light in response to neuronal activity offer a powerful approach to understand the spatial and temporal relationship of calcium dynamics to the release of neurotransmitter in defined neuronal populations. To simultaneously image synaptic vesicle recycling and changes in cytosolic calcium, we developed a red-shifted reporter of vesicle recycling based on a vesicular glutamate transporter, VGLUT1-mOrange2 (VGLUT1-mOr2), and a presynaptically localized green calcium indicator, synaptophysin-GCaMP3 (SyGCaMP3) with a large dynamic range. The fluorescence of VGLUT1-mOr2 is quenched by the low pH of synaptic vesicles. Exocytosis upon electrical stimulation exposes the luminal mOr2 to the neutral extracellular pH and relieves fluorescence quenching. Reacidification of the vesicle upon endocytosis again reduces fluorescence intensity. Changes in fluorescence intensity thus monitor synaptic vesicle exo- and endocytosis, as demonstrated previously for the green VGLUT1-pHluorin. To monitor changes in calcium, we fused the synaptic vesicle protein synaptophysin to the recently improved calcium indicator GCaMP3. SyGCaMP3 is targeted to presynaptic varicosities, and exhibits changes in fluorescence in response to electrical stimulation consistent with changes in calcium concentration. Using real time imaging of both reporters expressed in the same synapses, we determine the time course of changes in VGLUT1 recycling in relation to changes in presynaptic calcium concentration. Inhibition of P/Q- and N-type calcium channels reduces calcium levels, as well as the rate of synaptic vesicle exocytosis and the fraction of vesicles released.
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