Abstract
Among the hypomyelinating leukodystrophies, Pelizaeus–Merzbacher disease (PMD) is a representative disorder. The disease is caused by different types of PLP1 mutations, among which PLP1 duplication accounts for ∼70% of the mutations. Previous studies have shown that PLP1 duplications lead to PLP1 retention in the endoplasmic reticulum (ER); in parallel, recent studies have demonstrated that PLP1 duplication can also lead to mitochondrial dysfunction. As such, the respective roles and interactions of the ER and mitochondria in the pathogenesis of PLP1 duplication are not clear. In both PLP1 patients’ and healthy fibroblasts, we measured mitochondrial respiration with a Seahorse XF Extracellular Analyzer and examined the interactions between the ER and mitochondria with super-resolution microscopy (spinning-disc pinhole-based structured illumination microscopy, SD-SIM). For the first time, we demonstrated that PLP1 duplication mutants had closer ER-mitochondrion interfaces mediated through structural and morphological changes in both the ER and mitochondria-associated membranes (MAMs). These changes in both the ER and mitochondria then led to mitochondrial dysfunction, as reported previously. This work highlights the roles of MAMs in bridging PLP1 expression in the ER and pathogenic dysfunction in mitochondria, providing novel insight into the pathogenicity of mitochondrial dysfunction resulting from PLP1 duplication. These findings suggest that interactions between the ER and mitochondria may underlie pathogenic mechanisms of hypomyelinating leukodystrophies diseases at the organelle level.
Highlights
Hypomyelinating leukodystrophies (HLD) is a large category of neurological disorders with a distinct characteristic of hypomyelination, among which Pelizaeus–Merzbacher disease (PMD; MIM 312080) is representative
PMD is a rare X-linked hypomyelination disorder caused by mutations in the proteolipid protein 1 gene (PLP1, NM_001128834.20), which is located on Xq21-q22 and encodes two major central nervous system (CNS) myelin proteins, including PLP1 and its spliced isoform DM20
Our results showed that the maximal oxygen consumption rate (OCR) was lower in fibroblast mitochondria of the PLP1 duplication mutants than the control group (Fig. 1d), which indicated that PLP1 duplication mutants might have a compromised mitochondrial integrity and impaired mitochondrial respiratory function
Summary
Hypomyelinating leukodystrophies (HLD) is a large category of neurological disorders with a distinct characteristic of hypomyelination, among which Pelizaeus–Merzbacher disease (PMD; MIM 312080) is representative. PMD is a rare X-linked hypomyelination disorder caused by mutations in the proteolipid protein 1 gene (PLP1, NM_001128834.20), which is located on Xq21-q22 and encodes two major central nervous system (CNS) myelin proteins, including PLP1 and its spliced isoform DM20. At the time of this work, our group had accumulated 94 PMD cases [2], giving us a unique opportunity to investigate the pathogenic mechanisms of this rare genetic disorder. Previous studies have shown different pathogenic mechanisms resulting from point and duplication mutations of PLP1 [3,4,5], respectively. Point mutations lead to misfolded protein accumulation in the endoplasmic reticulum (ER), triggering an unfolded protein response (UPR) pathway or defective trafficking and fusion of mutated PLP1 [5]. Our patient pool supports this distribution with 65 patients carrying PLP1 duplications and 29 carrying PLP1 point mutations [2]
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