All ETDs from UAB

Advisory Committee Chair

Steven J Pittler

Advisory Committee Members

Mark O Bevensee

Kristina M Visscher

Timothy W Kraft

Christianne Strang

Document Type

Dissertation

Date of Award

2018

Degree Name by School

Doctor of Philosophy (PhD) School of Optometry

Abstract

GARP2, found exclusively in retinal rod photoreceptors, has been proposed to function as a structural protein, a calcium binding protein, and a modulator of the phosphodiesterase regulating visual phototransduction cascade kinetics. GARP2 is a splice variant of the Cngb1 gene which also encodes the β-subunit of the phototransduction cyclic nucleotide-gated cation channel and another glutamic acid-rich protein, GARP1. Mutations of the β-subunit and, recently discovered regions shared with the GARP en-coding regions of Cngb1cause retinitis pigmentosa (RP), while overexpression of GARP2 in the absence of the β-subunit accelerates the observed Cngb1-mediated retinal degeneration in mouse β-subunit knockout disease models. In this study, we have used a selective knockout of murine GARP2 (GARP2-KO) to assess functional and structural changes associated with its absence and to assess its role in the retina. In the GARP2-KO mice, the morphology of the photoreceptors remained intact. However, regions of longer outer segments were sporadically observed that were misaligned relative to the retinal pigment epithelium. At one month, the GARP2-KO had normal electroretinogram responses under both light- and dark-adapted conditions. However, surprisingly, the GARP2-KO photoresponse was altered by three months of age showing both scotopic a- and b- wave reductions, reduced bipolar cell sensitivity to light, faster oscillatory potentials, and reduced scotopic critical flicker fusion responses. To assess changes in gene expression triggered by the absence of GARP2, transcriptomes of GARP2-KO, Cngb1-X1, and WT mouse retinas were compared using RNAseq analysis. Ten commonly differentially expressed genes between the GARP2-KO and Cngb1-X1, both of which lack GARP2, were confirmed by RT-PCR that function in cell cycle regulation, maintenance of the connecting cilium, circadian rhythms, or retinoid signaling. This work has shown that the length of the rod outer segment and the formation and transmission of electrical signals from the rod photoreceptor to the bipolar cell is somewhat dependent on the presence of GARP2. The GARP2-KO phenotype is a subtle, yet progressive, non-degenerative, functionally atypical model of vision.

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