Identification of RALDH2 as a visually regulated retinoic acid synthesizing enzyme in the chick choroid.

Strong evidence from clinical and experimental studies indicates the presence of a vision-dependent emmetropization mechanism that acts to minimize refractive error through the coordinated regulation of the growth of the ocular tissues.1,2 In humans, the emmetropization mechanism often breaks down, leading to the development of nearsightedness or myopia usually because of excessive growth of the posterior portion of the eye. Direct evidence of emmetropization has been provided by numerous animal studies in which modulation of the refractive target with plus and minus lenses results in changes in vitreous chamber depth to align the retinal photoreceptors with the focal plane of the eye. Interruption of the emmetropization process as a result of the distortion of visual image quality, either through ocular pathology in humans1–4 or application of translucent occluders in animal models (form deprivation5), results in axial elongation and the development of myopia. Form deprivation-induced myopia is reversible; restoration of unrestricted vision (and the resultant myopic defocus) results in a temporary cessation of axial growth, eventually leading to the reestablishment of emmetropia (recovery) in the formerly deprived eye.6 The response to deprivation and defocus is rapid, leading to detectable changes in vitreous chamber depth within hours.7 It has been well established that in chicks these visually induced changes in ocular growth are directly associated with changes in proteoglycan synthesis and proteoglycan accumulation in the sclera at the posterior pole of the eye.8–12 Unlike the sclera of placental mammals, the chick sclera is dominated by a cartilaginous layer that is responsible for approximately 90% of total proteoglycan synthesis, primarily as a result of increased aggrecan (the cartilage proteoglycan) core protein synthesis.8 The visually guided scleral response is rapid, with significant changes in scleral proteoglycan synthesis occurring within 6 hours of recovery from a previous period of form deprivation.13 However, despite the identification of several molecular events occurring in the retina, RPE, choroid, and sclera in association with visually guided ocular growth, the chemical signals directly responsible for regulating changes in scleral extracellular matrix synthesis during myopia development and emmetropization have not been identified. Studies in animal models suggest that all-trans-retinoic acid (atRA) may be an important molecular signal in the postnatal control of eye size.14–17 In an elegant study, Mertz and Wallman14 demonstrated that the choroidal synthesis of atRA was increased in chick eyes during recovery from induced myopia and during compensation for imposed myopic defocus. Moreover, atRA was shown to be decreased in eyes undergoing form deprivation myopia and compensation for hyperopic defocus compared with the fellow control eye, suggesting that choroidal atRA synthesis could represent a molecular basis for emmetropization. atRA is an attractive candidate for a visually regulated ocular growth regulator because it is readily diffusible, has pronounced effects on scleral extracellular matrix metabolism, and exerts its effects through highly regulated, locally controlled synthesis and degradation. Tissue concentrations of atRA are tightly controlled by the activities of synthesizing enzymes, retinol dehydrogenase (RDH), the retinaldehyde dehydrogenases (RALDH1, RALDH2, and RALDH3; also know as Aldh1A1, Aldh1A2, and Aldh1A3, respectively), and the atra-metabolizing enzyme CYP26, a member of the cytochrome P450 family.18 Cyp1B1 may also contribute to atRA synthesis in the chick embryo.19 Furthermore, the rate of these dehydrogenation reactions is regulated by substrate availability, enzyme accessibility, and enzyme catalytic activity.20 Through autocrine or paracrine (or both) interactions, atRA binds with nuclear retinoic acid receptor complexes to directly control the transcriptional activity of more than 100 target genes.21 The molecular events involved in regulating atRA synthesis during recovery from induced myopia have yet to be characterized. Therefore, this study was undertaken to identify the enzymes involved in the synthesis of atRA during visually guided ocular growth, to identify the cells involved in the modulation of atRA biosynthesis in the choroid, and to demonstrate the effect of choroid-derived atRA on scleral proteoglycan synthesis.