Abstract
BackgroundThe rod photoreceptor cGMP-gated cation channel, consisting of three α- and one β subunit, controls ion flow into the rod outer segment (ROS). In addition to the β-subunit, the Cngb1 locus encodes an abundant soluble protein, GARP2 that binds stoichiometrically to rod photoreceptor cGMP phosphodiesterase type 6 (PDE6). To examine the in vivo functional role of GARP2 we generated opsin promoter-driven transgenic mice overexpressing GARP2 three-fold specifically in rod photoreceptors.ResultsIn the GARP2 overexpressing transgenic mice (tg), the endogenous channel β-subunit, cGMP phosphodiesterase α-subunit, peripherin2/RDS and guanylate cyclase I were present at WT levels and were properly localized within the ROS. While localized properly within ROS, two proteins cGMP phosphodiesterase α-subunit (1.4-fold) and cGMP-gated cation channel α-subunit (1.2-fold) were moderately, but significantly elevated. Normal stratification of all retinal layers was observed, and ROS were stable in numbers but were 19% shorter than WT. Analysis of the photoresponse using electroretinography (ERG) showed that tg mice exhibit no change in sensitivity indicating overall normal rod function, however two parameters of the photoresponse significantly differed from WT responses. Fitting of the rising phase of the ERG a-wave to an accepted model of phototransduction showed a two-fold increase in phototransduction gain in the tg mice. The increase in gain was confirmed in isolated retinal tissue and by suction electrode recordings of individual rod photoreceptor cells. A measure of response recovery, the dominant time constant (τD) was elevated 69% in isolated retina compared to WT, indicating slower shutoff of the photoresponse.ConclusionsGARP2 may participate in regulating visual signal transduction through a previously unappreciated role in regulating phototransduction gain and recovery.Electronic supplementary materialThe online version of this article (doi:10.1186/s12964-014-0067-5) contains supplementary material, which is available to authorized users.
Highlights
The rod photoreceptor cGMP-gated cation channel, consisting of three α- and one β subunit, controls ion flow into the rod outer segment (ROS)
GARP2 transgenic mice overexpress GARP2 in the rod photoreceptors To investigate the role of GARP2, we developed a transgenic mouse model overexpressing GARP2 in the rod photoreceptors
The same size product was obtained with RT-PCR from Line 3 transgenic retina cDNA, demonstrating that the GARP2 transgene-specific mRNA was transcribed (Figure 1B)
Summary
The rod photoreceptor cGMP-gated cation channel, consisting of three α- and one β subunit, controls ion flow into the rod outer segment (ROS). In addition to the β-subunit, the Cngb locus encodes an abundant soluble protein, GARP2 that binds stoichiometrically to rod photoreceptor cGMP phosphodiesterase type 6 (PDE6). Via alternative splicing, the GARP region of the CNGB1 locus is expressed as two shorter soluble proteins. One of these proteins, GARP2 is a 32 kDa protein and is about 20-fold more abundant than the other soluble protein, GARP1. We have found an opposite effect of excess GARP2 in mouse rods: increased gain and slowed recovery. These results indicate a novel role of GARP2 in phototransduction where it acts to regulate phototransduction gain and response recovery
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