Loss of Usp14 results in reduced levels of ubiquitin in ataxia mice

Abstract

The ataxia (ax J ) mutation is a spontaneous recessive mutation that results in reduced expression of ubiquitinspecific protease 14, Usp14. Mice homozygous for the ax J mutation are retarded for growth and exhibit several behavioral disorders, including a resting tremor and hindlimb paralysis. Although pathological defects appear to be limited to the central nervous system, reduction of Usp14 expression was widespread in the ax J mice. Usp14 co-fractionated with proteasomes isolated from livers and brains of wildtype mice. Proteasomes isolated from the ax J brains still possessed deubiquitinating activity and were functionally competent to hydrolyze 20S proteasomal substrates in vitro. However, the levels of monomeric ubiquitin were reduced approximately 35% in most of the ax J tissues examined.These results indicate that Usp14 functions to maintain the cellular levels of monomeric ubiquitin in mammalian cells, and that alterations in the levels of ubiquitin may contribute to neurological disease. Keywords: ataxia, mutation, neurological, proteasome, ubiquitin, ubiquitin-specific protease 14. . These disorders are associated with changes in protein degradation that are manifested through the production of protein aggregates. However, it is unclear whether these alterations in the UPS are the primary cause or a secondary consequence of these disorders. Our recent studies on ataxia (ax J ) mice indicate that primary alterations in the UPS can lead to neurological dysfunction (Wilson et al. 2002). The ax J mutation is a spontaneously arising recessive neurological mutation (D'Amato and Hicks 1965). The ax J mice suffer from progressive motor system abnormalities that first appear as a resting tremor when the mice are 2-3 weeks old. At 4 weeks of age, ax J mice exhibit severe hind muscle wasting and ataxia. These mice are completely immobile by 6 weeks of age and death occurs at about 8 weeks of age. The neurological phenotypes observed in the ax J mice are the result of a genetic lesion in the gene encoding the deubiquitinating enzyme Usp14 (Wilson et al. 2002). The insertion of an intracisternal A-particle into intron 5 of Usp14 reduces the expression of Usp14 in the ax J mice to only 5-10% of the levels found in wild-type mice, indicating that the ax J mutation represents a hypomorphic allele of Usp14. A better understanding of the deficiencies in the ax J mice, and in other mice that harbor mutations in the UPS, should therefore provide valuable insights into how the UPS functions in the nervous system and how alterations in these pathways lead to disease.

Towards mapping phenotypical traits in 18p− syndrome by array-based comparative genomic hybridisation and fluorescent in situ hybridisation

“…This hypothesis is supported by two published patients with short stature and slightly larger terminal 18p deletions of 10.2 -10.7 Mb and 10.7 -11.8 Mb, respectively. 7 USP14 (ubiquitin-specific protease 14), located at 18p11.32, may be an interesting gene in the context of growth retardation and seizures as mice with a mutation resulting in reduced protein expression were growthretarded 20 and the protein has been implicated in regulating synaptic activity in mammals. 33 A second possible candidate gene for seizures, DLGAP1 (discs large-associated protein 1) located at 18p11.31, is part of the postsynaptic density in neuronal cells.…”

“…Towards mapping phenotype traits in 18pÀ syndrome CH Brenk et al Breakpoint analysis The breakpoints were characterised by hybridisation of 18p and 20p FISH probe panels onto metaphases of unbalanced cases 2 and/or 3 and their balanced mother (Table 1). For BAC RP11-931H21 (18p11.23), a split signal was detected on the mother's der (18) and der (20) (Figure 3c) and diminished signal intensities on the der(18) of cases 2 and 3 ( Figure 3b). Thus, RP11-931H21 was shown to span the 18p breakpoint located in 18p11.23.…”

“…The deletion size on 18p was determined to be 8.070.1 Mb in cases 2 and 3. For BAC RP11-649H22 (20p12.3), a split signal on the mother's der (20) and der(18) was detected and a diminished signal intensity on the der(18) in cases 2 and 3. The 20p breakpoint was thus shown to be spanned by clone RP11-649H22 and the size of the partial trisomy in cases 2 and 3 to be 6.270.1 Mb.…”

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

“…By releasing ubiquitin molecules from the substrate, USP14/Ubp6 helps to prevent the rapid degradation of ubiquitin molecules together with the substrate protein ( Hanna et al , 2007). A critical role of USP14 in stabilizing cellular ubiquitin levels was demonstrated in vivo in USP14 deficient ax J mice which display decreased ubiquitin levels in all tissues with the greatest loss observed at synaptic terminals ( Anderson et al , 2005; Wilson et al , 2002). …”

Deubiquitylases From Genes to Organism

“…Yeast cells respond to ubiquitin depletion by upregulating the USP14 ortholog Ubp6, which restores ubiquitin levels (85). The ataxia (ax j ) mouse exhibits severe tremors at 2-3 wk of age, reflecting defective synaptic transmission, which results from an intronic mutation, leading to loss of full-length USP14 expression (14,278). The observed phenotypes probably reflect depletion of the synaptic ubiquitin pool observed in ax j mice, as they can be rescued by either neuron specific expression of Usp14 or ubiquitin itself (32, 33, 49).…”