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Synapsin is an evolutionarily conserved presynaptic phosphoprotein. It is encoded by only one gene in the Drosophila genome and is expressed throughout the nervous system. It regulates the balance between reserve and releasable vesicles, is required to maintain transmission upon heavy demand, and is essential for proper memory function at the behavioral level. Task-relevant sensorimotor functions, however, remain intact in the absence of Synapsin. Using an odor-sugar reward associative learning paradigm in larval Drosophila, we show that memory scores in mutants lacking Synapsin (syn\(^{97}\)) are lower than in wild-type animals only when more salient, higher concentrations of odor or of the sugar reward are used. Furthermore, we show that Synapsin is selectively required for larval short-term memory. Thus, without Synapsin Drosophila larvae can learn and remember, but Synapsin is required to form memories that match in strength to event salience-in particular to a high saliency of odors, of rewards, or the salient recency of an event. We further show that the residual memory scores upon a lack of Synapsin are not further decreased by an additional lack of the Sap47 protein. In combination with mass spectrometry data showing an up-regulated phosphorylation of Synapsin in the larval nervous system upon a lack of Sap47, this is suggestive of a functional interdependence of Synapsin and Sap47.
Learning and memory is considered to require synaptic plasticity at presynaptic specializations of neurons. Kenyon cells are the intrinsic neurons of the primary olfactory learning center in the brain of arthropods – the mushroom body neuropils. An olfactory mushroom body memory trace is supposed to be located at the presynapses of Kenyon cells. In the calyx, a sub-compartment of the mushroom bodies, Kenyon cell dendrites receive olfactory input provided via projection neurons. Their output synapses, however, were thought to reside exclusively along their axonal projections outside the calyx, in the mushroom body lobes. By means of high-resolution imaging and with novel transgenic tools, we showed that the calyx of the fruit fly Drosophila melanogaster also comprised Kenyon cell presynapses. At these presynapses, synaptic vesicles were present, which were capable of neurotransmitter release upon stimulation. In addition, the newly identified Kenyon cell presynapses shared similarities with most other presynapses: their active zones, the sites of vesicle fusion, contained the proteins Bruchpilot and Syd-1. These proteins are part of the cytomatrix at the active zone, a scaffold controlling synaptic vesicle endo- and exocytosis. Kenyon cell presynapses were present in γ- and α/β-type KCs but not in α/β-type Kenyon cells.
The newly identified Kenyon cell derived presynapses in the calyx are candidate sites for an olfactory associative memory trace. We hypothesize that, as in mammals, recurrent neuronal activity might operate for memory retrieval in the fly olfactory system.
Moreover, we present evidence for structural synaptic plasticity in the mushroom body calyx. This is the first demonstration of synaptic plasticity in the central nervous system of Drosophila melanogaster. The volume of the mushroom body calyx can change according to changes in the environment. Also size and numbers of microglomeruli - sub-structures of the calyx, at which projection neurons contact Kenyon cells – can change. We investigated the synapses within the microglomeruli in detail by using new transgenic tools for visualizing presynaptic active zones and postsynaptic densities. Here, we could show, by disruption of the projection neuron - Kenyon cell circuit, that synapses of microglomeruli were subject to activity-dependent synaptic plasticity. Projection neurons that could not generate action potentials compensated their functional limitation by increasing the number of active zones per microglomerulus. Moreover, they built more and enlarged microglomeruli. Our data provide clear evidence for an activity-induced, structural synaptic plasticity as well as for the activity-induced reorganization of the olfactory circuitry in the mushroom body calyx.