Abstract
Synapse-associated protein 1 (Syap1/BSTA) is the mammalian homologue of Sap47 (synapse-associated protein of 47 kDa) in Drosophila. Sap47 null mutant larvae show reduced short-term synaptic plasticity and a defect in associative behavioral plasticity. In cultured adipocytes, Syap1 functions as part of a complex that phosphorylates protein kinase Bα/Akt1 (Akt1) at Ser473 and promotes differentiation. The role of Syap1 in the vertebrate nervous system is unknown. Here, we generated a Syap1 knock-out mouse and show that lack of Syap1 is compatible with viability and fertility. Adult knock-out mice show no overt defects in brain morphology. In wild-type brain, Syap1 is found widely distributed in synaptic neuropil, notably in regions rich in glutamatergic synapses, but also in perinuclear structures associated with the Golgi apparatus of specific groups of neuronal cell bodies. In cultured motoneurons, Syap1 is located in axons and growth cones and is enriched in a perinuclear region partially overlapping with Golgi markers. We studied in detail the influence of Syap1 knockdown and knockout on structure and development of these cells. Importantly, Syap1 knockout does not affect motoneuron survival or axon growth. Unexpectedly, neither knockdown nor knockout of Syap1 in cultured motoneurons is associated with reduced Ser473 or Thr308 phosphorylation of Akt. Our findings demonstrate a widespread expression of Syap1 in the mouse central nervous system with regionally specific distribution patterns as illustrated in particular for olfactory bulb, hippocampus, and cerebellum.Electronic supplementary materialThe online version of this article (doi:10.1007/s00418-016-1457-0) contains supplementary material, which is available to authorized users.
Highlights
The synapse-associated protein of 47 kDa (Sap47) in Drosophila represents the founding member of a family of synapse-associated proteins with a BSD domain
Since the tma1 insertion allele does not delete Syap1 coding sequences (Fig. 1), we determined whether any residual intact Syap1 transcript or protein was generated by exon-3 to exon-4 splicing (ignoring the closer engrailed-2 protein (En-2) splice acceptor site (SA) of the tm1a insertion). qRT-PCR of cDNA isolated from wild-type and Syap1-/mutant brain with primers connecting exon-3 to exon-4 showed a reduction in the mutant of transcripts expressing these exons to 5.68 ± 0.54 % of wild-type levels (n = 6 independent experiments) (Fig. 2a)
The wild-type signal of hippocampal homogenate was still detectable when a 1:100 dilution of the lysates was loaded (Fig. S2b). These results demonstrate that Syap1 is abundant in nervous tissue, but it is not a nervous system-specific protein
Summary
The synapse-associated protein of 47 kDa (Sap47) in Drosophila represents the founding member of a family of synapse-associated proteins with a BSD domain. The superfamily of proteins containing a BSD domain includes functionally diverse proteins such as BTF2-like transcription factors, Sap homologues, and DOS2-like proteins involved in ubiquitin metabolism and control of singlecopy DNA replication (Doerks et al 2002). Basic synaptic transmission is normal in Sap null mutants, but current clamp recordings at larval neuromuscular junctions reveal enhanced synaptic depression during high-frequency stimulation, indicating a defect in short-term synaptic plasticity. Sap null mutant larvae show a ~50 % reduction in the ability to learn and/or remember the association of an Histochem Cell Biol (2016) 146:489–512 odorant with a rewarding tastant (Funk et al 2004; Saumweber et al 2011)
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