The ubiquitin–proteasome system postsynaptically regulates glutamatergic synaptic function

Abstract

The ubiquitin proteasome system (UPS) actively controls protein dynamics and local abundance via regulated protein degradation. This study investigates UPS roles in the regulation of postsynaptic function and molecular composition in the Drosophila neuromuscular junction (NMJ) genetic system. To specifically impair UPS function postsynaptically, the UAS/GAL4 transgenic method was employed to drive postsynaptic expression of proteasome β2 and β6 subunit mutant proteins, which operate through a dominant negative mechanism to block proteasome function. When proteasome mutant subunits were constitutively expressed, excitatory junctional current (EJC) amplitudes were increased, demonstrating that postsynaptic proteasome function limits neurotransmission strength. Interestingly, the alteration in synaptic strength was calcium-dependent and miniature EJCs had significantly smaller mean amplitudes and more rapid mean decay rates. Postsynaptic levels of the Drosophila PSD-95/SAP97 homologue, discs large (DLG), and the GluRIIB-containing glutamate receptor were increased, but GluRIIA-containing receptors were unaltered. With acute postsynaptic proteasome inhibition using an inducible transgenic system, neurotransmission was similarly elevated with the same specific increase in postsynaptic GluRIIB abundance. These findings demonstrate postsynaptic proteasome regulation of glutamatergic synaptic function that is mediated through specific regulation of GluRIIB-containing glutamate receptors.

Roles of ubiquitination at the synapse

“…Genetic postsynaptically-targeted proteasome inhibition caused a rapid increase in B-class receptors over the course of a few hours, with little effect on A-class receptor levels (Fig. 3) [109]. The mechanism likely involves ubiquitin-mediated turnover of Discs Large (DLG), a PSD-95 (postsynaptic density-95) scaffold homolog.…”

“…The mechanism likely involves ubiquitin-mediated turnover of Discs Large (DLG), a PSD-95 (postsynaptic density-95) scaffold homolog. DLG was shown to be ubiquitinated and accumulated after genetic proteasome inhibition [109] and is a known determinant of B-class receptor localization/maintenance at the Drosophila NMJ [110]. Direct ubiquitination of Bclass receptors also remains as a possibility that needs to be explored.…”

The nonsense-mediated decay pathway maintains synapse architecture and synaptic vesicle cycle efficacy

“…Other mRNA-binding proteins, such as fragile-X mental retardation protein (FMRP), similarly have crucial roles in the regulation of synaptic mRNA stability, trafficking and translation (Gatto and Broadie, 2008;Pan and Broadie, 2007;Pan et al, 2004;Pan et al, 2008;Repicky and Broadie, 2009;Tessier and Broadie, 2008;Zhang et al, 2001;Zhang et al, 2005). Moreover, local protein degradation via the ubiquitin proteasome system (UPS) has also recently been established as a key mechanism that shapes synaptic structural development, neurotransmission strength and synaptic plasticity (Haas and Broadie, 2008;Haas et al, 2007a;Haas et al, 2007b;Speese et al, 2003).…”

“…At the synapse, FMRP has several functions, including the control of axonal and dendritic arbor size, bouton number and distribution, transmission strength, postsynaptic glutamate receptor trafficking and the regulation of presynaptic vesicle pools (Gatto and Broadie, 2008;Pan and Broadie, 2007;Pan et al, 2008;Pan et al, 2004;Repicky and Broadie, 2009;Tessier and Broadie, 2008;Zhang et al, 2001;Zhang et al, 2005). In balance with translation regulation, UPSmediated degradation has dynamic functions that control synapse architecture, neurotransmission strength and synaptic protein abundance, including postsynaptic glutamate receptors (Haas and Broadie, 2008;Haas et al, 2007a;Speese et al, 2003). The ubiquitin ligase highwire acts to restrict synaptic overgrowth by down-regulating the MAPKKK-Wallenda pathway, where mutants exhibit increased neuromuscular junction (NMJ) branch and bouton numbers (Wan et al, 2000;Wu et al, 2007).…”

The role of the ubiquitin proteasome system in synapse remodeling and neurodegenerative diseases

“…However, the precise roles of those proteins in synapse formation or function remain to be determined. (64) …”