RGS (regulator of G protein signaling) proteins containing the G protein ␥-like (GGL) domain (RGS6, RGS7, RGS9, and RGS11) interact with the fifth member of the G protein ␤-subunit family, G␤5. This interaction is necessary for the stability of both the RGS protein and for G␤5. Consistent with this notion, we have found that elevation of RGS9-1 mRNA levels by transgene expression does not increase RGS9-1 protein level in the retina, suggesting that G␤5 levels may be limiting. To examine further the interactions of G␤5 and the GGL domain-containing RGS proteins, we inactivated the G␤5 gene. We found that the levels of GGL domain-containing RGS proteins in retinas and in striatum are eliminated or reduced drastically, whereas the levels of G␥2 and RGS4 proteins remain normal in the absence of G␤5. The homozygous G␤5 knockout (G␤5 ؊͞؊ ) mice derived from heterozygous knockout mating are runty and exhibit a high preweaning mortality rate. We concluded that complex formation between GGL domain-containing RGS proteins and the G␤5 protein is necessary to maintain their mutual stability in vivo. Furthermore, in the absence of G␤5 and all four RGS proteins that form protein complexes with G␤5, the animals that survive into adulthood are viable and have no gross defects in brain or retinal morphology.F irst discovered functionally as negative regulators of G protein signaling in Saccharomyces cerevisiae (Sst2p) (1) and Caenorhabditis elegans (EGL10) (2), RGS (regulator of G protein signaling) proteins accelerate the hydrolysis of GTP by the ␣-subunits of heterotrimeric G proteins (3). They form a large gene family with a diagnostic Ϸ120-aa RGS domain in which the GTPase-accelerating activity resides (4). In addition to the RGS domain, most RGS proteins possess additional domains that enable them to interact with other cellular signaling molecules (5). A subgroup of the RGS family, namely RGS9, RGS11, RGS7, and RGS6, possesses a G ␥-like (GGL) domain that binds the fifth member of the heterotrimeric G protein ␤-subunit (G␤5) both in vitro and in vivo (6-10). There are five known members of the G protein ␤-subunit family (11, 12). The first four members, G␤ 1-4 , are highly similar, sharing 80-90% sequence identity. G␤5 is the most divergent member of this family, sharing only 50% sequence identity with G␤ 1-4 . G␤5 exists in two forms: the common, short-splice form (G␤5-S) and a unique, long-splice form (G␤5-L) that exists exclusively in retinal photoreceptors. The long form results from the addition of an N-terminal exon through alternative splicing (11). G␤5 complexes with either GGL domain-containing RGS proteins or with certain G protein ␥-subunits such as G␥2 (12, 13). Likewise, the GGL domain-containing RGS proteins not only interact with G␤5, they also can interact with other proteins such as polycystin (for RGS7) (14) and SCG10 (for RGS6) (15). Interestingly, these RGS proteins do not interact with G␤ 1-4 , indicating that their interaction with G␤5 is selective and may be important for their in vivo function (6, 1...

Retinal photoreceptors are highly differentiated postmitotic neurons that transduce photons into electrical signals. While the functions of many photoreceptor-specific genes can be evaluated by direct gene targeting, here we facilitate the studies of nonphotoreceptor-specific genes in these cells by developing an Opsin-iCre transgenic mouse line, iCre-75, in which a 4-kb mouse rod opsin promoter drives the expression of bacteriophage P1 Cre recombinase. Immunohistochemical analysis demonstrated that Cre recombinase is present exclusively in the outer nuclear layer of iCre75 mouse retina. Cre expression is found only in rods and not in cones. The expression level reached 188+/-44 ng per retina at postnatal day (pnd) 11 and increased to 687+/-56 ng at 2 months and older. Cre-mediated excision of floxed genomic DNA was absent at pnd 4, became detectable at pnd 7, and was completed by pnd 18. Retinal morphology and electroretinograms were normal in 8-month-old transgenic animals. The iCre-75 transgenic mice are thus suitable for future genetic studies of essential genes in retinal rod photoreceptors.

Excessive phototransduction signaling is thought to be involved in light-induced and inherited retinal degeneration. Using knockout mice with defects in rhodopsin shut-off and transducin signaling, we show that two different pathways of photoreceptor-cell apoptosis are induced by light. Bright light induces apoptosis that is independent of transducin and accompanied by induction of the transcription factor AP-1. By contrast, low light induces an apoptotic pathway that requires transducin. We also provide evidence that additional genetic factors regulate sensitivity to light-induced damage. Our use of defined mouse mutants resolves some of the complexity underlying the mechanisms that regulate susceptibility to retinal degeneration.