No abstract

The accumulation of misfolded proteins is associated with various neurodegenerative conditions. Peripheral myelin protein 22 (PMP22) is a hereditary neuropathy-linked, short-lived molecule that forms aggresomes when the proteasome is inhibited or the protein is mutated. We previously showed that the removal of pre-existing PMP22 aggregates is assisted by autophagy. Here we examined whether the accumulation of such aggregates could be suppressed by experimental induction of autophagy and/or chaperones. Enhancement of autophagy during proteasome inhibition hinders protein aggregate formation and correlates with a reduction in accumulated proteasome substrates. Conversely, simultaneous inhibition of autophagy and the proteasome augments the formation of aggregates. An increase of heat shock protein levels by geldanamycin treatment or heat shock preconditioning similarly hampers aggresome formation. The beneficial effects of autophagy and chaperones in preventing the accumulation of misfolded PMP22 are additive and provide a potential avenue for therapeutic approaches in hereditary neuropathies linked to PMP22 mutations.

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.

Western blot analysis (e) reveal that whereas the 30 kDa subunits remains unchanged (arrowheads), levels of the 25 kDa 20Sa and 20Sb subunits (arrows) are reduced in the TrJ, compared with Wt. GAPDH is shown as a constitutive marker. Molecular mass is given on the left, in kDa.