Stim2b knockout induces hyperactivity and susceptibility to seizures in zebrafish larvae

STIM proteins regulate store-operated Ca2+ entry (SOCE) in every cell, but in neurons they are also involved in other calcium signaling pathways. Many features of STIMs activities are unclear, including their participation in neurological diseases. These problems can be solved by studies in vivo rather than at the level of cultured cells in vitro. This work aimed to understand the role of neuronal Stim2b protein in zebrafish, which shares 59% sequence identity with human STIM2. We generated the Stim2b knockout fish, which was fertile and had regular lifespan length. Using a variety of behavioral tests, we found that stim2b-/- zebrafish larvae were hyperactive in comparison to wildtype fish. The mutants had higher mobility, higher thigmotaxis, and disrupted phototaxis. They were also more sensitive to pentylenetetrazol (PTZ) and glutamate treatments. Using lightsheet microscopy, oscillations in Ca2+ levels were analyzed in the optic tectum neurons of fish expressing cytosolic calcium sensor GCaMP5G. Higher average oscillation frequency and higher average amplitudes of Ca2+ oscillations were observed in unstimulated stim2b-/- larvae, which correlates well with the behavioral changes. In addition, using RNAseq we detected upregulation of some genes like annexin 3a and G protein-coupled receptor 39 (gpr39), and downregulation of ribonucleotide reductase regulatory subunit M2 (rrm2), neuroguidin (ngdn) and homer2. The latter gene encodes a protein involved in several processes involved in Ca2+ homeostasis in neurons, including metabotropic glutamate receptors. We propose that due to the lack of Stim2b in neurons, the process of SOCE is dysregulated, triggers changes in gene expression, what in turn causes abnormal behavior such as hyperactivity and susceptibility to seizures.