Phosphorylation of eIF2α promotes Schwann cell differentiation and myelination in CMT1B mice with activated UPR.

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, 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 (UPR). Under stress conditions, the ER-resident kinase PERK phosphorylates eIF2α to attenuate global translation, thus reducing the misfolded protein overload in the ER. Genetic and pharmacological inactivation of Gadd34, a subunit of the PP1 phosphatase complex that promotes the dephosphorylation of eIF2α, prolonged eIF2α phosphorylation and improved motor, neurophysiological and morphological deficits in S63del mice. However, PERK ablation in S63del Schwann cells ameliorated, rather than worsened S63del neuropathy, despite reduced levels of P-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. Morphological 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 reduction in myelin genes expression and a delay in Schwann cell differentiation. Our results support the hypothesis that eIF2α-phosphorylation is protective in CMT1B and unveil a possible crosstalk between eIF2α and the MEK/ERK pathway in neuropathic nerves. SIGNIFICANCE STATEMENT: In the P0S63del mouse model of CMT1B, the genetic and pharmacological inhibition of Gadd34 prolonged eIF2α phosphorylation leading to a proteostatic rebalance that significantly ameliorated the neuropathy. Yet, ablation of the kinase PERK also ameliorated the S63del neuropathy, despite reduced levels of 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 UPR.