Research ReportUnique nuclear vacuoles in the motor neurons of conditional ADAR2-knockout mice

- GluA2 is a subunit of the AMPA receptor involved in motor neuron degeneration.
- Unedited GluA2 mRNA at the Q/R site catalyzed by ADAR2 plays a role in sporadic ALS.
- Conditional ADAR2 -knockout mice is a very specific animal model for sporadic ALS.
- ADAR2-reduction was associated with progressive deterioration of nuclear architecture resulting in vacuolated nuclei.
- Nuclear vacuoles may be due to a Ca2+-permeable AMPA receptor-mediated mechanism.

A reduction in adenosine deaminase acting on RNA 2 (ADAR2) activity causes the death of spinal motor neurons specifically via the GluA2 Q/R site-RNA editing failure in sporadic amyotrophic lateral sclerosis (ALS). We studied, over time, the spinal cords of ADAR2-knockout mice, which are the mechanistic model mice for sporadic ALS, using homozygous ADAR2flox/flox/VAChT-Cre.Fast (AR2), homozygous ADAR2flox/flox/VAChT-Cre.Slow (AR2Slow), and heterozygous ADAR2flox/+/VAChT-Cre.Fast (AR2H) mice. The conditional ADAR2-knockout mice were divided into 3 groups by stage: presymptomatic (AR2H mice), early symptomatic (AR2 mice, AR2H mice) and late symptomatic (AR2Slow mice). Light-microscopically, some motor neurons in AR2 and AR2H mice (presymptomatic) showed simple neuronal atrophy and astrogliosis, and AR2H (early symptomatic) and AR2Slow mice often showed vacuoles predominantly in motor neurons. The number of vacuole-bearing anterior horn neurons decreased with the loss of anterior horn neurons in AR2H mice after 40 weeks of age. Electron-microscopically, in AR2 mice, while the cytoplasm of normal-looking motor neurons was almost always normal-appearing, the interior of dendrites was frequently loose and disorganized. In AR2H and AR2Slow mice, large vacuoles without a limiting membrane were observed in the anterior horns, preferentially in the nuclei of motor neurons, astrocytes and oligodendrocytes. Nuclear vacuoles were not observed in AR2res (ADAR2flox/flox/VAChT-Cre.Fast/GluR-BR/R) mice, in which motor neurons express edited GluA2 in the absence of ADAR2. These findings suggest that ADAR2-reduction is associated with progressive deterioration of nuclear architecture, resulting in vacuolated nuclei due to a Ca2+-permeable AMPA receptor-mediated mechanism.