Functional assessment of a kcnb1 knock-out zebrafish to model KCNB1-related neurodevelopmental and epileptic disorders

Abstract

KEY POINTSO_LIkcnb1 is expressed in distinct cell subtypes and various regions of the central nervous system in zebrafish C_LIO_LIBrain anatomy and neuronal circuits are not disrupted in the kcnb1 loss-of-function zebrafish model C_LIO_LILoss of kcnb1 leads to altered behavior phenotype, light and sound-induced locomotor impairments C_LIO_LIkcnb1 knock-out zebrafish exhibit increased locomotor sensitivity to PTZ and elevated expression of epileptogenesis-related genes C_LIO_LIkcnb1-/- larvae show spontaneous and provoked epileptiform-like electrographic activity associated with disrupted GABA regulation C_LI ObjectiveKCNB1 encodes an -subunit of the delayed-rectifier voltage-dependent potassium channel Kv2.1. De novo pathogenic variants of KCNB1 have been linked to developmental and epileptic encephalopathies (DEE), diagnosed in early childhood and sharing limited treatment options. Loss-of-function (LOF) of KCNB1 with dominant negative effects has been proposed as the pathogenic mechanism in these disorders. Here, we aim to characterize a knock-out (KO) zebrafish line targeting kcnb1 (kcnb1+/- and kcnb1-/-) for investigating DEEs. MethodsThis study presents the phenotypic analysis of a kcnb1 knock-out zebrafish model, obtained by CRISPR/Cas9 mutagenesis. Through a combination of immunohistochemistry, behavioral assays, electrophysiological recordings, and neurotransmitter quantifications, we have characterized the expression, function, and impact of this kcnb1 LOF model at early stages of development. ResultsIn wild-type larval zebrafish, kcnb1 was found in various regions of the central nervous system and in diverse cell subtypes including neurons, oligodendrocytes and microglial cells. Both kcnb1+/- and kcnb1-/- zebrafish displayed impaired swimming behavior and "epilepsy-like" features that persisted through embryonic and larval development, with variable severity. When exposed to the chemoconvulsant pentylenetetrazol (PTZ), both mutant models showed elevated locomotor activity. In addition, PTZ-exposed kcnb1-/- larvae exhibited higher bdnf mRNA expression and activated c-Fos positive neurons in the telencephalon. This same model presents spontaneous and provoked epileptiform-like electrographic activity associated with disrupted GABA regulation. In this KO model, neuronal circuit organization remained unaffected. SignificanceWe conclude that kcnb1 knock-out in zebrafish leads to early-onset phenotypic features reminiscent of DEEs, affecting neuronal functions and primarily inhibitory pathways in developing embryonic and larval brains. This study highlights the relevance of this model for investigating developmental neuronal signaling pathways in KCNB1-related DEEs.