Analysis of phosducin as a candidate gene for retinopathies

Phosducin rs12402521 polymorphism predicts development of hypertension in young subjects with overweight or obesity

BACKGROUND AND AIMS

The G-protein regulator phosducin has been shown to be associated with stress-dependent blood pressure, but whether obesity is a modulator of the relationship between phosducin and risk of hypertension is unknown. We studied the effect of two phosducin polymorphisms on risk of hypertension in 273 overweight or obese (Ov-Ob) young-to-middle-age participants from the HARVEST and 287 normal weight (NW) participants.

METHODS AND RESULTS

Genotyping of phosducin SNPs rs12402521 and rs6672836 was performed by real time PCR. For rs12402521, 64.6% of the participants were homozygous for the G allele, 27.9% heterozygous, and 7.5% homozygous for the A allele. During 7.7 years of follow-up, 339 subjects developed hypertension. In a Cox multivariable model, carriers of the A allele had a 1.28 (95% CI,1.00-1.63, p = 0.046) increased risk of hypertension. However, increased incidence of hypertension associated with A allele (AA + AG, 79% and GG, 59%, p = 0.001) was observed only among Ov-Ob individuals with a hazard ratio of 1.60 (95% CI, 1.13-2.21, p = 0.007) whereas in NW subjects the incidence of hypertension did not differ by genotype (56% in both groups). In the whole cohort, there was a significant interaction of phosducin genotype with body mass index on the risk of hypertension (p = 0.012). For SNP rs6672836 no association was found with incident hypertension. No haplotype effect was detected on the risk of hypertension.

CONCLUSION

These data suggest that phosducin rs12402521 polymorphism is an important genetic predictor of obesity-related hypertension. In Ov-Ob carriers of the A allele aggressive nonpharmacological measures should be implemented.

The physiological roles of phosducin: from retinal function to stress-dependent hypertension

In the time since its discovery, phosducin's functions have been intensively studied both in vivo and in vitro. Phosducin's most important biochemical feature in in vitro studies is its binding to heterotrimeric G protein βγ-subunits. Data on phosducin's in vivo relevance, however, have only recently been published but expand the range of biological actions, as shown both in animal models as well as in human studies. This review gives an overview of different aspects of phosducin biology ranging from structure, phylogeny of phosducin family members, posttranscriptional modification, biochemical features, localization and levels of expression to its physiological functions. Special emphasis will be placed on phosducin's function in the regulation of blood pressure. In the second part of this article, findings concerning cardiovascular regulation and their clinical relevance will be discussed on the basis of recently published data from gene-targeted mouse models and human genetic studies.

Phosducin - a candidate gene for stress-dependent hypertension

Repeated exposure to stress may favor, both in experimental animals and in humans, an increase in blood pressure, leading in some instances to a true hypertensive state. It is thought that stress-induced hypertension is mediated by sympathetic nervous system activation that in turn, by exerting vasoconstrictor effects and increasing heart rate (and thus cardiac output), may promote the development and progression of the hypertensive state. A new study by Beetz and colleagues in this issue of the JCI, which reports the results of experimental studies carried out in both mice and humans, reveals the potential role of the phosducin gene in modulating the adrenergic and blood pressure responses to stress (see the related article beginning on page 3597).

The pharmacology of phosducin

The discovery of phosducin (Phd) in photoreceptor cells of the retina and the further identification of phosducin-like proteins (PhdLP) emphasizes the existence of a family of proteins characterized as cytosolic regulators of G protein functions. The individual members represent phosphoproteins with distinct tissue distributions whose highest concentrations were in the retina and the pineal gland, while lower levels were reported for tissues such as liver, spleen, striated muscle, and the brain. Several functions of Phd and PhdLP have been suggested, but their most important ability appears to be their high affinity sequestration with G betagamma subunits of heterotrimeric G proteins. This finding suggests that neutralization of G betagamma by Phd effectively impedes G protein-mediated signal transmission, since G alpha cannot reassemble with G betagamma to provide a functional G protein trimer (G alphabetagamma). Thus, it is the scavenger quality of Phd that is hypothesized to diminish intracellular communication simply by reducing the number of G proteins. An additional important function of Phd relates to the inhibition of G alpha subunits' inherent GTPase. The ability of Phd to directly bind G alpha subunits is probably of minor significance as the affinity between both proteins is low. In general, similar mechanisms have been reported for PhdLPs. In the majority of investigations concerning the interference of Phd with physiological mechanisms, the dark/light adaptation of retinal photoreceptor cells has been the most frequently studied aspect of Phd. More recently, Phd was associated with the adenylyl cyclase of olfactory cilia, as in the presence of the phosphoprotein an increased concentration of cAMP is observed. This finding is in line with the experimental outcome of permanent cell lines transfected to overexpress Phd, which exhibit sensitization to excitatory acting PGE(1), and isoproterenol, respectively. Furthermore, Phd was found to effectively slow down the mechanism of internalization of G protein-coupled opioid receptors. Pathophysiological processes associated with Phd were found for certain eye diseases. Experimental evidence suggests the development of retinal inflammation as a consequence of an autoimmunization process triggered by Phd or shorter fragments thereof. Thus, our present knowledge regarding the functions of members of the Phd family is limited currently to their control of G protein-mediated intracellular signal transmission, the process of endocytosis, and certain autoimmune diseases of the uvea and the pineal gland. However, recent information regarding the presence of certain members of the Phd family in the cell nucleus may bear new insights into the function of these compounds.

Acute annular outer retinopathy: report of four cases

PURPOSE

To describe the clinical findings and course in four patients with acute annular outer retinopathy.

METHODS

Four patients were evaluated during the course of acute annular outer retinopathy, and the historical and clinical findings were retrospectively collected.

RESULTS

Four healthy patients developed the acute onset of visual field loss associated with a localized, white annular outer retinopathy. All patients were caucasian; two were women, aged 29 and 32 years, and two were men, aged 71 and 79 years. The mean follow-up was 3.9 years (range, 1 to 6 years). On presentation, four eyes had an irregular, incomplete, peripapillary, annular band of gray-white, deep retinal opacification with visual

CONCLUSIONS

These four cases of acute annular outer retinopathy expand our knowledge of this disorder. Acute annular outer retinopathy may be a distinct entity or it may represent a variant of acute zonal occult outer retinopathy retinopathy. As more cases are recognized, the characteristic features of the disease spectrum, the etiology, and treatment options may be better elucidated.

PhLPs and PhLOPs in the phosducin family of G beta gamma binding proteins

In this study, we identify new isoforms of the retinal phosducin and investigate the expression of the phosducin family, showing that an isoform, PhLP1, has sequence homology with Phd and Gbeta gamma binding capability, whereas two isoforms (phosducin-like orphan proteins, PhLOPs) share sequence homology with Phd but fail to bind Gbeta gamma. Original identification of PhLP1 and the PhLOPs was from a human retina cDNA library, using a PCR product for library hybridization screening that contained a predicted functional epitope domain. The screen identified Phd and three related, but distinct, recombinants (PhLP1, PhLOP1, and PhLOP2). By RT-PCR, all isoforms are expressed in either retina or forskolin-stimulated Y79 retinoblastoma cells; however, the new isoforms are below the level of detection on Northern blot analysis. The predicted amino acid translation of each homologue revealed major differences, arising from either splice variants or gene duplication of Phd. To test the functional interaction of all phosducin isoforms with Gbeta gamma in vitro, a glutathione S-transferase (GST) fusion protein was developed for each member. Biochemical interaction with purified retinal transducin Gbeta gamma was verified for GST-Phd and demonstrated for GST-PhLP1; however, neither GST-PhLOP1 nor GST-PhLOP2 bound Gbeta gamma. Comparable results were observed when the GST-phosducin fusion proteins selectively sequestered Gbeta gammas from retinal extracts or when functional Gbeta gamma interactions were assessed using surface plasmon resonance technology. Phosducin and its isoforms are widely distributed in body tissues where they may participate in signal transduction pathways. Phd and PhLP1 possess an 11-amino acid conserved epitope domain (TGPKGVINDWR) that controls the high-affinity binding of Gbeta gamma; these isoforms are implicated in the G-protein signaling pathway. The phosducin-like orphan proteins (PhLOPs) fail to bind Gbeta gamma, suggesting that the PhLOP isoforms may participate in still unidentified signaling pathways.

Identification of genes causing photoreceptor degenerations leading to blindness

At least 15 genes with defects responsible for various forms of inherited retinal disease involving photoreceptor loss have been identified over the past eight years. Several of the genes were first considered as candidates for study because of their involvement in murine retinal disease, others because of their chromosomal loci. In two cases, novel genes were uncovered by positional cloning. Based on reports of disease loci for which no gene has yet been found, more than twice as many genes remain to be identified in this genetically heterogeneous group of diseases.

Crystal structure at 2.4 angstroms resolution of the complex of transducin betagamma and its regulator, phosducin

The crystal structure of transducin's betagamma subunits complexed with phosducin, which regulates Gtbetagamma activity, has been solved to 2.4 angstroms resolution. Phosducin has two domains that wrap around Gtbetagamma to form an extensive interface. The N-terminal domain binds loops on the "top" Gtbeta surface, overlapping the Gtalpha binding surface, explaining how phosducin blocks Gtbetagamma's interaction with Gtalpha. The C-terminal domain shows structural homology to thioredoxin and binds the outer strands of Gtbeta's seventh and first blades in a manner likely to disrupt Gtbetagamma's normal orientation relative to the membrane and receptor. Phosducin's Ser-73, which when phosphorylated inhibits phosducin's function, points away from Gtbetagamma, toward a large flexible loop. Thus phosphorylation is not likely to affect the interface directly, but rather indirectly through an induced conformational change.