Examination of genetic and pharmacological tools to study the proteasomal deubiquitinating enzyme ubiquitin‐specific protease 14 in the nervous system

Abstract

Strategies for enhancing protein degradation have been proposed for treating neurological diseases associated with a decline in proteasome activity. A proteasomal deubiquitinating enzyme that controls substrate entry into proteasomes, ubiquitin‐specific protease 14 (USP14), is an attractive candidate for therapies that modulate proteasome activity. This report tests the validity of genetic and pharmacological tools to study USP14’s role in regulating protein abundance. Although previous studies implicated USP14 in the degradation of microtubule associate protein tau, tar DNA binding protein, and prion protein, the levels of these proteins were similar in our neurons cultured from wild type and USP14‐deficient mice. Neither loss nor over‐expression of USP14 affected the levels of these proteins in mice, implying that modifying the amount of USP14 is not sufficient to alter their steady‐state levels. However, neuronal over‐expression of a catalytic mutant of USP14 showed that manipulating USP14’s ubiquitin‐hydrolase activity altered the levels of specific proteins in vivo. Although pharmacological inhibitors of USP14’s ubiquitin‐hydrolase activity reduced microtubule associate protein tau, tar DNA binding protein, and prion protein in culture, the effect was similar in wild type and USP14‐deficient neurons, thus impacting their use for specifically evaluating USP14 in a therapeutic manner. While examining how targeting USP14 may affect other proteins in vivo, this report showed that fatty acid synthase, v‐rel reticuloendotheliosis viral oncogene homolog, CTNNB1, and synaptosome associated protein 23 are reduced in USP14‐deficient mice; however, loss of USP14 differentially altered the levels of these proteins in the liver and brain. As such, it is critical to more thoroughly examine how inhibiting USP14 alters protein abundance to determine if targeting USP14 will be a beneficial strategy for treating neurodegenerative diseases.

Can We Treat Neurodegenerative Proteinopathies by Enhancing Protein Degradation?

“…The role of USP14, and probably UCH37, is to prevent excessive degradation of proteins by deubiquitinating substrates before they are committed to the 20S proteasome. Interestingly, the USP14 inhibitor IU1 activates the proteasome and decreases the levels of disease proteins, including tau and TDP‐43 in mouse primary neurons, 19 although its specificity and ability to decrease disease proteins have been contested 20 …”

“…Interestingly, the USP14 inhibitor IU1 activates the proteasome and decreases the levels of disease proteins, including tau and TDP-43 in mouse primary neurons, 19 although its specificity and ability to decrease disease proteins have been contested. 20 The 19S and 20S proteasome activity is stimulated by cAMP-activated protein kinase A (PKA)-mediated phosphorylation. 21 Consistently, the adenylyl cyclase-stimulator forskolin promotes the degradation of TDP-43 and tau in transfected human embryonic kidney (HEK) cells.…”