Identification of Apolipoprotein A-I as a Retinoic Acid-binding Protein in the Eye

Abstract

All-trans-retinoic acid may be an important molecular signal in the postnatal control of eye size. The goal of this study was to identify retinoic acid-binding proteins secreted by the choroid and sclera during visually guided ocular growth. Following photoaffinity labeling with all-trans-[11,12-3 H]retinoic acid, the most abundant labeled protein detected in the conditioned medium of choroid or sclera had an apparent M r of 27,000 Da. Following purification and mass spectrometry, the M r 27,000 band was identified as apolipoprotein A-I. Affinity capture of the radioactive M r 27,000 band by anti-chick apolipoprotein A-I antibodies confirmed its identity as apolipoprotein A-I. Photoaffinity labeling and fluorescence quenching experiments demonstrated that binding of retinoic acid to apolipoprotein A-I is 1) concentration-dependent, 2) selective for all-trans-retinoic acid, and 3) requires the presence of apolipoprotein A-I-associated lipids for retinoid binding. Expression of apolipoprotein A-I mRNA and protein synthesis were markedly upregulated in choroids of chick eyes during the recovery from induced myopia, and apolipoprotein A-I mRNA was significantly increased in choroids following retinoic acid treatment. Together, these data suggest that apolipoprotein A-I may participate in a regulatory feedback mechanism with retinoic acid to control the action of retinoic acid on ocular targets during postnatal ocular growth.

Retinoic Acid in Ocular Growth Regulation

“…We have also shown that choroidal expression of ApoA-1 is transcriptionally regulated by atRA, and choroidal ApoA-1 mRNA and protein synthesis are upregulated during recovery from induced myopia, suggesting the presence of a regulatory feedback mechanism to regulate atRA transport and activity [15]. We postulate that ApoA-1 functions to transport atRA from its site of synthesis by RALDH2+ cells in the proximal choroid to the sclera for the regulation of scleral ECM remodeling.…”

“…Therefore, the requirement for carrier proteins capable of forming a soluble complex with atRA and transporting atRA to target cells is necessary to achieve high efficiency and specificity while avoiding toxicity associated with random diffusion. Mertz and Wallman [9] and our lab [15] identified a secreted protein of Mr = 27,000 that was the major atRA binding protein present in choroid and sclera conditioned medium. This Mr 27,000 protein did not correspond in size to any of the previously identified atRA binding proteins [59].…”

“…This Mr 27,000 protein did not correspond in size to any of the previously identified atRA binding proteins [59]. We subsequently determined that the Mr 27,000 protein was apolipoprotein a-1 (ApoA-1) [15] (Figure 3).…”

“…We and others have demonstrated that in response to visual stimuli, ocular atRA synthesis is regulated exclusively via choroidal expression of the atRA synthe sizing enzyme, retinaldehyde dehydrogenase 2 (RALDH2) by a unique population of cells [10,14]. Furthermore, choroidally derived retinoic acid is transported to the sclera (the outer connective tissue shell of the eye) by apolipoprotein A-1 (ApoA-1) which functions as a specific extracellular atRA-binding and transport protein in the eye [15,16]. Once delivered to the sclera, we speculate that atRA regulates the transcription of many genes in the sclera to effect changes in scleral extracellular matrix remodeling, ocular size, and refraction.…”

CImP: Cellular Imprinting Proteomics applied to ocular disorders elicited by Congenital Zika virus Syndrome

“…Six apolipoproteins were downregulated in the Zikv CZS condition compared to control. Recently, ApoA-I has been identified as an all-trans-retinoic acid-binding protein in the choroid and sclera conditioned media of chick tissue (116). Abetalipoproteinemia is an autosomal recessive disease caused by genetic mutations in the MTTP (microsomal triglyceride transfer protein) gene that result in the inability to synthesize the APOB.…”