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William C Gordon, Karin A Zemski Berry, Robert C Murphy, Nicolas G Bazan; MALDI imaging mass spectrometry of lipids in age-related macular degeneration. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5234.
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The purpose of this study was to identify and localize lipid species in normal and AMD human retinas using MALDI imaging mass spectrometry.
Sections from human eyes were obtained and some H&E-stained for orientation. After matrix deposition on unstained sections, images were acquired with a mass spectrometer with an orthogonal MALDI source, and MALDI mass spectra obtained. Images were obtained with Analyst software and TissueView. Collisional ion activation occurred after imaging on the same section. LC-MS/MS analysis was performed on retina samples to identify very long chain polyunsaturated fatty acyl groups (VLC-PUFAs).
The positive ion MALDI mass spectra of normal and AMD retinas was dominated by phosphatidylcholine (PC) molecular species. The positive ion MALDI images of common PC lipids, eg PC(34:1), indicated their presence in all retina layers. Some PCs, eg PC(38:4), were present in ganglion cells, while DHA-containing PCs were highly localized in photoreceptors. VLC-PUFA-containing PCs were present and localized in photoreceptors with MALDI imaging. The MALDI positive ion mass spectrum of AMD retinas indicated a large ceramide signal as compared to controls. MALDI imaging of ceramides in controls localized them to the retinal pigment epithelium (RPE), while AMD images showed ceramides to be present in RPE and the inner and outer retina. Negative ion MALDI mass spectra of normal and AMD retinas localized many phosphatidyl-serines (PS), -ethanolamines (PE), and -inositols (PI) throughout the inner and outer retinal layers but not specifically at any one site. These images indicated that certain lipids, eg PE(18:0p/22:6), were highly localized in ganglion cells while PE(18:0/22:6) was only found in photoreceptors.
MALDI imaging demonstrated unique localization of certain lipids in both normal and AMD retinas, but their distribution was not altered in normal vs AMD retinas. However, some lipids were elevated in the positive ion MALDI mass spectra of AMD retinas and showed unique localization compared to controls. DHA and VLC-PUFAs were identified. Ceramide localization in both RPE and photoreceptors of AMD retinas but, only in RPE of control retinas, was particularly intriguing since the production and accumulation of bioactive ceramides has been shown to be linked to the initiation of apoptotic cell death of photoreceptors, which may lead to AMD.
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