Impaired proteasome activity and accumulation of ubiquitinated substrates in a hereditary neuropathy model

Abstract

Accumulation of misfolded proteins and alterations in the ubiquitin-proteasome pathway are associated with various neurodegenerative conditions of the CNS and PNS. Aggregates containing ubiquitin and peripheral myelin protein 22 (PMP22) have been observed in the Trembler J mouse model of Charcot-Marie-Tooth disease type 1A demyelinating neuropathy. In these nerves, the turnover rate of the newly synthesized PMP22 is reduced, suggesting proteasome impairment. Here we show evidence of proteasome impairment in Trembler J neuropathy samples compared with wild-type, as measured by reduced degradation of substrate reporters. Proteasome impairment correlates with increased levels of polyubiquitinated proteins, including PMP22, and the recruitment of E1, 20S and 11S to aggresomes formed either spontaneously due to the Trembler J mutation or upon proteasome inhibition. Furthermore, myelin basic protein, an endogenous Schwann cell proteasome substrate, associates with PMP22 aggregates in affected nerves. Together, our data show that in neuropathy nerves, reduced proteasome activity is coupled with the accumulation of ubiquitinated substrates, and the recruitment of proteasomal pathway constituents to aggregates. These results provide novel insights into the mechanism by which altered degradation of Schwann cell proteins may contribute to the pathogenesis of certain PMP22 neuropathies.

Protein misfolding and clearance in demyelinating peripheral neuropathies

“…Proteasome impairment induced by misfolded and aggregated proteins was observed in several mouse models of demyelinating CMT1A. 7 Moreover, proteasome inhibition by the chemotherapeutic medication bortezomib causes demyelinating neuropathy and worsens the neuropathic symptoms of CMT patients. 21,22 These findings suggest that proteasome dysfunction could contribute to demyelinating CMT pathogenesis.…”

“…Recent studies by our laboratory and others have provided important insights into the molecular and cellular pathways involved in demyelinating CMT, which may help develop better treatments for this disease. [6][7][8] and MPZ. 9,10 How misfolding of these membrane proteins with different topologies contributes to demyelinating neuropathy remain unknown.…”

The formation of peripheral myelin protein 22 aggregates is hindered by the enhancement of autophagy and expression of cytoplasmic chaperones

“…It is also feasible to speculate that the collapsed vimentin cage around PMP22 aggresomes prevents the mobility of proteins, whereas small aggregates are free to associate with additional molecules. Indeed, small PMP22 aggregates could be detrimental for SCs by providing a core for trapping other ubiquitinated substrates, based on specific, or hydrophobic protein-protein interactions (Fortun et al, 2005). Although we did not detect SC death in the time frame used for proteasome inhibition (16 h) or in the neuropathic models (Fortun et al, 2003;, it is possible that PMP22 aggregates could lead to toxicity if not eliminated by autophagy.…”

“…The formation of protein aggregates is associated with an overall accumulation of unrelated proteasomal substrates (Johnston et al, 1998;Bence et al, 2001;Fortun et al, 2005). Therefore, a reduction in the number of aggregates upon starvation-induced autophagy should correlate with a decrease in the levels of accumulated UPS substrates.…”

“…Although the molecular mechanisms underlying CMT1A are not well understood, halted intracellular protein trafficking and formation of aggregates are believed to play roles (D'Urso et al, 1998; Tobler et al, 1999; Naef and Suter, 1999; Colby et al, 2000;Fortun et al, 2003;. In SCs of CMT1A mouse models, the spontaneous accumulation of misfolded PMP22 in aggregates correlates with impaired proteasome activity (Fortun et al, 2005;. These aggregates resemble PMP22 inclusions formed in cultured SCs upon pharmacological inhibition of the proteasome, termed aggresomes (Johnston et al, 1998;Notterpek et al, 1999).…”