Flies lacking all synapsins are unexpectedly healthy but are impaired in complex behaviour

Abstract

Vertebrate synapsins are abundant synaptic vesicle phosphoproteins that have been proposed to fine-regulate neurotransmitter release by phosphorylation-dependent control of synaptic vesicle motility. However, the consequences of a total lack of all synapsin isoforms due to a knock-out of all three mouse synapsin genes have not yet been investigated. In Drosophila a single synapsin gene encodes several isoforms and is expressed in most synaptic terminals. Thus the targeted deletion of the synapsin gene of Drosophila eliminates the possibility of functional knock-out complementation by other isoforms. Unexpectedly, synapsin null mutant flies show no obvious defects in brain morphology, and no striking qualitative changes in behaviour are observed. Ultrastructural analysis of an identified 'model' synapse of the larval nerve muscle preparation revealed no difference between wild-type and mutant, and spontaneous or evoked excitatory junction potentials at this synapse were normal up to a stimulus frequency of 5 Hz. However, when several behavioural responses were analysed quantitatively, specific differences between mutant and wild-type flies are noted. Adult locomotor activity, optomotor responses at high pattern velocities, wing beat frequency, and visual pattern preference are modified. Synapsin mutant flies show faster habituation of an olfactory jump response, enhanced ethanol tolerance, and significant defects in learning and memory as measured using three different paradigms. Larval behavioural defects are described in a separate paper. We conclude that Drosophila synapsins play a significant role in nervous system function, which is subtle at the cellular level but manifests itself in complex behaviour.

Synaptic Vesicle Mobility in Mouse Motor Nerve Terminals with and without Synapsin

“…During periods of nerve activity, synapsin becomes phosphorylated, which reduces its affinity for vesicles (Huttner et al, 1983;TorriTarelli et al, 1992) and results in synapsin dispersion (Chi et al, 2001). Studies on terminals from animals lacking all synapsin genes [Drosophila larval muscle (Godenschwege et al, 2004); cultures of mouse CNS neurons (Gitler et al, 2004b)] found reduced synaptic vesicle numbers and reduced release during intense stimulation [especially in inhibitory synapses (Gitler et al, 2004b)] and support a model in which synapsin is particularly involved in anchoring vesicles belonging to the reserve pool (Hilfiker et al, 1999). None of these studies, however, addressed directly the core tenet of the synapsin hypothesis, namely that synapsin is the glue that regulates synaptic vesicle mobility.…”

Synapsin is required to “boost” memory strength for highly salient events

“…The syn 97 mutant strain carries the reported 1.4 kb deletion in the synapsin gene, removing part of the promote region, exon 1, and a small part of the first intron; consequentially, it lacks all Synapsin protein (Godenschwege et al 2004;Michels et al 2005). In the wild-type (WT) strain, we confirm expected Synapsin protein isoforms between 70 and 80 kDa and a weaker and variable band at 143 kDa (Klagges et al 1996).…”

“…In Drosophila, Synapsin is encoded by only one gene and is expressed in most if not all neurons of both the larval and adult nervous system (coding gene: syn, CG 3985: Klagges et al 1996;Michels et al 2005). Both adult and larval Drosophila lacking Synapsin show associative memory scores that are reduced by about half as compared with wild-type animals, as do animals upon an RNAi-mediated knockdown of Synapsin (adult odor-punishment memory: Godenschwege et al 2004;Knapek et al 2010;Niewalda et al 2015;Walkinshaw et al 2015; larval odor-reward memory: Michels et al 2005Michels et al , 2011. Corresponding phenotypes in learning and memory tasks have been reported throughout the animal kingdom, including man (Silva et al 1996;Garcia et al 2004;Südhof, 2004;Gitler et al 2008;Greco et al 2013).…”

“…(F -H) Western blot of larval brains. (F ) In the wild-type WT strain the anti-Synapsin antibody SYNORF1 detects expected Synapsin bands, namely a double band at 72 kDa and a weaker band at 143 kDa, whereas the syn 97 mutant is lacking all Synapsin protein (Godenschwege et al 2004;Michels et al 2005). The anti-Sap47 antibody nc46 labels expected Sap47 bands at 47 kDa, 70 kDa, and 90 kDa in both the wild-type WT strain and the syn 97 mutant, showing that the Sap47 protein is intact.…”

“…Based on electrophysiology as well as behavioral analyses, it has been suggested that the regulation of synaptic transmission via Synapsin may be particularly important to maintain high levels of transmission upon continuous, heavy demand (Godenschwege et al 2004;Bykhovskaia 2011;Vasin et al 2014). Regarding our odor-reward learning paradigm, we therefore predicted that Synapsin is particularly critical for forming memories of highly salient events.…”