ABSTRACT
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.
Subject(s)
Retinal Cone Photoreceptor Cells/embryology , Retinal Cone Photoreceptor Cells/metabolism , Rod Opsins/metabolism , Thyroid Hormone Receptors beta/metabolism , Animals , Body Patterning , Female , Gene Expression Regulation, Developmental , Gene Silencing , Male , Mice , Mice, Inbred C57BL , Mice, Knockout , Mice, Transgenic , Pregnancy , RNA, Messenger/genetics , RNA, Messenger/metabolism , Retinal Cone Photoreceptor Cells/growth & development , Rod Opsins/genetics , Thyroid Hormone Receptors beta/deficiency , Thyroid Hormone Receptors beta/genetics , Triiodothyronine/metabolismABSTRACT
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.
Subject(s)
Retina/metabolism , Retinal Cone Photoreceptor Cells/metabolism , Rod Opsins/biosynthesis , Animals , Cell Count , Immunohistochemistry , In Situ Hybridization , Mice , Mice, Inbred BALB C , Mice, Inbred C57BL , Microscopy, Confocal , Molecular Sequence Data , Organ Specificity , Protein Isoforms/biosynthesis , Protein Isoforms/genetics , RNA, Messenger/genetics , RNA, Messenger/metabolism , Retina/cytology , Retinal Cone Photoreceptor Cells/cytology , Retinal Rod Photoreceptor Cells/cytology , Retinal Rod Photoreceptor Cells/metabolism , Rod Opsins/genetics , Species SpecificityABSTRACT
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.
Subject(s)
Guanylate Cyclase/genetics , Isoenzymes/genetics , Receptors, Cell Surface , Receptors, Peptide/genetics , Amino Acid Sequence , Animals , Base Sequence , Brain/enzymology , Cattle , DNA, Complementary , Lung/enzymology , Molecular Sequence Data , Myocardium/enzymology , Pituitary Gland/enzymology , Polymerase Chain Reaction , Promoter Regions, Genetic , Rats , Receptors, Enterotoxin , Receptors, Guanylate Cyclase-Coupled , Retina/enzymology , Sequence Homology, Nucleic AcidABSTRACT
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.
