Rationale:
Voltage-gated sodium channels (VGSCs) play a critical role in modulating neuronal excitability. SCN8A encodes the VGSC Nav1.6, which is expressed in both excitatory and inhibitory neurons. Mutations that increase SCN8A activity are associated with epilepsy and a range of behavioral abnormalities. Increased Nav1.6 expression has also been observed in brains from patients with Alzheimer’s disease (AD) and mouse models of AD. We hypothesize that increased Nav1.6 activity contributes to multiple clinically-challenging features of AD, including epileptiform discharges, cognitive impairment, and AD neuropathology.
Methods:
We recently published the characterization of the gain-of-function Scn8a R1620L mouse line. Heterozygous mutants (RL/+) are seizure susceptible and exhibit spontaneous seizures and numerous behavioral abnormalities. As increased Nav1.6 expression has been observed in AD brains, we hypothesize that further increasing SCN8A activity in an AD mouse model would exacerbate disease severity. To test this hypothesis, RL/+ mutants were crossed with hemizygous 5xFAD mutants to generate 5xFAD mutants that also harbored the SCN8A RL mutation (AD/RL). AD/+, RL/+, and wild-type (WT) littermates were also generated as controls. Mice were weighed weekly and survival was monitored. Spontaneous seizure frequency and behavior were also examined.
Results:
Consistent with our prediction, AD/RL mutants exhibited a dramatically more severe phenotype. We observed a striking sex difference in premature mortality in the AD/RL mutants. All of the female AD/RL mutants die by 4 months of age (N = 15), whereas only 20% of the males (N = 10) die prematurely. No death was observed among the AD/+, RL/+, or WT littermates over the same time period. We also compared locomotor activity and anxiety between male WT, AD/+, RL/+, and AD/RL littermates using the open field paradigm (N = 9-12/genotype). The distance traveled and average speed was comparable across all genotypes, indicating that locomotor activity is normal in AD/RL mutants. However, the AD/RL mutants spent significantly less time in the center of the open field apparatus compared to WT littermates, suggesting that the AD/RL mutants exhibit greater anxiety-like behavior. We also performed a preliminary analysis of spatial memory using the Morris water maze task (N = 5-7/genotype). We found that at five to six months of age, the RL/+ and AD/RL mutants exhibited impaired learning and memory compared to WT and AD/+ mice. Finally, we conducted a preliminary comparison of spontaneous seizure frequency in three to four month-old male and female WT, AD/+, RL/+, and AD/RL mutants. During four days of continuous observation, four of five AD/RL mutants exhibited spontaneous seizures, while no spontaneous seizures were observed in the controls.
Conclusions:
This study is the first to examine the effect of increased
SCN8A activity in a mouse model of AD. Our observations suggest that increased
SCN8A activity in AD may worsen disease severity, which could have important clinical implications. Ongoing studies include additional cognitive behavior tests, examination of AD neuropathology, and further characterization of spontaneous seizures and epileptiform activity.
Funding: None