Abstracts

Rationale: Many neurological diseases present with comorbid gastrointestinal (GI) abnormalities, suggesting parallel altered functionality of neurons in the central nervous system (CNS) and enteric nervous system (ENS) or aberrations in the nerve tracts constituting the gut-brain-axis. A potential mechanistic link between neurological and GI disorders is altered functionality of voltage-gated sodium channels (VGSCs), which are located in the CNS, ENS, and vagus nerve (VN), a major gut-brain-axis tract. VGSCs control the generation and propagation of action potentials in neurons and play critical roles in excitable cell function. VGSC β1/β1B subunits modulate channel trafficking, gating, and voltage-dependence. β1/β1B subunits also mediate cell-cell and cell-matrix adhesion, neuronal pathfinding, and axon fasciculation in the CNS. Variations in VGSC expression and activity have been implicated in GI and neurological diseases. Dravet syndrome (DS) is a severe, intractable developmental and epileptic encephalopathy that is linked to VGSC gene variants. A subset of DS patients has homozygous loss-of-function (LOF) variants in SCN1B, encoding the VGSC β1/β1B subunits. In addition to seizures and intellectual disability, SCN1B-linked DS patients present with GI dysfunction and failure-to-thrive (FTT). Similar to DS patients, Scn1b-null mice exhibit spontaneous generalized seizures, stunted growth, and reduced developmental weight gain. The objective of this work is to understand how LOF mutations in Scn1b contribute to GI abnormalities and FTT in mice. We hypothesize that β1/β1B subunits function to (1) regulate neuronal excitability in the ENS, thereby controlling GI motility and secretions and (2) mediate VN tract formation, transmission, and innervation of the GI tract. Methods: Postnatal-day (P) 13-21 Scn1b-null mice and age-matched or weight-matched wild-type (WT) mice were weighed and the GI tracts dissected. Each GI tract was measured and then separated into duodenum, jejunum, ileum, and colon, and samples were prepared for either immunostaining and confocal imaging or for qPCR. Confocal images were analyzed by an investigator blind to genotype. The body composition of age-matched Scn1b-null and WT mice was determined by NMR. Vagus nerves were dissected from Scn1b WT mice, immunostained, and analyzed via confocal imaging. Results: Scn1b-null mice recapitulate the FTT phenotype observed in SCN1B-linked DS patients, and are thus an ideal model system in which to study the role of β1/β1B subunits in GI dysfunction. Scn1b-null mice exhibit a stagnation in weight around P10, though their small intestines continue to increase in length as they age. Scn1b-null mice also exhibit a reduction in both total muscle mass (n=4-6, p<0.05) and intestinal lipid absorption (n=9). Preliminary results suggest that VGSCs are similarly localized to enteric neurons, smooth muscle cells, and interstitial cells of Cajal within the GI tract in WT and null mice and that there is an increase in neurotransmitter synthesis in Scn1b-null small intestines (n=3-4, p<0.05). Finally, we show that β1 is localized to nodes of Ranvier in WT vagus nerve. Conclusions: These results suggest that β1 may play a role in GI tissue formation and function, enteric neuron signaling processes, and gut-brain-axis transmission. Future work will investigate the effects of Scn1b deletion on GI cell functionality and absorptive capacity as well as on vagal nerve development, conduction, and patterns of innervation. Funding: NIH T32-GM008322 to VCB; NIH R37-NS076752 to LLI