Transient expression of thyroid hormone nuclear receptor TRbeta2 sets S opsin patterning during cone photoreceptor genesis.

Cone photoreceptors in the murine retina are patterned by dorsal repression and ventral activation of S opsin. TR beta 2, the nuclear thyroid hormone receptor beta isoform 2, regulates dorsal repression. To determine the molecular mechanism by which TR beta 2 acts, we compared the spatiotemporal expression of TR beta 2 and S opsin from embryonic day (E) 13 through adulthood in C57BL/6 retinae. TR beta 2 and S opsin are expressed in cone photoreceptors only. Both are transcribed by E13, and their levels increase with cone genesis. TR beta 2 is expressed uniformly, but transiently, across the retina. mRNA levels are maximal by E17 at completion of cone genesis and again minimal before P5. S opsin is also transcribed by E13, but only in ventral cones. Repression in dorsal cones is established by E17, consistent with the occurrence of patterning during cone cell genesis. The uniform expression of TR beta 2 suggests that repression of S opsin requires other dorsal-specific factors in addition to TR beta 2. The mechanism by which TR beta 2 functions was probed in transgenic animals with TR beta 2 ablated, TR beta 2 that is DNA binding defective, and TR beta 2 that is ligand binding defective. These studies show that TR beta 2 is necessary for dorsal repression, but not ventral activation of S opsin. TR beta 2 must bind DNA and the ligand T3 (thyroid hormone) to repress S opsin. Once repression is established, T3 no longer regulates dorsal S opsin repression in adult animals. The transient, embryonic action of TR beta 2 is consistent with a role (direct and/or indirect) in chromatin remodeling that leads to permanent gene silencing in terminally differentiated, dorsal cone photoreceptors.

The murine cone photoreceptor: a single cone type expresses both S and M opsins with retinal spatial patterning.

Mice express S and M opsins that form visual pigments for the detection of light and visual signaling in cones. Here, we show that S opsin transcription is higher than that of M opsin, which supports ultraviolet (UV) sensitivity greater than midwavelength sensitivity. Surprisingly, most cones coexpress both S and M opsins in a common cone cell type throughout the retina. All cones express M opsin, but the levels are graded from dorsal to ventral. The levels of S opsin are relatively constant. However, in the far dorsal retina, S opsin is repressed stochastically, such that some cones express M opsin only. These observations indicate that two different mechanisms control M and S opsin expression. We suggest that a common cone type is patterned across the retinal surface to produce phenotypic cone subtypes.

The bovine guanylate cyclase GC-E gene and 5' flanking region.

The gene encoding the bovine guanylate cyclase isoform E (GC-E) was isolated as a single 18 kb genomic clone and shown to have 20 exons and 19 introns. Comparison of the structure of the GC-E gene with structures of other membrane guanylate cyclase genes indicates that the GC-E is most closely related to the subfamily of sensory guanylate cyclases. Comparison of the GC-E structure with that of the more distantly related guanylate cyclase isoform A (GC-A) gene shows the most divergence in the extracellular and C-terminal regions, but general conservation of introns and exons in the intracellular kinase-like and catalytic domains. RT-PCR from several bovine tissues shows that GC-E is expressed only in the retina. Consistent with this pattern of expression, elements for the retinal-specific transcription factors RET-1, RET-2 and Talpha-1 are located in the 5' flanking promoter region.

Retinoic aid increases arrestin mRNA levels in the mouse retina.

Arrestin, which plays a role in the termination of the visual transduction cascade, is one of several photoreceptor proteins whose mRNA levels are increased by light. Retinoic acid, a by-product of photoreceptor signaling and a potent modulator of hormonal transcription control, is one candidate for regulating the arrestin mRNA levels. Here we show that retinoic acid, injected intraperitoneally into dark-adapted mice, increases the arrestin mRNA levels and mimics the effect of light. Injection of 1 mumol of retinoic acid produces a maximal increase in arrestin mRNA levels. The mRNA level reaches a maximum 3 h after injection and slowly declines thereafter. The observations suggest that retinoic acid may mediate the increase in arrestin mRNA produced by light.

The photoreceptor guanylate cyclase is an autophosphorylating protein kinase.

The photoreceptor membrane guanylate cyclase is a member of a family of proteins with a set of four structural motifs: an extracellular ligand binding domain, a transmembrane domain, an intracellular protein kinase-like domain, and an intracellular catalytic domain. Purified preparations of the photoreceptor guanylate cyclase have allowed us to explore the function of the protein kinase-like domain. ATP enhances the guanylate cyclase activity 2-fold in membranes stripped of peripheral proteins. The stimulation can be mimicked by ATPgammaS (adenosine 5'-O-(3-thiotriphosphate)), AMPPNP (5'-adenylyl beta,gamma-imidodiphosphate), and ADP, but not AMP. While this effect is lost by solubilizing guanylate cyclase, ATP binds the purified, solubilized enzyme in a site distinct from the catalytic GTP site as shown by specific labeling with 8-N3[alpha-32P]ATP. The enzyme has a protein kinase activity that is Mg2+-dependent and autophosphorylates serine residues. Myelin basic protein serves as a substrate for the kinase and enables further characterization of the kinase properties. The Km for ATP is 81 microM. The kinase activity is unaffected by calcium, cyclic nucleotides, and phorbol 12-myristate 13-acetate/L-alpha-phosphatidylserine/Ca2+ and is inhibited by high concentrations of staurosporine. These properties are distinct from other Ser/Thr kinases identified in rod outer segment preparations including protein kinase A, protein kinase C, and rhodopsin kinase. The observations offer the first biochemical evidence that a member of the receptor guanylate cyclase family has intrinsic protein kinase activity.

The bovine photoreceptor outer segment guanylate cyclase: purification, kinetic properties, and molecular size.

A simple protocol was developed to isolate the integral membrane guanylate cyclase from bleached bovine photoreceptor outer segments. Hypotonic and hypertonic washes strip the photoreceptor outer segment membranes of peripheral proteins. The guanylate cyclase activity is solubilized by dodecyl-b-D-maltoside in a low salt concentration buffer. Phosphatidylcholine, glycerol, and dithiothreitol are used to stabilize the activity during chromatography. GTP-affinity chromatography achieves a 250-fold increase in specific activity over that of membranes stripped of peripheral proteins. From 100 retinas, the protocol yields 100-140 mg of purified guanylate cyclase composed of a 115-kDa subunit. The molar ratio of the guanylate cyclase to rhodopsin is estimated to be 1:440. A significant portion of the freshly solubilized enzyme behaves as a monomer with a Stokes radius of 48.7 A, whereas the purified protein forms homooligomers ranging from dimers to tetramers. These properties are similar to those of ANP and guanylin receptors, indicating that the photoreceptor protein shares characteristics of the membrane receptor guanylate cyclase family. For the physiological substrate MgGTP, the Km and Vmax are 1.07 +/- 0.20 mM and 3262 +/- 514 nmol cGMP min-1 mg-1, respectively, generating a turnover rate of approximately 3.9 nmol cGMP s-1 at physiological substrate concentrations. The relatively high Km suggests that in vivo changes in GTP concentration might modulate the rate of cGMP synthesis. These properties indicate that the photoreceptor membrane guanylate cyclase can sustain a rate of cGMP synthesis comparable to the dark-adapted (basal) rate of cGMP degradation by the cGMP phosphodiesterase.

Pineal opsin: a nonvisual opsin expressed in chick pineal.

Pineal opsin (P-opsin), an opsin from chick that is highly expressed in pineal but is not detectable in retina, was cloned by the polymerase chain reaction. It is likely that the P-opsin lineage diverged from the retinal opsins early in opsin evolution. The amino acid sequence of P-opsin is 42 to 46 percent identical to that of the retinal opsins. P-opsin is a seven-membrane spanning, G protein-linked receptor with a Schiff-base lysine in the seventh membrane span and a Schiff-base counterion in the third membrane span. The primary sequence of P-opsin suggests that it will be maximally sensitive to approximately 500-nanometer light and produce a slow and prolonged phototransduction response consistent with the nonvisual function of pineal photoreception.

Relationships of G-protein-coupled receptors. A survey with the photoreceptor opsin subfamily.

Relationships among the G-protein-coupled receptors were evaluated using several distance matrices with the neighbor-joining method of Saitou and Nei (1987). The relationships generated vary depending upon alignment, length, or region of sequence compared, and the distance matrix used to score similarity. To provide a statistical level of confidence, bootstrap resampling was applied to the analysis of a selection of G-protein-coupled receptors and the subfamily photoreceptor opsins. A general consensus indicates that the opsins behave as a discrete subfamily among the superfamily of G-protein-coupled receptors. Their relationship to other subfamilies remains unresolved. Within the opsin subfamily, the retinochromelike opsins segregate as a discrete group, but are more closely related to the invertebrate than vertebrate opsins. Among vertebrate opsins, the long wavelength cone opsins, the blue/violet opsins, and the rod opsins (including a class of green cone opsins) form distinct subgroups, but their relationships to one another remain unresolved. For this superfamily of receptors, the confidence levels for many branch pairings are low. The application of methods complimentary to those used in this preliminary study will be necessary to resolve questions about appropriate pairing and evolutionary relationships.

Localization of a retroviral element within the rd gene coding for the beta subunit of cGMP phosphodiesterase.

Retinal degeneration in the rd mouse is inherited as an autosomal recessive trait and is caused by a defect in the gene encoding the beta subunit of cGMP phosphodiesterase. Recently, a close genetic association of the rd gene with an endogenous xenotropic murine leukemia virus (Xmv-28) was established by linkage analysis using recombinant inbred strains of mice. In this study, genomic DNA mapping and sequence analyses clarify the position of the proviral sequences in relation to the rd gene. We find that the Xmv-28 provirus is integrated into intron I of the rd gene 1511 bp downstream of the exon-intron boundary. The transcriptional orientation of the provirus is opposite to that of the gene for the beta subunit of cGMP phosphodiesterase. Reverse transcription-PCR demonstrates that the integrated Xmv-28 sequences are transcribed in the retina. The provirus is present in every strain of rd mouse tested.

Circadian regulation of iodopsin gene expression in embryonic photoreceptors in retinal cell culture.

A circadian clock regulates a number of diverse physiological functions in the vertebrate eye. In this study, we show that mRNA for the red-sensitive cone pigment, iodopsin, fluctuates with a circadian rhythm in chicken retina. Transcript levels increase in the late afternoon just prior to the time of cone disc shedding. Furthermore, iodopsin mRNA levels are regulated similarly by a circadian oscillator in primary cultures of dispersed embryonic chick retina. Nuclear run-on experiments show that the circadian regulation of iodopsin transcript abundance occurs at the level of gene transcription. Our results provide a demonstration of clock-regulated gene expression in a vertebrate preparation maintained in cell culture.

Retinal degeneration is rescued in transgenic rd mice by expression of the cGMP phosphodiesterase beta subunit.

The beta subunit of the cGMP phosphodiesterase (PDE) gene has been identified as the candidate gene for retinal degeneration in the rd mouse. To study the molecular mechanisms underlying degeneration and the potential for gene repair, we have expressed a functional bovine cGMP PDE beta subunit in transgenic rd mice. One transgenic mouse line showed complete photoreceptor rescue across the entire span of the retina. A second independently derived line showed partial rescue in which photoreceptors in the superior but not the inferior hemisphere of the retina were rescued. In the latter animals, intermediate stages of degeneration were observed in the transition zone between rescued and diseased photoreceptors. Pathologic changes in the retina ranged from vesiculation of the basalmost outer segment discs in otherwise structurally intact rod cells to photoreceptors with highly disorganized outer segments and intact inner segments. Totally or partially rescued retinas showed a corresponding restoration of cGMP PDE activity, whereas nonrescued retinas had minimal enzyme activity, characteristic of the rd phenotype. These transgenic animals provide models for studying the molecular basis of retinal degenerative disease and conclusively demonstrate that the phenotype of rd mice is produced by a defect in the beta subunit of cGMP PDE.

Assignment of the beta-subunit of rod photoreceptor cGMP phosphodiesterase gene PDEB (homolog of the mouse rd gene) to human chromosome 4p16.

The gene encoding the beta-subunit of rod photoreceptor cGMP phosphodiesterase (gene symbol PDEB, homolog of the mouse rd gene) is mapped to human chromosome 4 using somatic cell hybrids and further localized to the chromosome band 4p16 using in situ hybridization. A mutation in the mouse gene underlies the recessive trait of retinal degeneration in the rd mouse. Thus, the human homolog is a candidate for lesions causing retinal degeneration.

Molecular determinants of visual pigment function.

Mutagenesis studies and comparisons of natural variants of rhodopsin and related visual pigments have led to new insights concerning photoreceptor function. The studies identify domains important for receptor folding, the residues that set the wavelength of absorption for the ligand 11-cis retinal, and residues, that when mutated, trigger the cell death of photoreceptors.

Tissue-specific and developmental regulation of rod opsin chimeric genes in transgenic mice.

Chimeric gene fusions between 4.4 kb of rod opsin 5' flanking sequence fused to a diphtheria toxin gene and 4.4 kb or 500 bp of rod opsin 5' flanking sequence fused to the E. coli IacZ gene were used to generate transgenic mice for analysis of cell type-specific expression and temporal and spatial distribution of reporter gene product during retinal development. Opsin-diphtheria toxin transgene expression evoked photoreceptor-specific cell death. The 4.4 kb opsin-IacZ transgene followed temporal and spatial gradients of expression that approximate opsin expression. The 500 bp opsin fragment targeted expression to photoreceptors, but expression was weaker and nonuniform, suggesting that elements located upstream may be required for enhanced and uniform spatial expression.

Distinctive properties of the purified Na-Ca exchanger from rod outer segments.

The Na-Ca exchanger of rod outer segments plays an important role in the regulation of Ca levels in photoreceptor cells. While this transporter shares functional properties with other Na-Ca exchangers, it has several unique features. The purified ROS exchanger migrates as a single band at 220 kDa in SDS-polyacrylamide gels, indicating that the unit size of its polypeptide is larger than other known Na-Ca exchangers (and most transporters). A specific antiserum to the ROS exchanger does not bind to the Na-Ca exchangers found in sarcolemmal vesicles or brain synaptic plasma membranes. Similarly, polyclonal antiserum specific for the cardiac exchanger does not react with ROS or brain proteins. The ROS exchanger requires K for transport activity. By incorporating the purified exchanger into proteoliposomes and measuring the sequestration of K, the actual transport of K is demonstrated. A stoichiometry of 4Na:1Ca,1K for the exchanger of ROS has been measured.

Retinal degeneration in the rd mouse is caused by a defect in the beta subunit of rod cGMP-phosphodiesterase.

Mice homozygous for the rd mutation display hereditary retinal degeneration and the classic rd lines serve as a model for human retinitis pigmentosa. In affected animals the retinal rod photoreceptor cells begin degenerating at about postnatal day 8, and by four weeks no photoreceptors are left. Degeneration is preceded by accumulation of cyclic GMP in the retina and is correlated with deficient activity of the rod photoreceptor cGMP-phosphodiesterase. We have recently isolated a candidate complementary DNA for the rd gene from a mouse retinal library and completed the characterization of cDNAs encoding all subunits of bovine photoreceptor phosphodiesterase. The candidate cDNA shows strong homology with a cDNA encoding the bovine phosphodiesterase beta subunit. Here we present evidence that the candidate cDNA is the murine homologue of bovine phosphodiesterase beta cDNA. We conclude that the mouse rd locus encodes the rod photoreceptor cGMP-phosphodiesterase beta subunit.

Genetic mapping demonstrates that the alpha-subunit of retinal cGMP-phosphodiesterase is not the site of the rd mutation.

In the inherited degenerative retinal disease of the rd mouse, rod cGMP levels rise above normal due to depressed cGMP-phosphodiesterase (cGMP-PDE) function a few days before degeneration begins. The subnormal activity of the cGMP-PDE may be due to a lesion in the enzyme itself, or in any of several proteins that regulate it. We have used a bovine cDNA for the alpha-subunit of cGMP-PDE to map its gene Pdea to mouse chromosome 18 at a distance of 21 centimorgans (cM) from the Mbp locus. Since the locus of the rd mutation is on mouse chromosome 5, a defect in the Pdea gene is ruled out as the cause of this inherited retinal degeneration.