Mutations associated with Charcot–Marie–Tooth disease cause SIMPLE protein mislocalization and degradation by the proteasome and aggresome–autophagy pathways

Abstract

SummaryMutations in SIMPLE cause an autosomal dominant, demyelinating form of peripheral neuropathy termed Charcot-Marie-Tooth disease type 1C (CMT1C), but the pathogenic mechanisms of these mutations remain unknown. Here, we report that SIMPLE is an early endosomal membrane protein that is highly expressed in the peripheral nerves and Schwann cells. Our analysis has identified a transmembrane domain (TMD) embedded within the cysteine-rich (C-rich) region that anchors SIMPLE to the membrane, and suggests that SIMPLE is a post-translationally inserted, C-tail-anchored membrane protein. We found that CMT1C-linked pathogenic mutations are clustered within or around the TMD of SIMPLE and that these mutations cause mislocalization of SIMPLE from the early endosome membrane to the cytosol. The CMT1C-associated SIMPLE mutant proteins are unstable and prone to aggregation, and they are selectively degraded by both the proteasome and aggresome-autophagy pathways. Our findings suggest that SIMPLE mutations cause CMT1C peripheral neuropathy by a combination of loss-of-function and toxic gain-of-function mechanisms, and highlight the importance of both the proteasome and autophagy pathways in the clearance of CMT1C-associated mutant SIMPLE proteins.

Protein misfolding and clearance in demyelinating peripheral neuropathies

“…misfolded SIMPLE is not cleared by ERAD 3 indicates that CMT pathogenesis can be mediated by dysfunction in ERADindependent pathways. Proteasome impairment induced by misfolded and aggregated proteins was observed in several mouse models of demyelinating CMT1A.…”

“…9,10 How misfolding of these membrane proteins with different topologies contributes to demyelinating neuropathy remain unknown. We found that misfolded SIMPLE forms abnormal cytosolic aggregates and mediates erroneous interactions with cellular proteins, 3 which are pathogenic mechanisms characteristic of many protein-misfolding diseases. 11,12 These findings suggest that demyelinating CMT may be a proteinmisfolding disease of Schwann cells.…”

“…The aggresome-autophagy pathway has emerged as another crucial protein quality control system in Schwann cells that degrades misfolded and aggregated proteins. [14][15][16] Reports by our group and others indicate that Schwann cells handle misfolded PMP22 and SIMPLE by sequestering them into perinuclear aggresomes through a mechanism requiring microtubule-dependent retrograde transport 3,14 ( Fig. 1 and step 8).…”

“…1 and step 4) but through an ERAD-independent mechanism. 3 Whether this novel ERAD-independent proteasomal degradation is unique to rotranslocated to the cytosol ( Fig. 1 and step 3) and cleared by the proteasome ( Fig.…”

“…www.landesbioscience.com Communicative & Integrative Biologyfinding that autophagy activation by rapamycin promotes autophagic degradation of misfolded SIMPLE, 3 together with the reported role of rapamycin in promoting myelination in explant cultures from neuropathic mouse models of CMT1A, 30 suggest that autophagy activation could be efficacious for treating demyelinating peripheral neuropathy.…”