Activation of XBP1s attenuates disease severity in models of proteotoxic Charcot-Marie-Tooth type 1B
Mutations in myelin protein zero (MPZ) are generally associated with Charcot-Marie-Tooth type 1B (CMT1B) disease, one of the most common forms of demyelinating neuropathy. Pathogenesis of some MPZ mutants, such as S63del and R98C, involves protein misfolding and retention in the endoplasmic reticulum (ER) of myelinating Schwann cells. To cope with proteotoxic ER-stress, Schwann cells mount an unfolded protein response (UPR) characterized by activation of the PERK, ATF6 and IRE1/XBP1 pathways. Previous studies have reported that targeting PERK pathway can mitigate the neuropathy in CMT1B mice. To unravel the role of the XBP1 pathway in normal myelination and in CMT1B, we generated mouse models of in which XBP1 is deleted specifically in Schwann cells.  We observed that whereas XBP1 is dispensable for normal developmental myelination, myelin maintenance and remyelination after injury, the absence of XBP1 dramatically worsens the hypomyelination and the electrophysiological and locomotor parameters in young and adult CMT1B neuropathic animals. RNAseq analysis suggested that XBP1 exerts its adaptive function in large part via the induction of genes involved in misfolded protein degradation. Accordingly, the exacerbation of the neuropathy was accompanied by upregulation of ER-stress pathways and of IRE1-mediated RIDD signaling, suggesting that the activation of XBP1 plays a critical role in limiting mutant proteins toxicity, which cannot be compensated by other stress responses. Schwann cell specific overexpression of spliced XBP1 partially re-established Schwann cell proteostasis and attenuated CMT1B severity in vivo in both the S63del and R98C mouse models. In addition, the pharmacologic selective activation of XBP1 signaling ameliorated myelination in S63del dorsal root ganglia explants. Overall design: RNA-seq of mouse sciatic nerve samples, with 4 biological replicates per genotype.