Diseases of G-Protein-Coupled Signal Transduction Pathways: The Mammalian Visual System as a Model

Seminars in NEUROSCIENCE 9, 232–239 (1998) Article No. SN980126

Diseases of G-Protein-Coupled Signal Transduction Pathways: The Mammalian Visual System as a Model Janis Lem Department of Ophthalmology, New England Medical Center, Tufts University School of Medicine, Boston, Massachusetts 02111

G-protein-coupled signaling systems play a role in a diversity of normal physiological functions. Logically, one might predict that mutations in genes encoding any one of the G-protein subunits, G-protein-coupled receptors, or effector proteins of a given signaling pathway could lead to disease. Mutations of G-protein-coupled signaling proteins known to cause human diseases are reviewed here, with a primary emphasis on the mammalian phototransduction system. r 1998 Academic Press KEY WORDS: Phototransduction; disease; signal transduction; retinal degeneration.

INTRODUCTION G-protein-coupled signal transduction pathways play diverse roles in normal biological function, mediating the physiological actions of a broad array of hormones, neurotransmitters, growth factors, odorants, and light (1). In brief, signaling is initiated by the binding of a ligand to its associated seven transmembrane receptor. This produces a conformational change in the structure of the receptor, allowing it to bind and activate specific heterotrimeric G-proteins. G-proteins are composed of a-, b-, and g-subunits. In the inactive state, the G-protein exists in a heterotrimeric state, and GDP is bound to the a-subunit. Upon binding of a liganded receptor molecule to the Gprotein heterotrimer, GDP is displaced and replaced by GTP. The GTP-bound a-subunit dissociates from the bg-subunit complex (see Weng et al., this issue). The a-subunit and the bg-subunit complex are separately able to modulate the activity of downstream effector molecules such as enzymes or ion channels to effect physiological changes (see other articles, this issue). The role of G-protein signaling in normal physiological function predicts that mutations in any one of the components of the signaling pathway, i.e., receptors, G-protein subunits, or effector molecules, might lead to human disease. Mutations in G-protein-coupled

232

receptors and G-proteins have, in fact, been identified (2, 3). Identifying disease producing mutations and characterizing the mode of action of the mutant protein are critical for the development of appropriate therapies. For instance, treatment of a disease with a receptor agonist or antagonist is futile if a receptor mutation prevents agonist binding or if G-protein coupling is prevented by a mutation in the G-protein, thus preventing appropriate signaling. Disease related mutations in the various components of G-proteincoupled signaling systems are reviewed here. Primarily, genetic mutations associated with human disease will be discussed, focusing on the mammalian visual system as a model. Below, the biochemical pathway of phototransduction is described. This is followed by a discussion of disease-related mutations that have been identified in receptor, G-protein, effector, and molecules involved in receptor regeneration.

THE MAMMALIAN VISUAL SYSTEM The mammalian visual system is a prototypical G-protein-coupled signaling pathway (4). Phototransduction takes place in rod cells of the retina, the neurosensory tissue of the eye. In mammalian phototransduction (Fig. 1), the chromophore, 11-cis-retinal is covalently bound to the opsin protein. Light converts 1044-5765/98 $25.00 Copyright r 1998 by Academic Press All rights of reproduction in any form reserved.

233

G-Proteins in Disease

FIG. 1. Mammalian phototransduction cascade. A schematic of the biochemical pathway that is triggered upon exposure of a rhodopsin molecule to light is shown. See text for details. Proteins with disease-related mutations are enclosed in rectangles. R, rhodopsin; R*, light-activated rhodopsin; PDE, phosphodiesterase; GMP, guanosine monophosphate; cG, cyclic GMP.

the chromophore to all-trans retinal, thus producing a conformational change in the rhodopsin molecule. This increases its affinity for the photoreceptor-specific G-protein, transducin. Rhodopsin binding to the heterotrimeric transducin permits the substitution of GTP for GDP on the transducin a-subunit, followed by dissociation of the a-subunit from the bg-subunit complex. The activated a-subunit modulates the activity of the tetrameric effector molecule, cGMP phosphodiesterase, by removing the inhibitory phosphodiesterase g-subunit. This permits the catalytic aand b-phosphodiesterase subunits to hydrolyze cGMP, thus lowering cyclic GMP levels in the photoreceptor cell and closing the cyclic GMP-gated channels. While a regulatory function for G-protein bg complexes has

been shown in other G-protein-regulated signaling pathways (5), no such function has yet been identified in the visual signaling pathway.

Receptor Recovery In another biochemical loop, activated rhodopsin is restored to its original light-responsive state. The activated rhodopsin is inactivated by phosphorylation at a cluster of C-terminal serines (6). This significantly reduces the ability of the active rhodopsin molecule to interact with transducin, but does not completely quench it. Phosphorylation of three C-terminal serines is mediated by rhodopsin kinase, a member of the

Copyright r 1998 by Academic Press

234

Janis Lem

family of G-protein receptor kinases (GRKs) (7, 8). Complete rhodopsin inactivation occurs upon capping of the phosphorylated rhodopsin by retinal arrestin, a member of the family of arrestin molecules (7). This mechanism of receptor inactivation is common to a number of G-protein-coupled receptor signaling pathways, which often exhibit receptor serine or threonine residues that are phosphorylated by specific receptor kinases and capped by specific arrestins.

DISEASE MUTATIONS IN G-PROTEIN-COUPLED SIGNALING MOLECULES Defects in phototransduction signaling are most likely to present as a blinding disease or impaired visual function. Screening of individuals suffering from retinitis pigmentosa (RP), a heterogeneous group of blinding diseases, has proven to be a valuable tool for identifying mutations in G-protein-coupled genes of the phototransduction cascade. RP affects approximately 1 in 4000 individuals, with 50,000 to 100,000 affected individuals in the United States. Individuals suffering from RP often suffer from night blindness early in life. With increasing age, photoreceptor cells are progressively lost, ultimately leading to severely impaired vision or complete blindness. Genes encoding many of the phototransduction proteins have been cloned and sequenced, permitting screens for human mutations in individuals affected with RP. Mutations associated with human retinal degenerations or congenital stationary night blindness have been identified in many of these phototransduction genes, as described below.

tions. In vitro expression studies suggest that mutations can affect G-protein interactions, transport of the protein to the disk membrane, or regeneration of the rhodopsin protein with the retinal chromophore. Two recessively inherited point mutations have also been identified, presumably producing loss-of-function mutant proteins (10, 11). Two point mutations are associated with congenital stationary night blindness (12), a condition in which night vision is affected, but day light vision is essentially normal. In vitro studies suggest that these mutations cause a constitutive activation of the rhodopsin protein in the absence of light (13). However, for the constitutively activated K296E mutation, in vivo studies in transgenic mice show that, in fact, the mutant rhodopsin is predominantly present in a phosphorylated form and, hence, constitutively inactivated (14). This serves to remind us of the importance of performing in vivo studies. Mutations in the rhodopsin gene account for approximately 10 to 15% of all human retinal degenerations, and 30% of all inherited autosomal dominant retinal degenerations. Cone opsin mutations which lead to color blindness have also been identified (15). Mutations in a number of other G-protein-coupled receptors produce endocrine-related diseases (Table 1). Activating versus loss-of-function mutations can produce different disease phenotypes. Loss-of-function mutants of V2 vasopressin, adrenocorticotropic hormone (ACTH), growth hormone releasing-factor hormone (GHRH), follicle-stimulating hormone (FSH),

TABLE 1 Diseases Associated with G-Protein-Coupled Receptor Mutations Receptor Rhodopsin

Receptor Mutations The gene encoding rhodopsin is a member of the large family of G-protein-coupled receptors. Upward of 80 mutations in the rhodopsin gene have been associated with retinal degenerations (9). Mutations have been found in both the N- and the C-terminals, in all seven transmembrane domains, in the three cytoplasmic loops, and in two of the three intradiscal loops. No mutation has yet been identified in the third intradiscal loop, which is postulated to be involved in transducin coupling. A vast majority of the mutations are dominantly inherited point mutations, with just a handful of insertion, deletion, and frameshift muta-

Cone opsins Vasopressin V2 ACTH GHRH FSH LH TSH CaR

PTH Thromboxane A2 Endothelin B

Copyright r 1998 by Academic Press

Disease Autosomal dominant retinitis pigmentosa Autosomal recessive retinitis pigmentosa Congenital stationary night blindness Color blindness Nephrogenic diabetes insipidus Familial ACTH resistance Familial growth hormone deficiency Hypergonadotropic ovarian dysgenesis Male pseudohermaphroditism Familial male precocious puberty Familial hypothyroidism Sporadic hyperfunctional thyroid nodules Familial hypocalciuric hypercalcemia; Neonatal severe primary hyperparathyroidism Familial nonautoimmune hyperthyroidism Familial hypoparathyroidism Jansen metaphyseal chondrodysplasia Congenital bleeding Hirschsprung disease

235

G-Proteins in Disease

luteinizing hormone (LH), thyroid-stimulating hormone (TSH), and calcium (CaR) receptors have, respectively, been associated with nephrogenic diabetes insipidus, familial ACTH resistance, familial growth hormone deficiency, hypergonadotropic ovarian dysgenesis, male pseudohermaphroditism, familial hypothyroidism, and familial hypocalciuric hypercalcemia/neonatal severe primary hyperparathyroidism. Like the rhodopsin mutations, more than 70 mutations have been identified in the vasopressin V2 receptor, distributed throughout the receptor. In addition, loss-of-function mutations in the thromboxane A2 and endothelin B receptors have been associated with congenital bleeding and Hirschsprung disease (16), respectively. On the other hand, gain-of-function mutations in the LH, TSH, CaR, and PTH receptors have, respectively, been associated with familial male precocious puberty, sporadic hyperfunctional thyroid nodules, familial nonautoimmune hyperthyroidism, familial hypoparathyroidism, and Jansen metaphyseal chondrodysplasia. While mutations in adrenergic, dopamine, serotonin, histamine, muscarinic, and cholinergic receptors are attractive candidate genes for neurological diseases such as schizophrenia, bipolar affective disorder, and Alzheimer’s disease, to date, no disease-associated mutations have been identified in humans.

G-Protein Mutations In sharp contrast to the large number of diseaseassociated rhodopsin mutations, only a single diseaseassociated mutation has been identified in the photoreceptor-specific transducin gene (17). The mutation was found in a French family representing one of the largest and earliest recorded pedigrees of a dominantly inherited disease. The founder, Jean Nougaret, lived in Southern France in the 1600s and suffered from congenital stationary night blindness. Over 135 cases were identified from 2100 individuals over 11 generations. No other mutations in the a-transducin gene have been identified despite intensive screening of RP patients. The mutation occurred in a glycine residue that is conserved in all known G-proteins with the exception of Gz. The glycine residue is the equivalent of glycine 12 in the p21 ras protein, which, when mutated, becomes oncogenic. This mutation reduces GTPase activity in ras by interfering with GTP/GDP exchange. Consistent with this is the observation that transgenic mice with a knockout of the rod transducin a-subunit have morphologically normal retinas (18). It may be

that few mutations have been found because atransducin mutations are associated with nondegenerative visual dysfunctions. No mutations in the transducin b- or g-subunits have been reported. Disease-associated mutations have been identified in a- and b-subunits of other G-proteins as well (Table 2). To date, no mutations have been identified in G-protein g-subunit genes. Like the receptor mutations, activating versus loss-of-function mutations present with different disease phenotypes. Loss-of-function mutations in the Gs a-subunit (Gas ) are associated with pseudohypoparathyroidism. Activating mutations of Gas cause McCune–Albright syndrome (characterized by polyostotic fibrous dysplasia, cafe-au-lait skin hyperpigmentation, and endocrine hyperfunction) (19), acromegaly, and hyperfunctional thyroid nodules. Gain-of-function mutations in the Gai2 gene produce ovarian and adrenalcortical tumors (20). Unexpectedly, mice with the Gai2 gene knocked out exhibit gastrointestinal defects. These Ga mutations are typically somatic rather than germline events, producing their very localized effects, despite the general localization of Gas and Gai2 throughout many tissues of the body. Germline mutations, while probably occurring, are also likely to be lethal. Gene knockout studies of other G-protein a-subunits also suggest that they may play a role in human disease, although no mutations in these genes have yet been associated with human disease states. Mice with a homozygous knockout of the Gaq gene exhibit impaired motor coordination, ataxia, and abnormalities in Purkinje cell innervation, making the gene and other transduction proteins coupled to Gq potential candidate genes for human neurological disorders (21). There is also evidence that

TABLE 2 Diseases Associated with G-Protein Subunits G-protein

Subunit

Disease

Transducin

a

Gs

a

Gi2

a

Gq Golf Ggus

a a a

G13

a b3

Nougaret’s congenital stationary night blindness Pseudohypoparathyroidism McCune–Albright syndrome Acromegaly Hyperfunctional thyroid nodules Ovarian tumors Adrenalcortical tumors Neuromuscular/vascular disorders in mice Olfactory dysfunction in mice Bitter/sweet taste transduction deficits in mice Vascular defects in mice Hypertension

Copyright r 1998 by Academic Press

236

Janis Lem

these mice have platelet defects, with implications in cardiovascular disease (22). Mice with a null mutation in the a-subunit of the olfactory G-protein, Golf, are defective in olfactory function, which severely impacts the ability of affected pups to survive and the ability of mothers to care for offspring (23). Gustducin-deficient mice, which lack the a-subunit of the gustatory Gprotein in taste receptor cells, exhibit electrophysiological changes in bitter and sweet taste transduction (24). Mice with a knockout of Ga13 show no obvious effects in the hemizygous state, but the homozygous state is embryonic lethal, with effects on vascularization (25). Only one G-protein b-subunit has been associated with a disease phenotype thus far. Very recently, a polymorphic base substitution in the G-protein b3subunit has been described which is associated with a predisposition to hypertension (26). The polymorphism alters messenger RNA splicing upstream of the nucleotide change, deleting 122 amino acids of the protein and creating a gain-of-function mutation. How the G-protein b-subunit is involved in signal transduction in hypertension is unknown.

TABLE 3 Diseases Associated with Other G-Protein-Coupled Signaling Proteins Protein Effector proteins cGMP PDE a-subunit b-subunit

g-subunit cGMP-gated channel a-subunit GIRK2 Receptor recovery enzymes Rhodopsin kinase Retinal arrestin Guanylate cyclase GC activator 1A

Disease

Autosomal recessive retinitis pigmentosa Autosomal recessive retinitis pigmentosa Congenital stationary night blindness ?; Autosomal recessive retinitis pigmentosa in mice Autosomal recessive retinitis pigmentosa ?; Neurological defects in Weaver mice Oguchi congenital stationary night blindness Oguchi congenital stationary night blindness Leber’s congenital amaurosis Autosomal dominant cone dystrophy

Effector/Ion Channel Mutations (Table 3) The effector protein in the visual transduction cascade is a tetrameric cGMP phosphodiesterase (PDE) which hydrolyzes cGMP. The cGMP PDE enzyme is composed of catalytic a- and b-subunits and two inhibitory g-subunits. Screens of families for mutations in the a-subunit of PDE have revealed mutations in the gene associated with autosomal recessive RP (27). Both homozygous and compound heterozygous affected individuals have been identified. A mutation in the b-PDE gene was originally identified in the rd mouse, a naturally occurring mouse mutant (28). Degeneration of the retina of this mouse progresses quite rapidly and correlates with elevated cGMP levels. Subsequent to the rd mouse studies, examination of human pedigrees suffering from inherited retinal degeneration revealed mutations in the homologous human gene associated with recessive retinal degenerations (29) and congenital stationary night blindness (30). The defect has also been reported in Irish setter dogs with rod/cone dysplasia (31). Genetic screens for mutations in the PDE g-subunit have also been performed. Unlike the a- and bsubunits of PDE, no mutations of PDE g-subunit have been identified in humans. However, transgenic mice with a gene knockout of the PDE g-subunit have been produced. While mice carrying one nonfunctional

copy of the PDE g-subunit gene appeared histologically and physiologically normal, mice with two nonfunctional copies of the PDE g-subunit gene exhibited a rapid retinal degeneration (32). The mutation is recessively inherited. The rod cell cGMP gated channel is a member of the family of cylic nucleotide gated channels, which are also typically coupled to G-protein signaling. The rod cell cGMP gated channel is composed of an a- and a b-subunit. Thus far, genetic screens have identified mutations in the a-subunit of the cGMP gated channel, which are associated with recessively inherited retinal degenerations (33). Both homozygous and compound heterozygous mutations have been identified in affected humans. The naturally occurring weaver mouse mutant, which suffers from spontaneous seizures, has been shown to carry a mutation in the G-protein-gated inwardly rectifying potassium channel 2 (GIRK2) (34). The mutation produces a neurological defect resulting from degeneration of dopaminergic neurons in the substantia nigra and extensive cerebellar granule cell death. Studies of this animal model have been pursued with great interest because of similarities to human

Copyright r 1998 by Academic Press

237

G-Proteins in Disease

Parkinson’s disease. However, to date, no mutation has been reported in the human homologue of the GIRK2 gene.

Receptor Recovery Mutations Ligand-activated G-protein-coupled receptors typically utilize an inactivation mechanism which involves phosphorylation of the activated receptor mediated by a receptor-specific kinase. The activity of the phosphorylated receptor is significantly, but not completely, quenched by phosphorylation. Complete quenching comes with capping by an arrestin molecule. In the rod cell, rhodopsin kinase and a rod cellspecific arrestin sequentially quench activated rhodopsin molecules. Mutations in genes encoding both of these proteins have been associated with Oguchi disease (35, 36). This is a rare autosomal recessive form of congenital stationary night blindness characterized by difficulty with night vision, but in which visual acuity, visual field, and color vision are unaffected. Other receptor-specific kinases have also been cloned, sequenced, and characterized. Among the best studied, along with rhodopsin kinase, is the b-adrenergic receptor kinase (b-ARK). Transgenic mouse studies in which the protein is expressed as a gain-of-function or loss-of-function mutation demonstrate changes in cardiophysiology, suggesting a role in of b-ARK in heart disease (37). Similarly, transgenic mice with a knockout of the b2-adrenergic receptor strongly suggest a role in heart disease (38). However, despite physiological effects in mice, no mutation has yet been identified in humans.

Cyclic Nucleotide Mutants One common function of G-protein-coupled effectors is the regulation of cyclic nucleotide levels. In the retina, the effector molecule, phosphodiesterase, mediates the hydrolysis of cGMP. To restore the photoreceptor cell to its original light sensitive state, in addition to inactivating the receptor protein, it is necessary to restore the levels of cGMP. This is mediated by the enzyme guanylate cyclase, which converts 5’GMP to cGMP. Mutations in the guanylate cyclase gene are implicated in half the cases of Leber’s congenital amaurosis (39). This is an early and the most severe form of inherited retinopathy characterized by optic atrophy and attenuation of retinal vessels. Affected

infants are born totally blind or with severely impaired vision. In addition, a mutation in the guanylate cyclase activator 1A (GUCA1A) has been associated with cone dystrophy (40). The guanylate cyclase activating protein is a calcium-dependent protein which regulates the activity of guanylate cyclase in the retina, which in turn, regulates cellular levels of cGMP and, hence, the gating of the cGMP-gated channel. A mutation in a homologous gene has been identified in the retinal degeneration (rd) chicken model (41).

SUMMARY Elucidation of phototransduction pathways has lead to the cloning and sequencing of many rod-specific phototransduction genes. This has permitted screening of human pedigrees for mutations associated with retinal degenerations and other visual dysfunctions. As described in this paper, mutations in a number of phototransduction genes have been identified which produce a disease state. These include the rhodopsin seven transmembrane receptor, the transducin asubunit, the a- and b-subunits of cGMP phosphodiesterase, guanylate cyclase, guanylate cyclase activating protein, the a-subunit of the cGMP-gated cation channel, rhodopsin kinase, and retinal arrestin. We can apply our understanding of phototransduction mechanisms and disease-associated mutations to identifying potential disease-related mutations in transduction protein homologues in other signaling systems. For instance, in cardiac tissue, the seven transmembrane b-adrenergic receptor mediates its activity via the G-protein, Gas. The effector molecule is adenylyl cyclase, which catalyzes the production of cAMP to mediate physiological effects in the heart. The characterization of retinal arrestin lead to the identification of the homologous b-arrestin. Similar to phototransduction mechanisms, the b-adrenergic receptor is inactivated by phosphorylation with the receptorspecific b-adrenergic receptor kinase and b-arrestin. As in phototransduction signaling, mutations in any one of these proteins may produce physiopathological changes. At the present time, however, no mutations have been identified in humans. Mutations in signal transduction proteins of the brain are also potential candidates for neuropsychological disorders such as Alzheimer’s, schizophrenia, and bipolar affective disorder (42). G-protein-coupled receptors such as the adrenergic and cholinergic receptors interact with G-proteins in the brain, the most preva-

Copyright r 1998 by Academic Press

238

Janis Lem

lent being Gq and G11. These G-proteins activate the effector, phospholipase Cb, to mediate phosphoinositide signaling in the brain. Numerous studies of postmortem human brain tissue from Alzheimer patients clearly show altered signal transduction in diseased brain tissue (43), (44). However, these studies are only suggestive of G-protein or GPCR mutations in the human disease state. Thus far, no definitive mutation has been found associated with disease in humans. The Human Genome Project has lead to rapid progress in the identification and cloning of many signal transduction genes. As we piece together the roles of these genes in cell signaling, and the mechanism of action of mutations in these genes using models such as the visual system, it will be possible to identify homologous signaling components in other physiological systems and to look for similar dysfunctional mutations. Understanding how these mutations disrupt normal signaling mechanisms will permit development of rational drug therapies for the treatment of human disease.

ACKNOWLEDGMENTS J. Lem is the recipient of a Career Development Award from Research to Prevent Blindness and NIH Grant R01 EY12008-01. Thanks to D. Gee for helpful comments.

REFERENCES 1. Stryer, L., and Bourne, H. R. (1986) G proteins: A family of signal transducers. Annu. Rev. Cell Biol. 2, 391–419. 2. Spiegel, A. M. (1995) Defects in G protein-coupled signal transduction in human disease. Annu. Rev. Physiol. 58, 143–170. 3. Gordeladze, J. O., et al. (1994) G-Proteins: Implications for pathophysiology and disease. Eur. J. Endocrinol. 131, 557–574. 4. Stryer, L. (1986) Cyclic GMP cascade of vision. Annu. Rev. Neurosci. 9, 87–119. 5. Mu¨ller, S., and Lohse, M. J. (1995) The role of G-protein bg subunits in signal transduction. Biochem. Soc. Trans. 23, 141–148. 6. Ohguro, H., et al. (1993) Sequential phosphorylation of rhodopsin at multiple sites. Biochemistry 32, 5718–5724. 7. Freedman, N. J., and Lefkowitz, R. J. (1996) Desensitization of G protein-coupled receptors. Rec. Progr. Horm. Res. 51, 319–353. 8. Hausdorff, W. P., Caron, M. G., and Lefkowitz, R. J. (1990) Turning off the signal: Desensitization of b-adrenergic receptor function. FASEB J. 4, 2881–2889. 9. Gal, A., Apfelstedt-Sylla, E., Janecke, A. R., and Zrenner, E. (1997) Rhodopsin mutations in inherited retinal dystrophies and dysfunctions. Prog. Retinal Eye Res. 16(1), 51–79. 10. Kumaramanickavel, G., et al. (1994) Missense rhodopsin mutation in a family with recessive RP. Nature Genet. 8, 10–11.

11. Rosenfeld, P. J., et al. (1992) A null mutation in the rhodopsin gene causes rod photoreceptor dysfunction and autosomal recessive retinitis pigmentosa. Nature Genet. 1, 209–213. 12. Sieving, P. A., et al. (1995) Dark-light: Model for nightblindness from the human rhodopsin Gly-90 Asp mutation. Proc. Natl. Acad. Sci. USA 92, 880–884. 13. Rao, V. R., Cohen, G. B., and Oprian, D. D. (1994) Rhodopsin mutation G90D and a molecular mechanism for congenital night blindness. Nature 367, 639–641. 14. Li, T., et al. (1995) Constitutive activation of phototransduction by K296E opsin is not a cause of photoreceptor degeneration. Proc. Natl. Acad. Sci. USA 92, 3551–3555. 15. Deeb, S. S., et al. (1992) Genotype–phenotype relationships in human red/green color-vision defects: Molecular and psychophysical studies. Am. J. Hum. Genet. 51, 687–700. 16. Tanaka, H., et al. (1998) Novel mutations of the endothelin B receptor gene in patients with Hirschsprung’s disease and their characterization. J. Biol. Chem. 273(18), 11378–11383. 17. Dryja, T. P., Hahn, L. B., Reboul, T., and Arnaud, B. (1996) Missense mutation in the gene encoding the a subunit of rod transducin in the Nougaret form of congenital stationary night blindness. Nature Genet. 13, 358–360. 18. Lem, J., et al. (1998) Characterization of Rod a-transducin Knockout mice. Invest. Ophthalm. Vis. Sci. 39, S644. 19. Schwindinger, W. F., Francomano, C. A., and Levine, M. A. (1992) Identification of a mutation in the gene encoding the a subunit of the stimulatory G protein of adenylyl cyclase in McCuneAlbright syndrome. Proc. Natl. Acad. Sci. USA 89, 5152–5156. 20. Lyons, J., et al. (1990) Two G protein oncogenes in human endocrine tumors. Science 249, 655–659. 21. Offermanns, S., et al. (1997) Impaired motor coordination and persistent multiple climbing fiber innervation of cerebellar purkinje cells in mice lacking Gaq. Proc. Natl. Acad. Sci. USA 94, 14089–14094. 22. Offermanns, S., Toombs, C., Hu, Y., and Simon, M. (1997) Defective platelet activation in G alpha (q)-deficient mice. Nature 389, 183–186. 23. Belluscio, L., Gold, G., Nemes, A., and Axel, R. (1998) Mice deficient in G(olf) are anosmic. Neuron 20, 69–81. 24. Wong, G. T., Gannon, K. S., and Margolskee, R. F. (1996) Transduction of bitter and sweet taste by gustducin. Nature 381, 796–800. 25. Offermanns, S., Mancino, V., Revel, J.-P., and Simon, M. I. (1997) Vascular system defects and impaired cell chemokinesis as a result of Ga13 deficiency. Science 275, 533–536. 26. Siffert, W., et al. (1998) Association of a human G-protein b3 subunit variant with hypertension. Nature Genet. 18, 45–48. 27. Huang, S. H., et al. (1995) Autosomal recessive retinitis pigmentosa caused by mutations in the a subunit of rod cGMP phosphodiesterase. Nature Genet. 11, 468–471. 28. Bowes, C., et al. (1990) Retinal degeneration in the rd mouse is caused by a defect in the b subunit of rod cGMP-phosphodiesterase. Nature 347, 677–680. 29. McLaughlin, M. E., Ehrhart, T. L., Berson, E. L., and Dryja, T. P. (1995) Mutation spectrum of the gene encoding the b subunit of rod phosphodiesterase among patients with autosomal recessive retinitis pigmentosa. Proc. Natl. Acad. Sci. USA 92, 3249–3253. 30. Gal, A., et al. (1994) Heterozygous missense mutation in the rod cGMP phosphodiesterase b-subunit gene in autosomal dominant stationary night blindness. Nature Genet. 7, 64–68.

Copyright r 1998 by Academic Press

239

G-Proteins in Disease 31. Farber, D. B., Danciger, J. S., and Aguirre, G. (1992) The b Subunit of Cyclic GMP phosphodiesterase mRNA is deficient in canine rod–cone dysplasia 1. Neuron 9, 349–356. 32. Tsang, S. H., et al. (1996) Retinal degeneration in mice lacking the g subunit of the rod cGMP phosphodiesterase. Science 272, 1026–1029. 33. Dryja, T. P., et al. (1995) Mutations in the gene encoding the a subunit of the rod cGMP-gated channel in autosomal recessive retinitis pigmentosa. Proc. Natl. Acad. Sci. USA 92, 10177–10181. 34. Signorini, S., et al. (1997) Normal cerebellar development but susceptibility to seizures in mice lacking G protein-coupled inwardly rectifying K1 channel GIRK2. Proc. Natl. Acad. Sci. USA 94, 923–927. 35. Fuchs, S., et al. (1995) A homozygous 1-base pair deletion in the arrestin gene is a frequent cause of Oguchi disease in Japanese. Nature Genet. 10, 360–362. 36. Yamamoto, S., Sippel, K. C., Berson, E. L., and Dryja, T. P. (1997) Defects in the rhodopsin kinase gene in the Oguchi form of stationary night blindness. Nature Genet. 15, 175–178. 37. Koch, W. J., et al. (1995) Cardiac function in mice overexpressing the beta-adrenergic receptor kinase or a beta ARK inhibitor. Science 268(5215), 1350–1353.

38. Rockman, H. A., Koch, W. J., and Lelfkowitz, R. J. (1997) Cardiac function in genetically engineered mice with altered adrenergic receptor signaling. Am. J. Physiol. 272(4 Pt 2), H1553–H1559. 39. Perrault, I., et al. (1996) Retina-specific guanylate cyclase gene mutations in Leber’s congenital amaurosis. Nature Genet. 14(4), 461–464. 40. Payne, A., et al. (1998) A mutation in guanylate cyclase activator 1A (GUCA1A) in an autosomal dominant cone dystrophy pedigree mapping to a new locus on chromosome 6p21.1. Hum. Mol. Genet. 7(2), 273–277. 41. Semple-Rowland, S. L., et al. (1998) A null mutation in the photoreceptor guanylate cyclase gene causes the retinal degeneration chicken phenotype. Proc. Natl. Acad. Sci. USA 95(3), 1271–1276. 42. Pacheco, M. A., and Jope, R. S. (1996) Phosphoinositide signaling in human brain. Prog. Neurobiol. 50, 255–273. 43. Fowler, C. J., Garlind, A., O’Neill, C., and Cowburn, R. F. (1996) Receptor–effector coupling dysfunctions in Alzheimer’s disease. Ann. New York Acad. Sci. 786, 294–309. 44. Fowler, C. J., et al. (1995) Disturbances in signal transduction mechanisms in Alzheimer’s disease. Mol. Cell. Biochem. 149/150, 287–292.

Copyright r 1998 by Academic Press