# Transcriptomic identification and developmental mapping of nrg3b expression in zebrafish

## TL;DR

This study identifies when and where a zebrafish gene called nrg3b is active during development and in adults, suggesting it may help form brain connections.

## Contribution

The study provides the first detailed characterization of nrg3b expression patterns in zebrafish embryos and adults.

## Key findings

- nrg3b expression increases significantly during early zebrafish development.
- nrg3b is enriched in excitatory neurons and specific brain regions in adults.
- nrg3b is part of a gene network linked to neuronal function.

## Abstract

Neuregulin 3 (NRG3) is an epidermal growth factor-like ligand that is involved in neuronal circuit formation in mammals. However, the expression profile of its zebrafish ortholog, nrg3b, remains poorly defined. The transcriptomic analyses of zebrafish sco-spondin mutants, which display alterations in their neurodevelopmental pathways, have indicated potential dysregulation of Nrg3b. Guided by these insights, this study aimed to characterize the temporal and spatial expression patterns of nrg3b in vivo in embryonic and adult zebrafish.

Publicly available RNA-seq data from heterozygous and maternal-zygotic zebrafish sco-spondin mutants at 5, 15, and 30 days postfertilization (dpf) were analyzed to evaluate differential expression, coexpression network positioning, and functional enrichment. Temporal expression of nrg3b was validated by quantitative reverse transcription polymerase chain reaction (RT-qPCR) in embryos at 1, 3, and 5 dpf. Spatial nrg3b localization at 5 dpf was validated by whole-mount in situ hybridization (WISH), alongside neuronal subtype specificity using GABAergic and glutamatergic markers. Adult brain expression was further evaluated via ISH on telencephalon sections.

Developmental stage was the main driver of early transcriptomic variation, with minor genotype-specific differences detected at 5 dpf. Network analysis positioned nrg3b as a hub gene within a neuronal function associated module. RT-qPCR showed a significant increase in nrg3b expression on days 3 and 5 compared to day 1. WISH revealed strong nrg3b expression in anterior brain regions and the spinal cord at 5 dpf, with greater overlap observed with the glutamatergic marker compared to the GABAergic one. In adults, ISH suggested the expression of nrg3b in telencephalic nuclei, particularly in dorsomedial and ventral regions.

nrg3b exhibits stage-dependent upregulation and preferential enrichment in excitatory neuronal regions during development, with sustained expression in adult telencephalon. These findings suggest a potential role for Nrg3b in synaptic organization and provide a foundation for future studies on the Nrg3b–ErbB4 signaling axis in zebrafish neurodevelopment.