Axonal abnormalities in vanishing white matter.

Objective: We aimed to study the occurrence and development of axonal
pathology and the influence of astrocytes in vanishing white matter. Methods:
Axons and myelin were analyzed using electron microscopy and immunohistochemistry on Eif2b4 and Eif2b5 single- and double-mutant mice and patient brain tissue. In addition, astrocyte-forebrain co-culture studies were performed.
Results: In the corpus callosum of Eif2b5-mutant mice, myelin sheath thickness,
axonal diameter, and G-ratio developed normally up to 4 months. At
7 months, however, axons had become thinner, while in control mice axonal
diameters had increased further. Myelin sheath thickness remained close to normal, resulting in an abnormally low G-ratio in Eif2b5-mutant mice. In more
severely affected Eif2b4-Eif2b5 double-mutants, similar abnormalities were
already present at 4 months, while in milder affected Eif2b4 mutants, few
abnormalities were observed at 7 months. Additionally, from 2 months onward
an increased percentage of thin, unmyelinated axons and increased axonal density were present in Eif2b5-mutant mice. Co-cultures showed that Eif2b5
mutant astrocytes induced increased axonal density, also in control forebrain
tissue, and that control astrocytes induced normal axonal density, also in
mutant forebrain tissue. In vanishing white matter patient brains, axons and
myelin sheaths were thinner than normal in moderately and severely affected
white matter. In mutant mice and patients, signs of axonal transport defects
and cytoskeletal abnormalities were minimal. Interpretation: In vanishing white
matter, axons are initially normal and atrophy later. Astrocytes are central in
this process. If therapy becomes available, axonal pathology may be prevented
with early intervention.