Structures and biogenetic analysis of lipofuscin bis-retinoids

Abstract

Age-related macular degeneration (AMD) is still an incurable blinding eye disease because of complex pathogenic mechanisms and unusual diseased regions. With the use of chemical biology tools, great progress has been achieved in improving the understanding of AMD pathogenesis. The severity of AMD is, at least in part, linked to the non-degradable lipofuscin bis-retinoids in retinal pigment epithelial (RPE). This material is thought to result from the lifelong accumulation of lysosomal residual bodies containing the end products derived from the daily phagocytosis of rod outer segments by RPE cells. Here, we present previously recognized bis-retinoids with focus on structures and biosynthetic pathways. In addition to a brief discussion on the mutual conversion relationships of bis-retinoids, future perspectives and the medical relevance of such studies on these lipofuscin constituents are also highlighted.

Fig. 1

Retinoid cycle and bis-retinoid formation of lipofuscin in the eye According to the proposed scheme that was published previously (Sparrow et al., 2010d), we here expand the schematic diagram regarding the formation of lipofuscin bis-retinoids; the leading feature is the formation of a novel bis-retinoid compound N-retinylidene-N-retinyl-glycerophosphoethanolamine (A2-GPE) (Yamamoto et al., 2011). Biogenesis of the latter starts with one molecule of all-trans-retinal incubated with GPE, followed by a [1,6]H-shift to give rise to N-retinylidene-glycerophosphoethanolamine (NR-GPE). Via a multi-step cascade, a proposed intermediate, dihydropyridinium A2-GPE is generated, and undergoes auto-oxidation to yield A2-GPE. Phosphate hydrolysis of the latter by phospholipase D (PLD) produces N-retinylidene-N-retinyl-ethanolamine (A2E), indicating that A2-GPE, like N-retinylidene-N-retinyl-phosphatidylethanolamine (A2PE), may also serve as the precursor of A2E

Fig. 2

Correlation of the ABCA4 transporter with lipofuscin bis-retinoid biosynthesis All-trans-retinal (ATR) comes from the isomerization of 11-cis-retinal that is released from light-activated rhodopsin in the retina. Incubation of ATR with phosphatidylethanolamine (PE)/glycerophosphoethanolamine (GPE) would give rise to N-retinylidene-phosphatidylethanolamine (NR-PE)/N-retinylidene-glycerophosphoethanolamine (NR-GPE) that can be delivered to the cytoplasmic side of disk membranes by ABCA4. When the activity of this transporter attenuates or becomes extinct, NR-PE and NR-GPE have no choice but to reside in the rod outer segments, and react with a second molecule of ATR to form various bis-retinoids including N-retinylidene-N-retinyl-phosphatidylethanolamine (A2PE), N-retinylidene-N-retinyl-dihydropyridine-phosphatidylethanolamine (A2-DHP-PE), N-retinylidene-N-retinyl-glycerophosphoethanolamine (A2-GPE), ATR dimer, and ATR dimer-PE via a multi-step cascade. Rod outer segments are routinely discarded on a daily basis, and subsequently are phagocytosed by RPE cells, resulting in the transfer of these di-retinal pigments to RPE where PLD, a lysosomal acid enzyme, can cleave A2PE, A2-GPE, A2-DHP-PE, and ATR dimer-PE to produce N-retinylidene-N-retinyl-ethanolamine (A2E), N-retinylidene-N-retinyl-dihydropyridine-ethanolamine (A2-DHP-E), and ATR dimer-E, respectively. However, ATR dimer remains intact due to the inability to be further digested by the lysosomal enzymes

Fig. 3

Eleven bis-retinoids associated with formation of retinal pigment epithelial (RPE) lipofuscin Structures, UV-visible absorbance (nm), and electronic transition assignments (↔) are shown. All of these fluorescent di-retinal pigments have dual conjugation systems located in two side-arms, conferring light-induced absorbance in both the ultraviolet and visible regions

Fig. 4

Biosynthetic pathways of N-retinylidene-N-retinyl-ethanolamine (A2E) and N-retinylidene-N-retinyl-dihydropyridine-phosphatidylethanolamine (A2-DHP-PE) These two bis-retinoids form from a proposed uniform intermediate, dihydropyridinium N-retinylidene-N-retinyl-phosphatidylethanolamine (A2PE). The latter is enclosed by a rectangle and is not detectable in the eye and biomimetic chemical reactions, probably resulting from the facile auto-oxidation of a unique dihydropyridinium ring, from which one or double particular hydrogen is eliminated with the electronic rearrangements. Hydrogens in blue and green represent the active atoms involved in the shifting and eliminating processes in the biosynthesis of these two heterocyclic fluorophores (Note: for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article)

Fig. 5

Biosynthetic pathway of N-retinylidene-N-retinyl-glycerophosphoethanolamine (A2-GPE) N-retinylidene-glycerophosphoethanolamine (NR-GPE) and dihydropyridinium A2-GPE, like N-retinylidene-phosphatidylethanolamine (NR-PE) and dihydropyridinium N-retinylidene-N-retinyl-phosphatidylethanolamine (A2PE) in the N-retinylidene-N-retinyl-ethanolamine (A2E) biosynthetic route, ought to play important roles in the biogenesis of A2-GPE. Due to the susceptibility of the dihydropyridinium ring to auto-oxidation, it is concluded that dihydropyridinium A2-GPE, like dihydropyridinium A2PE, is not found in the eye and biomimetic chemical reactions, from which NR-GPE is, however, possible to be trapped by high-performance liquid chromatography (HPLC)

Fig. 6

Biosynthetic pathways of all-trans-retinal (ATR) dimer, ATR dimer-E, and ATR dimer-PE It is thought that ATR dimer series, N-retinylidene-N-retinyl-ethanolamine (A2E) and N-retinylidene-N-retinyl-dihydropyridine-phosphatidylethanolamine (A2-DHP-PE), form from the same intermediate N-retinylidene-phosphatidylenamine (see structure in red). Also, it is interesting to know that ATR dimer-E (λ _max 290, 510 nm) and saturated ATR dimer-PE (λ _max 290, 510 nm) exhibited a visible absorption maximum of about 80 nm red shifted from that of ATR dimer (λ _max 290, 430 nm) because of a protonated Schiff base linkage formed in the position of aldehyde moiety (Note: for interpretation of the references to color in this figure legend, the reader is referred to the web version of this article)

Fig. 7

Diagram of mutual conversion relationships among some bis-retinoids The symbols (?) represent the potential conversion relationships amongst dihydropyridinium N-retinylidene-N-retinyl-phosphatidylethanolamine (A2PE), N-retinylidene-N-retinyl-dihydropyridine-phosphatidylethanolamine (A2-DHP-PE), A2PE, N-retinylidene-N-retinyl-dihydropyridine-ethanolamine (A2-DHP-E), and N-retinylidene-N-retinyl-ethanolamine (A2E), for which more evidence is needed