To investigate whether myelin was associated with neuropathological markers of axonal distress in experimental autoimmune encephalomyelitis (EAE) mice treated with vehicle and with clemastine, a compound capable of enhancing remyelination.
It was reported that the presence of myelin may be associated with neuropathological evidence of axonal degeneration, introducing a provocative paradigm in which myelin, by serving as an immunological target at a cellular level, makes axons more susceptible to damage.
We induced EAE in 30 female mice. Vehicle or clemastine (10 mg/kg) was dosed daily. On day 30, mice were euthanized, and optic nerve ultrastructure analyzed using EM (10 images/optic nerve). Axons containing enlarged (EnM) and multiple mitochondria (MM) and presenting with darker axoplasm (DA) were quantified.
We found that the frequency of distressed axons was slightly higher in unmyelinated axons (27.8% displayed EnM,11.7% MM,14.8% DA) when compared with myelinated axons (25% showed EnM,11.6% MM,9.7% DA). We found that clemastine preserved the axon integrity and were able to identify axons by an increase of 32%.
The frequency of axons presenting EnM (12.7% of unmyelinated and 17.8% of myelinated axons) or MM (6.4% of unmyelinated and 6.6% of myelinated axons) was reduced by clemastine treatment, while the frequency of axons with DA increased in both unmyelinated and myelinated axons (28.8% of unmyelinated and 19.8% of myelinated axons).
We found that the signs of axonal degeneration were more prevalent in unmyelinated axons of mice treated with vehicle. The clemastine-treated group displayed a higher number of axons, a reduction in the percentage of axons with EnM and MM, but more DA axons. While this approach is limited in scope—our findings are consistent that remyelination therapy is beneficial to axonal integrity. These findings cast significant doubt on the previous claim that myelin is a risk factor for axonal degeneration in autoimmune demyelinating disease.