Sources and sink of retinoic acid in the embryonic chick retina: distribution of aldehyde dehydrogenase activities, CRABP-I, and sites of retinoic acid inactivation.

Previous experiments in mice and zebrafish led to the hypothesis that an asymmetric distribution of the transcriptional activator retinoic acid (RA) causes ventral-dorsal polarity in the vertebrate eye anlage. A high concentration of RA in the ventral retinal neuroepithelium has been suggested to induce developmental events that finally establish topographic order in the retinotectal projection along the vertical eye axis. In the present study we have investigated potential sources and sinks of RA during embryonic development of the chick retina. At embryonic day (E)1 to E2, when the spatial determination of the eye primordia takes place, no RA synthesis by aldehyde dehydrogenases was detectable, and neither immunoreactivity for retinaldehyde dehydrogenase RALDH-2 nor for cellular retinoic acid binding protein CRABP-I was observed. These components of RA signal transduction appeared in the eye between E3 and E5. At later stages, RA-measurements with a reporter cell line showed highest synthesis in the retinal pigment epithelium (RPE) and at the ventral and dorsal poles of the retina. RA degradation occurred mostly in a horizontal region in the middle of the retina with only small differences along the nasal-temporal axis. CRABP-I immunoreactivity appeared first in differentiating retinal ganglion cells with no indication of a spatial gradient across the ventral-dorsal eye axis. RA-production depended on three NAD+-dependent enzyme activities, which could be competitively inhibited by citral. One enzyme, located in the dorsal retina (corresponding to mouse RALDH-1), and one enzyme in the RPE (RALDH-2) were aldehyde dehydrogenases of the same molecular weight (monomers about 55 kDa) but with different isoelectric points (6.5-6.9; 4.9-5.4). The third RA-synthesizing activity (pI 6.0-6.3) was limited to the ventral retina, and likely corresponded to mouse RALDH-3. The restricted localization of retinoid-metabolizing activities along the dorsal-ventral axis of the embryonic chick retina does support the idea that RA is involved in dorsal-ventral eye patterning. However, the late time of appearance of aldehyde dehydrogenase activities and CRABP-I points to functions in cellular differentiation that are distinct from the initiation of the dorsal-ventral polarity.

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