Temporal and Isoform-Specific Expression of CTBP2 Is Evolutionarily Conserved Between the Developing Chick and Human Retina.

Elizabeth Gage,Karl J Wahlin,Zixiao Wang,Calvin Chenault,Kameron Washington-Brown,Ray A Enke,Nusrat Jahan,Melissa K Jones,Devansh Agarwal,Donald J Zack,Sarah Szvetecz

doi:10.3389/fnmol.2021.773356

Abstract

Complex transcriptional gene regulation allows for multifaceted isoform production during retinogenesis, and novel isoforms transcribed from a single locus can have unlimited potential to code for diverse proteins with different functions. In this study, we explored the CTBP2/RIBEYE gene locus and its unique repertoire of transcripts that are conserved among vertebrates. We studied the transcriptional coregulator (CTBP2) and ribbon synapse-specific structural protein (RIBEYE) in the chicken retina by performing comprehensive histochemical and sequencing analyses to pinpoint cell and developmental stage-specific expression of CTBP2/RIBEYE in the developing chicken retina. We demonstrated that CTBP2 is widely expressed in retinal progenitors beginning in early retinogenesis but becomes limited to GABAergic amacrine cells in the mature retina. Inversely, RIBEYE is initially epigenetically silenced in progenitors and later expressed in photoreceptor and bipolar cells where they localize to ribbon synapses. Finally, we compared CTBP2/RIBEYE regulation in the developing human retina using a pluripotent stem cell derived retinal organoid culture system. These analyses demonstrate that similar regulation of the CTBP2/RIBEYE locus during chick and human retinal development is regulated by different members of the K50 homeodomain transcription factor family.

Highlights

Visual perception is initiated in the retina when photons of light are absorbed by rod and cone photoreceptor (PR) neurons
RIBEYE staining in the IPL by ED11 and OPL by ED16 demonstrated that synapses in the inner retina developed first, a concept that is supported by our previous work (Wahlin et al, 2008)
The vertebrate retina is a heterogeneous collection of diverse neurons and neuronal support cells that can be molecularly characterized by their unique transcriptional profiles

Summary

Introduction

Visual perception is initiated in the retina when photons of light are absorbed by rod and cone photoreceptor (PR) neurons. Visual stimuli received at PRs are transmitted to bipolar cells (BCs) followed by retinal ganglion cells (RGC) that project to higher-order neurons in the brain. PR and BC ribbon synapses are distinguished from conventional synapses by their electron-dense horseshoe-shaped structures that lie perpendicular to the synaptic membrane (Morgans, 2000a,b). These synapses are somewhat rare within the CNS in that they are restricted to cochlear hair cells of the inner ear, pinealocytes of the pineal gland, and PRs and BCs of the retina. Unlike conventional synapses that have wide active zones, synaptic contacts beneath ribbons are uniquely channeled for the tonic release of glutamate using a large complex of proteins, including the scaffold proteins RIBEYE (CTBP2), PICCOLO, and BASOON which link actin filaments with synaptic vesicle release (Takamori et al, 2006)

Methods

Results

Conclusion