In Alzheimer's disease proteasome activity is reportedly downregulated， thus increasing it could be therapeutically beneficial. The proteasome-associated deubiquitinase USP14 disassembles polyubiquitin-chains， potentially delaying proteasome-dependent protein degradation. We assessed the protective efficacy of inhibiting or downregulating USP14 in rat and mouse (Usp14) neuronal cultures treated with prostaglandin J2 (PGJ2). IU1 concentrations (IU1>25μM) reported by others to inhibit USP14 and be protective in non-neuronal cells， reduced PGJ2-induced Ub-protein accumulation in neurons. However， IU1 alone or with PGJ2 is neurotoxic， induces calpain-dependent Tau cleavage， and decreases E1~Ub thioeste... More
In Alzheimer's disease proteasome activity is reportedly downregulated， thus increasing it could be therapeutically beneficial. The proteasome-associated deubiquitinase USP14 disassembles polyubiquitin-chains， potentially delaying proteasome-dependent protein degradation. We assessed the protective efficacy of inhibiting or downregulating USP14 in rat and mouse (Usp14) neuronal cultures treated with prostaglandin J2 (PGJ2). IU1 concentrations (IU1>25μM) reported by others to inhibit USP14 and be protective in non-neuronal cells， reduced PGJ2-induced Ub-protein accumulation in neurons. However， IU1 alone or with PGJ2 is neurotoxic， induces calpain-dependent Tau cleavage， and decreases E1~Ub thioester levels and 26S proteasome assembly， which are energy-dependent processes. We attribute the two latter IU1 effects to ATP-deficits and mitochondrial Complex I inhibition， as shown herein. These IU1 effects mimic those of mitochondrial inhibitors in general， thus supporting that ATP-depletion is a major mediator of IU1-actions. In contrast， low IU1 concentrations (IU1≤25μM) or USP14 knockdown by siRNA in rat cortical cultures or loss of USP14 in cortical cultures from ataxia (Usp14) mice， failed to prevent PGJ2-induced Ub-protein accumulation. PGJ2 alone induces Ub-protein accumulation and decreases E1~Ub thioester levels. This seemingly paradoxical result may be attributed to PGJ2 inhibiting some deubiquitinases (such as UCH-L1 but not USP14)， thus triggering Ub-protein stabilization. Overall， IU1-concentrations that reduce PGJ2-induced accumulation of Ub-proteins are neurotoxic， trigger calpain-mediated Tau cleavage， lower ATP， E1~Ub thioester and E1 protein levels， and reduce proteasome activity. In conclusion， pharmacologically inhibiting (with low or high IU1 concentrations) or genetically down-regulating USP14 fail to enhance proteasomal degradation of Ub-proteins or Tau in neurons.
