Abstract
Background: Ubiquitin-specific protease 14 (USP14) is one of three proteasome-associated deubiquitinating enzymes that remove ubiquitin from proteasomal substrates prior to their degradation. In vitro evidence suggests that inhibiting USP14's catalytic activity alters the turnover of ubiquitinated proteins by the proteasome, although whether protein degradation is accelerated or delayed seems to be cell-type and substrate specific. For example, combined inhibition of USP14 and the proteasomal deubiquitinating enzyme UCH37 halts protein degradation and promotes apoptosis in multiple myeloma cells, whereas USP14 inhibition alone accelerates the degradation of aggregate-prone proteins in immortalized cell lines. These findings have prompted interest in USP14 as a therapeutic target both inside and outside of the nervous system. However, loss of USP14 in the spontaneously occurring ataxia mouse mutant leads to a dramatic neuromuscular phenotype and early perinatal lethality, suggesting that USP14 inhibition may have adverse consequences in the nervous system. We therefore expressed a catalytically inactive USP14 mutant in the mouse nervous system to determine whether USP14's catalytic activity is required for neuromuscular junction (NMJ) structure and function. Results: Mice expressing catalytically inactive USP14 in the nervous system exhibited motor deficits, altered NMJ structure, and synaptic transmission deficits that were similar to what is observed in the USP14-deficient ataxia mice. Acute pharmacological inhibition of USP14 in wild type mice also reduced NMJ synaptic transmission. However, there was no evidence of altered proteasome activity when USP14 was inhibited either genetically or pharmacologically. Instead, these manipulations increased the levels of non-proteasome targeting ubiquitin conjugates. Specifically, we observed enhanced proteasome-independent ubiquitination of mixed lineage kinase 3 (MLK3). Consistent with the direct activation of MLK3 by ubiquitination, we also observed increased activation of its downstrea targets MAP kinase kinase 4 (MKK4) and c-Jun N-terminal kinase (JNK). In vivo inhibition of JNK improved motor function and synapse structure in the USP14 catalytic mutant mice. Conclusions: USP14's catalytic activity is required for nervous system structure and function and has an ongoing role in NMJ synaptic transmission. By regulating the ubiquitination status of protein kinases, USP14 can coordinate the activity of intracellular signaling pathways that control the development and activity of the NMJ.
Original language | English (US) |
---|---|
Article number | 3 |
Journal | Molecular neurodegeneration |
Volume | 10 |
Issue number | 1 |
DOIs | |
State | Published - Jan 10 2015 |
Funding
We would like to thank Drs. Alfred Goldberg, Michael Brenner, and Victoria Zhukareva for their input and evaluation during the preparation of this manuscript and Life Sensors for providing reagents used in this study. This work was supported by grants to UAB from the Howard Hughes Medical Institute through the Med into Grad Initiative (J.J.H), the Civitan Research Center, the Evelyn F. McKnight Brain Institute and the National Institutes of Health (R01 NS047533 and R21 NS074456 to S.M.W).
Keywords
- JNK
- K63-linked ubiquitin
- MLK3
- Motor endplate disease
- Motor neuron
- Neuromuscular junction
- Synapse
- USP14
- Ubiquitin
- Ubiquitin proteasome system
ASJC Scopus subject areas
- Molecular Biology
- Clinical Neurology
- Cellular and Molecular Neuroscience