Fbxo45, a Novel Ubiquitin Ligase, Regulates Synaptic Activity

Abstract

Neurons communicate with each other through synapses. To establish the precise yet flexible connections that make up neural networks in the brain, continuous synaptic modulation is required. The ubiquitin-proteasome system of protein degradation is one of the critical mechanisms that underlie this process, playing crucial roles in the regulation of synaptic structure and function. We identified a novel ubiquitin ligase, Fbxo45, that functions at synapses. Fbxo45 is evolutionarily conserved and selectively expressed in the nervous system. We demonstrated that the knockdown of Fbxo45 in primary cultured hippocampal neurons resulted in a greater frequency of miniature excitatory postsynaptic currents. We also found that Fbxo45 induces the degradation of a synaptic vesicle-priming factor, Munc13-1. We propose that Fbxo45 plays an important role in the regulation of neurotransmission by modulating Munc13-1 at the synapse.The nervous system stores and retrieves information via synapses, which are its primary means of communication. Synapses are specialized intercellular junctions dedicated to the transfer of information from a neuron to another neuron. Synaptic transmission is rapidly, dynamically, efficiently, and tightly regulated by several molecular mechanisms (1-4). Among these mechanisms, recent studies have shown that synaptic components are modified by protein activation (5) and degradation (6 -9).Protein degradation can be mediated by the ubiquitin-proteasome system (UPS) 4 (10 -12). The modification of proteins by the ligation of ubiquitin molecules is critical role in regulating the degradation of specific proteins, thereby controlling protein turnover. This control mechanism is extremely effective because it allows the rapid elimination of particular regulatory proteins, ensuring that the biological process regulated by the proteins can be shut down immediately. Protein ubiquitination is catalyzed by a cascade of reactions involving three enzymes: E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin-protein ligase). E1 activates ubiquitin in an ATP-dependent reaction and transfers it to E2 with the formation of a thiol ester bond between the C terminus of ubiquitin and a cysteine residue of E2. Next, E2, either by itself or together with E3, transfers the ubiquitin moiety to a lysine residue of the substrate protein. Various E3s have been reported, and the action of each is substrate-specific.

The Dual Estrogen ReceptorαInhibitory Effects of the Tissue-Selective Estrogen Complex for Endometrial and Breast Safety

“…Therefore, we analyzed whether different components of the ubiquitin proteasome complex might be involved in CEversus TSEC-mediated ERa degradation. ERE-DNA pulldown assays coupled with mass spectrometry analyses revealed that FBXO45, a component of an E3 ubiquitin ligase complex (Saiga et al, 2009;Tada et al, 2010), was significantly recruited to the ERa/ERE complex in the presence of TSEC compared with vehicle (Fig. 4, C and D).…”

The role of ubiquitylation in nerve cell development

“…Taken together, these finding are in nice accord with data obtained in C. elegans, further supporting the notion that Highwire/RPM-1 and FSN operate in the same pathway with DLK as a major target 122,125 . Even in mammals, the Highwire/RPM-1 and FSN orthologues, Phr1 and Fbxo45, seem to function in a complex at synapses 126,127 , but this complex may have neuronal targets other than DLK 128 .…”

Fbxo45-mediated degradation of the tumor-suppressor Par-4 regulates cancer cell survival

“…The peptide sequence was used to generate antisera as previously described. 10 Flow cytometry analysis. For measuring apoptosis, transfected cells were dual stained with 7-AAD (PE) and APC-Annexin V using an Alexa Fluor 488-annexin V Dead Cell apoptosis kit (Invitrogen) according to the manufacturer's protocol.…”

“…7,8 Fbxo45 has been linked to the proteasomal degradation of a few targets including p73 9 and Munc13-1. 10 Fbxo45 is one of only six F-box proteins that are conserved in metazoans and the only F-box protein known to contain a SPRY domain, 3 which was first identified as a sequence repeat in the dual-specificity kinase splA and ryanodine receptors. 11 SPRY domains are present in more than 150 human proteins that cover a wide spectrum of biological functions, including regulation of cytokine signaling and innate retroviral restriction.…”