Phosphorylation of eIF2α Promotes Schwann Cell Differentiation and Myelination in CMT1B Mice with Activated UPR

Abstract

Myelin Protein Zero (MPZ/P0) is the most abundant glycoprotein of peripheral nerve myelin. P0 is synthesized by myelinating Schwann cells, processed in the endoplasmic reticulum (ER) and delivered to myelin via the secretory pathway. The mutant P0S63del (deletion of serine 63 in the extracellular domain of P0), that causes Charcot-Marie-Tooth type 1B (CMT1B) neuropathy in humans and a similar demyelinating neuropathy in transgenic mice, is instead retained the ER where it activates an unfolded protein response. Under ER-stress conditions, protein kinase R-like endoplasmic reticulum kinase (PERK) phosphorylates eukaryotic initiation factor 2α (eIF2α) to attenuate global translation, thus reducing the misfolded protein overload in the ER. Genetic and pharmacological inactivation of Gadd34 (damage-inducible protein 34), a subunit of the PP1 phosphatase complex that promotes the dephosphorylation of eIF2α, prolonged eIF2α phosphorylation and improved motor, neurophysiological, and morphologic deficits in S63del mice. However, PERK ablation in S63del Schwann cells ameliorated, rather than worsened, S63del neuropathy despite reduced levels of phosphorylated eIF2α. These contradictory findings prompted us to genetically explore the role of eIF2α phosphorylation in P0S63del-CMT1B neuropathy through the generation of mice in which eIF2α cannot be phosphorylated specifically in Schwann cells. Morphologic and electrophysiological analysis of male and female S63del mice showed a worsening of the neuropathy in the absence of eIF2α phosphorylation. However, we did not detect significant changes in ER stress levels, but rather a dramatic increase of the MEK/ERK/c-Jun pathway accompanied by a reduction in expression of myelin genes and a delay in Schwann cell differentiation. Our results support the hypothesis that eIF2α phosphorylation is protective in CMT1B and unveil a possible cross talk between eIF2α and the MEK/ERK pathway in neuropathic nerves.

SIGNIFICANCE STATEMENT In the P0S63del (deletion of serine 63 in the extracellular domain of P0) mouse model of Charcot-Marie-Tooth type 1B (CMT1B), the genetic and pharmacological inhibition of Gadd34 (damage-inducible protein 34) prolonged eukaryotic initiation factor 2α (eIF2α) phosphorylation, leading to a proteostatic rebalance that significantly ameliorated the neuropathy. Yet, ablation of protein kinase R-like endoplasmic reticulum kinase (PERK) also ameliorated the S63del neuropathy, despite reduced levels of eIF2α phosphorylation (P-eIF2α). In this study, we provide genetic evidence that eIF2α phosphorylation has a protective role in CMT1B Schwann cells by limiting ERK/c-Jun hyperactivation. Our data support the targeting of the P-eIF2α/Gadd34 complex as a therapeutic avenue in CMT1B and also suggest that PERK may hamper myelination via mechanisms outside its role in the unfolded protein response.