] Understanding the mechanisms underlying memory formation is a fundamental focus in neuroscience. As a potential molecular and cellular basis of synapse plasticity, including memory formation, we have proposed a novel hypothesis, the """"""""local feedback"""""""" model (Yoshihara et al., 2005, Science 310: 858-863). In this model, we postulate that reciprocal strengthening of presynaptic and postsynaptic signals by a positive feedback loop at single synapses keeps individual synapses potentiated, leading to eventual morphological changes and perpetually strengthened synapses. This long-lasting change could form the basis by which the brain stores memories. At neuromuscular junctions (NMJs) of Drosophila embryos, we demonstrated that stimulating the motor axon at high frequency (100 Hz) induces a large (100-fold) prolonged increase in miniature release frequency. We have termed this phenomenon High Frequency Stimulation-induced Miniature Release (HFMR). Several lines of evidence suggest that HFMR and synaptic growth require local postsynaptic retrograde signaling, mediated by a postsynaptic Ca2+ sensor, Synaptogtagmin 4 (Syt 4). A key prediction of the local feedback hypothesis is that the acute synaptic plasticity revealed by HFMR is translated into long-term structural changes. To fully test this prediction, we have begun to establish a whole- embryo culture system in which synaptic growth will be visualized in real time while stimulating motor nerves and recording from the muscles. The goal of this proposal is to test important predictions of the local feedback hypothesis, and to understand the molecular and cellular mechanism of this process. In particular, we have shown that postsynaptic Ca2+ is essential for retrograde signaling and we have evidence that Syt 4 functions as a Ca2+ sensor for the release of the retrograde signal. The goals of this project will be accomplished in three aims. (1) We will determine the signaling pathways responsible for the acute plasticity, including describing the role of postsynaptic Ca2+ and identification of the retrograde signal. (2) We will examine a postsynaptic vesicle trafficking mechanism dependent on Synaptotagmin 4 as a postsynaptic Ca2+ sensor. (3) We will elucidate the molecular mechanisms of the transition from acute plasticity to morphological changes using a genetic analysis. We anticipate that the outcome of this study will shed considerable light on basic principles of memory mechanisms, and provide a framework to address memory-related diseases such as amnesia, dementia, and Alzheimer disease.

Public Health Relevance

] We are addressing a key question of nervous system function - How do we remember? by studying synaptic physiology in a powerful model system, the fruit fly Drosophila. This project is based on the hypothesis that memory is stored in neuronal circuits in which individual connections between individual neurons (synapses) have been strengthened, and in which sequential firing of the neurons allows recall of specific events. Therefore, deciphering the mechanisms by which synaptic connections can be strengthened or weakened is key to understanding the mechanisms underlying memory. Taking advantage of a combination of synaptic physiological methods and sophisticated Drosophila genetics, we have recently proposed a novel hypothesis, the local feedback model as a potential molecular and cellular basis of memory formation (Yoshihara et al., 2005, Science 310: 858-863). In this model, we postulate that mutual intensification of synaptic strength by the presynaptic and postsynaptic neurons, via a positive feedback loop, keeps individual synapses potentiated. This leads to changes in the structure of synapses and to a perpetual increase in synaptic strength, thereby storing a memory. We are testing this working hypothesis by electrophysiology (whole-cell patch clamping, two-electrode voltage clamping, current clamping), live imaging and live manipulations (Ca2+ imaging, Ca2+ uncaging), electron microscopy, and fly genetics.

National Institute of Health (NIH)
National Institute of Mental Health (NIMH)
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Synapses, Cytoskeleton and Trafficking Study Section (SYN)
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Asanuma, Chiiko
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University of Massachusetts Medical School Worcester
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Flood, Thomas F; Gorczyca, Michael; White, Benjamin H et al. (2013) A large-scale behavioral screen to identify neurons controlling motor programs in the Drosophila brain. G3 (Bethesda) 3:1629-37
Flood, Thomas F; Iguchi, Shinya; Gorczyca, Michael et al. (2013) A single pair of interneurons commands the Drosophila feeding motor program. Nature 499:83-7
Korkut, Ceren; Li, Yihang; Koles, Kate et al. (2013) Regulation of postsynaptic retrograde signaling by presynaptic exosome release. Neuron 77:1039-46
Yoshihara, Motojiro (2012) Simultaneous recording of calcium signals from identified neurons and feeding behavior of Drosophila melanogaster. J Vis Exp :
Yoshihara, Motojiro; Guan, Zhuo; Littleton, J Troy (2010) Differential regulation of synchronous versus asynchronous neurotransmitter release by the C2 domains of synaptotagmin 1. Proc Natl Acad Sci U S A 107:14869-74