Retinal-phospholipid Schiff-base conjugates and their interaction with ABCA4, the ABC transporter associated with Stargardt disease.

Abstract

N-retinylidene-phosphatidylethanolamine (N-Ret-PE), the Schiff-base conjugate formed through the reversible reaction of retinal (Vitamin A-aldehyde) and phosphatidylethanolamine, plays a crucial role in the visual cycle and visual pigment photoregeneration. However, N-Ret-PE can react with another molecule of retinal to form toxic di-retinoids if not removed from photoreceptors through its transport across photoreceptor membranes by the ATP-binding-cassette transporter ABCA4. Loss-of-function mutations in ABCA4 are known to cause Stargardt disease (STGD1), an inherited retinal degenerative disease associated with the accumulation of fluorescent di-retinoids and severe loss in vision. A larger assessment of retinal-phospholipid Schiff-base conjugates in photoreceptors is needed, along with further investigation of ABCA4 residues important for N-Ret-PE binding. In this study we show that N-Ret-PE formation is dependent on pH and phospholipid content. When retinal is added to liposomes or photoreceptor membranes, 40 to 60% is converted to N-Ret-PE at physiological pH. Phosphatidylserine and taurine also react with retinal to form N-retinylidene-phosphatidylserine and N-retinylidene-taurine, respectively, but at significantly lower levels. N-retinylidene-phosphatidylserine is not a substrate for ABCA4 and reacts poorly with retinal to form di-retinoids. Additionally, amino acid residues within the binding pocket of ABCA4 that contribute to its interaction with N-Ret-PE were identified and characterized using site-directed mutagenesis together with functional and binding assays. Substitution of arginine residues and hydrophobic residues with alanine or residues implicated in STGD1 significantly reduced or eliminated substrate-activated ATPase activity and substrate binding. Collectively, this study provides important insight into conditions which affect retinal-phospholipid Schiff-base formation and mechanisms underlying the pathogenesis of STGD1.

• Authors: Tongzhou Xu, Laurie L Molday, Robert S Molday
• Journal: The Journal of biological chemistry (J Biol Chem) Vol. 299 Issue 5 Pg. 104614 (05 2023) ISSN: 1083-351X [Electronic] United States
• PMID: 36931393 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)
• Copyright: Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.

Chemical References

• ABCA4 protein, human
• ATP-Binding Cassette Transporters
• N-retinylidene-phosphatidylethanolamine
• Phosphatidylserines
• Phospholipids
• Retinoids

Topics

• Humans
• ATP-Binding Cassette Transporters (metabolism)
• Phosphatidylserines
• Phospholipids
• Retinoids (metabolism)
• Stargardt Disease (metabolism)