Very-long-chain polyunsaturated fatty acids in the retina: analysis and clinical relevance in physiological and pathological conditions

The retina is one of the vertebrate tissues with the highest content in polyunsaturated fatty acids (PUFA). A large proportion of retinal glycerophospholipids, especially those found in photoreceptor membranes, are dipolyunsaturated molecular species. Among them, dipolyunsaturated phosphatidylcholine (PC) molecular species are known to contain very-long-chain polyunsaturated fatty acids (VLC-PUFA) from the n-3 and n-6 series having 24-36 carbon atoms and four to six double bonds. Recent interest in the role played by VLC-PUFA arose from the findings that a protein called Elongation of very-long-chain fatty acids 4 (ELOVL4) is involved in their biosynthesis and that mutations in the ELOVL4 gene are associated with Stargardt-like macular dystrophy (STGD3), a dominantly inherited juvenile macular degeneration leading to vision loss. In this context, PC molecular species containing VLC-PUFA in retina must be precisely characterized to improve our understanding of the pathogenesis of STGD3. For 25 years, several current approaches using gas chromatography or gas chromatography-mass spectrometry have been used for the characterisation and quantification of VLC-PUFA. These methods allowed characterizing with precision the structures (carbon chain length and double bond positions) of the different VLC-PUFA in bovine retina. Thus, retinal C28-C36 fatty acids are polyunsaturated and belong to the n-3 and n-6 families. The n-6 family of C28-C36 fatty acids has four or five double bonds, while the n-3 family of VLC-PUFA contains five or six double bonds. Moreover, some studies allowed determining the position of VLC-PUFA in the VLC-PC species. But most of these conventional approaches are time-consuming, requiring successive extraction, chromatographic steps and often a derivatization step before. Recently, a normal LC-ESI-MS/MS method was proposed for the structural characterization and the quantification of VLC-PC species in retinas from bovines and human donors. It directly analyses phospholipids as intact molecules and preserves the information based on the relative position of acyl radicals on the glycerol backbone. Since the results from molecular and biochemical studies led to the conclusion that VLC-PUFA are involved in the pathogenesis of STGD3, this specific and accurate method may be useful to more precisely investigate the molecular mechanisms leading to photoreceptor dysfunction and death in STD3.