Jane M. Deeley

Imaging of Human Lens Lipids by Desorption Electrospray Ionization Mass Spectrometry

The lipid composition of the human lens is distinct from most other tissues in that it is high in dihydrosphingomyelin and the most abundant glycerophospholipids in the lens are unusual 1-O-alkyl-ether linked phosphatidylethanolamines and phosphatidylserines. In this study, desorption electrospray ionization (DESI) mass spectrometry-imaging was used to determine the distribution of these lipids in the human lens along with other lipids including, ceramides, ceramide-1-phosphates, and lyso 1-O-alkyl ethers. To achieve this, 25 μm lens slices were mounted onto glass slides and analyzed using a linear ion-trap mass spectrometer equipped with a custom-built, 2-D automated DESI source. In contrast to other tissues that have been previously analyzed by DESI, the presence of a strong acid in the spray solvent was required to desorb lipids directly from lens tissue. Distinctive distributions were observed for [M + H]+ ions arising from each lipid class. Of particular interest were ionized 1-O-alkyl phosphatidylethanolamines and phosphatidylserines, PE (18:1e/18:1), and PS (18:1e/18:1), which were found in a thin ring in the outermost region of the lens. This distribution was confirmed by quantitative analysis of lenses that were sectioned into four distinct regions (outer, barrier, inner, and core), extracted and analyzed by electrospray ionization tandem mass spectrometry. DESI-imaging also revealed a complementary distribution for the structurally-related lyso 1-O-alkyl phosphatidylethanolamine, LPE (18:1e), which was localized closer to the centre of the lens. The data obtained in this study indicate that DESI-imaging is a powerful tool for determining the spatial distribution of human lens lipids.

Unsuccessful ageing: dramatic changes to the human lens lipidome with age

Purpose: In peripheral vision, the natural pupil is elliptical and Zernike circular aberration polynomials cannot be directly used to describe the associated wavefront. We compare two methods for solving this problem. Methods: The stretched elliptical approach uses an elliptical pupil matching actual pupil shape. This is stretched along its minor axis to become a circle, allowing circular polynomials to be used. Another approach involves circular pupils with diameter matching either the larger or smaller axis of the elliptical pupil, again with circular polynomials. We investigated advantages and disadvantages of these alternatives using real and model eye data. Results: For 5-mm entrance pupils, the approaches give similar aberration coefficients to 20° from fixation: beyond this, they depart considerably. Advantages of the stretched elliptical relative to the circular pupil approach include: more physiological; aberrations include all information from the true pupil (may not be true of circular pupils smaller than the elliptical pupil); root-mean-squared aberrations are given directly from aberration coefficients; it is compatible with optical design programs. Its disadvantages include: greater mathematical complication; distortion of wavefront shape by stretching; comparing coefficients for different angles may not be valid; and MTFs and PSFs are distorted if derived directly from wave aberration coefficients. There are ways to compensate for some of the disadvantages of each of the methods. Conclusions: In spite of theoretical merits of the stretched elliptical pupil approach, the simplicity of the circular pupil approach and the ease with which its results can be understood by clinicians may make it preferable. AOVSM Free Papers Abstracts