Patrick, R. L. | Search bioRxiv

bioRxiv2022

Transcriptional control of visual neural circuit development by GS homeobox 1

As essential components of gene expression networks, transcription factors regulate neural circuit assembly. GS homeobox 1 (gsx1) is expressed in the developing visual system; however, no studies have examined its role in visual system formation. In zebrafish, retinal ganglion cell (RGC) axons terminate in ten arborization fields (AFs) in the optic tectum (TeO) and pretectum (Pr). Pretectal AFs (AF1-AF9) mediate distinct and essential visual behaviors, yet we understand less about their development compared to AF10 in the TeO. Using gsx1 zebrafish mutants, immunohistochemistry, and transgenic lines, we observed that gsx1 is required for vesicular glutamate transporter, slc17a6b, expression in the Pr, but not overall neuron number. gsx1 mutants have normal eye morphology, yet exhibit impaired vision and a significantly reduced volume of RGC axons innervating the Pr and TeO, including loss of AF7. Consistent with this, prey capture is reduced in gsx1 mutants. Timed laser ablation of slc17a6b-positive neurons reveals that they aide directly in AF7 formation. This work is the first to implicate gsx1 in establishing cell identity and functional neural circuits in the visual system. SUMMARY STATEMENTThis is the first study in any vertebrate model to establish a requirement for the homeobox transcription factor encoding gene, gsx1, in visual neural circuit formation and function.

neuroscience↗

bioRxiv2021

Gsx2 but not Gsx1 is necessary for early forebrain patterning and long-term survival in zebrafish

Central nervous system (CNS) development is regulated by regionally expressed transcription factors that impart initial cell identity, connectivity, and function to neural circuits through complex molecular genetic cascades. genomic screen homeobox 1 and 2 (gsx1 and gsx2) encode homeobox transcription factors expressed in the developing CNS in multiple vertebrates examined to date. However, we have limited knowledge of the expression of these transcription factors and the gene networks that they regulate across developing brain regions in zebrafish. The objective of this study was to comprehensively examine gsx1 and gsx2 expression throughout neurodevelopment and characterize gsx1 and gsx2 mutants to study the essential roles of these closely related transcription factors. Using RT-PCR, whole-mount in situ hybridization (WISH), and fluorescence in situ hybridization, we examine gsx1 and gsx2 expression from early embryonic to late larval stages. gsx1 is expressed initially in the hindbrain and diencephalon and later in the optic tectum, pretectum, and cerebellar plate. Comparatively, gsx2 is expressed in the early telencephalon and later in the pallium and olfactory bulb. gsx1 and gsx2 are regionally co-expressed in the hypothalamus, preoptic area, and hindbrain, however rarely co-localize in the same cells. To identify forebrain target genes, we utilize mutants made with Transcription activator-like effector nucleases (TALEN). gsx1 mutant zebrafish exhibit stunted growth, however, they survive through adulthood and are fertile. gsx2 mutant zebrafish experience swim bladder inflation failure that prevents survival past larval stage. Using WISH and RT-qPCR we demonstrate altered expression of genes including, distal-less homeobox genes and forkhead box gene foxp2. This work provides novel tools with which other target genes and functions of Gsx1 and Gsx2 can be characterized across the CNS to better understand the unique and overlapping roles of these highly conserved transcription factors.

developmental biology↗