Distinct effects of ubiquitin overexpression on NMJ structure and motor performance in mice expressing catalytically inactive USP14

Abstract

Ubiquitin-specific protease 14 (USP14) is a major deubiquitinating enzyme and a key determinant of neuromuscular junction (NMJ) structure and function. We have previously reported dramatic ubiquitin depletion in the nervous systems of the USP14-deficient ataxia (axJ) mice and demonstrated that transgenic ubiquitin overexpression partially rescues the axJ neuromuscular phenotype. However, later work has shown that ubiquitin overexpression does not correct the axJ deficits in hippocampal short term plasticity, and that transgenic expression of a catalytically inactive form of USP14 in the nervous system mimics the neuromuscular phenotype observed in the axJ mice, but causes a only a modest reduction of free ubiquitin. Instead, increased ubiquitin conjugates and aberrant activation of pJNK are observed in the nervous systems of the USP14 catalytic mutant mice. In this report, we demonstrate that restoring free ubiquitin levels in the USP14 catalytic mutant mice improved NMJ structure and reduced pJNK accumulation in motor neuron terminals, but had a negative impact on measures of NMJ function, such as motor performance and muscle development. Transgenic expression of ubiquitin had a dose-dependent effect on NMJ function in wild type mice: moderate levels of overexpression improved NMJ function while more robust ubiquitin overexpression reduced muscle development and motor coordination. Combined, these results suggest that maintenance of free ubiquitin levels by USP14 contributes to NMJ structure, but that USP14 regulates NMJ function through a separate pathway.

Does inactivation of USP14 enhance degradation of proteasomal substrates that are associated with neurodegenerative diseases?

“…They found no changes in endogenous tau or ataxin-3 protein levels, but did observe a difference in phosphorylated tau ( Jin et al , 2012). They also generated mice expressing catalytically inactive USP14 and could not detect altered proteasomal function in these mice, although tau levels were not analyzed ( Vaden et al , 2015). However, whether pharmacological or genetic inhibition of USP14 could improve degradation of aggregate-prone proteins in a disease state is still unknown.…”

Meddling with Fate: The Proteasomal Deubiquitinating Enzymes

“…If this adaptive proteasome stress response is impaired, the C118A mutation produces a severe growth defect. Noncatalytic effects have been reported for Usp14 as well [41,48,49], and were recently characterized in some detail using murine proteasomes [50]. …”

“…Experiments in various model systems have revealed a multitude of processes in which Usp14 and Ubp6 may function [31–33,45,47,48,50,76–78,90,91,93,109,113,127–138]. An interesting phenotype of ubp6 null mutants in yeast is their exceptional and perhaps unique ability to tolerate aneuploid chromosomes [135–137].…”

“…Thus, Usp14 seems especially critical in motor neurons, or in their development, although the mechanistic basis of this phenotype is not known. The axj phenotype can be ameliorated by overexpression of ubiquitin, in accord with a reduction of ubiquitin levels in neurons of the axj mutant [48,130]. Thus, perturbation of ubiquitin pools may be required for the full effect of the axj mutation, though it remains possible that ubiquitin overexpression can suppress a defect that is independent of ubiquitin pools.…”

Neurotoxic mechanisms by which the USP14 inhibitor IU1 depletes ubiquitinated proteins and Tau in rat cerebral cortical neurons: Relevance to Alzheimer's disease

“…The deubiquitinase USP14 is required for the development and functioning of the nervous system [9,12,79,84]. Mice homozygous for the ataxia mutation ( ax J , 95% USP14 loss) exhibit developmental abnormalities including motor impairment, reduced brain mass and death by two months of age [14].…”