Enhanced proconvulsant sensitivity, not spontaneous rapid swimming activity, is a robust correlate of scn1lab loss-of-function in stable mutant and F0 crispant hypopigmented zebrafish expressing GCaMP6s

Abstract

Zebrafish models of genetic epilepsy benefit from the ability to assess disease-relevant knock-out alleles with numerous tools, including genetically encoded calcium indicators (GECIs) and hypopigmentation alleles to improve visualization. However, there may be unintended effects of these manipulations on the phenotypes under investigation. There is also debate regarding the use of stable loss-of-function (LoF) alleles in zebrafish, due to genetic compensation (GC). In the present study, we applied a method for combined movement and calcium fluorescence profiling to the study of a zebrafish model of SCN1A, the main gene associated with Dravet syndrome, which encodes the voltage-gated sodium channel alpha1 subunit (Nav1.1). We evaluated for spontaneous and proconvulsant-induced seizure-like activity associated with scn1lab LoF mutations in larval zebrafish expressing a neuronally-driven GECI (elavl3:GCaMP6s) and a nacre mutation causing a common pigmentation defect. In parallel studies of stable scn1labs552 mutants and F0 crispant larvae generated using a CRISPR/Cas9 multi-sgRNA approach, we find that neither stable nor acute F0 larvae recapitulate the previously reported seizure-like rapid swimming phenotype nor does either group show spontaneous calcium events meeting criteria for seizure-like activity based on a logistic classifier trained on movement and fluorescence features of proconvulsant-induced seizures. This constitutes two independent lines of evidence for a suppressive effect against the scn1lab phenotype, possibly due to the GCaMP6s-derived genetic background (AB) or nacre hypopigmentation. In response to the proconvulsant pentylenetetrazole (PTZ), we see evidence of a separate suppressive effect affecting all conspecific larvae derived from the stable scn1labs552 line, independent of genotype, possibly related to a maternal effect of scn1lab LoF in mutant parents or the residual TL background. Nonetheless, both stable and F0 crispant fish show enhanced sensitivity to PTZ relative to conspecific larvae, suggesting that proconvulsant sensitivity provides a more robust readout of scn1lab LoF under our experimental conditions. Our study underscores the unexpected challenges associated with the combination of common zebrafish tools with disease alleles in the phenotyping of zebrafish models of genetic epilepsy. Our work further highlights the advantages of using F0 crispants and the evaluation of proconvulsant sensitivity as complementary approaches that faithfully reflect the shared gene-specific pathophysiology underlying spontaneous seizures in stable mutant lines. Future work to understand the molecular mechanisms by which scn1lab-related seizures and PTZ-related hyperexcitability are suppressed under these conditions may shed light on factors contributing to variability in preclinical models of epilepsy more generally and may identify genetic modifiers relevant to Dravet syndrome.