Such perceptions suggest that local protein degradation could be one of the regulatory mechanisms of neural transmission, ,. Upregulation of synaptic vesicle release was induced by the proteasome inhibitor as well as in SCR-KO neurons. Through increased frequency of miniature excitatory post synaptic currents (mEPSCs) and reduced paired pulse facilitation, both of which are caused by increased release probability, we found that Scrapper knockout hippocampal neurons had increased levels of spontaneously released neurotransmitter.
Thus, SCRAPPER regulates proper synaptic transmission via RIM1 degradation.
The target molecule RIM1 is an important factor for synaptic vesicle release and synaptic plasticity. Analyses of Scrapper mutant mice demonstrated that SCRAPPER-dependent UPS contributes to the regulation of synaptic vesicle release probability via RIM1. SCRAPPER directly binds to and ubiquitinates the active zone protein Rab3-interacting molecule 1 (RIM1) both in vitro and in vivo. In the ubiquitin-mediated degradation pathway, E3 ubiquitin ligases play an essential role in the regulation of diverse biological processes by promoting degradation. Selective proteasomal degradation is conducted by addition of ubiquitin to the target proteins by the enzymes of the ubiquitin-proteasome system (UPS) followed by protease digestion in the 26S proteasome,. Recently, protein degradation has attracted attention as a mechanism to control the amount of synaptic proteins. Synaptic proteins are regulated by various sophisticated processes including control of transcription, , translation and translocation. Pre- and post-synaptic sites contain complexes of scaffolding proteins, neurotransmitter-releasing machinery, receptors, ion channels and signaling molecules,. SCRAPPER is a synapse-localized E3 ubiquitin ligase that was identified by in silico screening by us. The behavioral phenotypes of Scrapper mutant mice suggest that molecular degradation conferred by SCRAPPER play important roles in hippocampal-dependent fear memory formation.
In addition, SCR (+/−) mice altered in some tests reflecting anxiety, which implies the loss of functions of SCRAPPER in the hippocampus. On the other hand, heterozygotes of Scrapper knockout mice showed significant difference in the contextual but not cued fear conditioning test. Scrapper transgenic mice overexpressing SCRAPPER in the hippocampus did not show any significant difference in every test argued in this manuscript by comparison with wild-type mice.
We carried out a series of behavioral test batteries for Scrapper mutant mice. Because it is likely that the alteration of neural transmission in Scrapper mutant mice affect their systemic condition, we have analyzed the behavioral phenotypes of mice with decreased or increased the amount of SCRAPPER.
A defect in SCRAPPER leads to neurotransmission abnormalities, which could subsequently result in neurodegenerative phenotypes. SCRAPPER localizes synapses and directly binds to Rab3-interacting molecule 1 (RIM1), an essential factor for synaptic vesicle release, thus it regulates neural transmission via RIM1 degradation. SCRAPPER, an F-box protein coded by FBXL20, is a subunit of SCF type E3 ubiquitin ligase.