Authors :
Presenting Author: Kala Nair, PhD – Stanford University School of Medicine
Gustavo Chau, B.S. – Stanford University School of Medicine; Madeline McKean, B.A. – Stanford University School of Medicine; Veronica Alonso, B.S. – Stanford University School of Medicine; Pierce Popson, B.S. – Stanford University School of Medicine; Ankita Batra, B.S. – Stanford University School of Medicine; Tristan Saucedo, B.S. – Stanford University School of Medicine; Maria Perez-Ramirez, Ph.D. – Stanford University School of Medicine; Lijun Ni, B.S. – Stanford University School of Medicine; John Huguenard, Ph.D. – Stanford University School of Medicine; Jennifer McNab, Ph.D. – Stanford University School of Medicine; Juliet Knowles, M.D., Ph.D. – Stanford University School of Medicine
Rationale:
Activity-regulated myelin plasticity is an adaptive process in the healthy brain. We discovered that myelin plasticity is also induced in the corpus callosum in response to absence seizures. Induced myelination, in turn, promotes epilepsy progression (Knowles et al, Nature Neuroscience 2022). Given that absence seizures involve large territories of the bilateral hemispheres, we sought to determine the brain-wide extent and functional consequences of seizure-induced myelin plasticity.
Methods:
Scn8a+/mut mice exhibit spontaneous, progressive, bilateral frontoparietal absence seizures. We mapped g-ratios (myelin sheath thickness per axon diameter) in Scn8a+/mut mice or Scn8a+/+ littermates with qMTI (quantitative magnetization transfer in conjunction with diffusion MRI) in the corpus callosum, internal capsules, hippocampal fornix and anterior commissure. To determine functional consequences of myelin plasticity, we induced OPC-specific deletion of the TrkB receptor for BDNF during seizure progression in Scn8a+/mut mice (Scn8a+/mut;TrkBfl/fl;PDGFRa::Cre mice; "OPC cKO"), previously shown to prevent activity-regulated myelination.Results:
Figure 1: Using qMTI in
Scn8a+/mut mice with established seizures (post-natal day [P]45-51), we found decreased g-ratios (increased myelin sheath thickness per axon diameter) across the longitudinal extent of the anterior and middle corpus callosum and in the anterior commissure, findings we confirmed with gold standard histology. By contrast,
Scn8a+/mut and
Scn8a+/+ mice exhibited similar g-ratios in the posterior corpus callosum and hippocampal commissures, where absence seizures are minimal. The internal capsules connect thalamocortical regions involved in seizures, but do not exhibit dramatic post-developmental myelin plasticity, and did not show seizure-related differences. No g-ratio differences were observed prior to seizure onset (P21). Figure 2: We previously showed that genetic blockade of activity-dependent myelination in
Scn8a+/mut mice markedly decreased seizure progression. Here, we found that
Scn8a+/mut mice with intact myelin plasticity exhibit spontaneous absence seizures with peak frequency ~8Hz. At 6 months, myelin plasticity blockade dramatically reduced spectral power of seizures (Figure 2), inter-hemispheric ictal hypersynchrony in motor and somatosensory cortices, and intra-hemispheric ictal hypersynchrony between somatosensory and visual cortices. Blocking myelin plasticity did not affect visual cortical coherence.
Conclusions:
Absence seizure-induced myelination is wide-spread but seizure-network specific, and occurs markedly in commissural tracts involved in seizure propagation. Seizure-induced myelin plasticity is required for seizure progression, ictal EEG power and hypersynchony, but does not prominently affect interictal network function.Funding:
NIH-NINDS K08 NS119800; NIH-NINDS
K12 NS098482; Citizens United for Research in Epilepsy (CURE) Epilepsy and Taking Flight Awards;
Child Neurology Foundation/Pediatric Epilepsy Research Foundation Elterman Award; Stanford Maternal and Children's Health Research Institute Pilot Award