Neurons have highly specialized intracellular compartments that facilitate the development and activity of the nervous system. Ubiquitination is a post-translational modification that controls many aspects of neuronal function by regulating protein abundance. Disruption of this signaling pathway has been demonstrated in neurological disorders such as Parkinson’s disease, Amyotrophic Lateral Sclerosis and Angleman Syndrome. Since many neurological disorders exhibit ubiquitinated protein aggregates, the loss of neuronal ubiquitin homeostasis may be an important contributor of disease. This review discusses the mechanisms utilized by neurons to control the free pool of ubiquitin necessary for normal nervous system development and function as well as new roles of protein ubiquitination in regulating synaptic activity.

Key pointsr Mice carrying the ataxia (ax J ) mutation have a 95% reduction in the deubiquitinating enzyme USP14, which results in a reduction in hippocampal paired pulse facilitation, a form of short-term synaptic plasticity.r Hippocampal synapses in ax J mice have a 50% reduction in synaptic vesicles but no change in the initial release probability, which is a novel mechanism for regulating paired pulse facilitation.r USP14 modulates hippocampal short-term plasticity and structure independent of its deubiquitinating activity, as overexpression of a catalytically inactive form of USP14 restores hippocampal paired pulse facilitation and vesicle number to the ataxia mice.r Pharmacological inhibition of the proteasome also rescues the deficits in hippocampal short-term plasticity in ataxia mice, implying that the loss of USP14 causes increased protein degradation.r These results suggest that USP14 plays a modulatory role in regulating protein turnover by the proteasome that is independent of its canonical role in the disassembly of polyubiquitin conjugates.Abstract The ubiquitin proteasome system is required for the rapid and precise control of protein abundance that is essential for synaptic function. USP14 is a proteasome-bound deubiquitinating enzyme that recycles ubiquitin and regulates synaptic short-term synaptic plasticity. We previously reported that loss of USP14 in ax J mice causes a deficit in paired pulse facilitation (PPF) at hippocampal synapses. Here we report that USP14 regulates synaptic function through a novel, deubiquitination-independent mechanism. Although PPF is usually inversely related to release probability, USP14 deficiency impairs PPF without altering basal release probability. Instead, the loss of USP14 causes a large reduction in the number of synaptic vesicles. Over-expression of a catalytically inactive form of USP14 rescues the PPF deficit and restores synaptic vesicle number, indicating that USP14 regulates presynaptic structure and function independently of its role in deubiquitination. Finally, the PPF deficit caused by loss of USP14 can be rescued by pharmacological inhibition of proteasome activity, suggesting that inappropriate protein degradation underlies the PPF impairment. Overall, we demonstrate a L. E. Dobrunz and S. M. Wilson contributed equally to this work.

Numerous studies have suggested a role for ubiquitin-proteasome-mediated protein degradation in learning-dependent synaptic plasticity; however, very little is known about how protein degradation is regulated at the level of the proteasome during memory formation. The ubiquitin-specific protease 14 (USP14) is a proteasomal deubiquitinating enzyme that is thought to regulate protein degradation in neurons; however, it is unknown if USP14 is involved in learning-dependent synaptic plasticity. We found that infusion of a USP14 inhibitor into the amygdala impaired long-term memory for a fear conditioning task, suggesting that USP14 is a critical regulator of long-term memory formation in the amygdala.The ubiquitin-proteasome system is a complex network of different ubiquitin ligases that can conjugate the small protein modifier ubiquitin onto individual proteins to target them for degradation (Hegde 2010;Mabb and Ehlers 2010). Once ubiquitinated, target proteins can bind to the 26S proteasome and are subsequently deubiquitinated, unfolded, and degraded. Although the selection of proteins to be ubiquitinated is largely regulated by the ubiquitin ligases, the actual degradation of the protein is regulated by a variety of different subunits found on the regulatory particle (19S) of the proteasome. Deubiquitinating enzymes are a class of regulatory proteins that bind the 19S complex of the proteasome and they function to regulate the degradation process by removing ubiquitin tags from ubiquitinated proteins bound to the proteasome, which is thought to serve an important role in facilitating substrate entry into the proteasome as well as in maintaining ubiquitin pools.The ubiquitin-specific protease 14 (USP14) is one of the most extensively studied deubiquitinating enzymes of the proteasome complex and may play a critical role in activity-dependent synaptic plasticity (Kowalski and Juo 2012). For example, loss of USP14 expression results in significant physiological impairments in both the central and peripheral nervous systems in the ataxia (ax J ) mice (Wilson et al. 2002) and these deficits can be largely rescued by transgenic overexpression of USP14 in neurons (Crimmins et al. 2006). Additionally, loss of USP14 results in reduced ubiquitin levels, impaired neurotransmitter release, and increased surface expression of the GABA A receptor (Anderson et al. 2005;Chen et al. 2009;Lappe-Siefke et al. 2009;Bhattacharyya et al. 2012), supporting the idea that USP14 is a critical regulator of synaptic plasticity. Interestingly, some studies have shown that USP14 is a negative regulator of ubiquitin-dependent protein turnover in vitro (Lee et al. 2010), suggesting that USP14 could serve a unique function relative to other deubiquitinating enzymes which normally facilitate the degradation process. However, it is currently not known if USP14 plays an important role in long-term memory formation in the mammalian brain.The formation of long-term fear memories requires new gene transcription and de novo protein synthesis in the amy...