Abstract
Hypomyelinating leukodystrophies (HLDs) are a group of hereditary CNS disorders characterized by hypomyelination and, sometimes, repeated cycles of demyelination and remyelination. In HLDs, various genetic mutations in the responsible genes disrupt the morphogenesis of oligodendrocytes (oligodendroglial cells), which wrap neuronal axons with their differentiated myelin sheaths. A stop-loss mutation (c.622T-C or c.622T-G) in the gene encoding claudin family tetraspan plasma membrane protein claudin-11 (CLDN11) is associated with HLD22, which is characterized by incomplete differentiation and hypomyelination or delayed myelination in the brain. Herein, we describe for the first time that a CLDN11 mutant protein with an additional amino acid sequence due to the stop-loss mutation, but not the wild-type protein, leads to decreased expression of oligodendroglial differentiation marker proteins in the FBD-102b oligodendroglial progenitor cell line, the model undergoing its differentiation, at both the molecular and morphological levels. Consistently, mutant CLDN11 exhibited decreased morphological differentiation with a reduced ability to extend processes. These cells contained punctate structures that were partially localized in the endoplasmic reticulum (ER) and stimulated phosphorylation of c-Jun N-terminal kinase (JNK) and eukaryotic translation initiation factor 2A (eIF2A) kinase, ER stress-responsible kinases. Hesperetin, a neuroprotective flavonoid that can downregulate ER stress, recovered the differentiation abilities of these cells. Notably, the effects were related to decreased phosphorylation of ER stress-responsible kinases. JNK was found to be present in a co-precipitate with the hesperetin core, whereby hesperetin inhibited signaling through c-Jun as a negative regulator of differentiation. These findings indicate that the HLD22-associated mutant protein can cause an ER stress response, decreasing cell morphological differentiation. In addition, this study offers possible therapeutic implications for the as-yet-unexplored mechanisms involved in HLD22, at least at the molecular and cellular levels.