Making the gradient: Thyroid hormone regulates cone opsin expression in the developing mouse retina

Abstract

Most mammals have two types of cone photoreceptors, which contain either medium wavelength (M) or short wavelength (S) opsin. The number and spatial organization of cone types varies dramatically among species, presumably to fine-tune the retina for different visual environments. In the mouse, S-and M-opsin are expressed in an opposing dorsal-ventral gradient. We previously reported that cone opsin patterning requires thyroid hormone ␤2, a nuclear hormone receptor that regulates transcription in conjunction with its ligand, thyroid hormone (TH). Here we show that exogenous TH inhibits S-opsin expression, but activates M-opsin expression. Binding of endogenous TH to TR␤2 is required to inhibit S-opsin and to activate M-opsin. TH is symmetrically distributed in the retina at birth as S-opsin expression begins, but becomes elevated in the dorsal retina at the time of M-opsin onset (postnatal day 10). Our results show that TH is a critical regulator of both S-opsin and M-opsin, and suggest that a TH gradient may play a role in establishing the gradient of M-opsin. These results also suggest that the ratio and patterning of cone types may be determined by TH availability during retinal development.photoreceptor ͉ retinal development ͉ nuclear hormone receptor T hyroid hormone (TH) is secreted mostly as thyroxine (T4) from the thyroid gland and converted locally to a transcriptionally active form, 3,5,3Ј-triiodothyronine (T3), by iodothyronine deiodinating enzymes (1). T3 receptors bind to DNA regulatory elements, often as heterodimers with retinoid X receptors (RXRs), to activate or repress transcription of target genes (for review, see refs. 2 and 3). T3 regulates diverse developmental processes in multiple regions of the central nervous system including the hippocampus, cerebral cortex, inner ear, and cerebellum (4-8). Numerous developmental processes are TH-dependent, including cell differentiation, migration, and dendritic growth (2, 3, 9-11). Perturbations in maternal or fetal TH in humans can lead to clinical syndromes ranging from severe mental retardation and hearing loss to mild or moderate deficits in motor skills, language, memory, and attention (12)(13)(14).In the retina, the TH ␤2 receptor isoform (TR␤2) is restricted to cone photoreceptors (15, 16), and we previously showed that it is a critical regulator of late-stage cone differentiation. In the first stage of cone differentiation, cones exit the cell cycle and begin to express specific transcription factors, including cone rod homeobox (CRX), TR␤2,. In the second stage of differentiation, cones express opsin and form specific synaptic connections. Most mammalian cones express short wavelength (S-opsin) or medium wavelength (M-opsin) opsin. However, the spatial arrangement of cone types varies considerably among species. In mice, S-opsin expression begins just before birth and is expressed predominantly in ventral cones. By contrast, M-opsin expression begins at the end of the first postnatal week and is expressed predominantly in dorsal cones (21, 22...

Regulation of photoreceptor gene expression by Crx-associated transcription factor network

“…Addition of exogenous TH also promoted M-opsin expression and inhibited S-opsin in embryonic retina explant cultures from wild-type (WT) but not Trβ2 knockout mice (Roberts et al, 2006). Furthermore, daily injection of TH into WT mouse pups beginning on P0 drastically decreased the number of S-cones found in all parts of the retina three days later.…”

“…In the mouse retina, S-cones differentiate first, beginning shortly after birth, with most differentiating cells localized to the ventral retina. M-cones begin differentiating about a week later, mostly in the dorsal retina and coinciding with a decrease in the number of cells expressing S-opsin (Cepko, 1996;Roberts et al, 2006). Increasing evidence suggests that the cone gradients are generated during development by the action of diffusible growth factors or hormones (extrinsic signals) and their receptors, mainly nuclear receptor family transcription factors (intrinsic program).…”

“…Harpavat and Cepko reviewed the role of TH in retinal development in 2003 (Harpavat and Cepko, 2003). TH is distributed uniformly across the retina at birth, but during the period of M-cone differentiation between P4 and P10 forms a gradient with higher concentrations in the dorsal than ventral retina [ (Roberts et al, 2006); Figure 2B]. This implicates TH as the extrinsic signal responsible for establishing the M-cone gradient.…”

“…The thyroid hormone receptor Trβ2, a splice variant of the thyroid hormone receptor B (Thrb) gene, is implicated in photoreceptor development in chick and mouse, based on its expression in retinal progenitor cells and developing photoreceptors. In mouse eyes, expression of Trβ2 begins about E16, peaks around E18 as cone photoreceptors begin differentiating, then decreases (Ng et al, 2001;Yanagi et al, 2002), but the expression is uniform across the retina (Ng et al, 2001;Roberts et al, 2005;Roberts et al, 2006). However, during the time M-cones are developing, its ligand TH becomes distributed in a gradient with higher concentrations in the dorsal than ventral retina [ (Roberts et al, 2006); Figure 2B].…”

“…However, there is a significant delay, several days in rodents, for newly born photoreceptor precursors to begin expressing the specific type of opsin and other genes that confer the mature phenotype (Watanabe and Raff, 1990;Cepko, 1996). During this lag time, the photoreceptor precursors appear to be "plastic" and can be induced to differentiate into different photoreceptor subtypes, depending on intrinsic and extrinsic regulatory factors (Nishida et al, 2003;Cheng et al, 2006;MacLaren et al, 2006;Roberts et al, 2006). The intrinsic factors mainly consist of transcription factors of homeodomain, bZIP and nuclear receptor families.…”