Targeted lipidomic in elucidating the pathophysiology of age-related macular degeneration

Abstract

Age-related macular degeneration (AMD) is one of the most common ocular disease in elderly and the leading cause to visual impairment among developed countries. Due to the gradual degradation of retinal epithelial cells and photoreceptors around the central region of the retina (i.e. the macula), this disease is characterized with blurring and partial loss of central vision. Decades of research have shown that the pathogenesis of AMD is both complex and multifactorial, with its etiology remains to be elucidated. This thesis aims to expand the understanding of AMD pathophysiology through a new perspective in lipid research. The development and validation of a liquid chromatography time-of-flight tandem mass spectrometry (LC-QTOF-MS/MS) quantitation method for multiple lipid oxidation products demonstrates that the protocol has excellent linearity, selectivity, precision and reproductivity. Additionally, the availability of the complete MS/MS spectrum is invaluable for future analysis.

The metabolic changes of AMD (particularly lipid mediators) were evaluated in human case-control study (n=99 AMD subjects vs n=198 age-matched healthy controls) and in vitro retinal epithelial cell treatments using targeted lipidomic approach. First, we demonstrated a relationship between dietary habits, plasma fatty acids and carotenoids levels, and the occurrence of AMD in Hong Kong Chinese adults. The results suggest exudative AMD to be associated with lower circulatory levels of carotenoids and omega-3 polyunsaturated fatty acids (PUFA), and higher level of omega-6 PUFAs, oleic acid and short chain fatty acids. Through a food frequency questionnaire, AMD subjects had less omega-3 PUFA rich food (e.g. seafood and nuts) intake and higher saturated fat (e.g. meat) intake in the diet.

Secondly, we found that a high plasma omega-6/omega-3 PUFA ratio and low carotenoids level were related to the prevalence of oxidized omega-6 PUFA metabolites and down-regulation of oxidized omega-3 PUFA metabolites. All these aforementioned factors contributed to the development of exudative AMD and further enhanced the understanding of lipid metabolism and the importance of balanced nutrition in the pathogenesis of exudative AMD.

Thirdly, in vitro ROS-stressed retinal epithelial cells showed depleted cellular DHA and enhanced pro-inflammatory mediators derived from PUFA oxidation. On the other hand, a combination of lutein + zeaxanthin with DHA provided cellular protection by restoring retinal DHA and reduced the production of pro-inflammatory omega-6 PUFA mediators through PPAR and ALOX-5 inhibition and enhanced the production of glutathione and anti-inflammatory mediators of omega-3 PUFAs.

Together, in this thesis, we demonstrated the metabolic changes through the assessment of fatty acids and lipid mediators of AMD as represented by in vitro cell study and in vivo human study using targeted lipidomics. Particularly, we also identified the dietary impact of omega-3 PUFA and carotenoids on exudative AMD. These observations provided a background for future prospective studies using dietary supplements to regulate and to prevent the progression of AMD.