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Andrew Tsin, Joshua Mimun, Federico Gonzalez-Fernandez; Interphotoreceptor Retinoid Binding Protein Protects and Delivers Retinoids in the Cone Visual Cycle. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3765. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
In the cone cycle, the recycling of all-trans retinol (atROL) and 11-cis-retinol (11cROL), between Müller cells and cone photoreceptor cells occurs within the retina. Interphotoreceptor retinoid binding protein (IRBP), the most abundant soluble protein in the interphotoreceptor matrix, can bind to these photosensitive retinoids, suggesting a role for IRBP as a transfer protein in the cone visual cycle. Thus, we hypothesized that IRBP is able to protect atROL and 11cROL from light induced degradation and deliver/retrieve the retinoids to/from Müller cells.
Protection studies: atROL and 11cROL (3.0 µM each) were exposed to 2,000 Lux, in the presence and absence of 3.0 µM IRBP. Retinoids were extracted every 10 min for 30 min and analyzed by HPLC. Delivery/release studies: Primary chicken Müller cells (CMCs) were incubated for 24 hrs in serum free media (SFM) with 2.0 µM atROL and 2.0 µM IRBP or 2.0 µM BSA. To test if these proteins facilitated the release of retinoids from CMCs after an initial 24 hr incubation with atROL and BSA (2.0 µM each) in SFM, the CMCs were re-incubated with SFM, 2.0 µM IRBP in SFM, or 2.0 µM BSA in SFM for an additional 24hrs. Cellular homogenate and media were extracted for retinoids and analyzed via HPLC.
30 min of light exposure resulted in degradation of 11cROL (83%) and atROL (67%). IRBP reduced these levels to 21% (11cROL) and 18% (atROL). After incubation with atROL for 24 hrs, the CMCs contained 0.32 pm/mg cell protein(cp) of all-trans retinyl palmitate (atRP) when incubated without proteins, 6.3 pm/mg cp of atRP when incubated with BSA, and 0.85 pm/mg cp of atRP when incubated with IRBP. The addition of BSA or IRBP into culture media of CMCs pre-incubated with atROL resulted in a significant increase of atROL when compared to SFM (BSA: 31.7 pm/mg cp; IRBP: 40.1 pm/mg cp; SFM: 0.56 pm/mg cp).
IRBP protects retinoids in situ from light-induced degradation. IRBP delivery and retrieval experiments suggest that IRBP delivers retinoids to the CMCs as indicated by the increase of atRP present in the cellular homogenate and retrieves retinoids from the CMCs as indicated by the increase of atROL present in the culture media. IRBP could act as a transport protein of the cone visual cycle by protecting and facilitating transport of retinols to Müller cells.
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