Differential Aggregation Of The Trembler And Trembler J Mutants Of Peripheral Myelin Protein 22

Abstract

Mutations in the gene encoding the peripheral myelin protein 22 (PMP22), a tetraspan protein in compact peripheral myelin, are one of the causes of inherited demyelinating peripheral neuropathy. Most PMP22 mutations alter the trafficking of the PMP22 protein in Schwann cells, and this different trafficking has been proposed as the underlying mechanism of the disease. To explore this problem further, we compared the aggregation of wild-type Pmp22 with those of the two Pmp22 mutations found in Trembler (Tr) and Trembler J (TrJ) mice. All three Pmp22s can be crosslinked readily as homodimers in transfected cells. Wild-type Pmp22 also forms heterodimers with Tr and TrJ Pmp22, and these heterodimers traffic with their respective mutant Pmp22 homodimers. All three Pmp22s form complexes larger than dimers with Tr Pmp22 especially prone to aggregate into high molecular weight complexes. Despite the differences in aggregation of Tr and TrJ Pmp22, these two mutant Pmp22s sequester the same amount of wild-type Pmp22 in heterodimers and heterooligomers. Thus, the differences in the phenotypes of Tr and TrJ mice may depend more on the ability of the mutant protein to aggregate than on the dominant-negative effect of the mutant Pmp22 on wild-type Pmp22 trafficking.

Intermittent fasting alleviates the neuropathic phenotype in a mouse model of Charcot–Marie–Tooth disease

“…While there is a genotypic effect between Wt and TrJ mice (Notterpek et al, 1997), the dietary modulation further enhances the expression of these proteins, indicating elevated lysosomal activity. Together, these findings suggest that an induced autophagy-lysosomal pathway may contribute to the improved phenotype of TrJ neuropathic mice, likely by removing the misfolded, aggregation-prone mutated PMP22 (Tobler et al, 2002) and other poly-ubiquitinated proteins.…”

“…In the disease states, when one copy of PMP22 is mutated, the fraction destined for proteasomal degradation is increased, which overwhelms the proteasome and leads to protein aggregate formation (Fortun et al, 2003). Data from multiple laboratories support the involvement of protein mistrafficking and aggregation in PMP22 neuropathies (Isaacs et al, 2002; Ryan et al, 2002; Tobler et al, 2002; Fortun et al, 2003; Fortun et al, 2006), a finding that is supported by studies of nerve biopsies from patients with PMP22 gene duplication, and point mutations (Nishimura et al, 1996; Hanemann et al, 2000). Cytosolic mislocalization of PMP22 alters protein homeostasis within Schwann cells and leads to the accumulation of ubiquitin, myelin proteins, chaperones and components of the proteasome near and within the aggregates (Ryan et al, 2002; Fortun et al, 2003; Fortun et al, 2006).…”

The Peripheral Neuropathy-LinkedTremblerandTrembler-JMutant Forms of Peripheral Myelin Protein 22 Are Folding-Destabilized

“…When proteasomal function is inhibited, degradation-targeted PMP22 forms perinuclear/centrosome-proximal aggresomes that are believed to normally be engulfed by autophagosomes and then degraded following delivery to the lysosomes(20;22). It is known that most CMT1A-linked mutant forms of PMP22, including the Tr and TrJ mutants examined in this work, are targeted for degradation by ER quality control, with the efficiency of correct folding and trafficking approaching 0%(17;19;24;26;27). However, the nature of the defects in the disease mutants that are recognized by quality control are not well understood.…”

Molecular mechanisms of inherited demyelinating neuropathies

“…Immunostaining a nerve biopsy from a “severe CMT1” patient with the L16P ( TremblerJ ) mutation showed PMP22 (wild-type and mutant protein cannot be distinguished in patients) colocalized with an ER marker, BiP (Hanemann et al, 2000). Three additional, abnormal characteristics of the G150D and/or L16P mutants that may relate to their ability to cause dys/demyelination have been described: (1) They form abnormally stable aggregates with themselves and even with wild-type PMP22 (Tobler et al, 2002); (2) they have abnormally prolonged association with the ER chaparone calnexin, so that sequestered calnexin may contribute to the pathogenesis of demyelination (Dickson et al, 2002; Shames et al, 2003); (3) they reduce the proteasomal activity, which in turn increases the levels of polyubiquitinated proteins (including PMP22), and cause proteins (including PMP22) to accumulate in special intracellular structures known as aggresomes (Fortun et al, 2005). …”