Modulation of Mouse Rod Photoreceptor Responses by Grb14 Protein

Michael L Woodruff,Ammaji Rajala,Gordon L Fain,Raju V.S Rajala

doi:10.1074/jbc.m113.517045

Abstract

Previous experiments have indicated that growth factor receptor-bound protein 14 (Grb14) may modulate rod photoreceptor cGMP-gated channels by decreasing channel affinity for cGMP; however, the function of Grb14 in rod physiology is not known. In this study, we examined the role of Grb14 by recording electrical responses from rods in which the gene for the Grb14 protein had been deleted. Suction-electrode recordings from single mouse rods showed that responses of dark-adapted Grb14(-/-) mice to brief flashes decayed more rapidly than strain-controlled wild type (WT) rods, with decreased values of both integration time and the exponential time course of decay (τREC). This result is consistent with an increase in channel affinity for cGMP produced by deletion of Grb14. However, Grb14(-/-) mouse rods also showed little change in dark current and a large and significant decrease in the limiting time constant τD, which are not consistent with an effect on channel affinity but seem rather to indicate modulation of the rate of inactivation of cyclic nucleotide phosphodiesterase 6 (PDE6). Grb14 has been reported to translocate from the inner to the outer segment in bright light, but we saw effects on response time course even in dark-adapted rods, although the effects were somewhat greater after rods had been adapted by exposure to bleaching illumination. Our results indicate that the mechanism of Grb14 action may be more complex than previously realized.

Highlights

Growth factor receptor-bound 14 (Grb14) is a negative regulator of insulin receptor (IR)
We discovered that rod responses from Grb14Ϫ/Ϫ mice recover after illumination more rapidly than responses of wild type (WT) mouse rods, as expected if the channels are more sensitive to cGMP and open at a lower concentration after deletion of the Grb14 gene
Our experiments show that deletion of the Grb14 gene produces a clear effect on the response properties of mouse rods

Summary

Background

Growth factor receptor-bound 14 (Grb14) is a negative regulator of insulin receptor (IR). Previous experiments have indicated that growth factor receptor-bound protein 14 (Grb14) may modulate rod photoreceptor cGMP-gated channels by decreasing channel affinity for cGMP; the function of Grb in rod physiology is not known. Suction-electrode recordings from single mouse rods showed that responses of dark-adapted Grb14؊/؊ mice to brief flashes decayed more rapidly than strain-controlled wild type (WT) rods, with decreased values of both integration time and the exponential time course of decay (␶REC) This result is consistent with an increase in channel affinity for cGMP produced by deletion of Grb. There is convincing evidence of a negative role of Grb in insulin signaling [3, 4], experiments with Grb14Ϫ/Ϫ animals have revealed positive effects of Grb on receptor-tyrosine kinase signaling in a tissue-specific manner [5, 6] Preliminary results of this study were reported at a meeting of the Association for Research in Vision and Ophthalmology [11]

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION