Interactions between mitochondria and endoplasmic reticulum in demyelinated axons

Demyelination leads to axonal changes that involve the functions and dynamics of axonal mitochondria supporting metabolism and survival of axons. However, the changes in the physical interactions between mitochondria and endoplasmic reticulum, called mitochondria-associated membranes, are poorly understood in demyelinated axons. In this study, we investigated the three-dimensional ultrastructural changes in membrane juxtapositions between mitochondria and endoplasmic reticulum in axons of a chronic progressive demyelination mouse model caused by extra copies of proteolipid protein (PLP4e). In the optic nerve of PLP4e mice, most axons were ensheathed by myelin by age 1 month, but were demyelinated by age 5 months. At age 1 month, mitochondria in PLP4e mice were slightly larger than those in wild-type mice, while the size and frequency of juxtaposition were similar. At age 5 months, the sizes of mitochondria and size of juxtaposition in PLP4e mice were prominently larger than those in wild-type mice. In degenerating axons under demyelination, the enlargement of mitochondria was diminished, while the density and frequency of juxtaposition were similar to those of non-degenerating axons. These results suggest that interactions between mitochondria and ER are enhanced in chronically demyelinated axons and maintained during axonal degeneration in hereditary myelin diseases.