Abstracts

Rationale: Dravet syndrome (DS) is a severe and intractable pediatric epilepsy linked to variants in genes encoding voltage-gated sodium channels (VGSCs). VGSCs control the generation and propagation of action potentials in neurons and therefore play a critical role in determining neuronal signaling, brain excitability, and seizure susceptibility. While most DS patients have variants in SCN1A, encoding the VGSC a subunit Nav1.1,  a subset of DS patients has loss-of-function variants in SCN1B, encoding the VGSC ß1/ß1B subunits. ß1/ß1B modulate VGSC gating and voltage-dependent properties and regulate the level of VGSC expression. In addition, these subunits participate in cell adhesion, neuronal pathfinding, and axon fasciculation in the central nervous system (CNS). Scn1b-null mice model DS, exhibiting spontaneous generalized seizures, spinal deformities, stunted growth, developmental delay, and death in the third postnatal week. VGSCs are expressed in the epithelial cells, nerves, and excitable pacemaker cells in the mammalian GI tract, yet the mechanisms underlying the aversive GI symptoms observed in DS patients remain unknown.My overall objective is to understand how variants in SCN1B contribute to the GI abnormalities and failure-to-thrive phenotype observed in DS patients. GI functional and motility disorders may arise from motor dysfunction in the wall of the GI tract, from aberrant axonal pathfinding in enteric nervous system (ENS)-ENS or CNS-ENS connections and subsequent alterations in neuronal control of digestive processes, or from morphological abnormalities. Determining changes in GI VGSC expression and functionality, neuronal pathfinding and ENS/CNS innervation patterns, and developmental epithelial cell migration in the Scn1b-null GI tract, are essential to understanding the role of ß1/ß1B subunits in normal GI physiology and the aberrant GI physiology observed in DS patients. Methods: To characterize the aberrant GI phenotype in postnatal-day (P) 13-21 Scn1b-null mice, null and age-matched and/or weight-matched wild-type (WT) mice were weighed and the intestinal tract was dissected. After measuring the length of the small intestine and colon, the tract was separated into duodenum, jejunum, ileum, and colon and samples were prepared for paraffin-embedding and sectioning. A subset of sections were stained with hematoxylin and eosin (H&E) to enable villus height measurement. The remaining sections were stained with anti-TUJ1 antibody to determine the extent of neuronal innervation of the GI tract. All measurements were made by an investigator blinded to genotype. Results: Our data suggest that Scn1b-null mice display decreased developmental weight gain compared to age-matched WT mice. In spite of this, the small intestine of Scn1b-null mice continues to grow, even after weight stagnation, up until death in the third postnatal week. While all sections of the GI tract in null and WT mice contain neuronal innervation, the villi of null animals appear to be shorter compared to WT. Conclusions: Our results suggest that SCN1B may function in GI epithelial cell development, adhesion, and/or migration. This work therefore illustrates a potential mechanism by which loss-of-function DS variants in SCN1B may impair proper absorption of nutrients and induce the failure-to-thrive phenotype observed in DS patients. Future work will confirm these findings and investigate potential differences in neuronal activity between genotypes, which is vital for maintaining GI contractions as well as controlling secretion of hormones, enzymes, and neuropeptides necessary for digestion. Funding: NIH-T32 Systems and Integrative Physiology Training Grant GM008322-26, NIH R37NS076752 to LLI