Marie E. Burns

Slowed recovery of rod photoresponse in mice lacking the GTPase accelerating protein RGS9-1.

Timely deactivation of the α-subunit of the rod G-protein transducin (Gαt) is essential for the temporal resolution of rod vision. Regulators of G-protein signalling (RGS) proteins accelerate hydrolysis of GTP by the α-subunits of heterotrimeric G proteins in vitro. Several retinal RGS proteins can act in vitro as GTPase accelerating proteins (GAP) for Gαt. Recent reconstitution experiments indicate that one of these, RGS9-1, may account for much of the Gαt GAP activity in rod outer segments (ROS). Here we report that ROS membranes from mice lacking RGS9-1 hydrolyse GTP more slowly than ROS membranes from control mice. The Gβ5-L protein that forms a complex with RGS9-1 (ref. 10) was absent from RGS9-/- retinas, although Gβ5-L messenger RNA was still present. The flash responses of RGS9-/- rods rose normally, but recovered much more slowly than normal. We conclude that RGS9-1, probably in a complex with Gβ5-L, is essential for acceleration of hydrolysis of GTP by Gαt and for normal recovery of the photoresponse.

Rapid and Reproducible Deactivation of Rhodopsin Requires Multiple Phosphorylation Sites

Efficient single-photon detection by retinal rod photoreceptors requires timely and reproducible deactivation of rhodopsin. Like other G protein-coupled receptors, rhodopsin contains multiple sites for phosphorylation at its COOH-terminal domain. Transgenic and electrophysiological methods were used to functionally dissect the role of the multiple phosphorylation sites during deactivation of rhodopsin in intact mouse rods. Mutant rhodopsins bearing zero, one (S338), or two (S334/S338) phosphorylation sites generated single-photon responses with greatly prolonged, exponentially distributed durations. Responses from rods expressing mutant rhodopsins bearing more than two phosphorylation sites declined along smooth, reproducible time courses; the rate of recovery increased with increasing numbers of phosphorylation sites. We conclude that multiple phosphorylation of rhodopsin is necessary for rapid and reproducible deactivation.

Role of guanylate cyclase-activating proteins (GCAPs) in setting the flash sensitivity of rod photoreceptors

The retinas photoreceptor cells adjust their sensitivity to allow photons to be transduced over a wide range of light intensities. One mechanism thought to participate in sensitivity adjustments is Ca2+ regulation of guanylate cyclase (GC) by guanylate cyclase-activating proteins (GCAPs). We evaluated the contribution of GCAPs to sensitivity regulation in rods by disrupting their expression in transgenic mice. The GC activity from GCAPs−/− retinas showed no Ca2+ dependence, indicating that Ca2+ regulation of GCs had indeed been abolished. Flash responses from dark-adapted GCAPs−/− rods were larger and slower than responses from wild-type rods. In addition, the incremental flash sensitivity of GCAPs−/− rods failed to be maintained at wild-type levels in bright steady light. GCAP2 expressed in GCAPs−/− rods restored maximal light-induced GC activity but did not restore normal flash response kinetics. We conclude that GCAPs strongly regulate GC activity in mouse rods, decreasing the flash sensitivity in darkness and increasing the incremental flash sensitivity in bright steady light, thereby extending the rods operating range.

Absence of the RGS9·Gβ5 GTPase-activating Complex in Photoreceptors of the R9AP Knockout Mouse

Timely termination of the light response in retinal photoreceptors requires rapid inactivation of the G protein transducin. This is achieved through the stimulation of transducin GTPase activity by the complex of the ninth member of the regulator of G protein signaling protein family (RGS9) with type 5 G protein β subunit (Gβ5). RGS9·Gβ5 is anchored to photoreceptor disc membranes by the transmembrane protein, R9AP. In this study, we analyzed visual signaling in the rods of R9AP knockout mice. We found that light responses from R9AP knockout rods were very slow to recover and were indistinguishable from those of RGS9 or Gβ5 knockout rods. This effect was a consequence of the complete absence of any detectable RGS9 from the retinas of R9AP knockout mice. On the other hand, the level of RGS9 mRNA was not affected by the knockout. These data indicate that in photoreceptors R9AP determines the stability of the RGS9·Gβ5 complex, and therefore all three proteins, RGS9, Gβ5, and R9AP, are obligate members of the regulatory complex that speeds the rate at which transducin hydrolyzes GTP.

Control of rhodopsin activity in vision

Although rhodopsins role in activating the phototransduction cascade is well known, the processes that deactivate rhodopsin, and thus the rest of the cascade, are less well understood. At least three proteins appear to play a role: rhodopsin kinase, arrestin and recoverin. Here we review recent physiological studies of the molecular mechanisms of rhodopsin deactivation. The approach was to monitor the light responses of individual mouse rods in which rhodopsin was altered or arrestin was deleted by transgenic techniques. Removal of rhodopsins carboxy-terminal residues which contain phosphorylation sites implicated in deactivation, prolonged the flash response 20-fold and caused it to become highly variable. In rods that did not express arrestin the flash response recovered partially, but final recovery was slowed over 100-fold. These results are consistent with the notion that phosphorylation initiates rhodopsin deactivation and that arrestin binding completes the process. The stationary night blindness of Oguchi disease, associated with null mutations in the genes for arrestin or rhodopsin kinase, presumably results from impaired rhodopsin deactivation, like that revealed by the experiments on transgenic animals.

Phototransduction in a Transgenic Mouse Model of Nougaret Night Blindness

The Nougaret form of dominant stationary night blindness is linked to a G38D mutation in the rod transducin-α subunit (Tα). In this study, we have examined the mechanism of Nougaret night blindness using transgenic mice expressing TαG38D. The biochemical, electrophysiological, and vision-dependent behavioral analyses of the mouse model revealed a unique phenotype of reduced rod sensitivity, impaired activation, and slowed recovery of the phototransduction cascade. Two key deficiencies in TαG38D function, its poor ability to activate PDE6 (cGMP phosphodiesterase) and decreased GTPase activity, are found to be the major mechanisms altering visual signaling in transgenic mice. Despite these defects, rod-mediated sensitivity in heterozygous mice is not decreased to the extent seen in heterozygous Nougaret patients.